Journal of Catalysis 249 (2007) 162–173 www.elsevier.com/locate/jcat A detailed study of the activity and deactivation of zeolites in hybrid Co/SiO 2 -zeolite Fischer–Tropsch catalysts Agustín Martínez a,∗ , Joan Rollán a , Maria A. Arribas a , Henrique S. Cerqueira b , Alexandre F. Costa a , Eduardo Falabella S.-Aguiar b a Instituto de Tecnología Química, UPV-CSIC, Avda. de los Naranjos s/n, 46022 Valencia, Spain b Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (CENPES), Av. Jequitibá 950, Cidade Universitária, Ilha do Fundão, 21941-598 Rio de Janeiro, RJ, Brazil Received 12 March 2007; accepted 9 April 2007 Available online 4 June 2007 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 H 2 /CO = 2) has been studied. The zeolite cracked the primary C 13+ 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 dimen- sions, 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 reac- tion, 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.  2007 Elsevier Inc. All rights reserved. Keywords: Fischer–Tropsch; Hybrid catalysts; Silica-supported cobalt; Zeolite acidity; Zeolite pore structure; Deactivation; Coke 1. Introduction The Fischer–Tropsch (FT) synthesis is an attractive route to produce high-quality liquid fuels from alternative sources to pe- troleum, such as natural gas, coal, and biomass via conversion of syngas (mixture of CO and H 2 ) over cobalt or iron based cat- alysts. During the last decades significant improvements were made both on the reactor [1] and catalyst technologies [2,3]. With iron catalysts and process temperatures close to 350 ◦ C, the main products are premium petrochemical naphtha and α- olefins. With cobalt-based catalysts and reaction temperatures in the range of 210–250 ◦ C and after an upgrading (hydrocrack- ing and hydrodewaxing) step, the main products are premium * Corresponding author. Fax: +34 963 877 809. E-mail address: amart@itq.upv.es (A. Martínez). diesel (cetane number higher than 70 and virtually no S or aro- matic compounds), food grade paraffin and specialty lubricants. Despite all those advantages, the production of liquid fuels by the FT process suffers from two main constraints. On one hand, the polymerization-type kinetics of the FT reaction re- sults in a statistical distribution of hydrocarbons (from methane to waxy products) known as Anderson–Schulz–Flory or ASF distribution characterized by the parameter α (chain growth probability). The ASF kinetics of the FT reaction imposes a limit to the maximum selectivity attainable for a given hydro- carbon fraction (i.e., 48% for gasoline-range products). On the other hand, the linear structure of the FT hydrocarbons (mostly n-paraffins) results in a too low quality fuel to be directly blended into the gasoline and diesel pools due to their low oc- tane numbers and poor cold flow properties, respectively. There are different possibilities to improve the selectivity and qual- 0021-9517/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2007.04.012