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Improved CO2 sorption in freeze-dried functionalised amine mesoporous silica sorbent Wenjing Wang, Julius Motuzas, Xiu Song Zhao, and João Carlos Diniz da Costa Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00129 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Industrial & Engineering Chemistry Research

Improved CO2 sorption in freeze-dried functionalised amine mesoporous silica sorbent Wenjing Wang, Julius Motuzas, Xiu Song Zhao, João C. Diniz da Costa* The University of Queensland, FIM2Lab - Functional Interfacial Materials and Membranes Laboratory, School of Chemical Engineering, the University of Queensland, Brisbane QLD 4072, Australia. * Corresponding author - Ph: +61 7 33656960, Fax: +61 7 33654199, Email: [email protected]

Abstract This work investigates the structural and CO2 sorption properties of amine-functionalised mesoporous silica prepared by a freeze-drying method. It was found that the freeze-dried samples delivered significantly higher CO2 sorption capacities up to a factor of 18 times as compared to samples prepared by the conventional oven-drying evaporation method. The improved performance of the freeze-drying method was attributed to a higher surface area and access to active amine sites for CO2 sorption. The freeze-dried samples also exhibited a good stability at a relatively high sorption temperature (90 ℃) and flue gas CO2 partial pressure (15 kPa). The freeze-drying method reached the best CO2 sorption capacity of 4.71 mmol g-1 for samples with 70 wt% amine loading. Higher loading (80 and 90 wt%) led to excess amine not being incorporated into the mesoporous silica structure, thus covering active sites and limiting CO2 diffusion.

1. Introduction The continuous increase of atmospheric CO2 emissions from burning fossil fuels is an ongoing global concern owing to climate change effects. It is now estimated that weather change patterns are directly associated with 5-20% economic loss of the world’s gross domestic product.1 Worldwide efforts to limit climate change started as early as 1992 with the United Nations Framework Convention on Climate Change at the Earth Summit in Rio de Janeiro, which was followed by the Kyoto Protocol in 1997.2 Very recently, a third international gathering was held in 2016 where 175 nations signed the Paris Climate Change Agreement. Central to these global efforts is carbon capture technologies, particularly those related to capturing CO2 from coal power stations,3,4 which accounts for 28% of the global CO2 emissions.5. A number of approaches to reduce CO2 emissions have been considered in the last

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30 years including post carbon capture (PCC), coal gasification and oxy-fuel coal combustion with pilot trials already established in several places around the world. PCC is in the forefront, a first generation technology that can be retrofitted at the back end of coal power stations, with pilot plant development implemented6 and a full plant in operation (Boundary Dam Power Station) in Canada capturing 1 million tonnes of CO2 per year.7 PCC utilises conventional gas separation technology such as absorption using amine solvents and adsorption based on solid sorbents. There are a variety of solid sorbents that have attracted the attention of the research community for carbon capture. Examples include activated carbons,8,9 zeolites,10 silicas,11,12 hydrotalcite clays,13-15 and metal-organic frameworks (MOFs).16,17 Hybrid materials containing both solvents and sorbents have been developed, particularly focusing on increasing CO2 capacity and selectivity at low partial pressure, which is the case of flue gas from coal power stations containing CO2 at 10-15 kPa.18 A major strategy here is to couple organic amines containing numerous amine functional groups with high surface porous solid supports.19 Amine functionalized silica was firstly reported by Leal et al.20 for CO2 capture. Since then, several reports using this hybrid method have been published on silicas, including MCM-n,21 SBA-n,22 silica hollow spheres,23 mesocellular silica foam,24 mesoporous silica tube25 and aerogels.26 A number

of

appropriate

amine

precursors

have

been

used

such

as

polyethyleneimine (PEI),27,28 diethylenetriamine,29 tetraethylenepentamine (TEPA),30 mono-, di-, triaminosilanes31,32 to enhance the CO2 capture capacity, whilst the wet impregnation technique has been extensively used to functionalize silica with amine.33 For instance, Fujiki et al.34 reported 118% increase in CO2 uptake of TEPA impregnated silica as compared to nonmodified support. Structural features are also an important consideration, as the morphology of PEI within the mesoporous silica35,36 can affect the CO2 capture capacity and uptake efficiency of sorbents. For instance, high pore volume and diameter of mesoporous silica enhanced the quantity of amine loadings, resulting in higher CO2 sorption capacity. The wet impregnation process requires evaporation-drying where a variety of ancillary methods have been used to obtain the final hybrid amine/solid sorbent. The use of a rotary evaporator for the preparation of PEI-impregnated SBA-15 containing Zr heteroatom delivered the lowest CO2 2 ACS Paragon Plus Environment

