Carbon dioxide adsorption on nitrogen enriched carbon adsorbents: Experimental, kinetics, isothermal and thermodynamic studies

Journal of CO2 Utilization 16 (2016) 50–63

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Carbon dioxide adsorption on nitrogen enriched carbon adsorbents: Experimental, kinetics, isothermal and thermodynamic studies Chitrakshi Goela , Harleen Kaurb , Haripada Bhuniaa,* , Pramod K. Bajpaia a b

Department of Chemical Engineering, Thapar University, Patiala 147004, Punjab, India School of Energy and Environment, Thapar University, Patiala 147004, Punjab, India

A R T I C L E I N F O

Article history: Received 14 March 2016 Received in revised form 16 May 2016 Accepted 6 June 2016 Available online xxx Keywords: CO2 adsorption Isotherm Kinetics Mesoporous carbon Nanocasting Thermodynamics

A B S T R A C T

Nitrogen enriched porous carbons were prepared by nanocasting method using hexamethoxymethylmelamine (HMMM) as precursor and MCM-41 silica as template. Carbonization temperature was varied from 500
C to 800
C and was followed by physical activation with CO2 at the same temperature. These materials were evaluated as adsorbents for CO2 capture. Textural and morphological properties of these carbons show that they have mesoporosity derived from template removal. Carbonization and activation at 700
C produced carbon with highest surface area of 463 m2/g and total pore volume of 0.48 cm3/g with nitrogen content of 9.2 wt%. Both of these properties account for the highest CO2 uptake of 0.80 mmol/g at 30
C using pure CO2. CO2 uptake decreased with increase in temperature suggesting occurrence of physiosorption process. Additionally, these prepared carbons exhibited stable cyclic adsorption capacity. CO2 adsorption kinetics on these adsorbents follow pseudo-first order model with maximum error of ca. 5.4%. The adsorbent surface was found to be energetically heterogeneous as suggested by Temkin isotherm model. Thermodynamics suggested exothermic, random and spontaneous nature of the process. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Climate change, a vital global problem at present, is the result of increasing anthropogenic carbon dioxide emissions. Combustion of fossil fuels, for meeting the world’s energy requirements, contributes to a great extent to these emissions and they are expected to remain the primary energy source for this century [1– 3]. Hence, there is a need to develop technologies that will allow the fossil fuel usage while reducing anthropogenic CO2 emissions. Carbon dioxide capture and sequestration (CCS) appears to be a promising approach to mitigate increasing CO2 levels. In addition, it will buy time for cost reduction of renewable energy sources while facilitating cleaner use of fossil fuels [4,5]. Separation of CO2 based on adsorption seems to be a promising method in the field of CCS on account of its low equipment cost, low energy consumption and ease of application. Porous materials such as zeolites, carbons, metal-organic frameworks and amine supported silica have been evaluated as potential candidates for CO2 separation from flue gas [6–11]. A time consuming post treatment by the use of corrosive and toxic reagents is required to

* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Bhunia). http://dx.doi.org/10.1016/j.jcou.2016.06.002 2212-9820/ã 2016 Elsevier Ltd. All rights reserved.

prepare amine supported zeolite and silica based adsorbents. Moreover, these adsorbents lack stability over various adsorption cycles and need to be regenerated at high temperatures for longer duration [7,12]. In comparison to these adsorbents, porous carbon based adsorbents offer several advantages such as large surface area, tunable porosity, high adsorption capacity, and ease of regeneration. They can be synthesized from large variety of low cost sources and by various techniques like sol-gel method, carbonization of carbon material and nanocasting. Among the techniques mentioned, nanocasting offers the development of carbon materials with controlled pore structure. In this process, a carbon precursor is infiltrated into the pores of the template and heat treated under controlled atmosphere. Then the template is removed by selective dissolution in hydrogen fluoride or sodium hydroxide. Pore structure of the prepared materials can be tailored depending on the template’s pore structure [13,14]. Moreover, introduction of hetero atoms like nitrogen in the carbon matrix enhances its surface polarity and basicity thereby improving the interaction between acidic gas molecules like CO2 and carbon surface [15,16]. Up till now, porous carbon adsorbents were mainly prepared by impregnation of basic amine groups after the synthesis of carbon material or by ammonia treatment but leads to instability of the amine and hence lacks the reusability of the

