Catalytic hydrogenation of phenol, cresol and guaiacol over physically mixed catalysts of Pd/C and zeolite solid acids

Article abstract of DOI:10.1039/c5ra00367a

Highly reactive phenolic compounds of pyrolysis bio-oil are recognized as a major cause of the unpleasant properties of this biofuel. Catalytic hydrodeoxygenation of phenolic compounds of bio-oil is an efficient technique for improving the quality of bio-oil. Dual function catalysts consisting of metal and acid sites are usually used for transformation of bio-oil/bio-oil model compounds to high value hydrocarbons. Metal and acid sites are generally involved in hydrogenation/hydrodeoxygenation and dehydration/hydrocracking/dealkylation/alkylation reaction mechanisms, respectively. In this work, the product selectivity of hydrogenation of phenol, o-cresol, m-cresol and guaiacol was investigated over combined catalysts of Pd/C with zeolite solid acids of HZSM-5 (Si/Al of 30, 50 and 80) and HY (Si/Al of 30 and 60). Catalytic activity and product distribution in the hydrogenation process were affected by the density and strength of zeolite acid sites. HZSM-5 (30) with only weak acid sites showed lower cyclohexane selectivity compared with HZSM-5 (50) and HZSM-5 (80) which had both weak and strong acid sites. HY (30) and HY (60) containing only strong acid sites favored production of cycloketones.

Full text of DOI:10.1039/c5ra00367a

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Catalytic hydrogenation of phenol, cresol and
guaiacol over physically mixed catalysts of Pd/C
and zeolite solid acids†
Cite this: RSC Adv., 2015, 5, 33990
*
Hoda Shafaghat, Pouya Sirous Rezaei and Wan Mohd Ashri Wan Daud
Highly reactive phenolic compounds of pyrolysis bio-oil are recognized as a major cause of the unpleasant
properties of this biofuel. Catalytic hydrodeoxygenation of phenolic compounds of bio-oil is an efficient
technique for improving the quality of bio-oil. Dual function catalysts consisting of metal and acid sites
are usually used for transformation of bio-oil/bio-oil model compounds to high value hydrocarbons.
Metal and acid sites are generally involved in hydrogenation/hydrodeoxygenation and dehydration/
hydrocracking/dealkylation/alkylation reaction mechanisms, respectively. In this work, the product
selectivity of hydrogenation of phenol, o-cresol, m-cresol and guaiacol was investigated over combined
catalysts of Pd/C with zeolite solid acids of HZSM-5 (Si/Al of 30, 50 and 80) and HY (Si/Al of 30 and 60).
Catalytic activity and product distribution in the hydrogenation process were affected by the density and
strength of zeolite acid sites. HZSM-5 (30) with only weak acid sites showed lower cyclohexane
selectivity compared with HZSM-5 (50) and HZSM-5 (80) which had both weak and strong acid sites. HY
(30) and HY (60) containing only strong acid sites favored production of cycloketones.
Received 8th January 2015
Accepted 7th April 2015
DOI: 10.1039/c5ra00367a
www.rsc.org/advances
Since the phenolic fraction of bio-oil is highly reactive causing
bio-oil instability, phenols have been widely used as bio-oil
model compounds.11,15,16
1. Introduction
Bio-oil produced through biomass pyrolysis is considered as a
renewable and sustainable source of energy.1,2 Due to the high
oxygen content of biomass (around 50 wt%),3 bio-oil is highly
oxygenated consisting of reactive chemical components
(phenols, carboxylic acids, aldehydes, ketones, furfurals,
carbohydrates and alcohols)4–7 and water (around 30 wt%).8
High content of such oxygen-containing compounds in bio-oil
results in undesirable properties such as high viscosity, low
heating value, low thermal and chemical stabilities, corrosive-
ness and immiscibility with fossil fuels.4,5,7,9–11 Therefore, bio-oil
needs to be upgraded in order to be considered as a standard
fuel. One of the most applicable and effective techniques for
bio-oil upgrading is catalytic hydrodeoxygenation1,3,5,6,10 which
treats bio-oil at high hydrogen pressure (50–350 bar) and
moderate temperature (150–450 ꢀC).12–14 Due to the wide variety
of compounds which are present in bio-oil and involved in
upgrading process, it is difficult to predict the reaction mech-
anism of bio-oil hydrotreating. The hydrodeoxygenation of
different model compounds of bio-oil could be studied in order
to nd out the mechanisms of their transformation and to
predict the overall reaction pathway in bio-oil hydrotreating.
