Titanosilicates
with extra-large pores or cages are expected to
effectively release the diffusion constraints suffered by the bulky
substrates in the hydrogen peroxide-involved liquid-phase selective
oxidation reactions. A reversible 3D–2D–3D structural
transformation was developed to fabricate a highly active IWV-type
titanosilicate (Ti-IWV) with a two-dimensional intersecting 12-membered
ring (MR) channel system and extra-large 14-MR supercages. The IWV
germanosilicate was readily disassembled into a layered 2D material
(Hydro-IWV) in HNO3 aqueous solution, which was reconstructed
to Ti-IWV with various Ti contents (Si/Ti ratio of 40–∞)
through the (NH4)2TiF6-assisted isomorphous
substitution of Ti and structure repair. The fluoride anions were
critical to recover the interlayer double-four-ring (d4r) units, which were destroyed in the hydrolysis
process. Ti-IWV was extremely active in the liquid-phase epoxidation
reaction of cycloalkenes, especially showing a much higher conversion
(99%) for cyclooctene than conventional titanosilicates. Rather than
diffusion rate, the high capacity for the adsorption of bulky alkene
molecules of extra-large 14-MR cages contributed to the outstanding
activity of Ti-IWV.
Zeolites, a class of crystalline microporous materials, have a wide range of practical applications, in particular serving as key catalysts in petrochemical and fine-chemical processes. Millions of zeolite topologies are theoretically possible. However, to date, only 235 frameworks with various tetrahedral element compositions have been discovered in nature or artificially synthesized, among which approximately 50 topologies are available in pure-silica forms. Germanosilicates are becoming an important zeolite family, with a rapidly increasing number of topological structures having unusual double four-membered ring (D4R) building units and large-pore or extra-large-pore systems. The synthesis of their high-silica analogues with higher (hydro)thermal stability remains a great challenge, because the formation of siliceous D4R units is kinetically and thermodynamically unfavorable in hydrothermal systems. Herein, it is demonstrated that such D4R-containing high-silica zeolites with unexpected crystalline topologies (ECNU-24-RC and IM-20-RC) are readily constructed by a versatile route. This strategy provides new opportunities for the synthesis of high-silica zeolite catalysts that are hardly obtainable by conventional hydrothermal synthesis and may also facilitate a breakthrough in increasing the number and types of zeolite materials with practical applications.
A quasi‐pure CH polymorph of microporous zeolite beta, named ECNU‐36, was obtained as a highly crystalline silicate using 1,5‐bis(tetramethylimidazolium) hydroxide as organic structure‐directing agent (OSDA) in fluoride media. An appropriate concentration of free fluoride in the synthetic mother liquor was crucial to purify the CH‐phase. The framework structure of ECNU‐36 consists of polymorph CH (>95 %) and polymorph B, elucidated by a combination of PXRD data, DIFFaX simulation, EDT, and HRTEM techniques. For the first time, the framework structure of beta CH polymorph was directly confirmed and solved using electron diffraction data. The pure‐silica ECNU‐36 showed an unusual crystal morphology, composed of stacked nanosheets, with typical 17 nm thickness and exposed {100} facets, which exhibited attractive adsorption performance for hydrocarbons and aromatics.
A quasi‐pure CH polymorph of microporous zeolite beta, named ECNU‐36, was obtained as a highly crystalline silicate using 1,5‐bis(tetramethylimidazolium) hydroxide as organic structure‐directing agent (OSDA) in fluoride media. An appropriate concentration of free fluoride in the synthetic mother liquor was crucial to purify the CH‐phase. The framework structure of ECNU‐36 consists of polymorph CH (>95 %) and polymorph B, elucidated by a combination of PXRD data, DIFFaX simulation, EDT, and HRTEM techniques. For the first time, the framework structure of beta CH polymorph was directly confirmed and solved using electron diffraction data. The pure‐silica ECNU‐36 showed an unusual crystal morphology, composed of stacked nanosheets, with typical 17 nm thickness and exposed {100} facets, which exhibited attractive adsorption performance for hydrocarbons and aromatics.
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