(215g) Effect of Temperature in Methane Dehydroaromatization over Mo Supported on Sulfated Zirconia Catalysts

Shale gas
revolution is the significant increase in the production of natural gas
courtesy of the advances in horizontal drilling and hydraulic fracturing. This
increase has given a strong motive for research in methane conversion to higher-value
endproducts, rather than flaring, power generation, or heating.  Methane conversion
can be divided into two routes: non-oxidative and oxidative. One of the most
widely studied non-oxidative route is the dehydroaromatization (DHA) in which
methane is converted into a mixture of aromatics (benzene, toluene etc.) while
simultaneously producing H­_2­.

   (ΔG_298K = +433
kJ/mol, ΔH_298K = +530 kJ/mol)

Out of the
various catalysts studied, bifunctional catalysts that possess Methane activation
sites as well as Brønsted acid sites, have been the most prominent ones
especially Mo supported on HZSM5/HMCM22 catalysts [1]. The DHA reaction is typically carried out around 600
– 700 ^oC [2] using these most widely studied catalysts.

Recently in
our lab, we studied Mo supported on another support, Sulfated Zirconia (SZ)
that also possesses strong Brønsted acid sites. Loading of Mo was confirmed
using Raman spectroscopy and presence of acid sites was estimated using
pyridine DRIFTS. This catalyst was further evaluated for methane DHA and was
found to be active for this reaction. One limitation of SZ is that sulfate can
start decomposing at these temperatures [3-5], which will reduce the number of available Brønsted acid sites. Hence, it is important to study the effect
of temperature in the range of 600-700 ^oC. For this study, SZ with
5% loading of Mo was studied at three different temperatures of 600 ^oC,
650 ^oC, and 700 ^oC.

Here, the
results show the expected increase in methane conversion with temperature, while
deactivation was also rapid, apparently due to coke formation.  Selectivity
differed considerably. Aromatic selectivity was found to be higher for the
intermediate temperature of 650 ^oC. This could be due to the
presence of competing rates specifically that of aromatization of dimeric
intermediates and rate of sulfur evolution from surface. These rates are
significantly influenced by the temperatures studied here.  

Figure 1: H₂ generated
with time on stream at different temperatures

Figure 2: Time on stream selectivity towards ethylene

Figure 3: Time on stream
selectivity towards Benzene

In terms of ethylene
selectivity, reaction at 700 ^oC showed higher levels than 600 ^oC,
or 650 ^oC, possibly due to higher rate of methane activation on Mo
sites, which forms more dimers at the highest temperature, 700 ⁰C.  At
650 ^oC, ethylene selectivity is lowest, possibly due to higher rate
of aromatization of dimer as compared to 700 ^oC.  Eventually all three
catalysts reach similar levels of methane conversion due to coking (not shown).
H₂ concentration levels also correlated well with benzene
selectivity at 650 ^oC possibly due to faster rate of aromatization
that is often accompanied by H₂ evolution.

[1] J.J. Spivey, G. Hutchings, Catalytic aromatization of methane, Chemical Society
Reviews, 43 (2014) 792-803.

[2]
Z.R. Ismagilov, E.V. Matus, L.T. Tsikoza, Direct conversion of methane on
Mo/ZSM-5 catalysts to produce benzene and hydrogen: achievements and
perspectives, Energy & Environmental Science, 1 (2008) 526-541.

[3]
W.-H. Chen, H.-H. Ko, A. Sakthivel, S.-J. Huang, S.-H. Liu, A.-Y. Lo, T.-C. Tsai,
S.-B. Liu, A solid-state NMR, FT-IR and TPD study on acid properties of
sulfated and metal-promoted zirconia: Influence of promoter and sulfation
treatment, Catalysis Today, 116 (2006) 111-120.

[4]
K.M. Arata, H., Solid Superacids, Nova Science Publishers, New York, 2011.

[5] B.M. Reddy, M.K. Patil, Organic Syntheses and
Transformations Catalyzed by Sulfated Zirconia, Chemical Reviews, 109 (2009)
2185-2208.