Carbon Aerogels with Excellent CO2

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Carbon aerogels with excellent CO2 adsorption capacity synthesized from clay-reinforced biobased chitosan-polybenzoxazine nanocomposites Almahdi A. Alhwaige, Hatsuo Ishida, and Syed A Qutubuddin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01323 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 3, 2016

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ACS Sustainable Chemistry & Engineering

Carbon Aerogels with Excellent CO2 Adsorption Capacity Synthesized from Clay-Reinforced Biobased Chitosan-Polybenzoxazine Nanocomposites Almahdi A. Alhwaige†,‡,*, Hatsuo Ishidaǁ, Syed Qutubuddin†,ǁ,* †

Department of Chemical and Biomolecular Engineering, Case Western Reserve University Cleveland, Ohio 44106-7217, USA ‡

Department of Chemical and Petroleum Engineering, Al-Mergib University, Alkhoms, Libya

ǁ

Department of Macromolecular Science and Engineering, Case Western Reserve University Cleveland, Ohio 44106-7202, USA

ABSTRACT The present study reports for the first time the use of biobased chitosan-polybenzoxazine (CTSPBZ) as a precursor for high CO2 adsorbing carbon aerogels (CAs). Montmorillonite (MMT) is used to reinforce the CTS-PBZ aerogel. MMT-CTS-PBZ nanocomposite aerogels are synthesized

using

freeze-drying

technique,

and

then

cross-linked

via

ring-opening

polymerization of benzoxazine, followed by carbonization at 800 oC. Polybenzoxazine improves the structural stability for removing CO2 from the environment even at a high pressure. The properties of polymeric and nanocomposite aerogels have been evaluated using X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy. The microstructure of the CAs is characterized by N2 adsorption-desorption measurements. The CAs exhibit mesoporous materials with pore size in the range of 2 to7 nm and high BET surface area. The total pore volume of CAs is as large as 0.296 cm3 g-1 and the maximum BET surface area is 710 m2 g-1. Breakthrough curves of CO2 adsorption show high CO2 adsorption capacity at ambient conditions and excellent CO2 adsorption-desorption reversible performance with a maximum of

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5.72 mmol g-1. Adsorption isotherms and thermodynamic properties of CO2 adsorption are described. Keywords: Carbon aerogels; chitosan; polybenzoxazine; CO2 adsorption; thermodynamic properties. INTRODUCTION Carbon capture and storage (CCS) are being extensively studied due to the increasing interests in environmental protection and energy efficiency [1,2]. A wide range of approaches, including solvents, cryogenic techniques, membrane processes, and solid sorbents, have been documented in the literature for CO2 capture [3-6]. Among these, the adsorption technique offers several advantages such as easy and cost-effective technology, applicability in wide temperature range, and low energy requirement for regeneration [4,7,8]. Significant efforts have been made to discover and develop efficient amine-enriched supports for CO2 capture [2,7,9,10]. Loading amines on porous materials, including silica gel, mesoporous silica, and carbon, has proven to be promising for achieving the effective adsorption [1,4,5,1113]. However, oxidation and other side reactions possibly occur as the performance of these sorbents gradually decrease due to amine volatilization and urea formation [7,10]. Therefore, great attention has been given to develop materials with long-term thermal stability, high mechanical strength, and low chemical degradation induced by gaseous species (e.g., evaporation, decomposition) [10]. Recent studies report that porous carbon materials are excellent CO2 sorbents both at atmospheric and high pressure with good regeneration stability [14-16].


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Porous carbons have several specific features including abundant basic sites, large surface area, low cost, an easy-to-design pore structure, chemical stability and low energy requirements for regeneration [15-17]. However, most activated carbons adsorb less than 3 mmol g-1 CO2 at ambient conditions [18]. In order to increase the adsorption capacity, extensive research has been made to incorporate nitrogen atoms into the surface of pores [3,15,16,18,19]. Carbonization is an effective method for generating porous polymeric materials [3,4]. Polymer-derived carbons (PDCs) are attractive for CO2 capture [17], and have been documented in the literature, including poly(benzoxazine-co-resol)-based porous carbon monoliths [20], silica-template melamine– formaldehyde resin [3], resorcinol-formaldehyde copolymer [15], and mixed waste plastics [21]. The development of new carbon sorbents with superior performance for CO2 adsorption is still active due to the desired for economic and eco-friendly materials [4,5]. Use of natural renewable resources for development of porous carbon materials offers an attractive opportunity. Several sustainable and biomass-derived products have been used as precursors for the production of carbon sorbents, including lignin [22], phloroglucinol [23], polysaccharides (starch and cellulose) [18], biomass (sawdust) [18], alginate [24], and cellulose [25]. Although the presence of N atom on porous carbons as active species is favorable for CO2 adsorption, finding amine-containing natural polymers is limited [8,23,24].

