186
7 C–C Bond Formation via Carbocations in the Methane …
7.7.2 Generation of Hydrogen Species Over Ag + -Exchanged
Zeolites in the Presence of Hydrogen: Reversible
Heterolytic Dissociation of Hydrogen Molecules
As discussed in Sect. 7.6.1, the Ag
+ cations in Ag-zeolites are reduced by H 2 to
produce acidic protons (Brønsted acid sites) and Ag
0 metal, as shown in reaction
(7.39); this reaction is partially reversible. The acidic protons originating from the
surface O–H groups in the zeolites were observed using infrared spectroscopy [58].
Baba et al. investigated the surface hydrogen species of Ag-zeolites using
1 H
magic angle spinning NMR (
1 H MAS NMR) spectroscopy. The Ag-zeolites were
treated with hydrogen, and their
1 H MAS NMR spectra were then measured in
the presence of hydrogen [63–65]. Figure 7.2 shows the
1 H MAS NMR spectra
of Ag-A. The Ag-A was placed in contact with H 2 (40 kPa) at 313 K for 30 min
(first hydrogen reduction). The amount of hydrogen consumed was 3.02 × 10
−4
mol g
−1 , which corresponded to the reduction of 21% of the Ag
+ cations in the zeolite.
The
1 H MAS NMR spectrum of the reduced Ag-A was measured in the presence
of hydrogen (Fig. 7.2a), and two peaks were observed at 4.0 and −1.8 ppm. The
peak at 4.0 ppm was unambiguously attributed to the acidic protons (Brønsted acid
sites), because such protons have been observed at chemical shifts of 3.9–4.4 ppm
in various H
+ -exchanged zeolites [66–68]. The peak at −1.8 ppm was a quartet with
an intensity ratio of 1:3:3:1 and a coupling constant of (131 ± 1) Hz, indicating that
Fig. 7.2 1 H MAS NMR
spectra of AgA recorded at
298 K in the presence of
hydrogen (Reprinted with
permission from ref [63].
Copyright 2020 American
Chemical Society). a AgA
reduced with H 2 (40 kPa) at
313 K for 30 min. The
spectrum was recorded under
40 kPa of H 2 . 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 30 min. The
asterisks (*) show the
spinning sidebands
7 C–C Bond Formation via Carbocations in the Methane …
7.7.2 Generation of Hydrogen Species Over Ag + -Exchanged
Zeolites in the Presence of Hydrogen: Reversible
Heterolytic Dissociation of Hydrogen Molecules
As discussed in Sect. 7.6.1, the Ag
+ cations in Ag-zeolites are reduced by H 2 to
produce acidic protons (Brønsted acid sites) and Ag
0 metal, as shown in reaction
(7.39); this reaction is partially reversible. The acidic protons originating from the
surface O–H groups in the zeolites were observed using infrared spectroscopy [58].
Baba et al. investigated the surface hydrogen species of Ag-zeolites using
1 H
magic angle spinning NMR (
1 H MAS NMR) spectroscopy. The Ag-zeolites were
treated with hydrogen, and their
1 H MAS NMR spectra were then measured in
the presence of hydrogen [63–65]. Figure 7.2 shows the
1 H MAS NMR spectra
of Ag-A. The Ag-A was placed in contact with H 2 (40 kPa) at 313 K for 30 min
(first hydrogen reduction). The amount of hydrogen consumed was 3.02 × 10
−4
mol g
−1 , which corresponded to the reduction of 21% of the Ag
+ cations in the zeolite.
The
1 H MAS NMR spectrum of the reduced Ag-A was measured in the presence
of hydrogen (Fig. 7.2a), and two peaks were observed at 4.0 and −1.8 ppm. The
peak at 4.0 ppm was unambiguously attributed to the acidic protons (Brønsted acid
sites), because such protons have been observed at chemical shifts of 3.9–4.4 ppm
in various H
+ -exchanged zeolites [66–68]. The peak at −1.8 ppm was a quartet with
an intensity ratio of 1:3:3:1 and a coupling constant of (131 ± 1) Hz, indicating that
Fig. 7.2 1 H MAS NMR
spectra of AgA recorded at
298 K in the presence of
hydrogen (Reprinted with
permission from ref [63].
Copyright 2020 American
Chemical Society). a AgA
reduced with H 2 (40 kPa) at
313 K for 30 min. The
spectrum was recorded under
40 kPa of H 2 . 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 30 min. The
asterisks (*) show the
spinning sidebands