Micropore Filling of Supercritical Xe in Micropores of Activated Carbon Fibers

Analysis of NMR Adsorption Isotherms of Zeolite ZSM-5: Adsorption Profiles Derived from the Pressure and Temperature Dependences of 129Xe NMR Chemical Shift and Signal Intensity

¹²⁹Xe NMR spectroscopy of nanomaterials, such as zeolites, can provide valuable information on the nanostructure and physicochemical properties of adsorption. In the present study the pressure and temperature dependences of the ¹²⁹Xe NMR chemical shift and the signal intensity were investigated in detail with a zeolite ZSM-5. The pressure dependence of the signal intensity at constant temperature was analyzed based on the Langmuir and Dubinin-Radushkevich (D-R) models, from which the thermodynamic parameters and energetic profiles of adsorption were obtained together with information concerning the nanospace size. From this isotherm analysis the coverage, θ, was calculated and used for isotherm analysis of the chemical shift. The θ dependence of the chemical shift was successfully fitted by an exponential function, and the results were discussed in relation to the chemical shift at zero coverage, that at full coverage and the curvature of the exponential function. The chemical shift data reported with the zeolites NaA and KA, where separated signals were observed for the different number of encapsulated Xe atoms in the α cage, were analyzed and discussed collectively.

Separation of phenolic compounds with modified adsorption resin from aqueous phase products of hydrothermal liquefaction of rice straw

Hydrothermal liquefaction can be used to convert rice straw into an aqueous phase product that contains valuable phenolic compounds. In experiments, commercial adsorption resin XAD-4 was modified by a benzene ring - α,α'-dichloro-p-xylene (DCX) - in order to separate the phenolic compounds from the aqueous phase product; and, the optimal conditions for separation were explored. The results showed that, after modification of the resin, its adsorption capacity improved by 50%, due to increases in surface area, pore volume and micropore volume. The selectivity of the resin increased when the benzene ring was introduced as the ring formed hydrogen bonds with the compounds. The optimal conditions for separation were desorption agent of 40%, 45% and 55% ethanol solution, a flow rate of 2.5-5 mL/min, and a ratio of the sample volume to the column volume was 1:1. The total content of phenolic compounds in aqueous solution increased from 18% to 78% after separation.

Contribution of Ash Content Related to Methane Adsorption Behaviors of Bituminous Coals

Methane adsorption isotherms on coals with varying ash contents were investigated. The textural properties were characterized by N2adsorption/desorption isotherm at 77 K, and methane adsorption characteristics were measured at pressures up to 4.0 MPa at 298 K, 313 K, and 328 K, respectively. The Dubinin-Astakhov model and the Polanyi potential theory were employed to fit the experimental data. As a result, ash content correlated strongly to methane adsorption capacity. Over the ash range studied, 9.35% to 21.24%, the average increase in methane adsorption capacity was 0.021 mmol/g for each 1.0% rise in ash content. With the increasing ash content range of 21.24%~43.47%, a reduction in the maximum adsorption capacities of coals was observed. In addition, there was a positive correlation between the saturated adsorption capacity and the specific surface area and micropore volume of samples. Further, this study presented the heat of adsorption, the isosteric heat of adsorption, and the adsorbed phase specific heat capacity for methane adsorption on various coals. Employing the proposed thermodynamic approaches, the thermodynamic maps of the adsorption processes of coalbed methane were conducive to the understanding of the coal and gas simultaneous extraction.

2H NMR study of 2D melting and dynamic behaviour of CDCl3 confined in ACF nanospace

Two-dimensional melting of trichloromethane (chloroform) confined in activated carbon fibre was investigated using differential thermal analysis and (2)H NMR techniques. Differential thermal analysis revealed a thermal anomaly with an endothermic peak at 269 K, which was distributed from 250 K to 287 K on the heating direction. This anomaly was also observed upon cooling at the same temperature. Furthermore, (2)H NMR revealed that slow motion such as molecular hopping and/or diffusion of CDCl(3) in ACF affected the spectral line width. The temperature dependence (Arrhenius plot) of the spectral line width showed an inflection point at 227 K. The activation energy of molecular motion of CDCl(3) in ACF was 4 kJ mol(-1) at temperatures greater than 227 K and 7.7 kJ mol(-1) at temperatures less than 227 K. Reduction of the activation energy suggests that the average intermolecular distance between CDCl(3) molecules enlarges above the inflection point. The difference of activation energy (3.7 kJ mol(-1)) is close to the enthalpy of fusion in typical plastic crystals. These results reveal that the thermal anomaly and the transition of dynamic process correspond respectively to melting of CHCl(3) in ACF and the pre-melting phenomenon.

Testing isotherm models and recovering empirical relationships for adsorption in microporous carbons using virtual carbon models and grand canonical Monte Carlo simulations

Using the plausible model of activated carbon proposed by Harris and co-workers and grand canonical Monte Carlo simulations, we study the applicability of standard methods for describing adsorption data on microporous carbons widely used in adsorption science. Two carbon structures are studied, one with a small distribution of micropores in the range up to 1 nm, and the other with micropores covering a wide range of porosity. For both structures, adsorption isotherms of noble gases (from Ne to Xe), carbon tetrachloride and benzene are simulated. The data obtained are considered in terms of Dubinin-Radushkevich plots. Moreover, for benzene and carbon tetrachloride the temperature invariance of the characteristic curve is also studied. We show that using simulated data some empirical relationships obtained from experiment can be successfully recovered. Next we test the applicability of Dubinin's related models including the Dubinin-Izotova, Dubinin-Radushkevich-Stoeckli, and Jaroniec-Choma equations. The results obtained demonstrate the limits and applications of the models studied in the field of carbon porosity characterization.

Adsorption of argon from sub- to supercritical conditions on graphitized thermal carbon black and in graphitic slit pores: A grand canonical Monte Carlo simulation study

In this paper we consider the adsorption of argon on the surface of graphitized thermal carbon black and in slit pores at temperatures ranging from subcritical to supercritical conditions by the method of grand canonical Monte Carlo simulation. Attention is paid to the variation of the adsorbed density when the temperature crosses the critical point. The behavior of the adsorbed density versus pressure (bulk density) shows interesting behavior at temperatures in the vicinity of and those above the critical point and also at extremely high pressures. Isotherms at temperatures greater than the critical temperature exhibit a clear maximum, and near the critical temperature this maximum is a very sharp spike. Under the supercritical conditions and very high pressure the excess of adsorbed density decreases towards zero value for a graphite surface, while for slit pores negative excess density is possible at extremely high pressures. For imperfect pores (defined as pores that cannot accommodate an integral number of parallel layers under moderate conditions) the pressure at which the excess pore density becomes negative is less than that for perfect pores, and this is due to the packing effect in those imperfect pores. However, at extremely high pressure molecules can be packed in parallel layers once chemical potential is great enough to overcome the repulsions among adsorbed molecules.