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To handle huge amount of glycerol produced in biodiesel industry, glycerol is transformed to value-added products. In this regard, glycerol acetalization to solketal is industrially attractive. As in this process various by-products can be formed, designing highly selective catalysts is of great importance. In this line, we wish to report a novel catalyst that benefits from strong acidity, high specific surface area and thermal stability, which can selectively form solketal in glycerol acetalization. To prepare the catalyst, hierarchical zeolite was prepared via a novel method, in which partially dealuminated NaY was treated with PluronicF-127 and then reacted with NH4NO3 to furnish the H-form zeolite. Hierarchical faujasite was then achieved through calcination and template removal. Subsequently, it was functionalized with ionic liquid and used for the immobilization of heteropolyacid. The results indicated the importance of the mesoprosity of zeolite and the presense of ionic liquid functionality for achiveing high solketal yield. Moreover, among three investigated heteropolyacids, phosphomolybdic acid exhibited the highest catalytic activity. In fact, using 10 wt% catalyst at 55 °C and glycerol to acetone molar ratio of 1:20, solketal with yield of 98% was furnished under solvent-less condition. Besides, the catalyst was recyclable with low leaching of heteropolyacid. [ABSTRACT FROM AUTHOR]

• A triple composite, composed of bentonite, acidic polymer and ionic liquid is prepared. • The composite was used as a support for the immobilization of heteropolyacid. • The prepared catalyst was highly selective and efficient for conversion of glycerol to solketal. • The catalyst was recyclable up to four times. Acetalization of glycerol that is the by-product of biodiesel to fuel additives has received considerable attention. However, formation of a mixture of products in this acid-catalysed process is a challenging issue. In this research, a highly selective catalyst is devised that can selectively acetalize glycerol to solketal. To prepare the catalyst, bentonite clay was first decorated with an acidic polymer containing sulfonic acid and carboxylic acid functional groups. Then, the carboxylic acid groups of the polymer were changed to ionic liquids through reaction with triethanolamine. Finally, phosphotungstic acid was immobilized on the bentonite-polymer composite and the resultant catalyst was characterized. To achieve the highest catalytic activity, the catalyst amount and phosphotungstic acid loading have been optimized. It was found that 10 wt.% catalyst with 20 wt.% phosphotungstic acid loading led to 100% conversion and 99% yield to solketal under solvent-free condition at 55 °C. Moreover, the catalyst could be recycled up to four reaction runs without significant loss of its activity. Comparative studies with ionic liquid-free control catalyst approved that the presence of ionic liquid in the structure of the catalyst played an important role in the selectivity of the catalyst and ionic liquid-free counterpart led to less selectivity and formation of 2,2-dimethyl-1,3-dioxane-5-ol as by-product. [Display omitted] [ABSTRACT FROM AUTHOR]