Regular Article
Stability of Polyamine Based Adsorbents to Gas Impurities for CO2 Capture
2022 Volume 62 Issue 12 Pages 2442-2445
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2022 Volume 62 Issue 12 Pages 2442-2445
Amine based adsorbents are promising options for CO2 removal from industrial exhaust streams because they offer low regeneration energy and high CO2 capture performance. Tetraethylenepentamine (TEPA) is one of the most common commercial polyamines that has been used as a prototypical amine to develop effective amine based adsorbents for CO2 capture. For the viability of carbon capture based on amine adsorbents, the durability of amine sites during practical applications is an important criterion. This work focused on the stability of TEPA based materials under accelerated oxidizing conditions in the presence of O2, SO2, NO2, water vapor, and simulated flue gas. It was found that the presence of gas impurities caused a marked loss of the CO2 adsorption capacity of the adsorbents. The presence of water vapor and CO2 suppressed oxidative degradation of the adsorbents.
The vast majority of CO2 released from large stationary sources like power plants, cement, and steel factories contributes to the dramatic increase in CO2 emissions into the atmosphere which is associated with global climate. To reduce CO2 emissions and lessen their burden on the global climate, it is, therefore, urgent to find efficient and cost-effective ways to capture CO2. CO2 sequestration using liquid amines has been practically applied on large scales, however, volatilization, high energy consumption for regeneration, and equipment corrosion are current problems.1,2,3) These shortcomings led to a high cost in capture process and equipment maintenance. Compared to absorption-based amine scrubbing, CO2 capture using amine impregnated adsorbents has appealed as a beneficial option for CO2 capture due to low regeneration energy consumption, high adsorption performance, low corrosivity.2,4)
Tetraethylenepentamine (TEPA) is one of the most common commercial polyamines that has been used as a prototypical amine to develop effective adsorbents for CO2 capture as it has a high content of amino groups and low volatility. TEPA impregnated MCM-41 adsorbent developed by Zhu’s group5) captured up to 5.02 mmol CO2/g at 75°C. TEPA/SiO2 synthesized by Choi group6) exhibited the highest CO2 adsorption capacity of 5.36 mmol/g at 60°C under the adsorption conditions of 15% CO2, 2% Ar, 10% H2O in N2 balance. In another work,7) the CO2 adsorption capacity of hierarchical porous silica modified with 65 wt% TEPA can reach 5.91 mmol/g. Zhao8) synthesized a novel mesocellular silica foams (MCF). By functionalizing 60 wt% TEPA into MCF, the TEPA supported MCF can capture 4.75 mmol CO2/g at 75°C.
Besides the CO2 adsorption performance, the stability of sorbent is a crucial parameter for industrial-scale applications in the field of capture and sequestration of CO2, resulting in a decrease in cyclability and practical lifetimes of sorbents. Off gases from ironmaking, steelmaking, and flue gas contain not only CO2 but also water vapor, SO2, NO2, and O2.9,10,11) Both chemical and physical conditions can affect the stability of amine solid adsorbents, leading to chemical deterioration induced by gas components contained in exhaust gases such as O2,12) NOx,13) SO2,13) and thermal deterioration including amine decomposition and evaporation.14) The presence of CO2 also affects the degradation mechanism of amine based adsorbents.15) This work focused on the stability of TEPA based adsorbents under accelerated oxidizing conditions in the presence of O2, water vapor, SO2, NO2, and CO2. To characterize the sorbents, CO2 adsorption measurements, N2 physisorption isotherms, diffuse reflectance FT-IR spectroscopy, and thermal gravimetric analyses were carried out. The results show that the deterioration of amine based adsorbents depends on the temperature, gas components, and gas concentrations of the oxidative conditions. The presence of humidity and/or CO2 in the gas streams suppresses TEPA from oxidative deterioration. Under oxidative environments, the amine based adsorbents showed a marked loss of the CO2 capture ability, which was mainly caused by changes in the functional groups of the polyamine.
Synthesis of amine impregnated adsorbents: The TEPA adsorbent was prepared by a wet impregnation method which was described in our previous research.16) A mixture of 6 g TEPA (TCI), 4 g mesostructured cellular silica foam (MF, Sigma Aldrich), and 200 mL methanol (Sigma Aldrich) was agitated for 2 h. The solvent was then removed in a rotary evaporator to obtain the TEPA adsorbent.