(a)

(b) NH

H3C HN

CH3

N

C

O

H N

n N O

CTS

MCBP(BA-tepa)

Figure 1. Chemical structure of the polymers used in this work: (a) chitosan (CTS) and (b) the main-chain benzoxazine polymer (MCBP(BA-tepa)). 3 ACS Paragon Plus Environment


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Chitosan (Figure 1a) is a biopolymer rich with amine groups and is the second most available natural renewable resource after cellulose [8]. Very recently, it has received much interest for the application of CO2 adsorption [8,24,26]. The highest CO2-adsorption capacity for chitosan-based carbons (spherical) was reported as 5 mmol g-1 at 0 oC atmospheric pressure [24]. However, the appropriate conditions for industry and economic impact are adsorption at ambient conditions. More recently, we reported a maximum 4.15 mmol g-1 using GO-CTS monolith at 25 oC and 1 bar [8]. In addition, 50 wt% polyethyleneimine (PEI) impregnated chitosan-biotemplated silica monolith showed a maximum value of 3.8 mmol g-1 [26]. These results clearly indicated that porous carbons derived from natural polymers containing N-atoms are excellent sorbents for CO2 capture. In this paper, a novel route for improving chitosan-derived carbon aerogels as CO2 sorbents is presented. Polybenzoxazine (PBZ) has received much interest in academia and industrial communities as an advanced polymeric material. Since 1944, benzoxazine monomers have been synthesized by very simple, Mannich condensation reaction from inexpensive raw materials [27,28]. Benzoxazine (BZ) resins undergo ring-opening cationic polymerization without added catalyst due to the cationic initiating residual material [29,30]. BZ-based carbon materials have shown outstanding mechanical properties [20]. Recently, main-chain type benzoxazine polymer (Figure 1b) as a precursor for cross-linked PBZ has been developed [31], which provides excellent possibility of synthesizing amine-enriched benzoxazines. Such polybenzoxazines are attractive for many applications, including membranes for ethanol-water separation [31], and carbon aerogels for removal of heavy metal ions [32], and supercapacitor [33]. Additionally, aerogel can also be used as CO2 sorbents [20].


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In general, sorbents with more pores and good pore interconnection improve adsorption capacity. Development of three-dimensional (3-D) porous carbon could lead to significant advancements in the field of CO2 capture. Therefore, recently, we reported a new family of sorbents “GO-CTS-monolith aerogels”, which are promising sorbents for CO2 capture due to their attractive properties such as low density, large surface area, tunable pore structure, and high mechanical properties [8]. Thermal treatment of the freeze-dried aerogels results in the formation of engineered porous carbons with required active sites for the CO2 adsorption. Previously, we reported a detailed study on the properties of biobased CTS-PBZ cross-linked films [34]. To the best of our knowledge, the performance of the clay-CTS-PBZ-based porous materials for CO2 adsorption has not been reported. Herein, we report facile preparation of biobased carbon aerogels with excellent CO2 adsorption capacity from clay-reinforced biobased CTS-PBZ nanocomposites. The primary goal of this study is to investigate the dispersion of clay in CTS-PBZ cross-linked polymer, improving the sorbent morphology, and enhancing the CO2 uptake capacity.

EXPERIMENTAL Materials Chitosan, CTS, (≥ 98% deacetylated, from shrimp, Mw ~ 200,000 – 300,000), formalin (37 wt%), bisphenol-A, BA, (96 wt%) and tetraethylenepentamine, TEPA, were purchased from Sigma-Aldrich Chemicals (USA). Glacial acetic acid and 1,4-dioxane were purchased from Fisher Scientific (USA). Sodium montmorillonite (Na-MMT) was supplied by ECC America. It is a fine powder with an average particle size of 75 µm in the dry state, and with a cationic exchange capacity of 90 meq/100g. All reagents were used without any further purification. 5 ACS Paragon Plus Environment


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Sorbents preparation Preparation of Benzoxazine Precursor, MCBP(BA-tepa) The main-chain benzoxazine polymer MCBP(BA-tepa) was synthesized as described in a previous paper [34]. Fabrication of Engineered MMT-CTS-PBZ Nanocomposite Aerogels The freeze-drying method was used for preparing series of cylindrical nanocomposite aerogels from sodium montmorillonite (Na-MMT) colloidal dispersions in various ratios of CTSMCBP(BA-tepa) blends. Before freeze-drying, the formulation of the gels was designed to give aerogels with a regular cylindrical shape as seen in Figure S1 of the Supporting Information. Acetic acid aqueous solution (1 v/v %) and 5 wt% solid content were used for preparation of all aerogels. A desired amount of Na-MMT was dispersed in 30 ml deionized water using magnetic stirring at room temperature. After fully dispersing the clay, 1 v/v% acetic acid was dropped to the Na-MMT dispersion and a calculated amount of chitosan (CTS) was added with continuous stirring. At the same time, a desired amount of main-chain benzoxazine polymer, MCBP(BAtepa), was dissolved in aqueous acetic acid solution (1 v/v% ) using a magnetic stirrer at ambient conditions. In low pH, the amine groups get protonated (R3NH+) and become better soluble in water. MCBP(AB-tepa) is possible to be dissolved in aqueous 1 % acetic acid due to protonation of secondary amine on the structure of the benzoxazine monomer. These protonated moieties (cations) contribute for MCBP(BA-tepa) to be water soluble monomer. After completely dissolving MCBP(BA-tepa), the solution was added slowly to the Na-MMT-CTS suspension under vigorous agitation by a magnetic stirrer until the mixture turned to be highly viscous gel. Then, any bubbles present were removed from the gel under vacuum at room temperature. The gel was frozen using ethanol and solid carbon dioxide at -70 oC and ambient pressure. Finally,


