Palladium complex immobilized on graphene oxide–magnetic nanoparticle composites for ester synthesis by aerobic oxidative esterification of alcohols

• Magnetically separable palladium based nanocomposite materials.
• Environmentally benign methodology for ester synthesis.
• Graphene oxide immobilized palladium complex.
• Heterogeneous catalyst for oxidative esterification.
• Efficient recovery and recycling of the catalyst.

A well dispersed magnetically separable palladium complex immobilized to magnetite–graphene oxide nanocomposite has been synthesized via solvothermal route. The successfully decorated graphene oxide sheets with the homogeneously dispersed iron nanoparticles and palladium(II) complex were proved by transmission electron microscopy. X-ray diffraction pattern of the synthesized catalyst revealed that palladium(II) complex was successfully immobilized to the support. Further, X-ray photoelectron spectroscopy confirmed that palladium is present in +2 oxidation state. The synthesized heterogeneous material was found to be an efficient catalyst for the oxidative esterification of alcohols with methanol by using molecular oxygen as oxidant. The developed catalyst was found to be more reactive than its homogeneous analog, i.e. PdCl2(CH3CN)2. Similarly, no reaction occurred in the presence of Pd(0)/magnetite–graphene nanocomposite catalyst. These studies confirmed that the palladium(II) ions are the main catalyst and superior reactivity of heterogeneous palladium catalyst is attributed to the promotion role of graphene support in the adsorption of reactant alcohol and oxygen. After the reaction, the catalyst was easily separated by external magnetic effect and reused for several runs with consistent catalytic activity without any detectable leaching.

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