The Exploration of Humin Formation and Modification: Using Parallel Synthesis Methods to Valorize Humins

The use of fossil resources over the years has resulted in significant pollution and serious environmental concerns. As a result, green chemists have focused on finding renewable, bio-based alternatives. The use of biomass as a replacement, especially for chemical production and fuel, has gained increasing popularity. For example, the acid catalyzed dehydration of carbohydrate-containing biomass (along with sugars) has been used to form high value platform chemicals such as 5-(hydroxymethyl)furfural (HMF), levulinic acid, and furfural. Unfortunately, this process is hindered by high yields of a black solid waste material. This waste product, a complex polymeric material known as humins, has been the subject of many recent studies. While some applications exist such as for fuel via burning or gasification, as matrices for composites, and enhancers for soil, further valorization is difficult. This is due to the material’s insolubility, its rigid crosslinked structure, and the acidic conditions its formation requires. The objective of this research was to modify humins before they form a highly crosslinked material, in hopes of finding novel applications. The idea was to find a way to introduce flexibility into the polymer or to increase its solubility. A parallel synthesis approach was used where the effects of the following factors were tested: acid concentration, temperature, reaction time, starting material (e.g. fructose, xylose, HMF), and presence and type of additive (e.g. aldehydes, ketones, carboxylic acids, alcohols, esters). Emphasis was placed on using HMF derivatives as additives and analyzing xylose humin intermediates known as oligomers. To analyze the sample changes, solubility tests were performed using solvents of varying polarities and NMR, IR, MS, DSC, and optical microscopy technologies were used. While no clear method was found to decrease the rigidity of solid humins, patterns were explored, and a good baseline was established for future research.