A Ni-foam-structured MoNi4-MoO: X nanocomposite catalyst for hydrogenation of dimethyl oxalate to ethanol

Article abstract of DOI:10.1039/c9cc07389b

We report a Ni-foam-structured MoNi₄-MoO_x nanocomposite catalyst derived from NiMoO₄ spinel in situ grown on Ni-foam, which is highly active, selective (>93%) and stable for the gas-phase hydrogenation of dimethyl oxalate to ethanol. Such a reaction proceeds mainly through ethylene glycol formation, whereas a pathway through methyl acetate (MA) formation also occurs. Catalyst activity and selectivity are primarily governed by the MoNi₄ nanoalloy but can be further improved by an MoO_x modifier, due to the synergistic MoNi₄-MoO_x interaction that markedly promotes the MA hydrogenation to EtOH.

Full text of DOI:10.1039/c9cc07389b

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A Ni-foam-structured MoNi –MoO
4
x
nanocomposite catalyst for hydrogenation
Cite this:DOI: 10.1039/c9cc07389b
of dimethyl oxalate to ethanol†
Received 20th September 2019,
Accepted 10th December 2019
Jian Zhu, Weidong Sun, Song Wang, Guofeng Zhao,
Ye Liu
and Yong Lu
*
DOI: 10.1039/c9cc07389b
rsc.li/chemcomm
We report a Ni-foam-structured MoNi
4
–MoO
x
nanocomposite DME to methyl acetate (MA) and the subsequent hydrogenation
catalyst derived from NiMoO spinel in situ grown on Ni-foam, of MA to EtOH. Though the industrial demonstration of this
4
which is highly active, selective (493%) and stable for the gas-phase process has been realized in China, a durable zeolite-based
5
hydrogenation of dimethyl oxalate to ethanol. Such a reaction catalyst for the DME carboxylation still faces challenges. In
proceeds mainly through ethylene glycol formation, whereas a path- principle, EtOH can also be synthesized from syngas through a
way through methyl acetate (MA) formation also occurs. Catalyst route named DMO-to-EtOH, consisting of CO coupling to
activity and selectivity are primarily governed by the MoNi
4
nanoalloy obtain dimethyl oxalate (DMO) and subsequent DMO hydro-
6
but can be further improved by an MoO modifier, due to the genation to form EtOH. Notably, the first step was successfully
x
7
synergistic MoNi
4
–MoO
x
interaction that markedly promotes the MA commercialized in 2010. Hence, the exploration of catalysts with
high catalytic activity for DMO hydrogenation to EtOH is crucial
hydrogenation to EtOH.
for the industrial-scale DMO-to-EtOH process from syngas.
Ethanol (EtOH), a versatile commodity feedstock, has been
Currently, Cu-based catalysts are considered to be the most
extensively used as a solvent, chemical intermediate, disinfectant active materials for vapor-phase hydrogenation of DMO since
1
and so on. In particular, EtOH is also considered as a promising Cu species can efficiently hydrogenate carbon–oxygen bonds
6
,8
clean fuel blending component because it can reduce CO, NO
x
,
but are relatively inactive for carbon–carbon bond hydrogenation.
8
and SO emission and prevent groundwater pollution caused For instance, the traditional Cu/SiO2 catalyst obtained by an
by methyl tert-butyl ether additives. Conventionally, EtOH is ammonia evaporation hydrothermal method is capable of
dominantly produced from the fermentation of edible agricultural completely converting DMO with 83% EtOH selectivity at 280 1C
x
2
3
9
carbohydrates and the hydration of oil-derived ethylene. How- and the Ce–Cu/SiO catalyst prepared by a urea-assisted gelation
2
ever, the fermentation process and hydration of ethylene are still method offers 92% EtOH yield at 280 1C. Despite the above
facing great challenges owing to the lack of food supplies and the outstanding results over Cu-based catalysts, there are still many
1c,2b
worldwide oil crisis, respectively.
Thus, it is necessary to technical problems to be addressed in the practical applications.
explore new approaches to supplement the existing technologies Most of all, in order to facilitate EtOH formation, the reaction is
and meet the increasing demand of EtOH. At present, an indirect normally carried out at a high temperature (above 270 1C), which is
EtOH synthesis from syngas that can be cheaply produced from quite a bit higher than the H u¨ ttig temperature of Cu (134 1C),
1c
diverse resources including natural gas, coal, inedible biomass resulting in severe Cu agglomeration. Moreover, such a high
1c,4
and even organic wastes has drawn much attention.
