A Bipyridine-Based Conjugated Microporous Polymer for the Ir

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A Bipyridine-Based Conjugated Microporous Polymer for the Ir-Catalyzed Dehydrogenation of Formic Acid Cornelia Broicher, Severin Foit, Marcus Rose, Peter J.C. Hausoul, and Regina Palkovits ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02425 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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A Bipyridine-Based Conjugated Microporous Polymer for the IrCatalyzed Dehydrogenation of Formic Acid Cornelia Broichera, Severin R. Foitb, Marcus Rosea, Peter J.C. Hausoula* and Regina Palkovitsa* a

Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen (Germany) b Forschungszentrum Jülich Institut für Energie- und Klimaforschung Grundlagen der Elektrochemie (IEK-9) 52425 Jülich (Germany) Supporting Information Placeholder ABSTRACT: Formic acid is considered a promising energy storage medium and its selective dehydrogenation enables the generation of high purity H2. Herein we report a bipyridine-based conjugated microporous polymer (CMP) loaded with [Cp*IrCl2]2 for the base-free aqueous dehydrogenation of formic acid to H2/CO2. This catalyst exhibits high activity and selectivity at temperatures over 130°C and in formic acid concentrations as high as 10 M. Recycling tests demonstrate a low Ir leaching and a gradual increase in the activity over six runs and a low CO content in the gas phase about 138 ppm. TOF’s up to 123 894 h-1 were obtained using 0.1wt% Ir loading.

Keywords: formic acid, dehydrogenation , iridium, CMP, heterogeneous 1. INTRODUCTION Current energy production is mainly based on the combustion of fossil fuels, which causes the release of large amounts of CO2 into the atmosphere and contributes to global warming. To reduce emissions and avert climate change, the use of sustainable and CO2-neutral energy sources such as wind and solar energy is imperative. Nevertheless, due to the fluctuating nature of these energy supplies, efficient storage and conversion strategies are also necessary. A possible strategy is the conversion of excess electrical energy to chemical energy via e.g. the electrolysis of water to H2/O2. H2 is a promising energy carrier which can be used for combustion and for the chemical synthesis of liquid fuels.1 Nevertheless, storage of gaseous H2 in compressed form is energy intensive and introduces additional risks. Therefore, the conversion of H2 to a condensed state is preferred. In this regard, the direct hydrogenation of CO2 to formic acid (FA), formaldehyde/methane diol and methanol is of particular interest. Of these, FA is highly interesting as it can be easily synthesized from H2/CO2 and decomposed back by transition metal catalysts (Scheme 1). In addition FA can also be obtained by oxidative biomass processing (e.g. the Biofine process for levulinic acid production2 and OxFA process for the partial oxidation of biomass wastes to FA).3 The dehydrogenation of FA to H2/CO2 is well known and was already reported in 1967 by Coffey, who revealed that transition metal complexes based on Ir and Ru are active for the reaction.4 Over the past decades, several highly active catalysts have been identified.5

Scheme 1. Hydrogen storage and delivery though FA production and dehydrogenation

For example Beller et al. reported turnover numbers (TONs) up to 1,000 000 for [RuCl2(C6H6)]2 in the presence of DPPE.6 Recently, Huang et al. reported a TON of 1,100 000 for [(RuH(CO)(PN3pincer)].7 Iguchi et al. developed water-soluble IrCp* complexes and reported TONs up to 320 000.8 Onishi et al. achieved by the incorporation of a bidentate ligand that combines an imidazoline and a pyridine moiety a TON of 2,000 000.9 Celaje and co-workers presented a TON up to 2., 60 000 for [Ir(cod)2((di-tbutylphosphino)methyl)pyridine].10 Wang et al. achieved a maximum TON of 2,500 000 for bifunctional cyclometalated Ir(III) complexes with hydroxy substituted pyridine-azole and pyrimidine-azole/azoline ligands.11 Li and co-workers achieved a maximum TOF of 147 000 h-1 at 40°C with a bifunctional cyclometalated Ir(III) complex.12 The [Ir(Cp*)Cl2(thbpym)] (Cp* = pentamethylcyclopentadienyl) developed by Hull et al. showed a remarkable TOF of 228 000 h-1 at 90°C.13 Recently, Fink et al. reported a maximum TOF of 3300 h-1 for [(Cp*)M(N–N’)Cl] (M = Ir, Rh), which have been proven to be active in aqueous media at 60°C.14 These studies demonstrate that particularly Ruand Ir-based complexes with phosphorus and nitrogen-donor ligands provide the most active catalysts.15 The application of these catalysts in fuel cells has also been demonstrated.16 However from the point of view of continuous processing the use of heterogeneous catalysts is clearly preferred. To date several studies concerning the use of supported metal catalysts have been reported and mainly systems based on Au,17 Pd,18 and bimetallic Au/Pd,19 and Pd/Ag,20 show the best activities.21 For example, a Ag1Pd9/SBA-15-NR3 catalyst prepared by Wan et al. exhibited 100% H2 selectivity and an initial TOF of 964 h-1 at 70°C.22

