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Pure and strontium carbonate nanoparticles functionalized microporous carbons with high specific surface areas derived from chitosan for CO2 adsorption

  • Gurwinder SinghEmail author
  • Steffi Tiburcius
  • Sujanya Maria Ruban
  • Dhanush Shanbhag
  • C. I. Sathish
  • Kavitha Ramadass
  • Ajayan VinuEmail author
Original Article

Abstract

A simple one step chemical activation involving a relatively mild chemical potassium citrate and inexpensive chitosan as a carbon source is presented for the synthesis of microporous carbons with a high specific surface area. The synthesis avoids the use of a highly corrosive and unfriendly activating agent, KOH, which is generally used for the creation of the porosity. The obtained microporous carbons display a high specific surface area which can be tuned by varying the amount of activating agent (1784–2278 m2 g−1). The optimized material exhibits a high surface area of 2278m2 g−1 and a pore volume of 1.00 cm3 g−1. As high microporosity is beneficial for CO2 adsorption, the prepared materials are employed as adsorbents for the capture of CO2. The optimized sample displays excellent CO2 uptakes at 0 °C/0.15 bar (1.1–1.8 mmol g−1) and 0 °C/1 bar (4.3–6.1 mmol g−1). The high surface area of the materials allows for high CO2 uptakes at 0 °C/30 bar (17.3–22.0 mmol g−1). The microporosity of these high surface area carbons is further decorated with strontium carbonate nanoparticles. The adsorption capacity per unit surface area is increased significantly upon the incorporation of the nanoparticles, revealing the role of the nanoparticles on the enhancement of the CO2 adsorption capacity. A similar strategy could be extended for the fabrication of a series of microporous carbons derived from biomass for many applications including CO2 capture.

Keywords

Microporous carbons Potassium citrate Chitosan High surface area High CO2 uptakes 

1 Introduction

Power generation plants are largely driven by the combustion of fossil fuels which release high amounts of CO2 into the atmosphere, which causes global warming and climate change [1, 2, 3]. Owing to the rapid urbanization, population growth, and modern life style, the reliance on using fossil fuels is significantly increased. Therefore, there will be no immediate respite from the problem in the foreseeable future unless some immediate measures are undertaken to curb CO2 emissions. The CO2 emitted from the flue gas streams of industries not only needs to be isolated and captured efficiently in a cost-effective process but at the same time be stored and/or converted in a safe and economic manner [4, 5]. Currently, the most common method of CO2 capture used in industries involves the use of amine-based systems in which the flue gas stream is scrubbed to separate and capture CO2 through the formation of chemical bonding between the amine and CO2. However, such amine-based capture systems are not only expensive but also require high power consumption for regeneration and create a risk with their toxicity and corrosivity [6, 7]. Porous materials such as zeolites and MOFs have also been shown to possess great potential for CO2 capture owing to their microporous structure and the ability to incorporate metal cations to enhance their basicity [8]. A promising alternative to the traditional amine-based systems is the porous carbon materials which form weak physical bonding with the captured carbon and make the regeneration process a lot easier and faster [9]. Their low cost, hydrophobicity, thermal and chemical stability, and environmentally benign synthesis present additional advantages. Natural biomass is the most promising precursor for designing the porous carbon-based adsorbents for CO2 capture which can satisfy all the above criteria [10]. Additionally, the biomass-based porous carbon materials could be easily incorporated with basic moieties which can help in enhancing the adsorbent-adsorbate interactions [11]. However, the final textural properties and the chemical composition of the porous biomass-based carbons, which are directly linked to the final performance of these materials for adsorption, separation, or energy applications, are related with the type and nature of the biomass precursors and their composition.

The biomass chitosan is available in abundant quantities and was chosen as a precursor for synthesizing porous carbon materials. A conventional two-step carbonization and activation approach is usually employed to produce porous carbons from biomass. The biomass precursor is first carbonized at high temperature (500–600 °C) to yield non-porous carbon which is then chemically activated in the second step by employing carbonization at temperatures exceeding 600 °C. The most versatile chemical used for chemical activation is KOH as it not only induces high porosity in the desired micro or meso domains but also generates high surface area in the porous carbons. We have demonstrated this in our previous studies the synthesis of porous carbon material with a high microporosity and high surface area from biomass Arundo donax through KOH activation [12]. High porosity and high surface area are considered as ideal parameters for a CO2 adsorbent that control the adsorption of CO2 capture at different pressures and temperatures [13, 14]. However, the usage of KOH for chemical activation suffers from drawbacks such as toxicity and corrosiveness [15, 16]. This creates the need for finding an activating agent which is more environmentally friendly and at the same time can generate high porosity and surface area in a controlled manner in the porous carbon materials.

