Rapid room temperature conversion of hydroxy double salt to MOF-505 for CO₂ capture

Abstract

Given the fact that solvothermal synthesis is the most common synthetic method to obtain metal–organic frameworks (MOFs) on a gram scale, it still remains a great challenge to produce MOFs in a scalable and sustainable synthetic process. In this work, we develop a facile and rapid synthesis of copper-based MOF-505 by the exploitation of layered (Zn,Cu) hydroxy double salt as the intermediate for efficient MOF-505 crystallization at room temperature, in which [(Zn,Cu)(OH)NO₃] is synthesized by reacting ZnO with Cu(NO₃)₂. Under an optimized reaction time of 25 min, MOF-505-25 exhibits a Brunauer–Emmett–Teller (BET) surface area of 1076 m² g⁻¹ and the pore size is narrowly distributed at 5.9 and 8.0 Å. With respect to CO₂ adsorption and separation performance, MOF-505-25 shows a moderate CO₂ uptake of 3.51 mmol g⁻¹ at 298 K and 100 kPa and high CO₂/CH₄ and CO₂/N₂ adsorption selectivities of 7 and 29. Particularly, the formation of MOF-505 can be completed within a short reaction time (less than 30 min) whilst maintaining high quality. This facile and rapid room temperature synthetic procedure would pave the way for promoting the development of MOF materials large-scale production with minimizing the energy input.

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Introduction

Metal–organic frameworks (MOFs) are a prominent class of solid crystalline nanoporous materials, which are composed of metal ions or clusters coordinated with organic ligands through coordination bonds.¹ Owing to the special reticular chemistry that endows rationally controlled MOFs with diverse structural features, extraordinarily high porosity and tailorable functionality, MOFs have potential applications in the fields of gas storage, separation, catalysis, and chemical sensing.^2–9 In particular towards the application of gas purification and separation, MOFs have been widely investigated in many important separation processes including CO₂ capture, paraffin/olefin separation, capture of toxic gases, separation of hexane isomers, and Xe/Kr separation.^10–15 Particularly, some well-known MOF materials such as MAF-X25ox,¹⁶ HKUST-1,¹⁷ NJU-Bai21,¹⁸ mmen-CuBTTri,¹⁹ SIFSIX series²⁰ and MOF-74 series^21,22 have been demonstrated as outstanding adsorbents for CO₂ capture to mitigate massive CO₂ emission.

Furthermore, an ideal MOF material should have high synthesis rates (also termed as the space–time yield) to push forward the process of industrial implementation.²³ Currently, the solvothermal method remains the most common synthetic approach to obtain MOFs on a gram scale in the laboratory worldwide.²⁴ However, this synthetic approach tends to be an energy-intensive and time-consuming process, since it generally requires long reaction time, high temperature and pressure, and harsh reaction conditions.²⁵ Provided that MOFs are only obtained under laboratory-scale conditions, their industrial applications will be largely limited and even fail.²⁶ Before industrial application, large-scale production of MOFs is a necessary prerequisite. Thus, there still remains a great challenge for large-scale production, considering the cost of production and the impact on ecological systems.

To this end, several synthetic strategies have been developed to tackle this challenge, such as electrochemical synthesis,²⁷ microwave synthesis,²⁸ mechanochemical synthesis,²⁹ and spray-drying synthesis.³⁰ Although these synthesis routes can substantially reduce the reaction time and temperature for MOF synthesis, significant amounts of external energy are required to drive the chemical reactions.²³ Thus, it is imperative to develop an efficient and facile synthetic strategy that enables MOF formation under mild conditions (such as in short reaction time, under atmospheric pressure and at ambient temperature), minimizing the energy input whilst maintaining the MOF quality.³¹ From a sustainable perspective, room temperature synthesis can be a promising alternative method for MOF large scale synthesis without additional energy input during the reaction process. ZIF-8, HKUST-1 and UiO-66 have been subsequently realized through the conversion of reaction precursors at room temperature.^23,32,33 Recently, Farha's group has reported the room temperature synthesis of a Zr-based MOF, NU-901, by exploiting pre-synthesized Zr₆ nodes capped with benzoate ligands.³¹ For three-dimensional copper-based MOFs, open metal sites are generally generated within their frameworks, which is an effective method to improve CO₂ adsorption and separation performance.¹⁰ However, only Cu-MOF-74, HKUST-1 and the HKUST-1@GO composite can be prepared by the rapid room temperature synthesis so far.^23,34–36 Thus, it is imperative to explore further investigations on the rapid room temperature synthesis to expand the scope of obtaining new copper-based MOF materials for CO₂ capture.

