Supported Polytertiary Amines: Highly Efficient and Selective SO2

Article pubs.acs.org/est

Supported Polytertiary Amines: Highly Efficient and Selective SO2 Adsorbents Ritesh Tailor, Mohamed Abboud, and Abdelhamid Sayari* Department of Chemistry, Centre for Catalysis Research and Innovation (CCRI), University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 S Supporting Information *

ABSTRACT: Tertiary amine containing poly(propyleneimine) second (G2) and third (G3) generation dendrimers as well as polyethyleneimine (PEI) were developed for the selective removal of SO2. N-Alkylation of primary and secondary amines into tertiary amines was confirmed by FTIR and NMR analysis. Such modified polyamines were impregnated on two nanoporous supports, namely, SBA-15PL silica with platelet morphology and ethanol-extracted pore-expanded MCM-41 (PME) composite. In the presence of 0.1% SO2/N2 at 23 °C, the uptake of modified PEI, G2, and G3 supported on SBA-15PL was 2.07, 2.35, and 1.71 mmol/g, respectively; corresponding to SO2/N ratios of 0.22, 0.4, and 0.3. Under the same conditions, the SO2 adsorption capacity of PME-supported modified PEI and G3 was significantly higher, reaching 4.68 and 4.34 mmol/g, corresponding to SO2/N ratios of 0.41 and 0.82, respectively. The working SO2 adsorption capacity decreased with increasing temperature, reflecting the exothermic nature of the process. The adsorption capacity of these materials was enhanced dramatically in the presence of humidity in the gas mixture. FTIR data before SO2 adsorption and after adsorption and regeneration did not indicate any change in the materials. Nonetheless, the SO2 working capacity decreased in consecutive adsorption/regeneration cycles due to evaporation of impregnated polyamines, rather than actual deactivation. FTIR and 13C and 15N CP-MAS NMR of fresh and SO2 adsorbed modified G3 on PME confirmed the formation of a complexation adduct.

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Furthermore, as demonstrated by Uyanga et al.6 and Supap et al.,18 monoethanolamines deactivate during the FGD process via formation of heat stable sulfate. Although other methods including semidry FGD,19 dry FGD,20 ammonium FGD,10 and electron beam FGD21 have some unique advantages, they always suffer from low desulfurization efficiency, high operation cost, difficulty of SO2 recovery, or undesirable secondary pollution.22 Recently, room temperature ionic liquids (ILs) have drawn much attention as potential FGD materials due to their thermal stability and tunable properties.16,23−28 In the presence of 1% SO2/N2 at room temperature, the SO2 uptake was found to be 3.5 and 4.32 mol/mol for azole-based IL,24 and 1-(2diethylaminoethyl) 3-methylimidazolium tetrazolate,26 respectively. This is indicative of 1:1 stoichiometric interaction of SO2 and nitrogen. However, information on the effect of water vapor and other gaseous species, in particular, the selectivity of ILs for SO2 vs CO2 is scarce. Wu et al.28,29 found that the presence of O2 and H2O causes oxidation of absorbed SO2,

INTRODUCTION Combustion of fossil fuels results in emissions containing acidic gases like CO2, SOx, and NOx. Recently, unprecedented attention has been focused on the removal and sequestration of CO2 from flue gas, which is regarded by many as a pivotal step in the control of greenhouse gases.1,2 The presence of SO2 holds a competitive and detrimental effect on the CO2 removal efficiency of materials used in the treatment of industrial gases, especially on amine mediated CO2 removal technology.3−8 Moreover, SO2 is also an environmental pollutant that can cause respiratory illness, acid rain, smog, and atmospheric particle deposition.9 Scrubbing of sulfur containing gases using liquid absorbents is the traditional process for flue gas desulfurization (FGD).10,11 Many amine solutions12−14 and physical solvents15,16 were reported to be efficient for flue gas desulfurization. However, most of these organic solvents are volatile, toxic, prone to degradation, and as such are not environmentally friendly. In addition, absorption technology holds the inevitable problem of equipment corrosion, large amount of wastewater, and excessive energy consumption for regeneration. Another widely commercialized technology to control SO2 emissions is the wet limestone process,4,17 which suffers similar shortcomings as absorption, in addition to the production of large amounts of low market value byproducts such as calcium sulfate. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2025

