Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol

Article abstract of DOI:10.1021/jacs.6b11637

A novel hydrogen storage system based on the hydrogen release from catalytic dehydrogenative coupling of methanol and 1,2-diamine is demonstrated. The products of this reaction, N-formamide and N,N′-diformamide, are hydrogenated back to the free amine and methanol by a simple hydrogen pressure swing. Thus, an efficient one-pot hydrogen carrier system has been developed. The H₂ generating step can be termed as "amine reforming of methanol" in analogy to the traditional steam reforming. It acts as a clean source of hydrogen without concurrent production of CO₂ (unlike steam reforming) or CO (by complete methanol dehydrogenation). Therefore, a carbon neutral cycle is essentially achieved where no carbon capture is necessary as the carbon is trapped in the form of formamide (or urea in the case of primary amine). In theory, a hydrogen storage capacity as high as 6.6 wt % is achievable. Dehydrogenative coupling and the subsequent amide hydrogenation proceed with good yields (90% and >95% respectively, with methanol and N,N′-dimethylethylenediamine as dehydrogenative coupling partners).

Full text of DOI:10.1021/jacs.6b11637

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Communication
An Efficient Reversible Hydrogen Carrier
System Based on Amine Reforming of Methanol
Jotheeswari Kothandaraman, Sayan Kar, Raktim Sen,
Alain Goeppert, George A Olah, and G. K. Surya Prakash
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b11637 • Publication Date (Web): 02 Feb 2017
Downloaded from http://pubs.acs.org on February 2, 2017
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Journal of the American Chemical Society
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An Efficient Reversible Hydrogen Carrier System Based on
Amine Reforming of Methanol
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Jotheeswari Kothandaraman, Sayan Kar, Raktim Sen, Alain Goeppert, George A. Olah* and G. K.
Surya Prakash*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park,
Los Angeles, California 90089ꢀ1661, United States.
Supporting Information Placeholder
ABSTRACT: A novel hydrogen storage system based on the
hydrogen release from catalytic dehydrogenative coupling of
methanol and 1,2ꢀdiamine is demonstrated. The products of this
reaction, Nꢀformamide and N,N’ꢀdiformamide, are hydrogenated
back to the free amine and methanol by simple hydrogen pressure
swing. Thus, an efficient oneꢀpot hydrogen carrier system has
Steam reforming of CH
obtain H and is performed at high temperatures (240ꢀ260 ºC) and
high pressures over heterogeneous catalysts. Recently, it was
3
OH is generally the preferred method to
2
11
12
13
discovered that the use of homogeneous catalysts , mainly Ru
14
and Fe pincer complexes, could also enable aqueous CH OH
3
dehydrogenation at much lower temperatures (<100 ºC). Strongly
basic conditions are nevertheless required in most cases to achieve
been developed. The H generating step can be termed as “amine
2
reforming of methanol” in analogy to the traditional steam reꢀ
forming. It acts as a clean source of hydrogen without concurrent
high TON. In addition, CO
reported using similar pincer catalysts.
of our knowledge, aqueous reforming of CH OH and the reverse
2
reduction to CH
3
OH has also been
7
d,10bꢀe
However, to the best
production of CO (unlike steam reforming) or CO (by complete
3
2
methanol dehydrogenation). Therefore, a carbon neutral cycle is
essentially achieved where no carbon capture is necessary as the
carbon is trapped in the form of formamide (or urea in the case of
primary amine). In theory, a hydrogen storage capacity as high as
6.6 wt% is achievable. Dehydrogenative coupling and the subseꢀ
quent amide hydrogenation proceed with good yields (90% and
reaction (CO
homogeneous catalytic system has not yet been demonstrated
Scheme 1).
2
hydrogenation to CH OH) in the presence of same
3
(
Scheme 1. Aqueous reforming of methanol
12.1 wt% H
2
Cat. I
CO
2
+
3 H
2
>95%
respectively
–
with
methanol
and
N,N’ꢀ
CH
3
OH
+
2
H O
Cat. II
dimethylethylenediamine as dehydrogenative coupling partners).
Different catalytic system for forward and reverse reaction
The growing use of fossil fuels since the industrial revolution
Scheme 2. Amine reforming of methanol
has resulted in a significant increase in CO concentration in the
atmosphere, from 270 ppm in 1750 to over 400 ppm presently.
2
This work
1,2
H
NH
NH
2
2
6.6 wt% H
2
N
CO being a greenhouse gas, this has contributed to an increase in
CH OH
+
O
+
3 H2
2
3
º
Cat.
