Microporous TS-1 Research Articles

Computational Approaches to the Determination of Active Site Structures and Reaction Mechanisms in Heterogeneous Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Computational approaches to the determination of active site structures and reaction mechanisms in heterogeneous catalysts

We apply quantum chemical methods to the study of active site structures and reaction mechanisms in mesoporous silica and metal oxide catalysts. Our approach is based on the use of both molecular cluster and embedded cluster (QM/MM) techniques, where the active site and molecular complex are described using density functional theory (DFT) and the embedding matrix simulated by shell model potentials. We consider three case studies: alkene epoxidation over the microporous TS-1 catalyst; methanol synthesis on ZnO and Cu/ZnO and C-H bond activation over Li-doped MgO.

Synthesis, characterization and catalytic properties of mesoporous TiHMA molecular sieves: selective oxidation of cycloalkanes

Titanium substituted hexagonal mesoporous aluminophosphate (TiHMA) molecular sieves were synthesized and characterized. Various physicochemical studies, viz., X-ray diffraction, transmission electron microscopy, electron diffraction, environmental scanning electron microscopy, thermal analyses, inductively coupled plasma–atomic emission spectroscopy, diffuse reflectance ultraviolet–visible spectroscopy, Fourier transform Raman spectroscopy, temperature programmed desorption of ammonia and nitrogen absorption–desorption isotherms, confirm the substitution of Ti 4+ ions in the tetrahedral framework of HMA. The catalytic activity of TiHMA was evaluated for the oxidation reactions of cyclic saturated hydrocarbons such as cyclohexane, cyclooctane and cyclododecane. The catalyst exhibits excellent substrate conversion and product selectivity, and that it was found to be truly as a heterogeneous catalyst as the activity was practically unaffected upon recycling. In particular, the oxidation was highly successful for the bulkier cycloalkane molecules. Furthermore, the activity of TiHMA is also compared with mesoporous TiMCM-41 as well as microporous TiAPO-5 and TS-1 catalysts.

A new mesoporous titanium-silicate Ti-MMM-2: a highly active and hydrothermally stable catalyst for H2O2-based selective oxidations

Physicochemical characterisation and catalytic properties of a new hydrothermally stable mesoporous mesophase titanium-silicate, denoted as Ti-MMM-2, synthesised under moderately acidic conditions are reported. Structure and texture were characterised by using X-ray diffraction, N2 adsorption and TEM. The state of titanium was characterised using DRS-UV and XANES techniques. Catalytic properties of Ti-MMM-2 were tested in oxidation of three large organic substrates, 2,3,6-trimethylphenol, caryophyllene and methyl phenyl sulfide, and compared with catalytic properties of TS-1 and Ti-MMM (titanium-silicate of the MCM-41-type synthesised under weakly alkaline conditions). The catalytic activity of Ti-MMM-2 is considerably higher compared to the activity of microporous TS-1 and similar to that of Ti-MMM. The performed physicochemical study implicated that Ti-MMM-2 combines marked accessibility of titanium and its high dispersion that ensure high catalytic activity of this material. By contrast to Ti-MMM, Ti-MMM-2 is hydrothermally stable and can be used repeatedly without deterioration of catalytic properties. A blend of high catalytic activity and hydrothermal stability allows to consider Ti-MMM-2 as a promising catalyst for the selective oxidation of large organic molecules with aqueous H2O2.

A Novel Titanosilicate with MWW Structure: II. Catalytic Properties in the Selective Oxidation of Alkenes

The catalytic properties of Ti–MWW catalysts have been studied for the oxidation of alkenes using hydrogen peroxide or tert-butyl hydroperoxide (TBHP) as an oxidant. Ti–MWW shows superior activity independent of the nature of oxidant when compared to the other Ti-containing catalysts both of microporous TS-1 and Ti-MOR and of mesoporous Ti–MCM-41 in the oxidation of cyclohexene. Compared to large-pore Ti–Beta, Ti–MWW is less active than in the oxidation of cyclohexene using H2O2, but its shows comparable activity in the case of TBHP. Ti–MWW is very stable in both the structure and the states of Ti species in the actual cycle of reaction–regeneration. The framework B species remaining within the Ti–MWW catalysts seem to have little influence on the intrinsic activity of framework Ti species, but they act as Brønsted acid sites contributing to the hydrolysis of the oxide product to form glycols. The turnover number (TON) of Ti–MWW for the cyclohexene conversion decreases sharply with increasing Ti content, while the TON for the 1-hexene conversion shows volcanic behavior when related to the Ti content. It is clarified that the bulky reaction is catalyzed mainly by the Ti species within the supercages and the exterior pockets of the MWW structure, while in addition to these highly accessible Ti species, those within the channels of a 10-membered ring greatly contribute to the oxidation reactions not significantly sizedemanding.

Ti - Substituted Mesoporous Molecular Sieves for Catalytic Oxidation of Large Aromatic Compounds Prepared by Neutral Templating Route

AbstractA new synthesis route to open framework mesostructures based on H-bonding and self-assembly between neutral primary amine surfactants (S°) and neutral inorganic precursors (I°) has been used to prepare hexagonal mesoporous silicas containing site isolated titanium centers. These new titanosilicates, designated Ti-HMS, exhibit exceptional catalytic reactivity for the oxidation of substrates too large to access the pore structure of conventional titanosilicates, such as titanium silicalite, TS-1. The catalytic properties of Ti-HMS materials for the peroxide oxidation of 2,6- di- tert-butylphenol are compared with those of microporous TS-1 and a mesoporous Ti-MCM-41 analog prepared by an electrostatic templating mechanism using a liquid crystal quaternary ammonium cationic surfactant.

Chemical studies on tobacco smoke. II. Comparison of the yields of several selected components in the smoke from five major Turkish tobacco varieties

With the increasingly stringent standards for limiting sulfide content in liquid fuels, oxidative desulfurization (ODS) has become a promising ultra-deep desulfurization process in fuel desulfurization. TS-1 zeolites show great potential as catalysts for ODS reactions, due to its remarkable oxidation activity at low temperatures and pressure. However, the inherent microporous structure of conventional TS-1 zeolites restricts the mass transportation and renders the active sites in the microporous space of TS-1 zeolites inaccessible for bulky aromatic organosulfur compounds. Fabrication of hierarchical TS-1 zeolites by incorporating meso-/macropores into microporous TS-1 zeolites is an effective strategy to improve mass transportability. In recent years, abundant efforts have been dedicated to developing synthetic strategies of hierarchical TS-1 zeolite, thereby improving its catalytic performance in the ODS process. This mini-review addresses the synthetic methods of hierarchical TS-1 catalysts and their catalytic performance in the ODS reactions. In addition, some current problems and prospects of synthesis routes for constructing hierarchical TS-1 catalysts have also been revised. We expect this mini-review to shed light on the more efficient preparation strategies of hierarchical TS-1 zeolites for the ODS process.