In our present study, we synthesized a series of novel high surface area microporous carbons from biomass precursor of chitosan by using potassium citrate as a mild and versatile activating agent in a simple one-step solid state reaction approach. Chitosan is an inexpensive biomass and potassium citrate is far less corrosive as compared to KOH. This underlines the potential for these two precursors for the fabrication of porous carbon materials with high surface areas that could be suitable for CO2 capture. The optimized material CPC2 obtained with chitosan/potassium citrate ratio of 2 showed a high surface area of 2278 m2 g−1 and a pore volume of 1.00 cm3 g−1. When tested for its CO2 capture ability, this material showed excellent adsorption of 22.0 mmoles of CO2 g−1 at 0 °C/30 bar and 5.9 mmoles of CO2 g−1 at 0 °C/1 bar. We further demonstrated that strontium carbonate particles ranging in size from nm to μm can easily be anchored onto the surface of porous carbons via a simple one-step solid state reaction between porous carbon and strontium acetate. The introduction of SrCO3 on the surface of porous carbon helped in a higher CO2 adsorption per unit surface area. The synthesis scheme is quite simple and environmentally friendly which when coupled with their overall performance for CO2 capture presents an exciting opportunity in the field.

2 Experimental

2.1 Materials synthesis

2.1.1 Synthesis of high surface area porous carbons

Chitosan was used as the starting precursor for the synthesis of high–surface area porous carbons which were then modified with strontium acetate to decorate the surface of the formed porous carbons with strontium carbonate nanoparticles. The procedure adopted for the synthesis involves one-step solid state carbonization cum activation and is described as follows. A given amount of chitosan powder (1 g) was mixed in a pestle and mortar with different amounts of potassium citrate ranging from 0 to 4 g in a single-step solid state mixing procedure. These five individual mixtures were then transferred onto a carbonization boat and heated to 800 °C using a heating rate of 10 °C min−1 and held at this temperature for 2 h under N2 atmosphere in a tubular furnace. After the carbonization cum activation, all five materials were washed with 2 M HCl and distilled water to clear all the pores filled with left over inorganic constituents. The above five materials are labeled as CPCn, where CPC denotes chitosan-derived porous carbon and n stands for the amount of potassium citrate added for the activation reaction.

2.1.2 Modification of high surface area porous carbon using strontium acetate

For achieving the decoration of strontium carbonate on the surface of porous carbon, we chose to modify the porous carbon with the highest surface area (CPC2) with strontium acetate using a simple solid state mixing approach. Three different combinations with each containing 400 mg of CPC2 and different amount of strontium acetate as 40 mg, 120 mg, and 200 mg were mixed together in their solid form using a pestle and mortar. The different weights of the strontium acetate represent 10%, 30%, and 50% of the weight of CPC2. After thorough mixing, the three individual mixtures were put in a ceramic boat for carbonization at a temperature of 900 °C achieved using a heating rate of 5 °C min−1 and held at this temperature for 2 h in a tubular furnace. The carbonized black materials were then washed with 2 M HCl and distilled water to obtain strontium carbonate decorated porous carbons. A total of three materials were prepared and are labeled CPC2S-m, where CPC2 is the parent porous carbon, S stands for strontium carbonate modified carbon, and m denotes the integer value from 1 to 3. The schematic illustration of the synthesis is depicted in Scheme 1.
Scheme 1

Schematic illustration of the synthesis of pure and strontium carbonate nanoparticles functionalized microporous carbon

