Mesoporous KIT-6 supported highly dispersed silver nanoparticles and nano-networks were successfully synthesized in supercritical carbon dioxide and co-solvent solution using AgNO3 as precursor, followed by calcinations. The nanocomposites were characterized by XRD, TEM, ICP and N2 adsorption-desorption isotherms. Three kinds of co-solvent were investigated including ethanol, the mixture of ethanol and ethylene glycol as well as the mixture of ethanol and water. It was found that when only ethanol was used as co-solvent, a small amount of AgNO3 was deposited onto the complex nanochannels of KIT-6 even if the deposition time was 12h. However, it took a short deposition time of 1h to deposit a large amount of precursors onto the substrates when the mixture of ethanol and ethylene glycol as well as the mixture of ethanol and water were used as co-solvent. The large solubility of AgNO3 in ethylene glycol or water had a positive effect on the dissolution of precursor in scCO2, which was beneficial to the adsorption of precursor on the substrates.

Supercritical carbon dioxide (scCO2) combined with an ultrasound method using 15–25MPa and the temperature range of 120–170 C for 0.5–4h was proposed as a pretreatment for corn cob and corn stalk with moisture contents of 50%. The results showed that both scCO2 pretreatment and scCO2 combined with ultrasound pretreatment could improve the total reducing sugar yields of the two materials. Enzymatic hydrolysis of both corn cob and corn stalk sugar using scCO2 pretreatment was increased by 50% and 29.8% compared with the control samples, respectively, while they were increased by 75% and 13.4%, respectively， using the scCO2 combined with ultrasound pretreatment under our experimental conditions. Following treatment from the two pretreatment processes, X-ray diffraction results indicated no significant change in crystallinity of the two materials. However, scanning electron microscopy images demonstrated that the microscopic structure of lignocellulose changed considerably and the surface area of lignocellulose increased significantly.

Uniformly dispersed Ag nanowires were obtained in SBA-15 at a short deposition time of 5 minutes in supercritical CO2. The short deposition time relied not only on the near-zero surface tension of supercritical CO2 but also on the addition of ethylene glycol as co-solvent. It was found that a small amount of the precursors were reduced into Ag0 by ethylene glycol and acted as nuclei leading to a non-equilibrium sorption, which representing a rapid adsorption of precursor onto the substrate. In order to provide an overall understanding of the exact role played by ethylene glycol when preparing Ag@SBA-15 in supercritical CO2, the dissolution enhancement of precursor by the co-solvent, the diffusion properties of the precursor into the nano-scale channels and the interaction between the precursor and the substrates were all taken into consideration. Finally, this method was successfully extended to other porous substrates such as γ-Al2O3, SiO2 and KIT-6 offering its great opportunities of applications in industry.

In previous reports, organic solvents have usually been used as cosolvents in supercritical fluid applications. In this study, dilute acids were introduced into supercritical CO2 as cosolvents for the first time to improve the adsorption and dispersion of metal precursors on the substrate. Superhighly dispersed Co3O4 nanoparticles were confined in the nanochannels of SBA-15 after deposition and calcination when a small amount of nitric acid was added as the cosolvent. As the concentration of nitric acid increased, more individual small nanoparticles aggregated in adjacent nanochannels through the micropores in the walls of the substrates, which favored the formation of large nanoparticles. Furthermore, only Co3O4 nanoparticles were obtained when nitric acid was added to supercritical CO2−ethanol solution, whereas nanowires were found instead when hydrochloric acid was used. This indicates that H+ favors the adsorption and dispersion of precursors on the substrates and Cl− plays a key role in dictating the nanowire morphology.

Supercritical carbon dioxide (scCO2) was used to pretreat corncob, cornstalk and rice straw at the temperature of 80-160 °C and pressure of 5-20 MPa, with the raw materials moisture content of 0-75% for the durations of 15-60 min. It was found that the reducing sugar yield of all the three materials showed a maximum with the increase of the moisture content and pretreatment temperature. The optimum experiment result was obtained at 100 °C, 15 MPa with the duration of 30 min and moisture contents of 50%. The reducing sugar yield was 39.6%, 27.4% and 36.6% for corncob, cornstalk and rice straw, respectively, while for untreated materials it was 26.2%, 22.5% and 30.9%. High Performance Liquid Chromatography (HPLC) analysis for hydrolysates of corncob and rice straw indicated that scCO2 pretreatment had a positive effect on the hydrolysis of both cellulose and hemicelluloses. Among the three materials, the reducing sugar yield of corncob was improved the most after scCO2 pretreatment, which could be demonstrated by the X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses.