Palladium Nanoparticles Supported by ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Palladium Nanoparticles Supported by Carboxylate-Functionalized Porous Organic Polymers for Additive-Free Hydrogen Generation from Formic Acid Hong Zhong,† Yanqing Su,†,‡ Caiyan Cui,† Feng Zhou,† Xiaoju Li,*,‡ and Ruihu Wang*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ Fujian Key Laboratory of Polymer Materials, College of Materials Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350007, China S Supporting Information *

ABSTRACT: Facile hydrogen generation from formic acid (FA) is a promising way of hydrogen storage and release in the fuel-cell-based hydrogen economy; the development of efficient heterogeneous catalyst systems for ultrapure H2 generation from FA in the absence of additives remains a major challenge. Herein, we present a prefunctionalized porous organic polymer (POP) containing 2,6-bis(1,2,3-triazol-4yl)pyridyl (BTP) units and carboxylate groups. The terdentate BTP and hydrophilic carboxylate are homogeneously incorporated into the host framework of the POP. BTP units with strong chelating ability can effectively stabilize palladium nanoparticles for heterogeneous dehydrogenation of FA, whereas carboxylate not only increases polarity and dispersibility of the catalytic system in aqueous solutions but also functions as basic sites to facilitate the O−H bond dissociation. The catalytic system shows high catalytic activity, excellent stability and superior recyclability in H2 generation from aqueous FA without any additives. KEYWORDS: Porous organic polymers, Palladium nanoparticles, Additive free, Hydrogen generation, Sustainable Chemistry

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practical application.16−18 Therefore, it is highly desirable to develop a green and efficient catalyst for H2 generation from FA without any additives. Heterogeneous catalysts based on metal nanoparticles (NPs) have attracted tremendous attention because of their simple handling processes and excellent reusability.19−22 The supports of metal NPs are known to play crucial roles in limiting their agglomeration and improving their dispersion.23,24 Furthermore, the electron-donating coordination groups and/or basic sites could be incorporated easily into the supports, this not only is beneficial for synergetic immobilization of metal NPs but also can serve as proton scavengers to replace the additives of FA dehydrogenation, generating formate intermediates for further dehydrogenation. Recently, a few imine25 and aminefunctionalized26,27 inorganic solid materials as well as pyridinicnitrogen-doped carbons28 have been used for additive-free dehydrogenation of FA. However, nonuniform distribution of functional groups and modification difficulty of these support materials encourage us to explore a type of new support

INTRODUCTION With ever-increasing consumption of fossil fuels and negative environment impacts associated with burning fossil fuels, considerable efforts have been devoted to searching for sustainable, benign and renewable energy sources.1−4 Hydrogen is considered to be one of environmentally attractive fuels and the most promising clean energy carriers for future applications due to its high energy density, renewability and clean burning with water as byproduct.5−7 However, controlled storage and release of hydrogen remain significant bottlenecks for the development of fuel-cell-based hydrogen economy.8,9 Formic acid (FA, HCOOH), as a safe and convenient hydrogen carrier, which has been of remarkable interest in fuel cells designed for portable use because of its high hydrogen content (4.4 wt %), nontoxicity, liquid state, easy accessibility and outstanding stability under ambient conditions.10,11 Hydrogen stored in FA can be catalytically released through dehydrogenation and dehydration pathways. It is pivotal to avoid undesirable dehydration process and give ultrapure H2 for subsequent use in the fuel cells. Various homogeneous and heterogeneous catalysts have been developed for H2 generation from FA under mild conditions,12−15 but most catalytic systems have required the addition of additives, such as triethylamine and sodium formate, which greatly limits their large-scale © 2017 American Chemical Society

