Supramolecular Strategies for the Recycling of Homogeneous Catalysts

Supramolecular approaches are increasingly used in the development of homogeneous catalysts and they also provide interesting new tools for the recycling of metal-based catalysts. Various non-covalent interactions have been utilized for the immobilization homogeneous catalysts on soluble and insoluble support. By non-covalent anchoring the supported catalysts obtained can be recovered via (nano-) filtration or such catalytic materials can be used in continuous flow reactors. Specific benefits from the reversibility of catalyst immobilization by non-covalent interactions include the possibility to re-functionalize the support material and the use as “boomerang” type catalyst systems in which the catalyst is captured after a homogeneous reaction. In addition, new reactor design with implemented recycling strategies becomes possible, such as a reverse-flow adsorption reactor (RFA) that combines a homogeneous reactor with selective catalyst adsorption/desorpion. Next to these non-covalent immobilization strategies, supramolecular chemistry can also be used to generate the support, for example by generation of self-assembled gels with catalytic function. Although the stability is a challenging issue, some self-assembled gel materials have been successfully utilized as reusable heterogeneous catalysts. In addition, catalytically active coordination cages, which are frequently used to achieve specific activity or selectivity, can be bound to support by ionic interactions or can be prepared in structured solid materials. These new heterogenized cage materials also have been used successfully as recyclable catalysts.     

Synthesis of thermoregulated phase-separable triazolium ionic liquids catalysts and application for Stetter reaction

A series of polyether-substituted triazolium ionic liquids catalysts have been first synthesized for resolving the problem of separation and reuse of Stetter catalysts. The catalysts possess the properties of critical solution temperature (CST) and inverse temperature-dependent solubility in toluene/heptane solvents. Based on these properties, the catalysts can achieve the catalytic process named as thermoregulated phase-separable catalysis (TPSC) with the characteristic of homogeneous reaction at higher temperature and phase-separation at lower temperature. The novel TPSC system has been successfully applied for Stetter reaction of furfural or butanal with ethyl acrylate. The experimental results have showed that the novel catalysts exhibit excellent TPSC with high recycling efficiency.     

Synthesis Strategies towards Tagged Homogeneous Catalysts To Improve Their Separation

The recycling of homogeneous catalysts while keeping them in the homogeneous matrix is an ongoing challenge many reactions face if they are to find industrial applications. While a plethora of different synthetic approaches towards better, recyclable homogeneous catalysts exist, the literature shows a gap when one searches for a concise overview of the different catalyst modifications. This Review is designed to close that gap by summarising the existing synthesis pathways towards polar, non-polar, fluorous, and molecular-weight-enlarged catalysts and by examining their respective synthesis routes with a focus on modular and late-stage approaches. Furthermore, we map out the potential for a generally applicable tag library that allows straightforward catalyst modifications to tune them for each desired recycling strategy.     

甲烷无氧芳构化反应中钼基分子筛催化剂上积炭的表征

利用化学方法对钼基分子筛催化剂上的CH2Cl2可溶性积炭和不可溶性积炭进行分离，并通过质谱、红外光谱、核磁共振等手段对积炭进行了表征。结果表明，可溶性积炭的主组分为一种非芳烃物质，它的组成不随反应时间、反应温度和催化剂的酸性的变化而变化。可溶性积炭分子对催化剂的活性影响不大。高不饱和度的稠环芳烃型不可溶性积炭分子是催化剂失活的主要原因。     

Poly(ethylene glycol)-supported nitroxyls: branched catalysts for the selective oxidation of alcohols

Nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) are highly selective oxidation catalysts for the conversion of primary alcohols into the corresponding aldehydes. In this study, direct tethering of TEMPO units onto linear poly(ethylene glycol) (PEG) has afforded macromolecular catalysts that exhibit solubility in both aqueous and organic solvents. Recovery of the dissolved polymer-supported catalyst has been carried out by precipitation with a suitable solvent such as diethyl ether. The high catalyst activities and selectivities associated traditionally with nitroxyl-mediated oxidations of alcohols are retained by the series of "linker-less" linear PEG-TEMPO catalysts in which the TEMPO moiety is coupled directly to the PEG support. Although the selectivity remains unaltered, upon recycling of the linker-less polymer-supported catalysts, extended reaction times are required to maintain high yields of the desired carbonyl compounds. Alternatively, attachment of two nitroxyl radicals onto each functionalized PEG chain terminus via a 5-hydroxyisophthalic acid linker affords branched polymer-supported catalysts. In stark contrast to the linker-less catalysts, these branched nitroxyls exhibit catalytic activities up to five times greater than 4-methoxy-TEMPO alone under similar conditions. In addition, minimal decrease in catalytic activity is observed upon recycling of these branched macromolecular catalysts via solvent-induced precipitation. The high catalytic activities and preservation of activity upon recycling of these branched systems is attributed to enhanced regeneration of the nitroxyl species as a result of intramolecular syn-proportionation.     

