Improve in CO 2 and CH 4 Adsorption Capacity on Carbon Microﬁbers Synthesized by Electrospinning of PAN

: Carbon microﬁbers (CMF) has been used as an adsorbent material for CO 2 and CH 4 capture. The gas adsorption capacity depends on the chemical and morphological structure of CMF. The CMF physicochemical properties change according to the applied stabilization and carbonization temperatures. With the aim of studying the e ﬀ ect of stabilization temperature on the structural properties of the carbon microﬁbers and their CO 2 and CH 4 adsorption capacity, four di ﬀ erent stabilization temperatures (250, 270, 280, and 300 ◦ C) were explored, maintaining a constant carbonization temperature (900 ◦ C). In materials stabilized at 250 and 270 ◦ C, the cyclization was incomplete, in that, the nitrile groups (triple-bond structure, e.g., C ≡ N) were not converted to a double-bond structure (e.g., C = N), to form a six-membered cyclic pyridine ring, as a consequence the material stabilized at 300 ◦ C resulting in fragile microﬁbers; therefore, the most appropriate stabilization temperature was 280 ◦ C. Finally, to corroborate that the speciﬁc surface area (microporosity) is not the determining factor that inﬂuences the adsorption capacity of the materials, carbonization of polyacrylonitrile microﬁbers (PANMFs) at ﬁve di ﬀ erent temperatures (600, 700, 800, 900, and 1000 ◦ C) is carried, maintaining a constant temperature of 280 ◦ C for the stabilization process. As a result, the CMF chemical composition directly a ﬀ ects the CO 2 and CH 4 adsorption capacity, even more directly than the speciﬁc surface area. Thus, the chemical variety can be useful to develop carbon microﬁbers with a high adsorption capacity and selectivity in materials with a low speciﬁc surface area. The amount adsorbed at 25 ◦ C and 1.0 bar oscillate between 2.0 and 2.9 mmol / g adsorbent for CO 2 and between 0.8 and 2.0 mmol / g adsorbent for CH 4 , depending on the calcination treatment applicated; these values are comparable with other material adsorbents of greenhouse gases.


Introduction
In the last few years, carbon dioxide (CO 2 ) and methane (CH 4 ) emissions have become the most important greenhouse gases with the largest impact on climate change [1]. As a result, several researchers have focused on the development of efficient materials for CO 2

Materials
The polymer precursor, polyacrylonitrile (PAN), was acquired from Sigma-Aldrich, with a molecular mass average of 150,000 amu, T g = 358 K and transition temperature of 590 K. The precursor was used as received without additional purification. The solvent N,N-dimethylformamide (DMF) grade anhydrous (99.8%) was also acquired from Sigma-Aldrich.

CMF Preparation
PANMFs were synthesized using a concentration of 10% (w/w) of PAN, the electrospinning apparatus was set at a flow rate of 0.5 mL/h, a voltage of 15 kV, and 10 cm distance between the tip of the syringe and the collector. First, PANMFs were divided into four portions, each stabilized at 250, 270, 280, and 300 • C (air atmosphere), respectively. Subsequently, all samples were carbonized at 900 • C (nitrogen atmosphere). After the evaluation of these materials, another batch of PAN microfibers was prepared, stabilized at 280 • C and carbonized at five different temperatures (600, 700, 800, 900, and 1000 • C).
Samples labeling: PANMF corresponds to pristine material (PAN microfibers synthesized by electrospinning). The first letter corresponds to the stabilization, "S". This is followed by a number indicating the stabilization temperature. Samples, to which the letter C is added, correspond to materials carbonized at 900 • C. For example, S-250 is a sample stabilized at 250 • C, while S-250-C, was stabilized at 250 • C and carbonized at 900 • C. For the second batch of PANMFs, the letters mean that the samples were stabilized at 280 • C and carbonized at 600, 700, 800, 900, or 1000 • C, SC-600, SC-700, SC-800, SC-900, and SC-1000, respectively. For a better understanding of nomenclature, please see Figure S1 in Supplementary Material.

Characterization Techniques
The FTIR experiments were performed on a Perkin Elmer Paragon 1000 (Waltham, MA, USA), in a wavelength interval of 500 to 3000 cm −1 , averaging 16 scans. The purpose of this technique is to show qualitatively the transformation of C≡N to C=N. The morphology and size of the microfibers were observed in a scanning electron microscope JEOL JSM-6010LA (Akishima, Tokyo, Japan) at 20 kV of acceleration voltage in conditions of high vacuum to 2000× and 10,000×. The textural properties of samples were determined by N 2 and CO 2 adsorption: The first measurement was performed on a Micromeritics ASAP 2020 system (Norcross, GA, USA) at liquid nitrogen temperature (-196 • C), and the second measurement was made on a Quantachrome equipment at 0 • C (the adaptation of thermal re-circulator to control the temperature); in both cases, the samples were degassed at 200 • C under vacuum. The Brunauer-Emmett-Teller (BET) method was used for the determination of the specific surface area, while the software Autosorb 1 of Quantachrome equipment was utilized for the calculations of average pore size, considering the Non Localized Density Functional Theory (NLDFT) model for slit pores.