Catalysts including both metal and acid functions are highly
efficient for hydrotreating of bio-oil. Metal and acid sites
accomplish hydrotreating reaction through hydrogenation/
hydrogenolysis and dehydration/isomerization/alkylation/
condensation mechanisms, respectively.3,4,17 These dual func-
tional catalysts are generally prepared by embedding metals
into acidic supports. As an alternative to dual functional cata-
lysts, supported or unsupported metal catalysts such as Pd/C
and RANEY® Ni could be physically mixed with liquid
(H3PO4) and/or solid acids (HZSM-5, sulfated zirconia, Naon/
SiO2, Amberlyst 15) and used for hydrogenation of phenolic
compounds of bio-oil.1,3,8,16 Zhao et al.8 reported that
substituting liquid acid with solid acid enhances the hydro-
treating efficiency of the mixed catalyst system. Brønsted acids
are appropriate for hydrotreatment of oxygenated compounds
due to the presence of H+ sites in their structure which promote
dehydration reaction. Zeolites contain Brønsted acid sites and
have high shape selectivity and thermal/hydrothermal stability
which have been widely used in acid catalyzed reactions such as
isomerization, cracking and alkylation.18 Zeolites with optimum
density and strength of acid sites seem to be suitable for
hydrodeoxygenation of bio-oil/bio-oil model compounds. The
structural framework of zeolite includes the atoms of silicon
and/or aluminium having tetrahedral coordination to four
oxygen atoms.19,20 A zeolite consisting of only SiO4 is electrically
neutral but the presence of each AlO4 in zeolite framework
Department of Chemical Engineering, Faculty of Engineering, University of Malaya,
50603 Kuala Lumpur, Malaysia. E-mail: h.shafaghat@gmail.com; pouya.sr@gmail.
com; ashri@um.edu.my; Fax: +60 3 79675319; Tel: +60 3 79675297
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra00367a
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makes a negative charge which can be compensated by a
cation.19,20 The presence of AlO4 species and cations in zeolite
structure changes catalytic properties of zeolites. By compen-
sating the negative charges with protons, zeolites become
strong solid acids.
In this study, the reaction mechanisms of hydrogenation of
phenols over physically mixed catalysts containing metal and
acid active sites are investigated. Phenol, o-cresol, m-cresol and
guaiacol (the most abundant phenolic compounds of bio-oil)
were used as model compounds to investigate the hydrogena-
tion activity of 10 wt% Pd/C and combined catalysts of 10 wt%
Pd/C with HZSM-5 or HY zeolites. Effects of metal and acid
active sites on catalytic activity and product selectivity of
hydrogenation process were studied. The dependency of cata-
lytic properties of zeolite on its aluminium content was inves-
tigated by the use of HZSM-5 and HY with different Si/Al ratios
of 30, 50, 80 and 30, 60, respectively.
Fig. 1 Nitrogen adsorption–desorption isotherms of 10 wt% Pd/C, HY
(30), HY (60), HZSM-5 (30), HZSM-5 (50), and HZSM-5 (80).
2. Results and discussion
2.1. Catalyst characterization
strong acid sites, respectively.4,11 These acid sites could be both
types of Lewis and Brønsted acids.3,8,21–23 Jin and Li23 professed
Textural properties of 10 wt% Pd/C catalyst and zeolite solid that Lewis acid sites are dominant at desorption temperature
acids determined by N2 isothermal adsorption–desorption are range of 200–300 ꢀC while at 300–450 ꢀC, Brønsted acid sites are
presented in Table 1. The BET surface area of all catalysts is the major acid sites of ZSM-5. Desorption temperature is asso-
high (>290 m2 gꢁ1) and the micropore surface area of all cata- ciated to the strength of interaction between ammonia and acid
lysts predominates on external surface area. The increase of sites; high temperature is required for separation of ammonia
Si/Al ratio in all the zeolites led to higher BET surface area and adsorbed on strong acid sites. By the increase of Si/Al ratio from
pore volume. As depicted in Fig. 1, all the catalysts showed type 50 to 80, both the low- and high-temperature desorption peaks
IV of adsorption isotherm with hysteresis loop H4. The H4 shied to lower temperature which means that the acid
hysteresis loop and nearly horizontal curve of isotherms strength is decreased. HZSM-5 (30) has only one broad
demonstrate that Pd/C and zeolite solid acids are mostly desorption peak at low-temperature (221 ꢀC) indicating that its
microporous. Meanwhile, the distribution of meso scale pores acidic strength is low. Since standard condition volume of
in catalysts determined by BJH desorption pore size distribution ammonia desorbed per gram of catalyst typically signies the
(Fig. S1†) exhibits that Pd/C has a very broad pore size distri- concentration of acid sites,24 HZSM-5 (30) has higher density of
bution between 3 and 60 nm. HY zeolites have two pore size acid sites compared to HZSM-5 (50) and HZSM-5 (80) (Table 2).
