Activated carbons are high surface area materials, exhibiting good stability, good mechanical resistance and high porosity. Due to these properties, they are widely used in heterogeneous catalysis as catalysts and catalyst supports. It was observed, that the treatment with fuming sulfuric acid improves the catalytic performance of the carbon catalyst. All the oxidised carbon catalysts exhibit good selectivity to a-terpineol.
Mild fractionation/pretreatment processes are becoming the most preferred choices for biomass processing within the biorefinery framework. To further explore their advantages, new developments are
needed, especially to increase the extent of the hydrolysis of poly- and oligosaccharides. A possible way forward is the use of solid acid catalysts that may overcome many current drawbacks of other common methods. In this Review, the advantages and limitations of the use of heterogeneous catalysis for the main groups of solid acid catalysts (zeolites, resins, carbon materials, clays, silicas, and other oxides) and their relation to the hydrolysis of model soluble disaccharides and soluble poly- and oligosaccharides are presented and discussed. Special attention is given to the hydrolysis of hemicelluloses and hemicellulose-derived saccharides into monosaccharides, the impact on process performance of potential catalyst poisons originating from biomass and biomass hydrolysates (e.g., proteins, mineral ions, etc.). The data clearly point out
the need for studying hemicelluloses in natura rather than in model compound solutions that do not retain the relevant factors influencing process performance. Furthermore, the desirable traits that solid acid catalysts must possess for the efficient hemicellulose hydrolysis are also presented and discussed with regard to the design of new catalysts.
This review gives a critical idea on the importance of intramolecular reactions as models for enzymatic catalysis. Intramolecular lactonizations, ester and amide hydrolysis studies result in theories which try to explain the difference between intermolecular, intramolecular and enzyme reactions and rationalize the enhancement promoted by these biological catalyst.
Since its discovery, phase transfer catalysis (PTC) has grown considerably and nowadays is one of the most versatile preparative methods. The search for new catalysts, their use in PTC asymmetric synthesis and the attempts to understand their mechanistic role are modern and exciting topics of investigation. A review on main achievements in the last two decades is presented.
For economical and ecological reasons, synthetic chemists are confronted with the increasing obligation of optimizing their synthetic methods. Maximizing efficiency and minimizing costs in the production of molecules and macromolecules constitutes, therefore, one of the most exciting challenges of synthetic chemistry. The ideal synthesis should produce the desired product in 100% yield and selectivity, in a safe and environmentally acceptable process. In this highlight the concepts of atom economy, molecular engineering and biphasic organometallic catalysis, which address these issues at the molecular level for the generation of "green" technologies, are introduced and discussed.
The use of zeolites and other molecular sieves as catalysts is discussed at an introductory level. The text includes a brief historic background on the use of zeolites in catalysis, and a discussion of some chemical and physical properties of silicalite, aluminosilicate, and aluminophosphate molecular sieves. The strategies currently used to chemically modify zeolites and related materials to produce catalysts with increased activity and selectivity are discussed, including the use of redox molecular sieves for hydrocarbon oxidation and the leaching of the active metals from the support.
In the last 25 years the research in catalysis has passed from heterogeneous processes mainly applied to petrochemical industries to homogeneous processes applied to fine chemicals and green chemistry. In this context the Brazilian Chemical Society (SBQ) is playing an important role in the organization of this interdisciplinary area involving researches from different fields of physical sciences.
This review describes the use of catalytic asymmetric aldol reactions of silyl enol ethers and silyl (thio)ketene acetals with aldehydes (the Mukaiyama aldol reaction) in order to illustrate its synthetic utility. A variety of Lewis acid and basic reagents were employed for catalytic aldol reactions with high diastereo- and enantioselectivities. The origins of the selectivity of these reactions are discussed and some representative examples of their application in the synthesis of natural products are presented. New developments in chiral heterobimettalic lanthanoid catalysis and enantioselective aldol reactions in aqueous media are also included.