September 16, 2013 December 12, 2013 January 17, 2014 January 17, 2014 dx.doi.org/10.1021/es404135j | Environ. Sci. Technol. 2014, 48, 2025−2034

Environmental Science & Technology

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obtained from Sigma-Aldrich and used as received. Polypropyleneimine dendrimers of second and third generations, DAB-AM-8 (PPI-G2, Mw = 773.3) and DAB-AM-16 (PPI-G3, Mw = 1686.8) were obtained from SyMO-Chem (Netherlands). Nitrogen (99.999%) and SO2 (0.01%, 0.1%, and 1% + balance N2) gases were supplied by Linde, Canada. N-Methylation of Polyamines. The primary and secondary amines of branched PEI (Mw = 600), PPI-G2, and PPI-G3 were N-methylated into tertiary amines in accordance with the procedure described in the literature.57 In brief, PEI, PPI-G2, or PPI-G3 (1 g) was added to a solution containing water (0.4 mL), formic acid (14 mL), and formaldehyde (12 mL). The mixture was heated at 120 °C overnight under nitrogen atmosphere. Note that only PEI containing mixture was degassed for 1 h before heating. The resultant solution was cooled to 25 °C and any volatile component was removed using a rotary evaporator at 45 °C. The residual mixture was made basic using a 20% NaOH solution (30 mL). To the PEI-containing mixture, KOH pellets were added until the product was separated as an oily organic layer. The Nmethylated polyamine was extracted with Et2O (20 mL × 5), and then dried over Na2SO4. After removing the solvent by rotary evaporator, the N-methylated PEI was obtained as orange oil (yield: 96%). As for PPI G2 or G3, the white suspended residue was extracted into Et2O (30 mL × 3), and the organic extracts were dried over potassium hydroxide. The excess solvent was removed by rotary evaporation to finally obtain modified PPI G2 and G3 as yellowish oil (yield: 81%) and yellowish orange oil (yield: 80%), respectively. The modified PEI, PPI-G2, and PPI-G3 containing mainly tertiary amines will be referred to as mPEI, mG2, and mG3, respectively. Preparation of SO2 Adsorbents. The N,N-dimethylaminopropyl grafted pore-expanded MCM-41 was prepared according to our earlier report.58 Briefly, after vacuum drying (120 °C, 2 h), 1 g of PE-MCM-41 (PMC) silica was dispersed in 150 mL of toluene in a multineck glass flask until a homogeneous mixture was obtained. Distilled deionized water was added at the rate of 0.4 mL per gram of PMC and the mixture was stirred for 30 min. The flask equipped with a condenser was then submerged in a silicon oil bath set at 85 °C using a temperature-controlled stirring hot-plate. Subsequently, 2 mL of N,N-dimethylaminopropyltrimethoxysilane per gram of silica was added to the mixture and left under stirring for 16 h. The resultant product was filtered, washed with toluene and pentane, and then dried overnight at room temperature. The material obtained was referred to as PMC-TER. The modified polyamines, mPEI, mG2, and mG3, were loaded by impregnation on SBA-15PL and PME. The desired amount of polyamine was dissolved in methanol and the appropriate amount of SBA-15PL or PME support was added to the solution. The resulting slurry was stirred at room temperature until the solvent was evaporated, and then the material was further dried at 60 °C under slightly reduced pressure (700 mmHg). The obtained adsorbents were designated as SBA-15PL-X or PME-X with X = mPEI, mG2, or mG3. The actual amounts of amine loaded on PMC, SBA-15-PL, and PME were measured by thermogravimetric analysis (TGA) using a TA Q-500 instrument. In a typical experiment, the sample was dried at 120 °C for 90 min and heated to 800 °C under N2 at 10 °C/min and then to 1000 °C in air for 5 min. The weight loss was used to calculate the amine loading. The

