Evolution of the active species of homogeneous Ru hydrodeoxygenation catalysts in ionic liquids

Abstract

This work establishes structure-property relationships in Ru-based catalytic systems for selective hydrodeoxygenation of ketones to alkenes by combining extensive catalytic testing, in situ X-ray absorption spectroscopy (XAS) under high pressures and temperatures and ex situ XAS structural characterization supported by density functional theory (DFT) calculations. Catalytic tests revealed the difference in hydrogenation selectivity for ketones (exemplified by acetone) or alkenes (exemplified by propene) upon changing the reaction conditions, more specifically in the presence of CO during a pretreatment step. XAS data demonstrated the evolution of the local ruthenium structure with different amounts of Cl/Br and CO ligands. In addition, in the absence of CO, the catalyst was reduced to Ru⁰, and this was associated with a significant decrease of the selectivity for ketone hydrogenation. For the Ru-bromide carbonyl complex, selectivity towards acetone hydrogenation over propene hydrogenation was explained on the basis of different relative energies of the first intermediate states of each reaction. These results give a complete understanding of the evolution of the Ru species, used for the catalytic valorization of biobased polyols to olefins in ionic liquids, identifying the undesired deactivation routes as well as possibilities for reactivation.

Scheme 1. Simplified system to study the two key hydrogenation steps in the valorization of biobased alcohols with catalytic Ru and IL: (1) reduction of ketone intermediates to alcohols; (2) avoiding overhydrogenation of the desired olefin end products.

Fig. 1. Preservation of propene over multiple runs. ^aFraction of propene in the (propene + propane) fraction. Conditions: RuBr₃·xH₂O (2 mol%), propene (1 bar), Bu₄PBr (577 mg, 3.4 mmol). Pretreatment: CO gas (1 bar) 40 bar H₂, 0.5 h, 180 °C. Each hydrogenation uses a fresh charge of propene gas (1 bar), 40 bar H₂, 1 h, 220 °C. ^bCO pretreatment before first run. ^cCO pretreatment before each run.

Fig. 2. XANES (a) and magnitudes of Fourier-transformed (4–15 Å⁻¹) phase-uncorrected FT-EXAFS (b) data for RuBr₃ (red) and RuCl₃ (blue), before (dashed lines) and after (solid lines) dissolution in Bu₄PBr.

Fig. 3. XANES (a) and magnitudes of Fourier-transformed (4–15 Å⁻¹) phase-uncorrected FT-EXAFS (b) data for [RuBr₂(CO)₃]₂ (red) and [RuCl₂(CO)₃]₂ (blue), before (dashed lines) and after (solid lines) dissolution in Bu₄PBr.

Fig. 4. XANES (a) and phase-uncorrected FT-EXAFS (b) data for the reference RuBr₃ salt (dashed black) and [RuBr₂(CO)₃]₂ compound (dashed red), and RuBr₃ salt dissolved in Bu₄PBr without (dashed grey) and with (solid blue) addition of CO gas.

Fig. 5. Relative fractions of Br (shown in red) vs. CO (in blue) in the Ru coordination sphere, for different samples, as obtained by LCF analysis.

Fig. 6. XANES (a) and phase-uncorrected FT-EXAFS (b) data for RuBr₃ salt dissolved in IL before (solid red) and after (solid purple) reaction with propene. Dashed grey lines correspond to metallic Ru reference.

Fig. 7. (a) XANES spectra of pure Ru-species (solid coloured lines) extracted by MCR-ALS plotted together with the reference spectra of RuBr₃, [RuBr₂(CO)₃]₂, and Ru foil (dashed red, green and blue lines respectively). (b) Concentration profiles of the three Ru-species extracted from MCR-ALS. (c) A list of experimental conditions applied during in situ XAS data collection. Conditions were varied within the described boundary conditions (see ESI, “boundary conditions”): CO (0–5 bar), H₂ (0–30 bar) and temperature (180–220 °C). Dashed lines indicate when the sample was changed. ^aA high concentration of RuBr₃ was used, with 68 mg RuBr₃ in 2 g IL. ^bFor all other entries, low concentrations of Ru were measured (13.6 mg in 2 g IL, similar to catalytic results presented in Table 1). ^cFormaldehyde (100 μL) was thermally decomposed to generate in situ CO gas. ^dIsopropanol (IPA, 250 μL) was added as propene precursor.

Fig. 8. Relative energies in kcal mol⁻¹ of the most stable intermediates and first transition state determined for acetone (top) and propene (bottom) hydrogenation reactions. The corresponding relaxed structures and output files are attached in ESI.

Scheme 2. Evolution of Ru-species depending on reaction conditions.