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

Abstract

The main objective of this study was to gain a deeper understanding of oxidative valorization of humins using polyoxometalate (POM) catalysts. Based on experiments using individual monofuran derivatives as a substrate, evident reaction pathways for the conversion to maleic acid were postulated. By analogous routes, difuran derivatives or branched furan rings could also be converted to maleic acid and formic acid, respectively. Hereby, the monovanadium-substituted catalyst was found to be superior compared to other structures in terms of activity and selectivity due to its unique electrochemical properties. In detail, H 4 [PVMo 11 O 40 ] showed the lowest peak potential in Square-Wave-Voltammetry measurements, achieving the highest activity and carboxylic acid selectivity. Hereby, a deeper understanding of the selective oxidation of humin model substrates like furan derivatives of varying complexity could be achieved. Therefore, helpful conclusions can be drawn for the transformation of humins representing a decisive step toward efficient processes for their valorization.

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

“…Analogous findings could be demonstrated by previously published results on the oxidation of mono-furan derivatives and humin-like substances. 23 Here, higher substituted POMs showed significant higher activity for the oxidation of the mono-furan derivative furfural, which was ultimately converted to formic acid (FA) or to CO 2 with yields up to 43% and 30%, respectively. In comparison, the H 4 [PVMo 11 O 40 ] (HPA-1) catalyst was characterized by a much lower tendency to over-oxidation with CO 2 yields of only around 5%.…”

“…A comprehensive overview of further structural motifs of humins can be found in the review by Liu et al 9 Furthermore, a detailed discussion of various structural elements, important functional groups and possible humin-like model substances with regard to a catalytic conversion of humins can be found in previously published results. 23…”

“…As already described, higher vanadium-substituted POM catalysts exhibit increased catalytic activity due to the formation of peroxo-species. 23,25 Consequently, the use of the double-substituted HPA-2 catalyst led to a fivefold increase in the total yield to about 56% compared to the reaction without catalyst (Table 1, entry 3). Compared to that of the reaction catalyzed by HPA-1, the total yield increased by a factor of more than 2.…”

“…Based on previously published results dealing with SCO of humin-like model substances, oxygen functionalization is essential for efficient conversion of the structures by POM catalysts. 23 The simplest types of oxygen functions are free functionalities such as aldehydes and carboxylic acids (Fig. 10, pathway 2).…”

“…However, the results showed that vanadium-substituted Keggin-type POMs can be used to oxidize very complex aromatic structures like humins to carboxylic acids. Inspired by the aforementioned results, Esser et al 23 used individual monofuran derivatives like 5-hydroxymethylfurfural (HMF), furfural, 2-furoic acid, 2(5 H )-furanone, 2-furfuryl alcohol and 2-methylfuran to gain a deeper understanding of POM-catalyzed humin oxidation. Hereby, they found that the Keggin-type POM-catalyst H 4 [PVMo 11 O 40 ] (HPA-1) can oxidize both monofuran and difuran derivatives or even branched furan rings to maleic acid (MA) and FA, respectively.…”