Bifunctional catalysts and systems and methods for oxidative dehydrogenation of alkanes to olefins and high-valued products

Abstract

Bifunctional catalyst compositions, methods, and systems are provided for the use of CO.sub.2 as a soft oxidizing agent to effectively convert low-value small alkanes to high-value small olefins. The bifunctional catalyst comprises a metal oxide catalyst and a redox-active ceramic support.

Claims

1. A bifunctional oxidative dehydrogenation catalyst comprising a metal oxide and a redox active ceramic support, wherein the redox active ceramic support promotes a water-gas shift process, and wherein the redox active ceramic support comprises a doped perovskite ceramic.

2. The bifunctional oxidative dehydrogenation catalyst according to claim 1, wherein the metal oxide is selected from the group consisting of Cr.sub.2O.sub.3, V.sub.2O.sub.5, In.sub.2O.sub.3, Fe.sub.2O.sub.3, and combinations thereof, and wherein the metal oxide is mixed with or impregnated in the redox active ceramic support.

3. The bifunctional oxidative dehydrogenation catalyst according to claim 1, wherein the redox active ceramic support comprises one or more of Al.sub.2O.sub.3, Ce.sub.2O.sub.3, ZrO.sub.2, BaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Yb.sub.0.1O.sub.2.95, and BaZr.sub.1-xY.sub.xO.sub.3-δ (0≤x≤0.20).

4. The bifunctional oxidative dehydrogenation catalyst according to claim 1, wherein the metal oxide promotes alkane dehydrogenation via a Mars van Krevelen process with carbon dioxide as an oxidant.

5. A method for alkane dehydrogenation, comprising: providing a catalytic membrane reactor comprising a catalyst bed and a hydrogen selective membrane, wherein the catalyst bed comprises a bifunctional catalyst comprising a metal oxide and a redox active ceramic support, and wherein the redox active ceramic support comprises a doped perovskite ceramic, contacting an input gas stream comprising at least one alkane and carbon dioxide with the bifunctional catalyst to form a product gas stream comprising at least one olefin and hydrogen, and separating the hydrogen from the product gas stream.

6. The method according to claim 5, wherein the metal oxide is selected from the group consisting of Cr.sub.2O.sub.3, V.sub.2O.sub.5, In.sub.2O.sub.3, Fe.sub.2O.sub.3, and combinations thereof, and wherein the metal oxide is mixed with or impregnated in the redox active ceramic support.

7. The method according to claim 5, wherein the redox active ceramic support comprises one or more of Al.sub.2O.sub.3, Ce.sub.2O.sub.3, ZrO.sub.3, BaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Yb.sub.0.1O.sub.2.95, and BaZr.sub.1-xY.sub.xO.sub.3-δ (0≤x≤0.20).

8. The method according to claim 5, wherein the at least one alkane comprises ethane, propane, butane, or mixtures thereof.

9. The method according to claim 5, wherein alkane dehydrogenation occurs at least partially via a Mars van Krevelen process and wherein a water-gas shift process forms at least some of the carbon dioxide.

10. The method according to claim 5, wherein a molar ratio of the at least one alkane to carbon dioxide in the input gas stream is greater than 1.

11. The method according to claim 5, wherein the contacting step occurs at a temperature from about 450° C. to about 650° C.

12. The method according to claim 5, wherein the hydrogen selective membrane comprises a material selected from the group consisting of a metal, a ceramic, a polymer, and combinations thereof.

13. The method according to claim 5, wherein a rate of olefin production is about 1.0×10.sup.−2 mol gcat.sup.−1 h.sup.−1 or more.

14. A catalytic membrane reactor comprising: a. a bifunctional catalyst comprising a metal oxide and a redox active ceramic support, wherein the redox active ceramic support comprises a doped perovskite ceramic; and b. a hydrogen selective membrane, wherein the bifunctional catalyst is contained in a catalyst bed that is at least partially enclosed by the hydrogen selective membrane.

15. The catalytic membrane reactor of claim 14, wherein the hydrogen selective membrane comprises a material selected from the group consisting of a metal, a ceramic, a polymer, and combinations thereof.

16. The catalytic membrane reactor of claim 14, wherein the metal oxide is selected from the group consisting of Cr.sub.2O.sub.3, V.sub.2O.sub.5, In.sub.2O.sub.3, Fe.sub.2O.sub.3, and combinations thereof, and wherein the metal oxide is mixed with or impregnated in the redox active ceramic support.

17. The catalytic membrane reactor of claim 14, wherein the redox active ceramic support comprises one or more of Al.sub.2O.sub.3, Ce.sub.2O.sub.3, and ZrO.sub.2.

18. The catalytic membrane reactor of claim 14, further comprising an input gas stream and first and second output gas streams, the input gas stream comprising carbon dioxide and at least one alkane, the first output gas stream comprising at least one olefin, and the second output gas stream comprising hydrogen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the disclosed system and together with the general description of the disclosure given above and the detailed description of the drawings given below.

(2) FIG. 1: A schematic representation of MvK oxidative dehydrogenation over a redox active metal oxide catalyst.

(3) FIG. 2: A schematic representation of a catalytic membrane reactor (CMR) using a packed catalyst bed. The inset is a schematic representation of MvK oxidative dehydrogenation and WGS processes over the bifunctional catalyst.

(4) FIG. 3: A schematic representation of a catalyst test system using a packed bed reactor.

(5) FIG. 4: (A) Net C.sub.3H.sub.6 production rate and propane conversion and yield as a function of C.sub.3H.sub.8/CO.sub.2 ratio at 600° C.; (B) C.sub.3H.sub.8 and CO.sub.2 conversion and product selectivity as a function of temperature for a C.sub.3H.sub.8/CO.sub.2 ratio of 2.5.

(6) FIG. 5: (A) Impact of H.sub.2 addition on propane conversion and propylene selectivity at GHSV=33,900 h.sup.−1 and a C.sub.3H.sub.8/CO.sub.2 ratio of 2.5; (B) CO.sub.2 conversion as a function of temperature and H.sub.2/CO.sub.2 ratio.

(7) FIG. 6: Normalized catalytic activity for Cr.sub.2O.sub.3/BZY and Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalysts for propane conversion at T=600° C.

DETAILED DESCRIPTION

(8) An aspect of the invention is an oxidative dehydrogenation (ODH) catalyst for the conversion of low-value small alkanes to high-value small olefins using CO.sub.2 as a soft oxidizing agent. The ODH catalyst is a bifunctional catalyst, comprising a metal oxide catalyst and a redox-active ceramic support.

(9) The metal oxide catalyst component in the bifunctional catalyst converts alkanes to olefins according to a Mars van Krevelen (MvK) cycle using CO.sub.2. Alkane dehydrogenation via CO.sub.2 as a soft oxidant is known to proceed on redox-active metal oxide catalysts such as, by way of non-limiting example, Cr.sub.2O.sub.3, V.sub.2O.sub.5, In.sub.2O.sub.3, or Fe.sub.2O.sub.3. In some embodiments, the metal oxide may be a mixture of two or more metal oxides.

