A novel mesoporous hydrogen-bonded organic framework with high porosity and stability

Abstract

A highly stable hydrogen-bonded organic framework, HOF-14, has been successfully constructed and structurally characterized. It possesses a permanent three dimensional (3D) porous structure. The activated HOF-14 has a high BET surface area of 2573 m² g⁻¹ and a record large pore volume of 1.36 cm³ g⁻¹ among HOF materials. In addition, HOF-14 also exhibits high chemical and thermal stability and is capable of highly selective separation of light hydrocarbons under ambient conditions.

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As a class of newly developed porous materials, hydrogen-bonded organic frameworks (HOFs) constructed from the self-assembly of organic building units (OBUs) through intermolecular hydrogen-bonding interactions have attracted significant attention.^1,2 Similar to other famous porous materials of covalent organic frameworks (COFs)³ and metal–organic frameworks (MOFs),^4,5 HOFs also have a large surface area, high porosity and a tailored pore size/shape and can be potentially used in gas storage and separation,^6–12 sensing,¹³ biomolecule encapsulation,¹⁴ pollutant removal,¹⁵ proton conduction,¹⁶ heterogeneous catalysis^17,18 and so on. Compared with the porous materials mentioned above, however, HOFs possess some unique features such as mild synthetic conditions, solution processability and easy regeneration. Specifically, the metal free nature of HOFs makes them good candidates for biological applications.¹⁹ For practical applications of HOFs, however, one of the problems that needs to be addressed is their poor chemical and thermal stability, which is ascribed to the weak hydrogen bonding nature. Since the first report of the HOF material with permanent porosity, only few HOFs can survive harsh conditions.¹ Thus, the construction of stable HOFs is one of the most important challenges in this field.

Generally, there are two strategies to stabilize the framework of HOFs. One strategy is to create multiple hydrogen bonds. HOFs are constructed by hydrogen bonds between OBUs, and therefore more number of intermolecular hydrogen bonds will strengthen the entire structure. Under the guidance of this strategy, our group designed and synthesized a series of 2,4-diaminotriazinyl (DAT)-based OBUs featuring multiple amino groups. Several HOFs with permanent porosity were successfully constructed using these OBUs.^{7–11,20–23} The cooperation of hydrogen bonding interactions with other weak interactions such as π–π stacking is another strategy to stabilize the HOF structure. Recently, this strategy has been used to construct highly stable HOFs with a large surface area such as HOF-TCBP,²⁴ CPHAT-1a,²⁵ and PFC-1¹⁹ using carboxylic-based OBUs. The structure of the three HOFs is similar. In the three HOFs, the discrete OBUs connect with each other through intermolecular hydrogen bonds between carboxylic groups to form a single layer, and different layers are packed together through strong face to face π–π stacking interactions to form a 3D framework with a one-dimensional (1D) channel. In particular, for PFC-1, different layers are packed together in an AA packing mode, similar to those in the well-studied stable 2D COFs.²⁶ By selecting building units with different lengths, the pore size of 2D COF can be rationally tailored. We thus realized that the pore size of PFC-1 can also be tailored through the selection of OBUs with different lengths.

Herein, we synthesized an organic building unit with a larger size than that of H₄TBAPy which is used for the construction of PFC-1 (Scheme 1). Using this OBU, a robust one-component porous mesoporous hydrogen-bonded organic framework, HOF-14 isostructural to PCF-1 but with a larger surface area and pore size, was successfully constructed.²⁷ The Brunauer–Emmett–Teller (BET) surface area of HOF-14 is 2573 m² g⁻¹. In addition, HOF-14 also has a record large pore volume of 1.36 cm³ g⁻¹ among HOF materials. HOF-14 also exhibits high chemical and thermal stability and can easily be prepared and regenerated by simple addition of tetrahydrofuran (THF) to the dimethyl sulfoxide (DMSO) solution of H₄PTTNA under stirring. The activated HOF-14 turned out to be a promising candidate as to highly selectively separate CH₄ from C3 and C2 hydrocarbons under ambient conditions.

Scheme 1 Organic building units of H₄TBAPy and H₄PTTNA for the construction of PFC-1 and HOF-14, respectively.

