Supercritical Carbon Dioxide Enables Rapid, Clean, and Scalable

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Supercritical carbon dioxide enables rapid, clean and scalable conversion of a metal oxide into zeolitic metal–organic frameworks Joseph Marrett, Cristina Mottillo, Simon Alix Girard, Christopher W Nickels, Jean-Louis Do, Dayaker Gandarth, Luzia S. Germann, Robert E. Dinnebier, Ashlee J. Howarth, Omar K. Farha, Tomislav Friscic, and Chao-Jun Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00385 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Crystal Growth & Design

Supercritical carbon dioxide enables rapid, rapid, clean and scalable converconversion of a metal oxide into zeolitic metal– metal–organic frameworks Joseph M. Marretta, Cristina Mottilloa,b, Simon Girarda,b, Christopher W. Nickelsa,b, Jean-Louis Doa, Dayaker Gandrath,a Luzia S. Germann,c Robert E. Dinnebier,c Ashlee J. Howarth,d,e Omar K. Farha,d,f Tomislav Friščić*a and Chao-Jun Li*a a) Department of Chemistry, McGill University, 801 Sherbrooke St. W., Montreal, H3A 0B8, Canada, b) ACSYNAM, Inc., Montreal, Canada, c) Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany, d) Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA, e) Department of Chemistry and Biochemistry, Concordia University; 7141 Sherbrooke St W., Montreal, H4B 1R6, Canada, f) Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia KEYWORDS: Metal-organic frameworks, supercritical carbon dioxide, green chemistry ABSTRACT: Supercritical carbon dioxide (CO2) is an established medium for synthesizing molecular compounds and activation of microporous solids, but is much less explored for transformations of high-melting ionic reactants, such as metal oxides. We show that supercritical CO2 enables direct, clean and rapid transformation of zinc oxide into zeolitic imidazolate frameworks (ZIFs), without requiring toxic or corrosive metal nitrate or chloride precursors, organic or aqueous solvents, additives or auxiliaries. While recent syntheses of coordination polymers and metal–organic frameworks from supercritical CO2 highlighted the need for specialized metal reagents designed for solubility, we unexpectedly find that ZnO is quantitatively converted into grams of diverse ZIFs in minutes, with hundred grams of popular ZIF-8 accessible within an hour.

Introduction Metal–organic frameworks (MOFs)1-5 are versatile materials for applications from gas storage and separation,6-16 to proton conduction,17,18 catalysis,19-21 and light harvesting.22 The first commercial uses23 of MOFs have highlighted the need for rapid, efficient, but also clean, sustainable routes for their synthesis. Recent work has focused on MOF synthesis without toxic and/or corrosive chloride and nitrate reagents,24-26 avoiding solvothermal techniques and high-boiling solvents. Oxides are attractive in this context, by avoiding toxic or corrosive counter ions27-34 while being readily accessible and safe to handle. However, poor solubility of metal oxides has limited their use as MOF precursors to high-temperature thermochemical35-37 or additivedependent mechanochemical approaches.38-41 Here, we present a scalable, fast route for MOF synthesis from a metal oxide by using supercritical carbon dioxide (scCO2) as the reaction medium, avoiding the need for organic or aqueous solvents, or catalytic additives.42 While scCO2 has been extensively used for organic separations, synthesis,43-54 as well as drying and activation of microporous solids, 55-57 it is much less explored as a medium for reactions of metal oxides.58 We show that scCO2 enables conversion of ZnO, ionic material stabilized by lattice energy of >4 MJ/mol,59 into gram amounts of zeolitic imidazolate frameworks (ZIFs)60-63 within minutes, including open sodalite-topology (SOD) MOFs based on 2-methylimidazole (HMeIm) or 2-vinylimidazole (HVIm) (Figure 1).

Figure 1. a) Imidazoles used in this study; b) rapid, 5-minute synthesis of ZIFs in scCO2 directly from ZnO; c) structures of ZIFs synthesized using the presented scCO2-based route.