Chemical Characterization of PANMFs
To select the stabilization temperature interval, thermogravimetric analysis (TGA) was performed (Supplementary Material). The results showed that the most appropriate temperature before thermal degradation is between 250 and 300 • C (see Figure S2). Then, the effect of stabilization temperature on the chemical composition of PANMFs were analyzed by infrared spectroscopy. The infrared spectra ( Figure 1) show that the PANMFs precursors present particular vibrations in~2245,~1670, and~1454 cm −1 , characteristics of nitrile groups (-C≡N, carbonyl groups (-C=O), and aliphatic groups (CH, CH 2 , and CH 3 ), respectively [30,31,33]. After the stabilization process at temperatures between to 300 • C, the intensity of signal in 2245 cm −1 decreases significantly, while the signal intensity of carbonyl groups (-C=O,~1670 cm −1 ) increases, forming a wider signal corresponding to the overlapping of signals assigned to carbon-carbon double bonds (-C=C-,~1620 cm −1 ) and carbon-nitrogen double bonds (-C=N). In addition to these changes, a new signal appears in~810 cm −1 , this signal is characteristic of aromatic rings (C=C-H). The reduction of nitrile groups, loss of hydrogen, and formation of aromatic structures are the result of cyclization (-C=N-), dehydrogenation (-C=C-), and oxidation (-C=O) of PANMFs. When the stabilization temperature is 280 • C, the infrared spectroscopy technique is incapable of detecting the presence of nitrile groups, suggesting that most of these groups have been converted into aromatic structures; therefore, at a temperature above 280 • C, the stabilization process is complete. the signal intensity of carbonyl groups (−C=O, ~1670 cm -1 ) increases, forming a wider signal corresponding to the overlapping of signals assigned to carbon-carbon double bonds (−C=C−, ~1620 cm -1 ) and carbon-nitrogen double bonds (−C=N). In addition to these changes, a new signal appears in ~810 cm -1 , this signal is characteristic of aromatic rings (C=C−H). The reduction of nitrile groups, loss of hydrogen, and formation of aromatic structures are the result of cyclization (−C=N−), dehydrogenation (−C=C−), and oxidation (−C=O) of PANMFs. When the stabilization temperature is 280 °C, the infrared spectroscopy technique is incapable of detecting the presence of nitrile groups, suggesting that most of these groups have been converted into aromatic structures; therefore, at a temperature above 280 °C, the stabilization process is complete. TGA ( Figure S1) and FTIR ( Figure 1) characterization techniques allowed a preliminary analysis of PAN microfibers. Once the PAN microfibers were synthesized, the next step was to carbonize the materials at different temperatures. The results obtained for the two batches of materials are shown below: (i) Varying the stabilization temperature (250, 270, 280, and 300 °C) but keeping the carbonization temperature constant (900 °C); and (ii) keeping the stabilization temperature constant (280 °C) but varying the carbonization temperature (600, 700, 800, 900, and 1000 °C).

Impact of Stabilization Temperature
After the stabilization and carbonization of the PAN microfibers, the resultant CMF were analyzed by SEM ( Figure 2). Figure 2A presents the material stabilized at 250 °C, note that low stabilization temperatures do not allow the fibrous structure of the material to be preserved after carbonization, resulting in merged microfibers without interstitial spaces. The material S-280-C preserves both the fibrous structure and the length of microfibers ( Figure 2B). Meanwhile the material S-300-C, stabilized at temperatures closer to the PANMF degradation temperature, presents a break in the length; in this case, the fiber structure is also preserved, but the reduction in length causes the agglomeration of microfibers and, as a result, the interstitial space between the CMF is lost. Other characterization by TEM, XRD, XPS, and Raman was performed (more details can be found in previous publications [19,42]). TEM showed a random ordering of carbon sheets forming the structure of the microfiber, the distance between the different carbon sheets oscillate between 0.4 and TGA ( Figure S1) and FTIR ( Figure 1) characterization techniques allowed a preliminary analysis of PAN microfibers. Once the PAN microfibers were synthesized, the next step was to carbonize the materials at different temperatures. The results obtained for the two batches of materials are shown below: (i) Varying the stabilization temperature (250, 270, 280, and 300 • C) but keeping the carbonization temperature constant (900 • C); and (ii) keeping the stabilization temperature constant (280 • C) but varying the carbonization temperature (600, 700, 800, 900, and 1000 • C).

