Experimental and Theoretical Investigation of SO2 Adsorption over the

Article pubs.acs.org/JPCC

Experimental and Theoretical Investigation of SO2 Adsorption over the 1,3-Phenylenediamine/SiO2 System Duane D. Miller*,† and Steven S.C. Chuang*,‡ †

AECOM, 3610 Collins Ferry Road, Morgantown, West Virginia 26507-0880, United States FirstEnergy Advanced Energy Research Center, Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States

‡

ABSTRACT: The reversible adsorption of SO2 on 1,3-phenylenediamine was investigated using the step transient response technique coupled with operando infrared spectroscopy, mass spectrometry, UV−vis spectrometry, and density functional theory (DFT). At 50 °C, the reaction of SO2 at the amine site resulted in fixation of sulfur as hydrogen-bonded SO32− (sulfite) and SO42− (sulfate) species. Simulated infrared and UV−vis spectra at the DFT B3LYP/6-31G(d,p) level were compared to the experimental results to help characterize the infrared spectra, molecular interactions, and bonding of the adsorbing species. The theoretically calculated binding energies revealed the sulfite and sulfate species bind stronger at the ammonium sites as compared to the amine site, which agrees with the infrared spectroscopic observations. Temperature-programmed desorption showed a capacity of 1.39 mol SO2/mol sorbent for pure 1,3-phenylenediamine and 2.8 mol SO2/mol sorbent for the SiO2 supported sorbent. The presence of sulfite and sulfate in the sorbent layer at 50 °C resulted in the oxidative degradation of the amine site to produce −NO2 groups and deactivation of adsorption sites in the sorbent. The infrared data indicates that the adsorbed sulfite and sulfate species remained strongly bonded at the ammonium site, whereas the SO2, sulfite, and sulfate species at the amine site may be thermally desorbed from the sorbent. The retained SO32− and SO42− species led to the rapid deactivation of the sorbent during multicycle testing.

1.0. INTRODUCTION The separation of sulfur dioxide (SO2) from the flue gas in fossil fuel combustion processes are of particular interest for two primary reasons: (i) SO2 is harmful to human health and the environment and (ii) SO2 has the capability of poisoning the active site of various CO2 capture technologies.1,2 The mechanism of active site deactivation for amine-based sorbents is generally attributed to the irreversible adsorption of SO2 and its competitive adsorption with CO2. The sorbent’s capability to avoid irreversible reactions with SO2 is an important characteristic for postcombustion applications. A solid amine sorbent for SO2 removal having high adsorption capacity, longterm regeneration capacity, and low regeneration energy requirement may also improve plant operating costs. Our focus in investigating an amine-based sorbent for SO2 removal is to study the amine reactive site in order to gain a greater understanding of the SO2−amine interactions with the purpose of creating a more effective SO2 solid sorbent for the large-scale postcombustion scrubbing application. Recently, ionic liquids (ILs) have been proposed as adsorbents for acid gases such as SO2,3−9 CO2,10−14 and H2S,15−17 as compared to technologies developed over the last several decades for flue gas desulfurization (FGD), such as limestone scrubbing, ammonia scrubbing, and adsorption by organic solvents.18−20 These previous technologies have inherent disadvantages such as the production of large quantities of wastewater and environmentally hazardous © 2015 American Chemical Society

byproducts. Ionic liquids offer unique properties such as low vapor pressure, wide liquid temperature range, nonflammability, high stability, and tunable properties.21−24 However, the effective capture of SO2 from the flue gas requires strong chemical interactions because of the relatively low partial pressure (e.g., 0.2 vol % SO2) of the gas. ILs have demonstrated low SO2 capacity at low partial pressures,21,22 as well as high viscosity on adsorption and energy intensive requirements to regenerate the materials. These factors make ionic liquids inferior to solid sorbents for FGD applications. Solid absorbents remain very attractive for the FGD process.25 The Code of Federal Regulations (e-CFR) specifies SO2 emissions are not to exceed 0.80 lb/MMBtu gross output for liquid fossil fuel and wood residue and 1.2 lb/MMBtu for solid fossil fuels.26 The FGD technologies have been categorized as either throwaway or reusable, depending upon how the sorbent is treated following SO2 adsorption and may be further classified as solid (wet/dry) or liquid (ILs). The least expensive FDG technology is the wet limestone spray dry sorbent injection method.27 A regenerable sorbent is desirable for the FGD reaction. The thermal heat capacity for SiO2 (44.6 J/mol K)28 and the average heat capacity for 19 ILs (448.05 J/mol K)29 were used to estimate the energy (kJ) requirement per Received: February 3, 2015 Revised: March 6, 2015 Published: March 10, 2015 6713

DOI: 10.1021/acs.jpcc.5b01131 J. Phys. Chem. C 2015, 119, 6713−6727

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The Journal of Physical Chemistry C

Table 1. SO2 Content and Cost Per Ton of Coal and 1000 ft3 Natural Gas and the Estimated Sorbent Performance Criteria fuel

SO2 content

SO2 contentc

SO2

cost

solid sorbentd

Ilsd

(lbs/mmBtu)

(mmol/mmBtu)

(mol/kg)

($/mol SO2)

(kJ/kg K)

(kJ/$)

(kJ/kg K)

(kJ/$)

a

coal bituminous low med. sulfur med. sulfur med. high sulfur high sulfur subbituminous low med. sulfur medium sulfur lignite low med. sulfur med. sulfur med. high sulfur natural gasb

1.5 2.2 3 5

5.0 16.6 30.0 63.2

116.1 387.1 696.7 1470.8

0.76 0.23 0.13 0.06

5.2 17.3 31.1 65.6

58.7 195.8 352.4 744.0

52.0 173.4 312.2 659.0

590.5 1968.3 3543.0 7479.7

1.4 2.1

3.3 15.0

77.4 348.4

1.14 0.25

3.5 15.5

39.2 176.2

34.7 156.1

393.7 1771.5

1.4 2.1 2.9

3.3 15.0 28.3

77.4 348.4 658.0

1.14 0.25 0.13

3.5 15.5 29.3

39.2 176.2 332.9

393.7 1771.5 3346.2

0.9

0.29

34.7 156.1 294.8 0.0 0.4

0.001

0.0166

0.04

153.0

1538.0

a

U.S. coal fuel assumptions, http://www.epa.gov (accessed Dec 2014). bEmissions from the Combustion of Natural Gas http://naturalgas.org/ environment/naturalgas/ (accessed Dec 2014). cMinus the e-CFR allowable SO2 emission limits. dAssuming 100% efficiency (mol SO2/mol sorbent) for both solid sorbent and ILs.

study the stability of the amine−SO2 complex. In order to characterize the nature and strength of the interactions between the aromatic amine and SO2, we combined infrared spectroscopy, UV−vis spectroscopy, mass spectrometry, and computational chemistry calculations. In this study, research focused toward the characterization of the 1,3-phenylenediamine amine reaction sites during a series of sulfidation and regeneration cycles by the method of operando FTIR, UV−vis spectroscopy, mass spectrometry, and computational chemistry. These tests were conducted in order to understand the interactions of SO2 at the amine site during the adsorption and regeneration steps to help elucidate the deactivation mechanism. A fundamental understanding of the amine site could help guide the preparation of future aminebased sorbents for the flue gas desulfurization process.

dollar of SO2 generated via the combustion of fossil fuels, shown in Table 1. The calculated energy per dollar SO2 may be used as a measure of performance during the thermal regeneration of the sorbent. Comparison of the energy required per dollar SO2 generated for the solid sorbents versus the ILs indicates the solid SiO2 supported sorbent will significantly decrease the energy penalty for regeneration (by an order of magnitude) as compared to the IL sorbents. The SO2 capture concept is built on the integration of acid− base chemistry with a novel approach for manipulating the active basic site. Aromatic amines, which possess lower basicity than aliphatic amines, may offer an approach for SO2 capture with negligible CO 2 capture capacity, eliminating the competitive CO2 and SO2 reaction. An effective sorbent will be capable of adsorbing and desorbing SO2 in the same temperature range as aliphatic amines do during CO2 capture. The SO2 removal on amine-based sorbents is generally composed of two steps: (i) a fixation of SO2 from the flue gas by the formation of an amine−SO2 complex and (ii) thermal regeneration of adsorbing agents with SO2 separation and recovery of the sorbent at the same time. The mechanism for the amine−SO2 reaction follows the scheme reported in reaction step R1. RNH 2 + SO2 → RNH 2 − SO2

2.0. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of the Sorbents. The gases used in this study were supplied by Praxair, N2 (99.999 vol %), Ar (99.998 vol %), and SO2 (99.9 vol %). The 2 and 10% 1,3-phenylenediamine/SiO2 sorbents were prepared by depositing 1,3-phenylenediamine (Aldrich) onto SiO2 (CABOT) using the incipient wetness impregnation method. The solid sorbents were prepared from 0.148 and 0.195 M 1,3phenylenediamine ([diamine]) and ethanol solutions upon 5 g of SiO2. The resulting slurry was stirred at room temperature for 30 min. The solution was then heated at 80 °C until complete evaporation of the ethanol solvent. The resulting sorbent is further dried for 12 h in air at 80 °C. These sorbents were studied using Harrick Scientific DRIFTS reactor for FTIR and UV−vis spectroscopic analysis. A [diamine] thin film was prepared by adding dropwise 0.1 mL of 0.116 M [diamine]/ethanol solution upon a Harrick Scientific ATR ZnSe window. The thin film was allowed to sit stagnant for 15 min until complete evaporation of the ethanol solvent. The ZnSe window was then heated to 100 °C at 10 °C/min under 100% N2 flowing at 100 cm3/min to evaporate the remaining ethanol solvent and cleaning of the amine layer then cooled to 50 °C for the adsorption study. The single [diamine] molecule occupies 142 Å2 of the ATR ZnSe window

(R1)

The adsorption process consists of the formation of an electron donor−acceptor complex between the amine (the Lewis base) and SO2 (the Lewis acid). The structure and stability of such complexes between SO2 and various amines have been studied extensively, experimentally,30−36 and at the molecular level.37−39 The formation and stability of such complexes are a function of the basicity of the amine and acidic strength of the acid gas. Thus, the characterization on a molecular scale of the nature and the strength of these amines with respect to SO2 is of great importance. However, to our knowledge, there have been limited studies on the gas−solid interactions between aromatic amines and SO2. As the reaction between the amine and SO2 is expected to depend on the basicity of the amine and its size, we have been prompted to investigate the 1,3-phenylenediamine as a SO2 sorbent and 6714


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Figure 1. Experimental Apparatus.

(4.35 cm2); consequently, it is expected that a 4 μm [diamine] layer would consist of multiple layers of [diamine] on the ZnSe surface. 2.2. Fourier Transform Infrared Spectroscopy. The experimental apparatus, shown in Figure 1, consists of (i) a reactant metering system (Brooks Instrument 5850 mass flow controllers), (ii) a gas sampling system, including a 4-port valve, (iii) a diffuse reflectance infrared fourier transform spectroscopy (DRIFTS, HVC-DRP, Harrick Scientific) reactor containing 33 mg catalyst and an attenuated total reflectance accessory (ATR-IR, Harrick Scientific) with a custom reactor manifold mounted to the ATR-IR top plate placed inside of a Fourier Transform Infrared Spectrometer (FTS6700 FTIR, ThermoNicolet), and (iv) a mass spectrometer (MS, Pfeiffer Omnistar). The UV−vis diffuse reflectance spectra of the sorbents were recorded by a Hitachi U-3010 Spectrometer using the Harrick Scientific (HVC-DRP) diffuse reflectance accessory. The 4-port valve allows switching the inlet flow from 100% N2 to 0.33% SO2, while maintaining a total flow rate of 100 cm3/min over the sorbent. Changes in the concentration of IR observable complexes were monitored by the DRIFTS and the ATR-IR techniques. The IR absorbance spectrum of absorbed and gaseous species was obtained by A = −log(Io/I),40 where Io is the background IR single beam spectrum (32 coadded scans and a resolution of 4 cm−1) of the clean [diamine] sorbent and I is the IR single beam spectrum during the SO2 adsorption reaction. The MS responses corresponding to N2 (m/z = 28) and SO2 (m/z = 64) were monitored for the changes in the DRIFTS and ATRIR reactor effluent concentrations. 2.3. Theoretical Method. The sulfur dioxide (SO2), [diamine] structures, their complexes, and the calculated harmonic normal-mode frequencies were determined using density functional theory (DFT) in Spartan 14 (Wave function Inc.). DFT was utilized to determine the most stable conformations for the amines and their complexes with SO2. The optimized molecular geometries and binding energies (BE) were calculated using Becke’s three-parameter exchange functional and gradient corrected functional of Lee, Yang, and Parr (B3LYP),41−43 using the 6-31G**44,45 and 6-31+G*46 basis sets. The calculated infrared normal vibrational frequencies were determined at the B3LYP/6-31G** level47,48 and were not corrected with scaling factors prior to comparison to the experimental infrared spectra. The excited

states were calculated for the 1,3-phenyelendiamine and the SO2−[diamine] complexes using TDDFT/B3LYP/6-31+G*. The binding energies (BE) for the SO2−[diamine] complexes were obtained by eq 1. BE = E[SO2 − diamine]−E[SO2 ]−E[diamine]

(1)

where E[SO2-diamine] is the total energy of the [diamine] with the adsorbed SO2, E[SO2] and E[diamine] are the total energies of the SO2 and [diamine] molecules, respectively.

3.0. RESULTS AND DISCUSSION 3.1. Spectroscopic and Theoretical Characterization of the [diamine] Sorbent. The SO2 reaction with aminecontaining sorbents has been investigated by spectroscopic methods such as FT-IR, Raman, XRD, and NMR spectroscopies.9,49,50 Researchers have demonstrated that the chemical adsorption mechanism of amine ionic liquids is attributed to intermolecular hydrogen bonding between the hydrogen atom of the amine and the oxygen atom of the SO2,9 in the liquid phase. However, the details of the specific interactions between solid amine-based sorbents and gas phase SO2 have not been explored in the literature. Very few reports provide an understanding on the experimental analysis of the amine site interactions between solid amine sorbents and SO2 for the FGD reaction. Figure 2a shows the IR single beam spectra and Figure 2b the absorbance spectra of the pure [diamine] thin film (ATR-IR) and the 2 vol % [diamine]/SiO2 sorbent (DRIFTS) at 50 °C, respectively. The band assignments for the unsupported and SiO2 supported [diamine] are summarized in Table 2. The ATR-IR analysis of the pure [diamine] shows IR intensities corresponding to the symmetric νs(N−H) vibration at 3324 cm−1, the asymmetric νa(N−H) stretching vibration at 3396 cm−1, the overtone vibration at 3214 cm−1 for the δ(N−H) deformation vibration at 1580 and 1631 cm−1, ν(C−H) stretching vibration at 3042 cm−1, and δ(C−H) bending at 1483 cm−1 and ν(C−N) stretching vibration at 1322 cm−1. DRIFTS analysis of the impregnated of 2 vol % [diamine] on SiO2 (Figure 2, panels a and b) shows the presence of the [diamine] on SiO2 surface indicated by the appearance of the symmetric νs(N−H) vibration at 3342 cm−1, the asymmetric νa(N−H) vibration at 3420 cm−1, the overtone bending vibration at 3234 cm−1 for the δ(N−H) deformation at 1611 cm−1, the ν(C−H) stretching vibration at 3042 cm−1, and the 6715


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Table 2. Infrared Experimental and Theoretical Normal Modes of Vibration for the Optimized Structures at the B3LYP/6-31G** Level wavenumber (cm−1) (theoretical) 1,3-phenyelediamine 1195b, 1196, 1235a,b,e, 1257 1357a, 1362, 1365b, 1545b, 1549, 1550a, 1550c 1669I, 1671 3173b, 3184a, 3197a,b, 3194, 3204c 3528I, 3531a, 3574a, 3611d, 3650 3557e, 3623a,b, 3628I, 3635d, 3639a,b, 3659c, 3670a,b, 3736d, 3778 ([diamine])2 3394e, 3460b, 3497 3460a, 3467e, 3648, 3635e (a) SO2−[diamine] 1100a,b, 1151f, 1155I, 1276c 1271a,b, 1203c, 1306d, 1321e, 1327f 1126c, 1148e, 1158d 1238I, 1247I 1405I, 1479f

Figure 2. ATR-IR and DRIFTS (a) single beam spectra and (b) absorbance spectra of pure [diamine] and SiO2 supported [diamine] on at 50 C.

δ(C−H) deformation at 1515 cm−1. Comparison of the ATRIR spectra for the pure [diamine] with the DRIFTS reveals the IR intensities for the ν(N−H) stretching vibrations decreased and shifted the IR intensity to slightly higher wavenumbers (∼20 cm−1). The δ(N−H) deformation vibration also slightly decreases (∼20 cm−1), suggesting surface OH groups are interacting with the N in the NH2 functional groups. Researchers have shown that the IR spectrum of pure SiO251 exhibits an OH stretching bands consisting of the isolated ν(O−H) stretching vibration (3750 cm−1), the hydrogenbonded ν(O−H) vibration at 3660 cm−1, and the hydrogenbonded H2O broadband between 2700 and 3600 cm−1. The DRIFTS analysis of the freshly prepared [diamine]/SiO2 sorbent revealed a decrease in the IR intensity at 3750 cm−1, indicating that the [diamine] interacted with some surface H2O. However, the appearance of an IR broadband at 2500 to 3000 cm−1, following the impregnation of the [diamine] (Figure 2b), suggests that the [diamine] did not completely displace the H2O from the SiO2 surface. Figure 3a shows the DRIFTS difference spectra during SO2 adsorption at 50 °C, and Figure 3b shows IR spectra during thermal regeneration at 100 °C, over the [diamine]/SiO2 supported sorbent. The absorbance spectrum at 1.31 min or 60 °C was first obtained by subtracting the background spectra from the clean [diamine]/SiO2 sorbent prior to exposure to the SO2 reaction gas. The difference spectra were then obtained by subtracting the absorbance spectrum at 1.31 min SO2 exposure time from the remaining absorbance spectrum, as shown Figure 3. Exposing SO2 to the [diamine]/SiO2 sorbent resulted in an increase in IR intensity at 3147 cm−1 assigned to the formation of a stabilized ammonium ion, similar to what has been observed experimentally by others during the CO2 adsorption over aliphatic amines.51,52 SO2 adsorption at the amine site resulted in an increase in IR intensity at 1640 cm−1 for the δ(NH) deformation vibration and a decrease in IR intensities at 3323 and 3410 cm−1 for the symmetric and asymmetric ν(N− H) stretching vibrations, respectively. The IR spectra reveal that

1420c, 1495d, 1520c, 1554e, 1570d, 1587d 1628e, 1640, 1656b, 1661a,e, 1676d, 1680a, 1682f, 1693a,b, 1650c, 1676c, 1689e, 1697c, 1703d, 1709e, 1719d f 3058 , 3212f, 3245d, 3380I, 3389c, 3439f, 3520a, 3522b, 3547e, 3613b, 3669e 2− SO3 919, 981c, 1014e 957 SO42− 948, 992d 1205d, 1242, 1306d

(experimental)

description

1224g,h 1322g 1483g, 1515h 1580g, 1611h, 1631g 3042g,h 3214g, 3234h, 3324g, 3342h 3396g, 3420h

δ(C−H) ν(C−N) δ(C−H) δ(N−H) scissor νs(C−H) νs(N−H) νa(N−H)

νs(N−H···NH2) νa(N−H···NH2) νs(SO) νa(SO) δ(N−H) rocking δ(N−H···OSO) rocking ν(C−C) 1640h

δ(N−H) scissor

2905g, 3147h

δ(N−H···OSO) scissor ν(NH−H···OSO)

946h, 978g 1009g

νs(SO) νa(SO)

1077g,h 1176g,h, 1144g,h

νs(SO) νa(SO)

b Additional simulated complexes: SO2-[diamine]2. cAdditional simulated complexes: SO3-[diamine]. dAdditional simulated complexes: SO4-[diamine]. eAdditional simulated complexes: SO3-[diamine]2. f Additional simulated complexes: SO4-[diamine]2. IAdditional simulated complexes: SO42− [diamine]-(NH3+)2. gExperimental: ATR-IR, h Experimental: DRIFTS

the presence of SO2 at the amine site led to the oxidation of SO 2 to sulfite (946 cm −1 ) 53 and the sulfate species corresponding to the increase in IR intensities at 1041, 1077, 1144, and 1176 cm−1.54,55 The gas phase SO2 was not observed due to the difference spectra technique; the IR intensities corresponding to gaseous SO2 occur in the 1400−1300 cm−1 region for νa(OSO) vibration and in the 1200−1100 cm−1 region for the νs(OSO).56 The presence of the sulfur bearing species at the amine site also leads to the increase in IR intensities at 1341, 1386, 1483, and 1583 cm −1 , indicating the formation of an mdinitrobenzene species, which may result from the oxidation of the amine site to the −NO2 group.57 In addition to this, there was an increase in IR intensity at 1682 cm−1, after 3.15 min SO2 exposure, corresponding to the ν(CO) stretching 6716