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sorption capacity of 1.56 mmol g-1 (at 25 ℃ and 10 kPa CO2).37 Oven dried at ambient pressure was the method used in the preparation of a modified organic template impregnated TEPA which reached a CO2 sorption capacity of 7.61 mmol g-1 (at 75 ℃ and 100 kPa CO2).38 The application of a slight vacuum pressure (700 mmHg) oven dried method for the synthesis of MCM-41-PEI samples resulted in a CO2 sorption capacity of 5.85 mmol g-1 (at 75 ℃ and 100 kPa CO2).39 All these reports mainly focused on materials and amine modification rather than the effect of the drying method in the final performance for the hybrid sorbents.

There are other drying methods for the preparation of hybrid CO2 sorbents, which remain unexplored. One of the methods of interest is freeze-drying, which has been employed to avoid the structural collapse of aerogels.40,41 Therefore, this work investigates the preparation of hybrid CO2 sorbents prepared by the freeze-drying method. The hybrid sorbents contain a solid matrix based on mesoporous silica SBA-15 and a solvent based on TEPA as the amine source. The as-synthesised materials were fully characterised and tested for CO2 sorption under various regimes of sorption and desorption, in addition to cycling performance. The corresponding materials were prepared by an oven-drying method followed by materials characterisation and CO2 sorption for comparison purpose.

2. Experimental 2.1. Materials synthesis Pluronic P123 block copolymer (EO20PO70EO20, Sigma-Aldrich), tetraethylorthosilicate (TEOS, 98 wt%, Sigma-Aldrich), TEPA (technical grade, Sigma-Aldrich), aqueous ammonia (25 wt%, Merck) and HCl (32 wt%, Ajax Fimechem) were used as received without any further purification. SBA-15 was prepared using 2g P123, 15g H2O, 60g 2M HCl and 4.25g TEOS as per method published elsewhere.42 All mixtures were stirred for 24 h at 40 ℃ and then aqueous ammonia was added dropwise until the pH reached 8. The resultant mixtures were kept stirring for a further 2h before centrifugation and drying in an oven at 60 ℃ for 24h. The resultant dried powders were calcined in air atmosphere at 550 ℃ for 3 h to remove the P123 template.

The sorbents were prepared via the wet impregnation method where freeze-dried samples are denoted the prefix F as F-SBA-15/ TEPA (x) and oven-dried samples without the prefix as SBA-15/ TEPA (x), where x represents the weight percent of TEPA (30≤x≤ 90 wt%). The 3 ACS Paragon Plus Environment


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desired amount of TEPA was firstly dissolved in water (10 ml) under stirring for 10min. Subsequently, an amount of calcined SBA-15 was added to the mixture, which was kept under vigorous stirring at the room temperature for 24 h. In the case of the freeze-dried F-SBA15/TEPA samples, the resultant mixtures were exposed to liquid nitrogen (-196 ℃) for 30 min. Then the samples were housed in a freeze drier at -65 ℃ for 48 h with a vacuum pump, which allowed for the removal of frozen water by sublimation. To obtain the conventional SBA15/TEPA, the mixtures were kept in a drying oven at 60 ℃ until the samples were totally dried.

2.2. Materials characterization and CO2 sorption The structural properties of the sorbents were measured using liquid nitrogen at 77 K via a Micromeritics Tristar 3020. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method and the pore size distribution was determined by the non-local density functional theory (NLDFT) model from Micromeritics software. Prior to the measurements, the samples were degassed at 90 ℃ overnight to remove any adsorbed moisture. Small angle X-ray diffraction (XRD) patterns were collected at 40 kV and 20 mA with a graphite monochromators (Bruker D8 Advance) using filtered CuKα radiation. Fourier transform infrared (FTIR) was carried out using an attenuated total reflection (ATR) in an IRAffinity-1 Spectrometer (Shimadzu), with a wavenumber range of 600-4000 cm-1. The FTIR background was deducted from the spectra measurement of all samples. X-ray Photoelectron Spectroscopy (XPS) was carried out on a Kratos Axis Ultra photoelectron spectrometer with mono Al Kα (1486.6 eV) x-rays. A JEOL field emission scanning electron microscope (FE-SEM, JEOL JSM-7001F) operating at 5 kV was used to examine the morphology of freeze-dried and ovendried samples.