Impact of Stabilization Temperature
After the stabilization and carbonization of the PAN microfibers, the resultant CMF were analyzed by SEM ( Figure 2). Figure 2A presents the material stabilized at 250 • C, note that low stabilization temperatures do not allow the fibrous structure of the material to be preserved after carbonization, resulting in merged microfibers without interstitial spaces. The material S-280-C preserves both the fibrous structure and the length of microfibers ( Figure 2B). Meanwhile the material S-300-C, stabilized at temperatures closer to the PANMF degradation temperature, presents a break in the length; in this case, the fiber structure is also preserved, but the reduction in length causes the agglomeration of microfibers and, as a result, the interstitial space between the CMF is lost. Other characterization by TEM, XRD, XPS, and Raman was performed (more details can be found in previous publications [19,42]). TEM showed a random ordering of carbon sheets forming the structure of the microfiber, the distance between the different carbon sheets oscillate between 0.4 and 1.0 nm. XRD and Raman spectroscopy corroborates that the carbonization of PAN microfibers produces carbon structures with amorphous and graphitic character, and that the increase in the calcination temperature generates materials that are less amorphous. For illustrative purposes, the results of XRD and Raman for material stabilized at 280 • C and carbonized at 900 • C are shown in the Supplementary Material in Figure S3A,B, respectively. 1.0 nm. XRD and Raman spectroscopy corroborates that the carbonization of PAN microfibers produces carbon structures with amorphous and graphitic character, and that the increase in the calcination temperature generates materials that are less amorphous. For illustrative purposes, the results of XRD and Raman for material stabilized at 280 °C and carbonized at 900 °C are shown in the Supplementary Material in Figure S3 (A) and Figure S3 (B), respectively. The N2 adsorption isotherm of the PANMFs stabilized at 270 °C and carbonized at 900 °C is shown in Figure 3, the isotherms of the other three materials exhibited similar behavior; however, they presented a small hysteresis cycle, which are displayed in the figure frame. Based on the IUPAC classification [43], the isotherm for material S-250-C correspond to Type I, while for materials S-270-C, S-280-C, and S-300-C the isotherm type corresponds to a combination of Type I (due to the presence of microporosity) and Type IV (due to the presence of mesoporosity), with small hysteresis cycle The N 2 adsorption isotherm of the PANMFs stabilized at 270 • C and carbonized at 900 • C is shown in Figure 3, the isotherms of the other three materials exhibited similar behavior; however, they presented a small hysteresis cycle, which are displayed in the figure frame. Based on the IUPAC classification [43], the isotherm for material S-250-C correspond to Type I, while for materials S-270-C, S-280-C, and S-300-C the isotherm type corresponds to a combination of Type I (due to the presence of microporosity) and Type IV (due to the presence of mesoporosity), with small hysteresis cycle corresponding to Type H4, characteristic of micro-mesoporous carbon. At low relative pressures (less than 0.1 in the relative pressure interval) in all materials, nitrogen adsorption is mainly attributed to the presence of micropores. At relative pressures greater than 0.35, the materials S-270-C, S-280-C, and S-300-C present a hysteresis cycle, associated with the presence of mesopores (box at the bottom of Figure 3). While the S-250-C does not present the little loop, which implies the absence of mesopores. Therefore, to obtain a totally microporous material, a stabilization temperature of 250 • C is recommended. and S-300-C present a hysteresis cycle, associated with the presence of mesopores (box at the bottom of Figure 3). While the S-250-C does not present the little loop, which implies the absence of mesopores. Therefore, to obtain a totally microporous material, a stabilization temperature of 250 °C is recommended.
To explain the behavior of this material, it is considered that nitrogen adsorption in CMF can occur in two spaces: i) Between the carbon sheets that form the structure of microfibers, or ii) between the interstitial spaces that are formed by the arrangement of several microfibers. The material stabilized at 250 °C (S-250-C) does not present a hysteresis cycle due to the microfibers in this material completely fusing, as shown in Figure 3, forming a solid without interstitial spaces ( Figure 2(A1,A2)). The hysteresis cycle presented by the materials S-270-C, S-280-C, and S-300-C (box in the lower part of Figure 3) is reduced with the increase of stabilization temperature, confirming the observations made by SEM ( Figure 2). The material S-300-C shows shattered microfibers, which causes microfiber agglomeration and reduction in the interstitial spaces; these structural changes possibly aid the decrease in the adsorbed nitrogen capacity [42]. To obtain information from micropores, CO2 adsorption was utilized. Because of kinetic restrictions at the cryogenic temperature of nitrogen (-196 °C), the adsorption is of limited value for the characterization of very narrow micropores. One way of addressing this problem is to use CO2 (kinetic dimension 0.33 nm) as the adsorptive at 0 °C, where the saturation vapor pressure of CO2 is very high (∼35 bar) and hence the pressures required for micropore size analysis are in the moderate interval (∼0.001 to 1.000 bar). Because of these relatively high temperatures and pressures, diffusion is much faster and pores as small as 0.4 nm can be accessed [43].