distributions of 3–4 nm (very narrow) and 4–50 nm (very broad) TPD analysis of HY (30) and HY (60) zeolites resulted in only one
which the dominant part of pores have a diameter in the range ammonia desorption peak in the temperatures of 315 and
of 3–4 nm. HZSM-5 (30) and HZSM-5 (50) have also a bimodal 330 ꢀC, respectively. By increase of Si/Al ratio in HY zeolites,
distribution with maximum peaking at 3–10 and 3–4 nm, desorption peak moved to higher temperature indicating that
respectively. Substantial part of the mesopore volume found in acidity is strengthened. Based on a theoretic analysis, Brønsted
HZSM-5 (80) was in the range of 3–10 nm.
acid strength of zeolites (^Si–OH–Al^) depends on the
The NH3-TPD curves of HZSM-5 and HY zeolites are shown in composition of second coordination sphere around the span-
Fig. 2. It can be seen from this gure that HZSM-5 (50) and ning OH group; the strongest Brønsted acid site is the one with
HZSM-5 (80) have two ammonia desorption peaks at low and six Si4+ species in the second coordination sphere around OH
high temperature regions (Table 2) which belong to weak and group and replacing the Si4+ by Al3+ decreases the acid
Table 1 Textural properties of 10 wt% Pd/C and zeolite solid acids
HZSM-5
HZSM-5
HZSM-5
HY
HY
Catalyst
Pd/C
(Si/Al ¼ 30)
(Si/Al ¼ 50)
(Si/Al ¼ 80)
(Si/Al ¼ 30)
(Si/Al ¼ 60)
BET surface area (m2 gꢁ1
)
694
390
304
0.53
0.19
0.34
291
192
99
0.19
0.09
0.10
326
216
110
0.20
0.10
0.10
345
214
131
0.21
0.10
0.11
645
487
158
0.43
0.24
0.19
664
487
177
0.44
0.24
0.20
Micropore surface area (m2 gꢁ1
)
External surface area (m2 gꢁ1
Pore volume (cm3 gꢁ1
Micropore volume (cm3 gꢁ1
Mesopore volume (cm3 gꢁ1
)
)
)
)
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cyclohexane as oxygen-free product (path II). Except HY (30),
other zeolites dehydrated all produced cyclohexanol to cyclo-
hexene. Low hydrogen pressure of 15 bar in phenol hydroge-
nation stabilized the production of cyclohexanone
intermediate. Our result is in accordance with that of Zhao and
Lercher16 who noted that low hydrogen pressure in phenol
hydrodeoxygenation stabilizes cycloketone intermediate and
decreases cycloalkane formation. Meanwhile, our data reveal
that zeolites cannot be involved in hydrogenation mechanism;
no conversion of phenol was observed in hydrogenation of
phenol over HZSM-5 (80) and HY (60) catalysts without Pd/C,
indicating that zeolites do not possess proper active sites for
activation of hydrogen molecules to be involved in hydrogena-
tion reaction. In addition, benzene formation was only observed
over mixed catalysts of Pd/C-HY (30) and Pd/C-HZSM-5 (30). The
proposed reaction pathway for benzene production (path III) is
as follows: phenol decarbonylation to cyclopentadiene which is
converted to naphthalene via Diels–Alder mechanism,25,26
naphthalene hydrogenation to tetralin, tetralin ring-opening/
isomerization to alkyl benzene and dealkylation of alkyl
benzene to benzene. High density of acid sites in HY (30)
(4.822 ml gꢁ1) and HZSM-5 (30) (6.247 ml gꢁ1) resulted in
dealkylation of alkyl benzene and production of benzene.
Selectivity toward benzene in the presence of HY (30) and
HZSM-5 (30) was 2.34 and 3.12 mol%, respectively. As depicted
in Table 3, HZSM-5 was more efficient than HY for catalytic
transformation of phenol to cyclohexane. In the case of using
HZSM-5 (80), phenol hydrogenation occurred via two reaction
pathways; (i) the common pathway of ring hydrogenation of
phenol to cyclohexanone and cyclohexanol with subsequent
dehydration of cyclohexanol to cyclohexene followed by further
hydrogenation of cyclohexene to cyclohexane (paths I and II),
(ii) phenol hydrogenation to cyclohexanol and subsequent
production of 2-cyclohexylphenol through electrophilic
aromatic substitution (EAS) of phenol with cyclohexyl cation
followed by further hydrogenation of 2-cyclohexylphenol to
2-cyclohexylcyclohexanone and dicyclohexane (paths I and IV).27
In fact, Brønsted acid sites of zeolite catalyze the alkylation of
phenol with cyclohexanol to form 2-cyclohexylphenol.15 Gener-
ally, alkylation of phenol by cyclohexanol and production of
bicyclic compounds is possible over combined catalysts of acid
zeolite and palladium.15 One signicant factor for possibility of
alkylation reaction is molar ratio of phenol to Pd which should
be sufficient to proceed alkylation. At low ratios of phenol to Pd,
phenol undergoes hydrogenation rather than alkylation, while
at high phenol to Pd ratios and sufficient concentration of
cyclohexanol, phenol could be alkylated. Zhao et al.15 reported
that at low phenol to Pd molar ratio of 564, only 2.1 C%
Fig. 2 NH3-TPD curves of HZSM-5 and HY zeolites with different Si/Al
ratios.
