Titanium Silicalite-1 Zeolite Research Articles

Propylene Epoxidation Over Silver Supported on Titanium Silicalite Zeolite

Epoxidation of propylene to propylene oxide over Ag/TS-1 in the presence of oxygen and hydrogen was carried out in a fixed-bed, quartz flow reactor. The effects of Ag loading, Si/Ti ratio of TS-1 and calcination temperature and calcination method of Ag/TS-1 on the propylene epoxidation were investigated. The results show that Ag loading, the calcination temperature and calcination method of Ag/TS-1 as well as the Si/Ti ratio of TS-1 have a great effect on the catalytic properties. The optimum Ag loading, calcination temperature and calcination method of Ag/TS-1 and Si/Ti ratio of TS-1 are 2 wt%, 450 °C in air and 64 respectively. Over 2.0 wt% Ag/TS-1(nSi/nTi = 64) catalyst, at a space velocity of 3000 h-1, 0.92% propylene conversion with 91.21% selectivity to propylene oxide is obtained at 150 °C. The deactivation of Ag/TS-1 catalyst is not due to the changes of active species, but because of the coke of the catalyst, which can be easily regenerated by calcination at 450 °C in air.

Photocatalytic decomposition of 4-nitrophenol over titanium silicalite (TS-1) catalysts

Titanium silicalite zeolites (TS-1) having Si/Ti ratios of 3.3, 13.7, and 26.3 were synthesized and used as photocatalysts for decomposition of 4-nitrophenol (4-NP) in aqueous solution. In the dark, the adsorption of 4-NP on TS-1 changed greatly depending on the Si/Ti ratio in TS-1 and the pH of the solution. The amount of 4-NP adsorbed on TS-1 increased with increasing Si/Ti ratio and decreasing pH of solution. The photocatalytic activities of TS-1 zeolites for the decomposition of 4-NP were found to be greatly affected by H 2O 2. In the absence of H 2O 2, the TS-1 having lower Si/Ti ratio, and consequently containing a larger amount of extra framework titanium, exhibited higher photocatalytic activity. In contrast, in the presence of H 2O 2, the TS-1 with higher Si/Ti ratio, in which the titanium content is lower while the fraction of framework titanium is higher, showed a higher increase in photocatalytic activity. Especially in acid conditions, the addition of H 2O 2 to the reaction solution results in a remarkable enhancement of the photocatalytic activity of TS-1; the apparent rate constant increased with increasing Si/Ti ratio. Such results suggest that, upon UV illumination, OH radicals can be formed easily from the titanium-hydroperoxide species that are formed by the interaction of framework titanium in TS-1 with H 2O 2. The enhancement of photocatalytic activity of TS-1 for the decomposition of 4-NP by the addition of H 2O 2 can be attributed to both the efficient formation of OH radicals and the close proximity of OH radicals to the reactant molecules.

TS-1 zeolite microengineered reactors for 1-pentene epoxidation.

A zeolite-based microengineered reactor was fabricated and tested for 1-pentene epoxidation over titanium silicalite-1 (TS-1) catalyst, which has been selectively incorporated within the microreactor channel using a new synthesis procedure.

Photocatalytic oxidation and decomposition of acetic acid on titanium silicalite.

Transient reaction of adsorbed monolayers of acetic acid was used to characterize the photocatalytic properties of titanium silicalite zeolites (TS-1). The TS-1 zeolites having Si/Ti ratios of 5, 12.5, and 50 are effective catalysts at room temperature for both photocatalytic oxidation (PCO) and decomposition (PCD) of acetic acid. The rates of PCO are higher than the rates of PCD for each catalyst. Acetic acid oxidized photocatalytically in 0.2% O2 to form gas-phase CO2 and CH4 and adsorbed H2O on the TS-1 catalysts, whereas no CH4 formed on Degussa P25 TiO2. Isotope labeling showed that, on both TiO2 and TS-1 catalysts, the alpha-carbon formed CO2 whereas the beta-carbon formed CH4 and CO2. The rates of oxidation of the two carbons have different dependencies on UV intensity. The catalysts with higher Si/Ti ratios adsorbed significantly more acetic acid, and the PCO rates per gram of titanium are highest on the TS-1 catalyst with the lowest Ti content, apparently because a larger fraction of the Ti atoms are surface atoms on this catalyst. During PCD in an inert atmosphere, CO2, CH4, and C2H6 formed on TiO2 and on the catalyst with a Si/Ti ratio of 5, but C2H6 was not detected on the other catalysts. The CO2/CH4 selectivity during PCD increased with increasing Si/Ti ratio. The first step in PCO and PCD on TS-1 catalysts appears to be similar and involves formation of a CH3 radical.

