Analysis of the reasons why the hydrogenation catalytic reaction process is too slow(1)

The most common problem in hydrogenation catalysis is that the reaction is slow, or even stops, and the catalyst must be filtered out and the filtrate replenished with new catalyst to continue the reaction.

To sum up the experience, there are three main reasons for the slow reaction of catalytic reaction

Substrate structure

The ease of hydrogenation of the substrate structure is an intrinsic factor affecting the reaction rate. In the substrate structure, the ease of hydrogenation is mainly influenced by the functional groups. Among the common functionalities, the reduction of chlorides to aldehydes and nitro to amino is the easiest to occur, followed by the reduction of alkynes to alkenes, ketones to alcohols and nitrile to methylamines, etc., while the reduction of benzene rings to cyclohexanes and acids to alcohols is the most difficult. When the same functional group, other parts of the substrate structure also affect the ease of reaction. 

For example, when reducing a ketone to an alcohol, a substrate with fewer branched chains on the carbon near the ketone is easier to reduce. In process studies, the structure of the substrate depends on the reaction route and is generally not the focus of process optimization.

Catalyst activity

Different catalysts exhibit different activities in the reaction. The most commonly used catalysts in catalytic hydrogenation are palladium carbon and Raney nickel, both of which have relatively good activity and are suitable for most substrates. Palladium acetate and platinum carbon these are more commonly used in the laboratory and have high activity. Catalysts such as copper and chromium, which are commonly used in bulk industrialization, are generally less active and require higher reaction conditions. The commonly used and applied catalysts of value are limited, have a small selection, and are generally not used as a focus for process optimization, with some exceptions. 

For example, in the hydrogenation of acetophenones given as an example, the same 10% palladium carbon differs greatly between models, with reaction rates often varying between 2-20 times. As for the difference between each palladium carbon model, it should be related to the process used in the production of palladium carbon, where different models with the same content have a better catalytic effect for some compounds and a worse effect for others. The selection is based on experimental screening, in addition to consultation with the supplier, to filter out the most suitable catalyst.

Reaction conditions

The reaction conditions have a relatively simple effect on the reaction rate. High temperature and pressure and high speed stirring can speed up the reaction process. 

Due to equipment and safety restrictions, the reaction conditions cannot be increased indefinitely. The temperature generally cannot exceed the boiling point of the solvent, and the suitable range is between 25-80 degrees; the pressure is generally as low as possible, keeping it at 1-30 atm is more suitable; the stirring should not be too fast, the slower and smoother, generally at 150-300 rpm is more suitable; the solvent mostly uses methanol, ethanol, tetrahydrofuran, acetic acid, ethyl acetate and water, mainly to ensure that the products and raw materials are as fully soluble as possible in the solvent. 

The influence of solvent on the reaction speed is second to temperature, pressure and stirring speed, and the main consideration is the workability, safety and recovery of solvent; catalyst is the focus of cost control in catalytic hydrogenation reaction, which requires cheap and small amount of catalyst. Raney nickel, because of its cheap price, is generally used between 10-30% and can be recycled and used; palladium carbon, because of its expensive price, is generally used between 3-10% and must be recycled and used, and the recycled catalyst may need to be activated before each application, and some new catalysts need to be added when used.