The effect of pH on the hydrogenation of benzene in an aqueous biphase using a ruthenium catalyst

There is currently much controversy surrounding homogeneous arene hydrogenation catalysis since it has been suggested that the active catalysts are actually heterogeneous. Since many of the proposed homogeneous catalysts operate in water under biphasic conditions we have studied the effect of pH on a mononuclear ruthenium catalyst precursor. As the pH increases, turnover frequency also increases, which corresponds to an increase in nanoparticle formation. The method could be used as a screen to help assess homogeneous versus heterogeneous catalysis.     

Kinetic modeling of homogeneous catalytic processes

Homogeneous catalysis by soluble metal complexes is gaining considerable attention due to their unique applications and features like high activity and selectivity. In this paper, a brief review of kinetic modeling in homogeneous catalysis has been presented. Approaches using empirical as well as molecular level rate models have been discussed. Special features relevant to asymmetric catalysis and multiple rate controlling steps have also been addressed. A case study on kinetics of carbonylation of 1-(4-iso-butylphenyl)ethanol using a homogeneous palladium catalyst has been discussed.     

Supported ionic liquids: versatile reaction and separation media

The latest developments in supported ionic liquid phase (SILP) systems for catalysis and separation technology are surveyed. The SILP concept combines the advantages of homogeneous catalysis with heterogeneous process technology, and a variety of reactions have been studied where supported ionic liquid catalysts proved to be more active and selective than common systems. In separation applications the use of supported ionic liquids can facilitate selective transport of substrates across membranes.     

Mechanisms of hydrocarbon conversion reactions on heterogeneous catalysts: analogies with organometallic chemistry

Catalysis is central to most industrial processes for chemical manufacturing. As catalytic processes have become more complex and more demanding, selectivity has become the central issue in their design. Selectivity is defined by the relative rates of competing reaction pathways available to crucial intermediates, and can be controlled by subtle changes in the nature of the catalyst, the reactants, and/or the reaction conditions. In order to be able to do this in a systematic manner, a good understanding of the catalytic reaction mechanisms is needed. Here a connection is drawn between the key elementary steps comprising hydrocarbon conversion reactions on surfaces and those known to occur on discrete organometallic complexes. This way, the hydrogenation, dehydrogenation, hydrogenolysis, chain growth, and isomerization reactions typical in heterogeneous catalysis are redefined in terms of hydride elimination, oxidative addition, reductive elimination, migratory insertion, and 1, 2-shift elementary steps, among others. It is suggested that the knowledge already available from organometallic chemistry can be used to further advance the understanding of the surface science involved in heterogeneous catalysis. Thanks to the commonality of the chemistry involved, a better synergy could also be established between homogeneous and heterogeneous catalytic development. These ideas are discussed in this article in a critical and personal way.*Invited contribution to the special volume entitled The Interface between Heterogeneous and Homogeneous Catalysis, stemming from contributions at the recent International Symposium on Relations between Heterogeneous and Homogeneous Catalysis, and dedicated to the memory of Robert L. Burwell.     

Reaction Kinetics of Soybean Oil Transesterification Using Heterogeneous Metal Oxide Catalysts

Homogeneous acid or base catalysts dissolve fully in the glycerol layer and partially in the fatty acid methyl ester (biodiesel) layer in the triglyceride transesterification process. Heterogeneous (solid) catalysts, on the other hand, can prevent catalyst contamination making product separation much simpler. In the present work, the transesterification kinetics of five different solid catalysts with soybean oil is presented. It is found that heterogeneous catalysts require much higher temperatures and pressures to achieve acceptable conversion levels compared to homogeneous catalysts. Subsequent to preliminary investigations, transesterifications were conducted for selected high performance solid catalysts, i.e., MgO, CaO, BaO, PbO, and MnO₂ in a high pressure reactor up to a temperature of 215 °C. The yield of the fatty acid methyl esters and the kinetics (rate constant and order) of the reaction are estimated and are compared for each catalyst.     

