Abstract
Relaxation and spin-counting measurements have been used to probe the extent to which {sup 13}C NMR spectroscopy with cross polarization and magic-angle spinning is a useful technique for characterizing carbonaceous (coke) deposits in zeolite catalysts containing rare-earth cations. Such ions were anticipated to complicate this analysis as a result of paramagnetic shifts and/or relaxation effects. Contrary to expectations, no paramagnetic shifts were observed for carbonaceous deposits in the rare-earth-exchanged Y-type zeolites examined. Even more surprising was that the mixture of rare earths used in industrial catalysts has negligible effects upon the relaxation processes central to cross polarization. Small concentrations of lanthanides with long electron spin-lattice relaxation times (in particular gadolinium), however, have a deleterious effect on characterizing carbonaceous deposits by decreasing the proton rotating-frame spin-lattice relaxation time.
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