The effect of CO2 loading on alkanolamine absorbents in aqueous solutions

Post-combustion carbon capture by amine scrubbing is the most frequently used process to remove CO2 from pulverized coal-fired power plants and also biogas flue gas streams. The quest for novel absorbents for CO2 capture with improved properties requires insight into the properties of the CO2loaded mixed solutions. A comparative molecular dynamics study of the product state solutions, with chemically-bound CO2 of standard monoethanolamine (MEA) and the new alternative 4-diethylamino2-butanol (DEAB) at various CO2-loadings yields solvent properties in good agreement with experimental data. The concentration of all post-reaction species in solution was based on experimental equilibria distributions. The data generated provide detailed insight into the properties of reactive mixed alkanolamine solutions. The liquid structure of aqueous MEA solutions undergoes only minor changes when absorbing CO2. The diffusion coefficients of all molecular species, however, decrease significantly with increasing CO2-loadings. The large hydrophobic clusters formed in the reactant state by DEAB molecules in water prior to CO2 binding significantly decrease in size and structure upon CO2 absorption. The diffusion coefficients of all components decrease with increasing CO2-loading, whereas the pre-reaction alkanolamine DEAB shows an increase in diffusion coefficient. This structural and kinetic information supports the molecular design and further development of novel compounds and provides data for a global process simulation and optimization.


Introduction
2][3][4] Complementing engineering studies, computational approaches are essential to reveal the underlying reaction mechanisms of CO 2 absorption and to develop new absorbing compounds. 5,6Selectivity and reversibility of the absorbent, and its liquid structure properties such as diffusion coefficient and viscosities are important selection criteria.In computational solvent design, molecular dynamics (MD) simulations are able to yield information, which are difficult or sometimes impossible to obtain experimentally.For example, the diffusion coefficient of CO 2 in absorbing amine solution cannot be measured due to its reactivity and has to be extracted from nitrous oxide experiments.][9][10][11][12][13][14][15][16][17][18] However, all these studies were limited to the investigation of the pre-reaction state of CO 2 in aqueous amine solutions.During the chemical CO 2 absorption, carbamate and carbonate ions together with protonated alkanolamines are formed as products of the amine reaction with CO 2 , which then would eventually change the complex mixed solvent properties.In addition, at increasing CO 2 loadings both non-reacted carbon dioxide and partially reacted solvents are present.The design and control of such a chemical absorption process requires information about the solvent properties changes during the reaction.Data about pre-reaction solvents are scarce in comparison to unreacted solvents.0][21] Experimental viscosity studies of individual constituents do not allow an extrapolation of diffusion coefficients to the reactive solution.Therefore, structurebased simulations are indispensable to provide these missing data and only classical MD simulations allow a sufficient sampling and averaging over long trajectories and a large number of particles to give accurate results.
We here present results from MD studies of CO 2 -loaded aqueous alkanolamine compounds which were identified from a large screening of candidates. 22Here, we focus on the primary monoethanolamine (MEA), the standard compound currently mostly used for CO 2 capture, and the novel tertiary alkanolamine 4-diethylamino-2-butanol (DEAB) which were both fully characterized in binary aqueous alkanolamine solutions 14 and in the presence of CO 2 . 17The choice of these two compounds is motivated by their difference in mechanism upon CO 2 absorption and the formation of different intermediates in solution.MEA is fully hydrated and when chemically absorbing one CO 2 molecule the carbamate anion MEACOO À and the protonated MEAH + are generated. 23This sets the upper limit for a maximum capacity of MEA to 0.5 (i.e. the ratio of absorbed CO 2 and reacted MEA molecules). 24DEAB molecules, on the other hand, form large hydrophobic clusters in aqueous solution and during absorption of CO 2 the tertiary amine DEAB deprotonates water to give the protonated DEAB (DEABH + ) species, and the hydroxide ion reacts with carbon dioxide to give bicarbonate (HCO 3 À ), and carbonate (CO 3 2À ) ions. 25,26This sets the maximum capacity of DEAB to 1 (one bicarbonate ion HCO 3 À and one protonated DEABH + per CO 2 molecule are being formed).Artificial neural network based models were successfully applied to obtain equilibrium solubilities and mass transfer coefficients of CO 2 absorption in aqueous solutions of DEAB 27 from experimental data. 28nd to develop a modeling-optimization framework in order to assess the CO 2 absorption capacity for novel amine solutions. 29he molecular compounds in equilibrium are a mixture of reactant and product species and are presented in Fig. 1.Liquid structure properties of MEA and DEAB CO 2 absorbents in solution were determined in the product states over a wide range of CO 2 -loadings.This reveals the influence of increasing concentrations of post-reaction molecular species on the structure of the complex mixed five-and six-component mixed solution.Changes in solution viscosities and molecular diffusion constants during the chemical absorption process can be traced and give detailed molecular insight into the post-combustion capture of carbon dioxide.

