Ordered nanoporous carbon for increasing CO2 capture

Ordered nanoporous carbon for increasing CO2 capture

Journal of Solid State Chemistry 197 (2013) 361–365 Contents lists available at SciVerse ScienceDirect Journal of Solid State Chemistry journal home...

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Journal of Solid State Chemistry 197 (2013) 361–365

Contents lists available at SciVerse ScienceDirect

Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc

Ordered nanoporous carbon for increasing CO2 capture Hye-Min Yoo a,b, Seul-Yi Lee a,b, Soo-Jin Park a,b,n a b

Korea CCS R&D Center, Korea Institute of Energy Research, 152 Gajeongro, Yuseoung-gu, Daejeon 305-343, South Korea Department of Chemistry, Inha University, 100 Inharo, Nam-gu, Incheon 402-751, South Korea

a r t i c l e i n f o

abstract

Article history: Received 12 June 2012 Received in revised form 5 August 2012 Accepted 13 August 2012 Available online 7 September 2012

Ordered nanoporous carbons (ONCs) were prepared using a soft-templating method. The prepared ONCs materials were subjected to a controlled carbonization temperature over the temperature range, 700–1000 1C, to increase the specific surface area and total pore volume of ordered nanoporous carbon followed by carbonization of the phenolic resin. ONCs materials synthesized at various carbonization temperatures were used as adsorbents to improve the CO2 adsorption efficiency. The surface properties of the ONCs materials were examined by X-ray photoelectron spectroscopy. The structural properties of the ONCs materials were analyzed by X-ray diffraction. The textural properties of the ONCs materials were examined using the N2/77 K adsorption isotherms according to the Brunauer–Emmett–Teller equation. The CO2 adsorption capacity was measured by CO2 isothermal adsorption at 298 K/30 bar and 298 K/1 bar. The carbonization temperature was found to have a major effect on the CO2 adsorption capacity, resulting from the specific surface area and total pore volumes of the ONCs materials. Crown Copyright & 2012 Published by Elsevier Inc. All rights reserved.

Keywords: Carbon dioxide adsorption Carbonization temperature Ordered nanoporous carbons Soft-templating method

1. Introduction One of the technological challenges today concerns an examination of environmentally friendly, efficient and low cost industrial processes for the recovery of carbon dioxide (CO2). The removal of acidic gases is an important industrial operation. In particles, CO2 is produced in large quantities by many important industries, such as fossil fuel-fired power plants, steel production, chemical and petrochemical manufacturing, cement production and natural gas purification. On the other hand, CO2, as one of the greenhouse gases (GHG), is currently responsible for more than 60% of the enhanced greenhouse effect. Environmental concerns with global climate change are now considered as the most important and challenging environmental issues facing the world community and have motivated intensive research on CO2 adsorption and sequestration [1,2]. Over the past few decades, many types of porous materials have been used for CO2 adsorption, such as zeolites [3], porous silica (SBA-15 [4], MCM-41 [5], etc.), metal oxides [6], metalorganic frameworks (MOFs) [7], and carbon materials [8]. Zeolites suffer from a rapid decrease in adsorption capacity because of the instability caused by moisture. With mesoporous silica (SBA-15 and MCM-41), the pure silica surfaces do not interact very strongly with CO2 because the residual hydroxyl groups are unable to induce strong enough interactions and there are no n Corresponding author at: Korea Institute of Energy Research, Korea CCS R&D Center, 152 Gajeongro, Yuseoung-gu, Daejeon 305-343, South Korea. Fax: þ 82 32 860 8438. E-mail address: [email protected] (S.-J. Park).

