【作者】 刘丹；

【导师】 陈卓；

【作者基本信息】 北京理工大学 ， 材料科学与工程， 2024， 博士

【摘要】 有机催化反应在医药合成、化工生产、农药生产等领域具有重要的研究价值及应用前景。近年来,外场调控成为提高有机催化反应效率及选择性的有效途径,其中光场由于其强度、波长可控、条件温和而受到研究者的普遍关注。光对催化反应体系主要有以下两方面影响。首先,在催化剂制备方面,光化学还原可以降低金属元素价态、实现在半导体等光敏载体上均匀沉积金属纳米颗粒的目标,与传统的热及化学还原相比,光诱导金属离子还原具有显著优势:它能够在温和条件下进行,同时节约大量的能源,通过调控光照强度和波长范围,可以实现对所制样品的价态、分散度等参数的精确调控。其次,在催化反应过程中引入光照同样可以提升催化反应的速率及选择性。因此,将光照引入催化剂制备及催化反应过程中,精细调控主催化剂活性吸附位点和电子性质,从而高效、高选择性地完成催化反应,已经成为当前研究的热点领域。贵金属材料因为其独特的电子结构而具有优越的催化性能,可以应用于诸多有机催化反应体系。然而,高昂的价格限制了贵金属在催化反应中的应用。为此,针对目前贵金属基催化剂在催化有机反应过程中催化效率及原子利用率相对较低的问题,本论文通过在催化剂制备过程中引入光照,制备出了高效催化有机反应的均相与非均相贵金属基催化剂:针对Suzuki偶联反应,通过光还原法制备了低价Pd基催化剂,展现出了优异的催化性能;针对非均相硝基苯加氢还原反应,在通过光还原法制备了非均相催化剂后,通过光催化有效地提升了反应的催化效率。具体研究内容如下:一、设计合成低价态Pd基催化剂是实现其高催化Suzuki偶联反应性能的关键。通过对Pd（acac）₂溶液进行光辐照,获得了一种低价态的Pd基均相催化剂。室温下在制备过程中改变光照时间,发现Pd元素的聚集状态由光照时间不大于30 min时的单分散到50 min时的团簇再到60 min时纳米晶体。通过系统优化催化Suzuki偶联反应的实验条件,发现光照处理30 min后的催化剂在反应温度80 ^oC、反应溶剂为水醇混合液的条件下,催化Suzuki偶联反应的TOF（turnover frequency）值高达76000 h^-1,是未经光照处理的样品的4倍,远高于目前文献报道值。表征结果表明,30 min光照处理后的均相催化剂中Pd原子的价态降低,但催化剂仍保持单分散的状态而未出现团聚的现象。催化机理研究结果表明,处于单分散状态的Pd基均相催化剂在经过一个反应的诱导期后,其TOF值显著提升,但反应结束后出现团聚的Pd纳米颗粒,使得其催化性能大大衰减。光还原的Pd基催化剂因价态降低而使得催化性能明显提高,且光还原法对制备其他低价态贵金属乙酰丙酮盐具有普适性,为开发新型高效均相催化剂提供了新的技术途径和思路。将上述均相Pd基催化剂固载在SiO₂壳层内得到了非均相催化剂Pd@SiO₂;同样,光还原30 min后的Pd基催化剂固载在SiO₂壳层中得到非均相催化剂Re 30@SiO₂。通过实验发现非均相催化剂Re 30@SiO₂催化Suzuki偶联反应的性能优于非均相催化剂Pd@SiO₂的催化性能。然而,上述非均相催化剂不能实现催化剂的循环利用。二、采用光还原法制备了非均相催化剂Pd/TiO₂以实现催化剂的循环利用。控制制备过程中的光照强度和时间,调控了Pd基催化剂催化Suzuki偶联反应的活性。X射线光电子能谱（XPS）表征结果表明光还原法制备的催化剂表面的Pd元素价态低于投料时Pd元素的价态+2价。在水-醇溶剂中,催化剂Pd/TiO₂上的低价态Pd元素容易浸出到反应液中,使得催化剂具有优异的催化性能。然而,Pd浸出现象导致催化剂的活性稳定性较差。因此,通过对参与反应后的催化剂进行焙烧处理,实现了催化剂性能的保持。此外,XPS和X射线衍射分析（XRD）表征揭示了催化剂在反应前后元素价态及晶相结构的变化。这一研究为催化Suzuki偶联反应的均相催化剂的非均相化提供了可行的技术路径。三、采用光还原法制备了非均相催化剂Pt-Au/TiO₂,利用光诱导贵金属非均相成核,有效减少了Pt-Au纳米合金的单独成核,提高了金属半导体负载的成功率及金属均匀性。将催化剂应用于硝基苯加氢还原反应中,光照下催化性能明显提升。催化剂Pt-Au/TiO₂在光照下催化硝基苯加氢还原反应的TOF值达到了7315 h^-1,是暗态下反应的6.5倍(1122 h^-1)。动力学分析结果表明,引入光照后催化反应的反应级数改变,从暗态下的一级反应转变为引入光照后的零级反应;此外,活化能由暗态下86.5KJ/mol降低为引入光照的44.0 KJ/mol,活化能的降低有助于催化反应效率的提升。红外和瞬态吸收光谱的表征结果显示,引入光照有利于促进反应底物硝基苯分子的活化。此外,程序升温脱附（TPD）结果表明氢气分子可吸附在金属上,一部分被金属活化的活性H溢流到载体TiO₂上。XPS表征结果显示,光照可以促进更多的H₂分子活化并溢流。因此,引入的光照能够通过促进反应底物硝基苯分子和H₂分子的活化,大大提高催化反应速率。本研究结果不仅成功合成了异质结构催化剂用来高效催化硝基苯加氢还原反应,同时揭示了光促进的Pt-Au/TiO₂催化硝基苯加氢的机理,为开发高效贵金属-半导体异质结构光催化有机反应提供了借鉴及思路。更多还原

【Abstract】 The organic catalytic reaction has significant research value and application prospects in pharmaceutical synthesis,chemical production,pesticide production,and other fields.In recent years,external field control has become an effective way to improve the efficiency and selectivity of organic catalytic reactions,among which the light field has been widely concerned by researchers because of its intensity,wavelength control,and mild conditions.The effects of light on the catalytic reaction system are as follows.One is in the field of catalyst preparation,photochemical reduction can reduce the valence state of metallic elements and achieve the goal of uniformly depositing metal nanoparticles on the support.Compared with traditional thermal or chemical reduction methods,photoinduced metal ion reduction has obvious advantages.It can be carried out under mild conditions,save much energy,and control the valence state and dispersion of the prepared sample by controlling the light intensity and wavelength range.The other is that in the process of catalytic reaction,light can also improve the catalytic reaction rate and selectivity.Therefore,how to introduce light into the process of catalyst preparation and catalytic reaction to achieve the