Methylcyclohexane Transformation over HFAU, HBEA, and HMFI Zeolites:  II. Deactivation and Coke Formation

Coking and deactivation of HFAU, HBEA, and HMFI zeolites with an Si/Al ratio of 15 were investigated during methylcyclohexane transformation at 450 °C for several contact times. Coking was found to be much faster on HBEA than on HMFI and especially on HFAU, whereas the deactivating effect of coke is more pronounced on the latter zeolite. Coke was shown to be a secondary product resulting from the transformation of the apparent primary aromatic and olefinic products. At low coke contents, coke was constituted by polyaromatic species soluble in CH 2 Cl 2 . These species were trapped within the zeolite ultramicropores: their size was between the size of the cages and the size of the pore apertures. Above a certain coke content depending on the zeolite (1 wt % with HFAU, 2 wt % with HMFI, 8 wt % with HBEA), highly polyaromatic compounds insoluble in organic solvents appeared at the expense of soluble species. The mode of growth of coke molecules depends on the zeolite, with a large participation of aromatic products with HMFI and HBEA and of olefinic products with HFAU. With HMFI and HFAU, deactivation was shown to occur through pore blockage, whereas with HBEA, it occurs mainly through site poisoning. These differences in the mode of deactivation can be related to large differences in the crystallite sizes, specifically, 0.02 μm with HBEA compared to 0.5−1 μm with the other zeolites.