DOI:10.1002/cssc.201801585 Full Papers Very Important Paper Tuning Expanded Pores in Metal–Organic Frameworks for Selective Capture and Catalytic Conversion of Carbon Dioxide WeiMeng,[a] Yongfei Zeng,*[b] Zibin Liang,[a] WenhanGuo,[a] Chenxu Zhi,[c] Yingxiao Wu,[c] RuiqinZhong,[c] ChongQu,[a] and Ruqiang Zou*[a] Three Co-based isostructural MOF-74-III materials with expand- ment. In the catalytic conversion of CO2 with propylene oxide ed pores are synthesized,with varied extentoffused benzene to form propylene carbonate, the as-synthesized MOF-74- rings onto sidechain of same-length ligands to finely tune the III(Co) with desired properties of highly exposed and accessible pore sizes to 2.6, 2.4, and 2.2nm. Gas sorption resultsfor these open CoII centers, large mesoporeapertures and multi-interac- highly mesoporous materials show that alternately arranged tive sites, demonstrated highercatalytic activity compared fused benzene rings on one side of the ligand could serve as with other two MOFs, with benzene rings fused to ligands II extra anchoring sites for CO2 molecules with p–p interactions, hampering the functionality of Co centers as Lewis acid sites. conspicuously enhancing CO2 uptake and CO2/CH4 and CO2/N2 Our results highlight the viability of finely tuning the expanded selectivity;while more sterichindrance effect towards open pores of MOF-74isostructure and the effect of fused benzene II Co sites were imposed by ligands flankedwith fused benzene rings as functional groups onto selectiveCO2 capture and rings on both sides, compromising such extra-sites enhance- conversion. Introduction Metal–organic frameworks (MOFs)—a class of inorganic–organ- above applications, MOFs for sorption-based separationhave [9] ic hybrid crystalline materials—have sparkedmuch interest made the most progress, especially for selectiveCO2 cap- among researchers, owing to their high surfaceareas, ordered ture.[10] To address the issue of climate change caused by in- porousstructures,and the versatile tunability of their pore creasingemissions of anthropogenic CO2 into the atmosphere, environments and functionalities, all of whichcould be imple- carbon captureand storage(or sequestration;CCS) was pro- [11] mented by judiciousselection of variousbuildingblocks for posed as asignificant route to CO2 mitigation. Great efforts synthesis by self-assembly,[1] combinedoptionally with appro- have been made to develop more advanced solid adsorbents [2] [12] priate post-synthetic modifications. Generally,metal ions or for CO2 capture, in light to the easy operation conditions, clusters act as inorganic nodes, linked by organic bridging li- low energy cost for regeneration, and large working capacity gands containing elements such as oxygen, nitrogen, or sulfur. of MOFs.Given the intrinsicformationofunlimited metal– The highly desirable properties make MOFs promising materi- ligand combinationsinMOFs, they have been known to incor- als for awide variety of applications, with excellent performan- porate diversemetal-coordinatedclusters, open metal sites, ces in gas sorption/separation,[3] catalysis,[4] luminescence,[5] and many types of functional groupsand to allow facile chemi- sensing,[6] electrochemistry,[7] and magnetism.[8] Among the cal treatment to form composites, leading to easier design of targeted materials than for other traditional adsorbents, such as activatedcarbon,silica gel, and zeolites. [a] W. Meng, Z. Liang,W.Guo, C. Qu, Prof. Dr.R.Zou Among the many knowntypes of MOF,the well-known BeijingKey Laboratory for Theory and Technology of Advanced Battery Materials, Department of MaterialsScience and Engineering MOF-74family,composed of helical one-dimensional chains of College of Engineering, Peking University,Beijing100871 (China) edge-connected metal–oxygen octahedrabridged by 2,5-dihy- E-mail:rzou@pku.edu.cn droxy-1,4-benzenedicarboxylic acid or some analogue to fur- [b] Prof. Dr.Y.Zeng ther form honeycomb networks, has becomeone of the most Tianjin Key LaboratoryofStructure and Performance for Functional explored materials for many advanced applications,[13] because Molecules, Key Laboratory of Inorganic-Organic Hybrid Functional [14] MaterialChemistry (Ministry of Education) of these materials’ intriguing structuralfeatures. Upon re- College of Chemistry, TianjinNormalUniversity, Tianjin300387 (China) moval of solvent molecules at metal sites by heating under E-mail:yfzeng@nankai.edu.cn vacuum, the coordinationmode could be alteredfrom octahe- [c] C. Zhi, Y. Wu, Prof. Dr.R.Zhong dral to square pyramidal, leaving high-density open metal sites State Key Laboratory of Heavy OilProcessing lining the channel inside.[15] These materials show excellent po- China University of Petroleum, Beijing102249 (China) tentialasplatformswith Lewis acid sites for gas sorption,[16] Supporting informationand the ORCID identification number(s) for the [17] author(s) of this article can be found under: catalysis, or post-synthetic grafting positions of functional https://doi.org/10.1002/cssc.201801585. species.