Structural stability of Si-C bonds in periodic mesoporous thiophene-silicas prepared under acidic conditions

Periodic mesoporous thiophene-silicas with hexagonal (p6mm) symmetry were synthesized using a 2,5-bis(triethoxysilyl)thiophene (BTET) precursor in the presence of Pluronic P123 (EO₂₀PO₇₀EO₂₀) and PLGE (EO₁₇(L₂₈G₇)EO₁₇) triblock copolymers at different acidic conditions. P123-templated mesoporous thiophene-silicas with p6mm ordered structure were prepared in the presence of hydrochloric acid and iron(III) chloride hexahydrate used as acid catalysts. However, it was found that a relatively large fraction of the Si-C bonds in thiophene-bridging groups were decomposed during the synthesis process. On the other hand, thiophene-silicas synthesized at lower acidic conditions were disordered and nonporous structures. In contrast, PLGE-templated thiophene-silicas with p6mm ordered mesostructure were prepared using copper(II) perchlorate hexahydrate and boric acid as well as hydrochloric acid. Importantly, up to 97.3% of the Si-C bonds in mesoporous thiophene-silica prepared in the presence of boric acid were retained. Solid state ²⁹Si MAS NMR clearly showed that the structural stability of the Si-C bond is dependent on the acidity and time of the initial self-assembly stage. Also, the thermal stability of the thiophene-bridging groups was shown to be dependent on the acidity of the synthesis gel.