Received: May 27, 2017 Revised: July 15, 2017 Published: August 15, 2017 8061

DOI: 10.1021/acssuschemeng.7b01675 ACS Sustainable Chem. Eng. 2017, 5, 8061−8069

Research Article

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

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materials of metal NPs for additive-free ultrapure H2 generation from FA. As one class of emerging porous materials, porous organic polymers (POPs) have captured tremendous interest because of their large surface area, high stability, flexible synthetic strategy and ready functionality.29−31 They can serve as promising supports for metal NPs because they have provided a powerful confinement effect to restrain the agglomeration of metal NPs.32,33 Moreover, the coordination groups may be incorporated homogeneously into host frameworks of POPs for further stabilization and activation of metal NPs through coordination interactions,34−36 leading to uniform dispersion of metal NPs throughout POPs. However, most POPs are aromatic and show poor dispersibility in water, which limits their application in water phase reactions. The introduction of hydrophilic groups into POPs is regarded as one of feasible routes for improving dispersibility of POPs in water.37,38 As a basic group, carboxylate possesses outstanding coordination ability to metal species, its presence may increase polarity and dispersibility of catalytic systems in polar solvents, which accelerates mass transfer in the water-phase catalytic reactions. Attractively, the modular nature of POPs synthesis has allowed homogeneous incorporation of carboxylate group into organic networks by customizing building blocks containing carboxylate group at the molecular level.39,40 The carboxylate group in POPs not only can improve surface electronic structure of supported metal NPs but also can enhance catalytic activity of FA dehydrogenation through the interaction between carboxylate and FA. However, relative to extensive research in POPs functionality directed at gas sorption41−43 and effective immobilization of metal NPs,44−46 the predesignable functionalities of POPs for the improvement of catalytic performances have seldom been explored. In our continuous effort to develop highly efficient and recyclable catalytic protocols,47−49 herein, we present a functionalized POP containing terdentate chelating 2,6-bis(1,2,3-triazol-4-yl)pyridine (BTP) units and hydrophilic carboxylate groups, which can serve as an effective support of palladium NPs for additive-free FA dehydrogenation under mild conditions. The homogeneous-distributed carboxylate groups in POP both donate the electrons to palladium NPs and facilitate the O−H bond dissociation of FA, leading to high catalytic activity in H2 generation from aqueous FA without any additives.

EXPERIMENTAL SECTION

Synthesis of POP-1. A mixture of methyl-2,6-diethynylisonicotinate (0.370 g, 2.00 mmol), tetrakis(4-azidophenyl)methane (0.484 g, 1.00 mmol), CuSO4·5H2O (0.050 g, 0.20 mmol) and sodium ascorbate (0.081 g, 0.40 mmol) in dry DMF (50 mL) was stirred under nitrogen at 100 °C for 72 h to afford a brown-yellow powder. The solid was isolated by filtration, and subsequently washed with aqueous EDTA-2Na solution (0.250 g in 200 mL H2O), ethanol and CH2Cl2 to remove any unreacted monomers or residues. The brown powder was further treated by Soxhlet extraction in CH2Cl2 overnight and dried in vacuo at 60 °C for 12 h. Yield: 0.77 g (90%). Elemental analysis calculated (%) for C47H32N14O4: C, 65.89; H, 3.74; N, 22.90. Found: C, 61.29; H, 4.44; N, 19.36. FTIR (KBr cm−1): 3070 (w), 2948 (w), 2101 (w), 1726 (s), 1617 (m), 1568 (m), 1503 (s), 1432 (m), 1378 (m), 1286 (m), 1237 (s), 1122 (m), 1029 (m), 981 (m), 818 (s), 764 (m), 720 (m), 541 (w). Synthesis of POP-2. To a 50 mL aqueous NaOH solution (1.671 g, 41.79 mmol) was added POP-1 (500 mg), and the mixture was stirred at 80 °C for 24 h. The resultant solid was collected by filtration, washed with copious water to completely remove NaOH residues, and dried in vacuo to produce a brown powder. Yield: 0.42 g (83%). Elemental analysis calculated (%) for C45H26N14O4Na2: C, 61.93; H, 2.98; N, 22.48. Found: C, 53.78; H, 3.61; N, 15.62. FTIR (KBr cm−1): 3071 (w), 2937 (w), 2105 (w), 1610 (s), 1556 (s), 1502 (s), 1377 (s), 1334 (m), 1225 (m), 1046 (m), 986 (w), 828 (m), 789 (m), 725 (m), 545 (w). Synthesis of Pd/POP-1. To a CH2Cl2 solution (200 mL) of palladium acetate (0.210 g, 0.937 mmol) was added POP-1 (0.400 g), and the mixture was stirred vigorously at 60 °C for 24 h. The resultant product was washed thoroughly with CH2Cl2 to remove excess palladium acetate and dried in vacuo at 80 °C for 12 h. To a suspension of the above palladium polymer in water (200 mL) was added 10-fold excess of aqueous NaBH4, and the mixture was stirred at room temperature for 3 h. The resultant black powder was collected by filtration, washed with copious water, and dried in vacuo at 60 °C for 12 h. Yield: 0.44 g (88%). FTIR (KBr cm−1): 3416 (m), 3068 (w), 2948 (w), 2105 (w), 1726 (s), 1617 (m), 1568 (m), 1507 (s), 1433 (m), 1372 (m), 1281 (s), 1232 (s), 1122 (m), 1030 (s), 981 (m), 823 (s), 769 (m), 720 (m), 545 (w). Synthesis of Pd/POP-2. Pd/POP-2 was prepared using similar method to Pd/POP-1 except that POP-1 was replaced by POP-2. Yield: 0.41 g (83%). FTIR (KBr cm−1): 3377 (S), 3071 (w), 2100 (w), 1605 (s), 1551 (s), 1502 (s), 1377 (s), 1236 (m), 1046 (m), 986 (w), 910 (w), 823 (m), 725 (m), 546 (w). General Procedures for H2 Generation from FA Solution. Before starting the catalytic activity test, a jacketed reaction flask (25 mL) containing the catalyst and stirrer bar was placed on a magnetic stirrer and thermostated to reaction temperature. Then, a buret filled with water was connected to the reaction flask to measure the volume 8062


