By J. Spivey
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It was soon recognized8’# ’O that intracrystalline coke formation on zeolites is a shape-selective reaction, directly related t o t h e pore structure. 16* This proposal is true also for the conversion of methanol on zeolites. ’l showed that the deposition of carbonaceous residues on ZSM-5 occurs essentially on its external surface and that the active sites are not covered by coke. 17’ who studied the oligomerization of olefins on ZSM-5 and proposed that the coke precursors are formed inside the catalysts but have a high mobility, due to the low density of acid sites, so that they are readily desorbed to form coke on the outer surface.
One of the most important features of zeolite catalysts is their ability to act as a molecular sieve because the channels have molecular dimensions. Three types of shape selectivity can be distinguished:86f87 reactant, product , and restricted transition state selectivity, depending on whether reactants can enter, products can leave, or intermediates can be formed in the zeolite catalyst, respectively. e4t8e The high resistance toward coke formation on medium-pore zeolites has also been attributed to this type of shape Transition state selectivity and product selectivity.
The increase of SiO,/Al,O, ratio leads to the reduction of acid sites concentration and, consequently, to a change in catalytic properties Zeolite Catalysis in the Conversion of Methanol into Olejns 45 of mordenite. Zeolites X and Y have faujasite-like structure. The typical composition of these zeolites can be expressed as gH,O, NapAlpSi,,,-pO,,, where p varies between 74 and 96 for X zeolite and from 48 to 74 for Y zeolite. The value of q is between 250 and 270. The faujasite framework is an array of truncated, octahedra linked by D6R prisms (Figure 9).