Gallium-Containing Mesoporous Silica: Supported Catalysts with High Catalytic Activity for Oxidation of Benzene, Toluene ando-Xylene

Abstract

Benzene, toluene and xylene (BTX) are a particular class of volatile organic compounds, which are highly toxic pollutants. In this study, samples of gallium-containing mesoporous silica (MS-Ga7% and MS-Ga11%) were synthesized and their catalytic activity in the oxidation of BTX was investigated. The physicochemical characterization shows that the inclusion of gallium in the mesoporous silica structure leads to an increase in the number of oxygen vacancies in the structure of the MS-Ga system, which can result in an increase in the total and surface oxygen mobility. The results show the highest conversion for benzene (65%), with > 40% for toluene and > 28% for o-xylene. The high catalytic activity observed was attributed to a combination of several factors including a higher number of active sites (gallium and gallium oxide) being exposed, with a greater mobility of the active oxygen species on the surface of the catalyst promoting the catalytic activity. Keywords: gallium, mesoporous silica, catalytic oxidation, BTX IntroductionThe emission of high levels of volatile organic compounds (VOC) to the atmosphere during various industrial and commercial processes is considered detrimental to human health and the environment. [1][2][3][4][5][6] Benzene, toluene and xylene (BTX) are volatile organic compounds used as common solvents as well as raw materials in the production of other chemicals. [7][8][9] All three substances are known to be toxic. 10,11 Several VOC removal technologies, such as flame combustion, catalytic combustion, catalytic destruction using ozone and plasma, photocatalytic decomposition, adsorption processes and biological treatment, have been developed to eliminate VOCs. 2,6,[12][13][14] Catalytic oxidation is considered to be the most effective approach, mainly due to its high efficiency in the degradation to carbon dioxide and water, even in effluents with low concentrations of VOCs, and low energy cost.3,15 However, it is extremely difficult to achieve the complete combustion of VOCs at low temperatures. Thus, it is important to develop a novel active catalyst which can promote complete oxidation at the lowest possible temperatures. Conventional catalysts are supported on inorganic materials such as alumina and other metal oxides.However, the success of the catalyst is dependent on the nature of the support used. Common catalyst supports include activated carbons, silica, alumina, polymers and zeolites. Mesoporous silicas (MSs), for instance MCM-41, have been shown to be suitable supports for catalysts. 16,17 These mesoporous materials possess some important properties for catalytic purposes, such as high specific surface area, large pore volume and highly ordered pore structures with narrow pore size distributions, which are dependent on the synthesis chemicals and conditions. 18,19 The synthesis of these materials requires a source of silica, usually commercial, a structure directing agent, a solvent, a mineralizer agent, and these materials have been extensively investigated as promising cat...