Isosteric enthalpy behavior of CO₂ adsorption on micro-mesoporous materials: Carbon microfibers (CMFs), SBA-15, and amine-functionalized SBA-15

The isosteric enthalpy of adsorption (∆_ads ℎ) of CO₂ in three different micro and mesopo-rous materials was evaluated in this work. These materials were a microporous material with func-tional groups of nitrogen and oxygen (CMFs, carbon microfibers), a mesoporous material with si-lanol 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 ∆_ads ℎ was found. The reference value was compared with those ∆_ads 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, ∆_ads ℎ is worth 38, 30, and 29 KJ mol⁻¹ for SBA-15_APTES, CMFs, and SBA-15, respectively. So, phy-sisorption predominates in the case of CMF and SBA-15 materials, and the ∆_ads 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 ob-tain ∆_ads ℎ . This happens in such a way that the use of low temperatures (263–283 K) tends to pro-duce higher ∆_ads ℎ values, while the use of high temperatures (283–303 K) decreases the ∆_ads ℎ val-ues.