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CO2 adsorption in amine-grafted zeolite 13X
1. Introduction
A wide variety of adsorbents has been investigated lately for CO2 capture, which includes activated carbons, zeolites, mesoporous silicas, metal oxides, mixed hydroxides and metal-organic frameworks [1–4].
X Zeolites are crystalline microporous oxides with pore size in the range of 11–14 Å. Although their chemical composition is limited to aluminosilicate tetrahedra, cations (usually Na) should be introduced to compensate the negative charge of oxygen linked to Al atoms in the framework. This chemical versatility allows for the modification of some physicochemical zeolite properties (such as acidity, redox properties, or hydrophobic–hydrophilic nature) and, consequently, tunes them for a specific application [5].
Adsorbents functionalized with basic groups, such as amines, are claimed to be potentially effective materials for the capture of CO2 from flue gas mainly due to the relatively smaller decrease in adsorption capacity with rises in temperatures as compared to conventional physisorbents [1,4,6,7]. The optimal functionalization or modification of inorganic materials has received considerable attention in the last few years [8–13].
Modifying silicate and aluminosilicate surfaces with amino groups has been shown by some authors to lead to sorbents with increased capacity as compared to the pristine materials. Chatti et al. [9] have measured CO2 uptakes of 0.36 mmol/g for unmodified X zeolite and 0.45, and 0.52 mg/g for zeolite modified with monoethanolamine, and isopropanol amine at 1 bar and 25 °C. CO2 adsorption capacity of amine-grafted mesoporous silicas has been investigated by Vilarrasa-García et al. [14]. At 1 bar and 25 °C, CO2 uptakes of 2.4 mmol g−1or 0.64 mol CO2 per mol N were achieved. However, such CO2 uptakes are still below those reached by commercial zeolite samples under the same conditions of pressure and temperature. Bacsik et al. [15] also studied CO2 adsorption on amine-grafted mesoporous silica (AMS-6 and MCM-48) at two temperatures (273 and 294 K). The authors observed that CO2 adsorption capacity increases with temperature, which is distinct from what would be expected in physisorption alone. Such behavior has been attributed to the occurrence of chemisorption. As reported in a previous paper [4], the impregnation with dilute amine solutions and drying in inert atmosphere led to the same behavior. This study evaluated the adsorption capacity at two temperatures (298 and 348 K) and it was observed that, for a given impregnated sample with monoethanolamine, CO2 uptake increases with a rise in temperature.
On a recent comprehensive review about the subject, Ebner and Ritter [16] suggest that a near-term goal to be pursued concerning adsorbents for CO2 capture should be to develop materials which can operate at elevated temperatures in the presence of sulfur bearing compounds and possibly steam, with working capacities in the range of 3–4 mmol of CO2/g adsorbent, that is, 132–176 mg/g within the pressure range at which emissions occur.
Considerable experimental effort has been devoted in the last few years in the preparation of amine-functionalized adsorbents, however few studies discuss how each impregnation route affects CO2 capacity, how amine species are attached to the adsorbent surface and their long-term stability upon pressure and temperature swings. Proposing to perform this study, commercial X zeolite was functionalized by immersing it in monoethanolamine (MEA) methanolic solutions under different amine concentrations (0–10% v/v) and CO2 adsorption isotherms were measured for the thus-prepared solids. The pristine adsorbent and modified materials were analyzed by N2 adsorption/desorption isotherms at 77 K, thermo-gravimetric analysis (TGA), x-ray photoelectron spectroscopy (XPS) and CO2 adsorption microcalorimetry, in order to characterize the textural, structural and surface modifications induced on the adsorbents by the impregnation process. The adsorbents were tested for CO2 adsorption at two temperatures (298 and 348 K) under the pressure range of 10−2–10 bar using a gravimetric setup.
2. Experimental
2.1. Adsorbents
Zeolite Kostrolith 13X was supplied by Kostrolith (Germany) in the form of spherical pellets of approximately 2 mm diameter.
