钒系催化剂在乙苯混合脱氢过程中的应用

【作者】 张学彬；

【导师】 董文生；

【作者基本信息】 陕西师范大学 ， 应用化学， 2008， 硕士

【摘要】 烃类脱氢一直是石油化学工业最具挑战性的课题之一，其特点是反应转化率受到热力学平衡的限制。苯乙烯是一种重要的基本有机化工原料，广泛应用于生产塑料、树脂和橡胶等。目前，全球苯乙烯年生产能力在2500万吨以上，其中90％以上采用乙苯催化脱氢法生产。现有工艺的主要缺点是反应温度和水／烃比高、能耗大、平衡转化率低，在分离器中水蒸气所含的大量冷凝热难以回收，因此，研究开发新的替代脱氢工艺很有必要。乙苯氧化脱氢具有能耗低，不受平衡转化率限制等特点，自从20世纪70年代发现一些特定形态的碳对乙苯氧化脱氢表现出良好的催化活性以来，科研工作者对此倾注了极大的热情。然而，该新工艺对苯乙烯选择性较差，大量的副产物严重阻碍着该过程的工业应用。利用温室气体CO₂选择性氧化乙苯制苯乙烯是近年来的研究热点之一，在催化剂的制备、反应条件的选择和CO₂的作用机理等方面前人已做了不少工作。但该工艺仍为强吸热反应，能耗大，受热力学平衡限制且催化剂由于表面积炭失活严重。而若在系统中引入部分氧气，一方面通过氧化反应可供给体系部分能量，从而降低系统能耗；另一方面，可以消除催化剂表面积碳，从而改善催化剂稳定性。基于上述思考，本论文采用等体积浸渍法制备了一系列V₂O₅/γ-Al₂O₃（V/Al）及助剂改性的催化剂，考察催化剂在CO₂气氛下乙苯氧化脱氢反应性能和在系统中通入O₂对催化剂结构和催化性能的影响。主要研究结果如下：1．采用浸渍法制备不同V负载量的V／Al催化剂，并用于在CO₂气氛下乙苯温和氧化脱氢制苯乙烯的反应。考察了不同催化剂对乙苯和CO₂的转化率及苯乙烯选择性的影响，反应在常压、550℃、EB空速20．4 mmool/g·cat·h的条件下进行。实验结果表明，在不同V负载量的V／Al催化剂中，V（8％wt．）／Al催化剂的呈现出良好的乙苯脱氢活性。随着V负载量的增加，催化剂在CO₂氧化乙苯脱氢过程中的初活性增加，但催化剂的稳定性变差，在CO₂氧化乙苯过程中通入少量O₂可以有效抑制催化剂失活，维持催化剂较高的活性和稳定性。2．采用浸渍法制备了多种助剂改性的V／Al催化剂，实验证明助剂都对催化剂的乙苯脱氢活性起了积极的作用，都在一定程度上提高了EB转化率。在助剂改性的催化剂中V-Sb／Al乙苯脱氢活性最佳，有较高的苯乙烯选择性和乙苯转化率，并且有良好的稳定性，其中V（8％）Sb（9％）／Al催化剂最佳。在CO₂氧化乙苯过程中通入少量O₂，使催化剂的初活性有所降低，但减缓了催化剂的失活速度，提高了催化剂的稳定性。3．本文采用程序升温还原反应（XPR）、X射线光电子能谱分析（XPS）、粉末X射线衍射分析（XRD）和N₂物理吸附对反应前后的催化剂进行了表征，表征结果表明V／Al催化剂表面的V组分以高分散形式存在，造成催化剂失活的主要原因是催化剂表面组成结构改变和表面积炭，积炭造成催化剂表面积和孔体积的减少，尤其是微孔面积的完全损失。助剂的加入能够进一步提高活性组分在载体表面的分散度，并调变催化剂表面的酸碱度，从而有利于反应气体的吸附和活化，有利于催化剂反应性能的提高。助剂Sb改性的V／Al催化剂中形成的V_1.1Sb_0．9O₄中间相可能是V-Sb／Al催化剂的活性中心。在CO₂氧化乙苯过程中加入少量O₂可以有效抑制催化剂表面积炭，同时稳定催化剂表面组成结构，从而维持较高的活性和稳定性。更多还原

