Additive-Free Hydrogen Generation from Formic Acid Boosted by

Publication Date (Web): June 19, 2018 ... of formic acid (FA) is an efficient approach to store and release hydrogen in fuel-cell-based hydrogen econo...
0 downloads 0 Views 6MB Size
Download PDF
Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Additive-Free Hydrogen Generation from Formic Acid Boosted by Amine-Functionalized Imidazolium-Based Ionic Polymers Muhammad Asad Ziaee,†,‡ Hong Zhong,† Caiyan Cui,†,‡ and Ruihu Wang*,† †

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by STEPHEN F AUSTIN STATE UNIV on 07/20/18. For personal use only.

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, China S Supporting Information *

ABSTRACT: Catalytic dehydrogenation of formic acid (FA) is an efficient approach to store and release hydrogen in fuel-cell-based hydrogen economy; it is still a daunting challenge to the design and synthesis of the additive-free heterogeneous catalytic systems. In this contribution, we present an amine-functionalized main-chain imidazolium-based ionic polymer (ImIP-1) for boosting additive-free hydrogen generation from FA. The ultrafine palladium nanoparticles (NPs) with uniform dispersion over ImIP-1 were readily obtained through simple anion exchange between chloride in ImIP-1 and tetrachloropalladate and subsequent reduction with NaBH4. The palladium NPs are synergetically stabilized by coordination interaction and electrostatic effect from ImIP-1. The amine groups in the host backbone of ImIP-1 serve as basic sites to accelerate the cleavage of O−H bond in FA. The catalytic system shows outstanding catalytic activity, high stability, and excellent recyclability in additive-free heterogeneous FA dehydrogenation under mild conditions. The initial TOF values at 50 and 25 °C are as high as 1593 and 356 h−1, respectively, which are 10 times higher than those in its counterpart without amine groups. The impressive catalytic performance ranks it among the state-of-the-art of those in heterogeneous catalytic systems based on supported palladium NPs. KEYWORDS: Ionic polymers, Imidazolium, Palladium nanoparticles, Heterogeneous catalysis, Hydrogen generation

■

INTRODUCTION Given growing consumption of fossil fuels and negative environmental impacts related to the burning of fossil fuels, seeking sustainable, clean, and renewable energy sources is highly attractive.1−3 Hydrogen has been regarded as one of the most promising alternatives for fossil fuels,and has been largely used in fuel-cell-based technology owing to its advantages in terms of high gravimetric energy density, renewability, and benign burning.4−6 However, its safe storage and transport are still a major challenge for current development of hydrogen energy.7,8 Formic acid (FA, HCOOH) is a convenient and safe hydrogen carrier.9,10 Hydrogen stored in FA molecules could be efficiently released through either a dehydrogenation or a dehydration pathway in the presence of various catalysts.11 The reactivity and selectivity of FA dehydrogenation are closely associated with the catalysts used.12 It is a pressing requirement to explore the catalytic systems for efficient and selective hydrogen generation from FA under mild conditions. Supported metal nanoparticles (NPs) on the solid materials have captured tremendous interest in heterogeneous FA dehydrogenation. The supports have usually exerted crucial effects on the size, dispersion, and catalytic activity of metal NPs. Various materials, such as metal oxide,13 reduced graphene oxide,14 and macroreticular basic resin,15 have been © XXXX American Chemical Society

used as the supports of metal NPs in hydrogen generation from FA and shown promising catalytic activity, but these heterogeneous catalytic systems usually require the presence of extra additives, such as triethylamine and sodium formate, which not only lowers the gravimetric energy density of FA but also increases its cumber in processing. These disadvantages greatly mitigate the conception of green chemistry and sustainable development for hydrogen generation from FA.16−18 Therefore, it is highly desirable for developing new types of supports containing specific functional groups to replace the additives. The incorporation of basic groups into the matrix of the supports to serve as cocatalytic species is regarded as a feasible method in the additive-free hydrogen generation from FA.6,16,19 The main-chain imidazolium-based ionic polymers (ImIPs) have recently attracted considerable attention in heterogeneous catalysis because of their unique ionic structure, flexible synthetic strategy, and ready functionality. ImIPs are promising supports to the effective stabilization and uniform dispersion of metal NPs.21−23 The relatively weak electrostatic interaction Received: April 19, 2018 Revised: June 13, 2018 Published: June 19, 2018 A

