Chiral molecular fluoridosilicates and their twin polymerization for the preparation of fluorine-doped mesoporous silica and microporous carbon

Abstract

The synthesis of organic–inorganic hybrid materials, microporous carbon and fluorine-doped mesoporous silica by the twin polymerization of pentacoordinated fluoridosilicates is reported.

Twin Polymerization: A Unique Strategy towards Versatile Functional Materials

“…This will have direct influence on material properties including hydrophobicity, refractive index and elastic modulus. [108] In summary, the fluoride-initiated TP of spirocyclic silicon compounds 1 and 22 behaves differently as compared to common acid-and base-catalyzed TP (see earlier). [108] It should be noted that the added fluoride assists as a reactant rather than as a catalyst.…”

“…Consecutive calcination of the respective hybrid materials gave fluorine-doped silica with surface areas of up to 239 m 2 g À 1 . [108] Furthermore, within the studies it could be shown that also hybrid materials were accessible by applying the additive assisted TP of twin monomer 1 with [Bu 4 N]F or KF producing microporous carbon with a BET surface area of 1219 m 2 g À 1 and mesoporous silica with a surface area of 687 m 2 g À 1 . Oxidation of the potassium-Scheme 12.…”

“…Access possibility to fluorine-doped mesoporous silica and microporous carbon is given by TP of chiral molecular fluoridosilicates. [108] In this respect, twin monomers 1 and 8,8'di-tert-butyl-4H,4H'-2,2'-spirobi[benzo-4H-1,3,2-dioxasiline] ( 22) gave the appropriate fluoride species [(1,4,7,10,13,16-hexaoxacyclooctadecane-k 6 O 1,4,7,10, 13,16 )potassium] fluoridodi[2-(oxidomethyl)phenolate(2-)]silicate (23) or [(1,4,7,10,13,16-hexaoxacyclooctadecane-k 6 O 1,4,7,10, 13,16 )potassium] fluoridodi[6-tertbutyl-2-(oxidomethyl)phenolate(2-)]silicate (24), when potassium fluoride was added to 1 or 22 in presence of 18-crown-6. [108] It was found that the addition of fluoride lowers the activation barrier for the TP process and fluoridosilicates as isolated in this study represent intermediates in this process.…”

“…The penta-coordinated geometry at Si was confirmed by single crystal structure determination. [108] The formation of fluorine-doped phenolic resin-silica materials was proven by solid-state 1 H, 13 C{ 1 H}, 19 F and 29 Si{ 1 H} NMR spectroscopy. Carbonization of the as-obtained hybrid materials based on the highly cross-linked phenolic resins at 800 °C gave microporous carbon-silica hybrid materials and following treatment with HF resulted in the formation of microporous carbon (BET surface areas up to 1060 m 2 g À 1 ).…”

“…The authors also discussed within their studies that replacement of potassium fluoride by [Bu 4 N]F or [(Me 2 N) 3 S][F 2 SiMe 3 ] paves the way to materials with higher surface areas, while potassium silicate formation is prevented. [108] Furthermore, it was shown that preparation of hybrid materials with varying fluorine contents is possible by using different ratios of twin monomers of 1 and 23 in the polymerization process and converting these to F-doped silica materials (F : Si ratio 1 : 1.3 to 1 : 8.2 in hybrid materials and 1 : 7.5 to 1 : 281.9 in silica materials). This will have direct influence on material properties including hydrophobicity, refractive index and elastic modulus.…”