Study of catalytic hydrogenation and dehydrogenation of 2,3-dimethylindole for hydrogen storage application

Article abstract of DOI:10.1039/d1ra01552d

2,3-Dimethylindole (2,3-DMID), a candidate with a hydrogen storage capacity of 5.23 wt%, was studied as a new liquid organic hydrogen carrier (LOHC) in detail in this report. Hydrogenation of 2,3-DMID was conducted over 5 wt% Ru/Al2O3 by investigating the influences of temperature and hydrogen pressure. 100% of fully hydrogenated product, 8H-2,3-DMID can be achieved at 190 °C and 7 MPa in 4 h. Dehydrogenation of 8H-2,3-DMID was performed over 5 wt% Pd/Al2O3 at 180-210 °C and 101 kPa. It is found that dehydrogenation of 8H-2,3-DMID followed first order kinetics with an apparent activation energy of 39.6 kJ mol-1. The structures of intermediates produced in the 8H-2,3-DMID dehydrogenation process were analyzed by DFT calculations.

Full text of DOI:10.1039/d1ra01552d

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Study of catalytic hydrogenation and
dehydrogenation of 2,3-dimethylindole for
hydrogen storage application
Cite this: RSC Adv., 2021, 11, 15729
Yuan Dong, Haoming Zhao, Yinheng Zhao, Ming Yang, *ab Heshun Zhang
a
a
a
a
a
and Hansong Cheng
2,3-Dimethylindole (2,3-DMID), a candidate with a hydrogen storage capacity of 5.23 wt%, was studied as
a new liquid organic hydrogen carrier (LOHC) in detail in this report. Hydrogenation of 2,3-DMID was
conducted over 5 wt% Ru/Al
O
2 3
by investigating the influences of temperature and hydrogen pressure.
ꢀ
1
00% of fully hydrogenated product, 8H-2,3-DMID can be achieved at 190 C and 7 MPa in 4 h.
Received 26th February 2021
Accepted 8th April 2021
ꢀ
2 3
Dehydrogenation of 8H-2,3-DMID was performed over 5 wt% Pd/Al O at 180–210 C and 101 kPa. It is
found that dehydrogenation of 8H-2,3-DMID followed first order kinetics with an apparent activation
DOI: 10.1039/d1ra01552d
ꢁ1
energy of 39.6 kJ mol
. The structures of intermediates produced in the 8H-2,3-DMID
rsc.li/rsc-advances
dehydrogenation process were analyzed by DFT calculations.
20
hydrides ). High hydrogen storage densities can be achieved at
Introduction
ꢀ
relatively low temperature (ꢁ196 C) and high pressures (10–
Nowadays, the usage of renewable clean energy has become 100 bar) for physical adsorption for the former. In comparison,
unavoidable, aiming to reduce greenhouse gas emissions and to elevated temperature is required to release hydrogen stored by
1–3
compensate for declining fossil-fuel resources. A consensus chemical adsorption. LOHCs (liquid organic hydrogen carriers)
has been reached that hydrogen energy is one of the most have been considered as promising hydrogen storage substrates
4,5
21–26
attractive choices. However, it is difficult to store hydrogen in recent years.
Hydrogen was bonded to these compounds
without bulky, heavy equipment due to the low density and high through catalytic hydrogenation and released through catalytic
6
explosive risk. Apparently, the further transportation of dehydrogenation. As early as 1970s, aromatic hydrocarbon
hydrogen must be necessary depending on the method of compounds were initially proposed as hydrogen storage carriers
7,8
hydrogen storage. Thus, the major challenge towards the large such as benzene and toluene, which were mainly applied in
scale utilization of hydrogen is to develop a safe and practical industrial process (e.g., in oil reneries). However, with high
ꢁ1
9–11
method for the onboard storage.
In the past decades, enthalpy of dehydrogenation (>60 kJ mol H2), the operation
hydrogen was dominantly stored and transported in its temperatures of these hydrogenated cycloalkanes were required
ꢀ
elemental form as compressed hydrogen (200–700 bar) and over 300 C, which typically lead to the molecule cracking or
ꢀ
27–29
liqueed hydrogen (ꢁ253 C) for onboard applications, both of coking.
