Highly Efficient Method for the Synthesis of Activated Mesoporous

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Highly efficient method for the synthesis of activated mesoporous biocarbons with extremely high surface area for high pressure CO adsorption 2

Gurwinder Singh, Kripal Singh Lakhi, In Young Kim, Sungho Kim, Prashant Srivastava, Ravi Naidu, and Ajayan Vinu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08797 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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ACS Applied Materials & Interfaces

Highly efficient method for the synthesis of activated mesoporous biocarbons with extremely high surface area for high pressure CO2 adsorption Gurwinder Singhab, Kripal S. Lakhia, In Young Kima*, Sungho Kima, Prashant Srivastavaab, Ravi Naidubc* and Ajayan Vinua* a

Future Industries Institute, Division of Information Technology, Engineering and Environment,

University of South Australia, Mawson Lakes, South Australia 5095, Australia b

Cooperative Research Centre for Contamination Assessment and Remediation of the

Environment, University of South Australia, Mawson Lakes, South Australia 5095, Australia c

Global Centre for Environmental Remediation (GCER), ATC Building, The University of

Newcastle, Callaghan, New South Wales 2308, Australia *Corresponding author E-mail address: [email protected]; [email protected]; [email protected] KEYWORDS: Activated biocarbon, Arundo donax, Mesoporous, Microporous, One step solid state activation, CO2 adsorption,

ACS Paragon Plus Environment

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ABSTRACT A simple and efficient way to synthesize activated mesoporous biocarbons (AMBs) with extremely high BET surface area and large pore volume has been achieved for the first time through a simple solid state activation of freely available biomass, Arundo donax, with zinc chloride. The textural parameters of the AMB can easily be controlled by varying the activation temperature. It is demonstrated that the mesoporosity of AMB can be finely tuned with a simple adjustment of the amount of activating agent. AMB with almost 100% mesoporosity can be achieved using the activating agent and the biomass ratio of 5 and carbonization at 500 °C. Under the optimized conditions, AMB with a BET surface area of 3298 m2g-1 and a pore volume of 1.9 cm3g-1 can be prepared. While being used as an adsorbent for CO2 capture, AMB registers an impressively high pressure CO2 adsorption capacity of 30.2 mmolg-1 at 30 bar which is much higher than that of the activated carbon (AC), multiwalled carbon nanotubes (MWCNTs), highly ordered mesoporous carbons and mesoporous carbon nitrides. AMB also shows high stability with excellent regeneration properties under vacuum and temperatures of up to 250 °C. These impressive textural parameters and high CO2 adsorption capacity of AMB clearly reveal its potential as a promising adsorbent for high pressure CO2 capture and storage application. Also, the simple one step synthesis strategy outlined in this work would provide a pathway to generate a series of novel mesoporous activated biocarbons from different biomasses.


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INTRODUCTION Advanced porous carbonaceous materials derived from the agricultural biomass have been receiving a lot of attention in the recent years owing to their unique textural parameters including high surface area and large pore volume which make them available for various applications. Many agricultural biomasses including coconut or peanut shells, bamboo, cotton waste and crop residues have been widely used as the source of carbon for the preparation of a series of advanced porous carbon nanostructures including activated carbons and porous biocarbons.1-4 Although many advanced carbon materials have been prepared using these agricultural biomasses, their availability and the associated cost of preparation of carbon nanostructures is a major obstacle for their successful commercialization. Another major drawback of the agricultural biomasses is their variable carbon content which makes a significant impact on the final textural properties of the prepared biocarbon materials. Perennial grasses such as Arundo donax (giant reed) are rich in lignocellulosic content and can be used as precursors for the design of new advanced carbon nanostructures which may find applications in various fields including energy storage, carbon capture and sequesteration.5-7 These plants grow at a faster rate compared to other terrestrial plants and their invasive nature alters the ecological balance in the environment. By using these affordable but low cost plants as carbon source, not only novel advanced porous biocarbons could be developed but also a right balance could be maintained in the eco system.8-11 Among the advanced carbon nanostructures derived from biomass, the activated biocarbons prepared from natural biomass such as pineapple waste, coconut shell, coffee husk, corn cob etc. are quite attractive as they offer much better surface areas and pore


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volumes than the non-activated biocarbons.11-14 Conventionally, the activated biocarbons are produced using a two-step process involving carbonization and then activation by either physical or chemical agents. Chemical activation is generally preferred as it leads to the formation of hierarchical porous structure and high surface areas and KOH, H3PO4 and ZnCl2 are three commonly employed chemical activating agents.15-17 Activation using ZnCl2 is unique as it exerts a strong dehydrating effect which significantly reduces the overall carbonization temperature, suppresses the formation of tar and results in the formation of open micro and meso pores in the structure.18 The activation with low amount of ZnCl2 produces more micropores due to the mild dehydration of the carbon surface whereas large amounts of ZnCl2 offers much stronger dehydration effect which causes widening of the pores resulting in greater mesoporous character.19 Recent reports in the literature have shown that chemical activation using ZnCl2 can produce materials with surface area higher than 1800 m2g-1. For example, Luan et al. reported a surface area of 1841m2g-1 for ZnCl2 activated carbons prepared from loofah sponge.20 In another report, Sun et al. prepared carbon nanosheets with a surface area of 1874 m2g-1 from ZnCl2 activated coconut.12 However, the methods of synthesis of ZnCl2 activated carbons reported so far in the literature involved mixing of the aqueous solution of ZnCl2 and a two step activation process which make it quite time consuming and energy intensive processs. In the wake of above considerations, there is a drive towards simplifying the conventional preparation routes for activated biocarbons. The problems of wet-impregnation of activating agent and the two-step activation can be overcome by solid state mixing of activating agent together with the biomass followed by the high temperature carbonization cum activation in a


