The Effect of Electron-Donating Groups and Hydrogen Bonding on

Article pubs.acs.org/JPCC

The Effect of Electron-Donating Groups and Hydrogen Bonding on H2S Capture over Polyethylene Glycol/Amine Sites Duane D. Miller*,† and Steven S. C. Chuang*,‡ †

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

‡

ABSTRACT: The reversible adsorption of H2S on tetraethylenepentamine (TEPA) was investigated using attenuated total reflection infrared (ATR-IR) spectroscopy, mass spectrometry, and density functional theory (DFT). The in situ infrared method revealed H2S ability to selectively poison the TEPA amine active site by the formation of a strongly adsorbed species in the form of (HS−) NH3+-TEPA. In addition, the H2S−amine interaction, in the presence of oxygen, resulted in the oxidative degradation of the amine, indicated by the formation of an −NO2 species. The addition of polyethylene glycol (PEG) affects the H2S-amine interactions and prevents the amine site from oxidative degradation. Both experimental IR and DFT calculations revealed that PEG affected the electronic and geometric environment around the amine binding site through hydrogen bonding and electron-donating effects. The addition of PEG decreased the (+) charge of the ammonium ion site, effectively decreasing electrostatic anion−cation interaction of the strongly bound HS− species at the amine active site. This decrease in anion−cation interaction brought about by PEG resulted in lowering the binding energy of H2S on the amine site and decreasing the extent of oxidative degradation of the amine site. The addition of PEG also increased the adsorption capacity of the TEPA sorbent from 0.71 μmol H2S to 1.10 μmol H2S/g sorbent at 9.1 wt % PEG in the TEPA thin film, increasing the H2S adsorption capacity by 1.5 times. The qualitative aspect of this study suggests a way to tailor the amine active site using PEG to improve the adsorption capacity and regenerability of amine-based sorbents.

1. INTRODUCTION Removal of acid gas impurities such as hydrogen sulfide (H2S), a poisonous, corrosive, and odorous gas,1−6 from gas mixtures is very important in many chemical and fuel processes: natural gas processing, hydrogen purification, treating refinery off gases, synthesis gas for ammonia, and methanol synthesis. H2S removal prevents H2S derived sulfur dioxide (SO2) emissions in combustion processes which lead to acid rain, and/or deactivation of catalytic or amine active sites.7 The removal of H2S from gas streams has also been practiced in industrial gas processing for many specific purposes: corrosion prevention, waste minimization, resource utilization, and environmental pollution reduction.8,9 The separation of the low concentration H2S has been achieved by the method of catalytic oxidation of H2S over alkali-impregnated activated carbons10 and layered double hydroxides.11 This method has the advantage of high activity and fast reaction kinetics,12−14 where the H2S may be oxidized to the sulfite and sulfate species, which are fixed into the pore structure of the catalysts.15−17 The major disadvantage is the regeneration energy requirement (600−750 C) of the alkaliimpregnated sorbents.18 The metal oxide-based sorbents such as Fe2O3, ZnO, and CuO, have also been widely used where H2S is converted to SO2. Spectroscopic19−23 and kinetic24−28 measurements have been extensively performed to acquire insight into the details regarding the reaction mechanisms, © XXXX American Chemical Society

kinetics, and influences of the acidbase properties of metal oxide sorbents (e.g., γ-Al2O3) on H2S adsorption. The acid properties of H2S (similar to CO2) suggest the amine-based sorbent should be capable of reversibly adsorbing at the amine active site by an acid−base interaction. The aminebased sorbents capable of reversible reactions with H2S, coupled with its long-term regeneration capacity and lower energy regeneration requirements, make amines the ideal technology for H2S removal from various gas streams. The aqueous amine is a chemical absorbent used in amine scrubbing29,30 technology to remove hydrogen sulfide in refineries, where N-methyldiethanolamine (MDEA) was first proposed as an effective liquid H2S removal sorbent.31 The H2S removal on amine-based sorbents is generally composed of two steps: (i) a fixation of H2S from the process stream by the formation of an H2S−amine complex and (ii) thermal regeneration of adsorbing agents with H2S separation and recovery of the sorbent at the same time. The mechanism for the H2S−amine reaction could follow the proposed reaction step R1 and R2.17,32,33 The adsorption reaction process may also be qualitatively analogous to that of CO2 where the Received: December 2, 2015 Revised: December 30, 2015

A

DOI: 10.1021/acs.jpcc.5b11796 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Experimental Apparatus.

2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of the Sorbents. The gases used in this study were supplied by Praxair, N2 (99.999 vol %), and 1% H2S balance He (99.999 vol %). Hydrogen sulfide is one of the most toxic and malodorous gases having chronic toxicity below 1 ppm; therefore leak testing procedures and personal protection equipment were utilized during testing. A 4 μm thin film was prepared on the ATR ZnSe window from 0.0573 M tetraethylenepentamine (TEPA, Aldrich >30.0%) in ethanol (Pharmco-Aaper, 99.98%) solution. The thin film was created by adding dropwise 0.1 mL of TEPA/ethanol solution upon a ZnSe window. The thin film was allowed to sit stagnant for 30 min for evaporation of the ethanol solvent and heated to 100 °C at 10 °C/min under 100% N2 flow at 100 cm3/min to evaporate the remaining solvent and cleaning the TEPA layer, then cooled to 50 °C for the adsorption study. The single TEPA molecule (length: 15.716 Å, area: 271 Å2, DFT at B3LYP/6-31G**) was added to the ATR ZnSe window (area: 4.35 cm2), assuming an even distribution of TEPA on the ZnSe window, the evaporated solution produced a 4 μm layer consisting of multiple layers of TEPA on the ZnSe surface. Five TEPA-PEG thin films were created by adding polyethylene glycol (PEG, Aldrich, MW 200) to the TEPA solution at 9.1, 16.7, 23.0, 29.0, and 33.3 wt % PEG loading, respectively. The thin film was fabricated by adding dropwise the TEPA-PEG/ethanol solution upon the ZnSe window. The resulting thin film was allowed to set stagnant at 50 °C for 30 min. The ZnSe window was heated to 100 °C at 10 °C/min under 100 cm3/min N2 flow, to evaporate the ethanol solvent, cleaning and drying of the thin film, and then cooled to 50 °C for the adsorption study. Gases containing H2S are commonly referred to as sour gases or acid gases in the hydrocarbon processing industries. The typical operating range for the liquid absorber unit in the H2S amine gas treating process is between 35−50 °C;61,62 therefore, 50 °C was chosen for H2S adsorption over the TEPA and TEPA-PEG systems. 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 attenuated total reflectance accessory (ATR-IR, Harrick Scientific) with a custom reactor manifold mounted to the ATR-IR top plate placed inside of an Fourier transform infrared spectrometer (FTS6700 FTIR, Thermo-Nicolet), and (iv) a mass spectrometer (MS, Pfeiffer Omnistar). The 4-port valve allows switching the inlet flow from 100% N2 to 1% H2S while

adsorption of the CO2 molecule reacts with the amine adsorption sites according to the Step R3.34−44 RNH 2 + H 2S → RH 2N ··· H−S−H

