Lithiated primary amine - A new material for hydrogen storage

Article abstract of DOI:10.1002/chem.201402543

A facile method for synthesizing crystalline lithiated amines by ball milling primary amines with LiH was developed. The lithiated amines exhibit an unprecedented endothermic dehydrogenation feature in the temperature range of 150-250 °C, which shows potential as a new type of hydrogen storage material. Structural analysis and mechanistic studies on lithiated ethylenediamine (Li₂EDA) indicates that Li may mediate the dehydrogenation through an α,β-LiH elimination mechanism, creating a more energy favorable pathway for the selective H₂ release.

Full text of DOI:10.1002/chem.201402543

DOI: 10.1002/chem.201402543
Communication
&
Organometallic chemistry
Lithiated Primary Amine—A New Material for Hydrogen Storage
Juner Chen,[a, b] Hui Wu,[c, d] Guotao Wu,[a] Zhitao Xiong,[a] Ruiming Wang,[a] Hongjun Fan,[a]
Wei Zhou,[c, d] Bin Liu,[a] Yongshen Chua,[a] Xiaohua Ju,[a] and Ping Chen*[a]
(iPr)NHCH2CH2NH(iPr) transformed to [(iPr)NCH=CHN(iPr)]2ꢀ at
ambient temperature catalyzed synergistically by two distinct
metals of Li and Zn in the system.[6] It is worth pointing out
Abstract: A facile method for synthesizing crystalline lithi-
ated amines by ball milling primary amines with LiH was
that no matter which heteroatom is applied, dehydrogenation
developed. The lithiated amines exhibit an unprecedented
is normally occurs from the breaking of the CꢀH bond. Little
endothermic dehydrogenation feature in the temperature
investigation has been given to the direct dehydrogenation of
range of 150–2508C, which shows potential as a new type
of hydrogen storage material. Structural analysis and
aliphatic amines by cracking both NꢀH and CꢀH bonds. Our
experimental data show that ethylenediamine (EDA), for exam-
mechanistic studies on lithiated ethylenediamine (Li2EDA)
ple, undergoes decomposition giving rise to NH3, CHx, and
indicates that Li may mediate the dehydrogenation
CHxNy species at elevated temperatures (see Figure S1 in the
through an a,b-LiH elimination mechanism, creating
Supporting Information), which, similar to hydrocarbons, is rel-
a more energy favorable pathway for the selective H2 re-
evant to the relatively weaker CꢀC bond and CꢀN bond (~
lease.
331 kJmolꢀ1).[7] The situation changes when oxidative dehydro-
genation is implemented by a transition metal (Ru, Pt, Ni, Mo,
etc.) or a pincer complex catalyst by dihydrogen transfer to
proper acceptors (O2ꢀ, MnO2, tert-butylethylene, etc.).[8] Cata-
Thermal cracking of hydrocarbons normally leads to the break
of the CꢀC bond (~246 kJmolꢀ1) rather than CꢀH bond (~
lyzed by molten Zn, n-butylamine and benzylamine give off
363 kJmolꢀ1). For cyclic hydrocarbons, on the other hand, de-
gaseous hydrogen and convert to n-butyronitrile and benzoni-
trile.[9] H2 is regarded as a clean energy carrier, and aliphatic
hydrogenation can be achieved with minimal CꢀC bond rup-
ture with the aid of proper catalysts.[1] For example, in the
amines are H-rich compounds; however, little success has yet
presence of Pt or [IrH2(PCP)] (where PCP = C6H3(CH2PtBu2)2-2,6)
pincer catalysts, cyclohexane releases three equivalents of H2
to benzene at temperatures above 2008C.[2] The relatively high
temperature may be due to the unfavorable dehydrogenation
thermodynamics (DH8=205.9 kJmolꢀ1) and the high energy
barrier in activating the CꢀH bond.[3] Theoretical and experi-
mental studies show that partial substitution of carbon atoms
by heteroatom(s) (O, N, S, etc.) promotes low temperature de-
hydrogenation.[4] A series of key patents have been generated
by Pez et al.[5] More recently, Campbell et al. reported that
been made for their dehydrogenations. Therefore, it is of both
scientific and practical importance to investigate whether de-
hydrogenation can be achieved by a noncatalytic process
under mild conditions.
