Lw. Beck et al., H-1(AL-27) DOUBLE-RESONANCE EXPERIMENTS IN SOLIDS - AN UNEXPECTED OBSERVATION IN THE H-1 MAS SPECTRUM OF ZEOLITE HZSM-5, Journal of the American Chemical Society, 116(21), 1994, pp. 9657-9661
We report the existence of a previously unsuspected peak in the H-1 ma
gic angle spinning spectrum of commercially available HZSM-5 samples.
At 298 K, this resonance is a broad shoulder on the downfield side of
the Bronsted acid signal at 4.3 ppm. Cooling the sample caused the lin
e to narrow, and a clear peak at 6.9 ppm was visible at 123 K. As a fi
rst step in the assignment of this resonance, we report the results of
several H-1{Al-27} double-resonance experiments in the context of the
broad Al-27 resonance of the dehydrated zeolite, Reverse cross polari
zation (Al-27 --> H-1) failed as an assignment technique, but a relate
d indirect detection experiment in which H-1 transverse magnetization
was drained to Al-27 performed marginally better. A more successful ex
periment resembled H-1 observation with Al-27 decoupling; we call this
experiment ''quadrupole tickling'', because the Al-27 irradiation was
insufficient for true decoupling. This technique resulted in selectiv
e broadening of the 4.3 and 6.9 ppm resonances as a result of conflict
ing averaging of the H-1-Al-27 dipolar coupling. The silanol resonance
was unaffected by this procedure, as expected for a proton environmen
t remote from Al-27. Quadrupole tickling is shown to be related to a p
reviously described spin-echo double resonance experiment. H-1 2D expe
riments were carried out on HZSM-5 with a variety of temperatures and
mixing times. These show that site exchange between the 6.9 and 4.3 pp
m resonances is negligible on the time scale of the double resonance e
xperiments, but exchange occurs to a measurable extent at 250 ms. We c
onclude that the 6.9 ppm resonance corresponds to a novel aluminum-con
taining site in zeolite HZSM-5 and is not an artifact due to exchange
with the Bronsted site or an aluminum-rich impurity phase. A possible
interpretation of these results is a second Bronsted site for the zeol
ite.