Characterisation and application of montmorillonite-supported Fe Schiff base complexes as catalysts for the oxidation of n-octane

摘要

Context: Montmorillonite (Mt) has favourable characteristics which allow its use in many fields including as a support material in catalysis. Iron complexes of salen-type ligands are popular catalysts for many reactions. The oxidation of n-alkane under mild conditions is particularly challenging. Will supporting the complexes onto Mt afford active, selective and recyclable catalysts? Objective: This study presents results on the use of Mt-supported iron-salen complexes as recyclable catalysts for the liquid phase oxidation of n-octane under mild conditions. Method: Ion exchange was used to prepare Mt-Fe by first reacting Mt with NaCl followed by FeCl_3. Four salen-type ligands were synthesised and each was incorporated into Mt-Fe to afford corresponding clay-supported complexes. In conjunction with hydrogen peroxide (H_2O_2) and tert-butyl hydroperoxide (TBHP) as oxidants, the Mt-Fe and clay-supported complexes were tested as catalysts in the oxidation of n-octane. Results: All ligands and clay-supported complexes were fully characterised by spectroscopic and analytical techniques. The Mt-Fe and day-supported complexes showed moderate to good catalytic activities for n-octane oxidation. Generally, conversions were low (3.6-7.4%) with ketones as the major products (due to activation of internal carbons). In the series of reactions based on H_2O_2 as the oxidant, the clay-supported complexes when compared to Mt-Fe, displayed improved selectivity to alcohols (up to 38%) due to increased activation of primary carbons. Recyclability tests showed that the day-complexes were catalytically active over the course of 3 cycles, however leaching of active species was observed. Conclusion: Salen-type complexes of iron were successfully intercalated into Mt to yield catalysts that were active for the oxidation of n-octane under mild conditions. Selectivity to the more desired primary alcohols was better for the clay-supported complexes. However, drop in catalyst performance and leaching was observed over 3 cycles.