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CARBON MONOLITHS BY ASSEMBLING CARBON SPHERES FOR GAS ADSORPTION Shuai Gao, Lei Ge, Byron S. Villacorta, Thomas E Rufford, and Zhonghua Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04891 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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Industrial & Engineering Chemistry Research

CARBON MONOLITHS BY ASSEMBLING CARBON SPHERES FOR GAS ADSORPTION Shuai Gao1, Lei Ge1,2, Byron S. Villacorta1, Thomas E. Rufford1*, and Zhonghua Zhu1* 1School

2Center

of Chemical Engineering, The University of Queensland, St Lucia 4072, Australia

for Future Materials, University of Southern Queensland, Springfield 4300, Australia

Corresponding authors Z.H. Zhu Tel: +61 7 336 53528. E-mail: [email protected], and T.E.Rufford Tel: +61 7 3365 4165. E-mail: [email protected]

Abstract

We report high surface area, hierarchical pore structured, and robust activated carbon discs (ACD) prepared via the assembly of micron-sized carbon spheres with mesophase pitch in a low pressure foaming and carbonization process. The carbon disc ACDCS75 with the largest specific surface area (1338 m2·g-1) was obtained from a blend of 75 wt% carbon spheres and 25 wt% mesophase pitch, and this ACD had a bulk density of 0.62 g·cm-3 and a high compressive strength of 26.3 MPa. A nitrogen-doped ACDCS75 disc was prepared by post-carbonization ammonia treatment to study the effect of nitrogen containing surface functional groups on the uptake of CO2 on ACDs. The adsorption of pure fluids CO2, CH4, and N2 were measured at temperatures of 298 K, 308 K, and 318 K at pressures from 6 – 3496 kPa for CO2, pressures

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from 9 – 3996 kPa for CH4, and pressures from 7 – 3994 kPa for N2 using a high-pressure gravimetric apparatus (Belsorp-BG). The equilibrium adsorption capacities of ACDCS75 measured at 298 K and pressure close to 1000 kPa were 5.67 mmol·g-1 CO2, 3.60 mmol·g-1 CH4, and 2.09 mmol·g-1 N2; and at 3500 kPa were 6.16 mmol·g-1 CO2, 4.42 mmol·g-1 CH4, and 3.18 mmol·g-1 N2. After ammonia treatment the capacities of N-ACDCS75 at 298 K and 1000 kPa were 6.22 mmol·g-1 CO2, 3.70 mmol·g-1 CH4, and 1.98 mmol·g-1 N2; and at 3500 kPa were 7.16 mmol·g-1 CO2, 4.87 mmol·g-1 CH4, and 3.32 mmol·g-1 N2. The pure gas equilibrium adsorption capacities were regressed to Toth and Langmuir models; and the uptake of components from gas mixtures were predicted using an ideal selectivity to make a preliminary evaluation of the potential to use these ACDs for gas separation. After N-doping the predicted changes in selectivities at 298 K and 100 kPa were from 6.0 to 7.2 for CO2 over N2 and 2.8 to 3.3 for CH4 over N2 for ACDCS75 on N-ACDCS75 compared to ACDCS75.

Keywords

carbon spheres, mesophase pitch, carbon monoliths, adsorption, ammonia treatment


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1

Introduction Carbon spheres (CS) have been proposed for use in catalysis, adsorption, air and water

purification, and electrochemical energy storage1-4 because CS can be prepared with homogeneous geometry, adjustable porosity, and controllable particle size distribution5-9. One of the proposed applications of CS is as an adsorbent for CO2 capture10, purification12, 13. For example Wickramaratne and Jaroniec

10

11

and for biogas

reported KOH activated carbon

spheres with a specific surface area of 2400 m2·g-1 that adsorbed up 8.9 mmol·g-1 CO2 at 273 K and ambient pressure. Adsorption based processes based on novel adsorbents like CS could potentially lead to more energy efficient and cost effective gas separation and purification to the conventional technologies like amine scrubbing14. However, a challenge for use of nanosized and microsized CS as adsorbents is that CS powders cannot be used directly in large adsorbent beds but must be manufactured using binders into other forms such as extruded pellets, granules or structured adsorbents 15-17. Monolithic structures with higher specific surface areas, hierarchical pore structures, and high thermal conductivity have been reported to have substantial benefits over other adsorbent forms 18, 19.

For example, Luo et al.20 reported in a study of adsorption of organic vapors on activated

carbons bed using different forms of carbons that monolithic carbon beds exhibited lower pressure drop and higher permeability than beds packed with beads of carbon fiber cloths. In another example, Lin et al. developed a heat recuperator from blocks of carbon foam blocks21 that had better heat transfer rates and lower pressure drops than other recuperator bed designs. More advantages and possible drawbacks of monolithic adsorbents are summarized in Table 1. This study aims to use the attractive properties of CS for gas adsorption by inclusion of CS in monolithic activated carbon discs (ACD) prepared with mesophase pitch in a low pressure


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foaming and carbonization process. Importantly, we demonstrate that the high strength ACDs retain similar adsorption capacities to the powder form CS. In addition, CO2 adsorption capacities on carbons can be enhanced by introducing nitrogen functional groups to the carbon. In general there are two strategies used to increase the concentration of N-containing functional groups on porous carbon surfaces: (a) carbonization of nitrogen-rich carbon precursors4, 22, and (b) post-carbonization treatment of the carbon with a N-containing compound such as ammonia 23 24.

