Palladium nanoparticles encapsulated in MIL-101-NH2 catalyzed one-pot reaction of Suzuki-Knoevenagel reaction

Article abstract of DOI:10.1016/j.inoche.2019.02.042

Bifunctional Cr-MOF catalysts containing palladium nanoparticles (NPs) have been prepared. Combining the high activity of Pd NPs and base sites in MIL-101-NH₂, the catalysts (Pd?MIL-101-NH₂) exhibited distinct catalytic activity in a

Full text of DOI:10.1016/j.inoche.2019.02.042

InorganicChemistryCommunications103(2019)82–86
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Inorganic Chemistry Communications
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Short communication
Palladium nanoparticles encapsulated in MIL-101-NH2 catalyzed one-pot
reaction of Suzuki-Knoevenagel reaction
Jia-Xin Li, Xin Li, Hong Tang, Yu-Yang Zhang⁎, Zheng-Bo Han
College of Chemistry, Liaoning University, Shenyang 110036, PR China
G R A P H I C A L A B S T R A C T
In this paper, Pd@MIL-101-NH2 were synthesized, which was used as heterogeneous catalysts for one-pot Suzuki-Knoevenagel reactions. As expected that Pd@MIL-
101-NH2 exhibited remarkably catalytic activity and selectivity. Moreover, other reaction conditions have also been explored involving solvents, reaction tem-
peratures and other catalysts for the sake of achieving the best catalytic efficiency.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Palladium
MIL-101-NH2
Suzuki
Bifunctional Cr-MOF catalysts containing palladium nanoparticles (NPs) have been prepared. Combining the
high activity of Pd NPs and base sites in MIL-101-NH2, the catalysts (Pd@MIL-101-NH2) exhibited distinct
catalytic activity in a one-pot Suzuki-Knoevenagel reaction. Pd@MIL-101-NH2 has been characterized by X-ray
diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and fourier
transform infrared spectroscopy (FT-IR). Various substituted phenylboronic acids were used to research the one-
pot reaction. The results were highly satisfactory, not only got high yield, but reduced reaction time. Leakage
experiments further demonstrated that there is no leaching of active palladium nanoparticles during the reac-
tions. Catalysts could be reused for at least five cycles and activity is not significantly decreased.
Knoevenagel
One-pot
The Suzuki cross-coupling reaction is one of the most effective
methods for forming a CeC bond and engendering asymmetric biaryl
groups [1–3]. In the past few decades, this reaction has been in-
dustrially applied to the production of compounds such as biaryl and
alkene derivatives. Recently, this reaction has been succeeded in com-
pounding natural products [4–6], drugs [7–9] and conducting polymers
production of α,β-unsaturated carbonyl compounds which are widely
used in the synthesis of chemicals [12], biologicals [13,14], and phar-
maceuticals [15,16]. In general, Knoevenagel condensation reaction
was catalyzed by homogeneous catalysts, which have the disadvantage
that catalysts cannot be recycled. Therefore, it is quite desirable to
develop heterogeneous catalysts that achieve those reactions from an
environmental and economic standpoint. Compared with traditional
[10,11]. Knoevenagel condensation reactions are
a process for
⁎ Corresponding author.
E-mail address: yyzhang@lnu.edu.cn (Y.-Y. Zhang).
https://doi.org/10.1016/j.inoche.2019.02.042
Received 23 January 2019; Received in revised form 27 February 2019; Accepted 28 February 2019
Availableonline10March2019
1387-7003/©2019ElsevierB.V.Allrightsreserved.

J.-X. Li, et al.
InorganicChemistryCommunications103(2019)82–86
step-by-step reactions, one-pot [17–22] reactions have obvious ad-
vantages, such as less production waste, less energy consumption,
shorter reaction time and more environmental protection.
