CO₂ Absorption and Sequestration as Various Polymorphs of CaCO₃ Using Sterically Hindered Amine

One aspect of the attempt to restrain global warming is the reduction of the levels of atmospheric CO₂ produced by fossil fuel power systems. This study attempted to develop a method that reduces CO₂ emissions by investigating the absorption of CO₂ into sterically hindered amine 2-amino-2-methyl-1-propanol (AMP), the acceleration of the absorption rate by using the enzyme carbonic anhydrase (CA), and the conversion of the absorption product to stable carbonates. CO₂ absorbed by AMP is converted via a zwitterion mechanism to bicarbonate species; the presence of these anions was confirmed with ¹H and ¹³C NMR spectral analysis. The catalytic efficiency (<i>k</i>_cat/<i>K</i>_m), CO₂ absorption capacities, and enthalpy changes (Δ<i>H</i>_abs) of aqueous AMP in the presence or absence of CA were found to be 2.61 × 10⁶ or 1.35 × 10² M^–1 s^–1, 0.97 or 0.96 mol/mol, and −69 or −67 kJ/mol, respectively. The carbonation of AMP-absorbed CO₂ was performed by using various Ca²⁺ sources, viz., CaCl₂ (CAC), Ca­(OOCCH₃)₂ (CAA), and Ca­(OOCCH₂CH₃)₂ (CAP), to obtain various polymorphs of CaCO₃. The yields of CaCO₃ from the Ca²⁺ sources were found in the order CAP > CAA > CAC as a result of the effects of the corresponding anions. CAC produces pure rhombohedral calcite, and CAA and CAP produce the unusual phase transformation of calcite to spherical vaterite crystals. Thus, AMP in combination with CAA and CAP can be used as a CO₂ absorbent and buffering agent for the sequestration of CO₂ in porous CaCO₃

Carbonic Anhydrase Promotes the Absorption Rate of CO₂ in Post-Combustion Processes

The rate of carbon dioxide (CO₂) absorption by monoethanol amine (MEA), diethanol amine (DEA), <i>N</i>-methyl-2,2′-iminodiethanol (MDEA), and 2-amino-2-methyl 1-propanol (AMP) solutions was found to be enhanced by the addition of bovine carbonic anhydrase (CA), has been investigated using a vapor–liquid equilibrium (VLE) device. The enthalpy (−Δ<i>H</i>_abs) of CO₂ absorption and the absorption capacities of aqueous amines were measured in the presence and/or absence of CA enzyme via differential reaction calorimeter (DRC). The reaction temperature (Δ<i>T</i>) under adiabatic conditions was determined based on the DRC analysis. Bicarbonate and carbamate species formation mechanisms were elucidated by ¹H and ¹³C NMR spectral analysis. The overall CO₂ absorption rate (flux) and rate constant (<i>k</i>_app) followed the order MEA > DEA > AMP > MDEA in the absence or presence of CA. Hydration of CO₂ by MDEA in the presence of CA directly produced bicarbonate, whereas AMP produced unstable carbamate intermediate, then underwent hydrolytic reaction and converted to bicarbonate. The MDEA > AMP > DEA > MEA reverse ordering of the enhanced CO₂ flux and <i>k</i>_app in the presence of CA was due to bicarbonate formation by the tertiary and sterically hindered amines. Thus, CA increased the rate of CO₂ absorption by MDEA by a factor of 3 relative to the rate of absorption by MDEA alone. The thermal effects suggested that CA yielded a higher activity at 40 °C