C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

adsorbent after consecutive adsorption runs [17,18]. Alternatively, nitrogen enriched carbon materials can also be prepared by using nitrogen containing monomers e.g. melamine, aniline etc. with the templates like zeolite and mesoporous silica [19–21]. Xu et al. [22] synthesized mesoporous carbon materials by using sucrose as precursor and nano-CaCO3 as template and studied the effect of template to precursor ratio on textural properties. Nitrogen-doped porous carbon monoliths were produced by direct pyrolysis of the copolymer of resorcinol, formaldehyde, and lysine and they showed static CO2 uptake of 3.13 mmol/g at 25
C under pure CO2 flow [18]. Pevida et al. [19] reported the synthesis of nitrogen enriched carbon adsorbents from melamine-formaldehyde resin having CO2 uptake of 2.25 and 0.86 mmol/g at 25
C and 75
C respectively under pure CO2 environment. In another work, nitrogen enriched carbons were prepared from urea-formaldehyde and melamine-formaldehyde resins and chemical activation with K2CO3 and they exhibited CO2 uptake of 1.8 mmol/g and 1.03 mmol/g at 25
C respectively [23]. However, it is important to note that most of the reported literature includes development of carbon adsorbents by direct carbonization and chemical activation of various raw materials followed by their CO2 adsorption study under static flow conditions at room temperature, which does not have much relevance with respect to practical application in CO2 capture. CO2 capture performance under dynamic conditions is more important and the same has been carried out in this work. In the present study, we report the preparation of mesoporous carbons by employing a novel nitrogen-rich polymer i.e. hexamethoxymethylmelamine (HMMM) resin as the carbon precursor with mesoporous silica as template. Carbonization temperature was varied from 500 to 800
C to obtain a range of adsorbents. These adsorbents were evaluated for their textural, morphological and chemical properties followed by their dynamic evaluation as adsorbents for CO2 capture. Also, static equilibrium adsorption capacities of pure CO2 and N2 on prepared adsorbents were also measured. Cyclic adsorption-desorption studies were carried out to check their stability followed by kinetic, isotherm and thermodynamic studies. 2. Materials and methods 2.1. Materials Hexamethoxymethylmelamine (HMMM), procured from M/s Techno Waxchem Pvt. Ltd., India, was used as polymeric precursor. HMMM is methylated melamine-formaldehyde resin and is obtained from methylolation reaction between melamine and formaldehyde followed by methylation with methanol under acidic conditions [24]. Mesoporous silica (MCM-41) having surface area of 450 m2/g and average pore diameter of 3.5 nm was purchased from M/s Tianjin Chemist Scientific Ltd., Tianjin, China and was used as hard template. Ethanol (100% pure), used as solvent, and sodium hydroxide pellets (A. R. grade) were purchased from M/s S. D. Fine Chemicals India Ltd. Dry nitrogen and carbon dioxide gases of 99.999% purity and special gas mixtures of CO2 and N2 were procured from M/s Sigma Gases and Services, India. 2.2. Adsorbent preparation Carbon adsorbents were prepared by templating HMMM resin in the pores of mesoporous silica template followed by carbonization in N2 and activation in CO2 (each for 1 h) and then template removal as reported elsewhere [25]. In a typical procedure, around 80 g of HMMM resin was dissolved in pure ethanol followed by addition of 40 g of template (silica). This mixture was thoroughly mixed and then excessive solvent was removed by heating at

51

120
C for 3 h to obtain templated resin. Carbonization of these templated resins was performed in a quartz tubular furnace by heating the samples at a rate of 10
C/min up to 500–800
C for 1 h under N2 atmosphere. This was followed by physical activation with CO2 gas [26,27] by switching the gas from N2 to CO2 for 1 h under isothermal conditions. After physical activation, gas was switched back to N2 during cooling to room temperature to avoid excess gasification during cooling process. Template was removed by dissolution of samples in 40 wt% NaOH solution for at least 24 h and washing with excess amount of water. Samples were dried in oven at 100
C for 3 h and were designated as C-T, where T denotes the carbonization temperature. 2.3. Adsorbent characterization Elemental analysis (CHN) was performed using a Thermo Scientific Flash 2000 organic elemental analyzer (CHNS/O analyzer). Oxygen content was determined by difference. Kjeldahl method was also used to estimate the nitrogen content of the prepared adsorbents. Fourier transform infrared (FTIR) spectra of the prepared carbons were recorded on a PerkinElmer model 100 FTIR spectrometer in the wavelength range of 4000–625 cm1 with a resolution of 4 cm1. Surface area and pore volume of the adsorbents were measured from nitrogen sorption isotherms at 77 K on a Micrometrics ASAP 2020 sorption analyzer. Prior to any measurement, samples were outgassed at 200
C for 6 h under vacuum, to remove any gases or vapors that may have become physically adsorbed onto the adsorbent surface. Surface area (SBET) was determined using the Brunauer-Emmet-Teller (BET) equation and the total pore volume (Vtotal) was calculated from amount of N2 adsorbed at relative pressure of P/Po = 0.99. Mesopore size distribution was derived from the Barrett-Joyner-Halenda (BJH) method while micropore size distribution was obtained from Horvath-Kawazoe (HK) method. Transmission electron microscopy images were obtained with a Philips Tecnai 20 transmission electron microscope using an accelerating voltage of 200 kV. To classify the surface functional groups present on the prepared carbon adsorbents, X-ray photoelectron spectroscopy (XPS) was carried out by means of a SPECS system using Mg Ka X-ray source (energy 1253.6 eV). Both low and high resolution scans were carried out by using pass energies of 50 and 20 eV respectively. Temperature programmed desorption (TPD) of CO2 was carried out using a Micromeritics AutoChem II 2920 chemisorption analyzer equipped with a thermal conductivity detector. Carbon sample (ca. 100 mg) was first pretreated under pure He flow at 200
C and then the temperature was decreased to 30
C. Adsorption of CO2 was conducted by switching gas from He to CO2 for ca. 30 min. This was followed by desorption experiment by switching back to He gas and increasing the temperature to 250
C. 2.4. Dynamic CO2 capture performance Thermo gravimetric analyzer (TA Q500, TA Instruments, US) was used to measure the CO2 adsorption-desorption potential of the prepared carbon adsorbents at atmospheric pressure. In a typical experiment, ca. 20 mg of sample was loaded onto a platinum pan and temperature was raised from 30
C to 200
C, at a heating rate of 10
C/min, under pure N2 gas (flow rate = 50 ml/min) and held at this temperature for 2 h to remove any pre-adsorbed moisture. Temperature was then decreased to the desired adsorption temperature (i.e. 30
C) and the gas was switched to pure CO2 at a flow rate of 50 ml/min. On the complete saturation of the adsorbent, the gas flow was switched from CO2 to N2 and the temperature was increased to 200
C to carry out desorption study. This adsorption-desorption cycle was repeated ten times. Among

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2.7. Adsorption isotherm study

the prepared carbons, the optimized sample was evaluated for effect of adsorption temperature and CO2 inlet concentration on uptake of CO2 by varying the adsorption temperature from 30
C to 100
C and CO2 inlet concentration from 10% to 100% (balance N2). The change in adsorbent weight was recorded to estimate the adsorption capacity (mmol/g).