The H atom in the ꢀNH2 group is acidic, resembling to that
of NH3 and metal amides (LiNH2, Mg(NH2)2, etc.). There have
been over 10 years’ worth of activities in probing hydrogen re-
lease from the interaction of amides and hydrides, in which
the chemical potential of the combination of acidic H in amide
and basic H in hydride into H2 is regarded as the driving
force.[10] It is rational to extend such an interaction to amines
and hydrides composites.
[a] J. Chen, Prof. G. Wu, Prof. Z. Xiong, R. Wang, Prof. H. Fan, B. Liu,
Dr. Y. Chua, X. Ju, Prof. P. Chen
A few primary mono- and diamines, that is, ethylamine (EA),
ethylenediamine (EDA), propylamine (PA), 1,3-propanediamine
(PDA), benzylamine (BA), and p-xylylenediamine (PX) were
chosen as representatives and were ball milled with LiH in
a molar ratio of [ꢀNH2] group/LiH=1:1, separately. The gas
evolution from each sample was monitored by a pressure
gauge and then was identified as H2 by a mass spectrometer
(see Figure S2 in the Supporting Information). As shown in
Figure 1, with prolonged ball milling and in some cases post-
ball-milling heat treatment, nearly one equivalent H2 per LiH
was generated from each sample. The characterization of solid
residues reveals the formation of new phases (see Figure S3 in
the Supporting Information). The detailed crystallographic
study on lithiated EDA (Li2EDA for short) is given below while
the structural analyses on other lithiated amines are currently
underway. Summarizing the experimental results above, it can
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics
Chinese Academy of Sciences, Dalian 116023 (P. R. China)
Fax: (+86)411-8437-9583
E-mail: pchen@dicp.ac.cn
[b] J. Chen
University of Chinese Academy of Sciences
Beijing, 100049 (P. R. China)
[c] H. Wu, Dr. W. Zhou
NIST Center for Neutron Research
National Institute of Standards and Technology
Gaithersburg, Maryland, 20899-6102 (USA)
[d] H. Wu, Dr. W. Zhou
Department of Materials Science and Engineering
University of Maryland, College Park
Maryland, 20742-2115 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402543.
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Figure 2. Enthalpy changes for the formation of lithiated amines determined
theoretically and their dehydrogenation determined experimentally.
Figure 1. Time dependence of H2 release from the amine–LiH mixtures
during ball milling at 200 rpm. EDA, PA, and PDA were ball milled with LiH
at room temperature, separately; EA, BA, and PX were ball milled with LiH at
608C, separately; ball milled BA–LiH mixture was further heated to1008C for
reaction. In other words, a direct hydrogen restoration under
normal conditions is unlikely.
&
*
13.5 h to release the remaining hydrogen ( =EDA+2LiH; PA +LiH;
The “hydrogen story” does not stop here. The presence of Li
significantly changes the chemical nature of the amines. Mark-
edly different from the thermal decomposition of EDA, a rapid
H2 desorption from Li2EDA upon heating it to approximately
1778C was observed (see Figure S4 in the Supporting Informa-
tion). Moreover, all lithiated amines studied in this work exhibit
similar dehydrogenation behaviors around 150–2508C. Quanti-
tative analyses on hydrogen desorption are shown in Figure 3
~
~
&
*
=PDA+2LiH; =BA+LiH; =PX+2LiH; =EA+LiH).
be derived that Equation (1) takes place in the RNH2ꢀLiH mix-
tures giving rise to N-lithiated amines, which are normally used
as selective proton abstractors and convenient anion exchange
reagents.[11] The conventional synthetic method is by metathe-
sis of amines (R2NH, in which R=H or hydrocarbon group)
with n-butyllithium (LiBu) following Equation (2).[12] A recent
report by Mulvey and Robertson gave a systematic review on
the syntheses, structures, and applications of alkali hexame-
thyldisilazides, diisopropylamides, and tetramethylpiperidides
etc.[13]
RðNH2Þn þ nLiH ! RðNHLiÞn þ nH2
R2NH þ LiBu ! R2NLi þ BuH
ð1Þ
ð2Þ
It should be highlighted that the reaction in Equation (1)
can also be applied to other alkali and alkaline earth hydrides.
