Geoscience Reference
In-Depth Information
catalysts. These materials have been the most promising class of 3D host frame-
works with controlled microporosity. Atomic-scale engineering of catalytic func-
tions of isolated redox sites in the confined environments of zeolitic channels is
also of great interest.
Rabo et al.
[159,160]
showed that Na, Ca, Be, and La forms of Z,Y containing
residual protons reacted with NaCl, at 575
675 K, with the evolution of HCl.
Clearfield et al.
[161]
and Clearfield
[162]
demonstrated the introduction of
cations, such as Zn
2
1
,Cu
2
1
,Co
2
1
,Ni
2
1
,Mn
2
1
,Cr
3
1
,Cr
5
1
,Mo
5
1
,Ga
3
1
, and
V
4
1
, into zeolites A, X, and Y, by the reaction of their chlorides with the H forms
of the zeolites phosphates and molybdates.
Figure 6.47
shows the activity of Cr/H-
ZSM-5 in CH
4
burnt at 500
C
[164,165]
. The catalytic activity of the Cr/H-ZSM-5
system increased linearly with the rise of chromium content up to 1.5 wt%, show-
ing considerable site homogeneity. The activity of the catalyst reached a maximum
point when the number of isolated Cr
5
1
cations capable of changing valence state
upon redox treatment reached maximum. Stabilization of isolated Cr
5
1
ions in a
coordinatively unsaturated environment inside H-ZSM-5 may be a reason for such
unusually high intrinsic catalytic activity
[166]
. Similarly, a complete oxidation of
CH
4
over Mn/H-ZSM-5, Cu/H-ZSM-5, and V/H-ZSM-5 has also been studied in
detail by Kucherov et al.
[166]
. Ethane oxidation with N
2
O over H-ZSM-5 and Cu/
H-ZSM-5 has been studied by Kucherov et al.
[166,163]
. The C
2
H
6
oxidation by
N
2
O, as distinct from O
2
, proceeds on both H-ZSM-5 and Cu/H-ZSM-5. At
T
.
430
C, a stable 100% conversion took place on H-ZSM-5, but the decrease in
temperature to 410
400
C led to a drastic drop in C
2
H
6
and N
2
O conversion with
the fast cooking of zeolite. The oxidation of ethane with N
2
O was treated as a com-
plex heterogeneous
homogeneous process (catalytic decomposition of N
2
O and
subsequent gas-phase C
2
H
6
oxidation with the participation of excited molecules
and coupling products)
[163,167]
. Thus, the modification of pentasils by transition
Figure 6.47 Activity of Cr/H-ZSM-
5inCH
4
burnt at 500
C
[163]
.
SV per 1 Cr
5+
SV per 1 Cr
40
16
h
-1
h
-1
2
8
20
1
*
Cr/SiO
2
Cr (
wt%)
1
2
3
