This is the peer reviewed version of the following article: Andrés M.A., Carné-Sánchez A., Sánchez-Laínez J., Roubeau O., Coronas J., Maspoch D., Gascón I.. Ultrathin Films of Porous Metal–Organic Polyhedra for Gas Separation. Chemistry - A European Journal, (2020). 26. : 143 - . 10.1002/chem.201904141, which has been published in final form at https://dx.doi.org/10.1002/chem.201904141. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Ultrathin films of porous metal-organic polyhedra for gas separation Miguel A. Andrés[a,b], Arnau Carné-Sánchez[c], Javier Sánchez-Laínez[b,d], Olivier Roubeau[b], Joaquín Coronas[b,d], Daniel Maspoch*[c,e], Ignacio Gascón*[a,b] Abstract: Ultrathin films of a robust Rh(II)-based porous metal- air-liquid interface, and the subsequent transfer of such films to organic polyhedra (MOP) have been obtained. Homogeneous and permeable supports to fabricate multilayer membranes with compact monolayer films (ca. 2.5 nm thick) were first formed at the precise control on the thickness of the selective layer.[7] However, air-water interface, deposited onto different substrates and this potential remains under-exploited because the majority of characterized using spectroscopic methods, scanning transmission porous materials lack the solution processability required for the electron microscopy and atomic force microscopy. As a proof of LB method. Accordingly, the use of the LB technique to fabricate concept, the gas separation performance of MOP supported ultrathin membranes for CO2 capture has been restricted to membranes has been also evaluated. Selective MOP ultrathin films soluble organic molecules[7] and polymers of intrinsic porosity (thickness ca. 60 nm) exhibit remarkable CO2 permeance and CO2/N2 (PIMs).[8, 9] selectivity, demonstrating the great combined potential of MOP and Herein, we expand the scope of the LB method to discrete Langmuir-based techniques in separation technologies. metal-organic polyhedral molecules (MOPs) fabricating MOP- based ultrathin films. MOPs are nanosized molecules of 2-5 nm synthesised from the self-assembly of metal ions and organic The current technology for capturing CO2 at the emission points linkers, which can be soluble in different polar and non-polar relies on its absorption in alkanolamine aqueous solutions. This liquids and permanently porous in solid state.[10-14] In this solution-based method is highly selective but requires high communication, we show how robust Rh(II)-based MOPs can be regeneration temperatures, which reduces its recyclability and assembled at the air-water interface through the LB technique increases its cost making difficult its implementation at large forming homogenous 2-3 nm thick monolayer films. For this study, scale.[1] Alternatively, CO2 separation by membranes has we selected a cuboctahedral Rh-MOP functionalized with emerged as an environmentally friendly and more cost effective pendant aliphatic chains with formula: [Rh2(C12-bdc)2]12 (hereafter method,[2] and can be applied to other processes of interest such C12RhMOP; C12-bdc = 5-dodecoxybenzene-1,3-dicarboxylate; as the upgrading of biogas to biomethane.[3] However, current Figure 1a) due to its hydrolytic stability and good solubility in membranes lack the capacity to treat huge amounts of flue gas organic solvents such as dichloromethane (DCM).[15,16] High with the required selectivity for CO2.[4] Accordingly, membranes hydrolytic stability of MOPs is a prerequisite for the LB method able to achieve high gas flux (permeance) while maintaining a because the assembled monolayers are formed on top of the high selectivity for CO2 are intensively sought. One way of water phase. Thus, MOPs based on labile coordination bonds increasing gas flux is decreasing the thickness of the membrane such as Cu(II)-carboxylate were not considered for this study. because both parameters are inversely proportional. Therefore, Furthermore, the pendant aliphatic chains of C12RhMOP are ultrathin porous films with thicknesses below 100 nm have the expected to facilitate the formation of a compact monolayer at the potential to enhance membrane performance by increasing the air-water interface and to favour the cohesion among deposited flux of gas through the membrane without losing CO2 selectivity.[5, 6] multilayers. Moreover, previous studies[16] have shown that Langmuir-Blodgett (LB) is a technique especially suitable for this C12RhMOP displays a negligible N2 adsorption at 77 K (0.17 and purpose as it enables the self-assembly of monolayer films at the 0.82 N2 moles per MOP mol at P/P0 ≈ 0.1 and P/P0 ≈ 0.95, respectively) and a significant CO2 adsorption at 195 K (9 and 22 [a] M. A. Andrés, Dr. I Gascón mol of CO2 per MOP mol at P/P0 ≈ 0.1 and P/P0 ≈ 0.95, Departamento de Química Física and Instituto de Nanociencia de respectively), making it an interesting candidate for CO2/N2 Aragón (INA), Universidad de Zaragoza, 50009 Zaragoza (Spain). separation. Multiple C12RhMOP monolayer films were thus E-mail: igascon@unizar.es deposited on the permeable polymer poly[1-(trimethylsilyl)-1- [b] M. A. Andrés, Dr. J. Sánchez-Laínez, Dr. Olivier Roubeau, Prof. Dr. J. Coronas, Dr. I Gascón propyne] (PTMSP) to fabricate a multilayer membrane with the Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC and desired MOP film thickness. The CO2/N2 selectivity of the PTMSP Universidad de Zaragoza, 50009 Zaragoza, Spain. membrane at 1 bar and 35°C was significantly increased from 4.1 [c] Dr. A. Carné-Sánchez, Prof. Dr. D. Maspoch to 10.1 after 30 deposition cycles of C12RhMOP monolayers while Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, keeping high the CO2 permeance (195 GPU), thereby Campus UAB, Bellaterra, 08193 Barcelona, Spain. demonstrating the potential of molecularly thin MOP films for the E-mail: daniel.maspoch@icn2.cat CO2/N2 separation. [d] Dr. J. Sánchez-Laínez, Prof. Dr. J. Coronas Chemical and Environmental Engineering Department and Instituto de Nanociencia de Aragón (INA) Universidad de Zaragoza, 50018 Zaragoza, Spain [e] Prof. Dr. D. Maspoch ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain Supporting information for this article is given via a link at the end of the document. We initially prepared a diluted C12RhMOP solution in DCM substrates. Two different deposition methods were tested: vertical (2.8·10-6 M) to study Langmuir film formation. Then, 1000 - 1200 transfer (Langmuir-Blodgett films) and horizontal transfer µL of the C12RhMOP solution (i.e. the total mass of C12RhMOP (Langmuir-Schaefer, LS, films). Irrespectively of the deposition employed was 0.04 mg) were spread on the water subphase. method employed, the UV-Vis spectra of the C12RhMOP After solvent evaporation (ca. 10 min), the floating molecules were deposited on quartz reveal the presence of two bands at 265 and compressed to induce their assembly into a monolayer film 315 nm, which are in perfect agreement with the UV-Vis spectra (Figure 1b). The surface pressure - area isotherm (π-A) reveals of C12RhMOP in solution. This confirms the stability of C12RhMOP that the self-assembly process starts when C12RhMOP is throughout the whole assembly process (see Figure S4 in the compressed to an area of 8 nm2/molecule (Figure 1c). Further Supporting Information). compression causes a steep increase of the surface pressure However, the absorbance values obtained for the indicating the formation of the film. The maximum slope at ca. 5 monolayer films transferred horizontally are systematically higher nm2/molecule is in good agreement with the estimated lateral size than for those transferred vertically, at all pressures tested, of a single molecule with extended alkyl substituents at ca. 5.3 nm thereby suggesting a better quality and homogeneity for the LS (see Figure S1 in the Supporting Information), which would films (see Figure S5 in the Supporting Information). UV-Vis indicate that the film formation involves a partial interdigitation of measurements also indicate that the film absorbance values are the alkyl arms. Brewster angle microscopy images obtained lower when surface pressures above 25 mN·m-1 are employed. during C12RhMOP compression reveal that the water surface is These low absorbance values reflect a poorer film coverage, completely covered by a C12RhMOP monolayer film at a surface probably because the film becomes too rigid above this surface pressure of 25 mN·m-1 (see Figure S2 in the Supporting pressure, making the transfer less effective. These observations Information). Increasing the surface pressure above 37.5 mN·m-1 are in line with atomic force microscopy (AFM) measurements, as causes the collapse of the C12RhMOP film, which agrees with the complete substrate coverage is achieved when monolayer films change in the slope of the surface pressure in the π-A isotherm are transferred at a surface pressure of 25 mN·m-1 (Figure 1d and and the value determined from the surface potential Figures S6 and S7 in the Supporting Information). On the contrary, measurements (see Figure S3 in the Supporting Information). a lower film coverage is observed when transfers were performed at 35 mN·m-1, due to the presence of domains with different thicknesses. Altogether, these results confirm that horizontal transfer at a surface pressure of 25 mN·m-1 allows the formation of homogeneous LS C12RhMOP monolayer films on surfaces with an average height of ca. 2.5 nm (see Figure 1d and Figure S7 in the Supporting Information). Interestingly, no significant differences in terms of homogeneity are observed when using different types of flat substrate, i.e. the inorganic hydrophilic mica, quartz or the hydrophobic PTSMP organic polymer (see below). With the pressure and the deposition method of C12RhMOP monolayers optimized, a quartz crystal microbalance (QCM) was then used to study the multilayer deposition. A linear correlation between the deposited mass of C12RhMOP and the number of cycles is found (see Figure S8 in the Supporting Information), with an almost constant deposition of 0.31 µg·cm-2 for each MOP monolayer. This corresponds to 5.8 nm2/molecule thus very similar to the area observed at the air-water interface. This accounts for the film quality, as defects in one deposition would cause poorer deposition of the subsequent monolayers. The fabrication of MOP multilayer films with a controlled thickness offers several possibilities for the implementation of MOPs in membrane technology for gas separation. However, to our knowledge, MOPs have been only used in this field as fillers for mixed matrix membranes with micrometric thicknesses. [18-21] As a proof of concept, in order to demonstrate how the LS deposition can be used for the fabrication of MOP membranes Figure 1 (a) View of the C12RhMOP molecule based on the crystal structure with thicknesses in the nanometre range and supported on a of the related C12CuMOP and its schematic representation as a cuboctahedron.[17] Colour code: Rh (green), C (grey), O (orange). (b) porous substrate, we studied the multilayer deposition of Scheme of the formation of a continuous Langmuir film upon compression C12RhMOP monolayers onto PTMSP supports. PTMSP is an of C12RhMOP molecules expanded at the air-water interface. (c) π-A isotherm obtained from the compression of C12RhMOP. (d) AFM image of ultrapermeable polymer showing very high gas permeability and a single monolayer of C12RhMOP deposited on quartz substrate. Note here low CO2/N2 selectivity. Moreover, it can be easily processed into that the observed scratches correspond to defects on the surface of the very flat membranes by drop-casting and therefore, it is a suitable quartz plates used since phase images confirmed that the samples were homogeneous. support for the deposition of LB/LS films of different substances.[8, 9] Next, monolayer films compressed at surface pressures between 15 and 35 mN·m-1 were transferred onto mica and quartz To this end, different multilayer films were fabricated by in the PTMSP support. Moreover, the deduced thickness of the depositing 3, 5, 10 and 20 C12RhMOP monolayers onto PTMSP MOP layer derived from the EDS profiles is also consistent with supports using the optimised conditions (LS deposition; surface the STEM-HAADF image, at ca. 60 nm. pressure = 25 mN·m-1). The resulting multilayer C12RhMOP films The separation performance of C12RhMOP supported were then characterized by AFM and X-ray photoelectron membranes, consisting of 20 to 30 MOP monolayers deposited spectroscopy (XPS) (see Figure 2b, Figures S9, S11 and S12 and onto PTMSP supports was tested in CO2/N2 separation for post- Table S1 in the Supporting Information). Although some small combustion carbon capture (temperature 35°C, feed pressures 1 pinholes and C12RhMOP agglomerations are also present, an to 3 bar, CO2/N2 mixture composition 10/90 in volume). CO2 overall good coverage of the PTMSP support is observed, even permeance and CO2/N2 selectivity values for the different after the deposition of only three C12RhMOP monolayers. The membranes analysed are shown in Figure 3. Pure PTMSP thickness of a single monolayer is estimated to be ca. 2.5 nm membranes present a high CO2 permeance (415 GPU at 1 bar) measuring the height in different film defects and borders (see but low CO2/N2 selectivity (4.2) in agreement with previous Figure S7 in the Supporting Information). The size and number of works.[8, 9, 22] film defects diminishes gradually with the number of C12RhMOP monolayers transferred. An almost defect-free MOP multilayer is obtained after the deposition of ca. 20 LS C12RhMOP monolayers. Figure 2 (a,b) AFM images of a bare PTMSP substrate (a) and a PTMSP substrate covered with 20 C12RhMOP monolayers (b). (c) STEM-HAADF image from a lamella extracted from a PTMSP substrate covered with 30 C12RhMOP monolayers. (d) Typical EDS signal of the Rh Lα and Si Kα peaks along the lamella profile. Figure 3. (a,b) CO2/N2 selectivity (a) and CO2 permeance (b) of bare PTMSP (0 layers) and MOP supported membranes formed, respectively, To obtain a more detailed characterization of C12RhMOP by 20, 25 and 30 C12RhMOP monolayers deposited onto PTMSP. Error bars were determined from at least 2 independent measurements. multilayer films deposited onto PTMSP, a lamella of a sample obtained after depositing 30 C12RhMOP monolayers was The deposition of 20 to 30 C12RhMOP monolayers extracted for analysis by scanning transmission electron increases the CO2/N2 selectivity at 1 bar up to values of 9.1 and microscopy (STEM). The sequence of images of the lamella 10.1 (Figure 3a), respectively, while maintaining the CO2 thinning can be found in the Supporting Information (Figure S10). permeance above 195 GPU (Figure 3b). When the feed pressure Figure 2c shows a STEM-HAADF image of the lamella in which it increases from 1 to 3 bar, CO2 permeance values diminish by ca. is possible to distinguish the MOP multilayer with a different 6 % for MOP supported films. However, this reduction is ca. 13 % contrast to that of the PTMSP support at the bottom and the C/Pt for pure PTMSP supports. This difference is indicative of the coatings on top. According to the images obtained, the thickness formation of dense C12RhMOP multilayers, as shown in STEM of the MOP film is close to 60-65 nm. These values are in good images. agreement with the thickness derived from AFM measurements Comparing these data to previous studies,[8,9] where (each monolayer is ca. 2.5 nm thick). Moreover, the composition different polymers of intrinsic microporosity (PIMs) were of the lamella was analysed by energy-dispersive X-ray deposited onto PTMSP substrates, it can be highlighted that the spectroscopy (EDS). Figure 2d shows the Rh Lα (2.697 eV) and CO2 permeance values obtained with 30 C12RhMOP monolayers Si Kα (1.740 eV) peaks counts along the lamella profile, deposited onto PTMSP (195 GPU at 1 bar) are significantly higher confirming the presence of Rh in the MOP multilayer film and Si than those obtained when stacking the same number of PIM monolayers (134 GPU for PIM-TMN-Trip films and 118 GPU for (T43_17R and E31_17R research groups). ICN2 is supported by PIM-EA-TB(H2)). However, CO2/N2 selectivity values are slightly the Severo Ochoa program from Spanish MINECO (SEV-2017- lower for MOP supported films (10.1) than for PIM-TMN-Trip 0706). A.C.-S. thanks the Spanish MINECO for Juan de la Cierva (12.1)[9] and PIM-EA-TB(H2) (13.8)[8] supported films. This fellowship (IJCI-2016-29802), J. S.-L. and M.A.A thank the tendency is somehow expected as the higher is the permeability Spanish Education Ministry Program FPU2014 for their Ph.D. for a membrane, the lower is generally the selectivity. grants. The microscopy work was carried out in the Laboratorio In conclusion, we have demonstrated that the LB method de Microscopias Avanzadas at the Instituto de Nanociencia de can be employed to induce the self-assembly of C12RhMOP into Aragon (LMA-INA, Universidad de Zaragoza). dense and ultrathin porous films, and that these films can be used as selective materials in multilayer membranes for efficient CO2 Keywords: Metal-organic polyhedra, Langmuir-Blodgett separation. These results pave the way for the implementation of technique, Thin films, CO2 separation MOPs in separation technologies as the LB technique enables to fabricate large area membranes using minute amounts of active [1] B. Dutcher, M. Fan, A. G. Russell, ACS Appl. Mater. Interfaces 2015, 7, material. We believe that the versatility and ease functionalization 2137-2148. of MOPs will provide a wide catalogue of MOP-based membranes [2] N. Du, H. B. Park, M. M. Dal-Cin, M. D. Guiver, Energy Environ. Sci. 2012, 5, 7306-7322. with potential in gas separation and other applications. [3] V. Vrbová, K. Ciahotný, Energy Fuels 2017, 31, 9393-9401. [4] M. Wang, J. Zhao, X. Wang, A. Liu, K. K. Gleason, J. Mater. Chem. A 2017, 5, 8860-8886. [5] R. Selyanchyn, S. Fujikawa, Sci. Technol. Adv. Mater. 2017, 18, 816-827. Experimental Section [6] K. Xie, Q. Fu, G. G. Qiao, P. A. Webley, J. Membr. Sci., 2019, 572, 38- 60. [7] M. Wang, V. Janout, S. L. Regen, Acc. Chem. Res. 2013, 46, 2743-2754. Thin film formation at the air-water interface [8] J. Benito, J. Sánchez-Laínez, B. Zornoza, S. Martín, M. Carta, R. Malpass-Evans, C. Téllez, N. B. McKeown, J. Coronas, I. Gascón, ChemSusChem 2017, 10, 4014-4017. Film formation at the air-water interface (Langmuir films) was first [9] J. Benito, J. Vidal, J. Sánchez-Laínez, B. Zornoza, C. Téllez, S. Martín, optimized using a commercial Langmuir Teflon trough (NIMA, Model 702) K. J. Msayib, B. Comesaña-Gándara, N. B. McKeown, J. Coronas, I. Gascón, with a symmetrical double-barrier configuration and dimensions of 720 × J. Colloid Interface Sci. 2019, 536, 474-482. 100 mm. This device was used to register surface pressure vs. area (π-A) [10] J.-R. Li, H.-C. Zhou, Nat. Chem. 2010, 2, 893. [11] C. A. Rowland, G. R. Lorzing, E. J. Gosselin, B. A. Trump, G. P. A. Yap, isotherms and Brewster angle microscopy (BAM) images. These images C. M. Brown, E. D. Bloch, J. Am. Chem. Soc. 2018, 140, 11153-11157. were obtained using a KSV NIMA Micro BAM, equipped with a red laser [12] W.-H. Xing, H.-Y. Li, X.-Y. Dong, S.-Q. Zang, J. Mater. Chem. A 2018, 6, light source (50 mW, 659 nm) with a fixed incidence angle of 53.1º. Another 7724-7730. apparatus (KSV-NIMA, model KN 2003) with dimensions of 580 mm × 145 [13] M. Eddaoudi, J. Kim, J. B. Wachter, H. K. Chae, M. O'Keeffe, O. M. Yaghi, J. Am. Chem. Soc. 2001, 123, 4368-4369. mm, also equipped with a symmetrical double-barrier system, was used [14] S. Furukawa, N. Horike, M. Kondo, Y. Hijikata, A. Carné-Sánchez, P. for the fabrication of LB and LS films onto different substrates: quartz, mica, Larpent, N. Louvain, S. Diring, H. Sato, R. Matsuda, R. Kawano, S. Kitagawa, QCM discs and PTMSP dense membranes. Both troughs were kept inside Inorg. Chem. 2016, 55, 10843-10846. closed cabinets in a dedicated laboratory with special care to limit the [15] R. Kawano, N. Horike, Y. Hijikata, M. Kondo, A. Carné-Sánchez, P. Larpent, S. Ikemura, T. Osaki, K. Kamiya, S. Kitagawa, S. Takeuchi, S. presence of dust and at constant temperature (20 ± 1 ºC). Films were Furukawa, Chem 2017, 2, 393-403. compressed at constant speed (8 cm2·min-1). Ultra-pure Milli-Q water (ρ = [16] A. Carné-Sánchez, G. A. Craig, P. Larpent, T. Hirose, M. Higuchi, S. 18.2 MΩ·cm) was used as subphase in all the experiments. Surface Kitagawa, K. Matsuda, K. Urayama, S. Furukawa, Nat. Commun. 2018, 9, 2506. pressure was continuously registered in both devices using Wilhelmy [17] H. Furukawa, J. Kim, K. E. Plass, O. M. Yaghi, J. Am. Chem. Soc. 2006, 128, 8398-8399. balances with a filter paper plate. [18] E. V. Perez, K. J. Balkus, J. P. Ferraris, I. H. Musselman, J. Membr. Sci., 2014, 463, 82-93. Thin film deposition onto substrates [19] M. Kitchin, J. Teo, K. Konstas, C. H. Lau, C. J. Sumby, A. W. Thornton, C. J. Doonan, M. R. Hill, J. Mater. Chem. A 2015, 3, 15241-15247. [20] C. R. P. Fulong, J. Liu, V. J. Pastore, H. Lin, T. R. Cook, Dalton Trans. LB films were transferred onto solid substrates by vertical dipping at a 2018, 47, 7905-7915. constant speed of 1 mm·min-1. For LS film fabrication, substrates were [21] X. Liu, X. Wang, A. V. Bavykina, L. Chu, M. Shan, A. Sabetghadam, H. Miro, F. Kapteijn, J. Gascon, ACS Appl. Mater. Interfaces, 2018, 10, 21381- held horizontally and parallel to the water surface using a vacuum pump 21389. based horizontal dipping clamp. When the desired surface pressure was [22] S. D. Bazhenov, I. L. Borisov, D. S. Bakhtin, A. N. Rybakova, V. S. reached, the substrate was approached to the surface at a vertical speed Khotimskiy, S. P. Molchanov, V. V. Volkov, Green Energy Environ. 2016, 1, 235- of 1 mm·min-1. Once the substrate touched the water surface, it was 245. withdrawn at a vertical speed of 10 mm·min-1. After each transfer, LS films were dried with N2 at ambient temperature, and the procedure was repeated as many times as necessary to obtain films with the desired number of MOP monolayers. Acknowledgment This work was supported by the Spanish MINECO (projects RTI2018-095622-B-I00, MAT2016-78257-R, MAT2016-77290-R and MAT2017-86826-R), the Catalan AGAUR (project 2017 SGR 328), the ERC under the EU-FP7 (ERC-Co 615954), the CERCA Program/Generalitat de Catalunya and the Aragon Government COMMUNICATION The fabrication of compact Miguel A. Andrés, Arnau and dense porous metal- Carné-Sánchez, Javier organic polyhedra (MOP) Sánchez-Laínez, Olivier ultrathin films offers several Roubeau, Joaquín Coronas, advantages for the Daniel Maspoch*, Ignacio development of MOP-based Gascón* applications in gas Page No. – Page No. separation. MOP supported membranes obtained by Title successive Rh(II)-based MOP monolayer deposition can be applied for efficient CO2 separation.