A detailed study of the activity and deactivation of zeolites in hybrid Co/SiO2-zeolite Fischer–Tropsch catalysts

Abstract

The influence of the zeolite pore topology and acidity in hybrid catalysts comprising a physical mixture of a silica-supported cobalt (20 wt% Co) active in the Fischer–Tropsch (FT) synthesis and an acidic zeolite active in cracking under typical FT synthesis conditions (250 °C, 2.0 MPa, and H2/CO=2) has been studied. The zeolite cracked the primary C13+ long-chain n-paraffins formed on the Co catalyst to mainly gasoline-range branched products. The yield of branched products declined with TOS due to the accumulation of carbonaceous deposits (coke) on the zeolite. The amount of coke retained in spent zeolites correlated well with the observed deactivation rate and both increased with the zeolite pore dimensions, i.e. HZSM-5 < HMOR < HBeta < USY, but was little affected by zeolite acidity (USY-500 ∼ USY-720). Coke molecules predominantly comprised 2- and 3-ring aromatics in large pore zeolites, while it was mainly of paraffinic nature in the most stable HZSM-5. Aromatic coke is likely formed from light olefins produced in the FT synthesis through consecutive oligomerization, cyclization, and dehydrogenation reactions. Independent n-hexadecane cracking experiments performed under simulated FT conditions revealed that water, a primary product of the FT reaction, reduces the cracking activity of the zeolite by competing with the n-alkane feed molecules for adsorption on the Brønsted acid sites but has no appreciable effect on its stability with TOS.