The microwave oven became an important source of heating for many laboratory procedures including accelerating organic reactions. Reactions that require long reflux times can sometimes be carried out in a few hours or minutes in a conventional microwave oven. However, longer reflux times can be troublesome since domestic microwave ovens are not prepared for these harsh conditions. This technical note presents our finding on heterogeneous catalysis transesterification reactions between b-keto-esters and carbohydrate derivatives under heating or microwave irradiation using an adapted domestic microwave oven.
The field of chiral catalysis has experienced explosive growth over the last two decades. By now, many of the classical reactions in organic synthesis can be carried out efficiently in asymmetric manner. As one of the fundamental and powerful C-C bond-forming reactions, enantioselective catalytic allylation (ECA) and crotylation (ECC) of aldehydes has attracted considerable attention. In this article, we present an overview about the importance of chiral Lewis acids and bases in catalytic enantioselective addition of allyl- and crotyl metals to aldehydes and the application of this methodology in the total synthesis of natural and non-natural products.
Vanadium-containing molecular sieves are redox catalysts and are good candidates as substitutes for oxide-supported V2O5 in a number of reactions. These materials have the advantage of presenting better dispersion of vanadium species, as well as shape-selective properties and controllable acidities. They may be prepared by one-pot synthesis or by post-synthesis methods and a number of techniques such as diffuse reflectance UV-visible spectroscopy, 51V nuclear magnetic resonance and electron paramagnetic resonance, to name but a few, have been used to characterize these materials. In this review, methods of preparation of vanadium-modified molecular sieves, their characterization and applications in catalysis are discussed.
A review of most of the reported studies on the use of iron oxides as catalyst in specific processes, namely Haber-Bosch reaction, Fischer-Tropsch synthesis, Fenton oxidation and photolytic molecular splitting of water to produce gaseous hydrogen, was carried out. An essential overview is thus presented, intending to address the fundamental meaning, as well as the corresponding chemical mechanisms, and perspectives on new technological potentialities of natural and synthetic iron oxides, more specifically hematite (α-Fe2O3), goethite (α-FeOOH), magnetite (Fe3O4) and maghemite (γ-Fe2O3), in heterogeneous catalysis.
The bioactive 3,4-dihydropyrimidin-2(1H)-thione derivative known as Monastrol was synthesized under catalyzed and non-catalyzed conditions through the Biginelli multicomponent reaction under solvent-free conditions. The use of two Lewis acids (FeCl3 and CuCl2) and two Brønsted acids (HCl and CF3COOH) as catalysts improved the reaction yields of the transformation compared with the non-catalyzed reaction. The experiments investigated catalysis and its role, the importance of multicomponent reactions and their green features, and the application of these concepts to the synthesis of a biologically important structure.
The synthesis of gold nanoparticles (Au NPs) 15, 26, and 34 nm in diameter, followed by the investigation of their size-dependent optical and catalytic properties, is described herein as an undergraduate level experiment. The proposed experiment covers concepts on the synthesis, stabilization, and characterization of Au NPs, their size-dependent optical and catalytic properties at the nanoscale, chemical kinetics, and the role of a catalyst. The experiment should be performed by groups of two or three students in three lab sessions of 3 h each and organized as follows: i) synthesis of Au NPs of different sizes and investigation of their optical properties; ii) evaluation of their catalytic activity; and iii) data analysis and discussion. We believe that this activity enables students to integrate these multidisciplinary concepts in a single experiment as well as to become introduced/familiarized with an active research field and current literature in the areas of nanoparticle synthesis and catalysis.
Fonte: Sociedade Brasileira de QuímicaPublicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista CientíficaFormato: text/html
Publicado em 01/10/2001Português
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Metal-chiral crown ether complexes have been developed as efficient chiral Lewis acid catalysts for asymmetric aldol reactions of silyl enol ethers with aldehydes in aqueous media. While many excellent catalytic asymmetric reactions have been developed recently, most of them have to be carried out under strictly anhydrous conditions in organic solvents. This is probably due to the instability of many catalysts and/or intermediates in the presence of even a small amount of water. To address this issue, we searched for metal-crown ether complexes on the basis of our "multi-coordination" hypothesis, and found that lead(II) and lanthanide(III) catalysts worked well as chiral Lewis acids in aqueous media. To the best of our knowledge, these are the first examples of chiral crown-based Lewis acids that can be successfully used in catalytic asymmetric reactions. The catalysts have been characterized by X-ray diffraction, and their unique structures as chiral catalysts have been revealed. Use of water as a solvent is essential in these asymmetric catalysis, and the role of water on these reactions to explain the high reactivity and selectivity has been suggested. Another important point is that kinetic studies have shown the possibility that these types of crown ether complexes would be suitable as chiral catalysts employed in aqueous media. In addition...
