Synthesis of Stable Tetraethylenepentamine-Functionalized Mesocellular Silica Foams for CO 2 Adsorption

Various amounts of tetraethylenepentamine (TEPA) loading on mesocellular silica foams (TEPA-MSF-x) were fabricated via a chemical graing method in this study. e performance of dry 15% CO2 adsorption on the TEPA-MSF-x was tested by a microbalance at 348K under ambient pressure. Experimentally, CO2 adsorption capacities of the TEPA-MSF-x sorbents were found to be 28.5–66.7mg CO2/g-sorbent. In addition, TEPA-MSF-x sorbents exhibited enhanced durability during repeated adsorption-desorption cycles compared to the conventional sorbents prepared by a physical impregnationmethod.is signi�cant enhancement in the stability of CO2 adsorption-desorption process was most likely due to the decreased leaching of TEPA which is covalently tethered to the surface of MSF.


Introduction
It is well known that global warming of the earth is caused by the progressive increase of CO 2 concentration in the atmosphere due to the extensive utilization of fossil fuels [1][2][3].As a result, development of an efficient and economic method to capture CO 2 from large stationary sources, such as coal-�red power plants and cement and steel factories, is urgent.At present, absorption processes using amine-containing solvents have been generally utilized for postcombustion CO 2 capture.Nonetheless, this technology is restricted by some drawbacks including high capital and operation costs and high-energy consumption during regeneration of the sorbents.
For the past few years, extensive studies have been reported on technologies concerning physisorption by using porous solid sorbents for sequestration of CO 2 , such as microporous zeolites [4,5], nanoporous carbons [6][7][8], nanoporous coordination polymers [9], and organic nanostructures [10,11].However, these sorbents have low CO 2 adsorption capacities which are generally smaller than the criteria value of ca.88 mg g −1 sorbent for commercialized applications.Moreover, these materials also encounter difficulties such as poor selectivity, poor tolerance to water, and high-temperature regeneration/activation.
In the recent years, ordered mesoporous silicas (OMSs) are commonly used as gas sorbents because they possess unique properties including tunable pore size (2-50 nm), narrow pore size distribution, high surface area, large pore volume, and good thermal stability.For the purpose of CO 2 capture, surface modi�cation of OMSs with amine functional groups was further performed.At least two methods were reported to prepare amine-functionalized sorbents, including (i) liquid amines physically impregnated onto the OMSs and (ii) amines chemically bonded to the OMSs.In terms of the former method, different amino compounds and polyamines supported on various OMSs, such as MCM-48, KIT-6, MSF, and SBA-15, have been utilized as sorbents for CO 2 capture [12][13][14][15].For instance, ca.50 wt% polyethylenimine-(PEI-) incorporated MCM-41 with a high CO 2 adsorption capacity of 246 mg g −1 -PEI is 30 times higher than that of MCM-41 and also ca.2.3 times that of the pure PEI F 1: Scheme of preparation of TEPA-modi�ed MSF by a chemical graing method.[16,17].In terms of the latter method, the most common aminosilanes [18][19][20][21][22][23][24][25][26][27][28] used in functionalization of OMSs are including mono(3-aminopropyltrimethoxysilane), di-(3-(2-aminoethyl)aminopropyltrimethoxysilane), and triaminosilanes(3-[2-(2-aminoethyl)aminoethyl]aminopropyltrimethoxysilane).Furthermore, graing amines into the pore-expanded mesoporous silicas was capable of capturing CO 2 by greater amount of amine and also more resistant to moisture while compared to other supports such as activated carbon, silica gel, and pure silica [29][30][31].Cyclic adsorption-desorption process also indicated that the above adsorbents have good durability.Hicks et al. [32] prepared a covalently tethered hyperbranched aminosilica (HAS) sorbent which can perform CO 2 adsorption-desorption reversibly with a high capacity of 136.4 mg g −1 sorbents at 298 K and cyclic stability.A stepwise method was reported by Bhagiyalakshmi and coworkers [33] to synthesize amino dendrimers graed SBA-15 sorbents which are efficient for CO 2 capture with thermal durability aer seven cycles of adsorption-desorption process.
By far, a variety of amines loaded on various OMSs as sorbents for CO 2 adsorption were widely investigated.Among all sorbents, tetraethylenepentamine (TEPA) incorporated mesoporous silicas [34][35][36][37] demonstrate promising applications because of their high CO 2 adsorption capacities and facile preparation route.However, the TEPA-modi�ed sorbents have the shortage regarding the instability of cyclic adsorption-desorption process [38][39][40].In this study, a chemical graing route has been developed to synthesize TEPA-functionalized mesocellular silica foams (MSFs) sorbents as indicated in Figure 1.e resulting TEPAincorporated MSFs were characterized by a series of different analytical and spectroscopic techniques, including N 2 adsorption/desorption, small angle X-ray scattering (SAXS), and Fourier-transformed infrared (FTIR) spectroscopies.e synthesized sorbents show an extremely enhanced durability aer repeated adsorption-desorption cycles, revealing some opportunities for practical and cost-effective applications in industry.

