EPOXIDATION OF FUNCTIONALIZED OLEFINS OVER SOLID CATALYSTS

Heterogeneous catalytic epoxidation of functionalized olefins in the liquid phase has been reviewed, focusing on catalyst performance and its interrelation with the crucial parameters of the catalytic systems. Efficient catalysts include supported and mixed oxides, framework-substituted (“redox”) molecular sieves, layered-type materials, heterogenized homogeneous catalysts, and some others. Among the various substrates, allylic and homoallylic alcohols, and unsaturated carbonyl compounds have received most attention so far. The great variety of available catalysts enables selective epoxidation of most substituted olefins. The mechanistic understanding of heterogeneous catalytic epoxidation is still underdeveloped, rendering catalyst design rather empirical. A considerable potential for future development lies in the area of “heterogenization” of successful homogeneous catalysts especially for asymmetric epoxidation. Crucial requirements in the development of heterogeneous catalytic epoxidation catalysts are, besides good catalytic performance and cheap oxidant, recyclability and resistance to leaching of the active component. Some of the examples shown in the literature do not fulfill the latter requirement.     

Attempts to Fill the Gap Between Enzymatic, Homogeneous, and Heterogeneous Catalysis

Bio-inspired and single site metal complex catalysts have been discussed to direct towards a rational design of solid heterogeneous catalysts. When concepts derived from catalytic antibodies, molecular imprinting and molecular recognition, and site isolation and modification by appropriate ligands are combined, with new techniques to prepare, tailor made solid materials, catalysts can be prepared that improve reaction rate and selectivity by increasing the concentration and activation of reactants in the vicinity of the active sites, and by stabilizing transition states or intermediate products. It is also shown that enzymatic, homogeneous and hetergeneous catalysts can be combined to perform “one-pot” cascade reactions.     

Preparation of silica-immobilized titanium-containing silsesquioxane catalysts and activity for the epoxidation of alkenes

Silsesquioxanes containing tetrapodal titanium species and alkenylsilyl groups were immobilized onto dimethylsilyl-functionalized silica having mesopores by chemical tethering via hydrosilylative reaction in the presence of a Pt catalyst. The immobilized catalysts showed higher activity than their corresponding homogeneous catalyst towards the epoxidation of cyclooctene by tert-butylhydroperoxide, probably because of the improvement of the micro-environment around the titanium center. The reaction was found to be truly heterogeneous, since no further reaction proceeded after the separation of the catalyst by filtration. These catalysts also showed excellent catalytic activity for the epoxidation using hydrogen peroxide as an oxidant, while the parent homogeneous catalysts did not show any activity at all.     

“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.     

Heterogeneous catalyst mixtures for the polymerization of ethylene

Heterogeneous cocatalysts, catalysts, and catalyst mixtures for the polymerization of ethylene were prepared applying “fumed silica” and mesoporous MCM-41 support materials and zirconocene dichloride, titanocene dichloride, and a bis(arylimino)pyridine iron complex as catalyst precursors. The catalyst mixtures produced polyethylenes which exhibit the properties of two single polymers. Polyethylenes with the desired bimodal molecular weight distributions could be obtained with a series of ternary Zr/Ti/Fe catalysts. The ability of the zirconium and titanium species to copolymerize short-chain 1-olefins produced by the iron centers (“in situ” copolymerization) is useful for the production of copolymers from only one monomer (ethylene).     

钼基催化剂在烯烃液相环氧化反应中的应用研究进展

介绍了钼络合物催化剂、固载化的钼络合物催化剂及MoO₃基的固体钼催化剂在烯烃液相环氧化反应中的应用。以有机过氧化物为氧化剂时,不同的钼基催化剂均表现出很好的催化性能,其中络合物中的配体对其催化性能有很大影响,并且可以通过引入手性配体实现对烯烃的不对称环氧化,而固体钼催化剂的性能则与载体上钼物种的种类密切相关。但绝大多数钼催化剂对H₂O₂参与的环氧化反应没有活性,开发对H₂O₂有催化活性的钼基催化剂是一个重要的研究方向。     

