borate Ions - American Chemical Society

Subscriber access provided by UNIV OF DURHAM

Article 2

Computational Insights into Biomimetic CO Hydration Activities of (Poly)borate Ions Manju Verma, and Parag Arvind Deshpande

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02617 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Computational Insights into Biomimetic CO2 Hydration Activities of (Poly)borate Ions Manju Verma and Parag A. Deshpande∗ Quantum and Molecular Engineering Laboratory, Department of Chemical Engineering Indian Institute of Technology Kharagpur, Kharagpur 721302, India

Corresponding author: Parag A. Deshpande Email: [email protected]; Phone: (+91) 3222 283916

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 40

Abstract Borate-based compounds have been observed to be CO2 capture agents in natural and synthetic setups. Carbon capture in such systems has been proposed to take place via hydration of CO2 resulting in bicarbonate ion formation. Several experimental studies have reported borate-based catalysts for CO2 hydration reaction and the mechanism of the reaction has been proposed to follow the enzymatic carbonic anhydrase action. In view of the absence of detailed physical insights into the mechanistic aspects of borate-catalyzed CO2 hydration, computational investigations were carried out in this study under the density functional theory framework to explain the mechanism of the reaction over borate-based systems. Three borate-based systems viz., borate ions, triborate ions and tetraborate ions were tested as the catalysts for the aqueous phase carbon capture reaction. Free energy landscapes provided the details of the elementary steps of the reaction concluding the mechanism of the reaction to be indeed biomimetic consisting of parallel and bent CO2 complexation, intramolecular proton transfer, bicarbonate ion complex formation and displacement of bicarbonate ion by water molecule to form (poly)borate-water complexes. All three ions were found to be active for catalyzing the reaction. NMR and FTIR spectra of all the intermediates proposed during the biomimetic mechanism were computed and compared against the experimentally reported spectra. The comparative analyses proved the identities of the intermediates thus further confirming the mechanism of the reaction.


1

Page 3 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


Introduction

Carbon dioxide emission increment in the atmosphere is a major cause of climate change. Global atmospheric temperature is increasing primarily due to CO2 emission. 1 Fossil fuel-based power plants are the main sources of CO2 emission and consequently, reduction and control of CO2 emission are immediate requirements. Numerous techniques have been proposed to check CO2 concentration in the atmosphere. Naturally occurring carbonic anhydrase enzymes exhibit high CO2 hydration turnovers of the order of 106 s−1 at pH 9 and 25 ◦ C. 2,3 These metalloenzymes + catalyze the reversible hydration of CO2 to a bicarbonate ion (CO2 + H2 O ⇋ HCO− 3 + H ) in bi-

ological systems. 4,5 Detailed structural and functional relationships in different carbonic anhydrase enzymes have been studied and reported elsewhere. 6,7 Human carbonic anhydrase is a monomeric protein with 260 residues. The zinc ion present in the funnel-shaped active site coordinates to three histidine residues and a water/hydroxyl ion. 8–11 The active sites of carbonic anhydrases have a hydrophobic half 12 necessary for CO2 attachment and a hydrophilic half necessary for proton shuttling. The mechanism of CO2 hydration with these enzymes has been reported to consist of (i) nucleophilic attack on CO2 by zinc bound hydroxyl ion (ii) bending of CO2 molecule (iii) proton migration to form a bicarbonate ion (iv) zinc bound bicarbonate ion displacement by a water molecule and (v) water molecule deprotonation to regenerate zinc bound hydroxyl ion. 13 This mechanism serves as an ideal basis to explore synthetic biomimetic systems for reactive carbon capture applications. Aqueous CO2 absorption has been reported to be supported by borate, 14,15 arsenite, 16–19 phosphate, 19 silicate, 19 vanadate, 20–22 sulfate, 22 hypochlorite, 16,18 and nickel nanoparticles. 23–25 Further, K2 CO3 has been reported to improve CO2 aqueous absorption with implications to large scale carbon capture. 26–31 CO2 absorption has been observed to take place in lakes and oceans. 32 Seawater has trace materials like boron, sulfur and arsenic. 33 The concentration of boron in such water bodies is around 0.4 mM. 34 Alkaline saline lakes have significant amounts of such materials. 35 Borate ions have been reported to absorb CO2 36,37 on the surfaces of water bodies and CO2 capture by water bodies can be attributed to borate catalyzed CO2 hydration. Guo et al. 14 conducted detailed experimental studies on borate-catalyzed CO2 hydration and proposed that CO2 hydration catalyzed by borate ions followed the carbonic anhydrase mechanism. In their experimental study, B(OH)− 4 was found to be active for direct interaction with CO2 in aqueous medium. CO2 hydration with borate ions was possible at all relevant pH. This made an interesting ACS Paragon Plus Environment

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 40

case for a novel CO2 capture compound. Detailed insights into the mechanism of CO2 hydration are required and this study provides molecular details of the elementary steps of CO2 hydration over borate-based catalysts for biomimetic CO2 hydration.

Aqueous solutions of boric acid form polyborate ions in a pH range of 4 to 13. Boric acid − [B(OH)3 ] and triborate ions [B3 O3 (OH)− 4 ] exist upto pH 6 while borate ions [B(OH)4 ] and tetrab-

orate ions [B4 O5 (OH)4 2− ] appear in solutions with pH greater than 8. 38–40 B(OH)3 has a tendency to react with water molecules to form (poly)borate ions. 41–44 Da Silva and co-workers have worked extensively on solution-based CO2 capture techniques with special attention on borate-based catalytic systems. 14,30,31,45,46 The reaction kinetics of CO2 hydration was observed to be enhanced in presence of potassium carbonate by economically affordable boric acid. 45 Boric acid does not interact with sulphur dioxide and oxygen present in the flue gas of fossil fuel power stations thus posing borate-catalyzed CO2 hydration as an attractive CO2 capture technique. Phan et al. 47 have investigated the post combustion capture of CO2 by borate ions and proposed a new mechanism of CO2 hydration for inorganic oxoanions. Freeman et al. 48 prepared borate solution impregnated rayon cloth and tested its activity towards CO2 capture at different pH.

All the studies cited above showed the potential of borate-based compounds for carbon capture applications. While the interest of the community has been attracted by borate-based systems, as is evidenced by the literature cited above, detailed physical insights are sought and this study aimed at developing mechanistic insights into the successful working of borate-based systems for biomimetic carbon capture. Under the experimental conditions maintained by Guo et al., 14 boron species existed only as B(OH)− 4 and the investigators could successfully suppress the formation of polyborate ions. However, it is interesting to explore and compare the activities of (poly)borate ions towards CO2 hydration in order to develop a catalytic system which is robust towards changes in the solution pH. Computations carried out under the density functional theory (DFT) framework provided free energy landscapes for CO2 hydration catalyzed by borate, triborate and tetraborate ions. Correspondence between experimental and computational observations were established thereby providing the molecular reasoning behind experimental observations. This study is expected to provide rationale for further development of borate-based catalytic systems for carbon capture with potential applications to flue gas treatment.


3

Page 5 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2


Computational Details

Mechanistic details of CO2 hydration catalyzed by borate, triborate and tetraborate ions were obtained by developing free energy landscapes for the reaction. Equilibrium structures of reactants, products, intermediates and transition states were obtained by DFT calculations. All the calculations were carried out using Gaussian09 53 package. Hybrid exchange-correlation functional B3LYP was used in conjuntion with 6-31+G(d,p) basis sets to describe the molecular species. Frequency calculations were carried using the same level of theory to ensure correct extrema for intermediates (with no imaginary frequencies) and transition states (with one imaginary frequency). Frequency analysis also provided inputs for the Gibbs free energy calculations. All the free energy calculations reported in this study incorporated zero point energy corrections. Mulliken charge analysis was done to quantify the partial charges on different species involved during the reaction. The polarizable continuum model was used to compute the effect of solvation (water) on the free energy landscapes.