Hydroformylation of Olefins by a Rhodium Single‐Atom Catalyst with Activity Comparable to RhCl(PPh3)3

Homogeneous catalysts generally possess superior catalytic performance compared to heterogeneous catalysts. However, the issue of catalyst separation and recycling severely limits their use in practical applications. Single‐atom catalysts have the advantages of both homogeneous catalysts, such as “isolated sites”, and heterogeneous catalysts, such as stability and reusability, and thus would be a promising alternative to traditional homogeneous catalysts. In the hydroformylation of olefins, single‐atom Rh catalysts supported on ZnO nanowires demonstrate similar efficiency (TON≈40000) compared to that of homogeneous Wilkinson's catalyst (TON≈19000). HAADF‐STEM and infrared CO chemisorption experiments identified isolated Rh atoms on the support. XPS and XANES spectra indicate that the electronic state of Rh is almost metallic. The catalysts are about one or two orders of magnitude more active than most reported heterogeneous catalysts and can be reused four times without an obvious decline in activity.     

Hydrogen storage and delivery: immobilization of a highly active homogeneous catalyst for the decomposition of formic acid to hydrogen and carbon dioxide

The homogeneous catalytic system, based on water-soluble ruthenium(II)–TPPTS catalyst (TPPTS = meta-trisulfonated triphenylphosphine), selectively decomposes HCOOH into H₂ and CO₂ in aqueous solution. Although this reaction results in only two gas products, heterogeneous catalysts could be advantageous for recycling, especially for dilute formic acid solutions, or for mobile, portable applications. Several approaches have been used to immobilize/solidify the homogeneous ruthenium–TPPTS catalyst based on ion exchange, coordination and physical absorption. The activity of the various heterogeneous catalysts for the decomposition of formic acid has been determined. These heterogenized catalysts offer the advantage of easy catalyst separation/recycling in dilute formic acid, or for mobile, portable applications.     

Various methods for enhancement in heterogeneous catalysed biodiesel production reaction rate

The biodiesel can be produced by transesterification and esterification reaction with edible and non-edible oil respectively. These reactions are catalysed by both homogeneous and heterogenous catalyst. The transesterification reactions for heterogeneous catalysts proceed at a relatively slow rate. The heterogeneous reaction mixture constitutes a three-phase system, oil/alcohol/catalyst therefore the mass transfer limitation controls the reaction rate. In the present study Tetra-hydrofuran, Hexane and Heptane as co-solvent have been tested for the transesterification and esterification reaction. Tetra-hydrofuran, assists in decreasing the mass transfer between the oil and methanol phases. Tetra-hydrofuran was found to be the best co-solvent. It also represents that the presence of Tetra-hydrofuran only enhance the solubility of phases not final equilibrium yield. Results are compared with homogeneous and heterogeneous catalysed reaction. Ultrasonic irradiation influenced the mixing of the reaction mixture and resulted in higher yields, for each co-solvent case.     

A homogeneous, recyclable polymer support for Rh(I)-catalyzed C-C bond formation

A robust and practical polymer-supported, homogeneous, recyclable biphephos rhodium(I) catalyst has been developed for C-C bond formation reactions. Control of polymer molecular weight allowed tuning of the polymer solubility such that the polymer-supported catalyst is soluble in nonpolar solvents and insoluble in polar solvents. Using the supported rhodium catalysts, addition of aryl and vinylboronic acids to the electrophiles such as enones, aldehydes, N-sulfonyl aldimines, and alkynes occurs smoothly to provide products in high yields. Additions of terminal alkynes to enones and industrially relevant hydroformylation reactions have also been successfully carried out. Studies show that the leaching of Rh from the polymer support is low and catalyst recycle can be achieved by simple precipitation and filtration.     

准均相的铂钯合金纳米催化剂用于芳香族化合物脱卤反应

用一锅法合成的负载于聚酰胺酸盐上的铂钯纳米催化剂,可以通过调节溶液的p H值实现催化剂与反应体系的有效分离和循环利用.准均相的铂钯催化剂应用于水相中卤代芳香族化合物的氢化脱卤反应,转化率达到99%以上,并且在重复使用5次后仍然保持很高的活性.铂钯双金属催化剂拥有比单一金属铂或者钯更高的催化活性,这主要是由于铂钯合金在催化反应时具有协同效应.利用X射线衍射仪(XRD),透射电子显微镜(TEM)等方法对催化剂进行了表征.数据表明铂钯纳米粒子负载于聚酰胺酸上以后可以在水溶液中稳定存在并且处于均匀的分布状态,纳米粒子尺寸约为4 nm.     

