Continuous One-Step Synthesis of Porous M-XF6 -Based Metal-Organic and Hydrogen-Bonded Frameworks.

Metal-organic frameworks (MOFs) built up from connecting M-XF6 pillars through N-donor ligands are among the most attractive adsorbents and separating agents for CO2 and hydrocarbons today. The continuous, one-step spray-drying synthesis of several members of this isoreticular MOF family varying the anionic pillar (XF6 =[SiF6 ]2- and [TiF6 ]2- ), the N-donor organic ligand (pyrazine and 4,4'-bipyridine) and the metal ion (M=Co, Cu and Zn) is demonstrated here. This synthetic method allows them to be obtained in the form of spherical superstructures assembled from nanosized crystals. As confirmed by CO2 and N2 sorption studies, most of the M-XF6 -based MOFs synthesised through spray-drying can be considered "ready-to-use" sorbents as they do not need additional purification and time consuming solvent exchange steps to show comparable porosity and sorption properties with the bulk/single-crystal analogues. Stability tests of nanosized M-SiF6 -based MOFs confirm their low stability in most solvents, including water and DMF, highlighting the importance of protecting them once synthesised. Finally, for the first time it was shown that the spray-drying method can also be used to assemble hydrogen-bonded open networks, as evidenced by the synthesis of MPM-1-TIFSIX.


INTRODUCTION
The environmental impact associated to the energy demand is a major problem worldwide. [1] For example, CO2 emission caused by humanity -CO2 concentration at the South Pole recently passed the milestone of 400 ppm for the first time in the last 4 million years -[2] highly contributes to the climate change. In this sense, 2016 has also been the first year that the weekly CO2 concentration average monitored in the Mona Loa observatory did not go below this key value, meaning that the average global temperature is likely to increase more than the 1.5 oC warming threshold. [3] To address the current and future energy needs while mitigating the environmental impact, one of the strategies has been the development of efficient CO2 capture, storage and separation materials for achieving cleaner combustible supplies.
[4] These materials mainly include zeolites, activated carbons, metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs). [5] Among these innovative materials, an old-fashioned class of fluorinated materials [6] have recently been "This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." 3 brought back to the spotlights by Eddaoudi and Zaworotko groups thanks to their exceptional uptake and selectivity towards CO2 and hydrocarbons.
However, despite these great developments, scientific community and industrials still need to join their efforts for transferring these materials from the laboratory to industry. A very important step here is the optimization of their fabrication. [8] This fabrication must always envision fast and scalable one-step processes that produce ready-to-use products without the need of additional purification and drying steps. Here we report a synthetic method that allows producing several of the isoreticular M-XF6-based CO2 sorbents fulfilling all these requirements.
Our team recently introduced an industrially well-established spray-drying (SD) technique as a new synthetic way to prepare various MOFs (Scheme S1). [8l, 9] The SD technique is a scalable and fast method allowing continuous synthesis of MOFs in the form of spherical superstructures or beads based on the assembly of nanosized crystals.[9c] The strong expertise acquired from these previous studies, among several reports suggesting the capital importance of the formation of the inorganic secondary building unit (SBU) for the nucleation and growth of MOFs,[10] convinced us that premade pillars of the M-XF6 MOF platform would be ideal candidates for SD synthesis.
We therefore successfully embarked in the SD-based synthesis of several M-XF6 based materials, showing that this method is also compatible for reticular synthesis and metal tuning, ligand elongation and pillar substitution. Importantly, their rapid synthesis (few minutes versus few hours up to few days) does not negatively affect their sorption properties, demonstrating in most cases its ready-to-use character without the need of additional purification steps or time consuming, "This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." 4 repeated solvent exchange procedures. Moreover, we also demonstrate that SD can be used not only to synthesize porous materials based on coordination bonds but also based on hydrogen bonds.

SIFSIX-3-M Materials
"This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."

Synthesis and characterization
The one-step SD synthesis consisted on the combined atomisation of two methanolic solutions containing (i) M-SiF6 (M = Co, Cu, Zn) and (ii) pyz at 85 oC, which produced fine powders that were collected with a minimum amount of methanol (MeOH). Collection of these powders in methanol was a crucial protection step as we found that they were air-sensitive (vide infra). To assess the quality of the as-made SIFSIX-3-M MOFs, their CO2 sorption properties were compared with those of their bulk analogues. For this, the SD-synthesized SIFSIX-3-M collected in methanol were directly transferred from the spray drier collector to sorption cells, and dried and evacuated for 12 h at 65 oC. Then, their CO2 uptake at 298 K was measured. Notably, we confirmed that spray-dried SIFSIX-3-M did not require additional solvent exchange or purification step to exhibit remarkable CO2 capacities at low pressure and 298 K ( Figure 2h, Table 1), with an uptake less than 10 % lower than the reported bulk materials. The morphology of the materials was also investigated by field-emission scanning electron microscopy (FE-SEM), showing in all cases the occurrence of nanosized SIFSIX-3-M crystals assembled into spherical superstructures or beads; a shape that is typical from MOFs assembled by the SD method. The sizes of these superstructures were 7.9 ± 4.8 µm for SIFSIX-3-Co, 6.9 ± 3.4 µm for SIFSIX-3-Cu, and 3.5 ± 2.7 µm for SIFSIX-3-Zn (Figure 2 a-c and Figure S14a).[9, "This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." 6 11] Moreover, FE-SEM images confirmed the homogeneity of the materials, as already suggested by the absence of crystalline impurity peaks in the PXRD diagrams.
To finally determine the size of crystals composing the superstructures, they were disassembled by sonication and immediately transferred to a transmission electron microscopy (TEM) grid.

