Structure Tuning of Hafnium Metal–Organic Frameworks through a Mixed Solvent Approach

Abstract

The recent development of water-stable metal–organic frameworks (MOFs) has significantly broadened the application scope of this emerging type of porous material. Structure tuning of hafnium MOFs is less studied compared with zirconium MOFs. In this work, we report the synthesis of a mesoporous hafnium MOF, csq-MOF-1, through finely tuning the solvent mixture ratio. The successful synthesis of csq-MOF-1 also relies on the linker flexibility as linker bending and a symmetry decrease were observed in this framework as compared to its structural isomer NPF-300 (Hf). The mesoporous feature and permanent porosity were determined by the N₂ adsorption at 77 K. Such a hierarchical pore feature is expected to enable a variety of applications through encapsulation of large functional molecules. The synthetic strategy of utilizing a mixed solvent and flexible linker is expected to inspire the development of new hafnium MOFs with diverse topological structures.

1. Introduction

Metal–organic frameworks have attracted considerable attention in the past two decades. [1,2] Their highly tunable pore size [3], ultrahigh surface area [4], and facile functionalization of the pore surface [5,6] enable a variety of applications including in catalysis [7,8], gas storage [9,10], separation [11,12,13,14], and luminescence sensing [15,16]. In particular, the recent development of water-stable MOFs has further pushed this emerging type of porous material to more fields involving water, such as proton conduction [17], water purification [18,19], and water harvesting [20] from air. Among water-stable MOFs, zirconium and hafnium with +4 charge exhibit excellent stability [21,22] and also highly tunable topology structures [23,24,25,26,27,28], which greatly benefit performance optimization through delicate pore tuning.

The highly viable structure of Zr/Hf-MOFs can be attributed to the viable connections of the clusters. For the classical Zr₆O₄(OH)₄(COO⁻)₁₂ cluster, the maximum connection is twelve. Meanwhile, in MOF formation, the connection can be reduced to eight, six, and four, depending on the symmetry matching as well as the synthetic conditions. For example, diverse Zr-MOFs have been discovered with a tetracarboxyphenylporphyrin (TCPP) linker. In 2012, Yaghi et al. reported two structures: MOF-525 [29] with a twelve-connected cluster, and mesoporous MOF-545/PCN-222 [29,30] with an eight-connected cluster. The topology was ftw for MOF-525 and csq for MOF-545/PCN-222.

To purposely synthesize new Zr-MOF structures, several strategies have been developed. Through kinetically controlled synthesis, Zr-TCPP MOF (PCN-223) with the shp-a topology and a twelve-connected cluster was obtained. [31] In another case, Zr-TCPP MOF (PCN-224) was formed with a six-connected Zr cluster through a linker elimination strategy. [32] Interestingly, even with the same connection, different topologies can be formed. For example, a tetracarboxylate linker and an eight-connected Zr cluster can form the scu, csq, and flu topologies, and a systematic study revealed that such a topology difference is dependent upon the linker conformation. [33] Such a structure difference could lead to a significant change in application performance, such as for catalysis [34] and gas separation [35]. Therefore, it is of high significance to develop new structures and novel strategies for phase control.

Hafnium MOFs usually form isostructures with Zr-MOFs and exhibit different chemical properties and superior application performance in some cases [36,37,38]. However, the structural or topology tuning of Hf-MOFs has rarely been reported. In addition, although the structure tuning of Zr-MOFs has been explored, the linker flexibility has not been a consideration thus far. Herein, we report a mesoporous Hf-MOF, namely, csq-MOF-1, using a flexible tetracarboxylate linker, as a structural isomer of NPF-300 (Hf) [39]. The successful synthesis of csq-MOF-1 relies on a mixed solvent approach through a finely tuned DMF/DEF (DMF: dimethyl formamide; DEF: diethyl formamide) mixture ratio. The linker conformation of csq-MOF-1 is in a bending geometry, being different to that of NPF-300 (Hf). The rigid framework csq-MOF-1 also exhibits permanent porosity as revealed by N₂ uptake. This simple mixed solvent approach represents a new strategy to control Hf-MOF topology structures.

2. Materials and Methods

All solvents and reagents were purchased from commercial sources and, unless otherwise noted, used without further purification. The tetracarboxylic ligand linker 5′,5′′′′-(Buta-1,3-diyne-1,4-diyl)bis(([1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid)) was synthesized according to our previous method in the NPF-300 synthesis [39]. PXRD data were recorded with a PANalytical Empyrean diffractometer (Almelo, The Netherlands) with a PIXcel 3D detector. The copper target X-ray tube was set to 45 kV and 40 mA. Gas adsorption isotherms were collected using the surface area analyzer Micromeritics ASAP-2020 (Norcross, GA, USA). N₂ gas adsorption isotherms were measured at 77 K using a liquid N₂ bath.

