Amine-functionalization of multi-walled carbon nanotubesfor adsorption of carbon dioxide
Meei Mei Gui,1 Yan Xin Yap,1 Siang-Piao Chai1* and Abdul Rahman Mohamed2
1Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, 46150 Bandar Sunway, Selangor, Malaysia2School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia
Received 30 April 2012; Revised 4 September 2012; Accepted 4 October 2012
KEYWORDS: carbon nanotubes; CO2 adsorption; amine-functionalization; thermal treatment
INTRODUCTION
Global warming caused by greenhouse gases has trig-gered the awareness and aroused the tension on findingalternative technologies for carbon capture and storage.CO2 removal is an essential mitigation measure to over-come the rapid acceleration of global warming potentialand also to prevent market failure while exploring theeconomics of stabilizing greenhouse gases concentra-tions in the atmosphere. The current carbon captureand storage technology employs capture of CO2 fromstationary sources and injection of CO2 to undergroundfor long-term storage in a process called geologicalsequestration. Suitable sites for geological sequestrationof CO2 are pore spaces in rock below ground level ofmore than 800m such as depleted oil and gas reservoirsand unminable coal beds.[1]
In a conventional industrial operation, CO2 can bereadily separated from the emission sources usingreversible absorption with amine solution such asmonoethanolamine and diethanolamine. However,applicability and sustainability of this process is still
restricted because of the high operating cost and thegeneration of liquid waste. The use of amine solutionhas caused difficulty in the handling of the liquidabsorbent. Furthermore, the process becomes morecomplicated when involving regeneration and disposalof the liquid absorbents that are volatile. Due to thesereasons, recent research interests are focused on thedevelopment of efficient systems for separating CO2
from emission sources, such as adsorption, membraneseparation and many more. Intensive researches havebeen carried out to develop potential adsorbent materi-als that would effectively adsorb CO2 in a greatercapacity and with lower energy penalty in the regenera-tion process.[2] Various types of materials were reportedto be highly efficient for the adsorption of CO2, includ-ing activated carbon,[3] carbon nanotubes,[4] zeolites,[2,5]
mesoporous silica sorbents,[6] mesoporous sphericalparticles,[7] and mesoporous molecular sieve.[8]
CNTs have been receiving attention enthusiasticallyin exploring their potential in chemical, biologicaland biomedical applications because of their uniquestructural, electronic, optical and mechanical proper-ties. The remarkable physical and chemical propertiesof CNTs have led to various and versatile applicationsnot only as adsorbent support but also other applica-tions such as nanoelectronic devices, polymer compo-sites, chemical sensors, biosensors and many more.
*Correspondence to: Siang-Piao Chai, Chemical Engineering Dis-cipline, School of Engineering, Monash University, Jalan LagoonSelatan, 46150 Bandar Sunway, Selangor, Malaysia. E-mail: [email protected]
ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. (2012)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/apj.1693
The major limitation to employ CNTs for application isdue to the low solubility of CNTs in solvent. This pro-blem can be readily overcome by modification of thesurface of CNTs through covalent and non-covalentfunctionalization.Covalent functionalization schemes allow persistent
alteration of the electronic properties of the tubes, aswell as to chemically tailor their surface properties.In a covalent functionalization, functional groups canbe introduced at the bent parts, the part with defectsor on the sidewall of CNTs structure. Functionalgroups can be strongly attached to the surface of CNTsthrough these defects sides. However, this approachcould damage the surface and electronic structure ofCNTs by creating more defects on the surface ofCNTs.[9,10]
As for the noncovalent functionalization approach, itcan be done by employing relatively small molecules,which contain planar groups that irreversibly adsorb tothe nanotube surfaces by p-stacking forces and alsowrapping larger polymeric molecules around the tubes.Non-covalent functionalization includes ultra-sonica-tion, addition of surfactants and polymer wrapping. Thisapproach is commonly chosen if the electronic structureand specific surface of CNTs are to be retained. [11–13]
CNTs with functionalization appear to be the promis-ing adsorbents, providing a good affinity towards CO2
because of their porous structure, large surface area andexcellent thermal stability. The studies on the develop-ment of CNTs-based adsorbents for CO2, especially theeffects of functionalization conditions on the quality ofthe adsorbents, are still limited. In this study, MWCNTsfunctionalized with amine were investigated experimen-tally. Adsorbents were developed from various functio-nalization conditions with the aim to study the effectsof acid and thermal treatment as well as stirring durationon the quality of the adsorbents developed. Performanceof the adsorbents was also evaluated in a CO2 adsorptiontest rig.