The N2 and CO2 adsorption isotherms allow the calculation of the specific surface area and the average pore size distribution. The textural properties obtained from the N2 and CO2 isotherms are shown in Table 1. As observed in the isotherms of Figure 3, these materials present a high amount of micropores, which is reflected in a high specific surface area. The material stabilized at 250 °C presents a BET area of 966 m 2 /g; this value decreases when the stabilization temperature increases, decreasing to 635 m 2 /g for the material stabilized at 300 °C. In this way, the increment in the To explain the behavior of this material, it is considered that nitrogen adsorption in CMF can occur in two spaces: i) Between the carbon sheets that form the structure of microfibers, or ii) between the interstitial spaces that are formed by the arrangement of several microfibers. The material stabilized at 250 • C (S-250-C) does not present a hysteresis cycle due to the microfibers in this material completely fusing, as shown in Figure 3, forming a solid without interstitial spaces (Figure 2(A1,A2)). The hysteresis cycle presented by the materials S-270-C, S-280-C, and S-300-C (box in the lower part of Figure 3) is reduced with the increase of stabilization temperature, confirming the observations made by SEM ( Figure 2). The material S-300-C shows shattered microfibers, which causes microfiber agglomeration and reduction in the interstitial spaces; these structural changes possibly aid the decrease in the adsorbed nitrogen capacity [42].
To obtain information from micropores, CO 2 adsorption was utilized. Because of kinetic restrictions at the cryogenic temperature of nitrogen (−196 • C), the adsorption is of limited value for the characterization of very narrow micropores. One way of addressing this problem is to use CO 2 (kinetic dimension 0.33 nm) as the adsorptive at 0 • C, where the saturation vapor pressure of CO 2 is very high (∼35 bar) and hence the pressures required for micropore size analysis are in the moderate interval (∼0.001 to 1.000 bar). Because of these relatively high temperatures and pressures, diffusion is much faster and pores as small as 0.4 nm can be accessed [43].
The N 2 and CO 2 adsorption isotherms allow the calculation of the specific surface area and the average pore size distribution. The textural properties obtained from the N 2 and CO 2 isotherms are shown in Table 1. As observed in the isotherms of Figure 3, these materials present a high amount of micropores, which is reflected in a high specific surface area. The material stabilized at 250 • C presents a BET area of 966 m 2 /g; this value decreases when the stabilization temperature increases, decreasing to 635 m 2 /g for the material stabilized at 300 • C. In this way, the increment in the stabilization temperature plays an important role in the materials microporosity, which modifies the BET area by 35% [42]. Table 1. Structural properties by N 2 and CO 2 adsorption of PANMFs stabilized at 250, 270, 280, and 300 • C, and carbonized at 900 • C. S BET -BET area; D NLDFT -preferential pore diameter calculated by the NLDFT model; in the case of CO 2 adsorption, four pore sizes were observed (X1, X2, X3, and X4).

Adsorbate
Textural Properties S-250-C S-270-C S-280-C S-300-C The pore size distribution (PSD) obtained from the N 2 and CO 2 adsorption isotherms shows that all materials have a preferential pore diameter of~0.8 nm. The PSD obtained from CO 2 shows four preferential pore sizes (X1, X2...X4), one of which coincides with the average pore diameter obtained by N 2 adsorption isotherms, 0.8 nm. The CO 2 adsorption allows determining pore sizes that oscillate between 0.45 and 1.00 nm. The values obtained for the pore diameters in both cases, see Table 1, are comparable with the distances between the carbon sheets calculated by TEM micrographs [42].

Impact of Carbonization Temperature
The modification in the morphological structure of the carbon microfibers is mainly affected by a variation in the stabilization temperature (results shown above). While the modification in the chemical composition is due to the variation in the carbonization temperature (results shown below); therefore, a study was performed modifying this temperature. In this work, the selected stabilization temperature was fixed at 280 • C (because at this temperature the fibrous structure of the materials is preserved and, in addition, most of the nitrile groups have been eliminated, forming C=N structures). Then, carbonization of stabilized materials was performed at five different temperatures, 600, 700, 800, 900, and 1000 • C [42]. Table 2 shows a summary of the results on the morphological and chemical characteristics of these materials.