Hence, a large number of literatures concerning the
which require a large amount of energy in the storage catalysts development for dehydrogenation of cycloalkanes
9,12–14
30–37
process.
Besides, the safety and cost are another concern were reported in decades.
In 2004, a series of hetero-
for compressed hydrogen and liqueed hydrogen. Conse- aromatic molecules with relatively low heat of hydrogenation
quently, researchers are committed to search for materials screening as LOHCs were rstly studied through systematic
38,39
which can store hydrogen more safely and efficiently.
experimental and theoretical by Pez et al.
It is reported that
Hydrogen can be stored in form of hydrogen molecule by incorporation of N heteroatom into the aromatic ring favored
physical adsorption on materials with a large specic surface the dehydrogenation thermodynamics and kinetics. Carbazole-
1
5,16
ꢀ
area (e.g., carbon-based materials
and metal organic based compounds, such as N-ethylcarbazole (NECZ, mp: 70 C)
17,18
ꢀ
frameworks ) and in form of hydrogen atoms by chemical and N-propylcarbazole (NPCZ, mp: 48 C), were investigated
40–45
adsorption which are bonded with the substrates by chemical most widely as LOHCs by a lot of groups.
bonding (e.g., organic liquid compounds
With a hydrogen
19
and metal capacity of 5.79 wt%, NECZ can be completely hydrogenated
ꢀ
over Ru/Al
2
O
3
at the temperature of 150 C with a hydrogen
pressure of 5 MPa within 180 min. The fully reverse process,
a
Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China
University of Geosciences, Wuhan, 430074, P. R. China
ꢀ
dehydrogenation of NECZ, can be realized below 200 C over Pd/
47
b
Al O (ref. 46) and Pt/Al O . Wu et al. Reported a Co-based
2 3 2 3
Zhejiang Institute, China University of Geosciences, Hangzhou, 311305, China.
catalyst, which can achieve a reversible hydrogen uptake and
E-mail: yangming8180@gmail.com
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2021 The Author(s). Published by the Royal Society of Chemistry
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a
Table 1 Properties of selected indole-based LOHCs
Hydrogenation
conditions from
substrate to
hydrogenated
product
Dehydrogenation
conditions from
hydrogenated
Melting point
( C)
Hydrogen
capacity (wt%)
ꢀ
Substrate
indole
Hydrogenated product
product to substrate
2H-indoline
ꢀ
Pt/C, 3 MPa, 50 C,
3 h, 100%
1
6
.68
.4
—
2
conversion in H O
60
with p-TSA.
51
8H-indole
Pd/C, 0.7 MPa,
ꢀ
5
0 C, 8 h, 100%
—
conversion in HFIP
61
solvent.
NMID
NEID
8H-NMID
8H-NEID
ꢀ
2 3
O , 130 C,
Ru/Al
MPa, 2 h, 100%
ꢀ
Pd/Al
2
O
3
, 200 C,
6
ꢁ
29
5.76
5.23
101 kPa, 3 h, 100%
conversion in
hexane solvent.
57
57
conversion.
ꢀ
Ru/Al
2
O
3
, 180 C,
ꢀ
Pd/Al
2
O
3
, 190 C,
9
MPa, 2 h, 100%
ꢁ17.8
101 kPa, 6 h, 100%
conversion in
hexane solvent.
63
62
conversion.
7
-EID
8H-7-EID
ꢀ
Ru/Al
7
conversion in
2
O
3
, 160 C,
ꢀ
Pd/Al
2
O
3
, 190 C,
MPa, 1.5 h, 100%
ꢁ14.5
5.23
101 kPa, 270 min,
100% conversion.
59
59
hexane solvent.