(R1)

RNH 2 + H 2S ↔ RNH3+ + HS−

(R2)

CO2 + 2RNH 2 ↔ RNH3+ ··· RNCOO−

(R3)

R1 suggests that the H atom in the gaseous H2S interacts with the nitrogen atom at the amine site to form a hydrogen bonded H2S-amine complex. Amine sites may also act as base analogues in which H−S dissociates, protonating RNH2, according to Reaction Step (R2).45 The forward reaction, i.e., H2S adsorption at the amine site (Steps R1 and R2) and CO2 adsorption at the amine site (Step R3), occur at 25−75 °C46−49 followed by thermal regeneration at 100−225 °C,47,50−52 returning the amine sorbent to its original state. H2S adsorption on various sorbents have been studied using the in situ infrared technique by monitoring changes in the molecular surface species,13,53−55 revealing that H2S can adsorb on basic centers (reversibly and dissociatively) and proton centers as hydrogen bonded complexes.53,54 Few studies, however, have been conducted on the H2S−amine system.32,53,56 The formation and stability of the adsorbing species is expected to be a function of the basicity of the amine and the 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 H2S is of great importance. Hydrogen bonding has been proposed to play a major role in CO2 adsorption over N-doped porous carbon with KOH additives57,58 and in both the aqueous and solid amines.59,60 Motivated by these studies, five thin film preparations were created by blending PEG with TEPA on the ZnSe ATR-IR window at various PEG loadings. The resulting thin films were studied for H2S adsorption capacity using the ATR-IR coupled with mass spectrometry and density functional theory. We found PEG (polyethylene glycol) was able to modify the amine active sites through hydrogen bonding to further improve their H2S capture capacity and regenerability. The amine−PEG interaction modified the electronic and geometric environment of the adsorption site by decreasing the electrostatic charge of the ammonium ion and effectively decreasing the binding energy of the adsorbed H2S species improving the regenerability of the sorbent. These results suggest a fundamental understanding of the amine site interactions could help guide the preparation of future amine based sorbents for the acid gas separation processes. B


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The Journal of Physical Chemistry C maintaining a total flow rate of 100 cm3/min over the sorbent layer. Changes in the concentration of IR observable complexes were monitored by the ATR-IR technique. The IR absorbance spectrum of absorbed and gaseous species was obtained by A = −log(Io/I),63 where Io is the background IR single beam spectrum (32 coadded scans and resolution 4 cm−1) of the TEPA and TEPA-PEG thin films and I is the IR single beam spectrum collected during the H2S adsorption reaction. The MS responses corresponding to N2 (m/z = 28), O2 (m/z = 16, 32), H2S (m/z = 34), and SO2 (m/z = 48) were monitored for the changes in the ATR-IR reactor effluent concentrations. During the experimental testing, the MS analysis of the ATR-IR reactor effluent revealed a system leak where 1504.7 ppm of O2 was present in the reaction gas during the H2S adsorption cycles over the TEPA and TEPA-PEG thin films. Leakage of air to the DRIFT cell has been minimized to 1504.7 ppm. The absence of air emulates the practical condition in which a small amount of air in the stream cannot be completely removed. 2.3. Theoretical Method. The hydrogen sulfide (H2S), TEPA and TEPA-PEG structures, their complexes, and the calculated harmonic normal-mode frequencies were determined using density functional theory (DFT) in the Spartan 14 (Wave function Inc., USA) software package. DFT was utilized to study the most stable conformations for the amines and their complexes with H2S. The optimized molecular geometries and binding energies (BE) were calculated using Becke’s threeparameter exchange functional and gradient corrected functional of Lee, Yang, and Parr (B3LYP),64−66 using the 6-31G** and 6-31+G* basis sets. The calculated infrared normal vibrational frequencies were determined at the B3LYP/631G** level67,68 and were not corrected with scaling factors prior to comparison with the experimental infrared spectra. The binding energies (BE) for the H2S-TEPA complexes were obtained according to eq 1. BE = E[H 2S−TEPA] − E[H 2S] − E[TEPA]

Figure 2. Infrared spectra: (a) simulated spectra at the B3LYP/631G** level, (b) MMFF94 simulated spectra of 12 TEPA molecules, (c) ATR-IR single beam, and (d) ATR-IR absorbance, of pure TEPA on at 50 C.

(1)

where E[H2S-TEPA] is the total energy of the TEPA with the adsorbed H2S and E[H2S] and E[TEPA] are the total energies of the H2S and TEPA molecules, respectively. The Spartan 14 implementation of Merc Mechanics Force Field (MMFF94) was utilized to quickly investigate the molecular binding interactions to provide a qualitative account of the effects of symmetry (molecular geometry) on the infrared vibrational frequencies. A 12 TEPA molecule system was studied to simulate the TEPA thin film on the ZnSe window in the ATR-IR experiment. The starting geometries were designed to be highly nonsymmetrical based upon the orientations of the TEPA molecules on the ZnSe window. The MMFF94 infrared frequencies were compared to the theoretical DFT (B3LYP/6-31G**) and experimental infrared spectra.

value to 1.0. The simulated IR spectra at the DFT level are consistent with those of experimental spectra (Figure 2c and d): the w(N2−H) and w(N3−H) wagging vibrations contribute to IR intensity at 799 cm−1, w(N1−H), w(N2−H) wagging vibrations at 873 and 1503 cm−1, respectively, the δ(N1−H) deformation vibration at 1666 cm−1, and the ν(N1− C) vibration at 1174 cm−1, ν(C−H) vibrations at 2905, 3037, and 3090 cm−1, and the w(C−H) wagging vibrations at 1061 and 1323 cm−1, respectively. The simulated N−H stretching vibrations for νa(N1−H) at 3569 cm−1, νs(N1−H) at 3475 and νs(N5−H) at 3484 cm−1, ν(N2−H) at 3478 cm−1 and ν(N3− H) at 3481 cm−1, were relatively weak and were scaled by 250 times. Comparison of the simulated spectra at the DFT B3LYP/631G** level (Figure 2a) to the experimental spectra in Figure 2d, further revealed the DFT method significantly overestimated the ν(N1−H) stretching vibrations and underestimated the IR intensities. The B3LYP function has been found to overestimate the wavenumbers of the fundamental modes compared to the experimentally observed values due to the combination of electron correlation effects and basis set deficiencies69,70 and the geometries responsible for the calculated and observed vibrational modes. These differences are significant when the molecular symmetry is low. The DFT calculation with a single TEPA molecule (Figure 2a) produced

3. RESULTS AND DISCUSSION 3.1. Spectroscopic and Theoretical Characterization of the TEPA Thin Film. Figure 2 compares the simulated and experimental infrared spectra for tetraethylenepentamine. The infrared band assignments for the simulated and experimental data are listed in Table 1. The starting geometry for the simulated TEPA molecule, Scheme 1a, was first optimized at the HF/6-31G** level prior to being optimized at the B3LYP/ 6-31G** level. The simulated infrared intensities, shown in Figure 2a and b, are normalized by setting the largest intensity C


