Ultrahigh Hydrogen Uptake in an Interpenetrated Zn 4 O-Based Metal–Organic Framework

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Mar 2022Ultrahigh Hydrogen Uptake in an Interpenetrated Zn4O-Based Metal–Organic Framework Fu-Gang Li, Caiping Liu, Daqiang Yuan, Fangna Dai, Rongming Wang, Zhikun Wang, Xiaoqing Lu and Daofeng Sun Fu-Gang Li College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Caiping Liu Department State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Daqiang Yuan Department State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author , Fangna Dai College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Rongming Wang College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Zhikun Wang College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author , Xiaoqing Lu College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author and Daofeng Sun *Corresponding author: E-mail Address: [email protected] College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000738 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail As a highly promising candidate for hydrogen storage, crucial to vehicles powered by fuel cells, metal–organic frameworks (MOFs) have attracted the attention of chemists in recent decades. H2 uptake in an MOF is influenced by many factors such as pore size, ligand functionalization, and open metal sites. The synergistic effect of these factors can significantly enhance the H2 uptake in an MOF. Herein, we report a twofold interpenetrated MOF ( UPC-501) based on a Zn4O(COO)6 secondary building unit with the H2 uptake of 14.8 mmol g−1 (2.96 wt %) at 77 K and 0.1 MPa. This uptake is the highest among all the reported porous Zn-based MOF materials. Both experimental and theoretical results confirm that the reduced pore size derived from twofold interpenetration and the imidazole-functionalized ligand are responsible for the extremely high H2 uptake of UPC-501. Download figure Download PowerPoint Introduction Safe and energy-efficient hydrogen storage is crucial in the large-scale utilization of hydrogen energy. Traditional storage techniques such as high pressure or cryogenic tanks are either energy-intensive or ineffective. Therefore, development of new techniques for hydrogen storage with high volumetric and gravimetric densities is a great challenge.1 Since 2014, porous material-based hydrogen storage has received much attention due to its safety and efficiency when compared with traditional storage methods.2–5 Among all the porous materials, metal–organic frameworks (MOFs), as a new type of adsorbent, have been widely studied owing to their high surface area and structural diversity.6–8 Since the well-known MOF-5 based on Zn4O(COO)6, a secondary building unit (SBU) with a high surface area, was reported in 1999, the study of porous MOFs and their applications to gas storage and separation have been increasing steadily.9–15 With respect to hydrogen storage in an MOF, one of the most important factors that influence the H2 uptake is the adsorbate–adsorbent interaction.16 Based on current hydrogen storage research, the most facile and effective strategy to improve the adsorbate–adsorbent interaction is the thermal release of coordinated solvates from SBUs, such as the well-known paddlewheel SBU, to generate open metal sites.17–20 In the past decade, many porous copper MOFs based on paddlewheel SBUs have been explored.21–25 These porous MOFs normally exhibit high H2 uptake at 77 K and 0.1 MPa due to the high affinity of open metal sites for H2 molecules. An extremely high H2 uptake of 15.1 mmol g−1 (3.05 wt %) at 77 K and 0.1 MPa had been reported in 2008 in a microporous Cu-MOF with cuboctahedral cages (PCN-12, PCN = porous coordination network).26 The highest H2 uptake to date, reported in 2011, 15.2 mmol g−1 (3.07 wt %) at 77 K and 0.1 MPa was observed in a Cu-MOF of [Cu(Me-4py-trz-ia)].27 Simulations indicate there is no direct sorption of H2 molecules onto open metal sites in [Cu(Me-4py-trz-ia)] due to the low partial positive charges of Cu2+ ion in the framework; however, the narrow channels and the functional groups in [Cu(Me-4py-trz-ia)] are responsible for the ultrahigh H2 uptake.28 In contrast, although most porous MOFs based on Zn4O(COO)6 tetrahedral units exhibit high permanent porosity and thermal stability,29–32 the highest H2 uptake of Zn4O-based MOFs under the same conditions is 9.92 mmol g−1 (2.0 wt %).33 Therefore, construction of Zn4O-based porous MOFs with enhanced H2 uptake remains a great challenge. In addition to the open metal sites mentioned earlier, pore size and functionalization of organic ligands all have significant impacts on the H2 uptake. Theoretical predictions indicate that the ideal pore size with maximal attraction of H2 molecules to carbon materials is ∼6 Å at low pressure.15 Similar results have also been experimentally demonstrated by our group in two porous MOFs materials with the same fsc topology.34 In contrast, it has been observed in many porous MOFs materials21,32,35 that interpenetration of the framework can significantly reduce the pore size of an MOF, thus enhancing H2 uptake at low pressure and 77 K. Consequently, it is reasonable to suppose that porous MOF materials with enhanced H2 uptake based on SBUs, such as the Zn4O(COO)6 SBU, but lacking open metal sites can be achieved by accurately controlling the pore size through framework interpenetration. Herein, we report a twofold interpenetrated Zn-MOF ( UPC-501, UPC = China University of Petroleum (East China)) based on Zn4O(COO)6 SBUs and an imidazole-functionalized ligand, 4,4′,4″-(1H-imidazole-2,4,5-triyl)tribenzoic acid (H3ITTA; Figure 1a and Supporting Information Figures S1–S4). UPC-501 exhibits an ultrahigh H2 uptake (14.8 mmol g−1, 2.96 wt %) at 77 K and 0.1 MPa, which is slightly lower than those of [Cu(Me-4py-trz-ia)] (15.2 mmol g−1, 3.07 wt %) and PCN-12 (15.1 mmol g−1, 3.05 wt %), but is the highest among all the reported porous Zn-based MOF materials. This ultrahigh H2 uptake of UPC-501 is derived from the synergistic effects of pore size and the imidazole-functionalized ligand. Figure 1 | (a) The organic ligand H3ITTA. Gray, carbon; blue, nitrogen; red, oxygen; white, hydrogen. (b) The Zn4O(COO)6 SBU. (c) View of the 3-connected ITTA3− ligand with three Zn4O(COO)6 SBUs and the 6-connected Zn4O(COO)6 SBU with six neighboring ITTA3− ligands. (d) Topological representations showing the twofold interpenetration of two adjacent rtl nets viewed along the a axis. (e) The 1D channel viewed along the a axis. Download figure Download PowerPoint Results and Discussion The imidazole-functionalized linker (H3ITTA, Supporting Information Scheme S1) was synthesized with a condensation reaction between benzoin and methyl 4-formylbenzoate in acetic acid under reflux condition. UPC-501 was synthesized by solvothermal reaction of Zn(NO3)2·6H2O, and H3ITTA in a solvent mixture of dimethylformamide (DMF), H2O, and ethanol (EtOH), to which 25 μL of acetic acid was added. The structure of UPC-501 was characterized by single-crystal X-ray diffraction ( Supporting Information Table S1), and the formula of [Zn4O(ITTA)2]2·15DMF·10H2O was further confirmed by elemental and thermogravimetric analysis ( Supporting Information Figure S5). The powder X-ray diffraction patterns of the as-synthesized crystals match well with the simulated pattern based on single-crystal data, indicating the purity of the sample ( Supporting Information Figure S6). UPC-501 is a three-dimensional (3D) twofold interpenetrated porous framework based on Zn4O(COO)6 SBUs (Figure 1b). All the carboxylate groups of H3ITTA are deprotonated in the solvothermal reaction. Each ITTA3− ligand connects three Zn4O(COO)6 SBUs and each Zn4O(COO)6 SBU attaches to six ITTA3− ligands (Figure 1c), resulting in a 3,6-connected rtl network with a large one-dimensional (1D) channel along the a axis. With these large channels, two such 3,6-connected nets interpenetrate each other, narrowing the channel and further stabilizing the framework (Figure 1d). The pore limiting diameter and largest cavity diameter for UPC-501 were calculated using Zeo++ software,36 and it shows that interpenetration results in an effective aperture of the final channel in the range from 5.2 to 7.8 Å ( Supporting Information Figure S9). The Zn4O(COO)6 SBU in one net is located precisely in the center of the rhombus channel of another interpenetrating net, and the bonding interaction between the two N atoms of ITTA3− in different nets further stabilizes the interpenetration. Despite this interpenetration, UPC-501 remains porous (Figure 1e) with the calculated total solvent-accessible volume being approximately 62.5% using the probe radius of 1.2 Å. Thermogravimetric analysis reveals that UPC-501 has high thermal stability, being stable up to 400 °C. The permanent porosity of UPC-501 was confirmed by gas adsorption measurements. N2 adsorption measurement reveals that activated UPC-501 exhibits a Type I adsorption isotherm (Figure 2) with the Brunauer–Emmett–Teller (BET) and Langmuir surface areas of 2394 and 2553 m2 g−1 ( Supporting Information Figures S7 and S8), respectively. The total pore volume of UPC-501 is 0.935 cm3 g−1 with a micropore volume of 0.846 cm3 g−1, which is much higher than other MOFs such as IRMOF-2 (IRMOF = isoreticular MOF),37 IRMOF-9,38 and PCN-13.39 The pore size distributions (PSD) of UPC-501 were calculated using nonlocal density functional theory (NL-DFT) based on N2 isotherms, which shows the main PSD peaks are in the range of 6.5 to 8.5 Å ( Supporting Information Figure S9). Figure 2 | Nitrogen adsorption isotherm at 77 K. ADS, adsorption; DES, desorption Download figure Download PowerPoint Experimental and theoretical studies both show that the MOF with an aperture of ∼6 Å possesses the maximal attraction to H2 at the low-pressure range, and it can be speculated that UPC-501 may exhibit high H2 uptake due to its narrow PSD. The H2 adsorption isotherms for UPC-501 were recorded at 77 or 87 K and 0.1 MPa. As shown in Figure 3a, UPC-501 can adsorb 14.8 mmol g−1 of H2 at 0.1 MPa and 77 K, corresponding to a volumetric H2 uptake of 25.2 g L−1. The H2 adsorption of UPC-501 is significantly higher than other well-known Zn-based MOF materials (Table 1) such as ZIF-8 (6.35 mmol g−1, ZIF = zeolitic imidazolate framework),40 MOF-5 (6.60 mmol g−1),41 and SNU-4 (10.35 mmol g−1, SNU = Seoul National University),42 under the same conditions. To the best of our knowledge, UPC-501 shows the highest H2 uptake among all the reported Zn-based porous MOF materials. Meanwhile, the fraction of the pore volume filled by liquid H2 (ρ = 0.0708 g cm−3) at 0.1 MPa and 77 K for UPC-501 is 52%. This reflects that the H2 adsorption in UPC-501 is not exhausted and the uptake may continue to increase if the pressure increases. Upon increasing the pressure, the excess and total H2 adsorptions gradually increased to 29.5 and 35.5 mmol g−1, respectively, at 5.0 MPa ( Supporting Information Figure S10). The corresponding excess volumetric uptake of UPC-501 is as high as 48.6 g L−1 at 5.0 MPa. This remarkable volumetric H2 uptake, albeit at cryoscopic temperatures, compares favorably to the 2020 Department of Energy (DOE) target of 40 g L−143 and is slightly higher than those of other high hydrogen uptake MOFs ( Supporting Information Table S2). This relatively high volumetric storage density can be attributed to the higher density of UPC-501 compared with superlight MOFs such as MOF-5 and MOF-177.31 Figure 3 | (a) The H2 adsorption capacity of UPC-501 at 77 K (red curve) and 87 K (blue curve). (b) The Qst of UPC-501 for H2. ADS, adsorption; DES, desorption. Download figure Download PowerPoint Table 1 | BET Specific Surface Area, Pore Volume, H2 Uptake, and the Amount of H2 Adsorbed at 0.1 MPa and 77 K for Selected Zn-Based MOFs BET (m2 g−1) Pore Volume (cm3 g−1) H2 Uptake (wt %) n, H2 (mmol g−1) UPC-501 2394 0.935 2.96 14.8 MOF-5 (interpenetrated) 1130a NAb 2.00 10.0 MOF-5 3362 NA 1.32 6.60 IRMOF-9 1904 0.9 1.17 5.85 IRMOF-2 1722 0.88 1.21 6.05 MOF-177 4526 NA 1.25 6.25 IRMOF-6 2476 1.14 1.48 7.40 IRMOF-3 2446 1.07 1.42 7.10 IRMOF-8 1466 0.45 1.50 7.50 IRMOF-13 1551 0.73 1.73 8.65 SNU-77H 3670 1.52 1.79 8.95 IRMOF-20 3409 1.53 1.35 6.75 SNU-4 1463 0.53 2.07 10.35 ZIF-8 1630 0.64 1.27 6.35 MOF-74-Zn 783 0.39 1.77 8.85 aLangmuir surface area. bNot accessible. To better understand the H2 adsorption