Adsorption of carbon dioxide on naturally occurring solid amino acids

Accepted Manuscript Title: Adsorption of Carbon Dioxide on Naturally Occurring Solid Amino Acids Author: Sreedipta Chatterjee Sadhana Rayalu Spas D. Kolev Reddithota J. Krupadam PII: DOI: Reference:

S2213-3437(16)30218-4 http://dx.doi.org/doi:10.1016/j.jece.2016.06.007 JECE 1140

To appear in: Please cite this article as: Sreedipta Chatterjee, Sadhana Rayalu, Spas D.Kolev, Reddithota J.Krupadam, Adsorption of Carbon Dioxide on Naturally Occurring Solid Amino Acids, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.06.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Adsorption of Carbon Dioxide on Naturally Occurring Solid Amino Acids Sreedipta Chatterjeea, Sadhana Rayalua, Spas D. Kolev b, Reddithota J. Krupadam a* a

National Environmental Engineering Research Institute, Jawaharlal Nehru Marg, Nagpur

440020, India; bSchool of Chemistry, The University of Melbourne, VIC 3010 Australia.

E-mail: [email protected]; Tel: +91 712 2249 884; Fax: +91 712 2249 896

1

Abstracts Adsorption

of

CO2

on

solid

natural

amino

acids

(AAs)

D-arginine,

bis(2-

hydroxypropyl)amine, cysteamine, L-leucine, D-serine, L-valine, sarcosine, and taurine has been studied in the temperature range of 303 – 423 K. In a temperature programmed thermal reactor, CO2 adsorption was carried out in an isothermally controlled flow of CO 2 and the adsorbed CO2 was desorbed by shifting temperature to 523 K. Based on the experimental results, the amino acid taurine (TAU) was found to adsorb the highest quantity of CO2 (3.7 mmol g-1) among the studied AAs which was two-fold higher than the quantity of CO2 absorbed by activated carbon at 303 K. Theoretical calculations have agreed with the experimental results and have revealed that the interaction between the AAs and CO2 is non-covalent in nature. The polar side chains of AAs are responsible for their high binding ability with CO2. AAs with heteroatoms such as sulfur has great potential as ligands in developing selective adsorbents for CO 2 capture.

Keywords: CO2 capture; Adsorption; Amino acids; Thermogravimetric study; Molecular modeling; Climate change.

1.

Introduction The impact of increasing atmospheric CO2 concentration on global warming is

recognized as one of the key environmental issues facing human kind (D'Alessandro et al. 2010). The concentration of CO2 in the Earth's atmosphere is approximately 398 ppmv as of 2015 (NOAA, 2012) and increased with the rate of 2.0 ppmv yr-1 during the 2000-2009 period (Hedin et al. 2010). Various CO2 capture technologies including absorption, adsorption, cryogenics and membrane separation have been reported (Jassim and Rochelle,

2

2010). Among them, amine or ammonia-based absorption process is used in industrial separations and capture of CO2, which require relatively high energy for solvent regeneration, solvent evaporation losses are quire high and damage to the equipment due to corrosion restricts the scope of application of this method (Goff and Rochelle, 2004). A special report of Intergovernmental Panel on Climate Change (IPCC) has pointed out that the development of new generation materials with high adsorption capacities will undoubtedly enhance the competitiveness of adsorptive separation in flue gas applications (Herzog, 1999). On the other side, the success of this approach is dependent on the development of a low cost adsorbent with high CO2 adsorption capacity and selectivity, even at moderate (30-50 oC) to high (100-200 oC) temperatures. Materials with large surface area, such as zeolites and activated carbon have been widely reported in the literature (Rabbani and El-Kaderi, 2011). Recently, amine functionalized polymer adsorbents have attracted much attention because they are expected to offer the benefits over the liquid amines in a typical absorption process (Stuckert and Yang, 2011). Some of the polymer adsorbents reported for their high adsorption capacity for CO2 include (i) polyethylenimine (PEI) impregnated with MCM-41 adsorbent (2.55 mmol g-1) (Xu et al. 2002), (ii) amine-modified SBA-15 with N-β-(amino ethyl)-y-aminopropyl dimethoxy methylsilane (AEAPMDS) (1.27 mmol g-1) (Zhao et al. 2000) and (iii) PEI-modified glass fiber adsorbent (4.12 mmol g-1) (Liu et al. 2011). These adsorbents have limitations of low CO2 selectivity and high synthesis costs and difficulty in the production of large quantities required for industrial applications. CO2 capture applications of natural materials such as amino acids (AAs) have not been explored so far, and these materials could be attractive candidates for post-combustion CO2 capture because of their low environmental impact, low volatility compared with conventional amines, low ecotoxicity, and high biodegradability (Mello et al. 2011). Importantly, AAs can be used as ligands to functionalize materials/polymers to achieve

