Titanium Silicate Catalysts Research Articles

Engineering surface framework TiO6 single sites for unprecedented deep oxidative desulfurization.

Catalytic oxidative desulfurization (ODS) using titanium silicate catalysts has emerged as an efficient technique for the complete removal of organosulfur compounds from automotive fuels. However, the precise control of highly accessible and stable-framework Ti active sites remains highly challenging. Here we reveal for the first time by using density functional theory calculations that framework hexa-coordinated Ti (TiO6) species of mesoporous titanium silicates are the most active sites for ODS and lead to a lower-energy pathway of ODS. A novel method to achieve highly accessible and homogeneously distributed framework TiO6 active single sites at the mesoporous surface has been developed. Such surface framework TiO6 species exhibit an exceptional ODS performance. A removal of 920ppm of benzothiophene is achieved at 60°C in 60min, which is 1.67 times that of the best catalyst reported so far. For bulky molecules such as 4,6-dimethyldibenzothiophene (DMDBT), it takes only 3min to remove 500ppm of DMDBT at 60°C with our catalyst, which is five times faster than that with the current best catalyst. Such a catalyst can be easily upscaled and could be used for concrete industrial application in the ODS of bulky organosulfur compounds with minimized energy consumption and high reaction efficiency.

Molecular Structure Effect on the Epoxidation of 1-Butene and Isobutene on the Titanium Silicate Catalyst under Transient Conditions in a Trickle Bed Reactor.

Epoxidation of two butane isomers (1-butene and isobutene) on the commercial titanium silicate (TS-1) catalyst was studied in a laboratory-scale trickle bed reactor. The transient step response technique was used as the main tool in the investigation. The transient responses revealed different dynamics of product formation in continuous operation. The study of isomers showed the impact of the molecular structure on the transient and stationary states of the system. The four-carbon chain present in 1-butene displayed a dynamic behavior with a prominent maximum of the conversion as a function of time-on-stream. On the contrary, the behavior of isobutene was displayed to be closer to ethene and propene under similar conditions reaching a steady state after ca. 2 h. The structure of the epoxide was an important factor in order to achieve a high epoxide selectivity. In isobutene epoxidation, the primary product 1,2-epoxy-2-methylpropane was highly reactive, giving a spectrum of parallelly formed byproducts. Therefore, the selectivity of the epoxide from isobutene was limited to ca. 70%. In the epoxidation of 1-butene, 1,2-epoxybutane was displayed to be a highly stable product with a selectivity close to 99%. Based on the transient and stationary data, a reaction mechanism was proposed for the epoxidation and ring-opening reactions present in the system.

Continuous Liquid-Phase Epoxidation of Ethylene with Hydrogen Peroxide on a Titanium-Silicate Catalyst

Ethylene epoxidation with hydrogen peroxide was studied in a laboratory-scale trickle bed reactor under a broad range of experimental conditions (15–80 °C, 2.5–8.5 bar) utilizing a commercial titanium-silicate catalyst (TS-1). The catalyst was very stable and selective over 150 h time-on-stream. The main reaction product was ethylene oxide, while 2-methoxyethanol and ethylene glycol were observed as kinetic byproducts. In most of the experiments, ethylene glycol was not detected at all. An increase in temperature and pressure affected negatively the ethylene oxide selectivity, while an increase in the hydrogen peroxide concentration improved both the ethylene oxide selectivity and ethylene conversion. Ethylene epoxidation was comparable with propylene epoxidation, displaying, however, important differences in activity and selectivity, which were attributed to the partial pressures studied in the present work. It was demonstrated that TS-1 is a very selective and active catalyst for the selective epoxidation of ethylene with hydrogen peroxide.

SYNTHESİS AND STUDY OF MESOPOROUS TİTANİUM-SİLİCATE CATALYSTS

The paper presents the results of studies on the development of synthesis methods and study of the properties of a catalyst based on titanium silicalite for the synthesis of MEKmethylethylketone. Samples of titanium silicalite were synthesized with different ratios of the starting reagents - tetraethoxysilane and tetrabutoxysilane. Tetra-n-propyl ammonium hydroxide was used as a structure-forming agent. The data of IR spectroscopy and powder Xray diffraction made it possible to establish the effect of the amount of the structure-forming agent on both the structure and morphology of the titanium silicalite obtained. The main product is a mixture of TiO2 and amorphous SiO2. The dependence of the average particle size on the initial ratio of the initial reagents is established: with an increase in the initial molar ratio of the reactants, a change in the size of the catalyst particles is observed. The main indicators of the catalytic activity of the catalyst samples obtained at different initial ratios of reagents are compared. The carried out physicochemical studies on the synthesis of titanium silicalite made it possible to establish the composition that provides the maximum selectivity for the formation of methylethylketone. Keywords: methylethylketone, mesoporous titanium-silicate catalyst, catalytic activity.

