Thermally Stable Amine-Grafted Adsorbent Prepared by Impregnating

Article pubs.acs.org/IECR

Thermally Stable Amine-Grafted Adsorbent Prepared by Impregnating 3‑Aminopropyltriethoxysilane on Mesoporous Silica for CO2 Capture Dang Viet Quang,† T. Alan Hatton,‡ and Mohammad R. M. Abu-Zahra*,† †

The Institute Center for Energy (iEnergy), Masdar Institute of Science and Technology, P.O. Box 54224, Masdar City, Abu Dhabi, United Arab Emirates ‡ Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ABSTRACT: In this paper, an amino-functionalized adsorbent for CO2 capture was synthesized by impregnating 3aminopropyltriethoxysilane (APTES) on mesoporous silica. Amino functional groups were chemically bonded to silica support. Because of chemical bonds, the thermal stability of this adsorbent (205 °C) was greater than that of an adsorbent prepared by impregnating polyethylenimine on the same mesoporous silica (160 °C). The cyclic CO2 loading capacity was improved by increasing adsorption temperature, reaching a maximum of 89.3 mg CO2/g at 100 °C and then reduced at temperatures above 100 °C. Optimum regeneration temperature was determined at 120 °C. The adsorbent was stable for multiple adsorption− regeneration cycles operating at adsorption temperature of 100 °C in pure CO2 and regeneration temperature of 120 °C in atmosphere, respectively. The amino-functionalized mesoporous silica is a thermally stable adsorbent with relatively high CO2 loading capacity and high optimum adsorption and regeneration temperatures. These characteristics make the adsorbent suitable for CO2 capture application. (TEPA) on porous silica.5,15,20−22 Previous study indicated that the CO2 adsorption capacities of silica monolith impregnated with PEI and TEPA can reach 210 and 260 mg/ g, respectively.22 In the second method, amino functional groups are grafted onto silica substrate by the condensation of a coupling agent such as 3-aminopropyltriethoxysilane (APTES), N-[3-(trimethoxysilyl) propyl]-ethylenediamine (DAEAPTS) with silanol groups located on the surface of silica substrate.10,22−24 Grafting process is usually conducted by refluxing in an organic solvent. This type of adsorbent contains amino functional groups, which are chemically bonded to silica substrate by forming siloxane bridges (Si−O−Si); therefore, it would be more stable compared to the adsorbent prepared by the impregnation method. In the third method, silica substrate is simultaneously precipitated with coupling agent to form adsorbent. The adsorbent synthesized by this method contains high content of functional groups; however, the accessibility to those groups is very limited because they are conformed inside silica structure. This adsorbent has less accessibly active sites for an adsorption and thus, is not much interested in comparison with adsorbent prepared by the other methods.19 In the impregnation method, amines create physical bonds, i.e., van der Waals interaction and hydrogen bond, with the silica support. The bonding between amines and support is

1. INTRODUCTION The application of amine-based solid sorbent for CO2 capture has drawn significant attention due to its higher adsorption capacity, lesser corrosion, and possibly lower energy requirement compared to conventional CO2 capture process based on aqueous amine solution.1,2 Many amine-supported solid adsorbents has been prepared and investigated for capturing CO2 from flue gas at both laboratory and pilot scale.3−8 Aminesupported adsorbents are usually produced by introducing amino functional groups onto the surface and into porous structure of a substrate, e.g., porous silica, porous carbon, slag, or fly ash.7 Among those materials, porous silica is the most widely and thoroughly investigated.6 Porous silica is a structural inorganic polymer, which is chemically and thermally stable. Porous silica has numerous silanol groups (Si−OH) on the surface and these silanol groups act as bridges to connect and graft various functional groups such as amino or thiol groups onto the substrate. Those functional groups are beneficial to many application purposes such as acidic gas adsorption, heavy metal removal, and nanoparticle stabilization.9−13 Typically, there are three frequently introduced methods to prepare amine-based adsorbent for CO2 capture, i.e., (1) impregnating mono or polymer amines on a porous structure,14−16 (2) grafting amino functional groups onto a substrate by functionalization method,17,18 and (3) coprecipitation of amine compounds with silica during synthesis.19 The first method has been extensively investigated due to its easy preparation and resulting adsorbent with high CO2 adsorption capacity. The adsorbent is usually prepared by the impregnation of amine compounds such as polyethylenimine (PEI), diethylenetriamine, triethylenetetramine, 2-amino-2-methyl-1propanal, diethyleneamine, and tetraethylenepentamine © 2016 American Chemical Society

