Thermal, Oxidative, and CO2-Induced Degradation of Supported

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Thermal, Oxidative, and CO2-Induced Degradation of Supported Polyethylenimine Adsorbents Aliakbar Heydari-Gorji and Abdelhamid Sayari* Department of Chemistry, Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada S Supporting Information *

ABSTRACT: This work examines the stability of polyethylenimine(PEI)-impregnated mesoporous silica for CO2 removal over a wide range of conditions. The support used was a SBA-15 silica with platelet morphology and short pore channels (SBA-15PL). The effect of long-term exposure to different gaseous streams, including carbon-free air (CFair), simulated flue gas (SFG), and different CO2/O2/N2 mixtures on the carbon dioxide adsorption capacity was investigated. Extensive CO2 adsorption− desorption cycling using dry and humid streams at different adsorption and regeneration temperatures was also carried out. Based on adsorption data, as well as diffuse reflectance infrared Fourier transform (DRIFT) and 13C CP MAS NMR measurements, it was found that PEI-modified adsorbents exhibit (i) high thermal stability at moderate temperatures, (ii) highly stable CO2 uptake in the presence of moisture, (iii) extensive degradation in the presence of dry CO2, particularly at high temperature, (iv) fast degradation upon exposure to CFair even at moderate temperatures, (v) excellent stability in the presence of humidified gases containing both CO2 and O2.

1. INTRODUCTION Because, carbon dioxide (CO2) is a major greenhouse gas with significant impact on global warming, its separation and capture received significant attention in recent years.1 In addition, the removal of CO2 from different gas streams is an important step for many applications such as natural gas sweetening and air and hydrogen purification. CO2 removal using amine-containing adsorbents has been an increasingly active area of research.2−20 Recent literature data provide evidence of the great variety of adsorbents produced by supporting amine-containing species on porous materials. Amine groups may be bound to the silica surface via such techniques as grafting5,6 or in situ polymerization,3 or may be impregnated.7−19 Impregnation of polyethylenimine (PEI) is a popular strategy because this technique is simple, cheap, and affords high loadings. PEI-containing adsorbents exhibit high CO2 adsorption uptake and selectivity,9,10,13,16−19 tolerance to water vapor,13,14,16,17,25 reversibility13,17 and low energy requirements compared to aqueous amine scrubbing.20 The CO2 adsorptive properties of amine-containing materials in the presence of a number of gaseous compounds, such as hydrogen sulfide,9,21 sulfur oxide,22,23 and oxygen26−30 have been investigated. Nonetheless, most literature reports were dominated by measurements of equilibrium or near-equilibrium adsorption capacity of materials using pure CO2 or simulated industrial gases. However, despite its critical importance, the stability of PEI-impregnated materials has not been thoroughly investigated. The lifetime of adsorbents is a key property of equal importance as adsorption capacity, selectivity, and kinetics, with direct impact on the economics of commercialscale operations. As far as PEI-supported materials are concerned, their stability was evaluated using only a limited number of temperature swing adsorption−desorption cycles,8−12,16,17 which may not be sufficient to draw reliable conclusions. Moreover, the stability of amine-containing © 2012 American Chemical Society

adsorbents is a multifaceted issue, which encompasses thermal degradation (e.g., evaporation, decomposition), mechanical strength, and chemical degradation induced by gaseous species, such as SO2, NO2, O2, or CO2. Drage et al.24 carried out accelerated deactivation experiments on PEI-impregnated silica in the presence of pure CO2 at different temperatures. Beyond 135 °C, they observed a weight gain that they attributed to the formation of urea linkages. They also studied the thermal stability of the adsorbent in the presence of air and nitrogen and showed that above 135 °C, the mass of PEI (Mw = 1800)-impregnated adsorbent decreased rapidly, particularly in air atmosphere as a result of evaporation and/or thermal degradation. Using both grafted and impregnated amine-containing adsorbents, Sayari and Belmabkhout25 found that, in the presence of dry gases, deactivation via urea formation occurs even under mild conditions. They also demonstrated that the formation of urea groups may be completely inhibited in the presence of moisture. Bollini et al.26,27 and Heydari-Gorji et al.28 studied independently the stability of amine-functionalized CO2 adsorbents in the presence of air at different temperatures as a function of the nature of amine groups. Although the oxidative degradation of supported amines was not fully delineated on a molecular scale, several interesting findings were reported. Grafted primary monoamine was found to be exceedingly more stable in the presence of oxygen at high temperature than grafted secondary monoamine. In mixed amine-containing materials, such as grafted propyldiethylenetriamine, the degradation was more severe than for either monoamines, indicating the occurrence of faster oxidative Received: Revised: Accepted: Published: 6887

