Volatile fatty acids (VFAs) are produced in the anaerobic digestion of urban or food wastes and sewage sludge. The interest in these compounds has increased because of their use as platform chemicals. In this paper, the application of natural terpenoids and eutectic solvents to sustainably recover VFAs from aqueous solutions by extraction has been evaluated. First, a screening of solvents has been done using COSMO-RS, selecting three terpenoids: geraniol, citral, and eugenol, and two hydrophobic eutectic solvents formed by octanoic acid and menthol or thymol. These solvents have been tested in the extraction of acetic, propionic, butyric, and valeric acids. The highest yields were obtained using geraniol and the (octanoic acid + thymol) eutectic solvent, being comparable or higher than those for organic solvents from the literature. The back-extraction of the VFAs from both solvents has been studied using NaOH and Na 2 CO 3 solutions, concluding that the eutectic solvent is not chemically stable to be reused, whereas the reusability of geraniol was confirmed in consecutive cycles of extraction. The technical viability of the proposal was confirmed in scale-up experiments using geraniol in a packed column, and VFA extraction yields higher than 98% were obtained in the simulation of a countercurrent extractor.

No abstract

Furfural comes from lignocellulosic biomass that, together with its derived products, has many useful applications in several industries. Furfural is usually obtained via pentose dehydration using a biphasic reactor to extract furfural in situ from its reaction medium. However, the conventional solvents used so far, mainly toluene and methyl isobutyl ketone (MIBK), are harmful to health and the environment. Therefore, hydrophobic natural solvents have been successfully tested in this work to improve the performance of the process from a sustainable point of view. First, 30 natural solvents were screened using the conductor-like screening model for real solvents (COSMO-RS) method to select solvents with high affinity to furfural. From the results obtained in the screening, 14 natural and 2 conventional solvents were selected for experimentation, including thymol, eugenol, toluene, MIBK, and several hydrophobic eutectic solvents formed by thymol. Then, the liquid–liquid extraction of furfural was carried out both in vials and in a reactor, simulating the usual temperature and pH conditions for obtaining furfural. Thymol and eugenol showed extraction yields of 95 and 91%, significantly higher than those of conventional solvents MIBK and toluene, which were 85 and 81%, respectively. Finally, the in situ reaction and extraction of furfural from xylose were performed using the natural solvents eugenol and thymol and the conventional solvent MIBK. Under operating conditions, namely, microwave heating to 443.2 K, 10 min of reaction, and a solvent-to-feed ratio of 1.00, xylose conversion of 96.7%, furfural selectivity of 75.3%, and a furfural production yield of 72.8% from xylose were obtained using eugenol as the organic solvent, with improved outcomes over MIBK and thymol cases, pointing an adequate approach to improve both the effectiveness and the sustainability of the process.

No abstract

Diverse and abundant applications of the eutectic solvents have appeared in the last years. Their promising tunable properties, eco-friendly character and the possibility of being prepared from numerous compounds have led to the publication of numerous papers addressing their use in different areas. Terpenes and terpenoids have been employed in the formulation of eutectic solvents, though they also have been applied as solvents in extraction processes. For their hydrophobic nature, renewable character, low environmental impact, cost and being non-hazardous, they have also been proposed as possible substitutes of conventional solvents in the separation of organic compounds from aqueous streams, similarly to hydrophobic eutectic solvents. The present work reviews the application of eutectic solvents in liquid–liquid extraction and terpenes and terpenoids in extraction processes. It has been made a research in the current state-of-the-art in these fields, describing the proposed applications of the solvents. It has been highlighted the scale-up feasibility, solvent regeneration and reuse procedures and the comparison of the performance of eutectic solvents, terpenes and terpenoids in extraction with conventional organic solvents or ionic liquids. Ultimately, it has been also discussed the employ of predictive methods in extraction, the reliability of thermodynamic models in correlation of liquid–liquid equilibria and simulation of liquid–liquid extraction processes.