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.