Optimal Binding of Acetylene to a Nitro-Decorated Metal-Organic Framework.

We report the first example of crystallographic observation of acetylene binding to -NO2 groups in a metal-organic framework (MOF). Functionalization of MFM-102 with -NO2 groups on phenyl groups leads to a 15% reduction in BET surface area in MFM-102-NO2. However, this is coupled to a 28% increase in acetylene adsorption to 192 cm3 g-1 at 298 K and 1 bar, comparable to other leading porous materials. Neutron diffraction and inelastic scattering experiments reveal the role of -NO2 groups, in cooperation with open metal sites, in the binding of acetylene in MFM-102-NO2.

Acetylene is an important chemical for the production of polymers and other advanced materials.[1] However, it is highly explosive and cannot be compressed beyond 2 atm at room temperature, thus making its storage and transport a challenge. Acetylene storage is achieved currently by dissolving acetylene gas in acetone under high pressure (equivalent to ∼10 atm) in a heavy-duty tank filled with porous materials, such as firebrick.[2] This enables the transport of acetylene in liquid form. However, such processes are not only costly but also limit the purity of stored acetylene to ∼95%, making it unsuitable for many applications unless a secondary purification process is applied. Porous metal–organic frameworks (MOFs) are emerging sorbents for a variety of gases owing to their high porosity, well-defined pore size, their design flexibility and ability to incorporate active binding sites.[3] Recently, there has been an increasing interest in the study of MOFs for acetylene storage,[4] and the introduction of organic functional groups is an efficient approach to increase the gas binding within pores. MOFs incorporating −OH,[5] −NH2,[6] −CONH–,[7]–C≡C–,[8] R—CO—R,[9] −R,[10] −OR,[11] pyridine-,[12] pyrimidine-,[13] pyrazine-,[13] pyridazine-,[13] and naphthalene[14] groups have been tested for acetylene adsorption. However, molecular insights into the precise role of these moieties on the binding of acetylene is largely lacking, thus restricting the design of improved materials.

Acetylene is relatively electron-rich due to its triple bond, and we argued that MOFs incorporating electro-positive or electron withdrawing groups would be a promising approach to facilitate C2H2 binding. The nitro group (−NO2) is one of the most powerful electron-withdrawing groups. However, the direct visualization of gas binding to −NO2 groups in MOFs has not been reported to date and adsorption of acetylene in NO2-decorated MOFs remains unexplored.[15] Here, we report the study of acetylene adsorption in a family of four iso-structural MOFs (the MFM-102 series) bearing different functional groups, including nitro, amine, and alkane groups. The selection of these functional groups gives a wide coverage in terms of their electron donating and withdrawing power. We found that functionalization of the parent MFM-102 with NO2 groups leads to a 16% reduction in the BET surface area in MFM-102-NO2, but a 28% improvement in acetylene adsorption to 192 cm3 g–1 at 298 K and 1 bar, comparable to the leading materials for acetylene storage. The binding domains for adsorbed acetylene molecules in MFM-102-NO2 have been studied by in situ neutron powder diffraction and inelastic scattering experiments. In comparison, other functional groups (i.e., amine and alkane groups in MFM-102-NH2 and MFM-111, respectively) have neutral or detrimental effects on acetylene adsorption compared to MFM-102-NO2. Importantly, we describe the first example of observation of direct binding of acetylene molecules to the −NO2 groups in a MOF material, leading to the optimal adsorption of this substrate.

H4L1 and H4L2 were synthesized using our previously reported methods.[16] Introduction of nitro and amine groups to H4L1 yielded H4L3 and H4L4, respectively. Single crystals of MFM-102, MFM-111, MFM-102-NO2, and MFM-102-NH2 were synthesized from Cu(NO3)2·6H2O and the corresponding ligand in DMF or DMF/DMSO under solvothermal conditions. Single crystal X-ray diffraction analysis indicated that all these MOFs are iso-structural, crystallize in the hexagonal space group R3̅m, and adopt a NbO-type structure (Figure ).

Figure 1

Views of organic linkers and the crystal structure for MFM-102, MFM-111, MFM-102-NO2, and MFM-102-NH2. The BET surface area for each MOF is shown at the bottom (C, dark gray; O, red; N, blue; Cu, turquoise; all coordinated waters and hydrogen atoms are omitted for clarity, except for the hydrogen atoms on the −NH2 group).