In this study we used the second approach of post-carbonization ammonia-treatment to add

N-groups to the surface of a prepared ACD, we observed that the selectivity of N-doped ACDs for CO2 from mixtures of CO2 + N2 and CO2 + CH4 increased.

We report high pressure

adsorption isotherms of CO2, CH4, and N2 single gases on the ACD, which are fitted into Toth and Langmuir isotherm models25, 26, and used to predict the ideal selectivity of the ACDs to capture CO2 from binary gas mixtures. 2

Experimental

2.1 Synthesis of carbon spheres We synthesized carbon spheres (CS) using the extended Stöber method described by Liu et al. 27.

The first step in CS preparation was to dissolve at room temperature (296 K) 1.2 g of

resorcinol (≥99.0%, Sigma Aldrich) in a mixture containing 0.5 mL ammonia solution (28-30 volume %, Emsure), 48 mL ethanol, and 120 mL distilled water. Then, to this mixture we added 1.8 mL of 37 wt % formaldehyde solution (Sigma Aldrich) dropwise. The reagents were stirred for 24 h, transferred to a 250 mL Teflon-lined sealed autoclave, and then hydrothermally treated in an oven at 373 K for 24 h. Solid polymer spheres (PS) produced in the hydrothermal treatment were recovered by centrifugation and dried at 373 K for 12 h.


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The polymer spheres were carbonized in a tube furnace at 873 K under argon to produce carbon spheres (CS). The heating program for carbonization of PS was to ramp the furnace temperature at 1 K·min-1 to 623 K, hold at 673 K for 2 h, ramp again at 1 K·min-1 to 873 K, and finally hold at 873 K for 4 h. 2.2 Preparation of activated carbon discs The CS were mixed in 20 mL ethanol with mesophase pitch (MP, 074608, Bonding Chemical, USA) and KOH pellets (85%, Chem-supply) at CS:MP:KOH mass ratios of 1:9:5, 1:3:2, 1:1:1, 3:1:2, and 9:1:5. The MP is the same pitch we used in our previous study that reports activated carbond discs from MP and coal powders28. The manufacturer’s technical product sheet reports that the MP has a softening point in the range 533 -553 K and mesophase content of about 85%wt, and other properties of this pitch are available in our previous report28. The precursor mixture was stirred at 350 rpm for 10 mins, sonicated for 30 mins in an ultrasonic bath (Model FXP12DH Unisonics, Australia), stirred again on a hot plate at 353 K to evaporate the ethanol, and then dried in an oven at 373 K for 12 h. The dried CS + MP + KOH blends were ground to powders by hand in a mortar and pestle. We mixed blends and prepared ACDs of CS + MP using the same methods we reported to prepare ACDs from coal powders and tar pitch28. Discs of 14 mm diameter were formed from about 1 g of the CS + MP + KOH blends in a cold isostatic press (YLJ-CIP-15 MTI, USA) at 2.4 T. Carbonization was performed under argon (50 mL·min-1) in a tube furnace at a heating rate of 10 K·min-1 up to 1073 K with a dwell time of 1 h. After carbonization the activated carbon discs (ACD) were washed in 0.1 M HCl, dried overnight at 353 K, and rinsed with distilled water until a filtrate pH of 7 was achieved. We label the ACDs containing carbon spheres as ACDCSx where x% is the mass percentage of CS to CS+MP in the precursor blends.


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The nitrogen doped ACD (N-ACDCS75) was prepared by the treatment of ACDCS75 in 10 mL·min-1 ammonia at 1073 K for 1 h. We prepared three discs at each set of synthesis conditions to confirm reproducibility of the method. The carbon yields of ACDs were calculated from the weight of the carbon spheres (mCS), mesophase pitch weight (mMP), and the weight of the washed ACD end product (mACD) according to Equation 1. The ACD yields at different CS to MP ratios ranged from 78.9 % (ACDS75) to 90.2 % (ACDCS10) as shown in Table 2, and these yields are higher than yields of other carbon foams recently reported from sucrose (42.1-60 %)

19,

coal and tar pitch (43.58-

48.17 %)29, CNTs and tar pitch (28.2-48.0 %)30, and coal and mesophase pitch (75.1-77.1 %)28 due to the high yield of mesophase pitch (78%-87%)

31, 32.

However, without enough MP as a

binder we did not produce the monolithic ACDs and ACDCS90 did not retain the disc shape. 𝑚𝐴𝐶𝐷

𝑌𝑖𝑒𝑙𝑑 (%) = 𝑚𝐶𝑆 + 𝑚𝑀𝑃 × 100

(1)

2.3 Materials characterization For materials characterization we used most of the same standard analytical instruments that were used in our lab’s other studies reporting ACDs prepared from different carbon precursors and filler particles28-30,

33, 34.