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higher selectivity of CO2 adsorption from mixture of gases. The dual functionality of AAs due to their carboxylic acid and amino groups works effectively in binding CO2 in gas phase. This paper reports on a thermogravimetric study of the influence of temperature, pressure and flow rate on CO2 adsorption by small AAs (i.e. D-arginine-ARG, bis(2hdroxypropyl)amine-BHA, cysteamine - CYS, leucine - LEU, D-serine-SER, L-valine-VAL, sarcosine-SAR and taurine-TAU). These AAs can be used as ligands for functionalization of high surface area materials. The potential regeneration of AAs and their reusability for application in cyclic processes have been also evaluated. The nature of interactions between AAs and CO2 has been investigated using computer simulation. The theoretical predictions based on computer simulations have provided an information about nature of binding and CO2 accommodation capacity. 2.

Experimental Section

2.1.

Chemicals All AAs (ARG, BHA, CYS, LEU, SER, VAL, SAR, and TAU) used in the study were

purchased from Sigma-Aldrich (St. Louis, USA) and were of analytical grade. The gases used in this study, He, N2 and CO2 of purity higher than 99.999%, and were supplied by Nikita Enterprises (Nagpur, India). 2.2.

Description of thermal reactor and CO2 adsorption experiments CO2 adsorption experiments were carried out in a simultaneous thermal analyzer (STA,

Perkin-Elmer STA 6000) equipped with a SaTumATM sensor. The analyzer contains a microbalance (0.1 μg maximum sensitivity) in a stainless steel high-pressure housing (210 bar pressure), and feed and exit pumps (ISCO 500D). The balance and housing were positioned on an active air vibration isolation surface, and the whole system was placed inside a temperature-controlled chamber. Pressure in the chamber was measured by a digital pressure gauge (±0.035%) attached to the microbalance housing.

4

An inbuilt data-acquisition board was used to continuously monitor the pressure, the temperature and the microbalance output. The chamber allowed fast cool down (<10 min) of the sample from 1273 to 303 K. The cooling system was integrated with a mass flow gas controller to pump accurately CO2 into the chamber, which was sealed and purged with a steady flow of He gas. The programmable temperature of the chamber was ramped from room temperature to 373 K at a heating rate of 276 K min-1, to degas and dehydrate the sample. After the mass of the AA sample reached a constant value in He environment, and was shifted to CO2 gas and CO2 flow was maintain with the rate 20 mL min-1. An increase in the sample mass was observed indicating that the AAs were adsorbing CO2. The CO2 flow was continued until constant mass was attained. The CO2 gas was hold in the chamber at a given temperature to record adsorption capacities. By varying the holding times the CO2 adsorption capacities were determined. Data acquisition, storage, and numerical treatment of the CO2 adsorption experiments were carried out with the software PyrisTM (Ver. 11, Perkin-Elmer).

2.3.

FTIR analysis of AAs Fourier Transform Infra Red (FTIR) was used to verify the formation of bonds between

AAs and CO2 (Bruker, France; Model, Vertex 70). AAs were initially treated with potassium bromide (Merck, Kenilworth, USA) approximately 1-3 wt% and the resulting powder was pressed into a transparent pellet using a hydraulic press. The pelleted sample was used to record IR spectra. The EZOMNIC software (Ver.7.3, Thermo Electron Corp. Madison MI, USA) was used to collect the peak intensities of the IR absorption bands. 2.4

