Molecular sieve manufacturer explains the relationship between acid density and catalytic activity of ZSM-5 molecular sieves

A batch of ZSM-5 zeolite molecular sieves with different silicon aluminum ratios were synthesized and modified with Co2+and Fe3+ions. Their performance and selectivity in the synthesis of pyridine by aldol ammonia condensation reaction were measured; The relationship between the acid density of the catalyst and the catalytic activity of aldehyde nitrogen condensation was studied by comparing their NH3-TPD and pyridine adsorption infrared spectra. The study showed that when the silicon aluminum ratio was small, there were more acid centers in ZSM-5. However, excessive acid centers could trigger other cracking and condensation reactions, thereby reducing the activity and selectivity of the catalyst; When the silicon aluminum ratio is around 120, the catalytic activity reaches its maximum and the pyridine base yield reaches 60%. If the silicon aluminum ratio is continued to increase, there are not enough acid centers for the reaction. ZSM-5 is modified with Co2+and Fe3+ions, and the change in its B acid center is not significant. The decrease in L acid center results in a decrease in the adsorption capacity for ammonia, ensuring appropriate acid center exposure, which is beneficial for the reaction. Using CoZSM-5 catalyst, the pyridine base yield can reach 78%.
Due to its high specific surface area, shape selective catalytic performance, and unique acid density, ZSM-5 molecular sieves have attracted increasing attention and have been widely used in adsorption separation, cation exchange, fine chemical synthesis, and other fields. ZSM-5 molecular sieves have also achieved good results in catalyzing aldehyde ammonia condensation reactions, such as formaldehyde, acetaldehyde and condensation to form pyridine and 3-methylpyridine, If ZSM-5 molecular sieve catalyst is used, the yield can be increased by about 30%.
The production of pyridine from aldol ammonia condensation was initially carried out using amorphous aluminosilicate catalysts, but the yield rapidly decreased after a few cycles of use. This was mainly due to the disordered pore structure of amorphous aluminosilicates, the tendency to accumulate carbon on the surface, and the difficulty in recovering activity through regeneration. Using amorphous aluminosilicate catalysts, the yield of pyridine and its derivatives from aldol ammonia condensation reaction did not exceed 50%. After gradual exploration, crystalline aluminosilicates were adopted, especially ZSM-5 molecular sieves with cross straight channel structures, which greatly improved the problem of carbon deposition and significantly increased the total yield of pyridine base. This may be due to the unique microporous structure of ZSM-5 molecular sieves, which is not conducive to the generation of most aromatics and reduces the formation of carbon deposition; The pore size of ZSM-5 zeolite is comparable to that of pyridine molecules, thus improving the selectivity of pyridine. In addition, ZSM-5 zeolite also has excellent thermal stability, and its structure only begins to be destroyed after calcination at 1050 ℃.
In recent years, research on the production of pyridine through aldehyde ammonia condensation reaction has mainly focused on the modification of zeolites, such as selecting ZSM-5 molecular sieves with a certain silicon aluminum ratio, or introducing Pb, Co, Fe, Pt, etc. into ZSM-5 molecular sieves through cation exchange to improve the catalytic activity of zeolites. The use of modified ZSM-5 molecular sieves can achieve a yield of about 80%, which has been reported in many patents. However, the relationship between the acid density of ZSM-5 molecular sieves and the catalytic activity of aldoammonia condensation has not been reported yet. We synthesized a batch of ZSM-5 molecular sieves with different silicon aluminum ratios and modified them through cation exchange. The activity and selectivity of their aldehyde-ammonium condensation synthesis of pyridine were measured, and the relationship between the acid density of the catalyst and the catalytic activity of aldehyde-ammonium condensation was studied through NH3-TPD and pyridine adsorption infrared spectroscopy, providing a theoretical basis for the synthesis of efficient and highly selective catalysts. 
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