Abstract
ConspectusThe 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.
Highlights
With the depletion of fossil resources and the rise of alternative renewable energy sources and chemical feedstock, the permanent evolution and adaptation of catalysts are of critical importance.[5−7] Following the recognition of the importance of well-defined small metal nanoparticles in heterogeneous catalysis, the field literally exploded, generating a plethora of nanoparticle-based catalytic systems applied to myriad transformations.[8,9]
This opened the way for the production of hybrid organic−inorganic materials consisting of molecularly modified surfaces (MMSs).[7,13−19] In particular, the molecular imprinting of solid supports,[13,14] the development of Supported Ionic Liquid Phase (SILP) and SCILL catalysts,[15,16] and the immobilization of polymeric structures[17,18] received great attention, leading to novel design options for solid catalyst materials
We have shown that bimetallic NPs immobilized on imidazolium-based SILPs are very promising candidates to combine these features (Figure 8).[2,31]
Summary
With the depletion of fossil resources and the rise of alternative renewable energy sources and chemical feedstock, the permanent evolution and adaptation of catalysts are of critical importance.[5−7] Following the recognition of the importance of well-defined small metal nanoparticles in heterogeneous catalysis, the field literally exploded, generating a plethora of nanoparticle-based catalytic systems applied to myriad transformations.[8,9] In particular, catalysts comprising metal nanoparticles immobilized on various support materials were found to be extremely versatile.[9,10] While the supports were initially used primarily to stabilize the nanoparticles and to allow for easy recycling, it became obvious that they are rarely innocent and can influence the nanoparticles stability and reactivity through various types of interactions. Bimetallic catalysts were more active and selective for the hydrogenolysis of glycerol to 1,2-propanediol than the monometallic Cu and Ru versions with good results obtained for the Cu25Ru75 catalyst (100% conversion, 87% selectivity for 1- and 2propanol).[21] These examples demonstrate that the selectivity of the hydrogenation reactions, and especially the catalyst’s ability to hydrogenate polarized C O units versus aromatic rings, can be effectively and finely controlled through the preparation of bimetallic NPs with tunable metal ratios on SILP-type materials. This third part will cover monometallic nanoparticles immobilized on MMSs where the molecular modifier possesses a functionality that is directly involved in the catalysis These materials fall in the class of multifunctional catalysts, which currently attract considerable attention for the production of fine chemicals and pharmaceuticals and for the conversion of oxygen-containing renewable feedstocks and platform chemicals into value-added chemicals and fuel components.[44,45] We used this approach to develop catalytic systems able to perform selective hydrodeoxygenation reactions. This provides access to a broad range of ethylphenol derivatives that are important building blocks for the production of fine chemicals, polymers, and pharmaceuticals.[60,61]
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