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SO-Resistant Amine-Containing CO Adsorbent with a Surface Protection Layer Chaehoon Kim, Woosung Choi, and Minkee Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02831 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019
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ACS Applied Materials & Interfaces
SO2-Resistant Amine-Containing CO2 Adsorbent with a Surface Protection Layer Chaehoon Kim, Woosung Choi and Minkee Choi* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea KEYWORDS CO2 capture, SO2-induced degradation, amine-based solid adsorbent, polyethyleneimine, stability
ABSTRACT Amine-containing solids are promising adsorbents for CO2 capture, but they suffer from irreversible poisoning by the highly acidic SO2 in a flue gas. Here, we demonstrate a facile strategy to inhibit SO2 poisoning. We first prepared an amine-containing adsorbent by impregnating polyethyleneimine (PEI) into a porous silica. The PEI located at the external surface of the adsorbent was selectively alkylated with epoxide so that amines were fully converted to tertiary amines. As opposed to primary and secondary amines, SO2 adsorption onto tertiary amines is fully reversible. Therefore, during the flue gas adsorption, SO2 is reversibly captured by the tertiary amine-rich layer and then desulfurized CO2 is adsorbed onto PEI beneath this layer. The resultant adsorbent showed insignificant loss of CO2 adsorption capacity (8.52%) even after 1000 CO2 adsorption-desorption cycles in the presence of 50 ppm 1
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SO2, while a conventional PEI/silica showed severe capacity loss (65.1%) due to irreversible SO2 poisoning.
INTRODUCTION Significant scientific attention has been paid to carbon capture and sequestration (CCS) technologies because of increasing atmospheric CO2 concentration causing rapid climate change.1,2 Among the various CCS technologies, post-combustion CO2 capture has been most widely investigated because of the possibility of retrofitting existing power plants.3 Absorption using aqueous amine solutions has been considered as a benchmark technology for postcombustion CO2 capture,3,4 but it still suffers from inherent limitations such as large regeneration heat, volatile amine loss, and reactor corrosion.3,5,6 To overcome these limitations, solid adsorbents that are noncorrosive and require low regeneration heat have emerged as promising alternatives.3,7,8 Among the various types of solid adsorbents, amine-containing solids have been most extensively investigated because they can selectively adsorb lowconcentration CO2 (10–15%) in a flue gas and can be regenerated under relatively mild conditions.3,7,8 These adsorbents can be prepared by the impregnation of amine polymers,9-28 surface grafting of amines,23-32 and in situ polymerization of amine monomers25-28 within porous supports. For practical application of these amine-containing adsorbents, long-term stability must be ensured. Such adsorbents can degrade by various chemical pathways including i) urea formation under a CO2-rich atmosphere,13,14,23,24,29-31 ii) steam-induced structural collapse of porous supports,17-19,25 iii) oxidative degradation of amines,13,14,22,25,34 and iv) SO2 poisoning.3336
Earlier studies have successfully demonstrated various material design strategies for
suppressing the first three degradation pathways. In the case of SO2 poisoning, however, the 2
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ACS Applied Materials & Interfaces
only plausible solution thus far suggested is to thoroughly remove SO2 from a flue gas by adopting costly pre-treatment processes. Flue gas from power plants generally contains 500– 2000 ppm of SO2, which is produced by the combustion of sulfur-containing coals.33,37-39 Among the various flue gas desulfurization (FGD) technologies, wet FGD using limestone (CaCO3) slurry has been most widely used because of its cost-efficiency.39-42 It can typically remove ~90% of SO2 in a flue gas, which can reduce SO2 concentration to the 50–200 ppm level.41-43 Even though the resultant SO2 concentration is fairly low, SO2 is a much stronger Lewis acid than CO2 and can still irreversibly poison the basic amine sites of adsorbents.33-36,44 This can cause the gradual deactivation of amine-containing adsorbents, requiring significant adsorbent make-up.37 Extensive FGD can reduce SO2 levels further down to the few ppm level, but it is more cost-intensive than general FGD processes.44 Therefore, the development of amine-containing adsorbents with high SO2-resistance can substantially reduce the overall CO2 capture cost. To design SO2-resistant adsorbents, it is essential to understand the interaction between SO2 and different amine species. Primary (1º) and secondary (2º) amines are known to react with SO2 to form heat-stable salts,34,35 which are stable up to 200 ºC. At typical regeneration temperatures (
