Function of metals and supports on the hydrodeoxygenation of phenolic compounds

Article abstract of DOI:10.1002/cplu.201402145

Hydrodeoxygenation (HDO) is an important process for removing oxygen from lignin-derived phenolic monomers to obtain chemicals that can be used as fuel or fuel additives. A systematic study is performed to check the effects of supports (acidic, neutral, basic) and noble metals (Pd, Pt, Ru) on the HDO of phenol, guaiacol, and eugenol. Evaluation of the combinations of metals and supports under the similar reaction conditions shows that the metals supported on a highly acidic support (SiO₂-Al₂O₃) yield complete hydrogenation products with the possibility of C-O bond cleavage to achieve a real HDO activity, whereas on a mildly acidic support (γ-Al₂O₃), a complicated product distribution is achieved, and neutral (C) and basic (HT) supports give restricted hydrogenation activity but yield the products with very high selectivity. On the basis of the results, reaction pathways are suggested and deliberated. The catalysts show reproducible activity in recycle runs. The catalysts are characterized by various techniques (XRD, TEM, TPD, ICP-OES) to establish the catalyst activity-property relationship.

Full text of DOI:10.1002/cplu.201402145

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DOI: 10.1002/cplu.201402145
Function of Metals and Supports on the
Hydrodeoxygenation of Phenolic Compounds
[
a]
Ayillath K. Deepa and Paresh L. Dhepe*
Hydrodeoxygenation (HDO) is an important process for remov-
ing oxygen from lignin-derived phenolic monomers to obtain
chemicals that can be used as fuel or fuel additives. A system-
atic study is performed to check the effects of supports (acidic,
neutral, basic) and noble metals (Pd, Pt, Ru) on the HDO of
phenol, guaiacol, and eugenol. Evaluation of the combinations
of metals and supports under the similar reaction conditions
shows that the metals supported on a highly acidic support
possibility of CꢀO bond cleavage to achieve a real HDO activi-
ty, whereas on a mildly acidic support (g-Al O ), a complicated
2
3
product distribution is achieved, and neutral (C) and basic (HT)
supports give restricted hydrogenation activity but yield the
products with very high selectivity. On the basis of the results,
reaction pathways are suggested and deliberated. The catalysts
show reproducible activity in recycle runs. The catalysts are
characterized by various techniques (XRD, TEM, TPD, ICP-OES)
to establish the catalyst activity–property relationship.
(
SiO -Al O ) yield complete hydrogenation products with the
2 2 3
Introduction
As a result of the drawbacks associated with the fossil feed-
stock, researchers are focusing increasingly on the develop-
ment of new methods for the conversion of abundantly avail-
able lignocellulosic biomass (mainly crop waste) into chemicals
examining the activity of catalysts, because similar types of
compounds are obtained upon lignin depolymerization.
Several attempts have been made to design stable and reus-
able catalysts with the aim of improving the hydrodeoxygena-
tion activity. The literature suggests that above 573 K, hydro-
deoxygenation of phenolic compounds is possible over sulfid-
ed CoMo and NiMo catalysts, but drawbacks associated with
these catalysts, such as coke formation, low water tolerance,
[
1]
and fuels. Lignin, a phenolic polymer, is one of the major
components (15–25%) of lignocellulosic biomass, and is ac-
knowledged to be high in energy density compared with
other components of lignocellulosic biomass, namely, cellulose
and hemicellulose. Upon depolymerization, lignin can produce
bio-oils rich in phenolic compounds, and hence, it has been at-
[4,5]
and sulfur contamination, hamper their catalytic activities.
Therefore, instead of these catalysts, several other metal (Fe,
Cu, Ni, Co, W, Re, Mo, Pt, Ru, Pd) catalysts under different reac-
[
2]
tracting a lot of attention recently. In many cases, lignin de-
polymerization reactions are not very selective, so several
products are formed depending on the reaction conditions
used. The phenolic bio-oils obtained from lignin depolymeriza-
tion have higher “O” contents, and thus, it is desired for them
to be upgraded to make them suitable for use as transporta-
[5c,6–38]
tion conditions
have been evaluated for their HDO activi-
ty. Reports suggest that compared with noble metal catalysts,
other metals show poor activities, and hence, recent works
have been devoted mainly to the utilization of noble-metal-
based catalysts. Acidic conditions may enhance the HDO activi-
[
2]
[12–15,39]
tion fuels. For an increase in its fuel efficiency, this phenolic
bio-oil mixture has to be subjected to hydrodeoxygenation
ty,
so the use of a bifunctional combination of a stable
carbon-supported noble metal catalyst (Pd/C) and a homogene-
ous acid (H PO ) to convert phenolic bio-oil components (phe-
[
3]
(
HDO) reactions. Most of the lignin-derived phenolic com-
3
4
pounds contain several substitutions such as, alkyl, alkoxy, hy-
droxyl, olefinic double bonds, and so on; hence, in the upgrad-
ing reactions of these types of compounds, there is typically
a competition between HDO and the hydrogenation (reduc-
tion) of double-bond/ring or aldehyde/ketone groups. It is
widely assumed that phenolic compounds such as guaiacol,
phenols, syringol, eugenol, and so on are best compounds for
nols, guaiacols, and syringols) with the possibility of forming
[12,13]
cycloalkanes and methanol has been reported.
The draw-
backs associated with the use of homogeneous acids were
overcome by replacing them with a solid acid catalyst such as
Nafion/SiO , but its lower thermal stability was proved to be
2
detrimental, as these reactions are generally conducted above
[15]
473 K. Furthermore, C6–C9 cycloalkanes and methanol have
been produced over a combination of Pd/C and H-ZSM-5 cata-
[7]
lysts. Recently, Pt/C catalysts were reported, to avoid the use
of acidic supports, which can lead to coke formation, but to
[a] A. K. Deepa, Dr. P. L. Dhepe
Catalysis and Inorganic Chemistry Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pune 411008 (India)
Fax: (+91)20-25902633
[11]
achieve HDO activity. It is evident from the published reports
that several researchers have devoted much effort to the eval-
uation of the catalytic activities of different metals impregnat-
ed on a variety of supports for HDO activity under different re-
action conditions; hence, through direct comparison of their
E-mail: pl.dhepe@ncl.res.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402145.