Asymmetric epoxidation of unfunctionalized olefins catalyzed by Mn(III) salen complex immobilized on MCM-48

Chiral Mn(III) salen complexes were supported on mesoporous molecular sieve MCM-48 and employed as catalyst in the asymmetric epoxidation of some unfunctionalized olefins. The as-synthesized catalysts showed excellent enantioselectivity than homogeneous catalysts for α-methylstyrene and the highest ee value (up to >99%) was obtained. Furthermore, these catalysts were also effective for bulkier olefins such as indene and 1-phenylcyclohexene. Compared to homogeneous counterparts, the heterogeneous catalysts are more stable and can be recycled three times without loss of enantioselectivity.     

Plasmachemical synthesis and regeneration (activation) of nanostructured catalysts for steam conversion of methane

Under the conditions of thermal electric-arc plasma in a plasmachemical reactor (PCR) with “cold” (CW) and “warm” (WW) walls, catalysts were synthesized and/or regenerated to be used for conversion of methane with steam. The “plasma” samples thus produced were characterized by means of electron microscopy, X-ray diffraction, and chemical analyses.

It was established that the plasma samples are reduced 2–4 times as fast as their conventional analog G56A. Upon application of an external high-frequency (HF) discharge, the reduction rate of the plasmachemically synthesized samples rose by about 10%. The catalysts synthesized in a CW PCR had lower activity than their industrial analog G56A, while those synthesized in a WW PCR had activity comparable to that used in the industry (HIMKO Company, Vratsa, Bulgaria); moreover, under certain technological conditions, they were more active (by up to 10%), e.g., in the case of heterogeneous catalytic interaction between methane and water steam with applied HF discharge.     

Modification of the preparation procedure for increasing the hydrodesulfurisation activity of the CoMo/γ-alumina catalysts

In this work we have examined whether the re-impregnation of CoMo/γ-alumina catalysts or the replacement of the conventional non-dry impregnation step by “equilibrium deposition filtration” (EDF) may be used for improving their surface characteristics and thus their catalytic activity.

Two samples were prepared. In the first sample (EDF) the molybdenum species were mounted by “equilibrium deposition filtration” whereas in the second sample these species were mounted by non-dry impregnation (NDI). In both cases the Co was deposited on the calcined Mo/γ-Al₂O₃ precursor solid by simple dry impregnation. An aliquot of each sample was impregnated again with an amount of pure water equal to its pore volume and then it underwent drying and calcination.

The catalysts prepared were characterized using N₂ adsorption measurements (BET), UV–vis diffuse reflectance spectroscopy (DRS), laser Raman spectroscopy (LRS) and NO chemisorption. The hydrodesulfurization (HDS) activities over the catalysts studied were determined using a continuous-flow tubular fixed-bed microreactor operating in a differential mode at atmospheric pressure.

It was confirmed that the replacement of the conventional impregnation by equilibrium deposition filtration results to catalysts with relatively high active surface and high portion of the well-dispersed octahedral cobalt and thus, to catalysts with 30% higher HDS activity. The re-impregnation resulted to partial dissolution and re-dispersion of the Mo and Co supported oxidic phases. Concerning the NDI catalyst re-impregnation resulted to an increase of the active surface and of the portion of the well-dispersed octahedral cobalt and thus to 25% higher catalytic activity. The opposite effects were observed for the EDF catalyst which exhibited almost 7% lower activity after re-impregnation.     

The impact of the “memory effect” on the catalytic activity of Mg/Al; Mg,Zn/Al; Mg/Al,Ga hydrotalcite-like compounds used as catalysts for cycloxene epoxidation

The epoxidation of cyclohexene by hydrogene peroxide and benzonitrile, in methanol as solvent on solid base catalysts, hydrotalcites (HT) and corresponding mixed oxides (C-HT) was investigated. The employed catalysts were as-synthesized Mg/Al; Mg,Zn/Al; Mg/Ga,Al hydrotalcites, their derived mixed oxides and the corresponding reconstructed hydrotalcite forms (R-HT) obtained via the “memory effect” upon a rehydration treatment of C-HT. The catalysts were characterized by XRD, CO₂-TPD and N₂ adsorption–desorption measurements.

