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 TiCl 4 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 TiCl 4 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 TiCl 4 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 18 O-exchange reaction between C 18 O 2 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 TiCl 4 treatment. From these results, a reaction mechanism was proposed for the reaction of TiCl 4 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 18 O exchange with C 18 O 2 shifted the 963 cm -1 band for Ti-M to 928 cm -1 . On the basis of the 18 O 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.