Development of nitrogen enriched nanostructured carbon adsorbents for CO2 capture

Development of nitrogen enriched nanostructured carbon adsorbents for CO2 capture

Journal of Environmental Management 162 (2015) 20e29 Contents lists available at ScienceDirect Journal of Environmental Management journal homepage:...

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Journal of Environmental Management 162 (2015) 20e29

Contents lists available at ScienceDirect

Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman

Research article

Development of nitrogen enriched nanostructured carbon adsorbents for CO2 capture Chitrakshi Goel, Haripada Bhunia*, Pramod K. Bajpai Department of Chemical Engineering, Thapar University, Patiala 147004, Punjab, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 February 2015 Received in revised form 15 July 2015 Accepted 17 July 2015 Available online xxx

Nanostructured carbon adsorbents containing high nitrogen content were developed by templating melamine-formaldehyde resin in the pores of mesoporous silica by nanocasting technique. A series of adsorbents were prepared by altering the carbonization temperature from 400 to 700  C and characterized in terms of their textural and morphological properties. CO2 adsorption performance was investigated at various temperatures from 30 to 100  C by using a thermogravimetric analyzer under varying CO2 concentrations. Multiple adsorptionedesorption experiments were also carried out to investigate the adsorbent regenerability. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the development of nanostructured materials. Fourier transform infrared spectroscopy (FTIR) and elemental analysis indicated the development of carbon adsorbents having high nitrogen content. The surface area and pore volume of the adsorbent carbonized at 700  C were found to be 266 m2 g1 and 0.25 cm3 g1 respectively. CO2 uptake profile for the developed adsorbents showed that the maximum CO2 adsorption occurred within ca. 100 s. CO2 uptake of 0.792 mmol g1 at 30  C was exhibited by carbon obtained at 700  C with complete regenerability in three adsorptionedesorption cycles. Furthermore, kinetics of CO2 adsorption on the developed adsorbents was studied by fitting the experimental data of CO2 uptake to three kinetic models with best fit being obtained by fractional order kinetic model with error% within range of 5%. Adsorbent surface was found to be energetically heterogeneous as suggested by Temkin isotherm model. Also the isosteric heat of adsorption for CO2 was observed to increase from ca. 30e44 kJ mol1 with increase in surface coverage. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Nanostructured carbon Nanocasting CO2 capture Adsorption kinetics Isotherm Isosteric heat of adsorption

1. Introduction Increasing carbon dioxide (CO2) concentration in the atmosphere from the burning of fossil fuels has resulted in the global climate change. Concentration of CO2 has augmented from a value of ~280 ppmv at the preindustrial level to ~395 ppmv at present and is estimated to reach to a level of ~570 ppmv by 2100. Also annual CO2 emissions because of fossil fuel burning has increased to ~38 Gt in 2004 from ~21 Gt in 1970 (Samanta et al., 2012; Wang et al., 2011). Hence, capture of CO2 from large point sources like fossil fuel fired power plants, synthetic and natural gas processing plants has received a great consideration to alleviate the increasing CO2 levels followed by long term storage in the coal and/or oil beds (Drage et al., 2009). Economic sequestration of CO2 needs relatively

* Corresponding author. E-mail addresses: [email protected] (C. Goel), [email protected] (H. Bhunia), [email protected] (P.K. Bajpai). http://dx.doi.org/10.1016/j.jenvman.2015.07.040 0301-4797/© 2015 Elsevier Ltd. All rights reserved.

pure stream from a cost-effective capture process and then compressing it to the high pressures. Various approaches available for CO2 capture are precombustion, post-combustion and oxy-combustion capture. Among these approaches post-combustion capture approach can be retrofitted to the existing power plants and hence has the maximum near-term potential in order to decrease CO2 emissions (Figueroa et al., 2008). Amine based absorption process for postcombustion CO2 capture is the most commonly used technology but at the same time it has large thermal losses and requires high energy consumption. Also the aqueous alkanolamine solutions used in this technology are corrosive in nature and prone to oxidative degradation (Aaron and Tsouris, 2005). Suitable nanostructured materials like adsorbents need to be developed to decrease the cost associated with the CO2 capture process (Baxter et al., 2009). In this regard, various adsorbents such as porous carbons (Sevilla et al., 2011; Wahby et al., 2010), zeolites (Chatti et al., 2009; Siriwardane et al., 2005), metal organic frameworks

