Showing papers in "Topics in Catalysis in 2004"

Recent Catalytic Advances in the Chemistry of Substituted Furans from Carbohydrates and in the Ensuing Polymers

Abstract: In this review, an overview is given on the last development of catalytic methods for the preparation of substituted furans from carbohydrates and ensuing polymers. The review starts with the recent aspects in the synthesis of some key furan monomers in the presence of solid catalysts. In the second part, selected examples are given of polymerization systems leading to furan-based materials with promising properties, thus constituting a serious alternative to petroleum-based counterparts. Finally, a short examination is given on what could be the future of furan chemistry with the recent development of ionic liquids as solvents.

TEMPO-Mediated Oxidation of Polysaccharides: Survey of Methods and Applications

Abstract: This review deals with TEMPO as a catalyst in oxidation of alcohol functions in polysaccharides. Synthesis of TEMPO and derivatives and the mechanism of the oxidative cycle in which TEMPO is involved in oxidation of alcohols are discussed. Results of oxidation of various polysaccharides with respect to yield, and introduction of the functional groups (aldehyde and/or carboxylate) are presented. Most of the primary oxidants are not ideal, as they produce large amounts of salts, e.g., sodium chloride from sodium hypochlorite. Results and perspectives are given to change the salt-based oxidative systems for much cleaner oxygen or hydrogen peroxide/enzyme-based TEMPO systems. Moreover, several immobilized TEMPO systems have been developed.

Esterification and transesterification of renewable chemicals

Abstract: An overview is given of newly developed Lewis or Bronsted acid and base catalysts for esterification, transesterification and ester interchange reactions. The most relevant applications of these catalysts and reactions in the domain of renewable resources, in particular, oils and fats, are discussed.

Catalytic Conversion of Terpenes into Fine Chemicals

Abstract: This work reviews the use of terpenes available from renewable resources in the fine chemicals industry. Different products and processes are discussed. Transformations of terpenes involving isomerization, hydration, condensation, hydroformylation, hydrogenation, cyclization, oxidation, rearrangement, and ring contraction/enlargement are routes to produce a variety of compounds of interest to the pharmaceutical, agrochemical, and flavors and fragrances industries.

Catalysis by Gold Nanoparticles: Epoxidation of Propene

Abstract: Supported nano-gold catalysts have attracted rapidly growing interest due to their potential applicability to various reactions of industrial and environmental significance. In this article, we focus on the advances related to the direct vapor-phase oxidation of propene to propene oxide in the presence of molecular oxygen and hydrogen over gold catalysts supported on Ti-incorporated silica materials prepared by different methods. The importance of catalyst preparation and pretreatment method, nature of support material, Au particle size and loading amount is emphasized. Possibilities to enhance the catalyst performance by using promoters and by silylation are also discussed.

Oxidation of Glycerol Using Supported Gold Catalysts

Abstract: A series of supported gold catalysts (0.25, 0.5, 1.0 wt% Au/graphite) have been investigated for the oxidation of glycerol and propan-1,2-diol. The 1 wt% Au/graphite catalyst is found to give 100% selectivity to the mono acid product, isolated as the sodium salt, as long as NaOH is present. The catalysts are characterized by TEM and cyclic voltammetry. By TEM, active catalysts all comprise fairly broad-size distributions (5–50 nm diameter) for the gold nanoparticles, although most are ca. 25 nm in diameter. An inactive 1 wt% Au/graphite is shown to have considerably larger particle sizes (>50 nm) and this indicates that there may be an optimum particle size for the desired catalysis. Characterization using cyclic voltammetry of active Au/graphite catalysts carried out in NaOH reveals the presence of an oxide species that may be responsible for the observed catalysis. In contrast, the inactive 1 wt% Au/graphite catalyst shows no oxidation in the cyclic voltammetry experiments.

