188
7 C–C Bond Formation via Carbocations in the Methane …
Fig. 7.3 Changes in the 1 H
MAS NMR spectrum of
AgY during a hydrogen
adsorption–desorption cycle
(Reprinted from ref. [4] and
[64]. Copyright 2020, with
permission from Elsevier).
a AgY reduced with H 2
(40 kPa) at 313 K for 60 min.
b The same sample after
being evacuated at 313 K for
2 h. c The sample after being
re-exposed to H 2 (40 kPa) at
313 K for 1 h. The asterisks
(*) show the spinning
sidebands
to a silver hydride (Ag n –H) species. The silver cationic cluster (Ag
+
n ) of Ag-Y may
be larger (n > 3) than the Ag
+
3 cationic clusters of Ag-A.
Ag n
+
+ H 2
Ag n –H + H + (n > 3)
(7.46)
The sample from the first hydrogen reduction was further evacuated at 373 K for
1 h. This operation greatly diminished the intensities of the three peaks, as shown
in Fig. 7.3b. The peak at 3.9 ppm due to the bridged hydroxyls (O–H groups) in the
supercage nearly disappeared, even though the intensity of the peak at −0.1 ppm
(silver hydride) decreased by half. Re-exposing the sample to H 2 (40 kPa) at 333 K
(the second hydrogen reduction), mostly recovered the intensities of the three peaks,
as shown in Fig. 7.3c. These results mentioned above demonstrate that the reduction
of the Ag
+ cations and the heterolytic dissociation of H 2 molecules on Ag-Y proceed
reversibly. However,
1 H MAS NMR studies of the zeolites Ag-Y and Ag-A have
shown that the reversibility depends on the degree of reduction of the Ag
+ cations
[4, 64], as the value of n in silver cationic clusters Ag
q+
n decreases with increasing
number of silver atoms per cationic cluster at higher degrees of Ag
+ cations reduction.
The phenomenon of reversible inter-conversion between Ag
+ cations and Ag
metallic clusters was also investigated in detail for the zeolites Ag-ZSM-5 [71] and
Ag-mordenite [72]. It was concluded that the heterolytic dissociation of hydrogen
molecules in Ag-zeolites is reversible, with the reversibility depending on the degree
of Ag
+ ion reduction. Thus, reaction (7.46) should be expressed as the following
7 C–C Bond Formation via Carbocations in the Methane …
Fig. 7.3 Changes in the 1 H
MAS NMR spectrum of
AgY during a hydrogen
adsorption–desorption cycle
(Reprinted from ref. [4] and
[64]. Copyright 2020, with
permission from Elsevier).
a AgY reduced with H 2
(40 kPa) at 313 K for 60 min.
b The same sample after
being evacuated at 313 K for
2 h. c The sample after being
re-exposed to H 2 (40 kPa) at
313 K for 1 h. The asterisks
(*) show the spinning
sidebands
to a silver hydride (Ag n –H) species. The silver cationic cluster (Ag
+
n ) of Ag-Y may
be larger (n > 3) than the Ag
+
3 cationic clusters of Ag-A.
Ag n
+
+ H 2
Ag n –H + H + (n > 3)
(7.46)
The sample from the first hydrogen reduction was further evacuated at 373 K for
1 h. This operation greatly diminished the intensities of the three peaks, as shown
in Fig. 7.3b. The peak at 3.9 ppm due to the bridged hydroxyls (O–H groups) in the
supercage nearly disappeared, even though the intensity of the peak at −0.1 ppm
(silver hydride) decreased by half. Re-exposing the sample to H 2 (40 kPa) at 333 K
(the second hydrogen reduction), mostly recovered the intensities of the three peaks,
as shown in Fig. 7.3c. These results mentioned above demonstrate that the reduction
of the Ag
+ cations and the heterolytic dissociation of H 2 molecules on Ag-Y proceed
reversibly. However,
1 H MAS NMR studies of the zeolites Ag-Y and Ag-A have
shown that the reversibility depends on the degree of reduction of the Ag
+ cations
[4, 64], as the value of n in silver cationic clusters Ag
q+
n decreases with increasing
number of silver atoms per cationic cluster at higher degrees of Ag
+ cations reduction.
The phenomenon of reversible inter-conversion between Ag
+ cations and Ag
metallic clusters was also investigated in detail for the zeolites Ag-ZSM-5 [71] and
Ag-mordenite [72]. It was concluded that the heterolytic dissociation of hydrogen
molecules in Ag-zeolites is reversible, with the reversibility depending on the degree
of Ag
+ ion reduction. Thus, reaction (7.46) should be expressed as the following