A new Cu-based system for formic acid dehydrogenation

Article abstract of DOI:10.1039/c4ra11031e

The production of H₂ from HCOOH was achieved by using simple Cu compounds and different HCOOH/amine adducts. The activity is strongly dependent on the amine, more basic and bulky ones giving better results whereas the use of chelating amines is detrimental.

Full text of DOI:10.1039/c4ra11031e

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A new Cu-based system for formic acid
dehydrogenation†
Cite this: RSC Adv., 2014, 4, 61514
*
Nicola Scotti, Rinaldo Psaro, Nicoletta Ravasio and Federica Zaccheria
Received 23rd September 2014
Accepted 6th November 2014
DOI: 10.1039/c4ra11031e
www.rsc.org/advances
The production of H2 from HCOOH was achieved by using simple Cu
In this paper we wish to report our results in the production
compounds and different HCOOH/amine adducts. The activity is of H2 from formic acid by using simple copper complexes and a
strongly dependent on the amine, more basic and bulky ones giving HCOOH/amine adduct.
better results whereas the use of chelating amines is detrimental.
In the absence of catalyst a 5 : 2 HCOOH/NEt3 adduct (NEt3
¼ triethylamine) gave no gas evolution, but as soon as Cu(OAc)2
is added formation of H2 and CO2 starts giving in 3 h a total
volume of 20 ml. GC analysis of the gas phase of all the exper-
iments showed that H2 and CO2 were formed in a 1 : 1 ratio,
together with traces of CO (<150 ppm).
By decreasing the HCOOH/NEt3 ratio we observed the same
trend reported by Beller and coworkers,5a,d the higher the amine
concentration, the higher the conversion, although for ratio
lower than 1 the increase was quite limited (Table 1).
On the contrary the use of different copper compounds for
the reaction did not lead to important differences in conversion
and total volume produced. This could be due to the rapid
replacement of pristine ligands by formate and amine, due to
their high concentration in the reaction medium. Only CuCl2
was signicantly less performant while Cu powder was almost
inactive.
New catalytic systems for the hydrogenation of CO2 to formic
acid followed by its decomposition to give back CO2 and H2
are extremely interesting as the combination of the two
reactions would allow one to valorize CO2 by its trans-
formation in a highly useful instrument for hydrogen
storage.
The development of improved technologies for H2 gener-
ation and H2 storage in a safe and reversible manner is a
prerequisite for the utilization of hydrogen as fuel.1 Moreover
formic acid is one of the major by-products of levulinic acid
production with the Bione process.2 Compared to H2, for-
mic acid is liquid and easy to store, transport and handle.3
Besides, HCOOH is considered less hazardous than meth-
anol (its rst competitor as hydrogen carrier) and therefore a
valuable alternative in spite of its lower hydrogen density
(43 vs. 125 g kgꢀ1).4
In the last years some papers, especially from Beller and
Laurenczy groups, on formic acid dehydrogenation have been
published, reporting promising results, but using expensive
catalysts based on noble metals, such as [{RuCl2(p-cymene)}2] or
proton-switchable Ir complexes.5 Others works report an inter-
esting activity using various heterogeneous systems mainly
based on Pd, Au and Ag.5b,6 As far as non-noble metals are
concerned, only a few homogeneous iron catalysts are reported
to be active in the reaction, in the presence or in the absence of
visible light1,7,8 while the potential of Cu based system is still
largely unexplored.
Table 1 Decomposition of formic acid using different HCOOH/NEt3
ratio and different Cu precursorsa
C (%) TON TOF
V (ml)
Cu precursor HCOOH/NEt3 ratio 3 h 6 h 22 h 22 h
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OOCH)2
Cu(acac)2
Cu(NO3)2
CuCl2
5/2
1/1
2/5
1/1
1/1
1/1
1/1
1/1
1/1
20
28 56
4.1
4.4 0.20
58 103 234 17.2 18.6 0.85
63 112 273 20.1 21.6 0.98
47
64 121 253 18.6 20.3 0.92
66 116 231 17.0 18.6 0.85
17
47
8
88 229 16.8 18.4 0.84
26 71
66 249 19.6 19.6 0.89
21 1.5 1.7 0.08
5.2
5.7 0.26
Institute of Molecular Science and Technology CNR, Via Golgi, 19, 20133 Milano, Italy.
