CO2 Uptake and Adsorption Kinetics of Pore-Expanded SBA-15

Nov 4, 2013 - Walter Christopher Wilfong , Brian W. Kail , Christopher W. Jones , Carlos Pacheco , and McMahan L. Gray .... David Madden , Teresa Curt...
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CO2 Uptake and Adsorption Kinetics of Pore-Expanded SBA-15 Double-Functionalized with Amino Groups Raúl Sanz,*,† Guillermo Calleja,† Amaya Arencibia,† and Eloy S. Sanz-Pérez‡ †

Department of Chemical and Energy Technology, ESCET. Universidad Rey Juan Carlos, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain ‡ Department of Chemical and Environmental Technology, ESCET. Universidad Rey Juan Carlos, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain ABSTRACT: The CO2 adsorption properties of amine double-functionalized SBA-15 adsorbents were investigated, focusing on the adsorption kinetics of these samples as compared to those prepared by grafting and impregnation. The doublefunctionalization method, recently developed to obtain amino-containing mesoporous solids as CO2 adsorbents with a high adsorption capacity and selectivity, consists of the impregnation of previously grafted materials. The adsorbents were prepared by grafting the pore-expanded SBA-15 with aminopropyl-trimethoxysilane (AP), followed by impregnation with either polyethyleneimine (PEI) or tetraethylenepentamine (TEPA). The influence of the amount of the organic molecule incorporated and the functionalization method on the CO2 adsorption results was studied, particularly on the kinetics and diffusion behavior. Equilibrium adsorption isotherms showed that the CO2 uptake was enhanced for materials prepared by the doublefunctionalization technique due to both the higher nitrogen content and the amino group efficiency. On the other hand, CO2 adsorption maximum rates were found to be significantly different for AP-grafted and PEI- or TEPA-impregnated materials, being higher for the latter adsorbents. The adsorption rate for impregnated materials depended on the type of organic molecule used and also on the amount loaded, especially for PEI. The adsorption rate was slightly lower for the double-functionalized samples than the simply impregnated sorbents, regardless of the organic compound used for impregnation (TEPA or PEI). However, the results showed that all double-functionalized sorbents exhibited a maximum adsorption rate of about 0.28 mol of CO2 per active nitrogen and minute, which was unchanged with the variation in the nitrogen content (9.8−15.3 %N) or the free pore volume of the sample after impregnation (0.05−0.47 cm3/g).

1. INTRODUCTION Among the technologies available for postcombustion CO2 capture in fossil-fuel-based thermal power plants, the adsorption on mesostructured silica materials functionalized with amino groups stands out, as it presents remarkable advantages for practical purposes.1 These sorbents are interesting candidates due to their very high selectivity toward CO2,2 their noticeable CO2 adsorption capacity under working conditions (45−75 °C, 0.15 bar CO2),3 and their lower energy requirement compared to amine solvent systems, that can be reduced up to 30−50% when the sorbent CO2 uptake is at least 2−3 mmol of CO2/g.4 Numerous research works have focused on the development of these valuable adsorbents with the aim of obtaining high CO2 adsorption capacities.1,5 The methods commonly used to incorporate aminocontaining molecules into mesostructured supports are based either on forming covalent bonds with the surface (grafting, cocondensation, and hyperbranching) or on generating weaker physical interactions (impregnation).5−8 Grafting of organosilanes usually yields a moderate organic incorporation but a high efficiency of amino groups in CO2 capture (measured as mol of CO2/mol of N),9−12 since tethered groups are homogeneously dispersed and easily reachable by CO2 molecules.13 On the other hand, the impregnation with aminated molecules (with tetraethylenepentamine, TEPA, and polyethyleneimine, PEI, being the most common chemicals) leads to a much higher organic incorporation.2,13−15 However, © XXXX American Chemical Society

these adsorbents achieve lower efficiencies due to the dense packing and the distribution of impregnated compounds in the porous structure, which limit the diffusion of CO2 molecules and their accessibility to every amino group.15 When highly loaded adsorbents are prepared, significant adsorption capacities can be expected as a result of the larger number of amino active sites. However, at the same time, a much lower void space inside the pores is available, so that diffusion and accessibility limitations are critical.15 Some alternatives have been proposed to overcome the above-mentioned disadvantages. The use of pore expanded materials such as PE-MCM-41 has already been proven more useful for CO2 capture than their conventional counterparts when functionalizing them either by grafting11 or impregnation.16 Similarly, grafted and impregnated PE-SBA-15 presented higher adsorption efficiencies of the amino groups17 in comparison with conventional SBA-15 sorbents.18,19 This was mainly due to the better dispersion of the organic groups, resulting in more amino moieties being easily accessible by CO2 molecules, and also in an easier CO2 diffusion through the filled pores. Another approach was to synthesize supports with a higher void volume, such as mesoporous silica capsules, which have been reported to enhance the CO2 adsorption efficiency of Received: August 1, 2013 Revised: November 4, 2013