Computational studies Density Functional Theory (DFT) approach was used to study the interactions

between CO2 and AAs in gaseous phase. Initially, geometrically optimized CO2 and AAs

5

were built and configured. DFT computations have been applied to calculate the interaction energies between the AAs and CO2 in vacuum. All the AAs and CO2 were parameterized using the procedure described by Seminario (1996) involving quantum chemistry calculations, where bonding force parameters were estimated from analysis of Hessian Force Matrix (HFM) and atomic electric charges were determined from an Electrostatic Potential (ESP) fit. The bond lengths between AAs and CO2 of linear structures were calculated by using a Linear Constraint Solver (LINCS) algorithm. During formation of a complex between an AA and CO2, it was hypothesized that each site interacts with all sites of different molecules via Lennard-Jones (LJ) and columbic interactions. The cut-off distances of columbic and van der Waals forces used to describe the interactions mentioned above were 0.9 nm and 1.0 nm, respectively. The LJ parameters used in the calculations were σ = 3.40 Å and ε = 0.086 kcal/mol for carbon and σ = 2.60 Å and ε = 0.015 kcal mol -1 for hydrogen corresponding to values from the standard General AMBER Force Field (GAFF). Simulations were conducted with a time step of 0.5 fs for 2000 ps trajectories. The Particle Mesh Ewald (PME) summation method was used to calculate the electronic interactions between the molecules of AA and CO2 complex. The analysis of the energy and trajectory of the simulated system were carried out with Gaussian 9.0 software (Gaussian Inc. Wallingford, USA). The methodology followed for the molecular binding energy and distances computation is presented in Fig. SI-1 (Supporting Information) and the geometrically optimized structures of AAs and CO2 are shown in Fig. SI-2 (Supporting Information). The activation energy, reaction energy, and binding energy between AAs and CO2, computed by DFT/ab-initio/6-31G*, are presented in Table SI-1 (Supporting Information).

6

DELL Precision T7500 with Windows XP operating system, CPU - Intel (R) Xeon (R) 2.80 GHz/2.79 GHz Dual Processors, and 24 GB of RAM (memory), and 4 TB hard disk was used to run Gaussian 9.0 software.

3.

Results and Discussion

3.1.

CO2 adsorption onto AAs The adsorption isotherms of CO2 at 303 K up to 15 bar were constructed by monitoring

the increase in mass of the AA sample exposed to a constant flow of CO2. The isotherms in Fig. 1 show saturation at 12 bars and the increasing order of the adsorption capacity at 12 bars follows the order: ARG < BHA < CYS < LEU < SER < VAL < SAR < TAU. The amino acids TAU and SAR showed the highest adsorption capacity, and this could be due to the strong CO2 adsorption affinity towards amine functionalities. At low CO2 flow rates, all AAs showed low adsorption capacity for CO2. However, the adsorption capacities were better than those for adsorbents such as amine functionalized silica, fly ash, zeolites, and activated carbon. The functionalization of these adsorbents by different ligands/grafting agents such as polyethylenimine (PEI) and 3-aminopropyltriethoxysilane (APTS) further improved their adsorption capacity for CO2. The AAs studied in this work showed better adsorption capacities than those of the amines used in functionalization of zeolites and polymers. The adsorption capacities of selected amine functionalized adsorbents are compared with those of the best performing amino acids, i.e. VAL, SAR and TAU in Table 1. In the present study, kinetically faster CO2 adsorption was achieved with the AAs. The maximum adsorption capacity was achieved by TAU and CYS for 7 min and 5 min, respectively. The adsorption capacities of TAU and CYS were 0.52 and 0.41 mmol g-1 at 303 K and 1 bar, respectively (Fig. 2). The presence of amine-functionality and electronegative atoms in AAs (e.g. S in TAU) could be responsible for the observed fast adsorption rate. The

