Development of novel catalytic materials based on fine structure analysis of novel multi-dimensional, large-pore zeolite

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High-silica MSE-type zeolite YNU-2 has a three-dimensional micropore structure composed of 12×10×10-membered ring (MR). It has been synthesized by dry-gel conversion, using N,N,N',N'-tetraethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium cation (TEBOP^<2+>) as an organic structure-directing agent (SDA). The precursor YNU-2 (i.e. YNU-2P) includes a large number of Si atom defects (over 10% of all the Si atoms in a unit cell) in the framework. SDA molecules are located in a 12-MR straight channel and the supercage. Although the MSE topology consists of eight distinct T-sites, four probable site defects (T1, T3, T6, and T7 positions) in the framework were identified by structure analysis. Due to the significant site defects, the YNU-2P structure readily collapse upon calcination. We overcame this problem by vapor-phase silylation using tetramethoxysilane (TMOS) under acidic conditions. Afterwards, it was found that only the steam treatment without acid also helped stabilize the framework. Several findings suggest that the steam treatment causes Si-atom migration and condensation between silanols of site defects and Si atom fragments. We investigated the effect of steam treatment on the framework stability of YNU-2P. The corresponding structural changes were also investigated using powder XRD, solid-state NMR, TG-DTA, and nitrogen gas adsorption experiments. As a result, it was con- firmed that the site defects in microporous YNU-2P were partially restored by steam treatment at 250℃ for 24 h without any additional Si source. This may afford new insights into the understanding of structural stability of high-silica zeolites.