The catalytic effects of sulfur in ethane dehydroaromatization

Article abstract of DOI:10.1039/d0cc00408a

In this work, we investigated the catalytic effect of adding sulfur on Zn/ZSM-5 catalyst for direct conversion of ethane to aromatics. We show that the continuous addition of hydrogen sulfide (H₂S) effectively stabilizes zinc, prevents coking and results in a highly selective and stable catalyst. Considering the high content of sulfur in shale gas resources, these results highlight the importance of investigating catalysts under realistic operating conditions.

Full text of DOI:10.1039/d0cc00408a

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The catalytic effects of sulfur in ethane dehydroaromatization
Farnoosh Goodarzi,a Lars P. Hansen,b Stig Helveg,b Jerrik Mielby,a Thoa T.M. Nguyen,b Finn
Joensen b and Søren Kegnæs*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In this work, we investigated the catalytic effect of adding sulfur on alkenes. The most studied catalyst, which has shown high
Zn/ZSM-5 catalyst for direct conversion of ethane to aromatics. We catalytic performance in this process, is zinc-containing ZSM-5,
show that the continuous addition of hydrogen sulfide (H2S) providing both dehydrogenating and acidic sites.3–7 However,
effectively stabalizes zinc, prevents coking and results in a highly the two most important challenges for this catalyst deactivation
selective and stable catalyst. Considering the high content of sulfur are coke deposition and zinc leaching under the reducing
in shale gas resources, these results highlight the importance of conditions of the process. Based on previous studies,8,9 the
investigating catalysts under realistic operating conditions.
dissociation energy of C−C bonds is lower than that of C−H
bonds; therefore, cracking reactions and rapid coke formation
occur as a result. Consequently, an efficient active site is
required to selectively activate the C−H bond and suppress
undesired C−C bond cleavage.
The population growth and general progress in living standards
have resulted in a gap between the production and demand of
chemicals. As a consequence of the gradual depletion of oil
reserves and the so-called ‘shale gas revolution’, the direct
conversion of light alkanes such as methane and ethane into
value-added chemicals has therefore attracted increasing
interest. Ethane is the second most abundant compound in
shale gas and the over-supply of ethane has decreased the
prices over the last few years.1 In particular, ethane may be
converted into valuable aromatic products such as benzene,
toluene and xylene (BTX products), which are important
intermediates in the production of several bulk chemical
products.
The reaction pathway for the conversion of light alkanes to
aromatics on pure acid HZSM-5 zeolite, proposed by Guisnet et
al. (1992)2, shows that the first step is dehydrogenation and
demonstrated that the limiting step is the formation of alkenes
from alkanes. Afterward, aromatics are formed through
oligomerization of the alkenes, followed by, cyclization and
hydrogen transfer. However, on pure ZSM-5, cracking is a
competing reaction; therefore, a bi-functional catalyst helps to
improve the direct conversion of alkanes to aromatics by first
dehydrogenating the alkane with minimized cracking,
subsequently oligomerizing and, eventually, aromatizing the
In an industrial perspective sulfur compounds are generally
considered a poison to most heterogeneous catalysts.10
However, in some processes sulfur has been proven to enhance
the activity or modify the selectivity such as Fischer–Tropsch
synthesis,11,12 catalytic reforming,13,14 and hydrogenation of
hydrocarbons15–17 as well as hydrocarbon oxidation18,19
.
Nowadays, different sulfide catalysts are widely used in
hydrogenation processes.20,21 Natural gas typically contains
sulfur, ranging from ppmv to percent levels. Therefore, in
catalytic processing of natural gas liquids (C2+ hydrocarbons),
sulfur tolerant catalysts would be preferred, thus saving the
cost associated with the removal of sulfur contaminants such as
hydrogen sulfide and mercaptans from the feed stream. In case
of direct conversion of sulfur-containing light alkanes to
aromatics, sulfur converts to H2S that is easily separated (and
recycled to the reactor) from the liquid aromatic product.
