Preparation of cellulose aerogels using supercritical drying: Influence of process variables on aerogel porosity

Resumen

Aerogels (AE) are lightweight solid materials with an open interconnected structure, high porosity, and specific surface areas, which are drawing increasing attention in many scientific fields such as biomedical, energy storage, and others. They can be prepared using different technologies and a wide range of polymers, therefore, their chemistry, pore size, and surface area can be tailored. Cellulose is a biopolymer of great interest due to its renewable character, commercial availability, biocompatibility, and biodegradability, emerging as a promising source to obtain biobased and biocompatible AE. Cellulose AE can be prepared in 3 steps: dissolving/dispersing cellulose, forming a hydrogel by the sol-gel process, and gel drying. The latter step can be performed at ambient evaporation, solvent sublimation by lyophilization, or under supercritical conditions. The main goal is to avoid pores collapse due to capillary pressure, retaining the 3D porous structure. In this sense, drying by supercritical CO2 (scCO2) is considered an appropriate alternative, being a safe and green method suitable for most kinds of gels. Before scCO2 drying, the liquid phase of the gel is replaced by organic solvents miscible with CO2 such as ethanol to form alcogels. This process avoids the liquid-gas surface tension and liquid-solid adhesive forces preventing the gel collapse. Furthermore, during scCO2 drying, single-phase fluid conditions can be achieved by setting the process variables, avoiding pore collapse due to surface tension forces.1 In this work, AE based on cellulose microcrystalline (MCC) were prepared using static scCO2 drying. The effect of different process variables on the final AE structure was studied aiming to contribute to the knowledge of the basis of scCO2 drying and its optimization.The MCC hydrogels were obtained after thermal gelation of alkaline solutions of 7 wt.% using two aqueous solvent mixtures, NaOH:urea 12/7 wt.% and NaOH:urea:thiourea 8:8:6.5 wt.% according to the literature.2 MCC alcogels were obtained after a gradual solvent exchange using 10-100 % v/v ethanol solutions. Finally, they were dried for 2 h using scCO2 under different conditions in batch mode. Four process variables were studied, CO2 density (689 kg m-3 at 12 MPa/40°C, and 604 kg m-3 at 15 MPa/60°C), depressurization rate (0.4 and 1.5 MPa min-1), drying cycles (1 cycle of 2 h and 2 cycles of 1 h each), and composition (AU= MCC-NaOH/urea and AUT= MCC-NaOH/urea/thiourea). The textural properties (SBET= specific surface area, and DP= pore diameter), morphology, crystallinity, and typical functional groups were determined by N2 adsorption/desorption isotherms, SEM, XRD, and FT-IR, respectively.Among all studied variables, porosity (%P) was affected significantly.