Current Organic Chemistry - Volume 18, Issue 10, 2014

Heterogeneous catalysis is an important field of organic chemistry and has some obvious advantages such as easy separation, no corrosion, and negligible waste production. It is well known that heterogeneous catalysis has played a significant role in the synthesis of organics and fine chemicals. In recent years, however, the technique of heterogeneous catalysis has also been extended to the field of energy and environment, for example, catalytic conversion of carbon dioxide to fuels as well as photocatalytic degradation of pollutants. There is no doubt that catalysts are the key point of heterogeneous catalysis. The last decade has witnessed incredible advances in novel nanoporous materials, which open up new opportunities to catalytic processes. Nanoporous materials with abundant pore structures (e.g. zeolites and mesoporous silicas) provide an ideal accommodation for catalytically active sites. Great progress has been made in the development of highperformance heterogeneous catalysts based on nanoporous materials. This special issue will cover recent advances and new trends in the field of nanoporous materials for heterogeneous catalysis. The nanoporous materials vary from zeolites with micropores to mesoporous silicas and organosilicas with mesopores. In addition to inorganic nanoporous materials, organic-inorganic hybrids will be covered in this special issue as well. Particular interest will be paid to rational, novel, and systematic development of materials with particular catalytic properties for organic reactions. Due to the importance of chiral catalysts in organic chemistry, the fabrication of nanoporous materials using chiral building blocks and their applications in asymmetric catalysis were firstly reviewed by Silva et al. Next, Wu et al. dealt with enantioselective hydrogenation catalyzed by chiral nanoporous materials. Thereafter, several useful advanced functional materials for heterogeneous catalysis were covered. Kaskel et al. systematically summarized the design of functional nanostructured carbons for heterogeneous catalysis. Romero-Salguero et al. reviewed periodic mesoporous organosilicas as catalysts for organic reactions. Sun et al. dealt with the preparation of mesoporous solid superbases by using metal oxide interlayers, which can catalyze diverse organic reactions under mild conditions. Wang et al. reviewed catalytic applications of the zeolite ZSM-5 with hierarchical pore structure. It is known that pore engineering techniques can tailor pore sizes of nanoporous materials precisely, which can optimize different reactions through diffusivity control. In Tsai’s review, they summarized pore engineering methods for the enhancement of zeolite catalytic performances on aromatics conversion. The use of heterogeneous nanoporous catalysts in energy and environment-related organic chemistry has received increasing attention recently. In this special issue, Yao et al. reviewed the conversion of carbon dioxide, a greenhouse gas, to valuable fuels. Lu et al. summarized the applications of carbon-based materials in the photocatalytic fields. Also Yao et al. presented recent advances in liquid-phase heterogeneous photocatalysis for organic synthesis.

Asymmetric catalysis is unique in the sense that a minute quantity of a chiral catalyst is sufficient to produce large amounts of the desired chiral product [1]. With the increased demand for chiral products a number of efficient asymmetric homogeneous catalysts based on transition metal complexes with versatile chiral privileged ligands have been developed that work for a wide range of organic reactions. Their application in industry has been however hindered by their high cost and the product metal contamination issue. The immobilization of asymmetric homogeneous catalysts on porous supports has been a quite explored strategy to make them recyclable and economical. More recently, the construction of innovative hybrid porous materials such as MOFs and PMOs using a modular approach starting from conveniently di-derived chiral privileged ligands has been a reliable strategy to prepare efficient asymmetric heterogeneous catalysts with higher metal loadings than the post-synthetic techniques. The privileged ligands that have been used as MOFs or PMOs chiral building blocks are the BINOL/ BINAP, salen and bis(oxazoline). Herein focus will be given to the bis-functionalized chiral privileged ligands which have been used to synthesize this innovative nanoporous materials and the outcome of their application as asymmetric heterogeneous catalysts in organic transformations.