Metal-organic frameworks (MOFs) [23–26] are composed of metal
clusters or ions and organic ligands. Owing to their adjustable high pore
structure and large specific surface areas, they have extensive appli-
cation fields, such as catalysis [27–36], gas storage [37], adsorption
[38], biomedicine [39,40]. For decades, MOFs have proved their en-
ormous potentiality as heterogeneous catalysts, especially as a support
carrier for metal nanoparticles. Although plenty of researches have
been conducted on the role of MOFs in catalysis, the number of studies
on bifunctional metal nanoparticles@MOFs for catalyzed one pot re-
actions is limited. Although single-step reactions have been reported in
the literature, the use of bifunctional MOFs to simultaneously catalyze
the Suzuki-knoevenagel one-pot reaction has hardly been reported, so
there is no suitable catalyst to compare. It is worth mentioning that
metal nanoparticles@MOFs catalysts have many advantages than
homogeneous catalysts, such as reuse and separate. Moreover, com-
pared with nanoparticles, metal nanoparticles@MOFs catalysts have
good stability and not easy to be aggregated. Here, we developed a
bifunctional one-pot heterogeneous catalysts by encapsulating Pd na-
noparticles (NPs) into cavities of MIL-101-NH2. They can catalyze Su-
zuki cross-coupling reaction of aldehydes and subsequent Knoevenagel
condensation reaction of malononitrile. For all we know, this is the first
example of applying MOF-based heterogeneous catalysts to a one-pot
Suzuki-Knoevenagel reactions. Additionally, and they can be reused
multiple times without distinct catalytic activity.
Fig. 1. PXRD patterns of simulated (black), MIL-101-NH2 (red), Pd@MIL-101-
NH2 (blue), recovered after five runs (green). (For interpretation of the refer-
ences to colour in this figure legend, the reader is referred to the web version of
this article.)
consistent with the XRD result. TEM images illustrated that Pd nano-
particles were adequately dispersed into MIL-101-NH2 (Fig. 2a), which
increased activity of the catalyst. It can be found in Fig. 2b that it has a
The X-ray powder diffraction measurements of samples were de-
tected by Bruker AXS D8 advanced automated diffractometer with Cu-
Kα radiation. Fourier transform infrared spectroscopy (FT-IR) spectra
were recorded with the Nicolet 5DX spectrometer in range
4000–400 cm−1 using KBr pellets. Thermogravimetric analyses (TGA)
were taken on a Perkin-Elmer Pyrisl (25–800 °C, 5 °C min−1, flowing
N2). Transmission electron microscopy (TEM) graphics of samples was
acquired on JEOL-2010 electron microscope with an operating voltage
of 200 kV. Energy-dispersive X-ray (EDX) spectra were collected at a
field-emission instrument of the type SU8010. Inductively coupled
plasma atomic emission spectroscopy (ICP-AES) quantified the content
of Pd in Pd@MIL-101-NH2. Gas absorption isotherms were performed
with a Belsorp-Max automatic volumetric adsorption apparatus. The
catalytic reaction products were analyzed and identified by SP-2100A
gas chromatograph (GC, equipped with a Kromat-Bond Series capillary
column and a flame ionization detector).
uniform size of 1.5
1 nm. Energy-dispersive X-ray spectroscopy
(EDX) analysis showed a pattern containing element of Pd (Fig. 2a). The
content of Pd which was determined by ICP-AES was 2.02 wt%. To
reveal thermal stability of Pd@MIL-101-NH2, Thermogravimetric Ana-
lysis (TGA) experiment was carried out under N2 atmosphere conditions
at a rate of 10 °C min−1 over a range of 25–800 °C. As illustrated in Fig.
S2, the first main weight-loss was due to decomposition of the solvent
molecule DMF, unreacted 2-aminoterephthalic acid in cage of MOF to
ca. 100 °C. Weight losses in 393 °C attributed to decomposition of or-
ganic skeleton. This decomposition temperature compared well to the
value reported in the literature [44]. BET surface areas of MIL-101-NH2
and Pd@MIL-101-NH2 were determined by N2 physisorption measure-
ments at 77 K (Fig. S3). The BET surface area of MIL-101-NH2 was
2154 m2 g−1. The BET surface area of Pd@MIL-101-NH2 was reduced
to 1973 m2 g−1, mainly due to the fact that Pd NPs occupy the pores of
MIL-101-NH2, which was close to the previous report [45].
In order to confirm the successful synthesis of MIL-101-NH2 and
Pd@MIL-101-NH2, TEM, XRD, EDX, ICP-AES and FT-IR characteriza-
tions were performed. XRD of MIL-101-NH2 and Pd@MIL-101-NH2 is
displayed in Fig. 1, the XRD pattern of as-synthesized MIL-101-NH2
matched well with the already published XRD patterns, which suggest
that MIL-101-NH2 was obtained. After loading of Pd NPs, there was no
apparent loss of crystallinity in XRD patterns (Fig. 1), which demon-
strated that the frameworks of MIL-101-NH2 are mostly maintained.