A

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Scheme 1. Reaction between CO2 and Amino Groups under Anhydrous Conditions

amino groups.20 It was also described that the use of “as made” rather than calcined supports yielded a better dispersion of the impregnated molecules and an enhanced efficiency of amino groups in CO2 capture.16,21 Also, along this line, Choi et al. prepared adsorbents based on the impregnation of a mixture of PEI and aminopropyl-trimethoxysilane (AP), resulting in a better adsorption and desorption kinetics, although no improvement of the CO2 efficiency was registered.22 The impregnation of PEI together with aminated and non-aminated surfactants, with the aim of enhancing CO2 diffusion through the bulk of PEI, was recently studied. This last approach provided better results of CO2 uptake (up to 142 mg of CO2/g of ads), amine efficiency, adsorption kinetics, and reutilization behavior compared to simply impregnated supports.23 Our group has recently developed a new functionalization method based on the impregnation of previously grafted materials. This novel route, denominated double-functionalization, was applied to pore-expanded SBA-15 using AP and DT (diethylenetriaminopropyl-trimethoxysilane) for grafting and PEI and TEPA for impregnation. Interesting CO2 adsorption capacities, up to 235 mg of CO2/g of ads, were obtained,24 due to the high efficiency of the amino groups achieved, 0.45 of mol CO2/mol of N, very close to the theoretical maximum of 0.5 mol of CO2/mol of N (anhydrous conditions). The higher efficiency compared to simply impregnated materials was originated by the presence of tethered (grafted) and mobile (impregnated) amino groups. Mobile groups promote the CO2−NH2 reaction, specifically the second step of the adsorption mechanism, that consists of the formation of ammonium carbamate from the zwitteronic structure (see Scheme 1), as they favor the closeness of amino groups.26,27 Another contribution to the higher adsorption efficiency lies in the improved CO2 accessibility to the adsorption amino sites, since the presence of organosilanes may break the bulk PEI layer, resulting in scattered PEI aggregates rather than a uniform compact film, as previously suggested.23 The aim of the present work is to study the doublefunctionalization method more in depth, with special emphasis on the adsorption kinetics. The influence of the functionalization method (grafting, impregnation, and double-functionalization) and the kind and amount of the incorporated molecules on the CO2 adsorption properties are investigated. For this purpose, several single- and double-functionalized sorbents with a different amount of amino groups incorporated by grafting (AP) and impregnation (PEI or TEPA) were considered.

