CATALYSTS FOR THE OXIDATIVE DEHYDROGENATION OF HYDROCARBONS

Abstract: The present invention provides a catalyst for the oxidative dehydrogenation of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin. In one embodiment, the catalyst includes a nonstoichiometric rare earth oxycarbonate of the formula MxCyOz having a disordered and/or defect structure, wherein M is at elast one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; X = 2; Z = 3 + AY; A is less than about 1.8, and Y is the number of carbom atoms in the oxycarbonate. When used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above bout 100 psig. the catalyst has a selectivity of at least about 40 percent to at least one higher hydrocarbon and/or lower olefin. Methods for preparing catalysts taught by the invention and processes for using the catalysts for the oxidative dehydrogenation of lower hydrocarbons are also provided.

Description: CATALYSTS FOR THE OXIDATIVE DEHYDROGENATION OF HYDROCARBONS This invention relates, in general, to the oxidative dehydrogenation of hydrocarbons. More particularly, the present invention relates to rare earth catalysts that provide unusually high selectivity to higher hydrocarbons and/or lower olefins when used for the oxidative dehydrogenation of a lower hydrocarbon at elevated pressure. Accordingly, the rare earth catalysts of the invention are particularly useful for coupling methane by oxidative dehydration to form ethane, ethylene and higher hydrocarbons, and for the oxidative dehydrogenation of ethane to form ethylene. Methane is an attractive raw material because it is widely available and inexpensive, but it is used mainly as a fuel. Natural gas liquids (ethane, propane, butane and higher hydrocarbons) are the major raw material for ethylene and propylene, from which many petrochemicals are produced. But the supply of natural gas liquids has not kept pace with increasing demand for olefins, so more costly cracking processes that use naphtha from petroleum are being commercialized. Therefore, the development of economical processes for manufacturing olefins and other hydrocarbons from methane is highly desirable. Methane has low chemical reactivity, so severe conditions are required to convert it to higher hydrocarbons. Oxidative dehydrogenation is favored because conversion is not thermodynamically limited and reactions are exothermic. But selectively producing ethylene, ethane, and higher hydrocarbons by partial oxidation while avoiding complete oxidation to carbon oxides is difficult to achieve. Accordingly, those skilled in the art have expended much effort in attempts to develop selective catalysts for methane coupling. Rare earth oxycarbonate and oxide catalysts have been of particular interest. U. S. Patent No. 4,929,787 discloses a catalyst for oxidative coupling that contains at least one rare earth metal carbonate, which is defined to include simple carbonates and oxycarbonates and which comply approximately with the stoichiometric formulas M 2 (CO 3 ) 3 , M 2 O 2 CO 3 , M 2 O(CO 3 ) 2 , or M(OH)(CO 3 ), which may be characterized by elementary analysis, where M is at least one rare earth metal. The rare earth oxycarbonates, M 2 O 2 CO 3 , are preferred, with lanthanum oxycarbonate, La 2 O 2 CO 3 , being most preferred. Only lanthanum, neodymium, and samarium are used in the examples. The catalysts may be prepared in several ways by thermal decomposition of a rare earth metal compound: carbonates may be directly decomposed; hydroxides, nitrates, carbonates, or carboxylates may be added to a solution of poly carboxylic acid (citric), dried, and roasted under vacuum or in air; carbonates, hydroxides, or oxides may be added to an acid (acetic), dried, and decomposed in air; carbonates or carboxylates (acetates) may be dissolved into aqueous carboxylic acid (formic or acetic), impregnated onto a carrier, and heated in air; or oxides may be contacted with carbon dioxide. These methods all specify decomposing the precursors at a temperature of 300° to 700°C, but the examples all use 525° to 600°C. The decomposition may be done outside or inside the reactor before passing the reacting gas mixture over the catalyst. In one example, the La 2 θ 2 CO 3 catalyst was prepared by heating at 120°C an acetic acid solution containing lanthanum acetate, reducing the volume of the solution by aspiration, drying the material at 150°C under high vacuum, crushing the resultant foam to fine powder, and roasting the powder in air at 600°C for two hours. In another example, the reactor was charged with anhydrous lanthanum acetate and treated with helium at 525°C for one hour to form the La 2 θ 2 CO 3 catalyst. The catalyst may also contain one or more alkaline earth metal (Be, Mg, Ca, Sr, Ba) compounds to improve selectivity and a Group IVA metal (Ti, Zr, Hf) to increase activity. The reaction temperature specified is 300° to 950°C, preferably 550° to 900°C; the examples are mainly at 600° to 750°C, but the catalysts are selective at temperatures exceeding 900°C as well. The reaction pressure specified is 1 to 100 bars, particularly 1 to 20 bars, but the examples are all at atmospheric pressure. Carbon dioxide may be beneficially added (up to 20 percent) to the reaction gases as a diluent to increase yield by moderating the bed temperature and as a constituent to maintain a high activity of the carbonate catalyst. These catalysts are utilized in the related processes disclosed in U. S. Patent Nos. 5,025,108 and 5,113,032. The effect of reaction pressure on a catalyst disclosed in U. S. Patent No. 4,929,787 was studied in M. Pinabiau-Carlier, et al., "The Effect of Total Pressure on the Oxidative Coupling of Methane Reaction Under Cofeed Conditions", in A. Holmen, et al., Studies in Surface Science and Catalysis, 61, Natural Gas Conversion, Elsevier Science Publishers (1991). The catalyst (A) was a mechanical mixture of lanthanum oxycarbonate and strontium carbonate that was calcined in air at 600°C for two hours. Increasing the pressure substantially decreased the selectivity to C 2 + hydrocarbons (reaction temperature of 860°C) from 72 percent at 1 bar to 39 percent (constant flow rate) or 35 percent (increased flow rate for constant conversion) at 7.5 bar (94 psig). Another catalyst (B) was a magnesia support impregnated with aqueous lanthanum and strontium nitrates and then calcined at 800°C for two hours. This calcination temperature is above the maximum specified calcination temperature of 700°C disclosed in U. S. Patent No. 4,929,787 for producing oxycarbonate, and is a temperature at which predominantly lanthanum oxide, La^, is expected to form. The preparation furthermore did not include a carbon source from which oxycarbonate could be formed from the nitrate. Increasing the pressure significantly decreased the C 2 + selectivity (900°C) from 79 percent at 1.3 bar to 65 percent at 6 bar (72 psig) with constant flow rate. The study concluded that the reaction should be operated at pressures below 3 bar (29 psig). Clearly, there is a need for improved catalysts for the oxidative dehydrogenation of hydrocarbons and, in particular, for producing ethylene, ethane, and higher hydrocarbons from methane by oxidative dehydrogenation coupling. Such catalysts would provide high selectivity for oxidative dehydrogenation reactions and would enable these reactions to be carried out at elevated pressure instead of at atmospheric pressure. Improved catalysts would also have high activity at low temperature, operate at economical conversion levels, and remain stable during long-term operation. These catalysts must also be suitable for large-scale commercial production. The present invention meets the above-noted objects by providing, in one aspect, catalysts which are highly selective for the oxidative dehydrogenation of lower hydrocarbons to produce higher hydrocarbons and/or lower olefins. The invention further provides methods for preparing such catalysts and processes for using the catalyst in the oxidative dehydrogenation of lower hydrocarbons. As used herein, the term "lower hydrocarbon" includes lower alkanes (typically C r C 4 alkanes), alkyl aromatics (typically aromatics having C r C 4 alkyl appendages), and cyclic compounds. The term "higher hydrocarbon" means a hydrocarbon having a greater number of carbon atoms than the lower hydrocarbon which undergoes oxidative dehydrogenation (for example, the coupling of methane to form ethane, ethylene and other higher hydrocarbons). The term "lower olefin" refers to an olefin having the same number of carbon atoms as the lower hydrocarbon which undergoes oxidative dehydrogenation (for example, the oxidative dehydrogenation of ethane to form ethylene). In one embodiment, the catalyst taught by the invention comprises a nonstoichiometric rare earth oxycarbonate of the formula M x CγO z having a disordered and/or defect structure, wherein M is at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; X = 2; Z = 3 + AY; A is less than 1.8; and Y is ' the number of carbon atoms in the oxycarbonate. When used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, the catalyst has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. The catalyst may further comprise a cocatalyst containing at least one metal selected from V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. The cocatalyst may also include at least one alkali metal or alkaline earth metal. In another embodiment, a catalyst according to the invention comprises an oxycarbonate, hydroxycarbonate, and/or carbonate of at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm. When used for the oxidative dehydrogenation of a lower hydrocarbon, the catalyst exhibits higher selectivity to at least one higher hydrocarbon and/or lower olefin at a pressure above 100 psig than the catalyst or a precursor of the catalyst exhibits at a pressure in the range of atmospheric pressure to 25 psig. When operating at a pressure above 100 psig, the catalyst has a selectivity of at least 40 percent. In still another embodiment, the catalyst taught by the invention comprises: (1) an oxycarbonate, hydroxycarbonate and/or carbonate of at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. When used for the oxidative dehydrogenation of a lower hydrocarbon the catalyst has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. In yet another embodiment, the catalyst of the invention comprises: (1) an oxide of at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from V, Nb, Ta, Cr, Mo, W, Re, Fe, Co, and Ni. The catalyst, when used for the oxidative dehydrogenation of said lower hydrocarbon, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. Figure 1 is a plot of the elemental mole ratios of the catalyst Z/X (O/M) versus Z/Y (O/C), which compares measured ratios of nonstoichiometric oxycarbonate compositions of the present invention (parameter A less than 1.8) with stoichiometric oxycarbonates (or mixtures thereof) of the prior art (A = 2.0). Figure 2 is a schematic diagram illustrating order, disorder and defects, as well as crystalline vs. amorphous composition in a catalyst structure. Figure 3 is a schematic diagram illustrating disorder and defects at a catalyst surface. Figure 4 is a schematic diagram illustrating long range order in a catalyst structure which is not in accordance with the present invention for catalysts that do not exhibit such long-range order. Figure 5 is a plot of C 2 selectivity versus reaction temperature for the oxidative coupling of methane with a nonstoichiometric lanthanum oxycarbonate catalyst at a pressure of 125 psig. Figure 6 is a plot of C 2 selectivity and methane conversion versus time for long-term oxidative coupling of methane by a lanthanum oxycarbonate catalyst having manganese, tantalum, and antimony cocatalysts at a temperature of 575-600°C and pressure of 125 psig. Figure 7 is a plot of C 2 selectivity and methane conversion versus time for long-term oxidative coupling of methane by a lanthanum oxycarbonate catalyst having iron and Na 2 CO 3 cocatalysts with acetic acid treatment, at a temperature of 575-600°C and pressure of 125 psig. Figure 8 is a plot of C 2 + selectivity versus time for long-term oxidative coupling of methane by a lanthanum oxycarbonate catalyst having manganese and tungsten cocatalysts and supported by α-Al 2 O 3 either with binder (circles) or without binder (triangles) at a temperature of 550-600°C and pressure of 125 psig. Figure 9 is a plot of C 2 selectivity and ethylene/ethane ratio versus time for long- term oxidative coupling of methane by a sodium chloride-promoted lanthanum oxycarbonate catalyst at a temperature of 500°C and pressure of 125 psig. Figure 10 is a low resolution electron microscope micrograph of a nonstoichiometric lanthanum oxycarbonate catalyst prepared by treating lanthanum oxide with aqueous acetic acid at pH 4 and calcining the material at 400°C for one hour in flowing air (scale is 47 nm per cm). Figure 11 is a high resolution electron microscope micrograph of the catalyst in Figure 10 (scale is 1.1 nm per cm). Figure 12 is a plot of C 2 + C 3 selectivity versus reaction temperature for oxidative coupling of methane by a nonstoichiometric lanthanum oxycarbonate catalyst with an iron oxide cocatalyst. Figure 13 is a plot of C 2 + selectivity versus reaction temperature for oxidative coupling of methane by a nonstoichiometric lanthanum oxycarbonate catalyst with a manganese oxide cocatalyst. Figure 14 is a plot of C 2 + selectivity versus reaction temperature for oxidative coupling of methane by a conventional lanthanum oxide catalyst not in accordance with the present invention. The catalysts and processes of the present invention are used for the oxidative dehydrogenation of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin. They are particularly suitable for the oxidative dehydrogenation coupling of methane to form ethylene, ethane, and higher hydrocarbons such as propylene, propane, and other higher alkanes and olefins, which are produced in progressively lesser amounts as the carbon number increases. Ethylene and ethane are therefore the main products from methane coupling, but significant amounts of propylene and propane can also be produced. Other hydrocarbons may also be used as the feedstock, such as ethane to produce butylene and butane, or propane to produce hexene and hexane, or a mixture of hydrocarbons may be used, such as natural gas (typically a mixture of 90+ percent methane, and the balance being ethane, propane and butane), or a mixture of ethane and propane. In the case where the higher hydrocarbon is an olefin, that is, an olefin having a higher number of carbon atoms than the lower hydrocarbon undergoing oxidative dehydrogenation, it should be understood that the olefin can be formed directly from the lower hydrocarbon or in a secondary oxidative dehydrogenation reaction. For example, where the lower hydrocarbon is methane, ethylene can be formed directly from methane via oxidative dehydrogenation. Alternatively, ethane is formed first in a coupling reaction, and then the ethane undergoes a secondary oxidative dehydrogenation reaction to form ethylene. As noted above, the catalysts of the present invention are also useful for the oxidative dehydrogenation of a lower hydrocarbon to form a lower olefin, that is, an olefin having the same number of carbon atoms as the lower hydrocarbon. Accordingly, the catalysts of the invention have particular utility for forming ethylene from ethane and propylene from propane. This is particularly advantageous because, in general, olefins such as ethylene and propylene are the most desired products. Thus, byproduct alkanes such as ethane and propane can be recycled and converted to the desired olefins. The hydrocarbon feedstock may be obtained from any suitable source. The hydrocarbon may be pure or present in a mixture, such as with other hydrocarbons, inert gases such as nifrogen and argon, and/or other components, such as water. Undesirable impurities, such as poisons for the catalyst, preferably are at low levels that permit economical operation of the oxidative dehydrogenation reaction. Undesirable impurities include hydrogen sulfide and other sulfur compounds, mercury, phosphorous and acetylenes. Inert gases should not be at excessive levels. Hydrogen and carbon monoxide are preferably present at low levels because they consume the reactant oxygen to undesirable H 2 O and CO 2 . Although carbon dioxide may be present, it is preferably at a low level below 5 percent by volume, more preferably below 2 percent, because carbon dioxide decreases reaction selectivity with some of the catalysts of the present invention. When the hydrocarbon is methane, the methane may be obtained from any suitable source, such as natural gas, refinery gas, and synthetic natural gas, preferably with methane being the primary component. Processed natural gas is preferred because impurities are at acceptably low levels. The processed natural gas may be used without removing ethane, propane, and higher hydrocarbons. The necessary oxygen may be obtained from any suitable source, including without limitation, oxygen, ozone, and oxides of nitrogen. Preferably, oxygen is used to carry out the reaction. The O 2 may be fed at any concentration by mixing with N 2 He, or other inert gases. A convenient and safe source of oxygen is air. High purity oxygen from an oxygen plant or oxygen-enriched air may also be used as the source of this reactant. First Catalyst Embodiment One embodiment of the catalyst taught by the invention comprises a nonstoichiometric rare earth oxycarbonate of the formula M x C Y O z having a disordered and/or defect structure, wherein M is at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; X = 2; Z = 3 + AY; A is less than 1.8; and Y is the number of carbon atoms in the oxycarbonate. When used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, the catalyst has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. The nonstoichiometric rare earth oxycarbonate catalyst of the formula M x C Y O z , wherein X = 2, C is carbon, and O is oxygen, can be formed conceptually from the corresponding rare earth oxide, M 2 O 3 , according to the following equation. M 2 O 3 + Y CO A - M 2 C Y O 3+AY The parameter value of A = 2 generates all of the stoichiometric rare earth oxycarbonate compounds, mixtures, and intermediates of the prior art as the parameter Y increases from zero, according to the following equation. M 2 O 3 + Y CO 2 -> M 2 C Y O 3+2Y This corresponds to adding carbon dioxide in increasing amounts to the rare earth oxide. This is a standard method of preparing the stoichiometric rare earth oxycarbonate compounds, as well as mixtures of the stoichiomefric oxycarbonates with each other and the oxide, as is known to one skilled in the art. The parameter value of Y = 1 generates the rare earth dioxymonocarbonate, M 2 O 2 CO 3 ; Y = 2 generates the monooxydicarbonate, M 2 O(CO 3 ) 2 ; and Y = 3 generates the carbonate, M 2 (CO 3 ) 3 , all of which, as used herein, are considered to be stoichiometric rare earth oxycarbonates, according to the following equations. M 2 O 3 + CO 2 -» M 2 CO 5 (M 2 O 2 CO 3 ) Dioxymonocarbonate M 2 O 3 + 2 CO 2 - M 2 C 2 O 7 (M 2 O(CO 3 ) 2 ) Monooxydicarbonate M 2 O 3 + 3 CO 2 -» M 2 C 3 O 9 (M 2 (CO 3 ) 3 ) Carbonate Noninteger values of Y < 3 correspond to mixtures of the oxycarbonates with each other or with the oxide. For example, Y = 0.5 is an equimolar mixture of dioxymonocarbonate and oxide, Y = 1.5 is an equimolar mixture of dioxymonocarbonate and monooxydicarbonate, and Y = 2.5 is an equimolar mixture of monooxydicarbonate and carbonate. Other mixtures are also possible. Values of Y > 3 correspond to carbonate containing excess carbon