The crystal sample was taken from the mother liquid, transferred to oil, and mounted by a loop. Single-crystal X-ray diffraction data were collected using synchrotron radiation. Indexing was performed using APEX2 (difference vector method). Data integration and reduction were performed using SaintPlus 6.0 (Bruker, WI, USA). Absorption correction was performed using a multi-scan method implemented in SADABS. Space groups were determined using XPREP implemented in APEX2. The structure was solved using SHELXS-2014 (direct methods) and refined using SHELXL-2014 within Olex 2 (full-matrix least squares on F²). Hf, C, and O atoms were refined with anisotropic displacement parameters, and H atoms were placed in geometrically calculated positions and included in the refinement process using a riding model with isotropic thermal parameters: Uiso(H) = 1.2Ueq(-CH). The contribution of disordered solvent molecules was treated as diffuse using the SQUEEZE procedure implemented in PLATON. [40] Crystal structure images were produced using Diamond software by our collaborator at Sun Yat-Sen University.

To activate MOFs for N₂ gas adsorption measurement, as-synthesized MOF samples were exchanged with fresh DMF three times and heated in oven at 80 ⁰C for 4 h. Then, the samples were exchanged with fresh DMF 3 times and subsequently exchanged with anhydrous acetone 6 times in 3 d to remove the DMF completely. The acetone-exchanged samples were decanted and dried in a vacuum oven for 6 h. The dry samples were further de-gassed with the surface area analyzer ASAP 2020 at 80 °C to remove trace amounts of acetone trapped in the pore.

NPF-300-Hf synthesis: Here, 16 mg of HfCl₄ and 200 mg of benzoic acid were mixed in a mixed solvent (DMF/DEF = 0.8 mL:0.1 mL) in a glass vial and ultrasonically dissolved. The clear solution was heated in an oven at 80 °C for 1 h. After cooling down to room temperature, 4 mg of ligand was added to this solution, and the mixture was sonicated for 5 min to dissolve all of the ligand. The yellow solution was heated in an oven at 120 °C for 48 h. After cooling down to room temperature, light-yellow, block-shaped single crystals were obtained. As-synthesized NPF-300-Hf was activated by soaking in fresh DMF at 80 °C for 4 h to remove the unreacted ligand and cluster.

csq-MOF-1 synthesis (single crystal): Here, 16 mg of HfCl₄ and 200 mg of benzoic acid were mixed in a mixed solvent (DMF/DEF = 0.1 mL:0.8 mL) in a glass vial and ultrasonically dissolved. The clear solution was heated in an oven at 80 °C for 1 h. After cooling down to room temperature, 4 mg of ligand was added to this solution, and the mixture was sonicated for 5 min to dissolve all of the ligand. The yellow solution was heated in an oven at 120 °C for 48 h. After cooling down to room temperature, light-yellow, block-shaped single crystals were obtained. As-synthesized NPF-300-Hf was activated by soaking in fresh DMF at 80 °C for 4 h to remove the unreacted ligand and cluster.

For the surface area measurement, the csq-MOF-1 sample was synthesized with 15 mg of HfCl₄, 5 mg of ligand, 100 mg of benzoic acid, and 0.1 mL of formic acid in a mixed solvent (DMF/DEF = 0.2 mL:0.8 mL), as an optimized synthetic method with a higher yield.

3. Results and Discussion

As it is known that diverse structures of Zr/Hf MOFs can be formed with the same linker, we explored the Hf-MOF structural diversity using a tetracarboxylate linker of NPF-300 due to its flexibility. As discovered in our previous work of linker insertion in NPF-300, the tetracarboxylate linker with a dialkynyl unit could form in-plane and out-of-plane bending to accommodate secondary linkers of different lengths. We were interested in exploring if such flexibility of the linker could affect the formation of diverse topological structures. After extensive attempts under synthetic conditions, we discovered a new structure using solvothermal conditions with a mixed solvent of DMF/DEF at 120 °C for 48 h. The effect of the solvent may be attributed to the different solubilities of precursors or nucleates which alter the crystallization kinetics. The formation of csq-MOF-1 needs a higher amount of DEF with a DMF/DEF ratio of 1:8, in comparison with the DMF/DEF ratio of 8:1 for NPF-300 synthesis. Benzoic acid was used for both cases as a modulator.