“HETEROGENIZING” HOMOGENEOUS CATALYSTS

For many years, it has been customary to classify catalysts as “homogeneous” or “heterogeneous.” The former commonly operate through the formation of “intermediate compounds,” and the latter, by adsorption of the reactants on the catalyst surface. The line between the two is a fine one, for the distinction between adsorption and compound formation is not at all clear, and seems to be becoming less and less clear as we learn more about adsorption. In recent years, several writers [l-7] have stressed the point that there is a good deal of overlap between homogeneous and heterogeneous catalysis. Experimental evidence supporting this point of view is accumulating, and while we are not prepared to say that there is no distinction, we can say with certainty that many homogeneous catalysts can be converted into heterogeneous ones, retaining the advantages of great activity and selectivity inherent in homogeneity and, at the same time, assuming the ready recovery which is the great advantage of heterogenity. In practice, of course, the matter is not quite that simple, for other factors must be considered. On the whole, however, many advantages have been found in the use of heterogenized homogeneous catalysts.     

Reverse flow adsorption: integrating the recovery and recycling of homogeneous catalysts

Homogeneous catalysts are not applied often when compared to the use of heterogeneous catalysis due to various drawbacks of the usual recovery methods. In this paper, a novel concept is proposed for the integrated recovery and recycling of homogeneous catalysts. It combines an adsorptive separation with the reverse flow technology: reverse flow adsorption. Na⁺ loaded Amberlyst 15 proved to be one of several suitable adsorbents for the reversible adsorption of homogeneous catalysts. A simplified plug flow model for packed beds showed that the implementation of the reverse flow adsorption concept into an oxo-synthesis process would only require two adsorption beds with a very small relative volume of only 1% compared to the volume of liquid reaction phase in the reactor. The adsorption temperature, which was chosen to be equal to the reaction temperature, was well within the stability constraints of the homogeneous catalyst and therefore, decomposition of the catalyst is not expected. Reverse flow adsorption is a promising concept that overcomes the drawbacks of the standard recovery methods and therefore, has a high potential to succeed.     

Polymerization of ethylene: Some aspects of metallocene catalyst stabilization under homogeneous and heterogeneous reaction conditions

The development of metallocene‐based catalysts is an important advance on the study of polyolefinic materials. However, due to the rather different conditions that are established in actual applications, only around 3% of these polymers are obtained from metallocene technology. In view of this, novel strategies must be developed to produce metallocene‐based catalysts that are more thermally stable, which is a fundamental requirement to establish metallocene technologies. Homogeneous and heterogeneous polymerizations of ethylene were compared, using the Ph₂C(Cp)(Flu)ZrCl₂/MAO system. Homogeneous polymerizations were more active than the corresponding supported reactions. At low ethylene pressure, the addition of 1‐hexene increases the activity under homogeneous conditions. Nevertheless, this is not observed on the respective supported systems. At higher pressure conditions, all polymerizations attained higher yields. However, when the reaction temperature increases the activity significantly decreases under homogeneous conditions. Furthermore, when the polymerization was performed under heterogeneous conditions the deactivation was lower. The homogeneous and supported catalytic systems show different characteristics and, in all attempted reactions, immobilization of the molecular catalyst reduces the activity. However, the deactivation ratio was lower when the polymerization was performed under heterogeneous conditions. This means that immobilization of Ph₂C(Cp)(Flu)ZrCl₂ on silica can improve the thermal stability of the catalytic species. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Easily separable molecular catalysis

Homogeneous metal catalysts offer greater selectivity and controllability because their molecular nature ensures that only one type of active site is present. However, the majority of large-scale, industrial chemical processes employs heterogeneous catalysts because of ease of their separation from products, thermal stability, and amenability to continuous processing. Heterogenization of homogeneous catalysts explores the cross-fertilization of two systems to combine most of their advantages. Some examples are discussed of heterogenized palladium complex catalysts applied for low-temperature CO oxidation, carbonylation for the synthesis of ibuprofen or naproxen, oxidative carbonylation for the synthesis of diphenyl carbonate, and functionalization of methane. There appears no significant change in reaction mechanisms when active centers are transferred from solution to the solid surface. In addition to the easy recovery, heterogenized homogeneous catalysts can have advantages in the productivity over homogeneous catalysts when a suitable support material is selected considering its hydrophilicity and surface reactivity.     