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1630 cm−1 corresponding to the deformation vibration of δ(NH3+), at 1512 cm−1 for the w(N−H) wagging vibration, at 1097 cm−1 for the ν(N1−C) vibration, and at 980 cm−1 for the w(N1−H) vibration. The IR intensity at 3365 cm−1 correspond to the N1 and N5 primary amine locations, and the IR vibration at 3309 cm−1 is partly attributed to the ν(N−H) vibration at the N2, N3, and N4 amine locations. The decrease in IR intensities at 3184, 3309, and 3365 cm−1 corresponding to the νs(N−H) symmetric vibration, the overtone band for the δ(N1−H) deformation vibration, and the ν a (N1−H) asymmetric vibration, respectively, upon H2S adsorption further indicated the direct interaction of H2S with the amine functional group and conversion of amine to ammonium ions. The IR bands at the amine site during H2S adsorption are similar to those during CO2 adsorption which produced an alkyl-ammonium carbamate species, i.e., a carbamate and ammonium ion pair (Step R3).74,75 Flowing nitrogen over the H2S saturated TEPA layer (Figure 3b) resulted in a decrease in the IR intensity indicating the removal of a weakly adsorbed H2S. These weakly adsorbed H2S desorb under flowing nitrogen without heating the TEPA film. The absorbance spectrum was obtained by subtracting the background spectra from the H2S saturated TEPA thin film prior to flowing nitrogen. These results are similar to what others have observed during H2S adsorption and evacuation over 2,6-dimethyl-pyridine on Al2O3.23 The IR intensity ratio of the weakly adsorbed H2S to the thermal regenerated H2S reveal an H2S fraction 0.72 was weakly adsorbed. This fraction corresponds to a stoichiometric ratio of 3.5 amine sites to five being capable of weakly adsorbing the gaseous H2S species. The availability of a large fraction of weakly adsorbed species suggests that the amine sorbent could be used in a vacuumswing process for H2S separation. Figure 3c shows the thermal regeneration of the TEPA thin film resulted in an increase in IR intensities for the symmetric νs(N1−H) stretching vibration at 3184 cm−1, the asymmetric νa(N1−H) stretching vibration at 3365 cm−1, and the overtone band for the δ(N1−H) deformation vibration at 3309 cm−1, corresponding to the removal of the adsorbing species from the amine site. We classify the species desorbed during TPD as strongly adsorbed species in contrast to those weakly adsorbed species which desorbed at 50 °C under N2 flow. It is important to note that weakly adsorbed species will play a key role in vacuum or pressure swing adsorption processes while strongly adsorbed species in a temperature swing adsorption process. The increase in IR intensity at 1695 cm−1 for ν(CO) vibration and decrease in IR intensity at 2931 cm−1 for the ν(C−H) vibration during the thermal regeneration cycle revealed that part of TEPA was oxidized to an amide species during temperature-programmed desorption. The source of the oxygen is due to the presence of 1504 ppm of O2 during the adsorption and thermal regeneration cycles. Following the multicycle testing, the color of TEPA gradually turned yellow, an indication of the oxidative degradation of TEPA.76 The quantity of O2 in the flow was determined by MS. Comparison of the IR intensity during the H2S adsorption and thermal regeneration cycle reveals the presence of both a strongly bonded and weakly bonded H2S species. The decrease in magnitude of the IR intensity at 2559 cm−1 during thermal regeneration (Figure 3c) suggests the strongly bonded H2S species is associated with the ammonium adsorption site.

Table 1. Infrared Experimental and Theoretical Normal Modes of Vibration for the Optimized Structures at the B3LYP/6-31G** Level wavenumber (cm‑1) (theoretical) 1174 1061, 1323 799b, 873a, 1503b 1666 2905, 3037, 3090 3475, 3484 3569 3478b 3481c

(ATR-IR) TEPA 1127 1308 1454 1666, 1594 2931 3184 3365

description ν(N1−C) w(C−H) w(N2−H) δ(N1−H) scissor ν(C−H) νs(N1−H) νa(N1−H) ν(N2−H) ν(N3−H)

H2S νs(S−H) νa(S−H) δ(S−H) scissor

2753 2769 1221 913, 873 1446c, 1537a 2379b, 2390a 1673a, 1665a 2439, 2722

3047 3176 3444 3445 3508 2378a a

H2S−TEPA 980 1512 1630 3365 3309 HS−−H−NH3+−TEPA 2559a,b,c

3309 3365 (HS−) NH3+−TEPA 2559

w(N1−H) w(+N1−H) ν(S−H)···N1−H δ(NH2) scissor ν(SH) for H2S ν(N1−H) ν(N1−H) ν(NH3+···SH−) ν(+N3−H)···SH ν(+N3−H) νs(N1−H) νa(N1−H) ν(+N1−H···HS−)

Notes: Amine site locations on TEPA: N1 or N5. bN2 or N4. cN3.

higher ν(N1−H) vibrations in the 3475−3484 cm−1 region, while those calculated from a 12 TEPA molecule model by MMFF94 produced IR bands in the 3184−3475 cm−1 region, and overestimate the IR intensities. (Figure 2b). The latter produced from a 12 TEPA molecule system are more consistent with the experimental spectra in ν(N1−H) vibrations occurring at 3365 and 3184 cm−1, and the δ(N1− H) overtone band at 3309 cm−1 than those obtained for a single TEPA molecule. These observations highlight the limitations of the theoretical IR calculations. 3.2. IR Studies during H2S Adsorption over the Pure TEPA Thin Film. Figure 3a shows the evolution of ATR-IR absorbance spectra during H2S adsorption on the TEPA film at 50 °C; adsorption of H2S produced a prominent IR band at 2559 cm−1. This band and its associated broad band can be assigned to a stabilized ammonium ion ν(NH3+···SH−), similar to what has been observed experimentally in our previous work during SO2 adsorption over aromatic amine,7 and by others during CO2 adsorption over aliphatic amines.23,34,71,72 The broadness of this band is a manifestation of hydrogen bonding. The growth of this band was accompanied by the suppression of the C−H intensity at 2931 cm−1, a manifestation of the formation of ammonium ion on the C−H chain.73 H2S adsorption also resulted in an increase in IR intensity at 1604 cm−1 for the δ(NH2) deformation vibration and the shoulder at D


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Scheme 1. Optimized Structures at the B3LYP/6-31G** Level for the H2S and TEPA Interaction for (a) H2S−TEPA (N1), (b) H2S−TEPA (N2), (c) H2S−NH3+−TEPA (N1), (d) H2S−NH3+−TEPA (N3), (e) H2S−TEPA2 (N1), and (f) (HS−) NH3+− TEPA (N1)

