Comparative study of microwave-assisted versus conventional heated reactions of biomass conversion into levulinic acid over hierarchical Mn₃O₄/ZSM-5 zeolite catalysts

Conversion of delignified cellulose from rice husk biomass, and model compounds of cellobiose and glucose to levulinic acid (LA) over hierarchical Mn₃O₄/ZSM-5 catalyst was carried out using a household microwave method, and then compared to the established conventional thermos-reaction method. The hierarchical ZSM-5 was prepared using a double template method, aiming for micro and mesoporous systems developed in the structure. The as-prepared ZSM-5 were modified with Mn₃O₄ through incipient wetness impregnation with Mn²⁺ solution followed by calcination at 550 °C. The catalysts were characterized using various techniques such as powder XRD, SEM, BET, AAS, and FT-IR which indicated the hierarchical structure of MFI zeolite (Si/Al of 30–34) with Mn loading of 2.14 wt%. The conversion products were analyzed using HPLC, ¹H NMR, and ¹³C NMR instruments. The microwave-assisted reaction using 600 W for 180 s using delignified cellulose, cellobiose, and glucose gave conversion of 37.27%, 46.35%, and 54.29%, respectively which is close to the conversion given by the conventional reaction carried out at 130 °C for 4 h (36.75%, 55.62%, and 60.9%, respectively). Interestingly, the LA yield from the microwave-assisted reaction (4.33%, 6.12%, and 9.57%) is higher than the yield from the conventional reaction, which only produced 5.2%, 4.88%, and 6.93% respectively. The microwave-assisted method is also shown to give less by-products compared to the thermochemical reaction. Therefore, it could be considered an alternative method for converting cellulose to LA.