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a field emission JSM-7500F FESEM (JEOL) instrument, operated at 1 KeV. 2.2. Adsorption Measurements. CO2 adsorption and cyclic adsorption−desorption measurements in the presence of dry and humidified feed and purge gases were carried out using a Rubotherm magnetic suspension balance. Details about the experimental setup and procedure are found in the Supporting Information. To investigate the deactivation of the adsorbent in the presence of dry CO2, after nitrogen pretreatment at 100 °C for 90 min, the sample was exposed to dry 5% CO2 in N2 for 10 h at different temperatures in the range 85−120 °C, then to nitrogen at the same temperature for 1.5 h to remove gaseous and adsorbed CO2. Subsequently, the CO2 uptake was measured at 75 °C in order to compare with the fresh sample. For cycling experiments, the sample was pretreated in the same way as described above, then cooled down to the adsorption temperature before switching to a pure CO2 or 15:85 CO2/N2 stream. After 30 min of exposure, the regeneration took place at the desired temperature under flowing nitrogen at atmospheric pressure. To assess the effect of moisture, the same experiment was performed, except that the feed and purge gases used for adsorption and desorption were bubbled through a temperature-controlled water saturator maintained at 20 °C. The procedure was repeated over several cycles for both dry and humid conditions. 2.3. Long-Term Exposure to CO2- and O2-Containing Gases. The effect of long-term exposure to CFair, simulated flue gas (SFG), and different CO2/O2/N2 mixtures on the CO2 adsorption properties of the materials was investigated using a Rubotherm magnetic suspension balance. In a typical run, the adsorbent was first exposed to dry N2 at 100 °C for 90 min, then brought to the treatment temperature (Ttre), before switching to CFair, CO2/O2/N2 (1−20% CO2, 4.5−17% O2, balance N2), or SFG (12% CO2, 4.5% O2, 83.5% N2). After 30 h exposure, the sample was cooled to 75 °C and then exposed to pure dry CO2 for 30 min to measure the CO2 uptake. Regarding the thermal stability measurements, the weight loss of the samples upon treatment in flowing N2 (110 mL/ min) at different temperatures for 30 h was monitored using a thermogravimetric analyzer (Q500 TA Instruments). 2.4. Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy and Nuclear Magnetic Resonance (NMR). A Nicolet Magna-IR 550 spectrometer equipped with a MCT detector and a Thermo diffuse reflectance cell was used to collect DRIFT spectra. About 15 mg of powder sample was placed in the cell and pretreated in flowing ultrahigh purity He at 100 °C for 2 h. The DRIFT spectra were then recorded under He atmosphere for fresh and treated materials. The spectrum for KBr was used as background. The 13C CP/MAS NMR experiments were conducted on a Bruker AVANCE 500. The spinning frequency was set to 10 kHz. The contact time was 2 ms, with recycle delays of 2 s.

