Bimetallic MetalâOrganic Frameworks for Gas Storage and Separation

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Perspective

Bimetallic metal-organic frameworks (MOFs) for gas storage and separation Xinchun Yang, and Qiang Xu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00166 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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Crystal Growth & Design

Bimetallic metal-organic frameworks (MOFs) for gas storage and separation Xinchun Yanga,b and Qiang Xua,b* a

National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-

8577, Japan b

Graduate School of Engineering, Kobe University, Nada Ku, Kobe, Hyogo 657-8501, Japan

KEYWORDS: MOFs, Bimetallic nodes, Gas storage, Gas separation ABSTRACT: Emerging as a new family of hybrid crystalline materials, bimetallic porous metalorganic frameworks (MOFs) have received great attention in gas storage and separation. We present a critical perspective in the construction of bimetallic MOFs, involving one-step synthesis and post-synthetic modification (PSM), and their applications in gas storage and separation. Especially, several examples of bimetallic MOFs have been provided to well understand why such MOFs are so unique for these applications. We hope that the present perspective will inspire the chemists working in this area to rationally design/develop new bimetallic MOFs for advanced applications.

ACS Paragon Plus Environment

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1. INTRODUCTION Metal-organic frameworks (MOFs), as a new class of highly promising porous materials, have gained increasing interest in gas storage and separation, owing to their advantages of high capacity and selectivity.1-4 Typically, MOFs are crystalline solids that are constructed from organic linkers with corresponding metal-based nodes, possessing high porosity, large internal surface areas, and tailorable chemistry.5-7 Since it came into being in the 1990s, numerous fascinating MOFs have been synthesized for gas storage and separation by choosing appropriate ligands or functionalizing organic ligands, and several important reviews have been published in this field.8-13

In order to further improve the uptakes of gases within MOFs, another important consideration is to incorporate new secondary metal nodes into the frameworks, that is, to prepare bimetallic MOFs.13-15 In principle, the incorporation of bimetallic nodes in the same framework will allow the generation of defects in MOFs as well as the excellent synergistic


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effects between different metals through the intimate form of integration, which may benefit their intrinsic properties. Actually, bimetallic systems have already been proven to be effective, for instance, bimetallic nanoparticles often exhibit higher catalytic activities than their monometallic counterparts.16-20

Meanwhile, the doping of secondary metal ions into the

crystalline lattices of metal oxides usually leads to the enhancement of optical, electronic and magnetic properties.21-23 Thus, it is easy to understand that introducing bimetallic nodes into MOFs may lead to MOFs with high porosity and rich adsorptive sites. Although the concept of bimetallic MOFs seems simple and feasible, there are considerable challenges: (a) the incorporation of secondary metals often yields fragile frameworks, (b) the incorporated secondary metals are easily separated by organic linkers, which make them possess the similar structures and intrinsic properties as the same in monometallic frameworks, and (c) the topologies and functions of bimetallic MOFs are difficult to predict or control.

In the last decade, thanks to the great efforts of many researchers, the one-step synthesis approach has made significant progress toward introducing new secondary metal nodes to MOFs for enhancing the gas uptakes. On the other hand, post-synthetic modification (PSM), which is usually used to functionalize the organic ligands,24 has been extended to incorporate bimetallic nodes in the same framework. Nevertheless, a critical perspective focused on this field is lacking. The main objective of this perspective is to summarize the recent progress in synthesis of bimetallic MOFs by one-step synthesis and post-synthetic modification (PSM), and illustrate why they are so unique for gas storage and separation. We hope that the present perspective will benefit the researchers working in this area to rationally create novel bimetallic MOFs for advanced applications.