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the frozen gel was transferred to a VirTis AdVantage@ EL-85 freeze-drier for five days. Thus, the aerogel was formed. The obtained aerogels of chitosan/benzoxazine/clay are abbreviated as MMTxCTSyPBZz, where x:y:z represents the weight ratio of CTS, MCBP(BA-tepa), and NaMMT, respectively. Synthesis of Engineered Carbon Aerogels Firstly, the aerogels obtained via freeze drying were cross-linked using step-wise thermal treatment as follows: 100, 125, 150, 175 oC for 2h each. While the initial physical gel forms via hydrogen bonding among amines and phenols and partial covalent bonds formed between the primary amines of CTS and PBZ, this thermal treatment allows full polymerization of the residual oxazine groups to improve the structural integrity.

Subsequently, all cross-linked

aerogels were carbonized up to 800 oC at a heating rate of 10 oC min-1 under nitrogen. The carbonized aerogels based on CTS-PBZ and MMT-CTS-PBZ nanocomposites are abbreviated as CA-x and MMT-CA-x, respectively, where x represents the weight ratio of MCBP(BA-tepa) in the aerogel. A summary of specific weight ratios and abbreviations is listed in Table 1.

Table 1. Raw materials composition and abbreviations of the studied aerogels. Sample code Polymerized aerogel o

[a]

Weight ratio

Carbon aerogel o

Na-MMT

CTS

MCBP(BA-tepa)

at 175 C

at 800 C

(x)

(y)

(z)

MMT0CTS1PBZ1

CA-1[a]

0

1

1

MMT1CTS1PBZ1

MMT-CA-1[b]

1

1

1

MMT1CTS1PBZ2

MMT-CA-2 [c]

1

1

2

CA: CTS-PBZ-based carbon aerogel. MMT-CA: MMT-reinforced CTS-PBZ nanocomposite carbon aerogel.

[b],[c]



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CO2 Adsorption Measurements The gravimetric method was used to measure the dry CO2 adsorption capacities against applied pressure. The mass increase determined by the isothermobalance Intelligent Gravimetric Analyzer (IGA), Hiden Isochema, corresponds to the adsorbed CO2 and is expressed as the amount of CO2 per mass unit of dry solid sorbent in mmol g-1 [3]. This system is able to perform adsorption measurements with gas pressure up to 2 bar. The change in the weight of the adsorbent sample as well as the pressure and temperature were measured continuously until equilibrium was reached. Prior to the adsorption analysis, the sample was degassed at 150 oC for 3 h. The CO2 adsorption experiments were performed at temperatures of 25, 50, 75, and 100 oC. Characterization X-ray diffraction (XRD) was used to characterize the d-spacing change of montmorillonite using Cu-Kα radiation (λ= 0.15418 nm) with scanning rate of 0.2o/min at room temperature. XRD was performed on dried aerogel samples which were cut into thin disks prior to X-ray measurements. Bragg’s equation (nλ=2dsinθ, where d is the layer spacing and θ is the angle of diffraction) was used to compute the d-spacing for montmorillonite (MMT). Surface morphology of obtained aerogels was observed using scanning electron microscopy (SEM) with an acceleration voltage of 15kV. The pore structure of the aerogels, assessment of the CO2 capture capacities, and regeneration potential of the sorbents were determined by nitrogen adsorption method at -196oC using Intelligent Gravimetric Analyzer (IGA) as previously reported [8]. The thermal properties of the aerogels were evaluated by thermogravimetric analysis (TGA) using a TA Instruments High Resolution 2950 thermogravimetric analyzer. A sample (5-10 mg) was placed in a platinum pan and heated from room temperature to 900 oC at a ramp rate of 10oC min-1 under nitrogen atmosphere with a gas flow rate of 60 mL min-1. 8 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Design and characterization of sorbents Engineered Geometry of Sorbents Solid porous materials are widely employed in CO2 adsorption, most of which are used as welldefined powder or spheres. The features of the sorbents can be tailored to such an extent that one is justified in using the term engineered porous carbon materials. In recent years, substantial progress has been made in enhancing the processability of porous solids for CO2 adsorption [8,15,26]. Chitosan-based carbon aerogel which was pyrolyzed to 800 oC under nitrogen atmosphere showed weak mechanical properties due to the absence of cross-linking. However, this work confirms the positive effect of polybenzoxazine on the geometrical stability of chitosan-based cylindrical carbon aerogel. X-ray Diffraction Analysis XRD is useful for characterizing the dispersion of nanofillers in polymeric matrix [35]. Figure 2(a) displays the XRD diffractogram of Na-MMT which displays the characteristic peak of the (001) plane at 2θ = 7.5° (basal spacing of 1.21 nm). This compares with the d001 spacing of 1.24 nm documented in the literature [36,37]. The XRD pattern of neat Na-MMT shows additional peaks at 2θ = 19.6° and 22.8°, which are assigned to the (100) and (020) reflections. Furthermore, the peak at 2θ = 28.5° can be assigned to (Mg, Fe) SiO3 salts [38]. In nanocomposite aerogels (curves b and c), the diffraction peak shifted to lower angles suggesting intercalation of polymer in the silicate gallery. This change is due to the replacement of the Na+ of Na-MMT by the positively charged amine groups on the cross-linked CTS-PBZ in an acidic medium [11,35]. The intercalation increased the interlayer distance by 0.35 nm for



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CTS1PBZ1MMT1 and 0.40 nm for CTS1PBZ2MMT1. The polymer near the nanofiller surface is reported to form rarefied material [39], which might result in an increase in the specific area of the carbon aerogel after pyrolysis at 800 oC.