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ꢀ
1
activities it is almost impossible to determine which factors are
important in deciding the selectivity toward product forma-
tion. It is learnt from earlier works that in the guaiacol HDO re-
action over a Ru catalyst, the product distribution changes de-
pending on the reaction conditions and support. For example,
the Ru/HZSM-5 catalyst after 4 h at 473 K gives cyclohexane as
sites, 0.46 mmolg strong acid sites) and AL has an acidity of
ꢀ
1
ꢀ1
ꢀ1
0.10 mmolg (0.09 mmolg weak acid sites, 0.01 mmolg of
strong acid sites), as measured by the TPD-NH method. TPD-
3
NH of the carbon catalyst showed that the material is neutral
3
in nature, owing to the absence of both strong and weak acid
ꢀ
1
sites. The HT catalyst has a basicity of 0.88 mmolg calculated
by the TPD-CO method. The surface area (N sorption), metal
[
18]
a main product (93%), whereas over Ru/C at 473 K after 4 h
2
2
[
21]
the major product is cyclohexanol (70%). It was also found
that the addition of base (MgO) along with the Ru/C catalyst
in the guaiacol HDO reaction yields cyclohexanol in 80% yield
particle size, and dispersion (HRTEM) were determined and are
summarized in Table S2.
Phenol (1) was chosen as a model compound to explore the
activities of catalysts in HDO reactions (Scheme 1). Table 1 sum-
[
19]
under relatively milder conditions (433 K after 2 h). Addition-
ally, over the Ru/C catalyst at 523 K, methoxy cyclohexanol was
[
5c,37]
formed as a major product (60%).
Even for a simple sub-
strate such as phenol subjected to the HDO reaction over vari-
ous Pd catalysts, a difference in the products formed was ob-
[
18,19]
served.
The above results reveal how changes in the tem-
perature, support, and time can alter the course of the reac-
tion, and hence, it is impractical to compare the activities of
these catalysts reported in assorted papers to arrive at a defi-
[
40]
nite conclusion.
Considering this, it is very important to
check the activities of different catalysts under similar reaction
conditions.
The chief role of the support is to stabilize the metal parti-
cles, but it is evident from the available results that the sup-
ports play an active role in these reactions. It is well under-
stood from the existing literature that the course of the reac-
tion to yield either partially hydrogenated or fully hydrogenat-
ed products are decided by the combinations of metals and
supports used in the work. Notwithstanding the current flurry
of consideration of lignin, there is still a limited understanding
of the reaction networks of the HDO of lignin-derived chemi-
cals. Moreover, in the literature, there is a lack of systematic
studies verifying which factors influence the course of the reac-
tion under similar reaction conditions. Hence, we synthesized
various catalysts with combinations of different metals (Pt, Pd,
Ru) and various supports chosen from acidic [Al O (AL), SiO -
Scheme 1. Representative phenol hydrogenation products.
[
a]
Table 1. Phenol hydrogenation reactions over supported metal cata-
lysts.
2
3
2
[b]
[c]
Support
SA
Metal
Conversion[%]
Yield [%]
Al O (SA)], neutral [carbon (C)], and basic [hydrotalcite (HT)]
2
4
2
3
types, and evaluated their activities in phenol, guaiacol, and
eugenol HDO reactions and HDO reactions of mixtures of
these compounds under similar reaction conditions, to paint
a picture of the factors responsible for driving the reaction in
a particular direction.
Pd
Pt
95
ꢁ99
0
0
90
99
C
Pd
Pt
ꢁ99
ꢁ99
99
99
0
0
[
2
a] Reaction conditions: phenol (2 mmol), catalyst (substrate/metal=
00 mol), hexadecane (30 mL), H pressure at RT=3 MPa, 523 K, 1 h. All
2
Results and Discussion
the reactions were performed at least twice and an analysis error of
ꢂ2% was observed in all the reactions. [b] Determined by GC analysis.
[c] Determined by GC analysis.
All the catalysts were synthesized through the impregnation
method, and were calcined and reduced before evaluation of
their catalytic activities. The catalytic reactions were performed
in an autoclave. The XRD characterization (Figures S1–S3 in the
Supporting Information) showed the characteristic peaks for all
the metals and supports. In some cases, owing to higher dis-
persion, the metal peaks are not clearly visible. Table S1 sum-
marizes the results on the metal loading by ICP analysis; there
is only a marginal difference between the theoretical and prac-
tical loading of metal on the supports. From the acidity and
basicity data of the supports, it emerges that the SA support
marizes the HDO results, and we observe that with an acidic
support (SA), both Pd and Pt catalysts gave cyclohexane (4) as
a major product, but with a neutral support (C) the major
product was cyclohexanol (2). Notably, phenol does not yield
benzene irrespective of the support or metal used under these
reaction conditions.
It is observed from the results that both metals, Pd and Pt,
supported on a neutral support favor the formation of 2,
which means that ring hydrogenation is a first step in phenol
ꢀ1
ꢀ1
has an acidity of 0.63 mmolg (0.17 mmolg of weak acid
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HDO reactions. Thus, it is presumed that cyclohexanol (2),
once formed, undergoes dehydration to yield cyclohexene (3),
which is hydrogenated immediately to yield cyclohexane (4).
Similar observations were also made in a few of the earlier
[
12,21]
studies.
The overall reaction pathway for phenol HDO
passes through the formation of cyclohexanol (2), but interest-
ingly, it stops at that stage over the C support, which is not
acidic.
This observation showed us that the first step of ring hydro-
genation is a metal-catalyzed reaction, but further cleavage of
the CꢀO bond occurs only if the support is acidic (SA). The
complete HDO activity achieved on these supports is far supe-
rior to that in earlier works, in which the selectivity (yield) for
cyclohexane is lower (4% cyclohexane) even if the reactions
[
3,4]
are performed at 673 K over sulfided CoMo catalysts.