Asymmetric Cu-catalyzed Henry reaction using chiral camphor Schiff bases immobilized on a macromolecular chain

Immobilized catalysts have attracted chemists’ attention for long time because of convenient recycling, which is very important for some special catalyst even immobilizations accompanying the decrease of catalytic activity and selectivity. Our group focused on the developing chiral catalysts with camphor framework to catalyze various asymmetric reactions for long time. For easily recycling the unique chiral catalysts, a series of polymer catalysts with chiral camphor unit were synthesized by RAFT polymerization. Herein, the performance of the synthesized polymer chiral catalysts was reported by catalyzing asymmetric Henry reaction. After optimizing the reaction conditions, the synthesized chiral polymer catalyst provides a good yield and enantioselectivity to the reaction of p-nitrobenzaldehyde and nitromethane. In the meantime, the recycles and reused properties of synthesized polymer catalysts were examination by the model asymmetric Henry reaction. The catalyzed activities and enantioselectivities did not show obviously decrease until recycling five times.     

可溶性高分子负载催化剂*

均相催化剂的负载化是解决催化剂分离与回收的一条有效途径,也是绿色化学研究的重要内容。可溶性高分子,尤其是树状大分子作为另一类催化剂载体近年来受到了越来越多的关注。通过选择合适的反应介质,可溶性高分子负载催化剂可以在均相条件下催化有机反应,反应结束后通过外加不良溶剂的固/液相分离、温度等调控的液/液相分离以及膜过滤等方法进行催化剂的分离与回收。本文概述了在可溶性高分子负载催化剂研究中取得的新进展,重点介绍了负载手性催化剂在不对称催化反应中的应用。     

Thermomorphic Multiphase Systems: Switchable Solvent Mixtures for the Recovery of Homogeneous Catalysts in Batch and Flow Processes

Over the past 20 years, thermomorphic multiphase systems (TMS) have been used as a versatile and elegant strategy for the recovery and recycling of homogeneous transition-metal catalysts, in both batch-scale experiments and continuously operated processes. TMS ensure a homogeneous reaction in a monophasic reaction mixture at reaction temperature and the recovery of the homogeneous transition-metal catalyst through liquid–liquid separation at a lower separation temperature. This is achieved by using at least two solvents, which have a highly temperature-sensitive miscibility gap. The suitability of commercially available solvents makes this approach highly interesting from an industrial point of view. For the first time, herein, all studies in the area of TMS are reviewed, with the aim of providing a concise and integral representation of this approach for homogeneous catalyst recovery. In addition to the discussion of examples from the literature, the thermodynamic fundamentals of the temperature-dependent miscibility of solvents are also presented. This review also gives key indicators to compare different TMS approaches, for instance. In this way, new solvent combinations and in-depth research, as well as improvements to existing approaches, can be addressed and promoted.     

Supported ionic liquid asymmetric catalysis. A new method for chiral catalysts recycling. The case of proline-catalyzed aldol reaction

A new method for chiral catalysts recycling, based on the supported ionic liquid asymmetric catalysis concept, has been developed. This concept involves the treatment of a monolayer of covalently attached ionic liquid on the surface of silica gel with additional ionic liquid. These layers serve as the reaction phase in which the homogeneous chiral catalyst is dissolved. As first application of this concept the L-proline-catalyzed aldol reaction has been carried out. Good yields and ee values, comparable with those obtained under homogeneous conditions have been obtained. Moreover, this material shows high regenerability.     

Catalytic reactions in ionic liquids

The chemical industry is under considerable pressure to replace many of the volatile organic compounds (VOCs) that are currently used as solvents in organic synthesis. The toxic and/or hazardous properties of many solvents, notably chlorinated hydrocarbons, combined with serious environmental issues, such as atmospheric emissions and contamination of aqueous effluents is making their use prohibitive. This is an important driving force in the quest for novel reaction media. Curzons and coworkers, for example, recently noted that rigorous management of solvent use is likely to result in the greatest improvement towards greener processes for the manufacture of pharmaceutical intermediates. The current emphasis on novel reaction media is also motivated by the need for efficient methods for recycling homogeneous catalysts. The key to waste minimisation in chemicals manufacture is the widespread substitution of classical 'stoichiometric' syntheses by atom efficient, catalytic alternatives. In the context of homogeneous catalysis, efficient recycling of the catalyst is a conditio sine qua non for economically and environmentally attractive processes. Motivated by one or both of the above issues much attention has been devoted to homogeneous catalysis in aqueous biphasic and fluorous biphasic systems as well as in supercritical carbon dioxide. Similarly, the use of ionic liquids as novel reaction media may offer a convenient solution to both the solvent emission and the catalyst recycling problem.     

