Abstract

The physicochemical and catalytic properties of 6%Ni/Al₂O₃ catalysts in the gas-phase hydrodechlorination of chlorobenzene (CB) are studied. The catalysts are synthesized by supporting nickel nitrate on two types of alumina—A (synthesized by aluminum isopropoxide hydrolysis) and E (manufactured by Engelhard)—with different morphologies 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 and reduction of the modified materials are conducted at relatively low temperatures (250 and 330°C, respectively). To provide 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 conversion of the original nickel nitrate to oxide; nickel nitrates and hydroxynitrates are present in the catalyst precursors; their content decreases upon modification with the HPA. Differences in the composition and strength of Lewis acid sites 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 reduction somewhat more readily than nickel in Ni/Al₂O₃(A) does; however, the conditions used in this study provide the reduction of only a small portion of nickel in the two catalysts. According to in situ XPS, TPR, and DRIFT spectroscopy with CO adsorption, the modification of Ni/Al₂O₃ with the HPA leads to a further change in the acidic properties and the coordination of nickel nitrate during impregnation and an increase in nickel reducibility; it prevents nickel from migration from the surface into the bulk of the sample and leads to the formation of new active sites owing to the strong nickel–tungsten interaction in the HPA. Depending on the nature of the support, modification with the HPA leads to an improvement (Ni/HPA/Al₂O₃(A)) or deterioration (Ni/HPA/Al₂O₃(E)) of the catalytic efficiency of the samples. At high temperatures, the benzene selectivity of the HPA-modified catalysts decreases 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 in temperature. The effect of nickel reducibility on the catalyst efficiency in CB hydrodechlorination is more significant than the effect of differences in texture and nickel content.

中文翻译：

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氧化铝负载的镍催化剂上氯苯的气相加氢脱氯：载体结构和杂多酸HSiW改性的影响

摘要

研究了6％Ni / Al ₂ O ₃催化剂在氯苯气相加氢脱氯中的物理化学和催化性能。催化剂是通过将硝酸镍负载在两种形态和织构不同的氧化铝（A（由异丙醇铝水解合成）和E（由Engelhard制造））上而合成的。一些样品的未修改的，有些是通过沉积20％的杂多酸（HPA）H改性₈的Si（W ₂ ö ₇）₆  ⋅  Ñ ħ ₂O.为防止HPA分解，在相对较低的温度（分别为250和330°C）下进行空气煅烧和还原改性材料。为了提供足够的比较，对不含HPA的催化剂进行类似处理。程序升温还原（TPR）表明，在250°C的温度下煅烧并不能完全将原始的硝酸镍转化为氧化物。硝酸盐和硝酸镍存在于催化剂前体中。通过HPA修改后，它们的含量会减少。两种类型的Al ₂ O ₃表面路易斯酸位的组成和强度的差异TPR，具有CO吸附的漫反射红外傅里叶变换（DRIFT）光谱和原位X射线光电子能谱（XPS）揭示了硝酸盐与镍还原性差异的不同配位。载镍以Ni / Al ₂ ö ₃（E）经历还原稍微更容易地比以Ni / Al镍₂ ö ₃（A）确实; 但是，本研究中使用的条件只能减少两种催化剂中镍的一小部分。根据原位XPS，TPR和DRIFT光谱及CO吸附，对Ni / Al ₂ O ₃进行改性HPA导致浸渍过程中酸性性质的进一步变化和硝酸镍的配位作用以及镍还原性的提高；它可防止镍从表面迁移到样品的大部分中，并由于HPA中强烈的镍-钨相互作用而导致新的活性位点的形成。根据载体的性质，用HPA进行改性会导致催化剂的催化效率提高（Ni / HPA / Al ₂ O ₃（A））或变差（Ni / HPA / Al ₂ O ₃（E））。样品。在高温下，由于形成环己烷，HPA改性催化剂的苯选择性降低。催化剂效率按以下顺序增加：Ni / HPA / Al ₂O ₃（E）<Ni / Al ₂ O ₃（A）<Ni / Al ₂ O ₃（E）<Ni / HPA / Al ₂ O ₃（A）。活性最高的催化剂Ni / HPA / Al ₂ O ₃（A）在长期测试中表现出最高的稳定性，随温度升高和随后降低。镍还原性对CB加氢脱氯催化剂效率的影响远大于结构和镍含量差异的影响。