A new Cu-based system for formic acid dehydrogenation

Abstract

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.

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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 transformation in a highly useful instrument for hydrogen storage.

The development of improved technologies for H₂ generation 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 Biofine process.² Compared to H₂, formic acid is liquid and easy to store, transport and handle.³ Besides, HCOOH is considered less hazardous than methanol (its first competitor as hydrogen carrier) and therefore a valuable alternative in spite of its lower hydrogen density (43 vs. 125 g kg⁻¹).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 interesting 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 light^1,7,8 while the potential of Cu based system is still largely unexplored.

In this paper we wish to report our results in the production of H₂ from formic acid by using simple copper complexes and a HCOOH/amine adduct.

In the absence of catalyst a 5 : 2 HCOOH/NEt₃ adduct (NEt₃ = 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 experiments 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).

Table 1 Decomposition of formic acid using different HCOOH/NEt₃ ratio and different Cu precursors^a

Cu precursor	HCOOH/NEt3 ratio	V (ml)			C (%)	TON	TOF
		3 h	6 h	22 h	22 h
a Reaction conditions: Cu = 0.26 mmol, 95 °C, HCOOH = 28 mmol (1.31 g).
Cu(OAc)2	5/2	20	28	56	4.1	4.4	0.20
Cu(OAc)2	1/1	58	103	234	17.2	18.6	0.85
Cu(OAc)2	2/5	63	112	273	20.1	21.6	0.98
Cu(OOCH)2	1/1	47	88	229	16.8	18.4	0.84
Cu(acac)2	1/1	64	121	253	18.6	20.3	0.92
Cu(NO3)2	1/1	66	116	231	17.0	18.6	0.85
CuCl2	1/1	17	26	71	5.2	5.7	0.26
CuO	1/1	47	66	249	19.6	19.6	0.89
Cu powder	1/1	8	9	21	1.5	1.7	0.08

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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 significantly less performant while Cu powder was almost inactive.

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 pK_b (9.37 and 8.75) gave negligible conversion after 22 h (0.6% and 2%) whereas highly basic dibutylamine and triethylamine, allowed to reach a conversion around 17% in 22 h (V = 221).

Table 2 Decomposition of formic acid with Cu(OAc)₂ using different amines: effect of basicity^a

Amine	pKb	V (ml)			C (%)
		3 h	6 h	22 h	22 h
a Reaction conditions: Cat = 0.26 mmol, 95 °C, HCOOH = 28 mmol (1.31 g, 99 wt%), HCOOH/amine = 1.b pKb1.
Dibutylamine	2.75	53	94	221	16.2
Piperidine	2.88	59	99	164	12.0
Triethylamine	2.99	58	103	234	17.2
Tributylamine	3.11	115	186	349	25.6
Ethylenediamine	3.29^b	8	8	8	0.6
Tripropylamine	3.35	160	241	312	22.9
Benzylamine	4.67	13	17	28	2.1
Pyridine	8.75	24	26	28	2.1
Aniline	9.37	11	10	10	0.7
Diphenylamine	13.21	4	5	5	0.4

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However, Fig. 1 shows that there is no linear dependence of the activity on basicity. On the contrary a very narrow range of pK_b grants high activity while outside this range activity collapses. Moreover there is no linearity even in the pK_b range ensuring activity, e.g. piperidine is more basic than triethylamine or tributylamine (pK_b = 2.88 vs. 2.99 and 3.11) but the volume of gas produced is much lower (t = 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 ∼85° and thus inhibiting coordination of the HCOO⁻ species and forming a very stable and inactive complex.

Fig. 1 Conversion vs. pK_b for the different amines. The inset shows the region between 2.5–3.5 pK_b.

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.

Table 3 Decomposition of formic acid: effect of nucleophylicity^a

Amine	n^b	pKb	θ^c	6 h	22 h	22 h
				V (ml)		C (%)
a Reaction conditions: Cat = 0.26 mmol, 95 °C, HCOOH = 28 mmol (1.31 g, 99 wt%), HCOOH/amine = 1.b n = nucleophilic constant.c conic angle.
Piperidine	5.59	2.88	121	99	164	12.0
Dibutylamine	4.77	2.75	158	94	221	16.2
Triethylamine	4.09	2.99	150	103	234	17.2
Tripropylamine	—	3.35	160	241	312	22.9

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A reaction mechanism may be suggested in which the presence 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 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. After a few minutes (5 min) the mixture turns back to blue (Cu(II)) and an increase in activity is observed (Fig. 2).

Fig. 2 Re-activation of Cu(OAc)₂, by exposing the reaction mixture to air after 22 h.

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 significant 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.

Table 4 Stabilization of Cu catalyst using stoichiometric amount of ethylenediamine or TMEDA^a

Cu/en Ratio	3 h		6 h		22 h
	V (ml)	C (%)	V (ml)	C (%)	V (ml)	C (%)
a Reaction conditions: Cu(OAc)2 = 0.26 mmol, 95 °C, HCOOH = 28 mmol (1.31 g, 99 wt%), HCOOH/NEt3 = 1.
Cu/en = 1/2	60	4.4	105	7.7	234	17.2
Cu/en = 1/1	121	8.9	189	13.9	299	22.0
Cu/en = 2/1	59	4.3	98	7.2	223	16.4
Cu/TMEDA = 1/1	70	5.2	111	8.2	224	16.4

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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 profile during the first hours of reaction, but the long term stability increased very much (V = 320 vs. 220, C = 24.5 vs. 16.4) as shown in Fig. 3.

Fig. 3 Stabilization of the Cu catalyst by the addition of CH₃COOH to the starting mixture.

Finally we tested some precursor with very low potential reduction to metallic copper (Table 5).

Table 5 Decomposition of formic acid using different Cu precursors^a

Cu precursor	V (ml)			C (%)
	3 h	6 h	22 h	22 h
a Reaction conditions: Cu = 0.26 mmol, 95 °C, HCOOH = 28 mmol (1.31 g), HCOOH/NEt3 = 1.
CuCl	32	59	189	14.9
Cu2O	79	124	217	16.0
CuI	110	209	493	36.2
[(PPh3)CuH]6	28	47	59	4.3

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Cu₂O showed high initial activity but it slowed down reaching 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 Ru^5a that are up to 4 order of magnitude higher.

In conclusion, copper compounds appear as promising catalysts for H₂ production from HCOOH/amine adducts although, so far, neglected by the scientific community.

Their activity can be finely 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.

Acknowledgements

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.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental conditions (apparatus, catalytic experiments and gas-phase analysis) and reaction profiles. See DOI: 10.1039/c4ra11031e

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