Infrared and Raman imaging of heterogeneous catalysts

The miniaturization of in situ spectroscopic tools has been recognized as a forefront instrumental development for the characterization of heterogeneous catalysts. With the multitude of micro-spectroscopic methods available fundamental insight into the structure-function relationships of catalytic processes can be obtained. Among these techniques vibrational spectroscopy is one of the most versatile methods and capable to shed insight into the molecular structure of reaction intermediates and products, the chemical state of catalyst materials during reaction as well as the nature of interactions between reactants/intermediates/products and the catalyst surface. In this tutorial review we discuss the recent developments in the field of infrared (IR) and Raman micro-spectroscopy and illustrate their potential. Showcase examples include (1) chemical imaging of spatial heterogeneities during catalyst preparation, (2) high-throughput catalyst screening, (3) transport and adsorption phenomena within catalytic solids and (4) reactivity studies of porous oxides, such as zeolites. Finally, new in situ spectroscopy tools based on vibrational spectroscopy and their potential in the catalysis domain are discussed.     

Combining in situ characterization methods in one set-up: looking with more eyes into the intricate chemistry of the synthesis and working of heterogeneous catalysts

Several in situ techniques are known which allow investigations of catalysts and catalytic reactions under real reaction conditions using different spectroscopic and X-ray methods. In recent years, specific set-ups have been established which combine two or more in situ methods in order to get a more detailed understanding of catalytic systems. This tutorial review will give a summary of currently available set-ups equipped with multiple techniques for in situ catalyst characterization, catalyst preparation, and reaction monitoring. Besides experimental and technical aspects of method coupling including X-ray techniques, spectroscopic methods (Raman, UV-vis, FTIR), and magnetic resonance spectroscopies (NMR, EPR), essential results will be presented to demonstrate the added value of multitechnique in situ approaches. A special section is focussed on selected examples of use which show new developments and application fields.     

A Fully Integrated Continuous‐Flow System for Asymmetric Catalysis: Enantioselective Hydrogenation with Supported Ionic Liquid Phase Catalysts Using Supercritical CO2 as the Mobile Phase

A continuous‐flow process based on a chiral transition‐metal complex in a supported ionic liquid phase (SILP) with supercritical carbon dioxide (scCO₂) as the mobile phase is presented for asymmetric catalytic transformations of low‐volatility organic substrates at mild reaction temperatures. Enantioselectivity of >99 % ee and quantitative conversion were achieved in the hydrogenation of dimethylitaconate for up to 30 h, reaching turnover numbers beyond 100 000 for the chiral QUINAPHOS–rhodium complex. By using an automated high‐pressure continuous‐flow setup, the product was isolated in analytically pure form without the use of any organic co‐solvent and with no detectable catalyst leaching. Phase‐behaviour studies and high‐pressure NMR spectroscopy assisted the localisation of optimum process parameters by quantification of substrate partitioning between the IL and scCO₂. Fundamental insight into the molecular interactions of the metal complex, ionic liquid and the surface of the support in working SILP catalyst materials was gained by means of systematic variations, spectroscopic studies and labelling experiments. In concert, the obtained results provided a rationale for avoiding progressive long‐term deactivation. The optimised system reached stable selectivities and productivities that correspond to 0.7 kg L ^?1 h^?1 space–time yield and at least 100 kg product per gram of rhodium, thus making such processes attractive for larger‐scale application.     

The Role of Vanadia for the Selective Oxidation of Benzyl Alcohol over Heteropolymolybdate Supported on Alumina

A series of 12-molybdophosphoric acid (MPA) supported on V2O5 dispersed γ-Al2O3 catalysts with different vanadia loadings were prepared by impregnation and characterized by N2 adsorption-desorption,X-ray diffraction,temperature-programmed reduction,in situ laser Raman spectroscopy,UV-Vis diffused reflectance spectroscopy,scanning electron microscopy,and temperature-programmed desorption of NH3 techniques.Their catalytic activities were evaluated for the vapor phase aerobic oxidation of benzyl alcohol.The catalysts exhibited high catalytic activity and the conversion of benzyl alcohol depended on the vanadia content while the catalyst with 15 wt％ V2O5 content showed optimum activity.The characterization results suggest the presence of well-dispersed V2O5 and partially disintegrated Keggin ions of MPA on the support.In situ Raman studies showed a reduced Mo(IV) species when the catalysts were calcined at high temperatures.The high oxidation activity of the catalysts is related to the synergistic effect between MPA and V2O5.     

