Evaluation of a green pressurized reaction media (subW-CO2) and pressurized microwave-assisted reaction for furfural production from corn stover and its derivatives sugars

Abstract

Corn stover is a highly produced agricultural waste worldwide, accounting with more than 1 billion tons per year [2]. Corn stover is a lignocellulosic biomass, so its main fractions are cellulose, hemicellulose and lignin. This composition makes of corn stover an ideal biomass for the production of value-added chemicals, highlighting in this work the production of furfural. Furfural (C5H4O2), is an aromatic aldehyde obtained by the hydrolysis of pentoses through their subsequent dehydration. It is a versatile chemical with many applications in the chemical, pharmaceutical or fuel industries. For its production from lignocellulosic biomass, the pentoses present in its hemicellulose fraction first need to be hydrolized into their monomeric form, and then follow their conversion into furfural. To increase furfural yield and selectivity in this reaction, different catalysts and organic solvents are usually added. These techniques are pollutant agents for the environment, and should be replaced. In this work, two green technologies were proposed for the production of furfural from xylose and corn stover. The first one was a green, organic solvent-free, subcritical water-CO2 (subW-CO2) system. Under subcritical conditions, the water acquires new properties, such as an increase of its ionic product, giving the water acidic and basic catalyst properties [3]. However, to optimize furfural production, CO2 was selected as the pressurizing agent due to its Brønsted acid effect after its dissolution in water as carbonic acid. Brønsted acids have shown to promote xylose and its isomers dehydration into furfural [4]. To further improve furfural production, the addition of CrCl3, which acts as Lewis acid, was tested. The results were promising, with no catalyst, a furfural yield of 41 % was obtained when treating xylose, showing the potential of the subW system itself for furfural generation. When adding CrCl3, the yield increased to 50 % after 1.7 hour treatment at 180 °C (Figure 1). Corn stover was treated under the same conditions, although in this case it only achieved a 25 % furfural yield. The second tested technology was a microwave system. In this case, xylose and corn stover with CrCl3 were added into the pressure vessels. The addition of the Lewis acid catalyst played a key role in MW treatments. When no catalyst was added, a maximum furfural yield of 8 % was reached after 25 minutes at 180 °C, while it was 30 % when CrCl3 was added, proving the effect that CO2, and therefore Brønsted acids present for furfural production. Corn stover also showed a much lower furfural yield in MW, achieving a 15 % after a 90 minutes treatment. Comparing both technologies at 180 °C (Figure 1), it is possible to observe how both xylose and corn stover show same furfural yield values between the two technologies at a fixed treatment time of 40 minutes. However, when time increases, furfural yield is further increased in the subW system samples in a greater extent than in the MW ones. After treating xylose and corn stover, side reactions took place, being the main byproducts lactic, acetic, and formic acids. The concentration of these byproducts was higher in the MW treated samples, highlighting lactic acid, which concentration was 10 times more in the MW assisted reaction than in the subW system in the case of xylose. Therefore, the selectivity towards furfural when treating xylose or lignocellulosic biomass was higher in the subW-CO2, and therefore, it can be concluded that the presence of a Brønsted acid, in this case carbonic acid produced from CO2, significantly affects to the selectivity of the reaction to furfural, and consequently, to its yield. Both green technologies proved to be promising alternatives for furfural production, making possible to stablish a starting point for the use of these techniques to produce furfural from lignocellulosic biomass.