2.2 Characterization and CO2 adsorption experiments

2.2.1 Characterization

The textural parameters of all the synthesized carbon materials were determined from nitrogen sorption carried out at a temperature of − 196 °C using a Micromeritics ASAP analyzer model number 2420. The analysis samples were outgassed at a temperature of 200 °C under constant vacuum. The total surface area was obtained using the Brunauer-Emmett-Teller (BET) method, and the total pore volume was obtained at a relative pressure (P/Po) of 0.99. The micropore area and micropore volume were obtained using the t plot method. As the materials are highly microporous in nature, the MP method was used for plotting the pore size distribution (PSD), and the pore size was taken as the maxima of the PSD curves. The X-ray diffraction (XRD) patterns of all samples were obtained using Panalytical Empyrean XRD instrument operating with CuKα1 and Kα2 radiations at a wavelength of λ = 1.540598 Å and 1.544426 Å. The copper radiation was produced using a generator voltage of 40 kV and a tube current of 40 mA. Each individual sample placed at the center of a circular sample holder was scanned within a 2θ angle range of 5 to 80° using a scan step size of 0.006 and a time per step of 97.92. The surface functional groups of the synthesized materials were determined using Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra were acquired using a Perkin Elmer instrument and an average of 32 scans were performed for each sample in the spectral range of 4000–400 cm−1 at a resolution of 4 cm−1. All samples were mixed with KBr and designed into thin pellets before the FTIR measurements. TG/DSC thermal behavior of the synthesized materials was investigated using the Perkin Elmer thermogravimetric analyzer (Model no. STA-8000) wherein both thermogravimetric (TG) and differential scanning calorimetry (DSC) were performed at the same time. For each measured sample, weight closer to 10 mg was taken in a tiny crucible which was heated in a confined compartment at the rate of 10 °C min−1 to raise the temperature from 30 to 1000 °C under N2 atmosphere. Zeiss Merlin Scanning electron microscope was used to investigate the surface morphology of the synthesized materials. An operating voltage of 2 kV was employed for obtaining the SEM images.

2.2.2 CO2 adsorption experiments

The high pressure CO2 adsorption of the chitosan-derived porous carbons and the strontium carbonate-decorated porous carbons was carried out using a Quantachrome high pressure sorption instrument. Prior to analysis, all samples were degassed for overnight under constant vacuum and constant heating at a temperature of 200 °C for complete removal of any adsorbed species or moisture. All samples to be analyzed were measured within a pressure of 0 to 30 bar at a constant temperature of 0 °C. For CPC2 and CPC2S1 samples, further measurements at temperatures of 10 °C and 25 °C were also carried out in order to ascertain the effect of temperature on adsorption and for the calculation of isosteric heat of adsorption (Qst). The value of Qst was calculated by employing the Clausius Clapeyron equation to adsorption isotherms obtained at three temperatures of 0 °C, 10 °C, and 25 °C.

3 Nitrogen adsorption-desorption analysis

Two series of samples were studied in our current investigation, which include chitosan-derived porous carbon and strontium carbonate-modified porous carbon. Figure 1 shows the N2 adsorption-desorption isotherms and pore-size distribution of all the synthesized samples. The quantification of the textural parameters is summarized in Table 1. As per Fig. 1, the general shape of sorption curves of all the studied samples belongs to type I according to the International Union of Pure and Applied Chemistry (IUPAC) classification [17]. There is high adsorption of nitrogen in the initial stages when the relative pressure is less than 0.2 and thereafter a near saturation is achieved. This indicates that the synthesized materials are microporous in nature which is the desired property for CO2 adsorption at low pressures. The sample CPC0 which is synthesized without any potassium citrate activation adsorbs the lowest amount of nitrogen while all other samples prepared using potassium citrate activation display a much higher amount of nitrogen adsorption (Fig. 1A). This demonstrates the successful implication of the solid state chemical activation of chitosan using potassium citrate in generating porosity in the carbon structure. It is to be noted that the porous carbon CPC2 was further modified and decorated with strontium carbonate, and all these modified samples CPCS1–3 show the presence of H4 hysteresis loop in their sorption isotherms (Fig. 1C) [18]. According to the modern classification of the hysteresis loops, such isotherms identify the materials which contain slit-shaped pores arising from the plate-like particles in the carbon. It is surmised that the reaction of strontium-based species generated during carbonization prove to be beneficial for the pore widening process.
Fig. 1

A, B N2 sorption isotherms and pore-size distribution curves of (a) CPC0, (b) CPC1, (c) CPC2, (d) CPC3, and (e) CPC4. C, D N2 sorption isotherms and pore size distribution curves of (a) CPC2, (b) CPC2S1, (c) CPC2S2, and (d) CPC2S3

Table 1

Textural properties of the prepared pure microporous carbons and strontium carbonate functionalized porous carbons

Material

SABETa

(m2 g−1)

SAmicrob

(%)

V total c

(cm3 g−1)

V micro d

(%)

PWe

(nm)

