Acid
zeolites are important materials for a variety of petrochemical reactions,
including cracking of alkanes to olefins. Cracking of short n-alkanes has been
used as a model reaction in numerous studies attempting to understand the
relationship between zeolite acidity, adsorption, and apparent reaction rates.
We have developed a first principles computational approach to predict the
apparent first-order rate coefficient, kapp, for cracking C3-C6
n-alkanes in zeolites MFI, FAU, MOR, and BEA. The value of kapp is
represented by the product of the intrinsic rate coefficient for alkane
cracking, kint, and the equilibrium constant for alkane adsorption,
Kads. Density functional theory and absolute rate theory are used to
determine kint, whereas statistical mechanics is used to predict Kads.
Our computational approach reproduces the observed dependence of kapp
on the carbon chain length in MFI. Agreement with experimental data is good for
all zeolite frameworks studied. In particular, there is good agreement between
our approach and experimental data from several authors for n-hexane cracking
in FAU between 673 and 823 K. Our work supports the hypothesis that kint
is independent of zeolite framework structure and differences in kapp
are due primarily to differences in Kads.