The crystal structure of MFM-102-NO2 is described here in detail. Two Cu(II) ions are bridged by four carboxylate groups to form a [Cu2(OOCR)4(OH2)2] paddlewheel node {Cu2}. This serves as a 4-connected node that is further linked to other 4-connected nodes to construct a 3D NbO-type open structure. All these MOFs are constructed by the alternative packing of 2 types of metal–ligand cages (A and B) (Figure ). Cage A, constructed by six linkers and six {Cu2} paddlewheels, has a cylindrical shape with a diameter of 14 Å and length of 19 Å. Cage B (length of 32 Å) has a shuttle-shape with 12 {Cu2} paddlewheels residing at the vertices and 6 ligands on the faces. It is noteworthy that the cages in MFM-111, MFM-102-NH2, and MFM-102-NO2 are decorated with alkane, −NH2 and −NO2 groups, respectively, pointing into both cages, thus providing additional binding sites for gas molecules. Phase purity of each complex has been confirmed by PXRD data (see SI). The coordinated water molecules and solvent molecules in the pores can be removed under heating to generate open Cu(II) sites in the desolvated materials.

N2 isotherms at 77 K confirm that desolvated MFM-102, MFM-111, MFM-102-NH2, and MFM-102-NO2 show BET surface areas of 3412, 2930, 2928, and 2893 m2 g–1, respectively (Figures , 2) with introduction of functional groups leading to reduction in porosity. These values are higher than other reported MOFs with NbO-topology such as NJU-Bai-17 (2423 m2 g–1),[17] UTSA-88 (1771 m2 g–1),[18] and ZJU-7 (2209 m2 g–1).[19]

Figure 2

Adsorption isotherms for desolvated MFM-102, MFM-111, MFM-102-NO2, and MFM-102-NH2. (a) N2 at 77 K; (b) C2H2 at 298 K; (c) CH4 at 298 K. Solid and open symbols represent adsorption and desorption, respectively.

Uptakes of C2H2 at 273 K and 1 bar were recorded as 292, 261, 251, and 241 cm3 g–1 for MFM-102-NO2, MFM-102-NH2, MFM-102, and MFM-111, respectively (Figure S8). Significantly, the C2H2 uptake of MFM-102-NO2 (292 cm3 g–1) is among the best-performing MOFs under the same conditions (Table S3), such as NJU-Bai17 (295 cm3 g–1),[17] MFM-188a (297 cm3 g–1),[7] and FJI-H8 (277 cm3 g–1).[20] Interestingly, although the introduction of NO2-groups to MFM-102 leads to a 15% reduction of BET surface area in MFM-102-NO2, it results in a 28% enhancement in C2H2 adsorption under ambient conditions, demonstrating the positive effect of NO2-groups on C2H2 binding. With similar BET surface areas, MFM-102-NO2, MFM-102-NH2, and MFM-111 provide an excellent platform to directly examine the role of functional group on C2H2 adsorption. Compared to the parent MFM-102, introduction of the amine group shows a small increase (5.3%) in adsorption of C2H2, while the alkane group leads a moderate reduction (−12%) in C2H2 adsorption. The isosteric heats of C2H2 adsorption (Q) for these MOFs are around 31 to 33 kJ mol–1, comparable to reported MOFs with open Cu(II) sites (Table S3). The Q plots shows little variation as a function of uptake, indicating the presence of cooperativity of host–guest and guest–guest binding.[21]

To establish the role of these functional groups on C2H2 binding, adsorption of CH4, as a noninteracting gas probe, was also measured. At 298 K and 20 bar, MFM-102 shows a CH4 uptake of 236 cm3 g–1, which is higher than that for MFM-102-NO2, MFM-102-NH2, and MFM-111 (188, 179, and 173 cm3 g–1, respectively). The trend in uptake of CH4 is consistent with the variation of BET surface areas of these materials, and suggests potential binding interactions between the −NO2 functional group and unsaturated C2H2.