Scheme 2 represents the reaction network of guaiacol (6) re-
actions. On the basis of the results obtained in the phenol re-
action, it is likely that first, ring hydrogenation occurs to yield
Figure 1. Effect of support on supported Pd catalysts in guaiacol hydrogena-
tion reactions. Guaiacol (12 mmol), catalyst (0.2 g), hexadecane (30 mL), H
2
pressure at RT=3 MPa, 523 K, 1 h. All the reactions were performed at least
twice and an analysis error of ꢂ2% was observed in all the reactions.
a 52% yield is observed for 12, with a similar conversion of
guaiacol (54%), whereas on the Pd/C catalyst, ꢁ99% yield is
seen for 7. Additionally, with Pd/AL as a catalyst, the formation
of 10 (33%) and 12 (3%) along with 7 (57%) is observed. Pd
supported on a basic support (HT) gave 7 in 86% yield, and 8
and 12 were also formed in minor quantities. These results
show that Pd behaves differently if supported on different sup-
ports. To assess these results appropriately, we will consider
the acidity or basicity of these supports. It is understood from
the TPD-NH3 and TPD-CO2 characterizations that SA has
ꢀ
1
ꢀ1
a higher acidity (0.63 mmolg ) than AL (0.10 mmolg ), and
ꢀ
1
the HT support has a basicity of 0.88 mmolg . The catalytic re-
sults imply that with increasing acidity from AL to SA, the for-
mation of 12 increases, which means that acidic supports are
active for CꢀO bond cleavage. A careful look at the AL-sup-
ported catalyst showed formation of compounds in the se-
quence 7!10!12. This goes against the general belief in the
literature, wherein it is suggested that guaiacol (6) gives 8,
from which 10 and 12 (8!10!12) can be formed subsequen-
[5d,e,35,36,41–43]
tly.
In our work, the absence of 8 and 9 and the for-
mation of 7, 10, and 12 clearly shows that the reactions pro-
ceed by guaiacol, first undergoing ring hydrogenation, and
later, cleavage of the CꢀO bond. This can be explained on the
basis of the phenol HDO results, wherein first, phenol (1) un-
dergoes ring hydrogenation to yield cyclohexanol (2) over
a neutral (C) support, and later over the SA support can yield
cyclohexane (4) through the cleavage of the CꢀO bond. In ad-
dition, we can comment that over an acidic support, the ꢀ
Scheme 2. Reaction network of guaiacol hydrogenation reaction.
methoxycyclohexanol (7); otherwise the reaction may proceed
with the removal of either the ꢀOCH or ꢀOH groups to yield
3
products such as phenol (8) or anisole (9), respectively. Further,
products 8 and 9 can yield the compounds cyclohexanol (10)
and methoxycyclohexane (11), respectively. Finally, complete
hydrodeoxygenation occurs, resulting in the formation of cy-
clohexane (12). Similarly, once methoxycyclohexanol (7) is
formed, it can yield methoxycyclohexane (11) or cyclohexanol
OCH group is the first to leave, followed by the ꢀOH group,
3
as we did not observe product 11 (methoxycyclohexane), but
did observe product 10 (cyclohexanol). The difference in prod-
uct formation on both Pd/SA and Pd/C might be caused by
(
10), which can be further converted to cyclohexane (12).
ꢀ
1
Figure 1 represents the guaiacol HDO results obtained over
the presence of a lower acid amount on AL (0.10 mmolg )
ꢀ
1
various supported Pd catalysts. It is found from the data that
the support plays an important role in deciding the course of
the reaction. For example, over the SA-supported Pd catalyst,
than on SA (0.63 mmolg ). Moreover, as mentioned earlier, SA
ꢀ
1
contains a higher number of strong acid sites (0.46 mmolg )
ꢀ
1
than AL (0.01 mmolg ), which may also play an important
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role in deciding the course of the reaction. Contrary to Pd sup-
ported on acidic supports, Pd impregnated on the neutral sup-
port, C, is very selective in giving only methoxy cyclohexanol
showed larger particles sizes: SA (20 nm), AL (10 nm), and HT
(15 nm). However, it seems that the particle size does not play
a major role in these reactions, because all the catalysts
showed >99% conversions within 1 h of reaction, and the
product distribution is different for all the catalysts. The effect
of particle size is discussed further during the time study (see
below).
(
7), and no further conversion to 10 or 12 is possibly, unlike
that seen over SA and AL supports. Similarly to the phenol
HDO reactions, the metal in the presence of an acidic support
gives the complete hydrodeoxygenated product (12), and the
neutral support gives only the ring hydrogenated product (7).
The activity with Pd/HT also showed 7 as a main product, with
much lower concentrations of products 8 (10%) and 12 (1%).
The absence of 10 and much lower concentration of 12 indi-
cates that over the base support there is almost no possibility
Moreover, a particle size difference is also observed for Pd,
Pt, and Ru metals impregnated on the same support, but
almost no difference in activity was seen (Table S2). We pur-
posefully checked the activity of all the catalysts at a definite
time period (1 h) and did not change the time intervals of the
reactions, because we were interested in the activities of differ-
ent catalysts under similar reaction conditions. Though there
might be differing opinions on the effect of time on the prod-
uct formation, we wish to draw attention to the fact that com-
plete conversions were achieved on C- and HT-supported Pd
catalysts, and in both of these reactions we obtained the
major product as methoxycyclohexanol, 7, which can still be
converted to 10 or 12. We did not observe any of these prod-
ucts on the Pd/C and Pd/HT catalysts, but did observe them on
the Pd/AL and Pd/SA catalysts, so we were assured that the
conversion levels are not important but the support in which
the metal is supported is a crucial factor for achieving the
products. This is because, despite the lower conversions on
the Pd/AL and Pd/SA catalysts, further hydrogenation products
are observed.
of CꢀO bond cleavage (ꢀOCH or ꢀOH). Notably, with the for-
3
mation of 8 seen only with the Pd/HT catalyst, it seems that
the catalyst is capable of taking another route for product for-
mation compared with the other catalysts.
The above results provide the information that with neutral
(C) and acidic (SA, AL) supports, path A is chosen by guaiacol,
but with a basic support, although the main path is A, guaiacol
[
12,18,22]
can also choose path B.
Unlike in an earlier report,
wherein under acid-free conditions (much like using the C sup-
port) with reactions at 473 K, hydrolysis of the ꢀOCH group
3
and formation of 1,2-cyclohexanediol by methyl group migra-
[
21]
tion is observed.
