Solventless Synthesis of MOFs at High Pressure - ACS Sustainable

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Solventless synthesis of MOFs at high pressure Lorena Paseta, Gregory Potier, Sara Sorribas, and Joaquin Coronas ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00473 • Publication Date (Web): 30 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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ACS Sustainable Chemistry & Engineering

Solventless synthesis of MOFs at high pressure Lorena Pasetaa, Grégory Potierb, Sara Sorribasa†, Joaquín Coronasa,* a

Chemical and Environmental Engineering Department and Instituto de Nanociencia de Aragón

(INA), Universidad de Zaragoza, 50018 Zaragoza, Spain b

Département Sciences des Matériaux, Polytech Nantes, 44306 Nantes, France

*Corresponding author: [email protected] KEYWORDS: Solventless process, High pressure, Metal organic frameworks, Zeolitic imidazolate frameworks, ZnO.

ABSTRACT: Besides the substitution or minimization of the use of harmful solvents, one essential goal of chemistry is to try to avoid their use altogether whenever possible. In the case of the synthesis of MOFs (metal-organic frameworks), this can only be achieved by finding alternatives to conventional processes. An example is the approach described here which involves working at high pressure (at 0.31 GPa) without using a solvent. This has evident advantages over mechanochemical synthesis by grinding or milling (also a solventless process) where the sample is submitted to attrition. The present paper reports the simple high pressure synthesis of the ZIF (zeolitic imizadolate framework) ZIF-8. This methodology enables fast synthesis of MOF materials and offers new insights into their industrial implementation. In addition, this technique could be applied to the synthesis of other MOFs and even COFs (covalent organic frameworks).

ACS Paragon Plus Environment

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INTRODUCTION Solvent free chemistry, and in particular its application to the synthesis of metal-organic frameworks (MOFs),1 constitutes a challenge of great environmental importance due to the possibility of achieving the wasteless production of substances of interest.2 This solid-state chemistry usually relates to mechanochemistry or mechanosynthesis which, besides being a sustainable process, has the advantage of short reaction times and the possibility of working at room temperature with low solubility precursors (which in turn would require of high solvent volumes).3 The required mechanical impact is generated by grinding, milling, shearing, scratching, polishing or rapid friction.4 High pressure contact does not enter into the description of mechanosynthesis, even though high pressure is able to provoke transitions between different polymorphs of certain drugs,5 with the possibility of altering their nature during tableting, typically carried out at 0.1-0.4 GPa.6 However, high pressure has never before been used to provide intimate contact between static moieties in the solventless synthesis of MOFs. This constitutes the aim of this work and will add a new perspective to the production and application of MOFs. MOFs are crystalline and porous materials formed by the assembly of metal ions or clusters with organic ligands resulting in neutral 1D, 2D or 3D structures.7 The two most important families of MOFs are those based on carboxylate and imidazolate ligands. Some of the earliest known MOFs based on Zn, Cu and Cr carboxylates, using terephthalic or trimesic acids, were MOF-5,8 HKUST-19 and MIL-53,10 respectively. The large pore volume and the high surface area of these materials11, along with the possibility of modifying their porosity and changing the


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functionality of the organic linker8, make them suitable for use in a large number of applications including catalysis,12 the separation and storage of gases,13 encapsulation,14 medicine15 and membranes.16 The so-called zeolitic imidazolate frameworks (ZIFs) are MOFs formed by metal ions (e.g., Zn2+, Co2+) with tetrahedral coordination geometry bridged by imidazolate ligands. The name given to these MOFs derives from the fact that the angle formed between the ion metal and the ligand is similar to the angle formed between the aluminum or silicon and the oxygen atoms in zeolites (145°).17 The best known ZIF is ZIF-8 which has a SOD type structure, in which the metal is Zn and the linker is 2-methylimidazolate.17 This MOF, with large pores of 11.6 Å connected through small apertures of 3.4 Å, outstanding high thermal stability (up to 400 ºC) and of a hydrophobic character, has been widely studied for different applications, including gas separation,18 encapsulation,19 H2 storage20 and catalysis.21 Like many other porous materials, MOFs are conventionally synthesized under solvothermal conditions, sometimes using organic solvents (e.g. N,N-dimethylformamide, methanol, etc.) due to the poor water solubility of the corresponding moieties,22 which are expensive and unhealthy for humans. In addition, this kind of synthesis may require long reaction times (hours or days), parameter that needs to be taken into account for scaling up the process to take advantage of the multiple potential applications that these materials have, for example in the manufacture of powders and coatings. For these reasons, a number of new synthetic techniques have been proposed: a hydrothermal process,23 microwave-assisted synthesis,24 sonocrystallization,25 electrochemical synthesis26 and the above mentioned solventless mechanochemical synthesis.27 The latter approach can be considered as a green methodology as no solvents are used in the


