Spin probes of chemistry in zeolites Page: 4 of 14
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adsorbed species. Finally, extension of our methods to acidic zeolites has afforded a
significant new window onto mechanisms of zeolite catalysis.
1.2. Radiolyis/EPR Studies in Zeolites. Our interest in zeolites was initiated
by our desire to utilize the constrained spaces of these nanoporous solids to control the
chemistry of radical cations. Zeolites are crystalline materials made up of (SiO4) and
(A104 )5 tetrahedra. From these basic building blocks arise myriads of lattice architectures
consisting of channels and cages with different types of connectivity. One of these, ZSM5,
has a three-dimensional network of intersecting channels with diameters of approximately
5.5 A. Exchangeable cations are present in stoichiometric amounts to balance the negative
charge introduced into the lattice by trivalent aluminum. We have used both the Na' and H
forms. NaZSM5 is inert while HZSM5 is a strong solid acid.
Zeolite samples were radiolyzed with gamma rays from a Co-60 gamma source or 3
MeV electrons from a Van de Graaff accelerator, and the radiolyzed sample is interrogated
using X-band EPR. Variations include pie- and/or post-irradiation exposure to probe
molecules, use of more than one probe molecule, sequentially or in mixtures, use of inert
diluents, etc. In all of our work, probe molecules were delivered to the zeolite via a glass
vacuum manifold and were pre-purified by freeze-pump-thaw cycling. Only dehydrated
zeolites were used in the experiments described here. The zeolite powders were first heated
to 450 C under vacuum (<10' torr) for 4 h. Oxygen-free conditions must be maintained if
stabilized radical cations are to be detected.
2. RADICAL CATIONS IN ZEOLITES
Radical cations of adsorbed molecules are generated radiolytically and stabilized in
zeolites according to eqs (1) and (2). Energy loss from the ionizing particles occurs to the
matrix; direct ionization of the guest species is insignificant. Transfer of the positive charge
from the lattice (Z') to guest species is efficient for a wide range of donor molecules, M.
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Werst, D.W. & Trifunac, A.D. Spin probes of chemistry in zeolites, article, September 1, 1997; United States. (digital.library.unt.edu/ark:/67531/metadc703132/m1/4/: accessed November 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.