Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and

Nov 15, 2014 - CO 2 capture using triamine-grafted SBA-15: The impact of the support pore structure. Masoud Jahandar Lashaki , Abdelhamid Sayari. Chem...
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Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and Water: Spectral Deconvolution of Adsorbed Species Stephanie A. Didas, Miles A. Sakwa-Novak, Guo Shiou Foo, Carsten Sievers, and Christopher W. Jones* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Three primary amine materials functionalized onto mesoporous silica with low, medium, and high surface amine coverages are prepared and evaluated for binary CO2/H2O adsorption under dilute conditions. Enhancement of amine efficiency due to humid adsorption is most pronounced for low surface amine coverage materials. In situ FT-IR spectra of adsorbed CO2 on these materials suggest this enhancement may be associated with the formation of bicarbonate species during adsorption on materials with low surface amine coverage, though such species are not observed on high surface coverage materials. On the materials with the lowest amine loading, bicarbonate is observed on longer time scales of adsorption, but only after spectral contributions from rapidly forming alkylammonium carbamate species are removed. This is the first time that direct evidence for bicarbonate formation, which is known to occur in liquid aqueous amine solutions, has been presented for CO2 adsorption on solid amine adsorbents. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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conditions are to be developed. To date, there have been a handful of studies of supported amine adsorbents for air capture under humid conditions.7−17 Typically, these studies have been conducted in a very narrow range of conditions, usually at one CO2 partial pressure and one to two relative humidities (RHs). Furthermore, the relative humidities explored have generally been quite high and have often resulted in water existing on the sorbent surface and in the sorbent pores in a condensed, liquid-like state. There remains little understanding of the behavior of coadsorbed water and CO2 under low RH and low CO2 coverage conditions. Because the energy penalty for removing adsorbed water can be high,15 it is worthwhile to explore dilute CO2 capture under low RH conditions in a model material with varying degrees of amine surface coverage. Therefore, the goal of this work was to explore the effect of amine surface coverage on adsorption of dilute CO2 in the presence of low partial pressures of water. Additionally, in situ FT-IR spectroscopy was used to observe differences in the nature of adsorbed species between low and high amine surface coverage materials. Three sorbent materials were prepared using a primary aminosilane (3-aminopropyltrimethoxysilane, APS) that was grafted to silica SBA-15 with varying degrees of surface coverage, as depicted in Scheme 1. SBA-APS-low has a submonolayer surface coverage of amines, SBA-APS-medium has approximately monolayer coverage and SBA-APS-high has

upported amine adsorbents are now widely studied for CO2 capture from flue gas as well as ambient air and are promising materials for use in adsorption-based separation processes.1−4 One key metric by which these materials are evaluated is the amine efficiency, defined as the moles of CO2 captured per mole of amine. Higher amine efficiencies translate to higher adsorption capacities and reduced amounts of sorbent required to capture a given amount of CO2. There have been two general mechanisms proposed for CO2 adsorption onto supported amines that have been extrapolated from the aqueous amine literature.1 The first mechanism can occur in dry and humid conditions and results in the formation of alkylammonium carbamate ion pairs, and suggests a maximum amine efficiency of 0.5 mol CO2/(mol N).5 The second mechanism can occur only in humid conditions and results in the formation of alkylammonium bicarbonate, with a hypothetical amine efficiency that can approach 1 mol CO2/ (mol N).6 Though it is known that bicarbonate species form during CO2 absorption in aqueous solutions, compelling evidence for bicarbonate has not been observed for CO2 adsorption with supported amine adsorbents. Despite this, it is still routinely observed experimentally that humid adsorption conditions enhance CO2 uptake, or amine efficiency, of supported amine materials.1,7 The cause for this increase is sometimes suggested to be due to bicarbonate formation (without direct experimental evidence of such species), whereas other times, an increase in carbamate formation is suggested. For CO2 capture under ultradilute conditions, such as from ambient air (referred to here as “air capture”), the adsorption of CO2 and water in the very low surface coverage regime must be understood if improved sorbents tuned for use under these © XXXX American Chemical Society

