Electrochemical Synthesis of Some Archetypical Zn2+, Cu2+, and

Article pubs.acs.org/crystal

Electrochemical Synthesis of Some Archetypical Zn2+, Cu2+, and Al3+ Metal Organic Frameworks Alberto Martinez Joaristi, Jana Juan-Alcañiz, Pablo Serra-Crespo, Freek Kapteijn, and Jorge Gascon* Catalysis Engineering, Chemical Engineering Department, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Several archetypical metal organic frameworks (MOFs), namely, HKUST-1, ZIF-8, MIL-100(Al), MIL-53(Al), and NH2-MIL-53(Al), were synthesized via anodic dissolution in an electrochemical cell. The influence of different reaction parameters such as solvent, electrolyte, voltage− current density, and temperature on the synthesis yield and textural properties of the MOFs obtained was investigated. The characterization of the samples involved X-ray diffraction, gas adsorption, atomic force microscopy, diffuse reflectance infrared Fourier transform spectroscopy, and scanning electron microscopy. In the present article, we demonstrate that electrochemical synthesis is a robust method offering additional degrees of freedom in the synthesis of these porous materials. The main advantages are the shorter synthesis time, the milder conditions, the facile synthesis of MOF nanoparticles, the morphology tuning and the high Faraday efficiencies. The synthesized MIL-53 and NH2-MIL-53 samples exhibit suppressed framework flexibility compared to samples synthesized solvothermally.

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applications have focused on adsorption/separation,5−7 storage,8 and catalysis.9 When it comes to the synthesis of porous materials, high temperature and extreme pH conditions are usually needed to overcome the frequently high enthalpies of formation and/or to account for the slow kinetics of nucleation.10 Moreover, when thinking of the application and functionalization of these materials (i.e., the incorporation of active organo-molecules via encapsulation11,12 or the synthesis of coatings),13 extreme synthetic conditions constitute a tremendous drawback. In this regard, biomimetic synthesis of crystalline inorganic frameworks at room temperature and neutral pH has been demonstrated to work. Morse and his co-workers14 showed that silicateins involved in the biomineralization of sponge silica spicules are able to catalyze in vitro the formation of silica and amorphous or partially crystallized, nonbiological inorganic materials (titanium dioxide, titanium phosphates, gallium oxohydride, and gallium oxide). Later, Corma’s group showed that simple molecules such as tromethamine, cysteamine, and ethanolamine, functional mimics of the protein silicatein α, act as catalysts for the synthesis of Si-based molecular sieves at neutral pH and at room temperature.15 Although it is possible to synthesize large MOF crystals under certain mild synthetic conditions and to accelerate their precipitation by creating changes in the pH/solvent at room

ynthetic crystalline micro- and mesoporous materials have been extensively researched during the last few decades. They have applications in many different fields such as catalysis, adsorption/separation/storage, electronics, health, semiconductors, the food industry, and in detergents.1 Several unique aspects of these materials are responsible for their success: since they have a very high and tunable adsorption capacity, active sites of different strengths can be generated in the frameworks, the size of their channels and cavities falls within the range of that of many molecules of interest, and many materials present excellent ion exchange capabilities and exciting electronic properties, ranging from insulators to conductors and semiconductors.2,3 In addition, owing to their periodic nature, these materials are excellent playgrounds for scientists, since macroscopic events may be explained on the basis of interaction at the molecular level. Among the different classes, metal organic frameworks (MOFs) bridge micro- and mesoporous materials and present unprecedented topological richness. The combination of organic and inorganic building blocks offers an almost infinite number of combinations, enormous flexibility in pore size, shape, and structure, and plenty of opportunities for functionalization, grafting, and encapsulation. These materials hold very high adsorption capacities, specific surface areas, and pore volumes. Their porosity is much higher than that of their inorganic counterpart zeolites (up to 90% higher). Their thermostability is sometimes unexpectedly high, reaching temperatures above 400 °C. Obviously, MOFs have attracted much attention, the major studies have dealt with the synthesis of new structures,4 and most © 2012 American Chemical Society