dioxide. In contrast, the rare earth oxycarbonate catalysts of the present invention are nonstoichiometric compounds having the parameter A less than 1.8. , The parameter value of A = 1 corresponds to nonstoichiometric oxycarbonates being formed conceptually by the addition of carbon monoxide to the rare earth oxide, according to the following equation. M 2 O 3 ÷ Y CO → M 2 C Y O 3+Y Similarly, the parameter value of A = 0 corresponds to adding carbon to the oxide, according to the following equation. M 2 O 3 + Y C - M 2 C Y O 3 Increasing values of the parameter Y correspond to adding increasing amounts of carbon monoxide or carbon to the oxide. Noninteger values of 0 < A < 1.8 correspond to adding a mixture of carbon monoxide, carbon dioxide, and/or carbon to the oxide. For example, A = 1.5 corresponds to adding an equimolar mixture of carbon monoxide and carbon dioxide to the oxide, whereas A = 0.5 corresponds to adding an equimolar mixture of carbon monoxide and carbon to the oxide. Other mixtures are also possible. The parameter A for a given nonstoichiometric oxycarbonate can be readily calculated from measured values of the elemental ratios O/M and O/C for the material according to the following equation. A = [(O/M) - 1.5] (O/C) / (O/M) Examples of measured compositions of nonstoichiometric oxycarbonate catalysts of the present invention (parameter A < 1.8) are given in Figure 1 as a plot of the elemental mole ratios Z/X (O/M) versus Z/Y (O/C). The compositions are for the rare earths lanthanum and gadolinium and were prepared by the methods of the present invention. They include catalysts both as prepared and after reaction, and also without and with a cocatalyst. The overall average parameter value is A = 1.08, so on average the nonstoichiometric oxycarbonate compositions correspond approximately to adding carbon monoxide in different amounts to the rare earth oxide. The compositions of the present invention are also compared with the stoichiometric oxycarbonates, or mixtures thereof, of the prior art (curve with A = 2.0), which in contrast correspond to adding carbon dioxide in different amounts to the rare earth oxide. Therefore the nonstoichiometric compositions are richer in carbon and deficient in oxygen compared to the stoichiometric oxycarbonates. Figure 1 also shows curves corresponding to parameter A values of 1.5, 1.0, 0.5, and 0. The curves approach the composition of the rare earth oxide in the limit of very large ratio of Z/Y, as the parameter Y goes to zero. In the opposite limit, as the parameter Y becomes very large, the curves asymptotically approach Z/Y = A. The nonstoichiometric oxycarbonate catalysts of the present invention preferably have a parameter A value less than 1.7, more preferably less than 1.6, still more preferably less than 1.5, and most preferably less than 1.3. The parameter A value preferably is greater than 0.2, more preferably greater than 0.4, still more preferably greater than 0.5, and most preferably greater than 0.7. The parameter Y is preferably in the range of 0.5 to 10, more preferably in the range of 0.6 to 8, still more preferably in the range of 0.8 to 6, and most preferably in the range of l to 4. Preferably the ratio Z/X is in the range of 1.5 to 4.5 and the ratio Z/Y is in the range of 1.0 to 6.0. When the parameter A is in the range of 0.4 to 1.6, preferably the ratio Z/X is less than 3.75 and the ratio Z/Y is in the range of 1.5 to 4.5. When the parameter A is in the range of 0.5 to 1.5, preferably the ratio Z/X is less than 3.5 and the ratio Z/Y is in the range of 1.75 to 4.25. The elemental mole ratios Z/X (O/M) and Z/Y (O/C) of the catalyst may be measured by using electron energy loss specfroscopy (EELS) on a scanning transmission electron microscope, which is known to one skilled in the art. This technique was used to determine measured values in Figure 1 by crushing the catalyst sample and collecting between 10 and 50 individual spectra from each sample in order to obtain a representative average sampling of the material. The individual spectra measurements exhibit variation in the elemental ratios that is reflective of variation in the nonstoichiometric composition within the catalyst material. As used herein, the elemental mole ratios Z/X and Z/Y, and therefore values of parameter A, are understood to mean values that are representative of the catalyst material. The elemental mole ratios may also be determined by using wave-length dispersion x-ray fluorescence, x-ray photoelectron specfroscopy, or other methods known to those skilled in the art. As used herein, it is understood that the nonstoichiometric oxycarbonate catalysts of the present invention, in addition to the at least one rare earth element, carbon, and oxygen, may also contain hydrogen as a secondary component, including but not limited to such forms as hydroxyl or hydroxide groups, -CH X groups, and hydrides. Hydrogen may become incorporated into the catalyst from water during preparation, from oxidative reaction of the hydrocarbon, or as a remnant of starting materials. The catalyst may also contain halogen as a secondary component, especially as a consequence of optionally feeding trace quantities of halocarbons to enhance olefin formation. The catalyst may also contain impurities present in starting materials. The nonstoichiometric rare earth oxycarbonate catalysts of the present invention have a disordered and/or defect structure. All materials of commercial interest, with exceptions such as diamonds and semiconductors, are disordered at some level. One limit is perfect single crystals that contain no structural or chemical defects, disruptions, or randomness and therefore are considered to be perfectly ordered. The other limit is a perfectly random structure, such as a glass, that is completely amorphous. In between these limits lies the region that at some level is disordered. The degree of disorder is related to the structure and chemistry of the material and the frequency with which disruptions and randomness occur in the perfect structure and chemistry. Long range order is typically ascribed to structures that lack disruptions and randomness for several hundreds or thousands of angstroms. Short range order typically refers to lacking disruptions and randomness for tens of angstroms. As used herein, the term "disordered structure" is understood to mean the absence of long range order in regions of the catalyst material. The frequency of the disruptions and randomness can vary from one location to another in the catalyst material, such that one location can have very few disruptions and have long range order and another location can have a high frequency of disruptions and randomness and be limited to short range order. A high frequency of disruptions and randomness can create a very disordered region with locations that have no order and are amorphous. As used herein, the term "defect structure" is understood to mean the presence of defects within regions of the catalyst material. The defects may be structural defects and/or chemical defects and include, but are not limited to, the following types of defects, which are known to those skilled in the art: grain boundaries, stacking faults, twin boundaries, inversion boundaries, crystallographic shear planes, antiphase phase boundaries, point defects (vacancies/interstitials), dislocations, shear planes, and polytypoids. Defects that cause disruption in the crystal structure can be readily observed in high resolution transmission electron micrographs. These are often, but not always, associated with changes in the local chemistry around the fault region. Point defects such as vacancies and interstitials are defects that cause nonstoichiometry; this form of chemical disorder cannot be readily distinguished visibly in micrographs. However, in structural terms, these disordered nonstoichiometric regions may appear to have long range order, because the vacancies do not necessarily disrupt the crystal structure. Electron diffraction can suggest the presence of nonstoichiometry and local chemical disorder, but only through quantitative chemical analysis can the nonstoichiometry be confirmed. Therefore, a full analysis of disorder of a material is based upon the chemical fluctuations within the material, which can be measured spectroscopically, and the frequency of disruption in the crystal structure, which can be observed visually in the high resolution transmission electron microscope. The extent of disorder is a subjective measure based on the frequency of structural disruptions and the chemical fluctuations. Disorder and defects in a catalyst structure as viewed in a high resolution electron microscope are illustrated in the schematic diagram of Figure 2. The series of parallel hatched lines represent the atomic planes of the crystal structure as viewed under a given crystallographic projection. The types of order in the material cover the spectrum from completely disordered (amorphous) regions to highly ordered (long range order) regions. The disordered regions frequently are present as an assemblage of nanocrystalline domains of various orientation, size, and degree of order. The diagram also illustrates several examples of structural defects: twin boundaries, stacking faults, grain boundaries, and dislocations. Disorder and defects at a catalyst surface are illustrated in the schematic diagram of Figure 3. The circles represent the atom columns. Amorphous regions, faults, and strain which result in lattice distortions and surface reconstruction are indicated as they may appear in a high resolution electron microscope image. Surfaces of the type illustrated do not possess long range order and exhibit disordered structure. Vacancies are indicated but they would not be readily apparent in the image unless a large vacancy cluster were present or an entire column of atoms were missing. In contrast, long range order in a catalyst structure which is not in accordance with the present invention as viewed in a high resolution electron microscope is illustrated in the schematic diagram of Figure 4. The series of parallel hatched lines represent the atomic planes of the crystal structure as viewed under a given crystallographic projection. In the compositions of the present invention, preferably at least 5 percent, more preferably at least 10 percent, still more preferably at least 20 percent, and most preferably at least 30 percent of the nonstoichiometric rare earth oxycarbonate catalyst has a disordered and/or defect structure, as shown by high resolution electron microscopy. The disordered structure of the catalyst preferably has short range order that is mainly limited to being less than 300 angstroms, more preferably less than 200 angstroms, and most preferably less than 100 angstroms. The defect structure of the catalyst preferably has a high spatial frequency of defects wherein the defects mainly occur more frequently than one defect per 300 angstroms, more preferably one defect per 200 angstroms, and most preferably one defect per 100 angstroms, as shown by high resolution electron microscopy. Unlike catalysts in the prior art, the catalysts of the present invention exhibit high selectivity for the oxidative dehydrogenation of lower hydrocarbons at elevated pressure. When used for the oxidative dehydrogenation of a lower hydrocarbon, the catalyst should have a selectivity to at least one higher hydrocarbon and/or lower olefin of at least 40 percent, preferably at least 45 percent, more preferably at least 50 percent, still more preferably at least 55 percent, and most preferably at least 60 percent when at a pressure above 100 psig. Unlike catalysts in the prior art, which can be utilized at the elevated temperatures that are necessary to obtain high activity at low pressure, which are generally in the range of 600° to 900°C, the nonstoichiometric rare earth oxycarbonate catalysts of the present invention have the property that the catalyst becomes unselective for the oxidative dehydrogenation reaction at elevated temperature, which generally occurs in the range of 600°C to 750°C, and frequently occurs in the range of 650°C to 700°C. As used herein, the term "unselective" is understood to mean that the selectivity of the oxidative dehydrogenation reaction is below 20 percent or decreases substantially. This property is illustrated for a nonstoichiometric lanthanum oxycarbonate catalyst in Figure 5 at an elevated pressure of 125 psig, which shows a relatively constant selectivity of 60-62 percent for temperatures of 400 to 550°C, a slight decline to 56 percent at 600°C, but a severe decline to 8 percent at 650°C when the temperature becomes too high and the catalyst becomes unselective. Furthermore, after the temperature is subsequently lowered from the elevated temperature, the catalyst has lower selectivity for the oxidative dehydrogenation reaction than prior to elevating the temperature, generally having a selectivity to at least one higher hydrocarbon and/or lower olefin that is substantially lower than 40 percent or is unselective. Without wishing to be bound by theory, it is believed that such elevated temperatures destroy the selective catalyst composition and structure, which are not regenerated by simply cooling the material. In addition to exhibiting high selectivity at elevated pressure, the catalysts of the present invention have been discovered to be able to maintain high selectivity for the long operating times that are necessary for commercial application. This long-term stability is illustrated in Figures 7-10 for four different catalysts of the present invention. Generally after an initial decline, selectivity asymptotically approaches a steady level over time. The temperature may be adjusted incrementally after a period of operation to reestablish a higher and/or more stable selectivity. Operating parameters such as flow rate may be similarly adjusted. Conversion and other reaction characteristics are similarly stable. This long-term stability is unlike prior art catalysts, which generally suffer from a decline in selectivity to low values over a relatively short time, which frequently occurs quite rapidly. When used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, the catalyst preferably maintains a selectivity to at least one higher hydrocarbon and/or lower olefin of at least 40 percent, more preferably at least 50 percent, for at least 7 days, more preferably for at least 14 days, still more preferably for at least 21 days, and most preferably for at least 28 days. Although not critical to the catalyst compositions of the present invention, higher catalyst surface area can be beneficial to producing higher selectivity. While not wishing to be bound by theory, it is believed that higher surface area can indicate greater disorder and a higher frequency of defects in the catalyst structure, which produces a higher concentration of active sites. This increases activity at lower temperature and can increase selectivity by depleting gas phase oxygen more quickly, which reduces unselective gas phase oxidation. Higher surface area can also promote heat transfer at the active sites, which keeps the catalyst surface cooler. The catalyst should generally have a surface area of at least 3 m 2 /g, preferably at least 5 m 2 /g, more preferably at least 10 m 2 /g, still more preferably at least 15 m 2 /g, and most preferably at least 20 m 2 /g. The catalysts of the present invention contain at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm. These rare earths have been discovered to produce catalysts having at least 40 percent selectivity at pressures above 100 psig. The rare earth element is preferably selected from La, Pr, Nd, Sm, Eu, Tb, and Tm, which have been discovered to produce high selectivity. The rare earth element is more preferably selected from La, Nd, Sm, Eu, and Tb, which produce the highest selectivity. The rare earth element is most preferably selected from La, Sm, and Tb. The rare earths Ce, Yb, and Lu may be used in combination with the aforementioned rare earths, but they produced low selectivity at elevated pressure when used by themselves. When the rare earth element is selected from La, Pr, Nd, Sm, and Eu, it has been discovered that the catalyst can have a porous microstructure that contains pore sizes in the range of 10 to 1000 angstroms. As used herein, the term "porous microstructure" is understood to mean that the catalyst structure contains a three-dimensional system or network of microscopic pores, channels, and/or voids. The term "pore size" is understood to mean the characteristic diameter or dimension of the microscopic pore, channel, or void. The porous microstructure can be observed and the pore size measured by using an electron microscope, particularly at high resolution. The tendency of the rare earths to form the porous microstructure morphology diminishes in the order of La, Pr, Nd, Sm, and Eu, from a maximum for La to a minimum for Eu. The porous microstructure has not been observed for rare earths beyond Eu. The porous microstructure generally is formed and remains stable only at temperatures below 650°C. The porous microstructure is preferably formed by calcination of a catalyst precursor, in an atmosphere that contains oxygen, at a temperature in the range of 300°C to 600°C, more preferably in the range of 400°C to 500°C. The porous microstructure generally does not form below 300°C. Catalysts that have become unselective by heating them to a temperature that is too high, which is generally above 700°C, show a collapse of the porous microstructure and possess an annealed (smoother) surface. The porous microstructure is desirable, but not critical, for forming catalysts having high surface areas above 20 m 2 /g, preferably above 30 m 2 /g. The typical pore size is preferably below 500 angstroms, more preferably below 300 angstroms, still more preferably below 200 angstroms, and most preferably below 100 angstroms. Electron microscope micrographs of nonstoichiometric lanthanum oxycarbonate catalysts which have a disordered and defect structure and which also have a porous microstructure are illustrated in Figures 10 to 11. The catalysts were prepared by treating lanthanum oxide with aqueous acetic acid at pH 4 and calcining the material in flowing air. The highly porous nature of the catalysts is illustrated by the low resolution image of Figure 11. A catalyst calcined at 400°C for one, four, and eight hours, which has an average parameter A value of 0.9, is shown in Figure 12, respectively. These high resolution images show that the materials are disordered and lack long range order, which is evident in the images as wavy or irregular lattice fringes, displace fringes, pockets of amorphous contrast, jogs in lattice fringes, moire fringes, and constantly varying image contrast. Amorphous contrast is often observed within pits on the surface. The frequency of structural faults is quite high, with defects occurring every 10 to 100 angstroms in locations, with some regions being amorphous. The porous microstructure is readily evident, with the typical diameter of the pores being between 50 to 100 angstroms. Where pores have not fully penetrated the material to form a hole or channel, the surface is pitted with voids. At a higher calcination temperature of 550°C, the disordered catalyst structure has become more ordered. The pores have also become better defined and faceting is preferred. At a high calcination temperature of 700°C, the disordered catalyst structure has become still more ordered. The pores are gradually disappearing, leaving ghost images of their location. The nonstoichiometric rare earth oxycarbonate catalyst may further comprise a cocatalyst containing