CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402426

Efficient and Selective Hydrogen Generation from Bioethanol using Ruthenium Pincer-type Complexes Peter Sponholz,[a] Dçrthe Mellmann,[a] Christoph Cordes,[a] Pamela G. Alsabeh,[a] Bin Li,[b] Yang Li,[a] Martin Nielsen,[a] Henrik Junge,[a] Pierre Dixneuf,[b] and Matthias Beller*[a] Catalytic generation of hydrogen from aqueous ethanol solution proceeds in the presence of pincer-type transition metal catalysts. Optimal results are obtained applying a [Ru(H)(Cl)(CO)(iPr2PEtN(H)EtPiPr2)] complex (catalyst TON 80 000) in the presence of water and base. This dehydrogenation reaction provides up to 70 % acetic acid in a selective manner. For the first time, it is shown that bioethanol obtained from fermentation processes can be used directly in this protocol without the need for water removal. The produced hydrogen can be directly utilized in proton exchange membrane (PEM) fuel cells, since very low amounts of CO are formed.

ample, recently Grtzmacher and our group independently reported on the use of ruthenium pincer complexes in aqueous methanol reforming reactions for the selective production of hydrogen.[7] In addition, we have shown that ruthenium PNPpincer complexes catalyze the production of ethyl acetate from ethanol with concomitant H2 generation (dehydrogenative coupling) reaching high catalyst turnover numbers (TONs) up to 15 000.[5a] In this respect, also the recent synthesis of butanoic acid from aqueous 1-butanol using ruthenium pincertype complexes is noteworthy.[8] Among the different renewable alcohols, bioethanol is an especially promising hydrogen carrier.[9] Bioethanol is produced on large scale by fermentation from biomass,[10] such as biodegradable waste from agriculture and food industries.[11] It also possesses low toxicity and is readily available in high quality at

Complementary to previous work using renewables for a more sustainable and practical energy supply,[1] liquid organic hydrogen carriers (LOHCs)[2] have since recently attracted increasing interest for hydrogen generation and storage.[3] Apart from the catalytic dehydrogenation (DH) of formic acid,[4] biobased alcohols constitute interesting hydrogen donors.[4b] However, their use has been mainly limited to transfer hydrogenations (THs) for synthetic organic applications.[5] Depending on the reaction conditions (i.e., water, base, acid), DHs allow for the transformation of alcohols into valuable products, including Figure 1. Pincer-type complexes investigated in the dehydrogenation of aqueous ethanol. esters, aldehydes, or carboxylic acids. So far, only few examples an affordable price.[12] Advantageously, bioethanol contains of acceptorless DH concentrated on H2 generation.[5a, c, 6] For exwater, allowing it to be used directly for aqueous reforming reactions without the need for stringent purification methods.[13] [a] P. Sponholz,+ D. Mellmann,+ C. Cordes, Dr. P. G. Alsabeh, Dr. Y. Li, Dr. M. Nielsen, Dr. H. Junge, Prof. M. Beller Ideally, using bioethanol it should be possible to perform cataLeibniz-Institut fr Katalyse e.V. an der Universitt Rostock lytic H2 release under ambient conditions and to directly use it Albert-Einstein-Str. 29a, 18059 Rostock (Germany) in H2/O2 fuel cells without concomitant production of waste or Fax: (+ 49) 381-1281-5000 fuel-cell poisons such as CO.[14] E-mail: [email protected] Herein, we report a highly efficient ethanol reforming pro[b] B. Li, Prof. P. Dixneuf Centre of Catalysis and Green chemistry-OMC cess under mild conditions using molecular-defined pincerInstitut Sciences Chimiques de Rennes type complexes. Initially, based on the experience in dehydroUMR 6226-CNRS-universit de Rennes 1 genation and transfer hydrogenation of alcohols,[5d, 6, 15] various Campus de Beaulieu, 35042 Rennes (France) + ruthenium- and iridium-based complexes[16] were selected for [ ] These authors contributed equally to this work. catalytic screening experiments (Figure 1). Notably, the coordiSupporting Information for this article is available on the WWW under nated ligands in complexes 1–8 comprise an aliphatic or arohttp://dx.doi.org/10.1002/cssc.201402426.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemSusChem 2014, 7, 2419 – 2422