Received: January 6, 2012 Revised: May 25, 2012 Published: May 30, 2012 3489

dx.doi.org/10.1021/cg300552w | Cryst. Growth Des. 2012, 12, 3489−3498

Crystal Growth & Design

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temperature,16 in the case of certain applications and for the sake of the fast and reproducible synthesis of large amounts of homogeneous samples, the development and understanding of new mild and rapid synthetic methods allowing continuous production is still imperative. Three different alternatives have been proposed: Son et al., who applied sonochemical methods, concluded that ultrasound can lead to homogeneous nucleation and to a substantial reduction in crystallization time compared to conventional oven heating in the synthesis of MOF-5.17 Pichon et al. demonstrated that mechanochemical, solvent-free synthesis of a Cu-based MOF is possible when ball-milling the framework precursors.18 Finally, using a different approach, BASF pioneered the electrochemical room temperature synthesis of the wellknown HKUST-1.19 Electrochemical synthesis facilitates metal salt-free and continuous production, which is a major advantage in any industrial environment. The principle relies on supplying the metal ion by anodic dissolution to a synthesis mixture that contains the organic linker and an electrolyte (see Figure 1).

2. EXPERIMENTAL RESULTS 2.1. General Considerations about Electrochemical Synthesis. As was already anticipated by Müller and his co-workers,19,23 electrochemical synthesis of MOFs can be performed in batch mode or in continuous flow operation. In the case studies presented in this paper, the experiments were performed in batch mode. In electrochemical synthesis, the metal ion is not supplied as salt but by oxidation of the electrode. The energy required to oxidize the anode can be supplied in amperometric or potentiometric mode. In the first case the voltage is fixed and current can be used to measure the reaction rate expressed as the speed at which the metal ions are dissolved. In potentiometric mode, the current through the cell is fixed and variations in voltage are related to the energy needed to dissolve the electrode. Both methods can be applied continuously over time and in wave or square functions. A general advantage of electrochemical synthesis is that it allows synthesis under milder conditions than typical solvothermal or microwave synthesis. It also reduces the time required for certain synthesis: whereas solvothermal synthesis might take hours or days, electrochemical methods can produce the material within minutes or hours. Electrochemical synthesis allows more control to be exercised over the reactant concentration in the synthesis over the course of time since it is performed without building pressure and so not only the metal can be added at different rates by controlling the anodic oxidation, but the linker can also be continuously added to the solution. In addition, it should be possible to carefully control the oxidation state of the metal simply by adjusting the voltage provided to the electrode. Several key points have to be carefully considered in the electrochemical cell configuration, as presented below. The main drawback with the electrochemical synthesis of MOFs is that in order to dissolve the organic building blocks, organic solvents are frequently needed, while little is known about the electrochemistry of such media. Moreover, organic solvents display a higher resistivity than aqueous solutions, and while it is clear that oxidation of the metal takes place at the anode, it is not easy to understand the chemical processes or processes taking place at the cathode of the cell. The physical location and orientation of the anode and cathode and the use of the appropriate reference electrode and/or potential therefore become crucial. It is also convenient to use a corrosion cell when conducting fundamental studies on metal oxidation in organic solvents.24 Reference electrodes can be used, but standard commercial electrodes use a saturated KCl solution, and KCl is not soluble in most organic solvents. To overcome this limitation, the reference electrode can be connected to the synthesis solution by a glass frit or a Luggin capillary that will only allow through it a very slow diffusion of water or solvent. When this approach is followed in prolonged experiments, a high junction potential difference is likely to develop in the glass frit which, in combination with the low conductivity of the organic solvent, will make it very difficult to correlate the voltage applied with theoretical data. Li et al.22 used a Ag/Ag(cryptand)+ as a reference electrode in the reductive electrosynthesis of MOF-5. Other reference electrodes for organic solvents can be found elsewhere.24−27 Another matter of concern is the limitation of standard reference electrodes for organic solvents at relatively high temperatures (>60 °C). In this paper, we have optimized reference voltages for a given electrochemical cell geometry. When a reference electrode was used, Ag/AgCl was chosen because of its broader application temperature range (−5−110 °C).28 Current distribution along the cell is the next factor to consider. It is important that the cell geometry leads to uniform current distribution, especially during the synthesis of thin MOF films. When no reference electrode is used, it is strongly recommended to maintain a constant distance between electrodes, since the current−voltage is determined by the cell geometry. Ideally, the cathode and the anode should be separated by a porous glass so that the products from the cathode do not affect the MOF synthesis at the anode; however, if the system conductivity is too low, this might not be possible. Because of the likely production of H2 at the cathode, electrochemical cells in aqueous solutions are usually equipped with a nitrogen bubbler in order to deaerate the solution and avert the formation of explosive

Figure 1. Schematic view of an electrochemical synthesis cell. Legend: WE = working or indicating electrode (red) GND = ground connection, protecting earth (green) RE = reference electrode (blue) CE = auxiliary or counter electrode (black) S = sense electrode.

