Communication pubs.acs.org/cm
Magnetic Induction Framework Synthesis: A General Route to the Controlled Growth of Metal−Organic Frameworks Haiqing Li,*,†,∥ Muhammad Munir Sadiq,‡ Kiyonori Suzuki,‡ Paolo Falcaro,§ Anita J. Hill,∥ and Matthew R. Hill*,†,∥ †
Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia CSIRO, Clayton, Victoria 3168, Australia ‡ Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia § Institute of Physical and Theoretical Chemistry, Graz University of Technology, Graz 8010, Austria ∥
S Supporting Information *
M
To hasten the utilization of MFCs and aid the production of MOFs with improve reaction efficiency and controllability of MOF growth, we herein present a new synthetic route, magnetic induction framework synthesis (MIFS). Distinct from the existing MOF synthetic strategies including hydrothermal,17 microwave,18 electrochemistry,19 mechanochemistry,20 continuous flow,21 and sonochemical synthesis,22 MIFS is based on the magnetic induction heating capacity of MNPs. In response to an applied alternating magnetic field, MNPs can act as “nanoheaters” to rapidly generate heat locally due to the power loss induced by hysteresis, relaxation, and/or resonance processes.23 Upon exposing a MOF mother liquor containing carboxylic acid-decorated MNPs to an alternating magnetic field, each MNP “nanoheater” generates heat locally to create a solvothermal microenvironment (Figure 1). Additionally, the MNPs act as a nucleation surface for MOF growth. As a result, MOFs are produced at orders of magnitude increases in speed compared with conventional solvothermal chemistry. Moreover, the resulting MFCs demonstrated well controlled size and MNPs distribution depending on the reaction conditions. We report here the successful MIFS preparation of five different MOFs, and exemplify the performance enhancement of improved MNP distribution with Mg-MOF-74, from which a remarkable 98.4% of adsorbed CO2 could be released. Carboxy-terminated MgFe2O4 MNPs with 45 nm of average diameter were synthesized via a hyperbranched polyglycerol mediated reaction,24 followed by surface modification with succinic acid (Figure S1). The resulting MNPs (20 mg) were then homogeneously dispersed into 10 mL of Mg-MOF-74 mother liquor. The resulting reaction mixture was then exposed to an alternating magnetic field of 94.8 mT to promote the growth of MOF crystals. The formation of MFCs was monitored by powder X-ray diffraction (XRD). As shown in Figure 2a, no obvious MOF phase was identified until 1.0 h of reaction time. As the reaction further proceeded, the characteristic XRD peaks of Mg-MOF-74 became more prominent. The calculated average yields of those produced MOF crystals were 12.9, 40.9, 66.2, 79.2, and 91.1% based on the organic ligand after the reaction times of 1.0, 1.5, 2.5, 5.0, and 8.0 h,
etal−organic frameworks (MOFs), periodically connected metal centers joined by organic linker molecules to produce microporous materials, have attracted tremendous interest in a variety of areas ranging from storage, separation, sensing, to catalysis, owing to their exceptionally high internal surface areas and chemical versatility.1,2 To access a wider range of applications, MOFs imparted with additional functionality have also been actively pursued.3−6 One such example is magnetic framework composites (MFCs), which are synthesized by incorporating magnetic nanoparticles (MNPs) as seeds for the MOF growth. They have found potential use in drug delivery, chemical sequestration, and microdevice fabrication.7−9 In particular, we recently proved that MFCs are ideal gas adsorbents because they not only have exceptionally high gas capture capacity but also can be regenerated with minimal energy usage.10−13 Regeneration occurs through MNPsdelivered magnetic induction heating upon the exposure of MFCs to an alternating magnetic field.10−13 The utilization of such localized heat can effectively overcome the thermally insulating nature of MOFs, making the heating process more homogeneous, rapid, and energy-efficient in comparison to the traditional heating methods. Most MFCs are typically produced by dispersing MNPs into a MOF mother liquor, followed by standard hydrothermal techniques, where the solvated metal and organic precursors assemble at elevated temperatures to form the crystalline product.14 This can be a slow process and makes the large scale commercial production of MFCs difficult.15 Reducing reaction time is aggressively pursued as it would lead to more energyefficient processes, less demanding synthesis equipment, and suitability for scaled-up materials production. In addition to the rapid synthesis, the controlled growth of MOFs is also of significance as it plays important roles in determining the distribution of MNPs in MFCs as well as the morphology and size of MOF crystals.16 However, the use of the conventional solvothermal chemistry for MFC synthesis did not deliver sufficient control over the MOF growth, which unavoidably resulted in the formation of free MOF crystals that did not contain MNPs, and also MNP aggregates.10−13 The presence of MNP aggregates in MFCs could cause localized overheating when they are exposed to an alternating magnetic field such that the porous structure of MOFs could be damaged. © 2017 American Chemical Society
Received: May 3, 2017 Revised: June 12, 2017 Published: June 12, 2017 6186
DOI: 10.1021/acs.chemmater.7b01803 Chem. Mater. 2017, 29, 6186−6190
Communication
Chemistry of Materials
Figure 1. Schematic illustration for MOF growth in a MIFS process, and the crystal structures of MOFs synthesized with MIFS.
