In situ synthesis of carbon nanotube doped metal–organic frameworks for CO₂ capture

Abstract

Metal organic–frameworks (MOFs) with intriguing structural motifs and unique properties are potential candidates for carbon dioxide (CO₂) storage. Although structures with the single functional constructions and micropores were demonstrated to capture CO₂ with high capacities at low temperature, their feeble interactions still limit practical applications at room temperature. Herein, we report in situ growth observation of hierarchical pores in copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC) doped MOFs which gives high adsorption and enhances the CO₂ binding ability. Thus, understanding this CO₂-capturing mechanism, which has been causing controversy, is crucial for further development toward advanced study. The doped MOFs exhibit high specific surface areas of 1180 m² g⁻¹ and show good capacity to store CO₂, which is mainly due to the presence of acid and amine functionalized CNTs and a large amount of narrow micropores (<1.0 nm).

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The mitigation of CO₂ emissions has attracted considerable attention due to the fact that this gas is the main anthropogenic contributor to climate change.^1–3 Amongst various sources, fossil fuel combustion causes most of the emissions. The challenge of limiting CO₂ concentration has attracted extensive efforts of researchers devoted to it.⁴ Many methods and mechanisms have been developed for capturing and separation of CO₂ which include sundry separation, adsorption and absorption methods.⁵

MOFs are microporous crystalline structures composed of central cation molecules linked together by organic linkers (ligands) to form a three-dimensional structure. The high surface area, porosity, thermal stability, tunable pore size and topography together with low densities makes them especially interesting material for applications in catalysis, nonlinear optics, adsorption, storage and separation of gases.^6,7 Recent studies about the composites of MOFs and different substrates (graphite oxide,^8–10 polymers,^11,12 alumina, silica^13,14 and carbon nanotubes^15,16 etc.) that address the problems related to MOFs. In particular, incorporation of CNTs into MOFs can obtain better crystals to enhance the composite performance because of the unusual mechanical, thermo conductive, electro conductive and hydrophobic properties of the CNTs.^17,18 Specific MOFs have shown superseding adsorption capacities for CO₂ and H₂ uptake compared to the activated carbon and zeolites under the same conditions of adsorption.^19–21

Although a great deal of attention has been focused on developing a MOF for capturing flue-gas i.e., CO₂ it is clear that simply targeting high capacities is quite inadequate.^22–28 Addition of multiwall carbon nanotubes (MWCNTs) in the synthesis process of Zn MOF-74 and Cu-BTC will affect the structure and properties. However Zn MOF-74 has been chosen for present study because of high porosity and adsorption capacity. In scenario of CO₂ capture from flue emissions, CNTs functionalized with amine have gained consideration due to their exclusive physicochemical characteristics in addition to great thermal and chemical strength.^29–31 The reported literature states them as promising CO₂ capturing solid materials.^32–37 Recently, Z. Xiang et al. reported new materials name covalent organic frameworks (COFs) having huge surface area and porosity and high CO₂ absorption. However, complex porosity control process of COFs and costly raw materials limit the easy adaptability of COFs for commercial purposes. Therefore, it is strongly needed to explore materials having easier porosity control.³⁸

Cu-BTC is commonly used for gas adsorption capacity but its extremely water sensitive even under atmospheric conditions and shows sharp reduction in surface area after exposure to humid air.³⁹ Cu-BTC can be used as very promising target for commercialization if its water stability is increased. The use of CNTs can control Cu-BTC hydrophilicity and maximize its CO₂ adsorption. Present work is focused to examine the local coordination of copper centers in the structure in order to modify Cu-BTC and Zn MOF-74 properties by inducing controlled amount of modified linker or any adsorptive material in the structure. Here we developed an in situ technique by tuning amounts of acid and amine functionalized CNTs for the synthesis of CNT doped MOFs. We precisely controlled the hierarchical composite nanostructures that increased CO₂ capturing through solvothermal and nano-doping technique.

Fig. 1a and b presents FE-SEM images of particle shape and microstructure of as-synthesized Cu-BTC MOFs. The developed MOF samples possess nearly identical particle size with octahedral morphology (1–10 μm), and demonstrate porous structure and rough surface which favours CO₂ adsorption. The morphology of Zn MOF-74 indicates needle like yellow crystals with approximately the size 1–50 μm, in Fig. 1c and d (low magnification images in ESI S2†).

