Highly microporous polymer-based carbons for CO₂ and H₂ adsorption

Abstract

A series of microporous carbons has been obtained through carbonization and KOH activation of a commercially available styrene divinylbenzene resin with sulfonate functional groups, Amberjet 1200 H. The resulting carbons featured very high specific surface areas: from 725 m² g⁻¹ to 3870 m² g⁻¹, large total pore volumes: from 0.44 cm³ g⁻¹ to 2.07 cm³ g⁻¹, and exceptionally developed microporosity: from 0.2 cm³ g⁻¹ to 1.16 cm³ g⁻¹. The controlled activation process afforded high amounts of ultramicropores (micropores < 0.75 nm) reaching 0.32 cm³ g⁻¹. Physisorption measurements showed very high uptakes of CO₂ and H₂ reaching 356 mg g⁻¹ of CO₂ (0 °C, 800 mmHg), 209 mg g⁻¹ of CO₂ (25 °C, 850 mmHg), and 39 mg g⁻¹ of H₂ (−196 °C, 850 mmHg).

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Introduction

Gas adsorption on nanoporous materials plays an important role in solving energy-related issues such as CO₂ capture and H₂ storage. CO₂ is a greenhouse gas that is excessively emitted to the atmosphere by burning fossil fuels, whereas H₂ is considered as an environmentally-friendly fuel of the future.¹ Thus, there is a great interest in the development of suitable nanoporous adsorbents for gas capture/storage that can be used for controlling the greenhouse effect and solving energy-related issues. Potential environmental benefits arising from this type of research may in a long run result in economic benefits and importantly, enhance the national energy security (independence from fossil fuel-based energy).^2–5 Notably, nanoporous adsorbents may also be used in adsorption of different greenhouse (e.g., CO₂, CH₄) and environmentally-harmful gases (e.g., SO₂, NO_x).²

Numerous nanoporous materials such as carbons,^6–8 ordered mesoporous silicas and amine-functionalized organosilicas,^9–11 metal organic frameworks,^12,13 porous polymers,¹⁴ and metal oxides^15,16 were successfully employed as CO₂ adsorbents. Recently, however, novel advanced materials such as microporous carbons,^6,17–19 carbon–metal composites,²⁰ biomass-derived carbons,^17,21 carbon nanotubes,²² ordered mesoporous carbons,²³ and N-doped carbons²⁴ gained a significant interest. These carbon-based materials are favourably employed in CO₂ adsorption, unsurprisingly, considering their availability and low cost as well as their good thermal, mechanical, and chemical stability. It appears that these carbon materials can be utilized not only in environmental remediation but in energy sector as well. One of such applications is H₂ adsorption and storage. Hydrogen is considered a zero-emission fuel with the highest energy density per unit mass. Hydrogen storage can be achieved by adsorption, liquefaction, compression, or chemical bonding as metal hydrides.²⁵ Adsorption of hydrogen on nanoporous carbon materials has significant advantages such as complete reversibility and fast kinetics of the process; carbon materials are also lightweight and, hence, the impact on the energy density is small. Importantly, carbon materials used in hydrogen storage have to provide extremely high surface area,^26,27 which is often achieved by the presence of micropores with very small sizes that effectively increase material's H₂ storage capability.^28–30 Based on experimental data, Ströbel et al.³¹ predicted, using an extrapolation, that the specific surface area exceeding 4000 m² g⁻¹ is required to achieve 6 wt% of adsorbed hydrogen. Achievement of such high surface area is extremely challenging. Typically, the amount of adsorbed gas is associated with the specific surface area: the larger surface area is the larger adsorption is. However, recent studies showed that H₂ adsorption can also be enhanced by the presence of ultramicropores (micropores < 0.75 nm).³²

Among various polymeric-type carbon precursors, styrene divinylbenzene ion-exchange resins with sulfonate functional groups are promising polymers for preparation of microporous carbons.^33–35 Carbonization of these resins at temperatures between 500 and 900 °C in nitrogen atmosphere resulted in the decomposition of sulfonate groups and creation of microporosity. In particular, carbonization at 500 °C gave carbons with narrow pore size distribution in the micropore range with the average pore size between 0.38 nm and 0.45 nm.³⁵

