N.F. Dummer et al. / Catalysis Today 160 (2011) 50–54
51
Over a range of chromium MCM-41 materials low conversion of
cyclohexane (ca. 4%) and selectivities to benzene (20–40%) were
reported at 573 K [7]. It is therefore apparent that catalysts typ-
ically displaying activity for oxidation reactions require excessive
temperatures to achieve appreciable conversions with cyclohexane
and consequently CO2 formation becomes dominant. If this prob-
lem of purification is to be solved in an energy efficient manner it
is important that improved catalysts are designed.
ient wetness method using aqueous solutions of PdCl2 (Johnson
Matthey) and/or HAuCl ·3H O (Johnson Matthey). For the 2.5 wt%
4
2
Au–2.5 wt% Pd/TiO2 catalyst the detailed procedure used was as
described below. An aqueous solution of HAuCl ·3H O (10 ml, 5 g
4
2
dissolved in water (250 ml)) and an aqueous solution of PdCl2
(4.15 ml, 1 g in water (25 ml)) were simultaneously added to TiO2
(3.8 g). The paste formed was ground and dried at 353 K for 16 h
and calcined in static air, typically at 673 K for 3 h.
Recently, Oyama and co-workers [8] showed that supported
gold catalysts, and Au/TiO2 in particular, could be used for oxida-
tive hydrogenation of propane. This was used to provide propene
which subsequently could be epoxidised. They, therefore, used a
combination of different supported gold catalyst in series; firstly;
Two other catalysts were also used for comparison. 5 wt%
CuO/Al O3 was prepared by impregnation method with
2
Cu(NO ) ·3H O at the desired concentrations. ␥-Al O (Pur-
3
2
2
2
3
2
lox 200 m /g) was used as the support. The sample was dried at
383 K (24 h) and calcined at 673 K (4 h). (VO) P O7 coated with
2
2
Au/TiO followed by Au/TS-1. Interestingly, it was reported that the
silica was a commercial sample supplied by DuPont and has been
previously described and characterised extensively [19].
2
oxidative dehydrogenation of propane was able to occur at tem-
peratures <473 K with these gold-based catalysts and that zeolites,
titania, MgO and Ni/Mo catalysts all required higher temperatures
2.2. Catalyst testing
[
8]. This is not the first report for the potential of gold as a dehy-
drogenation catalyst, since two earlier studies over 40 years ago
alluded to this possibility. Gold was reported to dehydrogenate
cyclohexene by Erkelens et al. [9] in 1963 with gold films, and
Chambers and Boudart [10] demonstrated the reaction with gold
powder in the 1966. The intended target reaction in these two
studies was the hydrogenation to cyclohexane, however, benzene
was observed. The amount of benzene was found to increase with
increasing temperature and decreasing hydrogen pressure. Hence,
it is apparent that gold could act as a dehydrogenation catalyst.
Selective dehydrogenation of cyclohexene has been reported by
Borade et al. [11] with Pd-exchanged ␣-zirconium phosphate. They
report a conversion of ca. 85% with a benzene selectivity of ca. 80%
at 473 K in the absence of oxygen. However, cyclohexane was found
to be secondary product. In addition, palladium based membranes
have recently been used in the production of hydrogen from the
dehydrogenation of cyclohexane [12,13].
Reactions were conducted in a glass flow micro-reactor (i.d.
mm). The reactor temperature was varied and monitored by
a thermocouple located inside the micro-reactor. Catalyst sam-
ples (0.05 g) were pre-conditioned in a He flow (50 ml/min at
3
atmospheric pressure) for 10 min. Followed by a 20% O /He flow
2
(
50 ml/min). Reactant (5 ml) was then injected (@ 1.05 ml/h) into
◦
a pre-heater to vaporise (250 C) and mix with the incoming gas
stream to direct the vapour to the catalyst. Cyclohexane (Chro-
mosolv, ≥99.7%, Riedel de Haën), cyclohexene (purum, ≥99.0%,
Aldrich) and methylcyclohexene (97%, Aldrich) were used as
received. Analysis was conducted with online Varian 3400 GC fit-
ted with FID and DB-Wax capillary column and offline gas samples
with a Varian 3800 GC fitted with TCD and FID detectors. Product
identification was carried out with a Waters GCT premier GC/MS
system fitted with an Agilent HP 5MS capillary column operating
in electron ionisation mode.