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Scheme 2. Synthesis of CMP and Ir@CMP; Conditions: i) 0.2 eq (1), 1.0 eq (2), 0.8 eq. (3), 10 mL DMF, 10mL NEt3, 0.35 mol% CuI, 3.5 mol% [Pd(PPh3)4], 70°C, 24 h; ii) [Ir(Cp*)Cl2]2, MeOH, RT, 24 h.

A Schiff base modified Au catalyst by Liu et al. showed a TOF of 4368 h-1 at 50°C.17b Bi et al. reported a TOF of 5530 h-1for Pd on N-doped carbon at 25°C.17e This demonstrates that supported metal catalysts, although active, generally exhibit lower activities than homogeneous catalysts. To combine the advantages of heterogeneous and homogeneous catalysis, metal complexes immobilized on solid supports (e.g. SiO2, polystyrene (PS)) have been proposed. Immobilized catalysts should retain the high activity and selectivity of metal complexes and allow facile separation and recycling.23 Typically N- and P-donor based ligands are tethered to the solid support and loaded with metal complexes via coordination. For example Laurenzcy et al investigated Ru/TPPTS immobilized on PPh3-PS, DOWEX and Na-BEA.24 An initial TOF of 427 h-1 at 90°C was reported for Ru/TPPTS@PPh3-PS. Gan et al. reported a TOF of 2750 h-1 at 90°C for Ru/[email protected] Deng et al. reported TOF`s of 7357 h-1, 3295 h-1 and 1428 h-1 for RuCl3 supported on SiO2-SH, SiO2PPh3 and SiO2-NH2, respectively.26 The development of functional porous polymers opened up new opportunities in heterogeneous catalysis.27 Tailor made porous organic polymers (POPs) featuring high surface areas and extraordinary chemical and thermal stability have attracted considerable attention. A recent study by us on P-containing porous polymers demonstrated that they are promising catalysts for on-site hydrogen production in the biorefinery. The highest TOF (22900h-1) could be obtained using Ru@pDPPE at 160°C.28 Nevertheless, contrary to P-based ligands, N-based ligands are insensitive to oxidation and allow handling and storage of the catalyst in air. In addition, Ir systems are more efficient in FA dehydrogenation compared to Ru systems. Recently, Bavykina et al.29 reported an initial TOF of 27 000 h-1 at 80 °C using a [IrCp*(OH)](OTf)2 loaded covalent triazine framework (CTF). This shows that Ir/N based porous polymers have a high potential for FA dehydrogenation. Besides the cyclotrimerization method used for CTF materials, N-donor ligands can also be incorporated into porous polymers using cross-coupling reactions. E.g. conjugated microporous polymers (CMP) are a wellknown class of materials and can be prepared via the Sonogashira-Hagihara reaction.30 Here we propose the use of a bipyridine-based CMP loaded with [IrCp*Cl2]2 as catalyst for the aqueous, base-free dehydrogenation of FA. 2. RESULTS AND DISCUSSION 2.1 Catalyst synthesis and characterization CMP was prepared using the monomers, 5,5‘-Dibromo-2,2bipyridine (1), 1,3,5-triethynylbenzene (2), 1,4-dibromobenzene (3) (Scheme 2). Polymerization is catalyzed by [Pd(PPh3)4] and CuI in a mixture of N,N-dimethylformamide and NEt3 overnight at 70°C. After washing with CHCl3, water, MeOH and acetone, CMP was obtained as a yellow-brown powder. N2 physisorption of the CMP revealed a BET surface area of 706 m2g-1.