leading to the formation of H2SO4 with decreased absorption capacity. Compared to absorption, adsorption is a relatively simple technology with little or no water consumption, lower energy requirement, and smaller footprint. Numerous adsorbents for flue gas desulfurization, including activated carbons,30−32 and carbon fibers,33,34 oxides,4,35,36 zeolites,37−39 supported polymers,40 supported metals,41,42 and metal−organic frameworks43,44 have been reported in the literature. Among these adsorbents, activated carbons and carbon fibers showed good selectivity toward SO2 versus CO2. However, they are also adversely affected in the presence of O2 and H2O due to the formation of H2SO4.34,45 Similarly to absorption in amine solutions,6,17 adsorption of SO2 on amine-supported solid materials containing primary and secondary amines led to irreversible deactivation.7−9,46−48 Thus, separation of sulfur dioxide prior to CO2 capture is mandatory, especially when amine-containing adsorbents are used. However, not only is the concentration of SO2 in many industrial gases much lower than CO2, but both are acidic in nature, making it particularly challenging to develop materials for selective and reversible adsorption of SO2. One possible route is the use of tertiary amines, which are known to have very low affinity toward CO2, yet they readily interact with SO2.48−51 Surprisingly, despite the explosive growth of research on CO2 removal over supported amines,52−56,58 there are very few reports on reversible SO2 adsorption on amine-containing materials.48−51 Zhou et al.50 reported that a 4.62 mmol/g triethnolamine-impregnated SBA-15 adsorbed 2.76 mmol/g SO2 in the presence of 1340 ppm in N2 at 25 °C, corresponding to SO2/N = 0.6. Moreover, the SO2 uptake increased to 207 mg/g under humid conditions. In an earlier communication, we found that N,N-dimethylpropylamine grafted on pore expanded MCM-41 silica was highly selective toward SO2 vs CO2.51 Its adsorption capacity at room temperature for 1% SO2/N2 was 2.19 mmol/g (SO2/N = 0.67). Moreover, the presence of water vapor enhanced the SO2 adsorption 3-fold at 50 °C. Nonetheless, the strategy of using supported tertiary amines for the purpose of flue gas desulfurization before CO2 capture has not been fully explored. Building on our preliminary findings,51 the current work is focused on the development of novel polytertiary amine supported nanoporous adsorbents instead of a tertiary monoamine. The effects of water vapor on the adsorptive properties of the materials as well as the nature of the SO2 interactions with tertiary amines were investigated. Three support materials were selected based on earlier screening. Amine grafting was carried out using calcined pore-expanded MCM-41 (PMC), which exhibits a unique combination of high surface area, large pores, and high pore volume.52 As for polyamine impregnation, two supports were used, namely, SBA-15PL with platelet morphology and ethanol-extracted PEMCM-41 (PME). Because of its short diffusion path, SBA15PL was found to be an excellent support for impregnated polyethylenimine (PEI) as CO2 adsorbent.53 Likewise, because of the layer of cetyltrimethylammonium (CTMA+) cations on the internal surface of PME, impregnated PEI was highly dispersed, leading to enhanced CO2 adsorption efficiency.54

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EXPERIMENTAL SECTION Materials. Except for N,N-dimethylaminopropyltrimethoxysilane (>95%, Gelest), all other chemicals used in the synthesis of the adsorbents and N-methylation of polyamines were 2026