strength.20 According to the high-temperature desorption peak,
it can be concluded that the acid strength of zeolites follows the
trend: HZSM-5 (50) ꢂ HZSM-5 (80) > HY (60) > HY (30).
2.2. Catalytic hydrogenation of phenol
Reactant conversion and product distribution obtained from
catalytic hydrogenation of phenol over Pd/C catalyst and
combined catalysts of Pd/C and zeolite solid acids are presented
in Table 3. Using mixture of Pd/C and zeolite led to higher
phenol conversion compared to the use of Pd/C alone.
Maximum phenol conversion of 96.89 mol% was achieved over
mixed catalyst of Pd/C-HZSM-5 (80) while phenol was converted
only 48.05 mol% over Pd/C. By increase in density of acid sites
in both zeolites of HZSM-5 and HY (decrease of Si/Al ratio),
phenol conversion decreased. This could be due to the fact that
higher density of acid sites causes higher formation of coke and
zeolite deactivation.4
Reaction mechanisms of phenol hydrogenation on Pd/C or
mixed catalysts of Pd/C and zeolite is depicted in Scheme 1.
Over Pd/C, hydrogenation of benzene ring of phenol was the
main reaction pathway producing cyclohexanone and cyclo-
hexanol (path I); cyclohexanol is the end product since it is not
dehydrated in the absence of Brønsted acid sites. The addition
of zeolite solid acids to Pd/C led to dehydration of cyclohexanol
to cyclohexene (not detected in product samples) and further
hydrogenation of cyclohexene on metal catalyst producing
Table 2 Ammonia desorption temperature and amount of desorbed ammonia from zeolite acid sites obtained by TPD analysis
Zeolites
HZSM-5 (30)
HZSM-5 (50)
HZSM-5 (80)
HY (30)
HY (60)
Desorption peak temperature (ꢀC)
Volume of desorbed ammonia
(ml gꢁ1 STP)
221
6.247
225
1.592
382
3.219
203
1.276
378
2.810
315
4.822
330
3.136
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Table 3 Hydrogenation of phenol over mixed catalysts of Pd/C and zeolite solid acids
Pd/C-HZSM-5
(80)
Pd/C-HZSM-5
(50)
Pd/C-HZSM-5
(30)
Pd/C-HY
(60)
Pd/C-HY
(30)
Catalyst
Pd/C
Conversion (mol%)
Product compounds
96.89
68.24
49.00
53.15
50.27
48.05
Product selectivity (mol%): (mole of product compound/mole of total product) ꢃ 100
Benzene
Cyclohexanone
Cyclohexanol
—
70.71
—
22.83
3.25
0.85
0.57
1.80
—
75.99
—
22.14
—
—
3.12
76.86
—
19.97
—
—
—
0.20
—
87.09
—
11.11
—
—
2.34
85.94
1.14
10.40
—
—
—
0.24
—
83.86
14.35
—
—
—
Cyclohexane
Dicyclohexane
2-Cyclohexylcyclohexanone
2-Cyclohexylphenol
Tetralin
—
1.88
—
1.90
—
1.97
selectivity of bicyclic products was achieved. In our work, the agreement with the results reported by Massoth et al.28 who
ratio of phenol to Pd is very low (ꢂ227) which prevents the showed that higher aromatic yield is achieved from hydroge-
alkylation mechanism and only by the use of Pd/C mixed with nation of methyl-substituted phenols rather than phenol over
HZSM-5 (80), very low concentrations of bicyclic products were sulded CoMo/Al2O3 catalyst. Generally, conversion of o-cresol
observed. Furthermore, the possibility of phenol alkylation has was lower than that of m-cresol; maximum conversion of m-
reverse dependency on density of acid sites; higher density of cresol was 76.23 mol% which was achieved over Pd/C-HY (60)
acid sites favors dehydration reaction rather than alkylation.15 while maximum conversion of o-cresol was only 50.65 mol%
Therefore, bicyclic compounds were only produced over obtained by Pd/C-HZSM-5 (50). Lower conversion of o-cresol,
HZSM-5 (80) which has lower density of acid sites compared to with the methyl group adjacent to hydroxyl group, could be due
HZSM-5 (50) and HZSM-5 (30) (Table 2).