3 3.1

Results and Discussion Analysis of Elementary Steps of Biomimetic CO2 Hydration Catalyzed by (Poly)borate Ions

Experimental studies on the activity of borate ions for CO2 hydration by Guo et al. 14 indicated the mechanism of the reaction to be the carbonic anhydrase action. Since details of mechanistic aspects of borate-catalyzed CO2 hydration was the prime motive of this computational study, Guo et al.’s proposition of carbonic anhydrase action was tested under the DFT framework. Elementary reactions equivalent to the carbonic anhydrase action but catalyzed by (poly)borate ions are shown in Figure 1 and described briefly here. In all the intermediates shown in Figure 1, other coordinations of O atoms coordinated tetrahedrally to B atoms have not been shown to highlight that the catalytic species is a (poly)borate ion and can be any of the borate, triborate or tetraborate ions. In the enzyme, the active species is a hydroxyl group coordinated to the metal ion. Similar is the case here and the hydroxyl centres of (poly)borate ions act as active sites for the reaction. Interaction of CO2 with the catalyst takes places via the hydroxyl centres resulting in CO2 complexation first in a linear fashion followed by its bending, as shown in Figure 1. Intramolecular ACS Paragon Plus Environment

4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 40

proton transfer/internal rotational has been reported for carbonic anhydrase action which has also been considered in the present case. This step results in the formation of the product HCO− 3 which is displaced by H2 O molecule to form catalyst-H2 O complex deprotonation of which completes the catalytic cycle. All these elementary steps are shown in the reaction mechanism of Figure 1 and we discuss in detail the associated geometries and energetics in this section.

In borate compounds, boron oxoanions coordinate to adjacent boron oxoanions by hydrogen bonds, or they interconnect by boron-oxygen bonding into infinite sheets, chains or 3-dimensional frameworks. 54 However, boron oxoanions are also present in isolation in solutions. Borate minerals and their synthetic compounds can contain one to six boron atoms. However, isolated boron oxoions containing six boron atoms are rare and have not been reported. Therefore, borate [B(OH)4 ]− , triborate [B3 O3 (OH)4 ]− and tetraborate [B4 O5 (OH)4 ]2− ions were chosen in this study to analyze the catalytic activities towards CO2 hydration.

DFT optimized geometries of (poly)borate ions are shown in the inset of Figure 1. Borate ion had bond lengths of B-O of 1.48 ˚ A while O-H bond lengths were 0.96 ˚ A. Triborate ion was a result of condensation of two boric acid molecules and one borate ion. B-O bond lengths in a triborate ion were in the range of 1.38 − 1.51 ˚ A and O-H bond lengths were similar to those in a borate ion. In a tetraborate ion, B-O bonds were formed between two borate ions and two boric acid molecules. B-O bond lengths in a tetraborate ion were in a range of 1.35-1.52 ˚ A while the O-H bond lengths were similar to those in borate or triborate ions. All the three (poly)borate ions, marked as IS in Figure 2a, 3a and 4a, were tested for CO2 hydration activity.

All (poly)borate ions had hydroxyl groups which directly interacted with linear CO2 as the first step of CO2 hydration. Parallel CO2 interacted with O-atoms of hydroxyl groups of the borate, triborate and tetraborate ions (see Figure 2b, 3b, 4b). Atomic distances between C atom of CO2 A for borate, A, 2.87 ˚ A, 2.84 ˚ molecule and O atom of (poly)borate hydroxyl groups were 2.82 ˚ triborate and tetraborate ions, respectively. C-O bond lengths of CO2 molecule were 1.16 ˚ A and 1.17 ˚ A while the O-C-O angles were in a range of 176.8 ◦ − 177.1 ◦ . During this step, B atom transferred electrons to O atoms and became more positively charged. Parallel CO2 complexation was observed as an endergonic process with free energy changes of 6.40 kcal/mol (borate ion), 6.82


5

Page 7 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kcal/mol (triborate ion), 7.08 kcal/mol (tetraborate ion) (see Figure 5).

As the next elementary step of biomimetic CO2 hydration, bending of CO2 took place. This can be seen from Figures 2d, 3d and 4d over all the (poly)borate ions. It can be observed that in this complex (IM2) a change in coordination of O atom of the hydroxyl group took place. O atom from CO2 coordinated to B atom in the intermediate IM2. The coordination was unidentate. As a result, the O atom from the hydroxyl group, coordinated originally to B atom, detached from B atom. Bond lengths of newly established B-O bonds were 1.64 ˚ A, 1.55 ˚ A and 1.53 ˚ A for borate, triborate and tetraborate ion, respectively. C-O bond lengths in this complex were 1.23 ˚ A, 1.28 ˚ A and 1.37 ˚ A for the borate ion with the O-C-O angles of 114.5 ◦ , 119.7 ◦ and 125.7 ◦ . C-O bond lengths and O-C-O bond angles in triborate ion and tetraborate ion were similar to those in borate ion. The bond lengths were 1.23 ˚ A, 1.29 ˚ A and 1.36 ˚ A with similar O-H (0.97 ˚ A) bond lengths. The O-C-O bond angles were 114.6 ◦ , 120.8 ◦ and 124.5 ◦ in triborate ion and 115.6 ◦ , 120.4 ◦ and 124.0 ◦ in tetraborate ion. CO2 bending was accompanied with small changees in free energies over the three catalysts. The reaction free energy for IM1-IM2 was −1.72 kcal/mol for borate ion while the reaction free energies were 1.40 kcal/mol and 0.54 kcal/mol for triborate ion and tetraborate ion, respectively (see Table 1). The catalyst attacked nucleophilically the C atom of CO2 and transition state (TS1) for CO2 bending was observed on (poly)borate ions (see Figures 2c, 3c, 4c). C-O interactions between C atom of CO2 and O atom of hydroxyl groups were created with bond distances of 1.90 ˚ A (borate ion), 1.72 ˚ A (triborate ion), 1.76 ˚ A (tetraborate ion). The C-O bond lengths of CO2 molecule increased to fall in a range of 1.20 − 1.21 ˚ A while the O-C-O angles reduced to fall in a range of 144.4 ◦ -150.9 ◦ in TS1 state. The activation free energies for CO2 bending were 7.49 kcal/mol, 10.74 kcal/mol and 9.32 kcal/mol with borate, triborate and tetraborate ions, respectively (see Table 2). Thus, the activation free energy barrier for CO2 bending was the least with borate ion, as can be seen from the free energy landscapes of Figure 5.

Lipscomb and Lindskog mechanisms have been reported for bicarbonate ion formation from OH− .CO2 complex during enzyme catalyzed CO2 hydration. 55–57 In Lipscomb mechanism, proton transfer takes place for bicarbonate ion formation while in Lindskog mechanism, OH− .CO2 complex rotates to change the metal-oxygen (M-O) coordination for bicarbonate formation. 58–60 Transition state for Lipscomb mechanism is represented as TS2 while transition state for Lindskog mechanism


6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 40

′

is represented as TS2 in our study. Proton moved away from O atom of IM2 complex to attain the transition state TS2 following the Lipscomb mechanism. In TS2 state, proton coordinated to both the O atoms with O-H bond lengths of 1.27 ˚ A. B-O and C-O bond lengths changed slightly. B-O bond length increased from 1.64 ˚ A to 1.77 ˚ A (borate ion), 1.55 ˚ A to 1.59 ˚ A (triborate ion) and 1.53 ˚ A to 1.55 ˚ A (tetraborate ion). C-O bond lengths for borate ion were 1.26 ˚ A, 1.31 ˚ A and 1.31 ˚ A with bond angles of 105.8 ◦ , 126.5 ◦ and 127.8 ◦ in TS2 state. Bond length variations were similar for triborate and tetraborate ions but the bond angles were slightly different. The C-O bond lengths in TS2 state were 1.27 ˚ A , 1.30 ˚ A and 1.30 ˚ A and the O-C-O angles for triborate ion were 107.8 ◦ , 124.5 ◦ and 128.3 ◦ and for tetraborate ion were 107.0 ◦ , 123.5 ◦ and 129.5 ◦ . The activation free energies for attaining TS2 state were 27.74 kcal/mol (borate ion), 29.86 kcal/mol (triborate ion) and 29.51 kcal/mol (tetraborate ion) as shown in Table 1 and depicted in Figure 5. For attaining a TS2´ state following Lindskog mechanism, OH− .CO2 complex rotated with C atom as a center and C-O-B angles changed in borate ion (127.7 ◦ to 122.8 ◦ ), triborate ion (127.6 ◦ to 125.2 ◦ ) and tetraborate ion (129.3 ◦ to 131.9 ◦ ). Unidentate B-O bonds were found to be unaltered in all the ions. C-O-C angles between C-O bonds changed slightly due to OH− .CO2 complex rotation while C-O bonds were not found to be changed. In rotational (TS2´) transition state, altered O-C-O angles were 111.3 ◦ , 120.6 ◦ and 128.2 ◦ in borate ion. Small change in O-C-O angles of OH− .CO2 complex in triborate ion (1.8 ◦ -2.6 ◦ ) and tetraborate ion (0.3 ◦ -6.5 ◦ ) were observed due to rotation. Significant reductions in free energy barriers were observed following the internal rotational pathway when compared to the proton transfer pathway. The free energy requirements were in a range of 0.2-4.4.26 kcal/mol for internal rotation against a requirement in a range of 27.74-29.89 kcal/mol for proton transfer.