To understand the effect of the acidic support on the prod-
uct formation, we performed guaiacol reactions using a physi-
cal mixture of Pd/C (neutral) and an acidic support (SA or AL).
Table S3 summarizes the results obtained with the Pd/C+AL
and Pd/C+SA catalytic systems. It is observed that if the reac-
tion is performed with Pd/C+SA, the formation of 7, 10, and
Guaiacol reactions were also studied using metals like Ru
and Pt on various supports (Table 2). The Pt/SA catalyst gave
12 as the only product, whereas Pt/C showed formation of 7
and Pt/HT also showed formation of 7 as a lone product. As
discussed earlier, the Pd catalysts also showed a similar prod-
uct formation pattern, which implies that Pt and Pd behave
comparably. However, there is a slight dissimilarity in the activi-
ty between the Pt/AL and Pd/AL catalysts. Over Pt/AL, the
12 is possible. Similarly, with the Pd/C+AL catalyst, the forma-
tion of 7, 10, and 12 is also observed. In contrast, with only
the Pd/C catalyst, 7 was formed selectively (Table S3). This
clearly indicates that the acidic supports play an important
role in deciding the selectivity of the product. Moreover, the
data presented in Table S3 also reveal that, depending on the
acid support used (either AL or SA), the selectivity toward the
formation of 7, 10, and 12 changes.
[a]
Table 2. Guaiacol hydrogenation reactions over supported metal cata-
We have observed transmethylation reactions in the pres-
ence of acidic supports such as SA and AL, giving products
such as methyl guaiacol. However, these products accounted
for only about 2–4% (Figure S4). A similar phenomenon was
lysts.
[
b]
[c]
Support
SA
Metal
Conversion[%]
Yield [%]
7
8
10
12
Ru
Pt
61
76
0
0
7
0
0
0
54
75
[
21,44]
also observed in earlier works.
It is also likely that after hy-
drolysis, guaiacol can form catechol, but in our work we did
not observe the formation of this product. Besides the ob-
served products in all these reactions, there is a possibility of
obtaining cyclohexene by loss of water from cyclohexanol (10).
Nevertheless, under our reaction conditions, we did not ob-
serve a peak for cyclohexene in GC, possibly because hydroge-
nation of this product can occur very quickly, and hence, the
immediate formation of product 12 is possible. As all the Pd
catalysts showed differences in their activity, we could discuss
the effect of the Pd particle size on these supports, which may
also influence the catalyst activity. Hence, we calculated the
average particle size of Pd on these supports (Table S2 and Fig-
ures S5–S7). The C-supported Pd showed the smallest metal
particles with an average size of 5 nm, and the other supports
AL
Ru
Pt
66
41
5
17
5
0
11
14
29
10
C
Ru
Pt
37
87
19
80
12
0
0
0
5
0
HT
Ru
Pt
38
49
25
47
6
0
0
0
3
0
[
d]
Noncatalytic
<5
0
0
0
0
[
(
a] Reaction conditions: guaiacol (12 mmol), catalyst (0.2 g), hexadecane
30 mL), H pressure at RT=3 MPa, 523 K, 1 h. [b] Determined by GC anal-
2
ysis. [c] Determined by GC analysis. [d] Time=6 h. All the reactions were
performed at least twice and an analysis error of ꢂ2% was observed in
all the reactions.
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major product observed is 7 (17%), but formation of 10 (14%)
and 12 (10%) is also observed with a marginal difference in
the concentrations. In comparison, with the Pd/AL catalyst, 7 is
formed in high concentration (57%), whereas the formation of
The results of the time study of guaiacol HDO reactions
(Table S4) showed the same selectivity toward the SA-support-
ed metal catalysts. The SA-supported metal catalysts did not
show any change in activity with increasing time as 12 was
always observed as a major product. With the AL support, for
all the catalysts, the concentration of 7 started increasing with
increasing time. As mentioned earlier, it is possible that 7 is
formed first and is later converted into 10 and then to 12. This
suggestion is supported by the time study, wherein the con-
centration of 7 is typically built up in the reaction mixture,
then eventually decreases as the concentrations of 10 and
later 12 increase as time progresses. A similar trend was also
observed for the Ru/HT catalyst, and in the case of the Pt/HT
catalyst, though the conversions increased with increasing
time, product 7 remained stable and was formed with >90%
selectivity. It should be noted here that the particle size of the
metal does not play an important role in deciding which prod-
uct will be formed predominantly. As observed from Table S4,
after 6 h reaction over Pd/SA (88% conversion, 85% yield for
12; 20 nm), Pt/SA (96% conversion, 90% yield for 12, 8 nm),
and Ru/SA (82% conversion, 73% yield for 12, 15 nm), similar
levels of conversions and selectivities (90–96%) for 12 are ob-
served, whereas the particle size of the metals changes drasti-
cally (8–20 nm). Similarly, with different Pd catalysts of particle
sizes from 5 to 20 nm (Table S2) at similar conversion levels
(88–100%) distinguished product formation is observed. More-
over, even with 100% conversion over the Pd/HT catalyst
(15 nm), the major product is 7 whereas with the Pd/SA cata-
lyst (20 nm) at 88% conversion the major product is 12. These
results clearly imply that particle size does not play a major
role in these reactions.
10 (33%) is lower, and product 12 (3%) is formed with minimal
concentration. These results indicate that although the activity
for the cleavage of the CꢀO bond for the formation of 10 and
12 over the Pd/AL catalyst is lower, the Pt/AL catalyst is active
for the same reaction. A careful look at Table 2 and Figure 1 re-
veals that, except for the SA-supported catalysts, all the Pd cat-
alysts are more active than the Pt catalysts in these reactions.
It is claimed in the earlier reports that over the Pt/C catalyst,
[
21]
guaiacol can give 10 as a major product, but under our reac-
tion conditions, only the formation of 7 is observed. Some re-
ports also argue that compared with Pd, Pt catalysts are more
[
11,21]
active in HDO reactions,
but our results show that the Pt
[33]
catalysts have lower activities than their Pd counterparts.