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(Li et al., 2012; Millward and Yaghi, 2005) and amine-modified silicas (Belmabkhout et al., 2011; Wei et al., 2010; Zhao et al., 2013) are being considered as prospective candidates for CO2 capture. Amongst these materials, carbon based adsorbents present several advantages such as large surface area, tunable pore structure, high adsorption capacity, low energy for regeneration, hydrophobic character and availability. Nanocasting or templating technique is used for the development of porous carbons with controlled porosity. This technique includes the infiltration of the carbon precursor into the template pores followed by heating in s-Solís and controlled atmosphere and template removal (Valde Fuertes, 2006; Zhao et al., 2010). To enhance the interaction of CO2 with the adsorbent surface, surface modification or incorporation of nitrogen functional groups into the carbon matrix is carried out (Gray et al., 2004; Plaza et al., 2007). Adsorbent surface is modified by impregnation or grafting with amine solutions but these materials lack stable capacities after regeneration because of amine degradation. Nitrogen can be directly incorporated into the adsorbent matrix by using nitrogen rich polymeric materials (melamine, aniline etc.) as precursor with various mesoporous templates like silica (MCM-41 and SBA-15), zeolites etc. (Huang et al., 2011; Pevida et al., 2008; Vinu et al., 2008). Mesoporous carbons were prepared by templating sucrose in the pores of nanoCaCO3 and the effect of template to precursor ratio on textural properties of the carbons was investigated (Xu et al., 2010). Huang et al. (2011) synthesized nitrogen enriched carbon materials by using melamine-formaldehyde resin as precursor and CaCl2 as template. Direct pyrolysis of copolymer of resorcinol and formaldehyde was carried out in presence of lysine to obtain nitrogendoped porous carbon monoliths which exhibited CO2 uptake of

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Fig. 2. Powder XRD patterns of nanostructured carbon adsorbents.

3.13 mmol g1 at 25  C and 100% CO2 flow under static conditions (Hao et al., 2010). In another work, melamine-formaldehyde resin and fumed silica were used as precursor and template respectively to produce nitrogen containing carbon adsorbents (Pevida et al., 2008). At adsorption temperatures of 25  C and 75  C under pure CO2 atmosphere, these materials showed CO2 uptake of 2.25 and 0.86 mmol g1 respectively. Drage et al. (2007) also synthesized

Fig. 1. (a) N2 adsorption (closed symbols) and desorption (open symbols) isotherms, (b) pore size distribution of nanostructured carbon adsorbents.

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carbon materials from urea-formaldehyde (UF) and melamineformaldehyde (MF) resins by chemical activation using K2CO3. MF based adsorbents demonstrated maximum CO2 uptake of 1.03 mmol g1 while UF based carbon showed uptake up to 1.8 mmol g1 at 25  C. Generally synthesis of porous carbon materials include post-synthesis treatments like chemical activation or physical activation in order to improve the pore structure but these process are time consuming and include large energy requirements. Moreover the obtained carbons are investigated for CO2 adsorption under static flow conditions at 0  C or 30  C which are not relevant for CO2 capture from flue gases. In this work, we report the development and characterization of nitrogen enriched nanostructured carbon adsorbents from nitrogen rich polymer as precursor and mesoporous silica template by nanocasting technique keeping in view their application to flue gases. Thermogravimetric studies were carried out to evaluate these materials for CO2 capture at various adsorption temperatures and CO2 concentrations. Three kinetic models were also used to analyze the experimental CO2 uptake data on these adsorbents followed by equilibrium study.

the relative pressure range of 0.05e0.2 was used to calculate the BET (BrunauereEmmetteTeller) surface area (SBET). Pore size distribution and total pore volume (VBJH) were obtained from the adsorption branch using the BarretteJoynereHalenda (BJH) model. X-ray powder diffraction (XRD) patterns were recorded for the synthesized carbons on a PANalytical X'Pert Pro diffractometer operating at 45 kV and 40 mA using CueKa radiation over the 2q range from 10 to 80 . Transmission electron micrographs (TEM) were taken on a Philips CM200 apparatus operating at an accelerating voltage of 200 kV. For specimen preparation, carbon materials were dispersed in toluene on carbon-coated copper grid and then the solvent was allowed to evaporate at room temperature. Fourier transform infrared (FTIR) spectra were acquired on a Perkin Elmer Spectrum 100 FTIR spectrometer in the attenuated total reflection (ATR) mode. The FTIR spectra were obtained over the range of 4000e625 cm1 at a resolution of 4 cm1. Elemental analysis of the samples was carried out on a Thermo Finnigan Flash EA 1112 Series elemental analyzer. The oxygen content was calculated by difference. Nitrogen content of the samples was also measured by Kjeldahl method.

2. Experimental

2.3. CO2 adsorption studies

Mesoporous silica having surface area of 450 m2 g1 and average pore diameter of 3.5 nm was used as hard template and was acquired from M/s Tianjin Chemist Scientific Ltd., Tianjin, China. All the chemicals were of analytical grade and were purchased from M/ s S. D. Fine Chemicals India Ltd. Dry nitrogen and carbon dioxide gases of grade-1 purity (99.999%) and special gas mixtures of CO2 and N2 were procured from M/s Sigma Gases and Services, India.

