Potassium salt-assisted synthesis of highly microporous carbon spheres for CO2 adsorption

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8 2 ( 2 0 1 5 ) 2 9 7 –3 0 3

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Potassium salt-assisted synthesis of highly microporous carbon spheres for CO2 adsorption Jowita Ludwinowicz, Mietek Jaroniec

*

Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA A R T I C L E I N F O

A B S T R A C T

Article history:

Highly microporous carbon spheres for CO2 adsorption were prepared by using a slightly

Received 19 September 2014

modified one-pot Sto¨ber synthesis in the presence of potassium oxalate. Formaldehyde

Accepted 23 October 2014

and resorcinol were used as carbon precursors, ammonia as a catalyst, and potassium oxa-

Available online 29 October 2014

late as an activating agent. The resulting potassium salt-containing phenolic resin spheres were simultaneously carbonized and activated at 800 C in flowing nitrogen. Carbonization of the aforementioned polymeric spheres was accompanied by their activation, which resulted in almost five-time higher specific surface area and total pore volume, and almost four-time higher micropore volume as compared to analogous properties of the carbon sample prepared without the salt. The proposed synthesis resulted in microporous carbon spheres having the surface area of 2130 m2 g 1, total pore volume of 1.10 cm3 g 1, and the micropore volume of 0.78 cm3 g 1, and led to the substantial enlargement of microporosity in these spheres, especially in relation to fine micropores (pores below 1 nm), which enhance CO2 adsorption. These carbon spheres showed three-time higher volume of fine micropores, which resulted in the CO2 adsorption of 6.6 mmol g

1

at 0 C and 1 atm.

 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

In the light of the report published by the Intergovernmental Panel on Climate Change (IPCC) [1], carbon dioxide (CO2) mitigation efforts have become of great importance to the scientific community. IPCC states that CO2 is the main gas contributing to greenhouse effect. CO2 is released to the atmosphere from various anthropological sources such as industrial processes, including cement production, and chemical, petrochemical, and refining processes [2,3]; however, the combustion of fossil fuels represents over 70% of the global CO2 emissions. Thus, the long-term goal is to reduce greenhouse gases emissions, especially CO2, and expand the low-carbon power generation. IPCC proposes to implement the carbon capture and storage program, which is intended to reduce the carbon concentration and then to stabilize, and recycle it to chemicals such as

* Corresponding author. E-mail address: [email protected] (M. Jaroniec). http://dx.doi.org/10.1016/j.carbon.2014.10.074 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

carbonates, urea, and fuels (e.g. methanol) [4,5]. The CO2 capture must be efficient but most of all economically rational. The efforts are made to find new technologies and improvements that could potentially aid economical implementation of CO2 capture. So far, absorption in amine solutions [6] is used in industry but adsorption on solids is proposed to replace it in the future as it is more cost effective and environmentally friendly. Materials investigated toward CO2 capture include metal–organic frameworks [7], porous polymers [8], zeolites [9], modified silicas [10] and organosilicas [11], and carbons [12,13]. Especially carbons possess high specific surface area, large volume of pores with the proper size, favorable surface chemistry, and exhibit high chemical (water, alkaline, and acidic media) and thermal stability. These sorbents can be produced from widely available inexpensive precursors such as coal, petroleum pitch, wood, and various biomass sources.

