Equipment used for hydrogenation
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Today’s bench chemist has three main choices of hydrogenation equipment:
* Batch hydrogenation under atmospheric conditions
* Batch hydrogenation at elevated temperature and/or pressure
* Flow hydrogenation
• Batch hydrogenation under atmospheric conditions
The original and still the most commonly practised form of hydrogenation, this process is usually effected by adding solid catalyst to a round bottom flask of dissolved reactant which has been evacuated using nitrogen or argon gas and sealing the mixture with a penetrable rubber seal. Hydrogen gas is then applied by fixing a balloon filled from a cylinder to a syringe and needle using laboratory tape and inserting the needle through the rubber seal, with the resulting three phase mixture being mechanically stirred until the reaction has gone to completion.
Some scientists prefer to measure hydrogen uptake to monitor the process of their reaction. This is achieved by either using a graduated tube containing a coloured liquid, usually aqueous copper sulfate, or investing in a hydrogenation laboratory equipped with gauges for each reaction vessel.
• Batch hydrogenation at elevated temperature and/or pressure
Many key hydrogenation reactions such as hydrogenolysis of protecting groups and the reduction of aromatic systems proceed extremely sluggishly (if at all) at atmospheric temperature and pressure, leading to the popularity of pressurised systems. In these cases, catalyst is added to a solution of reactant under an inert atmosphere in a pressure vessel. Hydrogen is added directly from a cylinder or built in laboratory hydrogen source and the system is mechanically rocked to provide agitation. Heat may also be used, as the pressure compensates for the associated reduction in gas solubility. This vastly increases the rate of reaction as described by the Arrhenius equation.
• Flow Hydrogenation
In recent times, flow hydrogenation has become a very popular technique at the bench and increasingly the process scale. This technique involves continuously flowing a dilute stream of dissolved reactant over a fixed bed catalyst in the presence of hydrogen. Using established HPLC technology, this technique allows the application of pressures from atmospheric to 1,450 PSI. Elevated temperatures may also be used. At the bench scale, systems use a range of pre-packed catalysts which eliminates the need for weighing and filtering pyrophoric catalysts.
* Batch hydrogenation under atmospheric conditions
* Batch hydrogenation at elevated temperature and/or pressure
* Flow hydrogenation
• Batch hydrogenation under atmospheric conditions
The original and still the most commonly practised form of hydrogenation, this process is usually effected by adding solid catalyst to a round bottom flask of dissolved reactant which has been evacuated using nitrogen or argon gas and sealing the mixture with a penetrable rubber seal. Hydrogen gas is then applied by fixing a balloon filled from a cylinder to a syringe and needle using laboratory tape and inserting the needle through the rubber seal, with the resulting three phase mixture being mechanically stirred until the reaction has gone to completion.
Some scientists prefer to measure hydrogen uptake to monitor the process of their reaction. This is achieved by either using a graduated tube containing a coloured liquid, usually aqueous copper sulfate, or investing in a hydrogenation laboratory equipped with gauges for each reaction vessel.
• Batch hydrogenation at elevated temperature and/or pressure
Many key hydrogenation reactions such as hydrogenolysis of protecting groups and the reduction of aromatic systems proceed extremely sluggishly (if at all) at atmospheric temperature and pressure, leading to the popularity of pressurised systems. In these cases, catalyst is added to a solution of reactant under an inert atmosphere in a pressure vessel. Hydrogen is added directly from a cylinder or built in laboratory hydrogen source and the system is mechanically rocked to provide agitation. Heat may also be used, as the pressure compensates for the associated reduction in gas solubility. This vastly increases the rate of reaction as described by the Arrhenius equation.
• Flow Hydrogenation
In recent times, flow hydrogenation has become a very popular technique at the bench and increasingly the process scale. This technique involves continuously flowing a dilute stream of dissolved reactant over a fixed bed catalyst in the presence of hydrogen. Using established HPLC technology, this technique allows the application of pressures from atmospheric to 1,450 PSI. Elevated temperatures may also be used. At the bench scale, systems use a range of pre-packed catalysts which eliminates the need for weighing and filtering pyrophoric catalysts.