Three isotherm models i.e. Langmuir, Freundlich and Temkin models (Eqs. (5)–(7)) were used to analyze the equilibrium CO2 uptake by prepared carbons [31–33]. qe ¼

2.5. Static CO2 and N2 capture performance Static volumetric analyzer (Micromeritics ASAP 2020, USA) was used to evaluate the static CO2 and N2 adsorption capacity of the prepared carbon (HMMM-700) at four different temperatures ranging from 30
C to 100
C under pure CO2 and N2 atmosphere respectively. The pressure was varied from 0 to 1 bar and degassing of the adsorbent was carried out at 200
C for 12 h under vacuum.

To investigate the kinetics of adsorption of CO2 onto synthesized adsorbents, pseudo-first and pseudo-second order models were considered in the present work. Pseudo-first order and pseudo-second order models can be written as Eqs. (1) and (2) respectively [28,29]:
 ð1Þ qt ¼ qe 1  ek1 t

k2 q2e t 1 þ k2 qe t

qe ¼ K F P1=n

ð6Þ

qe ¼ BlnðK T PÞ

ð7Þ

2.8. Thermodynamic studies The change in Gibbs free energy (DGo in J/mol) for CO2 adsorption on prepared carbons can be derived from the adsorption equilibrium constant by using the following equation:   DGo ¼ RTln K eq ð8Þ

ð2Þ

where, qt and qe (mmol/g) are adsorption capacities at time t (sec) and at equilibrium respectively and k1 (1/sec) and k2 (g/mmol.sec) are the kinetic rate constants of the pseudo-first order and pseudosecond order kinetic models respectively. Each model’s adequacy was determined by the coefficient of determination (R2) and an error (Error%) which is based on the normal standard deviation (according to Eq. (3)) [30]. Error% showed the fit between the experimental and model predicted data. vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uX h
 i2 u qtðexpÞ  qtðpredÞ =qtðexpÞ t Errorð%Þ ¼  100 ð3Þ N1

where Keq is the equilibrium constant for adsorption process, obtained from Langmuir or Temkin isotherm models, at temperature T. Dependency of adsorption equilibrium constant on temperature can be explained by the classical van’t Hoff equation. Assumption of very slight variation in DHo with temperature gives the following form of van’t Hoff equation:   DH o 1 þC ln K eq ¼  R T

ð9Þ

where DHo (J/mol) is the standard molar adsorption enthalpy at temperature T (K), C is the integration constant. DHo and standard entropy change (DSo ) can be obtained from the slope and intercept, respectively, of ln (Keq) vs 1=T [34]. After this, energy required for desorption of adsorbed CO2 was also calculated. It includes desorption heat (Qst) and sensible heat needed for heating the adsorbent to desorption temperature. Desorption heat is assumed to be equal to heat of adsorption i.e. isosteric heat of adsorption and is obtained from ClausiusClapeyron equation [35,36]. " # @lnP ð10Þ Q st ¼ R 1

where Error% is the error function, qt(exp) and qt(pred) are the experimental and calculated uptakes of CO2 at a given time respectively and N is the number of experimental points. Arrhenius equation was employed to determine the activation energy for adsorption from kinetic rate constant (k) as shown below: k ¼ AeðEa =RT Þ

ð5Þ

where qe and qm (mmol/g) are the equilibrium and maximum monolayer adsorption capacity respectively; KL (1/atm) is the Langmuir parameter related to free energy of adsorption; KF (mmol/g.atm1/n) and n are the Freundlich model parameters indicating the adsorption capacity and adsorption intensity respectively; B = RT/b with b (J/mol) and KT (1/atm) are the Temkin constants related to heat of sorption and equilibrium binding constant respectively; and P (atm) is the CO2 partial pressure.

2.6. Adsorption kinetic models

qt ¼

qm K L P 1 þ KLP

ð4Þ

where A and Ea (J/mol) are the pre-exponential factor of Arrhenius equation and activation energy respectively.

@

T

qe

Table 1 Chemical analysis, yield and ash content of carbon adsorbents. Sample

C-500 C-600 C-700 C-800

Elemental analysis (weight%) Carbon

Nitrogen

Hydrogen

Oxygen

63.9 63.1 61.7 64.2

21.2 15.9 13.6 9.2

1.9 2.6 1.4 1.9

12.9 18.4 23.2 24.7

Kjeldahl nitrogen (%)

Yield (%)

Ash content (%)

17.78 15.79 14.42 5.94

44.7 41.3 38.2 32.5

2.88 3.65 2.91 2.67

C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

Sensible heat depends on heat capacity of the adsorbent (Cp in J/ g.K), its adsorption capacity and temperature difference (DT) between adsorption and desorption processes [37,38] i.e. Sensible heat ¼

C p DT adsorption capacity

ð11Þ

Heat capacity of the adsorbent was obtained by using a differential scanning calorimeter (NETZSCH DSC 200F3, NetzschGeratebau GmbH, Germany) wherein adsorbent was heated from room temperature to 300
C at a heating rate of 10
C/min under pure N2 atmosphere. 3. Results and discussion 3.1. Adsorbent characterization Nitrogen content of the prepared carbons tends to decrease with the increase in carbonization temperature, from 21.2 wt% for the sample carbonized at 500
C to 9.2 wt% for the sample carbonized at 800
C, and same trend is followed by Kjeldahl nitrogen (Table 1). In contrast, oxygen content of adsorbents was found to increase with carbonization temperature which could be because of oxidation of carbon surface by carbon dioxide during activation [39]. However, there was a slight decrease in carbon content of the samples which is also due to activation with CO2. This decrease in carbon content was very small than nitrogen indicating easier removal of nitrogen, by oxidation, than carbon. Also, with increase in carbonization temperature, a decrease in the yield of carbonization and activation process was observed and can be attributed to lesser loss of volatile matter while carbonization and activation processes at lower temperatures. Ash content of the prepared carbons was also determined and was found to be in the range of 2.6–3.7%.

