Gibbs Dividing Surface and Helium Adsorption

All adsorption data is based on the definition of Gibbs dividing surface, which is a purely mathematical transformation. Adsorption measurements in microporous solids necessitate experimental determination of the dividing surface. An international protocol does not exist on how to perform this important measurement. Commonly, helium is assumed not to adsorb and used as a probe molecule for this measurement. Each experimentalist chooses an arbitrary set of conditions, often without even disclosing them, which adds to the confusion in adsorption literature. Here, a self-consistent method for the analysis of helium data is proposed which does not assume non-adsorbing helium. The method is compared to others using the extensive set of helium/silicalite data. The Gibbs dividing surface and hence the helium isotherms at all temperatures are determined

Net Adsorption: A Thermodynamic Framework for Supercritical Gas Adsorption and Storage in Porous Solids

The thermodynamic treatment of adsorption phenomena is based on the Gibbs dividing surface, which is conceptually clear for a flat surface. On a flat surface, the primary extensive property is the area of the solid. As applications became more significant, necessitating microporous solids, early researchers such as McBain and Coolidge implemented the Gibbs definition by invoking a reference state for microporous solids. The mass of solid is used as a primary extensive property because surface area loses its physical meaning for microporous solids. A reference state is used to fix the hypothetical hyperdividing surface typically using helium as a probe molecule, resulting in the commonly used excess adsorption; experimentalists measure this reference state for each new sample. Molecular simulations, however, provide absolute adsorption. Theoreticians perform helium simulations to convert absolute to excess adsorption, mimicking experiments for comparison. This current structure of adsorption thermodynamics is rigorous (if the conditions for reference state helium measurements are completely disclosed) but laborious. In addition, many studies show that helium, or any other probe molecule for that matter, does adsorb. albeit to a small extent. We propose a novel thermodynamic framework, net adsorption, which completely circumvents the use of probe molecules to fix the reference state for each microporous sample. Using net adsorption. experimentalists calibrate their apparatus only once without any sample in the system. Theoreticians can directly calculate net adsorption; no additional simulations with a probe gas are necessary. Net adsorption also provides a direct indication of the density enhancement achieved (by using an adsorbent) over simple compression for gas (e.g.. hydrogen) storage applications

Adsorptive Storage of Mechanical Energy (Work) for Transportation Applications

Hydraulic accumulators are commonly used in industry to store mechanical energy to prevent intermittent operation of hydraulic pump; a gas contained in a vessel is compressed to very high pressures (200-300 bar) when there is no demand for power as the pump is idling. Inclusion of a solid adsorbent in a hydraulic accumulator operating at such high pressures may first seem counter intuitive based on adsorptive gas storage studies mostly operating close to isothermal conditions. In contrast hydraulic accumulators, particularly for transportation applications, operate under adiabatic conditions making it possible to exploit adsorption phenomena in a different mode. The application and experimental data for several solid-gas pairs under realistic conditions up to 330 bar pressure is presented. This is an abstract of a paper presented at the 2006 AIChE National Meeting (San Francisco, CA 11/12-17/2006)

Adsorption Characteristics of Metal–Organic Frameworks Containing Coordinatively Unsaturated Metal Sites: Effect of Metal Cations and Adsorbate Properties

Metal–organic frameworks in the M/DOBDC series are known to contain a large number of coordinatively unsaturated metal (M) sites. In this work, we study the influence of various metal cations (M = Mg, Mn, Co, and Ni) in the framework on its gas adsorption characteristics. The probe gases (viz. CO₂, CO, CH₄, C₂H₆, N₂, and Ar) were carefully chosen to cover a wider range of polarity and polarizability. While a significant impact of metal atom in the framework is observed on adsorption of polar gases such as CO₂ and CO, it has a negligible effect on adsorption of other relatively nonpolar gases. On one hand, Henry’s constant of CO₂ for Mg/DOBDC is about 4–10 times higher than that for other frameworks; on the other, Henry’s constant for CO on Ni/DOBDC is about 100 times larger than that on Mn/DOBDC. The pore volume of the framework governs adsorption capacity at higher pressures. Each of the frameworks exhibits widely different adsorption enthalpies for polar gases such as CO₂ and CO. At pressures below 15 bar, the Ideal Adsorbed Solution Theory predicts very good selectivity for CO over all other studied gases on Ni and Co/DOBDC frameworks, while Mg and Mn/DOBDC frameworks exhibit preferential selectivity for CO₂