Based on the results shown in Table 2, the specific surface area rises as the carbonization temperature increases, from a BET area of 296 m 2 /g for SC-600 material to 822 m 2 /g for SC-1000 material. Considering the information provided by the nitrogen adsorption isotherms, there are some carbon sheets for the material S-250-C that present a separation of~1.15 nm, when the carbonization temperature is increased this separation decreases until it becomes practically constant at~0.80 nm. CO 2 adsorption isotherms allow for the determination of pore size in the ultramicropore interval, for all materials this size ranges from 0.5 to 0.8 nm. XPS analysis identifies the three main elements present in the CMF: Carbon 1s (C1s), Nitrogen 1s (N1s), and Oxygen 1s (O1s). At 600 • C, the carbon content is 79% and increases in parallel to the carbonization temperature up to 91% for the material carbonized at 1000 • C. Some investigations have demonstrated that, in specific applications, different species of nitrogen have a significant influence. For this reason, the results corresponding to high-resolution XPS spectra in the N1s are also shown in Table 2. These results were obtained from the deconvoluted spectra of XPS N1s, considering the following signals: i) residual PAN (397.  system, while the hydrogen atom is bound in the plane of the ring; therefore, within the accuracy of XPS measurements, pyridone cannot be distinguished from pyrrolic-N [44]. Table 2. Textural and chemical properties of PANMFs carbonized at 600, 700, 800, 900, and 1000 • C. S BET -BET area; D NLDFT -preferential pore diameter calculated by the NLDFT model; in the case of CO 2 adsorption, four pore sizes were observed (X1, X2, X3, and X4). XPS general for carbon, nitrogen, and oxygen. High-resolution XPS spectra in the N1s. As already mentioned, the stabilization of PAN microfibers causes the breaking of the C≡N bonds to form C=N; bonds; while with the carbonization temperature, the elimination of H 2 and N 2 takes place, generating different intermediate species. In the materials carbonized to 600 and 700 • C predominate pyridine nitrogen and pyrrole nitrogen; temperatures higher than 800 • C show a decrease in both species, probably due to the conversion of pyridine nitrogen to quaternary nitrogen, which are favored at high temperatures (> 800 • C) becoming the predominant groups [42]. Therefore, carbonization at temperatures above 800 • C induces the formation of new nitrogen species such as pyridone, quaternary nitrogen or graphite, pyridine nitrogen with oxidized species, and chemisorbed nitrogen [35,36,[44][45][46], and causes the disappearance of the original functional groups.

Impact of Stabilization Temperature
The CO 2 and CH 4 adsorption capacity was evaluated at 25 • C, the isotherms obtained are presented in Figures 4 and 5. Figure 4 shows the adsorption isotherms in units of mmol CO 2 adsorbed in one gram of adsorbent. These values are similar to the ones reported in the literature [14,15,47]. Note that the material stabilized at 250 • C adsorbs the most CO 2 and the material stabilized at 300 • C adsorbs the least. To eliminate the impact of the surface area on the CO 2 adsorption, the isotherms are normalized considering the BET area of each of the materials; the resulting behavior is shown in Figure 4B. The materials stabilized at 270, 280, and 300 • C adsorb similar quantities per square meter (3.4 µmol/m 2 , at 1 bar) that represent a greater adsorption capacity than material stabilized at 250 • C. Even though the material stabilized at 250 • C is the material with the largest surface area, it has the lowest CO 2 adsorption per square meter, this may be due to the different chemical composition shown by the carbon fiber. It was shown in FTIR that this material still has nitrile groups, which can cause a lower affinity with the CO 2 molecule on its surface. chemical composition shown by the carbon fiber. It was shown in FTIR that this material still has nitrile groups, which can cause a lower affinity with the CO2 molecule on its surface.  Figure 5 shows the CH4 adsorption isotherms at 25 °C (A) and the normalized isotherms (B) using the BET area. In both graphs, the material stabilized at 250 °C has the lowest adsorption capacity. This means that having a larger BET area does not guarantee higher CH4 adsorption. In this way, it was observed how the affinity to CH4 is conferred mainly by the chemical composition of the carbon microfiber. This carbon microfiber presented nitrile groups; however, which may be responsible for the lower affinity to CH4 per square meter, similar to the observed in the CO2 adsorption. In Table 3 (keeping the pressure constant at 1 bar), the CO2 adsorption capacity of CMF oscillates between 2.2 and 2.9 mmol CO2/g adsorbent (97 to 128 mg CO2/g adsorbent) at 25 °C. This value is comparable to those reported for carbon materials. For example, Kim et al. [14] obtain capacities of 2.5 mmol CO2/g for carbon microfibers activated with urea (the urea is added to increase nitrogen species). Meng et al. [15] capture between 50 and 150 mg CO2/g adsorbent on activated carbon, with treatments of KOH to increase the specific surface area. The BET area of these materials oscillates between 800 and 2200 m 2 /g. Comparing these materials with the CMF synthesized in this work that have similar areas (SC-900, 700 m 2 /g), the CO2 adsorption capacity is higher in the material SC-900 (110 mg CO2/g adsorbent) than in activated carbons (50 mg CO2/g adsorbent). Shafeeyan