Bronsted 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. 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 N-2 as probe molecules. This instrumental technique enables one simultaneous measurement of the bathochromic shift of the stretching O-H mode, Delta nu((OH)) of the Bronsted acid group interacting (through hydrogen bonding) with the probe molecule and the corresponding standard enthalpy change, Delta H-0, in the adsorption process. The results obtained clearly showed that the gallosilicates are distinctively less acidic than the aluminosilicates, whichever acidity indicator is used: Delta nu((OH)) either or Delta H-0. Nevertheless, no change of Bronsted acid strength was found when changing the Si/Ga ratio.