A Pressure Reactor, sometimes referred to as a pressure tube, or a sealed tube, is a chemical reaction vessel which can conduct a reaction under pressure. The pressure can be caused by the reaction itself or created by an external source, like hydrogen in catalytic transfer hydrogenation.
A pressure reactor can offer several advantages over the conventional round-bottom flask. Firstly, it can conduct a reaction above the boiling point of a solvent. Based on the Arrhenius equation, the reaction rate can double for every 10 degree Celsius increase in temperature. In other words, for every 10 degree Celsius increase, reaction time can be shortened in half. Secondly, the pressure can reduce the reaction volume, including the liquid phase, and in turn increase concentration and collision frequency, and accelerate a reaction.
Increase in temperature can speed up the desired reaction, but also speed up the decomposition of reagents and starting materials. However, pressure can speed up the desired reaction and has no impact on decomposition. When the desired reaction is accelerated, the competing reagent decompositions are minimized. Pressure enables faster reactions with cleaner reaction profiles.
The above benefits from a pressure reactor has been shown in microwave chemistry. E.g., if a Suzuki Coupling takes 8 hours at 80°C, it only takes 8 minutes at 140°C in a microwave synthesizer. Microwave effect is a controversial topic. Later experiments show some of these early reports to be artifacts and rate enhancement is strictly due to thermal effects[1][2][3].
If a pressure reactor is engineered properly, it can meet 4 out of 12 green chemistry principles (#1, less solvent and cleaner reaction profile result in less waste; #5, less solvent is needed; 6#, short reaction time can save up to 92 percent electricity and 200 gallons of cooling water per refluxed reaction; 12#, closed vessel can prevent releasing toxic gas and explosions).
One of the drawbacks of a standard glass pressure reactor is the potential explosions due to hard-to-predict excessive internal pressure and lack of relief mechanism.
The drawbacks of a metal pressure reactor (bomb) are set-up, maintenance, and corrosiveness.
The drawbacks of a microwave synthesizer are solvent limitation (it needs a polar solvent to absorb microwave energy), and localized overheat which causes decomposition.
There is a new pressure reactor, called Q-Tube[4]. It features a simple and safe pressure release and reseal mechanism which can prevent a glass pressure reactor from explosions and retain the solvent.