Synthesis and characterization of Ga-modified Ti-HMS oxide materials with varying Ga content

This work reports on the structure and surface characteristics of hexagonal mesoporous silica (HMS) modified with both Ti and Ga. Several techniques were used (low-angle XRD, S-BET, TPD-NH3, DRS, FT-IR of framework vibration, FT-IR of adsorbed pyridine, SEM and HRTEM). These systems were then used as catalysts in the o-xylene hydroisomerization carried out in a batch reactor at 300 degrees C and atmospheric H-2 pressure. The catalyst characterization demonstrated that the modification of Ti-HMS substrate with Ga by post-synthesis grafting yields high surface area substrates with good Ga dispersion and bimodal distribution of gallium species. On the basis of lattice expansion observed from XRD data together with the increase of absorption band due to vibration of Si-O-Ga linkages, it is inferred that Ga species are incorporated into the silica framework. Moreover, textural analysis revealed that highly dispersed Ga2O3 species are deposited within the inner pores of the substrate. The largest o-xylene conversion observed for the Ti-HMS-2.5Ga sample containing the largest Ga loading is related to the heterogeneous distribution of its Ga species and its large acidity. Upon reaction conditions employed, the o-xylene hydroisomerization over Ti-HMS-Ga proceeds exclusively via intramolecular mechanism (formation of m- and p-xylenes) and hydrogenation of benzene aromatic ring of o-, p- and m-xylenes did not occur. (C) 2014 Elsevier B.V. All rights reserved.