Controlled Synthesis of Mg-MOF-74 and Its CO₂ Adsorption in Flue Gas

Abstract

Yellow nanorod Mg-MOF-74 was obtained through a solvothermal synthesis method. The XRD, N₂ adsorption–desorption isotherms, SEM, and TGA results suggest that the particle size of Mg-MOF-74 reaches 400 nm after the introduction of sodium acetate in the precursor of Mg-MOF-74. Furthermore, the morphology of Mg-MOF-74 changed from cauliflower- to rod-shaped particles. At the same time, the BET specific surface area and pore volume of Mg-MOF-74 also increased a lot. Then, the CO₂ dynamic adsorption capacity of Mg-MOF-74 was measured with a self-made fixed bed at 30 °C and a 0.1 bar CO₂ partial pressure. The results show that the CO₂ adsorption capacity of Mg-MOF-74-N2 reaches 3.67 mmol/g, and its CO₂ adsorption capacity remains unchanged after 10 CO₂ adsorption–desorption cycles.

1. Introduction

Excessive CO₂ emissions caused by the burning of fossil fuels and human activities have led to increasing global warming, melting glaciers, frequent climate disasters, and a sharp decline in plant and animal diversity. The United Nations (IPCC) has issued a goal of reducing global CO₂ emissions by 41% by 2050 compared with 2010 [1]. In China, power plants are a concentrated source of CO₂ emissions, accounting for 40% of the total CO₂ emissions. Controlling CO₂ emissions in the flue gas of power plants is the key to achieving CO₂ emission reduction goals. At present, CO₂ capture and storage (CCS) technology is a key technology for mitigating and controlling CO₂ emissions, and reducing the cost of the capture stage is the top priority in implementing this technology. Based on the low CO₂ content in the flue gas of power plants, the commonly used CO₂ capture methods include absorption, adsorption, membrane separation, and low-temperature distillation. Among them, the adsorption method has the advantages of being a simple process, having mild operating conditions and low energy consumption, and incurring no corrosion to equipment, so it is the most promising capture method at present. At present, the most commonly used materials in the field of CO₂ adsorption are activated carbon, molecular sieves, metal oxides, and metal-organic frameworks (MOFs) [2,3,4,5]. A high CO₂ adsorption capacity, a fast absorption/desorption rate, good selectivity, good adsorption stability, and low regenerative energy consumption are the conditions for the preparation of excellent CO₂ adsorbents. MOFs are a new type of crystalline porous material composed of metal ions and organic ligands [6]. The high specific surface area, developed porosity and tunable chemical composition characters make MOFs widely used in gas adsorption and separation [7,8,9,10,11], drug carrier [12], catalysis [13], sensors [7,14], and other fields. With the development of nanotechnology, nanoscale MOFs have attracted the attention of scientists because of their special morphology, particle size, and other excellent characteristics. At present, the synthesis methods of nanometer MOFs include the direct precipitation method, the microwave-assisted method, the hydrothermal/solvothermal synthesis method, the mechanical method, and so on.

For example, Ahn, et al. [15] reported a method for the preparation of nanoscale Mg-MOF-74 using ultrasonic chemistry, which uses triethylamine as the deprotonation agent and greatly reduces the reaction time. Sun’s [16] research group used polyethylpyrrolidone and ammonia water to regulate the morphology and particle size of Mg-MOF-74. In addition, the N₂ adsorption and desorption results also show that the morphology and particle size of Mg-MOF-74 affect the adsorption properties of the gas. Chen’s [12] research group prepared Mg-MOF-74 with a particle size of less than 200 nm through a two-step method and explored its application as a drug carrier. Zhang [17] et al. reported that a Mg-MOF-74/sodium alginate composite aerogel was prepared by loading Mg-MOF-74 on sodium alginate sheets, followed by directional freeze-drying. The experimental data showed that the composite achieved a CO₂ adsorption of 2.461 mmol/g at 298 K. This may be due to the hierarchical pore structure formed by the composite aerogel. The micropore/mesopore structure of Mg-MOF-74 can improve the interaction between CO₂ and the adsorbent and enhance the gas mass transfer rate [1,18,19]. However, there are still few studies on CO₂ adsorption of nanosized Mg-MOF-74 in flue gas.

In this paper, nanorod Mg-MOF-74 particles were synthesized by a simple solvothermal method without surfactants. The morphology and particle size of the MOFs were adjusted by controlling the concentration of sodium acetate (NaAc). The dynamic adsorption performance of nanoscale Mg-MOF-74 under a simulated flue gas environment was tested by a self-made fixed bed, which provided a theoretical basis for the design and synthesis of a CO₂ adsorbent suitable for a power plant flue gas environment.

2. Experimental Part

2.1. Reagents and Characterization

2,5-dihydroxyterephthalic acid (H₄DOBDC), magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), N,N-dimethylformamide (DMF), anhydrous methanol (CH₃OH), and sodium acetate (NaAc) were bought from Shanghai Aladdin Reagent Co., Ltd. (No. 809, Chuhua Branch Road, Fengxian District, Shanghai). The reagents used were all analytically pure. CO₂ (high purity) was obtained from Beijing Beiwen Gas Factory, and N₂ (high purity) was obtained from TISCO Gas Factory.

