Reversible Hydrogen Uptake/Release over a Sodium Phenoxide–Cyclohexanolate Pair

Article abstract of DOI:10.1002/anie.201810945

Hydrogen uptake and release in arene–cycloalkane pairs provide an attractive opportunity for on-board and off-board hydrogen storage. However, the efficiency of arene–cycloalkane pairs currently is limited by unfavorable thermodynamics for hydrogen release. It is shown here that the thermodynamics can be optimized by replacement of H in the -OH group of cyclohexanol and phenol with alkali or alkaline earth metals. The enthalpy change upon dehydrogenation decreases substantially, which correlates with the delocalization of the oxygen electron to the benzene ring in phenoxides. Theoretical calculations reveal that replacement of H with a metal leads to a reduction of the HOMO–LUMO energy gap and elongation of the C?H bond in the α site in cyclohexanolate, which indicates that the cyclohexanol is activated upon metal substitution. The experimental results demonstrate that sodium phenoxide–cyclohexanolate, an air- and water-stable pair, can desorb hydrogen at ca. 413 K and 373 K in the solid form and in an aqueous solution, respectively. Hydrogenation, on the other hand, is accomplished at temperatures as low as 303 K.

Full text of DOI:10.1002/anie.201810945

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Accepted Article
Title: Reversible hydrogen uptake/release over sodium phenoxide-
cyclohexanolate pair
Authors: Yang Yu, Teng He, Anan Wu, Qijun Pei, Abhijeet Karkamkar,
Tom Autrey, and Ping Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810945
Angew. Chem. 10.1002/ange.201810945
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Angewandte Chemie International Edition
10.1002/anie.201810945
COMMUNICATION
Reversible hydrogen uptake/release over sodium phenoxide-
cyclohexanolate pair
[
a], [b]
*[a]
*[c]
[a], [b]
[d]
[d]
*[a],
Yang Yu,
[e], [f]
Teng He, Anan Wu, Qijun Pei,
Abhijeet Karkamkar, Tom Autrey, Ping Chen
Dedication to 70th anniversary of Dalian Institute of Chemical Physics, Chinese Academy of Sciences
[
4]
[5]
Abstract: Hydrogen uptake and release in arene-cycloalkane pairs
provide an attractive opportunity for on-board and off-board
hydrogen storage. However, the efficiency of arene-cycloalkane
pairs currently is limited because of unfavorable thermodynamics for
hydrogen release. In this paper, we propose a strategy for optimizing
the thermodynamics based on substitution of H in the -OH group of
cyclohexanol and phenol with alkali or alkaline earth metals. The
enthalpy changes of dehydrogenation decreases substantially, which
correlates with the electron delocalized from oxygen to the benzene
ring in phenoxides. Theoretical calculations reveal that replacement
of H with a metal leads to a reduction in the energy gap of HOMO
and LOMO and elongation of the C-H bond in the α site in
cyclohexanolate, which indicates the cyclohexanol is activated upon
metal substitution. Our experimental results demonstrate that
sodium phenoxide-cyclohexanolate, an air and a water stable pair,
can desorb hydrogen at ca. 413K and 373K in the solid form and in
toluene and dibenzene toluene, as hydrogen carriers to
mediate the energy storage, transportation, and use chain.
However, the technological implementation of those
hydrocarbons suffers in part from low round trip storage
efficiencies; that is, the large enthalpy change of
[6]
dehydrogenation (ΔH
d
=
60-70 kJ/mol-H
2
)
requires high
temperatures to release hydrogen. Several strategies for
circumventing or alleviating the thermodynamic constrain have
been discussed in literatures, such as: I. non-thermal driven
dehydrogenation via photocatalysis or electrocatalysis. Li et al.
showed encouragingly that cyclohexane released H
temperature by using Pt/TiO catalyst under visible light ;
dehydrogenation of N-heterocycles can also be realized at room
temperature through photocatalysis or electrocatalysis.
compositional alteration to optimize the thermodynamic
properties of hydrocarbons. Previous efforts by Pez, Crabtree
2
at ambient
[7]
2
[
8]
II.
an aqueous solution, respectively. Hydrogenation, on the other hand, and Jessop in tuning the thermodynamics of dehydrogenation
is accomplished at the temperatures as low as 303K.
focused on incorporating heteroatoms, such as N, into the
[
9]
aromatic ring or substituting electron-donating groups for the
[
10]
aromatic ring.