Catalytic Transformations of the Major Terpene Feedstocks

Abstract: A review covering some of the key catalytic transformations of the major monoterpene feedstocks. Monoterpenes are key ingredients in the flavor and fragrance industry, with α- and β-pinene (obtained from turpentine) being some of the most important. The review focuses on the hydrocarbons; pinene, limonene, carene, and the interconversion of the monooxygenates; geraniol/nerol, citronellol, citral, and citronellal. The major areas covered are catalytic hydrogenation/hydrogenolysis, dehydrogenation, rearrangement/isomerization/aromatization, reactions with carbon monoxide, epoxidation, and the addition of alcohols and acids.

On the Role of Oxygen Storage in Three-Way Catalysis

Abstract: The role of the so-called oxygen storage and release capacity (OSC) in promoting the activity of noble metals in the three-way catalysts (TWCs) is critically discussed. It is shown that the promoting effects of CeO2 in the TWCs cannot be attributed to a simple redox type of effect according to the reaction CeO2 ↔ CeO2 − x + x/2O2, since multiple and sometimes intriguing effects of CeO2 promoter have been observed, particularly in the latest generation of TWCs containing modified CeO2–ZrO2 mixed oxides.

Catalytic Metathesis of Unsaturated Fatty Acid Esters and Oils

Abstract: This article describes the catalytic routes that are available for converting unsaturated fatty acid esters and related compounds, obtained from renewable resources, into useful chemical products via the olefin metathesis reaction. These routes offer new possibilities for the oleochemical industry as a contribution to a sustainable chemical industry. Highly selective homogeneous as well as heterogeneous catalyst systems have been developed for these metathetical conversions. Supported rhenium oxide catalysts are already active at room temperature, can be easily separated from the reaction mixture, and can be regenerated many times. New well-defined homogeneous ruthenium complexes are very effective catalysts, showing high turnover numbers. The present knowledge about the catalyst systems and the possibilities for practical applications for the metathetical conversion of unsaturated fatty acid esters and oils will be discussed.

NOx Storage-Reduction Catalysts for Gasoline Engines

Abstract: The automotive catalyst technology is now faced with very difficult problems. As the result of automakers' efforts to produce more efficient and lower-emission vehicles, lean-burn gasoline engines have been introduced into the market. While these engines are much more efficient than conventional ones, NOx removal has become significantly more difficult. After enormous efforts, we succeeded in solving the problem, by inventing a new class of catalyst. We overcame two strict problems with the new type catalyst, which are sulfur poisoning and thermal deterioration.

Urea-SCR in Automotive Applications

Abstract: Stricter emission legislation for diesel vehicles will make exhaust after-treatment mandatory in the near future. Urea-SCR has been chosen for NO x reduction on trucks and busses. Since its effectiveness in reducing NO x is great, and since there is a trade-off between NO x emissions and fuel consumption, engines can be adjusted on fuel efficiency. This review article deals with subjects such as the description of SCR catalysts, catalytic systems, system performance, choice of reducing agent and durability.

Control of NOx emissions from diesel engine by selective catalytic reduction (SCR) with urea

Abstract: Among the catalysts screened, Cu-ion exchanged ZSM5 zeolite exhibited the highest NO removal activity, particularly at low reaction temperatures below 200 °C, maintaining a wide operating temperature window. The hydrothermal stability of the CuZSM5 catalyst can be improved by the optimization of metal content of the catalyst. Through the variation of reactor operating conditions, NO conversion of better than 90% could be achieved with a minimum NH3 slip. The decomposition of urea was also examined and a kinetic model for both thermal and catalytic decomposition of urea was developed. Urea-SCR over the CuZSM5 catalyst exhibited that the NO removal activity is competitive to that by NH3-SCR, indicating urea can be effectively utilized in SCR reactor system as the reducing agent.

State-of-the-Art in Zeolite Membrane Reactors

Abstract: This overview discusses the developments occurred in the field of membrane reactors during the last few years. Novel applications of existing reactor concepts as well as new reactor proposals are described. Related areas such as new zeolite and zeolite-related materials for membranes, alternative supports and scale up studies are also discussed. Finally, emerging uses of zeolite membranes and coatings in the micro-world (microreactors, micromembranes, sensors) are also presented.