2
H-2-MID
H-2-MID
Cobalt pincer
catalyst, 150 C, 101
kPa, 4 d, 100%
conversion in p-
ꢀ
Pt/C, 60 C, 4 MPa,
4
ꢀ
h, 97% conversion
in toluene with p-
2
-MID
1
5
.43
.76
64
TSA.
66
xylene solvent.
57
8
ꢀ
ꢀ
Ru/Al
MPa, 0.7 h, 100%
conversion in 1,4-
2
O
3
, 160 C,
Pd/Al
2
O
3
, 190 C,
7
101 kPa, 4 h, 100%
conversion in
decalin solvent.
65
65
dioxane solvent.
1
,2-DMID
8H-1,2-DMID
ꢀ
Ru/Al
7
2
O
3
, 160 C,
ꢀ
Pd/Al
1
1
2
O
3
, 200 C,
MPa, 30 min,
100% conversion in
01 kPa, 60 min,
00% conversion in
decalin solvent.
55
5.23
1,4-dioxane
58
5
8
solvent.
a
HFIP: hexauoroisopropanol; p-TSA ¼ p-toluenesulfonic acid.
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Table 2 Physicochemical properties of 2,3-DMID and 8H-2,3-DMID
Name
2,3-DMID
91-55-4
8H-2,3-2MID
N/A
CAS number
Molecula formula
Molecular weight
C
10
H
11
N
10 19
C H N
153.20
145.20
Structural formula
ꢀ
Melting point/ C
105
285
<ꢁ10
$285
ꢀ
Boiling point/ C
State
Yellow solid at room temperature
Store in cool, dry and well-ventilated areas;
stable under ambient conditions, but light
sensitive
Transparent liquid at room temperature
Store in cool, dry and well-ventilated areas; stable
under ambient conditions, light sensitive
Storage and stability
Safety
May cause irritation of skin and eyes. The
toxicity of this substance has not been
investigated completely
May cause irritation of skin and eyes. The toxicity of
this substance has not been investigated completely
48
release of NECZ in high efficiency. Fully dehydrogenated N- hydrogenation of 2,3-DMID can be realized at the temperature
ꢀ
ꢀ
propylcarbazole can be obtained at 200 C in 240 min with of 190 C within 240 min. The dehydrogenation reaction rate of
49
applied Pd/Al O catalyst. Arlt et al. investigated the thermo- 8H-DMID was rst order for 8H-DMID concentration. The
2
3
chemical properties of N-alkylcarbazoles (N-isopropylcarbazole, apparent activation barrier of 8H-DMID consumption was
5
0
ꢁ1
ꢀ
N-propylcarbazole, and N-butylcarbazole) and found the 39.6 kJ mol at temperature in the range of 180–210 C. The
melting point of eutectic systems composed of N-alkylcarba- dehydrogenation process of 8H-2,3-DMID was analysed by
ꢀ
51
zoles can be lowered dramatically to 24 C, which are suitable experimental and DFT calculations.
for large scale and long distance storage and delivery at ambient
conditions with existing infrastructures. In addition, Was-
Results and discussion
serscheid et al. proposed benzyltoluene (BT) and dibenzylto-
52,53
luene
(DBT)
as
promising
LOHCs.
Although The physicochemical properties of 2,3-DMID and 8H-2,3-
dehydrogenation of these hydrocarbon molecules was required DMID
ꢀ
54
at temperatures above 250 C, these mixtures were still
regarded as attractive LOHCs as the result of their low melting
As shown in Table 2, both of 2,3-DMID and 8H-2,3-DMID keep
high stability at ambient conditions and no severe hazard to
people health and environment. The melting point of 2,3-DMID
ꢀ
ꢀ
point (ꢁ34–36 C), high boiling point (390 C), low ammability
and low costs. Recently, indole-based compounds were also
ꢀ
is 105 C, which can be decreased by mixing with other LOHCs
5
5,56
suggested as LOHCs, such as indole,
(
N-methylindole
to form eutectic mixtures. The hydrogenated product, 8H-2,3-
DMID, was liquid at room temperature, which can be conve-
niently delivered by using existing gasoline-based
infrastructures.