Evidence for the formation of a TEPA−NH3+(HS−) species is indicated by the broadband IR intensity, and the weak ν(HS−) vibration centered at 2559 cm−1 (Figure 3a).46 The structure assignment for the strongly bonded TEPANH3+(HS−) species is consistent with the theoretically calculated infrared bands and the H2S binding energy at the B3LYP/6-31G** level as discussed in Section 3.3. The presence of a strongly bonded H2S species may also lead to the deactivation of the amine adsorption site; therefore, the ATR-IR spectra of the TEPA layer were studied under multicycle adsorption−regeneration tests. Figure 4 shows the difference spectra during three H2S adsorption cycles. The infrared spectra during adsorption cycles no. 2 and 3 revealed an increase in IR intensities at 1268, 1428, 1486, and 1531 cm−1 corresponding to the formation of the −NO2 group.77 The IR data in Figure 4 suggest the primary amines are oxidizing to the −NO2 species. This is consistent with studies on the electrochemical oxidation of aliphatic amines that show primary amines have higher redox potential than those of secondary and tertiary amines.78 The presence of the H2S species and O2 at the amine site also led to an increase in the IR intensity at 1695 cm−1 corresponding to a CO containing species in the second and third adsorption cycles. The formation of these species is accompanied by the broadening of the ν(N1−H) vibrational bands at 3184 and 3365 cm−1, and the disappearance of the δ(NH2) overtone band at 3309 cm−1, indicating the disappearance of the amine active sites. There was also a decrease in the IR intensity for the

band associated with ammonium ion (2559 cm−1) on increasing number of adsorption cycles. This data suggests the presence of H2S could enhance the oxidative activity of O2 at low concentrations. In addition, the spectra in Figure 4 also revealed the formation of H2O in the TEPA layer during the multicycle testing as indicated by the increase in IR intensities at 1628 cm−1 for the δ(H2O) deformation vibration, and at 3037 cm−1 for the ν(O−H) vibration (Figure 4, cycle no. 3). The formation of H2O supports the pathway for the dissociative adsorption of H2S in the TEPA layer (Step R2), similar to what was observed by previous studies over γAl2O3.19,25,79 The dissociative adsorption of H2S led to the appearance of the −OH and the SH− species.19,21,54,80,81 Further combination of two OH species could lead to H2O in the TEPA layer. 3.3. Theoretical Investigation of Adsorbing Species at the TEPA Amine Site. The starting geometries for the H2S and TEPA systems, shown in Scheme 1, were optimized at the HF/6-31G** level prior to proceeding to the B3LYP/6-31G** and B3LYP/6-31+G* levels. The most stable structures were determined by varying the starting geometries in the computational study. The H2S molecule was oriented at various locations around the TEPA molecule, the optimized structures show H2S interacts at three amine locations, as indicated in Scheme 1a and b, depended upon the H2S starting position relative to the respective amine sites (N1, N2, or N3). The optimized geometries for the interaction of one H2S and one TEPA molecule resulted in three unique and stable structures E


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2. Moving from left to right, the theoretical BEs show a trend indicated by the decrease in BE for the H2S-TEPA at the Table 2. Binding Energies Calculated at the B3LYP/631G** and 6-31+G* Level structure weakly adsorbed

strongly adsorbed

weakly adsorbed

Figure 3. IR absorbance spectra during (a) H2S adsorption over TEPA at 50 °C, the background spectra taken prior to H2S adsorption; (b) during flowing N2, the background spectrum was taken prior to flowing N2; and (c) temperature-programmed desorption at 50−100 °C, the background spectrum taken from the H2S saturated TEPA layer. weakly adsorbed

a

H2S−TEPA (N1) H2S−TEPA (N2) H2S−TEPA (N3) H2S−TEPA2 (N1) H2S−TEPA2 (N2) HS−−H−+NH2− TEPA (N1) HS−−H−+NH2− TEPA (N2) HS−−H−+NH2− TEPA (N3) (HS−) NH3+−TEPA (N1) (HS−) NH3+−TEPA (N2) (HS−) NH3+−TEPA (N3) H2S−TEPA−PEG (N1) H2S−TEPA−PEG (N2) H2S−TEPA−PEG (N3) HS−−H−+NH2− TEPA−PEG (N1) HS−−H−+NH2− TEPA−PEG (N2) HS−-H-+NH2-TEPAPEG (N3) (HS−) NH3+− TEPA−PEG (N1) (HS−) NH3+− TEPA−PEG (N2) (HS−) NH3+− TEPA−PEG (N3) (HS−) NH3+− TEPA−PEG2 (N1) (HS−) NH3+− TEPA−PEG2 (N2) (HS−) NH3+− TEPA−PEG2 (N3)

BE 6-31G** (kcal/mol)

BE 6-31+G* (kcal/mol)

TEPA −5.71 −5.53 −5.38 −13.0 −10.9 −5.89

−4.21 −3.97 −3.92 −8.75 −7.80 −4.32

−5.57

−4.02

−5.36

−3.83

−139

−130

−131

−122

−129

−120

TEPA-PEG −6.34

−4.95

−5.12

−3.42

−5.33

−3.71

−5.44

−5.22

−4.16

−4.65

−4.92

−4.76

−127

−118

−113

−105

−121

−115

−104

−97.7

−109

−100

−118

−108

BE exp.a (kcal/mol)

−10.2

−7.55b

BE calc. based on ref 84/ bCalculated for the 23 wt % PEG-TEPA.

B3LYP/6-31G** level (−5.17 kcal/mol) as compared to the B3LYP/6-31+G* level (−4.21 kcal/mol). The 6-31+G* basis set includes highly diffuse functions to more accurately describe hydrogen bonded complexes. Decreases in calculated BE could be the result of the charge transfer contribution to the BE decreasing as the size of the basis set increases.82 The optimized structures for H2S adsorption at the amine site (Scheme 1a and b) revealed that molecular interactions in the form of hydrogen bonding resulted in similar BEs, −5.71 kcal/mol (N1), −5.53 kcal/mol (N2), and −5.38 kcal/mol (N3), listed in Table 2, and are consistent with experimentally determined BE −5.0 kcal/mol.45 The calculated BE show a decreasing trend as the H2S adsorption moves from the N1 location, to the N2, and N3 locations for all the adsorbing species, consistent with the fact that secondary and tertiary amines has a lower basicity than the primary amine.83

Figure 4. IR spectra of adsorbed H2S in adsorption cycles no. 1, 2, and 3 over TEPA at 50 °C. The spectra were obtained by subtracting the absorbance spectrum at 0.81 min H2S exposure time on cycle no. 1 from the absorbance spectrum at 4.0 min for cycle no. 1, 2, and 3.

corresponding to H2S adsorption at the N1 location (Scheme 1a), the N2 location (Scheme 1b), and the N3 location (not shown). The BE for the H2S adsorption at the amine site for the various structures shown in Scheme 1 were compared at the B3LYP/6-31G** and 6-31+G* levels and are reported in Table F


