Ti Active Sites Research Articles

Tighter Confinement Increases Selectivity of d-Glucose Isomerization Toward l-Sorbose in Titanium Zeolites.

Aqueous-phase isomerization of d-glucose to d-fructose and l-sorbose is catalyzed in parallel by Lewis acidic Ti sites in siliceous frameworks. Glucose isomerization rates (per Ti, 373 K) are undetectable when Ti sites are confined within mesoporous voids (Ti-MCM-41, TiO2 -SiO2 ) and increase to detectable values when Ti sites are confined within the smaller 12-membered ring (12-MR) micropores of Ti-Beta. Isomerization rates decrease to lower values (by ≈20×) with further decreases in micropore size as Ti sites are confined within 10-MR pores (Ti-MFI, Ti-CON), likely because of intrapore reactant diffusion restrictions, and reach undetectable values within the 8-MR pores of Ti-CHA as size exclusion prevents glucose from accessing active sites. Remarkably, the selectivity toward l-sorbose over d-fructose increases systematically as spatial constraints around Ti sites become tighter, and is >10 on Ti-MFI. These findings demonstrate the marked influence of confinement around Ti active sites on the selectivity between parallel stereoselective sugar isomerization pathways.

Tighter Confinement Increases Selectivity of d‐Glucose Isomerization Toward l‐Sorbose in Titanium Zeolites

AbstractAqueous‐phase isomerization of d‐glucose to d‐fructose and l‐sorbose is catalyzed in parallel by Lewis acidic Ti sites in siliceous frameworks. Glucose isomerization rates (per Ti, 373 K) are undetectable when Ti sites are confined within mesoporous voids (Ti‐MCM‐41, TiO2‐SiO2) and increase to detectable values when Ti sites are confined within the smaller 12‐membered ring (12‐MR) micropores of Ti‐Beta. Isomerization rates decrease to lower values (by ≈20×) with further decreases in micropore size as Ti sites are confined within 10‐MR pores (Ti‐MFI, Ti‐CON), likely because of intrapore reactant diffusion restrictions, and reach undetectable values within the 8‐MR pores of Ti‐CHA as size exclusion prevents glucose from accessing active sites. Remarkably, the selectivity toward l‐sorbose over d‐fructose increases systematically as spatial constraints around Ti sites become tighter, and is >10 on Ti‐MFI. These findings demonstrate the marked influence of confinement around Ti active sites on the selectivity between parallel stereoselective sugar isomerization pathways.

Creation of Hierarchical Titanosilicate TS‐1 Zeolites

AbstractHierarchical TS‐1 zeolites that possess at least two levels of porosities have aroused growing interest in the past years. The enhanced mass transfer efficiency and increased accessibility of Ti active sites in the hierarchical TS‐1 zeolites greatly improve the catalytic performance by overcoming the steric and diffusional problems. In this review, recent important progress in the construction of hierarchical TS‐1 zeolites is presented, including mesoporogen, mesoporogen‐free, and demetallization strategies. Effective characterization methods for the Ti species and the hierarchical zeolite structures are described. The impacts of hierarchical structures on the creation of active Ti species and improvement of catalytic performances in the oxidative desulfurization and epoxidation reactions are highlighted. Finally, current challenges and future development perspectives for preparing hierarchical TS‐1 zeolite catalysts are addressed.

Depression effect of Al(Ⅲ) and Fe(Ⅲ) on rutile flotation using dodecylamine polyxyethylene ether as collector