MATERIALS AND METHODOLOGY
Materials and chemicals
Pristine MWCNTs with carbon purity of ca. 66.82%were used in this study. The high resolution transmis-sion electron microscopy image of the pristine MWCNTsis shown in Fig. 1. Sulfuric acid (Riendemanna SchmidtChemical, 95–97%) and nitric acid (R&M Chemicals,70%) were used for the pretreatment of MWCNTs priorto amine-functionalization. 3(aminopropyl)-triethoxysilane(APTS) (purity: >98%, Sigma Aldrich) was used asthe amine functional precursor and toluene (95%purity, PC Laboratory Reagent) was used as solventin the functionalization process.
Functionalization of MWCNTs
Carboxylation of MWCNTs was employed firstlythrough acid pretreatment to introduce carboxyl andhydroxyl groups on the MWCNT surface prior toamine-functionalization. In this step, 500mg ofMWCNTs was stirred with the solution containingHNO3 (30ml) and H2SO4 (90ml) premixed at avolume ratio of 1:3 with the concentration of 5M each,under the reflux condition of 100 �C for 3 h.The amine-functionalization was then carried out on
the pretreated MWCNTs with APTS. The pretreatedMWCNTs were continuously stirred with APTS andtoluene with a volume ratio of 1:9 under various condi-tions as described in Table 1. The mixture was thenfiltered and repeatedly washed with toluene in a vacuumfiltration unit to remove APTS in excess. The solid wascollected and dried overnight in an oven at 105 �C toremove excess solvent. The adsorbents obtained werecharacterized for their chemical and thermal propertiesand lastly the performance of the adsorbents in CO2
adsorption was studied.
Characterization of the developed adsorbents
Fourier transform infrared (FT-IR) spectra were obtainedfrom Nicolet iS10 FT-IR spectrometer using KBr DieModel 129 to determine the type of amine groupsattached on the surface of the MWCNTs. The APTSloading on the adsorbents can be quantified usingthermogravimetric analysis (TGA). TGA of the adsor-bents was performed by heating the adsorbents to800 �C at a heating rate of 10 �C/min in nitrogen environ-ment. Under continuous heating in nitrogen flow, most of
Figure 1. High resolution transmission electronmicroscopyimage showing the structure of the pristine multi-walledcarbon nanotube.
M. M. GUI ET AL. Asia-Pacific Journal of Chemical Engineering
the organic functional moieties, including carboxylic acid,amine precursors and APTS, which are thermallyunstable, are expected to be decomposed, leaving behindthe MWCNTs, which are thermally stable in this thermalcondition. The loading of APTS was then calculated fromthe percentage of weight loss obtained from thethermogram.
Adsorption studies
The adsorption capacity of the developed adsorbents wasfurther investigated in a custom-fabricated packed-bedadsorption system as shown in Fig. 2. The adsorption
system was equipped with mass flow controllers tocontrol the flowrate of the incoming gas at the desiredset point. The adsorbent was packed into the adsorptioncolumn with glass wool and placed in a well-insulatedlow temperature furnace that was being designed tocontrol the temperature of the adsorption process. Theadsorption test was carried out at the temperature of60 �C with the adsorbent loading of 0.3 g. CO2 wascontinuously fed into the adsorption column with nitro-gen used as a carrier gas, CO2:N2 ratio of 5:95(v/v),giving a total flowrate of 200mL/min. The outlet gasfrom the adsorption system was then collected in a gassampling bag, and the gas composition was determined
Table 1. Amine-functionalization conditions and sample code design for this research work.