Despite the promising features shown by the electrochemical method, only Hartmann et al.,20 Ameloot et al.,21 and more recently Li and Dinca22 have employed this synthetic approach. In the first case, HKUST-1 crystals were used in the adsorptive separation of butene from isobutene, and later electrochemical synthesis was used to manufacture high quality patterned coatings of HKUST-1 on copper electrodes.21 In the last example, electroreduction of oxoanions was shown to afford hydroxide equivalents that induce the selective deposition of MOF-5 on conductive Zn surfaces used as cathodes.22 In this work we present a detailed study of the main experimental variables governing the electrochemical synthesis of MOFs based on different metals (Cu, Zn, and Al) and linker connectivities (bi- and tridentate carboxylic acids and imidazoles), together with a critical assessment of the main advantages and limitations of the method. 3490



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atmospheres. Many organic solvents are too volatile to be deaerated. In many cases, the presence of water is even desired to increase conductivity, for counter electrode reaction, and even for the synthesis of metal species in solution prior to MOF synthesis. Ideally both the water and the organic solvent should be deaerated before being used and mixed in the corresponding amounts before the reaction. In addition, in view of the temperatures at which syntheses take place and the volatility of the organic solvents, the use of a reflux condenser is strongly recommended. Because of the low conductivity of the reaction media, electrolytes that enhance charge transport in solution need to be used. Tributylmethylammonium methyl sulfate (MTBS) has been recommended for syntheses carried out in organic media.29 As already mentioned above, organic solvents, water, and the electrolyte itself may be involved in the reduction reactions that take place at the counterelectrode. It is therefore important that the counterelectrode has a larger surface area than the working electrode: (i) in order to avoid rate limiting reactions at the counterelectrode and (ii) to minimize the effect of products from the cathodic reaction on the synthesis at the anode. The electrochemical cell used in this study has been designed in such a way that the electrodes are supported by PTFE holders, which only allow contact with the synthesis solutions through a circular opening of 25 mm in diameter. In this way, the exposed electrode surface and the current densities can be controlled. No reference electrode is used unless otherwise stated. Counter electrodes are stainless steel gauze electrodes so as to increase the surface area and to ensure that the counter electrode reaction does not limit the overall process. Following the ohmnic drop law:30 ΔV = IR = I

S aC

temperature simultaneously increases the solubility and the synthesis yield. In order to limit the influence of linker concentration during electrochemical synthesis, a saturated solution of the linker and an electrolyte is put in contact with two copper electrodes. By applying a given current, the Cu in the electrode is oxidized to Cu2+ in the solution. The rate of formation of HKUST-1 is rather high, and it already takes place in the double layer around the electrode where thin MOF films are formed (see video as a web enhanced object).21 The current density can be used to control the amount of Cu2+ present in the double layer, and therefore the formation of HKUST-1 crystals close to (or directly on) the surface. (a). Influence of Solvent. The ethanol−water ratio not only affects linker solubility, but also solution conductivity and the deprotonation of the BTC acid. The lower the ratio is, the higher the conductivity will be,and the higher the ratio is, the higher the solubility and the deprotonation will be.38 In order to produce HKUST-1, the ethanol content must be above 75 vol%. If less ethanol is present, a different coordination polymer called catena-triaqua-mu-(1,3,5-benzenetricarboxylate)-copper(II) is formed.13,39 Figure S2, Supporting Information shows the influence of the water content on the morphology of the products. At low ethanol content,the formation of other phases is promoted. (b). Influence of Temperature. Better solubility at a higher temperature yields higher linker concentration. As the synthesis occurs at atmospheric pressure, the temperature is limited by the boiling point of the solvent or the solvent mixture. In the case of HKUST-1 synthesized in EtOH/H2O mixtures, due to process and safety reasons, the higher synthesis temperature was 80 °C. Under hydrothermal conditions, temperatures above 120 °C have been reported which promote the formation of Cu2O.32,40 Because of the low temperatures used in this synthesis, no Cu2O was observed, as was the case with the room temperature synthesis previously mentioned.33 Standard synthesis solutions of 15 mmol (3.15 g) of BTC and 33 mmol (1.038 g) of MTBS dissolved in 100 mL 96%v/v EtOH are heated to three different temperatures in contact with two copper electrodes separated by 3 cm. Then, 50 mA are passed through the system for 1 h. On average ∼100 mg of dried HKUST-1 is obtained from each synthesis, showing low temperature influence in the synthesis yield. We speculate that in the case of HKUST-1, the synthesis velocity is clearly limited by the dissolution and diffusion of the metal and linker. The XRD patterns of