N
H
Earth’s average surface temperature of 0.8 C over the last 100
Primary
diamine
3
Cyclic Urea , A
years. According to scientific observations and predictions, the
H
ongoing global warming will be associated with severe environꢀ
4
,5
5.3 wt% H2
N
N
O
NH
mental and social changes in the near future. Renewable energy
sources, including solar, wind, geothermal and biomass, are inꢀ
creasingly being implemented to complement fossil fuels. Howꢀ
ever, the intermittent and fluctuating nature of some of these
sources, namely solar and wind, remains a problem for largeꢀscale
deployment. Storage of the generated energy in the form of chemꢀ
ical bonds, such as in hydrogen or methanol, is one of the promisꢀ
ing pathways and has led to the proposed “hydrogen economy”
4 H2
2
CH
3
OH
+
+
O
NH
Cat.
H
Secondary
diamine
N,N'-diformamide, B
Advantages (i) Carbon neutral cycle (ii) Liquid fuel at room
temperature (iii) Easily reversible (iv) Pure H gas produced.
2
Herein, we present a reversible hydrogen storage system
based on a CH OH/amine system, where H is generated by what
we call “amine reforming of CH OH”, in analogy with the steam
reforming of CH OH. CH OH and amine are regenerated in the
reverse hydrogenation reaction, thus closing the cycle. Both H
“loading” and “unloading” are performed in the presence of the
same Ruꢀpincer catalysts by a simple H pressure swing (Scheme
2). This process enjoys three main advantages over traditional
CH OH steam reforming in the context of sustainable H storage
6
,7
and “methanol economy”.
3
2
As a hydrogen carrier, liquid organic hydrogen carriers
LOHC) have gained significant attention recently as they are safe
3
(
3
3
to store and transport, have high wt% H storage capacities and
2
2
can offer fully reversible H loading and unloading. They can also
2
enable a relatively easy transition by allowing the utilization of
2
8
existing fuel infrastructures. Formic acid (HCO H), over the
2
years, has been explored thoroughly as a potential LOHC, and
3
2
highly efficient catalysts for both H loading and unloading have
and transportation — (1) it is reversible in the presence of the
same catalytic system, (2) the dehydrogenative coupling products,
2
9
been designed by us and others. However, a maximum H storꢀ
2
age of only 4.4 wt% is feasible in HCO H with the emission of
formamide (or urea), unlike CO
need to be recaptured as they remain in solution and are readily
available for the subsequent H loading step, and (3) pure H gas
is produced, which can be potentially used in H /Air fuel cells
2
from steam reforming, do not
2
stoichiometric amount of CO for each H .
2
2
Methanol (CH OH) is a good alternative because of its 12.6
2
2
3
10
wt% H content, ease of handling and convenient production.
2
2
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°
without purification. In 2016, Milstein et al reported an ethanol
based reversible hydrogen storage system in the presence of ethꢀ
relatively high boiling point (119 C) and at the same time low
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
3
3
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3
4
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4
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4
4
4
5
5
5
5
5
5
5
5
5
5
6
molecular weight, leading to a high w/w H storage potential (5.3
wt%) (Table 1). Diamine 7 in presence of CH OH and 1 mol% C-
2
15
ylenediamine. However, since CH OH has one fewer carbon, a
3
3
°
CH OH based hydrogen storage system could provide higher
1 catalyst loading at 140 C in toluene gave a H yield of 29%
3
2
1
hydrogen storage density when coupled with diamines.
after 24 h (entry 1, table 1). As the H NMR signals of 7a and 7b
overlap with each other, it was difficult to differentiate them from
the crude reaction mixture. However, after concentration, all H
1
Dehydrogenative Coupling of Amines and CH OH
3
(
a) Primary 1,2-diamine and CH OH: Activation of smaller
NMR signals of the rotamers of 7a (4 rotamers, 8 ppm, 7.96 ppm,
7.90 ppm and 7.89 ppm) and 7b (2 rotamers, 7.94 ppm and 7.97
ppm) were clearly assigned (Figure S5ꢀ7). A total 41% of the
corresponding formamide products 7a and 7b was obtained. Gas
evolved during dehydrogenation was collected and analyzed by
3
alcohols such as CH OH is considered challenging since the enerꢀ
3
gy barrier for their activation is much higher than for higher alcoꢀ
16
13b
17
hols. In 2013, Beller and Grützmacher showed independentꢀ
ly the dehydrogenation of CH OH (aqueous CH OH reforming) in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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0
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5
6
7
8
9
0
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2
3
4
5
6
7
8
9
0
3
3
presence of homogeneous ruthenium catalysts. Later, CH OH was
GC. A small amount of CO was detected along with H due to
3
2
2
0
used as a formyl source for the Nꢀformylation of amines and niꢀ
dehydrogenation of the formaldehyde intermediate to CO.