7

CO2 molecules are quite small with the critical diameter of approximately 0.32 nm. This allows CO2 to bind effectively to the AA binding sites. According to results published by Hernandez-Huesca et al. (1999), the adsorption of CO2 on zeolite erionite (ZAPS) occurred very fast and ~70% of the total capacity was achieved at 293 K. The linear driving force model could describe such faster kinetics of CO2 adsorption onto the zeolite (Martin et al. 2011). It was reported that the adsorption activation energy for CO2 on zeolite 13X decreased as the pressure increased (Ochoa-Fernandez et al. 2006). The results mentioned above reports conclude that temperature and pressure influence the rate and capacity of adsorption of CO2 on zeolitic surfaces. The adsorption data presented in Fig. 1 and Fig. 2, it could be concluded that the smaller the size of the AAs the better the adsorption rate and capacity. The presence of high electronegative atoms such as sulfur in the smaller-size AAs enhances the rate of adsorption compared with that of the larger AAs. The smaller AAs with high electronegative atoms will be the good candidates for functionalization of polymeric and zeolitic surfaces for CO2 capture. The FTIR spectra provide an evidence of CO2 adsorption on AAs (Fig. 3). The IR spectra of TAU showed distinct intense peaks at 1342 cm-1 (-C-N-), 1210 cm-1 (-N-H-) and 1043 cm-1 (S-O-). Presence of electronegative atom adjacent to the amine in AAs (such as taurine where sulfonic acid located near the amine functionality) improves affinity for CO2. This is evidenced by reduction in peak intensity of infra-red spectra at 1342 cm-1 and 1043 cm-1 for bonds of -C-N-, and -S-O-, respectively, while the peak at 1210 cm-1 is disappeared. In the case of SAR, three peaks were noticed at 1554 cm-1 (-N-H-), 1306 cm-1 (-C-O-) and 762 cm-1 (-O-H-) and after CO2 adsorption the peak at 1545 was shifted to 1645 cm-1 and an intense new peak was formed at 766 cm-1 (-O-H). Similarly, there was an intensification of the peaks at 1520 cm-1 (-N-H-) and 534 cm1 (-C-O) after CO2 adsorption onto VAL. These changes in

8

IR spectra after CO2 adsorption clearly evidence the formation of bond between the AAs and CO2. Selectivity of AAs for CO2 was determined by passing equimolar concentration of CO2/N2 mixture through a gas dilution system connected to the STA chamber at 303 K. Fig. 4 depicts the selective adsorption capacity of AAs - TAU, SAR, and VAL for CO2 in the presence of N2. CO2 is more strongly adsorb on AAs than N2 due to large quadruple moment of CO2. N2 sorption capacity of AAs is in the range between 0.18 and 0.3 mmol g-1 at 1 bar and 303 K. The important finding of this experiment is the high selectivity for CO2 (78) of TAU at lower pressures (<1 bar). At higher pressures (i.e. > 2 bar) lower selectivity values were reported. This may be due to the high polarity and smaller size of TAU, which should determine its higher affinity for CO2, compared to N2. N2 adsorption capacity on TAU and SAR was slightly higher than the other AAs studied herein. The selectivity experiments suggest that the functionality and charge of AAs are dominant characteristics in discriminating gases based on preferential uptake of CO2. The selectivity of TAU, SAR and VAL for CO2 compared to N2 is higher than that of MOFs, zeolite 4A, zeolite 13X, activated carbon reported in the literature (Chen et al. 2010). The effect of temperature on CO2 adsorption was studied by contacting CO2 gas at different temperatures ie., 303, 353 and 423 K to AAs. It was found that there was a decreasing trend of CO2 adsorption from 303 to 423, while there was a steep increase in adsorption of CO2 with increasing pressure from 0.5 to 2.0 bar (Fig. 5). At 303 K and 1 bar TAU adsorbed 3.25 mmol g-1 of CO2 while SAR adsorbed 2.63 mmol g-1. The adsorption capacity marginally declined for TAU and SAR at 353 K to 3.11 and 2.45 mmol g-1, respectively, compared to the corresponding values at 303 K. The adsorption capacity of TAU and SAR decreased about 50% (1.84 mmol g-1) and 60% (1.57 mmol g-1), respectively, when the temperature was increased from 303 to 423 K. The similar trend of CO2 adsorption

9

was shown by all other AAs. These results indicate that it would be possible to use AAs retain CO2 adsorption capacity at elevated temperature till 423 K, which is requirement of CO2 capture from industrial stacks, auto-exhausts and industrial separations during products manufacturing. Due to amorphous nature of AAs, their direct application would be difficult; however, they are potential candidates to improve both adsorption capacity and selectivity via functionalization of high surface area materials such as MOFs, zeolites and polymers. Another important aspect of CO2 capture is regeneration potential of adsorbents. To examine the suitability of AAs – TAU, SAR and VAL, adsorption-desorption cycles were performed by shifting temperature repeatedly heated to 473K in He environment and then cooled to ambient temperature (Fig. 6). The results showed that the CO2 adsorption capacity decreased marginally ~2% after each cycle indicating the reusability of these AAs for many cycles. 3.2.