Herein, we develop an effective and facile route for the rapid room temperature synthesis of a copper-based MOF under mild conditions, MOF-505. The key to this protocol is the employment of (Zn,Cu) hydroxy double salt [(Zn,Cu)(OH)NO₃] as the intermediate that is capable of facile and ultrafast conversion to MOFs reported by Parsons and co-workers.²³ Layered [(Zn,Cu)(OH)NO₃] is readily converted via the reaction of ZnO with Cu(NO₃)₂, comprising cationic sheets connected by interlayer anions to form interlamellar compounds.³⁷ In this work, we provide the successful example of MOF-505 by the conversion of [(Zn,Cu)(OH)NO₃] into MOF-505 within a short reaction time (less than 30 min) whilst maintaining high quality. By investigating the time-dependent evolution of MOF-505 microstructures, the classical growth pathway of MOF-505 crystallization is confirmed. Benefitting from the excellent anion exchangeability of [(Zn,Cu)(OH)NO₃], MOF-505 crystals can be rapidly formed at room temperature and also maintain high quality evaluated by their CO₂ adsorption and separation performance.

Experimental section

Materials

Zinc oxide (ZnO, 99%) was supplied by Tianjin Damao Chemicals Co. Ltd. Copper(II) nitrate trihydrate [Cu(NO₃)₂·3H₂O, 99.99%] was obtained from Aladdin. The organic ligand biphenyl-3,3′,5,5′-tetracarboxylic acid (H₄bptc, 98%) was purchased from Beijing HWRK Chem Co. Ltd. N,N-Dimethylformamide (DMF, 99.5%), ethanol (EtOH, 99.7%) and acetone (99.5%) were obtained from Guangdong Guanghua Sci-Tech Co. Ltd. All the materials were used as received without further purification.

Synthesis of the (Zn,Cu) hydroxy double salt [(Zn,Cu)(OH)NO₃] intermediate

The synthesis of [(Zn,Cu)(OH)NO₃] was achieved by following a previously reported procedure with slight modifications.²³ ZnO powder (0.293 g, 0.36 mmol) was dispersed in deionized water (4 mL) and sonicated for 5 min at ambient temperature to form a nanoslurry. Simultaneously, Cu(NO₃)₂·3H₂O (0.174 g, 0.72 mmol) was dissolved in EtOH (6 mL) and sonicated for 5 min to form a clear solution. Subsequently, the Cu(NO₃)₂ solution was added into the ZnO nanoslurry under moderate stirring for 5 min. Then the [(Zn,Cu)(OH)NO₃] intermediate was recovered by filtration and dried at 65 °C for 1 h.

Synthesis of MOF-505 using the [(Zn,Cu)(OH)NO₃] intermediate

H₄bptc (82.6 mg, 0.25 mmol) dissolved in 6 mL of DMF was added into the turbid solution of the [(Zn,Cu)(OH)NO₃] intermediate. After different times of stirring, the MOF-505 products were immediately filtered and washed with 10 mL of acetone. The products were labeled MOF-505-x, where x = 10, 15, 20, 25, and 30 represent the stirring times of 10, 15, 20, 25, and 30 min upon addition of H₄bptc. The corresponding MOF-505-x was immersed in 50 ml of acetone, which was replaced three times daily for 2 days, and then filtered and dried in a vacuum oven at 150 °C for 6 h. The yield is about 51% based on the H₄bptc ligand.

Characterization

The powder X-ray diffraction (PXRD) patterns of the MOF-505 samples were collected using a Bruker D8 Advance X-ray diffractometer with a monochromatized Cu Kα radiation, maintaining an operation voltage and current of 40 kV and 40 mA, respectively. Fourier transform infrared spectroscopy (FTIR) was carried out using a Bruker Vector 70 within the scope of 400–4000 cm⁻¹ to examine the characteristic functional groups. Scanning electron microscopy (SEM) images were recorded using a Hitachi SU-70. A thin Au layer was sputter-coated onto the samples before SEM measurement. Energy dispersive X-ray (EDX) analysis for MOF-505 was performed using the same Hitachi SU-70 equipped with an Oxford energy dispersive X-ray spectrometer to determine the content of Cu and Zn elements on the MOF-505 surfaces. The Cu and Zn element contents of MOF-505 were measured using a plasma mass spectrometer (ICP-Optima 8300). The porosity and textural properties were analyzed via N₂ sorption at 77 K using a Micromeritics ASAP 2046 surface area analyzer. Prior to analysis, about 100 mg MOF-505 samples were degassed under vacuum at 150 °C for 6 h. Thermogravimetric measurement was performed using a Netzsch TG 209F3 instrument under flowing N₂ gas conditions with a heating rate of 10 °C min⁻¹.