CPC0

598

93

0.24

88

0.56

CPC1

1824

64

0.87

55

0.56, 0.76

CPC2

2278

71

1.00

63

0.56, 0.73

CPC3

1996

80

0.88

72

0.56, 0.72

CPC4

1784

84

0.78

74

0.56, 0.66

CPCS1

1608

71

0.76

61

0.56, 0.72

CPCS2

1217

72

0.57

61

0.56, 0.72

CPCS3

1016

70

0.49

59

0.56, 0.72

aTotal specific surface area calculated using Brunauer, Emmet and Teller method

bPercentage of micropore surface area (t plot)

cTotal pore volume calculated at p/po = 0.99

dPercentage of micropore volume (t plot)

ePore width calculated using MP method

The quantification of the textural parameters is presented in Table 1 which shows that the material prepared without activation (CPC0) shows a surface area of 598 m2 g−1 whereas all other materials prepared with potassium citrate activation or strontium carbonate modification show a specific surface area in the range of 1016–2278 m2 g−1. The material CPC2 displays the highest surface area of 2278 m2 g−1 and pore volume of 1.00 cm3 g−1 which indicates that the potassium citrate/chitosan ratio of 2 is an optimum condition for converting chitosan into highly porous carbon. The same sample after modification with strontium acetate displayed a decrease in the surface area from 2278 m2 g−1 to a range of 1608–1016 m2 g−1 depending on the quantity of strontium acetate used for modification. It is possible that the strontium carbonate particles occupy some of the surface active sites which is evident from the SEM images. The material CPC0 shows the highest amount of microporous content in terms of surface area (93%) and micropore volume (88%). However, the micropore contribution subsides upon activation, and the overall porosity is represented by a mix of micro and non-micropores with micropores being the dominant contributor. The co-existence of such pores is conducive for CO2 adsorption process in two ways wherein the micropores play a crucial role for CO2 adsorption at low pressure while larger pores dominate CO2 adsorption at high pressures.

The generation of micropores with different sizes upon activation can also be generalized from the pore size distribution curves based on the MP method, which are presented in Fig. 1B and D. The non-activated sample CPC0 shows a single pore size maximum of 0.56 nm whereas the pores are distributed into a bimodal structure with pore size maxima located at 0.56 nm and 0.66–0.76 nm upon activation. The overall volume of the pore size maxima depends on the quantity of potassium citrate used for activation or the quantity of strontium acetate used for modification. The pore volume increases with the increasing amount of potassium citrate used for activation and decreases with an increasing amount of strontium acetate used for modification.

4 X-ray diffraction (XRD) analysis

The crystalline nature and the purity of the synthesized materials were investigated by XRD measurements. Fig. S1 shows the XRD patterns for all samples except the ones modified with strontium carbonate which are shown in Fig. 2A. The non-activated sample CPC0 and all the activated samples ranging from CPC1 to CPC4 were black in color and exhibited typical shoulder peaks for an amorphous carbon in their XRD patterns. These two XRD peaks occur at the 2θ angle positions of ca 25° and 43° and correspond to the 002 and 100 reflection modes in an amorphous carbon [19]. Interestingly, the lower angle peak is more pronounced in the non-activated sample CPC0 and it also shows several small crystalline peaks arising from the impurities. The porous carbon materials obtained after the activation with potassium citrate show no such crystalline peaks and hence considered as pure carbon. The disappearance of lower angle peak in activated samples CPC1–2 is evident of diminished interplanar spacing of carbon layers lying in 002 reflection plane [20]. The reappearance of lower angle peak in samples CPC3 and CPC4 indicates that a high activation amount of potassium citrate is beneficial in widening the interplanar spacing of the amorphous carbon layers. The higher angle peak corresponding to a 100 reflection plane shows a similar trend in terms of intensity and position in all the samples and represents the randomly oriented layers in the carbon structure [21]. The XRD patterns of porous carbons decorated with strontium carbonate are shown in Fig. 2B which shows highly crystalline peaks corresponding to strontium carbonate as per the previous literature [22, 23]. The intensity of the SrCO3 peaks in these samples varies as per the amount of strontium acetate used for modification, and as such CPCS3 which was synthesized using the highest amount of strontium acetate shows the much higher intensity of the strontium carbonate peaks than those of other samples. The major crystalline peaks of SrCO3 are observed at 2θ angle positions of 25.2, 26, 28.3, 29.6, 31.3, 35, 36.2, 43.9, 45.5, 46.4, 47.6, and 49.8. The decoration of flower-like SrCO3 nanoparticle is also confirmed from the SEM images.
Fig. 2

A X-ray diffraction patterns and B TG/DSC profiles of (a) CPC2, (b) CPC2S1, (c) CPC2S2, and (d) CPC2S3

