Synthesis of ZSM-5 zeolites with different morphology and their catalytic performance in methanol to propylene reactions

Using cheap raw materials, ZSM-5 zeolites with different morphology were hydrothermally synthesized by adding urea and adjusting alkalinity in initial solutions. The effects of zeolite morphology on the catalytic performance in methanol to propylene reactions were studied. The results show that the contents of urea and alkalinity have a great effect on the crystal morphology and growth directions. Specifically, ZSM-5 crystal shows the slowest growth rate along b-axis direction and the crystals of product appear as flakes of about 130 nm in thickness when urea/SiO₂(mol ratio)=0.28 and Na₂O/SiO₂(mol ratio)=0.035. As the content of alkalinity increases within a certain range, the products gradually convert to the congeries of nano-size particles. The characterizations of ICP, NH₃-TPD, and N₂-adsorption show that the SiO₂/Al₂O₃ mol ratios, acidity, and pore structures for all the products are much near to each other. The products of flake shape show many advantages in catalytic reactions: good selectivity in methanol to propylene conversions, high over 60% of olefin (ethylene + propylene), about 8.4 of propylene/ethylene ratio, and superior catalytic stability, being over 95% of methanol conversion for 200 h continuous reaction. The excellent performance may be attributed to the short diffusion path length along b-axis direction and high crystallinity.