Hydrogen generation from methanol reforming under unprecedented mild conditions

Article abstract of DOI:10.1016/j.cclet.2017.03.038

A homogeneous catalyst [Cp*Rh(NH₃)(H₂O)₂]³⁺ has been found for the clean conversion of methanol and water to hydrogen and carbon dioxide. The simple and easily available reaction steps can circumvent the formati

Full text of DOI:10.1016/j.cclet.2017.03.038

Accepted Manuscript
Title: Hydrogen generation from methanol reforming under
unprecedented mild conditions
Authors: Yu-Lu Zhan, Yang-Bin Shen, Shu-Ping Li, Bao-Hua
Yue, Xiao-Chun Zhou
PII:
S1001-8417(17)30126-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2017.03.038
CCLET 4033
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
21-1-2017
3-3-2017
27-3-2017
Pleasecitethisarticleas:Yu-LuZhan, Yang-BinShen, Shu-PingLi, Bao-HuaYue, Xiao-
Chun Zhou, Hydrogen generation from methanol reforming under unprecedented mild
conditions, Chinese Chemical Lettershttp://dx.doi.org/10.1016/j.cclet.2017.03.038
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Original article
Hydrogen generation from methanol reforming under unprecedented mild conditions
a,b
b,c
b,c
a
b,*
Yu-Lu Zhan , Yang-Bin Shen , Shu-Ping Li , Bao-Hua Yue , Xiao-Chun Zhou
a
Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, China
b
Division of Advanced Nanomaterials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, China
University of Chinese Academy of Sciences, Beijing 100049, China
c

Graphical abstract
Original article
Hydrogen generation from methanol reforming under unprecedented mild conditions
a,b
b,c
b,c
a
b,*
Yu-Lu Zhan , Yang-Bin Shen , Shu-Ping Li , Bao-Hua Yue , Xiao-Chun Zhou
a
Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, China
b
Division of Advanced Nanomaterials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, China
University of Chinese Academy of Sciences, Beijing 100049, China
c
3
+
A homogeneous catalyst [Cp*Rh(NH
3 2
)(H O)
2
]
has been found for the clean conversion of methanol and water to hydrogen and carbon dioxide. The
, therefore, making it possible to avoid inactivating catalysts and
simple and easily available reaction steps can circumvent the formation of CO
2
contaminating the hydrogen fuel. Different from conventional reforming method for hydrogen production, no additional alkaline or organic substances
are required in this method.

ARTICLE INFO
ABSTRACT
3
+
Article history:
A homogeneous catalyst [Cp*Rh(NH
3
)(H
2
O)
2
]
has been found for the clean conversion of
Received 23 January 2017
Received in revised form 25 February 2017
Accepted 1 March 2017
Available online
methanol and water to hydrogen and carbon dioxide. The simple and easily available reaction
steps can circumvent the formation of CO, therefore, making it possible to avoid inactivating
catalysts and contaminating the hydrogen fuel. Different from conventional reforming method
for hydrogen production, no additional alkaline or organic substances are required in this
method. Valuable hydrogen can be obtained under ambient pressure at 70 ℃, corresponding
-1
TOF is 83.2 h . This is an unprecedented success in reforming methanol to hydrogen. Effects of
reaction conditions, such as reaction temperature, initial methanol concentration and the initial
pH value of buffer solution on the hydrogen evolution are all systematically investigated. In a
certain range, higher reaction temperature will accelerate reaction rate. The slightly acidic
condition is conducive to rapid hydrogen production. These findings are of great significance to
the present establishment of the carbon-neutral methanol economy.
Keywords:
Homogeneous catalysts
Methanol reforming
Hydrogen
Low temperature
Low CO
1
. Introduction
water. However, limited by the catalytic activity and selectivity
With the increase of global energy demand and the attentions to
the depletion of fossil energy, the development of large-scale
alternatives to fossil fuels becomes more important nowadays [1,
of the reported catalysts, the MSR always perform at high
temperatures (> 200 ℃). In addition, the co-generation of carbon
monoxide limits its further applications in fuel cells which only
bear a low CO concentrations [7, 21]. Moreover, the subsequent
purification of the reforming gas would increase the costs of
hydrogen generation. To meet the requirements of applications
and reducing cost, catalysts with high activity and selectivity are
needed.
2
]. Hydrogen can be used as an energy source for fuel cells with
the release of water as the only side product and tremendous
potential to decrease greenhouse gas emissions. The advantages
of hydrogen as an energy source include: 1) high mass energy
density (123 MJ/kg), 2) facile conversion to electrical energy as
well as mechanical energy, and 3) generation of only water upon
conversion to energy. Approximately 50 million metric tons of
hydrogen produced from fossil fuels annually are used mainly for
oil refining and ammonia synthesis. However, current
thermochemical production options are prohibitively expensive
Herein, we described an aqueous phase methanol reforming
process under low temperatures (<100 ℃) and mild conditions.
With the existence of homogeneous rhodium catalyst, no carbon
monoxide could be detected in the reforming gas. Hydrogen
generation was observed with excellent turnover frequencies
-
1
3+
[
3]. To unlock the tremendous potential of the hydrogen
(TOF) of 83.2 h by [Cp*Ru(NH )(H O) ] at 70 ℃without any
3 2 2
economy, it is highly important to develop an efficient and
practical method for the hydrogenation from renewable and easy-
to-handle resources.
addition of alkaline or organic materials.
. Results and discussion
Several organo-rhodium complex catalysts were tested for the
2
In recent years, one-carbon molecules constantly arise as good
carriers of renewable hydrogen [4‒10]. CH OH is a high
methanol reforming reaction. All the experiments were carried
out at 70 ℃. The activities of various catalysts for methanol
reforming were listed in Table 1. The chemical equation has been
confirmed as follows:
3
hydrogen storage medium (12.5 wt% H ) and a convenient
2
chemical feedstock to produce a myriad of chemicals and
products [7, 11‒14]. It is already one of the most important
building blocks in the chemical industry with an annual
production in excess of 70 million tons. Furthermore, it can be
directly used as a drop-in liquid fuel for internal combustion
engines and direct methanol fuel cells (DMFC). Besides,
compared with the difficulties of selective activation of methane
and the inferior energy balance of formic acid, methanol is the
most promising one-carbon molecule in the world currently [8,
Rh
CH
OH+H
O
3H
+CO
2
(1)
3
2
2
7
0 ℃
As shown in Fig. 1, different from other reported catalysts
which bear the pincer-type ligands [22‒25], the central rhodium
of the four catalysts all bears one Cp* ligand, and ligands on the
remaining empty coordination numbers are different from each
other. Three null coordination of cat.1 are filled with one NH3
1
5‒20].
and two H O molecules, respectively. 4-(1H-Imidazol-1-yl)

Corresponding author.
E-mail address: xczhou2013@sinano.ac.cn (X.C. Zhou).
2
benzoic acid relies on the nitrogen of the imidazolyl to connect
with Rh-Cp*, and the rest of the coordination is filled with one
water molecule. N,N-Chelate bond were generated by the Rh-
Cp* and ligands of cat.3 and cat.4. 2,2′-Bipyrimidine is
connected to rhodium by two nitrogen on the same side, and
water molecule acts as another ligand. The 2,2′-biimidazole and
2,2′-bipyrimidine are all chelating reagent, which could connect
to Rh-Cp* strongly.
There are mainly three methods for hydrogen production from
methanol, that is, methanol steam reforming for hydrogen
production (MSR), partially catalytic oxidation of methanol
reforming (POR), and auto-thermal reforming of methanol
(ATR). Among them, the MSR is the most economic method, by
which the molar ratio of H and CO in gaseous product is about
2
2
3
:1 theoretically. The so-called methanol-reforming process
allows for the production of H from a mixture of methanol and
2