These characterization methods included scanning electron

microscope (SEM), transmission electron microscope (TEM), thermogravimetric analysis (TGA) and differential thermogravimetry (DTG), X-ray photoelectron spectroscopy (XPS), uniaxial compressive strength (UCS), mercury intrusion porosimetry (MIP), helium pycnometer, and standard gas sorption analysis with N2 at 77 K and CO2 at 273 K. The instruments and details of measurement conditions are listed in the Supporting Information (SI).


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The particle size distributions of PS and CS were measured by a dynamic light scattering (DLS) method in a particle size range from 0.1 to 5000 nm. Samples were suspended and treated by ultrasound for 30 sec in ethanol before analysis on a Malvern Zetasizer nano ZS.

2.4 High pressure gravimetric gas adsorption measurements As in our other ACD studies33, 34, high pressure adsorption equilibrium capacities of CO2, CH4, and N2 were measured at 298 K, 308 K, and 318 K at pressures up to 4000 kPa on a Belsorp-BG apparatus (BEL, Japan) equipped with a magnetic floating balance (Rubotherm, Germany). Samples were degassed in situ at a temperature of 423 K and a pressure of 0.005 kPa for 5 h prior to the high pressure adsorption measurements. Details of the Belsorp-BG apparatus and its operation are described in our lab’s previous articles 33, 34.

3. Result and Discussion 3.1 Formation of activated carbon discs The TEM images of polymer spheres (PS) in Figure 1a and carbon spheres (CS) in Figure 1b demonstrate that we successfully synthesized smooth spheres from resorcinol and formaldehyde using Liu, et al’s extended Stöber method27. The SEM image in Figure 1c and the DLS particle size distribution data in Figure 1d shows a uniform distribution of CS sizes centered around 1000 nm. Figure 2 shows (a) the weight loss and (b) rate of weight loss with temperature change (i.e. DTG curves) measured under N2 gas flow from CS and KOH, MP and KOH, and the 9:1:5 mixture of CS + MP + KOH used to produce ACDCS75. Both the CS and MP were treated with KOH for the TGA measurements to normalize any activation effects from the KOH. In Figure 2b


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all three curves show peaks around 368 K that correspond to the evaporation of physical adsorbed water on the surface of carbon and KOH. The increased rate of weight loss observed at temperatures around 1014 K may due to the dehydration of KOH to K2O and the consumption of carbon by reaction with K2O35-37. The MP began to melt at its softening point between 533 K and 553 K, then a more rapid weight loss associated with evolution of volatiles from the MP was observed at around a temperature of 683 K (peak in Figure 2b DTG curve). The CS exhibited a weight loss peak around 431 K which may be caused by desorption of water, CO2, and N2 from the surface of the CS. The effect of CS on the morphology of the ACDs is observed in Figure 3 where we see that at low CS contents (10%) the structure of the ACD is dominated by the MP, and in this case the CS only work as a type of filler or modifier particle 38. With a further increase to the CS proportion from 25% to 50%, then to 75%, we infer that the pitch foaming process has been stabilised by the extra CS and the deformation of the structure is minor. This observation could be confirmed from the pore size distribution of ACDs in Figure 4 (b), which shows that with extra CS, ACDCS50 and ACDCS75 feature fewer large pores (1-100 µm) than ACDCS10 and ACDCS25. And in our previous work 29, we found that without any filler particles, the pitch will overflow due to an intense foaming process. In contrast, at a CS loadings of 75% more carbon spheres are observed in the ACD, and these carbon spheres are tightly arranged together with mesophase pitch as binder 39. 3.2 Activated carbon disc bulk properties A photo of a typical monolithic disc (ACDCS75) prepared from carbon spheres and mesophase pitch with KOH activation is shown in Figure 5. The summary of bulk properties of the activated carbon discs (ACDs) with 10% to 90% of CS in precursor is listed in Table 2. The


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bulk density and porosity of the specimen (ACDCS90) with 90% CS is not shown, as the monolithic shape could not be maintained after carbonization. The skeletal density of ACDs with different CS proportions ranged from 1.72 to 2.11 g·cm-3 and these results are consistent with the range of densities achieved in our previous works with other ACD precursors29,

30.

As the

concentration of CS in the ACD was increased from 10% to 50% the bulk density of ACDs first increased from 0.74 g·cm-3 to 0.80 g·cm-3, but at higher CS loading of 75% the bulk density fell back to 0.62 g·cm-3. Correspondingly, the porosity of ACDs first decreased from 64.8% to 59.1% and then rose to 70.8%, which is consistent with the cumulative pore size distribution in Figure 4 (a). We propose these observation result because at a low concentration of CS in the foam precursor the bubbles that form in the MP phase are not stabilized by enough CS filler particles, which leads to large pores (greater than 10 µm) as seen in ACDCS10 in Figure 4 (b). At higher CS loadings, the growth of pitch bubbles was stabilized by CS particles and the resultant ACD has fewer large pores; for example ACDCS75 in Figure 4 (b) has a peak pore size distribution between 0.1 and 1 µm. Figure 6 shows three measurements of compressive strength of three ACDCS75 discs, and the mean failure strength of these three measurements is 26.3 MPa±2.2 MPa. This CS + MP ACD is much stronger than most other reported carbon foams prepared from, for example, phthalonitrile polymer (2.0-12.3 MPa)

