摘要
本文以气态环己烷选择性催化氧化制取琥珀酸酐为目标反应,采用溶胶—凝胶的方法,在Al_2O_3和ZRP—5分子筛两种常规载体上负载纳米TiO_2,从而制得一系列的复合载体。并以V_2O_5为主组分,添加不同含量的P、Mo、Sb、K等助剂,制得一系列探索目标反应的催化剂。用正交实验法对催化剂载体、组分以及制备条件等进行优化筛选。催化反应在常压微型固定床反应器中进行。实验结果表明,复合载体、催化剂组成、反应条件均对目标反应产生较大的影响。
实验结果表明:纳米复合载体上TiO_2的含量对载体性能影响较大,其负载量以10%为宜;活性最好的催化剂组成为:V_2O_5负载量为载体质量的5.5%、P与V的原子比为1.5、Mo含量为1.15%、Sb含量为0.5%、K含量为3.8%。
借助差热—热重分析(DTA—TG)、X—射线衍射(XRD)、傅立叶—红外(FT-IR)和TEM等表征手段,研究了催化剂和载体热处理过程中的物理化学变化、表面结构、晶体结构、表面形貌。结果表明:TiO_2在600℃左右就开始晶型转变,最大失重范围与相变温度一致:转变过程中纳米粒子在载体上分散良好,负载的纳米粒子和载体之间存在一定的作用,催化剂对环己烷的主要吸附位为V=O和Mo=O。XRD和TEM测试结果显示,复合载体仍保留着Al_2O_3和ZRP—5的骨架,负载在Al_2O_3上的TiO_2粒径约为35 nm、而负载在ZRP—5分子筛上的粒径约为26 nm,并且两种载体中TiO_2都以锐钛矿形式存在。
催化剂活性评价结果表明,溶胶—凝胶法比浸渍法和混凝胶法制得的催化剂能显示更好的催化活性。适当的氧含量、环己烷浓度和气相中CO_2浓度均能提高催化剂对环己烷的转化率和琥珀酸酐的选择性。实验范围内的最佳反应条件为:反应温度290~310℃,空速1400 h~(-1),环己烷浓度2%,氧含量15%~20%。
In this thesis, butanedioic anhydride preparation acted as the target reaction by selectively catalytic oxidation of gaseous cyclohexane, nano-TiO2 was loaded on the general supporters of Al2O3 and ZRP-5 molecular sieves based on sol-gel method and a series of composite supporters were obtained. A series of catalysts probing into the target reaction were obtained with main active ingredient of V2O5 and doped reagent of different content such as P, Mo, Sb and K. The catalyst supporters, active ingredients and the conditions of preparation were optimized by the orthogonal test. The catalytic reactions were processed in the bed-secured micro-reactor under normal pressure. It showed that the composite supporters, the catalyst component and the reaction conditions could have great influence on the target reaction.
The results showed that the content of TiO2 on the composite supporters had great effect on the performance of the supporters. The appropriate content of TiO2 was 10wt%. The catalysts containing 5.5 wt% V2O5, 1.15 wt% Mo, 0.5 wt% Sb, 3.8 wt% K and P/V atomic ratios of 3/2 exhibited high activity.
Techniques, such as differential thermal analysis thermogravimetry (DTA-TG), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscope (TEM) were performed to study the physicochemical change during the heat treatment process, the surface structure, the crystalline structure and surface morphology of the catalysts and supporters. The DTA-TG results showed that the transformation from anatase phase to rutile phase started at 600. The temperature scope that samples lost weight was in conformity to its transformation temperature. The nanoparticals could be dispersed on the composite supporters very well during the transformation. There was interaction between nanoparticals and the supporters. The cyclohexane was mainly adsorpted on V=O and Mo = O of the catalysts. The results of XRD and TEM showed that the composite supporters still reserved the framework Al2O3 and ZRP-5, the particle sizes of TiO2 loaded on the Al2O3 and ZRP-5 were around 35 nm and 26 nm respectively, and Ti
O2 of the two supporters was present in an anatase state on the supporters.
The evaluation results of the catalyst activity showed that catalyst obtained by sol-gel method exhibited higher catalytic activity than by impregnation method and mixing gel method. The proper content of O2. cyclohexane and CO2 could increase the conversion rate of cyclohexane and the selectivity of butanedioic anhydride. The optimal reaction conditions in the target reaction were reaction temperature 90-310,
7
GHSV=1400h-1, the concentration of cyclohexane 2 vol%, and the content of O2 15-20 vol %.
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