Chemisorption of Probe Molecules on Metal Oxides

A more complete understanding of the structural and mechanistic details of a catalyzed heterogeneous reaction leads both directly and indirectly to the development of new and better catalysts. For catalyst technology, the most sensitive probe of catalysts performance will continue to be the rate and selectivity of a chemical reaction. However, these macroscopic observations, adequate for determining how good a catalyst is, require supplementary microscopic information to remove ambiguity in the deduction of a catalytic mechanism. This information, almost down to the atomic level, concerning the structure and reactivity of the intermediates, the nature of adsorption sites (and sometimes the active sites) and their number, is the main objective of the science of catalysis. The most promising approach to this problem is the use of suitable probe molecules for the quantitative titration of site density and qualitative characterization of their nature by means of surface spectroscopies of the chemisorbed probe molecules [1, 21. This framework of action is schematically represented in Fig.1.     

Rational approach to polymer-supported catalysts: synergy between catalytic reaction mechanism and polymer design

Supported catalysis is emerging as a cornerstone of transition metal catalysis, as environmental awareness necessitates "green" methodologies and transition metal resources become scarcer and more expensive. Although these supported systems are quite useful, especially in their capacity for transition metal catalyst recycling and recovery, higher activity and selectivity have been elusive compared with nonsupported catalysts. This Account describes recent developments in polymer-supported metal-salen complexes, which often surpass nonsupported analogues in catalytic activity and selectivity, demonstrating the effectiveness of a systematic, logical approach to designing supported catalysts from a detailed understanding of the catalytic reaction mechanism. Over the past few decades, a large number of transition metal complex catalysts have been supported on a variety of materials ranging from polymers to mesoporous silica. In particular, soluble polymer supports are advantageous because of the development of controlled and living polymerization methods that are tolerant to a wide variety of functional groups, including controlled radical polymerizations and ring-opening metathesis polymerization. These methods allow for tuning the density and structure of the catalyst sites along the polymer chain, thereby enabling the development of structure-property relationships between a catalyst and its polymer support. The fine-tuning of the catalyst-support interface, in combination with a detailed understanding of catalytic reaction mechanisms, not only permits the generation of reusable and recyclable polymer-supported catalysts but also facilitates the design and realization of supported catalysts that are significantly more active and selective than their nonsupported counterparts. These superior supported catalysts are accessible through the optimization of four basic variables in their design: (i) polymer backbone rigidity, (ii) the nature of the linker, (iii) catalyst site density, and (iv) the nature of the catalyst attachment. Herein, we describe the design of polymer supports tuned to enhance the catalytic activity or decrease, or even eliminate, decomposition pathways of salen-based transition metal catalysts that follow either a monometallic or a bimetallic reaction mechanism. These findings result in the creation of some of the most active and selective salen catalysts in the literature.     

Preparation,characterization, and kinetic evaluation of dendrimer-derived bimetallic Pt–Ru/SiO2 catalysts

A series of silica-supported Pt, Ru, and Pt–Ru catalysts has been synthesized using dendrimer–metal nanocomposite (DMN) precursors prepared by both co- and sequential complexation with metal salts. The catalysts have been characterized by several techniques, including electron microscopy, temperature-programmed titration of adsorbed oxygen, and X-ray diffraction. Liquid-phase selective hydrogenation of 3,4-epoxy-1-butene (EpB) was used as a probe reaction to evaluate their catalytic performance. The bimetallic catalyst prepared by the co-complexation method exhibits a superior catalytic activity compared to the sequential one, and is much more active than a conventional catalyst prepared by incipient wetness. The activity enhancement is attributed to a bifunctional performance of the PtRu alloy sites created, based on a strong correlation between turnover frequencies, and both the alloy compositions and metal surface site distributions. In addition, the co-complexation catalyst is selective toward crotonaldehyde, suggesting that this reaction pathway is favored on the PtRu sites.     

助剂改性对Ni-Cu/γ-Al_2O_3催化剂催化月桂腈加氢性能的影响

对浸渍法制备的负载型Ni-Cu/γ-Al_2O_3催化荆,用Na和Cr进行酸碱性调节,并通过H_2,NH_3和CO_2程序升温脱附(TPD)技术表征了催化剂H_2吸附能力和酸碱性质.结果显示,Cr和Na改性后催化剂的酸碱性质得到了调节,H_2吸附能力显著增强,其中Cr与Na联合改性后的样品Ni-Cu-Cr-Na/γ-Al_2O_3具有最小的低强度酸量、最大的低强度碱量和最大的H_2吸附能力.催化剂月桂腈加氢性能表明,在氢分压2.0 MPa、反应温度70℃、反应时间30 min时,Ni-Cu-Cr-Na/γ-Al_2O_3催化剂具有最大活性,月桂腈的转化率为98%,月桂伯胺的选择性为99.2%.催化剂稳定性好,重复使用10次后,月桂腈的转化率由98%降为94.6%,月桂伯胺的选择性仍维持在98%以上.     

