Cite this: RSC Advances, 2013, 3, 16011 Biobased chitosan hybrid aerogels with superior adsorption: Role of graphene oxide in CO 2 capture3 Received 24th April 2013, Accepted 1st July 2013 DOI: 10.1039/c3ra42022a www.rsc.org/advances Almahdi A. Alhwaige,{ ab Tarek Agag, c Hatsuo Ishida b and Syed Qutubuddin* ab Currently extensive research is focused on developing and designing novel porous materials for clean energy and environmental applications such as reducing the emission of carbon dioxide (CO 2 ). In this work, hybrid monolith aerogels of chitosan (CTS), an environmentally-benign biopolymer, with different amounts of graphene oxide (GO) are prepared using freeze-drying. The sorption performance of the developed aerogels for CO 2 capture is studied. The aerogels are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and nitrogen adsorption– desorption measurements. Homogeneous dispersion of GO in CTS is studied beyond the particle concentration where agglomeration takes place. The effects of GO on the specific surface area of the aerogels and CO 2 capture are investigated and shown to increase with GO content. The BET surface area is dramatically increased from 153 to 415 m 2 g 21 by loading 20 wt% GO into the CTS adsorbent. The amount of CO 2 adsorbed at 25 uC increases from 1.92 to 4.15 mol kg 21 with the addition of 20 wt% GO. Adsorption–desorption cycles exhibit the stability of the hybrid aerogels during prolonged cyclic operations, suggesting excellent potential for CO 2 capture technology. Introduction Gas purification represents a major challenge in a time of concerns for growing air pollution based on emission of gases (especially toxic and greenhouse gases (GHG)) from various industrial sources. 1–3 In particular, the subject of carbon dioxide (CO 2 ) capture, utilization, and storage (CCUS) has received widespread attention because of the interest in reducing the amount of released CO 2 as a greenhouse gas (GHG). 2–9 Carbon dioxide is considered by some to be the most important contributor to the increase in average temperatures since the mid-20th century. 10 The amount of CO 2 present in the atmosphere contributes to 60% of global warming effects. 11,12 Currently, one-fourth of the energy needed in the world comes from natural gas, and it is expected to grow by 50% over the next 20 years. 3,13 As an extreme of impurities present in a natural gas source, the effluent natural gas from a well may contain 4–50% CO 2 . 14,15 Many CO 2 capture technol- ogies, such as absorption, cryogenic, adsorption, and mem- branes, have been investigated. 2,4,7 The selective and efficient capture and separation of CO 2 has been given much attention due to the economical and energy impacts. 16 Because CO 2 is an acidic gas, basic groups such as amino-functional groups can be active sites for CO 2 sorption. 3,10,17–19 Polymers with high content of nitrogen atoms, such as primary, secondary, and tertiary amine groups, are good candidates for affinity towards CO 2 . 19,20 Liquid amines have been widely used for a long time to absorb CO 2 from flue gas in aqueous solutions. Unfortunately, liquid amines are highly corrosive toward equipment and pipelines, and thereby complicate their maintenance. In addition, regeneration of liquid amine is highly energy intensive due to the high heat capacity of liquid amine and necessity for large amount of water. 20,21 Therefore, at present, adsorption on regenerable porous solids including activated carbon, zeolites, silica, hybrid crystalline solids, single-walled carbon nanotubes, and metal–organic frame- works is considered as a potential alternative to liquid absorption for capturing CO 2 from gas mixtures. 3,20 Adsorption on solid surfaces enjoys high efficiency, selectivity, and cost-effectiveness. 3,22–27 There has been growing interest in developing new porous solids that have high efficiency for capturing large quantities of CO 2 and ability to be regenerated with low energy. 7,8,12 The efficiency of these porous sorbents for CO 2 capturing may be improved by promoting chemisorp- tion through impregnation or immobilization of chemicals that react reversibly with CO 2 , such as amines. 10,23 During recent years, solid amine sorbent surfaces have been intensively researched for adsorption of CO 2 . 3,8,16,23 Amine-enriched sorbents can be obtained by (i) covalently a Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7217, USA. E-mail: [email protected]b Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA c Lord Corporation, Erie, PA 16509, USA 3 Electronic supplementary information (ESI) available: Detailed GO preparation and discussion of the confirmation results for GO synthesis. See DOI: 10.1039/ c3ra42022a { On leave from El-Mergib University, Libya ([email protected]). RSC Advances PAPER This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 16011–16020 | 16011 Published on 02 July 2013. Downloaded by Brown University on 06/09/2013 14:07:55. View Article Online View Journal | View Issue
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Cite this: RSC Advances, 2013, 3,16011
Biobased chitosan hybrid aerogels with superioradsorption: Role of graphene oxide in CO2 capture3
Received 24th April 2013,Accepted 1st July 2013
DOI: 10.1039/c3ra42022a
www.rsc.org/advances
Almahdi A. Alhwaige,{ab Tarek Agag,c Hatsuo Ishidab and Syed Qutubuddin*ab
Currently extensive research is focused on developing and designing novel porous materials for clean
energy and environmental applications such as reducing the emission of carbon dioxide (CO2). In this
work, hybrid monolith aerogels of chitosan (CTS), an environmentally-benign biopolymer, with different
amounts of graphene oxide (GO) are prepared using freeze-drying. The sorption performance of the
developed aerogels for CO2 capture is studied. The aerogels are characterized by X-ray diffraction (XRD),
scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and nitrogen adsorption–
desorption measurements. Homogeneous dispersion of GO in CTS is studied beyond the particle
concentration where agglomeration takes place. The effects of GO on the specific surface area of the
aerogels and CO2 capture are investigated and shown to increase with GO content. The BET surface area is
dramatically increased from 153 to 415 m2 g21 by loading 20 wt% GO into the CTS adsorbent. The amount
of CO2 adsorbed at 25 uC increases from 1.92 to 4.15 mol kg21 with the addition of 20 wt% GO.
