Amine Modification of Binder-Containing Zeolite 4A Bodies for Post

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General Research

Amine Modification of Binder-Containing Zeolite 4A Bodies for Post-combustion CO2 Capture Debashis Panda, E. Anil Kumar, and Sanjay Kumar Singh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03958 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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Industrial & Engineering Chemistry Research

Amine Modification of Binder-containing Zeolite 4A Bodies for Post-Combustion CO2 Capture Debashis Panda,† E. Anil Kumar, ‡ and Sanjay Kumar Singh*,§,⊥ †Discipline

of Mechanical Engineering, §Discipline of Chemistry, ⊥Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Simrol, Indore 453552, India ‡Department

of Mechanical Engineering, Indian Institute of Technology Tirupati, Tirupati 517506, India E-mail address: [email protected] (SKS), [email protected] (EAK)

Abstract In the current study, a new type of composite adsorbent was synthesized by amine modification of binder-containing zeolite 4A bodies and its potential application in the postcombustion CO2 capture was evaluated. A wide range of aliphatic straight chain amines such as propylamine (PA), butylamine (BA), pentylamine (PEA), and their respective branched chain amines, iso-propylamine (IPA), iso-butylamine (IBA), and iso-pentylamine (IPEA) were used in a smaller fraction to modify binder-containing zeolite 4A bodies. The synthesized materials were characterized by various spectro-analytical techniques to elucidate the effect of amine modification on physico-chemical properties of bindercontaining zeolite 4A bodies and its reactivity for CO2 capture. Among all the studied hybrid adsorbents, the iso-butyl amine-modified binder-containing zeolite 4A bodies (IBA-Z4A) exhibited excellent CO2 adsorption performance with a maximum adsorption capacity of 2.56 mmol g-1 at 25 °C and 1 bar pressure. Notably, IBA-Z4A also demonstrated excellent purity (98%) and remarkably high CO2/N2 selectivity (335) as compared to the pristine binder-containing zeolite 4A bodies (24). Such enhanced CO2 adsorption capacity and high CO2/N2 selectivity values for IBA-Z4A can be attributed to the symbiotic interactions between CO2 and amines governed by the basicity, electron density at N-atom of amines, and the steric effect of adsorbing molecules (CO2 and N2) at the adsorbent surface. Notably, IBA-Z4A also displayed marginal isosteric heat of adsorption for CO2 (51 kJ mol-1) along with the encouraging thermochemical cyclic stability over five consecutive CO2 adsorptiondesorption cycles at 25 °C and 1 bar, believed be to the best suited for post-combustion CO2 capture.

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1. Introduction Emission of anthropogenic CO2 due to uncontrolled burning of fossil fuels has become an important environmental problem, which contributes up to 60% of the global warming effects.1,2 Therefore, different preferential carbon sequestration strategies such as absorption, adsorption, membrane separation, wet scrubbing, and cryogenic separation are extensively explored as frontier processes.2-4 Among these well-established strategies for post-combustion CO2 capture, the adsorption technique has gained considerable interest due to its minimal energy requirements and low capital cost investment.5 In particular, Pressure Swing Adsorption (PSA), Vacuum Swing Adsorption (VSA), and Thermal Swing Adsorption (TSA) techniques are being widely employed for CO2 capture from power plants.5 For postcombustion CO2 capture, VSA and PSA techniques are mostly used due to their easy process control and low-cost equipment.6 Moreover, considering the importance of CO2, which is no longer regarded as a waste product, and found to be an essential source for the production of various value-added chemicals,7a and fuel (such as methanol),7b the selection of economically viable Carbon Capture Storage (CCS) technique is expected to have a significant impact on the future energy solution. While dealing with flue gas composition (c.a. 70-90% N2, 10-15% CO2, and remaining moisture), the choice of adsorbents plays a vital role in efficient CO2 adsorption and separation.3,4,8 In addition to adsorption capacity, working capacity under the required pressure window, the selectivity of CO2 over N2, facile regenerability, adsorption/desorption kinetics, hydrophobicity, and prolonged thermochemical stability are few of the essential criteria which make the materials suitable for post-combustion CO2 capture.8 In recent years, various adsorbent materials such as zeolite,3,4,7 mesoporous silica,3 activated carbon (AC),4 metal-organic frameworks (MOF),9 and covalent organic frameworks (COF),10 are extensively explored for the selective capture of CO2 from power-plant flue gas. Compared with the other microporous adsorbents, zeolites offer most of the features mentioned above, especially in the low-pressure regime along with superior CO2/N2 selectivity.11,12 Further, zeolites are cost-effective, can be readily be synthesized, and some of the zeolites (such as 4A, 5A) are produced from industrial waste.13,14 Several zeolites such as zeolite 13X, Y, A, β, chabazite, and ZSM, are also used for separation of CO2 from N2.11 Due to the high working capacities, zeolite 13X and NaY are widely utilized in post-combustion CO2 capture, 2 ACS Paragon Plus Environment

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whereas, zeolite NaA offers excellent CO2-over-N2 selectivity compared to FAU due to its narrow pore window.11,15 Further, amine modification of zeolites have also attracted substantial scientific recognition as promising candidates for CO2 adsorption in the low-pressure regime, presumably due to symbiotic interaction of amine-CO2 and CO2-metal site of zeolites.3,4,16-20 However, pore blockage and diffusion hindrance of amine molecules into the zeolite pores are the major hurdles in achieving high CO2 adsorption capacity.17b,18,19c,20a This effect is more severe for small pores containing zeolites such as zeolite 4A due to the steric hindrance of amines with the pores of the zeolite.21 Therefore, zeolites containing larger pores such as 13X, NaY and β are widely used for the amine modification, as these pores are easily accessible for amine modification.16-19 In this context, the binder-containing commercial zeolite 4A can be a better alternative for enhanced CCS properties due to the presence of heterogeneous pore size distribution developed by the binders

22

which can be utilized for

amine modification. Notably, the binder (such as kaolin, attapulgite) itself contains a network (arteries) of mesopores which provides an additional pathway for the diffusion of CO2 into the mouth of zeolite crystal pores.23 Since micropores of the crystals present inside the commercial binder-containing zeolite 4A bodies cannot accommodate amine molecules within it, the amine modification of the additional larger pore created by binder, may advantageously enhance CO2 adsorption property and CO2-over-N2 selectivity.24 Recent studies have shown the usage of various amines such as monoethanolamine (MEA), iso-propanolamine (IPPA), tetraethylenepentamine (TEPA), polyethylenimine (PEI), to modify zeolites and other porous adsorbents for CO2 capture, albeit in high loading ca.1040 wt%.3,16-19 Although MEA is well-known for the CO2 capture process, other aliphatic amines (such as iso-butylamine, butylamine, iso-propylamine, and propylamine) have also been reported for their high affinity towards CO2 molecule for the post-combustion CO2 capture.25,26 Notably, the absorption capacity of iso-butylamine (0.78 mol of CO2/mol of amine) for the post-combustion CO2 capture in amine-based CO2 absorption systems (also known as amine scrubbing process) is higher than the MEA (0.72 mol of CO2/mol of amine).25a In particular, primary and secondary amines interact favorably with CO2 to form ammonium carbamate, where the basicity and electron density at N-atom plays a vital role in the selection of these amines for CO2 capture.26a In addition to that, a few studies have focused on amine modification of binder containing solid sorbents (mesoporous silica) for CO2 capture, but they are limited to pre-synthesis only (aqueous binder solution mixed with 3 ACS Paragon Plus Environment