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration for the Synthesis of Pd/ImIP-1 and Pd/ImIP-2

thoroughly using H2O and dried under vacuum at 80 °C for 12 h. The suspension of the above palladium polymer in water (20 mL) was added into NaBH4 (0.60 mmol, 22 mg); the mixture was stirred at 25 °C for 2 h. The resultant powders were collected by filtration, washed using copious water, and dried under vacuum at 80 °C for 12 h. Yield: 351 mg (81%). FTIR (KBr cm−1): 3111 (s), 2948 (w), 1617 (s), 1504 (s), 1420 (w), 1292 (s), 1254 (s), 1116 (m), 1081 (m), 1017 (s), 907 (w), 824 (w), 759 (m), 652 (w). Synthesis of ImIP-2. A mixture of 1,3,5-tri(1H-imidazol-1yl)benzene (1.0 mmol, 276 mg) and 1,5-dichloropentane (1.5 mmol, 191 μL) in DMF (15 mL) was stirred at 100 °C for 24 h to give rise to light yellow precipitates. The precipitates were collected by centrifugation, washed using DMF (3 × 30 mL), MeOH (3 × 30 mL), and H2O (3 × 30 mL), Soxhlet extracted with CH2Cl2 for 12 h, and dried under vacuum at 80 °C for 12 h. Yield: 396 mg (81%). Anal. Calcd (%) for C42H60N12Cl6: C, 53.34; H, 6.40; N, 17.77. Found: C, 47.59; H, 5.96; N, 15.58. FTIR (KBr cm−1): 3083 (w), 1621 (s), 1525 (s), 1506 (m), 1454 (m), 1363 (w), 1206 (m), 1093 (m), 1007 (m), 859 (m), 751 (m), 671 (w), 652(w). Synthesis of Pd/ImIP-2. A mixture of ImIP-2 (430 mg) and Na2PdCl4 (0.057 mmol, 17 mg) in water (20 mL) was vigorously stirred at 25 °C for 10 h. The resultant product was washed thoroughly using H2O and dried under vacuum at 80 °C for 12 h. The suspension of the above palladium polymer in water (20 mL) was added into NaBH4 (0.60 mmol, 22 mg); the mixture was stirred at 25 °C for 2 h. The resultant powders were collected by filtration, washed using copious water, and dried under vacuum at 80 °C for 12 h. Yield: 371 mg (86%). FTIR (KBr cm−1): 3083 (w), 1621 (s), 1525 (s), 1506 (m), 1454 (m), 1365 (w), 1206 (m), 1093 (m), 1007 (m), 861 (m), 751 (m), 671 (w), 652 (w). General Procedures for Dehydrogenation of Formic Acid. Hydrogen generation from aqueous FA solution was conducted in a 10 mL vessel at a preset temperature (25−50 °C) under ambient atmosphere. In general, 30 mg of catalyst was first placed in the vessel with H2O (1.0 mL); then aqueous formic acid solution (1.0 mL, 2.0 M) was injected quickly. The volume of released gas was measured through connecting the reaction vessel to a gas burette filled with water. Durability Testing of Pd/ImIP-1. After FA dehydrogenation was finished, the catalytically active species in the aqueous phase was separated by simple filtration, washed with water and ethanol, dried under vacuum at 80 °C, and then directly used in the next run reaction with the fresh aqueous FA solution.

between their host cationic backbones and charge-balanced halide ions allows for facile exchange with metal-containing anionic precursors,24 which provides the possibility for tunable loading and homogeneous distribution of metal NPs in ImIPs. In the meantime, the coordination and electrostatic effect based on positive-charged imidazolium moieties and counteranions could interact with metal NPs, resulting in the suppression of agglomeration during preparation and subsequent application in heterogeneous catalytic reactions.25 The most attractive approach, the flexible synthetic strategy of ImIPs, allows the basic groups to be integrated evenly into the host backbones of ImIPs through judicial selection of building units containing electron-donating groups, which could deprotonate FA to replace the additives, facilitating cleavage of the O−H bond in FA.20,26 For the sake of these attractive merits, main-chain ImIPs have been highly recommended as heterogeneous supports of metal NPs in the additive-free hydrogen generation from FA. As a proof of concept, herein, we present an aminefunctionalized ImIP (ImIP-1), which is used as a support to immobilize ultrafine palladium NPs (1.5 ± 0.7 nm) for additive-free hydrogen generation from FA under mild conditions. Amine groups in ImIP could deprotonate FA, which significantly boosts FA dehydrogenation without any additives. ImIP-1 shows much higher catalytic activity than its counterpart without amine groups (ImIP-2).

■

EXPERIMENTAL SECTION

Synthesis of ImIP-1. 1,3,5-Tri(1H-imidazol-1-yl)benzene (1.0 mmol, 276 mg) and bis(2-chloroethyl)amine (1.5 mmol, 213 mg) were dissolved in DMF (50 mL). White precipitates were obtained after the mixture was stirred at 100 °C for 24 h. The precipitates were collected by centrifugation, washed using DMF (3 × 30 mL), MeOH (3 × 30 mL), and H2O (3 × 30 mL), Soxhlet extracted using CH2Cl2 for 12 h, and dried under vacuum at 80 °C. Yield: 408 mg (83%). Anal. Calcd (%) for C39H57N15Cl6 : C, 49.38; H, 6.06; N, 22.15. Found: C, 48.72; H, 5.16; N, 21.45. FTIR (KBr, cm−1): 3111 (s), 2942 (w), 1617 (s), 1504 (s), 1420 (w), 1292 (s), 1249 (s), 1116 (m), 1081 (m), 1011 (s), 907 (w), 824 (w), 759 (m), 652 (w). Synthesis of Pd/ImIP-1. A mixture of ImIP-1 (430 mg) and Na2PdCl4 (0.057 mmol, 17 mg) in water (20 mL) was vigorously stirred at 25 °C for 10 h. The resultant product was washed B