On crossing the activation barrier for intramolecular proton transfer, the proton coordinated to the O atom coming from CO2 molecule and formed the IM3 intermediate of CO2 hydration. This formed the product bicarbonate ion coordinated to the (poly)borate ions (see Figures 2g, 3g and A in A in borate ion, 1.55 ˚ 4g). B-O atomic distances decreased from those in TS2 state to 1.63 ˚ triborate ion and 1.53 ˚ A in tetraborate ion. The C-O bond lengths of bicarbonate ion over borate ion were 1.28 ˚ A, 1.36 ˚ A and 1.23 ˚ A with 111.5 ◦ , 120.2 ◦ and 128.4 ◦ O-C-O bond angles. The C-O bond lengths in triborate and tetraborate ions were similar. In triborate ion, the bond lengths of C-O were 1.22 ˚ A, 1.30 ˚ A and 1.36 ˚ A with O-C-O bond angles of 110.3 ◦ , 121.2 ◦ and 128.5 ◦ . The


7

Page 9 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O-H bond length was 0.97 ˚ A in bicarbonate ion was the same in borate, triborate and tetraborate ions. Reaction free energies for bicarbonate ion formation were slightly exergonic over all three catalysts. Reaction free energies calculated between IM2-IM3 were −1.47 kcal/mol (borate ion), −1.25 kcal/mol (triborate ion) and −1.18 kcal/mol (tetraborate ion), as shown in Table 1. The product release in carbonic anhydrase action takes place via the displacement of HCO− 3 by H2 O. Product detachment and H2 O coordination in the current system took place as a simultaneous one-step process over all the three catalyts. In presence of H2 O, B-O bond elongated to enable desorption of the product to the aqueous solution of water solvent. The B-O bond length increased from 1.63 ˚ A to 1.69 ˚ A in borate ion, from 1.55 ˚ A to 1.57 ˚ A in triborate ion and from 1.53 ˚ A to 1.54 ˚ A in tetraborate ion (see Figures 2h, 3h and 4h). Increment of B-O5 bond length indicated the desorption of bicarbonate. The reaction free energies were positive for desorption of bicarbonate ion. Reaction free energies between IM3 and IM4 steps were 4.53 kcal/mol with of borate ions, 4.96 kcal/mol with triborate ions and 5.09 kcal/mol with tetraborate ions. After bicarbonate desorption, borate ion converted to boric acid while triborate ion converted to 2, 4, 6-boroxinitriol and H2 O molecule complexed with B atom of boric acid and 2, 4, 6-boroxinitriol. In case of tetraborate ion, H2 O molecule absorbed immediately on B atom as it had larger positive charge than borate and triborate ion as obtained by mulliken charge analysis. Spontaneous H2 O deprotonation is expected to take place because of solution pH thereby completing the biomimetic CO2 hydration cycle.

3.2

Computational Nuclear Magnetic Resonance Spectroscopy Analysis

Computational nuclear magnetic resonance spectroscopy (NMR) was used to confirm the molecular structures of (poly)borate ions and their complexes present in the system under study. Computed NMR shielding (σ) and NMR shifts (δ) of

11

B and

13

C have been tabulated in Table 3. We

calculated NMR shifts of boron containing compounds with boron trifluoride diethyl etherate (BF3 O(C2 H5 )2 ) as the reference. For carbon containing compounds, tetramethylsilane (Si(CH3 )4 ) was taken as reference. NMR shieldings of boron and carbon containing compounds were calculated with B3LYP functional and 6-31+G(d,p) basis set.

NMR shieldings for B(OH)− 4 and H2 O·B(OH)3 were obtained computationally as 106.52 ppm and 92.59 ppm, respectively. Similar NMR shieldings have been reported experimentally for ACS Paragon Plus Environment

8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 40

B(OH)3 and H2 O·B(OH)3 by Tossell et al. 61 The shielding difference between B(OH)− 4 and H2 O·B(OH)3 was 13.94 ppm which was comparable to 15.80 ppm reported by Tossell et al. 61 Qiu et al. 62 reported a range from −3 to 3 ppm of NMR shifts for B(OH)− 4 and a range of 15-20 ppm for H2 O·B(OH)3 . The shift calculated for B(OH)− 4 was 1.20 ppm and it was close to 1.00 ppm reported by Schott et al. 63 H2 O·B(OH)3 had NMR shift of 15.13 ppm which was close to the NMR shift reported by Qiu et al. 62 B3 O3 (OH)− 4 had NMR shielding of 97.95 ppm and H2 O·B3 O3 (OH)3 had shielding of 92.34 ppm. NMR shielding difference for triborate ion was 5.61 ppm which was lesser than that of the borate NMR shielding difference. The NMR shift difference for triborate compounds of boron − also decreased to 5.61 ppm. NMR shielding peaks of B4 O5 (OH)2− 4 and H2 O·[B4 O5 (OH)3 ] were

observed at 99.51 ppm and 98.09 ppm, respectively. NMR shielding differences and NMR shift differences of tetraborate compounds were calculated to be 1.42 ppm and 1.50 ppm, respectively. From all of these observations, the proposed primary step of the mechanism of CO2 hydration shown in Figure 1 consisting of (poly)borate ion formation and borate-water complexation were validated.

NMR analysis was also carried out for carbon compounds complexed with (poly)borate ions, as shown in Table 4. NMR shieldings of CO2 in presence of OH− ion were obtained in a range of 72.11 − 72.50 ppm in (poly)borate solutions. The resonances due to CO2 (124.5 ppm) and 64 Gallagher et al. 65 have reported HCO− 3 ion (160.9 ppm) have been reported by Merritt et al.

the resonance peak for CO2 at 125 ppm and the peak for HCO− 3 at 161 ppm experimentally. The resonance shifts of

13

C for CO2 ·OH− complexes were observed in a range of 120.11 − 120.30 ppm,

as shown in Table 4 and the shifts were reasonably close to the experimental values. The spectral shifts of NMR for HCO− 3 were in a range of 149.94 − 153.92 ppm in (poly)borate solutions. NMR shifts calculated for HCO− 3 by DFT calculations were observed to be close to the experimental NMR − shifts. The resonance shifts of H2 O·HCO− 3 complex reduced for HCO3 ion and were in a range of

148.49 − 152.70 ppm (Table 4). A good agreement of computed and experimental NMR spectral properties of (poly)borate-OH− ·CO2 and (poly)borate-HCO− 3 systems further strengthened the borate-catalyzed biomimetic CO2 hydration mechanism proposition.


9

Page 11 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.3


Computational Infrared Spectroscopy Analysis

Fourier transform infrared spectroscopy (FTIR) was used to further confirm the possibility of different intermediates proposed on the basis of free energy landscapes for borate-catalyzed CO2 hydration. Vibrational bond stretchings of reactants, intermediates and products of CO2 hydration are discussed in detail here and their comparative FTIR spectra are shown in Figures 6-8 and comparative frequencies are given in Table 5. Vibrational analysis of B(OH)− 4 and B(OH)3 has been done by several investigators. 66,67 Computed spectra of B(OH)− 4 showed intensities at 475, 825, 847, 865, 974, 1045, 1087, 1146 and 3812 cm−1 which are shown in Figure 6(a). O-B-O bending deformations occured at 475 cm−1 and Balachander et al. 66 have experimentally observed the O-B-O bendings to occur at 470 cm−1 . B-O tetrahedral (BO4 ) bond stretchings were observed from 825-865 cm−1 and 1045-1146 cm−1 which was supported with a range of 807-918 cm−1 and 1020-1156 cm−1 for B-O tetrahedral bond stretchings quoted by Zeebe. 67 B-O-H bending defor−1 mations were observed at 974 cm−1 and O-H bond stretchings of B(OH)− 4 occurred at 3812 cm .

