Abstract:
The present invention includes a sol-gel supported catalyst for the dehydrogenation of lower alkanes, the catalyst comprising at least one active metal and at least one promoter metal attached to a sol-gel mixed oxide support. The sol-gel mixed oxide support arises from the polymerization of at least one precursor. The active metal and the promoter metal have been attached to the sol-gel mixed oxide support by the active metal and the promoter metal having been co-precipitated with the precursor of the sol-gel mixed oxide support. The invention also includes a method of making the above mentioned catalyst and a method of using the catalyst to dehydrogenate lower alkanes to produce lower alkenes.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is in the field of dehydrogenation catalysts using a modified sol-gel/co-precipitation technique. 
     BACKGROUND OF THE INVENTION 
     Over the past 15 years, many academic and industrial research efforts have focused on the conversion of lower alkanes (C 1 -C 5 ) to petrochemical feed stocks. These catalytic reactions include: methane partial oxidation to formaldehyde and oxidative coupling to C 2  hydrocarbons; ethane and propane oxidative dehydrogenation to olefins and oxygenates (acetic acid, acrolein, acrylic acid); and butane and pentane oxidative dehydrogenation to maleic and phthalic anhydrides. Thus far, there are no industrially practical operations for such applications except for the production of maleic anhydride from butane. 
     The research drive to develop an oxidative dehydrogenation process for propane and ethane comes from the fact that the chemical industry depends heavily on propene and other alkene feed stocks. For example, propene demand is estimated to grow 4.5% per year between 1991 and 2000. Catalytic oxidative dehydrogenation (ODH) is an attractive alternate route for the production of alkenes compared to conventional cracking and dehydrogenation processes. This is because ODH is thermodynamically favored at lower temperatures and usually does not lead to the formation of coke and smaller hydrocarbons. Recent literature has focused on selective, high surface area catalysts active below 823K that can limit the amount of carbon oxides formed. In our recent work, an observed yield of propene of 30% was obtained in the oxidative dehydrogenation of propane. This is among the highest yields ever reported for this type of reaction. Furthermore, certain formulations of the catalyst lead to little or no carbon monoxide production. These facts make this catalyst a viable option for an industrial process. 
     Some of the most selective oxidative dehydrogenation catalysts reported in, recent literature consist of vanadium and molybdenum compounds. In particular, promising results have been obtained when molybdate-based catalysts are promoted or supported. For example, Ni—Co—Mo, V—Nb—Mo/TiO 2 , K—MnMoO 4 , and K 2 MoO 4  have shown promise in ODH and other partial oxidation reactions. The positive effect of alkali dopants (Li, Na, K, Rb, and Cs) has been discussed in many oxidation reactions and is becoming more and more applicable to different catalysts. However, the effect is still not well characterized. Alkali doping can have the effect of increasing selectivity and activity while preventing phase transformations, inhibiting sintering, and creating basic centers on the catalyst surface. Abello et al. in  Catal. Letters  53, 53 (1998) have shown a significant increase in selectivity on Mo/MgO-γ-Al 2 O 3  with the addition of potassium. On this catalyst, an interesting trend was noticed in catalyst activity, redox behavior, and surface acidity. Furthermore, past work from our group has shown that potassium can largely affect oxygen exchange between bulk MnMoO 4  catalysts and gas phase oxygen as well as adsorption/desorption behavior of the catalyst. These parameters are the most common features used to describe ODH catalysts. 
     As previously mentioned, the study of silica-titania mixed oxides has gained much attention because of their high activity for epoxidation reactions of olefins with hydroperoxides. It has been cited that TiO 2  in mixed oxides of silica and titania can be present not only as anatase, but in the form of very small domains in which the normal octahedral coordination of TiO 2  has changed to tetrahedral (see Notari, B.,  Adv. Catal.  41, 253 (1996)). This leads to the unique structural and chemical properties of this material. Silica-titania mixed oxide supports, through sol-gel preparations, can provide advantages that the respective single oxides cannot. These benefits include stronger metal-support interactions, hindering reduction of the active metal, and smaller particle size that leads to better dispersion and higher surface area. Silica-titania mixed oxides have been studied extensively for attributes such as acidity, porosity, Ti—O—Si connectivity, and phase separations. However, few studies have been done on their use as active metal supports. 
     Baiker et al. in  Appl. Catal  A 35, 365 (1987) and Vogt et al. in  J. Catal.  114, 313 (1988) have used vanadia supported on silica-titania mixed oxides for the reduction of nitric oxide with ammonia. Baiker et al. has shown that the addition of titania causes an interaction that prevents agglomeration of surface vanadia species. Udomsak et al. in  Ind. Eng. Chem. Res.  35, 47 (1996) have shown a significant difference in isobutane dehydrogenation activity on chromia/silica-titania catalyst with different preparation methods. Hydrogen and carbon monoxide interaction with titania promoted palladium on silica was studied by Rieck and Bell in  J. Catal.  99, 262 (1986). Here, it was shown that TiO, species decorate the palladium, causing a notable difference in the CO adsorption behavior. Feng et al. in  J. Catal,  136, 423 (1993) have shown the hydrogen abstracting ability of the weakly acidic silica titania mixed oxide supported palladium catalysts was the dominating factor for non-oxidative dehydrogenation of propane. 
     Sol-gel science is well summarized well by Brinker and Scherer in “Sol-Gel Science”, Academic Press, New York, 1990, but the use of sol-gel preparations for supported metal catalysts is sparse. The use of the technology combined with active metal dispersion is limited to conventional techniques, such as wet impregnation (making the Si:Ti support first, then dispersing active components). Combining the positive effects of alkali doping and sol-gel science in a way to disperse the active component as the support network is forming has never been attempted in known literature. 
     Study of silica-titania mixed oxides have gained much attention because of their high activity for epoxidation reactions of olefins with hydroperoxides. Silica-titania mixed oxide supports, through sol-gel preparations, can provide advantages that the respective single oxides (SiO 2 , TiO 2 ) cannot. Silica-titania mixed oxides have been studied extensively. However, few studies have been done on their use as active metal supports. Furthermore, catalysts containing active metals supported over Si:Ti mixed oxides have not been prepared in the manner of this invention nor do they use the same materials. 