[18] Previous studies have demonstrated exceptionally ChemSusChem 2018, 11,3751 –3757 3751 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Papers II high CO2 adsorption of Mg-MOF-74 with open Mg sites at a Results and Discussion 1 [19] concentration of more than 8mmol gÀ at 298 Kand 1bar, and amine-appended derivatives have shown even better tun- [18,20] Characterization of MOF-74-III(Co) materials ability for CO2 capture under differentconditions. Further- more, the unexpected mechanism of CO2 interaction was veri- The as-synthesized MOF-74-III(Co) materials were obtained fied by comprehensive experimental analysis.[21] Recent works through one-pot solvothermal synthesis, and the crystallinity is have structurally resolved some of the sorption sites and confirmed by the powder X-ray diffraction (PXRD) patterns (see [20d,22] dynamics of CO2 with frameworks. However,although the SupportingInformation, Figure S1). The patterns of all systematic expansionofMOF-74 pores has already been repor- three products matched well with reportedsimulated val- ted,[14b] along with some examples with functional amine ues,[14b] with no undesired peaks, indicating the high purity of groups,[20a,23] little work has been focusedonstrategies to the materials. Taking account of the size of fused benzene finely tunepore size in expanded MOF-74 isostructures. ring(s)that form the polycyclic part of ligands along with the Herein, for the first time, we have employed the ligand 3,3’’- reported structure,[14b,20a] the pore sizes of MOF-74-III(Co) mate- dihydroxy-[1,1’:4’,1’’-terphenyl]-4,4’’-dicarboxylicacid (denoted rials can be presumably deduced as 2.6, 2.4, and 2.2 nm for 1, 1 as L )and another two analogues with different polyacenes as 2,and 3 (Scheme 1) respectively.From N2 sorptionmeasure- the middle strut,incorporating different numbersoffused ben- ments (Figure 1), each of the three activated materials demon- zene rings (Scheme1,L2 and L3), together with CoII cations to strates atype IV isotherm,which is typical of mesoporous ma- [24] construct MOFs with IRMOF-74-III structure featuring finely terials. At P/P0 < 0.003, the isotherms exhibit steep uptake, tuned pores. We then carefully examined their selectiveCO2 followed by asecond uptake step at P/P0 =0.05–0.14, after captureand conversionperformance to unravel the pore-size which aplateau is reached. The isotherms of MOF-74-III(Co) tuning effect broughtaboutbysidechain fused benzene rings. species 1 and 2 both have distinct two-stepprofiles, whereas This also exemplifiesaCo-based expanded-pore MOF-74,with in that for 3,the two steps are connected by an almost the fascinatingelectronic configuration of Co2+,compared smooth transition, making the distinction between the steps with Mg2+,Zn2+ and Ni2+ ,tohelp elucidate its intrinsic effect unclear and more like acontinuous pore filling, as is seemingly on Lewis acidity-related applications. characteristic of micropores. Since monolayer–multilayer ad- sorbed N2 occupied the narrow mesopore space, this altered the effective potential for pore filling.[25] The evolutionofthe isotherm profiles and the shift of the second step position (with startingpoints at P/P0 = 0.11, 0.068, and 0.048 for 1, 2, and 3,respectively)reflect well the trend in pore size tuning. Moreover, the DFT fitting-derived pore size distributions fur- ther validate the pore apertures of MOF-74-III(Co) materials as 2.6, 2.4, and 2.2 nm, showingsingle peaks (Figure 1, inset). The second steps of the three isotherms, which are due to pore condensation, are accompanied by fully reversible desorption branches with no hysteresis, providing furtherevidence for the narrow mesopores of these materials. The BET surface areasof 2 1 MOF-74-III(Co) are calculated as 2941, 2712, and 2065 m gÀ , 3 1 and the pore volumes are 1.77, 1.38, and 1.15 cm gÀ for 1, 2, and 3,respectively (Table S1). The morphologyofeach sample was observedwith TEM to be one-dimensional rod-like micro- crystalline particles (Figure S2). To test the stability, thermogra- vimetricanalysis of the materials showed that desolvation oc- curred upon initial heatinguntil 140–160 8C, after which the frameworks lost no weight up to about 260–280 8C, indicating the thermal stability of the materials (Figure S3). Gas sorption by MOF-74-III(Co) materials These highly porousstructures with exposed CoII sites may endow the materials with excellent potentialfor gas uptake. We performed gas sorptiontests on 1, 2,and 3 with ultrahigh purity CO ,CH,and N .From the obtained isotherms, all the Scheme1.Synthesis of Co-based MOF-74-III materials 1, 2,and 3 (gray =C, 2 4 2 red=O, olive =Co, white =H). materials present selectivecaptureofCO2 over CH4 and N2.As shown in Figure 2a, 1 demonstrates CO2 uptake of 3 1 3 1 101.0 cm gÀ at 273 Kand 1bar,and 75.1 cm gÀ at 298 Kand 1bar,with selectivities for CO2/N2 mixtures (CO2/N2 =15:85 v/v) ChemSusChem 2018, 11,3751 –3757 www.chemsuschem.org 3752 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Papers Figure 2. CO2,CH4,and N2 adsorption isothermsat273 and 298 Kfor MOF- 74-III(Co) materials 1 (a), 2 (b), and 3 (c).