First-principles modeling combined with experimental methods were used to study hydroxyapatite in which Sr2+ is substituted for Ca2+. Detailed analyses of cation-oxygen bond distributions, cation-cation distances, and site 1-oxygen polyhedron twist angles were made in order to provide an atomic-scale interpretation of the observed structural modifications. Density functional theory periodic band-structure calculations indicate that the Ca2+ to Sr2+ substitution induces strong local distortion on the hydroxyapatite lattice: the nearest neighbor Sr-O bond structures in both cationic sites are comparable to pure SrHA, while Sr induces more distortion at site 2 than site 1. Infrared vibrational spectroscopy (FTIR) and extended X-ray absorption fine structure (EXAFS) analysis suggest increasing lattice disorder and loss of OH with increasing Sr content. Rietveld refinement of synchrotron X-ray diffraction patterns shows a preference for the Ca1 site at Sr concentrations below 1 at.%. The ideal statistical occupancy ratio Sr2/Sr1=1.5 is achieved for approximately 5 at.%; for higher Sr concentrations occupation of the Ca2 site is progressively preferred.

Gallium-modified ZSM5 zeolites, containing 2, 3, and 4 wt % Ga, were prepared by incipient wetness impregnation with gallium(III) nitrate solutions of HZSM5 samples with SAR values of 24 and 35. The modified zeolites were subjected to two consecutive treatment cycles of reduction in hydrogen and reoxidation in air. The catalysts were characterized by nitrogen physical adsorption, 29Si and 27Al MAS NMR, temperature-programmed reduction, temperature-programmed desorption of ammonia, X-ray absorption spectroscopy, and infrared spectroscopy in the OH stretching region and in the region of skeletal vibrations of adsorbed pyridine. Catalytic activity and selectivity were measured in propane conversion reactions at 703 K. Results were rationalized in terms of correlations among the framework Ga/Al ratio, density, strength and type of acidity, gallium cluster sizes, reducibility of gallium, and propane aromatization activity. It was found that the dispersion of the gallium species was a function only of the framework Ga/Al ratio. Lewis acidity and reducibility of the gallium species were positively correlated with each other and with their degree of dispersion. A correlation was found between propane aromatization activity and the product of the concentrations of strong Lewis and Brønsted acid sites in the oxide form of the catalysts.

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