Effective Gas Separation Performance Enhancement Obtained by

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Effective Gas Separation Performance Enhancement Obtained by Constructing Polymorphous Core-Shell Metal-Organic Frameworks Yingdian He, Mingzhe Sun, Qinghu Zhao, Jin Shang, Yuanmeng Tian, Penny Xiao, Qinfen Gu, Liangchun Li, and Paul A. Webley ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08592 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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ACS Applied Materials & Interfaces

Effective Gas Separation Performance Enhancement Obtained by Constructing Polymorphous Core-Shell Metal-Organic Frameworks Yingdian Hea,1,†, Mingzhe Sunb,c,†, Qinghu Zhaoa, Jin Shangb,c,*, Yuanmeng Tianc, Penny Xiaoa,e, Qinfen Gud,*, Liangchun Lie, Paul A. Webleya a. Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia. b. School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR c. City University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen HiTech Industrial Park, Nanshan District, Shenzhen, P.R. China d. Australian Synchrotron (ANSTO), 800 Blackburn Rd, Clayton, Victoria 3168, Australia. e. School of Chemical Science and Engineering, Tongji University, Shanghai, No. 67 Chifeng Road, Shanghai 200092, P.R. China

KEYWORDS: gas adsorption; metal-organic frameworks; core-shell; CO2 separation; in-situ synchrotron powder X-ray diffraction.

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ABSTRACT

We reported a new polymorphous core-shell metal-organic framework (MOF) in the form of a 3D-MOF core wrapped in a 2D layered MOF shell by applying a general acid-solvent synergy synthesis. This hybrid material can achieve high adsorptive selectivity/capacity simultaneously, which is validated by the unary isotherms of CO2 and N2 conducted at 273 K (0~1 bar). The MOFS@MOF-C with 7-day exchange showed the highest CO2/N2 selectivity (32.7) among our samples and a moderate CO2 capacity (2.3 mmol/g), which is 3 times and 1.6 times of those of the MOFC and MOF-S, respectively. We attributed the enhanced selective adsorption performance to the negligible N2 uptake exhibited by the outer shell of MOF-S@MOF-C. This study provides a new route for elevating gas separation performance by constructing multi-functional core-shell materials.

INTRODUCTION Porous materials, such as metal-organic frameworks (MOFs) or porous coordination polymers (PCPs), have attracted increasing attention due to their applications in adsorptive gas separation over the past few decades thanks to the ultra-high surface area and tuneable pore chemistry.1-7 Most porous MOFs feature large pore volume and high surface area and thus can provide relatively high adsorption capacity for all components in the gas mixture, but result in relatively low selectivity based on equilibrium adsorption.8-10 A few MOFs possessing uniform pore/channel size similar to the gas molecules could offer very high selectivity based on the molecular sieving mechanism, but lack of sufficiently high capacity due to the reduced overall pore volume.11-13 Particularly, flexible MOFs as a unique family of MOFs, generally exhibit high


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adsorption selectively attributed to their structural responsiveness to the inclusion of certain guest molecules.14-16 It is highly desirable to develop MOFs with both high adsorption selectivity and high capacity. In order to take full advantage of the merits of MOF materials for gas separation, a core-shell MOF composite was constructed from a MOF-Core (MOF-C) featuring high adsorption capacity as the gas storage core and a MOF-Shell (MOF-S) featuring high gas selectivity as the outer shell. Generally, construction of the core-shell structure in MOF families is a challenging and less studied area. Several pioneering studies have proven the possibility of constructing the core-shell MOFs and demonstrated their exceptional capabilities for versatile applications.17-22 For example, [Zn2(ndc)2(dabco)]n and [Zn2(ndc)2(dpndi)]n with different pore surface functionality were integrated into one single crystal via face-selective epitaxial growth strategy (where ndc = 1,4naphthalene dicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane and dpndi = N,N’-di(4pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide).23 The heterogeneous crystals offer potential to integrate sorption, separation, and reaction properties in one functional material. Another coreshell product was obtained by a seeded growth technique on MOF-5 (Zn4O clusters with BDC) with IRMOF-3 (Zn4O clusters with ABDC), where BDC = 1,4-benzene dicarboxylate and ABDC = 2-amino benzene-1,4-dicarboxylate. Interestingly, the N2 uptake of IRMOF-3@MOF-5 and MOF-5@IRMOF-3 are approximately 820 cm3/g which is between that of MOF-5 (920 cm3/g) and IRMOF-3 (750 cm3/g). The identical topology of the two structures, multi-layered crystals in forms of MOF-5@IRMOF-3@MOF-5 and IRMOF-3@MOF-5@IRMOF-3, were produced successfully by applying the same technique.24 A core-shell architecture built from two isostructural compounds, Co2(ad)2(CH3CO2)2 (named bio-MOF-11, where ad = adeninate) and Co2(ad)2(C4H9CO2)2 (bio-MOF-14), can integrate the benefits of two structures for selective


