Gas-Phase Hydrodechlorination of Chlorobenzene over Alumina-Supported Nickel Catalysts: Effect of Support Structure and Modification with Heteropoly Acid HSiWстатья
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Аннотация:The physicochemical and catalytic properties of 6%Ni/Al2O3 catalysts in the gas-phase hydrodechlorinationof chlorobenzene (CB) are studied. The catalysts are synthesized by supporting nickel nitrate on two typesof alumina—A (synthesized by aluminum isopropoxide hydrolysis) and E (manufactured by Engelhard)—with differentmorphologies and textures; some of the samples are unmodified, and some are modified by depositing 20%heteropoly acid (HPA) H8Si(W2O7)6⋅nH2O. To prevent the HPA from decomposition, the air calcining andreduction of the modified materials are conducted at relatively low temperatures (250 and 330°C, respectively). Toprovide an adequate comparison, the catalysts containing no HPA are subjected to a similar treatment. Temperature-programmed reduction (TPR) reveals that air calcining at 250°C does not provide the complete conversionof the original nickel nitrate to oxide; nickel nitrates and hydroxynitrates are present in the catalyst precursors; theircontent decreases upon modification with the HPA. Differences in the composition and strength of Lewis acidsites on the surface of two types of Al2O3 lead to dissimilar coordination of nitrate and differences in nickel reducibility,as revealed by TPR, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy with CO adsorption,and in situ X-ray photoelectron spectroscopy (XPS). Nickel contained in Ni/Al2O3(E) undergoes reductionsomewhat more readily than nickel in Ni/Al2O3(A) does; however, the conditions used in this study provide thereduction of only a small portion of nickel in the two catalysts. According to in situ XPS, TPR, and DRIFT spectroscopywith CO adsorption, the modification of Ni/Al2O3 with the HPA leads to a further change in the acidicproperties and the coordination of nickel nitrate during impregnation and an increase in nickel reducibility; it preventsnickel from migration from the surface into the bulk of the sample and leads to the formation of new activesites owing to the strong nickel–tungsten interaction in the HPA. Depending on the nature of the support, modificationwith the HPA leads to an improvement (Ni/HPA/Al2O3(A)) or deterioration (Ni/HPA/Al2O3(E)) of thecatalytic efficiency of the samples. At high temperatures, the benzene selectivity of the HPA-modified catalystsdecreases owing to the formation of cyclohexane. The catalyst efficiency increases in the following order:Ni/HPA/Al2O3(E) < Ni/Al2O3(A) < Ni/Al2O3(E) < Ni/HPA/Al2O3(A). The most active catalyst—Ni/HPA/Al2O3(A)—exhibits the highest stability in long-term tests with an increase and subsequent decrease intemperature. The effect of nickel reducibility on the catalyst efficiency in CB hydrodechlorination is more significantthan the effect of differences in texture and nickel content.