such as poor chemo-catalytic performances, harsh reaction conditions, a high processing cost, and low catalyst stability. The development and implementation of sustainable and effective technologies for the conversion of biomass into biofuels and chemicals is vital for the emerging bioeconomy. Among different strategies, the use of catalysis for the valorization of biomass of different types (lignocellulosic biomass, cellulose, lignin, vegetable oils, algal biomass, etc.) into fuels, platform chemicals, and other value-added products, are promising. This special issue concentrates on the pretreatment of biomass using enzyme, anaerobic digestion of naive source, various factors on the thermal pyrolysis, value-added chemicals synthesis and degradation, covering the following themes:• The recent progress in the bio-and chemo-catalytic conversion of model/real biomass components to value-added chemicals and biofuels with different catalysts (biocatalytic, homogeneous, and heterogeneous),• Molecular insights in the processing of biomass with different catalysts (homogeneous, enzymatic, and heterogeneous),• Research related to catalytic reaction engineering, industrial utilization, biocatalytic system, and eco-friendly catalytic processes for biomass valorization,• Research comparing various catalytic approaches including those involving biotechnology.
Saddler and coworkers reported the enhancing enzyme-mediated cellulose hydrolysis by incorporating acid groups onto lignin during biomass pretreatment. They found that pretreated mechanical pulp (MP) derived from Aspen wood chips was incubated with either 16% sodium sulfite or 32% sodium percarbonate to incorporate similar amounts of sulfonic and carboxylic acid groups onto the lignin (60 mmol/kg substrate) present in the pulp without resulting in significant delignification (Fig. 1a). When Simon's stain was used to assess potential enzyme accessibility to the cellulose, it was apparent that both post-treatments enhanced enzyme accessibility and cellulose hydrolysis. To further elucidate how acid group addition might influence potential enzyme binding to lignin, Protease Treated Lignin (PTL) was isolated from the original and modified mechanical pulps and added to a cellulose rich, delignified Kraft pulp. As anticipated, the PTLs from both the oxidized and sulfonated substrates proved less inhibitory and adsorbed less enzymes than did the PTL derived from the original pulp. Subsequent analyses indicated that both the sulfonated and oxidized lignin samples contained less phenolic hydroxyl groups, resulting in enhanced hydrophilicity and a more negative charge which decreased the non-productive binding of the cellulase enzymes to the lignin.
During the anaerobic digestion of biomass, many factors would influence the efficiency.Various efforts have been focused on optimization of the reaction parameters. Li and coworkers demonstrated that the addition of zero valent iron (ZVI, 37 μm, 10 g/L) could hamper the anaerobic digestion (AD) of cow manure and lignocellulose (Fig. 1b). They found that proper ZVI addition promoted the AD process depending on the feeding materials. An interesting finding of their study was that ZVI could largely enhance the methanogenic rate of cow manure in 6 days since the start, but decreased the accumulative methane yield and volatile fatty acids yield by 10.3% and 12%, respectively. While studying the varying effects of ZVI in diverse anaerobic systems, they also found that the effects were influenced by the characteristics of substrate and inoculum-substrate ratio.
Pyrolysis is one of crucial tools to upgrade biomass into value-added products. Bi and coworker studied the synergistic effects of catalyst mixtures on biomass catalytic pyrolysis in comparison with the single catalyst in a microwave reactor and a TGA (Fig. 1c). They investigated the deoxygenation behavior and the reaction kinetics including the kinetics of catalytic pyrolysis of biomass component (i.e., cellulose, hemicellulose, and lignin). They found 10KP/10Bento (a mixture of 10% K3PO4 and 10% bentonite) increased the mass loss rate by 85 and 45% at a heating rate of 100 and 25°C/min, respectively, compared with that obtained without catalyst. The activation energy for 10KP/10Bento and 10KP/10Clino (a mixture of 10% K3PO4 and 10% clinoptilolite) was slightly lower or similar to those of other catalysts at 30 wt.% loading. The reduction in the activation energy by the catalyst mixture was greater at 100°C/min than 25°C/min due to the improved catalytic activity at a higher heating rate. Synergistic effects were also reflected in the improved properties of bio-oil, as acids, aldehydes, and anhydrosugars were significantly decreased, whereas phenol and aromatic compounds were substantially increased. 30KP (30% K3PO4) and 10KP/10Bento increased the content of alkylated phenols by 341% and 207%, respectively, in comparison with that from pyrolysis of switchgrass without catalyst. Finally, the use of catalyst mixtures improved the catalytic performance markedly, which shows the potential to reduce the production costs of bio-oil and biochar from microwave catalytic pyrolysis.
Bisphenol A (BPA) is a typical endocrine disrupting compound that is carcinogenic and genotoxic and can be accumulated in water due to its extensive use and difficulty of degradation. Yan and coworkers reported a photocatalytic route to degrade BPA using a porous ZnO photocatalyst (Fig. 1d). The effects of various parameters: calcination temperatures, BPA concentrations, ZnO dosages, pH and inorganic ions on the degradation performance were systemically studied. The results showed that 99% degradation of BPA was achieved in 1 h using the porous ZnO calcined at 550°C under the conditions of 30 mg/L BPA, 1 g/L ZnO, and pH of 6.5. Besides, the inhibition effects of anions on the photocatalytic removal of BPA was also investigated in details. The results of scavenging experiments confirmed that h + , •O −2 , and e − played a key role in the photocatalytic degradation process. This study offers a photocatalytic route to upgrading biomass-derived chemicals.
A variety of biomass-derived chemicals can be selectively produced from biomass, among which cyclopentanol can be used as a versatile eco-friendly solvent in various applications. Zhang and coworkers fabricated a bimetallic Ru-Mo catalyst for selective hydrogenation-rearrangement of furfural-like compounds, achieving 89.1% cyclopentanol yield using 1%Ru-2.5%Mo/CNT catalyst pre-reduced at 600 °C (Fig. 1e). The weak acidity and strong hydrogenation activity of the bimetallic Ru-Mo catalyst were found to be important for the synthesis of cyclopentanol from furfurallike compounds.
