ucts in methanol. The effect of solvent on the activity
and selectivity of hydrogenation reactions has been
reported previously.[12] However, there is little infor-
mation related to TTMA hydrogenation. Therefore,
we further investigated TTMA hydrogenation reac-
tions in six different solvents, including methanol,
ethanol, 1-butanol, acetone, water, and toluene. A
lower temperature setting, 1208C,[7] was used to
compare the hydrogenation efficiency (Table S2–S3).
It was found that acetone exhibited the highest se-
lectivity among the solvents tested (Figure S2). The
selectivity toward the formation of 1 was also in-
creased along with the increase in alcohol chain
length from methanol to both ethanol and butanol.
The observed solvent effect could be attributed to a
number of factors, such as thermal interaction be-
tween TTMA and the solvent molecules,[12a] competitive ad-
sorption onto the catalytic sites,[12b] and H2 dissolvability in sol-
vents,[12a] or others. We also found that the Re/TiO2 catalyst did
not hydrogenate aromatic rings in toluene or phenolic com-
pounds, such as phenol and benzene ethanol. This finding
demonstrates that rhenium has the potential to selectively
convert lignin-derived muconic acid to adipic acid without de-
stroying aromatic structure.
Scheme 1. Formation of three esters from hydrogenation of TTMA in methanol.
ca. 30% ester 3 and ca. 57% partial hydrogenated ester 2 with
a slightly higher production rate of ester 1 than the blank. The
hydrogenation yields obtained from rhenium supported on
g-Al2O3, ZSM-5, SBA-15, and MgO were similar with ca. 30%
ester 1, ca. 60% ester 2, and ca. 10% ester 3. Significantly, a
high hydrogenation activity and selectivity were obtained over
the Re/TiO2 catalyst. The yields of esters 1 and 2 were 88%
and 12%, respectively, with no ester 3 detected after 5 h of re-
action. Such hydrogenation yields are similar to results ob-
tained with precious metal catalysts,[6,7] and are the highest
yields achieved with non-precious metal catalysts so far report-
ed.[3,4] These supported rhenium catalysts did not catalyze hy-
drogenation/hydrodeoxygnation of the carbonyl (C=O) in
TTMA under the reaction conditions tested.
In conclusion, we have demonstrated that metal oxide-sup-
ported rhenium catalysts can catalyze hydrogenation of TTMA
and produce adipic acid in high yield. The hydrogenation is
highly selective toward alkene double bonds while no hydro-
genation on aromatic rings or hydrogenation/hydrodeoxyge-
nation of carboxylic group was observed. The results from this
study also open up several interesting questions related to sol-
vent effects, and mechanisms of supported rhenium in hydro-
genation reactions (i.e., support effect, the intrinsic nature of
catalytic sites) for further investigation.
The rhenium loading on Re/TiO2 was found to be much
lower than on Re/g-Al2O3 or Re/SBA-15 (Supporting Informa-
tion, Table S1). The higher hydrogenation selectivity of Re/TiO2
toward C=C bonds in TTMA is likely due to the different distri-
bution of active sites of rhenium on TiO2 compared to other
oxide supports. The distribution of active sites of rhenium on
supports has been shown to play a dominant role in the hy-
drogenation of ethyl acetate.[8d] However, little is known about
the influence of this parameter on the hydrogenation of C=C
bonds. For supported rhenium catalysts, the structure of sur-
face rhenium sites is generally recognized as a monolayer, irre-
spective of the rhenium loading. This structure consists of
three metal oxo-bonds and one Re-O-support bridge bond, as
proposed by Lee and Wachs.[10] They also discovered that the
bonding of rhenium to alumina is much stronger than that of
rhenium to TiO2.[10c] The weak binding of rhenium to the TiO2
support led to a high ratio of reduced rhenium sites formed
on the surface, which confers onto the catalyst an improved
hydrogenation activity. Beside this effect, a number of other
factors can also affect the activity and selectivity of Re/TiO2, in-
cluding surface area of the support, crystalline phase of the
TiO2, and its semiconductor propensity.[11] The exact mecha-
nism contributing to the high selectivity and activity of Re/TiO2
in catalyzing TTMA hydrogenation deserves further
investigation.
Experimental Section
Supported rhenium catalysts were prepared by the incipient wet-
ness impregnation method using an aqueous solution of ammoni-
um perrhenate (NH4ReO4, 99+%, STREM chemicals) as precursor.
The catalytic performances of the supported rhenium catalysts for
hydrogenation of muconic acid, phenol, and benzene ethanol
were evaluated by using a combinatorial high throughput reactor
system (detail see supporting information).
Acknowledgements
This work was supported by Environment Canada under the Asia
Pacific Partnership on Clean Development & Climate Program,
and Washington State’s STAR Researcher grant. We thank Sasol
North America for the g-Al2O3 sample, Dr. Kake Zhu for the SBA-
15 sample, and Dr. Haiying Wan for the meso-ZSM5 samples. We
also thank Karl Albrecht for valuable suggestions to the manu-
script.
TTMA has a low solubility in many solvents. The esterifica-
tion reaction likely improves the solubility of the reaction prod-
1072
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ChemSusChem 2011, 4, 1071 – 1073