Few literatures pay emphasis on investigating the effect brought out by metal ions on flotation in the case of cationic collector. In this work, the effect of Al(Ⅲ) and Fe(Ⅲ) on rutile floatability using dodecylamine polyxyethylene ether (AC1203) as collector was evaluated. In addition to the flotation tests, zeta potential measurements, solution chemistry calculation, surface tension measurement and XPS analysis were conducted to elucidate the detailed depression mechanism. The results indicate that the floatability of rutile seriously inhibited by Al(Ⅲ) and Fe(Ⅲ), and the depression effect of Al(Ⅲ) is much stronger than that of Fe(Ⅲ). Moreover, on the basis of zeta potential and solution chemistry calculation tests, Al(OH)3 and Fe(OH)3 precipitates are believed to be the principal components that act with rutile surface, and the preferential adsorption of Al(OH)3 precipitate could further deteriorate the interaction of collector with rutile. It is worth mentioning that Al(Ⅲ) and Fe(Ⅲ) are capable of acting with AC1203 to form complexes, consequently affecting the flotation environment. Furthermore, XPS analysis reveals that Fe(OH)3 precipitate tend to adsorb on rutile surface through physical interaction, while Al(OH)3 precipitate could react with Os and bridging O on rutile surface to form Ti-O-Al bonds, resulting in less active Ti sites for collector adsorption.

Fabrication of titanosilicate pillared MFI zeolites with tailored catalytic activity

Titanosilicate pillared MFI zeolite nanosheets were successfully synthesized by infiltrating the mixed tetraethyl orthosilicate (TEOS)/tetrabutyl orthotitanate (TBOT) solvent into the gallery space between adjacent MFI zeolite layers. The obtained zeolite catalysts were characterized using powder X-ray diffraction, N2 adsorption/desorption isotherms, scanning electron microscopy, transmission electron microscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy techniques. The H2O2 oxidation of dibenzothiophene (DBT) was used to evaluate the catalytic performance of the obtained titanosilicate pillared MFI zeolites. The conversion of DBT and selectivity of dibenzothiophene sulfone (DBTS) were most affected by the textural properties of the zeolites. This was attributed to the DBT and DBTS molecules being larger than micropores of the MFI zeolites. The conversion of DBT and yield of DBTS could be systematically tailored by tuning the molar ratio of the TEOS/TBOT solvent. These results implied that a balance between the meso- and microporosity of zeolites and tetrahedrally coordinated Ti (IV) active sites of titanosilicate pillars can be achieved for the preparation of desired catalysts during the oxidation of bulk S compounds.

Highly selective 1-pentene epoxidation over Ti-MWW with modified microenvironment of Ti active sites

The hexa-coordinated Ti active sites with the piperidine molecules as ligand significantly accelerate H2O2 activation, which effectively improve the catalytic performance of Ti-MWW in the 1-pentene epoxidation reaction.

Fast synthesis of hierarchical Al-free Ti-BEA plate-like nanocrystals from low-templated dry gel via a steam-assisted conversion method

A hierarchical nanosized Ti-BEA zeolite prepared via a green steam-assisted conversion (SAC) method.

Nitrogen doping and titanium vacancies synergistically promote CO2 fixation in seawater.

The electrocatalytic generation of useful chemicals from CO2, H2O, and sustainable energy resources offers a promising strategy for the carbon cycle. However, the current CO2 electrolysis system is mainly operated in artificial electrolytes (e.g. ionic liquids and inorganic salt solutions), of which the high cost and impractical working conditions hinder its large-scale development. In this case, seawater represents an attractive alternative due to its abundance and good conductivity. Herein, we show that N-doping and titanium vacancies (VTi) can be introduced in Ti3C2 MXene nanosheets via a facile NH3-etching pyrolysis approach. These nanosheets demonstrate impressive CO2 reduction reaction (CO2RR) performances in seawater with a remarkable 92% faradaic efficiency and a partial current density of -16.2 mA cm-2 for CO production, being close to those of noble metal electrodes. Mechanistic studies reveal that the existence of N dopants and VTi synergistically modulates the electronic structure of the active Ti site, on which the free energy barriers for the key *COOH formation and desorption of *CO are greatly reduced, thereby leading to a notable CO2RR improvement. This study provides an opportunity for developing an active and cost-effective CO2 electrolysis system by using seawater as the electrolyte.

A one-step rapid synthesis of TS-1 zeolites with highly catalytically active mononuclear TiO6species

The modulation and determination of the coordination environments of Ti active sites in titanosilicate zeolites are key challenges in the rational design of high-performance heterogeneous catalysts.

