Porous Solids Impregnated with Task-Specific Ionic Liquids as

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The Journal of Physical Chemistry

Porous Solids Impregnated with Task-Specific Ionic Liquids as Composite Sorbents

K. Neil Ruckart† , Richard A. O’Brien‡ , Seth M. Woodard† , Kevin N. West† , T. Grant Glover†,∗

†

University of South Alabama Department of Chemical and Biomolecular Engineering 150 Jaguar Dr. Mobile, AL, 36688 ‡

University of South Alabama Department of Chemistry 6040 USA Drive South Mobile, AL, 36688

∗

Author to whom correspondence should be addressed: University of South Alabama Department of Chemical and Biomolecular Engineering 150 Jaguar Dr. Mobile, AL, 36688 Tel.: (251) 460-7462 FAX: (251) 461-1485 e-mail: [email protected]

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Abstract

This work examines six task-specific ionic liquids (TSILs), comprised of the taurinate anion paired with five tetraalkylammonium cations (where alkyl = methyl, ethyl, propyl, butyl, and hexyl) and a tetrabutylphosphonium cation, impregnated in ordered mesoporous silica, SBA-15. The composites showed significantly increased CO2 uptakes relative to the parent SBA-15 materials. The surface area of these materials varies from approximately 5 to 1000 m2 /g depending on the amount of IL loaded into the silica pores. The presence of n-hexyl side chains on the TSIL significantly reduces water loading, indicating that judicious IL selection may provide a means of controlling water uptake. After exposure to water vapor, three of the six cation-taurinate composites displayed an increase in CO2 capacity. X-ray diffraction data of the composite of tetramethylammonium taurinate and SBA-15 shows that the ionic liquid is crystalline inside the pores of the silica. Isotherms are measured at several different temperatures and the results show that storage at ambient humidity significantly impacts the capacity of these materials. By comparison, the TSILs supported on an amorphous porous support, BPL activated carbon, showed no increase in CO2 adsorption capacity. The results provide physical insight into the synthesis, structure, porosity, and sorption capacity of composites of adsorbents and ionic liquids that need to be considered prior to application. Keywords: adsorption, carbon dioxide, task-specific ionic liquid, SBA-15

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1. Introduction The development of novel adsorbent materials as solutions to complex separations problems has increased significantly over the past decade with a multitude of different types of materials introduced to the field, such as MOFs, ZIFs, GOFs, and PIMs.1–4 Because porous materials can be tailored for particular applications, and because porous materials are being considered for a variety of processes, many materials have been reported in the literature. For example, MOFs have been examined as a CO2 capture technology, as means to store hydrogen, as alternatives to activated carbon for gas filtration, and for a variety of other applications.1, 5 These separations processes take place at highly diverse conditions. For example, CO2 capture from flue gas requires evaluation of the novel materials at humidity and temperatures common to post combustion flue gas streams.6, 7 Whereas, CO2 capture in processes such as acid gas removal in natural gas processing, biogas upgrade, and to maintain life in closed cabin environments, such as spacecraft and submersible vessels, take place at very different conditions.8–11 While high CO2 capacity is a critical evaluation parameter for novel sorbent materials regardless of application, other properties including mass transfer, CO2 selectivity, water stability, heat of adsorption, and long-term chemical stability must also be considered.12 In regards to CO2 capture research, amine functionalized materials, as shown in Scheme 1A, are of particular interest.13–15 Amines have shown the ability to chemically fix CO2 through carbamate formation. Upon initial reaction with CO2 , the amine forms a zwitterion which can then protonate a second amine resulting in a theoretical maximum capacity of 1:2 mole of CO2 : mole of amine, or an amine efficiency of 0.5 mole CO2 per mole of amine. The presence of water has been shown to influence this ratio though the formation of bicarbonate, as shown in Scheme 1B. Amine-CO2 capture technologies commonly employ liquid amine solutions, primar-



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ily mixtures of monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and diisopropanolamine (DIPA), in various water concentrations. These applications have well documented drawbacks including insufficient CO2 capture capacity, high energy input requirements during regeneration, high solvent losses due to evaporation, chemical degradation and corrosion problems.16–19 In addition to liquid amine absorbents, a wide range of solid adsorbent materials have been studied. Of these, activated carbons and zeolites have been the most prevalent in a variety of industrial applications. Zeolite 13X, an aluminosilicate material consisting of a microporous network with high surface sites and high partial charge densities, provides capacities greater than 3 mol per kg at a low, ≈ 10 kPa, CO2 partial pressures. However, the capacity of these materials is drastically decreased in the presence of water and therefore in a post combustion CO2 capture or air revitalization processes, the CO2 rich stream must be dehumidified prior to adsorption.20, 21 Since their discovery, ordered mesoporous silicas with long-range ordered pore structures and high surfaces areas, such as MCM-41 and SBA-15, have been extensively studied as solid adsorbents.22, 23 While these materials have shown high CO2 capacities at high pressures, up to 35 bar, weak adsorbate-adsorbent interactions at low pressures, coupled with low selectivity, inhibit the use of ordered mesoporous silica in many adsorption and separation applications.7, 24 Therefore, research on ordered mesoporous materials has examined the modification of the internal pore surface by various substrates to increase selectivity and adsorptive capacity. To increase CO2 capture capacities by MCM-41, SBA-15, and many other porous materials, impregnation with amine containing compounds by physical adsorption or covalent grafting has been examined. Li et al. classified these materials into three classes: Class 1 adsorbents encompass materials in which polymeric amines such as poly(ethyleneimine), PEI, are physically impregnated onto the outer surface or inner pores of various solid supports;25–28


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Class 2 adsorbents consist of materials that covalently bind alkyl amine chains to oxide surface sites of various porous and nonporous materials through silane bonds;9, 21, 29–35 and Class 3 materials, first reported by Hicks et al., combine both Class 1 and 2 materials through the acid catalyzed ring opening polymerization of aziridine tethering polymeric amines to mesoporous silicas.12, 36–39 With negligible vapor pressures and relatively high molar capacities for CO2 , ionic liquids (ILs) have received considerable attention in CO2 absorption processes research.7, 40 In their most simplistic form, ILs are molten salts often comprised of a polyatomic cation and/or anion with melting points below 100 o C. In addition to their nonvolatile nature, conventional ionic liquid (IL) properties commonly include wide liquidus range, low melting points, high thermal and chemical stability, low energies of regeneration for CO2 capture, and have tunable properties based on cation-anion pair selection. The process by which CO2 is captured by conventional ionic liquids is a purely absorptive dissolution process with the specific cation-anion pairings and higher degrees of fluorination influencing CO2 solubility. Furthermore, Brennecke et al. have stated the physical solubility of CO2 in ionic liquids can be improved by increasing the alkyl chain length on either the anion or cation components of the designed materials.7, 41–44 Task-specific ionic liquids (TSILs) with a covalently bound amine functionality for CO2 capture were first reported by Bates et al. and are analogous to alkanolamine liquids.45 Utilizing the alkyl side chains of the cation derivatives, amine functionalities can increase the CO2 solubility through the same carbamate formation mechanism seen in Scheme 1A while maintaining the desirable properties of their conventional IL counterparts.45, 46 In addition to cation tethering, a variety of amino acid derivatives have been studied as ionic liquid anions for CO2 capture.46–48 Under dry conditions, certain amine containing anions have been shown to chemically fix CO2 as carbamic acid rather than the carbamate.49, 50 This mechanism seen in Scheme 1C, indicates a 1:1 ratio of CO2 to TSIL is theoretically possible.



