Copper based catalysts in the selective dehydration of polyols
The goal of current work is to design, develop and understand a catalytic system for the dehydration of 1,2,6 hexanetriol, optimising the selective removal of the secondary hydroxyl group. The nature of biorenewable resources, in particular the existence of multiple functional groups poses a huge challenge for their catalytic upgrading to useful commodity chemicals. We have successfully demonstrated copper modified silica alumina as well as copper modified silica catalysts as viable catalysts for this reaction. In order to understand the catalytic conversion of biorenewable feedstocks, 1,2,6 hexanetriol has been selected as the platform molecule and the catalytic routes to known commodity chemicals over copper based catalysts have been investigated. A collection of these catalytic rules can in future be used in the development of a catalytic toolbox which will provide some insights into the general catalytic conversion of biorenewables. The ultimate goal is to develop a comprehensive catalytic toolbox that will facilitate the logical design of catalytic systems from given starting biorenewable platforms to desired commodity end product.
Despite the fair amount of research that has been done on the catalytic dehydration of diols and mono-ols not much work has been done on dehydration of triols. There is therefore a lack of kinetic, mechanistic and active site identification work in the catalytic dehydration of higher order polyols reported in literature. This work investigates the role of the metal functionality in copper based composites for the dehydration of 1,2,6 hexanetriol. We found that the copper catalysts that were active in flows of H2 were virtually inactive in flows of inert gas, a discovery that led us to the proposal of 3 hypotheses based on the experimental data. We propose that the copper metal surface facilitates the dissociative adsorption of H2 followed on by some events (extensively discussed in chapters 3 and 4), which ultimately result in the production of protonic active sites which participate in the catalytic reaction. A second hypothesis was proposed concurrently with the first one, stating that the H2 played the role of keeping the copper metal in its reduced state, as Cu0 and we proposed that Cu0 was the active site for the reactions proceeding. Correlations between catalyst specific activity and Cu0 surface concentrations revealed a strong dependency between the two variables suggestive of the fact that Cu0 was indeed the active site for the reactions proceeding. Lastly, we propose that H2 is preventing the catalysts from coking, by working together with the copper metal to remove product from the catalyst surface, thus preventing product polymerization which leads to coke formation.
We established that copper metals supported on high acid strength materials had the propensity to drive ring closing reactions, thereby increasing selectivity toward pyranic and cyclohexyl products under those conditions. On the other hand, copper catalysts supported on moderate to low acid strength materials yielded higher selectivities toward linear products such as 1,6 hexanediol and its precursors. Higher H2 partial pressures produced increases in catalytic activity and these conditions steered selectivities towards 1,6 hexanediol and its precursors. Strong synergic effects between Cu0, acid support and choice of carrier gas were exhibited by all catalysts; therefore experimental work is mostly focused on decoupling the effects of the variables so as to determine the individual roles played by each component in the system.
Finally, we proposed some reaction mechanisms for the formation of ring products obtained in this study.