title: Development and Application of a Liquid Core Waveguide Membrane Microreactor
Creators: name: Ponce, Sebastian
Publisher: name: UNSPECIFIED
Year: 2020

description: Optofluidic devices are unique as they combine properties of microreactors with an enhanced light-sample interaction. This can be advantageous for developing in situ spectroscopy as also photoactivation. However, up to now, their application in gas-liquid reactions can only be studied to a limited extend, as gas bubbles impede light transmission. To overcome this limitation in this work, a novel liquid core waveguide membrane (LCWM) microreactor combining intense light matter interaction for in situ sensing and/or photoactivation and excellent gas-liquid mass transfer is designed, realized, characterized and applied in three case studies. Basis is a liquid-filled Teflon amorphous fluoropolymer (Teflon AF) tube, which provides light transmission within the liquid core and gas permeation through the wall. Characterization of the new reactor was carried out from an optical and engineering point of view. The LCWM microreactor allows operating in a wide spectral range (240-1500 nm) with relatively low optical losses (UV/Vis regime:1.67 dB m-1). A working regime preventing gas bubble formation for CO2, N2 and synth. air was deduced for semi-batch and flow operation for pressures up to 8 bar. Furthermore, it was shown that residence time distribution (RTD) experiments can be carried out using the optical equipment of the reactor. The capabilities of the LCWM setup are demonstrated for three case studies: i. deducing redox kinetics of different heteropolyacids during the oxidation of biomass to formic acid (the ‘OxFA process’), ii. disentangling the complex network of (photo)degradation of methylene blue (MB) in glucose-alkaline solutions, and iii. studying ultra-fast (photo)degradation of silver nanoparticle (AgNP) colloids. For the first application, the potential of this reactor concept to derive kinetics in gas-liquid reactions is demonstrated. For vanadium-substituted, Keggin-type heteropoly acids (HPA-n, n represents the degree of V substitution), the kinetics of the reduction and reoxidation step of the catalyst cycle was deduced using in situ UV/Vis spectroscopy. Reduction was studied from 40 to 80 oC under anaerobic conditions: a strong dependency of the reduction kinetics was deduced. In combination with additional cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) characterization, it can be assumed that two active centers promote this reaction, the dissolved vanadium species, and the HPA itself, while the existence and proportions of both types depends on the degree of vanadium substitution. Being the HPA-5 complex the most active catalyst for biomass oxidation. On the other side, as demonstrated in the characterization section, the rapid mass transfer allowed transient response experiments to be carried out for the reoxidation step, which was studied up to 150 oC. Switching from anaerobic to aerobic conditions, an even stronger dependency of the reoxidation kinetics on the degree of vanadium substitution was revealed. Higher V-substituted catalysts (e.g. HPA-5 and HPA-2) showed to be able to reoxidize at mild temperature conditions (&lt; 90 oC) while non- and mono-substituted catalyst (HPA-0 and HPA-1) needed higher temperatures (&gt; 100 oC), making the reoxidation step rate determining. For the MB degradation study, the potential of the LCWM optical setup for revealing the complex interplay of several reaction and degradation pathways in gas-liquid photocatalytic systems is demonstrated. By combining long path lengths, intense light matter interaction for in situ characterization and/or photoactivation, and transient response studies, the photoenhanced methylene blue redox cycle in alkali is studied. Deconvolution of high resolution UV/Vis spectra, together with common liquid-phase characterization techniques, gives rise to several photo and non-photoactivated competing reaction pathways taking place. Accounting for all studied effects, it can be assumed that in addition to the redox cycle, photoexcited methylene blue fastly degrades via N/demethylation and deamination steps, forming new species which subsequently deprotonate in alkali. Furthermore, in the presence of molecular oxygen, the formation of highly reactive singlet oxygen via a photosensitizer cycle is likely, revealing an additional path for further degradation of organic matter in solution. For the final application, the potential of the reactor to study photochemical transformations experienced in colloidal solutions is demonstrated. For highly diluted citrate-capped silver nanoparticles of average diameter ≈ 9 nm, the kinetics of the photooxidation by UV light irradiation was deduced. UV-induced photooxidation under anaerobic and aerobic conditions, variable initial citrate concentrations, and temperatures up to 80 oC was studied: a strong dependency of the photooxidation kinetics on the gas saturation was deduced following a pseudo-first order kinetic mechanism. Photoexcited NP surfaces seem to actively interact with dissolved molecular oxygen. Moreover, higher citrate concentrations seem to slow down the overall degradation process by protecting the nanoparticle surface against oxygen molecules. In addition, the effect of broader light (Vis) irradiation was studied. By applying the MIE model by fitting the UV/Vis surface-plasmon resonance (SPR) spectra recorded within the LCWM reactor, it can be assumed that in addition to the photooxidation process (fast), the indirect photooxidation of citrate (slow) induces nanoparticle growth. Being both two competing mechanisms. Finally, exemplary long-term stability studies under semi-batch and continuous operation were developed. Silver colloids in batch conditions and broad light irradiation tend to develop cyclic oxidation  reduction processes, which results in NPs degradation and re-nucleation and growth. On the other hand, flow conditions seem to enhance nanoparticles agglomeration, but also the indirect citrate oxidation, which results in metallic silver deposition to the formation of big flat surfaces. Interestingly. all above studied effects seem to happen in a shorter time scale (min to hours) compared to common batch photo-reactors (hours to months)