18,

tar pitch (5.95-11.72 MPa)

29, 30, 40,

sucrose (1.1-4.5 MPa)

19, 41

and

phenolic resin (3.78-6.39 MPa) 42. We attribute the excellent mechanical properties of ACDCS75 partly to the narrow pore size distribution of this ACD compared to the other carbon foams, which contain a larger volume of larger porous cells which may be weak structures in the foam than smaller sized cells. This ACDCS75 structure is related to both (i) the mesophase pitch properties of high carbon content and thermal rheology behaviour 28, and the CS acting as filler


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particles to stabilize foam development during heating reported for other types of filler particles33. We note that ACDCS75 was not as strong as an activated carbon disc prepared from MP and coal powders (56±3 MPa) we reported elsewhere28. However, the specific surface area and micropore volume of the ACDCS75 are much higher than those properties of the coal powder ACDs, and a strength of 26.3 MPa should be sufficient for most cyclic adsorption processes. Usually KOH activated of carbons leads to breakage of particles and to fine powders 43, 44, and to low density carbons at high ratios of KOH

45, 46.

Thus, most KOH activated carbons do not

exhibit the high compressive strength that we observed for ACDCS75. However, in our study we have used only 50%wt KOH to carbon precursors (carbon spheres + mesophase pitch) compared to much higher KOH ratios commonly used to prepare high-surface area activated carbon powders (up to 300%wt KOH to carbon precursors). Obviously, at the lower KOH ratios we did not achieve as high a surface area as that obtained in high ratio KOH activated carbons. 3.3 Pore texture characterization by low pressure N2 and CO2 sorption analyses The N2 sorption isotherms measured on the ACDs at 77 K using the TriStar II 3020 are shown in Figure 7(a) and CO2 isotherms measured at 273 K are shown in Figure 7(b). The N2 and CO2 isotherms measured on carbon spheres are included in the Supporting Information. All N2 isotherms on the ACDs were Type I isotherms with large volumes of N2 adsorbed at low pressure (P CH4 > N2, which is the expected trend for adsorption of these gases on a porous carbon. For instance, the CH4 uptake at 298 K rapidly increased to 3.60 mmol∙g-1 at 1000 kPa and then with further increasing pressure up to 4000 kPa, the CH4 uptake climbed to 4.46 mmol∙g-1. Isosteric heat of adsorption for different adsorbate loadings was calculated by the ClausiusClapeyron equation 29 to provide additional data for the evaluation of adsorption processes based on ACDCS75. Figure 11 displays the isosteric heat obtained for CO2, CH4 and N2 on ACDCS75 as a function of the amount adsorbed. Values of heat of adsorption were in the ranges 20.32 -


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89.48 kJ·mol-1 for CO2 at loadings of 1.05 - 5.5 mmol·g-1, 58.00 - 13.58 kJ·mol-1 for CH4 at loadings of 0 - 4.0 mmol·g-1, and 18.10 - 22.21 kJ·mol-1 for N2 at loadings of 0.3 - 2.1 mmol·g-1. These results show at low loadings there is relatively strong interactions between CO2 and CH4 with heterogeneous sites on the carbon surface, between N2 and the carbon surface

49.

48

but weaker and more uniforom interactions

The plateau in slope of the qst of CO2 at higher surface

loadings may be attributed to strong CO2–CO250. The absolute equilibrium adsorption capacities up to 3500 kPa of CO2, CH4 and N2, Toth model fitting parameters, and the selectivity of CO2/N2, CO2/CH4 and CH4/N2 on N-ACDCS75 are included in Figure 12, Table S3 (Supporting Information), Table 5, and Figure 13. At pressures measured at 700 kPa and greater the N-ACDCS75 capacity for CO2 was larger than ACDCS75 at all measured temperatures, but at pressure of 300 kPa and lower the CO2 adsorption capacity measured on N-ACDCS75 was not always higher than on ACDCS75. Similar observations for N-doped carbons with nitrogen concentrations close to that of NACDCS75 (4.45%at. N by XPS, Table 4) have been reported by others4, 11, 51, and it’s understood that micropores, especially ultramicropores, also play a key role in adsorption of CO251. The effect of ultramicropores on CO2 uptake will be more significant at lower pressures than higher pressures, and this mechanism may explain why the uptake of CO2 was greater on ACDCS75 than on N-ACDCS75 at low pressures. At 298 K and 3500 kPa the CO2 capacity of N-ACDCS75 was 7.16 mmol·g-1, and at 298 K and 100 kPa, N-doping increased the selectivity of CO2 over N2 from 6.0 on ACDCS75 to 7.2 on N-ACDCS75. The enhanced selectivity for CO2 over N2 on N-ACDCS75 is expected to be due to the stronger interactions of the N-doped carbon’s basic nitrogen functional groups with CO2 molecules that have a larger quadrapole moment (13.4 × 10−40 C·m2) than N2 molecules (4.7 ×


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10−40 C·m2), and this is a well-known phenomena reported in the carbon literature

22, 52.