Olefin Epoxidation over Metal-Organic Frameworks Modified with Transition Metals

Mixed aluminum-containing metal-organic frameworks (MOF) modified with Fe, V, and Ti are synthesized in two steps. At the first step, the amorphous precursors of metal-organic frameworks containing various amounts of transition metals are obtained by the electrochemical method. At the second step, the precursors are crystallized under solvothermal conditions. Study of the effect of synthesis conditions shows that Fe-containing metal-organic frameworks crystallize in the presence of dimethylformamide; V- and Ti-containing counterparts crystallize in the presence of water. The time and temperature of crystallization are varied in the range from 150 to 200°С and 24 to 72 h depending on the concentration of the transition metals. The resulting samples are investigated by low-temperature nitrogen adsorption, scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and thermogravimetric and differential thermal analysis (TGA-DTA). Research into the influence of catalyst composition on its activity in 1-octene epoxidation by tert-butyl hydroperoxide suggests that that the titanium-containing samples are catalytically inactive, while the vanadium-containing samples are more active than the iron-containing ones. Testing of the vanadium-containing samples in propylene epoxidation demonstrates that their activity is comparable with the activity of a commercial titanium silicate catalyst.

Titanium and oxygen pulled apart by light

Photocatalysis Light absorption often prompts extremely rapid chemical changes in the absorbing medium that in turn can stimulate chemical reactivity. Yoo et al. applied ultrafast electron microscopy to elucidate these changes in a class of titanium silicate catalysts that harness light to transform nitric oxide, CO2, and various other small molecules. The titanium centers start out in a square pyramid geometry with a double-bonded oxygen at the apex. Less than trillionths of a second after the light absorption, the microscopy data captured a lengthening of this bond that was consistent with reduction of the titanium to a Ti3+-O1− motif primed for ensuing reactivity. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1522869113 (2016).

The oxidation of limonene at raised pressure and over the various titanium-silicate catalysts

Abstract This work presents the studies on the oxidation of limonene with hydrogen peroxide and tert-butyl hydroperoxide (TBHP) in the presence of : TS-2, Ti-Beta, Ti-MCM-41 and Ti-MWW catalysts, at the autogenic pressure and atmospheric pressure. The examination were performed at the following conditions: the temperature of 140°C (studies in the autoclave) and 80°C (studies in glass reactor), the molar ratio of limonene/oxidant (H2O2 or WNTB) = 1:1, the methanol concentration 80 wt%, the catalyst content 3 wt%, the reaction time 3 h and the intensity of stirring 500 rpm. The analysis of the results showed that in process not only 1,2-epoxylimonene was formed but also: 1,2-epoxylimonene diol, carveol, carvone and perillyl alcohol but for 1,2-epoxylimonene obtaining the better method was the method at the autogenic pressure and in the presence of TBHP.

Epoxidation of Natural Limonene over the Ti-MCM-41 Catalyst with 60 Wt.% Hydrogen Peroxide

Limonene belongs to the group of the monoterpenes, which are a raw material for the production of very valuable compounds. On an industrial scale limonene is obtained from orange peels – biomass, which comes from the fruit juice industry. Thus, limonene is an easily available compound and also relatively cheap because it comes from renewable sources. This paper presents the process of limonene epoxidation using the titanium-silicate catalyst Ti-MCM-41 and 60 wt% hydrogen peroxide as an oxidant and methanol as a solvent. The catalytic activity of this mesoporous material was tested at the selected temperatures (110-140°C) and for the reaction time amounted to 6h. In this work we present different ways of the transformation of limonene, not only to 1,2-epoxylimonene, but also to carvone, carveol and perillyl alcohol, which are also very valuable oxygenated derivatives of limonene.

