CO2 Adsorption on Polyethylenimine-Functionalized SBA-15

Article pubs.acs.org/jced

CO2 Adsorption on Polyethylenimine-Functionalized SBA-15 Mesoporous Silica: Isotherms and Modeling Nicola Gargiulo,† Antonio Peluso,† Paolo Aprea,† Francesco Pepe,*,‡ and Domenico Caputo† †

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Napoli, Italy ‡ Dipartimento di Ingegneria, Università del Sannio, P.zza Roma 21, 82100 Benevento, Italy ABSTRACT: SBA-15 mesoporous silica was functionalized with polyethylenimine and was used as a substrate for CO2 adsorption. The synthesized material, denoted SBA-15-PEI, was characterized by means of X-ray diffraction, thermogravimetric analysis, and N2 adsorption/desorption at 77 K, in order to prove that polymer chains efficiently filled the pores of functionalized samples. CO2 adsorption isotherms on SBA15-PEI were evaluated at T = (298, 313, 328, and 348) K for pressures up to 100 kPa by means of a volumetric technique. The experimental data showed a significant dependence of the CO2 adsorption capacity on temperature, with the highest capacity encountered at the highest temperature explored. Despite this unusual behavior, CO2 adsorption on SBA-15-PEI was satisfactorily modeled by means of the Sips isotherm. The modeling effort allowed to evaluate the isosteric heat of adsorption as a function of the fractional coverage of SBA-15-PEI. The comparison between the results obtained in the present work and those relative to CO2 adsorption on “benchmark” microporous substrates, such as 13X zeolite and Cu-BTC metal organic framework, allowed us to highlight significant analogies and differences with those other solids, giving interesting hints on the possible applications of SBA-15-PEI.

1. INTRODUCTION

CO2 removal from gaseous streams has been historically carried out by physical or chemical absorption, using aqueous solvents, such as moderately concentrated amine solutions or potassium carbonate solutions. An important alternative technology is represented by adsorption,4 commonly performed as pressure swing adsorption (PSA) or, less frequently, as vacuum swing adsorption (VSA). The materials more often considered for PSA/VSA processes have been microporous adsorbents that are characterized by pore sizes below 2 nm. Among these, both conventional adsorbing materials (such as silica−gel, activated alumina, and activated carbon) and alumino−silicates of the class of zeolites have been considered.4 Concerning zeolites, 13X zeolite, that is characterized by a relatively high surface basicity, has demonstrated to be a very suitable adsorbent for CO2 capture by PSA processes.5 Despite the broad use of zeolites for CO2 capture, in the last two decades intense research efforts have been committed to adsorption-based CO2 separation processes, and more specifically to the identification of novel, more selective adsorbents, since their development has the potential of notably improving the performance of the adsorption process. As examples, various adsorbent materials belonging to the class of microporous metal organic frameworks (MOFs) revealed themselves

Carbon dioxide (CO2) is the most significant contributor to the greenhouse effect. However, more than 15 years after the Kyoto protocol (1997), emissions of CO2 and other greenhouse gases continue to increase.1 As a result, it is crucial to set up suitable technologies to avoid CO2 production in the first place and to prevent its release into the atmosphere when no other possible choice to its production exists. Anthropogenic CO2 emissions mainly arise from combustion processes, although also some other industrial processes, such as hydrogen production, give relevant contributions.2 One possible approach to prevent CO2 emissions from these processes is to remove this species from the gaseous streams in which it is contained, in order to subsequently dispose of it in deep underground areas, such as exhaust oil reservoirs or ocean sediments. Furthermore, it is important to observe that the problem of CO2 removal from a gaseous stream also occurs in other situations in addition to the control of CO2 release into the atmosphere. Indeed, in cryogenic air separation plants, air sent to liquefaction must be de facto CO2-free; otherwise a blockage due to possible deposition in the heat exchange apparatus could result.3 Likewise, in NH3 synthesis, CO2 is significantly present in the H2 stream that originates from the steam reforming or the partial oxidation steps, and its level must be cut down to parts per million levels in order to prevent complications with the synthesis reactor.3 © 2014 American Chemical Society

