Gas-phase hydrogenation of 4-tert-butylphenol over Pt/SiO2

Article abstract of DOI:10.1016/j.jcat.2004.06.027

Gas-phase hydrogenation of 4-tert-butylphenol (4-TBP) over 1% Pt/SiO ₂ to cis and trans 4-tert-butylcyclohexanol (4-TBCHOL), via the intermediate 4-tert-butylcyclohexanone, was studied in a differential reactor at atmospheric pressure and at temperatures between 127 and 227°C. The formation of by-products due to hydrogenolysis played an important role in the reaction at temperatures over 200°C. The rates of 4-t-butylcyclohexanol and 4-t-butylcyclohexanone formation passed through a maximum at 187°C. The catalyst deactivation was also considerable. Being thermodynamically more stable the trans form of the alcohols isomers is produced in separate experiments by epimerization in the presence of hydrogen as an astoichiometric compound. The reaction network for the reaction is proposed.

Full text of DOI:10.1016/j.jcat.2004.06.027

Journal of Catalysis 227 (2004) 60–67
www.elsevier.com/locate/jcat
Gas-phase hydrogenation of 4-tert-butylphenol over Pt/SiO2
Ahmad Kalantar a,b, Henrik Backman a, Jose H. Carucci a, Tapio Salmi a, Dmitry Yu. Murzin a,∗
a
Laboratory of Industrial Chemistry, Process Chemistry Centre, Åbo Akademi University, FIN-20500 Turku-Åbo, Finland
b
Nynäs Naphthenics AB, SE-149 82, Nynäshamn, Sweden
Received 7 April 2004; revised 23 June 2004; accepted 30 June 2004
Available online 21 July 2004
Abstract
Gas-phase hydrogenation of 4-tert-butylphenol (4-TBP) over 1% Pt/SiO to cis and trans 4-tert-butylcyclohexanol (4-TBCHOL), via
2
the intermediate 4-tert-butylcyclohexanone, was studied in a differential reactor at atmospheric pressure and at temperatures between 127
◦
◦
and 227 C. The formation of by-products due to hydrogenolysis played an important role in the reaction at temperatures over 200 C. The
◦
rates of 4-t-butylcyclohexanol and 4-t-butylcyclohexanone formation passed through a maximum at 187 C. The catalyst deactivation was
also considerable. Being thermodynamically more stable the trans form of the alcohols isomers is produced in separate experiments by
epimerization in the presence of hydrogen as an astoichiometric compound. The reaction network for the reaction is proposed.
2004 Elsevier Inc. All rights reserved.
Keywords: Hydrogenation; Gas phase; 4-tert-butylphenol; Pt/SiO
2
1. Introduction
pending on the product of interest. The highest selectivity
to cyclohexanones is achieved by palladium whereas a mix-
ture of cis and trans isomers is obtained from hydrogenation
of alkyl-substituted phenols over nickel-based catalysts [3].
For the hydrogenation of 4-tert-butylphenol a stereoselectiv-
ity up to 100% has been achieved using a rhodium catalyst
with an acidic cocatalyst [4]. As noted, these considerations
have been studied in the liquid phase. The liquid-phase re-
action places restrictions on temperature and pressure and
therefore the equipment is more complicated. The catalyst
must be separated from the products, creating additional
complications [5]. A promising alternative could be gas-
phase hydrogenation, therefore the gas-phase hydrogena-
tion of 4-tert-butylphenol over Pt/SiO2 was the focus of the
present study. To the best of our knowledge no report on gas-
phase hydrogenation of TBPH is given in the literature.
Alkylated cyclohexanols, like 2- and 4-tert-butylcyclo-
hexanol, are important intermediates for the fragrance and
perfumery industry [1], conventionally prepared by cat-
alytic liquid-phase hydrogenation of alkyl-substituted phe-
nols. The problem of stereoselectivity of catalytic reactions
has been encountered for many years. Alkyl-substituted phe-
nols can be selectively hydrogenated to corresponding cyclo-
hexanols (cis and trans) or they can be selectively directed to
cyclohexanones, which are sometimes preferable products.
The enol form of this intermediate (alkylcyclohexen-1-ol) is
predominantly hydrogenated to the cis isomer [2]. It partic-
ipates apparently in an increase of this isomer in the final
mixture. Industrial interests, in a number of cases, are lim-
ited only to the cis stereoisomer form of the corresponding
alkylcyclohexanols as they serve as important intermediates
for the industry.
2. Experimental
In general hydrogenation of alkylphenols is carried out
in the liquid phase over palladium or nickel catalysts, de-
2.1. Catalyst preparation
*
The Pt/SiO2 catalysts were prepared by impregnation of
a silica support (Merck, 0.063–0.2 mm), having a BET sur-
Corresponding author. Fax: +358-2-2154479.
E-mail address: dmurzin@abo.fi (D.Yu. Murzin).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.06.027
2004 Elsevier Inc. All rights reserved.

