In the pursuit to replace fossil resources with more sustainable alternatives, the chemocatalytic conversion of carbohydrates provides an important method for producing new functional chemicals. This PhD project focuses on the use of a Sn-Beta zeolite catalyst for the conversion of glucose, xylose and glycolaldehyde in methanol, to form α-hydroxy esters, such as methyl lactate and methyl vinyl glycolate (MVG), with interesting potential as building blocks for polymer production.
The project is concerned with the development and use of NMR spectroscopic methods to identify and quantify several new products in the catalytic reaction without having to purify samples or isolate new substances. Among other things, two new interesting molecules were identified from the carbohydrate conversion, namely methyl 2,5-dihydroxy-3-pentenoate (DPM) and methyl 2,4,5-trihydroxy-3-pentanoate (THM). The yields were optimized by varying several reaction conditions, yielding 19.4% of THM and 42% of DPM. It was subsequently shown that DPM could be used to prepare polymers.
Alkali salts such as potassium are known to modify the reaction selectivity of the Sn-Beta catalyzed carbohydrate conversion so that the transformation follows a different reaction pathway and primarily yields methyl lactate. This influence was investigated further, leading to three findings: 1; the effect was proportional to the ratio of alkali salt and tin atoms in the catalyst, 2; the reaction selectivity is under kinetic control, and 3; alkali salts facilitate breaking of carbon-carbon bonds. The reaction mechanistic pathways were investigated using carbohydrates with isotope-labeled hydrogen and carbon atoms. The key findings from this were that isomerization competes with dehydration to determine the selectivity for DPM / THM. In addition, it indicates that all carbohydrates remain connected to the tin in the catalyst until fully reacted.
Overall, this project has contributed many additional details about the Sn-Beta catalyzed conversion of carbohydrates through the study of intermediates, product distributions and the tracking of atoms during the reactions.
1H-13C spectral region of secondary alcohol CH-groups (indicated by small spheres) adjacent to carboxylic groups, showing the signals of both known and new α-hydroxy esters formed from conversion of glucose.