Magnesium oxide (MgO) as a solid, strong base has shown potential as a catalyst and as a catalyst support. We have attempted to maximise the catalytic potential of this oxide via magnesium hydroxide prepared under hydrothermal conditions with NaOH or tetra-propyl ammonium hydroxide as precipitating agents and a common surfactant; sodium dodecyl sulphate (SDS). This was then calcined to form magnesium oxide and subsequently used as a base catalyst in a Knoevenagel condensation reaction (benzaldehyde and ethyl cyanoacetate). Analysis of nitrogen adsorption indicated that in general surface areas could be improved by adding SDS, however, close to the critical micelle concentration of SDS (1-2 mmol) surface areas dramatically decreased. Concurrent adsorption of benzaldehyde through its C=O group on Mg2+ sites and adsorption of ethyl cyanoacetate via H-bonding to O2- sites will improve the catalytic activity. SDS can influence the crystal growth of the forming Mg(OH)2 in the presence of NaOH; such that the surface is more suitable to such condensation reactions. The highest activity of ca. 18 mol of product per Kg cat per hour was achieved over a catalyst precipitated by NaOH with 4 mmol of SDS added (Fig. 1). According to NMR analysis of the MgO surface hydroxyl density, increasing the surfactant concentration resulted in an increase of basic -O2- sites with NaOH precipitated Mg(OH)2. Conversely, the combination of TPAOH and SDS resulted in materials with a lower density of hydroxyl sites. This approach may find wider application in other catalytic reactions where fine control of basic surface sites is desirable.

Surface modification of basic sites on MgO by varying surfactant and precipitating agent concentrations

251st National Meeting (2016)

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