     The above-cited references are hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     The present invention includes a catalyst, a method for its preparation and a method for using the catalyst. 
     The present invention includes a sol-gel supported catalyst for the dehydrogenation of lower alkanes, the catalyst comprising at least one active metal and at least one promoter metal attached to a sol-gel mixed oxide support. The sol-gel mixed oxide support arising from the polymerization of at least one precursor. The active metal and the promoter metal having been attached to the sol-gel mixed oxide support by the active metal and the promoter metal having been co-precipitated with the precursor of the sol-gel mixed oxide support. 
     Lower alkanes, as used herein, may typically include those alkanes of C( 1-5 ) carbons. 
     The active metal, as used herein, may be any metal adapted to bring about oxidative dehydrogenation activity and may preferably include molybdenum or vanadium. The active metal may be present in any amount that is effective and may include anywhere from 1 to 70% by weight of the finished catalyst and more particularly from 1 to 20% by weight. 
     The promoter metal (also called the alkali promoter), as used herein, is selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals, and mixtures thereof. The promoter metal is preferably selected from metals from Group IA of the Periodic Table, in particular, the promoter metal may include alkali metals such as Li, Na, K, Rb, or Cs. 
     The active metal is supported over a mixed metal oxide support and may preferably include a silica-titania mixed oxide support (i.e. SiO 2 —TiO 2 ). The support mixed metal oxide molar ratio may range from 0:1 to 1:0, with a Si:Ti ratio of 1:1 being the most preferred. 
     Furthermore, promoter metal loading has been on a molar basis and has ranged from 0 to above 2-alkali/active metal molar ratios. 
     Also included within the scope of the present invention is a method of producing a sol-gel supported catalyst. The method comprising: obtaining a sol-gel precursor solution comprising at least one silicon alkoxide and at least one titanium alkoxide in a solution, adding to the sol-gel precursor solution at least one active metal-containing precursor in aqueous solution and at least one promoter metal-containing precursor in solution; and allowing the silicon alkoxide and the titanium alkoxide to become polymerized to form a sol-gel while allowing the active metal-containing precursor and the promoter metal-containing precursor to precipitate. 
     Active metal-containing precursors, as used herein, may be selected from the group consisting of ammonium heptamolybdate, molybdenum isopropoxide, molybdic acid, silicomolybdic acid, molybdenum chloride, molybdenum oxide, vanadium chloride, vanadium oxyalkoxides, vanadium acetylacetonate, vanadium pentoxide, vanadium acetate, pure V and Mo powders and other molybdenum or vanadium precursors. 
     Promoter metal-containing precursors, as used herein, may be selected from the group consisting of carbonates, nitrates, hydroxides, chlorides and molybdates of alkali, alkaline earth and rare earth metals (i.e. X 2 MoO 4  where X is any of the above-mentioned metals). 
     The catalyst may be prepared in a solvent. The solvent may be any solvent that can dissolve titanium or silicon alkoxides. The solvent may be selected from alcohol, hexane, benzene or other polar-aprotic solvents to name a few. If alcohol is used, it may comprise pure or mixed alcohols selected from the group consisting of methanol, ethanol, propanol, iso-propanol, and butanol. 
     Finally, the present invention includes a method of dehydrating lower alkanes to produce lower alkenes using a catalyst. The method comprising the steps: (a) obtaining a sol-gel supported catalyst, the catalyst comprising at least one active metal and at least one promoter metal attached to a sol-gel mixed oxide support. The sol-gel mixed oxide support arising from the polymerization of at least one precursor thereof, the active metal and the promoter metal having been attached to the sol-gel mixed oxide support by the active metal and the promoter metal having been co-precipitated with the precursor of the sol-gel mixed oxide support; and (b) bringing into contact with the catalyst at least one lower alkane for sufficient time and at sufficient temperature so as to allow the lower alkane to be dehydrogenated. 
     The precursor of the sol-gel mixed oxide support, as used herein, may be any liquid alkoxide compound and is preferably silicon and titanium alkoxides. These include Si(OR) 4  and Ti(OR) 4 , where R can be CH 3 , C 2 H 5 , linear C 3 H 7 , branched C 3 H 7  (isopropyl) or C 4 H 9 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a lower alkane partial oxidation/oxidative dehydrogenation reactor system, in accordance with one embodiment of the present invention. 
     FIG. 2 shows Raman spectra of Si:Ti 1:1 support, in accordance with one embodiment of the present invention. 
     FIG. 3 shows Raman spectra of 10% (K/Mo)/Si:Ti 1:1 catalysts, in accordance with one embodiment of the present invention. 
     FIG. 4 shows Titania 2p 3/2  binding energies for 10% (K/Mo)/Si:Ti 1:1 catalysts, in accordance with one embodiment of the present invention. 
     FIG. 5 shows the variation of the conversion of propane for 10%Mo/Si:Ti 1:1 catalysts with different K/Mo ratios, equal surface area reactions (65m 2 ), %N 2 /C 3 /O 2 :61%/26%/13%, 25 cc/min, in accordance with one embodiment of the present invention. 
     FIG. 6 shows the selectivity to propylene for 10%(K/Mo)/Si:Ti 1:1 catalysts at equal propane conversion (˜5% and ˜10%), 450° C., %N 2 /C 3 /O 2 :61%/26%/13%, 25 cc/min. (*-did not reach 10% Conversion), in accordance with one embodiment of the present invention. 
     FIG. 7 shows the effect of Si:Ti molar ratio of 10% (K/Mo=2)/Si:Ti 1:1 catalysts on yield, constant Mo loading (0.1 g), 550° C., % N 2 /C 3 /O 2  92.5/5/2.5, 25 cc/min, in accordance with one embodiment of the present invention. 
     FIG. 8 shows adsorbed NH 3  IR bands over 10% (K/Mo)/Si:Ti 1:1 catalysts and bare support, in accordance with one embodiment of the present invention. 
     FIG. 9 shows the temperature programmed reduction profiles for 10% (K/Mo)/Si:Ti 1:1 catalysts, 10% H 2 /N 2 , in accordance with one embodiment of the present invention. 
     FIG. 10 shows propane temperature programmed desorption: propene desorption profiles over 10% (K/Mo)/Si:Ti 1:1 catalysts, in accordance with one embodiment of the present invention. 
     FIG. 11 shows propane temperature programmed desorption: carbon monoxide desorption profiles over 10% (K/Mo)/Si:Ti 1:1 catalysts (*=ethylene desorption), in accordance with one embodiment of the present invention. 
     FIG. 12 shows (a) In situ DRIFT spectra of 10% (K/Mo)/Si:Ti 1:1 catalysts, 450° C. surface temperature, % N 2 /C 3 /O 2 :61/26/13 (b) DRIFT spectra of 10% (K/Mo)/Si:Ti 1:1 catalysts after quenching under N 2 , in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with the foregoing summary, the following presents a detailed description of the preferred embodiment of the invention that is currently considered to be the best mode. 