The reconstructed HT samples exhibit higher activity in comparison to the as-synthesized carbonated HT samples. A tentative correlation of the catalytic properties with the base strength of the active sites, the structural and textural characteristics of the catalysts, has been explored.     

Synthesis, Characterization and Catalytic Applications of Organized Mesoporous Aluminas

The synthesis of organized mesoporous aluminas has opened a very interesting area for application of this type of materials, particularly as catalysts or catalyst supports. This review focuses on the individual synthesis routes to produce organized mesoporous aluminas with large surface areas and narrow pore size distributions, and on the evaluation of their textural, chemical and thermal properties and outlines examples of catalytic applications of organized mesoporous alumina-based catalysts. We tried to rationalize the synthetic approaches to prepare organized mesoporous aluminas, to relate their properties to synthetic procedures used as well as to their catalytic behavior in different reactions. Utilization of various structure-directing agents for “cationic,” “neutral,” “anionic,” “nanocasting,” and special approaches leading to scaffolding and lathlike organized mesoporous aluminas is discussed in the first part of this review, as well as textural and structural characterization and thermal stability of mesoporous aluminas synthesized by different synthetic approaches. In the second part, catalytic applications of organized mesoporous aluminas described in the open literature are evaluated from the standpoint of the importance of these reactions for technological applications.     

Modification of faujasites to generate novel hosts for “ship-in-a-bottle” complexes

Zeolites X, Y, and DAY have been modified by a post-synthetical dealumination procedure to generate mesopores that are completely surrounded by micropores. In these novel host materials several bulky transition metal salen complexes have been occluded via the “ship-in-a-bottle” synthesis approach. Both the host materials and the “ship-in-a-bottle” catalysts have been characterized by FTIR spectroscopy and nitrogen adsorption. Additionally, the “ship-in-a-bottle” catalysts have been characterized by thermogravimetric analysis and ICP-AES spectroscopy. The catalysts have been tested in the stereoselective epoxidation of R-(+)-limonene and (−)--pinene with molecular oxygen as oxidant. The best results so far — 100% conversion, 96% selectivity and 91% de — were achieved with the immobilized (R,R)-(N,N′)-bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethylene-1,2-diaminocobalt complex in the epoxidation of (−)--pinene.     

Hydrogenation active sites of unsupported molybdenum sulfide catalysts for hydroprocessing heavy oils

The purpose of the present study was to elucidate the nature of the hydrogenation active sites on unsupported molybdenum sulfide catalysts, aimed at the improvement of the catalysts for the slurry processes. The number of hydrogenation active sites was found to relate to the “inflection” on the basal plane of the catalyst particles. The comparison of the catalytic activity to that of an oil-soluble catalyst in the hydroprocessing of heavy oils suggests that the performance of the oil-soluble catalyst was near the maximum, unless another component such as Ni or Co was incorporated.     

Chiral salen Mn(III) immobilized on sulfoalkyl modified ZSP-IPPA as an effective catalyst for asymmetric epoxidation of unfunctionalized olefins

Sulfoalkyl modified zirconium poly (styrene-isopropenyl phosphonate)-phosphate (ZPS-IPPA), with special structures of new type of organic–inorganic hybrid material were designed and synthesized for immobilization of the chiral salen Mn(III) Jacobsen’s homogenous catalyst by axial coordination. All the heterogeneous chiral salen Mn(III) catalysts with different linkage lengths obtained exhibited great catalytic activity and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins. The influence of the linkage lengths on the catalytic performance was investigated. What’s more, the catalysts were easily separated from the reaction systems and could be reused for several times without significant loss of catalytic activity.     