dx.doi.org/10.1021/es404135j | Environ. Sci. Technol. 2014, 48, 2025−2034


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N-methylated polyamines as well as the SO2 interaction with tertiary amine groups were investigated using IR and NMR spectroscopy. Attenuated total reflectance (ATR) spectra were recorded on a Nicolet 6700 instrument. Liquid-state 1H NMR spectra were obtained on a Bruker AVANCE 400 instrument. A standard 30° proton pulse was employed sequentially. The spectral width and acquisition time were 8400 Hz and 4 s, respectively. The relaxation delay was 0.01 s, and sixteen transients were signal-averaged. Deuterated chloroform (CDCl3) was used as solvent, and the chemical shifts (δ) are reported in ppm relative to tetramethylsilane (TMS) signal as internal reference. Solid-state 13C and 15N CP-MAS NMR were conducted on a Bruker AVANCE III 200 NMR instrument using ca. 300 mg of material at a resonance frequency of 50 and 20 MHz for 13C and 15N NMR, respectively. The 13C data were collected with a spinning rate of 4500 Hz using a H/X/Y triple resonance 7 mm probe in the double resonance mode. The 15N data were collected with a spinning speed of 4500 Hz using a H/X double resonance 7 mm stretch probe. The proton 90° pulses of 3.3 and 5 μs with 2 ms contact time were used for 13C and 15N CP-MAS. The relaxation delay was set at 2 and 4 s for 13 C and 15N, with a spectral width of 20 kHz in both cases. The 13 C CP-MAS NMR spectra were referenced to the carbonyl carbon of glycine at 176.5 ppm, whereas the 15N CP-MAS NMR chemical shifts were referenced to neat nitromethane (0 ppm).

amount of surfactant was also taken into account in computing the actual amine content of PME-supported materials (see section 1 in Supporting Information). SO2 Adsorption Measurements. Column-breakthrough measurements of SO2 adsorption capacity were carried out using a packed-bed column as schematically presented in Scheme 1. A 3.5-cm-long stainless steel column with an inner Scheme 1. Experimental Setup for SO2 Adsorption in a Packed-Bed Column

diameter of 0.42 cm was loaded with ca. 0.10−0.15 g of 20−40 mesh sized of adsorbent pellets and placed in a temperature controlled oven. Before each run, the adsorbent was activated at 115 °C for 90 min under a nitrogen flow of 80 mL/min. The temperature was then lowered to the desired level (room temperature ≈23 or 50 °C) and the flow was switched to a mixture containing 0.01−1% SO2 in N2 (flow rate: 50 or 80 mL/min). As for experiments under humid conditions, the N2 stream was passed through a water saturator maintained at 20 °C, then mixed with the SO2/N2 stream before entering the packed-bed. The column downstream was continuously monitored using a MKS Cirrus LM99 mass spectrometer (MS). The columnbreakthrough curves of SO2 were obtained from the MS signal corresponding to 64 amu. All experiments were performed at atmospheric pressure. The dynamic adsorption capacity (q) of the material was calculated using eq 1: q=

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RESULTS AND DISCUSSION Characterization of Modified Polyamines. Figure 1 shows the 1H NMR spectra of PEI (Figure 1a) and mPEI

FC0tq (1)

w

where F is the total molar flow, C0 is the concentration of the adsorbate in the feed stream, W is the mass of adsorbent loaded in the column, and tq is the stoichiometric time, which is estimated from the breakthrough profile using eq 2 tq =

∫0

C ⎞ ⎜1 − A ⎟ d t C0 ⎠ ⎝

Figure 1. 1H NMR spectra of PEI (a) and mPEI (b).

∞⎛

(2)

(Figure 1b) with the corresponding peak assignments. It is seen that upon N-methylation, the broad peak at 1.79 ppm attributable to NH/NH2 protons disappeared, whereas a prominent peak at ca. 2.23 ppm associated with methyl protons developed. Moreover, the peaks assigned to methylene protons adjacent to primary and secondary amines in Figure 1a (peaks b, c, and d) shifted to higher field due to the methylation process. Thus, Figure 1 provides strong evidence that both primary and secondary amine groups were fully methylated into tertiary amines. Similarly, Figure 2 shows that N-methylation of PPI-G2 gave rise to a strong peak at 2.21 ppm attributable to CH3 protons

where C0 and CA are the upstream and downstream concentrations of the adsorbate, respectively. Characterization. The PMC, PME, and SBA-15PL and their corresponding amine containing materials were characterized by N2 adsorption and desorption at 77 K using a Micromeritics 2020 instrument. The pore volume was calculated at relative pressure of 0.97, and the average pore size was determined using the BJH (Barrett−Joyner−Halenda) model with Kruk-Jaroniec-Sayari correction.59 The amineloaded materials were degassed at room temperature for 4 h under vacuum. 2027