at least one metal selected from V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. Cocatalysts containing at least one of these metals have been discovered to be beneficial for oxidative dehydrogenation of hydrocarbons at pressures above 100 psig. The benefits include increased selectivity, improved product distribution, lower operating temperature, and longer catalyst life. Different metals can provide different benefits, so using two or more metals can improve overall catalyst performance, which will depend upon the particular application. Without wishing to be bound by theory, it is believed that these cocatalyst metals stabilize the nonstoichiometric and disordered structure of the catalyst. As used herein, the term "cocatalyst" will be understood to include both materials that catalyze oxidative dehydrogenation as well as promoters that improve or modify catalyst performance. In addition to the aforementioned metals, the cocatalyst may contain additional elements, such as oxygen, carbon, halides, nitrogen, sulfur, phosphorous, as well as other metals, provided that they do not unsatisfactorily degrade catalyst performance. Suitable forms of the cocatalyst include but are not limited to oxides, carbonates, nitrates, phosphates, sulfates, halides, hydroxides, acetates. The cocatalyst is preferably an oxide or carbonate. The catalyst and/or cocatalyst may further comprise at least one alkali metal or alkaline earth metal, preferably at least one alkali metal, which have been found to be beneficial in suppressing combustion. In contrast, metals from Rh, Pd, Pt, Ag, and Au have been found to be generally unsuitable because they increase combustion, although they may be used in combination with other metals if desired. The cocatalyst preferably contains at least one metal selected from V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Sn, Pb, Sb, and Bi; more preferably at least one metal selected from Nb, Ta, W, Mn, Re, Fe, Pb, Sb, and Bi; and most preferably at least one metal selected from W, Mn, Fe, Pb, and Bi. Cocatalyst metals that improve selectivity include Mn, Fe, W, Pb, Bi, Nb, and Sb. Cocatalyst metals that improve catalyst life include Re, Mn, Bi, Fe, and Ta. Cocatalyst metals that give lower operating temperature include Bi, Sb, Fe, Mn, Re, Nb, and Ta. Although not critical to the catalyst composition of the present invention, the cocatalyst metal is preferably present in the catalyst in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.005 to 0.400, still more preferably in the range of 0.010 to 0.200, and most preferably in the range of 0.020 to 0.100. The optimal amount will depend upon the actual cocatalyst composition chosen and it will generally have to be determined by systematic experimentation. Suitable forms of the alkali metal or alkaline earth metal include but are not limited to halides, oxides, carbonates, hydroxides, nitrates. The alkali metal is preferably sodium, potassium, or cesium, most preferably sodium or potassium. Although lithium is particularly beneficial in prior art catalysts, it has been found to be detrimental with the catalysts of the present invention at elevated pressure, although it may be used if desired. The alkali metal compound is preferably selected from the group consisting of NaF, NaCl, NaBr, Nal, KC1, KBr, KI, CsCl, CsBr, Csl, sodium oxide, potassium oxide, cesium oxide, Na 2 CO 3 , K 2 CO 3 , CsCO 3 , NaNO 3 , KNO 3 , CsNO 3 , NaOH, KOH, and CsOH, and most preferably selected from NaCl, NaBr, KC1, sodium oxide, potassium oxide, Na 2 CO 3 , and K 2 CO 3 . The alkaline earth metal is preferably calcium, magnesium, or barium. Although strontium is particularly beneficial in prior art catalysts, it has been found to be ineffective or detrimental with the catalysts of the present invention, although it may be used if desired. The alkaline earth metal compound is preferably selected from CaCl 2 , MgCl 2 , BaCl 2 , calcium oxide, magnesium oxide, barium oxide, CaCO 3 , MgCO 3 , BaCO 3 , Ca(NO 3 ) 2 , Mg(NO 3 ) 2 , and Ba(NO 3 ) 2 . The at least one alkali metal or alkaline earth metal may be present as a compound with the at least one cocatalyst metal. Although not critical, the alkali metal or alkaline earth metal is preferably present in the catalyst in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.010 to 0.600, still more preferably in the range of 0.020 to 0.300, and most preferably in the range of 0.040 to 0.200. The optimal amount will depend upon the actual composition chosen and it will generally have to be determined by systematic experimentation. Excessively high levels are to be avoided because they can lower catalyst activity. Suitable combinations of cocatalyst components include but are not limited to Fe Na 2 CO 3 , K/Fe/SO 4 , W/N-^CO^ MnWO 4 , Pb/WO 4 , MnMoO 4 , Sn/ReO 4 , NajCr ,, Mn/Na 2 WO 4 , Na/MnWO 4 , Cs/Fe/WO 4 , Na/MnMoO 4 , Mn/Na-CrO 4 , K/Pb/ReO 4 , Rb/Pb/SO 4 , Na/Sb/ReO 4 , Mn Sb/TaO 3 , K/Bi/TaO 3 , Na/Ca/Fe/ReO 4 , K/Mn/Bi/NbO 3 , K Mg/Sn/PO 4 , Cs/Ca/Pb/PO 4 , Na/Mn/Bi/NbO 3 , K Ba/V/NbO 3 , K/Fe/Cr/ReO 4 , K Mn/Ni/ZrO 3 , Rb/Mg/Bi/ReO 4 , Rb/Fe/V/TaO 3 , Rb/Mn/Cr/MoO 4 , Cs/Ba/Bi/MoO 4 , Cs/Fe/Sb/NbO 3 , Cs/Mn/V/ReO 4 , K/Mg/Fe/ReO 4 , K/Mn/NaNbO 3 /Sb 2 O 3 , and Mn/Li/NaTaO 3 /Sb 2 O 3 . The form in which the cocatalyst is combined with the catalyst is not critical, provided that the combination is effective. The cocatalyst may be a surface deposit or intimately mixed with the catalyst material. The physical form of the catalyst is not critical to the compositions of the present invention. The catalyst may be a powder, pressed or pelletized powder, particulates, or a bulk or formed mass. The catalyst is preferably in a form that is suitable for use in a commercial reactor, as is known to one skilled in the art. The catalyst may further comprise a support material. Using a support material can be beneficial to shape the catalyst, to enhance physical properties, such as strength, durability, and abrasion resistance, and to utilize or disperse the catalyst material more efficiently, such as to reduce cost. Suitable support materials include but are not limited to α-alumina, γ-alumina, silica, titania, magnesia, calcium oxide, and zinc oxide. The support material may have a binder or be binderless. The supported catalyst preferably has a formed shape. Suitable formed shapes include spheres, pellets, rings, extrudates, monoliths. The manner in which the catalyst is combined with the support material is not critical, provided that the combination is effective, as is known to one skilled in the art. Second Catalyst Embodiment The present invention is also directed to a catalyst for the oxidative dehydrogenation of a lower hydrocarbon which comprises an oxycarbonate, hydroxycarbonate, and/or carbonate of at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm. When used for the oxidative dehydrogenation of a lower hydrocarbon, the catalyst exhibits higher selectivity to at least one higher hydrocarbon and/or lower olefin at a pressure above 100 psig than the catalyst or a precursor of the catalyst exhibits at a pressure in the range of atmospheric pressure to 25 psig. When operating at a pressure above 100 psig, the catalyst has a selectivity of at least 40 percent. The higher selectivity is preferably higher by at least 2 percentage points, more preferably by at least 4 percentage points, and most preferably by at least 6 percentage points. The higher selectivity typically occurs at a lower temperature when at the pressure above 100 psig than when at the pressure in the range of atmospheric pressure to 25 psig. The catalyst furthermore has the property that it becomes unselective for the coupling reaction at an elevated temperature, which typically occurs in the range of 600°C to 750°C, and after the temperature is subsequently lowered from the elevated temperature, the catalyst has lower selectivity for the oxidative dehydrogenation reaction than prior to elevating the temperature, usually having a selectivity that is substantially lower than 40 percent or is unselective. As before, the catalyst may also comprise a cocatalyst containing at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. The catalyst and/or cocatalyst may likewise further comprise at least one alkali metal or alkaline earth metal. In contrast to prior art catalysts, we have unexpectedly discovered catalysts that can actually produce higher selectivity at elevated pressure. The table below compares selectivities obtained for oxidative coupling of methane at 125 psig for a nonstoichiometric lanthanum oxycarbonate catalyst, with NaCl to reduce combustion, and the selectivities obtained at 25 psig for the lanthanum oxide catalyst precursor, with NaCl. Selectivity Selectivity Δ Selec. at Temperature at 25 psig at 125 psig 125-25 psig 500°C 1 percent 0 percent -1 550°C 5 percent 1 percent -4 600°C 18 percent 65 percent +47 650°C 35 percent 65 percent +30 700°C 49 percent 2 percent -47 The selectivity at 125 psig is considerably higher and the catalyst becomes unselective at 700°C, whereas the catalyst precursor at 25 psig remains selective. Figure 12 shows a plot of C 2 + C 3 selectivity versus temperature for oxidative coupling of methane by a nonstoichiometric lanthanum oxycarbonate catalyst with an iron oxide cocatalyst. When the catalyst precursor is reacted at atmospheric pressure, the selectivity increases continually with higher temperature from 45 percent at 450°C to 49 percent at 650°C. But when the catalyst is reacted at 125 psig, the selectivity is considerably higher, 57-58 percent, and is relatively constant over the temperature range of 450 to 590°C. But at 600°C, the temperature becomes too high, and the selectivity declines progressively at higher temperature until the catalyst becomes unselective at 650°C. For another comparison, a nonstoichiometric lanthanum oxycarbonate catalyst (parameter A of 1.0) with an iron oxide/Na 2 CO 3 cocatalyst, which was prepared by treating lanthanum oxide with iron nitrate, Na 2 CO 3 , and aqueous acetic acid at pH 4 and calcining it at 400°C, was reacted at both 125 psig and 15 psig over a wide range of temperature. The maximum selectivity at 125 psig was 61 percent at 500°C, but the maximum selectivity at 15 psig was only 54 percent at 650°C. Figure 13 shows a plot of C 2 + selectivity versus temperature for oxidative coupling of methane by a nonstoichiometric lanthanum oxycarbonate catalyst with a manganese oxide cocatalyst. When the catalyst precursor is reacted at atmospheric pressure, the selectivity passes through a maximum of 45 percent at 775°C, and gives no indication of becoming unselective at a higher temperature of 850°C. But when the catalyst is reacted at 125 psig, the selectivity is considerably higher, with a maximum of 58 percent, at a much lower temperature of 540°C. The comparison kept the gas composition and residence time approximately the same. For comparison, Figure 14 shows a plot of C 2 + selectivity versus temperature for oxidative coupling of methane by a conventional lanthanum oxide catalyst, which is not in accordance with the present invention. When the catalyst is reacted at atmospheric pressure, the selectivity increases with temperature and reaches 60 percent at 800°C. But at 125 psig, the catalyst is unselective with a very low selectivity of 5 percent. Third Catalyst Embodiment The present invention is also directed to a catalyst for the oxidative dehydrogenation of a lower hydrocarbon which comprises (1) an oxycarbonate, hydroxycarbonate and/or carbonate of at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. The catalyst, when used for the oxidative dehydrogenation of said lower hydrocarbon, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. The cocatalyst preferably contains at least one metal selected from Nb, Ta, W, Mn, Re, Fe, Pb, Sb, Bi, and most preferably at least one metal selected from W, Mn, Fe, Pb, and Bi. The cocatalyst metal is preferably present in the catalyst in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.005 to 0.400, still more preferably in the range of 0.010 to 0.200, and most preferably in the range of 0.020 to 0.100. The catalyst and/or cocatalyst may further comprise at least one alkali metal or alkaline earth metal, preferably at least one alkali metal. The rare earth oxycarbonate is a preferably a nonstoichiometric rare earth oxycarbonate of the formula M x C Y O z , wherein M is the rare earth element; X = 2; Z = 3 + AY; the parameter A is less than 1.8; and Y is the number of carbon atoms in the oxycarbonate. The rare earth oxycarbonate, hydroxycarbonate, and/or carbonate preferably has a disordered and/or defect structure. Fourth Catalyst Embodiment The present invention is also directed to a catalyst for the oxidative dehydrogenation of a lower hydrocarbon, which comprises (1) an oxide of at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from V, Nb, Ta, Cr, Mo, W, Re, Fe, Co, and Ni. The catalyst, when used for the oxidative dehydrogenation of said lower hydrocarbon, has a selectivity of at least 40 percentto at least one higher hydrocarbon and/or lower olefin. The cocatalyst preferably contains at least one metal selected from V, Nb, Ta, Cr, Re, and Fe. The cocatalyst metal is preferably present in the catalyst in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.005 to 0.400, still more preferably in the range of 0.010 to 0.200, and most preferably in the range of 0.020 to 0.100. The catalyst and/or cocatalyst may further comprise at least one alkali metal or alkaline earth metal, preferably at least one alkali metal. The rare earth oxide preferably has a disordered and/or defect structure. The disordered structure of the catalyst preferably has short range order that is substantially limited to being less than 100 angstroms. The catalyst structure preferably is substantially characterized by defects that occur with a frequency of more than one defect per 100 angstroms. When used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, the catalyst preferably has a selectivity to at least one higher hydrocarbon and/or lower olefin of at least 40 percent, more preferably at least 50 percent. First Method Embodiment One method for preparing a nonstoichiometric rare earth oxycarbonate catalyst having a disordered and/or defect structure comprises, in general, the steps of first forming a catalyst precursor and then forming a catalyst from the catalyst precursor at elevated pressure. The catalyst precursor is formed from at least one rare earth compound that includes at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm and in addition includes at least oxygen. The rare earth element is preferably selected from La, Pr, Nd, Sm, Eu, Tb, and Tm, and more preferably from La, Nd, Sm, Eu, and Tb. In addition to oxygen, the rare earth compound may include other elements, such as carbon, hydrogen, nitrogen, sulfur, halides, phosphorous. The rare earth compound may be selected from rare earth oxides, hydroxides, acetates, chloroacetates, oxalates, carbonates, stoichiometric oxycarbonates, nitrates, sulfates, and phosphates. Other oxygenated compounds may also be used. The rare earth compound is preferably selected from rare earth oxides, hydroxides, acetates, carbonates, and nitrates; more preferably selected from rare earth oxides, hydroxides, and acetates; and most preferably is a rare earth oxide. The at least one rare earth compound is treated with at least water and/or an organic compound that contains a hydroxyl group. The organic compound is preferably an alcohol, such as methanol, ethanol, propanol, isopropanoi, or butanol. As used herein, the terms "treated" and "treating" are understood to mean that the rare earth compound and a fluid material are combined with intimate contact such that the fluid material can act upon the rare earth compound, and includes forming a hydrate of the rare earth compound. Generally the rare earth compound is simply either mixed with or added to the water and/or organic compound so that the rare earth compound is wetted or immersed. The rare earth compound may also be treated with an acid, preferably an organic acid. The organic acid may be acetic acid, formic acid, propionic acid, lactic acid, citric acid, or butyric acid, and is preferably acetic acid. The rare earth compound is preferably treated with the organic acid to form an aqueous mixture having a final pH in the range of 2 to 6, more preferably in the range of 3 to 5. The treated rare earth compound is then dried. The method is not critical to the present invention, and drying methods may be used that are known to one skilled in the art. Generally the material is dried at low temperatures in the range of from ambient temperature to 90°C to 150°C, preferably at 100°C to 140°C. The drying may be done in air, under vacuum, or in an inert atmosphere such as nitrogen. The drying may be done under a flowing atmosphere, which may include the solvent below its saturation level at ambient conditions to control the rate of drying. In the case of water this is referred to by those skilled in the art as controlled humidity drying. When an organic compound is dried, the drying atmosphere should be kept below flammable limits for safety. The drying atmosphere preferably contains a low concentration of carbon dioxide, preferably below 1 percent, and most preferably does not exceed atmospheric level of carbon dioxide. The drying time or degree of dryness is not critical. Generally the material is dried until free liquid has evaporated. The treated rare earth compound may be dried during calcination if desired. The treated rare earth compound is then calcined at a temperature in the range of 300°C to 1000°C in an atmosphere containing oxygen. The calcination temperature is preferably in the range of 350°C to 900°C, more preferably in the range of 400°C to 800°C, and most preferably in the range of 400°C to 600°C. The calcination time is not critical, provided that sufficient calcination is achieved, but preferably should be in the range of a few minutes (1-30 minutes) to 12 hours, more preferably in the range of 45 minutes to 8 hours, still more preferably in the range of 45 minutes to 6 hours, and most preferably in the range of 1 hour to 4 hours. Unlike prior art preparations, calcination atmospheres that have no oxygen have been found to be detrimental and to produce catalysts having lower selectivity. The calcination atmosphere preferably contains oxygen in the range of 5 percent to 100 percent, more preferably in the range of 10 percent to 70 percent, still more preferably in the range of 15 percent to 50 percent, and most preferably in the range of the oxygen content of air to 30 percent. The atmosphere containing oxygen is preferably inert and is generally air, but it may also be oxygen-enriched air or oxygen. The catalyst precursor should be calcined in such manner that the bulk of the calcined material is in effective contact with the atmosphere containing oxygen. A flowing atmosphere, such as flowing air, is desirable to maintain a supply of oxygen during the calcination, particularly when the catalyst precursor is prepared in bulk. The flow rate of the air is not critical, provided that an adequate oxygen concentration is maintained. Any effective method may be used, such as providing fresh atmosphere to the calcination chamber, blowing the atmosphere onto or through the material, conveying the material such as on a conveyor belt, bubbling the atmosphere through the material, or using a fluidized bed or riser bed. Other continuous belt dryer/roasters known in the art may be used, such as those disclosed in Siles, A. and Koch, T., Catalyst Manufacture. Marcel Decker, Inc., 2 nd ed., pp. 47-48 and 68-69 (1995). Unlike prior