Imparting Superhydrophobicity to Porphyrinic Coordination Frameworks Using Organotin

Abstract

Open AccessCCS ChemistryCOMMUNICATION14 Jul 2022Imparting Superhydrophobicity to Porphyrinic Coordination Frameworks Using Organotin Liang He, Yu-Jun Guo, Yi-Hong Xiao, Er-Xia Chen, Ming-Bu Luo, Zeng-Hui Li and Qipu Lin Liang He State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Yu-Jun Guo State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Yi-Hong Xiao State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Er-Xia Chen State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 Google Scholar More articles by this author , Ming-Bu Luo State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Zeng-Hui Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author and Qipu Lin *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101378 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail It is important to find a general strategy to construct water-stable metal–organic frameworks (MOFs). In this work, we report for the first time the synthesis of a series of water-stable MOFs based on porphyrin and organotin moieties. The organic part of organotin protects the coordination bond from attack effectively using water molecules. The structures and properties of three new materials with different types of topological networks (SnTCPP-SQL, SnTCPP-PTS, and SnTCPP-SHE) are described. SnTCPP-PTS exhibited exceptional superhydrophobicity with a water contact angle of 170° and high chemical stability in an aqueous solution at pH ranging from 1 to 13. SnTCPP-SHE had a superhydrophobicity with a water contact angle of 165°, ranked as one with the highest Brunauer–Emmett–Teller (BET) surface area of 3940 m2 g−1 among all reported hydrophobic frameworks. These results reveal a facile approach to impart hydrophobicity to MOFs. We have also described the preparation of a unique hydrophobic and functionality fabric coated by nanoscale crystallites of SnTCPP-PTS. These coated fabrics have highly efficient oil–water separation capability. This work describes the first effort of applying organotin-driven engineering for superhydrophobic MOFs, advancing a novel concept for establishing a strategy for MOF design with controlled wettability for practical applications. Download figure Download PowerPoint Introduction Metal–organic frameworks (MOFs) are a class of crystalline inorganic–organic hybrid porous materials, with the tunability of their metal nodes or clusters, their secondary building units (SBUs), and their organic linkers, allowing the construction of MOFs with versatile pore environments and surface characteristics.1,2 Notwithstanding the great promise of MOFs, the susceptibility of most SBUs to hydrolysis limits their applications in humid conditions.3 Improving their metal-ligand bonding strength has been a commonly used method for enhancing the stability of MOFs.4,5 Moreover, the geometry and connectivity of organic spacers also greatly influence the stability of MOFs.4,5 Recently, porphyrins or metalloporphyrins with a high number of linkages have been chosen to serve as ligands. These ligands not only improve the controllability of robust MOFs but also contribute to a variety of functions to MOFs.6 Despite this advancement, however, most MOFs display poor stability in water. It is crucial to find a general strategy with which to construct water-stable MOFs. Unlike strengthening the bonding of MOFs, internal surface hydrophobicity tends to prevent water capturing into pores and/or condensation of water around SBUs; thus, enduring hydrolytic attack.7 The integrated attributions of hydrophobic MOFs with respect to porosity and stability have led to good prospects in their applications, including humid CO2 trapping,8 oil–water separations,9–11 substrate-selective catalysis,12 anti-corrosion coatings,13 and self-cleaning.14 To construct hydrophobic MOFs in a one-pot reaction, researchers usually utilize organic linkers with polyfluoro or aliphatic chains.15,16 The complexity of these ligands increased the preparation time and cost significantly. Superhydrophobic MOFs have also been realized via postsynthetic modification (PSM) by fluoroalkyl chains or other hydrophobic groups on the metal-oxo nodes.17–19 Compared with the PSM of ligands,20 the route via metal-oxo node engineering of MOFs have readily attainable advantages. However, the PSM method often exhibits limitations due to challenges involved in achieving precise control of the position and content of the introduced components. Tin chemistry has experienced enormous advances since the discovery of organotin compounds.21 One of the reasons for the popularity of tin chemistry is the many applications of tin compounds, including their use as biocidal agents,22 extreme UV lithography,23–25 and as catalysts in some industrial processes, for