| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 02.04.2002 |
| PATENT TITLE |
Process for co-production of dialkyl carbonate and alkanediol |
| PATENT ABSTRACT | A method is provided for co-producing dialkyl carbonate and alkanediol by reacting alkylene carbonate with alkanol in the presence of a zeolite catalyst which contains alkali metal, alkaline earth metal, or a combination thereof present in excess of a stoichiometric amount. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | August 20, 1999 |
| PATENT REFERENCES CITED |
Knifton, J.F. and Duranleau, R.G., "Ethylene Glycol-Dimethyl Carbonate Cogeneration," J. of Molecular Catalysis 67:389-399(1991). Watanabe, Y. and Tatsumi T., "Hydrotalcite-type Materials as Catalysts for the Synthesis of Dimethyl Carbonate from Ethylene Carbonate and Methanol.sup.1," Microporous and Mesoporous Materials 22:399-407(1998). Chang, C.D., Handbook of Heterogenous Catalysis, Wiley-VCH:Weinheim, Germany, vol. 4, Chapter 3.7 (1997). Yagi, F., Kanuka, N., Tsuji, H., Nakata, S., Kita, H. and Hattori, H., ".sup.133 Cs and .sup.23 Na MAS NMR studies of zeolite X containing cesium," Microporous Materials 9:229-235(1997). Skibsted, J., Vosegaard, T., Bilds.o slashed.e, H. and Jakobsen, H.J., ".sup.133 Cs chemical Shielding Anisotropics and Quadrupole Couplings from Magic-Angle Spinning NMR of Cesium Salts," J. Phys. Chem., 100:14872-14881(1996). |
| PATENT CLAIMS |
We claim: 1. A method for co-producing dialkyl carbonate and alkanediol comprising reacting alkylene carbonate with alkanol in the presence of a zeolite catalyst under process conditions, said catalyst comprising alkali metal, alkaline earth metal, or a combination thereof present in excess of a stoichiometric amount. 2. The method of claim 1 wherein said alkylene carbonate is ethylene carbonate. 3. The method of claim 1 wherein said alkanol is methanol. 4. The method of claim 1 wherein said alkali metal is cesium. 5. The method of claim 1 wherein said zeolite is selected from the group consisting of ZSM-5, zeolite beta, ZSM-22, ZSM-23, ZSM-48, ZSM-35, ZSM-11, ZSM-12, Mordenite, Faujasite, Erionite, zeolite USY, MCM-22, MCM-49, MCM-56, and SAPO. 6. The method of claim 5 wherein said zeolite is ZSM-5. 7. The method of claim 1 wherein said alkali metal, alkaline earth metal, or combination thereof present in excess of a stoichiometric amount is at least partially located within a zeolite pore. 8. The method of claim 7 wherein said alkali metal, alkaline earth metal, or combination thereof located within said zeolite pore is an oxide. 9. The method of claim 1 wherein said alkali metal, alkaline earth metal, or combination thereof is incorporated in said zeolite through impregnation. 10. The method of claim 1 wherein said process conditions comprise a reaction temperature of about 20.degree. C. to about 300.degree. C., a reaction pressure of about 14 to about 4000 psig, a liquid hour space velocity of about 0.1 to about 40 hr.sup.-1, and a molar ratio of alkanol to alkylene carbonate of about 1-20. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
BACKGROUND This invention relates to a method of co-producing dialkyl carbonate and alkanediol, and, in particular, to a method for enhancing the efficiency of the co-production by the use of zeolite supported alkali and/or alkaline earth metal present in excess of a stoichiometric amount. Various homogeneous catalysts have been proposed for carbonate transesterification. For example, U.S. Pat. Nos. 3,642,858 and 4,181,676 disclose the preparation of dialkyl carbonates by tranesterifying alkylene carbonates with alcohols in the presence of alkali metals or alkali metal compounds without the use of a support material. U.S. Pat. No. 4,661,609 teaches the use of a catalyst selected from the group consisting of zirconium, titanium and tin oxides, salts or complexes thereof. Commercial use of homogeneous catalysts is restricted because separation of the catalyst from the reactants can be difficult. Because the transesterification is an equilibrium reaction, in an attempt to isolate the intended dialkyl carbonate by distillation of the reaction liquid without advance separation of the catalyst, the equilibrium is broken during the distillation and a reverse reaction is induced. Thus, the dialkyl carbonate once formed reverts to alkylene carbonate. Furthermore, due to the presence of the homogenous catalyst, side reactions such as decomposition, polymerization, or the like concurrently take place during the distillation which decrease the efficiency. Various heterogenous catalysts have also been proposed for carbonate transesterification. The use of alkaline earth metal halides is disclosed in U.S. Pat. No. 5,498,743. Knifton, et al., "Ethylene Glycol-Dimethyl Carbonate Cogeneration," J. Molec. Catal. 