Studies on the crystallization process of titanium silicalite-1 (TS-1) synthesized using tetrapropylammonium bromide as a template

tetrapropylammonium bromide (tpabr) has been used as a template for the synthesis of titanium silicalite-l (ts-l), and the involved crystallization mechanism and the characteristics of the incorporated ti in the zeolites were investigated by applying a variety of methods, e.g. solid-state nmr, ir, uv-vis, xrd and sem, as well as ph-meter techniques. it is revealed that the crystallization process performed with tpabr follows the well-known solution-mediated transformation mechanism, and that the ti atoms were introduced into the frameworks synchronously with the formation of the zeolite structures. in addition, the formation of less amount of inactive tio2 crystals with an anatase structure is not in the initial stage of the crystallization as reported by some authors, but through the whole process of the zeolite synthesis, which suggests that a careful control of the synthesis parameters throughout the whole crystallization process is quite important for obtaining the anatase-free titanium-silicalite zeolite. (c) 2000 elsevier science s.a. all rights reserved.

Studies on superoxide O 2− species on the interaction of TS-1 zeolite with H 2O 2

The interactions of silicalite-1 (S-1), titanium silicalite-1 (TS-1), Ti–Si gel and anatase with H 2O 2 were investigated by ESR technique. It was found that different Ti–O 2 − species could be formed over TS-1 zeolite and Ti–Si gel, which are dependent on the local structures of Ti atoms. The decay of O 2 − species on these samples depicted that O 2 − species on framework Ti sites are much more unstable than O 2 − species on dispersed extra framework Ti sites. The reaction of O 2 − species with phenol confirmed that O 2 − species on framework Ti sites are highly active.

Highly effective Cu-HMS catalyst for hydroxylation of phenol

Transition metals copper and titanium substituted mesoporous silicas (Cu-HMS and Ti-HMS) were synthesized at ambient temperature by using dodecylamine (DDA) surfactant as templating agent. XRD measurements prove that incorporating titanium and especially copper into the mesostructures causes the d100 peaks of mesoporous silicas to become shifted to lower angles, indicating progressive expansion of the lattice d-spacings upon heteroatoms Ti and especially Cu incorporating. FT-IR measurements indicate that the calcined Cu-HMS and Ti-HMS samples all exhibit a weaker absorption band near 960 cm-1 which may be rather a fingerprint of the heteroatom on the matrix of [SiO4] units whatever its crystallization state. Cu-HMS possesses relatively high catalytic activity for the hydroxylation of phenol with 30% aq. H2O2 in aqueous solution (about 36% phenol conversion and more than 95% selectivity for dihydroxybenzene isomers), but Ti-HMS has no catalytic activity under the same reaction conditions. The product distribution obtained from Cu-HMS is completely different from that of the microporous titanium silicalite zeolites (TS zeolites). This is attributed to the porous structural differences between Cu-HMS and TS zeolites. The catalytic activity of the Cu-HMS is strongly dependent on the nature of the solvent; the Cu-HMS does not have any catalytic activity in the presence of organic solvents such as methanol or acetone instead of water. A reusing test of the recovered Cu-HMS indicates that the recovered catalyst suffers almost loss of activity and must be regenerated by calcination in air at 873 K in order to recover its activity.

Synthesis, characterization and catalytic properties of titanium and boron co-substituted silicalite zeolites

Titanium and boron co-substituted silicalite-2 (TBS-2) has been synthesized by using TBAOH as a templating agent in hydrothermal conditions. The main factors affecting Ti and B co-incorporated into the framework of silicalite-2 (S-2) such as the used amounts of H 2O 2 (for dissolving hydrous Ti oxides) and H 3BO 3 (used as boron source) and the addition of TMAOH were studied in detail. XRD, FT-IR, SEM, and NH 3-TPD, were used to characterize the influences of Ti and B co-incorporation on the unit cell parameters of S-2, framework IR bands, crystalline morphology, and acid properties, respectively. The epoxidation of styrene and the rearrangement of styrene oxide were respectively used to check the oxidative and acidic catalytic properties of TBS-2.

