Catalytic conversion of bio-derived platform molecules into useful chemicals via selective oxidation

Abstract

Green Chemistry regards the design of products and processes that minimise the use and generation of hazardous substances. Heterogeneous catalysis facilitates such energy and atom efficient processes, affording simple and low cost product isolation methods, and catalytic materials that can be easily recovered and reused. This thesis reports on the selective aerobic oxidation of 5-hydroxymethyl-2-furufural (HMF), a potential platform chemical that may be derived from cellulose, into 2,5-furandicarboxylic acid (FDCA) over noble metal nanoparticles dispersed on a solid base support. FDCA has been touted as a potential replacement for polyethyleneterephtalate (PET) and is also an interesting synthetic building block. Au nanoparticles are extremely active and selective oxidation catalyst for a range of environmental and fine chemical transformations, however they require a homogeneous base, such as NaOH, to work. Au NPs dispersed on hydrotalcites (HT), anionic microporous clays, have shown promise in HMF oxidation to FDCA, hydrotalcite acting as both the support and the source of base. However, key questions remained regarding the nature of active site, potential role of homogeneous contributions and importance of reaction basicity upon activity and selectivity. Kinetic profiling of HMF and its intermediates HMFCA and FFCA over Na-free Au/MgAl HT catalysts highlight the role of base in achieving high FDCA yields. The order of reaction in Au, in oxygen and in HMF were found, determination of the Arrhenius Ea for the R-OH and the R-CHO functions allowed to find the rate-determining step. Ex situ and operando XAS were performed to detect Au oxidation state and Au chemical environment in the catalyst, enlighting the true active site during the selox. Eventual changes in Au oxidation state were investigated to find how the precursor HAuCl4 evolves during the calcination. As the reaction was found to be pH-sensitive and as the calcination of HTs convert them into stronger bases, the impact of calcination temperature was studied; also a comparison between calcination and calcination rehydration protocols was done. The incorporation of Pd into Au catalysts improved activity and lifetime for these AuPd bimetallic formulations. AuPd NPs were prepared via DP method on HT support, varying Au:Pd ratio, then kinetic studies for the selox of HMF to FDCA were carried out, followed by accurate characterisations.