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2. ONE-STEP SYNTHESIS One-step synthesis, also called as one-pot reaction, has been widely used for preparing MOFs.25-28 However, it is very difficult to apply one-step synthesis for preparing bimetallic MOFs, because the incorporation of new secondary metal nodes to MOFs often yields fragile frameworks as well as unpredictable topologies and functions. Recently, under the great efforts of many researchers, much progress has been made in this area. Generally, the classical MOFs are ideal candidates for bimetallic MOFs by one-step synthesis owing to their large surface areas, exceptional pore volumes and high thermal stability. The Zn2+ ions in MOF-5 [Zn4O(BDC)3] (BDC = 1,4-benzene dicarboxylate) have been substituted by Co2+ ions during the solvothermal crystallization.29

Although it is a great

challenge to replace Zn2+ ions in MOF-5 by Co2+ ions, with which no more than 25 % of Zn2+ can be replaced, the obtained bimetallic MOF CoZn-MOF-5 materials showed higher adsorption capacities for H2, CH4 and CO2 at high pressures than that of their mother Zn-MOF-5. As MOF5 possesses unexposed metal sites that are less accessible to gas molecules, such contributions were assigned to certain flexibility of framework produced by Co incorporation. How about the effects of exposed metal sites in MOFs? As containing coordinatively unsaturated metal sites, MOF-74 may be a good candidate for studying the relationship between the substituted metal nodes and gas molecules. By varying the Zn/Co molar ratios from 0 to 100 %, a series of bimetallic Zn1-xCoxMOF-74 materials have been successfully prepared under one-step synthesis.30 Unlike CoZn-MOF-5, H2, CO2 and CH4 adsorption capacities of CoZnMOF-74 materials were improved even at low pressure. Moreover, such MOF-74 based bimetallic MOFs exhibited high isosteric heats of H2 adsorption. As further evidence, bimetallic


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MOFs of CoMg-NOF-74 and NiMg-MOF-74 with different amounts of Co and Ni were prepared via a one-pot solvothermal reaction.31 Similarly, all CoMg-MOF-74 and NiMg-MOF74 samples showed the enhanced uptakes for CO2 molecules. Clearly, the introducing of secondary metals into exposed sites of MOF-74 would give rise to a stronger affinity for the adsorbate as the new metal ion directly interacts with the gas molecules. It is very important to choose appropriate secondary metals for constructing bimetallic MOFs. HKUST-1, also known as Cu3(btc)2 (btc = 1,3,5-benzenetricarboxylate), is another MOF which has high surface area and pore volume.32 By using a slightly modified solvothermal method, bimetallic Cu3-xZnx(btc)2 MOFs have been successfully prepared with Zn2+ ions replacing Cu2+ ions in the paddle wheel units up to 21 %. Unfortunately, N2 uptakes and the specific surface areas of Cu3-xZnx(btc)2 MOFs decreased with increasing the incorporation of Zn2+ ions. In this regard, Wang et al. demonstrated that the bimetallic MOF ZnCu-HKUST-1 had decreased N2 uptakes and specific surface areas with the increase of Zn content.33 Additionally, by using a direct preparation route under mild reaction conditions, bimetallic MOF Cu2.75Ru0.25(BTC)2· xH2O has been achieved in which only 8 % Cu centers can be replaced by Ru.34 Similarly, the N2 uptake and surface area of such bimetallic MOFs decreased in comparison with the pure HKUST-1. The main reason was that there are unfavorable coordination environments of Zn2+ or Ru3+ ions in HKUST-1. Another possible reason was the increased lattice disorder from the large size difference. Metals which have similar ionic radii and coordination geometries may be firstly considered. In one instance, Cu2+ doped ZIF-8 crystals have been successfully prepared by reaction of Cu(NO3)2, Zn(NO3)2, and 2-methylimidazole in methanol at room temperature.35 Introducing Cu


