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Metal Effects on the Framework Stability and Adsorption Property of a Series of Isoreticular MOFs based on an in-situ Generated T-Shaped Ligand Ying Xiong, Tao Yang, Sha Chen, Chang-He Zhang, Cheng-Xia Chen, Zhang-Wen Wei, Dawei Wang, Ji-Jun Jiang, and Cheng-Yong Su Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01423 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018
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Crystal Growth & Design
Metal Effects on the Framework Stability and Adsorption Property of a Series of Isoreticular MOFs based on an in-situ Generated T-Shaped Ligand Ying Xiong,[b] Tao Yang,[c] Sha Chen,[a] Chang-He Zhang, [a] Cheng-Xia Chen, [a] Zhang-Wen Wei, [a] Dawei Wang, [a] Ji-Jun Jiang,* [a] and Cheng-Yong Su[a, d] [a] MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China. [b] Life science institute, Jinzhou Medical University, Jinzhou 121001, P. R. China. [c] Guangxi University of Chinese Medicine, Nanning 530001, P. R. China. [d] State Key Laboratory of Applied Organic Chemistry Lanzhou University, Lanzhou 730000, P.R. China.
Supporting Information Placeholder ABSTRACT: An in-situ approach has been undertaken to synthesize a series of metal-organic frameworks (MOFs) with the pore surface functionalized via the organic linkers of multi Nheterocycles. This was done so as to explore their stability, flexibility and adsorption behaviors on the basis of the nature of metal centers (M = Zn, Mg, Co, Ni). Four isoreticular MOFs based on in-situ synthesized T-shaped ligands were obtained, and their phase purity and stability were investigated in conjunction with the adsorption properties for a wide range of gases (Ar, N2, H2, CO2, CH4, C2H6, C2H4, C3H8, and C3H6). The results revealed that the stability of MOF based on Ni2+ is better than those of Mg, Zn, and Co analogues, and that the multi N-heterocycles ligand endows these MOFs with higher affinity for CO2, C2H6, C2H4, C3H8, and C3H6, leading to an excellent selectivity for these gases over CH4.
Within the last few decades, the discovery and development of metal-organic frameworks (MOFs) have been one of the most inspiring events in chemical world. Plenty of research results have witnessed the diverse structures and striking application of MOFs on guest sorption and separation 1-5, luminescence properties 6-9, catalysis 10-11, sensors 12-13, and so on. One of the most intriguing properties of some MOFs is their structural flexibility, which is hardly observed in other porous materials like zeolites and activated carbons. The phenomenon involves a diverse range of conformational factors that result in the reversible adjustment of pore size and shape to external stimuli such as physisorption 14, temperature 15, mechanical force 16, and light 17. We mainly focus on the guest-induced flexibility, which grants enhanced application for flexible MOFs in gas separation 18-19. For some guests that can induce structural transformation from a closed aperture to open aperture caused by specific host–guest interactions, it is
usually accompanied by an abrupt rise of adsorption amount of the guests and resulting stepwise behavior in measured isotherms. On the contrary, non-target guests cannot induce the aforementioned structural transformation for the reason of having a lack of the weak affinity to the guest, thus causing a wide gap between the adsorption amount of the specific guests and non-target guests and leading to splendid guest selectivity properties 20. Such behavior is one of the most effective ways to enrich desired gas molecules, creating a bright prospect in industrial application. Although fruitful results have been obtained in relation to the structural details during the flexible transformation 21-23, it is still unclear why some MOFs possess flexibility. In other words, the inherent factors that cause flexibility of some specific framework are still an important goal in the field of researching MOFs. Insitu ligand reactions are another field that have been extensively investigated because they can provide a onestep synthetic method for organic
Scheme 1. In-situ reaction from TIT to dimto, and the formation of dimto based LIFM-13(M) with various flexible (or rigid) behavior, M=Mg, Co, Ni, Zn.
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compounds that require multiple reactions under conventional conditions or are even inaccessible in direct preparation 24. The in-situ reactions include reduction, oxidation, and displacement to form a new ligand with modified functional groups. The generated ligand is then formed in crystalline coordination products 25-28. By consciously utilizing the in-situ ligand reactions strategy, some MOFs with novel network topologies and specific function can be constructed. However, owing to sensitive reaction conditions and unclear mechanism, in-situ ligand reactions are still rare and difficult to be extended to other synthetic conditions. In this paper, four isostructural MOFs that were named as LIFM-13(M) (M=Mg, Co, Ni, Zn, LIFM means Lehn Institute of Functional Materials), were formulated as M(dimto)solvent series, and were synthesized based on an unexpected in-situ synthesized ligand of 4,6-di(1-imidazolyl)-1,3,5-triazine-2-one (dimto) under heating in dimethylacetamide (DMAC) (Scheme 1). Single-crystal analyses have unambiguously verified their isostructures with quadrate pore shape and multi-nitrogen heterocyclic pore surface. Varianttemperature powder X-ray diffraction (VT-PXRD) reveals that the frameworks exhibit different structural features varying from rigid to flexible by changing the metal centers, highlighting that metal centers play a crucial role in structural flexibility (or rigidity). Moreover, high gas adsorption selectivities are benefit from the multi N-heterocycles ligand of the MOFs. Three MOFs of LIFM-13 have been structurally characterized by single-crystal X-ray diffraction analyses excluding LIFM-13(Ni) (Crystallographic data is shown in Table S1 and S2 in supporting infromation). LIFM-13(Zn), LIFM-13(Mg) and LIFM-13(Co) crystallize in space group I41/amd, I41/a, I41
Figure 1. Representation of the crystal structure of LIFM13. (a) asymmetric unit and (b) 1D apertures of LIFM13(Zn), N sites are amplified to show the functional pore surface of the pore.