pathways, possibly involving both primary and secondary amines.28 However, whether these accelerated air-degradation experiments are relevant to CO2 adsorption in oxygencontaining gases is yet to be explored. The idea that, in the presence of O2 and CO2, the amine would react faster with CO2, leading to much more air-resistant species is worth investigating. A similar issue has been addressed in aqueous amine scrubbing. It was found that, in the presence of CO2, both O2 and SO2 were much less detrimental to aqueous monoethanolamine (MEA) solution than in the absence of CO2, presumably because of a salting out effect; that is, due to the higher solubility of CO2, both O2 and SO2 exhibit diminished solubility in the MEA solution, thus less interaction with amine molecules.31 Although there are some reports dealing with thermal degradation of PEI-modified adsorbents, 24 as well as oxidative26−28 and CO2-induced degradation25 of amine-grafted materials, there are no comprehensive studies on the stability of supported PEI under different conditions. Thus, the objective of this work was to investigate the behavior of PEI-impregnated adsorbents in the presence of dry CO2, oxygen, and moisture at different temperatures and under lengthy isothermal and nonisothermal adsorption−desorption cycling. A SBA-15 mesoporous silica with platelet morphology and short pore channels, referred to as SBA-15PL, was used as support. In an earlier investigation,19 this material was found to be an effective support for PEI.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis of Adsorbents. All chemicals were obtained from Sigma-Aldrich. Carbon dioxide (99.99%), nitrogen (99.999%), carbon-free air (CFair), and carbon dioxide (5% and 15%) in nitrogen were supplied by Linde Canada. The preparation of SBA-15 platelets was carried out according to Zhang et al.32 Typically, 2.4 g of Pluronic P123 was dissolved in 84 mL of HCl solution (1.07 M) and stirred at room temperature until the solution became clear. Then, 13.9 g of decane was added while stirring the solution at room temperature for 1 h. Finally, 0.027 g of NH4F was added under stirring as a hydrolysis catalyst, followed by 5.1 g of tetraethyl orthosilicate. The mixture was stirred at 40 °C for 20 h, then transferred into an autoclave for further treatment at 100 °C for 48 h. The material was calcined at 550 °C to remove the template. Regarding the preparation of CO2 adsorbents, the required amount of branched PEI (Aldrich, average Mn ∼ 423 or Mn ∼ 600) or linear PEI (Polysciences Inc., Mw ∼ 2500) was dissolved in methanol before adding the mesoporous silica support. The resultant slurry was stirred at room temperature until the solvent is evaporated, then the sample was further dried at 50 °C under reduced pressure (700 mmHg). The adsorbents were denoted as SBA-15PL-x(y), where x is the average molecular weight of the impregnated PEI (i.e. 400, 600, or 2500), and y represents the weight percent of PEI in the adsorbent. The PEI loading used in this work was 55 wt %, which was below the maximum allowable amine loading (59 wt %) of SBA-15PL, which can be determined based on the pore volume of the support (1.4 cm3/g) and the density of PEI (1.05 g/cm3). All materials were characterized by nitrogen adsorption and desorption at −196 °C using a Micromeritics 2020 apparatus. Scanning electron microscopy (SEM) images were collected on

3. RESULTS AND DISCUSSION 3.1. Material Characterization. A typical SEM image for the support material is shown in Figure 1. As seen, SBA-15PL consisted of ca. 200 nm thick particles with ca. 1 μm diameter. The nitrogen adsorption−desorption isotherm for SBA-15PL support (not shown) corresponded to type IV according to the IUPAC classification, which is characteristic of mesoporous materials.19 The surface area of the silica support was 590 m2/g, whereas the pore volume and diameter were 1.4 cm3/g and 13.6 nm, respectively. 6888

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Figure 1. SEM image of SBA-15PL.

3.2. Thermal Degradation Measurements. Table 1 shows the PEI loss for SBA-15PL-x(y) after 30 h exposure to Table 1. PEI Weight Loss after 30 h Exposure to Dry N2a PEI weight loss (wt %)

a

temp. (°C)

SBA-15PL-400(55)

SBA-15PL-600(55)

SBA-15PL-2500(55)

75 105 120 150

2.6 n.d.b 22.5 n.d.

0.3 1.0 4.0 10.0

0.0 n.d. 1.3 5.0

Losses (wt %) are based on total PEI content. bn.d.: not determined. Figure 2. CO2 uptake during nonisothermal CO2 adsorption− desorption cycling over SBA-15PL-600(55) in both dry and humid conditions (adsorption at 75 °C and 1 bar using pure CO2 for 30 min and desorption at (a) 105 °C and (b) 120 °C using pure N2 for 30 min, dew point for humid condition was 20 °C).