醇类选择性催化氧化的研究进展

醇类选择性氧化制备相应羰基（醛或酮）化合物是有机合成中的重要反应。综述醇类选择性催化氧化的研究现状,主要有液均相氧化、液多相氧化和水/有机两相催化氧化,对所用催化剂发展状况和反应机理分别进行阐述。均相催化氧化催化剂难于从反应体系分离,造成成本过高,而且污染环境。大部分多相催化剂来自均相催化剂的负载,活性中心分布不均匀,结构不明确,存在活性组分易从载体上脱落和流失的现象,导致催化剂活性下降。以水作溶剂,不仅清洁无污染,且产物和催化剂容易分离,催化剂可以循环使用,从经济和环保角度值得大力推广,但该体系价格昂贵,反应条件不够温和,还需进一步改进。因此,多相催化氧化和水/有机两相催化氧化相对于均相催化剂有更广阔的发展和应用空间,是今后的研究方向。     

Anwendungsmöglichkeiten der homogenen Übergangsmetallkatalyse in der Fettchemie

Homogeneous Transition Metal Catalysis in Oleochemistry Numerous processes in industrial oleochemistry are carried out using transition metal catalysts. However, in most cases the catalysts applied are heterogeneous. The present review considers the huge number of possible applications of homogeneous transition metal catalysis in oleochemistry. The advantages of homogeneous transition metal catalysis are the high selectivity and the mild reaction conditions. These advantages are often used in petrochemistry, but in industrial oleochemistry exist no similar developments. The results of the laboratory research in the last two decades prove that the special properties of homogeneous transition metal catalysts can be applied in oleochemistry too. Well known chemical reactions could be improved and new reactions could be developed. The present review refers some of the most interesting reaction types, e. g. the selective hydrogenation of manifold unsaturated fatty acids or the possibilities to isomerize the double bonds selectively. Oxidation reactions, reactions with carbon monoxide, metathesis, CC-linkage reactions, telomerizations and additions to the double bonds of unsaturated fatty compounds will also be presented.     

π-Complex Intermediates in Homogeneous and Heterogeneous Catalytic Exchange Reactions of Hydrocarbons and Derivatives with Metals

The exchange of unsaturated and aromatic hydrocarbons and their derivatives with isotopic hydrogen on group VIII transition metals has been reviewed. Both classical and π-complex heterogeneous mechanisms proposed to account for the observed behavior have been compared. Evidence to distinguish between the application of the heterogeneous associative and dissociative π-complex exchange mechanisms is discussed. The recently discovered homogeneous equivalent of the above heterogeneous reactions is described in detail. Homogeneous associative and dissociative π-complex mechanisms are proposed to explain aromatic hydrogen exchange. Mechanisms are also proposed to explain the pattern of exchange observed homogeneously with platinum in the alkyl groups of alkyl aromatics. Possible mechanistic relationships between heterogeneous and homogeneous systems are considered using as representative unsaturated and aromatic series, the polycyclic hydrocarbons, the monohalogenated benzenes, the alkyl benzenes (and relevant simple alkanes), steroids, and synthetic hormones. The role of π-complexes in relating the present heterogeneous and homogeneous exchange reactions to the chemistry of inorganic coordination complexes is also considered.     

Catalysis by molecular design

The possibilities of molecular design in the development of new catalysts and catalytic technologies are discussed with the data obtained recently at the Boreskov Institute of Catalysis as particular examples. Examples from the following areas are presented: homogeneous catalysis with metal complexes, heterogeneous catalysis with anchored metal complexes, heterogeneous catalysis with catalysts prepared via anchored metal complexes and organometallics, catalysis of olefin polymerization, catalysis by metals, catalysis by oxides, catalysis by zeolites, catalysis by heteropolyacids, catalysis with nontraditional oxidants and biomimetic catalysis.     