Simulation details
All non-bonded and bonded interactions for alkanolamine molecules, their protonated forms MEAH + and DEABH + and the MEACOO À carbamate and also (bi-)carbonate ions were treated with help of the all-atom optimized potentials for liquid simulations (OPLS-AA) forcefield. 30Water molecules were represented by the charge/extended (SPC/E) model, 31 and CO 2 molecules by the transferable potentials for the phase equilibria (TraPPE). 32The accuracy of the use of these forcefields for the simulation of CO 2 solvation in aqueous alkanolamines was validated before. 14,17Partial charges for the carbamate MEACOO À and protonated MEAH + were taken from literature, 18 and for the protonated DEABH + from ref. 30.For a consistent set of charges, the Lowdin atomic charges for HCO 3 À and CO 3 2À were re-calculated with Jaguar v. 8.7, 33 using the M06-2 hybrid functional, 34 an implicit SM8 solvation model for water 35,36 and a 6-31G*+ basis set for all atoms.The charge distribution was symmetrized for chemically equivalent atoms and retains the full molecular charge.The calculated partial charges are given in the ESI.† Long-range electrostatic interactions were treated with the particle-mesh Ewald algorithm.Non-bonded interactions were modeled with a 12-6 Lennard-Jones potential.
A cutoff radius of 1.4 nm was used for all interactions.Lennard-Jones parameters for unlike interactions were calculated using the geometric average rule.MD simulations were carried out with Gromacs 5.1.2 37at 313 K.An isothermal-isobaric (NPT) ensemble 2 ns simulation run (preceded by 500 ps of equilibration) was carried out to define the solvent density.The liquid solution structures and molecular diffusion coefficients were acquired in a canonical (NVT) ensemble 40 ns simulation run (preceded by a 1 ns equilibration run).Such long trajectories were needed in order to obtain reliable and converged results for the diffusion coefficient of CO 2 molecules at a low concentration of 0.1 M. The number of molecules and simulation box sizes for all studied molecular ensembles are provided in the ESI † (Tables S1 and S2).The appropriate simulation box dimensions were checked for absence of finite size effects.Diffusion coefficients were calculated from the relation between the mean square displacement and the observation time (over a 100-500 ps time interval). 38The shear viscosities were calculated using the non-equilibrium method 39 by applying external forces, from which the energy is dissipated through viscous friction.The generated heat is removed by coupling to a heat bath.This journal is © the Owner Societies 2019