5 Thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis

The thermal stability of the SrCO3-decorated porous carbons was investigated using thermogravimetric measurements carried out within a temperature range of 30–1000 °C. The thermal behavior of parent porous carbon CPC2 and the strontium carbonate-decorated porous carbons CPC2S1–3 was compared. As shown in Fig. 2B, there are two noticeable stages observed for the thermal decomposition in all the samples. The initial weight loss at a temperature of < 100 °C could be attributed to the removal of the slightest amount of the moisture that has been adsorbed on the surface of the material due to their exposure to the atmosphere. The second weight loss does not start until the temperature exceeded to 800 °C. In the case of CPC2, it could be attributed to the loss of the functional groups on the surface whereas the decomposition of SrCO3 is responsible for this weight loss in samples CPC2S1–3 [24]. Previous literature on the thermal decomposition of Sr(CH3COO)2 suggests its conversion into SrCO3 takes place between a temperature range of 400–480 °C [25]. The SrCO3 starts decomposing at temperatures close to 925 °C; however, the full decomposition into SrO and CO2 requires temperatures > 925 °C. In our investigations, SrCO3 is deposited on the surface of the porous carbon, and the content of the SrCO3 nanoparticles is directly proportional to the amount of Sr(CH3COO)2 added for the modification as per the XRD investigations. The TG investigations, however, revealed that the SrCO3-decorated porous carbon undergoes decomposition at temperatures > 800–900 °C which means that even though the synthesis process was carried at 900 °C, the surface-deposited SrCO3 particles are not stable at temperatures exceeding 800 °C. This could be happening since they are merely supported on the porous carbon and are susceptible to decomposition at high temperatures. The overall weight loss of all the samples at temperatures < 800 °C is 20–25% which signifies that these materials are quite stable to the heating effects and could be used for CO2 capture under a wide range of temperature. The DSC curves for these samples show that the overall heat flow during the thermal treatment is positive and the process is endothermic in nature.

6 Scanning electron microscope (SEM) and electron dispersive X-ray spectroscopy (EDS) analysis

The surface morphology of the porous carbons and the size and shape of SrCO3 nanoparticles were investigated using the SEM analysis. Figure 3 shows a comparison of the sample CPC2 which has the highest surface area among the porous carbons and the sample CPC2S1 which has the highest surface area among the strontium carbonate-decorated porous carbons. Both samples display a high porosity and sponge-like morphology which is typically associated with the porous carbons. A closer look at the Fig. 3 a–c reveals that potassium citrate acts as a two-way agent as it creates porosity and also acts as a structure-directing agent resulting in hollow porous carbon. The presence of strontium carbonate anchored to the surface of porous carbon can be easily detected in Fig. 3 c and d. These images reveal the formation of flower-shaped SrCO3 particles of varying sizes in the nanometer scale on the surface of porous carbon.
Fig. 3

SEM images of a, b CPC2 and c, d CPC2S1

The presence of different elements in the porous carbon CPC2 and strontium carbonate-decorated carbon CPC2S1 was further confirmed by EDS analysis, and the results are shown in Fig. S2 and summarized in Table S1. The sample CPC2 shows the presence of carbon as the main element followed by oxygen and nitrogen. The sample CPC2S1 shows the presence of carbon as a dominant element which is followed by oxygen, strontium, and nitrogen. The EDS spot chosen for elemental analysis confirms the presence of SrCO3 in CPC2S1 (Fig. S2C).

7 Fourier transform infra-red (FTIR) analysis

The FTIR spectra were recorded to ascertain the nature of the functional groups present in the as-synthesized materials. Figure 4A shows the FTIR spectra of the non-activated sample CPC0 and all the activated samples ranging from CPC1–4. An O-H vibrational stretch is recorded at the position of 3398 cm−1, and carbonyl group stretching is observed at 1589 cm−1 [26]. The other bands appearing at 1224 cm−1 and 1090–796 cm−1 represent amide/amine C-N stretching and aromatic C-H stretching, respectively [27]. The reduced intensity of all the bands in porous activated carbon samples indicates that the number of functional groups gets reduced when carbonization is done in the presence of activating agent. Similar kind of bands are also observed for the strontium carbonate-decorated porous carbons except a very prominent band appearing at 1460 cm−1 which may be attributed to the C-O stretching vibration in carbonate groups [28].
Fig. 4

A FTIR spectra of (a) CPC0, (b) CPC1, (c) CPC2, (d) CPC3, and (e) CPC4 and B (a) CPC2, (b) CPC2S1, (c) CPC2S2, and (d) CPC2S3