The thermal decomposition properties of the samples were analysed from 25 to 500 ℃ at a rate of 10 ℃ min-1 in air using a thermogravimetric analyser (TGA/DSC 1, Mettler-Toledo). The CO2 sorption capacities of the sorbents were measured using the same TGA equipment. Initially, the samples were exposed under pure N2 (60 ml min-1) at 90 ℃ for 30 min to remove any adsorbed CO2 and moisture. Subsequently, the feed gas was changed to a mixture of CO2/N2 at 15:85 volume ratio at a flow rate of 60 ml min-1. The samples were kept under this simulated flue gas condition for 30min also at 90 ℃. Several sorption and desorption experimental tests were carried out from 25 to 120 ℃, in addition to sorption/desorption cycling testing between 75-120 ℃.


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3. Results and discussion 3.1. Material Characterization Initially, the samples were tested for CO2 sorption and the results are displayed in Fig. 1. It is observed that the freeze-dried samples always had a higher CO2 sorption capacity as compared to the conventional oven-dried samples. The difference in CO2 capacity was very large at 70 and 80% TEPA content, though small at low (30%) and high (90%) TEPA. These results are very interesting and suggest that the use of different drying methods may have influenced the structural formation of composite sorbents, or even changed the chemical make-up of the materials, thus affecting the final CO2 sorption properties.

Fig. 1. Difference of CO2 capacity as the freeze-dried (qfd) F-SBA-15/TEPA minus the oven dried (qod) SBA-15/TEPA at 90 ℃ and 15 kPa CO2.



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Fig. 2. (a) N2 adsorption/desorption isotherms, (b) BET surface areas and pore volumes, and (c) pore size distribution based on the NL-DFT model.

Subsequently, the samples were analysed using N2 sorption to determine their structural properties with full pore characteristic details listed in Table S1 (supplementary information). The isotherm of a pure SBA-15 sample in Fig. 2a exhibit an initial uptake at lower relative pressures, and a hysteresis at 0.650%), the spectra started losing their SBA-15 features, and gain TEPA characteristics. These results suggest that TEPA was mainly imbibed into the SBA-15 7 ACS Paragon Plus Environment


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mesoporous structure for samples loaded with TEPA ≤50% whilst higher loading (70% or higher) resulted in the excess TEPA covering the external surface of SBA-15. The differences between both drying methods are mainly noticeable for samples with TEPA >50%. In the case of the oven-drying method, the bands at 1604 and 1458 cm-1 of TEPA corresponding to N-H bending vibration shifted to 1576 and 1465 cm-1, respectively. These shifts can be attributed to the formation carbamate (C-N bond),46 indicating that reaction of CO2 with the amine groups took place during the oven drying method. Furthermore, it is interesting to notice an intense peak appearing at 1309 cm-1 assigned to O-C bonds. These changes are not observed in the spectra of the freeze-drying method.

Fig. 4. FTIR spectra of (a) SBA-15/TEPA and (b) F-SBA-15/TEPA as a function of TEPA concentration.

Fig. 5. FTIR spectra of (a) SBA-15/TEPA (70) and (b) F-SBA-15/TEPA (70) as prepared and after exposure to N2, CO2 sorption and desorption. 8 ACS Paragon Plus Environment

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Further tests in Fig. 5a show the peak at 1309 cm-1 of the as-prepared SBA-15/TEPA (70) decreased after keeping this sample in N2. The same peak does not appear for the as-prepared freeze-dried sample (Fig. 5b). These results clearly indicate that the bonds at 1309 cm-1 formed because of atmospheric CO2 sorption/reaction during evaporation by the oven-drying method. FTIR spectra of both samples were also analysed after CO2 sorption and desorption. The peak at 1309 cm-1 remained in the case of the oven-dried sample. However, this peak appeared at a small intensity value for the freeze-dried sample during sorption, then disappeared during desorption. These results suggest that the oven-drying method formed an irreversible strong OC bond. The increase in the intensity of the peak at 1309 cm-1 is attributed to the reactions occurred during CO2 sorption as follows:47 2RNHଶ + COଶ ↔ RNHଷା + RNHCOOି

(1)

2Rଵ R ଶ NH + COଶ ↔ Rଵ R ଶ NHଶା + Rଵ R ଶ NCOOି

(2)

where R, R1 and R2 represent alkyl groups. As the intensity of the peak at 1309 cm-1 reduced during the desorption process, then amine functional groups were regenerated and these reactions were reversible, though not the case for the oven-dried samples.