In FTIR analysis of B3 O3 (OH)− 4 , IR intensities were observed at 500, 690, 797, 894, 957, 1030, 1087, 1117, 1287, 1434 and 3505 cm−1 (see Figure 7(a)). Bond stretching peaks of [B4 O5 (OH)4 ]2− were observed at 472, 680, 741, 752, 763, 781, 893, 948, 954, 1012, 1016, 1025, 1049, 1090, 1157, 1260, 1293, 1421 and 3803 cm−1 (see Figure 8(a)). Jun et al. 68 quoted a range for out of plane bendings of B-O in BO3 trigonal plane (620-750 cm−1 ), B-O symmetric bond stretchings in BO4 unit (740-890 cm−1 ) and B-O asymmetric bond stretchings in BO3 unit (1300-1450 cm−1 ). Balachander et al. 66 also reported a range of B-O stretching vibrations in BO4 unit (950-980 cm−1 ) and B-O bond stretchings in BO3 unit (1176-1500 cm−1 ). B-O bond stretchings in BO4 unit were present in range of 797-957 cm−1 and 1087-1117 cm−1 for B3 O3 (OH)− 4 while B-O stretchings in BO3 unit were present at 690 cm−1 and 1287-1487 cm−1 , as shown in Figure 7(a). B-O-H bending deformations due to O-H symmetric bond stretching were identified at 1030 cm−1 in B3 O3 (OH)− 4. O-H stretching vibrations were present at 500 and 3805 cm−1 in B3 O3 (OH)− 4 . Similarly, O-H bond stretching vibrations were observed at 472 cm−1 and 3803 cm−1 in [B4 O5 (OH)4 ]2− . B-O bond stretchings in BO4 of tetraborate ions occurred in a range of 752-932 cm−1 and 1025-1090 cm−1 while B-O stretchings in BO3 unit occurred at 680 cm−1 , in the range of 948-954 cm−1 and 1260-1421 cm−1 . B-O-B bending deformations in tetraborate ion were observed at 741 cm−1 and 1157 cm−1 while the B-O-H bending deformations were seen at 1012 cm−1 and 1157 cm−1 also. As can be observed from the comparative experimental and computational observations marked ACS Paragon Plus Environment

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 40

in Figures 6-8, the existance of (poly)borate ions and their water complexes, as suggested in the reaction mechanism of Figure 1 and observed computationally in Figures 2-4, is confirmed.

IR analysis of CO2 complexes of borate compounds have been reported previously. Therefore, analysis of CO2 and HCO− 3 complexes of (poly)borate ions was also carried out to confirm the reaction mechanism. A peak of C-O asymmetric bond stretching was observed at 2343 cm−1 and a low intensity peak of O-C-O bending was observed at 610-612 cm−1 for CO2 molecule in (poly)borate solutions, as shown in Figures 6(b)-8(b). Similar vibrational frequencies for C-O bond stretchings (2343 cm−1 ) and O-C-O bendings (655 cm−1 ) have been reported by Moore and Khanna. 69 They have also reported the IR intensities of C-O stretchings for CO3 plane of a bicarbonate ion at 1367 cm−1 , 1705 cm−1 , and 812 cm−1 . In Figure 6(c) of borate ion, peaks for C-O bond stretchings in CO3 plane with C-O-H bending deformations were observed at 1358 cm−1 and 1691 cm−1 and those for O-C-O bending deformations in CO3 plane were observed at 697 cm−1 signifying the formation of CO3 plane with CO2 ·OH− complex formation. IR peaks at 761, 1178, 1700, 3736 cm−1 were seen for bond stretchings in CO2 ·OH− complex over triborate ion and peaks at 1188, 1378, 1689, 3735 cm−1 were seen correspondingly over the tetraborate ion. In the triborate ion, C-O bond stretchings in CO3 trigonal plane as well as B-O bond stretchings in BO4 unit took place at 761 cm−1 . C-O bond stretchings in CO3 plane are highlighted at 1700 cm−1 that was found similar to 1705 cm−1 (C-O bond stretching in CO3 plane). IR intensities of C-O-H bending deformations in CO2 ·OH− occurred at 1178 cm−1 and O-H stretchings were observed at 3736 cm−1 in the triborate ion. Bond stretching peaks at 1188, 1378 and 1683 cm−1 in Figure 8(c) ion were identified for C-O asymmetric bond stretchings in CO3 plane as well as for C-O-H bending deformations in CO2 ·OH− complex. All these observations confirmed the complexation of CO2 with (poly)borate ions forming intermediates IM1 and IM2 of Figures 2-4.

The last sets of intermediates to be tested and confirmed for biomimetic mechanism were the bicarbonate-complexed (poly)borate ions. Bicarbonate ion complexed with B(OH)3 showed IR intensities at 528, 794, 1006, 1206, 1372 and 1671 cm−1 . IR intensities at 528 cm−1 were observed due to O-H bond stretchings in HCO− 3 ion and while C-O-H bending deformations took place at 1206 cm−1 . Vibrational intensities of 794 cm−1 were due to out of plane stretchings of C-O in CO3 plane thus confirming the formation of product bicarbonate ions. O-C-O bending deformations


11

Page 13 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of CO3 planes were also observed at 1372 and 1671 cm−1 , as shown in Figure 6(d). Baldassarre et al. 70 have also reported similar peaks at 1366 cm−1 and 1627 cm−1 for bicarbonate ions. For bicarbonate formation, Figure 7(d) and 8(d) showed similar peaks at 1353 cm−1 for C-O bond stretchings in CO3 plane and C-O-H bending deformations in triborate and tetraborate ions. O-H bond stretching peaks of H2 O molecule in H2 O·HCO− 3 complex were observed at 602, 1617 and 3527 cm−1 . At 3527 cm−1 , vibrations for O atom of H2 O molecule migrating HCO− 3 by O-H stretchings were observed. Schott et al. 63 have reported the frequencies of H2 O molecule at 1631 cm−1 .

After the desorption of bicarbonate ions, the vibrational intensities were observed at 643, 990, 1388 and 1584 cm−1 (Figure 6(f)). B-O symmetric bond stretchings of B(OH)3 trigonal plane were identified at 643 cm−1 (out of plane) and 1388 cm−1 (in plane). At 990 cm−1 , B-O asymmetric bond stretchings as well as B-O-H bending deformations in BO3 trigonal plane were observed and O-H symmetric bond stretchings of H2 O molecule were observed at 1584 cm−1 . Zeebe 67 have reported similar IR intensities for B-O stretchings at 642 cm−1 and Schott et al. 63 have observed them at 645 cm−1 and 1388 cm−1 . Peaks at 1584 cm−1 were observed correspondingly for O-H symmetric bond stretchings in H2 O. Figure 7(f) shows peaks in a range of 1345-1381 cm−1 of B-O bond stretching vibrations in B3 O3 ring of B3 O3 (OH)3 while small intensity peaks were seen for O-H bond stretching in B3 O3 (OH)3 (499-515 cm−1 , 3799-3801 cm−1 ), O-B-O bending deformations (677 cm−1 ), B-O-H bending deformations in B3 O3 (OH)3 (909-1025 cm−1 ), and O-H bond stretchings in H2 O (1594 cm−1 , 3747 cm−1 , 3849 cm−1 ). In Figure 8(f) of H2 O·[B4 O5 (OH)3 ]− , a peak of O-H bond stretching in H2 O was observed at 631 cm−1 and the other O-H vibrations of O-H bond stretching in H2 O were present in a range of 3600-3750 cm−1 as are also reported by Gautam et al. 71 Thus, the comparative analyses of experimental and computed NMR and FTIR spectra confirmed the presence of various intermediates of the mechanism given in Figure 1 and provide a strong evidence for biomimetic CO2 hydration mechanism catalyzed by borate, triborate and tetraborate ions.


12


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.4

Page 14 of 40

Basis Set and Density Functional Effects on the Free Energy Landscapes

Free energies of the molecular systems calculated under DFT framework are often sentitive to the choice of the functional and the basis set. Therefore, it was important to ascertain that the free energy landscapes of Figure 5 provided a correct picture and the conclusions drawn were not the artefacts of the choice of the computational technique. Therefore, a range of calculations were carried out identifying the sensitivity of the energy landscapes towards the choice of basis sets and functionals.

