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.
The adsorption and tautomerization reaction of acetone in H-FER, H-ZSM-5, and H-MCM-22 zeolites has been studied using full quantum calculations at the M06-2X/6-311+G(2df,2p) level of theory. The combination of a large quantum cluster and this meta-hybrid density functional results in reasonably accurate adsorption energies of -26.9, -28.1, and -23.9 kcal/mol for acetone adsorption in H-FER, H-ZSM-5, and H-MCM-22, respectively. Due to the acidity of the zeolite and the framework confinement effect, the tautomerization of acetone proceeds through a much lower activation barrier than in the isolated gas phase or in the presence of water molecules alone. The activation energies are calculated to be 24.9, 20.5, and 16.6 kcal/mol in H-FER, H-ZSM-5 and H-MCM-22, respectively. The endothermic reaction energy decreases with increasing of the zeolite pore sizes and amounts to 22.7, 17.6, and 15.9 kcal/mol for the reaction in H-FER, H-ZSM-5 and H-MCM-22, respectively. In addition, the adsorbed acetone enol is found to be highly unstable in the zeolite framework and readily reverse-transforms to adsorbed acetone with a very small activation energy. The activity trend and relative stabilities of the adsorbed keto and enol forms are well correlated with the interactions within the Brønsted acid site.
Nanostructured Fe‐ZSM‐5: Structures and reactivities of an Fe‐exchanged ZSM‐5 zeolite (see picture, the blue and red spheres represent Fe and O atoms, respectively) for decomposition of nitrous oxide and oxidation of methane to methanol were investigated using density functional theory calculations and a two‐layered ONIOM (our own n‐layer integrated molecular orbital and molecular mechanics) scheme that explicitly takes into account the extended zeolitic framework.
The effects of the zeolite framework on the mechanism of n-hexane monomolecular cracking have been investigated with M06-2X/6-311+G(2df,2p)//M06-2X/6-31G(d,p) calculations. M06-2X is a recently developed hybrid-meta functional that is parametrized to include the London dispersion energy. The 38T H-FAU and 34T H-ZSM-5 nanocluster models where T atoms are either Si or Al atoms are used to represent H-FAU and H-ZSM-5 zeolites. The adsorption energies of hexane are predicted to be -10.8 and -18.2 kcal/mol for H-FAU and H-ZSM-5, respectively, in good agreement with experimental measurements. This indicates that the confinement effects on different types of zeolites can be well represented by the M06-2X functional. The reaction is assumed to proceed in two steps. In the first step, the central C-C bond of adsorbed n-hexane is protonated to form a hexonium intermediate. The adsorbed 3-C-hexonium is highly unstable and can be rapidly decomposed to produce the products. The first step is found to be the rate-determining step with activation energies of 45.7 and 45.8 kcal/mol for H-FAU and H-ZSM-5, respectively. For step two, the activation energies are calculated to be 8.6 and 9.9 kcal/mol for H-FAU and H-ZSM-5, respectively. The results clearly demonstrate that the reaction of n-hexane cracking is intrinsically the same in these large-and medium-pore zeolites. The different apparent activities can be explained by the different adsorption energies which are mainly due to the van der Waals interactions with the zeolite walls.
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.
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.
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