Few disagreements are observed upon comparison of our re-
sults with earlier reports on similar studies, as in those studies,
different methods for the synthesis of the catalysts and differ-
ent HDO reaction conditions to test their activities are used.
For this reason, we have attempted to compare all the cata-
lysts under similar reaction conditions. Without a catalyst, less
than 5% conversion was observed, but we could not identify
any products as they were below the detection limits of the
GC instrument.
From the results of guaiacol HDO (Table 2) it was found that,
compared with the Pd and Pt catalysts, the supported Ru cata-
lysts showed a difference in activity. For Ru supported on SA
(7%), AL (5%), C (12%) and HT (6%) supports, formation of 8
is observed, which was absent for Pd and Pt supported on
these supports (except for the Pd/HT catalyst). Consequently,
the Ru catalyst may also follow path B along with path A. The
other major activity difference was observed with the Ru/AL
catalyst. Similarly to the AL-supported Pd and Pt catalysts, Ru/
AL also showed formation of 7, 10, and 12, but the difference
lies in the fact that with Ru/AL, yield of 12 was higher than
with the other two catalysts. This is particularly important, be-
cause over the Pd/AL catalyst only 3% of product 12 is formed
after 93% conversion, but over the Ru/AL catalyst, with 66%
conversion, a 29% yield for product 12 is achieved. The guaia-
col HDO over Ru/C showed the formation of 8 and 12 along
with 7. This is in stark contrast to the Pd/C and Pt/C catalysts,
with which selective formation of 7 was accomplished. The for-
mation of 8 over Ru/C can be explained on the basis of guaia-
To probe the effect of temperature on the hydrogenation of
guaiacol, we performed reactions at 3 MPa hydrogen pressure
over Pd/SA as a catalyst; the results are tabulated in Table S5.
It is apparent from the results that at 473 K no conversion is
possible, but with an increase in temperature to 503 K guaiacol
starts to be converted. In all the reactions, the formation of 12
as a product is observed. However, at 503 K, a small amount of
10 is observed after reaction for 3 h. Our results are consistent
[40]
with the literature; however, there are few reports in which
it is shown that Pd can show activity for the HDO of guaiacol
even at 473 K. This may be because in some of these works,
large quantities of additional acids (HZSM-5, H PO ) are used,
3
4
and high hydrogen pressures (5 MPa) are also mentioned,
[7,13,21]
which may influence the catalytic behavior.
Another pos-
[21]
col undergoing demethoxylation, as discussed earlier. Typi-
cally, cleavage of the CꢀO bond is observed over acidic sup-
ports, but rather surprisingly, Ru/C showed an activity toward
bond cleavage. Logically, ring hydrogenation of 8 should give
sible reason is that most of these studies were made using
water as a solvent, and at high temperatures, in situ proton
[11]
generation is reported to yield dehydration products. In this
work, we have performed HDO reactions in a solvent (hexade-
[
19]
+
product 10, the formation rate of which may increase with
increasing temperature; however, in our work we did not ob-
serve this product even at 523 K.
cane) that will not give any H ions to catalyze the reaction.
Considering this, it becomes more difficult to assess whether
the activity observed in earlier works originated only from the
supported metal catalysts or if there was any possible contri-
bution from the solvent; this is ruled out in our work.
Moreover, it is shown that with the Ru/C+MgO system, the
[
19]
major product is 10, but in our system with the Ru/HT cata-
lyst we observed the major product as 7 (25%) followed by 8
To explore the consequences of a change in hydrogen pres-
sure on the product formation, we performed reactions at
pressures of 1, 2, and 3 MPa (Table S6). It was observed that
(6%) and 12 (3%).
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with the increase in pressure from 1 to 2 MPa, the rate of prod-
uct formation (12) is enhanced, but remains almost constant
upon a further increase of the pressure to 3 MPa (Table S6).
This reflects the fact that there is no need to use the high pres-
sures (5 MPa) that are reported in some publications.
Furthermore, we also checked the risk of leaching of the metal
by ICP-OES , and found that no metal is leached out from the
catalyst.
From the above set of data, we propose the reaction net-
work for the Pd, Pt, and Ru catalysts (Figures S13–S15). The
support plays an important role in deciding the course of the
reaction. Metals supported on a strong acidic support (SA)
yielded cyclohexane (12) as the main product. Owing to the
weak acidity of the AL support, the acid-catalyzed CꢀO bond-
cleaved products were reduced, and methoxycyclohexanol (7)
and cyclohexanol (10) were observed along with 12. With
a neutral support, in contrast to the Ru catalyst, both the Pd
and Pt catalysts gave product 7 exclusively. These results indi-
cate that a higher acidity is required for CꢀO bond cleavage,
and that under the reaction conditions used, it is possible to
synthesize product 12 with ꢁ99% selectivity. In the absence
of any acidity (C), the catalysts gave product 7 with ꢁ99% se-
lectivity. The mildly acidic support (AL) would certainly give
mixed products, and hence, may not be the support of choice.
The basic support (HT) can also yield 7 selectively as a major
The reusability of the Pd/SA and Ru/SA catalysts was scruti-
nized for the HDO reaction of guaiacol, and the results are pre-
sented in Figure 2. After the first reaction, the catalysts were
[19]
product. It is reported in the literature that in the presence
of base, the major product obtained is cyclohexanol (10), and
that hydrolysis of the ꢀOCH group and cyclohexane (12) for-
3
mation can be suppressed. It is also proposed that in the pres-
ence of base, the reaction can proceed through phenol forma-
tion to cyclohexanol. However, our results are somewhat con-
tradictory to these observations. In our work, as mentioned
earlier, the major product is not 10 but 7 (methoxycyclohexa-
nol). Nevertheless, our observations agree with the possibility
of the formation of phenol (8) as a product in the presence of
[19]
base, as reported earlier, through path B.
In an extension to the work on guaiacol, another complex
monomeric phenolic compound derived from lignin, namely
eugenol, was tested for hydrogenation over several supported
metal catalysts (Scheme 3). The reason for the selection of eu-
genol for this study was that it contains several key functional
groups similar to those of lignin and coniferyl alcohol. The cat-
alytic activities of the Pd catalysts are shown in Figure 3; nota-
bly, with all the catalysts, >99% eugenol conversion was ach-
ieved.
