Shuping Yuan

Location of Si Vacancies and [Ti(OSi)4] and [Ti(OSi)3OH] Sites in the MFI Framework: A Large Cluster and Full Ab Initio Study

Titanium silicalite-1 (TS-1) is an important catalyst for selective oxidation reactions. However, the nature and structure of the active sites and the mechanistic details of the catalytic reactions over TS-1 have not been well-understood, leaving a continuous debate on the genesis of active sites on the TS-1 surface in the literature. In this work, the location of Si vacancies and [Ti(OSi)(4)] and [Ti(OSi)(3)OH] sites in the MFI (Framework Type Code of ZSM-5 (Zeolite Socony Mobile-Five)) framework has been studied using a full ab initio method with 40T clusters with a Si:Ti molar ratio of 39:1. It was shown that the former four energetically favorable sites for Si vacancies are T6, T12, T4, and T8 and for Ti centers of [Ti(OSi)(4)] are T10, T4, T8 and T11, being partially the same sites. Whether by replacing Si vacancies or substituting the fully coordinated Si sites, the most preferential site for Ti is T10, which indicates that the insertion mechanism does not affect the favorable sites of Ti in the MFI lattice. For the defective [Ti(OSi)(3)OH] sites, it was found that the Si vacancy at T6 with a Ti at its neighboring T9 site (T6-def-T9-Ti pair) is the most energetically favorable one, followed by a T6-def-T5-Ti pair with a small energy gap. These findings are significant to elucidate the nature of the active sites and the mechanism of reactions catalyzed by TS-1 and to design the TS-1 catalyst.

Origins of Opposite Syn−Anti Diastereoselectivities in Primary and Secondary Amino Acid-Catalyzed Intermolecular Aldol Reactions Involving Unmodified α-Hydroxyketones

The effects of different amino acid catalysts on the stereoselectivity of the direct intermolecular aldol reactions between alpha-hydroxyketones and isobutyraldehyde or 4-nitrobenzaldehyde have been studied with the aid of density functional theory methods. The transition states of the crucial C-C bond-forming step with the enamine intermediate addition to the aldehyde for the proline and threonine-catalyzed asymmetric aldol reactions are reported. B3LYP/6-31+G** calculations provide a good explanation for the opposite syn vs anti diastereoselectivity of these two kinds of amino acid catalysts (anti-selectivity for the secondary cyclic amino acids proline, syn-selectivity for the acyclic primary amino acids like threonine). Calculated and observed diastereomeric ratio and enantiomeric excess values are in good agreement.

Removal of nitric oxide by the highly reactive anatase TiO2 (0 0 1) surface: A density functional theory study

In this paper, density functional theory (DFT) calculation was employed to study the adsorption of nitric oxide (NO) on the highly reactive anatase TiO2 (001) surface. For comparison, the adsorption of NO on the (101) surface was also considered. Different from the physical adsorption on the (101) surface, NO molecules are found to chemisorb on the TiO2 (001) surface. The twofold coordinate oxygen atoms (O2c) on the anatase (001) surface are the active sites. Where NO is oxidized into a nitrite species (NO2(-)) trapping efficiently on the surface, with one of the surface Ti5c-O2c bonds adjacent to the adsorption site broken. Our results, therefore, supply a theoretical guidance to remove NO pollutants using highly reactive anatase TiO2 (001) facets.

On the Origin of the Visible-Light Activity of Titanium Dioxide Doped with Carbonate Species

Plane-wave-based pseudopotential density functional theory (DFT) calculations are used to elucidate the origin of the high photocatalytic efficiency of carbonate-doped TiO(2). Two geometrically possible doping positions are considered, including interstitial and substitutional carbon atoms on Ti sites. From the optical absorption properties calculations, we believe that the formation of carbonates after doping with interstitial carbon atoms is crucial, whereas the contribution from the cationic doping on Ti sites is negligible. The carbonate species doped TiO(2) exhibits excellent absorption in the visible-light region of 400-800 nm, in good agreement with experimental observations. Electronic structure analysis shows that the carbonate species introduce an impurity state from Ti 3d below the conduction band. Excitations from the impurity state to the conduction band may be responsible for the high visible-light activity of the carbon doped TiO(2) materials.

Density Functional Study of Organocatalytic Cross-Aldol Reactions between Two Aliphatic Aldehydes: Insight into Their Functional Differentiation and Origins of Chemo- and Stereoselectivities

The chemo-, diastereo-, and enantioselectivities in proline and axially chiral amino sulfonamide-catalyzed direct aldol reactions between two enolizable aldehydes with different electronic nature have been studied with the aid of density functional theory (DFT) method. The potential energy profiles for the enamine formation between each aliphatic aldehyde and the catalyst confirm that two subject catalysts can successfully differentiate between 3-methylbutanal as an enamine component and α-chloroaldehydes as a carbonyl component. Transition states associated with the stereochemistry-determining C-C bond-forming step with the enamine intermediate addition to the aldehyde acceptor for proline and chiral amino sulfonamide-promoted aldol reactions are reported. DFT calculations not only provide a good explanation for the formation of the sole cross-aldol product between two aliphatic aldehydes both bearing α-methylene protons but also well reproduce the opposite syn vs anti diastereoselectivities in the chiral amino sulfonamide and proline-catalyzed aldol reactions.