Fig. 6a depicts the TGA curves of TEPA, F-SBA-15/TEPA and SBA-15/TEPA loaded with various amount of TEPA calcined in air. The mass loss curves of F-SBA-15/TEPA and SBA15/TEPA resembled each other up to 70 wt% TEPA loading. For the samples with higher TEPA loadings of 80 and 90 wt %, the mass loss curves diverged for temperatures below 250 ℃, and then converged for temperatures above 250 ℃. Overall, the total mass loss of all samples were very similar, indicating that the amount of loaded TEPA was very similar irrespectively of the drying method. The only minor variation is for 90% TEPA high loading. These results suggest that the wet impregnation method was responsible for obtaining the appropriate homogeneous loading of TEPA to SBA-15, which was not affected by the drying method used. In addition, mass losses in Fig. 6a are attributed to H2O and/or CO2 (up to 100 ℃), partial decomposition of loaded TEPA (100-255 ℃), and the combustion of residual amine species (>255 ℃). The DSC results in Fig. 6b show an exothermic peak corresponding to TEPA decomposition for all samples. It is interesting to note that this peak was similar at the same 9 ACS Paragon Plus Environment


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temperature (~195 ℃) for both freeze-drying and oven-drying method for samples loaded with TEPA between 30 and 70 wt %. For higher TEPA loading (80 and 90 wt %), the peaks of SBA15/TEPA shifted to higher temperature, and much closer to that of pure dense TEPA. These results clearly indicate that excess TEPA may not interact with the SBA-15 mesoporous structure.

Fig. 6. (a) TGA curves and (b) DSC curves of TEPA, F-SBA-15/TEPA and SBA-15/TEPA samples calcined in air.

Fig. 7 shows the ratio of Si to N for both samples, which decreased as a function of TEPA content. This ratio was determined from XPS analyses, which gives an indication of surface coverage up to 4 nm.48,49 As this ratio reduces, it means that the concentration of the nitrogen containing TEPA coverage increases on the external surface of the silica rich SBA-15. When the Si/N ratio reached zero, then TEPA coverage was higher than a thickness of 5 nm. It is interesting to observe that the Si/N ratio of the freeze-dried samples was always higher than that of the analogue oven-dried samples. In other words, the latter had a higher thickness coverage of TEPA on the external surface of SBA-15 than the former. As the amount of TEPA for both samples was almost the same as determined by TGA analyses (Fig. 6a), these results strongly suggest that the freeze-dried method provided superior distribution of the TEPA on both internal and external surfaces of SBA-15. Contrary to this, the limited access of TEPA into the SBA-15 by the oven-dried method may be caused by the partial structural collapse of the mesopores during evaporation of water. These two morphological features for both freeze-dried and oven-dried methods are schematically shown in the inset of Fig. 7. 10 ACS Paragon Plus Environment

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Fig. 7. The ratio of Si to N as a function of TEPA content.

Fundamentally, the structural collapse of the oven-dried samples is attributed to the hydrolytic attack of water on silica surfaces,50, 51 resulting in silica dissolution and re-deposition, and causing structural changes as reported for SBA-15.52 This is evidenced by the reduction of total pore volume (Fig. 2a) and surface area (Fig. 2b) as compared to the freeze-dried method. In addition, representative SEM images in Fig. 8 show that different morphological features for each drying method. For instance, the ovendried SBA-15 (30) sample (Fig. 8a) lost its rod-like shape resulting in irregular morphology. Contrary to this, the freeze-dried F-SBA-15 (30) sample maintained its regular rod-like shape, thus avoiding structural changes and confirming the proposed structural schematics in the inset of Fig. 7. Further support is given by the XRD results (Fig. 3) where the oven drying method delivered less intense XRD peaks. This could be attributed to the partial loss of mesopore ordering together with the reduction of X-ray signal caused by the excess TEPA on the outer surface of the oven-dried SBA-15. Wide-scan XPS spectra containing specific surface elemental compositions are provided in Fig. S1 (supplementary information). Samples were also prepared using a rotary evaporator for comparison purpose. Table S1 (supplementary information) shows that the oven-dried and rotary evaporated/dried samples resulted in similar BET surface areas and pore volumes. This is a further proof that the freeze-dried method conferred superior structural integrity of the TEPA loaded SBA-15 sorbents.



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Fig. 8. Representative SEM images of SBA-15 (30) samples prepared by (a) the oven-drying and (b) freze-drying methods.

