Formation mechanism microporous zeolites

Yamazaki S and Tsutsumi K. Synthesis of A-type zeolite membrane using a plate heater and its formation mechanism. Micropor Mesopor Mater 2000 37 67-80. 

NaX (13X) zeolite is the catalyst of choice for benzylic chlorinations while zeolites with high Bronsted acidity (ZF520) affected ring chlorination, even though X-ray diffraction studies have later shown that the zeolite lattice collapses under the reaction conditions127. In both instances the mechanism involves active site outside the channel network of the microporous solid. Contradictory to the latter authors, Delude and Laszlo suggest that aluminum-rich zeolites would preferably initiate radical chain reaction via formation of siloxy radicals. Both the reaction medium and substituents on the aromatic substrate have a profound effect on the rate and selectivity of these reactions. Interestingly, the catalyst applied in the radical chlorinations can be easily recycled and reused. The opposite has been observed in the ionic chlorinations where the catalyst has rapidly lost its activity. 

However, whilst chemical differences between the different classes of microporous materials clearly do exist, it is not yet clear to what extent these affect the mechanism of synthesis. Possibly the products do derive directly from PNBUs but there seems no reason at present to reject the alternative possibility that much of this chemistry goes on at the liquid-solid interfacial growth points rather than (as is implied) independently of the growing crystal. Tt seems unlikely that any completely new concepts will be necessary to explain the formation patterns of zeotypes. However, just as the range of behaviour observed for zeolites requires a flexible (but coherent) mechanistic scheme, this spectrum will need to be further extended in order to allow for the differences in composition, structure, polarity and solution chemistry found in zeotype synthesis. 

The transition states leading to the cis and trans alcohols differ substantially in size and the way in which they can be accommodated in the pores of zeolite Beta. That for the cis isomer is more or less linearly aligned with the pore axis and can easily be accommodated within the straight channels of the zeolite. The transition state for the formation of the trans isomer is more or less perpendicular to the channel wall and cannot be well accommodated within the micropores (Scheme 5). As required by this mechanism, the cis alcohol was found to undergo Oppenauer oxidation over zeolite Beta whereas activity for the trans isomer being negligible. 

An acid treated zeolite beta with a 6 M HNO3 acid solution is a shape-selective catalyst for the alkylation of biphenyl with propene. This is however not the case for the alkylation of naphthalene. The reaction is highly mass transfer limited and the selectivity mechanism is attributed to a product selectivity. The major effect of the acid treatment is to deactivate the external surface area so that the intrinsic micropore properties can come out. Contrary to zeolite mordenite, the acid treatment does not reduce deactivation and the formation of highly aromatic coke remains important at high temperatures. 

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