Mass transfer and adsorption equilibrium for low volatility alkanes in BPL activated carbon.

The structure of a molecule and its concentration can strongly influence diffusional properties for transport in nanoporous materials. We study mass transfer of alkanes in BPL activated carbon using the concentration-swing frequency response method, which can easily discriminate among mass transfer mechanisms. We measure concentration-dependent diffusion rates for n-hexane, n-octane, n-decane, 2,7-dimethyloctane, and cyclodecane, which have different carbon numbers and geometries: straight chain, branched chain, and cyclic. Micropore diffusion is determined to be the controlling mass transfer resistance except at low relative saturation for n-decane, where an external mass transfer resistance also becomes important, showing that the controlling mass transfer mechanism can change with system concentration. Micropore diffusion coefficients are found to be strongly concentration dependent. Adsorption isotherm slopes obtained from measured isotherms, the concentration-swing frequency response method, and a predictive method show reasonably good agreement.

Organoalkoxysilane-grafted silica composites for acidic and basic gas adsorption.

With the prevalence of air quality issues in our society, the ability to remove toxic gases from air is a necessity. This work addresses the development of biphasic, nanostructured, organoalkoxysilane-grafted, siliceous materials for use in single pass filters of various types for the removal of acidic and basic gases from humid air. Materials exhibit high single pass capacities for sulfur dioxide, a representative acid-forming gas, or ammonia, a representative basic gas. The nanostructured siliceous support provides initial ammonia capacity, and grafted amine and carbonyl groups provide desired functional chemistries for sulfur dioxide and enhanced ammonia capacities. Methacryloxypropyltrimethoxysilane (MAPS)-MCM-41 has the highest ammonia capacity at about 7 mol/kg at 1500 ppmv and 23 °C, and 3-aminopropyltriethoxysilane (APTES)-MCM-41 has the highest sulfur dioxide capacity at 0.85 mol/kg at 500 ppmv and 23 °C. These biphasic materials exhibit high adsorption capacity for two distinct gases and are promising candidates as adsorbents for protection from toxic industrial gases.

Stability effects on CO2 adsorption for the DOBDC series of metal-organic frameworks.

Metal-organic frameworks with unsaturated metal centers in their crystal structures, such as Ni/DOBDC and Mg/DOBDC, are promising adsorbents for carbon dioxide capture from flue gas due to their high CO(2) capacities at subatmospheric pressures. However, stability is a critical issue for their application. In this paper, the stabilities of Ni/DOBDC and Mg/DOBDC are investigated. Effects of steam conditioning, simulated flue gas conditioning, and long-term storage on CO(2) adsorption capacities are considered. Results show that Ni/DOBDC can maintain its CO(2) capacity after steam conditioning and long-term storage, whereas Mg/DOBDC does not. Nitrogen isotherms for Mg/DOBDC show a drop in surface area after steaming, corresponding to the decrease in CO(2) adsorption, which may be caused by a reduction of unsaturated metal centers in its structure. Conditioning with dry simulated flue gas at room temperature only slightly affects CO(2) adsorption in Ni/DOBDC. However, introducing water vapor into the simulated flue gas further reduces the CO(2) capacity of Ni/DOBDC.