Enhanced catalytic oxidation performance of K+-modified Ti-MWW through selective breaking of interfacial hydrogen-bonding interactions of H2O2

Cationic modification was applied to Ti-MWW zeolite via the treatment of potassium chloride (KCl). Compared to the original Ti-MWW zeolite, the K-modified Ti-MWW sample (Ti-MWW-K) exhibited much improved catalytic activity, selectivity and stability in the liquid-phase epoxidation of alkenes. Our investigation shows that the K+ modification has negligible effect on the physicochemical properties, the state of the Ti active sites and the Lewis acidic strength of the Ti-MWW catalyst. Our result has further indicated that breaking of the hydrogen-bonding interactions between the Oβ atom of H2O2 (Hα-Oα-Oβ-Hend) and the H atom of the silanols adjacent to the Ti-OH (SiOH(HOTi)) in the Ti-MWW-K zeolite promotes the activation of H2O2, which is responsible for its superior catalytic oxidation activity. This finding helps shed light on utilizing the hydrogen-bonding interactions based on the titanosilicates/H2O2 system to achieve the excellent catalytic oxidation performance.

Ti-sites environment-mediated hierarchical TS-1 catalyzing the solvent-free epoxidation: The remarkably promoting role of alcohol modification

The solvent-free epoxidation of vegetable oils-derived compounds is in great demand due to its green initiative and economic efficiency. Herein, we showcase a persuasive strategy for functionalizing hierarchical TS-1 (HTS-1) with alcohol to achieve remarkably enhanced catalytic activity and selectivity of methyl oleate (MO) epoxide with H2O2 as an oxidant. The effects of alcohol types, treatment temperature and time on the catalytic performance of modified HTS-1 were investigated. The functionalization of HTS-1 with ethanol was proven to be more beneficial to enhance catalytic performance than methanol and steric effect should be taken into account when using bulkier alcohol such as isopropanol. The optimal catalytic property was achieved over ethanol-functionalized HTS-1 through a facile treatment procedure (ambient temperature, 10 h). As a result, the MO conversion could reach as high as 92.7%, overwhelming that afforded by pristine HTS-1 (only 56.9%). This strategy unfolds the following multiple merits: (i) reducing non-framework Ti species and creating accessible channel for MO diffusion; (ii) increasing hydrophobicity of catalyst accompanied by the defect-mending; (iii) elevating electrophilicity of Ti active sites to obtain more electrophilic O as the active oxygen-donating species. Such merits are responsible for significantly improved catalytic activities of HTS-1.

Novel Cr/Ti bimetallic polyethylene catalysts synthesized through introduction of chromium species into the (SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta catalyst

A series of novel Cr/Ti bimetallic polyethylene catalysts were successfully synthesized for the first time through introduction of chromium species into the (SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta catalysts. The bimetallic catalysts, which showed high activity, were simply prepared by refluxing titanium choloride with chromium modified silica gel/alkyl Mg adducts. Moreover, the effects of chromium content and calcination temperature were investigated in detail. The results obtained in the study were used to systematically discuss a clear relationship between the catalyst preparation condition and the relative performance of different metal active centers. Factors including surface hydroxyl content, catalyst performance in homo-polymerization, ethylene/1-hexene copolymerization were systematic investigated. Comparably, the chromium modified catalyst shows catalytic performance with 89% higher activity, 41.5% higher hydrogen response and 46.3% higher 1-hexene incorporation than the original (SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta catalyst. A new schematic model was proposed to explain the accelerated activation process of the bimetallic catalysts with kinetic and SEM results: chromium active sites on the outer surface of support react 10 times quicker than Ti active sites initially and help to crack the catalyst particle in the first stage; then the released inner surface active sites start to accelerate the polymerization reaction in the second stage.