Received: September 24, 2014 Accepted: November 15, 2014

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Scheme 1. Hypothetical representation of amine materials used in this study with low (SBA-APS-low, top), medium (SBA-APS-medium, middle), and high (SBA-APS-high, bottom) surface coverage

a dense, multilayer array of amines, as is evidenced by the loss of mesoporosity deduced from the N2 isotherm shown in Figure S1 in the Supporting Information. The materials were evaluated for binary CO2−H2O adsorption properties in the low partial pressure region (up to 0.01 bar CO2) and were also subjected to in situ FT-IR spectroscopic measurements under dry and humid conditions to observe the nature of the adsorbed CO2 species. For binary isotherm measurements, experiments were done such that the amount of adsorbed water was effectively constant between the three materials. By holding the water loading constant for the humid adsorption studies, the effect of surface coverage of amines could be more clearly observed with respect to CO2 uptake. This type of measurement can only work if the adsorption of water onto the amine adsorbent is independent of CO2 partial pressure and CO2 adsorption; this was verified and is presented in Figure S2 in the Supporting Information. The measured binary adsorption isotherms for the amine adsorbents are displayed in Figure 1 along with the corresponding dry CO2 adsorption data. From the binary adsorption isotherms, it can be seen that water serves to enhance adsorption of CO2 the most for SBAAPS-low, followed by SBA-APS-medium and then SBA-APShigh, especially in the ultralow pressure region that corresponds to air capture conditions (ca. 0.004 bar). This could suggest that a monolayer to multilayer coverage of amines on the surface in some way hinders the ability of CO2 to interact with water as favorably in the adsorption process or that the presence of surface silanols on SBA-APS-low offers an additional route to CO2 adsorption through amine−silanol− water interaction that does not occur to the same extent with the other materials. This concept can be further examined by comparing the amine efficiency of each material between dry and humid adsorption conditions. To do so, the efficiency enhancement was calculated for each material and compared as a function of CO2 partial pressure using numerical fits obtained

Figure 1. Binary adsorption isotherms for (a) SBA-APS-low, (b) SBAAPS-medium, and (c) SBA-APS-high. Dashed lines are from numerical fitting with a single site Toth model.

by fitting the measured isotherms to a single site Toth model.18−20 Efficiency enhancement is defined as follows: efficiency enhancement =

amine efficiency|nH2O > 0 amine efficiency|nH2O = 0

The efficiency enhancement for the amine adsorbents is shown in Figure 2. This figure demonstrates the effect of amine surface coverage with respect to humid adsorption in low RH conditions, where the greatest degree of enhancement is observed for the material with the lowest amine loading. It is interesting to note that although the low and medium loading 4195

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literature is that the dominant adsorbed CO2 species are ammonium carbamate ion pairs, with a lesser degree of carbamic acid and surface bound carbamates also observed (with the latter species only forming in dry CO2, low amine surface coverage conditions).25−34 These species are presented in Scheme 2 along with the corresponding IR bands that are Scheme 2. Reported Adsorbed Species for CO2 on Supported Amine Adsorbents in Dry and Humid Conditions along with IR Band Assignments25−43a

Figure 2. Efficiency enhancement of supported amine adsorbents during coadsorption of CO2 and water as compared to dry adsorption conditions.