example, the stabilization of polyvinyl chloride or the preparation of polyurethane.26–28 Tin has also been used to fabricate MOFs, but only a few examples have been reported to date, most of which are pore-free structures.29–37 As opposed to the introduction of hydrophobic linkers or via PSM by fluoroalkyl chains or other hydrophobic groups on the metal-oxo nodes, theoretically, organotin species could be introduced directly into the SBUs of MOFs to improve the environment of their inner cavities. Herein, we attempted to fabricate hydrophobic porphyrinic MOFs using organotin precursors. Based on dimethyltin Me2SnCl2 and dibutyltin n-Bu2SnCl2, we developed a two-dimensional (2D) net and a three-dimensional (3D) net, respectively, viz, SnTCPP-SQL and SnTCPP-PTS, both of which were built from tetra-organotin clusters and ligands of tetrakis(4-carboxylphenyl)porphyrin (TCPP). Among them, SnTCPP-PTS exhibited high chemical stability in aqueous solution, superhydrophobicity with a water contact angle of 170°, and excellent oil–water separation performance. Replacement of n-Bu2SnCl2 by monobutyltin n-BuSnO(OH) in the synthesis of SnTCPP-PTS gave another 3D net, viz, SnTCPP-SHE, comprising hexa-butyltin SBUs, having superhydrophobicity with a water contact angle of 165° and one of the highest Brunauer–Emmett–Teller (BET) surface areas of 3940 m2 g−1 among all known hydrophobic MOFs. Results and Discussion All the organotin MOF derivatives could be formed in capped vials (see Supporting Information for details). The method used for SnTCPP-PTS could be adapted readily to a large-scale (>50 fold) production with a nanomorphology (∼100 nm). Their phase purities and formulas were confirmed by X-ray diffraction (XRD), elemental and thermogravimetric analyses (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), Fourier transform infrared (FT-IR), and ultraviolet-visible (UV–vis, diffuse reflection spectroscopy, and fluorescence spectroscopy ( Supporting Information Figures S2–S6, S13–S31, S39, and S40). The structures of SnTCPP-SQL and SnTCPP-PTS were determined by single-crystal XRD. SnTCPP-SQL comprised uniform tetra-dimethyltin clusters of {(Me2Sn)4O4(O2CR)4} – [Sn4]. Each TCPP ligand of SnTCPP-SQL was coordinated to four [Sn4] nodes, and each [Sn4] node was coordinated to four TCPP linkers, producing a coordination network topological motif, 2D (4,4)-connected “sql net,” as shown in Figures 1a–1c. The SBU in SnTCPP-PTS was similar to that of SnTCPP-SQL but with more distortion and n-butyl substituent instead of methyl species. Distinct from the bridging coordination mode of all carboxyl groups in SnTCPP-SQL, two of four carboxyl groups in SnTCPP-PTS displayed a monodentate coordination geometry. Topologically, the framework could be classified as a 3D (4,4)-connected pts net with the point symbol {4^2.8^4} (Figures 1d–1f). SnTCPP-PTS contained one type of rhombic channel with a dimension of ca. 28 Å × 16 Å along the c axis. The n-butyl groups pointed directly into the channels. Figure 1 | (a–c) Assembly of SnTCPP-SQL with a (4,6)-connected sql net. (d–f) Assembly of SnTCPP-PTS with a (4,4)-connected pts net. (g–i) Assembly of SnTCPP-SHE with a (4,6)-connected “she net.” Cyan, red, blue, dark, and green spheres represent Sn, O, N, C, and H atoms, respectively. Download figure Download PowerPoint Our successful syntheses of organotin-MOFs based on TCPP prompted us to explore the construction of TCPP-MOFs based on other organotin clusters, like one with a higher symmetric hexamer {(n-BuSn)6O6 (O2CR)6} – [Sn6].38 The [Sn6] could be simplified into a hexagonal unit with planar 6-coordination. The configuration of accessibly 12-connected [Zr6] units with isomorphic modes differed from the six Sn4+ ions above and below the hexagonal plane occupied fully by the n-butyl groups, which had only 6-connected modes.39,40 Theoretically, the [Sn6] clusters were connected to six TCPP ligands forming a (4,6)-connected “she net” with the point symbol {4^4.6^2}3 {4^6.6^6.8^3}2 and the space group Im-3m (Figures 1g–1i and Supporting Information). Using n-BuSnO(OH) instead of n-Bu2SnCl2 in the preparation of SnTCPP-PTS, the target net SnTCPP-SHE with microcrystallinity and high yield was finally obtained. The phase purity was confirmed by SEM ( Supporting Information Figure S6). The bulk powder sample mainly consisted of ball-shaped crystals with an average size of ∼1 μm. The experimental powder XRD (PXRD) was refined using the Pawley algorithm, resulting in good agreement factors (Rp = 4.98%, Rwp = 7.09%, Figure 2c). The structural model for SnTCPP-SHE was also validated with the Rietveld algorithm ( Supporting Information Figure S8). Figure 2 | (a) PXRD patterns of SnTCPP-PTS after different treatments. (b) CO2 adsorption–desorption isotherms (195 K) of SnTCPP-PTS. (c) Pawley refinement based on PXRD of