The CO2 adsorptionedesorption performance of the synthesized carbon materials was assessed by using a thermogravimetric analyzer (TA Q500 TGA). About 20 mg of the sample was placed on the sample pan and heated to 200  C under a flow of dry nitrogen gas at a flow rate of 50 ml min1. It was held isothermally for 2 h to ensure removal of any remaining pre-adsorbed gases or moisture. Subsequently the temperature was decreased to the desired adsorption temperature and the gas flow was switched from N2 to pure CO2 at a flow rate of 50 ml min1. Weight change of the adsorbent was recorded with respect to time to calculate its adsorption capacity. Once the adsorbent was saturated with CO2 adsorbate, desorption study was carried by switching the gas back to N2 and raising the temperature to 200  C. Three adsorptionedesorption cycles were carried out to evaluate the reusability of the adsorbents. For the optimized sample, the effect of CO2 concentration and adsorption temperature on CO2 uptake was studied by varying the CO2 concentration from 10% to 100% by using special gas mixtures of CO2 and N2 and adsorption temperature from 30 to 100  C.

2.1. Synthesis of adsorbent Nitrogen enriched nanostructured carbon adsorbents were synthesized via nanocasting route by templating melamineformaldehyde resin in the pores of mesoporous silica (hard template) followed by carbonization under N2 atmosphere and template removal in the end. In a typical experiment, 46.6 g of melamine was added to 200 ml of 5 wt % methanol solution under continuous stirring and heated to 70  C. 200 ml of 37 wt % formaldehyde solution was added slowly to this reaction mixture in 3 h while maintaining pH in the range of 8e9 by adding potassium carbonate. Next, temperature was increased to 75  C to carry out advanced polymerization and stirring was continued for next 2 h. During this phase, pH was maintained in the range of 7e8 by the addition of sodium hydroxide solution and borax. Finally, the reaction mixture was cooled to room temperature and 6 ml of 48% sulfuric acid solution was added followed by addition of 15 g of mesoporous silica. Templated resin was the cured for 2 h at 60  C and was then allowed to solidify at room temperature. In order to obtain nitrogen enriched nanostructured carbon materials, templated resin samples were placed in quartz tubular furnace and heated to temperatures between 400 and 700  C under N2 environment for 1 h followed by template removal with 40 wt% sodium hydroxide solution and washing with large amount of water. The obtained materials were dried in oven at 100  C and were labeled as MF-x with x referring to the carbonization temperature (400e700  C). 2.2. Characterization of adsorbent Nitrogen sorption isotherms were measured at 196  C on Micromeritics ASAP 2010 volumetric adsorption analyzer. The samples were degassed at 220  C for 6 h under vacuum before adsorption measurements. Nitrogen adsorption isotherm data in

2.4. Adsorption kinetics Various models are available in the literature that explains the kinetics of adsorption. In the present study, kinetics of CO2 adsorption on synthesized nanostructured carbon adsorbents have been studied by using three kinetic models namely pseudo-first order, pseudo-second order and fractional order kinetic models. Lagergren's equation is one of the most widely used model describing adsorption kinetics based on adsorption capacity. The pseudo-first order model can be written as (Lagergren, 1898):

dqt ¼ k1 ðqe  qt Þ dt

(1)

where qt and qe (mmol g1) are the adsorption capacities at time t (minutes) and equilibrium respectively and k1 (sec1) is the pseudo-first order rate constant. Integrating Eq. (1) with boundary conditions of qt ¼ 0 at t ¼ 0 and qt ¼ qt at t ¼ t gives the following equation:

  qt ¼ qe 1  ek1 t

(2)

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Fig. 3. TEM images of (a) MF-600 and (b) MF-700.

The pseudo-second order model can be expressed as (Ho and McKay, 1999):

dqt ¼ k2 ðqe  qt Þ2 dt

(3)

where k2 (g mmol1 sec1) is the rate constant of pseudo-second order model. The integration of Eq. (3) for the above mentioned boundary conditions leads to the following form of the equation:

qt ¼

k2 q2e t

1 þ k2 qe t

(4)

qt ¼ qe  

1 ððn  1Þkn

=mÞt m

  1=n1 þ 1 qn1 e

(6)

Nonlinear regression was used to determine the parameters of the above adsorption kinetic models with the help of OriginPro 8 software. Adequacy of the kinetic models is assessed by calculating an error function based on the normal standard deviation (SernaGuerrero and Sayari, 2010).