Received: December 10, 2013 Accepted: February 24, 2014 Published: March 4, 2014 896

dx.doi.org/10.1021/je401075p | J. Chem. Eng. Data 2014, 59, 896−902

Journal of Chemical & Engineering Data

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aim of having the amount of polymer approximately capable of filling the total pore volume of the hosting system (the value of which is reported in the Results and Discussion section). The resulting solid was labeled as SBA-15-PEI. 2.2. Characterization of SBA-15 and SBA-15-PEI. The mesoporous materials were characterized before and after modification by X-ray diffraction (XRD, Philips X’Pert diffractometer, Cu Kα rad.). Thermogravimetric (TG) analysis was carried out with a Netzsch STA 409 Luxx device using samples with masses of about 20 mg, which were heated in a nitrogen flow from ambient temperature up to 873 K at a rate of 10 K·min−1 and using α-alumina as reference. Microporosimetric characterization was carried out by N2 adsorption/ desorption cycles at 77 K using a Micromeritics ASAP 2020 volumetric instrument. The instrument controls both operating temperature (± 0.1 K) and pressure (± 0.5 Pa), and adsorbed amounts are indirectly measured on the basis of pressure variations inside the adsorption chamber (estimated error in the calculation of the adsorbed amount: ± 0.1 mg). An accurately weighted sample (± 0.1 mg) of the material under investigation was used for adsorption/desorption experiments. The specific surface area was evaluated by means of the BET method, the total pore volume was estimated from N2 adsorbed amounts at p/p0 = 0.99, and the pore size distribution was obtained by applying the BJH method to the desorption branches of the isotherms. Prior to characterization, nonfunctionalized SBA-15 was degassed at 423 K for 15 h, while SBA-15-PEI, in light of the low thermal stability that usually characterizes molecular baskets,23 was degassed at 348 K for 15 h. 2.3. CO2 Adsorption on SBA-15-PEI. CO2 adsorption isotherms on SBA-15-PEI at four different temperatures, namely, T = 298, 313, 328, and 348 K, and with p ≤ 100 kPa, were obtained using the ASAP 2020 instrument mentioned above. The experimental uncertainties were those mentioned in section 2.2. The highest operating temperature was selected considering the fact that for T > 350 K incipient sorbent degradation occurs. It has to be observed that, since ASAP-series devices were mainly designed to work at the boiling temperatures of noble/inert gases, the Dewar flask in which the sample tube is usually immersed was substituted by an ad hoc container, whose outer shell was filled with flowing thermostatted water. Prior to each adsorption experiment, SBA15-PEI was degassed under high vacuum at 348 K for 15 h. Furthermore, in order to verify the reversibility of the adsorption process, the sample tube was weighed after every re-degassing step that followed the single adsorption run.

as good candidates for improving performances of adsorptionbased CO2 sequestration processes.6−8 In recent years, the use of immobilized amines9 and, in particular, of polyethylenimine (PEI) stabilized on a highsurface-area solid to adsorb carbon dioxide was proved to give very good results even in situations involving life support, such as the “Regenerable CO2 Removal System” (RCRS) of the Space Shuttle.10 The main characteristic of this CO2 capture method relies on a process that evolves through the formation of ammonium carbamates as reported in the following reaction equations: CO2 + 2RNH 2 → RNHCOO− + RNH3+

(1)

CO2 + 2R 2NH → R 2NCOO− + R 2NH 2+

(2)

CO2 + R 2NH + R′NH 2 → R 2NCOO− + R′NH3+

(3)

Starting from this concept, Xu et al. developed the so-called “molecular baskets”, which consist of mesoporous silicas functionalized with PEI chains,11 and this pioneering work kicked off a very intense investigation activity on the CO2 adsorption properties of such materials in the past decade.12−25 As an example, Son et al. synthesized and impregnated with PEI a series of mesoporous silicas, finding a maximum CO2 adsorption capacity higher than 3 mol·kg−1 for PEI-loaded KIT-6 mesoporous silica.14 Among the different substrates used for preparing molecular baskets, SBA-15 mesoporous silica that is one of the most extensively studied mesostructures so far was obviously taken into account.12−16,18,19,21,24,25 However, there is still a lack of a thorough description of the thermodynamics of CO2 adsorption on such material in its PEI-functionalized form. On this basis, the aim of this study is the modeling of CO2 adsorption isotherms on PEI-functionalized SBA-15 mesoporous silica. The isotherms were obtained at four different temperatures between (298 and 348) K at pressures up to 100 kPa, and the experimental data were then treated by means of the Sips model to find the values of the isosteric heat of adsorption (i.e., the ratio of the infinitesimal change in the adsorbate enthalpy to the infinitesimal change in the amount adsorbed) and other relevant parameters such as CO 2 maximum adsorption capacity, adsorbent affinity toward CO2 and heterogeneity level of the adsorption process.

