【作者】 祝良芳；

【导师】 胡常伟；

【作者基本信息】 四川大学 ， 物理化学， 2007， 博士

【摘要】 长期以来,人们对化学合成更看重的是化学反应的高选择性和高收率,而忽视了反应物中原子的有效利用,有时难免造成既浪费资源又污染环境的尴尬局面。随着现代工业的发展,尤其是近年来绿色化学的发展,人们对环境和资源越来越关心和重视,并将绿色化学的基本思想应用于化学化工的众多领域。高原子利用率的反应越来越受到重视,希望在节约资源的同时减少或消除环境污染。C?H键的选择性活化和定向功能化,尤其是苯及相关芳香族化合物C?H键在温和条件下的选择性活化和定向功能化,是合成化学和绿色化学所面临的重大挑战之一。直接将氨基引入苯环合成苯胺属于这类研究的课题。以苯为原料,选择适当的胺化剂和氧化剂在一定的条件下一步直接合成苯胺,不但减少了中间产物和副产物的生成,而且提高了反应的原子利用率,副产物氢或水都对环境无害,是一种符合绿色合成化学理念的新方法。进行一步法合成苯胺新工艺的研究开发可望使苯胺工业朝着简单、清洁和绿色环保的方向发展。本文采用苯为起始原料,以氨水为氨基源,过氧化氢为氧化剂,在温和条件(低温、常压)下研究了由苯直接氧化氨基化合成苯胺的反应。由于苯环的特殊稳定性,要实现温和条件下苯环的C–H键、氨中的N–H键、过氧化氢中的O–O键的同时活化或优化活化三个中的1–2个以引发反应,催化剂的选择和制备是关键。本文在已有研究的基础上,采用Ni、Mo和/或Mn为主要催化活性组分,制备了系列负载型Ni/Al2O3、Mo/Al2O3、Mn/Al2O3、Mo–Ni/Al2O3和Mn–Ni/Al2O3催化剂,将其用于温和条件下苯的直接氧化氨基化反应。实验结果表明,单组分金属催化剂(Ni/Al2O3、Mo/Al2O3、Mn/Al2O3)对苯的胺化反应有一定催化活性,但对苯胺选择性很低;Ni物种可以实现苯环上C–H和氨的N–H键的同时活化,且还原形式的Ni物种能够相对提高对N–H键的活化能力;MoO3或还原形式的Mo物种也能够同时活化原料分子的C–H键和N–H键,且还原态的Mo物种更有利于提高C–H键的活化;还原态的Mn物种对胺化反应有一定活性,但Mn的氧化物没有催化活性。向Ni基催化剂中引入金属Mo或Mn后,其催化活性和选择性都有很大提高。不同Mo/Ni或Mn/Ni原子比的Mo–Ni/Al2O3和Mn–Ni/Al2O3催化剂对生成苯胺的活性和选择性不同:对Mn–Ni/Al2O3来说,在Mn/Ni原子比为0.16的催化剂上获得了最大的苯胺收率;对Mo–Ni/Al2O3来说,Mo/Ni原子比为1.7的还原的催化剂上获得了本文研究条件下最大的苯胺收率。催化剂的性能主要决定于催化剂的结构,本文对制备的各种催化剂进行了X射线衍射(XRD)和程序升温还原(TPR)表征。实验表明,Mo–Ni/Al2O3上还原态的Mo物种有利于苯环C–H键的活化,而还原态的Ni物种有利于活化氨中的N–H键,催化剂上NiMoO4的形成导致其催化活性下降;Mn–Ni/Al2O3催化剂上Ni和Mn物种之间或两者与载体之间的相互作用形成能使原料分子中C–H键和N–H键同时活化的催化活性相。本文研究条件下的苯、氨水、过氧化氢体系的苯的氧化氨基化反应是一个固体催化剂作用下的多相反应。苯酚是本实验条件下的主要副产物,通过催化剂的调变,可增大苯胺的选择性,降低副产物苯酚的选择性。本文还采用盐酸羟胺为胺化剂,考察了温和条件下醋酸-水介质中苯与盐酸羟胺合成苯胺的反应。选取NaVO3为均相催化剂,考察了介质的酸性、空气(氧)、NaVO3用量、进料比(nNH2OH/ nbenzene)、反应温度、反应时间、加料次序、Na+离子等对胺化反应的影响。实验结果表明,酸性的介质,尤其是有机酸介质(如醋酸的水溶液),有利于胺化反应的进行,醋酸不但为反应提供酸性环境,而且还起到使反应体系均相化的作用,另外也可能与钒中心发生配位,参与胺化反应,进而影响胺化反应机理。通过在开放和密闭体系中的反应对比,证实开放体系比密闭体系更有利于胺化反应进行,空气(氧气)的存在有利于提高目标产物苯胺的收率和选择性。在不引入其他杂阴离子的情况下,催化剂中的金属Na+离子对胺化反应有一定促进作用。对反应条件进行了优化,优化后的反应条件为:22.5 mmol苯,0.3 mmol NaVO3,n benzene/n NH2OH 1:1,VHOAc: VH2O = 4:1,80°C,反应4小时。在此条件下,得到的苯胺收率为64 mol%,苯胺选择性为95.6%,转化数为48摩尔苯胺/摩尔V,此结果好于已有文献报道值。研究了盐酸羟胺在反应条件下的分解反应,考察了温度、醋酸浓度、NaVO3用量、钒离子价态等因素对盐酸羟胺分解过程的影响,发现其适当的分解速度利于胺化反应的进行,其分解促进了活性氨基化物种的形成。采用51V核磁共振(51V NMR)、电子顺磁共振(EPR)、紫外-可见光谱(UV–VIS)等谱学手段对胺化反应过程中钒催化剂的状态变化进行了监测,对紫外-可见吸收光谱进行了理论模拟以帮助解谱,结合活性测试结果,提出并阐明了可能的苯氨基化反应的机理(自由基机理)。即:反应体系pH值低于1时,钒物种首先以VO2+形式存在,加入盐酸羟胺后,VO2+与NH2OH发生相互作用,VV被还原并主要生成VIV,部分NH2OH被氧化生成N2O从体系中排出;体系中可能存在VIII,但其很容易被空气氧化生成稳定的VIV物种;而VIV物种在羟胺、醋酸及氧气存在下可能被再氧化成VV;由于低价V较强的还原能力,体系中的NH2OH于是扮演氧化剂的角色,并被还原成质子化氨基-钒络合物(HO–VV–?NH3+),并作为活性氨基化试剂进攻苯环,生成质子化的氨基环己烯自由基中间体,该中间体被VV夺取一个电子后形成质子化的苯胺;VV最终被还原成VIV,完成一个催化循环。体系中的羟胺消耗完后,胺化反应就结束了,钒最终以四价形式存在。体系中VV的存在不仅导致低价钒物种的产生,而且直接参与中间体的芳构化并生成苯胺;低价的V与羟胺相互作用使得HO–VV–?NH3+得以形成。因此,体系中VV和VIV的共存才能使催化剂发挥最大的催化活性,而空气氧的存在能够使低价钒被再氧化为VV,因此有利于胺化反应进行。以钒为主要活性组分,设计制备了各种负载型的钒基催化剂,对其结构进行了X射线衍射(XRD)、X射线光电子能谱(XPS)以及漫反射紫外-可见吸收光谱(DRUV–VIS)等表征,将这些催化剂用于苯与盐酸羟胺合成苯胺的反应,对胺化反应条件进行了优化。实验表明,不同载体负载的钒基催化剂的活性不同,相同制备条件和反应条件下各催化剂的活性顺序为:γ-Al2O3 > TiO2> SiO2 > AC > HZSM-5 > CeO2,各催化剂上的V负载量与载体的性质有关,γ-Al2O3上负载的V含量最高,且该V/γ-Al2O3催化剂的活性也最高。以V/γ-Al2O3为催化剂,其上V的负载量影响催化剂的催化活性:V负载量越高,催化活性越高。当载体上V的实际含量低于5%时,V物种在载体表面上均匀分散;高于5%时,V物种在载体上堆积成少量V2O5和VO2晶相。