Selective Oligomerization of Glycerol Over Mesoporous Catalysts

Abstract: In the general context of the development of the use of agricultural products for nonfood applications, particularly in the field of glycerol valorization (coproduct of triglyceride hydrolysis and methanolysis processes), the selective etherification of glycerol to di- and triglycerol was studied.

Brief Review of Steam Reforming Using a Metal Membrane Reactor

Abstract: Recent studies and development trends of a membrane reformer, which incorporates a metal membrane for hydrogen separation and purification into fuel reformers for polymer electrolyte membrane fuel cells, are outlined. The challenges for fabricating a dense and thin palladium film by compositing to an inorganic porous substance and the research and development of a new and cheaper membrane material in place of the palladium membranes are also briefly introduced.

Exceptional activity for NOx reduction at low temperatures using combinations of hydrogen and higher hydrocarbons on Ag/Al2O3 catalysts

Abstract: The effect of the addition of hydrogen on the SCR of NOx with a hydrocarbon reaction was investigated. It was found that hydrogen had a remarkable effect on the temperature range over which NOx could be reduced during the SCR reaction with octane. Reduction of NOx was initiated at as low a temperature as 100 °C and >95% NOx conversion was achieved over a temperature range of 200–450 °C. Hydrogen has the effect of activating octane at lower temperatures and also promotes the oxidation of NO to NO2 in the absence of hydrocarbon. Transient kinetic and in situ DRIFTS measurements indicated that hydrogen has a direct role in the reaction mechanism by either promoting the formation and storage of an organic C = N species which can then readily reduce NOx and/or removing a species which acts as a poison to the SCR reaction at low temperatures.

Base catalysis in the synthesis of fine chemicals

Abstract: The catalytic properties of hydrotalcites are investigated for reactions of aldolisation, Meerwein–Ponndorf–Verley hydrogenations, epoxidations and oxidations, Michael and Wadsworth–Emmons additions. Excellent yields have been reported using mild conditions and catalysts which can be easily separated from the reaction medium and could be regenerated. The catalytic properties are controlled by the activation of the solid, and traces of chlorine appear to be deleterious. In many cases rehydration increases activity dramatically. Solid bases give a very specific catalysis for instance with Michael or Wadsworth–Emmons reactions of aldolisable compounds due to the exclusion of carbonyls from the surface by the other reactant.

Controlling Particulate Emissions from Diesel Vehicles

Abstract: Diesel engines move the world Their excellent fuel economy makes them the power source of choice in a multitude of applications, such as transportation, excavation and mining They are used extensively in commercial transport, being employed in trucks, buses, trains and ships The Diesel engine is also becoming increasingly popular in the passenger car sector, especially in Europe where around 30% of the market is made up of Diesel vehicles; this proportion is expected to increase to 50% by 2010 However, increasing attention is being paid to the particulate matter (PM) emissions from Diesel vehicles, which consists mostly of carbonaceous soot and a volatile organic fraction (VOF) of hydrocarbons that have condensed on the soot There is growing concern about these emissions, since emerging evidence potentially implicates PM as a major pollutant associated with acute health effects This article reviews the strategies currently available to control the PM emissions from Diesel vehicles, and outlines the operating principles of the current state-of-the-art systems

Influence of the type of reducing agent (H2, CO, C3H6 and C3H8) on the reduction of stored NOX in a Pt/BaO/Al2O3 model catalyst

Abstract: In this investigation, a comparative study for a NOx storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H2, CO, C3H6 and C3H8) and different temperatures (350, 250 and 150 degrees C), for a Pt/BaO/Al2O3 catalyst. Transient experiments show that H2 and CO are highly efficient reductants compared to C3H6 which is somewhat less efficient. H2 shows a significant reduction effect at relatively low temperature (150 degrees C) but with a low storage capacity. We find that C3H8 does not show any NOx reduction ability for NOx stored in Pt/BaO/Al2O3 at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed.

