摘要
环氧环己烷是一类非常重要的化学反应中间体,在有机合成领域中有着重要的学术价值。其制备方法主要分为均相和非均相两大催化体系,均相反应具有选择性好,转化率高等优点。但其缺点主要表现为催化剂回收困难,导致生产成本较高。为了克服均相反应所存在的弊端,近年来,这类反应的研究热点主要集中在开发非均相催化反应体系。以MCM-41为代表的介孔分子筛的出现,为人们定向合成结构可控和稳定的催化剂提供了丰富的理论基础和技术保证。
本文以双模型介孔分子筛(BMMs)为载体、通过嫁接法将二氯二茂钛负载在BMMs的孔道表面,从而制备出一系列Ti/BMMs介孔分子筛催化剂。以环己烯的环氧化反应为探针,结合多种表征手段,详细考察了影响反应的诸多因素,从而筛选出性能最优的催化剂,并找出最佳的反应条件。同时,本文对催化反应机理也进行了探讨。主要获得以下结果:
1.利用嫁接法将二氯二茂钛负载在BMMs的孔道表面制备出一系列Si/Ti摩尔比不同的Ti/BMMs分子筛催化剂,采用XRD、SEM、TEM、TG、UV-vis、ICP、FT-IR等测试手段对样品进行了结构分析和性能表征。结果表明,当n(Si)/n(Ti)分别为:82.9,61.5和47.9时,Ti主要以四配位状态存在于分子筛的孔道结构中。随着Ti含量的进一步增加(n(Si)/n(Ti)小于38.1),UV-vis光谱图上出现了八配位钛的特征吸收峰。ICP测试结果显示,在Si/Ti摩尔比为61.5时,所制备的Ti/BMMs分子筛催化剂中Ti的实际含量为0.47 (wt) %。该催化剂不仅能够保持BMMs分子筛的双模型等级孔道结构,且Ti在Ti/BMMs分子筛的孔道表面主要以四配位结构存在,并得到较好分散。
2.以环己烯的环氧化反应为探针反应,详细考察了Si/Ti摩尔比、煅烧温度、反应温度、溶剂种类和反应时间等诸多因素对催化反应的影响规律。结果表明,最佳反应条件为:n(环己烯)/n(TBHP)=1: 1.1,氯仿为溶剂, Si/Ti摩尔比为61.5的Ti/BMMs分子筛催化剂,反应温度为60°C,反应时间为6 h。在此条件下,环己烯的转化率达到61.75 %,环氧环己烷的选择性与频率转化因子(TOF)分别为73.54 %和87.88 h-1。
3.考察了催化剂的煅烧温度对其孔道结构和Ti的配位环境的影响规律。结果表明,当Ti/BMMs分子筛催化剂在450°C煅烧时,BMMs分子筛的双模型孔道结构保持完好,且Ti在分子筛孔道表面主要以四配位状态存在。在温度低于450°C时,由于二氯二茂钛在Ti/BMMs分子筛孔道内分解不完全导致孔道堵塞;温度高于450°C时,分子筛的孔道结构出现坍塌。
4.进一步探索了催化剂回收以及再利用问题。初步结果表明催化剂经过3次循环后,仍能够保持比较完整的双模型孔道结构,且具有较高的催化活性与选择性,其中环己烯的转化率达到57.37 %,环氧环己烷的选择性为72.61 %。
5.结合TG表征结果以及环己烯环氧化反应机理,造成Ti/BMMs分子筛催化剂失活的原因主要有以下3点:(1)Ti/BMMs分子筛催化剂回收利用多次后,催化剂的孔道结构遭到一定程度的破坏。特别是经过第4次回收后,Ti/BMMs分子筛催化剂不再保持BMMs分子筛独特的双模型孔道结构;(2)环己烯环氧化反应所产生的有机分子吸附在分子筛孔道表面,阻止了反应物分子与活性中心的接触,降低了四配位钛与反应物分子之间的碰撞几率,从而降低了Ti/BMMs催化剂的活性;(3)随着催化剂回收次数的不断增加,Ti在回收过程中的流失或配位环境的变化(由四配位转变为八配位)也是造成催化活性下降的原因。总之,造成催化剂失活的主要原因还需要深入细致的考察。
Cyclohexene oxide, one of the most important organic intermediates, has attracted great academic interest in the field of organic synthesis. At present, its synthesis methods are divided into the homogeneous and the heterogeneous system, the former possesses many advantages, such as good conversion and high selectivity, however, there are many shortcomings, for example, the catalysts are difficult to be reused, and therefore cost too much. To overcome these limitations, researches focus on the field of heterogeneous reaction system. In the early 1990s, a new theory and method has been carried out to synthesize stabilized catalysts with controllable struture after the MCM-41 mesoporous molecular sieves have been reported.