Highly Active Layered Titanosilicate Catalyst with High Surface Density of Isolated Titanium on the Accessible Interlayer Surface

AbstractMetallosilicate catalysts with tetrahedrally coordinated transition metals are attractive because of their high selectivity in the oxidation of hydrocarbons. Here, we report highly active titanosilicate catalysts featuring an isolated titanium species with a uniform tetrahedral coordination state, unusually high surface density, and high accessibility of the Ti active sites. The layered titanosilicate prepared by grafting of TiIV acetylacetonate onto the surfaces of the crystalline layered silicate, HUS‐7, showed excellent catalytic performance in the epoxidation of cyclohexene with tert‐butyl hydroperoxide as an oxidant (turnover frequency of 230 h−1 and reaction rate of 314 mmol g−1 h−1).

Fluorine Modified TAPO-5: Synthesis and Catalytic Performance in Cyclohexanone Ammoximation

A series of fluorine modified TAPO-5 molecular sieves was synthesized by one-pot method using ammo-nium hexafluorotitanate as titanium and fluorine sources. The XPS and DRS UV-Vis results indicated that the exis-tence of F could promote the formation of tetrahedrally coordinated framework Ti and Al, meanwhile, inhibit the formation of anatase TiO2 in TAPO-5 sieves due to the unique role of F in the sol-gel process of metal ions. Further-more, TG, contact angle test, Py-FTIR and 27Al MAS NMR results revealed that fluorine modification increased the surface hydrophobicity and the Lewis acidity of Ti active sites through forming Al x F y units in the neighborhood. All these factors make these fluorine modified TAPO-5 sieves exhibit good catalytic performance in the ammoximation of cyclohexanone.

Anionic polyelectrolytes in titanosilicate molecular sieve synthesis towards simultaneously accomplishing low production cost and high catalytic activity.

Assisted by an anionic polyelectrolyte of poly(acrylic acid) (PAA), high-performance titanium silicalite-1 (TS-1) could be facilely synthesized at very low usage of expensive organic templates (tetrapropylammonium hydroxide). The presence of PAA helped to incorporate more active Ti species into the TS-1 framework and change the morphology to a plate-like shape, which was beneficial to molecular diffusion among its micropores to access the Ti active sites. Therefore, TS-1 synthesized with PAA showed much higher catalytic activity than that synthesized using the traditional synthesis without polyelectrolytes, and only a 30% usage amount of organic template was used. Moreover, this ultra-cheap catalyst also displayed a better catalytic activity than commercial TS-1 synthesized via a series of complicated preparation processes including alkene epoxidation with hydrogen peroxide as a green oxidant.

Active Sites of M(IV)-incorporated Zeolites (M\u2009=\u2009Sn, Ti, Ge, Zr)

M(IV)-incorporated zeolites have recently aroused wide interest due to outstanding catalytic effects while their active sites remain largely elusive. Here periodic density functional theory calculations are conducted finding that active sites are determined jointly by identity of M(IV) ions, topology of zeolites, type of framework species and choice of T sites. All M2(IV) active sites in BEA zeolites are penta-coordinated with chemisorption of one water while subsequent water molecules that form only H-bonds promote chemisorption of the first water, especially the second water possessing comparable or even higher adsorption strengths as the first water; Ti(IV) and Ge(IV) active sites at the intersection remain penta-coordinated and Sn(IV) and Zr(IV) active sites prefer to hexa-coordination although potentially expanded to hepta-coordination. Different from other zeolites, Ti(IV) active sites in FER zeolites are hexa-coordinated as Sn(IV) active sites, due to the promoting effect of the first water. Lewis acidic defects expand Ti(IV) active sites to hexa-coordination while inhibit the formation of hepta-coordinated Sn(IV) species. Two forms of Brϕnsted acidic defects exist for Sn(IV) sites instead of only one for Ti(IV) sites, and all M(IV) Brϕnsted acidic defects, regardless of different acidic forms and M(IV) ions, can chemisorb only one water.