Fullerene-like Soot from EURO-IV Diesel Engine: Consequences for Catalytic Automotive Pollution Control

Abstract: Soot particulates from an EuroIV diesel engine are sampled and investigated by high-resolution electron microscopy (HRTEM) and thermal gravimetry. The experiments reveal a drastic reduction of primary particle size down to less than 20 nm, much smaller than that emitted by earlier diesel engines. HRTEM reveals primary particles with deformed fullerenoid structures. The defective fullerenoid soot is more prone to oxidation than the soot of a black smoking diesel engine. Our findings may initiate a critical review of the current strategy for the reduction of soot emission from diesel engines. The newly developed engines reduce the quantity of soot emitted, they also emit smaller soot particles with a fullerene-like structure into the exhaust.

The potential for use of gold in automotive pollution control technologies: a short review

Abstract: Recent studies relevant to the future use of gold catalysts in the automotive industry are summarized. Gold catalysts have been examined for their potential in low temperature activity to combat cold-start emission problems and removal of NO x from leanburn gasoline and diesel engines. The justification for developing gold catalyst technologies is based both on their promising technical performance and the relatively stable price and greater availability of gold compared with the platinum group metals (PGMs). Use of gold catalysts under mild conditions could also produce lower proportions of CO in the hydrogen streams used for automotive fuel cells and they could be used for the selective oxidation of carbon monoxide in these hydrogen streams in order to increase the efficiency of these fuel cells. Technical barriers are indicated which still remain to be overcome before application of gold catalysts is successful in the automotive sphere.

The preparation of fatty acid esters by means of catalytic reactions

Abstract: A review of actually available techniques for the preparation of esters derived form natural oils and fats is presented. A wide choice of catalysts, either homogeneous or heterogeneous, is currently available. This paper evaluates in detail the possibilities for the preparation of fatty acid esters with different alcohols and discusses the criteria for a correct choice. The different problems related to the preparation of esters with low molecular weight monoalcohols and of polyolesters with polyalcohols are discussed. The last part of the paper is devoted to the evaluation of ion-exchange resins as effective catalysts for the preparation of fatty acid esters with low molecular weight monoalcohols.

Mono- and Bifunctional Heterogeneous Catalytic Transformation of Terpenes and Terpenoids

Abstract: Selective reactions of terpenes, catalyzed by different heterogeneous systems, are reported. α-pinene and limonene epoxides can be effectively isomerized to carbonyl compounds (selectivity >70%) over silica aluminas that appear to be good alternatives to homogeneous ZnBr2. The different reactivity of geometric isomers can be used to separate them. Unsaturated ketones can be converted through a two-step one-pot reaction in cyclic or bicyclic ethers through a bifunctional process involving a hydrogenation and an acid-catalyzed step. However, the choice of the solvent allows to inhibit the acidic sites, thus obtaining a selective hydrogenation reaction. On the other hand, the acidic reaction alone can be useful for alcohol epimerization and epimer separation. Grafting of Ti in a siliceous matrix gives rise to a material with both redox and acidic properties. Solids obtained in this way are active and selective in the epoxidation of terpenic alcohols but can also promote bifunctional reactions.

Catalysis in Membrane Reactors: What About the Catalyst?

Abstract: Catalytic membrane reactors (CMRs) combine a membrane that controls transfers and a catalyst that provides conversion This paper focuses on the catalyst itself Depending on the application, the environment of the catalyst in the CMR may be quite different from that existing in conventional reactors This could originate changes of the catalyst properties In some cases, catalysts for CMRs might require a specific design

A Review of Catalytic Membrane Layers for Gas/Liquid Reactions

Abstract: This paper summarises the main ideas of how to use membranes with built-in catalytic function for performing catalysed gas/liquid reactions. Mass transfer effects on catalyst performance and other decisive factors for the design of industrial gas/liquid and gas/liquid/solid reactors are briefly reviewed. Various concepts for applying catalytic membrane layers in multiphase reactors are introduced and the motivation behind their use is explained. What follows is an assessment of the present achievements in using such concepts for gas/liquid reactions. The results available in literature are critically evaluated, focussing mainly on the field of liquid-phase hydrogenations and oxidations, which constitute the major areas where catalytic membranes have been applied to date to gas/liquid systems. The aim of such treatment is to point out the targets, to elaborate on how far the whole field has developed and what challenges remain, rather than to cover a multitude of specific aspects of the many different studies reported in literature.