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adsorption CO2 over N2 with high water stability.25 However, for these reported successful examples, the structures of the shell and the core component are topologically identical, predominately on a basis of the growth of the shell out of the core by an epitaxial mechanism. This limits the development of core-shell MOF composites to certain MOF families. Therefore, a new synthetic strategy is required to construct core-shell MOF composites which feature structural diversity between the shell and the core materials, so as to extend the structural multiformity of the composites. Herein, we report a new polymorphous MOF composite constructed from a 3D rigid MOF as the core and a 2D layered flexible MOF as the shell obtained by crystal phase-converting in an “outside-in” fashion. Importantly, this core-shell MOF composite integrates the high adsorption capacity of the core MOF and the ideal selectivity of the shell MOF, which demonstrates the desired properties for the potential practical application in gas separation. Inspired from our previous studies on structural conversion of a layered MOF to achieve enhanced selectivity,11 we designed a new core-shell composite to realize both high adsorption capacity and selectivity in the system. To be specific, we fabricated a “gate-keeping” shell around the “storage” core so that undesired gas molecules (e.g., N2) can be excluded from entering the internal pores within the core. This goal can be achieved by ligand exchange of the MOF-C into MOF-S using similar reported method,26-28 which we have accomplished as shown below.

EXPERIMENTAL 1. General Information


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All chemicals were purchased from commercial sources (Sigma Aldrich and Merck). Zinc acetate dehydrate Zn(CH3COO)2·2H2O, 4,4 ′ -biphenyldicarboxylic acid (BPDC), 4,4 ′ Dipyridyl (BiPY) (98%), 1,3-Bis(4-pyridyl)propane (BPP) (98%), Dimethyl sulfoxide-d6 (DMSOd6)(99.9 atom %D), Sulfuric acid-d2 solution (D2SO4/D2O)(96-98 wt. % in D2O) were purchased from Sigma-Aldrich. Glacial Acetic acid AR (CH3COOH) (100%), and Ethanol (C2H5OH) (100%) were purchased from chem-supply. N,N-dimethylformamide (DMF) (99.9%) was purchased from PROLABO CHEMICALS. 2. Materials synthesis Preparation of parent MOFs: MOF-C (Zn3(BPDC)3(BiPY)) was synthesized following the reported method29 except for a few minor modifications. To be specific, 0.2195 g of Zinc (II) acetate dehydrate, 0.242 g of 4’-biphenyldicarboxylic acid, and 0.0788g of 4,4’-Dipyridyl were mixed in DMF (20 mL) followed by stirring for 3 hours. The mixture was then transferred into a 40 mL Teflon-lined stainless-steel autoclave and well-sealed before placing it in a pre-heated oven at 120 °C for 3 days. Afterwards, the colourless products were filtered and washed with 15 mL of DMF 3 times and stored in DMF solution. The as-prepared samples were filtered and dried at 60 °C in a vacuum oven for 2 hours before further characterization or reaction. Preparation of core-shell MOF materials Zn(BPDC)(BPP) @ Zn3(BPDC)3(BiPY): Based on the single crystal of MOF-C as precursor, the core-shell structures MOF-C @ MOF-S (MOF-C = Zn3(BPDC)3(BiPY), MOF-S = Zn(BPDC)(BPP))30 were synthesized by the exchange method via acid-solvent synergy for Metal-Organic Framework Synthesis (EASY-MOFs). To be specific, a concentrated 1,3-Bis(4-pyridyl)propane (BPP) solution was prepared as follows: 0.1983 g of BPP was mixed with 8 mL DMF, 2 mL Ethanol, and 200 µl acetic acid, followed by sonicating