were carried out by placing the same reaction systems on a hot plate at 115 °C. Determined by XRD measurement, crystalline Mg-MOF-74 became evident after 8.0 h of reaction (Figure S4). The average yield of MOF was 7.3%, which is even lower than for just 1 h of MIFS (12.9%, Figure 2b). A continuous reaction at 115 °C for up to 23 h resulted in 64.4% of MOF yield. Similarly, hot plate heating of MOF mother liquor in the absence of MNPs for 23 h results in bare Mg-MOF-74 with 61.1% yield. In contrast, it took only 1.5 h to achieve the similar MOF yield (61.7%) with MIFS. Therefore, the use of localized magnetic induction heating is able to dramatically accelerate the reaction compared with traditional solvothermal chemistry. Figure 2c,d,e shows typical images of Mg-MOF-74 MFCs synthesized with MIFS for 1.5 h, magnetic Mg-MOF-74 and bare Mg-MOF-74 synthesized with hot plate heating at 115 °C for 23 h after decanting the mother liquors, respectively. All these products have a similar yield of MOF crystals. It can be observed that with hot plate heating, MOF crystals prefer to grow on the inner wall of the vial reactors, whereas with magnetic induction heating the as-prepared MFCs would naturally precipitate after switching off the magnetic field. Therefore, magnetic induced localized heating can effectively suppress the growth of MOF crystals on vial reactors, which is important for practical production of MFCs. Brunauer−Emmett−Teller (BET) measurements confirmed that all produced MFCs have similar microporous features to the bare MOF (Figure S5). With the increase in the reaction time from 1.5 to 8.0 h, BET surface areas of the resultant MFCs increased from 603 to 942 m2 g−1, whereas they were significantly lower than that of bare Mg-MOF-74 (1413 m2 g−1) due to the integration of dense MNPs in the composites.25 Density functional theory (DFT) analysis revealed that compared to the bare MOF, the resulting MFCs presented additional small pores that were constructed by the links between MNPs and MOF units (Figure S6). The similar phenomenon was also observed in our previous reported MFCs obtained with conventional hydrothermal reaction.10 Determined by scanning electron microscopy (SEM) observation, bare MNPs have 45 nm of average diameter (Figure 3a). In comparison, MFCs produced with magnetic induction heating for 1.0, 1.5, and 2.5 h present as spheres with increased average diameter of 112, 210, and 445 nm, respectively (Figure 3b−d). Transmission electron microscopy
Figure 2. Powder XRD patterns of bare MNPs, bare Mg-MOF-74, and MFCs obtained with different reaction time (a); MOF yields as a function of reaction time (b); typical photographs of as-prepared products after the mother liquor was decanted: MFCs obtained with magnetic induction heat for 1.5 h (c), MFCs (d), and bare Mg-MOF74 (e) obtained with hot plate heating at 115 °C for 23 h, respectively.
respectively (Figure 2b). Obviously, with magnetic induction heating, MOF crystals began to form within 1.0 h. Thereafter, the MOF production underwent an acceleration period during 1.0−2.5 h of reaction time, contributing to 53.3% of the MOF yield. Further prolonging reaction time led to the formation of more MOF crystals. In addition, all the produced MFCs showed strong magnetic responses to an alternating varying magnetic field (Figure S2). With the application of 94.5 mT of magnetic field to 10 mL of Mg-MOF-74 mother liquor containing 20 mg of MgFe2O4 MNPs, the temperature of the reaction system stabilized at 115 °C after 10 min (Figure S3). Therefore, control experiments 6187
DOI: 10.1021/acs.chemmater.7b01803 Chem. Mater. 2017, 29, 6186−6190
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Chemistry of Materials
yields of MOF components in MFCs, respectively. It can be seen that the increase in the reaction time, MNPs concentration, and magnetic field strength can be used as effective handles to enhance MOF yield with MIFS. SEM observations indicate that like control over the reaction time, MFCs obtained with the varied MNP concentration and magnetic field strength present similar spherical shapes after 1.5 h of reaction (Figure S9). Moreover, longer reaction time and the application of higher magnetic field increased the size of the resulting MFC particles (Figure S10). In particular, with the application of 112 mT of magnetic field, the average diameter of the resulting MFC particles was significantly increased to 374 nm after 1.5 h. However, variation in the concentration of MNPs ranging from 1.5 to 2.5 mg mL−1 rendered minor effects on the size of MFCs. With the increase in the applied magnetic field strength, the induction heating capacity of MNPs is effectively enhanced, which is favorable for the accelerated growth of MOF crystals. These results suggest that the size of MFCs can be well controlled by manipulating the