Investigation of the Formation, Characterization, and Oxidative Catalytic Valorization of Humins

Abstract

The industrial use of biomass, e.g., for the production of platform chemicals such as levulinic acid, became increasingly important in recent years. However, the efficiency of these processes was reduced by the formation of insoluble solid waste products called humins. Herein, the formation of humins from various carbohydrates was investigated under different process conditions, in order to obtain information about the structure and the formation mechanism. During this process, new potential structural fragments of humins were identified. Subsequently, the produced humins were oxidatively converted to low-molecular-weight carboxylic acids with the use of polyoxometalate catalysts. The experiments showed that the use of sugars in acetic acid and ethanol only lead to the formation of a small amount of humins, which were also structurally most suitable for conversion to carboxylic acids. The main products of the oxidative valorisation of these humins were acetic acid, formic acid, and CO2, respectively, and our results indicate that certain functional groups were converted preferentially. These findings will help to improve processes for the valorisation of biomass by enabling an overall more efficient use of thermo-sensitive feedstock such as carbohydrates.

Catalytic Valorization of Humins by Selective Oxidation Using Transition-Metal-Substituted Keggin-Type Polyoxometalate Catalysts

“…Both linkages via C–O–C and C–C bonds are plausible and coexistence in the real humin structure cannot be excluded either. An exemplary structural motif that allows both types of bonds was recently suggested by Wassenberg et al and corroborated via mass spectroscopic analyses (Table , entry 5). Nevertheless, we can conclude that the complex structure of humins has not been fully elucidated to date.…”

“…As mentioned and discussed above as well as supported by a large number of studies, the humin structure exists of a furan-rich polymer network. ,,,,,, Numerous results indicate a linkage of these furan rings via oxygen-functionalized C–O–C bonds. − ,,,, However, aliphatic C–C bonds are also under discussion. ,,, Hoang et al and Shi et al suggest that the furan rings are also linked via aliphatic acetyl groups as well as via CH 2 bonds. Both linkages via C–O–C and C–C bonds are plausible and coexistence in the real humin structure cannot be excluded either.…”

“…In addition to the already existing functionalities, a modification of the humin structure by catalytic processes is possible. Wassenberg et al 29 demonstrated by CHNS analysis and IR spectroscopy that oxygen is incorporated into the humin structure during their catalytic oxidation. Hence, introducing a certain degree of functionalization at the furan ring by oxidative cleavage of C−C bonds in the humin structure is also within the realm of possibility.…”

“…4,6,7,27,28,30,31 Numerous results indicate a linkage of these furan rings via oxygen-functionalized C−O− C bonds. [4][5][6]23,25,28,29 However, aliphatic C−C bonds are also under discussion. 7,25,27,31 Hoang et al 7 and Shi et al 30 suggest that the furan rings are also linked via aliphatic acetyl groups as well as via CH 2 bonds.…”

Selective catalytic oxidation of humins to carboxylic acids using the H4[PVMo11O40] Keggin-type polyoxometalate enhanced by alcohol doping and solubilizer

“…5,14,19,20 Another promising idea is the chemical valorization of humins by catalytic oxidation. 5,21 Pioneering work in this field was done by Maerten et al , 5 who demonstrated for the first time the chemical valorization of humins by selective catalytic oxidation (SCO) using polyoxometalate (POM) catalysts to short-chain carboxylic acids. POMs are a unique class of inorganic compounds having a wide variety of geometric and electronic structural properties.…”