Improved dehydrogenation catalyst

Abstract: This invention relates to a dehydrogenation catalyst containing iron and promoted with potassium and cerium which has been shown to have improved activity when a copper compound is added thereto as a component. The improvement in activity is obtained without substantially sacrificing selectivity. While copper has been employed as a stabilizer in some iron dehydrogenation catalysts, its combination with cerium in the presently disclosed catalyst composition is novel.

Description: This invention relates to improved catalysts for the dehydrogenation of hydrocarbons to corresponding more-unsaturated hydrocarbons, more particularly, to the production of vinyl aromatic hydrocarbons from alkyl aromatic hydrocarbons and to the production of olefins from the corresponding more-saturated aliphatic hydrocarbons. The vinyl benzenes and butadienes play a particularly important role in the preparation of synthetic rubbers, plastics and resins. The polymerization of styrene, for example, with various comonomers such as butadiene to produce synthetic rubbers is well known as is the polymerization of styrene to produce polystyrene resins. Styrene and butadiene are typically produced from ethylbenzene and butylene, respectively, by dehydrogenation over solid catalysts in the presence of steam and at temperatures ranging from 500° to 800°C. The class of catalysts found to be the most effective for this process contains a predominant amount of iron oxide, promoted by potassium carbonate, and stabilized by chromium oxide. The activity of a catalyst is measured by the amount of starting material converted at a given temperature to another material and the selectivity is the percent of the converted material which is the desired product. Any improvement which results in either increasing the selectivity or the activity without lowering the other is economically attractive since the result is that the yield of the product (moles of desired product produced per mole of reactant) has been increased. An increase of only a few tenths of a percent in the selectivity can result in a substantial savings of starting materials while an increase in activity can substantially reduce capital expenditure and energy consumption. The prior art catalysts used in the dehydrogenation of alkyl aromatics to alkenyl aromatics, for example, ethylbenzene to styrene as previously discussed, are widely known. Early dehydrogenation patents describe the use of cerium oxide as the major active ingredient. Thus, for example, U.S. Patent 1,985,844 discloses the use of cerium oxide precipitated on broken pieces of clay to dehydrogenate ethylbenzene below atmospheric pressure. Another, U.S. Patent 2,036,410, discloses the use of cerium oxide promoted with oxides of tungsten, uranium, and molybdenum. Various catalysts, dehydrogenation conditions and other operating data are disclosed by Pitzer in U.S. Patent 2,866,790 relating to the use of a catalyst composition including potassium carbonate, chromium oxide, and iron oxide. Other catalysts and procedures are also shown by Gutzeit, U.S. Patent 2,408,140; Eggertsen, et al., U.S. Patent 2,414,585; Hills, et al, U.S. Patent 3,360,579; and Riesser, U.S. Patent 4,098,723. O'Hara, in U.S. Patent 3,904,552, discloses the use of cerium and molybdenum in alkali promoted iron oxide dehydrogenation catalysts. MacFarlane, in U.S. Patent 3,223,743, teaches that it is very difficult to obtain a catalyst which has both high selectivity and high activity since as one increases, the other usually decreases. To overcome this he teaches the use of two layers of catalyst, the first layer with high selectivity (lower activity) and a second layer with high activity (lower selectivity). The first layer may contain 20-80% by weight iron and 0.5-5% by weight cerium oxide. Riesser, in U.S. Patent 4,144,197, discloses the use of vanadium to improve the selectivity of a potassium promoted dehydrogenation catalyst containing 20-95% by weight ferric oxide and 0.01-50% by weight cerous oxide. Data in the '197 patent is consistent with the teaching of MacFarlane with respect to vanadium addition since increasing amounts increase the selectivity at the sacrifice of activity. Also disclosing the use of cerium as a promoter in iron oxide based catalysts is U.S. Patent 4,467,046. U.S. Patent 4,758,543 discloses a catalyst formulation containing low concentrations of iron in combination with high concentrations of potassium and cerium. Patents which show the use of copper as a stabilizer for dehydrogenation catalysts include U.S. Patents 2,395,875 and 2,395,876 which disclose catalysts containing a major amount of magnesium oxide, a minor amount of iron oxide together with an alkali or alkaline earth promoter and a copper oxide stabilizer. A zinc oxide-based catalyst containing a minor amount of iron oxide, promoted with potassium oxide and stabilized with copper oxide is disclosed