d d
For example, ΔH and T , defined as the
The development of cost-effective, stable, and efficient energy
carriers to store and transport energy, especially those
generated from intermittent and distributed renewable resources,
is vitally important. Hydrogen has long been viewed as an ideal
energy carrier; however, storing hydrogen effectively is
temperature at which the Gibbs free energy change for
dehydrogenation at 1 bar hydrogen back pressure equals zero
(
d 2
i.e., ΔG = 0) decrease from 73.6 kJ/mol-H and ca. 599K for
the cyclohexane-benzene pair to 67.3 kJ/mol-H
2
and ca. 546K
and ca. 532K
for piperidine-pyridine pair, to ca. 65.6 kJ/mol-H
2
[
1]
considered a grand challenge. Over the past two decades,
significant research effort has focused on materials development
[9b]
for the cyclohexylamine-aniline pair. Density functional theory
(
DFT) calculations also show that increasing the number of N
[
2]
for chemi- and physi-sorption of hydrogen. In particular, storing
hydrogen in chemical bonds by the reduction of arenes offers
atoms especially in the 1,3- or 1,3,5-arrangement of the ring or
by incorporating a stronger electron-donating capability will
[
3]
the opportunity for using mass-produced chemicals, such as
[9b, 10]
reduce ΔH
d
effectively.
d
The ΔH can be manipulated by
strengthening the aromatic character of the  systems to
stabilize arenes and/or by weakening C-H or N-H bonding to
destabilize the cycloalkanes (or hetero-cycloalkanes).
In this paper, we describe a new approach in materials
development and thermodynamic alteration that involves
introducing alkali and alkaline earth metal into the arene-
[
a]
Y. Yu, Q. Pei, Dr. T. He, Prof. P. Chen.
Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian, 116023, China.
E-mail: heteng@dicp.ac.cn; pchen@dicp.ac.cn
Y. Yu, Q. Pei.
University of Chinese Academy of Sciences, Beijing 100049, China.
Dr. A. Wu
Fujian Provincial Key Laboratory of Theoretical and Computational
Chemistry, College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen, 361005, China.
[
b]
c]
cycloalkane pairs. The phenol-cyclohexanol pair (ΔH
d
of ca. 64.5
2 d
kJ/mol-H and T of 516 K ) was chosen as the parent pair that
[
11]
[
subsequently was converted to alkali (or alkaline earth)
phenoxide-cyclohexanolate pairs. DFT calculations provide
insight into the manipulation of the electronic properties and
thermodynamics in hydrogen storage of the pairs. Experimental
studies show three equivalents (equiv.) of hydrogen can be
reversibly stored in the sodium phenoxide-cyclohexanolate pair
at 373K. To the best of our knowledge, this is the first report on
the metallation of the cycloalkane-arene pair to achieve
reversible hydrogen storage under moderate conditions.
E-mail: ananwu@xmu.edu.cn
A. Karkamkar, T. Autrey.
Pacific Northwest National Laboratory, Richland, Washington,
[
d]
e]
99352, United States.
[
Prof. P. Chen.
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian, 116023, China.
E-mail: pchen@dicp.ac.cn
[f]
Prof. P. Chen.
The combined annual global production level of phenol and
cyclohexanol is 1.1 billion tons. Cycling between phenol and
cyclohexanol with a ~6 wt% or ~57 g/L reversible hydrogen
capacity following R (1) provides an attractive route for hydrogen
Collaborative Innovation Centre of Chemistry for Energy Materials
(iChEM·2011), Xiamen University, Fujian 361005, China.
Supporting information for this article is given via a link at the end of
the document
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Angewandte Chemie International Edition
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COMMUNICATION
storage. Little attention, however, has been focused on this pair,
Because the sodium cyclohexanolate-phenoxide pair is more
possibly resulting from the unfavorable ΔH
d
, (ca. 64.5 kJ/mol-H
and side reactions during
the other hand, the
2
appealing in terms of hydrogen density (4.9 wt%), ΔH
d
(50.4
[
11]
corresponding to T
dehydrogenation
d
of 516 K)
etc. On
kJ/mol-H ), and T (385 K) (ΔG =0, assuming entropy change in
2
d
d
dehydrogenation contributed mainly by hydrogen release) (see
Figure 1a), we analyzed their electronic properties and
compared the results with those of the parent cyclohexanol-
phenol pair (see Figure 1c). Evidently, the replacement of H with
Na redistributes the electron density. Compared to cyclohexanol,
the energy level of highest occupied molecular orbital (HOMO)
rises while lowest unoccupied molecular orbital (LUMO) drops,
respectively, leading to a narrower HOMO-LUMO energy gap,
which indicates sodium cyclohexanolate is easily activated.