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Recently, highly microporous carbons were prepared by carbonization of organic salts. Such carbons [14,15] have been proposed as good CO2 sorbents because of their well-developed porous structure. Atkinson and Rood [16] carbonized lithium, sodium, and potassium dichloroacteates to obtain highly microporous carbons with specific surface area as high as 740 m2 g 1. Sevilla and Fuertes [17] showed that pyrolysis of sodium, potassium, and calcium gluconates, citrates, and alginates can produce carbons with surface area up to 1960 m2 g 1 and micropore volume up to 0.85 cm3 g 1. Also, Adeniran et al. [18] carbonized potassium hydrogen phthalate to produce highly microporous carbons. The resulting material adsorbed 4.8 mmol g 1 of CO2 at 1 bar and 25 C. Post-synthesis activation (chemical or physical) has been widely used to develop extra surface area and porosity and improve sorption properties of carbons. Our previous studies showed that this process can be used to enhance the microporosity in phenolic resin-based carbons; Gorka and Jaroniec [19] used CO2 and water vapor post-synthesis activation of these carbons, whereas Souza et al. [20] utilized potassium hydroxide (KOH) as an activating agent. Wickramaratne and Jaroniec [21] reported that CO2 activation of carbon spheres obtained by extended Sto¨ber method can produce materials with CO2 capacity as high as 8.05 mmol g 1 at 0 C and 1 atm. Therefore, activated phenolic resin-based carbon spheres are attractive for adsorption of CO2 because they possess intrinsic microporosity with high fraction of small micropores (<1 nm) that according to recent studies [22–25] enhance CO2 uptake at ambient conditions. Especially potassium compounds such as KOH [26], potassium carbonate (K2CO3) [27], and potassium oxalate (K2C2O4) [28], have been used for chemical activation of carbons; however, this process involves post-synthesis impregnation of carbons, which can be time consuming and somewhat difficult due to the hydrophobic nature of these materials. Physical mixing can be used instead of impregnation, but it often leads to an uneven distribution of the activating agent, resulting in poor activation. Overall, the post-synthesis activation is an additional step in the preparation of carbons. This work shows a simple strategy to enhance microporosity in the carbon spheres prepared by Sto¨ber method, which involves the addition of potassium oxalate to the ethanol– water–ammonia solution of resorcinol and formaldehyde. This one-pot modified Sto¨ber synthesis produces the saltcontaining polymer spheres, the carbonization of which in the presence of potassium species is accompanied by their activation, resulting in the development of additional microporosity. This is the first report showing the effect of direct addition of potassium organic salt during formation of phenolic resin spheres, which plays the role of an in-situ activating agent and additional carbon precursor. The proposed method affords carbons with spherical morphology, high surface area and high volume of micropores.

2.

Experimental

2.1.

Chemicals

Resorcinol (C6H4(OH)2, 98%), formaldehyde (HCHO, 37 wt% solution in water, stabilized with 10–15% methanol) and

potassium oxalate monohydrate (K2C2O4ÆH2O, 99%) were purchased from Acros Organics (Geel, New Jersey, USA). Ammonium hydroxide (ACS grade, 29 wt%) was purchased from Fischer Scientific (Fair Lawn, New Jersey, USA). Technical grade ethanol and deionized water were used in all experiments.

2.2.

Synthesis of carbon spheres

Carbon spheres were prepared by a suitably modified recipe reported by Liu et al. [29]; namely, 0.20 g of resorcinol was added to the mixture consisting of 20 mL of water, 8 mL of ethanol, and 0.10 mL of ammonium hydroxide under magnetic stirring for 10 min at room temperature. Next, 0.75 g, 1.20 g, or 1.65 g of K2C2O4ÆH2O were added to the synthesis mixture under stirring for 30 min to achieve the potassiumcarbon weight ratio equal to 3:1, 5:1, and 7:1, respectively. Afterwards, 0.28 mL of formaldehyde was added and the mixture was stirred for 24 h and then subjected to the hydrothermal treatment in an autoclave at 100 C for 24 h. Subsequently, the solution was transferred to a Petri dish and dried at room conditions overnight. The dried materials were carbonized in nitrogen atmosphere at 350 C for 2 h (1 C min 1 heating rate); then, temperature was ramped to 800 C (1 C min 1 heating rate) and kept at that temperature for 2 h. The carbonized materials were washed with 0.01 M HCl solution and deionized water until pH 7 to remove salt residue. Finally, the materials were dried at 100 C for 12 h. The resulting carbon materials were labeled C-K-r, where r denotes the potassium-carbon weight ratio. For the purpose of comparison, one carbon sample was prepared without salt addition and labeled as C.

2.3.

Measurements

Nitrogen adsorption–desorption isotherms were measured at 196 C, carbon dioxide adsorption isotherms were measured at 0, 25, 50 and 120 C on an ASAP 2020 surface area and porosity analyzer manufactured by Micromeritics (Norcross, GA, USA). All samples were degassed using vacuum at 200 C for 2 h prior to each measurement. Scanning electron microscopy (SEM) images were taken on a Hitachi S-2600N scanning electron microscope using 25 or 30 kV accelerating voltage.

2.4.