(1)

where ΔV is the voltage difference in V, I is the current in A, R is the resistance in ohm, S is the distance between the electrodes in cm, a the area in cm2, and C the conductivity of the solution in S cm−1; the expected voltage required to send 50 mA through a solution of 1 mS cm−1 conductivity is 30.56 V. The voltages used range from 2 to 30 V (the maximum ΔV from our system is 35 V) and the current intensities range from 1 to 20 mA/cm2. Our cell configuration and agitation allow us to reach ca. 30 V by passing 50 mA with a conductivity of only 530 μS cm−1 (Figure S1, Supporting Information). 2.2. Case Study I: Electrochemical Synthesis of HKUST-1. Copper benzene tricarboxylate was first reported in 1999 and named HKUST-1.31 Since then, it has become one of the most studied MOFs. It is frequently prepared under hydrothermal conditions with a copper salt (usually nitrate) and 1,3,5-benzenetricarboxylic acid (BTC). The precursors are dissolved and mixed in mixtures of ethanol (EtOH) and water, and finally synthesized at temperatures between 25 and 80 °C,31,32 although synthesis at room temperature is also possible.33 It was also the first MOF to be electrochemically synthesized.19,23 The nucleation and crystallization of HKUST-1 have been studied by means of dynamic light scattering,34 atomic force microscopy (AFM)35 and X-ray diffraction.36,37 It was concluded that crystallization is dominated by homogeneous nucleation and growth, with continuous nucleation during all syntheses at all the temperatures studied. The initial formation of nuclei is effectively instantaneous without induction time. The activation energy for nucleation is 71.6 kJ mol−1, whereas the activation energy for crystal growth is 63.8 kJ mol−1.37 Looking at the relationship between cell potential and Gibbs free energy:30

Figure 2. XRD patterns (Co-Kα radiation) of samples electrochemically synthesized at different temperatures compared to the simulated pattern of HKUST-1. N2-BET surface areas (calculated between 0.01 and 0.05 p/p0) are indicated for each sample.

(2)

ΔG = − nF ΔV −

the reaction products are shown in Figure 2. The patterns fit in very well with the theoretical calculated pattern, and the calculated surface areas correspond well with those given in the literature (BET area = 1400 m2/ g, pore volume = 0.6 cm3/g).31,32 No Cu2O is observed. Scanning electron microscopy micrographs show a wide distribution of sizes in the resulting HKUST-1 powder. The crystal sizes range from several hundred nanometers to several micrometers (Figure S3, Supporting Information). Because of the fast formation of HKUST-1, it is

where n is the moles of electrons transferred (6 mol e per mol of Cu3(BTC) 2); F is the Faraday constant (96485 C mol−1 e−); and V is the cell potential. For a standard value of 30 V in the cell, ΔG = −17.4 MJ mol−1. So, to overcome the nucleation activation energy of 71.6 kJ mol−1, only 0.124 V are necessary. The main limitation for the formation of HKUST-1 is the linker solubility (∼3 g/L in 96% ethanol at room temperature). Increasing the 3491



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reasonable to argue that large crystals are formed on the surface of the electrode, while small crystals are directly formed in solution. (c). Influence of Conductivity and Current Density on Kinetics. By controlling the current density, the Cu2+ concentration close to the electrode can be tuned. However, current density and voltage are interrelated through the system geometry and the solution conductivity. A standard synthesis solution with 15 mmol of BTC and 33 mmol of MTBS in 100 mL of 96%v/v EtOH displays a conductivity of ca. 530 μS/cm. By increasing the electrolyte concentration 5 and 15 times, conductivity can be increased to 1.86 and 3.6 mS/cm, respectively. Increasing conductivity improves the yield because less energy is thus required to overcome the ohmnic drop in the solution and can therefore be used to dissolve the electrode. As expected, higher conductivity results in higher yield. The high impact of the ohmnic drop is remarkable because a 3.5 times increase in conductivity only increases the yield by a factor 1.6; and a 7 times increase in conductivity only increases the yield by a factor of 2.

