Xinwen Guo

A high‐resolution solid‐state NMR study on nano‐structured HZSM‐5 zeolite

Variations in the structure and acidity properties of HZSM‐5 zeolites with reduction in crystal sizes down to nanoscale (less than 100 nm) have been investigated by XRD, TEM and solid‐state NMR with a system capable of in situ sample pretreatment. As evidenced by a combination of 27Al MAS NMR, 29Si MAS, CP/MAS NMR and 1H MAS NMR techniques, the downsize of the zeolite crystal leads to an obvious line broadening of the 27Al, 29Si MAS NMR spectrum, an increasing of the silanol concentration on the external surface, and a pronounced alteration of the acidity distribution between the external and internal surfaces of the zeolite. In a HZSM‐5 zeolite with an average size at about 70 nm, the nonacidic hydroxyl groups (silanols) are about 14% with respect to the total amount of Si, while only 4% of such hydroxyl groups exist in the same kind of zeolite at 1000 nm crystal size. The result of 1H MAS NMR obtained using Fluorinert® FC‐43 (perfluorotributyl amine) as a probe molecule demonstrates that most of the silanols are located on the external surface of the zeolite. Moreover, the concentration of Brønsted acid sites on the external surface of the nano‐structured zeolite appears to be distinctly higher than that of the microsized zeolite.

Titanium species in titanium silicalite TS-1 prepared by hydrothermal method

The titanium species existing in titanium silicalite TS-1, which is prepared by hydrothermal method, were investigated using chemical analysis, XRD, FT-IR, Si-29 MAS NMR, UV-VIS, ESR. It has been observed that several kinds of titanium species may exist in titanium silicalite. The form that titanium atoms incorporate into the framework of titanium silicalite synthesized using tetrapropylammonium bromide (TPABr) as template differs from that using the classical method. But, the symmetry of titanium silicalite, changes from monoclinic to orthorhombic with the increase of titanium content in both methods. The Ti-O-2(-) originated from framework titanium and H2O2 has the moderate stability and may be active site in oxidation reaction. TS-1 synthesized using TPABr as template does not contain anatase, but contains a kind of partly condensed titanium species with six-fold coordination. The titanium species may correspond to 270-280 mn band in UV-VIS spectra and also can form Ti-O-2(-). But, this kind of Ti-O-2(-) is very stable and cannot be catalytic active site. So, the six-fold coordination titanium species may be inactive in both the oxidation reaction and the decomposition of H2O2. The hypothesis has been further proved by the phenomena that the titanium species is easily washed off using acid, and acid treating will not influence the catalytic performance of TS-1

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.

Preparation and characterization of titanium-containing MFI from highly siliceous ZSM-5: effect of precursors synthesized with different templates

Titanium-containing MFI zeolites (Ti-ZSM-5) have been prepared by gas-solid reaction between highly siliceous ZSM-5 precursors synthesized with different organic molecules, and TiCL4 vapor at elevated temperature. XRD, FT-IR, UV-Vis, Si-29 and Al-27 solid NMR techniques are used to characterize precursors and Ti-ZSM-5 zeolites. Highly siliceous ZSM-5 synthesized with different templates possesses different amount of internal silanol groups clustered as silanol nests in framework. When the precursor is contacted with TiCl4 vapor at elevated temperature, titanium atoms are incorporated into the tetrahedral sites by the reaction between TiCl4 and silanol nests, not by the isomorphous substitution of titanium for framework aluminium or/and silicon. Highly siliceous ZSM-5 precursor synthesized with NEA, TEABr or HDA template possesses relatively large amount of internal silanol groups, and then obtains high concentrations of framework titanium after contacted with TiCl4 vapor, compared to those synthesized with TPAOH or TPABr template. In addition, Ti-ZSM-5 with precursors obtained by using NEA, TEABr or HDA template can occupy higher catalytic activity in the epoxidation of propylene with diluted hydrogen peroxide

Preparation of titanium-containing zeolites with MEL structure from B-ZSM-11 and their characterization

Titanium-containing MEL zeolites (Ti-ZSM-11) are prepared by gas-solid reactions between TiCl4 vapor and B-ZSM-11 precursors. XRD, FTIR, UV-Vis, B-11 and Si-29 MAS NMR techniques are used to characterize these materials. The results show that Ti atoms are incorporated into the silanol nests that were previously occupied by B atoms. The precursor deboronated by acid reflux possesses a large amount silanol nests and so can obtain a high concentration of titanium in the framework, compared to the directly calcinated precursors. Ti-ZSM-11 zeolites are used to catalyze the epoxidation of propylene with dilute hydrogen peroxide. The results show that the catalytic activities are related to the physicochemical properties of these materials