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ARTICLE IN PRESS
B. Güvenatam et al. / Catalysis Today xxx (2013) xxx–xxx
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may be that cyclohexenol is converted directly to CyO=. This inter-
mediate has not been observed in our experiments. The subsequent
hydrogenation of the keto-group to the main reaction product,
CyOH, is slow, especially over Pd/C. This step is inhibited at low
temperature, even in the presence of an acid and might be the
rate-determining step for HDO of PhOH. The acid promoter, how-
Only under acidic conditions, complete HDO of PhOH to CyH
was achieved. CyH= (cyclohexene) is typically considered as an
intermediate product in CyOH conversion to CyH under acidic con-
ditions [4,5,7,25]. Our study further supports this proposition. Small
amounts of CyH= were observed during PhOH (Fig. 2c and d) as well
as CyOH conversion on Pt/C under acidic conditions at 473 K (see
supporting information, Table S1). For most of the other catalysts,
the hydrogenation of CyH= was so fast that it was not possible to
detect this intermediate compound.
3.1. HDO of phenol
conditions (Fig. 2a and b), PhOH is converted to cyclohexanol
(CyOH) via intermediate formation of cyclohexanone (CyO=). The
with previous studies [7], the combination of acidic conditions
and elevated reaction temperature resulted in almost complete
(Fig. 2d). Comparison of results presented in Fig. 2c and d sug-
gests that hydrogenation of the CyO= intermediate is inhibited at
423 K and low pH. Deoxygenation of CyOH is not catalyzed at this
ate reaction times (see Fig. 2c). In line with previous findings [15],
our results demonstrate the necessity of Brønsted acidity for phe-
nol HDO and that high selectivity can be achieved at the elevated
temperature (473 K).
Zhao et al. [7] reported that Pd/C, Pt/C, Ru/C and Rh/C show
similar activity and selectivity in PhOH conversion at 473 K and
a H2 pressure of 50 bar. However, the present results obtained at
lower H2 pressure (20 bar) point to pronounced differences in the
catalytic behavior of noble metal catalysts (Table 1, Figs. S1–S3).
While the reaction over Ru/C proceeds via CyOH formation (Fig.
S2) similar to Pt/C, for Pd/C CyO= is identified as a major reaction
intermediate under acid-free as well as acidic conditions (Fig. S1).
The high intermediate yield of CyO= observed in the course of the
reaction under acid-free conditions points to low activity of Pd/C
toward hydrogenation of the polar carbonyl group in CyO=. This
conclusion also follows from the observation that when the reac-
S1). For Ru/C, on the other hand, the dehydroxylation of the CyOH
is slow, even at elevated temperature and under acidic conditions.
After 2 h of reaction, a lower PhOH conversion to CyOH is achieved
for Ru/C (Table 1, entry 10) as compared to Pt/C (Table 1, entry 5).
It is important to note that in the former case the selectivity to the
CyO= intermediate is much lower, suggesting faster hydrogenation
of the polar C=O moiety by Ru/C than by Pt/C and Pd/C (see sup-
porting information). Wildschut et al. [24] also showed that Ru/C
promotes fast hydrogenation of CyO=, which was not observed as
an intermediate component in the conversion of PhOH to CyH at
relatively high temperature and pressure (523 K, 200 bar).
We further considered HDO of guaiacol, which is a more realistic
model compound for lignin, over the carbon-supported Pt, Pd and
Ru catalysts. The main results of the catalytic tests are summarized
in Fig. 4. Similar to PhOH conversion, the main reaction products are
due to fast hydrogenation of the aromatic ring. The highest hydro-
resulting in a conversion of 75% after 2 h. For Ru/C a reaction time
of 4 h was needed to reach this conversion level (Fig. 4c). Pd/C was
the least active catalyst. Only 50% guaiacol conversion was reached
after 4 h reaction time with Pd/C (Fig. 4b). Guaiacol HDO over Pt/C
is strongly enhanced in the presence of H3PO4 (Fig. 4d). In this
case, the maximum 75% conversion of guaiacol was reached already
within 0.5 h reaction time. In all cases, the conversion of guaiacol
was lower than for HDO of phenol. The lower reactivity of guaiacol
has previously been attributed to the electron-donating hydroxyl
group that stabilizes the transition state carbocation decreasing the
hydrolysis rate of the methoxy group [7].
The main reaction products include products from aromatic
ring hydrogenation, demethylation, demethoxylation and dehy-
droxylation of guaiacol. The reaction selectivity strongly depends
on the nature of transition metal catalyst and the presence of
the Brønsted acid promoter (Fig. 4). Under acid-free reaction
conditions, substantial selectivity toward the ring hydrogenation-
product, methyl-1,2-cylohexanediol, is observed for all catalysts.
This product is formed by ring hydrogenation and methyl group
transfer reactions of guaiacol [27]. When the reaction is performed
under acid-free conditions over Pt/C catalyst, the main product
is CyOH formed with a selectivity of around 70%. The selectivity
to the second most abundant product, methyl-1,2-cylohexanediol,
is ca. 25%. In addition, minor amounts of PhOH, CyO= and cyclo-
hexylmethyl ether were observed as intermediates (see supporting
information, Fig. S4). It is important to note that although guaia-
col conversion over Pt/C steadily increased from nearly 40% to 80%
from the beginning of the reaction (0.5 h) till the end (4 h), the prod-
uct distribution remained largely unchanged. This suggests that the
main reaction products, methyl-1,2-cyclohexanediol and CyOH, are
formed over Pt/C via different reaction routes.
Ru/C is the only catalyst for which the product of direct deoxy-
genation of PhOH was observed under acidic conditions. Nearly
7% selectivity to benzene was obtained after 4 h reaction at 473 K
low for this catalyst and did not depend on the pH. This may
be due to structural transformation of the support under hydro-
Methylcyclopentane formation under acidic condition on Ru/Al2O3
can be explained via ring–contraction and proton transfer isomer-
ization of CyH. Such a reaction sequence requires metallic and
acidic sites [7,26]. Its reversibility [26] explains the decrease in
selectivity of 1-methylcylopentane and increase of CyH for pro-
longed reaction times (Fig. S3b).
With Pd/C, the predominant reaction product formed with
selectivity of ca. 60% is methyl-1,2-cyclohexanediol (Fig. 4b).
Although the selectivity toward CyOH increases in the course of the
reaction, it reached only 25% after 4 h. The saturated ketone prod-
45% to 2-methoxycyclohexanone is observed after 0.5 h of reaction,
this intermediate is completely converted, most likely to CyOH,
via demethoxylation and hydrogenation during the reaction. CyO=,
PhOH and methylcyclohexyl ether (Fig. 5) were also observed in
A mechanism of PhOH hydrodeoxygenation is proposed in Fig. 3.
The first reaction step is rapid hydrogenation of the aromatic ring
over the noble metal resulting in the CyO= intermediate. In fact,
CyO= can be formed via fast isomerization of the cyclic keto/enol
transformations pathway between cyclohexenol and CyO= [7]. It
Please cite this article in press as: B. Güvenatam, et al., Hydrodeoxygenation of mono- and dimeric lignin model compounds on noble metal