Hydrothermal Stability of ZSM-5 Zeolite
Alex Maag and Michael T. Timko
Department of Chemical Engineering
Worcester Polytechnic Institute
100 Institute Road Worcester MA 01609
Hydrothermal water has potential as a reaction environment for many important reactions. While hydrothermal water alone can be sufficient to promote desirable chemistries, addition of a suitable catalyst may be required to achieve acceptable reaction rates. Catalyst stability under hydrothermal conditions is a key barrier for solid acid catalysts. One common solid acid catalyst used industrially is ZSM-5, an active catalyst prone to coking in the vapor phase. Operation under hydrothermal conditions has promise to improve selectivity and reduce coking by inhibiting oligomerization reactions. The hydrothermal stability of the catalyst framework in liquid water conditions at temperatures greater than 150OC has only been characterized in part. In this work, we exposed ZSM-5 to hydrothermal catalyst stability tests at temperatures ranging from 200 to 350OC. Postrun catalyst analysis using XRD, DRIFTs, Raman and NH3 desorption evaluated catalyst crystallinity and acid site breakdown. Decrystallization of the ZSM-5 framework due to hydrothermal treatment is temperature dependent and most rapid at 350OC. Representative diffraction data are shown in the figure. We postulate as to the de-silication/de-alumination breakdown mechanisms and propose ways to stabilize the zeolite.
Figure 1: XRD diagram comparing: [A] calcined ZSM-5 catalyst (Si/Al=38) to hydrothermally treated ZSM-5 in a continuous flow reactor for 6 hours at 3600 psi and [B] 200OC, [C] 300OC and [D] 350OC.
See more of this Group/Topical: Catalysis and Reaction Engineering Division