3.2. CO2 sorption The CO2 sorption performance of F-SBA-15/TEPA and SBA-15/TEPA loaded from 30 to 90 wt% TEPA was tested at 90 ℃ (see Fig. 9a). The CO2 capacity of conventional oven-dried SBA-15/TEPA reached the highest value for 50 wt% TEPA at 2.96 mmol g-1. It then dropped dramatically with further increase of TEPA content above 50 wt%. This result is consistent with the Si/N ratio in Fig. 7, thus indicating poor CO2 accessibility to the active sites for sorption. However, the CO2 capacity of F-SBA15/TEPA kept a significant sustained growth with increasing TEPA from 30 to 70 wt%, peaking at 4.71 mmol g-1. For comparison purpose, Fig. 3 shows that the CO2 sorption capacity of the oven-dried and rotary evaporator-dried samples were very similar with no significant differences. These results strongly suggest that the freeze-drying method provided a more homogeneous distribution of TEPA within the SBA-15 structure and access to the active sites for CO2 sorption. The active sites are those related to the band at 1604 and 1458 cm-1 of TEPA corresponding to N-H bending vibration as determined by FTIR (see Fig. 2) which reacted with CO2 to form carbamate (C-N bond). As the TEPA increased to >70 wt%, the DSC results in Fig. 6b shows that the excess TEPA in the composite sorbents caused a shift in the exothermic peak towards the pure TEPA peak. These results may imply that excess TEPA is blocking the access of CO2 to the active sorption sites in the F-SBA-15/TEPA. The CO2 uptakes of F-SBA-15/TEPA (70, 80) were 5.3 and 18 times higher than those of SBA-15/TEPA (70, 80), respectively. The normalised capacity of CO2 sorption per N content in the amine, also known as amine efficiency53 as mmolCO2 mmolN-1, is presented in Fig. 9b. The N content was determined from the amount of TEPA determined from the TGA mass loss curves (Fig. 6). For the TEPA content in the range of 30-50 wt%, 12 ACS Paragon Plus Environment

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the amine efficiency for both samples was around ~0.25 to ~0.30. These values are in line with amine efficiencies of 0.3 reported for amine functionalised SBA-15.54 The freeze-dry F-SBA-15/TEPA consistently delivered higher amine efficiencies than the oven-dried SBA-15/TEPA by around 20% (30 to 50 wt% TEPA), though significantly higher by a factor 5.4 (70 wt% TEPA) and 17.8 (80 wt%TEPA) times. The low amine efficiency of SBA-15/TEPA, particularly for TEPA ≥ 70 wt%, suggests that many of the amine active sites for the oven-dried sample were no longer accessible to CO2 molecules. Compared to conventional oven-dried SBA-15/TEPA, the freeze-dried F-SBA-15/TEPA showed a remarkable increase in CO2 capacity and amine efficiency, especially at higher loadings of 70 and 80 wt% TEPA. The lower capacities of the oven-dried samples could be associated with lower surface area (Fig. 2b) coupled with blocking the active sites, which were determined by FTIR analyses (Fig. 5a). The higher surface area of the freeze-dried samples together with superior access to the active sites conferred superior CO2 sorption capacity and amine efficiency.

Fig. 9. Effect of TEPA content on (a) CO2 sorption and (b) amine efficiency at 90 ℃ and 15 kPa CO2.

The sorption curves in Fig. 10 show that both F-SBA-15/TEPA and SBA-15/TEPA were characterised by a fast CO2 uptake stage followed by a slow sorption stage towards equilibrium. The CO2 capacities of freeze-dried F-SBA-15/TEPA (≤ 70) reached ~88% of total values within the first 2 min. This means a large amount of CO2 immediately reacted with the active sites of TEPA, a classical effect of composite mesoporous/amine sorbents.55 In contrast, the proportion of initial stage capacity dropped to ~70% for the freeze-dried sample with 80 wt% TEPA. This result clearly indicates that there is a hindrance of CO2 accessing the active sorption sites. This may be caused by the excess TEPA, as determined by the DSC analysis, which does not interact with the SBA-15 structure and covering some of the active sorption sites. It is also interesting to note that initial 13 ACS Paragon Plus Environment


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stage capacity of ~62% for the oven-dried SBA-15/TEPA (70 wt%), demonstrating similar diffusion behaviour as the freeze-drying sample loaded with excess TEPA (80 wt%). These results point out that the oven-drying method is delivering inhomogeneous composite structures, in addition to blocking the active sorption sites. Further, the maximum CO2 sorption capacity was achieved for TEPA loading of 70% for the freeze-dried samples, whilst a lower 50% TEPA loading only for the oven dried samples. This is attributed to the superior impregnation of TEPA as the freeze drying method retained the SBA-15 mesoporous structure (see Fig. 7). Contrary to this, the oven drying method caused the partial collapse of the SBA-15 ordered mesoporous structure, where the excess TEPA saturated the SBA-15 external surface, resulting in mass transfer limitations of CO2 to the active sites.

