462550 The Role of Atmosphere, Solvents and Acids on the Production of 5-Hydroxymethylfurfural from Biomass Derived Fructose
As global energy demands and atmospheric CO2 levels continue to rise, sustainable green energy sources will become necessary solutions. One such alternative energy source is corn Stover, a domestically produced, cellulosic biomass which is low enough in cost and of sufficient abundance for production of liquid fuels on a large scale to be competitive with petroleum. Sugars, such as fructose, can be derived from biomass through pretreatment of biomass to glucan enriched solids, these solids can undergo saccharification and isomerization to produce fructose-enriched streams. This fructose stream can be processed to produce platform chemical feedstocks, like 5-hydroxymethylfurfural (5-HMF). 5-HMF is known as a platform chemical, as it is easily converted to biofuels or high valued commodity chemicals.
Past studies have demonstrated that it is difficult to improve the selective and/or conversion of biomass sugars to 5-HMF in aqueous solutions. The majority of these studies rely on increasing the catalyst loadings or using high-temperature processes. Furthermore, numerous studies have explored the use of organic solvents to improve the selective conversion of biomass sugars to furanic compounds. However, these studies are often times inconsistent and have not conclusively demonstrated the combined effects of acids, solvents, and atmosphere on the conversion of fructose to 5-HMF.
Presently, the highest reported yields of HMF obtained, arises from acid-catalyzed dehydration of fructose in dimethyl sulfoxide (DMSO). A combination of Brønsted acids and DMSO has been proposed to provide an effective catalytic pathway and intermediate stabilization that maximizes HMF yields. However, significant contradictions exist in literature regarding the roles of DMSO, O2, and H2O in the mechanism of fructose conversion. It is also unclear whether the fructose conversion mechanisms are radically different between: 1) various acids in DMSO or 2) different aprotic solvents vs. DMSO. To address these questions we utilized a 96 well plate high-throughput batch reactor system loaded and sealed under inert or atmospheric conditions. The reactor was loaded into a high pressure vessel and a Fulton electric steam boiler was utilized to rapidly heat the vessel to systematically and accurately explore the influence of pH, O2, H2O, solvent, time and temperature on fructose conversion and 5-HMF selectivity.
We report three primary results: 1) DMSO decomposes in O2 at elevated temperatures to produce Brønsted acids that drive fructose conversion. (2) In O2 free environments, increasing H2O content does not negatively influence the HMF yield. 3) Lastly, 5-HMF production follows an almost identical, negative linear relationship between conversion and selectivity, which universally captures the behavior regardless of solvent, reaction time, reaction temperature and acid loading. These systematic studies have allowed us to identify realistic conditions where fructose conversion to 5-HMF can be executed with high yields, >80% at reasonable pH (~2), water contents, temperatures, and times. Furthermore, we utilize these findings to propose a universal mechanism for the fructose conversion pathways driven by homogeneous Brønsted acids and present insights into reactor design and conditions to optimize 5-HMF yields at minimal cost.