Effect of temperature and time in the hydrothermal treatment of HY zeolite

Zeolite HY was steamed at 1 bar water pressure in steps of 100 K from 473 K to 1073 K in dependence of the time (0.5–20 h). Three routes of hydrothermal reactions can be outlined: At low temperature (473 K), a partly framework dissolution occurs accompanied by the formation of X-ray amorphous material. At intermediate temperatures (773–873 K), framework dealumination takes place with the occurrence of different non-framework species. Thereby, with increasing steaming time, saturation of the Si/Al ratio occurs on low levels (Si/Al ratios of about 4 and 5, respectively). At 973 K, a crossover from a low level saturation in Si/Al (6) towards higher values (11) is observed, which is accompanied by a partial amorphization of the zeolite with formation of aluminium oxide and silica gel and a significant decrease in crystal size as well. At high temperatures (1073 K), a fast thermal decomposition occurs. The IR spectra directly reveal an aluminosilicate phase as decomposition product of the framework. At 573 K, the zeolite framework remains unchanged.