This is the peer reviewed version of the following article: Garzón-Tovar L., Pérez-Carvajal J., Yazdi A., Hernández-Muñoz J., Tarazona P., Imaz I., Zamora F., Maspoch D.. A MOF@COF Composite with Enhanced Uptake through Interfacial Pore Generation. Angewandte Chemie - International Edition, (2019). . : - . 10.1002/anie.201904766, which has been published in final form at https://dx.doi.org/10.1002/anie.201904766. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Enhanced Uptake in a MOF@COF Composite by Interfacial Pore Generation Luis Garzón-Tovar,[a],# Javier Pérez-Carvajal,[a],# Amirali Yazdi,[a] Jose Hernández-Muñoz,[b] Pedro Tarazona,[b] Inhar Imaz,*[a] Félix Zamora[c,d] and Daniel Maspoch*[a,e] Abstract: Herein we describe a new class of porous composites Researchers have recently developed a class of crystalline comprising metal-organic framework (MOF) crystals confined in porous polymers known as covalent-organic frameworks single spherical matrices made of packed covalent-organic (COFs),[8] which enable use of COFs as components in porous framework (COF) nanocrystals. These MOF@COF composites are composites. For example, Fu et al. recently developed a new synthesized via a two-step method of spray-drying and subsequent class of membranes by interfacing a MOF (Zn(bdc) 2(dabco) or amorphous (imine-based polymer)-to-crystalline (imine-based COF) ZIF-8) layer on top of a COF (COF-300) layer. The resulting transformation. This transformation around the MOF crystals membranes showed synergic enhancement of selective generates micro- and mesopores at the MOF/COF interface that separation of H2 over CO2 relative to the summed separation provide far superior porosity compared to the constituent MOF and capacity of the MOF and COF components.[9] Additionally, COF components taken together. We report that water sorption in various groups have developed a series of core-shell MOF/COF these new pores occurs within the same pressure window as in the composites that exhibit remarkable photocatalytic properties, COF pores. Our new MOF@COF composites, with their additional mainly in H2 evolution.[10] pores at the MOF/COF interface, should have implications for the Herein we present a new MOF@COF composite that we development of new composites. developed by confining MOF crystals in single spherical COF beads. In these beads, the MOF (UiO-66-NH2 or Zr-fumarate) crystals are dispersed into a spherical matrix made of packed Combining different types of porous materials into composites nanocrystals of COF-TAPB-BTCA, a two-dimensional COF can yield materials that, relative to their constituent components, assembled two trigonal building blocks: 1,3,5- exhibit greater porosity, new synergic properties, higher stability benzenetricarbaldehyde (BTCA) and 1,3,5-tris-(4- and novel shapes for myriad applications, including separation,[1] aminophenyl)benzene (TAPB). Structurally, the beads are zero- catalysis,[2] sensing,[3] pollutant removal[4] and gas capture.[5] dimensional versions of MOF-MMMs, in which the organic Such composites tend to show hierarchical porosity. Most of polymer matrix has been substituted with a crystalline COF them are formed by embedding one porous material into another matrix. Figure 1a shows the synthesis of these MOF/COF one or by interfacing two porous materials.[6] Archetypal composite beads. First, spray-drying is used to encapsulate the examples are mixed-matrix membranes (MMMs) in which a MOF crystals into an amorphous, imine-based, TAPB-BTCA porous material (e.g. zeolite or metal-organic framework [MOF]) polymer bead. Then, the beads are subjected to dynamic is incorporated into an amorphous porous polymeric film.[7] covalent chemistry conditions, which transform the amorphous polymer into COF-TAPB-BTCA crystals.[11] We discovered that this amorphous-to-crystalline transformation around the MOF [a] Dr. Luis Garzón-Tovar, Dr. Javier Pérez-Carvajal, Dr. Amirali Yazdi, crystals generates pores at the MOF/COF interface, leading to a Dr. I. Imaz, Prof. D. Maspoch Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC ca. three times higher N2 BET surface area relative to the value and BIST expected from simply summing the respective contributions of Campus UAB, Bellaterra, 08193 Barcelona, Spain each component (MOF and COF). In addition, we have proven E-mail: inhar.imaz@icn2.cat, daniel.maspoch@icn2.cat that water adsorption in these pores occurs at the same P/P0 as # Both authors contributed equally [b] Jose Hernández-Muñoz, Prof. Pedro Tarazona in the COF pores, which enables a three to four times increase Departamento de Física Teórica de la Materia Condensada, IFIMAC of the water uptake step. Condensed Matter Physics Center, Universidad Autónoma de Madrid, Madrid, 28049, Spain [c] Dr. Félix Zamora Departamento de Química Inorgánica and Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, Madrid, 28049, Spain [d] Dr. Félix Zamora Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA Nanociencia), Cantoblanco, 28049, Madrid, Spain [e] Prof. D. Maspoch ICREA Pg. Lluís Companys 23, Barcelona, 08010, Spain Supporting information for this article is given via a link at the end of the document We began synthesis of the first composite by preparing SEM cross-sectional image of these UiO-66-NH2@COF-TAPB- UiO-66-NH2@COF-TAPB-BTCA beads. A solution of BTCA (63 BTCA beads confirmed that the UiO-66-NH2 particles were well mg) in a mixture of DMSO and acetic acid (9 : 1 v/v, Vtotal = 15 dispersed within the COF matrix (Figure 1f). However, we Figure 1 a) Schematic of the MOF@COF composite synthesis. b,c) Representative FESEM and HAADF-STEM images of microspherical UiO-66- NH2@COF-TAPB-BTCA beads. d,e) FIB-SEM images of an imine-based polymeric bead containing encapsulated UiO-66-NH2 crystals (d), and of a crystalline UiO-66-NH2@COF-TAPB-BTCA bead (e). f) 3D reconstruction of a representative UiO-66-NH2@COF-TAPB-BTCA bead. g) XRPD diffractogram for UiO-66-NH2@COF-TAPB-BTCA (green), compared with the simulated powder patterns for UiO-66-NH2 (blue) and COF-TAPB-BTCA (orange). Scale bars: b) 2.5 µm, c-e) 1 µm, and f) 3.5 µm. mL) and a mixture of TAPB (143 mg) and UiO-66-NH2 [40 mg; observed a COF shell, and a certain degree of hollowing of the synthesized under solvothermal conditions; crystal size = 217 ± dense matrix, during the COF crystallization (Figures 1d,e). We 32 nm; obtained as pure phase as confirmed by X-ray powder hypothesized that the COF shell formation and the hollowing diffraction (XRPD), field-emission scanning electron microscopy that generated macroscale pores could have been due to inside- (FE-SEM), N2 sorption isotherm (SBET = 1151 m2·g−1; Supporting out Ostwald ripening, whereby COF crystallization on the Information, Figures S1-S4)] in DMSO (15 mL) were surface of the sphere would be thermodynamically more independently atomized using a two-fluid nozzle (T-mode) at a favorable than inside the sphere, given the lower surface energy feed rate of 3.0 mL min-1, a flow rate of 336 mLmin-1 and an inlet of surface crystals compared to internal ones.[12] Thus, we temperature of 200˚C, using a B-290 Mini Spray Dryer (BṺCHI reasoned that formation of the shell by self-assembly of the COF Labortechnique). This atomization immediately afforded a yellow crystals was due to the continuous dissolution of the core and powder. High-angle annular dark-field scanning transmission subsequent migration to the shell. A similar phenomenon was electron microscopy (HAADF-STEM), FE-SEM and XRPD observed by Banerjee et al. during formation of hollow spherical performed on this intermediate solid revealed the successful COFs._ENREF_5[13] In our composite beads, we estimated encapsulation of UiO-66-NH2 crystals into amorphous spherical the content of Zr by digesting the powder (previously outgassed imine-based polymer beads (diameter: 0.5 µm to 10 µm; at 150 oC) in a mixture of HNO3/HF, and then analyzing it by Supporting Information, Figures S5,S6). Additionally, focused inductively-coupled plasma optical-emission spectroscopy (ICP- ion beam scanning electron microscopy (FIB-SEM) cross- OES). We found a UiO-66-NH2 content of 15% wt%. The sectional images of these beads revealed a homogeneous maximum achievable MOF loading in these COF-TAPB-BTCA distribution of UiO-66-NH2 crystals within the dense, imine- beads was 15 wt%, as a higher quantity of UiO-66-NH2 used in based polymer matrix (Figure 1d). Nitrogen adsorption the synthesis precluded its complete encapsulation inside the measurements performed at 77 K revealed that this intermediate beads. Note also that the optimum heating time for the solid was non-porous (Supporting Information, Figure S7), amorphous-to-crystalline transformation was found to be 72 suggesting that the encapsulated MOF was not accessible to N2 hours, as longer heating times resulted in etching of the UiO-66- and confirming that the UiO-66-NH2 had been incorporated into NH2 crystals (Supporting Information, Figure S8). the polymer beads. Once we had optimized the synthesis of UiO-66- In the second synthetic step, we subjected the solid NH2@COF-TAPB-BTCA, we studied its N2 adsorption product from above to an amorphous-to-crystalline properties (Figure 2a). We performed N2 adsorption transformation by dispersing it into a mixture of 1,4- measurements at 77 K on the individual composite components dioxane/mesitylene/water/acetic acid, and then heating under (COF-TAPB-BTCA beads and UiO-66-NH2; Supporting reflux the resulting dispersion at 80 ˚C for 72 h.[11] Remarkably, Information, Figures S3,S4,S12); a physical mixture of UiO-66- the XRPD pattern of the so-formed solid (hereafter, UiO-66- NH2 and COF-TAPB-BTCA beads (15% and 85% w/w, NH2@COF-TAPB-BTCA) confirmed that the UiO-66-NH2 had respectively); and UiO-66-NH2@COF-TAPB-BTCA beads not been affected, and it showed a new diffraction peak at 5.5o, (Figure 2a). As reported, UiO-66-NH2 showed a type I isotherm, confirming the desired transformation into COF-TAPB-BTCA with a SBET of 1151 m2·g−1. The isotherms of the other three (Figure 1g). HAADF-STEM and FE-SEM images revealed samples follow a complex type I trend, exhibiting a steep confinement of UiO-66-NH2 nanoparticles into beads created by increase at low P/P0 values, followed by a continuous and the close packing of COF nanoparticles (Figures 1b,c). A FIB- monotonic increase showing an H3 type hysteresis loop.[14] This (qmax) of 0.39 g·g-1 at 0.90 P/P0 (Figure 2b). There are two interesting features in this isotherm. First, UiO-66-NH2@COF- TAPB-BTCA beads also exhibited a much higher qmax at 0.90 P/P0 than that expected from the mass contribution of both components (expected qmax = 0.25 g·g-1; measured qmax of the physical mixture = 0.22 g·g-1). This behavior resembles that observed for the higher N2 uptake attributed to the enhanced porosity generated in the MOF/COF interface (vide supra). Secondly, the main steep water uptake in UiO-66-NH2@COF- TAPB-BTCA beads occurred at the same P/P0 range than in the pure COF-TAPB-BTCA beads and in the physical mixture. Moreover, this steep water uptake is more than three times (0.26 g·g-1) that of the physical mixture (0.07 g·g-1) and nearly three times that of the COF-TAPB-BTCA beads (0.09 g·g-1). Based on the above observations, we hypothesized that the COF pores, and the pores generated at the MOF/COF interface, rapidly fill with water molecules within the same pressure window. To corroborate our hypothesis, we performed a theoretical calculation of water molecules filling the pores, using a Landau-Ginzburg density functional representation of water, and a simple geometrical model for the hierarchical structure of interfacial pores between COF-like and MOF-like walls. The thermodynamic condition for the filling of a pore of radius 𝑟 is 𝑃/𝑃𝑜 ≅ 1 − 𝑤 cos(𝜃) /𝑟, where 𝜃 is the contact angle of the liquid on the walls, and the length 𝑤 ≡ 2𝛾/(𝑅𝑇 𝜌𝑙𝑖𝑞 ) is ≈ Figure 2. a) N2 adsorption isotherms for UiO-66-NH2 (blue), COF-TAPB- 1 𝑛𝑚 with the surface tension, 𝛾, and the molar density of water. BTCA beads (orange), the physical mixture of these two components (grey) and UiO-66-NH2@COF-TAPB-BTCA beads (green). b) Water Interfacial (irregular) pores would be represented by a broad adsorption isotherms for UiO-66-NH2 (blue), COF-TAPB-BTCA beads distribution of effective values for 𝑟, which would gradually fill at (orange), the physical mixture of these two components (grey) and UiO-66- increasing vapor pressure, at a rate depending on cos(𝜃), thus NH2@COF-TAPB-BTCA (green). describing the mean hydrophilicity of the confining COF walls. At complex trend with a non-distinctive plateau is common in 𝑃 = 𝑃𝐶𝑂𝐹 , the nanopores of COF-TAPB-BTCA fill and there is a microporous adsorbents comprising small agglomerated sudden jump ∆〈cos(𝜃)〉𝑒𝑓 ≈ 𝑎𝑝 , in the effective contact angle of particles, which exhibit micropores and mesopores, and an water on these surfaces, where 𝑎𝑝 ≈ 0.07 is the ratio of the external surface.[15] From these isotherms, we found SBET values nanopore normal section across the COF-TAPB-BTCA crystal. of 319 m2·g−1 for the COF-TAPB-BTCA beads, 420 m2·g−1 for the All the interfacial spaces within a width ∆𝑟 ≈ physical mixture, and 1153 m2·g−1 for the UiO-66-NH2@COF- 𝑎𝑝 𝑤⁄(1 − 𝑃𝐶𝑂𝐹 ⁄𝑃𝑜 ), for their effective radii, would be filled at the TAPB-BTCA (Supporting Information, Figures S10-S12). Note same humidity as the COF pores. We employed this approach here that the SBET of the physical mixture was close to the to extract a smooth distribution of (effective) size for the expected value (445 m2·g−1) calculated by considering the interfacial pores from the experimental adsorption curves. isotherms for the COF-TAPB-BTCA beads and the UiO-66-NH2 As a proof-of-principle for the validity of the mesoscopic according to their respective mass proportions. thermodynamic analysis, we explored the effect at molecular By comparing the four isotherms from above, we found scale, through a Landau-Ginzburg Density Functional analysis of that UiO-66-NH2@COF-TAPB-BTCA beads exhibited a the grand potential energy considerably higher SBET (1153 m2·g−1) than the summed SBET 𝑏 2 for the two components (420 m2·g−1). Given the similar Ω[𝜌] = ∫ 𝑑𝑟 (𝑓(𝜌(𝑟)) + 2 (∇𝜌(𝑟)) + (𝑉(𝑟) − 𝜇) 𝜌(𝑟)), structures of COF-TAPB-BTCA and UiO-66-NH2@COF-TAPB- BTCA beads (i.e. a COF shell with internal hollowing), we in terms of the bulk free energy density of a Lennard-Jones fluid attributed the higher surface area in the latter to the formation of and a square gradient term, with parameters to reproduce the pores of quasi-microscale (also known as supermicropores) in phase coexistence and surface tension of water, in a confining the interfaces between the MOF and COF crystals.[16] Formation external potential 𝑉(𝑟) with the geometrical structure of cavities of these pores was also reflected by the pore-size distribution confined between porous walls. The results confirm that all the (PSD; Supporting Information, Figure S13). The PSD of UiO-66- interfacial pores, within a range of sizes (in our calculations: NH2@COF-TAPB-BTCA showed the characteristic pore from 26 Å to 34 Å), are collectively filled at the same pressure at populations of both UiO-66-NH2 (pore size: ca. 9 Å and 11 Å) which the regular pores of the COF-like wall are filled and COF-TAPB-BTCA (pore size: ca. 17 Å) attributed to the (Supporting Information, Section 4). The theoretical analysis fully intrinsic porosity of their frameworks. Additionally, we observed corroborates a generic concept important for increasing the formation of a pore population ranging from ca. 18 Å to 32 Å in sorption uptake step of adsorbents for water-sorption size, and a wider and larger population of pores (in comparison applications, including water harvesting, humidity control, and to COF-TAPB-BTCA) sized from 32 Å to beyond ca. 100 Å. adsorption heat pumps and chillers. Having observed the formation of a hierarchical porous To demonstrate the generality of our approach, we UiO-66-NH2@COF-TAPB-BTCA composite, we then studied its synthesized a second composite under similar conditions, water sorption properties. The collected water isotherm revealed substituting UiO-66-NH2 with Zr-fumarate [synthesized under an “S”-type sorption isotherm, exhibiting a main steep uptake solvothermal conditions; crystal size = 20 ± 12 nm; obtained as between 0.35 P/P0 and 0.50 P/P0, and a maximum capacity pure phase as confirmed by XRPD, HAADF-STEM and N2 materials: for example, for water-sorption applications and especially, in the fabrication of composite membranes. Acknowledgements This work was supported by the Spanish MINECO (projects PN MAT2015-65354-C2-1-R, MAT2016-77608-C3-1-P, FIS2017- 86007-C3 and FPU2015/0248), the Catalan AGAUR (project 2014 SGR 80), the ERC, under EU-FP7 (ERC-Co 615954), and Figure 3. a) FIB-SEM images of Zr-fumarate@COF-TAPB-BTCA the CERCA Program/Generalitat de Catalunya. ICN2 is beads. Scale bar = 200 nm. b) XRPD diffractogram for Zr- supported by the Severo Ochoa program from Spanish MINECO fumarate@COF-TAPB-BTCA (pink), compared with the simulated (Grant No. SEV-2017-0706). IFIMAC is supported by the Maria powder patterns for Zr-fumarate (blue) and COF-TAPB-BTCA (orange). de Maeztu program (Grant No. MDM-2014-0377). c) N2 adsorption isotherms for Zr-fumarate (blue), COF-TAPB-BTCA beads (orange) and Zr-fumarate@COF-TAPB-BTCA beads (pink). d) Water sorption isotherms for Zr-fumarate (blue), COF-TAPB-BTCA Conflict of interest (orange) and Zr-fumarate@COF-TAPB-BTCA (pink). The authors declare no conflict of interest. sorption isotherm (SBET = 830 m2·g−1; Supporting Information, Keywords: metal-organic framework • covalent-organic Figures S20-22]. After the two-step synthesis, FIB-SEM cross- framework • composite • hierarchical porosity • interface sectional and HAADF-STEM images, and XRPD, of the resulting solid revealed formation of crystalline beads of COF-TAPB- [1] T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro, A. Corma, F. BTCA in which the Zr-fumarate particles were encapsulated Kapteijn, F. X. Llabrés i Xamena, J. Gascon, Nat. Mater. 2014, 14, 48. (Figure 3a,b; Supporting Information, Figure S20). In this case, [2] H.-W. Liang, X. Zhuang, S. Brüller, X. Feng, K. Müllen, Nat. the optimum heating time for the amorphous to crystalline Comm. 2014, 5, 4973. transformation process was determined to be 18 hours; again, [3] M. Tubaishat, S. Madria, IEEE Potentials 2003, 22, 20-23. further heating caused etching of the Zr-fumarate crystals. 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Huang, L.-H. from the collective contribution of the two components (Figure Chen, Y. Li, X.-Y. Yang, Z.-Y. Yuan, B.-L. Su, Chem. Soc. Rev. 3c,d; Supporting Information, Figures S22-S24). Thus, the 2016, 45, 3479-3563. expected SBET value was calculated to be 240 m2·g−1 and the [7] a) B. Zornoza, C. Tellez, J. Coronas, J. Gascon, F. Kapteijn, Microp. Mesoporous Mater. 2013, 166, 67-78; b) Y. Cheng, Y. expected qmax value, 0.19 g·g-1, based on the mass proportion of Ying, S. Japip, S.-D. Jiang, T.-S. Chung, S. Zhang, D. Zhao, each component: 18 wt% of pristine Zr-fumarate nanoparticles Adv. Mater. 2018, 30, 1802401; c) T. Ma, E. A. Kapustin, S. X. (SBET = 830 m2·g−1; qmax = 0.39 g·g-1) and 82 wt% of COF-TAPB- Yin, L. Liang, Z. Zhou, J. Niu, L.-H. Li, Y. Wang, J. Su, J. Li, BTCA beads (SBET = 110 m2·g−1; qmax = 0.14 g·g-1; heated for 18 X. Wang, W. D. Wang, W. Wang, J. Sun, O. M. Yaghi, Science 2018, 361, 48-52. hours). Likewise, the water isotherm revealed an “S”-type [8] a) A. P. Côté, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J. sorption isotherm exhibiting a steep uptake between 0.35 P/P0 Matzger, O. M. Yaghi, Science 2005, 310, 1166-1170; b) B. J. and 0.50 P/P0 ca. four-fold (0.17 g·g-1) that expected from the Smith, L. R. Parent, A. C. Overholts, P. A. Beaucage, R. P. mass contribution of COF-TAPB-BTCA beads (0.04 g·g-1). Bisbey, A. D. Chavez, N. Hwang, C. Park, A. M. Evans, N. C. Together, our results confirmed the formation of additional pores Gianneschi, W. R. Dichtel, ACS Cent. Sci. 2017, 3, 58-65. [9] J. Fu, S. Das, G. Xing, T. Ben, V. Valtchev, S. Qiu, J. Am. at the MOF/COF interfaces. Chem. Soc 2016, 138, 7673-7680. In conclusion, we have synthesized microspherical [10] a) D. Sun, S. Jang, S.-J. Yim, L. Ye, D.-P. Kim, Adv. Funct. MOF/COF composite beads by encapsulating MOF crystals Mater. 2018, 28, 1707110; b) Y. Peng, M. Zhao, B. Chen, Z. within a crystalline COF matrix, through a two-step process of Zhang, Y. Huang, F. Dai, Z. Lai, X. Cui, C. 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These two observations suggest that the sizes of the 7535. newly formed pores are within the supermicropore regime and [12] Z. Wang, L. Zhou, X. W. Lou, Adv. Mater. 2012, 24, 1903- 1911. that they are similar to the COF pores. We expect that the ability [13] S. Kandambeth, V. Venkatesh, D. B. Shinde, S. Kumari, A. to generate additional pores at MOF/COF interfaces should Halder, S. Verma, R. Banerjee, Nat. Commun. 2015, 6, 6786. have implications in the development of new composite [14] K. S. W. Sing, F. Rouquerol, J. Rouquerol, P. Llewellyn, in Adsorption by Powders and Porous Solids (Second Edition) (Eds.: F. Rouquerol, J. Rouquerol, K. S. W. Sing, P. Llewellyn, G. Maurin), Academic Press, Oxford, 2014, pp. 269-302. [15] K. S. W. Sing, F. Rouquerol, P. Llewellyn, J. Rouquerol, in Adsorption by Powders and Porous Solids (Second Edition) (Eds.: F. Rouquerol, J. Rouquerol, K. S. W. Sing, P. Llewellyn, G. Maurin), Academic Press, Oxford, 2014, pp. 303-320. [16] a) R. Semino, J. C. Moreton, N. A. Ramsahye, S. M. Cohen, G. Maurin, Chem. Sci. 2018, 9, 315-324; b) Y. Cheng, Y. Ying, L. Zhai, G. Liu, J. Dong, Y. Wang, M. P. Christopher, S. Long, Y. Wang, D. Zhao, J. Memb. Sci. 2019, 573, 97-106. Entry for the Table of Contents (Please choose one layout) Layout 1: COMMUNICATION A new class of porous composites Luis Garzón-Tovar, Javier Pérez- comprising metal-organic framework Carvajal, Amirali Yazdi, Jose Hernández- (MOF) crystals confined in single Muñoz, Pedro Tarazona, Inhar Imaz,* spherical matrices made of packed Félix Zamora and Daniel Maspoch* covalent-organic framework (COF) nanocrystals is described. These Page No. – Page No. MOF@COF composites show the formation of micro- and mesopores at the MOF/COF interface that provide Enhanced Uptake in a MOF@COF Composite by Interfacial Pore superior porosity compared to the Generation constituent MOF and COF components taken together.