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Herein described reactivity is especially surprising as the recent explorations of scCO2 as a medium for making metal-organic materials have established the inability to use conventional metal salts as precursors.64-67 As a result, the use of supercritical CO2 in making MOFs and coordination polymers has relied on solubilizing additives, such as ionic liquids, or specialized reagents such as zinc hexafluoroacetylacetonate, that enable solubilization of metal species.64-67

Experimental Details General details Zinc oxide was calcined overnight at 400°C in a Thermolyne muffle furnace prior to use. Reagents zinc oxide (>99%), imidazole (HIm) (98%), (HMeIm) (99%), and 2ethylimidazole (HEtIm) (98%) were purchased from Sigma Aldrich and used without modification. Reagent HVIm was synthesized as previously described.68 Methanol (reagent grade) was purchased from Fisher Scientific. Powder XX-ray diffraction (PXRD) Powder X-ray diffraction patterns of reaction products were collected using a Bruker D2 powder diffractometer equipped with a CuKα (λ=1.54060 Å) source and Lynxeye detector with a lower and upper discriminant of 0.110 V and 0.250 V respectively. Alternatively, powder X-ray diffraction patterns were collected on a PROTO Manufacturing AXRD Benchtop Powder Diffractometer, equipped with a CuKα (λ=1.54060 Å) source and a Dectris Hybrid Pixel Detector. The patterns were collected in the 2θ-range of 3° to 40°. Analysis of PXRD patterns was conducted using Panalytical X’Pert Highscore Plus software. Experimental patterns were compared to simulated patterns calculated from published crystal structures using Mercury crystal structure viewing software. A high-resolution PXRD pattern of SOD-Zn(VIm)2 was collected on a Stoe Transmission Powder Diffraction System (STADI-P) with CuKα1 radiation, equipped with a MYTHEN 1K detector (Dectris Ltd.) and a Ge(111) Johann monochromator (STOE & CIE). SOD-Zn(VIm)2 was gently ground and filled into a ∅ 0.5 mm glass capillary (No. 0140, Hilgenberg). The PXRD pattern was measured at room temperature in the 2θ-range from 4o to 90° over 13 hours. FourierFourier-transform infrared attenuated total reflection (FTIR--ATR) spectroscopy (FTIR All FTIR-ATR spectra were collected in the solid state using a Bruker Vertex 70 FTIR-ATR spectrometer in the range of 400 cm-1 to 4000 cm-1. FTIR spectra were analyzed using Bruker OPUS software. Thermogravimetric analysis (TGA) Thermograms were collected on a Mettler Toledo TGA/DSC 3+ with high temperature furnace using an alumina pan. Samples were heated under dynamic atmosphere of N2 to an upper temperature limit ranging from 600 °C to 800 °C at a rate of 10 oC/min. The balance and purge flow were 40 mL/min and 60 mL/min respectively.

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13 SolidSolid-state C MAS NMR spectroscopy Natural abundance 13C solid-state NMR (ssNMR) spectra were collected on a 400 MHz Varian VNMR equipped with a 7.5 mm CP-MAS probe. All spectra were collected at a spin rate of 5 kHz with a contact time of 2 ms and a recycle delay of 2 s. NMR spectra were analyzed using MestreNova software.

Supercritical upercritical CO2 setup In a typical reaction, the solid reagents were placed in a cone-shaped filter paper and placed inside a 500 mL Waters TharSFC Supercritical Fluid Extraction vessel which had been pre-heated to the required temperature using an external heating jacket. The extraction vessel was filled with cylindrical stainless steel blocks to reduce the vessel volume to approximately 250 mL. Upon insertion of the reactants, the vessel was filled with CO2 heated to the reaction temperature using a heat exchanger, at a flow rate of 20 g/min. The reaction was conducted under static conditions at the pre-determined temperature and pressure. A typical pressurization and depressurization profile for a 5 minute reaction can be found in the Supplementary Information.

Results and Discussion Our first target was the commercially-relevant ZIF-8, a sodalite (SOD) topology framework composed of Zn2+ nodes and 2-methylimidazolate (MeIm-) linkers. Initial experiments consisted of placing a pre-milled 1:2.2 stoichiometric mixture of ZnO (0.326 g, 4 mmol) and HMeIm (0.723 g, 8.8 mmol) into a cone-shaped filter paper situated in a 0.5 L scCO2 extraction vessel (Figure 2a) that had been pre-heated to 65 °C. The vessel was pressurized to 90 bar with CO2 at a rate of 25 g/min, after which the vessel was sealed off for 1 minute and, subsequently, depressurized at a rate of approximately 20 bar/min (see Supplementary Information for a typical reaction pressure profile).