phenol/cyclohexanol pair has a distinct feature—the labile H of
the OH group allows facile chemical modification. For example,
phenol reacts with NaOH to yield sodium phenoxide and water
as products of the acid-base reaction. We employed the Xls
[
12]
method , which is based on DFT and neutral network, to
simulate a number of alkali (or alkaline earth) phenoxides and
cyclohexanolates and investigate the effect of replacing H of -
OH with those basic elements. As shown in Figure 1a, the
calculated dehydrogenation enthalpy change of cyclohexanol to
phenol is 64.5 kJ/mol-H
change derived from known thermodynamic data,
2
is in good agreement with enthalpy
[
11]
which
validates the accuracy of the calculation method used in this
study. Forming phenoxides by reacting phenol with metals (M=
Li, Na, K, Mg and Ca) is thermodynamically favorable （Table
S1）. Analyses of the natural bond orbital charges show that
electrons on O (Figure 1b and Table S2) of phenoxides are
more delocalized resonating with the arene ring, thus forming p-

conjugation and consequently stabilizing the phenoxides.
Formation of corresponding cyclohexanolates is also exothermic
Table S1), while a general observation is elongation of C-H
(
bonding especially at the  site (Figure 1b and Table S3). As the
magnitude of the conjugation effect is significantly greater than
the hyper-conjugation effect of the same type, the exothermicity
of phenol to phenoxide is greater than that of cyclohexanol to
cyclohexanolate. The important consequence is the substantial
decrease in ΔH
d
of alkali (alkaline earth) cyclohexanolate-
phenoxide pairs (Figure 1a). For example, dehydrogenation of
sodium or potassium cyclohexanolate to corresponding
phenoxide has ΔH
d
2
values of 50.4 or 49.3 kJ/mol-H ,
respectively, which are ca. 21.9% and 23.6% less than that of
cyclohexanol to phenol. Alkali or alkaline earth metals donate
more electrons than H on O, and the electrons delocalize
through the p- conjugation and stabilize. Therefore, we refer to
the charge delocalized from O atom to the C-ring in phenoxides
as an indication of the metallation effect (Table S2). A nonlinear
d
relation between the ΔH and charge delocalized from the O
atom to a C-ring in phenoxide was obtained (Figure 1a). The
molecular structures of the Li, Mg, and Ca phenoxides are
nearly of C2 symmetry, while Na and K cations bent over to the
ring, which may cause charge redistribution within the system. It
is worth mentioning that the lower ΔH
gas phase simulation. However, the ΔH
d
calculated here is from a
in the solid phase may
d
be even lower because of the - interaction among molecules
in phenoxides, which stabilizes the phenoxides more than
cyclohexanolates. We also note the findings of Jessop and co-
[10]
workers
d
in which the ΔH of cycloalkane scales with the
Hammett parameter (σ) of substitute group, where the Hammett
parameter is a measure of the electron-donating ability of the
-
substituent group. Because the Hammett parameter of -O (-
[13]
d
Figure 1. (a) Theoretical calculations on the ΔH in gaseous phase of
0.81) is more negative than that of -OH (-0.37) , the substantial
metallated cyclohexanolates to phenoxides as a function of charge in the
benzene ring and the length of C-H bond in α-site cyclohexanolate compared
to that of cyclohexanol. The lines are guide to eye. (b) Charge in the benzene
ring of phenoxide compared to that of phenol and the lengths of C-H bonds in
cyclohexanolate compared to that of cyclohexanol. (c) The HOMO-LUMO
energy gaps (a.u.) for cyclohexanol and sodium cyclohexanolate and the
interfacial plots of the orbitals.
decrease in ΔH of alkali (alkaline earth) cyclohexanolate-
d
phenoxide also is understandable from their viewpoint. Because
δ-
different alkali or alkaline earth metals may lead to different -O
d
values, the ΔH reduction would vary depending on the metal
involved.