to the presence of steric hindrance to adsorption of o-cresol on
catalytic active sites. Hydrogenation of o-cresol (Scheme 2) was
proceeded through pathways of direct hydrodeoxygenation of
cresol to toluene (path III), cresol alkylation to 2,6-dimethyl-
phenol (path IV), and hydrogenation of benzene ring to
2-methylcyclohexanone/2-methylcyclohexanol (path I) followed
by dehydration of 2-methylcyclohexanol to methylcyclohexane
(path II). In addition to the above mentioned reaction
2.3. Catalytic hydrogenation of cresol
Results obtained from hydrogenation of o-cresol and m-cresol
over 10 wt% Pd/C and zeolite solid acids are presented in Tables
4 and 5. Compared to phenol molecule, the presence of methyl
group in cresol favors aromatic production. This is in
Scheme 1 Reaction pathways of phenol hydrogenation over Pd/C and Pd/C mixed with HZSM-5 or HY zeolites; HDO: hydrodeoxygenation,
HYD: hydrogenation, DHY: dehydration, EAS: electrophilic aromatic substitution.
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Table 4 Hydrogenation of o-cresol over mixed catalysts of Pd/C and zeolite solid acids
Pd/C-HZSM-5
(80)
Pd/C-HZSM-5
(50)
Pd/C-HZSM-5
(30)
Pd/C-HY
(60)
Pd/C-HY
(30)
Catalyst
Pd/C
Conversion (mol%)
Product compounds
38.16
50.65
35.90
39.38
47.75
35.20
Product selectivity (mol%): (mole of product compound/mole of total product) ꢃ 100
Toluene
29.58
2.59
66.24
—
29.52
2.09
67.33
—
29.84
1.83
66.92
—
19.98
—
79.07
—
18.24
—
80.63
0.33
0.80
16.23
—
79.80
3.96
—
Methylcyclohexane
2-Methylcyclohexanone
2-Methylcyclohexanol
2,6-Dimethylphenol
1.58
1.06
1.41
0.95
mechanisms for hydrogenation of o-cresol, m-cresol seems to be
hydrogenated through some more reaction pathways resulting
in some different products (Table 5). The proposed reaction
mechanisms for hydrogenation of m-cresol are depicted in
Scheme 3. The results obtained in this work indicate that the
effects of addition of zeolite solid acids to Pd/C on product
selectivity are dependent on the methyl group position in cresol
molecule. For instance, zeolite addition did not have negative
effect on toluene selectivity in hydrogenation of o-cresol while in
m-cresol hydrogenation, zeolite addition led to the decrease of
toluene production (except HZSM-5 (30)) due to dehydration,
demethylation and alkylation reactions which are competitively
occurred to produce methylcyclohexane, phenol and 2-ethyl-
phenol, respectively.
In o-cresol transformation, catalytic activity of mixed cata-
lysts of Pd/C and zeolite solid acids decreased in the order
Pd/C-HZSM-5 (50) > Pd/C-HY (30) > Pd/C-HY (60) > Pd/C-HZSM-5
(80) > Pd/C-HZSM-5 (30) > Pd/C. Hydrogenation of benzene ring
Scheme 2 Mechanisms of hydrogenation of o-cresol over combined
catalysts of Pd/C and zeolite solid acids.
Table 5 Hydrogenation of m-cresol over mixed catalysts of Pd/C and zeolite solid acids
Pd/C-HZSM-5
(80)
Pd/C-HZSM-5
(50)
Pd/C-HZSM-5
(30)
Pd/C-HY
(60)
Pd/C-HY
(30)
Catalyst
Pd/C
Conversion (mol%)
Product compounds
48.02
66.95
39.63
76.23
42.00
39.58
Product selectivity (mol%): (mole of product compound/mole of total product) ꢃ 100
Toluene
Methylcyclohexane
Phenol
15.30
1.48
6.80
2.33
73.61
—
—
—
0.64
0.34
—
9.26
2.87
11.19
3.16
73.40
—
—
—
—
—
20.57
0.22
1.24
1.00
76.50
—
0.38
—
—
14.47
0.20
1.00
1.25
80.79
—
0.08
0.12
—
18.63
0.97
—
19.51
—
—
Cyclohexanone
—
—
3-Methylcyclohexanone
3-Methylcyclohexanol
2-Ethylphenol
2-Ethylcyclohexanone
1,10-Bicyclopentyl
1-Butylcyclohexene
1,10-Bicyclohexyl, 4,40-
dimethyl
79.73
0.16
0.52
—
—
—
80.00
1.29
—
—
—
—
—
—
0.20
—
—
—
—
1-Phenyl-1-
—
—
—
0.14
—
—
cyclohexylethane
4,40-Dimethylbiphenyl
Cyclohexylbenzene
—
—
—
—
—
0.12
1.76
—
—
—
—
—
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Scheme 3 Mechanisms of hydrogenation of m-cresol over combined catalysts of Pd/C and zeolite solid acids.