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However, CO2 capture from a rigorously dried stream is impractical in many applications.51 While TSILs maintain the desirable properties of conventional ILs, most importantly non-volatility, chemical stability, and reusability, the addition of functional groups often leads to a significant increases in IL viscosity, a phenomena that can be further increased during the chemical fixation of CO2 .52, 53 Molecular simulations by Gutowski and Maginn concluded that the glassy or gel-like substance formed by amine functionalized ILs upon exposure to CO2 was attributed to the formation of hydrogen bonding networks during the ammonium carbamate fixation of CO2 .54 Goodrich et al. have shown that the presence of water in phosphonium based amino acid ILs decreases the viscosity of both the neat and CO2 -complexed TSIL while only significantly decreasing CO2 capacities when large amounts of water are present.55 In addition to viscosity limitations, a significant number of quaternary ammonium salts are crystalline solids at room temperature due to the symmetric nature of the cations, hindering CO2 capture rates due to diffusion limitations. One method to overcome these limitations is to immobilize viscous ILs onto solid supports to produce supported ionic liquids (SILs).56, 57 SILs have been produced using a variety of porous and nonporous solid supports and have been primarily studied as novel catalysts.58 Only a few works have examined supported task-specific ionic liquids as composite adsorbents.59–64 Specifically, Zhang et al. first reported the use of tetrabutylphosphonium amino acid ILs supported on silica gels with fast, reversible CO2 adsorption with high amine efficiency, nearly 1:1 molar ratio CO2 :TSIL in the presence of 1% water by weight.65 Absent from these works is a detailed study examining CO2 sorption as a function of the amount of TSIL contained in the porous support and the TSIL structure. Additionally, water isotherms and details about the impact of water exposure on CO2 capacity for these materials are limited. Many of the porous materials mentioned previously have been extensively studied, which makes it easier to identify which separations challenges the ma-


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terials are well suited to solve. However, the lack of details about the sorption behavior of composites of ionic liquids and silicas prevents identifying where they are best applied. Therefore, this work provides a preliminary and thorough study of the utilization of the ordered mesoporosity of SBA-15 as a structural support to overcome crystallinity, high viscosities and mass transfer limitations associated with amine functionalized task-specific ionic liquids for CO2 sorption. Five tetraalkylammonium cations, where alkyl = methyl, ethyl, propyl, butyl, and hexyl, were selected as the cation species and paired with taurine. Additionally, tetrabutylphosphonium taurinate was examined as a cation due to the higher thermal stabilities and decreased viscosities commonly associated with phosphonium based TSILs. This work provides a detailed study of these composite adsorbent materials by examining the pure CO2 capacity as a function of amount of impregnated TSIL and cation alkyl chain length and sorption temperature. The impact of exposing these composites to water vapor prior to CO2 sorption is also examined. 2. Experimental 2.1 Materials Methanol (99.9% HPLC), Pluronic-123, hydrochloric acid ( 37 wt %), taurine ( 99%), tetramethylammonium hydroxide (25 wt % in H2 O), tetrabutylammonium hydroxide (40 wt % in H2 O), and tetraethyl orthosilicate (GC, 99%) were purchased from Sigma-Aldrich and used as delivered. Tetraethylammonium hydroxide (25 wt % in H2 O) and tetrapropylammonium hydroxide (25 wt % in H2 O) were purchased from Acros Organics. SBA-15 was purchased from ACS Materials and also synthesized in house following the procedure below. UHP nitrogen, research grade carbon dioxide, and Zero air were used for isothermal adsorption and thermogravimetric analysis experiments. 2.2 Task-Specific Ionic Liquid Synthesis The TSILs were prepared by the neutralization of the tetraalkylammonium hydroxide



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and tetrabutylphosphonium hydroxide solutions with 1:1 stoichiometric ratios to taurine. The solutions were stirred for 24 hours at ambient conditions. After ensuring the mixture was homogenous, the material was placed on a rotary evaporator and the excess solvent was removed under vacuum at 100 o C. The synthesized ionic liquids and acronyms are shown in Fig. 1. Chemical purity was verified using H1 and

13

C NMR and are reported in the

supplemental information (SI). 2.3 SBA-15 Synthesis Procedure Following an established procedure, 3.0 g of Pluronic-123 was heated to approximately 60 o C.66 Next, 22.4 g of deionized water and 90 g of 2M HCl were added and the solution mixed vigorously. When the solution appeared as a cloudy mixture, 6.5 g of tetraethylorthosilicate (TEOS) as a silica source was added and the reaction solution was stirred for 24 hours at 40 o C. The solution was transferred to a Teflon-lined autoclave and placed in an oven at 100 o C for 48 hours. The product was vacuum filtered, washed with deionized water, and allowed to dry in air overnight. The as synthesized product was calcined in a continuous flow of Zero Air at a ramp rate of 1 o C per minute to 540 o C and held for 10 hours. Post calcination, the material was stored in a desiccator. 2.4 Composite Material Synthesis 2.4.1 SBA-15 Supported Ionic Liquid Composite SBA-15 TSIL materials were prepared using the incipient wetness technique for impregnation. Two impregnations were completed to produce the composite materials. In a typical synthesis, 200 mg of SBA-15 was weighed out in a 20 mL scintillation vial. Then 2 mL of methanol was added to the specified amount of TSIL and the solution was gently stirred until the TSIL was dissolved. The total amount of ionic liquid used for the impregnation varied between 10 and 500 mg depending on the targeted amount of IL required in the resulting composite. The amount of IL used to produce various composites at different