C
6
H
5
OH +3H
2
= C
6
H11OH
R (1)
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This finding agrees well with the elongated αC-H bond (Figure
hydrogenation. The hydrogenated sample then was
dehydrogenated under vacuum. However, dehydrogenation
1b and Table S3) where the first C-H bond dissociation is likely
to occur. Because of the correlation of the kinetic barrier with the
2 3
proceeded very slowly at 150°C. We replaced the Ru/Al O
reaction heat (Brønsted–Evans–Polanyi relations), the increase
catalyst with 5%Pt/C catalyst and obtained appropriate
dehydrogenation at temperatures above 100°C, about 5 equiv. H
were desorbed at 140°C (Figure 3b), where the starting
dehydrogenation temperature is around 90°C and sodium
in C-H bond length at the
dehydrogenation enthalpy change (Figure 1a and Table S4).
With the substantial decrease in ΔH and promising kinetic
α site also scales with the
d
feature, we synthesized sodium cyclohexanolate-phenoxide pair
and evaluated its hydrogen uptake and release properties.
Sodium phenoxide and cyclohexanolate can be synthesized
by mechanical ball milling phenol and cyclohexanol with NaH
upon evolution of one equiv. hydrogen, respectively (details are
given in experimental section and Supporting Information). We
analyzed the products using X-ray diffraction (XRD), nuclear
magnetic resonance (NMR), and Fourier transform infrared
(
FTIR) spectroscopy. The XRD pattern of synthesized sodium
phenoxide matches well with that in the database (Figure 2a).
The synthesized sodium cyclohexanolate, on the other hand,
has poor crystallinity and presents a set of weak diffraction
peaks at 7.6°, 16.8°, 21.3°, and 41.5° that cannot be assigned to
1
any known material containing Na, C, O, and H. The H NMR
and FTIR spectroscopy measurements showed the absence of
resonance of
H of OH and -OH vibration from both
cyclohexanolate and phenoxide (Figure 2b and Figure S1). On
1
3
the other hand, the C NMR spectrum of phenoxide, shows an
obvious downshift compared with that of phenol, which is
consistent with the increase in electron density in the C-ring
determined from calculations (Figure S2). All these
measurements suggest the formation of sodium phenoxide and
sodium cyclohexanolate via reactions R (2) and R (3).
Figure 2. (a) X-ray diffraction patterns of synthesized sodium phenoxide and
C
C
6
H
5
OH (s) + NaH (s) → C
6
H
5
ONa (s) + H
11ONa (s) + H
2
(g)
(g)
R (2)
R (3)
1
sodium cyclohexanolate. (b)
H NMR spectra of sodium phenoxide and
6
H11OH (s) + NaH (s) → C
6
H
2
sodium cyclohexanolate compared with that of phenol and cyclohexanol in
DMSO.
To confirm the thermodynamic data (ΔH
d
) calculated from the
[
12]
Xls method,
experiments were designed to measure the
enthalpy change for hydrogenation of sodium
phenoxide to cyclohexanolate using C80
[
14]
Calvet calorimeter . The enthalpy change for
the hydrogenation measured (R (4)) is
approximately -156 kJ/mol (-52 kJ/mol-H
which agrees well with the calculated result
50.4 kJ/mol-H for dehydrogenation). Details
are given in Supporting Information.
2
),
(
2
6 5 2 6
C H ONa (s) + 3H (g) → C H11ONa (s) R (4)
Because of the decrease in ΔH
release from the sodium
d
, hydrogen
phenoxide-
cyclohexanolate pair would occur under
moderate conditions. As cyclohexanolate and
phenoxide are solid, both were ball milled with
2 3
commercial catalysts (5% Ru/Al O and 5%
Pt/C) to assist the activation of hydrogen and
the C-H bond during hydrogenation and
dehydrogenation. As shown in Figure 3a,
hydrogenation of sodium phenoxide under 30
bar hydrogen occurred slowly at ambient
Figure 3. (a) Hydrogen uptake by sodium phenoxide catalyzed by 5%Ru/Al O3 under 30 bar
hydrogen. (b) Hydrogen release from sodium cyclohexanolate catalyzed by 5%Pt/C under
vacuum. The molar ratios of metals to phenoxide (or cyclohexanolate) are 1:10. (c)
2
2 3
temperature in the presence of 5% Ru/Al O
2 3
Hydrogenation of sodium phenoxide in aqueous solution catalysed by 5%Ru/Al O with the
catalyst, ca. 6 equiv. H were absorbed when
the temperature was increased to 150°C where
the product was characterized as sodium
cyclohexanolate (Figure S9), showing full
molar ratio of Ru to phenoxide of 1:30. *: Cphenoxide = 0.148 mol/L, the molar ratio of Ru to
phenoxide of 1:15. (d) Dehydrogenation of cyclohexanol in NaOH aqueous solution catalysed
by 5%Pt/Al
2 3
O with different pH for 20 h or 72 h with the molar ratio of Pt to cyclohexanol of
1:50. Detail information can be found in Supporting Information.
