Using metrics and sustainability considerations to evaluate the use of bio-based and non-renewable Brønsted acidic ionic liquids to catalyse Fischer esterification reactions

Starch valorization: Direct conversion of starch to hexyl levulinate over SO4/ZrO2-KIT5 composite

Valorization of biomass for the synthesis of valuable chemicals is a promising toolbox for replacing fossil fuel consumption. Long-chain hexyl levulinate (HL) is one of the attractive high-value chemicals obtained from biomass valorization. The current paper investigates the synthesis of KIT5-supported SO4/ZrO2 and its application in the successive hydrolysis and dehydration of starch to HL. The acidity of the prepared catalyst was modified, and its effect on the conversion of starch and HL yield was thoroughly studied. The parameters effective on the reaction yield and selectivity were optimized, and the possibility of 5-((hexyloxy)methyl) furan-2-carbaldehyde formation was explored. The prepared SO4/ZrO2-KIT5 can be used at least in four successive runs with a slight decrease in its reactivity. The HL yield was increased to a maximum of 28 %, while the starch conversion increased to a maximum of 100 % by conducting the reactions at 220 °C for 10 h. The accessibility and low cost of the starting materials as well as the method's simplicity, can give a practical outlook of its possible industrial application.

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

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).

Core-shell assembly of ZrO2 nanoparticles with ionic liquid: a novel and highly efficient heterogeneous catalysts for Biginelli and esterification reactions

Imidazolium sulfonic acid chloride grafted ZrO₂ nanoparticles (ZrO₂-IL) were synthesized through facile post-treatment of the nanoparticles with the imidazolium-sulfonic acid chloride ionic liquid. The immobilization of the ionic liquid over the ZrO₂ nanoparticles was evident from the XRD, SEM, TEM, Raman, BET, and XPS analysis. The results obtained from the XRD analysis clearly show that the catalyst has an orthorhombic structure and from the BET analysis it is evident that the surface is mesoporous with uniform pore sizes and pore distribution. Further evidence of immobilization of ionic liquid over the ZrO₂ NPs was obtained from the SEM, TEM, XPS, and Raman analysis. Under mild conditions, the synthesized heterostructure was used in the acid-catalyzed esterification of different acids. The ZrO₂-IL catalyst converts 99% of the acid to ester with a 98.9% yield in 1h. The material was also shown to be highly efficient as catalyst for the Biginelli reaction under solvent-free conditions, with the catalyst for dihydropyrimidin-2(1H)-one (DHPMs) in 1h with 99.2% conversion and 99% yield. The synergy between the ionic liquid catalyst and the substrates increased the catalytic efficiency and resulted in high-yield product conversion. The mechanism of both transformation reactions was investigated, as well as the synergy between ionic liquid and ZrO₂ nanoparticles for better catalytic efficiency was established.

Production of xylose, levulinic acid, and lignin from spent aromatic biomass with a recyclable Brønsted acid synthesized from d-limonene as renewable feedstock from citrus waste

This work aimed to develop a green protocol for chemical processing of spent aromatic biomass to obtain xylose, levulinic acid, and lignin in good yields via treatment with p-cymene-2-sulphonic acid (p-CSA), a Brønsted acid synthesised from d-limonene as a renewable feedstock from citrus waste. Chemical processing of palmarosa biomass with p-CSA under heating in an autoclave resulted in hydrolysate containing xylose (~16% yield). Further processing of pre-treated biomass with p-CSA in presence of aq. HCl under refluxing caused a selective degradation of cellulose to levulinic acid (~22% yield with respect to biomass). The residual biomass was used to afford lignin in good yields.

Processing of wet Kinnow mandarin (Citrus reticulata) fruit waste into novel Brønsted acidic ionic liquids and their application in hydrolysis of sucrose

In citrus processing industries, where up to 60% of the whole fruit is discarded, generates citrus waste (peel, seeds, membrane, and pulp) on a massive scale. The waste does not currently have high-value applications, instead the majority is disposed of or pelletized for animal feed. Therefore, a concise and efficacious protocol for processing of Kinnow mandarin (Citrus reticulata) fruit waste to numerous novel Brønsted acidic ionic liquids (3a-k) have been developed. BAILs were characterized using spectroscopic techniques (FT-IR and NMR). Water immiscibility of ILs 3a, 3g and 3h, a property never observed with sulfonic acid ILs, allowed the catalytic application of BAIL 3a in hydrolysis of sucrose/table sugar, giving a mixture of d-glucose and d-fructose in excellent yields. The ionic liquid could be recycled for >3 times without significant loss of activity.