Oxygen-Functionalized Mesoporous Activated Carbons Derived from

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Oxygen functionalized mesoporous activated carbons derived from casein and their superior CO adsorption capacity at both low and high pressure regimes 2

Gurwinder Singh, Kripal S. Lakhi, C I Sathish, Kavitha Ramadass, Jae Hun Yang, and Ajayan Vinu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00059 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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ACS Applied Nano Materials

Oxygen Functionalized Mesoporous Activated Carbons derived from Casein and their superior CO2 Adsorption Capacity at both Low and High Pressure Regimes Gurwinder Singh,* Kripal S. Lakhi, CI Sathish, Kavitha Ramadass, Jae-Hun Yang, and Ajayan Vinu*

Global Innovative Centre for Advanced Nanomaterials, Faculty of Engineering and Built Environment, The University of Newcastle, University Drive, Callaghan NSW 2308 Australia. Email: [email protected]

Keywords: Casein, KOH activation, mesoporous carbon, high surface area and CO2 capture

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ABSTRACT We report on a simple approach for converting casein into oxygen functionalized mesoporous carbons (MPC) with an extremely high surface area and carbon content (~ 94 %). The materials synthesized by KOH activation of non-porous casein based carbon precursor (NPC) at a carbonization temperature of 800 °C displayed mixed micro and mesoporosity which can be easily controlled by simply adjusting the amount of the KOH. The optimized material MPC3, which was prepared with the KOH/carbon source ratio of 3, exhibits highest specific surface area of 3617 m2 g-1, pore volume of 2.16 cm3 g-1 and pore diameter of 2.7–3.1 nm. Owing to these excellent textural features, MPC3 achieved excellent CO2 uptake at 1 bar/ 0 °C (4.12 mmol g-1) and 30 bar/0 °C (39.1 mmol g-1) which is higher than that of mesoporous carbons and silica, activated carbon, carbon nitrides, and carbon nanotubes. The high performance is linked with the combination of excellent textural parameters and unique oxygen functionalities on the surface. It is also demonstrated that these materials are highly stable and can be used repeatedly without much loss of the adsorption capacity. Therefore, the presented materials can be a good candidate for pre and post-combustion CO2 capture.


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INTRODUCTION Global warming is a serious issue that has indisputably resulted from the increasing concentration of greenhouse gases (GHGs) in the atmosphere with carbon dioxide being the prominent contributor.1 Industrial emissions from large point sources such as power generation plants are major contributors to overall CO2 emissions to the globe.2 Therefore, it becomes important to control and minimize the amount of CO2 emissions from such sources. Advanced materials based technologies that will help to capture and convert the CO2 molecules are the possible solutions to tackle these environmental issues. However, from industrial perspective, advanced materials based adsorbents with a high CO2 capture efficiency integrated with a low cost and availability would be an appealing solution for curtailing the CO2 emissions through a simple capture process.3 To achieve this, advanced materials with properties such as large specific surface area, high robustness, high regeneration abilities, high hydrophobicity and fast adsorption kinetics etc. are desired.4 Recently, several advanced porous materials including zeolites, metal organic frameworks (MOFs), amine functionalized MOFs, coordination polymer networks (CPNs), ordered mesoporous carbons (OMCs), mesoporous carbon nitrides and activated porous carbons have been proposed as solid adsorbents for CO2 adsorption.5-10 Among these materials, activated porous carbons derived from low cost starting precursors emerged as alternative materials for efficient CO2 adsorption.11 One of the major factors for the promising behavior of activated porous carbons in CO2 adsorption is the physical nature of the adsorption process which allows for easier regeneration of the adsorbent and contributes heavily in cutting the overall cost of the process.12 The procedure for the synthesis of activated porous carbon is quite simple, less tedious and environmentally friendly.13 Moreover, their CO2 adsorption capabilities are on par with the



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conventional materials. Several kinds of CO2 adsorbents materials derived from precursors such as resorcinol and formaldehyde14, pluronic F127 and dicyandiamide15, phloroglucinol, formaldehyde and pluronic F12716, polyaniline17 have been previously reported for either low or high-pressure CO2 adsorption. The starting precursors are expensive, and the synthesis of adsorbents involves complicated and non-environmentally friendly procedures. However, the fabricated materials adsorb a high amount of CO2 g-1 which is a direct result of the presence of nitrogen containing functional groups in their structure. High surface area and microporosity are considered as important factors for enhancing the adsorption of CO2 on porous solid adsorbents.18 The former implies the exposure of higher number of active sites on the surface for maximum number of interactions with the incoming gas molecules which would result in greater mass transport of CO2 whereas the latter is an important index for enhanced CO2 adsorption at low pressure conditions.19 It is also strongly believed that the change in the surface charge of the porous adsorbents makes a huge impact on the total adsorption capacity of the porous adsorbents. However, the fabrication of porous adsorbents with a combination of both these parameters together with the tunable surface charge for the industrial CO2 adsorption at both low pressure and high-pressure system is a challenging task. Although activation of the carbon containing precursors generates materials with a varying degree of porosity, it is required to adopt a new synthesis methodology to design adsorbents with both high specific surface area and surface functionalities.20-24 In this work, we have demonstrated for the first time the use of casein as a starting precursor for generating a series of porous activated carbon materials with a high specific surface area and large mesopores as well as oxygen functionalized surface through a liquid phase activation with KOH. It should be mentioned that KOH is the preferred choice for designing activated materials with 4 ACS Paragon Plus Environment

