Mesoporous aluminas synthesis using carboxylic acids to enhance performance in CO2 adsorption

This work evaluated for the first time the effect of carbon chain size of different carboxylic acids (stearic and caprylic acid) and crystallization temperatures (60, 120, and 180 °C) on the structural, textural, morphological, basic, and mechanism properties of reaction in the synthesis of mesoporous aluminas (meso-Al2O3). Regarding the structure-directing agents (SDA) traditionally used in the synthesis of meso-Al2O3 (i.g. strong acids, strong bases, and synthetic polymers), carboxylic acids are non-toxic, biodegradable compounds of low commercial value, and can be obtained from renewable sources. The physical-chemical characterizations performed, including low and high-angle X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermal analysis (TG/DTG), nitrogen adsorption-desorption, temperature-programmed desorption of carbon dioxide (CO2-TPD), scanning electron microscopy (SEM), and transmission (TEM), confirmed the formation of mesostructured γ-Al2O3. No pore organization was observed, but a system of interconnected channels was provided by the wormhole-like structure. Mesoporous aluminas synthesized at 180 °C showed an increase in structural, textural (higher of 68 % in pore volume and 36% in pore diameter), and basic properties (around 19 %) about materials synthesized at 60 °C. This can be attributed to the increased degree of coordination of the carboxylic acid with aluminum during the hydrolysis-condensation reactions. Furthermore, stearic acid (longer carbon chain carboxylic acid) tends to bend during micellar aggregation, leading to pore sizes 33% smaller than caprylic acid (shorter carbon chain acid). Therefore, the best condition for meso-Al2O3 synthesis was obtained at 180 °C and using caprylic acid (CAP-180 °C) as SDA. The results of CO2 adsorption showed that all materials correlate better with the pseudo-first order model in the entire temperature range studied (10 – 50 °C), suggesting that diffusion through the interface gas-solid (external diffusion) is the limiting step of the adsorption process. The adsorbent CAP-180 °C showed high adsorption capacity (around 134.45 mgCO2/gads at 10 °C under atmospheric pressure) and elevated stability after eight cycles of adsorption-desorption, reaching adsorption capacity superior to other meso-Al2O3.