E-mail: n.ravasio@istm.cnr.it
CuO
Cu powder
9
† Electronic supplementary information (ESI) available: Experimental conditions
(apparatus, catalytic experiments and gas-phase analysis) and reaction proles.
See DOI: 10.1039/c4ra11031e
a
Reaction conditions: Cu ¼ 0.26 mmol, 95 ꢁC, HCOOH ¼ 28 mmol (1.31 g).
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A more interesting and evident effect was observed by
changing the amine (Table 2). Here the basicity plays an
important role. In particular, the lower the basicity of the
amine, the lower the activity. Aniline and pyridine, e.g., with
high pKb (9.37 and 8.75) gave negligible conversion aer 22 h
(0.6% and 2%) whereas highly basic dibutylamine and
triethylamine, allowed to reach a conversion around 17% in
22 h (V ¼ 221).
However, Fig. 1 shows that there is no linear dependence of
the activity on basicity. On the contrary a very narrow range of
pKb grants high activity while outside this range activity
collapses. Moreover there is no linearity even in the pKb range
ensuring activity, e.g. piperidine is more basic than triethyl-
amine or tributylamine (pKb ¼ 2.88 vs. 2.99 and 3.11) but the
volume of gas produced is much lower (t ¼ 22 h, 156 ml vs. 221
and 349). The particular case of ethylenediamine is worth
noting: notwithstanding its high basicity the reaction does not
almost proceed. This behavior suggests a coordination effect of
the amine on the copper center. Ethylenediamine is well known
for its chelating properties and could strongly coordinate the Cu
atom blocking all the four square planar coordination sites with
its bite angle9 of ꢂ85ꢁ and thus inhibiting coordination of the
HCOOꢀ species and forming a very stable and inactive complex.
Therefore we took into consideration other parameters such
as nucleophilicity and steric hindrance that can play a role in
the coordination chemistry involved in this reaction. Table 3
reports catalytic data related with the nucleophilic constant, n
(ref. 10) and with the conic angle11 of the amines. It is apparent
from this comparison that activity increases by increasing steric
hindrance and by decreasing nucleophilicity, thus suggesting
that the role of the amine is not limited to HCOOH
deprotonation.
Fig. 1 Conversion vs. pKb for the different amines. The inset shows the
region between 2.5–3.5 pKb.
weak reducing agents such as reducing sugars, citric and
ascorbic acid12 is well known.
However the catalyst could be easily reactivated, before
complete reduction to metallic phase, simply by opening the
reactor to air. Aer a few minutes (5 min) the mixture turns back
to blue (Cu(II)) and an increase in activity is observed (Fig. 2).
To stabilize copper in solution we tried to exploit the
chelating ability of ethylenediamine, as we already observed
that in the presence of such amine neither decomposition of
HCOOH nor discoloring of the solution or separation of solids
took place. Results reported in Table 4 show that in fact a
stoichiometric amount of ethylenediamine with respect to Cu
gave a signicant improvement in the performance of the
HCOOH/NEt3 adduct. On the contrary, Cu/en ratio other than 1
or a different diamine such as TMEDA had no effect.
Another chance is to avoid alkalinization of the medium as
reduction of copper to form Cu(0) nanoparticles only occurs
under basic conditions. Therefore we added an acid other than
HCOOH to keep a low pH during the reaction, without affecting
the HCOOH/amine ratio. Addition of acetic acid to the
1HCOOH/1NEt3 mixture did not modify the reaction prole
during the rst hours of reaction, but the long term stability
increased very much (V ¼ 320 vs. 220, C ¼ 24.5 vs. 16.4) as shown
in Fig. 3.
A reaction mechanism may be suggested in which the pres-
ence of two bulky amines could favour the elimination of
hydrogen and CO2 from a square planar intermediate.