(10) The redox-active ceramic support promotes water-gas-shift (WGS) chemistry. Suitable redox-active ceramic support materials are, by way of non-limiting example, alumina (Al.sub.2O.sub.3), Ce.sub.2O.sub.3, and ZrO.sub.2. In preferred embodiments, the redox-active ceramic support is a doped-perovskite ceramic. A general formula of a doped perovskite material may be AB.sub.xM.sub.1-xO.sub.3-δ, where x and δ represent cations molar ratio and oxygen-ion vacancy, respectively. Suitable doped-perovskite ceramics are, by way of non-limiting example, BaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Yb.sub.0.1O.sub.2.95, and BaZr.sub.1-xY.sub.xO.sub.3-δ (0≤x≤0.20) (BZY). It has previously been shown that some doped-perovskite ceramics (e.g., BZY) can avoid solid coke formation under ethane and propane steam-reforming environments. For example, in the presence of steam (an ODH byproduct), BZY becomes hydrated, providing surface hydroxyls. While not wishing to bound to a particular theory, these surface hydroxyls may spill over to the metal oxide catalyst to oxidize coke-precursors that may accumulate on the metal oxide. This information is set forth in: D. M. Jennings, C. Karakaya, H. Zhu, C. Duan, R. P. O'Hayre, G. S. Jackson, I. E. Reimanis, and R. J. Kee, “Measurement and characterization of a high-temperature, coke-resistant bi-functional Ni/BZY15 water-gas-shift catalyst under steam-reforming conditions,” Catal. Lett., 148:3592-3607, 2018; C. Duan, R. J. Kee, H. Zhu, C. Karakaya, Y. Chen, S. Ricote, A. Jarry, E. J. Crumlin, D. Hook, R. Braun, N. P. Sullivan, and R. P. O'Hayre, “Highly durable, coking and sulfur tolerant, fuel-flexible protonic ceramic fuel cells,” Nature, 557:217-222, 2018; and Z. Zhang, S. Liguori, T. F. Fuerst, J. D. Way, and C. A. Wolden, “Efficient ammonia decomposition in a catalytic membrane reactor to enable hydrogen storage and utilization,” ACS Sust. Chem. Eng., 7:5975-5985, 2019 (each of which are incorporated herein by reference in their entirety). In some embodiments, the ceramic support may be a mixture of two or more ceramic support materials.

(11) The bifunctional catalyst may be formed by known methods in the art. In some embodiments, the metal oxide is dispersed within the redox-active ceramic support by physically mixing the two materials together or the metal oxide may by impregnated into the ceramic support material. For instance, the bifunctional catalysts may be prepared by an incipient wetness impregnation technique using the corresponding metal nitrate or metal carbonate as a precursor, or using another suitable precursor, to form the metal oxide. A solution of the metal nitrate or metal carbonate precursor is prepared, the ceramic support is added to the solution and the mixture is diluted with water. The mixture is dried and then the metal oxide/ceramic support mixture is calcinated to remove any residual solvent and convert the metal nitrate or metal carbonate to a metal oxide. The calcination temperature depends upon the specific metal nitrate. Typical calcination temperatures range from about 450° C. to about 650° C., preferably 500° C. to about 600° C., and may be performed in air for several hours (e.g., 4-7 hours).

(12) In some embodiments, the bifunctional catalyst comprises less than about 50 wt. % of the redox-active metal oxide catalyst, less than about 45 wt. % of the redox-active metal oxide catalyst, less than about 40 wt. % of redox-active metal oxide catalyst, less than about 35 wt. % of the redox-active metal oxide catalyst, less than about 30 wt. % of the redox-active metal oxide catalyst, less than about 25 wt. % of the redox-active metal oxide catalyst, less than about 20 wt. % of the redox-active metal oxide catalyst, less than about 15 wt. % of the redox-active metal oxide catalyst, or less than about 10 wt. % of the redox-active metal oxide catalyst. In some embodiments, the bifunctional catalyst comprises at least 1 wt. % of the redox-active metal oxide catalyst, or at least 5 wt. % of the redox-active metal oxide catalyst. In some embodiments, the bifunctional catalyst comprises about 50 wt. % of the redox-active metal oxide catalyst, about 45 wt. % of the redox-active metal oxide catalyst, about 40 wt. % of the redox-active metal oxide catalyst, about 35 wt. % of the redox-active metal oxide catalyst, about 30 wt. % of the redox-active metal oxide catalyst, about 25 wt. % of the redox-active metal oxide catalyst, about 20 wt. % of the redox-active metal oxide catalyst, about 15 wt. % of the redox-active metal oxide catalyst, about 10 wt. % of the redox-active metal oxide catalyst, about 5 wt. % of the redox-active metal oxide catalyst, or about 1 wt. % of the redox-active metal oxide catalyst, or any range within any two of these values. In preferred embodiments, the bifunctional catalyst comprises between about 5 wt. % to about 20 wt. % of the redox-active metal oxide catalyst. In embodiments, the bifunctional catalyst comprises between about 1 wt. % to about 50 wt. %, preferably about 5 wt. % to about 20 wt. %, of the redox-active metal oxide catalyst, with the remainder of the bifunctional catalyst consists essentially of the redox-active ceramic support.

