Molecular Modeling and Experimental Study of Catalysts for CO2 Reaction in Amine Solution

W. Shi, C.A. Lippert, J.A. Steckel, D.P. Hopkinson, D.E. Alman, K. Liu
National Energy Technology Laboratory,
United States

Keywords: molecular modeling, experiment, catalyst, CO2 absorption, aqueous amine solution

Summary:

It has recently been reported that catalysts enhance the CO2 mass transfer coefficient (KG) by 35% in the 30 wt% monoethanolamine (MEA) aqueous solution [1]. This has significant impacts on the cost of CO2 capture [1]. Note that the experimentally determined KG value is a lumped parameter which depends on the solution physical properties such as viscosity and surface tension, and other properties such as CO2 diffusivity and CO2 chemical reaction in the solution. Consequently, it is very important to understand how the catalyst affects the MEA solution properties in order to develop more efficient and effective catalysts. In the present work, molecular simulations and experimental studies were used to provide insight on the effect of adding one specific catalyst, C3P, to MEA. Both simulations and experimental data show that adding the C3P catalyst to concentrations of 2.3 -27 g/L in 30 wt % MEA aqueous solution does not change the solution viscosity and surface tension. Additionally, simulations show that adding C3P catalyst in the MEA solution does not affect CO2 physical diffusivity in the solution. The simulations and experiments suggest that the enhanced KG for CO2 when CP3 is added to MEA is not due to C3P changing physical properties but altering the CO2 reaction mechanism. Furthermore, the simulations show that the CO2 and MEA local concentrations around C3P molecules are increased and water concentrations are decreased compared to concentrations in the bulk MEA aqueous phase. Due to higher CO2 and MEA concentrations around the C3P molecule, CO2 reaction rate in the region around the C3P catalyst molecule is expected to be larger compared with CO2 reaction rate in the bulk MEA aqueous phase. This finding may partly contribute to the enhanced KG for CO2. Finally, simulations show that the C3P catalyst molecules tend to aggregate to form a large cluster. This aggregation may block the accessibility of some C3P molecules and hence decrease the catalytic behavior. Similarly, our experimental data also show that even after adding the C3P catalyst concentration by 10 times larger, the CO2 reaction rate is not significantly increased. All the simulation and experimental findings suggest that dispersing the catalyst in MEA aqueous solution may improve the C3P catalytic performance. In addition to the above classical atomistic modeling, quantum ab initio calculation results will also be shown in this presentation. [1] Cameron A. Lippert, Kun Liu, Moushumi Sarma, Sean R. Parkin, Joseph E. Remias, Christine M. Brandewie, Susan A. Odom and Kunlei, Liu, ”Improving Carbon Capture from Power Plant Emissions with Zinc- and Cobalt-based Catalysts”, Catalysis Science & Technology 2014, 4, 3620-3625.