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Morphological Map of ZIF-8 Crystals with Five Distinctive Shapes: Feature of Filler in Mixed-Matrix Membranes on C3H6/C3H8 Separation Fan Yang, Hao Mu, Chongqing Wang, Long Xiang, Ke Xin Yao, Lingmei Liu, Yang Yang, Yu Han, Yanshuo Li, and Yichang Pan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01073 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Chemistry of Materials

Morphological Map of ZIF-8 Crystals with Five Distinctive Shapes: Feature of Filler in Mixed-Matrix Membranes on C3H6/C3H8 Separation Fan Yang,‡ Hao Mu,‡ Chongqing Wang, Long Xiang, Ke Xin Yao, Lingmei Liu, Yang Yang,* Yu Han, Yanshuo Li,* and Yichang Pan* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering Nanjing Tech University, Nanjing 210009, P.R. China The School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, P.R. China Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 239556900, Saudi Arabia Abstract: Morphology-tailoring of metal-organic frameworks (MOFs) is essential for their specific application in molecularseparation, catalysis, and drug-delivery. However, precision syntheses of uniform MOFs crystals with anisotropic shape are still challenging. Herein, a morphological map for monodisperse ZIF-8 crystals with five distinctive shapes was constructed in terms of the synthetic conditions. It was revealed that the formation mechanism for ZIF-8 crystals with various shapes is strongly dependent on the concentration of cetyltrimethylammonium bromide (CTAB) added, not limited to the traditional capping mechanism for MOFs and metal nanocrystals. The generality of this strategy was also verified through the successful morphology-controllable synthesis of ZIF-67 crystals. The effect of ZIF-8 morphology on separation performance of resulting cross-linked poly(ethylene oxide) (XLPEO)-based mixed-matrix membranes for C3H6/C3H8 mixtures was also evaluated.

Metal-organic frameworks (MOFs), crystalline coordination polymers consisting of metallic nodes linked by organic ligands, are an attractive class of crystalline porous materials that have extensive application in molecular separation, catalysis, sensing, drug-delivery, etc.1-8 Inspired by the principles of “structure dictates function” in inorganic nanomaterials,9, 10 the tailoring of morphology, especially anisotropic shape, for MOFs is also essential to feature their specific applications.11-17 For example, Pang et al. found that the electro-catalytic glucose activity of MOFs Cu3(btc)2 nanocrystals was strongly dependent on the number of {100} facets.18 The nano-cubes with six {100} facets exhibited higher activity than other shapes (truncated cube, cuboctahedron, and octahedron). In addition to the fine adjustment of the thermodynamically and/or kinetically controlled conditions or post-synthetic chemical etching,19-21 anisotropic MOF nanocrystals are also achieved with the help of structure-directing agents, surfactants and modulators.22-26 To date, the fabrication of anisotropic MOF nanocrystals with high uniformity and purity is highly desired, but still remains challenging. As one of the most important MOFs, ZIF-8, constructed from zinc ions and 2-methylimidazole (Hmim), has been widely studied due to its facile synthesis, exceptional hydrothermal and chemical stability.27, 28 Furthermore, ZIF-8 is also considered as a promising membrane material for propylene/propane separation, which is one of the most energy-consuming processes in petrochemical industry by traditional cryogenic distillation.29-32 In view of its high-

symmetry bcc crystal structure, even under the help of modulators, ZIF-8 crystals with rhombic dodecahedral or cubic shapes are always prepared.23 This phenomenon is similar to the synthesis of metal nanocrystals (e.g. Au nanocrystals), where high-symmetry crystal structure tends to form high-symmetry and compact morphology.33, 34 Herein, we first report the successful fabrication of highly uniform ZIF-8 nanocrystals with anisotropic morphologies (nano-plate and nano-rod) using surfactant cetyltrimethylammonium bromide (CTAB). The morphology of the as-synthesized ZIF-8 crystals is found to strongly depend on the concentration of CTAB. The formation of anisotropic ZIF-8 crystals is attributed to the competitive interaction between CTAB and ligands with zinc ions. The generality of this morphology-control strategy is also verified through the successful fabrication of anisotropic ZIF67 nanocrystals, which are constructed from cobalt ions and Hmim. In addition, mixed-matrix membranes derived from anisotropic ZIF-8 with nano-rods morphology exhibits highest C3H6/C3H8 separation performance than the other morphologies. Employing zinc acetate as metal source, 2methylimidazole as organic ligands, and water as solvent, ZIF-8 rhombic-dodecahedron (RD) with an average size of ~2 μm was synthesized at 120 oC for 24 h (Figure 1a). The composition of the synthesis solution in molar ratio is 1 Zn2+: 30 Hmim: 1800 H2O. In the presence of a small amount of CTAB (0.08 wt.%), monodisperse ZIF-8 nanocubes (NC) with an average size of 200 nm were generated