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matic bridge between the C, P, or N donor atoms, among which one arm can be hemi-labile (e.g., NEt2).[15a] In general, the catalytic activity of complexes 1–8 (3.9 mmol) was investigated in aqueous ethanol/water (9:1) solutions with 2 m base (NaOH) under reflux conditions (Table 1). The internal temperature of the reaction solutions was determined to be

Table 1. Testing of different pincer catalysts for aqueous ethanol reforming.[a] Entry

Catalyst

Vgas 1 h [mL]

TOF 1 h [h 1]

Vgas 3 h [mL]

TOF 3 h [h 1]

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

9.6 32 42 113 110 131 119 118

101 335 440 1184 1152 1372 1246 1236

n.d. 45 70 243 247 347 335 314

n.d. 157 244 848 862 1212 1170 1096

[a] Reaction conditions: 3.9 mmol (25 ppm) catalyst in 10 mL 9:1 EtOH/ H2O mixture (v/v), 2 m NaOH, 81 8C inner temperature, thermostat was set to 90 8C, gas volume measured with burettes and analyzed via GC. n.d. = not determined.

Table 2. Variation of the base concentration.[a] Entry

nNaOH [mol]

inner T [8C]

Vgas 1 h [mL]

TOF 1 h [h 1]

Vgas 3 h [mL]

TOF 3 h [h 1]

1 2 3 4 5 6[b]

– 2 4 6 8 8

79 81 83 86 87 81

4.8 119 141 157 185 60

50 1246 1477 1644 1938 628

5.8 335 387 432 504 163

20 1170 1351 1508 1760 569

[a] Reaction conditions: 3.9 mmol (25 ppm) catalyst 7 in 10 mL 9:1 EtOH/ H2O mixture, variation of NaOH concentration, thermostat temperature set to 90 8C, gas volume measured with burettes and analyzed via GC. [b] Inner temperature was set to 81 8C.

the same base concentration had a detrimental effect on the overall activity (entry 6). While the dehydrogenation of structurally diverse alcohols has been studied by several research groups in the past, the effect of water in such reactions is basically unknown. Hence, also the influence of water concentration was evaluated (Table 3). More specifically, the water content was increased

Table 3. Variation of the EtOH/H2O ratio.[a]

81 8C. With the exception of iridium-based complex (1), all pincer-type complexes 2–8 exhibited good to high catalytic activity for the DH reaction. Notably, N-ligated pyridine complexes 2 (TOF3h 157 h 1) and 3 (TOF3h 244 h 1) showed poorer performance compared to the aliphatic Ru-MACHO 4 (TOF3h 848 h 1) and Ru-MACHO-BH 5 (TOF3h 862 h 1) complexes. A further increase in DH activity was observed when the phenyl substituents on the phosphorous atom of the aliphatic pincer ligands were replaced with iPr (6, 7) or Cy (8). Thus, the dihydride complex 6 (TOF3h 1212 h 1) and the hydrochloride complex 7 (TOF3h 1170 h 1) proved to be optimal in this initial screening study. Considering the increased stability of 7 in comparison to 6, the former complex was chosen for further optimization of the reaction conditions. In basic solution complex 7 probably forms a free coordination site under chloride dissociation and is ready to enter the catalytic cycle.[7b] Next, the influence of the nature of the base was investigated in more detail (see Supporting Information, Table S2). Decreased catalytic activities were obtained in the presence of other bases including NaPO4, Na2HPO4, Na2CO3 and NaOAc. Compared to NaOH, all these bases gave approximately one order of magnitude lower activity (TOFs3h < 160 h 1). The influence of the NaOH concentration is shown in Table 2. In the absence of base (entry 1), complex 7 exhibited only minimal activity that diminished over time (1–3 h). On the other hand increased base concentrations of 4, 6, and 8 m (entries 3–5) led to improved catalyst activities (TOFs3h 1351, 1508, and 1760 h 1, respectively). However, this trend may be also a result of the concomitant rise of the reaction temperature with higher concentration of base applied. In fact, decreasing the internal temperature of the reaction solution to 81 8C with  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Entry

