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.


Experimental Safety warning
High-pressure experiments with compressed H2 must be carried out only with appropriate equipment and under rigorous safety precautions.

General
If not otherwise stated, the immobilization of nanoparticles on the PGS-material (Ru@PGS) was performed out under an inert atmosphere using standard Schlenk techniques or in a glovebox as previously reported. Furfuralacetone (IUPAC name: 4-(2-Furyl)but-3-en-2-one) was purified by sublimation prior to use (white crystals).
[Ru(2-methylallyl)2(cod)] was obtained from Umicore. Synthetic air (20.5 Vol-% O2, rest N2, no hydrocarbon) was purchased from Westfalen AG. Catalyst solutions and substrate were prepared under air, but were flushed with H2 and/or CO2 prior to catalysis. All other chemicals and solvents were purchased from commercial sources and used without purification. 1 H and 13 C NMR spectra were obtained using a Bruker Avance 400 TM spectrometer at 400 MHz ( 1 H) and 100 MHz ( 13 C) at 25 °C. Solid-state 29 Si & 13 C CP-MAS NMR spectra were obtained using a Bruker AVIII-500 spectrometer. The GPC analysis was performed using THF as the eluent. Samples were prepared at 5.0 mg/mL and passed through a 0.2 μm PTFE filter prior to injection. The samples were analysed on a Waters 2695 separation module using a refractive index detector (Waters 410 differential refractometer) at 32 °C and 1 mL/min flow rate. The GPC was calibrated using polystyrene standards. Brunauer-Emmett-Teller (BET) measurements were performed on a Quadrasord SI automated Surface Area and Pore Size Analyser from Quantachrome Instruments and the data analysis using QuadraWin 5-04. Inductively . Anhydrous anisole (100 mL) was added via cannula to the mixture followed by L-ascorbic acid (10 eq., 16.7 mmol) Cu(II)Br2 (0.66 eq., 1.1 mmol), N,N,N′,N′′,N′′pentamethyldiethylenetriamine (PMDETA, 3.29 eq., 5.6 mmol) and ethyl 2-bromo-2methylpropionate (EBIB, 1 eq., 1.7 mmol). The mixture was slowly heated to 40 °C and left to react for 20 h. The polymerization solution changed colour from blue (oxidized copper) to colourless and then to yellow/orange over the course of 20 h.

Analysis
The polymer-grafted silica (PGS) was collected in a vacuum funnel. The PGS were washed multiple times with THF followed by sonication 3 times in THF. The particles were stirred in a 1 M EDTA solution (pH~10) for 1 h to remove any residual copper, followed by drying under reduced atmosphere at 60 °C for 18 h and stored under argon until further use.

Synthesis of Ru@PGS
[Ru(2-methylallyl)2(cod)] (128 mg, 0.401 mmol) was dissolved in DCM (10 mL) and added to a suspension of PGS (500 mg) in DCM (10 mL). The reaction mixture was stirred at room temperature for 1 h. After solvent removal at room temperature and in vacuo drying of the impregnated PGS, the powder was loaded into a 20 mL highpressure autoclave and subjected to an atmosphere of H2 (25 bar) at 100 °C for 18 h.
Under this reducing environment, the impregnated PGS transformed from a light orange to a black colour indicating the immobilization of the Ru NPs onto the PGS.
The reaction mixture was stirred at room temperature for 1 h. After solvent removal at room temperature and in vacuo drying of the impregnated SiO2, the powder was loaded into a 20 mL high-pressure autoclave and subjected to an atmosphere of H2 (25 bar) at 100 °C for 18 h. Under this reducing environment, the impregnated SiO2 transformed from a white to a black colour indicating the immobilization of the Ru NPs onto the SiO2.

Titration method
The procedure for the titration method was previously reported by Boniface et al. 1,3 All vessels were plastic and they were rinsed thoroughly with Millipore water to have pH value of > 6. Stock solutions of sodium hydroxide (NaOH) 9.006 mM, and hydrochloric acid (HCl) 10.5 mM were prepared. In a typical experiment, ca. 10 mg of PGS or Ru@PGS was added to 20 mL of Millipore water in a 50 mL centrifuge vial.

Hydrogenation of 1 with CO2/H2
In a typical experiment, Ru-catalyst (35 mg, 0.026 mmol), 1-butanol (0.5 mL) were combined with 1 (90 mg, 0.65 mmol, 25 eq.) in a glass insert and placed in a highpressure autoclave. After purging with CO2 and left to stir for a couple of minutes, the autoclave was further pressurized first with15 bar CO2 and then with enough H2 to raise the total pressure to 30 bar (CO2/H2 ratio ~ 1:1). The reaction mixture was stirred 80°C in an aluminium heating block under desired pressure of H2 and CO2.
Once the reaction was finished, the reactor was cooled in an ice bath and carefully vented. After filtration, a sample of the reaction mixture was taken and analysed via GC-FID using tetradecane as an internal standard.

Hydrogenation of 1 with various additives
In a typical experiment, Ru-catalyst (35 mg, 0.026 mmol), 1-butanol (0.5 mL) and the additive were combined with 1 (90 mg, 0.65 mmol, 25 eq.) in a glass insert and placed in a high-pressure autoclave. After purging the autoclave with H2, the reaction mixture was stirred at 80°C in an aluminium heating block under 15 bar of H2. Once the reaction was finished, the reactor was cooled in an ice bath and carefully vented.
After filtration, a sample of the reaction mixture was taken and analysed via GC using tetradecane as an internal standard. Conversion > 99%, product yields were determined by GC-FID using tetradecane as an internal standard. [b] Heptane as solvent. [c] Methyl tert-butyl ether as solvent.

D2-experiment: 13 C NMR of the catalyst suspension
Ru@PGS (35 mg, 0.026 mmol) was combined with deuterated methanol (0.5 mL) without furfuralacetone in a glass insert and placed in a high-pressure autoclave.
After purging with an argon atmosphere, 2 bar labelled CO2 were pressurized and left to stir for a couple of minutes. The autoclave was further pressurized then with CO2 up to 15 bar and then with enough deuterium (D2) to raise the total pressure to 30 bar. The reaction mixture was stirred at 80°C in an aluminium heating block under H2 and CO2. Once the reaction was finished, the reactor was cooled in an ice bath and carefully vented. The mixture of catalyst and solution was removed with a syringe, transferred into an NMR tube and analysed by 13 C NMR spectra.
Supplementary Figure 5: 13 C NMR spectrum (400 MHz, MeOD) of the reaction mixture after catalysis using deuterium gas instead of H2.

Solid-state 13 C CP-MAS NMR of the Ru@PGS-catalyst
Ru@PGS (35 mg, 0.026 mmol) was combined with deuterated methanol (0.5 mL) without furfuralacetone in a glass insert and placed in a high-pressure autoclave.
After purging with an argon atmosphere, 2 bar labelled CO2 were pressurized and left to stir for a couple of minutes. The autoclave was further pressurized then with CO2 up to 15 bar and then with enough H2 to raise the total pressure to 30 bar. The reaction mixture was stirred at 80°C in an aluminium heating block under H2 and CO2.
Once the reaction was finished, the reactor was cooled in an ice bath and carefully vented. The reaction mixture was left to dry under air at room temperature for 48 h.
The dried catalyst was then analysed by solid-state NMR spectroscopy. The parameters of the measurements are the same as stated above for solid-state NMR spectroscopy.    Composition of the reaction mixture was determined by GC using tetradecane as an internal standard.