6,6′,6′′,6′′′-(Pyrene-1,3,6,8-tetrayl)tetrakis(2-naphthoic acid) (H₄PTTNA) was synthesized through a Suzuki coupling reaction (see the Supporting Information). Diffusion of THF vapor to DMSO solution where H₄PTTNA was dissolved at room temperature for about three weeks afforded yellow stick-shaped crystals suitable for X-ray analysis. HOF-14 can also be obtained in large scale by simple addition of THF to the DMSO solution of H₄PTTNA under stirring. Single crystal X-ray diffraction analysis reveals that HOF-14 is isostructural to PFC-1 and crystallizes in a monoclinic C2/m space group. In its crystal structure, each H₄PTTNA OBU interacts with four adjacent OBUs through intermolecular O–H⋯O hydrogen bonds to form a two-dimensional (2D) layer (Fig. 1a). The O–H⋯O distance and the angle are 2.7 Å and 170°, respectively, falling into a strong hydrogen bond range. From a topological view point, the H₄PTTNA ligands can be seen as a 4-connected node, and the 2D layer mentioned above can thus be simplified as a 4-connected uninodal net with a point symbol of {4⁴·6²} which corresponds to the sql topology according to Topos software. The 2D square layers are further packed together in an AA mode along the c axis through strong face-to-face π–π stacking interactions (the interlayer distance is 3.7 Å) to form a 3D structure with one-dimensional square channels (Fig. 1b and c). The channel size of HOF-14 is 31.2 × 24.1 Å, larger than that of PFC-1 (18 × 23 Å).

Fig. 1 Crystal structure of HOF-14. (a) View of the connection of the adjacent OBUs highlighting the hydrogen-bond length and the angle between the adjacent carboxylic groups; (b) the stacking of 2D layers highlighting the face-to-face π–π stacking interactions; and (c) representation of the porous framework.

The permanent porosity of HOF-14 has been confirmed by N₂ sorption at 77 K (Fig. 2a). Saturated N₂ uptakes of 881 cm³ (STP) g⁻¹ as to HOF-14 are achieved, and its evaluated BET and Langmuir surface areas are 2573 and 3204 m² g⁻¹, respectively (Fig. 2a). The experimental total pore volume is 1.36 cm³ g⁻¹, which is the highest among HOF materials to date (Table 1).

Fig. 2 Structural stability of HOF-14. (a) PXRD patterns and (b) N₂ adsorption isotherms at 77 K of the as-synthesized HOF-14 and samples treated with different solutions; (c) PXRD patterns and (d) N₂ adsorption isotherms at 77 K of HOF-14 activated at different temperatures.

Table 1 Summary of adsorption results of some well-known HOFs calculated from the N₂ uptakes at 77 K

HOFs	N2 uptake [cm^3 g^−1]	SABET [m^2 g^−1]	Pore volume [cm^3 g^−1]
a Calculated by the CO2 uptake at 195 K.
TTBI^28	754	2796	1.02
PFC-1^24	613	2122	0.95
HOF-TCPB^24	535	2066	0.83
HOF-5a^9	—	1101^a	0.44
PFC-2^29	415	1014	0.64
CBPHAT-1a^25	362	1288	0.56
HOF-19^17	287	685	0.45
tcpb^30	266	1095	0.42
IISERP-HOF1^31	405	1025^a	0.26^a
SOF-7a^32	—	900	0.23
HOF-14	881	2573	1.36

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Considering the relatively strong hydrogen bonding interactions between carboxylic groups and the presence of strong face to face π–π stacking interactions in HOF-14, we tried to explore its chemical stability. Firstly, the HOF-14 samples were treated in water, concentrated HCl aqueous solution, and NaOH aqueous solution (pH = 10) at room temperature, as well as in boiling water. Upon soaking in these solutions for 24 h, HOF-14 exhibited maintained crystallinity and an unchanged structure as confirmed by PXRD measurements (Fig. 2a). Furthermore, the N₂ sorption isotherms for the samples after being immersed in water, boiling water, HCl aqueous solution (pH = 1), and NaOH aqueous solution (pH = 10) for 24 h were measured and almost the same uptakes were found (Fig. 2b). In order to further confirm the excellent chemical stability of HOF-14, the morphologies of the samples before and after acid/base aqueous solution treatment were characterized by scanning electron microscopy (SEM). As shown in Fig. S3, ESI,† after being treated in HCl (pH = 1) or NaOH (pH = 10) aqueous solutions for 24 h, the surface of the sample still remained smooth and no obvious flaw was spotted. All the characterization mentioned above together confirmed the excellent chemical stability of HOF-14, rarely observed in HOF materials.