Fig. 2. (a) Effect of the initial water methanol ratio on the hydrogen evolution with
time increase. (b) Effect of the initial water methanol ratio on the TOF1h. T=70 ℃.
As can be seen from Fig. 2a, when the ratio of methanol to
water was between 1:4.5~3:1, the reaction activity gradually
increased with the increase of the ratio, namely, the methanol
conversion rate was gradually increased. There is a sharp
decrease upon increasing the ratio to 5:1. Overall, the catalytic
activity decreased over time. Fig. 2b shows the effect of the
initial methanol to water ratio on the initial dehydrogenation
reactivity (TOF ). In brief, 3:1 was the optimum ratio of
Fig. 1. Corresponding structures of the four catalysts.
Table 1
Catalytic activity of methanol reforming for hydrogen production
by Rh catalysts bearing different ligands.
1
h
-
1
-1
-1
Entry
Cat.
TOF [h ]
TOF [h ]
TOF [h ]
methanol to water for methanol dehydrogenation. The reaction
mechanism of the methanol dehydrogenation was proposed and
1
h
2h
3h
discussed as follows:
1
2
3
Cat.1
Cat.2
Cat.3
20
7
10.1
1.67
0.3
3.7
1.8
0.3
Θ
CH OH
HCHO + H2 Δ G = 63.74 kJ/mol
(2)
3
r
m
Θ
HCHO + H O
CO + 2H Δ G = ‒54.69 kJ/mol (3)
2 2 r m
2
For the first step, the process of C-H bonds of methanol
breaking into hydrogen and formaldehyde was an up-hill
reaction, which made the methanol dehydrogenation very
difficult and determined the whole reaction rate. The following
step (3) was the thermodynamically down-hill reaction, which
could be carried out quickly in the presence of catalysts. It was
also the renowned “aldehyde-water shift (AWS)” reaction, in
which water played the role of the effective oxidant [30, 31].
During this process, the aldehyde group was firstly oxidized into
carboxyl group, meanwhile, releasing hydrogen as the valuable
by-product. Subsequently, formic acid was decomposed into
hydrogen and carbon dioxide rapidly [32, 33]. Obviously, as the
reactant and oxidant, water content had a great influence on the
reaction rate.
The addition of some basic or organic substances into the
system were typically used to accelerate the reaction rate,
especially for the first step [7, 34, 35]. Considering the high
requirements on the container and delivery process caused by
alkaline and organic materials, aqueous solution without extra
alkaline or organic substances is desired for industrial
applications nowadays [36, 37]. The reaction system we
described here can meet the above requirements well.
0.3
4
Cat.4
0.22
0.06
0.06
Reaction conditions: CH
3
2
OH (1 mL), H O (1 mL), Cat. (1 mg), T = 70 ℃.
As we all know, it is very difficult to break N,N-chelate bond
of the catalysts, which as a result, greatly hampered the hydrogen
evolution [26, 27]. Table 1 shows that the TOF values of cat.3
and cat.4 were below 0.5, which was determined by their similar
structures. The chelating ligand could increase the steric
hindrance during the catalytic reaction [28, 29]. The catalytic
activity of cat.2 increased to 7~8 times compared to the former
two. Remarkably, the catalytic activity of cat.1 with NH as a
ligand quickly upgraded to about 20 times, and TOF value
3
-
1
reached 20 h . Apparently, the difference in activities lies in the
difference of coordination environment. Catalysts with multiple
active sites can greatly enhance the catalytic activity, and
synchronously, reactants are more likely to be exposed to the
active transition metal. In general, the catalytic activity decreased
with the increase of time for the methanol dehydrogenation.
Effects of reaction temperatures, molar ratio of water to
methanol and reaction time on the performance of the reaction
should be investigated. The concentration of carbon monoxide in
the reforming gas has a strongly negative effect on the efficiency
of proton exchange membrane fuel cell (PEMFC). Optimization
of the reaction conditions is beneficial to improve the conversion
of the reactant and the selectivity of the product. After
demonstrating the reactivity of the four catalysts for methanol
reforming, cat.1 was chosen to assess the influence of the
reaction conditions (pH, temperature and water content) on the
performance of the catalyst.
We used GC to measure the composition of the evolved gas
from the methanol reforming reaction. No CO was detected
(below detection limit) in the generated gas (Fig. 3c). Besides
hydrogen, the only detectable gas was CO (Fig. 3a and Fig. 3b).
Hydrogen generation without CO is especially useful to the high
performance of PEMFC.
2