富勒烯类炭材料作为新型载体在多相催化中的应用

评述了富勒烯类新型炭材料作为金属催化剂的载体在多相催化领域中的潜在应用途径。首先概要介绍这些炭材料在均相和多相催化中一些应用实例,然后分别详细分绍C60分子化合物、单壁纳米炭管和多壁纳米炭管等富勒烯类新型炭材料的催化性能及其应用。     

The Clicked Pyridyl‐Triazole Ligand: From Homogeneous to Robust,Recyclable Heterogeneous Mono‐ and Polymetallic Palladium Catalysts for Efficient Suzuki–Miyaura,Sonogashira, and Heck Reactions

Various mono‐ and polymetallic palladium complexes containing a 2‐pyridyl‐1,2,3‐triazole (pyta) ligand or a nonabranch‐derived (nonapyta) ligand have been synthesized by reaction of palladium acetate with these ligands according to a 1:1 metal‐ligand stoichiometry and used as catalysts for carbon‐carbon cross‐coupling including the Suzuki–Miyaura, Sonogashira and Heck reactions. The unsubstituted monopalladium and nonapalladium complexes were insoluble in all the reaction media, whereas tri‐ and tetranuclar palladium complexes were soluble, which allowed conducting catalysis under either homogeneous or heterogeneous conditions. The organopalladium complexes were characterized by standard analytical and spectroscopic methods and by thermogravimetry showing decomposition above 110 °C. Both types of catalysts showed excellent activity for these cross carbon‐carbon bond formations involving aryl halides including activated aryl chlorides or acyl chloride. Besides the comparison between homogeneous and heterogeneous catalysis, the key feature of these catalysts is their remarkable robustness that allowed recycling at least ten times in the example of the Heck reaction with excellent yields and without significant reduction of the conversion.     

New carbon- and sulfur-based ligands in catalysis

Homogeneous catalysis is a field of research that has gained central importance in both organic and inorganic chemistry and the use of well-defined ligand systems in the synthesis of transition metal complexes has had an enormous impact on the development of such catalysts. Neutral, two-electron donor ligands based on phosphorous and nitrogen have been tremendously successful as ancillary entities for late-transition metal (LTM) catalysts, whereas ligands based on anionic nitrogen, oxygen and the cyclopentadienyl motif (Cp(-)) have propelled early-transition metal (ETM) catalysis forward. We believe that expanding the ligand families capable of acting as successful entities in metal-mediated reactivity and catalysis is crucial for future discoveries in this field. Research in our group therefore tries to identify new non-chiral and chiral ligands for late-transition metal chemistry that are based on neutral, two-electron carbon and sulfur donor atoms. In particular, we have until now focused on the development of modular, monodentate N-heterocyclic carbene ligands (NHCs) that can serve as a basis for the development of chiral ligand frameworks for the application to asymmetric catalytic transformations. In the second major research project developed over the last six years, we have started an investigation on the use of chelating sulfoxide-based ligands in asymmetric late transition-metal based catalysis.     

π-Complex Intermediates in Homogeneous and Heterogeneous Catalytic Exchange Reactions of Hydrocarbons and Derivatives with Metals

The exchange of unsaturated and aromatic hydrocarbons and their derivatives with isotopic hydrogen on group VIII transition metals has been reviewed. Both classical and π-complex heterogeneous mechanisms proposed to account for the observed behavior have been compared. Evidence to distinguish between the application of the heterogeneous associative and dissociative π-complex exchange mechanisms is discussed. The recently discovered homogeneous equivalent of the above heterogeneous reactions is described in detail. Homogeneous associative and dissociative π-complex mechanisms are proposed to explain aromatic hydrogen exchange. Mechanisms are also proposed to explain the pattern of exchange observed homogeneously with platinum in the alkyl groups of alkyl aromatics. Possible mechanistic relationships between heterogeneous and homogeneous systems are considered using as representative unsaturated and aromatic series, the polycyclic hydrocarbons, the monohalogenated benzenes, the alkyl benzenes (and relevant simple alkanes), steroids, and synthetic hormones. The role of π-complexes in relating the present heterogeneous and homogeneous exchange reactions to the chemistry of inorganic coordination complexes is also considered.     