Error ð%Þ ¼

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP h . i2 u qtðexpÞ  qtðpredÞ qtðexpÞ t N1

 100

(7)

The fractional order kinetic model is based on the assumption that rate of adsorption is affected by nth power of driving force and mth power of the adsorption time (Heydari-Gorji and Sayari, 2011; Zhao et al., 2013) and it can be written as:

where Error (%) is the error function, qt(exp) and qt(pred) are the experimental and model predicted amounts of CO2 uptake respectively at a given time and N is the total number of experimental points.

dqt ¼ kn ðqe  qt Þn t m1 dt

2.5. Adsorption isotherm study

(5)

where kn is the fractional order rate constant and m and n are the model constants. Integrated form of Eq. (5) for the above stated boundary conditions is:

Equilibrium uptake of CO2 on nanostructured carbons is evaluated by using three isotherm models namely Langmuir, Freundlich and Temkin models. Langmuir isotherm assumes monolayer adsorption occurring on finite number of energetically equivalent

Fig. 4. FTIR spectra of nitrogen enriched nanostructured carbon adsorbents.

Fig. 5. Elemental composition and Kjeldahl nitrogen with carbonization temperature.

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adsorbent sites (Langmuir, 1916). It can be written as:

qe ¼

qm KL P 1 þ KL P

(8)

where qe and qm (mmol g1) are the equilibrium and maximum monolayer adsorption capacity respectively, KL (atm1) is the Langmuir parameter related to free energy of adsorption and P (atm) is the CO2 partial pressure. Freundlich isotherm is applicable for adsorption on energetically heterogeneous adsorbent surface (Freundlich, 1906) and can be expressed as:

qe ¼ KF P 1=n

(9)

where KF (mmol g1 atm1/n) and n are the Freundlich model parameters indicating the adsorption capacity and adsorption intensity respectively. Temkin isotherm also explains adsorption on heterogeneous surfaces but with the assumption that adsorption heat of the molecules in the layer decreases linearly rather than logarithmic with coverage because of adsorbenteadsorbate interactions (Temkin and Pyzhev, 1940) It can be represented as:

qe ¼ B lnðKT PÞ

(10)

where B ¼ RT/b with b (J mol1) and KT (atm1) are the Temkin constants related to heat of sorption and equilibrium binding constant respectively. R (8.314 J mol1 K1) is the universal gas constant and T (K) is the temperature. 2.6. Isosteric heat of adsorption The isosteric heat of adsorption is defined as the difference between the activation energy required for adsorption and desorption process and was obtained by using ClausiuseClapeyron equation (Guo et al., 2006).

" Qst ¼ R

vln P

v T1

# (11) qe

where Qst (kJ mol1) is the isosteric heat of adsorption at a given qe. Isosteric heat of adsorption is calculated from the slope of the plot between ln P against 1/T at a constant amount of CO2 adsorbed (qe) (called isosteres). 3. Results and discussion 3.1. Characterization of adsorbent Fig. 1 illustrates the nitrogen sorption isotherms and corresponding pore size distributions for the synthesized carbons. Textural parameters of these carbon materials are reported in Table S1. The isotherms of synthesized carbons other than MF-700 are of typical type IV with a hysteresis loop at P/Po > 0.8, characteristic of mesoporous materials whereas MF-700 shows a steep rise in N2 adsorption at low relative pressure (P/Po < 0.1) corresponding to filling of the micropores. Thus carbon carbonized at 700  C exhibits a combination of type I and IV isotherms. As seen from Table S1, both SBET and VBJH increased with increase in carbonization temperature and were found to be the highest for MF-700. Both these parameters have very low values for samples carbonized up to 600  C showing insufficiency of this temperature in porosity development. With increase in carbonization

Fig. 6. CO2 uptake of prepared carbons at 30  C under pure CO2 flow.

temperature from 400 to 700  C, BET surface area increased from 6 to 266 m2 g1 and the pore volume increased from 0.02 to 0.25 cm3 g1. Also among the prepared adsorbents, only MF-700 showed a small micropore volume of 0.03 cm3 g1. Pore size distribution (PSD) of the prepared carbons is derived from the adsorption branch of the nitrogen isotherm because it is free of tensile strength effect (TSE). TSE is the phenomenon of forced closure of the hysteresis loop which occurs due to the sudden drop in the adsorbed volume along the desorption branch of the nitrogen isotherm in the relative pressure range of 0.41e0.48 (Groen et al., 2003). Application of BJH model to the adsorption branch suggests that all the prepared carbon materials except MF400 are made up of mesopores with a size of around 9e10 nm. Fig. 2 shows the powder XRD patterns of the synthesized carbon adsorbents. The most intense peak is observed at 2q ¼ 26e27 for all the samples and this broad peak corresponds to the (002) diffraction planes of graphitic carbon (JCPDS X-ray Powder Diffraction Database No. 75-1621). The d-spacing is around 0.34 nm and is very near to the value that for ideal graphite. Also the diffraction patterns are not very sharp thereby suggesting the synthesized carbon materials to be semi-crystalline in nature. Further broadening of the most intense peak indicates very small crystallite size of the synthesized carbons thereby indicating the formation of nanostructured carbon adsorbents. Formation of nanostructured materials can be clearly seen in the transmission electron micrographs (Fig. 3) of the prepared carbons (MF-600 and MF-700). The average particle size of the prepared carbons is 30e50 nm. The FTIR spectra obtained for all the synthesized carbon samples are presented in Fig. 4. All the samples have the similar functional groups. Peak at 1059 cm1 is attributed to stretching vibration of

