Supercritical fluids (SCFs), compounds heated and pressurized beyond the critical point, have many unusual properties. Homogeneous molecular catalysts, which have far greater control over selectivity than heterogeneous solid catalysts, are now being tested in SCFs, and early results show that high rates, improved selectivity, and elimination of masstransfer problems can be achieved. As industry moves away from toxic or environmentally damaging solvents, supercritical carbon dioxide may be an ideal replacement medium for nonpolar or weakly polar chemical processes. More than simply substitutes for nonpolar solvents, SCFs can radically change the observed chemistry. Supercritical carbon dioxide is also an excellent medium for its own fixation, as demonstrated by studies of its hydrogenation.

Homogeneous Catalysis in Supercritical Fluids.

Isokinetic relationship, isoequilibrium relationship, and enthalpy-entropy compensation.

We have studied the dispersion and exfoliation of four inorganic layered compounds, WS2, MoS2, MoSe2, and MoTe2, in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersibility. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However, the dispersed concentration varied greatly from solvent to solvent. For all four materials, the concentration peaked for solvents with surface energy close to 70 mJ/m2, implying that all four have surface energy close to this value. Inverse gas chromatography measurements showed MoS2 and MoSe2 to have surface energies of ∼75 mJ/m2, in good agreement with dispersibility measurements. However, this method suggested MoTe2 to have a considerably larger surface energy (∼120 mJ/m2). While surface-energy-based solubility parameters are perhaps more intuitive for two-dimensional materials, Hansen solubil...

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Solvent exfoliation of transition metal dichalcogenides: dispersibility of exfoliated nanosheets varies only weakly between compounds.

Vapor-phase adsorption and separation of the C8 alkylaromatic components p-xylene, m-xylene, o-xylene, and ethylbenzene on the metal−organic framework MIL-47 have been studied. Low coverage Henry adsorption constants and adsorption enthalpies were determined using the pulse chromatographic technique at temperatures between 230 and 290 °C. The four C8 alkylaromatic components have comparable Henry constants and adsorption enthalpies. Adsorption isotherms of the pure components were determined using the gravimetric technique at 70, 110, and 150 °C. The adsorption capacity and steepness of the isotherms differs among the components and are strongly temperature dependent. Breakthrough experiments with several binary mixtures were performed at 70−150 °C and varying total hydrocarbon pressure from 0.0004 to 0.05 bar. Separation of the different isomers could be achieved. In general, it was found that the adsorption selectivity increases with increasing partial pressure or degree of pore filling. The separation ...

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Pore-filling-dependent selectivity effects in the vapor-phase separation of xylene isomers on the metal-organic framework MIL-47

Inverse gas chromatography as a source of physiochemical data.

Diffusion, adsorption and catalytic studies by gas chromatography

Measurement of diffusion coefficients by reversed-flow gas chromatography instrumentation

Rate coefficients for evaporation of pure liquids and diffusion coefficients of vapors

Measurement of activity coefficients by reversed-flow gas chromatography

If the internal wall of the diffusion column used in the reversed-flow gas chromatography (RFGC) technique is covered with an adsorbent material or a supported liquid, then isotherm determination becomes much simpler. Diffusion and resistance to mass transfer are not neglected, the sorption effects is non-existent and the pressure gradients are negligible along the bed. The method leads directly to independent experimental isotherms over a wide range of concentrations, without specifying a priori an isotherm equation. The isotherm can also be determined in the presence of a surface reaction of the adsorbate. The necessary theoretical analysis has been developed, and the equations derived were applied to benzene and cyclohexane adsorbed on graphitized carbon black, to ethene, ethyne, propene and sulfur dioxide on marble powder, and to propene on silica gel.

Nicholas A. Katsanos

Papers

Diffusion, adsorption and catalytic studies by gas chromatography

Measurement of diffusion coefficients by reversed-flow gas chromatography instrumentation

Rate coefficients for evaporation of pure liquids and diffusion coefficients of vapors

Measurement of activity coefficients by reversed-flow gas chromatography

Simple determination of experimental isotherms using diffusion denuder tubes