(13) Another aspect of the present invention is a method for ODH of alkanes. In some embodiments, the alkane is ethane, propane, and/or butane, although larger alkanes may also be reduced. In some embodiments, the alkane is a C.sub.2-C.sub.6 alkane or a mixture thereof. The method for alkane dehydrogenation comprises contacting an input gas stream comprising at least one alkane and carbon dioxide with the bifunctional catalyst to form a product gas stream comprising the corresponding olefin(s). The product gas stream may further comprise water, carbon monoxide, carbon dioxide, and hydrogen as reaction products. Water and carbon monoxide are produced from the ODH of alkanes, and the GWS process converts these products to carbon dioxide and hydrogen (reaction 7). Hydrogen may also be formed from non-oxidative reduction of alkanes on the metal oxide catalyst (reaction 6). The formation of carbon dioxide is beneficial as it is used as a reactant in the MvK process. The formation of hydrogen, however, may impede the overall process by shifting the GWS reaction towards the reactants. Because of this, in preferred embodiments, hydrogen is removed from the product gas, for example using a hydrogen selective membrane.

(14) In some embodiments, the method for alkane dehydrogenation, comprises: providing a catalytic membrane reactor (CMR) comprising a catalyst bed and a hydrogen selective membrane, wherein the catalyst bed comprises the bifunctional catalyst; contacting an input gas stream comprising at least one alkane and carbon dioxide with the bifunctional catalyst to form a product gas stream comprising the corresponding olefin(s); and separating hydrogen from the product gas stream. The product olefin(s) may be captured and purified down stream of the reactor.

(15) In some embodiments, the input gas stream is contact with the bifunctional catalyst at a temperature in the range of about 450° C. to about 700° C. In some embodiment, the input gas stream is contact with the bifunctional catalyst at a temperature of about 450° C., about 475° C., about 500° C., about 525° C., about 550° C., about 575° C., about 600° C., about 625° C., about 650° C., about 675° C., or about 700° C., or any range within any two of these values. In general, the rate of the ODH reactions increase with increasing temperature; however, at higher temperatures the olefin product(s) dehydrogenate to form coke or polyaromatic hydrocarbons (PAHs) which are detrimental for the catalyst stability. Removal of hydrogen from the catalyst bed helps to mitigate or alleviate the formation of undesirable coke and PAHs. In embodiments where hydrogen is removed from the product gas, the temperature may be lower, compared to cases where hydrogen is not removed, typically the temperature may be in the range of about 450° C. to about 550° C.

(16) The ratio of alkane to CO.sub.2 in the input stream may impact the alkane conversion and olefin production rate. In some embodiments, the input stream has a ratio of alkane/CO.sub.2 of less than about 1 (i.e., lean condition). In other embodiments, the amount of alkane/CO.sub.2 is greater than about 1 (i.e., rich condition). In some embodiments, the input stream has a ratio of alkane/CO.sub.2 of less than about 3.0, less than about 2.9, less than about 2.8, less than about 2.7, less than about 2.6, less than about 2.5, less than about 2.4, less than about 2.3, less than about 2.2, less than about 2.1, less than about 2.0, less than about 1.9, less than about 1.8, less than about 1.7, less than about 1.6, less than about 1.5, less than about 1.4, less than about 1.3, less than about 1.2, less than about 1.1, less than about 1.0, less than about 0.9, less than about 0.8, less than about 0.7, less than about 0.6, less than about 0.5, less than about 0.4, or less than about 0.3. In some embodiments, the input stream has a ratio of alkane/CO.sub.2 of greater than about 1.0, greater than about 1.1, greater than about 1.2, greater than about 1.3, greater than about 1.4, greater than about 1.5, greater than about 1.6, greater than about 1.7, greater than about 1.8, greater than about 1.9, greater than about 2.0, greater than about 2.1, greater than about 2.2, greater than about 2.3, greater than about 2.4, or greater than about 2.5. In some embodiments, the input stream has a ratio of alkane/CO.sub.2 of about 3.0, about 2.9, about 2.8, about 2.7, about 2.6, about 2.5, about 2.4, about 2.3, about 2.2, about 2.1, about 2.0, about 1.9, about 1.8, about 1.7, about 1.6, about 1.5, about 1.4, about 1.3, about 1.2, about 1.1, about 1.0 (i.e., stochiometric conditions), about 0.9, about 0.8, about 0.7, about 0.6, about 0.5, about 0.4, about 0.3, or about 0.2, or any range within any two of these values.

