【作者】 晋梅；

【导师】 程振民；

【作者基本信息】 华东理工大学 ， 化学工程， 2010， 博士

【摘要】 由于环己烷氧化反应制备环己酮和环己醇工艺存在转化率和选择性较低、三废污染严重和能耗大的问题,自Noyori等从环己烯直接氧化合成己二酸获得成功后,环己烯被认为是合成环己酮、环己醇和己二酸的最佳原料,该工艺被认为是环境友好的绿色化工工艺。环己烯的获得途径主要有两条工艺路线。一是通过苯选择加氢制取,主要采用日本旭化成公司专利技术。但是在生产过程中,不但苯的转化率较低、环己烯的选择性不高,同时还副产30%左右的环己烷。另外一条工艺路线为环己烷脱氢反应。但是在热力学体系中,环己烷脱氢生成苯为热力学稳定体系。为此,本文提出采用环己烷经气相氧化脱氢制取环己烯工艺,不仅可作为制取环己烯的一条新途径,同时也是环己烷—苯—环己烯三者循环利用的切实可行的绿色工艺路线。本论文主要从以下方面对Mg-V-O催化剂上环己烷气相氧化脱氢反应进行研究。以Mg3(VO4)2制备为例,采用柠檬酸溶胶-凝胶法在缓和的焙烧条件下制备Mg-V-O催化剂。在制备过程中提出干凝胶前驱体结构为(NH4)2[VO2(C6H6O7)]2-3MgC6H6O7,并得到制备过程中的主要影响因素：柠檬酸加入量和焙烧温度。在最优制备条件下可以得到高度均一性的纯晶相Mg3(VO4)2催化剂,且该催化剂在80 h活性稳定性反应中表现出了好的活性稳定性和热稳定性。Mg-V-O氧化物通常以Mg3(VO4)2、Mg2V2O7和MgV2O6三种晶形稳定存在,通过对三晶相结构、表面酸碱性及催化剂表面活性组分可还原性进行深入研究,并结合各自在环己烷氧化脱氢制取环己烯反应中的催化性能,由于Mg3(VO4)2具有“孤立活性位”、表面弱碱性及较低的金属可还原性,因而可作为环己烷氧化脱氢反应的催化活性相。一般Mg-V-O催化剂为混合晶相催化剂,由于存在不同晶相间的协同催化效应,因而对环己烷氧化脱氢反应产生了不同于纯晶相催化剂的影响：(1)MgO+Mg3(VO4)2催化剂体系：当MgO含量大于10 wt.%时,两者之间通过形成的内聚界面产生晶相间协同催化效应,从而提高了环己烷转化率而降低了环己烯选择性；(2)Mg3(VO4)2+Mg2V2O7催化剂体系：通过溢流氧的遥控机理对反应产生积极的影响,Mg3(VO4)2在催化剂体系中充当溢流氧供体还是受体作用主要取决于催化剂体系中的主要晶相组分；(3)MgV2O6+V2O5体系：V2O5含量小于60 wt.%时,两晶相通过遥控机理产生的协同催化效应对反应产生消极影响；V205含量大于60 wt.%时,由于V205具有的金属氧化物表面离子迁移作用而在催化剂外表面形成包覆层,整个催化剂性能类似于V2O5。为了进一步提高Mg3(VO4)2催化剂在环己烷氧化脱氢反应中的催化性能,分别对催化剂进行了碱／碱土金属改性和添加共进料两方面的研究：(1)在碱／碱土金属改性研究中发现当碱／碱土金属与钒的原子比为0.1时,碱／碱土金属的加入对催化剂比表面积和晶相结构未产生较大影响,而是对其表面物理化学和微观性质产生了较大的影响：一方面减少了反应活性位和吸附活性位,另一方面降低了催化剂上活性物种的可还原程度、提高了钒物种活性组分周围的电子云密度分布以及改变了催化剂表面上氧物种的类型和含量,引起了环己烯选择性的提高而降低了环己烷的转化率；(2)采用水蒸气、醋酸和四氯化碳(TCM)作为共进料物质,对环己烷氧化脱氢反应的影响进行研究以揭示催化剂表面结构、表面化学性质与催化性能之间的关系：采用水蒸气共进料时,对催化剂的主要影响在于降低了反应物的停留时间且形成了更为孤立的活性位,引入水蒸气并不能提高环己烷氧化脱氢反应中环己烯的产率；采用TCM共进料时,对催化剂表面氧物种迁移程度产生了较大的影响而影响了催化活性；采用醋酸共进料时,不同的醋酸加入量会产生两种不同的催化结果：当醋酸量较少时,醋酸虽然占据了催化剂表面环己烷分子的吸附活性位和氧化脱氢反应活性位,但是提高了环己烷分子的活化,因而显示了较好的催化性能,而较高的醋酸分压一方面大幅度降低了催化剂的比表面积和结晶度,另一方面提高了催化剂表面酸性,不利于环己烯的脱附,从而导致催化剂在反应中催化性能的下降。更进一步,以Mg3(VO4)2为催化剂研究了主要工艺操作条件对反应的影响,在排除外扩散的基础上提出Power-Law的宏观动力学模型,并采用四阶龙格库塔法对参数进行估值。在此基础上提出,若是要想得到更为精确的Mars-van Krevenlen动力学模型参数,则需要更进一步的研究。通过本文的研究表明了Mg-V-O作为环己烷氧化脱氢反应催化剂的应用潜力,对催化剂制备方法、催化活性相、晶相间协同效应进行研究以获得较好催化性能的催化剂。通过碱／碱土金属对催化剂进行改性和采用共进料的方法以期得到更高的环己烯选择性和产率,并将以上方法对催化剂微观结构和表面化学性质与催化性能进行关联以揭示催化剂微观性质的改变对反应的影响。在此基础上,采用Mg3(VO4)2催化剂,研究了主要工艺操作条件对环己烷氧化脱氢反应的影响,并建立了初步的宏观动力学模型,以期对环己烷氧化脱氢反应有更进一步的了解。更多还原

【Abstract】 The process of the oxidation of cyclohexane to cyclohexanone and cyclohexanol suffers from several problems, such as the lower cyclohexane conversion, lower product selectivity, higher energy consumption and the environmental pollution. Therefore, since the process of direct oxidation of cyclohexene to adipic acid proposed by Noyori et al. has been put forward and used in practice, cyclohexene is considered as the optimal raw material and the direct oxidation of cyclohexene process can be considered as the environmentally benign route in producing cyclohexanone, cyclohexanol and adipic acid. Cyclohexene can be obtained from two ways. One is the commercial process of partial hydrogenation of benzene to cyclohexene, which suffers from major problems of both low yield and low selectivity to cyclohexene. The other is the process of cyclohexane dehydrogenation. Since cyclohexane dehydrogenation to benzene is a thermodynamically favorable reaction, this process results in both