Catalytic Activity of Ti-based MXenes for the Hydrogenation of Furfural

Article abstract of DOI:10.1002/cctc.202000977

Herein we report on the catalytic activity of Ti-based MXenes (Ti₃CNT_z and Ti₃C₂T_z) for biomass transformation. MXenes were found to be active catalysts for the hydrogenation of furfural using either

Full text of DOI:10.1002/cctc.202000977

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Catalytic Activity of Ti-based MXenes for the Hydrogenation of
Furfural
Authors: Michael Naguib, Wenjie Tang, Katie L. Browning, Gabriel M.
Veith, Vineet Maliekkal, Matthew Neurock, and Alberto Villa
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ChemCatChem
10.1002/cctc.202000977
Catalytic Activity of Ti-based MXenes for the Hydrogenation of Furfural
Michael Naguib,*[a, b] Wenjie Tang, Katie L. Browning, Gabriel M. Veith,
[c]
[b]
[b]
Vineet Maliekkal , Matthew Neurock, and Alberto Villa,*[d]
[
c]
*[c]
[a]
Prof. M. Naguib
Department of Physics and Engineering Physics,
Tulane University,
6823 St Charles Ave
New Orleans LA 70118 (USA)
E-mail: naguib@tulane.edu
[b]
Prof. M. Naguib, K. L. Browning, Dr. G. M. Veith
Chemical Sciences Division,
Oak Ridge National Laboratory,
1
Bethel Valley Rd.
Oak Ridge TN 37831 (USA)
[c]
Dr. W. Tang, V. Maliekkal, Prof. M Neurock
Department of Chemical Engineering and Materials Science,
University of Minnesota,
421 Washington Ave. SE,
Minneapolis MN 55455 (USA)
E-mail: mneurock@umn.edu
[d]
Prof. A. Villa
Dipartimento di Chimica,
Università degli Studi di Milano,
Via Camillo Golgi, 19
Milan MI 20133 (Italy)
E-mail: alberto.villa@unimi.it
Abstract: Herein we report on the catalytic activity of Ti-based MXenes (Ti
3
CNT and
z
Ti
3
C
2
z
T ) for biomass transformation. MXenes were found to be active catalysts for the
hydrogenation of furfural using either gaseous hydrogen or 2-propanol as solvent and hydrogen
source. Both catalysts showed good activity in the conversion of furfural to furfuryl alcohol,
with furfuryl ether as the main by-product. Stability tests indicated that Ti
than Ti against deactivation. Ab initio calculations were used to examine the
hydrogenation and etherification reactions pathways and their corresponding reaction
energetics on the Ti CNT MXene. The results indicate that the hydrogenation of the carbonyl
3
CNT
z
is more stable
3
C
T
2 z
3
z
bond efficiently proceeds via the heterolytic activation of hydrogen over the metal-oxygen site
pair followed by the addition of the proton and hydride to the C and O atoms of the carbonyl.
The subsequent hydrogenation of the unsaturated furan ring via metal bound hydrogen,
however, is calculated to be much more difficult. Protons bound to the oxygen of the MXene
are acidic and can also catalyze the etherification of the resulting alcohols. The bifunctional
acid-metal site pairs also selectively catalyze the hydrogenolysis of the furfuryl alcohol to form
2
-methyl furan.
1
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10.1002/cctc.202000977
1
. Introduction
There has been an increasing interest in the selective transformation of lignocellulosic biomass
fine chemicals and fuels over the past decade.[1] In particular, furfural, derived from the
hemicellulosic fraction of lignocellulose, is considered to be a promising platform molecule.[1b,
2
]
Indeed many of the value added chemicals such as furfuryl alcohol, an intermediate for
[3]
producing resin or 2-methylfuran, a promising fuel additive for gasoline can be obtained, via
selective hydrogenation and hydrodeoxygenation reaction, respectively (Scheme 1).[4] Noble
metal catalysts based on Ru, Pd and Pt[2c, 3b, 5], and bimetallic catalysts such as PdAg have
been reported to convert furfural to the above mentioned products. However, efficient catalytic
materials based on non-noble-metals are preferable from an economic point of view.