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phenoxide was obtained finally (Figure S10). Therefore, the
metallation strategy proposed here successfully decreased the
relatively low temperature (refluxing of solvents such as toluene,
[
21]
xylene or 2,2,2-trifluoroethanol) , only the N-containing rings
were involved in the hydrogen release due to the delocalized π
bond in the whole molecule after dehydrogenation. Furthermore,
sodium phenoxide can be prepared by reacting phenol and
NaOH; therefore, it can be mass produced at low cost. It is
stable in air and water and has a low vapour pressure. All these
characteristics make the sodium phenoxide-cyclohexanolate pair
d
ΔH through forming phenoxide-cyclohexanolate pair, leading to
lower T
d
.
The slow kinetics in hydrogen uptake and release at lower
temperatures shown in Figure 3a and 3b implies inefficient
catalysis of solid Ru/Al O or Pt/C on solid-state reactants.
2 3
Therefore, we dissolved and dispensed the reactant and catalyst
in water and re-investigated the properties of hydrogenation and
dehydrogenation. It should be noted that
a
very attractive hydrogen storage material for practical
applications. Further efforts should focus on materials
engineering including developing more efficient
sodium
cyclohexanolate hydrolyzes into NaOH and cyclohexanol under
the conditions applied in the experiment. Under these conditions, hydrogenation/dehydrogenation catalyst and reaction medium.
hydrogenation and dehydrogenation actually react via R (5),
d 2
which has a ΔH of 182 kJ/mol (ca. 61 kJ/mol-H obtained from
C80 in the Supporting Information). Although with a higher ΔH
d
,
Acknowledgements
the entropy contribution of water may compensate such an
enthalpy loss. As shown in Figure 3c, ca. 100% hydrogenation
of sodium phenoxide can be achieved in 7.5 minutes at 100°C
under 40 bar of hydrogen and in the presence of commercial
T.H. and P.C. acknowledge the supports provided by the
National Natural Science Foundation of China (21875246,
51671178, 51472237), DICP (DICP ZZBS201616), Sino-
5%Ru/Al
2 3
O catalyst. Under a milder condition of 30°C and 2 bar
Japanese Research Cooperative Program of the Ministry of
Science and Technology (2016YFE0118300) and iChEM·2011.
A.W. would like to acknowledge the financial support provided
by the National Science Foundation of China (NSFC)
hydrogen, hydrogenation of sodium phenoxide also can be
1
achieved within 90 minutes. Cyclohexanol was detected by
H
NMR (Figure S11). Previous studies also showed that the
[
15]
[16]
hydrogenation of hydrocarbons such as phenol and toluene
(
21773193) and the fundamental research Funds for the Central
can be facilely carried out under mild conditions. However,
dehydrogenation is usually kinetically problematic. Therefore,
much attention is given to dehydrogenation.
Universities (20720160031). T.A. and A.K. gratefully
acknowledge support from the Hydrogen Materials − Advanced
Research Consortium (HyMARC), established as part of the
Energy Materials Network under the U.S. Department of Energy,
Office of Energy Efficiency and Renewable Energy, Fuel Cell
Technologies Office. Pacific Northwest National Laboratory is a
multi-program national laboratory operated by Battelle for the
U.S. Department of Energy under Contract DE-AC05-
C
6
H
5
ONa (aq.) + H
2
O (l) + 3H
2
(g) ⇌ C
6
H11OH (l) + NaOH (aq.)
R (5)
Because cyclohexanol and NaOH are the hydrogenation
products, we conducted the dehydrogenation of cyclohexanol
and NaOH in aqueous solution catalyzed by commercial 5%Pt/C.