Results and discussion
Simulations were carried out for starting compositions of alkanolamine/water solvent ratios of 30/70 w/w MEA/W and 22.5/77.5 w/w DEAB/W which are typical for the absorption process 2,4 and at a typical process temperature of 313 K. 22,40 The mole fractions of species in mixed solutions at various CO 2 loadings were obtained from NMR spectroscopic studies. 41,42he absolute compositions for each system are given in the ESI † (Tables S1 and S2).A comparison with experimental 43,44 data such as densities and viscosities is given in Fig. 2.There is a very good agreement for liquid densities over the entire range of CO 2 loading (deviations are below 1% for MEA and 2.5% for DEAB).The calculated viscosities are also in good agreement with experiment and correctly reproduce the trend of increasing viscosities with increasing loading.According to our findings, the TIP4P/2005 water model 45 performs slightly better for solvent viscosities of DEAB, but less so for the diffusion coefficients of complex mixed aqueous alkanolamine solutions. 14The large changes in density and viscosity with CO 2 loading for MEA with respect to DEAB can be rationalized by the fact that the number of absorbed CO 2 molecules in MEA at 0.5 loading is about 2 times larger than that in DEAB even at 0.815 loading.
In CO 2 -loaded solution, the primary amine MEA molecules are fully solvated by water and the CO 2 molecules are equally distributed between water and alkanolamine molecules. 17A simulation snapshot of an entire system at 0.3 CO 2 -loading is provided in Fig. S1 (ESI †).Compared to the pre-reaction system at 0 loading, in the fully reacted product system the MEA-MEA (see Fig. 3A), MEA-CO 2 (see the Fig. S3, ESI †) and likewise the MEA-water interactions are only marginally affected by increase of CO 2 -loading.The slight increase of the first and second peak amplitudes in the radial distribution function (RDF) of MEA C-C (first and second solvation shells) at increasing CO 2 -loading is an evidence for a small increase of the mutual aggregation of MEA molecules.This feature can be rationalized by an increase of selfassociation of the more hydrophobic MEA molecules separating from the MEACOO À and MEAH + ionic species and polar water molecules.MEACOO À and MEAH + form a persistent ion pair in solution with only an occasional exchange of molecules.The characteristic lifetime for the ion pair is about 100 ps across the entire range of CO 2 -loading.The strong MEACOO À -MEAH + pair is fully solvated by water molecules, uniformly distributed in solution and does not form larger clusters.As an example for the charged species interactions in the mixed five-component system, the RDF of the carbamate MECOO À with the other species in solution is given in Fig. 4.
The RDFs for the interactions of MEA and MEAH + s with other compounds in solution are given in the ESI.† The solvation of CO 2 in MEA solutions is not significantly affected by increasing the CO 2 -loading.The RDFs of CO 2 interactions with other molecular species in solution at 0.3 loading are shown in Fig. 3B.As also found in the pre-reaction state, CO 2 is equally found in proximity of water and MEA molecules (similar peak heights in Fig. 3B) but only slightly less probable to interact with MEACOO À and MEAH + .Analysis of interactions between CO 2 -MEA molecules at different loadings is given in the ESI † (Fig. S3).Upon an increase of MEACOO À -MEAH + pair formation at higher CO 2 -loadings, the presence of CO 2 molecules close to MEA species also increases slightly (see Fig. S3, ESI †).
Fig. 5 shows the molecular diffusion coefficients of all species present in solution after covalent CO 2 binding to MEA.The diffusion coefficients of all species significantly decrease with increasing CO 2 -loading (numerical data can be found in the ESI, † Table S4).It has to be noted that the diffusion  coefficient of CO 2 in the fully reacted system (at 0.5 loading) is two times smaller than in the pre-reaction state since all MEA molecules have reacted with CO 2 .As expected, the mobility is critically dependent on the molecular mass: the heaviest molecules MEACOO À and MEAH + are least mobile whereas water and CO 2 diffuse fastest.The significant decrease of molecular diffusion coefficients is consistent with the experimentally measured increase of viscosity of the mixed solution (see above).Since hydrogen bond formation of water molecules does not change with CO 2 -loading (data not shown), the increase of viscosity of CO 2 -loaded MEA solution can only be explained by an increase of ion pair formation of MEACOO À and MEAH + with higher supramolecular masses and lower diffusion coefficients.Compared to the primary amine MEA, the tertiary amine DEAB displays a different behavior upon an increase of CO 2 loading.In the pre-react state, DEAB molecules tend to self-aggregate and form large clusters (see a snapshot of an entire system in Fig. S2, ESI †).The CO 2 molecules show a preferred localization within these hydrophobic aggregates. 14,17n contrast to MEA, absorption of CO 2 in aqueous solutions by DEAB significantly affects the liquid structure.The effect of increasing CO 2 loading on DEAB cluster formation can be seen in The product DEABH + molecules form strong and stable complexes with oppositely charged HCO 3 À and CO 3 2À species (see Fig. 7).The lifetime of such an electrostatic interaction is about 100 ps for the DEABH + -bicarbonate and 1-1.5 ns for the DEABH + -CO 3 2À pair.The ionic ion pairs are uniformly distributed within each solution.
The localization of CO 2 molecules in close proximity to DEAB nitrogen atoms follows the same tendency as the DEAB-DEAB interactions upon increasing CO 2 -loadings.At low loading, the local CO 2 enrichment increases while with further CO 2 -loading it   starts to decrease and finally disappears (see the CO 2 -DEAB RDFs in the Fig. S6, ESI †).This can be rationalized when considering the decrease of unreacted DEAB molecules in number at higher CO 2 -loading.Thus, the molecular clusters of DEAB become smaller in size, water molecules are beginning to penetrate these clusters and displace CO 2 molecules close to the amine nitrogen atoms.The CO 2 -DEAB interaction is clearly the most prevalent and CO 2 interactions with water, DEABH + , HCO 3 À and CO 3 2À (see Fig. 6B) are less prominent.This is due to the hydrophobic nature of the interaction between CO 2 and the alkanolamine solvent. 17ig.8 shows the diffusion coefficients of all molecular species in DEAB at various loadings (for numerical data see Table S5 in the ESI †).With increasing CO 2 -loading, the mobility of all compounds in the mixed solutions decreases.This is in agreement with the observed increase of viscosity (see above).The diffusion coefficients for DEABH + and CO 3 2À are identical expect for very high loading when the strong electrostatic interaction is persistent (with a lifetime of 1 ns) and the complex diffuses as a supermolecule.At low and intermediate CO 2 loadings, the carbonate ions are preferably solvated by water molecules.The electrostatic interaction between the bicarbonate HCO 3 À and DEABH + is weaker and thus it shows a faster diffusion.The slower diffusion of CO 3 2À ions is determined its larger charge due to which the interactions with surrounding water molecules and DEABH + are stronger and responsible a slower mobility.
The hydrophobic DEAB molecules within the self-aggregate clusters diffuse slower than charged DEABH + molecules even when the latter are bound to anionic species such as carbonate and bi-carbonate.At very low CO 2 -loading, the DEAB molecules move even slower than in case of the pre-reaction solvent.As the CO 2 -loading increases, the DEAB cluster size becomes smaller and the diffusion coefficients of DEAB molecules thus increase with CO 2 concentration and finally approach that of DEABH + .The free CO 2 molecules are mostly localized within the DEAB clusters and are moving much faster than the DEAB molecules.The load-dependence of CO 2 diffusion reflects on the one hand the acceleration of DEAB molecule diffusion and on the other hand the slower diffusion of other species in solution.Therefore, the CO 2 diffusion coefficient is less sensitive to the CO 2 -loading than other mixture components.At a maximum loading of 0.815, the CO 2 diffusion coefficient is still at 79% of the non-reacted ternary system.Water diffusion slows down upon increase of CO 2 loading since the number of charged species DEABH + /HCO 3 À /CO 3 2À increased which possess a tight solvation shell.