C 1s Intensity / u.a.

4000

Intensity / u.a.

1 2 3 4 5 6 7 8 9 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

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3000

200

3 eV Ir(III)

150

Ir 4f

100 50 68

2000

N 1s

66

64

62

60

Binding energy / eV

Cl 2p Br 3p

1000

Ir 4f

0 400

300

200

100

0

Binding energy / eV Figure 1. XPS survey spectrum of 4 wt.% Ir@CMP, spectra of C 1s; Ir 4f spectra, N 1s and O 1s spectra. The adsorption isotherm exhibits a hysteresis which, based on DFT pore size distribution, can be attributed to micropores and mesopores (c.a. 5 nm). This is comparable to the data of Cooper et al. who reported a BET surface area of 822 m2/g and similar isotherms.30a TGA shows that the polymer is thermally stable up to 350°C (Figure S1) and the XRD pattern shows very broad and weak reflexes at 25° and 45° 2θ, suggesting that the polymer is amorphous (Figure S2). ICP analysis of the material shows that ca. 0.1wt% Cu and 0.1 wt% Pd from the cross-coupling procedure are still present in the polymer matrix. The polymers were washed several times with strong acids (such as glacial acetic acid and 1 M HCl) however ICP confirmed that Cu and Pd still remained in the polymers. The CMP was loaded with different Ir concentrations (0.1wt% - 4.0 wt.%) by stirring with [IrCp*Cl2]2 in methanol over 2 days at room temperature. The Ir-loaded polymer (1 wt% Ir@CMP) possesses a significantly reduced specific surface area (482 m2/g, Figure S3) which is attributed to the partial pore filling. TEM image (Figure S4) shows that the polymers are composed of polydisperse particles. Ir nanoparticles were not observed suggesting a molecular dispersion of metal complex. The 13 C SS-NMR spectrum (Figure S5) exhibits signals at 150 ppm for the C=N of bipyridine, at 125-135ppm for the aromatic groups and at 90 ppm for the C≡C of ethynyl, confirming the proposed structure. X-ray photoelectron spectroscopy (XPS) of 4wt.% Ir@CMP revealed the presence of Ir, O, C, Br, Cl and N (Figure 1). The presence of Br is attributed to incomplete conversion of Ar-Br end groups of the CMP polymer. Signals corresponding to Ir 4f3/2 and Ir 4f1/2 are observed at 62.5 and 65.5 eV (spinenergy separation: 3 eV), indicating the presence of Ir(III) species.31 This is comparable to the Ir 4f signals observed for the complex [[IrCp*Cl2]2] (Figure S6). This together with the Cl signals confirms that the [IrCp*Cl2]2 is successfully immobilization onto the CMP support without reduction. 2.2 Dehydrogenation of FA The Ir@CMP material as well as CMP, Ir/C, [IrCp*Cl2]2 and the homogeneous analog [IrCp*Cl(2,2-bipy)]Cl were tested in the dehydrogenation of aqueous FA (10 wt%, 2.17 M) at 160°C. The results are summarized in Figure 2 and Table 1. A blank test gave only 2.2 % conversion after 60 minutes, indicating that thermal decomposition is negligible. Commercial Ir/C gave a similar conversion confirming that Ir nanoparticles are not active in the reaction. ICP analysis revealed that ca. 15 % of the introduced Ir had leached into the reaction solution. Also the precatalyst [IrCp*Cl2]2 gave only 4.1 % conversion, demonstrating the need for bipyridine ligands to achieve high activity. [IrCp*Cl2/2,2bipy] reached full conversion within 25 minutes and a TOF of 43051h-1 could be obtained.