art preparations, the presence of more than a few percent of carbon dioxide during calcination has been found to be detrimental and to produce catalysts having lower selectivity. Therefore the calcination atmosphere preferably contains a low concentration of carbon dioxide, preferably below 1 percent, and most preferably does not exceed atmospheric level of carbon dioxide. At least one cocatalyst compound containing at least one metal selected from V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi may also be added to the at least one rare earth compound and/or the catalyst precursor. The manner in which the cocatalyst compound is added is not critical. The cocatalyst compound may be added directly to the rare earth compound, such as in a finely divided form. The cocatalyst compound may be added to the water and/or organic compound that contains a hydroxyl group that is used to treat the rare earth compound, such as by forming a solution, dispersion, or suspension. The cocatalyst compound may be added to the catalyst precursor, such as by dissolving or finely dispersing or suspending the cocatalyst compound in water, an organic compound that contains a hydroxyl group, or another medium; applying the mixture to the catalyst precursor, such as by immersion or incipient wetness; and then drying and calcining the combination by using the procedures and conditions discussed for preparation of the catalyst precursor. The cocatalyst metal is preferably added in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.005 to 0.400, still more preferably in the range of 0.010 to 0.200, and most preferably in the range of 0.020 to 0.100. Suitable cocatalyst compounds include but are not limited to nitrates, oxides, carbonates, phosphates, sulfates, halides, hydroxides, acetates, hydrates, salts. The cocatalyst may further comprise at least one alkali metal or alkaline earth metal, preferably at least one alkali metal. Nitrates, hydrates, oxides, sodium salts, and ammonium salts are particularly preferred. Examples are Fe(NO 3 ) 3 , Fe(NO 3 ) 3 '9H 2 O, Mn(NO 3 ) 2 , Mn(NO 3 ) 2 -6H 2 O, Bi(NO 3 ) 3 , Bi(NO 3 ) 3 -5H 2 O, MnWO 4 , MnMoO 4 , Sb 2 O 3 , NaNbO 3 , Na 2 WO 4> sodium rhenate, sodium niobate, ammonium tungstate, and ammonium rhenate. In the same manner, at least one alkali metal or alkaline earth metal compound may be added to the at least one rare earth compound and/or the catalyst precursor. Suitable alkali metal or alkaline earth metal compounds include but are not limited to halides, oxides, carbonates, hydroxides, nitrates. The alkali metal compound is preferably selected from NaF, NaCl, NaBr, Nal, KC1, KBr, KI, CsCl, CsBr, Csl, sodium oxide, potassium oxide, cesium oxide, N-^CO,, K 2 CO 3 , CsCO 3 , NaNO 3 , KNO 3 , CsNO 3 , NaOH, KOH, and CsOH, and most preferably selected from NaCl, NaBr, KC1, sodium oxide, potassium oxide, Na 2 CO 3 , and K 2 CO 3 . The alkaline earth metal is preferably calcium, magnesium, or barium. The alkaline earth metal compound is preferably selected from CaCl 2 , MgCl 2 , BaCl 2 , calcium oxide, magnesium oxide, barium oxide, CaCO 3 , MgCO 3 , BaCO 3 , Ca(NO 3 ) 2 , Mg(NO 3 ) 2 , and Ba(NO 3 ) 2 . The sulfate and phosphate salts of the alkali and alkali earth metals may also be used. The alkali metal or alkaline earth metal is preferably added in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.010 to 0.600, still more preferably in the range of 0.020 to 0.300, and most preferably in the range of 0.040 to 0.200. Other materials, such as a cerium compound, for example cerium nitrate, may also be added. The catalyst precursor may be formed on or mixed with a support material. Suitable support materials include but are not limited to α-alumina, γ-alumina, silica, titania, magnesia, calcium oxide, and zinc oxide. The support material may have a binder or be binderless. The supported catalyst preferably has a formed shape. Suitable formed shapes include spheres, microspheres (for fluid bed reactor use), pellets, rings, extrudates, monoliths. The method in which the catalyst precursor is formed on or added to the support material is not critical, and any method known to one skilled in the art may be used. The at least one rare earth compound and optionally at least one cocatalyst compound, at least one alkali metal or alkaline earth metal compound, and/or other materials are generally added to the support material as a solution, dispersion, or suspension prior to and/or during the drying step. More than one application of the materials to the support material may be used if desired, such as to build up the catalyst precursor in more than one layer. The materials may be applied together or sequentially. The material may be dried or dried and calcined between applications. One method is to combine the catalyst precursor materials, liquid treatment agent such as water and/or alcohol, and the support material, and to then dry the mixture to deposit the materials onto the support, such as by using a rotary evaporator. Another method is to put the support material into a vessel, fill the vessel with a mixture of catalyst precursor materials and liquid treatment agent, optionally put the vessel under vacuum and repressurize it several times to provide good contacting, drain the liquid, and dry the impregnated support material. These procedures may be repeated to build up the amount of deposited material to the desired level, or to apply the materials sequentially, without or with calcination between each impregnation. The amount of catalyst precursor applied to the support material is not critical provided that the combination is effective. Generally it is economically beneficial to apply the minimal amount that provides desired performance, whereas selectivity generally increases with catalyst loading until a maximum level is obtained which is similar to that obtained for an unsupported catalyst. The amount of rare earth metal, when measured as the corresponding oxide, in the combined catalyst precursor and support material by weight is preferably in the range of 5 percent to 90 percent of the combination, more preferably in the range of 10 percent to 70 percent, still more preferably in the range of 20 percent to 60 percent, and most preferably in the range of 25 percent to 50 percent. The combination of catalyst precursor and support material may also be formed by coprecipitating or comixing the catalyst precursor materials with the support material and optionally an inorganic binder such that particulates of the support material form a continuing support linkage after calcination to provide robust catalyst particles. The combined mixture may be formed into a shaped form and into a size that is suitable for a commercial reactor. This procedure may also be used to prepare the catalysts of the present invention that comprise rare earth oxide and cocatalyst containing at least one metal selected from V, Nb, Ta, Cr, Mo, W, Re, Fe, Co, and Ni. The nonstoichiometric rare earth oxycarbonate catalyst having a disordered and/or defect structure is then formed by (a) pressurizing the catalyst precursor to a pressure of at least 100 psig with a flowing gas that contains at least one hydrocarbon and oxygen and (b) heating the catalyst precursor and holding the catalyst precursor for at least 20 minutes at one or more temperatures within the temperature range of 300°C to 600°C wherein oxygen conversion is below 70 percent. The at least one hydrocarbon in the flowing gas is not critical and is generally a lower hydrocarbon such as methane, ethane, propane, butane. The hydrocarbon is generally the hydrocarbon feedstock to be used for oxidative dehydrogenation, but another hydrocarbon may be used. The hydrocarbon is preferably methane or ethane, and is most preferably methane. The source of the oxygen in the flowing gas is not critical. High-purity oxygen is preferred, but air, oxygen-enriched air, or another oxygenated gas may be used if desired. The oxygen level must be maintained sufficiently below the explosive limit to provide safe operation. Generally the oxygen concentration is maintained at 10 percent to 13 percent or lower by volume. The oxygen level is generally the same level used for oxidative dehydrogenation. The mole ratio of hydrocarbon to oxygen is preferably in the range of 4/1 to 12/1, more preferably in the range of 5/1 to 9/1. The flowing gas may also contain inert gases such as nitrogen, helium, argon, if desired, but the levels should not be excessive, and preferably are below 30 percent by volume, more preferably below 20 percent. Undesirable impurities, such as poisons for the catalyst, are preferably present at the low levels that are acceptable for oxidative dehydrogenation. The presence of carbon dioxide has been found to be detrimental and to produce catalysts having lower selectivity. Therefore the flowing gas should contain a low concentration of carbon dioxide that is below 5 percent by volume, preferably below 2 percent, more preferably below 1 percent, and most preferably below 0.5 percent. The catalyst precursor is pressurized by the flowing gas within a pressure vessel, which may be the reactor used for oxidative coupling. The type of pressure vessel, the method of contacting the flowing gas and catalyst precursor, and the flow rate are not critical provided that the flowing gas effectively contacts the catalyst precursor and the temperature or temperatures and oxygen conversion are maintained within the specified limits. The pressure vessel may be a tube, a tank, or another configuration. The pressure vessel may have a means for heating, such as a heater or a heat exchanger, and/or the flowing gas may be preheated. The flow rate is preferably in the range of 100 to 10,000 cc/min/g of catalyst precursor, more preferably in the range of 200 to 5,000 cc/min/g, and most preferably in the range of 300 to 2000 cc/min/g. The catalyst precursor is pressurized by the flowing gas to a pressure of at least 100 psig. The pressure is generally the pressure at which the oxidative dehydrogenation reaction is done. The pressure is preferably less than 600 psig, more preferably less than 400 psig, and still more preferably less than 300 psig. The pressure is most preferably in the range of 125 to 250 psig. The catalyst precursor is heated and while pressurized the catalyst precursor is held for at least 20 minutes at one or more temperatures within the temperature range of 300°C to 600°C at which oxygen conversion is below 70 percent. Under these conditions the catalyst precursor is converted to a nonstoichiometric rare earth oxycarbonate catalyst having a disordered and/or defect structure. Temperatures below 300°C are generally too low for the conversion to occur, and the nonstoichiometric oxycarbonate catalyst tends to degenerate and become unselective at temperatures in the range of 600°C to 750°C. Within the temperature range of 300°C to 600°C, the catalyst precursor is held at a temperature or temperatures at which oxygen conversion is below 70 percent. The incorporation of reaction intermediate species instead of just carbon dioxide is believed to produce the carbon rich and oxygen deficient composition. This realignment of the composition of the catalyst precursor is also believed to produce a realignment of the morphology as well, which produces disorder and defects in the structure. The elevated pressure is believed to have a beneficial effect by substantially increasing the concentration of reaction intermediate species and enabling the conversion to occur at moderate temperatures at which the catalyst composition is stable and selective. The manner in which the catalyst precursor is held at the temperature or temperatures at which oxygen conversion is below 70 percent within the temperature range of 300°C to 600°C is not critical. The temperature may be continually ramped at a slow rate, held at a steady value, stepped incrementally, or any other suitable temperature schedule may be used, or a combination thereof. Selectivity has in general been found to increase asymptotically with the time interval over which the catalyst transformation is allowed to occur. The catalyst precursor is therefore preferably held for at least 30 minutes, more preferably for at least 40 minutes, and most preferably for at least 50 minutes at the temperature or temperatures at which oxygen conversion is below 70 percent within the temperature range of 300°C to 600°C. Long times of 4 hours, 8 hours, or longer may also be used. The oxygen conversion level is also not critical and may change during the time interval. The oxygen conversion level is preferably below 50 percent. The catalyst transformation has been found to occur even at very low oxygen conversion levels. However, it is generally beneficial to use a longer time interval when at a lower temperature that gives a lower oxygen conversion level. The oxygen conversion is preferably above 1 percent, more preferably above 2 percent, and most preferably above 4 percent. The temperature or temperatures within which the oxygen conversion is held below 70 percent is preferably in the range of 350°C to 550°C, more preferably in the range of 400°C to 550°C, and most preferably in the range of 400°C to 500°C. S econd Method Embodiment A similar method for preparing a nonstoichiometric rare earth oxycarbonate catalyst having a disordered and/or defect structure and which also includes a cocatalyst generally comprises the steps of first forming a catalyst precursor and then forming the nonstoichiometric catalyst from the precursor at elevated pressure. The catalyst precursor is formed from a mixture comprising at least one rare earth compound which has been combined with at least one cocatalyst compound. The rare earth compound includes at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm, and, in addition, includes at least oxygen. The cocatalyst compound includes at least one metal selected from Fe, Mn, W, and Mo. The rare earth compound may be the same as those described above in connection with the first method embodiment, and it has been found that cocatalyst compounds which contain at least one metal selected from Fe, Mn, W, and Mo enhance catalyst formation. Without wishing to be bound by theory, these metals are believed to aid formation of the nonstoichiometric and disordered structure of the catalyst. They allow selective catalysts to be formed more rapidly and at lower temperature. The cocatalyst metal is preferably added in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.005 to 0.400, still more preferably in the range of 0.010 to 0.200, and most preferably in the range of 0.020 to 0.100. Suitable cocatalyst compounds include but are not limited to nitrates, oxides, carbonates, phosphates, sulfates, halides, hydroxides, acetates, hydrates, salts. The cocatalyst may further comprise at least one alkali metal or alkaline earth metal, preferably at least one alkali metal. As before, at least one alkali metal and/or alkaline earth metal compound, or other materials may also be added, and the catalyst precursor may be formed on or mixed with a support material. The mixture of the rare earth compound and cocatalyst compound is treated with at least water and/or an organic compound that contains a hydroxyl group and then dried and calcined at a temperature in the range of 300°C to 1000°C in an atmosphere containing oxygen, as before. The nonstoichiometric catalyst is then formed by (a) pressurizing the catalyst precursor to a pressure of at least 100 psig with a flowing gas that contains at least one hydrocarbon and oxygen, as before, and (b) heating the catalyst precursor at one or more temperatures at which oxygen conversion occurs within the temperature range of 300°C to 700°C, preferably within the temperature range of 350°C to 650°C, and more preferably within the temperature range of 400°C to 600°C. Third Method Embodiment A nonstoichiometric rare earth oxycarbonate catalyst having a disordered and/or defect structure and a surface area greater than 20 m 2 /g is prepared by a method which comprises the following three general steps. In the first step, at least one finely divided solid rare earth compound that includes at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm, and oxygen is treated with water and organic acid such that the final pH of the aqueous mixture is in the range of 2 to 6 and obtains a substantially constant value. The finely divided solid provides high surface area for treatment by the water and organic acid. As used herein, the phrase "finely divided solid" is understood to mean powder or fine particulates. The finely divided solid preferably has a particle size below 30 mesh, more preferably below 50 mesh. The rare earth compound is preferably selected from the group consisting of rare earth oxides, hydroxides, nitrates, sulfates, and phosphates, and is most preferably rare earth oxide. The organic acid is preferably selected from the group consisting of acetic acid, formic acid, propionic acid, and butyric acid; more preferably acetic acid and/or formic acid; and most preferably acetic acid. The method of combining the rare earth compound, water, and organic acid is not critical provided that at least the final pH of the aqueous mixture is in the desired range of 2 to 6 and obtains a substantially constant value. Generally, the rare earth compound is mixed with at least enough water to provide a fluid mixture when stirred, such as 5 ml of water per gram of rare earth compound, and then organic acid is added. During the acid treatment, the acid is generally added incrementally as in a titration. It is not critical that the pH remain within the desired range during the entire time that the acid is added until the pH obtains a substantially constant value. Interaction with the rare earth compound tends to neutralize the acid and to swing the pH towards basic, which indicates that more acid needs to be added, until the treatment of the rare earth compound is completed, after which the pH obtains a substantially constant value, preferably a constant value. The rate of addition of the acid is determined by the rate at which the acid interacts with the rare earth compound. The amount of acid that needs to be added is generally proportional to the amount of rare earth compound. The pH of the mixture preferably is maintained within the desired range for at least the final 25 percent of the acid addition, more preferably at least the final 50 percent, and most preferably for at least the final 75 percent. The mixture is preferably well mixed during the acid treatment to provide good contact between the rare earth and the organic acid and to maintain a uniform pH. The concentration of the acid added to the mixture is not critical. The acid preferably is dilute enough to maintain adequate pH control but concentrated enough to not overly dilute the mixture. The acid concentration is preferably in the range of 10 percent to 50 percent by weight, and the acid is preferably added slowly or in small increments. The final pH of the aqueous mixture is preferably in the desired range of 2.5 to 5.5, more preferably in the range of 3 to 5, still more preferably in the range of 3.5 to 4.5, and most preferably is 4. After the pH obtains a substantially constant value, the freated rare earth compound may remain in contact with the acid medium for a longer period of time if desired, such as to confirm that substantially constant pH has been obtained. The acid medium generally is not drained from the treated rare earth compound and the treated rare earth compound is generally not washed before drying. At least one cocatalyst compound including at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi may also be added to