Furthermore, we did not find any X-ray diffraction peaks from Pd na-
noparticles, probably due to the low Pd content. The FT-IR spectrums of
as-synthesized samples with a spectral range of 400–4000 cm−1 were
illustrated in Fig. S1. Strong bands exhibited by MIL-101-NH2 in the
range of 1750–1300 cm−1 were COO– symmetric, asymmetric
stretching vibration, CeC stretching vibration, which proved the ex-
istence of carboxylate linker in MIL-101-NH2 [41]. The CeH in-plane
and out-of-plane bending vibration of the aromatic ring corresponds to
a weak peak at 973 cm−1 and a peak at 759 cm−1 [42]. In addition, a
clear peak at 1619 cm−1 was NeH bending vibration, and NeH
asymmetric and symmetric stretching vibration of the amino groups
correspond to weak double peaks at 3378 cm−1 and 3461 cm−1 [43].
All of peaks of MIL-101-NH2 can be found correspondingly in spectrum
of Pd@MIL-101-NH2. The above evidence demonstrated that the
structure of MOF remained intact after attached Pd NPs, which was
Next we discuss the plausible mechanism of one-pot Suzuki-
Knoevenagel reaction. The plausible mechanism of Pd@MIL-101-NH2
in Suzuki reaction was depicted in Scheme 1.The catalytic cycle of the
Suzuki coupling reaction is generally considered to be an oxidation-
addition reaction of Pd(0) with a aryl halide aromatic hydrocarbon to
form a complex of Pd(II), followed by metal transfer reaction with ac-
tivated boric acid to form another complex of Pd(II). The complex is
finally subjected to reduction-elimination to form a product and Pd(0)
[46]. Scheme 2 is the illustration of the possible catalytic mechanism of
Pd@MIL-101-NH2 in Knoevenagel condensation of benzaldehyde and
malononitrile. In the first step, protons are taken from malononitrile by
a Lewis basic site of the catalyst to form a carbon anion. In the second
step, the carbon anion attacks the carbonyl carbon atom of benzalde-
hyde in the solution and forms a CeC bond by transferring a negative
charge to the oxygen atom. In the third step, the oxyanion acquires
protons from the Lewis basic site, forming water molecules. At the same
time, the Lewis basic site of the catalyst is regenerated [47].
To characterize possible catalytic behavior of Pd@MIL-101-NH2, a
series of one-pot Suzuki-Knoevenagel reactions were carried out in
different solvents and catalysts. Corresponding product yields were
obtained by GC. Firstly, we tested several different solvents for the
reaction catalyzed by Pd@MIL-101-NH2. When the first step solvents
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J.-X. Li, et al.
InorganicChemistryCommunications103(2019)82–86
Fig. 2. TEM image of Pd@MIL-101-NH2 (a) and corresponding size distribution of Pd nanoparticles (b). The inset in (a)is the EDX pattern.
were ethanol and methanol, excellent yields (99.9% and 98.1%, re-
spectively) were acquired (Table 1, Entries 1, 2). When solvents were
replaced by DMF and toluene, poor yields were obtained (Table 1,
Entries 3, 4). From these results, it can be seen that ethanol was the
most optimum solvent for the one-pot reaction. Because the boiling
point of ethanol is 78.4 °C, we researched effects of 60 °C and 70 °C for
the reaction. In entry 5, we found that the reaction time was prolonged
at 60 °C, so 70 °C is the best temperature. Next we verified that the
bifunctional catalysts Pd@MIL-101-NH2 were essential for the one-pot
reactions. In the first stage, we demonstrated that Pd NPs are critical to
Suzuki reaction part of the one-pot reaction. We used MIL-101-NH2 and
Pd@MIL-101-NH2 catalysts to catalyze the one-pot reaction (Table 1,
Entries 1, 7). In entry 7, we could find that no reaction occurred
without the Pd NPs. Entry 6 and entry 8 also demonstrated the necessity
of Pd NPs. In the second stage, it was proved that eNH2 group was
indispensable to Knoevenagel reaction part of one-pot reaction. The
one-pot reaction was catalyzed by Pd@MIL-101 and Pd@MIL-101-NH2.
