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

1. Introduction

Excessive CO₂ emissions caused by the burning of fossil fuels and human activities have led to increasing global warming, melting glaciers, frequent disaster weather, and sharp decline in plant and animal diversity. The United Nations (IPCC) has issued a goal of reducing global CO₂ emissions by 41% to 72% by 2050 compared with 2010[1]. In China, power plants are the 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 achieve CO₂ emission reduction goals. At present, CO₂ capture and storage (CCS) technology is the key technology to mitigate and control CO₂ emissions, and reducing the cost of capture stage is the top priority in implementing this technology. Based on the low CO₂ content in the flue gas of power plant, the commonly used CO₂ capture methods include absorption, adsorption, membrane separation and low temperature distillation. Among them, adsorption method has the advantages of simple process, mild operating conditions, low energy consumption and no corrosion to equipment, so it is the most promising capture method at present. At present, many materials used in the field of CO₂ adsorption are activated carbon, molecular sieve, metal oxides, metal-organic skeleton materials (MOFs) [2,3,4,5]. High CO₂ adsorption capacity, 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 is a new type of crystalline porous material composed of metal ions and organic ligands[6]. Its high specific surface area developed void structure and adjustable chemical composition make this material widely used in gas adsorption and separation[7,8,9,10,11], drug carrier[12], catalysis[13], sensor[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 direct precipitation method, microwave assisted method, hydrothermal/solvothermal synthesis method, mechanical method and so on.

For example, the Ahn[15] research group reported a method for the preparation of nano-scale Mg-MOF-74 by ultrasonic chemistry, which uses triethylamine as the deprotonation agent and greatly reduces the reaction time. The Sun[16] research group used polyethylpyrrolidone and ammonia water to regulate the morphology and particle size of Mg-MOF-74. In addition, 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[12] research group prepared Mg-MOF-74 with particle size less than 200 nm by two-step method and explored its application in drug carrier. Zhang[17] et al. reported that a Mg-MOF-74/sodium alginate composite aerogel were prepared by loading Mg-MOF-74 on sodium alginate sheets followed by directional freeze-drying. The experimental data showed that the composite achieved 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 micropores/mesopores 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, Up to now, in the reported MOFs, there are still few studies on CO₂ adsorption in flue gas for nanoscale Mg-MOF-74.

In this paper, nanorod Mg-MOF-74 particles were synthesized by a simple solvothermal method without surfactants. The morphology and particle size of MOFs were adjusted by controlling the concentration of sodium acetate (NaAc). The dynamic adsorption performance of nano-scale Mg-MOF-74 under simulated flue gas environment was tested by a self-made fixed bed, which provided a theoretical basis for the design and synthesis of CO₂ adsorbent suitable for 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), sodium acetate (NaAc) were used in Shanghai Aladdin Reagent Co., LTD. The reagents used were all analytically pure. CO₂ (high purity, Beijing Beiwen Gas Factory), N₂ (high purity, TISCO Gas Factory)

X-ray diffraction analysis was performed by D8 Advance X-ray diffractometer of Nikaku, using Cu Kα radiation (λ=0.154 nm), metal Ni filter, tube pressure 40 kV, tube flow 30 mA. The N₂ adsorption and desorption isotherm of the sample at 77 K was determined by ASAP 2020 automatic physical adsorption instrument. The sample was vacuumed and degassed at 200 ℃ for one night before the test. The microstructure and morphology of the sample were observed on JSM-7001 thermal field emission scanning electron microscope. The sample was coated with conductive adhesive and scanned after gold spraying. During the test, the acceleration voltage is 5-10 kV. Rigaku TG thermogravimetric analyzer was used to study the thermal stability of the sample. Under 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 ℃, and the heating rate was 10 ℃/min.

2.2. Synthesis of Mg-MOF-74

0.674 g H₄DOBDC and 2.8 g Mg(NO₃)₂•6H₂O were added to 300 mL DMF/EtOH/H₂O mixed solution with a volume ratio of 15:1:1, and stirred until dissolved. 0-4 equal amounts of NaAc were added to the mixture, then the reaction liquid was placed in a crystallization kettle and reacted in an oven at 125 ºC 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 placed in a 200 ºC vacuum drying oven to remove the solvent and obtain Mg-MOF-74. The resulting sample is named Mg-MOF-74-N(0-4), where N represents NaAc and 0-4 represents the equivalent amount added to NaAc.

2.3. Evaluation of adsorbents

The CO₂ dynamic adsorption breakthrough experiment of the adsorbents was measured on a self-built fixed bed. 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 Ar atmosphere at 200℃ for 6 h with a gas volume flow rate of 60 mL/min and a constant inlet concentration. After the activation, drop to the required temperature, switch the gas valve to the mixture of CO₂ and N₂ (CO₂/N₂=1:9), adjust the mass flowmeter to the required gas flow, use gas chromatography to detect the concentration of the air outlet, record the concentration of the air outlet every 10 s, until the CO₂ concentration of the air outlet is close to or even the same as the concentration of the air inlet, then stop the detection. Then, in argon atmosphere, desorption at high temperature. 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 concentration 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. Conclusion

Mg-MOF-74 was synthesized by solvothermal synthesis method. After adding NaAc into 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 under 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℃ and 0.1 barCO₂ partial pressure.