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ACS Sustainable Chemistry & Engineering of evolved hydrogen gas from the reaction. Two mmol aqueous FA solution was transferred into the reaction flask under 400 rpm stirring rate for the appropriate time. The volume of evolved hydrogen gas was measured by recording the displacement of water level every minute. The reaction was stopped when no hydrogen generation was observed. General Procedures for Recyclability Test. After H2 generation from aqueous FA solution was finished, the catalyst powder in the aqueous phase was separated by filtration and washed with water and ethyl alcohol. The recovered solid was directly used for the next run with the addition of fresh aqueous FA solution.

disappear in the FTIR spectra of POP-1 and POP-2, which suggests complete azide−alkyne cycloaddition and the formation of terdentate BTP unit.52 The characteristic bands for −COOMe at 1726 cm−1 and carboxylate at 1606 cm−1 are observed in the FTIR spectra of POP-1 and POP-2, respectively.53 In solid-state 13C NMR spectra of POP-1 and POP-2 (Figure 2c,d), the appearance of resonance peak around 148 ppm for C4-triazolyl carbon atom, combined with the disappearance of characteristic alkynyl peaks in the range of 70−100 ppm, further confirm successful polymerization between MDEN and TAPM.47,53 The signals at 140−110 and 64 ppm correspond to aromatic carbon atoms and central carbon atom of the tetraphenyl-methane core, respectively.54 The resonance peaks at 166 and 169 ppm are assigned to carbonyl carbon atom of POP-1 and POP-2, respectively.53 The peak for methyl carbon atom of −COOMe in POP-1 is observed at 52 ppm, which disappears in solid-state 13C NMR spectrum of POP-2. Notably, the FTIR spectra and solid-state 13 C NMR spectra of Pd/POP-1 and Pd/POP-2 are almost identical with that of POP-1 and POP-2, respectively, indicating their structural frameworks are well maintained after palladium NPs loading. In thermal gravimetric analysis (TGA) curve of POP-1, the initial weight loss of 6.8% before 110 °C is observed, whereas POP-2 gives a 16.3% weight loss before 130 °C (Figure S2). Thermal stabilities of Pd/POP-1 and Pd/POP-2 are slightly lower than that of POP-1 and POP-2, respectively. Powder Xray diffraction (PXRD) patterns indicate both POP-1 and POP2 are amorphous (Figure S3), which is attributed to the kinetics-controlled irreversibility of click reaction.55,56 In PXRD patterns of Pd/POP-1 and Pd/POP-2, a characteristic peak at 39.7° is assigned to palladium(111) crystal plane.57 SEM images show POP-1 and POP-2 show granular morphology with a size of 30−100 nm (Figure S4), and palladium NPs loading has no obvious effect on morphology of POP-1 and POP-2. The porous properties of POP-1 and POP-2 were investigated by physisorption of nitrogen at 77 K. Both POP1 and POP-2 give type I adsorption/desorption isotherm according to IUPAC classification (Figure S5).58 The sharp uptake of N2 at very low relative pressure (P/P0 < 0.01) indicates the presence of extensive micropores in POP-1 and POP-2.47,48 The Brunauer−Emmett−Teller (BET) surface areas of POP-1 and POP-2 are 620 and 336 m2 g−1, respectively. The pore size distribution reveals that the predominant pores in POP-1 and POP-2 are in the range of micropores (Figure S6), which is consistent with the results of nitrogen adsorption−desorption isotherm. After palladium loading, BET surface areas of Pd/POP-1 and Pd/POP-2 are decreased to 265 and 103 m2 g−1, respectively, which is attributed to both partial pore filling and mass increment after palladium loading.59 Transmission electron microscopy (TEM) analyses show that well-dispersed palladium NPs in Pd/POP-1 and Pd/POP-2 could be observed clearly, and their average diameters are 3.6 ± 0.5 and 3.7 ± 0.5 nm, respectively (Figure 3a,b,e,f). The size and distribution of palladium NPs in Pd/POP-1 and Pd/POP-2 were further confirmed by high-annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) mapping images (Figures S7, 8). The HRTEM images and corresponding fast Fourier-transform (FFT) patterns show that the intervals between two lattice fringes of palladium NPs in Pd/POP-1 and Pd/POP-2 are 0.219 and 0.221 nm (Figure

