Abstract
Translation of redox biocatalysis into a commercial hydrogenation flow reactor, with in-built electrolytic H2 generation, was achieved using immobilized enzyme systems. Carbon-supported biocatalysts were first tested in batch mode, and were then transferred into continuous flow columns for H2-driven, NADH-dependent asymmetric ketone reductions. The biocatalysts were thus handled comparably to heterogeneous metal catalysts, but operated at room temperature and 1–50 bar H2, highlighting that biocatalytic strategies enable implementation of hydrogenation reactions under mild–moderate conditions. Continuous flow reactions were demonstrated as a strategy for process intensification; high conversions were achieved in short residence times, with a high biocatalyst turnover frequency and productivity. These results show the prospect of using enzymes in reactor infrastructure designed for conventional heterogeneous hydrogenations.
Highlights
As sustainability increases in importance in the chemical and pharmaceutical industries, new technologies that lower energy demands, use renewable resources and aid productivity and automation are critical (Jiménez-González et al, 2011; Bryan et al, 2018)
Stability is highly desirable in flow as reactions can run continuously for as long as the catalysts are active, in contrast to batch conditions under which biocatalysis reactions are typically run for 24 h until high conversion is achieved
In our previous batch studies, we focused on co-immobilizing a separate NAD+ reductase with a more robust hydrogenase (E. coli hydrogenase 1, “Hyd1”) onto carbon particles; this system yielded much higher stability and cofactor regeneration activity compared with the soluble hydrogenase (SH)
Summary
As sustainability increases in importance in the chemical and pharmaceutical industries, new technologies that lower energy demands, use renewable resources and aid productivity and automation are critical (Jiménez-González et al, 2011; Bryan et al, 2018). The most well-established methods for asymmetric hydrogenations use precious-metal-based organometallic catalysts (such as Rh, Ru and Ir) (Noyori and Hashiguchi, 1997; Noyori, 2002; Zanotti-Gerosa et al, 2005) These homogeneous catalysts provide high enantioselectivity and total turnover numbers (TTN), they are expensive, toxic and difficult to recover and reuse. Attempts to mitigate this by catalyst immobilization (for example, onto mesoporous silica or organic polymers) are often associated with loss in catalyst stereoselectivity and efficiency
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