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loadings can be found in the SI. The solution was added drop wise to the SBA-15 and the composite was allowed to dry at ambient conditions for 24 hours. The TSIL was added over a two day period with the total amount of ionic liquid divided evenly both days. Before the second impregnation, the material was dried at 65 o C for 2 hours to remove any remaining solvent. The composite was allowed to cool and the impregnation process was repeated. After an additional 24 hours at ambient conditions, the material was recovered by vacuum filtration and washed with approximately 10 mL of methanol to remove any excess ionic liquid on the external surface of the composite material. Washing should be completed carefully as excessive washing, or use of large amounts of methanol, was found to remove portion of the TSIL from the silica mesopore. The impregnated material was dried at 80 o C for 2 hours to remove any remaining solvent. Composites of adsorbents and ionic liquids (or CAsILs) were designated with the following acronyms, CASIL-Solid Support-Batch-TSIL-wt% of TSIL. The solid supports presented in this work are designated as SBA or BPL. Three batches of SBA-15 were used for impregnation; one purchased from a commercial supplier with a batch designation of ACS and two synthesized from the procedure outlined in the experimental section, designated B3 and B4. The type of TSIL in the composite was also included in the name as combinations of taurinate ([Tau]) and one of the five tetra-n-alkylammonium cations, where n= methyl ([TMA]); ethyl ([TEA]); propyl ([TPA]); butyl ([TBA]); hexyl ([THA]); or a tetrabutylphosphonium ([TBP]) cation. As an example, CASIL-SBA-B3-[TBA][Tau]-71 indicates SBA-15, Batch 3, impregnated with 71% by weight tetrabutylammonium taurinate. The weight percent of TSIL was determined by thermogravimetric analysis. 2.4.2 BPL Activated Carbon Supported Ionic Liquid To produce the carbon composites, 1.0 g of BPL carbon was weighed out in a 40 mL vial. Then a solution of a specified amount of tetrabutylammonium taurinate in 5 mL of



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methanol was added to the BPL carbon. The material was placed on a rotary evaporator and rotated for two hours. After two hours the temperature was increased to 65 o C under vacuum and left overnight. The following day, 5 additional mL of methanol was added, rotated under vacuum for 4 hours at 65 o C to evaporate the solvent and 2 hours at 110 o C to remove adsorbed water. The BPL carbon captured all of the IL. 2.5 Differential Scanning Calorimetry Differential scanning calorimetry (DSC), using a TA Q2000, determined the phase change transition temperatures for the neat TSILs. Initially, all TSIL were tested with ramps from -20 o C, for solids at room temperature, and -80 o C, for liquids at room temperature, to 200 o C at a ramp rate of 10 o C per minute in a continuous 50 mL per min flow of nitrogen. This was done to provide a baseline evaluation of the thermal effects for each IL. Once a baseline for each IL was evaluated, more controlled runs were completed to determine thermal transition events. The adaptation of these runs consisted of pre-treatments at temperatures between 90 o C and 150 o C to remove absorbed water. The DSC TSILs experiments were completed in triplicate to ensure reproducibility. DSC curves for the TSILs can be found in the SI. 2.6 Thermogravimetric Analysis Thermogravimetric Analysis (TGA) was performed on a Netzsch TG 209 F1 Iris. For thermal decomposition points of the TSILs, the sample was heated in Zero air from 25-700 o

C at a ramp rate of 1 o C per minute. Using Netzsch Proteus software, the thermal de-

composition onset was determined using the Marsh method.67 Experiments were completed in duplicate and the temperature of onset of decomposition (Td,onset ) were averaged. TGA curves for the neat TSILs can be found in the SI. TGA data also determined the amount of TSIL impregnated in the SBA-15 composites. In a flow of Zero air, the composite was heated from 25-720 o C at a rate of 10 o C per min. Due


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to the extremely hygroscopic nature of several of the TSILs, a 15 minute isothermal hold at 130 o C was performed to ensure remove preadsorbed water. Weight percent of impregnated TSIL was determined by the following equation:

% T SIL Loading = MLost /MT otal = MLost /(MLost + MResidual )

(1)

Where MLost is the weight lost from 130-700 o C and MResidual is the mass remaining at 700 o C. Because many of the TSILs examined are hygroscopic materials and with thermal decompositions above 130 o C, the temperature range ensures preadsorbed water had been removed and the weight lost is attributed solely to the impregnated TSIL. While SBA-15 has a high thermal stability, up to approximately 850 o C, the upper limit of 700 o C ensures the TSIL has been fully oxidized without the SBA-15 support attributing to mass losses.68 The BPL impregnated samples were not analyzed via TGA due to the oxidation of the carbon support. 2.7 Adsorption Isotherms Nitrogen isotherms at 77 K were collected on a Micromeritics ASAP 2020 porosimeter. Prior to analysis, samples were degassed under vacuum for a minimum of 16 hours at 200 o C for pure SBA-15 and 110 o C for pure BPL activated carbon and all composite materials. IL composite samples were out gassed at 110 o C under vacuum overnight. N2 isotherms were used for characterization of pore volume, BET surface area (range 0.05-0.30 P/Po ), and pore widths utilizing the ASAP 2020 software. For pristine SBA-15 and SBA-15 composites, pore widths were determined using the BJH method with KJS correction. CO2 and H2 O isothermal data were collected at 25 o C unless otherwise stated. The sample tube was submerged in a recirculating water bath to maintain a constant temperature. Water isotherms were analyzed using the Micromeritics ASAP 2020 vapor adsorption option. Analysis water was provided by a Milli-Q, 18 MΩ purification system and degassed by the freeze-pump-thaw method. 10 ACS Paragon Plus Environment


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2.8 Powder X-ray Diffraction Powder X-ray diffraction data was gathered using a Rigaku Ultima-III In-Plane Theta:Theta diffractometer equipped with a long-fine focus Copper X-ray tube, parafocusing optics, Ni filter and D/Tex Ultra 250 1D strip detector spanning 0.5 to 10 2-theta degrees at 0.02 degree per step and 6 seconds per step. The PXRD data confirm the structure of SBA-15 and that structure is retained after impregnation with the ionic liquid. The primary SBA-15 peak is present and the remaining peaks are reduced in intensity consistent with other impregnated materials.69–71 3. Results and Discussion 3.1 Physical Properties of Neat TSILs Relatively high melting points, greater than 100 o C, are common properties of short nalkyl chain ammonium based ionic salts due to their symmetric nature and localized charge distribution allowing for the formation of a crystal lattice structure. These materials are often categorized as task-specific onium salts as their melting points are above 100 o C, the widely recognized upper limit for classification as an ionic liquid. For the TSILs of interest in this work, it should be noted that [TMA][Tau], [TPA][Tau], and [TBA][Tau] are crystalline solids at room temperature after the rotary evaporation step and subsequent cooling to ambient temperatures during synthesis. However, each of these salts are extremely hygroscopic and will readily deliquesce in ambient humidity. As prepared, the [TEA][Tau], [THA][Tau], and [TBP][Tau] TSILs appear as liquids at room temperature. Pure TSIL melting points are presented in the SI. While the [TMA][Tau] and [TBA][Tau] TSILs melting points of 103 and 102 o C, respectively, are slightly above the IL limit, the melting points of [TEA][Tau] and [TPA][Tau] are significantly lower 53 and 72 o C. Interestingly, the melting point for [TEA][Tau], 52 o C, indicates this TSIL to be a solid at room temperature. However, the hygroscopic nature of this material readily adsorbs ambient humidity and thus appears as a liquid. In this work all of these materials will be referred to as simply ionic liquids. 11 ACS Paragon Plus Environment