the at least one rare earth compound. The manner in which the cocatalyst compound is added is not critical, and it may be added before, during, or after the acid treatment, and it may be added to form a solution, dispersion, or suspension. The cocatalyst metal is preferably added in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.005 to 0.400, still more preferably in the range of 0.010 to 0.200, and most preferably in the range of 0.020 to 0.100. Suitable cocatalyst compounds include, but are not limited to, nitrates, oxides, carbonates, phosphates, sulfates, halides, hydroxides, acetates, hydrates, salts. The cocatalyst may further comprise at least one alkali metal or alkaline earth metal, preferably at least one alkali metal. The cocatalyst compound is preferably soluble in water or aqueous organic acid. Nitrates, hydrates, sodium salts, and ammonium salts are particularly preferred. Examples are Fe(NO 3 ) 3 , Fe(NO 3 ) 3 »9H 2 O, Mn(NO 3 ) 2 , Mn(NO 3 ) 2 » 6H 2 O, Na 2 WO 4; Na 2 WO 4 «2H 2 O. In the same manner, at least one alkali metal or alkaline earth metal compound may be added to the at least one rare earth compound. Suitable alkali metal or alkaline earth metal compounds are those aforementioned as being suitable for the other preparations. Na 2 CO 3 and K 2 CO 3 are particularly suitable. The alkali metal or alkaline earth metal is preferably added in an amount such that the mole ratio of the metal to the rare earth is in the range of 0.001 to 1.000, more preferably in the range of 0.010 to 0.600, still more preferably in the range of 0.020 to 0.300, and most preferably in the range of 0.040 to 0.200. Other materials, such as a cerium compound, may also be added. The catalyst may be formed on or be mixed with a support material. Suitable support materials and supports are those aforementioned as being suitable for the other preparations. The method in which the catalyst is formed on or added to the support material is not critical. The at least one rare earth compound and optionally at least one cocatalyst compound, at least one alkali metal or alkaline earth metal compound, and/or other materials are generally combined with the support material as a solution, dispersion, or suspension prior to and/or during the drying step. The materials may be applied together or sequentially. The material may be dried between applications. One method is to combine the catalyst materials, water, and support material, which may be done before, during, or after the acid treatment but before drying. The incipient wetness method may be used. Another method is to put the support material into a vessel, fill the vessel with a mixture of treated catalyst materials, drain the liquid, and dry the impregnated support material, which may be repeated. The amount of catalyst applied to the support material is not critical provided that the combination is effective, and the aforementioned amounts may be used. In the second step, the acid-treated rare earth compound, and optionally other materials, is dried to a substantially dry state, preferably to a dry state, such that the material does not form a foamed material. The method of drying is not critical to the present invention, and drying methods may be used that are known to one skilled in the art, provided that the material does not foam appreciably during drying. Foaming has been found to be detrimental and to produce catalysts having low selectivity. The material should be dried at least until it is essentially free of liquid and is not a paste. Generally the material is dried at low temperatures of 70°C to 120°C, preferably at 80°C to 110°C. The drying may be done in air, under partial vacuum, or in an inert atmosphere such as nitrogen. The drying may be done under a flowing atmosphere. The drying atmosphere preferably contains a low concentration of carbon dioxide, preferably below 1 percent, and most preferably does not exceed atmospheric level of carbon dioxide. In the third step, the dried material is calcined in a flowing atmosphere that contains oxygen, at a temperature in the range of 300°C to 600°C, such that the catalyst forms a surface area greater than 20 m 2 /g. Prior to calcination, if the dried material is not on or mixed with a support material, the dried material is preferably crushed into a finely divided solid or powder. The calcination temperature is preferably in the range of 350°C to 550°C, more preferably in the range of 400°C to 550°C, and most preferably in the range of 400°C to 500°C. The calcination time is not critical, provided that sufficient calcination is achieved and the material is not over calcined. The calcination time is preferably in the range of 30 minutes to 12 hours, more preferably in the range of 45 minutes to 8 hours, still more preferably in the range of 1 hour to 4 hours, and most preferably in the range of 1 hour to 2 hours. Calcination atmospheres that have no oxygen have been found to be detrimental. The calcination atmosphere preferably contains oxygen in the range of 5 percent to 100 percent, more preferably in the range of 10 percent to 70 percent, still more preferably in the range of 15 percent to 50 percent, and most preferably in the range of the oxygen content of air to 30 percent. The atmosphere containing oxygen is preferably inert and is generally air, but it may also be oxygen-enriched air. The catalyst material must be calcined in such manner that the bulk of the calcined material is in effective contact with the atmosphere containing oxygen. A flowing atmosphere, such as flowing air, is necessary to maintain an adequate supply of oxygen, particularly when the catalyst is prepared in bulk. The flow rate of the air is not critical, provided that an adequate oxygen concentration is maintained. Any of the aforementioned methods may be used. Carbon dioxide is detrimental and produces catalysts having lower selectivity. Therefore the calcination atmosphere preferably contains a low concentration of carbon dioxide, preferably below 1 percent, and most preferably does not exceed atmospheric level of carbon dioxide. The method is particularly useful for producing catalysts having a high surface area, which is preferably above 25 m 2 /g, more preferably above 30 m 2 /g, and most preferably is above 35 m 2 /g. When the rare earth element is selected from the group consisting of La, Pr, Nd, Sm, and Eu, the method can also produce catalysts having a porous microstructure that contains pore sizes in the range of 10 to 1000 angstroms. First Process Embodiment The present invention is directed to a process for the oxidative dehydrogenation of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin, which comprises contacting the lower hydrocarbon with oxygen and a catalyst comprising a nonstoichiometric rare earth oxycarbonate of the formula M x C Y O z having a disordered and/or defect structure, wherein M is at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm, X = 2, Z = 3 + AY, A is less than 1.8, and Y is the number of carbon atoms in the oxycarbonate. When used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, the catalyst has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. The catalyst may further comprise a cocatalyst containing at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. The method of contacting the lower hydrocarbon with oxygen and the catalyst is not critical to the practice of the present invention, and any suitable method may be used which is known to those skilled in the art. The lower hydrocarbon and oxygen are preferably mixed and contacted with the catalyst in a cofeed mode in a reactor suitable for commercial operation, but a sequential mode of operation may be used if desired. The reactor design should minimize void volume outside of the catalyst bed in order to minimize uncatalyzed gas phase reactions. The reactor should allow adequate heat transfer and permit desired temperature control, such as a tubular reactor, fluidized bed reactor, riser reactor. The lower hydrocarbon is generally methane, ethane, propane, or butane, but another hydrocarbon may be used. The lower hydrocarbon is preferably methane or ethane, and most preferably is methane. The source of oxygen is not critical and may include any of the oxygen sources discussed above. High-purity oxygen is preferred, but air or oxygen- enriched air may be used. The oxygen level must be maintained sufficiently below the explosive limit to provide safe operation. Generally the oxygen concentration is maintained at 10 percent to 13 percent or lower by volume. Higher oxygen concentration is desirable to increase hydrocarbon conversion and reactor productivity, but lower oxygen concentration may be desirable to increase selectivity. The mole ratio of lower hydrocarbon to oxygen is preferably in the range of 4/1 to 12/1 , more preferably in the range of 5/1 to 9/1. Unlike the prior art, carbon dioxide in the feed has been found to be detrimental and to lower reaction selectivity, so carbon dioxide is preferably at a low level below 5 percent by volume, more preferably below 2 percent, still more preferably below 1 percent, and most preferably below 0.5 percent. Furthermore, the catalyst must not be treated with carbon dioxide either before or during processing, because unlike the prior art, carbon dioxide treatment degenerates the catalyst in the present invention instead of regenerating it. Trace quantities of halocarbons may be fed with the hydrocarbon to enhance olefin formation, as is known to one skilled in the art. The process is preferably conducted at a pressure greater than 100 psig and a temperature less than 700°C. The pressure should be less than 600 psig, preferably less than 400 psig, and more preferably less than 300 psig. The pressure is still more preferably in the range of 125 psig to 250 psig. The temperature is preferably in the range of 300°C to 650°C, more preferably in the range of 400°C to 600°C. Generally a higher flow rate is beneficial because it minimizes uncatalyzed homogeneous reaction. Therefore, a high flow rate is preferably used that is consistent with high oxygen conversion, which is preferably above 80 percent, more preferably above 85 percent, still more preferably above 90 percent, and most preferably above 95 percent, in order to maximize hydrocarbon conversion. The reactor preferably does not become oxygen depleted to any significant extent. Second Process Embodiment The present invention is also directed to a process for the oxidative dehydrogenation of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin, which comprises contacting the lower hydrocarbon with oxygen and a catalyst comprising an oxycarbonate, hydroxycarbonate, and/or carbonate of at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm. When used for the oxidative dehydrogenation of a lower hydrocarbon, the catalyst exhibits higher selectivity to at least one higher hydrocarbon and/or lower olefin at a pressure above 100 psig than the catalyst or a precursor of the catalyst exhibits at a pressure in the range of atmospheric pressure to 25 psig. When operating at a pressure above 100 psig, the catalyst has a selectivity of at least 40 percent. As before, the process is preferably conducted at a pressure greater than 100 psig and a temperature less than 700°C. The lower hydrocarbon is most preferably methane, and the contacting may be done as aforementioned. Third Process Embodiment The present invention is also directed to a process for the oxidative dehydrogenation of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin, which comprises contacting the lower hydrocarbon with oxygen and a catalyst comprising (1) an oxycarbonate, hydroxycarbonate and/or carbonate of at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi . The catalyst, when used for the oxidative dehydrogenation of said lower hydrocarbon, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. Fourth Process Embodiment The present invention is also directed to a process for the oxidative dehydrogenation of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin, which comprises contacting the lower hydrocarbon with oxygen and a catalyst comprising (1) an oxide of at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Re, Fe, Co, and Ni. The catalyst, when used for the oxidative dehydrogenation of the lower hydrocarbon, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. The lower hydrocarbon is most preferably methane, and the contacting may be done as aforementioned. EXAMPLES The reactor was a 4-inch OD 304 stainless steel tube within a Lindberg furnace. The reactor temperature was controlled by Beckman 7200 controllers with thermocouples attached to the reactor wall. Temperature ramping was controlled by a Macintosh computer or set manually. The temperatures given were reactor temperatures, measured by a thermocouple in contact with the tube wall. The reactor was charged with catalyst sandwiched between a combination of quartz wool/quartz chips/quartz wool. Gas composition was measured by gas chromatography. A small volume of nitrogen was included in the methane-oxygen feed as an internal standard. In the catalyst preparation, unless indicated otherwise, the material was dried overnight in a vacuum oven at 100 to 140°C, calcined at atmospheric pressure, and pressed into a pellet under mechanical pressure for 15 minutes. The pellet was then broken, screened to the desired particle size, and loaded into the reactor tube with quartz chips/wool at the ends to hold the catalyst in place. EXAMPLE 1 The catalyst precursor was prepared by mixing 8.0 grams of commercial La^ with 50 ml of deionized water. The mixture was then slowly heated with stirring to evaporate most of the water, dried, calcined in air at 400°C for one hour, pressed, and broken into 14/30 mesh particles. The surface area was 13.9 m 2 /g. The catalyst precursor (1.0 gram) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen in a ratio of 9: 1 at a flow rate of 500 cc/min and a gas hourly space velocity (GHSV) of 30,000 hr "1 . It was then heated to 450°C and held for four hours, during which reaction occurred with low oxygen conversion. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 700°C. The results were: C 2 + C 3 Methane Oxygen Temperature Selectivity Conversion Conversion 450°C 0 percent 0.3 percent 6 percent 500°C 46 percent 10 percent 100 percent 550°C 46 percent 9 percent 100 percent 600°C 39 → 32 percent 8 percent 100 percent 650°C 2 percent 5 percent 100 percent 700°C 2 percent 5 percent 100 percent The catalyst had stable selectivity at 500 and 550°C, but selectivity declined during the four hours at 600°C, and the catalyst was unselective at 650 and 700°C. For analysis of the active catalyst, the procedure was then repeated with fresh catalyst precursor, except that the catalyst was cooled down in flowing reaction gases after reacting at 550°C and analyzed. The surface area was 4.9 Vg. Elemental analysis (x-ray photoelectron specfroscopy) showed that the catalyst had an oxygen/carbon ratio of 1.9 and an oxygen/lanthanum ratio of 2.84, which was a parameter A value of 0.90. EXAMPLE 2 This example is not in accordance with the present invention. Commercial La^ as received, which had a surface area of 1.5 m 2 /g, was placed in a tubular reactor (1.0 grams) and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . The LaA was then heated to 450°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 650°C. The results are set forth in Table 1 : Table 1 2 + c 3 Methane Oxygen Temperature Selectivity Conversion Conversion 450°C 0 percent 0.4 percent 7 percent 500°C 0 percent 0.6 percent 10 percent 550°C 0 percent 2 percent 36 percent 600°C 1 percent 5 percent 100 percent 650°C 2 percent 5 percent 100 percent The lanthanum oxide did not give total oxygen conversion until 600°C and it was unselective for methane coupling at all temperatures. EXAMPLE 3 The catalyst precursor was prepared by mixing.60.0 grams of La 2 O 3 with 100 ml of distilled water. The mixture was then heated slowly to evaporate most of the water, dried, and calcined in air at 800, 1000, or 1200°C for eight hours. The surface areas were 3.8, 1.1, and 0.2 m 2 /g, respectively. The catalyst precursor was placed in a tubular reactor (2.0 grams, 10/20 mesh) and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 15,000 hr "1 . It was then heated gradually to 550°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 700°C. The results are set forth in Table 2: Table 2 Calcination Reactor C 2 + C 3 Oxygen Temperature Temperature Selectivity Conversion 800°C 550°C 2 percent - 800°C 600°C 15 percent 100 percent 800°C 650°C 45 percent 100 percent 800°C 700°C 19 percent 100 percent 1000°C 550°C 1 percent - 1000°C 600°C 2 percent - 1000°C 650°C 5 percent 100 percent 1000°C 700°C 18 percent 100 percent 1200°C 550°C 3 percent - 1200°C 600°C 4 percent - 1200°C 650°C 8 percent - 1200°C 700°C 11 percent 100 percent The material calcined at 800°C gave 45 percent selectivity at a reactor temperature of 650°C but was unselective at 700°C, in accordance with the present invention. The material calcined at 1000 or 1200°C was unselective at all the reactor temperatures, and was not in accordance with the present invention. EXAMPLE 4 The catalyst precursor was prepared and reacted as in Example 3, except La(NO 3 ) 3 -H 2 O was used. The surface areas were 3.4, 1.5, and 1.0 m 2 /g, respectively. The results are set forth in Table 3 : Table 3 Calcination Reactor C 2 + C 3 Oxygen Temperature Temperature Selectivity Conversion 800°C 550°C 3 percent _ 800°C 600°C 3 percent 100 percent 800°C 650°C 45 percent 100 percent 800°C 700°C 18 percent 100 percent 1000°C 550°C 9 percent - 1000°C 600°C 49 percent 100 percent 1000°C 650°C 39 percent 100 percent 1000°C 700°C 9 percent 100 percent 1200°C 550°C 10 percent - 1200°C 600°C 10 percent 100 percent 1200°C 650°C 16 percent 100 percent 1200°C 700°C 21 percent 100 percent The material calcined at 800°C gave 45 percent selectivity at a reactor temperature of 650°C but was unselective at 700°C, in accordance with the present invention. The material calcined at 1000°C gave 49 percent selectivity at a reactor temperature of 600°C but was unselective at 700°C, in accordance with the present invention. The material calcined at 1200°C was unselective at all the reactor temperatures, and was not in accordance with the present invention. EXAMPLE 5 The catalyst precursor was prepared by precipitating lanthanum hydroxide from a mixture of lanthanum nitrate and ammonium hydroxide in water. The precipitate was then washed to a pH of 8.5, dried, and calcined in air at 650°C for five hours. The catalyst precursor (1.0 gram, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9 : 1 ) at a flow rate of 500 cc/min and a GHSV of 25,000 hr "1 . It was then slowly heated to 550°C over a period of two hours and held. The C 2 selectivity was 41 percent and the C 2 + selectivity was 43 percent, with an ethylene/ethane ratio of 0.67, a methane conversion of 10.1 percent, and an oxygen conversion of 100 percent. The reactor was then depressurized and the catalyst was purged with flowing carbon dioxide for five hours at 550°C. The reactor was then repressurized and the flow of reactants resumed. The treatment with carbon dioxide substantially reduced the C 2 selectivity to 24 percent, the C 2 + selectivity to 25 percent, and the ethylene/ethane ratio to 0.34, with a lower methane conversion of 8.0 percent but an oxygen conversion of 100 percent. EXAMPLE 6 The catalyst precursor was prepared by first dissolving 20.8 grams of La(NO 3 ) 3 -6H 2 O in 100 ml of methanol and 40 ml of ammonium hydroxide (30 percent NH 3 in H 2 O) in 20 ml of methanol. The solutions were then mixed dropwise into 30 ml of methanol with stirring. The