Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Silicon Oxycarbide (SiOC)-Supported Ionic Liquids: Heterogeneous Catalysts for Cyclic Carbonate Formation

Silicon oxycarbides (SiOCs) impregnated with tetrabutylammonium halides (TBAX) were investigated as an alternative to silica-based supported ionic liquid phases for the production of bio-based cyclic carbonates derived from limonene and linseed oil. The support materials and the supported ionic liquid phases (SILPs) were characterized via Fourier transform infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption, X-ray photoelectron spectroscopy, microscopy, and solvent adsorption. The silicon oxycarbide supports were pyrolyzed at 300-900 °C prior to being coated with different tetrabutylammonium halides and further used as heterogeneous catalysts for the formation of cyclic carbonates in batch mode. Excellent selectivities of 97-100% and yields of 53-62% were obtained with tetrabutylammonium chloride supported on the silicon oxycarbides. For comparison, the catalytic performance of commonly employed silica-supported ionic liquids was investigated under the same conditions. The silica-supported species triggered the formation of a diol as a byproduct, leading to a lower selectivity of 87% and a lower yield of 48%. Ultimately, macroporous monolithic SiOC-SILPs with suitable permeability characteristics (k1 = 10-11 m2) were produced via photopolymerization-assisted solidification templating and applied for the selective and continuous production of limonene carbonate with supercritical carbon dioxide as the reagent and sole solvent. Constant product output over 48 h without concurrent catalyst leaching was achieved.

Reworking Organic Synthesis for the Modern Age: Synthetic Strategies Based on Continuous-Flow Addition and Condensation Reactions with Heterogeneous Catalysts

While organic synthesis carried out in most laboratories uses batch methods, there is growing interest in modernizing fine chemical synthesis through continuous-flow processes. As a synthetic method, flow processes have several advantages over batch systems in terms of environmental compatibility, efficiency, and safety, and recent advances have allowed for the synthesis of several complex molecules, including active pharmaceutical ingredients (APIs). Nevertheless, due to several reasons related to the difficulties arising from byproduct formation during the flow process, such as lower yields, poor selectivities, clogging of columns due to poor solubility, catalyst poisoning, etc., successful examples of continuous-flow synthesis of complex organic molecules are still limited. In order to solve this bottleneck, the development of selective and atom-economical continuous-flow organic transformations are needed. This perspective highlights examples of atom-economical addition and condensation reactions with heterogeneous catalysts under continuous-flow conditions and their applications for the synthesis of complex organic molecules such as natural products and APIs. In order to realize new continuous-flow methodologies, based on addition and condensation reactions, in place of substitution reactions, the development of novel reactions and heterogeneous catalysts is required.

Recent Advances in Applications of Supported Ionic Liquids

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The supported ionic liquids have shown immense potential for numerous applications in catalysis and separation science. In the present review, the remarkable contribution of supported ionic liquids has been highlighted. The main emphasis has been laid on describing the facile separation of gas from binary gas mixtures owing to the capability of selective transport of permeable gases across supported membranes and removal of environmentally hazard sulfur compounds from fuels. The catalytic action of supported ionic liquids has been discussed in other applications such as biodiesel (biofuel) synthesis by transesterification/esterification processes, waste CO2 fixation into advantageous cyclic carbonates, and various chemical transformations in organic green synthesis. This review enclosed a maximum of the published data of the last ten years and also recently accomplished work concerning applications in various research areas like separation sciences, chemical transformations in organic green synthesis, biofuel synthesis, waste CO2 fixation, and purification of fuels by desulfurization.

Molecular design of ionic liquids as novel non-metal catalysts for the acetylene hydrochlorination reaction

Theoretical prediction of catalytic performance is crucial for the rational design of novel catalysts. In this study, density functional theory (DFT) simulations were carried out to predict the catalytic performance of four ionic liquids (ILs) used as novel non-metal catalysts in the acetylene hydrochlorination reaction, and the obtained catalytic performances were verified via our experimental tests; moreover, both the theoretical and experimental results showed that the catalytic performance of the four IL catalysts followed the order tetraphenylphosphonium bromide (TPPB) > tetraphenylphosphonium chloride (TPPC) > butyltriphenylphosphonium bromide (BuTPPB) ≫ tetraphenylphosphonium tetrafluoroborate (TPPT), and the 15%TPPB/SAC catalyst exhibited efficient catalytic performance when compared with the recently reported non-metal catalysts for the acetylene hydrochlorination reaction. Furthermore, the catalytic mechanisms of the four ILs with different cations and anions were revealed via theoretical Mulliken, partial density of states (PDOS) and electron density difference (EDD) analyses combined with the experimental XPS and XRD characterizations. The results showed that the effects of the anions on the catalytic activity were much significant than those of the cations. A good IL non-metal catalyst for acetylene hydrochlorination would mainly donate electrons to Cl to activate the H-Cl bond, and then, the electrons would be donated back to the IL catalyst in the transition state. This study provides new insights into the design of efficient nonmercuric catalysts for the acetylene hydrochlorination reaction.

Carbon-based SILP catalysis for the selective hydrogenation of aldehydes using a well-defined Fe(ii) PNP complex

In this work, the supported ionic liquid phase (SILP) method was applied for the immobilization of a newly developed, well-defined hydride Fe(ii) PNP pincer complex dissolved an in ionic liquid (IL) onto polymer-based spherical activated carbon.

Supported ionic liquid phase (SILP) facilitated gas-phase enzyme catalysis – CALB catalyzed transesterification of vinyl propionate

The supported ionic liquid phase (SILP) technology has been used to immobilize Candida Antarctica Lipase B (CALB) within a hybrid monolith.