It can be seen from Figure 5 that the there were two pathways possible to obtain IM3 from IM2. While the pathway sampling TS2 involved large energy barriers of the order of 30 kcal/mol over ′

the three catalysts, the second pathway sampling TS2 required significantly lesser amount of free energies in a range of 0.2-4.3 kcal/mol. Therefore, the sensitivity of the energy landscape was tested for this step over the triborate catalyst.

The effect of the size of the basis set was tested first with B3LYP as the functional. The basis sets tested were 6-31+G(d,p), 6-311++G(d,p), 6-31++G(2d,p) and AUG-cc-pVTZ. 6-31+G(d,p) basis set, which was used for the entire energy landscape of Figure 5, had a single first polarization function and a diffuse function. An additional diffuse function was present in 6-311++G(d,p), 6-31++G(2d,p) with multiple polarization functions in 6-31++G(2d,p). AUG-cc-pVTZ basis set provided one diffuse function for every atom with polarization functions included in the definition of the basis set family. This choice of basis sets provided a clear way of identifying the effects of size of the basis sets on the energy landscape. ′

Free energy barriers for obtaining TS2 and TS2 over triborate catalyst were calculated using the above basis sets and B3LYP functional. It can be seen from Figure 9a that the energy barriers remained unchanged with an increase in the size of the basis set. No trend for the change in free energy barriers was observed with a change in the size of the basis set and the ranges of 29.9-31.1 ′

kcal/mol for TS2 and 4.2-4.3 kcal/mol for TS2 could be considered to be well within computational accuracy of adopted DFT methods.


13

Page 15 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Considering the insensitivity of the free energy barriers to the choice of the basis sets tested and the computational time requirements, 6-31+G(d,p) was used to test the effect of functional on the free energy barriers. B3LYP, BMK, M06, M06L and wB97XD were tested. B3LYP, BMK, M06 and WB97XD are hybrid exchange-correlation functionals while M06L is a pure functional. Since the intermediates under study contain non-bonded interactions, dispersion corrections could be of significance. We had two choices of incorporating the dispersion corrections: employing empirical corrections to the aforementioned functionals or using dispersion-corrected functionals. We chose the second method by using wB97XD which had the empirical correction included in the definition. The effect of functional on free energy barriers can be seen from Figure 9b. The ′

free energy barriers were in a range of 29.9-30.5 kcal/mol for TS2 and 4.3-6.4 kcal/mol for TS2 . The free energy barriers with dispersion corrected wB97XD/6-31+G(d,p) were observed to 30.2 ′

and 4.7 kcal/mol for TS2 and TS2 , respectively, against 29.9 and 4.3 kcal/mol observed with B3LYP/6-31+G(d,p). This gives us the confidence in the computational methodology and the conclusions drawn from the energy landscapes of Figure 5.

4

Conclusions

Borate, triborate and tetraborate ions followed the reaction mechanism of CO2 hydration reaction catalyzed by carbonic anhydrases. Hydroxyl centres in the borate compounds acted as the active sites for CO2 hydration. The elementary steps of the reaction involved parallel CO2 complex formation which was endergonic, bent CO2 complex formtion which was an activated process, bicarbonate ion formation, and bicarbonate displacement by water molecule. Bicarbonate ion formation followed the Lindskog mechanism involving bicarbonate ion formation following internal rotation. This pathway was found to preferable over the internal proton transfer with smaller activation barriers (0.20-4.26 kcal/mol) against activation barriers in a range of (27.74-29.51 kcal/mol) for proton transfer. All the intermediates were confirmed by a comparative experimental and computed NMR and FTIR and the spectroscopic analyses proved the reaction mechanism to be biomimetic and all three borate ions to be active for CO2 hydration. Acknowdgement: This work was supported by the Department of Biotechnology of Ministry of Science and Technology, Government of India (Bioinformatics, computational and systems biology programme BT/PR7054/BID/7/422/2012).


14


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

References [1] Edenhofer, O.; Madruga, R. P.; Sokona, Y.; Seyboth, K.; Kadner, S.; Zwickel, T.; Eickemeier, P.; Hansen, G.; Schlömer, S. et al. Renewable energy sources and climate change mitigation: special report of intergovernmental panal on climate change; New York, USA, 2012. [2] Steiner, H.; Jonsson, B. H.; Lindskog, S. The catalytic mechanism of carbonic anhydrase: Hydrogen-isotope effects on the kinetic parameters of the human C isoenzyme. Eur. J. Biochem. 1975, 59, 253-259. [3] Silverman, D. N.; Lindskog, S. The catalytic mechanism of carbonic anhydrase: implications on the rate-limiting protolysis of water. Acc. Chem. Res. 1998, 21, 30-36. [4] Khalifah, R. G. The Carbon dioxide hydration activity of carbonic anhydrase. J. Biol.Chem. 1971, 246, 2561-2573. [5] Jo, B. H.; Kim, I. G.; Seo, J. H.; Kang, D. G.; Cha, H. J. Engineered escherichia coli with periplasmic carbonic anhydrase as a biocatalyst for CO2 sequestration Appl. Environ. Microbiol. 2013, 79, 6697-6705. [6] Liljas, A.; Kannan, K. K.; Bergsten, P. C.; Waara, I.; Fridborg, K.; Strandberg, K.; Carlbom, U.; Järup, L.; Lövgren S.; Petef, M. Crystal structure of carbonic anhydrase C. Nature New Biol. 1972, 235, 131-137. [7] Hassan, M. I.; Shajee, B.; Waheed, A.; Ahmad, F.; Sly, W. S. Structure, function and applications of carbonic anhydrase isozymes Bioorg. Med. Chem. 2013, 21, 1570-1582. [8] Hakansson, K.; Carlsson, M.; Svensson L. A.; Liljas, A. Structure of native and apo carbonic anhydrase II and structure of Some of its anion-ligand complexes. J. Mol. Biol. 1992, 227, 1192-1204. [9] Esbaugh, A. J.; Tufts, B. L. The structure and function of carbonic anhydrase isozymes in the respiratory system of vertebrates Respir. Physiol. Neurobiol. 2006, 154, 185-198. [10] Gilmour, K. M. Perspectives on carbonic anhydrase Comp. Biochem. Phys. Part A 2010, 157, 193-197.


15

Page 17 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[11] Tripp, B. C.; Smith, K.; Ferry, J. G. Carbonic anhydrase: new insights for an ancient enzyme J. Biol. Chem. 2001, 276, 48615-48618. [12] West, D.; Kim, C. U.; Tu, C.; Robbins, A. H.; Gruner, S. M.; Silverman, D. N.; McKenna, R. Structural and kinetic effects on changes in the CO2 binding pocket of human carbonic anhydrase II Biochemistry 2012, 51, 9156-9163. [13] Lindskog, S. Structure and mechanism of carbonic anhydrase. Pharmacol. Ther. 1997, 74, 1-20. [14] Guo, D.; Thee, H.; Da Silva, G.; Chen, J.; Fei, W.; Kentish, S.; Stevens, G. W. Boratecatalyzed carbon dioxide hydration via the carbonic anhydrase mechanism. Environ. Sci. Technol. 2011, 45, 4802-4807. [15] Shen, S.; Yang, Y.; Ren, S. CO2 absorption by borate-promoted carbonate solution: promotion mechanism and vapor liquid equilibrium Fluid Phase Equilib. 2014, 367, 38-44. [16] Sharma, M. M.; Danckwerts, P. V. Catalysis by Brönsted bases of the reaction between CO2 and water. Trans. Faraday Soc. 1963, 59, 386-395. [17] Kumar, N.; Rao, D. P. Design of a packed column for absorption of carbon dioxide in hot K2 CO3 solution promoted by arsenious acid. Gas Sep. Purif. 1989, 3, 152-155. [18] Sharma, M. M.; Danckwerts, P. V. Fast reactions of CO2 in alkaline solutions- (a) carbonate buffers with arsenite, formaldehyde and hypochlorite as catalysts (b) aqueous monoisopropanolamine (1-Amino-2-Propanol) solutions. Chem. Eng. Sci. 1963, 18, 729-735. [19] Roberts, D.; Danckwerts, P. V. Kinetics of CO2 absorption in alkaline solutions- (I) transient absorption rates and catalysis by arsenite. Chem. Eng. Sci. 1962, 17, 961-969. [20] Le, Q.; Tu, J.; Shi, Y. Catalytic activity of vanadium pentoxide for the absorption of carbon dioxide by potassium carbonate solution. Acta Phys.-Chim. Sin. 1992, 8, 753-759. [21] Sch¨ afer, D.; Kamps, A. P.-S.; Rumpf, B.; Maurer, G.; An experimental investigation of the influence of vanadium pentoxide on the solubility of CO2 in aqueous solutions of potassium carbonate J. Chem. Eng. Data 2012, 57, 2902-2906.


