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The synthesis of micro-/mesoporous beta zeolite by employing a solid precursor was described and its activity in the isopropylation of naphthalene was tested. The precursor was prepared by spray-drying of a homogenous mixture of silica sol, polyaluminum chloride solution and acidophilus milk, the presence of the latter induced formation of mesopores. The volume of mesopores of the synthesized beta zeolites ranged from 0.13 to 0.6 cm3/g and strongly depended on the concentration of tetraethylammonium hydroxide (TEAOH) in the reaction mixtures at the hydrothermal synthesis. Micro-/mesoporous beta zeolites exhibit positive effects of mesopores on the naphtalene conversion in its isopropylation by isopropyl alcohol, by improving the availability of active sites. With an increasing volume of mesopores, the conversion of naphthalene proportionally increases from 12 up to 27% for the volume of mesopores 0.6 cm3/g, while the selectivity to isopropylnaphthalene decreases from 92 to 55–66% for beta zeolites with 0.15–0.45 cm3 of mesopores/g, followed by selectivity increase up to mesopore volume 0.6 cm3/g reaching 81% at conversion 27%. A yield of isopropylnaphthalene is increasing in the whole range, particularly when the volume of mesopores exceeds 0.4 cm3/g. A zeolite with the highest volume of mesopores provides 1.8 times higher yield of isopropylnaphthalene then a microporous beta zeolite without mesopores. The increasing conversion of naphthalene is accompanied by a decrease in the selectivity to isopropylnaphthalene for zeolites with a low volume of mesopores, but an increasing selectivity to isopropylnaphthalene in the case of zeolites with a volume of mesopores above 0.4 cm3/g. This unusual effect can be explained by the mesoporosity of zeolites. Mesopores improve the availability of active centers, so they react faster, but the by-products are also formed faster and later undesired reactions run more easily as active centers are gradually blocked by by-products. Zeolites with the largest pores provide not only better availability of active centers, but the reaction products can also leave the zeolitic phase more easily and undesirable reactions have less chance to occur. This method of micro/mesoporous beta zeolite synthesis is advantageous for its low cost.