Síntesis y estudio espectroscópico de zeolitas protónicas del tipo ZSM-5 con sustitución isomórfica de aluminio por galio

Abstract

Aiming at studying both, their acidity and their thermal stability, protonic gallosilicates having the structure type ZSM-5 (which are isomorphous of the corresponding [Al]-ZSM-5 zeolites) were synthesized following a hydrothermal procedure from gels having Si/Ga ratios of 25, 50 and 75. Likewise, for comparison, protonic zeolites [Al]-ZSM-5 having Si/Al ratios of 25 and 50 were also prepared. The obtained materials were characterized by power X-ray diffraction, scanning electron microscopy, MAS-NMR and IR spectroscopy. Brønsted acidity of the structural Si(OH)Ga groups in the gallosilicates and Si(OH)Al groups in the aluminosilicates was quantified by means of IR spectroscopy at a variable temperature using CO and N2 as probe molecules; this instrumental technique enables simultaneous measurement of the bathochromic shift of the stretching O−H mode (∆ ̅OH) of the Brønsted acid group interacting (through hydrogen bonding) with the probe molecule and the corresponding ΔH⁰ value of the CO and N2 adsorption process. For the aluminosilicates, the corresponding values were found to be ∆ ̅OH(CO)=−305 cm-1 and ΔH⁰=−29.3 (±2) kJ mol-1 for CO, and ∆ ̅OH(N2)=−115 cm-1 and ΔH⁰=−19.7 (±2) kJ mol-1 for N2; regardless of Si/Al ratio. The gallosilicates showed ∆ ̅OH(CO)=−280 cm-1 and ΔH⁰=−23.6 (±2) kJ mol-1 for CO and ∆ ̅OH(N2)=−103 cm-1 and ΔH⁰=−15.4 (±2) kJ mol-1 for N2, independently of their Si/Ga ratio. These results clearly showed that the gallosilicates are significantly less acidic than the aluminosilicates, whichever acidity indicator is used; ∆ ̅OH or ΔH⁰. Thermal stability of the gallosilicates (which is relevant to their use as catalysts) was studied by heating them in a vacuum for 2 hours at 723 K, followed additional 2 hours a 973 K. These thermal treatments led to very significant loss of framework gallium and simultaneous formation of extra-framework gallium oxide species. Nevertheless, a further thermal treatment under oxygen led to considerable regeneration of the framework structure. Finally, a careful analysis of IR spectroscopic results obtained using CO as probe molecule led to the proposal of precise mechanisms explaining gallium loss from de gallosilicate framework, and its reincorporation during a subsequent thermal treatment under oxygen.