Three dimensional transition metal carbides have shown a great potential as non-noble-
[6]
[7]
metal catalysts for biomass conversion . In hydrodeoxygenation reaction, transition metal
carbides can efficiently convert furfural to 2-methylfuran at moderate temperatures.[8]
Nanostructuring transition metal carbides increases their active surface area and enhances their
[9]
catalytic activity . Carbides are well known to catalyze hydrogenation, dehydrogenation,
hydrogenolysis and isomerization[8c, 10] and have been used effectively in the hydrogenolysis
of biomass.[4, 11] Initial studies by Boudart showed that W-carbide has properties similar to Pt
and can readily carry out dehydrogenation and hydrogenation reactions.[10a] The oxygen present
on many of the carbide surfaces is strongly bound and in the presence of hydrogen can readily
form Brønsted acid sites that can carry out acid catalyzed reactions and can promote along with
metal sites hydrogenolysis. Bhan[7b], for example, showed the presence of both metal and acid
sites on Mo
selective addition of oxygen.
Recently, a new family of two-dimensional (2D) transition metal carbides and
2
C and demonstrated how these sites and their reactivity can be tuned by the
carbonitrides, so-called MXenes, has been developed by the selective etching of atomically
thin metal layers from layered ternary carbides and carbonitrides called MAX phases.[12]
MXenes have a general composition of Mn+1
Ti, V, Cr, Nb, Mo, etc.), X stands for carbon and/or nitrogen, T
surface terminations (O, OH, F) that resulted from the etching in F-contained aqueous solution,
and n =1-3. With more than 15 unique members synthesized so far (e.g. Ti , Nb CT
TiNbT , Mo CT ), MXenes are the fastest growing family of 2D material. Similar to transition
X
n
T
z
where M stands for an early transition metal
(
z
stands for a mixture of the
3
C
2
T
z
2
z
,
z
2
z
metal carbides, MXenes are electrically conductive.[12c, 13]
2
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ChemCatChem
10.1002/cctc.202000977
Scheme 1 Proposed pathways for furfural hydrogenation.
These materials are similar to the carbides and may offer significant promise in terms
of controlled functionalization and tunability. Thanks to their surface terminations, MXenes
are hydrophilic.[12c] This combination of hydrophilicity and electronic conductivity is very rare
in 2D materials. MXenes also exhibit a highly negative zeta potential in neutral and high pH
values which allows them to disperse easily in water.[14] MXenes were found to be promising
for many applications such as electrochemical energy storage,[15] water purification[16] and
sensing[17]
In the catalysis area, MXenes have been explored as supports for catalytic materials
like Pt,[18] Ru,[19] RuCo,[20] RuNi,[21] and CuO .[22] MXenes were also found to help promote or
co-catalyze some reactions.[23] For example, adding 5 wt% of Ti
to rutile resulted in 4
.
2
3
C T
2 z
times enhancement in visible-light photocatalytic hydrogen evolution compared to pure
rutile.[23b] Liu et al. reported on a significant reduction in the dehydrogenation onset
temperature of MgH
2
(reduced from 278 °C to 185 °C) by adding 5 wt.% of Ti
3
C
2
T
.[24]
z
Combining carbon nitride nanosheets with Ti
3
C
T
2 z
resulted in a high electrocatalytic activity
when tested for oxygen-evolution reaction.[25] MXenes were also found to be promising
electrocatalyst for hydrogen evolution reaction.[25-26] Moreover, it has been demonstrated that
TiO
2
are active in the transfer hydrogenation of C=O bond.[27] Diao et al., reported that Ti
3
C
T
2 z
can catalyze ethylbenzene dehydrogenation with a reactivity of 92 μmol m h–1[28]. Recently,
–2
3
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10.1002/cctc.202000977
Blanco et al. showed that can be used for hydrodeoxygenation (HDO) of guaiacol resulting in
phenol and methylanisole[29]. Interestingly, they showed that the MXene surface has both weak
acid and basic sites.
In the present work we report for the first time on the catalytic activity of MXenes in
[4]
the selective hydrogenation of furfural, a molecule derived from hemicelluloses. In particular
we show that Ti-MXenes (viz. Ti and Ti CNT ) promote the hydrogenation to furfuryl
alcohol, ethers and the hydrodeoxygenation to 2-methyl furan. Ti and Ti CNT were
3
C
2
T
z
3
z
3
C
2
T
z
3
z
selected for this study since the former was the first reported MXene[12b] and the most studied
MXene. The latter is the only carbonitride MXene reported so far, which would allow us to
study the role of nitrogen in the catalytic reaction for the same transition metal MXene.
2
. Results and Discussion
Ti MAX phases and their corresponding MXenes were prepared following the procedure
reported by Naguib et al..[12a, 12b] Synthesis of both Ti
and Ti CNT was confirmed by
3
C
2
T
z
3
z
using X-ray diffraction. As shown in Figure 1, etching Al from MAX phases resulted in
vanishing the 100% peaks (around 2θ of 39°) and broadening of the (0002) peaks.
