When molecules of low-medium proton affinity (B = N-2, CO, C2H4, C2H2, propene, methylacetylene, acetonitrile and H2O) are adsorbed on acidic zeolites, the resulting host-guest interactions in the cavities and in the channels are dominated by hydrogen bonding in neutral Z-H ... B adducts between the Bronsted groups Z-H (Z = zeolite framework) and the base. If the interaction is sufficiently strong and the protonated products stable (ethylene, acetylene, propene and methylacetylene) the hydrogen bonded precursors are slowly consumed with formation of olygomeric products entrapped into the zeolite cavities. Acetonitrile gives a stronger hydrogen bonding interaction not followed by protonation. Water initially gives neutral complexes; however when more than one water molecule per protonic site is present, H5O2+. nH(2)O species are formed. When molecules of high proton affinities are adsorbed (NH3 and Py), B-H+... Z(-) ionic pairs readily appear. The modifications induced on the spectrum of B-H+ by interaction with Z(-) can be used to probe the basicity of the negatively charged framework. At higher filling conditions B-H+... B dimers are also observed. The basic IR spectroscopy of all these hydrogen bonded systems is discussed in detail.
Host-guest interactions in zeolite cavities.
ZECCHINA, Adriano;BORDIGA, Silvia;SCARANO, Domenica;RICCHIARDI, Gabriele;SPOTO, Giuseppe
1995-01-01
Abstract
When molecules of low-medium proton affinity (B = N-2, CO, C2H4, C2H2, propene, methylacetylene, acetonitrile and H2O) are adsorbed on acidic zeolites, the resulting host-guest interactions in the cavities and in the channels are dominated by hydrogen bonding in neutral Z-H ... B adducts between the Bronsted groups Z-H (Z = zeolite framework) and the base. If the interaction is sufficiently strong and the protonated products stable (ethylene, acetylene, propene and methylacetylene) the hydrogen bonded precursors are slowly consumed with formation of olygomeric products entrapped into the zeolite cavities. Acetonitrile gives a stronger hydrogen bonding interaction not followed by protonation. Water initially gives neutral complexes; however when more than one water molecule per protonic site is present, H5O2+. nH(2)O species are formed. When molecules of high proton affinities are adsorbed (NH3 and Py), B-H+... Z(-) ionic pairs readily appear. The modifications induced on the spectrum of B-H+ by interaction with Z(-) can be used to probe the basicity of the negatively charged framework. At higher filling conditions B-H+... B dimers are also observed. The basic IR spectroscopy of all these hydrogen bonded systems is discussed in detail.
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