Oxidative pretreatment of lignocellulosic biomass for enzymatic hydrolysis: Progress and challenges.

Deconstruction of cell wall structure is important for biorefining of lignocellulose to produce various biofuels and chemicals. Oxidative delignification is an effective way to increase the enzymatic digestibility of cellulose. In this work, the current research progress on conventional oxidative pretreatment including wet oxidation, alkaline hydrogen peroxide, organic peracids, Fenton oxidation, and ozone oxidation were reviewed. Some recently developed novel technologies for coupling pretreatment and direct biomass-to-electricity conversion with recyclable oxidants were also introduced. The primary mechanism of oxidative pretreatment to enhance cellulose digestibility is delignification, especially in alkaline medium, thus eliminating the physical blocking and non-productive adsorption of enzymes by lignin. However, the cost of oxidative delignification as a pretreatment is still too expensive to be applied at large scale at present. Efforts should be made particularly to reduce the cost of oxidants, or explore valuable products to obtain more revenue.

Synthesis and Computational Investigation of Antioxidants Prepared by Oxidative Depolymerization of Lignin and Aldol Condensation of Aromatic Aldehydes.

Novel antioxidants are synthesized by CuSO4 -catalyzed oxidative depolymerization of lignin to form aromatic aldehydes followed by aldol condensation with methyl ethyl ketone (MEK). Aldol condensation greatly improves the antioxidation ability of lignin depolymerized products. Three lignin monomeric aromatic aldehydes, - p-hydroxybenzaldehyde, vanillin, and syringaldehyde - are further employed for aldol condensation with MEK, resulting in successful synthesis of new antioxidants 1-(4-hydroxyphenyl)pent-1-en-3-one (HPPEO), 1-(4-hydroxy-3-methoxyphenyl)pent-1-en-3-one (HMPPEO), and 1-(4-hydroxy-3,5-dimethoxyphenyl)pent-1-en-3-one (HDMPPEO), respectively. Kinetic modeling illustrates that p-hydroxybenzaldehyde has the highest rate of reaction with MEK, followed by vanillin and then syringaldehyde, which is probably affected by the presence of methoxy groups. The syringaldehyde-derived product (HDMPPEO) displays the best antioxidation ability. As revealed by density functional theory calculations, electron-donating groups, such as methoxy, and conjugated side chains effectively improve the antioxidation ability. A hydrogen atom transfer (HAT) mechanism tends to occur in nonpolar solvents, whereas a sequential proton-loss electron transfer (SPLET) mechanism is favored in polar solvents. This work thus can inspire new pathways for valorization of lignin to produce high value-added products.

Model-based process intensification of dilute acid pre-hydrolysis of oil palm empty fruit bunch biomass for pretreatment and furfural production.

A novel process for simultaneous production of furfural and pretreatment of oil palm empty fruit bunch (EFB) by dilute acid pre-hydrolysis was developed based on non-isothermal kinetic modeling. Mass transfer analysis suggested that the internal diffusion could be neglected as diffusion time of sulfuric acid in EFB particles was significantly shorter than the pre-hydrolysis period, whereas the heating stage could not be neglected due to a significant part of xylan was solubilized at the stage. A strategy for increasing furfural yield was developed by intermittent discharging of steam, resulting in 71.4 % furfural yield. The pretreated solids showed good enzymatic digestibility. 136.3 g/L glucose corresponding to 81.6 % yield was obtained by high-solid loading hydrolysis. 95.4 g furfural and 212 g glucose could be obtained from 1 kg dry EFB. Therefore, non-isothermal effects on polysaccharide hydrolysis and pentose decomposition should be considered carefully for an efficient process design of EFB biorefining.

All-iron ions mediated electron transfer for biomass pretreatment coupling with direct generation of electricity from lignocellulose.

A coupled process of biomass pretreatment for increasing cellulose digestibility and direct conversion of biomass to electricity has been developed with ferric or ferricyanide ions as the anode electron carriers, and Fe(NO3)3 activated by HNO3 as the cathode electron carriers. Pretreated substrates are subjected to enzymatic hydrolysis for release of fermentable sugars, while the pretreatment liquor is employed as anolyte for electricity generation in a liquid flow fuel cell (LFFC). Pretreatment of sugarcane bagasse with 2 M FeCl3 in anode reactor removes âˆ¼ 100% hemicelluloses and obtains 76% enzymatic glucan conversion (EGC), while pretreatment with 0.1 M K3[Fe(CN)6] in 0.5 M KOH achieves 78% lignin removal, 95.8% EGC and 85.1% xylan conversion. From 1000 g bagasse, 171.3 g fermentable sugars is produced with co-generation of 101.4 W·h electricity based on FeCl3 pretreatment, while 519 g fermentable sugars and 28.9 W·h electricity are obtained based on K3[Fe(CN)6] pretreatment.

Light-driven lignocellulosic biomass conversion for production of energy and chemicals.

The depletion of fossil fuels and the increasingly severe environmental pollution caused by massive fossil fuel consumption has driven the quick development of emerging renewable energy technologies. As the most extensive renewable carbon resource, lignocellulose is the potential substitute of fossil resources because of its sustainability and carbon-neutral features. Efficient lignocellulose conversion based on photocatalysis is a promising topic because of sustainable solar energy and the mild condition. This review highlights state-of-the-art photocatalytic technologies for lignocellulosic biomass conversion, focusing on the electricity generation, hydrogen production, and high-value-added biomass derivatives production. Moreover, the progress, challenge, and perspectives of related photocatalytic technologies are specifically discussed. It is recommended that developing more robust and efficient photocatalysts suitable for the complex structure of lignocellulose is necessary to promote the oxidation the biomass. Design and development of novel photochemical reactors and photoelectrochemical cells are also important for demonstration of light-driven lignocellulose conversion at larger scale.