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. FTIR spectra for (a) BCEA, TImB, ImIP-1, Pd/ImIP-1, and Pd/ImIP-1−5run and (b) DCPT, TImB, ImIP-2, and Pd/ImIP-2. Solidstate 13C NMR spectra for (c) ImIP-1 and Pd/ImIP-1 and (d) ImIP-2 and Pd/ImIP-2. (e) N 1s XPS spectra for ImIP-1, Pd/ImIP-1, Pd/ImIP-1− 5run, ImIP-2, and Pd/ImIP-2. (f) Pd 3d XPS spectra for Pd/ImIP-2, Pd/ImIP-1, and Pd/ImIP-1−5run.

■

transform infrared spectroscopy (FTIR), solid-state 13C NMR, and elemental analysis. As displayed in Figure 1a and 1b, the moderate peaks at 680 and 721 cm−1 correspond to the stretching vibration of the C−Cl bond in BCEA and DCPT, respectively. These peaks disappear in the FTIR spectra of ImIP-1 and ImIP-2. In addition, the typical peaks of the methylene groups at 1420 cm−1 in BCEA and at 1454 cm−1 in DCPT are greatly attenuated in ImIP-1 and ImIP-2. These results suggest the occurrence of a quaternization reaction of TImB with BCEA and DCPT. In the solid-state 13C NMR spectrum of ImIP-1, the characteristic peak of the methylene carbon atom occurs at 55 ppm, while the typical peaks appear at 48 and 26 ppm in ImIP-2 (Figure 1c and 1d). The other signals at 108−138 ppm are assigned to the carbon atoms of the benzyl and imidazolium rings. Noticeably, the FTIR and solid-state 13C NMR spectra of Pd/ImIP-1 and Pd/ImIP-2 are similar to those of ImIP-1 and ImIP-2, respectively, revealing that their structural backbones are retained after the loading of palladium NPs. Elemental analyses of ImIP-1 and ImIP-2 indicate that the experimental N and C values are slightly lower than their respective theoretical values, which is attributed to the existence of trapped solvent molecules. However, the experimental C/N molar ratios of ImIP-1 and ImIP-2 are

RESULTS AND DISCUSSION The synthetic route for task-specific ImIP-1 is presented in Scheme 1. The facile quaternization reaction between 1,3,5tri(1H-imidazol-1-yl)benzene (TImB) and bis(2-chloroethyl)amine (BCEA) in DMF generated ImIP-1. The formed precipitates were separated by centrifugation and washed with DMF, water, and methanol. The obtained solid was further treated by Soxhlet extraction with dichloromethane. In order to validate the role of amine groups in hydrogen generation from FA, ImIP-2 was synthesized by the reaction of TImB and 1,5-dichloropentane (DCPT), in which the amine group is replaced by a methylene group. As expected, both ImIP-1 and ImIP-2 show excellent dispersibility in water due to the abundant imidazolium group (Figure S1), which is conducive for mass transfer for catalytic reactions in water. The treatment of ImIP-1 and ImIP-2 with aqueous Na2PdCl4 solution and subsequent reduction by NaBH4 afforded Pd/ ImIP-1 and Pd/ImIP-2, respectively. Palladium contents in Pd/ImIP-1 and Pd/ImIP-2 were determined by inductively coupled plasma (ICP) analysis; they are 0.51 and 0.56 mmol g−1, respectively. The chemical compositions and structures of ImIP-1, ImIP2, Pd/ImIP-1, and Pd/ImIP-2 were defined by FourierC