Adsorption–desorption cycles exhibit the stability of the hybrid aerogels during prolonged cyclic
operations, suggesting excellent potential for CO2 capture technology.
Introduction
Gas purification represents a major challenge in a time ofconcerns for growing air pollution based on emission of gases(especially toxic and greenhouse gases (GHG)) from variousindustrial sources.1–3 In particular, the subject of carbondioxide (CO2) capture, utilization, and storage (CCUS) hasreceived widespread attention because of the interest inreducing the amount of released CO2 as a greenhouse gas(GHG).2–9 Carbon dioxide is considered by some to be the mostimportant contributor to the increase in average temperaturessince the mid-20th century.10 The amount of CO2 present inthe atmosphere contributes to 60% of global warmingeffects.11,12 Currently, one-fourth of the energy needed in theworld comes from natural gas, and it is expected to grow by50% over the next 20 years.3,13 As an extreme of impuritiespresent in a natural gas source, the effluent natural gas from awell may contain 4–50% CO2.14,15 Many CO2 capture technol-ogies, such as absorption, cryogenic, adsorption, and mem-branes, have been investigated.2,4,7 The selective and efficient
capture and separation of CO2 has been given much attentiondue to the economical and energy impacts.16 Because CO2 isan acidic gas, basic groups such as amino-functional groupscan be active sites for CO2 sorption.3,10,17–19 Polymers withhigh content of nitrogen atoms, such as primary, secondary,and tertiary amine groups, are good candidates for affinitytowards CO2.19,20 Liquid amines have been widely used for along time to absorb CO2 from flue gas in aqueous solutions.Unfortunately, liquid amines are highly corrosive towardequipment and pipelines, and thereby complicate theirmaintenance. In addition, regeneration of liquid amine ishighly energy intensive due to the high heat capacity of liquidamine and necessity for large amount of water.20,21 Therefore,at present, adsorption on regenerable porous solids includingactivated carbon, zeolites, silica, hybrid crystalline solids,single-walled carbon nanotubes, and metal–organic frame-works is considered as a potential alternative to liquidabsorption for capturing CO2 from gas mixtures.3,20
Adsorption on solid surfaces enjoys high efficiency, selectivity,and cost-effectiveness.3,22–27 There has been growing interestin developing new porous solids that have high efficiency forcapturing large quantities of CO2 and ability to be regeneratedwith low energy.7,8,12 The efficiency of these porous sorbentsfor CO2 capturing may be improved by promoting chemisorp-tion through impregnation or immobilization of chemicalsthat react reversibly with CO2, such as amines.10,23
During recent years, solid amine sorbent surfaces havebeen intensively researched for adsorption of CO2.3,8,16,23
Amine-enriched sorbents can be obtained by (i) covalently
aDepartment of Chemical Engineering, Case Western Reserve University, Cleveland,
Ohio 44106-7217, USA. E-mail: [email protected] of Macromolecular Science and Engineering, Case Western Reserve
University, Cleveland, OH 44106-7202, USAcLord Corporation, Erie, PA 16509, USA
3 Electronic supplementary information (ESI) available: Detailed GO preparationand discussion of the confirmation results for GO synthesis. See DOI: 10.1039/c3ra42022a{ On leave from El-Mergib University, Libya ([email protected]).
