Model Systems in Heterogeneous Catalysis: Selectivity Studies at the Atomic Level

Reaching 100% selectivity is the theme of the twentyfirst century in heterogeneous and heterogenized homogeneous catalysis. To study chemical reactivity at the atomic scale, model systems have been prepared and characterized. We discuss selectivity of hydrogenation and dehydrogenation reactions on supported Pd particles, methanol oxidation at vanadium oxide model catalysts and, at last, the design of model catalysts with a well defined charge state of the metal, i.e., Au catalyst model systems.     

固载杂多酸催化剂PW_12／SiO_2的制备及其对酯化合成乳酸正丁酯的催化作用

研制了固载杂多酸催化剂ＰＷ１２／ＳｉＯ２，并用其复相催化酯化合成乳酸正丁酯，发现该固载杂多酸催化剂对酯化合成乳酸正丁酯具有催化活性高、催化剂容易回收并可多次重复使用等优点。讨论了催化剂活化温度、用量及重复使用次数对催化剂活性的影响     

A Quantitative Approach to the Ensemble Model of Catalysis by Metals

The concept of ensembles is based on the idea that adsorbed species and intermediates in catalytic reactions require more than one surface atom. This model can probably be traced to the mul-tiplet theory of Balandin [1], who, as early as 1929, made the assumption that a reacting molecule can be simultaneously adsorbed on several atoms.     

A Quantitative Approach to the Ensemble Model of Catalysis by Metals

The concept of ensembles is based on the idea that adsorbed species and intermediates in catalytic reactions require more than one surface atom. This model can probably be traced to the mul-tiplet theory of Balandin [1], who, as early as 1929, made the assumption that a reacting molecule can be simultaneously adsorbed on several atoms.     

Crossing the Borders Between Homogeneous and Heterogeneous Catalysis: Developing Recoverable and Reusable Catalytic Systems

Homogeneous and heterogeneous catalysis have developed independently as two separate disciplines. Homogeneous catalysis is characterized by the use of highly active, well-characterized compounds. In contrast, heterogeneous catalysis exhibits the advantage of easy separation of the catalyst from the products and can be easily adapted to continuous flow processes. In recent years, there is an emerging trend in catalysis that is bridging these two fields. On one hand, some of the complexes used in homogeneous catalysis are really precursors of nanoparticles that are species that have been traditionally subject of study in heterogeneous catalysis. On the other hand, the use of novel media allows the recovery and reuse of homogeneous catalysts, a hallmark of heterogeneous catalysis. Also, powerful experimental techniques can characterize the active sites in solids up to a much higher level of detail. In this review, we have selected two reaction types that are attracting much current interest, namely the enantioselective addition to aldehydes by chiral metallosalen complexes and the palladium catalyzed C–C cross coupling, and used them to illustrate a series of strategies based on new concepts that can serve to impart the advantages of homogeneous catalysis into heterogeneous catalysis and vice versa.     

Pd/SiO2 as Heterogeneous Catalyst for the Heck Reaction: Evidence for a Sensitivity to the Surface Structure of Supported Particles

The contact of Pd(AcO)₂ in solution with thiouric tethered to a silica led to the formation of supported nanoparticles (ca. 2 nm in size) active in the Heck reaction, quite sensitive to the palladium leaching. A stable supported phase was obtained by subsequent calcination, but the catalytic activity resulted strongly dependent on the calcination temperature. Such a behaviour was related to differences in the surface structure of the supported particles, as those monitored by IR spectroscopy of CO adsorbed on catalysts reduced at various temperatures.     

Oxidation of Cyclohexane with Molecular Oxygen Catalyzed by SiO2 Supported Palladium Catalysts

The di(ethylthio)ethane palladium (A) and di(ethylthio)propane palladium (B) complexes have been immobilized on the surface of silica gel and these catalyst systems are shown to serve as effective supported Pd catalysts for cyclohexane oxidation with O₂ under solvent free condition. The supported catalyst A provides best results (overall ca. 16.2% conversion, with a selectivity of 72 or 24% for cyclohexanol or cyclohexanone, respectively, which are further promoted in the presence of 2-pyrazinecarboxylic acid which acts as a co-catalyst (overall ca. 21.2% conversion).     