respectively. Single crystal structure analysis reveals that all the frameworks exhibit a similar structure. Herein we take LIFM-13(Zn) as the example of all the analogues to show their structural details (Fig. 1). Similarly as compound of [Zn(dimto)2]n · χ (DMF)28, in LIFM-13(Zn), one of the imidazole groups in the tripod ligand of 2,4,6-tri(1-imidazolyl)-1,3,5-triazine (TIT) was broken and was replaced by a carbonyl group to afford a new T-shaped ligand (dimto). The feature of C O double bond rather than single bond is confirmed by bond length of around 1.25 Å in each crystallographic data, and the existence of dimto was also confirmed by IR spectra (Fig. S1~S4). Thus, the C3 symmetry in TIT disappears and transfers into C2 symmetry (Fig. S5). The asymmetric unit of LIFM-13(Zn) contains one Zn2+ and two ligands (solvent molecules are omitted) (Fig. 1a). The Zn2+ performs a hexahedral six coordinated mode, with two axial sites coordinating to two carbonyl groups, and the other four equatorial sites coordinating to four imidazole groups. Each dimto links three Zn2+, and the two imidazole groups in one ligand have the opposite coordination direction of 180°, also forming 90° with the coordination direction of the carbonyl group (Fig. S5). In view of the ligand-to-axial approach 29, Zn2+ ions connect the imidazole groups of dimto forming sql 2D layers, which are further pillared by the carbonyl groups to afford the 3D porous frameworks with considerable void spaces of 62.3 % (void spaces of LIFM-13(Mg) and LIFM-13(Co) are 62.4 % and 62.7 %, respectively), as calculated by PLATON (Fig. S6a-c) 30. In view of topology, the Zn2+ ions serve as 6-connected nodes and the T-shaped ligands serve as 3-connected nodes, thus generating a (3,6)-connected framework of ant topology, calculated by Topos 4.0 (Fig. S6d) 31-32. The purity of the as-synthesized compounds was confirmed by PXRD studies (Fig. 2 and Fig. S7). These similar patterns also indicated that those four MOFs have a very similar structure except LIFM-13(Mg). The thermal stability of MOFs is evaluated by thermal
Figure 2. PXRD patterns of LIFM-13 series frameworks.
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Crystal Growth & Design
gravimetric analyses (TGA) (Fig. S8). The framework stability is tested by VT-PXRD measurements under N2 atmosphere (Fig. S9-S12). The chemical decomposition process is related to bond breaking and the corresponding temperature can be estimated by TGA measurements, while the framework collapse is correlated with the crystallinity of the solids that can be characterized by VT-PXRD. As seen from Fig. S7, TGA curves of the as-prepared crystals show considerable weight loss before 200 C, demonstrating that all MOFs contain large amounts of solvents within their pores. After desolvation, the TGA measurements of the dried samples indicate that the chemical decomposition of Ni-, Co-, and Zn-based MOFs mainly occur in a similar temperature range of 300-350 C, which is significantly lower than that of Mg-based MOFs (450-500 C). In contrast, the analysis of the VT-PXRD patterns collected for the four MOFs lead to another trend regarding their framework collapse (Fig. S9-S12). LIFM-13(Mg), LIFM-13(Co), and LIFM-13(Zn) retains their crystallinity up to 120 C. LIFM-10(Ni) shows a much better stability of up to 400 C. These results suggest that Mg-, Co-, and Zn-based MOFs lose their porosity after the solvents are removed. Nevertheless, LIFM13(Ni) can maintain its crystallinity at a slightly higher temperature. This confirms that the Ni-based MOFs are more thermally stable when compared with the Mg-, Co, and Zn-based analogues. Based on the Pearson’s hard/soft acid/base (HSAB) principle, the in-situ obtained ligand dimto offers both coordination sites of imidazole group as a soft Lewis base and oxygen group as a hard lewis base, thus We hypothesize that Ni2+ which is either too hard acid like Mg2+ or too soft acid like Co2+/Zn2+, acts as a borderline acid that fits well with the hybrid coordination environment, giving LIFM13(Ni) the highest stability among its analogues. After all, this whole set of observations unveils that the thermal stability of a MOF may be different in regard to the chemical decomposition and the framework collapse. Successful activation of these MOFs upon removal of the guest solvents helps us to carry out their gas adsorption properties. As shown in Fig. 3, the Ar(87 K) adsorption of LIFM-13(Zn) apparently performs a multistep isotherm which is often elucidated as gate open/closed phenomenon, and related with structural flexibility 33-34. The first step of the isotherm exists at P/P0