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did not alter the high stability of ZIF-8 crystals even with a percentage up to 50 %, and had no significant influence on the surface area. Lah and co-workers have prepared a series of bimetallic MOFs, NixM1-x-ITHDs, by using a conventional solvothermal reaction with mixed metal ions varying Ni2+/Zn2+ and Ni2+/Co2+ molar ratios. The bimetallic MOF Ni0.11Co0.89-ITHD exhibited a very high CO2 uptake at 195 K and ambient pressure among all the other samples.36 Additionally, bimetallic CoZn-ZIF-8 frameworks have been obtained via the room temperature reaction of Zn2+, Co2+ and 2-methylimidazole in methanol.37 The N2 sorption isotherms suggested that CoZn-ZIF-8 with the Zn/Co molar ratio of 17.8 had similar microporous features to those in ZIF-8 and ZIF-67. Unexpectedly, bimetallic CoZn-ZIF-8 exhibited markedly better water stability than ZIF-67. More recently, a series of highly robust bimetallic CoxZn100-x-ZIF-8 frameworks has been synthesized by a simple one-pot reaction in water at room temperature.38 By varying the ratios of Co/Zn, the as-prepared CoxZn100-xZIF-8 frameworks showed tuned pore sizes, pore volumes and surface areas. Notably, Co75Zn25-ZIF-8 showed the highest CO2 and H2 uptakes at 298 and 77 K among all the CoxZn100-xZIF-8 frameworks (Figure 1). The difference in ionic radii of Zn and Co might result in defects in the framework and thus contribute to the enhancement of porosity, while the chemical heterogeneity and enhanced porosity were suggested as the reason for the excellent gas adsorption performances.


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Figure 1. (a) CO2 uptake isotherms at 298 K, (b) H2 uptake isotherms at 77 K, and (c) comparative CO2 and H2 uptakes of Zn-ZIF-8, Co-ZIF-8 and CoZn-ZIF-8. Reproduced from Ref. [38] with permission. Copyright Royal Society of Chemistry, 2016.

Bimetallic MOFs may have a strong ability of selective adsorption of gas molecules. In this regards, Zhong et al. have doped the alkali metal cations including Li+, Na+, and K+ in MOF HKUST-1, which strongly improved the CO2 adsorption properties.39 The highest adsorption of CO2 obtained by 1K-HKUST-1 was ca. 11% increase in adsorption capacity at 298 K and 18 bar as compared to mother HKUST-1. Moreover, the 1K-HKUST-1 exhibited CO2/N2 selectivity higher than that of pure HKUST-1. A novel bimetallic CrMg-MIL-101 was also successfully prepared by doping Mg during the solvothermal synthesis of MIL-101 (Cr).40 Notably, the doping of Mg not only resulted in a higher surface area, but also created rich adsorptive sites for CO2 adsorption. CO2 uptake of CrMg-MIL-101 reached 3.28 mmol/g at 298 K and 1 bar, which indicated an increase of 40 % compared to MIL-101(Cr). More importantly, the CO2/N2 adsorption selectively of MIL-101 (Cr, Mg) was significantly enhanced up to 86 at 100 kPa, 3


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times higher than MIL-101 (Cr). Based on above-mentioned MOF-74 and MIL-101 (Cr), Mg ions seem to have priority for enhancing the CO2 uptake. This may be extended to other MOFs while the great challenge may be the preparation of bimetallic MOFs including Mg. Additionally, Serre and co-workers reported the synthesis of bimetallic FeCr-MIL-53 by controlling the reaction rate of metals to ligands.41 CO2 adsorption experiments exhibited that the breathing behavior has been improved by the presence of mixed Cr-Fe chains. Ce0.05Zr0.95-UiO-66 was also prepared, which possess a high N2 uptake and surface area comparable to UiO-66 (Zr).42

As described above, similar metals are common in the construction of bimetallic MOFs. However, compared to similar metals, dissimilar metals in the same framework may exhibit stronger synergistic effect through the most intimate form of integration. Although it is not easy to integrate dissimilar metals into the same structural units, Bu and co-workers have described some typical examples of dissimilar metals in bimetallic MOFs.43-47 In one instance, his group synthesized five series of bimetallic MOFs, CPM-18-M (M = Nd, Sm), CPM-19-M (M = Nd, Pr), CPM-20, CPM-21-M (M = Mn, Co, Cu), and CPM-23, based on indium heterometallic clusters, with differing nuclearity, metal-to-metal ratios, geometry and charge.43 Among all of these bimetallic MOFs, CPM-20 exhibited significant H2 uptake at 77 K and 1atm as well as CO2 uptake at 273 K and 1 atm. Moreover, such CPM-20 possessed a high CO2/N2 selective adsorption at 273 K. More recently, they reported a family of bimetallic MOFs by adjusting the metal nodes in MOF CPM-200 with divalent (Mg2+, Mn2+, Co2+, Ni2+) and trivalent metals (In3+, Ga3+, Fe3+, V3+, Sc3+) (Figure 2).47 Significantly, among all the as-prepared bimetallic MOFs, FeMg-CPM-200 showed the highest uptake of CO2, while the VMg-CPM-200 showed the highest isosteric heat of CO2 adsorption.


