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Cite this: DOI: 10.1039/c5cc01928a Received 6th March 2015, Accepted 13th March 2015

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A polyhedral oligomeric silsesquioxane functionalized copper trimesate† E. S. Sanil,ab Kyung-Ho Cho,a Do-Young Hong,ab Ji Sun Lee,a Su-Kyung Lee,a Sam Gon Ryu,c Hae Wan Lee,c Jong-San Changad and Young Kyu Hwang*ab

DOI: 10.1039/c5cc01928a www.rsc.org/chemcomm

A metal–organic framework (MOF), copper trimesate (Cu3(BTC)2), was selectively functionalized with aminopropylisooctyl polyhedral oligomeric silsesquioxane (O-POSS) to make the external surface of Cu3(BTC)2 hydrophobic and thereby enhance the stability of the material against humidity. POSS modification was also successfully applied to other MOFs such as MOF-74 and MIL-100.

Metal–organic frameworks (MOFs) are unique porous hybrid materials that not only possess a large surface area and high pore volume, which makes them especially attractive for applications in gas storage, gas separation, drug delivery, and catalysis, but can also be tailored for targeted chemical interaction.1 Thus, researchers have devoted enormous effort toward the development of MOFs. Meanwhile, several MOFs have been reported to lose their crystallinity and pore ordering upon exposure to chemicals.2 In particular, water can easily penetrate the pores and destroy the MOF structures either by hydrolysis or by the replacement of the ligands coordinated to the metal centers.3 As a result of this low resistance against humidity, MOFs find limited applications despite their potential for use in various fields. Copper benzenetricarboxylate (Cu3(BTC)2; BTC = benzene-1,3,5tricarboxylate), which is also known as HKUST-1, has been widely studied because it has a large surface area as well as high pore volume, and exhibits relatively high chemical stability and good liability of coordinated solvents or water molecules in the pores of the framework.4 Coordinatively unsaturated metal sites (CUS) are generated by the removal of coordinated water molecules from the Cu3(BTC)2 structure.4 The presence of CUS in MOFs leads to diverse applications in catalysis after the grafting of organic molecules and metal

nanoparticles.5 Nonetheless, the presence of CUS may result in characteristics that are undesirable for several applications: the Cu3(BTC)2 structure undergoes slow degradation in the presence of moisture;6 thus, Cu3(BTC)2 has limited industrial applicability. Thus far, only few studies have addressed the stability issues of MOFs with respect to moisture.2–3,6–7 In a previous study, the stability of MOFs against moisture was improved either by the pre- or post-modification of ligands containing hydrophobic functional groups.7 Although this approach improves their stability against moisture, the pore volume and surface area of the MOFs decreases considerably; these properties are of utmost importance in the catalytic and sorption applications of MOF materials. For example, Decoste et al.8 have reported excellent results using a perfluorohexane plasma-enhanced chemical-vapor-deposition method for enhancing the stability of Cu3(BTC)2 against moisture; however, using their method, the surface area and total pore volume decreased by 25.9% and 31.7%, respectively. Recently, Son et al.9 have reported a method to improve the hydrophobicity of UiO-66NH2 by coating with polymers via the Sonogashira coupling reaction. However, the coating process is very complicated, thereby limiting large-scale production. Herein, we propose a new method, as shown in Scheme 1, to enhance the stability of Cu3(BTC)2 against moisture by the selective functionalization of the outer Cu sites on the surface of Cu3(BTC)2 crystals with aminopropylisooctyl polyhedral oligomeric silsesquioxane (hereafter referred to as

a

Catalysis Center for Molecular Engineering (CCME), Korea Research Institute of Chemical Technology (KRICT), Gajeong-Ro 141, Yuseong, Daejeon 305-600, Korea. E-mail: [email protected] b Department of Green Chemistry, University of Science and Technology (UST), 217 Gajeong-Ro, Yuseong, Daejeon 305-350, Korea c Agency for Defense Development, Yuseong P.O. Box 35-52, Daejeon 305-600, Korea d Department of Chemistry, Sungkyunkwan University, Suwon 440-476, Korea † Electronic supplementary information (ESI) available: Table S1 and Fig. S1–S10. See DOI: 10.1039/c5cc01928a

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Scheme 1

Schematic of the surface modification of Cu3(BTC)2 with O-POSS.

