Kinetics of combustion of carbonaceous deposits on HZSM-5 zeolites

Abstract

ZSM-5 zeolites have two kinds of very fine micro pores inside their crystallites: straight and zig-zag pores. Acid sites are distributed inside and outside of the crystallites. The effective acid sites are different depending on the kinds of chemical reaction, resulting in different manners of coke formation. Kinetic model was developed for each of burning regenerations of coked HZSM-5 zeolites in two extreme cases: coke formed mainly within crystallite and most of coke deposited on the outer surface of the crystallite. A typical example of the former case in the coke deposition was alkylation of toluene by methanol. Coke in the straight pores was first oxidized and then coke in the zig-zag pores reacted with oxygen. Thus, the coke within the crystallite was burned heterogeneously. The example of the later case was reaction of methanol to gasoline. Coke was burned homogeneously by the same way to the case of the coked silica alumina catalyst. These two proposed models were found to represent well the whole coke burning reaction in the preceding cases.