Cordierite honeycomb monoliths coated with Al(III)/ZrO2 as an efficient and reusable catalyst for the Knoevenagel condensation: A faster kinetics

Abstract

Al(III)/ZrO2 with 5–25 wt% Al(III) was coated on honeycomb monoliths by dip & dry technique and were also prepared in their powder forms for the purpose of comparison. These prepared catalysts were characterized for their total acidity, crystallinity, and morphology. The catalytic activity of Al(III)/ZrO2 was examined in 3 different modes of heating, i.e., conventional, ultrasonic, and microwave. The effect of calcination temperature of the catalyst, poisoning of acid sites of the catalyst by adsorbing pyridine adsorption on the physico-chemical properties and the catalytic activity was also studied. The results indicated that the microwave heating mode shows highest catalytic activity when compared to either ultrasonic or conventional modes. The honeycomb form of the catalysts shows almost 1.2 fold time increase in their catalytic activity when compared to their powder forms. Pre-adsorption studies disclose that the Knoevenagel condensation reaction obeys the Langmuir-Hinshelwood type which involves chemisorptions of both of the reactants. A faster kinetic study confirms that monolithic catalysts [15Z-A (CHM)] possess the least energy of activation (35.6 kJ/mol) while their powder forms analogs [15Z-A (PFM)] posses highest activation energy (59.2 kJ/mol) correspondingly with a shorter reaction time indicating that monolithic catalysts are highly efficient compared to their powder forms. Reusability tests pertaining to both calcined and uncalcined catalysts authenticates that calcination enhances catalytic activity with the augmentation of catalytically active sites. Keywords: Knoevenagel condensation reaction, Conventional method of condensation, Ultrasonic-assisted condensation, Microwave-assisted condensation, Al(III)/ZrO2, Kinetics