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

interaction of amine groups with palladium NPs. The size and distribution of these palladium NPs are comparable with those supported by pyridinic-nitrogen-doped carbon11 and poly(ionic liquids) with the similar palladium loading,22 but they are much smaller and more uniform than those supported by silica nanosphere,31,32 mesoporous La2O3,33 and metal− organic frameworks,34,35 which mainly originate from abundant imidazolium moieties, amine groups, and counteranions in ImIP-1. The palladium NPs were synergetically stabilized through electrostatic and coordination interactions from them. The small size and excellent dispersion of palladium NPs in Pd/ImIP-1 and Pd/ImIP-2 have been further demonstrated in their high-annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) mapping images. Notably, no apparent characteristic diffraction peaks are detected in the powder X-ray diffraction (XRD) patterns of Pd/ImIP-1 and Pd/ImIP-2, which is probably due to small size and low content of palladium combined with the amorphous nature of ImIPs (Figure S3). To identify the surface interactions between palladium species and the supports, X-ray photoelectron spectroscopy (XPS) of ImIPs and Pd/ImIPs was performed. In the highresolution N 1s XPS spectrum of ImIP-1, N 1s peaks at 400.30 and 398.16 eV are attributed to imidazolium and amine nitrogen atoms,6 respectively (Figure 1e). The binding energy peak of the amine nitrogen atoms in Pd/ImIP-1 shifts toward a higher binding energy of 398.29 eV when compared with that in ImIP-1, which suggests electron donation from the amine groups to palladium species owing to their coordination. In the high-resolution Pd 3d XPS spectra, the Pd 3d region is composed of two spin−orbital pairs assigned to Pd 3d5/2 and Pd 3d3/2 (Figure 1f). The Pd 3d5/2 peaks at 335.20 and 335.32 eV are ascribed to Pd(0) species of Pd/ImIP-1 and Pd/ImIP-2, respectively.33 The ratios of Pd(0) to Pd(II) in Pd/ImIP-1 and Pd/ImIP-2 are 0.84 and 0.71, respectively. In comparison with that of Pd/ImIP-2, a negative shift of 0.12 eV in Pd/ImIP-1 further reveals the coordination of amine groups with palladium species in Pd/ImIP-1. The catalytic performance of Pd/ImIP-1 and Pd/ImIP-2 was examined by FA dehydrogenation under additive-free conditions. The dehydrogenation reaction was initially performed using different FA concentrations in the range of 0.25−6.0 M by fixing the molar ratio of [Pd]/FA at 0.0076 and the temperature at 50 °C. The volume of generated CO2 + H2 versus time over Pd/ImIP-1 at different FA concentrations was shown in Figure 4a. The gas generation volume enhances as FA concentration increases from 0.25 to 1.0 M, but decreases when FA concentration is increased over 1 M, revealing that a large amount of water is conducive to the catalytic dehydrogenation of FA. Impressively, the complete conversion of FA in the aqueous FA solution of 1.0 M is achieved within 16 min, and the initial turnover frequency (TOF) value is 1593 h−1 (Figure 4c). Pd/ImIP-2 also shows the highest catalytic activity in 1.0 M (Figure 4b), but the initial TOF value is 157 h−1, which is as low as one-tenth of that in Pd/ImIP-1. The much higher catalytic activity in Pd/ImIP-1 is mainly originated from the presence of amine groups, which deprotonates FA molecules. The selectivity for FA decomposition was analyzed by gas chromatography (GC); there is no evidence of CO formation in Pd/ImIP-1 when the produced gas was detected (Figure 4d), revealing superior selectivity of this catalytic system toward FA dehydrogenation. The effect of temperature on FA dehydrogenation was also

2.60 and 3.50, respectively, which are similar to the corresponding theoretical values of 2.65 and 3.55, respectively. Thermogravimetric analyses (TGA) further confirm the existence of trapped solvent molecules.27−29 The initial weight losses before 150 °C in ImIP-1 and ImIP-2 are 2.8% and 6.9%, respectively. ImIP-1 and ImIP-2 are stable before 260 and 240 °C, respectively (Figure S2). In scanning electron microscopy (SEM) images, ImIP-1 shows a regular wire-shaped morphology due to the orientation function of hydrogen bonds between the amine group of BCEA and the imidazolyl group of TImB (Figure 2a),30 while ImIP-2 is composed of tiny

Figure 2. SEM images of ImIP-1 (a), Pd/ImIP-1 (b), ImIP-2 (c), and Pd/ImIP-2 (d).

particles (Figure 2c). Notably, the loading of palladium NPs has not exerted any obvious effects on the morphology of ImIP-1 and ImIP-2 (Figure 2b and 2d). In the transmission electron microscope (TEM) images of Pd/ImIP-1 and Pd/ImIP-2, well-dispersed palladium NPs could be observed clearly (Figure 3). The average diameter of palladium NPs in Pd/ImIP-1 is 1.5 ± 0.7 nm, which is much smaller than those of 2.3 ± 0.7 nm in Pd/ImIP-2. The smaller size of palladium NPs in ImIP-1 is ascribed to the coordination

Figure 3. TEM, HRTEM, HAADF-STEM, and EDX mapping images for (a−c) Pd/ImIP-1 and (d−-f) Pd/ImIP-2. D

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. Gas generated from different concentrations of aqueous FA solution catalyzed by (a) Pd/ImIP-1 and (b) Pd/ImIP-2. (c) Initial TOF values of Pd/ImIP-1 at different concentrations. (d) GC spectra for (I) commercial H2, (II) gas generated from aqueous FA solution in the presence of Pd/ImIP-1 at 50 °C, and (III) commercial pure CO. (e) Gas generated from aqueous FA solution in the presence of Pd/ImIP-1 at different temperatures. (f) Arrhenius plots of Pd/ImIP-1 with initial TOF values. Reaction conditions: 2 mmol of FA, 15.2 μmol of Pd at 50 °C.

