Multilayered Two-Dimensional V2CTx MXene for Methane Dehydroaromatization

Article abstract of DOI:10.1002/cctc.201902366

We report a thermally stable multilayered two-dimensional vanadium carbide (V₂CT_x) MXenes catalyst for the direct conversion of methane (CH₄) into benzene (C₆H₆). The multilayered carbide structure shows state-of-the-art CH₄ conversion 11.8 % with a C₆H₆ formation rate of 1.9 mmol g_cat^?1h^?1 (4.84 % C₆H₆ yield) at 700 °C, which is comparable to the benchmark Mo/ZSM-5 catalyst. The structure-activity relationship was explored by numerous characterization techniques including in-situ/operando Raman-MS, ex-situ X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and ammonia temperature programmed desorption (NH₃-TPD). This work provides a new platform to design and explore multilayered two-dimensional catalysts demonstrating confinement effect to convert CH₄ into *C₂H₃ intermediates which further oligomerize inside multilayered structures producing C₆H₆ as the final product.

Full text of DOI:10.1002/cctc.201902366

Accepted Article
Title: Multilayered Two-Dimensional V2CTx MXene for Methane
Dehydroaromatization
Authors: Raj Thakur, Megan Hoffman, Armin VahidMohammadi, Justin
Smith, Mingyang Chi, Bruce Tatarchuk, Majid Beidaghi, and
Carlos Carrero
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To be cited as: ChemCatChem 10.1002/cctc.201902366
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10.1002/cctc.201902366
ChemCatChem
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Multilayered Two-Dimensional V2CTx MXene for Methane
Dehydroaromatization
Raj Thakur, [a] Megan Hoffman, [a] Armin VahidMohammadi, [b] Justin Smith, [a] Mingyang Chi, [a] Bruce
Tatarchuk, [a] Majid Beidaghi, [b] and Carlos A. Carrero*[a]
[a]
[b]
R. Thakur, M. Hoffman, J. Smith, M. Chi, Dr. B.Tatarchuk, Dr. C.A.Carrero*
Department of Chemical Engineering
Auburn University
212 Ross Hall
E-mail: cac0134@auburn.edu
Dr. A. VahidMohammadi, Dr. M. Beidaghi
Department of Materials Engineering
Auburn University
279 Wilmore laboratories
Supporting information for this article is given via a link at the end of the document.
Abstract: We report a thermally stable multilayered two-dimensional
vanadium carbide (V2CTx) MXenes catalyst for the direct conversion
of methane (CH4) into benzene (C6H6). The multilayered carbide
structure shows state-of-the-art CH4 conversion 11.8% with a C6H6
formation rate of 1.9 mmol gcat-1h-1 (4.84% C6H6 yield) at 700 0C, which
is comparable to the benchmark Mo/ZSM-5 catalyst. The structure-
activity relationship was explored by numerous characterization
techniques including in-situ/operando Raman-MS, ex-situ X-ray
diffraction (XRD), scanning electron microscopy (SEM), X-ray
photoelectron spectroscopy (XPS), and ammonia temperature
programmed desorption (NH3-TPD). This work provides a new
platform to design and explore multilayered two-dimensional catalysts
demonstrating confinement effect to convert CH4 into *C2H3
intermediates which further oligomerizes inside multilayered
structures producing C6H6 as a final product.
(MDA). Recently, we reported m-V2CTx as an active, selective
and coke-resistant catalyst for the dry reforming of methane at
800 0C.[10] We proposed an in-situ redox mechanism that
demonstrates the catalytic attractiveness of MXenes for reactions
where deactivation by carbon deposition is still a present issue.
Taking advantage and simultaneously combining the i) structure
of multilayered MXenes, that eventually provides the confinement
effect that has been proposed as responsible to enhance the
selectivity toward aromatics when using zeolites,[11] ii) the oxy-
carbide species coexisting on the surface of MXenes,[12] and iii)
the relatively good thermal and structural stability of m-V2CTx
under various conditions,[9] paved the path to study the MDA
reaction over this promising m-V2CTx MXenes catalyst (Reaction
1).
