Investigation of Deep Eutectic Solvent-Based Microwave-Assisted Extraction and Efficient Recovery of Natural Products

A systematic study of the principles of deep eutectic solvent-based microwave-assisted extraction (DES-MAE) was performed. It was found that the heating rates of most DESs decreased (heat capacity increased) under microwave irradiation with increasing water content, allowing high-efficiency extraction for thermally sensitive compounds. In addition, DESs containing carboxylic acids reacted with hydroxyl groups of sugar and choline chloride, resulting in cell wall destruction and inhibition of cellulose, hemicellulose, and lignin reconnection in cell walls through hydrogen bonds, thus leading to better extraction performance. This was verified by extracting anthraquinones from Rheum palmatum using DES-MAE and optimizing extraction conditions. DES with citric acid as the hydrogen bonding donor gave the highest extraction efficiency under the optimized conditions. In addition, anthraquinones in the DES extract were recovered using three kinds of silica modified by different functional groups. The results showed that material containing a phenyl group is beneficial to the recovery of anthraquinones in acid-based DESs because it can facilitate strong hydrophobic and π–π interactions. This study showcases the green chemistry applications of DES-MAE in a laboratory and industry alike, and demonstrates the recovery of natural products from DES extracts. The findings also provide valuable information for green extraction, modification, and application of cellulose, hemicellulose, and lignin.