silicate) as a silica source, TIPB (1,3,5-triisopropylbenzene) as a swelling agent, and ammonium fluoride as a solubility enhancer (all from Sigma-Aldrich). A P123:TIPB molar ratio of 2.4:1 was employed, and the hydrolysis step was performed at 17 °C for 24 h. An aging step was then carried out at 100 °C for 48 h. The surfactant was removed from the mesostructure of the obtained material by ethanol extraction. 2.2. Functionalization of SBA-15. 2.2.1. Grafting. PE-SBA-15 was functionalized with aminopropyl-trimethoxysilane, AP (SigmaAldrich), following the procedure of grafting under toluene reflux. The amount of AP used was 2.29 mmol of AP/g of silica for PE-SBA-AP (2) and 6.87 mmol of AP/g of silica for PE-SBA-AP (6). These values correspond to the complete tethering with AP molecules of a siliceous surface with a hypothetical surface silanol concentration of 2 or 6 SiOH/nm2,19 with the stoichiometry of the reaction between AP organosilane molecules and silanol groups being of 1:1. Grafted material, named PE-SBA-AP (2), was dried at room temperature overnight, since the drying in air at mild temperatures (110 °C) leads to the degradation of the loaded amines and the subsequent loss of reactivity toward CO2.19,30 2.2.2. Impregnation. PE-SBA-15 was also functionalized by impregnation with PEI (branched polyethyleneimine, average molecular weight 800, ρ = 1.05 g/mL; molar ratio of primary, secondary, and tertiary amino groups of 1:1.1:0.7) and TEPA (tetraethylenepentamine, ρ = 0.998 g/mL) (Sigma-Aldrich). The amount of PEI and TEPA incorporated was either 30 or 50 wt % of organic content in the final sorbents. Methanol was used as a solvent with a methanol:silica weight ratio of 8:1.18 The impregnated materials were named PE-SBA-PEI (y) and PE-SBA-TEPA (y), with y referring to the organic percentage. These adsorbents were also dried at room temperature overnight. 2.2.3. Double-Functionalization. PE-SBA-AP (2) sample prepared by grafting of AP over PE-SBA-15 was further functionalized by impregnation with 30% PEI and TEPA, following the original procedure recently reported by our group with a methanol:silica weight ratio of 8:1. 24 The resulting solids were named PE-SBA-AP (2)-PEI (30) and PE-SBA-AP (2)-TEPA (30). Samples denoted as PE-SBA-AP (6)-PEI (30) and PE-SBA-AP (6)TEPA (30) were also prepared using an amount of AP grafting agent 3 times higher than mentioned above. The adsorbents synthesized with 50% PEI and TEPA (PE-SBA-AP (6)-PEI (50) and PE-SBA-AP (6)TEPA (50)) have been included for comparison purposes. 2.3. Characterization. 2.3.1. Physicochemical Characterization. Prepared materials were characterized by nitrogen adsorption− desorption at 77 K using a Micromeritics Tristar-3000 sorptometer to acquire N2 isotherms. Prior to the analysis, the samples were outgassed in N2 flow for 8 h at 200 °C (siliceous materials) and 150 °C (amino-containing materials). The BET surface was determined by applying the linearized BET equation in the range of relative pressures from 0.05 to 0.20. The pore size distribution was obtained by means of the BJH model from the adsorption branch of the isotherm (assuming cylindrical pore geometry), and the total pore volume was determined at a relative pressure around 0.97.31 Organic content, namely, organic nitrogen, was determined by elemental analysis in a Vario EL III Elementar Analyzer System GMHB. 2.3.2. Adsorption of CO2. Pure CO2 adsorption−desorption isotherms were obtained at 45 °C and with pressures ranging from 0 to 6 bar using a VTI Scientific Instruments HVPA-100 apparatus. Prior to the isotherms, degasification at 110 °C for 2 h under a vacuum

2. EXPERIMENTAL SECTION 2.1. Synthesis of Pore Expanded SBA-15 Mesostructured Silica. PE-SBA-15 material was prepared according to the procedure reported by Cao et al.,28 that modified the original method by Zhao et al.29 The detailed synthesis of pore-expanded SBA-15 (PE-SBA-15) can be found elsewhere.24,28 Briefly, the procedure involves the use of Pluronic P123 as a structure directing agent, TEOS (tetraethylortoB

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Figure 1. Nitrogen adsorption−desorption isotherms at 77 K for siliceous PE-SBA-15 and materials functionalized by grafting and impregnation: (a) experimental isotherms and (b) BJH pore size distributions.

Table 1. Textural Properties and Pure CO2 Uptake at 45 °C and 1 bar for Siliceous PE-SBA-15 and Materials Functionalized by Grafting and Impregnation CO2 adsorption capacity adsorbent

SBET (m2/g)

DPORE (nm)

VPORE (cm3/g)

N (wt %)

q (mg of CO2/g of sample)

qm (mol of CO2/mol of N)

PE-SBA-15 PE-SBA-AP (2) PE-SBA-PEI (30) PE-SBA-TEPA (30)

428 258 187 221

15.2 13.3 13.5 13.2

1.18 0.81 0.63 0.72

n/a 2.8 9.0 9.4

11.4 31.6 80.5 94.9

n/a 0.36 0.28 0.32

(5 × 10−3 mbar) and free volume measurement with helium were conducted. Free volume and isotherm points were aquired using the Sievert method considering two equilibrium criteria: a maximum equilibration time of 50 min or a pressure drop below 0.2 mbar/3 min. Kinetic data of CO2 adsorption were obtained from the mass gain curves acquired by thermogravimetric analyses with a 100 mL/min pure CO2 flow in a DSC-TGA thermobalance model SDT Simultaneous 2960 from TA Instruments. A degasification step at 110 °C for 2 h in 100 mL/min N2 was applied before each analysis. After this step, a CO2 flux of 100 mL/min was put in contact with the sample and the CO2 adsorption experiment was maintained for 3 h.