Special Issue: International Conference on Carbon Dioxide Utilization 2015 Received: Revised: Accepted: Published: 7842

October 30, 2015 December 21, 2015 January 13, 2016 January 13, 2016 DOI: 10.1021/acs.iecr.5b04096 Ind. Eng. Chem. Res. 2016, 55, 7842−7852

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Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration of the CO2 adsorption experiment.

weak causing adsorbent less stable. In fact, impregnated amines are easily to be vaporized while being regenerated at high temperature and this accelerates the deactivation of adsorbent.17,25,26 Moreover, impregnated amines may also be leached out during operation. The leaching phenomenon is more severe at high impregnated amine content and particularly in a fluidized bed reactor based CO2 capture process. The amino functionalized silica prepared by grafting method may overcome the vaporization problem facing by the impregnated method due to the formation of chemical bonds. Czaun et al. prepared amino-functionalized silica by mixing 7.0 g of silica in 280 mL of toluene following by adding a silane coupling agent then stirring at 110 °C for 12 h.12 Huang et al. immobilized amine functional groups on the silica surface by refluxing 1.0 g (silica xerogel and MCM-48) and 5.0 mL of APTES with 50 mL of toluene at 70 °C for 18 h.18 Knowles et al. functionalized silica gel by dispersing 1.0 g of silica in 100 mL toluene then treated with APTES at various temperature.23 In another study, Sanz et al. synthesized amino-functionalized SBA-15 silica by refluxing 1.0 g of silica in 250 mL of toluene with APTES for 24 h.27 Recently, Wang et al. have reported a method to prepare amino-functionalized silica using ethanol solvent. Accordingly, 6 g of MCM-41 was mixed with 350 mL anhydrous ethanol followed by adding 1 mL distilled water and different amount of APTES and then the mixture was refluxed for 10 h.28 These methods successfully grafted amino functional groups onto the silica surface; however, the critical issue of these functionalization methods is the use of organic solvent and reflux at relatively high temperature.12,18,23,27−31 Using a large amount of organic solvent is not environment-friendly and causes an increase in the cost of adsorbent. Therefore, these methods are not suitable to synthesize adsorbent for CO2 capture. To address this issue, a facile and novel method has been proposed to synthesize amino-functionalized adsorbent. This method allows one to prepare amino-functionalized mesoporous silica with minimizing the quantity of organic solvent or eliminating organic solvent and under a milder conditions without solvent reflux at high temperature. APTES is used as a functionalization

agent whereas mesoporous silica synthesized by acidizing sodium silicate solution is utilized as a porous support material. The resulting amino-functionalized mesoporous silica will be used as an adsorbent for CO2 capture and their CO2 adsorption performance and thermal stability will be investigated. The thermal stability of this amino-functionalized mesoporous silica will be compared with PEI-impregnated adsorbent prepared from the same mesoporous silica. In particular, a thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TGA-FTIR) technique will be used to study the gases evolved during adsorbent degradation for a further understanding of degradation mechanism.

