7.7 Unique Properties of Silver Cations …
209
(7.66)
(7.67)
(7.68)
C
H 2 + C 5 H 10
CH 4 + C 4 H 8
C 2 H 6 + C 3 H 6
H 3 C
CH 2 CH 3
CH 3
H
When H-ZSM-5 was used as a catalyst, the ratio of these reaction rates was 10:9:3.
The C–H bond cleavage reaction rates over both Ag- and Zn-ZSM-5 were almost 20
times faster than that over H-ZSM-5, with dehydrogenation being the most predominant among the three primary reactions. Notably, the formation rate of CH 4 (reaction
(7.67)) was also enhanced over Zn-ZSM-5. However, Ag-ZSM-5 completely suppressed both the reactions (7.67) and (7.68), which give CH 4 and C 2 H 6 via C–C
bond cleavage, indicating that Ag-ZSM-5 selectively cleaved the C–H bond of isopentane. On the other hand, Zn-ZSM-5 simultaneously dissociated both the C–H and
C–C bonds of iso-pentane
(5) neo-Pentane cracking
The cracking of neo-pentane proceeds via reaction (7.69) below.
C
CH 3
CH 3
CH 3
H 3 C
CH 4 + C 4 H 8
(7.69)
The primary products in this reaction can be accounted for using only reaction (7.69).
No C–H bond cleavage reaction occurs because the formation of the primary carbenium ions
+ CH 2 C(CH 3 ) 3 by the abstraction of H
− ions via Mechanism 2 hardly
proceeds. Therefore, as shown in Table 7.10, Ag-ZSM-5 depressed neo-pentane
cracking. However, this cracking reaction easily proceeds over Zn-ZSM-5 because
the Zn
2+ cations can cleave the C–C bond of neo-pentane via Mechanism 1. The
formation rate of CH 4 over Zn-ZSM-5 was 28 times as fast as that over H-ZSM-5.
This result shows that Zn
2+ cations directly cleave the C–C bonds of alkanes.
Based on the results shown in Table 7.10, the absolute rates of dehydrogenation
and those of C–C bond cleavage in lower alkanes were determined, and are shown
in Tables 7.11 and 7.12, respectively. These results can be summarized as follows
[98].
(1) Ag-zeolites selectively cleave the alkane C–H bond to produce H 2 more effectively than H-zeolites. For example, in iso-butane cracking, the hydrogen formation rate over Ag-ZSM-5 was 54 times greater than that over H-ZSM-5.
Furthermore, Ag-ZSM-5 hardly catalyzed neo-pentane cracking.
(2) Zn-zeolites directly cleave both the C–H and C–C bonds of alkanes, and enhance
C–C bond cleavage to a greater extent than C–H bond cleavage. For example,
209
(7.66)
(7.67)
(7.68)
C
H 2 + C 5 H 10
CH 4 + C 4 H 8
C 2 H 6 + C 3 H 6
H 3 C
CH 2 CH 3
CH 3
H
When H-ZSM-5 was used as a catalyst, the ratio of these reaction rates was 10:9:3.
The C–H bond cleavage reaction rates over both Ag- and Zn-ZSM-5 were almost 20
times faster than that over H-ZSM-5, with dehydrogenation being the most predominant among the three primary reactions. Notably, the formation rate of CH 4 (reaction
(7.67)) was also enhanced over Zn-ZSM-5. However, Ag-ZSM-5 completely suppressed both the reactions (7.67) and (7.68), which give CH 4 and C 2 H 6 via C–C
bond cleavage, indicating that Ag-ZSM-5 selectively cleaved the C–H bond of isopentane. On the other hand, Zn-ZSM-5 simultaneously dissociated both the C–H and
C–C bonds of iso-pentane
(5) neo-Pentane cracking
The cracking of neo-pentane proceeds via reaction (7.69) below.
C
CH 3
CH 3
CH 3
H 3 C
CH 4 + C 4 H 8
(7.69)
The primary products in this reaction can be accounted for using only reaction (7.69).
No C–H bond cleavage reaction occurs because the formation of the primary carbenium ions
+ CH 2 C(CH 3 ) 3 by the abstraction of H
− ions via Mechanism 2 hardly
proceeds. Therefore, as shown in Table 7.10, Ag-ZSM-5 depressed neo-pentane
cracking. However, this cracking reaction easily proceeds over Zn-ZSM-5 because
the Zn
2+ cations can cleave the C–C bond of neo-pentane via Mechanism 1. The
formation rate of CH 4 over Zn-ZSM-5 was 28 times as fast as that over H-ZSM-5.
This result shows that Zn
2+ cations directly cleave the C–C bonds of alkanes.
Based on the results shown in Table 7.10, the absolute rates of dehydrogenation
and those of C–C bond cleavage in lower alkanes were determined, and are shown
in Tables 7.11 and 7.12, respectively. These results can be summarized as follows
[98].
(1) Ag-zeolites selectively cleave the alkane C–H bond to produce H 2 more effectively than H-zeolites. For example, in iso-butane cracking, the hydrogen formation rate over Ag-ZSM-5 was 54 times greater than that over H-ZSM-5.
Furthermore, Ag-ZSM-5 hardly catalyzed neo-pentane cracking.
(2) Zn-zeolites directly cleave both the C–H and C–C bonds of alkanes, and enhance
C–C bond cleavage to a greater extent than C–H bond cleavage. For example,