TEPA (30) sample). Figure 1 shows the nitrogen adsorption− desorption isotherms for these samples, and Table 1 summarizes their textural properties. As expected, a reduction in the adsorbed amount of N 2 was observed after functionalizing PE-SBA-15 by grafting or impregnation, e.g., the BET surface was reduced from 428 m2/g in the siliceous support to 258 m2/g for the grafted sample and to 187 and 221 m2/g for the impregnated materials (PEI and TEPA, respectively). A decrease in the pore volume was also observed, although the value is still significant for all functionalized samples, being higher than 0.60 cm3/g. In general, the decrease observed in the textural properties was proportional to the nitrogen content incorporated: 2.8% for PE-SBA-AP (2) grafted sample and 9.0 and 9.4% for PEI- and TEPAimpregnated materials, respectively. The amount of AP incorporated into PE-SBA-AP (2) sample corresponds to a grafting efficiency of 85%. Pure CO2 adsorption−desorption isotherms for all samples were carried out at 45 °C, between 0 and 6 bar (Figure 2). Two kinds of curves corresponding to different behaviors can be

3. RESULTS AND DISCUSSION 3.1. Single Functionalization of PE-SBA-15. Poreexpanded SBA-15 was chosen instead of the conventional material in order to provide a structure with a high pore volume still available after the organic functionalization, since it has already been demonstrated that pore-expanded-based adsorbents are more efficient in CO2 capture.17 In particular, a poreexpanded SBA-15 material was obtained by performing the hydrolysis step at 17 °C and the surfactant removal by ethanol extraction. This material was previously selected among a series of PE-SBA-15 samples as the best support for further organic incorporation.17 The characterization of PE-SBA-15 by low angle X-ray diffraction, N2 adsorption−desorption at 77 K, and transmission electron microscopy has already been reported,17 and just a brief description is shown here. This support exhibited a well-defined bidimensional hexagonal structure with a large cell parameter a0 of 16.3 nm due to the pore-expansion process. Also, noticeable textural properties were observed (Figure 1, Table 1), such as a BET surface area of 428 m2/g, a mean pore diameter of 15.2 nm, and a pore volume of 1.18 cm3/g, as well as a smaller particle size than conventional SBA-15. First, pure silica PE-SBA-15 was functionalized by grafting with aminopropyl (PE-SBA-AP (2) sample) and by impregnation with PEI (PE-SBA-PEI (30) sample) and TEPA (PE-SBA-

Figure 2. CO2 adsorption−desorption isotherms at 45 °C for siliceous PE-SBA-15 and materials functionalized by grafting and impregnation. C

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Figure 3. Nitrogen adsorption−desorption at 77 K for adsorbents obtained by double-functionalization: (a) experimental isotherms and (b) BJH pore size distributions.

functionalized materials.10,19 From the reversibility and pressure dependence of the CO2 isotherm of PE-SBA-AP (2), it can be inferred that the contribution of physical adsorption to the overall CO2 adsorption capacity is significant, in accordance with its low nitrogen content (2.8%, Table 1). This contribution has been previously reported to be around onethird in a similar AP-grafted material with 2.4% N.33 Thus, the calculation of the CO2/N ratio, that assumes that all the adsorption capacity is due to chemical adsorption, overestimates the efficiency of amino groups in samples with low nitrogen content. The adsorbents impregnated with 30% PEI and TEPA achieved efficiency values of 0.28 and 0.32 mol of CO2/mol of N, respectively (Table 1), which are higher than those obtained for conventional SBA-15 impregnated with the same organic amount (0.25 for PEI and 0.21 for TEPA).18,24 A similar increment was also found for materials loaded with 50% amines.15,17 In fact, the incorporation of 50% PEI to conventional and pore expanded SBA-15 yielded adsorbents with similar nitrogen contents (13.5 and 13.2%, respectively) but with different efficiencies, 0.19 mol of CO2/mol of N for conventional SBA-15 and 0.33 mol of CO2/mol of N for the pore-expanded material.17 At first sight, these results could be ascribed to diffusion and accessibility problems in the conventional support, as pore volumes were different, 0.09 cm 3 /g for SBA-PEI (50) and 0.22 cm 3 /g for PE-SBA-PEI (50).17,18 However, the impregnation of these two siliceous materials with only 30% PEI or TEPA resulted in adsorbents with pore volumes higher than 0.25 cm3/g in all cases. In this case, CO2 diffusion was not expected to be hindered, but still, samples prepared from pore-expanded supports showed a higher CO2 adsorption efficiency. These results indicate that the remaining pore volume is not a decisive factor in the amine efficiency, but rather the better dispersion of the organic groups over the surface of pore expanded support seems to allow a better accessibility of CO2 to the amino groups. When comparing efficiency values of AP-grafted and impregnated materials, it should be taken into account that used PEI is formed by primary, secondary, and tertiary amines in a 1:1.1:0.7 molar proportion. Tertiary amines are not reactive with CO2 under anhydrous conditions (see Scheme 1), so they should not be considered for the calculation of adsorption efficiencies. With this criterion, a more realistic value of efficiency for PE-SBA-PEI (30) is obtained: 0.37 instead of 0.32 mol of CO2/mol of N. However, since this calculation is not

