Synthesis and characterization of tungstophosphoric acid-modified mesoporous sponge-like TUD-1 materials

Abstract

Mesoporous sponge-like siliceous materials (TUD-1) were synthesized using TEOS (tetraethylorthosilicate) as a precursor of silica and TEA (triethanolamine) as a mesostructure-directing agent. Different TEOS/TEA molar ratios and hydrotreatment times were used, and their influence on the textural and morphological properties of the solids was evaluated. They were characterized by transmission electron microscopy (TEM), X-ray diffraction, and N2 adsorption/desorption isotherm analysis. By changing the TEOS/TEA molar ratio, micro-mesoporous and mesoporous (with a narrow or wide pore size distribution) materials were obtained. The increment of hydrotreatment times increased the average pore size of TUD-1 and significantly reduced the specific surface area. TEM images showed that the solids were formed by agglomerates of rather spherical particles whose size increased with the increment of the TEA amount used. The structure and morphology of TUD-1 solids modified with tungstophosphoric acid (TPA) were similar to those of the mesoporous silica used as support. Their diffraction patterns did not display any of the characteristic peaks of TPA or its more common hydrates, suggesting that TPA was well dispersed on the support as a noncrystalline phase. In addition, the characterization of all the solids impregnated with TPA by FT-IR and 31P NMR indicated the presence of undegraded [PW12O40]3− and [H3−XPW12O40]X− species interacting electrostatically with the ≡Si–OH2+ groups, and by potentiometric titration it was estimated that the solids presented very strong acid sites. The TUD-1 solids modified with TPA gave excellent yield in the solvent-free synthesis of 2,4,5-triphenyl-1H-imidazole, without formation of by-products resulting from competitive reactions or decomposition products. In summary, they are good candidates to be used in multicomponent reactions catalyzed by acids.