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The Journal of Physical Chemistry C Comparing the H2S adsorption on the single TEPA (Scheme 1a and b) to the H2S adsorption on the multilayer TEPA adsorption site (Scheme 1e) shows an increase in BE from −5.7 to −13.0 kcal/mol at the B3LYP/6-31G** level, and is comparable to the experimental H2S adsorption over γ-Al2O3 (13 kcal/mol).76,80 The increase in the BE corresponds to the increase in molecular stability of the absorbed H2S at the amine adsorption site. The H2S adsorption at the N2 location in H2S−TEPA2 also resulted in an increase in BE (−10.9 kcal/ mol) as compared to the single TEPA molecule case (−5.53 kcal/mol). This is consistent with the BE for H2S adsorption in ionic liquids (−31.6 kcal/mol).2 The H2S desorption energy was also calculated from the spectral features of the H2S MS profile during TPD by the Redhead method. 84 The experimental BE was determined to be −10.19 kcal/mol (Table 2) which is in good agreement with the theoretical BE for the H2S−TEPA2 species. The association of the ammonium ion with the weakly adsorbed and strongly adsorbed species is proposed as the HS−−H−+NH2−TEPA (Scheme 1c and d) and (HS−) NH3+− TEPA (Scheme 1f) structures, respectively. The TEPA structures containing the ammonium ion (protonated amine site) are indicated by the label NH3+−TEPA. The calculated BE listed in Table 2 shows that the weakly adsorbed HS−-H-+NH2TEPA species slightly increased (−5.89 kcal/mol) as compared to the H2S-TEPA structure (−5.71 kcal/mol). The increase in BE could be caused by the electrostatic interactions resulting from the induced partial positive and negative charges on the nitrogen and sulfur atoms, respectively. In contrast, the strongly adsorbed H2S could be in the form of (HS−) NH3+-TEPA species (Scheme 1f) which gave a significantly high BE (−139 kcal/mol, Table 2). The optimized structure in Scheme 1f revealed the interaction of the HS− species with the ammonium ion site (+N−H)···SH− gave the intermolecular distances, 3.397 Å (N+···S) and 2.026 Å (N−H), correspond to a strong hydrogen bond. The hydrogen bonding of this type (N−H···O) has an approximate strength of 1.91 kcal/mol85 in a neutral cluster. The hydrogen bonding coupled with the electrostatic attraction between the oppositely charged ions could lead to stronger BE for the adsorbed HS− species as compared to the H2S species. Thus, we conclude the strongly adsorbed species observed in Figure 3c is in the form of HS− binding at the ammonium sites (N1: −139 kcal/mol, N2: −131 kcal/mol, N3: −129 kcal/mol listed in Table 2). These strongly adsorbed species have the capacity to deactivate the TEPA molecule for further H2S adsorption. Although the calculated BE for the strongly adsorbed species is higher than the BE of those species desorbed (Figure 4), at a regeneration temperature of 100 °C. These results are qualitatively consistent which shows the species adsorbed at the N2 and N3 sites desorb more readily than those at N1 sites, as shown in Figure 3b. The theoretical vibrational modes for the TEPA, H2S− TEPA, HS−−H−NH3+−TEPA, and (HS−) NH3+−TEPA complexes in Figure 5a−f reveals that the type of adsorbing species did not significantly alter the simulated IR bands of the amine adsorption sites (N1, N2, or N3). Comparing the BE for the adsorbed H2S on these amine sites showed minor variation of approximately 0.33 kcal/mol. The similarity in the simulated IR spectra (Figure 5b and c) may be due to the weaker interaction of H2S as a hydrogen bonded complex. The broad band at 2559 cm−1 in Figure 5g, replotted from in situ ATR-IR in Figure 3a, is assigned to the (HS−) NH3+-TEPA species of which simulated IR gives the band at 2378 cm−1 (Figure 5f).

Figure 5. Simulated infrared spectra for the optimized structures at the B3LYP-6-31G** level for (a) TEPA, (b) H2S−TEPA (N1), (c) H2S− TEPA (N2), (d) H2S NH3+−TEPA (N1), (e) H2S NH3+−TEPA (N3), (f) (HS−) NH3+−TEPA (N1), (g) ATR-IR absorbance spectra during H2S adsorption over pure TEPA at 50 °C.

The broad 2559 cm−1 band reflected the presence of hydrogen bonding from neighboring adsorbed species while the single sharp simulated IR peak at 2378 cm−1 could be the result of the isolated adsorbing species, e.g., a lack of neighboring adsorbed species in the simulated structures in Scheme 1. The presence of the (HS−) species at the amine active site may lead to the increased amine reactivity for oxidation deactivation in the TEPA layer, as proposed in Scheme 2. The ammonium ion site which is associated with an ionized (HS−) species could be liable to oxidation during thermal regeneration, producing the −NO2 species. In addition, the redox potential for quaternary ammonium86 is higher than those for aliphatic amines,78 and so it is reasonable to conclude the ammonium is more likely to oxidize. At the same time, C−H could be oxidized to form a species containing CO at 1695 cm−1, a species that has been reported to be formed during the oxidative degradation of TEPA.34 Thus, we can conclude that deactivation of amine sites can occur via both the oxidation and the strongly adsorbed H2S species. 3.4. IR Studies during H2S Adsorption over the Blended TEPA−PEG System. Figure 6 shows the simulated G


Article

The Journal of Physical Chemistry C Scheme 2. Mechanism for Amine Deactivation during H2S

The absorbance spectra for adsorbed H2S, following 4.0 min H2S exposure to the TEPA−PEG thin films with varying compositions, are shown in Figure 7. It should be noted that 4.0 min H2S exposure resulted in a nearly steady H2S coverage where the intensity of adsorbed H2S remained constant. The spectral features of adsorbed H2S on the blended TEPA−PEG thin films (Figure 7b−f) are very similar to TEPA (Figure 7a) in wavenumber with varying intensities. Increasing the PEG loading of the TEPA−PEG film to 23 wt % increased the IR intensities of adsorbed H2S species at 980, 1097, 1482, 1548, 1604, and 2559 cm−1. The positive effect of PEG on increasing the H2S adsorption capacity is similar to what has been shown in other studies during CO2 adsorption using PEG blended with the aliphatic amine.36,60,76 Further increasing PEG loading beyond 23 wt % led to a decrease in the IR intensities of adsorbed H2S and amine efficiency. The simulated infrared spectra for the optimized structures for TEPA−PEG and adsorbed H2S at the B3LYP/6-31G** level, are shown in Figure 8. The theoretical vibrational modes are summarized in Table 3. H2S adsorption at the N1 location and N2 location in the TEPA−PEG system produces IR bands at 2524 cm−1 and 2223 cm−1 for the ν(S−H)···O−H and ν(S− H)···N2−H vibration, respectively, shown in Figure 8b and c, respectively. The wavenumbers of these two sharp peaks fall in the range of the ATR-IR experimentally determined broad band centered at 2559 cm−1. The pure TEPA and blended TEPA−PEG thin films were evaluated for the multicycle adsorption and thermal regeneration (at 100 °C) testing to study the H2S adsorption capacity and regeneration efficiency of the sorbent. Mass spectroscopic analysis of the ATR-IR reactor effluent during regeneration of the pure TEPA layer produced 0.71 μmol H2S/g sorbent

