Coffee Ground Lignocellulosic Fractionation Using Natural Deep Eutectic Solvents Coupled with CO2

The capacity of processing biomass into marketable products and energy is known as biorefinery. By-products from several agro-industrial industries can be used as raw materials, from where added-value products with vast applications in many industries can be extracted. Lignocellulosic materials are a typical example of a waste product that is widely applied in biorefinery. However, in order to follow the green chemistry principles, sustainable techniques must be applied. The use of supercritical fluids (SCF), such as sCO2 or subcritical water, have been widely used for the separation, extraction or hydrolysis of lignocellulosic material. Still, there are other green solvents such as ionic liquids or deep eutectic solvents that can be used in combination with supercritical fluids.
In this work, deep eutectic solvents were used for fractionation and extraction of lignocellulosic material of spent coffee grounds. According to USDA, and Sakuragi et al. (2016), annually 6 million tons of spent coffee grounds are produced, which is mainly composed by hemicelluloses (about 50 % w/w), lignin (15 % w/w) and cellulose (8 % w/w). The solubility and fractionation of lignin, cellulose and hemicelluloses has been studied in different DES. Hemicelluloses presented good solubility in DES, as well as lignin. Cellulose did not present any dissolution, in any of the DES tested. Nevertheless, it was possible to see a reduction in cellulose crystallinity that was confirmed by FTIR and X-RAY analysis. From X-RAY it was possible to observe a reduction in crystallinity of about 20 % and FTIR showed cellulose associated crystallinity bands shifting. Furthermore, the physical properties of these DES, as viscosity and conductivity in the temperature range of 293.15 K to 338.15 K were determined.
One the main disadvantages of DES is their high viscosity. This was circumvented by the combination of scCO2 and DES. The enzymatic hydrolysis of DES treated and untreated spent coffee grounds was performed in DES media and in biphasic DES/scCO2 media.