**2. Instrumentation**

Unlike batch reactions where gas-solvent contact is limited by diffusion of gas into the bulk solvent, flow hydrogenation rapidly saturates the solvent with hydrogen using two different approaches [5]. The first, used by the ThalesNano H-cube®, employs in-line mixing of hydrogen with the solvent under pressure, which prevents outgassing and rapid solvent stream saturation (**Figure 1A**) [6]. The second approach, used by the Vapourtec Gas/Liquid reactor (**Figure 1B**) and Gastropod Gas Liquid Module, employs gas permeable membranes in a tubein-tube reactor. These systems enable solvent stream saturation by passing hydrogen gas under pressure through a gas porous polymer and into the solvent [7–10].

**Figure 1.** Schematic of the mechanical mixing setup in (**A**) the ThalesNano H-Cube® and (**B**) schematic of the gas permeable membrane (tube-in-tube) technology [1].

The Thalesnano H-cube® was the first commercial flow hydrogenator. Together with the use of in-line gas mixing, the H-cube uses exchangeable 30 or 70 mm heterogeneous catalyst cartridges and a HPLC pump. Hydrogen gas is generated in situ through water electrolysis. The system is capable of heating to 100°C and 150 bar, with a flow rate range from 0.5 to 5.0 mL min−1 [11]. Tube-in-tube reactors require specialised materials displaying high gas permeability while being impermeable to (nonfluorinated) liquids and corrosive chemicals, e.g., Teflon AF-2400 [12]. The Vapourtec Gas/Liquid system uses 'plug-in reactors' and the Gastropod Gas Liquid Module can be equipped with a small gas cylinder or attached to any custom flow systems [9, 10].