Gas sorption measurements

CO₂, CH₄ and N₂ sorption isotherms were determined by a volumetric method using a Micromeritics 3Flex instrument. The measured pressures ranged from 0 to 100 kPa. Gas adsorption data for CO₂, CH₄ and N₂ were collected at 278, 288 and 298 K, using a Julabo water circulator for precise temperature control. Before each gas measurement, 80–100 mg of MOF-505 was degassed under vacuum conditions at 423 K for 6 h. Extra-high purity gases (CO₂, CH₄ and N₂, 99.99%) were used for the adsorption measurements.

Results and discussion

Physical characterization

The bulk purity of the MOF-505 samples is determined from the PXRD patterns, as shown in Fig. 1. By comparison, it can be seen that the experimental PXRD patterns for the MOF-505 products obtained from rapid room temperature synthesis match well with the simulated pattern,³⁸ confirming that MOF-505 products can be synthesized by exploiting layered [(Zn,Cu)(OH)NO₃] as the intermediate within such short time at room temperature. Meanwhile, the characteristic peaks at 12.7, 25.7 and 33.5° are consistent with prior reports for (Zn,Cu) hydroxy nitrate [(Zn,Cu)(OH)NO₃],^23,37 indicating that [(Zn,Cu)(OH)NO₃] can be rapidly converted. However, these peaks for [(Zn,Cu)(OH)NO₃] are not observed in the corresponding patterns of the MOF-505 products. This is because the content of [(Zn,Cu)(OH)NO₃] in the resultant MOF-505 products is quite small, as will be discussed later.

Fig. 1 PXRD patterns of the [(Zn,Cu)(OH)NO₃] intermediate and MOF-505 synthesized by the rapid room temperature method and simulated MOF-505 pattern.

FTIR spectra were recorded to characterize the corresponding chemical groups of the [(Zn,Cu)(OH)NO₃] intermediate and MOF-505 products, as illustrated in Fig. 2. For [(Zn,Cu)(OH)NO₃], the broad band around 3546 cm⁻¹ is attributed to O–H stretching vibration.²³ The bands at 1420, 1360 and 1047 cm⁻¹ reveal the distinct NO₃⁻ group modes.³⁹ The peak at 516 cm⁻¹ is assigned to Cu–O bending vibration while the Zn–O stretching vibration is located at 425 cm⁻¹.^40,41 These spectra clearly confirm the formation of (Zn,Cu) hydroxy nitrate via the reaction of ZnO with Cu(NO₃)₂ to form [(Zn,Cu)(OH)NO₃]. Upon subsequent addition of the H₄bptc solution over several minutes, these characteristic vibrational bands for O–H, NO₃⁻ and Zn–O diminish, which is consistent with the PXRD analysis that [(Zn,Cu)(OH)NO₃] is not observed in the resultant MOF-505 products. The asymmetric and symmetric stretching modes of the carboxylate groups (C=O) in the H₄bptc appear in the series of MOF-505 products.⁴² Meanwhile, the Cu–O vibration shifts from 516 to 493 cm⁻¹, indicating a change in the coordination environment of Cu from [(Zn,Cu)(OH)NO₃] to MOF-505. This spectrum change explicitly demonstrates the fast anion exchange between OH⁻ and NO₃⁻ in the [(Zn,Cu)(OH)NO₃] and bptc⁴⁻, yielding rapid formation of MOF-505 at room temperature.

Fig. 2 FTIR spectra of the [(Zn,Cu)(OH)NO₃] intermediate and MOF-505 synthesized by the rapid room temperature method.

