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Effect of Humidity on the CO Adsorption of Tertiary Amine Grafted SBA-15 Jason J Lee, Chia-Hsin Chen, Daphna Shimon, Sophia E. Hayes, Carsten Sievers, and Christopher W Jones J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07930 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017
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The Journal of Physical Chemistry
Effect of Humidity on the CO2 Adsorption of Tertiary Amine Grafted SBA-15 Jason J. Lee†, Chia-Hsin Chen§, Daphna Shimon§, Sophia E. Hayes§, Carsten Sievers†*, and Christopher W. Jones†* †School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States §Department of Chemistry, Washington University, One Brookings Drive, Saint Louis, Missouri 63130, United States Abstract Aminosilica materials are promising candidates for CO2 capture from dilute streams such as ambient air and flue gas. Most aminosilica sorbents are constructed using primary and/or secondary amines, which have been shown to primarily react with CO2 to form alkylammonium carbamates and related structures. While ammonium bicarbonate formation is known to occur in aqueous amine solutions in the presence of CO2, there has been conflicting evidence of its formation on solid supported analogues. To probe if the ammonium bicarbonate species can exist on solid supported amines, tertiary amines, which are known to form bicarbonates in aqueous solution, are grafted onto mesoporous silica SBA-15, and the materials are further characterized using in-situ FTIR spectroscopy and solid-state NMR spectroscopy in the presence of humid and dry CO2. Dry and humid CO2 capacities for these sorbents are also evaluated using fixed bed experiments and thermogravimetric analysis. This work shows that ammonium bicarbonates can exist on solid supported amines, but also demonstrates that tertiary amines are poor CO2 sorbents under the conditions employed. Introduction Fossil fuels are heavily relied on as the main energy source today, and forecasts do not indicate that fossil fuel usage will diminish in the near future.1 The use of fossil fuels results in the release of large amounts of CO2 into the atmosphere, which has been linked to climate change over the past century.1 As a result, there has been a significant research focus on developing technologies to capture CO2 from both flue gas and ambient air.1–5 The most common technology used for post-combustion capture CO2 is absorption by aqueous amine solutions.2,4 While technically feasible, the use of aqueous amines comes at a high energy requirement for desorption of CO2 due to the high heat capacity of water.2 To this end, contemporary research has focused on using solid supported amine sorbents as potential 1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
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replacements for aqueous amine solutions.3,4,6 Processes based on solid amine sorbents could use less energy to strip CO2 in a temperature swing operation due to the smaller heat capacities of solid sorbents compared to aqueous sorbents.4,6 To rationally optimize the CO2 capture performance of solid amine sorbents, an understanding of the nature of the chemisorbed species is needed. Two primary chemisorbed products have been proposed for CO2 adsorption onto supported amines by extrapolation from literature on aqueous amine solutions. Alkylammonium carbamate ion pairs can form only on primary and secondary amines in dry and humid conditions. Furthermore, the maximum amine efficiency of a sorbent that only adsorbed CO2 as alkylammonium carbamates would be 0.5 mol CO2/mol N (i.e., 1 mole of CO2 is captured by 2 moles of nitrogen) (Scheme S1).7 Tertiary amines are reported to only capture CO2 as alkylammonium bicarbonate in the presence of water (Scheme 1). The formation of such species would give a maximum amine efficiency of 1 mol CO2/mol N.8 It should be noted that in solution, primary and secondary amines can also form bicarbonate species in the presence of water, though carbamate formation is kinetically faster than bicarbonate formation for primary and secondary amines.9 Carbamic acid, a species generally not seen in aqueous amine solutions, have been reported to exist on the solid supported primary and secondary amines under both humid and dry conditions (Scheme S1).10–16 Bicarbonate formation is known to occur in aqueous amine solutions in the presence of CO2, but there has been conflicting evidence regarding the formation of such species on solid supported analogues during CO2 capture. Ammonium bicarbonate formation could be beneficial for solid supported amines because of its lower heat of adsorption compared to the ammonium carbamate species.17,18 Moreover, a low heat of adsorption could help reduce energy costs associated with the desorption process. Furthermore, the ammonium carbamate species that dominate adsorption on sorbents containing primary and/or secondary amines have been shown to slow down diffusion of CO2 by crosslinking aminopolymer chains.19 The formation of ammonium bicarbonate species would not lead to crosslinking because only one nitrogen site is needed to capture a molecule of CO2. In-situ FTIR spectroscopy is the most common technique used to study the mechanisms of CO2 adsorption on supported amines.10–14,16,20–27 Previous studies have shown both an increase and decrease in CO2 capacity on supported amine sorbents when comparing humid conditions to dry conditions, respectively.12,28–31 In studies, in which the CO2 capacity decreased, it was claimed that if too much water was adsorbed onto the sorbents, water can block amine sites. In most FTIR spectroscopy studies, increases of the CO2 capacity under humid conditions are thought to be due to the formation of more carbamate species in the presence of humidity, rather than formation of bicarbonate species. 5,10,32,33 Bacsik et. al. claimed that water liberated 2 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
nitrogen groups that were hydrogen bonded to surface hydroxyl groups and, thus, increased the CO2 uptake and amine efficiency by enabling the formation of more carbamates.10 Water is also thought to improve the kinetics of CO2 adsorption by helping CO2 diffuse through the sorbent and/or increasing aminopolymer chain mobility.12,28,34,35 Few reports have provided experimental evidence for bicarbonate formation over solid amine sorbents. Didas et. al. observed a new band assigned to bicarbonate formation using in-situ FTIR spectroscopy on supported primary amines under humid CO2 capture conditions, but only in (apparently) small amounts.12 Furthermore, bicarbonate formation was observed only when the bands associated with fast forming species were subtracted from the FTIR spectrum. Foo et. al. recently claimed that bicarbonates are formed on tertiary amine [N,N-dimethyl-3-aminopropyltrimethoxysilane (DMAPS)] grafted SBA-15 under nominally dry conditions.11 Residual physisorbed water after pretreatment at 110 oC for 3 h under helium flow was thought to allow for the formation of bicarbonate species in the presence of otherwise dry CO2. However, the reported CO2 capacity of