the amine-modified sorbent to make pellet).27 For instance, Klinthong et al.27a used polyallylamine and NaOH for making binder solution to construct high durable pellet from powdered amine-functionalized mesoporous silica and able to achieve 90% CO2 recovery

than the powder adsorbent. So these findings have helped to explore the above mentioned primary aliphatic amines for modification of commercial binder-containing zeolite 4A bodies by post-synthesis technique (dipping in amine solution) and investigate its applicability towards post-combustion CO2 capture. In this context, herein, the work presents a study on the synthesis of amine-modified binder-containing zeolite 4A bodies using a wide range of primary aliphatic amines, with good adsorption performances for post-combustion CO2 capture. The effect of steric hindrance and the carbon chain length of amines on CO2 adsorption performance in aminemodified binder-containing zeolite 4A bodies are investigated. Single gas adsorption of CO2 and N2, selectivity of CO2 over N2 and thermo-cyclic stability of these amine-modified binder-containing zeolite 4A bodies are critically explored. Moreover, the effects of pore size and surface area on CO2 adsorption properties have also been meticulously scrutinized in this work. Several characterization techniques such as P-XRD, SAXS, BET, FE-SEM, TGA, XPS, 15N NMR, and FTIR have been used to investigate the proposed structural and chemical modification, amine reactivity and their effect on CO2 adsorption properties for postcombustion applications. 2.

Experimental details

2.1.

Materials Commercially available binder-containing zeolite 4A bodies in the form of cylindrical

pellets of approximately 3.2 mm diameter were purchased from Sigma-Aldrich (batch no. 334294). Aliphatic amines such as propylamine (PA), iso-propylamine (IPA), butylamine (BA), iso-butylamine (IBA), pentylamine (PEA), iso-pentylamine (IPEA) were procured from Sigma-Aldrich and used as received without any further purification. Anhydrous ethanol was used as solvent purchased from analytical CSS. The capacity of gas adsorption measurement was conducted using high purity CO2 (99.99 %) and N2 (99.99 %) supplied by Inox air products Pvt Ltd., India. 2.2.

Preparation of amine-modified binder-containing zeolite 4A bodies 4 ACS Paragon Plus Environment

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Prior to amine incorporation, 300 mg of binder-containing zeolite 4A bodies (preheated under vacuum at 350 °C for 3 h) were soaked in 30 mL of anhydrous ethanol under slight agitation for 20 min. After that, the respective amines (linear chain amines, PA, BA, PEA, and branched chain amines, IPA, IBA, IPEA) in a fixed concentration (0.3 wt%) were added in the above suspension, and the resultant suspension was vigorously stirred for 24 h at room temperature. Finally, the solid adsorbent was filtered, repeatedly washed with ethanol, and subsequently dried under vacuum at 120 °C for 3h. The obtained samples were designated as (x)-Z4A, where x represents the respective amine. For example, butylaminemodified binder-containing zeolite 4A bodies were represented as BA-Z4A, whereas pristine binder-containing zeolite 4A bodies were represented as Z4A throughout the manuscript. The quantitative analysis of amine content on amine-modified binder-containing zeolite bodies was calculated by back titration technique.28 About 5 mg of amine-modified bindercontaining zeolite 4A bodies were dispersed in 100 mL of 1.0 mM HCl solution under sonication for 30 minutes. After centrifugation for 10 min, 20 mL of the supernatant was titrated with standardized 1.0 mM NaOH solution, in the presence of phenolphthalein indicator, to estimate the amine content in the synthesized amine-modified binder-containing zeolite 4A bodies. 2.3.

Characterization Powder X-ray diffraction patterns (P-XRD) of the binder-containing zeolite 4A bodies

and amine-modified binder-containing zeolite 4A bodies were recorded on a Rigaku SmartLab advanced diffractometer with monochromatic Cu Kα radiation ( λ= 0.154 nm) at a step size 0.03° over a 2θ range of 5° to 80°. The small angle X-ray scattering (SAXS) measurements were performed using Anton Paar SAXSpoint 2.0 with monochromatic Cu Kα radiation (λ= 0.154 nm). The distance between sample and detector was fixed at 1076 mm, and the region of the scattering vector (q = 4πsin θ/λ) lay between 0.02-3 nm-1 corresponding to a 2θ range of approximately 0° to 4.2°. The surface morphology and pore structure of the synthesized amine-modified binder-containing zeolite 4A bodies was determined by Field emission scanning electron microscope (FE-SEM) using Carl ZEISS Supra-55. Thermogravimetric study (TGA) was performed to analyze the thermal stability and dehydration characteristics of the studied adsorbents using Mettler-Toledo TGA/SDTA851 thermal analyzer. Nitrogen sorption isotherm and the physicochemical properties (surface area, pore volume, pore size) of the adsorbent materials were measured at -196 °C using a 5 ACS Paragon Plus Environment


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Quantachrome Autosorb iQ2 TPX automated gas sorption system. Samples were degassed at 120 °C for 3 h under high vacuum (10-4 to 10-5 bar) before analysis. The surface area of the adsorbents was calculated using the Brunauer- Emmett-Teller (BET) equation applied to adsorption data in the relative pressure (P/P0) range of 0.05 to 0.30. The micropore surface area of binder-containing zeolite 4A bodies was calculated by subtracting Langmuir surface area [obtained from CO2 adsorption isotherm at 0 °C in the relative pressure (P/P0) range of 0.0002-0.008] and BET surface area as reported by Akhtar et al.11 The total pore volume was calculated from the amount of adsorbed N2 at P/P0= 0.99 using single point adsorption method and non-local density functional (NLDFT) algorithm (considering spherical pores kernel) respectively.29 Similarly, the mesopore volume (Vmeso) and mesopore diameter (Dp) was calculated from the BJH method using desorption data. The above physicochemical properties are evaluated using Quantachrome® ASiQwin™ data processing software. Fourier transform infrared spectra (FTIR) of the studied adsorbents were recorded using a spectrometer equipped with an attenuated total reflectance (FTIR/ATR Model FTIR-STD-10, Perkin-Elmer, MA, U.S.A) in the wavenumber range 4000−500 cm-1.

15N

NMR spectra

[retrieved from Heteronuclear Multiple Bond Correlation (HMBC) experiments] were recorded in CDCl3 using a Bruker Avance 400 (400 MHz) spectrometer. The detailed experimental procedure of 15N NMR and SAXS is given in the Supporting Information. 2.4.

