Synthesis, Crystallization Mechanism, and Catalytic Properties of Titanium-Rich TS-1 Free of Extraframework Titanium Species

Abstract

A new route to the synthesis of TS-1 has been developed using (NH4)2CO3 as a crystallization-mediating agent. In this way, the framework Ti content can be significantly increased without forming extraframework Ti species. The prepared catalyst had a Si/Ti ratio as low as 34 in contrast to the ratio of 58 achieved with the methods A and B established by the Enichem group (Clerici, M. G.; Bellussi, G.; Romano, U. J. Catal. 1991, 129, 159) and Thangaraj and Sivasanker (Thangaraj, A.; Sivasanker, S. J. Chem. Soc., Chem. Commun. 1992, 123), respectively. The material contained less defect sites than the samples synthesized by the other two methods. As a result, it showed much higher activity for the oxidation of various organic substrates, such as linear alkanes/alkenes and alcohols, styrene, and benzene. The crystallization mechanism of TS-1 in the presence of (NH4)2CO3 was studied by following the whole crystallization process with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermogravimetry/differential thermal analysis (TG/DTA), inductively coupled plasma atomic emission spectrometry (ICP), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-vis spectroscopy, and (29)Si MAS (magic-angle spinning) NMR spectroscopy techniques. It was shown that the presence of (NH4)2CO3 not only drastically lowered down pH, slowing down the crystallization process and making the incorporation of Ti into the framework match well with nucleation and crystal growth, but also modified the crystallization mechanism. It seems that the solid-phase transformation mechanism predominated in the crystallization process initiated by dissociation, reorganization, and recoalescence of the solidified gel although a small amount of nongelatinated Ti shifted to the solid during the crystal growth period. In contrast, a typical homogeneous nucleation mechanism occurred in the method A system. Thus, although in the method A system most of Ti cations was inserted into the lattice after the crystallization was nearly completed, the inclusion of Ti started at the earlier nucleation period in the presence of (NH4)2CO3. This is favorable for the incorporation of Ti into the framework, resulting in a more homogeneous distribution of Ti in the framework. Oxidation of 1-hexene and 2-hexanol over the samples collected during the whole crystallization process indicated that condensation of Ti-OH and Si-OH proceeded even after the crystallization was completed. This resulted in an increase in hydrophobicity and an overall improvement in microscopic character of Ti species and consequently a great increase in the catalytic activity with further progress of crystallization.

UV Raman Spectroscopic Characterization of Catalysts and Catalytic Active Sites

“…The catalytic performance of these transition metal-containing materials depends largely on their composition and structure. A deep understanding of the active sites and the formation mechanism in these materials is helpful for the development of catalysts with high selectivity and activity [36,37]. However, due to the low concentration of transition metal ions (\2 %), it is difficult to obtain definite information on the transition metal ions in these materials [31], such as their coordination environments and distributions.…”

“…As a highly active and selective catalyst, TS-1 has been widely used in a number of oxidation reactions with H 2 O 2 as oxidant under mild conditions [32,33,37]. However, there are still many issues under debate, such as what's the exact nature of the active site [36,37,68,69] and the reaction pathways [70][71][72][73].…”

“…However, there are still many issues under debate, such as what's the exact nature of the active site [36,37,68,69] and the reaction pathways [70][71][72][73]. Recently, we have conducted a thorough investigation of the active titanium species in TS-1 zeolite [74].…”

Metal-catalyzed Hydrogen Peroxide Oxidation for Synthesis of Value-added Chemicals

“…TS-1 was chosen as a standard titanosilicate zeolite catalyst with an MFI-type structure containing three-dimensional 10-membered ring (3D 10-MR) pores 22) . Ti-MWW was prepared as an MWW-type structure containing two types of ring: 12-MR pores with a system of independent 10-MR channels 23) .…”

Direct Synthesis of H2O2over Ti-Containing Molecular Sieves Supported Gold Catalysts: A Comparative Study for In-situ-H2O2-ODS of Fuel

“…Herein, two lines of available evidence can be presented to explain the different behavior between Au/Ti-HMS and Au/TS-1. For one thing, it has been reported that TS-1 is a type of hydrophobic material, 30 which exhibits an obvious wettability and adsorbability for methanol over water. Therefore, in methanol/water solvent or pure methanol, a number of the active centers on Au/TS-1 would be covered by the adsorbed methanol molecule, resulting in the decrease of catalytic performance.…”