Experimental Section
2.1.Materials Preparation.e pure MSF samples were synthesized based on the method reported earlier [41].In a typical run, 2 g of neutral triblock copolymer surfactant (Pluronic 123; EO 20 PO 70 EO 20 , MW = 5800, Aldrich) was dissolved in 75 mL of aqueous 1.6 N HCl at room temperature, followed by adding 12 mg of NH 4 F (ACROS) and 1.5 g of 1,2,4-trimethylbenzene (TMB; 98%, ACROS) into the mixture.Aer stirring for 1 h at 313 K, 4.3 g of TEOS was added to the above mixture.e resultant reaction mixture was stirred at 313 K for 20 h followed by aging at 373 K for 24 h.e solid products of as-synthesized MSF were recovered by �ltration and dried at room temperature overnight followed by removal of organic template by calcination at 823 K. TEPA-functionalized MSF adsorbents were prepared by a chemical graing method.0.5 g of calcined MSF was dispersed in 25 mL of dry toluene with 50 mM (3-chloropropyl)trimethoxysilane (CPTMS; 98%, ACROS).e reaction mixture was re�uxed at 383 K for 24 h, and the �nal product was �ltered, washed with toluene and then alcohol, and dried under a vacuum at 343 K for 8 h.is resultant sample is denoted MSF-Cl.Subsequently, various amounts of TEPA were added to 0.5 g of MSF-Cl to perform surface functionalization via the above-mentioned procedure developed for CPTMS graing.e obtained samples are denoted as TEPA-MSF-x (x is weight ratios of TEPA/MSF, x = 0.5-4.0).

Characterization Methods.
Small angle X-ray scattering (SAXS) was carried out on a Nanostar U system (Bruker, AXS Gmbh).Nitrogen adsorption isotherms were obtained at 77 K on a Micromeritics ASAP 2020 analyzer.Pore size distribution curves were evaluated by the BJH method from the adsorption branch.e Brunauer-Emmett-Teller (BET) speci�c surface area was calculated from nitrogen adsorption data in the relative pressure ( 0 ) range from 0.05 to 0.2.e total pore volume was estimated from the amount adsorbed at the  0 of 0.99.Fourier transform infrared (FTIR) spectra were obtained on a Bio-rad 165 spectrometer with 4 cm −1 resolution using KBr pellets at room temperature.

CO 2 Adsorption.
A modi�ed thermogravimetric analyzer (TGA, PerkinElmer Pyris 6) with a H 2 O saturator [42] was applied to evaluate the adsorption and desorption performance of various adsorbents.Typically, about 10 mg of sorbent placed in a sample cell was heated to 373 K under N 2 �ow (50 mL min −1 ), followed by maintaining at that temperature for at least 30 min until no weight loss was observed.Aerward, the sample was cooled down to 348 K, then by introducing 15% dry CO 2 into the TGA cell at a �ow rate of 50 mL min −1 .Aer 42 min of adsorption process, the gas was switched to pure N 2 �ow (50 mL min −1 ) to achieve desorption procedure at the same temperature for 78 min.e sensitivity and accuracy of TGA microbalance are 10 g and 0.1%, respectively.e cyclic adsorption/desorption measurements were also studied to evaluate the stability of the sorbents.