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highly microporous structure till date.25 However, our present investigation revealed that the activated porous carbon materials with a high mesoporosity can also be developed using KOH activation when casein is used as a carbon source. We propose that the unique chemical structure of the starting precursor with different functionalities and the choice of the activation agent are the main factors which control the textural parameters of the synthesized materials. The role of the porosity and the specific surface area as well as the surface functional groups of the prepared materials on the total CO2 adsorption capacity has been investigated. The mesoporous activated carbon MPC3 with the highest specific surface area showed the CO2 adsorption capacity of 39.1 mmol g-1 at 0 °C/30 bar which is the highest value ever reported in the literature for similar kind of activated carbon materials. These findings suggest the immense potential of mesoporous activated carbons derived from casein for CO2 capture under variable circumstances encountered in industrial operations. Another underlying importance of current findings is that they can be employed for the fabrication of high surface area metal/metal oxides functionalized composites that will find immense potential in various electrochemical processes. MATERIALS AND METHODS Synthesis of mesoporous activated carbons (MPCs) from casein MPC materials were fabricated via a one-step carbonization cum activation of the non-porous carbon (NPC) derived from pyrolysis of casein (Scheme 1). At first, a given amount of casein was carbonized at 600 °C for 2 hours under a heating rate of 10 °C min-1 to obtain non-porous carbon which is labelled as NPC. Another set of samples were prepared by mixing NPC with different amounts of potassium hydroxide (KOH) and activated at a high temperature of 800 °C. In a typical preparation, 1 g of NPC was mixed with 2 g of KOH in separate small beaker containing 40 – 50 ml of water to disperse both the constituents. The mixed solution was stirred for overnight and



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evaporated at 90 °C. The solid mixture was then placed in a 100 °C oven for 5 -6 hours to allow complete removal of water. Subsequently, the dried composite was placed onto a ceramic boat and transferred into the tubular furnace for activation at 800 °C for 5 hours under a heating rate of 5 °C min-1 to achieve the maximum effect of the KOH activation and temperature. The cooled down carbonized sample was then treated with different amounts of 2M HCl (50 – 150 ml) depending upon the amount of activating agent used to remove the residues and then washed with water until the pH of the supernatant was ~ 6 - 7. The black solid material was then kept in a 100 °C oven for overnight drying. A series of samples were prepared by using the above procedure but with different amount of KOH ranging from 1g to 5g. These materials are denoted as MPC1-5, where MPC stands for mesoporous activated carbon and numbers 1-5 denotes the amount of KOH in grams used for activation. Before the adsorption of CO2 measurements, the adsorbents were stored in airtight containers for further experimentation. Characterization of MPCs The powder X-ray diffraction (PXRD) patterns of all samples were recorded using Panlytical Empyrean XRD instrument. CuKα1 and Kα2 radiations with respective wavelength of λ =1.5406 Å and 1.5444 Å produced at a voltage of 40 kV and a current of 40 mA were used to irradiate the sample placed at the center of a circular sample holder. The XRD measurements parameters were set to a high angle range of 2θ = 10 - 70° and a scan step size of 0.006° was used for each measurement. Thermal stability of the synthesized materials was carried out using simultaneous thermogravimetric (TG) and differential scanning calorimetric (DSC) measurements using Perkin Elmer model STA-8000. About ~10 mg of each sample was placed in an aluminum crucible and heated from 30 °C to 1000 °C using a heating rate of 10 °C min-1. Fourier Transform Infra-red spectra were obtained for all samples using Perkin Elmer instrument. A small amount of MPC


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samples was taken for each measurement and the scanning was performed in the range of 400 cm-1 - 4000 cm-1 using a resolution of 4 cm-1 and 32 scans for each sample. Nitrogen sorption isotherms were measured at a temperature of -196 °C using Micromeritics 2420 instrument. All samples were degassed under a constant vacuum of ~ 5 µm Hg at a temperature of 200 °C for 15 hours before analysis. The total specific surface area was determined using the Brunauer-Emmett-Teller (BET) method and the reported BET surface area results have the correlation coefficient R2 value > 0.9999 for all the samples. The total pore volume was determined using Barrett-Joyner-Halenda (BJH) method from the amount of nitrogen adsorbed at a relative pressure (P/Po) of 0.99. The external surface area, micropore surface area and micropore volume were calculated using t-plot method. Non-local original density functional theory method (NLDFT) method was employed for determining the pore size distributions (PSDs) and the narrow slit shaped pores. The surface morphological features of the investigated samples were observed using scanning electron microscope (SEM) and an operating voltage of 2kV was used for each measurement. The X-ray photoelectron spectroscopy (XPS) measurements were performed to ascertain the surface functional groups and the heteroatoms and the nature of their bonding in the carbon materials. Kratos Axis Ultra XPS instrument operating at a power of 225 W, a voltage of 15 kV and a current of 15 mA was used for recording the spectra. The analyzed samples were degassed before the actual measurements. A pass energy of 160 eV, binding energy range of 1200 – 0 eV, step size of 1.0 eV and a dwell time of 100 ms were the employed parameters for recording survey spectral scans. A lower pass energy of 20 eV was used for recoding the high resolution spectra. CO2 adsorption experiments The CO2 adsorption isotherms of the synthesized carbon samples were measured by using Quantachrome gas adsorption instrument that is capable of handling pressures up to 200 bar.