“HETEROGENIZING” HOMOGENEOUS CATALYSTS

For many years, it has been customary to classify catalysts as “homogeneous” or “heterogeneous.” The former commonly operate through the formation of “intermediate compounds,” and the latter, by adsorption of the reactants on the catalyst surface. The line between the two is a fine one, for the distinction between adsorption and compound formation is not at all clear, and seems to be becoming less and less clear as we learn more about adsorption. In recent years, several writers [l-7] have stressed the point that there is a good deal of overlap between homogeneous and heterogeneous catalysis. Experimental evidence supporting this point of view is accumulating, and while we are not prepared to say that there is no distinction, we can say with certainty that many homogeneous catalysts can be converted into heterogeneous ones, retaining the advantages of great activity and selectivity inherent in homogeneity and, at the same time, assuming the ready recovery which is the great advantage of heterogenity. In practice, of course, the matter is not quite that simple, for other factors must be considered. On the whole, however, many advantages have been found in the use of heterogenized homogeneous catalysts.     

Chiral reactions in heterogeneous catalysis (1975-1999)

The preparation of pure chemical compounds has been one of the most rapid growth areas in chemistry over the last decade with heterogeneous catalysis being recognised as providing new possibilities. A number of different strategies have been developed to obtain enantioselectivity through heterogeneous catalysis: the use of chiral species to modify the solid surface; the grafting of a chiral catalytic complex on a solid - the so called heterogenised homogeneous catalyst - and homogeneous formation of a chiral complex before reaction at the solid surface. Because the interaction between functional groups is complex, the influence the surface has on reaction mechanisms is, so far, not well developed. Hydrogenation reactions have dominated the field with α- or β-ketoesters being the preferred substrates and nickel modified by tartaric acid and platinum modified by cinchona alkaloids as preferred catalytic systems with modified zeolites playing a crucial role. Some examples of the leading groups are in Switzerland (Blaser and Baiker), Japan (Nitta, Osawa, Izumi and Harada), USA (Sachtler, Augustine), United Kingdom (Webb, Wells, Thomas, Catlow, Hutchings and Whyman) and the Netherlands (Sheldon).In spite of the present hyper-activity the use of metal catalysts to effect asymmetric reactions is not new with Schwab in Germany and Lipkin and Stewart in the USA active in the 1930s. In 1995 at the ChiCat Symposium both Reisse and Ghosez suggested that those in catalysis should “not reinvent wheels that organic chemists had laboured over many years” and “that goals already achieved by organic chemists will not be easy to obtain through heterogeneous catalysis”. We await future developments with much anticipation! This revised version was published online in July 2006 with corrections to the Cover Date. This revised version was published online in July 2006 with corrections to the Cover Date.     

Can the Observed Changes in the Size or Shape of a Colloidal Nanocatalyst Reveal the Nanocatalysis Mechanism Type: Homogeneous or Heterogeneous?

The surface energy of metallic nanocrystals is relatively high compared to bulk materials due to the metal–metal bond deficiency of the surface atoms. This results in an insufficient chemical valency. In addition, smaller nanoparticles possess a higher degree of curvature, weakening the bonding of their surface atoms. This is especially true for non-spherical shapes, which are comprised of a large number of sharp corner and edge sites. These atomic sites possess higher surface energies due to the lower number of shared bonds with the nanoparticle, resulting in instability of the surface atoms and rendering them physically unstable and chemically active. In many instances, the constant “bombardment” of these surface atoms by the solvent molecules as well as by the reactant molecules when these nanocrystals are in colloidal solution could lead to surface atom dissolution, both physically and/or chemically. This phenomenon could alter the functionality of the metallic colloidal nanoparticle from supplying catalytically active sites (in heterogeneous catalysis) to serving as a reservoir of catalytically active species to the solution (in homogeneous catalysis). In the latter type, if the atoms of the nanocatalyst appear in the products, the nanoparticle is no longer a catalyst but a reactant. In this review we attempt to answer the question raised in the title by examining our previous work on the changes in size, shape, and other physical and chemical properties of colloidal transition metal nanoparticles during the nanocatalysis of two fundamentally different and important reactions: (1) the gentle electron-transfer reaction at room temperature involving the reduction of hexacyanoferrate (III) ions with thiosulfate ions and (2) the more harsh Suzuki cross-coupling reaction between phenylboronic acid and iodobenzene that takes place at 100 °C for 12 h. Changes in the nanoparticle dimensions were followed with TEM and HRTEM. Raman and FTIR spectroscopies were used to follow the chemical changes. For each change, we will use the above definition to see if the observed change can help us determine whether the catalysis is homogeneous or heterogeneous.