COMPARATIVE-STUDY OF COKING AND REGENERATION OF HEMT AND HFAU ZEOLITES

The formation of coke from propene (P = 13 kPa) was investigated in a microbalance at 450 degrees C over a series of HEMT and HFAU zeolites with various framework Si/Al ratios (from 4 to 100). For identical fra mework compositions coke formation is faster with HEMT than with HFAU samples. For both series the initial rate of coking is roughly proport ional to the number of protonic sites which retain pyridine adsorbed a t 150 degrees C. The turnover frequency values are 1.7 times higher wi th HEMT than with HFAU samples, which can be attributed to the stronge r acidity of the protonic sites of HEMT. The composition of coke was d etermined for different amounts of coke deposited on the zeolite sampl es. With both series of zeolites, the same families of coke components were found. However, large differences in distribution were observed, the coke of HEMT samples containing a greater proportion of the less bulky components (alkyl cyclopentapyrenes) and a smaller proportion of the most bulky polyaromatic components. These differences can be rela ted both to the presence of hypocages and to a better distribution of coke molecules in the crystallites of HEMT samples. The removal of cok e by treatment under oxygen was investigated in the microbalance. What ever the framework composition and the coke content the oxidation of c oke is faster with HFAU samples. This easier oxidation of coke may be related to easier access of oxygen to the coke molecules which in HFAU zeolites would be preferentially located near the outer surface of th e zeolite crystallites. (C) 1998 Elsevier Science B.V. All rights rese rved.