Óxidos binários à base de alumina com elevadas propriedades texturais aplicados na esterificação do glicerol

Abstract

The synthesis of aluminum-containing porous oxides has been achieved by the sol-gel method. This synthetic route involved the use of the cetyltrimethylammonium bromide as cationic surfactant and inorganic titanium and silicon alkoxides precursors to produce the as-synthesized TiO2-Al2O3 and SiO2-Al2O3 binary oxides. After calcination procedures, the solids had elevated textural properties with pores in the micro and mainly mesopores regions. To increase the porosity, polyvinylpyrrolidone micelle size expander was used during the step synthesis route enabling the adjustment control of the porosity. All solids were characterized by appropriate physicochemical characterizations such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Scanning Electron Microscopy coupled to Energy-dispersive X-ray spectroscopy (SEM-EDS) analyses, N2 adsorption isotherms and Temperature-programmed desorption with ammonia (NH3-TPD). The catalytic performance was through the esterification of glycerol in the presence of acetic acid (EG) reaction to produce esters of glycerol such as mono, di and triacetin. The catalytic evaluation of the solids obtained have been investigated and discussed in terms of comparison with those of porous Al2O3. Structural characterizations by XRD revealed that the Si-containing materials are amorphous whereas the Ti-based solids are semi-crystalline with the latter composed of TiO2 rutile, TiO2 anatase, γ-Al2O3 phases. The as-synthesized solids had low textural properties due to the surfactant and organic compounds pore occlusions, as observed by FTIR analyses. Contrary to the other solids, the calcined TiO2-Al2O3 and SiO2-Al2O3 binary oxides exhibited mesopores, especially for those expanded with polyvinylpyrrolidone that showed an ordered porous structure with interconnectivity between these pores. Among all the catalysts, the as-synthesized solids demonstrated very low performance in the EG reaction at 80 °C and glycerol to acetic acid molar ratio of 1:1 for 6h under stirring speed of 370 rpm. The meso-macroporous of the calcined solids depicted best performances in the reaction with the conversion up to 40% and the formation of the glycerol esters. Moreover, the porosity and acidic features of Ti-based catalysts play an essential role in terms of achieving superior catalytic activity and selectivity towards triacetin, in comparison to the benchmark γ-Al2O3 catalyst.