Zeolites for Cleaner Processes: Alkylation of isobutane and n-butene

Large pore beta zeolite with straight and zig-zag channels and USY zeolite with super cages have shown potential to replace toxic and hazardous mineral acids as catalysts in alkylation processes. However, due to rapid deactivation of these zeolites, their acceptance in industrial alkylation has been delayed. In this study we seek a deeper understanding of the kinetics, transport, and sorption of organic molecules used in alkylation processes in nano-porous zeolites. A six lump kinetic model that considers the key reactions for the zeolite catalyzed alkylation process is presented. The influence of different reactions and rate limiting steps on catalyst, and reactor performance is examined by coupling an appropriate reactor scale model, that accounts for different back-mixing on the reactor scale, with a zeolite particle model, which accounts for the diffusion inside the zeolite pores, the alkylation reactions, and zeolite deactivation. Single pulse TAP experiments are employed for obtaining estimates of intra-particle diffusion coefficients and better insight into adsorption-desorption dynamics and equilibria for species in beta and USY zeolite. This technique provides a unique way of directly estimating transport and sorption processes at extremely low surface coverage, in the absence of an inert carrier stream, with no external mass transfer resistance, and with a negligible thermal effect. The results of this study suggest that higher alkylate yield and longer zeolite activity are achieved by increasing the intrinsic hydride transfer rate and the ratio of isobutane to n-butene: P/ O) concentration in the vicinity of the catalyst. For a given P/O feed ratio, achieving close to plug flow for isobutane and high back-mixing for n-butene further enhances local P/O ratio and yield. Furthermore, optimal zeolite catalyst design should consider the egg shell type of Br├╕nsted acid site distribution and a low silicon to alumina: Si/ Al) ratio. It is suggested that the ideal zeolite morphology should feature characteristics of both beta and USY zeolites. Specifically, it should have straight and zig-zag channels with an average pore diameter of 0.74 nm, for decreasing the deactivation rate. Further, to enhance the formation of 2,2,4-trimethylpentane it should have small cages in between these channels, with average pore diameter less than 1.2 nm but more than 0.74 nm