Adsorption Measurements The gas sorption measurements were carried out in Quantachrome Autosorb iQ2 TPX

automated gas sorption system equipped with highly accurate pressure transducers and a thermostatic bath. The static volumetric mechanism measured the amount of gas adsorbed in the adsorbents. Before adsorption experiments, the samples were degassed at 120 °C for 8 h under a turbomolecular vacuum pump using a customized heating programme, allowing slow removal of moisture at low temperature, without structural changes to samples. The CO2 and N2 adsorption capacities of binder-containing zeolite 4A bodies and an amine-modified binder-containing zeolite 4A bodies were evaluated at 1 bar and 25 °C respectively. Further, an additional CO2 adsorption experiment was carried out for binder-containing zeolite 4A bodies and the best performing amine-modified binder-containing zeolite 4A bodies at 1 bar and 25 °C with high degassing condition (350 °C for 12 h). 2.5.

Adsorption isotherm modeling


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To evaluate the adsorption affinity between adsorbate - adsorbent in the pressure range (0-1 bar), the CO2 and N2 adsorption data was modeled by fitting it to Toth and Sips equation (Equations 1 and 2) respectively. The Toth isotherm is an empirical modification of the Langmuir equation, whereas Sips isotherm is the combined form of Langmuir and Freundlich equation developed to reduce the error between experimental data and the predicted value of equilibrium data. These models are used to describe the heterogeneous adsorption systems, subjected to low and high adsorbate concentration.10,30 q=

q=

qtbtP (1 + (btP)t)

1/t

qs(bsP)1/s 1 + (bsP)1/s

(1)

(2)

where, P is the equilibrium adsorbate pressure (bar), q is the adsorption capacity (mmol g-1), qt, qs are the saturated CO2 adsorption capacity (mmol g-1), and bt, bs are the affinity constant for both Toth and Sips model respectively. The parameter t and s are usually less than unity and characterize the heterogeneity of the adsorption system.30 The values of parameters of the Toth and Sips model can be evaluated by nonlinear curve fitting of the respective isotherm data. Henry constant (Kh) calculations were based on the Virial plots of the CO2 and N2 adsorption isotherm at 1 bar and 25 °C respectively as mentioned in Equation 3.31 q

P = KHexp (A1q + A2q2 +…)

(3)

where A1, A2 are the Virial coefficients and q is the adsorption capacity (mmol g-1). A plot of ln(P/q) versus q, should approach the axis linearly as q →0 with the intercept –ln(KH). The Henry constant depicted the quantitative information about the interaction strength of the adsorbate with the adsorbent at very low coverage.31 2.6.

Adsorption thermodynamics The thermodynamic property such as isosteric heat of adsorption at a given CO2

adsorption capacity (q) under isothermal condition was calculated from the isotherm data at two different temperatures (0 °C and 25 °C) by applying Clausius-Clapeyron equation as represented in Equation 4.32



∂(ln P) (∂(1/T) )q =

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qst

(4)

R

Where qst and R are the isosteric heat of adsorption and universal gas constant respectively. The difference between the activation energy for adsorption and desorption is called isosteric heat of adsorption. It depends upon the temperature, and surface coverage signifying the interacting binding strength between adsorbate and adsorbent.32 2.7.

Adsorbent evaluation parameters for CO2 capture As flue gas streams emitting from coal-fired power plants contains relatively low CO2

and high N2 concentration, the adsorbent must be capable of selectively adsorbing CO2 component from the gas mixture. Further, the purity of CO2 is also desired for developing a suitable transport and storage infrastructure.33 Thus, in this study, adsorbents were evaluated regarding few essential parameters such as selectivity of CO2 over N2, 31 and purity of CO2 captured,33 retrieved from their adsorption data. The evaluation of the adsorption performance of an adsorbent from the adsorption isotherms data points for studied gas is a well-established methodology and has been used by various groups.12,31-33 Pure component adsorption selectivity (KhCO2/KhN2) was estimated as the ratio of Henry constant for respective CO2 and N2 at 25 °C (Equation 5), whereas, the CO2/N2 selectivity and purity of CO2 under adsorption conditions were calculated at 25 °C using the following equations (Equations 6 and 7).12,31,33 Pure component adsorption selectivity= qCO2/qN2

Selectivity (αCO2/N2) = PCO2/PN2 qCO2

Purity = qCO2 + qN2 × 100 (%)

KhCO2 KhN2

(5)

(6)

(7)

where qCO2, qN2 were the amount of CO2 and N2 adsorbed at their respective equilibrium partial pressures (PCO2 and PN2). KhCO2, KhN2 are the corresponding Henry constant for CO2 and N2 at 25 °C. It was assumed that the flue gas was generated at a total pressure of approximately 1 bar having a CO2 concentration of 15% and N2 concentration of 75%. So under these conditions, the corresponding partial pressure was found to be 0.15 bar for CO2 and 0.75 bar for N2.33 8 ACS Paragon Plus Environment

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2.8.

CO2 adsorption capacity over prolonged cyclic testing For the post-combustion CO2 capture, adsorbents should not only possess high

adsorption capacity but also display a stable cyclic adsorption performance to avoid their frequent replacement during long-term cyclic operation. Therefore, cyclic CO2 adsorption and desorption experiments were conducted for the best performing adsorbent at 25 °C. After attaining the adsorption equilibrium, the sample was degassed at two different conditions; (i) via vacuum desorption at different pressures (0.01 bar, 0.1 bar) and combination of vacuum and thermal desorption (0.01 bar at 120 °C) for 30 min.34 The results were illustrated in the form of adsorption index (abbreviated as AI. %), which is defined as a percentage ratio of adsorption capacity of the regenerated samples to fresh samples.34 Thus, 100% AI implies that the adsorbent has not deteriorated at all during these cyclic adsorption-desorption study. 3.

Results and discussion

3.1.

Synthesis and characterization of amine-modified binder-containing zeolite 4A

bodies. Amine-modified binder-containing zeolite 4A bodies are synthesized by treating a suspension of binder-containing zeolite 4A bodies in ethanol with a fixed concentration of 0.3 wt% of respective amines (linear chain amines, PA, BA, PEA, and branched chain amines, IPA, IBA, and IPEA) as elaborated in the experimental section. The schematic diagram of amine modification and the molecular structure of used amines are illustrated in Scheme S1 and S2 respectively. The amine content in the synthesized amine-modified binder-containing zeolite 4A bodies (Table 1) is estimated by titration technique, and it is inferred that the amine loading is equivalent in all the cases (using a different concentration of HCl) as listed in Table 1.



(a)

(b)

Z4A

BA-Z4A

200 nm

200 nm

1µm

(c)

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1µm

(d)

IBA-Z4A

200 nm

1µm

Figure 1. (a-c) FE-SEM images and (d) P-XRD patterns of Z4A, BA-Z4A, and IBA-Z4A. The P-XRD and FESEM results suggest the intactness of structural and morphological integrity for binder-containing Z4A bodies after amine modification (Figure 1 and Figure S1). The effect of amine (s) on physico-chemical properties of binder-containing zeolite 4A bodies is also evaluated and listed in Table 1. Table 1. Physicochemical properties of the amine-modified binder-containing zeolite 4A bodies Material SBET a Dpc Pore volume Amine content (m2 g-1) (nm) (mmol g-1) Vtotald Vmesoe (cc g-1) (cc g-1) Z4A 37, 337 b 3.4 0.140 0.138 PA-Z4A 36 3.4 0.135 0.132 0.89 IPA-Z4A 33 3.4 0.133 0.130 0.90 BA-Z4A 35 3.9 0.139 0.136 0.96 b IBA-Z4A 32, 255 3.8 0.125 0.123 0.98 PEA-Z4A 35 3.4 0.135 0.132 0.94 IPEA-Z4A 32 3.4 0.127 0.118 0.91 aBET surface area, bMicropore surface area, cMesopore diameter, dTotal pore volume, eMeso pore volume.


