Cobalt, copper and nickel-embedded chitosan/PAAS composite nanofiber mats as efficient heterogeneous catalysts for air oxidation of benzoin to benzil

We have applied electrospinning technique to embed cobalt, copper and nickel catalysts inside chitosan (CS) and polyacrylic acid sodium salt (PAAS) composite nanofiber mats (M@CS/PAAS) (M = Co²⁺, Cu²⁺ and Ni²⁺) as stable heterogeneous catalysts. Scanning electron microscopic analysis shows that the resultant M@CS/PAAS composite mats are bead-free and cylindrical nanofibers with mean diameter ranging from 474 to 504 nm, and the incorporated transition metal catalysts are homogeneously distributed inside the nanofibers in tight chelation with the chitosan amine groups. The M@CS/PAAS nanofiber mats exhibited excellent catalytic activity for atmospheric air oxidation of benzoin to benzil, and are much more stable than those prepared by conventional wetness impregnation (M-CS/PAAS nanofiber mats). Moreover, their catalytic activities and stabilities could be further enhanced by introduction of additional ligands through Schiff base transformation of the chitosan pendant amine groups. The free volume holes of the nanofibers were estimated to be considerably larger than the reactant and product molecules, allowing them to freely migrate around the internal catalytic sites. Taken together, we have demonstrated a straightforward approach to prepare efficient and recyclable heterogeneous catalysts derived from the first-row transition metal catalysts by entrapping them inside the chitosan/PAAS composite nanofibers.