8 CO2 adsorption and isosteric heat of adsorption

The synthesized carbon materials were investigated for their CO2 adsorption behavior within a pressure range of 0–30 bar, and the corresponding results are shown in Fig. 5 and summarized in Table 2. We have recently reported Arundo donax-derived activated porous biocarbons which recorded a large CO2 capture ability at high pressures [12, 21, 29]. In our current investigation, an inexpensive precursor of chitosan and a relatively mild activating agent of potassium citrate is used. All the samples were recorded for CO2 adsorption in the pressure range of 0–30 bar at a temperature of 0 °C. The samples CPC2 and CPC2S1 were also investigated for high-pressure CO2 adsorption at temperatures of 10 °C and 25 °C. As shown in Fig. 5A, the non-activated sample adsorbs the least amount of CO2 in the whole pressure range of 0–30 bar which is due to the lowest surface area. However, all the activated samples showed a higher amount of CO2 adsorption due to the better developed textural parameters such as surface area and pore volumes. The sample CPC2 with the highest surface area of 2278 m2 g−1 and pore volume of 1.00 cm3 g−1 and containing hierarchical porous network of different sized micropores exhibits exceptional CO2 uptake under both low pressure (5.9 mmol g−1 at 0 °C/1 bar) and high pressure (22.0 mmol g−1 at 0 °C/30 bar) conditions. The same sample also records impressive CO2 adsorption at a low pressure of 1.6 mmol g−1 at 0 °C/0.15 bar. Similarly, the CO2 adsorption at the other two temperatures of 10 °C and 25 °C is also highest for this material (Fig. 5B). The lowering of CO2 adsorption with increasing temperature suggests that the nature of the adsorption process is exothermic. It needs to be mentioned that the same batch of the samples CPC2 and CP2S1 was used for measuring the CO2 adsorption at 0 °C, 10 °C, and 25 °C which indicates that it is easier to liberate the adsorbed CO2 by degassing, and the materials can be reused on a repeated cycle. The material recyclability for CO2 adsorption was tested by using CPC2 in four cycles for the repetitive measurements carried out at 0 °C, and the results are plotted in Fig. S5. The different cycles exhibit almost similar CO2 adsorption which was recorded as 22.1, 21.9, 21.8, and 21.9 mmol g−1 at 30 bar. There is no significant alteration in the CO2 adsorption plots, and the overall CO2 adsorption values suggest that these materials are recyclable. Overall, these results demonstrate the suitability of the synthesized materials for both pre- and post-combustion CO2 capture processes. CO2 capture performance shown by CPC2 is the highest among several porous carbon-based adsorbents and comparable with other materials such as zeolites reported in the literature (Table 3). Moreover, the involved precursors are far less expensive than zeolites and MOFs, and the synthesis procedure is relatively simpler. The anchoring of strontium carbonate on the surface of porous carbon CPC2 does not result in any significant enhancement in the overall CO2 capture performance; however, there is a considerable increase in the CO2 adaption per unit surface area which is attributed to the increased acid-base interactions (Table S2). As shown in Fig. 5C, the sample CPC2 shows the highest amount of CO2 adsorption whereas the strontium carbonate-decorated porous carbon shows CO2 adsorption in the order of CPC2S1 > CPC2S2 > CPC2S3 at all pressure values. This is compelling evidence that the overall surface area is the main deciding factor that controls the CO2 adsorption in these materials but at the same time, the basicity of SrCO3 can increase the CO2 adsorption per unit volume.
Fig. 5

A CO2 adsorption isotherms of (a) CPC0, (b) CPC1, (c) CPC2, (d) CPC3, and (e) CPC4. B CO2 adsorption isotherms of (a) CPC2, (b) CPC2S1, (c) CPC2S2, and (d) CPC2S3. C CO2 adsorption isotherms of CPC2 and D CPC2S1 observed at three temperatures

Table 2

CO2 adsorption of the prepared pure microporous carbons and strontium carbonate-functionalized porous carbons

Material

CO2 adsorption (mmol g−1)

T a

T b

T c

P a

P b

P c

P a

P b

P c

P a

P b

P c

CPC0

1.4

2.7

5.6

      

CPC1

1.1

4.3

17.3

      

CPC2

1.6

5.9

22.0

1.2

4.7

19.7

3.4

17.5

CPC3

1.7

6.0

19.6

      

CPC4

1.8

6.1

17.8

      

CPCS1

1.4

5.2

19.4

3.6

17.6

2.8

14.2

CPCS2

1.2

4.1

15.7

      

CPCS3

1.0

3.4

13.3

      