A. Kalantar et al. / Journal of Catalysis 227 (2004) 60–67
61
face area of 485 m2/g, with a solution of Pt(NH3)4Cl2. After
2.3. Catalytic activity measurement
the impregnation the samples were step-washed with deion-
ized water, dried at 90 ◦C, and stored prior to their use. The
catalyst metal content was determined by the direct current
plasma (DCP) technique (Spectraspan IIIA, Spectrometrics).
A calcium-doped Pt/SiO2 was also prepared. SiO2 sup-
port was doped with Ca by ion exchange with nitrate salt.
Pt/alumina catalysts were prepared by wet impregnation of
a γ -alumina support (LaRoche, Versal GL25, BET surface
area of 219 m2/g) using a solution of H2PtCl6 and Pt(NO3)2.
These two salts were used to study the effects of increased
carrier acidity due to the presence of surface chlorine anion
and its influence on the electric properties of small platinum
particles.
The catalytic experiments were carried out mainly with
1% Pt/SiO2 in a continuous flow tube reactor, operating
at atmospheric pressure, around which a heating element
was mounted. A K-type thermocouple was inserted coaxially
into the catalyst bed and was used to simultaneously measure
the catalyst bed temperature and control the power supply to
the heating element via a programmed temperature control
unit. Since 4-TBP (Acros Organics) is a solid substance, in
order to avoid the crystallization problems, the reactor and
the saturator were placed in an oven kept at 130 ◦C, which
effectively prevented the crystallization of the reactant and
the products. All the lines after the oven were heated. The
products were analyzed by a GC (HP 5890) equipped with
flame ionization detector and a DV-WAX column. The GC
analyses were carried out isothermally at 170 ◦C. In order
to determine the light products (hydrogenolysis/cracked at
higher temperatures) the separation was carried out at am-
bient temperature after which the ramping and final dwell
at 170 ◦C were applied. Helium was used as the carrier
and the injector and detector temperatures were 250 and
240 ◦C, respectively. The products were further confirmed
by GC-MS (HP 6890–5973 Instrument). The gas hour space
velocity (GHSV) was set at 125.76 h−1 (124.8 mL/min)
and the partial pressures of hydrogen and 4-TBP were var-
ied from 0.058 to 0.13 bar and from 0.0088 to 0.0015 bar,
respectively, using argon as a make-up gas keeping a con-
stant GHSV. Prior to each catalytic experiment, the catalyst
(60 mg, 125–150 µm) was reduced in situ in a flow of hy-
drogen at 400 ◦C for 2 h followed by cooling in hydrogen
flow to the desired temperature of the reaction. All the gases
were of 99.999 vol% purity. Preliminary experiments were
carried out to establish the testing conditions, and to ensure
that the kinetic experiments are measured in the absence of
external and internal diffusion limitation problems.
Mass balance study was performed at both lowest and
highest temperatures where the hydrogenolysis/cracking
products were formed. The products were identified by
GC-MS and special calibrations were carried out to deter-
mine the reactants/products concentration with a high degree
of accuracy. The amount of light hydrogenolysis/cracking
products (although in small amounts) was determined by
GC and special calibration gases. The GC analysis of light
and heavier products was determined both isothermally and
with temperature ramping from ambient temperature. The
results indicated that almost all the carbon was accounted
for, and the amount that could be attributed to carbon depo-
sition on the catalyst as well as the error was in the range of
acceptable experimental error.
2.2. Catalyst characterization
The dispersion and mean particle diameters of the metal
were determined by hydrogen adsorption, using a Sorp-
tomatic 1900 (Carlo Erba Instruments). The adsorption
isotherms were recorded at 298 K and pressures of 0.0013–
0.133 bar. Extrapolation of adsorption isotherms to zero
pressure was used to determine the amount of irreversibly
adsorbed hydrogen. The amount of reversibly adsorbed hy-
drogen was determined by the back-sorption method. Prior
to the H2 adsorption, the catalysts were reduced in situ under
hydrogen flow at 400 ◦C for 2 h. Dissociative adsorption of
hydrogen was adopted and the metal particle sizes were de-
termined by assuming spherical particle geometry. The BET
specific surface area of the fresh and used Pt/SiO2 catalyst
was measured by nitrogen adsorption using a Sorptomatic
1900 (Carlo Erba Instruments). All the samples were freed
of all adsorbed materials by outgassing in vacuum at 300 ◦C
for 3 h, before starting the N2 adsorption.
The surface and subsurface composition of the catalysts
was investigated by a scanning electron microscope (Leica
Cambridge, Stereoscan 360) equipped with energy disper-
sive X-ray analyzer (EDXA) and X-ray photoelectron spec-
troscopy (Perkin-Elmer 5400). The calibration of the XP
spectrometer was checked with a test sample containing Au,
Ag, and Cu metal, on top of a two-sided tape. Binding ener-
gies (BE) were referenced against carbon 1s impurity peak at
284.6 eV [6]. Measurements were carried out using Mg-Kα
source and 35 eV pass energy. The accuracy of the measured
binding energies was ca. 0.15 eV. The catalyst powder was
placed on top of a two-sided tape, and the reduced sam-
ples were transferred in a N2 protective atmosphere with a
Perkin-Elmer Vacuum Transfer Assembly unit. The Shirley
background removal method was applied to all of the spec-
tra. The intensity ratios and energy separations were kept
fixed at their theoretical values. In the analysis, no impurities
except carbon were observed. Charging of the electrically in-
sulating samples during the measurement, due to the X-ray
bombardment, was rather stable ( 0.1 eV). The sensitivity
factors used for Pt 4f, Pt 4d, and Al 2p were 5.575, 3.50, and
0.234, respectively [7].
Isomerization and dehydrogenationof 4-TBCHOL (Acros
Organics; 70% trans, 30% cis) as well as hydrogenation of
4-TBCHONE (Acros Organics) were also studied in the
same experimental setup as for hydrogenation of 4-TBP.
The mass and particle size of the catalyst and reduction