Figure 1. X-ray diffraction patterns of samples before and after reaction for a) Ti
Ti CNT , detailed (0002) peak areas are presented in the insets. The blue diamonds in the
Figure b represent the peaks position of Anatase [PDF#02-0406].
3
C
2
z
T and b)
3
z
The catalysts were tested for the hydrogenation of furfural under hydrogen or using 2-
propanol as hydrogen source (453 K, 5 bar H
Table S1 and Figure 2.
2
or N ). The results are presented in Table 1 and
2
4
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10.1002/cctc.202000977
Table 1. Furfural hydrogenation in 2 propanol. [a]
Pressure
bar), gas
Catalyst
Activity[b] Conv
%)
Selectivity (%)[c]
Mass
Balance
(
(
Furfuryl 2-methyl-
alcohol furan
Ethers
(%)
1
2
5
5
5
5
5
5
, N
, N
, N
, H
, H
, H
2
2
2
2
2
2
No-cat
-
2
-
-
-
-
9
-
Ti
Ti
3
3
CNT
z
72
88
-
46
36
3
49
52
17
10
23
23
98
C
2
T
z
11
97
No-cat
Ti
Ti
3
3
CNT
z
126
145
59
62
43
49
25
13
23
27
5
6
96
95
C T
2 z
[
a] Reaction conditions: Furfural = 0.3 M; F/catalyst ratio = 50 wt/wt, 453 K, 5 bar N
2
, reaction time 48 h. [b]
-
1
-1
Activity (Converted mmolfurfural
gcatalyst h ), calculated after 3 h of reaction. [c] Selectivity at 35% of conversion.
Both catalysts showed higher activity when H
2
was used instead of N
2
, but the selectivity is not
showed a higher initial
significantly influenced by the hydrogen source (Table 1). Ti
3
C
T
2 z
catalyst h-1, using N
-
1
and H
respectively)
(
after 3 h) activity (88 and 145 converted mmolfurfural
compared to Ti CNT (72 and 126 converted mmolfurfural
respectively). However, comparing the conversion after 48 h, Ti
Ti (46% and 36% conversion for Ti CNT and Ti at bar 5 N
). On the contrary, in presence of H a similar conversion over the Ti CNT
achieved (59% and 62% conversion for Ti CNT and Ti , % bar H ).
Indeed, by examining the reaction profiles (Figure 2a), it can be observed that Ti
underwent evident deactivation when N was used. The resistance against deactivation was
investigated performing stability tests in presence of N (Figure 2b). Recycling experiments
were carried out by filtering and using the catalyst in the next run without any further
purification. Ti CNT exhibits good stability for six consecutive runs, whereas Ti
confirmed the lower resistance to deactivation.
g
2
2
catalyst h-1, using N
-
1
and H
3
z
g
2
2
3
CNT
z
is more active than
, respectively, Table
3
C
2
T
z
3
z
3
C
2
T
z
2
1
2
3
z
and Ti
3
C
2
z
T was
3
z
3
C
2
T
z
2
3
C T
2 z
2
2
3
z
3
C
T
2 z
5
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10.1002/cctc.202000977
Figure 2 Catalytic performance of Ti
test.
3
C
2
T
z
and Ti
3
CNT
z
a) reaction profiles, and b) stability
The selectivity of Ti
showing a similar trend, regardless of the utilization of H
product for Ti and Ti CNT , respectively (Table 1), indicating a very similar active site.
-methylfuran and ethers were also obtained as by-product (Table 1). Ti-based MXenes also
3
C
T
2 z
and Ti
3
CNT
z
were compared at iso-conversion (35%)
2
or N . Furfuryl alcohol was the main
2
3
C
2
T
z
3
z
2
showed the ability to promote the hydrodeoxygenation reaction to 2-methyl furan. Ethers,
deriving from the etherification of furfural with the solvent (2- propanol) or furfuryl alcohol
were also found (Table 1). Only traces of tetrahydrofurfuryl alcohol (THFA) were obtained,
evidence that Ti-based MXenes are not able to hydrogenate the furanic ring under these
reaction conditions. By analyzing the selectivity at different reaction times (Table S1), we
observed a decreased selectivity to furfuryl alcohol with the formation of higher amount to 2-
methylfuran and increasing the conversion. The result confirmed that methyl furan is derived
from the successive hydrogenolysis of furfuryl alcohol (Scheme 1).