Table 1. The HKUST-1 Yield for a System with 15 mmol BTC in 100 mL of 96% v/v Ethanol and Different Amounts of electrolytea current [mA]

conductivity [mS/cm]

time [h]

Faraday efficiencyb (%)

productivity [mg/h]

50 50 50

0.530 1.860 3.670

1 1 1

37 57 68

103 ± 20 160 ± 25 215 ± 25

a

Syntheses were performed at 40°C. bFaraday efficiency is determined with ±10% accuracy. (d). Influence of the Current Density on the Anodic Deposition of HKUST-1. To study the influence of the current density on the electrode surface, a corrosion cell was used. It was only after a short period of time that HKUST-1 crystals formed on the electrode. Figure 4 shows the electrode surface of systems where the same number of Faradays have been applied at varying current densities. Unlike with nucleation theory, higher current density does not lead to a large number of nuclei and small crystals.13 A formation of HKUST-1 films directly on the surface of the electrode via electrochemical synthesis is also possible. At the beginning of the experiment, when fresh copper electrode is available and exposed to the synthesis solution, the formation of the MOF layer takes place at the electrode. As the synthesis continues, new crystals grow on the uncovered metal surface. As the electrode surface is fully covered, the HKUST film thickens, and the charge transfer and transport of both Cu2+ and the linker become more difficult. If a complete film of about 10 μm is formed (one or few crystals of wide film), further dissolution of copper would cause the MOF film to detach and the new electrode surface would be available for further reaction. A video of a synthesis in the corrosion cell showing the quick formation of HKUST-1 crystals on the surface of the electrode is available. When square wave functions are used to control Cu2+ release (Figure 3), the best results are obtained in terms of electrode coverage. In this way the inhibiting effect of concentration polarization is alleviated. In experiments performed for the same period of time, it was observed that low current densities result in a large number of small crystals (Figure 3). Ex-situ AFM studies (Figure 4) confirmed this observation. In addition, terrace-like crystal growth was observed in the [111] direction, similar to in solvothermal synthesis.35 2.3. Case Study II: Electrochemical Synthesis of ZIF-8. ZIF-8 was first reported by Huang et al.41 and a more efficient solvothermal synthesis was described by Park et al.42 ZIF-8 has proven to be one of the easiest MOFs to be synthesized. Cravillon et al.43 reported rapid roomtemperature synthesis in methanol (MeOH), and recently Pan et al.44 reported its synthesis in aqueous solution. The different stages in ZIF-8 formation (nucleation, crystallization, and growth) have been studied by X-ray diffraction (XRD)45 and small-angle X-ray scattering.46 In the latter case, an excess of the ligand is used (in a similar way to the situation close to the electrode that produced Zn2+ ions). Nanometer-size cubical

Figure 3. SEM micrographs of the HKUST-1 layers obtained on top of copper electrodes under different electrochemical conditions. EtOH/ H2O 75:25 vol%. 3.15 g (15 mmol) of BTC and 1 g (33 mmol) of MTBS per 100 mL solution. (a) 30 mA/cm2 60 s; (b) 10 mA/cm2 200 s; (c) 3 mA/cm2 600 s; (d) 30 mA/cm2 33 s; (e) 10 mA/cm2 100 s; (f) 1 mA/ cm2 1000 s; (g) 5 mA/cm2 5 s, 0 mA/cm2 5 s; (h) 10 mA/cm2 5 s, 0 mA/ cm2 5 s; (i) 0.5 V/5 s; 0 V/5 s.

Figure 4. AFM micrographs of HKUST-1 coatings synthesized using different current densities after 60 s. The areas represented are 5 × 5 μm2. crystals are obtained after some minutes, and the change of crystal shape from cubes to rhombic dodecahedra only occurs in later stages of growth. By XRD analysis it was deduced that ZIF-8 crystallinity increases slowly at short synthesis times (70 45 40