Deterioration of adsorbents: Oxidative deterioration tests were carried out in a packed-bed reactor.17) Typically, an adsorbent was pretreated in N2 at 100°C for 1 h to remove pre-adsorbed gases. Subsequently, the adsorbent was subjected to the oxidative deterioration by heating the adsorbent to a temperature in the range of 60–100°C for 18 h under various mixture gases such as O2 (5%, 21%, and 100%), simulated flue gas SFG (13% CO2, 5% O2, N2 balance), and 200 ppm SO2/NO2 balance N2. After the course of deterioration, the gas was switched to N2 at room temperature, the degraded adsorbent was then collected. For deterioration tests in wet conditions, relative humidity (RH) was introduced to the specified gases up to 50%. The oxidative condition was denoted as the gas used for the oxidative step-temperature (°C)-time of oxidation (h)-RH (%). For example, 5%O2-100°C-18h-RH5 represents the adsorbent exposed to 5% O2 balance N2 at 100°C for 18 h at a relative humidity of 5%.
Measurement of CO2 adsorption capacity: CO2 adsorption performance of the adsorbent before and after the deterioration was measured using a surface area and porosimetry measurement system ASAP 2020, Micromeritics Co. About 0.1 g of a measurement sample was weighed into a sample tube. Before the measurement, the adsorbent was conducted in 6-hour vacuum evacuation at 40°C to remove preadsorbed gases. The measurement was then performed by introducing CO2 gradually The CO2 adsorption isotherms were obtained at 40°C.
Characterization of materials: Nitrogen adsorption/desorption isotherms were measured at −196°C with a surface area and porosimetry analyzer ASAP 2420, Micromeritics Co. Before the measurement, the sample was pretreated by a 6-hour vacuum evacuation to remove preadsorbed gases. The specific surface area was calculated by the multipoint Brunauer–Emmett–Teller (BET) method. The pore size distributions were obtained by using the Barrett–Joyner–Halenda (BJH) desorption branch of the isotherms. The single-point total pore volume was calculated at a relative pressure of 0.97.
Thermogravimetric measurements were carried out to calculate the organic content of the adsorbents. The measurements were operated on a Rigaku STA8122 thermogravimetric analyzer in a temperature range increasing from 25 to 100°C with a heating rate of 10°C/min under N2 flow of 100 mL/min.
The IR spectrometer (Shimadzu, Tokyo, Japan) was conducted with a resolution of 4 cm−1 to obtain the IR spectra of the samples.
After the modification with TEPA, the surface area and pore volume of the silica support decreased significantly (Fig. 1). The total pore volume was reduced from 2.03 to 0.19 cm3/g and the specific surface area was reduced from 507 to 39 m2/g, which confirms that TEPA has been successfully impregnated on/into the support.
(a) N2 adsorption desorption isotherms and (b) pore distributions of MF and MF impregnated with TEPA. (Online version in color.)
To evaluate the stability of the TEPA based adsorbents, the adsorption capacities of the materials before and after the treatment to oxidizing conditions were measured. Figure 2 shows the effect of oxidation temperature on the CO2 adsorption capacities of the adsorbents. In this study, treatment temperatures were chosen such that they are in the range of desorption conditions typically reported for amine based materials in post combustion CO2 capture applications.4,18,19,20,21) It is evident that TEPA adsorbent was not affected much by the oxidative treatment in low-temperature conditions (60°C) with a 1% loss in CO2 adsorption capacity. However, in the range of elevated temperature (80–100°C) under pure O2, the TEPA adsorbent showed significant decreases in adsorption capacity. After the treatment under pure O2 at 100°C for 18 h, the TEPA adsorbent was incapable to capture CO2, indicating that TEPA adsorbent deteriorated completely. After the thermal treatment in pure N2 at 100°C for 18 h, the adsorption capacity of TEPA adsorbent still retained 99% of its original capacity. Therefore, the loss of CO2 adsorption capacity of adsorbents during the treatments under O2 containing environments was mainly due to the oxidative deterioration of the amine.
Effect of oxidation temperature on stability of TEPA adsorbent. (Online version in color.)
Effect of O2 concentration and the presence of CO2 along with O2 on stability of TEPA adsorbent were evaluated and shown in Fig. 3. The presence of a gas containing O2 reduced the life of the adsorbent, significantly high with high O2 concentrations. After the treatment under dilute O2 concentration of 5%, the TEPA adsorbent still lost its CO2 capture ability, indicating that it is impossible to avoid oxidative deterioration of amine adsorbents if O2 is present in the gas stream. It is noteworthy that the presence of CO2 along with O2 enhanced the oxidative stability of the adsorbent. The degraded adsorbent obtained after the treatment under SFG (13% CO2, 5% O2) can capture more CO2 than the one obtained after the treatment under 5% O2. This may be attributed to the fast reaction between amino groups and CO2 to form carbamate and protonated amine, which exhibited much better oxidative stability compared to amines.