In situ synthesis of mixed-valent manganese oxide nanocrystals: an in situ synchrotron X-ray diffraction study

Phase transformations of materials can be studied by in situ synchrotron X-ray diffraction. However, most reported in situ synchrotron XRD studies focus on solid state/gel systems by measuring phase/structure changes during application of pressure or heat. Phase transformations during material synthesis and their applications, especially in wet chemistry processes with different media, have not drawn much attention. Here, using manganese oxides as examples, we report the successful characterization of phase transformations in in situ hydrothermal synthesis conditions by the in situ synchrotron XRD method using a quartz/sapphire capillary tube as the synthesis reactor. The results were used for better design of materials with controlled structures and properties. This method can be generally used for synthesis of manganese oxides as well as for in situ characterization of other material syntheses using hydrothermal, sol-gel, and other methods. In addition, catalytic processes in liquid-solid, gas-solid, and solid-solid systems can also be studied in such an in situ way so that catalytic mechanisms can be better understood and catalyst synthesis and catalytic processes can be optimized.     

Design and applications of a home-built in situ FT-Raman spectroscopic cell

In the field of heterogeneous catalysis, in situ spectroscopy is one of the topics with growing interest. The characterization of a catalyst under working conditions is essential to identify the catalytic active site and to study the relation between the surface structure of a catalyst and its catalytic performance. For the first time, the design of an in situ spectroscopic cell for FT-Raman is presented and its performance is demonstrated by monitoring the thermal conversion of as synthesized mesoporous titanium and by characterizing the molecular surface structure of the vanadium oxides grafted on MCM-48 after exposure to a probe molecule. The results in both cases indicate that the in situ FT-Raman cell is a promising technique for characterizing the molecular surface structure of catalyst materials.     

二氧化铈催化剂的氧化还原和酸碱性质的原位光谱表征

二氧化铈作为催化剂、催化剂载体和助剂被广泛应用于各类氧化还原的催化反应中,是多相催化领域中至关重要的金属氧化物.氧化铈因具有丰富的缺陷结构、较强的氧化还原能力以及异常的酸碱功能等独特性质,在催化领域中非常重要.在分子层面上理解氧化铈的储氧能力、氧化还原效应和酸碱性质对建立催化构效关系尤为重要,是有效合理地改善和设计铈基催化材料的关键.在诸多的表征手段中,光谱在氧化铈结构和表面性质的研究中显示出无可争议的优势,可以提供原子和分子层面的化学信息.本文总结了各种光谱方法(包括光学、X射线、中子、电子和核磁谱学)对氧化铈表面性质表征的研究进展.分析了直接光谱表征及其与探针分子耦合两种方法在氧化铈表征中的应用;归纳了预处理条件、氧化铈纳米粒子的形貌和尺寸对其表面位点的性质、强度和密度的影响.最后展望了如何利用反应条件下的原位光谱来更好地理解和揭示铈基材料的催化作用机制的可能性.     

Novel ultrasonic-modified MnOx/TiO2 for low-temperature selective catalytic reduction (SCR) of NO with ammonia

A novel ultrasonic-modified MnO(x)/TiO(2) catalyst was prepared and compared with two different kinds of MnO(x)/TiO(2) catalysts in the process of low-temperature selective catalytic reduction of NO with NH(3). The physicochemical properties of the catalysts were studied by using various characterization techniques, such as Brunauer-Emmett-Teller (BET) surface measurement, X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and in situ Fourier transform infrared spectroscopy (in situ FT-IR). The ultrasonic-modified process introduced ultrasound in the solution impregnation step of traditional impregnation method for MnO(x)/TiO(2) catalyst preparation. In this study, ultrasonic process significantly improved the dispersion behavior and surface acid property of manganese oxide on TiO(2) as well as the catalytic activity, especially at temperature below 120°C. The NO conversion could reach 90% at 100°C. For the novel ultrasonic-modified catalyst, the combination analysis of XRD and HRTEM confirmed that manganese oxide was in a highly dispersed state and Ti and Mn had strong interaction. Furthermore, in situ FT-IR studies revealed that there were significant amounts of Lewis acidity and high Mn atom concentration on the surface of the novel catalysts.     

