5-Hydroxymethylfurfural (HMF) has become a benchmark chemical as it can be obtained by the processing of renewable biomass, making its downstream modification an important topic both in industry and academia. Finding catalytic processes for the selective derivatization of HMF is challenging due to its reactivity, hence mild conditions are needed. Here is reported the selective oxidative esterification of HMF to methyl 5-(hydroxymethyl)furan-2-carboxylate and to dimethyl furan-2,5-carborxylate (FDCM) under mild conditions using oxygen (from air) as oxidant (in batch). A synergetic effect between the used catalysts, cobalt oxide over nitrogen-doped carbon, and ruthenium over carbon catalysts was observed by catalytic and electron paramagnetic resonance measurements. Kinetic analysis revealed a first order dependence on substrate for the cobalt catalyst and an apparent zero order for the ruthenium catalysts. The conversion rates were improved by the use of a perfluoroether as solvent, proving that the oxygen diffusion is the limiting factor in the system.
To run a continuous flow reactor and due to technical problems, a new series of nitrogen doped bimetallic Co/Ru heterogeneous catalysts had to be prepared. Two different supports were used. XRD, XPS and BET measurements showed that porosity of the materials and order of impregnation of metals to the carbon substrate lead to materials with different catalytic activities. CoxOy-N + RuOx-N@C-irregular, which showed a mesoporous character, was the most suitable catalyst; achieving 98% conversion of HMF with selectivity of 57% to FDCM was obtained.
The synthesis of a new type of furan-based monomers was studied. Coupling of HMF and diols was made through NHC catalysis, more specifically by oxidative esterification. Catalysts, base, oxidant and solvent screening was made. NHC catalyzed upgrade of HMF proved to be challenging; the results and mechanistic studies suggest that different mechanisms operate at the same time, therefore control of the reaction is a difficult task. 6-hydroxyhexyl 5-(hydroxymethyl)furan-2-carboxylate was synthesized with 28% isolated yield and 6-hydroxyhexyl 5-methylfuran-2-carboxylate could be identified as a side product. A mechanistic proposal is made showing the cooperative catalysis between the NHC catalyst and the MnO2. The formation of the side product is mostly due to the possible formation of a manganese ester. In the esterification of furfural, the reaction mechanism showed a dependence to the atmosphere used; oxygenative and oxidative mechanisms were identified. Esterification of benzaldehyde showed that electron withdrawing substituents on the aromatic ring foster the reaction also dependence on the alcohol used.
Exploring experiments on NHC catalyzed amidation of aldehydes showed, in general, low yields. In the case of primary amines, amidation and condensation to imines are competitive reactions. Tetrahydroisoquinoline as substrate gave the best amide yield (75%), on the contrary to pyrrolidine (0%), a secondary amine reduces suppress the formation of imine by condensation.