6CH4 ⇄ C6H6 + 9H2
(1)
∆H0298K = 434 kJ mol-1
Multilayered MXenes are
a relatively new family of two-
The MDA reaction has gained interest essentially due to i) the
ever-growing demand of aromatics, ii) the lack of on-purpose
technologies for the production of aromatics, driven by the recent
trends of using shale gas as feedstocks in cracker units,[11d, 13] and
iii) the notable discrepancies in the literature regarding the nature
of active site(s) and the possible reaction mechanism when using
the benchmark Mo/ZSM-5 catalysts.[14] The MDA process is far
from industrial implementation due to stability-related issues as a
result of various reasons such as i) rapid catalyst deactivation
caused by carbonaceous deposits obstructing the catalytically
active sites,[15] ii) extraction of framework Al species above
700⁰C,[11c, 13a] iii) migration of the active site (Mo species) from
internal to external framework in zeolites,[13a] iv) rapid increase in
graphitic-amorphous carbon over TOS demanding severe
regeneration processes,[16] and v) the lack of consensus about the
nature of active sites whether they are oxidic (MoOx), carbidic
(MoCx) and/or oxy-carbidic (MoCxOy).[14]
dimensional (2D) metal carbides and nitrides.[1] They are named
by their graphene-like morphology, in accordance with the
general chemical formula Mn+1XnTx, where n can be 1,2 , or 3, M
is an early transition metal, X is carbon and/or nitrogen, T
indicates various surface terminations (-OH, O- and F-), which
depend on the preparation method, and x is the number of surface
functional groups per unit formula (Scheme S1).[2] Already, about
20 different MXenes have been synthetized, while the properties
of several have been theoretically predicted.[1a, 3] Their potential
and attractiveness reside on both their unique electronic and
mechanical properties, and the bonding between interlayers
which exhibits a combination of ionic, metallic, and covalent
character.[1a, 4]
Only few studies have been reported on the catalytic activity of
MXenes at relatively high temperatures, such as the ammonia
perchlorate decomposition,[1a, 5] water gas shift reaction,[6] ethyl
benzene dehydrogenation,[7] and propane dehydrogenation.[8]
Based on the good thermal and chemical stability of m-V2CTx,
especially under inert and reducing atmospheres,[9] we decided to
further explore the catalytic properties of this material for the
production of liquid aromatics via methane dehydroaromatization
Herein, we report the use of m-V2CTx MXene as an active,
selective, and potentially regenerable catalyst for MDA. We
synthesize m-V2CTx by selectively etching Al from the
corresponding precursor, so-called MAX phase (V2AlC), using
hydrofluoric acid (Scheme 1).[9, 17]
1
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Initially, C-H bond activation occurs inside the channels of ZSM-5
forming methyl radicals, that subsequently dimerize forming C2
species and further undergoing oligomerization and cyclization
over Bronsted acid sites to finally produce benzene (Scheme
S2).[11a, 13a]
.
Scanning electron microscopy (SEM) confirms the formation of m-
V2CTx MXene (Figure 1b) after HF-treatment of V2AlC Max phase
(Figure 1a). The precursor, V2AlC, shows its typical morphology
analogous to a tightly stacked un-exfoliated graphite as observed
by scanning electron microscopy (SEM) (Figure 1a).[9, 17a] After
the HF treatment, the tightly stacked layers transform to loosen
accordion-like morphology with stacked layers well separated
from each other due to the etching of Al atoms from MAX phase
(Figure 1b).[9, 17a]
Scheme 1. Schematic representation of the V2AlC MAX phase transformation
into V2CTx MXene after HF treatment.
