A Study of MgO Aerogels As CO2 Adsorbents for Use In Adsorption Enhanced Production Processes

CO₂ capture on solid sorbents is an important technique in the context of greenhouse gas emission reduction as well as adsorption enhanced reaction processes. Traditional CO₂ adsorbents such as hydrotalcites, alumina and zeolite have been extensively investigated.  This work will focus on the study of CO₂ adsorption on sol-gel derived MgO aerogels, which could have a great potential because of their large surface area (about 500 m² g^-1).

In this study amorphous MgO aerogels where compared with Alumina, commercial (lower surface-area, crystalline) MgO and lower surface-area, amorphous MgO xerogels. A thermogravimetric analyzer (TGA) was employed where the weight gain of the sample due to CO₂ adsorption was measured. All the experiments where performed at 250^oC (this temperature is suitable for adsorption enhanced water-gas-shift and steam reforming of methanol for hydrogen production) and all the samples were pretreated under the flow of N₂ as the carrier gas. Pretreatment time and temperature varied for each material to ensure that all the materials were free from any other gases that might have been previously adsorbed on the surface. After pretreatment CO₂ was introduced at different partial pressures starting from 0.0204 atm and going on to 0.0417 atm, 0.0638 atm, 0.087 atm, 0.111 atm, 0.5 atm, and finally 1 atm. From these data adsorption isotherms and the adsorption capacity of each material were determined. Data showed that the adsorption capacity of the MgO aerogel at 1 am partial pressure of CO₂ was 0.314 mmol CO₂/g adsorbent which was much higher than the adsorption capacity of alumina (0.0268 mmol CO₂/g aerogels), commercial (lower surface-area, crystalline)  MgO (0.150 mmol CO₂/g adsorbent), and the lower surface-area, amorphous MgO xerogel (0.144 mmol CO₂/g adsorbent). It was also observed that MgO aerogel has faster CO₂ adsorption kinetics and is less prone to N₂ adsorption as compared to Alumina. Tentative explanations will be presented to explain the contributing factors of CO₂ adsorption.

Ongoing experimental studies are also pursued to compare MgO aerogels with other commercial adsorbents such as Na doped MgO and Na doped alumina. The reason behind this specific comparison is that it has been shown through past studies that alkali salts when supported to metal catalysts increase the selectivity of the catalysts, depress side reactions and increase adsorptivity. A small packed-bed reactor will also be used to test the studied materials under more realistic conditions primarily involving steam in the process since it has been shown that its presence in the adsorption process enhance even further the CO₂ adsorption especially when the adsorbents are promoted with alkali salts.