Multistep Bioelectrochemical Reaction Cascade in Continuously Operated Flow Reactors

The main objective of this project is to elucidate the key scientific questions to enable multistep bioelectrochemical reaction cascades in continuously operated flow reactors. The transition from AiO electrode setup in a batch to a continuously operated bioelectrochemical process starts from the established one-step bioelectrochemical system in the first project phase. This will be extended to a three-step bioelectrochemical reaction cascade, the oxidative valorization of 5-hydroxymethylfurfural (HMF) to valuable 2,5-furandicarboxylic acid (FDCA) catalyzed by two different unspecific peroxygenases (UPO). This cascade setup will allow the detailed study of different steady-state conditions through different reactor configurations, operating points as well as systematic properties of enzyme/electrode interactions. The reactor cascade can be operated with immobilized UPOs on the electrode surface as a sequence of plug flow reactors (PFR) as well as circulation loop reactors in continuously operated stirred tank reactor mode (CSTR). Alternatively, homogeneously solubilized UPOs can be applied that are recycled via an additional ultrafiltration membrane unit in the recirculation stream. By these two fundamental different reactor cascade configurations, a deepened understanding on affecting key performance parameters will be generated. Furthermore, challenges pointed out in the previous project will be addressed by designing improved porous Globugraphite (GG) electrodes. Key objective is to improve H₂O₂ productivity and Faradaic efficiency (F.E.). Several approaches will be explored, such as varying the polyvinyl butyral (PVB) content to influence the porosity of the GG and implementing multiple segmented GG modules. The latter ones might be separated by isolation or connected as units of different porosity. This will allow for a gradient of the H₂O₂ generation rate needed in a PFR setup to minimize H₂O₂ accumulation and enzyme deactivation. The following key scientific questions will be addressed:
• How do the flow reactor geometry and increased flow rate influence mass transfer across/through the electrode, possible diffusion limitation as well as enzyme stability?
• How does the morphology of the GG need to be varied by polyvinyl butyral (PVB) content, ZnO particle size and wall thickness to affect pore size in a way to maximize / tailor H₂O₂ productivity and F.E.?
• How can the durability of GG electrodes be increased by thermal treatment and/or wall thickness to withstand the internal gas pressure?
• How is a segmented graphite electrode to be designed to enable a length gradient in H₂O₂ generation rate?
• How do different reactor operation modes (e.g. batch, PFR, CSTR) and resulting different linear flow rates influence performance indicators such as total turnover number (TTN), turnover frequency (TOF), enzyme deactivation constants and productivity?

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