PolyE-IL, an Efficient and Recyclable Bronsted Acid Catalyst for Conversion of Rice Straw into Levulinic and Other Organic Acids

The lignocellulosic biomass has been identified as a potential renewable feedstock for production of levulinic acid (LA). In the present study, the catalytic thermo liquefaction (CTL) process for liquefaction of rice straw into LA was studied by integrating the polyethyleneimine functionalized acidic ionic liquid (PolyE-IL) catalyst. The screening of various PolyE-IL catalysts with variable counter ions showed the remarkable formation of carbohydrates, LA, formic acid, and acetic acid (AA). Among the tested PolyE-IL catalysts, PolyE-IL with a [HSO4]− counter ion showed the maximum conversion efficiency. The process intensification study to evaluate the influence of reaction parameters such as catalyst concentration, reaction temperature, time, and slurry concentrations was conducted to achieve the maximum conversion and yield of organic acids. Moreover, the influence of feedstock pretreatment was also studied. The pretreated rice straw provided maximum yields of LA and FA. The intensified CTL process for untreated rice straw resulted in 49.8% LA, 50.5% FA, and 100% AA, while CTL of pretreated rice straw resulted in 65.5% LA, 75.8% FA, and 26.7% AA of the theoretical maximum at 210 °C for 120 min. The separation of the catalyst and other liquefaction products was achieved using adsorption followed by membrane separation processes. The adsorption process leads to separation of the undesired polymeric side products, while membrane separation provides efficient separation of the catalyst and organic acids with >98% efficiency. The undesired side products separated in the adsorption process were concentrated as CTL-Oil and characterized for physiochemical properties. CTL-Oil is C-, H-, and O-rich feedstock with a caloric value of 24–26 MJ/Kg and thus could be explored for multiple fuel and energy applications. Thus, an efficient valorization of rice straw was achieved using a recyclable, robust, and efficient catalyst.