Hydro- temperature is favorable to form undesired C –C diols (e.g.,
3 4
genation of syngas-derived acetate acid (HOAc, by carboxylation of propanediol and butanediol) and ethers (e.g., 2-methoxyethanol
4
MeOH) to EtOH has been extensively studied in the last decade. The and 2-ethoxyethanol) over the Cu/SiO catalysts, which severely
2
1
0
economy of this route is strongly dependent on the HOAc market; on deteriorate the selectivity of EtOH. Furthermore, the local
the other hand, it suffers more severely from the corrosion problem overheating due to the exothermic DMO hydrogenation to
as well as the lack of a high-performance catalyst. EtOH can be EtOH (DH = À145.9 kJ mol ) and poor thermal conductivity
synthesized through the MOR-zeolite-catalyzed carboxylation of of the oxide support easily aggravates the above problem.11
Therefore, it is highly desirable to design a novel catalyst with
4
À1
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
an excellent EtOH yield, and especially long-term stability and
School of Chemistry and Molecular Engineering East China Normal University,
enhanced thermal conductivity.
Shanghai 200062, China. E-mail: ylu@chem.ecnu.edu.cn
Herein, we attempt to use a promising foam-structured
MoNi -MoO nanocomposite catalyst derived from NiMoO
4
†
Electronic supplementary information (ESI) available: Experimental details and
supplementary characterization. See DOI: 10.1039/c9cc07389b
4
x
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À1
1
(
87, 288, 340 and 369 cm , which are well assigned to MoO
x
1
5
Fig. 1E). The high angle annular dark-field scanning trans-
mission electron microscopy (HAADF-STEM) mapping in Fig. 1F
and G visually illustrates the Mo-rich but Ni-barren locals (marked
by dashed circles: orange for Ni, blue for Mo). The above results
solidly evidence that the MoNi
derived from the NiMoO compounds. It is easy to understand
that there is not enough Ni in the NiMoO for fully consuming the
Mo atoms by forming an MoNi alloy. As a result, during the
reduction treatment, the Mo atoms in the NiMoO except the part
4 x
–MoO nanocomposite structure is
4
4
4
4
alloying with Ni atoms extricated themselves to form MoOx
segments around the MoNi4 nanoalloys. Our representative
MoNi –MoO /Ni-foam-400 catalyst is characteristic of the entirely
4
x
open three-dimensional network structure (90 vol% voidage; low-
magnitude SEM image in Fig. 1H). Moreover, the flexibility of
such a Ni-foam-structured catalyst permits the tailoring of its
geometry on demand, for example, a circular chip (8 mm dia-
meter by 1.5 mm thick; the optical photograph (insert) in Fig. 1H).
The high-magnitude SEM image in Fig. 1I shows that the MoNi4–
MoO catalytic component mounted on the Ni-foam strut exhibits
x
a nanorod (0.3 mm diameter) array morphology. As a result, the
Fig. 1 (A) Schematic illustration of the fabrication of the representative
2
À1
specific surface area (SSA) is increased from 0.3 m g of the pure
MoNi
collected at different stages. TEM image (C), XPS spectra (D), Raman Ni-foam substrate to 39 m g of the as-obtained catalyst. We are
spectrum (E), HAADF-STEM images (F and G), optical photograph (insert H), confident that the nanorod is MoNi
4 x
–MoO /Ni-foam-400 catalyst. XRD patterns (B) of the samples
2
À1
4
alloy, which is consistent
and SEM images (H and I) of the representative MoNi –MoO /Ni-foam-400
4
x
with the XRD phase of only MoNi
4
(except Ni phase of Ni-foam;
catalyst. (J) DMO conversion and EtOH selectivity against time on stream
in the MoNi –MoO is believed to
Fig. 1B). In addition, the MoO
x
4
x
over the MoNi
4
–MoO
x
/Ni-foam-400 catalyst. Reaction conditions: 230 1C,
À1
be ultra-highly dispersed on the MoNi nanorods, because of the
4
2
.5 MPa, WHSVDMO of 0.22 h , and n(H
2
)/n(DMO) of 180.
