Abstract
Abstract Protonic gallosilicates H-[Ga]-ZSM-5 were synthesized, following a hydrothermal procedure, from gels having Si/Ga ratios of 25, 50 and 75. Likewise, for comparison, protonic zeolites H-[Al]-ZSM-5 having Si/Al ratios of 25 and 50 were also prepared. Bronsted acidity of the structural Si(OH)Ga groups in the gallosilicates was studied by means of IR spectroscopy at a variable temperature (VTIR) using CO and N2 as probe molecules. This instrumental technique enables one simultaneous measurement of the bathochromic shift of the stretching O H mode, Δυ(OH), of the Bronsted acid group interacting (through hydrogen bonding) with the probe molecule and the corresponding standard enthalpy change, ΔH⁰, in the adsorption process. The results obtained clearly showed that the gallosilicates are distinctively less acidic than the aluminosilicates, whichever acidity indicator is used: Δυ(OH) either or ΔH⁰. Nevertheless, no change of Bronsted acid strength was found when changing the Si/Ga ratio.
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