In this work, a series of microporous carbons was prepared by carbonization and activation of a commercially available styrene divinylbenzene resin with sulfonate functional groups, Amberjet 1200 H. The activation process conducted at 700 °C by using different amounts of KOH (1 : 1 to 6 : 1 KOH to carbon ratio) resulted in highly microporous and high surface area carbons. The resulting carbons exhibited high CO₂ uptake at 0 °C and 25 °C, and H₂ adsorption at −196 °C. The results presented in this work prove the dominant role of ultramicropores (pores below 0.75 nm) in CO₂ adsorption, and the requirement of the extremely high specific surface area (∼4000 m² g⁻¹) for effective H₂ adsorption.

Experimental

Preparation of polymer-based carbon

Monodisperse carbon spheres (C) were obtained from commercially available styrene divinylbenzene ion-exchange resin spheres (d = 0.5 mm), Amberjet 1200 H (Rohm and Haas, Philadelphia, PA, USA). Namely, 120 g of the resin spheres was dried at 100 °C for 3 hours and subsequently, immersed in 300 cm³ of 50% orthophosphoric acid (H₃PO₄, POCh, Poland). The swollen resin was evaporated in a rotary evaporator at 180 °C. The product was allowed to cool to room temperature and washed with 250 mL of water. Next, the resin was dried at 80 °C for 75 minutes. Finally, the resin was carbonized in static atmosphere in a furnace at 350 °C for 90 minutes using 20 °C min⁻¹ heating ramp. The carbonized product was washed with 500 mL of distilled water and dried at 100 °C for 60 minutes.

Activation of carbon material

The activation process was carried out in nickel crucibles using 1 g of the carbon material and different amounts to KOH (POCh, Poland) to achieve the following KOH/carbon weight ratios: 1 : 1, 2 : 1, 3 : 1, 4 : 1, 5 : 1, and 6 : 1. The crucibles were placed in a furnace in nitrogen atmosphere and heated from room temperature up to 700 °C using 20 °C min⁻¹ heating ramp and dwelled for 1 hour. The cooled product was immersed in 30 mL of 35 wt% of HCl and sonicated for 1 hour. Subsequently, the activated carbons were filtered and washed until natural pH, and dried at 100 °C for 24 hours. The resulting activated carbons were labelled according to the KOH/carbon ratios as: C-1, C-2, C-3, C-4, C-5, and C-6. The starting carbon material, obtained by carbonization of the resin, was denoted as C.

Measurements and calculations

Scanning electron micrographs were taken on a LEO 1530 scanning electron microscope manufactured by Zeiss (Germany) operated at 2 kV acceleration voltage.

All physisorption isotherms: N₂ at −196 °C, CO₂ at 0 °C and 25 °C, and H₂ at −196 °C, were measured on an ASAP 2020 volumetric surface and porosity analyzer manufactured by Micromeritics (Norcross, GA, USA). Prior to measurements, each sample was outgassed at 200 °C for 2 hours.

Specific surface area, S_BET, was calculated using Brunauer–Emmett–Teller (BET) method based on low-temperature nitrogen adsorption isotherm in a relative pressure range of 0.05–0.2, assuming a cross-section area of nitrogen molecule equal to 0.162 nm².³⁶ Total pore volume, V_t, was calculated by converting the volume of nitrogen adsorbed at relative pressure p/p_o = 0.99 to the volume of liquid nitrogen.³⁷ Pore size distribution function (PSD) was calculated from low-temperature nitrogen adsorption isotherm using DFT method implemented in the ASAP 2020 instrument software. Ultramicropore volume, V^DFT_mi1, was calculated from the corresponding PSD in a pore size range 0–0.75 nm. Micropore volume, V^DFT_mi2, was obtained for pores below 2 nm. Total pore volume, V^DFT_t, and specific surface area, S^DFT_t, were calculated using DFT algorithm for pores within range 0.4–50 nm. Ultramicropore volume, V_mi1^α_s, and micropore volume, V_mi2^α_s, were calculated using comparative α_s-plot method,³⁷ for α_s values in range 0.25–0.5 and 0.9–1.3, respectively (were α_s is a reduced adsorption defined as the ratio of adsorption at particular relative pressure to the adsorption at p/p_o = 0.4). The reference material used in the comparative studies was Cabot BP280 carbon black reference material.³⁸ External surface, S_ext^α_s, was calculated in α_s range of 3–8. Mesopore volume, V_me^α_s, was calculated by subtraction of V_mi2^α_s from V_t. Additionally, PSD was calculated using Kruk–Jaroniec–Sayari (KJS) method,³⁹ which is based on Barrett–Joyner–Halenda (BJH) algorithm,⁴⁰ and uses the corrected Kelvin equation and the experimentally obtained statistical film thickness for nitrogen on the Cabot BP280 material. Micropore width, w^KJS_mi2, and mesopore width, w^KJS_me, were calculated as maxima of the PSD curve in the corresponding pore size ranges. Microporosity was calculated as the ratio of V_mi2^α_s to V_t and expressed in %.