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Thermal decomposition points, reported in the SI, are of particular importance in understanding maximum operating parameters and appropriate regeneration temperatures. While melting point temperatures vary based on cation selection, thermal decomposition temperatures display a clear trend of increasing Td,onset with increasing cation alkyl chain length. Due to the polyatomic nature of ILs, these materials display stepwise decomposition with increasing temperature. While all TSILs contained a small amount of water, the weight lost in the 25-150 o

C range for [TEA][Tau] was between 4.1-4.5 wt% water by weight seen in the thermal

decomposition water removal step. These values are not significantly higher than other TSILs and show that the neat [TEA][Tau] is hygroscopic. Compared to ammonium based ILs, phosphonium based ILs have higher thermal stabilities and lower viscosities.72 Therefore, as expected, the thermally stability of [TBP][Tau] was significantly higher than the ammonium based TSILs. 3.2 Characterization of SBA-15 Impregnated with [TBA][Tau] at Varying Loadings Nitrogen isotherms for the CASIL-SBA-B3-[TBA][Tau] materials at varying TSIL wt% are shown in Fig. 2. It should be noted that graphical figures throughout this manuscript displaying lines are acting as guides for the eye and are not indicative of mathematical fits. Additionally, although numerous isotherm data points were gathered, a limited number of data markers are displayed in the figures throughout the manuscript to facilitate review of the trends in the data. As expected, with increasing TSIL loadings, N2 adsorption is correspondingly decreased. Likewise, Table 1 displays the pore characteristics with decreases in pore volumes and BET surface areas as TSIL loadings increase. The BJH-KJS method was used to determine the pore size of the composite materials as shown in Fig. 2.73, 74 Interestingly, composite material pore width decreases from 9.6 nm to 7.8 nm while the surface area decreases from 459 to 5 m2 /g as shown in Table 1. This trend can likely be attributed to a small number of empty and partially filled pores created 12 ACS Paragon Plus Environment


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by blockages at the pore openings during the impregnation step and subsequent removal during the washing procedure. Mathematically, the BJH-KJS method determines the pore width value at the maximum adsorbate uptake over the smallest change in relative pressure, the mesoporous step. Therefore, unfilled and partially filled SBA-15 pores would provide a significant pore volume contribution in the mesoporous range of the N2 isotherms. While shifts in the mesoporous step to lower relative pressure values in Fig. 2 and shifts in pore size shown in Fig. 3 may indicate some pore lining or partial pore filling effects resulting in smaller pore widths, decreases in the overall pore volume and BET surface area are likely a more accurate representations of TSIL loading. Fig. 4 displays the equilibrium CO2 adsorption isotherm at 25 o C for the SBA-15-B3[TBA][Tau] composites. As can be seen in the isotherm plot, low [TBA][Tau] loadings of up to 22 wt% show minimal increases in CO2 capacity at pressures lower than 10 kPa and less CO2 capacity at higher pressures relative to the parent SBA-15. As the adsorption mechanism for SBA-15 is based on adsorbent-adsorbate surface site interactions, the lower loaded samples inhibit these interactions without adding a significant number of amine groups to impact the overall CO2 uptake. However, as the amount of impregnated TSIL increases from 35 wt% and above, higher uptake at partial pressures less than 10 kPa, can be seen. Additionally, samples of 49-58 wt% display an appreciably higher overall capacity than the neat SBA-15 with a maximum capacity slightly above 1.0 mmol CO2 per gram. TSIL loadings of 55 and 58 wt% reach nearly two-thirds of the equilibrium capacity by 1 kPa. As can be seen by the change in the CO2 isotherm of [TBA][Tau] composites, relative to the pristine SBA-15, at high loadings, where pore volume reductions are 95% and greater, CO2 uptake can be almost entirely attributed to the chemical fixation by the amine functionality with the physical adsorption of remaining SBA-15 surface sites providing no significant contribution for this pressure range. Also, by examining these adsorbent composites on a


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per mass basis, longer cation alkyl chains inherently decrease composite CO2 capacity due the increase in molecular weight of the TSILs. 3.3 Effects of Cation Selection and Alkyl Chain Length on CO2 Adsorption in TSIL Supported Composites Examining the effects of cation chain length, the CO2 isotherms for the composites impregnated with 1 of 6 six different TSILs can be seen in Fig. 5a-f. In all six graphs, the low pressure CO2 uptake of the composite materials have significantly increased due to the amine functionality present in the taurinate anion. For these samples, the N2 isotherms and corresponding pore characterization, as well as TGA data, can be found in the SI. From Fig. 5c, the [TEA][Tau] composites display the highest adsorption capacity of these materials reaching more than 1 mmol/g of composite at a pressure of 10 kPa in CASILSBA-B4-[TEA][Tau]-55. The [TPA][Tau] composites, Fig. 5d, and the [TBA][Tau] composite, Fig. 5e, also show high uptakes at low CO2 pressures with equilibrium capacities near 1 mmol/g at 1 atm of CO2 pressure. Similar to the CASIL-SBA-B3-[TBA][Tau] materials, increasing the amount of impregnated TSIL increased the CO2 capacity of the [TBP][Tau] composite materials; however, this was not seen with the [TMA][Tau] impregnated samples. The tetramethylammonium cations are appealing as CO2 sorbents because the materials contain more amine groups per mass than the higher alkyl chain cations. However, as displayed in Fig. 5a and b, the more highly loaded CASIL-SBA-B3-[TMA][Tau]-61 and CASIL-SBA-B4-[TMA][Tau]-56 displayed significantly lower uptakes relative to the [TEA][Tau] composites. Additionally, the amine efficiencies of these materials, reported in the SI, are 0.11 and 0.31, respectively. Interestingly, the lower loaded sample of CASIL-SBA-B4-[TMA][Tau] shows a significant increase in CO2 capacity, a behavior not displayed by other TSIL materials where capacity increases with increasing IL loading. The [TBP][Tau] composites CASIL-SBA-B4-[TBP][Tau]-55 and 50, displayed in Fig. 5f, 14 ACS Paragon Plus Environment


