Use of monolithic silicon carbide aerogel as a reusable support for development of regenerable CO₂ adsorbent

Abstract

Monolithic silicon carbide aerogel (MSiCA) was firstly used as a reusable support to develop a novel CO₂ adsorbent, i.e. amine functionalized monolithic silicon carbide aerogel (AFMSiCA). Used or exhausted AFMSiCA was separated, treated and re-functionalized by amine to form RF-MSiCA. CO₂ capture performance of the resulting adsorbents was investigated in a 1% CO₂ flow stream. The results revealed that CO₂ adsorption capacities of AFMSiCA at different temperatures ranging from 25 to 75 °C showed little change, indicating that the resulting adsorbent is well-adapted to temperature. The adsorption rate is affected by the combined effect of CO₂ adsorption and desorption. Benefiting from the highly unique properties of the aerogel (e.g. high surface area and large pore volume), MSiCA-based adsorbents are dynamic in their CO₂ adsorption process. MSiCA could be readily recovered and reused at least 12 times without significant loss of CO₂ adsorption performance. The resulting adsorbents presented good stability during cyclic adsorption–regeneration tests. MSiCA is exceptional in practical application for CO₂ capture due to its excellent reusability and the regenerability of its amine functionalized counterparts.

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Introduction

Concentrations of CO₂ in the atmosphere are increasing at an accelerating rate, a phenomenon that has drawn significant attention because of its link to climate change. CO₂ capture and sequestration (CCS) is a promising way to control CO₂ emissions.¹ Among available techniques, amine scrubbing is the current state-of-the-art technology for CO₂ capture on an industrial scale. However, this approach shows significant shortcomings, including corrosion, oxidative degradation, high energy consumption for regeneration, and secondary pollution resulting from its high volatility.² Hence, porous adsorbents functionalized with amines have been extensively studied for CO₂ capture because they can overcome the shortcomings of an aqueous amine scrubbing technique.³ Recent investigations have focused on using aerogels as new CO₂ capture materials, due to their unique physical properties such as low densities, high surface areas and porosities.⁴ Moreover, the microstructures and elemental compositions of aerogels can be tailored using solution chemistry via a process known as the sol–gel method.⁵ Therefore, aerogel-based CO₂ adsorbents have been extensively developed recently. Cui et al. achieved a high CO₂ adsorption capacity in 10% CO₂ mixture stream by using silica aerogel modified with 3-aminopropyltriethoxysilane (APTES).⁶ Begag et al. developed hydrophobic amine functionalized aerogels by grafting amine groups onto silica backbone and achieved 1.43 mmol g⁻¹ CO₂ sorption capacity in simulated flue gas.⁷ Linneen et al. found that 1 g silica aerogel immobilized with tetraethylenepentamine (TEPA) could adsorb 3.5 mmol CO₂ under a dry 10% CO₂/Ar stream.⁸ Wang et al. found that 1 g silica aerogel can adsorb 3.3 mmol CO₂ from pure CO₂ at 25 °C and 1 atm.⁹ Masika developed a carbon aerogel for CO₂ capture, the CO₂ adsorption capacity obtained in pure CO₂ at 25 °C and 1 bar was only 2.2 mmol g⁻¹ although the carbon aerogel has a high surface area of 1100 m² g⁻¹.¹⁰ Alhwaige et al. developed chitosan hybrid aerogels for CO₂ capture, the CO₂ adsorption capacity in pure CO₂ at 25 °C is up to 4.15 mmol g⁻¹.¹¹ Lin et al. successfully developed hydrophobic silica aerogel membranes for CO₂ capture.¹² We have developed a novel aerogel adsorbent based on amine hybrid silica aerogel via facile sol–gel route and supercritical drying, its CO₂ adsorption capacity of at 25 °C and 1 atm in a 1% CO₂ mixture stream is as high as 5.55 mmol g⁻¹.¹³ All these reports indicate that aerogels have a promising prospect in CO₂ capture.