The recent progress, particularly in this decade, in enantioselective hydrogenation using nanoporous materials supported catalysts was reviewed. The main content consists of the enantioselective hydrogenation or asymmetric transfer hydrogenation of prochiral carbonyl compounds or other prochiral substrates containing C=C or C=N double bonds using the immobilized chiral metal complex catalysts onto nanoporous materials, and the enantioselective hydrogenation of prochiral carbonyl compounds using nanoporous materials supported noble metal nanoparticles/clusters after chirally modified with cinchona alkaloids or other chiral molecules. The nanoporous materials discussed in this review involve ordered mesoporous silicates or aluminosilicates, alumina-mesoporous silica composites, nanoporous carbons, periodic mesoporous resols and metal-organic frameworks. Many examples about the enantioselective hydrogenation using nanoporous materials supported catalysts show that the enantioselectivities are similar to or even higher when compared with the corresponding homogeneous counterparts. In particular, the heterogenization of well-established soluble transition metal-based asymmetric catalysts on inorganic oxide or polymer supports allows easy separation and hence recycling and reuse of these expensive asymmetric catalysts. Nanoporous materials supported noble metal nanoparticles/clusters are also efficient in the relevant asymmetric hydrogenation reactions.

The research and development of functional nanostructured carbons (FNCs) or FNC-based catalysts for heterogeneous catalysis is a highly dynamic and important topic in academia and industry because FNCs show integrated advantages: accessible pore system, regular pore structure, controlled bulk composition and surface chemistry, rapid ionic and electron mobility, etc as compared to their conventional analogs. Rather than a detailed treatise into every aspects of FNCs, this short review attempts to highlight some critical and important issues of a series of FNCs based on some representative reactions: (1) the pathways in rational fabrication of the structure and chemistry controlled FNCs, (2) the strategies to make the most of structural merits in design of high-performance catalysts based on FNCs, (3) the possible roles of textural parameters and surface heterogeneities of FNCs on catalytic behaviors. Finally, the challenges in the design of more tailored FNCs and FNC-based catalysts are pointed out, and the need of advanced characterizations and computational predication for in-depth understanding of carbon physics and chemistry of FNCs as well as catalysis mechanisms are suggested.

Periodic Mesoporous Organosilicas (PMOs) emerged on the scene of Materials Chemistry in 1999. Since then, the literature concerning their synthesis, characterization and applications has rapidly grown. From the very beginning, one of their main applications has been for heterogeneous catalysis. In order to impart them a catalytic behaviour, they have been functionalized to create catalytic sites on their pore walls, sometimes following well-established strategies reminiscent from mesoporous silicas but other times carrying out new methods based on the existence of organic bridges in their framework. Herein, we will give an overview on their use as catalysts, but being focused on the properties that distinguish them from mesoporous silicas and on those processes where their application involves a real advantage over other materials.

Mesoporous solid superbases have high potentials for applications as environmentally friendly catalysts in diverse reactions. However, the generation of strong basicity on mesoporous silica is quite difficult because of the weak guest-host (precursor-silica) interaction and the poor alkali-resistant capacity of host. In this paper, a method was developed to generate strong basicity on mesoporous silica SBA-15 by precoating a metal oxide layer prior to modification by base precursor KNO3. The results show that the guest-host interaction is enhanced by the introduction of metal oxides including Al2O3, MgO, ZrO2, and CeO2, which can be quantitatively characterized by hydrophilicity and electronegativity of the surface of host. As a result, the decomposition of KNO3 can proceed at much lower temperatures as compared with KNO3 supported on pure SBA-15. The alkali-resistance of mesoporous silica can also be tailored by the introduction of metal oxides, while mesostructure and basicity of resultant materials are strongly dependent on the dispersion state of metal oxides. Al2O3 and MgO can form smooth layers on the surface of SBA-15, which prevents the host from corroding by resultant strongly basic species. However, ZrO2 and CeO2 tend to aggregate, and thus the reaction of basic species with siliceous frameworks is still inevitable. We demonstrate that in the presence of an Al2O3 interlayer, materials possessing both ordered mesostructure and superbasicity (H_ = 27.0) are successfully fabricated, which is impossible to realize on pure mesoporous silica. The present method may open a new way for the design and synthesis of functional materials with strong basicity.

