GUO Lian et al. / Chinese Journal of Catalysis, 2012, 33: 595–600
Table 2 Catalytic activity of different catalysts
ƻ
ƻ
Ƹ Cubic Pb
Orthorhombic PbO
EC conversion DEC yield
DEC selectivity
Catalyst
2
(
%)
(%)
1.9
11.3
13.4
13.9
15.7
(%)
15.7
32.7
49.8
41.9
43.0
Pure metal Pb
12.1
34.6
26.9
33.2
36.5
PbO
Metal lead sample
Pb + PbO
Fresh lead oxide
2
*
ƻ
2
ƻ
ƻ
ƻ
Ƹ
*
Prepared by reducing lead oxide.
Ƹ
ƻ
ƻ
Reaction conditions: n(ethanol):n(EC) = 10, catalyst concentration 1
wt% , 180 °C, 7 h, 2 MPa.
10
20
30
40
50
ꢀ/( )
60
70
80
90
o
2
pared metal lead sample showed an excellent activity and
selectivity. DEC yield decreased slightly while DEC selectivity
increased over the metal lead sample. Since the prepared metal
Fig. 3. XRD pattern of lead oxide reacted with DEC.
lead sample was composed of metal Pb and PbO , the catalytic
indicating that the presence of DEC was the main reason for the
reduction of PbO.
2
performance of Pb/PbO mixture (mass ratio 1:1) prepared by
2
physical mixing was also evaluated. DEC yield and selectivity
were 13.9% and 41.9%, respectively, indicating that the cata-
To investigate the reaction of DEC and lead oxide thor-
oughly, the gaseous products were collected and analyzed after
reaction and the results showed the presence of hydrogen,
ethylene, and carbon dioxide. Furthermore, the liquid products
were qualitatively analyzed by GC-MS. The result showed that
besides DEC, the main products in this reaction system were
ethanol, diethylether, ethylacetate, and water. Notario and
coworkers [18] found that DEC could decompose to ethanol,
ethylene and carbon dioxide at temperatures of 280–440 °C. In
our blank test, DEC did not decompose at 180 °C but did par-
tially decompose at 180 °C in the presence of lead oxide. So we
suppose that lead oxide can promote the decomposition of
DEC.
lytic activity of Pb/PbO mixture was close to that of the fresh
2
lead oxide catalyst. Hence, we are convinced that the mixture
of metal Pb and PbO is the catalytic agent and the synergistic
2
action between metal Pb and PbO is responsible for the high
2
catalytic activity.
From Fig. 1 we know that tetragonal PbO was transformed
into cubic metal Pb during the DEC synthesis reaction. In order
to study the reason for the reduction of PbO in this reaction
system, the following experiments were designed and con-
ducted.
Experiment No.1: the reaction of ethanol and lead oxide
(
1
reaction conditions: mass ratio of ethanol to lead oxide = 60:1,
80 °C, 7 h). Acetaldehyde and ethylacetate were detected in
Experiment No. 4: to test whether PbO could be reduced to
metal Pb by the reaction products of experiment No. 3, the
reaction between lead oxide and a mixture of diethylether,
ethylacetate, acetaldehyde, and ethanol in the presence of
gaseous mixture of hydrogen, ethylene, carbon dioxide, and
nitrogen studied (reaction conditions: mass ratio of lead oxide
to diethylether to ethylacetate to acetaldehyde to ethanol =
1:1:1:1:60, 180 °C, 7 h) was carried out. The XRD analysis of
the solid sample after reaction revealed that PbO was not re-
duced to metal Pb, indicating that the above reaction system
does not effect this reduction.
the liquid phase after the reaction of ethanol and the lead oxide
sample. Idriss et al. [17] investigated the reaction of ethanol
with a series of metal oxides and found that the main reaction
products in all cases were acetaldehyde and ethylacetate. They
supposed that ethanol tended to dehydrogenate to acetaldehyde
which further reacted to form ethylacetate via the Tishchenko
reaction. The result of the reaction between ethanol and lead
oxide was in agreement with Idriss’s work [17]. Furthermore,
the XRD characterization of the solid sample after the reaction
revealed that no metal Pb was observed, indicating that the
presence of ethanol was not the reason for the reduction of PbO
to metal Pb.
Based on the above experiments, it can be inferred that the
reaction between DEC and PbO is the main reason for the
reduction of PbO to metal Pb.
Experiment No. 2: the reaction of EC and lead oxide (reac-
tion conditions: water as solvent, mass ratio of EC to PbO =
To further analyze the reason for the reduction of PbO to
metal Pb, experiment No. 5 was designed to test the effect of
reaction time on the reaction between DEC and PbO (reaction
conditions: mass ratio of DEC to lead oxide = 60:1, 180 °C, 3
h). GC analysis showed that acetaldehyde appeared at 3 h but
disappeared at 7 h (experiment No. 3) in the reaction solution,
and other products did not change at 3 and 7 h. This means that
the acetaldehyde formed must be further converted to other
products such as ethylacetate [17]. Additionally, the DEC
15:1, 180 °C, 7 h). The XRD analysis of the solid sample re-
vealed that no metal Pb was observed either, indicating that the
presence of EC was not the reason for the reduction of PbO.
Experiment No. 3: the reaction of DEC and lead oxide (re-
action conditions: mass ratio of DEC to PbO = 60:1, 180 °C, 7
h). The XRD analysis of the solid sample illustrated that
tetragonal PbO was transformed into cubic metal Pb (Fig. 3),