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.
Highlights
During the course of the last couple of centuries, our easy access to fossil fuels has allowed the development of industry and led to the advanced society we know today
By centering our analysis of the studies in the reaction media and catalyst used, we examine the trends in published studies and draw attention to and criticize the metrics used to evaluate these studies
While the results reported in these studies are usually centered on the yield and the product distribution in the bio-oil obtained, it is hard to say with certainty which combination of factors is the best performing, with multiple, sometimes very distinct studies reporting bio-oil yields of above 80%
Summary
During the course of the last couple of centuries, our easy access to fossil fuels has allowed the development of industry and led to the advanced society we know today. Biomass conversion technologies relied heavily on simple sugars, starches and vegetable oils as raw materials to produce biofuel and led to a debate about the use of food as a source of biofuel synthesis, and its impact on food prices; these were called “first-generation biofuels”. Understanding this problem, attention was paid to non-edible plant matter that could be collected postharvest at relatively low cost; this led to the development of what is called “second-generation biofuels”, which rely entirely on non-edible plant matter, in the production of cellulosic ethanol. Studies employing solvolysis to depolymerize lignin suffered from high reaction temperature, long reaction time or high hydrogen pressure [10]; catalyst development and better understanding of the process have led to the development of less severe processes
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