Up to now, the member of zeolite
family has expanded to more than
230. However, only little part of them have been reported as catalysts
used in reactions. Discovering potential zeolites for reactions is
significantly important, especially in industrial applications. A
carbonylation zeolite catalyst Al-RUB-41 has special morphology and
channel orientation. The 8-MR channel of Al-RUB-41 is just perpendicular
to its thin sheet, making a very short mass-transfer distance along
8-MR. This specific nature endows Al-RUB-41 with efficient catalytic
ability to dimethyl ether carbonylation reaction with beyond 95% methyl
acetate selectivity. Compared with the most widely accepted carbonylation
zeolite catalysts, Al-RUB-41 behaves a much better catalytic stability
than H-MOR and a greatly enhanced catalytic activity than H-ZSM-35.
A space-confined deactivation mechanism over Al-RUB-41 is proposed.
By erasing the acid sites on outer surface, Al-RUB-41@SiO2 catalyst achieves a long-time and high-efficiency activity without
any deactivation trend.
Reduction process is a key step to fabricate metal-zeolite catalysts in catalytic synthesis. However, because of the strong interaction force, metal oxides in zeolites are very difficult to be reduced. Existing reduction technologies are always energy-intensive, and inevitably cause the agglomeration of metallic particles in metal-zeolite catalysts or destroy zeolite structure in severe cases. Herein, we disclose that zeolites after ion exchange of ammonium have an interesting and unexpected self-reducing feature. It can accurately control the reduction of metal-zeolite catalysts, via in situ ammonia production from ‘ammonia pools’, meanwhile, restrains the growth of the size of metals. Such new and reliable ammonia pool effect is not influenced by topological structures of zeolites, and works well on reducible metals. The ammonia pool effect is ultimately attributed to an atmosphere-confined self-regulation mechanism. This methodology will significantly promote the fabrication for metal-zeolite catalysts, and further facilitate design and development of low-cost and high-activity catalysts.
Deposition of carbonaceous compounds was used to improve the propylene selectivity of ZSM-5 by deactivating some acid sites meanwhile maintaining the high activity for methanol conversion. The carbonaceous species of pre-coked samples before and after MTP reactions were investigated by elementary analysis and thermogravimetric analysis (TGA). The results showed that pre-coke formed at low temperature (250°C) was unstable and easy to transform into polyaromatics species at the high reacting temperature, while combining 5% pre-coking process with 95% steam treatment at high temperature (480°C) was effective in inhibiting the formation of coke deposits and presented a significant improvement in the propylene selectivity.
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