0.042 0.775 0.125 0.010

Figure 7. XRD analysis of electrochemical NH2-MIL-53 for different H2O/DMF volume ratios. Elemental analyses were performed after every synthesis in order to determine whether some electrolyte was present in the crystalline structure after synthesis. In the case of MIL-53(Al) and NH2-MIL53(Al), amounts of potassium below 0.1 wt.% were found. The electrolyte was KCl except in the case of pure DMF, when MTBS was used. NH2-MIL-53(Al) was formed in every case as pure phase, according to Al/C ratios and XRD. Different pore configurations were obtained at different H2O/DMF ratios. It was predominantly the np phase that was obtained when the synthesis was carried out in pure water, while an almost completely open structure (lp) was obtained for the experiments in the presence of DMF. Apparently, the presence of DMF seems to stabilize the lp configuration of the material. Figure S9, Supporting Information shows the nitrogen adsorption isotherms for the different samples. The results correspond well with the XRD data. When only H2O is used as a solvent, the highly crystalline material shows an isotherm with only micropores. As the structure is partially closed, the amount of N2 adsorbed is lower. When DMF is added to the system microporosity is preserved. As the DMF/H2O ratio increases the crystallinity of the material becomes less and particles become smaller, while the presence of interparticle mesopores is clearly observed at high P/P0. Figure 8 shows the diffuse reflectance infrared Fourier transform (DRIFT) spectra of the regular NH2-MIL-53(Al)np (prepared by hydrothermal synthesis), and electrochemically synthesized samples synthesized using an H2O/DMF (lp). When focusing on the hydrothermally synthesized sample, the broad band centered around 3680 cm−1 along with a shoulder at 3693 cm−1 is assigned to the bridging hydroxyl group of the MIL-53, that is, Al-OH-Al (μ-hydroxo groups). The shoulder position is very close to that reported for unfunctionalized MIL-53, while the main band is slightly red-shifted. We attribute the main band to hydroxyl groups which directly interact with the neighboring amines, while the shoulder at 3693 cm−1 corresponds to surface -OH groups not engaged in hydrogen bonding.61,62 The two sharp bands at 3385 and 3495 cm−1 in the spectrum of the hydrothermally synthesized NH2-MIL-53(Al) represent the symmetric and asymmetric vibrations of the respective NH2 groups. For samples synthesized electrochemically, a clear shift is observed in the amine vibrations along with a change in the ratio and a split between the two μ2hydroxo vibrations. We attribute this change to the opening of the structure in the smaller electrochemically synthesized NH2-MIL-53(Al) particles: as the structure opens, H-bonding between NH2 groups and pending hydroxyls is lost. On the other hand, the DRIFT spectra of the

Faraday efficiency is determined with ±10% accuracy.

Table 3 summarizes the different syntheses performed. The use of KCl or NaOH results in a conductivity of ∼9 mS cm−1 compared to 1.70 mS cm−1 of MTBS. As the synthesis advances, protons are reduced to H2 and substituted in the solution by Al3+, thus increasing the basicity of the solution. The final pH can be as high as 12. By applying a current of 2 mA/cm2 for 3 h, 125 mg of material is recovered (6.2% yield with respect to the initial amount of linker); when the current is set at 100 mA (20 mA/cm2), ca. 750 mg of material is obtained per hour (38% yield during the first synthesis hour). It has to be stressed that an increase in current does not translate into a proportional increase in yield because of the intrinsic synthesis kinetics (the linker concentration is similar in both cases). When MTBS is used as the electrolyte, conductivity drops to 1.70 mS/cm with a subsequent decrease in productivity (1% yield of initial linker or 10 mg product per hour). In all the syntheses high Faraday efficiencies were obtained, with crystals smaller than 1 μm, surface areas of ∼1200 m2/g and pore volumes of 0.5−0.8 cm3/g (Figure S8, Supporting Information) compared to 716 m2/g and 0.4 cm3/g of Basolite A100 (commercial MIL-53 (Al)). The XRD pattern corresponds with the large pore configuration of the material (MIL-53lp), and no other structures like MIL-101, MIL-68, or MIL-88 are observed. Figure S7, Supporting Information compares the XRD patterns of the large (lp) and narrow pore (np) configurations with the powder XRD of MIL-53 obtained hydrothermally and electrochemically. The hydrothermally synthesized sample shows an np configuration in its hydrated form (room temperature), in contrast to the electrochemically synthesized sample, which has an lp configuration. As is illustrated by Figure S7, Supporting Information and below, all MIL-53(Al) samples (functionalized and nonfunctionalized) synthesized under electrochemical conditions showed a certain degree of pore opening. (b). NH2-MIL-53. (b.1). Influence of the Solvent. One of the main advantages in the synthesis of NH2-MIL-53 is the higher solubility of the linker (aminoterephthalic acid) in water compared to other linkers. To study the influence of the solvents in the synthesis of NH2-MIL-53, four different solutions (100 mL) at varying H2O-DMF volume ratios were prepared 3494



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Figure 9. CO2 adsorption isotherms at 25 °C for different NH2-MIL-53 synthesized electrochemically compared with the adsorption isotherm of the hydrothermally synthesized material in the presence of KCl. Closed symbols represent adsorption, and open symbols show the desorption isotherm. form and finally to an almost pure lp form. In contrast, a single step isotherm is found for all materials synthesized under electrochemical conditions, with clear differences between the samples: the np displays a lower CO2 capacity than samples with a more pronounced lp configuration. The saturation loading for the np sample is slightly higher than that of the NH2-MIL-53(Al)np, while the more open framework shows a capacity slightly lower than the breathing material in its lp configuration. Furthermore, no hysteresis in the adsorption isotherm is found for the electrochemically synthesized samples, confirming the absence of cell expansion/contraction during CO2 adsorption. Similar, nonbreathing behavior was found with the electrochemically synthesized MIL-53(Al) (see Supporting Information).

