Redox behavior and selective oxidation properties of mesoporous titano- and zirconosilicate MCM-41 molecular sieves

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Abstract

Mesoporous titano- and zirconosilicate molecular sieves, Ti-MCM-41 and Zr-MCM-41, respectively, with Si/M ratios in the range from 11 to 96 (M=Ti or Zr), have been synthesized by the hydrothermal method and characterized by XRD, XRF, N2 adsorption and diffusive reflectance UV–Vis (DRUV–Vis), FT-IR and electron spin resonance (ESR) spectroscopic techniques. The redox behavior and selective oxidation properties of these materials have been investigated. ESR of samples reduced with LiAlH4 (298 K) and H2 (673–873 K) reveals two types of metal ion species: species I located inside the pore walls and species I′′ located at the pore surface. The reduced species I′′ are highly reactive towards oxygen and form M(O2−·) radicals. The M(O2−·) radicals were also observed when the samples were reacted with aqueous H2O2 or tert-butylhydroperoxide (TBHP). ESR studies reveal that Ti-MCM-41 is easier to reduce and reoxidize than Zr-MCM-41. The DRUV–Vis spectra are consistent with a monoatomic dispersion of the metal ions. In the samples with high metal loading the presence of a nanocrystalline metal oxide phase cannot be ruled out. Both Ti-MCM-41 and Zr-MCM-41 catalyze the hydroxylation of 1-naphthol with aqueous H2O2 and the epoxidation of norbornylene with TBHP.

Introduction

Metallosilicate molecular sieves, obtained by isomorphous substitution of Si by metal ions in silicate structures, exhibit catalytic activity in selective oxidation reactions using hydrogen peroxide and alkylhydroperoxides at mild conditions. Among the microporous metallosilicates, titanium-substituted silicalite-1 (TS-1; MFI topology; pore diameter∼5.5 Å) is unique in catalyzing a variety of selective oxidation reactions [1], [2]. In recent years, attention has increasingly been directed towards the study of metal-containing mesoporous M41S type molecular sieves with large pores (20–100 Å diameter) suitable for the transformation of bulky organic compounds [3], [4], [5], [6], [7], [8]. Metal ions such as Al [9], [10], [11], [12], Ti [13], [14], Fe [15], Mn [16], V [17], [18], and Ga [19], [20], [21] have been successfully incorporated into the MCM-41 framework. The catalytic activity of these modified MCM-41 materials has been examined in the selective oxidation of substrates such as hex-1-ene, α-terpineol, norbornylene and 2,6-di-tert-butylphenol. Titanium- and zirconium-containing MCM-41 materials have been reported to be useful as catalysts and catalyst supports [22], [23], [24], [25]. Kevan and co-workers [26], [27], [28] reported on the electron spin resonance (ESR) spectroscopic characterization of Ti-MCM-41 samples irradiated with γ-rays. Recently, we have reported detailed spectroscopic investigations on the incorporation of Sn(IV) [29] and Zr(IV) [30] ions in MCM-41. The present study deals with the redox behavior and catalytic activity of mesoporous titano- and zirconosilicate molecular sieves (Ti-MCM-41 and Zr-MCM-41) with varying metal contents (Si/M=11–96) prepared by hydrothermal methods. Characterization of the samples by XRD, XRF, N2 adsorption, FT-IR, diffuse reflectance UV–Visible (DRUV–Vis) and ESR prove the substitution of metal ions in the framework of the MCM-41 structure.

Section snippets

Materials

The MCM-41 samples were synthesized hydrothermally, using fumed silica (99%, Sigma), tetramethylammonium silicate (TMA silicate; 10 wt.% silica solution, TMA/SiO2=0.5; Sachem Inc., USA), cetyltrimethylammonium chloride/hydroxide (CTMACl/OH; 17.9 wt.% Cl and 6.7 wt.% OH) prepared in the laboratory by partial exchange of CTMACl (25 wt.% aqueous solution, Aldrich) over an ion exchange resin, TMA hydroxide (TMAOH; 99%, Aldrich) and titanium butoxide and zirconium butoxide (80 wt.% solution in

XRD

The XRD patterns of the as-synthesized Ti-MCM-41 and Zr-MCM-41 samples resembled those of Si-MCM-41 [3], [4]. The long-range order in hexagonal symmetry was retained in the calcined forms of silicious and metal-incorporated MCM-41 samples. A typical XRD profile of calcined Ti-MCM-41 is shown in Fig. 1, and the d100 values for all the samples are listed in Table 1. The XRD peaks shift to higher d-values with increasing metal content consistent with a probable incorporation of Ti and Zr into the

Conclusions

ESR studies on reduced samples of Ti- and Zr-MCM-41 reveal the presence of metal ions located inside the pore walls (species I) and at the surface of the pores (species I′′). The reduced ions, especially species I′′ are highly reactive towards aerial oxygen and form M(O2−·) species, an active intermediate invoked in the oxidation reactions involving metallosilicates. The Ti samples are easier to reduce than the Zr samples. Both Ti- and Zr-MCM-41 are catalytically active in

Acknowledgements

KC and RB acknowledge CSIR, New Delhi, for financial support in the form of senior research fellowships.

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