This

explanation is supported by the much higher areal CO2 adsorption capacity of N-ACDCS75 (5.47 μmol·m-2) than that of ACDCS75 (4.10 μmol·m-2). Furthermore, Figure S3 shows that the heat of CO2 adsorption on N-ACDCS75 is higher than that of ACDCS75. The high value of qst of N-ACDCS75 at low surface coverage is related to adsorption of CO2 on active Ncontaining functional groups,

53

and at higher surface loadings once these N-groups have been

saturated with CO2 the difference between heat of adsorption for N-ACDS75 and ACDS75 are not as large. We tested a temperature-dependent Toth isotherm model25 (Equation 2) and, at the request of a manuscript reviewer, a temperature dependent Langmuir model26 (SI, Equation S1) to determine the ability of this model to predict the experimentally measured adsorption capacities on ACDCS75. The fitting results of the Langmuir model are included in the SI Figure S6 and Table S4. = 𝑄𝑇𝑜𝑡ℎ 𝑄𝑇𝑜𝑡ℎ 𝜇𝑖 𝜇𝑠𝑖

𝑏𝑖𝑃 𝑡𝑖 1/𝑡𝑖

[1 + (𝑏𝑖𝑃) ]

with 𝑏𝑖 = 𝑏0,𝑖exp (

―∆𝐻𝑇𝑜𝑡ℎ,𝑖 𝑅𝑇

)

(2)

where P and T are the measured pressure and temperature, respectively; R is the molar gas constant, and ―∆𝐻𝑇𝑜𝑡ℎ,𝑖 is the isosteric heat of adsorption at zero loading. In the regression of the Toth model, ―∆𝐻𝑇𝑜𝑡ℎ,𝑖 and other empirical parameters (𝑄𝑇𝑜𝑡ℎ 𝜇𝑠𝑖 , 𝑏0,𝑖 and 𝑡𝑖) were treated as adjustable parameters. The best-fit parameters for Equation 2 determined by a least-squares regression analysis to 1/2

2 ― 𝑄𝑇𝑜𝑡ℎ (𝑁1 )∑(𝑄𝑚𝑒𝑎𝑠 𝜇𝑖 𝜇𝑖 ) )

minimize the standard deviation (𝑆𝐷 = (

) between the measured

capacities (𝑄𝑀𝑒𝑎𝑠 ) and the calculated capacities with Toth model (𝑄𝑇𝑜𝑡ℎ µ 𝜇𝑖 ), where N is the number of data points regressed, are listed in Table 5. The regressions was performed in MS Excel using the GRG nonlinear algotherim in the Solver tool. The predicted gas adsorption capacities using


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Page 16 of 37

the best fit parameters are shown as solid lines in Figure 10 a, b and c for CO2, CH4 and N2, respectively. Figure 10 (e-f) display the deviation of the Toth model calculated capacities from the measured data (𝑄𝑚𝑒𝑎𝑠 ― 𝑄𝑇𝑜𝑡ℎ 𝜇𝑖 𝜇𝑖 ) for CO2, CH4 and N2. As can be seen from Figure 10, the Toth model provides a good fit to the adsorption data for all three adsorbates with a deviation ( -1 ― 𝑄𝐶𝑎𝑙𝑐 𝑄𝑀𝑒𝑎𝑠 µ µ ) in the range of ±0.15 mmol·g at 298-318 K at pressures up to 3000 kPa. At

pressure levels above 3000 kPa, the adsorption capacities for N2 and CH4 predicted by Toth model are still reasonable, whereas for CO2 adsorption capacities from the modeling are less reliable. To evaluate the potential of the activated carbon disc ACDCS75 for the separation of three gas mixtures: CO2/N2, CO2/CH4 and CH4/N2, the calculation of ideal selectivity 𝛼𝑖𝑗 at three temperatures and pressure up to 3000 kPa was carried out using the fitted Toth model. The ideal equilibrium selectivity (𝛼𝑖𝑗) was defined as: 𝛼𝑖𝑗 =

( )( ) 𝑥𝑖

𝑦𝑗

𝑥𝑗

𝑦𝑖

𝑖𝑓 (𝑦𝑖 = 𝑦𝑗)

𝛼𝑖𝑗 =

( ) 𝐶𝜇𝑖

(3)

𝐶𝜇𝑗

where x and y are the mole fraction of component i and j in the adsorbed phase and vapor, respectively.

By assuming the equimolar gas mixtures(𝑦𝑖 = 𝑦𝑗), the selectivity values of

CO2/CH4, CH4/N2 and CO2/N2 as a function of pressure at 298 and 318 K are shown in Figure 14. It can be seen that the CO2 selectivity decreases with increasing pressure increment. In particularly, at 318 K, the selectivity of ACDCS75 for CO2 from N2 was 7.1 at pressure close to 100 kPa and decreased to 3.0 at pressure about 1000 kPa. At 298 K and 100kPa, the selectivity of ACDCS75 for CO2 over N2 was 6.0, which is comparable to selectivities reported for other carbon adsorbents like carbon monoliths33 (5.94), honeycomb monoliths54 (6-7) and carbon foams55 (7-8), but not as high as some other type of materials such as MOF (37.2)

56.