An X-ray diffraction analysis was performed with a D8 Advance X-ray diffractometer from Nikaku (Rigaku Corporation, Woodlands, TX, USA) using Cu Kα radiation (λ = 0.154 nm), a metal Ni filter, a tube pressure of 40 kV, and a tube flow of 30 mA. The N₂ adsorption and desorption isotherms of the sample at 77 K were determined with an ASAP 2020 automatic physical adsorption instrument (Micromeritics, Norcross, GA, USA). The sample was vacuumed and degassed at 200 °C for one night before the test. The microstructure and morphology of the sample were observed using a JSM-7001 thermal field emission scanning electron microscope (JEOL Ltd., Tokyo, Japan). The sample was coated with a conductive adhesive and scanned after gold spraying. During the test, the acceleration voltage was 5–10 kV. A Rigaku TG thermogravimetric analyzer (Rigaku Corporation, Woodlands, TX, USA) was used to study the thermal stability of the sample. Under an N₂ inert atmosphere, the weight of the sample was 10 mg. The thermogravimetric curve of the sample was tested in the range of 25–600 °C, and the heating rate was 10 °C/min.

2.2. Synthesis of Mg-MOF-74

H₄DOBDC (0.674 g, 3.4 mmol) and Mg(NO₃)₂·6H₂O (2.8 g, 10.9 mmol) were added to 300 mL of a DMF/EtOH/H₂O mixed solution with a volume ratio of 15:1:1 and stirred until dissolved. Then, 0–2 equivalent amounts of NaAc were added to the mixture, and then the reaction liquid was placed in a crystallization kettle and reacted in an oven at 125 °C for 20 h. Finally, the mother liquid was poured out to obtain yellow microcrystals. The obtained crystals were impregnated into methanol solution for replacement once every two days for three consecutive times, and the crystals were washed with N, N-dimethylformamide (DMF) three times and then placed in a 200 °C vacuum-drying oven to remove the solvent and obtain Mg-MOF-74. The resulting sample is named Mg-MOF-74-N(0-2), where N represents NaAc and 0–2 represents the equivalent amount added to NaAc.

2.3. Evaluation of Adsorbents

The CO₂ dynamic adsorption breakthrough experiment of the adsorbents were carried out on a self-made fixed bed. An appropriate amount of adsorbent was pressed through the screen, and 20–40 mg of adsorbent was put into a U-shaped quartz tube with an inner diameter of 5 mm. The adsorbent was activated in an Ar atmosphere at 200 °C for 6 h with a gas volume flow rate of 60 mL/min and a constant inlet concentration. The sample temperature dropped to the required temperature and maintained it during the CO₂ adsorption process after activation. Switch the gas valve of CO₂ and N₂ (CO₂/N₂-1:9) and adjust the gas flow. Gas chromatography was used to detect the concentration of the air outlet every 10 s until the CO₂ concentration of the air outlet was the same as the concentration of the air inlet, which suggests the adsorption process is over. The CO₂ desorption experiment was carried out at a high temperature in an argon atmosphere. After desorption, the temperature was lowered to the specified adsorption temperature, and then the CO₂ adsorption experiment was carried out.

According to the concentration of CO₂ at the air outlet, the CO₂ penetration curve of the material can be obtained, and the CO₂ adsorption capacity can be calculated according to the penetration curve. The specific calculation formula is as follows:

$$q=\frac{Q\left(t_s-t_0\right)C_{in}}{22.4W}$$

(1)

$$t_s={{\displaystyle \int }}_0^t\left(1-\frac{C_{out}}{C_{in}}\right)d_t$$

(2)

where t_s(s) is the average residence and adsorption time, t₀(s) is the average residence and adsorption time of the empty bed, C_in and C_out are the CO₂ concentrations of the import and export, respectively, that is, the volume fraction of CO₂ in the gas, q is the CO₂ equilibrium adsorption capacity of the material (mmol/g), and Q is the volume flow rate of the mixture (mL/s). W is the mass of the adsorbent.

3. Results and Discussion

To investigate the effect of the NaAc concentration on the structure of Mg-MOF-74, 0–2 equivalent amounts of NaAc were added to the precursor solution of Mg-MOF-74, and the resulting products were named Mg-MOF-74-N0, Mg-MOF-74-N0.5, Mg-MOF-74-N1, and Mg-MOF-74-N2, respectively. The obtained samples were characterized by XRD, and the results are shown in Figure 1. It can be seen from Figure 1 that the XRD diffraction peak of the samples is consistent with the XRD pattern reported in the literature [20]. After adding NaAc, the diffraction peak of Mg-MOF-74 did not change, indicating that the introduction of NaAc did not destroy the crystal structure. However, after adding NaAc, the diffraction peak intensity decreased rapidly, and with the increase in NaAc concentration, the diffraction peak intensity decreased gradually. The results showed that the crystallinity of the crystals decreased after the introduction of NaAc into the precursor solution of Mg-MOF-74.