magnetic induction heating capacity of MNPs rather than the concentration of MNPs. As demonstrated, MNPs can serve as nanoheaters to generate heat locally to construct a solvothermal microenvironment where MOF crystals prefer to heterogeneously nucleate and rapidly grow around MNPs, forming magnetic framework composite particles. As the reaction proceeds, continuous growth of MOF crystals on the as-formed MFCs particles results in the further increase in their yields and sizes. As MNP delivered magnetic induction heat is rapidly dissipated to the immediate surroundings, the temperature of the overall reaction system is elevated such that the homogeneous nucleation and growth of free MOF crystals is triggered, following the traditional solvothermal reaction mechanism.26,27 Even so, MNPs act as heating centers to present much higher temperatures on their surfaces than their surrounding media. As such, energy-favorable MOF growth prefers to take place on MNP surface. In addition, the heterogeneous nucleation on the MNP surface has much shorter reaction time than the homogeneous nucleation.14 These two effects collaboratively determine that in the MIFS process, MFC nanoparticles are rapidly synthesized prior to the formation of free MOF crystals. In addition to the magnetic Mg-MOF-74, magnetic CoMOF-74, PCN-250, ZIF-8, and HKUST-1 nanoparticles were also successfully achieved by exposing the corresponding MOF mother liquors containing MNPs to an alternating magnetic field (Figure 4 and Figure S11). This confirmed that the currently developed MIFS process provides a general methodology to synthesize MFCs. In a traditional solvothermal strategy for MOF synthesis, the heating is conducted with an external heating source (e.g., hot plate and oil bath), causing slow, nonuniform, and inefficient energy transfer in the reaction systems.16 In contrast, a magnetic field can fully penetrate the entire reaction system and is concentrated in the magnetically responsive MNPs, resulting in immediate generation of localized induction heat in the MIFS reaction mixture. Such localized heating makes the heat transfer in the reaction mixture more rapid, uniform and energy efficient.10−13 In comparison, microwave heating has similar heating features. Moreover, the use of microwave heat usually leads to more rapid MOF synthesis.28 Even so, microwave heating is only selectively applicable to polar reaction media.29 It also could arouse serious safety concerns owing to the formation of local hot spots induced by the
Figure 3. SEM images of bare MNPs (a), MFCs obtained with magnetic induction heating for 1.0 h (b), 1.5 h (c), 2.5 h (d), and 5.0 h (f); back scattered electrons (BSE) image and elemental mapping analysis of MFCs particles obtained with 2.5 h of reaction (e); SEM image of bare MOF crystals obtained with hot plate heating for 23 h (g); insets of panels a−g are TEM image of the corresponding samples.
(TEM) observation revealed that in the resulting MFCs, MNPs were embedded in MOF crystals and no MNP aggregates were observed (inset of Figure 3b−d). A typical elemental analysis on MFC particles obtained with 2.5 h of MIFS further confirmed the well distribution of MNPs in MFCs (Figure 3e). However, with the reaction continued for 5.0 h, microsized free MOF crystals appeared along with the as-formed MFC spheres (Figure 3f). MFCs can be easily separated with an external magnet after a sonication treatment. Control experiments indicated that microsized MOF crystals were generated upon heating the same reaction mixture (2 mg mL−1 of MNPs in MOF mother liquor) with a traditional hot plate at 115 °C for 8 and 23 h, respectively (Figure S7). Similarly, hot plate heating of pure MOF mother liquor for 23 h lead to the formation of microsized bare MOF crystals with irregular shapes (Figure 3g). Therefore, MNP-delivered magnetic induction heating is favorable for the formation of regular MFC nanoparticles with reduced size. The impacts of MNP concentration and the applied magnetic field strength upon the yield and morphology of the resulting MFCs were also investigated. Upon exposure of 1.5, 2.0, and 2.5 mg mL−1 of MNP dispersion in MOF mother liquors to an alternating magnetic field of 94.8 mT for 1.5 h, the yield of MOF component in the resulting MFC is 24.8%, 40.9%, and 66.7%, respectively (Figure S8). Further investigation showed that the application of 66.1, 94.8, and 112.0 mT of magnetic field to MOF mother liquor containing 2.0 mg mL−1 of MNPs for 1.5 h resulted in 9.1% to 40.9% and 71.3% 6188
DOI: 10.1021/acs.chemmater.7b01803 Chem. Mater. 2017, 29, 6186−6190
