High Volumetric Hydrogen Adsorption in a Porous Anthracene-Decorated Metal–Organic Framework

We report an unprecedented ligand-based binding domain for D2 within a porous metal–organic framework (MOF) material as confirmed by neutron powder diffraction studies of D2-loaded MFM-132a. A tight pocket of 6 Å diameter is formed by the close packing of three anthracene panels, and it is here rather than the open metal sites where D2 binds preferentially. As a result, MFM-132a shows exceptional volumetric hydrogen adsorption (52 g L–1 at 60 bar and 77 K) and the highest density of adsorbed H2 within its pores among all the porous materials reported to date under the same conditions. This work points to a new direction for H2 storage in porous materials using polyaromatic ligand-based sites.


■ INTRODUCTION
Hydrogen (H 2 ) is a promising energy carrier for mobile applications due to its abundance and the absence of CO 2 output at the point of use. 1 Although H 2 has a high energy density by mass (120 MJ kg −1 vs 44.5 MJ kg −1 for gasoline), it has low volumetric energy density because of its volatility at ambient conditions. 2 As a result, a safe and cost-effective storage system with high volumetric capacity is needed to enable the on-board use of H 2 . Compared to the state-of-theart storage techniques based upon liquefaction or high-pressure compression, adsorptive H 2 storage has attracted significant attention. 2−4 Among the wide range of porous materials, such as porous carbons, 5 zeolites, 6 and porous polymers, 7 metal− organic frameworks (MOFs) 8 show great promise for H 2 storage because of their large surface area and tunable and well-defined crystal structures. H 2 adsorption in MOFs follows a physisorption mechanism, where H 2 molecules bind to MOF surfaces through weak dispersive interactions. Thus, MOFs with high surface area usually show high H 2 gravimetric adsorption capacities, albeit at low temperatures. Strategies for enhancing the binding energy between the framework and H 2 are still being actively pursued. These include the synthesis of frameworks containing open metal sites 9 and framework catenation/interpenetration for creating small pore size for enhancing the overlapping potential from opposite pore walls for H 2 . 10−12 However, the former is unable to enhance the H 2 adsorption overall because the open metal sites reach saturation quickly at low surface coverage, while the latter often suffers from unpredictable control and significant reduction in the material porosity, and hence reduced total uptake capacity.
Large conjugated aromatic systems can not only provide strong affinity for guest molecules based on specific van der Waals interactions 13 but also significantly enhance the rigidity and stability of the resulting materials. 14 Anthracene has been widely used as a versatile molecular building unit for the construction of a variety of molecular capsules, cages, tubes, and ring structures. 15 The flat, panel-like aromatic system results in cages having enclosed shells, allowing the encapsulated guest molecules to be segregated from the external environment, thus providing unique potential for the encapsulation of guest molecules in these supramolecular hosts. 16 Here, we report an investigation of the adsorption and molecular binding of H 2 in an anthracene-decorated (3, 24)connected framework (denoted MFM-132). Significantly, desolvated MFM-132a exhibits exceptionally high volumetric H 2 capacity of 52 g L −1 at 60 bar and 77 K, among the highest values for porous materials reported under the same conditions. Neutron powder diffraction (NPD) studies of D 2 -loaded MFM-132a reveal that a cleft formed by the anthracene moieties within the pores of MFM-132a is directly responsible for its excellent H 2 adsorption performance by creating specific ligand-based binding domains.

■ RESULTS AND DISCUSSION
MFM-132 was prepared by the solvothermal reaction of H 6 BTAT [H 6 BTAT = 5,5′,5″-(benzene-1,3,5-triyltris-(anthracene-10,9-diyl))triisophthalic acid; Figure 1b] and Cu(NO 3 ) 2 ·2.5H 2 O in DMF and isolated as blue-green microcrystalline powder. 17 As-synthesized MFM-132 was solvent-exchanged with acetone and then subjected to thermal evacuation under dynamic vacuum at 100°C for 16 h to afford the desolvated material MFM-132a as blue-purple powder. Rietveld refinement of the NPD data confirms that MFM-132a has a (3, 24)-connected 18−20 open structure, comprising four types of metal−organic cages (denoted A, B, C, and D, Figure  2). Cage A (13 Å in diameter), constructed by 12 {Cu 2 } paddlewheels and 24 isophthalates from 24 independent BTAT 6− units, is heavily shielded by 24 anthracene units from different linkers on the exterior of this cage. In addition, cage A also contains 12 open Cu(II) sites internally. With a large number of anthracene panels protruding into the central voids, cages B and C in MFM-132a show pore sizes of 6.5 and 13 Å (measured by the diameter of the largest sphere which can be fitted into the cavity), respectively. In the void between fused cages B and C exists the fourth cage, cage D, which is a spherical compartment of 10 Å in diameter and has small apertures of 5 Å × 5 Å, enclosed by eight anthracene panels from four linkers and two {Cu 2 } paddlewheels. Thus, the cages in MFM-132a show a rich combination of micropores in the range 6−13 Å with varying functionalities, affording a unique pore environment for binding of small guest molecules.