【Abstract】 Dehydrogenation of hydrocarbons is one of the challenges in the petroleum chemical industry, since it is often confined by the thermodynamic equilibrium. Styrene （ST） is one of the most important fundamental chemicals, which is used for the production of plastic, resin and synthetic rubber. The total output of styrene in the world is above 25 billion ton/a, and 90% of styrene is produced by catalytic dehydrogenation of ethylbenzene （EB） in the presence of steam, a high-energy-consuming and equilibrium-limited process.Oxidative dehydrogenation of EB has attracted much attention since the discovery of certain catalysts for the reaction in the early 1970s. Nevertheless, a considerable decrease in styrene selectivity owing to deep oxidation of hydrocarbons to carbon oxide makes it unpractical in economical point of view. The dehydrogenation process for the production of ST based on the selective oxidation of EB with the major global warming gas CO₂, has aroused widespread interest recently for its lower energy consumption and higher equilibrium yield of styrene. The usage of CO₂ instead of steam could provide several advantages such as reduction of the reaction temperature, remarkable energy saving in the distillation process of ST, restraining deactivation of catalysts to some degree, and so on. Numerous research works on this subject have been reported and the possibility of dehydrogenation of EB in the presence of CO₂ instead of O₂ has been acknowledged. Although the application of CO₂ is very effective, the catalysts deactivation mechanism and suitable measures to enhance their catalytic stability must be further investigated.Reaction coupling is one of the best substitutes of the commercial dehydrogenation, which can solve such problems and has been studied extensively in recent years. The mechanism of CO₂ oxidation is also discussed. It can be concluded that the remarkable promoting effect of CO₂ on the dehydrogenation of EB is due to both redox cycle of oxide catalyst and coupling of EB with reversed water gas shift reaction.In the present work, V₂O₅/γ-Al₂O₃ （V/Al） catalysts with different V loading, with different promoters were prepared by impregnation method. The catalysts were tested in the dehydrogenation of EB. The main contents of this thesis are as follows:1. V/Al catalysts were prepared by impregnation method, the catalysts were used in mild oxidative dehydrogenation of EB to ST in presence of CO₂. Effects of different catalysts on oxidative dehydrogenation were investigated, under reaction conditions: atmospheric pressure, 550℃, EB space velocity 20.4 mmol/g·cat·h. The results show that with increasing V loading the initial activity of V₂O₅/γ-Al₂O₃ catalysts in the dehydrogenation of ethylbenzene with CO₂ increases; whereas, the stability of these catalysts decreases. V（8%wt.）/Al has the best activity for dehydrogenation of EB. The addition of O₂ to the process improves the stability of the V/Al catalysts.2. The V/Al catalysts with different promoters were prepared by impregnation method. The results show that: alkali metal oxide and rare earth oxide were found being suitable promoters for V/Al catalysts. The V-Sb/Al catalyst affords the highest dehydrogenation activity, and good stability. The V（8%）Sb（9%）/Al catalyst has the highest ethylbenzene conversion and styrene selectivity. The initial activity of the catalysts in the dehydrogenation of ethylbenzene decreases by addition of O₂, whereas, the stability of these catalysts increase.3. The fresh and used catalysts were characterized using temperature-programmed reduction （TPR）, X-ray photoelectron energy spectroscopy （XPS）, X-ray diffraction （XRD）, N₂ physical adsorption. The active center of dehydrogenation is belived to be related with V⁵⁺. Catalyst deactivation is mainly caused by coke deposition, which results in the decrease of the BET surface and pore volume. The addition of promoters can increase the BET surface and lead to higher dispersion of the active species and adjust the surface acidity and basicity. The V_1.1Sb_0.9O₄ phase was actually found to be the main component in the crystalline V-Sb bulk oxide system, this phase may be the active center of the V-Sb/Al catalysts. The addition of O₂ to the process improves the stability of the catalysts due to suppressing the coke deposition and keeping the vanadium species at high valence.更多还原