16


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

[22] Phan, D. T.; Maeder, M.; Burns, R. C.; Puxty, G. Catalysis of CO2 absorption in aqueous solution by vanadate and sulfate and their application to post combustion capture Int. J. Greenh. Gas Control 2015, 36, 60-65. ˇ [23] Bhaduri G. A.; Siller, L. Nickel nanoparticles catalyse reversible hydration of carbon dioxide for mineralization carbon capture and storage. Catal. Sci. Tech. 2013, 3, 1234-1239. ˇ [24] Bhaduri, G. A.; Alamiry, M. A. H.; Siller, L. Nickel nanoparticles for enhancing carbon capture. J. Nanomater. 2015, 2015, 1-13. ˇ [25] Bhaduri, G. A.; Muzaffar, B.; Alamiry, M. A. H.; Yuan, J.; Shangguan, W.; Siller, L. Photochemical enhancement in catalytic activity of nickel nanoparticles for hydration of CO2 , ChemistrySelect 2016, 1, 2091-2095. [26] Gao, S.; Guo, D.; Jin, H.; Li, S.; Wang, J.; Wang, S. Potassium carbonate slurry-based CO2 capture technology. Energy Fuels 2015, 29, 6656-6663. [27] Li, S.; Jin, H.; Gao, L.; Mumford, K. A.; Smith, K.; Stevens, G. Energy and exergy analyses of an integrated gasification combined cycle power plant with CO2 capture using hot potassium carbonate solvent. Environ. Sci. Technol. 2014, 48, 14814-14821. [28] Nicholas, N. J.; da Silva, G.; Kentish, S.; Stevens, G. W. Use of vanadium(V) oxide as a catalyst for CO2 hydration in potassium carbonate systems. Ind. Eng. Chem. Res. 2014, 53, 3029-3039. [29] Smith, K. H.; Harkin, T.; Mumford, K.; Kentish, S.; Qader, A.; Anderson, C.; Hooper, B.; Stevens, G. W. Outcomes from pilot plant trials of precipitating potassium carbonate solvent absorption for CO2 capture from a brown coal fired power station in Australia. Fuel Process. Technol. 2017, 155, 252-260. [30] Thee, H.; Nicholas, N.; Smith, K.; Da Silva, G.; Kentish, S.; Stevens, G. A kinetic study of CO2 capture with potassium carbonate solutions promoted with various amino acids: glycine, sarcosine and proline. Int. J. Greenh. Gas Control 2014, 20, 212-222. [31] Smith, K.; Xiao, G.; Mumford, K.; Gouw, J.; Indrawan, I.; Thanumurthy, N. R.; Quyn, D. M.; Cuthbertson, R.; Rabindran, A. V.; Nicholas, N. et al. Demonstration of a concentrated potassium carbonate process for CO2 capture. Energy Fuels 2014, 28, 299-306. ACS Paragon Plus Environment

17

Page 19 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[32] Adams E. E.; Caldeira, K. Ocean storage of CO2 . Elements 2008, 4, 319-324. [33] Quinby-Hunt, M. S.; Turekian, K. K. Distribution of elements in sea water. EOS 1983, 64, 130-132. [34] Miller, E. P.; Wub, Carrano, Y. C. J. Boron uptake, localization, and speciation in marine brown algae. Metallomics 2016, 8, 161-169. [35] Wanninkhof, R.; Knox, M. Chemical enhancement of CO2 exchange in natural waters. Limnol. Oceanogr. 1996 41, 689-697. [36] Lackner, K. S. Carbonate chemistry for sequestering fossil carbon. Annu. Rev. Energy Environ. 2002, 27, 193-232. [37] Orr, J. C.; Fabry, V. J.; Aumont, O.; Bopp, L.; Doney, S. C.; Feely, R. A.; Gnanadesikan, A.; Gruber, N.; Ishida, A.; Joos, F. et al. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 2005, 437, 681-686. [38] Duffin, A. M.; Schwartz, C. P.; England, A. H.; Uejio, J. S.; Prendergast, D.; Saykally, R. J. pH-dependent x-ray absorption spectra of aqueous boron oxides. J. Chem. Phys. 2011, 134, 154503-7. [39] Smith Jr., H. D.; Wiersema, R. J. Boron-11 nuclear magnetic resonance study of polyborate ions in solution. Inorg. Chem. 1972, 11, 1152-1154. [40] Ay, A. N.; Z¨ umreoglu-Karan, B.; Temel, A.; Mafra, L.; Layered double hydroxides with interlayer borate anions: a critical evaluation of synthesis methodology and pH-independent orientations in nano-galleries. Appl. Clay Sci. 2011, 51, 308-316. [41] Endo, K.; Nguyen, Q. S.; Kentish, S. E.; Stevens, G. W. The effect of boric acid on the vapour liquid equilibrium of aqueous potassium carbonate fluid Fluid Phase Equilib. 2011, 309, 109-113. [42] Ghosh, U. K.; Kentish, S. E.; Stevens, G. W. Absorption of carbon dioxide into aqueous potassium carbonated promoted by boric acid Energy Procedia 2009, 1, 1075-1081. [43] Ahmadi, M.; Gomes, V. G.; Ngian, K. Advanced modelling in performance optimization for reactive separation in industrial CO2 removal Sep. Purif. Technol. 2008, 63, 107-115. ACS Paragon Plus Environment

18


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

[44] Smith, K. H.; Anderson, C. J.; Tao, W.; Endo, K.; Mumford, K. A.; Kentish, S. E.; Qader, A.; Hooper, B.; Stevens, G. W. Pre-combustion capture of CO2 -results from solvent absorption pilot plant trials using 30 wt% potassium carbonate and boric acid promoted potassium carbonate solvent Int. J. Greenh. Gas Control 2012, 10, 64-73. [45] Thee, H.; Smith, K. H.; Da Silva, G.; Kentish, S. E.; Stevens, G. W. Carbon dioxide absorption into unpromoted and borate catalyzed potassium carbonate solutions. Chem. Eng. J. 2012, 181-182, 694-701. [46] Thee, H.; Smith, K.; Da Silva, G.; Kentish, S.; Stevens, G. Carbonic anhydrase promoted absorption of CO2 into potassium carbonate solutions. Greenhouse Gas. Sci. Technol. 2015, 5, 108-114. [47] Phan, D. T.; Maeder, M.; Burns, R. C.; Puxty, G. Catalysis of CO2 absorption in aqueous solution by inorganic oxoanions and their application to post combustion capture. Environ. Sci. Technol. 2014, 48, 4623-4629. [48] Freeman, J. J.; Gimblett, F. G. R. Studies of activated charcoal cloth- (II) influence of boron containing impregnants of the rate of activation in carbon dioxide gas. Carbon 1987, 25, 565-568. [49] Deshpande, P. A. Computational investigation of Cu7 as a model biomimetic CO2 capture catalyst, Chem. Eng. Sci. 2016, 145, 294-298. [50] Verma, M.; Sravan Kumar, K. B.; Deshpande, P. A. Computational insights into the activity of transition metals for biomimetic CO2 hydration. J. Phys. Chem. C 2016, 120, 5577-5584. [51] Verma, M.; Deshpande, P. A. Computational design of novel heterofullerene-based biomimetic α-carbonic anhydrase analogues. ChemPhysChem 2016, 17, 3120-3128. [52] Verma, M.; Deshpande, P. A. Mechanistic insights into biomimetic carbonic anhydrase action catalyzed by doped carbon nanotubes and graphene. Phys. Chem. Chem. Phys. 2017, 19, 8757-8767. [53] Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09 (Revision D.01), Gaussian Inc., Wallingford CT, 2009. ACS Paragon Plus Environment

