CATALYST AND PROCESS FOR PREPARING AMMONIA

Abstract: A catalyst for synthesis of ammonia from N2 and H2, containing a basic zeolite support; Group VIII metal clusters supported on the basic zeolite support; and divalent and alkali metal ions incorporated into the zeolite support, and a process for the production of ammonia using such a catalyst.

Description: TITLE OF THE INVENTION CATALYST AND PROCESS FOR PREPARING AMMONIA The research leading to the invention described herein was supported in part by funds from Grant #CTS-9257306 from the National Science Foundation. As such, the United States Government may have certain rights in the present invention. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a novel supported catalyst comprising a Group VIII transition metal supported on a basic molecular sieve, for providing improved synthesis of ammonia from nitrogen and hydrogen gases and a process for using the same in preparing ammonia. Discussion of the Background Conventional industrial ammonia synthesis processes use a triply promoted iron catalyst operating at temperatures of 400-700°C and pressures in excess of 300 atm. However, at such extreme temperatures, the equilibrium reaction of N 2 and H 2 to give ammonia is not especially favored, hence the need for the extreme pressures. In U.S. Patent No. 3,770,658, Ozaki et al disclosed a transition metal based catalyst, preferably of ruthenium, which contained alkali metal, for the preparation of ammonia from nitrogen and hydrogen under temperatures less than 400°C. In U.S. Patent No. 4,142,993, Elofson et al also disclose a Group VIII transition metal based catalyst containing alkali metal, which is supported on an activated carbon support for synthesis of ammonia at temperatures of 375°C or higher and pressures of 27-67 at . Such traditional ruthenium-based ammonia synthesis catalysts consist of ruthenium clusters supported on carriers like carbon and magnesium oxide. In addition, as shown by Ozaki et al. alkali metal promotors such as potassium or cesium are often added to enhance the catalytic activity of the ruthenium. In U.S. Patent 4,600,571, McCarroll et al disclose the use of ruthenium based ammonia synthesis catalyst which contain an alkali metal and barium, all supported on a carbon support. Recently published work by Cisneros and Lunsford [J. Catal . 141 (1993) 191-205] and ellenbuscher et al (Catal. Letters 25.(1994) 61-74) shows that ruthenium clusters supported on alkali-containing zeolites also catalyze the synthesis of ammonia from nitrogen and hydrogen at atmospheric pressure. From the results of Cisneros and Lunsford, ammonia synthesis at 650 K and atmospheric pressure over ruthenium clusters supported on potassium-loaded zeolite X occurs at a rate of 1.7 x 10" s mol NH 3 /g Ru/sec. However, each of the prior art catalysts still do not provide the desired level of activity and reaction rate. Accordingly, an improved ammonia synthesis catalyst is desired. SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a catalyst which provides improved ammonia synthesis rates under industrially useful conditions. A further object of the present invention is to provide an improved process for the preparation of ammonia from nitrogen and hydrogen gases. These and other objects of the present invention have been satisfied by the discovery of a catalyst for ammonia synthesis, comprising Group VIII transition metal clusters supported on a basic zeolite, which further comprises alkali metal ions and divalent metal ions, which provides markedly improved rates of reaction of N 2 and H 2 to give ammonia. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a Group VIII transition metal/basic zeolite catalyst for the synthesis of ammonia from N 2 and H 2 . The catalyst of the present invention comprises Group VIII transition metal clusters supported on a basic zeolitic support. The Group VIII transition metal cluster is preferably made of Fe, Ru or Os, with Ru being most preferred. The Group VIII transition metal cluster can be prepared using conventional methods (see "Catalyst Manufacture", 2nd Ed., A.B. Stiles and T.A. Koch, Marcel Dekker, New York, 1995). For example, ruthenium clusters can be formed from various ruthenium compounds, such as Ru (NH 3 ) 6 C1 3 . In particular, the Group VIII metal compounds are ion exchanged or impregnated onto the zeolite, following which the resulting material is reduced, for example, by hydrogenation, thus providing the resulting clusters in an oxidation state of the corresponding metal (such as Ru°) . The zeolite used in the present catalyst as a support for the Group VIII metal clusters is preferably a Faujasite-type zeolite (or Faujasitic zeolite) , such as Zeolite X, Zeolite Y, EMT, ZSM-3, ZSM-20, Zincophosphate X or SAPO-37. Preferably the zeolite is a icroporous crystalline aluminosilicate, preferably having a Si:Al ratio of from 1:1 to 6:1, more preferably 1:1 to 2.5:1. The catalyst of the present invention further contains Group I alkali cations and divalent metal cations. The divalent metal cations can be alkaline earth ions or divalent transition metal ions. Preferably the alkaline earth ions are used, with Ba +2 being most preferred. The divalent metal ions can be incorporated into the Group VIII metal/zeolite catalyst by conventional processes, such as ion exchange or impregnation. Commercially available zeolites often already contain alkali metal ions, such as Na. For example. Zeolite X is available containing Na ions and is conventionally called NaX zeolite. These zeolites can be used as supplied, or can first be subjected to modification, such as ion exchange or impregnation, to replace Na with another alkali metal, such as K, with the preferred alkali metals being K, Rb and Cs. The alkali metal containing zeolite is then subjected to modification with divalent cations, such as alkaline earth metal ions, by ion exchange or a combination of ion exchange and impregnation. Prior to or after this divalent cation modification, the Group VIII metal can be incorporated into the zeolite using conventional techniques to form the metal clusters and provide the Group VIII/M* 2 /basic zeolite catalyst. While the above sequence of steps can be used to prepare the present catalyst, the steps can be performed in any order, to provide incorporation of the divalent metal ions and the Group VIII metal clusters, to provide the catalyst of the present invention which is active for the production of ammonia from N 2 and H 2 . The Group VIII metal based catalyst of the present invention provides its advantages in reaction rate upon incorporation of even minute quantities of Group VIII metal into the basic zeolite. However, it is preferred that the loading be in the range of 0.1 to 10%, most preferably in the range of 1-5% by weight, based on the amount of zeolite. In the case of the more expensive Group VIII metals, the preferred loading is in the range of 0.1 to 2.0%. The divalent metal ions also provide their advantages even upon incorporation of very small quantities into the catalyst. Preferably, the molar ratio of divalent metal ions to alkali metal ions is in the range from 0.01 to 100, most preferably from 10 to 100. Once the divalent ions and Group VIII metals have been incorporated into the zeolite, it is important to render the zeolite basic in nature. This is preferably done by impregnating the composition with a basic compound such as divalent or alkali metal hydroxides, alkali alkoxides, alkali oxides, alkali metals, etc. In using the catalyst of the present invention, the catalyst is contacted with N 2 and H 2 gas in a N 2 :H 2 molar ratio of from 10:1 to 1:10, preferably from 1:3 to 1:6. The reaction is performed at a temperature and pressure sufficient to provide excellent yield per unit time. Preferably the temperature of ammonia synthesis is from 200-600°C and the pressure is from atmospheric to 400 atmospheres. These parameters can be adjusted to provide the optimum reaction rate, depending primarily on the conditions achievable in the reaction vessels used. The contact time, or weight hourly space velocity (g feed/g catalyst/hour) , is adjusted to achieve the desired yield of ammonia (i.e., longer contact gives more ammonia and vice versa) . Further, the reaction can be performed either batchwise or continuously. By using the catalyst of the present invention, it is possible to obtain surprisingly improved rates of ammonia 5 synthesis