Knoevenagel condensation catalyzed by amino-functionalized mesoporous silica materials of MCM-41 type
Sylwia Jarmolińska, Agata Wawrzyńczak and Izabela Nowak
Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, 61-614, Poznań, POL
Knoevenagel condensation is a well-known reaction in the organic chemistry, since it facilitates creation of new C–C bonds. It can be promoted by basic catalysts and among various nanomaterials, mesoporous silicas are promising inorganic supports allowing the attachment of various organic functionalities through silanol groups1. MCM-41 mesoporous silica functionalized with amino groups was found to be very useful for basic-catalyzed reactions. Introduction of the amino groups onto/into the siliceous matrix generates active centers, that enables the use of those materials in the Knoevenagel condensation2.
The purpose of the presented study was to investigate the catalytic properties of amino-modified MCM-41 materials obtained by two different synthesis methods. In order to examine the catalytic activity of the obtained samples Knoevenagel condensation between benzaldehyde and ethyl acetoacetate was used.
In the first stage of the study ordered mesoporous materials of MCM-41 type were synthesized in the basic conditions with tetraethyl orthosilicate (TEOS) as a silica source and cetyltrimethylammonium bromide (CTAB) as structure directing agent (template). Materials containing functional groups were prepared using two different aminoorganosilanes as sources of the amino groups, namely: Z-6032 (N-(2-(vinylbenzyl-amino)-ethyl)-3-aminopropyltrimethoxysilane) and Z-6020 (N-(2-amino-ethyl)-3-aminopropyltrimethoxysilane). All syntheses were carried out with two different procedures: direct synthesis (co-condensation) and post-synthetic functionalization (grafting).
In the next stage of the study several techniques were applied in order to evaluate structural and textural properties and confirm mesoporosity of the synthesized materials. The samples were characterized by a small-angle X-ray diffraction (XRD) analysis, low-temperature N2 sorption measurements and FT-IR spectroscopy. Selected materials were tested additionally by transmission electron microscopy (TEM) and elemental analysis.
Catalytic tests showed that all synthesized amino-modified MCM-41 materials, obtained both by a co-condensation and grafting method, may serve as efficient heterogeneous basic catalysts in the solvent-free Knoevenagel condensation between benzaldehyde and ethyl acetoacetate. In most cases the desired reaction product, which was ethyl 2-benzylideneacetylacetate, was obtained with selectivity of 96-98%.
National Science Centre is kindly acknowledged for the financial support (project no: DEC-2013/10/M/ST5/00652).