Theoretical Predictions One of the important requirement of CO2 capture materials is they should have multiple

adsorption sites. The experimental data provides that AAs have the potential to adsorb reversibly CO2 from gaseous phase and offer advantages over amines by being environmentally friendly and non-corrosive. The CO2 – AA configuration is stabilized by two interactions, one is the stronger H(COOH-O(CO2) and other is the weaker C=O-C(CO2) for CO2 capture (Fig. 7). The binding energies and distances between AAs and CO2 were computed using Density Functional Theory (DFT) with the basis set 6-31G*. The interactions between AA and CO2 minimize the binding energy due to quadruple moments developed because of charge separation in the -C=O bonds. This moment also allows CO2 to act both as a Lewis acid and base. When CO2 acts as an acid, weak non-bonded interactions take place between the nucleophilic centers in the AAs and the electron deficient carbon atom of CO2 and lead to the formation of electron donor-acceptor complexes. These weak non-bonding interactions are responsible for increase in the binding distance. There is also an interaction

10

of CO2 with the nitrogen atom of the AA. The activation energy (Ea) was computed at 303 K using Eq. (1). Ea = ΔH# (T) + RT

(1)

where ΔH# is the change in enthalpy, T is the absolute temperature and R is the universal gas constant. Activation energy, reaction energy, and binding energy of interaction between CO 2 and AAs are given in Table 2, based on MM-AM1/ 6-31*G level computations. The amino acid, ARG contains 4 nitrogen atoms and it would be hypothesized that CO2 preferentially binds at four different sites. The stabilized confirmation of ARG is dominated by the electrostatic interaction between the lone pair of electrons on the nitrogen atom of the amino group and the electron deficient carbon of the CO2 molecule. The AA (CO2)…H (αβγα1carbon) or H(NH2) binding distances of 3.73 Å and  3.71Å respectively, and the binding energy calculated as 4.39 kcal mol-1 suggest weak hydrogen bond (HB) interactions. This AA showed the lowest affinity towards CO2 among the AAs studied. TAU is one of the smallest AAs and it showed the highest binding capacity for CO2. This could be due to the presence of sulfur atom along with the amine functionality causing more electronegativity and thus favoring stronger interactions with CO2. Nitrogen contains two bonding pairs of electrons and a single lone electron, all in the same plane. Sulfur contains two bonding pair of electrons and one non-bonding lone pair of electrons. These groups shield the three bonding electrons of CO2 and minimize the interaction. The binding energy and binding distance computed for TAU and CO2 interaction was 2.41 kcal mol-1 and 1.19 Å, respectively. In the presence of more CO2 molecules, it was found that there was an increase in binding distance between TAU and CO2. For example the binding distance increased from 1.19 to 5.49 Å and binding energy decreased from 2.41 to 1.49 kcal mol-1 as a result of the presence of five molecules of CO2. This increase in the binding distance and decrease in the binding energy facilitates CO2 adsorption-desorption phenomena.

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The three AAs, TAU, SAR and VAL, showing the highest CO2 adsorption capacity have the lowest binding energies 2.41, 2.47 and 3.17 kcal mol-1, respectively, compared with the other AAs studied in this work. The binding distances between CO2 and TAU, SAR, and VAL were calculated as 1.19, 1.83 and 1.98 Å, respectively. These binding properties (i.e. distance and energy) are primarily determined by the strong H(COOH)-O(CO2) and C=O-C(CO2) interactions. When CO2 acts as an acid, a weak non-bonded interaction takes place between the nucleophilic centers in the AAs and the electron deficient carbon atom of CO2 forming electron donor-acceptor complexes and increasing the binding distance. The number of CO2 molecules accommodated by each AAs were determined by adding CO2 molecules to one molecule of AA and computing the interaction energy scores for each CO2 molecule added (Table 3). TAU can accommodate about eight CO2 molecules while SAR, VAL and CYS can accommodate 5 molecules at maximum. The multiple co-operative interactions exhibited by AAs with CO2 provide higher binding capacities compared to those of other CO2 capture materials.