et al. [47] chemically modified the activated carbon with ammonium, the results obtained fluctuate between 40 and 80 mg CO2/g adsorbent, values lower than those obtained by CMF. Although activated carbon has larger BET areas than CMF, the CMF adsorbs the same or higher amounts of CO2. This may be because microfibers promote the diffusion of CO2 molecules to be absorbed or an adequate amount   Figure 5 shows the CH4 adsorption isotherms at 25 °C (A) and the normalized isotherms (B) using the BET area. In both graphs, the material stabilized at 250 °C has the lowest adsorption capacity. This means that having a larger BET area does not guarantee higher CH4 adsorption. In this way, it was observed how the affinity to CH4 is conferred mainly by the chemical composition of the carbon microfiber. This carbon microfiber presented nitrile groups; however, which may be responsible for the lower affinity to CH4 per square meter, similar to the observed in the CO2 adsorption. In Table 3 (keeping the pressure constant at 1 bar), the CO2 adsorption capacity of CMF oscillates between 2.2 and 2.9 mmol CO2/g adsorbent (97 to 128 mg CO2/g adsorbent) at 25 °C. This value is comparable to those reported for carbon materials. For example, Kim et al. [14] obtain capacities of 2.5 mmol CO2/g for carbon microfibers activated with urea (the urea is added to increase nitrogen species). Meng et al. [15] capture between 50 and 150 mg CO2/g adsorbent on activated carbon, with treatments of KOH to increase the specific surface area. The BET area of these materials oscillates between 800 and 2200 m 2 /g. Comparing these materials with the CMF synthesized in this work that have similar areas (SC-900, 700 m 2 /g), the CO2 adsorption capacity is higher in the material SC-900 (110 mg CO2/g adsorbent) than in activated carbons (50 mg CO2/g adsorbent). Shafeeyan et al. [47] chemically modified the activated carbon with ammonium, the results obtained fluctuate between 40 and 80 mg CO2/g adsorbent, values lower than those obtained by CMF. Although activated carbon has larger BET areas than CMF, the CMF adsorbs the same or higher amounts of CO2. This may be because microfibers promote the diffusion of CO2 molecules to be absorbed or an adequate amount  Figure 5 shows the CH 4 adsorption isotherms at 25 • C (A) and the normalized isotherms (B) using the BET area. In both graphs, the material stabilized at 250 • C has the lowest adsorption capacity. This means that having a larger BET area does not guarantee higher CH 4 adsorption. In this way, it was observed how the affinity to CH 4 is conferred mainly by the chemical composition of the carbon microfiber. This carbon microfiber presented nitrile groups; however, which may be responsible for the lower affinity to CH 4 per square meter, similar to the observed in the CO 2 adsorption.
In Table 3 (keeping the pressure constant at 1 bar), the CO 2 adsorption capacity of CMF oscillates between 2.2 and 2.9 mmol CO 2 /g adsorbent (97 to 128 mg CO 2 /g adsorbent) at 25 • C. This value is comparable to those reported for carbon materials. For example, Kim et al. [14] obtain capacities of 2.5 mmol CO 2 /g for carbon microfibers activated with urea (the urea is added to increase nitrogen species). Meng et al. [15] capture between 50 and 150 mg CO 2 /g adsorbent on activated carbon, with treatments of KOH to increase the specific surface area. The BET area of these materials oscillates between 800 and 2200 m 2 /g. Comparing these materials with the CMF synthesized in this work that have similar areas (SC-900, 700 m 2 /g), the CO 2 adsorption capacity is higher in the material SC-900 (110 mg CO 2 /g adsorbent) than in activated carbons (50 mg CO 2 /g adsorbent). Shafeeyan et al. [47] chemically modified the activated carbon with ammonium, the results obtained fluctuate between 40 and 80 mg CO 2 /g adsorbent, values lower than those obtained by CMF. Although activated carbon has larger BET areas than CMF, the CMF adsorbs the same or higher amounts of CO 2 . This may be because microfibers promote the diffusion of CO 2 molecules to be absorbed or an adequate amount of chemical species (nitrogen). Bao et al. [6] measured the capture capacity of CH 4 in two materials, considering the conditions at 25 • C and 1 bar: i) Metal-organic frameworks (MOFs) with an area of 1174 m 2 /g adsorb an approximate amount of 1 mmol/g; and ii) zeolites with an area of 330 m 2 /g adsorb~0.35 mmol/g of adsorbent; this corroborates that the area does not impact, in a transcendental way, the capture of gases; but morphology and chemical composition do change the amount of CH 4 adsorbed. For his part, Liu et al. [9] studied CO 2 and CH 4 capture in SBA-15 materials, adsorbing 1.3 mmol/g and~0.2 mmol/g (25 • C and 1 bar), respectively; these amounts show an improvement in CMF as gas adsorbents. To evaluate the reusability of CMF, a collection of CO 2 and CH 4 adsorption isotherms at 0 and 25 • C are shown in Figure 6. After ten analyses, the CMF preserve their adsorption capacity. For the first experiment, the CMF was degassed at 200 • C by 8 h, and then the CO 2 adsorption analysis was performed at 0 • C. For the second analysis, the same sample was degassed at 200 • C by 2 h and again analyzed. The following analyses followed this procedure. For the isotherms shown in the methane adsorption, the same procedure was performed at 25 • C. This allows us to conclude that the synthesized materials can be used more than ten times whilst maintaining their adsorptive capacity.
