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Reaction Mechanisms of Aqueous Monoethanolamine with Carbon Dioxide: a Combined Quantum Chemical and Molecular Dynamics Study

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Reaction Mechanisms of Aqueous Monoethanolamine with Carbon Dioxide: a Combined Quantum Chemical and Molecular Dynamics Study Showcasing research from the laboratory of As featured in: Professor Gyeong S. Hwang at the University of Texas at Austin. Title: Reaction mechanisms of aqueous monoethanolamine with carbon dioxide: a combined quantum chemical and molecular dynamics study Molecular mechanisms underlying the reaction of aqueous MEA with CO2 are examined using ab initio and classical molecular dynamics simulations combined with static quantum chemical calculations, with special attention to the infl uence of solvent structure and dynamics. This work highlights the critical role of H2O molecules, particularly their availability and arrangement around zwitterionic intermediates, in the progression of competing CO2 capture and solvent regeneration processes. See Gyeong S. Hwang et al., This improved understanding can contribute to developing Phys. Chem. Chem. Phys., more comprehensive kinetic models for use in modeling and 2015, 17, 831. optimizing the CO2 capture process. www.rsc.org/pccp Registered charity number: 207890 PCCP View Article Online PAPER View Journal | View Issue Reaction mechanisms of aqueous monoethanolamine with carbon dioxide: a combined quantum chemical Cite this: Phys. Chem. Chem. Phys., 2015, 17,831 and molecular dynamics study† Gyeong S. Hwang,*ab Haley M. Stowe,b Eunsu Paeka and Dhivya Manogaranc Aqueous monoethanolamine (MEA) has been extensively studied as a solvent for CO2 capture, yet the underlying reaction mechanisms are still not fully understood. Combined ab initio and classical molecular dynamics simulations were performed to revisit and identify key elementary reactions and intermediates in 25–30 wt% aqueous MEA with CO2, by explicitly taking into account the structural and dynamic effects. Using static quantum chemical calculations, we also analyzed in more detail the funda- mental interactions involved in the MEA–CO2 reaction. We find that both the CO2 capture by MEA and solvent regeneration follow a zwitterion-mediated two-step mechanism; from the zwitterionic inter- mediate, the relative probability between deprotonation (carbamate formation) and CO2 removal (MEA regeneration) tends to be determined largely by the interaction between the zwitterion and neighboring H2O molecules. In addition, our calculations clearly demonstrate that proton transfer in the MEA–CO2–H2O solution primarily occurs through H-bonded water bridges, and thus the availability and arrangement of H2O Received 6th October 2014, molecules also directly impacts the protonation and/or deprotonation of MEA and its derivatives. This Accepted 4th November 2014 improved understanding should contribute to developing more comprehensive kinetic models for use in DOI: 10.1039/c4cp04518a modeling and optimizing the CO2 capture process. Moreover, this work highlights the importance of a detailed atomic-level description of the solution structure and dynamics in order to better understand www.rsc.org/pccp molecular mechanisms underlying the reaction of CO2 with aqueous amines. I. Introduction MEA system for commercial-scale applications tends to be impeded by MEA degradation, corrosion problems, and moreover the high 2,4,7 Published on 04 November 2014. Downloaded by University of Texas Libraries 06/05/2015 02:26:00. The ever-increasing rate of carbon dioxide (CO2) emissions, mainly parasitic energy consumption during solvent regeneration. as a result of growing fossil fuel consumption, has become a Several experimental and modeling studies have been undertaken 1 widespread concern. At present, aqueous alkanolamine solvents to better understand the CO2 capture process with aqueous MEA, are the predominantly used method to remove CO2 from flue gas but some fundamental aspects of the MEA–CO2 reaction in an and natural gas.2,3 Particularly, monoethanolamine (MEA) has been aqueous solution, particularly the role of water molecules, still the most extensively studied for decades and is commonly used as remain uncertain despite its importance in designing more the benchmark solvent.2,4–6 However, scaling up of the aqueous efficient MEA-based solvents and processes. It has been thought that two MEA molecules react with one a CO molecule to form carbamate and protonated MEA, perhaps Department of Chemical Engineering, University of Texas at Austin, Austin, 2 Texas 78712, USA. E-mail: [email protected] via a single-step termolecular (direct) or two-step zwitterion b Materials Science and Engineering Program, University of Texas at Austin, Austin, mechanism. The two-step process involves the formation of a Texas 78712, USA zwitterion as an intermediate which undergoes deprotonation c Departments