Topotactic Conversion of Alkali‐Treated Intergrown Germanosilicate CIT‐13 into Single‐Crystalline ECNU‐21 Zeolite as Shape‐Selective Catalyst for Ethylene Oxide Hydration

Abstract

The conversion of the alkali‐treated intergrowth germanosilicate CIT‐13 into the single‐crystalline high‐silica ECNU‐21 (named after East China Normal University) zeolite, with a novel topology and a highly crystalline zeolite framework, has been realized through a creative top‐down strategy involving a mild alkaline‐induced multistep process consisting of structural degradation and reconstruction. Instead of acid treatment, hydrolysis in aqueous ammonia solution not only readily cleaved the chemically weak Ge(Si)−O−Ge bonds located within the interlayer double four ring (D4R) units of CIT‐13, but also cleaved the metastable Si−O−Si bonds therein. This led to extensive removal of the D4R units, and also generated silanol groups on adjacent silica‐rich layers, which then condensed to form a novel daughter structure upon calcination. Individual oxygen bridges in the reassembled ECNU‐21 replaced the germanium‐rich D4R units in CIT‐13, thereby eliminating the original intergrowth phenomenon along the b axis. With an ordered crystalline structure of 10‐ring (R) channels as well as suitable germanium‐related Lewis acid sites, ECNU‐21 serves as a stable solid Lewis acid catalyst for the shape‐selective hydration of ethylene oxide (EO) to ethylene glycol (EG) at greatly reduced H2O/EO ratios and reaction temperature in comparison with the noncatalytic industrial process.

A Cationic Oligomer as an Organic Template for Direct Synthesis of Aluminosilicate ITH Zeolite

“…As an important type of zeolites, germanosilicatebased zeolites provide many new structures, exhibiting excellent performance in various catalytic reactions. [23][24][25][26][27][28][29][30][31][32][33][34] For example, BEC zeolite displays good catalytic performance in the isomerization of glucose to fructose; [24] ECNU-21 zeolite is an efficient shape selective catalyst for ethylene oxide hydration; [25] ITH zeolite shows excellent performance in the catalytic cracking and methanol-to-propylene (MTP) reaction. [26][27][28][29] ITH zeolite, firstly discovered by Cormas group, [30] has a special framework structure (three dimensional 9 10 10membered ring channels; aperture size of 4.0 4.8, 4.8 5.1, and 4.8 5.3 ).…”

Synthesis and Post‐Synthesis Transformation of Germanosilicate Zeolites

“…UTL)a nd its derivative, which contain only oxygen bridges as interlayer connecting units (PCR,F igure 1). [62] Other techniques,s uch as "inverse sigma transformation" [31] or "topotactic conversion", [63] recently applied to germanosilicates,a re based on similar principles and individual steps.…”

New Trends in Layered Zeolites

“…More detailed characterization indicated that the presence of Si▬O▬Si bonds vertical to the Si-rich layers may prohibit the successful selective removal of D4R subunits [64]. Wu et al reported a mild alkaline treatment to hydrolyze both Ge▬O▬Ge(Si) and Si▬O▬Si bonds in the D4R subunits, giving a layered intermediate ECNU-21P (Figure 5), which was transformed to 3D ECNU-21 zeolite upon calcination [26]. In addition, the selective removal of D4R subunits from CIT-13 zeolite eliminates the intergrowth phenomena and resulted in a single crystalline zeolite framework.…”