2.2. Wet amine impregnation
MEA (VETEC, 99,9%, Brazil) impregnation on zeolite type X was carried out following an experimental procedure reported by Chatti et al. [9] and Bezerra et al. [4] with different amine concentrations. About 2.0 g adsorbent, previously regenerated at 623 K, were soaked in 50 mL MEA solution in different concentrations (from 0.2 to 10% (v/v)) in methanol (VETEC, 99,9%, Brazil). The zeolites remained in contact with the MEA solution under slight agitation for 72 h and at 298 K. The solids were separated from the amine solution by filtration. One of the samples (blank) was prepared with methanol only, following the same procedure except for the absence of MEA. The modified sorbents were dried at 423 K under inert atmosphere (nitrogen flow). Table 1 shows the different MEA concentrations used for amine impregnation and the code to identify each of the samples.
| Support | Monoethanolamine/Methanol (concentration in % vol.) | Code |
|---|---|---|
| Zeolite 13X | No impregnation | ZX |
| 0% (methanol only) | ZX0 | |
| 0.2% | ZX0.2 | |
| 0.5% | ZX0.5 | |
| 0.8% | ZX0.8 | |
| 1.0% | ZX1.0 | |
| 1.5% | ZX1.5 | |
| 10.0% | ZX10 |
2.3. Characterization of the amine-functionalized adsorbents
Nitrogen adsorption/desorption isotherms were measured using an Autosorb-1 MP apparatus (Quantachrome, U.S.A.) for the determination of textural properties such as surface area (SBET), total pore volume (VTOTAL) and micropore volume (VMICRO). The impregnated adsorbents were initially outgassed at 423 K, to avoid amine volatilization (boiling point 443 K), and then subjected to stepwise N2 relative pressure increases and decreases at 77 K. The specific surface area was calculated using the BET equation and micropore volume was determined by using the Dubunin–Radushkevich equation [17].
The identification of the crystalline phases of the zeolite samples was performed by x-ray diffractometry in a Rigaku (DMAXB) X-ray Powder Diffractometer by using a Bragg–Brentano geometry. X-ray photoelectron spectroscopy (XPS) was carried out to determine the atomic concentrations (%) of the elements on the surface of the pristine and amine-impregnated samples, especially the atomic concentration % of nitrogen induced by the chemical modification. Spectra were obtained at ultra high vacuum (10−12 bar) using a Physical Electronics spectrometer (model 5700). Thermogravimetric analyses (TGA) were carried out using as Shimadzu TGA 50 equipment, with a heating rate of 10 K/min, in a dynamic N2 atmosphere with approximately 5.0 mg sample. The temperature range of 340–1000 K was used for TGA analyses. FTIR spectra were obtained using a Bruker IFS88 spectrometer and specially designed cells which were permanently connected to a gas or vacuum line. Spectra were obtained at two temperatures (298 and 348 K) and two pressures (vacuum and 1 bar pure CO2).
CO2 adsorption microcalorimetric experiments were carried out using an isothermal Tian–Calvet type microcalorimeter (model CA-100 from ITI Company, Del Mar, CA) combined with a homemade volumetric device. Thus, simultaneous measurement of isotherms and calorimetric curves for the adsorption of CO2 were performed at 298 K up to 101 kPa. In order to evaluate the adsorption differential enthalpy from calorimetric data, the so-called discontinuous procedure was employed, which consists of injecting discrete quantities of gas in successive steps to the adsorbent. The adsorbed amount was calculated from mass balance of the gas phase by using pressure, volume and temperature measured data. Further details on the experimental procedure can be found in Rouquerol et al. [17] and Silva et al. [18].
2.4. CO2 adsorption isotherms
Adsorption equilibrium isotherms for pure CO2 were measured using a magnetic suspension balance by Rubotherm (Bochum, Germany). The adsorbents were degassed in situ at 423 K (except for pristine zeolite 13X, which was regenerated 623 K) until no mass variation in the system was observed. After that, the measuring chamber was cooled down to the experiment temperature (298 or 348 K) and the gas pressure (CO2) was increased stepwise (UHV until approximately 10 bar).
