Mesoporous silica-supported new metallosalphen complexes: Effective photochromic catalysts for styrene epoxidation

The epoxidation of styrene to produce styrene oxide is a crucial industrial reaction, as styrene oxide is an important building block for valuable chemicals and polymers.This work presents new heterogeneous catalysts including Co(II), Mn(II), Ni(II), and Cu(II) salphen-azobenzene complexes, [ML₂] , which have been incorporated amino-functionalized mesoporous silica (SBA-15, MCM-41, MCM-48) via grafting/coordination method. In the first step, the salphen-azobenzene ligand was synthesized and then complexed with various metal ions. In the second step, aminopropyltriethoxysilane (APTES) was grafted onto the surfaces of different types of commercial mesoporous silica then bond to the [ML₂] complex through coordination bond. The successful immobilization of [ML₂] on the mesoporous silica surfaces, as well as the thermal stability of the resulting materials, were confirmed using various characterization techniques, including FT-IR, powder XRD, SEM, TEM, BET, and TGA. The results indicated a high dispersion of [ML₂] across the silica surfaces. The catalytic performance of the obtained catalysts Silica-N-ML₂, was evaluated for the epoxidation of styrene to form styrene oxide using different oxidants. Notably, the cis – trans isomerization of the azobenzene upon UV irradiation was found to influence the catalytic activity of Silica-N-ML₂ . Among the materials, the cis isomer of SBA-15-N-CoL₂ demonstrated the highest styrene conversion (90 %) and styrene oxide selectivity (92 %) under mild conditions (60 °C, 4 h) when tert-butyl hydroperoxide (TBHP) was used as the oxidant. Furthermore, SBA-15-N-CoL₂ exhibited excellent stability over four consecutive cycles.