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Table 1. Structural Properties of Materials Materials

SBET, m2/g

Vp, cm3/g

dp, nm

PMC SBA-15PL PME PMC-TER SBA-15PL-mG3 SBA-15PL-mPEI PME-mG3 PME-mPEI

1153 658 570 79 46 22 92 133

2.81 0.88 1.59 0.11 0.08 0.05 0.23 0.34

11 7.5 10.9 5.6 5.9 6.2 9.9 9.5

PME supports obtained from N2 adsorption isotherms exhibited high surface area, high pore volume, and high pore diameter before amine modification. After amine loading the surface area, pore volume and pore size decreased due to the presence of amines within the pore systems. The amine content measured by TGA were as follows: PMC-TER (28 wt %, 3.26 mmol N/g), SBA-15PL-mG2 (41 wt %, 5.81 mmol N/g), SBA15PL-mG3 (38 wt %, 5.32 mmol N/g), SBA-15PL-mPEI (41 wt %, 9.53 mmol N/g), PME-mPEI (49 wt %, 11.29 mmol N/ g), and PME-mG3 (38 wt %, 5.32 mmol N/g). SO2 Adsorption. Figure 4 shows the adsorption capacity of all materials, measured in the presence of 1000 ppm SO2/N2 at room temperature. As shown in Figure 4a, the PMC-TER

Figure 2. 1H NMR spectra of liquid G2 (a) and mG2 (b).

(Figure 2b). Moreover, the broad signal corresponding to NH2 protons at ca. 1.39 ppm (peak a in Figure 2a) almost disappeared, whereas the peak associated with methylene protons attached to primary amines (signal g in Figure 2a) shifted from 2.71 to 2.25 ppm. This indicates that all terminal primary amines were methylated into tertiary amines. Figure S1 provides similar findings regarding the PPI-G3 dendrimer, including the disappearance of the NH2 signal at 1.67 ppm (peak a, Figure S1a), the appearance of CH3 peak at 2.22 ppm (peak a, Figure S1b), and the high field shift of the protons in methylene groups attached to primary amines (peak i in Figure S1b). Furthermore, the N-methylation of PEI and PPI-G2/G3 was confirmed by FTIR spectroscopy. Figure 3 shows that the

Figure 3. FTIR spectra of fresh and modified liquid PEI, PPI G2, and PPI G3.

bands associated with N−H stretching at around 3280 and 3357 cm−1, NH2 scissoring at around 1596 cm−1, and N−H wagging at 820 cm−1 observed in the starting materials disappeared in the methylated polyamines. The band attributed to C−N stretching also shifted from ca. 1070 to 1040 cm−1. In addition, the systematic development of new bands in the C−H stretching regions at 2760 cm−1 and 2970 cm−1 (shoulder) are also indicative of N-methylation.60,61 Structural Properties of the Adsorbents. As shown in Table 1, the BET surface areas (SBET) of SBA-15PL, PMC, and

Figure 4. (a) Saturation capacity and (b) SO2/N ratio of SO2 adsorption on SBA-15PL, PME, and PMC supported amines (1000 ppm SO2/N2, T = 23 °C, total flow rate = 50 mL/min). 2028



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Figure 5. Effect of water vapor on SO2 adsorption over PME-mG3 and PME-mPEI using 0.05% SO2/N2, 19% RH, total flow rate = 50 mL/min, T = 50 °C.