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(Figure 1b). By increasing the amount of CTAB to 0.16 wt.%, octagonal plates (OP) with an average lateral dimension of 1.2 μm and an average thickness of 200 nm, were achieved (Figure 1c). As shown in Figure 1d and e, with further increasing the content of CTAB to 0.24 and 0.35 wt.%, respectively, uniform ZIF-8 crystals with the shapes of interpenetration twin (IT) and nano-rod (NR) were also successfully fabricated. The particle size of IT crystals ranges from 1 to 1.8 μm, and the average crystal size is 1.2 μm. For the NR crystals, the average length and width are 1.2 μm and 200 nm, with an average aspect ratio of 6. X-ray diffraction (XRD) characterization indicates that all synthesized samples are pure-phase ZIF-8 structure with high crystallinity (Figure 1f). It is worth noting that fabrication of ZIF-8 nanocrystals with morphologies of OP, IT and NR was rarely reported in literature.

Figure 1. (a-e) SEM images and (f) XRD patterns of ZIF-8 crystals with different morphologies: (a) rhombic-dodecahedron (RD), (b) nano-cubic (NC), (c) octagonal plates (OP), (d) interpenetration twin (IT) and (e) nano-rod (NR). The scale bar in all SEM pictures is 2 μm. The insets in (a-e) are TEM images of the corresponding samples.

The control of the crystal growth kinetics, via adjustment of the synthesis time, was also investigated by timedependent SEM analysis. Without or with small amount of CTAB (0.08 wt.%), both the shape and particle size of ZIF-8 crystals (RD and NC shapes) from the synthesis solution at early state are similar to those prepared for a prolonged durations (Figure S1 and S2). In contrast, with addition of high loadings of CTAB (> 0.16 wt.%), the crystal growth of as-synthesized ZIF-8 crystals (OP, IT and NR shapes) is strongly dependent on the synthesis kinetics (Figure S3S5). For the OP crystals, the increase in lateral dimension of crystals is sharper than the thickness of the crystals with the increase in synthesis time, resulting in the increase of

the aspect-ratio of OP crystals. The particle size of IT crystals with high symmetry also increases with the synthesis time. For the NR crystals with anisotropic shape, the length increases with the synthesis time, but the thickness almost maintains. As a result, the aspect-ratio of NR crystals increases as the reaction proceeds. This slow-down of the crystal growth with the addition of CTAB is possibly due to the competitive interaction between ligands (Hmim) and surfactants (CTAB) with metal source (zinc ions). In order to further verify the anisotropic growth of OP and NR crystals, high-resolution transmission electron microscopy (HRTEM) and the selected-area electron diffraction (SAED) measurements were conducted. As shown in Figure 2, the SAED patterns of both crystals well match the simulated SAED patterns along the [001] zone axes that obtained from SingleCrystal version 2.4 software (Figure 2b). This result indicates that successful synthesis of single crystals with high crystallinity. Furthermore, the spacing between adjacent lattice planes in HRTEM images corresponds to the distance of (002) crystal planes, indicating that [001] is the preferential growth direction of both anisotropic crystals.

Figure 2. (a) Crystal structure of ZIF-8 projected along the [001] zone axis, (b) simulated SAED pattern of ZIF-8 along the [001] zone axis using SingleCrystal version 2.4 software, (c and d) HRTEM images of the ZIF-8-NR crystals, (e and f) HRTEM images of the ZIF-8-OP crystals. The insets in c and e are the experimental SAED patterns of corresponding samples along the [001] zone axis.

The scope of the morphology-controllable synthesis for ZIF-8 crystals was further investigated by assessing the impact of the compositions in synthesis solutions. The morphological map as a function of the concentration of CTAB and molar ratio of H2O/Hmim in the synthesis solution is shown in Figure 3. All the reactions were conducted at 120 oC for 24 h, and the morphological purities of samples in each region were all higher than 95%. Without CTAB, uniform ZIF-8 crystals with RD shape were easily fabricated when the molar ratio of H2O/Hmim is lower than 100. With gradually increasing the amount of CTAB in synthesis solution, ZIF-8 crystals with morphologies of NC, NP, IT and NR occurred in sequence. When the concentration of CTAB and the molar ratio of H2O/Hmim are ranged in 0.04~0.125 wt.% and 40~200, respectively, monodispersed ZIF-8 crystals with NC shape


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Chemistry of Materials were successful synthesized. In addition, the simultaneous increase of CTAB concentration and decrease of H2O/Hmim ratio leads to the increase in particle size of assynthesized NC crystals (Figure S6). For the successful synthesis of high-pure OP crystals, it is also requisite that simultaneous increase in the concentration of CTAB (0.075~0.2 wt.%) and decreases in the molar ratio of H2O/Hmim (40~200). Both the lateral dimension and thickness of nano-plates increase with the CTAB concentration while decrease with the H2O/Hmim ratio thus maintaining a similar aspect-ratio of ~6 (Figure S7).

phiphilic long carbon chain of the CTAB molecules, rather than the halides.

Figure 4. SEM images of ZIF-67 crystals with various morphologies synthesized with aid of CTAB: (a) nano-cubic (NC), (b) octagonal plates (OP), (c) interpenetration twin (IT) and (d) nano-rods (NR). The scale bar in all SEM pictures correspond to 2 μm.

Figure 3. Morphological map of ZIF-8 crystals synthesized as a function of the concentration of CTAB and the molar ratio of H2O/Hmim. The synthesis conditions were fixed at 120 oC for 24 h, and zinc acetate was used as the zinc source.

ZIF-8 crystals with IT morphology can be obtained in pure phases over a wide range of the gel compositions (H2O/Hmim = 50~200 and content of CTAB ranged from 0.08 to 0.4 wt.%). We found that the H2O/Hmim ratio, rather than the concentration of CTAB, determine the particle size of IT crystals (Figure S8). The particle size of samples obviously decreases with the increase of H2O/Hmim ratio. In addition, ZIF-8 crystals with NR shapes are always synthesized under the moderate concentration of CTAB (0.2~0.4 wt.%) and molar ratio of H2O/Hmim (40~100). The aspect-ratio of NR crystals is mainly dependent on the H2O/Hmim ratio, and obviously increases with the increase of H2O/Hmim ratio (Figure S9). It is worth noting that this map is generated by using zinc acetate as the metal source and may not be directly applicable to composition in which another zinc source (e.g., zinc nitrate and zinc chloride) is used. Furthermore, the same synthesis methodology can be successfully extended to tune morphology of ZIF-67 frameworks, constructed from cobalt ions and Hmim, which is of isostructural SOD topology with ZIF-8 (Figure 4). Control experiments indicate that ZIF-8 crystals with above mentioned shapes were successfully prepared with the addition of cetyltrimethylammonium chloride (CTAC), as shown in Figure S10. However, only ZIF-8 crystals with RD shape were formed with addition of KBr (Figure S11). Therefore, the morphology change of ZIF-8 crystals is mainly attributed to the am-

In order to further explore the formation mechanism of ZIF-8 with various shapes, in the present of CTAB, FTIR characterization for all the samples was conducted. As shown in Figure 5a, all the spectra are in good agreement with the previous reports on ZIF-8 crystals,35, 36 with the exception of the appearance of peaks in the region of 2800~3200 cm−1 assigned to the special vibrations of CTAB molecules.37 The absorption at 2918 and 2846 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of C−CH2 in the methylene chains, respectively. Furthermore, compared to the as-synthesized ZIF-8 crystals with other shapes, IT crystals exhibit the highest absorptive intensity of characteristic peaks from CTAB molecules. This phenomenon is attributed to the existence of abundant CTAB molecules in IT crystals, which was verified through the thermogravimetric analysis (TGA) in air atmosphere. As shown in Figure 5b, the decomposition temperature windows for Hmim and CTAB are 150~200 and 230~300 oC, respectively, and the temperature for structural degradation of ZIF-8 structure ranges from 330 to 430 oC. Prior to the decomposition of ZIF-8 framework, crystals with various shapes exhibit different weight-loss and can be categorized into two groups. For the first-group crystals (RD, NC and OP), the weight-loss is mainly due to the release of embedded Hmim molecules. On the contrary, the weight-loss on the second-group crystals (IT and NR) is the release of both Hmim and CTAB, suggesting the existence of non-negligible amounts of CTAB inside the IT and NR crystals. In addition, N2 sorption measurements at 77 K also indicate that NR and IT crystals exhibit relative lower adsorption uptake than other shapes (Figure 5c). All samples exhibit Type I isotherms profiles due to the presence of micro-pores. Compared with the adsorption isotherms of other crystals, NC crystals exhibits a continuous rising plateau due to the significant external area from small agglomerated particles.38 As shown in Table S1, the Brunau-