and experimental ATR-IR spectra of TEPA blended with PEG. The vibrational modes for the TEPA, TEPA−PEG, H2S− TEPA−PEG, HS − −H−NH 3 + −TEPA−PEG, and (HS − ) NH3+−TEPA−PEG complexes are summarized in Table 3. The optimized TEPA-PEG structures at the B3LYP/6-31G** level in Scheme 3a reveal their intermolecular interaction occurs via hydrogen bonding. PEG’s hydroxyl group interacts with the long pair electron at the amine site: O−H···NH2 at N1 location and N2−H···O for N2 and N4 locations; where the oxygen atoms in the aliphatic backbone of the PEG molecule interact with H of the amine site. The calculated BE for the TEPA-PEG interaction is −16.36 kcal/mol which is consistent with the formation of three strong hydrogen bonds.76 The simulated IR spectra for the optimized TEPA-PEG structure, shown in Figure 6c, reveals N1 in TEPA−PEG interaction produced a strong IR intensity for the ν(O−H)···NH 2 interaction at 3457 cm−1, further confirming our previously proposed hydrogen bonding between the NH2/NH groups of TEPA and the −OH groups of PEG.76 The strong IR intensity for the simulated ν(O−H)···NH2 IR peak at 3547 cm−1 is also consistent with the observation of the broad band at 3180 cm−1 in experimental ATR-IR absorbance spectra for the five TEPAPEG thin films, shown in Figure 6d. The high wavenumber of the simulated IR peaks compared to those experimental IR peak is caused by the overestimation of the calculated normal mode, as discussed in Section 3.1. The DFT models (not shown) also showed PEG entanglement with TEPA, where the N2/N3/N4 sites are stereocenters, where PEG interaction was dependent upon the TEPA conformation. PEG entanglement and conformation may affect the H2S adsorption capacity due to a loss in specific capacity by diluting amine sites with the polyether. H


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Table 3. Infrared Experimental and Theoretical Normal Modes of Vibration for the Optimized Structures at the B3LYP/6-31G** Level wavenumber (cm−1) (theoretical)

(experimental)

description

PEG 1167 3023, 3106 3841 799b,c 873a 1167 1167 1503 1666 2928 3457 3841

1666 2223 2524 3274 3431b, 3468a b

1487 , 1592

a

3172a, 3265b 3182 3483 3487 2222a 3494a 3664a

Figure 6. Simulated infrared spectra for the optimized structures at the B3LYP-6-31G** level for (a) TEPA, (b) PEG, (c) PEG−TEPA, and (d) IR absorbance spectra, effect of PEG on the 4 μm TEPA thin film. The PEG Loading at 323 K, (i) 9.1 wt %, (ii) 16.7 wt %, (iii) 23.0 wt %, (iv) 29.0 wt %, and (v) 33.3 wt % PEG/TEPA. Background spectra taken from clean ATR-IR window. The absorbance spectrum was obtained by subtracting the background spectra of the clean ZnSe ATR-IR window from the IR spectra during injection of the TEPA and PEG ethanol solutions onto the ZnSe window.

a

ν(C−O) ν(C−H) ν(O−H)

1119 2928 3180−3400 TEPA−PEG 980 1119 1119 1590 1672 2928 3180−3365 3180−3500 H2S−TEPA−PEG 1302 1630

w(N2−H) w(N1−H) ν(C−O) ν(N1−C) w(N2−H) δ(N1−H) scissor ν(C−H) ν(O−H)···NH2 ν(O−H) w(+N2−H)···SH2 δ(N−H) ν(S−H)···N2−H ν(S−H)···O−H ν(O−H)···NH2 ν(N2−H)···O−H

HS−−H−NH3+−TEPA−PEG 1548a w(+N1−H) b,c 1482 w(+N2−H) 2559 ν(+N1−H)···SH− ν(+N2−H)···O−C ν(O−H)···NH2 ν(+N1−H) (HS−) NH3+−TEPA−PEG 2559 ν(+N−H)···SH− ν(O−H)···NH2 ν(O−H)···SH−

Notes: Amine site locations on TEPA: N1 or N5. bN2 or N4. cN3.

capacity for H2S adsorption, similar to what was observed during the CO2 solubility studies. Examination of the results listed in Table 4 further revealed that the addition of PEG at the amine site increased both the amount of strongly adsorbed H2S, improved the amine efficiency, and decreased the fraction of weakly adsorbed species. Increasing the PEG loading beyond 23.0 wt % also resulted in a decrease in both the H2S adsorption capacity, decreased the amine efficiency, and significantly increased the fraction of weakly adsorbed species. The MS analysis reveals the maximum amine efficiency of 0.38 (23 wt % PEG) is in good agreement with the literature for both the H2S and CO2 adsorption over amine based sorbents. As a result, the high amine efficiency case at 23 wt % PEG loading was further studied using the ATR-IR spectroscopy. In addition, comparison of the literature for CO2 capture (Table 4) the PEG/PEI sorbents with similar OH/amine ratio (0.08) to the 9.1 wt % PEG/TEPA (0.04) produced very similar capture capacities. Figure 9a show the ATR-IR absorbance spectra during H2S adsorption over the 23 wt % TEPA−PEG thin film at 50 °C for cycle no. 1, Figure 9b during flowing N2, and Figure 9c during H2S thermal regeneration from 50 to 100 °C. Exposure of the TEPA-PEG thin film to H2S led to a decrease in IR intensity for

(Table 4). The amount of H2S/g sorbent was obtained by dividing the amount of H2S in the reactor effluent during regeneration by the weight of the TEPA and PEG loaded onto the ZnSe window. The amount of weakly adsorbed H2S was determined to be 1.85 mmol/g sorbent, revealing an H2S fraction 0.72 was weakly adsorbed, and consuming approximately 34% of the amine sites in the TEPA layer. The amount of H2S adsorbed and regenerated on the TEPA thin film is consistent with others during H2S adsorption over amine-based sorbents. In addition, the H2S amine efficiencies for the strongly adsorbed H2S are very similar to those in the literature for CO2 adsorption.53,87,88 CO2 solubility studies on TEPA (1.916 mol-CO2/·molabsorbent) have shown to be higher than those into MEA (0.512 mol-CO2/·mol-MEA) and MDEA (0.478 mol-CO2/· mol-MDEA).89 H2S solubility studies report 0.362 mol-H2S/ mol-MEA90−92 in a 15.3 wt % MEA solution. These results suggest that TEPA having multiple amino groups improved I


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Scheme 3. Optimized Structures at the B3LYP/6-31G** Level for the H2S and TEPA−PEG Interaction for (a) PEG−TEPA, (b) H2S−TEPA−PEG (N1), (c) H2S−TEPA−PEG (N2), (d) H2S−NH3+−TEPA−PEG (N1), (e) H2S−NH3+−TEPA−PEG (N2), (f) HS−−NH3+−TEPA−PEG (N1), and (g) HS−−NH3+−TEPA−PEG2 (N1)

