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Adsorption of carbonyl sulfide on propylamine tethered to porous silica Zoltan Bacsik, and Niklas Hedin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01371 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018
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Adsorption of carbonyl sulfide on propylamine tethered to porous silica Zoltán Bacsik, Niklas Hedin* Department of Materials and Environmental Chemistry, Berzelii Center EXSELENT on Porous Materials, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden Keywords: carbonyl sulfide, COS, adsorption, carbon dioxide capture, amine-modified silica
Abstract
Carbonyl sulfide (COS) reacts slowly with amines in the aqueous solutions used to absorb CO2 from natural gas and flue gas and can also deactivate certain aqueous amines. The effects of COS on amines tethered to porous silica, however, have not been investigated before. Hence, the adsorption of COS on aminopropyl groups tethered to porous silica was studied using in situ IR spectroscopy.
COS
chemisorbed
mainly
and
reversibly
as
propylammonium
propylthiocarbamate ion pairs (R-NH(C=O)S- +H3N-R) under dry conditions. In addition, a small amount of another chemisorbed species formed slowly and irreversibly. Nevertheless, the CO2 capacities of the adsorbents were fully retained after COS was desorbed.
Introduction
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Cost-effective technologies to capture CO2 from flue gas are needed for carbon capture and storage (CCS) and are important for natural gas upgrading. Amine scrubbing is commonly used to remove CO2 from gas mixtures but has drawbacks related to the high energy of amine regeneration and the possible leaching of toxic amine degradation products.1,2 To overcome these drawbacks, adsorption-based CO2 capture with immobilized amines is researched.3 Immobilized amines retain the advantages of the aqueous amines but have a potential to be regenerated with a lower heat input. Evaporation of large amounts of water during the regeneration can be avoided and solid adsorbents have a lower heat capacity than amine-water mixtures.4 Immobilized amines are also expected to significantly lower or totally eliminate the risk of emissions of amines or amine decomposition products. The immobilization of the amines can be performed by chemical tethering of the amines to the internal surfaces or by partly filling the pores of the support with amines or polyamines. The amine–CO2 chemistry for immobilized amines has been studied in great detail.3 Commonly, the capture of CO2 is studied from simulated flue gas, consisting of 5–30 vol.% of CO2 and 70–95 vol.% of N2 (or another inert gas), but flue gas also contains significant amounts of other components such as water, SO2, etc. The effect of co-adsorbed water on amine-rich CO2 adsorbents has been studied in great detail but fewer studies have considered the effects of other trace components in the flue gas. Several trace components degrade, deactivate or interfere with the effects of aqueous amines during the reactions with CO2.5 In relation to the integrity of amine-rich CO2 adsorbents, Fan et al.6 studied the effect of SO2 and NO on amine-modified adsorbents embedded in hollow fibers. They observed that NO had no effect on the CO2 uptake capacity of the adsorbents after 150 cycles but that SO2 degraded the primary-amine-modified adsorbent significantly and reduced its CO2 uptake capacity by 55 %. Dramatic decreases in the
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CO2 uptake capacities were observed in two separate investigations of amine-modified adsorbents exposed to SO2 and NO2.7,8 In two other studies, it was shown that SO2 did not degrade adsorbents modified with tertiary amines,9,10 while adsorbents modified with propyldiethanolamine did degrade under cyclic SO2 exposure under moist conditions.11 Huang et al. showed that H2S reacted with propylamine-modified silica and formed R-NH3+-HS- ion pairs under both dry and moist conditions.12 A reversible adsorption of H2S was especially interesting for us as carbonyl sulfide (COS) can be hydrolyzed to H2S. Carbonyl sulfide is relevant to the capture of CO2 from both flue gas and natural gas as the emission requirements of sulfur-containing gas molecules are strict. To remove COS from CO2 mixtures is also challenging and especially important for the production of food-quality CO2, as was noted by Wang et al. in their study on the removal of COS with an AgNO3-modified NaZSM-5 zeolite.13 The amine–COS chemistry in aqueous amines has been studied in detail and the formation of alkylammonium alkylthiocarbamate ion pairs was proposed by Sharma and Danckwerts.14 Such ion pairs were identified by 1H and