3. Results and discussion
3.1. Adsorbent characterization
3.1.1. Nitrogen adsorption/desorption isotherms
The textural properties of the pristine and amino-impregnated zeolites are shown in Table 2. A progressive reduction of the specific surface area and pore volume were observed as the amine concentration increases, which indicates that MEA tends to fill and obstruct micropores, preventing nitrogen from being adsorbed. Only the textural characteristics for the pristine zeolite and that loaded with the lowest amine solution concentration were nearly the same. Note that, when MEA is absent (sample ZX0), rinsing with methanol (only) tends to increase surface area and micropore volume possibly due to solubilization of impurities (e.g. pellet binder). Therefore, the slightly higher nitrogen uptake observed for ZX0.2 (Fig. 1) suggests that the added MEA was little enough so that the solvent effect (methanol) in opening up porosity was nearly compensated by the obstruction effect of MEA.
| Adsorbent | SBET (m2/g) | VTOTAL (cm3/g) | VMICRO (cm3/g) |
|---|---|---|---|
| ZX | 515 | 0.454 | 0.298 |
| ZX0 | 608 | 0.400 | 0.343 |
| ZX0.2 | 491 | 0.456 | 0.277 |
| ZX0.5 | 462 | 0.378 | 0.237 |
| ZX0.8 | 436 | 0.312 | 0.227 |
| ZX1.0 | 429 | 0.198 | 0.190 |
| ZX1.5 | 419 | 0.236 | 0.168 |
| ZX10 | 121 | 0.148 | 0.054 |
The nitrogen adsorption isotherms at 77 K for pristine and amine-impregnated X zeolite are shown in Fig. 1. All isotherms may be classified as Type I, in accordance with the IUPAC classification, with predominant microporous characteristics as evidenced by the high volume of nitrogen adsorbed within very low relative pressures. A second sharp increase in relative pressures close to 1.0 present in some isotherms is probably due to the presence of macropores as a result of pellet conformation and addition of binding agent.
For the other amine-impregnated zeolites loaded with different amine concentrations, nitrogen adsorption isotherms showed a progressive reduction in uptake as the concentration of the impregnating solution increases. The isotherm shapes indicate that ZX1.0 and ZX1.5 practically have no macropores, possibly because they were completely filled with amine. The samples impregnated with increasing amine concentration (up to 1.5% MEA) also showed a progressive decrease in the final uptake rise, which leads to the hypothesis that MEA fills not only micropores, but also larger pores to a certain extent.
3.1.2. X-ray diffraction
X-ray diffraction technique was used to determine the crystallinity of impregnated and fresh samples. The XRD patterns of the parent (ZX) and MEA-modified zeolite (ZX10) are shown in Fig. 2. The reflection lines are well resolved and it can be seen that the location of the reflection lines remains constant, indicating that the structure of zeolite 13X was well preserved even with the highest load of MEA. Sample ZX10 was chosen because it was the most intensely affected sample in terms of its textural characteristics. Nevertheless, the crystalline zeolitic structure remained nearly intact, which suggests that the impregnation procedure did not affect the crystallinity of any of the other impregnated samples.
3.1.3. TGA
Mass loss profiles, as determined by TGA, are shown in Fig. 3 for ZX and ZX10. For both samples, there is the same continuous and smooth mass loss up to 443 K. Beyond this temperature, a higher variation in the mass loss profile was observed for the amino-loaded zeolite, evidencing the presence of retained MEA (boiling temperature of 443 K). The adsorption sites in the pristine sample, usually occupied by water molecules, were mainly loaded with MEA in ZX10. This changes the hydrophilic character of the zeolite. In the inset of Fig. 3, it is possible to observe the first peak at lower temperature (attributed to desorption of moisture) in ZX10 sample. The second peak, at 520 K, is ascribed to the volatilization of the entrapped organic material, MEA [4]. This peak is absent for the pristine ZX.
3.1.4. XPS
The surface atomic concentrations of elements nitrogen and carbon, as obtained by XPS measurements, are summarized in Table 3 for the pristine and amino-loaded samples ZX and ZX10. The atomic concentration percentages of C and N were 4.43% and 0.89%, respectively for sample ZX, and 18.99% and 4.34%, respectively for sample ZX10, respectively. These results are in accordance with TGA results shown previously. The observed increase in the carbon and nitrogen contents upon modification of the zeolite is due to the incorporation of the monoethanolamine. The presence of a small amount of N on the surface of the pristine zeolite is due to impurities (i.e., from the binder) containing nitrogen. These results suggest the successful incorporation of MEA (CH2(NH2)CH2OH) in the solid. However, the observed percentage of carbon in sample ZX10 is higher than that expected if the percentage of nitrogen is considered. This can be explained by the presence of residual methanol (solvent) or by adsorption of atmospheric CO2.