Calculations

Specific surface area (SBET) was evaluated by Brunnauer– Emmett–Teller (BET) equation [30] in a relative pressure range of 0.05–0.2 using a nitrogen cross-section area of 0.162 nm2 [31]. Total pore volume (Vt) was evaluated by converting the amount adsorbed at a relative pressure of 0.99 to the volume of liquid nitrogen at experiment conditions [31]. Pore size distributions (PSD) were calculated from adsorption branches of nitrogen adsorption–desorption isotherms using the 2DNLDFT Heterogeneous Surface model for carbon materials implemented in SAIEUS program provided by Micromeritics [32,33]. The volume of micropores, pores below 1 nm, and ultramicropores (Vmi, Vmi<1nm and Vumi respectively), were calculated on the basis of the PSD curves obtained by the

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DFT method. The isosteric heat of adsorption qst was calculated by using the Clausius–Clapeyron equation: qst = RT2(oln p/oT)a where p is the equilibrium pressure, a is the amount adsorbed, T denotes absolute temperature, and R is the universal gas constant [34].

3.

Results and discussion

3.1.

Morphology

Morphology of the obtained carbons was studied using scanning electron microscopy. Fig. 1 presents SEM images of all carbon materials, which possess spherical morphology with sizes between 0.5 and 1 lm; however, the carbon spheres prepared without salt addition were more uniform and ca. 500 nm in size, whereas the activated spheres had somewhat disturbed shapes and larger diameters. These irregularities and non-uniformity may have resulted from the salt addition, which affected precipitation, polymerization, and/or condensation rates through pH changes (as discussed in details in reference [35]). Most importantly, the spherical morphology, which is important for some applications, was preserved despite the salt addition and the activation process accompanying carbonization of polymeric spheres. Note that postsynthesis KOH activation of polymeric spheres destroyed the original morphology [36,37]. The recipe proposed in this work allows for simultaneous chemical activation with retention of spherical morphology thanks to uniform distribution of potassium species throughout phenolic resin spheres, which was facilitated by organic nature of oxalate moiety. In addition, neither pre- nor post-activation was required in this strategy because the activating agent (potassium salt) was incorporated directly into polymeric matrix allowing

Fig. 1 – SEM images of the carbon spheres prepared without salt addition (A), with 3:1 (B), 5:1 (C) and 7:1 (D) potassium– carbon weight ratios. (A color version of this figure can be viewed online.)

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simultaneous carbonization and activation during thermal treatment. Yet, this strategy afforded materials with extremely well-developed porous structure (see the next section).

3.2.

Nitrogen adsorption studies

Nitrogen adsorption data were used to evaluate the specific surface area and porosity of the carbons studied. Fig. 2 shows low-temperature nitrogen adsorption–desorption isotherms and the corresponding pore size distributions for all carbons. All adsorption isotherms are of Type I according to the IUPAC classification [38], which is characteristic for microporous materials. For the sample C-K-7, the isotherm does not flatten at a relative pressure ca. 0.1, indicating the carbon has large micropores and/or small mesopores as well [31]. Table 1 lists the calculated structural parameters for all carbons. The specific surface area ranges from 460 to 2130 m2 g 1, pore volume ranges from 0.24 to 1.10 cm3 g 1, and micropore volume from 0.20 to 0.78 cm3 g 1. Importantly, all these parameters increase with potassium oxalate amount, showing the effectiveness of structure development in carbon materials obtained in the presence of potassium organic salts. In the case of C-K-7 activation resulted in almost five-fold increase of SBET and Vt and almost four-fold increase of Vmi. The goal of this study was the development of materials featuring high CO2 uptake at ambient pressures. In this case the presence of microporosity, especially small micropores with sizes below 0.7 nm, is important [21–25]. The calculated PSDs confirm that all carbons possessed well-developed microporosity. The volume of ultramicropores Vumi (pores < 0.7 nm) increases from 0.15 to 0.36 cm3 g 1 throughout the series except the C-K-7 sample. The maximum value of Vumi is observed for C-K-5, while the Vumi value for C-K-7 is much smaller and analogous to that of C-K-3. However, the volume of micropores below 1 nm (fine micropores) increases from 0.16 to 0.46 cm3 g 1 throughout the entire series. For the C-K-7 sample, however, the fraction of fine pores in the total porosity is only 42%, whereas it is almost twice higher (75%) for C-K-5. In addition, the C-K-7 sample possesses some additional mesoporosity, as well. The presence of mesoporosity can be attributed to a high amount of the activating agent that caused the excessive pore widening during activation. As a result, the ultramicropores grew into supermicropores (pores from 0.7 to 2 nm) and supermicropores grew into small mesopores [39–42]. Overall, all carbons were highly microporous, which projected well on their CO2 adsorption properties. All data show that potassium organic salt is an excellent activating agent. Moreover, the degree of activation can be easily controlled by adjusting amount of the salt added. Interestingly, the volume of ultramicropores reached a maximum at the weight ratio = 5:1 under the conditions used. Further salt addition caused a gradual improvement of the structural parameters (SBET, Vt, and Vmi) but, unfortunately, did not improve volume of ultramicropores. Thus, for CO2 adsorption and other applications requiring large volumes of ultramicropores the 5:1 potassium-carbon ratio is optimal; however, higher ratios can be used to achieve the larger values of SBET, Vt, and Vmi, which are suitable for other applications such as adsorption of larger molecules, e.g., benzene vapor.

