Preparation, structure and thermal stability of onium- and amino-functionalized silicas for the use as catalysts supports

We explored and compared several synthetic methods of grafting silica with strong (alkyltriphenylphosphonium, tetralkylammonium, propylpyridinium and dialkylimidazolium) and weak (γ-aminopropyl, γ-(N-imidazolyl)propyl) anion-exchanging groups starting with commercially available chloroalkyl- and γ-aminopropylsilanes. Structure of the intermediate and final materials was investigated by elemental analysis, titration, 13C, 29Si, 31P MAS NMR, DRIFT, and TPD MS. The derivatives of alkyltriphenylphosphonium, propylpyridinium and dialkylimidazolium cation can be prepared with satisfactory quaternisation yields (ca. 30–100%) via the nucleophilic substitution of γ-chloropropyl groups either in the silane or in chloropropylsilica, resulting in bonded phases with moderate densities: 0.2–1.0 group nm−2 (onium salts) and 0.2–1.5 group nm−2 (amines). Parallel one-pot end-capping/hydrophobization can be done if a mixture of target silane with end-capping reagent or γ-chloropropylsilane is used. The grafted layer is highly stable at the level of Si–C bonds and decomposes at ca. 400 °C, while the onium functions begin to decompose at ca. 250 °C, lowering the thermal stability of materials. Thus, anion-exchanging silicas can be envisaged for the use as catalyst supports at moderate temperatures.

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