XRD, SEM and XPS investigation of the catalysts after reaction were performed and
compared to the fresh ones, to address the different stability during the reaction of the two
catalysts (Figure 1,3 and 4). Figure 1 shows the XRD patterns for both Ti
before and after testing. In both cases, MXene peaks were preserved but with changes in c
lattice parameter (LP). As shown in the inset of Figure 1a, the 0002 peak of pristine Ti at
θ of 8.96º (c-LP 19.7 Å) was replaced by 3 peaks after testing. One of the peaks was located
3
C
2
T
z
and Ti
3
CNT
z
3
C
T
2 z
2
at higher angle (2θ of 9.24º) that corresponds to c-LP of ~ 19.1 Å. The other two peaks were
shifted to lower angles of 3.94º and 5.56º (corresponding to c-LP 44.8 and 31.8 Å, respectively)
compared to pristine MXene. Assuming that they all have similar chemical composition, by
comparing the area under the three new 0002 peaks it reasonable to suggest that the Ti
3
C
T
2 z
with c-LP 44.8 is the major MXene (areas of 0002 peaks for c-LP 44.8 : 31.8 :19.1 Å ~ 3.5 :
6
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10.1002/cctc.202000977
1
.0 : 1.0) after testing. The increase in the MXene lattice parameter can be explained by
intercalating some of the catalytic reaction by-products that trapped between the layers and
pillared the structure. Similar phenomena were reported upon cycling MXenes in Li,[30] Na,[31]
and K ion batteries[15i]. The presence multiple intercalations states after the reaction (evidenced
by the 3 different 0002 peaks instead of only before the reaction) is most likely due to presence
of various surface terminations on the MXenes surface that interact differently with the
reactants. This suggests that there is a large room for tuning the MXenes surface chemistry to
target certain reaction. A new sharp peak at 2θ of 19.61º appeared after the reaction but it could
not be identified. Unlike Ti
3
C
2
T
z
, Ti
3
CNT showed no change in the c-LP (~ 21.9 Å) after the
z
reaction, but new peaks corresponding to anatase emerged after the reaction. Nitrogen
contained MXenes are predicted to be less stable than their carbides counterparts,[32] which
may explain the presence of anatase in case of Ti
3
CNT
z
and its absence in case of Ti
3
C
2
z
T after
being exposed to the same testing conditions. Nitrogen in carbonitride MXene render the layers
to be much more highly negatively charged (with zeta potential of -65 mV around neutral pH)
compared to their carbide counterpart,[14] that lead to spontaneous intercalation of aluminum
ions from the etching process of the parent MAX phase during the synthesis resulting in a larger
c-LP of Ti
the reaction compared to the three of Ti
termination. For example, our recent neutron scattering study showed that Ti
significantly lower content OH terminations compared to Ti
T [33].
By comparing the SEM images of MXenes before (Fig. 3a and 3c for Ti
CNT respectively) and after the reaction (Fig. 3b and 3d for Ti and Ti
respectively), it is clear that nano-features appeared on the MXenes surfaces and in-between
the layers after the reaction. For Ti the layers were fully covered with small uniform nano-
features. In case of Ti CNT , these features were larger, and the particles surfaces were more
3
CNT
z
compared to Ti
3
C
2
T [
z
33]. The presence of a single Ti
can be explained by the less diverse surface
3
CNT
z
0002 peak after
3
C
T
2 z
3
CNT
z
has
3
C
2 z
3
C T
2
z
and
Ti
3
z
C
3 2
T
z
3
CNT
z
3
C
T
2 z
3
z
charging which agrees with the presence of anatase detected by XRD.
7
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10.1002/cctc.202000977
Figure 3 SEM images of Ti
3
C
2
T
z
a) before and b) after the reaction, and Ti
3
CNT c) before
z
and d) after the reaction. The insets are higher magnifications for the surfaces.
Moreover, XPS of Ti
3
C
2
T revealed a weaker Ti 2p signal compared to the pristine
z
surface (Figure 4a), evidencing a lower amount of Ti groups on the surface after the reaction.
This phenomenon can be ascribed to the irreversible adsorption of reactants and reaction
products on the surface as suggested by SEM and XRD analyses. Partial leaching of Ti into the
solution (1%, measured by atomic absorption spectrometry, AAS) may contribute to the
weaker Ti 2p signal (Figure 4a). Oxidation of Ti cannot be excluded as one of the deactivation
mechanisms of Ti
Ti (Figure 2). On the contrary, the more stable behavior of Ti
the absence of any changes in the c-LP after the reaction (Figure 1b). As shown in Figure 4b,
Ti 2p XPS peak for Ti CNT after the reaction was found to be strong which suggests that Ti
3
C
2
T
z
. These phenomena can be responsible of the evident deactivation of
3
C
2
T
z
3
CNT can be justified but
z
3
z
sites were not covered by reaction products. Moreover, no leaching occurred as confirmed by
AAS. Though there is a loss of low energy Ti 2p and C 1s species (456 and 282 eV) indicate
that carbides on the initial surface are reacted away in solution resulting in Ti4+ surface
termination.