2. EXPERIMENTAL SECTION 2.1. Porous Silica Preparation. Details of the sample preparation are described elsewhere.1,2,32 Typically, desired amount of sodium silicate solution (2.5SiO2·Na2O, 26.5 wt % SiO2, Sigma-Aldrich) was added to a 1 L beaker containing the solution of NaCl 3.5 wt %. The beaker was placed in a heating mantle and heated to 40 °C. Then, H2SO4 10% was added by a dropwise method until the first silica aggregates were observed. After mixing for 5 min, addition of the acid was continued until the pH of the solution reached 5. The temperature was raised to 80 °C, and the solution was aged for 30 min. The slurry was filtered, washed with water, and dried at 130 °C for 3 h to produce mesoporous precipitated silica (PS). 2.2. Adsorbent Preparation. Functionalized mesoporous silica was prepared by directly impregnating coupling agent onto mesoporous silica and following by aging at 40 °C. Accordingly, 4.0 g of functionalization solution was added to 1 g of precipitated silica in a 20 mL beaker and mechanically mixed using a stainless steel spatula. The functionalization solution was prepared by mixing a desired amount of APTES (99%, Sigma-Aldrich) with ethanol or water as a dilute solvent to generate adsorbents with APTES-impregnated concentration of 40, 50, 60, 70, and 80 wt %. APTES-impregnated PS (APTES-PS) was aged at room temperature for 48 h, then at 40 °C for 12 h, and finally dried at 105 °C for 3 h to generate 7843

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Industrial & Engineering Chemistry Research APTES-PS-x adsorbent in which x is weight percentage of APTES. In this procedure, APTES-PS was not filtered or washed before drying; therefore, all solvents (ethanol and water) and byproduct (ethanol) were removed by evaporation during drying adsorbent at 105 °C. Polyethylenimine-impregnated PS (PEI-PS) was prepared by adding desired amount of PEI 30.6 wt % in water (branched polyethylenimine, Mw 1200, Sigma-Aldrich) to a 50 mL beaker containing PS with a weight ratio of 4:1 (PEI solution to silica) and mechanically mixed using a steel stainless spatula. The resulting wet sample was dried at 105 °C for 3 h to generate PEI-PS adsorbent. 2.3. Adsorbent Characterization and Testing. 2.3.1. Adsorbent Characterization. The surface area and porosity were measured by a nitrogen adsorption−desorption method using Quantachrome instruments (Nova Model 25). Samples were degassed at 110 °C for 3 h before analyzing. Pore volume was calculated by the Barrett−Joyner−Halenda (BJH) method using desorption isothermal data. The morphology of adsorbent was studied by scanning electron microscopy (SEM) using a Quanta 250 microscope. Thermogravimetric analysis (TGA) was performed from room temperature to 800 °C in atmosphere at a heating rate of 5 °C/min using a thermogravimetric analyzer (Netzsch STA 449 F3). Fourier transform infrared spectroscopy (FTIR) measurements were conducted on a vertex 80 spectrometer (Bruker). The nitrogen content in the adsorbent was analyzed using a TOC/TN analyzer (Vario TOC Cube, Elementar). 2.3.2. CO2 Adsorption−Regeneration Studies. CO2 adsorption performance of adsorbents was studied on a flow Micro Reaction Calorimeter (URC) provided by Thermal Hazard Technology (UK) as described in a previous publication.1 A schematic illustration of experimental setup is shown in Figure 1. Typically, an approximately 0.3−0.5 g of adsorbent, which had been activated at a regeneration temperature for 30 min, was fed into analysis cell and mounted on the calorimeter. The flow rate and volume fraction of feed gas are controlled by mass flow controllers (MFC1 and MFC 2). The gas can be directed to moisturizer (A) and then enter a makeup vessel (B) or directly enter makeup vessel by controlling valves (V1, V2, and V3). Before entering the analysis cell or reactor, the gas can be flowed through a desiccant column (C) to remove moisture if needed by controlling valves (V4 and V5). The flow rate of CO2 introducing into reactor was adjusted by MFC3. The valve V6 was used to control the total gas pressure of ̀ the system that was indicated by a transducer (T4). The CO2 concentration in the gas out of reactor was monitored by a CO2 Transmitter Series GMT220 (Vaisala, Finland). Tests were run under the isothermal mode, and the adsorption progress was monitored by the variation of power (mW) with time (s). When the power signal stabilized, pure CO2 was introduced into the analysis cell at the rate of 0.5 mL/ min and ambient pressure. Because CO2 adsorption is an exothermic process, the power signal increased whenever the adsorption occurred. The test was completed when the power signal came back to the initial level and stabilized again. The CO2 loading capacity of adsorbent (mg/g) was calculated by dividing the mass of CO2 adsorbed per the mass of lean adsorbent as described in eq 1. CO2 loading =