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have amine efficiencies above 0.6. While these values are similar to the highly loaded [TBA][Tau] composites supported by SBA-15-B3 and B4, the overall uptakes are lower due to the higher molecular weight of the phosphorous atom. To examine the effects of pore characteristics of the support material, a commercial batch of SBA-15 with smaller pore volumes and pore widths was impregnated by three TSILs. As seen in Fig. 6, the CO2 results are consistent with the previous isotherms. The trend of the CO2 loading versus ionic liquid loaded in the CASIL-SBA-ACS-[TMA][Tau] samples is also consistent with the previous results. 3.4 H2 O Adsorption and Impact on CO2 Capacities As mentioned previously, water stability is an important factor of adsorbent materials design as humid streams are common in CO2 removal processes. Therefore, H2 O adsorption isotherms for selected composites from 0.01-85 wt % relative humidity at 25 o C were measured. Fig. 7a and b illustrate the 25 o C water vapor adsorption for CASIL-SBA-B4[TMA][Tau]-56, [TEA][Tau]-56, and [TPA][Tau]-54 (a) and CASIL-SBA-B4-[TBA][Tau]-54, [THA][Tau]-49, and [TBP][Tau]-55 (b). To more easily interpret the low end adsorption and hysteresis effects the data in Fig. 7a and b are displayed over a smaller range in Fig. 7c and d. As expected, the [TMA][Tau], [TEA][Tau], [TPA][Tau], and [TBA][Tau] composites display relatively high water capacities (≈ 25-30 mmol H2 O/g) due to their hydrophilic nature. Interestingly, while the [TBP][Tau] composite displayed lower capacities than the short chain tetraalkylammonium TSILs at approximately 17 mmol/g, the [THA][Tau] composite showed drastically lower H2 O uptakes at approximately 6 mmol/g. This is likely due to the longer hexyl side chains preventing H2 O molecules from associating with the polar ammonium and taurinate ions. These results show that selection of IL provides a means of tailoring the water adsorption properties of the composites. The ability to alter the side chains of the ionic liquid and adjust water adsorption capacity may be significant in processes where the 15 ACS Paragon Plus Environment

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competitive coadsorption of CO2 and H2 O occurs. The need to tailor sorbents to account for coadsorption of water has also been examined in other materials such as MOFs.75 In regards to specific applications, water co-adsorption must be considered when filtering toxic chemicals from air or removing CO2 from closed environments.5, 76 The SBA-15 material displays the expected limited interactions with water in the 0-50 wt% saturation pressure region and steadily increase over the 50-75 wt% range. However, a sharp mesoporous step is seen at approximately 75-80 wt% saturation where the adsorbateadsorbate interactions begin to dominate, condensation occurs, and the H2 O uptake increases by approximately 40 mmol/g. The desorption branch of the parent material also shows that once condensed on the silica surface, the H2 O molecules do not readily desorb from the surface under vacuum. After H2 O adsorption, the composites were degassed under vacuum at 85 o C for 16 hours to remove the remaining water. Subsequently, CO2 isotherms at 25 o C were measured to determine the effects on water vapor the composite materials. Displayed in Fig. 8, the CO2 capacities of the composite materials, relative to the initial work in the previous section, increased in CASIL-SBA-B4-[TMA][Tau]-56, [TEA][Tau]-56, and [TBA][Tau]-55 composites. While these effects were minimal in the [TEA][Tau] sorbent, the uptakes were nearly doubled in the [TMA][Tau] composite and increased by approximately 30 wt% in the [TBA][Tau] material. The increase in the [TBA][Tau] material increases the amine efficiency from 0.66 to 0.88 mole CO2 per mole of TSIL. As the regeneration temperature was kept relatively low, it is likely that a small of remaining water influences the viscosity, melting point, or bicarbonate formation during CO2 fixation and thus increases the equilibrium capacity of the confined TSIL closer to the 1:1 theoretical capacity limit. Additionally, the CASIL-SBAB4-[TBP][Tau]-55 composite showed slight decreases in CO2 capacity post H2 O adsorption. However, more appreciable decreases can be seen in the CASIL-SBA-B4-[TPA][Tau]-54 and -[THA][Tau]-49 composites.



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3.5 CASIL-SBA-15 Composites: Adsorption Cycles and Temperature Effects Fig. 9, displays four CO2 adsorption cycles for CASIL-SBA-B3-[TBA][Tau]-55 at 25 o

C. After each cycle, the material was degassed for 16 hours under vacuum at 110 o C. As

can be seen, the CO2 capacity is slightly decreased with each recurring cycle. This can likely be attributed to the inability to remove all captured CO2 at these regeneration conditions, and each subsequent regeneration step likely continues to remove small amounts of water from the composites, which decreases the capacity of the materials consistent with the data shown in Fig. 8. As seen in the previous section, water concentration appears to significantly impact CO2 capacity in these composites. While the majority of the water is likely removed during the degas process, the removal of trace amounts may need higher temperatures and/or longer degas hold times. In addition to overall CO2 capacity, water stability, long-term material stability, and regeneration energies are of significant importance in potential application of novel CO2 sorbent materials. As displayed in the previous section, while the [TMA][Tau] TSILs are advantageous due to their lower molecular weight and thus a higher number of amine groups impregnated per composite, the CO2 equilibrium isotherms displayed much lower capacities per amine present than the impregnated longer alkyl chain TSILs composites. However, as displayed in Fig. 10, elevated temperatures are shown to increase CO2 capacities of three CASIL-[TMA][Tau] materials. Interestingly, temperature effects of the other four TSIL composites displayed in Fig. 11 follow a more expected trend with decreasing CO2 adsorption with increasing isothermal temperature. Although, the trend of increasing temperature resulting in an increase in adsorption capacity is observed on all three CASIL-[TMA][Tau] samples, a general trend reflecting an optimum temperature for adsorption is absent. The lack of a clear trend of capacity as a function of temperature is somewhat expected as the CO2 capture mechanism for these composites is complex. For example, it has been shown that increasing the temperature


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only 20 degrees in neat tetra-n-alkylammonium amino acid salts decreases the viscosity by up to a third of the initial 25 o C viscosity.46, 77 Additionally, the hydrogen bonding formed during the ammonium carbamate formation during CO2 complexation by the amino acid TSIL has been shown to significantly increase the viscosity. Ren et al. found the neat dual amine containing TSIL swelled and solidified, resulting in an increase molar volume upon the complexation of CO2 .61 It is also possible that the isotherm temperature, coupled with any trace water in the ionic liquid, may melt the ionic liquid contained in the pores. The impact of temperature is observed on each of the three CASIL-[TMA][Tau] materials all of which were made with different sized porous supports indicating that the effect is not limited to one sample. Therefore, one possible explanation for the low CO2 capacities and amine efficiencies of the [TMA][Tau] composites, displayed in the SI, is due to a combination of the crystalline nature of the neat TSIL and the increase in viscosity upon CO2 complexation at the most available active amine sites. The increased uptake at elevated temperatures, displayed in Fig. 10, coupled with the higher uptake displayed by lower loaded [TMA][Tau] composites in Fig. 5b and 6, could indicate that the supported TSIL at the pore openings may exhibit an increase in viscosity and/or swelling upon the complexation of CO2 that induces diffusion limitations of the CO2 molecules through the remaining supported TSIL phase. Additionally, PXRD data, shown in Fig. 12, confirm the crystalline nature of the ionic liquid contained in the [TMA][Tau] composites. These results are significant because other similar composites of adsorbents and ionic liquids containing amine anions may have viscosity changes, swelling, chemical capture, and adsorption phenomena each contributing to the measured sorption capacity of the material. To confirm that the hygroscopic nature of the IL can influence the measured isotherms, the sample of CASIL-SBA-B4-[TMA][Tau] used for the nitrogen porosity measurements was stored in an ambient air vial with a cap for approximately 9 months, regenerated at 120 o C