1
8
triles with Ru(NHC) complexes. Recently, Hong et al reported
Table 1. Condition screening for the dehydrogenative cou-
the synthesis of urea compounds from amines using CH OH as
3
pling of 7 and CH OH.
3
the C1 source in the presence of a ruthenium pincer catalyst, proꢀ
19
ducing H as a byꢀproduct. Inspired by these independent studꢀ
2
CHO
H
N
Cat. (C-1)
Toluene
ies, we envisioned a reversible and practical H storage system
N
2
N
+ n CH
3
OH
+ m H2
N
R
H
based on amine and CH OH.
3
(1 mmol)
7
20 oC , 24h
1
R=CHO, 7a; n=2, m=4
R=H, 7b; n=1, m=2
Scheme 3. From primary amines to urea
2
.4 wt%
H
2
b
CH
(mmol)
3
OH
K
3
PO
4
7a yield
(%)
7b yield
(%)
H
2
yield
CO
H
H
1
40 oC, 24h
Entry
a
Ph
N
N
Ph
+
3 H2
2
Ph
NH
2
+
CH
3
OH
(mol%)
(%)
29
30
49
56
82
66
79
86
76
65
nd
(%)
2.8
0.2
2.8
0.4
3.3
2.6
2.7
2.8
0.2
3
A
6
0 bar H2, 145 oC, 24h
Toluene
O
c
1
2
2
2
4
4
4
4
4
4
3
4
ꢀ
ꢀ
16
15
23
40
70
52
67
75
56
40
ꢀ
25
29
52
31
21
28
24
22
39
49
ꢀ
1
1a
H
H
Ph
2
3
4
5
N
P
Ph
CO
Ru
H
25
ꢀ
P
Dehydrogenation reaction: 88% yield
Hydrogenation reaction: >99% yield
Ph
Ph
BH
3
Catalyst (C-1)
25
25
10
5
d
NH
2
NH
6
Catalyst (C-1)
O
+
3 H
2
+
3
CH OH
140 oC, 24h
Toluene
7
B
NH
NH
2
8
NH
NH
2
2
e
NH
NH
2
2
9
5
H
2
N
NH
2
H
2
N
NH
2
Substrates:
NH
2
NH
2
10
5
f
1
1
5
nd
2
3
4
5
6
awt% H
:
6.6
5.7
52
4.1
3.6
3.6
2
Reaction conditions: 7 (1 mmol), C-1 (1 mol%), toluene (1.5 mL), reacꢀ
a
tion time (24 h) The theoretical H
2 2
yield is taken as 4 mmol and the H
Urea yield (%): traces
33
*
*
yield was calculated indirectly from the amount of –NCHO (from 7a +
*
Insoluble white solid crashed out.
1
7
b) formed, which is determined by H NMR using TMB as an internal
b c d
Reaction conditions: (A) 1 (2 mmol), CH
C-1 (10 ꢁmol); (B) substrate (1 mmol), CH
toluene (1.5 mL), reaction time (24h). Yields based on H NMR using
3
OH (2 mmol), toluene (3 mL),
standard. Determined by GC. 140 °C. 1,4ꢀdioxane (1.5 mL) was used as
e
f
3
OH (2 mmol), C-1 (10 ꢁmol),
a solvent instead of toluene. 100 °C. Ethylenediamine (2) was used inꢀ
stead of 7. nd=not detected. NMR yield calculations error = +/ꢀ 5%.
1
a
1
,3,5ꢀtrimethoxybenzene (TMB) as an internal standard. Maximum theoꢀ
°
retical wt% H obtainable. NMR yield calculations error = +/ꢀ 5%.
2
Decreasing the reaction temperature to 120 C produced a
similar H yield (30%), with a significant decrease in CO forꢀ
2
The dehydrogenative coupling of benzylamine (1) and
mation (entry 2, table 1). In reactions catalyzed by RuꢀPNP comꢀ
plexes, K PO is often used as a base additive to enhance catalytic
CH OH with RuꢀmachoꢀBH catalyst (C-1) in a closed reactor
3
3
4
formed N,N’ꢀdibenzylurea (1a) in 88% yield (Scheme 3A). The
7d,21,10c
activity via a favorable ꢀNH assisted pathway.