observed: siliceous PE-SBA-15 showed a physisorption-like CO2 isotherm, i.e., highly dependent on pressure, with almost no CO2 uptake at low pressure, and a complete reversibility. Functionalized materials presented an important increase in the CO2 adsorption capacity due to their content in amino groups, which react selectively with CO2. The presence of amino groups resulted in isotherms with features assigned to chemical adsorption: a lower dependence with the pressure, a higher CO2 uptake at low pressures, and noncomplete reversibility. Thus, impregnated materials show chemisorption-like CO2 isotherms, while the curves for grafted samples, with lower nitrogen contents, exhibit a joint contribution of chemical adsorption on the amino groups and physical adsorption over the remaining sorbent surface. The amount of CO2 adsorbed by PE-SBA-AP (2) grafted sample at 45 °C and 1 bar was 31.6 mg of CO2/g of ads, half the value registered for PE-SBA-AP (6) (65.7 mg of CO2/g of ads), in line with their relative nitrogen contents (2.8 and 4.4%, respectively).24 The best result reported for AP-grafted samples was obtained by Sayari and co-workers using pore-expandedMCM-41 silica. A 5.9% N was incorporated into the support, yielding a CO2 uptake as high as 90.2 mg of CO2/g of ads at 25 °C with 5% CO2.25 The CO2 uptake for PEI- and TEPA-impregnated samples at 1 bar, shown in Table 1, reached values of 80.5 and 94.9 mg of CO2/g of ads, which are higher than those corresponding to similarly impregnated samples with conventional SBA-15.18,24 This result is ascribed to the larger pore diameter of PE-SBA-15, which favors PEI and TEPA diffusion and avoids pore blockage, resulting in a better accessibility to the amino groups. Also, the smaller particle size of PE-SBA-15 (smaller total pore length) compared to SBA-15 favors CO2 diffusion and increases the adsorption uptake, as previously reported.32 There is a wide variety of solid structures in the literature where PEI or other amine-containing compounds have been impregnated. Considering just mesostructured supports, the CO2 uptakes obtained range between 44 and 138 mg of CO2/g of ads (at 1 bar and ca. 45 °C), with the latter corresponding to the impregnation of 50% PEI in the same pore-expanded SBA15 support presented here.17 The efficiency of amino groups in CO2 capture is estimated from the CO2/N molar ratio, as previously mentioned, with 0.5 mol of CO2/mol of N being the maximum achievable value under anhydrous conditions.26 AP-grafted PE-SBA-15 material presented a value of 0.36 mol of CO2/mol of N, in agreement with the high efficiency generally observed for aminopropylD

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Table 2. Textural Properties and Pure CO2 Uptake at 45 °C and 1 bar for Double-Functionalized Samples from PE-SBA-15 CO2 adsorption capacity adsorbent PE-SBA-AP (2)-PEI PE-SBA-AP (6)-PEI PE-SBA-AP (2)-TEPA PE-SBA-AP (6)-TEPA PE-SBA-AP (6)-TEPA PE-SBA-AP (6)-PEI

(30) (30) (30) (30) (50) (50)

SBET (m2/g)

DPORE (nm)

VPORE (cm3/g)

N (wt %)

q (mg of CO2/g of sample)

qm (mol of CO2/mol of N)

