Nano-sized mesoporous phosphated tin oxide as an efficient solid acid catalyst

Abstract

Herein, we prepared a mesoporous tin oxide catalyst (mSnO 2 ) activated with phosphate species by the adsorption of phosphate ions from a phosphoric acid solution onto tin oxyhydroxide (Sn(OH) 4 ) surface. The phosphate content ranged from 3 to 45 wt%. The nonaqueous titration of n-butylamine in acetonitrile was used to determine the total surface acidity level. FTIR of chemically adsorbed pyridine was used to differentiate between the Lewis and Brönsted acid sites. Thermal and X-ray diffraction analysis indicated that the addition of phosphate groups stabilized the mesostructure of mSnO 2 and enabled it to keep its crystalline size at the nanoscale. FTIR analysis indicated the polymerization of the HPO 42− groups into P 2 O 74− , which in turn reacts with SnO 2 to form a SnP 2 O 7 layer, which stabilizes the mesoporous structure of SnO 2 . The acidity measurements showed that the phosphate species are distributed homogeneously over the mSnO 2 surface until surface saturation coverage at 25 wt% PO 43− , at which point the acid strength and surface acidity level are maximized. The catalytic activity was tested for the synthesis of hydroquinone diacetate, where it was found that the % yield of hydroquinone diacetate compound increased gradually with the increase in PO 43− loading on mSnO 2 until it reached a maximum value of 93.2% for the 25% PO 43− /mSnO 2 catalyst with 100% selectivity and excellent reusability for three consecutive runs with no loss in activity.