Synthesis of polyoxymethylene dimethyl ethers over different microporous and mesoporous zeolites: the effects of acidity and pore size

Abstract

Polymethoxydimethyl ether (PODE_n) is an ideal blending component of diesel oil. As a typical acid catalyzed reaction, studies on the structure–activity relationship of catalysts, especially the influence of acidity and pore size on PODE_n 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 PODE_n. 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 effect of varying acidity on the reaction under the conditions of constant pore size, it was found that ZSM-5, Al-SBA-15 and Al-MCM-41 all showed the same changing trend, and the synthesis process of PODE_n required a suitable amount of acid, and 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 too high, a relatively large pore size would be more conducive to the diffusion of formaldehyde monomers, thus reducing the formation rate of the by-product methyl formate and obtaining a larger yield and selectivity of the product. However, when the acidity was moderate or too low, the yield and selectivity of the product and the conversion of feedstock catalyzed by three kinds of porous catalysts reached basically the same value, illustrating that the pore size had no effect on the adsorption and diffusion process of products with relatively low local density of formaldehyde monomer. In addition, the pore size of the catalyst had no effect on the product distribution and chain length under suitable acidic conditions.