Silicon carbide (SiC) has high hardness, excellent thermal stability and superior chemical inertness,¹⁴ therefore, its aerogel counterpart can be promisingly used for CO₂ capture. Bhatia et al. developed SiC-derived carbon for CO₂ capture.¹⁵ However, Monolithic SiC aerogel (MSiCA) has never been used for CO₂ adsorbent or support of a CO₂ adsorbent. In this work, MSiCA was firstly used as a reusable support to develop a novel CO₂ adsorbent, i.e. amine functionalized monolithic silicon carbide aerogel (AFMSiCA). As most of the porous adsorbents for CO₂ capture are particulate, they have to be dispersed in sand to prepare a fixed bed to avoid blocking of the reactor when performing CO₂ capture process.¹⁶ Monolithic aerogels with compact integral framework and porous microstructure can give lower backpressure, higher permeability and better performance compared to particles when they are used in the fixed bed CO₂ separation processes.¹⁷ Furthermore, researches mostly focus on developing novel adsorbents and improving their CO₂ capture performances. Few work pays attention on developing reusable supports or adsorbents, which can reduce the overall CO₂ capture costs. The separation and collection of monolithic CO₂ adsorbents should be much easier than their particulate counterpart, avoiding secondary pollution and improving utilization efficiency.

Experimental

Preparation of AFMSiCA

Preparation method of MSiCA was reported elsewhere.^14a Sol–gel process followed by supercritical drying (SCD) and thermal treatment are used to prepare MSiCA used for the preparation of AFMSiCA. Resorcinol (R), formaldehyde (F, 37% aqueous solution), anhydrous sodium carbonate (C), deionized water (W), and ethanol (EtOH) were mixed at 50 °C with a molar ratio of R : F : EtOH : W : C = 1 : 2 : 3.43 : 2 : 0.01. The mixture was stirred for 30 min to form RF sol. Tetraethoxysilane (TEOS), EtOH, W and hydrochloric acid (HCl) were mixed at room temperature (25 °C) with TEOS : EtOH : W : HCl prepared at a molar ratio of 1 : 10.3 : 2 : 0.002. The mixture was stirred for 40 min to form silica sol. The RF sol was poured into silica sol with a molar ratio of R : TEOS = 1 : 2 to form resorcinol-formaldehyde/silica composite (RF/SiO₂) sol. The pH value of RF/SiO₂ sol was regulated to 9.4 ± 0.1 by ammonium hydroxide. Then RF/SiO₂ sol was poured into a polypropylene mould to form RF/SiO₂ gel at room temperature (25 °C). RF/SiO₂ gel was aged at 60 °C for 24 hours and simultaneously washed with ethanol (3 times, 8 h per wash cycle) to remove water and residual chemicals. The resulting alcogel was dried by SCD to form RF/SiO₂ aerogel. RF/SiO₂ aerogel was transformed to MSiCA by carbothermal reduction. Thermal treatment of RF/SiO₂ aerogel was performed in a tube furnace (72 mm inner diameters of tube). The temperature was first raised to 800 °C with a rate of 2 °C min⁻¹ under argon flow (150 ml min⁻¹), and maintained at that level for 3 h. Subsequently, the flow rate was lowered to 100 ml min⁻¹ and the temperature was raised further to 1500 °C with a rate of 2 °C min⁻¹ and it was maintained at that level for 5 h. At the end of that period, the temperature was lowered to 550 °C, argon was changed to air (200 ml min⁻¹), and excess free carbon was burned off by maintaining the temperature at that level for 3 h.

To prepare AFMSiCA, MSiCA was immersed in a mixture of water (W), APTES and ethanol (EtOH) at 50 °C for 24 h, the volume ratio of APTES, W and EtOH was prepared at 1 : 0.2 : 2. Then MSiCA was taken out from the mixture and dried in an oven at 50 °C to obtain AFMSiCA. To re-functionalize used or exhausted AFMSiCA, AFMSiCA was thermally treated in a furnace at 600 °C for 2 h to burn off the organic moieties, the resulting sample was washed by water under ultrasonic (6 times, 0.5 h per wash cycle) and re-impregnated with APTES via the same procedure for preparing AFMSiCA to obtain re-functionalized MSiCA, denoting as RF-MSiCA-x in which x represents the amine functionalization time. In this paper, x represents 6, 12 or 18.

