Gasoline selective ZSM-5 FCC additives:: effects of crystal size, SiO2/Al2O3, steaming, and other treatments on ZSM-5 diffusivity and selectivity in cracking of hexene/octene feed

ZSM-5 catalysts with silica/alumina ratios of 55/1 and 450/1, and with varying crystal size, were studied to better understand the operation of "gasoline selective" FCC additives. Most materials were steamed for 5 h at 1400 degreesF, and some were subjected to other treatments such as mechanical milling, P addition, and HCl reflux. These materials were characterized for total Al content, tetrahedral Al, and sorption capacity and rate. The ratios of octene isomerization to octene cracking, and of octene cracking to hexene cracking, for a mixed hexene/octene feed at 538 degreesC were used to assess the potential FCC gasoline selectivity of these catalysts. Steaming generally led to a decrease in effective diffusivity, as calculated from uptake rates. HCl reflux of two steamed samples removed substantial quantities of non-framework alumina, resulting in increased diffusivities and higher selectivities. Density of acid sites did not appear to be important in determining the gasoline selectivity of the catalysts examined here. The effects of most catalyst treatments on reaction selectivities could be related to diffusional blockage, especially by non-framework alumina. However, not all results could be explained by a simple reaction/diffusion model. Factors other than diffusion, such as narrowing of available reaction space in the ZSM-5 channels, may also play a role in determining selectivities. (C) 2001 Elsevier Science B.V. All rights reserved.