(17) In some embodiments, the olefin production rate increases as the ratio of alkane/CO.sub.2 is increased due the non-oxidative dehydrogenation pathway (reaction 6). In some embodiments, the olefin production rate may be greater than about 0.5×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 0.6×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 0.7×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 0.8×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 0.9×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 1.0×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 1.1×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 1.2×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 1.3×10.sup.−2 mol gcat.sup.−1 h.sup.−1, greater than about 1.4×10.sup.−2 mol gcat.sup.−1 h.sup.−1, or greater than about 1.5×10.sup.−2 mol gcat.sup.−1 h.sup.−1, at a temperature of between 450° C. to about 700° C. In some embodiments, the selectivity of olefins is greater than 50%, greater than 60%, greater than 70%, greater than 80%, greater than 90%, greater than 95%, greater than 98%, or greater than 99% at a temperature of between 450° C. to about 700° C.

(18) Another aspect of the invention is a system for ODH of alkanes. In some embodiments, ODH of alkanes is performed using a catalytic membrane reactor (CMR). The CMR comprises the bifunctional catalyst, disclosed herein, and a hydrogen selective membrane. In some embodiments, the reactor may be a fixed bed reactor or a packed bed reactor, where the catalyst bed is at least partially enclosed by a hydrogen selective membrane. Alternatively, the bifunctional catalyst may be wash-coated on the hydrogen selective membrane wall.

(19) FIG. 2 shows a schematic of an embodiment of a CMR. The reactor comprises a packed catalyst bed that is enclosed in a hydrogen selective membrane. The reactor is housed in an enclosure to accommodate the H.sub.2 that passes through the hydrogen selective membrane. The enclosure may be swept with a gas or a light vacuum may be applied to remove the hydrogen from the enclosure. The ODH step to produce olefin(s) takes place on metal oxide catalyst (e.g., Cr.sub.2O.sub.3). The unwanted co-products CO and H.sub.2O are converted to CO.sub.2 and H.sub.2 over the WGS active ceramic support (e.g., BZY). The H.sub.2 is removed from the catalyst bed through a H.sub.2-selective membrane thus shifting the WGS equilibrium to convert more CO to CO.sub.2, but also producing H.sub.2. After separation, the hydrogen may be captured and stored in a container or on a sorbent (contained in the outer enclosure or downstream of the outer enclosure) or it may used in other downstream processes. Likewise, the other reaction products, other than hydrogen, may be captured and stored, or used in other downstream processes.

(20) In some embodiments, the hydrogen selective membrane fully or at least partially encloses the catalyst bed. The hydrogen selective membrane may be selected from a metal, a ceramic, a polymer, and combinations thereof. Suitable hydrogen selective membranes are thin-film Pd and Pd alloys that are supported in or on porous-ceramic structures. In these membranes, H.sub.2 flux is driven by H.sub.2 partial pressure differences across the membrane. This information is set forth in H. W. Abu El Hawa, S. N. Paglieri, C. C. Morris, A. Harale, and J. D. Way, “Application of a Pd—Ru composite membrane to hydrogen production in a high temperature membrane reactor,” Sep. Purif. Technol., 147: 388-397, 2015 (incorporated herein by reference in its entirety). Other suitable membrane are protonic ceramic membranes, such as for example, membranes based on doped barium zirconates and cerates that are proton conductors, where the effective H.sub.2 flux is controlled by combinations of H.sub.2 partial pressure and an imposed voltage. Ohmic heating associated with proton conduction could play a role in the reactor thermal balance. This information is set forth in H. Zhu, S. Ricote, C. Duan, R. P. O'Hayre, D. S. Tsvetkov, and R. J. Kee, “Defect incorporation and transport with BaZr.sub.0.8Y.sub.0.2O.sub.3-δ (BZY20) proton-conducting membranes,” J. Electrochem. Soc., 165: F581-F588, 2018; and H. Zhu, S. Ricote, C. Duan, R. P. O'Hayre, and R. J. Kee, “Defect chemistry and transport within dense BaCe.sub.0.7Zr.sub.0.1Y.sub.0.1Yb.sub.0.1O.sub.3-δ (BCZYYb) proton-conducting membranes,” J. Electrochem. Soc., 165: F845-F853, 2018 (each of which are incorporated herein by reference in their entirety).