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RESULTS AND DISCUSSION The synthetic route for prefunctionalized POP containing BTP units and carboxylate groups was shown in Scheme 1. The facile reaction of methyl 2,6-diethynylisonicotinate (MDEN) and tetrakis(4-azidophenyl)methane (TAPM) under the standard click conditions gave rise to POP-1, subsequent hydrolysis of POP-1 in aqueous NaOH solution formed POP-2. The treatment of POP-1 and POP-2 with a CH2Cl2 solution of Pd(OAc)2 in a 1:1 molar ratio of BTP to Pd(OAc)2, followed by NaBH4 reduction generated black Pd/POP-1 and Pd/POP2, respectively. Both Pd/POP-1 and Pd/POP-2 are insoluble in water or common organic solvents. Inductively coupled plasma spectroscopy (ICP) analyses show palladium contents in Pd/ POP-1 and Pd/POP-2 are 0.42 and 0.29 mmol g−1 , respectively. The good dispersity of the catalysts in reaction media is one of important factors for high catalytic activity in heterogeneous catalytic systems, which is conducive to interfacial contact between catalytical active sites and substrates.50 As expected, POP-2 and Pd/POP-2 show better dispersibility in water than POP-1 and Pd/POP-1 owing to the presence of hydrophilic carboxylate group (Figure S1). The hydrophilicity of POP-1, Pd/POP-1, POP-2 and Pd/POP-2 was further characterized by water contact angle (CA) measurement using pellets of each material (Figure 1).51 POP-1 and Pd/POP-1 show moderate

Figure 1. Water contact angle measurement for (a, e) POP-1, (b, f) Pd/POP-1, (c, g) POP-2 and (d, h) Pd/POP-2.

water wettability with CA of 56° and 53°, respectively. Interestingly, rapid swell and crack are observed as soon as water drop touches the pellets of POP-2 and Pd/POP-2, suggesting strong wettability of POP-2 and Pd/POP-2, thus, they can act as promising candidates of supports and catalysts for water-phase catalytic reactions. Chemical structures and compositions of POP-1, POP-2, Pd/ POP-1 and Pd/POP-2 were identified by Fourier-transform infrared spectroscopy (FTIR) and solid-state 13C NMR. As shown in Figure 2a,b, the stretching vibration peaks of terminal alkynyl in the FTIR spectrum of MDEN occur at 3262 and 2112 cm−1, and the characteristic peak of azido group in the FTIR spectrum of TAPM is at 2121 cm−1, these peaks totally 8063


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Figure 2. (a) FTIR spectra for starting materials, POP-1 and Pd/POP-1; (b) starting materials, POP-2 and Pd/POP-2; (MDEN: methyl-2,6diethynylisonicotinate, TAPM: tetrakis(4-azidophenyl)methane); (c) solid-state 13C NMR spectra for POP-1 and Pd/POP-1, (d) POP-2 and Pd/ POP-2.