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Before the CO2 measurements, the samples were outgassed for > 20 hours to completely remove any adsorbed gaseous components. The weight of the samples after degassing was close to 200 ± 20 mg for each sample. All the MPC samples were subjected to CO2 adsorption in the pressure range of 0-30 bar and at a temperature of 0 °C. Because of its excellent textural parameters, the optimized material MPC3 was also recorded for CO2 adsorption in the same pressure range at two other temperatures of 10 °C and 25 °C. For MPC3, the same lot was degassed for measurement at each of the three temperatures. The isosteric heat of adsorption was calculated for MPC3 by applying the Clausius Clapeyron equation to three sets of CO2 adsorption isotherms obtained at three different temperatures of 0 °C, 10 °C and 25 °C.

RESULTS AND DISCUSSION Nitrogen sorption analysis Figure 1A shows the N2 adsorption-desorption isotherms for the activated carbon materials MPC1-5. Various textural parameters calculated from these isotherms are summarized in Table 1. The adsorption isotherm of MPC1 and MPC2 show type I isotherm which is typically observed for the microporous materials.26 The type of the isotherms and the absence of any hysteresis loop in the isotherms of these samples confirm that they are microporous in nature. The microporosity in these samples is also evident from the t-plot analysis. The micropore volume of these samples is in the range of 0.394 to 0.526 cm3 g-1 with the corresponding specific micropore surface area in the range of 1017-1324 m2 g-1 (Table 1). On the other hand, MPC3, MPC4 display a type IV isotherm with a broad capillary condensation step and a H4 hysteresis loop27, revealing that these samples are predominantly mesoporous in nature. The broadness of the capillary condensation step is indicative of wide range of mesopores in the samples. It can also be seen that the capillary


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condensation step is shifted to higher relative pressure as the amount of the activating agent is increased. As the capillary condensation step is directly linked with the pore size of the sample, this observation concludes that the pore diameter of the materials can be controlled with the simple adjustment of the amount of the activating agent. It can be correlated with the high magnification TEM images which shows tangled graphitic layers which could be associated with narrow slit like pores developed in between those layers.28 These observations are a strong evidence of the dominant mesoporous character of the synthesized high surface area activated carbons prepared using a large amount of activating agent. As per the Table 1 samples MPC1 and MPC2 display 44 % and 47 % contribution from micropore surface area whereas it gets diminished to 11 % and 14 % for MPC3 and MPC4 and respectively. The optimized sample in terms of textural parameters is MPC3 as it shows a significantly high specific surface area of 3617 m2 g-1 and pore volume of 2.16 cm3 g-1. All these observations are again a strong evidence that the porosity of the synthesized materials can be suitably tailored from micro to meso character by the simple adjustment of the impregnation amount of KOH at a given temperature. It should also be noted that the directly carbonized casein NPC without any activation displays a very low surface area of 5 m2 g-1 and is highly non-porous which is also clear from SEM images as shown in Figure S2a. These observations are also in agreement with the pore size distribution calculated using the density functional theory model. As shown in Figure 1B, the samples MPC1 and MPC2 show a high volume of the pores in the size range of 1.9-2.1 nm which indicates the presence of a large amount of micropores in these samples. However, in case of the MPC3 and MPC4, there is higher occupancy of the pores of size in the range 2.7 – 3.4 nm which is an evidence for their dominant mesoporosity. The large amount of mesoporosity for these samples results from widening of the micropores during the chemical



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reaction of KOH with carbon. The oxidation of the carbon nanostructures and the diffusion of the volatile gases generated during the activation process contributes to this pore widening effect. It is also believed that the formation of metallic potassium and K2O that were intercalated into the carbon matrix during the activation process and the subsequent removal of the same with HCl also support the formation of large amount of mesopores in the samples that were activated with a large amount of KOH. It is surmised that the loosely bonded carbon nanostructures interact more firmly with the activating agent owing to the presence of large number of functional groups. This helps to create higher content of mesopores and high specific surface area for the samples MPC3 and MPC4. However, the addition of large amount of KOH gave detrimental effect as it oxidizes most of the carbon nanostructures. Powder X-ray Diffraction (PXRD) analysis The crystallinity of the NPC and MPC materials was analyzed by the powder X-ray diffraction (PXRD) measurements. The NPC sample exhibits two broad peaks centered around 2 = 25 ° and 2 = 43 ° as shown in Figure S1. These diffraction peaks correspond to the (002) and (101) reflection planes and represent partial graphitization of the carbon in the non-activated material.29 However, upon activation with KOH, MPC materials undergo a significant alteration in their carbon framework which leads to considerable loss in the degree of graphitization as compared to non-activated sample. The PXRD patterns of the MPC1-4 materials (Figure 2) show that the intensity of the higher angle peaks decreases with increasing the KOH/NPC impregnation ratio from 1 to 4. Moreover, the peak at 25 ° was completely disappeared in all the samples after the activation process, suggesting lesser degree of graphitization due to the damage of the graphite layers during the activation.30 These findings are in consistent with the other similar reports existing in the published literature.31,32


