Computer-Free Group-Addition Method for pKa Prediction of 73

Dec 21, 2016 - The pKa value of an amine aqueous solution is directly related to the rate of its chemical reaction with CO2. A quick and accurate pKa ...
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Computer-Free Group-Addition Method for pKa Prediction of 73 Amines for CO2 Capture Juan Qian,* Rui Sun, Shaozeng Sun, and Jihui Gao School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China ABSTRACT: The pKa value of an amine aqueous solution is directly related to the rate of its chemical reaction with CO2. A quick and accurate pKa predicted method can provide a shortcut for efficient screening of CO2 absorbents. The original and updated Perrin−Dempsey−Serjeant (PDS) pencil-and-paper group-addition methods were used to predict the pKa. The errors in pKa predictions of some cyclic, tertiary, and multihydroxy amine solutions were larger than that of the linear primary and secondary amines whether the original PDS or the updated PDS method was used. Another computer-free group-addition method (called QSSG) was proposed to predict the pKa values of 73 CO2-relevant alkanolamines/amines and cyclic amines, including piperazine and morpholine derivatives. Intramolecular hydrogenbonding, hyperconjugation, solvent, and steric effects were analyzed. Six extra parameters were added, and 10 parameters were changed to improve the accuracy of the predicted pKa values, based on a linear free energy relationship, of a training set of 73 amines for industrial CO2 capture. The root-mean-square errors (rms) were 0.13 pKa units for a set of 73 relevant industrial CO2-capture amines. The rms errors in the pKa values of nine multihydroxy amines were 0.45 for the original PDS, 0.28 for Sumon’s updated PDS method, and 0.12 for our QSSG method; for cyclic tertiary amines the corresponding values were 0.47, 0.22, and 0.12, respectively. The 10 amines outside the training set agreed well with the experimental results with QSSG predictions.

1. INTRODUCTION CO2 contributes to global warming and climate change. Many techniques are currently used for the separation and capture of CO2 from gas streams. Aqueous solutions of amines, especially alkanolamines, are generally used in commercial postcombustion CO2 capture. However, the energy consumed, environmental impact, and capture costs of this technique need to be reduced. The most suitable amines should have high cyclic capacities, fast absorption rates, high equilibrium temperature sensitivities, and low heats of regeneration. Many amines have been screened as absorbents, including alkanolamines (primary/secondary/tertiary amines), cyclic amines (substituted piperazines and morpholines), and polyamines. Chowdhury et al.1 investigated CO2 capture with 24 tertiary amine absorbents, including 3-(dimethylamino)-1,2propanediol (DMA-1,2 -PD), 4-ethyl-methylamino-2-butanol (4EMA-2B), 3-piperidino-1,2 -propanediol (3PP-1,2-PD), 1-ethyl-3-hydroxypiperidine (1E-3HPP), and N-isopropyldiethanolamine (IPDEA), etc. Gas scrubbing, vapor−liquid equilibrium (VLE), and reaction calorimetry experiments were used to evaluate the performance for CO2 capture, including the absorption rate, the amount of CO2 absorbed, cyclic CO2 capacity, and heat of reaction for each absorbent. Rochelle and Chen measured the absorption/desorption rate at typical rich/lean CO2 loading and provided practical ranges of CO2 loading for 1-methylpiperazine (1-MPZ), 2-methylpiperazine (2-MPZ), N-(2hydroxyethyl) piperazine (HEP), 1-(2-aminoethyl)piperazine © 2016 American Chemical Society

(AEP), 2-piperidine ethanol (2-PE), and trans-2,5- dimethylpiperazine (2,5-DMPZ) using a wetted wall column for CO2 capture.2 Perinu et al.3 studied chemical absorption of CO2 by amine absorbents, including primary alkanolamines at varying chain lengths, including 2-amino-1-ethanol (MEA), 3-amino-1propanol (3A1P), 4-amino-1-butanol (4A1B), 4-amino-1-pentanol (5A1P), 1-amino-2-propanol (1A2P), and isobutylamine (IsoBA), etc. 13C and 15N NMR spectroscopies were used to investigate the relationship between the chemical properties of amines and their tendency to form amine carbamates. The pKa is an important parameter in amine screening. The alkalinity is a fundamental property of an alkanolamine, and this can be quantified based on the pKa value of an aqueous solution of its conjugate acid. The alkalinity is important because it affects the kinetics and mechanism of the capture process.4−6 The pKa is defined as − log Ka, where Ka is the equilibrium constant for the deprotonation reaction. There have been many reports7,8 of a Brønsted relationship between the rate constant of the reaction between an amine and CO2 and the amine alkalinity. A linear relationship between the pKa of an acid or base and the reaction rate was reported by Brønsted et al.9,10 The secondorder reaction rate constants (k2) of cyclic amines are higher than those of other types of amine; this has been confirmed based on Received: June 12, 2016 Accepted: December 8, 2016 Published: December 21, 2016 111