As observed over the acidic support, SA, a complete hydro-
genation product, 17, is possible, similarly to that observed in
the case of the guaiacol substrate. However, unlike in the
guaiacol case, with ꢁ99% selectivity toward cyclohexane, 12,
in the case of eugenol, almost equal amounts of products 14,
Figure 2. Recycle study with supported metal catalysts for guaiacol hydroge-
nation reaction. a) Ru/SA, b) Pd/SA; guaiacol (12 mmol), catalyst (0.2 g), hex-
adecane (30 mL), H pressure at RT=3 MPa, 523 K, 6 h. All the reactions
2
were performed at least twice and an analysis error of ꢂ2% was observed
in all the reactions.
15, and 17 are formed. A cautious look at the data suggests
that double bond hydrogenation occurs first, with subsequent
further conversion to 15 and then to 17. Along with these
products, isoeugenol, 20, an isomerization product of eugenol,
and 21 are also formed in minor concentrations. Comparable
observations were also made in earlier studies using the Pt/
recovered, washed with acetone, dried in an oven, and re-
duced under a hydrogen flow at 473 K, and were then subject-
ed to the next run. As observed from Figure 2a, the Ru/SA cat-
alyst showed a similar activity with the yield of product 12 re-
maining almost constant in all the recycle runs. Corresponding-
ly, over the Pd/SA catalyst, a similar activity was also observed
in all the runs (Figure 2b). These catalytic observations imply
that the catalysts are recyclable and stable. Nevertheless, to
prove our point, we analyzed fresh and spent catalysts by XRD,
and found no difference in the peak pattern (Figures S8–S12).
[44]
Al O catalyst.
2
3
Reactions conducted over Pd supported on AL gave a com-
plicated product pattern. Similarly to the SA catalyst, this cata-
lyst also formed products 14, 15, and 17; however, the concen-
tration of 14 is quite high compared to those of 15 and 17.
This is because AL is mildly acidic compared to SA, and cleav-
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Similar observations have also been noted by other research-
[44,50]
ers.
It is proposed that the acid-catalyzed isomerization may in-
volve carbenium ion chemistry (Figure S16). However, it is
learnt from the previous studies and from alkane isomerization
reactions that these carbenium ions may also act as a precursor
[44,51–55]
for coke formation on the catalyst surface.
Thus, it is ap-
parent from the catalytic results that in our reactions we ob-
served no complete carbon balance (without taking into ac-
count coke deposited on the catalyst). Moving away from the
acidic support to the neutral support, C, the selective forma-
tion (90%) of 18 is achieved. This implies that over the C sup-
port, the reaction proceeds through 14 to yield 18 without
conversion of the product 14 to any other product such as 15
or 21. This is expected, because the formation of 15 and 21 re-
quires acidity and the support in question is neutral. For the
same reason, the formation of 20, an isomerization product of
eugenol, is also not possible over the C support. Reactions
were also performed with the basic support (Pd/HT catalyst),
and the data reveal the predominant formation of 14 along
with products 15 and 18. Because acidity is required to obtain
1
5 (to cleave CꢀO bond), its yield is restricted to a minimum
Scheme 3. Reaction network of eugenol hydrogenation reaction.
(4%). Surprisingly, 21 is formed in a much lower quantity (8%),
as the basic support should be able to isomerize eu-
[45–48]
genol.
The low carbon balance and change of
color of the catalyst to dark brown/black suggests
that either product 14 or the reactant or another
product is strongly adsorbed on the catalyst.
For the identification and comparison of the activi-
ties of Ru and Pt impregnated on various supports,
eugenol reactions were conducted under similar re-
action conditions to those used for the Pd catalysts,
and the results are summarized in Table 3. In hind-
sight, it is clear that in these reactions, the Pt cata-
lysts have mostly shown higher activities than the Pd
and Ru catalysts. Moreover, it is clearly visible that
the Pt catalysts are more selective in yielding 14 as
a major product if acidic supports are used. However,
if the neutral support is used, the formation of 18 is
observed in 95% yield. Also, the carbon balance on
the Pt catalyst is much higher than that on the other
metals. This may be because Pt generally shows
lower hydrogenation activity than Pd. This explana-
tion is reasonable, because over the SA support, Pt
showed 75% higher formation of 14 than Pd (26%),
and a further hydrogenation product, 17, was formed
in higher quantity on Pd (21%) than on Pt (16%). On
Figure 3. Effect of support on supported Pd catalysts in eugenol hydrogenation reac-
tions. Eugenol (2 mmol), catalyst (Substrate/Metal=200 mol), hexadecane (30 mL), H
2
pressure at RT=3 MPa, 523 K, 1 h. All the reactions were performed at least twice and an
analysis error of ꢂ2% was observed in all the reactions.
age of the CꢀO bond is difficult over AL although cleavage of
the relatively weak CꢀO bond is observed to yield 15. In the
case of guaiacol, 7 was formed preferentially over AL, which is
equivalent to product 18 in the case of eugenol. Compared
with 18 here, the formation of 17 was higher because of the
higher initial concentration of 15 from 14. Typically, isomeriza-
AL, a partially hydrogenated product, 14, is also formed in
higher concentration over Pt (92%) than over Pd (45%), and
a further hydrogenation product, 17, is formed in higher con-
centration on Pd (10%) than on Pt (0%). As expected, the Ru
catalysts showed activities between those of Pd and Pt.
The Pd/SA and Pd/C catalysts were used repeatedly (three
times) to check whether they show similar activities after wash-
ing, drying, and reducing them under a hydrogen flow after
each run. It is observed that the Pd/SA catalyst shows reprodu-
cible activity for the formation of products, 14, 15, and 17
[45]
tion of eugenol is catalyzed by a homogeneous base
or
[
46–48]
[49]
solid base
or with metal complexes. However, in this
work, we observed, albeit in lower concentrations, the forma-
tion of isoeugenol, 21, over Pd on acidic supports (SA and AL).
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changing the support from acidic to neutral (C), the product
[
a]
Table 3. Eugenol hydrogenation reactions over supported metal cata-
lysts.
distribution changed, and we observed formation of product 7
(
98%), probably originating from guaiacol. Besides this, we
[
b]
Support
SA
Metal
Yield [%]
17
also found the formation of 18 (80%) derived from eugenol.