Sample characterization

Thermogravimetry analysis (TGA) was performed using a Netzsch STA 449C thermogravimetric analyzer under an air flow of 30 ml min⁻¹ at a heating rate of 20 °C min⁻¹. The morphology of the specimens was surveyed by LEO 1530VP scanning electron microscope (SEM) and JEOL JEM-2010 transmission electron microscope (TEM). N₂ adsorption–desorption tests were conducted using a Quantachrome Autosorb-iQ analyzer. Samples were treated under vacuum at 90 °C for 3 h before tests. Specific surface areas were calculated using Brunauer–Emmett–Teller (BET) model. Pore-size distributions were obtained using the non-local density functional theory (NLDFT) model. Pore volumes were estimated from the adsorbed amount of N₂ at a relative pressure of P/P₀ = 0.99. X-ray diffraction (XRD) was performed on a ARL ARLX'TRA X-ray diffractometer with a Cu-Kα radiation to evaluated the phase composition and confirm the formation of SiC nanocrystal.

CO₂ capture test

The setup for the CO₂ capture test at atmospheric pressure is presented in Fig. S1 (ESI†). A fixed bed and a CO₂ + N₂ mixture gas with a CO₂ concentration of 1% were used for CO₂ adsorption. Pure N₂ was used for desorption through the temperature swing. Adsorbent was placed in the glass tube for CO₂ capture. A flow meter (part 9 of Fig. S1†) was used in the apparatus to detect the pressure drop of the fixed bed, i.e. the backpressure of the apparatus. The gas flow rates for adsorption and desorption were calibrated using an Elster American Meter AL17-1 Wet Test Meter and switched to standard state. The adsorption and desorption temperatures were controlled by a MTI GSL-1100X tube furnace. The blank or background of the apparatus was measured when there was no adsorbent in the reactor. All of the calculated capacities in this paper have subtracted the background. Before the CO₂ adsorption test, the fresh adsorbents were treated by N₂ at 130 °C for 30 min to remove adsorbed moisture and CO₂ from the air. For CO₂ capture, the temperature of the furnace was stabilized to the temperature for CO₂ adsorption. Meanwhile, water vapor (1%) was generated and maintained at a steady state. N₂ flow was then switched to the CO₂ + N₂ mixture gas and water vapor for CO₂ adsorption. After finishing CO₂ adsorption, the CO₂ + N₂ mixture gas was changed to N₂ to purge the pipes of the apparatus until the outlet CO₂ concentration was 0 or very close to 0. The water vapor was cut off, after which the temperature of the furnace was raised to 100 °C for CO₂ desorption. For cyclic experiments, the CO₂ adsorption process was repeatedly performed on the desorbed adsorbents using the procedure mentioned above. Detail of the calculation method of CO₂ adsorption capacity was reported elsewhere.¹³

Results and discussion

It should be noted that APTES moieties in AFMSiCA are physically adsorbed on the surface of SiC nanocrystal or in the pore space of MSiCA. Actually, APTES can be chemically grafted onto the surface of SiC particle. As reported by Williams et al.,¹⁸ SiC was first treated in oxygen environment to formation a surface silica layer, the resulting product was exposed to air to obtain moist silanol-terminated SiC, the surface silanol is necessary for the surface graft reaction. The following procedures (reaction, wash and drying) are similar as the surface modification of SBA. The grafted APTES moiety on SiC particle is single layer; hence, the amine loading is low. However, high amine loading should be achieved in order to obtain a desirable CO₂ adsorption capacity. Therefore, we used a facile method to deposit APTES onto the surface of SiC particle or into the pore space of SiC network to achieve relatively higher amine loading. It should be also noted that MSiCA is not the adsorbent; it plays an important role of support. AFMSiCA is the CO₂ adsorbent.