改变V/γ-Al2O3催化剂前体浸渍溶液的钒原料及浸渍溶剂(水或草酸的水溶液)对催化剂的结构和催化活性影响不大;提高V/Al2O3催化剂前体的煅烧温度(高于700 oC),载体晶相结构发生相变(γ-Al2O3→θ-Al2O3),使得钒物种在载体表面发生堆积并形成聚合钒物种,提高了催化剂的催化活性和稳定性。将V10/γ-Al2O3反复使用5次,其催化活性降低至初始活性的一半,除了活性物种的流失外,固体催化剂在介质溶液中的破碎也是一个重要的原因。另外,酸性的反应介质有利于胺化反应的进行,但强酸的加入会导致固体催化剂的溶解和产物苯胺收率的降低。开放体系比密闭体系更有利于胺化反应进行;空气氧的存在有利于胺化反应。以V10/γ-Al2O3为催化剂,在2 mL苯为底物的情况下(nNH2OH?HCl: nbenzene= 1:1),在VHOAc:VH2O = 4:1的醋酸-水溶液中于80 oC反应4小时,得到了此条件下最大的苯胺收率(64%),此时的苯胺选择性为95.8%,此结果与均相反应结果相当。V/γ-Al2O3催化下的苯与盐酸羟胺的反应主要是一个多相催化下的反应,但溶液中溶脱的钒离子也对胺化反应有催化活性。将所制备的V10/γ-Al2O3催化剂用于其他芳香族化合物(如乙苯、二甲苯、氯苯、硝基苯)的氨基化反应,发现苯环上的C–H键均能被活化,但活化能力受苯环上取代基团的性质的影响。更多还原

【Abstract】 For a long time, more attentions have been paid on the high selectivity and high yield of chemical reactions than atomic efficiency of the reactants. Thus the wastes of resource and environment pollution were simultaneously encountered. With the development of modern industry, especially the development of green chemistry, people care more about the environment and the energy consumption, and tend to apply the basic ideas of green chemistry to every fields of chemistry and chemical industry. Therefore, increasing attentions is paid to better atomic efficiency, and the“greening”trends of global chemical manufacturing requires new processes that economize resource and reduce or even eliminate environmental pollutions. The selective activation and directional functionalization of C–H bonds, especially those of benzene and other aromatics under mild conditions, remains one of the most significant challenges in both synthetic and green chemistry. The introduction of amino-group into benzene ring to produce aniline is involved in this research. By using suitable aminating reagent and oxidant, the direct amination of benzene to aniline under certain conditions is a green synthetic process because it reduces the formation of intermediate products and by-products, and increases the atomic efficiency of the reactants. The exploitation of new method for the one-step synthesis of aniline will lead the development of aniline industry to a simple, clean, and green energy-conservative direction. The present thesis deals with the study of the direct oxy-amination of benzene to aniline with aqueous ammonia using hydrogen peroxide as the oxidant under mild reaction conditions (low temperature and atmospheric pressure). Because of the peculiar stability of benzene ring, the selection and preparation of the catalyst are the key to the co-activation of C–H bond of benzene, N–H bond of ammonia, and O–O bond of H2O2, or one or two of them under mild conditions to initiate the reaction. Based on the previous research results, Ni, Mo and/or Mn were used as the active components and a series of supported catalysts (i.e. Ni/Al2O3, Mo/Al2O3, Mn/Al2O3, Mo-Ni/Al2O3, and Mn-Ni/Al2O3) were prepared and used in the direct oxy-amination of benzene to aniline under mild reaction conditions.It was found that the one-component catalysts (Ni/Al2O3, Mo/Al2O3, and Mn/Al2O3) are active with lower selectivity to aniline. Nickel species on Ni/Al2O3 are active for the co-activation of C–H bond of benzene and N-H