(621r) Hydrothermal Stability of ZSM-5 Zeolite

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 150^OC has only been
characterized in part.  In this work, we exposed ZSM-5 to hydrothermal
catalyst stability tests at temperatures ranging from 200 to 350^OC. 
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 350^OC. 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] 200^OC,
[C] 300^OC and [D] 350^OC.