Imidazole immobilization in nanopores of silicas and niobiosilicates SBA-15 and MCF—A new concept towards creation of basicity

Abstract

Mesoporous silicas and niobiosilicates of SBA-15 and MCF types were modified by anchoring of imidazole, characterized (XRD, N2 adsorption/desorption, XPS, TEM, UV–vis, FTIR, TG/DTA, TG/MS) and tested as catalysts in the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate, the reaction involving the basic active centres of catalysts. Moreover, the behaviour of functionalization agent used before imidazole loading, (3-chloropropyl)trimethoxysilane (ClPTMS), was studied in detail. The use of mesoporous materials of different structures as supports allowed the discussion of the structure effect on imidazole loading and stability. Imidazole was preferentially located in the windows of MCF structure. The novelty in the formation of new catalysts was the use of niobium containing mesoporous silicas as the supports for imidazole. Niobium species in NbSBA-15 and NbMCF interacted weakly with imidazole anchored and much stronger with ClPTMS enhancing the effectivity of imidazole anchoring. In this paper, for the first time, the reaction pathway of transformation of both modifiers, ClPTMS and imidazole, upon thermal treatments was established and documented. In this way, the influence of different bonding of imidazole to mesoporous silica on the activity in the Knoevenagel reaction was explained. For the higher activation temperature, the lower conversion of benzaldehyde was observed. The catalytic activity of imidazole species was enhanced by the niobium presence in the support, working as Lewis acid site and taking part in the reaction (the carbonyl of the aldehyde is activated by acidic sites) and by the more open structure of MCF supports than that of SBA-15 one.