原位拉曼光谱与X射线吸收光谱研究能源转换电催化反应

近年来,水分解、氧气/二氧化碳还原等电化学能源转换技术为解决全球能源短缺及环境问题提供了新的思路和方向.然而,对这些能源转换技术的反应机理及其催化剂的活性位点目前仍缺乏深刻的认识和理解,这限制了高效、稳定催化剂的开发,以致阻碍该类电化学技术的进一步发展.原位光谱技术的快速发展为解决上述问题提供了坚实的基础,其中拉曼光谱...     

Supercritical hydrogenation and acid-catalysed reactions "without gases"

The high temperature catalytic decomposition of HCO2H and HCO2Et are used to generate the high pressure H2 and the supercritical fluids needed for micro-scale hydrogenation of organic compounds; our approach overcomes the problems and limitations of handling high pressure gases on a small-scale and opens the way to the widespread use of continuous supercritical reactions in the laboratory.     

Exploring Between the Extremes: Conversion‐Dependent Kinetics of Phosphite‐Modified Hydroformylation Catalysis

The kinetics of the hydroformylation of 3,3‐dimethyl‐1‐butene with a rhodium monophosphite catalyst has been studied in detail. Time‐dependent concentration profiles covering the entire olefin conversion range were derived from in situ high‐pressure FTIR spectroscopic data for both, pure organic components and catalytic intermediates. These profiles fit to Michaelis–Menten‐type kinetics with competitive and uncompetitive side reactions involved. The characteristics found for the influence of the hydrogen concentration verify that the pre‐equilibrium towards the catalyst substrate complex is not established. It has been proven experimentally that the hydrogenolysis of the intermediate acyl complex remains rate limiting even at high conversions when the rhodium hydride is the predominant resting state and the reaction is nearly of first order with respect to the olefin. Results from in situ FTIR and high‐pressure (HP) NMR spectroscopy and from DFT calculations support the coordination of only one phosphite ligand in the dominating intermediates and a preferred axial position of the phosphite in the electronically saturated, trigonal bipyramidal (tbp)‐structured acyl rhodium complex.     

Vibrational Spectroscopy in Supercritical Fluids: From Analysis and Hydrogen Bonding to Polymers and Synthesis

Supercritical fluids are beginning to be used widely in chemistry. Applications range from extraction and chromatography in analytical chemistry to solvents for reaction chemistry and preparation of new materials. Spectroscopic monitoring is important in much of supercritical chemistry, and vibrational spectroscopy is particularly useful in this context because the vibrational spectrum of a given molecule is usually quite sensitive to the environment of that molecule. Thus, vibrational spectra are excellent probes of conditions within the fluid. In this review, we describe a variety of techniques and cells for IR and Raman spectroscopy in supercritical fluids and illustrate the breadth of applications in supercritical fluids. The examples include: the use of supercritical Xe as a spectroscopically transparent solvent for chemistry and for supercritical fluid chromatography with FTIR detection of analytes; Raman spectroscopy as a monitor for gases dissolved in supercritical CO₂; the effect of solvent density on hydrogen bonding in supercritical fluids and the formation of reverse micelles; IR as a monitor for the supercritical impregnation/extraction of polymers and the reactions of organometallic compounds impreganated into polymers; reactions of organometallic compounds in supercritical fluids; and finally, the use of miniature flow reactors for laboratory-scale preparative chemistry. Overall, our aim is to provide a starting point from which individual readers can judge whether such measurements might usefully be applied to their own particular problems.     