| Sample | N | C | Binding energies (eV) | |
|---|---|---|---|---|
| a.c. % | a.c. % | C 1s | N 1s | |
| ZX | 0.83- | 4.43 |
284.8 (adventitious C C)
|
401.0- |
| ZX10 | 4.34 | 18.99 |
284.8 (adventitious CC)
|
399.8 (RNH2 (N-alkylamine))
|
|
286.1 (CN) and (CO)
|
401.4 (RNH2CO2 (carbamate), RNH3+ (Nalkylammoniun))
|
|||
287.8 (OC O)
|
||||
Table 3 shows the binding energy values for C 1s and N 1s for ZX10 sample. For the pristine sample, which is essentially an aluminosilicate, only a nearly symmetric C 1s peak was observed corresponding to adventitious carbon. In modified zeolites, the C 1s signal can be decomposed into three contributions (spectra not shown) and the corresponding binding energies (BE) allowed us to identify the presence of aliphatic carbons (284.8 eV), C
XPS analysis was also performed for sample ZX10 after contact with CO2 in high pressure gravimetric experiments. The corresponding atomic concentrations (%) are summarized inTable 4 and spectra decomposition are shown in Fig. 4 for the N 1s and C 1s signals in the case of the impregnated sample after CO2 adsorption at high pressure.
| Element | ZX10 | ZX10 after CO2 capture | ||||
|---|---|---|---|---|---|---|
| a.c. (%) | BE (eV) | a.c. (%) | BE (eV) | |||
| Total | Deconvolution | Total | Deconvolution | |||
| C1s | 18.99% | 37% | 284.8 | 24.00% | 63% | 284.8 |
| 52% | 286.1 | 23% | 286.4 | |||
| 11% | 287.8 | 8% | 288.5 | |||
| – | – | 6% | 291.4 | |||
| O1s | 48.50% | 100% | 531.7 | 45.79% | 100% | 531.2 |
| N1s | 4.34% | 72% | 399.8 | 4.30% | 80% | 399.4 |
| 28% | 401.4 | 20% | 401.0 | |||
Upon MEA modification (sample ZX10), the N 1s signal was examined in a higher resolution XPS spectrum (Fig. 4a). The asymmetrical and broad features of N 1s signal suggest the coexistence of distinguishable nitrogen bonds. The peak decomposition shown in Fig. 4indicates the presence of more than one N-containing species in the impregnated sample, with their binding energies at 399.8 and 401.4 eV. The former corresponds to primary alkylamines (R
The percent atomic concentration of nitrogen for the sample ZX10 before and after CO2adsorption at 10 bars (4.34% and 4.30%, respectively) are essentially the same and this fact suggests that nitrogen is firmly anchored to the adsorbent surface (Table 4). In addition, there is an increase in the amount of fixed carbon after CO2 adsorption at high pressure (from 18.99% to 24.00%), which is likely to be due to the interaction between the amine and CO2. The analysis of the C 1s spectra is very complex. In the case of sample ZX10, the main contribution at 286.1 eV is derived from the presence of monoethanolamine (C
3.1.5. FTIR
In Fig. 5, in situ FTIR spectra are shown for the pristine zeolite 13X and amine-grafted sample ZX10. The pristine sample presented peaks in the range of 3750–3500 cm−1attributed to hydroxyl groups belonging to the surface of the adsorbent or physisorbed water. The spectrum also suggests the formation of hydrogen bonds, judging from the peaks observed in the range of 3250–3400 cm−1. Note that at high temperature (pristine sample), the hydrogen bonds are not found. A large peak near 2330 cm−1 is related to physisorbed CO2 in both samples [20,21], which is present in both temperatures with different intensities.
The peaks found in the region from 1200 to 1700 cm−1 (Fig. 5b) are relevant because they provide important evidence of amine impregnation and formation of carbamate species (characteristic of chemical adsorption). By comparing the spectra of the pristine and grafted samples under vacuum, the presence of an additional peak is observed for ZX10 at 1615 cm−1, relative to the incorporation of amine groups (protonated form) on the adsorbent surface[22,23].
The pristine sample in the presence of CO2 presented peaks in the region from 1681 to 1650 cm−1, which may be assigned to adsorbed CO2 coordinated to the compensation cation (Na+) in bidentate form, and around 1361 cm−1 in monodentate form [24]. In the case of ZX10, other observed bands account for unimpeded carbonate ion in the range of 1420–1450 cm−1[21].
