The effects of lattice composition on the physicochemical properties of H-offretite crystals

Several spectroscopic techniques have been used to investigate the acidic properties of a set of galliosilicate molecular sieves with the offretite structure. IR spectra in the OH stretching region contain well-defined bands at or near 3735, 3640, 3600 and 3550 cm⁻¹ that can be attributed to OH groups associated with SiOH, and with M(IV)OH groups located in secondary building units, in the main channels (or in the gmelinite cages), and with M(IV)OH groups associated with oxygens shared by the main channels and the cancrinite cages; M = Al,Ga. Spectra of chemisorbed pyridine show that all crystals contain both Bronstedt (B) and Lewis (L) acid sites, and that the presence of Ga seems to decrease B-acid strength and to increase the L-sites' density and strength. Microcalorimetry experiments with ammonia and pyridine as probe molecules show that Ga insertion into the aluminosilicate framework increases the strength of the strongest acid sites and that the presence of Ga generate a heterogeneous distribution of acid site strengths absent in the parent Al-analogs. Moreover, in H,Ga-offretites, there is a drastic reduction in acid site density while the initial acid site strength remains high. NMR results suggest that the observed effects can be attributed, in part, to the presence of extra-framework Al and Ga atoms. Levels of Al(VI) and Ga(VI), as well as partial lattice degradation, strongly depend on the NH₄-exchange and calcination procedure used to activate these offretite crystals.