Detecting Cage Crossing and Filling Clusters of Magnesium and Carbon Atoms in Zeolite SSZ-13 with Atom Probe Tomography

The conversion of methanol to valuable hydrocarbon molecules
is
of great commercial interest, as the process serves as a sustainable
alternative for the production of, for instance, the base chemicals
for plastics. The reaction is catalyzed by zeolite materials. By the
introduction of magnesium as a cationic metal, the properties of the
zeolite, and thereby the catalytic performance, are changed. With
atom probe tomography (APT), nanoscale relations within zeolite materials
can be revealed: i.e., crucial information for a fundamental mechanistic
understanding. We show that magnesium forms clusters within the cages
of zeolite SSZ-13, while the framework elements are homogeneously
distributed. These clusters of just a few nanometers were analyzed
and visualized in 3-D. Magnesium atoms seem to initially be directed
to the aluminum sites, after which they aggregate and fill one or
two cages in the zeolite SSZ-13 structure. The presence of magnesium
in zeolite SSZ-13 increases the lifetime as well as the propylene
selectivity. By using operando UV–vis spectroscopy
and X-ray diffraction techniques, we are able to show that these findings
are related to the suppression of aromatic intermediate products,
while maintaining the formation of polyaromatic compounds. Further
nanoscale analysis of the spent catalysts showed indications of magnesium
redistribution after catalysis. Unlike zeolite H-SSZ-13, for which
only a homogeneous distribution of carbon was found, carbon can be
either homogeneously or heterogeneously distributed within zeolite
Mg-SSZ-13 crystals as the magnesium decreases the coking rate. Carbon
clusters were isolated, visualized, and analyzed and were assumed
to be polyaromatic compounds. Small one-cage-filling polyaromatic
compounds were identified; furthermore, large-cage-crossing aromatic
molecules were found by isolating large coke clusters, demonstrating
the unique coking mechanism in zeolite SSZ-13. Short-length-scale
evidence for the formation of polyaromatic compounds at acid sites
is discovered, as clear nanoscale relations between aluminum and carbon
atoms exist.