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 (gamma-aminopropyl, gamma-(N-imidazolyl)propyl) anion-exchanging groups starting with commercially available chloroalkyl- and gamma-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 gamma-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 gamma-chloropropylsilane is used. The grafted layer is highly stable at the level of Si-C bonds and decomposes at ca. 400 degrees C, while the onium functions begin to decompose at ca. 250 degrees C, lowering the thermal stability of materials. Thus, anion-exchanging silicas can be envisaged for the use as catalyst supports at moderate temperatures.

Synthesis, structure, and acidic properties of MCM-41 functionalized with phosphate and titanium phosphate groups.

Mesoporous molecular sieves Si-MCM-41 (purely siliceous) and Ti-MCM-41 (partly covered with a surface layer of TiO2) were functionalized with phosphate groups by treatment with POCl3 (denoted -MCM-41(P)and Ti-MCM-41(P), respectively). With the use of TEM, X-ray diffraction, and N2 adsorption, it was shown that the initial hexagonal structure, the high specific surface area, and porosity are retained in the functionalized materials but are not as good as in the starting materials. 1H MAS NMR and 31P MAS NMR revealed that the surface of Si-MCM-41(P) consists of silicon phosphate and pyrophosphate species. That of Ti-MCM-41(P) additionally contains titanium dihydro-, hydro-, and pyrophosphate species, the latter being predominant. TPD of adsorbed ammonia for Si-MCM-41(P) and Ti-MCM-41(P) showed that functionalization leads to the creation of moderate and strong acid sites. A combination of mesoporous structure with acidic properties makes the MCM-41 functionalized with phosphate groups promising for use as solid acid catalysts.