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single step. This approach has been recently receiving much attention owing to less energy intensive process and time of the activation.21,

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Recently, Vinu and co-workers realized this

fantastic approach and reported a single step activation of biomass carbon using solid KOH as the activating agent for biocarbon derived from Arundo donax.23 The prepared activated biocarbon exhibits a BET surface area of 1122 m2g-1, pore volume of 0.50 cm3g-1 and high microporosity in the region of 0.56 nm and registers CO2 adsorption capacity of 15.4 mmolg-1 at 0 °C and 30 bar. Although activation via KOH is a relatively economical, the final textural parameters of the obtained activated biocarbons are not much impressive which is reflected in the lower CO2 adsorption capacity. It is expected that the replacement of solid KOH with solid ZnCl2 and a single step activation would offer activated biocarbon with both micro and mesoporosity. However, no studies on the preparation of activated mesoporous biocarbons from freely available biomass Arundo donax with this solid state and single step activation approach have been reported in the open literature so far. Herein we report on the effective way to prepare activated mesoporous biocarbons (AMBs) with extremely high surface area and mesoporosity through a single-step and solid ZnCl2 activation method from freely available biomass Arundo donax. To the best of our knowledge, this is the first report on the preparation of AMBs from Arundo donax using this approach. The effect of carbonization temperature on the structural and textural parameters of these AMBs has been systematically investigated and the micro and mesoporosity of the AMBs have been controlled with the simple adjustment of the ratio of the biomass and solid ZnCl2. The optimized AMB material exhibits a high surface area of 3298 m2g-1 and records CO2 adsorption capacity of 30.2 mmolg-1 at 0 °C/30 bar which is much higher than any biomass derived activated carbon materials, activated carbons,


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carbon nanotubes, mesoporous carbon, silica and mesoporous carbon nitrides reported till now. EXPERIMENTAL Synthesis of activated biocarbons Arundo donax was obtained from a local farm in the outskirts of Adelaide, South Australia and converted to activated biocarbons using ZnCl2 as the chemical activating agent (Scheme 1). The cane was cut into several small pieces which were then bone dried for an extended period in an oven at 100 °C to ensure complete removal of moisture. The dried biomass was then pulverized into fine powder using a grinder. Powdered biomass was thoroughly mixed with different amounts of solid ZnCl2 chosen for chemical activation. Afterwards, the mixture was carbonized in a tubular furnace at different temperatures (400-700 °C) and a heating rate of 10 °Cmin-1 under a constant flow of nitrogen. After carbonization, the carbon black powder was washed 2-3 times with 1M hydrochloric acid and then deionised water to remove all residues and obtain a final pH close to neutral. Finally, the activated materials were dried overnight at 100 °C in a hot air oven. The obtained materials were labelled as AMBn-T where n = 1-5 and denotes the impregnation ratio of ZnCl2 to biomass and T = 400-700 °C and indicates the carbonization temperature. Characterization of activated biocarbons The structure of all activated biocarbon materials was investigated by using XRD diffraction patterns which were obtained on a PANalytical Empyrean XRD instrument. The samples were put at the centre of a circular holder and irradiated with Cu Kα radiation (λ =1.5418 Å) produced at 40 kV and 40 mA. XRD patterns were recorded at a high angle


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range of 2θ=10-80° under a continuous scan using 0.06° step size. XRD patterns were also compared before and after HCl washings to ensure complete removal of inorganic residues. Surface morphology of the samples was investigated using field emission gun scanning electron microscope (FE-SEM, Zeiss Merlin) used at an operating voltage of 2 kV and high resolution transmission electron microscope (HR-TEM, JEOL, JEM 2100F). The BET surface area and pore volume were determined by using nitrogen adsorption/desorption isotherms at -196 °C on Micromeritics ASAP 2420 analyzer. All samples were degassed for 10 h at 250 °C prior to analysis. BET surface area was measured using BET (Brunauer Emmett-Teller) method in relative pressure range of P/Po 0.05-0.20 and total pore volume (Vt) was taken at P/Po =0.99. Total micropore area (Smicro), external surface area (Sext) and micropore volume (Vmicro) were determined using t-plot method. Mesopore volume is calculated as the difference of total (Vt) and micropore volume (Vmicro). CN bulk analysis was performed using a Leco analyser. X-ray photoelectron spectroscopic (XPS) spectra were recorded on Kratos Axis ULTRA X-ray photoelectron spectrometer equipped with a monochromatic Al Kα X-rays source of incident radiation operating at 225 W (15 kV, 15 mA). Survey spectra scans were recorded at a pass energy of 160 eV within a binding energy range of 1200–0 eV with 1.0 eV steps and a dwell time of 100 ms. High resolution scans were recorded at pass energy of 20 eV. Prior to XPS measurements, all samples were degassed for overnight under vacuum. The chemistry of surface functional groups was investigated using Fourier Transform Infrared (FT-IR) spectra obtained with Newport FTIR spectrometer. A reference carbon material was used to set up a background spectrum every 30 minutes and then the sample was analysed in the infrared region ranging from 4000 cm-1 to 400