     The preferred method of preparation of the catalyst is as follows. This method takes advantage of sol-gel chemistry, or in other words, inorganic polymerization. During slow addition of water (hydrolysis), the alkyl group leaves and forms an alcohol by nucleophilic addition of water. The next three types of reactions are condensation reactions in which the support structure forms; alcoxolation, oxolation, and olation. The reaction type the alkoxide precursors use to form depends on the charge of the metal, the leaving group, and the type of solvent used. There are many parameters that affect the final structure of the sol-gel prepared silica-titania oxide, including, the charge on metal, the coordination chemistry, the composition, the electronegativity, the pH, what alkoxy groups are used, the water/precursor ratio, the rate at which the water is added, the addition of catalyzing agent, the concentration, the solvent employed, the temperature, and the drying /aging methods. As they pertain to surface area, final catalyst activity, and selectivity several parameter have been studied. Important information has been obtained regarding hydrolysis conditions, solvent effects, pH control, and drying conditions that have been used in further catalyst preparations. 
     One of the features believed to be unique in the preparation method of the present invention comes from the addition of the active metal and promoter metal. Aqueous solutions of active metal precursor and promoter metal precursors are contained in the hydrolyzing water and are added slowly to the solution containing Si and Ti alkoxide precursors. 
     For instance, alkali molybdates precipitate in the solution as Si and Ti alkoxides and are hydrolyzed to form Si—O—Si, Ti—O—Ti, and Ti—O—Si networks. This preparation method has been termed a modified, “one-pot” sol-gel/co-precipitation technique. Some of the advantages of this preparation method may include stronger metal-support interactions, hindering reduction of the active metal, and smaller particle size that leads to better dispersion and higher surface area. 
     The catalyst may be used for the oxidative dehydrogenation of propane in a catalytic, gas-phase reaction in which the feed consists of propane/oxygen/nitrogen at varying low rates and propane/oxygen molar ratios. The concentration of the feed stream should be maintained outside the flammability limits of propane-oxygen-nitrogen mixtures. The reaction temperature may preferably range from 300 to 550° C. The main products of this type of dehydrogenation reaction are propene, ethylene, carbon dioxide, carbon monoxide, and water. Acrolein was the only oxygenated product observed, and when observed, was at the ppm level. 
     EXAMPLES 
     Catalysts may be prepared using a modified sol-gel, co-precipitation technique. The method that may be used involves the reaction of metal alkoxide precursors in an alcohol solvent when contacted with water. During hydrolysis, the alkyl group leaves and forms an alcohol by nucleophilic addition of water. The next three types of reactions are condensation reactions; alcoxolation, oxolation, and olation. The type of reaction that the alkoxide precursors condense by depends on the charge of the metal, the leaving group, and the type of solvent used. There are many parameters that affect the final structure of the silica-titania that is prepared by the sol-gel method. These include the charge on metal, the coordination chemistry, the composition, the electronegativity, the pH, the number and type of alkoxy groups, the water/precursor ratio, the addition of catalyzing agent, the concentration, the solvent used, the temperature, and the drying/aging methods. 
     Ammonium heptamolybdate, (AHM) (Mallinkrodt) and KOH (Fisher) were used for molybdenum and potassium precursors respectively. For silica-titania mixed oxides, tetraethylorthosilicate (TEOS) (Aldrich) and titanium(IV) isopropoxide (TIPO) (Aldrich) were used. 
     The solvent was isopropyl alcohol. In this modified sol-gel method, calculated amounts of the silica and titania alkoxide precursors were placed in 100 mL of solvent to yield, after calcination, SiO 2 —TiO 2  mixed oxides with the desired Si:Ti molar ratio. This solution was left stirring while an aqueous solution containing the necessary amount of AHM (10% Mo loading) was then added drop-wise with a syringe pump. The aqueous solution added contained the stoichiometric amount of water necessary to hydrolyze all of the alkoxide precursors. The aqueous solution was added at a rate of 0.5 cm 3 /minutes for all catalysts with one exception. For the catalyst prepared with fast addition, the aqueous solution was added at a rate of 2 cm 3 /minutes. For catalysts containing potassium, KOH was added to the aqueous solution to give the desired K/Mo molar ratio. For catalysts denoted as pre-hydrolyzed, the stoichiometric amount of water necessary to hydrolyze the entire silica precursor was added to the silica precursor first and stirred for 15 minutes. Then, the titania precursor was added followed by the addition of the aqueous solution of potassium and molybdenum. For acidic and basic preparations, the effective pH of the alcohol solution was maintained at pH=3 (acidic) and pH=11 (basic) during preparation using HNO 3  and NH 4 OH respectively. Resulting gels were stirred for an additional 15 minutes after all of the aqueous solution had been added and dried at room temperature for less than 3 hours. They were then placed into an oven at 110° C. for overnight drying and solvent removal. After drying, the catalysts were ground to a fine powder and calcined under oxygen at 550° C. for 5 hours. This method is referred to as a “one-pot” sol-gel/co-precipitation because as the silica and titania precursors are hydrolyzed and precipitate out of solution, potassium molybdate species, that are insoluble in alcohol, also precipitate. 
     Synthesized catalysts are listed in Table 1 below. Catalysts numbered 1 through 7 are a series of molybdate catalysts with increasing K/Mo molar ratio at constant (10 weight %) loading of Mo and a Si:Ti molar ratio of 1. Catalysts 8 through 11 were prepared keeping K/Mo constant at 2 and varying the silica and titania content. Since TEOS hydrolyzes much slower than TIPO a series pre-hydrolyzed catalysts were also prepared. Catalysts 12 through 15 were synthesized using different hydrolysis methods (pre-hydrolysis of silica precursor, fast or slow addition, and acidic or basic conditions). Catalysts 16 and 17 refer to a bare-silica titania support and a potassium-doped silica titania support, respectively. 
     BET surface area measurement and nitrogen adsorption-desorption isotherms were recorded using a Micrometrics AccuSorb 2100E instrument. X-ray diffraction was performed with a Scintag PAD-V diffractometer using, Cu—Kα radiation. Raman spectra were recorded with a Dilor spectrometer using the 514.5 nm line of an Innova 300 Ar Laser. Spectra were taken in the range 200-1800 cm −1  in 180° back-scattering mode with a Spectrum One CCD detector. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Sol-gel catalysts and supports 
               
