Molecular simulations of the siting locations and
energetics of Mo(CO)6 and C6H6
cosorbed in faujasitic
zeolites Na
n
FAU (n = 0−96,
Si/Al = 100−1) have been presented in combination with Diffuse
Reflectance Infrared
Fourier Transform Spectroscopy (DRIFTS). The in situ
DRIFTS technique was found to be an efficient tool to
monitor the cosorption at low coverage as well as the reaction of
Mo(CO)6 and C6H6 under thermal
activation within
the void space of the Na
n
FAU zeolites.
The molecular simulations are based on Monte Carlo calculations
using the
grand Canonical ensemble and are derived from a suitable zeolite-metal
carbonyl-hydrocarbon potential set. From
the present experimental and theoretical results as well as earlier
experiments related to the reagents sorbed alone,
a coherent picture of the cosorption and chemical behavior of
Mo(CO)6 and C6H6 within the
void space of the
faujasitic zeolites has been drawn as a function of the aluminum
content. In siliceous faujasite (Si/Al = 100) the
Mo(CO)6 and C6H6 molecules are
randomly distributed within the void space and the molecular motions
approach
the rapid isotropic limits of liquids. The chemical behavior upon
thermal activation is found to be analogous to that
observed in solution. In Na56FAU (Si/Al = 2.5)
the reagents are trapped in well-defined sorption sites in
close
proximity. Upon gentle thermal activation a fast reaction occurs
to form
Mo(CO)3(η6-C6H6)
inside the supercage
through a concerted mechanism including the electrostatic field and the
basicity of the framework oxygens. In
Na(85
-
96)FAU (Si/Al =
1.25, 1) the Mo(CO)6 and C6H6
molecules are not encapsulated in close proximity.
Mo(CO)6 reacts thermally in the void space like in the absence
of added C6H6 to lose sequentialy three CO
ligands and
form predominently a
Mo(CO)3(O
z
)3
species in which the three vacant coordination sites are occupied by
three O
z
framework oxygens.
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