Fig. 10. CO2 sorption isotherms of F-SBA-15/TEPA and SBA-15/TEPA samples at 90 ℃ and 15 kPa CO2. The two best samples F-SBA-15/TEPA (70) and SBA-15/TEPA (50), which reached the highest CO2 capacities for each drying method, were further tested in the temperature range of 25 to 120 ℃. Again, the freeze-dried F-SBA-15/TEPA (70) achieved in the highest sorption capacity over this temperature range as compared to the oven-dried SBA-15/TEPA (50) (see Fig. 11). It is interesting to note that the CO2 capacity of F-SBA-15/TEPA (70) remained as high as 4.66 mmol g-1 at 105 ℃. The poor capacity below 90 ℃ of F-SBA-15/TEPA (70) is caused by low activity of TEPA25 as well as the incomplete desorption of adsorbed CO2, which is proved by the desorption process in Fig. S2 (supplementary


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information). The decrease in CO2 uptake at the temperature above 105 ℃ is mainly attributed to TEPA loss due to evaporation or pyrolysis.56

Fig. 11. Effect of sorption and desorption temperature on F-SBA-15/TEPA (70) and SBA-15/TEPA (50).

To gain further insight into the cycle performance of freeze-dried sorbents, the sorption/desorption performance of F-SBA-15/TEPA (70) is presented in Fig. 12. This freezedried sample showed outstanding cycle stability with a high average CO2 capacity of 4.64 mmol g-1 at 90 ℃ over 10 cycles. By decreasing the temperature to 75℃, the CO2 capacity of FSBA-15/TEPA (70) reduced to 3.47 mmol g-1 on average, but remained stable during the cycling tests. Contrary to this, the CO2 capacity at higher temperatures of 105 and 120 ℃ gradually declined by 14% and 39% after 10 cycles, respectively. These higher temperatures are associated with the evaporation or pyrolysis of TEPA, thus explaining the lack of cycling stability.



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Fig. 12. Cyclic CO2 sorption performance of F-SBA-15/TEPA (70).

4. Conclusions The freeze-drying method proved to be more effective to prepare composite amine silica sorbents for CO2 sorption than the conventional evaporation oven-drying method. The freezedried samples, containing a mesoporous silica SBA-15 support functionalized by TEPA, reached a sorption capacity of 4.71 mmol g-1. This CO2 sorption capacity was up to 18 times higher than the best oven-dried sample for the same TEPA loading. The superior sorption capacity was attributed to higher surface area and access to the amine’s active sorption sites. In other words, the freeze-drying method conferred superior structural homogeneity than the conventional oven-dried analogue. TEPA loading on SBA-15 reached the best CO2 sorption capacity at 70 wt%. Higher TEPA loading (80 and 90 wt%) lead to excess TEPA not being incorporated in the SBA-15 structure. The excess TEPA then behaved more like a pure TEPA material based on DSC analysis, thus hindering CO2 access and limiting CO2 diffusion to the active sorption sites of the composite sorbent. CO2 sorption stability was achieved at a relative high sorption temperature (90 ℃) at 15kPa CO2 partial pressure, showing good stability over 10 cycles of sorption/desorption.

ASSOCIATED CONTENT Supporting Information. Table S1 lists the pore characteristics of the sorbents, and Table S2 lists the band assignments of FTIR in Fig. 4 and 5. Fig. S1 shows the wide-scan XPS spectra of (a) F-SBA16 ACS Paragon Plus Environment

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15/TEPA and (b) SBA-15/TEPA as a function of TEPA content. Fig. S2 displays the desorption of FSBA-15/TEPA (70) in N2 with increasing temperature. Fig. S3 shows the CO2 capture capacity for oven-dried and rotary evaporator-dried samples. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: +61 7 3365 6960. Fax: +61 7 3365 4199.

Notes The authors declare no competing financial interest.

Acknowledgements W. Wang gratefully acknowledges The University of Queensland scholarship. J.C. Diniz da Costa thanks the support from the Australian Research Council (ARC) via the Future Fellowship Program (FT130100405).

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