(21) The removal of H.sub.2 from the pore volume of the bifunctional catalyst is important because H.sub.2 inhibits olefin formation. Removal of H.sub.2 from the catalyst bed enhances both the WGS and non-oxidative dehydrogenation pathway. In a typical ODH process over a redox active metal-oxide catalyst, the rate limiting step is the activation of alkane via MvK cycle. In other words, the catalytic activity is determined by the available lattice oxygen supplied by the metal-oxide catalyst. This step is kinetically controlled, and lattice oxygen concentration increases with increasing redox activity at high temperatures. Thus, the ODH operating temperature is highly controlled by the available lattice oxygen in metal-oxide catalyst. The catalytic membrane reactor increases the lattice oxygen concentration by shifting the equilibrium of reaction 4; more lattice oxygen is generated at lower reaction temperatures where redox activity is limited (e.g., T≤450° C. for Cr.sub.2O.sub.3). Hence, the membrane integrated ODH process can operate at lower temperatures compared to a packed-bed operation. Because use of a catalytic membrane reactor can decrease the ODH reaction temperature, the catalyst durability or stability is improved. At relatively low temperatures (of about 500° C. to about 550° C.) olefin dehydrogenation to coke, via the Boudouard reaction (reaction 8), and aromatization pathways to undesirable PAHs can be impeded or avoided. Further, use of a catalytic membrane reactor produces two valuable product streams, olefin(s) and pure H.sub.2.

(22) The CMR reactor may comprise at least one input gas stream and at least two output gas streams, one for the separated hydrogen gas and the other for at least one of the other reaction products. The flow through the reactor may be characterized as plug flow or parabolic flow. The Gas Hourly Space Velocity (GHSV) may range from about 10,000 h.sup.−1 to about 50,000 h.sup.−1. In some embodiments, the GHSV may be about 10,000 h.sup.−1, about 15,000 h.sup.−1, about 20,000 h.sup.−1, about 25,000 h.sup.−1, about 30,000 h.sup.−1, about 35,000 h.sup.−1, about 40,000 h.sup.−1, about 45,000 h.sup.−1, or about 50,000 h.sup.−1, or any range within any two of these values.

(23) In some embodiments, the reactor may comprise one or more reactor tubes and/or one or more hydrogen selective membranes. The reactor may further comprise a means for controlling and/or measuring the input gas flow rate, the output gas flow rate, the temperature, and the pressure. The reactor may comprise a means for measuring the presence of and/or the concentration of one or more components of the input gas stream and/or output gas stream (e.g., FTIR, HPLC, or other analytic means). The reactor may comprise a means for collecting the hydrogen gas stream and a means for collecting the olefins.

