Influence of water vapor and acid gases on CO2 adsorption using N,N-dimethylethylenediamine decorated Cu-BTC
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N,N-Dimethylethylenediamine (mmen) was incorporated as a ligand on the Cu2+ of Cu-BTC by a protophilic solvent-assisted solvothermal method for improved performance. X-ray diffraction, scanning electron microscopy and low temperature N2 adsorption were used to investigate the structural stability of the synthesized material under the influence of flue gas components, such as water vapor, NO and SO2. The thermal stability was determined through thermal gravimetric (TG) analysis. The influence of the main flue gas components on the CO2 adsorption capacity of the material were discussed as well. Finally the SO2/NO resistance mechanism were deduced. Both Cu-BTC-raw and Cu-BTC-mmen were very stable in the presence of H2O, while the former could be destroyed by the continuous NO/N2 and SO2/N2 mixture flow significantly. This situation has been improved on Cu-BTC-mmen sample, probably because the mmen on the surface of Cu-BTC react with SO2/NO preferentially and protect the Cu2+ sites to some extent. Cyclic regeneration test of the CO2 adsorption process on fresh and disposed adsorbents was also investigated.
KeywordsCu-BTC N,N-Dimethylethylenediamine CO2 adsorption Water vapor Acid gases
At present, postcombustion carbon capture and sequestration from power power plant flue gas is considered to be an effective way to mitigate the impact of CO2 emissions on climate change [1, 2, 3, 4]. Capturing CO2 from flue gas is extremely challenging due to the high flow rates, the low partial pressure of CO2 and the need to consider the separation costs . Adsorption CO2 with solid media is considered to be a more energy efficient technology for CO2 capture than traditional absorption method with alkaline solvents [6, 7].
Since coal is commonly burned in air, N2 is the highest content in the flue gas. The typical composition of the coal-fired flue gas was 12.5–12.8% CO2, 6.2% H2O, 4.4% O2, 50 ppm CO, 420 ppm NOx, 420 ppm SO2, and 76–77% N2 . Therefore, the CO2/N2 selectivity is as important as the CO2 adsorption capacity for CO2 adsorbents [9, 10]. In the past decades, selective adsorption of CO2 over N2 has been extensively studied, both theoretically and experimentally. In particular, other components in flue gas, such as H2O, SO2 and NO, are crucial for the applications of a sorbent in CO2 capture due to the considerable content and particular interaction with the adsorbents . The effect of H2O on CO2 adsorption depends on specific situation. When water molecules compete with CO2 for the adsorption sites, it has an inhibiting effect on CO2 adsorption. Contrarily, water vapor in the flue gas promotes the CO2 adsorption on the amine-based sorbents because the formation of bicarbonate under a wet condition can enhance the amine efficiency . What’s more, the effect of moisture on CO2 adsorption also depends on moisture content . In general, SO2 inhibit CO2 adsorption in amine-based sorbents due to the formation of the heat-stable and irreversible amine salts . NOx on the other hand was predicted to have a significantly smaller effect on limiting CO2 adsorption .
Metal–organic frameworks (MOFs) are a novel type of porous crystal materials with ultrahigh porosity, enormous internal surface areas and the extraordinary degree of variability for both the organic and inorganic components of their structures [16, 17, 18]. Due to these properties, MOFs have proved to be the hottest material for CO2 capture in recent years and displayed superior CO2 adsorption performance [19, 20, 21]. The influence of the impurities, such as water, SO2, and NOx, in flue gases on CO2 separations in MOFs were also explored by some researchers. Decoste reported the water resistance property by characterizing the crystal structure and morphology of Cu-BTC before and after exposure to 90% relative humidity at 25 °C . Xie investigated the PXRD patterns of Cu-BTC before and after being kept in moist air, SO2/N2 mixture and NO/N2 mixture .
In this work, the N,N-dimethylethylenediamine (mmen) was incorporated as a ligand on the Cu2+ cation sites of Cu-BTC for improved performance. A thorough investigation of the effects of the impurities in flue gas on the structure, such as crystal structure, morphology, surface area, as well as CO2 capture was carried out.
2 Experimental methods
2.1 Synthesis of adsorbents
The power X-ray diffraction (XRD) (Beijing Purkinje General Instrument Co., Ltd., China) pattern were recorded using nickel-filtered Cu Kα radiation at 36 kV and 40 mA (2θ ranging from 5° to 80°, λ = 0.15418 nm and 0.02° step size). The morphologies and composition of samples were observed on the JSM 6380 SEM (Japanese Electronics Co., Ltd., Japan) at an acceleration voltage of 30 kV. The BET surface areas (SBET) and pore volumes (Vpore) of given samples were determined by surface area and porosimetry analyzer (V-Sorb 2008P).
