Synthesis, Fine Structural Characterization, and CO2 Adsorption Capacity of Cobalt and Nickel-Based Metal Organic Framework-74

Abstract: Two metal organic frameworks of MOF-74 group (cobalt and nickel-based) were synthesized, characterized, and evaluated for CO₂ adsorption. The BET specific surface areas of MOF-74(Co) and MOF-74(Ni) MOFs were 1,406 and 1,418 m²g^-1, respectively with the identical pore volume of 0.84 cm³g^-1. CO₂ adsorption capacities of MOF-74(Co) and MOF-74(Ni) were 3.35 and 4.07 mmol·g^-1, respectively measured at 273 K and 1.1 bar. The oxidation state of central atoms in MOF-74(Co) was composed of Co(II) and Co(III) confirmed by XANES spectra while MOF-74(Ni) was Ni(II) central atoms. The bond distances of Co„ŸO and Ni„ŸO were 1.95 and 1.97 Å, respectively.

 

1. Introduction

Global population and the industrialization are increasing and hence, the energy consumption is also increasing swiftly. Till now, the main source of energy is the burning of fossil fuels. However, burning of fossil fuels releases a vast amount of CO₂ greenhouse gas into atmosphere which causes the raise of average surface temperature of the earth. As the present developments in CCS technology are not sufficiently effective by means of cost and energy consumption, it has prompted to improve the existing technologies and find some new alternatives [1]. Metal organic frameworks (MOFs) are a new class of materials that have attracted substantial attention in past few years [2]. MOFs are synthesized using organic linkers and metal joints that self-assemble to form different dimensional networks. It can generate unusually large diameter channels and cavities which is particularly attractive for CO₂ gas storage application. Here we report the CO₂ gas adsorption capacity on same type of two MOFs; such as MOF-74(Co) and MOF-74(Ni).

2. Experimental

In a typical experiment, 0.75 g of 2,5-dihydroxyterephthalic acid (DHTA) and 3.0 g of Co(NO₃)₂·6H₂O  or Ni(NO₃)₂·6H₂O were dissolved in 150 mL of DMF with sonication in a 250 mL Duran bottle. The bottle was capped tightly and placed in a 110°C oven for 20 h. After cooling to room temperature, the mother liquor was decanted and the crystals were washed with DMF. The product was immerged into methanol for six days. The products were then evacuated and heated under vacuum to150°C over a period of 12 h.

3. Results and discussion

The FE-SEM images of MOF-74(Co) and MOF-74(Ni) samples are shown in Fig.1a and 1b, respectively. It shows that MOF-74(Co) has cauliflower type structure with the particle size of about 4-6 mm and MOF-74(Ni) has uniformly distributed spherical shape with the average diameter of 2-4 mm, although some particles are aggregated. The XRD patterns (Fig.1c) shows that both samples have the unique crystalline phase indexed on the basis of a face-centered hexagonal unit cell of MOF-74. Both samples indicate almost similar diffraction patterns at identical 2Į values ascribed the formation of similar crystallographic phases.

Figure 1. -SEM images of (a) MOF-74(Co), (b) MOF-74(Ni), and (c) XRD patterns of synthesized MOF-74 samples.

MOF-74(Co) was thermally stable up to 258°C while in case of MOF-74(Ni) the value was 344°C confirmed from the TGA analysis shown in Fig. 2. The thermogravimetric curves of both samples are consistent with prominent weight-loss steps. It can be seen that the thermal stability is not similar for these two samples although they follow the same trend of loosing weight. In case of MOF-74(Co), a fully hydrated molecular sieve contains about 27 wt.%  of water and other guest molecules. and in case of MOF-74(Ni) this value is almost similar, but there is one additional step of weight loss of about 13 wt%.

