Sinnathambi, Chandra Mohan
(2005)
Mo- Modified HZSM -5 Zeolite for the Dehydrooligomerisation
and Aromatisation of
Methane in the Absence of Oxygen.
PhD thesis, THE UNIVERSITY OF LIVERPOOL.
Abstract
Methane dehydrogenation aromatisation (MDA) has been highly pursued
since 1993 as a route of converting methane to aromatics. It is believed to
proceed via a by bi-functional catalyst namely Mo modified HZSM
where by methane is activated by the Mo carbide by dehydrogenation
process and the intermediate species are oligomerised and
dehydrocyclised over the acid site of the HZSM-5 zeolite. The aim of this
study is to optimise catalytic activity with high yield and selectivity to
aromatics and low coke yield and selectivity. This will include basic
principles behind catalyst design, activation and stabilisation. The MDA
analysis was carried out on a quartz micro reactor operating at 1 atm. and
700°C using an on-line GC and a list of catalyst characterisation
equipments to explain the findings.
The optimum catalytic activities were investigated using methane on Mo
modified HZSM5 (Si/Al =41). Mo wt% loadings used were 3, 5 and 10
prepared by wet impregnation of ammonium heptamolybdate on HZSM-
5. The preliminary micro reactor test showed that a GHSV of 1500 h-1
was the optimum flow rate for the process.
Based on investigation on the hcp ~-Mo2C and fcc a-MoC 1-x , Mo
loading of 10wt% is found to be the optimum catalytic loading for MDA
for both. Re-dispersion of the Mo03 species over the catalyst surface and
into the channels of the zeolite or pore is governed by calcination. Based
on this study for the hcp ~-Mo2C catalyst 600°C was the optimum
calcination temperature while for the a-MoC 1-x it was 500°C.
Activation of the Mo03 species results in the formation of hcp ~-Mo2C
and fcc a-MoC1.x. the active Mo carbide phase which is responsible for
methane activation and follow different route of preparation. It was found
the hcp ~-Mo2C catalyst has higher catalytic activity than the fcc aMoC1.
x counterpart. This higher catalytic activity for the hcp ~-Mo2C is
attributed by the higher coke deposition as compared fcc a-MoC1_x which
has lower coke yield and selectivity but higher aromatic yield and
selectivity at lower calcinations temperature.
Catalytic activity and stability was found to be improved by co-dosing
with CO onto both the hcp ~-Mo2C and MoC1-x catalysts. CO dosing was
found to prevent coke build up during MDA reactions. From this
investigation, it was found CO dosing is more significant towards higher
Mo loading of 10wt% rather than for the lower loading of 3wt%. The
most suitable CO for the !Owt% a-MoC 1-x was 4 v/v%, while for 10
wt % hcp ~-Mo2C it is 12 v/v%
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