Kinetics of Zeolite Dealumination: Insights from H‑SSZ-13

When
zeolite catalysts are subjected to steam at high temperatures,
a permanent loss of activity happens, because of the loss of aluminum
from the framework. This dealumination is a complex process involving
the hydrolysis of four Al–O bonds. This work addresses the
dealumination from a theoretical point of view, modeling the kinetics
in zeolite H-SSZ-13 to gain insights that can extend to other zeolites.
We employ periodic density functional theory (DFT) to obtain free-energy
profiles, and we solve a microkinetic model to derive the rates of
dealumination. We argue that such modeling should consider water that
has been physisorbed in the zeolite as the reference state and propose
a scheme for deriving the free energy of this state. The results strongly
suggest that the first of the four hydrolysis steps is insignificant
for the kinetics of zeolite dealumination. Furthermore, the results
indicate that, in H-SSZ-13, it is sufficient to include only the fourth
hydrolysis step when estimating the rate of dealumination at temperatures
above 700 K. These are key aspects to investigate in further work
on the process, particularly when comparing different zeolite frameworks.