53

FTIR spectra of prepared carbons are shown in Fig. 1. All samples showed a small peak at 1059 cm1 attributed to stretching vibrations of CO bond in alcohol, phenol, ether or ester groups. The peaks at 1246 and 1770 cm1 can be assigned to stretching vibrations of aromatic C N and C¼O bonds respectively [40]. The band at 2300 cm1 is due to the presence of CN bonds in the prepared carbon adsorbents while the small peak around 3000 cm1 is assigned to C H stretching vibration of  CH2 NH CH2  or  CH2NH CH3 group. Fig. 2a shows the nitrogen sorption isotherms at 196
C of carbons prepared at different carbonization temperatures. According to the IUPAC classification, all the isotherms could be classified as type IV with the H3-type hysteresis loop at relative pressures of 0.45–1.0. This is attributed to the phenomenon of capillary condensation, which is characteristic of the mesoporous materials. As carbonization temperature increases, BET surface area (SBET) of the synthesized carbon materials increases and reach a maximum for sample carbonized at 700
C (Table S1). On the other hand, total pore volume (Vtotal) of the adsorbents carbonized at 500
C and 600
C is almost equal but further increase in carbonization temperature to 700
C lead to an increase in Vtotal. C-700 sample exhibited maximum BET surface area of 463 m2/g and total pore volume of 0.48 cm3/g among the prepared carbons. Further increase in carbonization temperature to 800
C resulted in considerable decrease (around 75%) in both these parameters. SBET decreased from 463 m2/g to 112 m2/g while Vtotal decreased from 0.48 cm3/g to 0.12 cm3/g. This could be ascribed to extensive gasification at 800
C which resulted in widening of the pores and partial collapse of the pore structure [41,42]. Fig. 2b presents the pore size distribution (PSD) derived from adsorption isotherms for the prepared carbons by using BJH and HK methods. Adsorption branch of the isotherm was selected for this analysis rather than desorption branch because PSD derived

Fig. 1. FTIR spectra of synthesized adsorbents.

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C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

Fig. 2. (a) Nitrogen adsorption-desorption isotherms, (b) pore size distribution of carbon adsorbents derived from the adsorption branch of the nitrogen isotherm.

from the desorption branches of isotherms are reported to show entirely different results as compared to those obtained from the adsorption branches because of tensile strength effect (TSE).

However, the adsorption branch is not affected by TSE. PSD derived from the adsorption branch suggests that all the prepared carbon materials except C-700 exhibit the peak centered at around 2 nm.

C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

On the other hand, PSD of C-700 is centered at ca. 1 nm with a small peak at ca. 1.3 nm indicating the development of micropores in this sample. Fig. S1 presents the transmission electron micrographs of the prepared carbons, confirming the formation of nanostructured carbon materials. The particle size of the prepared adsorbents is in the range of 25–40 nm. X-ray photoelectron survey scan spectra of the obtained carbon adsorbents is shown in Fig. S2. Only three peaks were observed at ca. 285, 400 and 532 eV which could be assigned to C, N and O respectively. However, no peak for Si was observed which suggests that the mesoporous silica template was completely removed from the carbon matrix. Further, high resolution scan for N1s was carried out in order to determine the various nitrogen moieties present on the adsorbent surface. The core level spectra for N1s was deconvoluted into four peaks, for all the prepared carbons, using XPS peak 4.1 software (Fig. 3). Peak 1 was observed at binding energy of 398.6 eV corresponding to pyridinic form of nitrogen whereas peak 2 at ca. 400.1 eV corresponds to pyrrolic and pyridonic nitrogen. The presence of various nitrogen groups such as amides, amines or imides are also indicated by a peak around this binding energy only. Due to similarity in the chemical environments of pyrrolic and pyridonic nitrogens, it is not possible to distinguish them within the accuracy of XPS measurements.

55

Peak 3 at 401.1 eV and peak 4 at ca. 405.3 eV can be attributed to quaternary nitrogen and pyridinic nitrogen oxides respectively [43,44]. It can be seen that preparation of carbon adsorbents by carbonization and activation up to 700
C produced carbons having similar nitrogen functionality, as indicated by similar N1s core level envelopes. In these prepared carbons, peak 1, representing pyridinic nitrogen, showed the maximum contribution among the nitrogen functional groups present on the carbon surface. However, pyrrolic and pyridonic nitrogens (peak 2) and quaternary nitrogen (peak 3) were found to be present in similar amounts in these carbons. But in case of C-800 carbon, there is a decrease in the area percentage of peak 1 and 2 while an increase in peak 3 thereby suggesting the formation of more stable species (quaternary nitrogen) under severe thermal conditions. Pyridinic, pyridonic and pyrrolic nitrogen groups are reported to exhibit Lewis basicity whereas quaternary nitrogen demonstrates Lewis acidity [8,45]. Thus, all carbons except C-800 will display basic character and will have higher affinity towards slightly acidic CO2 molecules. On the other hand, C-800 carbon will show acidic character on account of quaternary nitrogen moieties present on its surface.

Fig. 3. High resolution XPS spectra of N1s region of (a) C-500, (b) C-600, (c) C-700, and (d) C-800.

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C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

Fig. 4. Temperature programmed desorption profile of CO2 on C-700.