57
58
59
NMID), 1,2-dimethylindole and 7-ethylindole (7-EID).
Table 1 gives an overview of properties and reaction conditions
for indole derivatives hydrogenation and dehydrogenation
which have been reported in relative literatures. It is indicated
that indole derivates with different alkyl chains substituted in
different positions exhibit markedly difference in hydrogena-
tion and dehydrogenation kinetics.
In fact, we have published a number of hydrogen storage 200 C and 7 MPa. The hydrogen absorption curves at the
molecular articles, including carbazole series, indoles series, selected temperatures have been displayed in Fig. 1. The
etc. The purpose of this work is to build a database of organic hydrogenation uptake rate accelerates as the reaction temper-
Hydrogenation of 2,3-DMID
The hydrogenation reaction of 2,3-DMID was performed at 160–
ꢀ
ꢀ
ꢀ
hydrogen storage molecules and to obtain a certain construc- ature was increased from 160 C to 190 C. Aer reaction for 1 h,
tion rule by studying the inuence of molecular structure (such the hydrogen storage gravimetric density reaches 0.57 wt% at
as the number of side groups, types and number of aromatic 160 C, 2.91 wt% at 170 C, 3.21 wt% at 180 C and 4.67 wt% at
ꢀ
ꢀ
ꢀ
ꢀ
rings) on the dehydrogenation performance of organic mole- 190 C, respectively. Specially, the hydrogen uptake rate
ꢀ
ꢀ
cules. In this work, the disubstituted 2,3-dimethylindole (2,3- increases notably with temperature rising from 160 C to 170 C,
DMID) was well studied as a new LOHC candidate. The inu- which can be explained that most reactant molecules can be
ences of temperature and hydrogen pressure on the hydrogen actived to overcome the hydrogenation reaction barrier as the
ꢀ
uptake rate of 2,3-DMID were investigated. Complete temperature exceeded 160 C. The theoretical hydrogen capacity
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ꢀ
Fig. 1 Hydrogen uptake of 2,3-DMID hydrogenation at 160–200
C
and 7 MPa over the 5 wt% Ru/Al
2 3
O catalyst with a ratio to 2,3-DMID of
Fig. 3 Hydrogen uptake of 2,3-DMID hydrogenation under 5–8 MPa
0
.72 mol%.
ꢀ
at 190 C over a 5 wt% Ru/Al
O
2 3
catalyst with a ratio to 2,3-DMID of
0
.72 mol%.
ꢀ
of 2,3-DMID (5.23 wt%) can be reached at 190 C aer 4 h.
However, the reaction rate decreased when the temperature was
Fig. 2 shows the evolution of product distribution with
ꢀ
further raised to 200 C. The reasons for the decrease may be its reaction time during 2,3-DMID hydrogenation over Ru/Al
2
O
3
.
exothermic nature of hydrogenation reaction, which causes the Only 4H-2,3-DMID was detected as the intermediate along with
equilibrium to be shied towards the reactants with increasing the fully hydrogenated product, 8H-2,3-DMID. The consump-
ꢀ
temperature. In addition, high temperature led to the difficulty tion rate of 2,3-DMID increased signicantly in 170–200 C as
ꢀ
in dissolving more hydrogen in the liquid though the pressure compared with that in 160 C, which explained the obvious
ꢀ
was increased, which limited the hydrogen transfer at the decrease of the hydrogen uptake rate (Fig. 2) in 160 C at the
67
surface of the catalyst.
beginning of the reaction. The concentration of 4H-2,3-DMID
rapidly reached maximum in the initial 30 minutes at 170–
ꢀ
2
00 C. Almost 100% hydrogenation of 2,3-DMID was realized
ꢀ
ꢀ
ꢀ
in 240 min at 190 C, 540 min at 180 C and 660 min at 170 C.
Increase temperature to 200 C, the consumption of 4H-2,3-
ꢀ
DMID become sluggish which lead to the decrease of
ꢀ
hydrogen uptake rate compared with that of at 190 C. No 2H-
and 6H- intermediates were detected. It is likely that they were
kinetic unstable and immediately further converted to the nal
product upon formation.