Results and discussion

Morphology

Scanning electron microscopy (SEM) was used to investigate morphology of the styrene divinylbenzene resin, and the resulting carbon material and activated carbons. Fig. 1 shows SEM micrographs of a single resin sphere, a single carbon sphere, a collection of the carbon spheres, and grains of the activated carbon C-4. As can be seen in this figure the resin material – used as a carbon precursor – has spherical morphology with extremely uniform diameters of ca. 500 μm.

Fig. 1 Scanning electron micrographs of the styrene divinylbenzene resin (A), the carbon material (B) and (C), and the activated carbon C-4 (D).

The carbon material, obtained through carbonization of the resin, retained the spherical morphology; however, the spheres are noticeably cracked with diameters ca. 450 μm. It is possible that the cracking is related to the volume change, which occurred when the swollen resin impregnated with orthophosphoric acid underwent the thermal treatment at 350 °C. Finally, the subsequent activation process, conducted at 700 °C with KOH, destroyed the spherical morphology and the resulting activated carbon was in the form of irregularly shaped grains of different sizes. Activation conditions were used as in our previous work, which shows that 700 °C is sufficient temperature for effective KOH activation.⁴¹

Porosity

The aim of our work was to obtain carbon materials with high specific surface area, well-developed porosity, and with possibly large amount of ultramicropores. Fig. 2 shows low-temperature nitrogen adsorption–desorption isotherms of all carbon materials. All isotherms can be classified as type IV with H4 hysteresis loop according to the IUPAC classification.⁴² Table 1 lists all of the structural parameters calculated based on the nitrogen adsorption data.

Fig. 2 Low-temperature nitrogen adsorption–desorption isotherms measured on all carbon materials.

Table 1 Structural parameters for all carbon materials^a

Carbon	S^BET (m^2 g^−1)	Vt (cm^3 g^−1)	V^DFTmi1 (cm^3 g^−1)	V^DFTmi2 (cm^3 g^−1)	V^DFTt (cm^3 g^−1)	S^DFTt (m^2 g^−1)	Microporosity (%)
a C – carbon, C–X – X denotes the KOH/carbon weight ratio; SBET – BET specific surface area; Vt – total (single-point) pore volume obtained from the amount adsorbed at p/po ≈ 0.99; V^DFTmi1 – volume of ultramicropores (pores < 0.75 nm) obtained on the basis of DFT PSD; V^DFTmi2 – volume of micropores (pores < 2 nm) obtained on the basis of DFT PSD; V^DFTt – total pore volume estimated by the DFT method; S^DFTt – total specific surface area obtained by the DFT method; microporosity – percentage of volume of micropores (V^αsmi2) in the total pore volume (Vt).
C	218	0.12	0.01	0.07	0.10	101	67
C-1	725	0.44	0.08	0.20	0.34	489	68
C-2	1510	0.83	0.20	0.55	0.66	1480	77
C-3	3180	1.70	0.27	0.97	1.25	2120	75
C-4	3870	2.07	0.32	1.16	1.60	2600	77
C-5	3630	1.95	0.25	0.99	1.53	2240	75
C-6	2900	1.54	0.18	0.75	1.25	1760	77