ν(N1−H) vibrations at 3184, 3309, and 3365 cm −1 , corresponding to the H2S reaction at the amine site, similar to H2S adsorption in the pure TEPA thin film (Figure 3a). The presence of H2S at the amine site also led to an increase in IR intensity at 1630 cm−1 for the δ(N1−H) deformation vibration, at 1548 cm−1 for the w(+N1−H) wagging vibration, at 1482 cm−1 for w(+N2−H) vibration, at 1302 cm−1 for the w(N2− H)···SH2 vibration, at 1097 cm−1 for the ν(N1−C) vibration,

and at 980 cm−1 for the w(N1−H) vibration, which is characteristic for the H2S interactions at all of the TEPA amine sites. In addition, the H2S exposure to the TEPA−PEG layer also resulted in a broadband IR intensity centered at 2559 cm−1 (Figure 9a), similar to but with greater intensity, as compared to the pure TEPA layer in Figure 3a. Switching the reactant gas flow from 1% H2S balance He to 100% N2 (Figure 9b) also resulted in a decrease in the IR J


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Figure 7. IR absorbance spectra during H2S adsorption at 323 K over (a) TEPA, (b) 9.1 wt % PEG/TEPA, (c) 16.7 wt % PEG/TEPA, (d) 23.0 wt % PEG/TEPA, (e) 29.0 wt % PEG/TEPA, and (f) 33.3 wt % PEG/TEPA sorbents. The absorbance spectrum was obtained by subtracting the background spectra from the clean TEPA and TEPA− PEG layers prior to H2S exposure.

intensity indicating the removal of a weakly adsorbed H2S species, similar to the pure TEPA case. Decreasing the partial pressure of H2S over the H2S saturated TEPA-PEG (23 wt %) layer resulted in a decrease in IR intensity at 2559 cm−1 also suggesting that the weakly adsorbed H2S had a role in forming a HS−-H-NH3+-TEPA-PEG ammonium ion. The comparison of the IR intensity for the weakly adsorbed H2S to the IR intensity during thermal regeneration reveals a fraction of weakly adsorbed H2S of 0.61. The regeneration of the H2S saturated TEPA-PEG layer was facilitated by heating the ATR-IR window to 100 °C (Figure 9b). Heating the thin film produced a greater decrease in IR intensities in the 1600−1100 cm−1 region as compared to the pure TEPA (Figure 3c) indicating PEG ability to improve the regeneration of the amine site. Figure 10 shows the difference spectra during H2S adsorption for cycle no. 1, 2, and 3. These results suggest the PEG interactions at the amine site facilitate the regeneration of the amine site returning the amine back to its original state. The calculated BE for the weakly adsorbed and strongly adsorbed species listed in Table 2 are generally consistent with the experimentally observed spectroscopic ATR-IR data, shown in Figure 9. The effect of PEG on the amine site is consistent with the qualitative trend in calculated BE for adsorbed H2S and the formation of a hydrogen-bonded weakly and strongly adsorbed H2S on TEPA-PEG. The H2S−TEPA−PEG structure (Scheme 3b and c) resulted in BE of −6.34 kcal/mol and −5.12 kcal/mol. The experimental BE was determined to be −7.55 kcal/mol (Table 2), which is in good agreement with the theoretical value. The presence of PEG at the amine site has the effect of decreasing the binding energy of the strongly adsorbed (HS−) (−127 kcal/mol) as compared to the pure TEPA (−139 kcal/mol). Similar to the pure TEPA case, the BE is affected by

Figure 8. Simulated infrared spectra for the optimized structures at the B3LYP-6-31G** level for (a) TEPA−PEG, (b) H2S−TEPA−PEG (N1), (c) H2S−TEPA−PEG (N2), (d) H2S NH3+−TEPA−PEG (N1), (e) H2S NH3+−TEPA−PEG (N2), (f) (HS−) NH3+−TEPA− PEG (N1), (g) ATR-IR absorbance spectra during H2S adsorption over 23 wt % TEPA−PEG at 50 °C.

the electrostatic interactions of the neutral optimized (HS−) NH3+-TEPA-PEG structure. PEG influences the adsorption mechanism by decreasing BE of H2S at the ammonium ion site and thus making the strongly adsorbed species less thermally stable. The simulation results suggest by the +N−H···OH interaction, the OH in PEG acts as an electron-donating group decreasing the electrostatic charge of the ammonium ion. The DFT simulation results reveal the decrease in BE for the (HS−) NH3+−TEPA−PEG system (Scheme 3f) as compared to the (HS−) NH3+−TEPA (Scheme 1d), is the result of the decrease in the electrostatic charge at the ammonium ion adsorption site. The presence of PEG has the effect of decreasing the electrostatic cation−anion attraction, resulting in a decrease in BE of the strongly adsorbed species. The experimental data also show a trend in the effect of PEG on TEPA decreasing the BE from −10.19 to −7.55 kcal/mol, consistent with the theoretical results. The PEG effect on the amine active site was further simulated by adding a second PEG molecule as shown in Scheme 3g, the (HS−) NH3+-TEPA-PEG2 structure. The optimized TEPA-PEG2 structure shows two PEG molecules K


Article

The Journal of Physical Chemistry C Table 4. Literature Comparison for H2S or CO2 Capacity and Amine Efficiency experimental cond. support

amine type

PEG (mmol)

ZnSe ZnSe ZnSe ZnSe ZnSe ZnSe SiO2 MCM-41 SBA-15

TEPA TEPA TEPA TEPA TEPA TEPA PEI PEI PEI

0.00 1.15 2.29 3.44 4.58 5.73

ZnSe ZnSe ZnSe ZnSe ZnSe ZnSe

TEPA TEPA TEPA TEPA TEPA TEPA

0.00 1.15 2.29 3.44 4.58 5.73

SIO2 SiO2 SBA-15 SiO2 MSU-F beta zeolite SiO2 Al2O3 MCM-48 MCM-48 PE-MCM41 PE-MCM41 SBA-15 SBA-15b SBA-15b SBA-15 SBA-15 SBA-15 SBA-15 SBA-15 SBA-15 SBA-16 SBA-12 HMSd HMSd CNTs CNTs

TEPA TEPA PEI PEI TEPA TEPA TEPA TEPA APTES APTES DAEAPTS

6.27 27.84 1.78 0.4 × 10−3

a

× × × × ×

× × × × ×

−4

10 10−4 10−4 10−4 10−4

10−4 10−4 10−4 10−4 10−4

amine (mmol) 1.09 1.09 1.09 1.09 1.09 1.09 23.3 1.77 1.18

× × × × × ×

1.09 1.09 1.09 1.09 1.09 1.09

× × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3

10−3 10−3 10−3 10−3 10−3 10−3

6.27 62.12 5.75 0.4 × 10−3 14.16 1.32 0.50 0.29 2.3 2.3 7.95

H2S adsorption capacity (mmol/g sorbent)a H2S Strong Ads. 0.71 1.10 1.06 1.06 0.38 0.34 1.27 1.84 1.98 H2S Weakly Ads. 1.85 1.60 1.59 1.63 3.76 3.76 CO2 Ads. 2.436 445.14 0.79 1.53 5.39 0.82 0.40 0.07 2.05 1.14 2.65

p* (atm)

ads. T (°C)

peak T (°C)

amine efficiency (H2S/TEPA)

ref

0.011 0.011 0.011 0.011 0.011 0.011 0.01 0.04 0.04

50 50 50 50 50 50 22 22 22

100 100 95 97 75 72 75 100 100

0.15 0.28 0.32 0.38 0.16 0.15 0.01 0.26 0.42

87 53 88

0.011 0.011 0.011 0.011 0.011 0.011

50 50 50 50 50 50

50 50 50 50 50 50

0.34 0.30 0.30 0.30 0.70 0.70

1 0.22 0.04 1 1 0.25 0.25 0.25 1 0.05 0.05

25 55 30 45 40 30 30 30 25 25 25

100 115 110 45 225 104 135 125 90 90 100

0.08 0.93 0.14

34 60 93

0.08 0.12 0.16 0.05 0.89 0.50 0.11

51 94 94 94 46 46 95

DAEAPTS

7.8

2.28

0.05

70

150

0.10

47

DAEAPTS AEAPSf DAEAPTS AEAPTS APTES aziridine aziridine aziridine aziridine AEAPTS APTES APTS DAEAPTS APTES AEAPTS