flowing nitrogen at different temperatures for PEI with three different molecular weights (Mn ∼ 400, 600 and Mw ∼ 2500). As seen, the heavier PEI showed limited weight loss due to evaporation. At 75 °C, the PEI(600 and 2500)-containing samples did not suffer any loss, whereas SBA-15PL-600(55) and SBA-15PL-2500(55) underwent 10 wt % and 5 wt % loss at 150 °C, respectively. As for PEI(400)-containing material, the weight losses were 2.6% and 22.5% upon heating in nitrogen for 30 h at 75 and 120 °C, respectively. This is mostly due to the evaporation of tetraethylenepentamine, which represents ca. 20% of PEI-400, as indicated by the supplier (Aldrich). Therefore, the PEI-modified adsorbents are thermally stable under mild conditions. Higher molecular weight PEIs are understandably more thermally stable, but they may lead to higher diffusion resistance because of higher viscosity. 3.3. CO2-Induced Degradation. 3.3.1. Cyclic Adsorption−Desorption Measurements. Using SBA-15PL-600(55), extensive adsorption−desorption measurements were carried out in the presence of pure CO2 and 15:85 CO2/N2 mixture at different temperatures in both dry and humid conditions. Figure 2 presents the temperature swing adsorption data in the presence of pure CO2 at two different desorption temperatures under dry and humid conditions. As seen, after 66 cycles, using dry feed and purge gases, the adsorbent lost ca. 40% and 52% of CO2 uptake for regeneration under dry condition at 105 and 120 °C, respectively, which was mainly due to loss of amine groups via urea formation (vide inf ra).25 In contrast, using prehumidified gases (with 6%, 2%, and 1.2% relative humidity

(RH) at 75, 105, and 120 °C, respectively) under otherwise the same conditions, the adsorbent was stable, confirming the effective role of water vapor in preventing the loss of amine groups via the formation of urea. The small amount of uptake loss (ca. 2 and 3.5%) in humid condition was consistent with PEI evaporation loss at 105 and 120 °C, respectively. Most CO2-containing industrial gases are dilute. For instance, flue gas is a water-saturated mixture with typically 10−15% CO2. Therefore, the performance of SBA-15PL600(55) was investigated in the presence of humidified 15% CO2 in N2 at two typical flue gas temperatures (i.e., 50 and 75 °C), and the results are shown in Figure 3 (temperature swing adsorption at 50 °C for adsorption and 85 °C for desorption) and Figure 4 (concentration swing process at 75 °C for both adsorption and desorption). As seen in Figure 3, the adsorbent was stable over 66 cycles. Moreover, because of the favorable properties of the support, the obtained CO2 working capacity (ca. 13.8 wt %) was one of the highest uptakes reported for 15% CO2 at 50 °C.4,12,17−19 Figure 4 provides evidence that, under mild and humid conditions, PEI (Mn ∼ 600)-containing adsorbents are stable over hundreds of cycles. Upon 300 cycles totalling approximately 350 h of CO2 exposure with both 6889