Both phenoxide and cyclohexanone were found in the
dehydrogenation products as shown in Figure 3(d) and Figure
S12. Selectivity to phenoxide is pH dependent, i.e., the
dehydrogenation of cyclohexanol in the presence of one equiv.
of NaOH (pH = 13.3) has a conversion of cyclohexanol and
selectivity to phenoxide of 56.9 % and 81.0 % after 20 hours
reaction; with excess NaOH (pH = 14.0), both the conversion
and selectivity reach 60.0 % and 91.7 % within the same periods
of time. Prolonging the reaction leads to >99% conversion of
cyclohexanol and selectivity to sodium phenoxide in aqueous
7
6RL01830.
Keywords: reversible hydrogen storage • metallation • organic
hydride • thermodynamic modification
[1]
a) L. Schlapbach, A. Zuttel, Nature 2001, 414, 353-358; b) T. He, P.
Pachfule, H. Wu, Q. Xu, P. Chen, Nat. Rev. Mater. 2016, 1, 16059; c)
US
Department
of
Energy
(DOE)
targets,https://energy.gov/eere/fuelcells/downloads/target-explanation-
document-onboard-hydrogen-storage-light-duty-fuel-cell; d) J. Yang, A.
Sudik, C. Wolverton, D. J. Siegel, Chem. Soc. Rev. 2010, 39, 656-675;
e) U. Eberle, M. Felderhoff, F. Schüth, Angew. Chem., Int. Ed. 2009, 48,
phase. As H
is > 99%. As hydrogen is the only detectable gaseous product
Figure S13), the reversed R (5) occurs at a temperature as low
as 100°C. It is noteworthy that aqueous sodium phenoxide and
cyclohexanol are stable in the air for weeks (Figure S14), which
is a desirable property for hydrogen storage materials.
2 2
is only detectable gaseous product, the yield of H
6
608-6630.
[2]
a) B. Bogdanović, M. Schwickardi, J. Alloy. Compd. 1997, 253-254, 1-9;
b) P. Chen, Z. Xiong, J. Luo, J. Lin, K. L. Tan, Nature 2002, 420, 302-
(
304; c) S.-i. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel, C. M. Jensen,
Chem. Rev. 2007, 107, 4111-4132; d) M. Chandra, Q. Xu, J. Power
Sources 2006, 156, 190-194; e) D. Teichmann, K. Stark, K. Muller, G.
Zottl, P. Wasserscheid, W. Arlt, Energ Environ. Sci 2012, 5, 9044-9054;
f) S. Ott, Science 2011, 333, 1714-1715; g) N. L. Rosi, J. Eckert, M.
Eddaoudi, D. T. Vodak, J. Kim, M. O'Keeffe, O. M. Yaghi, Science 2003,
300, 1127-1129; h) M. P. Suh, H. J. Park, T. K. Prasad, D.-W. Lim,
Chem. Rev. 2012, 112, 782-835.
There are reports demonstrating that metallated (by alkali or
[
17]
[18]
alkaline earth metals) amidoboranes,
and primary amines
hydrazinoboranes,
[
19]
have reduced dehydrogenation
temperatures and enhanced selectivity to hydrogen from their
neat forms. In the present work, such a prominent effect of
metallation also was achieved. The sodium modified
cyclohexanol-phenol pair can undergo dehydrogenation at
[
3]
a) Q.-L. Zhu, Q. Xu, Energ Environ. Sci 2015, 8, 478-512; b) P.
Preuster, C. Papp, P. Wasserscheid, Accounts Chem. Res. 2017, 50,
74-85.
[4]
5]
A. Shukla, S. Karmakar, R. B. Biniwale, Int. J. Hydrogen. Energ 2012,
37, 3719-3726.
1
(
00°C, which is substantially lower than the T
generally >300°C for thermal dehydrogenation) and for N-
heterocycles and substituted cycloalkanes (generally >170°C for
d
for cycloalkanes
[
6]
[
a) H. Jorschick, P. Preuster, S. Durr, A. Seidel, K. Muller, A. Bosmann,
P. Wasserscheid, Energ Environ. Sci 2017, 10, 1652-1659; b) D.
Geburtig, P. Preuster, A. Bösmann, K. Müller, P. Wasserscheid, Int. J.
Hydrogen. Energ 2016, 41, 1010-1017.
[
10, 20]
thermal dehydrogenation).
Although the dehydrogenation of
1,2,3,4-tetrahydroquinolines or indolines can be achieved at
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Angewandte Chemie International Edition
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COMMUNICATION
[
[
[
6]
7]
8]
R. B. Biniwale, S. Rayalu, S. Devotta, M. Ichikawa, Int. J. Hydrogen.
Energ 2008, 33, 360-365.