In line with the literature, we carried out the MDA reaction at
700⁰C using 50 mol% of both methane (CH4) and inert, primarily
producing benzene and hydrogen.[18] As stated in various studies
using the benchmarked Mo/ZSM-5 catalyst, the first step in MDA
reaction involves the carburization of the catalyst,[11d, 19] wherein
the molybdenum oxide, anchored in the internal framework of the
zeolite, is converted to carbide/oxy-carbide, which then acts as
the active site for converting methane.[11b, 11c, 13a, 14, 20]
Interestingly, the well-defined layered structure provides an
ͦ
interlamellar space of ~ 7.0 A,[9] which is slightly larger than the
ͦ
kinetic diameter of benzene (~ 6.0 A).[11d, 18] These interlamellar
spaces provide the confinement necessary for the oligomerization
and cyclization of intermediates (C2 species) to then form
benzene. X-ray diffraction (XRD) (Figure 1c) shows
a
characteristic peak at 2θ of 8.99o which corresponds to (0002)
plane for m-V2CTx, along with traces of unreacted V2AlC MAX
phase (13.5o and 41.3o). The latter appears in almost all MXenes
synthesized by HF etching, because more severe etching
procedures lead to the extraction of V as well, resulting in forming
carbide-derive-carbon (CDC) materials.[1a, 9] The Raman spectra
(Figure 1d) for the V2AlC MAX phase exhibits sharp peaks at 158
(E2g), 239 (E2g), 258 (E1g), and 360 (A1g) cm-1,[21] while for the m-
V2CTx exhibits boarder peaks at 379, 495, 630, 800, 1040, 1364,
and 1594 cm-1. This broadening of peaks for m-V2CTx is attributed
to the large interlayer spacing,[21c] and the assignment of all the
Raman vibrations can be found elsewhere.[9] Amongst the various
vibrations, the Raman peak at 800 cm-1 corresponds to two
dimensional V2CTx MXene with terminal functional groups (F,
OH),[21c] which typically appears around 750 cm-1 for bulk VC
carbide.[21c] Also, we track the changes on the surface of m-V2CTx
using X-ray photoelectron spectroscopy (XPS) (Figure 1e). The
peak centered at ~513.3 eV (V2+) evidences both the presence of
unreacted V2AlC MAX phase and the V-C phase from the V2p
and C1s region, respectively. The peak centered at ~ 516.3 eV
(V4+) is attributed to the existence of a monolayer of vanadium
oxide on the surface of vanadium carbide.[17a] Details of the
deconvoluted XPS spectra in the C1s and O1s region be found in
Figure S1. The N2-physisorption study (Figure 1f) of m-V2CTx
shows a surface area of 10.25 m2/g and H3-type adsorption-
desorption trajectory with a hysteresis loop (P/P0 = 0.5-1.0)
distinctive for macroporous and layered materials.[7] While the
V2AlC (MAX phase) has a surface area of 1.50 m2/g and no
porosity.
a
b
1 µm
1 µm
After confirming the formation of m-V2CTx, we explored its
catalytic activity in MDA under industrially attractive conditions
and specifically focused on benzene formation (Figure 2). As
shown in Figure 2a, the rate of formation of C6H6 initially has a
maximum formation rate of 1.9 mmol C6H6/gcat.h at 11.8 % CH4
conversion, which decreases as a function of time up to 0.4 mmol
C6H6/gcat.h in about ten hours. Similar deactivation trend is
observed when using Mo/ZSM-5 catalyst (Figure S2), although
with higher magnitude (2.3 to 1.1 mmol C6H6/gcat.h within seven
hours). The C6H6 yield for both catalysts can be found in Figure
S3.
Figure 1. SEM micrograph of (a) V2AlC MAX and (b) m-V2CTx MXene. (c-d)
XRD diffractogram and Raman of V2AlC MAX phase and the m-V2CTx MXenes.
(JCPDS No. 29-0101). (e) XPS V2p spectra of as prepared m-V2CTx MXenes.
(f) N2 physisorption of V2AlC MAX and m-V2CTx MXene. SA=Surface Area.
2
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a
Figure 2. (a) CH4 conversion and C6H6 formation rates over m-V2CTx MXene.
