Immobilized Metal Complexes On Montmorillonite-K10 As Heterogeneous Catalyst For Epoxidation Of Cyclohexene
Epoxides are one of the important intermediates for the synthesis of oxyoencontaining organic compounds as they undergo ring-opening reactions easily to form bifunctional compounds. Homogeneous catalysts have been used in producing epoxide compounds but due to the demand for environmentally clean...
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| Summary: | Epoxides are one of the important intermediates for the synthesis of oxyoencontaining
organic compounds as they undergo ring-opening reactions easily to form
bifunctional compounds. Homogeneous catalysts have been used in producing
epoxide compounds but due to the demand for environmentally clean reactions,
heterogeneous catalysts have attracted considerable attention to replace homogeneous,
polluting and corrosive acid catalysts. In this study, metal complexes of MoO, (acac),
and VO(acac), at various concentration were immobilized onto montmorillonite-K 10
(MMT-K10) and characterized using X-ray diffractogranl (XRD), atomic absorption
spectroscopy (AAS), N, adsorption analysis. Fourier transform infrared spectroscopy
(FTIR), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA).
The materials were labeled as KI 0-Mo or K IO-V (1-4), where numerical I. 2,3 and 4
indicate 0.05 M, 0.1 M, 0.15 M and 0.2 M of metal complexes concentration
respectively. The immobilized MMT-K 10 samples were then tested for epoxidation of'
cyclohexene with tert-butylhydroperoxide (TBHP). XRD analysis showed that MMTK10
retained its structure even alter metal complexes were introduced. This was
proven by the diffractogranl patterns Ihr immobilized MMT-K10 samples with door
basal spacing at 89° were 9.9 A- 10.0 A similar to the unmodified MMT-K 10. The
intensities of 19.9° and 35° peaks were found to decrease that results in reducing
crystallinity for immobilized MMT-K 10 samples. Characterization using N2
adsorption revealed a lower Brunauer-Emmett-Teller (BET) surface area for samples
with high concentration of metal complexes relative to the unmodified MMT-K10.
The pore size calculated from Barrett-Joyner-Halenda (B. IH) equation showed
increasing pore size in the presence of higher metal elements, due to the substitution
of shorter Si-O bond with a longer Mo-O or V-O bond lengths. A lot of deposition of
the small particles of metal complexes on the surface of modified clays was observed
from SEM images. Large particle size on the immobilized clay resulted in less
crystalline material, as obtained from XRD dilfragtogram. More random shape of
particles and high degree of dispersion of Mo or V species on clay surface enhances
the catalyst performance in the epoxidation reaction. From the TGA results, high
concentration of metal complexes Immobilized on MMT -K 10 structure make them
being better hydrophobic nature and thus, lower the loss of physisorbed water
molecules weakly bound to the material. MMT-K 10 gives a lower selectivity in
epoxidation reaction because the opening of oxirane may be accelerated due to
existence of higher -OH compared to immobilized MMT-K 10. The selectivity
increased as the metal complexes used increased in both homogeneous (32 °o for Moo
and 76 % I'm- Mo4) and heterogeneous (20 °o fir KIO-M04 and 70 (a Im- KIO-V4)
catalytic system. This is because K10-Mo and K IO-V in higher concentration of metal
complexes have bigger pore size although in smaller surface area, that resulted in the
optimum interactions between the reactant and Mo or V active sites. |
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