Towards sustainable production of value-added chemicals and materials from lignocellulosic biomass: carboxylic acids and cellulose nanocrystals

Abstract

Industrial production of many chemicals and chemical products depends primarily on fossil resources. Lignocellulosic biomass, which is the most abundant and sustainable biomass on earth, is a potential renewable resource for the production of a wide range of products (e.g. chemicals, fuels, materials) that could be used to replace products currently produced by the petrochemical industry. Large amounts of lignocellulosic materials are generated as waste byproducts of various industrial and agro-based processes. Efficient valorization of such materials would lead to lower greenhouse gas emissions and waste volumes and would bring considerable economic benefits.

Various approaches are available for the conversion of lignocellulosic materials into highvalue products. In this work, partial wet oxidation by molecular oxygen is proposed as a potential industrial process for the production of chemicals, particularly low-molecular carboxylic acids, from lignocellulosic biomass waste. The study investigated the catalytic potential of two homogenous heteropolyacid catalysts for the partial wet oxidation of lignin based on their effectiveness as regards carboxylic acid yield and lignin conversion. Recovery of the carboxylic acids produced was studied using solvent extraction. Additionally, an enzyme-mediated approach for coproduction of cellulose nanocrystals and fermentable sugars is proposed. In this work, alkaline lignin and cellulose (wood pulp and filter paper) were used as a model of lignocellulosic feed materials for the partial wet oxidation and enzymatic hydrolysis experiments, respectively.

Formic acid, acetic acid and succinic acids are among the major products produced by the partial wet oxidation of lignin in alkaline and neutral aqueous solutions. In this work, optimum reaction conditions to obtain the maximum yield of products were investigated. An important finding is that in an alkaline medium, the overall yield of products from partial wet oxidation decreased at higher lignin concentration. It was found that this reduction could be attributed to repolymerization/condensation side reactions of lignin fragments that compete with oxidative lignin depolymerization reactions.

Of the two catalysts studied, the phosphomolybdate catalyst (H3PMo12O4) provided the best results in terms of both the lignin conversion rate and the total yield of carboxylic acids. Although both heteropolyacids showed different catalytic behaviors, the reaction pathway of the lignin oxidation seems to be determined by the type of addenda atom in the HPA catalyst, with Mo favoring a selective oxidation reaction. In addition, it was found that recovery of the two catalysts could be easily achieved, which would make partial lignin wet oxidation an environmentally friendly and potentially economically viable process.

In the solvent extraction studies, various types of extractants were evaluated. Different factors were considered in the solvent selection process. Of the tested solvents, Alamine 336 and 2-methyltetrahydrofuran (2-MTHF) solvents showed the most promising results and both solvents can be used to recover the studied components from aqueous solutions. It was found that the use of toluene as a diluent for Alamine 336 prevented the formation of a third intermediate liquid phase.

The impacts of introducing a cellulase-cocktail treatment system before an acid hydrolysis process for isolating cellulose nanocrystals (CNCs) from a cellulose-based model feedstock was investigated. It was found that for a given weight of feedstock, the enzyme-mediated approach improved CNC yield (8-86 wt%) from acid hydrolysis. Another important finding was that significant recovery of fermentable sugars (20-60 wt%), which would otherwise have entered the acid waste stream, is achievable with this method. These findings indicate the potential of enzymatically mediated acid hydrolysis processes for the co-production of fermentable sugars, thus providing additional revenue, and for improvement of the acid hydrolysis efficiency, thus offsetting CNC production costs.