EtOH/H2O ratio

Vgas 1 h [mL]

TOF 1 h [h 1]

Vgas 3 h [mL]

TOF 3 h [h 1]

1 2 3 4[b]

9:1 7.5:2.5 5:5 1:9

119 99 59 12

1246 1037 618 126

335 268 168 33

1170 936 587 115

[a] Reaction conditions: 3.9 mmol catalyst 7 in different 10 mL EtOH/H2O (v/v) mixture, 2 m NaOH, 81 8C inner temperature, thermostat was set to 90 8C, gas volume measured with burettes and analyzed via GC. [b] 88 8C inner temperature.

from the initial 9:1 mixture of EtOH/H2O to 7.5:2.5 (v/v), which corresponds to a 1:1 molar ratio of EtOH and H2O. Moreover, combinations of EtOH/H2O of 5:5 and 1:9 were dehydrogenated. When using less water, the main reaction was dehydrogenative coupling of ethanol (i.e., the formation of ethyl acetate).[5a] Changing the molar ratio of EtOH/H2O from 9:1 to 7.5:2.5 or 5:5 resulted in 25 % and 50 % decreased activities after 3 h, respectively (Table 3, entries 1–3). Despite the elevated internal reaction temperature (88 8C) measured for this dilute ethanol mixture (1:9 EtOH/H2O, entry 4), the corresponding catalyst activity was one order of magnitude lower than that of the 9:1 mixture. The resulting DH plots of H2 evolution volume versus the different EtOH/H2O ratios over 3 h are shown in Figure S3 (Supporting Information). Notably, complex 7 demonstrated high stability at EtOH/H2O 9:1 (TON24h = 13 600) and 7.5:2.5 (TON22h = 10 312) ratios as well as at 5:5 (TON17h = 6540). This is an important requirement for practical H2 generation and its direct conversion into electric energy using fuel cells. ChemSusChem 2014, 7, 2419 – 2422

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CHEMSUSCHEM COMMUNICATIONS Analysis of the different catalytic experiments by gas chromatography (GC) revealed the presence of only trace amounts of CO2 and CO (below 10 ppm). Using lower concentrations of water, small quantities of acetaldehyde and ethyl acetate were observed as well. The major dehydrogenation product in solution in all the catalytic experiments is acetic acid, which was proven by quantitative 13C NMR analysis of the reaction mixture (Supporting Information, Figure S4). However, using lower water concentrations (9:1 EtOH/H2O) the formation of sideproducts increased to ca. 30 %. Here, 1-butanol, via the Guerbet reaction (Scheme 1),[17] and ethyl acetate were also detect-

www.chemsuschem.org Table 4. Hydrogen evolution from industrially produced bioethanol.[a] inner T Vgas 1 h TOF 1 h Vgas 3 h TOF 3 h [8C] [mL] [h 1] [mL] [h 1]

Entry

sample

1 2

EtOH/H2O 9.5:0.5 88 industrial bioEtOH[b] 88

163 169

1707 1770

462 483

1613 1686

[a] Reaction conditions: 3.9 mmol (25 ppm) catalyst 7 in 10 mL solution from industry, 8 m NaOH, thermostat temperature 90 8C, inner temperature 88 8C, gas volume measured with burettes and analyzed via GC. [b] 95 % wet ethanol from Anklam Bioethanol GmbH.

concentrations and even fermented bioethanol can be successfully applied as hydrogen source. In addition, this process can be used for a benign production of acetic acid, which represents an alternative method compared to known high-temperature processes based on fossil feedstocks.

