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hexanol as the major product. The anDalOyIs:i1s0.o10f39th/Ce7CrCe0a4c7t5io9nB
progress over time for the Ni-Re (1:2)/TiO2 catalyst (Fig. S13)
showed that at lower reaction times of 1 to 4 h, the selectivity
to 1-hexanol is over 80% and it gradually decreases as the
reaction progresses to approximately 60% at full conversion.
This selectivity loss is due to the secondary conversion
processes dominated by hexanol hydrogenolysis.
In hydrogenation of methyl hexanoate in octane, the
performance of high-loading Re/TiO2 catalysts was comparable
to that of Ni-Re (1:2)/TiO2, whereas the Ni-only catalyst was
completely inactive (Table 1, Table S2). The use of hexane as a
solvent appeared to be crucial for the hydrogenation activity of
the monometallic Ni/TiO2 catalyst as well as for the enhanced
performance of the bimetallic catalysts. We tentatively
attribute this to the effects related to competitive adsorption
from the liquid phase to the surface.
Table 1 Evaluation of Ni-Re (x:y)/TiO2 catalysts with varying compositions in the
hydrogenation of methyl esters in different alkane solvents.
X,
%
0
Selectivity to , %
alcohol alkane other
Ni:Re
Substrate
Solvent
Octane
1:0
5:2
1:1
-
-
-
10
74
70
67
65
5
81
56
37
11
16
63
34
92
67
76
76
80
20
74
87
57
2
2
6
7
Methyl
Hexanoate
25
12
14
6
76
16
4
29
95
5
20
32
1:2
12
10
14
4
10
9
14
3
2
1:2[a]
0:1[b]
1:0
5:2
1:2
0:1
1:0
5:2
1:2
0:1
Methyl
Octanoate
Hexane
Hexane
Methyl
Decanoate
93
69
61
11
7
The substrate scope was next extended to methyl octanoate
and methyl decanoate to investigate the influence of the
substrate chain-length. Ni-Re (5:2) and Ni-Re (1:2)/TiO2
catalysts were exceptionally active for the hydrogenation of
both methyl octanoate and methyl decanoate in hexane,
respectively. In terms of conversion, Ni-Re (5:2)/TiO2 was found
to be critically sensitive to the length of substrate side-chain (X
= 81 % with Me-octanoate vs. 16% with Me-decanoate) while
selectivity towards the alcohol product peaked at 93%. Both
high conversion and selectivity could be obtained over Ni-Re
(1:2)/TiO2. Critically, the synergy between the two metals is
clear for the hydrogenation of both methyl octanoate and
methyl decanoate esters in hexane solvent when moving from
pure Ni or pure Re to the bimetallic Ni-Re/TiO2 systems.
Next, Ni-Re (1:2)/TiO2 catalyst was evaluated for the
hydrogenation of hexanoic acid under mild reaction conditions.
The efficient hydrogenation of hexanoic acid in octane solvent
(Table 2) can be achieved at temperatures as low as 120 °C (X =
35%, S = 82%). Upon raising the reaction temperature to 150 °C
the yield of 1-hexanol increased at the expense of the product
selectivity. The hydrogenation of carboxylic acids proceeds at
lower temperatures relative to that of their corresponding
methyl esters. In fact, hydrogenation of hexanoic acid at 180 °C,
that is the temperature used for ester hydrogenation, is
Conditions: Catalyst (18 mg), Substrate (0.44 mmol), Solvent (3.5 mL), n-decane
[a]
(35.2 µL), 180 °C, 50 bar H2, 8 h, 1000 rpm.
Catalyst recycling with an
intermediate reduction of the reused catalyst in 10% H2 flow at 300 °C for 1 h; [b]
18 wt% Re/TiO2
Temperature programmed reduction (TPR) shows that all
bimetallic Ni-Re/TiO2 catalysts could be reduced at
temperatures below 350 oC (Fig. S13). Among the Ni-Re
catalysts dispersed on different supports, the reducibility of the
TiO2-supported ones was highest. A somewhat lower H2-to-
metal ratio observed for the bimetallic catalysts (Table S1) is
attributed to the prevalence of sub-nanometre-sized cluster
species strongly interacting with the oxide support.
X-ray photoelectron spectroscopy (XPS) analysis of the
reduced catalysts (Fig. 3, S14,15) showed the co-existence of Ni0
and Ni2+ species and a high heterogeneity of Re species in
different oxidation states on the catalyst surface. For the
monometallic Re/TiO2 catalysts, (Fig. 3(a,b), S15) the increase in
Re loading resulted in the increase of the percentage of metallic
Re from 17 to 40% at the expense of the overall contribution
from the oxidized species (ReOx, Re2+, Re4+, Re5+ and Re6+),
which relative abundance was not affected by the Re contents.
For the bimetallic Ni-Re (1:1)/TiO2 system with a nominal Re
loading of 9.5 wt.%, the fraction of Re0 (37%) is ca. twice higher
than that in the corresponding 9 wt.% Re/TiO2 catalyst (17%).
Interestingly, the addition of Ni eliminated the Re2+ fraction in
the bimetallic sample, while the fractions of all other Re valance
states were unchanged relative to each other. The XPS data
suggest that surface enrichment of metallic Re species,
particularly in bimetallic Ni-Re catalysts, gives rise to an
enhanced performance in methyl ester hydrogenation.
detrimental to the selectivity (S
= 60%) due to the
accompanying side-reactions such as decarboxylation,
dehydration and trans re-arrangement reactions.
Table 2. Hydrogenation of hexanoic acid with Ni-Re (1:2)/TiO2 catalyst.
Selectivity to , %
T, °C
X, %
hexanol
82
70
hexane
4
9
other
14
21
120
150
180
35
72
100
Further catalytic tests reveal that the chain length of both
solvent and substrate impact strongly the activity trends as well
as the extent of the Ni-Re synergy. Table 1 shows that for
hydrogenation of methyl hexanoate in octane the addition of
60
26
14
Conditions: Catalyst (18 mg), substrate (0.44 mmol), octane (3.5 mL), n-decane
small amount of Re to Ni/TiO2 made the catalyst active with a (35.2 µL), 180 °C, 50 bar H2, 8 h, 1000 rpm.
high selectivity to the 1-hexanol product (S = 92%). Further
The catalyst could be successfully re-used after reduction in
a flow of 10% H2 (300 °C / 1 h). No significant loss of conversion
and no change in product selectivity were observed with the
addition of Re was beneficial in terms of conversion (Xmax
=
74%), but was accompanied by some variation in product
distribution. Small amounts of alkane (hexane) and trans-ester
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