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It has been well established that, for the binder-containing zeolite bodies, the BET surface area represents the total surface area including internal surface (pore) and the external surface (crystal, binder).35 On the other hand, for binder-free microporous zeolites such as 4A, the surface area represents the external surface area only, which is accessible for the N2 molecule.35 In this context, the studied binder-containing zeolite 4A bodies exhibit quite different physico-chemical behavior than the conventional binder free microporous zeolite 4A due to the presence of the binder. As shown in Figure 2a and Figure S2, all the individual isotherms of binder-containing zeolite 4A bodies before and after amine modification, are classified as type IV having a well-defined plateau with modest hysteresis; whereas, binderfree microporous LTA type zeolite (4A, 5A) generally displays a type I isotherm.36 Moreover, it has been observed that the binder-containing zeolite 4A bodies display higher BET surface area (37 m2 g-1) and DFT total pore volume (0.074 cc g-1) as compared to the other binder-free microporous zeolite 4A (typically ~ 5‒32 m2 g-1

11,37a-d

and DFT total pore

volume ~ 0.029 cc g-1),37a reported in the literature. The binder also increases the total pore volume of the system.37e Since the kinetic diameter of nitrogen molecule (0.36 nm) is comparable to the effective pore opening of zeolite 4A (about 0.4 nm), it is unable to enter into pore (as shrinkages of 8-membered rings (8-MR) pore window) at a temperature of -196 °C, and hence consequently exhibits a low BET surface area and pore volume.37a-d Therefore, the obtained surface area of the binder-containing zeolite 4A bodies is not the true surface area of microporous zeolite 4A powder (crystal) rather it is the surface area of the binder itself. Alternatively, CO2, having a kinetic diameter (0.33 nm) less than N2, will more easily enter into the pore opening of zeolite 4A, thus the Langmuir surface area is calculated for the binder-containing zeolite 4A bodies from CO2 adsorption isotherm at 0 °C, and found to be 374 m2/g. In addition to that, the micropore surface area is calculated as 337 m2 g-1, which is comparable to the findings of other research groups on microporous zeolite 4A powder.11 It is further noted that, all the samples display a closed hysteresis loop (Figure 2a) in the relative pressure region of 0.45-1.0, could be related to the capillary condensation in mesopores.38 The presence of mesopores is probably due to the binder (existence of non-rigid aggregates of plate-like particles), which is in accordance with some existing reports.22a,23 It is clear from Table 1, that the textural properties of binder-containing zeolite 4A bodies seemed to be heterogeneous and decreased after incorporation of amines. The reduction of surface area and pore volume (for IBA-Z4A: total pore volume from 0.140 cc g-1 to 0.125 cc g-1 and mesopore volume from 0.138 cc g-1 to 0.123 cc g-1 in comparison to Z4A bodies) are supposed to fill the external pores (as generated by the binder) during amine modification process. Due to the 11 ACS Paragon Plus Environment


difference in critical diameters between the amine and the window aperture, it is assumed that amine molecules fail to insert themselves into the small micropores of the zeolite 4A units inside the binder, and thus fill or clog the inter-particle mesopores present in the binder or zeolite pore space.20a It is anticipated that in zeolite systems, the amine will first clog on to pore walls rather than filling the pores as suggested by Cogswell et al. 20a and Holewinski et al.39 Since most of the amine molecules can be preferentially accommodated into the large mesopores network available in the binder of the zeolite bodies, the mesopore volume is proportionately decreased (Figure 2b).

80 60 40 20 0 0.0 5

0.2 0.4 0.6 0.8 Relative Pressure (P/P0)

10 (c) 4 10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 0.0

0.005 0.004

1.0

Z4A BA-Z4A IBA-Z4A

0.5

1.0 -1 q (nm )

1.5

(b)

3.4 nm 3.8 nm 3.9 nm

Z4A BA-Z4A IBA-Z4A

0.003 0.002 0.001 0 3 6 9 12 15 18 21 24 27 30 Pore diameter (nm) 1.2

Pore volume (relative)

100

0.006 -1 -1

Z4A BA-Z4A IBA-Z4A

3

(a)

dV/dD(cmnm g )

Adsorbed Volume (cmg

3 -1

)

120

Intensity (a.u.)

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

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2.0

Z4A BA-Z4A IBA-Z4A

(d)

1.0 0.8 0.6 0.4 0.2 0.0 0

2

4 6 8 10 12 14 16 Pore diameter (nm)

Figure 2. (a) N2 adsorption-desorption isotherm, (b) BJH pore size distribution curves, (c) small-angle X-ray scattering patterns, and (d) pore-size distribution curve of obtained from small angle X-ray scattering of Z4A, BA-Z4A, IBA-Z4A, (Solid and open symbols represent adsorption and desorption isotherms respectively). It is evident that the pores which are smaller than the size of N2 will not be detected and therefore conventional micropore model such as HorvÁth-Kawazoe (HK) may underestimate the size accuracy of the micropores smaller than 0.36 nm.37d Consequently; the 12 ACS Paragon Plus Environment

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Kelvin equation is not appropriately applicable for small pores less than 1 nm.40a In this context, a small-angle X-ray scattering (SAXS) is quite fruitful in overlapping the range of 1‒2 nm, and can provide a reasonable estimate of the pore size distribution of binder containing zeolite 4A bodies after amine modification (bearing both micro and mesopores).40b In SAXS technique, Guinier’s law is used to calculate the mean pore radius (referred as structural sizes) of the particle by applying the method of tangents (Figure S3a).37c The Guinier equation describes the mean intensity of the radiation scattered by a particle of any shape, averaged through all its possible orientations in space. As shown in Table S1, the values of surface fractal dimension (pore diameter) of binder containing zeolite 4A bodies expressed as the radius of gyration is in mesoporous or slightly macroporous range (51 nm), which indicates the primary particle of the sample have a rough pore-solid interface, presumably due to the presence of binder.37c It means, the binder circumscribed the microporous particles within it. The observed and calculated SAXS patterns (I(q) vs. (q)) and the resultant broad pore size distributions are shown in Figure 2c,d. The broad pore size distribution obtained by SAXS is indicating the region between micro to meso and includes a contribution from every pore size within the entire range.41 The observation is in good agreement with the finding of commercial zeolite 4A reported by Du et al.37c From the Figure 2c; it is quite clear that IBA-Z4A display higher scattering intensity compared to Z4A, which is attributed to its high porosity.40a,42 The development of new and smaller pores (< 2 nm) after amine modification can be observed in the pore size distribution (Figure 2d). The new pore space generation is apparently due to the decrease in pore number density observed for the same pore sizes by the filling of amine, 41 which is in line with the reported results.20 Further, to verify the prejudicial impact of amine modification on textural properties, FTIR and TGA analyses are also performed for these amine-modified Z4A bodies (Figure S4 and S5 respectively). As compared to Z4A, FTIR spectra of BA-Z4A and IBA-Z4A display increasing intensity and broadness of bands ranging from 3600 cm-1 to 2800 cm-1, which could be due to the overlapping of several bands including 3270 cm-1 (N-H stretching) and 1470 cm-1 (NH2 bending ) in the single region (Figure S4b).19,43 Z4A, BA-Z4A, and IBAZ4A exhibit analogous thermograms with a significant weight loss (~17%) in the temperature range of 100-160 °C, which is attributed to the removal of moisture,17a,24 and the elimination of amine. With further increase in temperature, the weight loss becomes insignificant, and no weight loss is observed for the remaining weight of 83% up to 650 °C, indicating the high thermal stability of Z4A/amine-modified binder-containing zeolites bodies (Figure S5). 13 ACS Paragon Plus Environment