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The activated carbons studied exhibited extremely high values of the specific surface area S_BET (up to 3870 m² g⁻¹), total pore volume V_t up to 2.07 cm³ g⁻¹, micropore volume V^DFT_mi2 up to 1.16 cm³ g⁻¹, and ultramicropore volume V^DFT_mi1 up to 0.32 cm³ g⁻¹. Notably, all these parameters change as a function of the KOH/carbon ratio: initially, the values increase from C-1 to C-4 and decrease thereafter for C-5 and C-6 samples.

Interestingly, in all cases the values of the total pore volume V^DFT_t, calculated using DFT method, were considerably smaller as compared with the total pore volumes V_t, calculated using volume of nitrogen adsorbed at p/p_o ≈ 0.99. The same is apparent when comparing the values of the specific surface area S^DFT_t, calculated using DFT method, with S_BET, obtained by the BET method. This behaviour is known because the BET method overestimates the specific surface area of microporous carbons.⁴³

The aforementioned quantities, specific surface area and total pore volume, reach their maximum values for the C-4 material; the same apply to the other structural parameters listed in Table 1. Unsurprisingly, the values of the structural parameters obtained for the carbon material C are considerably lower as compared to those evaluated for the activated carbons. Fig. 3 shows pore size distribution functions (PSDs), calculated using DFT method, for all carbon materials. All activated carbons show trimodal PSDs with three maxima appearing in the following ranges: 0–1 nm, 1–2 nm, and 2–3 nm. The carbon material C, on the other hand, does not have any porosity in the range of smallest pores: 0–1 nm. Based on this observation, one can conclude that KOH activation was shown to be a successful way for introducing ultramicroporosity (pores below 0.75 nm) in carbons, which was found to be the key factor controlling CO₂ uptake at ambient conditions.^19,44

Fig. 3 Incremental pore size distribution functions for all carbon materials (calculated by the DFT method).

In addition, the KOH activation increased the amount of larger micropores (1–2 nm) in addition to those already present in non-activated carbon. Interestingly, the fraction of microporosity in the total porosity is ca. 75% on average for all of the activated carbons. The amount of introduced ultramicroporosity, and larger micropores (supermicropores), follows the same trend as all other structural parameters and reach their maxima for the C-4 material.

In addition, the structural parameters have been calculated using the α_s-plot method.³⁷ Fig. 4 shows α_s plots for the selected activated carbons: C-1, C-3, and C-4. The solid straight lines, calculated in the α_s range 0.25–0.5, were used to estimate volumes of ultramicropores V_mi1^α_s; whereas, the dotted straight lines, calculated in the α_s range 0.9–1.3, were used to estimate volumes of micropores V_mi2^α_s. Table S1 (ESI†) lists all structural parameters calculated using the α_s-plot method. Based on the obtained values, one can conclude that KOH activation resulted in the development of both, ultramicropores and large micropores (supermicropores). Similarly, the highest values of the structural parameters were recorded for the activated carbon C-4: ultramicropore volume V_mi1^α_s = 0.31 cm³ g⁻¹, micropore volume V_mi2^α_s = 1.59 cm³ g⁻¹, and mesopore volume V_me^α_s = 0.54 cm³ g⁻¹.

Fig. 4 α_s plots for the selected activated carbons: C-1, C-3, and C-4. The solid straight lines calculated in the α_s range 0.25–0.5 (V_mi1^α_s), the dotted straight lines calculated in the α_s range 0.9–1.3 (V_mi2^α_s).

Low-temperature nitrogen adsorption isotherms were used to calculate PSD functions using Kruk–Jaroniec–Sayari (KJS) method.³⁹ Fig. S1 (ESI†) shows the calculated PSDs for all activated carbons studied. The PSD curves show two distinct maxima: one present in the micropore range w^KJS_mi2 ca. 0.8 nm and the other at the border range of micropores and mesopores. The values of these maxima for each material are listed in Table S1 (ESI†). Notably, the PSD peaks in the micropore range are narrower than the peaks present in the mesopore range. The obtained PSD curves illustrate the effect of the KOH activation on the evolution of microporosity in the activated carbons studied.