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for 5 minutes to obtain a clear solution. The parent crystals of MOF-C (ca. 200 mg) were immerged in the BPP solution and transferred into a 20 mL Teflon lined autoclave. Every 72 hours, the obtained product was washed thoroughly with hot DMF (3*20mL) and the BPP solution was refreshed. The reaction was undertaken at 105 °C with various times (3, 4, 6, 7, 8, and 9 days) (Schematic 1, Fig.S1).

Schematic 1. Schematic illustration of acid-solvent synergy synthesis for the core-shell MOF-S@MOF-C composite with enhanced gas separation performance.

3. Characterization measurements

NMR: 1H NMR experiment was conducted on a Bruker AvanceIII 400 spectrometer, operating at 400 MHz. The samples for 1H spectral measurements were prepared by the following procedure: around 5 mg of a sample was washed thoroughly by DMF and dried at 60 °C in a vacuum oven. The sample was digested by 10 min sonication in 1 mL deuterated dimethyl sulfoxide (DMSO) solution mixed with 3 drops of D2SO4. The digestion solution was used directly for 1H-NMR. SEM: Scanning electron microscopy (SEM) was used to investigate the morphology of the as-prepared samples on a JEOL 7001F FEG (5 - 15 kV) microscope. The sample was ground into


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fine powders and sprinkled on carbon tape mounted on a metal stub, followed by coating with a 1 nm layer of platinum metal. TEM: Sample preparation: a suitable cross section sample was prepared using focused ion beam (FIB) cutting under SEM imaging (Fig. S2). Transmission electron microscopy studies on the as-prepared cross-sectioning sample of the MOF were performed using a TECNAI F20 Transmission Electron Microscope (TEM) at 100 kV. Thermogravimetric analyses TGA: The stability and composition of the products were investigated by thermogravimetric analyses on a Mettler Toledo TGA-SDTA851 analyzer (Switzerland) from 30 to 800 °C (2 °C/min) under nitrogen (40 mL/min). N2 sorption at 77K and CO2 sorption at 273K: To understand the porosity of the asprepared samples, we measured the N2 isotherms at 77K. However, the pore volume and surface area of the core-shell composites cannot be obtained due to the negligible N2 adsorption capacity (Fig. 3) by BET method. Thus, we calculated the porosity using Dubinin-Radushkevitch equation through the CO2 adsorption isotherms (Fig. 4) conducted at 273 K (Table S1). Synchrotron X-ray Powder Diffraction: Phase identification and structure of both parents and MOF-C@MOF-S composites were investigated using high-resolution synchrotron X-ray powder diffraction (XRPD) by a Mythen-II detector at powder diffraction beamline (the Australian Synchrotron). Prior to the XRPD measurements, the as-prepared materials were dried under vacuum (60 °C) and loaded into 0.7 mm glass capillaries, which were then sealed with wax to avoid the exposure to air. The sample temperature was well controlled (100 – 500 K) by an Oxford cryostream instrument. A flow cell setup was used for in-situ gas flow XRPD experiment. The wavelength was calibrated by NIST LaB6 660b standard of 0.6887 Ǻ.


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Gas adsorption/desorption: The adsorption performance of as-prepared samples was evaluated by conducting unary isotherms using volumetric apparatus ASAP 2010 (Micromeritics, USA). Before the measurement, about 0.12 g of each sample (0.12 g) was degassed at 95 °C (0.1 °C/min) under vacuum (below 10–2 Pa) for 10 hours to obtain the desolvated samples.