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Figure 2. M2+ and M3+ combinations for CPM-200s. Reproduced from Ref. [47] with permission. Copyright American Chemical Society, 2016.

3. POST-SYNTHETIC MODIFICATIONS (PSMs) Broadly defined, PSM is the partial or complete replacement of a metal ion at the site of another, which has been demonstrated as an effective strategy for obtaining modulated and functionalized MOFs.48-51 For example, the synthesis of novel MOFs by using PSM functionalized organic ligands has been well established,52, 53 and more and more researches concerning PSMs of exchanging organic ligands to construct extended MOFs have also been reported.54,55 Recently, by completing exchange of metal nodes in MOFs, PSMs have been exploited to produce new MOFs which may not be yielded by one-step synthesis.56-60 Notably, as the exchange of metal nodes within MOFs needs to overcome the large kinetic barriers and diffusion problems caused by ultrafine pores, steric hindrance as well as confinement in unusual coordination geometries, in most of cases, such approach results in partial exchanges, meaning the formation of bimetallic MOFs. Unfortunately, very few works have been aware of the


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potential of PSMs on bimetallic MOFs. Here, we summarize MOF architectures in which metal nodes have been partially exchanged to form bimetallic MOFs for gas storage and separation. Although MOFs emerged decades ago, the first reports on the synthesis of bimetallic MOFs by using PSM appeared in 2007.61 A three-dimensional cubic tetrazolate-based MOF Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 (1-Mn2+, BTT = 1,3,5-benzenetristtrazolate), in which chloride-centered square-planar [Mn4Cl]7+ units were linked via BTT ligands, has been selected as an ideal platform for metal nodes exchange. By soaking 1-Mn2+ in methanol solutions of Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Li+ and Cu+ salts, the authors obtained a series of isostructural bimetallic frameworks. Notably, such bimetallic MOFs were stable after desolvation and possessed relatively high gas adsorption performances. For instance, H2 uptakes for 1-Fe2+ and 1-Ni2+ were 2.21 and 2.29 wt % at 77 K and 900 torr, respectively (Figure 3a), while the highest initial adsorption enthalpy 10.5 KJ mol-1 was showed for 1-Co2+ (Figure 3b). The much strong binding of H2 molecules to the unsaturated metal centers within each framework was suggested as the reason for the high adsorption performances. In this respect, Hou and co-workers observed the earliest examples of partial exchanges of Cd nodes within the porous pillared bilayer MOF {[Cd (bpy)2(O3SFcSO3)]·(CH3OH)4}n (2, Fc = ferrocene) with Cu2+ ions to form the bimetallic MOF {[Cd0.5Cu0.5(bpy)2(O3SFcSO3)]·(CH3OH)4}n (3).62 Although gas adsorption of MOF 3 was not reported, the authors realized the potential of bimetallic MOFs in storage and separation. In their successive work, they reported that the partial exchange of the central Cu2+ ions in the crystal of [Cu8L16]

(4,

HL

=

4′-[4-methyl-6-(1-methyl-1Hbenzimidazolyl-2-group)-2-n-propyl-1H-

benzimidazolyl methyl]) with Co2+ and Zn2+ ions resulted in the formation of bimetallic MOFs of [Zn1.6Cu6.4L16] (5) and [Co1.2Cu6.8L16] (6) for H2 storage.63 In contrast to the mother monometallic MOF 4 (6.38 cm3/g), H2 uptake of MOF 6 was much higher (40.78 cm3/g).


