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. 2024 Sep 17;124(19):10701–10876. doi: 10.1021/acs.chemrev.4c00087

Table 31. Overview of Reductive Depolymerization of Isolated Lignin with a Modified Structure.

Entry Source and scale Catalyst and ratio of cat./substrate Solvent Conditions Yield Major products Ref
1 Beech and F5H-poplar lignin under formaldehyde stabilization; lignin solution 20 mL 5 wt% Ru/C; 100 mg cat. THF 250 °C, H2 4 MPa, 15 h 47% Phenolic monomers for beech lignin and 78% for F5H-poplar lignin 4-n-Propylsyringol and 4-n-propanolsyringol (605)
2 Birch spruce and F5H-poplar lignin extracted with acetaldehyde (AA) and propionaldehyde (PPA) protection; lignin solution 20 mL Pd/C; 250 mg Ru/C; 100 mg Ni/C; 100 mg 1,4-dioxane 200 °C, H2 4 MPa, 20 h 48% Phenolic monomers for birch lignin, 70% for F5H-poplar lignin, and 20% from spruce lignin; the yields listed here are based on Pd/C High selectivity (80%) to 4-n-propanolsyringol for poplar lignin (648)
3 Endocarp lignin under propanal-assisted fractionation; 100 mg Ru/C; 1:1 THF 250 °C, H2 4 MPa, 3 h Up to 120 g monomers/kg biomass Ca. 75% selectivity to propylguaiacol/syringol (649)
4 Propylidene acetal-stabilized lignin from birch, 2.5mg/mL in 1,4-dioxane/methanol mixture Ni/C, Ru/C 1,4-dioxane/methanol (2:8, v/v) 180 °C, H2 6 MPa, 20 h; flow reactor: lignin, feed: 0.1mL/min, lignin solution, flow rate of H2: 50 mL/min 45% Phenolic monomers over Ni/C and 40% phenolic monomers over Ru/C Approximately 39% 4-n-propylsyringol over Ni/C and approximately 15–20% 4-n-propylsyringol over Ru/C (650)
5 Ethyl glycol-incorporated lignin (α-alkoxylated lignin); 200 mg Ru/C; 1:1 MeOH 220 °C, H2 4 MPa, 18 h 24% Monophenolic monomers 49% Selectivity to 4-n-propylsyringol (654)
6 Catechyl lignin from enzymatic hydrolysis (C-lignin) and C-lignin containing biomass; 200 mg Pt/C, Pd/C, Ru/C; 0.5:1 MeOH, 1,4-dioxane, or THF 200 °C, H2 4 MPa, 15 h Up to 85 mol% catechyl monomers with isolated C-lignin Approximately 60 mol% catechylpropane with Pd/C; 55% mol catechylpropanol with Ru/C (655)
7 Isolated C-lignin by ChCl/LA; 50 mg Pd/C, Ru/C; 0.3:1 MeOH 200 °C, H2 3 MPa, 4 h Catechyl monomers in 29.6 wt% 24 wt% Catechylpropanol with Pd/C (656)
8 C-lignin sample from endocarp of castor seed coats; coats extracted by enzymatic and mild acidolysis treatment; 50 mg Ru/ZnO/C; 0.3:1 MeOH 200 °C, H2 3 MPa, 4 h Catechyl monomers in 66 mol% 77% High selectivity to propenylcatechol (657)