62
A. Kalantar et al. / Journal of Catalysis 227 (2004) 60–67
temperature were the same as in the 4-TBP hydrogenation
experiments.
3. Results and discussion
3.1. Catalyst characterization
The dispersion of the metal in 1% Pt/SiO2 was 86.06%
with a metal particle diameter of 1.2 nm. Calcination of the
catalyst (calcined at 600 ◦C for 3 h) resulted in a particle size
of almost three times of the size of the noncalcined one. For
the 1% Pt/SiO2 catalyst both EDXA and XPS analyses in-
dicated that chlorine is practically disappearing after the re-
duction of the catalyst. After reduction at 673 K, the Cl/Si as
well as the Cl/Pt ratio decreased below detection limit from
2.1×10−3 (XPS)/3.2×10−3 (EDXA), and 0.44 (XPS)/0.62
(EDXA), respectively. This is important to know, since the
residual chlorine, an electronegative species, could affect the
properties of the active metal and hence the reaction rate and
product selectivity. The presence of chlorine at the surface
has been shown to lower the activity of the Al2O3-based
catalysts in aromatic hydrogenation [8,9]. The XPS analysis
also indicated that the platinum particles are slightly posi-
tively charged (Pt2+, BE 72.3 eV) [16]. This could be the
effect of the carrier, since its acidity could result in e− with-
drawal from small Pt particles. This is comparable to the BE
of Pt on alumina, 314 eV. The differences in the values re-
ported by XPS and EDXA is probably due to the depth of
the two techniques, with XPS being more surface sensitive
than EDXA.
Fig. 1. Temperature dependency of the rates of 4-tert-butylcyclohexanol and
4-t-butylcyclohexanone formation over 1% Pt/SiO . p
= 0.0034 bar,
2
4-TBP
p
= 0.13 bar (ꢀ), 0.1084 bar (ꢁ), 0.078 bar (ꢂ), 0.052 bar (•).
H
2
Fig. 2. Selectivities of the products vs time on stream. 4-TBP hydrogena-
◦
tion. 1% Pt/SiO , T = 177 C, p = 0.1048 bar, p
= 0.0034 bar.
H
2
4-TBP
(∗) Toluene, (×) TBBZ, (•) TB2CH, (ꢀ) cis-4-TBCHOL, (ꢁ) trans-
4-TBCHOL, (ꢂ) 4-TBCHONE.
3.2. Hydrogenation of 4-tert-butylphenol
of tert-butylbenzene (TBBZ) that can be further be hy-
drogenated to tert-butylcyclohexane (TBCH). The alcohol
product (4-TBCHOL) can be dehydrated as well to form
TBCH. At the beginning of the reaction formation of by-
products is high but decreases rapidly after 20 min. At the
end of the experiment the production of the by-products
was minor compared to the production of 4-TBCHOL and
4-TBCHONE (Fig. 2). The observed pattern indicates that
more ketone was formed at longer times on stream, under-
going less hydrogenation to the alcohols. The cis to trans
ratio was also studied and compared at different times on
stream. The ratio rises, beginning from 0.79, and closer
to quasi-steady-state conditions the ratio tends to stabilize
around 0.88.
The main products from the hydrogenation of 4-TBP
were 4-tert-butylcyclohexanone, cis- and trans-4-tert-butyl-
cyclohexanol (4-TBCHOL); see Scheme 1. Similarly to
alkylbenzene hydrogenation reactions, the rates of 4-tert-
butylcyclohexanol and 4-t-butylcyclohexanone formation
passed through a reversible maximum at 187 ◦C (Fig. 1).
The hydrogenolysis of 4-TBP results in the formation
The hydrogenation experiments were carried out with a
prereduced catalyst. A longer run at 177 ◦C was made to re-
veal the behavior of the catalyst in a period of time (Fig. 3).
As can be seen a continuous slow deactivation took place
and a steady state was not reached even after 4 h. Catalyst
deactivation is commonly observed for reactions involving
hydrocarbons and is assumed to be a consequence of car-
bon deposition [10]. The BET surface area of the fresh and
spent catalyst indicated that the used catalyst retained 75%
of its initial surface area. The deactivation was found to be
Scheme 1. Hydrogenation of 4-TBP to cis and trans 4-TBCHOL.

