Abstract
As the supply of fossil feedstocks is diminishing, and the consequences thereof are increasing, also the urgency for research into alternative feedstocks is increasing. To provide building blocks for the chemical industry in sufficient amounts and in a sustainable manner, only one alternative resource is available: biomass. The major source
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of biomass is lignocellulosic biomass. Chemically, lignocellulosic biomass is completely different from fossil-based feedstocks. Where fossil feedstocks are under-functionalized and virtually oxygen-free, lignocellulosic biomass has a high oxygen content and is over-functionalized with hydroxyl groups. To obtain suitable building blocks for the chemical industry from lignocellulosic biomass, these hydroxyl groups need to be (at least partially) removed or converted. The most direct way to remove hydroxyl groups is via a dehydration reaction, wherein an alcohol is converted into an olefin, expelling one equivalent of water. The traditional way to perform a dehydration reaction is by using Brønsted acid catalysts. Due to their corrosive nature, however, many of these catalysts pose safety, environmental, and reactor technological issues. These catalysts also cope with selectivity issues in dehydration reactions. Therefore research into alternative catalysts is desirable. In this thesis, the use of rhenium- and molybdenum-based catalysts for selective dehydration reactions has been investigated and the application of these catalysts in the dehydration of bio-based alcohols is described. The use of several commercially available rhenium-based catalysts in the dehydration of a broad scope of different alcohols to their corresponding olefins has been explored. Benzylic, allylic, aliphatic, and homoallylic alcohols, either tertiary or secondary, were tested as substrate under relatively mild reaction conditions, using technical toluene as the solvent, in ambient atmosphere and at 100 – 150 °C. Amongst the complexes tested, rhenium(VII) oxide (Re2O7) was found to be the most active catalyst, surpassing the benchmark catalyst, sulfuric acid, in both activity and product selectivity with most alcohols. Using this protocol, bio-based terpene alcohols can be dehydrated to terpenes, and a proof-of-principle for the catalytic upgrading of essential oils was demonstrated. The mechanistic aspects of this dehydration reaction were investigated using experimental and theoretical investigations. These approaches correlate well and point to the involvement of a carbenium ion intermediate in the catalytic cycle. As an alternative to rhenium-based catalysts, complexes based on the cheaper and more abundant molybdenum were investigated. These complexes showed to be active in the dehydration reaction, with their activity and selectivity intermediate between Re2O7 and sulfuric acid. For specific substrates, however, results comparable to Re2O7 were obtained, showing the potential of these complexes as dehydration catalysts. Finally, the deoxygenation of lactic acid was investigated as a relevant bio-based alcohol deoxygenation. Here, molybdenum-based catalysts showed very good activity in the conversion of lactic acid to propionic acid, using reactive distillation conditions at 220 – 270 °C. Overall, we have demonstrated that transition-metal-based catalysts can replace and even surpass the classical Brønsted acid catalysts in the dehydration reaction of various types of alcohols. This research has thereby contributed to the sustainable application of this fundamental reaction step in the processing of biomass and its derivates.
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