Struct Chem (2011) 22:537–549 DOI 10.1007/s11224-010-9719-2 ORIGINAL RESEARCH Insights into amine-based CO2 capture: an ab initio self-consistent reaction field investigation Phil Jackson • Ariana Beste • Moetaz Attalla Received: 28 May 2010 / Accepted: 17 December 2010 / Published online: 30 December 2010 Ó The Author(s) 2010. This article is published with open access at Springerlink.com Abstract Ab initio many-body perturbation theory (MP2/ and (iii) the amine-catalysed reaction between OH- and 6-311??G(,dp)), density functional theory (B3LYP/6- CO2 to generate bicarbonate correlates with amine pKa. 31??G(d,p)) and self-consistent reaction field (IEF-PCM Thermodynamic data from the ab initio computations UA HF/6-31G(d)) calculations have been used to study the predicts that the heterocycles PD and PZ and the acyclic CO2 capture reagents NH3, 2-hydroxyethylamine (MEA), sorbent EN are good choices for a capture solvent. AMP diaminoethane (EN), 2-amino-1-propanol (2A1P), diethanola- and tris perform poorly in comparison. mine (DEA), N-methyl-2-hydroxyethylamine (N-methylMEA), 2-amino-2-methyl-1-propanol (AMP), trishydroxymethyl- Keywords Post-combustion capture Á Amine Á Ab initio Á aminomethane (tris), piperazine (PZ) and piperidine (PD). Piperazine Á Alkanolamine This study involved full conformational searches of the capture amines in their native and protonated forms, and their carbamic acid and carbamate derivatives. Using this Introduction data, we were able to compute Boltzmann-averaged ther- modynamic values for the amines, carbamates and car- Anthropogenic CO2 emissions must be reduced over the bamic acid derivatives, as well as equilibrium constants for coming decades in order to avoid climate change. To a series of ‘universal’ aqueous capture reactions. Important minimize global environmental impacts, atmospheric CO2 findings include (i) relative pKa values for the carbamic needs to be stabilized at a level less than or equal to acid derivatives are a useful measure of carbamate stabil- 450 ppm [1], which should limit average global tempera- ity, due to a particular chemical resonance which is also ture rises to less than 2 °C. To achieve such emission manifest in short computed N–CO2H bonds at both levels reductions, technologies such as solvent-based post-com- of theory, (ii) the computational results for sterically hin- bustion carbon capture (PCC), which can be retro-fitted to dered amines such as AMP and tris are consistent with existing coal-fired electricity generators, have to be these species forming carbamates which readily hydrolyse developed and deployed within a relatively short time frame. Now that greenhouse gas emissions from develop- ing nations have finally surpassed those of developed Electronic supplementary material The online version of this article (doi:10.1007/s11224-010-9719-2) contains supplementary nations [2], there is an even greater imperative for CO2 material, which is available to authorized users. capture to succeed. Currently, the large-scale roll-out of solvent-based PCC is hampered by economic factors, & P. Jackson Á M. Attalla ( ) which are a deterrent for owners of generator infrastructure Coal Portfolio, CSIRO Energy Centre, 10 Murray-Dwyer Circuit, Mayfield West, NSW 2304, Australia (e.g., PCC plant capital and solvent on-costs, see below), e-mail: [email protected] and government coercion via emission trading schemes will be widely used to stimulate uptake [3]. A. Beste Aside from economics, a further impediment to the Computational Chemical Sciences Group, Oak Ridge National Laboratories, PO Box 2008 immediate deployment of PCC is the flue gas scrubbing MS6367, Oak Ridge, TN 37831-6367, USA performance of alkanolamine capture solvents. Whereas a 123 538 Struct Chem (2011) 22:537–549 number of solvents can efficiently remove acid gases (CO2, protonated amines, and their carbamate- and carbamic SO2) from natural and industrial gas streams at higher acid-derivative was undertaken in order to predict trends pressures ([5 bar), the lower pressures typical of coal-fired for future studies of more complex alkanolamine candi- power station flue gas streams pose separate difficulties. dates. Relative carbamate stability has been successfully Problems may also be encountered for individual CO2 studied previously [15] using computational techniques. scrubbing solvents, such as monoethanolamine (MEA) and Typically, the time and expense incurred with the genera- piperazine (PZ), the former being prone to oxidation by tion of this data is low compared to the in-house experi- flue gas components [4, 5], whereas the latter is a sec- mental efforts expended to date. ondary amine that can react with NOx-containing flue gas to form hazardous nitrosamines [6], [7, p. 1100]. As part of