Abstract
In this review article, we first discussed the development of silica monoliths with hierarchical macro-/mesopore structure and their potential figures of merit as continuous-flow micro-/mesoreactors of up to 30 ml working volume. Making use of the flow hindrance of different pore structures seen from the Darcy law perspective, we discriminated four structures of the monoliths (M1–M4). We then summarized the most important results, mainly from our studies of continuous-flow structured monolithic reactors and rotating bed reactors (RBRs) filled with structured pellets, activated with various catalytic entities and enzymes. The results show that an increase in the flow rate and thus velocity in reactors activated with more conventional catalytic sites has no or a minor positive effect on the apparent reaction rate. On the contrary, in those with the most open structure (M1) and functionalized with enzymes, it could increase by more than two orders of magnitude even at low overpressures. The production systems worked stably for at least 200 h. To conclude, the synthetic system made of the hierarchically structured monoliths, or RBRs filled with structured catalytic pellets, lay the foundation for a new platform for the high-yield production of a wide variety of specialty chemicals, even on a multikilogram scale, in a safe and sustained manner.
Highlights
AND BACKGROUNDFor decades, the synthesis of fine chemicals and active pharmaceutical ingredients (APIs) was carried out in batch operations until in the early nineties that was about to start to change, sparked by the observation that chemical reactions are better controlled and run much faster in a continuous flow, especially in channels with submillimeter diameters (Ehrfeld et al, 2000; Stankiewicz, 2001)
We summarized the most important results, mainly from our studies of continuous-flow structured monolithic reactors and rotating bed reactors (RBRs) filled with structured pellets, activated with various catalytic entities and enzymes
In addition to most typical monolithic reactors designed for continuous-flow synthesis, we report on the performance of rotating bed reactors (RBRs, nicknamed SpinChem), with baskets filled with monolithic beads, exhibiting the same pore structure as monolithic microchannel reactors (MMRs)
Summary
The synthesis of fine chemicals and active pharmaceutical ingredients (APIs) was carried out in batch operations until in the early nineties that was about to start to change, sparked by the observation that chemical reactions are better controlled and run much faster in a continuous flow, especially in channels with submillimeter diameters (Ehrfeld et al, 2000; Stankiewicz, 2001). Monolithic Microreactors in Flow Synthesis than fine chemical synthesis on a multi-g/h scale To overcome this limitation, polymeric, hybrid macrocellular (HIPE), or inorganic (silica) monolithic microchannel reactors (MMRs) and their carbon replicas were proposed (Xie et al, 1999; Kawakami et al, 2005; Roucher et al, 2019; Brun et al, 2011; van den Biggelaar et al, 2019; Baccour et al, 2020). When the enzyme (lipase) was attached to crushed monolith grains, the particulate catalysts obtained appeared to be three times more active than when they were attached to mesoporous silica (MPS) from the SBA-15 family, and much more active than the commercial catalyst of the same lipase (Novozym 435) (Drożdż et al, 2013) These studies, carried out over a decade ago, clearly demonstrated the scale of the benefits that can be expected from the use of silica materials with a multimodal pore structure. Combining all advantages of MMRs and RBRs allows the residence time to be extended to achieve target conversion
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