Surface behavior of supported 12-heteropoly acid as revealed by nuclear magnetic resonance, X-ray photoelectron spectroscopy, and Fourier transform infrared techniques

Bulk, silica-, and zirconia-silica-supported 12-molybdophosphoric acid (HPMo) catalysts have been characterized by P-31 and H-1-magic angle spinning NMR spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared (FTIR) spectroscopy of pyridine adsorption. Two species, bound to the support surface and acid-free components, were found in both types of supported catalysts. The relative amount of these species varied depending on the type of the support and on the degree of hydration of the samples. Thermally stable molybdophosphate species up to higher temperature (723 K) were formed on silica support compared to bulk. The mixed zirconia-silica support causes the destruction of HPMo at significantly lower temperature, due to a stronger interaction of the zirconia with supported molybdophosphates. The FTIR of pyridine adsorption showed two types of acidity on the supported samples: Lewis and Bronsted. A correlation between the change in the intensities of the Bronsted acidity band and H-1 NMR signal of acidic protons with temperature was found for supported HPMo: high intenstity of H-1 NMR signal for HPMo on the mixed zirconia-silica support corresponds to high Bronsted acidity band caused from the residual protons involved in bridging OH groups.