CO₂ Adsorbents: Synthesis, Characterization, And Their Adsorption Properties

CO2 is one of main greenhouse gases. As a continuous increase in CO2 concentration in the atmosphere, the contribution of CO2 to the greenhouse effect is considered to be severe. Therefore, the capture and sequestration of CO2 are an important issue that has drawn global attention. Adsorption-based separation is considered to be one of the most effective technologies to capture CO2 from industrial emissions. It is of great importance to develop an efficient CO2 sorbent with a high CO2 adsorption capacity, a high selectivity, and a long-term regeneration property.In recent years, among various types of CO2 capture sorbents, amino-functionalized mesoporous silicas (AFMS) and metal-organic frameworks (MOFs) have attracted much attention due to their high surface area, large pore volume, tunable pore size, and high selectivity for CO2 adsorption.This MSc thesis mainly consists of three parts as follows:1) Amino-functionalized mesoporous silicas (AFMS) were directly synthesized using anionic surfactant N-lauroyl sarcosine sodium as the template, tetraethoxysilane (TEOS) as the silica source, and 3-aminopropyltrimethoxysilane (APMS) as the co-structure directing agent. The effects of the used doses of APMS and hydrochloric acid in the synthesis system on the structure and CO2 adsorption properties of the synthesized AFMS were characterized by XRD and N2 adsorption-desorption techniques as well as by CO2 adsorption/desorption experiments. It is found that an increase in amine content, specific surface area, and pore volume of the synthesized AFMS can result in a high CO2 adsorption capacity. The best AFMS adsorbent synthesized under the optimized conditions shows a CO2 adsorption amount as high as 1.95 mmol·g-1 at 30℃and 100 kPa and the adsorbent is rather stable, confirmed by ten cycles of CO2 adsorption-desorption. Additionally, under the same conditions, the adsorption of N2 and CH4 on the adsorbent is negligible. Therefore, the synthesized adsorbent has a high selectivity for CO2 over N2 and CH4, implying its potential application in CO2 separation from CO2/N2 and CO2/CH4 mixtures. 2) MIL-53(Al) and NH2-MIL-53(Al) were successfully synthesized by a hydrothermal method. The effects of the synthesis conditions, including reaction temperature, time, and reaction medium and the dose of acid or base into the reaction mixture, on the morphology and crystal structure of the synthesized materials were investigated in detail in combination with SEM and XRD characterizations. The results show that MIL-53(Al) crystals with uniform size (4-6μm) and a BET surface area of 1149 m2/g can be synthesized under the following conditions:water as the solvent, Al(NO3)3 and 1,4-benzenedicarboxylic acid (1,4-H2BDC) (an Al(NO3)3/1,4-H2BDC molar ratio of 2) as the reactants, reaction for 4 d at 190℃, then calcined at 330℃for 3 d. NH2-MIL-53(Al) crystals can be obtained under the following synthesized conditions: water as the solvent, Al(NO3)3 and 2-amino-1,4-benzenedicarboxylic acid (2-NH2-1,4-H2BDC) (an Al(NO3)3/2-NH2-1,4-H2BDC molar ratio of 1) as the reactants, reaction for 5 h at 150℃, followed by an activation procedure. The synthesized NH2-MIL-53(Al) crystals have a size of 2-3μm and a BET specific surface area of 1107m2/g.3) A comparative study on the adsorption of CO2 on MIL-53(Al) and NH2-MIL-53(Al) was investigated by means of volumetric techniques. The "breathing effects" of MIL-53(Al) was observed during CO2 adsorption at low pressures and temperatures lower than the critical temperature of CO2 (31℃). This is due to the fact that the host-guest interactions force the framework to close and the cell to shrink, resulting in the transition from the large-pore (1p) form into the narrow-pore (np) one. However, from the isotherms of CO2 obtained at temperatures higher than the critical temperature of CO2, no sign of the phase transition was observed in the pressure range investigated and the framework of MIL-53(Al) remains in the lp form. A similar observation as CO2 on MIL-53(Al) holds for CO2 on NH2-MIL-53(Al) at the same experimental conditions, but there are tiny hysteresis loops in their desorption branches during CO2 adsorption at low pressures and temperatures lower than the critical temperature of CO2. This spectacular difference between MIL-53(Al) and NH2-MIL-53(Al) could result from an enhanced affinity of NH2-MIL-53(Al) for CO2 due to the presence of amino groups in NH2-MIL-53(Al). Both adsorbents show some superb adsorption properties for CO2.