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Table 1. Parameters for pKa Prediction Using Empirical PDS, Updated PDS, and QSSG Methodsa terms

functional group

PDS values12

updated PDS values13

QSSG values

base value

primary amine NH2R secondary amine NHR2 tertiary amine NR3 each CH3 on tertiary N each CH3 on primary and secondary N each β OR each β NH2 each β NHR each β NR2 each β OH each γ group each δ group each ε OH group ring effect if two equivalent N sites β- CH(CH3)2 β-C(CH3)3 solvent effects((CH2CH2OH)2) solvent effects ((CH2CH2OH)3) steric effects of cyclic tertiary amine intramolecular H bondingc

10.77 11.15 10.50 −0.2 −0.2 −1.2 −0.8 −0.9 −0.9 −1.1 +0.4Δβ +0.4Δγ 0 +0.2 +0.3 b − − − − −

10.60 11.10 10.60 −0.2 −0.2 −1.4 −0.9 −1.0 −1.0 −1.0 +0.4Δβ +0.4Δγ 0 0 +0.3 − − − − − −

10.60 10.80 10.60 −0.2 0 −1.3 −0.9 −0.8 −1.0 −1.0 +0.6Δβ +0.6Δγ +0.6Δγ +0.2 +0.3 −0.3 −0.45 +0.3 +0.6 −0.5 +0.2

ΔpKa shifts

a (i) Terms for aliphatic N- and O-containing amines; (ii) β effect added twice for ringed compounds such as morpholine and piperazine. If two different types of amino groups are present, the amino group attached to the most H atoms is considered to be the base amine. b“−” represents no assignment. cOnly intramolecular hydrogen bonding between R1R2N- and -OH/-NH2 groups can been considered for secondary and tertiary amines.

implicit solvent methods. In comparison with explicit solvent method, implicit solvation models could accelerate phase-space sampling by replacing discrete solvent molecules by a dielectric continuum. The accuracy of implicit solvation models ∼ 5 kJ/mol for typical neutral solutes, whereas monovalent ionic species have larger uncertainties of 15 kJ/mol or higher. Khalili et al. intended that their main goal was to develop an improved technique that enables aqueous pKa values to be predicted with an accuracy < 1 pKa unit for a set of similar industrially relevant amines for CO2 capture using a quantum-chemical technique.11 As a result, the quantum chemistry method is not competitive with the addition of functional groups in the prediction of amine pKa. The pKa value of aqueous amine solutions containing polyamines, multiple -OH groups, and cyclic amines cannot be predicted accurately. The purpose of this study was to develop another computer-free group-addition method that makes better predictions for multihydroxy and cyclic amines in particular based on the methods used by Perrin et al. and Sumon et al. for pKa prediction. The studied compounds consisted of primary, secondary, and tertiary mono-, di-, and trialkanolamines; secondary amines, including heterocyclic species; and -CH2OHand -CH2CH2OH- substituted piperidines. A quick and accurate pKa predicted method can provide a shortcut for screening CO2 absorbents.

stopped-flow experiments conducted at the same concentrations, and the Brønsted plots of amines with similar pKa values.11 Versteeg et al.6 reported that the correlation between the secondorder rate constants for the formation of zwitterions and the dissociation constants of amines used for CO2 capture conformed to a Brønsted relationship. They reported the following Brønsted relationship for aqueous primary and secondary alkanolamines: ln k 2 = pK a + 17.60 −