Table 6

compares the CO2 adsorption capacity and CO2/N2 selectivity of N-ACDCS75 with ACDCS75,


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and with other porous carbon spheres and carbon monoliths reported in the literature. Furthermore, the selectivity of CO2 over N2 is very important in CO2 capture from post combustion flue gases that typically contain 70% N2, 15% CO2 and other gases such as O2 and H2O

55, 57.

The predicted selectivity of CO2 over N2 was calculated from equation 3 where yi

(CO2) =15% and yj (N2) =70% and the result are presented in Figure S4 and S5 in Supporting Information.

4. Conclusions This study demonstrates the preparation of high-strength (26.4±2.2 MPa) and high surface area (1338 m2·g-1) monolithic activated carbon discs (ACDs) by direct foaming of carbon spheres and mesophase pitch with KOH. The concentration of CS in the foam precursor mixture has a strong effect on the pore structure that develops in the ACD, and on the compressive strength of the material. We found that N-doping of the ACDs enhanced CO2 adsorption capacities up to 6.25 mmol·g-1 at 273 K and 100 kPa, and up to 7.16 mmol·g-1 at 298 K and 3495 kPa. Furthermore, the selectivity of CO2 over N2 increase from 6.0 on ACD to 7.2 on N-doped ACD at 298 K and 100 kPa. Therefore, the ACDs presented here show a potential use of CS adsorbents to develop practical and robust adsorbents with large CO2 adsorption capacities that approach performance of other activated carbon spheres (6.2-8.9 mmol·g-1, 273 K and 100 kPa) and metal organic frameworks (up to 8.5 mmol·g-1). This study has focused on the ACD preparation and measurement of equilibrium adsorption capacities, but we recommend that future work on these types of ACDs include dynamic breakthrough adsorption experiments to determine kinetic sorption parameters and evaluate separation of gas mixtures.


17


Page 18 of 37

Supporting Information A table of the instruments and methods used to characterize the activated carbon discs; adsorption isotherm of N2 and CO2 on carbon spheres; pore size distribution of ACDs; Isosteric heat of adsorption of CO2, CH4 and N2 on N-ACDCS75; Predicted selectivity of 15% CO2/ 70% N2 on ACDCS75 and N-ACDCS75 as a function of pressure at 298 and 318 K; A temperaturedependent Langmuir model fitting and fitting parameters; CO2, CH4 and N2 adsorption equilibrium data on ACDCS75 and N-ACDCS75. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments This research was funded by the Australian Research Council (FT120100720 and DE140100569). S. Gao received financial support from the China Scholarship Council. We thank Jinxuan Zhang and Linzhou Zhuang for their assistance with SEM and TEM, respectively. The authors acknowledge the facilities and technical assistance of the Australian Microscopy & Microanalysis Research facility at the Centre for Microscopy & Microanalysis at the University of Queensland.


