Yongzhuang Liu

Facile extraction of cellulose nanocrystals from wood using ethanol and peroxide solvothermal pretreatment followed by ultrasonic nanofibrillation

Cellulose nanocrystals (CNCs) were successfully extracted from wood flour by a two-step process that comprised ethanol and peroxide solvothermal pretreatment and an ultrasonic disintegration process. Characterization results showed that 97% of the total lignin and 70% of the hemicellulose could be fractionated in a single ethanosolv pretreatment step. Additional treatment with alkaline hydrogen peroxide removed the residual lignin and hemicellulose and resulted in high purity cellulose. The CNCs obtained after ultrasonication displayed a similar yield, size, morphology, and crystallinity but had better thermal stability and film forming properties than those produced by concentrated acid hydrolysis. Overall, the solvothermal treatment using ethanol and its combination with peroxide is an ideal substitute method for pretreatment of lignocellulose. Further integration of such pretreatments with ultrasonication provides a promising efficient process with low environmental impact for production of CNCs.

Testing of the superhydrophobicity of a zinc oxide nanorod array coating on wood surface prepared by hydrothermal treatment

Abstract Natural wood surface is hydrophilic and can easily absorb moisture. Inorganic nanometer-scale metal oxides can be controlled and fabricated on wood surface to serve as a protective coating. In the present study, a superhydrophobic surface consisting of zinc oxide (ZnO) nanorod array was successfully attached on wood via a cosolvent hydrothermal method at low temperature. By means of X-ray diffraction pattern and Fourier transform infrared spectroscopy, the presence of the hexagonal wurtzite phase of ZnO was detected and characterized. Field-emission scanning electron microscopy showed uniform, large-scale rod-like ZnO crystal whiskers on the wood surface. The wettability of the wood surface after treatment became superhydrophobic, with a water contact angle of 153°. The prepared coating also showed an anti-contamination effect against milk, cola, soya sauces and coffee.

Efficient Flame-Retardant and Smoke-Suppression Properties of Mg–Al-Layered Double-Hydroxide Nanostructures on Wood Substrate

Improving the flame retardancy of wood is an imperative yet highly challenging step in the application of wood in densely populated spaces. In this study, Mg-Al-layered double-hydroxide (LDH) coating was successfully fabricated on a wood substrate to confer flame-retardant and smoke-suppression properties. The chemical compositions and bonding states characterized by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed the coating constituents of Mg-Al LDH. The coating evenly covered the sample wood surfaces and provided both mechanical enhancement and flame-retardancy effects. The limiting oxygen index of the Mg-Al LDH-coated wood increased to 39.1% from 18.9% in the untreated wood. CONE calorimetry testing revealed a 58% reduction in total smoke production and a 41% reduction in maximum smoke production ratio in the Mg-Al LDH-coated wood compared to the untreated wood; the peak heat release rate and total heat release were also reduced by 49% and 40%, respectively. The Mg-Al LDH coating is essentially hydrophilic, but simple surface modification by fluoroalkyl silane could make it superhydrophobic, with a water contact angle of 152° and a sliding angle of 8.6°. The results of this study altogether suggest that Mg-Al LDH coating is a feasible and highly effective approach to nanoconstructing wood materials with favorable flame-retardant and smoke-suppression properties.

Efficient Cleavage of Lignin–Carbohydrate Complexes and Ultrafast Extraction of Lignin Oligomers from Wood Biomass by Microwave-Assisted Treatment with Deep Eutectic Solvent