fact of no MoO diffraction (Fig. 1B) but a MoO -enriched surface
x
x
(Fig. 1D). Our foam-structured catalyst shows pleasing mechanical
spinel directly grown on a Ni-foam (8.0 mm diameter; 1.5 mm robustness: the nanorod arrays adhere firmly to the Ni-foam strut,
thick; 100 pores per inch (PPI)), aiming to achieve a unique evidenced by a low weight loss of 5.8 wt% (including the mass of
combination of high catalytic performance with enhanced heat/ Ni-foam, Fig. S1, ESI†) after an ultrasonic treatment in methanol
mass transfer for application in strongly exothermic reaction even for 5 h. Undoubtedly, our approach works effectively and
12
processes, such as the DMO-to-EtOH process. Fig. 1A schematically efficiently for engineering MoNi
4 x
–MoO nanocomposites at
illustrates the synthetic route of our representative MoNi –MoO / a ‘‘nano–mico–macro’’ triple-scale level in one-step. Most
4
x
Ni-foam-400 catalyst engineered from macro- to micro-scales. importantly, the representative MoNi –MoO /Ni-foam-400 catalyst
4
x
The NiMoO
Ni-foam strut surface via a facile hydrothermal process. The being capable of fully converting DMO at a high EtOH selectivity
as-obtained NiMoO O/Ni-foam could be transformed, for of about 93%, and particularly, is stable for at least 220 h without
ÁxH
example, into the representative MoNi –MoO /Ni-foam-400 any sign of deactivation at 230 1C and 2.5 MPa, using a H /DMO
catalyst after calcining at 350 1C (dehydrated to form NiMoO molar ratio of 180 and a weight hourly space velocity (WHSVDMO
Ni-foam) in air and reduction at 400 1C in H (XRD, Fig. 1B). of 0.22 h (Fig. 1J and Table S1 for comparison with literature
Notably, after reduction treatment, only one weak XRD peak is results, ESI†).
detected at 2y of 43.51, assigned to the (121) plane of MoNi4 In general, the DMO-to-EtOH process comprises several
JCPDS No. 65-5480) except Ni from the Ni-foam substrate ( JCPDS sequential reactions, including DMO hydrogenation to methyl
4 2
ÁxH O nanorod arrays were initially grown onto the is highly active, selective and stable in the DMO-to-EtOH process,
4
2
4
x
2
4
/
)
À1
2
(
No. 65-2865). The high-resolution transmission electron micro- glycolate (MG) and MG hydrogenation to ethylene glycol (EG)
scopy (HRTEM) image shows well-resolved lattice fringes with followed by EG hydrogenation to EtOH as well as MG hydro-
interplanar distances of 0.208 nm and 0.181 nm, which can be genation to methyl acetate (MA) followed by MA hydrogenation
6
c,9
indexed to the MoNi
(
(
4
(121) plane and (310) plane, respectively to EtOH (Fig. 2A).
Fig. 1C). Fig. 1D displays the X-ray photoelectron spectroscopy dominantly follows over our MoNi
XPS) spectra in the Mo3d region, which could be deconvoluted the reaction rates and apparent activation energies (E ) of the
To determine which pathway the reaction
13
4
–MoO /Ni-foam-400 catalyst,
x
a
via a Gaussian peak fitting method into eight peaks. The peaks MG-to-EG, EG-to-EtOH, MG-to-MA, and MA-to-EtOH processes
0
observed at 228.2 and 231.5 eV are assigned to Mo species in were calculated, with the results shown in Fig. 2. As shown in
MoNi
4
, and the peaks detected at 229.2/232.5, 230.5/233.8 and Fig. 2B, the MG-to-EG reaction rate is only slightly higher than
4+
5+
6+
232.1/235.4 eV are attributed to Mo , Mo and Mo in MoO
x
,
that of the MG-to-MA process at each temperature, indicating that
13,14
respectively.
The Raman spectrum shows five peaks at 147, the above two reactions almost keep pace with each other.
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(93% EtOH selectivity with full DMO conversion under identical
reaction conditions). It is rational to infer from the above
results that the MoNi nanoalloy plays a key part in the DMO-
4
to-EtOH process and the MoO modification leads to a further
x
improvement in EtOH selectivity. A possible explanation for
such improvement in the EtOH selectivity is that the MA-to-
EtOH reaction is markedly promoted by MoO
evidenced by a significant reduction of MA selectivity.