their relative peak areas, which further validates the formation of BTP unit. In comparison with POP-1, the binding energy peaks of N 1s in POP-2 shift negatively by 0.2 eV, which is ascribled to electron-donating effect of carboxylate group. To illustrate the interactions between the supports and incorporated palladium species, the existing states of surface palladium in Pd/POP-1 and Pd/POP-2 were investigated by XPS. XPS survey spectra of Pd/POP-1 and Pd/POP-2 are almost identical with that of POP-1 and POP-2 except the appearance of characteristic peaks of Pd 3d. The deconvolution of Pd 3d spectra presents two sets of double peaks corresponding to Pd 3d5/2 and Pd 3d3/2 (Figure 4b). The binding energy peaks of Pd 3d5/2 in Pd/POP-1 and Pd/POP-2 are 335.68 and 335.08 eV, respectively, which are assigned to Pd(0) species, whereas Pd 3d5/2 peaks at 337.28 and 337.68 eV correspond to Pd(II) species. The ratios of surface Pd(0) to Pd(II) in Pd/POP-1 and Pd/POP-2 are 0.95 and 1.13, respectively, as determined by the ratio of their relative peak areas. The presence of Pd(II) species is probably related to the residual Pd(II) and/or reoxidation of Pd(0) during air contact,62,63 which shifts negatively by 1.12 and 1.22 eV, respectively, when compared with that of 338.4 eV for free Pd(OAc)2.64 This negative shift mainly results from strong coordination interaction of Pd(II) with chelating terdentate BTP units, in which electron donation from BTP to Pd(II) makes the Pd(II) species less electron-decient.52,53 The Pd 3d5/2 binding energy of Pd(0) species in Pd/POP-2 shifts negatively by 0.60 eV in comparison with that of Pd/POP-1 because the electron-donating effect of carboxylate group makes Pd(0) species more electron-rich in Pd/POP-2 than that in Pd/POP-1.55 To further confirm the interactions between BTP units and palladium species, N 1s XPS spectra of Pd/ POP-1 and Pd/POP-2 were also investigated (Figure 4a). The binding energy peaks of N1, N3 and pyridyl N atoms in Pd/ POP-1 and Pd/POP-2 are observed at 399.18 and 399.13 eV, respectively, which shifts positively by 0.10 and 0.08 eV,

Figure 3. TEM, HRTEM images and corresponding FFT patterns for (a−d) Pd/POP-1 and (e−h) Pd/POP-2.

3c,d,g,h), respectively, corresponding to (111) lattice spacing of the face-centered cubic palladium.57 These results agrees well with PXRD analysis of Pd/POP-1 and Pd/POP-2 (Figure S3). X-ray photoelectron spectroscopy (XPS) survey spectra of POP-1 and POP-2 are shown in Figure S9; the binding energy peaks around 285.0, 399.9 and 532.0 eV correspond to C 1s, N 1s and O 1s, respectively. An extra peak at 1070.3 eV is observed in XPS survey spectrum of POP-2, which is assigned to Na 1s.60 High-resolution XPS spectra of N 1s in POP-1 and POP-2 are shown in Figure 4a; N 1s regions are divided into two spin−orbital peaks. The peak around 400.9 eV is ascribed to N2 atom of 1,2,3-triazolyl, the other peak around 399.1 eV corresponds to N1, N3 and pyridyl N atoms,48,61 The intensity ratio of such two peaks is 2:5, as determined by the ratio of 8064


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Figure 4. (a) N 1s XPS spectra for POP-1, POP-2, Pd/POP-1, Pd/POP-2, Pd/POP-1-4run and Pd/POP-2-4run; (b) Pd 3d XPS spectra for Pd/ POP-1, Pd/POP-2, Pd/POP-1-4run and Pd/POP-2-4run.

Figure 5. (a) Plot of conversion versus FA concentration; (b) plot of H2 generation from FA as a function of time; (c) H2 generation from FA catalyzed by Pd/POP-2 at different temperatures; (d) Arrhenius plot and TOF values of FA dehydrogenation catalyzed by Pd/POP-2. Reaction conditions: 2.0 mmol aqueous FA solution, molar ratio of [Pd]/FA = 0.03.