T temperature, Ta 0 °C, Tb 10 °C, Tc 25 °C, P pressure, Pa 0.15 bar, Pb 1 bar, Pc 30 bar

Table 3

Comparison of CO2 uptake of pure porous carbons prepared in this study with that of the materials reported in the literature

Material

CO2 uptake

Reference

Porous carbon CPC4

6.1 mmol g−1 at 0 °C/1 bar

This work

Zeolite-templated carbon

6.9 mmol g−1 at 0 °C/1 bar

[30]

Porous polymer PPN-6-CH2DETA

4.3 mmol g−1 at 22 °C/1 bar

[31]

Hyper-cross-linked organic polymer HCP1

1.7 mmol g−1 at 25 °C/1 bar

[32]

Amine-attached MCM-48

0.8 mmol g−1 at 25 °C/1 bar

[33]

Porous carbon CPC2

22.0 mmol g−1 at 0 °C/30 bar

This work

Mesoporous Cu-SBA-15

17.1 mmol g−1 at 0 °C/30 bar

[34]

NEPB carbon

14.1 mmol g−1 at 0 °C/30 bar

[35]

Norit R1 extra carbon

10.2 mmol g−1 at 25 °C/40 bar

[36]

Hyper-cross-linked organic polymer HCP1

13.3 mmol g−1 at 25 °C/30 bar

[32]

A detailed analysis of the relationship between micropore volume vs CO2 adsorption at low pressure and surface area vs CO2 adsorption at high pressure of the studies samples is illustrated in Fig. 6 A–C. Among the porous carbons CPC1–4, there is a direct correlation observed between the micropore volume and the CO2 adsorption recorded at 0 °C/1 bar. The sample CPC1 with the lowest contribution of micropore volume of 55% adsorbs 4.3 mmol g−1 of CO2 whereas the sample CPC4 with the highest micropore volume of 74% adsorbs 6.1 mmol g−1 of CO2. The remaining two sample CPC2 and CPC3 also show a similar relationship between the micropore volume and the CO2 adsorption. These materials could potentially be applied for post combustion CO2 capture in various sectors such as power generation industries. We also found a direct relationship between the surface area and the high pressure CO2 adsorption for all the studied materials and the results are displayed in Fig. 6 B and C. Among the porous carbons, sample CPC2 adsorbs the highest amount of CO2 g−1 (22.0 mmol g−1) at 30 bar which is due to its high surface area of 2278 m2 g−1. The materials CPC1 and CPC4 having almost similar surface areas of 1824 m2 g−1 and 1784 m2 g−1 adsorb 17.8 mmol g−1 and 17.3 mmol g−1 of CO2. A similar observation is noticed for the strontium carbonate-modified porous carbons wherein the material CPC2S1 with the highest surface area of 1608 m2 g−1 adsorbs the largest amount of CO2 (19.4 mmol g−1), and sample CPC2S3 adsorbs only 13.3 mmol g−1 owing to its low surface area of 1016 m2 g−1. All these materials have the potential to be suitably applied for pre-combustion capture of CO2 from the flue gases in systems such as natural gas wells where CO2 needs to be adsorbed at very high pressures [37]. The strength of the interactions between the porous carbon CPC2 and strontium carbonate-decorated porous carbon CPC2S1 was ascertained from the values of isosteric heat of adsorption (Qst) which was calculated using Clausius Clapeyron’s equation. The heterogeneity of the surface with respect to isosteric heat of adsorption (energetic heterogeneity) is a common issue encountered with the microporous CO2 adsorbents. This results in a high value of Qst at low CO2 coverage; however, it decreases considerably as the CO2 loading is increased. This issue has been addressed through the incorporation of alkali metal or alkaline earth metal cations in the microporous framework in materials such as zeolites [38, 39]. However, in our current investigations, we found that the porous carbon or the SrCO3-decorated porous carbon shows a quite stable value of Qst at low-pressure loading and at high coverage of CO2 up to pressure of 8 bar (Fig. 6D). The Qst lies in the range of 25 to 28 kJ mol−1 which signifies that the adsorption of CO2 on the surface of these materials is physical in nature, and the adsorbed CO2 can be easily removed using mild heating (100–200 °C), thereby making the process of regeneration of these materials cost effective [40, 41]. The Qst value of samples CPC2 and CPCS2 is comparable with other CO2 adsorbents such as zeolites, MOFs, and silica. For example, zeolite ITQ-6 and silica SBA-15 show Qst value of ~ 26 kJ mol−1, which increases to 47 and 65 kJ mol−1, respectively, after functionalization with amine ligands [42]. Similarly, the MOF SNU-100 shows a Qst value of 29.3 kJ mol−1, which increases to 37.4 k J mol−1, when the MOF is impregnated with calcium metal cation [43]. In the materials discussed, Qst increased for the functionalized materials as the overall CO2 adsorption increased upon the incorporation of functionalization onto zeolites and /or MOFs. Previous literature has established that in case of such materials, the CO2 adsorption is mainly chemisorption which means any surface-functionalized moieties with a basic character would help increase the Qst; however, in case of porous carbonaceous materials, a similar functionalization might not increase the Qst as the nature of adsorption is physical which is mainly controlled by surface area rather than basic moieties [44]. This is illustrated in the currently studied materials as the sample CP2S1 which has a lower surface area (1608 m2 g−1) shows lower Qst value than the CPC2 which has higher surface area 2278 (m2 g−1). However, the margin of difference between their Qst values is not that large (~ 3 mmol g−1) despite a large difference in their surface area. The presence of SrCO3 does enhance the CO2 adsorption per unit surface area which could be due to its basic character being a carbonate; however, no significant increase in CO2 adsorption as compared to CPC2 is observed due to its low surface area and hence the Qst value is also low. It is always a challenge to synthesize a porous carbon material having suitable functionalization with basic moieties without compromising the overall surface area. Furthermore, a nearly constant value of Qst for either of the samples indicates that the surface of these materials is offering homogenous energetic conditions for the CO2 adsorption.
Fig. 6