Fig. 1 (a) FE-SEM image of crystalline structure of Cu-BTC MOF, (b) FE-SEM image of porous structure of Cu-BTC MOF, (c) FE-SEM image of crystalline structure of Zn MOF-74, and (d) FE-SEM image of porous structure of Zn MOF-74.

N₂ adsorption–desorption measurement was conducted to investigate the porous structure of the as-prepared doped MOFs. Fig. 2a shows that the isotherms of all samples demonstrate a series of typical adsorption/desorption behaviour including micropores filling, monolayer adsorption, multilayer adsorption and capillary condensation.¹⁵ For pure MOF, most of nitrogen adsorption occurs at a low relative pressure (P/P₀ < 0.1), and a plateau appears at a middle relative pressure, demonstrating a typical microporous structure with few mesopores. A slight decrease of the adsorption plateau could be attributed to the distortion of micropores.

Fig. 2 (a) N₂ adsorption–desorption isotherms, and (b) 2D-NLDFT pore size distribution curves of pure and doped Cu-BTC MOF.

For detailed and more specific examination of porous structure of samples, specific surface area, total pore volume, average pore diameter and pore size distribution, were determined using BET surface area method (Fig. 2a and b). The specific surface area of conventional samples for pure MOF, amine and acid functionalized Cu-BTC MOF was found out to be 725, 951, and 1180 m² g⁻¹ respectively. This result indicates a major contributing role of functionalized CNTs on the specific surface area.

The solvothermal method gives good porous structure and CO₂ adsorption shown as in results. Doped Zn MOF-74 and Cu-BTC with acid as well as amine functionalized CNTs developed higher surface area and resulted in higher CO₂ adsorption.

In particular, the pore size distribution and pore volume in active materials are important characteristics for CO₂ adsorption. The enhanced mesopores volume provides low resistance pathway and short diffusion route for gas adsorption because of the admission of larger amount of CO₂ into the pore volumes (Fig. 2b). Therefore, increase the amount of CNTs directly effects the performance of CO₂ adsorption. Generally, these doped MOFs materials show type IV isotherms with an H1 hysteresis loop, indicating the existence of some mesoporous structures (Fig. 2a). A sharp capillary condensation step can be observed in agreement with their well-ordered structures.

Due to its structure, Zn MOF-74 can be considered as not a highly porous MOF, functionalized CNTs and other polar solvent molecules preferably bind to the open Zn coordination sites that face the centre of the larger pores. Cu-BTC shows crystals structure which are highly porous compounds having large affinity to adsorb CO₂. SEM images of Cu-BTC show its size range in microns. Cu-BTC can be considered more porous hydrophilic MOF, as compared to Zn MOF 74.

Fig. 3 shows the XRD patterns of the pure MOF and doped MOFs. It can be seen that the positions and relative intensities of the diffraction peaks matched well with standard patterns, indicating a successful synthesis. The XRD pattern of Cu-BTC clearly shows the crystalline nature of the sample. The main peaks at 200, 220, 222 and 400 correspond to successfully synthesized crystal planes as compared to reported Cu-BTC.⁴¹ CNTs represent the highest crystallinity among carbon allotropes, and the diffraction peaks appears at 222 and 511 planes. Sharpness of peaks also implies that amorphous contents are very less which alone is in accordance with SEM images. XRD patterns of as-prepared samples show that Zn MOF-74 is the unique crystalline phase. All XRD patterns were indexed on the basis of a face-centred hexagonal unit cell, which are in good agreement with their ionic radii (Zn²⁺ > Cu²⁺ > Mg²⁺ > Co²⁺). The differences in the unit cell parameters obtained qualitatively support the metal incorporation into the Zn MOF-74 framework. The main peaks (110, 300, 101, and 131) are corresponded to the octahedral geometry that is similar to reported Zn MOF-74 (ref. 40) (ESI S3†).

Fig. 3 XRD pattern of (a) pure Cu-BTC, (b) acid, and (c) amine functionalized CNTs modified Cu-BTC MOF.

There are several potential advantages of using MOFs for gas storage compared to gas cylinders. Firstly, there can be an increased storage density when compared with gas cylinders, zeolites, and activated carbons. Secondly, in applications where only a small amount of gas is required, it could be easier to handle the gas in the form of solid pellets with adsorbed gas whose capacity can be tailored for application.²⁸ Thirdly, it might be safer to handle a gas when it is adsorbed on a solid, especially if higher pressures can be avoided. This is particularly applicable in the case of biological gases, where controlled delivery of the gas is required.