RSC Advances
PAPER
This journal is � The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 16011–16020 | 16011
bonding an amine to a support, or (ii) wet impregnation i.e.immobilizing liquid amines within the pores of a sup-port.3,8,20,25 For example, studies of CO2 capture were reportedusing tetraethylenepentamine,4,8,24,28 aminopropyltriethoxysi-lane,7,9 3-aminopropyl-trimethoxysilane,11 polyethylenei-mine,4,12,20,25,29 monoethanolamine,16,30 ethanolamine,30 anddiethanolamine.31 However, a reduction in CO2 capturecapacities was observed after several cycles, which is attributedto the leaching of the physisorbed, low molecular weightamines from the support.28 Alternatively, porous carbonadsorbents with high nitrogen content can be developeddirectly from nitrogen-rich precursors, such as copolymer ofresorcinol, formaldehyde, and lysine17 and primary benzylamine.32
Porous materials have recently gained much interest due totheir wide range of applications in the fields of medicine,petrochemical industry, tissue engineering and environmentalprotection.33–35 An aerogel is a continuous pore material withultra-low density and high surface area that has been recentlyinvestigated for CO2 adsorption.36 Aerogels are obtained bydrying gels using various techniques, such as freeze drying andCO2 supercritical drying.33,35,37,38 The freeze-drying techniqueis an attractive method because water is used as solvent tosafely produce porous materials.34 Gelation can occur throughphysical cross-linking, such as poly(vinyl alcohol)-clay aero-gels,37 or chemical cross-linking, such as polysaccharide-basedaerogels.34,39 Since Kistler40 explored aerogels from polysac-charides, these materials have found many potential applica-tions. These include solid supported catalysis, hydrogen fuelstorage, supercapacitors, gas diffusion electrodes, watertreatment, insulation, and filters. Their advantages includeultra-low density, extremely large surface area, and openporosity that allows loading with active compounds.35,39
Porous carbon materials are obtained by pyrolysis oforganic aerogels in an inert atmosphere such as nitrogen orargon to produce carbon materials containing macropores andmicropores with extremely high surface areas.15,35 Examplesfor such materials include poly(benzoxazine-co-resol) carbonparticles,16 chitin carbon aerogel,38 and resorcinol-formalde-hyde, melamine-formaldehyde, and combined aromatic-ali-phatic-diamine carbon aerogels.41 Carbonization is aneffective method for generating porous polymeric materi-als.35,41 However, the nitrogen content decreases with increas-ing pyrolysis temperature while the surface area of pyrolyzedmaterial increases up to a certain temperature; thus, anoptimal heat treatment should be applied to achieve porousmaterials with high nitrogen content and surface area that areefficient CO2 sorbents.17,41 Development of porous carbonsfrom renewable sources for CO2 capture has been reported.21
The chemical activation of hydrothermally carbonized poly-saccharides (starch and cellulose) and biomass (sawdust)shows promising results for CO2 capture. Furthermore, theresulting N-doped activated carbons prepared from beandregs, a biomass waste, showed high performance for CO2
capture at ambient conditions.5 In this work, bio-basedchitosan sorbents for CO2 capture will be discussed.
Among the natural polymers enriched with amine groups,chitosan has been used for the past few decades in variousapplications such as, for waste water treatment, drug delivery,and heterogeneous catalysis, due to its advantages, such aslow-cost, environmental friendliness and useful functionalgroups, –NH2 and –OH.34,42–44 On the other hand, the physicaland mechanical properties of chitosan are insufficient for awide range of applications. Furthermore, chitosan is notsuitable to be used as adsorbents because its flake and powderforms have low surface area and there is no porosity. Thepreparation of hybrid porous materials through incorporationof nanofillers, such as graphene, graphene oxide, carbonnanotubes, carbon black, and clay, is an effective approach forimproving the physical and mechanical properties of chit-osan.44
Xie et al.43 were the first to study chitosan for CO2 removalusing ionic liquid of 10% wt chitosan/1 L solution under mildconditions (30 uC, 1 atm CO2). Chitosan is soluble only inacidic solutions to form polyelectrolyte which may be highlycorrosive toward equipment and pipelines. The gels areviscous and hence inconvenient for processing. Chitosansolutions exhibit high level of gelation even at low concentra-tion (y2 wt%). Furthermore, the reactive functional groupsmay be affected by acid which influences inter- and intra-molecular hydrogen bonding. Therefore, a new processingstrategy for developing porous sorbents based on chitosan isneeded to mitigate these drawbacks for CO2 capture.
Graphene is a monolayer of sp2-hybridized carbon atomsarranged in a two-dimensional lattice.45,46 Graphite, whichconsists of a stack of flat graphene sheets, is an inexpensivenatural resource available for production of graphene andgraphene oxide.46–48 Graphene oxide (GO) is a layered materialobtained by the treatment of natural graphite using strongmineral acids and effective oxidizing agents, which provide avariety of oxygen-based chemical functional groups to thebasal planes and edges of graphene.46–51 These functionalmoieties, such as epoxide, hydroxyl, and carboxylic groups,increase the spacing between the graphene layers from about0.34 to 0.6–1.2 nm depending on the oxidation efficiency andthe hydration level.49 Due to the presence of oxygen-basedfunctional groups, graphene oxide is an interesting materialthat has high dispersibility in water, unlike graphite.47,48 It isalso a nanofiller with functional groups available on the basalplane for compatibilization and reinforcement. Very recently,Wood et al.52 theoretically investigated the binding of CO2 withfunctional groups of graphene oxide. The results indicate thatgraphene oxide may be a good candidate for CO2 capture.