The csq-MOF-1 framework crystallizes in a hexagonal crystal system with the P 6/mmm space group, as shown in

Table 1. Crystal data and structure refinement for csq-MOF-1.

Identification Code	csq-MOF-1
CCDC	2167258
Empirical formula	C44H22Hf3O16
Formula weight	1342.08
Temperature/K	271 (2)
Crystal system	hexagonal
Space group	P 6/mmm
a/Å	40.2473 (12)
b/Å	40.2473 (12)
c/Å	20.4333 (11)
α/°	90
β/°	90
γ/°	120
Volume/Å3	28,664 (2)
Z	6
ρcalc/g cm−3	0.466
μ/mm−1	0.395
F(000)	3780
Crystal size/mm3	0.5 × 0.1 × 0.1
Radiation	Synchrotron (λ = 0.41325)
2θ range for data collection/°	0.672 to 14.069
Index ranges	−47 ≤ h ≤ 47, −47 ≤ k ≤ 47, −24 ≤ l ≤ 24
Reflections collected	588,792
Independent reflections	9026 [R(int) = 0.1355]
Data/restraints/parameters	9026/0/159
Goodness of fit on F2	1.496
Final R indexes [I ≥ 2σ (I)]	R1 = 0.0810 wR2 = 0.2449
Final R indexes (all data)	R1 = 0.0985, wR2 = 0.2589
Largest diff. peak/hole/e Å−3	1.345 and −1.915

. Each Hf₆ cluster is coordinated with eight carboxylates from different linkers and eight terminal H₂O/OH⁻ groups in the equatorial plane. The overall formula of csq-MOF-1 is Hf₆O₄(OH)₈(H₂O)₄L₂. The structure has a csq topology, being the same as PCN-222 [30] and NU-1000 [25]. In comparison with NPF-300 with a single type of channel, csq-MOF-1 exhibits two types of channels with triangular and hexagonal geometries, as shown in Figure 1. Such a structural feature is quite interesting due to the intrinsic hierarchical pore structure. The phase purity of csq-MOF-1 was confirmed using PXRD, which showed excellent matching with the patterns simulated from the single-crystal data (Figure 2).

A close examination of csq-MOF-1 revealed that the ligand flexibility is quite important in the structure formation. As shown in Figure 3a, the structure view along the b axis clearly shows out-of-plane bending of the tetracarboxylate linker. In NPF-300, the linker is planar with a C2h symmetry, while the out-of-plane bending of the linker causes its symmetry to decrease to the Cs point group in the csq-MOF-1 structure. The linker bending also leads to a decrease in the cluster distance along the c axis from 13.198 Å of NPF-300 to 12.618 Å of csq-MOF-1. Such a difference in the cluster distance is potentially more favorable for the insertion of a shorter linker. The hexagonal channel exhibits the largest dimension of ~32.238 Å, being much larger than that of NPF-300 (12.660 Å), as shown in Figure 3c,d.

The mesoporous feature was further validated by the N₂ adsorption isotherm at 77 K. As shown in Figure 4a, a sudden increase in uptake was observed, which can be attributed to the capillary condensation of the N₂ gas in the mesopore. Based on the adsorption isotherm, the surface area of csq-1 was determined as 1735 m²/g using the BET (Brunauer–Emmett–Teller) method and 1986 m²/g using the Langmuir method. Further, the total pore volume was determined as 1.1 cm³/g, and all these results demonstrate the high permanent porosity of csq-MOF-1. The pore size distribution showed two major pore sizes: micropores of ~15 Å, and mesopores of ~30 Å, being consistent with the hierarchical pore feature determined from the single-crystal structure. Such a mesoporous Hf-MOF is expected to be useful for the encapsulation of large guest molecules such as enzymes, protein drugs, and metal nanoparticles for a broad scope of applications.

4. Results and Discussion

In summary, we report a new mesoporous Hf-MOF using a mixed solvent approach to control the topology. The resulting csq-MOF-1 exhibits a csq topology and large hexagonal channels of ~ 32 Å. The ligand linker in csq-MOF-1 is in a bending geometry with a decreased symmetry, as compared with the isostructural MOF NPF-300-Hf. The permanent porosity and mesoporous feature were demonstrated by a N₂ adsorption experiment, and a high BET surface area of 1735 m²/g was calculated based on the adsorption isotherm. The new MOF structural feature and structure tuning strategy are expected to inspire the development of diverse Hf-MOF structures and further broaden their application scope.