Recently, Professor Yu Haipeng from Northeast Forestry University and his team have proposed a new method for preparing carboxymethyl cellulose nanofibers, which solves the previous problem of the difficulty in achieving low-cost, large-scale, and sustainable preparation of this type of nanocellulose. This is conducive to the green and large-scale production of nanocellulose.

According to the introduction, this method can retain the natural structure of cellulose nanofibers. The prepared cellulose nanofibers not only have an ultra-fine diameter, high aspect ratio, and rich carboxyl groups, but also inherit the natural excellent properties of cellulose.

Therefore, it has great potential to become a complementary method to TEMPO (2,2,6,6-Tetramethylpiperidoxyl, 2,2,6,6-tetramethylpiperidine oxide) oxidation and become another classic preparation strategy for cellulose nanofibers.

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The solvent used in the study has a recycling capacity of more than 10 times, which highlights the recyclability and economic feasibility of this study, and will also bring certain help to the preparation of bio-macromolecules and sustainable materials.

Yu Haipeng said: "The next step is to actively cooperate with enterprises, continuously combine with the actual production to develop and interpret this method, and strive to form a mature technical solution and process regulations."At present, we are in discussions with a biomass new energy company and a paper company for cooperation, hoping to accelerate the incubation process through collaboration.

"Forty Years of Trials and Tribulations of Nanocellulose"

As is well known, today's society is facing the depletion of fossil resources and the demand for carbon neutrality. One of the solutions is to convert biomass resources into high-value bio-based materials, energy, and chemicals, and other high-value products.

Nanocellulose is widely found in various plants such as wood, bamboo, cotton, hemp, straw, wheat straw, reeds, and pulp, and is a fibrous form of plant cellulose composition. Because its diameter size is usually within 100nm, it is called "nanocellulose".The molecular structure, geometric shape, chemical properties, physical and mechanical properties, and biological characteristics of nanocellulose are all highly distinctive, and it has found a wide range of applications.

As early as 1983, academia had already obtained nanocellulose in the laboratory, but it was not until after 2004 that people began to intensively study this field, and a global research boom has been maintained since 2012.

In recent years, academia has developed many preparation methods, including chemical methods, mechanical methods, biological methods, and hybrid methods.

In terms of preparation effects and popularity, strong acid hydrolysis, TEMPO (2,2,6,6-Tetramethylpiperidoxyl, 2,2,6,6-tetramethylpiperidine oxide) oxidation, and chemical-mechanical methods are the most representative.

Strong acid hydrolysis is mainly used to produce cellulose nanocrystals with a relatively small aspect ratio. Chemical-mechanical methods are mainly used to produce cellulose nanofibers with a relatively high aspect ratio.The TEMPO oxidation method produces products with an aspect ratio that falls between the two mentioned above, but is more refined, with carboxyl functional groups on the surface, making it more stable in nature and more outstanding in functional applications.

The preparation methods for these nanocelluloses, although relatively mature in laboratory-scale research, are not widely commercialized.

The practical production of nanocellulose not only requires efficient methods and processes but also needs to comprehensively consider the issues of industrial scale-up equipment, energy consumption, reagent costs, reagent recycling, waste liquid discharge, and environmental impact. Otherwise, it will restrict the development of its production and application.

Therefore, the development of a new method with industrial prospects must use solvents that comply with the "Twelve Principles of Green Chemistry" and meet the development elements highly compatible with sustainable development.Continuously Tackling the Challenges in the Preparation of Nanocellulose

In 2009, Yu Haipeng heard about the concept of nanocellulose. After reading a few articles, even though the laboratory did not have professional equipment at the time, he still began to explore.

Overcoming the rudimentary conditions, he and his student Chen Wenshuai used the laboratory's method of making paper pulp, utilizing sodium hypochlorite and dilute alkali solution to remove lignin and hemicellulose from wood fibers.

Then, high-frequency ultrasonic treatment was applied to the purified cellulose to extract nanocellulose from wood powder, obtaining a cellulose nanofiber with a diameter of 2-20nm and an aspect ratio of several hundred.

The method they used is a mild chemical treatment combined with mechanical preparation, which is characterized by the structure and function of the components of the wood cell wall.With the aid of chemical reagents, cellulose is extracted by breaking the anti-depolymerization barrier, and then ultrasonic vibration in the liquid phase is used to break the hydrogen bonds between the swollen fibers, thereby achieving the separation of microfibrils and elementary fibrils, and thus obtaining cellulose nanofibers.

Extended experimental results have proven that this method is applicable to various agricultural and forestry biomass raw materials and can be connected with different mechanical devices such as high-pressure homogenization, high-speed stirring, and nano-grinding.

At the same time, this method has the characteristics of original state separation, energy saving, and simple preparation, which is very easy to connect with the existing industrial system.

In 2011, after the relevant papers of the research group were published, this method was quickly adopted by many external research institutions and has now become a representative preparation method.

In 2016, another student of Yu Haiping, Liu Yongzhuang, began to study the refining of wood biomass based on deep eutectic solvents.During this period, Liu Yongzhuang discovered that a low eutectic solvent composed of choline chloride and oxalic acid could produce an excellent degradation effect on lignin and hemicellulose at 110°C. Moreover, high-purity cellulose could be retained and separated as an insoluble substance.

If this solvent is combined with 800W microwave irradiation, the treatment effect can be achieved in just 3 minutes, and the extraction efficiency of cellulose will be greatly improved.

