Synthesis of Polyoxymethylene Dimethyl Ethers over different microporous and mesoporous zeolites: The effects of acidity and pore size
Polymethoxydimethyl ether (PODEn) is an ideal blending component of diesel oil. As a typical acid catalyzed reaction, studies on the structure-activity relationship of catalyst, especially the influence of acidity and pore size on PODEn synthesis, are of vital importance. Herein, three kinds of catalysts with different pore sizes: ZSM-5, Al-MCM-41 and Al-SBA-15 with variable Si/Al ratios were chosen to investigate the effects of acidity and pore sizes on the yield, selectivity and product distribution of PODEn. X-ray diffraction, N2 adsorption-desorption, transmission electron microscopy, X-ray fluorescence and ammonia temperature-programmed desorption were executed to characterize the structures and acid properties of obtained catalysts. By comparing the effects of different acidity on the reaction under the condition of constant pore size, it was found that ZSM-5, Al-SBA-15 and Al-MCM-41 all showed a same changing trend, and the synthesis process of PODEn required a proper amount of acid, insufficient or excessive acid would lead to a low yield of the product. By checking the catalytic activity of catalysts with different pore sizes under the same acidity conditions, it was demonstrated that when the acidity of the catalyst was excessive, a relative large pore size would be more conducive to the diffusion of formaldehyde monomers, thus reducing the formation rate of by-product methyl formate and obtaining a larger yield and selectivity of the product. However, when the acidity was moderate or inadequate, the yield and selectivity of the product and the conversion of feedstock catalyzed by three kinds of porous catalysts reached a basically same value, illustrating that the pore size had no effect on the adsorption and diffusion process of products with relative low local density of formaldehyde monomer. In addition, the pore size of catalyst had no effect on the product distribution and chain length under suitable acidic conditions.