             
          
           
               
                   
                   
                   
                 Surface 
               
               
                   
                   
                   
                 Area 
               
               
                 # 
                 Composition 
                 Preparation 
                 (m 2 /g) 
               
               
                   
               
               
                  1 
                 10% Mo/Si:Ti 1:1 
                 sol-gel 
                 229 
               
               
                  2 
                 10% (K/Mo = 0.07)/Si:Ti 1:1 
                 co-precip., sol-gel 
                 136 
               
               
                  3 
                 10% (K/Mo = 0.14)/Si:Ti 1:1 
                 co-precip., sol-gel 
                 121 
               
               
                  4 
                 10% (K/Mo = 0.3)/Si:Ti 1:1 
                 co-precip., sol-gel 
                 166 
               
               
                  5 
                 10% (K/Mo = 0.6)/Si:Ti 1:1 
                 co-precip., sol-gel 
                  65 
               
               
                  6 
                 10% (K/Mo = 1)/Si:Ti 1:1 
                 co-precip., sol-gel 
                  17 
               
               
                  7 
                 10% (K/Mo = 2)/Si:Ti 1:1 
                 co-precip., sol-gel 
                 106 
               
               
                  8 
                 10% (K/Mo = 2)/SiO 2   
                 co-precip., sol-gel 
                 156 
               
               
                  9 
                 10% (K/Mo = 2)/TiO 2   
                 co-precip., sol-gel 
                  43 
               
               
                 10 
                 10% (K/Mo = 2)/Si:Ti 2:1 
                 co-precip., sol-gel 
                  75 
               
               
                 11 
                 10% (K/Mo = 2)/Si:Ti 1:2 
                 co-precip., sol-gel 
                  17 
               
               
                 12 
                 10% (K/Mo = 2)/Si:Ti 1:1 
                 acidic prehydrolyzed sol- 
                 271 
               
               
                   
                   
                 gel 
               
               
                 13 
                 10% (K/Mo = 2)/Si:Ti 1:1 
                 fast prehydrolyzed sol-gel 
                 149 
               
               
                 14 
                 10% (K/Mo = 2)/Si:Ti 1:1 
                 prehydrolyzed sol-gel 
                 178 
               
               
                 15 
                 10% (K/Mo = 2)/Si:Ti 1:1 
                 basic prehydrolyzed sol-gel 
                 179 
               
               
                 16 
                 Si:Ti Support only 
                 sol-gel 
                 320 
               
               
                 17 
                 K doped Si:Ti support 
                 sol-gel KOH (for K/Mo = 
                 380 
               
               
                   
                   
                 2) 
               
               
                   
               
             
          
         
       
     
     Temperature programmed reduction (TPR) of catalysts was performed using a laboratory-made gas flow system described in detail elsewhere. Catalyst samples (100 mg) were placed in a ¼-in.-i.d. U-tube quartz reactor flow at 550° C. for thirty minutes followed by cooling to room temperature under nitrogen. The reduction was performed with 10% hydrogen in nitrogen (25 cm 3 /minutes). The thermal conductivity detector (TCD) was operated in differential mode and the signal transferred to a data acquisition computer. The outlet of the reactor was passed through a column of silica gel to remove moisture formed during the reduction. The temperature program was as follows: 10 minutes at room temperature, 10°/minutes ramp rate to 850° C., and holding at 850° C. for 10 minutes. 
     X-ray photoelectron spectroscopy (XPS) of catalysts was performed with Physical Electronics/Perkin Elmer (model 550) ESCA/Auger Spectrometer operated at 15 kV, 20 mÅ, and using Mg—Kα radiation. Spectra were corrected using the C 1 s signal, located at 284.6 eV. Relative percentages of K 2 MoO 4  and MoO 3  in the samples were calculated using the integrals of the de-convoluted Mo 3d spectra. The de-convolution of Mo 3d spectra was accomplished using linked lets of equal FWHM, an intensity ratio of 2/3, and a splitting of 3.15 eV. 
     Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) of the catalysts was performed using a Bruker IFS66 equipped with a DTGS detector and a KBr beamsplitter. Catalysts were placed in a sample cup inside a Spectratech diffuse reflectance cell equipped with KBr windows and a thermocouple mount that allowed direct measurement of the surface temperature. Room temperature spectra for each catalyst were averaged over 1000 scans in the mid-IR range (400-4000 cm −1 ) to a nominal 2 cm −1  resolution. Prior to collecting spectrum, catalysts were pretreated under 10% oxygen in helium for thirty minutes at 400° C. surface, temperature to remove adsorbed water and carbon dioxide. For NH 3  adsorption experiments, background was taken under helium at room temperature. Following background measurement, NH 3  adsorption (0.5%NH 3 /He) was performed for 1 hour. Spectra were taken after evacuation for thirty minutes under helium. For in situ reaction spectra, background spectrum was taken at room temperature and with the sample surface temperature at 450° C. Gas phase spectra were taken at 450° C. The spectrum was taken after 15 minutes to 1 h of exposure to reaction mixture (%N 2 /C 3 /O 2 :61/26/13) afterwards a spectrum was taken once the reaction chamber was quenched to room temperature under nitrogen flow. 
     Propane Temperature Programmed Desorption (TPD) of catalysts was performed using the same laboratory-made gas flow system described for TPR experiments. Catalyst samples (100 mg) were placed in a ¼-in.-i.d. U-tube quartz reactor and pretreated under oxygen flow at 550° C. for thirty minutes followed by cooling to room temperature under helium. Samples were flushed with helium for 1 h followed by 1 h of propane adsorption. After adsorption, desorbed species were followed by a HP5890GC-MS under helium carrier gas. For these studies, the G.C. columns were replaced by an empty capillary column. The mass spectrometer was equipped with a quadrupole mass analyzer that allows tracking of up to 20 mass-to-charge ratios (m/z) simultaneously in the selected ion mode. Identification of species with equal m/z ratios was accomplished by following characteristic mass fragments of the species. The temperature program was as follows: 10 minutes at room temperature, 10°/minutes ramp rate to 700° C., and holding at 700° C. for 20 minutes. 
     Steady state reaction experiments were carried out in a fixed-bed, quartz reactor, operated at ambient pressure. The reactor system, shown in FIG. 1, is such that gaseous feed may be directed through ⅛-in. stainless steel lines into the reactor and furnace assemblies. An HP 5890 series II gas chromatograph equipped with FID and TCD detectors performed separations and analysis of reaction products online. Separations of hydrocarbon and inorganic species were performed using three columns: (1) Hayesep D (8-ft.×⅛-in.) (2) Porapak Q (6-ft-×⅛-in.) and (3) molecular sieve 5 Å (6-ft.×⅛-in.). Catalyst samples, ranging from 0.1 g to 1.5 g, were held in place by a quartz frit. The dead volume of the quartz microreactor was filled with quartz wool and/or ceramic beads to minimize effects from any homogeneous reaction/surface-assisted gas phase reaction and to provide a short residence time for propene formed. Reaction temperatures ranged from 723° K to 823° K. The quartz reactor, both empty and filled with quartz wool/ceramic beads, showed no activity up to 823° K. The feed consisted of propane/oxygen/nitrogen at flows between 20-200, usually at 25 cm 3 /minutes. The amount of nitrogen was varied for some runs. However, the propane/oxygen molar ratio was held constant at 2. The main products of the dehydrogenation reaction were propene, ethylene, carbon dioxide, carbon monoxide, and water. Acrolein was the only oxygenated product observed, and when observed, was at the ppm level. The product distributions maintained a carbon balance of 100% (+/−5%). Conversion is defined as the moles of carbon converted divided by the moles of carbon present in the feed. Selectivity is defined as the moles of carbon in the product divided by the moles of carbon reacted. 
     Molybdate catalysts show a general decrease in surface area with increasing amounts of potassium added. The catalyst with the K/Mo ratio of 2, however, is somewhat outside this trend. Furthermore, all catalysts containing potassium exhibited lower surface area than the “molybdenum only” catalyst. Comparing catalysts of different Si:Ti contents, it is seen that the highest surface area is achieved with a silica-titania molar ratio 1:1. Pre-hydrolyzed catalysts all show higher surface areas than the catalyst prepared using stoichiometric hydrolysis of both precursors. The effect of potassium on the support is shown to increase the surface area from 320 to 380 m 2 /g. 
     The nitrogen adsorption-desorption isotherm of the Si:Ti 1:1 indicated a micro to meso porous structure. The pore size distribution was calculated using the desorption isotherm. This yielded an average pore diameter of 2.1 nm and a pore volume of 0.34 cm 3 /g. 
     X-ray diffraction of the Si:Ti 1:1 support yielded a pattern typical of a silica titania sample. One broad peak with center located at a d spacing of 3.59 was observed, which is the most intense diffraction line from anatase structure. A broad peak is indicative of a finely dispersed, small x-ray particulate anatase structure supported over amorphous silica. With the addition of molybdenum and potassium species, this band becomes less narrower. The presence of potassium doping also has an effect on the width of this line, making it narrower with increasing K/Mo ratio. This suggests a change in the dispersion and segregation of titania in the Si:Ti matrix. No molybdenum species can be detected in the 10%(K/Mo) loaded Si:Ti 1:1 catalysts. This indicates that molybdena species are more finely dispersed on these mixed oxide supports than on silica or titania alone. Although quite weak, the two most intense peaks from crystalline molybdenum oxide become noticeable when Mo loading level is increased to 20%. 
     Raman spectra of the Si:Ti 1:1 support are shown in FIG.  2 . The bands associated with anatase structure are shifted to lower wavenumbers than that of pure anatase and appear at 618, 488, and 370 cm −1 . The shoulder at 955 cm −1  provides some evidence for the Si—O—Ti connectivity as reported in the literature. A second band that is associated with the SI—O—Ti bond is at 1100 cm −1 . However, this band overlaps with the asymmetric Si—O—Si stretching vibration of 1070 cm −1  and is difficult to resolve. Raman Spectra of catalysts with different K/Mo ratios are presented in FIG.  3 . Present in the spectra are the 3 bands associated with anatase at ˜643, ˜523, and ˜404 cm −1 . These bands are shifted to lower wavenumbers than those of pure anatase and seen to grow in intensity with increasing K/Mo ratio. An important feature of these spectra is that there is little or no evidence of crystalline MoO 3  since the most intense band characteristic of Mo—O—Mo stretching vibrations in MoO 3  is not present, except as a very weak band on the catalyst with K/Mo=0.6. The bands associated with isolated terminal Mo═O stretching vibrations are visible in the 970-999 cm −1  region. Broad bands arising from surface-coordinated Mo—O—Mo vibrations are located around 850 cm −1 . For catalysts K/Mo=0.6 and 1, there is evidence for higher crystallinity of potassium molybdate species (K 2 MoO 4 , K 2 Mo 2 O 7 ) indicated by sharper bands around 900-950 cm −1 . 
     Molybdenum 3d 5/2  binding energies of catalysts K/Mo supported catalysts are presented in Table 2 below. In potassium containing catalysts, molybdenum may exists in two distinct environments, one that corresponds to a MoO 3  matrix and the other to a K 2 MoO 4  matrix. Binding energies of bulk MoO 3  and K 2 MoO 4  are presented for comparison. When on a Si:Ti 1:1 support, our work has shown that Mo 3d binding energies shift to a lower value when compared to bulk MoO 3 . The Mo3d 5/2  peak for the K/Mo=0.6 shows the nearest binding energy to that of bulk MoO 3  possibly indicating the presence of three-dimensional MoO 3  regions on this catalyst. The percentages of molybdenum in the MoO 3  matrix, calculated using the deconvoluted peak areas, closely match the “as prepared” compositions. It appears that all of the potassium added to these catalysts exists in a K 2 MoO 4  type matrix. Furthermore, K 2p 3/2  spectra show one peak at an average location of 292.5eV corresponding to that of K 2 MoO 4 . 
     