Table 1 CO2 uptake (mmol g1) of MF-700 at different adsorption temperatures and CO2 concentrations. CO2 concentration (%)

Temperature ( C) 30

50

75

100

10 20 50 100

0.484 0.591 0.698 0.792

0.396 0.488 0.572 0.667

0.303 0.349 0.443 0.507

0.201 0.256 0.309 0.354

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Fig. 7. CO2 uptake on MF-700 at different adsorption temperatures under (a) 10% CO2 flow, and (b) 100% CO2 flow.

CeO bond of alcohol, ether or ester groups; peak at 1246 cm1 corresponds to stretching vibration of aromatic CeN bond and peak at 1770 cm1 can be assigned to C]O bond stretching vibration (Chen et al., 2012). A small peak around 3000 cm1 is also observed and can be assigned to stretching vibration of CeH bond of eCH2eNHeCH2e or eCH2eNHeCH3. Weak bands at 3400 cm1 demonstrate the presence of NeH symmetric stretching vibration (Hao et al., 2010). Presence of nitrogen in the carbon matrix of the adsorbents is also confirmed by elemental analysis and Kjeldahl nitrogen. For all the prepared adsorbents, it is observed from Fig. 5 that both elemental nitrogen and Kjeldahl nitrogen tend to decrease with increase in carbonization temperature. On the other hand, carbon content increased continuously with carbonization temperature from 41 wt% to 52 wt%. 3.2. CO2 adsorption studies CO2 uptake of the prepared carbon adsorbents at 30  C and 100% CO2 flow is shown in Fig. 6. Fast adsorption kinetics is exhibited by all the samples with major CO2 uptake within ca. 100 s and reaching

Fig. 8. CO2 adsorptionedesorption cycles obtained for MF-700 at 30  C.

a plateau. CO2 uptake increased from 0.209 to 0.792 mmol g1 with increase in carbonization temperature from 400 to 700  C. Adsorption capacity increased significantly on increasing carbonization temperature from 600 to 700  C because of considerable improvement in textural properties (BET surface area and pore volume). Carbon adsorbents from melamine-formaldehyde resin and K2CO3 as chemical activation agent were also synthesized and sample obtained at 700  C exhibited CO2 uptake of 0.86 mmol g1 at 30  C (Drage et al., 2007). Uptake of CO2 on these nanostructured materials is comparable to the values mentioned in the literature (Sayari et al., 2011). As MF-700 showed the highest CO2 uptake at room temperature, the effect of adsorption temperature and CO2 inlet concentration on CO2 uptake was studied only for this adsorbent by conducting adsorption experiments at four different CO2 concentrations i.e. 10%, 20%, 50% and 100% CO2 in N2 at different adsorption temperatures ranging from 30 to 100  C. The weight change of the adsorbent was monitored to determine the CO2 uptake and results are displayed in Fig. 7. With increase in temperature from 30 to

Fig. 9. Experimental CO2 uptake (symbols) on prepared carbons and corresponding fit to kinetic models (solid lines) at 30  C under pure CO2 flow.

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Table 2 Kinetic model parameters for CO2 uptake on prepared carbons at 30  C and 100% CO2 atmosphere. Kinetic model

Pseudo-first order

Pseudo-second order

Fractional order

Parameters

1

k1 (sec ) qe (mmol g1) R2 Error % k2 (g mmol1 sec1) qe (mmol g1) R2 Error % kn qe (mmol g1) n m R2 Error %

Sample MF-400

MF-500

MF-600

MF-700

0.030 0.210 0.987 4.129 0.236 0.223 0.916 10.943 0.012 0.209 0.975 1.305 0.997 1.698

0.033 0.272 0.984 4.153 0.203 0.288 0.907 11.592 0.012 0.271 0.981 1.354 0.996 2.133

0.029 0.475 0.989 4.211 0.096 0.507 0.887 12.925 0.004 0.472 0.991 1.686 0.996 3.396