2. MATERIALS AND METHODS 2.1. Preparation of PEI-functionalized SBA-15. SBA-15 mesoporous silica was synthesized according to a recipe from literature:26 A total of 2.0 g of Pluronic P123 (Aldrich) was dissolved in 15 g of ultrapurified water and 60 g of 2 M HCl solution with stirring at room temperature. Then, 4.25 g of tetraethylorthosilicate (TEOS, Aldrich) was added into that solution with stirring at room temperature for 20 h. The mixture was aged at 353 K for 24 h without stirring. The solid product was filtered, washed, and air-dried at room temperature. Calcination was carried out at 773 K for 6 h (heating rate: 1 K·min−1). Samples of SBA-15 were functionalized with PEI using the wet impregnation method reported by Xu et al..11 Typically, 0.75 g of PEI (Aldrich) were dissolved in 4 mL of methanol (Aldrich). After stirring for 15 min, 1 g of SBA-15 was added to the mixture, which was kept under stirring for about 30 min and then dried at 343 K for 16 h under reduced pressure (about 90 kPa). The (PEI)/(SBA-15) weight ratio of 0.43 which was used in the wet impregnation procedure was chosen with the

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of nonimpregnated and PEIloaded SBA-15 mesoporous silicas. The pattern of the nonimpregnated SBA-15 sample reports, as typical for this material, an intense diffraction peak corresponding to the (100) plane and two weak diffraction peaks corresponding to (110) and (200) planes. On the contrary, only one broad (100) peak is presented by the PEI-functionalized SBA-15 samples; moreover, the impregnation with PEI gave rise to a strong decrease in the intensity of the aforementioned peak, together with a shift of its position toward higher 2θ values: this can be ascribed to the pore-filling effects that can reduce the scattering contrast between the pores and the silica walls27 and was already observed in other cases of impregnation of mesoporous structures with organic compounds.28 897



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m2·g−1, while the total pore volume is 0.70 cm3·g−1, and the main peak of the BJH pore size distribution, as reported in Figure 3b, is centered around 7.5 nm. To verify that PEI is contained inside the pores of the mesostructured carrier, a N2 adsorption/desorption cycle at 77 K, whose results are reported in Figure 4, was also performed on SBA-15-PEI: as a

Figure 1. X-ray diffraction patterns (Cu Kα radiation) of nonimpregnated and PEI-loaded SBA-15. Solid line: nonimpregnated sample. Dashed line: PEI-loaded sample.

Figure 2 shows the results of the TG analysis performed on SBA-15-PEI: two main weight losses are observable, the first

Figure 4. N2 adsorption (circles)/desorption (squares) isotherm at 77 K of PEI-functionalized SBA-15 mesoporous silica.

consequence of the successful functionalization process, the registered isotherm is classifiable as type II, which is typical of nonporous materials.29 The very negligible hysteresis loop that is still visible in Figure 4 is pretty similar to that pointed out for MCM-41 samples whose mesopores were completely filled with PEI;30 in similar accordance with the aforementioned case, BET specific surface area of SBA-15-PEI turned out to be 5 m2· g−1, that is a value typical of nonporous particles. CO2 adsorption isotherms on SBA-15-PEI at (298, 313, 328, and 348) K for CO2 pressures ranging from (0 to 100) kPa were obtained. As mentioned above, the highest operating temperature was selected considering the fact that for T > 350 K incipient sorbent degradation occurs. Figure 5 reports the four adsorption isotherms, together with fits to the Sips

Figure 2. Thermogravimetric (TG) analysis of PEI-functionalized SBA-15 (continuous curve) and PEI (dashed curve).

one occurring between (350 to 450) K and the second one occurring between (500 to 700) K. Since nonfunctionalized SBA-15 is thermally stable at least up to the calcination temperature of 773 K and the SBA-15-PEI sample used for the TG analysis was almost moisture-free, the two weight losses of Figure 2 can be interpreted in terms of a two-step decomposition of PEI chains, as also pointed out for similar materials.23 Figure 3a shows the N2 adsorption/desorption isotherm at 77 K on nonfunctionalized SBA-15: the shape of the isotherm is clearly classifiable as IUPAC type IV, which is typical of mesoporous materials;29 moreover, a H1-type hysteresis loop is highlighted. The BET specific surface area turned out to be 752

Figure 3. (a) N2 adsorption (circles)/desorption (squares) isotherm at 77 K and (b) BJH differential pore size distribution of nonfunctionalized SBA-15 mesoporous silica.

Figure 5. CO2 adsorption isotherms on SBA-15-PEI at 298 K (a), 313 K (b), 328 K (c), and 348 K (d). Symbols: experimental data. Continuous lines: best fitting Sips theoretical isotherms. 898



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aminosilanes), the effect of the pore curvature on the CO2 adsorption efficiency should be considered negligible.32 In order to have a clearer understanding of the phenomena examined, a modeling effort was undertaken using the semiempirical three parameter Sips isotherm.33 The Sips isotherm (sometimes called the “Langmuir−Freundlich isotherm”) is a semiempirical model which contains mathematical aspects of both the Langmuir and Freundlich isotherms: even if its thermodynamic consistency shows limits in the very low pressure region (it does not reduce to Henry’s law), its simple expression does not require the definition of the saturation pressure for the adsorbate, thus being suitable to process both subcritical and supercritical isotherms, even when a strong deviation from purely physical adsorption exists. According to this equation, the pressure dependence of the adsorbed amount takes the following form:

equation (see below). Figure 6 shows a magnification of the low pressure range of the isotherms reported in Figure 5, in