⎛ mg ⎞ m1 − m0 × 1000⎜ ⎟ m0 ⎝ g ⎠

Where m0 is the mass of lean adsorbent, which is pretreated at a desired regeneration temperature in a drying oven for 30 min, m0 is the mass difference between the analysis cell containing lean adsorbent and empty cell, and m1 is the mass of CO2 loaded adsorbent that is the mass difference between the analysis cell containing adsorbent after CO2 adsorption and empty cell. To investigate the recyclability, CO2 loaded adsorbent was regenerated by thermal treatment under atmospheric conditions at a desired temperature for 30 min followed by another adsorption test as described above. The adsorption−regeneration test was repeated for at least 10 cycles or until the loss in the cyclic CO2 loading was observed.

3. RESULTS AND DISCUSSION 3.1. Adsorbent Characterization. The FTIR spectra of mesoporous silica and adsorbents are presented on Figure 2.

Figure 2. FTIR spectra of prepared adsorbents.

Vibration bands at 3444, 1633, 1101, 788, and 463 cm−1 were observed on the spectra of both silica substrate and adsorbents. These are characteristic vibrations of the major bonds in silica structure. The vibrations at 3444, 1633, and 788, cm−1 are assigned to stretching and bending of the O−H bond in the sample (water or O−H group attached to the silica structure), whereas the peaks at 1101 and 463 belong to asymmetric stretching and bending of Si−O−Si.33 After impregnation with APTES, new peaks emerged can be seen on the spectra of adsorbent. Vibrations at 3358, 3282, and 1598 are corresponding to asymmetric stretching, symmetric stretching of N−H bonding, and NH2 deformation, respectively.34−36 The peaks at 2931, 2880, 1474, and 1412 cm−1 are attributed to the vibrations of the C−H bond. The vibration at 695 and small shoulder at 1300−1600 cm−1 can be designated to Si−C and C−N bonds.11,35 The emergence of the new peaks belonging to C−H, N−H, Si−C, and C−N bonds indicated the existence of the organic moiety, which would be 3-aminopropylsilane groups (SiCH2CH2CH2NH2), in the structure of the adsorbent. Moreover, the peak observed at 965 cm−1 on the spectrum of silica substrate due to the vibration of silanol group (Si−OH) was completely attenuated after impregnation of APTES indicating disappearance or significant reduction in the density of silanol groups on the silica surface. The

(1) 7844

DOI: 10.1021/acs.iecr.5b04096 Ind. Eng. Chem. Res. 2016, 55, 7842−7852

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Industrial & Engineering Chemistry Research Scheme 1. Functionalization Route of Mesoporous Silica

materials. The hysteresis loops on the isotherms were contracted and the volume of the nitrogen adsorbed reduced, suggesting the decrease in the surface area and pore volume of materials, with the increase in APTES-impregnated concentration. Further details on the variation in the BET surface area and pore volume with the amount of APTES impregnated are described in Figure 4. As can be seen in this figure, both surface

disappearance of silanol groups suggests that the condensation of those groups with hydrolyzed products of APTES or APTES molecules occurred and resulted in the attachment of the amino function groups onto the surface of mesoporous silica as described in Scheme 1. Accordingly, the impregnated APTES was first hydrolyzed due to the absorbed water or added water in functionalizing solution to produce 3-aminopropyltrihydroxylsilane (reaction 2). Then, the hydrolyzed molecules were condensed with silanol groups on silica substrate to form siloxane bridge (Si−O−Si) (reactions 3 and 4). The APTES may also directly condense with the silanol groups as shown in reaction 5; however, this reaction could occur with a negligible rate because APTES is more sensitive with water.37−39 The results from FTIR study indicated that mesoporous silica was successfully functionalized by impregnating APTES and the amino functional groups covalently bond to the silica substrate. The nitrogen adsorption−desorption isotherms of samples shown in Figure 3 are of type IV according to the IUPAC classification, corresponding to characteristic of mesoporous

