Synthesis and characterization of zinc porphyrin complex heterogenized in functionalized SBA-15 for liquid phase epoxidation of limosine

Abstract

Long-range ordered mesoporous silica SBA-15 of high surface area and uniform pore size was obtained by hydrothermal synthesis using tetraethyl orthosilicate (TEOS) as silica source and Pluronic P123 copolymer as surfactant in acidic media. Functionalization of SBA-15 with 3-aminopropyltriethoxysilane (APTES) via sol-gel reaction gave NH2-SBA-15 which was qualitatively confirmed with FTIR spectroscopy. Meanwhile, tetra-(p-chlorophenyl)porphyrin (TClPP) was synthesized using modified Alder-Longo method. Insertion of zinc(II) into metal-free TClPP was achieved using zinc(II) acetate to obtain ZnTClPP. The successful complexation of zinc(II) in porphyrin molecule was supported by the disappearance of two Q bands and the presence of Soret band in the UV-Vis spectra. ZnTClPP was also characterized using 1H NMR spectroscopy, MALDI-TOF MS and elemental CHN analysis. Heterogeneous catalyst NH2-SBA-15-ZnP was synthesized by incorporating various loadings of the ZnTClPP into NH2-SBA-15 and characterized by DR-UV Vis spectroscopy, N2 adsorption-desorption isotherm, SAXS, FESEM and TEM. The DR-UV Vis spectra of NH2-SBA-15-ZnP exhibit absorption peaks similar to free ZnClTPP indicating that porphyrin molecule is confined in the SBA-15. Furthermore, the intensity of Soret and Q bands increases with increasing of ZnTClPP loading. The hexagonal mesopore structure of SBA-15 was still intact after the modification treatments as shown by the SAXS patterns, FESEM and TEM images. The catalytic activity of homogeneous catalysts ZnClTPP was compared with the heterogeneous NH2-SBA-15-ZnP in the epoxidation of limonene in the presence of H2O2 oxidant. GC-FID and GC-MS detection analysis showed that the activity of heterogeneous catalysts NH2-SBA-15-ZnP were active in the formation of limonene oxide as main product. The limonene conversion (34.72%) when catalyzed by free ZnTClPP is comparable to NH2-SBA-15-ZnP (26.24%) with ~95% selectivity. The catalytic activity of NH2-SBA-15-ZnP increases with the increase of ZnTClPP loading on the SBA-15 support. For the liquid-phase epoxidation of limonene over NH2-SBA-15-ZnP catalyst in acetonitrile, the optimum conditions for the highest percentage of limonene conversion was at reaction temperature of 80°C during 24 h and using H2O2 as oxidant.