in U.S. Patent 2,418,888, while U.S. Patent 2,418,889 discloses a similar beryllium oxide-based catalyst. U.S. Patent 2,426,829 which discloses an alkali promoted iron oxide dehydrogenation catalyst indicates that oxides of aluminum, zinc or copper may be added as stabilizers. Other patents teaching the use of copper oxide as a stabilizer for alkali-promoted iron oxide catalysts are U.S. Patent Patents 3,387,053; 3,448,058; 3,542,897; 4,064,187 and 4,134,858. Other catalysts in which copper is the primary or major component include those disclosed in U.S. Patent Patents 4,279,777; 4,334,116 and 4,590,324. It has now been discovered that an alkali promoted iron/cerium dehydrogenation catalyst can be improved with respect to its activity, with little or no effect on selectivity, by the addition of a copper compound. While copper has been employed as a stabilizer in some iron dehydrogenation catalysts, as above indicated, its combination with cerium in the presently disclosed catalyst compositions is novel and provides unexpected benefits. The catalyst compositions of the present invention contain, by weight, from 5 to 70 percent iron as Fe₂O₃, preferably from 15 to 60 percent; from 10 to 60 percent potassium as K₂CO₃, preferably from 20 to 55 percent; from 10 to 60 percent cerium as Ce₂O₃, preferably from 12 to 30 percent and from 0.1 to 50 percent, copper as CuO, preferably from 1 to 15 percent, based on the total weight of the finished calcined catalyst. Iron is generally added to the compositions of the invention as the oxide. While various forms of iron oxide can be employed in the compositions of the present invention, the preferred form is a mixture of red iron oxide (Fe₂O₃) and yellow iron oxide (Fe₂O₃·H₂O). When the mixture of oxides is employed, from 30 percent to 70 percent of the total moles of iron oxide is added as yellow iron oxide. The amount preferred is 40 to 60 molar percent of the yellow oxide and the most preferred is 50 molar percent. Particularly suited are pigment grade red and yellow iron oxides. The catalyst compositions of the present invention may also contain as a catalyst promoter, an alkali metal compound, e.g. a potassium compound. The potassium promoter may be added to the catalyst in various forms. For example, it may be added as the oxide, hydroxide, carbonate or bicarbonate. The potassium compound is preferably present in the catalyst as potassium carbonate or as a mixture thereof with potassium oxide. Cerium as used in the catalyst compositions of the present invention can be added to the catalyst in the form of cerium oxide or in the form of other cerium compounds which decompose upon calcination to form cerium oxide, as for example, cerium carbonate, cerium nitrate and cerium hydroxide. Copper can be added to the catalyst formulations of the invention as its oxides, hydroxides, carbonates, acetates, nitrates or sulfates. Other known catalyst additives can be included in the catalysts of the invention, but are not essential. Thus, an optional component of the catalyst composition of the invention is a chromium compound which serves as a stabilizer for the active catalytic components. Chromium compounds have, in fact, typically been added to alkali-promoted iron oxide catalysts to extend their life. Chromium, as used in the compositions of this invention, can be added to the catalyst in the form of a chromium oxide or in the form of chromium compounds which decompose upon calcination to chromium oxides as, for example, chromium nitrates, hydroxides and acetates. If potassium chromates are used, such materials can, of course, also contribute to the requisite concentration of potassium present in the dehydrogenation catalyst compositions as hereinbefore discussed. Catalyst compositions of the invention optionally contain up to 5 percent by weight, preferably (when employed) from 0.5 percent to 3 percent, of a chromium compound, expressed as Cr₂O₃. A second optional component, used to improve the selectivity of the catalyst, is molybdenum which can be added as its oxide or as a molybdate. The molybdenum component is present (when employed) in an amount up to 2.5 percent by weight, preferably from 0.2 to 1.5 percent molybdenum, calculated as MoO₃. The physical strength, activity and selectivity of the catalyst compositions of the present invention can be improved by adding certain binding agents. Binding agents can include, for example, calcium aluminate or Portland cement (a registered trademark). These cements can be added individually or in combination. The catalyst should contain from 3 percent to 20 percent by weight hydraulic cement, preferably from 5 percent to 15 percent. Calcium also can be added as its sulfate and/or its carbonate as a supplement to, or replacement for a part of, the binder component. The density of the catalyst composition herein can likewise be modified by the addition of various filler substances, for example, combustible