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O-POSS). O-POSS is a hybrid molecule composed of well-defined cubic octameric silica cages surrounded by eight organic functional groups at the corners (an isooctyl group in the case of O-POSS). The hydrophobic nature of the O-POSS material is attributed to the long aliphatic chains.10 Typically, Cu3(BTC)2 crystals are synthesized using a coordination modulation method according to details reported elsewhere (see ESI†).11 First, the synthesized crystals were activated at 150 1C for 12 h under vacuum to generate the CUS of copper; next, they were functionalized with O-POSS by refluxing in hexane under nitrogen for 48 h (see ESI†). The resulting POSS-modified Cu3(BTC)2 is hereafter referred to as O-POSS@Cu3(BTC)2. The powder X-ray diffraction (XRD) patterns of Cu3(BTC)2 and O-POSS@Cu3(BTC)2 are shown in Fig. S1, ESI;† both materials exhibited well-resolved prominent diffraction peaks, which are characteristic of cubic Cu3(BTC)2. This observation indicates the complete crystalline nature of the samples. The intensity of the diffraction peaks remained unchanged even after modification with O-POSS, clearly indicating that O-POSS@Cu3(BTC)2 and Cu3(BTC)2 have the same order of crystallinity. Contact angle measurements were conducted to investigate the hydrophobic nature of O-POSS@ Cu3(BTC)2. As shown in Fig. 1a, O-POSS@Cu3(BTC)2 and Cu3(BTC)2 exhibited water–air contact angles of 831 and 461, respectively. The hydrophobic nature of O-POSS@Cu3(BTC)2 was further evidenced by dispersing the material in water. Even after storage for 2 weeks in water, O-POSS@Cu3(BTC)2 remained as a layer on the top surface without any dispersion, whereas Cu3(BTC)2 dispersed well (Fig. 1b). As can be observed from the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Fig. S2, ESI†), both Cu3(BTC)2 and O-POSS@Cu3(BTC)2 formed cubic crystals with an average particle size of 750 nm. From the above results, it is evident that modification with O-POSS does not impart any change to the crystal morphology of Cu3(BTC)2. Inductively coupled plasma spectroscopy results (Table S1, ESI†) indicated the presence of 1.4 wt% Si in O-POSS@Cu3(BTC)2. The curves obtained from thermogravimetric analysis (Fig. S3, ESI†) indicated that both samples exhibit the same thermal degradation pattern. However, O-POSS@Cu3(BTC)2 exhibited a higher weight loss (4.8%), corresponding to a Si loading of 1.3 atomic wt%. To gain insight into the effect of modification with POSS on the texture, the samples were characterized by N2 and Ar adsorption isotherms. Fig. S4 (ESI†) shows the N2 adsorption isotherms of Cu3(BTC)2 and O-POSS@Cu3(BTC)2. A typical Type IV pattern was observed, which is characteristic of mesoporous materials; however, this observation is in contrast to those reported elsewhere for Cu3(BTC)2 materials. As Cu3(BTC)2 is known to be microporous,

Fig. 1 (a) Images of a water droplet on the surface of Cu3(BTC)2 and O-POSS@Cu3(BTC)2. (b) Images of Cu3(BTC)2 and O-POSS@Cu3(BTC)2 in water.

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it typically exhibits a Type I sorption isotherm. From the pore size distribution curves of the samples (plotted from the adsorption branch of Barrett–Joyner–Halenda, as well as by the Horvath– Kawazoe method in Fig. S5 and S6, ESI†), most of the pores were microporous (6–7.5 Å) and very few were mesoporous (20–25 Å). This clearly indicates that both samples have a hierarchical dualpore structure with a majority of micropores. Meanwhile, mesopores originate from the interstitial grain voids within the cubic nanocrystals of Cu3(BTC)2, which was observed using TEM measurements (Fig. S2, ESI†). Table S1 (ESI†) lists the Brunauer–Emmett–Teller (BET) surface areas and pore volumes of Cu3(BTC)2 before and after modification with O-POSS. Cu3(BTC)2 had a BET surface area of 1661 m2 g 1 and a total pore volume of 0.57 cm3 g 1. After treatment, O-POSS@Cu3(BTC)2 had a surface area of 1514 m2 g 1 with a total pore volume of 0.55 cm3 g 1. There was no considerable decrease in the surface area, and the total pore volume was almost unchanged after the modification of Cu3(BTC)2 with O-POSS. These observations are attributed to the selective functionalization of the CUS of Cu on the outer surface of the Cu3(BTC)2 crystals with O-POSS. The above results solidify the hypothesis that the larger molecular size of O-POSS (ca. 1.3 nm) relative to the window size of Cu3(BTC)2 (ca. 0.9 nm) prevents the penetration through the micropores of Cu3(BTC)2. To confirm their stability against moisture, the samples were exposed to 90% relative humidity (RH) at RT for a week in a saturated K2SO4 solution. Fig. 2 and 3 show the XRD patterns and N2 sorption isotherms of samples before and after exposure to humid conditions,

Fig. 2 Comparison of the XRD patterns of Cu3(BTC)2 and O-POSS@Cu3(BTC)2 before (a and c) and after (b and d) exposure to 90% RH at room temperature for 1 week in a saturated K2SO4 solution, respectively.