⎛ 7188 ⎞ 3 −1 −1 ⎜ ⎟ m · mol · s ⎝ T ⎠

The relationship for tertiary amines is ln k 2 = 1.3pK a + 11.48 −

⎛ 8270 ⎞ 3 −1 −1 ⎜ ⎟ m · mol · s ⎝ T ⎠

The protonation constants of many alkanolamines/amines under standard-state conditions have been determined experimentally or based on model predictions, and are available in the literature.1,12,15 In 1981, Perrin, Dempsey, and Serjeant12 described a computer-free group-addition method (the PDS method) for pKa prediction; the accuracy of the method was a few tenths for amines. The pKa prediction was based on the assumption that, within particular classes of acids and bases, substituents produce free energy changes which are a linear addition. Sumon et al. developed an updated PDS method and investigated 32 CO2-relevant amines. The reaction rates of polyamines and cyclic amines were higher than those of linear primary and secondary amines. However, the prediction errors of the amines containing polyamines, multiple -OH groups, and cyclic amines were larger than that of the linear primary and secondary amines with the updated PDS model.13 Errors in the predicted pKa values for amine aqueous solutions hinder efficient screening of amine absorbents. Recently, Ho14 comprehensively summarized the computational pKa prediction methods with particular attention on the

2. METHODS 2.1. PDS and Updated PDS Methods. The original empirical pencil-and-paper PDS method uses pKa base values and ΔpKa additive functional-group corrections. In 2012, Sumon et al.13 updated the parameter values relevant to amines for CO2 capture; the updated PDS enables much better predictions to be made. The PDS method considers all amines as structural variations of aliphatic amines, and the pKa values are predicted by linearly adding the substituent effects of all the groups to a fixed base 112

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Table 2. Predictions of pKa Values of 73 CO2-Relevant Amines, Obtained Using PDS, Updated PDS, and QSSG Method1,3,8,15−21

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Table 2. continued

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Table 2. continued

value; these depend on whether the amino moiety is primary, secondary, or tertiary. Base-weakening (or base-strengthening) effects of ring structures, n-methylation, and substituents such as -OH and -OR in the β or γ position are considered. 2.2. QSSG Method. The amine basicity depends on the degree of difficulty of combining nonshared electron pairs on

N atoms and protons. The ability of the N atom to accept a proton is related to the electron cloud density on the N atom and the steric hindrance around it. The pKa of the amine aqueous solution is affected by the solvent. The PDS and updated PDS methods do not consider the effects of steric hindrance, solvent, σ−p hyperconjugation, and intramolecular hydrogen bonding. 115

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by interpolation from existing experimental data. This analysis method has been demonstrated effective in a previous study.12 Our QSSG method can be represented as follows:

Using the prediction method of the base value coupling functional-group contribution, the pKa is predicted based on linear free energy relationships, by analogy, by extrapolation, and

pK a = base value + 0.3(nequivalent N − 1) + ( −0.2)ntertiary N ‐ CH3 + ( −1)nα NR 2 + ( −1)nβ NR 2 + ( −0.8)nβ NHR + ( −0.9)nβ NH2 + ( −1)nβOH + ( −1.3)nβOR + 0.6[(− 1)nγ NR 2 + (− 0.9)nγ NH2 + (− 1)nγ OH(− 1.3)nγ OR ] + 0.6 × 0.6 × (− 1)nδOH + 0.6 × 0.6 × 0.6 × (− 1)nεOH + 0.3(nβOH − 1)//solvent effect// + 0.2nring //ring effect// + ( −0.5)nterteriay amine ring //steric hindrance effects of cyclic tertiary amine// + 0.2n intramolecular H bonding //H‐bonding effect// + [(− 0.3)nβ tertiary C + (− 0.45)nβ quaternary C]//σ − p hyperconjugation effect// = base value + 0.3(nequivalent N − 1) + ( −0.2)ntertiary N ‐ CH3 + ( −1)nα NR 2 + ( −1)nβ NR 2 + ( −0.8)nβ NHR + ( −0.9)nβ NH2 + ( −1)nβOH + ( −1.3)nβOR + ( −0.6)nγ NR 2 + (− 0.54)nγ NH2 + ( − 0.78)nγ OR + (− 0.6)nγ OH + (− 0.36)nδOH + ( −0.216)nεOH + 0.3(nβOH − 1) + 0.2nring + (− 0.5)n terteriay amine ring + 0.2n intramolecular H bonding + (− 0.3)nβ tertiary C + ( −0.45)nβ quaternary C

Base value = 10.6 for primary and tertiary amines; base value = 10.8 for secondary amines. If two different types of amino group are present, the amino group attached to the most H atoms is considered to be the base amine. The β effect is incorporated twice for cyclic compounds such as morpholine and piperazine. Table 1 lists the parameter values obtained using the PDS, updated PDS, and QSSG methods. There are differences among the values obtained using the old PDS method and the new empirical QSSG prediction model.

