Catalytic Depolymerization of Lignin and Woody Biomass in Supercritical Ethanol: Influence of Reaction Temperature and Feedstock

Abstract

The one-step ethanolysis approach to upgrade lignin to monomeric aromatics using a CuMgAl mixed oxide catalyst is studied in detail. The influence of reaction temperature (200–420 °C) on the product distribution is investigated. At low temperature (200–250 °C), recondensation is dominant, while char-forming reactions become significant at high reaction temperature (>380 °C). At preferred intermediate temperatures (300–340 °C), char-forming reactions are effectively suppressed by alkylation and Guerbet and esterification reactions. This shifts the reaction toward depolymerization, explaining high monomeric aromatics yield. Carbon-14 dating analysis of the lignin residue revealed that a substantial amount of the carbon in the lignin residue originates from reactions of lignin with ethanol. Recycling tests show that the activity of the regenerated catalyst was strongly decreased due to a loss of basic sites due to hydrolysis of the MgO function and a loss of surface area due to spinel oxide formation of the Cu and Al components. The utility of this one-step approach for upgrading woody biomass was also demonstrated. An important observation is that conversion of the native lignin contained in the lignocellulosic matrix is much easier than the conversion of technical lignin.

Scaling-Up Catalytic Depolymerisation of Lignin: Performance Criteria for Industrial Operation

“…Lignin depolymerization into smaller, aromatic molecules, often requires solvents, catalysts, and hydrogen for deoxygenation. Solvents including methanol [6][7][8] ethanol [3,[9][10][11] ethanol/water 1 3 [12,13] methanol/water [14] propanol [15] or dioxane [16,17] have been investigated in detail as a medium for tandem solvolysis and hydrogenolysis reactions. These latter reactions can be catalyzed both by transition (Pt, Pd, Ru, Rh, etc.)…”

“…In most of the aforementioned literature, the lignin-tosolvent ratios going into the reactor were chosen between 1:30 and 1:60 w/v, with higher lignin loadings generally manifesting in prohibitively high char formation and/or low conversion towards mono-aromatics [3,6,7,[9][10][11][12]. Here, we refer to our early work as a case in point, wherein we demonstrated an effective approach for lignin depolymerization to mono-aromatics in supercritical ethanol, using a Cu-Mg-Al mixed oxide catalyst [3,[9][10][11]. In this work, a high solvent to lignin ratio was used with the aim to obtain a high yield of lignin monomers.…”

Catalytic Transfer Hydrogenolysis Reactions for Lignin Valorization to Fuels and Chemicals

“…Considering the repolymerization via the phenolic hydroxyl groups, the authors suggested that ethanol not only acts as a hydrogen-donor solvent, but also as a capping agent and formaldehyde scavenger, as shown in Figure 10 [121,123]. In a following paper, they discussed the influence of reaction temperature on the products [124]. At lower temperatures in the range of 200-250 • C, recondensation reactions are dominant whereas at higher temperatures of 380-420 • C, the char formation due to carbonization played a major role.…”

Alternatives for Chemical and Biochemical Lignin Valorization: Hot Topics from a Bibliometric Analysis of the Research Published During the 2000–2016 Period

“…Simple noncatalytic solvolysis of biorefinery lignin in supercritical ethanol can produce a heptane soluble bio-oil without the need for the addition of a catalyst or a reducing agent [257]. The catalytic ethanolysis of lignin proposes the use of different catalysts: precious group metals (Pd, Pt, Ru, Rh …) and more abundant metals (Cu, Ni, Mo …) supported over oxides (alumina, ceria, zirconia, magnesia …) have been frequently reported [258][259][260][261], but alternative catalysts have been tested as well, such as silicon carbon nanoparticles [262] or ionic liquids [263]. Methanol have replaced ethanol in aqueous mixtures for lignin solvolysis with very similar results [264], while propanol and butanol have also been proposed [265].…”

“…. ) have been frequently reported [258][259][260][261], but alternative catalysts have been tested as well, such as silicon carbon nanoparticles [262] or ionic liquids [263]. Methanol have replaced ethanol in aqueous mixtures for lignin solvolysis with very similar results [264], while propanol and butanol have also been proposed [265].…”