The N 2 isotherm at 77 K for MFM-132a reveals a BET surface area of 2466 m 2 g −1 and total pore volume of 1.06 cm 3 g −1 . 17 H 2 sorption isotherms for MFM-132a were recorded using both gravimetric and volumetric methods at 77 K ( Figure 3). MFM-132a shows a very high H 2 uptake of 2.83 wt % at 1 bar and 77 K ( Table 1). The high H 2 adsorption at low pressure in MFM-132a is comparable to the high-performing Cu(II)-based MOFs UTSA-20 21 and SUT-5. 22 This is due to the combination of the open Cu(II) sites and the geometrically hindered pores with anthracene units in MFM-132a which can provide strong overlapping potential to H 2 molecules. 26 Virial analysis 27 was applied to calculate the coverage-dependent adsorption enthalpies based on the H 2 adsorption isotherms at 77 and 87 K (see Supporting Information). The value of Q st at   the low loading limit is estimated to be 6.8 kJ mol −1 , gradually decreasing to 4.6 kJ mol −1 as the H 2 coverage increases to 2.8 wt %. The high-pressure measurement up to 60 bar revealed that the saturated excess H 2 uptake reached 63 mg g −1 at 35 bar and 77 K, and the total uptake amounts to 80 mg g −1 at 60 bar and 77 K. Although these values are lower than those for MFM-112a (saturated excess uptake of 76.1 mg g −1 at 45 bar and 77 K; total uptake of 108 mg g −1 at 60 bar and 77 K) due to the reduced surface area and pore volume in MFM-132a, the density of the adsorbed H 2 in MFM-132a reaches a remarkable value of 0.075 g cm −3 at 60 bar, the highest observed in a porous material under the same conditions (Table 1). This is because of the geometrically restricted pore system in MFM-132a, which can enable efficient packing of H 2 molecules within the pores.
Volumetric capacity is of critical importance for a given storage system if it is to find practical applications. As shown in Figure 3, although MFM-112a 18 shows comparably higher gravimetric H 2 uptakes than MFM-132a at high pressures (>10 bar), MFM-132a exhibits higher volumetric total H 2 uptake of 52 g L −1 at 60 bar and 77 K than that of MFM-112a (49 g L −1 under same conditions). It is worth noting that, through this report, the volumetric uptake is derived from the bulk material density on the basis of single crystals, and when the efficiency  of powder packing is considered, the uptakes will be reduced accordingly. Significantly, the total volumetric H 2 uptake for MFM-132a is comparable with the best-behaving MOFs to date such as MOF-177, 23,24 anhydrous MOF-5, 25 and NU-100 19b (Table 1).
In order to evaluate the favored adsorption sites within the cages, in situ NPD studies on D 2 -loaded MFM-132a at low surface coverage of 0.5 and 1.0 D 2 /Cu (1.0 D 2 /Cu corresponds to 0.47 wt % of H 2 uptake in MFM-132a) have been performed. Le Bail analyses of the NPD patterns indicated that the unit cell parameters of MFM-132a at different gas loadings remain essentially constant, confirming the structural rigidity of MFM-132a. The positions of adsorbed D 2 molecules were identified in the difference neutron scattering Fourier maps derived from Rietveld refinements of the NPD patterns for both the bare and gas-loaded samples.