It was found that the first-step, Suzuki reaction parts, were completed
totally, but the catalysis of entry 6 could hardly catalyze the second-
step, Knoevenagel reaction parts, due to absence of -NH2 group. Pd/C
catalyst with poor repeatability could also catalyze the first-step reac-
tion, but could not catalyze the second-step reaction due to the same
reason (Table 1, Entry 9). Entry 10 showed that the one-pot reaction did
not occur without any catalyst. Following this, a number of substituted
Scheme 1. A plausible mechanism for the Suzuki condensation catalyzed by
Pd@MIL-101-NH2.
Scheme 2. A plausible mechanism for the Knoevenagel condensation catalyzed by Pd@MIL-101-NH2.
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InorganicChemistryCommunications103(2019)82–86
Table 1
The one-pot Suzuki-Knoevenagel reaction of different substrates in different solvents using different catalysts.
Entry
R
Catalyst
Solvent
Time (h)
Step1
Yield (%)
Step2
2.5
3
4
1a
H
Pd@MIL-101-NH2
Ethanol
1
4
99.9
98.1
84.5
63.1
99.9
99.9
–
99.9
53.7
42.4
38.2
99.9
–
2a
3a
H
Pd@MIL-101-NH2
Pd@MIL-101-NH2
Pd@MIL-101-NH2
Pd@MIL-101-NH2
Pd@MIL-101
MIL-101-NH2
MIL-101
Methanol
DMF
2.5
2.5
2.5
3.2
2.5
2.5
2.5
2.5
2.5
2.5
0.5
2.5
1
H
4
4a
H
Toluene
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
4
5b
H
H
1.6
1
6c
7d
H
4
–
8e
H
4
–
–
9f
H
Pd/C
4
99.9
–
–
10g
11a
12a
13a
14a
H
–
4
–
4-NO2
4-CH3O
4-Cl
3-CH3O
Pd@MIL-101-NH2
Pd@MIL-101-NH2
Pd@MIL-101-NH2
Pd@MIL-101-NH2
4
30.9
99.9
81.7
99.9
13.1
99.9
6.5
99.9
1
1
4
Reation conditions: 4-iodobenzaldehyde (1 mmol), phenylboronic acids (1.5 mmol), solvent (5 mL), K2CO3 (0.7mmol), N2 (0.1 MPa), 70 °C. b60 °C, other conditions
are the same as a. Catalyst: a50 mg Pd@MIL-101-NH2 (Pd, 1 mmol%) was used. c50 mg Pd@MIL-101 (Pd, 1 mmol%) was used. d50 mg MIL-101-NH2 was used. e50 mg
f
MIL-101 was used. 0.4 mol% commercial 5% Pd/C was used. gNo catalyst was used.
substrates. It is easy to separate the catalyst from reaction mixture after
catalytic reaction by simple centrifugation. The catalyst could be reused
several times, and activity hardly declines. Further work is how to ex-
plore new one-pot catalytic reactions with Pd@MIL-101-NH2 and op-
timize the performance of Pd@MIL-101-NH2.
Acknowledgment
This work was granted financial support from the National Natural
Science Foundation of China (Grant 21671090). The authors gratefully
thank the Collaborative Innovation Group of Clean Energy Resources
for kind help in catalyst characterization.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.inoche.2019.02.042.
References
Fig. 3. The recyclability studies performed for Pd@MIL-101-NH2.
[1] P. Phukan, P.R. Boruah, P.S. Gehlot, A. Kumar, D. Sarma, Appl. Organomet. Chem.
32 (2018) e4009.
[2] L. Pedersen, M.F. Mady, M.O. Sydnes, Tetrahedron Lett. 54 (2013) 4772–4775.
[3] N. Şahin, J. Mol. Struct. 1177 (2019) 193–198.
phenylboronic acid reactants were used to investigate the effect on the
yield (Table 1, Entries 1, 11–14). The results illustrated that phe-
nylboronic acids with electron withdrawing groups such as nitro (-NO2)
(Table 1, Entry 11) and chlorine (eCl) (Table 1, Entry 13) had low
yields, while the electron donating group (-OCH3) had moderate to high
yields (Table 1, Entries 12, 14).
