Characterization and catalytic hydrolysis of lignocellulosic Castor plants for bioethanol production

Second generation non-food lignocellulosic biofuels have great potential as renewable fuels. While castor oilseeds have been used to produce biodiesel [1], the lignocellulose in Castor plant (Ricinus communis) has never been... [ view full abstract ]

Second generation non-food lignocellulosic biofuels have great potential as renewable fuels. While castor oilseeds have been used to produce biodiesel [1], the lignocellulose in Castor plant (Ricinus communis) has never been used for bioethanol production. This study characterizes the stems and leaves of Castor oil plants to evaluate their suitability as biofuel feedstock and catalytically hydrolyzes them to biofuel precursors such as glucose, furfural and HMF, along with Levulinic and Formic Acids.

Characterization studies show that Castor leaves and stems are composed of cellulose (40.5%, with Degree of Polymerization of 1040), hemicellulose (20%) and lignin (13%). The Crystallinity Index (CI) is measured as 37% for the raw lignocellulose, and as 60% for its cellulose-rich component (Figure 1(a)). CI increases to 49% upon pre-treatment with Ionic Liquid (1-butyl-3-methyl-imidazolium chloride) and again reduces to 38% upon hydrolysis (Figure 1(b)). FESEM images of raw (Figure 1(c)), cellulose-rich, pre-treated (Figure 1(d)) and hydrolyzed (Figure 1(e)) substrates suggests that pretreatment breaks the bonds between the cellulose, hemicelluloses and lignin polymers in the pores of the lignocelluloses, exposes the cellulose to catalytic hydrolysis, while increasing the crystallinity of the substrate. FTIR Spectroscopy and CHNS Analysis (Figure 2) are performed on Raw and Cellulose-rich substrates to ascertain their bonds and the elemental compositions, while particle and pore-size analyses are performed to determine their pore structures and porosities. 

Upon pretreatment with Ionic Liquid followed by catalytic hydrolysis, temporally oscillatory yields of glucose and HMF are observed for various water addition rates, with the glucose yield maximizing at 3.5 hours to 53% (w.r.t. the total carbohydrate content) for a water addition rate of 35 µl/gm of Ionic Liquid/hr (Figure 3). The oscillatory dynamics of the product yields are attributed to a reaction mechanism [2], which shows that while water catalyzes the hydration of cellulose to glucose and of HMF to Levulinic and Formic Acids, the production of HMF from glucose is a dehydration process. Thus water addition rates can be optimized to control the product distribution in the reactor.

[1] Dias, J.M., et al., Energy, 53, 58-66 (2013)

[2] Paul, S.K., Chakraborty. S., Bioresource Technology, 253, 85-93 (2018)