Abstract
Abstract Pd-H4SiW12O40 supported on SiO2 with different loadings of H4SiW12O40 were systematically characterized by X-ray diffractometry (XRD), infrared (IR) spectroscopy, and NH3-temperature programmed desorption (TPD) and benzonitrile adsorption analyses. X-ray diffraction and infrared spectroscopy revealed that very few H4SiW12O40 crystallites formed at less than 20 wt.% loading and that the Keggin structure was retained for all these catalysts. The density of acid sites on the top surface of catalysts determined by benzonitrile adsorption proved the average number of H4SiW12O40 layers on SiO2 to be 2.2, 2.6 and 4.0 at 10, 20 and 40 wt.% acid loading, respectively. NH3 temperature programmed desorption showed that the acid strength increased markedly with the loading of H4SiW12O40, attributed to suppressed interactions between the protons of H4SiW12O40 and the OH groups on SiO2. The selectivity to isopentane in skeletal isomerization of n-pentane was found to be highly sensitive to the loading of H4SiW12O40, decreasing with increasing load, associated with the change in acid strength by the supporting on SiO2. A catalyst with the composition 1 wt.% Pd-20 wt.% H4SiW12O40 on SiO2 exhibited very high selectivity (99.1%) and high activity.
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