(b-c) Operando Raman -MS plot (d) Relative rate of D (1351 cm-1) and G (1592
cm-1) carbon formation as a function of TOS at 700 0C.
Figure 3. (a) SEM, (b) XRD, and XPS of spent m-V2CTx MXenes, evidencing
the excellent structural and thermal stability.
Additionally, the C1s spectra (Figure 3d) shows prominent C-C
(~284.8 eV) along with V-C (~282.2 eV) peaks. The C-C (~284.8
eV) peak increases relative to V-C (~282.2 eV), confirming the
deposition of carbon on the surface. The N2 physisorption profile
for the spent catalyst can be found in Figure S5, where the typical
H3-type N2 adsorption-desorption trajectory is observed.
As for Mo/ZSM-5 catalyst,[11, 13a, 14] we propose that first the CH4
is activated over metal (V-C) sites of m-V2CTx MXene, thereby
forming CHx species, which further undergoes oligomerization
and dimerization through a series of gas phase reactions in
between the interlayers yielding C6H6, C2H4 and H2. (Figure 4).
Methyl and ethyl oligomerization primarily occurring in gas phase
to yield benzene have already been reported when using
Fe©SiO2 catalysts.[13b]
On the other hand, the V2AlC MAX phase exhibits no reactivity in
MDA (Figure S4), thus proving the need of confinement to
produce benzene.[11d]
Operand Raman spectroscopy, evidences i) the presence of the
2D structure of m-V2CTx (peaks at 800 cm-1 and 1040 cm-1
assigned to 2D-VC and 2D-V=O,[9] respectively) (Figure 2b), ii)
the formation of C2H4 and H2 as the initial products followed by
the formation of C6H6 (Figure 2c), and iii) the development of solid
amorphous and graphitic carbon on the surface (eventually in
between the layers) of m-V2CTx (Figure 2b&d). Such
carbonaceous species limit the accessibility of CH4 inside the
channels, explaining the deactivation. As the reaction proceeds,
the amount of C6H6 starts to decrease (Figure 2a and c), which
coincides with the rise of D/G carbon Raman peaks (Figure 2b).
The D-carbon peak corresponds to the sp3 amorphous carbon
while the G-carbon peak is attributed to the sp2 graphitic carbon.[9]
The gradual drop in C6H6 can be attributed to precipitation of
carbon (amorphous and graphitic) in between the layers, thereby
blocking the active sites and ultimately reducing the activity of the
catalyst. Figure 2c shows that the formation of coke starts right
from the onset of MDA reaction as the C6H6 and H2 signals starts
to decre-ase.
We performed SEM, XRD, and XPS to the spent material to
further explore the deactivation process. The SEM micrographs
still shows up to certain extent the multilayered structure of the
material (Figure 3a), while XRD reveals that the (0002) MXene
peak remarkably decreases (noise level) and shift to a slightly
higher angle (Figure 3b). The almost disappearance of this
specific diffraction peak can be attributed to the deposition of
carbon between the layers, while its shift is primarily due to a
decrease in the lattice parameter because of dehydration.[9]
Finally, the V2p (Figure 3c) XPS spectra shows the presence of
516.3 eV (V4+) and ~513.3 eV (V2+) peaks, similarly to Figure 1e.
Figure 4. MDA reaction scheme over m-V2CTx MXene forming C6H6 and H2 as
the main reaction products.
However, the need of (weak) Lewis acid sites to catalyze the
oligomerization of C2 species to form benzene has been also
proposed.[22] m-V2CTx contains less, but still some, Lewis and
Brönsted sites when comparing to Mo/ZSM-5 (Figure S6). Such
acidity can be attributed to the presence of terminal functional
groups (e.g. OH and O-) and/or due to unreacted MAX Phase
within the MXene. At this point, we cannot conclusively state that
3
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solely the proposed gas-phase mechanism for MDA occurring in
a confinement environment is taking place when using m-V2CTx,
because it also contains acid sites, which might significantly
contribute during the oligomerization of the ethyl radicals (*C2H3)
to form benzene as proposed when using zeolite-based
catalysts.[11a, 11d, 13a]
milled, and sieved through a 400 mesh to obtain powders with an
average particle size of ∼32 μm.
