CO2 capture in aqueous ammonia solutions: a computational chemistry perspective

Area of Science:

• Computational Chemistry
• Chemical Kinetics
• Biogeochemistry

Background:

• Carbon dioxide (CO2) fixation is a critical process in various chemical and biological systems.
• Understanding the mechanisms and energetics of CO2 reactions with nucleophilic bases is essential for catalysis and environmental science.
• Previous studies have explored CO2 capture, but detailed computational insights into transition states and activation energies are often limited.

Purpose of the Study:

• To computationally investigate the transition structures (TSs) for CO2 fixation by ammonia and ammonia-water mixtures.
• To determine the relative activation energies for carbamate (NH2CO2-) and bicarbonate (HCO3-) formation pathways.
• To elucidate the role of proton transfer and ammonium ions in CO2 fixation and carbamate decomposition.

Main Methods:

• Quantum chemical calculations using the M06-2X/6-311++G(d,p) level of theory to locate 25 transition structures.
• High-level single-point energy calculations at CCSD(T)/maug-cc-pVTZ//M06-2X/6-311++G(d,p).
• Inclusion of solvation effects using the SM8 model to obtain best-estimate aqueous activation energies.

Main Results:

• Carbamate (NH2CO2-) formation exhibits significantly lower free energy of activation (44-45 kJ mol-1) compared to bicarbonate (HCO3-) formation (86 kJ mol-1).
• Proton transfer from an ammonia molecule to the nucleophilic base effectively lowers the activation free energies for CO2 fixation.
• Computational results support a crucial role for ammonium ions in the observed decomposition of NH2CO2- around pH 9.

Conclusions:

• The carbamate pathway is kinetically favored over the bicarbonate pathway for CO2 fixation by ammonia.
• Proton transfer mechanisms are vital for enhancing the efficiency of CO2 capture by basic solutions.
• The findings provide theoretical support for the involvement of ammonium ions in carbamate stability and decomposition dynamics.