K. Vasanth Kumar

Nanoporous Materials for the Onboard Storage of Natural Gas

Climate change, global warming, urban air pollution, energy supply uncertainty and depletion, and rising costs of conventional energy sources are, among others, potential socioeconomic threats that our community faces today. Transportation is one of the primary sectors contributing to oil consumption and global warming, and natural gas (NG) is considered to be a relatively clean transportation fuel that can significantly improve local air quality, reduce greenhouse-gas emissions, and decrease the energy dependency on oil sources. Internal combustion engines (ignited or compression) require only slight modifications for use with natural gas; rather, the main problem is the relatively short driving distance of natural-gas-powered vehicles due to the lack of an appropriate storage method for the gas, which has a low energy density. The U.S. Department of Energy (DOE) has set some targets for NG storage capacity to obtain a reasonable driving range in automotive applications, ruling out the option of storing methane at cryogenic temperatures. In recent years, both academia and industry have foreseen the storage of natural gas by adsorption (ANG) in porous materials, at relatively low pressures and ambient temperatures, as a solution to this difficult problem. This review presents recent developments in the search for novel porous materials with high methane storage capacities. Within this scenario, both carbon-based materials and metal-organic frameworks are considered to be the most promising materials for natural gas storage, as they exhibit properties such as large surface areas and micropore volumes, that favor a high adsorption capacity for natural gas. Recent advancements, technological issues, advantages, and drawbacks involved in natural gas storage in these two classes of materials are also summarized. Further, an overview of the recent developments and technical challenges in storing natural gas as hydrates in wetted porous carbon materials is also included. Finally, an analysis of design factors and technical issues that need to be considered before adapting vehicles to ANG technology is also presented.

Effect of pore structure on the selectivity of carbon materials for the separation of CO2/H2 mixtures: new insights from molecular simulation

Molecular simulations were performed to study the effect of the nanoporous structure on the selectivity of carbon materials for the adsorption of carbon dioxide from mixtures of carbon dioxide and hydrogen at 298 K and for fluid compositions: xCO2/xH2 = 1/9 and 2/8. Both carbon dioxide and hydrogen were studied using classical Lennard-Jones intermolecular potentials. Typical pore geometries such as slit-shaped pores and nanotubes were considered, along with a hypothetical foam-like structure and a carbon model exhibiting a random porous structure with a wide pore size distribution. Simulation results show that selectivity for carbon dioxide is sensitive to pore structure and composition; the solid/fluid interactions play a decisive role in the selectivity and most of the effects can be explained by the independent analysis of the interactions of carbon dioxide with the pore walls. In the range of pressure and composition studied, nanotubes have the highest selectivity towards carbon dioxide (100–313), followed by slit (9–63), foam-like (29–35) and random porous carbon (8–30). Molecular simulations further indicate that predicting the adsorption behavior for a CO2/H2 mixture from pure component isotherms is inadequate due, to the competing effects of the molecules with the pore walls.

Co-adsorption of N2 in the presence of CH4 within carbon nanospaces: evidence from molecular simulations

Molecular simulations were performed to study the separation of CH(4) and N(2) from mixtures of composition x(CH(4))/x(N(2)) = 5/95 and x(CH(4))/x(N(2)) = 10/90 at 50 bar and 298 K on prototype carbon materials with different pore structures. The studied carbon structures include a slit and a tubular pore, that represent the simplest form of activated carbon and carbon nanotubes, respectively, in addition to a realistic porous carbon model with disordered pore structure and a recently introduced carbon foam model, which has a three-dimensional pore structure. The results indicate that, depending on the pressure and composition, the pore structure influences both the CH(4)/N(2) selectivity and the adsorption behaviour of the fluid molecules. The selectivity was decided by the interactions between CH(4) and N(2) molecules within the pore structure, in addition to the solid-fluid interactions. The simulation results indicate that, at least for the case of activated carbons (slit and random pores), it would not be appropriate to predict the binary adsorption behaviour of methane and nitrogen by means of pure component information. Regardless of the pore structure, the simulation results indicate that carbon materials show a CH(4)/N(2) (thermodynamic) selectivity of only 2-3 up to 2 bar at 298 K, and above this pressure, at equilibrium, none of the carbon materials is adequate for the efficient separation of this mixture.

