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.
Selective and durable fixed-bed catalysts are highly desirable for developing eco-efficient HPPO (hydrogen peroxide propylene oxide) process. The powder titanosilicate catalysts must be shaped before being applied in industrial processes. As the essential additives for preparing formed catalysts, binders are usually the catalytically inert components, but they would cover the surface and pore mouth of zeolite, thereby declining the accessibility of active sites. By recrystallizing the binder (silica)/Ti-MWW extrudates with the assistance of dual organic structure-directing agents, the silica binder was converted into MWW zeolite phase to form a structured binder-free Ti-MWW zeolite with Si-rich shell, which enhanced the diffusion efficiency and maintained the mechanical strength. Meanwhile, due to the partial dissolution of Si in the Ti-MWW matrix, abundant silanol nests formed and part of framework TiO4 species were transferred into open TiO6 ones, improving the accumulation and activation ability of H2O2 inside the monolith. Successive piperidine treatment and fluoridation of the binder-free Ti-MWW further enhanced the H2O2 activation and oxygen transfer ability of the active Ti sites, and stabilized the Ti-OOH intermediate through hydrogen bond formed between the end H in Ti-OOH and the adjacent Si-F species, thus achieving a more efficient epoxidation process. Additionally, the side reaction of PO hydrolysis was inhibited because the modification effectively quenched numerous Si-OH groups. The lifetime of the modified binder-free Ti-MWW catalyst was 2400 h with the H2O2 conversion and PO selectivity both above 99.5%.
The epoxidation of allyl chloride (ALC) to epichlorohydrin (ECH) with H2O2 using piperidine (PI) modified Ti-MWW catalyst (Ti-MWW-PI) in a continuous slurry reactor was investigated to develop an efficient reaction...
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