materials show favorable enhancement throughout the adsorption isotherm range when water is coadsorbed, the high loading material initially displays slightly lower amine efficiencies compared to dry adsorption and then surpasses the amine efficiency of the dry conditions at higher partial pressures of CO2.21 Reductions in CO2 capacity under humid conditions have been reported and are often rationalized using kinetic arguments. Goeppert et al. reported an 18% decrease in amine efficiency when comparing CO2 capacities measured at 67% RH and 400 ppm of CO2 compared to dry conditions at the same CO2 partial pressure for a fumed silica material impregnated with 52 wt % PEI.13 Their explanation for this behavior was that the adsorbed water blocked access to some of the more difficult to reach amine groups, thereby reducing the CO2 capacity. Higher partial pressures of CO2 were not evaluated in that study to assess if the behavior continued under such conditions. Subagyono et al. also observed a similar trend, where CO2 isotherms measured with 2.8% RH only started to surpass dry adsorption capacities in the range of 5−30% CO2 for highly loaded (∼70 wt %) branched and linear PEI adsorbents.22 Those experiments were performed in a higher partial pressure range, from 2.5 to 50% CO2 at an adsorption temperature of 75 °C. However, it has also been argued that adsorbed water can have a positive effect on the amine efficiency through reduced kinetic restrictions, either acting as a diffusive intermediate to transport CO2 or increasing the mobility of the amine chains via a plasticization effect.23,24 It is not clear why these differences are observed, although the relative humidity and adsorption temperature may have an impact on the observed behavior. These studies, combined with the proposed difference in dry and humid adsorption products discussed above, suggest that water can alter CO2 capacities and amine efficiencies in kinetic or thermodynamic ways and that laboratory data may reflect a convolution of these effects. The findings from the binary adsorption studies show a difference in humid adsorption behavior for supported amine adsorbents with varying degrees of amine loading. One possible cause for these differences could be the result of a different adsorption mechanism occurring within the different materials. To investigate this, in situ FT-IR spectroscopy was employed for SBA-APS-low and SBA-APS-high under dry and humid CO2 adsorption conditions. The current consensus in the

a

See Supporting Information Table S2 for more detailed assignments.

characteristic of these species. As noted above, the most recent literature suggests that bicarbonates and carbonates do not form upon humid CO2 adsorption and that any increase in amine efficiency is the result of one or more of the following: (i) more carbamate ion pairs forming, (ii) formation of hydrogen bond stabilized carbamic acid species, or (iii) the release of additional hydrogen bonded amines.29,31,33 In situ FT-IR experiments were conducted at a fixed CO2 or CO2/H2O pressure and monitored as a function of time to examine if the nature of the adsorbed species changed during the course of adsorption, as it has been shown that different species can form over different time scales (ammonium carbamate pairs and carbamic acid form more rapidly, whereas silylpropylcarbamate forms more slowly).31 All materials were initially activated by heating at 110 °C under vacuum to remove preadsorbed water and CO2 (see Supporting Information). The absorption difference spectra for SBA-APS-low and SBA-APShigh are presented in Figure 3. It should be noted that these spectra represent the difference between the IR spectra after a given time of exposure to CO2 (or CO2 and water for humid experiments) and the activated spectra (or spectra equilibrated with water before dosing of CO2). In this way, contributions from CO2 adsorption can be more clearly observed as the water signal is removed from the spectrum. Results shown in Figure 3 are fairly consistent with what has been observed previously in the literature.25−34 SBA-APS-low appears to form ammonium carbamate ion pairs, carbamic acid and bound carbamate under dry CO2 conditions (further evidence for bound carbamate is discussed later) and ammonium carbamate ion pairs in humid conditions, while SBA-APS-high only forms ammonium carbamate pairs in both dry and humid conditions. It is clear that SBA-APS-high should not form bound carbamate during adsorption because there are no or relatively few surface silanols.32 However, carbamic acid 4196

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Figure 3. In situ FT-IR difference spectra as a function of adsorption time for (a/b) SBA-APS-low and (c/d) SBA-APS-high in (a/c) dry CO2 conditions (0.01 bar CO2) and (b/d) humid CO2 conditions (1.5 × 10−3 bar CO2 and 5 × 10−3 bar H2O vapor).