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Additionally, by similarly suspending the bilayered open framework [Zn(4,4’-bpy)2(FcphSO3)2]n (10, FcphSO3Na = m-ferrocenyl benzenesulfonate) in concentrated solutions of Cd2+, Pd2+ and Cu2+, Hou’s group reported other bimetallic MOFs such as [Cd0.4Zn0.4(4,4’-bpy)2(FcphSO3)2]n

(7),

[Zn0.75Pb0.25(4,4’-bpy)2-(FcphSO3)2]n

(8)

and

[Cu0.5Zn0.5(4,4’-bpy)2-

(FcphSO3)2]n (9), respectively.64 Unquestionably, PSM can be served as a powerful tool to create previously unknown bimetallic MOFs for gas storage and separation.

Figure 3. (a) Low-pressure region of the H2 adsorption isotherms for 1-M (77K) and (b) Zero-coverage values for the enthalpy of H2 adsorption in compounds 1-M. Error bars represent one standard deviation. Reproduced from Ref. [61] with permission. Copyright American Chemical Society, 2007.

Notably, metal exchanges are more prone to occur between the related metals which possess similar ionic radii and preferred coordination geometries. In this respect, exposing the MOF NU505-Zn (11) to a dimethyl sulfoxide (DMSO) solution of Ni2+ resulted in a Zn/Ni ratio of 6/94 in the pillared-paddlewheel MOF structure.65 Compared with the mother MOF NU-505-Zn, the obtained bimetallic NiZn-NU-505 became more stable with a higher N2 uptake. More recently, a new strategy (AIM-ME) combined the atomic layer deposition in MOFs (AIM) and metal exchange (ME) for introducing secondary metal nodes into MOFs. MOF NU-1000 (12), which


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possesses Zr6(µ3-OH)4(µ3-O)4(OH)4(OH2)4 nodes and tetratopic 1,3,6,8-(p-benzoate)pyrene (TBAPy4-) linkers, has been used for installing dispersed metal atoms.66 By complete exchange of the Zn2+ ions within the MOF Zn-AIM-NU-1000 with cations of Cu2+, Ni2+ and Co2+, a series of CuZr, NiZr and CoZr-based bimetallic MOFs have been yielded. Interestingly, N2 uptake of the NiZr-NU-1000 samples was higher than that of the CuZr-NU-1000, CoZr-NU-1000 and the pristine Zn-AIM-NU-1000. By modifying the pore environments, increasing the adsorptive sites as well as defects in MOFs, PSMs can enhance the gas adsorption performances of bimetallic MOFs. The original MOF Mn(H3O)[(Mn4Cl)3(hmtt)8] (13, POST-65), which contains Mn2+ centers as secondary building units (SBUs) at the vertices of octahedral cages, has been examined in N, Ndimethylformamide (DMF) solutions of Fe2+, Co2+, Ni2+ and Cu2+ for 12 days.67 Interestingly, partial exchange of Mn2+ ions in POST-65 (Mn) (14) only occurred with Cu2+ ions, which resulted in the formation of a bimetallic MOF CuMn-POST-65. However, compared to the bimetallic MOF CuMn-POST-65 and the mother POST-65 (Mn), the complete exchanged POST-65 (Fe) showed the highest H2 adsorption capacity because of the different degrees of interaction of the H2 molecules with open metal species. As a further evidence, the central Cr nodes in MIL-101 (Cr) (15) have been partially substituted by Al3+ and Fe3+ ions using a solventassisted cation substitution method (SACS) without any structure change.68 Interestingly, the obtained bimetallic AlCr-MIL-101 and FeCr-MIL-101 showed the higher H2 uptakes than the mother MIL-101 (Cr). PSMs exhibit different effects on different gases in the same kind of bimetallic MOFs. Recently, in order to explore the effects of metal exchange on intrinsic properties of bimetallic MOFs, an ideal MOF [(Zn4O)2(Zn2)1.5(L)6(H2O)3] (L = 10-(4-carboxy-penyl)-10-phenoxazine-