Figure 8. DRIFTS spectra of hydrothermally synthesized NH2-MIL53(Al) and electrochemically synthesized NH2-MIL-53(Al). The bottom graph zooms in on the amine-OH region of the top spectra.

3. DISCUSSION Through the different case studies, we have shown that electrochemical synthesis offers many possibilities for the synthesis of metal organic frameworks. According to our results, highly reactive metal species are formed upon anodic dissolution. As a result, both synthesis time and temperature can be strongly reduced when compared to regular hydro- or solvothermal synthesis protocols. In the literature, it has been shown that by changing the metal source, MOF formation kinetics can be tuned. One clear example is the use of Cu(NO3)2 vs Cu(OAc)2 in the synthesis of HKUST-1.33 In the case of electrochemical synthesis, it is easy to envisage that the absence of counteranions further facilitates the formation of the MOF phase, avoiding the generation of waste in the form of, for instance, nitrates or residual impurities in the MOF. On the other hand, the solubility of the linker plays a major role. As most syntheses are carried out under quite mild temperature conditions, this might be a drawback, due to the relatively low linker concentrations achievable in batch mode. However, if a continuous synthesis protocol can be adopted, high synthesis yields with almost full linker conversion can very easily be envisaged. Moreover, when highly soluble linkers are used, as was the case for ZIF-8, the production of large quantities of very pure MOF is possible. The Faraday efficiencies can be high in all the systems studied, although a clear interplay between kinetics and linker availability affects the overall efficiency of the process. The production of MOF coatings on electrodes is also an attractive feature of electrochemistry. In the case of HKUST-1, the formation of high quality MOF films is possible, even when different electrodes such as copper meshes are used (see Figure 11). In this respect, electrochemical synthesis holds much promise for optoelectronic and sensing applications, where high

electrochemically synthesized MIL-53 samples rule out the presence of strong defects. For example, free terephthalic acid presents its main vibration at 1700 cm−1, associated with free carboxylic acid groups, and this vibration is shifted to 1640 cm−1, when carboxylic groups are coordinated. The coordination of DMF is not likely either, since no significant vibrations are observed around 2950 cm−1. (b.2). Effect of Electrochemical Synthesis on the NH2-MIL-53(Al) Breathing. The flexibility of the MIL-53 network is still a matter of debate. The removal of guest molecules, such as linker or solvent, by heating leads either to an expansion (in the case of Al, Ga, and Cr) or to a contraction of the unit cell (in the case of Fe).63 Although several attempts to rationalize the breathing dynamics of these frameworks have been published,64,65 very little is as yet known. Recently we discovered that the enhanced performance of the NH2-MIL-53(Al) framework in CO2 capture and separation is mostly due to the specific flexibility that results from a delicate interplay of weak dispersion forces controlling the flexibility of the framework. In contrast to its unfunctionalized counterpart, the np form is preferred at low adsorbate pressures in the NH2-MIL-53(Al). Only at high CO2 partial pressures does the framework expand to its lp form.61,66,67 In order to discover whether the synthesis procedure has any effect on the breathing behavior of the electrochemically synthesized MIL-53(Al) samples, high pressure adsorption isotherms of CO2 were measured and compared with a NH2-MIL-53(Al) sample synthesized hydrothermally in the presence of a similar concentration of KCl. (Figure 9). The regular HT-NH2-MIL-53(Al)KCl displays a two-step CO2 isotherm due to the breathing properties of the NH2-MIL-53(Al) framework, which is identical to samples synthesized under standard conditions (in the absence of KCl). Because of the strong interaction with the material, CO2 adsorbs efficiently in the np, accommodating around 2 mmol g−1 CO2. The first plateau in the CO2 isotherm extends to about 1.5 MPa. A step in adsorption capacity is visible at higher pressure; at 3 MPa the material adsorbs up to 8 mmol of CO2 per gram. This step thus corresponds to the transition from the np form to a mix of the np and lp 3495