User-friendly synthesis of highly selective and recyclable mesoporous titanium-silicate catalysts for the clean production of substituted p-benzoquinones

Mesoporous titanium-silicates have been prepared following the evaporation-induced self-assembly (EISA) methodology and characterized by elemental analysis, XRD, N2 adsorption, SEM, DRS UV–Vis and Raman techniques. The use of acetylacetone during synthesis allowed the formation of highly dispersed dimeric and/or small oligomeric Ti species, within the mesostructured silica network, to be realized. The materials catalyse oxidation of alkylsubstituted phenols to corresponding p-benzoquinones with 100% selectivity using the green oxidant – 30% aqueous hydrogen peroxide. The titanium-silicates prepared by the convenient and versatile EISA-based procedure reveal the true heterogeneous nature of the catalysis and do not suffer from titanium leaching. They show advantages over other types of mesoporous Ti,Si-catalysts, such as TiO2–SiO2 mixed oxides and grafted Ti/SiO2, in terms of the catalyst stability and reusability.

Zeolite Confined Ti(OH)4 Nanoparticles in Highly Active and Selective Oxidation Catalyst

Some highly active, hydrophilic, MFI structured titanium silicate catalysts are characterized by an intense UV absorption centered in the 213–233 nm (43000–47000 cm−1) range. The assignment of this band to a specific molecular structure has remained unclear unlike the well published 233 nm UV bands of higher coordinated Ti-atoms at lattice defects or in variously agglomerated extra lattice Ti x O y (OH) z species. To decipher the structural origin of this intermediate UV band, we used Time-Dependent Density Functional Theory (TD-DFT) to compute the UV spectra of various model compositions and show in this paper that only one model, in which isolated tetrahedral Ti(OH)4 molecules reside in the zeolite channels without any covalent bond to the framework, fits the experimental UV spectrum. Hence these confined tetrahedral Ti(OH)4 nanoparticles may account for the enhanced activity and selectivity of some MFI type titanium silicate catalysts.

How to reach 100% selectivity in H 2O 2-based oxidation of 2,3,6-trimethylphenol to trimethyl- p-benzoquinone over Ti,Si-catalysts

A comprehensive study of structure/activity/selectivity relationships and mechanistic aspects of the oxidation of 2,3,6-trimethylphenol (TMP) with aqueous H 2O 2 over a wide variety of titanium-silicate catalysts allowed us to infer requirements to an optimal catalyst and optimal reaction conditions for this reaction and to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, Vitamin E precursor) with nearly 100% selectivity at 100% substrate conversion. The main by-products in the TMP oxidation are C–C and C–O dimers, formed by coupling of intermediate phenoxyl radicals. The formation of TMBQ is favoured by (1) a poor coordinating solvent (MeCN), (2) elevated temperature (80 °C), (3) low TMP concentration (not higher than 0.1 M), (4) high H 2O 2/TMP molar ratio (ca. 3.5), and (5) low TMP/Ti ratio (<10–20). The crucial factors which determine the selectivity of Ti,Si-catalysts in TMP oxidation to TMBQ are mesoporosity and an optimal surface concentration (ca. 0.7–1.0 Ti atoms/nm 2) of accessible highly dispersed, probably, dimeric Ti(IV) species. The catalysts prepared by a simple, affordable and cheap synthesis methodology via grafting titanium(IV) precursors onto the surface of commercial mesoporous silica completely fulfil these requirements and thus can be viewed as promising catalysts for environmentally benign TMBQ production.

H2O2‐Based Oxidation of Functionalized Phenols Containing Several Oxidizable Sites to p‐Quinones Using a Mesoporous Titanium‐Silicate Catalyst.

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H2O2-Based oxidation of functionalized phenols containing several oxidizable sites to p-quinones using a mesoporous titanium-silicate catalyst

The oxidation of 2-allylphenol and different phenols bearing alcohol functional groups with aqueous H2O2 as oxidant and heterogeneous titanium-silicate Ti-MMM-2 as catalyst exhibits an unusual selectivity affording the corresponding p-quinones with good to moderate yield and keeping the other oxidizable sites intact. The use of hydrogen peroxide and heterogeneous titanium-silicate catalyst is a green alternative to the stoichiometric oxidation with hypervalent iodine compounds that provides better yields of structurally complex quinones in one step.

Oligodeoxyribonucleotide analogues with bridging dimethylene sulfide, sulfoxide, and sulfone groups. Toward a second-generation model of nucleic acid structure.