when compared to the catalysts of the prior art. These improved rates of reaction allow the use of milder conditions, which can prove industrially advantageous financially, as well as in the area of industrial safety. Having generally described this invention, a further 10 understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. EXAMPLES 15 Catalyst Synthesis Examples Example 1. Ru/NaX NaX zeolite (15 g) from Union Carbide (elemental analysis: Si - 20.48%, Al - 16.55%, Na - 12.6%) was ion- exchanged with 0.936 g of Ru(NH 3 ) 6 Cl 3 in 300 mL of distilled, 20 deionized water. The resulting solids were filtered, dried and reduced in flowing H 2 at 723 K. The final solid contained 1.85% of Ru by weight. I Example 2. Ru/KX First, NaX zeolite from Union Carbide (elemental analysis: Si - 20.48%, Al - 16.55%, Na - 12.67%) was ion- exchanged three times with 1 molL" 1 aqueous KN0 3 (75, 230 and 350 mL, respectively) and dried to produce KX zeolite. 15 g of KX zeolite was ion exchanged with 0.936 g of Ru(NH 3 ) 6 Cl 3 in 1.2 L of water. The resulting solids were filtered, dried and reduced in flowing H 2 at 723 K. The reduced solid was then impregnated with 310 L of a 0.2 molal aqueous solution of KOH and dried. Elemental analysis: Ru - 2.04%, Si - 19.70%, Al - 13.03%, Na - 0.58%, K - 16.08%. Example 3. Ru/CsX 1.24 g of sample in example 2 (before impregnation with KOH) were ion-exchanged three times with 1 molL "1 aqueous cesium acetate solutions (12.5, 12.5 and 30 mL, respectively), filtered and dried. The resulting solids were impregnated with 0.2 molal aqueous solution of Cs(OH). Elemental analysis: Ru - 2.01%, Si - 13.88%, Al - 7.9%, Na - 0.93%, K - 2.50%, CS - 24.53%. Example 4. Ru/BaX 1.22 g of sample in example 2 (before impregnation with KOH) were ion-exchanged two times with l molL "1 aqueous barium acetate solutions (10 and 15 mL) , filtered and dried. The resulting solids were impregnated with 50 mL of a 0.2 molal aqueous solution of Ba(OH) z . Elemental analysis: Ru - 2.05%, Si - 13.12%, Al - 9.48%, K - 1.32%, Ba - 24.4%. Example 5. Ru/BaX(2) First, NaX zeolite from Union Carbide (elemental analysis: Si - 20.48%, Al - 16.55%, Na - 12.67%) was ion- exchanged three times with 1 molL "1 of aqueous KN0 3 and dried to produce KX zeolite. 17.68 g of KX zeolite were ion exchanged with 1.1048 g of Ru(NH 3 ) 6 Cl 3 in water. The resulting solids were filtered, dried and reduced in flowing H 2 at 723 K. 3.285 g of the solids were ion-exchanged two times with 1 molL" 1 aqueous barium acetate solutions (10 and 10 mL) , filtered and dried. The resulting solids were impregnated with 30 mL of a 0.2 molal aqueous solution of Ba(OH) 2 . Elemental analysis: Ru - 2.10%, Si - 14.38%, Al - 9.04%, K - 1.47%, Ba - 21.6%. Reactivitv Examples All samples were sieved to 170 mesh (90 μm) before reaction. Catalysts were loaded into a constant volume recirculation reactor and reduced in-situ at 723 K before reaction. A 3:1 molar ratio of H 2 :N 2 reactant mixture at a total pressure of 1 atmosphere was introduced into the system. The product ammonia was condensed into a liquid nitrogen trap thus preventing the product from inhibiting the reaction. Rates were calculated from the total pressure drop in the system as a function of time and are reported as moles of ammonia produced per gram of Ru per time. The fraction of Ru exposed to the surface (dispersion) was evaluated by a standard hydrogen chemisorption measurement and results were used to calculate the specific rate of reaction per surface Ru atom. Reaction Results: Ammonia Synthesis over Ruthenium Catalysts Fraction of Temperature Reaction Rate Specific Rate Example Sample Ru exposed /K(±5 K) /10-' molNHj /IO-S- 1 (gRuP's- 1 1 Ru/NaX 0.57 650 4.48 7.94 2a Ru/KX 0.93 650 2.84 3.09 2b Ru/KX 0.93 700 14.2 15.4 3a Ru/CsX 0.72 650 7.62 10.7 3b Ru/CsX 0.72 700 25.9 36.4 4a Ru/BaX 0.92 650 34.5 37.9 4b Ru/BaX 0.92 700 105 115 5a Ru/BaX(2) 0.79 650 33.6 43.0 5b Ru/BaX(2) 0.79 700 127 163 At a given temperature, the Ba-containing Ru catalyst was more active for ammonia synthesis than catalysts containing alkali cations (Na, K, Cs) without Ba. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: WHAT IS CLAIMED AS NEW AND DESIRED TO BE SECURED BY LETTERS PATENT OF THE UNITED STATES IS: 1. A catalyst for synthesis of ammonia from N 2 and H 2 , comprising: a basic zeolite support; Group VIII transition metal clusters supported on said zeolite support; and divalent and monovalent metal ions incorporated into said zeolite support. 2. The catalyst of Claim 1, wherein said zeolite support is an aluminosilicate. 3. The catalyst of Claim 2, wherein said zeolite support has a Si:Al ratio of from 1:1 to 6:1. 4. The catalyst of Claim 2, wherein said zeolite support has a Si:Al ratio of 1:1 to 2:1. 5. The catalyst of Claim 1, wherein said zeolite support is a Faujasitic zeolite. 6. The catalyst of claim 5, wherein said Faujasitic zeolite is a member selected from the group consisting of Zeolite X, Zeolite Y, EMT, ZSM-3, ZSM-20, SAPO-37 and Zincophosphate X. 7. The catalyst of Claim 1, wherein said Group VIII transition metal clusters are Ru metal clusters. 8. The catalyst of Claim 1, wherein said divalent metal ions are alkaline earth metal ions. 9. The catalyst of claim l, wherein said divalent metal ions are transition metal ions. 10. The catalyst of Claim 1, wherein said divalent metal ions are Ba +2 . 11. The catalyst of Claim 1, further comprising alkali metal ions within said zeolite support. 12. The catalyst of Claim 11, wherein said alkali metal ions are selected from the group consisting of potassium, rubidium and cesium. 13. The catalyst of Claim 1, wherein said Group VIII metal clusters are present in an amount of from 0.1 to 10 wt%, based on the weight of the zeolite. 14. The catalyst of Claim 1, wherein said divalent metal ion to alkali metal ion molar ratio is from 0.1 to 100. 15. The catalyst of claim 14, wherein said divalent metal ion to alkali metal ion molar ratio is from 10 to 100. 16. The catalyst of Claim 1, wherein said catalyst is rendered basic by impregnation with a solution of a basic compound selected from the group consisting of divalent metal hydroxides, alkali metal hydroxides, alkali metal alkoxides, alkali metal oxides, and alkali metals. 17. The catalyst of Claim 16, wherein said divalent metal hydroxide is a hydroxide of a divalent metal which is identical to the divalent metal ion present in the catalyst. 18. A method for the production of ammonia comprising: contacting a mixture of N 2 and H 2 gases with a catalyst comprising (i) a basic zeolite support (ii) Group VIII metal clusters supported on said zeolite support, (iii) divalent metal ions incorporated into said zeolite support and (iv) alkali metal ions incorporated into said zeolite support; at a temperature and pressure sufficient to provide reaction of N 2 and H 2 gases to give NH 3 . 19. The method of claim 18, wherein said zeolite support is an aluminosilicate. 20. The method of Claim 19, wherein said zeolite support has a Si:Al ratio of from 1:1 to 6:1. 21. The method of Claim 19, wherein said zeolite support has a Si:Al ratio of from 1:1 to 2:1. 22. The method of Claim 18, wherein said zeolite support is a Faujasitic zeolite. 23. The method of claim 22, wherein said Faujasitic zeolite is a member selected from the group consisting of Zeolite X, Zeolite Y, EMT, ZSM-3, ZSM-20, SAPO-37 and Zincophosphate X. 24. The method of Claim 18, wherein said Group VIII transition metal clusters are Ru metal clusters. 25. The method of Claim 18, wherein said divalent metal ions are alkaline earth metal ions. 26. The method of claim 18, wherein said divalent metal ions are transition metal ions. 27. The method of Claim 18, wherein said divalent metal ions are Ba* 2 . 28. The method of Claim 18, further comprising alkali metal ions within said zeolite support. 29. The method of Claim 28, wherein said alkali metal ions are selected from the group consisting of potassium, rubidium and cesium. 30. The method of Claim 18, wherein the temperature is from 200 to 600°C. 31. The method of Claim 18, wherein the pressure is from 1 atm to 400 atm.

Inventors: DAVIS ROBERT J, FISHEL CHRISTOPHER T, GARCES JUAN M, DAVIS, ROBERT, J., FISHEL, CHRISTOPHER, T., GARCES, JUAN, M.
Applicants: DOW CHEMICAL CO, UNIV VIRGINIA, THE DOW CHEMICAL COMPANY, UNIVERSITY OF VIRGINIA PATENT FOUNDATION
Application year: 1996
Main classification: B01J8
IPC codes: B01J29, C01C1