As shown in Figure 1, methyl or ethyl substitution of a primary amine N atom has little effect on the pKa value. However, similar substitution of a tertiary amine N atom significantly affects the pKa, and the effect is greater with an ethyl substituent. As shown in Figure 2, the errors in the experimental pKa values and those predicted using the QSSG model for single-chain alkyl

3. RESULTS Although use of the updated PDS method for predicting the aqueous pKa values of amines for CO2 capture reduces the rootmean-square (rms) error from 0.33 to 0.18 for 32 specific amines,13 the rms errors can be larger for other amines. In this study, we used a computer-free method, i.e., the QSSG method, to investigate 73 amines, including alkanolamines, diamines, sterically hindered amines, substituted piperazines, and amines with two different functional groups. The errors of 73 CO2-relevant amines for the PDS, updated PDS, and QSSG methods are 0.34, 0.22, and 0.13, respectively; the results are shown in Table 2.

Figure 2. Experimental and QSSG pKa values for single-chain alkyl primary amines.

primary amines are within the range ±1%. However, for secondary and tertiary amines, the induction and steric effects of the alkyl group on the N atom should be taken into account. 4.2. Electronic, Solvent, and Steric Hindrance Effects of β-Hydroxy-Substituted Alkyl Groups Attached to N Atom of Tertiary Amine. The QSSG method considers solvent effects and steric hindrance by substituents as well as electronic effects. The effects of steric hindrance, the number of -OH groups, and alkyl chain length on the dissociation constants were examined. Figure 3 shows that the addition of -OH groups at various locations on tertiary amines decreases the pKa. The pKa

4. DISCUSSION 4.1. Effects of Alkyl Substituent on pKa of Aliphatic Amine Aqueous Solutions. Usually the more -CH3 groups on the N atom, the more electronegative the N atom is and the stronger the alkalinity is.22 However, the pKa values of H2NCH3, NH(CH3)2, and N(CH3)3 lead to the opposite conclusion. This is because the pKa of an aliphatic amine aqueous solution is affected by steric hindrance at the N atom as well as electronic effects.

Figure 1. Experimental and QSSG pKa values for amines with different degrees of alkyl substitution. 116

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Figure 3. Trends in experimental pKa values of tertiary amines at 298 K on addition of -CH3 and -OH groups.

Figure 4. Effect of σ−p hyperconjugation on experimental pKa values and values for alkylamines predicted using QSSG method.

In general, the introduction of -OH functional groups reduces the pKa of an amine aqueous solution. 4.3. σ−p Hyperconjugation Effects. The molecular system is stabilized by σ−p hyperconjugation involving a C−C σ bond and the p orbitals of a β N atom. This is because the interaction generates a larger molecular orbital. The more stable the C−C σ bond is, the less likely the N atom is to supply electrons; therefore the pKa is lower. Experimental results show that the pKa values for amines with one, two, and three -β-CH3 groups, respectively, are 10.54, 10.34, and 10.15 (Figure 4). The pKa values of β tertiary carbons and quaternary carbons, predicted using the QSSG model, are −0.3 and −0.45, respectively. The predicted pKa values of NH2CH2CH3, NH2CH2CH(CH3)2, and NH2CH2C(CH3)3 are 10.60, 10.30, and 10.15, respectively. The values predicted using the QSSG model are consistent with the experimental values. 4.4. Solvent Effects. Replacement of the H atoms on carbon atoms adjacent to amine groups in primary alkylamines by -CH2OH groups produces primary alcohol amines. The pKa of a primary alcohol amine aqueous solution is mainly affected by the number of -OH groups; other substituents on the α carbon atom have little effect. As shown in Figure 5, the experimental pKa values of aqueous solutions of organic amines with two β-OH groups, namely, BIS, AMPD, and AEPD, are 8.81, 8.84, and 8.82, respectively; the predicted pKa values are around 8.90. The experimental values are in agreement with the QSSG predicted values.