RESULTS and DISCUSSION Interestingly, the morphologies of the samples with the 4-day reaction time exhibited same columnar-like single crystals and remained the shape similar to the parent MOF-C, as evidenced by our microscope observations (Fig. 1a, 1b). In addition, the core-shell composite showed considerably rougher surface compared with that of the pristine MOF-C crystals (Fig. S4). This may be due to a dramatic difference in the structural topologies between MOF-S and MOF-C crystals. In this case, with the ligand exchange proceeding to certain extent, the surface of parent single crystals (MOF-C) cracked into smaller crystals as indicated from the optical microscope images (Fig. S3) and the SEM images of MOF-S (Fig. S2d). Basically, the structural transformation from MOF-C to MOF-S involves a substantial change of the crystal morphology at a critical point when the intactness of the parent crystals cannot remain upon the ligand exchange. In this case, the crystals of the sample after 7-day exchange time lost their single crystal nature caused by crystal micro strain and lattice mismatch. In-situ optical microscope observation of the synthesis process indicates the polymorphous structure (for the MOF-S@MOF-C composites having the same crystal morphology as the parent MOF-C) of the samples with certain exchange time. To further confirm the formation of the core-shell structure, we conducted TEM experiments on the sample with 6-day exchange time. The cross-section of one typical core-shell


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MOF single crystal was prepared by focus ion beam (FIB) cutting method (Fig. 1c, S2) and the TEM image clearly shows the interface between the core and shell crystal phases (Fig. 1d).

Figure 1. (a) SEM image of MOF-C crystals, (b) core-shell MOF-S@MOF-C composite after 4-day exchange reaction, and (c) SEM image of a sample crystal (core-shell MOF-S@MOF-C composite with 6-day exchange) prepared by focused ion beam (FIB) cutting, (d) TEM image of a cross-section of the MOF-S@MOF-C composite (6-day exchange time) crystal cut from Fig1 (c).

To better understand the mechanism of the formation of the polymorphous core-shell MOF composite and the structural conversion from MOF-C to MOF-S in atomic scale, we conducted synchrotron X-ray powder diffraction (XRPD) experiment for the samples collected after various reaction periods, i.e. 3, 4, 6, and 9 days, to monitor the crystal phase transformation or MOF-S shell growth during the ligand exchange process (Fig. 2a). The parent MOF-C sample remained almost unchanged with 3-day exchange time, and a complete pure phase of MOF-S was formed after 9 days. The XRPD patterns indicated that the core-shell structure displayed an increasing ratio of MOF-S phase in MOF-S@MOF-C with increased ligand exchange time. For example, the


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MOF-S@MOF-C composite with 4-day reaction time contained approximately 40.1 wt % of parent MOF-C phase and 59.9 wt % of MOF-S phase on basis of the Rietveld refinement results from the XRPD data (Fig. 2b). After 7-day exchange, phase composition reached 9.2 wt % of parent MOF-C phase and 90.2 wt % of MOF-S phase (Fig. S6). It is worth noting that the ratio of the MOF-S and MOF-C is an average value considering the crystal size distributions due to inevitable small crystal defect (Fig. S3, S9). In addition, the ratio between MOF-S and MOF-C phases can be precisely controlled by changing reaction time, temperature, and acid concentration.

Figure 2. (a) Synchrotron X-ray powder diffraction (XRPD) patterns of pristine parent MOF-C, and samples with 3-, 4-, 6- (MOF-S@MOF-C) and 9-day (MOF-S) exchange time; (b) Rietveld refinement profile for the MOF-S@MOF-C composite (4-day exchange time).

To verify our prediction of the enhanced gas separation performance on our rationally designed core-shell MOF composite, we conducted N2 sorption experiments at 77K on the 4-day exchanged sample as well as pure MOF-C and pure MOF-S for comparison (Fig. 3). The MOFC showed a microporous structure, evidenced by the sharp increase in uptake of N2 at


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low pressure (Fig. 4a), which is distinct from that of the MOF-S or MOF-S@MOF-C composite – negligible uptake suggesting non-porous structure. Notably, such a seemingly non-porous feature toward N2 suggests that the shell layer of core-shell structure, MOF-S, blocks N2 uptake at 77 K (Fig. 3, Fig. S10). In contrast, the core material of the core-shell structure, MOF-C, adsorbed a substantial amount of N2 (approximately 270 cm3/g at 1 bar and 77 K) at the same conditions (Fig. S11). As anticipated, the coreshell structure, MOF-S@MOF-C composite, displayed negligible N2 uptake similar to MOF-S. These results prove that our MOF-S@MOF-C composite inherits the property of the shell, that is, blocking N2 due to molecular sieving.