To further investigate the effect of the NaAc concentration on the crystal morphology of Mg-MOF-74, the samples were characterized by SEM. As can be seen from Figure 2, without adding NaAc, the obtained Mg-MOF-74 showed the morphology of a cauliflower, consisting of rod-like particles aggregated into a cauliflower shape, which was consistent with the literature reports. The particle size of Mg-MOF-74-N0 was about 17 μm, and the size of the cauliflower particles was decreased to 13 μm after adding 0.5 equivalent amounts of NaAc. When the concentration of NaAc increased to 1 equivalent, Mg-MOF-74-N1 mainly presented rod-like particles, and the length and the diameter of rod particles were 5 μm and 1 μm. When the concentration of NaAc was further increased to two equivalent amounts, the obtained Mg-MOF-74-N2 still had rod-like particles with a diameter of about 3μm and a uniform distribution, and the diameter of the particles was about 400 nm. With the increase in NaAc concentration, the particle size of Mg-MOF-74 decreased from microns to nanometers.

To further test the pore structure properties of nanoscale Mg-MOF-74, N₂ adsorption and desorption experiments were performed on the sample, and the results are shown in Figure 3. It can be seen from the N₂ adsorption and desorption isotherms of the sample that the adsorption isotherm of Mg-MOF-74 synthesized without adding NaAc conforms to a type I adsorption isotherm, indicating that the pores in Mg-MOF-74 are micropores. The H3 hysteresis ring in the adsorption isotherm of Mg-MOF-74-N2 indicates the presence of mesoporous pores in the sample. This may be due to a reduction in the size of the crystals, resulting in the accumulation of holes. The pore volume, specific surface area, and pore diameter of Mg-MOF-74 and Mg-MOF-74-N2 are listed in

Table 1. Pore volume, specific surface area, and pore size of Mg-MOF-74 and Mg-MOF-74-N2.

Samples	Specific Area (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
Mg-MOF-74	570.3	0.218	0.91
Mg-MOF-74-N2	624.7	0.264	0.92

. It can be seen from the data in the table that the pore volume increases with the increase in the specific surface area of Mg-MOF-74-N2, and the pore size remains almost unchanged. This shows that the addition of NaAc helps to increase the specific surface area of the crystal, and the increase in pore volume is due to the formation of many accumulation holes due to the reduction in crystal size, which leads to an increase in the total pore volume of the sample [21].

To further test the thermal stability of the Mg-MOF-74 crystal, we performed a thermogravimetric analysis of the sample, and the results are shown in Figure 4. The weight loss from room temperature to 100 °C is attributed to the moisture adsorbed in the Mg-MOF-74 hole, and the weight loss from 100 to 250 °C is attributed to the solvent adsorbed in the hole [22]. It can be seen from the thermogravimetric curve that the weight of Mg-MOF-74-N2 no longer decreases at 500 °C, while Mg-MOF-74 continues to lose weight at 600 °C, so the thermal stability of Mg-MOF-74-N2 is reduced compared with Mg-MOF-74.

A self-made fixed bed was used to test the dynamic CO₂ adsorption performance of nanoscale Mg-MOF-74 in the simulated flue gas environment, and the dynamic adsorption penetration curve is shown in Figure 5. According to Formulas (1) and (2), the dynamic CO₂ adsorption capacity of Mg-MOF-74-N2 under 30 °C and a 0.1 bar CO₂ partial pressure is 3.63 mmol/g, which is much higher than the 1.62 mmol/g absorption capacity of Mg-MOF-74-N0 under the same conditions. This may be due to the increase in the specific surface area and pore volume of the synthesized Mg-MOF-74-N2 nanoparticles after the addition of NaAc, so their CO₂ adsorption performance is also improved.

To test the regeneration performance of Mg-MOF-74-N2, we used a fixed bed to carry out the CO₂ cycling absorption and desorption experiments on the adsorbent at 30 °C and a 0.1 bar CO₂ partial pressure, and the results are shown in Figure 6. When the CO₂ adsorption is saturated, the temperature rises to 200 °C for desorption under an Ar gas atmosphere, and then the temperature drops to 30 °C for CO₂ adsorption under a 0.1 bar CO₂ partial pressure. After 10 cycles of adsorption and desorption, the CO₂ adsorption capacity of the adsorbent under 30 °C and a 0.1 bar CO₂ partial pressure almost remains unchanged. The results indicated that the regeneration performance of Mg-MOF-74-N2 was high.

4. Conclusions

Mg-MOF-74 was synthesized through a solvothermal synthesis method. After adding NaAc to the precursor solution, the crystal morphology changed from cauliflower-type to rod-like particles, and the particle size changed from micron to nanometer particles. The dynamic CO₂ adsorption performance of Mg-MOF-74 in a simulated flue gas environment was tested. The results showed that the CO₂ adsorption performance of nanoscale MG-MOF-74-N2 synthesized by adding NaAc was significantly improved, and the CO₂ adsorption capacity reached 3.63 mmol/g at 30 °C and a 0.1 bar CO₂ partial pressure.