19

Page 21 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[54] Schmitz, J.-M. et al., Nonaborate compositions and their preparation. WO Patent WO 2001/053201, 2007. [55] Solà, M.; Lledós, A.; Duran, M.; Bertrán, J. Ab initio study of the hydration of CO2 by carbonic anhydrase. a comparison between the Lipscomb and Lindskog mechanisms J. Am. Chem. Soc. 1992, 114, 869-877. [56] Tautermann, C. S.; Loferer, M. J.; Voegele, A. F.; Liedl, K. R. About the kinetic feasibility of the Lipscomb mechanism in human carbonic anhydrase II J. Phys. Chem. B 2003, 107, 12013-12020. [57] Hartmann, M.; Merz, K. M.; van Eldik, R.; Clark, T.; The important role of active site water in the catalytic mechanism of human carbonic anhydrase II - a semiempirical MO approach to the hydration of CO2 J. Mol. Model. 1998, 4, 355-365. [58] Ma, R.; Schuette, G. F.; Broadbelt, L. J. Toward understanding the activity of cobalt carbonic anhydrase: a comparative study of zinc- and cobalt-cyclen Appl. Catal., A 2015, 492, 151-159. [59] Ma, R.; Schuette, G. F.; Broadbelt, L. J. Microkinetic modeling of CO2 hydrolysis over Zn-(1, 4, 7, 10-tetraazacyclododecane) catalyst based on first principles: revelation of ratedetermining step J. Catal. 2014, 317, 176-184. [60] Ma, R.; Schuette, G. F.; Broadbelt, L. J. Insights into the relationship of catalytic activity and structure: a comparison study of three carbonic anhydrase mimics Int. J. Chem. Kinet. 2014, 46, 683-700. [61] Tossell, J. A., Boric acid, carbonic acid, and N-containing oxyacids in aqueous solution: ab initio studies of structure, pKa, NMR shifts, and isotopic fractionations. Geochim. Cosmochim. Acta 2005, 69, 5647-5658. [62] Qiu, X.; Sasaki, K.; Osseo-Asare, K.; Hirajima, T.; Ideta, K.; Miyawaki, J. Sorption of H3 BO3 /B(OH)− 4 on calcined LDHs including different divalent metals, J. Colloid Interface Sci. 2015, 445, 183-194. [63] Schott, J.; Kretzschmar, J.; Acker, M.; Eidner, S.; Kumke, M. U.; Drobot, B.; Barkleit, A.; Taut, S.; Brendlera, V.; Stumpfa, T. Formation of a Eu(III) borate solid species from a weak Eu(III) borate complex in aqueous solution. Dalton Trans. 2014, 43, 11516-11528. ACS Paragon Plus Environment

20


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

[64] Merritt, M. E.; Harrison, C.; Storey, C.; Jeffrey, F. M.; Sherry, A. D.; Malloy, C. R. Hyperpolarized

13

C allows a direct measure of flux through a single enzyme-catalyzed step by NMR,

PNAS 2007, 104, 19773-19777. [65] Gallagher, F. A.; Sladen, H.; Kettunen, M. I.; Serrao, E. M.; Rodrigues, T. Q.; Wright, A.; Gill, A. B.; McGuire, S.; Booth, T. C.; Boren, J. et al. Carbonic anhydrase activity monitored In vivo by hyperpolarized

13

C-magnetic resonance spectroscopy demonstrates its importance

for pH regulation in tumors. Cancer Res. 2015, 75, 4109-4118. [66] Balachander, L.; Ramadevudu, G.; Shareefuddin, Md.; Sayanna, R.; Venudhar, Y. C. IR analysis of borate glasses containing three alkali oxides. ScienceAsia 2013, 39, 278-283. [67] Zeebe, R. E. Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)− 4. Geochim. Cosmochim. Acta 2005, 69, 2753-2766. [68] Jun, L.; Shuping, X.; Shiyang, G. FT-IR and Raman spectroscopic study of hydrated borates. Spectrochim. Acta 1995, 51A, 519-532. [69] Moore, M. H.; Khanna, R. K. Infrared and mass spectral studies of proton irradiated H2 O+CO2 ice: evidence for carbonic acid. Spectrochim. Acta 1991, 47A, 255-262. [70] Baldassarre, M.; Barth, A. The carbonate/bicarbonate system as a pH indicator for infrared spectroscopy. Analyst 2014, 139, 2167-2176. [71] Gautam, C.; Yadav, A. K.; Singh, A. K. A Review on infrared spectroscopy of borate glasses with effects of different additives. ISRN Ceramics 2012, 2012, 1-17.


21

Page 23 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


List of Figures 1

Proposed biomimetic CO2 hydration mechanism catalyzed by (poly)borate ions. The inset shows the DFT optimized structures of (a) borate ion, (b) triborate ion, and (c) tetraborate ion. Colour key: boron - pink; oxygen - red; hydrogen - white . 30

2

DFT-optimized structures of biomimetic CO2 hydration intermediates and transition states catalyzed by borate ions. (a) borate ion, (b) parallel CO2 adsorption complex, (c) transition state for CO2 bending, (d) bent CO2 complex, (e) transition state of proton migration, (f) transition state of bicarbonate formation through OH− .CO2 complex rotation, (g) bicarbonate ion complex, (h) bicarbonate ion displacement by H2 O addition (i) H2 O adsorption complex. Colour key: same as that of Figure 1; carbon - black . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3

DFT-optimized structures of biomimetic CO2 hydration intermediates and transition states catalyzed by borate ions. (a) triborate ion, (b) parallel CO2 adsorption complex, (c) transition state for CO2 bending, (d) bent CO2 complex, (e) transition state of proton migration, (f) transition state of bicarbonate formation through OH− .CO2 complex rotation, (g) bicarbonate ion complex, (h) bicarbonate ion displacement by H2 O addition (i) H2 O adsorption complex. Colour key: same as that of Figure 1; carbon - black . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4

DFT-optimized structures of biomimetic CO2 hydration intermediates and transition states catalyzed by borate ions. (a) tetraborate ion, (b) parallel CO2 adsorption complex, (c) transition state for CO2 bending, (d) bent CO2 complex, (e) transition state of proton migration, (f) transition state of bicarbonate formation through OH− .CO2 complex rotation, (g) bicarbonate ion complex, (h) bicarbonate ion displacement by H2 O addition (i) H2 O adsorption complex. Colour key: same as that of Figure 1; carbon - black . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5

Free energy landscapes for biomimetic CO2 hydration reaction catalyzed by borate, triborate and tetraborate ions) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34


22


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6

Page 24 of 40

Computational FTIR spectra for various intermediates of CO2 hydration of Figure 2 − − catalyzed by borate ions. (a) B(OH)− 4 , (b) CO2 ·B(OH)4 , (c) CO2 ·OH ·B(OH)3 , (d) − HCO− 3 ·B(OH)3 , (e) H2 O·HCO3 ·B(OH)3 , (f) H2 O·B(OH)3 . The computed wavenum-

bers are shown in black colour while the experimentally reported 66–71 values are shown in red colour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 7

Computational FTIR spectra for various intermediates of CO2 hydration of Figure 3 − − catalyzed by triborate ions. (a) B3 O3 (OH)− 4 , (b) CO2 ·B3 O3 (OH)4 , (c) CO2 ·OH ·B3 O3 (OH)3 , − (d) HCO− 3 ·B3 O3 (OH)3 , (e) H2 O·HCO3 ·B3 O3 (OH)3 , (f) H2 O·B3 O3 (OH)3 . The com-

puted wavenumbers are shown in black colour while the experimentally reported 66–71 values are shown in red colour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 8

Computational FTIR spectra for various intermediates of CO2 hydration of Figure 4 catalyzed by tetraborate ions. (a) [B4 O5 (OH)4 ]2− , (b) CO2 ·[B4 O5 (OH)4 ]2− , (c) − − − CO2 ·OH− ·[B4 O5 (OH)3 ]− , (d) HCO− 3 ·[B4 O5 (OH)3 ] , (e) H2 O·HCO3 ·[B4 O5 (OH)3 ] ,

(f) H2 O·[B4 O5 (OH)3 ]− . The computed wavenumbers are shown in black colour while the experimentally reported 66–71 values are shown in red colour . . . . . . . . . . . . 37 9