Figure 2. XRD diffractograms of neat Na-MMT and nanocomposite aerogels: (a) Neat NaMMT, (b) MMT1CTS1PBZ1, and (c) MMT1CTS1PBZ2. Morphological and Microstructure Analysis Scanning electron microscopy (SEM) was used to reveal the pore morphology of the nanocomposite aerogels clearly. The micrographs of the MMT0CTS1PBZ1 aerogel and MMT1CTS1PBZ1 nanocomposite aerogel are compared in Figure 3. As seen in the SEM images 3a and b, the nanocomposite aerogel has more layered structure than the clay-free aerogel and the layers are oriented parallel to each other with high density of pores. A similar morphology has been widely observed in nanocomposite aerogels [40-44]. The SEM image 3c shows a crosssection of the MMT1CTS1PBZ1 nanocomposite aerogel. The SEM micrograph indicates that the aerogel is highly porous material with random pore structure. The increase in pore volume facilitates the gas transport to the sorbent; subsequently, it also enhances the adsorption performance. This SEM observation is consistent with the previously discussed possible cause 10 ACS Paragon Plus Environment

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due to higher void formation by the rarefied layer near the nanofiller surface [37]. However, the porous observed in SEM images correspond to external porosity. SEM does not give any information about micro- or mesoporosity of the solids. Therefore, nitrogen adsorptiondesorption method was used to investigate the textural properties of polymerized and carbonized aerogels.

Figure 3. SEM micrographs of samples: (a) pristine MMT0CTS1PBZ1 polymeric aerogel, (b) and (c) images from side direction and cross section of MMT1CTS1PBZ1 nanocomposite aerogel, respectively. Nitrogen Gas Adsorption-Desorption The N2 adsorption-desorption method was used to find the specific surface area (SBET) of uncarbonized nanocomposite aerogels. The BET Surface area (SBET) of the polymerized, crosslinked, aerogels was found to be very low approximately 16 m2 g-1; the adsorbed CO2 capacity is ~ 0.7 mmol g-1 at 25 oC and 1 bar. Many reports indicate that SBET and CO2 adsorption capacity significantly increase with carbonization of the polymer-based porous solids [3,8]. Hereinafter, the adsorption results are for the polymer-based carbon aerogel (CA-1) and nanocompositebased carbon aerogels (MMT-CA-1 and MMT-CA-2). 11 ACS Paragon Plus Environment


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Figure 4. (a) Nitrogen adsorption–desorption isotherms at -196 oC for CAs (full and open symbols, respectively) and (b) pore size distribution (PSD). The N2 adsorption-desorption isotherms and pore size distribution (PSD) of the carbon aerogels are shown in Figure 4. The values of BET surface area (SBET), total pore volume (Vt), micropore volume (Vmicro), pore diameter (Dp), and adsorption capacity are summarized in Table 2. As seen in Figure 4a, the hysteresis of the isotherms indicates the mesoporous characteristics of the sample [14], while the adsorption isotherms relate to Type I, indicating microporous isotherm with significant adsorption below relative pressure (P/Po) = 0.1 [15,16,45,46]. The present study is in agreement with literature that some carbon porous materials present 12 ACS Paragon Plus Environment

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adsorption isotherms between type I and IV, indicating that not only micropores, but also smallsized mesopores were developed in these porous materials [47]. The Dubinin–Astakhov (D–A) model was used for obtaining the micropore volumes (Vmic) [47,48]. Details of the method for estimations of Vmicro and Vmeso are given in the Supporting Information, S2. Figure 4b shows the pore size distribution curve for the carbon aerogels using Barrett-Joyner-Haenda (BJH). The average pore size Dp was estimated assuming slit-shaped pores by means of the expression 2Vt /SBET. The total pore volume was determined from the amount of nitrogen adsorbed at P/Po = 0.95 [45]. Table 2. Textural properties and CO2 adsorption capacity of carbon aerogels. Sample code

SBET

Vt

Vmicro

Vmeso

Dp

CO2 uptake

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

(cm3 g-1)

(nm)

(mmol g-1 )