Abstract Lignocellulosic biomass is an abundant and renewable resource for the production of biobased value‐added fuels, chemicals, and materials, but its effective exploitation by an energy‐efficient and environmentally friendly strategy remains a challenge. Herein, a facile approach for efficiently cleaving lignin–carbohydrate complexes and ultrafast fractionation of components from wood by microwave‐assisted treatment with deep eutectic solvent is reported. The solvent was composed of sustainable choline chloride and oxalic acid dihydrate, and showed a hydrogen‐bond acidity of 1.31. Efficient fractionation of lignocellulose with the solvent was realized by heating at 80 °C under 800 W microwave irradiation for 3 min. The extracted lignin showed a low molecular weight of 913, a low polydispersity of 1.25, and consisted of lignin oligomers with high purity (ca. 96 %), and thus shows potential in downstream production of aromatic chemicals. The other dissolved matter mainly comprised glucose, xylose, and hydroxymethylfurfural. The undissolved material was cellulose with crystal I structure and a crystallinity of approximately 75 %, which can be used for fabricating nanocellulose. Therefore, this work promotes an ultrafast lignin‐first biorefinery approach while simultaneously keeping the undissolved cellulose available for further utilization. This work is expected to contribute to improving the economics of overall biorefining of lignocellulosic biomass.

Formation and properties of polyelectrolytes/TiO2 composite coating on wood surfaces through layer-by-layer assembly method

Abstract A transparent and protective multilayer coating composed of poly(allylamine hydrochloride) (PAH), poly(styrene sulfonic acid) sodium salt (PSS), and nano-TiO2 films was fabricated on wood surfaces by layer-by-layer assembly method. The coating was formed through pre-immobilization of cationic PAH layer on wood substrate, followed by alternate soaking cycles in pH-controlled anionic PSS solution and TiO2 colloidal solution. The structure and properties of the assembled coating were characterized through scanning electron microscopy (SEM), energy disperse X-ray analysis (EDXA), UV reflection and absorption, colorimetry, and contact angle (CA) measurement. Results revealed that the coverage uniformity and thickness were improved with increasing number of PSS/TiO2 bilayers. The coating masked the cell wall ultrastructure while leaving the microscale features intact. The anatase TiO2 in the assembled coating enhanced the UV stability of wood and resulted in a lowered photochromism. Furthermore, the photocatalytic capability of the nano-TiO2 films in degrading dyes of rhodamine B and methylene blue was verified. The nano-TiO2 film on the top surface of the coating embodied a superhydrophilicity, showing self-cleaning and anti-fog capabilities. Stearic acid modification altered the superhydrophilicity to hydrophobicity with CA of 140°.

Process of in situ forming well-aligned Zinc Oxide nanorod arrays on wood substrate using a two-step bottom-up method

A good nanocrystal covering layer on wood can serve as a protective coating and present some new surface properties. In this study, well-aligned ZnO nanorods (NRs) arrays were successfully grown on wood surface through a two-step bottom-up growth process. The process involved pre-sow seeds and subsequently their growing into NRs under hydrothermal environment. The interface incorporation between wood and ZnO colloid particles in the precursor solution during the seeding process was analyzed and demonstrated through a schematic. The growth process of forming well-aligned ZnO NRs was analyzed by field-emission scanning electron microscopy and X-ray diffraction, which showed that the NRs elongated with increased reaction time. The effects of ZnO crystal form and capping agent on the growth process were studied through different viewpoints.

Synthesis of aluminum hydroxide thin coating and its influence on the thermomechanical and fire-resistant properties of wood.

Abstract Aluminum hydroxide [Al(OH)3] film on wood substrate has been synthesized by means of the hydrothermal (HT) method for improvement of wood’s mechanical properties and resistance to combustion. The HT temperature was set to 100°C and 120°C, and the reaction time varied from 4 h to 10 h. X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS) results show that the thin film grown on wood surface was amorphous Al(OH)3 and the relative crystallinity of treated woods slightly improved. SEM observation revealed that the Al(OH)3 film is composed of regular micro/nano-sized spheres, whose production and size increase with the reaction time, and some AlO(OH) structures emerged at 120°C with reaction times longer than 6 h. The storage modulus of the wood treated at 100°C and 120°C for 8 h can be improved by 30%, while the viscoelastic properties are also influenced by the HT treatment and Al(OH)3 coating. The limiting oxygen index of raw wood rose from 24.7% to 27.9% after the treatment, which can be interpreted as a moderate improvement of the fire resistance.