properties of UPC-501, the isosteric heat of adsorption (Qst) for H2 was calculated.36 The Qst of UPC-501 is 5.29 kJ mol−1 at lower coverage (Figure 3b), which can be compared with those of other Zn4O-based MOFs.29 The Qst is almost constant as the H2 loading increases, indicating the homogeneity of the H2 adsorption sites in UPC-501. The relatively stable and low Qst values reflect the nature of the lack of open metal sites in UPC-501; however, it fails to reasonably explain the remarkably high H2 uptake. There are two major factors including suitable pore size and the weak interactions between H2 molecules and functional groups of linkers that contribute to the high H2 uptake of UPC-501. Initially, UPC-501 possesses twofold interpenetrating nets, and previous reports show that interpenetration can enhance H2 uptake at 77 K in some MOFs.32 To further explore the possible H2 sorption site, the adsorption isotherms of H2 for UPC-501 were produced by means of Grand Canonical Monte Carlo (GCMC) simulations using the RASPA2 package.44 As shown in Supporting Information Figure S9, the simulated excess H2 sorption isotherms are compared with the experimental data. During the simulation, the fourth-order Feynman–Hibbs quantum correction is considered, and the simulated H2 isotherm matches the experimental data well ( Supporting Information Figure S11). However, exclusion of the Feynman–Hibbs correction leads to a situation in which the simulated isotherm will overestimate the experimental isotherm. Based on the GCMC results ( Supporting Information Figure S12), the H2 adsorption density plot is produced in Figure 4a, and shows that there are two types of adsorption sites in UPC-501 for H2. Furthermore, DFT calculations were used to identify the H2 adsorption sites and to estimate the strength of the interactions between the adsorbed H2 molecules and UPC-501. The results of the calculation show that site A is around the edge of the Zn4O(COO)6 SBU (Figure 4b) and the adsorbed H2 molecule is close to the two oxygen atoms from two nearby carboxylate groups. The H−H···O distances are 2.97 and 3.31 Å, respectively. As shown in Figure 4c, site B is in the middle of two interpenetrated nets, where one H atom of the adsorbed H2 molecule is close to the oxygen atom of a carboxyl group with an H···O distance of 3.06 Å and the other one is close to the nitrogen atom of the ITTA3− ligand with an H···N distance of 3.01 Å. Using the DFT calculations, the static binding energy of H2 molecules at 0 K is −11.66 and −11.05 kJ mol−1 for the two binding sites, respectively. Obviously, the weak interactions between the H2 molecules and oxygen atom and nitrogen from the atom ITTA3− ligand are responsible for the enhancement of H2 uptake. Figure 4 | (a) The simulated adsorption density plot of H2 absorbed in UPC-501 at 77 K and 0.001 MPa. The DFT optimized binding positions site A (b) and site B (c) of H2 molecules in UPC-501. Download figure Download PowerPoint Conclusion We report an imidazole-functionalized MOF based on Zn4O(COO)6 SBU, exhibiting an ultrahigh H2 uptake (14.8 mmol g−1, 2.96 wt %), which is highest in all reported Zn-based MOFs, resulting from the existence of narrow pores derived from interpenetration as well as the ligand functionality. Our work presented here provides an intriguing strategy through linker design for the development of new MOF materials with high hydrogen storage capability. Supporting Information Supporting Information is available and includes the detailed synthetic method and characterizations of the linker, thermogravimetric analysis, powder X-ray diffraction, crystal data, high-pressure uptake, and simulation method. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the NSFC (grant no. 21875285), Taishan Scholar Foundation (grant no. ts201511019), Key Research and Development Projects of Shandong Province (grant no. 2019JZZY010331), the Strategic Priority Research Program of CAS (grant no. XDB20000000), the Key Research Program of Frontier Sciences, CAS (grant no. QYZDB-SSW-SLH019), and the Fundamental Research Funds for the Central Universities (grant no. 18CX02047A).