3.2. Adsorption isotherms
Taking into account the results obtained from the characterization of the pristine and grafted adsorbents, the samples ZX, ZX0.2 and ZX10 were chosen to be assessed for CO2adsorption (Fig. 6). As expected, the isotherms of conventional ZX supports show a sharp increase in CO2 uptake at low partial pressures reaching a constant plateau at higher pressures (typical of strongly favorable isotherms).
Functionalized adsorbents show different trends regarding their CO2 adsorption isotherms as compared to the pristine zeolite 13X, some of which had already been previously observed [4]. ZX0.2 and ZX10 presented a lower CO2 uptake in the whole pressure range under study as compared to ZX. The reduction in textural characteristics can justify the lower CO2 uptake. However, ZX10 presented two distinct behaviors: CO2 uptake increases at higher temperatures (348 K) and, at high pressures, CO2 adsorbed concentration keeps rising steadily. These features are consistent with the hypothesis of CO2 chemisorption, as suggested in XPS and FTIR results previously shown.
3.2.1. Effect of regeneration temperature
Because impregnated moieties are usually weakly attached to the adsorbent, it is important to examine the effect of the regeneration temperature of the amino-loaded sample on the CO2 uptake. This aspect was addressed by measuring two isotherms for the same amine-loaded sample after being regenerated at two different temperatures. In Fig. 7, the CO2adsorption isotherms obtained for the amino-loaded zeolite ZX0.2 at 348 K are shown after the sample was regenerated at 423 and 473 K. For the sake of comparison, the isotherm of the pristine zeolite ZX at this temperature was also included.
It may be observed that the CO2 uptake increases when a higher temperature of regeneration is applied and approaches that of the pure zeolite ZX. These results suggest that part of the loaded amine was removed upon heating at 473 K, a higher temperatures than MEA boiling point (443 K) even though TGA analysis indicates that most of the amine volatilizes only beyond 500 K (Fig. 3, inset). Nevertheless, in real PSA separation units, cycles are performed under nearly isothermal conditions (at least not higher than 373 K) and regeneration is performed under pressure swings. Adsorption uptakes should ideally be constant after pressure swings.
3.2.2. Adsorption reversibility
CO2 adsorption and desorption isotherms were measured for the amino-loaded zeolite (ZX10) at two temperatures, 298 and 348 K, which are shown in Fig. 8(a) and (b), respectively.
The adsorption and desorption CO2 isotherms at 298 K do not follow the same path and such behavior cannot be attributed to physisorption equilibrium hysteresis under the studied P and T conditions. Such behavior may be due to an irreversible reaction between CO2 and amine sites present in the adsorbent. On the other hand, the apparent irreversibility observed in Fig. 8a may also be attributed to diffusion limitations imposed by the pore blocking with MEA. This is consistent with the adsorption/desorption isotherms at 348 K, a higher temperature which will eventually enhance intraparticle mass transfer. In fact, both adsorption and desorption isotherms overlap at this temperature. The diffusion resistance and intraparticle mass transfer of CO2 through filled pores may prevent the achievement of a strict equilibrium in the adsorption branch [25], particularly at the lower temperature (Fig. 8a). At this stage, however, none of these two hypotheses may be ruled out. It is likely that both mechanisms (chemisorption and hindered diffusion) occur.
3.2.3. On the occurrence of chemisorption
In order to collect experimental evidence as to whether chemisorption of CO2 on amine-loaded samples would take place, as suggested in a previous publication [4] and in the XPS analysis, the behavior of adsorption isotherms for sample ZX10 at different temperatures was observed. As can be seen in Fig. 9, for zeolites loaded with high concentrations of amine, the usual physisorption behavior with temperature does not occur. That is to say, CO2 uptakes apparently increase for increasing temperatures in some amine loaded samples. Nevertheless, from the discussion in the previous sections, it is also reasonable to attribute such behavior also to diffusion resistances caused by the clogging of the porous structure of the adsorbent with MEA.