Example 1: Preparation of Bifunctional Catalyst

(24) The BZY (BaZr.sub.0.85Y.sub.0.15O.sub.3-δ) support was prepared using calcination of nitrate precursors. The metal precursors were Ba(NO.sub.3).sub.2 (Alfa Aesar, 99% purity), ZrO(NO.sub.3).sub.2 (Sigma Aldrich, 99% purity), and Y.sub.2O.sub.3 (Alfa Aesar, 99.9% purity). Complexing agents were ethylenediaminetetraacetic acid (EDTA, Alfa Aesar, 99.4% purity), citric acid (Alfa Aesar, 99% purity), nitric acid (Sigma Aldrich, 99.999% purity), and ammonium hydroxide (VWR Analytical, 38-30% concentration). Powders were produced utilizing a slightly modified EDTA-citrate complexing synthesis method. In this method, stoichiometric amounts of nitrate metal cation precursors were combined with a sufficient amount of EDTA and citric acid to ensure complete cation mixing. The molar ratio of EDTA to citric acid to BZY powder was 2.5:1.2:1. After adding yttria that was dissolved in a heated solution of water and nitric acid, the solution was heated to 325° C. while adding ammonium hydroxide to reduce the pH of the solution. The solution was then stirred and heated continuously until a sticky gel was formed. The gel was subsequently transferred to a drying oven at 150° C. for 12 hours. The result was a BZY char that was then calcined at 900° C. for 10 hours, producing as-calcined powder with an average particle diameter of 40 nm.

(25) Bifunctional catalysts were prepared using the incipient wetness impregnation technique. Metal nitrate salts and either gamma alumina powder or BZY powder were premixed and diluted with water. The resulting solution was dried overnight at 80° C. while stirring. Finally, the catalyst was calcined at 500° C. for 5 hours in air.

Example 2: Oxidative Dehydrogenation of C.SUB.3.H.SUB.8 .Using a Cr.SUB.2.O.SUB.3./Al.SUB.2.O.SUB.3 .Catalyst

(26) The oxidative dehydrogenation performance of a Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst was evaluated using a laboratory-scale packed-bed reactor, a schematic of which is shown in FIG. 3. The catalyst particles were sieved into the 125-250 μm range. Then 0.5 grams of the Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst was mixed with 0.5 grams quartz sand (125-250 μm range) and packed in a 10 mm OD, 7 mm ID quartz reactor. The catalyst was sandwiched between quartz wool and housed in a horizontal furnace. The reaction temperature was controlled via two K-type thermo-couples, positioned before and after the catalyst bed. The flow rates of the feed gases (C.sub.3H.sub.8, CO.sub.2, H.sub.2, N.sub.2, and O.sub.2) were established via mass flow controllers (MFC, Bronkhorst) and the gasses were premixed before entering the reactor. The inlet and outlet lines were heated to 130° C. to avoid any condensation. The reactor outlet gas composition was measured by an online Fourier-transform infrared spectrometer (FTIR, Multigas MG2030, MKS). The experiments were conducted over a range of flow rates, temperature, and C.sub.3H.sub.8/CO.sub.2/H.sub.2/N.sub.2 ratios. The reported conversions, rates, and selectivities are average steady-state values within 10-minute measurement intervals.

(27) FIG. 4A shows propane dehydrogenation at 600° C. using a Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst, as a function of C.sub.3H.sub.8/CO.sub.2 ratio. The data show two distinct regions where high C.sub.3H.sub.8 conversion is possible. Under very C.sub.3H.sub.8-lean conditions (i.e., C.sub.3H.sub.8/CO.sub.2≤0.3), high levels of C.sub.3H.sub.8 conversion are obtained, but overall olefin production rates are low. The process behavior changes as the feed mixture becomes more C.sub.3H.sub.8 rich. The conversion and yield initially drop sharply as C.sub.3H.sub.8/CO.sub.2 is increased before undergoing an abrupt recovery and stabilizing in the regime 1≤C.sub.3H.sub.8/CO.sub.2≤2.5. A benefit of operating under propane rich conditions is the increased C.sub.3H.sub.6 production rate, which is approximately 0.01 mol gcat.sup.−1 h.sup.−1, which is an order of magnitude more than previously reported in the literature.

(28) FIG. 4B displays the temperature dependence of C.sub.3H.sub.8 and CO.sub.2 conversion and product selectivity for a fixed C.sub.3H.sub.8/CO.sub.2 ratio of 2.5. Across the entire range examined, the selectivity to C.sub.3H.sub.6 is excellent (80-90%). Moreover, catalytic activity is negligible below 450° C., before increasing significantly with temperature. However, simply increasing temperature is detrimental for catalyst stability because above 600° C. as the olefin product(s) continue to dehydrogenate to form coke or polyaromatic hydrocarbons. A final important observation is that CO.sub.2 conversion lags propane conversion. The data in FIG. 4B infers that the WGS and thermal cracking side reactions likely compete with the desired ODH reaction.

