Recently, the biomass conversion team in the State Key Laboratory of Microbial Technology (SKLMT) of Shandong University has made new progress in the enzymatic production of nanocellulose from commercial pulp, which has been published inChemical Engineering Journal with the title of “Microwave-associated ZnCl2pretreatment followed by enzymatic hydrolysis for high-efficiency production of nanocellulose from Eucalyptus Dissolving Pulp”. Li Huiwen, a master's degree candidate from SKLMT, is the first author of this paper. Prof. Zhao Jian and Assoc. Prof. Lu Xianqin is the co-corresponding author.

Cellulose, the most abundant biopolymer on earth, is the primary component of plant lignocellulosic biomass. Structurally, cellulose consists of glucose units linked into chains by β-1,4-linkages. These chains are aligned parallelly to form the elementary fibrils (d: 2-5 nm), arranged into bundles surrounded by lignin and hemicellulose, forming larger microfibrils with a width of 10-30 nm. These microfibrils collectively constructed the cell wall structure. Through enzymatic, chemical, or mechanical treatments, cellulose fibers can be defibrillated into nanoscale particles known as nanocellulose (NC). Compared to chemical and mechanical treatments, the enzymatic treatment shows many advantages, such as environmentally friendly and milder operating conditions, higher selectivity, and lower energy cost. However, the stable hydrogen bond and the compact crystalline structure of cellulose make enzymatic hydrolysis difficult, resulting in the low yields of CN obtained by the enzymatic method.

To solve this problem, Prof. Zhao's group has recently developed a novel process, which involves microwave-associatedZnCl₂ pretreatment followed by enzymatic hydrolysis,for high-efficiency production of nanocellulose from Eucalyptus Dissolving Pulp(EDP). During this ZnCl₂ pretreatment, ZnCl₂ interacts with the hydroxyl groups on the C2 and C6 positions of cellulose chains, which weakens the hydrogen bonds between cellulose chains and disrupts the fiber structure, thus enhancing the hydrolysis efficiency of the subsequent enzymatic treatment and increases NC yield. After optimization of the treatment process, the ultrafine CNC and CNF/CNC blends were produced in high yield (total yield of 93.3%). It was also testified that theZnCl₂pretreatment solution can be recycled and reused, and the NC yield can be obtained above 90% even after five cycles. The produced NC was characterized by SEM, XRD, FTIR, and TGA, which suggested that the NC has abundant functional groups, such as -COOH, free -OH, and -NH₂, and comprised approximately 20 wt% Zn ions, which gave the NC potential antimicrobial capabilities, ion conductivity, and freeze-tolerance. This study highlights the viability and sustainability of microwave-assistedZnCl₂pretreatment for efficient nanocellulose production, offering a sustainable and cost-effective route for industrial-scale application.

Fig 1 Chemical and structural characteristics NCs produced by microwave-associated ZnCl₂ pretreatment followed by enzymatic hydrolysis

Fig 2 The morphological characteristics and size distribution of NC produced through microwave-associated ZnCl₂ pretreatment followed by enzymatic hydrolysis process under optimized pretreatment conditions

In recent years, the Biomass Conversion Team in SKLMT of Shandong University has made a series of progress in the enzymatic production of nanocellulose from various lignocellulosic biomass (including microcrystalline cellulose, commercial dissolving pulp, and commercial bleached kraft pulp), and they had been published respectively in Carbohydrate Polymers(2020, In 234:115862；2023, 301: 120291), Industrial Crops and Products(2020, 155:112755), International Journal of Molecular Sciences（2023, 24：10676）, and Bioresources and Bioprocessing (2023,10:42). A Chinese invention patent has been obtained(ZL 202010026664.0).