8
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10.1002/cctc.202000977
Figure 4. XPS core levels of Ti 2p and C 1s for a) Ti
reaction.
3
C
2
T
z
and b) Ti
3
CNT
z
before and after the
It is worth noting that when different TiO materials (rutile, anatase and P25) were
2
tested under the same conditions as MXenes, they showed very low activity (Table 2). The
activity of the prepared Ti MXenes was compared to the activity of metal carbides and noble
metal free catalysts reported in the literature, showing comparable or even better activity (Table
2
).
9
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Table 2. Comparison of the activity different Ti based catalysts and metal carbides for furfural (FA)
hydrogenation
Catalyst
T
Pressure
FA/catalyst Activity[a]
ratio wt/wt
Selectivity (%)
Ref
(
K) (bar), gas
Furfuryl 2-methyl-furan
alcohol
Ti
3
3
CNT
z
453
453
453
453
453
423
423
423
5, N
5, N
5, N
5, N
5, N
1, N
1, N
2
2
2
2
2
2
2
50
50
50
50
50
3.2
3.2
3
72
88
7
49
52
40
45
30
57
97
70
17
10
-
This work
This work
This work
This work
This work
Ti
2 z
C T
TiO
TiO
TiO
TiO
2
2
2
2
P25
Rutile
anatase
8
-
3
-
[
27a]
10
18
7
-
[
27a]
NiO
Ni0.5MoC–
-
[
34]
60, H
2
25
SiO
2
[
35]
36]
36]
37]
38]
39]
MoC
423
353
353
493
493
493
3, H
34, H
34, H
2
6.6
10
10
4
15
2
10
49
53
90
71
92
89
-
[
[
[
[
[
Ni/SiO
2
2
2
Co/SiO
2
2
-
Cu/MgO−Al
NiFe
Fe
2
O
3
1, N
2
42
0.3
1.0
-
2
O
4
n.d.
10
3
-
2
O
3
@HAP
2.4
-
-
1
-1
[
a] Activity (Converted mmolfurfural
g
catalyst h )
First principle density functional theory (DFT) calculations were carried out to help
elucidate the reaction pathways, mechanisms and the nature of the active sites that control the
hydrogenation of furfural. The calculations predominantly examined the use of H as a
2
hydrogen source. The synthesized MXene structures are thought to be covered with oxygen
or hydroxyl species (Figure 5c). Under the strongly reducing condition used to carry out
catalysis, however, the surface OH species can be removed as water with a barrier of 1.05 eV
(Figure S1). As such, the MXene surface is thought to be covered with O*, OH* and H* species
under reducing conditions. This is consistent with the presence of metal and acid (OH) sites
[4]
on metal carbide surfaces under reaction conditions . The elementary adsorption, surface
reaction and desorption steps involved in the selective hydrogenation, hydrogenolysis and
etherification paths shown in Scheme 1 were calculated on the MXene surface covered in OH*
and O*. The detailed reactant, transition and product state structures for the reactions
1
0
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10.1002/cctc.202000977
considered are shown in Figures S1-S3. At the high coverages of OH*, O* and H* found under
operating conditions, the rate is ultimately controlled by the elementary step with the highest
intrinsic activation barrier where the adsorption and desorption steps are quasi-equilibrated.
We therefore highlight the highest intrinsic barrier steps in the potential energy cycles for the
hydrogenation and hydrogenolysis reactions reported in Figures S5-S6 and the etherification
reactions reported in Figures S7. We simplify these energy cycles in the in Schemes 2-3 which
report the highest intrinsic barriers for the particular hydrogenation, hydrogenolysis and
etherification paths. We discuss each of the different paths below.
Figure 5. a) Top view and b) side view of a monolayer of Ti CN MXene in the absence of OH
3
termination; c) top view of MXene surface covered with OH species. The Ti, C, N, O, H atoms
are represented by green, black, cyan, pink and white, respectively.
Vacancy sites were formed via the reaction between two OH* groups to eliminate water.
Hydrogen is readily activated over metal-metal site pairs to form two M-H bonds with a very
small barrier of 0.08 eV or over metal-oxygen site pairs to form M-H and O-H bonds with a
barrier of 0.48 eV as shown in Figure S2. A charge analysis for the dissociation of hydrogen
over the M-O bond shows that H splits heterolytically resulting in a charge of +0.65e for the
2
proton bound to the oxygen and a charge of -0.55e for the hydride bound to the metal. This
proton and hydride combination is of importance for the subsequent hydrogenation of furfural
discussed later.