99 38 144 80 19 n/a

12.7 10.6 11.5 9.7 n/a n/a

0.34 0.12 0.47 0.23 0.05 n/a

9.8 10.7 10.6 11.1 15.3 15.0

103.4 111.1 128.9 139.1 215.0 123.1

0.41 0.33 0.39 0.40 0.45 0.26

(0.34 and 0.47 cm3/g) than for their impregnated counterparts (0.63 and 0.72 cm3/g). Thus, the small influence of the available pore volume by itself on the amine efficiency during CO2 capture is again revealed. As previously proposed, the second step of the reaction between CO2 and amino groups (Scheme 1) is very likely favored by the presence of tethered (AP) and mobile groups (either TEPA or PEI) in the same adsorbent, as the proximity of two amino groups seems much more probable.24 Also, the better dispersion of impregnated molecules in the PE-SBA-AP (2) material in contrast with PE-SBA-15 may be playing a significant role in the increase of the CO2 capture efficiency.16,34 In order to study more in depth the performance of these adsorbents, the kinetics of CO2 adsorption were studied by carrying out dynamic analyses in a thermobalance under a pure CO2 flow. Figure 5 shows the CO2 mass uptake for samples functionalized (a) by grafting and impregnation independently and (b) by double-functionalization. As seen, the adsorption capacity increased very fast in the early stages of the process, being followed by a much slower approach to equilibrium for all samples. This tendency is frequently observed for aminofunctionalized silicas,21,35−39 with the slow second step being attributed to several possibilities: (i) a hindered diffusion through the amino layer of the adsorbent,15,35 (ii) a slower reaction of CO2 with secondary amino groups compared to primary amino groups,37 and (iii) the chemical rearrangement from carbamate to carbonate species in the presence of water,25,39 although the latter only applies when moisture is present. The comparison of the kinetic profiles for materials prepared by conventional one-step functionalization or double-functionalization procedures shows small differences in the overall process. PE-SBA-AP (2), PE-SBA-PEI (30), and PE-SBA-TEPA (30) achieved CO2 uptakes after 30 min of 30.7, 69.0, and 83.8 mg of CO2/g of ads, respectively, which correspond to 88, 92, and 90% of the final uptake values after 180 min. However, the CO2 uptake of double-functionalized samples kept increasing very slowly in the long time region. In this case, the kinetic behavior could also be the result of the continuous reorganization of mobile impregnated molecules in order to react with CO2 molecules in cooperation with the tethered amino moieties from the organosilanes. The adsorption rate, calculated as the derivative of the CO2 uptake versus time, is also plotted in Figure 5c and d. Impregnated materials presented a higher adsorption rate compared to the AP-grafted adsorbent (Figure 5c) due to their higher nitrogen content, the maximum rate values being of 96 mg of CO2/g of ads·min for PE-SBA-TEPA (30), 80 mg of CO2/g of ads·min for PE-SBA-PEI (30), and 22 mg of CO2/g of ads·min for PE-SBA-AP (2).

common in publications based on PEI adsorbents for CO2 capture,1,5 it will not be considered in this paper. 3.2. Double-Functionalization of PE-SBA-15. The double-functionalization technique consists of the impregnation of previously grafted materials. PE-SBA-AP (2) sample with 2.8% N presented a high pore volume (0.81 cm3/g) available for a further functionalization by impregnation. Thus, PE-SBAAP (2) was loaded with 30% of either PEI or TEPA. The organic amount selected for the impregnation was relatively low in order to obtain samples with a sufficient remaining pore volume, with the aim of favoring CO2 diffusion. The nitrogen adsorption−desorption isotherms for these materials and their pore size distributions are shown in Figure 3, while their textural properties are listed in Table 2. The reduction in the N2 adsorbed amount was higher for doublefunctionalized samples than for impregnated materials. This reduction is in agreement with the higher organic content incorporated by the double-functionalization technique (9.8 and 10.6 wt % N for PEI and TEPA samples, respectively, Table 2) compared with impregnated materials (9.0 and 9.4 wt %, Table 1). However, the most important difference between the impregnated and double-functionalized samples was their behavior in CO2 adsorption. Pure CO2 isotherms at 45 °C are shown in Figure 4, and uptake values at 1 bar are

Figure 4. CO2 adsorption−desorption isotherms at 45 °C for adsorbents obtained by double-functionalization using AP (2) and PEI (30) or TEPA (30).

listed in Table 2. Double-functionalized samples presented adsorption capacities of 103.4 and 128.9 mg of CO2/g of ads for PEI and TEPA, respectively, at 1 bar, while impregnated materials were just 80.5 and 94.9 mg of CO2/g of ads (Table 1). This fact is due to the higher nitrogen content of doublefunctionalized samples and also due to their higher efficiency in CO2 capture (0.41 for PEI and 0.39 mol of CO2/mol of N for TEPA) compared to impregnated samples (0.28 for PEI and 0.32 mol of CO2/mol of N for TEPA). It is worth noting that pore volume is much smaller for double-functionalized samples E

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Figure 5. Mass of CO2 uptake in a pure CO2 flow and adsorption rate for (a, c) grafted and impregnated PE-SBA-15 and (b, d) doublefunctionalized PE-SBA-15.

