Coking, aging and regeneration of zeolites XV. Influence of the composition of HY zeolites on the mode of formation of coke from propene at 450°C

Abstract

The formation of coke from propene was investigated at 450°C on a series of HY zeolites differing by the number of framework and extraframework aluminium atoms per unit cell ( N Al and N EFAL ). A constant turnover value for coking was found for samples with low N al while lower values were obtained for high N Al numbers. Extraframework aluminium species participated in coke formation but to a limited extent. The coke composition was established for various coke contents. Whatever the zeolite sample, relatively small polyaromatic (C n H 2n − 26 , C n H 2n − 32 and C n H 2n − 36 ) molecules were formed in the zeolite supercages at low coke contents. These molecules can be transformed into highly polyaromatic molecules either through successive reactions of alkylation, cyclization, hydrogen transfer—or through coupling. The first mode occurs mainly for coke molecules trapped in supercages close to the outer surface of the zeolite crystallites or close to mesopores; the resulting highly polyaromatic molecules overflow onto the outer surface or into the mesopores. The second mode consists of coupling of coke or coke precursor molecules trapped in adjacent supercages. The relative significance of these two modes depends on the rate of coking, hence on the density of the acid sites. The lower the density the more significant the formation of highly polyaromatic molecules through coupling because of the longer residence time of the coke molecules in the pores.