Enhanced hydrolysis of mechanically pretreated cellulose in water/CO2 system

CO2 -Enabled Biomass Fractionation/Depolymerization: A Highly Versatile Pre-Step for Downstream Processing

Lignocellulosic biomass is inevitably subject to fractionation and depolymerization processes for enhanced selectivity toward specific products, in most cases prior to catalytic upgrading of the three main fractions-cellulose, hemicellulose, and lignin. Among the developed pretreatment techniques, CO₂ -assisted biomass processing exhibits some unique advantages such as the lowest critical temperature (31.0 °C) with moderate critical pressure, low cost, nontoxicity, nonflammability, ready availability, and the addition of acidity, alongside easy recovery by pressure release. This Review showcases progress in the study of sub- or supercritical CO₂ -mediated thermal processing of lignocellulosic biomass-the key pre-step for downstream conversion processes. The auxo-action of CO₂ in biomass pretreatment and fractionation, along with the involved variables, direct degradation of untreated biomass in CO₂ by gasification, pyrolysis, and liquefaction with relevant conversion mechanisms, and CO₂ -enabled depolymerization of lignocellulosic fractions with representative reaction pathways are summarized. Moreover, future prospects for the practical application of CO₂ -assisted up- and downstream biomass-to-bioproduct conversion are also briefly discussed.

Reversible covalent chemistry of carbon dioxide unlocks the recalcitrance of cellulose for its enzymatic saccharification

To overcoming the natural recalcitrance of cellulose for glucose production via enzymatic hydrolysis, a new strategy of destroying hydrogen bond donor to reconstruct cellulose's hydrogen bonding network was developed via a mild reversible reaction of cellulose with CO₂ catalyzed by organic bases. The reaction dynamics of cellulose with CO₂ in the presence of organic bases was studied by using in situ IR. Investigation also included how the organic bases in pretreatment media and pretreatment parameters including CO₂ pressure, pretreatment temperature and time affected the physical-chemical structure of cellulose by Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Atomic force microscopy (AFM) and subsequent enzymatic scarification of cellulose. The findings showed that dissolution activation efficiency significantly correlated to various parameters, that can be optimized to be the tetramethyl guanidine (TMG)/CO₂/DMSO solvent system at 50 °C, 2 MPa of CO₂ for 2 h, by which a complete transformation the cellulose crystalline structure from I to II, and 100% glucose yield were achieved. The recyclability and usability are also investigated.

Bacterial-mediated recovery of copper from low-grade copper sulphide using acid-processed rice straw

Bacteria community and copper recovery in presence of acid-processed rice straw (ARW) were explored during low-grade copper sulphide bioleaching. The results indicated a strongly promoting response of appropriate-quality ARW with improved bacteria concentration and enhanced copper recovery. The highest bacteria concentration reached 9.54 × 10⁷ cells·mL^-1 with an increase by 69.15%. And a maximum of 95.32% copper leaching rate with a relatively low Fe³⁺ concentration (329.00 mg·L^-1) was obtained in presence of 1.0 g powdered ARW compared to only 83.40% in its absence. That is due to less development of passivation layer formed by Fe³⁺ hydrolysis, which is contributed by reducing ARW. 16S rDNA analysis illustrated the dominant leaching bacteria (Acidithiobacillus ferrooxidans) was influenced significantly, whose proportion reached 40.38% to the total bacteria when the ARW was added compared to 15.92% in its absence. And Stenotrophomonas accounted for the highest proportion of the bacteria community throughout bioleaching process.

Coupled Pretreatment with Liquid Nitrogen and Ball Milling for Enhanced Cellulose Hydrolysis in Water

A key problem in the conversion of cellulose into chemicals and fuels is the low product yield from cellulose due to its robust structure. In this work, for the first time, cellulose was pretreated with coupling of liquid nitrogen and ball milling (LN-BM) for cellulose hydrolysis. After the LN-BM treatment, the glucose yield from cellulose by HCl in water increased by almost 2 times and yield of formic acid catalyzed by H₂SO₄-NaVO₃ was more than 3-fold that obtained from untreated cellulose. The yields were also much higher than that from the individually ball-milled cellulose. The structure variation of cellulose indicated that reduction of both crystallinity index and molecular weight contributed to improving the conversion efficiency, but the former was the dominant factor. The combination of liquid nitrogen and ball milling developed in this work is an effective and environment-friendly approach for cellulose pretreatment.