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cm−1, the νs(N−H) at 3650 cm−1, and the νa(N−H) at 3778 cm−1. The ATR-IR absorbance spectra is compared alongside of the simulated spectra, shown in Figure 4a, and revealed the theoretical normal modes of vibration, for the [diamine] molecule, are predicted at higher wavenumbers as compared to the experimental results. This suggests the wave functions describing the atoms and their interactions, as determined by the Spartan software, did not precisely predict the molecular structure for the [diamine] molecule in the ATR-IR thin film. The optimized structure that determines the atomic interactions in the molecules result in bond lengths differing somewhat from the actual (experimental) values, and thus the simulated spectra show some degree of variation as compared to the experiment. During the sample preparation, it is expected that the [diamine] loading will consist of multiple layers of [diamine] on the surface of the ZnSe window (ATR-IR) and on the surface of the SiO2 support (DRIFTS). The [diamine]− [diamine] structure was chosen to model the layering interactions. The [diamine]−[diamine] complex resulted in the symmetric νs(N−H···NH2) vibration at 3497 cm−1 and asymmetric νa(N−H···NH2) vibration at 3648 cm−1, indicating that intermolecular hydrogen bonding of the amine sites is occurring within the [diamine] layers. From a joint analysis perspective of calculated and experimental molecular geometries, the different physical meaning of the two sets of data should be taken into account.58 The quantum chemical calculations provide a straightforward picture of the geometry and bonding phenomenon of the molecular structures, whereas, the experimental observations pertain instead to a dynamic average of behaviors of many molecules. In other words, the experimental vibrational spectra can be sufficiently complex, and the theoretical model can be overly simplified making a discussion and comparison between theory and experiment difficult. Furthermore, the experimental data reflect the effects of intermolecular interactions in the solid phase with the gaseous reacting gas, while our computational models reflect the gas phase free molecules since we were not able to include a large number of molecules for computation. There are significant differences between the theoretical and experimental setup for these two sets of data. In addition, the theoretical normal modes vary significantly from the experimentally determined spectra. In this case, the experimental results are preferentially chosen as the true infrared data and literature values should be used to substantiate the infrared band assignment versus theory. With this in mind, the theoretical geometries may be correlated to the infrared measurements during adsorption by studying the trends in the molecular geometries during the SO2 interaction. The simulated infrared spectra reflect the characteristics of the experimental spectra providing additional support for the interpretation of the IR spectra at the amine site during the SO2 reaction. The starting geometries for the SO2 and [diamine] systems, shown in Scheme 1, were optimized at the HF/6-31G** level prior to proceeding to the B3LYP/6-31G** and B3LYP/631+G* levels. The most stable structures were determined by varying the starting geometries in the computational study. The geometries with the SO2 molecule oriented at the farthest point from the amine sites failed to converge at the HF level. Only the SO2 molecules oriented near the amine sites resulted in the self-consistent field (SCF) convergence. The SCF convergence and the number of iterations required to converge was

Figure 3. DRIFTS difference spectra during (a) SO2 adsorption over the [diamine]/SiO2 at 50 C and (b) SO2 desorption at 100 C.

vibration. The adsorbing sulfur species may be active for the oxidation of the aromatic ring to produce a Quinone type species. The formation of CO is accompanied by a decrease in the intensities for ν(C−H) vibration at 3042 cm−1. The decrease in the IR intensity for ν(N−H) are comparable in intensity to that of ν(C−H) vibration, suggesting that a fraction of the sorbent degradation is occurring via oxidation of a C−H site as compared to the amine site. The effect of temperature on the IR intensity of the adsorbed species is shown in Figure 3b. The background spectrum was taken from the SO2 saturated [diamine]/SiO2 sorbent. Subtraction of the background spectrum during thermal regeneration produced negative intensities, indicating the removal of the sulfur-bearing species from the [diamine]/ SiO2 sorbent. The regeneration of the [diamine] layer was facilitated by heating the DRIFTS reactor to 100 °C. Heating the [diamine]/SiO2 sorbent resulted in a slight increase in IR intensities for the asymmetric and symmetric ν(N−H) stretching vibrations at 3410 and 3323 cm−1, a decrease in intensity for the δ(N−H) vibration at 1640 cm−1, for the δ(C− H) vibration at 1496 cm−1, and for the ν(SO) vibration at 1268 cm−1, corresponding to the removal of the SO2 species from the amine site. The removal of the adsorbed SO2 from the [diamine]/SiO2 sorbent also resulted in a decrease in IR intensity for the ammonium ion at 3147 cm−1. The subsequent decrease in the broad band intensity centered at 3147 cm−1 indicates the dependence of the ammonium ion on the presence of the adsorbed sulfur-bearing species. A computational investigation was performed to theoretically study the interactions between the solid [diamine] system and SO2 reaction gas, as shown in Scheme 1. The theoretical infrared band assignments for the [diamine] and the various SO2−[diamine] complexes are summarized in Table 2. The calculated spectrum for the [diamine] molecule (Figure 4a) shows the δ(C−H) deformation vibrations at 1196, 1257, 1362, and 1549 cm−1, the δ(N−H) deformation vibration at 1671 6717


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Scheme 1. Optimized Structures at the B3LYP/6-31G** Level for the SO2 and [Diamine] Interaction for (a) SO2-[Diamine], (b) SO2-[Diamine] H-Bonded, (c) SO2-[Diamine]2, (d) SO32−-[Diamine], (e) SO42−-[Diamine], (f) SO32−-[Diamine]2, (g) SO42−-[Diamine]2, and (h) SO42−-[Diamine]-(NH3+)2

dependent upon the quality of the initial guess for the molecular orbitals (e.g., the molecular structure). The optimized geometries for the interaction of one SO2 and one [diamine] molecule resulted in two stable structures. Scheme 1a represents the Lewis acid−base interaction of the SO2 at the amine site. The other stable structure (Scheme 1b) resulted in the SO2 molecule hydrogen bonding with the ortho- and paraamine sites (N1−H1···OSO···H3−N2). The stability of these complexes are indicated by their equilibrium BE, −7.07 kcal/ mol (Scheme 2a) and −7.60 kcal/mol (Scheme 2b), shown in Table 3, which are consistent with previous studies.59,60 The BE for SO2 adsorption at the amine site for the various structures shown in Scheme 1 were compared at the B3LYP/631G** and 6-31+G* levels and are reported in Table 3. Moving from left to right, the theoretical binding energies, listed in Table 3, show a trend indicated by the decrease in BE for the SO2-[diamine] at the B3LYP/6-31G** level (−7.72 kcal/mol) as compared to the B3LYP/6-31+G* level (−7.60 kcal/mol). The 6−13+G* basis set includes highly diffuse functions to more accurately describe hydrogen-bonded complexes.61 Researchers have reported that decreases in BE are the result of the charge transfer contribution to the binding energy decreasing as the size of the basis set increases.62 Comparing the SO2 adsorption on [diamine] to the SO2

adsorption at the multilayer [diamine]2 adsorption site show an increase in BE from −7.60 to −11.64 kcal/mol, respectively. The increase in BE corresponds to an increase in molecular stability of the absorbed SO2. The trend in stability is attributed to adsorbed SO2 interacting with neighboring amine sites in the [diamine] layer leading to the stronger SO2-[diamine]2 bond (Scheme 1c). IR studies have shown that SO2 adsorption on metal oxides have led to the formation of the sulfite (SO32−) and sulfate (SO42−) species.53,63−66 Therefore, the optimized structures SO3-[diamine] (Scheme 1d) and SO4-[diamine] (Scheme 1e) were studied at the B3LYP/6-31G** and 6-31+G* levels. The optimized structures for SO32− and SO42− adsorption on the [diamine] revealed that molecular interactions occur at the amine site through hydrogen bonding. In both cases, a hydrogen bond is formed between the hydrogen in the amine site and the oxygen in SO32− or SO42−. As the number of oxygen atoms bonded to the sulfur atom increases, the interactions become stronger as shown in Table 3 indicated by the trend in the binding energy. Increasing the number of [diamine] molecules slightly increased the binding energy indicative of the more stable species. Studies of SO2 adsorption on nickel−alumina surfaces have shown that SO2 adsorbs as both SO32− and SO42− with some degree of variation in the 6718