Experimental Section General information: All compounds were purchased from commercial suppliers if not otherwise stated. Complexes 1,[15b] 2,[15a] 3,[17] 4,[6] and 5[7b] have been previously reported. Complexes 1,[15b] 6,[5d] 7,[7b] and 8[7b] were synthesized according to literature protocols. A sample of freshly prepared bioethanol was purchased from Anklam Bioethanol GmbH. The volume of gas liberated was measured by an automatic gas burette (S1) or manual gas burette.[18] The relative composition of the evolved gas was determined by GC (gas chromatograph HP6890N, carboxen 1000, TCD or TCD + methanizer/FID, external calibration). Details on the equipment, the experimental set-up as well as the calculations can be found in the Supporting Information. All experiments were performed at least twice, with a maximum variance of 15 %. Typical procedure for dehydrogenation of ethanol/water mixtures: A reaction vessel equipped with a thermostatic compartment was alternately evacuated and purged with argon several times. Then the reactor was charged with ethanol, water, and base. The solution was heated while stirring to the desired temperature and allowed to reach thermal equilibrium over 30 min. The catalyst complex was then added under argon counterflow. The evolved gases were measured using either manual or automatic burettes (S1) and thus, reaction and gas evolved were recorded by the operator or via the computer, respectively. NMR spectroscopy: 1H NMR and quantitative 13C NMR spectra were obtained at 300 MHz (Bruker AV-300) or 400 MHz (Bruker AV400). NMR chemical shifts (d) are reported in parts per million (ppm) downfield from tetramethylsilane and were referenced to the residual proton resonance and the natural abundance 13C resonance of the solvents.[19] 1,4-Dioxane was used as an internal standard.

Scheme 1. Major and minor reaction pathways in the acceptorless dehydrogenation of aqueous ethanol.

ed by use of 13C NMR and GC/MS techniques (Figure S4-1). Already at an EtOH/H2O ratio of 7.5:2.5, side-product formation is significantly reduced. Gratifyingly, a 5:5 EtOH/H2O ratio furnished H2 gas and acetic acid as the only products (Figure S42). Quantitave 13C NMR analysis of the reaction solution confirmed that the amount of ethanol converted to acetic acid (or 1-butanol) is congruent with the amount of evolved gas (average low error of 6 %). Scheme 1 displays all the different reaction pathways observed for ethanol dehydrogenation depending on the reaction conditions. It should be noted that this reaction can also be used for a novel process for acetic acid production. In fact, under optimized conditions the 5:5 mixture of EtOH/H2O (v/v) gave a highly selective 70 % conversion of ethanol to acetic acid within 20 h. Additionally, a long-term experiment demonstrated that complex 7 dehydrogenated an aqueous ethanol solution with excellent catalyst turnover numbers (80 000). After 98 h reaction time, the resulting hydrogen volume exceeded 7.8 L (Supporting Information, Figure S5). Finally, we used aqueous solutions of bioethanol (5 % H2O), which were produced by fermentation. To our delight, this fermented bioethanol showed similar catalyst activity (TOFs3h 1686 h 1) compared to the aqueous ethanol solutions (Table 4, entries 1–2). Notably, the bioethanol sample needed no further purification prior to use. This demonstrates the tolerance of the catalyst towards “real” fermented bioethanol. In conclusion, ruthenium PNP-pincer-type complexes are versatile catalysts for hydrogen release from aqueous ethanol and bioethanol. Aqueous-phase reforming of ethanol using molecularly defined catalysts proceeds under mild conditions (

Efficient and selective hydrogen generation from bioethanol using ruthenium pincer-type complexes.

Catalytic generation of hydrogen from aqueous ethanol solution proceeds in the presence of pincer-type transition metal catalysts. Optimal results are...
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