(350a) On the Origin of Catalytic Activity of Zirconia in Aqueous-Phase Partial Oxidation of Cellulose to Levulinic Acid
The production of levulinic acid from cellulosic biomass has been known for more than 150 years through acid catalysis with mineral acids as the catalyst. A well-accepted reaction pathway of converting cellulose to levulinic acid is that cellulose is hydrolyzed to glucose which is dehydrated to 5-hydroxymethyl furfural (HMF), and finally HMF is rehydrated to equal molar levulinic acid and formic acid. However, the commercialization of this conventional acid catalyzed process is hindered by strong corrosivity, high separation cost, and significant waste disposal. Our group has reported that a high yield of levulinic acid was produced by directly converting cellulose over a ZrO2 catalyst in a catalytic aqueous phase partial oxidation (APPO) process. We proposed an alternative pathway to synthesize levulinic acid from cellulose: cellulose is partially oxidized to gluconic acid which is then transformed into levulinic acid via consecutive decarboxylation, dehydration, and rehydration reactions.
Herein, the investigation of APPO reaction mechanism was performed via radical scavenger, probe reaction, and isotopic labeling studies, as well as electron paramagnetic resonance (EPR) spectrometry. We found that the in-situ generated superoxide radical anions may play an important role in the partial oxidation of cellulose. The influences of ZrO2 morphology on the catalyst performance were also investigated. Electron paramagnetic resonance (EPR) and pyridine-FTIR results show that, relative to tetragonal zirconia (t-ZrO2), monoclinic zirconia (m-ZrO2) has a higher amount of Zr3+ centers and higher strength of acid sites, which leads to better catalytic performance: the yield of LA is more than 50% and lactic acid yield is 8% (on carbon molar basis) under optimized reaction conditions. Moreover, experimental results show that the reaction APPO of cellulose follow a radical mechanism and the Zr3+ centers are more active than F-centers in the formation of superoxide radical anions in subcritical water.