Adsorption and Separation of Carbon Dioxide Using MIL-53(Al) Metal-Organic Framework

In this work, we report adsorption isotherms of various industrially important gases, viz. CO₂, CO, CH₄, and N₂ on MIL-53­(Al) metal organic framework (MOF). The isotherms were measured in the range of 0–25 bar over a wide temperature range (294–350 K). The structural transformation of the adsorbent and the resulting breathing phenomenon were observed only in the case of CO₂ adsorption at 294 and 314 K. Adsorption of CO (another polar gas), N₂ and CH₄ did not induce any structural transformation in this adsorbent for the experimental conditions considered in this work. Since the CO₂ isotherms at 294 and 314 K involve structural transformation and show a distinct step, a conventional isotherm model cannot be used to describe such behavior. In order to model these isotherms, a dual-site Langmuir-type equation (one site each for the two structural forms, i.e., large pore phase and narrow pore phase) that includes a normal distribution function to account for structural transformation is proposed. This model successfully mimics the Type-IV isotherm behavior of CO₂ on MIL-53­(Al). Henry’s constants and adsorption enthalpies of CO₂ on the two structural forms were calculated using this model. The Ideal Adsorbed Solution Theory (IAST) was used to predict the selectivity of CO₂ at 350 K over other gases studied in this work

Hydrogen Adsorption on Zn-BDC, Cr-BDC, Ni-DABCO, and Mg-DOBDC Metal–Organic Frameworks

This work reports hydrogen adsorption properties of four different metal–organic frameworks (MOFs) namely Zn-BDC, Cr-BDC, Ni-DABCO, and Mg-DOBDC. Gravimetric hydrogen adsorption measurements are performed over a wide range of temperature (90 K to 298 K) and pressure (0 bar to 100 bar). At the lowest experimental temperature (90 K to 100 K) all the isotherms are saturated and the adsorption capacity is governed by pore volume. On the other hand, at room temperature the isotherms closely follow Henry’s law. Modeling of the excess isotherms is also done. Net adsorption isotherms, which can directly indicate the efficiency of porous adsorbent for storage, are also presented. In terms of volumetric efficiency, Mg-DOBDC MOF exhibits best storage capacity out of all the MOFs considered in this study

Effect of Adsorbent History on Adsorption Characteristics of MIL-53(Al) Metal Organic Framework

Structural transformation of MIL-53­(Al) metal organic framework from large pore to narrow pore form (lp → np) or vice versa is known to occur by adsorption of certain guest molecules, by temperature change or by applying mechanical pressure. In this work, we perform a systematic investigation to demonstrate that adsorbent history also plays a decisive role in the structural transitions of this material (and hence on its adsorption characteristics). By changing the adsorbent history, parent MIL-53­(Al) is tuned into its np domain at ambient temperature such that it not only exhibits a significant increase in CO₂ capacity, but also shows negligible uptake for CH₄, N₂, CO, and O₂ at subatmospheric pressure. In addition, for the high pressure region (1–8 bar), we propose a method to retain the lp form of the sample to enhance its CO₂ uptake

Measurement and Modeling of Adsorption of Lower Hydrocarbons on Activated Carbon

This work reports adsorption isotherms for C₁ to C₆ hydrocarbons on activated carbon at three different temperatures (293 K, 318 K, and 358 K) over a wide range of pressure (0 bar to 100 bar). The isotherms were measured using a standard gravimetric method. The experimental data were correlated and compared using Toth, modified virial, and potential theory models. On the basis of the adsorption potential, characteristic curves were also generated for methane, ethane, propane, isobutane, <i>n</i>-pentane, and <i>n</i>-hexane on activated carbon over broad ranges of pressure and temperatures. The micropore volume of the activated carbon predicted from potential theory was in good agreement with those obtained using a N₂ isotherm measured at 77 K. The enthalpy of adsorption at zero loading was found to increase linearly with the carbon number