Recent advances in vanadium catalyzed oxygen transfer reactions

Vanadium complexes have proven to be effective catalysts for the activation of peroxides and the selective oxidation of substrates like bromides, sulfides and alkenes. Besides their capability to form metalloperoxo species, which effectively transfer oxygen atoms to the substrate, these systems are synthetically useful for obtaining valuable oxidized molecules on a preparative scale, with a high degree of selectivity and TONs. Furthermore, the use of environmentally friendly oxidants like hydrogen and alkyl hydroperoxides increases significantly their potential application at an industrial level.Here we report a critical survey on the most effective homogeneous vanadium catalysts reported in the last decade concerning their synthetic application in oxygen transfer reactions (sulfoxidation, epoxidation, haloperoxidation) using hydrogen peroxide or alkyl hydroperoxides, demonstrating the different classes of ligands and complexes, their catalytic performances, their reactivity, chemo, stereo and substrate selectivity. Some examples of the use of non conventional reaction media or techniques and catalyst recycling studies will be also discussed.     

用十聚钨酸季铵盐催化过氧化氢氧化环己烯为己二酸

己二酸是重要的化学品.开发绿色、洁净的氧化方法合成己二酸,一直吸引着许多化学工作者的注意.1998年,Noyori等[1]报道了使用钨酸钠和甲基三辛基硫酸氢铵作为催化剂把环己烯氧化为己二酸的方法.1999年邓友全等[2,3]报道了在钨酸钠和草酸等形成原位过氧钨酸有机配合物催化剂存在下,使用30%过氧化氢由环己烯合成己二酸的方法.姜恒等[4,5]先后在甲基三辛基硫酸氢铵替代物的选择上和过氧钨酸有机配合物有机配体的作用方面进行了较为系统的工作.这些研究工作虽然都能直接获得高质量和高产率的己二酸,但是都存在着催化剂难于分离的问题.重复…     

Asymmetric heterogeneous catalysis

Limited natural resources and an increasing demand for enantiomerically pure compounds render catalysis and especially heterogeneous asymmetric catalysis a key technology. The field has rapidly advanced from the initial use of chiral biopolymers, such as silk, as a support for metal catalysts to the modern research areas. Mesoporous supports, noncovalent immobilization, metal-organic catalysts, chiral modifiers: many areas are rapidly evolving. This Review shows that these catalysts have more to them than facile separation or recycling. Better activities and selectivities can be obtained than with the homogeneous catalyst and novel, efficient reaction mechanisms can be employed. Especially fascinating is the outlook for highly ordered metal-organic catalysts that might allow a rational design, synthesis, and the unequivocal structural characterization to give tailor-made catalysts.     

单分子层保护纳米 Au 粒子表面配位催化剂的制备及其应用

 贵金属纳米粒子由于其小尺寸效应而表现出特殊的催化性能. 综述了纳米 Au 粒子表面配位催化剂的制备方法及其在催化中的应用. 由于 Au 可与硫化物形成配位键, 所以硫化物可在 Au 表面形成有序单分子膜. 单分子膜保护的 Au 纳米粒子具有非常好的溶解性、分散性、稳定性, 以及由不同的表面功能团而导致的不同的催化性能. 该催化体系兼具均相催化剂和多相催化剂的特点, 这对开发新型催化剂具有重要的理论和实际意义.     

Recent developments of heterogeneous solid catalysts for liquid-phase Friedel–Crafts type benzylation reaction

Liquid-phase Friedel–Crafts type benzylation of aromatics has been effected traditionally with catalysis by homogeneous protonic acid or Lewis acid. However, heterogeneous catalysts have the advantages, compared to their homogeneous counterparts, of facile recovering and recycling. This short article describes the recent advances in the liquid-phase Friedel–Crafts type benzylation by benzyl chloride of aromatics over redox metal: gallium (Ga), indium (In) and thallium (Tl) containing novel heterogeneous solid catalysts. Unlike conventional acidic catalyst, the benzylation activity of the Ga-, In- or Tl-based solids does not depend solely on their acidic properties, even present; these solids in their non-acidic or basic form also shows high benzylation activity. The catalytic activity order of Ga, In and Tl containing solid catalysts supported on chemically similar inert catalyst carrier is as follows: thallium/support > indium/support > gallium/support, which is quite similar to their redox potential values indicating the role of redox function in the benzylation process. A plausible reaction mechanism for the benzylation reaction over these catalysts is proposed. These heterogeneous solids are highly efficient for the Friedel–Crafts type benzylation, even in the presence of moisture, than the conventional strongly acidic solid acid catalysts.Dedicated to Dr.Vasanth R. Choudhary, National Chemical Laboratory, Pune, India