In sharp contrast, Pd/ImIP-2 gave significantly enhanced catalytic activity in the presence of equivalent SF. The reaction finished in 28 min; the initial TOF is enhanced from 157 to 956 h−1. These results reveal that the basic amine groups in Pd/ImIP-1 are effective and sufficient to deprotonate FA with the concomitant formation of metal−formate species, thus validating the importance of the basic functional group in ImIPs in the additive-free hydrogen generation from FA. Durability of Pd/ImIP-1 in hydrogen generation from FA was carried out using 1.0 M aqueous FA solution under additive-free conditions (Figure 5b). After the reaction was finished, the catalyst was separated from solution and directly used for the next run using fresh aqueous FA solution. Remarkably, the complete dehydrogenation of FA was performed at least for 5 runs; no apparent decrement of catalytic activity was detected. After the first run, the filtrate was examined by ICP analysis; negligible leaching of palladium was detected. This catalytic system was further investigated after consecutive catalytic reaction for five runs, and the recovered sample was assigned to Pd/ImIP-1−5run. The SEM image shows that the morphology of Pd/ImIP-1−5run is the

investigated in the absence of any additives. The timedependent hydrogen generation in the aqueous FA solution of 1 M at different temperatures is recorded in Figure 4e. The hydrogen generation amount was greatly enhanced when the reaction temperatures were increased from 25 to 50 °C, revealing that FA dehydrogenation could be accelerated at a high reaction temperature. Impressively, Pd/ImIP-1 could catalyze the additive-free FA dehydrogenation at 25 °C; the initial TOF of 356 h−1 is comparable with the state-of-the-art of those in reported heterogeneous palladium catalytic systems (Table S1). The apparent activation energy (Ea) in the reaction is determined as 46.50 kJ mol−1 based on the Arrhenius plot (Figure 4f), which is lower than the majority of the reported catalytic systems for the hydrogen generation from FA.6,14,20,32,36−,38 It is well established that FA dehydrogenation over heterogeneous catalysts could be promoted by the use of extra additives, for example, sodium formate (SF). The effect of the additives on Pd/ImIP-1 was tested under optimized conditions. Figure 5a illustrates that the use of equivalent SF has no detectable effect on the catalytic activity of Pd/ImIP-1. E

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. (a) Gas generated from aqueous FA solution with SF catalyzed by Pd/ImIP-1 and Pd/ImIP-2. Reaction conditions: FA (2 mmol, 1 M), Pd (15.2 μmol, Pd/FA = 0.0076), SF (2 mmol, 1 M) or without SF at 50 °C. (b) Recyclability test of Pd/ImIP-1 for FA dehydrogenation. Reaction conditions: FA (2 mmol, 1 M), Pd (15.2 μmol, Pd/FA = 0.0076) at 50 °C. (c) SEM, (d), and (e) TEM images for Pd/ImIP-1−5run.

same as that of Pd/ImIP-1 (Figure 5c). TEM images show that palladium NPs in Pd/ImIP-1−5run maintain high dispersion, their average size slightly enhances to 2.4 ± 0.8 nm, but no apparent agglomeration is observed (Figure 5d and 5e), suggesting that synergetic coordination and electrostatic interactions in Pd/ImIP-1 could effectively immobilize palladium NPs. The surface palladium NPs in Pd/ImIP-1− 5run was analyzed by XPS spectra (Figure 1); there is no appreciable binding energy difference in the high-resolution Pd 3d and N 1s XPS spectra when compared with as-synthesized Pd/ImIP-1, but the Pd(0)/Pd(II) ratio slightly enhances to 0.89, which is attributed to further reduction of residual Pd(II) during catalytic dehydrogenation of FA (Figure 1f). On the basis of the aforementioned results and previously related reports,39−41 a plausible mechanism of FA dehydrogenation over Pd/ImIP-1 in the absence of additives has been proposed (Scheme 2). The initial step of this reaction is involved in the dissociation of O−H bond in FA. Pd/ImIP-1 possesses a large amount of amine groups with periodical distribution; they function as Lewis basic sites to cleavage of the O−H bond of FA, forming protonated amine and metal− formate species. Subsequently, the C−H bond of formate species is catalytically activated by the adjacent palladium NPs supported by ImIP-1, generating CO2 and H2 with the recovery of catalyst for deprotonation and catalytic dehydrogenation of next FA molecule. The elimination of β-hydride from the palladium−formate species is known to be a ratedetermining step in FA dehydrogenation; the amine groups in ImIP-1 are enough to deprotonate FA and guarantee smooth implementation of the catalytic reaction, thus resulting in high catalytic activity under additive-free conditions.