In fact, NaH or KH readily react with the amines listed above
giving rise to hydrogen and Na or K-amines. MgH2 and CaH2,
on the other hand, would require longer times and higher
temperatures to achieve the complete conversion. Although
the synthetic parameters are the subjects for further optimiza-
tion, the advantages of the present route lie in the following:
1) the overall process is solvent-free, 2) the only solid product
is metalated amines, and 3) metal hydrides are cheaper, more
stable and less reactive than LiBu etc.
Figure 3. Volumetric releases of lithiated amines. Samples were heated to
the desired temperature with a ramping rate of 28Cminꢀ1. It needed ap-
proximately 1.2–1.7 h to reach the specified temperatures.
and S5 in the Supporting Information. Generally, dilithiated dia-
mines samples (Li2EDA and Li2PDA) gave off approximately
two equivalents of H2 at 180 and 2008C, respectively. Monoli-
thiated monoamines, on the other hand, released approxi-
mately one equivalent of H2. It appears that the amount of the
desorbed H2 depends on the number of [NHꢀCH2] units
[Eq. (3)]. In other words, direct cracking of CꢀH and NꢀH
bonds with H2 formation may be achieved. Our hypothesis is
supported by the observation of N=C=N stretches at around
2100 cmꢀ1 (the FTIR spectrum is shown in Figure S6 in the Sup-
porting Information) as well as solid-state 13C CPMAS NMR
spectroscopic signals at around d=173 ppm belonging to C in
C=N or N=C=N environments (see Figure S7 in the Supporting
Information).
The next question is whether the released H2 can be
charged back, that is, the reversibility of Equation (1) under
normal conditions. As direct thermal analyses can hardly be
done under the present reaction conditions, we employed mo-
lecular calculations to determine the heat of formation of
those lithiated amines. As shown in Figure 2 and Table S1 in
the Supporting Information, all the lithiated amine molecules
are more stable than the starting amines plus LiH, taking into
consideration that the crystallization energy of those ionic lithi-
ated amines will add weight to the exothermicity, the reaction
in Equation (1) is thus a thermodynamically favorable one-way
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H (C) distance is 2.419 ꢁ calculated from the relaxed structure.
It is very likely that the b-H would first transfer from C to Li
forming a ꢀ(H)NꢀLiꢀH moiety, and then, it reacts with the a-H
on the nearby N atom giving rise to H2 and [LiN=CHR].
RðCH2NHLiÞn ! RðHC¼NLiÞn þ nH2
ð3Þ
As shown in Table S1 in the Supporting Information, the
lithiated amines are stabilized by approximately 150 kJmolꢀ1
from the corresponding amines. It is thus interesting to look
into the thermal effect of the dehydrogenation of these stable
lithiated amines. As all the post-dehydrogenated solid residues
are essentially amorphous (see Figure S8 in the Supporting In-
formation), calculation can hardly be done without any defined
structures. Differential scanning calorimetry (DSC) was em-
ployed to measure the heat flow during the dehydrogenation.
Figure S9 in the Supporting Information shows the DSC curves
manifesting endothermic nature of dehydrogenation. The mea-
sured heat of desorption ranges from 8 to 22 kJmolꢀ1 H2. Com-
paratively, monolithiated monoamines require more energy to
release hydrogen than the dilithiated diamines.
A similar mechanism was proposed by Kim et al. and Lee
et al. in the interpretation of hydrogen release from LiNH2BH3,
in which b-H(B) transfers to Li forming a LiH intermediate. LiH,
then, reacts with the a-H atom (from ꢀNH2) giving rise to H2
and [LiNHBH2].[17] Lithiated primary amines may follow a similar
dehydrogenation pathway as that of LiNH2BH3 by forming
a [LiN=CHR] unit, which may have chance to partially dispro-
portionate to LiH and NꢁCR. Therefore, the a,b-LiH elimination
mechanism also manifests the important role of lithium in de-
hydrogeantion of lithiated primary amines. Both [LiN=CHR]
and [NꢁCR] are likely to polymerize into amorphous matter at
elevated temperatures. At temperatures higher than 3008C,
Li2CN2 and amorphous [LiCNH] species were observed (see
Table S2 and Figure S12 in the Supporting Information).