Sample code
Functionalization conditions
With toluene solvent Stirring duration (h) Reflux duration (h) Reflux temperature (�C)
with a gas chromatography (Agilent 7890A) equippedwith thermal conductivity detector.
Adsorption kinetic model
Kinetic model of the CO2 adsorption was performed torelate the quantity of CO2 adsorbed in the function oftime. Pseudo first-order equation of Lagergren andpseudo second-order were employed in this study.[14,15]
The equations are expressed as follows:
Pseudo first order :@qt@t
¼ k qe � qtð Þ (1)
Pseudo second order :@qt@t
¼ k qe � qtð Þ2 (2)
Where; qt and qe are the adsorption capacity at the timeof t and at equilibrium point, respectively and k is themasstransfer constant for the adsorption. Integration of Eqns(1) and (2) thus give Eqns (3) and (4), respectively:
lnqe � qtqe
� �¼ �kt (3)
1qe � qt
¼ kt þ 1qe
(4)
Linear lines of CO2 adsorption, qt to the function oftime can be constructed by data fitting of the experi-mental data into Eqns (3) and (4), and subsequentlythe mass transfer constant, k, can be determined fromthe slope of the plot.
RESULTS AND DISCUSSION
Effect of acid pretreatment of pristine MWCNTs
The organic groups that presented on the pristine and acidpretreated MWCNTs were studied and the respective FT-
IR spectra were shown in Fig. 3. The peaks observed at3422 and 3448 cm�1 for both pristine and acid pretreatedMWCNTs, respectively are corresponding to the –OHstretching as a result of moisture content absorbed byKBr during the FT-IR analysis. Both pristine and acid pre-treated MWCNTs exhibit peaks at 2930 and 2918 cm�1,corresponding to the presence of –CH2-stretching groups,and these peaks are identical for MWCNTs.Referring to the FT-IR spectrum of acid pretreated
MWCNTs (trace b), a peak at 1697 cm�1 was observed,representing –C=O stretching contributed by –COOHgroup attached on the surface of MWCNTs after theacid pretreatment.[16,17] The presence of –COOH groupcan be further supported by the broadening of the3448 cm�1 peak which corresponds to the –OH contrib-uted by the carboxylic group on the external surface ofMWCNTs.[18] Peaks observed at 1655 and 1638 cm�1
indicate the presence of C=C in the structure ofMWCNTs after acid pretreatment. Meanwhile, the bandsat 1051 and 672 cm�1 correspond to S=O groups fromthe sulfoxide group formed during H2SO4 oxidation.The introduction of carboxylic groups via oxidation withH2SO4 and HNO3 mixture is expected to create defectsin the hexagonal or pentagonal structure of MWCNTswhich act as active sites for the functional groups toattach on during the amine-functionalization process.[19]
Effect of solvent
Amine-functionalization was carried out on the acidpretreated MWCNTs by continuously stirring theMWCNTs at room temperature (ca. 25 �C) for the dura-tions of 1, 5 and 24 h. The experiments were conductedwith and without toluene with the aim to evaluate theeffect of solvent on the functionalization process. Thepresence of amine functional groups on the adsorbentswas investigated with FT-IR. Fig. 4 shows the FT-IRspectra of the adsorbents obtained from the functionali-zation of MWCNTs with APTS without toluene. FromFig. 4, peaks observed at 2361 and 2342 cm�1 can beidentified as CO2 molecules present in the surrounding
Figure 3. Fourier transform infrared spectra of (a) pristine multi-walled carbonnanotubes and (b) acid pretreated multi-walled carbon nanotubes.