Insight into Green Phase Transfer Catalysis

Abstract: There is a need to develop green, clean and smart chemical technologies. Waste reduction through clever strategies and catalysis are at the heart of green chemistry. In the case of over 600 industrial phase transfer catalyzed (PTC) processes in several industries, mostly practised as liquid–liquid PTC, the catalyst is not recovered and disposed as a waste. Liquid–liquid PTC can be replaced by solid–liquid (S–L), liquid–liquid–liquid (L–L–L), solid (catalyst)–liquid–liquid, solid–liquid–omega liquid, gas–liquid–solid (G–L–S) and capsule membrane PTC to recover and reuse the catalyst and also to enhance selectivity, thereby advocating the realm of green PTC. Use of microwaves and ultrasound will also help in increasing rates. MILL–PTC and MISL–PTC are attractive techniques to enhance rate and selectivities under mild input of microwave irradiation (MI). In cascade-engineered PTC, several steps are combined in a reactor using the same catalyst and solvent, if used at all, without separation, or with partial replenishment of the reactants for all the steps to get substantial waste minimization. This paper provides a new insight into PTC for some reactions of industrial importance.

Solid Catalysts for the Synthesis of Fatty Esters of Glycerol, Polyglycerols and Sorbitol from Renewable Resources

Abstract: Partial esters of polyols such as glycerol, polyglycerols and sorbitol are nonionic surfactants used as emulsifiers in the food, detergents and cosmetics industries. The commercial large-scale production of these chemicals relies on traditional homogeneous catalysis processes making use of strong mineral acids or alkalis. This technology possesses severe drawbacks, related to the generation of large amounts of by-products, high energy demand, and heterogeneity of the ester mixtures obtained. The development of new processes based on more selective solid catalysts has, therefore, a great economical interest. This contribution reviews the scientific and technical literature in this field. It is proposed that solid catalysts based on MCM-41 and other mesostructures could give rise to the development of new, more efficient processes for the large-scale synthesis of fatty acid esters of polyols.

The Use of NO x Adsorber Catalysts on Diesel Engines

Abstract: NO x adsorber catalysts (NACs) can be applied in lean-burn gasoline and diesel engines to reduce NO x emissions. Typically NACs are formulated using platinum as an oxidation catalyst, barium as a storage component and rhodium for NO x reduction. There has been intense research to optimise these catalysts for use on diesel engines aimed at increasing their efficiency, durability, sulfur tolerance and operating temperature window. This paper describes these developments and outlines the current level of knowledge of NO x adsorber catalyst systems using results from small-scale laboratory tests as well as engine-bench work.

Silver/Alumina Catalyst for Selective Catalytic Reduction of NOx to N2 by Hydrocarbons in Diesel Powered Vehicles

Abstract: Ag/alumina has been found to be a promising catalyst for the selective catalytic reduction of NOx to N2 by hydrocarbons (HC-SCR) in laboratory tests as well as in full-scale diesel engine operation. The steps in developing an active silver catalyst for practical applications involve knowledge of the mechanism, choice of the right metal content and correct support, adjustment of the concentration of the hydrocarbon to be used for reduction reactions etc. As gas phase reactions seem to play a significant role in obtaining high NO to N2 conversion, also reactor aspects have to be included in the optimisation procedure of a full size Ag/alumina catalytic converter.

CO Oxidation and NO Reduction on Metal Surfaces: Density Functional Theory Investigations

Abstract: This article reviews the accumulated theoretical results, in particular density functional theory calculations, on two catalytic processes, CO oxidation and NO reduction on metal surfaces. Owing to their importance in automotive emission control, these two reactions have generated a lot of interest in the last 20 years. Here the pathways and energetics of the involved elementary reactions under different catalytic conditions are described in detail and the understanding of the reactions is generalized. It is concluded that density functional theory calculations can be applied to catalysis to elucidate mechanisms of complex surface reactions and to understand the electronic structure of chemical processes in general. The achieved molecular knowledge of chemical reactions is certainly beneficial to new catalyst design.