纳米Au、Ag催化剂的合成及其在液相巴豆醛选择加氢制巴豆醇反应中的研究

【作者】 田莉；

【导师】 乔明华；

【作者基本信息】 复旦大学 ， 物理化学， 2011， 博士

【摘要】 α,β不饱和醇是重要的有机合成中间体,在香料、药物及其他精细化学品生产中有着广泛的应用。目前,工业上制备α,β-不饱和醇的方法多是采用四氢化锂铝、硼氢化钠或异丙醇铝等化学还原剂直接还原α,β-不饱和醛。虽然这种方法可以得到较高收率的α,β-不饱和醇,但反应条件苛刻、产物与还原剂、溶剂分离困难、产品的后处理繁琐、产生的三废多,不符合现代绿色化工的要求。但催化选择加氢α,β-不饱和醛为α,β-不饱和醇的难点在于C=C键的键能小于C=O的键能,所以C=C键加氢在热力学上更有利。因此,研制高效、环境友好的α,β-不饱和醛选择加氢催化剂,通过动力学途径来提高C=O键的加氢活性和选择性,具有重要的经济价值和学术价值。前人大量的研究工作表明,在绝大多数金属催化剂上,α,β-不饱和醛倾向于C=C键加氢生成饱和醛,而对α,β-不饱和醇的选择性不高。在近些年的研究发现,纳米Au、Ag催化剂,在一些α,β-不饱和醛的气相和液相加氢反应中,均显示出优于传统贵金属加氢催化剂的催化性能,因而受到广泛的关注。本论文设计和制备了新型的纳米金属催化齐(?)(Au、Ag)，以巴豆醛催化选择加氢为探针反应,通过各种表征手段,深入地研究了催化剂的结构、电子性质、活性物种尺寸以及金属和载体之间的相互作用对催化剂性能的影响,催化性能与活性中心的微观结构及电子性质进行了关联。论文的主要工作及结果如下：1.氨基硅烷功能化介孔SBA-15负载金催化剂上的巴豆醛选择加氢性能研究以介孔分子筛SBA-15为载体,通过对其进行不同种类氨基硅烷(APTS、TPED、TPDT)的表面修饰,制备了高度分散的纳米Au催化剂,并应用于巴豆醛催化选择加氢制取巴豆醇反应,研究了活性物种尺寸及金属与载体之间的相互作用对催化性能的影响。采用嫁接方法制备的金催化剂上有机官能团与金颗粒之间存在较强的互作用,这种相互作用一方面使得金粒子高度分散在介孔分子筛的孔道中,另一方而对纳米金粒子的电子性质起到修饰作用。在Au5.0/APTS-SBA-15、Au5.0/TPED-SBA-15和Au5.0/TPDT-SBA-15催化剂上,金的粒径和电子性质存在着差异,Au5.0/APTS-SBA-15上金最富电子,而后二者依次降低。由理论计算可知,若金属原子上的电子密度越高,α,β-不饱和醛中的C=C键在表面上的吸附就会因泡利四电子排斥作用增强而削弱,使其不易加氢；而富电子的金属原子更容易把电子反馈到C=O键的反键轨道上,这不仅增强了C=O键在表而的吸附,而且使得C=O键活化,从而有利于不饱和醇生成。研究了Au的负载量对催化性能的影响,发现当Au负载量为10wt%,在APTS-SBA-15载体上,巴豆醇的收率和选择性达到最大值(分别为60%和64%)。同时研究了APTS嫁接量对催化性能影响,发现嫁接量对改变活性中心与载体的相互作用的影响并不显著。2. Au/MWCNTs及氨基硅烷功能化的Au/APTS-MWCNTs催化剂上的巴豆醛选择加氢性能研究以MWCNTs为载体,采用浸渍法制备了纳米Au催化剂,在巴豆醛选择加氢中,表现出对C=C键强的加氢性能。通过改变MWCNTs载体的外管径,系统研究了活性中心粒子的尺寸及电子性质对巴豆醛加氢选择性的影响。研究发现,C=C键的初始的加氢速率随着外管径的增大而减小,而C=O键初始加氢速率随着外管径的增大而增大。通过TEM表征：该方法制备的金催化剂,其金的粒径较大(>10nm),分散性不好。通过加氢产物与粒径进行关联,发现随着金的粒径增大,C=C加氢产物的选择性是下降的。布此基础上,采用嫁接法制备了一系列APTS修饰的不同外管径MWCNTs负载纳米Au催化剂。通过TEM表征发现,金颗粒的粒径明显小于浸渍法制备的催化剂,并随着外管径的增大而增大,而且在巴豆醛加氢反应中表现出较高C=O加氢选择性。通过XPS表征发现,在这类催化剂上Au 4f的结合能均小于体相84.0 eV,说明Au是表而富电子的。Au表而电子是由嫁接在MWCNTs上的氨基提供的,而富电子的Au有利于C=O加氢。这些研究表明不同载体导致金颗粒的粒径及金属与载体之间相互作用的变化,是影响金催化剂催化性能的主要因素。此外,对嫁接法制备的金催化剂上的反应动力学研究,发现在该催化剂上,对巴豆醛为零级,对氢气为一级,反应的表观活化能为60kJmol-1。3.高效Au/AlOx和Au/CeAlOx催化剂上的巴豆醛选择加氢性能研究以蒸发溶剂诱导法制备的AIOx为载体,用沉积-沉淀法负载纳米Au催化剂,应用于巴豆醛催化选择加氢反应中,并与商业γ-AI2O3作为载体负载的金催化剂作比较。XPS结果表明,Au在AIOx载体上主要以Au0的形式存在,同时有少量的Auδ+孙存在,并且二者存在一定的协同作用。考察了载体的焙烧温度对催化性能的影响,发现载体的焙烧温度升高,金的粒径增大,在金的粒径为3.1 nm时,巴豆醛的收率和选择性达到最大值,分别为77%和78%,达到了文献中的较好水平。进一步升高焙烧温度使得金粒径明显增大,目标产物的选择性也随之明显降低。在催化剂的套用过程中,发现AlOx-N-773作为载体的负载金催化剂表现出较好的催化稳定性。进而我们制备催化剂载体过程中,添加了Ce,发现Ce有助于巴豆醇选择性的进一步提高,巴豆醇的得率可达到81%,对应的选择性为83%。其原因：一方面,Ce离子能够稳定更多的Auδ+；另一方面,Ce离子和AUδ+离子均能作为路易斯酸,促进C=O键的吸附和活化。4.Ag及AgIn/SBA-15催化剂上的巴豆醛选择加氢性能研究尽管Ag催化剂是一种重要的氧化反应催化剂,但研究者发现其在加氢中能够表现出催化活性。我们采用浸渍法、双溶剂法和不同载体(SBA-15, SiO2)制备了纳米Ag催化剂,在巴豆醛选择制加氢取巴豆醇的反应中,表现出了较高的催化活性。在本实验室前期工作和文献工作的基础上,通过添加第二组分In,制备了高选择性的Agln/SBA-15催化剂。在组成为Ag9.oln3.o/SBA-15的催化剂上巴豆醇的收率和选择性分别为86%和87%,是目前文献中报道的巴豆醛选择加氢制取巴豆醇中的较好结果。通过表征,发现In的作用主要有两个方而：(1)使得银纳米粒子能够高度分散在SBA-15上,从而为加氢提供更多活性位。由于小晶粒的配位不饱和的表面原子数多,有利于氢气的解离化学吸附；(2)In的存在极大地的抑制了C=C键加氢,对C=O键的加氢也有较好的促进作用。此外,还对Ag/SBA-15和Ag9.oln3.o/SBA-15催化剂对巴豆醛加氢中间产物(丁醛、巴豆醇)的加氢性能进行了研究,发现在前者上比在后者上更容易发生中间产物的后续加氢,表明In的加入有利于巴豆醇的稳定存在。更多还原