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cm-1 using 8 cm-1 resolution. High pressure Quantachrome Isorb HP1 instrument fitted with an external water circulator was used to carry out CO2 adsorption measurements. Measurements were made at three different temperatures of 0 °C, 10 °C and 25 °C under a pressure range from 0 to 30 bar. The adsorbate-adsorbent interactions were established using measured values of isoelectric heat of adsorption calculated using Clausius Clapeyron equation. RESULT AND DISCUSSION Yield of activated biocarbon samples From an economic point of view, the overall yield of activated carbons is of particular interest if the process were to be scaled up for industrial scale production. The wt% yield of all carbon samples calculated as a ratio of the weight of the final product to the weight of the original mass is shown in Figure S1. It is evident that the yield of biocarbon materials prepared by ZnCl2 activation process falls in the range of 6-16 wt%. The acid washing of inorganic residues after the carbonization process results in a huge loss in mass which is reflected in lower yields. Higher the amount of ZnCl2 used, greater is the number of inorganic residues generated which in turn leads to a decrease in overall mass and yield. Percentage yields are higher at the lowest carbonization temperature of 400 °C as the effect of the activating agent on the carbon structure at this temperature is not very strong. However, at higher carbonization temperature of 700 °C, the effect of the activating agent is the strongest leading to higher tar volatilization, widening of micropores into mesopores and shrinkage of structure. These observations suggest that a lower carbonization temperature and lower impregnation ratio of ZnCl2 to biomass are needed for producing higher yield of biocarbons. Crystal structure and morphology of activated biocarbons


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All AMB materials exhibit a low density and are fluffy in appearance when compared to nonactivated biocarbon, indicating the formation of porous structure after the activation process. The graphitic structure and the purity of AMB materials were analyzed by powder X-ray diffraction (XRD) analysis. Figure 1A shows the XRD patterns of the AMB prepared with different ZnCl2 to biomass ratios at the carbonization temperature of 500 °C. All the AMBn-500 materials (where n is the ratio of ZnCl2 to biomass) are found to be devoid of any zinc metallic residues and exhibit two broad peaks centered at 2θ = 23° and 43° which typically represent amorphous carbon.24 These peaks correspond to (002) and (101) reflection planes in a turbostratic carbon structure, indicating randomly oriented graphitic layers in the material.18,

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It is interesting to

note that the interlayer distance of the carbon layers is much higher than that for graphite (2θ = 26.4°) which is typically observed for the porous biocarbons and may be attributed to the presence of large number of functional groups on the surface of carbons. However, the peaks at 2θ = 23° and 43° are shifted towards higher angles as the amount of the activating agent is increased, revealing a reduction in the interlayer spacing of the turbostratic graphitic carbon layers. This may be ascribed to the higher dehydrating effect on the carbon structure due to the large amount of activating agent, resulting in a progressive decrease of the functional groups in the samples which is linked with the interlayer spacing between the graphitic layers.26 As shown in Figure S2 (see the Supporting information), non-activated biocarbon, which was treated at 500 °C without adding any activating agent, shows two broad peaks at 2θ = 23° and 43° which typically correspond to turbostratic carbon24 and several sharp peaks which correspond to sylvite mineral that may have accumulated in the parent biomass during its growth period over time.27 The effect of temperature on the crystallinity of the AMBs was also analyzed. Figure 1B shows the XRD patterns of samples carbonized at


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different temperatures. All AMB3-T samples show two broad peaks which are assigned to (002) and (101) reflection planes in a graphitic structure.28 The intensity and the sharpness of the peaks at 2θ = 23° and 45° increases as the carbonization temperature is increased, indicating the improvement of ordering of graphitic carbon layers at a high temperature.29 The interlayer spacing of the samples treated at a high temperature approaches that of carbon with highly graphitic layers, indicating that the graphitization of the AMB can be enhanced with the simple adjustment of the carbonization temperature. Surface morphology of AMB materials was investigated using FE-SEM. Figure 2 and Figure S3 (see Supporting Information for Figure S3) show the SEM micrographs of AMBn-500 and AMB3-T, respectively. The surface morphology of all the AMB samples exhibited the presence of macropores (pore diameter 2-70 µm) which are originated due to the decomposition and depolymerisation reactions of cellulose, hemicellulose and lignin components of the biomass during the activation process. As shown in Figure 3, it was observed that the size of these macropores varies from 5 µm to 70 µm in all the activated samples synthesized at 500°C and the pore size is dependent on the amount of activating agent used in the synthesis. This observation has been illustrated on a statistical basis in Figure S4. AMBn-500 samples show larger proportion of cavities with size 5-10 µm at lower activation ratio of 1-3 which eventually decreases at higher activation ratios of 4-5. This happens due to the varying degree of reaction between biomass and ZnCl2. AMB3-T samples record similar observations as illustrated in Figure S3. At high carbonization temperature, a large amount of volatile or decomposed components of the biomass may be liberated from the carbon composites with the activating agent that


