The 29Si T1 and T2 NMR relaxation in porous paramagnetic material SiO2–MnO–Al2O3

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Abstract

The 29Si spin–lattice relaxation in porous silica-based material 1, doped by ions Mn2+ at a Si/Mn ratio of 3.5, is non-exponential, independent of magic-angle spinning (MAS) rates and governed by direct dipolar coupling between electron and nucleus where an electron relaxation time is estimated to be about 10−8 s. In the absence of mutual energy-conserving spin flips (spin diffusion) in 1, the 29Si T2 time increases linearly with spinning rates. None was observed in diamagnetic porous system 2. The unexpected 29Si T2 dependence has been interpreted in terms of the large bulk magnetic susceptibility (BMS) effects. It has been shown that editing the 29Si Hahn-echo MAS NMR spectra eliminates wide lines, belonging to 29Si nuclei in the proximity of paramagnetic centers, and reduces the BMS broadenings in sideband patterns for nuclei remote from these centers.

Introduction

Porous materials constantly attract the attention of chemists due to their industrial applications as catalysts, ion exchangers, etc. Generally, these materials are doped with diamagnetic transition metal ions distributed in the matrix or/and cavities and their distributions can be successfully determined by the multinuclear solid-state magic-angle spinning (MAS) NMR techniques [1], [2]. In contrast, NMR applications for materials, doped by manganese, copper, nickel and other paramagnetic ions, are difficult because of paramagnetic effects, which are especially strong at high metal-ion contents. NMR relaxation is one of the promising approaches to such studies [3], [4], [5]. However, interpretations of relaxation data in structural terms require detailed knowledge of relaxation mechanisms that depend on the nature of nuclei and their natural abundance, the nature of paramagnetic ions and their concentrations, the nature of chemical interactions between paramagnetic ions and structural elements of materials and molecular mobility in solids [6]. This paper reports on the T1 and T2 29Si NMR relaxation times measured in static and spinning samples of porous silica-based material SiO2–MnO–Al2O3 (1) obtained by the sol/gel method at a Si/Mn2+ ratio of 3.5. Diamagnetic material SiO2–Al2O3 (2), prepared by the same method in the absence of ions Mn2+, has been studied for comparison. These systems are typical representatives of new supermicroporous materials, described by Clearfield and Shpeizer, and their structural features and porosity characteristics can be found, for example, in Ref. [7]. Ions Mn2+ in such systems are accumulated mainly in the pores, as clusters Mn–O–Mn [8], and only 2–4% of ions Mn2+ can be incorporated into the silica matrix [5].

Section snippets

Experimental

The room-temperature 29Si NMR experiments were performed with a Bruker Avance-400 spectrometer equipped with a standard 4 mm MAS probe head. The 29Si Hahn-echo MAS NMR spectra were obtained at echo-delays synchronized with rotor periods where FIDs were collected immediately after 180° pulses. Spinning rates were controlled with an accuracy of 1–2 Hz. The carrier frequencies in the Hahn-echo [9] were varied between −1200 and +1200 ppm relatively to TMS at a 400 ppm steps. The NMR probe was tuned

Results and discussion

The 29Si Hahn-echo NMR spectrum of static sample 1 shows a resonance with a line broadening of about 40 kHz in agreement with the high Mn2+ content. In the 29Si Hahn-echo MAS NMR spectra, collected at various carrier frequencies, this resonance transforms to a combination of a very broad line and an isotropic resonance with δ around −110 ppm, accompanied by an intense sideband pattern (Fig. 1) typical of paramagnetic systems [11], [12]. The latter has been assigned to 29Si nuclei in the silica

Conclusions

Porous silica-based paramagnetic material 1, doped with ions Mn2+ at a Si/Mn ratio of 3.5, has been studied by solid-state NMR as static and spinning samples. The 29Si Hahn-echo MAS NMR spectra recorded with synchronized spinning rates and at different carrier frequencies, exhibit a very broad line and an isotropic resonance at −110 ppm accompanied by an intense sideband pattern. The sideband pattern belongs to 29Si nuclei of the silica matrix, which are remote from paramagnetic centers. The

Acknowledgments

VIB is grateful to Prof. A. Clearfield and Dr. B. Shpeizer (Texas A&M University) for the synthesis of materials 1 and 2.

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