Indeed,
reaction was performed. To our surprise, at a H pressure of 60
2
when 25 mol% of K PO was added to the reaction mixture, the
3
4
bar, 1a was completely converted back to CH OH and 1 (Figure
3
H yield increased to 49% (entry 3, table 1). Using more CH OH
2 3
S8). However, the high molecular weight of 1 makes it an ineffiꢀ
(
4 mmol) induced better H yield (82%) (entry 5, table 1). In 1,4ꢀ
2
cient H storage material. An ideal amine for this application must
2
dioxane, a low H yield was observed (entry 6, table 1). Lower
2
have low carbon content for efficient H storage along with low
2
amounts of K PO gave similar yields and 5 mol% was found to
3 4
volatility for easy handling. We therefore turned our focus to diaꢀ
mines which satisfy both criteria. However, when reacted with
be an optimum for our reaction conditions with 86% H yield
2
(
entry 8, table 1). When 3 mmol of CH OH was used instead of 4,
3
CH OH, the yields of corresponding cyclic ureas were low in the
3
the H yield dropped to 65% (entry 10, table 1). Ethylenediamine,
2
presence of primary diamines as can be seen in Scheme 3B. With
xylylenediamines, 5 and 6, the intermolecular polymeric urea
products crashed out as white solids, which could not be hydroꢀ
genated back to free amines and CH OH under H pressure. These
2
, was screened under these optimized condition but only traces of
Nꢀformyl and urea products were obtained (entry 11, table 1).
3
2
white solids were insoluble in water and in most organic solvents.
b) Secondary 1,2-diamine and CH OH: In light of someꢀ
(
3
what unsatisfying results obtained with primary diamines, we
decided to screen a secondary 1,2ꢀdiamine for the dehydrogenaꢀ
tive coupling reaction. N, N’ꢀdimethylethylenediamine (7) has a
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H
H
H
H
H
Ph
Ph
Me
Ph
Ph
tBu
P t
Bu
H
iPr
iPr
N
P
N
P
1
2
3
4
5
6
7
8
9
N
N
P
Ru
Cl
Ru
Ru
P
CO
P
CO
Ru
Ph
Ph
N
CO
P
CO
Ph
Ph
Et
iPr
Et
iPr
Cl
Cl
C-2
C-3
C-4
C-5
H
xyl
xyl
H
H
N
2
H
iPr
iPr Ts
Cl
N
P
O
Ru
N
P
H
N
2
Ru
Fe
Br
N
Cl
P
P
CO
xyl
iPr
Ph
iPr
H
xyl
H
3
CO
OMe
Ph
Hydrogenation of In-situ Formed 7a/7b
C-6
C-7
C-8
Formamides are reported to be generally amenable to reducꢀ
25
tion under moderate H pressures with metalꢀpincer complexes.
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
To optimize the reverse reaction, the reactor was charged with H2
upon completion of the dehydrogenation reaction, and heated to
120 °C (in the presence of same catalyst (C-5)) (Table 2). When a
H pressure of 40 bar was applied, 92% of 7 formed, 4% of 7b
2
remained unreacted and no trace of 7a was observed (entry 1,
table 2). At 60 bar pressure, 95% of 7 formed after 24 h and no
1
trace of 7a/7b were observed by H NMR (entry 2, table 2). In
these reactions (entry 1ꢀ2, table 2), lower CH OH yields (~75%)
3
26
are due to the loss of CH OH during the hydrogen release. The
3
recyclability of the catalyst (C-5) was studied on 1 mmol scale
(Figure S12) and the catalyst was recycled three times. More than
80% of its initial activity was retained after three cycles with a
total production of 230 mL H2.
Table 2. Hydrogenation of in-situ formed 7a.
CHO
H
Cat. (C-5)
Figure 1. Catalysts screening for the dehydrogenative coupling of 7 and
CH OH. Reaction conditions: 7 (1 mmol), CH OH (4 mmol), catalysts (1
mol%), K PO (5 mol%), toluene (1.5 mL), time (24 h), and T=120 °C. H
2
N
m H2
N
+
3
n CH OH
N
R
+
N
H
3
3
Toluene
120 oC , t
3
4
R=CHO, 7a; n=2, m=4
R=H, 7b; n=1, m=2
7
K
3
PO
4
yields are based on the amount of –NCHO (from 7a + 7b) formed, which
1
is determined by H NMR using TMB as an internal standard. CO content
determined by GC. In case of C-3, C-4, C-6 and C-7, an insufficient
amount of gas was produced to collect and analyze by GC. NMR yield
calculations error = +/ꢀ 5%.