Digital photos of MSiCA, AFMSiCA and RF-MSiCA-x are presented in Fig. S2 (ESI†). All samples remain monolithic shape; revealing that the network of MSiCA is robust and immune to water and high temperature during re-functionalization process. The color of RF-MSiCA-18 is hoary while RF-MSiCA-6 and RF-MSiCA-12 remain the color of MSiCA, which should be due to the oxidation of SiC during thermal treatment and the residual silica particles derived from APTES in the pore space of SiC network, as proved by XRD patterns.

Phase composition of samples

XRD patterns of the samples are obtained, as presented in Fig. S3 (ESI†). The peaks with 2θ values of 35.6°, 41.4°, 60° and 71.8° correspond to the diffraction from the 111, 200, 220 and 311 crystal planes of β-SiC, respectively (PDF#29-1129). A weak peak, which is observed at 2θ = 34.1°, is characteristic of α-SiC (PDF#29-1128).^14,19 It can be seen that all the samples remain the characteristic of SiC crystal. No other crystalline phases of silica, carbon or other impurities were obviously detected in the as-prepared MSiCA and AFMSiCA. However, there are amorphous moieties in RF-MSiCA-x, which should derive from the oxidation of SiC framework and residual APTES-based SiO₂.

Pore structure of the adsorbents

N₂ adsorption–desorption isotherms and pore-size distributions of MSiCA, AFMSiCA and RF-MSiCA-x are presented in Fig. 1. Pore structure data of these samples are summarized in Table 1. All samples show type IV isotherms with type H1 hysteresis loops in the IUPAC classification, suggesting that they are typical mesoporous materials with cylindrical pores.^19,20 The adsorption of P/P₀ < 0.05 attributed to the contribution of micropores (<2 nm) is low, indicating that there is hardly micropores in the adsorbents, which is proved by the pore-size distribution curves. The adsorption from P/P₀ = 0.05 to the closed loop is ascribed to the multilayer adsorption on the framework's external surfaces; the abrupt increase of adsorption quantity results from the capillary condensation of N₂ in pores. There is no obvious saturation adsorption in the isotherms of MSiCA whereas the saturation adsorption appears in the isotherms of AFMSiCA and RF-MSiCA-x. It reveals that MSiCA has more macropores (>50 nm) than AFMSiCA and RF-MSiCA-x, which can be also proved by pore-size distributions. It can be seen from the pore-size distribution curves that smaller pores disappear and the most probable pores move left from MSiCA to AFMSiCA, due to the blocking of pore space after impregnation.⁸ Hence, surface area and pore volume present an obvious decrease from MSiCA to AFMSiCA. As observing from the pore-size distribution curves and Table 1, the most probable pores move left and the pore structure data (surface area and pore volume) decrease after amine functionalization and re-functionalization. It is explained by the effect of residual silica particles in the SiC network and the oxidation of SiC network. The residual silica particles which derive from APTES could block the pore space of SiC network. Also, the oxidation of SiC framework results in the decrease of the pore spaces of the adsorbents.

Fig. 1 (a) N₂ adsorption–desorption isotherms and (b) pore-size distributions of samples.

Table 1 Pore structure data of MSiCA, AFMSiCA and RF-MSiCA-x

Sample	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)
MSiCA	332	2.35
AFMSiCA	108	1.27
RF-MSiCA-6	99	1.18
RF-MSiCA-12	72	0.92
RF-MSiCA-18	43	0.56

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TGA of samples

TG curves of AFMSiCA and RF-MSiCA-x are presented in Fig. 2. TG curve of AFMSiCA suggests that the organic moieties of AFMSiCA can be completely removed after thermal treatment at 550 °C for 2 h. Weight losses of AFMSiCA and RF-MSiCA-x below 155 °C are attributed to moisture and CO₂ absorbed from air. Weight losses at 155–250 °C and 250–700 °C result from bonded water and organic moieties, respectively. Increases of weight at high temperature are assigned to the oxidation of SiC. It can be calculated from TG curves that weight loading of organic moieties of APTES in MSiCA is in the range of 38.1–15.7% (Table S1, ESI†). TG curves of RF-MSiCA-x are also measured. The results indicate that the APTES (amine) loading gradually decreases along with the increase of functionalization time. TG curves of MSiCA presented in Fig. S4 (ESI†) reveal that MSiCA is thermally stable in air at under a temperature up to 600 °C.