lanthanum hydroxide precipitate was filtered, washed with methanol, dried, and calcined in air at 700°C for five hours. The catalyst precursor (0.4 grams, 10/20) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 43,000 hr "1 . It was then gradually heated to 450°C over a period of 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 600°C. The results are set forth in Table 4: Table 4 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 450°C 39 percent 41 percent 0.71 10.3 percent 100 percent 500°C 45 percent 48 percent 0.82 10.9 percent 100 percent 550°C 51 percent 55 percent 0.92 11.6 percent 100 percent 600°C 49 percent 53 percent 0.91 11.2 percent 100 percent The reactor was then depressurized and the catalyst purged with flowing carbon dioxide (500 cc/min) for three hours at 600°C. The reactor was then repressurized and the flow of reactants resumed. The treatment with carbon dioxide substantially reduced the C 2 selectivity to 32 percent, the C 2 + selectivity to 34 percent, and the ethylene/ethane ratio to 0.51, with a lower methane conversion of 9.0 percent but an oxygen conversion of 100 percent. EXAMPLE 7 The catalyst precursor was prepared by precipitating lanthanum hydroxide from a mixture of lanthanum nitrate and ammonium hydroxide in isopropanoi. The precipitate was then washed with water, dried, and calcined in air at 650°C for five hours. The surface area was 28.6 m 2 /g. The unpressed catalyst precursor (1.0 gram) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 33,300 hr "1 . It was then gradually heated to 450°C over a period of 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 600°C. The results are set forth in Table 5: Table 5 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 450°C 17 percent 18 percent 0.32 1.1 percent 100 percent 500°C 28 percent 29 percent 0.45 8.4 percent 100 percent 550°C 42 percent 45 percent 0.72 9.6 percent 100 percent 600°C 50 percent 53 percent 0.82 10.5 percent 100 percent EXAMPLE 8 The catalyst precursor was prepared by mixing 13.3 grams of La(NO 3 ) 3 -6H 2 O and 27.7 grams of urea in 200 ml of water and heating the mixture to 75°C and then gradually to 100°C over four hours, to precipitate lanthanum hydroxide and generate ammonium nitrate and carbon dioxide. The precipitate was washed with water, dried, and calcined in air at 650°C for five hours. The surface area was 6.2 m 2 /g. The catalyst precursor (0.8 grams, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 25,000 hr "1 . It was then gradually heated to 450°C over a period of 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 650°C. The results are set forth in Table 6: Table 6 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 450°C - - - - 3 percent 500°C - - - - 10 percent 550°C 19 percent 20 percent 0.33 1.1 percent 100 percent 600°C 32 percent 35 percent 0.56 8.7 percent 100 percent 650°C 42 percent 45 percent 0.87 9.1 percent 100 percent EXAMPLE 9 The catalyst precursor was prepared by precipitating a mixture of 70 percent lanthanum hydroxide and 30 percent cerium hydroxide from a mixture of lanthanum nitrate, cerium nitrate, and ammonium hydroxide in water. The precipitate was washed, dried, and calcined in air at 550°C for five hours. The catalyst precursor (0.5 gram, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9: 1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then gradually heated to a temperature of 400°C over a period of 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 700°C. The results are set forth in Table 7: Table 7 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 400°C 36 percent 39 percent 0.46 10.0 percent 100 percent 450°C 46 percent 49 percent 0.59 10.8 percent 100 percent 500°C 52 percent 57 percent 0.70 11.3 percent 100 percent 550°C 55 percent 60 percent 0.78 11.4 percent 100 percent 600°C 52 percent 56 percent 0.74 11.0 percent 100 percent 650°C 45 → 41 percent 49 → 44 percent 0.64 → 0.51 10.3 → 9.9 percent 100 percent 700°C 41 → 18 percent 44 → 18 percent 0.49 → 0.18 9.7 → 7.5 percent 100 percent For comparison, the preparation and reaction was repeated by substituting zirconium nitrate for the lanthanum nitrate. The material was unselective (<15 percent) over the entire temperature range. EXAMPLE 10 The catalyst precursor was prepared by precipitating lanthanum oxalate by combining aqueous solutions of lanthanum nitrate and oxalic acid (20 percent excess). The precipitate was washed with water several times, dried overnight in a vacuum oven at 120°C, and calcined at 550°C for 4.5 hours in flowing air (200 cc/min). The catalyst precursor (0.5 grams, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then gradually heated to 400°C over 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 700°C. The results are set forth in Table 8: Table 8 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 400°C - - - - - 450°C - - - - 1 percent 500°C - - - - 3 percent 550°C - - - 1 percent 10 percent 600°C 47 percent 51 percent 0.94 10.9 percent 100 percent 650°C 46 percent 50 percent 0.84 10.3 percent 100 percent EXAMPLE 11 The catalyst precursor was lanthanum acetate hydrate, La(CH 3 COO) 3 - 1.5 H 2 O, which was used either uncalcined or calcined at temperatures of 400 or 800°C in flowing air for two hours. The catalyst precursor (14/30 mesh) was placed in a tubular reactor, and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then heated to 450°C and held for four hours, during which reaction occurred with low oxygen conversion. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 700°C. The results are set forth in Table 9: Table 9 Calcination Initial Initial Max. Ethylene/ Peak Catalyst Temp. Selec. Temp. C+ Selec. Ethane Temp. Unselective None 36 percent 550°C 49 percent 0.9 650°C 700°C 400°C 28 percent 500°C 57 percent 1.2 650°C 700°C 800°C 53 percent 500°C 55 percent 1.1 550°C 650°C The initial selectivity was the selectivity at the initial temperature at which the catalyst reacted with 100 percent oxygen conversion. The peak temperature was the temperature of maximum C 2 + selectivity. The last column was the temperature at which the catalyst became unselective. EXAMPLE 12 This example is not in accordance with the present invention. Lanthanum acetate was charged to a tubular reactor and heated at 525°C for one hour with flowing helium (900 cc/min) at atmospheric pressure. It was then pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and heated to 400°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 750°C. The maximum C 2 selectivity was 29.6 percent at a reactor temperature of 700°C, with a methane conversion of 10.0 percent. For comparison, the catalyst was reacted in the same manner but in a quartz tubular reactor at a low pressure of 15 psig and up to 800°C, which gave a much higher maximum C 2 selectivity of 51 J percent at 550°C, with a methane conversion of 12.1 percent. EXAMPLE 13 The catalyst precursor was lanthanum carbonate hydrate, La-(CO 3 ) 3 -8 H 2 O, which was used either uncalcined or calcined at temperatures of 400, 450, or 500°C in flowing air for two hours. The catalyst precursor (14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9: 1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then heated to 450°C and held for four hours, during which reaction occurred with low oxygen conversion. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 700°C. The results are set forth in Table 10: Table 10 Calcination Initial Initial Max. Peak Temp. C + Selec. Temp. C,+ Selec Temp. None 35 percent 500°C 49 percent 550°C 400°C 48 percent 500°C 56 percent 550°C 450°C 50 percent 500°C 59 percent 550°C 500°C 44 percent 500°C 52 percent 550°C EXAMPLE 14 The catalyst was prepared by mixing 10.0 grams of La^ in 50 ml of water and adding dropwise a mixture of 10 ml of acetic acid (concentrated) and 10 ml water with active stirring to maintain the pH at 4 until the pH remained constant at 4 for five minutes. The mixture was then heated with stirring to evaporate most of the water and dried overnight in a vacuum oven at 80°C. No foamed material was produced. The dry dense- cake material was crushed to a powder and calcined in flowing air at atmospheric pressure at 400°C for one hour. The catalyst was white in color with a powder density of 0.4 g/ml. The calcined catalyst was pressed and broken into 14/30 mesh particles. The prepared catalyst had a surface area of 35.7 m 2 /g. Elemental analysis of the catalyst (electron energy loss specfroscopy) gave an oxygen/carbon ratio of 3.15 and an oxygen/lanthanum ratio of 2.39, which is a parameter A value of 1 J 7. The low-resolution electron microscope micrograph (Figure 10) shows the highly porous nature of the catalyst, and the high- resolution micrograph (Figure 11) shows the disordered and porous microstructure. The catalyst (1.0 grams) was then placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then heated to 400°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 650°C. The results are set forth in Table 11: Table 11 C 2 C + C 3 Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio 400°C 0 percent 0 percent - 0.2 percent 4 percent 450°C 37 percent 40 percent 0.6 9 percent 100 percent 500°C 40 percent 43 percent 0.7 9 percent 100 percent 550°C 54 percent 59 percent 0.9 11 percent 100 percent 600°C 54 percent 58 percent 0.9 10 percent 100 percent 650°C 52 → 48 perce nt 57 → 51 perc sent 0.8 9 percent 100 percent The procedure was then repeated with fresh catalyst, except that the catalyst was cooled down after reacting at 500°C and analyzed. The reacted catalyst had an oxygen/carbon ratio of 3.92 and an oxygen/lanthanum ratio of 2.41, which is a parameter A value of 1.48. The procedure was then repeated again with fresh catalyst, except that the catalyst was cooled down after reacting at 600°C and analyzed. The reacted catalyst had an oxygen/carbon ratio of 3.25 and an oxygen/lanthanum ratio of 2.30, which was a parameter A value of 1.13. For comparison, the preparation was repeated except that the catalyst was calcined for 16 hours. Elemental analysis gave an oxygen/carbon ratio of 3.81 and an oxygen/lanthanum ratio of 2.25, which was a parameter A value of 1.27. EXAMPLE 15 The same procedures were used as in Example 14, except that the acetic acid was added to hold the pH at 6. The density of the calcined material was 1.0 g/ml. The results are set forth in Table 12: Table 12 c 2 2 + C 3 Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 400°C 0 percent 0 percent 0.1 percent 1 percent 450°C 0 percent 0 percent 0.5 percent 8 percent 500°C 33 percent 35 percent 0.5 8 percent 100 percent 550°C 38 percent 40 percent 0.5 8 percent 100 percent 600°C 38 percent 40 percent 0.5 8 percent 100 percent 650°C 35 → 3 percent 37 → 3 percent 0.5 → 0.1 8 → 1 percent 100 percent EXAMPLE 16 The catalyst was prepared and reacted the same way as in Example 14, except that formic acid was substituted for the acetic acid. The maximum C 2 selectivity was 45 percent at 550°C. EXAMPLE 17 The catalyst precursor was prepared by mixing 1.0 gram of NaCl and 8.0 grams of La 2 O 3 in 50 ml of water. The mixture was then heated with stirring to evaporate most of the water, dried, and calcined in air at 400°C for one hour. The catalyst precursor (1.0 gram, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then heated to 500°C and held for four hours, during which reaction occurred with low oxygen conversion. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 700°C. The results are set forth in Table 13: Table 13 2 + C 3 Ethylene/ Methane Oxygen Temperature Selectivity Ethane Ratio Conversion Conversion 500°C 0 percent - 0.4 percent 6 percent 550°C 1 percent - 2 percent 20 percent 600°C 65 percent 1.1 12 percent 100 percent 650°C 65 percent 1.1 12 percent 100 percent 700°C 2 percent - 5 percent 100 percent For comparison, the procedure was repeated with fresh catalyst precursor but at a low pressure of 25 psig, with a flow rate of 90 cc/min and a GHSV of 5,400 hr "1 . The results are set forth in Table 14: Table 14 C 2 + C 3 Ethylene/ Methane Oxygen Temperature Selectivity Ethane Ratio Conversion Conversion * 500°C 1 percent — 2 percent 25 percent 550°C 5 percent - 3 percent 42 percent 600°C 18 percent 0.3 5 percent 100 percent 650°C 35 percent 0.2 8 percent 100 percent 700°C 49 percent 0.3 9 percent 100 percent The maximum selectivity and ethylene/ethane ratio at low pressure were substantially lower than at elevated pressure. EXAMPLE 18 The same catalyst and procedure were used as in Example 17, at 125 psig, except the oxygen content of the mixture of methane and oxygen was increased to a ratio of 5.4:1, at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 , and the initial temperature was 450°C. The results are set forth in Table 15: Table 15 Reactor — ' "" * "" ^ Ethylene/ Methane Oxygen Temperature ; Selectivity Ethane Ratio Conversion Conversion 500°C 53 percent 2.2 18 percent 100 percent 550°C 53 percent 2.2 18 percent 100 percent 600°C 50 percent 2.2 17 percent 100 percent 650°C 49 → 20 percent 2.2 → 0.5 17 → 12 percent 100 percent Selectivity was lower but the ethylene/ethane ratio and methane conversion were higher. EXAMPLE 19 The same catalyst and procedure were used as in Example 17, but at a flow rate of 700 cc/min and a GHSV of 42,000 hr "1 , with reaction maintained for eight days at 500°C and 125 psig. The C 2 selectivity and ethylene/ethane ratio obtained are given in Figure 9. After the initial loss, the selectivity loss was 1 percent per day and the ratio loss was 3 percent per day. During the seventh day, both selectivity and ratio were regained (to the levels obtained after the initial loss) by increasing the flow rate to 900 cc/min. Analysis showed that the aged catalyst had a 34 percent loss of sodium and an 82 percent loss of chlorine, and that other lanthanides were present in the catalyst precursor, with Gd 2 O 3 (1.7 percent) being in largest amount, with lesser amounts of Pr 2 O 3 , Nd 2 O 3 , Eu^, and Tb 2 O 3 . EXAMPLE 20 A variety of catalyst precursors containing an alkali chloride, alkaline earth chloride, or sodium halide was prepared by mixing an amount equimolar to 1.0 gram of NaCl with 8.0 grams of La^ in 50 ml of water. The procedure and reaction conditions were then the same as in Example 17, except the temperature range was 450 to 650°C. The maximum C 2 + C 3 selectivity for each compound at 100 percent oxygen conversion is set forth in Table 16: Table 16 Compound Maximum Selectivity LiCl 8 percent KC1 61 percent RbCl 40 percent CsCl 20 percent MgCl 2 41 percent CaCl 2 45 percent SrCl 2 37 percent BaCl 2 35 percent NaF 37 percent NaBr 63 percent Nal 50 percent None 46 percent The amounts of these compounds were not individually optimized. However, if the compounds were present in optimized amounts, it is expected that maximum C 2 + C 3 selectivity for all of the above-listed compounds would be at least 40 percent. Moreover, as noted earlier, Li and Cs are not particularly preferred alkali metals, but may be used in combination with other materials which improve catalyst stability, prolong catalyst life, or provide a lower reaction temperature. Accordingly, if LiCl and CsCl are combined with other cocatalysts, such as W, Pb, Fe, Mn, or Bi, then catalyst systems that provide good results can be obtained. EXAMPLE 21 The catalyst precursor was prepared by mixing 1.0 gram of Fe(NO 3 ) 3 -9H 2 O and 10.0 grams of La-O 3 in 50 ml of water. The mixture was then heated with stirring to evaporate most of the water, dried, and calcined in air at 400°C for one hour. The surface area was 14.4 m 2 /g. The catalyst precursor (1.0 gram, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9: 1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was heated to 500°C and held for four hours. This gave a constant C 2 + C 3 selectivity of 66 percent, an ethylene/ethane ratio of 0.7, a methane conversion of 11 percent, and an oxygen conversion of 100 percent. The temperature was then increased to 550°C for four hours, which gave a selectivity of 63 percent. The selective catalyst was then cooled and analyzed. The surface area was 7.2 m 2 /g. Elemental analysis gave an oxygen/carbon ratio of 2.1 and an oxygen/lanthanum ratio of 2.8, which is a parameter A value of 1.0. The procedure was then repeated with fresh catalyst precursor, except that the catalyst was taken to 600°C, which gave a selectivity of 60 percent. At 650°C, the catalyst became unselective over time, and at 700°C, the catalyst was totally unselective because the temperature was too high. A second catalyst precursor was then prepared and reacted in the same manner, except that 0.5 grams of Fe(NO 3 ) 3 -9H 2 O and 8.0 grams of La^ were used, calcination was at 800°C for six hours, and it was heated to 450°C in the reactor before the temperature was increased in 10°C increments to 650°C. The C 2 + C 3 selectivity was constant at 57-58 percent over the temperature range of 450 to 590°C, and then decreased to 37 percent at 640°C and 3 percent at 650°C (Figure 12). EXAMPLE 22 The catalyst was prepared by the same procedure as in Example 14, except that 1.0 gram of Fe(NO 3 ) 3 -9H 2 O was added. The prepared catalyst had a surface area of 43.5 m 2 /g. Elemental analysis gave an oxygen/carbon ratio of 1.9 and an oxygen lanthanum ratio of 3.1, which is a parameter A value of 0.97. The catalyst (1.0 gram, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then heated to 500°C and held for four hours. This gave a C 2 + C 3 selectivity of 64 percent, an ethylene/ethane ratio of 0.7, a methane conversion of 11 percent, and an oxygen conversion of 100 percent. The temperature was then increased to 550°C for four hours, which gave a selectivity of 60 percent. The catalyst was then cooled and analyzed. The surface area was 10.4 m 2 /g. The oxygen/carbon ratio was 1.9 and the oxygen/lanthanum ratio was 3.2, which is a parameter A value of 1.00. The procedure was then repeated with fresh catalyst, except the catalyst was taken to 600°C, which gave a selectivity of 57 percent. At 650°C, the catalyst became unselective over time, and at 700°C, the catalyst was totally unselective because the temperature was too high. EXAMPLE 23 The catalyst was prepared and tested the same way as in Example 16, except that 1.0 gram of Fe(NO 3 ) 3 -9H 2 O was added to the La^. The maximum C 2 selectivity was 62 percent at 450°C. EXAMPLE 24 The catalyst was prepared by mixing 1.0 gram of Fe(NO 3 ) 3 -9H 2 O, 0.25 grams of Na j CO^ and 8.0 grams of La 2 O 3 in 50 ml of water and following the procedure of Example 14. The catalyst (0.5 grams, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 700 cc/min and a GHSV of 84,000 hr "1 . The catalyst was gradually heated to 500°C and held for two days, during which the C 2 selectivity decreased from 60 to 56 percent. The temperature was then increased to 575°C, which increased the selectivity to 62 percent, and held for thirteen days. It was then increased to 600°C, which did not change the selectivity, and held for fifteen days. During the 30-day run (Figure 7), the C 2 selectivity dropped to a steady level of 54 percent, with a steady methane conversion of 10-12 percent and a steady ethylene/ethane ratio of 0.93. EXAMPLE 25 The catalyst was the same as in Example 24. The catalyst (0.5 grams) was placed in a -inch OD tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 60,000 hr "1 . It was then heated to 400°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 750°C. The maximum C 2 selectivity was 61.2 percent at 500°C, with a methane conversion of 12.4 percent. For comparison, the catalyst was reacted in the same manner but in a quartz tubular reactor at a low pressure of 15 psig and up to 800°C, which gave a lower maximum C 2 selectivity of 53.9 percent at 650°C, with a