Figure 4. Variation in BET surface area and pore volume of adsorbents as a function of increasing APTES-impregnated concentration.

area and pore volume of the adsorbents were abruptly reduced with the rise of APTES-impregnated concentration. The BET surface area and pore volume of mesoporous silica varied from 133 m2/g and 0.45 cm3/g to 3.3 m2/g and 0.01 cm3/g as the APTES-impregnated concentration increased from 0 to 80 wt %, respectively. Obviously, the APTES percolated into the pores causing the pore blockage as PS was impregnated with 80 wt % APTES. The pore blockage by APTES impregnation can be further confirmed by the observation of representative adsorbent’s SEM images shown in Figure 5. It is evident from these SEM images that the porosity of adsorbent decreased with the increasing APTES-impregnated concentration and it

Figure 3. Nitrogen adsorption−desorption isotherms of prepared adsorbent. 7845

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Figure 5. SEM images of silica substrate (A), and adsorbent samples impregnated with APTES 40 wt % (B), 70 wt % (C), and 80 wt % (D).

could be clearly observed at 80 wt % APTES-impregnated concentration. At the APTES-impregnated concentration of 80 wt %, most of PS pores was disappeared due to the occupation of APTES. 3.2. Thermal Stability. Thermal stability is one of the most important properties for the real applicability of adsorbent used for CO2 capture and is usually determined by TGA analysis. TGA analysis provides information about the weight loss as a function of heating temperature. This technique provides the weight loss at a wide temperature range without informing how the absorbent being decomposed.40,41 To understand further the thermal degradation of the adsorbent, thermogravimetric analyzer was coupled with FTIR spectrometry (TGA-FTIR) for real-time monitoring of the gases evolved during TGA analysis. This technique allows one to determine major gas components, which is generated during TGA analysis as thermal degradation products at the investigating temperature. Therefore, the use of TGA-FTIR will provide more accurate thermal degradation temperature at the molecular level. In this study, the TGA profiles of all PS, PEI-PS, and APTESPS samples were collected, whereas only PEI-PS and APTES were analyzed by TGA coupled with FTIR to evaluate and compare the thermal stability of different amine-based adsorbents. The TGA curves of the samples are presented in Figure 6. The weight loss from room temperature to about 145 °C observed on both silica substrate and adsorbents is due to desorption of absorbed water and CO2. The weight loss of PS is negligible in the range from 145−800 °C; it was less than 5% mostly accounted for vaporization of bonding water and the condensation of silanol groups. The TG curves of PEI-PS and

Figure 6. TGA profiles of PS, PEI-PS, and APTES-PS.

APTES-PS show a second weight loss at 160−450 °C and 285−670 °C, respectively. The second weight loss is associated with the decomposition of organic species in adsorbent. Obviously, APTES-PS is much more thermally stable compared to PEI-PS. The higher thermal stability of APTES-PS is likely due to the formation of chemical bonds between amino functional groups and silica substrate, which require more energy to break in comparison with physical bonds between PEI and silica substrate. The 3D graph and FTIR spectra of gases evolved during TGA of APTES-PS at representative temperatures (150, 300, 7846