Ni-Cu/g-Al2O3催化剂上月桂腈的加氢性能

用Cr和Na对浸渍法制备的g-Al2O3负载Ni-Cu催化剂进行改性,用H2-TPD和CO2-TPD技术表征催化剂H2吸附能力和碱性质,并考察了其对月桂腈加氢反应的催化性能. 结果表明,Cr和Na改性后催化剂H2吸附能力显著增强,碱性质得到调节,Ni-Cu-Cr-Na/g-Al2O3催化剂具有最大H2吸附能力和最大低强度碱量及最大活性,月桂腈转化率为98%,伯胺选择性为99.2%. 优化反应条件为：氢分压2.0 MPa,反应温度70℃,反应时间30 min,搅拌转速600 r/min. 反应10次后,月桂腈的转化率由98%降为94.8%,伯胺的选择性维持在98%以上.     

The Nature of the Catalytic Sites for H2 Dissociation

Scanning Tunneling Microscopy (STM) can reveal the nature of active sites on the surface of heterogeneous catalysts. This is shown for the case of the dissociation of molecular hydrogen on Pd(111), which has been studied recently both experimentally and theoretically. STM can image in real time to generate movies of adsorbed atoms diffusing on the catalyst surface and forming aggregates. Of particular interest is the behavior near saturation coverage, a situation that is common when catalysts operate under the gas pressures typical of many industrial reactions. Under these conditions, active catalyst sites are formed as a result of density fluctuations that free atoms at the catalyst surface of adsorbates, so that they become available for new reactions. Little is known about the structure of the sites generated in this process. While the end state of a dissociative adsorption of a diatomic molecule requires at least two empty sites to accommodate the reaction products, the initial state where the molecule adsorbs and dissociates, might be more complicated and its nature is unknown. The review shows how STM can provide an improved understanding of the nature of these initial sites.     

Heterogeneous Catalytic Oxidation of Acrolein to Acrylic Acid: Mechanism and Catalysts

Partial oxidation of acrolein is a commercially important reaction, its product—acrylic acid—being widely used industrially for producing resins, dyes, glues, nonwoven fabrics, etc.

Partial oxidation of acrolein is also a convenient model reaction because: (1) the number of reaction products is moderate (CO, CO₂, acrylic acid) and (2) their difference in acid-base properties from the starting material makes it possible to select desirable catalysts by applying directly and efficiently Boreskov's concept of intermediate chemical interaction of a catalyst with reaction mixture components. According to this concept [1], the transformation of surface intermediates (SI) formed in the interaction of reactants with a catalyst's surface is determined by the structure and bond energy of these SI.

The study of the reaction mechanism includes determination of structures and energy characteristics of the surface intermediates and the elucidation of their connection with catalyst chemical composition and reaction routes to particular products. This reliable information helps us to understand the nature of catalyst action and to elaborate the theory of catalyst selection. We have used this method to approach the problem of the systematic selection of catalysts for the oxidation of acrolein to acrylic acid. The review summarizes the research done in the lnstitute of Catalysis of the Siberian Branch of the Russian Academy of Sciences during recent years.     

A Facile and Effective Method for the Distribution of Mo/HZSM-5 Catalyst Active Centers

Post-steaming treatment of Mo/HZSM-5 catalysts results in more molybdenum species migrating into and residing in the HZSM-5 zeolite channels. This is confirmed by XRF and XPS measurements. ¹H MAS NMR and ²⁹Si MAS NMR also demonstrate that the number of free Brönsted acid sites decreases in the Mo/HZSM-5 catalysts that underwent post-steaming treatment, compared to untreated Mo/HZSM-5 catalysts. As a result, the deactivation rate constant (k _d) on the Mo/HZSM-5 catalyst after post-steaming treatment for 0.5h is much smaller, and the catalyst therefore shows remarkable stability in the probe reaction of methane dehydro-aromatization. The results suggest that a more beneficial bi-functional balance between active Mo species for methane activation and acid sites for the following aromatization is developed over those Mo/HZSM-5 catalysts that have experienced post-steaming treatment for 0.5h, in comparison with the untreated Mo/HZSM-5 catalysts.     

ISOMERIZATION OF N-BUTANE: CONVERSION AS A FUNCTION OF SULFATE LOADING ON SULFATED ZIRCONIA CATALYSTS

Sulfur loading on a sulfated zirconia catalyst was studied as a function of H 2 SO 4 concentration. The resulting catalytic activity was tested using the isomerization of n -butane as a reaction probe at 200°C. Optimum catalytic activity was observed for a catalyst prepared using 0.5 N H 2 SO 4 . The amount of carbon required to completely deactivate the catalyst was determined using a TGA/FTIR technique. For all of the catalysts synthesized in this study it was found that two active sulfate sites were poisoned by a single deposited carbon atom. Using this information, an active site composed of two coupled sulfate sites, perhaps a surface pyrosulfate site, is suggested. Using a TGA/FTIR technique it was found that the number of catalytically active sulfate sites never exceeded 14% of the total sulfate loading. These results were also supported by calorimetric studies.     

