The nonoxidative conversion of light alkanes over metal-loaded H-ZSM-5 zeolite catalysts
Date
2008-06-20T11:36:20Z
Authors
Ngobeni, Maropeng Walter
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Abstract
The study of the aromatisation of methane was conducted at 750oC over metalimpregnated
H-ZSM-5 catalysts with a feed flow rate of 13 ml/min and the
composition of the feed was 90% methane balance argon. Typical products that
were detected from the outlet stream were ethene, ethane, benzene and toluene. The
amount of coke produced was determined by using 10% argon as an internal
standard. The effects of different parameters such as the type of the support
material, the molybdenum content, the %XRD crystallinity and SiO2/Al2O3 ratio of
H-ZSM-5, the reaction temperature, the feed flow rate, the type of the molybdenum
precursor, the catalysts preparation method, the addition of dopants, silanation and
the regenerability of the catalysts were investigated.
The results obtained showed that H-ZSM-5 was a better support for the preparation
of catalysts used for the aromatisation of methane. Mo/H-ZSM-5 catalysts were
more active when the molybdenum loading was between 2 and 4 wt% and loadings
higher than 4% led to lower activities. The lower activities observed at higher
molybdenum loadings was related to the poor dispersion and decrease in the pore
volumes and surface areas observed due to the formation of MoO3 crystallites.
Furthermore, the zeolite structure collapsed under the reaction conditions when the
molybdenum loading was more than 4 wt%. The study showed that the conversion
of methane increased linearly with increasing reaction temperature and the apparent
activation energy of the reaction was found to be 64.5 kJ/mol.
The results of the effect of the %XRD crystallinity of H-ZSM-5 on the performance
of H-ZSM-5 catalysts showed that 2%Mo/H-ZSM-5 catalysts were more active
when the crystallinity of the zeolite was between 50 and 70%. The conversion of
methane decreased with an increase in the SiO2/Al2O3 ratio of H-ZSM-5. Higher
aromatisation activities were observed when the SiO2/Al2O3 ratio of H-ZSM-5 was
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60. The type of the molybdenum precursor used in the preparation of 2%Mo/HZSM-
5 catalysts did not have a significant influence on the conversion of the
catalysts, but higher selectivities for aromatics were observed when ammonium
heptamolybdate was used as a source of molybdenum. The catalysts prepared by
physical mixing of MoO3 and H-ZSM-5 catalysts were more active than those
prepared by impregnation with solutions of ammonium heptamolybdate.
The presence of dopants such as boron, silver and alkali metal ions (Li+, Na+ and K+)
in 2%Mo/H-ZSM-5 catalysts was also investigated. Boron (0.05-0.2 wt%) did not
affect the conversion level of the catalysts but changed their selectivity properties.
The selectivity for C2 hydrocarbons increased with boron content, while the
selectivity for aromatics decreased. The addition of silver ions (0.5 wt%)
significantly improved the conversion of the catalysts. This was attributed to the
enhancement of the acvidity of the catalysts upon addition of silver ions which was
observed by temperature programmed desorption of ammonia and pyridine
adsorption studies of the infrared spectra of the catalysts. The addition of alkali
metal ions in the Mo:Metal ratio of 0.5 led to decreased catalytic activities, due to the
lowered acidities of the catalysts.
The silanation of H-ZSM-5 improved the conversion of methane but lowered the
selectivity for aromatics. A comparative study of the W-based and Mo-based
catalyst at equivalent molar contents showed that molybdenum-based catalysts were
more active than tungsten based catalysts. The study also showed that the catalytic
performance of 2%Mo/H-ZSM-5 catalysts could be regenerated to appreciable levels
by treatment of the catalysts in air at 600oC.
The possibility of using Mo/H-ZSM-5 catalysts for the aromatisation of propane was
also evaluated at 530oC, with consideration of three variables, namely, the
molybdenum loading, the reaction temperature and %XRD crystallinity. The results
indicated that impregnation H-ZSM-5 catalysts with molybdenum led to lower
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propane aromatisation activities. This lower activity was attributed to the lower
Brønsted acid sites in the Mo/H-ZSM-5. The activities of the catalysts could be
improved by operation at higher temperatures, but the rate of deactivation was also
improved at higher temperatures. In line with the observations from the conversion
of methane, higher activities were observed when the %XRD crystallinity of the
catalyst was 61%.
Description
Keywords
Catalysts, Methane, Zeolites, H-ZSM-5