Plasma-chemical synthesis and regeneration of catalysts for reforming natural gas

This study describes a pioneering investigation of plasma-chemical synthesis (PCS) and/or regeneration of natural gas-reforming catalysts under the conditions of electric arc, low temperature plasma (LTP), as a function of plasma-chemical process (PCP) parameters and plasma-chemical reactor (PCR) type (with “cold walls” or “warm walls”). Based on the model calculations, a plasma-chemical installation was designed, built, and used to study the processes of catalyst preparation and regeneration of spent, deactivated catalysts for natural gas-reforming.

Plasma-chemically synthesized and/or regenerated samples were analyzed by means of complex physical-chemical, X-ray pattern structural, and electron-microscopic analyses. And the dynamics and kinetics of active surface formation by reduction of catalysts were studied.

A temperature range of 2000–3000 K has been established as optimal for synthesizing samples with maximum dispersion, producing a reduction rate 2–4 times faster than industrial analogues. Samples with high catalytic activity (CH₄ conversion rate, catalyst performance, catalyst efficiency) and thermal stability were obtained.

Thus, it has been found that LTP can be successfully used for catalyst synthesis and regeneration.     

烯烃环氧化钼系催化剂的多相化研究进展

介绍了以有机聚合物和无机硅酸盐为载体，负载钼催化剂及其对烯烃环氧化反应催化性能的研究进展；分析了硼酸树脂、乙烯-丙烯橡胶、螯合树脂、苯并吡咯树脂、聚酰亚胺树脂、聚硅氧烷树脂、阳离子交换树脂和聚苯乙烯等有机聚合物为载体的研究情况；评价了以无机硅酸盐为载体，采用溶胶-凝胶、离子交换、同晶取代等方法负载钼对催化性能的影响；展望了多相化烯烃环氧化钼系催化剂的发展前景。     

Olefin epoxidation with metal-based nanocatalysts

Alkene epoxidations are an important class of reactions carried out in industry; however, current methods are plagued by problems, including high cost, difficulty in recovering catalysts, and generation of large quantities of acidic and chlorinated waste. In recent years, nanocatalysts have been considered as robust, heterogeneous alternatives to homogeneous catalysts. This work evaluates silver- and base metal-containing nanocatalysts as olefin epoxidation catalysts, highlighting the industrial applicability and green aspects of these catalytic systems. The nanocatalysts discussed are mostly supported or composite materials that showed (generally) good activity and selectivity for various/multiple olefin epoxidation reactions.     

Optimisation of olefin epoxidation catalysts with the application of high-throughput and genetic algorithms assisted by artificial neural networks (softcomputing techniques)

An olefin epoxidation Ti catalyst has been optimised by means of high-throughput experimentation involving materials synthesis, postsynthesis treatments, and catalytic testing. Softcomputing techniques for advanced experimental design have been used. The variables explored in the hydrothermal synthesis of Ti-silicate-based catalysts were: concentration of OH⁻, titanium, and surfactant. The probe reaction employed for the optimisation was the solvent-free epoxidation of cyclohexene, with tert-butylhydroperoxide as oxidant. The different catalyst groups detected by clustering analysis were studied by XRD and UV diffuse reflectance. Ti-mesoporous MCM-41 and MCM-48 molecular sieves were among the most active catalysts. The best performing catalysts were tested for epoxidation of different linear olefins.     

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria

The effects of ZrO₂ content on the CO oxidation activity in a series of CuO_x/Ce_xZr_1−xO₂ (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H₂, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.     

Ketone-catalyzed asymmetric epoxidation reactions

This Account summarizes our efforts in developing organic ketone catalysts for enantioselective and diastereoselective epoxidation of olefins. We have developed an efficient and general protocol for epoxidation of olefins using dioxiranes generated in situ from activated ketones and Oxone (2KHSO5.KHSO4.K2SO4) in a homogeneous aqueous solvent system. We have also developed chiral ketone catalysts for highly enantioselective epoxidation of unfunctionalized trans-olefins and trisubstituted olefins. Excellent diastereoselectivities have been achieved for epoxidation of substituted cyclohexenes with ketone catalysts.