Furfural binds to the TiO-H Brønsted acid sites on the MXene surface with an
adsorption energy of -0.54 eV. This is similar to the adsorption of aldehydes and ketones to an
acid site in zeolites and other heterogeneous acid catalysts. The C=O bond is subsequently
hydrogenated via the addition of the surface hydride to the carbonyl carbon. The proton
transfer step occurs very rapidly and without a barrier upon the addition of the hydride (see
Figure S5). For simplicity, we therefore consider this as a single coupled hydride-proton
transfer step with a barrier of 0.68 eV to form the resulting furfuryl alcohol (FA), as shown in
Scheme 2 and Figures S3a. In the transition state shown in Figure S8, the hydride that transfers
from the metal is stabilized by its interaction with two metal sites and the carbon to which it
1
1
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10.1002/cctc.202000977
ultimately binds. The charge on the proton in the transition state is +0.69e whereas the charge
on the O is -0.63e.
For comparison, we also examine the transfer hydrogenation of furfural using 2-
propanol. The reaction is somewhat similar to that found using H in that the proton from the
2
alcohol readily transfers with a barrier to the bound furfural. The rate is dictated by the hydride
transfer from the bound 2-propoxy. The transition state for this reaction is shown in Figure S9.
The barrier for this step is 1.01 eV which is a somewhat higher than the 0.68 eV barrier for
hydrogenation using H
2
. This is consistent with the experimental results which show higher
activities when H was used as the hydrogen source over 2-propanol.
2
The subsequent hydrogenation of the C=C bond of FA to form 3,4 dihydrofurfuryl
alcohol, 4,5 dihydrofurfuryl alcohol and the fully saturated tetrahydrofurfuryl alcohol, which
are shown in detail in Figures S3c-g and S6 and summarized in Scheme 2, have activation
barriers of 1.68 eV, 1.89eV and 1.2 eV, respectively. These barriers are 2-3 times higher than
that for the hydrogenation of FA thus indicating that the formation of THFA is unlikely. The
high barriers for forming THFA agree very well with the fact that only traces of THFA were
found in the experiment.
Scheme 2. Furfural hydrogenation scheme. The details of atomic structures are shown in
Figure S3.
As shown in Figure S6c (see structures in Figure S3b) and Scheme 2, 2-methyl furan is
another major product that is formed via hydrogenolysis. This reaction selectively proceeds
via the coupling of metal and acid site pairs that aid in the addition of hydrogen and removal
of water via dehydration, respectively. Similar to the hydrogenation of the carbonyl of furfural,
hydrogen heterolytically dissociates to form a proton and a hydride that then add to the C-O
1
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ChemCatChem
10.1002/cctc.202000977
bond of FA and eliminate water to form 2-methyl furan. The step has a barrier of 1.08 eV that
is readily accessible at the reaction temperature 453 K. Alcohol dehydration over supported
heteropolyacid catalysts with similar barriers of 1.1 eV show experimental rates of dehydration
-1
[40]
of 0.001 [s H+] at 343 K The rates for such systems at 453 K would be as high as 20 [s
-1
H+] . The reaction proceeds via the simultaneous addition of a surface proton to OH group of
FA, the elimination of H O and a hydride addition to the methylene group of FA.
2
Beside 2-methyl furan, ethers are likely to form by the coupling of two alcohols. Based
on different alcohols present under experimental conditions, 3 different ethers can be generated,
as shown in Scheme 3 and Figure S7 (structures reported in Figure S4). All of the ethers form
via the same Brønsted acid catalyzed mechanism where a surface proton attacks the OH group
of the first alcohol to eliminate water. The proton on the second alcohol is then abstracted by
surface O site. The resulting alkoxide then attacks the alpha-C of the first alcohol to form the
C-O bond thus generating the ether. The activation barriers to form difurfuryl ether, furfuryl-
2
propyl ether, and furfuryl-tetrahydrofurfuryl ether formation are all similar falling in the range
from 0.95 to 1.00 eV. These ether formation barriers are all significantly lower than the barriers
for THFA formation, which explains the higher selectivity toward the ether products found in
experiment.