A Relation between micropore volume and CO2 adsorption of (a) CPC1, (b) CPC2, (c) CPC3, and (d) CPC4 at 0 °C/1 bar. B Relation between surface area and CO2 adsorption of (a) CPC1, (b) CPC2, (c) CPC3, and (d) CPC4 at 0 °C/30 bar. C Relation between surface area and CO2 adsorption of (a) CPC2S1, (b) CPC2S2, and (c) CPC2S3 at 0 °C/30 bar. D Isosteric heat of adsorption curves of (a) CPC2 and (b) CPC2S1 calculated from their respective adsorption isotherms obtained at 0 °C, 10 °C, and 25 °C

9 Conclusions

In summary, we have devised a simple and facile one-step approach to synthesize high–surface area porous carbons and strontium carbonate-anchored porous carbons from chitosan using chemical activation with potassium citrate as a mild activating agent. Chitosan and potassium citrate are non-expensive precursors, and the synthesis procedure involves their simultaneous carbonization and activation to yield porous carbons with high specific surface areas. We further demonstrated the successful anchoring of the strontium carbonate particles on the surface of porous carbon by one-step carbonization of parent porous carbon and strontium acetate. The porous carbon CPC2 displayed a high specific surface area of 2278 m2 g−1 and a pore volume of 1.00 cm3 g−1. The anchoring of strontium carbonate on the surface of porous carbon resulted in a surface area of 1608 m2 g−1 with a pore volume of 0.76 cm3 g−1. The textural parameters such as surface area, pore volume, and pore size can be varied by adjusting the amount of the potassium citrate used for activation or strontium acetate used for modification of porous carbons. The high surface area in combination with a controlled pore structure in the micropore region is considered as ideal for the CO2 capture at pre- and post-combustion conditions. The best sample CPC2 adsorbs 22.0 mmol g−1 of CO2 at 0 °C/30 bar and 5.9 mmol g−1 at 0 °C/1 bar. These values are high when compared to other reported activated carbons and comparable to non-biomass-based materials. The low cost of the starting precursors, tunable textural features, and excellent CO2 capture performance of the presented materials make them as attractive adsorbents for various industry-related adsorption and separation processes.

Notes

Acknowledgments

A. Vinu is grateful to Australian Research Council (ARC) for the future fellowship award and to the University of Newcastle for the start-up grants.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

42247_2019_50_MOESM1_ESM.docx (4.2 mb)
ESM 1 (DOCX 4307 kb)

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Copyright information

© Qatar University and Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Gurwinder Singh
    • 1
    Email author
  • Steffi Tiburcius
    • 1
  • Sujanya Maria Ruban
    • 1
  • Dhanush Shanbhag
    • 1
  • C. I. Sathish
    • 1
  • Kavitha Ramadass
    • 1
  • Ajayan Vinu
    • 1
    Email author
  1. 1.Global Innovative Centre for Advanced Nanomaterials, Faculty of Engineering and Built EnvironmentThe University of NewcastleCallaghanAustralia

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