Extra-Large Pore Titanosilicate Synthesized via Reversible 3D–2D–3D Structural Transformation as Highly Active Catalyst for Cycloalkene Epoxidation

Abstract

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.

Recent Advances in Tetra- (Ti, Sn, Zr, Hf) and Pentavalent (Nb, V, Ta) Metal-Substituted Molecular Sieve Catalysis

“…As a defined structural weakness, these germanium-rich D4R units can be used in 3D−2D−3D transformations of zeolites into different zeolites (via ADOR 26,135 ) or layered materials 24 or even to reconstruct the original structure with a different chemical composition. 136,137 Using this approach, the xgermanosilicate UTL was transformed into OKO zeolite (denoted as IPC-2) by replacing the original D4R units for single four-ring (S4R) units containing Al, Zn, Sn, Zr, V, Fe, Hf, and Ti (Figure 6). 116 The precise location of the heteroelements in the framework positions (e.g., the S4R unit) is questionable, but the procedure does lead to the formation of corresponding acid sites.…”

“…139 The same mechanism was also applied to prepare Ti-IWV from its germanosilicate IWV parent. 137 Layered zeolites combine the advantages of conventional zeolites and mesoporous molecular sieves. On the one hand, their crystalline layers ensure a defined local structure of active sites.…”

“…The presence of germanium-rich D4R units is a key feature of most germanosilicate zeolites (e.g., UTL, IWW, IWV, IWR, and UOV, among others). As a defined structural weakness, these germanium-rich D4R units can be used in 3D–2D–3D transformations of zeolites into different zeolites (via ADOR , ) or layered materials or even to reconstruct the original structure with a different chemical composition. , Using this approach, the xgermanosilicate UTL was transformed into OKO zeolite (denoted as IPC-2) by replacing the original D4R units for single four-ring (S4R) units containing Al, Zn, Sn, Zr, V, Fe, Hf, and Ti (Figure ). …”

Dealumination-controlled strategy mediates Ti–Y zeolite with cooperative active sites for selective oxidations

“…5,13,15 On the one hand, the direct synthesis encounters the drawbacks of being timeconsuming and having intricate synthesis procedures, effecting in limited zeolite topology frameworks, giving limited metal contents in the zeolite framework, occasionally producing large zeolite crystals and/or needing environmentally unfriendly fluoride. 9,10,13,16,17 Hence, the post-synthesis method comes into being as an alternative route in response to the above-mentioned issues, which includes dealumination followed by insertion of the metal atom. 11,13,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Aluminosilicate zeolite, as a starting material, is usually derived from acid treatment with the purpose of enabling the presence of silanol nests to accommodate metal atoms.…”

“…5,13,15 On the one hand, the direct synthesis encounters the drawbacks of being time-consuming and having intricate synthesis procedures, effecting in limited zeolite topology frameworks, giving limited metal contents in the zeolite framework, occasionally producing large zeolite crystals and/or needing environmentally unfriendly fluoride. 9,10,13,16,17…”