R. Ojeda-López1*, E. Vilarrasa-García1, Bianca F. dos Santos1, C. Felipe2, J. Muthuswamy-Ponniah2, D.
Azevedo1,
1 Universidade Federal do Ceará - Departamento de Engenharia Química
2 Instituto Politécnico Nacional - Departamento de Biociencias e Ingeniería
Abstract
Carbon microfibers (CMFs) were obtained by calcination of polyacrylonitrile microfibers (PANMFs) synthesized by the electrospinning method for their application in CO2 selectivity for the CO2:CH4 mixtures. The calcination process was carried out in two stages: i) stabilization at 280 °C in an air atmosphere and ii) carbonization at five different temperatures (600, 700, 800, 900, and 1000 °C) in a nitrogen atmosphere. The impact of carbonization temperature is reflected in the change of textural (specific surface area) and chemical (nitrogen and oxygen content) properties. The increase in carbonization temperature generates: i) increase in the specific surface area (microporosity) and in the carbon content, and ii) decrease in nitrogen content. Materials carbonized at 800 and 900 °C have high specific surface area and still preserve a proportion of nitrogen and oxygen functional groups, which makes them the most adequate materials for CO2 adsorption and CO2 selectivity in CO2:CH4 mixtures.
Keywords: carbon microfibers (CMFs), electrospinning, CO2 selectivity, microporosity.
1. Introduction
In the last years, the carbon materials have been studied for diverse applications as: adsorbent of CO2 [1], CH4 [2], H2 [3], electrocatalysts in fuel cells [4], cathode or anode in lithium ion batteries [5], and more. Particularly, as gas adsorbent, i) microporosity and ii) surface chemical composition (nitrogen or oxygen) have been reported to be of vital importance for better adsorption; the microporosity has been increment through chemical treatment (calcination at different temperatures or using KOH solutions) [6], while, the chemical composition surface by doping with oxygen or nitrogen groups [7]. Avoiding a post-synthesis modification for the addition of functional groups is possible by using carbon precursors that already contain nitrogen and oxygen functional groups and polyacrylonitrile (PAN) pyrolysis allows this process [8]. Therefore, this work focuses on the study of both parameters to select the most viable material for better i) CO2 adsorption and ii) CO2 selectivity in CO2:CH4 mixtures. CO2
capture is currently an important topic due to the effect of these gases on the environment, and the separation of CO2 from CO2:CH4 mixtures for the purification of natural gas and biogas.
The effect of the chemical composition and the microporosity on CO2 and CH4 adsorption it will be study principally by: i) CO2 and CH4 adsorption at 1 bar, and ii) ideal selectivity of CO2
in CO2:CH4 mixtures. The first study was performed on a volumetric equipment considering analysis temperatures from -10°C, -5, 0, 5, 10, 15, 20, 25, and 30 °C up to 1 bar, to obtain nine adsorption isotherms, which also allow the estimation of selectivity in gas mixtures.
The ideal selectivity of the CO2 for the CO2:CH4 gas mixture was calculated using the following equation:
𝑆𝑖/𝑗 = 𝑞𝑖/𝑞𝑗
𝑝𝑖/𝑝𝑗 (1)
where, Si/j is the selectivity, qi and qj represent the amount adsorbed of components i and j, PT
is the total pressure in the mixture, and pi and pj represent the partial pressure of components i and j, respectively. Considering the Dalton´s law and the definition of partial pressure, Eq. 1 is converted at:
𝑆𝑖/𝑗 =𝑞𝑖∗𝑝𝑗
The polymer precursor, polyacrylonitrile (PAN) was supplied by Sigma-Aldrich, with a molar mass average of 150,000 amu and it was used as received without additional purification.
The solvent was anhydrous N,N-dimethylformamide (DMF), with 99.8 % purity and density of 0.944 g/mL, also supplied by Sigma-Aldrich.
2.2. Methodology
PANMF were synthesize with a polymer concentration of 10 %. The electrospinning apparatus was set at a flow rate of 1.0 mL h-1, a voltage of 15 kV and 10 cm distance between the tip of the syringe and the collector. All microfibers were stabilized at 280 °C (air atmosphere) for 30 minutes and carbonized to five different temperatures at 600, 700, 800, 900, and 1000 °C (nitrogen atmosphere) for 90 minutes. The CMF samples will be identified as CY, where C is the reference to the carbonization and Y is the calcination temperature; for example, C800 is the sample calcined at 800 °C. CO2 and CH4 adsorption isotherms were obtained from temperatures between -10 °C and 30 °C up to 1 bar. CO2 selectivity in CO2:CH4 mixtures was evaluated considering two compositions: i) 50:50 and ii) 30:70.
