Synergy of Lewis and Brønsted acid sites for polyoxymethylene dimethyl ether synthesis from methanol and formaldehyde solution over Zr4+ modified sulfonated resin

Abstract

Polyoxymethylene dimethyl ether (PODEn) is a clean, effective and promising diesel additive. In this work, acidic sulfonated resin modified by zirconium (Zr4+-SR), which possess both Lewis and Brønsted acid sites, was employed to catalyze the synthesis of PODEn from methanol (MeOH) and formaldehyde (FA) solution. The catalysts were investigated by various characterization methods including SEM, BET, XPS, FT-IR, NH3-TPD, Pyridine FT-IR, TG-MS and ICP-OES. It was found that the introduction of Zr4+ into cationic exchange resin formed the Lewis acid sites and improvement of the catalytic performance in PODEn synthesis from methanol and formaldehyde solution was attributed to the synergistic effect of Lewis and Brønsted acid sites. With the increase of zirconium loading, the amount of Lewis acid sites and weak acidity of the Zr4+-SR catalyst increased gradually, and the catalytic activity of the catalysts for the PODEn synthesis reaction exhibited a trend of increasing first and then decreasing. In the methanol and formaldehyde solution, Brønsted acid sites were active for the acetalization of hemiformal and methanol, while Lewis acid sites were conducive to the activation of the methylene glycol. A possible reaction route for the PODEn synthesis from methanol and formaldehyde solution was proposed. 61.1% methanol conversion, 98.7% PODE1–6 and 22.4% PODE3–6 selectivity were achieved under optimal reaction conditions. The reusability investigation of the Zr4+-SR catalyst showed a stable catalytic activity for the synthesis of PODEn from methanol and formaldehyde solution and revealed that the decrease of catalyst activity was attributed to the partial loss of the zirconium.