TsOH-SiO2 as an efficient and eco-friendly catalyst for Knoevenagel condensation

Tosic acid on silica gel (TsOH-SiO₂) was synthesized and characterized using microscopic and spectroscopic techniques such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and FT-IR spectroscopy. Thermal behaviour of the catalyst was investigated by differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. TsOH-SiO₂ showed excellent catalytic activity for the Knoevenagel condensation and was recyclable for six cycles.     

Properties and reactions of charged species in nonpolar supercritical fluids

This review focuses on the properties and reactions of charged species in supercritical fluids. The techniques of pulse conductivity and transient absorption are used to follow the behavior of charged species. We begin with a discussion of the mobilities, yields, and energy levels of electrons. Studies of the pressure dependence of electron attachment reactions lead to information on the magnitude of activation volumes. This as well as diffusion and energetics are factors that influence the rates of electron attachment. The free energy changes in electron attachment-detachment equilibrium reactions decrease rapidly at pressures where the compressibility maximizes. The transport properties of ions in supercritical fluids are also discussed, as these studies provide a straightforward indication of the degree of interaction between ions and the medium. We conclude with a review of electron transfer reactions in supercritical fluids.     

Reevaluation of the mechanism of the amination of aryl halides catalyzed by BINAP-ligated palladium complexes

Two previous mechanistic studies of the amination of aryl halides catalyzed by palladium complexes of 1,1'-binaphthalene-2,2'-diylbis(diphenylphosphine) (BINAP) are reexamined by the authors of both studies. This current work includes a detailed study of the identity of the BINAP-ligated palladium complexes present in reactions of amines with aryl halides and rate measurements of these catalytic reactions initiated with pure precatalysts and precatalysts generated in situ from [Pd2(dba)3] and BINAP. This work reveals errors in both previous studies, and we describe our current state of understanding of the mechanism of this synthetically important transformation. 31P NMR spectroscopy shows that several palladium(0) species are present in the catalytic system when the catalyst is generated in situ from [Pd2(dba)3] and BINAP, and that at least two of these complexes generate catalytic intermediates. Further, these spectroscopic studies and accompanying kinetic data demonstrate that an apparent positive order in the concentration of amine during reactions of secondary amines is best attributed to catalyst decomposition. Kinetic studies with isolated precatalysts show that the rates of the catalytic reactions are independent of the identity and the concentration of amine, and studies with catalysts generated in situ show that the rates of these reactions are independent of the concentration of amine. Further, reactions catalyzed by [Pd(BINAP)2] with added BINAP are found to be first-order in bromoarene and inverse first-order in ligand, in contrast to previous work indicating zero-order kinetics in both. These data, as well as a correlation between the decay of bromobenzene in the catalytic reaction and the predicted decay of bromobenzene from rate constants of studies on stoichiometric oxidative addition, are consistent with a catalytic process in which oxidative addition of the bromoarene occurs to [Pd(BINAP)] prior to coordination of amine and in which [Pd(BINAP)2], which generates [Pd(BINAP)] by dissociation of BINAP, lies off the cycle. By this mechanism, the amine and base react with [Pd(BINAP)(Ar)(Br)] to form an arylpalladium amido complex, and reductive elimination from this amido complex forms the arylamine.     

In situ solid-state NMR for heterogeneous catalysis: a joint experimental and theoretical approach

In situ solid-state NMR is a well-established tool for investigations of the structures of the adsorbed reactants, intermediates and products on the surface of solid catalysts. The techniques allow identifications of both the active sites such as acidic sites and reaction processes after introduction of adsorbates and reactants inside an NMR rotor under magic angle spinning (MAS). The in situ solid-state NMR studies of the reactions can be achieved in two ways, i.e. under batch-like or continuous-flow conditions. The former technique is low cost and accessible to the commercial instrument while the latter one is close to the real catalytic reactions on the solids. This critical review describes the research progress on the in situ solid-state NMR techniques and the applications in heterogeneous catalysis under batch-like and continuous-flow conditions in recent years. Some typical probe molecules are summarized here to detect the Br?nsted and Lewis acidic sites by MAS NMR. The catalytic reactions discussed in this review include methane aromatization, olefin selective oxidation and olefin metathesis on the metal oxide-containing zeolites. With combining the in situ MAS NMR spectroscopy and the density functional theoretical (DFT) calculations, the intermediates on the catalyst can be identified, and the reaction mechanism is revealed. Reaction kinetic analysis in the nanospace instead of in the bulk state can also be performed by employing laser-enhanced MAS NMR techniques in the in situ flow mode (163 references).     