[4] J. Chakraborty, I. Nath, F. Verpoort, Chem. Eng. J. 358 (2019) 580–588.
[5] J. Chakraborty, I. Nath, F. Verpoort, Data Brief 21 (2018) 1157–1165.
[6] A.K. Koshvandi, M.M. Taheri, T. Heravi, Momeni, Appl. Organomet. Chem. 32
(2018) e4210.
[7] N.A. Dangroo, T. Ara, B.A. Dar, M.A. Khuroo, Catal. Commun. 118 (2019) 76–80.
[8] G.Y. Zhou, S.D. Chu, A. Nemati, C.S. Huang, B.A. Snyder, R.G. Ptak, M. Gochin, Eur.
J. Med. Chem. 74 (2018) 7009–7017.
[9] C. Hargitai, T. Nagy, J. Halász, G. Koványi-Lax, G. Németh, G. Simig, B. Volk,
Tetrahedron 74 (2018) 7009–7017.
Furthermore, it is obviously important to separate and reuse the
catalyst. After the catalytic reaction, Pd@MIL-101-NH2 can be re-
collected and reused easily. As illustrated in Fig. 3, the catalytic cap-
ability of Pd@MIL-101-NH2 hardly declines after five catalytic cycles.
Recycle XRD patterns of Pd@MIL-101-NH2 indicated the structure can
be retained after catalytic reaction (Fig. 1). Recycle TEM images of Pd@
MIL-101-NH2 further demonstrated the stability of the catalyst (Fig.
S4). Therefore, the integrity of the framework was confirmed.
In summary, we have developed a bifunctional heterogeneous one-
pot catalyst for Suzuki-Knoevenagel reaction, Pd@MIL-101-NH2, with
Palladium nanoparticles and -NH2 groups in its structure. It can be used
as solid catalyst for Suzuki-Knoevenagel reaction with different
[10] P. Pattanayak, S.P. Parua, S. Chattopadhyay, Polyhedron 157 (2019) 410–415.
[11] N.Y. Baran, J. Mol. Struct. 1176 (2019) 266–274.
[12] J. Li, X. Bai, H. Lv, Microporous Mesoporous Mater. 275 (2019) 69–75.
[13] S.F. Peter, P. David, B. Aliz, C. Evelin, K.S. Attila, T. Arpad, B. Istvan, Molecules 23
(2018) 3161.
[14] C. Remeur, S. Desrat, V. Gandon, F. Roussi, Eur. J. Org. Chem. 42 (2018)
5869–5877.
[15] B. Szilvia, U. Antal, V. Krisztina, J. Ferenc, J. Org. Chem. 83 (2018) 15486–15492.
[16] W.M. Cheng, X. Lu, J. Shi, L. Liu, Org. Chem. Front. 5 (2018) 3186–3193.
[17] J. Xi, H. Sun, D. Wang, Z. Zhang, X. Duan, J. Xiao, F. Xiao, L. Liu, S. Wang, Appl.
Catal., B 225 (2018) 291–297.
[18] J.G. Liu, W.W. Chen, C.X. Gu, B. Xu, M.H. Xu, Org. Lett. 20 (2018) 2728–2732.
[19] Y.T. Wang, Z. Fang, X.X. Yang, Y.T. Yang, J. Luo, K. Xu, G.R. Bao, Chem. Eng. J. 348
85

J.-X. Li, et al.
(2018) 929–939.
InorganicChemistryCommunications103(2019)82–86
[34] Z.J. Wang, J.J. Lv, J.J. Feng, N. Li, X. Xu, A.J. Wang, R. Qiu, RSC Adv. 5 (2015)
[20] X. Tian, M. Zhou, C. Tan, M. Li, L. Liang, K. Li, P. Su, Chem. Eng. J. 348 (2018)
775–785.
28467–28473.
[35] J.J. Lv, Z.J. Wang, J.J. Feng, R. Qiu, A.J. Wang, X. Xu, Appl. Catal. A Gen. 522
(2016) 188–193.
[21] Y. Zhang, Y.X. Wang, L. Liu, N. Wei, M.L. Gao, D. Zhao, Z.-B. Han, Inorg. Chem. 57
(2018) 2193–2198.
[36] Z.J. Wang, J.J. Lv, R.N. Yi, M. Xiao, J.J. Feng, Z.W. Liang, A.J. Wang, X. Xu, Adv.