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creates large cavities on the surface. This is an interesting observation that clearly reveals that a combination of high carbonization temperature and activating agent is responsible for the formation of large cavities. It should also be noted that the particles of AMB do not show uniform morphology which is also ascribed to the chemical activation with ZnCl2 which releases gaseous volatile components at a high carbonization temperature, resulting in the formation of a heterogeneous surface morphology. It is hard to observe the micro (pore diameter ≤ 2 nm) and mesopores (2 nm < pore diameter ≤ 50 nm) in activated biocarbons with SEM. The detailed surface topology of the optimised sample AMB3-500 was revealed further by using HR-TEM images taken at low and high magnification as illustrated in Figure 3. AMB3-500 exhibits both meso and micropores as shown in Figure 4A and Figure 4B and 4C respectively. This is consistent with previous reports on activated carbons derived from biomass based materials.30-32 HR TEM image also reveals that the sample is amorphous and becomes progressively thinner along the edges with many carbon layers as shown in Figure 3B and 3C. Textural parameters of activated biocarbons Textural parameters such as BET surface area, pore volume and pore width etc. were calculated from the N2 adsorption-desorption experiments as shown in Table 1. Figure 4A shows the N2 adsorption-desorption isotherms for AMBn-500. All activated samples prepared at 500 °C except AMB1-500 show type IV isotherm with a broad capillary condensation step in the relative pressure (P/Po) range of 0.30 to 0.65, revealing high mesoporosity in the samples. However, the broad capillary condensation step reveals that the samples exhibit disordered mesopores which is consistent with the data obtained from the TEM analysis.33 The mesoporous nature of the activated carbons is further confirmed


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by the experimental determination using the Barrett Joyner-Halenda (BJH) pore size distribution curves (Figure 4B) which provides an insight into the exact pore size developed as a result of ZnCl2 activation. On the other hand, the sample AMB1-500 exhibits a combination of type I and II isotherm with no well pronounced capillary condensation step that clearly reveals the presence of micropores or small mesopores. One of the most interesting features of this work is the total BET surface area of the AMB materials. The amount of nitrogen adsorbed in the monolayer region of the isotherms of AMB materials, which is directly related with the BET surface area, increases with increasing the ratio of ZnCl2 to biomass from 1 to 3 and then decreases as the ratio is increased further to 5. The BET surface area is 2007 m2g-1 (± 9.0 m2g-1), 3298 m2g-1 (± 15.0 m2g-1) and 2464 m2g-1 (± 10.0 m2g-1) for AMB1-500, AMB3-500 and AMB5-500, respectively (Table 1). The BET surface area of AMB3-500 is much higher than that of any activated biocarbon derived from biomass reported so far including Arundo donax. For example, Basar et al. reported the preparation of activated biocarbons by chemical activation of orange peel using ZnCl2 and H3PO4 solution at different temperatures ranging from 500 to 1000 °C.34 Although these researches did not use the dry salt and one-step activation approach, the best material reported in their work showed a BET surface area of 1215 m2g-1 which is much lower than that presented in this work. Under similar conditions, our material AMB3-500 outperforms in terms of the surface area by about 2085 m2g-1. In another comparison, activated carbons prepared from expensive precursor such as glucose exhibit a maximum surface area of 2560 m2g-1 which is less than that exhibited by the best sample in this work.35


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The high surface area values in the present study could be attributed to the nature and content of the biopolymers present in Arundo donax. Our materials also show better surface area results when compared to activated carbon produced from non-biomass materials such as highly expensive nitrogen rich azo-linked polymer (ALP-6).36 These polymers and their precursors used in the preparation of activated carbon require highly complex synthetic procedures, which involve significant costs and disposal of the generated waste must also be taken into consideration from environment perspective. However, our methodology is quite simple with a quick one step process to prepare activated carbons from almost freely available biomass Arundo donax using solid ZnCl2 as activating agent, whereas, most of the reports in the literature use a time-consuming two-step process and aqueous solution of activating agent. Despite slow rate of diffusion for solid phase mixing, the small particle size of Arundo donax (< 150 µm) and the low melting point of ZnCl2 (275 °C) together can produce homogeneous mixing for the solid phase mixture of biomass and ZnCl2. The molten ZnCl2 at 275 °C is freely diffused into the interspaces of biomass under the activation condition of 400-700 °C. Therefore, present method offers a facile, cost-effective and convenient way of preparing activated biocarbons from natural biomass without generation of any associated waste. Another interesting observation found in this work is the control of micro and mesoporosity of AMB materials. The mesoporosity is significantly increased as the amount of activating agent is increased. While AMB1-500 and AMB2-500 exhibit wide pore size distribution, the materials prepared using impregnation ratios of ZnCl2/biomass in the range 3-5, and carbonized at 500 °C show narrow mesopore size distribution with the pore size of 3.5 nm (Table 1). It is also interesting to note that the percentage of micropore area decreases from 70% to 4% as the amount of ZnCl2 to biomass ratio is increased from 1 to 3 and becomes 0% when the ratio was