methane conversion of 10.8 percent. EXAMPLE 26 The same catalyst and procedures were used as in Example 25, except that a mixture of methane, oxygen, and carbon dioxide in a ratio of 9 : 1 : 1 was used at a pressure of 125 psig. The C 2 selectivity was 52 percent at 500°C, with an ethylene/ethane ratio of 1.2. The catalyst became unselective at 650°C. The carbon dioxide decreased the selectivity. EXAMPLE 27 The catalyst precursor was prepared by mixing 1.0 gram of Fe(NO 3 ) 3 -9H 2 O, 0.25 grams of Na 2 CO 3 , and 16.8 grams of La(CH 3 COO) 3 -1.5 H 2 O in 50 ml of water. The mixture was then heated with stirring to evaporate most of the water, dried, and calcined in flowing air at 400°C for one hour. The catalyst (0.5 grams, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 60,000 hr "1 . It was then heated to 400°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 700°C. The maximum C 2 selectivity was 58.5 percent at 600°C. EXAMPLE 28 The catalyst precursor was prepared by mixing 1.0 gram of Fe(NO 3 ) 3 -9H 2 O, 0.25 grams of Na^O^ and 14.8 grams of La^CO^-δ H 2 O in 50 ml of water. The same procedures were then used as in Example 31. The maximum C 2 selectivity was 58.5 percent at a temperature of 600°C. EXAMPLE 29 A variety of catalysts were prepared by mixing 1.0 gram of the nitrates of either Pb, V, Re, W, Mn, or Cu and 8.0 grams of La^ in 50 ml of water and then following the acetic acid treatment and procedures of Example 14. The results obtained are set forth in Table 17: Table 17 Cocatalyst Maximum Peak Metal C + C ? Selectivity Temperature Pb 61 percent 450°C V 47 percent 500°C Re 41 percent 550°C W 56 percent 550°C Mn 57 percent 550°C Cu 48 percent 500°C The amounts were not individually optimized. EXAMPLE 30 Catalyst precursor A was prepared by mixing 1.0 gram of MnMoO 4 and 8.0 grams of La 2 θ 3 in 50 ml of water. The mixture was then slowly heated with stirring to evaporate most of the water, dried, and calcined in air at 400°C for one hour. Catalyst precursor B was prepared in the same manner but with 1.0 gram of sodium nitrate added. The catalyst precursor (1.0 gram, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then heated to 450°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 650°C. The results are set forth in Table 18: Table 18 Catalyst 2 + c 3 Methane Oxygen Precursor Temperature Selectivity Conversion Conversion A 450°C 52 percent 10 percent 100 percent A 500°C 54 percent 10 percent 100 percent A 550°C 51 percent 10 percent 100 percent A 600°C 41 → 8 percent 8 → 5 percent 100 percent A 650°C 2 percent 5 percent 100 percent B 450°C 0 percent 0 percent 3 percent B 500°C 1 percent 0 percent 8 percent B 550°C 1 percent 1 percent 11 percent B 600°C 58 → 48 percent 8 → 7 percent 100 percent B 650°C 54 → 2 percent 8 → 5 percent 100 percent EXAMPLE 31 Catalyst precursors A and B were prepared and reacted in the same manner as in Example 30, except that 1.0 gram of MnWO 4 was used instead of MnMoO 4 . The results are set forth in Table 19: Table 19 Catalyst Reactor C 2 + C 3 Methane Oxygen Precursor Temperature Selectivity Conversion Conversion A 450°C 38 → 40 percent 9 percent 100 percent A 500°C 44 percent 9 percent 100 percent A 550°C 50 percent 10 percent 100 percent A 600°C 50 → 45 percent 9 percent 100 percent A 650°C 36 → 2 percent 8 → 5 percent 100 percent B 450°C 0 percent 0 percent 3 percent B 500°C 0 percent 0 percent 8 percent B 550°C 62 percent 10 percent 100 percent B 600°C 58 → 40 percent 10 → 8 percent 100 percent B 650°C 2 percent 5 percent 100 percent EXAMPLE 32 The catalyst precursor (F) was prepared by dissolving 0.34 grams of ammonium tungstate, 0.99 grams of Mn(NO 3 ) 2 -6H 2 O, and 0.21 grams of sodium nitrate in 50 ml of water and adding 8.6 grams of La^. The mixture was then slowly heated with stirring to evaporate most of the water, dried, and calcined in air at 800°C for six hours. The procedure was repeated with either a 50 percent higher (B) or 50 percent lower (J) amount of cocatalyst materials. The catalyst precursor (2.0 grams, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 15,000 hr "1 . It was then heated to 450°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 650°C. The results are set forth in Table 20: Table 20 Catalyst 2 + C 3 Methane Oxygen Precursor Temperature Selectivity Conversion Conversion F 450°C 65 percent 11 percent 100 percent F 500°C 66 percent 11 percent 100 percent F 550°C 65 percent 11 percent 100 percent F 600°C 62 → 42 percent 10 → 8 percent 100 percent F 650°C 3 percent 5 percent 100 percent B 450°C 39 percent 7 percent 100 percent B 500°C 37 percent 7 percent 100 percent B 550°C 61 percent 10 percent 100 percent B 600°C 61 → 33 percent 10 → 7 percent 100 percent B 650°C 4 percent - 100 percent J 450°C - - 9 percent J 500°C 64 percent 11 percent 100 percent J 550°C 62 percent 11 percent 100 percent J 600°C 57 → 47 percent 10 → 7 percent 100 percent J 650°C 3 percent 5 percent 100 percent The preparation of catalyst precursor (F) was repeated except that calcination was at 400°C. The reaction was done in the same manner except that the methane:oxygen ratio was 8.5:1 and the initial temperature was 400°C. The results are set forth in Table 21 : Table 21 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 400°C 61 percent 67 percent 0.72 13.7 percent 100 percent 450°C 62 percent 67 percent 0.75 13.7 percent 100 percent 500°C 61 percent 66 percent 0.80 13.8 percent 100 percent 550°C 60 percent 65 percent 0.82 13.5 percent 100 percent 600°C 56 percent 61 percent 0.71 12.6 percent 100 percent 650°C 8 percent 8 percent 0.09 6.4 percent 100 percent EXAMPLE 33 The catalyst precursor was prepared by mixing 8.6 grams of La^ in 50 ml of water and adding 25 percent aqueous acetic acid dropwise with active stirring to maintain the pH at 4 until the pH remained constant at 4 for five minutes. Then 0.4 grams of Na 2 WO 4 -2H 2 O and 1.0 gram of Mn(NO 3 ) 3 -6H 2 O were added and the mixture was heated to evaporate most of the water, dried, and calcined at 600°C for five hours under flowing air (200 cc/min). The catalyst precursor (0.5 gram, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then gradually heated to 450°C over 1.5 hours and held for three hours. The temperature was then increased to 500°C and held. The C 2 selectivity was 54 percent and the C 2 + selectivity was 59 percent, with an ethylene/ethane ratio of 0.74, a methane conversion of 11.1 percent, and an oxygen conversion of 100 percent. Fresh catalyst precursor was then gradually heated to a temperature of 200°C over 1.5 hours and held for three hours. The temperature was then repeatedly raised by a 50°C increment and held for three hours, up to 500°C. The long heat up time of 20 hours gave a higher C 2 selectivity of 60 percent and C 2 + selectivity of 67 percent, and a higher ethylene/ethane ratio of 1.02, with a methane conversion of 12.7 percent and an oxygen conversion of 100 percent. EXAMPLE 34 Catalyst precursor A was prepared by dissolving 0.4 grams of Na 2 CrO 4 -4H 2 O and 0.99 grams of Mn(NO 3 ) 2 -6H 2 O in 50 ml of water and adding 8.6 grams of La^. The mixture was heated to evaporate most of the water, dried, and calcined in air at either 400°C for one hour or 800°C for six hours. Catalyst precursor B was prepared in the same manner but with the manganese nitrate omitted. The catalyst precursor (1.0 gram, 14/30 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then heated to 450°C and held for four hours. The temperature was then repeatedly raised by a 50°C increment and held for four hours, up to 650°C. The results are set forth in Table 22: Table 22 Catalyst Calcination Reactor C 2 + C 3 Methane Oxygen Precursor Temperature Temperature Selectivity Conversion Conversion A 400°C 450°C 41 percent 8 percent 100 percent A 400°C 500°C 49 percent 8 percent 100 percent A 400°C 550°C 52 percent 9 percent 100 percent A 400°C 600°C 44 → 21 percent 8 → 6 percent 100 percent A 400°C 650°C 3 percent 5 percent 100 percent A 800°C 450°C 44 percent 8 percent 100 percent A 800°C 500°C 51 percent 8 percent 100 percent A 800°C 550°C 56 percent 9 percent 100 percent A 800°C 600°C 54 percent 9 percent 100 percent A 800°C 650°C 36 → 3 percent 7 → 5 percent 100 percent B 400°C 450°C 0 percent 0 percent 3 percent B 400°C 500°C 0 percent 1 percent 17 percent B 400°C 550°C 42 percent 8 percent 100 percent B 400°C 600°C 44 percent 9 percent 100 percent B 400°C 650°C 44 → 2 percent 9 → 5 percent 100 percent B 800°C 450°C 0 percent 0 percent 2 percent B 800°C 500°C 0 percent 1 percent 10 percent B 800°C 550°C 27 percent 7 percent 100 percent B 800°C 600°C 31 → 22 percent 7 percent 100 percent B 800°C 650°C 1 percent 5 percent 100 percent For comparison, catalyst precursor (A) was prepared and tested in the same manner, but with α-Al 2 O 3 substituted for the La^. The catalyst was unselective under pressure (<15 percent) at all temperatures. EXAMPLE 35 A series of catalyst precursors was prepared using different cocatalysts and amounts in a modified 5 x 5 Latin Square design. The design used the metals Mn, Fe, Co, Pb, and Sn (five levels); the alkalis Li, Na, K, Rb, Cs (five levels); the alkaline earths Mg, Ca, Sr, Ba, and Zn (three levels); and the anions PO 4 , CI, SO 4 , WO 4 , and ReO 4 (three levels). The metal levels were: 0.0002 (1), 0.0011 (2), 0.0020 (3), 0.0029 (4), 0.0038 (5) moles. The alkali levels were: 0.00036 (1), 0.00198 (2), 0.00360 (3), 0.00522 (4), 0.00684 (5) moles. The alkaline earth levels were: 0.0009 (-1), 0.0018 (0), 0.0027 (1) moles. The anion levels were: 0.0009 (-1), 0.0018 (0), 0.0027 (1) moles. The numbers in parentheses are the codes for the levels of each cocatalyst. The compositions are given in the table. The metals, alkalis, and alkaline earths were added as nitrates and the anions were added as ammonium salts. The materials of each composition were mixed with 10.0 grams of La 2 O 3 and 50 ml of water. The mixture was then slowly heated with stirring to evaporate most of the water, dried, and calcined in air (muffle furnace) at 650°C for 6-8 hours. The catalyst precursor (2.0 grams, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 300 cc/min and a GHSV of 9,000 hr "1 . It was then gradually heated to 500°C over a period of 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment over 30 minutes and held for four hours, up to 650°C. The maximum C 2 selectivity ( percent), ethylene/ethane ratio, and peak temperature (°C) at which the maximum selectivity occurred for each case are in the table. Higher metal loading improved ethylene selectivity and total C 2 selectivity, whereas alkali, alkaline earth, and anion loadings show no correlation, as set forth in Table 23 : Table 23 EXAMPLE 36 A series of catalyst precursors was prepared using different cocatalysts and amounts in a modified 5 x 5 Latin Square design. The design used the first metals Bi, Sb, V, Cr, and Ni (five levels); the second metals Fe, Mn, Sr, Mg, and Ba (one level), the alkalis Li, Na, K, Rb, Cs (one level); and the polyatomic ions ZrO 3 , NbO 3 , TaO 3 , ReO 4 , and MoO 4 (five levels). The first metal levels were: 0.0002 (1), 0.0011 (2), 0.0020 (3), 0.0029 (4), 0.0038 (5) moles. The second metal level was 0.0040 moles. The alkali level was 0.0080 moles. The ion levels were: 0.0009 (1), 0.0018 (2), 0.0027 (3), 0.0036 (4), and 0.0045 (5) moles. The designed set compositions are in the table. The metals and alkalis were added as nitrates. The ions ZrO 3 , NbO 3 , and TaO 3 were added as sodium salts and ReO 4 and MoO 4 as ammonium salts. The materials of each composition were mixed with 10.0 grams of La^ and 50 ml of water. The aqueous mixture was then slowly heated with stirring to evaporate most of the water, dried, and calcined in air at 800°C for six hours. The catalyst precursor (2.0 grams, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 15,000 hr "1 . It was then gradually heated to 500°C over 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment over 30 minutes and held for four hours, up to 650°C. The maximum C 2 selectivity ( percent), ethylene/ethane ratio, and peak temperature (°C) for each case are set forth in the table 24: Table 24 EXAMPLE 37 The catalyst precursor was prepared by mixing 1.13 grams of NaTaO 3 , 0.42 grams of Sb 2 O 3 , 1.15 grams of Mn(NO 3 ) 2 -6H 2 O, and 0.55 grams of LiNO 3 with 70 ml of water and adding 10.0 grams of La^. The mixture was then slowly heated with stirring to evaporate the water until a paste remained and calcined in flowing air at 800°C for four hours. The catalyst precursor (0.5 grams, 10-20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 700 cc/min and a GHSV of 84,000 hr "1 . It was then gradually heated to 500°C over 1.5 hours and held for two days, during which the C 2 selectivity decreased from 60 to 48 percent. The temperature was then increased to 575 °C, which increased the selectivity back to 60 percent, and held for thirteen days. It was then increased to 600°C, which did not change the selectivity, and held for fifteen days. During the 30-day run (Figure 6), the C 2 selectivity dropped to a steady level of 54 percent, with a steady methane conversion of 10 percent and a steady ethylene/ethane ratio of 0.74. EXAMPLE 38 A series of catalyst precursors was prepared using different amounts of iron nifrate, potassium nitrate, magnesium nifrate, and ammonium rhenate in a 5 x 5 Latin Square design. The amounts used (moles) are in the table. The materials of each composition were mixed with 10.0 grams of either La^ or La(NO 3 ) 3 , or a 50/50 mixture of both, and 50 ml of water. The aqueous mixture was then slowly heated with stirring to evaporate most of the water, dried, and calcined in air at 800°C for six hours. The catalyst precursor (2.0 grams, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9: 1) at a flow rate of 500 cc/min and a GHSV of 15,000 hr "1 . It was then gradually heated to 500°C over 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 650°C. The maximum C 2 selectivity, ethylene/ethane ratio, and peak temperature for each case are set forth in Table 25: Table 25 C2 Ethylene/ Peak Base Run Fe Reθ4 Mg Selectivity Ethan Temperature Oxide 5 0.0156 0.0029 0.0054 0.0054 57 percent 0.95 500°C Oxide 14 0.0052 0.0029 0.0054 0.0018 56 percent 0.79 500°C Oxide 2 0.0156 0.0087 0.0054 0.0018 53 percent 0.89 550°C Oxide 9 0.0052 0.0087 0.0054 0.0054 53 percent 0.66 550°C Oxide 12 0.0052 0.0087 0.0018 0.0018 53 percent 0.71 500°C Oxide 3 0.0156 0.0087 0.0018 0.0054 49 percent 0.81 550°C Oxide 15 0.0052 0.0029 0.0018 0.0054 48 percent 0.64 500°C Oxide 8 0.0156 0.0029 0.0018 0.0018 47 percent 0.75 500°C Nitrate 13 0.0052 0.0029 0.0054 0.0054 56 percent 0.77 500°C Nifrate 7 0.0156 0.0029 0.0018 0.0054 55 percent 0.93 550°C Nitrate 4 0.0156 0.0087 0.0018 0.0018 54 percent 0.86 550°C Nifrate 10 0.0052 0.0087 0.0054 0.0018 52 percent 0.65 550°C Nifrate 1 0.0156 0.0087 0.0054 0.0054 50 percent 0.79 550°C Nifrate 16 0.0052 0.0029 0.0018 0.0018 48 percent 0.75 550°C Nitrate 11 0.0052 0.0087 0.0018 0.0054 46 percent 0.65 500°C Nifrate 6 0.0156 0.0029 0.0054 0.0018 43 percent 0.61 550°C 50/50 CP 0.0104 0.0058 0.0036 0.0036 56 percent 0.78 500°C EXAMPLE 39 A series of catalyst precursors was prepared using different amounts of manganese nitrate hexahydrate, potassium nitrate, bismuth nitrate pentahydrate, and sodium niobate in a 5 x 5 Latin Square design. The amounts used (moles) are in the table. The materials of each composition were mixed with 10.0 grams of La^ and 50 ml of water. The compositions with acetic acid treatment were prepared by first adding the LajO- to the water, adding acetic acid dropwise with active stirring to maintain the pH at 4 until the pH remained constant at 4 for five minutes, and then adding the materials. The mixture was slowly heated with stirring to evaporate most of the water, dried, and calcined in air at 800°C for six hours. The catalyst precursor (0.5 grams, 10/20 mesh) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 750 cc/min and a GHSN of 90,000 hr "1 . It was then gradually heated to 400°C over 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 650°C. Methane conversions were all 11 percent. The maximum C 2 + selectivity, ethylene/ethane ratio, and peak temperature for each case are set forth in Table 26: Table 26 C 2 + Ethylene Peak HOAc Run K Mn Nbθ3 Bi Selectivity Ethan Temperature No 2 0.024 0.012 0.0135 0.004 56 percent 1.03 600°C No 3 0.024 0.012 0.0045 0.012 58 percent 1.16 600°C No 5 0.024 0.004 0.0135 0.012 60 percent 0.98 600°C No 8 0.024 0.004 0.0045 0.004 63 percent 1.16 600°C No 9 0.008 0.012 0.0135 0.012 61 percent 1.20 600°C No 14 0.008 0.004 0.0135 0.004 62 percent 1.22 600°C No 15 0.008 0.004 0.0045 0.012 62 percent 0.95 600°C No CP 0.016 0.008 0.0090 0.008 61 percent 1.00 600°C Yes 1 0.024 0.012 0.0135 0.012 59 percent 1.08 600°C Yes 4 0.024 0.012 0.0045 0.004 59 percent 1.21 600°C Yes 6 0.024 0.004 0.0135 0.004 62 percent 1.43 600°C Yes 7 0.024 0.004 0.0045 0.012 64 percent 1.21 600°C Yes 10 0.008 0.012 0.0135 0.004 61 percent 1.04 550°C Yes 11 0.008 0.012 0.0045 0.012 62 percent 0.90 500°C Yes 13 0.008 0.004 0.0135 0.012 62 percent 1.02 550°C Yes 16 0.008 0.004 0.0045 0.004 61 percent 1.06 550°C Yes CP 0.016 0.008 0.0090 0.008 61 percent 0.98 550°C EXAMPLE 40 The catalyst precursor was prepared by mixing 2.95 grams of NaNbO 3 , 2.22 grams of Sb 2 O 3 , 4.59 grams of Mn(NO 3 ) 2 -6H 2 O, and 3.24 grams of KNO 3 with 150 ml of water and then adding 40.00 grams of La 2 O 3 . The mixture was then slowly heated with stirring to evaporate most of the water, dried, and calcined in air at 800°C for six hours. The catalyst precursor (0.25 gram, 10/20 mesh) was then placed in a tubular reactor and pressurized by methane and oxygen. It was then gradually heated to 500°C and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 650°C. The process variables of flow rate (500-1000 cc/min), pressure (112- 262 psig), and methane/oxygen ratio (6-12) were varied. The conditions used and the results obtained are set forth in Table 27: Table 27 Ethylene C 2 C 2 + C 3 Ethylene/ un Flow Rate Pressure CH 4 /O 2 Temp. Selec. Selec. Selec. .Ethane cc/m psig ratio °C percent percent percent ratio 1 1000 250 12 600 21 49 54 0.78 2 1000 250 6 550 26 38 42 2.23 3 1000 125 6 550 27 45 48 1.53 4 500 250 12 550 17 47 52 0.57 5 500 250 6 550 21 34 38 1.58 6 500 125 12 600 20 57 62 0.55 7 500 125 6 550 27 51 56 1.16 8 750 188 9 550 31 59 64 1.07 9 450 188 9 600 26 54 59 0.96 10 750 262 9 550 23 46 50 0.98 11 750 112 9 600 29 58 63 0.97 12 750 188 12 600 29 64 71 0.85 13 750 188 6 600 25 40 44 1.72 EXAMPLE 41 Catalysts of the present invention were prepared by using pre-formed α-alumina to provide supported catalysts for fixed bed or fluidized bed reactor use. The catalyst precursor was prepared by first adding 32.1 grams of 10/20 mesh α-Al 2 O 3 support having a surface area of 0.85 m 2 /g and a pore volume of 0.525 cc/g, and 19.3 grams of La(NO 3 ) 3 -6H 2 O to 30 ml of water, evaporating the water under vacuum at 75°C in a rotary evaporator, and drying the impregnated solid overnight in a vacuum oven at 150°C. The impregnation and drying procedure were then repeated twice using the previously impregnated support. The dried material (three times impregnated) was then calcined at 700°C for five hours under flowing air (200 cc/min). The impregnated support contained 24.8 percent La^. Then 0.065 grams of N-^WO^H and 0.15 grams of Mn(NO 3 ) 3 -6H 2 O were added to 5 ml of water and the solution was mixed with 4.8 grams of the impregnated support. The water was evaporated (85°C) and the material dried and then calcined at 800°C for eight hours using the previous procedures. The catalyst precursor (1 gram) was placed in a tubular reactor and pressurized to 125 psig by using methane and oxygen (9: 1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then gradually heated to 450°C over a period of 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 650°C. The maximum C 2 + selectivity was 62.5 percent at 500°C. EXAMPLE 42 The catalyst precursor was prepared by first filling a glass impregnation column with 70 grams of the same α-Al 2 O 3 support used in Example 40. A 30 percent aqueous solution of La(NO 3 ) 3 -6H 2 O was added to the column under vacuum for five minutes and then the system was repressurized and reevacuated several times, for a total contact time of fifteen minutes. The solution was then drained and the wet solid was dried overnight at 120°C in a vacuum oven. The impregnation and drying procedure was repeated six times. The impregnated material was then calcined at 650°C for five hours under flowing air (200 cc/min) directed over the material. The final-impregnated support (after seven depositions of the La component followed by drying and one final calcination) contained 43.4 percent La 2 O 3 by weight and had a surface area of 11 J m 2 /g. The material was then impregnated by the same technique with a 0.0658 g/cc aqueous solution of Na 2 WO 4 -2H 2 O and dried in a vacuum oven at 120°C two hours. The material was then impregnated with a 0.1645 g/cc aqueous solution of Mn(NO 3 ) 3 -6H 2 O and dried overnight as before. This deposited 0.0465 grams of the sodium tungstate composition and 0.H63 grams of the manganese nitrate composition per gram of lanthanum oxide. This final impregnated material was then calcined in a muffle furnace at 700°C for three hours under flowing air (200 cc/min). The catalyst precursor (1 gram) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen (9:1) at a flow rate of 500 cc/min and a GHSV of 30,000 hr "1 . It was then gradually heated to 450°C over 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, up to 650°C. The maximum C 2 selectivity was 66 percent and the C 2 + selectivity was 73 percent at 550°C, with a methane conversion of 8.4 percent and an ethylene/ethane ratio of 0.60. EXAMPLE 43 The catalyst precursor was prepared by the same method as in Example 41, except that 14/30 mesh α-Al 2 O 3 support (a binderless support of Norton, SA-5402, having a surface area of 0.85 m 2 /g and pore volume of 0.28 cc/g) was used, the impregnation was by an aqueous solution of 40 percent La(NO 3 ) 3 -6H 2 O, and calcination was at 700°C, which gave an impregnated support that contained 46.6 percent La 2 O 3 and had a surface area of 9.1 m 2 /g. Thus the surface area was increased from 0.85 m 2 /g for the support to 9.1 m 2 /g for the catalyst precursor, which gives higher activity. The reaction was done in the same manner, with the results as set forth in Table 28: Table 28 C + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 450°C . . 