【Abstract】 α,β-unsaturated alcohols are important intermediates in the production of perfumes, pharmaceuticals and organic synthesis reaction. Presently, the unsaturated alcohols are commercially achieved by using NaBH4 or AlLiH4 as reductant, but this process involves a lot of pollutants. Heterogeneous catalysis via direct hydrogenation ofα,β-unsaturated aldehydes are able to reduce the conjugated C=O double bond in the presence of molecular H2. This process provides a clean and economic approach to the production ofα,β-unsaturated alcohols. However, the manipulation of the selectivity in the hydrogenation ofα,β-unsaturated aldehydes is of considerable challenge, as it is thermodynamically more favored to produce saturated aldehyde or saturated alcohol than the unsaturated alcohol on metals. Therefore, development of the alternative green processes for the synthesis ofα,β-unsaturated alcohols has an important academic value and economic benefit.It have been documented that lots of metal-based catalyst is prone to hydrogenate C=C bond instead of C=O. In recent years, gold and silver have attracted growing interest in catalyst research since they have shown greater activity in liquid hydrogenation reactions than traditional hydrogenation catalysts. In this dissertation, the gold and silver were chosen as active species. The Au and Ag catalysts have been investigated systematically for the hydrogenation of crotonaldehyde. The influence of surface structure, electronic properties and the interaction between metal and support of the catalyst were studied in detail by various analytical and spectroscopic techniques. In addition, by correlation of the hydrogenation reaction results with above results, the active sites were discussed.1. Studies on the Aminosilane-Functionalized SBA-15 for chemoselective hydrogenation of crotonaldehydeSBA-15 was functionalized by three kinds of aminosilane (ATPS: 3-aminopropyltrimethoxysilane, TPED: N-[3-(trimethoxysilyl)-propylethylene] diamine, TPDT:trimethoxysilyl propyl diethylenetriamine). The highly dispersed gold nanoparticles in mesoporous silicas were applied into chemoselective hydrogenation of crotonaldehyde to crotyl alcohol, and the effect of the gold size and the interaction between metal and support were investigated. According to the catalytic results, we found the Au/amine-SBA-15 catalysts exhibit surprisingly high reaction activity and selectivity to crotyl alcohol compared with the Au/SBA-15. The excellent catalytic performance is mainly attributed to the strong interaction between gold nanoparticles and support. The role of interaction made the gold nanoparticles highly dispersed into the channel of SBA-15. On the other hand, it modified the surface charge of gold nanoparticles, which is favorable for the polarization of the C=O bond and enhances the hydrogenation of this functional group. Their different selectivity behaviors were tentatively interpreted on the basis of XPS measurements. The degree of binding energy shift was:Au/APTS-SBA-15> Au/TPED-SBA-15> Au/TPDT-SBA-15, suggesting that there were more electron on the surface of Au/APTS-SBA-15 catalyst. The increased electron density on metallic gold not only alter the interaction of the