Fig. 3 shows the SEM images of the [(Zn,Cu)(OH)NO₃] intermediate and MOF-505 crystals obtained for different reaction times to monitor their morphological evolution. The [(Zn,Cu)(OH)NO₃] particles in Fig. 3a show a uniform nanosheet morphology with a thickness of less than 0.1 μm. Meanwhile, the elemental mapping images of [(Zn,Cu)(OH)NO₃] nanosheets indicate the homogenous distribution of Zn and Cu (Fig. S2a†). As listed in Table 1, the content of Zn in [(Zn,Cu)(OH)NO₃] is 8.97% determined by EDX analysis. Interestingly, different morphologies of the MOF-505 products are clearly observed. For MOF-505-10 in Fig. 3b, only minor crystals with a cubic shape appear after 10 min, in which the classical cubic crystal shape is characteristic of MOF-505, consistent with a previous observation for MOF-505 crystals synthesized by the traditional solvothermal method.⁴³ Meanwhile, mostly two-dimensional sheet fragments are also simultaneously observed, meaning that the small aggregated crystals of MOF-505 grow nuclei. This is due to the fact that no characteristic peaks related to other impurities are observed in these results of the PXRD patterns and FTIR spectra, implying that only the MOF-505 crystalline phase exists throughout the whole formation process. In addition, the content of Zn in MOF-505-10 decreases to 6.04% since the formation of MOF-505 leads to partial release of Zn in [(Zn,Cu)(OH)NO₃]. With further increase in reaction time in Fig. 3c–f, the process of MOF-505 crystal growth proceeds by subsequent attachment of small sheet growth units which then evolves into three-dimensional cubic structures. Hence, less two-dimensional sheet fragments are observed while more cubic crystals simultaneously appear as the reaction time prolongs. By analyzing the Zn content in the resultant MOF-505 surfaces, we can observe that the Zn content gradually decreases with further increase in reaction time. For this reason, the formation mechanism of the MOF-505 material by using the [(Zn,Cu)(OH)NO₃] intermediate is tentatively proposed in Fig. 4, demonstrating that the MOF-505 crystallization pathway follows the classical growth model proposed by Cölfen and Mann.^26,44 As the reaction time increases to 25 min, the content of Zn in MOF-505-25 is lowered to 3.25%; while with further increase in reaction time, the Zn content in MOF-505-30 slightly decreases to 2.79%. There are still minimal amounts of fragments in the MOF-505-25 and MOF-505-30 products in Fig. 3e and f, because small amounts of aggregated [(Zn,Cu)(OH)NO₃] sheets still remain in the resultant MOF-505 products since they cannot be dissolved well in acetone, confirmed by the coexistence of the Zn element. Meanwhile, the elemental EDX analysis of Cu and Zn is also complemented by ICP experiments, the results of which are consistent with the results of the EDX analysis, demonstrating the similar trend that the Zn content gradually decreases with further increase in reaction time. Confirming this point, the crystal growth process of MOF-505 can be completed within such short time.

Fig. 3 SEM images of (a) the [(Zn,Cu)(OH)NO₃] intermediate and MOF-505 products synthesized by the rapid room temperature method for (b) 10 min, (c) 15 min, (d) 20 min, (e) 25 min, and (f) 30 min.

Table 1 Elemental analysis of Cu and Zn on MOF-505 determined from EDX and ICP analysis

Sample	Cu^a [%]	Zn^a [%]	Zn/Cu^a [wt]	Zn/Cu^b [wt]
a The ratio of Zn and Cu determined by EDX analysis. b The ratio of Zn and Cu determined by ICP analysis.
(Zn,Cu)(OH)NO3	91.03	8.97	9.85%	7.06%
MOF-505-10	93.96	6.04	6.43%	6.21%
MOF-505-15	95.47	4.53	4.74%	5.83%
MOF-505-20	95.71	4.29	4.48%	5.70%
MOF-505-25	96.75	3.25	3.36%	5.58%
MOF-505-30	97.21	2.79	2.87%	5.36%

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Fig. 4 Schematic illustration of the rapid synthesis of MOF-505 at room temperature.