The calculated results show that the proton coupled hydride addition to be especially
efficient for hydrogenation of polarized bond such as C-O bond. The C-C bonds of the furan
ring, on the other hand, are difficult to hydrogenate. The surface OH species act as acid that
readily catalyze ether formation. The presence of the metal and acid site pairs on the MXene
surface also drive the selective hydrogenolysis of the C-OH bond of furfuryl alcohol. This is
consistent with presence of bifunctional sites that drive the hydrogenolysis of oxygenates over
metal and metal carbide surfaces.[41]
1
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ChemCatChem
10.1002/cctc.202000977
Scheme 3. Summary of reaction scheme and barriers. The details of atomic structures are
shown in Figure S3 and S4.
3
. Conclusions
In conclusion, Ti-based MXenes, viz. Ti
hydrogenation of furfural using H
presence of H , Ti and Ti CNT
3
C
2
T
z
and Ti
3
CNT , were explored as catalysts for
z
2
as well as 2-propanol as the hydrogen source. In the
2
3
C
2
T
z
3
z
exhibited catalytic activities of 145 and 126 mmolfurfural
catalyst h-1, respectively. The activities in 2-propanol were somewhat lower at 88 and 72
-
1
g
catalyst h-1 for Ti
-
1
C
T
and Ti
CNT
, respectively. The main product of the reaction
Tz and Ti CNT
CNT , respectively
mmolfurfural
g
3
2
z
3
z
was found to be furfuryl alcohol with a selectivity of 52% and 49% for Ti
3
C
2
3
z
,
respectively for reactions with H and 49% and 43% for Ti Tz and Ti
2
3
C
2
3
z
for reaction in 2-propanol. Our results also provide evidence for oxygen removal and alcohol
coupling processes using Ti-MXenes as catalysts. Stability test evidenced a better stability of
Ti
3
CNT
z
than Ti
3
C
2
T
z
against deactivation. XRD suggested that the deactivation of Ti
3
C
2
z
T is
due to the intercalation of reaction products in MXenes. Ab initio calculations for the reaction
pathways and reaction energetics on the surface of MXene indicate that metal and Bronsted
1
4
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ChemCatChem
10.1002/cctc.202000977
acid sites on the MXene surface can heterolytically dissociate H
2
and open up efficient proton
and hydride transfer pathways that promote the selective hydrogenation as well as the
hydrogenolysis of polarized C-O bonds. The acid sites on the MXene also promote acid-
catalyzed etherification pathways. The hydrogen that forms on the exposed metal sites on the
MXene result in high barriers for the hydrogenation the unsaturated C=C bonds and shutdown
the hydrogenation of the furanic ring. These findings suggest exploring MXenes catalysts for
biomass conversions.
4
. Experimental and Methods Section
MAX phases synthesis. Ti AlC was synthesized by heating a mixture of TiC (2 µm, 99.5%,
Alfa Aesar) and Ti AlC (< 38 µm, >95%, KANTHAL), with 1:1 atomic ratio, to 1623 K with
a heating rate of 10 K/min. Then, it was held at that temperature for two hours. Afterward, it
was allowed to cool to room temperature (RT) in the furnace. For the synthesis of Ti AlCN, a
mixture of Ti (< 38 µm, 99.5%, Alfa Aesar), AlN (10 µm, ≥ 98%, Sigma-Aldrich), and C (7-
1 µm, 99%, Alfa Aesar), with an atomic ratio of 3:1:1, was heated to 1773 K with a rate of 10
3
2
2
3
1
K/min. followed by soaking at this temperature for 2 h. Then, the sample was allowed to cool
to RT in the furnace. The thermal cycles for the synthesis of MAX phases were carried out in
a tube furnace under a continuous flow of high purity argon. After cooling to RT, the MAX
phases were ground to produce powders using titanium nitride coated stainless steel milling bit,
and only powders of < 38 µm was used for synthesis of MXenes.
MXenes synthesis. Ti
an aqueous solution of hydrofluoric acid, HF (Macron Fine Chemicals – Avantor Performance
Materials) with a concentration 48% HF and a ratio of 1 g Ti AlC : 10 mL solution. The
3
C
2
T
x
was synthesized at RT by immersing slowly Ti
3
AlC powder in
2
3
2
mixture was stirred for 18 h at RT using a teflon coated magnetic stir bar. After the reaction
time, the mixture was centrifuged to separate the sediment from the HF solution and the
solution was decanted. Then, cycles of washing using deionized (DI) water and centrifugation
were repeated till the pH reached values more than 4. Afterward, the sediment was transferred
from the vial to vacuum-assisted filtration device to dry the powders at RT. For Ti
3
CNT , the
x
same procedure as described above was used but with solution of 30% HF instead of 48%. The
powders were washed and dried after the reaction as described above for the HF etching.