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(34) Arami-Niya, A.; Rufford, T. E.; Zhu, Z., Nitrogen-doped Carbon Foams Synthesized from Banana Peel and Zinc Complex Template for Adsorption of CO2, CH4, and N2. Energy & Fuels 2016, 30, 7298. (35) Romanos, J.; Beckner, M.; Rash, T.; Firlej, L.; Kuchta, B.; Yu, P.; Suppes, G.; Wexler, C.; Pfeifer, P., Nanospace engineering of KOH activated carbon. Nanotechnology 2012, 23. (36) Yang, J.; Shen, Z. M.; Hao, Z. B., Preparation of highly microporous and mesoporous carbon from the mesophase pitch and its carbon foams with KOH. Carbon 2004, 42. (37) Teng, H. S.; Weng, T. C., Transformation of mesophase pitch into different carbons by heat treatment and KOH etching. Microporous and Mesoporous Materials 2001, 50. (38) Li, S. Z.; Song, Y. Z.; Song, Y.; Shi, J. L.; Liu, L.; Wei, X. H.; Guo, Q. G., Carbon Foams With High Compressive Strength Derived from Mixtures of Mesocarbon Microbeads and Mesophase Pitch. Carbon 2007, 45, 2092. (39) Chen, M.-L.; Bae, J.-S.; Oh, W.-C., Characterization of AC/TiO2 Composite Prepared with Pitch Binder and Their Photocatalytic Activity. Bulletin of the Korean Chemical Society 2006, 27, 1423. (40) Liu, H. G.; Li, T. H.; Huang, T. T.; Zhao, X., Effect of multi-walled carbon nanotube additive on the microstructure and properties of pitch-derived carbon foams. J Mater Sci 2015, 50. (41) Narasimman, R.; Prabhakaran, K., Preparation of carbon foams by thermo-foaming of activated carbon powder dispersions in an aqueous sucrose resin. Carbon 2012, 50. (42) Wu, X. W.; Liu, Y. G.; Fang, M. H.; Mei, L. F.; Luo, B. C., Preparation and Characterization of Carbon Foams Derived from Aluminosilicate and Phenolic Resin. Carbon 2011, 49, 1782. (43) Wang, J.; Zhang, P.; Liu, L.; Zhang, Y.; Yang, J.; Zeng, Z.; Deng, S., Controllable Synthesis of Bifunctional Porous Carbon for Efficient Gas-Mixture Separation and HighPerformance Supercapacitor. Chemical Engineering Journal 2018, 348, 57. (44) Zhang, P.; Zhong, Y.; Ding, J.; Wang, J.; Xu, M.; Deng, Q.; Zeng, Z.; Deng, S., A New Choice of Polymer Precursor for Solvent-Free Method: Preparation of N-Enriched Porous Carbons for Highly Selective CO2 Capture. Chemical Engineering Journal 2019, 355, 963. (45) Ramos Fernández, J. M.; Martinez-Escandell, M.; Rodríguez Reinoso, F., Production of Binderless Activated Carbon Monoliths by KOH Activation of Carbon Mesophase Materials. 2008. (46) Maciá-Agulló, J. A.; Moore, B. C.; Cazorla-Amorós, D.; Linares-Solano, A., Activation of Coal Tar Pitch Carbon Fibres: Physical Activation vs. Chemical Activation. Carbon 2004, 42, 1367. (47) Sanchez-Sanchez, A.; Suarez-Garcia, F.; Martinez-Alonso, A.; Tascon, J. M., Influence of Porous Texture and Surface Chemistry on The CO2 Adsorption Capacity of Porous Carbons: Acidic and Basic Site Interactions. ACS Applied Materials & Interfaces 2014, 6, 21237. (48) Nandi, M.; Okada, K.; Dutta, A.; Bhaumik, A.; Maruyama, J.; Derks, D.; Uyama, H., Unprecedented CO2 Uptake Over Highly Porous N-Doped Activated Carbon Monoliths Prepared by Physical Activation. ChemComm 2012, 48, 10283. (49) Awadallah-F, A.; Al-Muhtaseb, S. A.; Jeong, H.-K., Selective Adsorption of Carbon Dioxide, Methane and Nitrogen Using Resorcinol-Formaldehyde-Xerogel Activated Carbon. Adsorption 2017, 23, 933. (50) Lu, Z.; Godfrey, H. G.; da Silva, I.; Cheng, Y.; Savage, M.; Tuna, F.; McInnes, E. J.; Teat, S. J.; Gagnon, K. J.; Frogley, M. D.; Manuel, P.; Rudic, S.; Ramirez-Cuesta, A. J.; Easun, T. L.;


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Table Table 1. Advantages and possible drawbacks of monolithic adsorbents. Monolithic adsorbents

Advantages

Drawbacks

Ref

Hierarchically organized porous structure

58 59

Rapid mass transfer and low pressure drop

21 60 61

Rapid heat transfer

21

High recyclability and easy regeneration

43 12 53

Adjustable thermal conductivity

19

Low active adsorbent loading

62

Possible high cost

17

High tortuosity and porosity (may decrease separation performance)

17


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Page 24 of 37

Table 2. Bulk density, skeletal density, porosity and carbon yield of the activated carbon discs prepared from carbon spheres and mesophase pitch. Carbon Carbon spheres: mesophase yield pitch: KOH mass ratio (%)

Skeletal density

Bulk density

Porosity

(g·cm-3)

(g·cm-3)

(%)

ACDCS10

1:9:5

90.2

2.10

0.74

64.8

ACDCS25

1:3:2

83.9

1.97

0.80

59.1

ACDCS50

1:1:1

82.2

2.01

0.77

62.0

ACDCS75

3:1:2

78.9

2.11

0.62

70.8

ACDCS90

9:1:5

74.8

1.72

-

-

Specimen


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Table 3. Surface area and pore volume of the obtained activated carbon discs prepared from carbon spheres and mesophase pitch. Total pore volume was evaluated at p/p0 ~ 0.99, micropore volume was evaluated by the t-plot method applied to N2 adsorption isotherms, and mesopore volume was evaluated by the BJH method applied to N2 adsorption isotherms at 77 K. DA Micropore surface area and narrow micropore volume determined from CO2 adsorption at 273 K. The BET specific surface area reported in this table is the mean of three N2 isotherm measurements on separate activated carbon discs, or carbon spheres, prepared in repeat experiments. The uncertainty in SBET shown is the standard deviation of the three measurements. Carbon spheres: mesophase pitch ratio

SBET

Vtotal

Vmicro

Vmeso

SD-A

Vmicro

(m2·g-1)

(cm3·g-1)

(cm3·g-1)

(cm3·g-1)

(m2·g-1)

(cm3·g-1)

CS

-

786±2.1

0.388

0.269

0.048

602

0.227

ACDCS10

1:9

848±125

0.296

0.230

0.036

720

0.293

ACDCS25

1:3

429±13

0.188

0.148

0.019

548

0.225

ACDCS50

1:1

500±27

0.218

0.169

0.020

716

0.324

ACDCS75

3:1

1338±136

0.549

0.487

0.022

922

0.357

ACDCS90

9:1

1025±83

0.473

0.349

0.061

740

0.294

N-ACDCS75

3:1

1142±4.1

0.472

0.410

0.020

1346

0.589

Specimen

N2 adsorption

CO2 adsorption


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Page 26 of 37

Table 4. Elemental surface analysis of the activated carbon discs prepared from carbon spheres and mesophase pitch before and after the ammonia heat treatment. Specimen ACDCS75

N-ACDCS75

Elemental content (at.%) C

O

N

92.94

6.50

0.56

C

O

N

93.81

1.74

4.45 Pyridinic

Amino

Pyrrolic

Graphitic

Oxidized N

2.20

0.66

0.99

0.50

0.10

Table 5. Fitting parameters of the Toth model for CO2, CH4 and N2 on ACDCS75 and NACDCS75.