67:389-399 (1991) disclose the use of free organic phosphines or organic phosphines supported on partially cross-linked polystyrene. U.S. Pat. No. 4,691,041 discloses the use of organic ion exchange resins, alkali and alkaline earth silicates impregnated into silica, and certain ammonium exchanged zeolites. U.S. Pat. No. 5,430,170 discloses the use of a catalyst containing a rare earth metal oxide as the catalytically active component. The use of hydrotalcites is disclosed in Japanese patent application 3[1991]-44,354. Zeolites ion-exchanged with alkali metal and/or alkaline earth metal, thereby containing a stoichiometric amount of metal, are disclosed in U.S. Pat. No. 5,436,362. Inorganic heterogenous catalysts generally possess thermal stability and easy regeneration. However, these catalysts, including the zeolites containing a stoichiometric amount of alkali or alkaline earth metal, generally demonstrate low activity and/or selectivity and are unsatisfactory for commercial application. Polymer supported organic phosphines and ion exchange resins show high activity and good to excellent selectivity in transesterification reaction between alkylene carbonate and alkanol; however, these polymeric materials do not appear very stable and gradually lose catalytic activity over a long period of time, especially at relatively high temperatures. Thus, there remains a need for a method of transesterifying alkylene carbonate with alkanol to co-produce dialkyl carbonate and alkanediol which will provide higher activity and selectivity over a wide temperature range. SUMMARY OF INVENTION A method is provided for co-producing dialkyl carbonate and alkanediol by reacting alkylene carbonate with alkanol in the presence of a zeolite catalyst which contains alkali metal, alkaline earth metal, or a combination thereof present in excess of a stoichiometric amount. The preferred alkylene carbonate is ethylene carbonate and the preferred alkanol is methanol. Cesium is the preferred alkali metal. The zeolite can be selected from the group consisting of ZSM-5, zeolite beta, ZSM-22, ZSM-23, ZSM-48, ZSM-35, ZSM-11, ZSM-12, Mordenite, Faujasite, Erionite, zeolite USY, MCM-22, MCM-49, MCM-56, and SAPO; ZSM-5 is most preferred. Alkali metal, alkaline earth metal, or combination thereof can be incorporated into the zeolite by any known means which will allow at least a portion of the excess metal to occupy the zeolite pore space; such as by impregnation. In a preferred embodiment, alkali metal and/or alkaline earth metal within the zeolite pore is in an oxide form. For example, when cesium is used as the alkali metal, the excess cesium occupies the zeolite pore in the form of cesium oxide. The process conditions include a reaction temperature of about 20.degree. C. (68.degree. F.) to about 300.degree. C. (572.degree. F.), a reaction pressure of about 14 to about 4000 psig, a liquid hour space velocity of about 0.1 to 40 hr.sup.-1, and a molar ratio of alkanol to alkylene carbonate of about 1-20. The transesterification catalysts of the current invention exhibit high activity and excellent selectivity in the reaction of alkylene carbonate with alkanol and are superior vs. zeolites in NH.sub.4.sup.+ -form or containing stoichiometric amount of alkali and/or alkaline earth metal. Unlike polymer catalysts such as ion exchange resins, the basic zeolite catalysts used in the method of the invention are thermally stable and regenerable. The combination of high catalytic activity and selectivity in a wide temperature range, and excellent thermal stability and regenerability of the catalyst render them suitable for commercial use in co-producing organic carbonate and alkanediol through ester exchange reaction. The organic carbonates produced by the method of the invention, dimethyl carbonate in particular, have potential application as "green" replacements for phosgene that is used mainly in manufacture of polyurethane and polycarbonate resins. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph demonstrating EC (ethylene carbonate) conversion vs. temperature for the MeOH/EC reaction between a stoichiometric Cs-ZSM-5 catalyst and a method of the invention using a Cs-ZSM-5 catalyst containing cesium in excess of a stoichiometric amount. FIG. 2 is a graph demonstrating DMC (dimethyl carbonate) selectivity vs. temperature for the MeOH/EC reaction between a stoichiometric Cs-ZSM-5 catalyst and a method of the invention using a Cs-ZSM-5 catalyst containing cesium in excess of a stoichiometric amount. FIG. 3 is a graph demonstrating EG (ethylene glycol) selectivity vs. temperature for the MeOH/EC reaction between a stoichiometric Cs-ZSM-5 catalyst and a method of the