The catalytic system tends to deactivate with time, as shown
by the solution discolouring and by the formation of a reddish
precipitate. Reduction of Cu(II) in basic medium by means of
Table 2 Decomposition of formic acid with Cu(OAc)2 using different
amines: effect of basicitya
C (%)
V (ml)
Amine
pKb
3 h
6 h
22 h
22 h
Table 3 Decomposition of formic acid: effect of nucleophylicitya
Dibutylamine
Piperidine
2.75
2.88
2.99
3.11
3.29b
3.35
4.67
8.75
9.37
13.21
53
59
58
115
8
160
13
24
11
4
94
99
103
186
8
241
17
26
221
164
234
349
8
312
28
28
16.2
12.0
17.2
25.6
0.6
22.9
2.1
2.1
6 h
22 h
22 h
Triethylamine
Tributylamine
Ethylenediamine
Tripropylamine
Benzylamine
Pyridine
Amine
nb
pKb
qc
V (ml)
C (%)
Piperidine
5.59
4.77
4.09
—
2.88
2.75
2.99
3.35
121
158
150
160
99
94
103
241
164
221
234
312
12.0
16.2
17.2
22.9
Dibutylamine
Triethylamine
Tripropylamine
Aniline
Diphenylamine
10
5
10
5
0.7
0.4
a
Reaction conditions: Cat ¼ 0.26 mmol, 95 ꢁC, HCOOH ¼ 28 mmol
a
b
Reaction conditions: Cat ¼ 0.26 mmol, 95 ꢁC, HCOOH ¼ 28 mmol (1.31 g, 99 wt%), HCOOH/amine ¼ 1. n ¼ nucleophilic constant.
(1.31 g, 99 wt%), HCOOH/amine ¼ 1. b pKb1
.
conic angle.
c
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Table 5 Decomposition of formic acid using different Cu precursorsa
C (%)
V (ml)
Cu precursor
3 h
6 h
22 h
22 h
CuCl
Cu2O
CuI
[(PPh3)CuH]6
32
79
110
28
59
124
209
47
189
217
493
59
14.9
16.0
36.2
4.3
a
Reaction conditions: Cu ¼ 0.26 mmol, 95 ꢁC, HCOOH ¼ 28 mmol
(1.31 g), HCOOH/NEt3 ¼ 1.
Fig. 2 Re-activation of Cu(OAc)2, by exposing the reaction mixture to
air after 22 h.
In conclusion, copper compounds appear as promising
catalysts for H2 production from HCOOH/amine adducts
although, so far, neglected by the scientic community.
Their activity can be nely tuned through the choice of the
amine while deactivation may be controlled. Work is in progress
to get a deeper insight into the mechanism and particularly into
the coordination geometry of the active site in order to develop
an effective system for hydrogen production from HCOOH in
the presence of a non-noble metal.
Table 4 Stabilization of Cu catalyst using stoichiometric amount of
ethylenediamine or TMEDAa
3 h
6 h
22 h
Cu/en Ratio
V (ml) C (%) V (ml) C (%) V (ml) C (%)
Cu/en ¼ 1/2
60
121
59
4.4
8.9
4.3
5.2
105
189
98
7.7
13.9
7.2
234
299
223
224
17.2
22.0
16.4
16.4
Cu/en ¼ 1/1
Cu/en ¼ 2/1
Acknowledgements
Cu/TMEDA ¼ 1/1
70
111
8.2
a
Reaction conditions: Cu(OAc)2 ¼ 0.26 mmol, 95 ꢁC, HCOOH ¼ 28
mmol (1.31 g, 99 wt%), HCOOH/NEt3 ¼ 1.
The Italian Ministry for University and Research (MIUR) is
acknowledged for funding the project: “Mechanisms of CO2
activation for the design of new materials for energy and
resource efficiency” PRIN 2010A2FSS9_001.
Finally we tested some precursor with very low potential
reduction to metallic copper (Table 5).
Cu2O showed high initial activity but it slowed down reach-
ing at 22 h a conversion comparable to the Cu species reported
in Table 1.
On the contrary CuI showed very high activity and stability
allowing to reach 36% conversion at 22 h and 66% at 45 h. This
matches with a TON value of 72 which is far away from the
values obtained with Fe8 or Ru5a that are up to 4 order of
magnitude higher.
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