values of the tertiary amines TMA, DMEA, MDEA, and TEA with zero, one, two, and three -CH2CH2OH groups, respectively, are 9.80, 9.22, 8.54, and 7.73, an approximate decrease for each additional -CH2CH2OH group of 0.58, 1.26, and 2.07. Clearly, the higher the number of -CH2CH2OH substituents, the sharper the decrease in the pKa. The three main factors affecting the pKa are as follows. (1) induction effects (β-OH is an electron-withdrawing group; when -CH3 is replaced by -CH2CH2OH(β−OH), the greater the number of β-OH substituents, the greater the decrease in the electronegativity of the N atom; the effect of a β-OH substituent on the pKa value is negative); (2) solvent effects (a β-OH substituent increases the solvent effect on an organic amine, and this increases the pKa of the amine; in this case, a β-OH substituent has a positive effect on the pKa); (3) steric hindrance effects (multiple β-OH substitution greatly increases the steric hindrance at the N atom, which hinders proton acceptance by the N atom, and this decreases the pKa of the organic amine). The effect of β-OH substitution on the pKa is negative. Overall, these three factors decrease the electronegativity of the tertiary amine N atom; therefore the pKa values of TMA, DMEA, MDEA, and TEA decrease gradually. The three β-OH substituents on the N atom in TEA occupy a large amount of space, and the impact of this steric hindrance is equivalent to −0.5 pKa units in the QSSG method. The effect of -CH3 or -CH2CH3 addition is less clear, but when these groups are directly attached to the N atom, converting the amine from a primary amine to a secondary amine, the pKa decreases. 117

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different substituents on the tertiary amine N atom, namely, MDEA, EDEA, t-BDEA, and TEA, are 8.54, 8.80, 9.03, and 7.73, respectively. The pKa increases slightly with increasing alkyl substitution at the N atom; however, the pKa value of TEA (7.73) is lower, because the third N substituent is β-OH instead of an alkyl group. In summary, the effect of -OH substituents on the pKa is greater than those of alkyl substituents.

For secondary and tertiary alcohol amines, the alkyl substituent on the N atom greatly affects the pKa. This is mainly because the alkyl substituent on the N atom significantly affects the electronegativity of N, because the alkyl substituent increases steric hindrance around the N atom. The pKa is less affected by an alkyl substituent on the N atom than by a β-OH. As shown in Figure 6, the experimental pKa values for various amines with

Figure 5. Experimental and QSSG predicted pKa values of primary amines with different alkyl group substituents on α carbon atom adjacent to primary amine group.

Figure 6. Experimental and QSSG pKa values of tertiary amines with different substituents on amine N atom.

Table 3. Errors in Predicted pKa Values for Amines Containing β-OH Groups

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amines decrease the pKa, but destabilize carbamate formation, resulting in higher than theoretically predicted CO2 absorption capacities. The rms errors in the pKa values of 32 specific amines in aqueous solution, predicted using the updated PDS method, were lower than those in the PDS method; the predicted values deviated greatly from the experimental values for some amines. In the QSSG method, the effect of intramolecular hydrogen bonds is introduced for cyclic tertiary amine structures because -NR1R2 functional groups and γ-OH groups tend to form hydrogen bonds. The R1R2N(CH2)3OH/R1R2N(CH2)3NH2 structure easily forms a six-membered ring, as shown in Figure 7. Typical examples are 2-PDE and 2-PZE; they contain the unit, which can generate intramolecular hydrogen bonds. Figure 8 shows a schematic diagram of hydrogen-bond formation in 2-PDE. The pKa values of 2-PDE and 2-PZE were calculated using the QSSG model as follows:

Figure 7. Formation of intramolecular hydrogen bond.

Figure 8. Intramolecular H bonding in 2-PDE.

Figure 9. Molecular structure of 2-PZE.

In the QSSG model, the PDS model is modified when the amine contains more than one β-OH. When two or three β-OH groups are present, the pKa is 0.3 or 0.6 pKa units, respectively, higher. The pKa values of nine amines with two or three -OH groups are listed in Table 3; the pKa rms errors are 0.45, 0.28, and 0.12 in the PDS, updated PDS, and QSSG models, respectively; the rms errors in the QSSG model are clearly lower. 4.5. Effects of Intramolecular Hydrogen Bonds between R1R2N- and -OH Groups for Secondary and Tertiary Amines. Puxty et al.23 reported that hydrogen bonds in

pK a,2‐PDE = base value + ring effect + γ OH + H‐bonding effect = 10.8 + 0.2 + 0.6(− 1) + 0.2 = 10.60

(experimental pKa = 10.628).

