High capacity CO₂ sorbents based on zinc-functionalized ionic liquid confined in morphologically diverse porous matrices

Abstract

We investigated the facile production of novel high capacity CO₂ sorbents based on a zinc-functionalized ionic liquid (EZT3) impregnated into several morphologically diverse porous supports: microporous alumino-silicate, non-porous nano-silica, and non-ordered mesoporous silica. These supported ionic liquid phase (SILP) hybrid sorbents with EZT3 loadings of 10 and 25 wt% were prepared via a solution intrusion-solvent evaporation method affording the deposition of a thin layer of IL on the surface of the supports. Textural, structural and thermal signatures of strong IL confinement on the surface were observed in 10 wt%-loaded sorbents, and multi-layers mimicking the bulk phase in 25 wt%-loaded sorbents. Adsorption of up to 3.4 mmol g⁻¹ was obtained at 313 K, a remarkable uptake capacity for low IL loading sorbents, significantly surpassing previously published details of similarly loaded materials. These new sorbents exhibit dramatic performance enhancement by factors of 1.6–16.0 (cf. bare support) and 3.6–5.3 (cf. bulk EZT3) highlighting the synergy between the available contact surface and pore network of the support, and the high affinity for CO₂ by the IL. Contrary to expectation, the materials based on non-porous nano-silica exhibited the highest uptake, rationalized by their unique core–shell configuration. In addition, the sorption kinetics were promoted by the external surface adsorption and facilitated diffusion into the pore network. The low IL loading and the relatively common support materials in these hybrid sorbents could potentially translate to cheaper and more sustainable CO₂ capture media.

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1. Introduction

The rise in atmospheric CO₂ concentration brought about by anthropogenic CO₂ emissions is now incontestable,¹ resulting in a measurable change in the global climate landscape. CO₂ emission abatement via carbon capture and storage^2,3 or utilization through conversion to value-added products^4,5 are promising near-term strategies, bridging the wait for more economically feasible access to renewable energy sources and realistic systematic reduction in global carbon footprints. The viability of these strategies relies on the production of cost-effective and sustainable capture media. A wide range of materials have been proposed to capture CO₂ from flue gas at large point sources including solvents,^2,6 solid sorbents,^7,8 membranes,^9,10 and their hybrids.^11–15

Although amine scrubbing continues to be the CO₂ capture technology of choice,¹³ when integrated into power plants, its inherent shortcomings¹⁶ result in 30% parasitic loss and a 60% price increase on the energy generated.¹⁷ These two factors are the major drivers for newer capture media and their development. Ionic liquids (IL), as a general class of materials, are being developed to address these shortcomings;⁶ actual application is impeded by their high viscosity, retarding CO₂ mass transport from the contact surface to their bulk.¹⁸ Solid sorbents with high porosities,^7,19,20 especially zeolites, metal–organic frameworks and activated carbons, exhibit seriously promising performances by physically adsorbing CO₂. Since many CO₂ emissions occur in a mixed gas stream, improvements in selectivity and recyclability continue to detract from wide application of these materials. Hybrid materials such as amine-modified sorbents and membranes,²¹ and supported ionic liquid phase (SILP) materials²² have attracted significant interest due to their ability to synergistically promote CO₂ sorption performance.

In contrast to bulk solvents and porous solid sorbents, hybrid materials in general offer significant advantages including lower cost of material production and lower energy penalties for regeneration,²³ higher selectivity and recyclability, and high tolerance to moisture and other impurities,²⁴ and overall reduction in industrial process cost.¹⁷

Our recent contributions showcase the production of high capacity hybrid sorbents prepared by impregnating ordered mesoporous silica (SBA-15)¹¹ and bio-templated porous silica beads²⁵ with our novel zinc-functionalized ionic liquid (EZT3).²⁶ Contacting a thin layer of EZT3 on the relatively high surface area porous support enhances the CO₂ uptake kinetics by exposing more accessible interaction sites, and facilitating diffusion through the pore network. This strategy effectively addressed the limitation of EZT3 due to its high viscosity and the formation of a semi-permeable layer at the gas/IL interface upon interaction with CO₂, retarding diffusion of CO₂ from the surface to the bulk.²⁷ More importantly, the performance of our hybrid sorbent materials is competitive with recently reported hybrid sorbents.^28–34 For example, our EZT3/SBA-15 sorbents readily achieved an uptake capacity of up to 4.7 mmol g⁻¹, and a ten-fold increase in the rate of uptake as compared with the bulk IL. The excellent wetting behaviour of EZT3 on the pore surface afforded the highly accessible pore network, unambiguously confirmed by SANS analyses. In the case of our bio-templated SiO₂ beads support, we have shown that the surface chemistry and pore morphology can be engineered, via post-synthetic pore modification, to produce highly efficient CO₂ sorbent materials.

