Polystyrene-based hybrid solid acids with tunable hydrophobicity for the synthesis of polyoxymethylene dimethyl ethers from formaldehyde and methanol

In the synthesis of polyoxymethylene dimethyl ethers (PODEn) from formaldehyde (FA) and methanol (MeOH) solution, the methanol conversion and PODEn selectivity are unsatisfactory because the presence of water limits the adsorption of reactants on acid sites and leads to the hydrolysis of some products. Herein, polystyrene-based hybrid solid acids with tunable hydrophobicity and high acid density were synthesized by post-synthesis grafting method in this work. The prepared catalysts were investigated by BET, SEM, TEM, FT-IR, and acid-base titration. The results demonstrated that the surface property of the catalyst could be changed from hydrophilicity to hydrophobicity with the increase of organic group chain length, which effectively improved methanol conversion and PODE3-6 selectivity. The correlation between contact angles and turnover frequency (TOF) was investigated, suggesting that the TOF of polystyrene-based hybrid solid acids treated with octadecyl trichlorosilane was 2.0 times higher than that of untreated ones. Moreover, a supposed reaction pathway for the synthesis of PODEn from formaldehyde and methanol was proposed, indicating that the improvement of hydrophobicity promoted the decrease of local water concentration in the catalyst, thus facilitating the stabilization of carbocations.