Aim and Objective:
Effect of two different modification methods for introducing Ni into ZSM-5 framework was
investigated under high temperature synthesis conditions. The nickel successfully introduced into the MFI structures at different crystallization conditions to enhance the physicochemical properties and catalytic performance.
Materials and Methods:
A series of impregnated Ni/ZSM-5 and isomorphous substituted NiZSM-5 nanostructure catalysts
were prepared hydrothermally at different high temperatures and within short times. X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Energy dispersive X-ray (EDX), Brunner, Emmett and Teller-Barrett, Joyner
and Halenda (BET-BJH), Fourier transform infrared (FTIR) and Temperature-programmed desorption of ammonia (TPDNH3) were applied to investigate the physicochemical properties.
Results:
Although all the catalysts showed pure silica MFI–type nanosheets and coffin-like morphology, using the isomorphous substitution for Ni incorporation into the ZSM-5 framework led to the formation of materials with lower crystallinity,
higher pore volume and stronger acidity compared to using impregnation method. Moreover, it was found that raising the
hydrothermal temperature increased the crystallinity and enhanced more uniform incorporation of Ni atoms in the crystalline
structure of catalysts. TPD-NH3 analysis demonstrated that high crystallization temperature and short crystallization time of
NiZSM-5(350-0.5) resulted in fewer weak acid sites and medium acid strength. The MTO catalytic performance was tested
in a fixed bed reactor at 460ºC and GHSV=10500 cm3
/gcat.h. A slightly different reaction pathway was proposed for the
production of light olefins over impregnated Ni/ZSM-5 catalysts based on the role of NiO species. The enhanced methanol
conversion for isomorphous substituted NiZSM-5 catalysts could be related to the most accessible active sites located inside
the pores.
Conclusion:
The impregnated Ni/ZSM-5 catalyst prepared at low hydrothermal temperature showed the best catalytic performance, while the isomorphous substituted NiZSM-5 prepared at high temperature was found to be the active molecular
sieve regarding the stability performance.
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