【Frontiers of HaiChuan Technology】Breakthrough achieved in efficient separation and high-value utilization of lignocellulose
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Lignocellulose is the most abundant renewable raw material in nature, primarily composed of cellulose, hemicellulose, and lignin. How to separate the three components with high quality and obtain raw materials suitable for large-scale use is a key challenge in utilizing lignocellulose as a renewable chemical feedstock. The research team led by Researcher Wang Feng at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has designed and developed the catalytic lignin arylation tri-hydroxyl group separation (CLAF) technology, achieving significant breakthroughs in the separation of the three components of lignocellulose and their value-added utilization. The relevant findings were published in the journal Nature on May 29. The key to efficient utilization – breaking free from the \"reinforced concrete\"-like structural constraints. Wang Feng explains that biomass is a renewable resource with both material and energy properties; it accounts for about one-tenth of the world’s total primary energy supply, with an estimated 170 billion tons of biomass available globally. Lignocellulose is an important component of biomass; China’s annual production amounts to approximately 1.18 billion tons, including forest resources and straws from agricultural crops. Lignocellulose is composed of three components: the hydrophobic lignin, the hydrophilic hemicellulose, and cellulose. Under an electron microscope, the microscopic structure of lignocellulose can be observed: cellulose, which contains glucose units, lignin with benzene rings embedded in it, and hemicellulose composed of pentose and hexose sugars, act as rebar, cement, and stirrups respectively. Cellulose fibers are interwoven into bundles dispersed within the hemicellulose and lignin components, forming a twisted structure similar to that of reinforced concrete. This structure plays a role in supporting and protecting plant growth, and it also makes it difficult to separate the three elements physically. The exploration of chemical techniques for the separation of the three components began in 1900, when the two-step acid hydrolysis method developed at that time became the most widely used approach for lignin quantification. This method can completely hydrolyze cellulose and hemicellulose into pentose and hexose sugars, while yielding lignin that has undergone condensation reactions. However, due to the limitations of detection technology at that time, structural changes in lignin were not detected. By 1957, the wood lignin extracted using dioxane had become the extractive lignin closest to its original structure, but the extraction yield was only 5% to 10%. The ancient pulp and papermaking technology in our country used a long-duration boiling process with lime to separate the fiber components from lignocellulose. In modern times, chemical pulping processes such as the caustic soda process, sulfate process, and sulfite process can remove lignin efficiently and at low cost, but they remain separation techniques that \"sacrifice\" lignin. “It can be said that today’s mature pulping process systems typically focus on only one type of component among these three elements. Such as printing paper, it is mainly produced by chemically treating lignocellulose to remove part of the lignin. During the preparation process, lignin undergoes uncontrolled condensation, which results in a significant reduction in the catalytic reaction activity; it is usually burned as industrial waste with very low added value. ”Wang Feng said regretfully that as the most valuable renewable carbon resource, if lignocellulose cannot be utilized fully, it will limit the economic viability and environmental friendliness of biomass chemical industry development. To achieve the utilization of various components or all components of lignocellulose, the key lies in the efficient and low-cost separation of these components.