On the effect of a non-ionic surfactant on the surface of sucrose crystals and on the crystal growth process by inverse gas chromatography

The effect of Hodag CB6, a widely used non-ionic surfactant in sugar crystallization process, on the surface properties of sucrose was studied in detail by inverse gas chromatography (IGC) experiments. IGC experiments were performed with pure sucrose crystals, surfactant coated sucrose crystals, and crystals grown in the presence of surfactant at 313.05 and 323.05 K. The surfactant promotes the specific interactions with the polar probes. The sorption of basic, acidic and amphoteric probes onto pure and surfactant coated sucrose was found to be endothermic and in the case of neutral probes was found to be exothermic. The surfactant increases both the acidity and basicity of the sucrose surface with the latter effect being significant. The role of interfacial tension on the growth kinetics of sucrose crystals was studied using IGC for different surfactant concentrations. IGC results with the surfactant coated sucrose were used to interpret the thermodynamic effect of surfactants during the crystal growth process. The dispersive component of the surface energy, gamma(s)(D), of surfactant coated sucrose crystals was found to be lower than that of pure sucrose crystals and was found to be in the range of 33.49-35.27 mJ/m(2).

Salt Templating with Pore Padding: Hierarchical Pore Tailoring towards Functionalised Porous Carbons

We propose a new synthetic route towards nanoporous functional carbon materials based on salt templating with pore-padding approach (STPP). STPP relies on the use of a pore-padding agent that undergoes an initial polymerisation/ condensation process prior to the formation of a solid carbon framework. The pore-padding agent allows tailoring hierarchically the pore-size distribution and controlling the amount of heteroatom (nitrogen in this case) functionalities as well as the type of nitrogen (graphitic, pyridinic, oxides of nitrogen) incorporated within the carbon framework in a single-step-process. Our newly developed STPP method offers a unique pathway and new design principle to create simultaneously high surface area, microporosity, functionality and pore hierarchy. The functional carbon materials produced by STPP showed a remarkable CO2 /N2 selectivity. At 273 K, a carbon with only micropores offered an exceptionally high CO2 adsorption capacity whereas a carbon with only mesopores showed promising CO2 -philicity with high CO2 /N2 selectivity in the range of 46-60 %, making them excellent candidates for CO2 capture from flue gas or for CO2 storage.

The required level of isosteric heat for the adsorptive/storage delivery of H2 in the UiO series of MOFs

The required level of isosteric heat of adsorption for efficient storage and delivery of H2 in the UiO series of MOFs was theoretically predicted using molecular simulations. Very high isosteric heats may lead to enhanced storage capacities, however the real H2 delivery capacity is practically reduced. In this respect, for maximum H2 delivery, there exists an optimum isosteric heat value (28–29 kJ mol−1).

A site energy distribution function for the characterization of the continuous distribution of binding sites for gases on a heterogeneous surface

A binding site energy distribution function for a Jensen-Seaton isotherm and its limiting cases was proposed and successfully applied to the adsorption of several gas molecules on different essentially microporous carbons. According to the proposed model the studied carbon materials exhibit two energetic states where the site energy is exponentially or unimodaly distributed depending on the adsorption pressure. Carbons with a larger contribution from micropores seem to be in general more heterogeneous, with a wider binding site energy distribution, than less microporous carbons.

Simple kinetic models to explain the change in dislocation activity of crystals during a growth process

Kinetic models are proposed from the concepts of the Burton–Cabrera–Frank (BCF) theory to explain the change in dislocation activity in crystals during their growth in the diffusion or in the kinetic regime. It is assumed that the change in dislocation activity decreases with time (i.e. there is a changing supersaturation), following first-order kinetics irrespective of the limiting conditions. The proposed models are very simple to use, and for the first time, incorporate a parameter to explain the change in dislocation activity during crystal growth. They have the advantage of being able to estimate the kinetic constant of the growth process and the rate of change in dislocation activity simultaneously.Kinetic models are proposed from the concepts of the Burton–Cabrera–Frank (BCF) theory to explain the change in dislocation activity in crystals during their growth in the diffusion or in the kinetic regime. It is assumed that the change in dislocation activity decreases with time (i.e. there is a changing supersaturation), following first-order kinetics irrespective of the limiting conditions. The proposed models are very simple to use, and for the first time, incorporate a parameter to explain the change in dislocation activity during crystal growth. They have the advantage of being able to estimate the kinetic constant of the growth process and the rate of change in dislocation activity simultaneously.