Figure 6. CO2 adsorption isotherms on SBA-15-PEI up to 10 kPa at T = 298 K (circles), 313 K (squares), 328 K (diamonds), and 348 K (triangles). Continuous lines: best fitting Sips theoretical isotherms.

q = qmax

which the abscissas are plotted on a logarithmic scale. Table 1 reports experimental values of the points shown in Figures 5 and 6. It is important to point out that the re−weighing procedure described above did not show any significant weight change, suggesting an essentially reversible phenomenon. Inspection of Figure 5 shows that the CO2 adsorption capacity progressively increases as temperature increases, rather than decreasing, as is the case of most of the adsorption processes, including those involving CO2 capture by activated carbons, zeolites, and even mesoporous silicas functionalized with nonpolymeric amines. In particular, the highest CO2 adsorption capacity, i.e., about 1.6 mol·kg−1, was observed at the highest temperature explored, i.e., 348 K. This result is comparable to others recently reported in the literature for similar materials24 and also for more traditional adsorbents, such as alkali metal exchanged FER zeolites with different Si/Al ratios, whose CO2 adsorption mechanisms are proved to be fairly different from those of PEI-functionalized mesoporous silicas.31 Indeed, according to the “molecular basket” theory,14 the progressive increase of CO2 adsorption capacity with temperature can be interpreted by assuming that, when temperature rises, PEI chains become more and more flexible, leading to an increase in the number of CO2-affine sites.14 Moreover, because wet impregnation is a functionalization technique that exploits the whole pore volume of SBA-15 and not just its pore surface (as in the case, e.g., of grafting with

(bp)1/ n 1 + (bp)1/ n

(4)

where qmax, b, and n are model parameters, and in particular qmax represents the maximum adsorption capacity, b is the affinity constant, and n is the heterogeneity coefficient (in particular for n = 1 the Sips isotherm reduces to the Langmuir isotherm, which applies to homogeneous adsorbent−adsorbate systems). Sips parameters are in general dependent on temperature, as reported by Do,34 but the value of n, depending on the nature of the adsorbent−adsorbate interaction, can presumably be considered independent from temperature.8,35 In this case, keeping qmax independent from temperature would clearly be inconsistent with the experimental results reported in Figure 5; therefore, considering that the choice of the temperature-dependence form of qmax can be arbitrarily chosen,34 the following exponential function was employed: qmax = qmax ,0exp[χ (1 − T0/T )]

(5)

in which qmax,0 is the value of qmax at a reference temperature T0 and χ is a dimensionless parameter, having positive value. As regards the dependence of the affinity constant on temperature, it is described by the following equation, reported by Do:34 ⎡ Q ⎤ (T0/T − 1)⎥ b = b0exp⎢ ⎣ RT0 ⎦

(6)

Table 1. CO2 Adsorption Isotherms on SBA-15-PEIa 298 Kb pc kPa 0.0022 0.0044 0.0107 0.1018 0.7882 7.5692 26.605 53.483 101.11

313 Kb qd mol·kg

pc −1

0.31717 0.41276 0.50047 0.68966 0.76607 0.77424 0.78902 0.80718 0.81315

kPa 0.0043 0.0063 0.0149 0.0454 0.7530 7.2532 26.763 53.379 101.24

328 Kb qd mol·kg

pc −1

kPa

0.30884 0.41252 0.49734 0.69898 0.96042 1.01154 1.02591 1.04273 1.05969

0.0076 0.0164 0.0251 0.0970 0.2454 7.5886 26.762 53.354 101.19

348 Kb qd mol·kg

−1

0.29098 0.41025 0.50214 0.71702 0.90280 1.18291 1.20465 1.23598 1.23599

pc

qd

kPa

mol·kg−1

0.0123 0.0439 0.1066 0.2588 0.7970 7.5315 26.814 55.320 101.35

0.26507 0.37246 0.48024 0.72727 1.05810 1.38100 1.53213 1.55207 1.55208

p: pressure. q: CO2 adsorbed amount. bMeasured with 0.1 K uncertainty. cMeasured with 0.5 Pa uncertainty. dCalculated with 2.3·10−3 mol•kg−1 uncertainty. a

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in which b0 is the value of b at T0, R is the gas constant, and Q is a measure of the adsorption heat. The experimental data concerning CO2 adsorption on SBA15-PEI were submitted to nonlinear regression, using the Matlab surface fitting toolbox, in order to calculate simultaneously the optimal values of the parameters of eqs 4−6, i.e., qmax,0, χ, b0, Q, and n, for the isotherms reported in Figure 5. The calculated values of the parameters, obtained using T0 = 298 K, are reported in Table 2, and the comparison between Table 2. Sips parameters for CO2 adsorption on SBA-15PEIa parameter