       
         
               
             
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Mo 3d  5/2  Binding Energies of Si:Ti 1:1 Supported K/Mo Catalysts 
               
             
          
           
               
                 Catalyst 
               
               
                   
               
             
          
           
               
                   
                 1st Mo 3d 5/2   
                 2nd Mo 3d  5/2   
               
               
                 MoO 3   
                 233.4 
                 — 
               
               
                 10% Mo/Si:Ti 1:1 
                 232.4 
                 231.7 
               
               
                 10% (K/Mo = 0.3)/Si:Ti 1:1 
                 232.6 
                 231.2 
               
               
                 10% (K/Mo = 0.6)/Si:Ti 1:1 
                 233.2 
                 232.0 
               
               
                 10% (K/Mo = 2)/Si:Ti 1:1 
                 232.3 
                 231.6 
               
               
                 K 2 MoO 4   
                 — 
                 231.8 
               
               
                   
                 % MoO 3  - XPS 
                 % MoO 3  - as prepared 
               
               
                 10% (K/Mo = 0.3)/Si:Ti 1:1 
                 83 
                 85 
               
               
                 10% (K/Mo = 0.6)/Si:Ti 1:1 
                 74 
                 70 
               
               
                 10% (K/Mo = 2)/Si:Ti 1:1 
                 5 
                 0 
               
               
                   
               
             
          
         
       
     
