Synthesis of polyoxymethylene dimethyl ethers catalyzed by sulfonic acid-functionalized mesoporous SBA-15

Abstract

Sulfonic acid-functionalized mesoporous SBA-15 (SO₃H-SBA-15) with different sulfur loading was synthesized via a post-synthesis method with 3-mercaptopropyltrimethoxysilane (MPTMS) and was used to investigate the catalytic performance for the synthesis of polyoxymethylene dimethyl ethers (PODE_n) from methylal (DMM) and trioxymethylene (TOX). X-ray diffraction, N₂ adsorption–desorption, Fourier transform infrared spectra, ¹³C-NMR and ²⁹Si-NMR, elemental analyses, and X-ray fluorescence were used to characterize the structures and sulfur loading of the obtained catalysts, and chemistry titration was carried out to investigate the acid amounts of the catalysts. Through the comparison of the catalysts with different sulfur loading, it was found that the acidity of the catalysts has a decisive effect on the product distribution and the chain length of the products: under the catalysis of the SO₃H-SBA-15(0.8) catalyst, which has an acid amount of 0.166 mmol g⁻¹, the highest DMM and TOX conversion and PODE_n yield and selectivity were achieved and the desired chain length of PODE_2–8 was obtained. The optimum reaction conditions such as reaction time, temperature, and molar ratio of DMM and TOX were investigated, and by comparison of different reaction temperatures, it was demonstrated that temperature played an important role in the TOX depolymerization rate and selectivity of the main products and by-products. By adjusting the molar ratio of DMM and TOX, it was observed that an increasing amount of TOX was beneficial for promoting the reaction to generate the products with a higher degree of polymerization. The comparison test of the reaction time confirmed that the reaction reached chemical equilibrium within 60 min and could not be promoted by further extending the time.