Advance in research on reaction mechanism and active sites for methanol synthesis

甲醇不仅是一种重要的有机化工原料,而且可以单独或与汽油混合作为汽车燃料,具有节能与环保的双重优势。甲醇合成反应研究尽管有四十多年的历史,但有关甲醇合成反应机理及催化剂活性中心类型等方面的问题仍存在争议。本文综述了近几年来在铜基催化剂上甲醇合成催化反应机理及催化剂活性中心的研究进展,以期提高人们对甲醇合成反应催化本质的认识,并为优良催化剂的开发提供一定借鉴。     

正十二烷在Pt/ZSM-22和Pt/ZSM-23上的加氢异构反应性能

采用常规水热合成法合成了ZSM-22和ZSM-23分子筛,进而制备了分别含有上述分子筛的催化剂,并借助XRD、SEM、NH3-TPD和Py-IR表征了这两种分子筛和催化剂的结构和酸性,同时以正十二烷为模型化合物,采用固定床反应器研究了Pt/ZSM-22和Pt/ZSM-23催化剂上正十二烷加氢异构反应性能。结果表明,在这种模型反应基础上,催化剂的反应活性和选择性主要取决于催化剂的酸量和酸强度以及酸分布,相对而言,ZSM-22分子筛催化剂由于其弱酸和中等强度酸的含量较高,具有更佳的异构化选择性。     

The nature of the active sites in cumene cracking on amorphous silica aluminas

We present evidence which indicates that the nature of the active sites is the same for amorphous catalysts as it is for crystalline alumina silicate cracking catalysts. The only differences we find in the various cracking catalysts lie in the total concentrations of active sites and in the ratio of Bronsted to Lewis sites. In view of this, and because even zeolites have only a fraction of their cationic sites active in cumene cracking, we suggest that it is possible that a silica alumina catalyst could exist that is more active by as much as an order of magnitude than any catalyst presently available for cumene cracking.     

Isomerization of n-butane: Conversion as a function of sulfate loading on sulfated zirconia catalysts

Sulfur loading on a sulfated zirconia catalyst was studied as a function of H 2 SO 4 concentration. The resulting catalytic activity was tested using the isomerization of n -butane as a reaction probe at 200°C. Optimum catalytic activity was observed for a catalyst prepared using 0.5 N H 2 SO 4 . The amount of carbon required to completely deactivate the catalyst was determined using a TGA/FTIR technique. For all of the catalysts synthesized in this study it was found that two active sulfate sites were poisoned by a single deposited carbon atom. Using this information, an active site composed of two coupled sulfate sites, perhaps a surface pyrosulfate site, is suggested. Using a TGA/FTIR technique it was found that the number of catalytically active sulfate sites never exceeded 14% of the total sulfate loading. These results were also supported by calorimetric studies.     

The impact of surface science on the commercialization of chemical processes

Surface science developed instruments for atomic- and molecular-scale studies of catalyst surfaces, their composition and structure, both in a vacuum and at high pressures, under reaction conditions (bridging the pressure gap). Surfaces ranging from single crystals, nanoparticles and thin films to porous high surface area catalytic materials have been studied. Classes of surface structure sensitive and insensitive reactions have been identified by surface science studies, including ammonia synthesis, hydrodesulfurization, reforming, combustion and hydrogenation. Rates of reactions often vary by orders of magnitude between using the right and the wrong surface structures. The roles of many promoters that modify the catalyst surface structures and bonding of adsorbates have been verified. Surface reaction intermediates could be identified and the mobility of adsorbates and the adsorbate induced reconstruction of the catalysts attest to the dynamic nature of the catalytic systems during the reaction turnover. The important active sites for catalysis include the low coordination surface step, kink, oxygen and chloride ion vacancies sites and sites at oxide-metal interfaces. Uncovering the molecular ingredients of heterogeneous catalysts will have a major impact on the understanding of reaction selectivity to help the evolution of green chemistry and selective reaction of many types.     