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further increased to 5. The change of the nature of the porosity in the AMB can be explained as follows: micropores are generated during activation process which offers an escape route for the volatiles inside the biomass structure. As the amount of activating agent is increased, there is a proportional increase in the volatiles escaping through the pores, thereby converting the tiny micropores into wide mesopores, resulting in a large mesopores that are responsible for the large mesopore volume.37 These results confirm that higher impregnation ratio is more effective in the development of mesoporous character in the activated biocarbons and the nature of the porosity of the AMB can be controlled with the simple adjustment of the amount of ZnCl2 during the activation process. To understand the role of carbonization temperature affecting the textural properties of the AMB, the materials were treated at different carbonization temperatures and analyzed by the nitrogen sorption analysis. Figure 4C and 4D show the N2 sorption isotherms and pore size distribution curves for the activated biocarbons synthesized at different temperatures from 400-700 °C using an impregnation ratio of 3. The ZnCl2 to biomass ratio of 3 was used in this study as it was found to be the best ratio to obtain excellent textural parameters. Among the samples studied, AMB3500 shows the highest amount of mesoporosity in terms of BET surface area and pore volume whereas all other materials possess reduced amounts of mesoporous content in their structure. Interestingly, the capillary condensation step of the isotherm of AMB3-400 is observed at a lower relative pressure, indicating many micropores together with the mesopores in the samples. This is also clearly reflected in the percentage of mesopore volume of this sample presented in Table 2. This could be since a lower temperature of 400 °C may not be sufficient to generate enough thermal effects to produce porosity and a high surface area. At higher temperatures of 600 and 700 °C, the stronger dehydrating effect of ZnCl2 tends to cause shrinkage of the carbon


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structure and surface heterogeneity, thereby reducing porosity and causing a decrease in the values of the BET surface area.28 Table 3 compares the reported values of BET surface areas of activated carbons produced from different sources of biomass using ZnCl2 as an activating agent. It can be observed that the value of the surface area reported in our present study is extremely high when compared to these activated carbons. Additionally, we have devised a quick one step process to prepare activated carbons from Arundo donax using ZnCl2 as activating agent, whereas, most of these reports used a time-consuming two-step process. The presence of high mesoporous character in our materials with narrow pore size distributions makes them highly conducive for adsorption based application. Surface composition and chemical bonding nature of activated biocarbons Surface composition and nature and co-ordination of carbon and other elements in AMB were investigated using XPS. Figure 5A and 5B shows the survey spectra of AMBn-500 and AMB3T, respectively. The survey spectra confirm that all the AMB materials prepared in this work predominantly contain carbon and along with trace amounts of other elements namely O, N and Si are present. The absence of peaks corresponding to Zn and Cl suggests that all the residual Zn and Cl ions were successfully removed during the washing step. It can be seen from Table S1 and S2 (see the Supporting Information) that all activated biocarbons are highly rich in carbon content ranging from 89-95%. XPS survey spectrum also reveals that approximately 0.80-1.1% of nitrogen and 3.8-7.0% of oxygen are bound to surface carbon atoms of the AMB, indicating the presence of functional moieties that orginated from the biomass attached to the aromatic carbon network. With increasing impregnation ratio, there is a progressive decrease in oxygen content of the samples which is ascribed to the higher dehydrating effect of ZnCl2 on the structure.


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The high resolution C1s, N1s and O1s spectra for AMB3-500 and AMBn-500 samples are shown in Figure 5C-E and Figure S5 (see the Supporting Information), respectively. All the samples show a sharp peak at 283.8 eV which is attributed to the sp2 hybridized carbon (C=C) from the turbostratic carbon layers of the samples41. This result is also consistent with the data obtained from XRD and TEM measurements. O1s spectrum of the samples is deconvoluted into two peaks that are centered at 530.8 eV and 532.4 eV. The peak at the lower energy contribution is attributed to carbonyl groups on the carbon surface whereas the higher energy contribution at 532.4 eV is assigned to phenolic or ether type groups attached on the carbons42. On the other hand, N1s spectrum reveals that nitrogen contents in the sample is low and both pyridinic (398.4 eV) and pyrrolic nitrogen (399.5 eV) groups are attached with the carbon structure43. These functional groups are key for providing the hydrophilic characteristic to the sample which may be essential for adsorption and energy storage applications. As per Figure 5B and Table S2 (see the Supporting Information), AMB3-T samples are also found to be highly carbonaceous and contain appreciable amounts of oxygen in their structure. With increasing temperature from 400 to 700 °C, the stronger thermal effects results in higher loss of oxygenated functional groups present on the surface, leading to a significant reduction in the content of N and O with a concomitant increase of C contents of the samples. The reduction in the number of functional groups for the samples treated at high carbonization temperature was also consistent with the data from the higher angle power XRD analysis. These results reveal that the nature and the density of the functional groups of AMB can be controlled with the simple adjustment of the carbonization temperature and 500 °C was the best condition to obtain AMB with optimized functional groups and exceptional textural properties.