4.

Conclusions The experimental adsorption study and theoretical predictions have shown that the

AAs, in particular TAU, SAR and VAL, are promising materials for CO2 capture. Some important findings related to AAs for CO2 capture are: (i) AAs have demonstrated high CO2 adsorption capacities i.e., >2 mmol g-1; (ii) adsorption capacities of AAs has been explained on the basis of their molecular architecture and availability of binding sties for accommodating CO2 and (iii) AAs can be regenerated and reused the capture of CO2. The multiple co-operative interactions proven via theoretical computations exhibited by AAs with CO2 explain their higher binding capacities compared to those of other CO2 capture materials. These findings together with the fact that AAs are natural materials which can be

12

safely discharged into the environment after denature makes AAs an efficient and environmentally friendly CO2 capture materials.

Acknowledgements Authors gratefully acknowledge grants from the Department of Science and Technology (DST), Government of India under the National Program on Carbon Sequestration Research (NPCSR), Grant No. DST/IS-STAC/CO2/162-13(G).

References D'Alessandro, D.M., Smit, B., Long, J.R., 2010. Carbon Dioxide Capture: Prospects for New Materials. Angew. Chem. Int. Ed. 49, 6058-6082. Dunne, J.A., Rao, M., Sircar, S., Gorte, R.J., Myers, A.L., 1996. Calorimetric heats of adsorption and adsorption isotherms. 2. O2, N2, Ar, CO2, CH4, C2H6 and SF6 on NaX, H-SZM-5, and Na-ZSM-5 zeolites. Langmuir, 12, 5896-5904. Goff, G.S., Rochelle, G.T., 2004. Monoehtanolamine degradation: CO2 mass transfer effects under CO2 capture conditions. Ind. Eng. Chem. Res. 43, 6400-6408. Hedin, N., Chen, L., Laaksonen, A., 2010. Sorbents for CO2 capture from flue gas - aspects from materials and theoretical chemistry. Nanoscale 2, 1819-1841. Hernandez-Huesca, R., Diaz, L., Aguilar-Armenta, G., 1999. Adsorption kinetics of CO2, O2, N2, and CH4 in cation-exchanged clinoptilolite. Sep. Purif. Technol. 15, 163-173. Herzog, H., 1999. An Introduction to CO2 separation and capture technologies. MIT Energy Laboratory, Cambridge University Press, UK. Jassim, M.S., Rochelle, G.T., 2006. Innovative absorber/stripper configurations for CO2 capture by aqueous monoethanolamine. Ind. Eng. Chem. Res. 2006, 45, 2531–2545 Kumar, P.S., Hogendoorn, J.A., Versteeg, G.F. 2003. Kinetics of the reaction of CO2 with aqueous potassium salt of taurine and glycine. AIChE J. 49, 203-213.