CA对Co-Mo/SiO_2加氢脱硫性能的影响

以柱层层析硅胶为原料,添加粘合剂制备了大孔SiO2载体,用等体积浸渍法制备了CoMo-CA/SiO2催化剂(CoO,MoO3质量分数分别为3%,12%,n(CA):n(MoO3)=1:1)。利用BET和XRD技术进行物化性能表征,以噻吩硫脱除率考察了柠檬酸(CA)对催化剂加氢脱硫性能的影响。结果表明:制备的大孔SiO2载体比表面积380m2/g,孔容0.39cm3/g,中孔(2~50nm)占据90.2%;添加CA后,活性组分均匀分散在载体表面,XRD谱无CoO和MoO3特征峰;在T为300℃、P为3.0MPa、LHSV为2h-1及氢油体积比为600时对噻吩硫脱除率达到96%以上。     

硅胶负载交联壳聚糖树脂的制备及吸附性能

以壳聚糖、硅胶为主要原料制备了一种硅胶负载交联壳聚糖树脂,采用红外光谱(IR)、X-衍射(XRD)和扫描电镜(SEM)对其进行结构性能表征,并研究了树脂对金、银离子的吸附行为。重点考察了交联剂用量、介质pH值和初始离子浓度等因素对树脂材料吸附率的影响,试验结果表明,该树脂对金银离子的吸附率均大于90%。对于金离子的吸附而言,控制pH值在4,吸附时间90 min时吸附率最高;对于银离子的吸附而言,控制pH值在4~6,吸附时间100 min时吸附率最高。离子初始质量浓度对树脂材料的吸附率影响不大。     

Model Apatite Systems for the Stabilization of Toxic Metals: I, Calcium Lead Vanadate

Apatites of composition (Ca_{10− x} Pb _x )(VO₄)₆F₂ with 0 ≤ x ≤ 9 were synthesized by sintering in air at 800°C the oxides of calcium, lead, and vanadium that were mixed with calcium fluoride. Variations in lattice parameters, grain size, and durability were monitored as a function of composition. The expected dilation in unit cell edges with increasing replacement of calcium (IR = 1.26 Å) by lead (IR = 1.45 Å) was observed. The a cell edge (in angstroms) varied linearly (0.0458 x + 9.7068) with x , while c followed the binomial function 0.0018 x ²+ 0.0279 x + 7.0118. Average grain sizes increased from approximately 5 μm ( x = 0) to 25 μm ( x = 9) as grain growth was enhanced due to incipient melting of the lead-rich compositions. For x < 5 the materials were biphasic. Overall dissolution, as determined by the toxicity characteristic leaching procedure (TCLP), was faster in calcium-rich apatites. Etching of well-crystallized apatite grains occurred preferentially on {10 1 0} faces. These leach losses were augmented through the more rapid removal of cryptocrystalline regions and intergranular thin films.     

Selective Nanocatalysis of Organic Transformation by Metals: Concepts, Model Systems, and Instruments

Monodispersed transition metal (Pt, Rh, Pd) nanoparticles (NP) in the 0.8–15 nm range have been synthesized and are being used to probe catalytic selectivity in multipath organic transformation reactions. For NP systems, the turnover rates and product distributions depend on their size, shape, oxidation states, and their composition in case of bimetallic NP systems. Dendrimer-supported platinum and rhodium NPs of less than 2 nm diameter usually have high oxidation states and can be utilized for catalytic cyclization and hydroformylation reactions which previously were produced only by homogeneous catalysis. Transition metal nanoparticles in metal core (Pt, Co)––inorganic shell (SiO₂) structure exhibit exceptional thermal stability and are well-suited to perform catalytic reactions at high temperatures (>400 °C). Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules. The facile rearrangement of NP catalysts required for catalytic turnover makes nanoparticle systems (heterogeneous, homogeneous and enzyme) excellent catalysts and provides opportunities to develop hybrid heterogeneous-homogeneous, heterogeneous-enzyme and homogeneous-enzyme catalyst systems.     