The effect of the pore properties of the support on the physical properties, and CO₂ sorption capacity and kinetics of hybrid sorbents has been explored by several groups. Son et al.³⁵ prepared hybrid sorbents based on polyethyleneimine (PEI) impregnated into SBA-15, SBA-16, MCM-41, MCM-48, and KIT-6. The uptake capacity and kinetics increase as a function of the pore size prompting the conclusion that pore diameter is a critical parameter in the design of hybrid sorbents for CO₂ capture. Similarly, Zeleňák et al.³⁶ observed that for pore diameters less than 3.5 nm, accessibility of amine sites within the pores is hindered resulting in low CO₂ uptake capacity. In contrast, Yan et al.³⁷ showed, by preparing PEI impregnated into SBA-15 with various pore diameter and pore volume, that a strong correlation exists between the CO₂ uptake capacity and the pore volume, but not with the pore size. Interestingly, Zhang et al.³⁸ demonstrated, in PEI/mesocellular foam silica hybrid sorbents, that for a given PEI loading, CO₂ sorption capacity increases with the pore volume and the pore size of the support. Overall, the use of highly ordered support with well-defined and controllable pore properties is a promising strategy to prepare hybrid sorbents. However, the high cost of such materials could limit the practical applicability of the resulting sorbents.

In addition to pore properties, the nature of the organic layer contacting the support (e.g. organo-amines, ILs, amidines) plays a crucial role in the overall performance of the hybrid sorbents. For example, increasing the number of amines (which readily forms carbamates upon reaction with CO₂), such as in high molecular weight PEI, does not necessarily result in enhanced capacity.³⁸ Complete filling of the nanopores was suggested to fully exploit the capacity of the IL in SILP materials,³⁹ but this is not suitable for highly viscous systems. Our group has shown that at a certain layer thickness, the IL starts to behave similar to the bulk, exhibiting the same high viscosity that retards CO₂ diffusion.^11,25 Moreover, the wetting behaviour of the organic layer onto the support is equally important. Heinze et al.⁴⁰ have shown that the surface-coverage of porous silica with IL strongly depends on the compatibility between the IL and the support; monotonic adsorption or aggregate formation, in general, will dictate the properties and performance of the resulting materials. Hence, the selection of the most appropriate combination of the IL and the support remains as a challenge in designing highly efficient CO₂ capture sorbents.

On the basis of our previous experiences,^11,25,41 with the aim of preparing hybrid sorbents exhibiting high uptake capacity and enhanced kinetics, and in consideration of the aforementioned challenges, we prepared new sorbents by impregnating EZT3 into three types of morphologically diverse supports with similar specific surface area (180–200 m² g⁻¹): (1) microporous alumino-silicate (ZSM-5), (2) non-porous nano-silica, and (3) non-ordered mesoporous silica. These relatively common supports are inexpensive and readily available, and the target hybrid sorbents could be prepared facilely. The effects of the nature of the supports and their interactions with EZT3 were systematically explored. These hybrid sorbents were found to exhibit very promising CO₂ uptake capacity, significantly surpassing similar materials reported previously. Contrary to expectation, the hybrid sorbents based on non-porous nano-silica exhibited the highest uptake. The uptake capacity and sorption kinetics of these materials were rationalized on the basis of their textural, structural and thermal properties, with the intent of designing highly efficient CO₂ capture media.

2. Experimental

2.1. Materials

Analytical grade reagents were purchased from Sigma-Aldrich (Newcastle, Australia) unless otherwise stated, and were used without further purification. H-ZSM-5 (S5, crystal size = 300 nm, ACS Material, Massachusetts, USA), nano-SiO₂ (S12, primary particle size = 12 nm), and non-ordered mesoporous SiO₂ (S60, particle size = 35–70 μm) were also used as received. High purity gases were acquired from Coregas (Adelaide, Australia) or BOC Gas (Adelaide, Australia).

2.2. IL synthesis

EZT3 (1-ethyl-3-methylimidazolium tri[bis(trifluoromethylsulfo-nyl)amide]zincate) was prepared by mixing equimolar amounts of ET (1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)amide) and Zn(TFSI)₂ (zinc di[bis(trifluoromethylsulfonyl)amide]) with stirring under a nitrogen atmosphere at 353 K for 7 days. The resulting IL was dried in vacuo for at least one week prior to use. ¹H NMR (500 MHz, DMSO-d₆, δ): 9.10 (s, 1H), 7.77 (t, 1H), 7.69 (t, 1H), 4.18 (q, 2H), 3.84 (s, 3H), 1.41 (t, 3H); ¹⁹F NMR (125 MHz, DMSO-d₆, δ): −78.8.

2.3. Preparation of SILP sorbents

Alumino-silicate (S5), nano-SiO₂ (S12) or fumed SiO₂ (S60) particles were suspended in dry methanol (5 mL) via sonication (43 ± 2 kHz, 30 min). In a separate container, a pre-defined, accurately measured mass of vacuum-dried EZT3 (10⁻³ kPa), was dispersed in dry methanol (5 mL) with soaking in an ultrasonic bath until a single phase was achieved. This methanolic EZT3 solution was quantitatively transferred to the silica suspension which was further sonicated (3 h). The solvent was subsequently removed using a rotary evaporator. The EZT3 impregnated sorbent was collected and dried completely in a vacuum oven (353 K, 10⁻³ kPa), then stored in a vacuum desiccator for at least 12 h prior to analysis. Samples were denoted by support-x, where support is S5, S12 or S60, and x corresponds to the EZT3 loading (wt%).