specific adsorption sites. Metal oxide adsorbents utilize chemical reactions with CO2 at high temperatures and generally suffer from either low CO2 adsorption capacities or severe energy penalties due to the need for high desorption temperatures. Yaghi et al. [9] reported that MOFs have high adsorption capacity but porous silica and MOFs are not utilized in thermal power plants because of their instability in moisture [10]. Carbon materials have attracted great scientific and technological interest in CO2 adsorption, and have been used for many years owing to their highly developed porosity, extended surface area, hydrophobicity of their surface chemistry, thermal stability and low cost [11,12]. Recently, porous carbon materials with high specific surface areas are the most common adsorbents materials used in CO2 adsorption [13,14]. On the other hand, conventional porous carbon has drawbacks, such as an irregular pore structure and wide pore size distribution. Therefore, ordered nanoporous carbon (ONC) has been proposed as a novel adsorbent for CO2 adsorption [15,16]. The discovery of ONCs in 1999 has opened a new chapter in research into ordered nanoporous materials [17]. ONCs materials have attracted considerable attention because of their potential applications in gas separation, catalyst supports, adsorption, and energy storage/conversion, etc. [18–23]. ONCs materials with a high surface area, large pore volume, well-tailored pore size, chemical inertness and electrical conducting property, have attracted increasing interest. A hard template method has been applied to synthesize ONCs materials using nanoporous silica as a template. This process generally involves the pre-synthesis of ordered nanoporous silica, repeated impregnation with carbon precursors, drying, carbonization and subsequent removal of the hard

0022-4596/$ - see front matter Crown Copyright & 2012 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.035

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templates by NaOH or HF solutions [24–30]. On the other hand, this synthesis route, although very useful for producing a variety of structures, is rather costly and less feasible from an industrial perspective. Although the process can be simplified, the use of hard templates is still unavoidable. On the other hand, the direct synthesis of ONCs using a soft-templating method was developed by the authors and several other research groups over the last few years. The soft-templating synthesis of ONCs [31,32], which involves the polymerization of carbon precursors (e.g., formaldehyde, phenol derivatives including resorcinol and phloroglucinol, and so on.) in the hydrophilic domains of a block copolymer template appears to be a very promising way for the large-scale preparation of ONCs. Several critical parameters (such as phenol/formaldehyde ratios, polymerization times, amount of catalyst, etc.) in the production of porous carbon materials can affect its structure. In addition, the type of nanoporous carbon can affect its structure and a high synthesis temperature would result in a larger amount of volatiles being released from the raw material, which can affect the porosity [33]. In practice, ONCs materials were prepared using the soft-templating method for CO2 adsorption. A series of ONCs materials samples was synthesized to monitor the effects of structural changes caused by the carbonization temperature (700–1000 1C) on the CO2 adsorption capacity.

purchased from Sigma-Aldrich Chemical Corporation. Formaldehyde (HCHO; 37%) and HCl were obtained from DUKSAN PURE CHEMICALS Co.. Ethanol was purchased from Burdick & Jackson Corporation. 2.2. Materials A slight modification of the methodology reported by Liang and Dai [34] recipe was used to synthesize the materials studied. Briefly, 2 g of resorcinol and 1.25 g of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolyme (Pluronic F127) were dissolved in 17 g of an ethanol and water solution at a 10:1 weight ratio and stirred at room temperature. After complete dissolution of the copolymer, 1.25 ml of 37% formaldehyde was added to the synthesis mixture. About 0.125 ml of a 37% HCl solution was added as a catalyst and stirred for an additional 30 min until a white color appeared. The solution turned cloudy after 30 min and separated into two layers after an additional 2 day. The top layer was composed of unnecessary solution for ONC, evaporated in the oven at 120 1C. The polymer-rich bottom layer was taken and stirred overnight. The carbonization temperature was in the range, 700–1000 1C. Carbonization of the resulting carbon was p1erformed in a tube furnace under a nitrogen flow and heating rate of 1 1C/min. After reaching the final temperature, the sample was kept in flowing nitrogen for 2 h.

2. Experimental 2.3. Characterization 2.1. Chemicals Resorcinol (C6H6O2) and poly(ethylene oxide)-poly(propylene oxide) triblock copolymer (EO106PO70EO106; Pluronic F127) were

All the samples were characterized by X-ray diffraction (XRD, Rigaku D/MAX 2200 V/PC) using a CuKa radiation. The low angle XRD patterns were collected over the scanning range, 2y ¼0.2–51,

Fig. 1. Schematic diagram of the preparation of ONCs.

