Selective catalytic synthesis methods
Dr. Eszter Baráth
Oil is the dominant feedstock for the chemical industry. In view of the limited supply of fossil resources, however, this era is coming to an end and research on the conversion of renewable feedstocks into bulk and fine chemicals has gained extraordinary importance, especially the catalytic conversion of inedible plant material, such as cellulose, lignin or lignocellulose. These materials can be converted into a number of platform chemicals, such as ethanol, 5-hydroxymethyl-furfural (HMF) or levulinic acid. Using mostly homogeneous catalysis we want to convert these platform chemicals into high-value chemicals and raw materials for polymers.
One of our projects, funded by Leibniz Foundation (SAW-LIKAT-1 523) focused on the preparation of 1-hydroxy-2,5-hexanedione (HHD) from 5-hydroxymethylfurfural (HMF). HMF is a platform chemical which is included in the “Top 10” list of renewables obtained from biomass. It can be produced from fructose in high yields. Alternative methods for its preparation from glucose and cellulose also exist. We did not only establish an optimized synthesis from HMF to HHD, but also a protocol for the isolation of the desired product in very good yields (Scheme 1).
The potential of HHD as a new platform chemical was demonstrated by its conversion to various industrially-relevant compounds, which can serve as the starting materials in the food, pharmaceutical, agricultural and fuel industries (see Scheme 1).
Another interest we have is to identify and to prepare affordable catalysts for important chemical transformations. Here, one approach is the replacement of noble metals with cheap ones. Currently, iron is the cheapest transition metal on the market. We found that the iron complex Fe-MACHO-BH is a suitable catalyst for transfer hydrogenations of ester groups (Scheme 2).
The catalyst turned out to be very versatile: Even a polyester was transformed to the corresponding diol(s), which opens up a new way to give used polymers a second life. On top of that, the transformation was performed with another platform chemical, ethanol, which not only acts as hydrogen source but also as the solvent.
Our second approach on the way to affordable catalysts is the design of cost-effective and robust ligands. An example of their application is a series of Co-NNS complexes, which we synthesized and tested successfully in various reductions (Scheme 3). Via mechanistic studies we revealed, that the cobalt complex dissociates and forms nanoparticles, which perform the catalysis. However, the selectivities obtained with these complexes are still influenced by the ligand.
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