er-Emmett-Teller (BET) surface areas of NR and IT crystals (1283 and 969 m2/g) are much lower than that of RD crystals (1539 m2/g). Especially for the IT crystals, the BET surface area and micro-pore volume are only ~60% to those of RD crystals. We speculate that the reduction of porosity is attributed to the inclusion of CTAB molecules inside ZIF-8 framework rather than adsorption of CTAB on the crystal surfaces. Therefore, the role of CTAB in the morphological-controllable synthesis of ZIF-8 crystals is not limited to the capping mechanism, which is the general observation for surfactant-assistant synthesis of both MOF and inorganic nanocrystals.10 Especially for the crystals (IT and NR) synthesized with high CTAB loading (>0.24 wt.%), CTAB molecules may first be anchored on the surface of nuclei, and further be embedded inside the crystals with the subsequent slow growth of crystals.

Figure 5. (a) FTIR spectra, (b) TGA curves in air atmosphere and (c) N2 sorption isotherms of as-synthesized ZIF-8 crystals with various shapes, (d) N2 sorption isotherms of ZIF-8 crystals heat-treated at 300 oC for 4 h under the N2 flow.

As a trial to remove the embedded CTAB molecules in IT crystals for releasing the porosity, traditional reflux in hot ethanol and acetone (80 oC) for 2 days were conducted. However, the BET surface area of the treated IT crystals is only slightly improved and still far lower than the targeted value (Figure S12). Thereafter, relying on the high thermal stability of ZIF-8 in inert atmosphere, the samples were heated in furnace at 300 oC for 4 h under the N2 flow to exclude the embedded CTAB molecules. It was found that all treated-samples exhibit Type I isotherm with a long horizontal plateau and similar adsorptive uptake (Figure 5d). As shown in Table S2, it is interesting that the BET areas of the treated-IT crystals is 1572 m2/g, which is very close to the value of RD crystals (1539 m2/g) and much higher than that of the original IT crystal (969 m2/g). The micro-pore volume of the treated-IT crystal is also improved from 0.48 to 0.65 cm3/g. In addition, the heattreatment also increases the BET areas of other crystals. The BET area of the NR crystals sharply increases from 1283 to 1909 m2/g. It is worth noting that the NR and NC crystals show significantly higher BET area than RD ZIF-8 (1400~1650 m2/g), which is commonly obtained in a surfactant-free solution as observed in this work and also by

other researchers.28 After heat-treatment, the crystallinity and morphology of all samples were all well preserved (Figure 6). The absorption peaks of CTAB molecules in FTIR spectra for all heat-treated samples were not found, suggesting the successful removal of CTAB molecules (Figure S13).

Figure 6. SEM images (a-e) and XRD patterns (f) of ZIF-8 crystals with different morphologies that were heat-treated at 300 oC for 4 h in N2 flow. The scale bar in all SEM pictures is 2 μm.

In view of the molecular-sieving effect of ZIF-8 framework, the heat-treated ZIF-8 crystals with five distinctive shapes were served as fillers (10~20 wt.%) in cross-linked poly(ethylene oxide) (XLPEO) to prepare mixed-matrix membranes (MMMs). The chelating effect between zinc nodes of fillers and ester groups of the polymer endows a well interfacial interaction between fillers and polymer matrices, as verified by the broadening of differential scanning calorimetry (DSC) for MMMs (Figure S14). In addition, the membrane fabrication procedures do not affect the crystallinity of ZIF-8 crystals in various MMMs (Figure S15). As shown in Table S3, the addition of ZIF-8 fillers, independent on the morphology, can obviously improve the separation selectivity of C3H6/C3H8 relative to the pristine XLPEO membrane. This result comes from the obvious increase of the permeability of C3H6 than C3H8, mainly due to the molecular sieving effect of ZIF-8 crystals.27 Compared with the MMMs derived from 6FDA-DAM polyimide,39 the separation performance for C3H6/C3H8 on XLPEO-based MMMs are relatively lower, mainly due to the intrinsic separation ability of polymeric matrix. The separation factor for C3H6/C3H8 on pristine XLPEO and 6FDA-DAM membrane are 3.4 and 12.4, respectively. However, the improvement of separation performance on XLPEO-based membrane is much higher than 6FDA-based membrane by blending same content (~20 wt%) of ZIF-8