However, unlike for the lone eugenol substrate, with which 14
was not formed, in this case we found 14 in low concentration
1
4
15
18
20
21
Ru
Pt
44
75
10
7
15
16
0
0
3
0
3
0
(
10%). Nevertheless, 14 can eventually be converted into 18.
These results assure us that the catalysts evaluated for the
lone substrates for hydrogenation activity are capable of deliv-
ering similar performances if a mixture of compounds is used.
For the determination of the oxidation state of Pd present
on different supports, X-ray photoelectron spectroscopy (XPS)
analysis was performed for the Pd/SA and Pd/C catalysts. In
both the catalysts, peaks at binding energies of 333.5 and
AL
Ru
Pt
59
92
11
2
4
0
0
0
1
0
13
0
C
Ru
Pt
77
0
6
0
0
0
0
95
0
0
2
0
HT
Ru
Pt
60
80
5
8
1
0
0
0
0
0
12
2
340.7 eV for Pd3d5/2 and Pd3d3/2 were observed (Figures S17
[c]
Noncatalytic
0
0
0
0
0
0
and S18). These peaks are characteristic of Pd metal, which im-
plies that irrespective of the nature of the support (acidic or
[
2
a] Reaction conditions: eugenol (2 mmol), catalyst (substrate/metal=
00 mol), hexadecane (30 mL), H pressure at RT=3 MPa, 523 K, 1 h.
2
[56–59]
neutral), Pd is present in the metallic state.
Moreover, it is
[b] Determined by GC analysis. [c] Time=3 h. Conversion was 100% for
all the catalytic reactions. All the reactions were performed at least twice
and an analysis error of ꢂ2% was observed in all the reactions.
suggested that under a hydrogen pressure of 3 MPa (during
catalytic reaction), the metal will be in the reduced state.
along with 20 and 21. Similarly, the Pd/C catalyst also gave
about 90% yield for product 18 in each run; the decrease in
yield of about 2% in each run may be caused by the loss of
catalyst through handling.
Conclusion
This study was performed to understand how different cata-
lysts show differences in their catalytic activities under similar
reaction conditions, because this type of work is lacking in the
literature. Noble metals such as Pd, Pt, and Ru impregnated on
various supports were observed to be highly active in the HDO
reactions of phenol, guaiacol, and eugenol. Irrespective of the
metal, the reactions are driven by the supports on which the
metals are impregnated after initial product formation. It is
proposed that the first step of ring hydrogenation in the case
of guaiacol or double-bond hydrogenation with eugenol is
a metal-driven reaction; however, further reactions are mostly
governed by the supports, with help from the metal to supply
hydrogen. The removal of oxygen by HDO is achieved mainly
on acidic supports such as AL and SA. Nevertheless, SA is
proven to be highly active owing to its high acidity compared
with AL. The reusability of the catalysts for both the substrates
(guaiacol and eugenol) has been proved beyond doubt by
subjecting the catalysts to the reaction at least three times.
To advance the knowledge generated by performing the re-
actions with lone substrates, we also performed the reactions
with a mixture of substrates. It was observed that the Pt/SA
and Pt/C catalysts perform similarly in these reactions. This
shows that these catalysts can exhibit similar catalytic activities
with the complex phenolic compounds obtained after lignin
depolymerization.
All the above reactions were performed using only a single
phenolic compound, so we now need to understand how the
catalysts will behave in the case of a phenolic mixture (similar
to lignin-derived bio-oils). Therefore, we performed reactions
with a mixture of guaiacol and eugenol over Pt catalysts.
Table 4 lists the results obtained for the reaction performed at
[
a]
Table 4. Hydrogenation of guaiacol and eugenol mixture over Pt cata-
lysts.
Support
Guaiacol
Eugenol
Yield [%]
14 15 17 18 20 21
[
b]
[c]
[b]
[c]
Conv. [%]
Yield [%]
Conv. [%]
7
8
10 12
SA
C
88
99
0
98
0
0
0
0
85
0
100
100
80
10
4
0
12
0
0
80
0
0
0
0
[
(
[
a] Reaction conditions: guaiacol (0.164 g) + eugenol (0.164 g), catalyst
0.065 g), hexadecane (30 mL), H pressure at RT=3 MPa, 523 K, 1 h.
b] Conversions calculated respective to the charge of the particular sub-
2
strate. [c] Yields calculated on the basis of the charge of the respective
substrate. All the reactions were performed at least twice and an analysis
error of ꢂ2% was observed in all the reactions.
523 K for 1 h. As expected, Pt supported over the acidic sup-
port, SA, showed a maximum yield for 12 (cyclohexane) may
be obtained from guaiacol, as it is evident from Table 2 (guaia-
col hydrogenation reactions) that the Pt/SA catalyst yields 12
as a major product if guaiacol is used as the only substrate.
Along with product 12, we also observed the formation of 14
The understanding of the catalyst functionalities gained
from this study will help in designing catalysts for HDO reac-
tions to enhance the yields of the desired products and to de-
velop active catalysts.
(80%), 15 (4%), and 17 (12%) in this reaction, which originated
from eugenol. These results are also in line with the observa-
tions discussed earlier (Table 3, eugenol hydrogenation). Upon
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3
h. The specific surface area was determined using the BET
Experimental Section
method, pore size data were obtained using the BJH method, and
pore volumes were found by using the t-plot method. The metal
contents in the prepared catalysts were analyzed by inductively
coupled plasma optical emission spectroscopy (ICP-OES) using
a SPECTRO ARCOS (Germany) FHS 12 instrument. NH -TPD was
performed with a Micrometrics Autochem-2910 instrument, USA.
The catalyst was activated at a temperature of 773 K in He gas
flow (25 mLmin ) and NH3 adsorption (30 mLmin ) was per-
formed at 373 K; desorption was started from 273 to 1096 K at the
rate of 10 Kmin . CO -TPD was performed using a Micromeritics
AutoChem 2910 instrument. Prior to measurement, the samples
were activated at 623 K for 15 min. The adsorption of CO was per-
formed at 323 K, and then the sample was kept at 373 K under He
flow. Next, the sample was heated from 373 to 873 K with a ramp-
ing rate of 4 Kmin . X-ray photoelectron spectroscopy (XPS) meas-
urements were performed on a VG Micro Tech ESCA instrument at
a pressure of 10 Torr.