CA-1

220

0.087

0.065

0.022

1.58

2.13

MMT-CA-1

679

0.371

0.319

0.052

2.18

5.72

MMT-CA-2

710

0.357

0.294

0.063

2.01

3.22

As seen in Table 2, SBET, Vt, and Dp of CA-1 are 220 m2 g-1, 0.087 cm3 g-1, and 1.58 nm, respectively. However, upon loading of clay to CA-1, the SBET area increased dramatically. The layered morphology and more porous structure of nanocomposite aerogels compared to the polymeric aerogel lead to enhance the surface area. The CA-1 shows a pore volume of 0.087 cm3 g–1, while it increases to 0.371 cm3 g–1 of MMT-CA-1 with the presence of clay. Therefore, presence of clay enhances the porosity of the aerogels. The pore volume depends on the growth ice crystal formed during the aerogel formation. For textural properties of aerogels, the structure and size of pores depend on the characteristics of the formulation solvent crystal ice during the 13 ACS Paragon Plus Environment


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freezing process [41,44]. In this study, during the gel formation, clay, CTS and MCBP were the components that form the solids, which controlled the aerogel structure. However, water is only included in the volume fraction for wet gel and does not contribute to the solid mass of the aerogel. During the freezing of the gel, the water will be solidified (ice crystal) and its volume fraction will form the pores after drying process. CO2 adsorption studies CO2 Adsorption Capacity The operating conditions for CO2 adsorption and nature of solid adsorbents can significantly affect the adsorption capacity. BET high surface area (SBET), micropore volume (Vmic), and the presence of basic N atoms are the most important three major parameters that influence the CO2 adsorption [20,45]. In this context, the effect of carbonization of the aerogels on CO2 capture has been investigated. Figure 5 shows the CO2 adsorption isotherms for MMT-CA-1 at 25 oC at various applied pressures. The slope of breakthrough curve for CO2 adsorption using the carbon aerogel is significantly higher than that for the non-carbonized aerogel. This indicates that the surface of CA has excellent CO2 adsorption capacity at low pressure. The maximum adsorption capacity of MMT-CA-1 is 5.72 mmol g-1, which is approximately 8 times higher than that of non-carbonized aerogel (MMT1CTS1BZ1). These results are consistent with the results reports in the literature that CTS porous materials without calcination showed CO2 adsorption capacity < 1 mmol g-1; however, remarkable increase in the adsorption capacity was achieved after calcination [8,24]. Pyrolysis of these nanocomposite aerogels leads to increase in pore interconnectivity and creates narrow pores [49]. Therefore, carbonization leads to the preparation of CAs with high BET surface area resulting in an increase in CO2 diffusion and adsorption


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capacity [24]. Naturally, the availability of the CO2 adsorption sites is assumed to be proportional to the surface area. In addition, significant high CO2 adsorption capacities have been achieved with small microporous carbon materials [50]. Any technique that increases the active sites on the surface of the adsorbent, will lead to higher adsorption capacity. For instance, Primo et al. [24] reported that the carbon aerogels prepared from neat CTS spherical aerogels at 800 oC have 11.61 wt% nitrogen content and show high CO2 adsorption capacity (5.0 mmol g-1 at 0 oC and 1 atm).

Figure 5. The effect of carbonization of the freeze-dried aerogel on CO2 adsorption capacity at 25 oC: MMT1CTS1PBZ1 (open circle) and MMT-CA-1(full circle).

Figure 6. The effects of polybenzoxazine and clay concentrations on CO2 adsorption capacity at 25 oC: CA-1 (triangular), MMT-CA-2 (square), and MMT-CA-1 (circle).



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Figure 6 shows the CO2 adsorption isotherm curves at 25 oC as a function of the applied pressure (up to 2 bar) for all the CAs. The adsorption of CO2 increases rapidly in beginning of the experiments, and then slows to reach an asymptotic value. Table 2 illustrates the CO2 adsorption capacities that were achieved for CAs at 25 oC and 1 bar. The highest CO2 adsorption capacity was found in CA with an equal ratio of CTS: MCBP(BA-tepa): MMT i.e. (MMT-CA 1); while, clay-free CA with the same concentration of CTS and MCBP(BA-tepa) i.e. (CA-1) demonstrated the lowest adsorption capacity. In other words, it is seen that the adsorption capacity of CO2 using CA-1 was 2.13 mmol g-1 and significantly increased to 5.72 mmol g-1 with clay-loading (MMT-CA-1). The different behavior of slope in the adsorption capacity curve for the CA-1 and MMT-CA-1 would confirm the detrimental effect of narrow pores in CO2 adsorption due to the diffusion resistance. Similar observation has been reported for other CO2 adsorption studies in the literature [7,51,52]. On the other hand, as an increase of PBZ content in nanocomposite aerogels, the adsorption capacity for CO2 decreased from 5.72 to 3.22 mmol g-1 for MMT-CA-1 and MMT-CA-2, respectively. It suggests that cross-linking of CTS with MCBP(BA-tepa) results in reduction of available active sites for CO2. Structure stability of the sorbents is one of the important factors especially for adsorption processes at higher pressure including fixed bed and backed bed adsorber as well as during particle movement in fluidized bed adsorber. Thus, a trade off might exist between the CO2 adsorption capacity and geometrical stability. Hoa et al. [20] reported that PBZ-based carbon spherical sorbents are 80 times stronger than other carbon monoliths due to the cross-linking by PBZ. The systems studied in the current paper offers synergistic relationship between CTS, PBZ, and clay for developing novel carbon aerogels as superior sorbents for CO2 capture by improving their mechanical strength and subsequent geometrical stability.


