Dip tube disintegration2/26/2023 4, 16 The direct aqueous-phase conversion of GA into EG is possible with H 2 as a reducing agent over supported metal catalysts. 15 EG is a highly demanded bulk chemical with several industrial applications, for example, as a polyester precursor, for which biomass-based pathways are of utmost research interest. To circumvent costly multi-step separation and purification of GA, 14, 15 its direct conversion into ethylene glycol (EG) is a viable alternative, following a suggestion by Huo and Shanks for similar carboxylic compounds. While biomass-based GA is becoming increasingly available, separation and purification poses an economical and technological challenge. Furthermore, the amounts of biological impurities are expected to be low compared to fermentation-based approaches. The conversion of CO 2 into GA is very efficient (up to 82 % of assimilated carbon is directed to glycolate synthesis), separation of GA-containing aqueous medium and algal cells is rather easy to achieve (as no extraction from the algal cells is needed) and the approach inherently offers the potential of continuous GA production. So far, reported accumulated GA concentrations did not exceed 3 g L −1 or 40 mmol L −1, 11 but further improvements can be expected. Consequently, the amount of cell biomass is kept constant and CO 2 is selectively funneled into the production of GA. Therefore, the GA-containing medium, but not the algal cells themselves are harvested. 11 The advantage of this GA-producing system is that the cells excrete GA directly into the aqueous medium. Importantly, the process of photorespiration is initiated by a high cultivation temperature (30–35 ☌) and a high O 2/CO 2 ratio of aeration. The absorbed light energy from the photosynthetic electron transport chain is used for glycolate formation through the process of photorespiration. The algal cells function as “photobioreactors” by exploiting the dual function of the Rubisco enzyme. In this study, GA was obtained using the approach of Wilhelm et al., 10- 13 in which algal cells ( Chlamydomonas reinhardtii) are not used as substrate feedstock, but as direct producers of GA. Fermentation with Escherichia coli bacteria is most advanced and can yield aqueous product solutions with GA concentrations >50 g L −1 (≈700 mmol L −1). only few studies report starting directly from raw biomass or cellulose 6) or on biotechnological processes (reviewed in Refs. The conversion of biomass into GA can either be based on (homo- or heterogeneously) catalyzed processes (reviewed in Refs. 3 “Conventional” biomass-based processes rely on the growth, harvesting and subsequent conversion of plant biomass. Currently, the α-hydroxycarboxylic acid GA, which is an important precursor for biodegradable polyester production, 1 is still produced either by carbonylation of formaldehyde 2 or by hydrolysis of monochloroacetic acid. In combination with a modification of the biotechnological process to mitigate the use of inhibitory compounds, and after acidifying the algal medium, over Ru/C a EG yield of up to 21 % even at non-optimized reaction conditions was achieved.Īs part of the concerted effort to find sustainable and biomass-based routes to industrially demanded chemicals, several strategies have been developed in recent years to explore such renewable pathways for the production of glycolic acid (GA). Moreover, pH adjustment by acidification is required, for which H 2SO 4 is found most suitable. Nitrogen- and sulfur-containing organic molecules can strongly inhibit the reaction. After identification of the key characteristics of the algal medium (compared to pure aqueous GA), the influence of pH, numerous salt additives, pH buffers and other relevant organic molecules on the catalytic GA reduction was investigated. The present study focuses on the conversion of an authentic algae-derived GA solution. In the second step, the GA-containing algal medium is used as feedstock for catalytic reduction with H 2 to EG over a Ru/C catalyst. GA is continuously excreted into the surrounding medium. First, microalgae are cultivated to photobiocatalytically yield glycolic acid (GA) by means of photosynthesis from CO 2 and water. Ethylene glycol (EG) is obtained by a novel, two-step approach combining a biotechnological and a heterogeneously catalyzed step.
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