Synthesis and characterization of acidic high-silica faujasite-type gallosilicates

Ultrastabilization of a (rare-earth-loaded) faujasite (FAU)-type gallosilicate appears to be the only viable route for synthesizing a gallosilicate with an acid site density comparable to that of ultrastable FAU-type aluminosilicates. Neither gallium insertion into a FAU-type silica nor substitution of framework gallium by silicon using ammonium hexafluorosilicate appears feasible. The ultrastable FAU-type gallosilicate shows no sites with enhanced Bronsted acidity. According to the shift observed by IIE spectroscopy when CO is adsorbed, the acid site strength of FAU-type gallosilicates is weaker than that of high-silica FAU-type aluminosilicates but stronger than that of FAU-type silicoaluminophosphates. Although the ultrastable FAU-type gallosilicate is (hydro)thermally stable in an oxidizing atmosphere up to 1150 K, it releases its gallium in a reducing atmosphere (starting at 525 K) and loses structural integrity increasingly rapidly above 635 K. This severely limits the potential use of catalysts based on gallosilicate framework structures in hydrocarbon hydroconversion reactions.