Long-term steam stability of MWW structure zeolites (MCM-22 and ITQ-1)

Steam stability of MCM-22 and its all siliceous analogue, ITQ-1, with MWW structure was investigated in a mixture of water vapor and nitrogen (95 mol% H2O, 5 mol% N-2) at 10 barg pressure and two different temperatures of 600 degrees C and 350 degrees C for a period of 84 days. Further, the effect of calcination temperature and silicon tetrachloride (SiCl4) treatment on the steam stability of ITQ-1 was studied. Behavior of these zeolites was monitored in 21-day intervals using XRD, SEM, TEM, Si-29 solid-state NMR, nitrogen and argon adsorption analyses. It was found that structural defects adversely affected the steam stability of the zeolites but SiCl4 treatment was effective to enhance the zeolite resistance to water vapor attack. It was observed that long-term steaming can generate holes in the crystals along with some amorphization; however, it also generated highly homogeneous short range order in the crystals. (C) 2014 Elsevier Inc. All rights reserved.