could, in principle, still form for the high loading material. The absence of these species, which would be characterized by an absorbance band at 1694 cm−1, suggests that alkylammonium carbamate ion pairs are the dominant species for adsorbed CO2 when amines are clustered. Also noteworthy is the apparent slower equilibration time of SBA-APS-high compared to SBAAPS-low. This can be directly observed by comparing band intensities as a function of time. For the high loading material there are obvious increases in intensity, whereas for the low loading material, this phenomenon is observed at a much smaller scale. By the end of the experiment at 10 h, it appears that equilibrium has been reached for the low loading material. However, the absorbance intensity of SBA-APS-high appears to be still increasing after 10 h. This is consistent with what was observed during binary adsorption experiments and can be explained by diffusional resistances that have been observed for highly loaded materials.44 One key difference can be observed for humid adsorption of SBA-APS-low in Figure 3b; the intensity of the asymmetric deformation of NH3+ at 1626 cm−1 gradually decreases with time and red-shifts to 1616 cm−1, which can be more clearly seen from the inset in the figure. Additionally, the adsorbed water band at 1658 cm−1 emerges in the shoulder. It is unclear whether this is the result of (i) a blue shift in the adsorbed water band at 1651 cm−1 due to interaction with some other species or (ii) the evolution of more adsorbed water because the pre-equilibration time for water before introduction of CO2

was only 1 h. Nevertheless, these observations suggest that the nature of the adsorbed species may be changing at longer time scales of adsorption over this material. A more detailed analysis of the dynamics of adsorption was performed to evaluate the notion that different species are forming later in the adsorption process. To do so, the contributions from the first 10 min or the first hour of CO2 adsorption were subtracted from the IR spectra. This approach allows for the observation of species forming at longer time scales and was used by Bacsik et al. to deconvolute rapidly forming carbamic acid from slower forming silylpropylcarbamate during CO2 adsorption on silica supported APS.31 The results of this analysis can be seen in Figure 4. For dry CO2 adsorption on SBA-APS-low, similar behavior as has been reported elsewhere is observed, where bands for bound carbamate are more clearly resolved after subtracting out the spectra for the initial 10 min of adsorption.31,32 However, the spectrum of humid SBA-APS-low looks very different from its dry counterpart as well as what is observed for the spectra of dry and humid SBA-APS-high. A blue shift in the band at 1562 cm−1 is likely due to overlap with a newly evolved band at 1595 cm−1 that corresponds with the liberation of amines from surface hydrogen bonds with silanols or silylpropylcarbamate in the presence of humidity.31 The appearance of this band does not necessarily reflect on any new CO2−amine species forming but rather indicates that more free amines are being made available upon humidification. It can also be seen that a new 4197

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Figure 4. Time-evolved FT-IR spectra at varying time intervals to display slow and rapid forming species for (a/b) SBA-APS-low and (c/d) SBAAPS-high in (a/c) dry CO2 conditions (0.01 bar CO2) and (b/d) humid CO2 conditions (1.5 × 10−3 bar CO2 and 5 × 10−3 bar H2O vapor).

band starts to evolve at 1350 cm−1 after the first several hours of adsorption and is most clearly seen in the “10h-1h” spectrum. This cannot be observed in Figure 3b because it overlaps with the NCOO− skeletal vibration around 1330 cm−1. However, by removing the contribution from the rapidly forming carbamate ion pairs in the initial hour of adsorption, it can clearly be seen that, on longer time scales, a different type of adsorption occurs that has not been widely discussed in the supported amine literature to date. In the aqueous amine absorption literature, the evolution of a band in the range of 1360−1350 cm−1 has been attributed to the symmetric C−O stretch of HCO3−.45−47 This band assignment has been validated in two ways. The first was from the observation of an IR band forming at 1360−1350 cm−1 during CO2 absorption of the aqueous tertiary amine N(2-hydroxyethyl)hydroxypiperidine, which is known to absorb CO2 solely through the formation of bicarbonate.45 The second validation came from the observation of the same IR band forming at 1360−1350 cm−1 during the gradual addition of ammonium bicarbonate to an aqueous piperidine solution.45 This band is observed for nonaromatic aqueous amines as well in the same region. A study by Richner and Puxty reported the IR spectra of aqueous monoethanolamine (MEA), diethanolamine (DEA), and aminomethyl propanol (AMP) during CO2 absorption and observed the formation of a bicarbonate band at 1360−1350 cm−1 as well.47 Therefore, it is suggested that the observed band at 1350 cm−1 could indicate formation of

