Piboon Pantu

Structure and dynamics of water confined in single-wall nanotubes.

The structure and dynamics of water confined in model single-wall carbon- and boron-nitride nanotubes (called SWCNT and SWBNNT, respectively) of different diameters have been investigated by molecular dynamics (MD) simulations at room temperature. The simulations were performed on periodically extended nanotubes filled with an amount of water that was determined by soaking a section of the nanotube in a water box in an NpT simulation (1 atm, 298 K). All MD production simulations were performed in the canonical (NVT) ensemble at a temperature of 298 K. Water was described by the extended simple point charge (SPC/E) model. The wall-water interactions were varied, within reasonable limits, to study the effect of a modified hydrophobicity of the pore walls. We report distribution functions for the water in the tubes in spherical and cylindrical coordinates and then look at the single-molecule dynamics, in particular self-diffusion. While this motion is slowed down in narrow tubes, in keeping with previous findings (Liu et al. J. Chem. Phys. 2005, 123, 234701-234707; Liu and Wang. Phys. Rev. 2005, 72, 085420/1-085420/4; Liu et al. Langmuir 2005, 21, 12025-12030) bulk-water like self-diffusion coefficients are found in wider tubes, more or less independently of the wall-water interaction. There may, however, be an anomaly in the self-diffusion for the SWBNNT.

NANOCAVITY EFFECTS ON N2O DECOMPOSITION ON DIFFERENT TYPES OF FE-ZEOLITES (Fe-FER, Fe-BEA, Fe-ZSM-5 AND Fe-FAU): A COMBINED THEORETICAL AND EXPERIMENTAL STUDY

Nitrous oxide decomposition on iron-exchanged zeolites (Fe-FER, Fe-ZSM-5, Fe-BEA, and Fe-FAU) has been studied both theoretically, by using the ONIOM (B3LYP/6-31G(d,p):UFF) method, and experimentally, by temperature programmed reaction, to determine the effect of different zeolitic nanostructured pore networks on the catalytic activity. The ONIOM quantum mechanical calculations show that the nitrous oxide molecule adsorbs with slightly stronger interactions energies on Fe-FER and Fe-ZSM-5 than on the larger pore Fe-BEA and Fe-FAU zeolites. In the transition state leading to the decomposition of nitrous oxide, the smallest pore ferrierite zeolite exerts the strongest van der Waals interactions on the reacting species and, thus, results in the lowest activation energy. Therefore, the predicted intrinsic activity trend is Fe-FER > Fe-BEA ∼ Fe-ZSM-5 ∼ Fe-FAU. On the other hand, the temperature programmed reaction on zeolites containing trace amounts of iron impurities shows an observed activity trend of Fe-FER > Fe-BEA > Fe-ZSM-5 > > Fe-FAU. The experimentally observed activity trend can be explained by the intrinsic activity of each zeolite except for Fe-FAU. Nitrous oxide decomposition in Fe-FAU could be limited by the mass transfer process and not governed by the intrinsic activity. It is known that cations are preferentially located on the six-membered ring in the sodalite cage of the faujasite, to which the reactants have a very limited access.

Glycine Peptide Bond Formation Catalyzed by Faujasite

The catalysis of peptide bond formation between two glycine molecules on H-FAU zeolite was computationally studied by the M08-HX density functional. Two reaction pathways, the concerted and the stepwise mechanism, starting from three differently adsorbed reactants, amino-bound, carboxyl-bound, and hydroxyl-bound, are studied. Adsorption energies, activation energies, and reaction energies, as well as the corresponding intrinsic rate constants were calculated. A comparison of the computed energetics of the various reaction paths for glycine indicates that the catalyzed reaction proceeds preferentially via the concerted reaction mechanism of the hydroxyl-bound configuration. This involves an eight-membered ring of the transition structure instead of the four-membered ring of the others. The step from the amino-bound configuration to glycylglycine is the rate-determining step of the concerted mechanism. It has an estimated activation energy of 51.2 kcal mol(-1). Although the catalytic reaction can also occur via the stepwise reaction mechanism, this path is not favored.

A quantum chemical study of the interaction of carbonyls with H-SZM-5 zeolite

Abstract The quantum cluster, ONIOM and embedded ONIOM models have been used to investigate adsorption properties of carbonyls in H-ZSM-5 zeolites. The active site has been modeled with realistic clustic cluster sizes up to 46 tetrahedra. The predicted adsorption energies of ZSM-5/carbonyls complexes for the embedded ONIOM2(MP2/6-311G(d,p):UFF) scheme are −119.0 and −139.0 kJ/mol for acetaldehyde and acetone, respectively, the latter value compared well with the experimental estimated of 130±4 kJ/mol, whereas the conventional quantum cluster yields an, underestimate value of −68.2 kJ/mol. The results obtained in this study suggest that the new embedded ONIOM scheme provides a more accurate method of studying the interaction of carbonyls with zeolites.

Structure and energetics of nitrous oxide and methane adsorption on the Fe-ZSM-5 zeolite: Oniom and density functional studies

Abstract The quantum cluster and ONIOM methods have been used to investigate interactions between adsorbed nitrous oxide and methane on the active iron center in Fe-ZSM-5, prior to its catalytic oxidation to methanol. A model of mononuclear iron complex as an active site of the highly dispersed state of Fe-SSM-5 was adopted in cluster models of 5T quantum cluster and 46T ONIOM2. The extended crystal framework included in the ONIOM model significantly enhances the interactions of both adsorbates with the active site. The adsorption energy of N 2 O on Fe-ZSM-5 calculated by the 5T quantum cluster is only −41.2 kJ/mol, whereas, the 46T ONIOM model gives the adsorption energy of −57.3 kJ/mol, which is comparable to the experimental estimate of −67 kJ/mol. After correction with single point calculation at MP2/6-31G ** :UFF//B3LYP/6-31G ** :UFF, the adsorption energy is calculated to be 66.9 kJ/mol, which is apparently identical to the experimental estimate. The ONIOM2 model also predicts the adsorption energy of −30.5 kJ/mol for the [CH4]/Fe-ZSM-5 complexes. These results demonstrate that the adsorption properties of Fe-ZSM-5 depend significantly on the specific environment of the zeolite crystal lattice.

Adsorption of toluene over faujasite zeolite investigated by the combined quantum mechanics/molecular mechanics method

Abstract The ONIOM (Our-own-N-layered Integrated molecular Orbital+molecular Mechanics) approach utilizing two-layer ONIOM2 schemes-ONIOM2(MP2/6-31G(d,p):HF/3-21G), ONIOM2 (B3LYP/6-31G(d,p):HF/3-21G), ONIOM2(B3LYP/6-31G(d,p):UFF), and ONIOM2(HF/3-21G:UFF)-have been used to investigate adsorption properties of aromatics in faujasite zeolites (H-FAU). The active site has been modeled with realistic clusters sizes up to 84 tetrahedra. The predicted adsorption energy of H-FAU/toluene complexes for the ONIOM2(B3LYP/6-311++G(d,p):UFF) scheme is −81.34 kJ/mol, which is in line with the experimental values of −58.5 and −85.3 kJ/mol for H-FAU/benzene and H-FAU/ethylbenzene, respectively. Where the conventional 3T quantum cluster yields an underestimated value of −32.52 kJ/mol. This finding clearly demonstrates that acidity does not depend only on the Bronsted group center but also on the lattice framework surrounding the Bronsted site. The results obtained in this study suggest that the ONIOM2 approach yields a more accurate for studying the adsorption of aromatics on zeolites.