bond of ammonia, and the activation of N–H bond of ammonia is enhanced by the reduced forms of nickel species. Both of MoO3 and the reduced forms of Mo species are active for the activation of C–H bond and N–H bond of the reactants and the reduced forms of Mo species on Mo/Al2O3 makes a more remarkable increase of C–H bond activation than N–H bond activation. The reduced forms of Mn species are responsible for the amination, but its oxides are inactive. The addition of Mo or Mn into Ni/Al2O3 results in an increase of both the yield and selectivity to aniline comparing to the one-component catalysts. Different Mo/Ni or Mn/Ni atomic ratios of the catalysts result in different activities and selectivities to aniline. For the Mn-Ni/Al2O3 catalysts, the highest aniline yield was obtained on the catalyst with a Mn/Ni atomic ratio of 0.16. The reduced Mo-Ni/Al2O3 catalyst with a Mo/Ni atomic ratio of 1.7 showed the best catalytic amination activity under the reaction conditions investigated.The catalytic performance of the catalyst is mainly determined by its structure, thus X-ray diffraction (XRD) and temperature-programmed reduction (TPR) were employed to characterize the catalysts prepared. The results showed that, the reduced forms of supported Mo species on Mo-Ni/Al2O3 makes a more remarkable increase of C–H bond activation than N–H bond activation, and the reduction of nickel species favors the activation of N–H bond of ammonia. The formation of NiMoO4 on Mo-Ni/Al2O3 decreases the amination activity. The interaction of Mn and Ni species or the interaction between the metals and the support leads to the co-activation of the C–H and N–H bonds of the reactants. The oxyamination was proved to be a heterogeneously catalyzed reaction. Phenol is the main by-product in the experiment. The amination activity and selectivity of the catalysts can be improved by properly optimizing the compositions of the catalysts and corresponding preparation method.The synthesis of aniline from benzene and hydroxylamine hydrochloride in acetic acid–water media under mild conditions was also studied. Using NaVO3 as the catalyst, its catalytic performance in the amination was investigated in detail. The influence of the acidity of the reaction media, the presence of air (oxygen), the amount of NaVO3 used, the feed ratio (nNH2OH/nbenzene), the reaction temperature, the reaction time, the feed order, and the presence of Na+ cation on the amination were investigated. It was found that, a relatively acidic reaction medium, especially an organic acid such as acetic acid-water medium, is advantageous for the present homogenously catalyzed amination. Acetic acid, used in the present study, actes not only as a good solvent for mixing the reactants into one phase, it also supplies an acidic surrounding, perhaps even coordinates with the vanadium center and affects the mechanism of the titled amination reaction. It was proved that the amination reaction takes place more efficiently in open air than in a closed system. The presence of air (oxygen) is favorable for the enhancement of the aniline yield and selectivity. Without the introduction of other anions, the Na+ cation in the catalyst favors the amination. The optimized reaction conditions investigated in the present work are: n benzene/n NH2OH 1:1, HOAc: H2O (v/v) 4:1, conducted at 353 K for 4 h under atmospheric pressure. Satisfactory aniline yield and turnover (64 mol%, 48 mol aniline per mol V), with a selectivity of 95.6% to aniline, were obtained under the optimized reaction conditions.The decomposition of hydroxylamine hydrochloride under the reaction conditions was studied to understand its function during the amination process. The effects of temperatures, the concentrations of acetic acid, the amount of NaVO3 used, the valences of vanadium, etc., on the decomposition of hydroxylamine hydrochloride were investigated in detail. It was found that an appropriate decomposition rate of hydroxylamine favors the amination, probably because the decomposition process is involved in the formation of active aminating agent.Several spectroscopic techiniques, including 51V NMR, EPR, and UV-VIS, were used to monitor the variation of the valence of vanadium in the amination process. A computational study on the UV-Vis spectra of some typical vanadium complexes was carried out using the time dependent DFT (TD-DFT) method to help explain the electronic spectra. A free radical mechanism is proposed based on both the results of spectroscopic and activity measurements as well as quantum chemical calculation. The proposed mechanism involves the interaction of a VO2+ cation with hydroxylamine to form the lower-valent vanadium species, mainly VIV. Gaseous N2O is evolved during this redox course. The probably formed VIII species is unstable in open air and is oxidized to the relatively stable VIV. The VIV species, however, would also be probably oxidized to VV in the presence of hydroxylamine, acetic acid, and atmospheric oxygen. On account of the relatively stronger reductive power of lower-valent vanadium species, the hydroxylamine present in the system then acts as oxidative agent and is reduced to a protonated amino-vanadium complex (HO-VV-?NH3+), then attacks the benzene ring to give a protonated aminocyclohexadienyl radical intermediate, which is subsequently oxidized by VV species to form protonated aniline accompanying the regeneration of lower-valent vanadium species, completing the catalytic cycle. The exhaustion of hydroxylamine stops the formation of HO-VV-?NH3+ which further results in the termination of the amination process. In addition, it is clear that the presence of the VV species is essential for the amination process, because they are not only involved in the production of lower-valent vanadium species, but also in the re-aromatization of the aminocyclohexadienyl radical intermediate to form aniline. The lower-valent vanadium species are further involved in the formation of the active protonated amino-vanadium radical species. Atmospheric air favors the co-existence of VV and