Acidic and catalytic properties of H-MCM-22 zeolites -: 2.: n-heptane cracking:: activity, selectivity and deactivation by coking

n-Heptane cracking was investigated at 450 degreesC over H-MCM-22 samples with Si/Al ratios of 14 and 30. With these zeolites which present two non-interconnected pore systems, there is a very fast initial deactivation followed by a plateau in activity. The initial deactivation is due to trapping of carbonaceous compounds (coke) in the large cages with small apertures, the activity at the plateau can be attributed to the protonic sites located in the sinusoidal channels. Very significant differences are found between the product distributions on the fresh sample and at the plateau, which indicates different modes of cracking on the protonic sites of the two pore systems. In the sinusoidal channels, cracking occurs through protolysis, whereas in the supercages it occurs through the classical carbenium ion chain mechanism. The composition of coke was established. Coke trapped in the supercages is constituted of methyl polyaromatic molecules, the size of which increases significantly with time-on-stream (TOS). In the sinusoidal channels, only naphthalene molecules are retained, these molecules being located and trapped in the small cages along the channels. In the supercages deactivation is due to pore blockage, in the sinusoidal channels to site poisoning. (C) 2001 Elsevier Science B.V. All rights reserved.