Fabrication of Brønsted acidic ionic liquids functionalized organosilica nanospheres for microwave-assisted fructose valorization

A series of Brønsted acidic ionic liquids (BAILs) functionalized hollow organosilica nanospheres ([C 3/4 Im][OTs/OTf]-Si( Et )Si, C 3/4  =  Pr / Bu SO 3 H) were synthesized by two steps. The process involved the preparation of hollow nanosphere supports via a toluene-swollen sol-gel co-condensation of 1,2-bis(trimethoxysilyl)ethane and 3-chloropropyltriethoxysilane in the presence of F127, and followed by a successive quaternary ammonization and protonation with imidazole , 1,3-propane/1,4-butane sultone and trifluoromethane sulfonic acid/ p -toluenesulfonic acid. The adjustable acid property, hollow inner diameter (5–15 nm) and shell thickness (5–9 nm) of [C 3/4 Im][OTs/OTf]-Si( Et )Si are achieved by introducing different organic acids and controlling toluene concentration, respectively. The [C 3/4 Im][OTs/OTf]-Si( Et )Si were applied in selective conversion of fructose to 5-hydroxymethylfurfural (HMF) and 5-ethoxymethylfurfural (EMF) under microwave heating. Under the optimized conditions, the [C 4 Im][OTs]-Si( Et )Si 3.0 nanospheres with the largest inner diameter and the smallest shell thickness exhibit the highest HMF yield (79.4%, 15 min) in fructose dehydration. And the [C 3 Im][OTf]-Si( Et )Si 0.5 nanospheres with the highest acid strength possess the highest EMF yield (70.4%, 30 min) in fructose ethanolysis. The high Brønsted acid-site density and acid strength of [C 3/4 Im][OTs/OTf]-Si( Et )Si catalysts accompanied by high microwave heating energy lead to excellent dehydration/ethanolysis activity. The product selectivity strongly depended on the BAILs structures and morphological characteristics of the catalyst. More importantly, the [C 3/4 Im][OTs/OTf]-Si( Et )Si can be reused three times without changes in leaching of BAILs, due to strong covalent bond between BAILs and silicon/carbon framework. This work will provide a simple strategy of chemically bonded BAILs on suitable supports as efficient solid acids, and an approach of combining morphology-controlled solid acids with microwave-heating for catalytic conversion of biomass/derivatives to fuels and value-added chemicals.