Catalytic tests. Furfural, F, (purity 99%, Sigma-Aldrich) hydrogenation was performed at 453
K, using a stainless steel reactor (30 mL capacity), equipped with heater, mechanical stirrer,
gas supply system and thermometer. Furfural solution (15 mL; 0.3 M in 2-propanol) was added
into the reactor and the desired amount of catalyst (F/catalyst ratio=50 wt/wt) was suspended
1
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ChemCatChem
10.1002/cctc.202000977
in the solution. The pressure of the hydrogen or nitrogen was 5 bar. The mixture was heated to
the reaction temperature, 453K, and mechanically stirred (1250 rpm). At the end of the reaction,
the autoclave was cooled down. Samples were removed periodically (0.2 mL) and HP 7820A
gas chromatograph equipped with a capillary column HP-5 30m x 0.32mm, 0.25 µm Film, by
Agilent Technologies. Authentic samples were analyzed to determine separation times.
Quantitative analyses with external standard method (n-octanol) was used. Recycling test: Each
run was carried out under the following reaction conditions: Furfural 0.3 M, and the catalyst
(
substrate/ catalyst = 50 wt/wt) 5 bar N , 453K. The catalyst was recycled in the subsequent
2
run after filtration without any further treatment.
Characterization. X-ray diffractions for samples investigated in this study were collected
using a Cu K-alpha Scintag X1 diffractometer (Scintag, Cupertino, CA, USA). Scanning steps
of 0.02° of 2Theta, and 1 s dwell time at each step were used. Zeiss Merlin VP (Carl Zeiss
Microscopy GmbH, Oberkochen, Germany) scanning electron microscopy (SEM) was used to
investigate the changes in the morphology of the MXene particles after the reactions. X-ray
photoelectron spectroscopy (XPS) data were collected using a PHI 3056 spectrometer
operating an Al X-ray source operated at 15 kV and an applied power of 350 W. Samples were
cold pressed between two pieces of indium, In, foil; the powder stuck to one piece of In which
was mounted to the XPS stage using carbon tape. Metal leaching was checked by ICP analysis
on the solution after reaction after filtration of the solid catalyst, on a JobinYvon JY24.
Theoretical calculations. All the DFT calculations reported herein were carried out using the
Vienna Ab Initio Software Package (VASP)[42] program with plane-wave basis sets and cutoff
energies of 400 eV. The core electrons were treated by pesudopotentials with the projector
augmented-wave (PAW) method.[43] The Perdew−Burke−Ernzerhof (PBE) form of the
gradient approximation (GGA) functional was used to model the exchange-correlation
effects.[44] Spin-polarization was employed for all calculations. The (3x3x1) unit cell of Ti
CN
3
MXene surface model was adapted from previous studies[45] and was simulated with a (2x2x1)
k-point mesh[46] with a 15Å vacuum gap in the z-direction. Geometries were considered
optimized when the forces on each atom were less than 0.03 eV/Å. The charge associated with
each atom was subsequently calculated using the Bader analysis.[47] The reaction pathway and
activation barriers were found by the climbing image nudged elastic band (NEB) method,[48]
combined with the dimer method.[49] The NEB band was used to find the minimal reaction path
between the reactant and product states to the point where the perpendicular forces on all of
the images along the band were lower than 0.1 eV/Å. The dimer method was subsequently used
1
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ChemCatChem
10.1002/cctc.202000977
to isolate the transition state to the point where the force acting on the transition state dimer
was lower than 0.03 eV/ Å.
Acknowledgements
MN was partially supported as a Wigner Fellow by the Laboratory Directed Research and
Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for
the U.S. Department of Energy. The catalysis testing studies was supported by the Università
degli Studi di Milano. XPS studies were supported by the US Department of Energy’s Office
of Basic Energy Sciences, Division of Materials Science and Engineering. All authors
contributed to analysis of results and preparation of the manuscript. Dr Jagjit Nanda is
acknowledged for the fruitful discussion.
Keywords:
Carbonitride; Furfural; Hydrogenation; MXenes; Two-dimensional
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Table of Contents:
Both titanium carbides and carbonitrides MXenes catalyzed the conversion of furfural into
-1
furfural alcohol by hydrogenation with catalytic activities of 145 and 126 mmolfurfural
g
catalyst
-1
h , respectively. The carbonitride showed better catalytic stability combined with no swelling
in the d-spacing compared to their carbide counterparts. Ab initio calculations were used to
explore the reaction pathway on the surface of MXene. The findings of this study suggest
exploring MXenes catalysts for biomass conversions.
2
1
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