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calculations also predicted that various stable SO2, sulfite, and sulfate hydrogen-bonded structures occurred around the ammonium ion. The effect of these structures upon the BE of the hydrogen-bonded species showed a minor variation in BE between 1 and 3 kcal/mol. Comparison of the BE, shown in Table 3, revealed that the presence of the ammonium ion resulted in an increase in BE for SO2 adsorption (−12.92 kcal/ mol) at the −NH3+ site, a decrease in BE for SO32− (−78.94 kcal/mol), and a slight decrease in BE for SO42− (−37.87 kcal/ mol) adsorbed species. The hydrogen-bonding adsorption interaction led to a significant vibrational shift of the ν(N−H) stretching vibration. The theoretical SO2−[diamine]−NH3+ structure produced an IR band at 3141 cm−1, corresponding to the ν(NH3+···OSO) hydrogen-bonded species, which is remarkably consistent with the experimental DRIFTS results (3147 cm−1). The SO2ammonium interaction by hydrogen bonding was not the only structure that exhibited the shift in vibration. The SO4-[diamine]2 species also produced an IR band in that region (3058 cm−1) due to the ν(NH−H···OSO) hydrogen bonding at the amine site. In addition to these structures, the SO2 interaction of the sulfite and sulfate species were studied in the case when both the ortho- and para-amine sites contain an ammonium ion [diamine]-(NH3+)2. The theoretical BEs for the SO32− [diamine]-(NH3+)2 (−288.55 kcal/mol) and SO42− [diamine]-(NH3+)2 (−314.44 kcal/mol), Scheme 1h, show a significant increase as compared to the single [diamine]-NH3+ structure. These results suggest that the diammonium species will strongly adsorb the sulfite and sulfate species at the amine adsorption site. In general, a lower value for the chemical BE will result in a decrease in the adsorption rate and equilibrium constants which would increase the size and capital costs of an absorber. On the other hand, increasing the chemical BE will increase the energy penalty in regeneration costs. It is reported that over 25% of the regeneration energy is the direct result of the BE in the CO2 adsorption monoethanolamine (MEA)-based processes.70 The most important development aim is to lower the chemical BE without affecting the adsorption rate, which was the motivation for using the 1,3-phenylenediamine molecule for the selective removal of SO2. The theoretical BEs shown in Table 3 for the SO2, sulfite, and sulfate species may be sufficient for the selective removal of SO2. The simulated infrared spectra for the optimized structures at the B3LYP/6-31G** level, are shown in Figure 4a for [diamine], Figure 4b for SO2−[diamine], Figure 4c for SO2− [diamine]2 structure, Figure 4d for SO3-[diamine], Figure 4e for SO4-[diamine]-(NH3+)2, and the experimental ATR-IR difference spectra are shown in Figure 4f and DRIFTS difference spectra in Figure 4g during SO2 adsorption over the [diamine] sorbents at 50 °C. The theoretical vibrational normal modes for the [diamine] and the SO2−[diamine] complexes are summarized in Table 2. Figure 4a shows the simulated IR spectra for the [diamine] molecule. The theoretical IR spectra show the vibrational modes of the amine site consist of the δ(N−H) scissor vibration at 1671 cm−1 and the degenerate stretching vibrations for νa(N−H) at 3778 cm−1 and νs(N−H) at 3650 cm−1. The SO2 reaction at the amine site, according to the Lewis acid− base adsorption mechanism (Figure 4b), caused an increase in the δ(N−H) scissor vibration from 1671 to 1680 cm−1 and produced ν(N−H) stretching vibrations at 3639 and 3670 cm−1 for νa(N−H) and 3531 and 3574 cm−1 for νs(N−H), shown in Figure 4b. The SO2 adsorption in the [diamine] layer is

Figure 4. Simulated infrared spectra for the optimized structures at the B3LYP/6-31G** level for (a) [diamine], (b) SO2−[diamine], (c) SO2−[diamine]2, (d) SO3-[diamine], (e) SO4-[diamine]-(NH3+)2, (f) ATR-IR difference spectra during SO2 adsorption over pure [diamine] sorbent at 50 °C, and (g) DRIFTS difference spectra during SO2 adsorption over [diamine]/SiO2 sorbent at 50 °C. The difference spectra are equivalent to the spectra of adsorbed SO2.

relative concentrations of the surface species that depended upon the physical properties of the oxide surface.53 The binding energies for the sulfite and sulfate species on metal oxide surfaces, determined by XPS spectroscopy also show an increase in binding energy comparing the BE for SO2 to the BEs for SO32− and SO42−. These previous experimental studies are consistent with the trend in our theoretical BEs shown in Table 3. The theoretical SO32− (92.60 kcal/mol) and SO42− (115.68 kcal/mol) adsorption at the multilayer [diamine]2 sites, shown in Scheme 1 (panels f and g), respectively, also show a corresponding increase in the BE. The formation of an ammonium species has been observed experimentally over the [diamine]/SiO2 sorbent; therefore, the SO2 interaction at the protonated amine site was investigated theoretically at the B3LYP/6-31G** and 6-31+G* levels; the BEs are reported in Table 3. Various starting structures were investigated with the SO2 molecule oriented at either the amine or the ammonium sites. The optimized structures revealed the most stable structures were those for SO2, SO32−, and SO42− interacting preferentially at the ammonium site. The DFT 6719


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The Journal of Physical Chemistry C Scheme 2. Mechanism for Amine Degradation during SO2 Adsorption

δ(N3−H5···OSO) scissor vibration. The experimental IR intensity at 1640 cm−1 in Figure 4g is assigned to the δ(N3− H5···OSO) vibration. SO2 adsorption also resulted in a shift in IR intensity for the νs(N−H) vibration from 3650 to 3522 cm−1 and νa(N−H) vibration from 3778 to 3613 cm−1. The SO2 exposure to the [diamine] layer also resulted in a broadband IR intensity centered at 2905 cm−1 (ATR-IR, Figure 4f) and 3147 cm−1 (DRIFTS, Figure 4g), indicating the presence of an ammonium ion. Previous studies51 have shown the CO2 adsorption at the amine site resulted in the formation of the NH3+ by protonation of the amine site. The IR spectra obtained from both the ATR and DRIFTS techniques reveal SO2 adsorption on the aromatic amine produces similar spectral features for the ammonium ion as compared to the IR spectra for CO2 adsorption over the aliphatic-amine sorbent.51,52 The experimental IR broadband intensity centered at 2905 cm−1 (Figure 4f) and 3147 cm−1 (Figure 4g) are assigned to the ν(NH−H···OSO) vibration. The simulated IR spectra for SO3-[diamine] and SO4[diamine]-(NH3+)2 are shown in Figure 4 (panels d and e, respectively). The interaction of SO32− and SO42− at the amine site occurred primarily by the NH−H···OSO (hydrogen bonding) indicated by the shift in IR intensity for the symmetric νs(N−H) vibration from 3650 to 3389 cm−1 for SO32− (Figure 4d) and to 3380 cm−1 for SO42− (Figure 4e). The optimized structures for the SO3-[diamine], and the SO4[diamine] molecules produced IR intensities for the νs(SO) vibrations at 981 and 1014 cm−1 for the SO32− adsorbed species and at 1151 and 1205 cm−1 for the SO42− adsorbed species, as shown in Table 2. Comparison of the simulated spectra to the difference IR spectra collected by ATR-IR (Figure 4f) and DRIFTS (Figure 4g), the experimental IR data show the formation of the sulfite (1009 and 978 cm−1) and the sulfate (1041, 1077, 1144, and 1176 cm−1) species at the amine

Table 3. Binding Energies Calculated at the B3LYP/631G** and 6-31+G* Level BEa structure

BEa

BEe

6-31G**

6-31+G*

literature

(kcal/mol)

(kcal/mol)

(kcal/mol)

SO2-[diamine]

−7.72

−7.60

SO2-[diamine] (H-bonded) SO2-[diamine]2 SO32− [diamine] SO32− [diamine]2 SO42− [diamine] SO42− [diamine]2 SO2-[diamine]-NH3+ SO2-[diamine]2-NH3+ SO32− [diamine]-NH3+ SO32− [diamine](NH3+)2 SO32− [diamine]-NH3+ X2 SO42− [diamine]-NH3+ SO42− [diamine]-(NH3+)2 SO42− [diamine]2-NH3+