of o-cresol to 2-methylcyclohexanone and 2-methylcyclohexanol acidity compared to other zeolites. Meanwhile, addition of
(Scheme 2, path I) was the dominant reaction mechanism over HZSM-5 zeolites and HY (60) resulted in demethylation of
Pd/C, while the addition of HZSM-5 led to dehydration of m-cresol to phenol followed by phenol hydrogenation and
2-methylcyclohexanol and formation of methylcyclohexane as cyclohexanone formation (path IV). Phenol alkylation to
oxygen-free product (path II). Lower density of acid sites in 2-ethylphenol and further hydrogenation of 2-ethylphenol to
HZSM-5 zeolites led to higher selectivity of methylcyclohexane 2-ethylcyclohexanone was also occurred (path V). High density
over mixed catalysts. However, HY zeolites were not selective to of acid sites in HZSM-5 (30) and HY (30) favored 2-ethylphenol
the production of methylcyclohexane. This is in agreement with production. Unlike m-cresol, demethylation mechanism was
the results obtained by Zhao et al.8 who demonstrated that not occurred in transformation of o-cresol illustrating that the
ketone was selectively formed over HY zeolite in hydro- methyl group adjacent to hydroxyl group prevents dealkylation
deoxygenation of 4-n-propylphenol. As is depicted in Scheme 2, reaction on zeolite acid site. Production of large molecule
HDO of o-cresol to toluene is another reaction mechanism over compounds over HY (60) might be due to the large pore size of
Pd/C and mixed catalysts (path III). The addition of HZSM-5 this zeolite which allows formation of such large molecules
strongly affected the selectivity toward toluene while HY did through aromatic ring condensation or isomerization reactions.
not have remarkable effect on HDO selectivity. It is also inferred
from Table 4 that toluene selectivity is not a function of density
and strength of zeolite acid sites. The results show that
Brønsted acid sites of zeolites were not efficient for dehydration
of 2-methylcyclohexanol to methylcyclohexane. Meanwhile, the
(Table 6). Compared to HY zeolite, HZSM-5 showed higher
combination of zeolite solid acids with Pd/C catalyst led to
alkylation of o-cresol (path IV) which occurs concurrently with
hydrogenation and hydrodeoxygenation reactions.
The study of catalytic hydrogenation of m-cresol over Pd/C
and mixed catalysts of Pd/C–zeolite revealed that conversion
of m-cresol decreased in the order Pd/C-HY (60) > Pd/C-HZSM-5
(50) > Pd/C-HZSM-5 (80) > Pd/C-HY (30) > Pd/C-HZSM-5 (30) >
guaiacol took place to form 2-methoxycyclohexanone (path I)
Pd/C. Direct hydrodeoxygenation (Scheme 3, path I) and
benzene ring hydrogenation (path II) were the only reaction
mechanisms in hydrogenation of m-cresol over Pd/C. Selectivity
towards toluene reduced in the order Pd/C-HZSM-5 (30) >
Pd/C-HY (30) > Pd/C > Pd/C-HZSM-5 (80) > Pd/C-HY (60) >
Pd/C-HZSM-5 (50). Dehydration of 3-methylcyclohexanol to
methylcyclohexane (path III) was activated by zeolite addition
and maximum selectivity of methylcyclohexane was obtained in
the presence of HZSM-5 (50) which has highest strength of
2.4. Catalytic hydrogenation of guaiacol
The addition of HZSM-5 and HY zeolites to Pd/C caused
enhanced catalytic performance in transformation of guaiacol
catalytic activity, and complete conversion of guaiacol was
achieved over mixed catalysts of Pd/C-HZSM-5 (80). Phenol,
cyclohexanone and cyclopentanone were the highly selective
products of guaiacol hydrodeoxygenation over Pd/C. Reaction
mechanisms of guaiacol hydrogenation on Pd/C is depicted in
Scheme 4. Benzene ring hydrogenation and dehydroxylation of
and anisole (path II), respectively. Concurrently, demethoxy-
lation of guaiacol occurred to produce phenol (path III) which
was transformed to cyclohexanone/cyclohexanol (path IV),
o-cresol/2-methylcyclohexanone (path V) and o-cresol/toluene
(path VI) through hydrogenation, methylation–hydrogenation
and methylation–dehydroxylation reactions, respectively.