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material. Assuming that this is due to the formation of urea, the weight increase would correspond to the consumption of 7 mmol of amine groups per gram PEI. Based on 13C NMR data (vide inf ra), the PEI has about 30% of primary amine and 40% of secondary amines, for a total amine content of 23.2 mmol/g PEI. Thus, the 7 mmol of amine consumed per gram PEI would correspond to about 43% of the 16.2 mmol of primary and secondary amines, assuming that tertiary amines (30%) are not involved in urea formation. Yet, the CO2 uptake loss compared to the fresh material was 68%, possibly because of the decrease in the surface density of the remaining free amines. Similarly, at 105 °C, the amount of amines involved in urea formation was ca. 3.5 mmol/g PEI, representing about 21% of the original number of primary and secondary amines, yet the decrease in CO2 uptake was 25%. Earlier work33 on CO2-deactivation of grafted propylamine also showed that the deactivated material exhibited a lower CO2/N ratio than expected based on its average surface density of amine groups, indicating the importance of local vs average surface density.34 3.3.3. DRIFT Spectroscopy and Solid State NMR Studies. As reported earlier, the CO2-induced degradation of the adsorbent is associated with the formation of urea.25 In a recent report,33 we demonstrated that contrary to grafted propylamine (primary amine), grafted N-methyl propylamine (secondary amine) did not undergo any adverse effect in the presence of dry CO2, even at 200 °C. The CO2-induced degradation of the primary amine was interpreted on the basis of urea formation via both carbamate dehydration and, most likely, isocyanate-amine coupling. The deactivation of mixedamine adsorbents such as grafted propyldiethylenetriamine was higher than expected based on complete transformation of the primary amine groups into the corresponding urea. To explain this behavior, it was proposed that the primary amine-derived isocyanate reacts with either primary or secondary amines, leading to di- or tri-substituted ureas, respectively. Such ureas could be cyclic or linear, depending on whether or not the two amine groups involved belonged to the same polymer branch or not. Interestingly, in a recent report, Wu et al.35 reported that, under high temperature and CO2 pressure (180 °C, 10 MPa), primary amines afford the corresponding disubstituted ureas in moderate to high yields without any catalyst. Using mixtures of primary and secondary amines, they obtained significant yields of primary amine-derived disubstituted ureas and mixed amine-derived trisubstituted ureas, but no tetrasubstituted ureas from secondary amines only. It was inferred that the isocyanate route, which requires the presence of primary amines, is the dominant mechanism toward the formation of ureas. These findings are fully consistent with the surface chemistry we reported elsewhere.33 Moreover, using a series of ethanolamines and ethylenediamines (R1R2N−CH2−CH2−NR3R4), Lepaumier at al.36 investigated the mechanisms of amine degradation in the presence of CO2 under the following conditions: amine, 4−5 mol/kg; CO2 pressure, 2 MPa; temperature, 140 °C; time, 15 days, stirring, 250 rpm. They reported that one of the main degradation pathways of ethylenediamines was the formation of imidazolidinones, which was particularly fast for diamines with the general formula R1NH−CH2−CH2−NHR2. It is thus inferred that, in the presence of dry CO2, PEI-containing adsorbents may deactivate through the formation of open chain and/or cyclic ureas. DRIFT and 13C NMR data for CO2-deactivated SBA-15PL600(55) provide strong support to the formation of different

Figure 3. CO2 uptake during CO2 adsorption−desorption cycling over SBA-15PL-600(55) in humid condition (adsorption at 50 °C and 1 bar using 15:85 CO2/N2 for 30 min with 19% RH and desorption at 85 °C using pure N2 for 30 min with 4% RH).

Figure 4. Isothermal adsorption−desorption cycling over SBA-15PL600(55) at 75 °C and 1 bar in humid condition (adsorption using 15:85 CO2/N2 for 30 min and desorption using pure N2 for 40 min with 6% RH).

adsorption and desorption at 75 °C, the uptake loss was only 2 wt %. 3.3.2. Continuous Exposure to Dry CO2. Table 2 shows the CO2 uptake on fresh SBA-15PL-600(55) and the correspondTable 2. CO2 Uptake Loss for SBA-15PL-600(55) after 10 h Exposure to Dry 5% CO2 in N2 5% CO2/N2 adsorption at 75 °C exposure temp. (°C)

CO2 uptake (wt %)

uptake loss (%)

fresh 85 105 120

12.8 12.5 9.5 4.1

2 25 68

ing capacity loss after 10 h exposure to dry 5% CO2/N2 at different temperatures. As seen, the degree of deactivation increases with increasing temperature. The weight gain for SBA-15PL-600(55) treated at 120 °C was approximately 5%, corresponding to 9.1% of PEI, which represents 55 wt % of the 6890

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urea linkages. The DRIFT spectrum for fresh adsorbent (Figure 5, trace a) showed absorption bands at 3360, 3285, and 1600

Figure 6. 13C CP MAS NMR spectra of fresh and CO2-degraded SBA15PL-600(55) at 120 °C for 10 h.

peaks at 160 and 163.5 ppm appeared, which may be assigned to CO in different open chain or cyclic ureas with different degrees of substitution. More detailed studies of urea formation and associated mechanisms for different types of amines will be the topic of future work. 3.4. Oxidative Degradation. Table 3 shows the CO2 uptake loss on SBA-15PL-600(55) after 30 h exposure to dry

Figure 5. DRIFT spectra for SBA-15PL-600(55) before and after exposure to dry 5% CO2/N2 at 120 °C for 10 h.