95 % confidence interval lower limit

qmax,0/mol·kg−1 χ b0/kPa−1 Q/J·mol−1 n regression coefficient

0.796 4.546 214 7.206 × 104 1.518 R2 = 0.995

best fitting value

95 % confidence interval upper limit

0.814 4.771 292 7.803 × 104 1.659

0.832 4.995 369 8.400 × 104 1.801

Figure 7. Isosteric heat of CO2 adsorption on SBA-15-PEI as a function of the fractional coverage of the adsorbent.

up to a negative vertical asymptote for θ = 1: this trend is similar to that of other expressions of the isosteric heat obtained from some of the most used semiempirical equilibrium adsorption models.23 This behavior is obviously a limit in the physical consistency of this kind of equations, which can be considered reliable for values of θ distant enough from unity.34 The fact that ΔH is significantly lower than zero points to the intrinsically exothermic nature of the adsorption process. This indicates that the rather unusual increase of the adsorption capacity with temperature does not depend on an (unaccountably for) endothermic adsorption process, but rather on the fact that the number of active sites increases with temperature, therefore leading to a substantial confirmation of the “molecular basket” theory by Xu et al.11 Moreover, by taking the derivative with respect to T of qmax in eq 5, and dividing such derivative for eq 5 itself, it can be obtained that:

a

qmax,0: maximum adsorption capacity qmax at the reference temperature T0 = 298 K (eq 5); χ dimensionless parameter relative to the dependence of qmax on temperature (eq 5); b0: affinity constant b at the reference temperature T0 = 298 K (eq 6); Q: measure of the adsorption heat (eq 6) n: heterogeneity coefficient (eq 4).

model and experimental results has been shown in Figures 5 and 6, in which symbols refer to experimental data and continuous curves refer to the best−fit Sips theoretical isotherms. Inspection of Figures 5 and 6 clearly indicates a very good fit between the model curves and the experimental points, even in the very low pressure region. This is also confirmed by the value of the regression coefficient R2 reported in Table 2. Using the Sips isotherm, it was possible to develop an expression for the isosteric heat of adsorption, i.e., the ratio of the infinitesimal change in the adsorbate enthalpy to the infinitesimal change in the amount adsorbed, as a function of the fractional coverage of the adsorbent θ = q/qmax. According to Do,34 the isosteric heat of adsorption (ΔH) can be calculated from the van’t Hoff equation: ⎛ ∂ln p ⎞ ΔH ⎟ = −⎜ 2 ⎝ ∂T ⎠q RT

χT 1 dqmax = 20 qmax dT T

(10)

Using eq 10, eq 9 can be written as follows: −ΔH = Q −

1 dqmax nRT 2 qmax dT 1 − θ

(11)

Inspection of eq 11 indicates that the decrease of (−ΔH) with the increase of θ depends on the increase of qmax with the temperature. If the maximum adsorption capacity were independent of temperature (as usually occurs for CO2 adsorption on zeolites and MOFs),8 the left-hand side of eq 10 would be zero, and therefore the isosteric heat of adsorption would be constant and equal to Q even when θ → 1. Eventually, analyzing Figure 7 it can be noted that, for low values of θ, the isosteric heat of adsorption is in the order of −50 kJ·mol−1: this result is comparable with both theoretical and experimental enthalpies of reaction between CO2 and typical alkanolamine aqueous solutions,36 and also with experimental CO2 adsorption heats at low coverage for ionexchanged zeolites of different structural types.37 It may also be useful to compare the calculated values of Sips parameters reported in Table 2 (i.e., qmax, b, and n) with those obtained for CO2 adsorption on two microporous substrates that can be considered benchmark adsorbents for this molecule, such as 13X zeolite and Cu−BTC MOF.8 Table 3 shows the values of qmax, b, and n for the three systems now mentioned: the reported value of qmax for SBA-15-PEI is corresponding to that of qmax,0, i.e. it has been calculated at T = 298 K, that is the

(7)

After rewriting eq 4 in terms of p versus q, substituting eqs 5 and 6 into it and then taking the derivative of its natural logarithm with respect to T, the following expression for the isosteric heat of adsorption is obtained: q −ΔH = Q − nχRT0 max qmax − q (8) Writing eq 8 in terms of the fractional coverage θ = q/qmax leads to: nχRT0 (9) 1−θ Figure 7 shows the plot of eq 9 using the best fitting values of the parameters n, χ, and Q reported in Table 2. Inspection of Figure 7 indicates that the isosteric heat of adsorption decreases as the fractional coverage of the adsorbent increases, reaching 0 for θ ≈ 0.75 and then becoming negative for higher values of θ, −ΔH = Q −