Synth. Catal. 360 (2018) 932–941.
[22] Y.Y. Zhang, J.X. Li, L.L. Ding, L. Liu, S.M. Wang, Z.B. Han, Inorg. Chem. 57 (2018)
13586–13593.
[37] A. Betard, R.A. Fischer, Chem. Rev. 112 (2012) 1055–1083.
[38] C.Q. Zhu, F.Q. Liu, C. Ling, H. Jiang, H.D. Wu, A.M. Li, Appl. Catal. B Environ. 242
(2019) 238–248.
[23] X. Gong, W.W. Wang, X.P. Fu, S. Wei, W.Z. Yu, B. Liu, C.J. Jia, J. Zhang, Fuel 229
(2018) 217–226.
[24] Y. Han, Y. Yu, L. Zhang, L. Huang, J. Zhai, S. Dong, Talanta 186 (2018) 154–161.
[25] S.M. Majhi, G.K. Naik, H.J. Lee, H.G. Song, C.R. Lee, I.H. Lee, Y.T. Yu, Sensors
Actuators B Chem. 268 (2018) 223–231.
[39] Y.C. Yang, H.S. Hou, G.Q. Zou, W. Shi, H.L. Shuai, J.Y. Li, X.B. Ji, Nanoscale 11
(2019) 16–33.
[40] K. Angelo, F. Liang, F.H. Drake, A.E. Joseph, H.C. Zhou, Chem. Soc. Rev. 47 (2018)
8611–8638.
[26] Y. Liu, R. Luo, Y. Li, J. Qi, C. Wang, J. Li, X. Sun, L. Wang, Chem. Eng. J. 347 (2018)
731–740.
[41] S. Y. Chong, T. T. Wang, L. C. Cheng, H. Y. Lv, M. Ji, doi:https://doi.org/10.1021/
acs.langmuir.8b03153.
[27] H. Zhang, X. Liu, Y. Wu, C. Guan, A.K. Cheetham, J. Wang, Chem. Commun. 54
(2018) 5268–5288.
[42] T.A. Vu, G.H. Le, C.D. Dao, L.Q. Dang, K.T. Nguyen, P.T. Dang, H.T.K. Tran,
Q.T. Duong, T.V. Nguyen, G.D. Lee, RSC Adv. 4 (2014) 41185.
[43] J.J. Wang, M. Yang, W.J. Dong, Z.K. Jin, J. Tang, S. Fan, Y.F. Lu, G. Wang, Catal.
Sci. Technol. 6 (2016) 161–168.
[28] P.P. Liu, H.L. Zhu, Y.Q. Zheng, J. Mater. Sci. 53 (2018) 11574–11583.
[29] Y. Gong, X. Zhao, H. Zhang, B. Yang, K. Xiao, T. Guo, J. Zhang, H. Shao, Y. Wang,
G. Yu, Appl. Catal. B 233 (2018) 35–45.
[30] J. Liu, J. Hao, C. Hu, B. He, J. Xi, J. Xiao, S. Wang, Z. Bai, J. Phys. Chem. C 122
(2018) 2696–2703.
[44] S.C. Pablo, R.F. Enrique, V.G. Jorge, K. Freek, Chem. Mater. 23 (2011) 2565–2572.
[45] D.M. Jiang, L.L. Keenan, A.D. Burrows, K.J. Edler, Chem. Commun. 48 (2012)
12053–12055.
[31] X. Duan, J. Liu, J. Hao, L. Wu, B. He, Y. Qiu, J. Zhang, Z. He, J. Xi, S. Wang, Carbon
130 (2018) 806–813.
[46] M. Jacquemin, D. Hauwaert, D.P. Debecker, E.M. Gaigneaux, J. Mol. Catal. A Chem.
416 (2016) 47–55.
[32] X. Duan, M. Xiao, S. Liang, Z. Zhang, Y. Zeng, J. Xi, S. Wang, Carbon 119 (2017)
326–331.
[47] J. Wei, W. Shen, J. Zhao, C. Zhang, Y. Zhou, H. Liu, Catal. Today 316 (2018)
199–205.
[33] J. Xi, J. Xiao, F. Xiao, Y. Jin, Y. Dong, F. Jing, S. Wang, Sci. Rep. 6 (2016) 21904.
86