5.1 4.61 5.8

1.1 1.36 1.58 1.95 1.53 5.55 4.0 1.98 3.11 1.4 1.04 1.59 1.34 1.32 2.59

0.15 0.15 0.15 1 0.1 0.1 0.1 0.1 0.1 1 0.1 0.9 0.9 0.15 0.5

60 60 60 22 25 25 75 75 25 27 25 20 20 20 20

150 150 150 150 110 115 115 130 130 120 110 150 150 120 100

0.07 0.30 0.09

96 74 74 97 98 48 48 52 52 99 98 49 100 101 50

2.72 9.78 9.78 7 7 0.76 2.76 2.29 4.57

0.56 0.57 0.14 0.28 0.15 0.92 0.38 0.69 0.10

mmol/g sorbent calculated using total weight of TEPA and PEG on ZnSe window.

DFT results which indicate a trend in decreasing theoretical BE on increasing PEG loadings, (HS−) NH3+−TEPA (−139 kcal/ mol) > (HS−) NH3+−TEPA−PEG (−127 kcal/mol) > (HS−) NH3+−TEPA−PEG2 (−104 kcal/mol). The better performance from increasing PEG exceeds the loss in specific capacity due to diluting amine sites with PEG as shown in Table 4 at 9.1, 16.7, and 23 wt % loading. The decrease in capacity on higher PEG loadings is believed to occur by both dilution of the amine active site and layering, indicated by FTIR analysis of the prepared sorbents in Figure 6d. The qualitative aspect of this

interacting at the ammonium ion through hydrogen bonding and electron donating effects. The second PEG molecule further decreased the electrostatic charge of the active site and further decreasing the BE (−104 kcal/mol, Table 2). These DFT results suggest PEG improves the regenerability of the strongly adsorbed species. Experimental multicycle testing confirmed PEG ability to increase the H2S adsorption capacity of TEPA (Table 4) .The MS H2S profiles in the reactor effluent (not shown) also indicated PEG had the effect of lowering the peak H2S regeneration temperature. This is consistent with the L


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4. CONCLUSIONS The ATR-IR spectroscopy revealed the reaction of H2S at the pure TEPA amine site resulted in the fixation of sulfur as hydrogen bonded HS− species. The deactivation of the TEPA thin film was indicated by the decrease in IR intensity for the ammonium species and the formation of −NO2 and CO species in the presence of oxygen through multicycle in situ ATR-IR testing. The presence of PEG modified the electronic and geometric environment around the amine binding site through hydrogen bonding and electron donating effects of the OH groups of neighboring PEG molecules with the amine site. The presence of PEG decreased the (+) charge of the ammonium adsorption site effectively decreased electrostatic anion−cation interaction and decreasing BE of the strongly bound HS− species at the amine active site. In addition, MS analysis during the temperature-programmed desorption revealed PEG increased the adsorption capacity of the TEPA sorbent from 0.71 μmol of H2S to 1.10 μmol of H2S at 9.1 wt % loading PEG in the TEPA thin film, increasing the H2S adsorption capacity by 1.5 times. The infrared, MS, and computational data suggests that the PEG is effective for modifying the amine active site and for improving the adsorption capacity of TEPA during the H2S adsorption process.

Figure 9. IR absorbance spectra during (a) H2S adsorption over 23.0 wt % TEPA−PEG sorbent at 50 °C, the background spectra taken prior to H2S adsorption, (b) during flowing N2, the background spectrum was taken prior to flowing N2, and (c) temperatureprogrammed desorption at 50−100 °C, the background spectrum taken from the H2S saturated TEPA−PEG layer.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: (+1) 304-285-5292. Fax: (+1) 304-285-4403. E-mail: [email protected]. *Tel: (+1) 330-972-6993, Fax: (+1) 330-972-5856. E-mail: [email protected]. 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 Grant DE-FE0001780 and DE-FC26-07NT43086.

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REFERENCES

(1) Razavi, S. M. R.; Shirazian, S.; Najafabadi, M. S. Investigations on the Ability of Di-Isopropanol Amine Solution for Removal of Co2 from Natural Gas in Porous Polymeric Membranes. Polym. Eng. Sci. 2015, 55, 598−603. (2) Huang, K.; Cai, D.-N.; Chen, Y.-L.; Wu, Y.-T.; Hu, X.-B.; Zhang, Z.-B. Dual Lewis Base Functionalization of Ionic Liquids for Highly Efficient and Selective Capture of H2s. ChemPlusChem 2014, 79, 241− 249. (3) Moioli, S.; Giuffrida, A.; Gamba, S.; Romano, M. C.; Pellegrini, L.; Lozza, G. Pre-Combustion CO2 Capture by Mdea Process in Igcc Based on Air-Blown Gasification. Energy Procedia 2014, 63, 2045− 2053. (4) Novochinskii, I. I.; Song, C.; Ma, X.; Liu, X.; Shore, L.; Lampert, J.; Farrauto, R. J. Low-Temperature H2S Removal from SteamContaining Gas Mixtures with Zno for Fuel Cell Application. 1. Zno Particles and Extrudates. Energy Fuels 2004, 18, 576−583. (5) Dalai, A. K.; Tollefson, E. L. Kinetics and Reaction Mechanism of Catalytic Oxidation of Low Concentrations of Hydrogen Sulfide in Natural Gas over Activated Carbon. Can. J. Chem. Eng. 1998, 76, 902− 914. (6) Ghosh, T. K.; Tollefson, E. L. A Continuous Process for Recovery of Sulfur from Natural Gas Containing Low Concentrations of Hydrogen Sulfide. Can. J. Chem. Eng. 1986, 64, 960−968.

Figure 10. IR difference spectra of adsorbed H2S in adsorption cycles no. 1, 2, and 3, over 23 wt % TEPA−PEG at 50 °C. The absorbance spectra during H2S adsorption was obtained from the clean TEPA− PEG layer prior to exposure to the H2S reactant gas. The absorbance spectrum from cycle no. 1 was subtracted from the absorbance spectrum at 4.0 min for cycle nos. 1, 2, and 3.

study suggests a way of using PEG to tailor the amine adsorption site of the TEPA sorbent. PEG has shown to be an effective additive for improving the TEPA sorbent and for preserving the amine active site during the H2S acid gas removal process. M


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