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under vacuum overnight, and the CO2 adsorption isotherm at 40 o C was measured. The CO2 isotherm was not the Type I shape shown in Fig. 10, but rather was linear with a loading of 0.18 mol/kg. To verify the linear isotherm results, the measurement was repeated on the same sample and a similar linear CO2 isotherm was produced. To confirm that isotherms shown in Fig. 10 were repeatable, a new sample of [TMA][Tau] IL was prepared, impregnated into a sample of B4 SBA-15 within 48 hours of IL synthesis, and the CO2 isotherm at 40 o

C was measured immediately after the impregnation was completed. The newly prepared

sample had a Type I isotherm shape consistent with the shape and loading of the other CASIL [TMA][Tau] samples. The isotherms from these experiment are shown in the SI. These results of 9 months of ambient air exposure are interesting because previously the [TMA][Tau] composite showed an increase in CO2 uptake after measuring a water isotherm as shown in Fig. 8. More generally, these results are important because it shows that if composites of ILs and adsorbents are applied to adsorption systems where the adsorption bed is regenerated after completing a separation, the stability of the composites will have to be examined to ensure that the capacities of the adsorbents will not diminish over time. It is also possible that the stability of the composite adsorbents will have to be examined over several cycles depending on the humidity of the inlet gas stream, and that the stability of the composite materials to adsorption and regeneration cycles will likely be a function of specific ionic liquid selected for impregnation. The adsorption isotherms show that water sorption in these materials has a significant impact on the adsorption capacity and preadsorbed water will need to be considered before any measurement or application. Measurement of multicomponent CO2 and water adsorption isotherms would be helpful in understanding the behavior of these materials. Studies discussing techniques and difficulties of measuring multicomponent isotherm data are available elsewhere, and although measurement of multicomponent isotherms is beyond the scope


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of this work, the single component isotherms have shown that any isotherm measurements will need to judiciously account for preadsorbed water.78, 79 The isotherm results also show that these materials are complex with the sorption behavior being a function of the solubility in the ionic liquid, the chemical reactivity of the ionic liquid, contributions from the adsorbent surface, and mass transfer resistances. Regarding mass transfer, the PXRD data shown in Fig. 12 show not only that the SBA-15 remains intact after impregnation, but also that the ionic liquid is crystalline inside the pores of SBA-15. This is important because efforts to model the diffusion of gases into these materials using techniques such as molecular dynamics, will need to consider the crystalline nature of the ionic liquid; however, such studies fall beyond the scope of the current work. It is also possible that the PXRD data of the ionic liquid inside the pore will be a function of preadsorbed water, and similar to the isotherms, may change over time if the materials are stored at ambient air. Changes to the ionic liquid structure may also impact the mass transfer resistances and the adsorption capacity of these materials. The impact of water on these composites is consistent with the water changing the physical properties of bulk ionic liquids80 Subsequent studies of these composite materials may need to consider how these challenges impact material design, characterization, and application. 3.6 BPL Carbon Based Composite Adsorbents To contrast the results produced using a mesoporous adsorbent support, composites were also synthesized using an amorphous porous support, BPL activated carbon, as a supporting adsorbent. BPL carbon nitrogen isotherms, shown in the SI, and pore characterization tabulated in Table 2, display a stepwise decrease in nitrogen uptake as TSIL loading increases. Correspondingly, CO2 isotherms for these composites, shown in the SI, display stepwise decreases in uptake for samples of 5.3 wt% to 22.3 wt% of IL loading. This is attributed to the porous nature of activated carbons and the crystalline structure of the tetrabutylammonium taurinate at atmospheric conditions. It is likely that the tetrabuty20 ACS Paragon Plus Environment


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lammonium taurinate is sterically hindered from entering the smaller micropores of the BPL activated carbon, the largest source of internal pore volume. The TSIL, in turn, fills the available macropores and mesopores blocking the micropore access and is likely crystallizing in the macropores, resulting in diffusion limitations. Interestingly, CASIL-BPL-[TBA][Tau]-2.5 exhibited decreases in pore volume and surface area, from N2 isotherms similar to that of CASIL-BPL-[TBA][Tau]-5.3; however, the CO2 adsorption isotherm showed the CO2 loading to be essentially equal to the pure BPL activated carbon isotherm. One possible explanation is a loading of the IL as a thin film. It should be noted that the high molecular weight of the ionic liquid increases the overall composite mass while decreasing the material surface area per gram. Therefore, viewing these uptakes as thin films may be more appropriately done on an uptake amount per BET surface area basis. In which case, the CASIL-BPL-[TBA][Tau]-2.5 CO2 uptake was increased 14% from the 0.00145 mmol per m2 for the pure BPL carbon to 0.00166 mmol per m2 in the composite. 4. Conclusions The work presented here provides a preliminary analysis of the use of TSILs supported on an ordered mesoporous silica as a composite CO2 sorbent. The ability to improve the CO2 capacity of crystalline and viscous TSILs by immobilization onto a solid support is significant as it addresses mass transfer limitations associated with pure ionic liquids and provides an effective route to utilize ionic liquids in a powdered form. Varying the alkyl chain length of the tetra-n-alkylammonium cation significantly impacts physical properties of the neat TSIL and directly influences the CO2 capacities of the composite materials. The presence of water has a profound effect on CO2 capacity of these composites. In several cases, exposure to water increases CO2 capture; however, long term exposure of the composites to ambient humidity decreased the capacity of the materials. Moreover, samples exposed to ambient humidity over a period of several months showed diminished CO2 capacity relative to the


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pristine material even after regeneration under vacuum and heat. The composite materials display long range order of the silica and crystallinity of the ionic liquid. The results show that these composite materials are complex and subsequent work tailoring the behavior of these materials for specific applications, such as closed cabin air revitalization, will require consideration of the absorption solubility, adsorption, reaction, crystalline structure of the ionic liquid and the impact of water absorption in the ionic liquid. 5. Acknowledgments We gratefully acknowledge the NASA EPSCoR program and Alabama Graduate Research Scholars Program for financially supporting this research. 6. Supporting Information Additional details including IL NMR data, DSC data, TGA data, nitrogen and CO2 isotherms, and other data can be found in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org