an experimental program within the CSIRO Computational method organization, more than 70 organic amines have been screened as potential CO2 capture solvents. This work has Gas phase calculations at the B3LYP/6-31??G(d,p) and been carried out over a two and a half year period, with MP2/6-311??G(d,p) levels were performed using the several dedicated technicians performing the experiments; GAMESS Revision 1 software package [16]. GAMESS these have included various kinetics studies (chemometric employs a B3LYP functional formula based on the VWN5 modelling [8], nuclear magnetic resonance spectroscopy local correlation functional, which may differ from other [9], thermogravimetric analysis [10] and Fourier transform B3LYP implementations [17]. The MacMolPlt 7.0 program infrared spectroscopy [11]) as well as loading or capacity [18] was used for visualization. measurements [12]. In addition, many of the CO2 capture Geometry optimizations were performed using a Newton– candidates were also synthesized in-house, at significant Raphson steepest descent algorithm until a stationary point cost. After completion of the laboratory investigations, was located, characterized by a gradient less than 0.0001. promising candidates were synthesized in bulk for further We have used the recommended density of grid points to lab-scale trials, and—contingent on scale-up performance reduce integration errors in the DFT exchange–correlation tests—for deployment at one of several pilot-scale PCC quadrature to less than 1 microhartree per atom, namely plants located at regional power-generating infrastructure NRAD = 96, NTHE = 36 and NPHI = 72, producing [13]. 248832 grid points per atom. Vibrational analysis was These laboratory studies have been useful for identify- performed for all stationary points. ing ‘‘high performers’’ amongst a broad suite of alkanol- Molar heat capacities (Cp, Cv), entropies, enthalpies and amines, however, our understanding of the properties that free energies at specified temperatures for both molecules confer excellent capture characteristics are still only rudi- and ions were provided by the GAMESS program. These mentary. There are inherent difficulties in deriving exper- values were scaled using factors proposed by Scott and imental thermodynamic values, i.e., reactions cannot be Radom [19]. All gas phase reaction energies were calcu- studied with laboratory techniques in isolation, due to the lated according to thermochemical conventions [20]. The interplay of numerous solvent equilibria, and several cap- hybrid DFT functional B3LYP was selected for its reli- ture products and intermediates are unstable under capture ability and widespread chemical application [21], in addi- conditions. This means that measurement uncertainty could tion to MP2 calculations. be greater than 50% of the experimental value. A knowl- The integral equation formalism-polarizable continuum edge of accurate thermochemical values is one key to model (IEF-PCM UA HF/6-31G(d)) [14, 22] was used for lowering overall PCC costs. the calculation of solvation free energies, from which To fast-track our research efforts, a first principles reaction energies for both the MP2 and DFT values in the investigation of a representative amine subset has been solution phase could be derived. The total free energy of undertaken using density functional theory and many-body solvation is expressed [23]: techniques. The primary goal of this study is the rapid DGsoln ¼ Gel þ Gcav þ Gdisp þ Grep ð1Þ identification of proven high-performing CO2 capture candidates, however, we also stand to gain a deeper where ‘el’, ‘cav’, ‘dis’ and ‘rep’ represent electrostatic, understanding of the structural features that impart desir- cavitation, dispersion and repulsion contributions, respec- able capture properties. After computing gas phase reaction tively. The usual parameters for water, i.e., dielectric energies and polarizable continuum model free energies of constant e = 78.39, solvent radius (H = 1.2 A˚ ,O= solvation (for a recent review of this methodology, see: 1.5 A˚ ) and solvent molecule density per unit volume [14]), aqueous equilibrium constants have been obtained (0.03348 A˚ 3), were used to define the continuum. via the use of thermodynamic cycles. A thorough investi- Within the PCM (UAHF) framework, DGsoln is derived gation of the conformational spaces of the amines, using a HF/6-31G(d) gas phase structure. Reasonably good 123 Struct Chem (2011) 22:537–549 539 agreement was found between experimental and calculated hydroxyethylamine (N-methylMEA); the sterically hindered DGsoln values for small ions and molecules involved in the primary amines 2-amino-2-methyl-1-propanol (AMP) and capture chemistry process (see Table S1, Supporting trishydroxymethylaminomethane