Characterization of Titanium Species Incorporated into Dealuminated Mordenites by Means of IR Spectroscopy and 18O-Exchange Technique

Abstract

Titanium mordenite, Ti-M, was prepared by the solid−gas reaction (atom-planting method) between highly dealuminated mordenites and TiCl4 vapor at elevated temperatures (473−873 K), and the coordination state of the incorporated Ti was studied comparing with that in titanosilicate with MFI structure (TS-1). The amount of Ti incorporated leveled off at 1.7 kPa of TiCl4 vapor for a process time of 1 h, and only trace amount of Al and Si was released during the treatment. IR spectra confirmed that TiCl4 reacted with internal SiOH groups in hydroxyl nests. The concentration of oxygen atoms bound to the framework Ti atoms in both TS-1 and Ti-M zeolites was measured by the 18O-exchange reaction between C18O2 and the zeolite. The number of oxygen atoms bound to a Ti atom was ca. 4 for TS-1 zeolites and varied from 5 to 4 for Ti-M zeolites with increasing temperature of the TiCl4 treatment. From these results, a reaction mechanism was proposed for the reaction of TiCl4 with hydroxyl nests in highly siliceous mordenites. Ti-M exhibited an IR band at 963 cm-1, while it was observed at 960 cm-1 for TS-1. The relationship between the relative intensity of the 963 cm-1 band and the Ti content also depended on the treatment temperature. The 18O exchange with C18O2 shifted the 963 cm-1 band for Ti-M to 928 cm-1. On the basis of the 18O exchange and IR measurements, we assign the characteristic IR band around 960 cm-1 for titanium zeolites to the stretching vibration of Si−O−Ti-bonds.