     To study the change in the nature of the support with K/Mo ratio, Ti 2p 3/2  XPS were taken for catalysts with K/Mo=0, 0.3, 0.6 and 2. The variation of Ti 2p 3/2  binding energies with increasing K/Mo ratio is presented in FIG.  4 . Here, the Ti 2p 3/2  binding energy is seen to shift to lower values with the addition of potassium. The binding energy for the K/Mo=0 catalyst appears at 459.7 eV indicating that titania is closely interacting with silica and in a state of very small anatase domains. The peaks shift to lower binding energies with increasing K/Mo ratios, reaching 458.2 eV for the K/Mo=2 catalyst. The Ti 2p 3/2  binding energy of pure anatase is around 458.0 eV. 
     Catalysts with K/Mo ratios ranging from 0 to 2 as well as the bare Si:Ti 1:1 support, were tested in the ODH reaction using equal surface area loading (65 m 2 ) in the reactor and at temperatures of 450° C. and 550° C. The feed percentages for these experiments were N 2 /C 3 H 8 /O 2 :61%/26%/13%. Variation of propane conversion with K/Mo ratio for these equal surface area tests is presented in FIG.  5 . At 450° C., conversion goes through a broad maximum reaching about 21% at K/Mo=0.07 and falls to zero at K/Mo=2. At 550° C., variation of conversion shows a similar same trend, but this time, reaching a maximum of about 33%. The Si:Ti support exhibits a higher conversion than catalysts with K/Mo=0.6 and above. Table 3 summarizes the product distribution obtained in these experiments. The yield of propylene, which is the major reaction product, mimics the trend observed for conversion, showing a broad maximum with increasing K/Mo ratio. The yield of propylene drops rather drastically at K/Mo ratios higher than 0.6. The support shows fairly high activity compared to high K/Mo-ratio catalysts. The yields of C 2 H 4 , C 2 H 6 , and CH 4  are all lower than 1%. At the low K/Mo ratios, the CO yield is higher than the CO 2  yield, but CO yield drops very rapidly with increasing K/Mo ratio. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Reaction Comparison for 10% Mo/Si:Ti 1:1 Catalysts With Different K/Mo Ratios 
               
             
          
           
               
                   
                 Yield 
               
             
          
           
               
                   
                 C 3 H 6 (%) 
                 CO 2 (%) 
                 CO(%) 
                 C 2 H 4 (%) 
                 CH 4 (%) 
                 C 2 H 6 (%) 
               
               
                   
                   
               
             
          
           
               
                 Si:Ti 1:1 
                   
                   
                   
                   
                   
                   
               
               
                 T (?C)-450 
                 6.9 
                 1.2 
                 0.7 
                 0.1 
                 0.0 
                 0.0 
               
               
                 T (?C)-550 
                 18.9 
                 3.6 
                 3.6 
                 0.5 
                 0.2 
                 0.0 
               
               
                 10%(Mo)/Si:Ti 1:1 
               
               
                 T (?C)-450 
                 11.4 
                 2.3 
                 4.9 
                 0.0 
                 0.1 
                 0.0 
               
               
                 T (?C)-550 
                 20.0 
                 3.1 
                 6.7 
                 0.4 
                 0.8 
                 0.0 
               
               
                 10%(K/Mo = 0.07)/Si:Ti 1:1 
               
               
                 T (?C)-450 
                 13.9 
                 2.3 
                 4.8 
                 0.0 
                 0.0 
                 0.0 
               
               
                 T (?C)-550 
                 21.2 
                 3.4 
                 6.0 
                 0.4 
                 0.8 
                 0.0 
               
               
                 10%(K/Mo = 0.14)/Si:Ti 1:1 
               
               
                 T (?C)-450 
                 11.3 
                 1.8 
                 3.4 
                 0.0 
                 0.0 
                 0.0 
               
               
                 T (?C)-550 
                 21.4 
                 4.1 
                 5.6 
                 0.0 
                 1.3 
                 0.0 
               
               
                 10%(K/Mo =0.3)/Si:Ti 1:1 
               
               
                 T (?C)-450 
                 5.0 
                 0.2 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 T (?C)-550 
                 19.9 
                 3.9 
                 2.9 
                 0.4 
                 0.4 
                 0.0 
               
               
                 10%(K/Mo =0.6)/Si:Ti 1:1 
               
               
                 T (?C)-450 
                 1.4 
                 0.2 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 T (?C)-550 
                 11.5 
                 2.5 
                 0.6 
                 0.6 
                 0.0 
                 0.1 
               
               
                 10%(K/Mo = 1)/Si:Ti 1:1 
               
               
                 T (?C)-450 
                 0.5 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 T (?C)-550 
                 7.2 
                 0.6 
                 0.0 
                 0.3 
                 0.2 
                 0.0 
               
               
                 10%(K/Mo = 2)/Si:Ti 1:1 
               
             
          
           
               
                 T (?C)-450 
                 no conversion 
               
             
          
           
               
                 T (?C)-550 
                 4.8 
                 0.6 
                 0.0 
                 0.1 
                 0.1 
                 0.0 
               
               
                   
               
               
                 Conditions: equal surface area reactions (65 m 2 ), % N 2 /C 3 /O 2 : 61%/26%/13%, 25 cc/min.  
               
             
          
         
       
     
     To better compare the selectivity of these catalysts, a series experiments were performed at 450° C. keeping the propane conversion constant at 5% and at 10%. Equal conversion levels were achieved by changing the mass of catalyst loaded. The feed concentrations were the same as that of the equal surface area experiments. FIG. 6 shows the variation of propylene selectivities with K/Mo ratio at 5% and 10% conversion levels. The propylene yield goes through a relatively sharp maximum reaching yield levels of about 96% and 92% for 5% and 10% conversion, respectively. Table 4 shows the overall product distribution for the equal-conversion experiments. It is seen that the carbon monoxide selectivity decreases with the increasing K loading, dropping to zero at K/Mo ratios 0.6 and above. While CO selectivity drops to zero at these higher K/Mo ratios, we begin to see formation of ethylene and methane. 
     The need for high yield catalysts is one of the most important considerations for using this process in propylene production. The previously mentioned reaction results were obtained with concentrated feed mixtures (N 2 /C 3 H 8 /O 2  61%/26%/13%) to provide a higher propylene concentration in the 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Reaction Comparison for 10%(K/Mo)/Si:Ti 1:1 Catalysts With 
               
               
                 Different K/Mo Ratios at Equal Propane Conversion 
               
             
          
           
               
                   
                 Selectivity 
               
             
          
           
               
                   
                 C 3 H 6 (%) 
                 CO 2 (%) 
                 CO(%) 
                 C 2 H 4 (%) 
                 CH 4 (%) 
                 C 2 H 6 (%) 
               
               
                   
                   
               
             
          
           
               
                 ˜5% Propane Conversion 
                   
                   
                   
                   
                   
                   
               
               
                 10%(Mo)/Si:Ti 1:1 
                 86.0 
                 5.6 
                 8.4 
                 0.1 
                 0.0 
                 0.0 
               
               
                 10%(K/Mo = 0.07)/Si:Ti 1:1 
                 92.3 
                 5.5 
                 2.2 
                 0.1 
                 0.0 
                 0.0 
               
               
                 10%(K/Mo = 0.14)/Si:Ti 1:1 
                 96.5 
                 3.2 
                 0.3 
                 0.1 
                 0.0 
                 0.0 
               
               
                 10%(K/Mo = 0.3)/Si:Ti 1:1 
                 96.1 
                 3.2 
                 0.5 
                 0.1 
                 0.1 
                 0.0 
               
               
                 10%(K/Mo = 0.6)/Si:Ti 1:1 
                 89.3 
                 10.2 
                 0.0 
                 0.5 
                 0.0 
                 0.0 
               
