Synthesized hierarchical mordenite zeolites for the biomass conversion to levulinic acid and the mechanistic insights into humins formation

The hierarchical Mordenite zeolites are synthesized by simple seed-assisted and OSDA-free method. The crystallinity, mesoporosity and the concentration of acid sites increase significantly with hydrothermal time. Formation and growth of the zeolite crystal are observed from electron microscopy. Based on the XRD and FESEM studies, the zeolite growth mechanism is postulated to be via seed surface crystallization. With the hydrothermal time, Si(0Al) species converts to Si(1Al) and Si(2Al), as observed from Si-29 and Al-22 SS-NMR. The maximum levulinic acid (LA) yields of 61, 56, 52 and 43% are obtained from bamboo sawdust, cellulose, glucose and fructose, respectively, over the hierarchical H-MOR catalyst. The humins are formed from the 5-Hydroxymethylfurfural with 2,5-dioxo-6-hydroxy-hexanal as an intermediate. Even though the yield of humins is lower in the biphasic system, the yield of LA (47%) is also lowered moderately. Lastly, the reaction mechanism is postulated for the conversion of biomass to LA and humins.