Metal sulfides are promising catalysts for catalytic
dehydrogenation of alkanes by improving catalytic activity and
suppressing coke formation22–26. Resasco et al.22 showed that
nickel sulfide catalysts improved isobutene selectivity and
decreased the rate of coke deposition; however, catalytic
activity was not high in the dehydrogenation of isobutane. In
addition, Wang et al.,23,25,27 investigated the performance of
different metal sulfide catalysts and demonstrated that sulfided
catalysts had higher catalytic performance in the first 30 min of
the reaction compared to metal oxide catalysts. Given this study
on metal sulfides, sulfided zinc-containing ZSM-5 may be the
a.DTU Chemistry, Technical University of Denmark, Kemitorvet 207, DK-2800 Kgs.
Lyngby, Denmark. E-mail: skk@kemi.dtu.dk.
b.Haldor Topsoe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark.
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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ideal stable catalyst for ethane dehydroaromatization by
utilizing the sulfur compound in the feed stream to stabilize zinc
and reduce coke formation.
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DOI: 10.1039/D0CC00408A
Therefore, in our work, the performance of sulfided Zn/ZSM-5
catalyst was investigated in an H2S containing ethane feed and
compared to Zn/ZSM-5 operating under sulfur-free conditions
at 550 °C. The ZSM-5 zeolite used in this work was synthesized
conventional HZSM-5 zeolite (synthesis procedure is in
supporting information), designated as ZSM-5. The zeolite was
impregnated with zinc nitrate to achieve 5 wt% Zn in the
catalyst, designated as ZSM-5-Zn. The ZSM-5-Zn catalyst was
sulfided in the reactor under H2S/N2 flow (500 ppmv H2S in N2)
and assigned as ZSM-5-Zn-S.
Fig. 2. TEM images of ZSM-5-Zn a) as prepared and b) after 2 hours sulfidation in
dedicated electron microscope under exposure to 0.7 mbar H2S at 350 ºC.
X-ray powder diffraction patterns of ZSM-5-Zn and ZSM-5-Zn-S
samples show the characteristic diffraction peaks of the MFI
zeolite structure (Fig. S1). Nitrogen physisorption isotherms of
ZSM-5 is also presented in Fig. S1. Type I isotherm was observed
in N2 adsorption-desorption isotherm study of the zeolite
sample, which confirms the microporous structure of the
synthesized zeolite. Textural properties of the synthesized ZSM-
5 is summerized in Table S1. SEM-EDS elemental mapping of Zn
and S for ZSM-5-Zn-S shows a uniform distribution of the zinc
and sulfur in the sample (Fig. S2).
To determine the effect of sulfidation on zinc, XPS
characterization was performed on ZSM-5-Zn and ZSM-5-Zn-S.
Fig. 1 shows the deconvoluted spectra of Zn 2p3/2 for the two
samples. The characteristic peak in ZSM-5-Zn sample at 1022.4
eV is assigned to the Zn 2p3/2 peak of Zn2+. The observed binding
energy for the aforementioned peak is shifted 0.3 eV towards
higher energy in the case of ZSM-5-Zn-S. In a previous study
done by Ali et al.,28 it was demonstrated that this shift is caused
because of ZnS species. Furthermore, deconvolution of Zn 2p3/2
peak for ZSM-5-Zn-S shows the presence of two zinc species.
The peaks at lower and higher binding energy are attributed to
Zn-S and Zn-O bond, respectively29. Therefore, the XPS result is
evidence to the formation of zinc sulfide species.
image in Fig. 2a reveals parts of two zeolite crystals of uniform
contrast with the crystal to the left showing lattice fringes of
spacing 0.95 nm in accordance with (111) planes of the ZSM-5
(MFI) structure. Fig. 2b shows that, after sulfidation, the zeolite
crystals are decorated by dark contrasted nanoparticles ranging
from 1.5-12.0 nm in size with an average diameter of 3.8 nm
(based on 775 particles from 9 image areas. In addition, particle
size distribution is provided in Fig.S4). Considering the
comparable sulfidation conditions between TEM and samples
applied for XPS, these nanoparticles are likely zinc sulfide
species since their presence was confirmed by XPS. We noted
that the exposure to the electron beam prior to or during
sulfidation more than a couple of minutes appeared to prevent
or affect the formation of highly dispersed nanoparticles in 6
out of 6 measured image areas (Supplementary Information Fig.