Figure 2. a) Reaction vessel used for the synthesis of ZIFs in scCO2; b) a shaped 100 g sample of ZIF-8 made in scCO2 (Canadian 25 cent coin shown for size comparison).

Analysis of the resulting solid by powder X-ray diffraction (PXRD, Figure 3a-j) revealed Bragg reflections of ZIF-8 and the absence of those of ZnO, consistent with full oxide conversion. However, the PXRD patterns also exhibited additional reflections (Figure 3d) corresponding to a zinc 2-methylimidazolate carbonate phase (1), previously observed upon degradation of ZIF-8 in moist CO2.69 Parasitic formation of 1 could be avoided by adding a stoichiometric


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Crystal Growth & Design excess (50% mol) of HMeIm with respect to ZnO (Figure 3e). Although in this case 1 minute was not sufficient to result in the full conversion of ZnO, extending the reaction time to 5 minutes led to complete consumption of the oxide (Figure 3f). The known role of moisture at promoting the formation of 1 led us to investigate whether meticulously dried reagents could yield pure ZIF-8, without needing a large excess of HMeIm. Indeed, using ZnO and HMeIm that had been thoroughly desiccated enabled formation of pure ZIF-8 after 5 minutes in scCO2 at 65 °C and 130 bar (Figure 3g). Such fast reactivity of ZnO in scCO2, on a gram scale, in the absence of additives or mechanical force, is unprecedented and contrasts previous reports citing the insolubility of metal precursors in pure scCO2 as a limitation to its use for MOF synthesis.64-67 Importantly, scCO2 appears to be critical for the reaction, as it was shown that thermal reactivity of ZnO with HMeIm is negligible below 120 °C, and even then takes 24 hours to complete.35 It is also notable that activators or catalytic additives, required for most solution and solid-state syntheses70-73 of ZIFs from ZnO, were not needed in scCO2 (see Supplementary Information for a more detailed assessment of the Green Chemistry metrics of methods for MOF synthesis).

Figure 3. PXRD patterns illustrating the synthesis of ZIF-8 and zni-Zn(Im)2 in scCO2 from ZnO: (a) ZnO reagent; (b) the zinc

2-methylimidazolate carbonate phase (1); (c) simulated pattern for the structure of ZIF-8 (CSD code VELVOY); (d) reaction mixture after 1 minute at 130 bar and 65 oC (a characteristic Bragg reflection of 1 is denoted by *); (e) reaction mixture containing 50% stoichiometric excess of HMeIm after 1 minute at 130 bar and 65 oC; (f) reaction mixture containing 50% stoichiometric excess of HMeIm after 5 minutes at 130 bar and 65 oC; (g) stoichiometric mixture of carefully dried HMeIm and ZnO after 5 minutes at 130 bar and 65 oC, washed with MeOH; (h) simulated pattern for the structure of zniZn(Im)2 (CSD code IMIDZB03); (i) stoichiometric mixture of ZnO and HIm after 1 minute at 130 bar and 65 oC; (j) mixture of ZnO and 20% stoichiometric excess HIm after 2 hours at 130 bar and 65 oC, washed with MeOH; (k) solid-state 13C CPMAS NMR spectrum of a 1 g batch of ZIF-8 synthesized in scCO2; (l) SEM micrograph, and (m) nitrogen BET sorption isotherm of a representative sample extracted from a 100 g batch of ZIF-8 synthesized in scCO2.