H2
(bar)
time
(h)
Unreacteda
a
Entry
Catalyst
7 (%)
7a/7b (%)
1
2
C-5
C-5
40
24
0/4
92
95
60
24
0/0
Catalysts C-2 ꢀ C-8 were also screened under the optimized
conditions from entry 8, table 1 (Figure 1). RuꢀMacho C-2 gave a
Reaction conditions: After the dehydrogenation, the reaction mixture from
C-5, figure 1 contained no trace of 7 and 1.9 mmol CH OH and this mixꢀ
ture was used to check the reversibility under high H pressure. CH OH
3
lower H yield (75%) compared to RuꢀMachoꢀBH C-1 (86%).
2
2
3
Interestingly, Nꢀmethylated pincer catalyst C-3 showed low cataꢀ
lytic activity, further demonstrating the involvement of a NꢀH
assisted mechanistic pathway (outerꢀsphere bifunctional mechaꢀ
a
1
yield for both entry 1 and 2=~75%. determined by H NMR using TMB
as an internal standard. NMR yield calculations error = +/ꢀ 5%.
2
2,7d
nism).
Milstein’s catalyst C-4 gave 10% of H2 yield. Ruꢀ
In order to further extend the scope of this hydrogen storage
ipr
PNP C-5 gave good H yield (90%), and to our delight no CO
system (7/CH OH), a neat reaction was performed without solvent
3
2
was observed in the gas mixture (GC: CO detection limit=0.099
v/v%) (Figure S4). When the evolved gas mixture was collected
by scaling up the reaction 5ꢀfold (5 mmol). Excitingly, both dehyꢀ
drogenation and hydrogenation gave good to moderate yield (76%
2
7
in a gas burette, a H yield of 87% (85 mL) was obtained, which
and 60% respectively) even in the absence of any solvent.
2
is in close accordance with the NMR yield (90%).
In conclusion, a novel reversible hydrogen carrier system
based on the dehydrogenative coupling of 1,2ꢀdiamine and
ipr
FeꢀPNP C-6 produced no CO but the H yield was poor
2
(9%). Addition of K CO additive (2 mol%) along with K PO (5
CH OH is demonstrated, where an overall carbon neutral cycle is
3
2
3
3
4
mol%), further increased the H yield to 26%. No trace of 7a/7b
achieved by trapping the carbon in the form of Nꢀformamides (or
urea). One of the major challenges was the CO contamination of
the gas mixture, which was overcome by using a wellꢀdefined
2
1
appeared in H NMR with a transfer hydrogenation catalyst, (R,
R)ꢀTsꢀDENEB (C-7). On the other hand, the hydrogenation
catalyst (R)ꢀRUCYꢀxylBINAP (C-8) gave small amounts (12%)
2
3
homogenous RuHCl(CO)HN(CH
absence of any solvent, this system exhibited good catalytic activꢀ
ity. Our future efforts in the context of CH OH/amine hydrogen
CH PiPr ) catalyst. Even in the
2
2 2 2
24
of H2. As seen in Figure 1, C-5 was the best catalyst for the
dehydrogenative coupling of 7 and CH OH, both in terms of seꢀ
3
3
lectivity and yield of H2.
CO detected by GC in the dehydrogenation reactions is assoꢀ
ciated with a competing mechanistic pathway, where the formalꢀ
storage systems will be directed towards broadening the substrate
scope to high boiling polyamines.
dehyde formed after initial dehydrogenation of CH OH is rapidly
ASSOCIATED CONTENT
Supporting Information
3
dehydrogenated further before the nucleophilic addition of amine
to form the αꢀamino alcohol can take place (Scheme 4).
The Supporting Information is available free of charge on the
ACS Publications website.
Scheme 4. Proposed mechanism based on metal-ligand
cooperation
AUTHOR INFORMATION
Corresponding Author
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gprakash@usc.edu
olah@usc.edu
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Jotheeswari Kothandaraman:0000ꢀ0001ꢀ6306ꢀ9468
Sayan Kar:0000ꢀ0002ꢀ6986ꢀ5796
Alain Goeppert:0000ꢀ0001ꢀ8667ꢀ8530
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ACKNOWLEDGMENT
Catal. B Environ. 2010, 99, 43–57. (b) Li, D.; Li, X.; Gong, J. Chem.
Rev. 2016, 116, 11529–11653. (c) Iulianelli, A.; Ribeirinha, P.;
Mendes, A.; Basile, A. Renew. Sustain. Energy Rev. 2014, 29, 355–
Support of our work by the Loker Hydrocarbon Research Instiꢀ
tute, USC is gratefully acknowledged. JK is thankful to the Staufꢀ
fer and the Morris S. Smith foundations for providing endowed
Graduate Fellowships.
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