Biomass is the most abundant renewable carbon resource, and it is most likely that fuels and chemicals obtained from biomass will play a major part in tomorrow’s society. However, for the necessary conversion of biomass, new and improved technology has to be developed. A major difference between fossil and biomass feedstock is the presence of oxygen in the biomass. The focus of the project was the study of reactions which remove oxygen from molecules in order to increase the value of biomass components.
More specifically, vanadium catalyzed deoxydehydration (DODH) was applied to conversion of vicinal diols (characteristic of biomass materials) to alkenes. Initial experiments found the DODH reaction to proceed in an autoclave at 230 °C, when 1,2-decanediol was allowed to react with 5 mol% ammonium metavanadate (NH4VO3) in isopropanol. After 17 hours, a yield of 1-decene that corresponded to 50 mol% of the starting substrate was achieved.
A further goal was to use an alcohol as both the solvent and as the reductant. Secondary alcohols were found to give much better yields that their primary analogues, and isopropanol was selected for further research.
Most of the screened catalysts gave the same yields, and NH4VO3 was judged to be a good choice based on its availability and ease of handling. Further, this catalyst proved to be more sustainable if reused before it gets deactivated.
Vanadium catalyzed DODH was found to have unique selectivity towards substrates containing exactly one primary and one secondary hydroxyl group. In contrast, internal diols such as 3,4-hexanediol gave no yield of alkene and both stereoisomers of 1,2-cyclohexandiol were almost completely unreactive. However, substrates that are stabilized by conjugation, such as hydro-benzoin, were found to undergo oxidative cleavage to form two aldehydes. The reactivity of the diols also depends strongly on the orientation of the hydroxyl groups. A sharp decline in product yield was thus observed when cis-1,2-cyclohexanediol was added to a reaction of 1,2-hexanediol, which was in contrast to addition of the trans stereoisomer. A possible reason is that the trans isomer cannot simultaneously coordinate to the metal with both hydroxyl groups and thereby inhibit the reaction. Glycerol proved to be unreactive as well, whereas 1,2-propanediol and 3-isopropoxy-1,2-propanediol did undergo DODH to yield propene and 3-isopropoxy-1-propene, respectively.
In a related but separate project, vanadium catalyzed reactive distillation of glycerol was developed. The reaction setup and conditions were optimized to ensure a quick separation of the products from the mixture. Here, just 1 mol% NH4VO3 was enough to achieve up to 22 mol% of allyl alcohol and 4 mol% acrolein from 23 g of glycerol, when heated to 275 °C for 5 hours. This was done in an optimized setup in which a short-range distillation centerpiece allowed for a fast separation of the alcohol.
The catalytic performance of MeReO3, (NH4)6Mo7O24・4H2O and NH4VO3 was compared, and vanadium was found to give more allyl alcohol than rhenium, which is surprising due to the increased DODH reactivity of rhenium.
Illustration:
Proof of concept: Use of vanadium catalysts for conversion of biomass-related molecules to value-added organic products