Activity, coking, and acidity of amorphous silica-alumina, ZSM-5 zeolite, and their composite

An investigation of the activity, coking behavior, and acidity of three heterogeneous catalysts was completed. Amorphous silica-alumina, ZSM-5, and a composite of the two were synthesized and studied with the methanol to hydrocarbons reaction. The composite, a 10 wt% ZSM-5 and 90 wt% amorphous mixture, displayed activity and deactivation resistance similar to pure ZSM-5. The increased activity was found to be an additive effect based on the pure components. The composite's product yields were dependent upon a complex reaction-diffusion process in which both catalytic components were involved at all times on stream. The amount of coke deposition onto the composite and amorphous catalyst was the same, but coke composition was affected by the presence of ZSM-5. The composite's coke was more easily hydrogenated and oxidized than the pure amorphous sample's coke. Site coverage and pore blockage were responsible for activity decay on the amorphous and composite catalysts, while ZSM-5 experienced external coke formation. Relative acidity characterization showed that isomerization of 2-methyl-2-pentene only measured the external acidity of ZSM-5 due to reactant shape selectivity. Step-wise thermal desorption of ammonia revealed the presence of acidic coke sites on both the amorphous and composite catalysts. The desorption was characteristic of both strong Bronsted and Lewis acidity. ZSM-5's coke developed acidic character only after long times on stream, indicating that its coke was less oxygenated than the other catalysts.