Preparation and characterization of bimodal porous alumina–silica and its application to removal of basic nitrogen compounds from light oil

Abstract

Aluminium nitrate and aluminium isopropoxide were used as aluminium precursors to prepare bimodal porous alumina–silica mixed oxides via the sol–gel method with the addition of nonionic surfactant C₁₆EO₁₀ as the structure-directing agent. The characterization of the mixed oxides was performed using scanning electron microscopy (SEM), nitrogen adsorption/desorption measurements, chemical composition measured by X-ray fluorescence (XRF), and surface acidity was investigated by the Hammett indicator method and Fourier transform infrared spectroscopy (FT-IR) of adsorbed pyridine. It is found that the resultant mixed oxides possess micrometer-range interconnected macropores and nanometer-range mesopores. Studies have shown that relatively large mesopore size materials can be obtained with aluminium nitrate precursor, but relatively high Al content materials can be synthesized using aluminium alkoxide as aluminium precursor. Surface acidity analysis results indicate that the alumina–silica mixed oxides synthesized by present sol-gel method have Lewis acid sites and Brønsted acid sites, and these acid sites exhibit mild strong acidity. Moreover, two different synthesis pathways have little influence on the acidity of alumina–silica prepared by the present method. In addition, the adsorption capacity of such acidic mixed oxides, with bimodal pore structures, for pyridine from diesel oil was studied. The adsorption experiments indicate that these mild strong acidic adsorbents with high surface areas and large macropores are effective in removing basic nitrogen compounds (BNC) such as pyridine from diesel oil, and are easily regenerated by washing with polar solvents.