S5-S6). This effect was observed as absence of particles in
previously imaged areas or disappearance of nanoparticles over
time; however, the particle size distribution was not affected
due to post-mortem imaging after sulfidation from areas not
previously exposed to the electron beam. Therefore, Fig. 2
shows images from two different areas to represent stages and
insights into the sulfidation process.
Table S2 summarizes the results obtained from ammonia TPD
for measuring the concentration of acid sites in ZSM-5, ZSM-5-
Zn and ZSM-5-Zn-S. Results obtained from ammonia
temperature-programmed desorption (TPD) at temperatures
between 180-550 ºC show that the introduction of zinc reduces
the concentration of strong acid sites, which are attributed to
Brønsted acid sites, from 0.09 to 0.04 mmol/g. However,
sulfidation has almost no effect on the total acidity of the
treated ZSM-5-Zn sample.
Sulfidation of ZSM-5-Zn sample was studied using transmission
electron microscopy (TEM). Fig. 2 shows TEM images of the as-
prepared ZSM-5-Zn and after 2 hours of exposure to 0.7 mbar
H2S (equivalent to ~700 ppmv H2S under standard 1atm
experiments) at 350 ºC in the microscope dedicated to
sulfidation experiments30 (Supplementary Fig. S3). The TEM
The catalytic performance of ZSM-5-Zn was evaluated under
sulfur-free conditions. Whereas ZSM-5-Zn-S catalyst was tested
in ethane with 60 ppmv H2S in the feed. The catalytic activity
and BTX selectivity of the sulfided catalyst is compared to the
relevant catalyst in the absence of sulfur, presented in Fig. 3, at
550 °C and 2 barg with gas hourly space velocity (GHSV)= 0.3 h-
1. Fig. 4a shows the conversion of ethane over ZSM-5-Zn without
sulfur in the feed and ZSM-5-Zn-S in the presence of 60 ppmv
sulfur in ethane feed. The deactivation trends under sulfur-free
conditions for ZSM-5-Zn and ZSM-5-Zn-S are very different. The
Fig. 1. XPS deconvolution spectra of Zn 2p3/2 level for a) ZSM-5-Zn and b) ZSM-5-Zn-S.
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initial conversion under sulfur-free conditions decreased rapidly Fig. 4 for ZSM-5-Zn and ZSM-5-Zn-S catalysts. In general,
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DOI: 10.1039/D0CC00408A
from around 40% to almost 11% after 30 h of operation. On the sulfidation of the catalyst has a profound influence on
contrary, the catalytic activity of ZSM-5-Zn-S in the presence of product yields (see Fig.4 and Fig. S7): the sulfided catalyst
sulfur showed improved stability and slightly increased from 8% produces virtually no methane (Fig. 4a) and much less heavy
to 11% after 30 h on stream. Furthermore, the conversion over aromatics (Fig. 4c). Particularly interesting is the fact that, in the
ZSM-5-Zn-S remains almost 11% after 40 h of time on stream presence of H2S, ethylene selectivity decreases (Fig. S8) while
while ZSM-5-Zn catalyst continues to deactivate (Fig. 3(a)). This BTX selectivity (Fig. 3) increases over time. To increase the
may indicate that, in the presence of sulfur, this catalyst conversion, ethylene must be converted by further reactions
provides improved longevity. As Fig. 3b shows, selectivity over strong acid sites to form aromatics to overcome the
towards BTX has an increasing trend in the ZSM-5-Zn-S sample thermodynamic limitation in ethane dehydrogenation.