Synthesis of V2CTx MXene
To synthesize V2CTx MXene, V2AlC MAX phase was treated with
48-50% concentrated hydrofluoric acid (ACS grade, BDH) in a
ratio of 1g powder to 20 mL etchant and stirred with a Teflon-
coated magnetic bar at 200 rpm for 92 h at room temperature. For
more details, the detailed procedure can be found elsewhere.[4a,
17a, 23] Briefly, the etched powder was then washed several times
using DI water and centrifuged at 3500 rpm for about 5 minutes
until the pH of the supernatant was higher than four. The MXene
powder was then filtered using a Celgard porous membrane,
In summary, we have shown that m-V2CTx MXene catalyzes the
formation of benzene via MDA at 700⁰C with a maximum
formation rate of 1.9 mmol C6H6/gcat.h (4.84% C6H6 yield), which
is comparable to state-of-the-art Mo/ZSM-5 catalysts (2.3 mmol
C6H6/gcat.h). We hope that this catalytic performance potentially
encourages the heterogeneous catalysis community to explore
the ever-growing family of MXenes in the upgrading of natural gas. rinsed with DI water and absolute ethanol, collected, and dried
The interesting properties observed by in-situ/operando Raman,
ex-situ XRD, XPS and SEM for the pristine and spent m-V2CTx
MXene, corroborate the remarkable and attractive stability of this
material when comparing to metal-containing zeolite catalysts,
which typically suffer from structural changes due to, for instance,
the expulsion of aluminum ions from the zeolite lattice forming
both octahedral non-framework aluminum ion and Al2(MoO4)3.[11d,
under vacuum for 24h.
Acknowledgements
The authors acknowledge financial support from the Departments
of Chemical and Materials Engineering at Auburn University. RT
and A.V.M. acknowledge the support from Auburn CO2 center and
Alabama EPSCoR Graduate Research Scholar Program (GRSP
Round 11 and 12) doctoral fellowships respectively. M.B.
acknowledges the support from Auburn University’s IGP program.
13a]
However, m-V2CT also deactivates due to the deposition of
amorphous/graphitic carbon, which remains as one of the
challenges for potential industrial application. The regeneration
protocol required to remove carbon from spent catalysts,
becomes an issue when using transition metal carbides materials
because of their total-bulk oxidation.
Ongoing research in our group on the use of MXenes for catalytic
applications at relatively high temperatures (>400 0C) has resulted
in developing a protocol to remove carbonaceous species and
avoiding total-bulk oxidation of the layered carbide phase.[10]
Playing with various regeneration protocols, tuning the acidity of
m-V2CTx, controlling the interlayer distance, and testing other
MXenes phases (Scheme S1) are the follow-up strategies we are
currently working on to gain new insights into the genesis of an
alternative, suitable, and regenerable no-zeolite catalyst for MDA.
Furthermore, since scaling-up the synthesis of MXenes is still an
issue, primarily due to the high exothermicity of the process and
the use of strong acids such as HF, we are currently applying a
factorial experimental design to increase the production of
multilayered MXenes per batch without altering the intrinsic
material structure and properties.
Conflict of interest
The authors declare no conflict of interest.
Keywords: heterogenous catalysis • carbides • kinetics •
methane • 2D material • MXenes
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Experimental Section
Synthesis of V2AlC MAX phase
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5
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Entry for the Table of Contents
R. Thakur,M. Hoffman, A
The transformation of CH4 into C6H6 and H2 over
highly thermally stable multilayered two-
dimensional structure, where the multilayer
structure provides necessary confinement for
the intermediate C2 radicals to produce C6H6.
VahidMohammadi, J. Smith, M. Chi,
B. Tatarchuk, M. Beidaghi, and C.
A. Carrero*
Multilayered Two-Dimensional
V2CTx MXenes for Methane
Dehydroaromatization
6
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