Fig. 3 displays the hydrogen adsorption curves of 2,3-DMID
ꢀ
under different hydrogen pressures at 190 C over 5 wt% Ru/
Fig. 2 Time-dependent product distribution for 2,3-DMID hydroge- Fig. 4 Hydrogen release of 8H-2,3-DMID dehydrogenation at 180–
ꢀ
ꢀ
ꢀ
ꢀ
2 3
nation at different temperatures. (a) 160 C, (b) 170 C, (c) 180 C, (d) 210 C and 101 kPa over the 5 wt% Pd/Al O with a ratio to 8H-2,3-
ꢀ
ꢀ
1
90 C, (e) 200 C.
DMID of 1.44 mol%.
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Al
RSC Advances
2
O
3
. It is found that the increase of pressure is in favor of Table 3 The conversion, selectivity and hydrogen release content
under different dehydrogenation temperatures in 5 h
accelerating hydrogenation rates, which can be attributed to the
improvement in hydrogen solubility and the shi in the direc-
tion of hydrogenation of the chemical equilibrium.
Temperature
Conversion
(%)
Selectivity
(%)
Hydrogen release
content (wt%)
ꢀ
(
C)
1
1
80
90
81.5
89.2
94.1
93.6
62.3
75.6
85.2
88.7
3.71
4.27
4.67
4.75
Dehydrogenation of 8H-2,3-DMID
The dehydrogenation reactions were carried out at the 200
210
ꢀ
2 3
temperature of 180–210 C over 5 wt% Pd/Al O . The hydrogen
release curves of 8H-2,3-DMID at different temperatures were
shown in Fig. 4. The dehydrogenation rate accelerates obviously
ꢀ
ꢀ
with the temperature raised from 180 C to 200 C. 96% dehy-
ꢀ
drogenation was obtained aer 600 min at 200 C. Further
increase temperature to 210 C, the time for achieving the
Specific product ½molꢂ
Sum of products ½molꢂ
Selectivity ¼
ꢃ 100%
(2)
ꢀ
theoretical hydrogen storage gravimetric density (5.23 wt%) was
shortened to be 480 min.
Fig. 5 shows the evolution of product distributions with
reaction time at selected temperatures during the process of 8H-
In fact, a disubstituted indole derivative, 1,2-DMID has been
studied in detail previously. Only 4H-1,2-DMID was identied as
the intermediate in the dehydrogenation reaction of 8H-1,2-
DMID and completely dehydrogenated product can be ob-
2
,3-DMID dehydrogenation. 8H-2,3-DMID was consumed as
soon as the reaction was started. However, the conversion of 8H-
,3-DMID was sluggish and the complete conversion of 8H-2,3-
ꢀ
tained in 60 min at 200 C with applied Pd/Al O . In compar-
2
3
2
ison, three intermediates, 6H-, 4H- and 2H- were detected and
the consumption of these intermediates were sluggish under
designed conditions in 8H-2,3-DMID dehydrogenation. The
reason may be that the steric hindrance produced by the methyl
connected to the N atom in 1,2-DMID benets the sorption of
,2-DMID on the catalyst, which leads to a faster dehydroge-
nation rate.
The dehydrogenation kinetics of 8H-2,3-DMID were also
investigated. Previous investigations on dehydrogenation of
several LOHCs such as N-ethylcarbazole, 1,2-dimethylindole
and 2-methylindole have shown that the reaction follows rst
order kinetics. Hence, the dehydrogenation of 8H-2,3-DMID
was assumed to follow rst order kinetics. Fig. 6(a) presents
DMID was not obtained until the reaction proceeded in 8 h at
ꢀ
2
10 C. Three intermediates, 6H-, 4H- and 2H- were detected
and no other side products were found. The accumulation of
6
4
H-2,3-DMID from 8H-2,3-DMID was converted rapidly to form
H-2,3-DMID and 2H-2,3-DMID, resulting in the maximum
1
concentration of 6H-2,3-DMID to be reached quickly. Obviously,
the dehydrogenation of 8H-2,3-DMID was a consecutive four
step reaction, sequentially forming 6H-2,3-DMID, 4H-2,3-
DMID, 2H-2,3-DMID and the fully dehydrogenated product,
2,3-DMID. The conversion of 8H-2,3-DMID and selectivity to 2,3-
DMID aer 5 h were presented in Table 3, which were calculated
by the eqn (1) and (2) bellow, respectively.