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Fourier Transform-Infra Red (FTIR) analysis FT-IR spectroscopic analysis is a useful tool for qualitative determination of the characteristic functional groups present in a material. Figure 3 shows quite similar FTIR spectra for activated carbon products ranging from MPC1 to MPC4. This is an expected result and correlates with the XPS elemental quantification which revealed that the surface of these materials contains almost similar content of carbon, oxygen and nitrogen elements. The weak band appearing at 760 cm-1 could be assigned to the out of plane C-H bending vibration.33 The higher intensity peak caused by bending vibrations of C-H bond occurs at around 865 cm-1.34 All samples show the presence of C-O stretch at 1230 cm-1 that is characteristic of carboxylic acids.35 The presence of a weak shoulder at 1820 cm-1 and 2540 cm-1 is attributed to C=O stretch in acid anhydrides and O-H stretch, respectively. The occurrence of two wide dips around 3000 cm-1 and 3660 cm-1 can be attributed to the presence of free and hydrogen bonded O-H groups on the surface of the activated materials.36 These results reveal that casein precursor helps to introduce a large amount of oxygen functionalities on the surface of the MPC materials which are helpful in achieving a high adsorption of CO2 molecules. Thermogravimetric/Differential Scanning Calorimetry (TG/DSC) analysis The thermal behavior of the synthesized materials was investigated using thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) measurements. As shown in Figure 4a, the TGA curves of MPC1 and MPC2 reveal an initial weight loss of around 8 - 10 % (~100 °C) which is attributed to the desorption of adsorbed moisture. MPC3 and MPC4 also display a water loss stage at 100 °C but the weight loss is ca. 5% which is comparatively smaller than that of MPC1 and MPC2. These two materials encounter another weight loss step around 180 °C – 200 °C which could be assigned to the removal of intercalated water and the possible decomposition



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of any hydroxyl groups on the surface.37 The final stage of weight loss lies in the temperature range 500 °C - 900 °C in all the materials and is attributed to the decomposition of carbonyl groups resulting in the formation of possible gases such as CO and CO2.38 This can also be correlated with the FTIR measurements where in all materials are found to be rich in carbon and oxygen containing functional groups. Overall, the TGA profiles of these materials synthesized using different KOH/NPC impregnation ratios suggest that 60 -76 % of the weight is still present in all the samples at temperatures close to 1000 °C. As shown in inset of Figure 4b, the DSC profiles of synthesized materials MPC1-4 exhibit two distinct endothermic peaks between 0 °C – 500 °C. The first peak occurring at a range of 80 °C – 120 °C corresponds to desorption of water. The second peak in all samples in positioned at different values of temperature in the range 200 °C – 500 ° C and it could be attributed to heat consumption resulting in the breakage of weak hydroxyl groups. The overall heat flow is exothermic in nature indicating that the synthesized materials do not absorb high amount of heat from the supplied heat and hence can be considered quite stable from the thermodynamic point of view. It is worthwhile mentioning that one of the key parameters of the adsorbents during precombustion CO2 capture operations is their stability at elevated temperatures in the range 200 °C500 °C. Apparently, the materials synthesized in this study show good thermal behavior around this temperature range without any significant changes occurring in the porous carbon skeleton. Hence, these materials are viewed as good adsorbents for high pressure CO2 capture. Scanning electron microscopic (SEM) analysis The morphological structure of the optimized sample MPC3 was investigated using both SEM and TEM images and the corresponding images are displayed in Figure 5. The SEM images for all other materials are shown in the Figure S2 in the supporting information. The non-activated


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sample NPC shows particles of size > 30 µm that display a smooth morphology and non-porous structure. However, in all the activated materials, there is a considerable reduction in the particle size accompanied by the creation of pores on the surface of these particles upon reaction with KOH at high temperature. When the amount of the KOH is increased, the number and size of the pores on the surface of the particles also increases. This could be attributed to the loss of biomass carbon species due to the reaction with the KOH and the subsequent release/removal of volatiles and the potassium species as they result in the formation of macro sized cavities and irregularly shaped cracks on the external surface. TEM images of MPC3 reveal the graphitic layers interwoven together resulting in an amorphous structure. It also shows the presence of a highly porous carbon structure with mixed porosity resulting from micro and mesopores domains. X-ray Photoelectron (XPS) analysis The surface elemental analysis of MPC1-4 materials performed using X-ray photoelectron spectroscopy reveal the presence of carbon as the major element (93 – 94 %) followed by oxygen (5.1 – 5.8 %) as the next element as shown in Table 2. Surprisingly, a very small amount of nitrogen is present in all samples even though the thermodynamic stability of N in the carbon matrix is very low at the activation temperature used for the preparation of the samples (800 °C). KOH is a well-known agent for generating porosity and oxygen containing functional groups on the surface of activated carbons synthesized from biomass based materials. The increase in oxygen content while going from MPC1 to MPC3 could be directly related to the amount of the KOH used for activation. MPC1 is synthesized by using 1 g, MPC2 with 2 g and MPC3 with 3 g of KOH. Therefore, higher is the activation amount, larger is the content of the oxygen containing functional groups generated on the surface. However, when much higher amounts of KOH (4g for MPC4) are used for activation, severe chemical reactions with the carbon are occurred which may result



