Selective hydrodeoxygenation of hydroxyacetophenones to ethyl-substituted phenol derivatives using a FeRu@SILP catalyst

From Lignin to Value-Added Pharmaceutical Intermediates Based on a Benzylic Oxidation Method with O2

We present a catalytic strategy for converting lignin into various pharmaceutical intermediates based on a highly selective lignin depolymerization method and a green benzylic oxidation method employing O2. Selective depolymerization of lignin first afforded 4-ethylphenol, which then efficiently generates several pharmaceutical intermediates with a simple 5-step process, resulting in substantial economic benefits. The study provides an innovative solution for the efficient utilization of lignin and the green acquisition of pharmaceutical intermediates.

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.

Synthesis, Characterization, and Catalytic Application of Colloidal and Supported Manganese Nanoparticles

Colloidal and supported manganese nanoparticles were synthesized following an organometallic approach and applied in the catalytic transfer hydrogenation (CTH) of aldehydes and ketones. Reaction parameters for the preparation of colloidal nanoparticles (NPs) were optimized to yield small (2-2.5 nm) and well-dispersed NPs. Manganese NPs were further immobilized on an imidazolium-based supported ionic phase (SILP) and characterized to evaluate NP size, metal loading, and oxidation states. Oxidation of the Mn NPs by the support was observed resulting in an average formal oxidation state of +2.5. The MnOx@SILP material showed promising performance in the CTH of aldehydes and ketones using 2-propanol as a hydrogen donor, outperforming previously reported Mn NPs-based CTH catalysts in terms of metal loading-normalized turnover numbers. Interestingly, MnOx@SILP were found to lose activity upon air exposure, which correlates with an additional increase in the average oxidation state of Mn as revealed by X-ray absorption spectroscopic studies.

Decarboxylation and Tandem Reduction/Decarboxylation Pathways to Substituted Phenols from Aromatic Carboxylic Acids Using Bimetallic Nanoparticles on Supported Ionic Liquid Phases as Multifunctional Catalysts

Valuable substituted phenols are accessible via the selective decarboxylation of hydroxybenzoic acid derivatives using multifunctional catalysts composed of bimetallic iron-ruthenium nanoparticles immobilized on an amine-functionalized supported ionic liquid phase (Fe25Ru75@SILP+IL-NEt2). The individual components of the catalytic system are assembled using a molecular approach to bring metal and amine sites into close contact on the support material, providing high stability and high decarboxylation activity. Operating under a hydrogen atmosphere was found to be essential to achieve high selectivity and yields. As the catalyst materials enable also the selective hydrogenation and hydrodeoxygenation of various additional functional groups (i.e., formyl, acyl, and nitro substituents), direct access to the corresponding phenols can be achieved via integrated tandem reactions. The approach opens versatile synthetic pathways for the production of valuable phenols from a wide range of readily available substrates, including compounds derived from lignocellulosic biomass.

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes were developed. The carbonyl substrates without β-CH functionality follow the hydrogenation-hydrogenolysis path, wherein the hydrogenolysis of the alkanol intermediates presents as...

Selectivity control in hydrogenation through adaptive catalysis using ruthenium nanoparticles on a CO2-responsive support

With the advent of renewable carbon resources, multifunctional catalysts are becoming essential to hydrogenate selectively biomass-derived substrates and intermediates. However, the development of adaptive catalytic systems, that is, with reversibly adjustable reactivity, able to cope with the intermittence of renewable resources remains a challenge. Here, we report the preparation of a catalytic system designed to respond adaptively to feed gas composition in hydrogenation reactions. Ruthenium nanoparticles immobilized on amine-functionalized polymer-grafted silica act as active and stable catalysts for the hydrogenation of biomass-derived furfural acetone and related substrates. Hydrogenation of the carbonyl group is selectively switched on or off if pure H₂ or a H₂/CO₂ mixture is used, respectively. The formation of alkylammonium formate species by the catalytic reaction of CO₂ and H₂ at the amine-functionalized support has been identified as the most likely molecular trigger for the selectivity switch. As this reaction is fully reversible, the catalyst performance responds almost in real time to the feed gas composition.

A multifunctional catalytic system composed of ruthenium nanoparticles immobilized on a silica surface decorated with an amine-functionalized polymer is used for the hydrogenation of biomass-derived furfural acetone and related substrates. The presence or absence of CO₂ in the gas feed alters the selectivity of the hydrogenation—producing either a ketone or a saturated alcohol, respectively—in a fully reversible manner.