Reactant converted½molꢂ
the plots of ln(C
0
/C8H-2,3-DMID) versus reaction time t at 180–
Conversion ¼
ꢃ 100%
(1)
ꢀ
Sum of reactants ½molꢂ
210 C, which indicates that the experimental points can be
readily tted to the rst order reaction kinetics model. The rate
constant derived from the slope of the tting lines increases
ꢁ1
ꢀ
ꢁ1
ꢀ
from 0.0050 mol L at 180 C to 0.0090 mol L at 210 C.
Particularly, the dehydrogenation rate constants increase obvi-
ꢀ
ꢀ
ously from 180 C to 200 C due to its endothermic nature.
ꢀ
Further increase temperature to 210 C, the rate constant only
Fig. 6 (a) First-order kinetic model fitted to the experimental data
Fig. 5 Time-dependent product distribution for 8H-2,3-DMID dehy- obtained for dehydrogenation of 8H-2,3-DMID. (b) Arrhenius plot
ꢀ
ꢀ
drogenation at different temperatures. (a) 180 C, (b) 190 C, (c) obtained from 8H-2,3-DMID dehydrogenation reactions at 180–
ꢀ
ꢀ
ꢀ
200 C, (d) 210 C.
210 C.
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Fig.
7 DSMS patterns of the released hydrogen in 2,3-DMID
Fig. 9 -Binding energy of each intermediates and products calculated
by MS.
dehydrogenation.
Scheme 1 The reaction pathway of 8H-2,3-DMID dehydrogenation.
ꢁ
1
(
Unit of Er: kJ mol ).
within 240 min in 3 cycles, indicating the high stability of Ru/
Al O catalyst in hydrogenation of 2,3-DMID. Similarly, the
2
3
same Pd/Al O was used to dehydrogenate 8H-2,3-DMID for 3
2
3
2 3
times. The obvious catalytic activity degradation of Pd/Al O
was observed. This may be because the carbon formation on the
surface of the catalyst and the metal agglomeration aer
dehydrogenation for a long time. The study on design and
preparation high activity and stability catalyst is under way
Fig. 8 Catalyst stability test for 2,3-DMID hydrogenation with Ru/
Al O and dehydrogenation with Pd/Al O .
2 3 2 3
(
Fig. 8).
During 8H-2,3-DMID dehydrogenation, three intermediates,
H-, 4H- and 6H-, were detected. The possible structures of the
increases slightly. The reason may be that the high temperature
is benecial for dehydrogenation kinetics meanwhile causes
carbon deposition on the catalyst which decreases the catalytic
activity. As shown in Fig. 6(b), the ln k was plotted verse the 1/T.
The estimated reaction activation barrier for 8H-2,3-DMID
dehydrogenation based on the Arrhenius equation is
2
isomers were designed and the corresponding energy for each
species has been displayed in Fig. 9.
The binding energy can indicate the stability of the structure
to a certain extent. It can be seen that the C]C double band was
ꢁ1
3
9.6 kJ mol , which is signicantly lower than that of 8H-1,2-

rstly formed in ve membered ring, leading to the 6H-2,3-
ꢁ1
DMID dehydrogenation (111.9 kJ mol ). Notably, the reaction
activation energy obtained from the experiments was inu-
enced by kinds of factors, such as reaction conditions, mass
DMID and 4H-2,3-DMID production. Subsequently, the
further dehydrogenation occurs in the six membered ring with
releasing another H molecule to form 2H-2,3-DMID. Hence,
the pathway for 8H-2,3-DMID dehydrogenation can be
expressed as follows (Scheme 1):
2
68
transfer and so on.