alkaline conditions counted against the reaction. Reacting below
0 ℃ opens the possibility to perform aqueous methanol
7
reforming with the waste heat from proton-exchange membrane
fuel cells. Furthermore, no CO was detected in the product gas,
which means that the gaseous product can be directly used for
fuel cells. In addition, slightly acidic conditions can avoid the
corrosion to the container caused by the strong acid or alkaline
solution, which extends its application range and makes it
capable to meet the requirements of industrial application. These
findings are very beneficial for the future industrial application
of hydrogen production.
Fig. 3. GC spectra of the gaseous product from methanol reforming reaction. (a)
; (b) CO ; (c) CO. Orange curves represent the standard gas samples. Reaction
conditions: methanol (1.71 mL), deionized H O (0.29 mL), cat.1 (3.4 μmol), T =
0 ℃. The evolved gas for gas chromatogram measurement was collected after
reacting 1 h.
4
. Experimental
H
2
2
2
4
.1. Materials and instruments
7
2
,2′-Bipyrimidine were obtained from J&K Chemical Co.,
(
Shanghai, China). 2,2′-Biimidazole were purchased from Alfa
Fig. 4a shows that higher temperature could increase the
reaction rate. The TOF value reached up to 20 without any
additional material required at 343K. Moreover, the apparent
activation energy (Ea) could be achieved by fitting the linear
correlation between ln(TOF) and 1/T accurately. The Ea for
methanol reforming was 96.0 kJ/mol. As exhibited in Fig. 4b, the
effect of buffer pH on the methanol dehydrogenation was
observed in the pH range from 5 to 8, which were adjusted by
phosphate buffer solution. At such relatively low temperatures,
the hydrogen evolution rate increased gradually with elevating
the buffer pH from 5 to 6 and presented a maximum TOF of 83.2
Aesar. Cp* was provided by Meryer (Shanghai) Chemical
Technology Co. 4-(1H-Imidazol-1-yl) benzoic acid was obtained
from Energy (Shanghai) Chemical Co. Other chemicals were
obtained from China National Pharmaceutical Group Co.
(Shanghai, China). All chemicals are analytical reagents. All
commercial materials were used as received unless specified.
The gaseous product generated from low temperature
methanol reforming was analyzed by GC-G5 (Beijing Persee
General Instrument Co., Ltd.), the gas chromatography
assembled with TCD, FID, TDX-1 column and methane
conversion reactor. N was used as carrier gas. The detection
limit of CO was below 10 ppm through a methane reformer.
TCD of the GC was used to detect H . The H NMR spectrums
2
-
1
h at pH 6. Further increasing the pH to 8 dramatically decreased
the reaction rate.
1
2
were characterized by NMR (Varian Plus 400 MHz). The UV-
Vis spectrum was collected by Shimadzu UV 1800.
4
.2. Synthesis of the four catalysts
3
+
Synthesis of [Cp*Rh(NH )(H O) ]
(cat.1): 200 mg
3
2
2
[
RhCp*Cl ] was stirred in 15 mL water at room temperature,
2 2
forming a pale yellow suspension. Then 200 mL ammonia was
dropped into the suspension slowly. Keep stirring at room
temperature for 8 hours, the solid will dissolve and turn into dark
red solution. Then reddish brown product will be gained by
evaporating solution under reduced pressure. Synthesis process
Fig. 4. (a) Effect of the temperature on the catalytic hydrogen generation; (b)
Effect of the initial buffer pH on the hydrogen evolution. Reaction conditions: (a)
1
refers to literature partially [38]. The corresponding H NMR
methanol (1.71 mL), deionized H
1.71 mL), deionized H O (0.19 mL), buffer solution (20 mmol/L, 0.1 mL), cat.1
3.4 μmol), T = 70 ℃.
2
O (0.29 mL), cat.1 (3.4 μmol). (b) Methanol
1
spectra is shown in Fig. S3 in Supporting information. H NMR
(
2
(
400 MHz, D O): δ 1.60 (s, 15H), 1.19 (t, 3H).
2
(
Synthesis
of
cat.2:
An
aqueous
solution
of
III
3
. Conclusion
In summary, we developed an unprecedented mild reaction
[Rh (Cp*)(H O) ](SO ) and equiv. of 4-(1H-pyrazol-1-
2
3
4
yl)benzoic acid was stirred under reflux for 12 h, and then the
solution was filtered with a membrane filter. The filtrate was
evaporated under reduced pressure to yield a yellow powder of
3
+
system for methanol reforming by [Cp*Rh(NH )(H O) ] in
3
2
2
aqueous phase, and no additional alkaline or organic substances
are required. Comparison of several different catalysts revealed
1
cat.2 [39]. H NMR spectra is shown in Fig. S4 in Supporting
1
information. H NMR (400 MHz, DMSO-d ): δ 0.35 (s, 15H),
6
that the coordinative H O ligand is labile, and the left vacant site
2
5
7
.56 (s, 1H), 6.44 (s, 1H), 6.66 (s, 1H), 6.79 (s, 1H), 6.84 (s, 1H),
.13 (s, 1H), 7.48 (s, 1H).
allows the metal center for catalysis, which finally accelerates
the reaction rate. Hydrogen with low CO content was obtained
-
1
Synthesis of cat.3: Reaction of the rhodium dimers
under 70 ℃and constant pressure with TOF of 83.2 h . The
optimum mole ratio of methanol to water was 3:1, the pH of
buffer had a great effect on the reforming process, and faintly
acidic conditions were conducive to the reaction. Similarly,
[
{(Cp*)RhCl } ] with 2 equiv. of biimidazole yielded the
2 2
cationic complexes, which were isolated as chloride salts. The
yellow compounds are air-stable, soluble in water and in most
1
organic solvents [40]. The corresponding H NMR spectra is