Polymer nanocomposites with exfoliated inorganic layershave been widely reported, for example.46,53 Polymers withfunctional groups that enhance the interactions with grapheneoxide can be used to form high-performance nanocompo-sites.44,46,54 The forces between the polymer and grapheneoxide are primarily dipole–dipole interactions and/or hydrogenbonding. These arise from the polar groups in the polymer andthe functional groups of COC, COOH, and COH on the surfaceof graphene oxide. For example, graphene-based nanocompo-
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sites were reported using poly(allylamine),55 poly(vinyl alco-hol),56 epoxy,45 and polystyrene-polyacrylamide copolymer.47
In addition, colloidal systems of chitosan-functional GO havebeen studied for metal-ions removal,34 drug and genedelivery,54,57 and bio-sensors.58,59
Development of carbon-based aerogels from graphene andGO have been reported with and without polymer binder.60,61
Polyvinyl alcohol (PVA) was added to the graphene oxide (GO)dispersion in water to achieve gelation.60 Freeze-drying andcritical point drying of GO-PVA gels resulted in highly porousnetworks having good mechanical integrity. Chitosan–GOorganic aerogels for metal-ion removal have also beenstudied;34 however, CO2 capture and effect of pyrolysis onCTS–GO hybrid aerogels have not yet been reported.
Although efficient CO2 sorbents can be obtained by highloading of low molecular weight amines onto a support, thelongevity of such a material appears limited due to leaching ofthe amines from the support.4,32 Polymeric sorbents based onamine-rich polymers, such as chitosan, are not efficient foradsorption applications due to material limitations. Herein, itis demonstrated that dispersing GO into chitosan matrix, inthe form of aerogel, leads to increased surface area withsubsequent increase in CO2 sorption. Addition of GO is furtherexpected to enhance the mechanical and physical properties ofthe aerogel. It is the purpose of this paper to develop GOreinforced chitosan aerogels using freeze-drying technique,followed by detailed study of the adsorption–desorptionbehavior of CO2 by CTS–GO hybrids.
Experimental
Materials
Graphite powder (micro 850) was kindly provided by AsburyGraphite Mills, Inc. Chitosan, CTS, (¢98% deacetylated, fromshrimp, Mw y 200 000–300 000, used without further pur-ification) was purchased from Aldrich. Concentrated sulfuricacid (H2SO4, 98%), potassium permanganate (KMnO4 crystal),hydrogen peroxide (H2O2, 30% aqueous solution), concen-trated hydrochloric acid (HCl), and glacial acetic acid(CH3COOH) were purchased from Fisher Scientific company.
Development of CTS–GO aerogel
Graphene oxide (GO) was synthesized from natural graphitepowder by modified Hummer’s method.34,50 GO, especially theone synthesized from natural graphite, can be easily dispersedin water and forms a stable colloidal dispersion.48,50,62 Thehybrid aerogels were developed from aqueous gels containinga total of 3 wt% solid content and various CTS–GO ratios.Samples are abbreviated as CTS–GO-x where x is the variableGO loading of 0, 1, 3, 5, 10, 15, and 20 wt%. For example, CTS–GO-5% sample was prepared by dispersing 30 mg of GO in 20ml of deionized water using sonication (10 min), leading to GOdispersion in water.44,63 Acetic acid was added to thedispersion to achieve 1% (v/v) aqueous solution and themixture stirred using magnetic stirrer at 300 rpm at roomtemperature. Subsequently, 570 mg of chitosan was added to
the mixture. The solution was then stirred at 300 rpm for 1 h,followed by sonication for 10 min. The solution was furtherstirred using a mechanical stirrer at 2000 rpm for 30 min,followed by sonication for 30 min to remove any bubbles. Thehomogeneous colloidal suspension was poured into a glassvial and aged for two days. The gel was then frozen usingethanol and dry ice at 270 uC and ambient pressure. Finally,the frozen gel was transferred to a VirTis AdVantage@EL-85freeze-drier for five days. Thus, the aerogel was obtained. Forcalcination of the adsorbents, calcination of all aerogels wascarried out up to 400 uC under nitrogen and at a temperatureramp rate of 10 uC min21.
Measurements
The X-ray diffraction (XRD) patterns of the CTS–GO hybridaerogels were measured using a Bruker GADDS diffractometerwith an area detector operating under a voltage of 40 kV and acurrent of 40 mA using Cu-Ka as the source (wavelength l =0.15418 nm).
The thermal stability of the organic aerogels was evaluatedby thermogravimetric analysis (TGA) using a TA InstrumentsHigh Resolution 2950. The TGA thermograms were obtainedin the temperature range of 25 to 900 uC under nitrogen andwith ramp rate of 10 uC min21.
The effects of GO loading and pyrolysis on the surfacemorphology of chitosan aerogel were visualized and evaluatedvia Scanning Electron Microscopy (SEM) at an accelerationvoltage of 15 kV.
The apparent bulk density (rb) of the aerogel was obtainedfrom the aerogel mass (m) and its volume (V) as described ineqn (1).
rb~m
V(1)
The values of skeletal density (rs) were obtained by usinghelium in gas-displacement pycnometry. The apparent poros-ity percentage of the aerogels was calculated according to thefollowing relation.34,38
Porosity%~ 1{rb
rs
� �|100 (2)
Nitrogen adsorption–desorption measurements and assess-ment of the CO2 capture capacities and regeneration potentialof the sorbents were carried out using Intelligent GravimetricAnalyzer (IGA), Hiden Isochema. The adsorption results wererecorded based on the change of the sorbent mass atequilibrium as a function of corresponding pressure whilethe temperature was fixed (isothermal adsorption).
