Efficient and reusable zeolite-immobilized acidic ionic liquids for the synthesis of polyoxymethylene dimethyl ethers

Abstract

A series of molecular sieve-immobilized Brønsted acidic ionic liquids (BAILs@MS) was prepared, characterized and utilized as efficient catalysts for the synthesis of polyoxymethylene dimethyl ethers (PODEn or DMMn) from methylal (DMM1) and trioxane (TOX). Combined characterization results from fourier transform infrared (FT-IR) spectroscopy, elemental analysis, thermogravimetry (TG), N2 adsorption-desorption (Brunauer-Emmett-Teller, BET) isotherms, X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed desorption of ammonia (NH3-TPD), suggested that the synthesized BAILs were successfully immobilized on the surface of molecular sieves through covalent bonds. Moreover, the catalytic performance tests demonstrated that NaZSM-5 immobilized SO3H-functionalized ionic liquids (ILs) i.e., BAILs@NaZSM-5, exhibited excellent activity for the acetalation of DMM1 with TOX, compared to the homogeneous catalysis of the precursors ([MIMBs]HSO4) as well as other molecular sieve-supported ILs. The influence of catalyst concentration, molar ratio of DMM1 to HCHO, temperature and reaction duration on the catalytic activity were investigated by employing [NaZSM-5IMBs]HSO4 as the catalyst. It was demonstrated that a superior conversion of formaldehyde (FA) (90.3%) and excellent selectivity for DMM3-8 (53.5%) has been achieved in mild conditions (110 °C, 2.5 MPa for 2 h). Additionally, the catalyst can be effortlessly separated by filtration and reused more than ten times without significant loss of activity.