1
[
6] R. Tanaka, M. Yamashita, K. Nozaki, Catalytic hydrogenation of carbon
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D O): δ 1.76 (s, 15H), 7.29 (s, 2H), 7.47 (s, 2H), 7.56 (s, 2H).
2
1
4168.
Synthesis of cat.4: The pentamethylcyclopentadienyl
complexes [Cp*RhCl2]2 react with 2 equiv. of 2,2′-bipyrimidine
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95 (2013) 85-89.
(
bpym)
in
methanol
to
form
complexes
the
cationic
[(η -C Me )
5 5
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1
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.3. General experimental procedures
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at a pre-set temperature (70 ℃) under ambient atmosphere. After
a period of time, a certain amount of catalyst solution was
injected into the reactor through a micro syringe. A semi liquid
filled U-shaped tube was connected to the reactor. The volume
content of the evolved gas could be calculated through changes
of liquid level in U-shaped tube. A camera was used to monitor
the changes of liquid level. The schematic diagram of the process
is shown in Fig. S1 in Supporting information.
[
[
[
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.4. Calculation of TOF and the volume of H2
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ꢃ
ꢉꢊꢋꢌꢃꢄꢍꢎꢏꢐ℃ꢑ
ꢄꢅꢆꢅꢇꢈ
ꢀ
ꢁꢂ
(4)
[
[
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ꢒꢄ
ꢓꢇꢅꢇꢈꢔꢕꢅ
The calculation of 푉m,H2,20 ℃ was carried out using van der
Waals equation (5); Vtotal is the total amount of gas produced;
t is the reaction time required to produce these gases; ncatalyst
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ꢚꢛ
ꢠ
푉ꢖꢎꢗꢘꢎꢘꢙꢄ℃ ꢂ ꢜ ꢝꢞꢟ ꢡꢢꢣꢄꢤꢉꢥꢦꢧꢄ
(5)
where R: 8.3145 m³ Pa/mol/K; T: 298.15 K; p: 101325 Pa; b:
ꢚꢛ
-
6
3
-10
3
2
2
6.7×10 m /mol; a: 2.49×10 Pa·m /mol .
[
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Acknowledgment
[
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Pt/In
2 3 2 3
O /Al O catalysts in methanol steam reforming, Int. J. Hydrogen
The authors are grateful for financial support granted by
Ministry of Science and Technology of the People's Republic of
China (Nos. 2016YFA0200700 and 2016YFE0105700), the
National Natural Science Foundation of China (Nos. 21373264
and 21573275), the Natural Science Foundation of Jiangsu
Province (No. BK20150362), Suzhou Institute of Nano-tech and
Nano-bionics (No. Y3AAA11004) and Thousand Youth Talents
Plan (No. Y3BQA11001).
Energy 41 (2016) 21990-21999.
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