DOI: 10.1021/acs.iecr.5b04096 Ind. Eng. Chem. Res. 2016, 55, 7842−7852

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°C. The characteristic peaks of CO2 for the second temperature range were intensified again at 330 °C and achieved another maximum at 595 °C. The characteristic peaks of NH3 at 930 and 965 cm−1 obviously arose at 205 °C with a low intensity (Figure 7C); it intensified at about 285 °C, and reached a maximum height at 427 °C. The emergence of new vibration peaks on the FTIR spectra at higher temperatures demonstrated that the adsorbent was decomposed with the increase in temperature. The degradation starts with the breakage of the CN bond to form NH3 (205 °C), followed by the rupture and the oxidation of hydrocarbon backbone (above 285 °C). At the beginning of the degradation process (205−285 °C), the breakage of the CN bond to form NH3 occurs with a relatively low rate and the mass loss is negligible; therefore, the change is insignificant on the TGA profile. The degradation is more severe above 285 °C with the rupture of both CN and hydrocarbon backbone resulting in significant loss on the TGA curve. These results confirm that APTES-PS started degradation at 205 °C; however, it was not seen in the TGA profile (Figure 6) because the mass loss at temperatures from 205 to 285 °C is negligible. The trend that amino functional groups were decomposed in this study is congruent with those in previous work where the surface NH groups were first decomposed at 270 °C to form NH3 and followed by the oxidation of the carbon backbone of silane at 325 °C.42 The FTIR spectra of gases evolved during TGA of PEI-PS at representative temperatures (150, 200, 300, 400, and 600 °C) and its 3D spectra are presented in Figure 8. At the earlier stage below 160 °C, the evolved gases are similar to those of APTESPS including H2O and CO2. However, the absorbance of CO2 at 2359 and 2322 cm−1 attained the first maximum at 112 °C (Figure 8A); this temperature is lower than that of APTES-PS (127 °C), indicating that the optimum CO2 desorption temperature of PEI-PS is lower than that of APTES-PS. The trace of NH3 at 930 and 965 cm−1 was found at the lower temperature 160 °C (Figure 8 C) and reached a maximum at 315 °C. The band at 2800−3100 cm−1 began at 185 °C and reached a maximum at 326 °C with distinctive peaks at 2833, 2878, 2954, and 3057 cm−1. A shoulder at about 3244 cm−1 is attributed to the symmetric stretching of the N−H bond in amine. These results suggest that the degradation of PEI-PS starts by the breakage of the C−N bond of the primary amine groups to form NH3 (160 °C), followed by the vaporization and the rupture of the PEI structure into smaller amine (above 185 °C), and the oxidation of amine finally occurs at above 280 °C. Obviously, PEI-PS is less thermally stable with the first traces of NH3 and C−H bonding components in the effluent gas found at 160 and 185 °C compared with 205 and 285 °C for APTES-PS, respectively. Results confirmed that TGA-FTIR is a useful technique to investigate the thermal degradation and stability of adsorbent. The technique allows one to determine a precise temperature at which degradation occurs, particularly for the adsorbent that has a degradation temperature that is not easy to be determined by normal TGA. In this research, for example, the TGA profile of APTES-PS showed a possible degradation at approximately 285 °C; however, a study by using the TGA-FTIR technique proved that thermal degradation actually began at 205 °C with the appearance of NH3 in the effluent gas. 3.3. CO2 Adsorption−Regeneration Study. Mesoporous silica has relatively low CO2 adsorption capacity due to physical adsorption.2 By introducing amino functional groups on a silica substrate, CO2 adsorption on the resulting adsorbent will be

450, 550, and 600 °C) are shown in Figure 7. As can be seen in this figure, there are two components observed in the effluent

Figure 7. FTIR spectra of the gases evolved during TGA of APTESPS; 3D graph (A) and FTIR spectra at representative temperatures (B), and early evolution of NH3 (C).

gas at 150 °C including H2O and CO2. The vibrations at 3400− 4000 cm−1 and 1300−1900 cm−1 belong to water, whereas the peaks at 669, 720, 2322, and 2358 cm−1 are assigned to the characteristic vibration of CO2. The significant vibrations of CO2 obtained at two temperature ranges, approximately 70− 285 °C and above 330 °C (Figure 7 A), in which the first one has a maximum at 127 °C. More vibration bands were observed with increasing temperature. The distinct peaks at 930, 965, and 3334 cm−1 are attributed to deformation and stretching vibrations of NH bond in NH3. The vibrational band from 2800−3100 cm−1 and peaks at 3016, 3086, 2958, and 1305 cm−1 are likely corresponding to the CH bond in degradation and oxidation products (CH, =CH, C CH, and CH4). The typical vibration band of the CH bond began at 285 °C; particularly, the peak at 3058 cm−1 together with the band for CO2 grew rapidly at temperature above 400 7847

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Figure 9. Variation in CO2 loading capacity as a function of APTESimpregnated concentration. Adsorption tests were conducted at 70 °C in pure CO2 at ambient pressure.

of 70 wt % and then reduced at higher APTES concentration. The variation of the CO2 loading with the increase in the APTES-impregnated concentration can be explained as below. By impregnation, APTES molecules were first filled into the pores of PS substrate and then a hydrolysis and condensation process occurred as described in Scheme 1. The hydrolysis and condensation process results in grafting amino functional groups on the PS structure. Certainly, the density of amino functional groups grafted should be proportional to APTES impregnated concentration. As experimental results shown in Figure 10, nitrogen content in adsorbent increased as a function

Figure 10. Variation in nitrogen content in adsorbent and functionalization efficiency as a function of APTES-impregnated concentration.