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(39) Drese, J. H.; Choi, S.; Didas, S. A.; Bollini, P.; Gray, M. L.; Jones, C. W. Effect of Support Structure on CO2 Adsorption Properties of Pore-Expanded Hyperbranched Aminosilicas. Micro. Meso. Mater. 2012, 151, 231-240. (40) Huang, J.; R¨ uther, T. Why are Ionic Liquids Attractive for CO2 Absorption? An Overview. Aust. J. Chem. 2009, 62, 298-308. (41) Anthony, J. L.; Anderson, J. L.; Maginn, E. J.; Brennecke, J. F. Anion Effects on Gas Solubility in Ionic Liquids. J. Phs. Chem. B 2005, 109, 6366-6374. (42) Muldoon, M. J.; Aki, S. N. V. K.; Anderson, J. L.; Dixon, J. K.; Brennecke, J. F. Improving Carbon Dioxide Solubility in Ionic Liquids. J. Phys. Chem. B 2007, 111, 9001-9009. (43) Aki, S. N. V. K.; Mellein, B. R.; Saurer, E. M.; Brennecke, J. F. High-Pressure Phase Behavior of Carbon Dioxide with Imidazolium Based Ionic Liquids. J. Phys. Chem. B 2004, 108, 20355-20365. (44) Nam, S. G.; Lee, B. C. Solubility of carbon dioxide in ammonium-based ionic liquids: Butyltrimethylammonium bis(trifluoromethylsulfonyl)imide and methyltrioctylammonium bis(trifluoromethylsulfonyl)imide. Korean J. Chem. Eng. 2013, 30, 474-481. (45) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. CO2 Capture by a Task-Specific Ionic Liquid. J. Am. Chem. Soc. 2002, 124, 926-927. (46) Jiang, Y.-Y.; Wang, G.-N.; Zhou, Z.; Wu, Y.-T.; Geng, J.; Zhang, Z.-B. Tetraalkylammonium Amino Acids as Functionalized Ionic Liquids of Low Viscosity. Chem. Commun. 2008, 505-507.



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(47) Zhang, Y.; Zhang, S.; Lu, X.; Zhou, Q.; Fan, W.; Zhang, X. P. Dual AminoFunctionalized Phosphonium Ionic Liquids for CO2 Capture. Chem. Eur. J. 2009, 15, 3003-3011. (48) Yu, H.; Wu, Y.-T.; Jiang, Y.-Y.; Zhou, Z.; Zhang, Z.-B. Low Viscosity Amino Acid Ionic Liquids with Asymmetric Tetraalkylammonium Cations for Fast Adsorption of CO2 . New. J. Chem. 2009, 33, 2385-2390 (49) Gurkan, B. E.; de la Fuente, J. C.; Goodrich, B. F.; Price, E. A.; Schneider, E. A.; Brennecke, J. F. Equimolar CO2 Adsorption by Anion-Functionalized Ionic Liquids. J. Am. Chem. Soc. 2010, 132, 2116-2117. (50) Mindrup, E. M.; Schneider, W. F. Computational Comparison of the Reactions of Substituted Amines with CO2 . Chem. Sus. Chem. 2010, 3, 931-938. (51) McDonald, J. L.; Sykora, R. E.; Hixon, P.; Mirjafari, A.; Davis, J. H. Impact of Water on CO2 Capture by Amino Acid Ionic Liquids. Environ. Chem. Lett. 2014, 12, 201-208. (52) Soutullo, M. D.; Odom, C. I.; Wicker, B. F.; Henderson, C. N.; Stenson, A. C.; Davis, J. H. J. Reversible CO2 Capture by Unexpected Plastic-, Resin-, and Gel-Like Ionic Soft Materials Discovered During the Combi-Click Generation of a TSIL Library. Chem. Mater. 2007 19, 3581-3583. (53) Goodrich, B. F.; de la Fuente, J. C.; Gurkan, B. E.; Zadigian, D. J.; Price, E. A.; Huang, Y.; Brennecke, J. F. Experimental Measurements of Amine-Functionalized Anion-Tethered Ionic Liquids with Carbon Dioxide. Ind. Eng. Chem. Res. 2010, 50, 111-118. (54) Gutowski, K. E.; Mafinn, E. J. Amine-Functionalized Task-Specific Ionic Liquids: A Mechanistic Explanation for the Dramatic Increase in Viscosity upon Complexation with CO2 from Molecular Simulation. J. Am. Chem. Soc. 2008, 130, 14690-14704. 29 ACS Paragon Plus Environment

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(55) Goodrich, B. F.; de la Fuente, J. C.; Gurkan, B. E.; Lopez, Z. K.; Price, E. A.; Huang, Y.; Brennecke, J.F. Effect of Water and Temperature on Absorption of CO2 by AmineFunctionalized Anion-Tethered Ionic Liquids. J. Phys. Chem. B. 2011, 115, 9140-9150. (56) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis. Wiley-VCH, Weinheim 2008. (57) Zhang, Z. M.; Wu, L. B; Dong, J.; Li, B. G.; Zhu, S. P. Preparation and SO2 Sorption/Desorption Behavior of an Ionic Liquid Supported on Porous Silica Particles. Ind. Eng. Chem. Res. 2009, 48, 2142-2148. (58) Selvam, T.; Machoke, A.; Schwieger, W. Supported Ionic Liquids on Non-porous and Porous Inorganic Materials-A Topical Review. Appl. Catal., A 2012, 445-446, 92-101. (59) Wan, M. M.; Zhu, H. Y.; Li, Y. Y.; Ma, J.; Liu, S.; Zhu, J. H. Novel CO2 -Capture Derived from the Basic Ionic Liquids Oriented on Mesoporous Materials. ACS Appl. Mater. Inter. 2014, 6, 12947-12955. (60) Kolding, H.; Fehrmann, R.; and Riisager, A. CO2 Capture Technologies: Current Status and New Directions Using Supported Ionic Liquid Phase (SILP) Adsorbers, Sci. China Chem., 2012, 55, 1648-1656. (61) Ren, J.; Wu, L.; Li, B.-G. Preparation and CO2 Sorption/Desorption of N-(3Aminopropyl)-Aminoethyl Tributylphosphonium Amino Acid Salt Ionic Liquids Supported into Porous Silica Particles. Ind. Eng. Chem. Res. 2012, 51, 7901-7909. (62) Wang, X.; Akhmedov, N. G.; Duan, Y.; Luebke, D.; Li, B. Immobilization of Amino Acid Ionic Liquids into Nanoporous Microspheres as Robust Sorbents for CO2 Capture. J. Mater. Chem. 2013, 1, 2978-2982.



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme Caption Scheme 1. Chemical fixation of CO2 by tethered amines.


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Figure Cations Fig. 1. Ionic structures of tetraalkylammonium taurinate TSILs. Fig. 2. Nitrogen isotherms at 77K for pure SBA-15-B3 and composites with varying [TBA][Tau] loadings. Fig. 3. Pore size distribution of SBA-15 and composites with varying [TBA][Tau] loadings. Fig. 4.

