Rapid tuning of ZSM-5 crystal size by using polyethylene glycol or colloidal silicalite-1 seed

Abstract

Two methods were used to control the crystal size of ZSM-5 synthesized with commercial silica sol. One is use of organic additives; the other is use of colloidal silicalite-1 seed. Addition of polyethylene glycol (PEG) to the synthesis gel favors the formation of relatively small uniform crystals because of promotion of nucleation and somehow hindering the growth of crystals, although it has little influence on the structure, crystallinity, acidity, and crystal shape. The prepared catalyst shows higher catalytic stability than the sample synthesized with the PEG-free gel in the methanol-to-hydrocarbon (MTH) reaction. Finely controllable synthesis of different-sized ZSM-5 crystals was realized by epitaxially growing ZSM-5 crystals around colloidal silicalite-1 seed. The crystal size could be tuned from 200 to 1000nm by every 100nm through adjusting the crystallization time and the amount of the added silicalite-1 seed. This makes it possible to systematically study the effect of zeolite crystal size on its catalytic performance and tune product distribution. A significant increase in the catalytic stability was observed with decreasing crystal size for ZSM-5 catalyzing MTH reaction. The crystals with a size of about 270nm showed a catalytic life of about 6 and 3 times as long as those of the samples synthesized in the absence and presence of PEG respectively.