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Figure 3. N2 adsorption isotherms at 77 K of MOF-C, MOF-S@MOF-C (the sample with 4-day exchange), and MOF-S (the sample with 9-day exchange)

Figure 4. (a) CO2 and N2 adsorption/desorption isotherms and (b) CO2/N2 selectivity and CO2 uptakes (273 K, 1 bar) of MOF-S@MOF-C with 0- (MOF-C), 3-, 4-, 7-, 8- and 9-day (MOF-S) exchange (N2 hollow symbols, CO2 solid symbols).

The enhanced CO2 adsorption capacity and the CO2/N2 selectivity were validated by further conducting the CO2 and N2 isotherms at 273 K on our core-shell adsorbents with different exchange time (supposed to be of different thickness in the shell). Fig. 4a shows that both CO2 and N2 adsorption capacity decreased with the increase in exchange time, indicating an increase in the thickness of the shell (MOF-S), as the shell has much lower N2 adsorption capacity. Note that the shell admits CO2 into its internal structure (though processing lower capacity than the core) but largely excludes N2. Examining the core-shell adsorbent with different shell thickness allows for reaching the sweet point, where the sacrifice of CO2 uptake is minimal while the exclusion of N2 is to the largest extent (considering the inevitable formation of defects in the shell). The MOFS@MOF-C with 7-day exchange shows a decent CO2 capacity (2.3 mmol/g) and the highest


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CO2/N2 selectivity (32.7), which is higher than that of the MOF-C and other reported MOF materials, i.e., UiO-66 (22.6) and UiO-NH2 (24.3)

31.

In this manner, the core-shell adsorbent

realized an optimal separation performance – substantially higher selectivity than the core and higher capacity than the shell. In other words, it results in an enhanced capacity and selectivity. It is worth mentioning that a small percentage of pure MOF-S crystals can be expected in the MOFS@MOF-C samples (Fig. S3 shows a large number of very small crystals present), since the parent MOF-C crystals with smaller crystalline size logically completed ligand exchange can be removed with a suitable manual sieve. This is likely the cause of the observed adsorption/desorption hysteresis from the flexible nature of the MOF-S structure.

CONCLUSIONS In summary, we applied a novel strategy to construct a core-shell architecture of MOFS@MOF-C composite which features the merits of both pure MOF-S and MOF-C materials in terms of high gas adsorption capacity and high gas selectivity, demonstrated by the enhanced CO2/N2 separation performance at 273 K. The 4-day exchanged MOF-S@MOF-C composite exhibited 212% higher CO2 adsorption capacity compared with the value for pure MOF-S, and 199% CO2/N2 higher selectivity compared with the value for pure MOF-C. This innovated synthesis route potentially offers a universal method for constructing advanced core-shell MOF structures as we have demonstrated a potential generalization route of the solvent-acid synergetic ligand exchange.32 Having overcome the limitation in topological similarities to construct coreshell MOFs, a numerous combination of the MOFs with distinct structures and properties via the


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strategy reported in this study would open up a new horizon to enhance gas separation performances by rationally designed polymorphous MOF composites.

ACKNOWLEDGMENT J.S. gratefully acknowledges the financial support from the National Natural Science Foundation of China (21706224), the Research Grants Council of Hong Kong (CityU 21301817), and City University of Hong Kong Start-Up grant (7200524). Authors acknowledge Australian Research Council (ARC) funding (DP2013000024). SEM experiment was conducted by Dr. Ranjeet Sigh. TEM was conducted by Dr. Sergey Rubanov. Part of work was undertaken at the PD beamline, Australian Synchrotron (ANSTO).

AUTHOR INFORMATION *Email: [email protected] (J. Shang) *Email: [email protected] (Q. Gu) Author Contributions †Y. He and M. Sun. are considered as co-first authors because they contributed equally to this work. Notes The authors declare no competing financial interest.


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Effective Gas Separation Performance Enhancement Obtained by

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