Short DNA analogues with bridging dimethylene sulfide, sulfoxide, and sulfone groups replacing the phosphate diesters (S-DNAs) were synthesized from building blocks prepared via two routes, both starting from D-glucose. Building blocks for RNA analogues were prepared by stereoselective introduction of nucleobase into a 2'-acylated ribose analogue. The ribose analogues were converted to deoxyribose analogues by replacement of a 3''-OH group by a thioacetyl unit, followed by photolytic deoxygenation or radical-based 2'-deoxygenation. DNA analogues joined via CH(2)(-)S-CH(2) units were prepared by S(N)2 displacement of a 6'-mesyl group on one building block using a thiolate nucleophile of another. 4,4'-Dimethoxytrityl protection and deprotection schemes were established for both the thiol and hydroxyl groups. The corresponding sulfoxide DNA analogues were obtained by oxidation with hydrogen peroxide. Sulfone DNA analogues were obtained by oxidation of the sulfide DNA with persulfate or hydrogen peroxide in the presence of a titanium silicate catalyst. The physical properties of several representative oligonucleotide analogues were examined, and interpreted in light of a "second-generation" model for DNA strand-strand recognition, a model that emphasizes the role of the polyanionic backbone in diminishing unwanted tendencies of highly functionalized molecules to form "structure" in solution. Even short sulfide-linked DNA analogues displayed association properties different from those displayed by standard DNA molecules. Complex formation observed with sulfide-linked tetramers by HPLC study in different solvents suggested that the complex is formed using hydrogen bonding. Sulfone-linked dinucleotides display Watson-Crick behavior; the tetramer, however, displayed self-structure. Self-structure and self-aggregation become more prominent as the length of the oligonucleotide analogues increases. The tendency to self-aggregate can be decreased by adding a charged sulfonate group to the 3''-end of the DNA analogue. Features of the second-generation model are important for many areas of nucleic acid chemistry, from the design of nucleic acid therapeutic agents to the search for life on other planets.

Oxidation of α-hydroxy containing monoterpenes using titanium silicate catalysts: comments on regioselectivity and the role of acidity

The regioselective epoxidation of monoterpenes in the liquid phase has been studied using the titanosilicates TS-1 and TiAlβ. A range of oxidants (hydrogen peroxide, tert-butyl hydroperoxide and urea–hydrogen peroxide complex) have been studied in detail. The allylic alcohols linalool and geraniol have been studied alongside the non-allylic alcohol citronellol and the diene dihydromyrcene to help determine the role of the hydroxy group in these reactions. Dihydromyrcene is selectively epoxidised at the more electron rich double bond regardless of the catalyst–oxidant–solvent system used. Geraniol can undergo allylic assisted epoxidation with TS-1–acetone–hydrogen peroxide and TiAlβ–acetonitrile–urea–hydrogen peroxide. With TiAlβ–hydrogen peroxide–methanol, the reaction shows an induction period in the conversion of geraniol which is considered to be characteristic of the autocatalytic removal of titanium from the catalyst framework. Reactions with citronellol show this titanium removal is entirely due to the presence of the allylic alcohol moiety. Finally, epoxidation of linalool and the subsequent in situ conversion of the epoxide to the furano- and pyrano-oxides were studied. The ratio of furano- and pyrano-oxides formed was considered to be due, in part, to the pore geometry and the Bronsted acidity of the catalyst.

A novel large-pore framework titanium silicate catalyst

Download options Please wait... Article information DOI https://doi.org/10.1039/B204135A Article type Paper Submitted 29 Apr 2002 Accepted 02 Oct 2002 First published 15 Oct 2002 Download Citation J. Mater. Chem., 2002,12, 3819-3822 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions A novel large-pore framework titanium silicate catalyst P. Brandão, A. Valente, A. Philippou, A. Ferreira, M. W. Anderson and J. Rocha, J. Mater. Chem., 2002, 12, 3819 DOI: 10.1039/B204135A To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Paula Brandão Anabela Valente Andreas Philippou Artur Ferreira Michael W. Anderson João Rocha

Vibrational structure of titanium silicate catalysts. A spectroscopic and theoretical study.

