Nanoestructuras de sílice, con diámetro y distribución de mesoporos variable, modificadas con ácido tungstofosfórico como catalizadores en la síntesis de quinoxalinas

Abstract

Tungstophosphoric acid supported on silica nanostructures (SNX#WPA) with variable diameter and mesopore size distribution were synthetized. Silica nanostructures (SNX) were prepared in octane/aqueous media using polystyrene and CTAB as organic templates. The materials were characterized by XRD, SEM, TEM and dinitrogen adsorption/desorption isotherm analysis. The octane/water ratio influenced the morphology and size of SNX prepared, as well as its pore size distribution. The SNX samples obtained using OCT/H 2 O ratios in the range 0.07-0.35 (SN4, SN5, and SN6 samples). present small (5-6 nm) and large (28-34 nm) mesopores (mainly generated by polystyrene). Large mesopores and their volume contribution were clearly higher than in the SN1, SN2, and SN3 samples. The structure and morphology of SNX#WPA samples were similar to those of the SNX. Furthermore, the characterization of all the SNX#WPA materials by FT-IR and 31 P NMR indicated the presence of undegraded [PW 12 O 40 ] 3− and [H 3-X PW 12 O 40 ] (3-X)− species. According to the potentiometric titration results, the solids presented very strong acid sites. The performance of SNX#WPA materials as catalysts in the synthesis of quinoxalines was evaluated. The yields achieved were high, without formation of by-products resulting from competitive reactions or decomposition products, so the prepared materials are highly selective and reusable catalysts.