Results and Discussion
As shown in Figure 2, the small-angle XRD pattern of the pure MSF samples indicates one main intensive (100) feature at 2 of ca.0.35 ∘ , suggesting the existence of well-ordered hexagonal arrays and two-dimensional (2D) channel structure.However, upon graing TEPA onto the MSF surface (TEPA-MSF-x), substantial decreases in diffraction peak intensities were observed, particularly for the TEPA-MSF-4.0 at which the (100) re�ection was almost vanished.e features at low 2 angles of TEPA-MSF-x were found to be decreased, which could be ascribed to the successful incorporation of TEPA compounds, leading to less order of hexagonal structure and/or partial collapse of the wall of mesostructured silica [43][44][45].As can be seen in Figure 3, N 2 adsorption/desorption curve of the pure MSF samples showed the typical signatures for mesoporosity, namely, type-IV isotherms with well-de�ned hysteresis loops, consistent with XRD results.However, the decrease of hysteresis loops may be caused by the blockage of mesopore channels upon incorporating a considerable amount of TEPA functional compounds onto the TEPA-MSF-x samples.As indicated in Table 1, notable decreases of total pore volume (V), BET surface area (S), and BJH pore size (D) were observed for TEPA-modi�ed samples compared to the pure MSF samples, again suggesting the successive functionalization of TEPA and/or CPTMS onto MSF.To further con�rm the presence of TEPA functional groups in the surface-modi�ed MSF samples, the FTIR spectroscopy was measured.As can been seen in Figure 4, the FTIR spectrum of the parent MSF showing characteristic peaks at 1079 and 968 cm −1 as assigned as Si-O-Si and Si-OH stretching, respectively.Upon incorporation of TEPA, the peak at 968 cm −1 vanished, and three weak peaks appear at 1667 cm −1 ( - ), 2931 cm −1 (−CH 2  as ), and 2818 cm −1 (−CH 2  s ) which were attributed to the NH 2 scissoring vibration and CH stretching vibration in the amine functional groups, respectively.ese results again con�rm the successful impregnation of TEPA onto MSF.CO 2 adsorption/desorption curves of pure MSF and TEPA-MSF-x are shown in Figure 5. e measurements were carried out at 348 K by using TGA under atmospheric pressure.e CO 2 adsorption capacities of various TEPA-MSF-x are also summarized in Table 1.Unlike the pure MSF samples, which show almost none CO 2 uptake, TEPA-MSF-x samples exhibit the adsorption capacities of ca.28.5-66.7 mg CO 2 /g-sorbent, respectively.As proposed previously [46], the mechanism of chemical adsorption pathway between amine active sites and CO 2 in anhydrous conditions is   the formation of ammonium carbamate, thus the isolated amine groups are ineffective in CO 2 adsorption due to the stoichiometric CO 2 /N ratio of 0.5 as indicated below: Among all TEPA-MSF-x samples, TEPA-MSF-2.0 sorbents have the maximum of CO 2 adsorption capacity.In terms of amine efficiency (CO 2 /N), all TEPA-MSF-x sorbents were below 0.5 mmol mmol −1 , which may be due to the fact that partial primary amino groups of TEPA were reacted with Si-Cl in the MSF-Cl samples during graing process, leading to the lower surface density of primary amines in the TEPA-MSF-T-x sorbents.In the earlier report [47], experimental results indicated that primary amino sorbents had the maximum CO 2 adsorption capacities among primary, secondary and tertiary amines.
In order to be realized for practical applications in industry, CO 2 capture sorbents with a cyclic adsorptiondesorption durability under long-term operation is desirable.As shown in Figure 6, it is worthy to note that CO 2 adsorption capacity of our prepared TEPA-MSF-1.0 sorbents exhibits a stable and reversible performance during �ve repeated runs of adsorption-desorption cycles (i.e., total operation period of ca.12.5 h) at 348 K under dry 15% CO 2 concentration.In contrast, the TEPA-MSF-1.0Psorbents, which were prepared by a conventional physically impregnation method, show a steady decrease with a capacity loss of approximately 54% aer the �h adsorption-desorption cycle.�is is probably owing to the extensive leaching of TEPA which is physisorbed on the surface of MSF.

Conclusion
In summary, CO 2 adsorption on TEPA-MSF-x sorbents prepared by a chemical graing route shows their CO 2 uptake capacities of ca.28.5-66.7 mg CO 2 /g-sorbent at 348 K under ambient pressure using dry 15% CO 2 .Among all TEPA-MSFx samples, TEPA-MSF-2.0 sorbents have the maximum of CO 2 adsorption capacity.Moreover, the TEPA-MSF-1.0 sorbents possess notably enhanced durabilities during repeated adsorption-desorption cycles compared to TEPA-MSF-1.0P sorbents.is enhancement of durability in CO 2 uptake is probably due to the decreased leaching of TEPA which is chemically modi�ed to the surface of MSF.ese TEPA-MSF-x sorbents demonstrate reversible and stable properties, revealing a capable CO 2 sorbent for the practical application during cyclic adsorption-desorption processes.

F 2 :
Small-angle XRD patterns of pure MSF and various TEPA-MSF-x samples.

T 1 :
Physicochemical properties and CO 2 adsorption performance of parent MSF and various TEPA-MSF-x samples.Sample V (cm 3 g −1 ) a S (m 2 g −1 ) b

F 4 :
FTIR spectra of pure MSF and various TEPA-MSF-x samples.