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in loss of some of the oxygen containing functional groups on the surface and hence a lesser oxygen content of 5.4 % is observed in this case. The survey XPS spectra as shown in Figure S3 present two distinct peaks centered around 284.0 eV and 532.0 eV which correspond to the binding energy of C1s and O1s respectively. C1s and O1s peaks were further deconvoluted to obtain a high-resolution spectrum. As shown in Figure 6, the high resolution C1s spectra of MPC1-4 materials are quite similar and show three peaks located at binding energy positions of ~284.0 eV, ~285.0 eV and ~289.0 eV which represent sp2-C, sp3-C and carbonyl groups respectively.39 The high resolution O1s spectra (Figure 7) of MPC1-4 samples display two peaks centered on binding energy ~531.0 eV and ~532.5 eV which correspond to oxygen bonding in carbonyl and hydroxyl groups respectively.40 The amount of oxygen content increases with increasing KOH/NPC ratio from 1 to 3 and then decreases when this ratio is increased to 4. KOH/NPC ratio of 3 gives the highest amount of oxygen functionalities of the samples, confirming that the optimization of the activation condition is crucial for not only to control the textural parameter of the samples but also the oxygen functionalities on the surface of the carbons. From these characterization result, it is clear that the surface chemistry of the oxygen functionalized mesoporous activated carbon incorporates carbon and oxygen containing functional groups, which are crucial for achieving a high CO2 adsorption capacity. Pre and post-combustion CO2 adsorption analysis The CO2 adsorption of the optimized material MPC3 was investigated at a pressure range of 0 30 bar and under three different temperatures of 0 °C, 10 °C and 25 °C (Figure 8a). Additionally, CO2 adsorption was also recorded for MPC1, MPC2 and MPC4 materials at the temperature 0 °C and compared with the CO2 uptake of MPC3 (Figure 8c). Table 3 summarizes the CO2 uptake of MPC3 at two pressures (1 bar and 30 bar) and three temperatures (0 °C, 10 °C and 25 °C) and the


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CO2 uptake of MPC1-4 at same values of pressure but under a single experimental temperature of 0 °C. As shown in Figure 8a, the material MPC3 which possess a highly porous structure and the highest specific surface area of 3617 m2 g-1 shows an exemplary CO2 uptake of 39.1 mmol g-1 at 0 °C and 30 bar. At the same pressure, this material also recorded an impressive CO2 uptake of 32.6 mmol g-1 and 24.7 mmol g-1 at temperatures of 10 °C and 25 °C, respectively. Adsorption at low temperature is exothermic and results in slower diffusion of CO2 molecules which is accompanied with reduced adsorption energy on the surface of the adsorbent and hence the amount of CO2 adsorption is decreased.41 The isotherm also clearly shows that the saturation for CO2 uptake was never reached at any of these temperatures at 30 bar which signifies that the porous carbon structure of these materials is robust and seize the ability to withstand pressures > 30 bar without any structural collapse. We compared the CO2 uptake of MPC3 with other similar materials reported in the literature (Table 4). It is encouraging to see that MPC3 registered much higher adsorption capacity that that of the similar reported materials under similar adsorption conditions. A closer look at the comparison of CO2 adsorption isotherms of MPC1-4 materials at a temperature of 0 °C as shown in Figure 8c is a perfect illustration of the role played by micro and mesoporosity in CO2 uptake as a measure of pressure. It is well known that the CO2 adsorption at low pressures mainly occurs through the filling of micropores in the carbon structure.42 The materials MPC1 and 2 possess greater contribution from the microporous structures as compared to other two materials as shown in Table 1. Hence, these two samples adsorb a predominantly large amount of CO2 valued at 5.01 mmol g-1 and 6.01 mmol g-1 respectively at low pressure of 1 bar. These two samples also show appreciable amount of CO2 adsorption at 0 °C/0.15 bar (1.20 mmol g-1 and 1.46 mmol g-1). The CO2 uptake performance shown by these two materials is indicative of their capability for their application in the post combustion capture of CO2 in relevant industries.



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When compared to nitrogen rich adsorbents14-17, our materials have relatively lesser nitrogen content but show considerably high CO2 adsorption at similar conditions. Additionally, the low cost of the starting precursor, casein and relatively simpler synthesis strategy makes our approach an attractive one. Furthermore, we demonstrated that the activation of casein with KOH generate high amount of mesoporosity which is quite unique and a novel thing as KOH is well known to produce high microporosity in the carbon. The CO2 capture performance of our material MPC2 at 1 bar/ 0 °C is 6.01 mmol g-1. This is also higher when compared to CO2 adsorption observed under similar conditions (5.01 mmol g-1) for biomass based porous carbons such as AC40 derived from olive stones.43 It is also quite high when compared to recently reported materials such as lead based MOFs (2.1 mmol g-1) under similar experimental conditions.44 Furthermore, the isosteric heat of adsorption (Qst) value of 19.7 kJ mol-1 suggests that the adsorption is physical and accounts for easier regeneration of the material. This value of Qst is less when compared to materials such as silica supported hindered amines (65-70 kJ mol-1), zeolites (43.7 kJ mol-1) and amine functionalized mesoporous silica (~90 kJ mol-1) which involve stronger forces between the adsorbent and adsorbate.45-47 It has to be mentioned that the topology and the extra framework cations in case of zeolites also play an important role for CO2 adsorption.48 However, our materials based on pure carbon are still showing enhanced CO2 adsorption without the aid of such factors. The materials MPC1 and MPC2 show higher amounts of CO2 adsorption in a wide pressure range of 0-10 bar as compared to MPC3 and MPC4. MPC3 and MPC4 materials are predominantly mesoporous and adsorb only 4.12 mmol g-1 and 2.52 mmol g-1 of CO2 at a low pressure of 1 bar. Additionally, MPC3 shows a good CO2 performance at 0.15 bar/0 °C (0.79 mmol g-1), 0.18 bar/10 °C (0.74 mmol g-1) and 0.20 bar/25 °C (0.55 mmol g-1). At 0 °C, the material MPC4 adsorbs 0.33 mmol g-1 of CO2 at 0.25 bar. In these materials the initial filling of the available micropores and 16 ACS Paragon Plus Environment