MgO/APT催化剂酸碱性质对1,4-丁二醇单醋酸酯转化反应的影响

以Mg(NO3)2为镁源,采用等体积浸渍法制备系列镁改性凹凸棒土催化剂Mg O/APT,利用XRD、BET、FT-IR和TPD对催化剂进行表征。结果表明,Mg O/APT催化剂具有与凹凸棒土相同的晶相结构;随着Mg O负载量的增加,Mg O/APT的碱强度和碱含量显著增加,较强酸量明显减少,弱酸酸位增加。在固定床反应装置上,考察催化剂气相催化转化1,4-丁二醇单醋酸酯反应性能。结果表明,催化剂表面的酸碱性对1,4-丁二醇单醋酸酯气相转化反应的产物分布有显著影响。酸性为主的催化剂APT上有利于1,4-丁二醇单醋酸酯发生分子内酯交换环化反应生成四氢呋喃;而适量酸-碱活性位有助于实现1,4-丁二醇单醋酸酯发生脱水-水解反应,生成3-丁烯-1-醇及3-丁烯-1-醇醋酸酯,过多碱性位有助于1,4-丁二醇单醋酸酯发生酯水解反应生成1,4-丁二醇。     

镁基固体催化剂催化酯交换反应制备生物柴油研究进展

生物柴油是一种重要的可再生能源,非均相催化甘油酯酯交换反应制备生物柴油是当前的研究热点,而高活性的非均相催化剂则是非均相催化工艺的核心。镁基催化剂由于其原料来源广泛、价格低廉,受到了广泛关注。综述近年来镁基催化剂催化酯交换反应制备生物柴油的研究进展,重点介绍了镁基催化剂制备方法、组成、结构、反应条件等对催化酯交换反应活性的影响,并探讨了镁基催化剂目前存在的不足以及今后的发展方向。     

丙烷芳构化催化剂活性组分浸渍方法优化

根据现有丙烷芳构化催化剂的特性,考察等体积浸渍、真空等体积浸渍、微波等体积浸渍3种不同浸渍方法对镓改性HZSM-5催化剂性能的影响,并结合多种技术表征,分析改性后催化剂内部性能变化情况。研究发现,浸渍方法对催化剂表面镓的分散度具有明显的影响,其中真空等体积浸渍具有更好的效果,使活性组分Ga分散更均匀,更有效地降低了分子筛的B酸位、增加了L酸位,进而抑制了丙烷的裂解,增强了催化剂的脱氢、芳构化能力,并且延长了催化剂的寿命。在反应温度为525℃,丙烷质量空速（WHSV）为1 h^-1的条件下,取样时间为丙烷通入后1.3 h,丙烷转化率达到93.8%,液相芳烃收率达到57.2%,BTX（苯、甲苯和二甲苯）收率达到48.6%。     

Platinum supported on carbon aerogels as catalysts for the n-hexane aromatization

A series of Pt-catalysts were prepared by impregnation of carbon aerogels. The supports were selected on the basis of a different porous texture. The dispersion and sintering resistance of the catalyst were evaluated by varying the Pt-loading and the pre-treatment conditions. XRD, XPS, HRTEM and H₂-chemisorption were used to determine the nature and dispersion of Pt-particles. The behavior of these catalysts on the n-hexane conversion was analyzed in different experimental conditions. The performance of catalysts was related to their porous, chemical and dispersion characteristics of the samples. Monofunctional catalysts were obtained in all cases. Isomerisation and cracking take place on the same active sites whereas the aromatization requires different sites. The selectivities for isomerization and hydrogenolysis change with the advance of the reaction while that for aromatization remains constant. Deactivation is related with the loss of the active sites responsible for the hydrogenolysis reactions.     

Microemulsion Prepared Pt in Ceria: Catalytically Active for Water Gas Shift Reaction but Totally Inert for Methanation

A core–shell Pt in ceria type of catalyst prepared by a microemulsion technique shows high activity for both water gas shift and reverse water gas shift reactions but it does not catalyze methanation from carbon oxides and hydrogen feeds at all, whereas Pt-ceria catalysts prepared by traditional methods are effective for all these reactions under comparable reaction conditions. This clearly reveals the fact that the nature of active sites for shift reactions is different from those of methanation reactions, where a special metal-support interface in catalysts created by microemulsion offers the differentiation.     

Advancements in Heterogeneous Catalysis for Biodiesel Synthesis

Heterogeneous catalysts are promising for the transesterification reaction of vegetable oils to produce biodiesel and have been studied intensively over the last decade. Unlike the homogeneous catalysts, heterogeneous catalysts can be easily separated from reaction mixture and reused for many times. They are environmentally benign and could be easily operated in continuous processes. This review classifies the solid catalysts into two categories based on their catalytic temperature, i.e. high temperature catalysts and low temperature catalysts. The nature of the catalysts can be specified into solid bases and solid acids. Three aspects, catalyst activity, catalyst life and oil flexibility, will be reviewed. Two kinds of heterogeneous catalysts, reported by IFP Inc. and by WSU, respectively, show a high catalytic activity, long catalyst life and low leaching of catalyst components. These two catalysts also show ability to simultaneously catalyze esterification and transesterification, and can be used in half-refined or crude oil system which provide a potential for greatly decrease the feedstock cost.