Use of aluminophosphate molecular sieves in CO hydrogenation

Abstract

The behaviour in syngas conversion of Me/AlPO4-5 and MeAPO-5 (Me=Fe,Co,-4wt%), as well as that of hybrid systems comprising an iron Fischer-Tropsch (FT) catalyst physically mixed SAPO-5, FAPO-5, SAPO-11 and FAPO-11, was studied at 1.2 MPa, 573 K and H2/CO=1. Conversion of CO was much lower for FE/AlPO4-5 and MeAPOs than for Co/AlPO4-5, which could be related to the presence of the metal in high-oxidation states on the former. Significant differences in selectivity were observed as a result of changes in the type of metal and in the way that the metal is introduced in the catalyst (added to the synthesis gel or impregnated on the molecular sieve). These differences in selectivity were explained in terms of the facility of forming alkenes (related to metal sites) and the presence of acid sites. Alkenes formed on the FT catalyst underwent oligomerisation, cracking, skeletal isomeritation and hydrogen transfer reactions in the presence of the physically mixed SAPO-5 and FAPO-5. Double bond shift and skeletal isomeritation were the only alkene transformations observed on the mixtures of FT with AEL-like catalysts (FT-FAPO-11 and FT-SAPO-11). Desactivation of acid sites with time-on-stream was evidenced for the physical FT-molecular sieves mixtures. However, their higher selectivity toward light hydrocarbons, as compared to that in the absence of molecular sieves, presisted. It was observed that the FAPO phase (FAPO-5 or FAPo-11) added to the FT catalyst led to an increase in the syngas conversion, probably due to the participation of iron- species associated with the molecular sieves.