It can be seen that this method not only can obtain the fractions of lignin and hemicellulose but also can use cellulose to prepare nanocellulose.

Particularly prominent is that cellulose nanocrystals prepared by microwave combined with low eutectic solvent can break through the shackles of having to use 65% concentrated sulfuric acid in traditional methods, making it an environmentally friendly and safer preparation method.

Yu Haipeng said, "After the publication of the paper on this method, it has also sparked a research boom in using low eutectic solvents for biomass refining, and the academic community has also begun to study the preparation and functionalization of nanocellulose."On this basis, they have further developed hydrated deep eutectic solvents, which not only reduce the cost of solvents to an extremely low level, but also can be used for processing pulp and achieving the preparation of nanocellulose.

So far, the research group has conducted a lot of research on the preparation of nanocellulose and has developed several new preparation methods, such as the chemical combination ultrasonic method, microwave deep eutectic solvent method, and hydrated deep eutectic solvent method, etc.

Each of these methods has its own advantages. However, compared with the TEMPO oxidation method, the latter's product characteristics are very prominent and widely used.

However, Yu Haipeng and others do not have a comparable preparation method. "So, can we develop a new method that can achieve the nanofiber preparation effect of the TEMPO method, while overcoming its shortcomings of high cost of TEMPO reagents, pH control dependence, and longer preparation time?" he once asked the students.

In 2019, Yu Haipeng arranged the above topic to the new student Shi Xiaochao. "Shi Xiaochao is a serious student who really took the teacher's words to heart, he conducted a lot of literature research and persevered in experiments," said Yu Haipeng.Ultimately, the new method they proposed for the preparation of carboxymethylated cellulose nanofibers can produce ultra-fine, ultra-long fibers, and has more advantages in terms of green environmental friendliness of solvents, low cost, recyclability, simplicity of preparation, industrialization, and sustainability.

"Without a doubt, this new method that can be compared with the TEMPO preparation method is also another milestone achievement of our team in the research of nanocellulose. Without being overly modest, I believe this technology has reached an internationally leading level," said Yu Haiping.

In the first round of peer review of the paper, two reviewers gave positive evaluations. Yu Haiping said: "I am particularly pleased that after the paper was published, we found that one of the reviewers was Professor Akira Isogai from the University of Tokyo.

This professor is the main promoter and the most representative figure in the preparation of nanocellulose by the TEMPO oxidation method, and has won the highest award in the field of international cellulose and renewable resource materials - the Anselm Payne Award." Yu Haiping and the students are very happy to get the recognition of this professor.

Under the guidance of another reviewer, the research group has once again supplemented a large number of experiments, such as the production of tons of nanocellulose, the evaluation of solvent performance and product performance in 10 cycles of recycling utilization, etc.Ultimately, the related paper was published under the title "Scalable production of carboxylated cellulose nanofibres using a green and recyclable solvent" in Nature Sustainability (IF 27.6).

Xiao Chao Shi is the first author, and Hai Peng Yu and Professor Kai Zhang from the University of Göttingen in Germany serve as co-corresponding authors[1].

Nature Sustainability featured a commentary article in the same issue, in which the journal's senior editor Yao Qing Zhang commented: "What impresses me about this work is that the authors have made significant progress in the sustainable preparation of nanocellulose. The recyclability of this solvent formulation can be used for the large-scale and economical production of carboxylated cellulose nanofibres."

"There is no reason it cannot be industrialized."It is also reported that people often ask Yu Haipeng: What is the industrialization prospect of nanocellulose? What are the main problems that limit its industrialization?

Yu Haipeng's view is: "Nanocellulose has the advantages of natural sustainable resources, many excellent properties, a very broad field of application, and a very solid research foundation. Such a good product must have a broad application prospect, and there is no reason why it cannot be industrialized."

But at the same time, he said: "The industrialization process of nanocellulose, especially the domestic process, is far behind the expectations of the academic community. But now it is just the darkness before dawn, and the dawn is just around the corner."

He believes that although there is a lack of industrial development in this field in China, China's research foundation and research ability in nanocellulose are not behind the foreign countries.

"And our industrial foundation is better, the application market is larger, so the prospect will also be better," said Yu Haipeng.Regarding the reasons for the limitations in the industrialization of nanocellulose, although the mainstream view is that high preparation costs and difficulty in mass scale-up are the main factors.

Although this is true to some extent, according to the technology and analysis discussed by the Haipeng research group, they believe that the current research foundation and technical solutions are sufficient to overcome this problem.

Especially new methods like those in this study can provide solutions to overcome the aforementioned issues. He stated, "I think the most important bottleneck is not in the upstream, but in the downstream. Without demand from the downstream, how can the upstream have the motivation to produce? Even if it is produced, it won't sell well."

So the core key lies in building an industrial cluster for downstream utilization, forming an industry chain that connects upstream and downstream. Once the industrial cluster framework is established, an industrial closed loop can be formed.

As for the establishment of the industrial cluster, Yu Haipeng believes that it should be integrated with existing light industry (papermaking, textile), chemical industry (coatings, polymers, materials), food, and pharmaceutical fields, making good use of the existing industrial foundation and leveraging the leading and exemplary effect of industry leaders.At the same time, he suggested that the development of "homogeneous substitution" and "similar substitution" of nanocellulose could take the lead. As long as it can partially play a role similar to that of new nanomaterials such as carbon nanotubes and precious metal nanowires, its industrial demand and scale will be considerable, and the related industries will inevitably be driven forward.