The isosteric enthalpy of adsorption (${\Delta }_{ads}\dot{h}$) of CO₂ in three different micro and mesoporous materials was evaluated in this work. These materials were a microporous material with functional groups of nitrogen and oxygen (CMFs, carbon microfibers), a mesoporous material with silanol groups (SBA-15, Santa Barbara Amorphous), and a mesoporous material with amine groups (SBA-15_APTES, SBA-15 amine-functionalized with (3-Aminopropyl)-triethoxysilane). The temperature interval explored was between 263 K and 303 K, with a separation of 5 K between each one, so a total of nine CO₂ isotherms were obtained. Using the nine isotherms and the Clausius–Clapeyron equation, the reference value for ${\Delta }_{ads}\dot{h}$ was found. The reference value was compared with those ${\Delta }_{ads}\dot{h}$ obtained, considering some arrangement of three or five CO₂ isotherms. Finally, it was found that at 298 K and 1 bar, the total amount of CO₂ adsorbed is 2.32, 0.53, and 1.37 mmol g⁻¹ for CMF, SBA-15, and SBA-15_APTES, respectively. However, at a coverage of 0.38 mmol g⁻¹, ${\Delta }_{ads}\dot{h}$ is worth 38, 30, and 29 KJ mol⁻¹ for SBA-15_APTES, CMFs, and SBA-15, respectively. So, physisorption predominates in the case of CMF and SBA-15 materials, and the ${\Delta }_{ads}\dot{h}$ values significantly coincide regardless of whether the isotherms arrangement used was three or five. Meanwhile, in SBA-15_APTES, chemisorption predominates as a consequence of the arrangements used to obtain ${\Delta }_{ads}\dot{h}$. This happens in such a way that the use of low temperatures (263–283 K) tends to produce higher ${\Delta }_{ads}\dot{h}$ values, while the use of high temperatures (283–303 K) decreases the ${\Delta }_{ads}\dot{h}$ values.

Carbon microfibers (CMF) has been used as an adsorbent material for CO₂ and CH₄ 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 effect of stabilization temperature on the structural properties of the carbon microfibers and their CO₂ and CH₄ adsorption capacity, four different 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 different 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 affects the CO₂ and CH₄ 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 CO₂ and between 0.8 and 2.0 mmol/g adsorbent for CH₄, depending on the calcination treatment applicated; these values are comparable with other material adsorbents of greenhouse gases.