Today, the vast majority of fuels and chemical raw materials rely on petroleum refining. However, petroleum is a non-renewable resource and also has an imbalance in geographical distribution.

Therefore, it is necessary to seek a widely distributed renewable resource and develop effective technical means to convert it into high-value-added products, achieving sustainable production of fuels and chemicals, in order to break away from dependence on fossil energy.

Lignocellulosic biomass, with cellulose, hemicellulose, and lignin as its main components, is a very good option. It is commonly found in agricultural and forestry waste, and if well utilized, it can turn waste into treasure.

People generally call the process of high-value transformation of this lignocellulosic raw material into bio-based products biorefining.

Advertisement

In the traditional biorefining model, it is usually necessary to use harsh pretreatment processes to break the dense cell wall structure of lignocellulose, making cellulose and hemicellulose easily accessible for enzymatic catalysis to obtain monosaccharides. Then, with the help of yeast, they are used as a carbon source and further fermented to produce ethanol and other products.In this process, the lignin component tends to undergo a significant increase in structural inertness due to intense recondensation reactions, making it difficult to further convert and utilize.

Only by solving this problem can the potential of lignin as the most abundant aromatic resource in nature be released.

To this end, in recent years, the academic community has developed a "lignin-first" conversion and utilization strategy to stabilize lignin active intermediates and improve the depolymerization efficiency of lignin macromolecules into aromatic monomers.

However, this strategy has to some extent compromised the enzymatic hydrolysis performance of the carbohydrate components. In other words, in the comprehensive utilization of lignocellulose, there seems to be a phenomenon where one cannot have both fish and bear's paw.

In response to this issue, Professor Lou Hongming and his team from South China University of Technology have developed a high-solid pre-treatment process based on organic amine aqueous solutions, which can break down corn straw into carbohydrates that are easy to enzymatically hydrolyze and lignin with good catalytic depolymerization activity.It is reported that this research project is based on the team's previous research results on the interaction between lignin and enzymes. In their previous research, they found that the residual lignin in the pretreated lignocellulose would adsorb cellulase onto the lignin through hydrophobic interactions, which would reduce the amount of enzyme binding to the target substrates (cellulose and hemicellulose) during the enzymatic hydrolysis process, leading to a decrease in enzymatic hydrolysis efficiency.

To address this issue, they wanted to make the pretreated lignin more hydrophilic, and thus they thought of organic amines as reagents - they have hydrophilic nitrogen and strong reactivity, which is very promising for introducing hydrophilic nitrogen into the lignin structure, thereby increasing its hydrophilicity.

Subsequently, they tried using diethylamine as a pretreatment reagent, and through some process adjustments, the pretreated corn straw showed a considerable enzymatic hydrolysis yield.

However, at this point, the highlights of the entire work were not enough, so they considered how to create a distinctive lignin utilization method under this pretreatment system?

Initially, they also wanted to try a similar approach as in existing literature, that is, to extract the lignin from the pretreatment liquid first, and then dissolve and depolymerize it.In their anticipation, the lignin treated with organic amines would retain the nitrogen-containing structures introduced during the pretreatment after depolymerization, resulting in nitrogen-containing depolymerized products.

However, after several attempts, the results were not satisfactory. So, they simply tried to catalyze the pretreatment solution directly, which might allow the unreacted organic amines remaining in the liquid during the pretreatment process to participate in the reaction and be utilized.

Facts have proven that this method is indeed effective; they achieved the successful transformation of lignin and solidified the foundation of nitrogen-containing product process technology.

In addition, by coupling the pretreatment and lignin depolymerization processes, the organic amines added during the pretreatment can be further utilized to nitrogen-functionalize the lignin depolymerization products.

Lignin structure does not contain nitrogen elements, so its high-value products generally only contain carbon, hydrogen, and oxygen elements. The team's method of "introducing nitrogen into wood" can expand the product pool of lignin.Through this, they efficiently convert cellulose, hemicellulose, and lignin from lignocellulose into monosaccharides, monophenols, and pyridine bases, providing a feasible path for the sustainable development of these high-value-added products.

At the same time, they unexpectedly discovered the "multifunctional" effect of diethylamine in the lignocellulosic biorefining process, which can remove lignin by dissolution in pretreatment, enhance the enzymatic hydrolysis effect of lignin, and improve the product yield in depolymerization.

In particular, after oxidizing the side chains of lignin and coupling them with diethylamine to prepare high-value-added pyridine base compounds, not only the utilization of the lignin benzene ring is achieved, but also the utilization of the lignin side chains.

It can be seen that for the preparation of aromatic compounds through lignin depolymerization, this achievement provides new ideas, not only improving the yield and value of depolymerization products, but also enhancing the economy and feasibility of preparing high-value-added chemicals or fuels from lignin.

Recently, the related paper was published in Nature Communications with the title "Aqueous amine enables sustainable monosaccharide, monophenol, and pyridine base coproduction in lignocellulosic biorefineries" [1].Xu Li is the first author, and Professors Hongming Lou and Xueqing Qiu from South China University of Technology serve as the co-corresponding authors.

It is anticipated that this achievement will lead to potential applications in two aspects:

On one hand, it can be used for the production of cellulosic ethanol.

The pretreatment technology used in this study can be conducted under high-solid conditions, and it has been proven that no enzyme inhibitors are produced during the pretreatment process. Therefore, enzymatic hydrolysis can be carried out directly after pretreatment without the need for washing and detoxification steps.

This reduces the number of process steps, and subsequently, by combining with simultaneous saccharification and co-fermentation technology, it is possible to achieve an efficient conversion from lignocellulosic materials to high-concentration fuel ethanol.On the other hand, it can be used for the black liquor in papermaking.

There is a large amount of industrial lignin in the black liquor of papermaking, and the current catalytic depolymerization method can be carried out in an aqueous solution system, so it may be possible to catalyze and convert the dissolved lignin into high-value-added products without additional treatment (such as lignin extraction) for the black liquor of papermaking.

The research group added: "Our team has been engaged in the study of lignin for nearly 30 years, and has more than ten years of accumulation in the directions of lignocellulose pretreatment, enzyme hydrolysis reaction enhancement, and lignin depolymerization."

Before, Lou Hongming repeatedly reminded the graduate students to pay attention to the hydrophilic substances in the depolymerization products, but it did not attract the attention of the students.

This time, because nitrogen elements were introduced into the depolymerization products, their hydrophilicity was significantly improved, and the team members finally began to pay attention to the aqueous phase products, and thus a gratifying result was achieved: that is, the lignin depolymerization products dissolved in water exceeded the organic phase, and a very high product yield was obtained.In the next step, they plan to delve into the underlying mechanisms, hence focusing on the study of the synthetic mechanisms and product regulation methods of pyridine compounds.

Secondly, they will optimize the process step by step, including optimizing the pre-treatment process with organic amines, the oxidation process of pre-treatment, and the process coupling of alkaline hydrogenolysis, to improve the yield and selectivity of mono-phenol products and pyridine products through process optimization.

Finally, they also plan to expand the innovative process to the depolymerization of black liquor in papermaking and enzymatic hydrolysis lignin, optimize the alkaline pulping and cellulosic ethanol process in papermaking, and achieve high-value utilization of lignin while realizing the efficient use of cellulose and hemicellulose, achieving high-value utilization of the whole components while improving economic efficiency.