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The FTIR spectra of the activated biocarbons AMBn-500 are shown in Figure 6A. All the samples show several overlapping peaks occurring at almost identical wavenumber and with almost similar intensities. These results suggest that varying the impregnation ratio of ZnCl2 to biomass has no significant effect on the nature of functional groups present in the samples. FTIR peaks suggest that all samples show aromatic character along with the development of various functional groups containing mainly oxygen and nitrogen. The band appearing at 795-805 cm-1 can be ascribed to C-H out of plane bending vibrations in substituted benzene rings.44 The appearance of two bands at 1250 cm-1 and 1268 cm-1 confirms the presence of C-N bonds in the structure of biocarbon. The carbonyl (C=O) stretching vibrations occurring at 1675 cm-1 also further confirms the presence of oxygen in the carbon structure.45 High carbon and oxygen content along with low quantities of nitrogen as observed in CN and XPS analysis can be correlated with the observations in FTIR measurements. As illustrated in Figure 6B, the intensity of the peak at 800-805 cm-1 increases as the temperature is increased from 400 to 700 °C, indicating the increase in aromatic character in the carbon structure. On the other hand, the intensity of carbonyl (C=O) stretching vibrations peak at 1675 cm-1 gradually decreases with increase in temperature and completely disappears at 700 °C. This could be attributed to the loss of oxygen in the form of volatiles such as CO, CO2, H2O, etc. at high temperature as the thermodynamic stability of oxygen or nitrogen in the carbon matrix is low at a higher temperature. The rapid decomposition of the biomass constituents and restructuring of the carbon layers at high temperature supports the loss of oxygen and nitrogen in the carbon matrix.34 CO2 adsorption experiments As AMB materials exhibit remarkable BET surface area and large pore volumes, these materials were used as adsorbents for the capture of CO2 molecules at different temperature and pressure


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conditions. Figure 7 shows the CO2 adsorption isotherms of AMB3-500 at different temperatures and pressures. AMB3-500 was chosen for CO2 uptake studies because it showed extremely high BET surface area and large mesopore volume among all the samples. CO2 adsorption isotherms were recorded at three different temperatures 0, 10 and 25 °C and pressure varying up to 30 bar. The highest CO2 adsorption capacity of 30.2 mmolg-1 was recorded at 0 °C and pressure of ca. 30 bar. At 10 °C, the adsorption capacity of AMB3-500 reduces to 24.9 mmolg-1 and a further reduction in CO2 adsorption capacity to 24.2 mmolg-1 is observed when the analysis temperature is increased to 25 °C. The reduction in CO2 adsorption capacity with increase in analysis temperature suggests that the CO2 adsorption process is exothermic in nature and is favoured at lower temperature. Similar trend has been observed in previous reports in the literature.46 Furthermore, it is observed that there is a sharp increase in CO2 adsorption indicated by the appearance of a shoulder in the pressure range of 0-4 bar at all three analysis temperatures used in this study. From 4 bar onwards, the isotherm is nearly a straight line which signifies that the quantity of CO2 adsorption is proportional to the pressure dose and increases as the pressure dose increases. At low pressures, the initial filling of active microporous centres on the surface takes place at a rapid pace. Afterwards, at higher pressures, the rest of the mesoporous centres inside the carbon structures are occupied by the circulating CO2 molecules. The CO2 adsorption capacity of ZnCl2 activated biocarbons prepared in this work was compared with high surface area containing highly ordered mesoporous carbon CMK-3, highly ordered mesoporous carbon nitrides (MCNs) with different nitrogen contents, and disordered activated charcoal (AC), and multi-walled carbon nanotubes (MWCNTs) as illustrated in Figure 8. Noteworthy, the adsorption capacity of AMB3-500 at high pressure of 30 bar is the highest ever value reported for any mesoporous or microporous


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activated biocarbons and is much higher than that of highly ordered two or three dimensional mesoporous carbons, mesoporous carbon nitrides, AC and MWCNTs. This is a remarkable result which makes a significant breakthrough in the field of CO2 capture. It can be easily seen that AMB3-500 outperforms all the other materials by a significant margin through the entire pressure range 0-30 bar. The higher CO2 adsorption capacity of AMB3-500 is predominantly attributed to its extremely large surface area of 3298 m2g-1 whereas CMK-3 and MCN show a maximum surface area of about 1200 m2g-1 and 655 m2g-1 respectively. Other materials such as AC and MWCNTs have lower surface area which explains their relatively poor performance in CO2 uptake studies. This observation is quite consistent with previous reports in the literature.47-49 It is also interesting to note that the CO2 adsorption capacity of AMB3-500 sample is obtained by a quick one step activation process from a bio-waste and is superior to other materials over the entire pressure range and the material is obtained by a quick one step activation process from a bio-waste whereas CMK-3, MWCNT, MCNs and AC are prepared by complex procedures and involve quite expensive processes. This shows that AMB3-500 based activated mesoporous carbons have tremendous potential to replace the existing expensive adsorbents for large scale industrial processes. In addition, AMB3-500 also shows a high value for CO2 adsorption measured at ambient pressure conditions when compared to materials such as alumina supported amine adsorbents (0.17-1.74 mmolg-1 at 25 °C).50 Isosteric heat of adsorption The strength and type of interactions between the adsorbent and adsorbate was studied by calculating the isosteric heat of adsorption using the Clausius Clapeyron relation.51