2.4. Characterization

For textural properties measurements, samples were degassed (393 K, 10⁻⁵ kPa, 12 h) prior to the N₂ adsorption experiments, with the isotherms measured at 77 K using a Micromeritics TriStar II 3020 system. Ultra high purity (>99.999%) helium and nitrogen from BOC Gases Australia were used for dead-space measurements and adsorption experiments, respectively. The specific surface area (SSA) was calculated using the linearized BET equation and the pore size distribution evaluated using a non-local density functional theory (NLDFT) macro from the suite of analytical software. DRIFTS spectra were taken using a Shimadzu FTIR-8400S spectrometer from 400–4000 cm⁻¹. For each spectrum, 100 scans were recorded at a spectral resolution of 1 cm⁻¹. Samples were mixed with KBr in a mass ratio of 1 : 100, and were vacuum dried for 24 h prior to analysis. Thermal stability and IL loadings were measured using a Setaram TG-DTA/DSC system. All samples were vacuum dried at room temperature for at least 48 h prior to analysis. Accurately measured samples (≈20 mg) were loaded in a cylindrical alumina crucible and were heated to 1073 K at a ramp rate of 10 K min⁻¹ under a stream of N₂ at 50 mL min⁻¹.

2.5. CO₂ uptake

CO₂ uptake was performed using the above TGA unit. Each sample analyzed (≈20 mg) was dried for eight hours at 393 K under nitrogen. After cooling to 313 K, the sample was exposed to a stream of high purity CO₂ (99.9%) at 50 mL min⁻¹. A similar measurement was performed wherein the sample was exposed to N₂ (99.99%, ex. Coregas, Adelaide, Australia) at 50 mL min⁻¹ to verify if the CO₂ uptake is due to CO₂ and not due to water or other impurities.

2.6. Adsorption kinetics

The CO₂ adsorption was modelled using the intra-particle diffusion model (IPDM) developed by Weber and Morris.⁴² The dynamic uptake capacity was plotted against t^0.5 following:

q_t = kt^0.5 + C

where q_t (mmol g⁻¹) is the sorption capacity at time t (s), k (mmol g⁻¹ s^−0.5) is the sorption rate constant, and C (mmol g⁻¹) is a constant.

3. Results

Novel sorbents based on zinc-functionalized IL (EZT3) impregnated into alumino-silicate or silica particles were prepared and evaluated as CO₂ capture media. Three types of support, based on their porosity, were used with nano- to micrometer particle sizes: microporous alumino-silicate (S5), non-porous nano-silica (S12), and non-ordered mesoporous silica (S60). For the adsorbed IL layer, incorporation of Zn(TFSI)₂ into ET results in a novel zinc-functionalized IL where the Zn acts as the metal center and the three TFSI⁻ ions act as ligands forming a homoleptic oxo-coordinated anion, with the Emim⁺ acting as the cation (Fig. S1†).⁴³

3.1. N₂ adsorption: textural properties

Fig. 1 compares the N₂ adsorption isotherms at 77 K and calculated pore size distributions (PSD) of the different sorbents. Textural properties and IL loading are summarized in Table 1. Each of the isotherms for the adsorbents (Fig. 1a, c and e) were type IV with a single H1-type hysteresis loop over the range of 0.50 < p/p_o < 0.99.

Fig. 1 N₂ adsorption isotherms at 77 K (left) and NLDFT pore size distribution (right) for the bare and SILP sorbents: (a and b) S5-x, (c and d) S12-x and (e and f) S60-x. S5-0 (●, cyan), S5-13 (♦, light blue), S5-25 (▲, blue), S12-0 (●, yellow green), S12-9 (♦, green), S12-25 (▲, dark green), S60-0 (●, yellow orange), S60-10 (♦, orange), S60-26 (▲, red). Insets are correlation plots of SSA and V_p with EZT3 loading.

Table 1 Textural properties and IL loading of bare and SILP sorbents

Sample	SBET^a (m^2 g^−1)	Vp^b (cm^3 g^−1)	dp^c (nm)	IL loading^d (wt%)
a Specific surface area by linear BET equation.b NLDFT pore volume.c Mean pore diameter.d TGA mass loss.e Inter-particle porosity.
S5-0	200	0.19	0.8, 5.5	0
S5-13	18	0.09	5.0	13.4
S5-25	11	0.06	—	25.0
S12-0	180	^e	^e	0
S12-9	142	^e	^e	8.9
S12-25	82	^e	^e	25.1
S60-0	201	0.73	10.0	0
			15.1
			21.3
S60-10	159	0.55	9.1	10.1
			15.0
			25.3
S60-26	99	0.39	15.2	26.2
			25.4

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For adsorbent S5-0, the micropores (0.8 nm) disappear after IL impregnation (Fig. 1b). Impregnation has a negative effect on the SSA of the support (Fig. 1a inset) manifested by the decrease from 200 m² g⁻¹ for S5-0 to 18 and 11 m² g⁻¹ for S5-10 and S5-25, respectively. The reduction in V_p correlates well with the IL loading (R² = 0.93, Fig. 1b inset), suggesting the IL has good wetting properties towards the mesopore surface. The IL dimension of ca. 0.9 nm results in the blocking of the micropores, which explains the change in SSA after impregnation.

Sorbents based on non-porous SiO₂ nanoparticles (S12-x) show a wide range of ill-defined pore sizes (Fig. 1d) attributed to void spaces when the primary nanoparticles agglomerate to reduce their surface energy.⁴⁴ Contact of EZT3 with this silica appears to have resulted in the development of surface-coated nanoparticles, with the SSA decreasing linearly with loading up to 25 wt% (R² > 0.99, Fig. 1c inset). No ordering in pore structure was observed suggesting that EZT3 does not promote formation of ordered agglomerates.