while it declines over time for the ZSM-5-Zn sample at a reaction Therefore, the lower conversion we observe on sulfided
temperature of 550 ºC. BTX selectivity that decreased to around samples may indicate that insufficient acidity leads to the
12% under sulfur-free conditions for ZSM-5-Zn, showed an accumulation of ethylene but as time passes, the conversion
increasing trend over time for the sulfided Zn/ZSM-5 catalyst. In
addition, already after 15 hours, the selectivity towards BTX for
the ZSM-5-Zn-S catalyst exceeds that of the ZSM-5-Zn operating
under sulfur-free conditions and reached 22% after 40 h.
Coke analyses on the spent catalysts confirm that less carbon
was deposited on the spent ZSM-5-Zn-S compared to spent
ZSM-5-Zn (Table S3). Furthermore, residual zinc in the formerly
spent catalyst is almost the same as in the fresh catalyst (only
10 % loss) whereas, the latter had lost more than 50% of the
zinc, indicating that the presence of sulfur effectively inhibits
the leaching of zinc under operating conditions. The yield of
other products such as C1, ethylene and heavy aromatics (C9+)
consisting of naphthalene derivatives over time are shown in
Fig. 3. a) Catalytic activity and b) BTX selectivity of ZSM-5-Zn in sulphur-free ethane feed
and ZSM-5-Zn-S in the presence of 60 ppmv H2S in ethane feed for ethane
dehydroaromatization at 550 °C, 2 barg and GHSV=0.3 h-1.
Fig. 4. a) C1, b) ethylene and C) heavy aromatics yield for ZSM-5-Zn in sulphur-free ethane
feed (filled triangles) and ZSM-5-Zn-S in the presence of 60 ppmv H2S in ethane feed
(empty triangles) for ethane dehydroaromatization at 550 °C, 2 barg and GHSV=0.3 h-1
.
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vol. 75, pp. 1707–1710.
T. Liang, H. Toghiani and Y. Xiang, Ind. Eng. Chem. Res.,
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increases and remains stable (Fig. 3). From our results, it looks
like that rate of dehydrogenation and aromatization reaches a
balance after some 10-15 hours. Since ethylene selectivity is
dependent on aromatization rate, the decrease of ethylene
selectivity for sulfided catalyst over time is due to increase of
ethylene conversion to aromatics.
Based on our previous results31, we propose that ZnOH+ species,
which are weak acid sites32, is responsible for dehydrogenation
of ethane to ethylene. Notably, the NH3-TPD of the sulfided and
non-sulfided catalysts show a similar density of weak acid sites.
Therefore, we speculate that the active ZnOH+ are largly
unaffected by the sulfidation. As previously emphasized, the 10
strong acid sites catalyze the oligomerization and cyclization, 11
which increases the conversion of ethylene. Despite the similar 12
concentration of strong acid sites presented in Table S3, we did 13
observe a 14 ºC shift towards lower temperatures in the NH3-
TPD (Fig.S9). This indicates that the sulfidation decreases the 14
strength of the strong acid sites, which may also explain less
coke formation and selectivity towards heavy aromatics. The 15
intimate mechanistic details remain unclear but will be the topic
of future studies.
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In conclusion, we investigated the influence of sulfur on the
18
catalytic performance and selectivity of ZSM-5 containing 5 wt%
Zn catalyst in the direct conversion of ethane to BTX. Presence
of zinc sulfide species was confirmed by XPS and the formation
of these species was observed by TEM. Catalytic results showed
that the presence of sulfur effectively reduces the selectivity
towards undesired methane and heavy aromatics products. In
addition, even though initial BTX selectivity decreases
compared to the sulphur-free analogue, it has an increasing
trend over time and seems to remain stable even after 40 hours
on stream. On the contrary, the sulfur-free catalyst selectivity
towards BTX shows a decreasing trend. Thus, we believe that
with optimizing the amount of sulfur in the feed, zinc sulfide
catalysts can be promising catalysts in dehydroaromatization of
ethane.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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