The conversion of CO2 and CH4 to acetic acid over the Au-exchanged ZSM-5 catalyst: a density functional theory study.

The direct conversion of methane and carbon dioxide to acetic acid is one of the most challenging research topics. Using the density functional theory (M06-L) the study reveals the catalytic activity of the Au(I)-ZSM-5 zeolite in this reaction. The Au(I)-ZSM-5 is represented by a 34T quantum cluster model. The activation of the C-H bond over the Au-ZSM-5 zeolite would readily take place via the homolytic σ-bond activation with an energy barrier of 10.5 kcal mol(-1), and subsequent proton transfer from the Au cation to the zeolitic oxygen, yielding the stable methyl-gold complex adsorbed on the zeolite Brønsted acid. The conversion of CO(2) on this bi-functional catalyst involves the Brønsted acid site playing a role in the protonation of CO(2) and the methyl-gold complex acting as a methylating agent. The activation energy of 52.9 kcal mol(-1) is predicted.

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.

Probing chirality of a lipid tubular by confocal Raman microscopy.

The chiral phospholipids 1,2-bis-(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9 PC) can self assemble into lipid nanotubules. This hollow cylindrical supramolecular structure shows promise in a number of biotechnological applications. The mechanism of lipid tubule formation was initiated by assembling of lipid bilayer sheets from amphiphilic solution. Upon cooling, small ribbons were detached from the sheets and rolled up into helical tubules. The lipid tubules obtained were 0.6-0.8 microm in diameter and approximately 50 microm in length. Raman spectra of individual polymerized lipid tubules were measured by focused laser excitation of 532 nm leading to intense and reproducible Raman spectra. The chirality of lipid tubules was investigated by atomic force microscopy (AFM) and confocal Raman microscopy. We report the Raman mapping images revealing helical tubular profiles of C=C stretching and C[triple bond]C stretching of lipid tubules. Circular dichroism property of lipid tubules has also been probed with a 532 nm laser.

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.

Structures and reaction mechanisms of propene oxide isomerization on H-ZSM-5: an ONIOM study.

The isomerization mechanisms of propene oxide over H-ZSM-5 zeolite have been investigated via the utilization of 5T and 46T cluster models calculated by the B3LYP/6-31G(d,p) and the ONIOM(B3LYP/6-31G(d,p):UFF) methods, respectively. The reactions are considered to proceed through a stepwise mechanism: (1) the epoxide ring protonation, and concurrently the ring-opening, and (2) the 1,2-hydride shift forming the adsorbed carbonyl compound. Because of the asymmetric structure of propene oxide, two different C-O bonds (more or less substituted carbon atom sides) can be broken leading to two different types of products, propanal and propanone. The ring-opening step of these mechanisms is found to be the rate-determining step with an activation barrier of 38.5 kcal/mol for the propanal and of 42.4 kcal/mol for the propanone. Therefore, the propanal is predicted to be the main product for this reaction.

Investigation of ethylene dimerization over faujasite zeolite by the ONIOM method.

Ethylene dimerization was investigated by using an 84T cluster of faujasite zeolite modeled by the ONIOM3(MP2/6-311++G(d,p):HF/6-31G(d):UFF) method. Concerted and stepwise mechanisms were evaluated. In the stepwise mechanism, the reaction proceeds by protonation of ethylene to form the surface ethoxide and then C--C bond formation between the ethoxide and the second ethylene molecule to give the butoxide product. The first step is rate-determining and has an activation barrier of 30.06 kcal mol(-1). The ethoxide intermediate is rather reactive and readily reacts with another ethylene molecule with a smaller activation energy of 28.87 kcal mol(-1). In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C--C bond formation. The activation barrier is calculated to be 38.08 kcal mol(-1). Therefore, the stepwise mechanism should dominate in ethylene dimerization.