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Effect of Operating Conditions While porous carbonaceous materials have been considered to be one of the most promising adsorbents for capturing CO2, the adsorption on these sorbents is mostly physisorption and possibly weak chemisorption that leads to temperature sensitivity and relatively poor selectivity [53,54]. It is important to understand the effect of the operating conditions on the performance of CAs for CO2 adsorption. Among the operating conditions, pressure and temperature dependency of the adsorption capacity have been evaluated in this study by plotting the CO2 uptake per gram of sorbent versus the pressure at various temperatures. As an example, Figure 7 represents the breakthrough curves of CO2 adsorption using MMT-CA-1 at various temperatures and pressures. As seen in the breakthrough curves, the amount of adsorbed CO2 increases with increasing the applied pressure at the same operating temperature; whereas, the adsorption capacity decreases with increase in the operating temperature at a fixed pressure. For instance, the results showed that CO2 adsorption capacity of 5.72 mmol g-1 at 25 oC and 1 bar was achieved, while increased to 6.25 mmol g-1 with increasing the applied pressure to 2 bar. However, a decrease was observed in the CO2 adsorption capacity from 5.72 to 3.30 mmol g-1 with increase in the temperature from 25 to 75 oC, respectively, at 1 bar. Therefore, temperature is negative to CO2 adsorption capacity and rate in the whole adsorption process. This point has been extensively investigated by monolithic aerogels, and we have also discussed the effect of temperature on CO2 adsorption performance elsewhere [8,54].The decrease in adsorption capacity with an increase in adsorption temperature indicates that the CO2 adsorption is an exothermic process [15]. On the other hand, the opposite finding was reported for CO2 adsorption using montmorillonite intercalated amines [11] and amine-modified mesocellular silica foams [4] due to the diverse nature of the sorbent surface interaction with CO2.



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Figure 7. Temperature and pressure dependency of CO2 adsorption capacity using MMT-CA-1. Regeneration Process The long term stability of sorbents for adsorption-desorption cycles is one of the important factors for practical applications and scale up [7,10]. In other studies, physically adsorbed low molecular weight amine on substrates has shown lower adsorption stability than N-rich polymeric sorbents. This decrease is attributed to volatilization of low molecular weight amine molecules during the regeneration process [10]. In this paper, adsorption-desorption cycles of the CTS-based carbon aerogels during the six temperature cycles between 25 oC adsorption and 130 o

C desorption has been investigated as shown in Figure 8. As seen, the performance of the

sorbent is fairly stable. Importantly, carbonization of CTS and CTS/nanofiller nanocomposite aerogels without benzoxazine structure lead to fluffy carbon-like ash with low mechanical properties that lose the structure and engineered shape of their carbon aerogels [8].


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Figure 8. Cyclical adsorption behavior of various aerogels: adsorption at 25 oC and desorption at 130 oC.

Figure 9. Multicycle profile of CO2 adsorption-desorption for MMT-CA-1.

Continuous adsorption-desorption cycles predict the ability of regeneration and stability of adsorption capacity using the obtained carbon aerogels [55,56]. Figure 9 shows the different adsorption capacities achieved under adsorption-desorption multicycles for MMT-CA-1. The



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adsorption capacity for the 1st run was 5.72 mmol g-1 and the 6th run showed an adsorption capacity of 5.61 mmol g-1. Therefore, the obtained sorbents can loss up to ~2 % of its adsorption capacity of first cycle after 6 runs. The loss of capacity is attributed to a small amount of CO2 irreversibly chemisorbed in the CAs [55]. The multicycle profiles observed are consistent with literature [11,18]. To summarize the CO2 capture results, the obtained CAs not only exhibit a high capacity of CO2 adsorption, but also show excellent regeneration.

Table 3. Comparison of the CO2 adsorption of related porous carbons. Sorbent

Adsorption conditions

Types of material

SBET m2 g-1

CO2 uptake mmol g-1

Reference

Carbon aerogel (800 oC)

25 oC, 1 bar

Chitosan and Polybenzoxazine

679

5.72

Present study

Carbon aerogel (800 oC)

0 oC, 1 bar

Chitosan and Polybenzoxazine

679

~6.70

Present study[a]

Carbon spheres (800 oC)

0 oC, 1 bar

Chitosan

384

~3.50

[24]

Carbon spheres (800 oC, KOH)

0 oC, 1 bar

Chitosan

969

~6.00

[24]

Carbon spheres (800 oC)

0 oC, 1 bar

Alginate

469

~5.00

[24]

Carbon material (800 oC)

0 oC, 1 bar

Furfuryl alcohol/ zeolite

3360

6.92

[16]

Carbon material

25 oC, 1 bar

Furfuryl alcohol/ zeolite

3360

4.38

[16]

80 oC, 1 bar

Polyethyleneamine 246 / chitosan/silica

3.8

[26]

(900 oC) Carbon spheres (800 oC) [a]

The adsorption capacity was empirically obtained at 0 oC from the values at 25, 50, 75, and 100 oC.