cm−1, which were assigned to asymmetric NH2 stretch, symmetric NH2 stretch (or NH stretch of secondary amine), and NH2 deformation of hydrogen bonded amino group, respectively.37 A weak shoulder at 1662 cm−1 may be assigned to NH3+ deformation of ammonium carbamate species associated with CO2 adsorption from air or the protonated amino group with silica surface hydroxyls.37 CH2 stretch showed absorption bands at 2939, 2883, and 2816 cm−1, and the band at 1458 cm−1 was assigned to CH2 deformation.37 The material obtained after 10 h exposure to dry 5% CO2/N2 at 120 °C (68% deactivated, Table 2) was analyzed using DRIFT (Figure 5, trace b). As seen, the disappearance of the 1600 cm−1 band and the appearance of four new bands at 1498, 1560, 1660, and 1702 cm−1 provide evidence for urea formation in dry condition and high temperature. It is believed that all these bands are attributable to different types of ureas. The bands centered at 1560 and 1660 cm−1 can be assigned to open-chain ureas,25 whereas the bands at 1498 and 1702 cm−1 are tentatively associated with cyclic ureas38 formed by two consecutive amine groups in a single polymer branch. Figure 6 shows the 13C CP/MAS NMR spectra for fresh and CO2-degraded SBA-15PL-600(55) at 120 °C for 10 h (68% deactivated, Table 2). For fresh PEI, the peaks between 39 and 58 ppm are associated with the methylene groups adjacent to primary (39−41 ppm), secondary (49−52 ppm), and tertiary (52−58 ppm) amine groups.39 In addition, there was a small signal at ca. 164.1 ppm corresponding to carbamate, due to CO2 adsorption from ambient air. Upon CO2-deactivation, the 13 C NMR spectrum underwent significant changes. In the aliphatic region, signals associated with carbon atoms adjacent to primary and secondary amines at 41 and 49.5 ppm, respectively disappeared; whereas signals at 39.3 and 46 ppm developed. Using the ACD/C+H NMR Predictors software, it is proposed to assign the peak centered at 39.3 to methylene groups adjacent to primary amines participating in the urea formation, while the peak at 46 is assigned to methylene groups adjacent to secondary amines involved in the formation of urea. Notice that the carbons adjacent to tertiary amines at 52 ppm remained unchanged, suggesting that such amines may not take part in the deactivation mechanisms. Moreover, upon CO2 degradation, the carbamate signal at 164.1 vanished, and two

Table 3. CO2 Uptake Loss for SBA-15PL-600(55) after 30 h Exposure to Dry CFair, Humidified SFG, and CO2/O2/N2 Mixturesa pure CO2 uptake loss (%) at 75 °C CO2/O2/N2 treatment temp. 50 75 90 100 120

CFair

SFG

1:17:82

5:14:81

7.5:10.5:82

20:17:63

b

0.0 0.1 1.8 n.d. n.d.

n.d. n.d. n.d. 70 n.d.

n.d. n.d. n.d. 37 n.d.

n.d. 0.1c n.d. 3.0d n.d.

0.0 n.d. 1.8 2.6 n.d.

n.d. 6 22 70 100e

a All streams except CFair were humidified using a water saturator controlled at 20 °C. bn.d.: not determined cAfter 120 h exposure. d 50% uptake loss in the presence of 10.5:89.5 O2/N2 stream, under otherwise the same conditions eAfter 20 h exposure.

CFair, humidified SFG, and CO2/O2/N2 mixtures, including CFair (0% CO2) at different temperatures. Except CFair, all other feed gases were humidified in order to prevent the urea formation, thus discriminating between CO2- and O2-induced degradations. As seen, the CO2 uptake decreased by 6% upon CFair exposure for 30 h at 75 °C, and by 22 and 70% at 90 and 100 °C, respectively. The sample was even more sensitive to air treatment at 120 °C, as it was completely deactivated upon 20 h exposure to CFair. It was also observed that the adsorbent lost 16 and 25% of the loaded PEI at 100 and 120 °C, respectively, compared to 1 and 4% upon N2 exposure, under otherwise the same conditions. In addition to temperature, the stability of SBA-15PL-600(55) depends on the oxygen partial pressure. For example using 10.5:89.5 O2/N2 at 100 °C for 30 h led to only 50% CO2 uptake loss. It should be noted that as the degree of deactivation increased, the adsorbent turned from white to brownish-yellow. Drage et al.24 also reported that the weight loss of PEI-1800 at about 135 °C was lower in the 6891