invention using a Cs-ZSM-5 catalyst containing cesium in excess of a stoichiometric amount. FIG. 4 is a graph demonstrating DMC and EG selectivity vs. temperature for the MeOH/EC reaction catalyzed by K-zeolite A. DETAILED DESCRIPTION OF INVENTION In accordance with the present invention, a method is provided for the catalyzed co-production of dialkyl carbonate and alkanediol through the transesterification of an alkylene carbonate with alkanol. The catalyst is a zeolite containing alkali metal, alkaline earth metal, or combination thereof in excess of a stoichiometric amount. Generally, all alkylene carbonates can be used as a reactant in this invention. However, lower alkylene carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate or the like is preferred; ethylene carbonate or propylene carbonate is most preferred. Generally, all alkanol reactants can be used, provided the alkanol reacts with cyclocarbonate to produce the dialkyl carbonate and alkanediol product. However, an aliphatic or aromatic alkanol having 1 to 10 carbon atoms is preferably used. For example, methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, secondary butanol, tertiary butanol, allyl alcohol, pentanol, cyclo-hexanol, benzyl alcohol, 2-phenyl ethyl alcohol, 3-phenyl propyl alcohol, 2-methoxy ethanol or the like can be used as the aliphatic or aromatic alcohol. A lower aliphatic alcohol such as methanol is most preferably used due to its reactivity and low cost. Further, a phenolic compound can be used in place of the alcoholic compound as the compound which has a hydroxyl (OH) group and reacts with cyclocarbonate to produce the carbonate. The support for the catalyst used in the method of the invention is a zeolite. It should be noted, however, that the zeolite acts as more than simply a support. Rather, the unique crystalline pore structure of the zeolite is important in retaining excess alkali and/or alkaline earth metal. Examples of zeolites which are suitable supports for the catalyst used in the method of the invention include ZSM-5, zeolite beta, ZSM-22, ZSM-23, ZSM-48, ZSM-35, ZSM-11, ZSM-12, Mordenite, Faujasite, Erionite, zeolite USY, MCM-22, MCM-49, MCM-56, and SAPO. ZSM-5, zeolite beta, and MCM-22 are preferred; ZSM-5 is most preferred. The catalyst contains alkali metal (Li, Na, K, Rb, Cs), alkaline earth metal (Be, Mg, Ca, Sr, Ba), or combination thereof belonging to IA group and/or IIA group in the periodic table of elements. Alkali metal and alkaline earth metal are both defined to include compounds containing these metals. Cesium is the preferred metal. The catalyst utilized in the method of the invention contains alkali and/or alkaline earth metal in excess of a stoichiometric amount. By "excess of a stoichiometric amount", it is meant that the molar ratio of alkali metal to structural aluminum in the zeolite support is greater than 1 or that the molar ratio of alkaline earth metal to structural aluminum is greater than 0.5. If a combination of alkali metal and alkali earth metal is used, "excess of a stoichiometric amount" can be represented by the equation: [(moles of alkali metal)+2(moles of alkali earth metal)]/(moles of zeolite A1)>1 (1) If the zeolite support is SAPO, the presence of alkali and/or alkaline earth metal in excess of a stoichiometric amount means that the metal content is greater than the maximum amount of metal which is exchangeable with the SAPO support. There is no upper limit to the amount of excess alkali and/or alkaline earth metal. However, the excess alkali and/or alkaline earth metal which is reactive preferably resides within the pore of the zeolite. If additional alkali or alkaline earth metal is added, it will reside outside the pore area and will most likely be flushed away by the reagents. Besides wasting the metal catalyst, this may cause an initial decrease in activity along with problems typically associated with homogeneous catalysts. Alkali and/or alkaline earth metal may be incorporated into the zeolite support by any known means, such as impregnation or ion exchange/impregnation combination; which will allow the metal cations to neutralize the acid sites within the zeolite as well as allow at least a portion of alkali and/or alkaline earth metal to occupy the zeolite pore space. Alkali and/or alkaline earth metal occupying the zeolite pore space can be part of a compound containing the metal. It is preferred that alkali and/or alkaline earth metal occupying the zeolite pore space be in oxide form. Ion exchange alone is not a preferred method because the metal cations will occupy the ion exchange sites within the zeolite, but this method will not permit excess alkali and/or alkaline earth metal to reside within the pore space. For example, a