CO₂ adsorption

CO₂ adsorption on the activated carbons was measured at 0 °C and 25 °C up to 800 and 850 mmHg, respectively. Fig. 5 and 6 show the measured CO₂ isotherms for all carbon materials for the respective temperatures. Complementarily, Fig. S2 and S3† show the same isotherms expressed as mmol of adsorbed CO₂ per unit mass of adsorbent (mmol g⁻¹) for the respective temperatures.

Fig. 5 CO₂ adsorption isotherms measured at 0 °C for all carbon materials.

Fig. 6 CO₂ adsorption isotherms measured at 25 °C for all carbon materials.

Initially, CO₂ uptake increases in order from the C-1 material to C-4 material, which correlates with the amount of the KOH used during activation. The maximum uptake is observed for the C-4 material and thereafter adsorption declines for the materials C-5 and C-6. Conspicuously, the highest CO₂ adsorption was observed for the material that has the highest values of the structural parameters: S_BET, V_t, V^DFT_mi1, V^DFT_mi2, V^DFT_t, S^DFT_t among all the activated carbons studied. The recorded values were: 356 mg of CO₂ (0 °C, 800 mmHg) and 209 mg of CO₂ (25 °C, 850 mmHg). Table 2 shows a comparison of CO₂ and H₂ adsorption data for various materials.

Fig. 7 shows the CO₂ uptake at 0 °C and 25 °C as a function of specific surface area S^DFT_t for all carbon materials. The solid lines represent a simple linear regression over the values at two temperatures with the correlation coefficients: R² = 0.918 for 0 °C, and R² = 0.879 for 25 °C. Although, the CO₂ uptake clearly increases with the specific surface area, the correlation is not strong enough to claim that the surface area is the parameter determining CO₂ adsorption.

Fig. 7 CO₂ uptake at 0 °C and 800 mmHg, and at 25 °C and 850 mmHg as a function of the specific surface area S^DFT_t for all carbon materials.

Table 2 Comparison of CO₂ and H₂ adsorption on various materials

Materials	CO2 at 1 bar and 0 °C (mg g^−1)	CO2 at 1 bar and 25 °C (mg g^−1)	H2 at 1 bar and −196 °C (mg g^−1)
This publication	73–343	42–189	5.3–37.5
Activated carbons	123–378^46	66–211^46	3–26^48
	222–380^47	131–207^47
Carbide-derived carbons	—	110–168^49	Up to 27^48
Zeolite-templated carbons	213–304^50	140–193^50	15.6–22.7^51
Various metal oxides	—	21–52^52	—
Zeolites	—	33–205^46	—
		120–231^52
MOFs	0.16–264^53	0.57–373^53	11.7–25.4^54

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Fig. 8 shows the CO₂ uptake at 0 °C and 800 mmHg, and at 25 °C and 850 mmHg as a function of the ultramicropore volume V^DFT_mi1 for all carbon materials. Similarly, the solid lines are linear regressions over two sets of points; however, the correlation coefficients are noticeably better in this case: R² = 0.986 at 0 °C and R² = 0.970 at 25 °C. These results suggest that the volume of ultramicropores has a pronounced effect on the CO₂ uptake at pressures up to 850 mmHg; the latter is almost linearly correlated with the volume of ultramicropores. Similar results were recently reported by Sevilla et al. in their work on microporous and N-doped carbon microspheres.⁴⁵

Fig. 8 CO₂ uptake at 0 °C and 800 mmHg, and at 25 °C and 850 mmHg as a function of ultramicropore volume V^DFT_mi1 for all carbon materials.

The above conclusion is additionally supported by Fig. S4–S6 included in ESI.† Fig. S4† shows the CO₂ uptake at 0 °C and 800 mmHg, and at 25 °C and 850 mmHg as a function of the specific surface area S_BET for all carbon materials; Fig. S5† shows the same variable as a function of the ultramicropore volume V_mi1^α_s and Fig. S6† as a function of the micropore volume V_mi2^α_s. In those cases, the specific surface area S_BET was calculated using BET method and the latter two values, V_mi1^α_s and V_mi2^α_s, were calculated using comparative α_s-plot method.

