Tailored Bronsted and Lewis acid sites of the phosphotungstic Wells-Dawson heteropoly-acid

S. E. COLLINS; S.R. MATKOVIC; L. E. BRIAND; M. BOSCO

Santa Fe

Conferencia; San Luis Conference on Surfaces, Interfaces and Catalysis; 2018

UNL-CONICET-INTEC

Previous investigations of the authors demonstrated the capability of the fosfotungtic Wells Dawson heteropoly-acid (HPA) H6P2W18O62 to catalyze the selective acylation of isobutylbenzene with acetic anhydride. The anhydride dissociatively chemisorbs on the Brønsted acid sites producing the intermediate acyl CH3CO+ species, adsorbed acetate species, and acetic acid. The former is known as the key intermediate species in the Friedel-Crafts acylation reaction. However, surface acetate species and acetic acid strongly adsorb on the active acid sites deactivating the heteropoly acid which prevented the further formation of p-isobutylacetophenone.Based on those observations, the present investigation is devoted to tune the acidity of the HPA in order to generate a material with both Lewis and Brønsted acid sites in order to overcome the problem of the irreversible adsorption of polar oxygenate species causing its deactivation.In this context, the effect of transition metal oxide supports (TiO2, ZrO2, Al2O3, SiO2) on the acidity of the HPA was investigated. Additionally, the protons of the HPA were replaced by transition metals [Ni(II), Cu(II), Mn(II), Cr(III) and Bi(III)] in order to obtain evidences of the influence of counter-cations of varying electronegativity in the concentration of Lewis and Brønsted acid sites of bulk HPA.The heteropoly-acid was dispersed onto the oxide supports through a conventional impregnation method at a loading that corresponds to the theoretical dispersion limit loading (18% HPA/TiO2, 13% HPA/ZrO2, 66% HPA/SiO2 and 33% HPA/Al2O3). However, the combination of in situ Raman and infrared studies demonstrates the presence of crystals of HPA suggesting that the dispersion limit loading was greatly exceeded. In fact, adsorption of pyridine on the HPA dispersed on silica and alumina demonstrated that the density of Lewis acid sites is similar to the bare oxide supports. This observation evidences that the surface of the oxide support is not completely covered and therefore, the acid sites are available to the probe molecule. The decreasing density of Brønsted acid sites from 18% HPA/TiO2, 13% HPA/ZrO2, 66% HPA/SiO2 to 33% HPA/Al2O3 could not be ascribed to the hydration degree of the HPA since the adsorption of pyridine was performed at R.T. without previous thermal treatment of the materials. The density of the Wells Dawson heteropoly-acid (11 tungsten atoms per nm2) is similar therefore the variation of the Brønsted acid sites is not ascribed to the amount of HPA. In fact, previous investigations reported by Bielański et al. demonstrated that the Wells Dawson HPA forms islands of nanocrystals (5 nm) when dispersed over TiO2. Therefore, according to that investigation it is possible to correlate the variation of the density of Brønsted acid sites to the size of the crystal of the HPA over the oxide supports.The heteropoly salts of the transition metals were synthesized by replacing the protons of the Wells-Dawson heteropoly acid. An aqueous solution of the HPA and the corresponding amount of the metal nitrate (or acetate in the case of manganese) were mixed and heated until dryness under continuous stirring. The adsorption of pyridine demonstrated that the addition of a transition metal in the heteropoly structure provides a concentration of Lewis acid sites of an order of magnitude higher than the heteropoly-acid.