Efficient processes for the production of chemicals from biomass will depend on achieving high selectivity. The use of heterogeneous catalysts in such processes is desirable due to their facile separation. To reach the desired selectivity, it is crucial to understand and control the way they interact with the biomass-derived oxygenates used as feed. Since most biomass-derived oxygenates have multiple functional groups and limited volatility, they are typically converted as solutions. This presentation will show how spectroscopy and DFT calculations can be used to identify surface species formed from polyols and sugars in the presence of solvents. This insight will be used to deduce reaction pathways for their conversion. Glycerol can favourably compete with water for binding sites on metal oxides like γ-Al2O3, TiO2, and N2O5 [1-3]. The most stable surface species is formed when one of the primary alcohol groups forms a bridging alkoxy bond on a Lewis acid site (LAS), while the other primary alcohol group interacts non-dissociatively with the same site. In addition, the secondary hydroxyl group forms a hydrogen bond with the surface [1]. Sufficient spatial separation between the alcohol groups is necessary for these multidentate surface species to form without excessive strain. Through the interaction with a LAS, the primary C-O bonds of this surface species are polarized making them more susceptible to cleavage. However, dehydration only occurs on metal oxide like Nb2O5 that also contain Brønsted acid sites (BAS) [3]. When LAS and BAS are involved, the primary alcohol group is removed preferentially, and acetol is formed. Without the involvement of a LAS, the secondary alcohol group is removed due to the higher stability of the secondary carbenium ion that is formed as the transition state. The intermediate 3-hydroxypropionaldehyde readily undergoes a second dehydration step to acrolein.

Surface reaction of biomass derived oxygenates on metal oxide catalysts

253rd National Meeting (2017)

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