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cracks with CO2 is accomplished at low pressures and then the larger pores in the meso and macro region control the CO2 uptake. These wider meso and macro pores and the overall surface area take the dominant role in the CO2 adsorption at high pressures. It is a well-documented in the existing literature that the CO2 adsorption at low pressures is attributed mainly to the microporosity.49 However, the materials MPC3 and MPC4 with very little contribution from microporosity also display an appreciable amount of CO2 adsorption at low pressures. This indicates that in addition to microporosity, the overall porosity in the region from micro to macro also has a role to play in the initial diffusion of the CO2 molecules into the carbon structure when the pressure is low.50 Additionally, the presence of surface oxygenated functional groups is also helpful in CO2 adsorption under low pressure conditions as discussed earlier. We believe that the materials synthesized in our study are quite unique owing to their high surface area, highly developed porous structure and surface oxygenated functional groups. As a result, highly promising and exceptional values for low and high-pressure adsorption of CO2 are observed. The overall CO2 uptake of MPC3 and MPC4 at 30 bar reaches a higher value as compared to those of MPC1 and 2 which can be attributed to their overall high surface area and superior mesoporosity of the former samples. Therefore, it can be concluded that the microporous material performs better at low pressures while the materials with both micro and mesopores give reasonable performance at low pressures but exceptionally high CO2 adsorption at high pressures. It also aligns with the reported literature wherein the materials with a low surface area but high microporosity excels with their CO2 adsorption at low pressures but records diminished CO2 adsorption at high pressures. This trend reverses for high surface area with reduced microporosity.51 The correlation between the overall CO2 uptake performance and textural parameters such as specific surface area and total pore volume of materials MPC1-4 is presented in Figure 8d. It is



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widely acknowledged that the specific surface area and the overall volume of the pores are the primary factors influencing the amount of CO2 adsorption in activated carbon type materials. The material MPC1 possesses the lowest specific surface area (1301 m2g-1) and the smallest pore volume (0.82 cm3g-1) and displays the lowest CO2 uptake (12.9 mmolg-1) at 30 bar and 0 °C among all the samples. Under similar conditions, the highest CO2 uptake of 39.1 mmol g-1 is exhibited by sample MPC3 which has the highest specific surface area (3617 m2 g-1) and pore volume (2.16 cm3 g-1) when compared to all other samples. The other three samples MPC1, MPC2 and MPC4 also show a similar kind of relationship. Even though the dominant factor responsible for low and/or high pressure CO2 uptake is the type of porosity in the synthesized materials, the role played by the oxygen containing functional groups on their surface cannot be ignored. It has been proven previously that the surface functional groups assist in the establishing chemisorption with the incoming CO2 molecules.52 Comparing the values in Table 2 and Table 3 for materials MPC1 and MPC2, it is observed that there is a direct relationship between their oxygen content and CO2 uptake at both 1 bar/0 °C and 30 bar/0 °C. MPC1 with a surface oxygen content of 5.1 % adsorbs 5.01 mmol and 24.6 mmol of CO2 g-1 at 1 bar /0 °C and 30 bar/0 °C, whereas MPC2 with a higher oxygen content of 5.5 % shows adsorption of 6.01 mmol and 27.6 mmol of CO2 g-1 at 1 bar /0 °C and 30 bar/0 °C. In comparison, MPC3 and MPC4 show lower CO2 uptakes at 1 bar but higher CO2 uptakes at 30 bar which lead to the conclusion that the surface oxygen functional groups also play a major role in CO2 adsorption at high pressures. It is proposed that the access to the surface functional groups that are present at the interior part of the pores is enhanced at a high pressure, resulting in an increase in the amount of CO2 adsorbed. Our group has previously reported several instances of high CO2 uptake by porous activated biocarbons synthesized from biomass Arundo donax and the best result stands at 30.2 mmol g-1 18 ACS Paragon Plus Environment