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ln p = -qst/RT + ln C where p is pressure, qst is the isosteric heat of adsorption, R is ideal gas constant, T is temperature and C stand for a constant. Adsorption isosteres are obtained by plotting ln p vs 1/T and qst is read as a slope of this graph. For greater accuracy, isotherms for each sample was recorded at three different temperatures and using an iterative process involving Clausius Clapeyron equation to obtain isosteric heat of adsorption. Figure 9a shows the variation of isosteric heat of adsorption for AMB3-500 sample with CO2 loading. The qst values were calculated over CO2 uptake of 0.1-14 mmolg-1 and found to be in a moderate range of 17-21.8 kJmol-1. The low values of heat of adsorption indicate that the interaction between adsorbate and adsorbent is predominantly Vander Waals in nature. Additionally, the decreasing heat of adsorption with increasing CO2 loading suggests the heterogeneous nature of the surface and the trend is consistent with previous reports in the literature. On the contrary, Tiwari et al. recently reported lower qst values of zeolite templated epoxy resin based carbon materials in the range of 7.79-11.10 kJmol-1 at low CO2 uptakes.52 A higher range of qst is shown by functionalized porous organic polymers (28.5-34.9 kJmol-1),53 nitrogen rich porous carbons (25-35 kJmol-1),54 and metal-organic frameworks such as MIL-53(Al)( 38.8 kJmol-1).55 Nachtigall et al. reported high adsorption enthalpies (20-60 kJmol-1) for a series of zeolites such as MFI, FER, FAU, LTA, TUN, IMF, and SVR.56 Similarly, the alkali metal exchanged zeolite Y was found to have a very strong adsorption of CO2.57 Both these studied concluded that the stronger adsoption of CO2 occurs due to the presene of extra framework cations in the zeolite structure. Although, our material is devoid of any such cationic functionalities, it still shows a high CO2 adsorption which mainly relies on the presnce of surface active


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sites and porous structure. Moreover, the absence of basic anchoring groups implies that there are no chemical interactions between the adsorbate and adsorbent. To check the reusability of the used sample, it was treated at 250 °C and used for the adsorption of CO2. It was found that the adsorption of CO2 on AMB3-500 sample is reversible and therefore regeneration of the adsorbent is relatively easy to accomplish by simply heating the samples at 250 °C under vacuum for 6-8 h. Figure 9b and 9c compares the isoelectric heat of adsorption of MWCNTs and AC with AMB-500. There is not a very large difference observed in isoelectric heat of adsorption among the three materials. However, a slightly higher value of 21.8 kJmol-1 for AMB3-500 is observed which indicates enhanced interactions between adsorbate and adsorbent.


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CONCLUSIONS In this paper, we have successfully demonstrated for the first time the effective way for the preparation of activated mesoporous carbons from low cost and freely available biomass Arundo donax using a single step and solid state activation using ZnCl2 salt. The characterization results reveal that the micro and mesoporous nature of the samples can be finely tuned with the simple adjustment of the amount of activating agent. It has been also demonstrated that nature of the functional groups and the graphitic character of the materials can be controlled by a simple fine tuning of the carbonization temperature. The ZnCl2/biomass ratio of 3 and carbonization temperature of 500 °C was found to be the best condition and the sample prepared under this condition (AMB3-500) registered an extremely high BET surface area of 3298 m2g-1 and a large pore volume of 1.9 cm3g-1 which is the highest ever value reported to date for any activated mesoporous biocarbons. The BET surface area of the prepared activated mesoporous carbon is much higher than that of highly ordered mesoporous carbons, activated carbons prepared from biomass and other non-biomass carbons. Additionally, we have demonstrated the excellent CO2 adsorption performance of these materials under conditions which mimic real life scenarios such as high pressure natural gas wells and in various industries generating CO2 as a waste. AMB3-500 registered the CO2 uptake of 30.2 mmolg-1 at 30 bar and 0 °C which is much higher than that of activated carbons, carbon nanotubes and other highly ordered mesoporous carbon materials measured under similar conditions. We surmise that the combination of low cost, single step and solid-state activation approach and the higher performance of carbon capture of these samples make them as extremely promising


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commercial adsorbents for various toxic and non-toxic adsorbates and further open the door for the generation of novel porous biocarbons from different biomasses.


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FIGURES

Scheme 1. Schematic representation of synthesis of activated biocarbons


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Figure 1. XRD patterns of (A) AMBn-500 and (B) AMB3-T


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Figure 2. SEM micrographs of A) AMB1-500 B) AMB2-500 C) AMB3-500 D) AMB4-500 E) AMB5-500


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Figure 3. A) LRTEM and B) and C) HRTEM images of AMB3-500. The circles in A and B indicate the meso and micropores, respectively. C is magnified image of a square section of B.