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A comparison of adsorption capacity of CO2 for various biopolymer systems reported in the literature is presented in Table 3. Assuming that the trend observed in the temperature range from 25 to 100 oC can be empirically obtained at 0 oC, the CO2 adsorption of the aerogel MMTCA-1 is estimated to be 6.7 mmol g-1 at 0 oC and 1 bar. Under similar conditions, the developed sorbent exhibits one of the best CO2 adsorption performance compared to many other types of biopolymer-based sorbents reported in the literature.

Analysis of Adsorption Isotherm Models The simplest and most commonly used adsorption isotherm models for CO2 adsorption study are Langmuir and Freundlich models which are defined in Eq. 1 and Eq. 3, respectively. These models were used to correlate the experimental isotherm data of CO2 adsorption [1,8]. ‫=ݍ‬

ܽ‫݌‬ … … … … … … … … … … . … … … … … … … … … … … … … … … … … … … … … … … … ሺ1ሻ ሺ1 + ܾ‫݌‬ሻ

where q is the amount adsorbed related to the pressure (P), and a and b are Langmuir adsorption parameters. The linear form of Eq. 1 is given by Eq. 2. 1 1 ܾ = + … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … . ሺ2ሻ ‫ܽ ܲܽ ݍ‬

‫݌ ݂ܭ = ݍ‬

1ൗ ݊

… … … … … … … … … … . … … … … … … … … … … … … … … … … … … … … … . … … … . ሺ3ሻ

where q is the amount adsorbed related to the pressure (P), n is the heterogeneity parameter, and Kf is Freundlich proportionality constant.



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Table 4. Constants of Langmuir and Freundlich models for CO2 adsorption on the CAs. Sample

Langmuir isotherm model

Freundlich isotherm model

a (mmol g-1 mbar-1)

b (mbar-1)

R2

Kf (mmol g-1 mbar1/n)

n

R2

CA-1

0.043

0.015

0.8949

0.134

2.50

0.9568

MMT-CA-1

0.139

0.024

0.8919

1.70

5.72

0.9958

MMT-CA-2

0.052

0.018

0.9001

0.209

2.55

0.9942

The collected data of CO2 adsorption for all CAs were fitted separately with Langmuir and Freundlich isotherms models. The results of the fitting parameters are summarized in Table 4. These parameters provide in-depth information on energetics of the sorbent surfaces [1]. The fit of each model with the experimental data is commonly evaluated by the correlation coefficient R2. According to the regression analysis, the Langmuir model gave a good fit for CA-1; while, Freundlich model describes the adsorption data well for MMA-CA-1 and MMA-CA-2. These results are in good agreement with the structure of the sorbents. Therefore, MMA-CA-1 and MMT-CA-1 show an energetically homogeneous surface of the active sites for CO2 adsorption; whereas presence of clay in CA shows a heterogeneous surface [1].

Thermodynamics of Adsorption To study the adsorption thermodynamics, CO2 adsorption at multiple-temperatures (25, 50, 75 o

C) for all CAs were investigated. The respective parameters of adsorption thermodynamics,

including the free energy, enthalpy, and entropy changes, were obtained from the following equations [9,57]:


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∆‫ = ݋ ܩ‬−ܴ݈ܶ݊‫ … … … … … … … … ݍ݁ܭ‬. … … … … … … … … … … … … … … … … … … … … … … … … ሺ4ሻ

∆‫ ݋ ܪ∆ = ݋ ܩ‬− ܶ∆ܵ ‫ … … … … … … … … … … ݋‬. … … … … … … … … … … … … … … … … … … … … … ሺ5ሻ ݈݊‫ݍ݁ܭ‬

∆ܵ ‫݋ܪ∆ ݋‬ = − … … … … … … … … … … … … … … … … … … … … … … … … … … … … … . … … ሺ6ሻ ܴ ܴܶ

where ∆Ho and ∆So are enthalpy and entropy changes, R is the universal gas constant (8.314 J mol-1 K-1), and T is the absolute temperature (in Kelvin). The equilibrium constant Keq is obtained from model parameter for the data using Langmuir equation [9]. The Gibb’s free energy (∆Go) was calculated from Eq. 4. The values of ∆Ho and ∆So were calculated from the slope and intercept of the van’t Hoff plot of ln Keq against 1/T, as shown in Figure S2 of the Supporting Information [9,57]. The calculated values of thermodynamic properties for the CO2 adsorption on the CAs are summarized in Table 5. The negative free energy values (∆Go) indicate a spontaneous adsorption process [9]. The equilibrium constant (Keq) provides important information of the temperature dependent CO2 interaction with sorbents. In other word, the Keq values indicate that the ratio of the adsorption/desorption rates (ka/kd) for each sorbent. As seen in Table 5, the adsorption rate (ka) is higher than the desorption rate (kd) of CO2 by about 5 - 60 times [57]. The negative values for ∆So indicate decreased randomness at the sorbent/CO2 gas interface during the adsorption process. Also, the negative values of ∆Ηo are compatible with exothermic adsorption of CO2 [9]. Figure 7 shows the temperature dependency of CO2 adsorption capacity using obtained carbon aerogels. The results showed that the adsorption capacities were decreased with an increase in the temperatures, which indicated that the adsorption of CO2 using these obtained carbon aerogels was an exothermic process. These observations are good experimental evidence for obtaining the negative ∆Ηo values. These findings are in good agreement with results of thermodynamic studies in literature [9,57]. 23 ACS Paragon Plus Environment