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presence of nitrogen than air. It is thus inferred that the loss of CO2 uptake upon air exposure is due to a combination of material loss and oxidative reactions. Moreover, since the material loss under air treatment was significantly higher than under nitrogen, it cannot be associated solely with PEI evaporation. Rather, some oxidative decomposition must take place. Lepaumier et al.40 reported that, under air (2 MPa, 140 °C, 15 days), ethylenediamines (similar to the repetitive units in PEI) undergo significant “dealkylation” (N−C bond splitting), leading to volatile compounds. Although no attempt was made to identify such species, it is inferred that, in the presence of air, even at moderate temperature, supported PEI undergoes significant weight loss due to the formation of low molecular weight amine-containing species. To study the effect of oxygen in CO2-containing mixtures, the material was treated with prehumidified SFG and different CO2/O2/N2 mixtures with O2 concentrations ranging from 4.5% (in SFG) to 17%. Prehumidification of the mixed gases was carried out to prevent the CO2-induced degradation in order to discriminate between the effect of O2 (oxidative degradation) and the effect of CO2 (urea formation). SBA15PL-600(55) treated with humidified 1:17:82 CO2/O2/N2 mixture at 100 °C for 30 h showed the same behavior as the CFair-treated material (i.e., ca. 70% uptake loss). In contrast, the material did not suffer any loss in CO2 uptake upon treatment with humidified SFG (12% CO2, 4.5% O2) below 75 °C; however, at 90 °C, a limited loss (1.8%) was observed, mostly because of PEI evaporation at this temperature (Table 1). SBA-15PL-600(55) also exhibited 3% uptake loss after exposure to humidified 7.5:10.5:82 CO2/O2/N2 for 30 h at 100 °C due to PEI evaporation loss, compared to as high as 50% loss in the presence of the same partial pressure of O2, but without CO2 (10.5:89.5 O2/N2), under otherwise the same conditions. The adsorbent was also stable even in the presence of higher O2 content (20:17:63 CO2/O2/N2) as only 1.8 and 2.6% CO2 uptake losses were observed at 90 and 100 °C, respectively, which are associated mostly with PEI evaporation. The foregoing discussion provides evidence that, in the presence of carbon dioxide, the material is much more stable toward oxygen and does not deactivate by oxidative degradation. It is inferred that the amine reacts faster with CO2 than with O2, leading to species such as carbamate and bicarbonate, with enhanced oxygen stability. Thus, the extent of oxidative degradation based solely on the effect of CO2-free air, as reported earlier,26−28 does not reflect the actual limited effect of O2 in the presence of CO2. Notice that Uyanga and Idem31 investigated the effect of O2 and SO2 on aqueous MEA in the presence and absence of CO2. In both cases, they found that CO2 inhibited the otherwise deleterious effect of O2 and SO2. They explained this behavior based on the salting out effect, whereby CO2 dissolves into the MEA solution in preference to the other gases, thus preventing the interaction of MEA with SO2 and O2. Although the explanation is different, the general phenomenon is similar for CO2 absorption in amine solutions, or adsorption on supported amine. The main practical implication of this finding is the inference that during CO2 removal from flue gas, the amine groups convert rapidly into carbamate (and bicarbonate) and, therefore, are preserved against O2 attack. DRIFT spectra for SBA-15PL-600(55) treated in dry CFair at 90 and 120 °C for 30 h are shown in Figure 7 (traces b and c). As seen, upon exposure to CFair for 30 h at 90 °C, a new IR band at 1679 cm−1 consistent with the occurrence of a CO

Figure 7. DRIFT spectra for SBA-15PL-600(55) before and after exposure to dry CFair at 90 and 120 °C for 30 h.

