Abstract
Rhodium nanoparticles immobilized on an acid‐free triphenylphosphonium‐based supported ionic liquid phase (Rh@SILP(Ph3‐P‐NTf2)) enabled the selective hydrogenation and hydrodeoxygenation of aromatic ketones. The flexible molecular approach used to assemble the individual catalyst components (SiO2, ionic liquid, nanoparticles) led to outstanding catalytic properties. In particular, intimate contact between the nanoparticles and the phosphonium ionic liquid is required for the deoxygenation reactivity. The Rh@SILP(Ph3‐P‐NTf2) catalyst was active for the hydrodeoxygenation of benzylic ketones under mild conditions, and the product distribution for non‐benzylic ketones was controlled with high selectivity between the hydrogenated (alcohol) and hydrodeoxygenated (alkane) products by adjusting the reaction temperature. The versatile Rh@SILP(Ph3‐P‐NTf2) catalyst opens the way to the production of a wide range of high‐value cyclohexane derivatives by the hydrogenation and/or hydrodeoxygenation of Friedel–Crafts acylation products and lignin‐derived aromatic ketones.
Highlights
The synthesis of alkyl cyclohexane derivatives has attracted considerable attention in the past decade owing to the importance of these compounds in the transportation sector[1] and as building blocks for the production of coating agents,[2] liquid crystals,[3] and pharmaceuticals.[4]
We report the synthesis of Rh nanoparticles immobilized on a triphenylphosphonium-based supported ionic liquid phases (SILPs)
The T2 and T1 signals correspond to the Si atoms of IL bound to the SiO2 surface and substantiate the covalent attachment of the IL on the silica support. 1H, 13C, 31P, and 19F solidstate NMR spectroscopy further confirmed the presence of the desired triphenylphosphonium–NTf2 ionic liquid in the SILP(Ph3-P-NTf2)
Summary
The synthesis of alkyl cyclohexane derivatives has attracted considerable attention in the past decade owing to the importance of these compounds in the transportation sector[1] (kerosene-type fuels) and as building blocks for the production of coating agents,[2] liquid crystals,[3] and pharmaceuticals.[4]. The traditional method for the synthesis of alkyl cyclohexanes consists of the hydrogenation of alkyl aromatic compounds[5] commonly produced through Friedel–Crafts alkylation reactions. This pathway suffers from the limited substrate scope and often low selectivity of Friedel– Crafts alkylation (overalkylation, carbocation rearrangements).[6]. The use of aromatic ketones as substrates gives the opportunity to access two classes of compounds (alkyl cyclohexanes and hydroxy-containing cyclohexane derivatives) and broadens significantly the range of possible products and potential applications (Figure 1). Emondts DWI-Leibniz Institute for Interactive Materials Forckenbeckstrasse 50, 52056 Aachen (Germany). The choice of the acid, and close proximity between the metal and acid sites were shown to be key factors in the development of effective hydrodeoxygenation catalysts.[16b–d]
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