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98.4% of CO2 desorption at 1 bar was possible by this method. In contrast, no obvious gas desorption was triggered from the bare MOF (Figure S13). These results exemplify the highly efficient CO2 desorption capacity of MFCs through the MISA process. A close observation showed that CO2 uptake capacity of MFCs were able to be fully recovered after each magnetic induction cycle, suggesting that the porosity of MFCs remains nearly intact after induction heating. Further investigation indicated that the XRD pattern of MFC3 remains little changes after exposing it to 39 mT of magnetic field for 8 min for 4 cycles (Figure S14). This result confirmed that under the current experimental conditions the magnetic induction heat did not cause obvious damages to the MOF structures. In conclusion, we have developed a magnetic induction process known as MIFS to rapidly synthesize magnetic MgMOF-74 composites driven by MNP-delivered localized magnetic induction heat. The yield and size of the resulting MFCs can be effectively regulated by the reaction time, MNPs concentration, and the strength of the applied magnetic field. In particular, the optimum yield of MOF components in MFCs can reach up to 91.1% after 8.0 h of reaction under the investigated conditions, 12.5-fold higher than that of the reaction performed with traditional hot plate heating for 23 h. All the resulting MFCs exhibited uniform distribution in their matrices and no MNP aggregates were observed. With the localized magnetic induction heating, the growth of Mg-MOF74 crystals on the inner walls of the reactor can be effectively avoided. The formation of free MOF crystals also can be effectively suppressed by simply controlling reaction time. In addition, MNP-delivered localized heating involved in MIFS makes MOF production more energy efficient compared with the traditional hot plate heating. The resulting magnetic MgMOF-74 showed up to 98.4% CO2 desorption capacity. Importantly, MIFS is applicable to the synthesis of a broad range of MFCs, with five other MOF composites also successfully prepared. Therefore, this first report of MIFS provides a general way to rapidly produce MFCs, and holds great potential for scaled-up MOF production.
Figure 4. SEM and TEM images of the MFC nanoparticles synthesized by MIFS: (a) Co-MOF-74, mixed solvent DMF:ethanol:H2O = 1:1:1, 2.0 mg mL−1 of MNPs; (b) PCN-250, solvent DMF, 2.0 mg mL−1 of MNPs; (c) ZIF-8, solvent H2O, 2.0 mg mL−1 of MNP; (d) HKUST, solvent ethanol, 1.0 mg mL−1 of MNP. All the reactions were performed in the presence of 94.8 mT of magnetic field for 1.5 h.
microwave irradiation.7 In contrast, the localized magnetic induction heating is more mild, controllable, and nonselective to reaction media, making MIFS a general and safe route to the synthesis of MFCs. Our previous research shows that MFCs are ideal gas adsorbents owing to their exceptionally high gas capture capacity and low-energy, efficient regeneration capacity through the MISA process.9−12 In this regard, CO2 uptake and desorption in the as-prepared magnetic Mg-MOF-74 composites with 39.8 wt % (MFC1), 50.6 wt % (MFC2), and 62.4 wt % of MNP content (MFC3) produced by exposing 2.0 mg mL−1 of MNPs in Mg-MOF-74 mother liquor to 94.8 mT of magnetic field for 8.0, 2.5, and 1.5 h were investigated, respectively. MFC1, MFC2, and MFC3 showed 104.8, 90.0, and 68.6 cm3 g−1 of CO2 gas capture at 298 K and at 1 bar (Figure 5). Upon exposure to 39 mT of magnetic field for 8 min, all the MFCs were rapidly heated up to 146, 184, and 219 °C, whereas no obvious temperature increase in bare MOFs was observed (Figure S12). Upon intermittently exposing MFCs to the same magnetic field during CO2 uptake measurement at 298 K, dynamic magnetic switching of CO2 uptake and release in MFCs was observed (Figure 5). Up to
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01803. Experimental details; FTIR, XRD, and magnetic hysteresis loop of MNPs and MFCs; magnetic induction heating profiles, gas adsorptions, SEM images of MFCs; yield and particle size of MFCs (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*H. Li. E-mail:
[email protected]. *M. R. Hill. E-mail:
[email protected]. ORCID
Figure 5. CO2 adsorption isotherms at 298 K (solid symbol line) and dynamic CO2 uptake and release (hollow symbol line) of magnetic Mg-MOF-74 composites: MFC1 (black), MFC2 (red), and MFC3 (green). Dynamic CO2 uptake and release in MFCs were obtained by intermittently exposing samples to 39 mT of magnetic field during CO2 uptake measurements at 298 K.
Haiqing Li: 0000-0002-1827-6625 Paolo Falcaro: 0000-0001-5935-0409 Notes
The authors declare no competing financial interest. 6189
DOI: 10.1021/acs.chemmater.7b01803 Chem. Mater. 2017, 29, 6186−6190
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Chemistry of Materials
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ACKNOWLEDGMENTS H. Li acknowledges the support from CSIRO OCE Fellowship. M. R. Hill acknowledges the ARC for support (FT130100345).
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