Adsorption experiments
The pore characteristics of the samples were determined bynitrogen adsorption method. Nitrogen adsorption–desorptionmeasurements were performed at 2196 uC by gravimetricmethod as reported.31 Typically, before introducing nitrogen,the samples were heated to 150 uC for 4–6 h under highvacuum (1 6 1025 millibar). Both N2 adsorption and
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desorption measurements were carried out using a bath ofliquid nitrogen to cool the adsorbate temperature to 2196 uC.The Brunauer–Emmett–Teller specific surface area (SBET) wascalculated from the linear part of the BET plot at P/Po in therange of 0.05–0.25. The total pore volume (Vp) and the averagepore size (Dp) were determined from the pore size distributiondata.
Carbon dioxide (CO2) sorption was determined by a static orgravimetric method carried out using IGA at various tempera-tures for all samples. Prior to each adsorption experiment, thesample was degassed at least for 4 h at 150 uC under highvacuum pressure (1 6 1025 millibar) to ensure that theresidual moisture was removed and fully dry sample wasachieved. Before the device was cooled to the adsorptiontemperature (25, 50, or 75 uC), it was set as dry mass, followedby the introduction of CO2 into the system. The CO2
adsorption capacity in terms of adsorbed mass underoperating conditions was recorded.
Results and discussion
Structure and thermal properties of the hybrid aerogels
Dispersion of GO into chitosan matrix was studied using X-raydiffraction (XRD). XRD patterns were obtained to investigate ifGO was exfoliated or intercalated. Fig. 1 displays the XRDdiffractograms for natural graphite, neat GO, pristine CTS, andCTS–GO aerogels. The XRD diffractogram of the naturalgraphite shows a characteristic (002) peak located at 2h =26.4u, corresponding to a d-spacing of 0.337 nm. GO exhibiteda (001) diffraction peak at 2h = 10.70u, indicating the d-spacingwas increased to 0.826 nm. This increase of interlayer spacingis a clear indication for the transformation of graphite toGO.49,50,57
The dispersion of GO into CTS matrix has been studied forCTS–GO hybrid aerogels. XRD of neat chitosan (CTS) shows acharacteristic peak at about 2h = 20u, which is attributed to thecrystalline structure of chitosan.34,63 This peak weakened athigher GO loading (10% GO and higher), indicating that thechitosan turned into more amorphous structure. XRD of CTS–GO with low GO loading up to 10% show disappearingdiffraction peak of the GO at 2h = 10.70u with a very weak andbroad peak between 6u and 7u. However, samples with 15%and 20% GO show a clearly defined peak near 6u, which isattributed to the formation of intercalated GO sheets. Theseresults imply that GO sheets with low nanofiller loading up to10% are partially exfoliated in the chitosan matrix, andintercalation of some GO layers is observed.46 The exfoliationof GO implies strong interactions between the functionalgroups of GO and chitosan, which lead to formation ofhomogeneous colloidal system. Homogenous dispersion ofthe nano-layers is expected to result in significant improve-ment in mechanical properties of CTS–GO hybrid aerogels.44
In nanocomposite studies, significant agglomeration ofnanofiller is typically reported above 3–5% concentra-tion.46,53,64
Morphological properties of the aerogels were studied usingscanning electron microscopy (SEM). Aerogels exhibited a well-
developed highly porous structure. Fig. 2 shows the SEMmicrographs of neat CTS and that containing 20 wt% GO(CTS–GO-20%).
Samples containing GO have more overlapping layers in themorphology than GO-free aerogels.34 This increase in thenumber of layers34 and the covalent bonding between chitosanand GO54,57 may lead to the observed increase in the BETsurface area with increasing GO content. GO seems to have asignificant influence on the morphological characteristics ofhybrid aerogels.34 Nanofillers directly affect pore structure,surface roughness and porosity of porous materials.22
Furthermore, calcination and carbonization enhance the porestructure of materials that could develop porous interconnec-tivity monolithic carbon aerogels.35,65 In this paper, theinfluence of calcination on porous structure of monolithCTS–GO aerogels is presented. Fig. 2b and 2c show that aftercalcination, the produced sorbents exhibit narrow pores onthe surface. The increased porosity of aerogels can be observedon the surface of the layers after the calcination possibly dueto the creation of very small pores without significantlyaltering the aerogel structure.
Fig. 3 illustrates the TGA thermograms of pristine chitosanat room temperature, neat chitosan aerogel, and CTS–GOhybrid aerogels after overnight heat treatment up to 100 uC.The thermal degradation of pristine chitosan at roomtemperature is different compared to the chitosan and CTS–GO aerogels. The first weight loss is observed from 50 to 125 uCdue to the volatilization of moisture and of residual aceticacid. The weight loss in the range of 120–250 uC is attributedto the loss of the adsorbed CO2 and the decomposition of thelabile oxygen-containing groups.34,57 This loss decreases withincrease in GO loading. For all samples, the significant suddenweight loss between the temperatures 250 and 400 uCrepresents the decomposition of the glucopyranose ring ofchitosan57 and also removal of hydroxyl and carboxyl groupsfrom GO.66
Fig. 1 XRD patterns of pristine graphite, neat GO, and chitosan aerogels withdifferent GO loading.
