Investigation of Si Atom Migration in the Framework of MSE-Type Zeolite YNU-2

The change in distribution of Si atom defects in the framework of zeolite YNU-2 by steam treatment was investigated using powder X-ray diffraction and solid-state NMR spectroscopy. The precursor of zeolite YNU-2 (abbreviated to YNU-2P) with a three-dimensional pore system has a large number of Si atom defects (more than 10% of all T sites in a unit cell) around the supercage. These defect sites were confirmed by observation of a Q(3) ((-SiO)(3)Si-OH) resonance peak by (29)Si magic angle spinning NMR spectroscopy. We have shown that steam treatment of YNU-2P at 523 K for 24 h significantly decreases the relative intensity ratio of the observed Q(3) resonance peak for the Q(4)((-SiO)(4)Si) peak. The Rietveld analysis of steam-treated YNU-2P (YNU-2P(ST)) shows a marked increase in the site occupancies of the defective Si sites. Furthermore, the Si atom defects in YNU-2P(ST) almost disappeared after calcination, yielding siliceous zeolite YNU-2. These results indicate that the defective framework structure was almost restored by steam treatment. Experimental results suggest that Si atom migration in the framework of YNU-2P takes place during steam treatment. The migrated Si atom fragment fills defect sites and is connected with adjacent silanol groups. In addition, the quantity of the half amount of structure-directing agent molecules was removed from the micropores in YNU-2P(ST) by steam treatment.