bicarbonate on SBA-APS-low during humid CO2 adsorption, as this matches well with what has been observed for aqueous amines. Formation of bicarbonate at longer time scales is not observed during humid CO2 adsorption for SBA-APS-high. In fact, no difference in adsorbed species during the later stages of adsorption can be observed for this material. It is possible that this is due to the longer equilibration time observed for the densely loaded material, and the fact that bicarbonate is a slowforming product. Nevertheless, on these time scales, it appears that the low surface coverage of SBA-APS-low allows for the adsorption of CO2 (in part) via a bicarbonate mechanism under humid conditions. This may occur with isolated amines that cannot form the more favorable carbamate species. Although bicarbonate formation for this material is observed, it is not suggested that this is the sole contribution to increased amine efficiency under humid conditions, as other studies have shown that humid CO2 adsorption can increase efficiency through the formation of more ammonium carbamate ion pairs as well as through the liberation of hydrogen bonded amines.31,33 In summary, it was found that amine surface coverage can have an impact on the enhancement of amine efficiency during low relative humidity CO2 adsorption. Materials with low surface amine coverage displayed the most improvement in adsorption efficiency during coadsorption with water. Adsorbents with multilayer amine coverage displayed the least amount of improvement upon humid adsorption. In situ FT-IR spectra 4198

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Notes

of low and high surface coverage amine adsorbents suggest one possible reason for the difference in amine efficiency between materials is a difference in the structure of the adsorbed species for humid CO2 conditions. By analyzing the IR spectroscopy data so that the spectral contributions from fast forming species were removed, it was possible to observe the formation of a band assigned here to bicarbonate on the supported amine adsorbent with lowest amine surface coverage at longer time scales. This is the first time that evidence for bicarbonate formation on supported amine adsorbents that matches with aqueous amine literature has been directly observed. Ammonium carbamate ion pairs were also observed on both low and high amine surface coverage materials in humid CO2 conditions, as has been previously reported. The results of these studies indicate that the effect of water coadsorption with CO2 is dependent on the choice of amine material and adsorption conditions. This further demonstrates the need to explore different operating conditions for adsorption so that an optimum in terms of operating costs and adsorption efficiency can be found for use of this class of materials in adsorptive gas separation processes. Use of additional spectroscopic techniques to probe the formation of bicarbonate species is warranted.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported in part by DOE-NETL under contract DE-FE0007804. The work was also supported in part by the Office of Naval Research, via ONR code 33. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the DOE or ONR. S.A.D. would like to thank Dr. Nicholas A. Brunelli for assistance in the design and construction of the adsorption system used in this work.