M. M. GUI ET AL. Asia-Pacific Journal of Chemical Engineering
atmosphere. Peaks at 1562 and 1057 cm�1 are relatedto the NH2 deformation contributed by the hydrogenbonded amino group and S =O, respectively.[20] Thebands that ranged from 1030 to 1090 cm�1 are resultedfrom overlapping of two different functional groups,which are S =O (at 1051 cm�1) and Si–O–Si (at1030 cm�1).[2,20] The intensity of 1057 cm�1 bandwas observed to be more significant as compared withthe FT-IR spectrum of MWCNTs after acid pretreat-ment, and it is believed to be contributed by Si–O–Sifrom APTS after the amine-functionalization. All threespectra show identical trends although with variedstirring durations, indicating that the stirring durationdoes not significantly affect the effectiveness of theamine-functionalization with the absence of toluene.Figure 5 shows the FT-IR results of the adsorbents
prepared with stirring with toluene. As shown in Fig. 5,the spectra of amine-functionalized MWCNTs showpeaks at 3422, 2917–2849, 1563, 1100 and1033 cm�1 of which corresponding to –OH, stretchingvibration of the alkyl chain, NH2 stretching,[20,21]
–C–NH2[17,22] and Si–O–Si[2,20] lattice vibrations,
respectively. Besides, with the increase of stirringduration from 1 to 24 h, significant increases of peak
intensity within the range 1100–1033 cm�1 (i.e. –C–NH2
and Si–O–Si groups) were noticed. This observationfurther confirms the presence of APTS on MWCNTsafter functionalization.The FT-IR spectra of adsorbents obtained from functio-
nalization with and without toluene were also comparedand shown in Fig. 6. The FT-IR spectra of the adsorbentsobtained from amine-functionalization with toluene exhi-bit higher peak intensity for –C–NH2 and Si–O–Si groupsas compared with the adsorbent obtained from amine-functionalization without toluene at the same stirringduration. This result reveals the importance of solvent inthe amine-functionalization process. Toluene is a com-mon solvent that can readily dissolve APTS and at thesame time disperse CNTs very well. It is believed thattoluene helped to improve the dispersion of MWCNTsin the APTS solution and thus increasing the contact areaof MWCNTs with the solution.
Effect of thermal treatment duringamine-functionalization
The adsorbents were also prepared with thermal treat-ment by refluxing the acid pretreated MWCNTs with
Figure 4. Fourier transform infrared spectra of amine-functionalized multi-walled carbon nanotubes without toluene with stirring durations of: (a) 1,(b) 5, and (c) 24 h.
Figure 5. Fourier transform infrared spectra of amine-functionalized multi-walled carbon nanotubes with toluene with stirring for (a) 1 and (b) 24 h.
Asia-Pacific Journal of Chemical Engineering AMINE-FUNCTIONALIZATION OF MWCNTS FOR ADSORPTION OF CO2
APTS and toluene at 105 �C for the durations of 1, 5and 10 h. The adsorbents obtained were analyzed withFT-IR to study the functional groups available on theadsorbents. Figure 7 shows the FT-IR spectra of thedeveloped adsorbents. It is observed that the trendsfor all three spectra are nearly identical. The broaden-ing of 3433 cm�1 peak is attributed to the N–H stretch-ing frequencies after the amine-functionalization.Peaks at ca. 2920 cm�1 can be related to CH2 stretchingfrom the amine group. NH2 from amine group posses-sing peaks around 1648 and 1637 cm�1 were observedin all three spectra, and appeared to be more significantwith the increase in reflux duration. The NH2 peak is
the most significant for the spectra of the adsorbentobtained from 10 h of reflux. Peaks at 1100 and1030 cm�1 which corresponding to –C–NH2 and Si–O–Si lattice vibrations were also observed, furtherconfirming the presence of APTS on MWCNTs. Theintensity of the peaks of all three spectra increased withthe increase in the reflux duration, indicating a positiveeffect of reflux duration on the APTS loading. Theintensity of the peaks such as –C–NH2, Si–O–Si, and–CH=CH2 were found to be the highest in Fig. 7(c),that was the adsorbent obtained with 10 h of reflux.Figure 8 shows the thermogram of the adsorbent
developed from amine-functionalization with the reflux
Figure 6. Comparison of the effect of toluene on the amine-functionalizationprocess, (a) CNT_APTS-1, (b) CNT_APTS-2, (c) CNT_APTS-3, (d) CNT_APTS-4,and (e) CNT_APTS-5.