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-8.8 2.25 2.3 2.35 2.4 2.45 2.5 -9.3 ln (k/T) (k-1)

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-9.8

-10.3 -10.8 -11.3

y = -10.827x + 15.645 R² = 0.993

-11.8

1000/T (K-1)

Figure 3. Reactor pressure profiles at different temperatures (left) and Eyring plot (right) Figure 2 Reactor pressure profiles of Ir-catalyzed dehydrogenation of aqueous formic acid at 160°C

Table 1. Ir-catalyzed dehydrogenation of aqueous formic acid at 160°C.[a] Catalyst

FA conv. [%]

TOF [h-1]

[CO](g) [ppm]

Ir loss [%]

Blank[b]

2.2

-

-

-

Ir/C[c]

2.2

-

-

15.0

[IrCp*Cl2]2[d]

4.1

-

-

-

[IrCp*Cl2]2/2,2-bipy[e]

99.9

43051

388

-

CMP[f]

13.1

n.d.

n.d.

-

1.0 wt% IrCl3@CMP[g]

99.8

19568

75

1.00

1.0 wt% Ir@CMP[a]

99.8

35246

117

1.10

[a] Conditions: 15 mL 10 wt% FA soln. (32.6 mmol FA), 0.008 mol% Ir, 160°C. [b] 90 min. [c] 50 mg Ir/C (1 wt%, 0.008 mol% Ir). [d] 2.25 mg, [e] 1.0 mg [IrCp*Cl2]2, 6.0 mg bipy, 0.008 mol% Ir. [f] 50 mg. [g] 50 mg IrCl3@CMP 0.008 mol% Ir, 160°C. Surprisingly the CMP material without Ir showed activity and resulted in 13.1 % conversion after 50 min. Figure 2 shows that the reactor pressure increases to 12 bar in 34 min, after which the pressure remains constant. As discussed above ICP data of the CMP materials showed traces of Cu and Pd. Hence it is proposed that the activity of CMP is likely caused by Pd.18 In contrast, Ir@CMP (1wt.% Ir) reached near full conversion (TON 12475) within 27 minutes with a maximum TOF of 35246 h-1. ICP of the reaction solution revealed that 1.1% Ir is leached into solution. A test with the filtered solution, to which fresh formic acid was added, gave no conversion, confirming that the leached species are inactive. GC analysis of the gas phase showed a high H2/CO2 selectivity with a CO impurity of 117 ppm. [IrCp*Cl2/2,2-bipy] demonstrated a similar catalyst performance compared to Ir@CMP (1wt.% Ir). Compared to P-containing porous polymers,28 the CO content of the gasphase was 8 times higher; however Ir@CMP showed a higher maximum TOF of 35246 h-1. To test if Ir(III) salts are also suitable for the reaction, we have loaded CMP with IrCl3 (IrCl3@CMP (1wt.% Ir). Also for this catalyst, the reaction reached full conversion (TON 12475) within 42 minutes with a maximum TOF of 19568 h-1.These results demonstrate that Ir@CMP is a highly active catalyst for FA dehydrogenation. The influence of reaction temperature was studied between 120 – 160 °C. The results are summarized in Figure 3 and Table 2.When the temperature is lowered to 130°C the TOF decreases to 3959 h-1. The CO concentration slightly increases to 152 ppm whereas the loss of Ir increases to 5.2%.

Table 2. Influence of reaction temperature on 1wt% Ir@CMP catalyzed FA dehydrogenation between 120 160°C.[a] Temp. (°C)

FA conv. [%]

TOF [h-1]

[CO](g) [ppm]

Ir-loss [%]

160

99.8

35246

117

1.1

150

99.8

13756

125

1.4

140

99.9

7488

144

1.7

130

99.9

3959

152

5.2

120

0.0

0

-

-

110[b]