o-Cresol could be produced via methylation of phenol with
methanol which is formed through demethoxylation of guaia-
col. Unusual production of cyclopentanone from guaiacol
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Table 6 Hydrogenation of guaiacol over mixed catalysts of Pd/C and zeolite solid acids
Pd/C-HZSM-5
(80)
Pd/C-HZSM-5
(50)
Pd/C-HZSM-5
(30)
Pd/C-HY
(60)
Pd/C-HY
(30)
Catalyst
Pd/C
Conversion (mol%)
Product compounds
100.00
93.95
92.08
62.79
56.43
37.38
Product selectivity (mol%): (mole of product compound/mole of total product) ꢃ 100
Phenol
61.07
30.90
—
0.24
1.11
2.91
3.36
0.39
0.01
—
66.87
23.62
—
0.37
1.21
2.88
4.44
0.62
—
71.08
20.50
—
0.09
1.22
3.12
3.15
0.78
—
33.61
37.03
—
0.21
2.20
16.37
9.89
—
33.03
40.84
—
0.20
3.32
17.35
5.02
0.16
0.08
—
31.91
27.23
4.62
—
2.88
17.97
8.85
0.20
0.15
6.21
Cyclohexanone
Cyclohexanol
Cyclohexane
Toluene
Cyclopentanone
Anisole
o-Cresol
2-Methylcyclohexanone
2-Methoxycyclohexanone
0.51
0.19
—
—
deoxygenation might be due to ring hydrogenation, ring toward phenol, and HZSM-5 with higher density of acid sites led
opening/closing and decarbonylation reactions of catechol to higher phenol selectivity. On the other hand, cyclopentanone
(path VII).29 Ring opening reaction is supposed to be catalyzed formation notably decreased by the addition of HZSM-5 to Pd/C
by metal catalyst.29,30 Since catechol was not detected in product while HY did not show remarkable effect on the selectivity of
analysis, it can be considered as an intermediate product which cyclopentanone. Except HY (60), addition of all other zeolites
was completely consumed during the reaction. In comparison caused a reduction in dehydroxylation reaction and anisole
with HY zeolite, HZSM-5 remarkably improved the selectivity production. Meanwhile, low concentration or absence of
Scheme 4 Reaction mechanisms of guaiacol hydrogenation over 10 wt% Pd/C catalyst and combined catalysts of Pd/C and zeolite solid acids.
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2-methoxycyclohexanone in product composition observed in reactor was pressurized with hydrogen gas to 15 bar and heating
guaiacol hydrogenation over Pd/C mixed with zeolites illustrates was started. Reaction was carried out at 275 ꢀC and stirring
that the addition of zeolite solid acids to metal catalyst resulted speed of 500 rpm. In the beginning of the reaction, total pres-
ꢀ
in the reduction of benzene ring hydrogenation of guaiacol.
sure of the system was decreased (from 70 to 66 bar at 275 C)
due to the high activity of catalysts for hydrogenation reaction
leading to high hydrogen consumption. Aer a short while, rate
of pressure reduction was decreased due to the decrease of
hydrogen consumption rate with the reaction time, and pres-
sure of the reactor was xed at 64 bar till the end of reaction.
Aer 2 h reaction, reactor was rapidly cooled to ambient
temperature and products were collected. Solid catalysts were
separated from liquid product by vacuum ltration. Organic
phase of product was separated by use of ethyl acetate as solvent
and analyzed by GC-MS (Shimadzu QP 2010, DB-5 30 m ꢃ
0.25 mm ꢃ 0.25 mm) coupled with ame ionization and mass
spectrometry detection. The products were identied by using
NIST (National Institute of Standards and Technology) library.
2-Isopropylphenol was used as internal standard for quantita-
tive analysis of product samples based on the equation of
Cx ¼ Cis(Ax/Ais) which Cx, Cis, Ax and Ais are concentration of
component x, concentration of internal standard, peak area of
component x and peak area of internal standard, respectively.
Conversion and product selectivity were calculated as follows:
conversion ¼ (mole of consumed reactant/mole of initial reac-
tant) ꢃ 100; selectivity ¼ (mole of a certain product/mole of
total product) ꢃ 100.
