https://doi.org/10.1016/j.egypro.2017.03.1277
“Recently, novel approaches including enzymes and catalysts have become research highlights in CO2 capture [2,
3]. Carbonic anhydrase (CA), a biocatalyst, has been mentioned recently as an activator for potassium carbonate
(K2CO3). K2CO3 has a high CO2 loading capacity and low heat of reaction. It seems to be a good solvent with low
energy requirement for regeneration; however, the rate of reaction is slow. In CO2 absorption by K2CO3, CA
catalyzed and increased the rate of reaction resulting in a smaller size of the absorber column [4, 5]. Although CA
has shown appreciable enhancement in CO2 separation properties, the limited lifetime of the enzyme and the loss of
activity by pH or temperature have been proven to be the major challenges in their commercial application [6].
The use of solid catalysts was first reported by Idem et al. [7]. Promising solid acid catalysts such as HZSM-5
DQGȖ-Al2O3 decreased the heat duty in the amine regeneration process. The catalysts were tested with several amine
solvents including MEA, MEA-MDEA and MEA-DEAB. The best scenario was MEA-DEAB solvent with HZSM-
5. It gave the lowest heat duty at 710.2 kJ/mol CO2 compared to the case of MEA without catalyst which gave the
heat duty of 1871.2 kJ/mol CO2. This constituted a 62% improvement [8]. Even though, the use of solid acid
catalyst in the CO2 desorption process was proven to be a promising approach to solving the challenges of energy
requirements in the CO2 desorption process, the data from the work Shi et al. [8] was obtained from a batch reactor.
Consequently, the absolute heat duty was not measured.
With the addition of a proton donating solid acid catalyst, the system now has the ability to supply protons even
without the deprotonation step. The MEACOO- in the rich MEA solution being fed to the desorber will pass over the surface of HZSM-5 catalyst and gets transformed into MEACOOH. Then, there is chemisorption of MEACOOH on
the surface of the catalyst through the bonding of O atoms and Al atoms. The H atoms attached to the O atoms
dislocate to the adjacent N atom. According to Shi et al. [9], this transition changes the MEACOOH into a
zwitterion ion. The N-C bond starts to stretch and weaken due to the H transfer. After the H transfer, the N-C bond
becomes weaker, resulting in the attachment of N+ to the second Al on the catalyst surface while the N-C bond starts
to break. After the bond breakage, the zwitterion becomes MEA and CO2. The solubility of CO2 at high
temperatures is quite low therefore CO2 will transfer to the gas phase. When the amount of HZSM-5 is increased,
the concentration of proton available significantly increases. This implies that the MEACOO- can react with more
H+ that is available on the catalyst surface instead of waiting for the MEAH+ deprotonation reaction to occur [8, 9].”