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shown by AMB3-500 which possess specific surface area of 3298 m2 g-1 and pore volume of 1.9 cm3 g-1.23 In this context, the current research involving the mesoporous dominant porous carbon materials obtained from casein displays much better efficacy for CO2 capture under diverse conditions of temperature and pressure. The current CO2 uptake performance shown by sample MPC3 at 30 bar (39.1 mmol g-1) and 1 bar (4.12 mmol g-1) presents an attractive prospect for pre and post combustion capture of CO2 using the same material. Alongside, the materials MPC2 and MPC4 also exhibit high CO2 uptakes at pre-combustion (30 bar/0 °C - 32.8 mmol g-1) and post combustion (1 bar/ 0°C - 6.01 mmol g-1) CO2 capture conditions, respectively. The diverse nature of porous carbons that exhibited excellent CO2 uptakes under different conditions puts them into the top class of other reported adsorbent materials. Isosteric heat of adsorption (Qst) analysis The isosteric heat of adsorption (Qst) is an important criterion that governs the regeneration of the adsorbents. In our studies, we calculated Qst using Clausius Clapeyron equation for best performance material MPC3 at three temperatures of 0 °C, 10 °C and 25 °C and the value of Qst obtained was 19.7 kJ mol-1 (Figure 8b). This value signifies that the process of adsorption is physical in nature which can facilitate easier regeneration of the adsorbent. There is no significant change observed in the value of Qst with increasing CO2 adsorption until 10 mmol g-1 which indicates that the surface of the MPC3 is homogenous in nature. However, a slightly higher value of Qst (~21 kJ mol-1) with further increase in CO2 uptake until 25 mmol g-1 (Inset figure 8b) could be due to better interactions between CO2 and oxygenated functional groups on the surface of the carbon. In summary, we have reported the synthesis of mesoporous dominant carbon materials that displayed a combination of micro and mesoporous domains in their highly porous structure with



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extremely high specific surface area that resulted in high CO2 adsorption at pre- and postcombustion CO2 capture conditions. The new materials named as oxygen functionalized mesoporous activated porous carbons (MPCs) were synthesized at a carbonization temperature of 800 °C by employing casein as a carbon source and KOH as the activation agent. It has been demonstrated that low activation amount of KOH produces a mix of micro and mesopores which transforms into mesopores at higher activation amounts. The material MPC3 exhibited a specific surface area of 3617 m2 g-1 and a specific pore volume of 2.16 cm3 g-1 with a mesoporosity of ~ 90 %. These are the highest ever reported values for the porous activated carbons prepared from liquid phase KOH activation. Owing to these excellent textural features, this material gave an attractive CO2 adsorption at pre-combustion conditions, namely 0 °C, 10 °C, 25 °C/30 bar (39.1 mmol g-1, 32.6 mmol g-1 and 24.7 mmol g-1). For the same material, the contribution from micropores resulted in reasonable CO2 capture under post combustion conditions at 0 °C, 10 °C, 25 °C/1 bar (4.12 mmol g-1, 3.20 mmol g-1 and 2.32 mmol g-1. The material MPC2 with a high microporosity and the material MPC4 with a high mesoporosity also responded very well for post (6.01 mmol g-1) and pre-combustion (32.8 mmol g-1) CO2 capture. A low value for heat of adsorption (19.7 kJ mol-1) is observed for MPC3 which signifies easier and inexpensive material regeneration as compared to conventional aqueous amine-based sorbents. Therefore, the synthesized materials present a highly promising case from the prospect of CO2 capture in either pre or post-combustion CO2 capture. The directions provided in this work would add significant knowledge to the concerned field and serve as a guidance for synthesis of new generation of hybrid materials using MPCs for various energy and environmental applications.


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ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publication website. Details include XRD pattern of NPC, non-porous carbon synthesized from casein, SEM images of NPC and porous activated materials MPC1, MPC2 and MPC4 and XPS survey spectra of MPC1-4. AUTHOR INFORMATION Corresponding author Email: [email protected] ORCID Ajayan Vinu: 0000-0002-7508-251X Notes The authors declare no competing financial interest ACKNOWLEDGEMENTS A. Vinu is grateful to and to the University of Newcastle for the start-up funds.



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54. Zhang, C.; Song, W.; Sun, G.; Xie, L.; Wang, J.; Li, K.; Sun, C.; Liu, H.; Snape, C. E.; Drage, T., CO2 Capture with Activated Carbon Grafted by Nitrogenous Functional Groups. Energy Fuels 2013, 27 (8), 4818-4823. 55. Zhang, Z.; Xu, M.; Wang, H.; Li, Z., Enhancement of CO2 adsorption on high surface area activated carbon modified by N2, H2 and ammonia. Chem. Eng. J. 2010, 160 (2), 571-577. 56. Bourrelly, S.; Llewellyn, P. L.; Serre, C.; Millange, F.; Loiseau, T.; Férey, G., Different adsorption behaviors of methane and carbon dioxide in the isotypic nanoporous metal terephthalates MIL-53 and MIL-47. J. Am. Chem. Soc. 2005, 127 (39), 13519-13521. 57. Millward, A. R.; Yaghi, O. M., Metal− organic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature. J. Am. Chem. Soc. 2005, 127 (51), 1799817999. 58. Thallapally, P. K.; Tian, J.; Radha Kishan, M.; Fernandez, C. A.; Dalgarno, S. J.; McGrail, P. B.; Warren, J. E.; Atwood, J. L., Flexible (breathing) interpenetrated metal− organic frameworks for CO2 separation applications. J. Am. Chem. Soc. 2008, 130 (50), 16842-16843. 59. Zhang, Z.; Zhang, W.; Chen, X.; Xia, Q.; Li, Z., Adsorption of CO2 on zeolite 13X and activated carbon with higher surface area. Sep. Sci. Technol. 2010, 45 (5), 710-719. 60. Ullah, R.; Salah Saad, M. A. H.; Aparicio, S.; Atilhan, M., Adsorption equilibrium studies of CO2, CH4 and N2 on various modified zeolites at high pressures up to 200 bars. Microporous Mesoporous Mater. 2018, 262, 49-58. 61. Min, J. G.; Kemp, K. C.; Hong, S. B., Zeolites ZSM-25 and PST-20: Selective Carbon Dioxide Adsorbents at High Pressures. J. Phys. Chem. C 2017, 121 (6), 3404-3409.



