COKING, AGING AND REGENERATION OF ZEOLITES .15. INFLUENCE OF THE COMPOSITION OF HY ZEOLITES ON THE MODE OF FORMATION OF COKE FROM PROPENE AT 450-DEGREES-C

The formation of coke from propene was investigated at 450 degrees C o n a series of HY zeolites differing by the number of framework and ext raframework 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 l ower values were obtained for high N-Al numbers. Extraframework alumin ium species participated in coke formation but to a limited extent. Th e coke composition was established for various coke contents. Whatever the zeolite sample, relatively small polyaromatic (CnH2n-26, CnH2n-32 and CnH2n-36) molecules were formed in the zeolite supercages at low coke contents. These molecules can be transformed into highly polyarom atic molecules either through successive reactions of alkylation, cycl ization, hydrogen transfer-or through coupling. The first mode occurs mainly for coke molecules trapped in supercages close to the outer sur face 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 pr ecursor molecules trapped in adjacent supercages. The relative signifi cance of these two modes depends on the rate of coking, hence on the d ensity of the acid sites. The lower the density the more significant t he formation of highly polyaromatic molecules through coupling because of the longer residence time of the coke molecules in the pores.