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Parallel spectral measurements allow comparison of intensities of the 1 H MAS-NMR peaks from the bridging and silanol hydroxyl groups for the same samples of HMOR, HY and HZSM-5 zeolites pretreated in vacuum at 400 C with those of the corresponding DRIFT bands. The results obtained indicate that the IR extinction coefficients of the acidic bridging hydroxyl groups are several times higher than for the less acidic silanol groups. This can be explained by the higher polarization ability of the former, resulting from the stretching of OH bonds upon vibrational excitation. It is suggested that the extinction coefficients of the O-H stretching vibrations be used as a measure of the intrinsic acidity.

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