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 rati o of 15 were investigated during methylcyclohexane transformation at 450 de greesC for several contact times. Coking was found to be much faster on HBE A than on HMFI and especially on HFAU, whereas the deactivating effect of c oke is more pronounced on the latter zeolite. Coke was shown to be a second ary product resulting fi om the transformation of the apparent primary arom atic and olefinic products. At low coke contents, coke was constituted by p olyaromatic species soluble in CH2Cl2. These species were trapped within th e 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 expens e of soluble species. The mode of growth of coke molecules depends on the z eolite, with a large participation of aromatic products with HMFI and HBEA and of olefinic products with HFAU. With HMFI and HFAU, deactivation was sh own to occur through pore blockage, whereas with HBEA, it occurs mainly thr ough site poisoning. These differences in the mode of deactivation can be r elated to large differences in the crystallite sizes, specifically, 0.02 mu m with HBEA compared to 0.5-1 mum with the other zeolites.