5 percent 500°C 65 percent 70 percent 0.58 9.6 percent 100 percent 550°C 61 percent 66 percent 0.51 8.9 percent 100 percent 600°C 62 percent 67 percent 0.52 9.1 percent 100 percent EXAMPLE 44 The catalyst precursor was prepared by the same method as in Example 48, except that the impregnation was by an aqueous solution of 40 percent La(NO 3 ) 3 -6H 2 O and calcination was at 700°C, which gave an impregnated support that contained 52.3 percent La 2 O 3 and had a surface area of 10.8 m 2 /g. The reaction was done in the same manner, with the results as set forth in Table 29: Table 29 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 450°C _ _ _ _ 6 percent 500°C 51 percent 56 percent 0.52 9.9 percent 100 percent 550°C 54 percent 59 percent 0.54 10.1 percent 100 percent 600°C 60 percent 66 percent 0.64 10.8 percent 100 percent EXAMPLE 45 The catalyst precursor and procedure were the same as in Example 43, except that the catalyst precursor was gradually heated directly to 550°C over 2 hours and then held at that temperature. The C 2 selectivity was 41 percent and the C 2 + selectivity was 44 percent, with an ethylene/ethane ratio of 0.40, a methane conversion of 9.8 percent, and an oxygen conversion of 100 percent. After one day of steady operation, the temperature was increased to 600°C and the flow rate was increased to 700 cc/min. This increased the C 2 selectivity to 54 percent, the C 2 + selectivity to 58 percent, the ethylene/ethane ratio to 0.53, and the methane conversion to 11.2 percent. The conditions were held constant for fifteen days, during which the C 2 + selectivity (circles in Figure 8) dropped asymptotically to a steady level of 51 percent. EXAMPLE 46 The catalyst precursor and procedure were the same as in Example 42, except that the catalyst precursor was gradually heated directly to 550°C over 2 hours and then held at that temperature. The C 2 selectivity was 37 percent and the C 2 + selectivity was 39 percent, with an ethylene/ethane ratio of 0.44, a methane conversion of 9.1 percent, and an oxygen conversion of 100 percent. After one day of steady operation, the temperature was increased to 600°C and the flow rate was increased to 700 cc/min. This increased the C 2 selectivity to 57 percent, the C 2 + selectivity to 61 percent, the ethylene/ethane ratio to 0.62, and the methane conversion to 11.6 percent. The conditions were held constant for fifteen days, during which the C 2 + selectivity (triangles in Figure 8) dropped asymptotically to a steady level of 48 percent. EXAMPLE 47 The catalyst precursor was prepared by slurry impregnation the ring form of the same α-Al 2 O 3 support used in Example 40 (5/16" O.D. x 5/16" length x 1/16" hole size) using a rotary evaporator. A solution was first prepared by mixing 125 ml of water with 0.91 and 2.28 grams of Mn(NO 3 ) 3 -6H 2 O with stirring, followed by reflux boiling for 15 minutes until the color changed to approximately yellow. Then 19.6 grams of La~O 3 was added, and the slurry was boiled under reflux and continuous stirring for three hours, during which it became off-white and then beige in color and more homogeneous in appearance. The resulting slurry was then mixed with 70 J grams of the α-Al 2 O 3 support in a rotary evaporator, and the impregnation was conducted at a temperature of 70 to 85°C under a partial vacuum of 19-inches Hg. The impregnated rings were dried overnight at 125°C under vacuum. Excess coating on the rings was removed by sieving the dried material on a 10-mesh screen. One portion of the dried material was calcined at 600° and another at 800°C, under 200 cc/min of flowing air for 5 hours. After calcination, the catalyst precursors contained 14.1 percent and 14.4 percent deposited solid by weight, respectively. The reaction with each was done in the same manner as in Example 44, except that 0.5 grams of catalyst precursor was used and the temperature range was 450 to 700°C. The results for the catalyst precursor calcined at 600°C are set forth in Table 30: Table 30 C + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 450°C _ 2 percent 500°C 41 percent 44 percent 0.49 10.6 percent 100 percent 550°C 47 percent 52 percent 0.59 10.8 percent 100 percent 600°C 48 percent 54 percent 0.64 11.5 percent 100 percent 700°C 24 percent 25 percent 0.20 8.2 percent 100 percent The results for the catalyst precursor calcined at 800°C are set forth in Table 31 : Table 31 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 500°C _ ._, _. _ 2 percent 550°C 50 percent 55 percent 0.66 12 percent 100 percent 600°C 53 percent 58 percent 0.65 12 percent 100 percent EXAMPLE 48 The preparation of the catalyst precursor was the same as in Example 46, except that 150 ml of aqueous acetic acid (25 percent volume concentration) was added to the solution. The impregnatedring catalyst precursors that were calcined at 600 and 800°C contained 21.1 percent and 12.6 percent deposited solid, respectively. The results for the catalyst precursor calcined at 600°C are set forth in Table 32: Table 32 C 2 C 2 + Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 550°C 22 percent 23 percent 0.23 1 percent 20 percent 600°C 42 percent 45 percent 0.63 8 percent 100 percent 650°C 41 percent 44 percent 0.61 8 percent 100 percent The results for catalyst precursor calcined at 800°C are set forth in Table 33: Table 33 C,+ Ethylene/ Methane Oxygen Temperature Selectivity Selectivity Ethane Ratio Conversion Conversion 550°C „ _ _ 1 percent 12 percent 600°C 47 percent 51 percent 0.68 8 percent 100 percent 650°C 32 percent 34 percent 0.38 5 percent 100 percent EXAMPLE 49 The catalyst was prepared by the same method as in Example 14, except that gadolinium oxide was used. Elemental analysis gave an oxygen/carbon ratio of 3.38 and an oxygen/gadolinium ratio of 2.10, which is a parameter A value of 0.97. The electron microscope micrograph showed that the catalyst had a disordered structure, but it did not have a porous microstructure. The surface area was 29 m 2 /g. EXAMPLE 50 Catalysts were prepared by the same method as in Example 14, except that the oxides of Pr, Nd, Sm, Eu, Tb, Dy, Er, and Tm were used individually. The surface areas (m7g) were: 37 for Pr, 44 for Nd, 26 for Sm, 27 for Eu, 42 for Tb, 56 for Dy, 54 for Er, and 56 for Tm. EXAMPLE 51 The catalyst precursor was prepared by mixing 8.0 grams of rare earth oxide with 50 ml of deionized water. The mixture was then slowly heated with stirring to evaporate most of the water, dried, calcined in air at 600°C for six hours, pressed, and broken into 10/20 mesh particles. The catalyst precursor (0.25 gram) was placed in a tubular reactor and pressurized to 125 psig by methane and oxygen in a ratio of 9: 1 at a flow rate of 700 cc/min. It was then gradually heated to 400°C over 1.5 hours and held for four hours. The temperature was then repeatedly ramped upward by a 50°C increment and held for four hours, at 650°C. The results for different rare earth elements at 100 percent oxygen conversion are set forth in Table 34: Table 34 Rare C 2 C 2 + Ethylene/ Methane Earth Temperature Selectivity Selectivity Ethane Ratio Conversion Ce 525°C 20 percent 20 percent 0.57 3.6 percent Pr 450°C 41 percent 44 percent 0.74 6.1 percent Nd 450°C 54 percent 59 percent 0.81 11.0 percent Sm 450°C 52 percent 56 percent 0.78 10.1 percent Eu 450°C 56 percent 60 percent 0.84 10.1 percent Tb 500°C 50 percent 54 percent 0.55 6.6 percent Ho 550°C 39 percent 42 percent 0.56 6.3 percent Tm 550°C 47 percent 50 percent 0.68 6.5 percent Lu 650°C 17 percent 17 percent 0.45 2.4 percent

Claims: What Is Claimed: 1. A catalyst for the oxidative dehydrogenation of a lower hydrocarbon characterized by an oxycarbonate, hydroxycarbonate and/or carbonate of at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm, and the oxycarbonate, hydroxycarbonate and/or carbonate having a disordered and/or defect structure, with the oxycarbonate being further characterized as having the formula M x C Y O z is at least one rare earth element as defined above, and X = 2, Z = 3 + AY, A is less than about 1.8, and Y is the number of carbon atoms in the oxycarbonate, and wherein the catalyst, when used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. 2. The catalyst of claim 1 , wherein the ratio Z/X is in the range of 1.5 to 4.5 and the ratio Z/Y is in the range of 1.0 to 6.0. 3. The catalyst of claim 1 , wherein the parameter A is in the range of 0.4 to 1.6. 4. The catalyst of claim 3, wherein the ratio Z/X is less than 3.75 and the ratio Z/Y is in the range of 1.5 to 4.5. 5. The catalyst of claim 1 , wherein the catalyst has a surface area greater than 5 m 2 /g. 6. The catalyst of claim 1 , wherein the rare earth element is selected from the La, Pr, Nd, Sm, Eu, Tb, and Tm. 7. The catalyst of claim 1 , wherein the rare earth element is selected from the La, Pr, Nd, Sm, and Eu, and the catalyst has a porous microstructure with pore sizes in the range of 10 to 1000 angstroms. 8. The catalyst of claim 1 further comprising a cocatalyst including at least one metal selected from the group consisting of N, Νb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Νi, Cu, Zn, Sn, Pb, Sb, and Bi. 9. The catalyst of claim 8 further characterized by having at least one alkali metal or alkaline earth metal. 10. The catalyst of claim 1 further characterized in that, when used for the oxidative dehydrogenation of a lower hydrocarbon, the catalyst exhibits higher selectivity to at least one higher hydrocarbon and/or lower olefin at a pressure above 100 psig than said catalyst or a precursor of said catalyst exhibits at a pressure in the range of about atmospheric pressure to 25 psig, and wherein at a pressure above 100 psig the catalyst has a selectivity to said the at least one higher hydrocarbon and/or lower olefin of at least 40 percent. 11. The catalyst of claim 1 further characterized in that said rare earth element is an oxide of at least one rare earth element selected from La, Pr, Νd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and containing a cocatalyst including at least one metal selected from N, Νb, Ta, Cr, Mo, W, Re, Fe, Co, and Νi; wherein the catalyst, when used for the oxidative dehydrogenation of said the lower hydrocarbon, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. 12. The catalyst of claim 11 further characterized in that the cocatalyst includes at least one metal selected from N, Νb, Ta, Cr, Re, and Fe. 13. A method for preparing a nonstoichiometric rare earth oxycarbonate catalyst having a disordered and/or defect structure, wherein the catalyst, when used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin, said method characterized by the steps of: (1) forming a catalyst precursor from at least one rare earth compound including at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm and oxygen, by treating the at least one rare earth compound with water and/or an organic compound that contains a hydroxyl group, drying the treated rare earth compound, and calcining the freated rare earth compound at a temperature in the range of 300°C to 1000°C in an atmosphere containing oxygen; and (2) forming catalyst by (a) pressurizing the catalyst precursor to a pressure of at least about 100 psig with a flowing gas including at least one hydrocarbon and oxygen, and (b) heating the catalyst precursor and holding the catalyst precursor for at least 20 minutes at one or more temperatures within the temperature range of 300°C to 600°C wherein oxygen conversion is below about 70 percent. 14. The method of claim 13 wherein the rare earth compound is selected from rare earth oxides, hydroxides, acetates, carbonates, and nitrates. 15. The method of claim 13 wherein the rare earth compound is freated with an organic acid to form an aqueous mixture having a final pH in the range of 2 to 6. 16. The method of claim 13 wherein the treated rare earth compound is calcined at a temperature in the range of 400°C to 800°C. 17. The method of claim 13 wherein the treated rare earth compound is calcined in an atmosphere of flowing air. 18. The method of claim 13 wherein at least one cocatalyst compound including at least one metal selected from V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi is added to the at least one rare earth compound and/or the catalyst precursor. 19. The method of claim 13 wherein at least one alkali metal or alkaline earth metal compound is added to the at least one rare earth compound and/or the catalyst precursor. 20. The method of claim 13 wherein the catalyst precursor is formed on or is mixed with a support material. 21. The method of claim 13, wherein the at least one hydrocarbon is methane. 22. A method for preparing a nonstoichiometric rare earth oxycarbonate catalyst having a disordered and/or defect structure and a surface area greater than 20 m 2 /g, wherein the catalyst, when used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above 100 psig, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin, in which the method is characterized by: (1) treating at least one finely divided solid rare earth compound comprising at least one rare earth element selected from La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm and oxygen with water and an organic acid to form an aqueous mixture such that the final pH of the aqueous mixture has a substantially constant value in the range of 2 to 6; (2) drying the aqueous mixture to a substantially dry state such that the treated rare earth compound does not form a foamed material; and (3) calcining the treated rare earth compound in a flowing atmosphere containing oxygen at a temperature in the range of 300°C to 600°C to provide a nonstoichiometric rare earth oxycarbonate catalyst. 23. The method of claim 22 wherein the rare earth compound is a rare earth oxide. 24. The method of claim 22 wherein the organic acid is selected from acetic acid, formic acid, propionic acid, and butyric acid. 25. The method of claim 22 wherein at least one cocatalyst compound containing at least one metal selected from N, Νb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Νi, Cu, Zn, Sn, Pb, Sb, and Bi is added to the at least one rare earth compound. 26. The method of claim 22 wherein at least one alkali metal or alkaline earth metal compound is added to the at least one rare earth compound. 27. The method of claim 22 wherein the catalyst is formed on or is mixed with a support material. 28. A process for the oxidative dehydrogenation of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin characterized by contacting the lower hydrocarbon with oxygen and a catalyst comprising a nonstoichiometric rare earth oxycarbonate, hydroxycarbonate and/or carbonate of at least one rare earth element selected from La, Pr, Νd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm, and the oxycarbonate, hydroxycarbonate and/or carbonate having a disordered and/or defect structure, with the oxycarbonate being further characterized as having the formula M x C Y O z is at least one rare earth element as defined above, and X = 2, Z = 3 + AY, A is less than about 1.8, and Y is the number of carbon atoms in the oxycarbonate; and wherein the catalyst, when used for the oxidative dehydrogenation of the lower hydrocarbon at a pressure above 100 psig, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. 29. The process of claim 28 conducted at a pressure greater than 100 psig and a temperature less than 700°C. 30. The process of claim 28, wherein the lower hydrocarbon is methane. 31. The process of claim 28 wherein the catalyst is further characterized by including a cocatalyst comprising at least one metal selected from N, Νb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Νi, Cu, Zn, Sn, Pb, Sb, and Bi. 32. A process for the oxidative coupling of a lower hydrocarbon to form at least one higher hydrocarbon and/or lower olefin characterized by contacting the lower hydrocarbon with oxygen and a catalyst comprising (1) an oxide of at least one rare earth element selected from La, Pr, Νd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from N, Νb, Ta, Cr, Mo, W, Re, Fe, Co, and Νi; wherein the catalyst, when used for the oxidative dehydrogenation of said lower hydrocarbon, has a selectivity of at least 40 percent to at least one higher hydrocarbon and/or lower olefin. 33. The process of claim 32 wherein the lower hydrocarbon is methane.

Inventors: BHASIN MADAN MOHAN, CAMBPELL KENNETH DWIGHT, CANTRELL RICK DAVID, GHENCIU ANCA, MINAHAN DAVID MICHAEL ANTHONY, NIELSEN KENNETH ANDREW, WESTWOOD ALISTAIR DUNCAN, BHASIN, MADAN, MOHAN, CAMBPELL, KENNETH, DWIGHT, CANTRELL, RICK, DAVID, GHENCIU, ANCA, MINAHAN, DAVID, MICHAEL, ANTHONY, NIELSEN, KENNETH, ANDREW, WESTWOOD, ALISTAIR, DUNCAN
Applicants: UNION CARBIDE CHEM PLASTIC, UNION CARBIDE CHEMICALS & PLASTICS TECHNOLOGY CORPORATION
Application year: 2001
Main classification: B01J27
IPC codes: B01J23, B01J27, B01J37, C01F17, C07B61, C07C2, C07C5, C07C9, C07C11