zeolite catalyst can be synthesized through ion-exchange with a metal sulfate in aqueous solution followed by removal of excess metal sulfate through washing with de-ionized water and calcining. The metal cations will then be predominantly associated with the polyanionic zeolite framework. However, applicants have discovered that if a zeolite is synthesized through appropriate means using excess alkali and/or alkaline earth metal, some of the metal will remain in the zeolite pore and help drive the transesterification reaction to completion. For example, if a zeolite is prepared through impregnation using cesium sulfate in aqueous solution; upon drying and calcination, preferably at a temperature greater than 1000.degree. F. (538.degree. C.), the zeolite acid sites are neutralized by a cesium cation, while some of the cesium sulfate is decomposed to a cesium oxide which resides in the zeolite pore. Without being bound by theory, the high activity of the method of the invention using a catalyst with excess alkali or alkaline earth metal is due to its content of strong base sites, e.g. alkali and/or alkaline earth metal oxides. The high selectivity is attributable to the high alkali and/or alkaline earth metal content of the catalyst which inhibits its reaction with methanol to form Bronsted acid sites, thus suppressing acid catalyzed side reactions such as dehydration of alkanol and subsequent hydrolysis/decomposition of organic carbonate product. To illustrate, during the reaction of ethylene carbonate (EC) with methanol (MeOH), especially at high temperatures, equilibrium methanolysis can occur as shown in following equation with "Metal" being alkali and/or alkaline earth metal: Metal-Zeolite+MeOH⇄H-Zeolite+Metal-OCH.sub.3 (2) Applicants have found that zeolites exchanged with a stoichiometric amount of alkali or alkaline earth metal gradually lose the metal when continuously treated with pure methanol at 60.degree. C. (140.degree. F.) under 50 psig pressure. It is anticipated that the resultant acid sites would catalyze and initiate dehydration of methanol to form dimethyl ether and water. Dimethyl ether can be further converted to hydrocarbons and water via MTG (Methanol To Gasoline) type reactions (See, Chang, C. D., Handbook of Heterogenous Catalysis, Wiley-VCH:Weinheim, Germany, Vol. 4, Chapter 3.7 (1997)). The resultant water can cause hydrolysis/decomposition of DMC/EC to generate carbon dioxide and corresponding alcohols. Dimethyl ether, C.sub.2 -C.sub.7 hydrocarbons, and CO.sub.2 have all been observed as byproducts during the EC/MeOH reaction. The following equations further illustrate the side reactions which can occur: ##STR1## Since the methanolysis reaction is an equilibrium reaction, high alkali and/or alkaline earth metal content inhibits the formation of acid sites and maintains the catalyst in its base form. As a result, the side reactions discussed above are minimized and the catalyst selectivity is improved. The acid initiated side reactions are more significant at lower temperatures due to the fact that gas phase dehydration of methanol to dimethyl ether is an exothermic equilibrium reaction and is more favorable at lower temperatures. In addition, lower feed conversion and higher methanol concentration at lower temperatures can further promote methanol dehydration. This explains why DMC selectivity decreases with decreasing temperature for EC/MeOH transesterification catalyzed by stoichiometric catalysts. The method of the invention using a catalyst containing excess alkali and/or alkaline earth metal, on the other hand, retains its selectivity with decreasing temperature. The reactor type in this invention can be any type generally known such as a continuous fluid bed, fixed bed or stirred tank, etc. With the heterogenous catalyst used in the method of the invention, it is preferred that a fixed bed be used so as to avoid the expense of having to recover the catalyst from the reagents. The reaction conditions of this invention include a reaction temperature of about 20.degree. C. to about 300.degree. C., preferably about 60.degree. C. to about 175.degree. C.; a reaction pressure of about 14 to about 4000 psig, preferably about 50 to about 400 psig; a liquid hour space velocity of about 0.1 to about 40 hr.sup.-1, preferably about 0.5 to about 10 hr.sup.-1 ; and a molar ratio of alkanol to alkylene carbonate of about 1 to 20, preferably about 2 to 8. The following comparative examples are provided to assist in a further understanding of the invention. The particular materials and conditions employed are intended to be further illustrative of the invention and are not limiting upon the reasonable scope thereof. |
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