To study the structural stability in water vapor, 300 mg of each sample was disposed with a saturation water vapor at 60 °C for 2 days. To study the structural stability in NO and SO2, 300 mg of each sample was disposed with a binary-gas flow of 400 ppm NO and SO2 in N2 for 2 days, respectively.
CO2 adsorption isotherms were measured using a surface area and porosimetry analyzer (V-Sorb 2008P) under low pressure (0–1.1 bar). The experiments were conducted at 0 °C and 20 °C by the dewar flask using ice water or circulated water in which the sample tube was immersed. Prior to each adsorption experiment, the samples were degassed at 120 °C overnight. The reversibility of the CO2 adsorption process on the samples studied was tested by multiple CO2 adsorption cycles. The corresponding adsorption isotherms have been acquired after heating at 120 °C for 12 h in vacuum in sequence.
3 Results and discussion
3.1 Structural characterization (XRD, SEM, BET)
When mmen is incorporated onto the adsorbents, secondary amino groups of mmen react with SO2 preferentially and protect the Cu2+ sites to some extent. As can be seen from Fig. 4b, the isotherm of all the four Cu-BTC-mmen samples remained typical I behavior. Therefore, the behavior of the Cu-BTC-raw is so different compared to the one of Cu-BTC-mmen.
Physical properties of Cu-BTC-raw and Cu-BTC-mmen
Pore volume (m3/g)
Pore volume (m3/g)
3.2 TG characterization
In Fig. 5a, the fresh Cu-BTC-raw displayed about 11% weight loss from 100 to 280 °C, corresponding to the removal of solvent molecules adsorbed on the surface and water molecules coordinated with Cu (II). With temperature increasing, Cu-BTC showed a sharp weight loss of 41% from 280 to 410 °C due to the decomposition of the trimesic acid, which confirmed the structural collapse of the sample . The decomposition temperature of Cu-BTC-raw is close to those of rht-MOF-9 and MOF-74(Co) [33, 34, 35]. The TGA curve of Cu-BTC-mmen almost remained that of Cu-BTC-raw but the weight loss increased, which could be attributed to the removal of N,N-dimethylethylenediamine. In Fig. 5b, the weight loss of disposed with water vapor was almost equal to that of fresh sample. However, the adsorbents disposed with a binary mixture of NO or SO2 in N2 displayed a higher weight loss, corresponding to the decomposition of nitrate and sulfate respectively. And the TGA curves of fresh Cu-BTC-mmen and the ones dealed with H2O/NO/SO2 in Fig. 5b resemble that of Cu-BTC-raw.
3.3 Adsorption properties
Amount of CO2 adsorbed at 0 °C and 20 °C
Amount of CO2 adsorbed (mmol/g)
At 0 °C
At 20 °C
The CO2 adsorption isotherms on the two samples studied at two temperatures follows the trend that adsorption capacity increased with the pressure increase and decreased with the temperature increase . At the same temperature, CO2 uptake of Cu-BTC-raw is larger than that of Cu-BTC-mmen, which is consistent with the values of surface area. Therefore, the CO2 adsorption on the synthesized adsorbents is mainly dependent on physisorption determined by the surface area.
3.4 Influence of flue gas components on CO2 adsorption capacity
3.5 SO2/NO resistance mechanism
3.6 Regenerability and multicycle stability
N,N-Dimethylethylenediamine (mmen) was incorporated as a ligand on the Cu2+ cation sites of Cu-BTC for improved CO2 adsorption performance. A thorough investigation of the effects of the impurities in flue gas on the structure, such as crystal structure, morphology, surface area, as well as CO2 capture was carried out. Cu-BTC-raw and Cu-BTC-mmen was very stable in the presence of H2O, but could be destroyed by the continuous NO/N2 and SO2/N2 mixture flow in varying degrees. It can be explained that the strong acidity of SO2 and NO competes with CO2 for the adsorption sites and forms heat-stable and irreversible salts. These salts could block the pore structures of the adsorbents, leading to a decrease in surface area, pore volume and CO2 uptake. It is notable that this situation has been improved on Cu-BTC-mmen sample, probably because the mmen on the surface of Cu-BTC react with SO2/NO preferentially and protect the Cu2+ sites to some extent. Cyclic regeneration test also reveals a more stable CO2 adsorption performance and excellent regenerability of Cu-BTC-mmen.
The Work was supported by Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Key Laboratory under Construction for Volatile Organic Compounds Controlling of Jiangsu Province.
Compliance with ethical standards
Conflict of interest
The author(s) declare that they have no competing interests.
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