Influence of Acidic (H₃PO₄) and Alkaline (NaOH) Additives on the Catalytic Reductive Fractionation of Lignocellulose

Abstract

Reductive catalytic fractionation of lignocellulose is a promising “lignin-first” biorefinery strategy wherein lignin is solvolytically extracted from the cell wall matrix and simultaneously disassembled, resulting in a stable lignin oil and a solid carbohydrate-rich residue. Herein, we report on the different influence of acidic (H3PO4) and alkaline (NaOH) additives on the Pd/C-catalyzed reductive processing of poplar wood in methanol (MeOH). It was found that the addition of small quantities of H3PO4 results in three rather than two product streams, since under acidic conditions both delignification and alcoholysis of hemicellulose are promoted, leaving behind a cellulose-rich pulp. The simultaneous acid-catalyzed fractionation of the carbohydrates into separate cellulose and hemicellulose streams provides opportunities for more efficient downstream conversion, as processing parameters can be tailored to the needs of both streams. Alkaline conditions, on the other hand, also enhance delignification, but additionally cause (i) the formation of lignin products other than those obtained under neutral and acidic conditions, (ii) a hampered degree of lignin depolymerization, and (iii) substantial loss of cellulose from the pulp. Further on, a modified process descriptor (LFFE: lignin first fractionation efficiency) was applied to evaluate the fractionation efficiency of lignocellulose in its three major constituents. According to this new efficiency measure, mildly acidic conditions performed best.

Hydrogen‐free catalytic fractionation of woody biomass

“…The rest of the lignin polymer is obtained as dimers or short oligomers. Moreover, the carbohydrate part of the biomass is retained to a large extent and can be processed by enzymatic hydrolysis or by a number of other chemical transformations ,,…”

“…The lignin undergoes partial solvolysis and the released lignin oligomers are depolymerized through transfer hydrogenolysis by the Pd/C catalyst to yield unsaturated monomers 1 a and 1 b , which undergo further reduction through a transfer hydrogenation reaction to generate saturated phenols 2 a and 2 b . These observations are in good agreement with previously reported results for similar catalytic fractionation processes utilizing heterogeneous transition‐metal catalysts …”

“…However, the lignin bio‐oil from this process has a complex composition of low‐molecular weight compounds. Sels and co‐workers reported a number of procedures utilizing Pd/C and Ru/C as catalysts in the hydrogenolysis of raw biomass (Scheme ) –,. The effect of solvents, hydrogen pressure, catalysts, and co‐catalyst were thoroughly studied and the impact on the lignin conversion, product selectivity, and retention of the carbohydrates was demonstrated, showing the feasibility of the catalytic biorefinery approach.…”

Effective Release of Lignin Fragments from Lignocellulose by Lewis Acid Metal Triflates in the Lignin‐First Approach

“…As delignification of lignocellulose is slow in the lignin‐first approach, we targeted ether and ester interlinkages between lignin and cellulose in woody biomass by selective catalysis. Diluted aqueous acids (e.g., HCl, H 2 SO 4 , and H 3 PO 4 ) used in organosolv pretreatment accelerate lignin release, but are problematic as they hydrolyze cellulose and degrade sugars into furfural and 5‐hydroxymethyl furfural, which can further condense with lignin fragments and also cause problems, such as corrosion . Metal‐catalyzed lignin deconstruction itself can be promoted by Lewis (Zn 2+ , Al 3+ ) and Brønsted acids …”

“…This product stems from etherification of the propanol side‐group with methanol catalyzed by Yb III ‐triflate or the slightly acidic character of subcritical methanol . GPC shows that some dimers and oligomers cannot be depolymerized, most likely owing to recalcitrant C−C (e.g., 5‐5) linkages . These results attest to the high effectivity of Yb III ‐triflate in releasing lignin fragments from the biomass.…”