Characterization and catalytic properties of cobalt supported on delaminated ITQ-6 and ITQ-2 zeolites for the Fischer-Tropsch synthesis reaction

摘要

Cobalt catalysts (ca. 20 wt% Co) supported on all-silica delaminated ITQ-2 and ITQ-6 zeolites have been prepared by impregnation with aqueous Co(NO3)(3) solutions. The catalysts have been characterized by X-ray diffraction (XRD), nitrogen adsorption, transmission electron microscopy (TEM), temperature-programmed reduction (TPR), and infrared spectroscopy of adsorbed CO. The catalytic properties for the Fischer-Tropsch synthesis (FTS) reaction under typical FTS conditions (493 K, 20 bar, H-2/CO = 2) have been evaluated, and the results compared with those obtained over a conventional CO/SiO2 and a mesoporous Co/MCM-41 catalyst with comparable cobalt loading. Co/ITQ-6 was the most active catalyst, with a FTS reaction rate which was about 1.5 and 1.8 times higher than that of Co/MCM-41 and CO/SiO2, respectively. The high activity of Co/ITQ-6 is ascribed both to a relatively good dispersion (as observed by TEM) and to a high reducibility (determined by TPR) of the supported Co3O4 particles. The dispersion and reducibility of cobalt particles in Co/ITQ-2 were very close to those in CO/SiO2. Consequently, both catalysts presented similar FTS reaction rates. In addition, Co/ITQ-6 and Co/ITQ-2 presented a higher selectivity toward the formation of C5+ hydrocarbons than CO/SiO2 and Co/MCM-41. The higher C5+ selectivity of the catalysts based on the delaminated zeolites was ascribed to a higher concentration of coordinatively unsaturated Co-0 sites which are characterized by a band at 1897 cm(-1) in the infrared spectrum of adsorbed CO. This type of site having an enhanced electron density might stabilize surface hydrocarbon intermediates favoring chain growth processes. (C) 2004 Elsevier Inc. All rights reserved.