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Improvement and Use of Empirical Estimates of 29Si Chemical Shifts in Zeolites

Published online by Cambridge University Press:  28 February 2011

J. M. Newsam ,
M. T. Melchior  and
R. A. BEYERLEIN
J. M. Newsam
Affiliation:
Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801
M. T. Melchior
Affiliation:
Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801
R. A. BEYERLEIN
Affiliation:
Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801
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Abstract

29Si chemical shifts, δ, are for aluminosilicate zeolites expressed as δSi-4Al (which can be estimated from the T-O-T angles observed crystallographically) plus terms involving b1 and b2, the quantitative influences of first and second neighbor aluminum atoms. The former, b1, is assumed to be the sum of a fixed electrostatic component, α, and a framework-specific angular term, β(0 < β < 0.9 ppm), that reflects the generally smaller Si-O-Al angles compared to Si-O-Si. The compositional dependence of δ is approximated by an equal influence, b2, per added second neighbor aluminum atom, the concentration of which is assumed to vary linearly with composition. Values of δS-nAl; n=0-3 for representative L (LTL-framework) and omega (MAZ) zeolites calculated on this basis agree well with those observed. Applying this description of the compositional dependence to as-synthesized and dealuminated CSZ-1 zeolites enables the measurement of framework aluminum contents in this four T-site case.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 111: Symposium O – Microstructure and Properties of Catalysts , 1987 , 125
Copyright
Copyright © Materials Research Society 1988

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