Figure 2. - TG/DTA curves of as-synthesized MOF-74(Co) and MOF-74(Ni)

From the N₂ adsorption isotherms it has been evaluated that both sample follow type-I isotherms with almost no hysteresis loop which corresponding to microporous material. The BET specific surface areas of MOF-74(Co) and MOF-74(Ni) MOFs were 1,406 and 1,418 m²g^-1, respectively with the identical pore volume of 0.84 cm³g^-1. DFT pore size distribution of MOF-74(Zn) and MOF-74(Cu) are shown in Fig. 4a and Fig. 4b, respectively while the values of average pore size and volume are presented in Table 1. These results suggest that these samples are microporous materials.

Figure 3. - N₂ adsorption/desorption isotherms of as-synthesized MOF-74(Co) and MOF-74(Ni). Filled and open symbols represent the adsorption and desorption of nitrogen gases, respectively.

 

Figure 4. DFT pore size distribution of (a) MOF-74(Co) and (b) MOF-74(Ni)

 

Fine structure and oxidation state of Zn and Cu species in MOF-74 were further investigated through X-ray absorption spectroscopy. The data were collected several times and standard deviation was also calculated. The XANES region of the X-ray absorption spectrum is very informative on both the oxidation and coordination states of metal ions. XANES spectra of MOF-74(Co) and MOF-74(Ni) samples are shown in Fig. 5a and 5b, respectively along with the standard materials. The comparison is designed to determine the oxidation states of center atoms in MOFs and understanding the effects of DHTA linkers and guest molecules on the crystal structures. It was found that the edge position (7709.46 eV) of as-synthesized MOF-74(Co) sample is very close to the Co(II). However, compared with the standard Co(II) the offset was +0.46 eV, revealed that Co ions were not entirely surrounded by oxygen atoms and hence formed hydroxides (OH^-) through covalent bond. This result reveals that the as-synthesized sample was not totally moisture or guest molecule free. The source of the OH^- ions is either adsorbed moisture or the organic linkers that cause the little distortion in the crystal structure. In case of MOF-74(Ni) the offset was +0.57 eV.  Fine structural parameters for both samples are shown in Table 2 where it can be seen that the bond distances of Co„ŸO and Ni„ŸO were 1.95 and 1.97Å, with the coordination number of 4.27 and 4.23, respectively.

 

Figure 5. (a) Co K-edge XANES spectra of MOF-74(Co) samples with Co(0) standard, (b) Ni K-edge XANES spectra of MOF-74(Ni) with Ni(0) standard, and (c) CO₂ adsorption isotherms of MOF-74 samples measured at 273 K. Filled and open circles represent the adsorption and desorption, respectively.

The CO₂ adsorption capacities of the synthesized samples are shown in Fig. 5c. The steady ascending slope of the CO₂ adsorption isotherms represents strong adsorption of CO₂ molecules having quadrupoles onto the open metal sites in MOF-74 samples. The CO₂ adsorption capacity of MOF-74(Ni) measured at 273 K and 1.1 bar was 4.07 mmol·g^-1; significantly higher than MOF-74(Co) which was 3.35 mmol·g^-1. However, both results are much higher than other prospective adsorbents of CO₂ such as Zeolite 13X [3] and ZIF-96 [4]. The elevation in CO₂ adsorption in MOF-74(Ni) may attribute to the higher surface area than MOF-74(Cu) and more importantly the strong affinity to CO₂ molecules due to different electronic configurations. It can be seen that the adsorption and desorption cycles are almost reversible for both samples. This integrity was also corroborated by the N₂ adsorption. The steady uphill slope of the adsorption curves indicates that higher adsorption capacity may obtain at higher pressure.

 

4. Conclusions

The MOF-74 samples were capable to store significant amount of CO₂ at relatively lower pressure. The difference in CO₂ adsorbed amount between two samples may attribute to the different affinity of cobalt and nickel ions to CO₂ molecules as well as the specific surface area. The moderate thermal stability confirms that these MOFs may apply to control the CO₂ gas from the emission of refinery or other industries.

 

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