A one-gram sample of ZIF-8 made in scCO2 was activated by stirring in MeOH overnight and drying in vacuo at 80 °C, and then analyzed by cross-polarization magic-angle spinning (CP-MAS) 13C solid-state NMR spectroscopy (ssNMR, Figure 3k), scanning electron microscopy (SEM, Figure 3l) FTIR-ATR spectroscopy, and thermogravimetric analysis (TGA) (see Supplementary Information). The CP-MAS 13C NMR spectrum revealed signals consistent with the aromatic and methyl carbon atoms of MeIm- linkers in ZIF-8. TGA in dynamic atmosphere of air revealed a single decomposition step at ca. 400 °C and a conversion of ZnO of 99.8%. To illustrate scalability, a 100 gram synthesis of ZIF-8 was achieved in 2.5 hours using a 50% stoichiometric excess of HMeIm. Such larger scale reactions gave a shaped product in the form of a cylindrical block (Figure 2b). Nitrogen sorption analysis of different sections of the 100 g batch sample after activation gave very high BET surface areas, from 1687 m2 g-1 to 1894 m2 g-1 (Figure 3m, also see Supplementary Information). This is on the higher end of literature values, which range between 800 m2 g-1 and 1900 m2 g-1.61, 74-82 A systematic screen revealed that varying the temperature and pressure did not significantly affect the reaction of ZnO and HMeIm in scCO2. PXRD analysis of reaction mixtures exposed to 45 °C, 55 oC, 65 oC, 75 oC, 85 oC or 90 °C at 130 bar revealed complete conversion of ZnO within 1 minute (see Supplementary Information) in all cases except the reaction at 45 °C, which exhibited partial conversion. Similarly, complete conversion of ZnO was observed within 1 minute for scCO2 pressures of 90 bar, 100 bar, 110 bar, 120 bar and 130 bar at a constant temperature of 65 oC. However, PXRD indicated no reactivity when ZnO and HMeIm were simply heated at 65 oC at 1 bar of CO2, or when the reactants were placed in liquid CO2 at 27 °C and 130 bar for 2 hours. These observations suggest that the supercritical phase is important to afford the reaction between ZnO and HMeIm (see Supplementary Information). The unexpectedly rapid formation of ZIF-8 in scCO2 led us to investigate the synthesis of ZIFs based on other linkers. We explored the reaction of ZnO (0.326 g, 4 mmol) and unsubstituted imidazole (HIm, 0.654 g, 8.8 mmol) in a 1:2.2 stoichiometric ratio, at 65 °C and 130 bar. After 1



minute, the formation of the close-packed zni-topology Zn(Im)2 was confirmed by comparing the product PXRD pattern with that simulated for the crystal structure of zniZn(Im)2 (Figure 3h,i). However, the presence of Bragg reflections of ZnO suggested the reaction was incomplete. Adding 20% excess HIm, in an attempt to offset any potential dissolution of ligand in scCO2, also did not result in complete ZnO conversion within a minute. However, extending the reaction time to 2 hours led to pure zniZn(Im)2, as evidenced by PXRD (Figure 3j). A washed, dried sample was analyzed by FTIR-ATR and TGA (see Supplementary Information). Next, we evaluated scCO2 as a medium for synthesizing a ZIF based on 2-ethylimidazole (HEtIm).83 It was previously shown that ZnO and HEtIm react under static, high humidity conditions to form either the open Zn(EtIm)2 framework of zeolite ρ (RHO) topology, or the close-packed structure Zn(EtIm)2 with a β-quartz (qtz) topology. Formation of the open RHO-Zn(EtIm)2 was favored by adding a space-filling agent, such as excess HEtIm.70,71 The reagents ZnO (0.326 g, 4 mmol) and a 150% excess of HEtIm were allowed to sit in scCO2 at 65 °C and 130 bar for 5 minutes. Analysis of the reaction mixture by PXRD and FTIR-ATR revealed the absence of new signals, suggesting no reaction (Figure 4a-c). A similar result was obtained upon extending the exposure time to 30 minutes (see Supplementary Information). Increasing the temperature to 110 °C, however, led to partial formation of RHOZn(EtIm)2 from a near-stoichiometric (1:2.2) mixture of ZnO and HEtIm after 2 hours, with diffraction lines of ZnO still evident in the PXRD pattern of the reaction mixture (Figure 4d). Next, we explored using an excess of HEtIm to compensate for possible inclusion of ligand in the pores of the nascent MOF.70,71 Adding a 50% stoichiometric excess HEtIm (1.54 g, 16 mmol) lead to full conversion of ZnO (0.326 g, 4 mmol) into a mixture of RHO-Zn(EtIm)2 and a small amount of qtz-Zn(EtIm)2. Conducting the reaction with 100% excess HEtIm at 80 bar for 1 hour gave pure RHO-Zn(EtIm)2 (Figure 4e). Lastly, we investigated scCO2 for synthesizing a microporous ZIF based on 2-vinylimidazole (HVIm). The sodalite (SOD) topology framework Zn(VIm)2 was recently reported by Sun et al., and is an attractive target due to vinyl groups available for covalent functionalization.68 We attempted the synthesis of SOD-Zn(VIm)2 by exposing a 1:2.2 stoichiometric mixture of ZnO (0.326 g, 4 mmol) and HVIm (0.829 g, 8.8 mmol) to scCO2 at 65 °C and 130 bar for 1 minute. While PXRD confirmed the formation of targeted SOD-Zn(VIm)2, the presence of Bragg reflections of ZnO indicated that the reaction was incomplete (Figure 4f,g). While increasing the reaction time to 30 minutes did not enable complete conversion of ZnO, using 20% stoichiometric excess of HVIm led to complete disappearance of reactant Bragg reflections and formation of SOD-Zn(VIm)2 within 5 minutes at 65 °C and 130 bar (Figure 4h). The reaction was readily conducted on a 1 gram scale and, after washing and activation in vacuo at 100 °C, the structure of evacuated SOD-Zn(VIm)2 was confirmed by 13C CP-MAS ssNMR (Figure 4i) and structure solution from PXRD data, yielding a result consistent to the recently reported structure which involved water molecules as guests (see Sup-

plementary Information). The material was fully characterized by SEM (Figure 4j), FTIR-ATR, TGA (see Supplementary Information), as well as by nitrogen sorption which gave a high68 BET surface area of 1091 m2 g-1 (Figure 4k). The mild conditions and speed of formation of ZIFs from an ionic substance like ZnO in scCO2 are remarkable. Zinc oxide does not dissolve noticeably in scCO2, melts only at 1975 oC and does not appreciably react with, for example, HMeIm below 120 oC.35

Figure 4. Powder X-ray diffraction patterns illustrating the synthesis of RHO-Zn(EtIm)2 and SOD-Zn(VIm)2 in scCO2 directly from ZnO: (a) ZnO reactant; (b) simulated pattern for the published structure of RHO-Zn(EtIm)2 (CSD code MECWOH); (c) mixture of ZnO and 150% stoichiometric excess of HEtIm after 5 minutes exposure to scCO2 at 130 bar and 65 oC (Bragg reflections corresponding to HEtIm are denoted by *); (d) stoichiometric mixture of ZnO and HEtIm after 2 hours exposure to scCO2 at 130 bar and 110 oC; (e) mixture of ZnO and 100% stoichiometric excess of HEtIm after 1 hour exposure to scCO2 at 80 bar and 110 oC; (f) simulated pattern for the structure of SOD-Zn(VIm)2; (g) stoichiometric mixture of ZnO and HVIm after 1 minute exposure to scCO2 at 130 bar and 65 oC (h) mixture of ZnO and 20% stoichiometric excess of HVIm after 5 minutes exposure to scCO2 at 130 bar and 65 oC; (i) solid-state 13C CP-MAS NMR spectrum (spinning side bands are denoted by *) (j) SEM micrograph, and (k) nitrogen BET sorption isotherm for a sample of SODZn(VIm)2 synthesized in scCO2.

We speculated whether scCO2 might be converting the oxide into a more reactive species, such as a carbonate, prior


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Crystal Growth & Design to reaction with the ligands. To do this, pre-milled ZnO was placed in scCO2 at 65 °C and 130 bar for 14 hours. The diffraction pattern of the sample after scCO2 treatment was identical to that of pristine ZnO. The absence of a chemical change was confirmed by FTIR-ATR spectroscopy, which revealed only one sharp absorption band at approximately 450 cm-1 (see Supplementary Information). Furthermore, other metal sources such as Zn(NO3)2·6H2O, ZnCl2, ZnSO4, and Zn(OAc)2·2H2O were also explored as precursors in the synthesis of ZIF-8 in scCO2. Subjecting the reaction mixtures to scCO2 at 65 °C and 130 bar either led to no change, or partial reaction to form so far unidentified materials (see Supplementary Information). Such inability to obtain ZIF-8 from other zinc precursors is in agreement with previous reports,64-67 and establishes ZnO as a simple, readily available metal source that is compatible with the presented scCO2-based methodology.

Conclusions The use of supercritical CO2 as a reaction medium enabled the synthesis of zeolitic imidazolate frameworks comprising HIm, HMeIm, HEtIm, and HVIm ligands directly from ZnO, at mild temperatures and at very short timescales. For all herein explored ligands, reactivity with ZnO in supercritical CO2 readily proceeds in the absence of catalytic additives, organic solvents, or auxiliary templating or metal complexation agents, in contrast to most reported routes for MOF synthesis from oxides.40,41,70-73 The reaction methodology is readily scalable to multi-gram amounts, as demonstrated by the synthesis of ZIF-8 in a 100 gram batch. Other reactions were readily conducted in gram amounts and appear to be readily scalable, the limiting factor being the cost and availability of the ligand. Importantly, both the commercially interesting microporous ZIF-8 and its vinyl-substituted analogue were readily accesible, and exhibited very high surface areas and thermal stability. The ability of supercritical CO2 to activate a highmelting ionic oxide for the synthesis of advanced functional MOF materials challenges the notion that using supercritical CO2 necessitates specially designed, soluble metal precursors.64-67 Whereas the specific mechanism that enables the observed high rates of ZIF synthesis in supercritical CO2 remains unknown, a tentative explanation might be in increased reactants diffusivity, as it is well known that diffusivities of small molecules in supercritical CO2 could be several orders magnitude higher than in conventional liquid phases.84,85 We believe that the speed, simplicity and scalability of the presented methodology provide a significant advance in developing cleaner, sustainable, more efficient routes for MOF manufacture from oxide precursors. We are now investigating supercritical CO2 for synthesizing materials based on other metals, notably copper.

ASSOCIATED CONTENT Supporting Information. Additional synthetic details, PXRD, NMR, FTIR-ATR and TGA data, and crystallographic data in CIF format. See DOI: 10.1039/x0xx00000x Crystallographic data for SOD-Zn(Vim)2 has been deposited with the Cambridge Crystallographic Data Center, CCDC code 1566964. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tomislav Friščić, Department of Chemistry, McGill University, 801 Sherbrooke St. W. H3A0B8 Montreal, Canada; E-mail: [email protected] * Chao-Jun Li, Department of Chemistry, McGill University, 801 Sherbrooke St. W. H3A0B8 Montreal, Canada; E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Funding Sources We acknowledge the support of the NSERC Strategic Grant program (grant no. STPGP 463405-14) and the NSERC I2I grant (grant no. I2IPJ 485231-2015). TF acknowledges the support of the NSERC E. W. R. Steacie Memorial Fellowship (NSERC SMFSU 507347-17). OKF is grateful for the financial support from Northwestern University. CM acknowledges the FRQNT Bourses de doctorate en recherche and the Sigma Xi Grants-in-Aid of Research for financial support. CJL is a Canada Research Chair (Tier I) in Green Chemistry.

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For Table of Contents use only Title: Supercritical carbon dioxide enables rapid, clean and scalable conversion of a metal oxide into zeolitic metal–organic frameworks Authors: Joseph M. Marrett, Cristina Mottillo, Simon Girard, Christopher W. Nickels, Jean-Louis Do, Dayaker Gandrath, Luzia S. Germann, Robert E. Dinnebier, Ashlee J. Howarth, Omar K. Farha, Tomislav Friščić* and Chao-Jun Li*

Synopsis: The use of supercritical carbon dioxide as a reaction medium enabled surprisingly rapid and scalable conversion of zinc oxide into highly crystalline, microporous zeolitic imidazolate frameworks. Quantitative synthesis of gram amounts of ZIF-8 was accomplished in minutes, while 100-200 gram syntheses were completed within a couple of hours, yielding shaped cylinders of a highly microporous material. Graphic entry for the Table of Contents (TOC)


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Supercritical Carbon Dioxide Enables Rapid, Clean, and Scalable

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