Infrared studies of reaction of ethylene with syngas on Ni/SiO2

Ni/SiO₂, a methanation catalyst, has been shown to exhibit CO insertion activity. In situ infrared studies of CO/H₂ and C₂H₄/CO/H₂ reactions on Ni/SiO₂ show that carbonylation of Ni/SiO₂ to Ni(CO)₄ leads to an inhibition of methanation in CO hydrogenation but an enhancement of formation of propionaldehyde in the C₂H₄/CO/H₂ reaction.     

An Improved Model for the Extraction of Multivalent Metals in Winsor II Microemulsion Systems

Abstract

Our earlier paper described a liquid-liquid extraction process for metal ions using a Winsor I1 microemulsion system (I). A Winsor I1 system is a biphasic system in which a water-in-oil microemulsion exists in equilibrium with a lower predominately aqueous phase (2). Complete details of the system, the experiments, and the modeling efforts have been given in our earlier work (1). Hence, to avoid redundancy, only the salient features are discussed here as a prelude to dealing with the extension of the model.     

活性炭负载磷钨酸催化合成丁酸丁酯的研究

以正丁醇和正丁酸为原料,用自制的PW12／C催化剂催化合成丁酸丁酯。通过考查合成丁酸丁酯的各影响因素,得出其最佳实验条件：磷钨酸负载量为21％左右,醇酸摩尔比为1．5：1,反应时间2．5h,催化剂用量为酸质量的2％,带水剂甲苯10111l,其酯化率可达94．5％。并对催化剂重复使用效果进行了实验,催化剂重复使用至第6次,其酯化率仍可达到90．7％。该法与传统浓硫酸作催化剂相比,具有催化剂用量少,活性高,不腐蚀设备,易分离,可重复使用等优点。     

Model Apatite Systems for the Stabilization of Toxic Metals: II, Cation and Metalloid Substitutions in Chlorapatites

Environmental chlorapatites, of the form A₁₀(B _x C_{1− x} O₄)₆Cl₂, where A=Ca, Cd, or Pb; B/C=P, V, or Cr and 0≤ x ≤1, were synthesized by solid-state reactions. Crystal structure variations, primarily lattice parameters, were examined through powder X-ray diffraction methods, while performance and long-term durability of these waste form apatites were assessed by a combination of the toxicity characteristic leaching procedure (TCLP) and the American Nuclear Society (ANS) method. As expected, an overall dilation of unit cell edges was observed with increasing ionic substitutions. However, a discontinuity in linearity was often observed, generally for x ≥0.5. The correlation of microstructural changes and leach testing shows that lead chlorapatite is found to be more suitable for waste stabilization than calcium and cadmium analogues.     

VO(acac)2 Supported on Titania: A Heterogeneous Protocol for the Selective Oxidation of Sulfides Using TBHP

The facile catalytic oxidation of sulfides to sulfoxides has been achieved with VO(acac)₂ supported on titania using tert-butyl hydroperoxide (TBHP) as an oxidant in dichloromethane in quantitative yields at room temperature. The catalyst is reusable and tested for five cycles without significant loss of activity and selectivity.     

Experimental phase equilibria studies of the PbO–SiO2 system

Phase equilibria of the PbO–SiO₂ system have been established for a wide range of compositions: (i) liquid in equilibrium with silica polymorphs (quartz, tridymite, and cristobalite) between 740°C and 1580°C, at 60‐90 mol% SiO₂; (ii) with lead silicates (PbSiO₃, Pb₂SiO₄, and Pb₁₁Si₃O₁₇) and lead oxide (PbO) between 700°C and 810°C. A high‐temperature equilibration/quenching/electron probe X‐ray microanalysis (EPMA) technique has been used to accurately determine the compositions of the phases in equilibrium in the system. Significantly, no liquid immiscibility has been found in the high‐silica range, and the liquidus in this high‐silica region has been accurately measured. The phase equilibria information in the PbO–SiO₂ system is of practical importance for the improvement of the existing thermodynamic database of lead‐containing slag systems (Pb–Zn–Fe–Cu–Si–Ca–Al–Mg–O).