13
C NMR spectroscopy on reactions of
COS with aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA) and diglycolamine.15 Alkylammonium alkylthiocarbamate ion pairs were also detected by Littel et al.16 and are widely accepted reaction products of COS and aqueous amines.17–19 Alkylammonium alkylthiocarbamate ion pairs have been shown to form ~100 times slower on reactions with COS than the corresponding alkylammonium alkylcarbamates on reactions with CO2.19–21 The slow reactions pose problems for the removal of COS using aqueous amines. In addition, COS has been shown to irreversible degrade MEA22 and DEA23, which are commonly used in amine scrubbers. Possibly the amine degradation occur on reactions with the hydrolysis product H2S but numerous degradation products and pathways have been proposed.22,23
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Even if COS has been shown to degrade amines and consequently reduce the CO2 uptake capacity of aqueous amines through irreversible and slow reactions, the amine–COS chemistry on amine-modified adsorbents have not been studied to the best of our knowledge. Here, this chemistry is studied under dry conditions on propylamine-modified silica with in situ IR spectroscopy.
Experimental Synthesis of the amine-modified silica. Porous silica (DavisilTM LC60, Grace Davison, US, called as DA) was post-synthetically modified with (3-aminopropyl)triethoxysilane (APTES) at two levels of amine density (Davisil with high and low amine density (DAH, DAL)). The samples were synthesized using the conditions of Aziz et al.24 Initially, silica was degassed at a temperature of 160 °C for 48 h under dynamic vacuum, after which two batches of 3.0 g of the silica and 180 ml of dry toluene were prepared in flasks in a glove box. The dispersions were heated up to a temperature of 50 °C for 30 min. Sequentially, 0.45 ml of H2O was added slowly for the DAH sample and no water was added for the DAL sample. The reaction suspensions were refluxed for 60–90 min. After this procedure, the temperature was reduced to 80 °C, and 15 g of APTES was added dropwise in both cases. The reactions were kept under a N2 atmosphere with reaction times of 7 and 72 h for the DAL and DAH samples. The colorless solid powders were filtered off and washed with toluene (50 ml x 2) and ethanol (50 ml x 3), extracted with dichloromethane, overnight, and dried under a flow of N2 at a temperature of 120 °C, overnight. Carbon dioxide (>99.9995 %) and carbonyl sulfide (0.1 vol.% in N2) were purchased by the Linde Gas Company and Scott Specialty Gases and used as received.
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N2 and CO2 adsorption. The adsorption-desorption isotherms of N2 were measured at a temperature of -196 ºC using a Micrometrics ASAP2020 volumetric adsorption analyzer. The surface area was calculated in the Brunauer-Emmett-Teller (BET) model, using the N2 uptake at relative pressures (p/p0) of 0.06–0.15. The samples were treated under dynamic vacuum for 10 hours at a temperature of 120 ºC before the experimentation. Adsorption and desorption of CO2 were measured at a temperature of 0 ºC, at pressures of 1– 101 kPa, using the volumetric adsorption analyzer mentioned above. Before experiments, the samples were treated under dynamic vacuum for six hours at a temperature of 120 ºC.
Thermogravimetric analysis (TG). The TG curves were recorded with a Perkin Elmer TGA7 instrument. The samples were subjected to a flow of air and the temperature was risen from 25 to 900 °C at a rate of 10 °C/min. The amounts of organics in the amine-modified silicas were estimated from the mass losses observed at temperatures between 200 and 600 ºC.
In situ IR spectroscopy was performed using a transmission mode configuration, with the IR cell interfaced to an in-house constructed vacuum system.25 Self-supporting pellets of DAL and DAH were prepared by pressing ~25 mg of the powders for 2 min using a press die and a pressing tool. The diameter of the die was 16 mm. The pellets were placed in the IR cell and connected to the vacuum system and treated under dynamic vacuum (