exhibited the lowest adsorption capacity; this can be well understood because of its lower amine content, which is limited by the grafting procedure. SBA-15PL-mPEI and mG2 showed higher uptake than PMC-TER due to higher amine loading, but lower adsorption than PME supported polyamines. In SBA15PL supported polyamines, the adsorption capacity was limited most likely because of inaccessibility of inner tertiary nitrogen atoms. The PME loaded with mG3 and mPEI exhibited the highest capacity of 4.34 and 4.68 mmol/g (Figure 4a), corresponding to SO2/N ratios of 0.82 and 0.41 (Figure 4b), respectively. The obtained SO2 uptakes are the highest ever reported in the literature for amine supported solid adsorbents. These data are to be compared with SO2/N ratio of 0.6 at 25 °C reported by Zhou et al.50 for 69 wt % triethnolamine on SBA-15 in the presence of 0.13% SO2/N2. In our investigation, the observed high capacity is due to the layer of alkyl chain (CTMA+) within the PME which plays an important role in enhancing the dispersion of mPEI and mG3, thereby decreasing the diffusion resistance and allows SO2 interaction with hindered amines groups. This is reminiscent of the very high CO2 adsorption capacity of PME-supported PEI reported earlier.54 Hence, further investigations were limited to SO2 adsorption over PME-mPEI and PME-mG3. Effect of Water Vapor on SO2 Adsorption. The influence of water vapor on the performance of adsorbents was examined by using a stream of N2 prehumidified in a water saturator maintained at 20 °C, which was then mixed with the SO2 containing stream before passing through the adsorbent bed. As seen in Figure 5, the breakthrough and saturation adsorption time for 500 ppm SO2/N2 over PME-mPEI and PME-mG3 at 50 °C increased significantly under a feed with 19% relative humidity (RH). Similarly to our previous report on PMC-TER,51 the SO2 saturation capacity increased 3-fold from 1.46 to 4.44 mmol/g and 1.26 to 3.78 mmol/g for PMEmPEI and PME-mG3, respectively, compared to dry conditions at 50 °C, all other conditions being equal. Similar findings were observed with SBA-15PL-mG2 (Figure S2), where SO2 adsorption capacity increased from 0.74 to 2.30 mmol/g under humid condition (19% RH) at 50 °C. SO2 uptake measurements at 25 °C with relative humidity from 0% to 83% were carried out in the sequence shown in Table 2. The data

Table 2. 0.05% SO2 Adsorption over PME-mG3 at Different Relative Humidity (RH) normalized uptake No

Temp (°C)

mmol SO2/g

SO2/N

% RH

1 2 3 4 5 6 7 8 9 10

25 25 25 25 25 25 25 50 50 50

3.79 8.88 7.27 5.75 3.76 7.61 3.93 1.59 4.67 1.63

0.72 1.67 1.37 1.09 0.71 1.44 0.75 0.30 0.89 0.31

0 57 38 25 10 83 0 0 19 0

indicate that within experimental errors, the weight normalized uptake increased with increasing relative humidity. Similar, but more robust, increases took place at 50 °C (entries 8−10). Regeneration of Adsorbents. As reported in the literature,62,63 the charge transfer complex of SO2-amine dissociates at high temperature, consistent with the decreasing SO2 adsorption capacity of all materials at increasing temperature (Figure S3). Desorption of SO2 from saturated PMEmG3 and PME-mPEI was carried out at 50, 85, 95, and 115 °C to determine the most appropriate desorption temperature for adsorption−desorption cycling. As shown in Figure 6, complete desorption was achieved at 95 and 115 °C within 90 min. Thus, for recycling purposes, the SO2 saturated PME-mPEI and PME-mG3 adsorbents were regenerated at 115 °C under a N2 flow of 80 mL/min for 90 min. The bed was then cooled to the adsorption temperature for further cycling. As shown in Figure 7, the measured working capacity of PME-mPEI and PME-mG3 decreased by ca. 11% after few cycles from 4.68 to 4.1 mmol SO2/g and from 4.3 to 3.8 mmol SO2/g, respectively. Similar results were observed with mPEI, mG2, and mG3 impregnated SBA-15PL (Figure S4). In separate experiments, different materials were exposed to flowing nitrogen at 115 °C to investigate the weight loss vs time via evaporation. This allowed us to normalize the working capacity in Figure 7 taking into account the total loss via 2029



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Figure 6. Desorption of adsorbed SO2 (0.25% SO2/N2) at different temperatures under N2 at 80 mL/min.