−8.76

−7.07

−12.45 −92.60 −97.91 −39.20 −111.64 −12.92 −17.79 −78.94 −288.55

−11.64 −31.72 −38.74 −52.77 −108.12 −11.95 −16.51 −19.56 −450.29

−92.48

−34.23

−37.87 −314.44

−37.15 −533.13

−134.06

−133.39

refs

5.69− 7.34b,c

60, 67, 68

8.12c 12.11c

68 68

86.92d,f

69

BE = E[SO2-diamine] − E[SO2] − E[diamine]. Level of theory: b HF/6-31G*. cB3LYP/6-31G(2df,p). dB3LYP/6-311+G**. eSimulated structures are similar: SO2 adsorption at amine site. fSO42− at nitrogen carrying the positive charge. a

modeled theoretically by the SO2-[diamine]2 structure shown in Scheme 1c. The optimized structure shows the oxygen atom of the adsorbed SO2 intermolecular hydrogen bonding (N3− H5···OSO) with the neighboring amine site resulting in a shift in IR intensity from 1671 to 1680 cm−1, corresponding to the 6720


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vibrations by the abstraction of the hydrogen from the amine site. The formation of SO32− and SO42− was accompanied by the IR broad band between 2500 and 3100 cm−1 and suggests that the formation of the ammonium species may proceed by a similar reaction mechanism as that during CO2 adsorption over aliphatic amines.51,52 The presence of SO2 species at the amine site also led to the increase in IR intensities at 1224, 1446, and 1531 cm−1, suggesting the formation of an m-dinitrobenzene species due to the oxidation of the amine site to the −NO2 group,57 similar to what was observed in the DRIFTS experiment (Figure 3a). The slower rate of oxidation of the amine sites and the rapid oxidation of SO2 to SO32− and SO42− species suggests that the [diamine] layer is enriched with the sulfite and sulfate anions. Studies have shown that SO2 cannot be oxidized to the sulfite and sulfate ions in solution without a catalyst present.72−74 In the FGD reaction, the SO2 adsorption on Ca-based sorbents,75 the SO2 in flue gas reacts with water to form an adsorbed bisulfite species. The bisulfate reacts with limestone or CaCO3 to form calcium hydrogen sulfite, and dissolved oxygen further oxidizes the sulfite to sulfate species and is fixed in the Ca sorbent. The rate controlling factors for converting SO2 to sulfite and sulfate are temperature, pH, and the concentration and interaction of the reactants. It is reported that the overall reaction rate is controlled by the absorption rate of oxygen into the reaction liquid.73,76 The mechanism for SO2 oxidation at the [diamine] reaction site is proposed to proceed in a similar pathway according to eqs 2−4.

adsorption sites on both the pure [diamine] and [diamine]/ SiO2 sorbents and are similar to the simulated spectra. 3.2. IR Studies during SO2 Adsorption over the Pure [Diamine] Thin Film. Figure 5 (panels a and b) show the

Figure 5. ATR-IR difference spectra of pure [diamine] thin film during cycle no. 1 (a) SO2 adsorption at 50 °C and (b) SO2 desorption at 100 °C.

SO2 + RNH 2 → RNH 2 − SO2

(2)

RNH 2 − SO2 + 1/2O2 → RNH − HSO3

(3)

RNH − HSO3 + RNH 2 + 1/2O2

ATR-IR difference spectra during SO2 adsorption at 50 °C for cycle no. 1 and the difference spectra during SO2 desorption at 100 °C, respectively. The absorbance spectrum at 1.01 min or 60 °C was first obtained by subtracting the background spectra from the clean [diamine] layer prior to exposure to the SO2 reaction gas. The difference spectra were then obtained by subtracting the absorbance spectrum at 1.01 min SO2 exposure time from the remaining absorbance spectrum as shown in Figure 5. Exposure of the [diamine] thin film to SO2 led to a decrease in IR intensity for ν(N−H) vibrations at 3242, 3324, and 3396 cm−1, corresponding to the SO2 reaction at the amine site. The IR spectra reveal that the presence of SO2 at the amine site led to the oxidation of SO2 to sulfite (1009 and 978 cm−1)53 and the sulfate species corresponding to the increase in IR intensities at 1041, 1077, 1144, and 1176 cm−1,54,55 similar to the SO2 adsorption over the [diamine]/SiO2 sorbent. The formation of the sulfate species also corresponded to the simultaneous increase in IR intensity at 915 cm−1, a characteristic δ(O−H) deformation vibration suggesting the presence of a sulfuric acid hydrate species71 at the amine site. The IR spectra shown in Figure 5a in the 1224−900 cm−1 region represent the overlapping vibrational modes for the HSO4− ↔ SO42− species; the IR intensity at 915 cm−1 indicates the presence of a hydrated species enabling elucidation of the overlapping core vibrational modes. In addition, the changes in the shape of the absorption IR profile in the sulfate region as a function of time is indicative of these evolving species. Hydration of SO bond occurring at the amine site may also have led to the further decrease in IR intensity for ν(N−H)

→ RNH − SO4 + RNH3

(4)

On exposure of the [diamine] layer to SO2, the gaseous SO2 adsorbs at the amine site according to reaction step (2). Proton (H+ amine) transfer and oxidation by oxygen occur on the adsorbed SO2 to produce adsorbed bisulfate species according to reaction step (3). Studies have shown the oxidation rate of Ca(HSO3)2 is 16 times higher than that of H2SO3.75 The adsorbed bisulfate species then reacts with oxygen coupled with simultaneous protonation of the amine site to produce the sulfate and the ammonium ions both of which are observed spectroscopically. The presence of the sulfate anion at the amine site may behave as an oxidizing agent, leading to the oxidation and formation of the −NO2 and CO species.77 The oxidation of the amine sites were also accompanied by an increase in IR intensity at 1682 cm−1, corresponding to the formation of a CO group, suggesting the oxidation of the aromatic ring produced a quinone type species. The carbon oxidation also corresponded to a decrease in the IR intensity for the ν(C−H) vibration at 3042 cm−1. The variation in IR intensities for the ν(N−H) is greater than that of ν(C−H), indicating only a fraction of the sorbent degradation is occurring by oxidation of the aromatic ring. The retained SO2 species in the form of sulfite and sulfate, coupled with the IR intensities at 1224, 1446, and 1531 cm−1 suggest the primary degradation and deactivation of the amine sorbent is occurring by the oxidation of the amine adsorption site. The infrared spectroscopic analysis indicates the oxidation of the amine site 6721


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IR window to 100 °C. Heating the [diamine] layer resulted in a decrease in IR intensities for the symmetric νs(N−H) stretching vibrations at 3242 and 3324 cm−1, the asymmetric νa(N−H) vibration at 3396 cm−1, and the δ(N−H···OSO) vibration at 1552 cm−1, corresponding to the removal of the SO2 species from the amine site. SO2 leaving the amine adsorption site corresponded to the decrease in IR intensities at 1474 cm−1 for the δ(N−H) vibration, at 1333 cm−1 for νa(S O) vibration, at 1552 cm−1 for δ(N−H···OSO), and at 1308 cm−1 for the νa(SO) vibration. The thermal regeneration of the [diamine] layer resulted in a decrease in the IR intensities at 969 and 1077 cm−1, corresponding to the SO2 desorption in the form of sulfite and sulfate species, respectively. These desorbing species may be those that are weakly absorbed at the amine site as compared to the diammonium sulfate species theory predicts to be more strongly bounded. The temperature program desorption IR data indicates that adsorbed SO2 desorbs predominantly as the sulfite and sulfate species. The removal of the adsorbed SO2 from the [diamine] layer also resulted in a decrease in IR intensity for the ammonium ion at 2905 cm−1. The subsequent decrease in the broad band intensity centered at 2905 cm−1 also indicates the dependence of the ammonium ion on the presence of the weakly adsorbed sulfite and sulfate species, adding further evidence that the ammonium ion may be reactive for oxidation in the [diamine] layer. Regeneration of the amine layer also intensified the C O band located at 1682 cm−1, shown in Figure 5b. Figure 7 (panels a and b) shows the IR absorbance spectra for cycles nos. 2 and 3 during the SO2 adsorption at 50 °C and

to the nitrite species occurred to a greater extent than the oxidation of the C−H sites. Plotting the IR intensities of the adsorbing species during the SO2 adsorption on cycle no. 1, shown in Figure 6, help to