N₂ adsorption–desorption isotherms at 77 K were measured to analyze the porous properties of the MOF-505 products. As shown in Fig. 5, all the isotherms of the MOF-505 samples are type I and have a steep increase in the N₂ adsorbed amount at a low relative pressure (P/P₀ < 0.01), corresponding to materials whose porosity is dominated by micropores. The porosity parameters of the corresponding MOF-505 structure are summarized in Table 2, including the Brunauer–Emmett–Teller (BET) and Langmuir surface areas and the total and microporous pore volumes. As the reaction time increases from 10 min to 25 min, the surface area (such as BET surface area ranging from 646 m² g⁻¹ to 1076 m² g⁻¹) and pore volume (such as microporous pore volume ranging from 0.218 cm³ g⁻¹ to 0.365 cm³ g⁻¹) of these MOF-505 samples increase. MOF-505-10 exhibits a remarkably lower surface area and pore volume than the other MOF-505 samples. This is associated with the insufficient crystallization process for 10 min. However, MOF-505-30 shows a slight decrease in surface area and pore volume when the reaction time prolongs to 30 min. This may be attributed to the generation of some defects caused by partial dissolution–recrystallization in the MOF-505 crystals during the prolonged crystallization process, which can be confirmed by the change of pore size. The detailed pore size was analyzed by density functional theory (DFT) to support our speculation. As can be seen from Fig. S3,† the pore sizes of all the MOF-505 samples are mainly distributed in the range of 5 to 9 Å, in which two dominating peaks are centered at 5.9 and 8.0 Å. As expected, an obvious decrease in the pore size to 5.9 Å for MOF-505-30 is observed while some pores with the size of 8.0 Å are slightly increased to 8.6 Å, which is due to the generation of some defects in MOF-505-30 crystals. A similar change in pore size is also observed in the case of PCN-160 that creates crystal defects through removing pro-labile-linker fragments by acid treatment.⁴⁵

Fig. 5 N₂ adsorption–desorption isotherms of the MOF-505 products at 77 K.

Table 2 Porous properties of the MOF-505 structures

Sample	S BET (m^2 g^−1)	S Langmuir (m^2 g^−1)	V tol (cm^3 g^−1)	V mic (m^3 g^−1)
MOF-505-10	646	748	0.218	0.218
MOF-505-15	904	1055	0.393	0.303
MOF-505-20	990	1147	0.434	0.332
MOF-505-25	1076	1231	0.460	0.365
MOF-505-30	1040	1227	0.459	0.353

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Application of CO₂ adsorption and separation for MOF-505

It is well demonstrated that copper-based MOFs with open metal sites are beneficial to CO₂ capture because such open metal sites behave as Lewis acidic sites, which can serve as strong binding sites for CO₂ molecules and thus form favorable affinity toward CO₂ molecules.^5,10,46 Hence, MOF-505 is a promising material for CO₂ capture, since open metal sites are generated within the MOF-505 framework by removing coordinated guest solvent molecules under vacuum,^29,38,43 accompanied with an obvious colour change of the MOF-505 material from light blue to deep blue.⁴⁷ Accordingly, CO₂ isotherms were measured under ambient conditions (at 298 K and 100 kPa) to investigate its CO₂ adsorption behavior. As depicted in Fig. 6, MOF-505-25 has the highest CO₂ uptake among these MOF-505 materials, reaching 3.51 mmol g⁻¹ at 298 K and 100 kPa. By comparison, the CO₂ adsorption capacities of the MOF-505 materials (except MOF-505-10, 2.42 mmol g⁻¹) synthesized by the rapid room temperature method in this work are higher than that of MOF-505 (2.87 mmol g⁻¹) synthesized with the solvothermal method. It can be explained by the lack of ultramicropores (5.9 Å) under solvothermal synthesis conditions.⁴³ It has been established that high CO₂ uptake closely depends on the ultramicropores (<0.7 nm) under ambient conditions, in which the smaller pore size in MOFs leads to higher CO₂ uptake.^48,49 By comparison with the pore size, it can be explained by the fact that the lack of ultramicropores (5.9 Å) for MOF-505 by the solvothermal method leads to lower CO₂ uptake. Particularly, it is observed that the CO₂ adsorption closely follows the order of the BET surface area. Although the difference in the BET surface areas of MOF-505-25 (1076 m² g⁻¹) and MOF-505-30 (1040 m² g⁻¹) is small, the difference in the CO₂ uptake values of MOF-505-25 (3.51 mmol g⁻¹) and MOF-505-30 (3.19 mmol g⁻¹) is remarkably higher. This can be explained by the decrease in the pore size to 5.9 Å for MOF-505-30, as described above. Therefore, apart from the presence of open metal sites, the CO₂ uptake of the resultant MOF-505 strongly correlates with the pore size. Based on the above results, MOF-505-25 is chosen as the representative material to study its CO₂/CH₄ and CO₂/N₂ separation performance in the following section.

Fig. 6 CO₂ adsorption isotherms of the MOF-505 samples at 298 K.