Scheme 2. Possible Mechanism of FA Dehydrogenation Catalyzed by Pd/ImIP-1

palladium NPs are ready available through conventional methods; they are effectively immobilized and uniformly dispersed in ImIP-1. The catalytic system shows outstanding catalytic activity, high selectivity, and excellent recyclability in heterogeneous FA dehydrogenation under additive-free conditions. The promising catalytic performance even could be achieved at room temperature, which can compete with those in reported supported palladium NPs. The amine groups in the backbone of ImIP-1 are validated to perform multiple functions in the catalytic system: (1) The amine groups have exerted a crucial effect on the formation of wire-shaped ImIP1; the morphology is much different from the reported spherical and tiny particle morphology, which is conducive for mass transfer of reaction substrates. (2) The amine groups promote the formation of ultrafine palladium NPs and uniform dispersion in ImIP-1. The size of 1.5 ± 0.7 nm is smaller than

■

CONCLUSION An amine-functionalized ImIP-1 has been prepared and used as a support to immobilize palladium NPs in the additive-free hydrogen generation from FA for the first time. The ultrafine F

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

(8) Jiang, K.; Xu, K.; Zou, S.; Cai, W. B. B-Doped Pd Catalyst: Boosting Room-Temperature Hydrogen Production from Formic Acid-Formate Solutions. J. Am. Chem. Soc. 2014, 136 (13), 4861. (9) Yan, J. M.; Wang, Z. L.; Gu, L.; Li, S. J.; Wang, H. L.; Zheng, W. T.; Jiang, Q. AuPd-MnOx/MOF-Graphene: An Efficient Catalyst for Hydrogen Production from Formic Acid at Room Temperature. Adv. Energy Mater. 2015, 5 (10), 1500107. (10) Wang, W. H.; Ertem, M. Z.; Xu, S.; Onishi, N.; Manaka, Y.; Suna, Y.; Kambayashi, H.; Muckerman, J. T.; Fujita, E.; Himeda, Y. Highly Robust Hydrogen Generation by Bioinspired Ir Complexes for Dehydrogenation of Formic Acid in Water: Experimental and Theoretical Mechanistic Investigations at Different pH. ACS Catal. 2015, 5 (9), 5496. (11) Bi, Q. Y.; Lin, J. D.; Liu, Y. M.; He, H. Y.; Huang, F. Q.; Cao, Y. Dehydrogenation of Formic Acid at Room Temperature: Boosting Palladium Nanoparticle Efficiency by Coupling with PyridinicNitrogen-Doped Carbon. Angew. Chem., Int. Ed. 2016, 55 (39), 11849. (12) Wang, Z. L.; Yan, J. M.; Ping, Y.; Wang, H. L.; Zheng, W. T.; Jiang, Q. An Efficient CoAuPd/C Catalyst for Hydrogen Generation from Formic Acid at Room Temperature. Angew. Chem., Int. Ed. 2013, 52 (16), 4406. (13) Karatas, Y.; Bulut, A.; Yurderi, M.; Ertas, I. E.; Alal, O.; Gulcan, M.; Celebi, M.; Kivrak, H.; Kaya, M.; Zahmakiran, M. PdAu-MnOx Nanoparticles Supported on Amine-Functionalized SiO2 for the Room Temperature Dehydrogenation of Formic Acid in the Absence of Additives. Appl. Catal., B 2016, 180, 586. (14) Song, F. Z.; Zhu, Q. L.; Tsumori, N.; Xu, Q. Diamine-Alkalized Reduced Graphene Oxide: Immobilization of Sub-2 nm Palladium Nanoparticles and Optimization of Catalytic Activity for Dehydrogenation of Formic Acid. ACS Catal. 2015, 5 (9), 5141. (15) Mori, K.; Dojo, M.; Yamashita, H. Pd and Pd-Ag Nanoparticles within a Macroreticular Basic Resin: An Efficient Catalyst for Hydrogen Production from Formic Acid Decomposition. ACS Catal. 2013, 3 (6), 1114. (16) Wen, M.; Mori, K.; Kuwahara, Y.; Yamashita, H. Plasmonic Au@Pd Nanoparticles Supported on a Basic Metal-Organic Framework: Synergic Boosting of H2 Production from Formic Acid. ACS Energy Lett. 2017, 2 (1), 1. (17) Sponholz, P.; Mellmann, D.; Junge, D. H.; Beller, M. Towards a Practical Setup for Hydrogen Production from Formic Acid. ChemSusChem 2013, 6 (7), 1172. (18) Wan, C.; An, Y.; Xu, G. H.; Kong, W. J. Study of Catalytic Hydrogenation of N-Ethylcarbazole Over Ruthenium Catalyst. Int. J. Hydrogen Energy 2012, 37 (17), 13092. (19) Yurderi, M.; Bulut, A.; Caner, N.; Celebi, M.; Kaya, M.; Zahmakiran, M. Amine Grafted Silica Supported CrAuPd Alloy Nanoparticles: Superb Heterogeneous Catalysts for the Room Temperature Dehydrogenation of Formic Acid. Chem. Commun. 2015, 51 (57), 11417. (20) Sun, J. K.; Antonietti, M.; Yuan, J. Y. Nanoporous Ionic Organic Networks: from Synthesis to Materials Applications. Chem. Soc. Rev. 2016, 45 (23), 6627. (21) Grygiel, K.; Kirchhecker, S.; Gong, J.; Antonietti, M.; Esposito, D.; Yuan, J. Main-Chain Polyimidazolium Polymers by One-Pot Synthesis and Application as Nitrogen-Doped Carbon Precursors. Macromol. Chem. Phys. 2017, 218 (18), 1600586. (22) Sun, J. K.; Kochovski, Z.; Zhang, W. Y.; Kirmse, H.; Lu, Y.; Antonietti, M.; Yuan, J. General Synthetic Route Towards Highly Dispersed Metal Clusters Enabled by Poly(Ionic Liquid)s. J. Am. Chem. Soc. 2017, 139 (26), 8971. (23) Serpell, C. J.; Cookson, J.; Thompson, A. L.; Brown, C. M.; Beer, P. D. Haloaurate and Halopalladate Imidazolium Salts: Structures, Properties, and Use as Precursors for Catalytic Metal Nanoparticles. Dalton Trans. 2013, 42 (5), 1385. (24) Pan, H.; Cheng, Z.; Xiao, Z.; Li, X.; Wang, R. The Fusion of Imidazolium-based Ionic Polymer and Carbon Nanotubes: One Type of New Heteroatom-doped Carbon Precursors for High-performance Lithium-sulfur Batteries. Adv. Funct. Mater. 2017, 27 (44), 1703936.