In the case of S60-x, sorbents supported by non-ordered mesoporous SiO₂, the hysteresis loops represent pore sizes in the range of 7 to 30 nm (Fig. 1e). Both the SSA (Fig. 1e inset) and V_p (Fig. 1f inset) are inversely proportional to EZT3 loading (R² > 0.98), again suggesting EZT3 uniformly spreads on the pore walls.

3.2. DRIFTS: structural signatures of confinement

Fig. 2 shows the DRIFTS absorption spectra of EZT3, S5-x, S12-x and S60-x in the range 500–2000 cm⁻¹ and 2500–4000 cm⁻¹ highlighting the EZT3 characteristic peak intensity variation with IL loading. Peak assignments are: 592 cm⁻¹ (*, σ_SO2), 625–680 cm⁻¹ (‡, σ_SNS), 740–750 cm⁻¹ (†, complex vibration of the TFSI anion), and 800 cm⁻¹ (●, ν_CS). The peak at 835 cm⁻¹ in the pure EZT3 (►, ν_a,SN) has merged with the Si–O vibration from the silica support (790 and 815 cm⁻¹). Other signature peaks arising from the IL are found around 1058 cm⁻¹ (▼, ν_SO2), 1138 cm⁻¹ (▲, ν_as,CN), 1237 cm⁻¹ (■, ν_as,SO2), 1320 cm⁻¹ (♦, ν_s,CN), 2930–3040 cm⁻¹ (ν_CH,arom) and 3050–3230 cm⁻¹ (ν_CH,aliph). The broad O–H peak arising from strongly surface-bound H₂O is manifested in 3540–3750 cm⁻¹ (□). Frequency shifts where observed in four vibrational modes (highlighted in Fig. 2): σ_SNS of the anion, and ν_s,CN, ν_CH,arom and ν_CH,aliph of the cation. The magnitude of the blue shifts in these vibrational modes are in the order of ν_CH,arom > ν_CH,aliph > ν_s,CN > σ_SNS, with the shift for the 10 wt%-loaded sorbent always greater than that for the 25 wt%-loaded sorbent. These hypsochromic shifts originate from the interaction of the adsorbed IL layer with the support. The IL-support interaction is primarily through the cation, as suggested by the large shifts in the CH and CN vibrations of the cation, consistent with observations deduced from ATR-IR⁴⁵ and sum frequency generation (SFG)⁴⁶ spectroscopy. In both studies, the imidazolium ring was shown to be oriented almost parallel to the silica surface. Minimal interaction of the anion via the TFSI⁻ ligands was observed as manifested by the shift in the σ_SNS.

Fig. 2 DRIFTS spectra for (a) EZT3, (b) S5-x, (c) S12-x and (d) S60-x (x = EZT3 loading, wt%). S5-0 (cyan), S5-13 (light blue), S5-25 (blue), S12-0 (yellow green), S12-9 (green), S12-25 (dark green), S60-0 (yellow orange), S60-10 (orange), S60-26 (red). Shaded regions indicate peaks incurring blue shifts. Symbols represent vibrational modes mentioned in the text.

3.3. TGA: thermal stability

Fig. S2† shows the TGA curves for the support, bulk EZT3 and impregnated materials. These curves were used to interpret the effect of confinement on the IL thermal stability, in addition to quantifying the IL loadings. The decomposition of the IL starts at the onset temperature, T_d,onset, of 623 K. In all systems, S5-x (Fig. S2a†), S12-x (Fig. S2b†) and S60-x (Fig. S2c†), the thermal stability of the confined IL is reduced upon contact with the support, similar to previous observations for ILs confined by non-conducting matrices (e.g. SiO₂).⁴⁷ The T_d,onset for the impregnated systems is summarized in Fig. 3 clearly showing that the thermal stability of 10 wt%-loaded sorbent is always lower than that in 25 wt%-loaded sorbent, showcasing the effect of confinement on thermal stability.

Fig. 3 Thermal stability determined by T_d,onset. Bulk EZT3 (black), S5-0 (cyan), S5-13 (light blue), S5-25 (blue), S12-0 (yellow green), S12-9 (green), S12-25 (dark green), S60-0 (yellow orange), S60-10 (orange), S60-26 (red).

3.4. CO₂ uptake

The dynamic CO₂ uptake of S5-x (Fig. 4a), S12-x (Fig. 4b) and S60-x (Fig. 4c) are shown in Fig. 4. The uptake capacities of the sorbents and bulk ILs after 2.0 × 10⁴ s are summarized in Fig. 4d. The adsorption was terminated after 2.0 × 10⁴ s because important regions to model the overall kinetics (vide infra) have been captured within this timeframe. A six-fold increase in the uptake capacity of EZT3 as compared with ET clearly shows that the presence of Zn promotes the CO₂ capture performance of this new IL; EZT3 interacts with CO₂ via chemisorption and physisorption, in contrast to purely physisorption in ET.²⁷ Impregnating EZT3 into the different supports leads to a dramatic increase in its CO₂ uptake capacity.

Fig. 4 Time-dependent capacities for (a) S5-x, (b) S12-x and (c) S60-x. (d) CO₂ uptake capacities after 2.0 × 10⁴ s. EZT3 (■, black), S5-0 (●, cyan), S5-13 (♦, light blue), S5-25 (▲, blue), S12-0 (●, yellow green), S12-9 (♦, green), S12-25 (▲, dark green), S60-0 (●, yellow orange), S60-10 (♦, orange), S60-26 (▲, red). Bulk ET (gray) is included for comparison.