3.2.

Page 14 of 33

CO2 and N2 adsorption studies of amine-modified binder-containing zeolite 4A

bodies The efficacy of the amine-modified zeolite bodies for post-combustion CO2 capture applications is extensively investigated through quantifying the CO2 and N2 adsorption capacities at low-pressure regime (up to 1 bar), pertaining to flue-gas-like conditions (Figure 3a and 3b). The results related to CO2 and N2 adsorption capacities of amine-modified binder-containing zeolite 4A bodies along with the parent binder-containing zeolite 4A bodies are listed in Table 2. Table 2. CO2 and N2 adsorption capacities of amine-modified binder-containing zeolite 4A bodies at 25 C Adsorbent

CO2 adsorption capacity

0.15 bar 1 bar -1 (mmol g ) (mmol g-1) Z4A 0.97 2.20 PA-Z4A 0.89 2.33 IPA-Z4A 0.96 2.43 BA-Z4A 1.14 2.48 IBA-Z4A 1.34 2.56 PEA-Z4A 0.37 1.90 IPEA-Z4A 0.68 1.91

N2 adsorption capacity

0.75 bar 1 bar -1 wt % (mmol g ) (mmol g-1) 9.69 0.20 0.25 10.27 0.04 0.05 10.68 0.07 0.09 10.89 0.04 0.06 11.26 0.02 0.02 8.38 0.10 0.13 8.39 0.09 0.12

Chemical shift 15N

δ (ppm) 21.58 43.74 21.95 17.14 20.97 22.06

The CO2 and N2 adsorption capacity of Z4A bodies is found to be comparably lesser than the reported binder-free microporous zeolite 4A (100% active material), presumably due to lower volumetric efficiency.11 Analogously, lower CO2 adsorption capacity was also observed by other researchers, while dealing with the binder -containing zeolite bodies.44 In general, the zeolite 4A based adsorbents are being degassed at a higher temperature (300-350 °C) to get rid of the moisture.12 However, enduring such high degassing temperature after each adsorption cycle is highly energy intensive. On the other hand, the low-temperature exhaust gases (70-230 °C) exiting from recovery devices in gas-fired boilers and ethylene furnaces remain largely unutilized for electricity generation due to their low Carnot efficiency.45 Hence, these unused low-temperature exhaust gases can be utilized to regenerate the adsorbents. In this context, a lower degassing temperature (120 °C) for binder-containing zeolite 4A bodies is selected for investigating the CO2 adsorption performance to utilize the waste heat generated from recovery devices. 14 ACS Paragon Plus Environment

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On the other hand, it is observed that the studied amine-modified Z4A bodies display enhanced CO2 adsorption properties at 0.15, and 1 bar respectively at 25 C. From Figure 3 and Table 2, it is evident that among several amines utilized for modification of bindercontaining zeolite 4A bodies; the branched chain amines display superior CO2 adsorption capacities as compared to the straight chain amine-modified binder-containing zeolite 4A bodies. Notably, among a wide range of utilized branched chain amines, the iso-butylaminemodified binder-containing zeolite 4A bodies (IBA-Z4A) display the steepest increase in CO2 adsorption capacity at 0.15 bar (1.34 mmol g-1, 5.89 wt %), which further increases to 2.56 mmol g-1 (11.26 wt %) at 1 bar. Notably, IBA-Z4A also exhibits the lowest N2 adsorption capacity (0.02 mmol g-1) among all the studied absorbents at 1 bar. In contrast to IBA-Z4A, pentylamine and iso-pentylamine-modified binder-containing Z4A bodies display lower CO2 adsorption capacity, presumably due to the bulky nature of pentylamine, and isopentylamine,25a and/or narrower adsorption energy distribution, which means amines are not able to modify the binder-containing zeolite 4A bodies properly.46a With this understanding, the loading of iso-butylamine in binder-containing zeolite 4A bodies is further varied (0.1 wt%, 0.3 wt%, and 1 wt%) to study the effect of amine loading on CO2 adsorption capacity at 25 °C and 1 bar (Figure S6a). Results showed that at minimal loading (0.1 wt% and 0.3 wt%) the CO2 adsorption capacity is better in comparison to the higher amine loading of 1 wt%. These findings are consistent with the earlier report of Bezerra et al.17a At optimal minimum loading (0.3 wt%) the CO2 adsorption capacity is found to be high in comparison to the loading of 0.1 wt% and 1 wt% in IBA-Z4A. Further study revealed the effect of loading (0.1 wt%, 0.3 wt%, and 1 wt%) of another amine, iso-pentylamine (IPEA), which is closest to iso-butylamine (IBA) in terms of its basicity, on the CO2 adsorption properties. Contrary to IBA-Z4A, results infer no significant enhancement in CO2 adsorption capacity for IPEA-Z4A with varying amine loading. Even with higher loading of 1 wt% IPEA, the CO2 adsorption is analogous to that of 0.3 wt% IPEA (Figure S6b). These results are consistent with the assumption that, due to the sterically bulky nature of IPEA and its narrow adsorption energy distribution, the pores of the binder-containing zeolite 4A bodies may be unable to be properly modified by the amine.25a,46a It is believed that the basicity of straight chain amines increase with the increase in chain length (to a certain limit) due to the increase in electron donating alkyl groups, and decreases due to steric hindrance effect, which adversely affects the interaction of the amines with CO2 molecules. This complexity occurs due to partial coverage of nitrogen atom by long organic chains, owing to the lower accessibility of the lone



pair of electrons, resulting in decreased interaction with CO2.16a,46b,46c Based on these findings, 0.3 wt% amine loading in binder-containing zeolite 4A bodies is believed to be optimal to achieve high CO2 adsorption capacity in binder-containing zeolite 4A bodies. In order to further investigate the adsorbate-adsorbent interaction, the measured pure component adsorption isotherm data for CO2 and N2 on the studied adsorbents are fitted with Toth and Sips model respectively (Figure 3a, b, and Figure S7). All the fitted parameters with the normalized standard deviation (NSD) are summarised in Table S2. The goodness-of-fit value (R2 ~ 0.99) for Toth model over entire pressure and temperature range in aminemodified binder-containing zeolite 4A bodies indicate that CO2 molecules are efficiently adsorbed on amine-modified Z4A bodies in multi-molecular layers due to a high degree of adsorbent surface heterogeneity.32a,33 Particularly, CO2 interacts with the lattice oxygen present inside the zeolite framework (Lewis basic site) which acts as an adsorption centre. Since the framework of the zeolite 4A crystal lies inside the binder, these broad ranges of pores present inside the binder act as micro reactors for the capture of CO2. Therefore when the amine modifies these pores, they exhibit superior CO2 adsorption performance, due to the symbiotic interaction between CO2 and amines.