Figure 6. CO2 molar adsorption rate for (a) grafted, impregnated and (b) double-functionalized PE-SBA-15.

The double-functionalized sample with TEPA, PE-SBA-AP (2)-TEPA (30), showed a similar adsorption rate (92 mg of CO2/g of ads·min) to the material only impregnated. On the contrary, the sample obtained by double-functionalization with PEI (30), PE-SBA-AP (2)-PEI (30), showed a maximum adsorption rate of 58 mg of CO2/g of ads·min, much lower than the value corresponding to PE-SBA-PEI (30). In both cases, double-functionalized samples presented a higher nitrogen content and a smaller pore volume than only impregnated samples (Tables 1 and 2). Therefore, it seems that the strongest argument to explain the differences found between PEI and TEPA materials could be based on the dissimilar disposition of both organic compounds. TEPA molecules are expected to be homogeneously spread out on the surface, without hindering CO2 diffusion, while more viscous PEI molecules cause poor diffusion40 and probably a certain blockage of pores.13 With the aim of normalizing the above-mentioned results taking into account the nitrogen content that is active for CO2

capture, the adsorption rate was estimated analyzing the molar CO2/N adsorption capacity instead of the net mass CO2 uptake. Also, in the case of PEI-containing samples, tertiary amino groups were not considered for this estimation, since they do not react with CO2 in the absence of water, as previously explained. Figure 6a shows the molar adsorption rate of samples prepared by grafting with AP and by impregnation with PEI and TEPA. Also, samples impregnated with 50% PEI and TEPA, i.e., with a high organic loading, have been included for comparison purposes. CO2 adsorption rates of impregnated materials referred to the nitrogen content were in the range 0.30−0.37 mol of CO2/ mol of N·min, higher than for the grafted sample, 0.27 mol of CO2/mol of N·min. This difference is probably due to the dissimilar contribution of the adsorption mechanisms that take place in grafted and impregnated materials. For the APfunctionalized adsorbent, there is a combination of relatively fast chemical adsorption on the amino groups and slower physical adsorption on the adsorbent surface, where there is not F

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Consequently, it is verified that the higher CO2 uptake obtained by adsorbents prepared by double-functionalization (Table 2) has its origin in the better performance of amino groups, that results in a higher efficiency in CO2 capture. Moreover, samples prepared by the double-functionalization technique have reached CO2 adsorption capacities up to 235 mg of CO2/g of ads at 45 °C and 0.15 bar CO2 with 5% moisture.24 This result is significant in comparison with other recent approaches, such as the joint impregnation of AP and PEI (99 mg of CO2/g of ads, 25 °C),22 the surfactantpromotion (142 mg of CO2/g of ads, 30 °C),23 the impregnation of surfactant-occluded supports (200−237 mg of CO2/g of ads, 75 °C),16,21 the hyperbranching technique (246 mg of CO2/g of ads, 25 °C),7 or the use of mesoporous capsules (349 mg of CO2/g of ads, 75 °C).20 Finally, several adsorption−desorption and regeneration cycles were carried out for double-functionalized samples prepared from PE-SBA-AP (2). The sample PE-SBA-AP (2)-PEI (30) successfully maintained its CO2 uptake after five consecutive runs, presenting a final value of 98.1 mg of CO2/g of ads, that represent 95% of the value obtained in the first cycle. PE-SBA-AP (2)-TEPA (30) material underwent a slight but sustained decrease along the five cycles, with a final adsorption capacity representing 87% of the initial value. These results are consistent with numerous publications, where PEI polymer is shown to be highly stable and CO2 uptake is maintained after several adsorption−desorption cycles.16,18,24,40,41 On the other hand, TEPA-containing materials generally suffer a certain decrease of CO2 uptake with the number of cycles,24,41,42 although some exceptions of highly stable TEPA-containing materials have been reported.43