dehydrogenation of FA using 1.0 M aqueous FA at 60 °C. As shown in Figure 5b, Pd/POP-2 shows a much higher catalytic activity than Pd/POP-1 in the entire catalytic process. A full dehydrogenation of FA is achieved within 12 min in the presence of 3.0 mol % Pd/POP-2, and no CO was detected when the compositions of the produced gas were analyzed by gas chromatography (GC), suggesting excellent selectivity of FA dehydrogenation in this catalytic system (Figure 6a). However, the use of Pd/POP-1 gives a 66% conversion of FA under the same conditions. FA dehydrogenation is still incomplete even when reaction time is elongated to 30 min. The better catalytic activity of Pd/POP-2 is mainly attributed to its superior dispersibility in solution and easier O−H bond dissociation of FA with the help of carboxylate group. The control experiments were also performed. When only POP-1 and POP-2 were used as catalysts, no hydrogenation product was detected. The commercial Pd/C provides 29% and 45% conversion of FA in 12 and 30 min, respectively, which are much inferior to that of Pd/POP-1 and Pd/POP-2. The turnover frequency (TOF) value of Pd/POP-2 is calculated to be 167 h−1. The TOF value is much higher than that of Pd/

respectively, when compared with that of POP-1 and POP-2, respectively. This shift toward higher binding energies mainly results from the coordination of nitrogen atoms to palladium,52,53 which further suggests election donation from BTP to palladium species after palladium loading. The catalytic performances of Pd/POP-1 and Pd/POP-2 were initially evaluated by H2 generation from different concentrations of aqueous FA in the absence of any additives. The plot of FA conversion and concentrations at 60 °C in 15 min shows a volcano-shaped relationship (Figure 5a). The catalytic activity of Pd/POP-1 and Pd/POP-2 increases as FA concentration increases from 0.1 to 1.0 M, Pd/POP-1 and Pd/ POP-2 give the highest conversion of 73% and 100%, respectively, in 1.0 M aqueous FA solution in 15 min. When FA concentration is higher than 1.0 M, the conversion in 15 min gradually decreases, indicating that large amount of water plays an indispensable role in the catalytic dehydrogenation of FA.65 To scrutinize the effects of −COOMe and carboxylate groups in POPs on the catalytic activities, we explored the kinetic curves of Pd/POP-1 and Pd/POP-2 in additive-free 8065


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Figure 6. (a) GC spectra for (I) commercial pure CO, (II) commercial pure H2, (III) the produced gas from aqueous FA solution catalyzed by Pd/ POP-2. Recyclability in H2 generation from FA for (b) Pd/POP-1; (c) Pd/POP-2 and (d) Pd/C. Reaction conditions: 1.0 M aqueous FA solution (2.0 mmol), molar ratio of [Pd]/FA = 0.03, 60 °C, 30 min.

POP-1 and Pd/C, but it is lower than imine-25 and aminefunctionalized inorganic solid materials26 as well as pyridinicnitrogen-doped carbons,28 which is probably ascribed to weaker Lewis basicity of the carboxylate group. Temperature plays an important role in FA dehydrogenation, the effect of temperature on the rate of FA dehydrogenation was measured in the presence of Pd/POP-2, Figure 5c depicts time-dependent H2 generation at different temperatures in 1.0 M aqueous FA solution in 15 min. H2 generation rate increases sharply with the rise of reaction temperature from 30 to 60 °C, which indicates that high reaction temperature is beneficial for accelerating FA dehydrogenation. According to the Arrhenius plot, 66,67 the apparent activation energy (E a ) of FA dehydrogenation is calculated to be 67.45 kJ mol−1 (Figure 5d). The recyclability of Pd/POP-2 and Pd/POP-1 was also examined by H2 generation in 1.0 M aqueous FA without any additives. As shown in Figure 6c, a full conversion of FA is achieved in 12 min in the first run in the presence of Pd/POP2. After the reaction was finished, the recovered catalytic species were directly used for next run with recharge of aqueous FA. The conversion in 30 min is gradually decreased to 89% in the fourth run. As a sharp comparison, the conversion in 30 min is decreased from 83% in the first run to 48% in the fourth run when Pd/POP-1 was used under the same conditions (Figure 6b). ICP analyses show palladium leaching for Pd/POP-1 and Pd/POP-2 after the first run reaction is 1.16 and 0.87 ppm, respectively, the negligible palladium leaching is probably ascribed to strong stabilization effect of palladium NPs by chelating BTP units. However, commercial Pd/C affords the conversions of 45% and 10% in the first run and the fourth run (Figure 6d), respectively, which are much lower than those in Pd/POP-1 and Pd/POP-2. To further understand this catalytic system, the resultant powders of Pd/POP-1 and Pd/POP-2 were isolated after consecutive reaction for four runs and were denoted as Pd/ POP-1-4run and Pd/POP-2-4run, respectively. SEM images