3.0 2.5

Z4A IPA-Z4A IBA-Z4A IPEA-Z4A

PA-Z4A BA-Z4A PEA-Z4A Toth fit

(a)

2.0 1.5 1.0 0.5 0.0 0.0

0.2

0.30 -1

3.5

N2 Adsorbed (mmol g )

-1

CO2 Adsorbed (mmol g )

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 33

0.4 0.6 0.8 Pressure (bar)

1.0

0.25 0.20

Z4A IPA-Z4A IBA-Z4A IPEA-Z4A

PA-Z4A BA-Z4A PEA-Z4A Sips fit

(b)

0.15 0.10 0.05 0.00 0.0

0.2


1.0

Figure 3. (a-b) CO2 and N2 adsorption isotherms for binder-containing zeolite 4A bodies and amine-modified binder-containing zeolite 4A bodies at 25 °C in the pressure range, (a) CO2 adsorption isotherms in 0-1 bar, and (b) N2 adsorption isotherms in 0-1 bar. After amine modification, the basicity of binder-containing zeolite 4A bodies increases due to the formation of extra adsorption centers by the presence of free -NH2 groups (Table 2). The mechanism of CO2 adsorption on amine-modified adsorbents has been 16 ACS Paragon Plus Environment

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described in recent literature reports.16a,26,47 Recently, Yaghi et al.47b investigated the interaction between CO2 and the –NH2 group using solid-state NMR and identified the formation of ammonium carbamate and carbamic acid as an important species. Perinu et al.26 also established a relationship between the reactivity of a series of primary straight chain alkyl amines (propylamine, butylamine) and branched chain alkyl amine (iso-propylamine, iso-butylamine) using 15N-NMR spectroscopy for CO2, where the formation of carbamate at equilibrium depends upon the basicity of the interacting amine. Therefore, the higher CO2 adsorption capacity observed for IBA-Z4A bodies is in accordance with the lowest 15N-NMR chemical shift of IBA (Table 2 and Figure S8) which may facilitate the efficient amine interaction with CO2. 3.3.

Adsorption thermodynamics and evaluation parameter of amine-modified

binder-containing zeolite 4A bodies for CO2 capture. Further, to study the energy penalty associated with the regeneration of the studied adsorbents, the isosteric heat of adsorption (Qst) is estimated for Z4A and IBA-Z4A. Figure 4a shows that there is a 10.2% increase in CO2 adsorption capacity, when the temperature rises from 0 to 25°C for IBA-Z4A, due to the effect of chemisorption (forming covalent C−N bonds), which is often seen in amine-modified adsorbents.16b,18a,19 Concurrently, the CO2 adsorption capacity decreases in Z4A with an increase in temperature because of the exothermic nature of physisorption (Figure 4a). Notably, the derived Qst value of Z4A bodies (38 kJ mol-1) is in line with the zeolite 4A series (ca. 35-50 kJ mol-1) reported in the literature.32b,c Furthermore, upon amine modification IBA-Z4A display a significantly higher value of Qst (51 kJ mol-1). The large Qst value of IBA-Z4A indicates the heterogeneity and higher affinity of IBA-Z4A for CO2 adsorption (Figure 4b). In fact, it has been observed that there is a reasonable increment in Qst (13 kJ mol-1) with a little rise in CO2 adsorption capacity in IBA-Z4A as compared to Z4A bodies, which is presumably due to high carbamate formation tendency of iso-butylamine during the chemical reaction with CO2.26b


3.5 3.0

60

(a)

Page 18 of 33

(b)

50

2.5

-1

Qst (kJ mol )

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 CO2 Adsorbed (mmol g )


2.0 1.5

Z4A/0 C

1.0

Z4A/25 C

0.5

IBA-Z4A/25 C

IBA-Z4A/0 C

0.0 0.0

0.2


(c) Adsorbent

Amine

Zeolite 13X Zeolite 13X Zeolite Y Mesoporous Foam

KIT-6 Silica IBA-Z4A

40 30 20 1.0

1.0

Z4A IBA-Z4A

1.2 1.4 1.6 1.8 2.0 -1 CO2 Adsorbed (mmol g )

Qst (kJ mol-1) 59.7 144.7 27.6-46.9

Ref.

MEA MEA TEPA

Loading (vol/wt%) 0.2 (v) 10 (v) 60 (w)

PEI PEI IBA

80 (w) 50 (w) 0.3 (w)

68 110 51

48a 48b This work

2.2

17b 17b 19

MEA: monoethanolamine, TEPA: tetraethylenepentamine, PEI: polyethyleneimine, IBA: iso-butylamine. Figure 4. (a) CO2 adsorption isotherms for Z4A and IBA-Z4A at 0 °C, 25 °C, (b) Isosteric heat of adsorption of CO2 for Z4A and IBA-Z4A, and (c) comparative chart for the isosteric heat of adsorption of CO2 of other amine-modified adsorbents. Figure 4b displays an initial decreasing and then almost stabilizing trend in Qst value with CO2 loading (indicating the preferential occupancy of high energy sites) followed by a plateau (saturation level). The Qst value (51 kJ mol-1) obtained for IBA-Z4A is lying in the very narrow range of weak chemisorption, which is at par with or even smaller than the established amine-modified adsorbents as reported in the literature (Figure 4c).48 The obtained lower Qst value of IBA-Z4A suggests that regeneration of the adsorbents will consume less energy.48c The above results are encouraging enough to calculate the Henry constant (Kh), CO2 over N2 selectivity, and purity of the amine-modified binder-containing zeolite 4A bodies to study their further applicability in selective CO2 capture under flue gas condition (Figure S9 as an example). 18 ACS Paragon Plus Environment

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Table 3. Adsorbent evaluation parameters for CO2 capture Adsorbents

Henry constant Kh (mmol g-1 bar-1) CO2 N2

Selectivity

Purity (%)

(KhCO /KhN )a

αCO /N b

2

2

2

2

Z4A

11

0.30

37

24

83

PA-Z4A

12

0.07

171

111

96

IPA-Z4A

17

0.17

100

69

93

BA-Z4A

18

0.04

450

143

97

IBA-Z4A

18

0.03

600

335

98

PEA-Z4A

3

0.12

25

19

79

IPEA-Z4A

6

0.15

40

38

88

aPure

component adsorption selectivity based on the adsorption conditions (qCO at 0.15 bar, qN at 0.75 bar)

bSelectivity

2

2

As listed in Table 3, it is found that the pure component adsorption selectivity of Z4A bodies (KhCO /KhN 2

2

=

37) is higher than the reported microporous powder zeolite 4A (ca.