materials such as graphite and methyl cellulose. Such materials can be added to the compositions during preparation, but are burned out after the catalyst pellets have been formed during the calcining step. These porosity promoting aids can also facilitate extrusion of catalyst pellets. These fillers generally comprise up to 10 percent by weight of the catalyst composition, preferably from 2 to 8 percent. The catalyst compositions of the present invention generally are prepared by admixing the essential and desired optional components heretofore described and by drying and calcining the resulting mixture. Calcination temperatures can range from 500°C to 800°C, preferably from 550°C to 750°C. The catalyst compositions of the present invention can be prepared in various ways known to the art. One method comprises ballmilling together a mixture of the desired compounds, adding a small amount of water, and extruding the composite to produce small pellets, which are then dried and calcined. Another method is mixing the components together with water, drying them to form a powder and tabletizing. Another procedure involves mixing the components together with an excess of water, partially drying, and then subsequently extruding, drying and calcining the resulting pellets. The catalysts of the present invention are especially effective in promoting the dehydrogenation of ethylbenzene to produce styrene. Such a dehydrogenation reaction is usually carried out at reaction temperatures of from 500°C to 700°C. However, higher or lower temperatures may be used as are known to the art. The use of subatmospheric, atmospheric, or superatmospheric pressures are suitable. However, since it is preferred to operate at as low a pressure as is feasible, atmospheric or subatmospheric pressure is preferred. The process of the invention preferably is carried out as a continuous operation. It is preferred to utilize a fixed bed which may consist of a single stage or a series of stages of the same catalyst in one or several reactors. With the use of the catalyst of this invention, it is desirable to add steam to the hydrocarbon reactant feed to aid in the removal of carbonaceous residues from the catalyst. Steam to hydrocarbon weight ratios of from 0.5:1 to 5:1 are desirable depending on the compound being dehydrogenated. Best results are obtained with steam to hydrocarbon ratios above 1:1. The contact time of the reactant-containing gas with the catalyst is usually expressed in terms of liquid-hourly-space velocity (LHSV), which is defined as the volume of liquid hydrocarbon reactant per volume of catalyst per hour. The LHSV of the organic reactants, eg. ethylbenzene, according to this invention may vary depending upon the reactant and is preferably adjusted within the range of from 0.3 to 10 to effect the degree of conversion desired for the particular feed in question. It should be noted that advantages resulting from increases in activity as indicated by a difference of a few degrees of temperature are extremely significant in a commercial process which may produce many hundreds of thousand pounds of product per day. The catalysts of the present invention and their use will be further described by the following illustrative examples which are representative of the invention. Throughout the specification the examples of the invention are numbered numerically, while comparative examples are identified by letters, ie. A,B,C, etc. General Procedure A catalyst formulation was made by blending red iron oxide (Fe₂O₃), yellow iron oxide (Fe₂O₃·H₂O), Ce₂(CO₃)₃·5H₂O, K₂CO₃, CuCO₃·Cu(OH)₂, MoO₃, CaSO₄·2H₂O (gypsum), CaCO₃, calcium aluminate cement (LUMNITE ), 20 g amorphous graphite and 10 g methyl cellulose. Approximately 116 mL of deionized water was added to the dry ingredients. The ingredients were thoroughly mixed, heated and dried to a consistency suitable for extrusion. The pellets, 5/32" (3.97 mm) in diameter, were extruded and then calcined at a temperature of from 550° to 650°C for a period of 2½ hrs. Examples 1 and 2 Two catalysts were prepared according to the general procedure, described above. Examples A, B and C (Comparative) The same procedure employed in the above examples was used to make catalysts similar to those of Examples 1-2, except that they contained no copper. The catalyst compositions of Examples 1-2 and A-C, expressed as shown, are given in Table I. Components (weight percent) Ex. No. Fe₂O₃ Ce₂O₃ K₂CO₃ MoO₃ CuO LUMNITE CaSO₄ CaCO₃ A 29.3 5.6 50.7 1.1 -- 6.8 3.4 3.0 B 39.7 13.2 33.1 1.1 -- 6.6 3.3 3.0 1 37.7 12.5 31.3 1.0 5.2 6.3 3.1 2.8 C 14.6 23.6 48.2 1.1 -- 6.4 3.2 2.9 2 11.5 23.0 47.0 1.0 5.2 6.3 3.1 2.8 Catalyst Test Procedure Each of the above catalysts of the invention and those prepared for comparison was tested for activity and selectivity in the reaction for dehydrogenating ethylbenzene to styrene by placing 70 mL of the above catalyst pellets in a fixed bed reactor and passing a preheated mixture of steam and ethylbenzene at a weight ratio of 1.5:1 through the bed which was maintained at a temperature to give the desired conversion of ethylbenzene, this temperature being dependent upon the activity of the particular catalyst. The LHSV was 1.0 and the pressure was maintained at atmospheric. The temperature was adjusted to produce a 50 percent conversion of ethylbenzene, that temperature being an indication of the activity of the particular catalyst. The results of the dehydrogenation reaction for Examples 1-2 and A-C are shown in Table II. Ex. No. Temp.