Solvolysis of Kraft Lignin to Bio-Oil: A Critical Review

Abstract

Lignin, a component of lignocellulosic biomass, is abundant and is produced extensively as a waste product of the Kraft pulping process, lignin obtained from this process is called Kraft lignin (KL). Lignin’s three-dimensional structure composed of aromatic alcohols (monolignols) makes it a potential source of renewable aromatic chemicals or bio-oil, if depolymerized. Among all the depolymerization methods for KL, solvolysis is the most popular, showing consistently high bio-oil yields. Despite the large number of studies that have been carried out, an economically feasible industrial process has not been found and comparison among the various studies is difficult, as very different studies in terms of reaction media and catalysts report seemingly satisfactory results. In this review, we compare and analyze KL solvolysis studies published, identify trends in bio-oil composition and give a comprehensive explanation about the mechanisms involved in the processes. Additional commentary is offered about the availability and future potential of KL as a renewable feedstock for aromatic chemicals, as well as logistical and technical aspects.

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“…Depolymerization of Kraft lignin to produce bio-oil has been extensively studied in the past decades, and various methods have shown excellent results [25]. This depolymerization process is shown in Figure 1, where the chemical bonds of lignin are severed, usually in presence of a catalyst and a solvent at relatively high temperatures.…”

NiCo and NiCo Decorated with Ru Nanoparticles for Magnetically Induced Hydroprocessing of Lignin Models

“…As a first valorization step, lignin oil or bio-oil can be obtained by depolymerizing lignin through reductive catalytic fractionation (RCF) of a lignocellulosic material. Additionally, many different treatments on native or isolated lignin, such as pyrolysis, , solvolysis, reductive depolymerization, , acid–base depolymerization, or oxidative alkaline depolymerization have been used to yield lignin oils, determining product distribution. Lignin oil comprises a wide range of smaller aromatics where phenols usually prevail, as well as acids, aldehydes, and ketones.…”

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“…8 Among these, thermochemical and catalytic methods have obtained the largest success, usually obtaining a mixture of gas, solid residue, and bio-oil as products from the reaction, in different proportions depending on the severity of the process, reaction media, and presence or absence of a catalyst. 9 While the merits of using transitionmetal-containing catalysts are well understood, resulting in a higher yield of bio-oil containing less oxygen and lower formation of solid residue, 10 their application at an industrial scale leaves much to be desired due to their relatively fast poisoning and low cost−benefit ratio when compared to simple strong alkali salts, such as NaOH or KOH in homogeneous reaction media. 11 Among the existing lignin depolymerization methods, basecatalyzed depolymerization in water as reaction media has attracted attention due to its economic feasibility, 12 short reaction times, 13 and reaction performance comparable to that seen in transition-metal-catalyzed processes, in terms of bio-oil yield, 14 particularly for pulping-derived lignins.…”

“… 8 Among these, thermochemical and catalytic methods have obtained the largest success, usually obtaining a mixture of gas, solid residue, and bio-oil as products from the reaction, in different proportions depending on the severity of the process, reaction media, and presence or absence of a catalyst. 9 While the merits of using transition-metal-containing catalysts are well understood, resulting in a higher yield of bio-oil containing less oxygen and lower formation of solid residue, 10 their application at an industrial scale leaves much to be desired due to their relatively fast poisoning and low cost–benefit ratio when compared to simple strong alkali salts, such as NaOH or KOH in homogeneous reaction media. 11 …”