Effect of O2 concentration and CO2 on oxidative stability of TEPA adsorbent. (Online version in color.)
The deactivation of materials under humid oxidation conditions is less severe compared with dry oxidation conditions (Fig. 4). Under the exposure of 50% RH SFG at 100°C for 18 h, an increased CO2 adsorption capacity of 0.46 mmol/g was obtained, which is ca. 13% higher than that obtained from dry SFG. A similar trend was observed for the deterioration in pure O2. After the exposure to dry pure O2 at 100°C for 18 h, the TEPA adsorbent lost 96% of its CO2 capture ability at 100 kPa, dropping from 4.0 mmol/g to 0.18 mmol/g. With the introduction of 5% RH to the oxidative condition in pure O2, the adsorbent retained the capture ability of 0.69 mmol/g. With the introduction of 50% RH, this increased ca. 2 times, suggesting that the CO2 capture ability of the adsorbent was not directly proportional to the RH.
Effect of water vapor on oxidative degradation of the TEPA adsorbent. (Online version in color.)
After exposure to humid O2 or CO2-containing environments, the adsorbent can capture more CO2 than the one treated without the presence of water vapor in oxidation conditions. The presence of humidity along with O2 could contribute to strong hydrogen bonding between the nitrogen atom of amine and water molecule. Such interaction could prevent the availability of the nitrogen atom to react with O2 species, reducing the deterioration of the amine adsorbent. Furthermore, the presence of moisture may inhibit the formation of open-chain and cyclic ureas which were found during the deactivation of amine in CO2.22)
The CO2 capacity of the adsorbent was influenced by exposure to SO2 and NO2 (Fig. 5). The adsorbent decreased in CO2 removal ability with the presence of these acidic gases. The adsorbent displayed a higher remaining CO2 capacity after being exposed to 200 ppm NO2 than 200 ppm SO2. After the exposure to SO2 at 100°C for 18 h, the adsorption capacity at 100 kPa of the adsorbent dropped from 4.0 mmol/g to 2.6 mmol/g whereas in the case of NO2 at the same condition the adsorbent still can capture 3.5 mmol/g.
Effect of SO2 and NO2 on CO2 capture ability of TEPA adsorbent. (Online version in color.)
Under oxidative conditions, the adsorbents exhibited a dramatic decrease in CO2 adsorption performance. FT-IR spectra of all the degraded adsorbents (Fig. 6) reveal degradation of the active sites as evidenced by the appearance of bands at 1668 cm−1 assigned to the formation of C=O/C=N. The intensity of this band in the degraded adsorbents depends on oxidative conditions, increasing as the following treatment conditions: CO2-containing environment < humid O2-containing environment < dry O2-containing environment. This trend of the intensity indicates the degree of deactivation of the adsorbents which was consistent with their CO2 capture ability (Fig. 4). The degraded adsorbent obtained after the treatment under SFG exhibited the highest adsorption capacity and the degraded adsorbent obtained after the treatment under dry O2 exhibited the lowest adsorption capacity.
FT-IR spectra of TEPA adsorbent before and after the oxidative deteriorations. (Online version in color.)
The organic content of the adsorbents after the treatment under oxidative conditions was assessed using thermogravimetric analysis (TGA) and the results are shown in Fig. 7. After the oxidative treatments, the adsorbent lost 2 wt% compared to the fresh adsorbent, and there is almost no difference in total change in weight at the end of all the degraded adsorbents. Therefore, the change in organic content did not appear to track the oxidative stability of the amine based adsorbents. However, the mass loss curves collapse onto a single curve at different temperatures for each oxidative condition, indicating a different degree of deactivation of the adsorbents.
TGA profiles of TEPA adsorbent before and after the oxidative deteriorations. (Online version in color.)
The oxidative conditions such as temperature, time duration, gas components, and gas concentration affect strongly oxidative stability of the polyamine based adsorbents. It is impossible to avoid the deterioration of the amine based adsorbent if gas components such as O2, SO2, and NO2 are present. The presence of moisture and CO2 in the O2 medium could enhance the stability of amine based adsorbents toward oxidative degradation. The presence of SO2 and NO2 must be avoided if the objective is CO2 capture from flue gas. The deactivation of basic amines into non-basic compounds in O2-containing environments mainly caused the marked loss in the CO2 adsorption capacity of the amine based adsorbents.
This work was financially supported by the Ministry of Economy, Trade, and Industry (METI) and the New Energy and Industrial Technology Development Organization (NEDO).