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Liu, L., Deng, Q.F., Ma, T.Y., Lin, X.Z., Hou, X.X., Liu, Y.P., Yuan, Z.Y., 2011. Ordered mesoporous carbons: citric acid-catalyzed synthesis, nitrogen doping and CO2 capture. J. Mater. Chem, 21, 16001-16009. Martin, C.F., Stöckel, E., Clowes, R., Adams, D.J., Cooper, A.I., Pis, J.J., Rubiera, F., Pevida, C., 2011, Hypercrosslinked organic polymer networka as potential adsorbents for pre-combustion CO2 capture. J. Mater Chem. 21, 5475-5483. Mello, M.R., Phanon, D., Silveira, G.Q., Llewellyn P.L., Ronconi, C.M, 2011. Aminemodified MCM-41 mesoporous silica for carbon dioxide capture. Microporous. Mesoporous. Mater. 143, 174-179 Metz, M., Davidson, O., de Coninck, H., Loos M., Meyer, L., 2005. IPCC Report on "Carbon dioxide Capture and Storage". Cambridge University Press, UK. NOAA Mauna Loa CO2 data (2015). Current data on Earth's atmospheric CO2; Scripps CO2now.org/current-CO2/CO2-now. Sept. 2015 Ochoa-Fernandez, E., Ronning, M., Grande, T., Chen, D. Nanocrystalline lithium zirconate with improved kinetics for high-temperature CO2 capture. Chem. Mater. 18, 13831385. Rabbani, M.G., El-Kaderi, H.M., 2011. Template-free synthesis of a highly porous benzimidazole-linked polymer for CO2 capture and H2 storage. Chem. Mater. 23, 1650-1653. Seminario, J.M., Zacarias, A.G., Tour, J.M., 1999. Molecular alligator clips for single molecule electronics. Studied of group 16 and isonitriles with Au contacts. J. Am. Chem. Soc. 121, 411-416. Stuckert, N.R., Yang, R.T., 2011. CO2 capture from the atmosphere and simultaneous concentration using zeolites and amine grafted SBA-15. Environ. Sci. Technol. 45, 10257-10264.

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Xu, X., Song, C., Andresen, J.M., Miller, B.G., Scaroni, A.W., 2002. Novel polyethylenimine-modified mesoporous molecular sieve of MCM-41 type as high capacity adsorbent for CO2 capture. Energy & Fuels. 16, 1463-1469. Zhao, D., Sun, J., Li, Q., Stucky, G.D., 2000. Morphological control of highly ordered mesoporous silica SBA-15. Chem. Mater. 12, 275-283. Zhang, X., Zhang, S., Yang, H., Feng, Y., Chen, Y., Wang, X., Chen, H., 2014. Nitrogen enriched biochar modified by high temperature CO2–ammonia treatment: Characterization and adsorption of CO2. Chem. Engg. J. 257, 20-27.

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4

CO2 adsorbed, mmol g-1

3.5

3 2.5 2 1.5 1

0.5 0 0

2

4

6

8

10

12

14

16

Pressure, bar TAU

SAR

VAL

SER

CYS

BHA

LEU

Figure 1. Adsorption isotherms of CO2 in the pressure range 0-15 bar. The quantity of each amino acid used was 50 mg in each experimental run and the temperature of the adsorption was 303 K.

16

0.6

CO2 adsorbed, mmol g-1

0.5

0.4

0.3

0.2

0.1

0 0

30

60

90

120

Time, min TAU

SAR

VAL

ARG

LEU

SER

CYS

BHA

Figure 2. Adsorption kinetics of AAs at pressure 0.2 bar and temperature 303 K. The quantity of each amino acid used was 50 mg.

17

1500

1400

1300

Wave number, cm-1 1200 1100 1000

TAU 900

800

700 Transmittance (%)

0 20 40 60

80 100 Wave number cm -1

SAR

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 20

40 60 80

Transmitance (%)

0

100

VAL Wave number, cm-1 1700 1600 1500 1400 1300 1200 1100 1000 900

800

700

600

500

20 40 60

80

Transmittance (%)

0

100

Figure 3. FTIR spectra of three AAS (TAU, SAR and VAL) with the highest CO2 adsorption capacities before (blue line) and after (dark brown) CO2 adsorption.

18

Figure 4. Pressure effect on the CO2 and N2 absorption of TAU, SAR and VAL at 303 K from an equimolar mixture of CO2:N2

19

(a)

TAU

4.5

(b) CO2 adsorbed, mmol g-1

CO2 adsorbed, mmol g-1

4 3.5 3 2.5 2

1.5 1

SAR

4

3.5 3 2.5 2

1.5 1 0.5

0.5 0

0 0

0.5

1

1.5

2

2.5

0

0.5

Pressure, bar 423 K

CO2 adsorbed, mmol g-1

(c)

353 K

1

1.5

2

2.5

Pressure, bar 303 K

423 K

353 K

303 K

VAL

3.5 3 2.5 2 1.5 1

0.5 0 0

0.5

1

1.5

2

2.5

Pressure, bar 423 K

353 K

303 K

Figure 5. Effect of temperature and pressure on CO2 adsorption by (a) TAU (b) SAR and (c) VAL. The quantity of each amino acid used was 50 mg.