Figure 7. Fourier transform infrared spectra of amine-functionalized multi-walled carbon nanotubes at reflux durations of (a) 1, (b) 5, and (c) 10 h.
M. M. GUI ET AL. Asia-Pacific Journal of Chemical Engineering
for 5 h. As shown in Fig. 8, three regions of weight losscan be observed. Approximately 3.75wt% of loss wasobserved in the first region (<100 �C), indicating theloss of moisture content and possible volatile compo-nents from the adsorbent. In the temperature range of200–350 �C, approximately 3.75wt% loss wasobserved, and it is attributed to the volatile compounds.The major weight loss observed in the temperaturerange of 400–800 �C indicates the decomposition ofAPTS from the adsorbent. The loading of the APTSwas quantified to be 12.5wt%.Figure 9 shows the comparison of APTS loading
between the adsorbents developed from amine-functionalization with and without reflux for 5 and24 h. Although it is found that the loading of APTSincreases with stirring from 5 to 24 h without heattreatment, the APTS loading is still low as comparedwith the adsorbent prepared with the reflux even for ashorter duration of 5 h. Figure 10 shows the thermogramof adsorbents obtained from the reflux for 1, 5 and 10hand Table 2 summarizes the APTS loading of theserespective adsorbents estimated from the thermograms.
From the thermograms, the APTS loading was foundto increase from 10.5 to 12.3 and 13.5 wt% with theincrease in the reflux duration from 1 to 5 and 10 h,respectively.
Adsorption studies
The capacity of CO2 adsorption (q, mg/g) over theadsorbent developed in this study was calculated usingEqn (5):
q ¼ 1m
Z t
0
Q� Cin � Ceff
� �dt (5)
Where;m is the mass of adsorbents (g), t is the contacttime (min), Q is the influent flow rate (l/min), and Cin
and Ceff are the influent and effluent CO2 concentra-tions (mg/l), respectively.Figure 11 shows the CO2 adsorption uptake of
selected adsorbents obtained from the adsorption test.A maximum CO2 uptake of 75.4mg CO2/g adsorbentwas achieved with the adsorbent prepared from amine-functionalization in a reflux for 5 h, which reveals thatthe developed amine-functionalized MWCNTs wereprominent for the adsorption of CO2. As shown inFig. 11, the pristine MWCNTs show weak adsorptionof CO2 via physisorption. The adsorption capacity was
75
80
85
90
95
100
0 200 400 600 800
Wei
ght
Per
cent
(w
t %
)
Temperature (oC)
3.75wt% moisture content
3.75wt% volatile compounds
12.5wt%APTS
Figure 8. Thermogravimetric analysis profile for amine-functionalized multi-walled carbon nanotubes with areflux for 5 h.
70
80
90
100
0 200 400 600 800
Wei
ght
Per
cent
(w
t %
)
Temperature (oC)
With reflux (5 hours)
Without reflux (stir 5 hours)
Without reflux (stir 24 hours)
(a)
(b)
(c)
(b)
(a)
(c)
Figure 9. Comparison between adsorbents developedfrom amine-functionalization with and without thermaltreatment at different functionalization durations.
70
75
80
85
90
95
100
0 200 400 600 800
Wei
ght
perc
ent
(%)
Temperature (oC)
Reflux 10 hours
Reflux 5 hours
Reflux 1 hour(a)
(b) (c)(c) (b)
(a)
Figure 10. Thermogram of adsorbents developed fromreflux durations of 1, 5 and 10 h.
Table 2. 3(aminopropyl)-triethoxysilane loading of theadsorbents developed from reflux durations of 1, 5and 10 h.