Significant work by Goodman and co-workers concerning
Au–Pd supported catalysts for vinyl acetate formation have
attempted to elucidate the effect of Au addition to Pd [14,15].
They report that ensembles of Au–Pd are crucial for activity and
selectivity to the acetate. In this case the separation of Pd atoms
on the surface of the supported particle into monomeric units
improves catalytic performance. Additionally, Au is considered to
decrease product decomposition and the formation of carbona-
ceous deposits.
In this paper we report our initial results for the use supported
gold and palladium catalysts to lower the temperature of oxidative
hydrogenation of cyclohexane and retain selectivity to benzene.
We have used the observations of Oyama and co-workers [8] as
a starting point. In previous studies we have shown that alloying
gold with palladium can lead to significantly enhanced reactivity
for selective redox reactions, in particular alcohol oxidation [16]
and the direct synthesis of hydrogen peroxide [17,18]. In this paper
we show that AuPd/TiO2 is a highly effective catalyst for cyclo-
hexane oxidative hydrogenation to give benzene at low reaction
3. Results and discussion
Our initial experiments were conducted using TiO , 5% Au/TiO ,
2
2
5% Pd/TiO2 and 2.5% Au–2.5% Pd/TiO2 catalysts with cyclohex-
ene and 1-methylcyclohexene as feedstocks. These catalysts were
selected as they have been fully characterised in our previous stud-
ies [16–18,20]. The catalysts comprise dispersions of nanoparticles
in the size range 2–50 nm with most particles in the 8–10 nm par-
ticle diameter range for the 5% Au/TiO2 and 2.5% Au–2.5% Pd/TiO2
catalysts. In addition, the 2.5% Au–2.5% Pd/TiO2 catalysts were
alloys and all the Pd and Au were present together in the same
nanoparticles which have a pronounced core shell structure with a
palladium-rich shell and a gold rich core. Cyclohexene was reacted
with oxygen in the flow reactor and the results are shown in
Table 1. TiO2 was found to be reactive at temperatures as low as
423 K. This is consistent with the earlier study by Oyama and co-
workers [8] where TiO2 was shown to be poorly active at 550 K
for propane oxidative dehydrogenation to propene and tempera-
tures of over 800 K were required to observe high rates of reaction.
As the temperature increases so does the activity of TiO2 but the
selectivity to benzene decreases markedly. Au/TiO2 was signifi-
cantly more active, although as the conversion approached 100%
the selectivity to benzene decreased and CO2 was formed. Addi-
tion of Pd increased the activity and the selectivity of the catalyst
significantly and >99% benzene selectivity could be achieved with
temperatures. However, the Pd/TiO catalyst demonstrates greater
2
activity. The catalyst shows selectivity in the presence of benzene
and so can be considered a potential candidate for the oxidative
hydrogenation of cyclohexane and structurally related impurities
in the commercial production of benzene.
2
. Experimental
1
00% conversion at 423 K. However, the use of the supported Pd
2.1. Catalyst preparation
catalyst exhibited similar levels of activity, although with slightly
reduced benzene selectivity. The secondary product in this case
5
wt% Pd, 5 wt% Au and a range of Au–Pd catalysts were prepared
was not only CO , but also a single oxygen containing hydrocar-
2
by impregnation of TiO (Degussa P25, mainly anatase) via an incip-
bon. This minor product was in trace amounts and at present we
2
Dummer, Nicholas F.