Six independent binding sites (I to VI) for adsorbed C2D2 molecules in MFM-102-NO2 have been determined by in situ neutron powder diffraction (NPD) (Figure ). Sites I and II (occupancies of 0.41 and 0.40, respectively) are located at the open Cu(II) sites with a side-on interaction between the C≡C bond and Cu(II) [Cu···C≡C(centroid) = 3.98(5) and 2.93(8) Å for site I and II, respectively]. Interestingly, sites I and II have very different bonding distance to the open Cu(II) site. A closer examination revealed that site I also forms supramolecular interactions to two aromatic −CH groups from two adjacent phenyl rings [−CH···C≡C(centroid) = 3.23(2), 3.48(3) Å] and becomes the most populated location owing to this cooperativity between sites. Site III (occupancy 0.31) is stabilized by a combination of three types of interactions to the host: (i) hydrogen bonds between the D(δ+) of C2D2 molecules and O(δ–) center of the −NO2 group [D···O = 1.99(9), 2.35(6) Å]; (ii) supramolecular interactions between the C≡C bond and the −CH group on adjacent NO2-decorated phenyl rings [−CH···C≡C(centroid) = 2.47(9), 2.80(4) Å]; (iii) intermolecular dipole interactions between π-electrons in the C≡C bond (site III) and D atoms of C2D2 at site IV. Site IV (occupancy 0.26) is also stabilized by hydrogen bonds between C2D2 molecules and −NO2 groups [D···ONO2 = 1.69(2) Å] and a −CH bond of the NO2-decorated phenyl ring [−CH···C≡C(centroid) = 3.11(4) Å]. Moreover, there is a weak π···π interaction between C≡C bond of C2D2(IV) and the NO2-decorated phenyl ring [ring centroid···C≡C(centroid) = 4.46(8) Å]. Site V (occupancy = 0.16) is at the window between the cylindrical and spherical cages involving similar side-on mode interactions with the −CH group of the NO2-decorated phenyl ring [−CH···C≡C(centroid) = 2.63(5) Å] and the hydrogen bond to the −NO2 group [D···ONO2 = 3.05(10), 3.17(7) Å]. Site VI (occupancy = 0.09) is located in the center of the elongated cage, interacting directly with the −NO2 group from three surrounding ligands with D···ONO2 distances ranging from 2.37(1) to 2.56(3) Å. Overall, C2D2 molecules at sites III–VI are all directly associated with the −NO2 groups in the pore, confirming the positive effect of −NO2 group in achieving optimal acetylene binding.

Figure 3

View of the binding sites for adsorbed C2D2 molecules in MFM-102-NO2. (C, dark gray; O, red; N, blue; Cu, turquoise and C2D2 molecules at site I (light blue), site II (orange), site III (light green), site IV (dark green), site V (purple) and site VI (sapphire).

The binding dynamics of C2H2-loaded MFM-102-NO2 were also studied by in situ inelastic neutron scattering (INS). The INS spectrum of bare MOF shows excellent agreement with that obtained from DFT calculations, thus allowing assignment of vibrational modes (Figure S12). The INS spectrum of C2H2-loaded MFM-102-NO2 shows a significant increase in intensity (Figures and S11). The INS peaks at 80 and 95 meV (assigned as the asymmetric and symmetric C–H bending mode of C2H2, respectively) show significant broadening on adsorption of C2H2 with the appearance of shoulders on both sides and a shift to lower energy. In addition, the translational modes of C2H2 molecules, represented by the INS peaks at low energy region (below 25 meV), show restricted motion on adsorption, with adsorbed C2H2 molecules well-ordered in the pore as these peaks all shift to lower energy but still remain a 3-fold feature as found in solid C2H2. Moreover, in the difference spectra, peaks at 164 and 189 meV (Figure S12) develop upon inclusion of C2H2 molecules in the pore, and this enhancement of both symmetrical and antisymmetrical stretching modes of −NO2 groups is consistent with the formation of host–guest interactions between the −NO2 groups and adsorbed C2H2 molecules. Overall, these results are highly consistent with the crystallographic studies showing various binding sites (I–VI) of C2D2 and the strong interaction between the −NO2 group and C2D2 molecules (sites III–VI) in the pore of MFM-102-NO2.

Figure 4

INS spectra for MFM-102-NO2 with C2H2 loading. Comparison of difference INS plots derived by subtracting INS spectra for C2H2-loaded MFM-102-NO2 and bare MFM-102-NO2 spectra (black), with condensed C2H2 in the solid state (red).

Achieving optimal gas adsorption and binding in porous materials requires integration of high porosity and appropriate binding sites. However, introduction of functional groups naturally reduces the porosity of decorated MOFs, leading to a trade-off between these two factors. In this study, the effect of amine, alkane and nitro groups on adsorption of C2H2 has been examined in a family of isostructural MOFs showing high BET surface areas. The first example of binding between electron-rich C2H2 molecules and electron withdrawing −NO2 groups in the MOF, MFM-102-NO2, at crystallographic resolution has been established. The combination of high BET surface area, high density of open Cu(II) sites and accessible NO2-groups leads to excellent adsorption capacity of C2H2 in MFM-102-NO2, making it one of the best-performing acetylene adsorbents to date.