(Figure S4) and FTIR spectra (Figure S10) show that the original chemical compositions and morphologies of Pd/POP-1 and Pd/POP-2 are intact after consecutive FA dehydrogenation. TEM analyses indicate that average diameters of palladium NPs in Pd/POP-1-4run and Pd/POP-2-4run are slightly increased to 4.1 ± 0.5 and 3.9 ± 0.5 nm (Figure 7a,b,e,f), respectively. The concomitant formation of a few large

Figure 7. TEM, HRTEM images and corresponding FFT patterns for (a−d) Pd/POP-1-4run and (e−h) Pd/POP-2-4run.

palladium NPs occurs owing to Ostwald ripening process during the catalytic reaction,68,69 but no obvious agglomeration is observed due to strong chelating ability of BTP units. These observations were further testified by HAADF-STEM and EDX mapping images (Figures S11, 12). HRTEM images reveal that the intervals between two lattice fringes in Pd/POP-1-4run and 8066


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POP-2 show higher catalytic activity than Pd/POP-1 and Pd/C but also endows Pd/POP-2 with good stability and durability in H2 generation from aqueous FA. In summary, this study not only widens the application scope of POPs in heterogeneous catalysis but also provides a new approach for customization of heterogeneous catalysts for additive-free selective dehydrogenation of FA under mild conditions.

Pd/POP-2-4run are similar to those in Pd/POP-1 and Pd/ POP-2 (Figure 7c,d,g,h), respectively, which is consistent with their PXRD results (Figure S3). To elucidate the properties of surface palladium NPs after consecutive FA dehydrogenation, Pd/POP-1-4run and Pd/POP-2-4run were analyzed by XPS (Figure 4b), the ratios of Pd(0)/Pd(II) are increased from 0.95 to 1.23 and from 1.13 to 1.19, respectively, which may be ascribed to further reduction of residual Pd(II) in the process of the catalytic reaction.70 The possible mechanism for FA dehydrogenation catalyzed by Pd/POP-2 in the absence of additives was proposed. The pKa values of benzoic acid and FA are 4.20 and 3.77, respectively,71 the conjugate base anion of benzoic acid can sequester proton from FA. Thus, the considerable quantity of carboxylate groups in Pd/POP-2 serve as efficient Lewis basic sites to dissociate O−H bond of FA, generating formate intermediates (Scheme 2). Simultaneosuly, palladium NPs

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Scheme 2. Plausible Mechanism for FA Dehydrogenation Catalyzed by Pd/POP-2

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01675. Dispersibility in water, TGA, XRD and SEM of POP-1, POP-2, Pd/POP-1 and Pd/POP-2, the N2 isotherms and pore size distribution of POP-1, POP-2, Pd/POP-1 and Pd/POP-2 at 77 K (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Xiaoju Li, Email: [email protected]. *Ruihu Wang, Email: [email protected]. ORCID

Ruihu Wang: 0000-0002-6209-9822 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21603228, 21671039, 21673241 and 21471151) and by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

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REFERENCES

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around them catalytically activate the C−H bond of formate species, resulting in the formation of CO2 and H2, which is a rate-determining step in the catalytic reaction.25,72 Besides, carboxylate group donates electron to palladium NPs, which is conducive to β-hydride elimination in the palladium-formate species, thus promoting the catalytic activity of palladium NPs in FA dehydrogenation.

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CONCLUSIONS A prefunctionalized POP-2 containing terdentate chelating BTP units and carboxylate groups has been prepared. POP-2 can serve as an effective support of palladium NPs for additivefree FA dehydrogenation. Palladium NPs are uniformly dispersed over POP-2, and are effectively immobilized by terdentate BTP units during dehydrogenation of FA. The carboxylate groups play multiple roles in the catalytic reaction: (1) improving polarity and dispersibility of Pd/POP-2 in water, which is beneficial for interfacial contact between catalytical active sites and substrates; (2) deprotonating FA to form formate intermediates, which enables H2 generation from FA in the absence of additives; (3) donating electrons to palladium NPs, which accelerates C−H bond cleavage for H2 generation. The predominance in design and structure not only makes Pd/ 8067


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