species was observed. Moreover, with increasing the airtreatment temperature to 120 °C, the intensity of this band increased sharply. Besides, the resolution of N−H stretching bands at 3300−3400 cm−1 and NH2 deformation at 1600 cm−1 decreased significantly with increasing temperature. Earlier work28 on the interaction of oxygen with different grafted amines showed that single primary amine (propylamine) was highly resistant to oxidative degradation, whereas single secondary amine (N-methylpropylamine) and propyldiethylenetriamine deactivated readily and showed a similar prominent IR band at ca. 1670 cm−1. Similarly, Calleja et al.41 monitored the behavior of SBA-15 grafted propylamine, propylethylenediamine, and propyldiethylenetriamine by DRIFT spectroscopy upon drying in air at 110 °C for different periods of time, up to 85 h. The DRIFT spectrum for the propylamine-containing material, as well as its CO2 adsorption capacity, did not show any significant change. On the contrary, the other two materials exhibited decreasing CO2 adsorption uptakes, paralleled by the development of an absorption band at 1667 cm−1, which was associated to CN species in oxime or imine groups. Figure 8 shows the 13C CP/MAS NMR spectra for SBA15PL-600(55) before and after CFair exposure at 90 and 120 °C for 30 h. Parallel to the CO2 uptake loss (Table 3), the intensity of the peaks associated with aliphatic carbons at 39− 41 ppm and 49−58 ppm decreased, and instead, a new peak at

Figure 8. 13C CP MAS NMR spectra of SBA-15PL-600(55) before and after exposure to CFair for 30 h at different temperatures. 6892

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Nanostructured Materials for Catalysis and Separation (2001− 2015).

45 ppm developed. Moreover, two peaks attributable to different CO species at 158 and 164.1 ppm developed. Notice that 164.1 ppm peak in the air-deactivated material at 120 °C for 30 h is very close to the peak corresponding to carbamate species in the fresh sample. However, this signal cannot be attributed to carbamate because the sample does not adsorb any CO2 (Table 3).These species are similar to those observed in air-deactivated grafted propyldiethylenetriamine.28 Lepaumier et al.40,42 reported that, in the presence of air, ethylenediamines afford different piperazinones through dealkylation/oxidation followed by cyclization. Based on our DRIFT and NMR data, there are strong indications that species containing different CO groups such as carboxylic acids and/or amides were formed upon oxygen deactivation of supported PEI. However, accurate identification of these species requires further investigation.

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4. CONCLUSION This work provides key data regarding the stability of PEIimpregnated adsorbents for CO2 removal applications. The effect of long-term exposure to carbon-free air, simulated flue gas (SFG), and different CO2/O2/N2 mixtures on the CO2 adsorption capacity as well as extensive CO2 adsorption− desorption cycling were investigated at different temperatures in both dry and humid conditions. It was revealed that PEI (Mn ≥ 600)-containing materials are thermally stable at mild temperatures. Long-term exposure to dry CO2 or extensive adsorption−desorption cycling using dry CO2-containing gases showed that the material deactivates, even under mild conditions. The dry CO2−induced degradation was attributed to the formation of different urea groups as revealed by NMR and DRIFT measurements. In contrast, in the presence of moisture, the formation of urea was strongly inhibited and the adsorbent showed high stability. Analysis of the adsorbents after exposure to CFair at 100−120 °C using 13 C CP MAS NMR, DRIFT spectroscopy and CO2 adsorption measurements showed that the PEI-impregnated materials degraded in the presence of CO2-free oxygen. On the contrary, the adsorbent was stable after long-term exposure to wet CO2 and O2-containing gas mixtures, indicating that, in the presence of CO2, amine groups are protected from oxygen attack, presumably because of their rapid conversion to carbamate and bicarbonate. These findings will play a significant role in the design of CO2 removal processes based on adsorption over amine-containing materials.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental setup and procedures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Canada Carbon Management (CMC-NCE) and the Natural Science and Engineering Council of Canada (NSERC) is acknowledged. A.S. thanks the Federal Government for the Canada Research Chair in 6893

dx.doi.org/10.1021/ie3003446 | Ind. Eng. Chem. Res. 2012, 51, 6887−6894

Industrial & Engineering Chemistry Research

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dx.doi.org/10.1021/ie3003446 | Ind. Eng. Chem. Res. 2012, 51, 6887−6894