Catalytic behavior of hybrid Co/SiO2-(medium-pore) zeolite catalysts during the one-stage conversion of syngas to gasoline

Abstract

The catalytic properties of 10-MR (membered ring) zeolites (ZSM-5, MCM-22, IM-5, ITQ-2, all with a similar Si/Al ratio of ca. 15) in hybrid Co/SiO2-zeolite catalysts for the direct conversion of syngas to mainly high-octane gasoline-range hydrocarbons has been studied under typical Fischer-Tropsch (FT) conditions: 250 °C, 2.0 MPa, and H2/CO = 2. Special emphasis has been given to the deactivation behavior and the characterization of the amount and nature of the carbonaceous deposits formed by a combination of techniques (elemental analysis, TGA (thermogravimetric analyses), GC–MS, and DR (diffuse reflectance) UV–vis spectroscopy). The presence of the medium-pore zeolite increases the gasoline yield by about 20–50%, depending on the particular zeolite, and enhances the formation of branched products with respect to the base Co/SiO2 catalyst, which is explained by the promotion of isomerization and cracking of long-chain (C13+) n-paraffins formed on the FT component. The initial zeolite activity is mostly determined by the surface acidity rather than by the total amount of Brønsted acid sites, pointing out to the existence of limitations for the diffusion of the long-chain n-paraffins through the 10-MR channels under FT conditions. Thus, ITQ-2 bearing the largest surface area presents the highest initial yield of branched gasoline-range products, followed by ZSM-5, IM-5, and MCM-22. All zeolites experience a loss of activity with TOS, particularly during the initial reaction stages. This deactivation is governed by the morphological and structural properties of the zeolite, which finally determine the amount and location of the coke species, and not by the acidity.