Figure 8. FTIR spectra of the gases evolved during TGA analysis of PEI-PS. 3D graph (A) and FTIR spectra at representative temperatures (B) and early evolution of NH3 (C).

of increasing APTES-impregnated concentration. The nitrogen content was increased from 2.95 to 7.37 wt % when APTESimpregnated concentration was raised from 40 to 80 wt %, respectively. Theoretically, the CO2 loading must be improved by raising the content of amino functional groups and APTESimpregnated concentration, however, at high concentration, APTES packed into pores causing the pore blockage and as result the pore volume decreased significantly (Figure 4). When the pores were blocked, CO2 gas could not access to hidden amine sites resulting in the reduction of the CO2 adsorption. In other words, the lower CO2 loading capacity at high APTESimpregnated concentration is due to the higher fraction of amino groups that do not react with CO2. This can be further

changed to chemical adsorption. Therefore, CO2 adsorption on APTES-PS is predominantly based on the chemical adsorption in which CO2 molecules react with amino groups located on the APTES-PS surface to form heat-regenerable carbamate or carbonate.31,43−45 The CO2 adsorption capacity, in principle, should increase with the increasing APTES-impregnated concentration. However, the results from this study exhibit a different scenario. As shown in Figure 9, initially the CO2 loading capacity increased with the rise of APTES-impregnated concentration, reaching a maximum CO2 loading at an APTES concentration 7848

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Industrial & Engineering Chemistry Research Table 1. Characteristics of the Adsorbenta sample PS APTES-PS40 APTES-PS50 APTES-PS60 APTES-PS70 APTES-PS80 APTES-PS70w a

APTES (wt %)

solvent

surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

nitrogen (wt %)

CO2 loading capacity (mg/g)

CO2/N molar ratio

functionalization efficiency (%)

0 40

EtOH

133.07 59.3

0.45 0.249

3.13 3.07

0 2.95

0.5 40.0

0.43

95.9

50

EtOH

48.23

0.134

3.5

3.90

61.6

0.50

84.7

60

EtOH

25.35

0.051

3.1

4.97

69.8

0.45

81.8

70

EtOH

11.21

0.017

3.4

5.85

77.0

0.42

74.2

80

EtOH

3.26

0.006

3.16

7.37

50.0

0.22

72.8

70

water

6.58

80.4

0.39

83.5

Adsorption tests were conducted at 70 °C in pure CO2 at ambient pressure and regeneration at 110 °C.

usage of water as a dilute solvent undoubtedly accelerated APTES hydrolysis and condensation reactions, resulting in the improvement of functionalization efficiency. To investigate the effect of adsorption temperature on the CO2 loading capacity, adsorption tests were conducted on APTES-PS-70 adsorbent at different temperatures ranging from 60 to 110 °C, and results are shown in Figure 11. The CO2

confirmed by CO2/N molar ratio shown in Table 1. When the APTES-impregnated concentration was 70 wt % and below, the CO2/N molar ratio was somewhere between 0.4 and 0.5, which is close to theoretical ratio of 0.5 according to the reaction between CO2 and amino groups in a dry conditions as shown in eq 6. However, the CO2/N molar ratio of the adsorbent impregnated with 80 wt % APTES was 0.22, which is much lower than that of adsorbent with APTES-impregnated concentration below 70 wt % indicating a large fraction of amino groups are not accessible and do not react with CO2. CO2 + 2 ≡SiCH 2CH 2CH 2NH 2 ↔ ≡SiCH 2CH 2CH 2NH3+ + ≡SiCH 2CH 2CH 2NHCOO−