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Chemistry of Materials fillers. For example, compared with the pristine polymeric membrane, the C3H6 permeability and C3H6/C3H8 selectivity increase by ~172% and ~170%, respectively, on the ZIF-8-NR/XLPEO MMMs (this work). In contrast, the C3H6 permeability and C3H6/C3H8 selectivity increase by ~100% and ~65%, respectively, on the ZIF-8/6FDA-DAM MMMs.40

lower Tg temperature of OP-XLPEO nanocomposites (Figure S14). It was found that the distortion of NR fillers facilitate the well interfacial structure between polymer and filler. On the contrary, rigid feature of OP fillers with relative large surface makes the polymer difficult to completely cover, resulting in the formation of non-selective defects. We speculate that the distortion of 2D nanosheets with high-aspect-ratio accounts for the improved separation selectivity on MMMs.13 In summary, we have reported the successful synthesis of monodisperse ZIF-8 nanocrystals with five distinctive morphologies, including anisotropic shapes, with the aid of surfactant CTAB. The morphological map shows that the morphology and purity of samples are strongly depended on both the concentration of CTAB molecules and the molar ratio of H2O/Hmim. The slow growth of anisotropic ZIF8 crystals (OP and NR) follows the [001] direction, due to the competitive interaction between ligands (Hmim) and surfactants (CTAB) with metal source (zinc ions). The CTAB molecules were not only found to cap on the surface of the synthesized NR and IT ZIF-8 crystals but also embedded inside the framework. The crystallinity, shape and porosity of all samples can be well recovered with the heat-treatment in inert atmosphere. As nano-fillers in XLPEO-based MMMs for C3H6/C3H8 separation, NR crystals exhibit the highest separation performance due to the intensification of the molecular-sieving effect of ZIF-8 framework. In the next step, composite membranes via deposition of ultrathin ZIF-8/XLPEO MMMs on porous flexible substrates will be designed for large-scale separation of C3H6/C3H8.

ASSOCIATED CONTENT Figure 7. (a) Mixed-gas permeation results for equal-molar C3H6/C3H8 mixture on the pristine XLPEO membrane and MMMs derived from 10-20 wt.% ZIF-8 fillers (OP and NR), (b) lower/higher magnification SEM cross-section images of MMMs containing 20 wt.% ZIF-8 fillers (OP and NR).

The separation performance for C3H6/C3H8 mixture on MMMs derived from ZIF-8 fillers with two anisotropic shapes (OP and NR) were also compared. Intuitively, MMMs with addition of OP fillers, 2D structure, would present the higher separation performance, as exemplified by the improved selectivity for CO2/CH4 on MMMs with effective cross-sectional occupation of CuBDC 2D nanosheets.13 However, in the current study, the MMMs derived from ZIF-8-OP showed relatively lower selectivity compared with that containing ZIF-8-NR. The separation selectivity for C3H6/C3H8 on the 20 wt.% ZIF-8-NR/XLPEO MMMs is ~60% higher than on the MMMs containing 20 wt.% ZIF-8OP crystals (Figure 7a). This phenomenon is possibly attributed to two main reasons. First, higher BET surface areas and microporosity for NR crystals than OP crystals would facilitate the transportation of C3H6 through the fillers inside the MMMs. In view of almost same C3H8 permeance, the MMMs derived from NR fillers exhibit higher C3H6/C3H8 selectivity. On the other hand, despite the homogeneous distribution of both fillers in the MMMs, more non-selective defects between polymer and fillers incline to form on MMMs derived from OP ZIF-8 than NR ZIF-8 (Figure 7b). This result was also reflected by the relatively

Supporting Information. Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

Materials and methods, SEM images showing the morphology evolution of various ZIF-8 crystals with time, morphological maps and SEM images of various ZIF-8 crystals synthesized under different conditions, tables showing textural properties of as-synthesized and heat-treated ZIF-8 crystals with various morphologies, BET surface areas of ZIF-8-IT crystals before and after solvent-reflux, FTIR spectra of heat-treated ZIF-8 crystals, DSC curves and XRD patterns of various ZIF-8/XLPEO MMMs, separation results for C3H6/C3H8 by various ZIF-8/XLPEO MMMs.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected]

[email protected];

li-

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. F.Y. and H.M. contributed equally to this work.

Notes



The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21406106, 21622607, 21776124), Jiangsu Provincial NSFC (BK20171459), Foundation of Jiangsu Edu-cational Committee of China (17KJA530004, 15KJA150005), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Financial support from “the Youth Thousand Talents Plan” of China and “the Shuang Chuang Plan” of Jiangsu province for Y.Y. is also gratefully acknowledged. Y.L. Appreciate the K.C. Wong Magna Fund in Ningbo University.

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