Conclusions
In conclusion, MD studies of CO 2 -loaded aqueous alkanolamine solutions for two representatives (MEA and DEAB solvents) were performed over a broad range of loading.Reaction of carbon dioxide with alkanolamines leads to complex five-and sixcomponent post-reaction mixed solutions with different behavior.CO 2 absorption does not significantly alter the liquid structure properties but the molecular diffusion coefficients change significantly for the standard primary amine MEA in aqueous solution.The MEA molecules and all charged product species are individually solvated; CO 2 shows no preference to approach either MEA or water molecules.The diffusion coefficients of all molecule species decrease with increasing CO 2 -loading since the number of charged species and the solvent viscosity increase during the reaction.
For the tertiary alkanolamine DEAB, an opposite behavior is observed.Self-aggregation and clustering of hydrophobic DEAB molecules are diminishing in presence of the newly formed charged and hydrophilic absorbent species; the local concentration of CO 2 molecules decreases in vicinity of DEAB molecules.At increasing CO 2 -loading, the diffusion coefficients decrease for all species in solution except for unreacted DEAB.Their mobility increases and finally approaches that of the protonated DEABH + product.The change of diffusion coefficient of CO 2 at increasing load is less prominent due to two compensating effects: the acceleration of mobility of the first solvation shell CO 2 molecules in close proximity to DEAB molecules at low loading and the disruption of DEAB selfaggregates at higher loadings.A clear understanding of molecular processes during carbon dioxide sequestration in post-combustion capture is an important pre-requisite to design and control novel CO 2 absorbing compounds.The results show that different, sometimes competing effects have an influence on the diffusional properties and viscosities of amine scrubbers in reactors at different CO 2 loadings.The physical and organic chemistry of alkanolamines is not facile to rationalize and requires careful analysis of novel data from structure-based simulations.These generated data form the basis of global process simulations and optimization of the entire carbon dioxide sequestration process in a chemical engineering approach.