Microporous MOFs are promising materials for CO₂ capture and separation. As we can rationally design and synthesize some highly porous doped MOFs, which can take up significant amount of CO₂ under high pressure (a few extremely highly porous doped MOFs with BET surface areas over 1000 m² g⁻¹ have been developed, the CO₂ storage capacities at room temperature and high pressure are basically proportional to the surface areas of the MOF materials), it is quite straight forward and feasible to realize doped MOF materials for high pressure CO₂ capture. CO₂ adsorption/desorption isotherms at STP of acid and amine functionalized Cu-BTC at high pressure and low pressure has been shown in Fig. 4. At higher pressure the amine functionalized Cu-BTC MOF has shown better adsorption capacity around 130 cm³ g⁻¹ as compared to acid functionalized Cu-BTC MOF, whose adsorption capacity is 108 cm³ g⁻¹.

Fig. 4 CO₂ adsorption/desorption isotherms at STP, (a) acid/amine functionalized Cu-BTC at high pressure (bar), and (b) acid/amine functionalized Cu-BTC at low pressure (mmHg).

The narrow hysteresis loop of adsorbed and desorbed curves for both types of acid and amine functionalized Cu-BTC MOF have shown that the same quantity CO₂ gas has been adsorbed and desorbed. The porosity enhancement of the MOFs incorporated with functionalized CNTs gives good result for CO₂ adsorption. It is found that surface texture of functionalized CNTs are one of the critical factors for CO₂ adsorption.

Fig. 5 shows the quantity of CO₂ adsorbed and desorbed for Zn MOF-74 by applying pressure between 0–5 bars. As the applied pressure increased, the rate of adsorption also increased and reached to the value of 128 cm³ g⁻¹ at 4.67 bars. The narrow hysteresis loop of adsorbed and desorbed curves has shown that almost same quantity of CO₂ gas has been desorbed.

Fig. 5 CO₂ adsorption/desorption isotherms at STP of amine functionalized Zn MOF-74.

The CO₂ storage capacities at room temperature depend on their pore size and surface areas which is clearly demonstrated in doped MOF materials. Generally, surface functional groups serve as anchoring sites for metal precursors, which influence metal loading. However, it cannot be ignored that not all the anchoring of metal species is equally related to the functional groups. Therefore, it cannot be easily deduced that there exists a quantitative correlation between the metal loading and functional groups. Besides, the amine functionalization can change the surface chemistry of the CNTs, e.g., hydrophobicity, polarity and proton affinity, which may influence the adsorption of the charged metal precursor in solution. Hence, all aspects should be taken into consideration.

Such a situation possess a great challenge to synthetic materials chemists on how to target some unique porous doped MOF materials, for which CO₂ capture can be significantly increased at ambient conditions. Ideally, we already have the resolution, that is, to immobilize specific sites such as open metal sites and –NH₂ sites to induce their strong interactions with CO₂, and to optimize the pore/cage sizes to maximize the van der Waals interactions between the pore surfaces and CO₂. However, only a few porous MOF have been realized for high CO₂ capture at ambient conditions. The fact that M-MOF-74 (M = Ni²⁺, Co²⁺, Zn²⁺ and Fe²⁺), and doped Cu-BTC with open metal sites exhibit high CO₂ uptake at room temperature at 5 bar, indicates that both the different types of open-metal sites and structures have important roles to enforce their strong interactions with CO₂ at ambient conditions. Doped Cu-BTC and Zn MOF-74 have successfully been employed for CO₂ adsorption. Their unique structure and properties are responsible for their current and future applications over diverse fields.

Conclusions

In conclusion, we successfully prepared the doped MOFs that are a new class of porous materials and have shown great CO₂ adsorption. While the linkers of MOFs influence the CO₂ adsorption performance greatly, the mechanism about it is still not clear. In Cu-BTC and Zn MOF-74 the effects of functionalized CNTs modifications increasing the specific surface area and CO₂ adsorption in MOFs in an effective way. This work will be helpful to design and synthesis of new materials that have high surface area and higher CO₂ adsorption abilities. There is margin for further research which can help in revealing more about the chemistry of these doped MOFs.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 51503028), the Fundamental Research Funds for the Central Universities, the “DHU Distinguished Young Professor Program”, and the project was funded by State key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (LK1513).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis procedure, materials and Xrd for Zn-MOF-74 are discussed. See DOI: 10.1039/c5ra25465e

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