The Catalyst Review January 2020 - 14

EXPERIMENTAL
Aromatization of Ethylene − Main Intermediate for MDA?...
Aromatics, such as benzene, toluene, and xylene (BTX), are in demand because they
enhance the octane number of fuels and are used as feedstocks for the chemical
industry. Driven by the high availability of cheap methane, there is significant
interest in optimizing the direct conversion of methane to aromatics. Methane
dehydroaromatization (MDA) over Mo/HZSM-5 has been hypothesized to proceed
via a two-step mechanism in which methane is first converted to ethylene on
the molybdenum (Mo) functionality followed by oligomerization, cyclization and
dehydrogenation of ethylene on the Brønsted acid sites (BAS) of the HZSM-5 support.
Herein, the authors test this assumption by studying the conversion of ethylene under
the same conditions as used for MDA, namely 700°C, atmospheric pressure, and by
co-feeding experiments with H2 and CH4. In addition, the coke formed during ethylene
conversion with different co-feeds and over different catalysts was characterized and
compared with the coke formed during MDA.
For ethylene aromatization (EDA) on bare HZ, only one coke oxidation peak is observed
at a high temperature (~650°C), which is associated with diffusion-limited combustion
of coke deposited in the micropores of the zeolite. When Mo is present on the catalyst,
two peaks are observed in the DTGA for both MDA and EDA. The peak at the lower
temperature is associated with Mo, while the high-temperature peak is associated
with coke in the micropores. Both types of coke oxidize at lower temperatures after
MDA than after ethylene feeding over MoHZ as well as over HZ. In addition, the coking
rate is higher during ethylene aromatization resulting in more coke deposits formed
for the same contact time, which also leads to higher pore filling as determined by N2
adsorption measurements.

Figure 1. Yield to aromatics when feeding
15 ml/min 5 mol%C2H4/95 mol% N2 or 5
mol%C2H4/10 mol%H2/85 mol%N2 to MoHZ
(closed symbols).

Figure 2. TGA (top) and DTGA (bottom)
profiles after reaction, feeding 15 ml/ min
95 mol%CH4/5 mol%N2 to MoHZ, 5 mol%
C2H4/95 mol% N2 to HZ and MoHZ and
5 mol%C2H4/10 mol% H2/85 mol%N2 to
MoHZ. TGA was performed in air with a heating
rate of 10°C/min. Total catalyst weight (mcat)
was the same for all experiments, 500 mg.
Coke amounts can be calculated by (1-w/
wo)*mcat.

During H2 co-feeding, the profiles of aromatic yields look almost identical to those
observed during an experiment solely with ethylene feed up to 200 min on stream,
but with co-feeding, the benzene yield stays stable for the duration of the experiment
and does not drop as in EDA (Figure 1). In addition, the final coke amount deposited
is lower, especially the more refractory coke (Figure 2). The oxidation of the Moassociated coke formed during the co-feeding experiment shifts to lower temperatures
and thus becomes more like the species observed after MDA, while the coke species
associated with Brønsted acid sites stay the same as during EDA (Figure 2). Thus, the
higher H/C ratio of methane leads to the formation of less coke which is slightly more
hydrogenated and reactive. Also, because the ratio of the reactive and refractory coke
is high, the experimental results cast doubt on the proposal of ethylene as the primary
intermediate of MDA. Firstly, because benzene and naphthalene selectivities are quite
different when feeding ethylene or methane at levels encountered in MDA even when
co-feeding H2. Secondly, thermodynamic limits are reached during MDA, while EDA
seems to be commandeered by kinetics. Thirdly, the carbonaceous deposits and the HCP species formed during EDA are much less
reactive than the ones forming during MDA resulting from the extended induction period encountered with ethylene feeding and
the higher oxidation temperature during TGA. Thus, when ethylene aromatization with high selectivity to BTX is desired, co-feeding
methane and or hydrogen should be beneficial enhancing aromatics selectivity and prolonging the lifetime of the catalyst. Source:
Vollmer I, Abou-Hamad E, et al. (2019). ChemCatChem, 11: 1-7.
Cobalt-Modulated Molybdenum−Dinitrogen Interaction in MoS2 for Catalyzing Ammonia
Synthesis...
Alternate strategies to produce ammonia via the energy-intensive Haber-Bosch process have focused, in part, on developing
artificial catalysts for electrochemical processes which mimic natural bacteria. The key concept in this approach is to counterfeit
well-designed nitrogenase structures with inorganic structures that are made of similar elements and configurations in order to
understand the impact of these structures in the nitrogen fixation catalysis process. These inorganic structures can be elaborate
copies of the molybdenum−iron−sulfur active site in nitrogenase, such as MoFe3S4 cubane and their bridged clusters,6−8 or simply
layered dichalcogenide MoS2. The advantages of using MoS2 as a raw catalyst for the nitrogen reduction reaction (NRR) include
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The Catalyst Review

January 2020

The Catalyst Review January 2020

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