98.9

3309

148

1.00

[a] Conditions: 15 mL 10 wt% FA soln. (32.6 mmol FA), 50 mg 1 wt% Ir@CMP, 0.008 mol% Ir, 160°C. [b] 3.24 mmol HCOONa, 50 mg Ir@CMP, 0.008 mol% Ir. Interestingly at 120°C no pressure build up was observed indicating that the reaction does not proceed or only very slowly. Using the Eyring equation the enthalpy of activation (∆H‡) was determined by linear regression from the plot of ln(k/T) vs 1000/T (Figure 3, right). The data indicate an apparent activation barrier of 90.0 kJ/mol with an entropy of activation (∆S‡) of -67.5 J/(mol·K). At a temperature of ca. 418 K this leads to an apparent free energy of activation (∆G‡) of 118.2 kJ/mol. The activation enthalpy found by for Ir/CMP is approx. 12 kJ/mol higher than that reported for [IrCp*(pyrazol pyrimidine)]+.11 The free energy of activation is in agreement with the energy spans calculated for [IrCp*(pyrazol pyrimidine)]+: 111.4-132.7 kJ/mol11 and [IrCp*(4,4-dihydroxy-2,2,-bipyridine)]+: 101.3 kJ/mol32 from reported DFT data. This suggests that Ir/CMP catalyzed FA dehydrogenation occurs via the same mechanism as the homogeneous analogs. It is known that the catalytic dehydrogenation of FA can be improved by the addition of formate.5 When the reaction at 130 °C is performed in the presence of 0.1 eq. sodium formate, the activity is increased by a factor of 1.8, yielding a TOF of 7291 h-1 (Figure S7). As such the temperature could be lowered to 110°C resulting a TOF of 3309 h-1 with 148 ppm CO in the the gas phase. At 100 °C, no pressure increase could be observed within the monitored time frame, again indicating a very slow reaction. The influence of FA concentration was tested while keeping the catalyst amount constant. The data (Figure S8 and Table 3) confirm an approximately linear dependence of the TOF on FA concentration between 10-50%. Interestingly, a low amount of CO was produced at 50% FA-solution (140 ppm) and a low Ir-catalyst (99.9%) and a negligible solubility of gasses. Pressure diagrams were converted to formic acid conversion diagrams using the formula: FA Conv. = 1-(P(t)/P(final)) The rate constants (k (min-1)) were determined by linear fitting. The turnover frequency (TOF) was calculated using the formula: TOF (h-1) = k * 60 min * NFA / NIr AUTHOR INFORMATION

Corresponding Author [email protected] [email protected] ASSOCIATED CONTENT

Supporting Information Characterization of CMP with and without metal loading and after catalysis including XRD, TGA, N2-physisorption, solid state NMR, ICP, XPS and STEM(-EDX); graphical representation of the impact of FA concentration: This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT The authors acknowledge financial support by the European Regional Development Fund (ERDF) and the state of NorthRhine Westphalia, Germany under the operational program ‘’Regional Competitiveness and Employment’’ Project ‘’Sustainable Chemical Synthesis’’ and the Federal Ministry of Education and Research (BMBF) for funding this work with the MANGAN research cluster BMBF-PTJ FKz 03SF0508.