Heck reactions with various types of palladium complex catalysts: application of multiphase catalysis and supercritical carbon dioxide

The application of multiphase catalytic systems for palladium catalyzed Heck reactions brings several benefits such as easy catalyst-product separation and catalyst recycling. The effective multiphase Heck systems can be prepared by using different types of catalyst phases, including biphasic catalysis and supported liquid phase catalysts, and a new generation solvent of supercritical carbon dioxide.     

从分子碎片到活性位：分子筛催化材料的原位、共振紫外拉曼光谱研究

在过去的近十年中,各种新型原位表征技术和反应器设计被应用于多相催化过程和催化材料的合成研究中,并获得了许多新认识．特别是最近几年,利用原位、共振拉曼光谱技术对分子筛合成关键物种检测、杂原子分子筛催化活性位的研究取得了一系列进展．这些技术的应用使得从分子水平认识复杂的多孔材料成为可能：从合成初期碎片基元检测、碎片相互连接的关键化学键到预组装类微孔结构;从高度隔离过渡金属中心到配位化学键断裂生成活性中间物种,再到完成催化反应循环．这为设计特定功能和结构的催化材料及高选择性的活性中心奠定了坚实的基础．     

Halogen‐Dependent Surface Confinement Governs Selective Alkane Functionalization to Olefins

The product distribution in direct alkane functionalization by oxyhalogenation strongly depends on the halogen of choice. We demonstrate that the superior selectivity to olefins over an iron phosphate catalyst in oxychlorination is the consequence of a surface‐confined reaction. By contrast, in oxybromination alkane activation follows a gas‐phase radical‐chain mechanism and yields a mixture of alkyl bromide, cracking, and combustion products. Surface‐coverage analysis of the catalyst and identification of gas‐phase radicals in operando mode are correlated to the catalytic performance by a multi‐technique approach, which combines kinetic studies with advanced characterization techniques such as prompt‐gamma activation analysis and photoelectron photoion coincidence spectroscopy. Rationalization of gas‐phase and surface contributions by density functional theory reveals that the molecular level effects of chlorine are pivotal in determining the stark selectivity differences. These results provide strategies for unraveling detailed mechanisms within complex reaction networks.     

Fabrication and Catalytic Properties of Films Based on Metal Ion-ligand Interaction between K_2PdCl_4 and 3-Amino-3-(4-pyridinyl)-propionitrile

This work presents a highly active and reusable heterogeneous film catalytic assembly for hydrogenation reduction of aromatic nitro compounds. The multilayer structures of PEI-(K2PdCl4-P1)n-film(PEI = polyethylenmine, P1 = 3-amino-3-(4-pyridinyl)-propionitrile) bound to quartz slides were fabricated by layer-by-layer(LbL) self-assembly method. Various characterization techniques including X-ray photoelectron spectroscopy(XPS), inductively coupled plasma OES spectrometer(ICP), UV-vis spectroscopy and atomic force microscopy(AFM) were employed to reveal the growth process of the resulting LbL multilayers in detail. Subsequent in situ reduction by H2 produced Pd nanoparticles embedded in such films were used as catalyst for the hydrogenation of nitroarenes. The catalytic performance test shows that the thin film catalyst can be applied to the hydrogenation reaction of various substituted nitroaromatics, and exhibits good catalytic activity and good catalyst stability. It is worth mentioning that our catalytic films can be easily removed from the reaction system in any time during the reaction, and the catalytic activity could be fully recovered when reused directly in another catalytic cycle for five times.