SnTCPP-SHE. (d) N2 adsorption–desorption isotherms (77 K) of SnTCPP-SHE. Download figure Download PowerPoint The permanent porosity of SnTCPP-SQL was confirmed by N2 adsorption–desorption isotherm at 77 K, showing an estimated apparent BET surface area of 613 m2 g−1 ( Supporting Information Figure S32). Due to the n-butyl chain occupancy in the pores, the BET value of SnTCPP-PTS was only 211 m2 g−1 ( Supporting Information Figure S33). Unexpectedly, SnTCPP-SHE revealed quite a high BET surface area, reaching 3940 m2 g−1 (Figure 2d), the highest among all MOFs based on tin SBUs or TCPP ligands, and among all reported hydrophobic materials ( Supporting Information Table S4).7 The CO2-uptake performance of all three structures at 273 and 298 K was also characterized ( Supporting Information Figures S34–S36). The two-step CO2-trap isotherm at 195 K (Figure 2b) and the main PXRD peaks shifted to lower 2θ angles with increasing temperature and shifted back with decreasing temperature ( Supporting Information Figure S24), indicating structural flexibility of SnTCPP-PTS, similar to MIL-53.41,42 The stability results obtained experimentally showed that all three structures have good chemical stability when immersed in various common solvents, including water for 24 h ( Supporting Information Figures S26–S28). It is significant to synthesize stable MOFs which are robust in both acid and alkaline environments.4,5 To further investigate the chemical stability of SnTCPP-PTS, we dispersed crystals of SnTCPP-PTS in different aqueous solutions over the pH range of 1–13 and observed that its PXRD patterns were almost sustained (Figure 2a and Supporting Information Figure S30). SnTCPP-PTS also maintained a very good crystallinity under hydrothermal conditions after immersing in water for 1 week and even after exposure to a humid environment for 5 months, indicating excellent stability performance compared with other hydrophobic MOFs.7 Compared with SnTCPP-PTS, the SnTCPP-SQL, protected by methyltin, presented reduced chemical stability (pH 4–11) ( Supporting Information Figure S29); also, SnTCPP-SHE exhibited lower chemical stability (pH 2–11) that might have been caused by its larger pores (ca. 19 Å × 19 Å) ( Supporting Information Figure S31). TGA and thermal PXRD ( Supporting Information Figures S23 and S25) showed that SnTCPP-SHE was stable up to at least 300 °C. To investigate the polarity of the channel surface, the H2O adsorption–desorption isotherms of SnTCPP-PTS were measured at 298 K (Figure 3e). We also carried out benzene, toluene, and n-hexane uptake experiments at 298 K to verify the hydrophobic nature of SnTCPP-PTS. We found that the hydrophobicity of SnTCPP-SHE was also characterized by the low uptake of water and much higher toluene adsorption capacity ( Supporting Information Figure S38) both at 298 K. Compared with the results of SnTCPP-PTS and SnTCPP-SHE, SnTCPP-SQL showed a less hydrophobic pore environment ( Supporting Information Figure S37), consistent with their structural discrepancy. Further, we assessed the external surface hydrophobicity of MOFs by water contact angle measurements (Figures 3a–3d). Water contact angles of 82°, 170°, and 165° were observed for SnTCPP-SQL, SnTCPP-PTS, and SnTCPP-SHE, respectively. These results revealed that as the length of the ester group increases, the materials become more hydrophobic. Superhydrophobicity, defined as displaying a contact angle exceeding 150° for a water droplet, was noted in SnTCPP-PTS and SnTCPP-SHE. These compounds presented one of the highest water contact angles among all the MOFs without PSM ( Supporting Information Table S4).7 The exposed lattice planes of SnTCPP-PTS were identified as {110}, {20-1}, and {1-20}, respectively, which were the crystallographic faces that water contacts directly (Figure 3f and Supporting Information Figure S7). The water contact angle of 84° was measured for PCN-224(Zr), which has a similar network as SnTCPP-SHE,39 further indicating that the organotin played a vital role in the superhydrophobicity of the organotin MOFs ( Supporting Information Figure S43). A water droplet adhered with the microcrystals of SnTCPP-PTS, showing high mechanical robustness, which could be picked up with tweezers ( Supporting Information Figure S44). Figure 3 | Photographs showing water droplets resting on a film of (a) SnTCPP-SQL, (b) SnTCPP-PTS, and (c) SnTCPP-SHE, and (d) their contact angle profiles of 82°, 170°, and 165° respectively. (e) Vapors of water, benzene, toluene, and n-hexane adsorption–desorption isotherms of SnTCPP-PTS. (f) Unit cell and crystal shape of SnTCPP-PTS, and its exposed lattice planes identified as {110}, {20-1}, and {1-20}, respectively. Download figure Download PowerPoint As one of the most important energy resources, the extensive use of fossil fuels has led to environmental pollution. Marine