In this dissertation, a series of Ti/BMMs samples were prepared by grafting titanocene dichloride into bimodal mesoporous materials. The catalytic properties of Ti/BMMs were investigated by using epoxidation reaction of cyclohexene as model reaction. Combining with some characterization techniques, the effects of many factors on the epoxidation properties have been studied in details, and the best catalyst and optimum reaction condition were obtained. In addition, the relationship between mechanism of epoxidation reaction and catalyst structure was also discussed.
Below are the main results:
1. A series of catalysts were prepared by incorporating titanocene dichloride on the surface of BMMs with different ratios and were characterized by XRD, SEM, TEM, TG, UV-vis, ICP and FT-IR, and so on. The results showed that the tetrahedral titanium complexes highly dispersed in the mesoporous surface of BMMs samples with Si/Ti molar ratio of 82.9, 61.5 and 47.9, respectively. However, with the increasing Ti content (Si/Ti molar ratio was lower than 38.1), besides tetrahedral titanium complexes, octahedral Ti species also presented on the basis of UV-vis spectra. Additionally, Ti amount for catalyst with the Si/Ti molar ratio at 61.5 was around 0.47(wt) % by ICP measurement, the resultant catalysts maintain a typical bimodal mesoporous structure, in which, titanium species exist in the form of tetrahedral and dispersed on the mesoporous surface.
2. The effects of Si/Ti ratio, heating temperature, reaction temperature, solvents and reaction time on the epoxidation reaction of cyclohexene were investigated. The best reaction conditions were as follow: cyclohexene/TBHP = 1:1.1(molar ratio), chloroform as solvent, Ti/BMMs catalyst with Si/Ti ratio at 61.5, reaction temperature at 60°C and reaction time for 6 hours. Ti/BMMs catalysts showed high catalytic activity with the conversion of cyclohexene up to 61.75 %, selectivity of cyclohexene oxide near 73.54 % and turnover frequency (TOF) achieved 87.88 h-1.
3. In addition, the effect of heating temperature on the coordinated environment of Ti ions of catalysts was studied. The results indicated that when calcined at 450°C, Ti/BMMs catalyst could maintain bimodal pore structure and tetrahedral titanium be dispersed on the mesoporous surface. In the case of lower temperature, the mesopores might be blocked since titanocene dichloride was not decomposed fully according to the TG analysis, when above 450°C, the pores structure was damaged on the basis of XRD pattern.
4. Meantime, recycling and reuse of Ti/BMMs catalysts were also performed. The results showed that after recycled three times, the catalyst could still maintain a typical bimodal mesoporous structure with high conversion and selectivity, that the cyclohexene conversion reached up to 57.37 % and selectivity of cyclohexene oxide was around 72.61 %.
5. Combining with the TG analysis, and the mechanism of cyclohexene epoxidation, some possible reasons for the decreasing of activity of Ti/BMMs were put forward, as follow: (1) The mesopore structure were damaged to some extent after series of recycling, especially at the forth time; (2) Some organic molecules which were by-produced in the cyclohexene epoxidation were absorbed on the mesoporous surface, resulting in that the reactants could not easily diffuse into the active sites; (3) With the increasing of recycling times, the tetrahedral titanium complexes were not only strongly leached, but also turned into other species. In a word, the investigations for decreasing of catalytic activity need to be further done.
引文
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