Locating Organic Guests in Inorganic Host Materials from X‑ray Powder Diffraction Data

Can the location
of the organic structure-directing agent (SDA)
inside the channel system of a zeolite be determined experimentally
in a systematic manner? In an attempt to answer this question, we
investigated six borosilicate zeolites of known framework structure
(SSZ-53, SSZ-55, SSZ-56, SSZ-58, SSZ-59, and SSZ-60), where the location
of the SDA had only been simulated using molecular modeling techniques
in previous studies. From synchrotron powder diffraction data, we
were able to retrieve reliable experimental positions for the SDA
by using a combination of simulated annealing (global optimization)
and Rietveld refinement. In this way, problems arising from data quality
and only partially compatible framework and SDA symmetries, which
can lead to indecipherable electron density maps, can be overcome.
Rietveld refinement using geometric restraints were then performed
to optimize the positions and conformations of the SDAs. With these
improved models, it was possible to go on to determine the location
of the B atoms in the framework structure. That is, two pieces of
information that are key to the understanding of zeolite synthesisthe
location of the organic SDA in the channel system and of the positions
adopted by heteroatoms in the silicate frameworkcan be extracted
from experimental data using a systematic strategy. In most cases,
the locations of the SDAs determined experimentally compare well with
those simulated with molecular modeling, but there are also some clear
differences, and the reason for these differences can be understood.
The approach is generally applicable, and has also been used to locate
organic guests, linkers, and ligands in metal–organic compounds.