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synthesis stage. Besides ZIF-8,27 mechanochemical synthesis has been applied to other MOFs such as ZIF-4,28 HKUST-1,29 MOF-1429a and [Cu(INA)2].1, 29b In the present work, MOFs (ZIF-8 (Figure 1a) and Zn(bIm)2) were synthesized without solvent under high pressure (Figure 1b). There are only a few reports dealing with the effect of high pressure on the structure of MOFs such as MOF-5 (which becomes amorphous at 3.2 GPa)30 and ZIF-8 (which undergoes a significant increase in its cavity volume at 1.5 GPa).31 In addition, it has been reported that compression to 1.6 GPa followed by decompression exerts a reversible effect on the ZIF-8 framework.32 The approach of solventless high pressure synthesis may be superior to that based on mechanochemical synthesis where the materials are submitted to attrition. In addition, as well as mechanosynthesis, this new eco-friendly methodology could be implemented in the synthesis of countless materials, bringing them closer to the requirements of industrial interest.

Figure 1. a) ZIF-8 structure with the ZnN4 tetrahedra in green and carbon atoms from ligand molecules in grey. This structure was builtwith Diamond 3.2. using the corresponding crystallographic data22 (Refcode: VELVOY01, CCDC: 602542). To clarify, the guest molecules have been omitted. b) Scheme of device used for the solventless high pressure synthesis of ZIF-8 from ZnO.


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EXPERIMENTAL SECTION Syntheses of ZIF-8. In a typical synthesis of ZIF-8, 4 mmol (0.405 g) of ZnO (Sigma-Aldrich, ≥99%) together with 8 mmol (0.820 g) of 2-methylimidazole (Sigma-Aldrich, 99%) were placed in a vial (8 mL) and mixed by hand shaking for about 1 min. This mixture was then placed at room temperature inside the metal cylinder (Supplementary Fig. S1) of a hydraulic press (Specac 25.011). After insertion of the metal piston, pills were compacted under a pressure of 0.31 GPa during different periods of time (2, 5, 10, 20 and 60 min). After washing with up to 60 mL ethanol and centrifugation, about 300 mg was recovered and dried at room temperature. Zn(bIm)2 was synthesized at 10 min following the same procedure but replacing the 2methylimidazole by benzylimidazole (8 mmol, 1.181 g, Sigma-Aldrich, 98%). Similarly to mechanosynthesis,28a the previous procedure was repeated adding 10 mg of NH4NO3 to the synthesis solid mixture, as a way of increasing the reaction yield. Two experiments were carried out for 10 min, one for the synthesis of ZIF-8 and the other for the synthesis of Zn(bIm)2. For the purposes of comparison, ZIF-8 was prepared by conventional solvothermal synthesis as follows.19 Two different solutions were prepared. First, 3.09 g (37.6 mmol) of 2methylimidazole was dissolved in 30 mL of methanol. Second, 0.96 g (3.2 mmol) of Zn(NO3)2•6H2O (Sigma-Aldrich, 98%) was dissolved in a mixture of 10 mL of methanol and 10 mL of distilled water. Both solutions were then mixed and stirred during 2 h at room temperature. The suspension obtained was then centrifuged at 10000 rpm for 10 min. The solid recovered was washed with methanol, centrifuged under the same conditions and then dried at 80 °C overnight.


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Characterization. Powder X-ray diffraction (XRD) was performed at room temperature in a D-Max Rigaku diffractometer with a copper anode and a graphite monochoromator so as to select Cu-Kα1 radiation (λ=1.5406 Å). Data were collected in the 2θ range=2.5-40°, and the scanning rate used was 0.03°/s. Thermogravimetric analyses (TGA) were carried out using Mettler Toledo TGA/SDTA 851e equipment. The samples were put in 70 µL alumina pans and heated up to 700 °C with a heating rate of 10 °C/min under air atmosphere. The ZnO yield to ZIF-8 was estimated from the thermogravimetric mass loss, considering ZnO and mIm dry basis (i.e. discounting both solvent and excess ligand) mass losses in pure ZIF-8 of 62.8% and 37.2% (Figure S2), respectively, as follows: % ·