most of the reported palladium NPs, which enables more catalytically active sites of palladium NPs available for the catalytic reaction. (3) The amine groups serve as proton scavengers to form formate intermediates, which function as the additives in FA dehydrogenation, which is pivotal for the catalytic reaction. In summary, this study not only has demonstrated the role of functional groups in the formation of ultrafine metal NPs and ImIPs with different morphology but also has provided one type of new heterogeneous catalyst for selective FA dehydrogenation under additive-free conditions. Further research for the synthesis of ImIPs with different functional groups and their catalytic application is in progress.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b01769.

■

Additional information on the dispersibility, TGA, XRD of ImIP-1, Pd/ImIP-1, ImIP-2, and Pd/ImIP-2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ruihu Wang: 0000-0002-6209-9822 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21471151, 21671039, and 21603228) and by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

■

REFERENCES

(1) Aresta, M.; Dibenedetto, A.; Angelini, A. Catalysis for the Valorization of Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2. Chem. Rev. 2014, 114 (3), 1709. (2) Qi, J.; Zhang, W.; Cao, R. Solar-to-Hydrogen Energy Conversion Based on Water Splitting. Adv. Energy Mater. 2018, 8 (5), 1701620. (3) Liu, P.; Wang, X.; Wang, Y. Design of Carbon Black/Polypyrrole Composite Hollow Nanospheres and Performance Evaluation as Electrode Materials for Supercapacitors. ACS Sustainable Chem. Eng. 2014, 2 (7), 1795. (4) Preuster, P.; Papp, C.; Wasserscheid, P. Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free Hydrogen Economy. Acc. Chem. Res. 2017, 50 (1), 74. (5) Zhao, P. P.; Cao, N.; Su, J.; Luo, W.; Cheng, G. Z. NiIr Nanoparticles Immobilized on the Pores of MIL-101 as Highly Efficient Catalyst toward Hydrogen Generation from Hydrous Hydrazine. ACS Sustainable Chem. Eng. 2015, 3 (6), 1086. (6) Liu, Q.; Yang, X.; Huang, Y.; Xu, S.; Su, X.; Pan, X.; Xu, J.; Wang, A.; Liang, C.; Wang, X.; Zhang, T. A Schiff Base Modified Gold Catalyst for Green and Efficient H2 Production from Formic Acid. Energy Environ. Sci. 2015, 8 (11), 3204. (7) Mellmann, D.; Sponholz, P.; Junge, H.; Beller, M. Formic Acid as a Hydrogen Storage Material-Development of Homogeneous Catalysts for Selective Hydrogen Release. Chem. Soc. Rev. 2016, 45 (14), 3954. G