(E)-1,2-vinylic dibromides 11a-f were stereoselectively prepared via bromination of acetylenic compounds with pyridinium tribromide in MeOH/CCl4 at low temperature and double cross-coupled with PhZnCl under Pd(0) catalysis (Negishi protocol) to afford tri- and tetrasubstituted olefins 14a-e. Tamoxifen, a selective estrogen receptor modulator clinically prescribed in breast cancer therapy, was prepared in 7 steps and 30% overall yield from 4-iodophenol (3) as a 2.3:1 mixture of (Z)- and (E)-isomers.
The Michael addition of ethyl acetoacetate 1 to chalcone 2 under Solid/Liquid Phase Transfer Catalysis (SL-PTC), solvent free, afforded Michael adduct 3 and an annulated compound 4 in good yield. The cyclic compound 4 was obtained in the majority via an annulation process of the Michael adduct 3. The proportion of 4 in the ratio of 3/4 was dependant on the structure of the quaternary ammonium cation. The results were interpreted on the basis of the organophilicity and accessibility of the catalysts as well as by invoking a 6-membered ring transition state for the formation of 4 due to the chelation level with Q+.
This work describes the preparation of babassu, castor oil biodiesel and mixtures in various proportions of these oils, using alkaline compounds of strontium (SrCO3 + SrO + Sr (OH)2) as heterogeneous catalysts. The mixture of oils of these oleaginous sources was used in the production of biodiesel with quality parameters that meet current legislation. The catalyst was characterized by X-ray diffractometry (XDR), physisorption of gas (BET method), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR). The viscometric technique was used to monitor the optimization. The transesterification reactions performed using strontium compounds reached conversion rates of 97.2% babassu biodiesel (BB), 96.4% castor oil biodiesel (COB) and 95.3% Babassu/Castor Oil Biodiesel 4:1 (BBCO41).
The need for a better carbon management and the reduction of CO2 emissions push away from the linear-carbon economy (LCE) towards a better carbon management including carbon recycling (CR). This implies the utilization of gaseous and liquid process streams so far under-utilized, and often either sent to combustion or disposed of. In this paper, four cases are discussed, namely: low-alkane streams, industrial CO2, ligno-cellulosic waste and salty-watery-glycerol. The role of catalysis in the valorization of such C-sources is discussed and examples of innovative processes are presented.
The idea that enzymes accelerate their reactions by entropic
effects has played a major role in many prominent proposals about the
origin of enzyme catalysis. This idea implies that the binding to an
enzyme active site freezes the motion of the reacting fragments and
eliminates their entropic contributions,
(ΔScat‡)′, to the
activation energy. It is also implied that the binding entropy is equal
to the activation entropy,
(ΔSw‡)′, of the
corresponding solution reaction. It is, however, difficult to examine
this idea by experimental approaches. The present paper defines the
entropic proposal in a rigorous way and develops a computer simulation
approach that determines (ΔS‡)′. This
approach allows us to evaluate the differences between
(ΔS‡)′ of an enzymatic reaction and of the
corresponding reference reaction in solution. Our approach is used in a
study of the entropic contribution to the catalytic reaction of
subtilisin. It is found that this contribution is much smaller than
previously thought. This result is due to the following:
(i) Many of the motions that are free in the
reactants state of the reference solution reaction are also free at the
transition state. (ii) The binding to the enzyme
does not completely freeze the motion of the reacting fragments so that
(ΔS‡)′ in the enzymes is not zero.
(iii) The binding entropy is not necessarily equal