Abstract
Effects of iron content on the structure and physical properties of Fe silicates were investigated by ESR and solid state NMR. Fe silicates having pentasil-type crystalline structure were synthesized by the rapid crystallization method.
The acid amount of Fe silicate increased with increase of iron content in Fe silicate. The rate of increase of the acid amount, however, decreased with increase of the Fe content.
It was found, by the peak at g=4 in the ESR spectra, that some iron atoms in the Fe silicate coordinated with the SiO4 tetrahedron. The iron in zeolitic framework increased with increase of iron content in the Fe silicate.
The intensity of Fe peak in the ESR spectra flattened at about 2 wt% of Fe content. This trend was also found with the acid amount. It was clear from these results that silicon atoms in the zeolitic framework were substituted for some iron atoms in the Fe silicate and the rate of iron substitution decreased with increase of iron content.
Two peaks at -112 and -103ppm were observed in the 29Si-NMR spectra of Na-Fe silicates. The peak at -103ppm disappeared or decreased in intensity in H-Fe silicates, although it was already confirmed from ESR that iron atoms in H-Fe silicate coordinated with the SiO4 tetrahedron. The two peaks at -112 and -103 ppm, therefore, were attributed to Si(0Fe) and -SiO-M+(M+=TPA+ or Na+), respectively. As -SiO-M+ sites were converted into -SiOH groups or SiOSi, the peak at -103ppm could no longer be detected in the spectrum of H-Fe silicate.
Spin-lattice relaxation times(T1) of 29Si were measured. T1 decreased from 700msec to 30msec with increase of iron content, which were extremely short as compared with T1 of ZSM-5. As close correlation was observed between 1/T1 and Fe content in Fe silicate, Fe in Fe silicate was markedly effective in shortening the T1.
