Activated vs. pyrolytic carbon as support matrix for chemical functionalization: Efficient heterogeneous non-heme Mn(II) catalysts for alkene oxidation with H2O2

Abstract

Two types of heterogeneous catalytic materials, Mn^II-L₃imid@Cox and Mn^II-L₃imid@PCox, have been synthesized and compared by covalent grafting of a catalytically active [Mn^II-L₃imid] complex on the surface of an oxidized activated carbon (Cox) and an oxidized pyrolytic carbon from recycled-tire char (PCox). Both hybrids are non-porous bearing graphitic layers intermixed with disordered sp²/sp³ carbon units. Raman spectra show that (I_D/I_G)_{activatedcarbon} > (I_D/I_G)_{pyrolyticcarbon} revealing that oxidized activated carbon(Cox) is less graphitized than oxidized pyrolytic carbon (PCox). The Mn^II-L₃imid@Cox and Mn^II-L₃imid@PCox catalysts were evaluated for alkene oxidation with H₂O₂ in the presence of CH₃COONH₄. Both showed high selectivity towards epoxides and comparing the achieved yields and TONs, they appear equivalent. However, Mn^II-L₃imid@PCox catalyst is kinetically faster than the Mn^II-L₃imid@Cox (accomplishing the catalytic runs in 1.5 h vs. 5 h). Thus, despite the similarity in TONs Mn^II-L₃imid@PCox achieved extremely higher TOFs vs. Mn^II-L₃imid@Cox. Intriguingly, in terms of recyclability, Mn^II-L₃imid@Cox could be reused for a 2th run showing a ∼20% loss of its catalytic activity, while Mn^II-L₃imid@PCox practically no recyclable. This phenomenon is discussed in a mechanistic context; interlinking oxidative destruction of the Mn-complex with high TOFs for Mn^II-L₃imid@PCox, while the low-TOFs of Mn^II-L₃imid@Cox are preventive for the oxidative destruction of the Mn-complex.