Abstract
Ti-containing zeolites showed extremely high activity and selectivity in numerous friendly environmental oxidation reactions with hydrogen peroxide as a green oxidant. It will be in high demand to synthesize Ti-containing crystalline extra-large-pore zeolites due to the severe restrictions of medium-pore and/or large-pore zeolites for bulky reactant oxidations. However, the direct synthesis of extra-large-pore Ti-zeolites was still challengeable. Here, we firstly report a strategy to directly synthesize high-performance Ti-containing CFI-type extra-large-pore (Ti-CFI) zeolites assisted with fluorides. The well-crystallized Ti-CFI zeolites with framework titanium species could be synthesized in the hydrofluoric acid system with seed or in the ammonium fluoride system without seed, which showed higher catalytic activity for cyclohexene oxidation than that synthesized from the traditional LiOH system.
Highlights
Zeolite, a type of crystalline microporous aluminosilicate, has wide applications in adsorption, separation, and catalysis, especially in oil refining and producing petrochemicals as solid-acid catalysts [1,2,3,4,5,6]
The broad peak around 20–25◦ implied the presence of amorphous raw materials in this sample. This indicated that well-crystallized Ti-CFI was difficult to achieve when hydrofluoric acid was used as a fluoride source, pure silica CFI-type zeolite could be fully crystallized in the HF system (Figure S1, Supplementary Materials)
This was evidence that the synthesis of titanium-containing zeolite was more difficult than synthesis of Ti-free pure silica zeolite
Summary
A type of crystalline microporous aluminosilicate, has wide applications in adsorption, separation, and catalysis, especially in oil refining and producing petrochemicals as solid-acid catalysts [1,2,3,4,5,6]. A series of Ti-containing zeolites with different topologies were synthesized including 10-ring medium-micropore (~0.55 nm in diameter) [12,13,14] and 12-ring large-micropore (~0.75 nm in diameter) zeolites [15,16,17,18,19,20,21]. Considering the severe restrictions for the oxidation of bulky reactants in these 10-ring and 12-ring micropores, it would be in high demand to synthesize. Ti-containing three-dimensionally crystalline extra-large-pore (larger than 12 rings) zeolites. It was very difficult to directly crystallize the starting gel (containing silicon source, titanium source, and structure-directing agents) to form Ti-containing extra-large-pore zeolites because the titanium ion proved no structure-directing ability and its ionic radius was larger than that of the silicon ion, which led to difficulties in the crystallization of raw materials and the incorporation of Catalysts 2019, 9, 257; doi:10.3390/catal9030257 www.mdpi.com/journal/catalysts
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