0.033 0.795 0.985 3.162 0.072 0.840 0.912 7.620 0.011 0.792 0.982 1.379 0.993 1.827

100  C, CO2 uptake decreased from 0.484 to 0.201 mmol g1 under 10% CO2 balance N2 and from 0.792 to 0.354 mmol g1 under pure CO2 atmosphere (Table 1). This indicates that favorable adsorption occurs at low temperatures. In physical adsorption process, both the adsorption energy of the surface and diffusion rate increase

with temperature causing instability of adsorbate on the adsorbent surface thus leading to desorption of the adsorbate and decrease in capacity. On the other hand, CO2 uptake was observed to increase from 0.484 to 0.792 mmol g1 with increase in CO2 concentration which is attributed to increased driving force at higher

Fig. 10. Experimental CO2 uptake on MF-700 and corresponding fit to kinetic models at different adsorption temperatures and CO2 concentrations.

C. Goel et al. / Journal of Environmental Management 162 (2015) 20e29 Table 3 Parameters of adsorption isotherm models. Model

Langmuir

Freundlich

Temkin

Temperature ( C)

Parameters

1

qm (mmol g ) KL (atm1) R2 KF (mmol g1 atm1/n) n R2 KT (atm1) b (kJ mol1) R2

30

50

75

100

0.830 13.112 0.966 0.798 4.899 0.984 415.686 19.166 0.996

0.698 12.202 0.952 0.669 4.640 0.987 326.948 23.437 0.993

0.535 11.252 0.928 0.510 4.395 0.995 264.086 31.966 0.996

0.378 10.815 0.978 0.358 4.321 0.977 230.701 47.512 0.994

concentrations. Synthetic activated carbon, prepared from coal tar pitch and furfural by carbonization and steam activation at 800  C, exhibited CO2 uptake of 0.61 mmol g1 at 30  C and 0.15 mmol g1 at 80  C under 15% CO2 gas flow (Balsamo et al., 2013), whereas at 75  C, MF-700 exhibited almost double CO2 uptake of 0.303 mmol g1 under 10% CO2 flow. In another work, adsorbents obtained from steam activation of fly ash exhibited CO2 uptake of 0.95 mmol g1 at 30  C higher than MF-700 (Maroto-Valer et al., 2008). But its CO2 uptake is lower than MF-700 at 75  C and for the flue gas application adsorption capacity at higher temperatures is of more importance. Porous carbon adsorbent from soybean by zinc chloride activation resulted in carbon with CO2 adsorption capacity of 0.93 and 0.51 mmol g1 at 30  C and 75  C respectively (Thote et al., 2010). But the adsorbent was not regenerated completely in the second adsorptioneadsorption cycle thereby showing zero capacity in the third cycle. Carbons obtained from carbonization and chemical activation with KOH of carpet material exhibited high CO2 uptake of 3.13 mmol g1 under 100% CO2 flow at 30  C. But its capacity under 15% CO2 at 100  C, which is more relevant for flue gas application, was 0.16 mmol g1 (Olivares-Marín et al., 2011). CO2 adsorption capacity of MF-700 under similar conditions was 0.201 mmol g1 (under 10% CO2 flow). Hence nanostructured carbon adsorbents, which are obtained from only carbonization process, have comparable CO2 uptakes at high temperatures under 10% concentrations which is more close to real flue gas composition. Fig. 8 demonstrates the multiple cycles of CO2 adsorption and desorption for MF-700 under pure CO2 atmosphere. It can be seen

27

that the CO2 adsorbed on the carbon surface was quickly desorbed on raising the temperature and switching the gas from CO2 to N2. This adsorptionedesorption cycle was repeated three times with no evident change in the CO2 uptake suggesting the reversible nature of the adsorption process. Hence the prepared nanostructured carbon adsorbents can be regenerated completely confirming the stability and reusability of the adsorbents.

3.3. Adsorption kinetics Adsorption kinetics of CO2 on prepared nanostructured carbon adsorbents was evaluated by using three kinetic models namely, pseudo-first order, pseudo-second order and fractional order kinetic models. Experimental uptake of CO2 and model predicted uptake with respect to time on the nanostructured adsorbents at 30  C under pure CO2 flow is illustrated in Fig. 9. The kinetic model parameters, correlation coefficients (R2) and associated errors are reported in Table 2. Higher values of error% for pseudo-first order and pseudo-second order models suggest that the experimental CO2 uptake does not follow these models strictly. Both these models overestimated the CO2 uptake during the initial phase of adsorption process (ca. up to 50 s). Pseudo-first order model followed the experimental data during the final phase of adsorption process but pseudo-second order model overestimated CO2 uptake in the final phase also. On the other hand, fractional order adsorption kinetic model was found to fit the experimental CO2 uptake over the whole range of the adsorption process. This was supported by higher values of R2 (>0.99) and significantly lower values of error% (<4%). Furthermore CO2 uptake at equilibrium predicted by fractional order model and experimental values are in good agreement with each other. Fig. 10 illustrates the experimental and model predicted CO2 uptake on MF-700 as function of CO2 concentration at different adsorption temperatures. Experimental CO2 uptake on MF-700 at all the adsorption temperatures and CO2 concentrations is also found to follow the fractional order adsorption kinetic model with maximum error of 3.7% (Table S2). Value of m indicates the fastness of adsorption process and n indicates the effect of driving force. An increase in value of m with concentration suggests that the adsorption process became fast at higher CO2 concentrations. Value of n is dependent on the apparatus being used for the adsorption study and hence remained almost constant.