3. Results and discussion
The textural and chemical properties are shown in Fig. 1A. The specific surface area increases when the carbonization temperature increases from 296 to 822 m2 g-1 for C600 and C1000. And by means of the general XPS spectra it can be observed that the increment in the carbonization temperature, increases the carbon content rises and decreases the nitrogen content, while the amount of oxygen does not follow a trend. The impact of these two properties is evident in Fig. 1B.
From the adsorption isotherms of CO2 and CH4, Fig. 1B shows the adsorbed volume at different temperatures and 1 bar. In overall, the materials present a greater affinity for the CO2. A peculiar behavior can be observed, the materials carbonized at 800 °C and 900 °C adsorb a greater amount of CO2, compared to the material carbonized at 1000 °C (with a greater surface area). This implies that in addition to the specific surface area, the surface chemical composition has a significant effect on the affinity of CMFs to CO2. When the analysis temperature is increased, C800 and C900 tend to adsorb very similar amounts of CO2. Generally, in an industrial application, temperatures above 25 °C are used, therefore, based on these results, it can be recommended carbonized materials in a temperature range between 800 and 900. To corroborate that a higher specific surface area but lower amount of nitrogen and oxygen functional groups, generates a decrease in CO2 capacity, in a previous work was performed a design of experiments producing a response surface and the results are reproducible [9]. These results demonstrate that improve the CO2 adsorption implicates a synergistic effect between the chemical composition and the specific surface area, because if it was only the chemical
not observed. Because the materials carbonized at 600 °C and 700 °C adsorb significantly less amount of CO2 and CH4, they are excluded for the following more detailed studies, remaining the three materials that adsorb similar quantities, i.e., C800, C900, and C1000.
Fig 1. (A) XPS general survey of precursor polymer (PAN) and the PANMF carbonized at different temperatures. (B) CO2 and CH4 adsorption in CMFs up to 1 bar.
The CO2 selectivity in the CO2:CH4 mixture is higher when it is considered a mixture 0.30:0.70, viz Figure 2A. But, in both cases, the behavior of three materials is similar and in the following order: C800 > C900 > C1000. In the mixture 0.30:0.70, at 0.1 bar, the theoretical selectivity is around 11.0, 10.0 and 8.4 mmol CO2/mmol CH4, respectively. As C800 material demonstrates the best selectivity, Fig. 2B shows the selectivity considering different temperatures of analysis (in an interval from -10 °C to 30 °C), where it is perceived that the selectivity is slightly higher at high temperatures, although the difference is not very significant.
This fact can be attributed to the CO2 diffusion through the smallest pores, which is improved at high temperatures.
Fig. 2. (A) Theoretical selectivity of CO2:CH4 at 25 °C of CMFs. (B) Theoretical selectivity of CO2:CH4 mixture of C800 at different temperatures.
Summarizing, a good CO2 adsorbent must comply with a high specific surface area and high content of functional groups of oxygen and nitrogen. The materials C600 and C700 have an interesting surface chemical composition but low surface area, and the materials C900 and C1000, have high specific surface area, but low density of functional groups, then, the material C800 turns out to be the most suitable, with a desirable surface area and surface chemistry.
4. Conclusions
The CO2 adsorption on CMFs synthetized by electrospinning using PAN as precursor polymer shows a synergic effect between the specific surface area and the chemical composition. The results shown that nitrogen and oxygen contend in CMFs played an important role for CO2 capture. A temperature of calcination lower than 800 °C leads to materials with major amount of nitrogen and oxygen groups, which are causing a greater disorder in the carbon layers of the CMFs, and consequently, they avoid the diffusion of CO2 into the micropores. In addition, they are the materials with the smallest specific surface area. Materials carbonized at temperatures above 800 °C have a higher specific surface area, however, the decrease in nitrogen functional groups causes a decrease in CO2 capture for the C1000 material. For these reasons, the C800 and C900 materials show better CO2 adsorption and consequently higher CO2 selectivity in CO2:CH4 mixtures. With respect to CH4 capture, the chemical composition has not a direct impact, being the surface area the most important property.
Acknowledgements
This research was funded by the Consejo Nacional de Ciencia y Tecnología (CONACyT) and the project SIP-IPN 20211384 “Procesos Capilares en Materiales Nanoporosos: un Estudio Experimental y de Simulación”. ROL acknowledges CONACyT for financial support with CVU: 442652. DCSA gratefully acknowledges program CAPES-PrInt (project 88887.311867/2018-0) from the Ministry for Education in Brazil.
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