Stability in different media
"This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which has been published in final form https://doi.org/10.1002/chem.201605507. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions." 7 The use of methanol to collect the fine powders produced with the SD method is capital to protect and use the as-made SIFSIX-3-M MOFs. [12] Without this precaution, all nanosized SIFSIX-3-M MOFs suffered a fast degradation and a loss of their CO2 sorption properties when they were exposed to ambient conditions. In addition, following the optimized washing procedure reported by Nugent et al., [7a] we observed that the SD-synthesized SIFSIX-3-Zn was instantaneously solubilized upon addition of N,N-dimethylformamide (DMF), leading to a clear solution. Re-spray drying attempts of this clear solution did not allow re-assembling the SIFSIX-3-Zn framework.
Instead, colourless single crystals (versus the yellow colour of SIFSIX-3-Zn crystals) appeared in the DMF solution after a period of ca. a month. This unknown structure, which crystallizes in P-1 space group, was solved by single crystal X-ray diffraction and appeared to be a cationic 1D It is important to mention here that a similar behaviour was found when immersing SIFSIX-3-Zn in water, also leading to a complete solubilisation of the crystals. The incubation of nanosized SIFSIX-3-Zn in other organic solvents, including acetonitrile, hexane, dichloromethane, chloroform, toluene, tetrahydrofuran and acetone, did not result in the solubilisation of the crystals but a fast phase transition into unidentified crystalline powders ( Figure S20).
The only tested solvent in which SIFSIX-3-Zn showed certain stability was MeOH. Initial incubation studies using microcrystals instead of nanocrystals -to easily follow the evolution by FE-SEM-showed that SIFSIX-3-Zn is also etched and finally solubilized in MeOH ( Figure S13).
However, we found that the minimum amount of MeOH needed for the complete solubilisation of spherical superstructures (size = 7.9 ± 3.6 µm; Figure 3c and Figure S14a) were collected and washed with MeOH. The material was obtained as a pure phase, as confirmed by PXRD ( Figure   3b). Also, it was found to be porous to N2 at 77 K, exhibiting an apparent BET area of 1300 m2.g-1 (Table 2, Figure S21). Again, the nanosized nature of the SIFSIX-1-Zn crystals was confirmed by TEM performed after disassembling the superstructures by sonication. The size of these nanocrystals was 20 ± 5 nm ( Figures S14b, S24).

Synthesis and characterization of TIFSIX-1-Cu
Recently, Nugent et al. [7f] reported the possibility to not only vary the metal in this type of pcu-

Materials
All the materials were synthesized using a Mini Spray Dryer B-290 (BÜCHI Labortechnik). All solvents and reagents were purchased from Sigma-Aldrich, City Chemicals and Scharlab and used as received.  Figure S1).  Figure S2).  Figure   S3).

Synthesis of SIFSIX-1-Zn
A 6 mL methanolic solution of 300 mg (1.45 mmol, anhydrous based) of ZnSiF6·xH2O and a 6 mL methanolic solution of 650 mg (4.16 mmol) of bpy were simultaneously spray-dried using a 3-fluid nozzle, a feed rate of 2.4 mL.min-1, a flow rate of 414 mL.min-1, an inlet N2 temperature "This is the peer reviewed version of the following article: V. Guillerm, L. Garzón-Tovar, A. Yazdi, I. Imaz, J. Juanhuix, D. Maspoch, Chem. Eur. J. 2017, 23, 6829, which  (w), 3101 (w), 3283 (w), 3357 (w) ( Figure S5). injected to a 6 mL methanolic solution of 325 mg (4.05 mmol) of pyz at room temperature, without stirring. After 1 h, 1 mL of the solution containing the resulting microcrystals was pipetted from the middle of the vial and mixed with 2 mL of MeOH. Aliquots of the sample were transferred immediately to the microscope to avoid contact with air, and the morphology of the crystals was studied by FE-SEM after 15 min ( Figure S13).

Crystallography
Crystallographic data for 1 were collected at 100 K at XALOC beamline at ALBA synchrotron [14] (λ = 0.79472 Å). Data were indexed, integrated and scaled using the XDS program. [15] Absorption correction was not applied. The structure was solved by direct methods and subsequently refined by correction of F2 against all reflections, using SHELXS2013 [16] and SHELXL2013 [17] within the WinGX package. [18] All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F2 using the program SHELXL2013.
Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters.