Valorization of biogenic raw materials by selective catalytic oxidation using polyoxometalate catalysts and efficient downstream processing via nanofiltration

Abstract

The selective catalytic oxidation of lignocellulosic biomass is a key technology for the bio-based production of chemicals and energy carriers. In so-called biorefineries, complex undesired by-products, known as humins, can form. This prevents the complete utilization of all raw material components. The material utilization of humins through selective catalytic oxidation could contribute to increasing raw material efficiency of biorefineries. In order to investigate the efficient conversion of humins, different generations of humin-like model substrates were initially used instead of the complex humins. In this context, furfural was converted as first-generation model substance to industrially relevant carboxylic acids using homogeneous Keggin-type polyoxometalate catalysts (POMs) under mild conditions (90 °C, 30 bar O2) in aqueous phase. The POM-catalysts used were modified by the substitution of molybdenum as an addenda atom in the Keggin-type phosphomolybdic acid (H3[PMo12O40]) by various other transition metals. A correlation between the electrochemical properties and the conversion of furfural was found and an adjustment of the redox potential was achieved. Due to this adjustment, it was possible to influence the conversion and selectivity. The singly vanadium-substituted H4[PVMo11O40] catalyst was found to be superior in terms of catalytic activity and yields of maleic and formic acid. Multiple substitution of addenda atoms with up to six vanadium atoms drastically increased the catalytic activity, but at the expense of selectivity. Accordingly, the H4[PVMo11O40] catalyst represented the best compromise between activity and selectivity. Serval commercially available monofuran derivatives, including 2-methylfuran, 2 furfuryl alcohol, 2(5H)-furanone, 5-hydroxymethylfurfural (5-HMF) and 2 furoic acid, were converted using the selected catalyst under otherwise identical conditions. Structure-activity-selectivity relationships were derived from these experiments. According to the relationships, a suitable functionalization of the substrates with oxygen is crucial for their efficient conversion. 5-HMF as a bifunctional substrate (high oxygen functionalization) could therefore be completely converted to the main products formic acid, maleic acid and CO2. From the results, reaction pathways were derived in which the elimination of formic acid by oxidative C-C bond cleavage and electron transfer to the catalyst represents the initial step. 5-Hydroxyfuran-2(5H)-one is formed as intermediate by reoxidation of the catalyst, which is ultimately converted to maleic acid. Analogous to the postulated reaction pathway, 2-furoic acid was also converted to maleic acid. However, the oxidative C-C bond cleavage can only occur by decarboxylation of 2 furoic acid, which is considerably less efficient causing significantly lower conversions. Both, 2-furoic acid and 2(5H)-furanone were selectively converted to maleic acid. The postulated reaction pathways could also be observed in oxidizing experiments using more complex model substances. These second-generation model substances consisted of differently linked or branched furan derivatives owning C-C or C-O-C bonds. The conversion of second-generation model substances also yielded in formic and maleic acid, although substrates with lower oxygen functionalization and C-C bonds represented a greater challenge. The knowledge gained was transferred to the oxidation of glucose-based humins in aqueous phase. It was shown that the H4[PVMo11O40] catalyst has a relative selectivity advantage over more highly substituted POM-catalysts as predicted by the preliminary tests on model substances. Various alcoholic additives were tested as inhibitors to further decrease CO2 formation. An addition of 5 vol% methanol proved to be the most efficient and enabled a strong inhibition of CO2 formation, whereby for the first time more value-added products (carboxylic acids) than CO2 were formed during the oxidation of humins. This remarkable effect could even be achieved at elevated reaction temperatures of 120 °C and thus at increased activity. As an alternative approach, the use of para-toluenesulfonic acid (pTSA) as a solubilizer and reaction promoter was investigated. Using an optimized addition of 1.5 mmol pTSA, the activity of the reaction was increased already at a temperature of 90 °C. Surprisingly, almost equivalent yields of carboxylic acids and CO2 were obtained even without the CO2-inhibiting effect of methanol. Increasing the reaction temperature to 120 °C allowed to drastically increase the activity, but the formation of CO2 dominated. For this reason, the synergistic combination of methanol to inhibit CO2 formation and pTSA to increase the activity at 120 °C was investigated. The new approach was superior to all previously developed reaction systems for the oxidation of humins in terms of activity and selectivity. Time-resolved experiments showed that the oxygen content in the solid residues of the reaction increases significantly in the first hours of the reaction. According to this observation and Fourier-transform infrared spectroscopy (FT-IR) analyses, oxygen-functionalized bonds are initially cleaved, forming short-chain products such as formic acid, acetic acid, and CO2. This process induces new oxygen functionalization on previously non-functionalized carbons. Consequently, the oxygen content increases and the carbon content decreases. Due to this mechanism, larger fragments of humin structure were not accessible. The efficient and selective conversion of serval humins based on different sugars was conducted applying the new reaction system. Furthermore, the new system enabled the use of the higher substituted H5[PV2Mo10O40] catalyst, which led to a higher activity and yield of value-added products even at a lower reaction temperature of 90 °C. For the recovery of the homogeneous POM-catalysts, a nanofiltration system was developed and optimized. The used system has an integrated stirrer within the membrane cell, which is responsible for sufficient cross-flow of the membrane and therefore differs from classic cross-filtration. Through statistical design of experiments following a Box-Hunter-Hunter experimental plan, stirring speed was identified as the most influential parameter. Applying the optimized parameters, a high rejection for the catalyst components vanadium (97 %) and molybdenum (99 %) as well as a low rejection for formic acid (3 %) and acetic acid (10 %) were achieved. Enrichment experiments showed that the catalyst could be enriched without loss of structural integrity. In this process, over 80 % of carboxylic acids were removed from the reaction solution. The developed nanofiltration process was successfully transferred to various POM-catalysts. To increase the efficiency of the process, serval commercial nanofiltration membranes were screened using a model solution based on the product solution of the selective catalytic oxidation of humins. The acid-stable XN45 membrane of Mann+Hummel proved to be promising, enabling high rejection of over 99 % for the catalyst components Molybdenum and Vanadium as well as a desirable low rejection for formic acid of 0 %. Duration tests showed that the selected membrane enabled efficient separation over an operating time of 168 h without loss in performance. In final recycling studies, the catalyst was successfully recycled several times in consecutive oxidation experiments of humins after processing using nanofiltration. Measurements using 51V-NMR showed that the catalyst structure and thus the catalytic activity are retained. This is due to the removal of the oxidation products and the stabilization of the pH value in an optimal range between 1.2 to 1.4. If the pH value was not stabilized and droped below pH 1, significant loss of catalytic activity could be determined due to the increased formation of less substituted catalyst isomers. Hence, the efficiency of the nanofiltration process was demonstrated and the basis for a continuous process was created.