Temperature programmed desorption profile of CO2 on C-700 carbon is displayed in Fig. 4. A broad peak in the temperature range of 100–150
C was observed, with a maximum at ca. 132
C, which suggests the occurrence of adsorption of CO2 by chemisorption on the surface of prepared nitrogen enriched carbon adsorbent [46,47]. This could be because of formation of covalent bonds between acidic CO2 molecules and basic surface functionalities present on carbon surface. Thus, desorption of CO2 at such a high temperature indicates presence of basic functional groups on carbon surface that are obtained from the nitrogen containing polymeric precursor. This has also been confirmed by X-ray photoelectron spectroscopy of these carbon materials [25]. 3.2. CO2 capture performance 3.2.1. Effect of carbonization temperature Fig. 5 presents the CO2 uptake at 30
C for the synthesized adsorbents under pure CO2 atmosphere. All the adsorbents showed fast adsorption kinetics with 95% of the CO2 being adsorbed in ca. 200 s, then reaching a plateau. On increasing the carbonization

Fig. 5. CO2 uptake of synthesized adsorbents at 30
C under pure CO2 flow.

temperature from 500
C to 600
C, there is a small decrease in CO2 uptake from 0.727 mmol/g to 0.685 mmol/g which can be attributed to the loss of nitrogen and small decrease in total pore volume. On further increase in carbonization temperature to 700
C, CO2 uptake increased to 0.80 mmol/g, the highest among the prepared adsorbents. This could be assigned to improvement in the textural properties with temperature. Furthermore, presence of basic nitrogen functional groups in these carbons mainly in the form of pyridinic, pyrrolic and pyridonic nitrogen, as suggested by XPS, provide active sites for adsorption of acidic CO2 adsorbate. It is worth mentioning that CO2 adsorption potential of an adsorbent is not directly linked with its total nitrogen content but depends on the type (i.e. nature) of functional groups present. Because of this reason, C-700 showed higher CO2 adsorption capacity than C-600 and C-500 carbons though the former contained lower nitrogen content. On the other hand, C-800 showed the least CO2 adsorption capacity, among the prepared carbons, of 0.602 mmol/g due to drastic decrease in surface area, pore volume and nitrogen content in addition to lesser affinity for CO2 molecules due to formation of acidic quaternary nitrogen. Hence, both the textural properties and surface chemistry play an important role in CO2 uptake performance of these adsorbents. As the highest CO2 uptake at 30
C is exhibited by C-700, only this adsorbent was studied for evaluating the effect of temperature on CO2 uptake. 3.2.2. Effect of adsorption temperature and CO2 concentration Effect of adsorption temperature and CO2 concentration on CO2 uptake was studied by carrying out CO2 adsorption experiments on C-700 at four different adsorption temperatures of 30, 50, 75 and 100
C under different CO2 concentrations (10% to 100%) and the results are illustrated in Fig. 6. It was observed that at a fixed CO2 concentration, the curve reached a plateau faster at higher adsorption temperatures. However, this trend was more prominent at lower CO2 concentration. At 10% CO2 concentration, saturation was achieved in ca. 150 s for adsorption at 30
C but adsorbent was saturated in ca. 100 s for adsorption at 75
C and 100
C. This can be attributed to faster adsorption kinetics at higher adsorption temperature that results in faster saturation of the adsorbent with CO2 adsorbate molecules. On the other hand, for a fixed adsorption temperature, adsorbent got saturated with the adsorbate faster at higher CO2 concentrations. For adsorption temperature of 75
C and 100
C, equilibrium was achieved in ca. 100 and 50 s for 10% and 100% inlet CO2 concentration respectively. Increased concentration gradient at higher CO2 concentration leads to faster adsorption process. With increase in adsorption temperature, CO2 uptake of C-700 continued to decrease showing the typical behavior of a physiosorption process (Table 2). Here, both rate of molecular diffusion and surface energy increase with temperature and hence the adsorbed gas becomes unstable on the adsorbent surface thereby resulting in desorption of adsorbed gas molecules [45]. The CO2 uptake of C-700 at 30
C (0.80 mmol/g) is around 3 times the value at 100
C (0.284 mmol/g) using pure CO2. For 10% CO2 flow, uptake of CO2 decreased from 0.531 mmol/g to 0.112 mmol/g with increase in temperature from 30
C to 100
C. By increasing CO2 concentration from 10% to 100%, CO2 uptake increased from 0.531 mmol/g to 0.80 mmol/g at 30
C. This could be assigned to increased driving force at high CO2 concentrations. The values for CO2 adsorption capacity obtained for the prepared carbons are comparable to the values for previously reported materials under dynamic conditions [48,49]. Porous carbon adsorbents were synthesized from melamine-formaldehyde resin with K2CO3 activation by Drage et al. [23] and the adsorbent carbonized at 700
C showed CO2 uptake of 0.86 mmol/g at 30
C. Arenillas et al. [50] prepared carbon adsorbents from

C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

57

Fig. 6. Effect of adsorption temperature on CO2 uptake of C-700 at (a) 10% CO2, and (b) 100% CO2 concentration.

polyethylene terephthalate by using various nitrogen functionalities like acridine, carbazole and urea and chemical activation with KOH. Carbon materials from acridine and urea showed CO2 uptake of around 0.60 mmol/g at 30
C while carbon obtained from carbazole (P-C) exhibited CO2 adsorption capacity of 1.09 mmol/g at 30
C. At 100
C, P-C showed CO2 uptake of 0.27 mmol/g which is similar to the values obtained for C-700. Steam activated carbon obtained from fly ash was found to have CO2 uptake of 0.95 mmol/g

at 30
C and 0.42 mmol/g at 75
C [51]. CO2 uptake of C-700 at higher temperatures is comparable with literature reported values which has more relevance to its application to CO2 capture from flue gases. Thote et al. [27] prepared carbon adsorbent from soybean by ZnCl2 activation and reported the maximum CO2 uptake of 0.93 mmol/g at 30
C and 15.4% CO2 flow. CO2 uptake at 75
C is 0.51 mmol/g but the prepared adsorbent was not regenerated completely in the second cycle thereby exhibiting

