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

Abstract 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, 13 C, 29 Si, 31 P 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.