KhCO /KhN = 19) at 25 °C (gas composition: 94.9% N2, 5.1% CO2).32b The reason may be due 2

2

to the selection of different gas phase composition while calculating the pure component selectivity at low pressure. The calculated Henry constant (KhCO ) of Z4A is in accordance 2

with the reported literature.44c It is worth noticing that, the observed higher KhCO2 for IBA-ZA as compared to the other amine-modified Z4A and parent Z4A bodies, presumably due to the strong interaction of CO2 with the adsorbent (Figure 5a). As a result, IBA-Z4A displays excellent pure component adsorption selectivity of CO2/N2 measured from the value of Henry constant ratio at 25 °C (Table 3). The observed higher CO2/N2 selectivity in IBA-Z4A could be a consequences of exceptionally large polarizability, and quadrupole moment of CO2 (29.11 × 10−25 cm−3; and 4.30 × 10−26 esu−1 cm−1, respectively) than N2 (17.40 × 10−25 cm−3; and 1.52 × 10−26 esu−1 cm−1 respectively), and steric effect of adsorbing molecules (CO2, N2) at the adsorbent surface.11a,33 Furthermore, IBA-Z4A exhibits excellent flue gas normalized selectivity (αCO /N2 = 335) along with the high VCO2/VN2 selectivity (128 at 1 bar), suggesting 2

that high purity CO2 (98%) could be recovered from the dry flue gas (Figure 5b). It is quite obvious that generating pure CO2 would support in the reduction of high capital investment for compression, transportation and storage/utilization of CO2 in power plant.33



(a)

(b)

(c)

(e)

(d)

Figure 5. (a) Semi-log plot of CO2 adsorption isotherm on amine-modified adsorbents at 25 °C. (b) CO2 over N2 Selectivity (αCO /N ) of studied adsorbents. (c,d) Cyclic CO2 adsorption2

2

desorption experiment for IBA-Z4A under different desorption conditions: (c) vacuum desorption at 0.01 bar, and (d) combined vacuum and thermal desorption at 0.01 bar, and 120 °C. (e) P-XRD patterns of spent IBA-Z4A before (i) and after consecutive five CO2 adsorption-desorption cycles under desorption conditions (ii) 0.01 bar, and (iii) 0.01 bar at 120 °C. For further investigations, additional CO2 and N2 adsorption experiments were conducted for the studied binder-containing zeolite 4A bodies and IBA-Z4A at 1 bar and 25 °C with higher degassing condition (350 °C for 12 h). The effect of degassing temperature on CO2 adsorption performance (Figure S10 and Tables S3) is estimated. From the results, an obvious enhancement in the CO2 and N2 adsorption capacity for both Z4A and IBA-Z4A is observed, due to the elimination of amine and water molecules from these adsorbents at the higher degassing temperature.12 Additionally, the higher degassing temperature for 20 ACS Paragon Plus Environment

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adsorbents may also adversely affect the CO2/N2 selectivity, due to the unhindered mobility of the adsorbing gases (CO2 and N2) in the pores of the binder-containing zeolite bodies. Results infer that the CO2/N2 selectivity of IBA-Z4A decreases significantly to 58 under degassing at 350 °C for 12 h, in comparison to the higher CO2/N2 selectivity of 335 observed under degassing at 120 °C for 8 h. Hence the preferential adsorption of CO2 over N2 on IBAZ4A makes it a suitable candidate for selective CO2 capture from flue gas with low energy penalty. Beside high CO2/N2 selectivity, IBA-Z4A also exhibits better adsorption performance during cyclic CO2 adsorption-desorption experiment at 25 ºC and 1 bar (Figure 5c,d). While operating the experiment at moderate desorption pressure (0.1 bar), the respective CO2 adsorption capacity and adsorption index of the spent IBA-Z4A is significantly reduced to 0.62 mmol g-1 (AI = 24%), after ten cycles of CO2 adsorption-desorption (Figure S11). However, when the experiment was conducted at low desorption pressure (0.01 bar), after initial drop the respective CO2 adsorption capacity and adsorption index shows better values and remains stable after 5th cycle (1.4 mmol g-1 and 55%) with only a marginal loss in CO2 adsorption capacity even up to 10th cycle (Figure S11). The decrease in CO2 adsorption capacity and adsorption index in prior both the cases could be attributed to the chemical interaction of CO2 and IBA-Z4A, where the adsorbed CO2 is unable to desorb completely from IBA-Z4A only by decreasing the desorption pressure. Hence, fresh cyclic experiment (five cycles) was conducted at 0.01 bar desorption pressure with thermal treatment (120 °C). It is worth noticing that, at combined vacuum and thermal desorption, the cyclic CO2 adsorption capacity is found to be almost unchanged (2.4 mmol g-1) with 96% adsorption index (Figure 5d). The stable adsorption capacity during the following five cycles is comparable with the other well-established amine-modified adsorbents.49 Notably, P-XRD analysis of the spent IBA-Z4A after cyclic adsorption-desorption displays no significant change in the characteristic diffraction patterns, confirming the excellent physico- and thermochemical stability of IBA-Z4A (Figure 5e). Hence the study suggests that the combined vacuum and thermal treatment (0.01 bar at 120 °C) is the most suitable desorption condition to regenerate the spent IBA-Z4A during CO2 adsorption experiments. 3.4.