such a strong chemical interaction. However, for impregnated materials, which have a higher organic loading, only the chemisorption mechanism occurs, since practically all the surface area of the adsorbent is covered by the organic compound, with no significant amine-free space. For impregnated samples, when the organic amount increases from 30 to 50%, the adsorption rate significantly decreased for PEI-based adsorbents (0.37−0.32 mol of CO2/ mol of N·min), pointing out that the maximum adsorption rate for these materials directly depends on the pore volume, as this value decreases from 0.63 to 0.22 cm3/g. By contrast, maximum velocity related to the active nitrogen content was not altered for TEPA-containing samples (0.30−0.32 mol of CO2/mol of N·min) although pore volume was reduced from 0.72 to 0.49 cm3/g, confirming that pore volume is not critical on the CO2 adsorption kinetics in this case. Thus, as previously stated, the differences found on the impregnated samples are probably due to the different physical properties, molecular size, and nature of PEI and TEPA rather than differences in available pore volume. Since PEI is a viscous polymer, the loading of a higher organic amount results in a steric hindrance for CO2 diffusion, and also, it may lead to the blockage of pore openings.13 On the contrary, no influence on the adsorption rate was observed for the impregnation of TEPA at high loadings like 50% because TEPA yields homogeneously covered materials and the lower viscosity of TEPA does not significantly affect the CO2 diffusion. The CO2 adsorption kinetics in terms of normalized rate for the double-functionalized samples are shown in Figure 6b. Samples prepared with AP (2) and AP (6) further impregnated with 30% and 50% PEI and TEPA are presented. As seen, virtually all samples presented the same maximum rate of adsorption, around 0.28 mol of CO2/mol of N·min, despite their different nitrogen content, porosity, and CO2 adsorption capacity, the former ranging from 9.8 to 15.3% N and the latter from 103 to 215 mg of CO2/g of ads (Table 2). The similarity among the maximum adsorption rates is remarkable when the remaining pore volume of the samples is analyzed. PE-SBA-AP (2)-TEPA (30) and PE-SBA-AP (2)-PEI (30) have pore volumes of 0.47 and 0.34 cm3/g, respectively, where CO2 is supposed to diffuse very easily. On the contrary, PE-SBA-AP (6)-PEI (30), PE-SBA-AP (6)-TEPA (30), and PE-SBA-AP (6)-TEPA (50) presented values of 0.12, 0.23, and 0.05 cm 3 /g, respectively, being even negligible for PE-SBA-AP (6)-PEI (50). Despite a significant hindrance in the CO2 diffusion might be expected for these samples, all of them showed very similar adsorption rates, indicating that this parameter is maintained for highly loaded double-functionalized samples and that the maximum adsorption rate does not have a direct correlation with the free pore volume. In the case of PEI, it is noteworthy that samples PE-SBA-AP (2)-PEI (30) and PE-SBA-AP (6)-PEI (30) presented a similar kinetic behavior despite the higher organic loading of the latter. Sample PE-SBA-AP (6)-PEI (50) achieved a maximum adsorption rate of 0.26 mol of CO2/mol of N·min, just slightly lower than the common value for the rest of the double-functionalized samples (0.28 mol of CO2/mol of N· min). It can be pointed that, despite the higher loading of doublefunctionalized materials and the subsequent reduction of porosity compared to just impregnated sorbents, the adsorption rate of the former was maintained even for the highest organic loadings and is similar to that of impregnated samples.

4. CONCLUSIONS The CO2 adsorption properties of a series of pore-expanded SBA-15-based materials synthesized by the double-functionalization technique were studied by comparing their adsorption capacity, amine efficiency, and kinetic behavior. Key differences were found among grafted, impregnated, and double-functionalized materials, the latter combining a high nitrogen content and a noteworthy efficiency of the amino groups. The CO2 adsorption rate was found to be higher for impregnated materials than for AP-grafted samples. The type of amine molecules influences the kinetics of the CO2 adsorption process, with faster kinetics being observed for the impregnation of TEPA compared to PEI. However, when only active amino groups are considered, the CO2 adsorption rate is higher for PEI-impregnated molecules at low nitrogen content, being reduced for higher PEI loadings. For TEPA-containing adsorbents, the maximum adsorption rate was found to remain constant. Double-functionalized materials present similar adsorption rates to their impregnated counterparts, so that the superior CO2 uptake and CO2 adsorption efficiency of the former are assigned to the better adsorption performance of the incorporated amino groups. Moreover, double-functionalized materials underwent small variations in their adsorption kinetics when changing the functionalization agent and the loaded amount, being independent of the available porosity of the material. Maximum adsorption rates were maintained even for adsorbents whose pores are completely filled with amino containing molecules, showing an extremely reduced pore volume, as a result of the improved dispersion of the organic compounds incorporated. G

dx.doi.org/10.1021/ef4015229 | Energy Fuels XXXX, XXX, XXX−XXX

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Article

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 91 488 8093. Fax: +34 91 488 7068. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors want to thank Madrid Government and Rey Juan Carlos University for supporting this research through the Regional Project URJC-CM-2010-CET-5618.

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dx.doi.org/10.1021/ef4015229 | Energy Fuels XXXX, XXX, XXX−XXX