CO₂, CH₄ and N₂ adsorption experiments were performed to evaluate the adsorption and separation performance of MOF-505-25 at three different temperatures. As shown in Fig. 7a–c, MOF-505-25 exhibits a steep adsorption isotherm for CO₂ at any temperature, which is an indication of a strong adsorption affinity toward CO₂ due to these open metal sites. At 100 kPa, the CO₂ uptake values of MOF-505-25 at 298, 288 and 278 K are 3.51, 4.36 and 5.48 mmol g⁻¹, respectively, which are much higher than the CH₄ and N₂ uptake values under the same conditions. However, a different adsorption behavior of CH₄ and N₂ for MOF-505-25 is observed compared with CO₂. The trend for CH₄ and N₂ adsorption is similar and almost linear in the measured pressure range, suggesting a much weaker adsorption affinity toward CH₄ and N₂ compared with CO₂. Thereby, the uptake of MOF-505-25 for CH₄ is 1.00 mmol g⁻¹ at 298 K and 100 kPa whereas the N₂ uptake is only 0.28 mmol g⁻¹.

Fig. 7 CO₂, CH₄ and N₂ adsorption isotherms of MOF-505-25 at (a) 298 K, (b) 288 K and (c) 278 K. (d) The isosteric heat of adsorption (Q_st) of CO₂ for MOF-505-25.

For quantitative evaluation of the adsorption affinity between CO₂ and the MOF-505 framework, the Clausius–Clapeyron equation was used to calculate the isosteric heat of adsorption (Q_st) of CO₂. Fig. 7d presents the Q_st values of CO₂ for MOF-505-25. The Q_st shows a declining trend as a function of adsorbed amount, confirming that these adsorption sites for CO₂ on the MOF-505-25 surface are heterogeneous.⁵⁰ The Q_st values are located in the range from 28.0 kJ mol⁻¹ to 31.2 kJ mol⁻¹, meaning that the CO₂ adsorption behavior of MOF-505-25 is a physical adsorption process, which are comparable to those of previously reported porous adsorbents.^51,52 At the initial stage, CO₂ molecules preferentially adsorb at the open metal sites due to the strong binding interactions of adsorbing CO₂. Limited by the number of open metal sites, only less favorable sites are available as the loading increases. As a result, the gradual decrease in the Q_st value reveals this adsorption process. Additionally, the modest Q_st value is an advantageous property, indicating that the regeneration can be conducted under mild conditions with less energy penalty.

Adsorption selectivity is an important factor to evaluate the separation performance of an adsorbent material. Therefore, we used ideal adsorbed solution theory (IAST) to calculate the adsorption selectivities for CO₂/N₂ (0.15 : 0.85) and CO₂/CH₄ (0.5 : 0.5) mixtures. As displayed in Fig. 8, both CO₂/N₂ and CO₂/CH₄ adsorption selectivities have a declining trend with the increase in pressure. Particularly, the declining trend is remarkably obvious in the low pressure region, in which the initial adsorption selectivities are determined to be 59 and 12 at 298 K for CO₂/N₂ and CO₂/CH₄, respectively. Even when the pressure is increased to 100 kPa, the CO₂/N₂ and CO₂/CH₄ selectivities are still high up to 29 and 7. These results indicate that MOF-505-25 exhibits excellent CO₂ separation performance, showing a potential material for CO₂ capture.

Fig. 8 IAST selectivities for CO₂/N₂ and CO₂/CH₄ mixtures for MOF-505-25 at 298 K.

Conclusions

In conclusion, we have provided a facile synthetic route for the rapid room temperature synthesis of MOF-505, in which the key is the exploitation of layered [(Zn,Cu)(OH)NO₃] as the intermediate that is capable of fast anion exchange between OH⁻ and NO₃⁻ in the [(Zn,Cu)(OH)NO₃] and bptc⁴⁻ and thus induces efficient MOF-505 crystallization. By investigating the effect of reaction time on the MOF-505 microstructure, we confirm the optimized reaction time of 25 min with respect to CO₂ uptake. Because of the coexistence of open metal sites and ultramicropores, MOF-505-25 has a moderate CO₂ uptake of 3.51 mmol g⁻¹ at 298 K and 100 kPa and high CO₂/CH₄ and CO₂/N₂ adsorption selectivities of 29 and 7. Additionally, the CO₂ adsorption heat is modest and advantageous for adsorbent regeneration. This facile and rapid room temperature synthetic procedure is promising for MOF large-scale production and acceleration of industrial applications. We believe that this facile synthetic route presented here can be extended to synthesize new MOF and MOF-based composite materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21576092, 21436005 and 21878101) and the Guangdong Province Science and Technology Project (No. 2017A020216016).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ce01489b

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