Comparison of adsorption performance of IBA-Z4A and other amine-modified

adsorbents



In the current study, it has been observed that, after successful amine modification of the binder-containing zeolite 4A bodies, it is possible to achieve superior CO2/N2 selectivity under the flue gas condition. Further, the low utilization of amine is able to address the conventional limitations like corrosion, and high regeneration cost for solid amine-based adsorption systems. The upgradation of CO2/N2 selectivity and purity of adsorbents upon amine modifications are in line with the previous literature reports (Table S4).24,46b,50 Several studies have been conducted by various researchers on amine-modified adsorbents to increase the CO2/N2 selectivity. For instance, Xu et al.24 reported the enhancement of CO2/N2 selectivity from 12.19 (parent zeolite β) to 25.67 upon 40 wt% MEA loading in zeolite β, predominantly due to the steric effect of adsorbate by reduction of adsorbent’s pore diameter and chemical adsorbate-adsorbent interaction. Especially in the flue gas, due to competitive adsorption, CO2 is preferentially adsorbed on amine-modified adsorbents by displacing the pre-adsorbed N2 over time, which increases CO2/N2 selectivity. This is in agreement with the results obtained by Jadhav et al.50a Demessence et al.50b They evaluated the CO2/N2 selectivity for Triazolate-bridged MOF (Cu-BTTri), before and after ethylenediamine modification (Cu-BTTri-en) at post-combustion CO2 capture condition and observed an enhancement in CO2/N2 selectivity of 21 to 25 after amine impregnation. Unlike amine impregnation, grafting of amine onto the surface of silica also helps in increasing CO2/N2 selectivity. Wang et al.50c reported the CO2/N2 selectivity enhancement from 46 (parent SBA15) to 131 for (3-aminopropyl) trimethoxysilane grafted SBA-15 in flue gas condition (0.15 bar CO2 and 0.85 bar N2). Besides, adsorbents with high N-donor molecules are also useful to achieve high CO2/N2 selectivity. Recently, Tekin et al. 50d found enriched CO2/N2 selectivity of 473 at 23 °C in microporous metal dicyanamide cluster [Co(hmt)(dca)2 (hmt: hexamethylenetetramine, dca: dicyanamide)] because of diffusion hindrance of N2 to small pore cavity of the adsorbent. Notably, apart from excellent CO2/N2 selectivity, we also observe a much higher CO2 adsorption capacity 2.56 mmol g-1 (11.26 wt%) even with ~10 times less amine loading (0.3 wt%) in binder-containing zeolite 4A bodies as compared to previously reported results (Table 4).17a,18b,24,48a,51 Earlier reports revealed that amine-modified zeolite Y exhibited enhanced CO2 adsorption capacities with high amine loading (20~40 wt%) of MEA, IPA, or TEPA.18b Further, with other adsorbents such as activated carbon or ordered MCM-41, at 25 °C, even with the high surface area and with high amine loading, the observed CO2 adsorption capacities are much lower than those obtained with IBA-Z4A in the present work 22 ACS Paragon Plus Environment

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(Table S5). Moreover, the adsorbent binder-containing zeolite 4A bodies and the respective modifying agent iso-butylamine (code I14150) are of lower cost than the commonly used microporous zeolite 13 X powder (or bodies) and MEA (code 398136). Hence, IBA modified binder-containing zeolite 4A bodies (IBA-Z4A) can be considered as a promising adsorbent for efficient CO2 capture. Table 4. Comparison of adsorption capacities of various amine-modified adsorbents Support

Zeolite β Zeolite-Y Zeolite-Y Zeolite-Y Zeolite-Y Zeolite-13X Zeolite-13X Zeolite-13X Zeolite-13X AC Si-MCM-41 Z4A bodies

Amine

MEA MEA TEPA MEA IPPA MEA MEA MEA MEA PEHA PEI BA

Loading (vol/wt %)

40 (w) 20 (w) 20 (w) 40 (w) 40 (w) 0.2 (v) 0.5 (w) 10 (v) 25 (w) 40 (w) 50 (w) 0.3 (w)

Operating conditions PCO2 T (bar) (°C) 1 30 1 25 1 25 1 25 1 25 0.1 25 1 30 1 25 1 30 1 25 1 25 1 25

Equilibrium CO2 adsorption capacity (mmol g-1) (wt %) 0.77 1.15 1.90 1.97 1.61 1.68 1.25 0.56 0.78 1.10 0.74 2.47

Ref.

3.38 5.06 7.08 8.66 8.36 7.39 5.50 2.46 3.43 4.80 3.29 10.89

24 18b 18b 18b 18b 17a 50a 50a 50a 51a 51b This work Z4A bodies IBA 0.3 (w) 1 25 2.56 11.26 This work MEA: monoethanol amine, TEPA: tetraethylenepentamine, IPPA: iso-propanol amine, PEHA: pentaethylenehexamine, PEI: polyethyleneimine, BA: butylamine, IBA: iso-butylamine, Z4A bodies: binder-containing zeolite-4A bodies.

4. Conclusions Efficient CO2 adsorption capacity over binder-containing zeolite 4A bodies is achieved by modification of a wide range of aliphatic amines (straight and branched chain amines) in binder-containing zeolite 4A bodies. In this study, various characterization techniques such as BET, SAXS, and FESEM have confirmed the presence of heterogeneous pore size distribution in binder-containing zeolite 4A bodies, which are utilized for amine modification. It is anticipated that amines are located in the bigger pore of zeolite bodies 23 ACS Paragon Plus Environment


created by the binder or filler of zeolite crystals owing to decrease in meso pore volume and, therefore, it facilitates enhanced interaction with CO2. Interestingly, the CO2 uptake behavior of amine-modified binder-containing zeolite 4A bodies is found to be substantially affected by the carbon chain length, straight/branched nature, and electronic behavior of the aliphatic amines. The findings have inferred that amine-modified binder-containing zeolite 4A bodies, with iso-butylamine (IBA-Z4A), demonstrate superior CO2 adsorption capacity (2.56 mmol g-1), high purity (98), and excellent CO2/N2 selectivity (335) as compared to various other amine-modified Z4A bodies. The high CO2/N2 selectivity values can be attributed to steric effect and strong chemical adsorbate-adsorbent interaction. Notably, IBA-Z4A exhibits marginal isosteric heat of adsorption (51 kJ mol-1) at 1 bar suggesting the facile regenerability of IBA-Z4A and prolonged thermo-cyclic stability over five consecutive cycles of CO2 adsorption-desorption. To the best of our knowledge, the obtained pure component CO2/N2 selectivity (600) and the CO2/N2 selectivity (335) derived from adsorption condition for IBAZ4A is comparable with the best values reported for other microporous materials. Thus, the study of amine modification of binder-containing zeolite 4A bodies to achieve superior CO2 adsorption capacity has demonstrated a better way to utilize the cost-effective material binder-containing zeolite 4A bodies as a promising adsorbent for Carbon Capture Storage (CCS) process. Acknowledgments The authors thank IIT Indore, CSIR-HRDG, New Delhi and SERB-DST, New Delhi for the financial support. SIC National facility of IIT Indore is acknowledged for instrumentation facilities. The authors thank Ms. Navjot Saini, and Mr Shyama Prasad Pal, Anton Paar Gurgaon, Delhi for their valuable support in SAXS and BET characterization. The authors would also like to thank Dr. Pratibha Sharma from DAVV Indore and ACMS, IIT Kanpur for providing FTIR and XPS facility respectively. D.P. thanks IIT Indore for the fellowship. Dr. Rohit Kumar Rai, Dr. Kavita Gupta, and Ms. Chinky Binnani, IIT Indore are acknowledged for their valuable suggestions to understand the chemical reaction mechanism. Dr. Lakshmi Iyengar, former visiting faculty, School of Humanities and Social Science, IIT Indore is highly acknowledged for her valuable support in proof correction. Appendix A. Supporting Information


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The physico-chemical characterization (P-XRD, BET, FTIR, TGA,

15N

NMR, and

SAXS) for studied adsorbents, fitting parameters for CO2 and N2 adsorption isotherms from respective Toth and Sips models, Effect of amine loading on CO2 adsorption properties, CO2 and N2 adsorption properties for Z4A and IBA-Z4A at high degassing temperature, comparison of pure gas adsorption selectivity of CO2/N2 and adsorption capacity for IBAZ4A with other porous materials are given in the Supporting information.

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For Table of Contents Only:

NH2

CO2 adsorption →

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


NH2

NH2 NH2

NH2

NH2

← Amines →


Amine Modification of Binder-Containing Zeolite 4A Bodies for Post

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