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(1) Bollini, P.; Didas, S. A.; Jones, C. W. Amine-Oxide Hybrid Materials for Acid Gas Separations. J. Mater. Chem. 2011, 21, 15100− 15120. (2) Jones, C. W. CO2 Capture from Dilute Gases as a Component of Modern Global Carbon Management. Annu. Rev. Chem. Biomol. Eng. 2011, 2, 31−52. (3) Goeppert, A.; Czaun, M.; Prakash, G. K. S.; Olah, G. A. Air as the Renewable Carbon Source of the Future: An Overview of CO2 Capture from the Atmosphere. Energy Environ. Sci. 2012, 5, 7833− 7853. (4) Hedin, N.; Andersson, L.; Bergström, L.; Yan, J. Adsorbents for the Post-Combustion Capture of CO2 Using Rapid Temperature Swing or Vacuum Swing Adsorption. Appl. Energy 2013, 104, 418− 433. (5) Caplow, M. Kinetics of Carbamate Formation and Breakdown. J. Am. Chem. Soc. 1968, 90, 6795−6803. (6) Donaldson, T. L.; Nguyen, Y. N. Carbon Dioxide Reaction Kinetics and Transport in Aqueous Amine Membranes. Ind. Eng. Chem. Fundam. 1980, 19, 260−266. (7) Gebald, C.; Wurzbacher, J. A.; Borgschulte, A.; Zimmermann, T.; Steinfeld, A. Single-Component and Binary CO2 and H2O Adsorption of Amine-Functionalized Cellulose. Environ. Sci. Technol. 2014, 48, 2497−2504. (8) Belmabkhout, Y.; Serna-Guerrero, R.; Sayari, A. Adsorption of CO2-Containing Gas Mixtures over Amine-Bearing Pore-Expanded MCM-41 Silica: Application for Gas Purification. Ind. Eng. Chem. Res. 2010, 49, 359−365. (9) Gebald, C.; Wurzbacher, J. A.; Tingaut, P.; Zimmermann, T.; Steinfeld, A. Amine-Based Nanofibrillated Cellulose as Adsorbent for CO2 Capture from Air. Environ. Sci. Technol. 2011, 45, 9101−9108. (10) Wurzbacher, J. A.; Gebald, C.; Steinfeld, A. Separation of CO2 from Air by Temperature−Vacuum Swing Adsorption Using DiamineFunctionalized Silica Gel. Energy Environ. Sci. 2011, 4, 3584−3592. (11) Choi, S.; Drese, J. H.; Eisenberger, P. M.; Jones, C. W. Application of Amine-Tethered Solid Sorbents for Direct CO2 Capture from the Ambient Air. Environ. Sci. Technol. 2011, 45, 2420−2427. (12) Stuckert, N. R.; Yang, R. T. CO2 Capture from the Atmosphere and Simultaneous Concentration Using Zeolites and Amine-Grafted SBA-15. Environ. Sci. Technol. 2011, 45, 10257−10264. (13) Goeppert, A.; Czaun, M.; May, R. B.; Prakash, G. K. S.; Olah, G. A.; Narayanan, S. R. Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent. J. Am. Chem. Soc. 2011, 133, 20164−20167. (14) He, L.; Fan, M.; Dutcher, B.; Cui, S.; Shen, X.; Kong, Y.; Russell, A. G.; McCurdy, P. Dynamic Separation of Ultradilute CO2 with a Nanoporous Amine-Based Sorbent. Chem. Eng. J. 2012, 189−190, 13− 23. (15) Wurzbacher, J. A.; Gebald, C.; Piatkowski, N.; Steinfeld, A. Concurrent Separation of CO2 and H2O from Air by a TemperatureVacuum Swing Adsorption/Desorption Cycle. Environ. Sci. Technol. 2012, 46, 9191−9198.

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EXPERIMENTAL METHODS Adsorbent materials were prepared by functionalizing (3aminopropyl)trimethoxysilane onto mesoporous silica SBA-15. Synthesis procedures as well as material characterization are provided in the Supporting Information. Single component CO2 as well as binary CO2−H2O adsorption isotherms were measured on a custom built volumetric adsorption system connected to an Agilent gas chromatograph with an HP-Plot U capillary column. Single component water isotherms were measured using a Hiden IGASorp. Operation procedures for both apparatuses are described in the Supporting Information. For binary adsorption, the amount of water injected was adjusted so that the quantity of adsorbed water on all materials would be approximately equivalent. The amount of water to be injected was estimated from the single component water isotherms. All adsorption experiments were performed at 30 °C. FT-IR spectroscopy was performed on a Nicolet 8700 IR spectrometer with a MCT/A detector. The difference spectra that are presented use the activated spectrum before addition of CO2 as the subtrahend for dry adsorption and the spectrum equilibrated with water for 1 h as the subtrahend for humid CO2 adsorption. Additional experimental details are in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Material synthesis and characterization, single-component CO2 and H2O adsorption isotherms, water adsorption capacity during binary adsorption isotherm measurements, Toth isotherm model description, experimental procedure for binary isotherms and in situ FT-IR, FT-IR spectra for activated materials, bare SBA-15, and full spectra for materials. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (404) 385-1683. 4199

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