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62. Lakhi, K. S.; Singh, G.; Kim, S.; Baskar, A. V.; Joseph, S.; Yang, J.-H.; Ilbeygi, H.; Ruban, S. J. M.; Vu, V. T. H.; Vinu, A., Mesoporous Cu-SBA-15 with highly ordered porous structure and its excellent CO2 adsorption capacity. Microporous Mesoporous Mater. 2018, 267, 134-141. 63. Llewellyn, P. L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.; Hamon, L.; De Weireld, G.; Chang, J.-S.; Hong, D.-Y.; Kyu Hwang, Y.; Hwa Jhung, S.; Férey, G., High Uptakes of CO2 and CH4 in Mesoporous Metal—Organic Frameworks MIL-100 and MIL-101. Langmuir 2008, 24 (14), 7245-7250.


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Scheme

Scheme 1: Schematic illustration of the synthesis of mesoporous activated porous carbons from casein and their application for CO2 adsorption



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Tables

Table 1 Textural properties of casein derived MPC1-5 materials Sample

SABETa (m2 g-1)

SAmicrob (m2 g-1)

Vtotalc (cm3 g-1)

Vmicrob (cm3 g-1)

PWd (nm)

NPC MPC1 MPC2 MPC3 MPC4

5 2311 2796 3617 3003

3 1017 1324 411 429

0.003 1.09 1.31 2.16 1.79

0.001 0.394 0.526 0.011 0.002

2.1 2.1 2.7 2.7

aTotal

surface area and correlation coefficient calculated using BET method surface area and micropore volume calculated using t-plot method cTotal pore volume calculated at P/P = 0.99 o dPore width calculated using NLDFT method bMicropore

Table 2 Surface elemental composition of MPC1-4 from XPS analysis Sample

Carbon (%)

Oxygen (%)

Nitrogen (%)

MPC1 MPC2 MPC3 MPC4

94 94 94 93

5.1 5.5 5.8 5.4

0.8 0.3 0.2 0.6

Impurities (Cl. Si, S) 0.1 0.2 0.0 1.0

Table 3 CO2 uptake of MPC1-4 at different conditions of pressure and temperature Material

Temperature ( °C)

Pressure (bar)

MPC1 MPC2 MPC3 MPC3 MPC3 MPC4

0 0 0 10 25 0

0.15/1/30 0.15/1/30 0.15/1/30 0.18/1/30 0.20/1/30 0.25/1/30

CO2 adsorption (mmol g-1) 1.20/5.01/24.6 1.46/6.01/27.6 0.79/4.12/39.1 0.74/3.2/32.6 0.55/2.3/24.7 0.33/2.5/32.8


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Table 4 Comparison of the CO2 adsorption capacity of MPC3 with other adsorbents Material Porous organic polymer N-doped activated carbon N-doped activated carbon Porous terephthalate MOF-177 Interpenetrated MOF Activated carbon Zeolite Y (CBV 300) Zeolite Na-ZSM-25 Mesoporous Cu-SBA15 Mesoporous MOF MIL-101 MPC3 MPC3

CO2 uptake temperature/pressure 25 °C/40 bar 25 °C/36 bar 25 °C/30 bar 31 °C/30 bar 25 °C/35 bar 25 °C/30 bar 25 °C /20 bar 35 °C/200 bar 25 °C/25 bar 25 °C/30 bar 30 °C/50 bar 0 °C/30 bar 25 °C/30 bar

CO2 uptake (mmol g-1) 15.3 19.1 20.0 10.0 33.5 7.10 18.3 39 3.9 7.7 40 39.1 24.7

Ref. 53 54 55 56 57 58 59 60 61 62 63

This work This work



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Figures

Figure 1 A) N2 adsorption (solid symbols) and desorption (open symbols) isotherms and B) NLDFT pore size distribution of a) MPC1, b) MPC2 c) MPC3 and MPC4


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Figure 2 Powder X-ray diffraction (PXRD) patterns of a) MPC1, b) MPC2, c) MPC3 and d) MPC4



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Figure 3 Fourier Transform Infra-Red (FT-IR) spectra of a) MPC1, b) MPC2, c) MPC3 and d) MPC4


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Figure 4a & 4b Thermogravimetric (TGA) and Differential Scanning Calorimetric (DSC) profiles of a) MPC1, b) MPC2, c) MPC3 and d) MPC4



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Figure 5 a-d) Scanning electron microscopic (SEM) images and e-h) HRTEM images of MPC3


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Figure 6 XPS high-resolution C1s spectra of a) MPC1, b) MPC2, c) MPC3 and d) MPC4



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Figure 7 XPS high-resolution O1s spectra of a) MPC1, b) MPC2, c) MPC3 and d) MPC4


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Figure 8 a) CO2 uptake and b) Isosteric heat of adsorption of MPC3 observed at 0 °C, 10 °C and 25 °C, c) Comparison of CO2 uptake of MPC1-4 materials at 0 °C and d) Relationship between CO2 uptake vs BET surface area and pore volume of MPC1-4. Note (K is a multiple of 1000)



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Oxygen-Functionalized Mesoporous Activated Carbons Derived from

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