In situ quantification of the active acid sites of H_6P_2W_(18)O_(62)·nH_2O heteropoly-acid through chemisorption and temperature programmed surface reaction of isopropanol

摘要

Isopropanol chemisorption and temperature programmed surface reaction analyses were used to determine the nature,amount and acid strength of the active sites of bulk WO_3,monolayer supported tungsten oxide over titania and phospho-tungstic Wells-Dawson and Keggin type heteropoly-acids.The temperature conditions were adjusted in order to cover the surface of the catalysts with a stable monolayer of intermediate chemisorbed isopropoxy species avoiding the adsorption of molecular isopropanol (non reactive) or surface reaction during the chemisorption process.Chemisorbed isopropoxy species dehydrate towards propylene (no other redox products were observed) over the investigated catalysts.The number of active sites (N_s) was determined as the amount of propylene desorbed during the temperature programmed reaction analysis.Heteropoly-anions possess the highest amount of active sites and acid strength among the investigated solid catalysts.Isopropanol chemisorbs both at the surface and the bulk of the heteropoly-anions due to the pseudo-liquid phase property of these materials.Moreover,the high acidity of the Bronsted and Lewis acid sites of the heteropoly-acids catalyze the reaction of the adsorbed isopropoxy species at a lower temperature (approx 100 deg C) than WO_3 and monolayer supported tungsten oxide species WO_x/TiO_2.Additionally,the influence of the degree of hydration on the amount and acid strength of the active sites of the Wells-Dawson acid was investigated.The results showed that the fully hydrated acid possesses a higher amount of accessible sites than the fully dehydrated acid therefore,the amount of propylene is almost double in the first condition.However,no modification of the temperature of reaction of the isopropoxy species was observed.