To further verify the promotion effect of MoO by synergistic
interaction with MoNi , a series of MoNi –MoO /Ni-foam catalysts,
x
modification,
x
4
4
x
with tuned contents of MoNi4 and MoO , were prepared from
x
the NiMoO /Ni-foam precursors by varying the H -reduction
4
2
temperature from 350 to 500 1C. Notably, all catalysts present
similar SEM surface morphology (Fig. S3, ESI†). The MoNi
MoO /Ni-foam-350 catalyst (i.e., reduced at 350 1C) possesses
4
–
x
2
À1
the highest specific surface area (SSA) of 92 m g , whereas the
SSAs of the other catalysts reduced at 400 1C, 450 1C and 500 1C
2
À1
are almost identical (38–42 m g ; Table S3, ESI†). Moreover,
as shown in Fig. 3A (deduced from deconvoluted XPS spectra in
Fig. S4, ESI†), the MoNi –MoO /Ni-foam-350 catalyst possesses
Fig. 2 (A) The reaction pathways for the DMO-to-EtOH process. The
reaction rates for (B) the MG-to-EG and MG-to-MA and (C) the EG-to-
4
x
4 x
EtOH and MA-to-EtOH processes over the MoNi –MoO /Ni-foam-400
0
the highest surface MoO content but an extremely low Mo
catalyst. (D) Activation energies of this catalyst for reaction steps involved
in the DMO-to-EtOH process.
x
content of 1.8 at%, indicating that the reduction temperature of
50 1C is too low to sufficiently reduce NiMoO spinel to form
an MoNi alloy, which is consistent with the observation of a
However, the reaction rate of the EG-to-EtOH process is quite a bit clear XRD peak of NiMoO on this sample (Fig. S4C, ESI†). Not
3
4
4
4
higher than that of the MA-to-EtOH process in the whole tem- surprisingly, this catalyst delivers a very low DMO conversion of
perature range studied (Fig. 2C), revealing that hydrogenation of 19.6% with an EtOH selectivity of only 3.1% (Fig. 3B). In
EG to EtOH is easier than MA to EtOH. Furthermore, the E of the contrast, when increasing the catalyst reduction temperature
a
À1
MG-to-EG process (40 kJ mol ) is close to that for the MG-to-MA to 400 1C, the DMO conversion and EtOH selectivity are
of the EG-to-EtOH process is dramatically improved to 100% and 93%, in line with the full
much lower than that for the MA-to-EtOH process (40 kJ mol vs. transformation of NiMoO
3 kJ mol ; Fig. 2D). Clearly, the formation of target EtOH is Fig. S4C, ESI†). In this case, Ni in the NiMoO
achieved through the hydrogenation of intermediate EG and MA, into MoNi
but the latter is more difficult to proceed over our catalyst.
À1
process (42 kJ mol ), whereas the E
a
À1
4
into MoNi
4
–MoO
x
(Fig. 3 and XRD in
À1
6
4
0
is fully transformed
0
4
alloy, confirmed by the Mo /Ni atom ratio of
0.26 (close to 1/4 for the MoNi ; Table S4, ESI†) and a Mo /Mo
4
0
n+
We then attempted to explore the synergistic catalysis of atom ratio of 0.30 (close to 1/3, one Mo alloyed with four Ni atoms
n+
the MoNi –MoO mounted on the Ni-foam, on the basis of the to form one MoNi4 and release three Mo during NiMoO4
4
x
above elucidated reaction pathway. Fig. 1 clearly shows the reduction; Fig. 3A and Table S4, ESI†). Interestingly, by further
coexistence of Ni, MoNi and MoO on our catalyst. Thus, to increasing the catalyst reduction temperature to 500 1C, the
shed light on the role of each species, four model catalysts EtOH selectivity is sharply decreased to 77.1% and the MA
including Ni/SiO , MoO /SiO , MoNi /SiO and MoNi –MoO selectivity is reversely increased to 21.3%, whereas the full
SiO (XRD in Fig. S2, ESI†) were prepared and examined in the conversion of DMO is retained (Fig. 3B). On such as-obtained
DMO-to-EtOH reaction. The Ni/SiO and MoO /SiO deliver low
4
x
2
x
2
4
2
4
x
/
2
2
x
2
DMO conversion of less than 34% with trace EtOH formation
under identical reaction conditions (Table S2, ESI†), indicating
that neither Ni nor MoO
EtOH reaction. In contrast, the MoNi
x
is active and selective for the DMO-to-
/SiO shows markedly
4
2
improved activity and selectivity, being capable of almost fully
converting DMO with EtOH/MA selectivity of 74.3%/19.8%
(
Table S2, ESI†). Most notably, once the MoO
together with MoNi formation on the SiO support, the as-
obtained MoNi –MoO /SiO becomes more selective and active
x
is formed
4
2
4
x
2
0
Fig. 3 (A) Surface atom fraction occupied by MoO
surface Mo atoms for the Ni-foam-structured MoNi –MoO
varied H -reduction temperatures, determined by XPS. (B) Conversion and
x
and Mo in total
in comparison with the MoNi /SiO , achieving a 99.9% DMO
conversion with further improved EtOH selectivity of 84.4%
while leading to a remarkable reduction of MA selectivity to
4
2
4
x
catalysts with
2
selectivity for the DMO-to-EtOH reaction against the catalyst reduction
À1
1
0.1%. Nevertheless, the MoNi
4
–MoO
–MoO
x
/SiO
/Ni-foam-400 catalyst n(DMO) of 180, 230 1C.
