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Abstract
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.
Highlights
The replacement of liquid acid or acid halide catalysts, such as H2SO4, H3PO4, HF and AlCl3, is a critical factor for the design of cleaner processes for green catalytic action.[1]
The rst exothermic effect with a weight loss of 8.9 wt% at around 312 C was attributed to the decomposition of the surfactant template and partial removal of the OH groups,[34,35] while the second exothermic peak at 423 C corresponded to a weight loss of 3.1 wt%, which may be due to the crystallization of tin oxyhydroxide into tetragonal cassiterite SnO2.36 In the case of PO43À/Sn(OH)[4] dried samples, the DTA in Fig. 2 shows exothermic peaks at 321 C, 334 C and 342 C as the phosphate content increases for the 8%, 25% and 45% PO43À/Sn(OH)[4] samples, respectively
These results show no effect that could be assigned to the decomposition of PO43À
Summary
The replacement of liquid acid or acid halide catalysts, such as H2SO4, H3PO4, HF and AlCl3, is a critical factor for the design of cleaner processes for green catalytic action.[1]. The selected area electron diffraction (SAED) pattern images (insets) of the samples exhibit diffraction rings attributed to the polycrystalline behaviour of the mSnO2, while the crystallite degrees of the 3%, 8% and 25% PO43À/mSnO2 catalysts decrease with the increase in PO43À content.[46,47] The TEM images of 3% PO43À/mSnO2 calcined at 550 C (Fig. 4b) illustrate that the mesoporous structure remains even a er calcination at 550 C, which re ects the role of the phosphate groups in improving the thermal stability of the tin oxide.[47] It is clear from the TEM
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