Analysis of the problem that catalytic hydrogenation reaction is too slow


The most common problem in catalytic hydrogenation is that the reaction is slow, or even the reaction stops, and the catalyst must be filtered out, and the filtrate can be supplemented with a new catalyst to continue the reaction. Summarizing experience, there are three main reasons for the slow reaction of catalytic reactions:
 


 

1. Substrate structure
  
The ease of hydrogenation of substrate structures is an intrinsic factor affecting the reaction speed. In the substrate structure, hydrogenation is mainly affected by functional groups. Among the common functions, the reduction of acid chloride to aldehyde and nitro to amino group is the most likely to occur, followed by alkyne reduction to alkene, ketone reduction to alcohol and nitrile reduction to methylamine, etc., while benzene ring reduction to cyclohexane and acid reduction to alcohol is the most difficult. With the same functional group, other parts of the substrate structure also affect the difficulty of the reaction. For example, when ketones are reduced to alcohol, substrates with fewer carbon branches near ketones are 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.
  
2. Catalyst activity
  
Different catalysts exhibit different activities in the reaction. The most commonly used catalysts in catalytic hydrogenation are palladium carbon and Raenenickel, both of which have relatively good activity and are suitable for most substrates. Palladium acetate and platinum carbon are used more in laboratories and have high activity. Catalysts such as copper chromium commonly used in bulk industrialization generally have poor activity and require higher reaction conditions. Catalysts that are commonly used and of value are limited, the selection is small, and they are generally not the focus of process optimization, but there are exceptions. For example, when the acetophenone compound given for example is hydrogenated, it is the same 10% palladium carbon, and the difference between different models is very large, and the reaction speed often varies between 2-20 times. As for the difference between each palladium carbon model, it should be related to the process of producing palladium carbon, and different models with the same content have a better catalytic effect on some compounds and a poor effect on some parts. In addition to consulting suppliers, the selection is mainly based on experimental screening to screen out the most suitable catalyst.
  
3. Reaction conditions
  
The influence of reaction conditions on the reaction speed is relatively simple, and high temperature and high pressure and high-speed stirring can speed up the reaction process. Due to equipment and safety limitations, it is not possible to increase the reaction conditions 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 lower as possible, and it is more appropriate to keep it at 1-30atm; The stirring should not be too fast, the slower the more stable, generally 150-300 rpm is more appropriate; The solvent is mostly methanol, ethanol, tetrahydrofuran, acetic acid, ethyl acetate and water, mainly to ensure that the product and raw materials are as completely soluble as possible in the solvent. The influence of solvent on reaction speed is second to temperature, pressure and stirring speed, and the operability, safety and recovery rate of solvent are mainly considered. Catalyst is the focus of cost control in catalytic hydrogenation reaction, and catalysts are required to be cheap and use small amounts. Because of its cheap price, the general dosage is between 10-30% and can be recycled and applied; Palladium carbon due to the high price, the general dosage between 3-10%, must be recycled and applied, each application may need to activate the recovered catalyst, use need to add some new catalyst.
  
The above three points are the conventional factors that affect the reaction speed, and the so-called problem of catalytic hydrogenation reaction is too slow generally refers to the problem that the reaction is still slow after adjusting the above factors to the limit, which is the focus of this problem. At these times, it is already difficult to solve the problem by only theoretical inferences, and through experimental summary, the main influencing factors include:
  
1. PH value is a key point that is easy to be overlooked when optimizing the catalytic hydrogenation process, and PH value affects the reaction speed by affecting the catalyst activity. Raininickel has high activity under alkaline conditions and poor activity under acidic conditions, so the alkalinity of the reaction solution can be improved by adding amines. Palladium carbon has high activity under acidic conditions and poor activity under alkaline conditions, and the acidity of the reaction solution can be improved by supplementing acetic acid. The above acidity and alkalinity should not be too high, can not use strong acid and alkali, need to consider the stability of the equipment and the impact on post-treatment.
  
2. Impurities, which refer to poisons, insoluble substances, metal ions and tar, excluding low molecular impurities, isomers and solvent residues. Raininickel generally has strong toxication resistance, and the poisoning is mainly for palladium carbon. The poisons are mainly thiophosphides, including various forms of substances containing thion elements. The routes of introduction of poisons include raw materials, solvents, reactors and gases; Insoluble substances are mainly inorganic substances and polymers, which are mainly deposited on the surface of the catalyst, which hinders the reaction site and slows down the reaction; Metal ions are mainly heavy metals, copper ions, mercury ions, lead ions and cadmium ions, etc., react with the catalyst, thereby slowing down the reaction; The mechanism of action of tar is similar to that of insoluble substances, and it may also be generated in the reaction, so the reaction conditions should generally not be too high; The factors of impurities are difficult to analyze by instrument, and must be verified experimentally to know, which is most easily ignored in catalytic hydrogenation.
  
3. Stirring type, mainly the mixing effect of stirring on the bottom catalyst. Due to the small solubility of hydrogen in solvent, most of the reaction systems exist in gaseous state; The raw material is dissolved in a solvent and is in a liquid state; The catalyst is solid. Microscopically, the reaction occurs on the surface of the catalyst, and good stirring can make the gaseous hydrogen and the raw material quickly pass through the surface of the catalyst and react, and at the same time take the product away. During the laboratory test, more magnetic stirring is used, because the amount is small and the magnetic force has the effect of grinding the catalyst, and the reaction is generally the fastest; In the pilot test, the push-type stirring paddle pressurized mixing reaction system has a better effect. Due to its heavierness, Raini nickel is the most affected by the stirring effect; Palladium carbon has little effect due to palladium metal adsorption on activated carbon; In industrial production, the scale is larger, the stirring effect is poor, and the reaction is generally slower than the pilot reaction; The traditional anchor stirring has no direction, and it can be stirred normally in both directions; The pusher mixing paddle has a direction, and the correct direction has a good mixing effect, and the reverse direction effect is poor.


 

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Reni nickel catalyst science knowledge(1)

Physical and chemical properties: Reni nickel catalyst before activation is silver-gray amorphous powder (nickel-aluminum alloy powder), with a moderate degree of flammability, partial activation in the presence of water and the production of hydrogen easy agglomeration, long-term exposure to air is easy to weather. Nickel-aluminum alloy powder is activated into gray-black particles, accompanied by active hydrogen, extremely unstable, oxidative combustion in the air, must be immersed in water or ethanol for preservation. It was first used by American engineer Murray Rainey as a catalyst in the hydrogenation of vegetable oils. The preparation process is to treat nickel-aluminum alloy with concentrated sodium hydroxide solution, in this process, most of the aluminum will react with sodium hydroxide and dissolve, leaving a lot of micropores of different sizes. In this way, the surface of Raininickel is a fine gray powder, but from a microscopic point of view, each tiny particle in the powder is a three-dimensional porous structure, this porous structure greatly increases its surface area, and the large surface area brings high catalytic activity, which makes Raininickel widely used as a heterogeneous catalyst in organic synthesis and industrial hydrogenation reactions. Since "Rainey" is a registered trademark of Grace Chemicals, strictly speaking, only products manufactured by the company's Davidson Chemical Division can be called "Lanny Nickel". The term "metal backbone catalyst" or "sponge-metal catalyst" is used to refer to catalysts with a microporous structure and physical and chemical properties similar to Raney nickel.