H₂ adsorption

H₂ adsorption was measured at −196 °C and pressure up to 850 mmHg. Fig. 9 shows H₂ adsorption isotherms measured at −196 °C for all carbon materials. Similarly to CO₂ adsorption, H₂ uptake at 850 mmHg pressure increases in order from C-1 to C-4, along with the amount of the KOH used during activation, and declines thereafter for the C-5 and C-6 materials. The H₂ uptake at 850 mmHg pressure was found to be highest for the C-4 material as well. The latter material adsorbed 39 mg g⁻¹ of H₂ at −196 °C and pressure of 850 mmHg. Fig. 10 shows H₂ uptake at −196 °C and 850 mmHg as a function of the specific surface area S^DFT_t for all carbon materials. The solid line represents a simple linear regression with the correlation coefficient: R² = 0.995.

Fig. 9 H₂ adsorption isotherms measured at −196 °C for all carbon materials.

Fig. 10 H₂ uptake at −196 °C and 850 mmHg as a function of the specific surface area S^DFT_t for all carbon materials.

The strong correlation between H₂ uptake and specific surface area proves that H₂ adsorption on activated carbons is mainly governed by their specific surface area. The conclusion is further supported by Fig. 11, which shows the same variable but plotted as a function of the ultramicropore volume, V^DFT_mi1. The solid line represents a simple linear regression with the correlation coefficient: R² = 0.979, which is high but noticeably lower than that for the linear dependence of H₂ uptake on the specific surface area (Fig. 10). Based on these observations, hydrogen is possibly adsorbed through film formation rather than by micropore filling mechanism. The strong correlation between H₂ uptake and ultramicropore or micropore volumes can be explained as indirect relation to high surface area of these pores (which is the main contribution to the overall surface area of the materials).

Fig. 11 H₂ uptake at −196 °C and 850 mmHg as a function of the ultramicropore volume V^DFT_mi1 for all carbon materials.

Additional plots showing the H₂ uptake as a function of the structural parameters calculated by using BET and α_s-plot methods are included in ESI.† Fig. S7† shows H₂ uptake at −196 °C and 850 mmHg as a function of the specific surface area S_BET for all carbon materials. Fig. S8 and S9† show the same variable but plotted as a function of the ultramicropore volume, V_mi1^α_s, and micropore volume, V_mi2^α_s, respectively. The correlation coefficients for these regressions are: 0.943, 0.936, and 0.950, respectively.

Conclusions

A series of microporous carbons has been obtained through carbonization and KOH activation of a commercially available styrene divinylbenzene resin with sulfonate functional groups, Amberjet 1200 H. The resulting carbons showed large amounts of ultramicropores (micropores < 0.75 nm) and high specific surface area. The well-developed microporous structure resulted in high CO₂ and H₂ uptakes. The best activated carbons adsorbed: 356 mg g⁻¹ of CO₂ (0 °C, 800 mmHg) and 209 mg g⁻¹ of CO₂ (25 °C, 850 mmHg); and 39 mg g⁻¹ of H₂ (−196 °C and 850 mmHg). KOH activation resulted in the development of highly microporous structure as compared to the corresponding non-activated carbon, see the following data: the specific surface area changed from 218 m² g⁻¹ to 3870 m² g⁻¹, total pore volume from 0.12 cm³ g⁻¹ to 2.07 cm³ g⁻¹, micropore volume from 0.07 cm³ g⁻¹ to 1.16 cm³ g⁻¹, and ultramicropore volume from 0.01 cm³ g⁻¹ to 0.32 cm³ g⁻¹. The extremely good adsorption properties of the resulting microporous carbons toward CO₂ and H₂ render them applicable in the environmental and energy-related applications such as CO₂ capture and hydrogen storage.

Acknowledgements

JC and LO acknowledge the National Science Centre (Poland) for support of this research under grant 2013/09/B/ST5/00076.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional table and nine plots included. See DOI: 10.1039/c3ra47278g

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