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3,6-dicarboxylic acid) (16), which possesses the unique structure of two kinds of SBUs including dimeric paddlewheel (Zn(COO)4) and tetrameric (Zn4(O)(CO2)6), was used to achieve singlecrystal to single-crystal metal ion exchanges that were inaccessible through MOFs which have only one type of SBUs (Figure 4).69 By exposing MOF 16 into DMF solutions of Cu2+ and Co2+ at room temperature, the authors observed that 41.8 % and 29.5 % of Zn2+ ions in the framework were exchanged by Cu2+ and Co2+ ions, respectively. They rationalized this result according to the high exchange activity of Zn2 paddlewheel. Interestingly, the Cu- and Co-exchanged bimetallic MOFs exhibited the higher N2 uptakes than that of mother MOF 16, owing to partial crystal defects after the process of metal exchanges. Meanwhile, Cu- and Co-exchanged bimetallic MOFs showed comparable CO2 adsorption capability and enthalpy to the mother MOF 16 at the same conditions, indicating that there was no obvious influence of different open metal sites on CO2 adsorption. A core-shell structure may improve the gas uptakes of MOFs. Based on a family of porous isostructural MOFs with the framework formula of [M6(BTB)4(BP)3] (17) (M = Zn2+, Co2+, Cu2+ and Ni2+, BTB = benzene-1,3,5-tribenzoate, and BP = 4,4’-bipridyl), Lah and co-workers have synthesized a series of bimetallic core-shell MOFs by controlling the duration of the metal exchange and the exchanged metals.70 Interestingly, after the formation of core-shell structures, such bimetallic MOFs have showed enhanced stability and N2 uptakes than their mother MOFs. More recently, Panda et al. have successfully synthesized a series of MOF alloys of Albdc/Ga-ndc, Zn-MOF-74/Mg-MOF-74, Zn-MOF-74/Mg-MOF-74, Zn-ZIF-8/Co-ZIF-8 and ZnZIF-8/Cd-ZIF-8 by a solvent-free mechanical milling and vapor exposure process (Figure 5).71 CO2 adsorption experiments indicated that those Al-bdc/Ga-ndc MOF alloys possess high CO2 uptakes comparable to the pristine Al-ndc MOF.


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Figure 4. (a) Illustration of fragmental cluster change via cation change in 1-Zn. (b) Crystal color changes before and after corresponding metal cation soaking. Reproduced from Ref. [69] with permission. Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2016.

Figure 5. A possible formation mechanism for the solid solution of Al-ndc and Ga-ndc by ball milling and vapor exposure processes. Reproduced from Ref. [71] with permission. Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2016.

CONCLUSIONS


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In this perspective, we have summarized the recent progress in the construction of bimetallic MOFs by one-step synthesis and PSMs, and their applications in gas storage and separation. Several given examples have indicated that the increased defects, rich adsorptive sites as well as enhanced synergistic effects within bimetallic MOFs resulted from the incorporation of new secondary metal nodes have great influences on their intrinsic properties. Although the research on bimetallic MOF is still in its infancy and it is still a great challenge to rationally construct bimetallic MOFs, we believe that more and more bimetallic MOFs will be developed and show their broad possibilities for the advanced applications such as gas storage, gas separation, catalysis, drug delivery and so on. We hope that the present perspective will inspire the chemists working in this area to rationally create new bimetallic MOFs for advanced applications. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: +81-72-751-9562. Fax: +81-72-751-9628. Author Contributions Q. Xu proposed the topic of the perspective. X. C. Yang investigated the literatures and wrote the manuscript. Q. Xu and X. C. Yang discussed and revised the manuscript. Notes The author declares no competing financial interest. ACKNOWLEDGMENT The authors thank the editors for the kind invitation and Japan Society for the Promotion of Science (JSPS) for financial support (KAKENHI No. 26289379). X. C. Yang is grateful to the


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Crystal Growth & Design Bimetallic metal-organic frameworks (MOFs) for gas storage and separation

Graphical abstract

This perspective presents the construction of bimetallic MOFs, involving one-step synthesis and post-synthetic modification, and their applications in gas storage and separation. Examples are provided for well understanding why such MOFs are so unique for these applications, which may inspire chemists working in this area to rationally design/develop new bimetallic MOFs for advanced applications.


Bimetallic MetalâOrganic Frameworks for Gas Storage and Separation

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