H.-M. Yoo et al. / Journal of Solid State Chemistry 197 (2013) 361–365

with a step width of 0.011 and a time per step of 0.5 s. X-ray photoelectron spectroscopy (XPS, VG Scientific Co., ESCA LAB MK II) was performed using a monochromatic MgKa X-ray source to examine the contents of elements on the surface of the carbon materials at 150 W. The porous texture of the samples was examined by N2/77 K adsorption/desorption isotherms using a gas adsorption analyzer (BERSORP, JAPAN, INC.). The specific surface area (SBET) was calculated using the Brunauer–Emmett–Teller (BET) equation at relative pressures between 0.03 and 0.22. The meso- and micro-pore size distribution were determined using the Barrett– Joyner–Halenda (BJH) method and Horvath–Kawazoe (HK) method, respectively. 2.4. Carbon dioxide adsorption properties analysis A CO2 adsorption test was carried out under ambient conditions of 298 K at both low (1 bar) and moderate pressures (30 bar, BEL, Japan). In each experiment, approximately 0.3 g of the sample was loaded into a stainless chamber. Before the measurements, the samples were degassed at 473 K for 5 h to obtain a residual pressure of o10–16 mmHg. After cooling the chamber to room temperature, CO2 was introduced until a pressure of 30 bar was reached. Ultrahigh purity grade (99.9999%) CO2 was used to exclude the effects of moisture and other impurities. Finally, a volumetric measurement method was used to determine the CO2 adsorption capacity.

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In the first part of this project, all samples were prepared by varying the synthesis temperature while keeping the copolymer to carbon precursor ratio the same. Figs. 2 and 3 show the nitrogen adsorption isotherms and corresponding meso- and micropore size distributions (PSDs) curves, respectively. The adsorption isotherms of all the samples showed type IV with H1 hysteresis loops observed for cylindrical nanopores [38]. This was identified as a slow rate of increase in N2 uptake at low relative pressures, corresponding to monolayer and multilayer adsorption on the pore walls. All the samples synthesized at various carbonization temperatures exhibited a much broader capillary condensation step as well as a stepwise desorption curve, which suggests the existence of pores with different openings. As shown in Table 1, all the samples possessed BET surface areas of 500– 800 m2/g. The total pore volume and mesopore volume showed a tendency to increase with increasing carbonization temperatures to the ONC900 sample with a decrease at higher temperatures (ONC1000 sample). The ONC900 sample exhibited large high total pore volume (0.764 cm3/g) with the main contribution coming from large mesopores (approximately 3.9 nm) and a high mesopore volume (0.566 cm3/g). An excessive carbonization temperature (i.e. ONC1000 sample) resulted in a gradual decrease in surface area and pore volume. Therefore, the latter temperature was excluded from further study. For better characterization of the change in the pore structure of the ONCs materials samples, the pore size distribution in the meso-/ micro-pore region of the ONCs samples obtained at different carbonization temperatures was examined. As shown

3. Results and discussion

2.5 ONC700 ONC800

2.0

ONC900 dV/dlogd0

Resorcinol–formaldehye resin is the most frequently used organic monomer for the synthesis of carbon gels and ONCs via an organic sol–gel process. The polymerization of resorcinol with formaldehyde can be catalyzed by acids or bases [35,36]. As an alternative approach, Zhang et al. [37] designed a new two step method for the rapid synthesis of ONCs materials. On the other hand, the present experiment was designed as a single step method except for the basic catalyst condition, as shown in Fig. 1. For this method, resorcinol and formaldehyde were polymerized in a single step in the presence of HCl to produce a resorcinol–formaldehyde resin. The addition of an acid catalyst prompted rapid self-assembly and condensation. Subsequently, the polymer gel was pyrolyzed at various temperatures to remove F127 and obtain the ONCs. ONCs materials can be synthesized under optical temperature conditions.

