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.

S2 hemispherical analyser, a multi-channel plate and delay line detector (DLD) with an X-ray incident angle of 30° and a collection angle of 0° (both relative to the surface normal). X-ray source was operated at 10 mA emission current and 12 kV anode potential. All spectra were recorded using an entrance aperture of 300 × 700 μm with pass energy of 80 eV for survey scans and 20 eV for high resolution scans. Charge neutralisation (used for solid samples only) was applied using a standard Kratos charge neutraliser consisting of a filament, coaxial with the electrostatic and magnetic transfer lenses, and a balance plate which creates a potential gradient between the neutraliser and sample. Charge neutralisation was applied at 1.9 A filament current and 3.3 V balance plate voltage. Sample stubs were earthed via the instrument stage using a standard BNC connector. Room temperature ionic liquid samples were prepared by placing a small drop (≈10 mg) of ionic liquid onto a stainless steel multi-sample bar; solid samples were fixed to the bar using double-sided adhesive tape. All samples were pre-pumped in a preparative chamber to pressures lower than 1 × 10 −6 mbar before transfer into the main analytical chamber. It is important to note that Rh scans should be measured first such that the native state of the metal is recorded before extended exposure to the X-ray illumination. In designing scan parameters, Rh should be measured early in sequences and the illumination area should be varied to ensure sample homogeneity and also to minimise the impact of the photochemistry to give rise to irradiative changes in oxidation state etc. All XPS data were analysed using the CASAXPS software. 1-octyl-3-(3-triethoxysilylpropyl)imidazolium bromide (5.6 g, 12.0 mmol) and bis(trifluoromethane)sulfonimide lithium salt (3.6 g, 12.6 mmol) were dissolved in water (20 mL) and stirred at rt for 1 h. DCM (50 mL) was added and the organic phase was washed with water (3x50 mL). The organic phase was dried over MgSO4 and the solvent was removed under reduced pressure. The product was dried in vacuo to yield a viscous, yellow/brown liquid (6.72 g, 84%).

General procedure for the synthesis of Supported Ionic Liquid Phases (SILPs)
[1-octyl-3-(3-triethoxysilylpropyl)imidazolium]NTf2 (5.49 g, 8.80 mmol) was dissolved in DCM (20 mL) and added to a suspension of dehydroxylated SiO2 (10.0 g in 50.0 mL toluene). The resulting mixture was refluxed for 18 h. Upon removal of the organic phase, the SILP was washed with DCM (3x25 mL) and dried in vacuo. The organic phases were combined and solvent removed to determine the residual quantity of IL not grafted onto the dehydroxylated SiO2 (Total IL Loading = Theoretical Loading -Recovered Residual IL).

2.3
General procedure for the synthesis of Rh@SILPs catalysts (theoretical loading: 0.1 mmolRh.g -1 ) The synthesis of Rh NPs immobilized on SILPs (Rh@SILPs) involved the wet impregnation of the SILP (0.50 g) with a solution of [Rh(allyl)3] (11.3 mg, 0.05 mmol) in dichloromethane (DCM) (5 mL). Upon addition of the [Rh(allyl)3] solution, the SILP transformed from a white to a bright yellow colour, which indicated the adsorption of the Rh precursor into the SILP. Upon evaporation of the solvent in vacuo, the impregnated SILP powder was subjected to an atmosphere of H2(g) (50 bar) at 100°C for 18 h. Under these conditions, the bright yellow powder turned black in colour and signified the formation of Rh NPs.

Catalysis in batch reactor
In a typical experiment, Rh@SILP (40 mg, 0.004 mmol Rh), furfuralacetone (0.4 mmol, 100 equiv.), and heptanes (0.5 mL) were combined in a glass insert and placed in a high-pressure autoclave. After purging the autoclave with H2, the reaction mixture was stirred at 100°C in an aluminum heating block under 20 bar H2. Once the reaction was finished, the reactor was cooled, carefully vented, and the reaction mixture was analyzed by GC using tetradecane as an internal standard.

Determination of TOFs
Turn over frequencies were determined using the following equation: The %(surface Rh) were estimated for each catalyst by calculating the volume of the Ru NPs as well as the volume of the shell containing the first layer of Ru atoms (approximation: spherical NPs): with rat Rh = 0.135 nm In case of Table 3-5, TOFs for the production of 5 were estimated for 1 h reactions, for which the yield of 5 was low.

Catalysis in continuous flow reactor
A 70 mm CatCart was filled with Rh@SILP (250 mg, 0.025 mmol Rh) and placed into a flow reactor (H-Cube Pro). Prior to catalysis, the catalyst was heated at 100°C under a flow of heptane (0.5 mL.min -1 ) and H2 (20 bar; gas flow rate under standard conditions = 35 NmL.min -1 ) for 30 min. The substrate solution (0.05 M furfuralacetone in heptane) was introduced into the system with a flow of H2 (20 bar) and the reaction parameters (temperature=100°C, substrate flow=0.5 mL.min -1 , H2 flow rate=35 NmL.min -1 ) were set up. The system was allowed to equilibrate under the desired reaction conditions for 20 min before approximately 2 mL of reaction solution was collected. The reaction mixture was analyzed by GC using tetradecane as an internal standard.