In addition, the purity of released hydrogen was tested by
DSMS (Dynamic Sampling Mass Spectrometer), which can
dynamically monitor the pressure of the released gas during
dehydrogenation. As shown in Fig. 7, the water vapor may come
from the atmosphere when the liquid was added to the dehy-
drogenation reactor. The result indicated that no side reactions
Experimental
Materials
occurred under designed dehydrogenation conditions and the 2,3-dimethylindole (AR) and Mesitylene (AR) were provided by
hydrogen gas was in a high purity (>99%).
Energy Chemical Company and Sinopharm Chemical Reagent
The catalyst stability was also tested in 2,3-DMID hydroge- Company Limited, respectively. 5 wt% Ru/Al O and 5 wt% Pd/
2
3
nation and dehydrogenation. The same Ru/Al O catalyst was Al2O3 catalysts were purchased from Shanxi Kaida Chemical
2
3
ꢀ
used to hydrogenate 2,3-DMID under 190 C and 7 MPa for 3 Engineering Company Limited. The BET surface areas and pore
2
ꢁ1
times. It is found almost 100% 8H-2,3-DMID was achieved sizes of the Ru/Al O and Pd/Al O are 85.24 m g , 13.41 nm
2
3
2 3
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hydrogenation and dehydrogenation kinetics, which can be
strongly inuenced by the steric hindrance which were created
by the different substituted alkin chains. 2,3-DMID was studied
as another new LOHC in detail in this report. Fully hydroge-
ꢀ
nated product can be obtained within 4 h at 190 C and 7 MPa.
Dehydrogenation of 8H-2,3-DMID ts rst order kinetics model.
The apparent activation barrier of 8H-DMID dehydrogenation
ꢁ
1
ꢀ
was estimated to be 39.6 kJ mol at 180–210 C. The dehy-
drogenation process can be written as: 8H-2,3-DMID / 6H-2,3-
DMID / 4H-2,3-DMID / 2H-2,3-DMID / 2,3-DMID.
Fig. 10 The experiment process of 2,3-DMID hydrogenation and
dehydrogenation.
Conflicts of interest
There are no conicts to declare.
2
ꢁ1
and 68.35 m g , 11.89 nm, which are measured by ASAP 2020.
Ultra-high purity H gas was obtained from Sichuan Ally High-
Tech Company. Gas chromatography and mass spectrometry
2
Acknowledgements
(Agilent 7890/5975C GC-MS) was used to analyse the reaction The authors gratefully acknowledge the nancial support from
liquid samples regularly. The test method was same as the National Key Research and Development Program of China
described in ref. 59.
(No. 2018YFB1502903), the Zhejiang Provincial Natural Science
Foundation of China (Grant no. LGG20B030001), the National
Natural Science Foundation of China (No. 21875225), Major
project of technical innovation of Hubei Province (No.
Hydrogenation reaction of 2,3-dimethylindole
The hydrogenation reaction was performed in a 250 ml auto-
clave batch reactor (Shanghai LABE Instrument LB250) over
2019AAA163), and China University of Geosciences (Wuhan) for
the program of Center for Advanced Energy Research and
Technologies.
5
wt% Ru/Al
hydrogen pressures. The reactor was loaded with 1 g of 2,3-
dimethylindole and 0.1 g of Ru/Al in a 40 ml of mesitylene
2 3
O by controlling the reaction temperatures and
2 3
O
before being sealed. The air in the reactor was purged by Notes and references
charging and discharging of hydrogen for 3 times. Aerwards,
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The 8H-2,3-DMID produced by complete hydrogenation was
used as initial reactant in dehydrogenation. The reaction was
12254–12269.
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ꢀ
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required temperature following by adding 1 g of 5 wt% Pd/Al O
2 3
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Conclusions
Indole-based derivatives have been selected as attractive LOHCs
recently. These indole-based compounds exhibit different
©
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