of chemical species (nitrogen). Bao et al. [6] measured the capture capacity of CH4 in two materials, considering the conditions at 25 °C and 1 bar: i) Metal-organic frameworks (MOFs) with an area of 1174 m 2 /g adsorb an approximate amount of 1 mmol/g; and ii) zeolites with an area of 330 m 2 /g adsorb ~0.35 mmol/g of adsorbent; this corroborates that the area does not impact, in a transcendental way, the capture of gases; but morphology and chemical composition do change the amount of CH4 adsorbed. For his part, Liu et al. [9] studied CO2 and CH4 capture in SBA-15 materials, adsorbing ~1.3 mmol/g and ~0.2 mmol/g (25 °C and 1 bar), respectively; these amounts show an improvement in CMF as gas adsorbents. To evaluate the reusability of CMF, a collection of CO2 and CH4 adsorption isotherms at 0 and 25 °C are shown in Figure 6. After ten analyses, the CMF preserve their adsorption capacity. For the first experiment, the CMF was degassed at 200 °C by 8 h, and then the CO2 adsorption analysis was performed at 0 °C. For the second analysis, the same sample was degassed at 200 °C by 2 h and again analyzed. The following analyses followed this procedure. For the isotherms shown in the methane adsorption, the same procedure was performed at 25 °C. This allows us to conclude that the synthesized materials can be used more than ten times whilst maintaining their adsorptive capacity. The CO2 adsorption isotherms at 25 °C of PANMF stabilized a 280 °C and carbonized at 600, 700, 800, 900, and 1000 °C are shown in Figure 7. It is observed that the increase of carbonized temperature produces an increase in the CO2 adsorption capacity. However, to observe the impact of the chemical composition of carbon microfibers on CO2 adsorption, the isotherms were normalized using the BET area, Figure 7B. The carbon microfibers decreased their CO2 adsorption capacity per square meter with the increase of carbonization temperature. This means that the increase in carbonization

Impact of Carbonization Temperature
The CO 2 adsorption isotherms at 25 • C of PANMF stabilized a 280 • C and carbonized at 600, 700, 800, 900, and 1000 • C are shown in Figure 7. It is observed that the increase of carbonized temperature produces an increase in the CO 2 adsorption capacity. However, to observe the impact of the chemical composition of carbon microfibers on CO 2 adsorption, the isotherms were normalized using the BET area, Figure 7B. The carbon microfibers decreased their CO 2 adsorption capacity per square meter with the increase of carbonization temperature. This means that the increase in carbonization temperature resulted in the loss of chemical groups able to interact with CO 2 but caused an increase in their BET areas. BET areas increase from 296 to 822 m 2 g −1 ; however, the CO 2 adsorption capacity does not increase proportionally to the surface area. In this way, the CO 2 adsorption capacity in carbon microfiber is mainly determined by their chemical composition. For the material SC-600 and SC-700, temperature resulted in the loss of chemical groups able to interact with CO2 but caused an increase in their BET areas. BET areas increase from 296 to 822 m 2 g -1 ; however, the CO2 adsorption capacity does not increase proportionally to the surface area. In this way, the CO2 adsorption capacity in carbon microfiber is mainly determined by their chemical composition. For the material SC-600 and SC-700, the nitrogen species pyridinic and pyrrolic are predominant, which means these species show a greater affinity for CO2. The CH4 adsorption isotherms at 25 °C of PANMF stabilized at 280 °C and carbonized at 600, 700, 800, 900, and 1000 °C are shown in Figure 8. Similarly to that observed in the CO2 adsorption, the increase of carbonized temperature produces an increase in the CH4 adsorption capacity. Once the isotherms are normalized, the material that adsorbs a greater amount of CH4 corresponds to the material with a smaller specific surface area, which implies that the interaction is also mainly due to the presence of the functional groups that exist in this material, primarily the pyridinic and pyrrolic groups, and the quaternary nitrogen content remains constant in all materials. It should be noted that, although the behavior is similar to CO2 adsorption, the amount adsorbed is lower in the case of CH4. Consider SC-600 material at 1 bar, for CO2 an amount of ~6.5 µ mol/m 2 is adsorbed and for CH4 ~2.8 µ mol/m 2 ; therefore, carbon microfibers present more affinity to the CO2 molecule, probably due to the presence of a polar momentum. The adsorption capacity of CO2 and CH4 per unit of adsorbent grams (shown in Table 4) rises slightly with the increase in carbonization temperature, however, when the results are normalized considering the BET area, the adsorption capacity diminishes with increasing temperature, which implies that the CO2 and CH4 capture capacity is directly related to the number of nitrogen species present in each material and not to the specific surface area. This behavior must be considered in the The CH 4 adsorption isotherms at 25 • C of PANMF stabilized at 280 • C and carbonized at 600, 700, 800, 900, and 1000 • C are shown in Figure 8. Similarly to that observed in the CO 2 adsorption, the increase of carbonized temperature produces an increase in the CH 4 adsorption capacity. Once the isotherms are normalized, the material that adsorbs a greater amount of CH 4 corresponds to the material with a smaller