Materials
The substrates, guaiacol (99%, Loba Chemie), phenol (99.5%, Al-
drich) and eugenol (99%, Aldrich), were purchased and used as re-
ceived. The supports, SiO -Al O , g-Al O , 3 wt% Pt/C, and 3 wt%
3
2
2
3
2
3
Pd/C, were purchased from Aldrich, and the carbon (BP2000) used
was from Cabot. Tetraamine platinum nitrate (99.99%) was pur-
chased from Alfa Aesar, and ruthenium chloride (RuCl , Ru content
ꢀ
1
ꢀ1
3
4
5–55%), and palladium chloride (PdCl Pd content ꢁ99.9%) were
2,
ꢀ1
2
purchased from Aldrich. The solvent, n-hexadecane (99%), was
procured from Aldrich. The products, 2-methoxy cyclohexanol
2
(
(
99%, Alfa Aesar), cyclohexane (99%, Loba Chemie), cyclohexanol
99.5%, Loba Chemie), 4-propyl phenol (TCI chemicals), and 4-
propyl cylohexanol (>98%, Loba Chemie) were purchased and
used for calibration. All other chemicals were procured locally and
used without any treatment.
ꢀ
1
ꢀ9
Synthesis of catalysts
Catalytic runs
Synthesis of hydrotalcite (HT)
All the reactions were performed in batch mode using a high-pres-
sure, high-temperature Amar autoclave with an internal volume of
Hydrotalcite (Mg/Al=3) was synthesized through the coprecipita-
5
0 mL. During heating, stirring was kept slow (100 rpm), and after
tion method. NaOH was used to maintain the pH, and Na CO was
2
3
the reaction temperature of 5 K less than the desired reaction tem-
perature was attained, the stirring rate was increased to 1000 rpm.
The reactions were repeated at least twice to check the reproduci-
bility.
used as a carbonate source. For the synthesis of the catalyst by the
coprecipitation method, an aqueous solution “A” (37.5 mL) of Mg-
(
NO )·6H O (0.279 mol) and Al(NO ) ·9H O (0.093 mol) were added
3 2 3 3 2
slowly into aqueous solution “B” (37.5 mL) of NaOH (0.4375 mol)
and Na CO (0.1125 mol) under vigorous stirring over a period of
2
3
2
h. The solution pH was maintained between 8 and 10. The white
precipitate thus obtained was kept in an autoclave at 333 K for
6 h. The precipitate was then washed with deionized water until
the pH became neutral. The solid thus obtained was then dried at
53 K for 18 h. The sample was calcined at 623 K in air for 8 h to
Hydrogenation of phenol and eugenol
1
Phenol (2 mmol) or eugenol (2 mmol), the catalyst (2–3.5 wt%
metal, S/C=200), and hexadecane (30 mL) were charged in an au-
3
toclave. H gas was charged to the autoclave with a pressure of
2
obtain HT (Mg/Al=3).
3
MPa (at RT). The reaction was performed at 523 K for 1 h.
Supported metal catalysts
Hydrogenation of guaiacol
Supported metals were prepared through the conventional im-
pregnation method. Before synthesis, the supports were evacuated
at 423 K for 6 h. An aqueous solution of metal precursor (corre-
sponding to metal loading on support) was added to the support
suspended in water. The solution was stirred for 16 h at room tem-
perature. Water was removed with a rotary evaporator, and the
powder obtained was dried in oven at 333 K for 16 h, and dried
further under vacuum at 423 K for 6 h. It was then subjected to cal-
Guaiacol (12 mmol), the catalyst (2–3.5 wt% metal, 0.2 g), and hex-
adecane (30 mL) were charged in an autoclave. The reactor was
heated to 523 K for 6 h with a H gas pressure of 3 MPa (at RT).
2
Online sampling was performed at 1 h and 3 h.
Recycle runs
ꢀ1
cination (673 K, O flow 20 mLmin ) and reduction (673 K, H flow
The catalyst was recovered by centrifugation, washed with ace-
tone, dried in an oven at 333 K for 2 h, and dried further in
a vacuum oven at 423 K for 2 h. Afterwards, the catalyst was re-
2
2
ꢀ
1
2
0 mLmin ) for 2 h each to yield the supported metal catalysts.
ꢀ
1
duced at 673 K (H , 20 mLmin ) and then used for the next reac-
2
Characterization
tion.
Powder XRD patterns were obtained using PANanalytical X’pert
Pro, Netherlands, with a duel goniometer diffractor instrument.
Analysis
The X-ray source was CuK (1.5418 ꢁ) radiation with a Ni filter, and
a
ꢀ
1
sample scanning was performed at the rate of 4.38min from 58
to 908. Transmission electron microscopy (TEM) images were taken
using an FEI TECNAI T30 Model instrument working at an acceler-
ating voltage of 300 kV. The sample was dispersed in isopropanol
and sonicated, and then a drop of the solution was deposited on
the grid to take the image. N2 adsorption/desorption was per-
formed with an Autosorb 1C Quantachrome instrument, USA. Prior
to the analysis, the samples were activated in vacuum at 523 K for
Reaction mixtures were analyzed with an Agilent Gas Chromato-
graph (GC) equipped with a capillary column (HP-5, 0.32 mm i.d. ꢂ
50 m). The gas chromatograph-mass spectrometer (GC-MS) analysis
was performed with a Varian GC-MS with a capillary column (VF-5,
0.25 mm i.d. ꢂ 30 m). Before analysis of the reaction mixtures, stan-
dard compounds were injected to draw a calibration curve with
ꢂ2% error. All the reaction samples were injected at least twice to
check the reproducibility of the analysis.
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The conversion, selectivity, and yields were calculated on the basis
of the GC analysis (mole basis) and the formulae are as follows:
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6383–6390.
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mole of Substrate)charged] ꢂ 100
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Keywords: eugenol · guaiacol · hydrodeoxygenation · lignin ·
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