Electron Spin Resonance Investigations on the Location and Reducibility of Zirconium in Mesoporous Zr−MCM-41 Molecular Sieves

Abstract

Zirconium containing mesoporous MCM-41 molecular sieves with Si/Zr ratios of 96 (A), 55 (B), 39 (C), and 23 (D) were synthesized and characterized by X-ray diffraction (XRD), surface area and pore size distribution measurements, and FTIR, diffuse reflectance UV−visible (DRUV−vis), 29Si MAS NMR, and electron spin resonance (ESR) spectroscopic techniques. The XRD patterns indicated an increase in the d100 spacing (from 36.77 to 40.88 A) with increasing Zr content. The samples showed a characteristic oxygen to zirconium charge-transfer band at 210 nm in the DRUV−vis spectra, consistent with monatomic dispersion of zirconium in Zr−MCM-41. Reduction of zirconium from the +4 to the +3 state was achieved by reaction with LiAlH4 at 298 K or with dry hydrogen at high temperatures. In samples A and B, Zr is located mostly in the pore walls, in substitutional locations (type I‘) and in C and D, it is present at the surface of the pores (type I‘ ‘) in addition to the substitutional locations. Zr(III) ions, in type I‘...