Figure 6. Difference spectra IR intensity data from difference spectra during cycle no. 1 SO2 adsorption over [diamine] at 50 °C.

further elucidate the reactions taking place at the amine site. The decrease in the intensity for the ν(N−H) vibrations (Figure 6a) correspond to the amount of adsorbed SO2 in the [diamine] layer. The oxidation mechanism of the amine site may be unraveled by examining the lead/lag behavior of the SO32− and SO42− species during the adsorption on the fresh amine site (Figure 6c). The difference spectra reveals SO2 interaction at the amine site resulted in the rapid formation of SO42−, indicated by the leading IR intensity profile for the sulfate species. The amount of retained SO42− species slightly decreased, indicated by the decrease in IR intensity profile shown in Figure 3c, while the intensity profile for SO32−, C O, and −NO2 (Figure 6b) continued to increase, suggesting the formation of these species occurred by the oxygen decomposition of the sulfate-to-sulfite anion. The retained SO42− species could convert the amine site to −NO2 and the carbon to CO. The SO2 adsorption and deactivation of the amine site proceeds by the reaction mechanism shown in Scheme 2. The oxidation occurred to a greater extent at the amine site than at the aromatic ring, as evidenced by the increasing IR intensity ratio of CO to −NO2. The presence of an ammonium ion indicated by the IR broadband intensity at 2905 cm−1, shown in Figure 5a, suggests that SO2 adsorption is active in the formation and stabilization of the ammonium species, as indicated according to reaction Scheme 2. The effect of temperature on the IR intensity of adsorbed species is shown in Figure 5b. The background spectrum was taken from the SO2 saturated [diamine] layer. Subtraction of the background spectrum during thermal regeneration produced negative intensities, indicating the removal of the sulfur-bearing species from the [diamine] layer. The regeneration of the [diamine] layer was facilitated by heating the ATR-

Figure 7. ATR-IR absorbance spectra during cycle nos. 2 and 3. (a) SO2 adsorption at 50 °C, and (b) SO2 desorption at 100 °C, over [diamine]. The background spectrum for Cyc no. 2 was taken from the regenerated [diamine] layer following Cyc no. 1. The background spectrum of Cyc no. 3 was taken from the regenerated [diamine] layer following Cyc no. 2.

the IR spectra during sorbent regeneration at 100 °C, respectively. Exposing SO2 to the [diamine] layer during cycle no. 2 produced a decrease in the ν(N−H) IR intensities due to SO2 interaction at the amine site similar to the cycle no. 1 data. Figure 7a shows the second SO2 adsorption cycle 6722


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The Journal of Physical Chemistry C produced more −NO2 indicated by the IR intensities at 1224, 1446, and 1531 cm−1. The IR spectra during SO2 adsorption consisted primarily of the IR intensities relating to the symmetric and asymmetric ν(SO) vibrations at 1098 and 1277 cm−1, respectively, for the gaseous SO2. An increase in the amount of −NO2 sites were observed during cycle no. 2, and the SO32− and SO42− species were not observed, suggesting the oxidation of SO2 had not occurred to a great extent. The amine layer may be saturated with the strongly bonded sulfite and sulfate species from the previous adsorption cycle. The retained sulfite and sulfate species leading to the deactivation of the amine layer is also consistent with the theoretical binding energies for SO32− (−92.60 kcal/mol) and SO42− (−115.68 kcal/mol), showing significantly higher BE than for the SO2 (7.72 kcal/mol). In addition to this, the formation of the SO32− [diamine]-(NH3+)2 (−288.55 kcal/mol) and SO42− [diamine](NH3+)2 (−314.44 kcal/mol) species may be a leading factor in the deactivation of the amine sites by the strongly adsorbed sulfite and sulfate. The higher BE facilitated the irreversible adsorption of these species at the amine site, leading to primarily gaseous SO2 being observed in the IR absorbance spectra. The cycle no. 3 IR data (Figure 7a) showed a further decrease in the IR intensities of N−H and the formation of the −NO2 species. The IR spectra of regenerated [diamine] layer for cycle nos. 2 and 3 consisted primarily of the IR intensities relating to the symmetric and asymmetric ν(SO) vibrations (1098 and 1277 cm−1, respectively) for the gaseous SO2. These IR data suggest that the [diamine] layer rapidly deactivates through saturation of the amine layer by the SO32− and SO42− species and the oxidation of the primary amines to the nitrite. It is important to observe that the ammonium ion broadband at 2905 cm−1 is less intense on Cyc no. 3, indicating the [diamine] layer is also saturated with ammonium ions, consistent with the theoretical results and a strongly irreversibly bonded sulfur species. The pure [diamine] and the [diamine]/SiO2 sorbent particles were evaluated for the multicycle adsorption− desorption reactions to study the regeneration efficiency of the sorbent at 100 °C. Mass spectroscopic analysis of the DRIFTS reactor effluent during regeneration of the [diamine]/ SiO2 sorbent produced 548 μmol/g of SO2 (2.8 mol/mol). The amount of SO2 obtained during multicycle testing is reported as the total moles of SO2 were removed and the molar ratio of moles SO2/mol sorbent (shown in Table 4). The cycle no. 1 data showed a SO2 capture capacity comparable to IL sorbents

reported by other researchers.78 The high SO2 adsorption capacity is attributed to the high surface area of the SiO2 support material. Increasing the number of adsorption/ regeneration cycles resulted in a significant decrease in the SO2 adsorption capacity, for both the pure [diamine] and the [diamine]/SiO2 supported sorbents (as shown in Table 4). During cycle 1, the [diamine] thin film (ATR) absorbed 1.39 mol/mol SO2, showing an amine efficiency of 0.82. Subsequent cycles show a decrease in SO2 capacity to 1.23, 0.88, and 0.74 mol/mol by the fourth cycle, corresponding to the rapid deactivation of the amine active site. Ethanol was added dropwise to redisolve the [diamine] on the ZnSe window, and the thin film was allowed to dry. Ethanol had the capability to regenerate the [diamine] sorbent indicated by an increase in absorption capacity (1.27 mol/mol), suggesting the deactivation mechanism may also result from the agglomeration of the amine sites during the SO2 adsorption reaction. The ethanol regenerated sorbent, however, showed a lower SO2 capture capacity, suggesting fewer amine sites were available due to the fractional degradation of the amine sites as indicated in the FTIR study. Further evidence for the sorbent degradation may be indicated by comparing the color change of the fresh [diamine]/SiO2 sorbent (white samples) to the samples following 4 adsorption−desorption cycles. The spent sorbent samples exhibited a blue-purple color. Previously, researchers have shown that the degree of degradation of the sorbent may be indicated by the extent of the color change in the sorbent.77 Both the chemical and morphological changes in the [diamine] layer may have led to the deactivation of the diamine sorbent. Researchers have studied the chemical and morphological changes due to degradation effect of SO2 for amines, ketones, nitriles, and esters using UV−vis spectroscopic methods.79−82 To investigate the agglomeration effect and the amount of retained SO2 species in the [diamine]/SiO2 sorbent, UV− visible diffuse reflectance spectroscopy was used to probe the surface chemistry during the adsorption process. 3.3. UV−vis Characterization of the [Diamine] Sorbent. Figure 8 presents the UV−vis diffuse reflectance