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Fig. 2 – Nitrogen adsorption–desorption isotherms measured at 196 C (left) and the differential pore size distributions calculated using DFT method (right) for all carbons studied. (A color version of this figure can be viewed online.)

Table 1 – Structural parameters and CO2 uptakes for all carbon materials.a Sample

SBET (m2 g 1)

Vt (cm3 g 1)

Vmi (cm3 g 1)

Vmi<1nm (cm3 g 1)

Vumi (cm3 g 1)

Microporosity (%)

CO2 uptake (mmol g 1)

C C-K-3 C-K-5 C-K-7

460 763 1160 2130

0.24 0.39 0.60 1.10

0.20 0.33 0.52 0.78

0.16 0.27 0.45 0.46

0.15 0.23 0.36 0.25

83 85 87 71

2.8 4.8 6.3 6.6

a Notation: SBET – specific surface area; Vt – single–point (total) pore volume at p/p0 = 0.99; Vmi – volume of pores below 2 nm obtained by DFT method; Vumi – volume of pores below 0.7 nm obtained by DFT method; Vmi<1nm – volume of pores below 1 nm obtained by DFT method; microporosity – percentage of the volume of micropores in the total pore volume, CO2 uptake at 0 C and 1 atm.

3.3.

CO2 adsorption studies

The synthesized samples were investigated in terms of their performance for CO2 adsorption. Fig. 3 shows CO2 adsorption isotherms and Table 1 lists CO2 uptakes measured at 0 C for all carbon materials. The CO2 uptakes range from 2.8 to 6.6 mmol g 1 and increase throughout the series. Other activated carbons show comparable properties [24,43–45] but these carbons require post-synthesis activation. The pre–activated carbons usually achieve up to 2 mmol g 1 uptakes of CO2 [20,44]. Importantly, in the proposed recipe neither prenor post-activation of the carbons is required, and the resulting samples show CO2 uptakes comparable to the carbons obtained by KOH post-synthesis activation [20,26]. These results may be attributed to the high volume of fine micropores created by simultaneous carbonization-activation of polymeric spheres with uniformly distributed potassium species. Previous reports show the strong correlation between ultramicropore volume and CO2 uptake [22–25]. The same is observed in the current study. Fig. 3 shows CO2 uptake at 1 atm as a function of volume of fine micropores. The straight line represents the linear regression with correlation coefficient R2 = 0.967. As a result, the carbons prepared with salt addition adsorbed more CO2 because of increase in Vmi<1nm, which resulted in over two-fold increase in the CO2 uptake between C-K-7 and C carbon.

C-K-7 sample shows significantly higher SBET, Vt and Vmi which resulted in the best CO2 uptake at 1 atm; however, it is C-K-5 sample that showed the best low-pressure performance because of its highest volume of ultramicropores. Thus, further CO2 sorption studies under flue-gas-like conditions were conducted using C-K-5 sample. CO2 partial pressure of 0.13–0.15 atm [46] was assumed as the representative value for CO2 post-combustion capture from flue gas. Fig. 4 shows CO2 adsorption isotherms at temperatures: 0, 25, 50 and 120 C, and Table 2 shows CO2 uptakes at these temperatures and equilibrium pressures of 0.15 atm and 1 atm. As expected for physical adsorption these uptakes decrease with increasing temperature from 2.4 to 0.2 mmol g 1 at 0.15 atm and from 6.3 to 1.1 mmol g 1 at 1 atm (see Fig. 4). Both sets of uptake data decrease exponentially with temperature (correlation coefficient R2 = 0.998; see right panel in Fig. 4), but this decrease is more pronounced at low pressures than in the case of higher pressures. The C-K-5 sample possesses impressive properties. It adsorbed maximum of 1.5 mmol g 1 at 0.15 atm and 25 C, which is twice more than the best value at low pressure reported in a highly cited review by Samanta et al., which is devoted to post-combustion CO2 capture by solid sorbents [47]. The C-K-5 sample showed cycle capacity of 0.4 mmol g 1 (17.4 mg g 1) if cycled through adsorption at 25 C and 0.15 atm and desorption at 120 C and 1 atm. Clearly, valuable

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Fig. 3 – CO2 adsorption isotherms measured for all carbon materials at 0 C (left) and CO2 uptake at 0 C as a function of the volume of pores below 1 nm (right). (A color version of this figure can be viewed online.)