Catalytic behavior of vanadium substituted mesoporous molecular sieves

A series of V-MCM-41 samples with different pore sizes has been systematically investigated with both gas and liquid phase reactions. Methanol oxidation has been performed in the gas phase on these catalysts. Turnover frequencies are normalized by oxygen uptake measured under reaction conditions. A strong effect of pore size on the catalytic activity was observed in the form of a “volcano curve” and may be correlated with a variation in the local bond angle of Si–O–V. Considering this effect, the catalytic activity may be `tuned' to maximum by varying the pore size. As expected, the same effect was observed in the liquid phase oxidation of cyclohexene. In order to interpret the pore size effect, the reaction rates of both the gas phase and liquid phase oxidation were then correlated with the variation of edge energies of V K-edge XANES on these MCM-41 catalysts. Additional reactions were performed in the liquid phase to compare the activity of V-MCM-41 with that of crystalline vanadium and titanium silicalite zeolites. The possible causes of the difference in the catalytic behaviors of these materials is also discussed.

Molecular models of catalytically active sites in zeolites. Quantum chemical approach

A brief review of quantum chemical studies of different active sites in zeolites is presented. Various factors that significantly affect the strength of Brønsted acid sites in zeolites are discussed. An interaction of zeolite protons with entrapped metal particles is considered as a reason of electron-deficiency of metal clusters in zeolite cavities on the Pd and Pt species as an example. Probable precursors of Lewis acid sites (LAS) and reliable molecular models of the LAS in zeolites are discussed on the basis of quantum chemical analysis. Transition metal ions can be catalytically active in the lattice or extra-lattice zeolite positions and the two possibilities are considered for selective oxidation site in titanium silicalite and FeZSM-5 zeolite catalysts, respectively.

Synthesis of fibrous titanium silicalite (FTS-1) zeolite

Fibrous titanium silicalite (FTS-1) zeolite has been prepared from titanium silicalite (TS-1) particles that were less than 120 nm in diameter. The formation of FTS-1 was mainly affected by the particle size of the TS-1 zeolite. FTS-1 is formed by both capillary force and dehydroxylation among the TS-1 particles during the drying and evaporation process of the dispersed TS-1 particles in aqueous solution. The average length and the aspect ratio (length/diameter) of FTS-1 are 2.2 and mm 50~70, respectively. The obtained FTS-1 shows a two-dimensionally aligned surface that is affected by the treatment of dispersed TS-1 particles in aqueous solution. 29Si MAS n.m.r. spectra showed that the concentration of the hydroxyl group on the surface of TS-1 particles was decreased after the formation of FTS-1 and was reduced continuously up to 750 °C with calcination. This suggests that the condensation reaction of hydroxyl groups on the surface of TS-1 particles occurs by drying and heat treatment. After calcination at 750 °C, FTS-1 still exhibited characteristics of MFI-type structure with orthorhombic symmetry. It showed distinctive secondary pores due to the aggregation of TS-1 particles. This method of FTS-1 formation may suggest a new method for the preparation of various types of zeolites.

Titanium incorporated ATS and AFI type aluminophosphate molecular sieves

Titanium aluminophosphate molecular sieves of ATS and AFI (with two different titanium levels in the framework) have been synthesized. These materials were characterized using XRD, SEM, BET, 27Al and 31P MAS NMR, XPS and EXAFS techniques. The results of XRD, SEM, BET and 27Al and 31P MAS NMR indicated the highly crystalline nature and phase purity of the samples. All these materials show a significant pre-edge peak at 4968 eV for Ti, characteristic of tetrahedral Ti. The average Ti—O coordination number for the ATS and AFI titanium aluminophosphates is 4.4 ± 0.3. XAFS obtained at the Ti K edge indicated the Ti incorporation in the ATS and AFI AlPO frameworks. In the ATS aluminophosphate, Mg and Ti are present in the framework. The catalytic performance of the ATS and AFI titanium aluminophosphate catalysts were investigated in the conversion of cyclohexanol to cyclohexanone with hydrogen peroxide in the presence of acetone at 353 K. The catalytic activity of these molecular sieves are compared with that of the titanium silicalite-1 zeolite.

Epoxidation process using titanium-rich silicalite catalysts

Olefins are epoxidized by hydrogen peroxide in the presence of a crystalline titanium silicalite zeolite catalyst having a Si:Ti molar ratio in the lattice framework of from 8:1 to 23:1. High yields of epoxides with minimal non-selective loss of either hydrogen peroxide or olefin are realized.

Hydroxylation of benzene and hexane by oxygen and hydrogen over palladium-containing titanium silicates

Palladium-containing titanium silicalite zeolites catalyse the hydroxylation of benzene and hexane by O2–H2 under mild conditions to give phenol and hexanols, respectively.