Study of Physical Mixtures of Cr 2 O 3 -ZnO and ZSM-5 Catalysts for the Transformation of Syngas into Liquid Hydrocarbons Javier Eren ˜ a,* Jose ´ M. Arandes, Javier Bilbao, and Andre ´ s T. Aguayo Departamento de Ingenierı ´a Quı ´mica, Universidad del Paı ´s Vasco, Apartado 644, 48080 Bilbao, Spain Hugo I. de Lasa † Chemical Reactor Engineering Centre, University of Western Ontario, London, Ontario N6A 5B9, Canada The effect of both the Cr/Zn atomic ratio of the Cr 2 O 3 -ZnO catalyst and the Si/Al ratio (different surface acidity) of the ZSM-5 catalyst on the activity and selectivity to gasoline is studied, when a physical mixture of both catalysts is used for transformation of syngas into gasoline in a single step. Furthermore, the composition of each catalyst required for a good compromise between activity and selectivity and for a high aromatic content in the gasoline has been determined. The kinetic behavior of these catalysts has been compared with that of others which have been prepared by combining conventional metallic catalysts in methanol synthesis with a ZSM-5 catalyst. 1. Introduction The catalytic transformation of syngas is an interest- ing process for the production of different hydrocarbons which can be used as fuel and is an alternative to that of oil consumption. Traditionally, all the processes for obtaining gasoline from syngas were based on the Fischer-Tropsch process (Anderson, 1984; Mills, 1988), and they were carried out in fixed- and fluidized-bed reactors (Sasol I, II, and III). Dry (1984, 1987, 1990) and Bussemeier et al. (1985) describe these processes in detail. Since the 1980s, processes have been developed for directly obtaining hydrocarbons from syngas with the aim of finding new ways for natural gas exploitation (Rao and Gormley, 1990; Sie et al., 1991) and for obtaining high octane gasoline, which would avoid the selectivity limitation of the Fischer-Tropsch process. The catalytic transformation of syngas into gasoline may be carried out in a single reactor by combining methanol synthesis with the transformation of the latter (intermediate product) into liquid hydrocarbons. The global reaction is By using a mixture of catalysts, one metallic and the other acidic, and carrying out the operation under suitable conditions, the methanol formed on the metallic catalyst is immediately transformed into gasoline by the acidic catalyst. ZSM-5 zeolite is the acidic catalyst used as it is the catalyst of the MTG (methanol-to-gasoline) process. Thus, the thermodynamic restrictions inherent to methanol synthesis (Marschner and Moeller, 1983) are avoided. Inui and Takegami (1982a,b) have shown the importance of the ZSM-5 zeolite, whose presence makes the conversion of CO 6 times higher than that reached when the Cr 2 O 3 -ZnO-Pd catalyst is used, which must be attributed to the displacement of the equilibrium in the methanol synthesis step of eq 1. In a pioneering paper in the field of combination of metallic and acidic catalysts, Chang et al. (1978), after trying different catalysts (ZnO/ZSM-5, ZnO-Al 2 O 3 /ZSM- 5, CuO-ZnO/ZSM-5, Cr 2 O 3 -ZnO/ZSM-5), determined the good behavior of the latter and proposed its suitable composition: 90 wt % of Cr 2 O 3 -ZnO and 10 wt % of ZSM-5 zeolite. Chang et al. (1980) determined that the atomic ratio Cr/Zn ) 1.5 (instead of 0.2, which is the optimum value for methanol synthesis) yields the high- est fraction of liquid hydrocarbons (close to 57 wt %, with 9 wt % durene). Costa-Novella et al. (1984) have determined that the maximum production of aromatics (40 wt %) is obtained with an atomic ratio of Cr/Zn ) 1.0. Simard et al. (1995) have determined that a catalyst with a high Cr/Zn atomic ratio (Cr/Zn ) 15.6) yields the maximum selectivity to liquid hydrocarbons, yet, on the other hand, this catalyst has a very low activity for transformation of syngas into hydrocarbons. Yashima et al. (1984) compared the behavior of Cr 2 O 3 -ZnO/ZSM-5 catalysts of different composition and found that the selectivity to aromatics increases with the content of the metallic catalyst (Cr 2 O 3 -ZnO), with the Si/Al ratio of the ZSM-5 zeolite, with pressure, and with CO/H 2 molar ratio. De Lasa et al. (1989) propose the following as the optimum formulation of the mixture: 63 wt % of Cr 2 O 3 -ZnO, 27 wt % of ZSM-5 zeolite, and 10 wt % of binder. This catalyst has a good behavior in an original fixed-bed reactor whose axial temperature profile is increased by means of heat transfer through the wall (Simard et al., 1991). Comelli and Fı ´goli (1993) used the Cr 2 O 3 -ZnO cata- lyst together with silica-alumina and carried out a complete study of the effect of the operating conditions on product distribution. The aforementioned papers clearly show the interest in the direct transformation of syngas in obtaining high octane gasoline. The versatility of the process is an indication of good perspectives for obtaining a product * Author to whom correspondence is addressed. Telephone: 34-4-4647700. Fax: 34-4-4648500. E-mail: iqparesj@lgdx04. lg.ehu.es. † Telephone: 519-661-2144. Fax: 519-661-3498. E-mail: hdelasa@eng-ntadmin.engga.uwo.ca. CO + H 2 h CH 3 OH h DME f olefins f paraffins + cycloparaffins + aromatics (1) 1211 Ind. Eng. Chem. Res. 1998, 37, 1211-1219 S0888-5885(97)00568-X CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998