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Figure 4. (A-B) N2 adsorption–desorption isotherms and BJH pore size distribution of AMBn500 and (C-D) N2 adsorption–desorption isotherms and pore size distribution of AMB3-T


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Figure 5. XPS survey spectra of (A) AMBn-500 and (B) AMB3-T and (C-E) High resolution C1s, N1s and O1s spectra for AMB3-500


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Figure 6. FTIR spectra of (A) AMBn-500 and (B) AMB3-T


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Figure 7. High pressure CO2 adsorption isotherms for AMB3-500 at a) 0 °C b) 10 °C and c) 25 °C


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Figure 8. Comparison of CO2 uptake between biomass and non-biomass derived carbons


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Figure 9. CO2 adsorption isostere for a) AMB3-500 b) Activated charcoal and c) MWCNT at 0, 10 and 25 °C


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TABLES.

Table 1: Textural properties of the activated carbons of AMBn-500 where n = 1 to 5

SABET

SAmicro

SAmeso

PVtotal

Vmicro

Vmeso

PW

(m2g-1 )

(%)

(%)

(cm3g-1)

(%)

(%)

(nm)

AMB0-500

0.06

-

-

-

-

-

-

AMB1-500

2007

70

30

0.9

62

38

2.2

AMB2-500

2298

32

68

1.2

25

75

2.9

AMB3-500

3298

4

96

1.9

0

100

3.5

AMB4-500

2458

1

99

1.6

0

100

3.5

AMB5-500

2464

0

100

1.7

0

100

3.5

Sample

SABET – Brunauer Emmett-Teller surface area, SAmeso – Mesopore surface area, SAmicro – Micropore surface area PVtotal – Total pore volume, Vmicro – Micropore volume, Vmeso – Mesopore volume and PW – Pore width


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Table 2: Textural properties of the activated carbons of AMB3-T

SABET

SAmicro

SAmeso

PVtotal

Vmicro

Vmeso

PW

(m2g-1)

(%)

(%)

(cm3g-1)

(%)

(%)

(nm)

AMB3-400

2070

29

71

1.1

20

80

2.2

AMB3-500

3298

4

96

1.9

0

100

3.5

AMB3-600

2067

13

87

1.2

6

94

3.5

AMB3-700

2284

18

82

1.23

11

89

3.5

Sample

SABET – Brunauer Emmett-Teller surface area, SAmeso – Mesopore surface area, SAmicro – Micropore surface area PVtotal – Total pore volume, Vmicro – Micropore volume, Vmeso – Mesopore volume and PW – Pore width


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Table 3: Comparison of textural parameters of ZnCl2 activated carbons produced from various biomass precursors Biomass Precursor

Carbonization Temp. (°C)

SABET

PV

PW

(m2g-1)

(cm3g-1)

(nm)

Ref.

Arundo donax

500

3298

1.9

2.9

This work

Grape stalk

700

1411

0.72

-

9

Hazelnut shell

250/700

737

-

-

10

Pineapple waste

500

915

0.56

2.4

11

Coffee husk

500

1530

0.99

-

13

Peanut shell

200/480

1642

0.42

-

18

Loofah sponge

800

1733

0.86

2.0

20

Pine wood chips

500

1625

1.16

-

24

Chestnut shell

700

927

0.49

2.1

25

Tomato waste

600

1093

1.57

5.9

28

Orange peel

500

1215

0.68

2.2

34

Glucose

800

2560

1.37

2.7

35

Ferula stalks

550

1476

0.14

1.8

38

Buriti shell

700

843

0.49

2.3

39

Biogas residue

700

517

0.24

-

40

SABET – Brunauer Emmett-Teller surface area Surface Area, PV – Pore Volume, PW – Pore Width


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ASSOCIATED CONTENT Supporting Information. Table S1: CN bulk and XPS elemental composition of AMBn-500 activated biocarbons Table S2:

CN bulk and XPS elemental composition of AMB3-T (400-700°C) activated

biocarbons Figure S1: Percentage yields for the synthesized activated biocarbon materials Figure S2: XRD pattern of non-activated sample made at 500 °C Figure S3: SEM micrographs of a) AMB3-400 b) AMB3-500 c) AMB-600 and d) AMB3-700 Figure S4: Statistical representation of pore size from SEM images for A) AMB1-500 b) AMB2-500 c) AMB3-500 d) AMB4-500 and e) AMB5-500 Figure S5: High-resolution XPS C1s, N1s and O1s spectra for a) AMB1-500 b) AMB2-500 c) AMB4-500 d) AMB5-500 The supporting information is available free of charge on the ACS publications.


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AUTHOR INFORMATION Corresponding Author E-mail address: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The work carried out in this manuscript was financially supported by the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE) through grant no. (3.3.02-15/16), whose activities are funded by the Australian Government's Cooperative Research Centre’s Program. A. Vinu also acknowledges the research support and laboratory facilities provided by Future Industries Institute, University of South Australia, and Australian Research Council for the Future Fellowship and Discovery Grants.


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Highly Efficient Method for the Synthesis of Activated Mesoporous

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