Fig. 2
Fig. 2 Comparison of liquid state properties with experiments at T = 313 K. Panel (A) shows the densities of the five-and six-component mixed solvents, panels (B) and (C) display the viscosities of the MEA/MEAH + /MEACOO À /W/ CO 2 and DEAB/DEABH + /W/CO 2 /HCO 3 À /CO 3 2À systems as a function of

Fig. 3
Fig. 3 Liquid solution structure of the five-component CO 2 -loaded aqueous MEA.(A) Radial distribution functions (RDFs) of alkanolamines MEA-MEA interactions at various CO 2 loadings, (B) RDFs for CO 2 interacting with all other molecular species (W/MEA/MEAH + /MEACOO À ) in solution at 0.3 loading at which all reaction products are present in significant amount.
Fig. 6A.The peak amplitudes of the first and the second peaks of the RDF characterize the tendency of DEAB molecules to form self-aggregates.At low CO 2 -loading, DEAB clustering even increases since the presence of only a small amount of charged particles DEABH + /HCO 3 À /CO 3 2À in solution enhances the hydrophobic/hydrophilic phase separation.Upon further increase of CO 2 -loading, more DEAB molecules have participated in the chemical reaction with CO 2 which then leads to the formation of an increasing number of protonated DEABH + molecules.Those tend to separate from the DEAB clusters due to their hydrophilic character and the number of hydrophobic DEAB-DEAB interactions decreases.This can be seen by the decrease of peak amplitudes in Fig. 6A at high loading.At a CO 2 -loading of 0.815, the second solvation shell of DEAB-DEAB aggregates almost vanishes indicating the formation of isolated molecular clusters.

Fig. 4
Fig. 4 RDFs of interactions of the carbamate MEACOO À with other mixture species at a CO 2 -loading of 0.3.The peaks numbered 1, 2, 3 at the top panel (A) refer to the long living MEACOO À -MEAH + species shown on the bottom panel (B).

Fig. 5
Fig. 5 Diffusion coefficients of molecular components as a function of CO 2 loading in aqueous MEA/CO 2 solution.

Fig. 6 (
Fig. 6 (A) Molecular interactions of DEABH + in the partially reacted mixed of solution DEAB/W/CO 2 with other species at a CO 2 -loading of 0.364.(B) Simulation snapshots displaying the most frequently occurring conformations of interactions in DEABH + /HCO 3 À and DEABH + /CO 3 2À ion pairs.