Tethering of Cu(II), Co(II) and Fe(III) tetrahydro-salen and salen complexes onto amino-functionalized SBA-15: Effects of salen ligand hydrogenation on catalytic performances for aerobic epoxidation of styrene

A variety of transition metal tetrahydro-salen complexes M-[H4]Salen (M = Cu, Co and Fe) and the corresponding metal salen complexes M-Salen were firstly tethered onto amino-functionalized mesoporous SBA-15 and screened as heterogeneous catalysts for aerobic epoxidation of styrene. The mesoporous structural integrity throughout the tethering procedure, the successful tethering of the organometallic complexes, the loadings of metal ions and organic ligands as well as the catalyst surface constitution and location of active organometallic species on the SBA-15 support were determined by comprehensive characterization techniques such as XRD, N2 adsorption/desorption, TEM, FT-IR, UV–vis spectroscopy, ICP-AES, XPS and TG/DSC. The tethered catalyst M-[H4]Salen-SBA (M = Cu or Co) exhibits improved reactivity than the corresponding M-Salen-SBA catalyst due to the incorporation of a modified coordination environment of the central metal cations by CN bond hydrogenation. However, Fe-[H4]Salen-SBA shows lower reactivity as compared to Fe-Salen-SBA possibly due to the easy formation of relatively inactive dinuclear iron(III) tetrahydro-salen complexes, verified by XPS and ESR. The recycling results of these heterogeneous catalysts exemplified by Co-Salen-SBA, Co-[H4]Salen-SBA and Fe-Salen-SBA, show good recoverability without significant loss of activity and selectivity within successive runs. Heterogeneity tests of three sample catalysts confirm the high stability of cobalt catalysts against leaching of active species into solution.

► Cu(II), Co(II) and Fe(III) tetrahydro-salen were firstly tethered onto SBA-15.
► Support and active species were fully characterized.
► Salen hydrogenation improved the activity of Cu(II) and Co(II) complexes.
► Formation of dinuclear Fe(III) tetrahydro-salen led to activity decline.