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Table 5. Thermodynamic parameters for the CO2 adsorption. Sample Code

Keq

∆Go

∆Ho

∆So

Qst

(kJ mol-1)

(kJ mol-1)

(J mol-1 K-1)

(kJ mol-1)

CA-1

5.8

-4.17

-8.54

-14.67

-17.2

MMT-CA-1

60.3

-10.33

-26.29

-53.58

-51.4

MMT-CA-2

20.9

-7.31

-18.90

-38.9

-24.6

Among the thermodynamic properties, the isosteric heat of adsorption (Qst) which is defined as the difference between the activation energy for adsorption and desorption, represents the strength of adsorbate–adsorbent interaction [58]. In order to calculate the heat of adsorption on CAs, the CO2 adsorption-desorption cycles were carried out at various temperatures. The isosteric heats of adsorption were calculated using the Clausis-Clapeyron equation as given in Eq. 7. Multiple isotherms at different temperatures (25, 50, 75, 100 oC) were used to evaluate the slope of ln(P) versus (1/T) for the same adsorption amount [2,58,59]. ቎

߲݈݊ܲ ܳ‫ݐݏ‬ ቏= … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … ሺ7ሻ 1 ܴ ߲ቀ ቁ ܶ

The isosteric heat of adsorption (Qst) for the carbon aerogels are listed in Table 5. In the case of CA-1, the value of Qst is 17.2 kJ mol-1, which indicates very weak interactions of CO2 molecules with sorbent-active sites (physisorption). This observation is consistent with our previous report that the interaction between CTS and PBZ reduces the available free amine groups [34]. On the other hand, the maximum value of Qst (~51 kJ mol-1) indicating stronger interactions of CO2 with MMT-CA-1 than that for CA-1, corresponding to chemisorption [2]. The negative values of Qst indicate that the adsorption of CO2 on the CAs is exothermic [7]. 24 ACS Paragon Plus Environment

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Upon increase the PBZ content in the aerogels, Qst decreases to ~ 25 kJ mol-1 demonstrating that a weak interaction was involved. Several reported data obtained for amine-modified porous substrates showed that the values of adsorption heat can range from 20–50 kJ/mol, which indicates a weak chemisorption [2,5,16,44,58,60]. Primo et al. [24] reported that the Qst values of CO2 adsorption using CTS-derived spherical carbons were in the range of 19-35 kJ mol-1. Herein, the obtained values of Qst are in good agreement with CTS-based sorbents for CO2 [8,24]. Very recently, we found that the values of Qst for CO2 adsorption using GO-CTS hybrid monoliths were in the range of 17-27 kJ mol-1 [8]. Low adsorption energy with superior CO2 adsorption capacity is an important goal due to the low energy requirement for the regeneration of CO2 adsorbent [9,18].

CONCLUSIONS Engineered nanoporous carbons are developed with both high adsorption capacities and excellent shape stability for removal of CO2 from gas streams. The products exhibit high surface areas (up to 710 m2 g-1) with excellent adsorption performance (adsorption capacity in the range of 2.21 mmol g-1 - 5.72 mmol g-1 at 25 oC at 1 bar). In addition, the carbon aerogels exhibit better sorption kinetics and regeneration performance. Geometrically more stable sorbents were obtained by increasing the CTS-PBZ cross-linking. The aerogels containing clay had a much more layered structure and higher porosity than without clay, revealing the increased surface area after carbonization. The Langmuir and Freundlich models show good fit for polymeric and nanocomposite carbon aerogels, respectively. The negative values of entropy changes show the interaction randomness at the solid/gas interface upon adsorption of CO2 molecules.



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The results presented here open a new opportunity for the rapid and large-scale industrial preparation of novel biobased porous carbons with high adsorption capacities for CO2 capture at various operating temperatures and can offer multicyclic adsorption-desorption stability.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Fabrication and optical images of the obtained nanocomposite aerogels; calculations of Micropore Volumes (Vmic); Adsorption thermodynamics.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Almahdi Alhwaige), [email protected] (Syed Qutubuddin). Tel: +1(216) 368-2764, Fax: +1(216) 368-3016.

Author Contributions The paper was written through contributions of all authors. All authors have given approval to the final version of the paper.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Authors gratefully acknowledge the Libyan Ministry of Higher Education for scholarship in the form of national financial for Amahdi Alhwaige to study abroad (PhD degree) in the USA. We thank Prof. David Schiraldi, Case Western Reserve University, for his kind help in the use of his freeze-drying equipment. 26 ACS Paragon Plus Environment

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Table of Contents (TOC) Graphic "For Table of Contents Use Only." Carbon Aerogels with Excellent CO2 Adsorption Capacity Synthesized from Clay-Reinforced Biobased Chitosan-Polybenzoxazine Nanocomposites Almahdi A. Alhwaige, Hatsuo Ishida, Syed Qutubuddin

For the first time, carbon aerogels derived from clay-reinforced biobased chitosanpolybenzoxazine, showed excellent performance in CO2 capture.


Carbon Aerogels with Excellent CO2

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