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4. CONCLUSIONS SBA-15 mesoporous silica samples were synthesized and then functionalized with PEI by the wet impregnation method, with the aim of testing their CO2 adsorption capacity. Characterization of produced material, denominated SBA-15-PEI, proved that polymer chains efficiently fill the pores of the mesostructured solid. SBA-15-PEI showed CO2 adsorption performances that are comparable to those of similar substrates. The CO2 adsorption capacity of SBA-15-PEI proved to be significantly dependent on temperature, with the highest capacity encountered at the highest temperature considered, i.e., T = 348 K. Moreover, an attempt to model CO2 adsorption isotherms on SBA-15-PEI at four different temperatures by means of the simple, still versatile Sips equation was successfully carried out. The modeling effort allowed to evaluate the isosteric heat of adsorption as a function of the fractional coverage of the PEI-functionalized mesoporous silica. The values that were found for the isosteric heat of adsorption are comparable with both theoretical and experimental enthalpies of reaction between CO2 and classic alkanolamine aqueous solutions. The comparison with similar modeling results for CO2 adsorption on benchmark microporous substrates (i.e., 13X zeolite and Cu-BTC metal organic framework) allowed to highlight significant analogies and differences in the process phenomenology (e.g., comparable CO2 affinity and process heterogeneity) with zeolitic adsorbents rather than with metal organic ones.

Table 3. Comparison of Sips parameters for CO2 adsorption on SBA-15-PEI, 13X zeolite and Cu-BTC MOF. Values for 13X zeolite and Cu-BTC MOF are extracted from the literature.8a

a b

parameter

SBA-15-PEI

13X zeolite

Cu-BTC MOF

qmax/mol·kg−1 b/kPa−1 n

0.81b 292b 1.66

7.06 278b 2.12

16.50 43b 0.94

qmax: maximum adsorption capacity (eq 4); b: affinity constant (eq 4). Values calculated at 298 K.

same temperature at which the three listed values of b have been computed (on the other hand the values relative to 13X and Cu-BTC are independent of temperature).8 From the analysis of the data reported in Table 3, it can be noted that the maximum adsorption capacity qmax for SBA-15-PEI is significantly lower than that calculated for both 13X and CuBTC. Furthermore, while the considered microporous substrates follow the typical behavior by which, at a fixed pressure, an increase in temperature leads to a decrease in the adsorption capacity, SBA-15-PEI shows an inversion of this phenomenon for CO2 pressures higher than about 1 kPa, as discussed above (and as already observed for other PEIimpregnated mesoporous adsorbents).4,23 Moreover, it also has to be noted that, while SBA-15-PEI can entirely exploit its adsorption capacity at very low CO2 pressures (p≈20 kPa), a noticeable fraction of the adsorption capacity of both 13X and Cu-BTC can only be used at CO2 pressures higher than 100 kPa. This is the reason why, even though PEI-functionalized mesoporous materials have been proposed for CO2 capture from gaseous streams that are significantly rich in this species (e.g., flue gas),38 they seem more well-suited for scenarios in which a thorough purification from CO2 of dilute gas mixtures is required.22 Additionally, it can be observed that the affinity coefficients b for SBA-15-PEI and 13X zeolite are quite similar, and both significantly higher than that relative to Cu-BTC: as already pointed out in a direct comparison between the two aforementioned microporous substrates,8 this can indicate a specific interaction between CO2 molecules and SBA-15-PEI active adsorption sites (i.e., the amino groups of PEI chains), whereas low−affinity adsorbents, such as Cu-BTC MOF, rely on high specific surface areas to achieve high nonspecific adsorption capacities. In regard to the heterogeneity parameter n, it is significantly higher than the unity for SBA-15-PEI: this result is clearly more similar to that obtained for13X (for which n is higher than 2) than to that of Cu-BTC, for which n is practically equal to unity, indicating a homogeneous (Langmuir-type) adsorption system. As already mentioned in the direct comparison between the two microporous substrates,8 this difference presumably depends on the different nature of the interactions between CO2 molecules and the inner porosity of the considered adsorbents. In particular, for CO2 adsorption on both SBA-15-PEI and 13X zeolite, it is interesting to note that this process develops through acid− base interactions that lead, in the first case, to the formation of carbamates (at least as initial stage)39 and, in the second case, to the occurrence of carbonate-like species starting from basic framework oxygen atoms and CO2 molecules polarized by neighboring Na+ ions.40

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AUTHOR INFORMATION

Corresponding Author

*Fax: +39 0824 325246. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the help provided by Dr. Ilaria Capasso (National Research Council, Institute of Composite and Biomedical Materials) and Dr. Barbara Liguori (Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II) in performing TG analyses.

