Kd. Schmitt et al., SPECTRAL EDITING - A QUANTITATIVE APPLICATION OF SPIN-ECHO NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY TO THE STUDY OF AL-27 IN ZEOLITE CATALYSTS, Zeolites, 14(2), 1994, pp. 89-100
Al-27 spin-echo nuclear magnetic resonance (n.m.r.) is used to measure
the spin-spin relaxation times, T2H, for a substantial number of mode
l compounds, and a theory (with no adjustable parameters) based on Al-
Al dipolar interactions combined with crystallographically determined
Al-Al distances is used to estimate T2H. The homonuclear magnetic dipo
le interaction explains the experimental data reasonably well for comp
ounds with high Al levels, but much less well for compounds with low l
evels of Al, where structure-specific interactions are important. Such
structure-specific interactions are exploited to edit zeolitic Al fro
m the background binder in alumina-bound ZSM-5 and in dealuminated zeo
lite-Y catalysts containing nonframework (NFW) Al. Editing allows quan
titative analysis of the zeolitic components. For dealuminated zeolite
Y, it is concluded that peaks assigned by others to ''pentacoordinate
'' Al may actually arise from NFW aluminum, based on the fact that the
ir T2H is short relative to framework (FW) Al. Theory and experimental
results for the technically more demanding measurement of T2H under c
onditions of ''magic-angle'' sample spinning (MAS) with synchronous sa
mpling are also reported. Spin-echo editing of synchronously sampled A
l-27 MAS n.m.r. spectra are shown to be useful for determining the FW
zeolitic Al content of realistically formulated (kaolinite bound) and
steamed/calcined fluidized bed cracking (FCC) catalysts. The loss of f
ramework Al in two series of steamed FCC catalysts is less precipitous
than the loss in catalytic activity, as measured by the hexane cracki
ng a parameter.