Catalytic conversion of biomass-derived platform molecules over zeolites

The conversion of lignocellulosic biomass to renewable fuels and chemicals has attracted significant attention as a key technology to enable the replacement of petroleum. Lignocellulosic biomass is the most promising renewable carbon energy source, as it is widely available around the world at a relatively low cost. Although it is the most abundant plant material resource, its conversion into chemicals faces a range of technological and economic challenges. In order to overcome these challenges, several different processes to obtain chemicals and biofuels from biomass are currently under development. The most promising is the multistep strategy using platform molecules as intermediates. The most important platform molecules that can be obtained from biomass in good yields are biomass-derived furans, such as furfural, 5-hydroxymethylfurfural (HMF), 2-methylfuran (MF), furan and 2,5-dimethylfuran (DMF). Recently, the Diels-Alder (DA) cycloaddition of biomass-derived furans, especially DMF, MF and furan, with dienes (mainly ethylene) has emerged as a prominent route to obtain aromatics, such as benzene, toluene and xylene (BTX). The BTX are essential for the petrochemical industry, since it is the raw material for the most consumed polymers and chemicals. As millions of tons of BTX are produced annually exclusively from fossil sources, it has now become extremely important develop technologies to obtain BTX from completely renewable sources.
In this context, the present thesis is focused on the investigation of a completely new/renewable facilitated process to convert DMF into aromatics over zeolites, which uses ethanol as a source of dienophile. The use of liquid ethanol instead of gaseous ethylene, as the source of dienophile in this one pot solvent free synthesis, renders the aromatics production in much simpler and renewable way, which avoids the use of ethylene at high pressure. More importantly, both our studies, experimental and theoretical, demonstrate that the use of ethanol instead of ethylene, results in significantly higher rates and higher selectivity to aromatics, due to lower activation barriers over the solid acid sites. The reaction mechanism was investigated by combining high quality synchrotron X-Ray diffraction (SXRD) data and Rietveld refinement analysis with detailed kinetic measurements and high-level first principles calculations. The results confirm that the reaction takes place via a facilitated process, where the preferential protonation of the ethanol molecule by the Brønsted acid site (BAS) and the presence of a water molecule formed by the ethanol dehydration inside the restricted pore of the zeolite catalyst lead to lower activation barriers over the BAS, higher rates and higher selectivity to aromatics.
In this study, it was also demonstrated how the Ethanol/DMF reaction is influenced by the strength of the acid sites in the zeolites. Using SXRD in combination with the basic probe molecule pyridine and 27Al NMR, it was possible to link the Brønsted sites in 3-D structure and global atomic arrangements with the acidity. After locating and determining the nature of the BAS, the selective site blockage strategy by K+ exchange was employed to control the product distribution for the DMF transformation reaction into aromatics. Comparing the product distribution obtained for the pure catalyst and for the catalysts with the partially and totally blocked BAS, it was possible to confirm that the Diels-Alder cycloaddition is catalysed by strong acid sites while hydrolysis of furan takes place over weak acid sites. This was the first time that a study correlates the nature of the BAS with product distribution for the complex reaction of DMF conversion into aromatics via Diels-Alder cycloaddition. This understanding is extremely important to allow more superior catalytic performances for a wider range of Biomass-derived furans.
Finally, the novel catalytic reaction presented in this thesis was investigated as a new strategy to convert biomass-derived furan into aromatics with high selectivity to aromatics, while reducing the production of the side product benzofuran. The use of ethanol as a source of dienophile in the reaction with furan showed high selectivity to aromatics, especially for the highly desired aromatic product ethylbenzene, while the formation of the side product benzofuran formation was considerably reduced. Further investigations by detailed kinetic experiments, SXRD and FTIR experiments showed that the key step to prevent the benzofuran formation and favour the aromatic products is the alkylation of the furan molecule by an ethyl species formed from the ethanol dehydration. This step occurs before the Diels-Alder (DA) reaction and has a double effect to promote the aromatic products. The substituent alkyl groups on the furan ring create greater steric hindrance, thus avoiding the direct coupling reaction between two furan molecules to form benzofuran. Furthermore, the alkylated furans (dienophile) undergo DA cycloaddition more readily, due to its higher energy HOMO orbital.
The novel catalytic reaction presented in this thesis is a breakthrough in the conversion of biomass derivatives into aromatics, making the reaction simpler and entirely renewable. Moreover, this novel approach has proved to be versatile and very useful to allow better yields for challenging biomass-derived furans, such as furan itself. More generally, further investigations into Diels–Alder cycloaddition reactions with alcohols as a source of dienophiles may allow activity and stereochemistry to be tailored, extending its application far beyond biomass conversion.

Advisor: Tsang, S