high yield and high selectivity to benzene. To obtain cyclohexene as much as possible and to deal with cyclohexane from the partial hydrogenation of benzene, the oxidative dehydrogenation of cyclohexane to cyclohexene and benzene will be of critical importance in forming a new technology as indicated below.The oxidative dehydrogenation of cyclohexane over Mg-V-O catalysts was investigated in this thesis as the following aspects.Citric acid complexation under mild condition was proposed to prepare monophasic and well crystallized Mg3(VO4)2 particle to be used as an active catalyst for the oxidative dehydrogenation of cyclohexane. The catalyst characterization and the catalytic test results suggest the gel precursor might be (NH4)2[VO2(C6H6O7)]2-3MgC6H6O7, and the amount of citric acid and the calcination temperature are critical to the purity and the structure of Mg3(VO4)2. Among the catalysts tested, Mg3(VO4)2 prepared with the (Mg+V)/citric acid molar ratio of 1:1.2 and calcined at 823 K for 6 h exhibits the best catalytic performance with an excellent thermal stability.The catalytic active phase was identified among Mg3(VO4)2, Mg2V2O7 and MgV2O6 pure magnesium vanadates. The characterization and evaluation results show that Mg3(VO4)2 has the isolated active sites, weakly basic surface and lower reducibility of the metal cations, and could be recognized as the catalytic active phase.A series of mechanically mixed catalysts were studied in the reaction in attempting to investigate the biphasic synergetic effect. The finding reveals the purity and the composition of Mg-V-0 catalysts play an important effect on the overall catalytic performance in the oxidative dehydrogenation of cyclohexane.(1) MgO+Mg3(VO4)2 mixed catalysts system:When MgO content is less than 10 wt.%, the excess MgO might in a highly dispersion into Mg3(VO4)2. There is no synergetic effect between them. When MgO content is higher than 10 wt.%, the epitaxial phenomena might lead to the formation of coherent interface between MgO and Mg3(VO4)2, therefore, the catalytic activity is improved with a decreasing selectivity to cyclohexene.(2) Mg3(VO4)2+Mg2V2O7 mixed catalysts system:The biphasic synergetic effect might arise from the remote control mechanism between Mg3(VO4)2 and Mg2V2O7, which plays a positive effect on the catalytic performance in the oxidative dehydrogenation of cyclohexane. Mg3(VO4)2 acting as donor or accepter of the mobile oxygen is depended on the main phase in the mixed catalyst. A better catalytic behavior could be obtained over the Mg3(VO4)2+Mg2V2O7 mixed catalyst when the content of Mg3(VO4)2 was more than 70 wt.%.(3) MgV2O6+V2O5 mixed catalysts system:When V2O5 content was less than 60 wt.%, the synergetic effect might arise from the remote control mechanism, which plays a negative effect on the catalytic performance. When V2O5 content was higher than 60 wt.%, V2O5 would form a contamination layer on the entire catalyst to result in a similar catalytic performance with that of V2O5.In order to improve the selectivity and the yield of cyclohexene, the effects of alkali/alkaline earth metals (Li, Na, K and Ca) introduced to Mg3(VO4)2 and the introduction of the additives (steam, acetic acid and TCM) into the feedstream on the physicochemical properties of the catalyst and the catalytic behaviors in the oxidative dehydrogenation of cyclohexane were investigated.(1) The addition of alkali/alkaline earth metal to Mg3(VO4)2 affects the physicochemical nature and the catalytic behavior in the reaction. During the coordination of the additive with the surface active species, the additive blocked the active sites and hindered the reducibility of the active species thus decreasing the catalytic activity. Moreover, the additive could improve the selectivity to cyclohexene by changing the acid-basic nature, the redox property as well as the type and number of the oxygen species as investigated by H2-TPR and XPS characterization.(2) The effects of the addition of the additives on the catalytic behavior were also investigated. The addition of steam to the reacting mixture has a negative effect on the conversion due to significant decrease of active sites on the catalyst surface and the short of the retention time. The addition of TCM to the feedstream affects the removability of lattice oxygen in the catalyst, which results a high conversion of cyclohexane and a low selectivity to cyclohexene. The addition of acetic acid to cyclohexane plays the interesting effects. A lower addition amount of acetic acid only affects the isolation of the active sites and the acid-base property of catalyst surface, while a higher addition amount of acetic acid affects not only the acid-base property of the catalyst surface but also the degree of crystallinity and the grain size of Mg3(VO4)2 catalyst.Furthermore, the effects of the reaction temperature, W/F and molar ratio of cyclohexane to oxygen on the oxidative dehydrogenation of cyclohexane over Mg3(VO4)2 catalyst were obtained. The kinetic of the reaction was investigated using the fixed reactor in the case of eliminating external diffusion. A Power-Law type model was proposed and the kinetic parameters were estimated by nonlinear regression of the experimental data, and the predicted concentration profiles were in agreement with the experimental results.The research of this thesis exhibits that the application potentiality of Mg-V-0 catalyst. The the preparation method, catalytic active phase, and the biphasic synergetic effect between the different phases were investigated in order to obtain the optimal composition of Mg-V-0 catalysts. In order to obtain the higher selectivity and the yield of cyclohexene, the effects of the addition of alkali/alkaline earth metal to Mg3(VO4)2 and the introduction of the additive into the feedstream on the physicochemical properties of the catalyst and the catalytic behaviors in the oxidative dehydrogenation of cyclohexane were studied. Furthermore, the effects of the reaction temperature, W/F and molar ratio of cyclohexane to oxygen on the oxidative dehydrogenation of cyclohexane over Mg3(VO4)2 catalyst were obtained and the kinetic model of the reaction was proposed in order to give a better understanding of the oxidative dehydrogenation of cyclohexane over Mg-V-0 catalysts.更多还原