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering (25) Gong, Y.; Zhong, H.; Liu, W.; Zhang, B.; Hu, S.; Wang, R. General Synthetic Route toward Highly Dispersed Ultrafine Pd-Au Alloy Nanoparticles Enabled by Imidazolium-based Organic Polymers. ACS Appl. Mater. Interfaces 2018, 10 (1), 776. (26) Koh, K.; Seo, J. E.; Lee, J. H.; Goswami, A.; Yoon, C. W.; Asefa, T. Ultrasmall Palladium Nanoparticles Supported on AmineFunctionalized SBA-15 Efficiently Catalyze Hydrogen Evolution from Formic Acid. J. Mater. Chem. A 2014, 2 (48), 20444. (27) Das, G.; Skorjanc, T.; Sharma, S. K.; Gándara, F.; Lusi, M.; ShankarRao, D. S.; Vimala, S.; KrishnaPrasad, S.; Raya, J.; Han, D. S.; Jagannathan, R.; Olsen, J.-C.; Trabolsi, A. Viologen-Based Conjugated Covalent Organic Networks via Zincke Reaction. J. Am. Chem. Soc. 2017, 139 (28), 9558. (28) Huang, K.; Liu, F.; Dai, S. Solvothermal Synthesis of Hierarchically Nanoporous Organic Polymers with Tunable Nitrogen Functionality for Highly Selective Capture of CO2. J. Mater. Chem. A 2016, 4 (34), 13063. (29) Wang, Y.; Zhao, H.; Li, X.; Wang, R. A Durable Luminescent Ionic Polymer for Rapid Detection and Efficient Removal of Toxic Cr2O72‑. J. Mater. Chem. A 2016, 4 (32), 12554. (30) Jordan, B. J.; Ofir, Y.; Patra, D.; Caldwell, S. T.; Kennedy, A.; Joubanian, S.; Rabani, G.; Cooke, G.; Rotello, V. M. Controlled SelfAssembly of Organic Nanowires and Platelets Using Dipolar and Hydrogen-Bonding Interactions. Small 2008, 4 (11), 2074. (31) Caner, N.; Bulut, A.; Yurderi, M.; Ertas, I. E.; Kivrak, H.; Kaya, M.; Zahmakiran, M. Atomic Layer Deposition-SiO2 Layers Protected PdCoNi Nanoparticles Supported on TiO2 Nanopowders: Exceptionally Stable Nanocatalyst for the Dehydrogenation of Formic Acid. Appl. Catal., B 2017, 210, 470. (32) Bulut, A.; Yurderi, M.; Karatas, Y.; Zahmakiran, M.; Kivrak, H.; Gulcan, M.; Kaya, M. Pd-MnOx Nanoparticles Dispersed on AmineGrafted Silica: Highly Efficient Nanocatalyst for Hydrogen Production from Additive-Free Dehydrogenation of Formic Acid Under Mild Conditions. Appl. Catal., B 2015, 164, 324. (33) Jiang, Y.; Fan, X.; Xiao, X.; Huang, X.; Liu, M.; Li, S.; Ge, H.; Chen, L. La2O3-Modified Highly Dispersed AuPd Alloy Nanoparticles and Their Superior Catalysis on the Dehydrogenation of Formic Acid. Int. J. Hydrogen Energy 2017, 42 (15), 9353. (34) Gu, X.; Lu, Z. H.; Jiang, H. L.; Akita, T.; Xu, Q. Synergistic Catalysis of Metal-Organic Framework-Immobilized Au-Pd Nanoparticles in Dehydrogenation of Formic Acid for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2011, 133 (31), 11822. (35) Ke, F.; Wang, L.; Zhu, J. An Efficient Room Temperature Coreshell AgPd@MOF Catalyst for Hydrogen Production from Formic Acid. Nanoscale 2015, 7 (18), 8321. (36) Zhang, S.; Metin, Ö .; Su, D.; Sun, S. Monodisperse AgPd Alloy Nanoparticles and Their Superior Catalysis for the Dehydrogenation of Formic Acid. Angew. Chem., Int. Ed. 2013, 52 (13), 3681. (37) Zhou, X.; Huang, Y.; Liu, C.; Liao, J.; Lu, T.; Xing, W. Available Hydrogen from Formic Acid Decomposed by Rare Earth Elements Promoted Pd-Au/C Catalysts at Low Temperature. ChemSusChem 2010, 3 (12), 1379. (38) Bulushev, D. A.; Zacharska, M.; Lisitsyn, A. S.; Podyacheva, O. Y.; Hage, F. S.; Ramasse, Q. M.; Bangert, U.; Bulusheva, L. G. Single Atoms of Pt-Group Metals Stabilized by N-Doped Carbon Nanofibers for Efficient Hydrogen Production from Formic Acid. ACS Catal. 2016, 6 (6), 3442. (39) Zhong, H.; Su, Y.; Cui, C.; Zhou, F.; Li, X.; Wang, R. Palladium Nanoparticles Supported by Carboxylate-functionalized Porous Organic Polymers for Additive-free Hydrogen Generation from Formic Acid. ACS Sustainable Chem. Eng. 2017, 5 (9), 8061. (40) Koh, K.; Jeon, M.; Yoon, C. W.; Asefa, T. Formic Acid Dehydrogenation over Pd NPs Supported on Amine-Functionalized SBA-15 Catalysts: Structure-Activity Relationships. J. Mater. Chem. A 2017, 5 (31), 16150. (41) Wang, Z.; Hao, X.; Hu, D.; Li, L.; Song, X.; Zhang, W.; Jia, M. PdAu Bimetallic Nanoparticles Anchored on Amine-Modified Mesoporous ZrSBA-15 For Dehydrogenation of Formic Acid Under Ambient Conditions. Catal. Sci. Technol. 2017, 7 (11), 2213. H

DOI: 10.1021/acssuschemeng.8b01769 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX