Ultrathin films of porous metal-organic polyhedra for gas separation

Ultrathin films of a robust Rh(II)-based porous metalorganic polyhedra (MOP) have been obtained. Homogeneous and compact monolayer films (ca. 2.5 nm thick) were first formed at the air-water interface, deposited onto different substrates and characterized using spectroscopic methods, scanning transmission electron microscopy and atomic force microscopy. As a proof of concept, the gas separation performance of MOP supported membranes has been also evaluated. Selective MOP ultrathin films (thickness ca. 60 nm) exhibit remarkable CO2 permeance and CO2/N2 selectivity, demonstrating the great combined potential of MOP and Langmuir-based techniques in separation technologies. The current technology for capturing CO2 at the emission points relies on its absorption in alkanolamine aqueous solutions. This solution-based method is highly selective but requires high regeneration temperatures, which reduces its recyclability and increases its cost making difficult its implementation at large scale.[1] Alternatively, CO2 separation by membranes has emerged as an environmentally friendly and more cost effective method,[2] and can be applied to other processes of interest such as the upgrading of biogas to biomethane.[3] However, current membranes lack the capacity to treat huge amounts of flue gas with the required selectivity for CO2. Accordingly, membranes able to achieve high gas flux (permeance) while maintaining a high selectivity for CO2 are intensively sought. One way of increasing gas flux is decreasing the thickness of the membrane because both parameters are inversely proportional. Therefore, ultrathin porous films with thicknesses below 100 nm have the potential to enhance membrane performance by increasing the flux of gas through the membrane without losing CO2 selectivity.[5, 6] Langmuir-Blodgett (LB) is a technique especially suitable for this purpose as it enables the self-assembly of a wide range of molecules/nanocrystals into dense monolayers.[7] The subsequent transfer of such films to permeable supports opens the possibility to fabricate multilayer membranes with precise control on the thickness of the selective layer.[8] However, this potential remains under-exploited because the majority of porous materials lack the solution processability required for the LB method. Accordingly, the use of the LB technique to fabricate ultrathin membranes for CO2 capture has been restricted to soluble organic molecules[8] and polymers of intrinsic porosity (PIMs).[9] Herein, we expand the scope of the LB method to discrete metal-organic polyhedral molecules (MOPs) fabricating MOPbased ultrathin films. MOPs are nanosized molecules of 2-5 nm synthesised from the self-assembly of metal ions and organic linkers, which can be soluble in different polar and non-polar liquids and permanently porous in solid state.[10-14] In this communication, we show how robust Rh(II)-based MOPs can be assembled at the air-water interface through the LB technique forming homogenous 2-3 nm thick monolayer films. For this study, we selected a cuboctahedral Rh-MOP functionalized with pendant aliphatic chains with formula: [Rh2(C12-bdc)2]12 (hereafter C12RhMOP; C12-bdc = 5-dodecoxybenzene-1,3-dicarboxylate; Figure 1a) due to its hydrolytic stability and good solubility in organic solvents such as dichloromethane (DCM).[15,16] High hydrolytic stability of MOPs is a prerequisite for the LB method because the assembled monolayers are formed on top of the water phase. Thus, MOPs based on labile coordination bonds such as Cu(II)-carboxylate were not considered for this study. Furthermore, the pendant aliphatic chains of C12RhMOP are expected to facilitate the formation of a compact monolayer at the air-water interface and to favour the cohesion among deposited multilayers. Moreover, previous studies[16] have shown that C12RhMOP displays a negligible N2 adsorption at 77 K (0.17 and 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 mol of CO2 per MOP mol at P/P0 ≈ 0.1 and P/P0 ≈ 0.95, respectively), making it an interesting candidate for CO2/N2 separation. Multiple C12RhMOP monolayer films were thus deposited on the permeable polymer poly[1-(trimethylsilyl)-1propyne] (PTMSP) to fabricate a multilayer membrane with the desired MOP film thickness. The CO2/N2 selectivity of the PTMSP membrane at 1 bar and 35°C was significantly increased from 4.1 to 10.1 after 30 deposition cycles of C12RhMOP monolayers while keeping high the CO2 permeance (195 GPU), thereby demonstrating the potential of molecularly thin MOP films for the CO2/N2 separation. [a] M. A. Andrés, Dr. I Gascón Departamento de Química Física and Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, 50009 Zaragoza (Spain). E-mail: igascon@unizar.es [b] M. A. Andrés, Dr. J. Sánchez-Laínez, Dr. Olivier Roubeau, Prof. Dr. J. Coronas, Dr. I Gascón Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de Zaragoza, 50009 Zaragoza, Spain. [c] Dr. A. Carné-Sánchez, Prof. Dr. D. Maspoch Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain. E-mail: daniel.maspoch@icn2.cat [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.

The current technology for capturing CO2 at the emission points relies on its absorption in alkanolamine aqueous solutions. This solution-based method is highly selective but requires high regeneration temperatures, which reduces its recyclability and increases its cost making difficult its implementation at large scale. [1] Alternatively, CO2 separation by membranes has emerged as an environmentally friendly and more cost effective method, [2] and can be applied to other processes of interest such as the upgrading of biogas to biomethane. [3] However, current membranes lack the capacity to treat huge amounts of flue gas with the required selectivity for CO2. [4] Accordingly, membranes able to achieve high gas flux (permeance) while maintaining a high selectivity for CO2 are intensively sought. One way of increasing gas flux is decreasing the thickness of the membrane because both parameters are inversely proportional. Therefore, ultrathin porous films with thicknesses below 100 nm have the potential to enhance membrane performance by increasing the flux of gas through the membrane without losing CO2 selectivity. [5,6] Langmuir-Blodgett (LB) is a technique especially suitable for this purpose as it enables the self-assembly of a wide range of molecules/nanocrystals into dense monolayers. [7] The subsequent transfer of such films to permeable supports opens the possibility to fabricate multilayer membranes with precise control on the thickness of the selective layer. [8] However, this potential remains under-exploited because the majority of porous materials lack the solution processability required for the LB method. Accordingly, the use of the LB technique to fabricate ultrathin membranes for CO2 capture has been restricted to soluble organic molecules [8] and polymers of intrinsic porosity (PIMs). [9] Herein, we expand the scope of the LB method to discrete metal-organic polyhedral molecules (MOPs) fabricating MOPbased ultrathin films. MOPs are nanosized molecules of 2-5 nm synthesised from the self-assembly of metal ions and organic linkers, which can be soluble in different polar and non-polar liquids and permanently porous in solid state. [10][11][12][13][14] In this communication, we show how robust Rh(II)-based MOPs can be assembled at the air-water interface through the LB technique forming homogenous 2-3 nm thick monolayer films. For this study, we selected a cuboctahedral Rh-MOP functionalized with pendant aliphatic chains with formula: [Rh2(C12-bdc)2]12 (hereafter C12RhMOP; C12-bdc = 5-dodecoxybenzene-1,3-dicarboxylate; Figure 1a) due to its hydrolytic stability and good solubility in organic solvents such as dichloromethane (DCM). [15,16] High hydrolytic stability of MOPs is a prerequisite for the LB method because the assembled monolayers are formed on top of the water phase. Thus, MOPs based on labile coordination bonds such as Cu(II)-carboxylate were not considered for this study. Furthermore, the pendant aliphatic chains of C12RhMOP are expected to facilitate the formation of a compact monolayer at the air-water interface and to favour the cohesion among deposited multilayers. Moreover, previous studies [16] have shown that C12RhMOP displays a negligible N2 adsorption at 77 K (0.17 and 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 mol of CO2 per MOP mol at P/P0 ≈ 0.1 and P/P0 ≈ 0.95, respectively), making it an interesting candidate for CO2/N2 separation. Multiple C12RhMOP monolayer films were thus deposited on the permeable polymer poly[1-(trimethylsilyl)-1propyne] (PTMSP) to fabricate a multilayer membrane with the desired MOP film thickness. The CO2/N2 selectivity of the PTMSP membrane at 1 bar and 35°C was significantly increased from 4.1 to 10.1 after 30 deposition cycles of C12RhMOP monolayers while keeping high the CO2 permeance (195 GPU), thereby demonstrating the potential of molecularly thin MOP films for the CO2/N2 separation.
We initially prepared a diluted C12RhMOP solution in DCM (2.8·10 -6 M) to study Langmuir film formation. Then, 1000 -1200 µL of the C12RhMOP solution (i.e. the total mass of C12RhMOP employed was 0.04 mg) were spread on the water subphase. After solvent evaporation (ca. 10 min), the floating molecules were compressed to induce their assembly into a monolayer film ( Figure 1b). The surface pressurearea isotherm (π-A) reveals that the self-assembly process starts when C12RhMOP is compressed to an area of 8 nm 2 /molecule ( Figure 1c). Further compression causes a steep increase of the surface pressure indicating the formation of the film. The maximum slope at ca. 5 nm 2 /molecule is in good agreement with the estimated lateral size of a single molecule with extended alkyl substituents at ca. 5.3 nm (see Figure S1 in the Supporting Information), which would indicate that the film formation involves a partial interdigitation of the alkyl arms. Brewster angle microscopy images obtained during C12RhMOP compression reveal that the water surface is completely covered by a C12RhMOP monolayer film at a surface pressure of 25 mN·m -1 (see Figure S2 in the Supporting Information). Increasing the surface pressure above 37.5 mN·m -1 causes the collapse of the C12RhMOP film, which agrees with the change in the slope of the surface pressure in the π-A isotherm and the value determined from the surface potential measurements (see Figure S3 in the Supporting Information).
Next, monolayer films compressed at surface pressures between 15 and 35 mN·m -1 were transferred onto mica and quartz substrates. Two different deposition methods were tested: vertical transfer (Langmuir-Blodgett films) and horizontal transfer (Langmuir-Schaefer, LS, films). Irrespectively of the deposition method employed, the UV-Vis spectra of the C12RhMOP deposited on quartz reveal the presence of two bands at 265 and 315 nm, which are in perfect agreement with the UV-Vis spectra of C12RhMOP in solution. This confirms the stability of C12RhMOP throughout the whole assembly process (see Figure S4 in the Supporting Information).
However, the absorbance values obtained for the monolayer films transferred horizontally are systematically higher than for those transferred vertically, at all pressures tested, thereby suggesting a better quality and homogeneity for the LS films (see Figure S5 in the Supporting Information). UV-Vis measurements also indicate that the film absorbance values are lower when surface pressures above 25 mN·m -1 are employed. These low absorbance values reflect a poorer film coverage, probably because the film becomes too rigid above this surface pressure, making the transfer less effective. These observations are in line with atomic force microscopy (AFM) measurements, as complete substrate coverage is achieved when monolayer films are transferred at a surface pressure of 25 mN·m -1 (Figure 1d and Figures S6 and S7 in the Supporting Information). On the contrary, 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 substrates, i.e. the hydrophilic inorganic mica or 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 nm 2 /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][19][20][21] As a proof of concept, in order to demonstrate how the LS deposition can be used for the fabrication of MOP membranes with thicknesses in the nanometre range and supported on a porous substrate, we studied the multilayer deposition of C12RhMOP monolayers onto PTMSP supports. PTMSP is an ultrapermeable polymer showing very high gas permeability and low CO2/N2 selectivity. Moreover, it can be easily processed into very flat membranes by drop-casting (Figure 2a) and, therefore, it is a suitable support for the deposition of LB/LS films of different substances. [8,9] To this end, different multilayer films were fabricated by depositing 3, 5, 10 and 20 C12RhMOP monolayers onto PTMSP supports using the optimised conditions (LS deposition; surface pressure = 25 mN·m -1 ). The resulting multilayer C12RhMOP films were then characterized by AFM and X-ray photoelectron spectroscopy (XPS) (see Figure 2b, Figures S9, S11 and S12 and Table S1 in the Supporting Information). Although some small pinholes and C12RhMOP agglomerations are also present, an overall good coverage of the PTMSP support is observed, even after the deposition of only three C12RhMOP monolayers. The thickness of a single monolayer is estimated to be ca. 2.5 nm measuring the height in different film defects and borders (see Figure S7 in the Supporting Information). The size and number of 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.
To obtain a more detailed characterization of C12RhMOP multilayer films deposited onto PTMSP, a lamella of a sample obtained after depositing 30 C12RhMOP monolayers was extracted for analysis by scanning transmission electron microscopy (STEM). The sequence of images of the lamella thinning can be found in the Supporting Information ( Figure S10). Figure 2c shows a STEM-HAADF image of the lamella in which it is possible to distinguish the MOP multilayer with a different contrast to that of the PTMSP support at the bottom and the C/Pt coatings on top. According to the images obtained, the thickness of the MOP film is close to 60-65 nm. These values are in good agreement with the thickness derived from AFM measurements (each monolayer is ca. 2.5 nm thick). Moreover, the composition of the lamella was analysed by energy-dispersive X-ray spectroscopy (EDS). Figure 2d shows the Rh Lα (2.697 eV) and Si Kα (1.740 eV) peaks counts along the lamella profile, confirming the presence of Rh in the MOP multilayer film and Si in the PTMSP support. Moreover, the deduced thickness of the MOP layer derived from the EDS profiles is also consistent with the STEM-HAADF image, at ca. 60 nm.
The separation performance of C12RhMOP supported membranes, consisting of 20 to 30 MOP monolayers deposited onto PTMSP supports was tested in CO2/N2 separation for postcombustion carbon capture (temperature 35°C, feed pressures 1 to 3 bar, CO2/N2 mixture composition 10/90 in volume). CO2 permeance and CO2/N2 selectivity values for the different membranes analysed are shown in Figure 3. Pure PTMSP membranes present a high CO2 permeance (415 GPU at 1 bar) but low CO2/N2 selectivity (4.2) in agreement with previous works. [9,22] The deposition of 20 to 30 C12RhMOP monolayers increases the CO2/N2 selectivity at 1 bar up to values of 9.1 and 10.1 (Figure 3a), respectively, while maintaining the CO2 permeance above 195 GPU (Figure 3b). When the feed pressure increases from 1 to 3 bar, CO2 permeance values diminish by ca. 6 % for MOP supported films. However, this reduction is ca. 13 % for pure PTMSP supports. This difference is indicative of the formation of dense C12RhMOP multilayers, as shown in STEM images.  Comparing these data to previous studies, [9] where different polymers of intrinsic microporosity (PIMs) were deposited onto PTMSP substrates, it can be highlighted that the CO2 permeance values obtained with 30 C12RhMOP monolayers deposited onto PTMSP (195 GPU at 1 bar) are significantly higher than those obtained when stacking the same number of PIM monolayers (134 GPU for PIM-TMN-Trip films and 118 GPU for PIM-EA-TB(H2)). However, CO2/N2 selectivity values are slightly lower for MOP supported films (10.1) than for PIM-TMN-Trip (12.1) [9b] and PIM-EA-TB(H2) (13.8) [9a] supported films. This tendency is somehow expected as the higher is the permeability for a membrane, the lower is generally the selectivity.
In conclusion, we have demonstrated that the LB method can be employed to induce the self-assembly of C12RhMOP into dense and ultrathin porous films, and that these films can be used as selective materials in multilayer membranes for efficient CO2 separation. These results pave the way for the implementation of MOPs in separation technologies as the LB technique enables to fabricate large area membranes using minute amounts of active material. We believe that the versatility and ease functionalization of MOPs will provide a wide catalogue of MOP-based membranes with potential in gas separation and other applications.

Experimental Section Thin film formation at the air-water interface
Film formation at the air-water interface (Langmuir films) was first optimized using a commercial Langmuir Teflon trough (NIMA, Model 702) with a symmetrical double-barrier configuration and dimensions of 720 × 100 mm. This device was used to register surface pressure vs. area (π-A) isotherms and Brewster angle microscopy (BAM) images. These images were obtained using a KSV NIMA Micro BAM, equipped with a red laser light source (50 mW, 659 nm) with a fixed incidence angle of 53.1º. Another apparatus (KSV-NIMA, model KN 2003) with dimensions of 580 mm × 145 mm, also equipped with a symmetrical double-barrier system, was used for the fabrication of LB and LS films onto different substrates: quartz, mica, QCM discs and PTMSP dense membranes. Both troughs were kept inside closed cabinets in a dedicated laboratory with special care to limit the presence of dust and at constant temperature (20 ± 1 ºC). Films were compressed at constant speed (8 cm 2 ·min -1 ). Ultra-pure Milli-Q water (ρ = 18.2 MΩ·cm) was used as subphase in all the experiments. Surface pressure was continuously registered in both devices using Wilhelmy balances with a filter paper plate.

Thin film deposition onto substrates
LB films were transferred onto solid substrates by vertical dipping at a constant speed of 1 mm·min -1 . For LS film fabrication, substrates were held horizontally and parallel to the water surface using a vacuum pump based horizontal dipping clamp. When the desired surface pressure was reached, the substrate was approached to the surface at a vertical speed of 1 mm·min -1 . Once the substrate touched the water surface, it was 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.