B. F. Sels et al.
tributed to changes in the properties of the Ru metal clusters
under the influence of the enhanced basicity of the lattice. It is
expected that these changes occur at the level of the residual
electron density on the clusters. How this mechanistically af-
fects CLA formation will be discussed below. The highest CLA
selectivity is obtained with the Ru/Cs-USY(40) catalyst, namely,
82% at a conversion of 82% (Table 2, entry 14), which is much
higher than that of the Ru/C catalyst, namely, 63% at a conver-
sion of 58% (Table 2, entry 1). Ru/C shows a higher selectivity
towards cis-9,trans-11- and trans-10,cis-12-CLA isomers, where-
as Ru/Cs-USY is more selective towards the formation of trans,-
trans-CLAs. This issue will be further discussed below.
In a next step, the Ru dispersion was determined by CO
chemisorption, TEM analysis and extended X-ray absorption
fine-structure (EXAFS) measurements. Ru dispersion from CO
chemisorption was very high, namely, 87%, which indicated
the presence of highly dispersed Ru. With TEM, no Ru clusters
could be detected, whereas EDX measurements at different
points on the zeolite confirmed the presence of Ru, implying
that Ru was indeed highly dispersed throughout the zeolite
crystal.
While ML isomerisation activity of such highly dispersed Ru
was not known, its existence has been described before.[18]
From the literature, it is known that activation conditions are
crucial to arrive at such high Ru dispersions.[18a,19] As described
in the Experimental Section, [Ru(NH3)6]3+-exchanged zeolite
was first heated under N2 to 3508C. MS analysis of the decom-
position gases revealed loss of H2O (from room temperature to
1508C) and NH3 (from 220 to 3508C), corresponding to zeolite
dehydration and decomposition of the Ru complex. In a fur-
ther step, the catalyst was reduced under a flow of H2 at
4008C (58CminÀ1). Almost no hydrogen was consumed, in
agreement with previous work using a Ru/Na-Y(2.4) catalyst.[18a]
Experimentally, it was confirmed that by omitting the reduc-
tion step, high conversions were also obtained, although the
CLA yield decreased from 75 to 55 wt% (Table 3, entries 2 and
3). Hence, no hydrogen was required when highly dispersed
Ru/USY catalysts were used for the production of CLA, or in
the pre-treatment procedure of the catalyst or during the iso-
merisation reaction.
It is important to mention that Ru/Cs-USY shows very low
selectivity for hydrogenated products due to the absence of a
hydrogen donor. In the presence of molecular hydrogen[12d–f]
or after pre-activation under hydrogen,[12b] higher selectivities
for hydrogenated products have invariably been reported.
Determination of active sites
From the above experiments it follows that superior CLA pro-
duction occurred with the Ru/Cs-USY(40) catalyst. NMR spec-
troscopy measurements of the solvent, namely, n-decane, after
the reaction, revealed that the solvent was inert under the re-
action conditions. To identify the active centres responsible for
the isomerisation reaction, tests were performed by using Cs-
USY(40) devoid of Ru. After reaction for 1 h, a CLA yield of only
1 wt% and a conversion of 5 wt% was obtained, compared
with a CLA yield of 75 wt% and a conversion of 94 wt% for
added Ru (Table 3, entries 1 and 2). Hence, the ML isomerisa-
tion activity towards CLAs can be assigned to the presence of
Ru. If no Ru is present, 3 wt% of coke is formed after 1 h,
whereas 5 wt% of coke is formed with the Ru/Cs-USY catalyst.
In agreement with the literature, this indicates that not only
the support, but also Ru contributes to the formation of
coke.[12a,14] However, it cannot be excluded that the higher
amount of coke for Ru/Cs-USY might also be associated with
the higher activity of this catalyst with respect to ML isomerisa-
tion to CLA.
From a previous report it is known that highly dispersed
nano-sized metallic Ru clusters in zeolite Y, are easily oxidised
at room temperature.[18a] This was confirmed with the Ru/Cs-
USY catalyst by means of an O2 titration experiment at room
temperature, immediately after the activation procedure, with-
out making contact with air. The high uptake of O2 at room
temperature (with Ru/O= ꢁ2) proved that metallic Ru was
rapidly converted to RuO2, in agreement with the literature.
EXAFS measurements also confirm the presence of Ru4+ spe-
cies, which are structurally similar to RuO2 species.[20] The
ruthenium K-edge X-ray absorption near-edge structure
(XANES) spectrum of Ru/Cs-USY(40) is compared with anhy-
drous RuO2, as well as (hydrated) RuCl3, in Figure 1. The ruthe-
nium K-edge energies and the detailed XANES features of the
sample are very similar to that of anhydrous ruthenium oxide,
but significantly different from ruthenium chloride, indicating a
very similar local geometry and similar electronic properties
between the sample and anhydrous RuO2. This indicates that
the ruthenium metal clusters in the catalyst after reduction are
oxidised to Ru4+, due to contact with air/oxygen at room tem-
perature. Fourier transform (FT) of the EXAFS data (Figure 2)
confirms the structural similarities between the anhydrous
RuO2 and the Ru/Cs-USY(40) sample. A similar radial distribu-
tion of the same neighbours is observed, albeit at longer dis-
tances (from ꢁ3.0 ꢂ) significantly fewer contributions are ob-
served for the catalyst sample. EXAFS analysis suggests octahe-
dral coordination of O around Ru with two RuÀO bonds with
distances of 2.01 ꢂ, four RuÀO bonds with distances of 1.95 ꢂ
and one Ru···Ru bond with a RuÀRu distance of 3.16 ꢂ. Where-
Table 3. Isomerisation of ML with various USY(40) catalysts, using differ-
ent activation procedures.[a]
Entry
Catalyst
Activation
XML
YCLA
YNC
YHP
YCP
1
2
3
4
5
Cs/USY
N2/H2/air[b]
N2/H2/air[b]
N2/air
5
94
83
85
100
1
75
55
57
55
0
10
19
15
20
0
2
3
2
7
3
5
5
9
15
Ru/Cs-USY
Ru/Cs-USY
Ru/Cs-USY
Ru/Cs-USY
N2/O2/air[c]
[d]
N2/H2
[a] Reaction conditions: 1658C, [ML]=7 mmolLÀ1, 0.8 g 0.5Ru/USY(40),
60 min; abbreviations are the same as those given in Table 2. [b] Under
N2 and H2 up to 350 and 4008C, respectively, followed by room-tempera-
ture transfer from a flow to a batch reactor in air. [c] Under N2 up to
3508C, followed by room-temperature contact with
a flow of O2.
[d] Transfer of the reduced catalyst from a flow to a batch reactor under
inert conditions.
760
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2011, 4, 757 – 767