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

Hassan, S. M., Mannaa, M. A., Ibrahim, Amr Awad
RSC advances 2019 v.9 no.2 pp. 810-818
Bronsted acids, Fourier transform infrared spectroscopy, X-ray diffraction, acetonitrile, acidity, adsorption, catalysts, catalytic activity, hydroquinone, ions, phosphates, phosphoric acid, pollution load, polymerization, porous media, pyridines, quinones, tin, tin dioxide, titration
Herein, we prepared a mesoporous tin oxide catalyst (mSnO₂) 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 mSnO₂ and enabled it to keep its crystalline size at the nanoscale. FTIR analysis indicated the polymerization of the HPO₄²⁻ groups into P₂O₇⁴⁻, which in turn reacts with SnO₂ to form a SnP₂O₇ layer, which stabilizes the mesoporous structure of SnO₂. The acidity measurements showed that the phosphate species are distributed homogeneously over the mSnO₂ surface until surface saturation coverage at 25 wt% PO₄³⁻, 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 PO₄³⁻ loading on mSnO₂ until it reached a maximum value of 93.2% for the 25% PO₄³⁻/mSnO₂ catalyst with 100% selectivity and excellent reusability for three consecutive runs with no loss in activity.