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The TGA results show that the CTS–GO aerogels are morethermally stable than the neat chitosan aerogel. The positiveeffect of nanofillers on the thermal stability of polymers thatare unstable is well known.46,67 Table 1 lists the residualweight of the CTS–GO aerogels at 200, 400, 600, and 800 uC.The highest weight loss occurs during heat treatment in therange of 300–400 uC, which corresponds to higher porosity andsurface area. The effect of calcination on the stability of GO inthe hybrid aerogels has been investigated by Raman spectro-scopy. The Raman spectrum of CTS–GO-10% shows amor-phous structure of CTS–GO hybrids. There is no signature forgraphene as observed in Fig. S1, ESI.3 The ratio of D and Gbands reveals that GO remains in the matrix after treatmentup to 400 uC.
The apparent density (rb) and porosity of the organicaerogels as listed in Table 2 are in the range of 0.038–0.074 g
cm22 and 93.9–96.2%, respectively. These results indicate thehighly porous structure of the hybrid aerogels. Nitrogen (N2)adsorption–desorption is a technique widely used to investi-gate the structural characteristics of porous materials.68 TheN2 adsorption–desorption isotherms at 2196 uC were used tostudy the aerogels. CTS–GO organic aerogels have low BETsurface area (16–40 m2 g21) depending on the GO content. Toincrease the surface area of the samples and generatemicroporous sorbents, all aerogels were calcined up to 400uC under N2 and temperature ramp rate of 10 uC min21. Theexperimental conditions were based on its thermal decom-position profiles as shown in Fig. 3. Table 2 shows thesummary of the properties of the adsorbents after calcinationat 400 uC. Hereinafter, the aerogels discussed are heat-treatedsamples unless otherwise noted. It is clear that calcination ofthe organic aerogels up to 400 uC leads to increased surfacearea (SBET). However, a decrease of nitrogen content in thesolid wall could also occur due to the partial decomposition ofthe amines.17 The calcined neat chitosan aerogel has a surfacearea of 153 m2 g21, a pore volume of 0.56 cm3 g21, and anaverage pore diameter of 3.28 nm. N2 sorption isothermresults show that the structural properties of neat chitosanaerogel are similar to these of chitin aerogel, as expected dueto similar chemical structure of the two polymers. Theproperties of chitin based organic and carbon aerogels havebeen reported by Tsioptsias et al.38
The effect of GO loading on the BET surface area (SBET) ofthermal-treated chitosan aerogel was studied. As shown inTable 2, the BET surface area increased from 153 to 412 m2 g21
with a loading of 20 wt% GO. The change in the BET surfacearea with GO loading is shown in Fig. 4. The dependence ofadsorption capacity on the surface area is linear as shown inthe insert in Fig. 4. During the thermal decomposition of thecomposite aerogel, the CO2 decomposes from the GO surfacecausing voids and interconnected tortuous pathways. Thisphenomenon for increasing the BET surface area agrees withreports that pyrolysis17,38 and carbon aerogels having a sheet-like nanocarbon-structure59 provide superior surface area.
In general, the pore size (Dp) of the aerogels depends on GO-loading and is in the range of 2.6–6.6 nm. The major change isthe increase in the pore volume (Vp). With 20 wt% GO-loading,the pore size, pore volume, and the surfaces area of the hybridsorbents increased from 3.28 nm, 0.56 cm3 g21, and 153 m2
g21 to 5.14 nm, 1.23 cm3 g21, and 412 m2 g21, respectively.Fig. 3 Thermal behavior of neat chitosan at room temperature and the studiedaerogels after thermal treatment up to 100 uC.
Table 1 Residual mass of chitosan and its hybrid aerogels at differenttemperatures under nitrogen and temperature ramp rate of 10 uC min21
Fig. 2 SEM micrographs of aerogels: (a) neat CTS aerogel before thermaltreatment; (b) and (c) CTS–GO-20% aerogel before and after thermal treatment,respectively; and (d) image (c) at higher magnification.
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CO2 capture capacities. The main purpose of this work is tohighlight the performance of nanostructured CTS–GO hybridaerogels as adsorbents for CO2 capture. The CO2 sorption byCTS–GO aerogels was studied in detail. Fig. 5 shows acomparison of the adsorption behavior using organic andcalcined neat chitosan (CTS–GO-0%) and samples containing5% GO loading (CTS-GO-5%). In the case of organic aerogels,the CO2 capture increased slightly with pressure initially andreached approximately low value of 0.40 and 0.60 mmol g21 forneat CTS and CTS–GO-5%, respectively. However, the adsorp-tion capacity of CO2 using calcined samples abruptly increasedto very high values compared to untreated samples, and thenincreased gradually until it reached the values of 1.92 and 2.78mmol g21 at 100 kPa.