. !, ##

% = $%&&' % ( ) *+ · 100

(1)

where mIm (methylimidazolate ligand) in sample corresponds to the TGA mass loss (%) above 300 ºC upon normalizing to 100% the total loss of mass. In other words, the mass loss below 300 ºC, related to unreacted ligand, was not considered for this calculation. Powder samples were characterized by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The spectra were recorded in the 4000-600 cm-1 wavenumber range with an accuracy of 4 cm-1. The equipment used was a Bruker Vertex 70 FTIR spectrometer equipped with a deuterated triglycine sulfate detector and a Golden Gate diamond ATR accessory. Scanning electron microscopy (SEM) was performed using an Inspect-F microscope (FEI) operated at 10 kW. The samples were prepared over a magnetic strip by coating with gold under vacuum conditions.


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Transmission electron microscopy (TEM) images were recorded on a FEI TECNAI F30 at an acceleration voltage of 300 kW. To prepare samples for TEM, 1 mg of the synthesized solid was suspended in a glass vial with 5 mL of ethanol and placed in an ultrasound bath for 2 min for good dispersion. A drop of this suspension was poured onto a copper grid (200 mesh) coated with a carbon film and allowed to dry in air.

Nitrogen adsorption-desorption isotherm were performed using a Micrometrics Tristar 3000 with N2 at 77 K. The samples were outgassed under vacuum for 8 h at 200 °C. The micropore volume was determined with the t-plot method.

RESULTS AND DISCUSSION Solventless synthesis of ZIF-8 at high pressure. In principle, solid-solid reactions (mechanosynthesis) require mortar and pestle or ball mills.33 We demonstrate here in the case of the synthesis of ZIF-8 from 2-methylimidazole (HmIm) and ZnO that a solventless, static reaction is also possible by working at 0.31 GPa high pressure (HP) at room temperature. In addition, the prolonged grinding required in mechanosynthesis may promote amorphization.34 An advantage of our proposed new methodology is that such grinding is avoided. Therefore, this study focuses on the synthesis of Zn imidazolates, and in particular on the combination of HmIm and Zn compounds. No apparent reaction was observed when Zn nitrate, acetate and chloride salts were used (e.g. after washing with ethanol the solids obtained at high pressure were totally dissolved), thus ZnO was chosen as the metal source for the Zn imidazolate synthesis. In fact, this matches with the use of ZnO as a Zn precursor to produce ZIFs by mechanosynthesis,27-28 and with the fact that the use of different metal salts may affect the


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reactivity of the ligand, as demonstrated for ZIF-835 and ZIF-67.36 As shown in Figure 2, X-ray powder diffraction revealed the presence of ZIF-8 (Zn(mIm)2) reflections together with those coming from unconverted ZnO. As shown in Table 1, the yield of ZnO to ZIF-8 was about 6.5±0.2% (average of three different samples, which highlights the reproducibility of the synthesis), as estimated from TGA analyses in air (Supplementary Fig. S2), giving rise to a concentration of MOF in the final solid of about 14 wt% in dry basis. This is consistent with the low intensity XRD peaks due to ZIF-8 diluted with unreacted ZnO, as shown in Figure 2, after 10 min of HP reaction. It is worth mentioning that in Figure S2, the TGA curves do not show mass loss below 300 ºC proving that activated material is obtained. The yield augmented to 14% (30 wt% of MOF in the final solid) by adding NH4NO3 salt to the reaction mixture. The role of the salt could be related to ion inclusion in the porous ZIF structure, which would enhance and direct the synthesis.28a It is worth mentioning that the yield using a reaction time of up to 60 min was in the 6.5-9.3% range (see Table 1, without NH4NO3), with evident XRD features related to ZIF-8, even though some small ligand intensity can be observed around 26º 2·theta value (in case of 10 mina sample), see Supplementary Fig. S3. The Fourier-transform infrared (FTIR) spectrum (Supplementary Fig. S4) also demonstrated that the HP-obtained sample corresponded to ZIF-8 material. The intensities assigned to C=N (1590 cm-1), ring stretching (1440 cm-1) and in-plane (990, 1130, 1300 cm-1) and out-plane (685, 760 cm-1) bending32 are observed in both ligand and ZIF-8 materials. The SEM observations shown in Figure 3 related to 2, 5, 10 and 60 min samples confirm the synthesis of ZIF-8 at high pressure with the typical shape of a rhombic dodecahedron37. Even though the size dispersion is large, as shown in Fig. S5, the average particle size increased from about 0.1 µm at 2 min to about 0.25 µm after 20 min, suggesting some kinetic effect in the high


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pressure synthesis. Even though there might be some contribution of the washing to the final ZnO yield to ZIF-8, Fig. S5 represents a clear evolution with time under high pressure. In any event, different washing times (1, 5 and 30 min) with ethanol were applied to the obtained pills. The peaks corresponding to ZIF-8 being present in all the three samples with no clear evolution of crystallinity with washing time (Fig. S6).