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C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

Table 2 Uptake of CO2 (mmol/g) on C-700 at different CO2 concentrations and adsorption temperatures. CO2 concentration (%)

Temperature (
C) 30

50

75

100

10 20 50 100

0.531 0.621 0.714 0.800

0.262 0.339 0.398 0.483

0.212 0.295 0.359 0.424

0.112 0.156 0.205 0.284

zero CO2 uptake in the third cycle. However, carbon adsorbents without nitrogen were prepared by nanocasting technique using resorcinol-formaldehyde resin as precursor [46]. Carbon obtained at 700
C (RF-700) exhibited comparable specific surface area as that of C-700 but showed higher CO2 adsorption capacity of 0.76 mmol/g at 30
C under 12.5% CO2 flow. This could be attributed to higher micropore volume of RF-700 sample than C-700 though former contained no nitrogen basic functional groups. Hence, it can be said that carbon adsorbents, from carbonization followed by activation of templated nitrogen enriched resin, in the present work exhibited comparable CO2 adsorption capacities under dynamic conditions at higher adsorption temperatures. This is believed to be a better approach in synthesizing the adsorbents, for CO2 capture, as compared to direct carbonization and activation of raw material (with no control over pore structure) or chemical activation using various chemicals, which further needs a thermal treatment thereby making it an energy intensive process. 3.2.3. Cyclic adsorption-desorption study Cyclic adsorption-desorption experiments were carried out to check the stability and reusability of the adsorbents. Desorption study was carried out by increasing the temperature from adsorption temperature to 200
C and switching the gas from pure CO2 to pure N2 at the same flow rate. As seen from Fig. 7, adsorbed CO2 was quickly desorbed from the adsorbent surface on

switching the gas and increasing the temperature. The adsorptiondesorption cycle was repeated ten times with no significant change being observed in the kinetics of CO2 uptake or desorption. Similar trend was observed for all the adsorbents. Thus, these prepared adsorbents could be easily regenerated over multiple cycles without any loss of adsorption performance confirming their stability. 3.2.4. Static CO2 and N2 capture performance As nitrogen enriched carbon obtained from carbonization and activation at 700
C exhibited the best dynamic CO2 uptake, pure component adsorption and desorption of CO2 and N2 on this carbon sample was carried out in order to evaluate its static volumetric adsorption capacity and results are presented in Fig. 8. It can be seen that for each adsorption temperature, CO2 and N2 uptake tend to increase with increase in pressure. However, CO2 and N2 uptake decreased with increase in temperature of adsorption indicating exothermic nature of adsorption process [52,53]. CO2 uptake decreased from 1.21 mmol/g to 0.46 mmol/g with increase in adsorption temperature from 30
C to 100
C respectively. On the other hand, equilibrium N2 adsorption capacity of C700 was very small as compared to CO2 uptake which suggests that the prepared nitrogen enriched carbon exhibits higher selectivity towards CO2 over N2. Uptake of N2 by C-700 at 30
C was only 0.09 mmol/g while at 100
C this value reduced to 0.03 mmol/g. Adsorption-desorption isotherms of CO2 and N2 exhibited a hysteresis loop for all adsorption temperatures indicating requirement of small energy for adsorbent reuse. Nitrogen doped carbons from pyrolysis of resorcinol-formaldehyde and lysine demonstrated higher CO2 uptake of 3.13 mmol/g at 30
C than C-700 but comparable CO2 uptake at higher temperatures [18]. It is important to note that static adsorption capacity is always higher than dynamic adsorption capacity of an adsorbent and dynamic adsorption capacity has more relevance for practical applications.

Fig. 7. Multiple CO2 adsorption-desorption cycles of C-700 at 30
C.

C. Goel et al. / Journal of CO2 Utilization 16 (2016) 50–63

59

Fig. 8. Pure adsorption (&)-desorption (&) of (a) CO2 and (b) N2 on C-700 at different adsorption temperatures.

3.2.5. Kinetic studies Pseudo-first and pseudo-second order models were used to evaluate the kinetics of CO2 uptake on prepared adsorbents at 30
C and at different temperatures for C-700. Fig. 9 depicts the experimental CO2 uptake as a function of time on prepared adsorbents and the corresponding profiles obtained from the two adsorption kinetic models. Table 3 shows the parameters of kinetic models at 30
C along with correlation coefficients (R2) and associated errors for the prepared adsorbents. Pseudo-second order model underestimated the CO2 uptake up to ca. 300 s and after this time it continually overestimated the CO2 uptake until the process approached equilibrium. On the other hand, pseudofirst order kinetic model was in good agreement with the

experimental data over the entire adsorption range. This was further supported by higher value of R2 and lower value of error% for pseudo-first order model as compared to that for pseudosecond order model for all the adsorbents. Also, equilibrium CO2 uptake predicted by pseudo-first order model is in close agreement with the experimental values. Transportation mode of CO2 molecules on carbon surface describes the first order nature of the adsorption process and according to first-order kinetics, the rate of diffusion of CO2 into the adsorbent surface is faster than the occurrence of chemical reaction on the surface. Pseudo-first order rate constant was highest for C-700 suggesting the fastest adsorption kinetics among the prepared adsorbents.

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Fig. 9. Experimental CO2 uptake on synthesized adsorbents and corresponding fit to kinetic models at 100% CO2 and 30
C.