Fig. 4 and Table 2 show the effect of GO loading on the CO2
capture at ambient conditions using calcined samples. TheCO2 uptake capacities of all CTS–GO adsorbents are signifi-cantly higher than that of the neat chitosan (CTS–GO-0%). Forinstance, the adsorption capacity for the sorbent containing 15wt% GO loading (CTS–GO-15%) is 3.96 mmol g21 while thevalue for neat aerogel (CTS–GO-0%) is 1.92 mmol g21 at thesame operating conditions. The presence of GO in aerogelsenhances the interfacial area between CO2 and pore surfaces.69
Furthermore, the functional groups of GO such as –COOH,
–OH, –NH2, –NO2, and –CH3 are good candidates for CO2
binding, as theoretically predicted.52 Such groups can sig-nificantly enhance CO2 binding with respect to a hydrogen-passivated edge, which is primarily attributed to the presenceof a quadruple moment in CO2. The theoretical results are inagreement with experimental data in the literature.70 Herein,the highest CO2 capture capacity of 4.15 mmol g21 wasachieved at 25 uC and 1 bar. This value of adsorption capacitycompares favorably with literature values which are in therange of 3–6 mmol g21 sorbent.29,71
Fig. 6 displays the effect of GO loading and temperature onCO2 capture. The adsorption capacity increases with theamount of GO at a constant operating temperature, while itdecreases with increasing temperature at a fixed loading. CO2
capture by neat CTS and CTS–GO-1% decreases linearly withoperating temperatures; however, it is non-linear for sorbentswith high GO content. Fig. 7 exhibits the effect of pressure onCO2 adsorption isotherm using CTS–GO-10 hybrid sorbent at25, 50, and 75 uC. The CO2 adsorption capacity decreases from3.48 to 2.14 mmol g21 with increasing the operatingtemperature from 25 to 75 uC. This can be explained bydesorption of CO2 at elevated temperatures. Physical adsorp-tion usually has a lower adsorbed amount at elevatedtemperature.21,32,41 The reduction in the adsorption capacityis attributed to the increased motion of CO2 molecules,
Table 2 Structural characteristics of the CTS–GO aerogels
resulting in reduced interactions at the adsorbent surface.Overall, the CO2 adsorption behavior is in agreement withprevious studies.17,21,24,71
The effect of applied pressure on the adsorption behaviorwas also investigated. Fig. 8 shows the typical experimentalbreakthrough curves of CO2 at 25 uC and at pressure up to 100kPa for 1 h. All the sorbents apparently follow similaradsorption behavior. The adsorption capacity of CO2 increaseswith increasing corresponding pressure.17,24,32 This increasedadsorption can be attributed to enhanced transport to thepreviously unavailable pores.30 In addition, the increasedapplied pressure reduces the physical desorption of adsorbedCO2 molecules.
To summarize the CO2 capture results, these biobasedsorbents derived from environmental-friendly, low-cost, andhighly available chitosan exhibit more efficient CO2 capturethan many other sorbents obtained from more expensive rawmaterials.
Apparent heat of adsorption
Isosteric heat of adsorption (Qst) is commonly defined as theenthalpy change on adsorption.8 The values of Qst for CO2
adsorption were calculated from the adsorption isotherms atvarious temperatures using the Clausius–Clapeyron equa-tion:8,11
Qst~DHad~RT2 L ln P
LT
� �n
~{RL ln P
L1=T
" #n
(3)
where R is the universal gas constant (8.314 J mol21 K), T is theabsolute temperature (K), n is the amount of CO2 adsorbed(mmol g21), and P is the pressure (Pa).
The isosteric heats of adsorption were calculated at 25, 50,and 75 uC, using eqn (3). Fig. 9 displays a plot of ln(P) versus (1/T) for the aerogel containing 10% GO. Herein, the isostericheats of adsorption are reported as the average of threecalculations for each sample. Fig. 10 shows a plot of the heatsof adsorption for all samples studied. The heat of adsorption isin the range of around 15 to 27 kJ mol21. The heat ofadsorption and the CO2 adsorption capacity are the lowest forthe neat chitosan while the heat of adsorption is highest for 10wt% GO loading.
The isosteric heat of CO2 adsorption increased from 14.9 ¡
1.8 to 26.3 ¡ 3.2 kJ mol21, while the CO2 adsorption capacityincreased from 1.92 to 3.47 mmol g21 by loading 10 wt% GO.The heat of adsorption and adsorption capacity depend onnitrogen content as well as specific surface area and pore size.It is interesting to note that CTS–GO aerogels show themaximum adsorption heat at 10 wt%, which also correspondsto the maximum in the pore diameter as illustrated in Fig. S3,ESI.3 The increase in the enthalpy is ascribed to stronginteractions between CO2 and the adsorbent active sites(chemisorption).8 The values are in good agreement withrecently reported predictions by Wood et al.52 for CO2
adsorption energy using graphene sheets and edge-functiona-lized graphene. The theory indicates that the CO2 adsorption
Fig. 7 Effect of temperature on the adsorption behavior of CTS–GO-10%aerogel.
Fig. 8 Influence of operating pressure on CO2 uptake by aerogels at ambienttemperature.
