Tuning of MgO's base characteristics by blending it with amphoteric ZnO facilitating the selective glucose isomerization to fructose for bioenergy development

Abstract

Fructose serves as an important intermediate in the preparation of liquid fuel compounds. Herein, we report its selective production via a chemical catalysis method over ZnO/MgO nanocomposite. The blending of an amphoteric ZnO with MgO reduced the latter's unfavorable moderate/strong basic sites that can influence the side reactions in the sugar interconversion, reducing fructose productivity. Of all the ZnO/MgO combinations, a 1 : 1 ratio of ZnO and MgO showed a 20% reduction in moderate/strong basic sites in MgO with ∼2–2.5 times increase in weak basic sites (overall), which is favorable for the reaction. The analytical characterizations affirmed that MgO settles on the surface of ZnO by blocking the pores. The amphoteric ZnO undertakes the neutralization of the strong basic sites and improves the weak basic sites (cumulative) by the Zn–MgO alloy formation. Therefore, the composite afforded as high as 36% fructose yield and 90% selectivity at 90 °C; especially, the improved selectivity can be accounted for by the effect of both basic and acidic sites. The favorable action of acidic sites in controlling the unwanted side reactions was maximum when an aqueous medium contained 1/5^th methanol. However, ZnO's presence regulated the glucose's degradation rate by up to 40% compared to the kinetics of pristine MgO. From the isotopic labelling experiments, the proton transfer pathway (or LdB–AvE mechanism by the formation of 1,2-enediolate) is dominant in the glucose-to-fructose transformation. The composite exhibited a long-lasting ability based on the good recycling efficiency of up to 5 cycles. The insights into the fine-tuning of the physicochemical characteristics of widely available metal oxides would help develop a robust catalyst for sustainable fructose production for biofuel production (via a cascade approach).