Table 3 Kinetic model parameters at 30
C for CO2 uptake on synthesized adsorbents. Kinetic model

Parameters

Sample C-500

C-600

C-700

C-800

Pseudo-first order

k1 (1/sec) qe (mmol/g) R2 Error%

0.017 0.730 0.999 2.181

0.016 0.690 0.997 2.765

0.029 0.795 0.981 2.850

0.015 0.609 0.994 4.265

Pseudo-second order

k2 (g/mmol.sec) qe (mmol/g) R2 Error%

0.030 0.810 0.970 8.564

0.029 0.770 0.965 10.323

0.056 0.850 0.968 3.194

0.029 0.686 0.956 10.972

Fig. S3 shows the experimental and kinetic order model for CO2 uptake of C-700 as a function of CO2 concentration at all adsorption temperatures and the results for the kinetic model parameters and error% are reported in Table S1. CO2 uptake of C-700 at different temperatures also followed the first order kinetics with maximum error of 5.4%. Experimental equilibrium values are in good agreement with pseudo-first order model predicted values. With increase in adsorption temperature, kinetics of CO2 uptake also increased as indicated by pseudo-first model constant values. This increase in rate constant was very small for adsorption at low CO2

concentrations while a significant increase in rate is observed for adsorption under pure CO2 flow. Adsorption at 30
C reached equilibrium in ca. 200 s while adsorption at 75
C achieved equilibrium within 100 s under 100% CO2 flow. This could be attributed to faster diffusion of CO2 gas molecules at higher temperatures and hence faster adsorption process. With increase in CO2 concentration, the rate constant was observed to increase which indicates faster diffusion of CO2 into the pores of the carbon adsorbent. Dependence of kinetic rate constants on temperature was evaluated from Arrhenius equation and activation energy for adsorption process are listed in Table 4. Activation energy for adsorption process was found to be in the range of 7.7–9.9 kJ/mol for CO2 concentration ranging from 10 to 100%. Positive values of Ea are due to increasing values of kinetic rate constant with adsorption temperature and these results are in agreement with literature reported values [54,55].

Table 5 Adsorption isotherm model parameters. Model

Parameters

Langmuir

qm (mmol/g) KL (1/atm) R2

Freundlich

KF (mmol/g.atm1/n) n R2

Temkin

KT (1/atm) b (kJ/mol) R2

Table 4 Parameters of Arrhenius equation for adsorption of CO2 on C-700 carbon adsorbent. CO2 concentration (volume%)

A

Ea (kJ/mol)

10 20 50 100

0.326 0.719 1.064 1.087

7.70 9.05 9.88 9.87

Temperature (
C) 30

50

75

0.823 16.845 0.948

0.511 9.850 0.932

0.464 8.330 0.949

0.332 4.356 0.925

0.803 5.817 0.991

0.483 4.009 0.976

0.430 3.661 0.959

0.281 2.545 0.978

1054.27 21.957 0.997

177.62 29.177 0.978

118.34 32.498 0.984

100

44.11 43.167 0.988

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Fig. 10. Experimental and Temkin model predicted equilibrium CO2 uptakes on C-700 at different adsorption temperatures.

3.2.6. Isotherm studies Non-linear regression was used to analyze the equilibrium data of CO2 uptake on C-700 and the parameters for three isotherm models, namely Langmuir, Freundlich and Temkin isotherm at different adsorption temperatures, reported in Table 5. Experimental data and Temkin model predicted equilibrium CO2 uptake is shown in Fig. 10 as function of adsorption temperature. Freundlich and Temkin isotherm models closely describe CO2 adsorption equilibrium data but the best fit is obtained by Temkin isotherm model as indicated by correlation coefficient (R2). Best fit by Temkin isotherm model suggests energetically heterogeneous adsorbent surface. Langmuir model parameter qm indicates maximum monolayer adsorption capacity and it decreased with increase in adsorption temperature because of the exothermic nature of adsorption process. Moreover, decrease in Freundlich parameter value (KF) with temperature and value of n > 1 imply that adsorption process is favorable at lower temperatures. 3.2.7. Thermodynamic studies Table 6 presents the thermodynamic parameters for CO2 adsorption on C-700. These parameters were obtained from the linear plot between ln (Keq) vs 1/T. The value of the correlation coefficient (R2) for this linear fit was found to be 0.952 suggesting the accuracy of these parameters. Values of DHo and DSo were found to be 39.556 kJ/mol and 0.0752 kJ/mol.K. The value of DGo for each adsorption temperature was found to be negative suggesting spontaneity and feasibility of the adsorption process. The negative value of DHo suggested exothermic nature of

Table 6 Thermodynamic parameters for CO2 adsorption on C-700. Temperature (
C) 30 50 75 100

 (kJ/mol)

DHo (kJ/mol)

DSo (kJ/mol K)