Fig. 6 Effect of GO on CO2 uptake by studied samples at various temperatures.
This journal is � The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 16011–16020 | 16017
energy is a function of the permanent dipole moment of thecorresponding functionalized benzene substrate and variesbetween 17 and 25 kJ mol21, close to the values reported here.
Adsorption isotherm models
The mechanism for adsorption of CO2 in the current study islikely chemisorption. According to the International Union ofPure and Applied Chemistry (IUPAC), type I isotherms areobserved in microporous solids having relatively small externalsurface area, where adsorption is controlled by the accessiblemicro-pore volume instead of the internal surface area.68
Therefore, IUPAC type I isotherms have a steep slope at lowpressure. The chemical reaction of CO2 with solid aminesorbents has been reported.23,30,72 The adsorption mechanismconsists of several steps: (i) CO2 diffusion to the surface; (ii)CO2 diffusion into the pores; (iii) interaction with the activesites; and (iv) formation of a product layer over the surface or
pore walls.68 Thus the pore structure of the adsorbent affectsthe type of adsorption observed.
In order to describe the behavior of the adsorptionisotherm, the experimental data of CO2 adsorption at 25 uCwere correlated using the following adsorption models.9,12
Langmuir isotherm model:
Q~aP
1zbP(4)
Freundlich isotherm model:
Q = aP(1/c) (5)
where P (kPa) is the gas pressure, and Q (mmol g21) is the CO2
adsorption amount on the adsorbent, a, b, and c are theparameters of adsorption isotherms. The parameters fitted bythe isotherm adsorption models are given in Table 3.
The regression coefficients (R2) show that the Freundlichisotherm model provides the best fit for the experimental datafor aerogels containing up to 15% GO. This model indicatesthat CO2 probes the adsorbent surface as energeticallyheterogeneous. However, the adsorption behavior of theaerogel containing 20 wt% GO is different and the Langmuirmodel provides a better fit than the Freundlich model whichsuggests that there is homogeneous dispersion of active sitesat high GO content.
Cyclic adsorption–regeneration behavior of sorbents
Regeneration of the adsorbents was investigated as it isessential for long-term operation. Pure CO2 can be producedduring desorption of sorbents using temperature-swing/pressure/vacuum-swing adsorption processes which are usedcommercially in large scale.20,21,32,71,73
The chemical bonds formed by the reactions between CO2
and the amine group are stronger than those in physisorption,requiring elevated temperatures to regenerate the sorbent andrelease the captured CO2.32 Amine-based sorbents are usuallygenerated between 120–170 uC.3 The saturated CO2 adsorbentwas heated up to 130 uC to desorb the CO2, thus completingone cycle of testing for the sorbent. Fig. 11 illustrates therepetitive cycles (adsorption–desorption) of CO2 captureperformance using CTS–GO-10% and CTS–GO-20%. TheCTS–GO aerogels not only exhibit a high capacity of CO2
adsorption, but also show excellent regeneration.Furthermore, the adsorbents are thermally stable. Theseadvantages make CTS–GO hybrid aerogels as one of the strongcandidates for energy-efficient CO2 capture.
Conclusions
Chitosan–graphene oxide (CTS–GO) hybrid aerogels withdifferent compositions were successfully prepared usingfreeze-drying method, and then assessed for CO2 capture byadsorption at different operating conditions. XRD results showintercalation of GO sheets in the chitosan matrix. The SEMimages of the CTS–GO aerogels indicate a very porous and
Fig. 10 The isosteric heats of adsorption and capacities of CO2 capture asfunction of GO loading.
Fig. 9 Plots of ln(P) vs. (1/T) at different CO2 adsorption amounts at lowpressure (,1 atm) for the CTS–GO-10% sample.
16018 | RSC Adv., 2013, 3, 16011–16020 This journal is � The Royal Society of Chemistry 2013
layered structure. Calcination of the hybrids affects the surfaceporosity and pore size depending on the loading of GO. N2
sorption measurements confirm that GO in the presence of CTSnot only influences the morphological characteristics, but alsoenhances the BET surface area of the aerogels. The BET surfacearea also increased significantly with calcination, primarily dueto decomposition of the soft segments and creation of tortuouspaths. This investigation is the first to demonstrate andquantify the strong effect of GO on CO2 capture using CTS–GO hybrid aerogels. The amount of CO2 adsorbed on CTS–GOaerogels doubled with the addition of 20 wt% GO at ambienttemperature and atmospheric pressure. Furthermore, thesesorbents are cost-effective, show easy regeneration and havegood stability over multiple cycles. Finally, the calcination of thefunctionalized chitosan hybrids and the highly porous structureof GO provide strong interactions with CO2. These physico-chemical interactions contribute to the enhanced performanceof the aerogels as a new class of CO2 sorbents.
Acknowledgements
Almahdi Alhwaige thanks the Ministry of Education of Libyafor the financial support in the form of a national scholarship.
The authors acknowledge Prof. D. Schiraldi and Prof. J. Mann,Case Western Reserve University, for their kind help in the useof their equipment.
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