controlled synthesis of catalyst,tune the active adsorption sites and electronic properties of the metal catalyst finely,and the completion of catalytic reaction with high efficiency and selectivity has become a frontier research hotspot.Because of its unique electronic structure,noble metal materials have superior catalytic properties and can be used in many organic catalytic reaction systems.However,the high price of precious metals limits their application.To this end,in view of the relatively low catalytic efficiency and atom utilization rate in the catalytic organic reaction process of precious metal-based materials,in this paper,by introducing light in the process of catalyst preparation or catalytic reaction,a high efficient noble metal based homogeneous or heterogeneous catalyst for organic reaction was prepared.For the heterogeneous nitrobenzene hydrogenation reduction reaction,the heterogeneous catalyst was prepared by photoreduction method,and the catalytic efficiency of the reaction was effectively improved by photocatalysis.For Suzuki coupling reaction,a low valence Pd-based catalyst was prepared by photoreduction method,which showed excellent performance of catalytic coupling reaction.The specific research contents are as follows:First,designing and synthesizing of low-cost Pd-based catalyst is the key to achieve its high catalytic performance for Suzuki coupling reaction.A low-cost Pd-based homogeneous catalyst was obtained by photoirradiation of Pd（acac）₂ solution.At room temperature,the irradiation time was changed during the preparation process,and it was found that the aggregation state of Pd elements changed from monodispersion when the irradiation time was less than 30 min to clusters at 50 min and then to nanocrystals at 60 min.By systematically optimizing the experimental conditions of catalytic Suzuki coupling reaction,it was found that the TOF（turnover frequency）value of the catalyst under 30 min irradiation treatment at room temperature for Suzuki coupling reaction was as high as 76000 h^-1 when the reaction temperature was 80 ^oC and the reaction solvent was water and alcohol mixture.The TOF value was 4 times that of the sample without light treatment.Much higher than the value reported in the literature.The characterization results showed that the valence of Pd atoms in the homogeneous catalyst was reduced after 30 min light treatment,but the catalyst remained in a monodisperse state without agglomeration.The results of catalytic mechanism study showed that the TOF value of Pd-based homogeneous catalyst in monodisperse state increased significantly after a reaction induction period,but the agglomeration of Pd nanoparticles appeared at the end of the reaction,which greatly attenuated its catalytic performance.The experimental results show that the catalytic performance of the photoreduced Pd-based catalyst is obviously improved due to the reduction of valence state,and the photoreduction method is universal for the preparation of other low-cost precious metal acetylacetonate,which provides a new technical approach and idea for the development of new efficient homogeneous catalysts.The homogeneous Pd-based catalysts were immobilized in the SiO₂ shell to obtain the heterogeneous catalyst Pd@SiO₂;similarly,the Pd-based catalysts after 30 min of photo-reduction were immobilized in the SiO₂ shell to obtain the heterogeneous catalyst Re30@SiO₂.Through experiments,it was found that the catalytic performance of the heterogeneous catalyst