Figure 7. SO2 adsorption (RT = 23 °C) and desorption (115 °C) cycles over PME-mG3 and PME-mPEI using 0.1% SO2/N2, total flow rate = 50 mL/min.

broad band at ca. 1140 cm−1 in Figure 8a) and two bands at 545 and 645 cm−1. Adsorbed SO2 is supposed to exhibit three main IR bands, corresponding to symmetrical, antisymmetrical, and bending vibrations.64 Since nitrogen occurs in different environments in mG3 and mPEI, one would expect different SO2-amine complexes. It is thus suggested that the new bands correspond to antisymmetric (∼1125 and ∼1170 cm−1) and bending (545 and 645 cm−1) vibrations of distorted SO2 molecule in different SO2-amine adducts. These findings are in general agreement with data reported by others.24,26,64−67 For example, Vasilieva et al.64 reported that the SO2-amine complex in a N,N-diethylpropylamine containing polysiloxane film exhibits three infrared bands at 643, 948, and 1245 cm−1. Other researchers using ionic liquids with nitrogen atoms in different environments reported the occurrence of two bands at ca. 1328 and 1146 cm−1, but a single band at ca. 950 cm−1,26,66 possibly indicative of multiple adducts. Similar results were observed with SBA-15PL-mG2, PMC-TER (Figure S6). In addition, as shown in Figure 8 and Figure S6, all bands associated with symmetric C−H stretching vibrations below

evaporation during the cumulative time for desorption at 115 °C. Thus, beyond the first cycle, almost all the apparent decrease in SO2 uptake was associated to gradual evaporation of impregnated mPEI and mG3 during the desorption step, rather than because of actual amine deactivation. Furthermore, FTIR spectra (Figure S5) of regenerated adsorbents after SO2 cycling were completely identical to the fresh materials, signifying that the amines did not undergo any chemical degradation. SO2−Tertiary Amine Interaction. Based on literature reports, SO2 interact with tertiary amines through the formation of noncovalent adducts. Because of the enhanced electron donating ability of amine groups due to their Nmethylation, the complexes with π-acceptor SO2 were stable enough to carry out IR measurements even after exposure to air. Figure 8 shows comparative FTIR spectra of PME-mPEI and PME-mG3 materials before and after exposure to 1% SO2/ N2 and after regeneration. Upon adsorption of SO2, a new absorption band developed at 944 cm−1 due to symmetric stretching of adsorbed SO2. In addition, there were two new bands at 1126 and 1170 cm−1 as shown in Figure 8b (or a 2030



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Figure 9. 15N CP-MAS NMR spectra of PME-mG3: (a) fresh, (b) exposed to SO2, and (c) regenerated.

(compare with 15N NMR in Figure S8 for PME containing only CTMA+ cations, without amines) and a very broad peak at ca. −342 ppm attributable to tertiary nitrogen of supported mG3.69 Upon SO2 adsorption, it is understandable that the signal associated with ammonium species did not change. However, a new peak appeared at ca. −305 ppm, most likely due to nitrogen of tertiary amine group interacting with adsorbed SO2 in noncovalent complexation. Upon SO2 desorption, the −305 peak vanished, whereas the −342 peak became better resolved than in the fresh material (Figure 9c). Similar results were observed with PMC-TER and SBA-15PLmG2 (Figure S9a,b). Since the peak at −342 ppm did not disappear upon SO2 adsorption, it is inferred that not all of the tertiary amine groups contributed in the formation of SO2amine adducts, which is consistent with the SO2/N ratio being lower than 1. Moreover, it is likely that upon exposure to air, partial desorption of SO2 took place. The solid state 13C CP-MAS NMR spectra of fresh PMEmG3 and after exposure to SO2 are shown in Figure 10. Upon SO2 adsorption, the NMR peaks became broad. The chemical shifts of CH2 carbons observed in fresh PME-mG3 at about

Figure 8. FTIR spectra of fresh PME-mPEI (a), and PME-mG3 (b), exposed to 1% SO2, and after regeneration.

