CO2 Capture in Lithium Cuprate (Li2CuO2) at Low Temperatures; Effect of the Water Vapour Addition

Carbon dioxide (CO₂) emissions are one of the most threatening pollution problems in the world. One approach to solve such environmental problem is the CO₂ capture and storage. Therefore, CO₂ capture in solid materials have been considered to be the most practical option as an efficient and cheap alternative to reduce greenhouse gases. In this way, different materials have been proposed as CO₂ captors such as zeolites, porous (active) carbons, metal−organic frameworks (MOFs), alkali metal-promoted alumina and carbonates, alkaline and alkaline-earth ceramics, and layered double hydroxides [1].

In addition, water vapour is commonly found to coexist with CO₂, during post-combustion. In a typical powder plant, the final flue gases contain approximately N₂ (72%), CO₂ (8-12%), H₂O (8-10%) and smaller concentrations of the other polluting species. Consequently, it is very important to understand the CO₂-H₂O system [2].

Lithium cuprate (Li₂CuO₂) has been studied as CO₂ chemisorbent at high temperature (>200ºC) presenting interesting results. The CO₂ capture, using Li₂CuO₂ as a solid absorbent, takes place according to the next reaction:

Li₂CuO₂(s) +CO₂(g) ↔ Li₂CO₃(s) +CuO(s)

For this reaction, the theoretical capacity corresponds to 9.1 mmol/g, while the maximal experimental reported is about 9.05 mmol/g, at 675 °C [3,4].  On the other hand, it has been reported that some lithium ceramic are able to trap CO₂, chemically, at low temperatures (30-80 °C) under the water vapour presence [2]. Hence, the objective of this work was the study the CO₂ capture on lithium cuprate under similar temperature and relative humidity conditions than those reported.

Experimental section

Lithium cuprate (Li₂CuO₂) was synthesized by solid-state method. Initially, lithium oxide (Li₂O, Aldrich) and copper oxide (CuO, Acros Organics) were mixed mechanically, in order to get a good homogeneity of the reagents. The reagent mixtures were prepared using a lithium excess of 10%, based on the stoichiometric lithium content on Li₂CuO₂. Then, the powders were calcined at 800 °C for 6 h in air.

X-ray diffraction was used to identify the phases obtained after the calcinations process. Additionally, N₂ adsorption-desorption technique was used to determine the sample surface area, using the BET model. Dynamic and isothermal experiments were carried out on a humidity-controlled thermobalance at different temperatures and relative humidity (RH). The experiments were performed using distilled water and two different gases: nitrogen (N₂, Praxair grade 4.8) and carbon dioxide (CO₂, Praxair grade 3.0). The total flow gas used in all the experiments was 100 mL/min and the RH percentages were controlled automatically by the equipment. Dynamic water vapor sorption/desorption experiments were generated at different temperatures (between 30 and 80 ◦C), varying the RH from 0 to 80% and then from 80 to 0%. Additionally, different isothermal experiments were performed at specific temperatures and RH for 180 min, using N₂ or CO₂ as carrier gases.

After the CO₂ capture experiments, the sample products were characterized to identify the hydration-carbonation products. The samples were analyzed using infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA).

Results

The XRD pattern of the pristine Li₂CuO₂ was fitted to JCPDS file 00-084-1971 and no other secondary phases were detected. Then, the corresponding N₂ adsorption-desorption curve was determined. The isotherm was type II, according to the IUPAC classification. This kind of N₂ adsorption-desorption isotherms are associated to macroporous or non porous materials. This result is in good agreement with the synthesis method used, solid-state reaction. Then, surface area was determined using the BET model (0.2 m²/g).

After the structural and microstructural characterization, different water sorption−desorption curves were obtained using N₂ as a carrier gas. In this case, the sorption isotherms corresponded to type III, according to the IUPAC classification. The water sorption varied as a function of the temperature, and it was not completed or limited to the sorption curves, because during some part of the desorption process, the samples continued gaining weight. The same isothermal trends were observed when CO₂ was used as carrier gas. Nevertheless, the weight increments were much higher in the CO₂ cases.

To further understand the influence of RH and temperature in the Li₂CuO₂−CO₂−H₂O system, different isothermal experiments were performed. The experiments performed at different temperatures (30, 40, 50, and 60 °C) and RH (20, 40, 60, and 80%). For each temperature, the weight increment rates increased as a function of the RH, as it could be expected.

To confirm and quantify that CO₂ was chemisorbed on Li₂CuO_2, the products were analyzed using infrared spectroscopy (FTIR) and termogravimetric analysis. The IR spectrums presented the metal–oxygen vibration bands (Li–O and Cu–O) between 400 and 750 cm⁻¹, and presented all vibration bands of lithium carbonate (1499, 1434, 1086, 862, 738 and 498 cm^-1) [5]. To quantify the carbonation and superficial hydration and/or hydroxylation on Li₂CuO₂, under the different thermal and humidity conditions, all of the isothermal products were characterized using TGA. The decomposition results confirm that lithium cuprate capture more CO₂ as a function of temperature and relative humidity. For example, at 80 °C and 80 % of humidity relative the Li₂CUO₂ capture was equal to 6.68 mmol/g, while at 80°C and 60% of RH the amount of CO₂ chemisorbed was 5.27 mmol/g. Additionally, at 60°C and 80% of RH, the CO₂ chemisorbed was equal to 4.39 mmol/g. These results showed that lithium cuprate works adequately as CO₂ captor at low temperatures in the presence of water vapour.

Reference

[1]Choi, S.; Dresse, J. H.; Jones, C. W. ChemSusChem. 2009, 2 (9), 796−854.

[2] Avalos-Rendon, T.;  Pfeiffer, H.  Energy & Fuels. 2012, 26 (5), 3110-3114

[3] Palacios-Romero, L. M.; Pfeiffer, H. Chem. Lett. 2008, 37, 862–863.

[4] Palacios-Romero, L. M.; Lima, E.; Pfeiffer, H. J. Phys. Chem. A 2009, 113, 193–198

[5] Ravikrishna, R.; Green, R.; Valsaraj, K. T. J. Sol-Gel Sci. Tech. 2005, 34 (2), 111-121.

Keywords

CO₂ capture, lithium cuprate, CO₂-H₂O sorption, low temperature captor.