The recent progress, particularly in this decade, in catalytic applications of mesoporous ZSM-5 was reviewed. Mesoporous ZSM-5 materials aim to combine the shape-selectivity from micropores of ZSM-5 with enhanced mass transportation from the additional mesoporosity in catalytic reactions. The reactions discussed in this review are classified into two types: (1) the reactions mainly occurred on the external surface or in the pore mouths of the mesoporous ZSM-5 and (2) the reactions mainly occurred in micropores of ZSM-5. The external acid sites of mesoporous ZSM-5 enable the reactions involving bulky reactants that exceed the size of ZSM-5 micropores; and the presence of mesoporosity which improves the mass transport of reactants as well as products, reduces diffusion limitation and accelerates catalytic reaction. Although the advantages of mesoporous ZSM-5 compared with the conventional one are closely related to high external surface area and large mesopore volume, the acidity, including the type, strength and amount of acid sites and Al distribution, should be taken into account when the mesoporous ZSM-5 is utilized as catalyst in a real reaction. Finally, future challenges and opportunities for mesoporous ZSM-5 materials are presented.

Several environmental benign processes have been developed for transformation of aromatics utilizing zeolite catalysts with controlled porosities and diffusivities. This review aims to summarize the available pore engineering techniques for zeolites and relevant enhancement in catalytic performances during aromatics conversion reactions, especially in terms of para-dialkylbenzene product selectivity and stabilit

Energy and environmental issues are the focus of our concerns, which have become increasingly important nowadays. There is no denying that carbon dioxide is the culprit of greenhouse effect due to the excessive consumption of fossil-based fuels in the past centuries. In order to mitigate the effect of CO2 on the environment and to realize the goal of sustainable development, it is of great importance to convert it into fuels. In this review, the main points will be concentrated on recent advances in designing efficient catalysts for the hydrogenation of CO2 to valuable fuels, such as hydrogenation of CO2 to methanol, conversion of CO2 to CO via reverse water-gas shift reaction and production of hydrocarbons through Fischer-Tropsch synthesis. Meanwhile, some key obstacles that need to be cracked to speed up the industrialization of CO2 fixation will be discussed as well.

Photocatalytic nanocomposites have attracted increasing attention due to their potential for solving energy and environmental problems. Carbon-based materials that are stable, highly electroconductive, and easy to be modified, are increasingly being used to form composites with semiconductors to enhance their photocatalytic activity. This review surveys recent literature and highlights the progress being made in the development of carbon-based materials for photocatalytic application, including carbon nanotubes, graphene, carbon nanodots, activated carbon, [60]-fullerene and C3N4. The synthesis methods and mechanisms for the enhancement of the photocatalytic performance are discussed, along with the challenges and opportunities for the future development of carbon-based photocatalysts.

Selective oxidation of organic compounds in liquid-phase heterogeneous photocatalysis is crucially important, and the promising reaction routes attracted great attention in the past few years. Many novel systems were designed to obtain high selectivity for organic synthesis and the mechanism was proposed. This paper reviews the recent developments of liquid-phase heterogeneous photocatalysis for organic synthesis by selective oxidation under ultraviolet or visible light. Alcohol oxidations, aromatic hydroxylations, alkene oxidations, cyclohexane oxidations and C-C coupling reactions are discussed by using various semiconductors, such as TiO2-based photocatalysts, ZnIn2S4, RGO-Ag3VO4, Bi2O3/TiO2-xNx, Fe/Al/silicate and Au/ZnO. In particular, benzyl alcohol and its derivatives to aldehydes, C-C coupling reactions to 2,3-butanediol and aliphatic pinacol, benzene to phenol, phenol to benzenediol, and cyclohexane to cyclohexanol or cyclohexanone were reviewed in details. The photooxidation processes discussed were carried out in aerobic or anaerobic condition, and the photocatalysis factors, such as the reaction condition, the solvent, and the structure of photocatalyst, played a key role in controlling the selective oxidation of organic compounds.