Re 30@SiO₂ in the Suzuki coupling reaction was superior to that of the heterogeneous catalyst Pd@SiO₂.However,the above-mentioned heterogeneous catalysts could not achieve the recycling of the catalyst.Second,heterogeneous catalyst Pd/TiO₂ was prepared by photoreduction method to realize the recycling of catalyst.The activity of Suzuki coupling reaction catalyzed by Pd-based catalyst was regulated by controlling the light intensity and time in the preparation process.The results of XPS（X-ray photoelectron spectroscopy）characterization show that the valence state of Pd element on the surface of the catalyst prepared by photoreduction method is lower than the+2 valence of Pd element when feeding.In water-alcohol solvent,the low-priced Pd element on the catalyst Pd/TiO₂ is easily leached to the reaction solution,which makes the catalyst have excellent catalytic performance.However,Pd leaching phenomenon leads to poor activity stability of the catalyst.Therefore,by roasting the catalyst involved in the reaction,the performance of the catalyst was maintained.In addition,XPS and XRD（X-ray diffraction）characterization revealed the changes of elemental valence and crystal structure before and after the reaction.This study provides a feasible technical path for the heterogeneization of homogeneous catalysts for Suzuki coupling reaction.Third,the heterogeneous catalyst Pt-Au/TiO₂ was prepared by photoreduction method,and the heterogeneous nucleation growth of precious metals was induced by light,which effectively reduced the single nucleation growth of Pt-Au nanoalloys,and improved the success rate and uniformity of metal semiconductor loading.When the catalyst is applied to the hydrogenation reduction of nitrobenzene,the hydrogenation performance of the catalyst is obviously improved under the condition of light.The TOF value of the catalytic nitrobenzene hydrogenation reduction reaction by Pt-Au/TiO₂ under light condition reached7315 h^-1,which was 6.5 times than that of the reaction under dark state(1122 h^-1).The kinetic analysis results show that the reaction order of the catalytic reaction changes from the first order reaction in the dark state to the zero order reaction after the introduction of light.In addition,it was found that the activation energy decreased from 86.5 KJ/mol in the dark state to 44.0 KJ/mol after the introduction of light,and the reduction of activation energy helped to significantly improve the catalytic reaction efficiency.The results of FTIR and transient absorption spectra showed that the introduction of light promoted the activation of nitrobenzene molecules.In addition,TPD（temperature programmed desorption）characterization results show that hydrogen molecules can be adsorbed on the metal,and a part of the active H activated by the metal spills onto the support TiO₂.The results of XPS characterization show that light can promote the activation and overflow of more H₂molecules.Therefore,the introduced light can greatly improve the catalytic reaction rate by promoting the activation of nitrobenzene and H₂ molecules as reaction substrates.The results of this study not only successfully synthesized heterostructure catalyst to efficiently catalyze nitrobenzene hydrogenation reduction reaction,but also revealed the mechanism of photopromoted Pt-Au/TiO₂ to catalyze nitrobenzene hydrogenation,providing reference and ideas for the development of efficient noble metal-semiconductor heterostructure photocatalytic organic reaction.更多还原