               
                 10%(K/Mo = 1)/Si:Ti 1:1 
                 88.5 
                 10.6 
                 0.0 
                 0.4 
                 0.1 
                 0.4 
               
               
                 10%(K/Mo = 2)/Si:Ti 1:1 
                 85.6 
                 10.0 
                 0.0 
                 2.5 
                 2.0 
                 0.0 
               
               
                 ˜10% Propane Conversion 
               
               
                 10%(Mo)/Si:Ti 1:1 
                 79.3 
                 7.0 
                 13.6 
                 0.1 
                 0.0 
                 0.0 
               
               
                 10%(K/Mo = 0.07)/Si:Ti 1:1 
                 88.5 
                 5.0 
                 6.4 
                 0.1 
                 0.0 
                 0.0 
               
               
                 10%(K/Mo = 0.14)/Si:Ti 1:1 
                 88.0 
                 7.5 
                 4.3 
                 0.1 
                 0.1 
                 0.0 
               
               
                 10%(K/Mo = 0.3)/Si:Ti 1:1 
                 91.7 
                 4.9 
                 3.3 
                 0.1 
                 0.0 
                 0.0 
               
               
                 10%(K/Mo = 0.6)/Si:Ti 1:1 
                 89.0 
                 7.4 
                 0.0 
                 2.4 
                 1.3 
                 0.0 
               
               
                 10%(K/Mo = 1)/Si:Ti 1:1 
                 87.3 
                 6.8 
                 0.0 
                 3.3 
                 2.6 
                 0.0 
               
             
          
           
               
                 10%(K/Mo = 2)/Si:Ti 1:1 
                 Did not reach 10% 
               
               
                   
               
               
                 Conditions: 450° C., % N 2 /C 3 /O 2 : 61/26/13, 25 cc/min.  
               
             
          
         
       
     
     product stream. Experiments were also performed using a more dilute propane concentration to obtain higher yields, (N 2 /C 3 H 8 /O 2 :92.5%/5%/12.5. Using this feed concentration, the performances of two of the catalysts with highest propylene selectivity (K/Mo ratios of 0.07 and 0.3) were compared to those of the potassium-free catalyst and of the bare Si:Ti support. 
     A set of pre-hydrolyzed catalysts with a constant K/Mo ratio of 2 were compared to the “stoichiometrically” hydrolyzed catalyst in regard to their ODH behavior. Selectivities obtained at an equal conversion of ˜3% and a temperature of 500° C. are presented in Table 5. When the silica precursor is prehydrolyzed keeping the other synthesis parameters the same, we observe an appreciable increase in propylene selectivity compared to the catalysts prepared without any prehydrolysis step. However, changing the preparation parameters, such acidity or the speed of addition of the aqueous solutions for the prehydrolyzed catalyst does not appear to have much effect on propylene 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Effect of Hydrolysis Conditions for 10% (K/Mo = 2)/Si:Ti 1:1 Catalysts 
               
             
          
           
               
                   
                 Selectivity 
               
             
          
           
               
                   
                 C 3 H 6   
                 CO 2   
                   
                 C 2 H 4   
                 CH 4   
               
               
                   
                 (%) 
                 (%) 
                 CO (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
          
           
               
                 Hydrolyzed 
                 83.2 
                 15.5 
                 0.0 
                 1.1 
                 0.1 
               
               
                 Prehydrolyzed 
                 90.0 
                 8.6 
                 0.0 
                 1.4 
                 0.0 
               
               
                 Prehydrolyzed Fast Addition 
                 73.5 
                 24.2 
                 0.0 
                 2.2 
                 0.0 
               
               
                 Prehydrolyzed Acidic (pH = 3) 
                 66.9 
                 28.2 
                 0.0 
                 4.9 
                 0.0 
               
               
                 Prehydrolyzed Basic (pH = 11) 
                 77.3 
                 13.3 
                 7.1 
                 2.0 
                 0.3 
               
               
                   
               
               
                 Conditions: ˜3% propane conversion, 500° C., % N 2 /C 3 /O 2  92.5/5/2.5, 25 cc/min.  
               
             
          
         
       
     
     selectivity. Another point worth noting about these comparisons is the fact that catalysts prepared without pH control (with or without prehydrolysis) show the lowest CO 2  and ethylene selectivities. 
     To investigate the effect of the support composition, catalysts with different Si:Ti ratios were compared using 10% Mo loading and a K/Mo ratio of 2. The comparison was based on equal mass of Mo metal in the reactor. The temperature for these reactions was 550° C. The yields of different reaction products obtained are presented in FIG.  7 . There was no C 2 H 6  observed in these runs. The highest propylene yield was obtained over the catalysts that had a Si:Ti ratio of 1:1. This catalyst showed no CO or CH 4  formation and very little CO 2  and C 2 H 4  formation. The propylene yield for the Si-rich catalysts also appear to give relatively high yields for propylene, but these are accompanied by high yields for CO, CO 2 , C 2 H 4  and CH 4 . 
     Homogeneous volume minimization downstream from reactor or free-radical quenching is necessary in propane ODH reaction to isolate catalytic activity from gas phase activity. Radicals, once formed on the surface of the catalyst, can desorb during reaction and contribute to gas phase pyrolysis downstream from the catalyst bed. To examine the contribution of homogeneous reaction, experiments were performed with and without dead volume packing downstream from the reactor. Our results showed this effect to be important even at 400° C. Experiments performed using the K/Mo=0.07 catalyst and a feed stream ((%N 2 /C 3 /O 2 :92.5%/5%/2.5%).) of 25 cm 3 /min flow rate showed that the propane conversion increased from 13 to 16% when packing downstream from the catalyst bed was removed. 
     The IR spectra of ammonia species formed on 10%Mo/Si:Ti 1:1 catalysts with different K/Mo ratios, are shown in FIG.  8 . The bands listed in Table 6 characterize the spectra. Bands commonly used to characterize Lewis and Brønsted acidity are those located at ˜1607 cm −1  (Lewis) and ˜1448 cm −1  (Brønsted). In Table 6, an attempt to quantify the Lewis and Brønsted characteristics of the catalysts is made using these peak areas and intensities. Compared to the Si:Ti support, the K/Mo=0 catalyst has considerably more Brønsted acid character and similar Lewis acid character. With the addition of potassium to the catalyst, the Lewis acid character decreases to a minimum at K/Mo=0.07 and sharply increases at K/Mo=2. 
     Temperature Programmed Reduction experiments were performed on catalysts with different K/Mo ratios. The results are compared to bulk MoO 3  and K 2 MoO 4  samples prepared from the same precursors as the synthesized catalysts. The results are plotted in FIG.  9 . The profiles for supported K/Mo catalysts are similar, consisting of one major temperature maximum in the 400-500° C. range. As the K/Mo ratio increases, the maxima begin to shift to higher temperatures and the peaks show considerable broadening. At K/Mo ratios of 0.6 and higher, we begin to see peaks becoming very noticeably asymmetrical, possibly representing two different reduction sites. 
     