Adsorbent

ACDCS75

N-ACDCS75

Qmax,i

B0,i

-ΔHToth,i

(mmol·g-1)

(kPa)

(J·mmol-1)

CO2

6.64

2.00 ·10-8

34.27

0.68

0.09

CH4

5.22

4.35 ·10-7

24.04

0.65

0.05

N2

4.67

2.60 ·10-7

21.56

0.69

0.03

CO2

8.23

2.00 ·10-8

32.85

0.64

0.10

CH4

6.24

4.35 ·10-7

22.75

0.63

0.07

N2

7.11

2.60 ·10-7

20.42

0.53

0.07

Adsorbate

ti

SD (mmol·g-1)


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Table 6. CO2 uptake and CO2/N2 selectivity of ACDs comparing with that of carbon spheres and carbon monolith in literature. CO2 uptake

CO2/N2 Selectivity

(mmol∙g-1, 273K)

(298 K)

-

5.1

6.0

Monolith

4.45

6.25

7.2

CS 4

Powder

3.55

5.5-6.2

-

CS11

Powder

-

5.1-8.05

-

CS10

Powder

-

6.9-8.9

-

KOH0.5TP50 33

Monolith

-

3.2

5.94

Carbon foam 55

Foam

2.87-3.37

7-8

Adsorbent

Type

N%

ACDCS75

Monolith

N-ACDCS75


27


Page 28 of 37

Figures

Figure 1. (a) TEM images of polymer spheres, (b) TEM images of carbon spheres, (c) SEM images of carbon spheres, and (d) DLS plot of the carbon spheres (Inlet: photograph of carbon spheres dispersed in ethanol.)


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Figure 2. (a) Weight (TGA) and (b) differential thermogravimetry (DTG) curves measured for specimens before carbonization. Heating rate 10 K·min-1, N2 flow rate 20 mL·min-1.

Figure 3. (a-f) SEM images of ACDs prepared from 10%, 50% and 75% of carbon spheres in precursor


29


Page 30 of 37

Figure 4. (a) Cumulative intrusion and (b) incremental intrusion as a function of pore size radius of ACDs prepared by 10%, 25%, 50% and 75% of carbon spheres in precursor with KOH activation determined by mercury porosimetry.

Figure 5. Photograph of a typical activated carbon disc (ACDCS75) prepared from carbon sphere and mesophase pitch.


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Figure 6. Stress-strain curves obtained from universal compressive strength (UCS) measurements of three KOH activated carbon discs prepared with 75% of carbon spheres in precursor.

Figure 7. Adsorption of (a) N2 77 K and (b) CO2 at 273 K on ACDs prepared by 10%, 25%, 50% and 75% of carbon spheres in precursor with KOH activation. Adsorption of N2 and CO2 on ammonia treated ACD with 75% of CS are also included.


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Figure 8. Micropore size distributions of ACDs prepared by 10%, 25%, 50% and 75% of carbon spheres in precursor with KOH activation calculated from equilibrium isotherms of CO2 sorption at 273 K. The PSD determined from the N2 sorption isotherms measured at 77 K are included in the Supporting Information.

Figure 9. (a) Wide XPS survey and (b) N1s XPS spectrum of activated carbon disc prepared from carbon spheres and mesophase pitch with ammonia treatment (N-ACDCS75).


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Figure 10. Measured and predicted absolute adsorption capacities of (a) CO2, (b) CH4 and (c) N2 on ACDCS75 at temperatures of 298 K, 308 K, and 318 K. The solid lines represent predicted adsorption capacities from the Toth model. Parts e) CO2, (d) CH4, and (f) N2 show deviations between the measured and the calculated adsorption capacities.


33


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Figure 11. Isosteric heat of adsorption of CO2, CH4, and N2 on ACDCS75 calculated via the Clausius-Clapeyron equation.


34

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Figure 12. Measured and predicted absolute adsorption capacities of (a) CO2, (b) CH4 and (c) N2 on N-ACDCS75 at temperatures of 298 K, 308 K, and 318 K. The solid lines represent predicted adsorption capacities from the Toth model. Parts e) CO2, (d) CH4, and (f) N2 show deviations between the measured and the calculated adsorption capacities.


35


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Figure 13. Predicted selectivity of CO2/N2, CH4/N2 and CO2/CH4 on N-ACDCS75 as a function of pressure at 298 and 318 K by ideal selectivity.

Figure 14. Predicted selectivity of CO2/N2, CH4/N2 and CO2/CH4 on ACDCS75 as a function of pressure at 298 and 318 K by ideal selectivity.


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37

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