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.

Ethene Hydroformylation Catalyzed by Rhodium Dispersed with Zinc or Cobalt in Silanol Nests of Dealuminated Zeolite Beta

Catalysts for hydroformylation of ethene were prepared by grafting Rh into nests of ≡SiOZn-OH or ≡SiOCo-OH species prepared in dealuminated BEA zeolite. X-ray absorption spectra and infrared spectra of adsorbed CO were used to characterize the dispersion of Rh. The Rh dispersion was found to increase markedly with increasing M/Rh (M = Zn or Co) ratio; further increases in Rh dispersion occurred upon use for ethene hydroformylation catalysis. The turnover frequency for ethene hydroformylation measured for a fixed set of reaction conditions increased with the fraction of atomically dispersed Rh. The ethene hydroformylation activity is 15.5-fold higher for M = Co than for M = Zn, whereas the propanal selectivity is slightly greater for the latter catalyst. The activity of the Co-containing catalyst exceeds that of all previously reported Rh-containing bimetallic catalysts. The rates of ethene hydroformylation and ethene hydrogenation exhibit positive reaction orders in ethene and hydrogen but negative orders in carbon monoxide. In situ IR spectroscopy and the kinetics of the catalytic reactions suggest that ethene hydroformylation is mainly catalyzed by atomically dispersed Rh that is influenced by Rh-M interactions, whereas ethene hydrogenation is mainly catalyzed by Rh nanoclusters. In situ IR spectroscopy also indicates that the ethene hydroformylation is rate limited by formation of propionyl groups and by their hydrogenation, a conclusion supported by the measured H/D kinetic isotope effect. This study presents a novel method for creating highly active Rh-containing bimetallic sites for ethene hydroformylation and provides new insights into the mechanism and kinetics of this process.

Rhodium Single-Atom Catalyst Design through Oxide Support Modulation for Selective Gas-Phase Ethylene Hydroformylation

A frontier challenge in single-atom (SA) catalysis is the design of fully inorganic sites capable of emulating the high reaction selectivity traditionally exclusive of organometallic counterparts in homogeneous catalysis. Modulating the direct coordination environment in SA sites, via the exploitation of the oxide support's surface chemistry, stands as a powerful albeit underexplored strategy. We report that isolated Rh atoms stabilized on oxygen-defective SnO₂ uniquely unite excellent TOF with essentially full selectivity in the gas-phase hydroformylation of ethylene, inhibiting the thermodynamically favored olefin hydrogenation. Density Functional Theory calculations and surface characterization suggest that substantial depletion of the catalyst surface in lattice oxygen, energetically facile on SnO₂ , is key to unlock a high coordination pliability at the mononuclear Rh centers, leading to an exceptional performance which is on par with that of molecular catalysts in liquid media.

Current State of the Art of the Solid Rh-Based Catalyzed Hydroformylation of Short-Chain Olefins

The hydroformylation of olefins is one of the most important homogeneously catalyzed processes in industry to produce bulk chemicals. Despite the high catalytic activities and selectivity’s using rhodium-based homogeneous hydroformylation catalysts, catalyst recovery and recycling from the reaction mixture remain a challenging topic on a process level. Therefore, technical solutions involving alternate approaches with heterogeneous catalysts for the conversion of olefins into aldehydes have been considered and research activities have addressed the synthesis and development of heterogeneous rhodium-based hydroformylation catalysts. Different strategies were pursued by different groups of authors, such as the deposition of molecular rhodium complexes, metallic rhodium nanoparticles and single-atom catalysts on a solid support as well as rhodium complexes present in supported liquids. An overview of the recent developments made in the area of the heterogenization of homogeneous rhodium catalysts and their application in the hydroformylation of short-chain olefins is given. A special focus is laid on the mechanistic understanding of the heterogeneously catalyzed reactions at a molecular level in order to provide a guide for the future design of rhodium-based heterogeneous hydroformylation catalysts.

Sustainable route for the synthesis of flower-like Ni@N-doped carbon nanosheets from bagasse and its catalytic activity towards reductive amination of nitroarenes with bio-derived aldehydes

Waste derived N-doped carbon for one pot domino catalytic transformation starting from nitroarenes and carbonyl compounds directed towards the preparation of imines and benzimidazole products.

Multi-walled carbon nanotube-based composite materials as catalyst support for water-gas shift and hydroformylation reactions

In times of depleting fossil fuel reserves, optimizing industrial catalytic reactions has become increasingly important. One possibility for optimization is the use of homogenous catalysts, which are advantageous over heterogeneous catalysts because of mild reaction conditions as well as higher selectivity and activity. A new emerging technology, supported ionic liquid phase (SILP), was developed to permanently immobilize homogeneous catalyst complexes for continuous processes. However, these SILP catalysts are unable to form freestanding supports by themselves. This study presents a new method to introduce the SILP system into a support made from multi-walled carbon nanotubes (MWCNT). In a first step, SILP catalysts were prepared for hydroformylation as well as low-temperature water-gas shift (WGS) reactions. These catalysts were integrated into freestanding microtubes formed from MWCNTs, with silica (for hydroformylation) or alumina particles (for WGS) incorporated. In hydroformylation, the activity increased significantly by around 400% when the pure MWCNT material was used as SILP support. An opposite trend was observed for WGS, where pure alumina particles exhibited the highest activity. A significant advantage of the MWCNT composite materials is the possibility to coat them with separation layers, which allows their application in membrane reactors for more efficient processes.

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.