Fig. 3 N2 sorption isotherms of (a) Cu3(BTC)2 and (b) O-POSS@Cu3(BTC)2 after exposure to 90% RH at room temperature for 1 week in a saturated K2SO4 solution.

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respectively. Even after exposure to 90% RH, the XRD patterns of O-POSS@Cu3(BTC)2 did not change significantly, while the structure of Cu3(BTC)2 was destroyed. In addition, the BET surface area of Cu3(BTC)2 drastically decreased from 1661 m2 g 1 to 81 m2 g 1 after exposure to moisture; however, only a negligible change in the BET surface area was observed in O-POSS@Cu3(BTC)2 (from 1514 to 1476 m2 g 1, Fig. 3). The enhanced humidity stability of O-POSS@Cu3(BTC)2 was also demonstrated by the SEM images of Cu3(BTC)2 and O-POSS@Cu3(BTC)2 recorded before and after exposure to 90% RH (Fig. S7, ESI†). No change was observed in the morphology of O-POSS@Cu3(BTC)2; however, the morphology of Cu3(BTC)2 changed from cubic particles to agglomerated particles having an undefined shape. From the above results, it is clear that surface modification of Cu3(BTC)2 with O-POSS dramatically enhances the stability of Cu3(BTC)2 against humidity under static conditions. To further confirm the stability against humidity, materials were subjected to continuous water sorption cycle experiments. Water adsorption–desorption cycle tests indicated that the hydrothermal stability of O-POSS@Cu3(BTC)2 was enhanced as compared with that of Cu3(BTC)2 under dynamic conditions (Fig. S8, ESI†). A marginal decrease in the sorption capacity was observed between the initial and last cycles (17 wt%) for O-POSS@Cu3(BTC)2, while a sharp decrease in the water sorption capacity was observed for Cu3(BTC)2 (30 wt%). To evaluate the generality of the suggested method in this study, O-POSS was further grafted onto the CUS on the surfaces of MOF-74, which has a low-symmetry rhombohedral structure (Ni and Co), and MIL-100(Fe), which has a high-symmetry cubic structure.12 As can be seen in the digital photo images in Fig. S9 (ESI†) and Fig. 4, O-POSSmodified MOFs were hydrophobic, implying that O-POSS has successfully modified the outer metal sites on the MOF-74(Ni and Co) and MIL-100(Fe) surfaces. Also, from the XRD patterns of O-POSSmodified MOF and pristine MOF, it is apparent that the crystallinity of the MOF crystals was unchanged after modification. As shown in Fig. 4(d), a negligible change in the mesoporous volume of MIL-100 was observed after modification with O-POSS, because the possibility

Fig. 4 Photo images in water (a), XRD patterns (b), N2 isotherms (c) of MIL-100 and O-POSS@MIL-100 at 77 K and pore size distribution profiles (d) calculated using the NLDFT method.

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of grafting of MIL-100(Fe) by O-POSS (1.3 nm) is believed to be eliminated owing to the small windows (5.5 and 8.6 Å) of MIL-100. In conclusion, we have demonstrated a simple, new method for enhancing the stability of Cu3(BTC)2 against moisture by the functionalization of the coordinatively unsaturated Cu sites on the outer surface with aminopropylisooctyl polyhedral oligomeric silsesquioxane (O-POSS); only marginal changes were observed in the surface area and pore volume of Cu3(BTC)2. The newly formed O-POSS@ Cu3(BTC)2 can be used for applications in the adsorption and separation of gases in the presence of relatively large amounts of moisture. The results indicate that this approach has potential application for the selective surface modification of the CUS on the MOFs by POSS. This study was supported by the Agency for Defense Development of Korea (UE135152ID) and partly by the Institutional Research Program (SKM1401-3). The authors thank Mr J.-Y. Bae and Dr U.-H. Lee for their beneficial contributions.

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Chem. Commun.

A polyhedral oligomeric silsesquioxane functionalized copper trimesate.

A metal-organic framework (MOF), copper trimesate (Cu3(BTC)2), was selectively functionalized with aminopropylisooctyl polyhedral oligomeric silsesqui...
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