Table 4. SO2 Adsorption on 4 μm Thin Film of [Diamine] cycle no. 1 2 3 4 add EtOH ethanol 1 2 3 4 5

SO2 (μmol)

molSO2/molsorbent

ATR (pure [diamine]) 16 1.39 14.2 1.23 10.1 0.88 8.52 0.74 14.6 1.27 DRIFTS (2 vol % loading [diamine]/SiO2) 548 2.80 252 1.29 183 0.93 130 0.67 78 0.40

Figure 8. UV−visible diffuse reflectance spectra of (a) ethanol, (b) 2 vol % [diamine]/ethanol solution, and (c) 10 vol % [diamine]/ethanol solution.

molSO2/molNH2 0.82 0.73 0.58 0.42 0.75

spectra for (a) ethanol, (b) 2 vol % [diamine]/SiO2, and (c) 10 vol % [diamine]/SiO2 sorbents. The UV−vis spectra of ethanol show no adsorption in the UV and visible portions of the spectrum, indicating ethanol is unresponsive to UV−vis excitations. The 2 vol % amine resulted in a weak broadband absorption in the 400−550 nm visible regions and may be the result of π-electron conjugation in adjacent (layered) aromatic molecules.83−85 The 10 vol % amine sorbent showed an increase in the broadband absorbance intensity (Figure 8c), corresponding to a greater amount of conjugated π-electrons, corresponding to the thicker [diamine] layer on the SiO2

0.98 0.45 0.33 0.23 0.14 6723


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respectively. Previous studies of solvatochromism of anionic solvatochromic dyes have shown that the increase in the polarizability of a solvent resulted in a bathchromatic shift of the UV−vis band for para-nitrosubstituted aromatics,89 similar to what is observed here during the SO2 adsorption at the amine site. The theoretical predictions also showed that the SO2 molecule strongly polarized the [diamine] layer (2.53 D) to 5.25 D. The bathchromatic shift of the UV−vis absorbance intensities are consistent with the SO2 polarizability of the [diamine] layer. In addition, the wavelength shift to lower frequencies suggests that the excited states for these compounds become more stable as SO2 reacts and strongly bonds at the amine site in the [diamine] layer. Flowing CO2 over the [diamine] sorbent (Figure 9b) resulted in a negligible increase in K−M absorbance intensity, which is attributed to the weak CO2 interaction at the amine site. A theoretical simulation for the CO2-[diamine] structure revealed CO2 weakly polarized the amine layer (2.54 D) due to the weak [diamine] basic site, thus the K−M absorbance intensity indicates mostly [diamine] character and resulted in no change in absorption intensity. The K−M absorbance spectra during SO2 adsorption over the [diamine] at 50 °C is shown in Figure 9c. The experimental data shows a similar trend to what was observed in the simulated UV−vis spectra. SO2 interacts with the [diamine], resulting in absorption bands as 266, 373, and 464 nm, following 5 min exposure to SO2. The theoretical excitation energies for the SO2−[diamine] system correspond to the HOMO → LUMO+1 (266 nm), HOMO+1 → LUMO (369 nm), and HOMO → LUMO (457 nm) transitions, respectively, are comparable to the experimental results. Increasing the SO2 exposure time produced the absorption band at 674 nm, which indicates a bathchromatic shift in the HOMO → LUMO transition to 586 nm, consistent with the trend in the theoretical results. The DFT results coupled with the experimental data indicate the SO2 adsorption at the amine site strongly polarizes the [diamine] layer. The strongly polarized diamine layer may have led to the agglomeration and deactivation of the amine sites where SO2 remains strongly bonded as the sulfite and sulfate species at the amine site. The blue-purple color of the sorbent following the multicycle testing suggests the sorbent layer is saturated with the SO32− and SO42− species, consistent with the operando spectroscopic studies. These results indicate that deactivation proceeds by the physical rearrangements of the sorbent molecules (agglomeration), the oxidative degradation of the amine site, and the strongly bonded sulfur bearing species remaining at the amine site during thermal regeneration at 100 °C.

surface. The linear increase in absorbance intensity shows the proportionality of intensity to concentration and may be used as a measure of agglomeration in the presence of highly conjugated molecular structures.86 Figure 9a shows the simulated UV−vis spectra, and Figure 9 (panels b and c) shows the experimental UV−vis diffuse

Figure 9. (a) Simulated UV−vis spectra of [diamine], SO2-[diamine] complexes at the B3LYP/6-31+G* level, and experimental UV−visible diffuse reflectance spectra of 1,3-phenyelediamine/SiO2 during adsorption of (b) CO2, and (c) SO2 at 50 C.

reflectance spectra during the adsorption of CO2 and SO2 over the [diamine]/SiO2 sorbent at 50 C, respectively. The experimental UV−visible absorbance spectrum was placed in Kubelka−Munk (K−M) units to highlight the changes in the chemical structures at the sorbent surface during exposure to SO2.87 The theoretical spectra for the [diamine] (X1) and [diamine]2 (X2) were simulated at the TDDFT/B3LYP/631+G* level; the [diamine] and the [diamine]2 are shown in Figure 9a. The excitation energies corresponding to the spectral lines shapes for the [diamine] X1 and X2 are similar, suggesting that layering and the intermolecular hydrogen bonding of the [diamine] molecules resulted in a localized and less mobile electronic charge over the [diamine] molecule.88 Following the multicycle experiments, the used sorbent samples produced blue-purple emission visible to the eye, which was not present in the freshly prepared samples. The simulated UV−vis spectrum during SO2 adsorption at the amine site showed two additional absorption peaks centered at 365 nm, corresponding to the HOMO+1 → LUMO excitation and 457 nm for the HOMO → LUMO transition, shown in Figure 9a. The adsorption of SO2 at the amine site resulted in the absorption band at 457 nm, which may correspond to the bluepurple color of the used sorbent and may be an indication that SO2 remains present and strongly bonded at the amine site. The SO2−[diamine]2 system further shifted the HOMO+1 → LUMO and HOMO → LUMO transitions to 543 and 586 nm,

4.0. CONCLUSIONS Temperature-programmed desorption showed the 1,3-phenylenediamine sorbent capacity of 1.39 mol SO2/mol sorbent for pure [diamine] thin film and 2.8 mol SO2/mol sorbent for the [diamine]/SiO2 supported sorbent. The adsorption of SO2 at 50 °C resulted in the formation of strongly adsorbed SO32− (sulfite) and SO42− (sulfate) species at the ammonium sites. The retained sulfite and sulfate species were remaining at the ammonium sites, whereas those adsorbed SO2 species at the amine sites were readily thermally removed at 100 °C. The strongly adsorbed sulfate species were able to oxidize the amine sites, producing the unreactive −NO2 groups and decreasing the number of adsorption sites in the sorbent. The infrared data indicates that the adsorbed sulfite and sulfate species remained 6724


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strongly bonded at the ammonium sites, whereas these species are more weakly adsorbed at the amine sites which may be thermally desorbed from the sorbent. The retained SO32− and SO42− species led to the rapid deactivation of the sorbent during multicycle testing.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (+1) 304-285-5292. Fax: (+1) 304-285-4403. *E-mail: [email protected]. Tel: (+1) 330-972-6993. Fax: (+1) 330-972-5856. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy under Grants DE-FE0001780 and DE-FC26-07NT43086.

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REFERENCES

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Experimental and Theoretical Investigation of SO2 Adsorption over the

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