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REFERENCES

(1) IPCC Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; IPCC: Geneva, 2007. (2) Halmann, M. M.; Steinberg, M. Greenhouse Gas Carbon Dioxide Mitigation: Science and Technology; CRC Press: Boca Raton, FL, 1999. (3) Rostrup-Nielsen, R. Catalytic Steam Reforming. In Catalysis Science and Technology Vol. 5; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, Germany, 1984; pp 1−117. (4) Gargiulo, N.; Pepe, F.; Caputo, D. CO2 Adsorption by Functionalized Nanoporous Materials: a Review. J. Nanosci. Nanotechnol. 2014, 14, 1811−1822. (5) Siriwardane, R. V.; Shen, M. S.; Fisher, E. P.; Poston, J. A. Adsorption of CO2 on Molecular Sieves and Activated Carbon. Energy Fuels 2001, 15, 279−284. (6) Millward, A. R.; Yaghi, O. M. Metal-Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature. J. Am. Chem. Soc. 2005, 127, 17998−17999. (7) Liang, Z.; Marshall, M.; Chaffee, A. L. CO2 Adsorption-Based Separation by Metal Organic Framework (Cu−BTC) Versus Zeolite (13X). Energy Fuels 2009, 23, 2785−2789. 901



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(8) Aprea, P.; Caputo, D.; Gargiulo, N.; Iucolano, F.; Pepe, F. Modeling Carbon Dioxide Adsorption on Microporous Substrates: Comparison Between Cu−BTC Metal Organic Framework and 13X Zeolitic Molecular Sieve. J. Chem. Eng. Data 2010, 55, 3655−3661. (9) Hao, S.; Xiao, Q.; Yang, H.; Zhong, Y.; Pepe, F.; Zhu, W. Synthesis and CO2 Adsorption Property of Amino-Functionalized Silica Nanospheres with Centrosymmetric Radial Mesopores. Microporous Mesoporous Mater. 2010, 132, 552−558. (10) Satyapal, S.; Filburn, T.; Trela, J.; Strange, J. Performance and Properties of a Solid Amine Sorbent for Carbon Dioxide Removal in Space Life Support Applications. Energy Fuels 2001, 15, 250−255. (11) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Preparation and Characterization of Novel CO2 “Molecular Basket” Adsorbents Based on Polymer−Modified Mesoporous Molecular Sieve MCM−41. Microporous Mesoporous Mater. 2003, 62, 29−45. (12) Gargiulo, N.; Caputo, D.; Colella, C. In From Zeolites to Porous MOF Materials − The 40th Anniversary of International Zeolite Conference, Proceedings of the 15th International Zeolite Conference, Beijing, P. R. China, August 12−17, 2007; Xu, R., Gao, Z., Chen, J., Yan, W., Eds.; Elsevier: Amsterdam, 2007; pp 1938−1943 (Studies in Surface Science and Catalysis, v 170B). (13) Gargiulo, N.; Aprea, P.; Caputo, D.; Eic, M.; Huang, Q.; Colella, C. In Special Topics on Materials Science and Technology - An Italian Panorama, Selected Papers presented at the 9th National Conference organized by the Italian Association of Materials Engineering, Piano di Sorrento, Italy, June 29 − July 2, 2008; Acierno, D., D’Amore, A., Caputo, D., Cioffi, R., Eds.; Brill: Leiden, 2009; pp 213−220. (14) Son, W.-J.; Choi, J.-S.; Ahn, W.-S. Adsorptive Removal of Carbon Dioxide Using Polyethyleneimine−Loaded Mesoporous Silica Materials. Microporous Mesoporous Mater. 2008, 113, 31−40. (15) Chandrasekar, G.; Son, W.-J.; Ahn, W.-S. Synthesis of Mesoporous Materials SBA-15 and CMK-3 from Fly Ash and Their Application for CO2 Adsorption. J. Porous Mater. 2009, 16, 545−551. (16) Dasgupta, S.; Nanoti, A.; Gupta, P.; Jena, D.; Goswami, A. N.; Garg, M. O. Carbon Di−Oxide Removal with Mesoporous Adsorbents in a Single Column Pressure Swing Adsorber. Sep. Sci. Technol. 2009, 44, 3973−3983. (17) Ma, X.; Wang, X.; Song, C. Molecular Basket” Sorbents for Separation of CO2 and H2S from Various Gas Streams. J. Am. Chem. Soc. 2009, 131, 5777−5783. (18) Badanicova, M.; Zelenak, V. Organo−Modified Mesoporous Silica for Sorption of Carbon Dioxide. Monatsh. Chem. 2010, 141, 677−684. (19) Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Perez, E. S. CO2 Adsorption on Branched Polyethyleneimine-Impregnated Mesoporous Silica SBA-15. Appl. Surf. Sci. 2010, 256, 5323−5328. (20) Chen, C.; Son, W.-J.; You, K.-S.; Ahn, J.-W.; Ahn, W.-S. Carbon Dioxide Capture Using Amine−Impregnated HMS Having Textural Mesoporosity. Chem. Eng. J. 2010, 161, 46−52. (21) Yan, X.; Zhang, L.; Zhang, Y.; Yang, G.; Yan, Z. Amine-Modified SBA-15: Effect of Pore Structure on the Performance for CO2 Capture. Ind. Eng. Chem. Res. 2011, 50, 3220−3226. (22) Choi, S.; Gray, M. L.; Jones, C. W. Amine−Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture Applications Including the Air Capture. ChemSusChem 2011, 4, 628−635. (23) Gargiulo, N.; Pepe, F.; Caputo, D. Modeling Carbon Dioxide Adsorption on Polyethylenimine-Functionalized TUD−1 Mesoporous Silica. J. Colloid Interface Sci. 2012, 367, 348−354. (24) Kuwahara, Y.; Kang, D.-Y.; Copeland, J. R.; Bollini, P.; Sievers, C.; Kamegawa, T.; Yamashita, H.; Jones, C. W. Enhanced CO2 Adsorption over Polymeric Amines Supported on Heteroatomincorporated SBA−15 Silica: Impact of Heteroatom Type and Loading. Chem.Eur. J. 2012, 18, 16649−16664. (25) Olea, A.; Sanz-Pérez, E. S.; Arencibia, A.; Sanz, R.; Calleja, G. Amino-Functionalized Pore-Expanded SBA-15 for CO2 Adsorption. Adsorption 2013, 19, 589−600. (26) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric

Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024− 6036. (27) Zhang, W.-H.; Shi, J.-L.; Chen, H.-R.; Hua, Z.-L.; Yan, D.-S. Synthesis and Characterization of Nanosized ZnS Confined in Ordered Mesoporous Silica. Chem. Mater. 2001, 13, 648−654. (28) Gargiulo, N.; Attianese, I.; Buonocore, G. G.; Caputo, D.; Lavorgna, M.; Mensitieri, G.; Lavorgna, M. α−Tocopherol Release from Active Polymer Films Loaded with Functionalized SBA−15 Mesoporous Silica. Microporous Mesoporous Mater. 2013, 167, 10−15. (29) Lowell, S.; Shields, J. E.; Thomas, M. A.; Thommes, M. Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2004. (30) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Novel Polyethyleneimine-Modified Mesoporous Molecular Sieve of MCM−41 Type as High Capacity Adsorbents for CO2 Capture. Energy Fuels 2002, 16, 1463−1469. (31) Zukal, A.; Pulido, A.; Gil, B.; Nachtigall, P.; Bludský, O.; Rubešc, M.; Č ejka, J. Experimental and theoretical determination of adsorption heats of CO2 over alkali metal exchanged ferrierites with different Si/ Al ratio. Phys. Chem. Chem. Phys. 2010, 12, 6413−6422. (32) Zukal, A.; Dominguez, I.; Mayerová, J.; Č ejka, J. Functionalization of Delaminated Zeolite ITQ-6 for the Adsorption of Carbon Dioxide. Langmuir 2009, 25, 10314−10321. (33) Sips, R. On the Structure of a Catalyst Surface. J. Chem. Phys. 1948, 16, 490−495. (34) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; Imperial College Press: London, 1998. (35) Peluso, A.; Gargiulo, N.; Aprea, P.; Pepe, F.; Caputo, D. Modeling Hydrogen Sulfide Adsorption on Chromium-Based MIL101 Metal Organic Framework. Sci. Adv. Mater. 2014, 6, 163−170. (36) Zhang, Y.; Chen, C.-C. Thermodynamic Modeling for CO2 Absorption in Aqueous MDEA Solution with Electrolyte NRTL Model. Ind. Eng. Chem. Res. 2011, 50, 163−175. (37) Grajciar, L.; Č ejka, J.; Zukal, A.; Otero Areán, C.; Turnes Palomino, G.; Nachtigall, P. Controlling the Adsorption Enthalpy of CO2 in Zeolites by Framework Topology and Composition. ChemSusChem 2012, 5, 2011−2022. (38) Xu, X.; Song, C.; Miller, B. G.; Scaroni, A. W. Adsorption Separation of Carbon Dioxide from Flue Gas of Natural Gas−Fired Boiler by a Novel Nanoporous “Molecular Basket” Adsorbent. Fuel Process. Technol. 2005, 86, 1457−1472. (39) Caplow, M. Kinetics of Carbamate Formation and Breakdown. J. Am. Chem. Soc. 1968, 90, 6795−6803. (40) Martra, G.; Coluccia, S.; Davit, P.; Gianotti, E.; Marchese, L.; Tsuji, H.; Hattori, H. Acidic and Basic Sites in NaX and NaY Faujasites Investigated by NH3, CO2 and CO Molecular Probes. Res. Chem. Intermed. 1999, 25, 77−93.

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CO2 Adsorption on Polyethylenimine-Functionalized SBA-15

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