Multiple hydrogen bond coordination in three-constituent deep eutectic solvents enhances lignin fractionation from biomass

As an emerging generation of green solvents, deep eutectic solvents (DESs) are promising for the pretreatment of lignocellulose and the production of biochemicals. However, not all DESs are effective for the cleavage of lignin–carbohydrate complexes (LCCs) in lignocellulose and the fractionation of lignin. In this study, we analyzed the nature of complex molecular interactions between choline chloride (ChCl) and glycerol in ChCl/glycerol (1 : 2) DES using density functional theory and the Kamlet–Taft solvatochromic method. The ChCl–glycerol DES exhibited weak competing interactions towards the linkages in the LCC network because the intramolecular hydrogen bonds (H-bonds) in ChCl–glycerol were constrained by mutually anionic H-bonds ([Cl(glycerol)]−) and cationic H-bonds ([Ch(glycerol)]+). Furthermore, because of the absence of active protons and acidic sites, the DES was unable to cleave ether bond linkages in the LCCs. Accordingly, we designed a three-constituent DES (3c-DES) by coordinating AlCl3·6H2O in ChCl/glycerol DES based on an acidic multisite coordination theory. The competition of anionic H-bonds and unidentate aluminum ligands was synchronized to form supramolecular complexes, allowing the multisite bridging ligands to cleave both the H-bonds and ether bonds in LCCs. Consequently, the lignin fractionation efficiency was significantly improved from 3.61% to 95.46%, and the lignin purity reached 94 ± 0.45%.

Assembly of Organosolv Lignin Residues into Submicron Spheres: The Effects of Granulating in Ethanol/Water Mixtures and Homogenization

The production of chemicals and various materials from black liquor lignin will greatly improve the economics of biomass refining. In the present work, organosolv lignin residues (OLRs) from the organosolv pulping process were used to fabricate submicron lignin spheres using ethanol/water mixtures as an antisolvent, in conjunction with homogenization. Both the ethanol content in the ethanol/water mixture and the applications of homogenization were investigated as key factors affecting the agglomeration of OLRs. The results show that the solubility of the amphiphilic OLRs in ethanol was approximately 28 and 32 times greater than those of alkali lignin and kraft lignin, respectively. Using the optimal percentage of ethanol together with homogenization enhanced the nucleation of lignin macromolecules, such that the colloidal spheres of OLR were spontaneously assembled via gradual hydrophilic–lipophilic aggregation. The resulting OLR colloidal spheres had a yolk–shell structure and a mean particle diameter of around 200 nm, when the ethanol content was 50% and the homogenization time was 15 min or more. This study demonstrates a simple means of utilizing OLRs to produce lignin-based spheres. The lignin spheres generated in this work are thought to have potential applications in many fields as porous carbon precursors for energy storage, sensitive functional materials, and controlled-release fertilizer carriers.

Comparison of ZnO nanorod array coatings on wood and their UV prevention effects obtained by microwave-assisted hydrothermal and conventional hydrothermal synthesis

Abstract ZnO nanorod array coatings were successfully synthesized on wood surfaces using a conventional hydrothermal (CHT) and a microwave-assisted hydrothermal (MWHT) method. The reaction time of the MWHT method at high temperature was only 1/12th that of the CHT method. The MWHT-ZnO nanorods exhibited better crystallinity than those produced by the CHT method. The ZnO nanorod had a wurtzite crystal structure oriented along the c-axis. The MWHT nanorods with their higher (002)/(101) ratio were more conducive and improved the orientation degree of the ZnO nanorods. Field emission scanning electron microscopy (FE-SEM) images showed that the MWHT-ZnO nanorods had smaller diameters and narrower size distributions than those produced by the CHT method. However, both methods formed well-aligned array coatings that covered the wood surfaces, and the two coatings had similar UV protecting effects and inhibited discoloration.