Table 1. ZnO yields to ZIF-8 for the different synthesis times in studio. To highlight reproducibility, three samples were repeated in the same conditions (10 min), giving rise to the errors shown. Mass loss, related to mIm (methylimidazolate ligand) in sample, was considered above 300 ºC upon normalizing to 100% the total loss of weight.

Synthesis time (min)

Mass Loss (%)

Yield (%)

5

11.3

7.4

10

9.9±0.2

6.5±0.2

20

13.6

9.3

60

13.3

9.1

10 (NH4NO3)

19

14


9


ZIF-8 HP 10'

(031) (222)

(112)

(022)

(011)

ZIF-8 solvothermal (002)

Intensity [a.u.]

ZIF-8 HP 10' NH4NO3

10

ZIF-8 simulated

°

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20

30

2θ [

40

]

Figure 2. XRD patterns of ZIF-8 from simulation with Diamond 3.2. using the corresponding CIF file (Refcode: VELVOY01, CCDC: 602542),22 conventional solvothermal synthesis, 10 min high pressure (HP) and 10 min HP reaction in the presence of NH4NO3. The peaks marked with asterisks correspond to ZnO. Up to 2·theta 30º, the XRD patterns for the HP syntheses have been multiplied by 5 (10’) and 10 (10’ NH4NO3).

High ZnO yields have been reported in previous mechanochemical syntheses of ZIF-8. Tanaka et al.27 reported increasing ZIF-8 TGA yields from 36% to 82% as a function of milling time from 3 h to 240 h. However, despite the relatively low ZnO yield to ZIF-8, we believe that our static high pressure is still of great interest and, of course, of general applicability. Not only is attrition avoided, but also the field is opened to other applications, for instance the easy coating of a surface by simple high pressure implementation. In fact, the attrition created during the synthesis process of the new reactant surface may be behind the high yields observed with ball milling. However, this may hinder the obtaining of shape-defined crystals of a given size.


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Moreover, the above-mentioned possible coating application at high pressure without any solvent would benefit from simple and fast processing, bringing MOFs closer to industrial conditions of interest. As shown, it took only 2 min to produce the synthesis of ZIF-8 from ZnO without the use of solvent (it is worth mentioning that ethanol was only used to remove excess ligand, mainly for characterization purposes). In addition, the high pressure was necessary. The mere mixture of the two solid reactants and their prolonged contact for about one month in the absence of high pressure did not produce the desired transformation (Supplementary Fig. S7).

Figure 3. SEM images of ZIF-8 obtained under solventless high pressure conditions: a) 2 min; b) 5 min; c) 10 min and d) 60 min.

High pressure synthesis mechanism of ZIF-8 from ZnO As previously described, the synthesis of ZIF-8 worked only in the presence of ZnO as a Zn precursor. Figure 4a illustrates


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the suggested mechanism for the process discovered here for ZIF-8. The high pressure driven synthesis of ZIF-8 involves an initial intimate contact between ZnO and the surrounding HmIm ligand. In fact, it has been demonstrated that high pressure may affect the intermolecular hydrogen bonds present in certain crystal structures.38 Here, hydrogen bonds can be established between the atoms in the imidazole group of the ligand19 and the hydroxyls present in the ZnO external surface (5.5 m2/g).39 Alternatively, oxygen vacancy sites in ZnO could also contribute to hydrogen bond interaction.40 with the imidazolate ligand. This initial interaction may contribute to the generation, in a very short time, of nanoparticles of ZIF-8 which would evolve through an Ostwald ripening process into the final 0.25 µm (on average) crystals observed by SEM. To corroborate this idea, high-resolution TEM images were taken (Figure 4b-c). These revealed a shell around the ZnO composed of an amorphous phase which can be considered as the MOF precursor, as observed in previous works.27 In this phase, ZIF-8 nanoparticles of around 5-20 nm were formed (Figure 4c, Supplementary Fig. S8). Their structure was confirmed by fast Fourier transform (FFT) data from high-resolution electron microscopy images. The FFT of the square in Figure 4c is shown in Figure 4d, where the dspacing values of the (1-1-1) and (01-2) planes are 9.8 Å and 7.6 Å, respectively. These values are in agreement with those reported for crystalline ZIF-8 particles (cubic lattice parameter of 16.99 Å).22 Some of these ZIF-8 nanoparticles evolve through the aforementioned Ostwald ripening process and are easily detached from the surface of the transformed ZnO particles (Figure 4b), while other ZIF-8 material remains strongly adhered, coating the ZnO (i.e. modifying its surface) and preventing subsequent reaction with the ligand. This explains why the obtained yield was relatively low in the case of ZIF-8. In mechanochemistry, the creation of