       
         
               
             
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
             
             
               
                 NH 3  IR Adsorption Bands 
               
             
          
           
               
                   
                 Band (cm −1 ) Acidity 
                 Assignment 
               
               
                   
                   
               
               
                   
                 1074-1082 H-Bonded NH 3   
                 NH 2  Rocking 
               
               
                   
                 1220-1248 Lewis 
                 N—N Streching 
               
               
                   
                 1450-1434 Brönsted 
                 NH 2  Wagging 
               
               
                   
                 1604-1607 Lewis 
                 NH 2  Scissoring 
               
               
                   
                 1670-1680 Brønsted 
                 NH 2  Scissoring 
               
               
                   
                   
               
             
          
           
               
                 baseline corrected peak areas 
               
             
          
           
               
                   
                 Lewis 
                 Brønsted 
                   
                   
               
               
                 K/Mo Ratio 
                 ˜1607 cm −1   
                 ˜1448 cm −1   
                 Area B/Area L 
                 % Lewis* 
               
               
                   
               
               
                 Si:Ti 1:1 
                 6.4 
                 26.1 
                 4.1 
                 20 
               
               
                 0   
                 6.8 
                 42.4 
                 6.3 
                 14 
               
               
                  0.07 
                 1.3 
                 32.7 
                 25.0 
                  4 
               
               
                 0.3 
                 5.1 
                 34.0 
                 6.6 
                 13 
               
               
                 0.6 
                 1.1 
                 9.4 
                 8.8 
                 10 
               
               
                 2   
                 5.7 
                 15.3 
                 2.7 
                 27 
               
               
                   
               
               
                 *calculated with peak areas  
               
             
          
         
       
     
     When these profiles are compared to that of bulk K 2 MoO 4 , we see that the temperature for this secondary feature, which appears as a large shoulder, coincides with the major reduction peak observed over the bulk K 2 MoO 4 . Table 7 summarizes the temperature maxima and the FWHM (Full Width at Half Maximum) of these major reduction peaks. Analysis of the desorbed species after propane adsorption showed propane, propylene, water, CO 2 , CO, O 2 , and trace amounts of methane, ethane, and acrolein desorbing from the surface. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Temperature Programmed Reduction of 10% (K/Mo)/Si:Ti 1:1 Catalysts 
               
               
                 With Different K/Mo Ratios 
               
             
          
           
               
                   
                 Catalyst 
                 1st peak Maxima (° C.) 
                 FWHM (° C.) 
               
               
                   
                   
               
               
                   
                 MoO 3   
                 459 
                 34 
               
               
                   
                 K/Mo = 0 
                 436 
                 58 
               
               
                   
                 K/Mo = 0.07 
                 450 
                 46 
               
               
                   
                 K/Mo = 0.14 
                 456 
                 53 
               
               
                   
                 K/Mo = 0.3 
                 512, 560 sh 
                 75 
               
               
                   
                 K/Mo = 0.6 
                 531, 566 sh 
                 104  
               
               
                   
                 K/Mo = 2 
                 520, 570 sh 
                 118  
               
               
                   
                 K 2 MoO 4   
                 531 
                 54 
               
               
                   
                   
               
               
                   
                 sh = shoulder  
               
             
          
         
       
     
     Propylene desorption profiles for the Si:Ti 1:1 support, 10%Mo/Si:Ti 1:1, and potassium containing catalysts of K/Mo=0.07 and 0.3 are plotted in FIG.  10 . Focusing on the “molybdenum only” catalyst, there is one desorption feature with peak maximum temperature around 200° C. An important aspect of these profiles is that the first desorption feature shifts to lower temperatures with increasing potassium loading. The water desorption profiles follow the propylene desorption closely, indicating that propylene formation takes place oxidatively, using the lattice oxygen. The shift of the propylene desorption peaks to lower temperatures also points to an ease of desorption from the surface for the potassium containing catalysts. With the addition of potassium, two additional sites for propylene desorption are formed on the K/Mo=0.3 catalyst at higher temperatures (˜260 and ˜300° C.). The desorption profiles for carbon monoxide for the same catalysts are plotted in FIG.  11 . Although ethylene has the same molecular weight as carbon monoxide, by following fragments created by both, we determined that the first desorption peaks (&lt;200° C.) are associated with ethylene desorption and the remainder of the profile belongs to carbon monoxide alone. There are three significant desorption features present on the “molybdenum only” catalyst at 360, 465, and 650° C. With the addition of potassium, these desorption features appear to be suppressed and less pronounced in a broad profile. 
     To ascertain the differences in reaction intermediates present on the catalyst during reaction conditions, IR spectra of adsorbed species were obtained at 450° C. surface temperature. After gas phase spectra subtraction, the results are plotted in FIG.  12 ( a ) in the range 1800-1300 cm −1 . Similar bands are observed on the K/Mo catalysts studied. Band observed around 1689 cm −1  is associated with adsorbed acetone. Bands located around 1540, 1503, 1430, 1360, and 1328-1336 cm −1  are associated with acetate, formate, π-allyl, and acrolein type species. Olefinic CH x  stretches are located at 1458 and 1390 cm −1 . Two interesting features to note in FIG.  12 ( a ) are that the highest yield ODH catalyst of this study, K/Mo=0.07, shows the most intense olefinic CH x  stretching bands. Furthermore, one of the most selective ODH catalysts, K/Mo=0.3, shows a lack of intensity from the acetate and formate species present on the other catalysts. This agrees well with the reaction results as these intermediates may lead to the formation of CO x  products. In FIG.  12 ( b ), the IR spectra are shown after the reaction has been quenched to room temperature under nitrogen. It is apparent that olefinic species are still present on the “molybdenum only” catalyst, indicated by the bands at 1441 and 1375 cm −1 , which have shifted to lower wavenumbers with the decrease in temperature of the sample surface. Over the potassium-containing catalysts, on the other hand, these bands have essentially disappeared. Again, this suggests a weaker adsorption of propylene on the surface, leading to easier desorption. 
     The preferred embodiments herein disclosed are not intended to be exhaustive or to unnecessarily limit the scope of the invention. The preferred embodiments were chosen and described in order to explain the principles of the present invention so that others skilled in the art may practice the invention. Having shown and described preferred embodiments of the present invention, it will be within the ability of one of ordinary skill in the art to make alterations or modifications to the present invention, such as through the substitution of equivalent compounds or through the use of equivalent process steps, so as to be able to practice the present invention without departing from its spirit as reflected in the appended claims, the text and teaching of which are hereby incorporated by reference herein. It is the intention, therefore, to limit the invention only as indicated by the scope of the claims and equivalents thereof.