A study of the structure and composition of an iron silicate catalyst, iron-ZSM-05, via electron microscopy

The purpose of this research is to understand the relationships between processing, structure, catalytic activity and selectivity of an iron silicate catalyst as a function of preparation and processing conditions. Iron silicate analogs of zeolite ZSM-5, referred to as FeZSM-5, represent the starting point for a novel method of dispersing catalytically active transition metals and transition metal oxides within the pores of a molecular sieve structure. The long term goal of iron silicate catalyst research is to synthesize a shape selective catalyst system; the promoted iron oxide is the active catalyst for the Fischer-Tropsch synthesis of hydrocarbons and the ZSM-5 structure would, by virtue of its pore size, restrict the products to the C{dollar}sb7{dollar}-C{dollar}sb{lcub}10{rcub}{dollar} "gasoline" range.; Scanning electron microscopy shows that stirring a gel of moderate to high iron concentration during crystal growth produces uniform, small iron silicate particles. Crystallization without agitation of the gel results in some small single crystals but predominantly large agglomerates of small crystallites are produced. Addition of alkali cations to the iron silicate gel during crystallization leads to a higher percentage of single, twinned and intergrown FeZSM-5 particles; however, most of the sample still consists of large particle agglomerates of small crystallites.; X-ray emission spectroscopy performed in the transmission electron microscope and in the scanning transmission electron microscope demonstrates that stirring the gel during crystal growth has a profound affect on the homogeneity of the iron distribution in the FeZSM-5 particles.; The particle size and spatial distributions of the catalytic iron phase, determined using conventional transmission electron microscope imaging, are directly linked to the overall iron concentration in the sample as well as to the local distribution of iron prior to hydrothermal treatment. High resolution lattice imaging and microdiffraction techniques are used to identify some of the large iron-rich second phase particles as iron silicates, oxides and hydroxides.; A probe reaction, the dehydrogenation of ethyltoluene to methylstyrene, shows the FeZSM-5 catalysts to be both activity and shape selective. Although the Fischer-Tropsch synthesis using these catalysts is the subject of future work, this research has taken the first steps toward a promising future of shape selective iron silicate catalysts.