ONC1000

1.5

1.0

0.5

0.0 0

10

20

30

d0(nm) 600 ONC700

2.0 ONC700

ONC900

ONC800

1.6

ONC900

ONC1000

400

ONC1000

1.2 dV/dd0

N2 adsorbed (cm3/g, 77 K)

ONC800

0.8

200 0.4 0.0

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 2. N2 adsorption/desorption isotherms at 77 K porosity parameters of the samples.

0.5

1.0 d0(nm)

1.5

2.0

Fig. 3. Meso-(a) and micro-(b) pore size distribution of the nanoporous carbon at carbonization temperatures of 700–1000 1C.

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Specimens

SBET (m2/g)a

VTotal (cm3/g)b

VMicro (cm3/g)c

VMeso (cm3/g)d

Dp e(nm)

ONC700 ONC800 ONC900 ONC1000

686 720 775 533

0.631 0.673 0.764 0.491

0.186 0.191 0.198 0.155

0.445 0.482 0.566 0.335

3.7 3.7 3.9 3.7

Intensity

Table 1 N2/77 K textural properties of the samples.

ONC700 ONC800

a SBET: Specific surface area calculated using the Brunauer–Emmett–Teller equation over a relative pressure range of 0.03–0.22. b VTotal: Total pore volume estimated at a relative pressure P/P0 ¼0.990. c VMicro: Micropore volume determined from the subtraction of mesopore volume from total pore volume. d VMeso: Mesopore volume determined from the Barret–Joyner–Halenda (BJH) equation. e Dp: Average pore diameter.

1

2 2 theta (deg.)

3

4

Fig. 4. Low angle XRD patterns of the nanoporous carbon materials synthesized at 700–1000 1C.

Table 2 Chemical compositions of the ordered mesoporous carbon materials measured by XPS. Element

O1s/C1s

Specimens

ONC700 ONC800 ONC900 ONC1000

C1s

O1s

N1s

86.65 91.29 93.51 93.79

11.9 7.78 5.77 5.72

1.45 1.74 0.72 0.49

13.73 8.52 6.17 6.09

Unit: %.

ONC700 ONC800 ONC900 ONC1000

160 CO2 adsorbed (mg/g, 298 K)

in Fig. 3a, all the samples consisted mainly of mesopores o50 nm in diameter. All samples gave a very sharp peak at approximately 7 nm because of the formation of pores after the pyrolysis of resorcinol– formaldehyde resin. The pore structures were enhanced predominantly by carbonization with a resorcinol precursor, and the pore size distributions showed that slight development can be observed around the meso region. As the carbonization temperature increased, a well-developed pore size distribution (o 10 nm) was clearly observed, except for ONC1000 sample. According to the relevant literature, carbonization of the polymer and combined pyrolysis temperature lead to great changes in the pore size distribution [39]. This suggests that the porous structure of the prepared carbon is dependent on the change in the carbonization temperature. The marked increase in the number of narrow mesopores was attributed to the more appropriate formation of pores from the precursors. As shown in Fig. 3b, the micropore structures were enhanced predominantly by the carbonization of the resorcinol precursor at 900 1C, whereas the distributions show that prominent development can be observed around the micro-pore region (0.5–0.75 nm). This clearly confirms that the micropore development of the prepared ONCs materials occurs mainly in the regions where the pore diameter is o1 nm. Moreover, a higher carbonization temperature favored the formation of a narrow micropore distribution. The XRD patterns in Fig. 4 concur with the BET results. The low angle XRD patterns of ONC700 (Fig. 4) revealed only one broad peak, indicating a less ordered structure. The samples pyrolyzed at 4800 1C showed two peaks, indicating a more ordered structure than ONC700. Moreover, there was a slight shift in the peaks towards a high angle with increasing carbonization temperature, suggesting that a higher degree of cross-linking of the resorcinol–formaldehyde resin increases the level of framework shrinkage caused by the high-temperature pyrolysis. On the other hand, ONC1000 showed considerably broader peaks, indicating an irregular structure due to the excessive pyrolysis temperature [37,40]. XPS was carried out to determine the elemental composition of the ONCs surfaces synthesized at carbonization temperatures ranging from 700 1C to 1000 1C. The results are summarized in Table 2. The oxygen content decreased from 13.73% to 8.52, 6.17 and 6.09% at carbonization temperatures of 700 1C, 800 1C, 900 1C and 1000 1C, respectively. This means that pore formation had been achieved by thermal decomposition at 900 1C. The ONC1000 sample had the carbon content highest (93.79%), which means a slightly crushed pore structure by pyrolysis the resin at high temperatures [41,42]. CO2 adsorption–desorption isotherms of the samples were measured using static equipment (Fig. 5). The ONCs materials exhibited a CO2 uptake of 107 and 158 mg/g of adsorbent, respectively, at 298 K and 1 atm. The CO2 adsorption capacity decreased in the following order: ONC9004ONC8004ONC7004ONC1000, as shown in Fig. 5. This suggests that the ONCs materials synthesized by the pyrolysis of