For the S5-x sorbents, the uptake capacities of S5-0, S5-13 and S5-25 are 1.62, 2.56 and 2.93 mmol g⁻¹, respectively. The relatively high uptake by S5-0 is due to its microporosity. Although the presence of EZT3 means the surface increased capacity by factors of 1.6 and 1.8 compared with the bare adsorbent, spreading it greatly enhanced its activity compared to the geometric area offered by the bulk IL; increases based on available IL were by factors of 4.0 and 4.6 for S5-13 and S5-25, respectively.

For the sorbents based on nano-SiO₂ support, the uptake capacities of S12-0, S12-9 and S12-25 are 0.21, 2.57 and 3.38 mmol g⁻¹, respectively. The bare support (S12-0) exhibited only nominal capacity probably due to the absence of any porosity, and the inter-particle porosity did not enhance any adsorption. Contacting EZT3 on the surface of these nanoparticles resulted in a dramatic enhancement of capacity: 12.3 and 16.0 (cf. bare support), and 4.0 and 5.3 (cf. bulk IL) for S12-9 and S12-25, respectively.

In the third set of sorbents, the uptake capacities of S60-0, S60-10 and S60-26 were 0.65, 2.29 and 2.74 mmol g⁻¹, respectively. The capacity of the bare support is equivalent to that of the bulk EZT3. As above, the capacity significantly increased after impregnation: 3.5 and 4.2 (cf. bare support), and 3.6 and 4.3 (cf. bulk IL) for S60-10 and S60-26, respectively.

Clearly, each of the SILP systems exhibited significant increases in their CO₂ uptake capacity as compared with their bare support and bulk EZT3, suggesting a synergy exists between the support and the adsorbed IL layer in enhancing the overall performance of these materials.

3.5. CO₂ sorption kinetics

Each of the SILPs' IPDM kinetic plots for S5-x (Fig. 5a), S12-x (Fig. 5b) and S60-x (Fig. 5c) exhibit up to three linear regions indicating that CO₂ adsorption mechanism involves up to three steps.⁴⁸ These steps include: (1) external surface adsorption, (2) gradual adsorption dominated by intra-particle diffusion, and (3) equilibrium adsorption. The sorption kinetics of the bulk EZT3 and the bare supports is characterized by a 2-step process. The absence of appreciable surface area or porosity in the bulk EZT3 implies CO₂ uptake is due to surface adsorption and reaction followed by diffusion into the bulk phase. Liu et al.²⁷ ascribed the gradual uptake to the high viscosity of the IL and the formation of a semi-permeable layer upon contact with CO₂, retarding mass transport.⁴⁹ In the case of the bare supports, the two regions were assigned to instantaneous adsorption and diffusion through the pore network, similarly observed in several zeolitic materials.⁵⁰

Fig. 5 IPDM kinetic plots of various sorbents and bulk EZT3: (a) S5-x, (b) S12-x, and (c) S60-x. Experimental data (symbols) and IPDM fitted curve (red solid line). Bulk EZT3 (■, black), S5-0 (●, cyan), S5-13 (♦, light blue), S5-25 (▲, blue), S12-0 (●, yellow green), S12-9 (♦, green), S12-25 (▲, dark green), S60-0 (●, yellow orange), S60-10 (♦, orange), S60-26 (▲, red). Insets are enlarged plot of the initial region.

The linear slopes corresponding to sorption steps 1, 2 and 3 yield rate constants k₁, k₂ and k₃, respectively. These have been correlated with EZT3 loading, SSA and V_p in Fig. S3.† Each of the sorbents exhibit a positive correlation with EZT3 loading, i.e., the rate of uptake increases with increasing EZT3 loading highlighting the role of the IL in the adsorption kinetics. The rate constants increase despite the decrease in SSA and V_p, suggesting that the presence of the adsorbed IL layer compensates for the decrease in the textural properties of the sorbents. For each sorbent, k₁ < k₂ indicating that surface adsorption is the rate-limiting step in the overall CO₂ uptake kinetics. Among the parameters correlated, the rate constants exhibit good to modest correlation in the order of V_p > EZT3 loading > SSA.

4. Discussion

The shortage of reports on the utilization of non-amine-functionalized IL for CO₂ capture prompted our group to explore transition metal-functionalized ILs.²⁶ Among the many novel ILs tested, EZT3 exhibited the most promising CO₂ uptake; enhancement of up to a factor of 10 can be achieved at 313 K and 1.0 bar as compared to the non-functionalized ET. However, having the general problem of functionalized IL, EZT3's high viscosity, and the formation of a semi-permeable layer on the surface upon its reaction with CO₂ results in very slow kinetics.²⁷ To fully realize the potential of this novel IL, we contacted a thin layer of EZT3 onto the pore surface of various supports. This strategy promotes the CO₂ uptake and sorption kinetics by exposing more sorption sites across the surface of the support, and by facilitating diffusion through the pore network. Three types of solid supports with comparable SSA were contacted with increasing amounts of EZT3 to afford SILP sorbents designed as CO₂ capture media. The supports were chosen to confront the challenge of finding the optimal IL-support combination by exploring the effect of pore properties on the nature of the IL layer-support interaction and the combined effect of the IL and the support on the performance of the sorbents towards CO₂ capture.