Fig. 11. (a) Adsorption isotherm on MF-700 at different adsorption temperatures, and (b) isosteric heat of adsorption of CO2 on MF-700.

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At low CO2 concentration (10%), pseudo-first order model closely described the adsorption process at all temperatures indicating diffusion controlled process when the concentration gradient is small. But at higher CO2 concentrations, pseudo-first order model overestimated the CO2 uptake during the initial phase followed by approaching the experimental data as the adsorption proceeded. Equilibrium CO2 uptake as predicted by pseudo-first order model is in close agreement with the experimental values but pseudo-second order model predicted higher equilibrium uptakes. 3.4. Adsorption isotherm study Equilibrium CO2 uptake of MF-700 has been investigated by using three isotherm models namely Langmuir, Freundlich and Temkin isotherm. Values of isotherm parameters at different temperatures are reported in Table 3 and the comparison of experimental CO2 uptake and Temkin model predicted data is presented in Fig. 11a. Both Freundlich and Temkin isotherm models predict the equilibrium data very closely but the best fit is obtained by Temkin isotherm model as suggested by coefficient of determination (R2). This indicates that the adsorbent surface is energetically heterogeneous. Maximum monolayer adsorption capacity (qm) tends to decrease with increase in adsorption temperature due to exothermic nature of adsorption process. Also decrease in value of KF with temperature suggests adsorption process to be favorable at lower temperatures. 3.5. Isosteric heat of adsorption Fig. 11b shows the change in isosteric heat of adsorption as a function of CO2 uptake for MF-700. The change in value of Qst with surface coverage indicates energetically heterogeneous adsorbent surface which has also been confirmed by best fitting of Temkin isotherm model. Isosteric heat of adsorption is found to increase from ca. 30 kJ mol1 to ca. 44 kJ mol1 with increase in surface coverage from 0.3 to 0.55 mmol g1. This is attributed to the presence of lateral interactions between the adsorbed gas molecules (Wahby et al., 2012). The value of Qst for the synthesized carbons is comparable to the literature reported values (30e50 kJ mol1) for nitrogen enriched carbon materials produced from various methods (Wang and Yang, 2012; Xia et al., 2011; Zhao et al., 2012). High values of isosteric heat of adsorption are due to strong interaction between CO2 gas molecules and nitrogen enriched carbon surface. 4. Conclusions A series of nanostructured carbon adsorbents were developed by nanocasting technique for CO2 capture. Melamineformaldehyde resin and mesoporous silica were used as the polymer precursor and hard template respectively. All the synthesized adsorbents showed mesoporous structure except MF-700 which exhibited small microporosity in addition to mesoporosity. Development of nitrogen enriched nanostructured carbon adsorbents was confirmed by XRD, TEM, FTIR and elemental analysis. These adsorbents were evaluated for CO2 capture performance in a thermogravimetric analyzer. Maximum CO2 uptake of 0.792 mmol g1 was exhibited by MF-700 at 30  C under pure CO2 flow. Also CO2 uptake decreased with increase in adsorption temperature demonstrating the occurrence of physical adsorption process. This study also presented the analysis of CO2 adsorption kinetics on these developed materials. Fractional order adsorption kinetic model successfully fitted the experimental CO2 uptake data at all adsorption temperatures and CO2 concentrations.

