(500b) Catalytic Conversion of Lignocellulosic Biomass to Carbochemicals In Carbonated Water

The concept of biorefinery entails the use of bio-based raw materials like abundant lignocellulosics biomass for the production of valuable products like fuels and biochemicals. Over the last decade, researchers have shown that bio-based raw materials can be effectively converted using various processes to value-added products like energy, fuels and carbochemicals like 5-HMF (5-hydroxymethylfurfural) and furfural (2-furaldehyde). Prior studies in our laboratory on the reactive pretreatment of biomass followed by enzymatic hydrolysis have been conducted with hot carbonated water  at temperatures from 160ºC and 180ºC showed that the production of glucose and xylose were one-half  that produced using dilute acid hydrolysis. The resultant pH of the reaction mixture produced via carbonated water catalysis was found to be optimal for subsequent enzymatic hydrolysis. Similarly switch grass with a particle size range of 75-106 µm was hydrothermally treated at temperatures ranging from 220ºC to 320ºC at varying flowrates from 0.5 and 1 ml/min in a semi-continuous stainless steel reactor system for 60 min. An Bio-Rad Aminex HPX-87H HPLC analysis of the hydrolyzate showed that the yields of 5-HMF was 0.95(g/100g raw biomass) at 320ºC and 0.5ml/min and 0.66(g/100g raw biomass) at 320ºC and 1ml/min whereas furfural yields were 1.66(g/100g raw biomass) at 280ºC and 0.5ml/min and 2.07(g/100g raw biomass) at 250ºC and 1ml/min. These results show that furfural yields increased with the increased level of hydration in the reaction mixture (i.e. at higher flowrates) whereas HMF which is produced from glucose required highest temperature for higher yields. This indicates that the dehydration of monomeric sugars to carbochemicals like HMF and furfural was influenced by temperature and flowrate. We also found that as the residence time of the water in the semi-continuous flow reactor increased, the concentration of 5-HMF and furfural decreased immediately within 20 min indicating an unstable nature of these carbochemicals towards forming other degradation products like levulinic, formic and acetic acids. Considering the results of high temperature water hydrolysis to produce sugars from biomass substrates, carbon dioxide was introduced into the system and the same set of experiments were performed as previously done with neat water. Carbon dioxide dissolves in water under the effect of temperature and pressure forming carbonic acid which eventually decreases the pH of the reaction mixture resulting in higher yields of 5-HMF and furfurals. The liquid carbon dioxide also acted as a catalyst enhancing the degradation of recalcitrant cellulose and hemicelluloses to sugars with further conversion to carbochemicals.