Metal Nanoparticles Immobilized on Molecularly Modified Surfaces: Versatile Catalytic Systems for Controlled Hydrogenation and Hydrogenolysis

The synthesis and use of supported metal nanoparticle catalysts have a long-standing tradition in catalysis, typically associated with the field of heterogeneous catalysis. More recently, the development and understanding of catalytic systems composed of metal nanoparticles (NPs) that are synthesized from organometallic precursors on molecularly modified surfaces (MMSs) have opened a conceptually new approach to the design of multifunctional catalysts (NPs@MMS). These complex yet fascinating materials bridge molecular ("homogeneous") and material ("heterogeneous") approaches to catalysis and provide access to catalytic systems with tailor-made reactivity through judicious combinations of supports, molecular modifiers, and nanoparticle precursors. A particularly promising field of application is the controlled activation and transfer of dihydrogen, enabling highly selective hydrogenation and hydrogenolysis reactions as relevant for the conversion of biogenic feedstocks and platform chemicals as well as for novel synthetic pathways to fine chemicals and even pharmaceuticals. Consequently, the topic offers an emerging field for interdisciplinary research activities involving organometallic chemists, material scientists, synthetic organic chemists, and catalysis experts.This Account will provide a brief overview of the historical background and cover examples from the most recent developments in the field. A coherent account on the methodological and experimental basis will be given from the long-standing experience in our laboratories. MMSs are widely accessible via chemisorption and physisorption methods for the generation of stable molecular environments on solid surfaces, whereby a special emphasis is given here to ionic liquid-type molecules as modifiers (supported ionic liquid phases, SILPs) and silica as support material. Metal nanoparticles are synthesized following an organometallic approach, allowing the controlled formation of small and uniformly dispersed monometallic or multimetallic NPs in defined composition. A combination of techniques from molecular and material characterization provides a detailed insight into the structure of the resulting materials across various scales (electron microscopy, solid-state NMR, XPS, XAS, etc.).The molecular functionalities grafted on the silica surface have a pronounced influence on the formation, stabilization, and reactivity of the NPs. The complementary and synergistic fine-tuning of the metal and its molecular environment in NPs@MMSs allow in particular the control of the activation of hydrogen and its transfer to substrates. Monometallic (Ru, Rh, Pd) monofunctional NPs@MMSs possess excellent activities for the hydrogenation of alkenes, alkynes, and arenes for which a nonpolarized (homolytic) activation of H2 is predominant. The incorporation of 3d metals in noble metal NPs to give bimetallic (FeRu, CoRh, etc.) monofunctional NPs@MMSs favors a more polarized H2 activation and thus its transfer to the C═O bond, while at the same time preventing the arrangement of noble metal atoms necessary for ring hydrogenation. The incorporation of reactive functionalities, such as, for example, a -SO3H moiety on NPs@MMSs, results in bifunctional catalysts enabling the heterolytic cleavage corresponding to a formal H-/H+ transfer. Consequently, such catalysts possess excellent deoxygenation activity with strong synergistic effects arising from an intimate contact between the nanoparticles and the molecular functionality.While many more efforts are still required to explore, control, and understand the chemistry of NPs@MMS catalysts fully, the currently available examples already highlight the large potential of this approach for the rational design of multifunctional catalytic systems.

Organometallic Synthesis of Bimetallic Cobalt-Rhodium Nanoparticles in Supported Ionic Liquid Phases (Cox Rh100- x @SILP) as Catalysts for the Selective Hydrogenation of Multifunctional Aromatic Substrates

The synthesis, characterization, and catalytic properties of bimetallic cobalt-rhodium nanoparticles of defined Co:Rh ratios immobilized in an imidazolium-based supported ionic liquid phase (Co_x Rh_{100- x} @SILP) are described. Following an organometallic approach, precise control of the Co:Rh ratios is accomplished. Electron microscopy and X-ray absorption spectroscopy confirm the formation of small, well-dispersed, and homogeneously alloyed zero-valent bimetallic nanoparticles in all investigated materials. Benzylideneacetone and various bicyclic heteroaromatics are used as chemical probes to investigate the hydrogenation performances of the Co_x Rh_{100- x} @SILP materials. The Co:Rh ratio of the nanoparticles is found to have a critical influence on observed activity and selectivity, with clear synergistic effects arising from the combination of the noble metal and its 3d congener. In particular, the ability of Co_x Rh_{100- x} @SILP catalysts to hydrogenate 6-membered aromatic rings is found to experience a remarkable sharp switch in a narrow composition range between Co₂₅ Rh₇₅ (full ring hydrogenation) and Co₃₀ Rh₇₀ (no ring hydrogenation).

Molecular Control of the Catalytic Properties of Rhodium Nanoparticles in Supported Ionic Liquid Phase (SILP) Systems

Rhodium nanoparticles (NPs) immobilized on imidazolium-based supported ionic liquid phases (Rh@SILP) act as effective catalysts for the hydrogenation of biomass-derived furfuralacetone. The structure of ionic liquid-type (IL) molecular modifiers was systematically varied regarding spacer, side chain, and anion to assess the influence on the NP synthesis and their catalytic properties. Well-dispersed Rh NPs with diameters in the range of 0.6-2.0 nm were formed on all SILP materials, whereby the actual size was dependent significantly on the IL structure. The resulting variations in catalytic activity for hydrogenation of the C=O moiety in furfuralacetone allowed control of the product selectivity to obtain either the saturated alcohol or the ketone in high yield. Experiments conducted under batch and continuous flow conditions demonstrated that Rh NPs immobilized on SILPs with suitable IL structures are more active and much more stable than Rh@SiO₂ catalyst synthesized on unmodified silica.

Selective hydrogenation of fluorinated arenes using rhodium nanoparticles on molecularly modified silica

Rh nanoparticles prepared on hydrophobic molecularly modified silica act as effective catalysts for the hydrogenation of fluoroarenes to fluorocyclohexane derivatives.