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Black plate (244,1) stretching frequencies were found to be related closely to the charge on the cations. For alkaline-earth metal cation (Mg2 þ , Ca2 þ , Sr2 þ , Ba2 þ )-exchanged forms of high-silica zeolites the frequency shifts were found to correlate well with the polarising power of the cations. The importance of any ‘‘hydrolysis’’ of the cations, i.e. the reaction Mnþ þ H2O ?MðOHÞðn1Þþ þ Hþ; in determining the extent of hydrogen polarisation is emphasised56. Various weakly interacting molecular probes have been used to characterise the acid and base sites in zeolites. Brønsted acidity can be determined by the effect of hydrogen-bonding (to the probe molecule) on surface O2H frequency shifts when N2 and CO are employed as the probes. Additionally, shifts in the C2O stretching frequency are found which generally parallel those of the O2H groups. In the case of N2, the interaction induces an absorption by the molecule, as noted above for homonuclear diatomics. In zeolites exchanged with small cations such as Li þ and Na þ , Lewis acidity was found to dominate the process, and as the cation increased in size, more basic properties became important. In all cases of CO, CO2, N2 and CH4 interacting with cations in zeolites, the interaction energy was found to decrease as the cation radius increased, in accord with the fall in polarising power (eyr) of the cation. At very low equilibrium pressures of CO, the siting of Naþ cations and the Al (Lewis site) distribution in the six-rings (SII-sites) could be investigated; CH4 was found to interact with cations in a C3v- symmetric configuration of the kind Mþ –H3CH. It is also shown that there is a potential use for C2H acids such as Cl3CH (Cl3CD), acetylene and methylacetylene as a probe for base sites at zeolite surfaces, on the basis of observed C2H frequency shifts when adsorption has taken place57. Variable-temperature IR measurements have been adapted successfully to the determination of the thermodynamic parameters for weak gas-solid interactions58. By equating the van’t Hoff equation, the Beer–Lambert law and the Langmuir isotherm, and measuring the temperature dependence of the intensity of an IR band characteristic of the adsorbed state (the band becomes more intense as the temperature is decreased), the enthalpy, DH and entropy, DS for the equilibrium between the gas-phase and adsorbed state of a probe molecule can be determined (Figure 10). Proton-exchanged zeolites often exhibit absorption bands from O2H groups associated with Brønsted sites Si(OH)Al, and they appear in two frequency ranges, 3650 – 3600 and 3580 – 3530 cm 1. The higher wavenumber 244 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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