Non-oxidative conversion of methane into aromatic hydrocarbons over molybdenum modified H-ZSM-5 zeolite catalysts
Date
2014-07-02
Authors
Tshabalala, Themba Emmanuel
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Dehydroaromatization of methane (MDA) reaction was investigated over platinum modified
Mo/H-ZSM-5 catalysts which were pre-carbided at 750 oC. The influence of platinum on the
catalytic performance and product selectivity of Mo/H-ZSM-5 catalysts for the MDA reaction at
700 oC was studied. The presence of platinum led to a slight decrease in methane conversion.
As the platinum loading increased, the methane conversion decreased further and the catalytic
stability increased with time-on-stream (TOS) during the MDA reaction. Aromatic selectivities
above 90% were obtained with catalysts containing low platinum loadings (0.5 and 1.0 wt.%),
with benzene being the most prominent product. A decrease in coke selectivity and coke
deposits was noted with the platinum modified Mo/H-ZSM-5 zeolite catalysts.
A comparative study was performed to compare platinum, palladium and ruthenium promoted
Mo/H-ZSM-5 zeolite catalysts with un-promoted Mo/H-ZSM-5. The ruthenium promoted
catalyst proved to be superior in catalytic performance, with a higher methane conversion
obtained than found for platinum promoted and palladium promoted Mo/H-ZSM-5 catalysts.
Benzene selectivity of about 60% was obtained for ruthenium and palladium promoted Mo/HZSM-
5 catalysts and the total aromatic selectivity was maintained at 90%. TGA results showed
a total reduction of 50% by weight of carbon deposited on the promoted Mo/H-ZSM-5 catalyst. Dehydroaromatization of methane was studied over tin modified Pt/Mo/HZSM-5 catalysts and
compared to Pt/Mo/H-ZSM-5 catalyst at 700 oC. Addition of tin decreased the activity towards
methane aromatization. However, the formation of aromatic compounds was favoured. The CO
FT-IR adsorption and CO chemisorption techniques showed that the catalyst preparation method
had an effect on the catalytic performance of tin modified Pt/Mo/H-ZSM-5 catalysts. High
aromatic selectivity and low coke selectivity were obtained with co-impregnated and
sequentially impregnated Pt/Sn catalysts. While a decrease in the formation rate of carbonaceous
deposits is mainly dependent on the availability of platinum sites for the hydrogenation of
carbon. The order of sequentially loading platinum and tin has an effect on the electronic and
structural properties of platinum as shown by XPS and FT-IR studies. CO chemisorption and the
FT-IR adsorption studies showed that addition of tin decreased the adsorption capacity of the
platinum surface atoms. Catalyst preparation methods and successive calcination treatments
affected the location of both tin and platinum atoms in the catalyst. Catalysts prepared by the coimpregnation
method showed a good platinum dispersion, better than found for the sequentially
impregnated catalysts.
The MDA reaction was carried out at 800 oC over manganese modified H-ZSM-5 zeolite
catalysts prepared by the incipient wetness impregnation method. The effect of a number of
parameters on the catalytic performance and product selectivity was investigated, such as
reaction temperature, manganese precursor-type, tungsten as promoter, manganese loading and
use of noble metals. The study of the effect of reaction temperature showed that the methane
conversion increased linearly with increase in reaction temperature from 700 to 850 oC. The selectivity towards aromatic compounds (of about 65%) was attained for the reactions performed
at 750 and 800 oC. Formation rate of carbonaceous deposits increased linearly with increase in
reaction temperature. The use of different manganese precursors to prepare Mn/H-ZSM-5
catalysts had an effect on both the catalytic behaviour and the product distribution. High
catalytic activities were obtained for the catalysts prepared from Mn(NO3)2 and MnCl2 salts.
However, the product distribution was significantly different, with the Mn(NO3)2 catalyst being
more selective towards aromatic compounds while the MnCl2 catalyst was more selective toward
coke. The effect of manganese loading was studied at 800 oC and an optimum catalyst activity was obtained at 2 and 4 wt.% manganese loadings. The aromatic selectivity above 70% and
coke selectivity of 20% were obtained for a 2 wt.% loaded catalyst.
Addition of tungsten as a promoter onto the 2 wt.% loaded catalyst (2Mn/H-ZSM-5) lowered the
catalytic activity but the catalyst remained fairly stable with increase in TOS. Tungsten modified
catalysts favoured the formation of carbonaceous deposits over aromatic compounds. TGA
results showed a coke deposit of 164 mg/g.cat, an 88% increase in coke deposit when tungsten
was used a promoter. Noble metals were added to reduce the total amount of coke on the
tungsten modified Mn/H-ZSM-5 catalysts. The presence of a noble metal favoured the
formation of aromatic compounds and suppressed the formation of coke. Platinum and
ruthenium promoted catalysts were the active catalysts and aromatic selectivity increased from
12% to 55% and 46% respectively. A reduction in the total amount of coke deposit on the
platinum promoted catalyst (42%) and the ruthenium promoted catalyst (31%) was noted.