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fresh surface is the third step to distinguish in the solid-state mechanism, after precursors transport and the phase transformation itself.2, 28b The amorphous phase surrounding ZnO particles and embedding ZIF-8 nanoparticles explains why the sample in Figure 4 has a microporous volume determined by t-plot method of 0.33 cm3/g, corresponding to the 60% of the microporous volume of ZIF-8 (0.55 cm3/g)41 when the amount of ZIF-8 in the final solid was estimated to be about 30%. The corresponding adsorptiondesorption isotherm is shown in Figure S9.

Figure 4. a) Mechanism for the solventless high pressure synthesis of ZIF-8 from 2methylimidazole (HmIm) and ZnO; b, c) high-resolution TEM images of ZIF-8 HP 5 min; d) FFT of the inset of image c) which corresponds to the [321] zone axis.

After demonstrating that the HP approach is valid for the solventless synthesis of ZIF-8, a final application of the process was devoted to obtaining Zn benzylimidazolate (Zn(bIm)2), a compound that, with the same chemical composition, can be synthesized either as ZIF-7 (SOD


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structure) or ZIF-11 (RHO structure).22 As in a recently published work reporting a study aimed at producing nanoparticles of ZIF-11,42 the material obtained here presented the TGA and FTIR features of Zn(bIm)2 (Supplementary Figs S10-S11) but not the crystallinity of either ZIF-7 or ZIF-11 (XRD not shown due to lack of ZIF intensities). Analogously to ZIF-8, the TGA reaction yield increased with the presence of NH4NO3. In any event, this can be considered as another example of a successful HP reaction, demonstrating that coordination was established between the ligand and the metal.

CONCLUSIONS In summary, we have demonstrated a solventless high pressure (HP) synthesis of imidazolate ZIF-8 from ZnO. The process is carried out at room temperature in a period as short as 2 min. The HP synthesis of ZIF-8 taking place through an Ostwald ripening mechanism is hypothesized starting at the intimately contacted ZnO-ligand interface. Then, ZIF-8 particles well-defined in shape and crystallinity detach from the transformed ZnO surfaces, and a ZIF-8-ZnO mixture is obtained as the final product in mechanosynthesis. Besides the obvious advantage of not requiring solvent, this static HP synthesis of MOFs avoids attrition (present in mechanosynthesis) and opens the field to industrial applications needing fast transformation such as, for instance, the simple coating of a surface. In addition, this technique could be applied to the synthesis of other MOFs and even COFs43 or cocrystals3 simplifying the procedures, minimizing the possibility of pollution with harmful chemicals and reducing waste. Finally, future work is required to obtain a deeper understanding of the MOF solventless high pressure crystallization, for instance by the in situ monitoring of the process, as has been done in


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the case of the mechanosynthesis of ZIF-4 and ZIF-8 by means of high-energy synchrotron Xray diffraction.28B ASSOCIATED CONTENT Supporting Information Experimental details, TGA, XRD, FTIR, particle size, TEM, FFT characterizations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses †

School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK.

Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the Aragón Government (T05) and the European Social Fund.

ACKNOWLEDGMENT We acknowledge the use of the facilities of the Laboratorio de Microscopías Avanzadas (LMA) at the Instituto de Nanociencia de Aragón, where the electron microscopy characterization was done, and the use of the Servicio General de Apoyo a la Investigación-SAI (Universidad de Zaragoza). J. A. Gómez, from the Departamento de Ciencia y Tecnología de Materiales y


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Fluidos (U. Zaragoza), is thanked for his help with the hydraulic press. A. Ibarra, from the LMA, is thanked for his help with the TEM characterization.

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Solventless synthesis of MOFs at high pressure Lorena Paseta, Grégory Potier, Sara Sorribas, Joaquín Coronas ZnO is transformed into ZIF-8 in absence of solvent by contacting at high pressure with 2methylimidazole (0.31 GPa)


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Solventless Synthesis of MOFs at High Pressure - ACS Sustainable

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