Zhenshan Hou

Effect of dissolved CO2 on the conductivity of the ionic liquid [bmim][PF6]

This work conducts the first study on the effect of compressed CO2 on the electric conductivity of ionic liquids (ILs), and the CO2/1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) system is investigated at 40.0 °C and 50.0 °C, and at pressures up to 130 bar. It is found that the conductivity increases with increasing pressure of CO2 in the pressure range studied, although CO2 is a non-polar compound.

Synthesis of dimethyl carbonate using CO2 and methanol: enhancing the conversion by controlling the phase behavior

The critical parameters and phase behavior of the multi-component system CO2-CH3OH–CH3I–H2O–CH3OC(O)OCH3 (dimethyl carbonate, DMC) were determined. The concentrations of the components were selected in such a way that they simulated the compositions of the reaction system at different conversions for synthesizing DMC using CO2 and methanol in a batch reactor. The critical density of the reaction system decreases with the conversion of methanol. The critical temperature and critical pressure of the reaction system increase with the conversion. Based on the determined critical parameters and phase behavior, DMC synthesis using CO2 and methanol was run at various pressures that corresponded to conditions in the two-phase region, the critical region as well as the single-phase supercritical region. The original ratios of the reactants CO2∶CH3OH were 8∶2 and 7∶3, and the corresponding reaction temperatures were 353.2 and 393.2 K, respectively, which were slightly higher than the critical temperatures of the reaction systems. The results indicate that the phase behavior affects the equilibrium conversion of methanol significantly and the conversion reaches a maximum in the critical regions of the reaction system. At 353.2 K, the equilibrium conversion in the critical region is about 7%, and can be about three times as large as those in other phase regions. At 393.15 K, the equilibrium conversion in the critical region is also much higher and can be twice as large as those in other phase regions.

Why Do Co-solvents Enhance the Solubility of Solutes in Supercritical Fluids? New Evidence and Opinion

The effects of two polar co-solvents, chlorodifluoromethane and acetone, on the solubility and enthalpy of a solution of 1,4-naphthoquinone in supercritical (SC) CO2 were studied. We found that the dissolution process becomes less exothermic in the presence of the co-solvents relative to that in pure CO2, although the solubility is enhanced significantly by the co-solvents. This indicates that the increase in the solubility by adding co-solvents results from the increase of the entropy of solution. On the basis of the unexpected results we propose a new mechanism for the solubility enhancement of the solute by the co-solvents in supercritical fluids (SCF); this should be applicable to cases in which the local density of the SC solvent around the solute and the co-solvent is larger, and the co-solvent associates preferentially with the solute. The results are also very important for the understanding of other fundamental questions of SCF science, such as the effect of co-solvents on the thermodynamic and kinetic properties of the reactions in SCFs.

Selective oxidation of cyclohexane in compressed CO2 and in liquid solvents over MnAPO-5 molecular sieve

The selective oxidation reaction of cyclohexane with MnAPO-5 molecular sieve catalyst to produce cyclohexanol and cyclohexanone was carried out in compressed CO2 at 398.2 K using oxygen as an oxidant, and the reaction mixture was controlled to be in the two-phase region, very close to the critical point, and in the supercritical (sc) region of the reaction system. The effect of a small amount of butyric acid cosolvent on the reaction in scCO2 was also studied. The reaction was also conducted in liquid butyric acid, benzene and CCl4, and in the absence of any solvent. It has been observed that the conversion and selectivity of the reaction in compressed CO2 change considerably with the phase behavior or the apparent density of the reaction system. Addition of a small amount (0.2 mol%) of butyric acid cosolvent to scCO2 enhances the conversion significantly, and the selectivity also changes considerably. The by-products of the reaction in compressed CO2 with and without the cosolvent are much less than that of the reaction in liquid solvents or in the absence of solvents. It is advantageous to conduct the reaction in supercritical CO2.

Measurement of critical points of the methylcyclohexane (MCH)–H2–CO2 system in the CO2-rich region

A simple apparatus for determining critical points is described. The apparatus was used to investigate pure CO2, the binary system of toluene–CO2, and the ternary system methylcyclohexane (MCH)–H2–CO2. The critical points were obtained on the basis of both critical opalescence and the appearance of the meniscus at equal volume of vapor and liquid. The critical points of pure CO2 and the toluene–CO2 system determined by this method agreed well with the literature values. The critical points of the ternary system MCH–H2–CO2 were determined in the CO2-rich region. The Peng–Robinson equation of state was used to predict the critical points and the results agreed reasonably well with the experimental data.

Preparation of mesoporous MCM-41/poly(acrylic acid) composites using supercritical CO2 as a solvent

The synthesis of host–guest composites consisting of entrapped poly(acrylic acid) (PAA) and inorganic host MCM-41 is described. The monomer acrylic acid (AA) and initiator azoisobutyronitrile (AIBN) were introduced into the channels of MCM-41 using supercritical CO2 as a solvent at 35.0 °C, followed by free radical polymerization at a higher temperature, resulting in MCM-41/PAA composites. The composites were characterized using thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption and desorption, scanning electronic microscopy (SEM) and X-ray diffraction (XRD). It was confirmed that PAA was indeed entrapped in the pores of MCM-41, and that the structure of MCM-41 was not destroyed. Composites with different compositions were obtained by controlling the processing parameters, such as the absorbing time, the concentration of AA in the fluid phase, and the pressure of CO2.

Small-angle X-ray scattering study on correlation length and density fluctuations in a supercritical CO2–water mixture

The microscopic inhomogeneity in a supercritical mixture of (CO2)(1-x)(H2O)(x) (x = 0.003) has been studied by small-angle X-ray scattering (SAXS) at 308.2 K and six different pressures/densities. The results show that the correlation length and density fluctuation is maximized near the critical point where the partial molar volume reaches a minimum