(6)

In contrast to nitrogen content in adsorbent, the functionalization efficiency (the percentage of actual nitrogen amount grafted on PS in comparison with theoretical nitrogen amount calculated from APTES impregnated) was decreased by increasing APTES-impregnated concentration (Figure 10). The functionalization efficiency reached 95.9% at 40 wt % of APTES impregnated, but reduced to 72.8% at APTES concentration of 80 wt %. The functionalization efficiency actually is the percentage of impregnated APTES bound to silica surface through condensation with silanol groups (Scheme 1). The density of silanol groups on the silica surface is constant. As APTES-impregnated concentration increased, the density of free silanol groups decreased and the silica surface became saturated with bound APTES. At this point, a much less APTES amount was grafted on the silica surface and therefore the functionalization efficiency was reduced. To evaluate the possible effect of dilute solvent on the functionalization efficiency and the loading capacity of prepared adsorbent, APTES-PS-70 was synthesized by replacing ethanol with water. Interestingly, upon solvent replacement the nitrogen content and functionalization efficiency increased from 5.85 to 6.58 wt % and from 74.2 to 83.5%, respectively. This improvement resulted in CO2 loading capacity increasing from 77.0 to 80.4 wt %. Results obtained here are similar to those reported in a previous study where the amino functional groups were grafted on mesoporous silica by reflux in toluene with the addition of water;39 it showed that amine loading can increase ca. 31% by employing a controlled amount of water. Obviously, water has a significant impact on the efficiency of the functionalization process. The addition of water and the

Figure 11. Variation in CO2 loading as a function of adsorption temperature. Tests were conducted with pure CO2 at ambient pressure on APTES-PS-70 sample, and the adsorbent was regenerated at 120 °C in atmospheric conditions.

loading of adsorbent was, initially, increased with the increase in adsorption temperatures from 60 to 100 °C and then decreased at higher temperatures. Accordingly, the optimal adsorption temperature was 100 °C with the CO2 loading of 89.3 mg/g. The trend in which CO2 loading of APTES-PS changes with the increase in adsorption temperature is similar to that in previous studies where various mesoporous silica and different amine precursors were used.4,20,21,46−50 The optimum temperature that results in maximum CO2 loading is, however, different among adsorbents depending on the characteristics of silica support and its corresponding adsorbent as shown in Table 2. Low-amine-loaded adsorbents have large surface area and high pore volume resulting in faster CO2 adsorption kinetics at low temperature. Whereas high-amine-loaded adsorbents have very small surface area and pore volume due to the package of amine in the silica pores causing slow CO2 diffusion, particularly at low temperature. Increasing the temperature reduces the CO2 diffusion resistance, resulting in faster CO2 adsorption kinetics and as result the optimum temperature is shifted to higher values. In a solid-sorbent-based CO2 capture process, it may require a cooling system to maintain the adsorption temperature if it increases to a value 7849

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Table 2. Optimum Adsorption Temperature and Porous Properties of Adsorbents Prepared from Different Silica Substrates and Amine Precursorsa BET (m2/g) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

o

PV (cm3/g)

PS (nm)

silica support

amine (wt %)

optimum temperature ( C)

support

adsorbent

support

adsorbent

support

adsorbent

SBA-15 SBA-15 SBA-15 SBA-15 SBA-15LP SBA-15SP SBA-15PLT PME PME PME PMC PE-MCM-41 PE-MCM-41 PE-MCM-41 MCM-41 HMS MPS PS

PEI-50 DAEAPTS-42 PEI-50 TEPA-60 PEI-50 PEI-50 PEI-55 PEI-55 PEI-30 PEI-20 PEP-55 PEI-55 PEI-55 PEI-30 TM-50 PEI-45 PEI-55 APTES-70

90 75−90 75−90 75 75 75 50−70 75 25 25 >100 >95 75 25 55 90 75 100

676 950 1099 580 746 590 570 570 570 1254 1254 570 570 451.5 952 230 133.0

15.9 80 121 no no no no 16.7 269 37.4 37.4