Biomedical subjects

Shaji Chempath

Publications and source records attributed to Shaji Chempath.

5 recordsLinked to original sources

PubMed2007

Storage and separation of CO2 and CH4 in silicalite, C168 schwarzite, and IRMOF-1: a comparative study from Monte Carlo simulation.

Storage of pure CO2 and CH4 and separation of their binary mixture in three different classes of nanostructured adsorbents--silicalite, C168 schwarzite, and IRMOF-1--have been compared at room temperature using atomistic simulation. CH4 is represented as a spherical Lennard-Jones molecule, and CO2 is represented as a rigid linear molecule with a quadrupole moment. For pure component adsorption, CO2 is preferentially adsorbed than CH4 in all the three adsorbents over the pressure range under this study, except in C168 schwarzite at high pressures. The simulated adsorption isotherms and isosteric heats match closely with available experimental data. A dual-site Langmuir-Freundlich equation is used to fit the isotherms satisfactorily. Compared to silicalite and C168 schwarzite, the gravimetric adsorption capacity of pure CH4 and CO2 separately in IRMOF-1 is substantially larger. This implies that IRMOF-1 might be a potential storage medium for CH4 and CO2. For adsorption from an equimolar binary mixture, CO2 is preferentially adsorbed in all three adsorbents. Predictions of mixture adsorption with the ideal-adsorbed solution theory on the basis of only pure component adsorption agree well with simulation results. Though IRMOF-1 has a significantly higher adsorption capacity than silicalite and C168 schwarzite, the adsorption selectivity of CO2 over CH4 is found to be similar in all three adsorbents.

Journal Article↗

PubMed2006

Quantum mechanical single molecule partition function from path integral Monte Carlo simulations.

An algorithm for calculating the partition function of a molecule with the path integral Monte Carlo method is presented. Staged thermodynamic perturbation with respect to a reference harmonic potential is utilized to evaluate the ratio of partition functions. Parallel tempering and a new Monte Carlo estimator for the ratio of partition functions are implemented here to achieve well converged simulations that give an accuracy of 0.04 kcal/mol in the reported free energies. The method is applied to various test systems, including a catalytic system composed of 18 atoms. Absolute free energies calculated by this method lead to corrections as large as 2.6 kcal/mol at 300 K for some of the examples presented.

Journal Article↗

PubMed2006

Density functional theory analysis of the reaction pathway for methane oxidation to acetic acid catalyzed by Pd2+ in sulfuric acid.

Density functional theory has been used to investigate the thermodynamics and activation barriers associated with the direct oxidation of methane to acetic acid catalyzed by Pd2+ cation in concentrated sulfuric acid. Pd2+ cations in such solutions are ligated by two bisulfate anions and by one or two molecules of sulfuric acid. Methane oxidation is initiated by the addition of CH4 across one of the Pd-O bonds of a bisulfate ligand to form Pd(HSO4)(CH3)(H2SO4)2. The latter species will react with CO to produce Pd(HSO4)(CH3CO)(H2SO4)2. The most likely path to the final products is found to be via oxidation of Pd(HSO4)(CH3)(H2SO4)2 and Pd(HSO4)(CH3CO)(H2SO4)2 to form Pd(eta2-HSO4)(HSO4)2(CH3)(H2SO4) and Pd(eta2-HSO4)(HSO4)2(CH3CO)(H2SO4), respectively. CH3HSO4 or CH3COHSO4 is then produced by reductive elimination from the latter two species, and CH(3)COOH is then formed by hydrolysis of CH3COHSO4. The loss of Pd2+ from solution to form Pd(0) or Pd-black is predicted to occur via reduction with CO. This process is offset, though, by reoxidation of palladium by either H2SO4 or O2.

Journal Article↗

PubMed2005

Simulated adsorption properties and synthesis prospects of homochiral porous solids based on their heterochiral analogs.

Molecular simulations of chiral molecules in porous heterochiral materials were performed to investigate fundamental adsorption properties and possibilities for production of homochiral porous solids. Zeolite BEA polymorph A and zeotype UCSB-7K each provide separated pores of opposite chirality. Single enantiomer and racemic mixture adsorption results are presented and indicate that significant equilibrium enantiomeric excesses of 40-70% in UCSB-7K and 10% in BEA can be achieved. Larger, better-fitting molecules display higher enantiomeric excesses. For dimethylallene, which moves on molecular dynamics time scales in UCSB-7K, self-diffusivities vary by almost an order of magnitude between the two opposite-handed UCSB-7K pores for a given enantiomer. The predicted properties indicate that equilibrium and nonequilibrium strategies using related homochiral materials for separations may be successful. To this end, a discussion of strategies for selectively blocking pores of one chirality on the basis of enantiomer segregation is provided.

Journal Article↗

PubMed2004

Adsorption of liquid-phase alkane mixtures in silicalite: simulations and experiment.

A combination of experimental and computational studies of adsorption from liquid-phase mixtures of linear alkanes in the zeolite silicalite is presented here. Configurational biased grand canonical Monte Carlo simulations combined with identity-swap moves are used to equilibrate the simulations in reasonable times. Interesting trends observed in experiments have been captured quantitatively by simulations. A siting analysis of the simulation data reveals that, during adsorption from a liquid mixture, shorter alkanes prefer the zigzag channels and longer alkanes concentrate in the straight channels of silicalite.

Adsorption↗