A breakthrough in the separation of the three components – making clever use of lignin condensation to turn obstruction into openness. “In our research, we found that a key reason for the inadequate utilization of lignocellulose is that lignin tends to undergo self-condensation during the reaction process, resulting in uncontrolled formation of intermolecular and intramolecular carbon-carbon bond crosslinks.” This is an inherent chemical property of natural lignin; just as children aged five or six are naturally curious and mischievous, it is part of their nature. For lignocellulose, it is inherent to lignin that it tends to self-condense during the reaction process. ”Dr. Li Ning from the Dalian Institute of Chemical Physics, a member of Wang Feng’s team, explained. To address this issue, most research teams have adopted a strategy to inhibit the carbon-carbon bond condensation of lignin itself. They stabilize the lignin components through chemical modification, catalytic depolymerization, and other methods to reduce the occurrence of self-condensation reactions. At first, Wang Feng’s team also considered following the approach used in previous studies, but it did not yield satisfactory results. After reevaluating the advantages and disadvantages of lignin condensation reactions, they came up with a new idea. “The self-condensation reaction of lignin can be chemically classified as an arylation reaction, and arylation reactions are not inherently bad. Rather than using a ‘blocking’ approach to suppress lignin condensation, it is better to take advantage of the presence of self-condensation sites in the lignin structure in order to address the issue of selectivity in arylation reactions. ”Wang Feng recalled his thoughts at the time when he set out to solve the problem. Therefore, Wang Feng’s team took advantage of the situation by introducing phenolic compounds that have a structure similar to lignin and high nucleophilic activity; during the separation process, these phenols undergo selective arylation reactions with lignin, thereby preventing the random self-condensation of lignin. “After arylation modification, the solubility of lignin increases significantly, allowing it to be separated efficiently from cellulose and hemicellulose components while retaining its own reactive aryl ether structure, which facilitates subsequent catalytic depolymerization. ”Li Ning explained. By shifting from blocking to facilitating the process and going with the flow of natural conditions, Wang Feng’s team took advantage of the tendency of lignin to condense, and by introducing lignin-derived phenols with high nucleophilic activity, they significantly increased the selectivity of the arylation reaction of lignin. On this basis, they developed the CLAF technology.
Achieving economic and environmental sustainability in technology – conducting innovative validation studies. After successfully isolating lignin, Wang Feng’s team focused on considering the catalytic conversion of lignin from the perspective of the end market, and identified the research direction of directly catalyzing the depolymerization of lignin to produce bisphenols. Based on the structural properties of arylated lignin, they developed a catalytic depolymerization route for aryl migration, which directly catalytically depolymerizes the lignin components treated by CLAF technology into lignin-based biphenols, thereby producing environmentally friendly renewable biphenols and oligophenols. Compared to bisphenol A, lignin-based bisphenols have essentially comparable mechanical properties, with a significant reduction in endocrine-disrupting activity, allowing their biological safety to be improved by more than 100 times. As an important precursor for both thermosetting and thermoplastic polymers, lignin-based bisphenols hold promise for providing renewable and environmentally friendly product solutions in the fields of coatings, adhesives, and general and engineering plastics. Furthermore, the cellulose and hemicellulose produced as a by-product of the CLAF technology can also be utilized for high-value applications. Cellulose can be used for native pulp, dissolving pulp, and non-food cellulose sugars. The cellulose purity in the dissolved slurry is over 95%, allowing it to replace cotton and serve as a raw material for textiles, pharmaceuticals, and other applications ； Hemicellulose can be used in the production of important platform compounds such as functional sugars, furfural, and its derivatives. “Last year, our country imported approximately 3 million tons of dissolved pulp, with an import dependence rate of nearly 90% ； The market demand for xylose and furfural-based products exceeds 500,000 tons ； The domestic demand for bisphenol A is also around 4 million tons. The market for downstream products of lignocellulose is clear; the main issue now is how to achieve the separation of these three components in an economical and environmentally friendly manner. There is still much to be done on this path; we need to continue innovating and making progress in areas such as the selection of lignocellulosic raw materials, reducing carbon emissions during the reaction process, the design of catalysts and reactors, and the purification and separation of products. ”Wang Feng continued. “Based on our laboratory-scale calculations, the production costs of dissolved pulp, xylose and furfural, as well as aromatic lignin, are much lower than market prices. Current research is focused on engineering scale-up, including the design and construction of pilot-scale facilities as well as optimization of the system to reduce energy consumption and carbon emissions. ”Li Ning added that he hopes the three-component separation technology will enable the full utilization of biomass materials from different regions, promote the local development of related industries, reduce their reliance on fossil resources, and address issues such as the underutilization of biomass materials in China.
It is a very popular industry, but despite being separated for over 100 years, there is still no mature solution