specific surface area, which implies that the interaction is also mainly due to the presence of the functional groups that exist in this material, primarily the pyridinic and pyrrolic groups, and the quaternary nitrogen content remains constant in all materials. It should be noted that, although the behavior is similar to CO 2 adsorption, the amount adsorbed is lower in the case of CH 4 . Consider SC-600 material at 1 bar, for CO 2 an amount of~6.5 µmol/m 2 is adsorbed and for CH 4~2 .8 µmol/m 2 ; therefore, carbon microfibers present more affinity to the CO 2 molecule, probably due to the presence of a polar momentum. temperature resulted in the loss of chemical groups able to interact with CO2 but caused an increase in their BET areas. BET areas increase from 296 to 822 m 2 g -1 ; however, the CO2 adsorption capacity does not increase proportionally to the surface area. In this way, the CO2 adsorption capacity in carbon microfiber is mainly determined by their chemical composition. For the material SC-600 and SC-700, the nitrogen species pyridinic and pyrrolic are predominant, which means these species show a greater affinity for CO2. The CH4 adsorption isotherms at 25 °C of PANMF stabilized at 280 °C and carbonized at 600, 700, 800, 900, and 1000 °C are shown in Figure 8. Similarly to that observed in the CO2 adsorption, the increase of carbonized temperature produces an increase in the CH4 adsorption capacity. Once the isotherms are normalized, the material that adsorbs a greater amount of CH4 corresponds to the material with a smaller specific surface area, which implies that the interaction is also mainly due to the presence of the functional groups that exist in this material, primarily the pyridinic and pyrrolic groups, and the quaternary nitrogen content remains constant in all materials. It should be noted that, although the behavior is similar to CO2 adsorption, the amount adsorbed is lower in the case of CH4. Consider SC-600 material at 1 bar, for CO2 an amount of ~6.5 µ mol/m 2 is adsorbed and for CH4 ~2.8 µ mol/m 2 ; therefore, carbon microfibers present more affinity to the CO2 molecule, probably due to the presence of a polar momentum. The adsorption capacity of CO2 and CH4 per unit of adsorbent grams (shown in Table 4) rises slightly with the increase in carbonization temperature, however, when the results are normalized considering the BET area, the adsorption capacity diminishes with increasing temperature, which implies that the CO2 and CH4 capture capacity is directly related to the number of nitrogen species present in each material and not to the specific surface area. This behavior must be considered in the The adsorption capacity of CO 2 and CH 4 per unit of adsorbent grams (shown in Table 4) rises slightly with the increase in carbonization temperature, however, when the results are normalized considering the BET area, the adsorption capacity diminishes with increasing temperature, which implies that the CO 2 and CH 4 capture capacity is directly related to the number of nitrogen species present in each material and not to the specific surface area. This behavior must be considered in the design of novel adsorbents materials, where the chemical composition of the surface is very important in order to obtain a high adsorbent material for greenhouse gases.

Conclusions
The stabilization temperature allowed the morphology of CMF to be modified, increasing or decreasing the specific surface area; whereas carbonization temperature changed the chemical composition, this variability being the one that directly affects the capture of CO 2 and CH 4 . The results corroborate that the specific surface area is not the determining factor for the adsorption of greenhouse gases. Thus, it is very important to take into account the chemical structure, especially for the enhancement of the interaction between CO 2 and CH 4 with pyridinic, pyrrolic, and quaternary groups. In both cases the amount adsorbed is similar and even improved compared to other materials such as zeolites, silica mesoporous, and activated carbon, oscillating from 2.0 to 2.6 mmol CO 2 /g and 0.8 to 1.2 mmol CH 4 /g.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2079-6439/7/10/81/s1, Figure S1: Nomenclature of PAN microfibers stabilized and carbonized, synthetized by electrospinning. Figure S2: TG/DTA analysis of polyacrylonitrile microfibers (PANMFs), synthesized by the electrospinning method. Reaching a temperature of 300 • C the material begins to degrade thermally, therefore, in this work, an interval between 250 • C and 300 • C was selected. Figure S3: PAN microfibers stabilized at 280 • C and carbonized at 900 • C: (A) XRD analysis, the high amplitude of the peak between 5o and 35 • in 2θ indicates a high degree of amorphicity on the graphene sheets. The signal in the plane (100) is attributed to the graphitic character of the material, and (B) Raman spectroscopy analysis, which exhibit two peaks, centered at 1360 cm −1 and 1600 cm −1 , they are known as D and G peaks, respectively. The first peak is attributed to disordered carbon films while the second one is attributed to the presence of graphitic type ordered structures. Funding: This research was funded by the Consejo Nacional de Ciencia y Tecnología (CONACyT) and the project SECITI/080/2017. The SEM images were made possible by Paty Castillo (UAM-I) and Anabel Gonzalez Diaz (UJAT). ROL thanks CONACyT for the support with the scholarship 268040.