Angewandte
Communications
Chemie
Hydrogen Storage System
Hot Paper
Rechargeable Hydrogen Storage System Based on the
Dehydrogenative Coupling of Ethylenediamine with Ethanol
Peng Hu, Yehoshoa Ben-David, and David Milstein*
Abstract: A novel and simple hydrogen storage system was
developed, based on the dehydrogenative coupling of inex-
pensive ethylenediamine with ethanol to form diacetylethyle-
nediamine. The system is rechargeable and utilizes the same
ruthenium pincer catalyst for both hydrogen loading and
unloading procedures. It is efficient and uses a low catalyst
loading. Repetitive reversal reactions without addition of new
catalyst result in excellent conversions in both the dehydrogen-
ation and hydrogenation procedures in three cycles.
are in fact hydrogen- and CO2-liberation systems, of which the
liquid carriers are consumed and cannot readily reload
hydrogen. Formic acid has been intensively investigated as
a hydrogen carrier; it can be decomposed to hydrogen and
CO2 under mild conditions and has a HSC of 4.4 wt%.[9] The
generated CO2, or mostly carbonates, can be transformed to
formic acid under hydrogen pressure. Recently, several
rechargeable systems based on formic acid were developed,
using the same catalyst for both hydrogenation and dehydro-
genation.[9d,e] However, apart from the moderate HSC of
formic acid, which is an inherent feature of these systems,
a stoichiometric amount of base was usually required to
capture the generated CO2.[10]
Besides formic acid, liquid organic hydrogen carriers
(LOHCs), which can unload and load hydrogen through
dehydrogenation and hydrogenation reactions, and can be
easily transported and stored, were investigated.[5b,6,11–13]
Much attention has been devoted to N-ethylcarbazole as
LOHC,[11] which has a HSC of 5.8 wt%; heterogeneous
catalysts were used and different catalytic systems were
required for the hydrogenation and dehydrogenation steps. In
addition, 2-methyl-1,2,3,4-tetrahydroquinoline[12a] and 2,6-
dimethyldecahydro-1,5-naphthyridine[12b] were also reported
as hydrogen storage materials. However, these systems suffer
from either relatively harsh conditions or high catalyst
loadings and both use relatively expensive materials. Until
now, most experimental and computational studies about
LOHCs emphasize heterocyclic aromatic hydrocarbons,
mostly N-heterocycles.[13] Considering this narrow material
scope, with its limitations, it is desirable to develop other
readily available inexpensive compounds as hydrogen carri-
ers.
T
he search for sustainable energy systems to replace the
current fossil-fuel-based technologies is an urgent issue.[1,2]
Construction of a sustainable energy supply chain requires
energy generation, storage, and release. Hence the develop-
ment of efficient and inexpensive energy storage and
liberation systems is needed. Through such systems, energy
can be stored during “energy-rich” periods and used during
“energy-lean” periods. Hydrogen, which holds the highest
energy density by weight, is viewed as an ideal candidate for
the future energy supply.[1,2] In fact, the hydrogen-powered
fuel cell has been intensively studied in recent years.[3]
However, hydrogen storage is a considerable challenge,
since its energy density by volume or weight is low when
stored as a pressurized gas or cryogenically as a liquid, and
safety issues are involved.[1,2,4,5] Hence hydrogen storage
materials have attracted much attention in the last decades
and many physical and chemical methods for hydrogen
storage have been developed. However, most of these
methods either suffer from low hydrogen storage capacity
or are too expensive for practical use.[1,2,5]
An attractive approach is the storage of hydrogen in
chemical bonds, and its release by dehydrogenation reactions,
using organic compounds.[2c,6] Especially interesting are
organic liquids with considerable hydrogen storage capacity
(HSC), which can be easily handled and transported, using
the existing infrastructure of the oil or gasoline industry.[6]
Recently, methanol reforming catalyzed by metal complexes
under relatively mild conditions was reported by Beller,
Grützmacher, and us.[7] In these promising systems, aqueous
solutions of methanol generate CO2 and afford three H2
molecules per one molecule of methanol and water. In
addition, a promising formaldehyde–water system was devel-
oped for hydrogen production by Prechtl.[8] These approaches
Based on Ru pincer catalysts (Figure 1), our group has
developed the acceptorless dehydrogenative amidation reac-
tion, using amines and primary alcohols as substrates,
resulting in formation of amides and dihydrogen
[*] Dr. P. Hu, Y. Ben-David, Prof. Dr. D. Milstein
Department of Organic Chemistry, Weizmann Institute of Science
Rehovot, 76100 (Israel)
E-mail: david.milstein@weizmann.ac.il
Supporting information for this article is available on the WWW
Figure 1. Structures of PNN ruthenium pincer complexes 1–5.
Angew. Chem. Int. Ed. 2016, 55, 1061 –1064
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1061