The enhancement of direct amide synthesis reaction rate over TiO 2 @SiO 2 @NiFe 2 O 4 magnetic catalysts in the continuous flow under radiofrequency heating

A series of TiO2@SiO2@NiFe2O4 composite magnetic catalyst with a core-double shell structure was synthesized by a sol-gel method. The morphology of the catalysts was studied by XRD, SEM, N2 physisorption and their magnetic properties were examined with magnetometry, and specific absorption rate measurements. The catalytic activity was determined in a direct amide synthesis reaction between aniline and phenylbutyric acid at 150 °C in a fixed bed flow reactor under radiofrequency heating. The intermediate silica layer of the catalyst increased the porosity of the outer titania layer and the specific absorbance rate of the catalyst. The initial reaction rate increased by 61% as compared to a similar core-shell TiO2@NiFe2O4 catalyst showing the detrimental effect of nickel ferrite on titania. The reaction rate was further increased by a factor of 3.5 after a sulfation treatment due to an optimum Lewis acid site strength. The highest specific reaction rate over TiO2@SiO2@NiFe2O4 was observed at a 7.5 wt% sulfate loading which was 2.6 times higher as compared to a mechanical mixture of the same composition. The initial reaction rate decreased by 36% after a period of 55 h on stream. The catalyst activity was restored after a treatment with a H2O2 solution.