active sites with the functional group and facilitate a partial transfer to theπc=O* orbital of the unsaturated bond, but also weaken the binding of the C=C group on the active sites. In addition, the gold loading and APTS content on the catalytic have been studied, it was found that the optimum yield of crotoyl alcohol was obtained on Au/APTS-SBA-15 with 10 wt% and there is no influence on interaction between active sites and supports.2. Preparation, characterization of Au/MWCNTs catalysts and their catalytic properties for the hydrogenation of crotonaldehydeThe Au/MWCNTs catalysts prepared by impregnation method have been investigated systematically for the hydrogenation of crotonaldehyde. And the effects of treatment condition and reduction method on catalytic performance in hydrogenation of crotonaldehyde were studied. The results showed that Au/MWCNTs catalysts exhibited strong hydrogenation of C=C bonds into butanal. The particle size of Au nanoparticles and electronic characterization were investigated systematically for hydrogenation crotonaldehyde. It was found that the original rate of hydrogenation of C=C decreased with the outer diameter of MWCNTs, however, the original rate over hydrogenation of C=O increased with the outer diameter of MWCNTs. Based on the TEM (transmission electron microscopy) characterization, we found the good relationships between the particle size of Au and the selectivity of butanal, that is, the biggher the particle size the lower the selectivity of C=C hydrogenation product. It was suggesting that the size effect is major factor influencing the products distribution.Besides above-mentioned method, the Au/APTS-MWCNTs catalysts were prepared by post-grafting method. The Au particle size was obviously smaller than the prepared by impregnation method with TEM (transmission electron microscopy) characterization. And the size of Au nanoparticles was became large increasing with the outer diameter of MWCNTs. The binding energy of gold catalysts was below 84.0 eV, manifesting that the rich-electron on the surface of gold nanoparticles. At the same time, N 1s binding energy of support showed that the electron on the surface of gold surface was supplied by amino of support. The effect of electron-rich metallic Au can account for the increased reactivity of the C=O group relative to the C=O group. Subsequently, the reaction conditions were optimized and displayed that zero-order for crotonaldehyde, one-order for hydrogen and 60 kJ mol-1 for the apparent energy of activation.3. Preparation, characterization of Au/AlOx and Au/CeAlOx catalysts and their catalytic properties for the hydrogenation of crotonaldehydeThe AlOx was prepared by sol-gel process associated with nonionic block copolymer as templates in ethanol solvent. Then, the Au/Alox and Au/y-Al2O3 were prepared by deposition precipitation method and applied into the hydrogenation of crotonaldehyde. It was found that Au/AlOx exhibited excellent performance on the hydrogenation of crotyl alcohol. Combined with kinds of characterizations, we