At 0.5 D 2 /Cu loading, the Rietveld analysis revealed three different D 2 binding sites ( Figure 4). Surprisingly, instead of the open Cu(II) site, the first adsorption site for D 2 (A1) was located within the central pocket created by the triangular {(Cu 2 ) 3 (isophthalate) 3 } window and three anthracene units. Site A1 is 3.260(11) Å away from the closest H atoms on the anthracene units at the bottom of the pocket and is in close contact with the surrounding isophthalate rings at a distance of 3.863(10) Å. This result is in contrast to all other NPD studies on MOFs, 28−33 which uniformly find that the open metal site is the strongest binding site for D 2 , with the exception of the rare earth material [Y(BTC)] (BTC 3− = 1,3,5-benzenetricarboxylate). 34 The next site to be occupied, A2, is the open Cu(II) site (Cu A ), lying inside the cuboctahedral cage with a close D 2 (centroid)−Cu A distance of 2.07(2) Å. Interestingly, this is the shortest D 2 −Cu distance observed so far for D 2 -loaded MOFs, 35 indicating the presence of a binding of H 2 molecules to the open Cu(II) sites. The other Cu(II) ion of the same paddlewheel unit (Cu B ) is identified as the third adsorption site A3, located outside the cuboctahedral cage and pointing toward the center of the cavity of cage D. Site A3 shows a longer D 2 −Cu distance of 2.329(15) Å, indicating a weaker D 2 −Cu binding interaction on the exterior of the cuboctahedral cage. Thus, the Rietveld analysis clearly indicates a difference for D 2 binding between the two Cu(II) sites of the same {Cu 2 } unit. This D 2 binding preference for the Cu(II) ions inside cage A in MFM-132a is consistent with the results of the NPD studies on D 2 -loaded MFM-112a with 0.5 D 2 /Cu loading, 29 indicating that the specific cuboctahedral configuration of Cu(II) cations can indeed modulate the binding interaction between the open metal site and the D 2 molecule. Incorporation of bulky anthracene groups in MFM-132a affords an enclosed pocket that can accommodate binding site A1, and creates a strong overlapping potential, thereby generating an unprecedented affinity for D 2 that is stronger than at the open Cu(II) sites.
Analysis of the site occupancies from the Rietveld refinements revealed that 54% of site A1 was occupied at the first loading. The other two sites A2 and A3 showed lower occupancies (0.2 and 0.29, respectively) than site A1, confirming the strong affinity for D 2 at site A1. When the loading is increased to 1.0 D 2 /Cu, additional D 2 molecules were distributed across the three adsorption sites, and a fourth D 2 binding site A4 (accounting for only 5% of the total adsorbed D 2 ) was located in the tight pocket of diameter 6.5 Å surrounded by 12 anthracene rings at the center of cage B.
The difference in D 2 binding between the two Cu(II) sites from the same {Cu 2 } paddlewheel was still observed at the higher loading. This is in contrast to the NPD study of MFM-112a with a D 2 loading of 1.0 D 2 /Cu, where two Cu(II) sites become equivalent in terms of D2 binding. Thus at higher loading of D 2 in MFM-132a, the Cu A site shows a D 2 −Cu A distance of 2.302(15) Å which compares with a D 2 −Cu B distance of 2.376(12) Å at 1.0 D 2 /Cu loading. This result further confirms the preferential binding with D 2 for the Cu(II) ions inside the cuboctahedral cage in MFM-132a.

■ CONCLUSIONS
In summary, we report here the adsorption properties of H 2 in an anthracene-functionalized (3, 24)-connected MOF material, MFM-132a, which is assembled by four different types of metal−ligand coordination cages. Despite the presence of a large number of bulky anthracene panels in the framework, MFM-132a still shows moderately high porosity with BET surface area of 2466 m 2 g −1 and pore volume of 1.06 cm 3 g −1 , which is attributed to the intrinsic advantage of the (3, 24)connected network constructed by the tessellation of individual metal−organic polyhedra possessing internal voids. MFM-132a shows remarkable H 2 adsorption over an extended pressure range, and an exceptionally high total volumetric H 2 capacity of 52 g L −1 is recorded at 60 bar and 77 K, among the highest values reported for MOFs to date under the same conditions. The high density of adsorbed H 2 confirms that functionalization with large aromatic panels can efficiently pack H 2 within the pores. NPD studies on D 2 -loaded MFM-132a confirm the first binding site to be within a tight pocket of 6 Å created by the triangular {(Cu 2 ) 3 (isophthalate) 3 } window and three anthracene units. Interestingly, this site has significantly higher D 2 occupancy than the subsequently occupied open Cu(II) sites, confirming that the tightly enclosed pocket created solely by organic units can generate binding affinity to D 2 . The two Cu(II) sites in the same {Cu 2 } paddlewheel show discrimination for D 2 binding due to the different chemical environment of these two open metal sites. The Cu(II) ions inside the cuboctahedral cage show a very short D 2 (centroid)···Cu distance of 2.07(2) Å. The strategy reported here, based upon the introduction of bulky functional groups on the organic ligand, provides a controllable method to create materials with specific enclosed ligand binding sites for gaseous substrates.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01607.
Synthesis, gas adsorption measurements, powder X-ray diffraction, neutron powder diffraction patterns, and Rietveld refinement results (PDF)