Novel Application of Polymer Networks Carrying Tertiary Amines as a Catalyst Inside Microflow Reactors Used forKnoevenagelReactions

Abstract

A novel application is described for utilizing hydrogel dots as organocatalyst carriers inside microfluidic reactors. Tertiary amines were covalently immobilized in the hydrogel dots. Due to the diffusion of reactants within the swollen hydrogel dots, the accessible amount of catalysts inside a microfluidic reactor chamber can be increased compared to the accessible amount of surface‐bound catalysts. To perform fast Knoevenagel reactions, important flow parameters had to be validated to optimize the reactor performance while keeping the dimensions of the reactor chamber constant; e.g. the height of the hydrogel dots had to be adjusted to the invariable dimensions of the reactor chamber, or an adjustment of organocatalysts in the hydrogel dots had to be validated to achieve the highest conversion rate during a certain residence time. To characterize the conversion, nuclear magnetic resonance (NMR) and UV/Vis‐spectroscopy were utilized as an offline and online method, respectively. With suitable hydrogel dots, the influence of different flow parameters (e.g., operating flow rate and reactant concentration) on the selected model reactions in the microfluidic reactor was investigated. Finally, a variety of reactants were screened with the optimized flow parameters. With these results, the turnover frequency was determined for the Knoevenagel reactions in a microfluidic reactor, and the results were compared with published data that were determined by other synthetic approaches.

Continuous Flow Synthesis of Azoxybenzenes by Reductive Dimerization of Nitrosobenzenes with Gel‐Bound Catalysts

“…For the recent study a gel composition comparable to that used to study the Knoevenagel reaction within a microfluidic reactor was used, with methacrylates as gel‐forming compound and using a proline‐based catalyst [11] …”

“…Using relatively low crosslinker contents enables reactions inside of swollen, sponge‐like, polymer networks, which, in contrast to solid particles, increases the amount of immobilized and accessible catalyst from only surface area to bulk volume [9] . The general applicability of surface‐bound gels with a high crosslinker content as carriers of catalysts and enzymes in MFRs, has already been shown recently [10,11] . In both cases, hexagonally ordered gel arrays, immobilized on glass slides, were obtained via photolithography, and used for various reactions within MFRs.…”

“…The necessary fast and efficient production of complex polymer gel‐based microfluidic chips on glass substrates can be achieved via a one‐step photo‐structuring [12] . Based on such arrays immobilization of tertiary amines as catalysts in surface‐bound gels is also possible and offers access to a wide range of Knoevenagel reactions under continuous flow conditions [11] …”

Direct Asymmetric Aldol Reaction in Continuous Flow Using Gel‐Bound Organocatalysts

“…Surface Modification : The surface modification of the slides was carried out as described previously [19] …”

“…An immobilization within crosslinked polymer networks enables reactions inside of swollen, three‐dimensional textures, which in contrast to solid particles, increases the amount of immobilized and accessible catalyst from surface area to bulk volume [16,17] . The general applicability of surface‐bound gels with a high crosslinker content as carriers of catalysts and enzymes in MFRs, has already been shown recently [18–20] . In all three cases, hexagonally ordered gel arrays, immobilized on glass slides, were obtained via photolithography, and used for various reactions within MFRs.…”