A thorough analysis of the vibrational features of the titanium silicalite-1 (TS-1) catalyst is presented, based on quantitative IR measurements, Raman and resonant Raman experiments, quantitative XANES, and quantum chemical calculations on cluster and periodic models. The linear correlation of the intensity of the IR and Raman bands located at 960 and 1125 cm(-1) and the XANES peak at 4967 eV with the amount of tetrahedral Ti are quantitatively demonstrated. Raman and resonant Raman spectra of silicalite and TS-1 with variable Ti content are presented, showing main features at 960 and 1125 cm(-1) associated with titanium insertion into the zeolite framework. The enhancement of the intensity of the 1125 cm(-1) feature and the invariance of the 960 cm(-1) feature in UV-Raman experiments, are discussed in terms of resonant Raman selection rules. Quantum chemical calculations on cluster models Si[OSi(OH)(3)](4) and Ti[OSi(OH)(3)](4) at the B3LYP/6-31G(d) level of theory provide the basis for the assignment of the main vibrational contributions and for the understanding of Raman enhancement. The resonance-enhanced 1125 cm(-1) mode is unambiguously associated with a totally symmetric vibration of the TiO(4) tetrahedron, achieved through in-phase antisymmetric stretching of the four connected Ti-O-Si bridges. This vibration can also be described as a totally symmetric stretching of the four Si-O bonds pointing toward Ti. The resonance enhancement of this feature is explained in terms of the electronic structure of the Ti-containing moiety. Asymmetric stretching modes of TO(4) units show distinct behavior when (i) T is occupied by Si as in perfect silicalite, (ii) T is occupied by Ti as in TS-1, or (iii) the oxygen atom belongs to an OH group, such as in terminal tetrahedra of cluster models and in real defective zeolites. Asymmetric SiO(4) and TiO(4) stretching modes appear above and below 1000 cm(-1), respectively, when they are achieved through antisymmetric stretching of the T-O-Si bridges, and around 800 cm(-1) (in both SiO(4) and TiO(4)) when they involve symmetric stretching of the T-O-Si units. In purely siliceous models, the transparency gap between the main peaks at 800 and 1100 cm(-1) contains only vibrational features associated with terminal Si-OH groups, while in Ti-containing models it contains also the above-mentioned asymmetric TiO(4) modes, which in turn are strongly coupled with Si-OH stretching modes. Calculations on periodic models of silicalite and TS-1 free of OH groups using the QMPOT embedding method correctly reproduce the transparency gap of silicalite and the appearance of asymmetric TiO(4) vibrations at 960 cm(-1) in TS-1. Finally, we demonstrate, for the first time, that the distortion of the tetrahedral symmetry around Ti caused by water adsorption quenches the UV-Raman enhancement of the 1125 cm(-1) band.

98/01095 Experimental evidence of α-olefin readsorption in Fischer—Tropsch synthesis on ruthenium-supported ETS-10 titanium silicate catalysts: Bianchi, C. L. and Ragaini, V. J. Catal., 1997, 168, (1), 70–74

We evaluated 3D interactions in highly Na-doped polyacetylene, (CH)x, reasonably based on the structure reported in an X ray experiment. Interchain transfers (ICTs) via dopants estimated quantum chemically are found to reach up to 0.93 eV and spread over five sites on each adjacent chain at doping concentration, 11.1%, Screened Coulomb interactions are evaluated as Thomas-Fermi potential for quasi ID conductors numerically, to estimate reasonable interchain Coulomb (ICC) and dopants' Coulomb (DC) potentials. As the intrachain Hamiltonian, SSH+extended Hubbard model is used. The Hartree-Fock (HF) ground state with geometry optimization is soliton lattices out-of-phase ordered. The gap little depends on chain length and is extrapolated to 1.3eV in infinite chain limit. So rather large ICT in itself cannot lead to metallic state in HF approximation, and electron correlation seems to be important.

Experimental Evidence of α-Olefin Readsorption in Fischer–Tropsch Synthesis on Ruthenium-Supported ETS-10 Titanium Silicate Catalysts

Fischer–Tropsch synthesis seems to develop the following two consecutive paths: a primary process that involves the formation of α-olefin products and a secondary process leading to the production of branched isomers and paraffins and requiring the readsorption of primary α-olefin products. It was already shown by Iglesiaet al.(E. Iglesia, S. C. Reyes, and R. J. Madon,J. Catal.129, 238 (1991)) that such readsorption steps are of fundamental importance for Ru catalysts and that they occur due to the slow diffusive removal of α-olefins when the molecular size increases, this resulting in a long intraparticle residence time. In the present paper α-olefins readsorption was enhanced by changing the metal distribution inside the pores of a titanium silicate (ETS-10), modified by ion exchange with alkali metal ions, used as a support for Ru-based catalysts.

A test reaction for titanium silicalite catalysts

The catalytic hydroxylation of phenol to hydroquinone and catechol can be used for checking the quality of titanium silicate catalysts since the selectivity of these catalysts is strongly affected by the presence of small amounts of non-framework titania. Titanium silicates prepared by modification of [Al]ZSM-5 exhibits the same catalytic properties as hydrothermally synthesized TS-1 of high purity.