20

CO2 adsorbed, (%)

100

90

80

70 0 1 2 3 4 5 6 7 8 9 10

Cycle TAU

SAR

VAL

Figure 6. Effect of repetitive CO2 adsorption-desorption on the % CO2 adsorption in the case of the three AAs with the highest adsorption capacities at 1 bar and 303 K.

21

(a)

(b)

VAL

SAR

TAU

Figure 7. (a) Types of interactions modelled between AAs and CO2 in vacuum and (b) structures of the VAL, SAR and TAU adducts with CO2 and computed binding distances between these AAs and CO2.

22

Table 1. Comparison of CO2 adsorption capacity of amine functionalized adsorbents Amine functionalized

CO2 adsorption

Conditions of

material

capacity, mmol

adsorption

Reference

g-1 CNTs grafted with

2.19

Cin=50%; T=298 K

Liu et al. (2011)

1.05

Cin=90%; T=29 3K

Mello et al. (2011)

0.43

Cin=99%; T=348 K

Xu et al. (2002)

0.45

Cin=15%; T=278 K

Zhao et al. (2000)

1.58

Cin=99%; T=303 K

Zhang et al. (2014)

Taurine

3.25

Cin=99%; T=303 K

Present study

Sarcosine

2.85

Cin=99%; T=303 K

Valine

2.29

Cin=99%; T=303 K

APTS Amorphous silica gel grafted with TA MCM 41 grafted with PEI SAB-15 grafted with EDA Fly Ash impregnated with monoethanolamine Amino Acids

23

Table 2. Activation energy, reaction energy and binding energy for carbomate formation when CO2 reacts with amino acids at DFT/ab-initio/ 6-311G level. All the energies are calculated in kcal mol-1. Amino acid

ΔEa

ΔEr

B.E

L/B

Binding distance, Å

Bis (2-hydroxypropyl) 88.16

27.54

4.01

2.28

4.43

amine Cysteamine

62.49

16.78

1.96

1.85

4.85

D-Arginine

105.43

33.47

4.39

1.51

3.35

DL-Serine

81.25

29.01

2.58

1.03

3.67

L-Leucine

86.96

32.32

3.77

1.79

4.45

L-Valine

81.31

28.06

3.17

1.69

3.38

Sacrosine

70.51

19.78

2.47

1.54

3.80

Taurine

83.73

31.78

2.41

1.63

3.49

MW, Molecular weight; ΔEa and ΔEr, are difference in activation energy and reaction energy, respectively, computed in vacuum; B.E, bonding energy and L/B, length -by-breadth computed by molecular dynamic simulations.

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Table 3. Optimized molecular ratio of AA to CO2 for accommodation. Interaction energy, E, kcal mol-1

Number of CO2 BHA

CYS

ARG

SER

LEU

VAL

SAR

TAU

1

4.02

1.97

4.44

2.68

3.86

3.41

2.51

2.49

2

4.62

2.57

5.03

3.28

4.46

4.01

3.11

3.11

3

5.22

3.17

5.63

3.88

5.46

4.61

3.71

3.63

4

5.51

3.77

6.23

4.47

5.65

5.71

5.31

4.27

5

6.41

4.37

5.67

5.07

5.34

5.72

6.02

4.83

6

7.43

4.97

5.07

5.07

5.63

6.74

6.07

4.97

7

6.91

6.87

4.65

6.78

6.95

6.17

6.37

6.01

8

6.66

6.85

4.51

6.62

6.38

6.11

6.49

6.87

9

5.21

5.10

4.43

6.19

5.22

6.13

5.87

5.33

10

5.01

5.03

4.41

5.71

5.17

6.45

5.81

5.31

BHA, Bis-(2-hydroxypropyl) amine; CYS, Cysteamine; ARG, Arginine, SER, DL-Serine; LEU, L-Leucine; VAL, L-Valine; SAR, Sarcosine; TAU, Taurine

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