Effect of alkali cations on the microstructure and composition of iron silicate catalysts

Abstract

Zeolites are shape selective catalysts widely used in industry. Selectivity arises from the fact that zeolites possess channels several angstroms in diameter which limits the size of species that can easily diffuse through. Promoted iron oxide is a highly active catalyst for the synthesis of hydrocarbons from CO and H2 (Fischer-Tropsch reaction) but researchers would like to narrow product selectivity to the C7-C10 gasoline range. Selectivity is improved by dispersing iron on zeolites and efforts are now directed at maximizing the iron distribution in the zeolite channels. One approach is to first incorporate iron directly into the zeolite framework during synthesis and then remove the iron from the zeolite framework and ideally form catalytically active iron oxide particles within the channels while still maintaining the pore structure of the zeolite. Iron silicate analogs of the zeolite ZSM-5 (FeZSM-5) have been synthesized and thermal treatments have been employed to produce a catalytic active material. The complete characterization by SEM and TEM of a series of FeZSM-5 catalysts with various Si/Fe ratios and preparation conditions has been achieved. This presentation is a continuation of that work. Three series of FeZSM-5 catalysts were synthesized, two had Si/Fe ratios of 45 with different Si/Al ratios and one had a Si/Fe ratio of 25. In each series, the synthetic conditions were modified by adding alkali metal cations (Li+, Na+ and K+) to the reaction mixture. It has been shown with ZSM-5 that the addition of alkali cations during synthesis produces large single crystals. The effect of alkali cations on the growth of zeolite analogs is interesting for several independent reasons ranging from basic studies of the mechanism of zeolite growth to practical experimental considerations. Single crystals are very desirable for electron microscopy studies: results are unambiguous for a single crystallite in one orientation compared to a region with many overlapping small particles.