Catalytic Conversion of Biomass-Based Platform Chemical: Triacetic Acid Lactone to Potassium Sorbate

Platform chemicals, also known as bulk or commodity chemicals, are chemicals that serve as building blocks or intermediates for the production of a wide range of other chemicals, materials, and products. The production of chemicals from renewable feedstocks is becoming increasingly attractive, driven especially by concerns over climate change and the rapid depletion of non-renewable fossil fuel sources. 4-hydroxy-6-methyl-2-pyrone, commonly denoted as triacetic acid lactone (TAL), has been highlighted as a platform chemical that enables the diverse conversion of renewable carbohydrates into families of commodity chemicals. Notably, it can be obtained from natural sources or synthesized from acetic acid. Potassium sorbate (KS) is a versatile chemical additive, primarily used as a food preservative.

In this research, a conversion of TAL to KS through three major steps was presented. The conversion process consists of: hydrogenation from TAL to 4-hydroxy-6-methyltetrahydro-2-pyrone (HMP), dehydration from HMP to 6-methyl-5,6-dihydro-2-pyrone (denoted as parasorbic acid or PSA), and ring-opening and hydrolysis from PSA to KS. During the hydrogenation step, it was found that a Ni/SiO₂ exhibited over 99.9% conversion of TAL and 99.3% selectivity to HMP. In the dehydration step, a reaction kinetics model was developed to maximize the PSA yield over Amberlyst 70 catalyst. Through the obtained kinetic data, the maximum yield to PSA was estimated to be 86.5%, and a subsequent experiment confirmed a yield of 84.2% of PSA with respect to HMP. In the ring-opening and hydrolysis step, KOH was used as a co-reactant and achieved >99.9% yield of KS from PSA in an ethanol (EtOH) solvent system.

Furthermore, the difference in THF solubility between the TAL-derived KS and its impurities resulted in a high level of purity (95.5%), which was confirmed by ¹H nuclear magnetic resonance (NMR) spectra. The overall yield of TAL-derived KS with respect to TAL was calculated to be 77.3%. A different geometrical configuration of TAL-derived KS compared to commercial KS was observed through NMR, but it was confirmed to have similarity in their antimicrobial properties against bacteria and fungi.

In conclusion, this study showed a new approach for producing KS from TAL, without the need for an intermediate step to yield sorbic acid. Through this research, the ability of TAL to serve as a potential platform chemical, transforming into multiple chemical derivatives with various downstream applications, was also confirmed.