TS-1 Crystals Research Articles

Preparation of Colloidal Suspensions of Discrete TS-1 Crystals

Stable colloidal suspensions containing discrete TS-1 crystals with average crystal sizes less than 100 nm and with a narrow particle size distribution have been synthesized from clear homogeneous solutions. The products were characterized with respect to crystallinity, crystal size and size distribution, Ti incorporation, specific surface areas, the nature of the Ti species in the products, and the colloidal crystals surface electric charge. The influence of several parameters including synthesis mixture composition and crystallization temperature on crystal size and Ti incorporation were investigated. The synthesis of colloidal crystals with an average size less than 100 nm is favored by low crystallization temperatures (100 °C), a highly basic environment, concentrated mixtures, and low Ti contents in the synthesis solutions. The maximum incorporation of tetrahedrally coordinated Ti was 0.84 mol %. High crystallization temperatures (>100 °C) favor products with a low Si/Ti molar ratio, but larger crystals are formed, >100 nm, and octahedrally coordinated Ti and extra-framework Ti are detected. A preliminary investigation of the clear hydrolyzed precursor solution and its freeze-dried amorphous equivalent showed that small particles with an average size of 4.8 nm exist in the solution and a certain degree of ordering resembling the MFI structure was present in the precursor particles.

Evidence of solid-solid transformations during the TS-1 crystallization from amorphous wetness impregnated SiO 2TiO 2 xerogels

The crystallization mechanism of TS-1 has been investigated when this Ti-containing material is obtained from amorphous SiO 2TiO 2 solids impregnated with TPAOH solutions followed by thermal treatment under autogenous pressure. Samples with different degrees of crystallinity have been prepared and characterized by elemental analysis, X-ray diffraction (XRD), infrared (IR) and diffuse reflectance ultraviolet visible (DR UV-Vis) spectroscopies, scanning and transmission electron microscopy (SEM and TEM), n-hexane adsorption and catalytic tests of n-hexane oxyfunctionalization. In contrast to the classical mechanism proposed to describe the zeolite synthesis through a mediated solution process, in the system studied here the TS-1 crystallization is mainly governed by solid-solid transformations, and a true crystal growth step is not observed. The major role of the solution is to provide the TPA + species necessary to complete the solid crystallization and to favour the migration of the secondary particles. During the first stages of the synthesis, a small amount of the raw material is dissolved yielding soluble Ti-rich species ( Si Ti = 5 ), which are no longer involved in the crystallization. Simultaneously, the starting polymeric SiO 2TiO 2 cogel is converted into a particulate amorphous material formed by a tight packing of primary particles with sizes around 50 nm. In a subsequent step, the latter are aggregated leading to secondary particles that become independent of the cogel as they reach a critical diameter of 0.5–0.7 μm. Once individualized, the secondary particles undergo a zeolitization process, being gradually transformed into TS-1 crystals of approximately the same size. In addition, the coordination of Ti sites become tetrahedral as the crystallization progresses due to the higher connectivity between Ti and Si atoms existing in the TS-1 network compared to the raw amorphous xerogel.

Ammoximation of Cyclic Ketones on TS-1 and Amorphous SiO2–TiO2

Ammoximation of cyclic ketones on TS-1 and amorphous SiO2–TiO2with H2O2and NH3was studied. The reactivity of cyclicketones over TS-1 and amorphous SiO2–TiO2showed the following order: cycloheptanone>cyclopentanone>cyclohexanone>cyclooctanone. Shape-selective poisoning studies revealed that the active sites of TS-1 for ammoximation of cycloheptanone are mostly inside the molecular sieve. The reactivity of cyclohexanone and methylcyclohexanones over TS-1 followed the order cyclohexanone>2-methylcyclohexanone=3-methylcyclohexanone>2,6-dimethylcyclohexanone, reflecting the difference in the diffusivity among the reactants or products inside TS-1. Besides the diffusivity the reactivity seemed to be affected by the difficulty in access of the carbonyl group to the Ti active site inside the zeolite; from the comparison of reactivity of dimethycyclohexanone isomers, the steric hindrance of the substituent methyl group to the access of carbonyl group was considered to decrease in the following order: β-equatorial>α-equatorial>β-axial>α-axial. Diffusion of cyclohexanone or its oxime has been found to play an important role in the ammoximation when the TS-1 crystals larger than 1.5 μm were employed as a catalyst. For the TS-1 crystals smaller than 0.3 μm, the ammoximation activity was independent of the diffusivity factor and was determined by the number of active Ti sites present in the zeolite framework.

Epoxidation of allyl alcohol to glycidol using titanium silicalite TS-1: effect of the method of preparation

The epoxidation of allyl alcohol with hydrogen peroxide catalysed by the microporous titanium silicalite TS-1 has been investigated with respect to the effect of the method of catalyst preparation. Three methods of TS-1 synthesis have been studied using the standard tetrapropyl-ammonium cation as template (i) using tetraethyl orthosilicate, tetraethyl orthotitanate as reagents, (ii) using a fluoride method and (iii) using tetrabutyl orthotitanate as the titanium source. The TS-1 samples were characterised by electron microscopy, X-ray diffraction and infrared spectroscopy. The method of preparation controlled the morphology of the TS-1 crystals and in particular the crystallisation time was found to be an important parameter. Data are presented that correlate the activity for the epoxidation of allyl alcohol with the morphology of TS-1. In addition it is found that the catalytic activity of TS-1 for this reaction is not related to the intensity or presence of the infrared absorption band at ca. 960 cm−1.

Synthesis and characterization of titanium silicalite TS-1 prepared using phosphonium ions

Titanium silicalites with the MFI structure have been synthesized using binary mixtures of tetrabutylphosphonium hydroxide (TBPOH) and tetraethylphosphonium hydroxide (TEPOH). Whereas the MEL structure was obtained with pure TBPOH, addition of TEPOH led to a modification of the structure, and TS-1, with the MFI structure, was obtained. The crystals had a specific habit, similar to that of standard TS-1 crystals. Their size decreased with the Ti content in the zeolite. A maximum Ti incorporation of about 1.35 wt% could be achieved following this synthesis route. X-ray diffraction, u.v.-vis, and i.r. spectroscopies have shown that the zeolites thus obtained possessed all the structural characteristics of TS-1. 13C and 31P solid-state n.m.r. revealed that both TBP + and TEP + cations were introduced into the structure.