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ACS Catalysis

Trans. 1981, 77, 2451–2463. (d) Hattori, M.; Shimamoto, D.; Ago, H.; Tsuji, M. J. Mater. Chem. A 2015, 3, 10666–10670. (e) Cho, J.; Lee, S.; Han, J.; Yoon, S. P.; Nam, S. W.; Choi, S. H.; Lee, K.; Ham, H. C. J. Phys. Chem. C 2014, 118, 22553. (f) Zhou, X.; Huang, Y.; Xing, W.; Liu, C.; Liao, J.; Lu, T. Chem. Commun. 2008, 3540–3542. (g) Wang, W.; He, T.; Liu, X.; He, W.; Cong, H.; Shen, Y.; Yan, L.; Zhang, X.; Zhang, J.; Zhou, X. ACS Appl. Mater. Interfaces 2016, 8, 20839–20848. (21) (a) Fellay, C.; Yan, N.; Dyson, P. J.; Laurenczy, G. Chem. Eur. J 2009, 15, 3752–3760. (b) Grasemann, M.; Laurenczy, G. Energy Environ. Sci. 2012, 5, 8171–8181. (c) Zhao, Y.; Deng, L.; Tang, S.-Y.; Lai, D.-M.; Liao, B.; Fu, Y.; Guo, Q.-X. Energy Fuels 2011, 25, 3693–3697. (22) Wan, C.; Yao, F.; Li, X.; Hu, K.; Ye, M.; Xu, L.; An, Y. ChemistrySelect 2016, 1, 6907–6913. (23) F.R. Hartley. Supported Metal Complexes: A New Generation of Catalysts, 1st ed.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1985. (24) Gan, W.; Dyson, P. J.; Laurenczy, G. React. Kinet. Catal. Lett. 2009, 98, 205–213. (25) Gan, W.; Dyson, P. J.; Laurenczy, G. ChemCatChem 2013, 5, 3124–3130. (26) (a) Deng, L.; Zhao, Y.; Li, J.; Fu, Y.; Liao, B.; Guo, Q. X. ChemSusChem 2010, 3, 1172–1175. (b) Deng, L.; Li, J.; Lai, D. M.; Fu, Y.; Guo, Q. X. Angew. Chem. Int. Ed. 2009, 48, 6529–6532. (27) (a) Rose, M. ChemCatChem 2014, 6, 1166–1182. (b) Jiang, J.-X.; Wang, C.; Laybourn, A.; Hasell, T.; Clowes, R.; Khimyak, Y. Z.; Xiao, J.; Higgins, S. J.; Adams, D. J.; Cooper, A. I. Angew. Chem. Int. Ed. 2011, 50, 1072–1075. (28) Hausoul, P. J. C.; Broicher, C.; Vegliante, R.; Göb, C.; Palkovits, R. Angew. Chem. Int. Ed. 2016, 55, 5597–5601. (29) Bavykina, V.; Goesten, M. G.; Kapteijn, F.; Makkee, M.; Gascon, J. ChemSusChem 2015, 8, 809–812. (30) (a) Jiang, J. X.; Su, F.; Trewin, A.; Wood, C. D.; Niu, H.; Jones, J. T. A.; Khimyak, Y. Z.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 7710– 7720. (b) Jiang, J.-X.; Su, F.; Trewin, A.; Wood, C. D.; Campbell, N. L.; Niu, H.; Dickinson, C.; Ganin, A. Y.; Rosseinsky, M. J.; Khimyak, Y. Z.; Cooper, A. I. Angew. Chem. Int. Ed. 2007, 46, 8574–8578. (c) Kuhn, P.; Forget, A.; Su, D.; Thomas, A.; Antonietti, M. J. Am. Chem. Soc. 2008, 130, 13333–13337. (d) Chen, L.; Honsho, Y.; Seki, S.; Jiang, D. J. Am. Chem. Soc. 2010, 132, 6742–6748. (e) Xu, Y.; Jin, S.; Xu, H.; Nagai, A.; Jiang, D. Chem. Soc. Rev. 2013, 42, 8012–8031. (31) J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben. Handbook of X-ray Photoelectron Spectroscopy; Corporation, Phys. Electr. Division. (32) Ertem, M. Z.; Himeda, Y.; Fujita, E.; Muckerman, J. T. ACS Catal. 2016, 6, 600–609. (33) Bower, J. F.; Krische, M. J. Iridium Catalysis; 2011; Vol. 34. (34) Road, M.; Krische, M. J. Top. Organomet. Chem. 2011, 34, 107– 138. (35) Beves, J. E.; Leigh, D. A.; Mcburney, R. T.; Rissanen, K.; Schultz, D. Nat. Chem. 2012, 4, 2–7.


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A bipyridine-based conjugated microporous polymer (CMP) loaded with [Cp*IrCl2]2 is proposed for the base-free aqueous dehydrogenation of formic acid to H2/CO2. This catalyst exhibits high activity and selectivity under a wide range of conditions and Recycling tests demonstrate facile catalyst separation with low Ir leaching. TOF’s up to 123 894 h-1 were obtained using 0.1wt% Ir loading.


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A Bipyridine-Based Conjugated Microporous Polymer for the Ir

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