3. Experimental section
3.1. Chemicals
Phenol (C6H6O, $99%), m-cresol (C7H8O, $98%), o-cresol
(C7H8O, $99%), guaiacol (C7H8O2, natural, $98%) and 2-iso-
propylphenol (C9H12O, $98%) were purchased from Sigma-
Aldrich. Ethyl acetate was purchased from R&M Chemicals.
All the chemicals were used as received without any purica-
tion. Puried hydrogen and nitrogen were supplied from Linde
Malaysia Sdn. Bhd.
3.2. Catalysts
Pd/C (10 wt% loading, matrix activated carbon support) was
obtained from Sigma-Aldrich. ZSM-5 (Si/Al of 30, 50 and 80) and
Y-zeolite (Si/Al of 30 and 60) were purchased from Zeolyst
International. Prior to catalytic activity measurements, all the
ꢀ
zeolites were calcined in air at 550 C overnight.
3.3. Catalyst characterization
BET surface area, pore volume and pore size distribution of
fresh samples of Pd/C, HZSM-5 (30), HZSM-5 (50), HZSM-5 (80),
HY (30) and HY (60) were determined from the N2 isothermal
(ꢁ196 ꢀC) adsorption–desorption using Micromeritics ASAP
2020 surface area and porosity analyꢀzer. Before textural analysis,
the samples were outgassed at 180 C under vacuum for 4 h.
The acidity of HZSM-5 and HY was studied by temperature
programmed desorption of ammonia (NH3-TPD) on Micro-
meritics ChemiSorb 2720 instrument using mixed gas of 10%
NH3/90%He. 200 mg of each sample was loaded in TPD cell and
heated from ꢁa1mbient temperature to 700 ꢀC with a heating rate
of 20 ꢀC min and was held at 700 ꢀC for 1 h in a stream of He
(20 ml minꢁ1). Then, the sample was cooled down to 210 ꢀC and
ammonia was adsorbed aer change of gas from pure He to
10%NH3/90%He for 30 min. The sample was purged with He
for 30 min to remove physisorbed molecules. Ammonia
4. Conclusion
The inuence of density and strength of acid sites on trans-
formation of different phenolic reactants was studied in a
physically mixed catalyst system. In hydrogenation of phenol,
o-cresol, m-cresol and guaiacol, the combination of 10 wt% Pd/
C catalyst with HZSM-5 (Si/Al of 30, 50 and 80) or HY (Si/Al of 30
and 60) zeolites resulted in enhanced catalytic activity
compared to the use of Pd/C alone. Zeolites containing strong
acid sites showed better catalytic performance for trans-
formation of phenol, o-cresol, m-cresol and guaiacol. Zeolite
solid acids led mostly to the reactions of alkylation, deal-
kylation and dehydration of cyclic alcohols. In phenol hydro-
genation over the combined catalysts, Brønsted acid sites were
mainly involved in dehydration of cyclohexanol to form
cyclohexane as an oxygen-free product. Hydrogenation reac-
tion selectivity of phenol molecules containing methyl
(o-cresol and m-cresol) or methoxy (guaiacol) groups is entirely
different from that of the simple phenol molecule. As can be
ꢀ
ꢀ
desorption measurement was carried ꢁo1ut from 70 C to 600 C
with the heating rate of 10 ꢀC min using He carrier gas
(20 ml minꢁ1).
3.4. Catalytic activity measurement
Hydrogenation of phenol, o-cresol, m-cresol and guaiacol was concluded from the data obtained in this research, the pres-
conducted in an autoclave batch reactor equipped with a ence of methyl and methoxy groups in cresol and guaiacol
magnetic drive stirrer and water cooling coil (1000 ml volume, structures favors direct hydrodeoxygenation mechanism in the
maximum temperature and pressure of 500 ꢀC and 100 bar, hydrogenation reaction. Meanwhile, the position of methyl
respectively). Before reaction, mixed catalysts of Pd/C (0.6 g) and group and spatial structure of cresol molecules affects the
calcined zeolite (1 g) were pretreated by hydrogen gas at 300 ꢀC product selectivity of hydrogenation reaction. In conversion of
and 20 bar for 1 h. Then, the system was cooled to ambient o-cresol, selectivity toward toluene was enhanced by the use of
temperature and pressure was released to atmosphere. Reactant mixed catalysts, while in m-cresol hydrogenation, zeolites led
(12 g) and H2O (140 ml) were loaded into the reactor under the to dealkylation and phenol production and decrease of
inert atmosphere of nitrogen. Hydrogen gas was charged–dis- hydrodeoxygenation. The presence of methoxy group in
charged for four times to displace the nitrogen. Aerward, guaiacol structure resulted in dehydroxylation reaction and
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The authors wish to acknowledge the Faculty of Engineering at
University of Malaya for nancial support through the HIR
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