Effects of basis sets and functionals on free energy barriers TS2-IM2 and TS2’-IM2. (a) Free energies calculated with B3LYP functional and different basis sets, (b) free energies calculated with 6-31+G(d,p) basis set and different functionals . . . . . . . 38


23

Page 25 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


List of Tables 1

Reaction free energies for elementary steps of biomimetic CO2 hydration (kcal/mol)

2

Activation free energies for elementary steps of biomimetic CO2 hydration (kcal/mol) 26

3

Computed and experimentally reported NMR shieldings and NMR shifts of

11

25

B for

intermediates of biomimetic CO2 hydration . . . . . . . . . . . . . . . . . . . . . . . 27 4

Computed and experimentally reported NMR shieldings and NMR shifts of

13

C for

intermediates of biomimetic CO2 hydration . . . . . . . . . . . . . . . . . . . . . . . 28 5

Computed and experimentally reported66−71 IR vibrational frequencies of different functional groups present in borate-based compounds (cm−1 ) . . . . . . . . . . . . . 29


24


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

Table 1: Reaction free energies for elementary steps of biomimetic CO2 hydration (kcal/mol) Borate ion

Triborate ion

Tetraborate ion

IM2 − IM1

-1.72

1.40

0.54

IM3 − IM2

-1.47

-1.25

-1.18

IM4 − IM3

4.53

4.96

5.09

IM5 − IM4

-12.05

-1.68

5.46


25

Page 27 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Activation free energies for elementary steps of biomimetic CO2 hydration (kcal/mol) Borate ion

Triborate ion

Tetraborate ion

T S1 − IM1

7.49

10.74

9.32

T S2 − IM3

27.74

29.86

29.51

0.20

4.26

1.90

′

T S2 − IM3


26


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

Table 3: Computed and experimentally reported NMR shieldings and NMR shifts of intermediates of biomimetic CO2 hydration Molecule

Computational

Experimental 61–65

Boron compound

σ

δ

σ

δ

B(OH)− 4

106.52

1.20

97.8

(-3)-(+3)

H2 O·B(OH)3

92.59

15.13

81.1

15-20

B3 O3 (OH)− 4

97.95

9.77

-

-

H2 O·B3 O3 (OH)3

92.34

15.38

-

-

B4 O5 (OH)2− 4

99.51

8.21

-

-

H2 O·[B4 O5 (OH)3 ]−

98.09

9.63

-

-

BF3 O(C2 H5 )2

107.72

0

-

-


27

11

B for

Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4: Computed and experimentally reported NMR shieldings and NMR shifts of

13

C for

intermediates of biomimetic CO2 hydration Molecule

Experimental 61–65

Computational Borate ion

Triborate ion

Tetraborate ion

Boron compound

σ

δ

σ

δ

σ

δ

σ

δ

CO2 ·OH−

72.31

120.30

72.11

120.50

72.50

120.11

-

124.5-125

HCO− 3

39.09

153.52

40.52

152.09

42.67

149.94

20.9

160.9-161

H2 O·HCO− 3

39.91

152.70

41.72

150.89

44.12

148.49

-

-

Si(CH3 )4

192.61

0

192.61

0

192.61

0

181.1

0


28


Table 5: Computed and experimentally reported66−71 IR vibrational frequencies of different functional groups present in borate-based compounds (cm−1 ) Functional group

Computational

Experimental

Borate ion

Triborate ion

Tetraborate ion

OH

528, 1584, 1617, 3527, 3812

499-515, 1594, 1617, 3527, 3736, 3747, 3799-3801, 3805, 3849

472, 631, 1617, 3527, 3803

OBO

475

677

-

470

CO

794, 1358, 1372, 1671, 1691, 2342

761, 1700, 2343

1188, 1353, 1378, 1683, 2343

812, 1366, 1627, 1705, 2343

1631, 3600-3750

OCO

610-612, 697

-

-

655

COH

1206, 1358, 1372, 1671, 1691

1178, 1353

1188, 1353, 1378, 1683

1366, 1705

BOH

900, 974

900-1025

1012, 1157

-

BO3

643, 990, 1388

690, 1287-1487

680, 948-954, 1260-1421

620-750, 740-890, 1176-1500, 1300-1450

BO4

825-865, 1045-1146

761, 797-957, 1087-1117

752-932, 1025-1090

807-918, 950-980, 1020-1156

B3 O 3

-

1345-1381

-

-

29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 30 of 40


Page 31 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: Proposed biomimetic CO2 hydration mechanism catalyzed by (poly)borate ions. The inset shows the DFT optimized structures of (a) borate ion, (b) triborate ion, and (c) tetraborate ion. Colour key: boron - pink; oxygen - red; hydrogen - white


30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

Figure 2: DFT-optimized structures of biomimetic CO2 hydration intermediates and transition states catalyzed by borate ions. (a) borate ion, (b) parallel CO2 adsorption complex, (c) transition state for CO2 bending, (d) bent CO2 complex, (e) transition state of proton migration, (f) transition state of bicarbonate formation through OH− .CO2 complex rotation, (g) bicarbonate ion complex, (h) bicarbonate ion displacement by H2 O addition (i) H2 O adsorption complex. Colour key: same as that of Figure 1; carbon - black ACS Paragon Plus Environment

31

Page 33 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3: DFT-optimized structures of biomimetic CO2 hydration intermediates and transition states catalyzed by borate ions. (a) triborate ion, (b) parallel CO2 adsorption complex, (c) transition state for CO2 bending, (d) bent CO2 complex, (e) transition state of proton migration, (f) transition state of bicarbonate formation through OH− .CO2 complex rotation, (g) bicarbonate ion complex, (h) bicarbonate ion displacement by H2 O addition (i) H2 O adsorption complex. Colour key: same as that of Figure 1; carbon - black


32


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 40

Figure 4: DFT-optimized structures of biomimetic CO2 hydration intermediates and transition states catalyzed by borate ions. (a) tetraborate ion, (b) parallel CO2 adsorption complex, (c) transition state for CO2 bending, (d) bent CO2 complex, (e) transition state of proton migration, (f) transition state of bicarbonate formation through OH− .CO2 complex rotation, (g) bicarbonate ion complex, (h) bicarbonate ion displacement by H2 O addition (i) H2 O adsorption complex. Colour key: same as that of Figure 1; carbon - black


33

Page 35 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5: Free energy landscapes for biomimetic CO2 hydration reaction catalyzed by borate, triborate and tetraborate ions)


34


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

Figure 6: Computational FTIR spectra for various intermediates of CO2 hydration of Figure 2 cat− − − alyzed by borate ions. (a) B(OH)− 4 , (b) CO2 ·B(OH)4 , (c) CO2 ·OH ·B(OH)3 , (d) HCO3 ·B(OH)3 ,

(e) H2 O·HCO− 3 ·B(OH)3 , (f) H2 O·B(OH)3 . The computed wavenumbers are shown in black colour while the experimentally reported 66–71 values are shown in red colour


35

Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7: Computational FTIR spectra for various intermediates of CO2 hydration of Figure 3 − − catalyzed by triborate ions. (a) B3 O3 (OH)− 4 , (b) CO2 ·B3 O3 (OH)4 , (c) CO2 ·OH ·B3 O3 (OH)3 , (d) − HCO− 3 ·B3 O3 (OH)3 , (e) H2 O·HCO3 ·B3 O3 (OH)3 , (f) H2 O·B3 O3 (OH)3 . The computed wavenumbers

are shown in black colour while the experimentally reported 66–71 values are shown in red colour


36


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8:

Page 38 of 40

Computational FTIR spectra for various intermediates of CO2 hydration of

Figure 4 catalyzed by tetraborate ions.

(a) [B4 O5 (OH)4 ]2− , (b) CO2 ·[B4 O5 (OH)4 ]2− ,

− − − (c) CO2 ·OH− ·[B4 O5 (OH)3 ]− , (d) HCO− 3 ·[B4 O5 (OH)3 ] , (e) H2 O·HCO3 ·[B4 O5 (OH)3 ] , (f)

H2 O·[B4 O5 (OH)3 ]− . The computed wavenumbers are shown in black colour while the experimentally reported 66–71 values are shown in red colour


37

Page 39 of 40


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Page 40 of 40

borate Ions - American Chemical Society

Recommend Documents