Wood Liquefaction: Role of Solvent

Abstract

The liquefaction of lignocellulose is strongly affected by the choice of the liquefaction solvent. This effect was studied by processing pinewood at 310 °C in various organic solvents, including some refinery streams. The impact on yields of char, biocrude, and gas was determined. The resulting biocrude was analyzed with GPC and GC-MS, the solid residue with FTIR and HR-SEM, and the gas with GC. The solvents appeared to influence the liquefaction through interaction with early carbohydrate degradation products rather than interaction with the final biocrude. Indeed, biocrude is soluble in all solvents, and char is not formed by degradation of biocrude but rather by degradation of early carbohydrate intermediates. Effective solvents show a good affinity with the biomass, e.g., as expressed by Hildebrand values of 25−40 MPa 1/2 or small Hansen distances (Ra) to cellulose and lignin. Protic solvents further slowed the liquefaction reaction.

A Review on Lignin Liquefaction: Advanced Characterization of Structure and Microkinetic Modeling

“…The information reviewed herein can be summarized in the physical models of lignin liquefaction (for pyrolysis and solvolysis) shown in Figure . − This model is used to guide our narrative. The reactions in solid phase lead to the formation of depolymerized oligomeric products that form a liquid intermediate.…”

“…Two recent works on the role of solvents in thermochemical biomass conversion can be found from Castellví Barnés et al and Shuai and Luterbacher. , Solvent effects can be divided into two categories: (1) the effect on feedstock solubility and (2) the effect on solvolysis chemical thermodynamics . Organic solvents can be divided into four groups: (1) polar protic (e.g., methanol and acetic acid), (2) polar aprotic (e.g., tetrahydrofuran (THF) and γ-Valerolactone (GVL)), (3) nonpolar (e.g., hexane and dimethyl ether), and (4) ionic liquids (e.g., [mmim]­[MeSO 4 ] and [bmpy]­[PF 6 ]) .…”

Lignocellulose Liquefaction to Biocrude: A Tutorial Review

“…Accordingly, high biocrude yield and minimum char yield were achieved with fairly polar solvents, with performance improving from alkanes to aromatics to phenolics. More specifically, the char yield was shown to decrease with decreasing the Hansen distance versus cellulose and to drop to nearly zero at R a (cellulose)<20 MPa 1/2 (Figure ). Water was evaluated for comparison and delivered lower biocrude yields than phenolic solvents.…”

“…The solvent was hypothesized to affect the selectivity of the liquefaction by solubilizing early, presumably carbohydrate‐like reaction intermediates, and preventing thereby their condensation and charing on the surface of unconverted biomass. The solvent effect seemed not to be related to the solubility of the final biocrude because all solvents of the study were shown to solubilize the biocrude at reaction temperature . Moreover, the biocrude was shown to be stable as it did not undergo condensation and charing reactions at extended reaction times.…”

Unexplored Areas of Direct Solvolytic Liquefaction of Lignocellulosic Biomass

“…HTL has been the subject of recent reviews , . Furthermore, more than 20 organic model solvents have recently been screened to develop an understanding of the influence of solvent properties on liquefaction yields, which were found to vary between 40 and 95 wt % . Fresh organic solvents can be classified as either non‐hydrogen donor solvents (e.g., phenol) or hydrogen‐donor solvents (mainly tetralin).…”