In order to further examine the hypothesis of chemisorption, adsorption calorimetric experiments were carried out in a Tian-Calvet microcalorimeter coupled to an adsorption volumetric device, as described in full detail elsewhere [18]. The calorimetric curves, which plot the differential adsorption enthalpy as a function of CO2 loading, are shown in Fig. 10 for the pristine zeolite 13X and some of the amine-loaded samples. As expected for microporous adsorbents, all curves start with a relatively high value of adsorption enthalpy at nearly zero loading, which decreases for increasing CO2 loadings and levels off at about 35 kJ/mol, which is in close agreement with reported values of adsorption enthalpy of CO2 in zeolites [18,26,27]. The most important piece of information in this kind of curve is the initial value, because it refers to the adsorption enthalpy in the stronger sites of the adsorbent and gives an idea of its surface heterogeneity. In Fig. 10, it may be observed that the zero-coverage adsorption enthalpy becomes higher as the MEA loading increases. At nearly-zero CO2 coverage, the initial heats of adsorption are 50.0, 59.7 and 144.7 kJ/mol for samples ZX, ZX0.2 and ZX10, respectively. These results are comparable to those reported for heats of absorption of CO2 in liquid amines [28]. On one hand, such values confirm that the introduction of MEA in the zeolite effectively causes new sorbate-sorbent interactions as compared to the pristine material. On the other hand, the excess MEA which fills completely the pores and deteriorates textural properties is possibly responsible for the little availability/accessibility of such stronger sites, which would explain the very rapid decrease in the calorimetric curves at relatively low CO2 uptakes of the amine-loaded samples.
This suggests that there are few available chemisorption surface sites owing to MEA impregnation. Above 1 mmol g−1 of loaded CO2, the differential enthalpy curves for the three samples reach similar values. These results show the order of magnitude of the adsorption enthalpy at zero-coverage is compatible with the occurrence of a chemical reaction between CO2 and the amine loaded surface.
According to Chatti et al. [9], the amino groups may act as a solvent in the confined pores of the zeolite which plays the role of microreactors for CO2 capture. Therefore, the reactions taking place in an amine-modified adsorbent would be essentially the same as those between the liquid amine and the CO2, wherein there is formation of carbamate species, as shown in reaction (1):
From the stoichiometry of this reaction, one CO2 molecule reacts with two amine molecules. From the adsorption isotherm of ZX10 at 298 K (Fig. 6), and considering that this sample has a molar concentration of 2.71 mmol N/g (XPS), such sample adsorbs 0.15 mol CO2/mol N at 1 bar. The hypothesis of diffusion resistance and intraparticle mass transfer of CO2through filled pores may justify this behavior. At 348 K, this figure rises to 0.57 mol CO2/mol N, probably because the temperature increase promotes/facilitates intracrystalline diffusion. This is approximately the same figure to be expected from the reaction stoichiometric in dry conditions (Eq. (1)).
At 10 bar, this sample (ZX10) retains 0.66 mol CO2/mol N at 298 K and 1.10 mol CO2/mol N at 348 K, which suggests that, besides chemisorption, physisorption also takes place. Note that these experiments were performed with dry CO2, however there is indication [29] that in the presence of water, reaction would proceed so as to form bicarbonates, in which case 1 mol CO2 would react with one mol N.
4. Conclusion
Zeolite 13X has been impregnated with monoethanolamine (MEA) from methanolic solutions with increasing concentrations. Texture and surface chemistry of the thus-prepared materials have been investigated by nitrogen adsorption/desorption isotherms, TGA, FTIR and XPS. It has been shown that MEA tends to obstruct the micropores of the zeolite. TGA and XPS analyses reveal that part of the loaded amine is firmly attached to the adsorbent surface. Nevertheless, not much of this covalently bonded amine is available for chemisorption. As a result, amine loaded zeolites adsorb less than the pristine material at a given temperature, but they tend to increase CO2 uptake as temperature rises. Such behavior together with measured calorimetric data confirms that there are chemisorption sites for CO2. The amine-grafted materials did not show superior adsorption capacities than the pristine sample, however they revealed important features and adsorptive properties that can lead to new studies. Therefore, though apparently discouraging, these results confirm that amine functionalization generates much stronger adsorption sites on the surface, which potentially leads to higher CO2 selectivity. Nevertheless, this advantage is overcast by the restricted accessibility of such sites in microporous matrices and it is likely mesoporous solids (or rather micro-meso structures) are better options for amine impregnation.
Acknowledgements
The authors thank CNPq for funding D.P. Bezerra (554049/2010-4), the financial support of Grant 295156, FP7-PEOPLE-2011-IRSES of the European Commission and project of excellence RNM 1565 of Junta de Andalucía.
Appendix A. Supplementary data
The following are Supplementary data to this article:
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