Research On Catalytic Activity Of Modified HZSM-5 Catalyst On Aldehyde-ammonia Condensation

There are two techniques for the production of pyridine bases. One is separation from coal tar. The other is chemical synthesis, which has been the major technique since its commerciallization in the 1950's. With the rapid development of chemical and feedstuff industries in China, the demand for pyridine bases has been increasing very fast. However, up to now, there is still not a mass production based on domestic technology. This work concentrates on the catalyst development for the condensation reaction of formaldehyde, acetaldehyde and ammonia. Since the reaction is a acid catalyzed one, the effect of the acidity of various catalysts on their performance was investigated by using NH₃-TPD,IR and so on.The Si/Al ratio in H-ZSM-5 zeolite was examined. It was found that a ratio of 70 is the best for the reaction. The effects of transition metal ion modification on the acidity of ZSM-5 and on the catalytic performance in aldehyde-ammonia condensation reaction were investigated. Cobalt has a positive effect and therefore, its suitable amount was explored. Modification of the zeolite with tetramethylsilane was introduced to improve its stability and selectivity.Reaction condition: SiO2/Al2O3 = 140, reaction temperature 450oC, normal pressure, formaldehyde/acetaldehyde ratio 2:3, ammonia/organic compound mole ratio 2:1, GHSV 1290 h-1,is found good for reaction and the total yield of alkylpyridines is 50.5%. The catalytic activity in the condensation reaction depends on the total acidity of the zeolite. The Lewis acidity and the total acidity of HZSM-5 increased with the increase of the cobalt content, while the BrÇ’nsted acidity decreased. The selectivity of 2-methylpyridine was improved while that of pyridine and 3-methylpyridine were reduced. The distribution curve of acid strength was also changed. The catalytic activities of Co/HZSM-5,Pb/HZSM-5 and Cd/HZSM-5 catalysts were all improved over the HZSM-5. With the treatment of tetramethylsilane [Si(OCH3)4], the external acid site of HZSM-5 was killed and the pore-opening size was reduced, whereas the internal surface properties of the zeolite kept unchanged. The catalytic performance of silicon-modified HZSM-5 was improved, however, the comparative selectivities of the pyridine bases were not changed.