Fig. 4 – CO2 adsorption isotherms for C-K-5 sample at 0, 25, 50 and 120 C (left); the dashed line indicates partial pressure in flue gas. CO2 uptakes at 0.15 atm and 1 atm for C-K-5 material as functions of temperature (right); ordinate axis is in a logarithmic scale. (A color version of this figure can be viewed online.)

Table 2 – CO2 uptakes at 0, 25, 50 and 120 C for C-K-5 carbon. Temperature (C) CO2 uptake at 0.15 atm (mmol g 1)

CO2 uptake at 1 atm (mmol g 1)

0 25 50 120

6.3 4.7 3.1 1.1

2.4 1.5 0.8 0.2

adsorption properties have to be complemented by good desorption properties (i.e. small uptake at 120 C) to yield a good sorbent. In our case, the C-K-5 material achieved impressive cycle capacity on the order of equilibrium adsorption reported for many activated carbons [47]. Further

reduction of CO2 uptake at 120 C and 1 atm could yield even better result. We suspect this could be achieved by reduction of volume of micropores >1 nm, which as shown in the case of C-K-7 related to higher CO2 uptake at high pressure. Fig. 5 shows the isosteric heat of CO2 adsorption for C-K-5 carbon calculated using adsorption isotherms measured at 0, 25, 50, and 120 C. The calculated values were in the range of 25.7–22.0 kJ mol 1 with the CO2 amount adsorbed in the range of 0.2–3.1 mmol g 1. Similar values for activated carbons were reported by other authors [20,48,49]. The isosteric heat of CO2 adsorption was calculated using four isotherms (left part of Fig. 5) and three isotherms (right part). Clearly, the significantly higher error was observed in the case of lower number of measurements. For the range of CO2 uptakes above 3.1 mmol g 1, only two isotherms were available and error significantly impacted the results.

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is crucial for applications in gas capture and/or storage, the reported strategy might be useful for production of activated carbons suitable for those purposes.

Acknowledgments The SEM data were obtained at the lab of the Characterization Facility of the Liquid Crystal Institute, Kent State University.

R E F E R E N C E S

Fig. 5 – Isosteric heat of CO2 adsorption for C-K-5 sample calculated from CO2 adsorption isotherms measured at 0, 25, 50, and 120 C. (A color version of this figure can be viewed online.)

Overall C-K-5 material showed an excellent equilibrium and practically useful CO2 sorption performance without need for pre- or post-activation, which is often used to enlarge porosity of various carbons [50].

4.

Conclusions

A simple one-pot modified Sto¨ber synthesis in the presence of potassium oxalate was proposed to create extra microporosity in carbon spheres. Highly microporous carbon spheres were obtained via thermal treatment of the K-containing phenolic resin spheres. The specific surface area of the resulting carbon spheres ranges from 460 to 2130 m2 g 1, pore volume varies from 0.24 to 1.10 cm3 g 1, and micropore volume ranges from 0.20 to 0.78 cm3 g 1. The adjustment of the amount of salt added was shown to be an effective strategy for tuning the volume of ultramicropores and fine micropores (<1 nm) and consequently, for improving the CO2 uptake at low pressures. Simply by controlling the amount of salt added one is able to alter porosity, both in the range of micropores and small mesopores. For CO2 adsorption and other applications requiring large volumes of ultramicropores the 5:1 potassium-carbon weight ratio is optimal; however, higher ratios can be used to achieve desired values of SBET, Vt, and Vmi for all other applications. Importantly, introduction of potassium oxalate to the synthesis did not affect the spherical morphology of carbons, which is desired in some industrial applications. The obtained materials showed an excellent CO2 uptakes ranging from 2.8 to 6.6 mmol g 1 at 0 C and 1 atm. The optimized material showed high lowpressure CO2 uptakes of 0.2–2.4 mmol g 1 in temperature range from 0 to 120 C, respectively. The proposed recipe for the preparation of polymericbased carbons is versatile and opens new possibilities for in-situ activation and pore tuning. Because the latter feature

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