C.E. Volckmar et al. / Journal of Catalysis 261 (2009) 1–8
7
Table 5
form at the metal–support perimeter, which might be able to co-
ordinate and activate different functional groups of the acrolein
molecule. The exchange of OH-groups of silica with deuterium to
form O–D, which was shown recently [4,11], was faster in the pres-
ence of silver nanoparticles. This also indicates the strong influence
of the metal–support perimeter and thus the kind of support ma-
terial on the activity of silver hydrogenation catalysts [19]. We
recently were able to show, that silver, which in its pure form in-
teracts only very weakly with hydrogen, needs positively charged
sites induced by subsurface oxygen or defects, which might be
stabilised at the metal–support perimeter to be able to dissociate
hydrogen and to be an active hydrogenation catalyst [5,11]. How-
ever, while such influences can not be excluded, it has to be stated
that the catalytic properties correlate with the acidity of the sup-
port, and not with its composition.
Catalytic properties in acrolein hydrogenation and support acidity of Ag/SiO2–Al2O3
catalysts.
Catalysta
20Ag/
20Ag/
20Ag/
20Ag/
20Ag/
20Ag/
SA100-P SA80-P SA60-P SA40-P SA20-P SA00-P
Al2O3 content in support [%]
0
20
1.64
20
34
7
40
2.0
24
29
7
60
1.83
39
32
12
80
1.64
63
38
24
100
1.0
90
42
32
Acidityb [10
mmol/m2]
0.6
89
43
25
−3
Conversionc [%]
Selectivityd [%]
Yielde [%]
a
Amount of Ag = 20 wt%.
b
c
Desorbed amount of NH3 referred to the BET surface.
W /FAC,0 = 15.3 g h/mol.
d
e
Selectivity to allyl alcohol with X = const. (25%).
Allyl alcohol.
A comparison of the catalysts at the same conversion of 25%
At this point, it is worthwhile to highlight some results from lit-
erature: Hájek and Murzin [25] experienced an increase in activity
in the cinnamaldehyde hydrogenation over Pt modified molecu-
lar sieves with increased support acidity. They also experienced a
simultaneously decrease of selectivity towards the unsaturated al-
cohol. Similar results were obtained by the authors over Ru/Al2O3
catalysts [26]. The support material plays an important role in
this process since the adsorption strength of the different com-
pounds is changed by the different supports. The group of Konings-
berger found, that the hydrogen chemisorption properties of sup-
ported metal particles depends on the support ionicity or acid/base
properties, however, they investigated much smaller particles then
those used in our studies [27–31]. A dependency of Lewis acidity
on the activity of metal catalysts supported on silica/alumina with
varied SiO2/Al2O3 ratio was also found by Yasuda et al. [32] and
Venezia et al. [33]. Reschetilowski et al. [34,35] explained metal–
support interaction of catalysts with varying support acidity by
the HSAB concept of Pearson [36] (HSAB: hard and soft acids and
bases). The authors observed an activity loss in hydrogenolysis and
isomerisation reactions with increasing support acidity [34,35].
From these studies it is clear that the acidity of the support
might be a possible explanation for the selectivity patterns that
were observed in the present study. Having in mind that the sil-
ver particles of our catalysts are in strong contact with the support
and probably partly encapsulated, it might be that the interaction
of the metal with the Lewis acid sites of the support leads to a
change in the hydrogenation properties of the silver by influenc-
ing hydrogen adsorption properties/coverage as well as acrolein
adsorption. This would in principle be an electronic interaction,
which is usually observed on smaller particles as those found in
this study. On the other hand, the striking correlation between
acidity and catalytic properties calls for an explanation of the ob-
served behaviour, which goes beyond pure effects of support com-
position.
(Fig. 8) showed that the samples with no or very high Al2O3 con-
tent in the support showed selectivity patterns, which are com-
monly observed in acrolein hydrogenation over supported silver
nanoparticles at high pressure [4,5,9,19]. The selectivity to allyl al-
cohol is approx. 40%, and the extent of consecutive reactions as
well as the formation of by-products is rather low. These sam-
ples are characterised by their low amount and decreased strength
of Lewis acid sites. The observation of a catalytic behaviour sim-
ilar to that of other Ag/support catalysts indicates, that the spe-
cial preparation procedure in the present case does not influence
the catalytic behaviour. However, for the catalysts with medium
alumina content (i.e., 40 or 60%), the selectivity to n-propanol be-
comes significant. At the same time, the selectivity to allyl alcohol
decreases, indicating that allyl alcohol formation and n-propanol
formation are related to each other. Consecutive reaction might
either occur by re-adsorbed allyl alcohol, which is then further
hydrogenated, or, more likely, further reaction of adsorbed semi-
hydrogenated intermediates. From Table 5 it can be seen that a
maximum of total acidity related to the BET surface was reached
for 20Ag/SA60-P and 20Ag/SA40-P with an alumina content of 40%
and 60%, respectively, which are exactly the catalysts that achieved
the lowest selectivity to the desired unsaturated alcohol or the
highest amount of consecutive reaction.
The results of pyridine-IR showed a similar trend of the Lewis
acidity as the results for the total acidity measured by NH3-TPD.
However, it has to be mentioned that the strongest Lewis acid sites
were achieved for 20Ag/SA80-P, 20Ag/SA60-P and 20Ag/SA40-P.
These samples also reached the lowest TOFs as well as the lowest
yields for allyl alcohol of all the samples. Thus, it can be concluded
that the amount and strength of the Lewis acid sites of the sup-
port material strongly influence the catalytic performance of the
catalysts with regard to selectivity and, probably, activity. If the
number and the strength of the Lewis acid sites decrease then an
increase of the yield for allyl alcohol and the TOF is obtained. It is
worth to mention that all Ag/SiO2–Al2O3 catalysts had in common
that there were no Brønsted acid sites detectable at the surface.
Therefore, the acidity of the different samples achieved lower val-
ues than the acidity of crystalline zeolite samples which contain
OH-groups on their surfaces which act as Brønsted acid sites [24].
Various mechanisms can be considered, in which the support
influences the catalytic properties of the deposited metal. While
different particle sizes stabilised by the different support materials
can be excluded from our TEM measurements, it may be that the
shape of the particles is different on the various support materials.
However, on one hand the differences in catalytic behaviour is too
large to be only due to particle shape effects, and from the avail-
able TEM images there is no hint on dominating particle shapes or
differences. Such indirect structural effects by the support can thus
be excluded.
5. Conclusion
Using the precipitation method for catalyst preparation led to
supported silver catalysts with varying SiO2/Al2O3 ratio but almost
equal Ag particle size. Changes in activity and selectivity could be
ascribed to the varying support properties of the catalysts which
were investigated by several characterisation methods.
From pyridine-IR and NH3-TPD it results, that the highest
amount of Lewis acid sites as well as the highest total acidity are
achieved at a medium Al2O3 content. These samples exhibit the
lowest selectivity to allyl alcohol. The strongest Lewis acid sites
were found at low and medium Al2O3 content. These samples also
obtained a low TOF and a low yield of allyl alcohol so that it is
concluded that these strong Lewis acid sites have a significant in-
fluence on the catalytic performance of the catalysts. Increasing
A second possibility would be a direct participation of the sup-
port material in the catalytic reaction. Special active sites might