In addition to the chemical stability, we further explored the thermal stability of HOF-14. As shown in Fig. S2, ESI,† thermo gravimetric analysis (TGA) shows that the activated HOF-14 is stable up to 673 K in an air atmosphere. Then the samples of HOF-14 were placed in ovens with different temperatures. It was found that even when exposed to a 573 K oven for 2 h, the crystallinity of HOF-14 still remained (Fig. 2c). Furthermore, we explored the changes in the N₂ uptake of HOF-14 at 77 K after being activated at high temperature and under high vacuum. As shown in Fig. 2d, initially, to fully remove the guest molecules in the channel of HOF-14, the sample was activated under high vacuum at 373 K for 10 h. The saturated N₂ uptake was 881 cm³ g⁻¹. Further activating the sample at 473 K for 2 h led to a decrease of the N₂ uptake amount (from 881 to 625 cm³ g⁻¹), indicating that the partial framework of the samples collapsed. Meanwhile, this also showed that the crystallinity of about 71% of the sample was retained, indicating the high thermal stability of HOF-14, rarely seen in HOFs. To recover the framework of HOF-14, the samples after being treated at high temperatrue or in strong base aqueous solutions were dissolved in a small amout of DMSO by heating, and then a large amount of THF was introduced to obtain a microcrystalline sample. The saturated N₂ uptake of this sample was 858 cm³ g⁻¹, which is almost the same as that of the crystal sample (881 cm³ g⁻¹, Fig. S4, ESI†).

Recently, natural gas purification has become an important research field in industry. The removal of C3 hydrocarbons from CH₄ can reduce the dew point of natural gas and prevent condensation during transportation. In addition, the separation of CH₄ over C2 hydrocarbons is also important. Acetylene is principally obtained from the cracking of natural gas. The separation of C₂H₂ from CH₄ is necessary to meet the requirements of Grade A C₂H₂ for organic synthesis. Besides, the oxidative coupling of CH₄ to C₂H₄ or C₂H₆ ultimately needs to separate CH₄ from them because of the incomplete conversion of CH₄.³³ We thus explored the separation of light hydrocarbons using HOF-14 under ambient conditions. Single-component gas sorption isotherms were recorded up to 100 kPa at 298 and 273 K, respectively (Fig. 3a and Fig. S5, ESI†). As can be seen from these isotherm data, HOF-14 shows the maximum C₃H₈, C₃H₆, C₂H₆, C₂H₄, and C₂H₂ uptake of 181.4, 154.9, 44.2, 32.5, and 25.6 cm³ g⁻¹, respectively, at 298 K and 100 kPa (Fig. 3a). In contrast, the CH₄ adsorption amount is only 7.8 cm³ g⁻¹ under the same conditions, which is much lower than those of C3 and C2 hydrocarbons. Such a difference between the adsorption amounts of higher hydrocarbons and that of CH₄ indicates that HOF-14 may enable highly selective separation of CH₄ from these C3 and C2 hydrocarbons. To analyse the gas separation abilities of HOF-14, ideal adsorbed solution theory (IAST) was used to predict the selectivity of binary mixtures of C3 and C2 hydrocarbons with CH₄ (50 : 50) at 298 K (Fig. 3b). As anticipated, the selectivities in C₃H₈/CH₄, C₃H₆/CH₄, C₂H₆/CH₄, C₂H₄/CH₄, and C₂H₂/CH₄ mixtures were estimated to be 28.6, 24.6, 6.3, 4.4 and 3.7 at 298 K and 100 kPa, respectively (Fig. 3b). In addition, the isosteric heats of adsorption of C3 and C2 hydrocarbons are larger than that of CH₄ (Fig. S6, ESI†), which indicates stronger interactions between the C3 and C2 hydrocarbons and the pore surfaces. Consequently, HOF-14 can highly selectively adsorb the light hydrocarbons under ambient conditions, thus separating them.

Fig. 3 (a) Single-component sorption isotherms for CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, and C₃H₈ of HOF-14 at 298 K up to 100 kPa; (b) a comparison of the IAST selectivities for the binary mixtures (50 : 50) of C₃H₈/CH₄, C₃H₆/CH₄, C₂H₆/CH₄, C₂H₄/CH₄ and C₂H₂/CH₄ at 298 K.

In summary, a novel chemically and thermally stable mesoporous HOF, HOF-14, was constructed. HOF-14 has a high surface area and a large pore volume and has potential applications for separation and purification of light hydrocarbons.

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21971038, 21975044, 21805039, 21673039, and 573042), the Fujian Science and Technology Department (2019H6012 and 2018J07001), the China Postdoctoral Science Foundation (Grant No. 2018M642556), and the Welch Foundation (AX-1730). Work at the Molecular Foundry and Advanced Light Source was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1955714. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9cc07802a

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