found that the morphology of supports, structure and crystal structure played important role in the surface electron-structure of Au nanoparticles. Based on the XPS characterization, the gold has two species, one is metallic Au0, the other is oxidized Auδ+. And there is synergy effect. At the same time, the calcination temperature has been investigated for the hydrogenation of crotonaldehyde. It was found that the size of Au naoparticles became large when the calcined temperature was raised. On the Au/A1Ox-N-773 catalyst with the 3.1 nm diameter of gold particles, the yield of crotyl alcohol reached 77%, and the selectivity of crotyl alcohol amounted to 78%. Finally, the Au/AlOx-N-773 catalyst has a good stability after five successive runs comparing Au/y-Al2O3 catalyst, confirming that the nature of the catalyst did not change during the reaction. Besides, the rare-earth element Ce was added during the preparation. It was found that the addition of Ce was favorable for improving the selectivity of crotyl alcohol, therefore, the yield of crotyl was 81% and the selectivity was up to 83%. There are two reasons:on one hand, the Ce ion were help for the much more Auδ+ ion, on the other hand, Ce ion and Auδ+ could be the Lewis acid site. Furthermore, the Lewis acid site can active C=O group and promote the produce the crotyl alcohol.4. Studies on the Ag and AgIn/SBA-15 catalysts for the chemoselective hydrogenation of crotonaldehydeSilver is widely used as an active component in oxidation reactions, surprizingly, it can also be used as a catalyst for reduction reactions, for instance, selective hydrogenation of unsaturated aldehydes such as acrolein and crotonaldehyde. Herein, we prepared and characterized the SBA-15-supported Ag and In-promoted Ag catalysts by modified "two solvents" strategy for crotonaldehyde hydrogenation to crotyl alcohol. On the optimum Ag9.0In3.0/SBA-15 catalyst, the initial selectivity amounts to 88%, and the maximum yield reaches 86%, which is one of the best results reported in the open literatures dealing with theα,β-unsaturated aldehydes hydrogenation toα,β-unsaturated alcohol. The investigations of the effect In on structures and properties of Ag catalyst showed that the addition of indium to Ag catalyst led to improvement the diepersion of Ag nanoparticles. Since the smaller Ag nanoparticles are favorable for dissociative adsorption of H2. The existing of indium greatly suppressed the hydrogenation of the C=C bond, and enhanced the hydrogenation of the C=C bond. On the other hand, the modified In3+ions function as Lewis acid sites, the crotonaldehyde molecule being adsorbed via the donation of a lone electron pair from the oxygen of the carbonyl group. Besides all, the hydrogenation of intermediate products was studied over Ag/SBA-15 and AgIn/SBA-15 catalysts. It was demonstrated that the monometallic Ag catalyst is prone to take place further hydrogenation comparing with bimetallic catalyst. Therefore, the addition of indium is contributory to high selectivity to crotyl alcohol.更多还原