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copy, TGA, elemental analysis, and XRD do not indicate the existence of defects like free linker or aluminum (hydr)oxide within the pores of the materials, we speculate that it is because of the rapid kinetics of formation (due to the presence of highly reactive Al3+ species in solution) that intergrown MOF particles containing a large density of grain boundaries are formed. Such grain boundaries within a single particle would stabilize the structure thus impeding the breathing. The fact that the kinetics of formation strongly influences the degree of structure opening confirms our hypothesis: samples synthesized in the presence of DMF and at high temperatures (higher linker solubility) show a high fraction of lp over np form, while samples synthesized in pure water and at low temperature are mostly in the np form. Although additional work is needed to fully understand this phenomenon, the results presented here demonstrate that nonbreathing analogues of breathing structures can be synthesized electrochemically and that unit cell parameters can be tuned by varying the synthesis conditions. These materials can be used as models of metastable phases for fundamental research and might offer several advantages for applications where crystal breathing is not desired (i.e., in coatings or even in the shaping of pellets, where the continuous expansion and reduction of crystals may result in the formation of cracks).

Figure 10. HKUST-1 coating electrochemically deposited on a copper mesh.

Figure 11. AFM micrograph of a NH2-MIL-53(Al) sample synthesized electrochemically.

quality coatings are required on conductive surfaces.68 However, as we show, electrochemical coating is not trivial for other MOF structures. Under the studied conditions, a certain induction time was observed for the synthesis of all other MOFs, so resulting in homogeneous nucleation in the liquid phase rather than in the formation of coatings on top of the electrodes. Indeed, the few in situ studies of the formation of ZIF-8 or MIL-53 point at separated nucleation and crystal growth processes.36,37,59 It goes without saying that if nucleation is the rate-determining step, then the growth of films at the double layer close to the electrode will not be trivial. A way to overcome this limitation could be to use elevated temperatures and to begin the electrochemical deposition after temperature stabilization. To this end, electrochemical pressure cells should be designed to withstand hydrothermal conditions and to cope with the gases generated during electrochemical synthesis. It is fair to admit that we are a long way from understanding the phenomena taking place during electrochemical synthesis. More fundamental research to unveil the different chemical processes happening at both the anode and the cathode during the formation of MOFs is needed. In order to realize such ideas, new reference electrodes, applicable under MOF synthesis conditions, need to be developed. The impact of synthesis on the flexibility of MIL-53(Al) and NH2-MIL-53(Al) deserves special attention. Pure phase MIL-53 can be produced electrochemically. However, synthesis conditions have a strong effect on the resulting unit-cell parameters. More importantly, in contrast to the hydrothermally synthesized MIL-53s, those that are electrochemically synthesized do not show any breathing through the adsorption of CO2. XRD collected after the adsorption of solvents like toluene or methanol did not show any cell expansion either, as would be expected for MIL-53 analogues.69 It has been proposed that particle size might have a marked effect on breathing behavior.64,65 As shown in Figure 11, NH2-MIL-53(Al) grains smaller than 100 nm are found after electrochemical synthesis. However, the complete absence of hysteresis in the CO2 isotherms together with the low loading obtained for the np configuration samples suggest that the small size of the synthesized particles is only indirectly responsible for the absence of breathing. Although AFM analysis, FTIR spectros-

4. CONCLUSIONS MOFs based on copper, aluminum, or zinc and linkers with different coordinating moieties and connectivities can be synthesized via anodic dissolution. Electrochemical MOF synthesis has various advantages: • Faster synthesis at lower temperatures than conventional synthesis • Metal salts are not needed and therefore separation of anions such as NO3− or Cl− from the synthesis solution is not needed prior to solvent recycle. No anionic residues end up in the MOFs • Virtual total utilization of the linker can be achieved, in combination with high Faraday efficiencies. Although this approach appears promising for the synthesis of coatings, it seems to be limited to situations where nucleation takes place almost instantaneously. A fine-tuning of the synthesis conditions will therefore be needed for MOFs like MIL-53, MIL100, or ZIF-8. Last but not least, the synthesis method has a clear effect on the breathing behavior. The electrochemically synthesized MIL-53s show that the breathing effect upon adsorption is absent.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. W Web-Enhanced Features *

A video of a synthesis in the corrosion cell showing the quick formation of HKUST-1 crystals on the surface of the electrode in AVI format is available.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3496



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Article

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ACKNOWLEDGMENTS Prof. Mark Koper, Dr. Gonzalo Garcia,́ and Dr. Paramaconi Rodriguez-Perez from Leiden University are acknowledged for the fruitful discussions they provided. Marcel Bus (TU Delft) is gratefully acknowledged for the AFM characterization. J.G. gratefully thanks the Dutch National Science Foundation (NWO-CW-VENI) for its financial support.

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