(°C) at 50% conversion Percent Selectivity A 597 96.4 B 595 95.5 1 587 96.1 C 589 96.3 2 577 96.6 Examples 3-5 Three additional catalysts were prepared according to the general procedure. The catalyst of Example 3 was calcined at a temperature of 700°C for one-half hour, the higher temperature requiring a shorter period of time for the calcination. The catalysts of Examples 4 and 5 were calcined at the same temperature and for the same period of time as were the catalysts of Examples 1-2. Examples 7-9 These three examples were also prepared according to the procedure of Example 1, except that the source of copper employed was Cu(NO₃)₂·2½H₂O. Example D (Comparative) For comparison, a catalyst was prepared without copper, but otherwise similar to that of Example 3. It was calcined at the same temperature and for the same period of time as were the catalysts of Examples 1-2. Example 9 The catalyst of this example was prepared according to the general procedure except that the catalyst contained no molybdenum. Example E (Comparative) The catalyst of this example was prepared in the manner of Example 9, except that it contained no copper, in order to compare with a similar catalyst which contained no molybdenum. The compositions of all of the above catalysts of Examples 3-9 and Examples D and E, expressed as shown, are given in Table III. Components (weight percent) Ex. No. Fe₂O₃ Ce₂O₃ K₂CO₃ MoO₃ CuO LUMNITE CaSO₄ 3 24.2 12.1 45.5 1.0 2.0 12.1 3.0 4 21.2 12.1 45.5 1.0 5.0 12.1 3.0 5 20.2 11.5 43.3 1.0 9.6 11.5 2.9 6 25.3 12.1 45.5 1.0 1.0 12.1 3.0 7 24.2 12.1 45.5 1.0 2.0 12.1 3.0 8 21.2 12.1 45.5 1.0 5.0 12.1 3.0 D 26.3 12.1 45.5 1.0 -- 12.1 3.0 9 21.4 12.3 45.9 -- 5.1 12.2 1.1 E 27.6 10.5 47.9 -- -- 12.8 3.1 The results of testing the catalyst compositions shown in Table III in the dehydrogenation reaction described above under "Catalyst Test Procedure" are shown in Table IV. Ex. No. Temp.(°C) at 50% conversion Percent Selectivity 3 588 96.8 4 585 96.6 5 587 96.3 6 591 96.2 7 587 96.3 8 585 96.9 D 592 96.6 9 595 95.2 E 605 95.8

Claims: A dehydrogenation catalyst composition which comprises from 5 to 70 percent iron as Fe₂O₃, from 10 to 60 percent potassium as K₂CO₃, from 10 to 60 percent cerium as Ce₂O₃ and from 0.1 to 50 percent copper as CuO. A composition as claimed in Claim 1 wherein the amounts of Fe₂O₃, K₂CO₃, Ce₂O₃ and CuO are from 15 to 60 percent, 20 to 55 percent, 12 to 30 percent and 1 to 15 percent, respectively. A composition as claimed in Claim 1 or 2 which contains a binder in an amount of 3-20 percent by weight of the catalyst. The composition as claimed in Claim 3 wherein at least a portion of the binder is a hydraulic cement. A composition as claimed in Claim 4 wherein the binder is present in an amount of from 3 to 20 percent by weight of the total composition. A composition as claimed in Claim 1 wherein the catalyst contains chromium or molybdenum promoters or a mixture thereof. A composition as claimed in Claim 6 wherein chromium is present in an amount of up to 5 percent, calculated as Cr₂O₃ and molybdenum is present in an amount of up to 2.5 percent, calculated as MoO₃. A process for dehydrogenating an alkylaromatic compound to prepare an alkenyl aromatic compound in which the alkylaromatic compound is heated together with steam in the presence of a catalyst promoted with an alkali metal compound, characterized in that said catalyst contains from 5 to 70 percent Fe₂O₃, from 10 to 60 percent K₂CO₃, from 10 to 60 percent Ce₂O₃ and from 0.1 to 50 percent CuO. A process as claimed in Claim 8 wherein the catalyst comprises from 5 to 70 percent iron as Fe₂O₃, from 10 to 60 percent potassium as K₂CO₃, from 10 to 60 percent cerium as Ce₂O₃ and from 0.1 to 50 percent copper as CuO, all by weight in the finished catalyst. A process for preparing a dehydrogenation catalyst composition which comprises admixing from 5 to 70 percent iron as Fe₂O₃, from 10 to 60 percent potassium as K₂CO₃, from 10 to 60 percent cerium as Ce₂O₃ and from 0.1 to 50 percent copper as CuO, and then drying and calcining the resulting admixture.

Inventors: LEWIS GREGG EVAN, SHERROD FRED ALLEN, SMITH ALLEN RAY, LEWIS, GREGG EVAN, SHERROD, FRED ALLEN, SMITH, ALLEN RAY
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1988
Main classification: B01J23
IPC codes: C07C5, C07C11, B01J27, B01J23, C07C15, C07C1, C07C67, C07B61