of Chemistry and Biochemistry, University of Texas at Austin, Austin, by another MEA to form carbamate and protonated MEA.8,9 The Texas 78712, USA † Electronic supplementary information (ESI) available: Additional available single-step mechanism assumes that amine, CO2, and base content includes the energy profiles of the AIMD simulations used to calculate molecules form a loosely-bound complex, rather than a zwitter- 10 DErxn (Fig. S1). Also provided are illustrations of the various configurations ion, which breaks up to form the products. Although recently considered in the proton binding energy calculations as reported in Fig. 4 the zwitterion mechanism is commonly adopted to explain the (Fig. S2) and the predicted lower-barrier pathway for deprotonation from a MEA–CO2 reaction, some fundamental aspects of the MEA–CO2 zwitterion into a well-connected water network (Fig. S3) and to a nearby MEA 11–20 molecule (Fig. S4). The functional forms and parameters of the force fields interaction in aqueous solution remain uncertain. used in the CMD simulations for MEA and its derivatives are also included. See Atomic-level characterization of the complex reaction-diffusion DOI: 10.1039/c4cp04518a behavior in aqueous solutions appears to be very challenging, This journal is © the Owner Societies 2015 Phys. Chem. Chem. Phys., 2015, 17,831--839| 831 View Article Online Paper PCCP in part because of the limited capabilities of common instru- We used the Gaussian 09 program27 for static QM calcula- mentation. A complementary computational effort has been made tions to investigate the detailed interactions of CO2 with MEA. in studying the fundamental issues related to CO2 capture and Geometry optimizations were performed with hybrid Becke solvent regeneration. The reaction of CO2 with aqueous amines 3-Lee–Yang–Parr (B3LYP) exchange–correlation functional has often been studied using an implicit solvent approach based with the 6-311++G basis sets for C, H, N, and O. All stationary on (static) quantum mechanical (QM) calculations;11–14 the points were verified as minima by full Hessian and harmonic implicit QM model may provide useful insight into the reaction frequency calculations. The self-consistent reaction field theory paths and energetics and the relative stability of reaction (SCRF) based on the polarisable continuum model (IEFPCM- intermediates. However, there are several less studied aspects UFF) implemented in the Gaussian program28,29 was employed like the microstructure of the solvent that may have a critical to account for solvation effects implicitly. influence on the MEA–CO2 reaction. Very recently, ab initio molecular dynamics (AIMD) has been applied to identify the B. Classical molecular dynamics simulation likely events and the reaction intermediates involved in the CO2 CMD simulations were performed using the Large-scale Atomic/ 17–19 capture by MEA. This method possibly better accounts for the Molecular Massively Parallel Simulator (LAMMPS) program.30 We solution structure and dynamics and their effects on the progress used a modified AMBER force field22,31 for MEA and its derivatives of the reaction, but is limited to small systems. While a quanti- with the SPC/E water model,32 and obtained the atomic charges tative understanding of the CO2 capture mechanisms is still for MEA–CO2 zwitterion, carbamate, and protonated MEA from lacking, the solvent regeneration process that may be the most QM calculations at the B3LYP/6-311++G level of theory; the force energy intensive and least understood seems to be forgotten in field parameters employed are available in ESI.† All the bonds most of the previous computational studies reported in the involving H atoms were constrained using the SHAKE literature thus far. In addition, there are relatively few studies algorithm.33 Spherical cutoffs of 10 Å and 12 Å were used for investigating how MEA and intermediates behave in bulk the Lennard-Jones and Coulomb interactions, respectively. 21–24 solution, and little understanding of how they may affect Electrostatic interactions beyond the cutoff were calculated the ability of CO2 to be captured or the solvent to be regenerated. using the Ewald summation method.34 Simulations were run In this work, we investigate molecular mechanisms underlying in the NVT ensemble with the temperature controlled by a the CO2 capture by aqueous MEA and the solvent regeneration Nose´–Hoover thermostat35 with a 100 fs damping parameter. using combined QM and force field calculations. Using AIMD, Each simulated system was first annealed at 1000 K and then we first identify the likely elementary reaction steps and inter- quenched to 323 K, followed by another anneal and quench mediates taking into account the structural and dynamics effects cycle. Production runs were carried out for 0.7 ns with a in the MEA–CO2–H2O solution. We then use a static quantum timestep of 1 fs. chemical approach at the
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