The process of impregnating the porous supports with dilute methanolic EZT3 solution aided by ultrasonication proved to be an effective methodology to introduce the IL onto the pore surfaces. As shown in Fig. 1, the SSA and V_p of the sorbents decrease with increasing EZT3 loading indicating the deposition of the IL on the surfaces of the support. Moreover, the shape of the PSD and the position of the mean were preserved, with the decrease in peak intensity indicating the presence of the adsorbed IL layer only. These EZT3 loading-dependent changes in the textural properties of the SILP systems indicate ultrasonication has no detrimental effect on the integrity of the pore structure. If we assume an ideal case where all the EZT3 penetrates the pores, the pore volume lost due to impregnation (V_p,lost) would be equal to the volume of the EZT3 (V_EZT3) loaded. The difference (ΔV = V_p,lost − V_EZT3) can be used to interpret the degree of pore inclusion.⁵¹ A negative value suggests that a certain amount of EZT3 is located on the external surface of the sorbent. A positive value may indicate some degree of pore blocking resulting in some pores becoming unavailable for N₂ adsorption. Of course, it is also possible to have both cases, wherein the sign of the ΔV will depend on the relative magnitudes.⁵² Drese et al.⁵¹ have demonstrated that the negative values of ΔV in hyperbranched amino-polymer immobilized into porous silica indicate the preferential external-surface polymerization. Chen et al.⁵² ascribed the negative ΔV, in PEI/polymer resin hybrid sorbents, to the amount of PEI not loaded on the pores of the resin, and the positive ΔV to the blocking of pores in the sorbents with high PEI loading. Table 2 summarizes the aforementioned volumetric parameters for our porous systems. No data were calculated for S12-x since the pore volumes measured corresponded to ill-defined inter-particle porosity. Only S5-25 has a negative ΔV suggesting that an excess of EZT3 creating an IL layer on the external surface. This is consistent with the results of the N₂ adsorption experiments where the mesopores were completely filled while the IL experienced molecular sieve effects with the micropores. For S5-13, S60-10 and S60-26, we speculate that both the deposition on the external surface and pore blocking occurred based on the positive value of ΔV. Small angle neutron scattering (SANS) studies are currently being explored to provide unequivocal resolution on which of the two cases predominates, and how much volume is occluded if pore blocking occurred. Results of these SANS analyses will be reported separately.

Table 2 Pore volume changes after EZT3 impregnation

Sample	Vp^a	VEZT3^b	Vp,lost^c	ΔV ^d
	(cm^3 g^−1)
a NLDFT pore volume.b Calculated as EZT3 loading divided by the density of EZT3 (1.824 g cm^−3).c Calculated by subtracting the Vp of the sorbent from the theoretical Vp of the bare support provided in the sorbent.d Calculated by subtracting VEZT3 from Vp,lost.
S5-13	0.09	0.07	0.08	0.01
S5-25	0.06	0.14	0.08	−0.06
S60-10	0.36	0.06	0.11	0.05
S60-25	0.39	0.14	0.16	0.01

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Significant shifts in several characteristics peaks of EZT3 were observed in all SILP systems. The larger shifts observed in S5-13, S12-9 and S60-10 as compared with S5-25, S12-25 and S60-26, respectively, indicate the existence of a highly confined IL phase which strongly interacts with the surface of the support.⁵³ These hypsochromic shifts are similarly present in [C₁₆mim][BF₄] and [C₁₆mim][CH₃SO₄] in mesoporous silica gels prepared by Zhang et al.,⁵⁴ who ascribed them to conformational disorder induced by the interaction of the IL with the support. As with any adsorption process, by increasing the loading, a thicker layer of the adsorbed IL phase develops, the effect of confinement is reduced and the layer begins to exhibit bulk phase properties. Our previous contribution has shown, via DRIFTS analyses, that EZT3 monotonically adsorbs on silica surface, developing multi-layer coverage at higher IL loading,¹¹ establishing the compatibility between EZT3 and SiO₂ surface, which conforms with the process optimization suggested by Heinze et al.⁴⁰ The presence of EZT3 characteristic peaks in the SILP sorbents strongly suggests that the energy provided by ultrasonication caused no IL decomposition, and the shifts observed are signatures of IL confinement on the surface. Comparing the shifts based on the support, the magnitudes are in the order of S12 > S60 > S5, suggesting that nano-SiO₂ has a greater capacity to surface-confine the adsorbed IL as compared with S60 and S5. We speculate that this arises from the development of an adsorbed IL layer on the agglomerates forming a silica core and an IL shell. At 25 wt% loading, the greater excess of IL, beyond a theoretical monolayer, resulted in a greater extent of particle agglomeration. In contrast, the internal porosity of S5 and S60 promotes spreading on the pore walls as opposed to external surfaces. These modes of pore-filling deduced from the evolution of textural properties and structural signatures as a function of EZT3 loading, as manifested in N₂ adsorption and DRIFTS, are illustrated in Fig. 6.

Fig. 6 Illustrative representation of bare (top) and SILP (bottom) sorbents: (a) S5-x, (b) S12-x and (c) S60-x.

TGA analyses (Fig. 3d) explicitly show that the reduction in thermal stability is more pronounced for the highly confined systems, i.e., S5-13, S12-9 and S60-10. It is possible that the wetting of the IL on the surface of the support, which exposes more IL molecules, makes the overall system more susceptible to degradation. Moreover, the interaction of the IL with the surface, as manifested in the observed shifts in characteristic peaks in the DRIFTS spectra of the SILP sorbents, weakens the interaction of the ionic components of the IL.^55,56 Singh et al.⁴⁷ and Gobel et al.⁵⁷ both observed the reduction in thermal stability upon confinement of IL into porous substrates. The confinement results in multi-step decomposition initiated by the decomposition of the exposed alkyl chain, followed by the imidazolium moiety. This mechanism is consistent with the reduction of electrostatic interaction due to the IL-surface interaction upon confinement. Developing a thicker layer, such as in the case of S5-25, S12-25 and S60-26, reduces the effect of surface confinement and promotes the interaction of the component ions leading to an increase in thermal stability approaching bulk behavior. Comparing the effect of confinement across the different supports, the magnitudes of ΔT_d,onset with respect to the T_d,onset of the bulk EZT3 are in the order of S12 > S60 > S5. This order agrees with the DRIFTS analyses. Together, these analyses strongly suggest the stronger confining ability of a core–shell support-IL configuration as compared with adsorbed IL on pore surfaces.

Silica and zeolites can interact with CO₂ via physisorption, prompting the exploration of these materials as CO₂ capture media.^7,19 ZSM-5 (S5-0) is a well-established zeolitic material with micropores that can efficiently trap CO₂, making it a good candidate for CO₂ capture.⁵⁸ Our result for S5-0 (1.62 mmol g⁻¹) is comparable with that reported by Ohlin et al.⁵⁸ (1.50 mmol g⁻¹). As shown, impregnation of ZSM-5 with EZT3 promotes CO₂ uptake by introducing high affinity IL–CO₂ interaction via chemisorption. In the case of non-porous nano-SiO₂ (S12-0), we only found two reports of their use: (1) PEI–SiO₂ hybrid sorbents,⁵⁹ and (2) inverse-SILP sorbents,³⁹ probably due to the lack of porosity in this material. On the other hand, the use of porous nano-SiO₂ is well-documented.^7,60 The effect of the lack of porosity is clearly manifested in S12-0 by the low CO₂ sorption capacity (0.21 mmol g⁻¹). Unfortunately, no comparison can be made since the performance of the bare support in the previously published studies is not reported. Interestingly, our SILP results showed enhancement by a factor of up to 16, making them the best performing materials in this contribution. For the non-ordered mesoporous silica (S60-0), the presence of the mesopores increased the uptake (0.65 mmol g⁻¹) as compared with our non-porous system, but still below the uptake of our microporous sorbent. This observation emphasized the importance of pore size in the design of CO₂ sorbents, as previously highlighted by Son et al.³⁵ and Zeleňák et al.³⁶

Dramatic enhancement in sorption capacities have been observed in all SILP systems produced as compared with their corresponding bare supports and the bulk EZT3. To put the current work into perspective of the previously published supported sorbents,^28–34 Fig. 7 showcases the outstanding performance of our hybrid sorbents. We benchmark the performance of our sorbents with a diverse set of organo-amines, amidines, and ILs which interact with CO₂ via chemisorption, similar to EZT3. Several materials with high organic loading clearly outperform our reported sorbents. However, for the hybrid sorbents with similar levels of loading (<30 wt%), our novel SILP sorbents significantly surpasses these materials. This remarkable high capacity achieved using low IL loading and relatively common support materials could probably translate to cheaper and more sustainable production of sorbent materials.

Fig. 7 Comparative CO₂ uptake capacity of various hybrid sorbents. S5-x (●, color), S12-x (▲, color), S60-x (♦, color), DBUOH/amorphous silica@298 K (ref. 28) (◊, ○), DBUOH/SBA-15@298 K (ref. 28) (Δ), CP/SBA-15@298 K (ref. 29) (●, black), AO/SBA-15@298 K (ref. 30) (♦, black), Arg/PSS-PMMA@298 K (ref. 30) (□), EmimGly/PMMA@313 K (ref. 31) (▲, black), AM-TEPA/DMS@298 K (ref. 32) (■, black), PEI/clay@348 K (ref. 33) (×), TEPA/SiO₂ aerogel@348 K (ref. 34) (+, *). The colored band shows range; red solid line indicates average. DBUOH – 1.8-diazabicyclo[5.4.0]undec-7-ene hydroxylamidine, CP – cyanopropyltriethoxysilyl, AO – amidoxime, Arg – arginine, PSS – polystyrene sulfonate, PMMA – polymethylmethacrylate, EmimGly – 1-ethyl-3-methylimidazolium glycinate, AM – acrylamide, TEPA – tetraethylenepentamine, DMS – disordered mesoporous silica, PEI – polyethyleneimine.

In general, the capacities increase with increasing EZT3 loading with good to modest correlation (Fig. S3†); S5-x (R² = 0.96), S12-x (R² = 0.82) and S60-x (R² = 0.80). This trend is expected considering that the long adsorption time rendered kinetic effects negligible, with the overall uptake capacity governed by accessibility of adsorption sites.⁵¹ The trend (Fig. 8a) highlights the positive dependence of the uptake with the concentration of EZT3, and its spreading as a thin layer (with strong affinity to CO₂) on a surface, resulting in the exposure of more adsorption sites. Aside from EZT3 loading, the CO₂ uptake is strongly correlated with V_p (Fig. 8b): S5-x (R² = 0.99) and S60-x (R² = 0.92). Exclusion of S12-x in the correlation is due to the absence of inherent porosity in these systems. These correlations in S5-x and S60-x suggest that both the IL and the pore properties of the support synergistically promote overall uptake capacity. The synergy is also present in S12-x, more pronouncedly, as evidenced by the highest uptake capacity observed in S12-25, but the enhancement originates from the unique core–shell configuration rather than the pore surface-mediated enhancement observed in S5-x and S60-x. In S12-x, the surface-coating of the nanoparticles with EZT3 effectively makes the sorption sites readily available for direct interaction with CO₂. In addition, the core–shell configuration addressed the high tortuosity in porous systems which has a negative impact on sorption capacity and kinetics.³⁹

Fig. 8 Correlation plots of CO₂ uptake with (a) EZT3 loading and (b) V_p, and (c) k₂ with V_p. S5-0 (●, cyan), S5-13 (♦, light blue), S5-25 (▲, blue), S12-0 (●, yellow green), S12-9 (♦, green), S12-25 (▲, dark green), S60-0 (●, yellow orange), S60-10 (♦, orange), S60-26 (▲, red). Fitted lines indicate trends. Crosshairs indicate mean capacity and standard deviation. S5-x (blue), S12-x (green) and S60-x (red). Extended correlations of CO₂ uptake, k₁ and k₂ with EZT3 loading, SSA and V_p are in Fig. S3.†

Sorption kinetics is an important parameter in evaluating the performance of CO₂ sorbent materials. The multi-step mechanism deduced using the IPDM kinetics model involved three steps; external surface adsorption, gradual adsorption and equilibrium adsorption, where external surface adsorption is the rate limiting step. The higher k₁ value for all SILP sorbents as compared with their corresponding bare supports and bulk EZT3 suggests the existence of a thin layer of IL on the external surface of the sorbents, corroborating with the calculated ΔV; the positive values in S5-13, S60-10 and S60-26 suggests the coexistence of external surface deposition and pore-blockage, and the negative value for S5-25 indicates completely filled pores with the excess EZT3 deposited on the external surfaces of the sorbents. k₁ has a modest correlation with EZT3 loading, SSA and V_p for S12-x and S60-x (Fig. S3†). However, excellent correlation between k₁ and EZT3 loading (R² = 0.99) was found in S5-x reinforcing the notion of externally deposited EZT3 layer on these sorbents. The second linear IPDM region is ascribed to the gradual adsorption dominated by intra-particle diffusion through the pore network, characterized by k₂. A positive correlation of k₂ with EZT3 loading can be established [S5-x (R² = 0.99), S12-x (R² = 0.84) and S60-x (R² = 0.79)], associated with the deposition of EZT3 on the pore surfaces of the support. Similar to k₁, SSA has a modest correlation with k₂ since the SSA corresponds to the overall surface of the support. k₂ correlates well with V_p [Fig. 8c, S5-x (R² = 0.90) and S60-x (R² = 0.91)], albeit inversely. The increase in k₂ despite the reduction in V_p showcases the chemisorptive nature of EZT3–CO₂ interaction as a critical factor in describing the sorption kinetics, which is similarly observed in porous solid-supported amine systems.^61–64 Lastly, the equilibrium step represented by k₃ varies minimally with respect to textural properties, as expected, considering that this slow adsorption is governed by physical adsorption to any residual active site. These results strongly suggest that by confining a thin layer of IL, with strong IL–CO₂ interaction, on the surface of a porous support, enhanced kinetics can be achieved originating from the exposition of more adsorption sites and the removal of the mass transfer retardation due to the high viscosity of the bulk IL. Moreover, the porous nature of the support contributes to the enhancement of kinetics by offering extensive pore networks where the adsorptive can diffuse and subsequently adsorb as implied by k₂ > k₁.

5. Conclusions

Novel SILP hybrid sorbents consisting of zinc-functionalized IL impregnated into silica and alumino-silicate substrates with diverse morphology are reported, and their CO₂ capture performance (capacity and kinetics) were systematically investigated. Remarkable features of these new materials include facile preparation, low cost of support, high sorption capacity and enhanced kinetics. Up to 16-fold enhancement of capacity was readily achieved compared with the bare support; 5.3-fold with respect to bulk IL. The CO₂ sorption capacity of 1.6–3.4 mmol g⁻¹ were in the top tier compared with similar low loading hybrid sorbent systems reported in the literature. Diffusion of CO₂ to access the active sites within the pores dominated the sorption kinetics. This contribution showcases a simple route towards the preparation of novel high capacity hybrid sorbents, with emphasis on the synergistic enhancement of CO₂ uptake capacity and sorption kinetics offered by the high affinity of CO₂ with EZT3, the exposition of more active sites on surface of the support, and the accessibility of these active sites via diffusion to the sorbent's pore network.

Acknowledgements

This work was supported by the University of South Australia and CSIRO Energy Flagship. I. H. A. acknowledges the awards of an International President's Scholarship from the University of South Australia and a Postgraduate Studentship from CSIRO. The authors thank Chee-Ling Tong (Flinders University) and S. Hadi Madani (University of South Australia) for their help in acquiring N₂ adsorption data.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Structure of EZT3, TGA curves for bulk IL, bare supports and hybrid sorbents, and correlation plots of CO₂ uptake and rate constants with EZT3 loading and textural properties. See DOI: 10.1039/c5ra12738f

‡ Present address: School of Chemistry, Monash University, Clayton, VIC 3800, Australia. Email: E-mail: jewel.huang@monash.edu.

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