Hierarchical Ti‐Beta Obtained by Simultaneous Desilication and Titanation as an Efficient Catalyst for Cyclooctene Epoxidation

AbstractA novel post‐synthesis route to hierarchical Ti‐beta zeolite and its performance as an efficient catalyst in heterogeneous cyclooctene epoxidation are presented. Catalytically active Ti sites as well as meso‐ and macropores are simultaneously introduced into microporous beta zeolite by hydrothermal treatment by using tetrabutyl orthotitanate as the titanation agent and tetraethylammonium hydroxide as the desilication base. Simultaneous desilication and titanation (SDT) offer the advantages of reduced experimental effort and chemical waste. The possibility to use a commercial zeolite available in high quantity as the starting material is attractive for industrialization. A significant mesopore volume (Vmeso=0.19 cm3 g−1) was introduced in the resulting hierarchical Ti‐beta zeolite without loss of microporosity and crystallinity compared to the purely microporous counterpart. The comparison clearly demonstrates a higher catalytic activity for the hierarchical Ti‐beta in cyclooctene epoxidation and accelerated desorption kinetics associated with the highly interconnected pore system.

Enhancement of Oxidative Desulfurization Performance over UiO‐66(Zr) by Titanium Ion Exchange

Oxidative desulfurization is considered to be one of the most promising methods for producing ultra-low-sulfur fuels because it can effectively remove refractory sulfur-containing aromatic compounds under mild conditions. In this work, the oxidative desulfurization performance over UiO-66(Zr) is greatly enhanced by Ti ion exchange. This strategy is not only efficient for UiO-66(Zr) with crystal defects but also for UiO-66(Zr) with high crystallinity. In particular, the performance of UiO-66(Zr) with high crystallinity in the oxidative desulfurization of dibenzothiophene can be improved more than 11-fold, which can be mainly attributed to the introduction of active Ti sites.

Immobilization of Ziegler–Natta Catalyst for Ethylene Polymerization on Macropores SiO2 with an Open-Framework Structure

A macroporous SiO2 (Macro-SiO2), which has uniformed macropores, an open-framework structure, and thin walls, was synthesized to support Ziegler–Natta catalysts. The immobilization behaviors of the Ziegler–Natta catalysts inside the Macro-SiO2 were investigated. Notable agglomerations of δ-MgCl2/titanium complex were observed because of the unconstrained environment of the Macro-SiO2. A highly superficial Ti content and a heterogeneously chemical structure of Ti active sites were thus achieved inside the Macro-SiO2. The ethylene polymerization results revealed that the Macro-SiO2/MgCl2/TiCl4 exhibited much higher catalytic activity (i.e., 12.1 × 106 g PE·(mol Ti·h)−1 or 17.8 × kg PE·(g cat·h)−1) than that of the traditional 955-SiO2 supported catalyst (i.e., 2.0 × 106 g PE·(mol Ti·h)−1 or 2.7 × kg PE·(g cat·h)−1). Finally, the fragmentation behavior of Macro-SiO2/MgCl2/TiCl4 was investigated during the polymerization. This unconstrained environment of Macro-SiO2 afforded fewer resistances to the diffusion...

Palladium Nanoparticles Encapsulated in Hollow Titanosilicate Spheres as an Ideal Nanoreactor for One-pot Oxidation.

One-pot reaction involving Pd-catalyzed H2 O2 production from H2 and O2 and Ti-catalyzed successive oxidation with H2 O2 in a single reaction vessel is an alluring strategy for the synthesis of targeted chemicals in terms of sustainability and economic competitiveness. In this study, a yolk-shell nanostructured catalyst, consisting of Pd nanoparticles (NPs) with core diameter ca. 4.0 nm and a porous titanosilicate shell of ca. 15 nm thickness, was fabricated by using an oil-in-water (O/W) microemulsion-based interfacial self-assembly approach. Compared with prototype titanosilicate-supported Pd NP catalysts and core-shell structured analogues, the yolk-shell nanostructured catalyst exhibited superior catalytic efficiency in the one-pot oxidation reaction of sulfides with 83 % H2 O2 utilization efficiency, because of the productive effect of the titanosilicate shell in limiting the diffusion of H2 O2 generated in situ over the encapsulated Pd NPs and the efficient access of the H2 O2 to the neighboring active Ti sites. This study provides promising avenues for the development of multifunctional nanostructured catalysts that are useful for one-pot reactions.