Modification of nitrogen doped carbon for SILP catalyzed hydroformylation of ethylene

Nitrogen-doped carbon is a new material for SILP catalysts that show improved performance as function of N-content and surface basicity.

Supported ionic-liquid-phase-stabilized Au(iii) catalyst for acetylene hydrochlorination

Using high-valent Au(iii) catalysis is highly desirable in many reactions; however it is plagued by the poor stability of Au(iii) complexes.

Unlocking the potential of supported liquid phase catalysts with supercritical fluids: low temperature continuous flow catalysis with integrated product separation

Solution-phase catalysis using molecular transition metal complexes is an extremely powerful tool for chemical synthesis and a key technology for sustainable manufacturing. However, as the reaction complexity and thermal sensitivity of the catalytic system increase, engineering challenges associated with product separation and catalyst recovery can override the value of the product. This persistent downstream issue often renders industrial exploitation of homogeneous catalysis uneconomical despite impressive batch performance of the catalyst. In this regard, continuous-flow systems that allow steady-state homogeneous turnover in a stationary liquid phase while at the same time effecting integrated product separation at mild process temperatures represent a particularly attractive scenario. While continuous-flow processing is a standard procedure for large volume manufacturing, capitalizing on its potential in the realm of the molecular complexity of organic synthesis is still an emerging area that requires innovative solutions. Here we highlight some recent developments which have succeeded in realizing such systems by the combination of near- and supercritical fluids with homogeneous catalysts in supported liquid phases. The cases discussed exemplify how all three levels of continuous-flow homogeneous catalysis (catalyst system, separation strategy, process scheme) must be matched to locate viable process conditions.

Nanocatalysis in Flow

Nanocatalysis in flow is catalysis by metallic nanoparticles (NPs; 1-50 nm) performed in microstructured reactors. These catalytic processes make use of the enhanced catalytic activity and selectivity of NPs and fulfill the requirements of green chemistry. Anchoring catalytically active metal NPs within a microfluidic reactor enhances the reagent/catalyst interaction, while avoiding diffusion limitations experienced in classical approaches. Different strategies for supporting NPs are reviewed herein, namely, packed-bed reactors, monolithic flow-through reactors, wall catalysts, and a selection of novel approaches (NPs embedded on nanotubes, nanowires, catalytic membranes, and magnetic NPs). Through a number of catalytic reactions, such as hydrogenations, oxidations, and cross-coupling reactions, the advantages and possible drawbacks of each approach are illustrated.

Chiral phosphine-phosphoramidite ligands for highly enantioselective hydrogenation of N-arylimines

The asymmetric hydrogenation of N-arylimines with the chiral phosphine-phosphoramidite ligand, (Sc,Sa)-PEAPhos 2b, has been developed, in which high turnover numbers and excellent enantioselectivity were achieved.

Highly enantioselective Rh-catalysed hydrogenation of 1-alkyl vinyl esters using phosphine-phosphoramidite ligands

MatPhos, a good mate for hard tasks: The asymmetric hydrogenation of 1-alkyl vinyl esters, thwarted so far by mediocre ee values and low activities, can now be achieved with MatPhos/Rh catalysts with ee values of 96-99% for a variety of substrates at low catalyst loadings (0.1-1 mol %) and under mild conditions (5-20 bar H2, room temperature). After hydrolysis, the corresponding chiral secondary alkyl alcohols can be obtained in high enantiopurities providing a general and practical route to this important product class.

Application of metal-based reagents and catalysts in microstructured flow devices

Over the years, organic synthesis has witnessed several improvements through the development of new chemical transformations or more efficient reagents for known processes. Likewise, technological advances, aiming at speeding up reactions and facilitating their work-up, have established themselves in academic as well as in industrial laboratories. In this Minireview, we highlight very recent developments in flow chemistry, focusing on organometallic reagents and catalysts. First, we describe reactions with homogeneous catalysts immobilized on different support materials using the concept of packed bed reactors. In the last chapter, we will discuss applications that utilize organometallic reagents.

Efficient and selective chemical transformations under flow conditions: The combination of supported catalysts and supercritical fluids

This paper reviews the current trends in the combined use of supported catalytic systems, either on solid supports or in liquid phases and supercritical fluids (scFs), to develop selective and enantioselective chemical transformations under continuous and semi-continuous flow conditions. The results presented have been selected to highlight how the combined use of those two elements can contribute to: (i) Significant improvements in productivity as a result of the enhanced diffusion of substrates and reagents through the interfaces favored by the scF phase; (ii) the long term stability of the catalytic systems, which also contributes to the improvement of the final productivity, as the use of an appropriate immobilization strategy facilitates catalyst isolation and reuse; (iii) the development of highly efficient selective or, when applicable, enantioselective chemical transformations. Although the examples reported in the literature and considered in this review are currently confined to a limited number of fields, a significant development in this area can be envisaged for the near future due to the clear advantages of these systems over the conventional ones.

Heterogeneous catalytic hydrogenation reactions in continuous-flow reactors

Microreactor technology and continuous flow processing in general are key features in making organic synthesis both more economical and environmentally friendly. Heterogeneous catalytic hydrogenation reactions under continuous flow conditions offer significant benefits compared to batch processes which are related to the unique gas-liquid-solid triphasic reaction conditions present in these transformations. In this review article recent developments in continuous flow heterogeneous catalytic hydrogenation reactions using molecular hydrogen are summarized. Available flow hydrogenation techniques, reactors, commonly used catalysts and examples of synthetic applications with an emphasis on laboratory-scale flow hydrogenation reactions are presented.