Table 4. Effects of Steric Hindrance on pKa Values of Cyclic Tertiary Amines

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Table 5. pKa Errors in QSSG Prediction Outside the Training Set

were 0.47, 0.22, and 0.12, respectively. In conclusion, since the steric effect is considered, the QSSG model is highly effective in solving the problem of estimating the errors for cyclic tertiary amines in pKa values predicted.

pK a,2‐PZE = base value + ring effect + additional amino + β NHR + δOH + H‐bonding effect = 10.8 + 0.2 + (2 − 1) × 0.3 + 1 × (− 0.8) + 0.6 × 0.6 × (−1) + 0.2

5. QSSG MODEL VALIDATION In order to demonstrate the validation of our QSSG methods further, 10 more types of amines were employed to predict the pKa values. As shown in Table 5, the results showed that the predicted values were all consistent with the experimental values, and the root-mean-square error was 0.17 pKa units, whether it is primary, secondary, tertiary, poly, or cyclic amines. To make it easier to understand our computing method, we added some calculation examples with the QSSG method in Table 6. The calculation methods of several typical amines were listed in Table 6, but not all functional groups were included in Table 6. Therefore, all effective functional groups should be taken into account when calculating the other amines.

= 10.34

(experimental pKa = 10.418). As shown in Figure 9, the effect of one base amino group (-HNCH2CH2NH-) on the amine ring is considered because the H atom of the sub amino group and -OH form an intramolecular hydrogen bond, and the effect of the second -HNCH2CH2NHgroup is neglected. The differences between the experimental and predicted pKa values of 2-PDE and 2-PZE were 0.02 and −0.06 pKa units for the QSSG model. 4.6. Effects of Steric Hindrance in Cyclic Tertiary Amines. Table 4 shows the changes in the pKa values of cyclic tertiary amine families (piperazines, piperidines, and morpholines) arising from functionalization with alkyl, amino, or alcohol groups. The steric hindrance in a cyclic tertiary amine greatly affects the pKa, equivalent to −0.5 pKa units in the QSSG model. The introduction of functional groups into a cyclic amine reduces the pKa. The pKa values of 1-MPD, 1,2-DMPD, 1-EPD, 1-MPD-3-ol, 4-MMOR, 4-EMOR, N-(2-HE)MOR, and 1,4-DMPZ, which are all cyclic tertiary amines, show that the effect of steric hindrance on the pKa by an -OH group is greater than that by a -CH3 group. The N atom in TEA is connected to three β-OH groups, and the steric hindrance is large. The effect of steric hindrance on the pKa value is therefore also considered in the QSSG model. A comparison of the experimental values for nine cyclic tertiary amines with those predicted using the PDS, updated PDS, and QSSG models shows that the rms errors in the model predictions

6. CONCLUSIONS Our QSSG model was established to amend many of the parameters in the PDS and updated PDS models. In the QSSG method, not only electronic effects but also σ−p hyperconjugation, solvent, steric, and hydrogen-bonding effects on the pKa values of organic amine aqueous solutions were considered. These relationships provide a effective method for high throughput screening of amine absorbents for CO2 capture. The main conclusions are as follows. (1) If two different types of amino group are present, the amino group attached to the most H atoms is considered to be the base amine. 120

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Table 6. Some Calculation Examples with QSSG Method

(2) When there are two or three β-OH groups attached to the N atom in the amine, the equivalent of 0.3 or 0.6 pKa units, respectively, is added to the pKa value. The rms errors in the estimated pKa values of nine amines with two or three -OH groups, obtained using the PDS, updated PDS, and QSSG models, were 0.46, 0.28, and 0.12, respectively. (3) Steric hindrance greatly affects the pKa values of cyclic tertiary amines; in our QSSG method, the difference is equivalent to −0.5 pKa units. A comparison of the experimental values for nine cyclic tertiary amines with the values predicted using the PDS, updated PDS, and QSSG models showed that the rms errors in the model predictions were 0.47, 0.22, and 0.12, respectively. (4) Intramolecular hydrogen bonding between -NR1R2 functional groups and γ-OH groups leads to easy formation of six-membered ring structures for secondary and tertiary amines; the effect of intramolecular hydrogen bonding on the pKa is equivalent to 0.2 pKa units. To sum up, the values predicted using our QSSG method were good in agreement with the experimental results, and the rms error in the predicted pKa values was 0.13 pKa units for a set of 73 amines for use in industrial CO2 capture. This QSSG method provides a shortcut for screening CO2 absorbents. In addition, the validation of our method for polyamines containing three or more amino groups is our next research focus.

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Funding

The authors are grateful for financial support from the National Natural Science Foundation of China (Grant Nos. 51306044 and 51421063). Notes

The authors declare no competing financial interest.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+86-451-86413231. E-mail: [email protected]. ORCID

Juan Qian: 0000-0002-1615-1052 121

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DOI: 10.1021/acs.jced.6b00481 J. Chem. Eng. Data 2017, 62, 111−122