x
2
is still inferior
temperature. Reaction conditions: 2.5 MPa, WHSVDMO of 0.22 h , n(H
2
)/
to that of our preferred MoNi
4
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Table 1 Activation energies of the MoNi
the reaction steps involved in the DMO-to-EtOH process against the
catalyst reduction temperature
4
–MoO
x
/Ni-foam catalysts for MoNi
to further improve the EtOH selectivity by speeding up
4
the MA-to-EtOH reaction step.
We are grateful for the financial support from the National
Natural Science Foundation of China (grants 21773069, 21703069,
À1
E
a
(kJ mol
)
21473057, U1462129, 21273075), the Key Basic Research Project
Reduction temp. (1C) DMO-MG MG-EG EG-EtOH MG-MA MA-EtOH
(
grant 18JC1412100) from the Shanghai Municipal Science and
4
4
5
00
50
00
38
38
39
41
42
41
40
43
43
43
45
46
63
75
89
Technology Commission, and the National Key Basic Research
Program (grant 2011CB201403) from the Ministry of Science and
Technology of the People’s Republic of China.
0
MoNi –MoO /Ni-foam-500, as shown in Fig. 3A, the surface Mo
4
x
Conflicts of interest
content (in total surface Mo atoms) is increased to 42 at%,
much higher than the theoretical value of 25 at% (according to
There are no conflicts to declare.
4 4
one MoNi formed from one NiMoO ). This observation
indicates that the catalyst is over-reduced at 500 1C, i.e., some
Notes and references
0
MoO
leading to a decline of the surface MoO
Fig. 3A). On the basis of the above results, we are thus
x
is reduced to free Mo (no more Ni for alloying) thereby
1
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2
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x
modifier with suitable amount.
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2
3
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x
selectivity improvement is owing to the MG-to-MA reaction
being inhibited or the MA-to-EtOH being promoted. To seek
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EtOH, MG-to-MA, and MA-to-EtOH), with the results shown
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6
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À1
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(
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À1
À1
500 1C) 4 75 kJ mol (450 1C) 4 63 kJ mol (400 1C). In nature,
this order links to the MoO content in the MoNi –MoO mounted
on the Ni-foam: the higher the MoO content, the lower the E
x
4
x
7
8
9
x
a
for the MA-to-EtOH reaction is. In addition, the reaction rate for
each reaction step involved in the DMO-to-EtOH process was
measured at 210 1C using the model catalysts of MoNi
and MoNi –MoO /SiO , with the results as shown in Table S5
ESI†). As expected, both of them achieved a similar reaction rate
for each reaction step of DMO-to-MG, MG-to-EG, EG-to-EtOH and
MG-to-MA; however, for the step of MA-to-EtOH, the MoNi
4 2
/SiO
2
4, 6441–6451.
4
x
2
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(
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–
2008, 257, 172–180.
À1 À1
MoO
x
/SiO
2
catalyst achieves a reaction rate of 0.55 mmol g
h
,
1
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À1 À1
almost 2 times as high as that (0.28 mmol g
h
) of the MoO -free
x
MoNi /SiO2 catalyst. Such similarity observed on both model
4
1
x
catalysts further confirms the conclusion that the MoO modifica-
tion makes the catalyst much more active for the MA-to-EtOH
reaction step without impact on other reaction steps.
(
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4
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1
1
thermal conductivity and high permeability, is developed for
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1
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4
220 h. The MoNi nanoalloy primarily governs the activity and
x
selectivity while the MoO modifier works synergistically with
Chem. Commun.
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