Literature reports show that lithiated alkanes, such as LiBu,
eliminate LiH, easily forming the corresponding alkenes.[14]
Lithiated secondary amines, such as lithium dimethylamide,
lithium diethylamide, and lithium diisopropylamide, follow the
same manner decomposing to LiH and N-methyenemethyla-
mine, N-ethylidenethyamine, and N-isopropylideneisopropyla-
mine, respectively.[15] Such an a,b-LiH elimination mechanism
manifests the important role of lithium in activating the CꢀH
bond in the thermal decomposition of lithiated alkanes and
secondary amines. In contrast, lithiated primary amines give off
H2, which is most likely due to the presence of a-hydrogen at
the N site. It is, therefore, of interest to figure out how dehy-
drogenation takes place in these lithiated primary amines. We
chose Li2EDA as a case for study. In a previous report, the mon-
omeric, dimeric, and trimeric structure of Li2EDA has been
studied by theoretical calculations.[16] In this work, we deter-
mined the crystal structure of Li2EDA from experimental data
for the first time. Li2EDA possesses a monoclinic structure
(space group C2/c) with lattice parameters of a=11.128(1), b=
12.518(1), c=8.069(1) ꢁ, b=134.022(3)8, V=808.3(1) ꢁ3 (see
Figure S11 in the Supporting Information), determined from
Rietveld refinement by using high-resolution XRD data (see
Figure S10 in the Supporting Information). Figure 4 shows the
coordination environment in Li2EDA in which the shortest Li–b-
It was found that the dehydrogenation was inhibited when
heating Li2EDA under a high initial H2 pressure (720 psi; see
Figure S13 in the Supporting Information) showing that at
least partial reversibility can be achieved. However, the direct
re-hydrogenation has not been performed successfully on the
amorphous solid residues, which is due to the kinetic and ther-
modynamic constraints. The relatively small endothermicity re-
flects that the hydrogenation can only be allowed at low tem-
peratures or under high H2 pressure. There are a few ways to
overcome this challenging task: 1) developing suitable cata-
lysts to facilitate the re-hydrogenation, 2) destabilizing the de-
hydrogenated product(s) by preventing them from polymeri-
zation, 3) altering the composition by forming bi- or multime-
tallic amines in the hope to stabilize the reactant(s). Those
strategies have been employed in complex hydrides,[18] amide–
hydride composites,[19] and amidoboranes.[20] With the diversity
of CꢀN chemistry and the large less-probed area of organome-
tallic compounds, the chance of materials development for hy-
drogen storage is considerably high.
In summary, a novel type of hydrogen carrier has been iden-
tified, which was facilely synthesized by ball milling primary
amines with LiH. Compared to the intensively studied BꢀN
containing hydrogenous compounds,[21] the thermodynamical-
ly favorable dehydrogenation in these lithiated amines can be
tailored by the presence of CꢀN and CꢀC bonds, whereas the
involvement of Li is the key to facilitate the unprecedented en-
dothermic dehydrogenation of NꢀH and CꢀH bonds under the
mild conditions. Continuous efforts are necessary in the under-
standing, design, and optimization of this new system.
Acknowledgements
The authors would like to acknowledge financial support from
the Project of National Natural Science Funds for Distinguished
Yong Scholars (51225206), 973 (2010CB631304), Postdoctoral
Science Foundation Funded Project, and Shanghai Synchrotron
Radiation Facility (SSRF) for providing the beam time.
Figure 4. Structure schematic diagram of Li2EDA.[22] Hydrogen atoms are rep-
resented by white spheres; nitrogen and carbon atoms are labeled by a and
b, respectively.
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Keywords: dehydrogenation · hydrogen storage · lithiated
amines · organometallic chemistry · a,b-elimination
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Received: March 11, 2014
Published online on April 15, 2014
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