ONC900 ONC1000

120

80

40

0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure (bar) Fig. 5. CO2 adsorption isotherms of nanoporous carbon measured at 1 bar and 298 K.

resorcinol-F127 resin increases the affinity of the porous carbon towards CO2 with increasing pyrolysis temperatures. This is because ONC900 has a higher volume of narrow nanopores (below 1.0 nm) than the other samples. In particular, ONC1000 showed the lowest CO2 adsorption capacity due to collapse of the pore structure by the excessive pyrolysis temperature. Overall, the ONCs materials are expected to be useful for CO2 adsorption because it is estimated that abundant mesopores can facilitate the diffusion of CO2 inside the pore channels to reach the micropores for effective CO2 trapping. The CO2 adsorption capacities of the samples increased with increasing pressure, as shown in Fig. 6. At 30 bar, ONC900 showed the maximum CO2 adsorption capacity of 686 mg/g at moderate temperatures. In addition, the result at 1 bar and 30 bar showed a consistent tendency in the order of the CO2 adsorption capacity.

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References

800 CO2 adsorbed (mg/g, 30bar)

365

ONC700 ONC800 ONC900 ONC1000

600

400

200

0 0

6

12 18 Pressure (bar)

24

30

Fig. 6. CO2 adsorption isotherms of nanoporous carbon measured at 30 bar and 298 K.

In the case of CO2 adsorption, the pore distribution is a key factor for developing a carbon material with a higher CO2 adsorption capacity. The number of micropores and a broad mesopore distribution had a significant effect on the CO2 adsorption capacity of ONCs materials. As the pressure was increased, the physical properties of the adsorbent, such as the specific surface area and meso- and micropore volume, become increasingly important for CO2 adsorption because a high mesopore volume means more adsorption sites available and a large pore volume means that there is more space available for CO2 adsorption. 4. Conclusion ONCs materials can be prepared readily using the directtriblock-copolymer-templating method. The one-pot synthesis method is an efficient route for the preparation of ONCs. Although ONCs materials exhibited a mean pore diameter of 3.7–3.9 nm, their specific surface areas were within 500–800 m2/g. The ONCs materials carbonized at 900 1C exhibited the highest CO2 adsorption capacity (158 mg/g at 1 bar and 685 mg/g at 30 bar), resulting from the higher specific surface area and narrower micropore size distribution at moderate temperatures. Overall, the softtemplating method provides opportunities for controlling the pore structure of ONCs materials. ONCs materials can be applied to adsorption technologies, which represent a new direction for capturing CO2 from thermal power plants. Acknowledgments The authors wish to acknowledge the financial support by Grants from Korea CCS R&D Center, funded by the Ministry of Education, Science and Technology of the Korean government.

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