Brønsted acidity of H-[Ga]-ZSM-5 zeolites as determined by variable-temperature IR spectroscopy

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. Brønsted 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 OH mode, Δυ(OH), of the Brønsted 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 Brønsted acid strength was found when changing the Si/Ga ratio.