Example 3: Impact of H.SUB.2 .Concentration on the ODH of C.SUB.3.H.SUB.8 .Using a Cr.SUB.2.O.SUB.3./Al.SUB.2.O.SUB.3 .Catalyst

(29) The impact of H.sub.2 concentration on the C.sub.3H.sub.8 conversion and selectivity was examined for a fixed C.sub.3H.sub.8/CO.sub.2 ratio of 2.5, using the reactor configuration and procedure described in Example 2. FIG. 5A shows of the impact of H.sub.2 addition to the C.sub.3H.sub.8/CO.sub.2 reactive gas mixture at 600° C., while keeping the total flow rate constant. Both the C.sub.3H.sub.8 conversion and selectivity to C.sub.3H.sub.6 decrease substantially as H.sub.2 is added to the system. In contrast, the CO.sub.2 conversion and CO selectivity increase. This is direct evidence that, in the presence of H.sub.2, the reverse water gas shift (rWGS) pathway dominates over the ODH pathway.

(30) If H.sub.2 addition decreases the ODH activity, it is expected that H.sub.2 removal via a H.sub.2-selective membrane can increase the olefin selectivity and conversion. The ODH process requires replenishing the lattice oxygen via rWGS step. Thus, the rWGS activity of the catalyst determines the overall ODH activity. FIG. 5B shows that the Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst is in fact a good rWGS catalyst as well. CO.sub.2 hydrogenation activity over Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst highly depends on the temperature. Below 450° C., the catalytic activity is independent of the CO.sub.2/H.sub.2 ratios and conversion is limited by the redox activity. However above 450° C., the catalytic activity highly depends on the partial pressure of H.sub.2. Under the desired operating conditions for ODH (450-550° C.) it is plausible that rWGS and ODH reaction co-exist. For all cases, H.sub.2O and CO are produced at equal rates. The CO selectivity is found to be ≥99.9%, and the only by-product detected is negligible amounts of CH.sub.4. Thus, the Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst is highly selective to CO formation when H.sub.2 co-exists with CO.sub.2.

Example 4: Oxidative Dehydrogenation of C.SUB.3.H.SUB.8 .Using a Cr.SUB.2.O.SUB.3./BZY Catalyst

(31) The oxidative dehydrogenation performance of a Cr.sub.2O.sub.3/BZY catalyst was evaluated in a laboratory-scale packed-bed reactor using the same procedure that was described in Example 2. FIG. 6 compares the catalytic activity of the Cr.sub.2O.sub.3/BZY catalyst versus the Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst at 600° C., at a C.sub.3H.sub.8/CO.sub.2 ratio of 2.5. Results are also shown for the Cr.sub.2O.sub.3/Al.sub.2O.sub.3 under C.sub.3H.sub.8-lean conditions of C.sub.3H.sub.8/CO.sub.2 of about 0.2. The BZY support delivers remarkable stability under C.sub.3H.sub.8-rich conditions. The Cr.sub.2O.sub.3/Al.sub.2O.sub.3 catalyst loses catalytic activity within a few hours, independent of the CO.sub.2 content. In fact, looking more carefully, under C.sub.3H.sub.8-lean operating conditions the fouling rate is further increased. The catalytic activity is slightly better within the first three hours for C.sub.3H.sub.8-lean conditions but then a sharp decay follows. This could be due to the high CO content, which enhances the Boudouard reaction.

(32) Accordingly, the compositions, apparatus, systems, and methods of the present disclosure have been described with some degree of particularity directed to the exemplary embodiments of the present disclosure. It should be appreciated though that modifications or changes may be made to the exemplary embodiments of the present disclosure without departing from the inventive concepts contained herein. Various modifications of the above-described invention will be evident to those skilled in the art. It is intended that such modifications are included within the scope of the following claims.