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plex on the coordinately unsatu-
3
+
[8]
rated Al ions and the pyri-
dine molecules coordinated to
+
[9]
the Na ions, respectively. In
zeolites, except for extra-frame-
work species such as coordin-
3
+
ately unsaturated Al
ions
the
[
10]
acting as Lewis acids,
charge-balancing cations that
act as electron acceptors can
also be considered as Lewis
[
11]
acids. In Figure 1a, the inten-
sity of the bands representing
the Lewis acidic sites (ꢀ1442
À1
and 1454 cm ) of the FAU zeo-
lites displays the shape of a vol-
cano with increasing SiO /Al O
3
Figure 2. The relationship between Lewis acidic sites and catalytic activities of the Pd/FAU catalysts with different
À1
SiO
2
/Al
2
O
3
ratios. Reaction conditions: T=383 K, P=0.2 MPa, gas hourly space velocity (GHSV)=8000 h , CO/MN/
2
2
N
2
=3.3:20:76.7 (vol.%).
ratio. For supported Pd catalysts,
changes in the Lewis acidic sites
still follow this trend, as indicated in Figure 1b. The concentra-
We further correlated the catalytic activities with the Lewis
sites of the catalysts (Figure 2). The turnover frequencies (TOFs,
calculated on the basis of CO conversion per mole Pd per
hour) increased linearly as the amount of Lewis acidic sites in
the Pd/FAU catalysts was increased (Figure 2a). The selectivity
for DMC on the basis of MN followed the same trend, as Fig-
ure 2b indicates. Moreover, a control experiment indicated
that the FAU zeolite itself is inactive in this reaction (Figure S2,
Supporting Information). Accordingly, we can deduce that the
Lewis acidic sites in the catalysts promoted the vapor-phase
carbonylation reactions. However, it is noteworthy that the Pd/
FAU catalysts with high SiO /Al O ratios possess a large
[
12]
tion of the Brønsted and Lewis acidic sites in the Pd/FAU cat-
alysts obtained from the intensity of the corresponding bands
is listed in Table 1. Additionally, in Figure 1a, FAU zeolites with
Table 1. Amount of acidic sites and the loading of Pd in the Pd/FAU cata-
lysts.
+
[a]
[b]
[c]
Catalyst
PyH
mmolg
PyL
[mmolg
Pd
À1
À1
[
]
]
[wt%]
Pd/FAU(2.5)
Pd/FAU(5.5)
Pd/FAU(10)
Pd/FAU(14)
0.09Æ0.02
0.08Æ0.02
0.41Æ0.02
0.35Æ0.02
0.28Æ0.02
0.57Æ0.01
0.13Æ0.02
0.10Æ0.02
0.84
0.85
0.83
0.83
2
2
3
amount of Brønsted acidic sites, which could result in the ex-
cessive decomposition of alkyl nitrite, which would lead to
[3c]
[
[
a] Pyridinium ion on Brønsted sites. [b] Pyridine complex on Lewis sites.
c] Loading of Pd was determined by ICP.
a sharp decrease in activity. Thus, the influence of Brønsted
acidic sites cannot be ruled out as a reason for the low activi-
ties of the Pd/FAU catalysts with high ratios of SiO /Al O . To
2
2
3
exclude the effects of the Brønsted acidic sites and to further
verify the unique role of the Lewis acidic sites, we designed ex-
periments to either eliminate the Brønsted acidic sites or to
keep the amount of Brønsted acidic sites consistent.
high SiO /Al O ratios were produced by hydrothermal de-alu-
2
2
3
minization of FAU(5.5) zeolites, which is widely believed to
À1 [13]
form extra-framework aluminum species (ꢀ1454 cm ) and
[
14]
a large amount of Brønsted sites. This is evidenced by the
results in Figure S1 (Supporting Information), which shows the
As Figure 1a shows, the FAU(5.5) zeolite has only Lewis-type
acidic sites. Additionally, Pd/FAU(5.5) showed the best perfor-
mance in the vapor-phase carbonylation of MN (Figure 2).
Therefore, we passivated the surface acidity of the FAU(5.5)
27
Al MAS NMR spectra of FAU zeolites with different SiO /Al O
3
2
2
ratios.
Moreover, upon loading 1 wt% of Pd (the optimum Pd load-
[16]
zeolite by deposition of a thin silica layer, resulting from hy-
drolysis of tetraethoxysilane (TEOS), to modify the amount of
Lewis acidic sites. Upon treatment with TEOS, the FAU(5.5) zeo-
lites were subjected to conventional ion exchange with aque-
[
5]
ing for this type of reaction ) to the FAU supports, the bands
À1
at approximately 1542 and 1454 cm appeared in the samples
with low SiO /Al O ratios and increased with increasing ratios
2
2
3
[
15]
+
2+
of SiO /Al O . Sato et al. reported that NH4 exchanged with
ous solutions of Pd(NH3)4 ions, the so-called Pd/iT-FAU(5.5)
2
2
3
+
Na to form Brønsted sites and that the zeolite framework par-
tial de-aluminization occurred during the ion-exchange
process.
samples (i indicates the amount of TEOS per gram of zeolite).
It was shown that the acidity distribution of the FAU zeolites
[17]
was homogeneous. Part of the Lewis acidic sites could be
coated with thin silica layers by chemical liquid deposition
The catalytic properties of the Pd/FAU catalysts with differ-
ent SiO /Al O ratios were examined under the same reaction
[16a]
(CLD) of TEOS.
Thus, we investigated the influence of Lewis
2
2
3
[
5]
conditions in a continuous fixed-bed microreactor. For
a given loading of Pd (Table 1), the Pd/FAU catalysts with dif-
ferent SiO /Al O ratios exhibited different catalytic activities.
acidity through quantification of the Lewis acidic sites.
After treatment with TEOS, no obvious differences were ob-
served in the physical properties of the FAU zeolites. In com-
2
2
3
ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 2174 – 2177 2175