Experimental CO2 uptake was fitted by using three equilibrium models with best fit obtained by Temkin isotherm model indicating heterogeneous adsorbent surface which was also indicated by change in isosteric heat of adsorption values. Acknowledgments The financial support from Science and Engineering Research Board under Department of Science and Technology (DST) (Scheme No.: DST/IS-STAC/CO2-SR-154/12(G)), and All India Council of Technical Education (AICTE), New Delhi (Scheme No.: 8023/RID/ RPS-66/2010-11), is acknowledged. Chitrakshi Goel acknowledges the financial support from DST-INSPIRE under its Assured Opportunity for Research Careers (AORC) scheme (Scheme No.: DST/ INSPIRE FELLOWSHIP/2012/398). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2015.07.040. References Aaron, D., Tsouris, C., 2005. Separation of CO2 from flue gas: a review. Sep. Sci. Technol. 40, 321e348. Balsamo, M., Budinova, T., Erto, A., Lancia, A., Petrova, B., Petrov, N., Tsyntsarski, B., 2013. CO2 adsorption onto synthetic activated carbon: kinetic, thermodynamic and regeneration studies. Sep. Purif. Technol. 116, 214e221. Baxter, J., Bian, Z., Chen, G., Danielson, D., Dresselhaus, M.S., Fedorov, A.G., Fisher, T.S., Jones, C.W., Maginn, E., Kortshagen, U., Manthiram, A., Nozik, A., Rolison, D.R., Sands, T., Shi, L., Sholl, D., Wu, Y., 2009. Nanoscale design to enable the revolution in renewable energy. Energy Environ. Sci. 2, 559e588. Belmabkhout, Y., Serna-Guerrero, R., Sayari, A., 2011. Adsorption of CO2-containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for CO2 separation. Adsorption 17, 395e401. Chatti, R., Bansiwal, A.K., Thote, J.A., Kumar, V., Jadhav, P., Lokhande, S.K., Biniwale, R.B., Labhsetwar, N.K., Rayalu, S.S., 2009. Amine loaded zeolites for carbon dioxide capture: amine loading and adsorption studies. Microporous Mesoporous Mater. 121, 84e89. Chen, C., Kim, J., Ahn, W.-S., 2012. Efficient carbon dioxide capture over a nitrogenrich carbon having a hierarchical micro-mesopore structure. Fuel 95, 360e364. Drage, T.C., Arenillas, A., Smith, K.M., Pevida, C., Piippo, S., Snape, C.E., 2007. Preparation of carbon dioxide adsorbents from the chemical activation of ureaeformaldehyde and melamineeformaldehyde resins. Fuel 86, 22e31. Drage, T.C., Blackman, J.M., Pevida, C., Snape, C.E., 2009. Evaluation of activated carbon adsorbents for CO2 capture in gasification. Energy Fuels 23, 2790e2796. Figueroa, J.D., Fout, T., Plasynski, S., McIlvried, H., Srivastava, R.D., 2008. Advances in CO2 capture technologydThe U.S. Department of Energy's carbon sequestration program. Int. J. Greenh. Gas. Control 2, 9e20. Freundlich, H.M.F., 1906. Over the adsorption in solution. J. Phys. Chem. 57, 385e471. Gray, M.L., Soong, Y., Champagne, K.J., Baltrus, J., Stevens, R.W., Toochinda, P., Chuang, S.S.C., 2004. CO2 capture by amine-enriched fly ash carbon sorbents. Sep. Purif. Technol. 35, 31e36. rez-Ramírez, J., 2003. Pore size determination in Groen, J.C., Peffer, L.A.A., Pe modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis. Microporous Mesoporous Mater. 60, 1e17. Guo, B., Chang, L., Xie, K., 2006. Adsorption of carbon dioxide on activated carbon. J. Nat. Gas. Chem. 15, 223e229. Hao, G.-P., Li, W.-C., Qian, D., Lu, A.-H., 2010. Rapid synthesis of nitrogen-doped porous carbon monolith for CO2 capture. Adv. Mater 22, 853e857. Heydari-Gorji, A., Sayari, A., 2011. CO2 capture on polyethylenimine-impregnated hydrophobic mesoporous silica: experimental and kinetic modeling. Chem. Eng. J. 173, 72e79. Ho, Y.S., McKay, G., 1999. Pseudo-second order model for sorption processes. Process Biochem. 34, 451e465. Huang, Y., Yang, F., Xu, Z., Shen, J., 2011. Nitrogen-containing mesoporous carbons prepared from melamine formaldehyde resins with CaCl2 as a template. J. Colloid Interface Sci. 363, 193e198. Lagergren, S., 1898. About the theory of so-called adsorption of soluble substances. Ksver. Veterskapsakad. Handl. 24, 1e39. Langmuir, I., 1916. The constitution and fundamental properties of solids and liquids. Part I. solids. J. Am. Chem. Soc. 38, 2221e2295. Li, J.R., Sculley, J., Zhou, H.C., 2012. Metal-organic frameworks for separations. Chem. Rev. 112, 869e932. Maroto-Valer, M.M., Lu, Z., Zhang, Y., Tang, Z., 2008. Sorbents for CO2 capture from high carbon fly ashes. Waste Manage. 28, 2320e2328. Millward, A.R., Yaghi, O.M., 2005. Metal-organic frameworks with exceptionally

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