VIV, which is crucial for the amination progress.Using vanadium as the main active component, various supported vanadium-based catalysts were designed and prepared. The structures of these catalysts were characterized by means of X-ray diffraction (XRD), X-photoelectron spectroscopy (XPS), and Diffuse reflectance UV-VIS (DRUV-VIS) spectroscopy. The as-prepared catalysts were used in the direct amination of benzene to aniline with hydroxylamine hydrochloride. The amination conditions were optimized. It was found that the activities of these catalysts with different supports are different. The activity of the supported catalysts with identical preparation conditions and reaction conditions are as follows:γ–Al2O3 > TiO2 > SiO2 > activated carbon > HASM-5 > CeO2. The actual vanadium loadings on the supports are related with the characters of the supports. The V/γ–Al2O3, with the highest vanadium loading, shows the best catalytic performance in the amination reaction. Using V/γ–Al2O3 as the catalyst, different vanadium loadings on the supports results in different catalytic activities, that is, the more of the vanadium loaded, the higher of the activity. The vanadium species are well dispersed onγ–Al2O3 when the vanadium content on the support is lower than 5%, and deposites on the support to form a small amount of V2O5 and VO2 when the vanadium content is higher than 5%. The variation of the impregnating solution has no obvious effect on the structure and performance of theγ–Al2O3 supported catalyst. The increase of the calcination temperature (above 700 oC) transforms theγ–Al2O3 phase toθ–Al2O3, and favors the formation of crystalline vanadium species on the support, and further results in the increment of the catalytic performance. The activity of V/γ–Al2O3 decreases to half after five consequent runs, this is attributed to the loss of active metals and the dissolution of the solid catalyst in the solution. In addition, an acidic reaction medium is favorable for the amination, but the addition of strong acid results in the dissolution of the catalyst and the decrease of the aniline yield. The amination takes place more efficiently in open air than in closed system. The presence of air (oxygen) favors the amination. Over the V/γ–Al2O3 catalyst, an aniline yield of 64%, with a selectivity of 95.8%, was obtained when using 2 mL of benzene in acetic acid-water solution (VHOAc: VH2O = 4:1) at 80 oC for 4 h. This value is comparable with that obtained in homogeneous NaVO3 catalyzed amination system. It was proved that the V/γ–Al2O3 catalyzed amination is mainly a heterogeneously catalyzed reaction, and the leaching vanadium ions are also responsible for the amination, although this contribution is not as high as the heterogeneous catalysis.The amination of other aromatics (i.e. ethylbenzene, dimethyl benzene, chlorobenzene, and nitrobenzene) was carried out over V/γ–Al2O3. It was found that the C–H bond in benzene ring could be activated but this activation is influenced by the nature of the substituted groups.更多还原