Reflux duration (h) APTS loading (wt%)
1 10.55 12.310 13.5
APTS, 3(aminopropyl)-triethoxysilane.
Asia-Pacific Journal of Chemical Engineering AMINE-FUNCTIONALIZATION OF MWCNTS FOR ADSORPTION OF CO2
greatly improved after amine functionalization (forCNT_APTS-6, CNT_APTS-7 and CNT_APTS-8)where the CO2 was adsorbed through physisorptionon the surface of MWCNTs and chemisorption viathe reaction between CO2 and amine groups thatattached on the surface of MWCNTs.
Kinetic study
Figure 12 shows the CO2 uptake in the function of timefor all three adsorbents developed in this study, represent-ing CNT_APTS-6, CNT_APTS-7 and CNT_APTS-8. Asshown in the figure, CNT_APTS-7 exhibited faster andgreater CO2 adsorption than that of CNT_APTS-6 andCNT_APTS-8. The mass transfer constant, k, for adsorp-tion using each adsorbent was estimated from Eqn (2) andsummarized in Table 3; meanwhile, Fig. 13 shows theaccuracy of the kinetic models by comparing the data pre-dicted from the models with the data collected from theexperimental works. Referring to Table 3, kinetic modelsdeveloped from pseudo first order of Lagergren equationexhibited better data fitting with the higher R2 value ascompared with the kinetic models developed from pseudo
0
1
2
3
4
5
0 5 10 15
CO
upt
ake,
q (
mm
ol/g
)
time (min)
CNT_APTS-6
CNT_APTS-7
CNT_APTS-8
Figure 12. CO2 uptake (q) for the adsorbents as afunction of time.
Figure 13. Comparison of the experimental data andthe data predicted from kinetic models: (a) CNT_APTS-6,(b) CNT_APTS-7, and (c) CNT_APTS-8.
0
10
20
30
40
50
60
70
80
PristineMWCNTs
CNT_APTS-6 CNT_APTS-7 CNT_APTS-8
33
63.8
75.4 71.5C
O2up
take
(mg
CO
2/g a
dsor
bent
)
Adsorbent
Figure 11. CO2 uptake of selected adsorbents:CNT_APTS-6 (reflux with toluene for 1 h), CNT_APTS-7(reflux with toluene for 5 h) and CNT_APTS-8 (refluxwith toluene for 10 h).
M. M. GUI ET AL. Asia-Pacific Journal of Chemical Engineering
second-order equation. Hence, pseudo first-order kineticmodels were chosen in this reported work.
CONCLUSIONS
Amine groups were effectively introduced to thesurface of MWCNTs via amine-functionalizationwith APTS and toluene. Roles of solvent, stirringtime and reflux duration in the functionalization wereinvestigated experimentally. It was found that solventplayed an important role in the dispersion of aminegroup to ease the functionalization of MWCNTs.Thermal treatment was found to be an importantfactor for the amine-functionalization as higher APTSloading was achieved in the adsorbents developedwith thermal treatment. It was also noted that anincrease in the reflux duration led to an increasein the amine loading. From the TGA, a maximumAPTS loading of 13.75wt% was obtained from thereflux heating with APTS and toluene for 10 h. Thehighest CO2 uptake of 75.4mg CO2/g adsorbentwas recorded for the amine-functionalized MWCNTsdeveloped in this work.
Acknowledgements
The authors would like to thank the funding providedby the Ministry of Higher Education Malaysia for theLong-term Research Grant Scheme (account number:2110226-113-00), Monash University for the MonashInternal Seed Fund (account number: E-4-11) andChemical and Sustainable Process Engineering (CSPE)research group, Monash University Sunway Campus,for the equipment and facilities support.
REFERENCES
[1] Carbon Dioxide Capture and Sequestration, USEPA. [Online].Available: http://www.epa.gov/climatechange/ccs/index.html.[Accessed: 03-Sept-2012].
[2] F. Su, C. Lu, W. Cnen, H. Bai, J. Feng. Sci. Total Environ.,2009; 407, 3017–3023.