Fig. 2 TG curves of AFMSiCA and RF-MSiCA-x.

Morphology of samples

SEM images of MSiCA, AFMSiCA, RF-MSiCA-x are presented in Fig. S5 (ESI†). MSiCA and AFMSiCA present disordered, porous structures of a typical colloidal gel, and nanoparticles are uniformly distributed to form a framework surrounded with irregular pores.²¹ Porosity gradually decreases along with the increase of functionalization time, which has been proved by the pore volume data presented in Table 1. Meanwhile, particle size increases with the increase of functionalization time. It may be attributed to the effect of residual silica which deposits on the surface of SiC framework and the oxidation of SiC nanocrystal.

CO₂ capture performance of adsorbents

The CO₂ adsorption kinetics and the corresponding CO₂ adsorption rates of AFMSiCA at different temperatures in the absence of water are presented in Fig. 3. The CO₂ adsorption behavior of AFMSiCA at different temperatures accords with that predicted by thermodynamic theories.²² The total CO₂ adsorption capacity of AFMSiCA varies from 1.81, 1.67, 1.53 to 1.00 mmol g⁻¹ when the adsorption temperature increases from 25, 50, 75 to 100 °C. The decrease of capacity from 25 to 75 °C is slow while the decrease of capacity from 75 to 100 °C is sharp. It results from the effect of desorption at 100 °C, indicating that AFMSiCA possesses a desorption temperature around 100 °C. Unlike CO₂ adsorption capacity, the CO₂ adsorption rate of AFMSiCA at different temperatures seems to follow no rule. The CO₂ adsorption rates at 25, 50 and 75 °C are very close, however, the adsorption rate at 100 °C is very different with these obtained at 25–75 °C. The effect of adsorption temperature on the adsorption rate is due to the combined effect of CO₂ adsorption and desorption. The CO₂ adsorption rate of the adsorbent depends on the competition of these factors. Obviously, higher temperature gives higher CO₂ adsorption rate as increasing temperature favors the reaction activity of amine groups. However, the effect of desorption gradually increase along with the increase of temperature. Therefore, adsorption–desorption equilibrium is achieved quickly and the adsorption rate sharply decreases with the adsorption progress when the adsorption temperature increases to 100 °C, indicating that the effect of desorption is dominant in the adsorption–desorption equilibrium at 100 °C. The CO₂ adsorption behaviors presented in Fig. 3 suggest that AFMSiCA is strong adaptability to the environment temperature, and therefore suitable for use in various fields and regions.

Fig. 3 CO₂ adsorption kinetics and the corresponding CO₂ adsorption rates of AFMSiCA at different temperatures in the absence of water (adsorption time of 20 min; adsorption gas flow rate of 310 ml min⁻¹; weight of adsorbent: 0.145 g): (a) CO₂ adsorption kinetics; (b) CO₂ adsorption rates.

The effect of water vapor on the CO₂ adsorption capacity and the corresponding CO₂ adsorption rates of AFMSiCA are presented in Fig. 4. The CO₂ adsorption capacities with and without water vapor at 50 °C are 2.12 and 1.67 mmol g⁻¹, respectively. Water is positive to CO₂ adsorption capacity and rate in the whole adsorption process. This point has been extensively proved by other amine functionalized adsorbents, and we have also discussed the effect of water on CO₂ adsorption performance elsewhere.^6,22 It should be noted that the CO₂ adsorption capacities obtained from the kinetic curves represent total capacity, while the useful CO₂ adsorption capacity in a practical separation process is determined by the breakthrough point (Fig. S6, ESI†).¹³ The useful CO₂ adsorption capacity at 50 °C with and without water are 0.95 and 0.57 mmol g⁻¹, respectively. Therefore, the effective fractions are 44.8% and 34.1%.

