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 Å) 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' locations (g₁ = 1.876, g₂ = 1.943 and g₃ = 1.970), were resistant to reoxidation while those in type I'' locations (g_||= 1.899 and g_⊥ = 1.961) readily oxidized and formed Zr^IV(O₂) species. Interaction with H₂O₂ or γ-ray irradiation also produced the superoxo radical species. Irradiation resulted in two types of superoxo radical species, type II' ions associated with surface Zr centers (g₁ = 2.0319, g₂ = 2.0093, and g₃ = 2.0024) and type II" ions associated with defect centers (g₁ = 2.0131, g₂ = 2.0098, and g₃ = 2.0045). The ESR results of Zr(III) ions are compared with those of the isoelectronic Ti(III) centers in silicalite structures.