17.535 13.909 13.811 11.743

39.556

0.0752

adsorption process and positive value of DSo indicated increased randomness at the adsorbent-adsorbate interface and also adsorbent’s affinity towards CO2 [56]. The small value of DSo indicated that there was no significant change in entropy. Finally, the energy required to desorb CO2 from the prepared carbon adsorbent was calculated from isosteric heat of adsorption and sensible heat. Isosteric heat of adsorption was found to increase with increase in surface coverage which suggests the heterogeneous nature of adsorbent surface (Fig. S4). Same inference has been obtained from best fit by Temkin adsorption isotherm model. Isosteric heat of adsorption increased from 38.7 kJ/mol to ca. 69 kJ/mol with increase in surface coverage from 0.2 mmol/g to 0.5 mmol/g respectively. Lateral interactions between the adsorbed CO2 molecules could be another reason for increase in isosteric heat of adsorption [57]. Average value of isosteric heat of adsorption was calculated to be 53.9 kJ/mol. Specific heat capacity of C-700 adsorbent was found to be 1.2 J/gK and this value was used to calculate sensible heat required for raising the temperature of adsorbent to desorption temperature. Sensible heat required was estimated to be 255 kJ/mol CO2. Assuming 75% heat recovery [37], net sensible heat required is 63.75 kJ/mol CO2. Hence, thermal energy required for desorption of CO2 from the surface of C-700 carbon is ca. 117.6 kJ/mol CO2 captured or 2.67 MJ/kg CO2 captured. This means that energy required to desorb 1 kg CO2 from the carbon surface is 2.67 MJ which is obtained from combustion of fossil fuels, assuming that bituminous coal is used for energy generation. The amount of CO2 produced is 0.0884 kg from 1 MJ energy generation [58]. For 1 kg adsorbent basis, CO2 adsorbed is 0.8 mmol or 0.0352 kg. From the preceding, the energy required to desorb this adsorbed CO2 is equal to 0.094 MJ which will result in the generation of 0.0083 kg CO2 from combustion of bituminous coal. 4. Conclusions Nanocasting technique was used to prepare a series of mesoporous carbon adsorbents from HMMM polymeric precursor

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and mesoporous silica template by varying carbonization temperature. Textural properties and chemistry of the adsorbents were strongly influenced by the carbonization temperature. Carbonization at 700
C resulted in the carbon adsorbent with highest surface area and pore volume. Further increase in carbonization temperature resulted in collapse of the pore structure as signified by its textural properties, XRD and SEM. Among the prepared carbons, C-700 exhibited the highest CO2 uptake of 0.80 mmol/g at 30
C under pure CO2 atmosphere; its uptake decreased with increase in adsorption temperature implying the occurrence of physiosorption process. All the prepared carbons were also stable over ten cycles of adsorption-desorption showing their reusability in long term use. Furthermore, these carbon materials exhibited fast adsorption kinetics with major CO2 uptake occurring within 200 s, following pseudo-first order kinetic model at all CO2 concentrations and adsorption temperatures. Temkin isotherm model explained the adsorption of CO2 on carbon surface and suggested energetically heterogeneous adsorbent surface. Thermodynamic parameters suggested exothermic and spontaneous nature of adsorption process. The thermal energy required for desorption of adsorbed CO2 on the surface of nitrogen enriched carbon was calculated to be 2.67 MJ per kg CO2, providing minor environmental footprint if derived from bituminous coal combustion (0.008 kg CO2). Acknowledgements The authors sincerely acknowledge the financial support from Department of Science and Technology, New Delhi. Chitrakshi Goel acknowledges the financial support from DST-INSPIRE under its Assured Opportunity for Research Careers (AORC) scheme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcou.2016.06.002. References [1] H. Yang, Z. Xu, M. Fan, R. Gupta, R.B. Slimane, A.E. Bland, I. Wright, Progress in carbon dioxide separation and capture: a review, J. Environ. Sci. 20 (2008) 14– 27. [2] A. Garg, P.R. Shukla, Coal and energy security for India: role of carbon dioxide (CO2) capture and storage (CCS), Energy 34 (2009) 1032–1041. [3] J.D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R.D. Srivastava, Advances in CO2 capture technology—the U.S. department of energy’s carbon sequestration program, Int. J. Greenh. Gas Control 2 (2008) 9–20. [4] D.M. D’Alessandro, B. Smit, J.R. Long, Carbon dioxide capture: prospects for new materials, Angew. Chem. Int. Ed. 49 (2010) 6058–6082. [5] M.G. Plaza, C. Pevida, A. Arenillas, F. Rubiera, J.J. Pis, CO2 capture by adsorption with nitrogen enriched carbons, Fuel 86 (2007) 2204–2212. [6] S. Sengupta, V. Amte, R. Dongara, A.K. Das, H. Bhunia, P.K. Bajpai, Effects of the adsorbent preparation method for CO2 capture from flue gas using K2CO3/ Al2O3 adsorbents, Energy Fuels 29 (2015) 287–297. [7] R. Chatti, A.K. Bansiwal, J.A. Thote, V. Kumar, P. Jadhav, S.K. Lokhande, R.B. Biniwale, N.K. Labhsetwar, S.S. Rayalu, Amine loaded zeolites for carbon dioxide capture: amine loading and adsorption studies, Microporous Mesoporous Mater. 121 (2009) 84–89. [8] M. Sevilla, P. Valle-Vigón, A.B. Fuertes, N-doped polypyrrole-based porous carbons for CO2 capture, Adv. Funct. Mater. 21 (2011) 2781–2787. [9] J.R. Li, J. Sculley, H.C. Zhou, Metal-organic frameworks for separations, Chem. Rev. 112 (2012) 869–932. [10] J. Wei, L. Liao, Y. Xiao, P. Zhang, Y. Shi, Capture of carbon dioxide by amineimpregnated as-synthesized MCM-41, J. Environ. Sci. 22 (2010) 1558–1563. [11] A. Zhao, A. Samanta, P. Sarkar, R. Gupta, Carbon dioxide adsorption on amineimpregnated mesoporous SBA-15 sorbents: experimental and kinetics study, Ind. Eng. Chem. Res. 52 (2013) 6480–6491. [12] X. Xu, C. Song, J.M. Andrésen, B.G. Miller, A.W. Scaroni, Preparation and characterization of novel CO2 molecular basket adsorbents based on polymermodified mesoporous molecular sieve MCM-41, Microporous Mesoporous Mater. 62 (2003) 29–45. [13] T. Valdés-Solís, A.B. Fuertes, High-surface area inorganic compounds prepared by nanocasting techniques, Mater. Res. Bull. 41 (2006) 2187–2197.

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