Fig. 4 CO₂ adsorption kinetics and the corresponding of AFMSiCA at 50 °C with and without water vapour (adsorption time of 20 min; adsorption gas flow rate of 310 ml min⁻¹; weight of adsorbent: 0.145 g; 1% water vapor): (a) CO₂ adsorption kinetics; (b) CO₂ adsorption rates.

Fig. 5 shows the CO₂ adsorption kinetics of AFMSiCA and RF-MSiCA-x at 50 °C. The CO₂ adsorption capacity decreases from 2.12 to 1.83, 1.57 and 0.83 mmol g⁻¹ after 6, 12 and 18 times amine functionalization, respectively. It is due to that the content of amine group plays an important role in CO₂ adsorption capacity. As presented in Table S1,† APTES loading gradually decreases with the increase of functionalization time, in other words, the surface amine content of the adsorbent decreases along with the increase of the functionalization time. CO₂ adsorption reaction of the amine-based adsorbents takes place between the amine group and CO₂. Therefore, amine content favors the CO₂ adsorption capacity of amine-based adsorbents. The adsorption rate (Fig. S7, ESI†) also presents an obvious decrease with the increase of the functionalization time. It results from the decrease of surface area, pore volume and surface amine content (APTES loading) of the adsorbent after re-functionalization, as presented in Table 1 and S1.† The CO₂ adsorption performance of the adsorbents reveals that MSiCA can be effectively recovered and reused as a support of CO₂ adsorbent for at least 12 times without fatal decrease of the CO₂ adsorption capacity of its amine functionalized counterpart.

Fig. 5 CO₂ adsorption kinetics of AFMSiCA and RF-MSiCA-x at 50 °C (adsorption time of 20 min; adsorption gas flow rate of 310 ml min⁻¹; weight of adsorbent: 0.140–0.150 g; 1% water vapor).

The regenerability of CO₂ adsorbents is important, as this directly affects their overall CO₂ capture costs. Fig. 6 shows the cyclic CO₂ adsorption capacities of AFMSiCA and AFMSiCA-12 at 50 °C in the presence of water. The capacities of the adsorbent do not noticeably diminish after 30 cyclic adsorption–desorption tests, decreasing from 2.12 to 2.03 mmol g⁻¹ and from 1.57 to 1.45 mmol g⁻¹ for AFMSiCA and RF-MSiCA-12, respectively. The regenerability of AFMSiCA and RF-MSiCA-12 in humid environment is better than that of amine hybrid silica aerogel and other excellent amine-based CO₂ adsorbents, due to their better structure stability with water.^13,23 The cyclic stability of the resulting MSiCA-based adsorbents in dry environment is more excellent, as presented in Fig. S8 (ESI†). This encouraging result shows that the adsorbent based on MSiCA is exceptional in regenerability.

Fig. 6 Cyclic adsorption capacities of AFMSiCA and RF-MSiCA-12 at 50 °C (adsorption time of 20 min; adsorption gas flow rate of 310 ml min⁻¹; weight of adsorbent: 0.150 g; 1% water vapor; desorption gas flow rate of 310 ml min⁻¹; desorption temperature of 100 °C).

Conclusions

MSiCA with high surface area and large pore volume is a robust and effective support for preparing amine functionalized CO₂ adsorbent. AFMSiCA was prepared by wet impregnation of MSiCA with APTES. Used or exhausted AFMSiCA was separated, treated and re-functionalized by APTES to reactivate and regenerate its CO₂ capture ability. MSiCA could be readily recovered and reused at least for 12 times without significant loss of CO₂ adsorption performance. The resulting MSiCA-based adsorbents are dynamic in CO₂ capture process and present good stability during cyclic adsorption–regeneration tests with and without water. The resulting adsorbent is strong adaptability to the environment temperature, and therefore suitable for use in various fields and regions. MSiCA is exceptional in practical application due to its excellent reusability and the regenerability of its amine functionalized counterparts.

Acknowledgements

The authors acknowledge the supports from Jiangsu Planned Projects for Postdoctoral Research Funds (1402016A), the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) and the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1146).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11261j

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