L. Li, X. Mu, W. Liu, Z. Mi, C.-J. Li, J. Am. Chem. Soc. 2015, 137, 7576-
7579.
a) K.-H. He, F.-F. Tan, C.-Z. Zhou, G.-J. Zhou, X.-L. Yang, Y. Li, Angew.
Chem., Int. Ed. 2017, 56, 3080-3084; b) S. Kato, Y. Saga, M. Kojima, H.
Fuse, S. Matsunaga, A. Fukatsu, M. Kondo, S. Masaoka, M. Kanai, J.
Am. Chem. Soc. 2017, 139, 2204-2207; c) Y. Wu, H. Yi, A. Lei, ACS
Catal. 2018, 8, 1192-1196; d) Q. Yin, M. Oestreich, Angew. Chem., Int.
Ed. 2017, 56, 7716-7718; e) M. Zheng, J. Shi, T. Yuan, X. Wang,
Angew. Chem., Int. Ed. 2018, 57, 5487-5491.
[
9]
a) G. P. Pez, A. R. Scott, A. C. Cooper, H. Cheng, F. C. Wilhelm, A. H.
Abdourazak,2008,US7351395B1; b) E. Clot, O. Eisenstein, R. H.
Crabtree, Chem. Commun. 2007, 2231-2233; c) R. H. Crabtree, Energ
Environ. Sci 2008, 1, 134-138.
[
[
10] Y. Cui, S. Kwok, A. Bucholtz, B. Davis, R. A. Whitney, P. G. Jessop,
New J. Chem. 2008, 32, 1027-1037.
11] From gas phase values of ΔHf (C6H5OH, g) = -96.4 kJ/mol and ΔHf
(C6H11OH, g) = -290 kJ/ mol, assuming a similar entropy change as
that of aniline-cyclohexylamine pair (125 J/mol K).
[
[
[
12] J. Wu, Z. I. Ying, X. Xu, Chemphyschem 2010, 11, 2561-2567.
13] C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165-195.
14] A. Karkamkar, K. Parab, D. M. Camaioni, D. Neiner, H. Cho, T. K.
Nielsen, T. Autrey, Dalton Trans. 2013, 42, 615-619.
[
[
15] D. Forberg, T. Schwob, R. Kempe, Nat. Commun 2018, 9, 1751.
16] M. Zahmakıran, Y. Tonbul, S. Özkar, J. Am. Chem. Soc. 2010, 132,
6541-6549.
[
[
17] Z. Xiong, et al., Nat. Mater. 2008, 7, 138-141.
18] H. Wu, W. Zhou, F. E. Pinkerton, T. J. Udovic, T. Yildirim, J. J. Rush,
Energ Environ. Sci 2012, 5, 7531-7535.
[
[
19] J. Chen, et al., Chem.-Eur. J. 2014, 20, 6632-6635.
20] a) Z. Wang, I. Tonks, J. Belli, C. M. Jensen, J. Organomet. Chem. 2009,
694, 2854-2857; b) M. Yang, C. Han, G. Ni, J. Wu, H. Cheng, Int. J.
Hydrogen. Energ 2012, 37, 12839-12845.
[
21] a) S. Chakraborty, W. W. Brennessel, W. D. Jones, J. Am. Chem. Soc.
2014, 136, 8564-8567; b) R. Yamaguchi, C. Ikeda, Y. Takahashi, K.-i.
Fujita, J. Am. Chem. Soc. 2009, 131, 8410-8412; c) C. Deraedt, R. Ye,
W. T. Ralston, F. D. Toste, G. A. Somorjai, J. Am. Chem. Soc. 2017,
139, 18084-18092; d) J. Wu, D. Talwar, S. Johnston, M. Yan, J. Xiao,
Angew. Chem., Int. Ed. 2013, 52, 6983-6987.
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Angewandte Chemie International Edition
10.1002/anie.201810945
COMMUNICATION
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COMMUNICATION
The metallation strategy is
*
*
Yang Yu, Teng He, Anan Wu, Qijun
successfully developed to optimize
the thermodynamic properties of
liquid organic hydrogen carriers.
Pei, Abhijeet Karkamkar, Tom Autrey,
*
Ping Chen
Page No. – Page No.
Reversible hydrogen uptake/release
over sodium phenoxide-
cyclohexanolate pair
This article is protected by copyright. All rights reserved.