2850 cm−1 disappeared upon SO2 interaction with tertiary amine. A new band developed at about 3030 cm −1 corresponding to C−H asymmetric stretching. It is instructive to compare these findings with the protonation of a tertiary amine. Figure S7 shows that the IR spectrum of primary alkylamine does not exhibit any band below ca. 2850 cm−1, whereas dimethylalkylamine exhibits three bands at 2813, 2780, and 2762 cm−1, almost the same wavenumbers as PME-mG3 and PME-mPEI (Figure 8). This indicates that these C−H vibrations are associated with the methyl groups. Moreover, upon protonation of dimethylalkylamine with HCl, the three bands below 2850 cm−1 vanished. It is thus inferred that the disappearance of such bands upon SO2 adsorption on supported tertiary amines is attributable to the nitrogen doublet sharing with SO2. All these results confirm our earlier report51 and others64,68 about the 1:1 interaction of SO2 with tertiary amine. The solid state 15N CP-MAS NMR spectra of PME-mG3 were recorded to confirm the interaction of SO2 with tertiary amine groups. As shown in Figure 9, the fresh PME-mG3 showed a sharp peak at ca. −331 attributable to CTMA+

Figure 10. 13C CP-MAS NMR spectra of PME-mG3: (a) fresh, (b) exposed to SO2, and (c) regenerated. 2031



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68.4, 55.7, 32.5, 28.3, 25.2, and 16.5 ppm hardly shifted upon SO2 adsorption. In contrast, the peak at 47.9 ppm attributed to CH3 carbon was significantly shifted to 41.3 ppm, consistent with the decreasing nitrogen electron density. Similar behavior was observed with PMC-TER and SBA-15PL-mG3 (Figure S10a,b). On regeneration, all spectra were identical to those of fresh materials, indicating that no chemical modification took place. The enhancement of SO2 capacity under humid condition might be due to salt formation as shown in eq 3, which decomposed on heating by eliminating H2O and SO2. In both cases, dry and humid, the adsorbent can be completely regenerated on heating at 115 °C. R3N + SO2 + H 2O ⇌ R3NH+HSO−3

(3)

These materials with high SO2 adsorption capacity and selectivity as well as their regeneration ability offer an excellent alternative to SO2 scrubbers for the desulfurization of industrial gases prior to CO2 removal over amine containing materials.

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ASSOCIATED CONTENT

S Supporting Information *

Calculation of amine content of PME supported polyamines. Structures of dendrimers G2/G3 and their 1H NMR interpretation upon N-methylation. Figures of SO2 adsorption−desorption cycling. FTIR spectra of mG2, mG3, and mPEI cycled materials, 15N NMR and 13C NMR spectra of TER-PMC and mG3 materials with 15N NMR spectra of PME support. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +1 (613) 562 5170; Tel: +1 (613) 562 5483; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge to Natural Science and Engineering Council of Canada (NSERC), Carbon Management Canada (CMC), and Alberta Innovates-Environmental and Energy Solutions (AI-EES) for financial support. A.S. thanks the Federal Government for the Canada Research Chair in Nanostructured Materials for Catalysis and Separation (20012015).

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ABBREVIATIONS PMC-TER, N,N-dimethylaminopropyl grafted pore expandedMCM-41; PME, ethanol extracted pore expanded-MCM-41; PPI G2/G3, polypropylenimine second or third generation dendrimer; mPEI, N-methylated polyethylenimine; mG2/mG3, N-methylated second or third generation dendrimer; CTMA+, cetyltrimethylammonium cation

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REFERENCES

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Supported Polytertiary Amines: Highly Efficient and Selective SO2

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