**3.9. New catalysts**

As further catalysts are developed for flow hydrogenation, the specificity and robustness of the chemical transformations achievable increase. Flow hydrogenation does require the use of a rare metal catalyst, which can be both expensive and potentially environmentally unsustainable. This negates the toxicity and disposal problems related to classical reducing agents and suggests that flow hydrogenation approaches may be expensive and not environmentally benign. This has led to the development of alternative hydrogenation catalysts such as carbonsupported iron-phenanthroline complexes, nickel nanoparticles, and FeNi alloys. These new catalysts show broad spectrum reductive capabilities (**Table 5**).


**Table 5.** Use of novel catalysts in flow reduction reactions.

Dehydrohalogenation is a significant and ongoing concern in flow (and batch) reduction [55], and thus the development of new catalysts that specifically avoid this outcome is a valuable research tool. Osako et al. used platinum nanoparticles dispersed on an amphiphilic polystyrene-poly(ethylene glycol) (ART-Pt) resin as a catalyst was specifically developed to avoid reduction of –Cl, −C=O, and –CN moieties, e.g., **55a**-**56c** (**Figure 19**) [51].

**Figure 19.** Alkene reductions via flow hydrogenation using the ARP-Pt catalyst. Reagents and conditions: X-cube®, H2 (5 vol%), ARP-Pt (0.073 mmol Pt), EtOH (50 mM), 5 bar, 2mL min−1.

Specialist catalyst development has often been targeted towards chemoselectivity. Fan et al. have shown that the Pd/triC catalyst was selectively reduced alkyne groups over nitro, bromo, and aldehyde groups [50]. While Rathi's palladium nanoparticles supported on maghemite were effective in reducing nitroarenes, azides, and alkenes in good to excellent yields [52].

Nagendiran et al. detail the use of aminofunctionalised mesocellular foam-supported nanopalladium in the conjugate reduction in a series of Michael acceptors [54]. Both the Vapourtec (1.0 mL min−1, 0.1 M, 1 bar H2, 20°C) and the H-cube (1.0 mL min−1, 0.1 M, 40 psi H2, 50°C) with this catalyst afforded chemoselective reduction in the olefin moiety. This approach was scalable enabling the selective reduction of cinnamaldehyde to 3-phenylpropanal on an approximately 20 g scale with no observed loss of catalysts' activity or selectivity.
