Studies on mesoporous silica ionogels prepared by sol–gel method at different gelation temperatures

Abstract

Ionogels (IGs), a novel group of materials having an ionic liquid entrapped in a mesoporous silica matrix, have been synthesized by a non-hydrolytic sol–gel process at different gelation temperatures (30, 0, −10 °C) using 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF₄] ionic liquid (IL), tetraethylortho-silicate (TEOS) as a metallic precursor and formic acid as a catalyst. The rate of gelation was lower at lower temperatures. Gelation temperature has also been found to affect (i) the densities of IGs, (ii) pore parameters of IGs like average pore diameter, pore surface area, pore volume and porosity, (iii) phase transition temperatures T_m, T_c, and T_g, (iv) thermal stability of IL and (v) vibrational spectra. The porosity of IG ingots, prepared at lower gelation temperatures (0, −10 °C) was higher. Unexpectedly, their densities were found to be higher than the IGs prepared at higher temperatures (say 30 °C). Possibly, one of the reaction products of the sol–gel process (namely, ethyl formate) could not escape the matrix due to its slow evaporation at low temperatures which resulted in their higher densities. The presence of ethyl formate in the IGs was confirmed by FTIR. The variation of the other properties of the IGs, listed above, synthesized at different gelation temperatures is also discussed and explained in the paper.