CO2 equilibrium isotherms in SBA-15-B3 composites with varied loadings of

[TBA][Tau]. Fig. 5. CO2 equilibrium isotherms for SBA-15-B3 and B4 composites impregnated with different TSILs. Fig. 6. CO2 equilibrium isotherms for SBA-15-ACS composites impregnated with [TMA][Tau], [TEA][Tau], and [TBA][Tau] measured at 25 o C . Fig. 7. H2 O equilibrium isotherms for selected SBA-15-B4 composites measured at 25 o C. Fig. 8. CO2 isotherms measured at 25 o C and after previous water vapor adsorption. Fig. 9. CO2 adsorption isotherms at various temperatures for CASIL-SBA-B3-[TBA][Tau]55. Fig. 10. Temperature effects on CO2 adsorption by selected CASIL-SBA-[TMA][Tau] composites. Fig. 11. CO2 isotherms for selected CASIL materials at various temperatures. Fig. 12 Powder X-ray Diffraction data for SBA-15 and CASIL-SBA-B4[TMA][Tau]-56 showing the structure of the silica adsorbent is retained after impregnation. Also, the pattern shows the ionic liquid as a crystalline material in the pores of the SBA-15.



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Table Captions Table 1. Characterization of pure and composite SBA-15-B3 materials impregnated with [TBA][Tau] at varying loadings. Table 2. Characterization of pure and composite BPL activated carbon materials.


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(A)

carbamate (B)

bicarbonate (C)

carbamic acid

Scheme 1 Scheme 1

Text to replace on page 2: “In turn, the delocalized carbamate charge forms a zwitterion with amine group and thus the theoretical maximum capacity is 1:2 mole CO2 per mole of amine.”

Replacement text:

36

“Upon initial reaction with CO2, the amine forms a zwitterion which can then protonate a second amine resulting in a theoretical maximum capacity of 1:2 Plus mole Environment of CO2: mole of amine, or an amine efficiency of ACS Paragon 0.5 mole CO2 per mole of amine.”


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Tetramethylammonium Taurinate [TMA][Tau]

Tetraethylammonium Taurinate [TBA][Tau]

Tetrapropylammonium Taurinate [TPA][Tau]

Tetrabutylammonium Taurinate [TBA][Tau]

Tetrahexylammonium Taurinate [THA][Tau]

Tetrabutylphosphonium Taurinate [TBP][Tau]

Figure 1


Page 38 of 52

Page 39 of 52

35

(a)

-1

Amount Adsorbed (mmol g )

30

SBA-15-B3 CASIL-SBA-B3-[TBA][Tau]-9 CASIL-SBA-B3-[TBA][Tau]-12 CASIL-SBA-B3-[TBA][Tau]-16 CASIL-SBA-B3-[TBA][Tau]-22 CASIL-SBA-B3-[TBA][Tau]-35

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Figure 2



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0.0020 SBA-15-B3 CASIL-SBA-B3-[TBA][Tau]-9 CASIL-SBA-B3-[TBA][Tau]-12 CASIL-SBA-B3-[TBA][Tau]-16 CASIL-SBA-B3-[TBA][Tau]-22 CASIL-SBA-B3-[TBA][Tau]-35 CASIL-SBA-B3-[TBA][Tau]-49 CASIL-SBA-B3-[TBA][Tau]-55 CASIL-SBA-B3-[TBA][Tau]-58

0.0015

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3

dV/dW (cm /g nm)

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Page 40 of 52

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Figure 3


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Page 41 of 52

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SBA-15-B3 CASIL-SBA-[TBA][Tau]-22 CASIL-SBA-[TBA][Tau]-35 CASIL-SBA-[TBA][Tau]-49 CASIL-SBA-[TBA][Tau]-55 CASIL-SBA-[TBA][Tau]-58

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Figure 4


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SBA-15-B3 CASIL-SBA-B3-[TMA][Tau]-61 0.1

SBA-15-B4 CASIL-SBA-B4-[TMA][Tau]-39 CASIL-SBA-B4-[TMA][Tau]-56

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Page 42 of 52

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Figure 5


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SBA-15- B4 CASIL-SBA-B4-[TBA][Tau]-54 CASIL-SBA-B4-[THA][Tau]-49 CASIL-SBA-B4-[TBP][Tau]-55

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Figure 7



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Figure 10 46 ACS Paragon Plus Environment

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(d)

(c)

0.8

0.90 0.7

-1


0.75

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 52

0.60

0.45

0.6

0.5

0.4

0.3

0.30 o

o

25 C CASIL-SBA-B4-[TBA][Tau]-54

25 C

0.2

o

45 C

CASIL-SBA-B4-[TBP][Tau]-55

o

o o

65 C

0.15

40 C 60 C

0.1

0

10

20

30

40


80

90

100

110

0

10

20

30

Figure 11


40


80

90

SBA-15-B4 CASIL-SBA-B4-[TMA][Tau]-56 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

(degrees)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity

Page 49 of 52

SBA-15-B4

CASIL-SBA-B4-[TMA][Tau]-56 0

10

20

30

40

50

(degrees)

Figure 12


60

70

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SBA-15-B3 CASIL-SBA-B3-[TBA][Tau]-5 CASIL-SBA-B3-[TBA][Tau]-10 CASIL-SBA-B3-[TBA][Tau]-20 CASIL-SBA-B3-[TBA][Tau]-33 CASIL-SBA-B3-[TBA][Tau]-50 CASIL-SBA-B3-[TBA][Tau]-63 CASIL-SBA-B3-[TBA][Tau]-67 CASIL-SBA-B3-[TBA][Tau]-71

Sample

1.13 0.87 0.82 0.75 0.62 0.25 0.05 0.02 0.01

[cm3 /g]

Pore Volume

22.31 27.24 33.65 44.96 77.40 95.85 97.96 99.22

Pore Volume Decrease [%] 792 459 412 370 316 136 22 10 5

BET Surface Area [m2 /g] 9.5 9.4 9.4 9.4 9.3 7.6 7.7 7.8 7.8

[nm]

Pore Diameter

0.094 0.119 0.161 0.217 0.364 0.488 0.554 0.578

Amount Impregnated IL (TGA) [weight % IL]

Table 1: Pore Characterization of SBA-15-B3 and Composites.


49


Page 50 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

BPL Carbon CASIL-BPL-[TBA][Tau]-2.5 CASIL-BPL-[TBA][Tau]-5.3 CASIL-BPL-[TBA][Tau]-12.2 CASIL-BPL-[TBA][Tau]-18.3 CASIL-BPL-[TBA][Tau]-22.3

Sample

0.63 0.54 0.51 0.40 0.25 0.08

[cm3 /g]

Pore Volume

13.7 18.2 36.1 60.4 87.5

Pore Volume Decrease [%] 1221 1065 1021 748 415 68

BET Surface Area [m2 /g]

0.025 0.053 0.122 0.183 0.223

Amount Impregnated IL (TGA) [weight % IL]

Table 2: Characterization of BPL Activated Carbon and BPL-Composite Materials.

Page 51 of 52 The Journal of Physical Chemistry

50



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Porous Solids Impregnated with Task-Specific Ionic Liquids as

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