pollution with crude oil and its products have globally become a major environmental concern.10 Developing super wetting materials for rapid and efficient separation of oil and water, industrially important due to the necessity of removing oil from industrial wastewater, contaminated seawater, and other sources of pollution. SnTCPP-PTS possess high porosity and hydrophobicity, and more importantly, excellent stability. This unique attribution makes SnTCPP-PTS very useful for oil–water separation. The nanoscale structure of SnTCPP-PTS makes it very easy to disperse in solution, and thus, easy to coat on basal materials. The coating of SnTCPP-PTS on a hydrophilic–lipophilic fabric was carried out using an ethanol dispersion (Figure 4a and Supporting Information Figures S45 and S46). Then the nanomorphology of SnTCPP-PTS was investigated using SEM (Figures 4b and 4c and Supporting Information Figure S47). The fabric finally changed from white to orange-red after coating, which realized the dyeing of the fabric and showed red fluorescence ( Supporting Information Figure S45). More importantly, it changed from hydrophilic to superhydrophobic, and the contact angle changed from 0° to 172.5° (Figure 4d and Supporting Information Figures S48 and S49). This finding was also verified by the weight absorption experiment of organic solvents for the fabrics with and without coating, which indicated that SnTCPP-PTS could impart superhydrophobicity to the fabric while still lipophilic ( Supporting Information Figures S52 and S53), providing optimal conditions for oil–water separation. A fabric coated with nano-SnTCPP-PTS was installed on the separation device, rendering a separator with a mesh window showing selective wettability. To increase the bearing capacity, a copper mesh was used as a support ( Supporting Information Figure S50). When the separator was placed into mixed liquor of dichloromethane (CH2Cl2) and water, the hydrophobic fabric allowed only the organic phase to flow immediately through the window and entered into the bottle (with separation efficiency >99.0%, Figures 4e and 4f, and Supporting Information Video S1). Moreover, the SnTCPP-PTS-coated fabric showed excellent recycling performance even after five cycles, demonstrating the as-prepared superhydrophobic-coated fabric’s good stability ( Supporting Information Figure S51). To further illustrate its excellent oil–water separation efficiency, we focused on experiments involving water separation from different gasoline sources, including various alkanes, cycloalkanes, and aromatic hydrocarbons. Altogether, our findings showed that their separation efficiencies were all >99.0% after three cycles (Figures 4g and 4h and Supporting Information Figure S54). The effect of efficient oil–water separation shows the potential of organotin MOFs in practical applications. Moreover, due to its hydrophobic properties, the fabric exhibited a self-cleaning function, providing a good reference for the preparation of functional fabrics ( Supporting Information Figures S55–S57 and Video S2). Figure 4 | (a) Schematic illustration of the preparation of a hydrophobic fabric coated with nanocrystals of SnTCPP-PTS. (b and c) SEM and TEM images of nanocrystals of SnTCPP-PTS. (d) Fabric before and after SnTCPP-PTS coating. (e and f) Photographs showing separation of dichloromethane (CH2Cl2) and water. (g and h) Photographs showing separation of gasoline and water (for easy observation, water is dyed blue). Download figure Download PowerPoint Conclusion We have prepared a series of new organotin-porphyrinic MOFs by a one-pot reaction under hydrothermal conditions. To the best of our knowledge, this is the first reported example of porphyrinic MOFs based on tin metal. These MOFs showed excellent stability and porosity and illustrated a general, facile, and effective approach to impart hydrophobicity to MOFs. We demonstrated that the fabric coated by SnTCPP-PTS exhibited superior performance in oil–water separation. To date, no other MOFs based on organotin have been studied for their hydrophobicity. Organotin MOFs with improved water stability could be emphasized in future research. The development of tin-MOFs could widen the scope of new technological applications. Therefore, the search for new tin-based MOFs and their potential uses could be considered an open field of investigation. Supporting Information Supporting Information is available and includes detailed experimental procedures and characterization data. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the Strategic Priority Research Program of Chinese Academy of Sciences (grant no. XDB20000000), the National Natural Science Foundation of China (grant no. 21501028), the Hundred-Talent Program of Fujian Province, and the Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (no. 2021ZR138).