Efficient and Mild Transfer Hydrogenolytic Cleavage of Aromatic Ether Bonds in Lignin-Derived Compounds over Ru/C

Abstract

Cleavage of aromatic ether bonds is crucial for the valorization of lignin and its fragments, which is challenging under mild conditions because the bonds are very stable. Herein, we found that Ru/C could efficiently catalyze the cleavage of the aromatic ether bonds in various lignin-derived compounds via a transfer hydrogenolytic route using isopropanol as the hydrogen resource. Various lignin-derived compounds could be efficiently cleaved over commercial Ru/C to generate the corresponding aliphatic alkanes, aliphatic alcohols and aromatic derivatives under milder conditions. A mechanism study indicated that the reaction occurred through the direct cleavage of aromatic ether bonds or the formation of the reaction intermediate cyclohexyl phenyl ether.

Selective Hydrogenation of Phenol to Cyclohexanol over Ni/CNT in the Absence of External Hydrogen

“…Therefore, the exploration of a cheap and effective catalyst for the hydrogenation of phenol in the absence of hydrogen is essential. Catalytic transfer hydrogenation (CTH) has been regarded as a good alternative to avoid the application of high-pressure hydrogen [22][23][24][25]. Galkin et al reported that Pd/C could catalyze transfer hydrogenolysis of β-O-4 model compound in lignin employing formic acid as a hydrogen-donor for the generation of acetophenone and phenol derivatives [22].…”

“…Paone et al reported the transfer hydrogenolysis of α-O-4 model compound in lignin conducted on Pd/Fe 3 O 4 under 240 • C [23]. Wu et al found that Ru/C could efficiently catalyze the cleavage of the 4-O-5 aromatic ether bond in a variety of lignin-derived compounds through the transfer hydrogenolytic pathway using isopropanol as the hydrogen-donor solvent [24]. Despite those achievements for the transfer hydrogenolytic transformation of biomass to value-added chemicals, the catalytic hydrogenation without the use of external hydrogen remains a great challenge [25,26].…”

Highly Efficient Cleavage of Ether Bonds in Lignin Models by Transfer Hydrogenolysis over Dual‐Functional Ruthenium/Montmorillonite

“…2‐Phenoxyacetophenone and α‐phenethyl alcohol phenyl ether, two typical model compounds containing the β‐O‐4 ether bond, could also be converted to generate the corresponding chemicals, but the reaction temperature had to be increased to 180 °C with a reaction time of 12 h (Table , entries 5 and 6). More interestingly, the compound containing the methoxy ortho ‐substituted β‐O‐4 bond (2‐(2‐methoxyphenoxy)‐1‐phenylethanol), which could be completely converted at 160 °C with a reaction time of 12 h, possessed higher reactivity (Table , entry 7) because the ortho ‐substituted methoxy group could assist in the cleavage of the β‐ether bond . Third, the α‐O‐4 ether bond (e.g., benzyl phenyl ether and 4‐benzyloxyphenol) could also be cleaved over Ru/MMT at 160 °C with a reaction time of 6 h (Table , entries 8 and 9).…”

“…As described in Figure C, the yield of cyclohexyl phenyl ether first increased and then decreased upon prolonging the reaction time, and no dicyclohexyl ether was generated throughout the whole reaction process, which implied that cyclohexyl phenyl ether was the reaction intermediate. The C−O bond in cyclohexyl phenyl ether would be cleaved to generate cyclohexane and phenol (not detected because of its rapid conversion rate at 150 °C), owing to the lower bonding energy of the C(sp 3 )−O bond compared with that of the C(sp 2 )−O bond . In another aspect, benzene would be detected at lower reaction temperature (Figure A), indicating the occurrence of the direct cleavage of the C−O bond in diphenyl ether.…”

Catalytic Transfer Hydrogenolysis as an Effective Tool for the Reductive Upgrading of Cellulose, Hemicellulose, Lignin, and Their Derived Molecules

“…Han and co-workers recently demonstrated the performing catalytic activity of the commercial Ru/C in CTH reactions of aromatic ethers using a variety of 4-O-5 type lignin model compounds using 2-propanol as a solvent/H-donor under mild conditions (at 120 °C for 10-26 h) [118].…”