A. Kalantar et al. / Journal of Catalysis 227 (2004) 60–67
63
Table 1
Reaction orders with respect to hydrogen and 4-TBP
◦
Temperature ( C)
H
4-TBP
2
127
147
167
187
207
1.47
1.71
1.82
1.89
1.72
−0.70
−0.64
−0.55
−0.52
−0.35
of the mode of adsorption of the molecule of 4-TBP over
the surface of the catalyst at higher temperatures or due to
weaker adsorption of the alkylphenol in this range.
Fig. 3. Overall hydrogenation rate as a function of time on stream.
◦
The reaction order with respect to hydrogen increased as
the temperature increased until a maximum was reached at
187 ◦C (Table 1). High reaction orders with respect to H2
along with an increasing tendency with temperature have
been previously reported for aromatic and substituted aro-
matic compounds [12,13]. The hydrogenation rate passed
through a maximum at approximately 187 ◦C. Similar tem-
perature dependencies have been reported for hydrogenation
of aromatic compounds [9,14,15]. The main reason for the
observed maximum in the rate versus temperature is proba-
bly a decreased surface coverage of the aromatic molecule,
analogously to o-xylene hydrogenation [8,20].
From a commercial point of view the cis isomer is the
most valuable product. By increasing the partial pressure of
hydrogen the selectivity toward alcohol increases and the
selectivity toward ketone and by-products declines. Fig. 4
shows the selectivity of the reaction to the main products,
i.e., 4-TBCHOL (cis and trans isomers) and 4-TBCHONE
at 177 ◦C. The same tendency was observed at other temper-
atures as well. Regarding stereoselectivity, a higher partial
pressure of hydrogen decreased the cis/trans ratio (Fig. 5)
which is contrary to what has been reported for hydrogena-
tion of xylenes [8,12]. The selectivities toward ketone and
alcohols were roughly the same up to 187 ◦C, thereafter
starting to decrease. Formation of TBBZ and TBCH can be
seen at higher temperatures (> 187 ◦C), with hydrogenoly-
sis playing an important role (Fig. 6). In particular formation
of TBBZ increased dramatically above 200 ◦C, reaching se-
lectivities of 30% (Fig. 7). The activation energy, calculated
4-TBP hydrogenation, 1% Pt/SiO , T = 177 C, p
= 0.1048 bar,
H
2
2
p
= 0.0034 bar. (ꢁ) Observed rate, (—) estimated rate.
4-TBP
reversible since the regenerated catalyst restored, and even
slightly improved, the activity. The regeneration of the cat-
alyst was carried out by heating the sample under hydrogen
flow to 400 ◦C where it was kept for 2 h, after which the
temperature was decreased to the desired temperature and
the reactants were introduced. The H2 treatment was done
to remove the loosely deposited carbonaceous species from
the surface. By approximating the reaction rate with an ex-
ponential function a mechanistic deactivation model for cat-
alyst fouling can be applied [11],
r = a1 · e−a ·t + a3,
2
where r is the reaction rate and ai are the constants esti-
mated in each case, as they depend on the temperature and
composition of the reactants stream,
ksr0
a1 =
,
a2 = ks + k
,
−s
ks + k
−s
r0
a3 = k
·
.
−s
ks + k
−s
ks and k are the rate constants for deactivation and self-
−s
regeneration, and r0 is the rate under deactivation-free con-
ditions. a1 plus a3 represents the initial rate of the catalyst,
when time is very close to zero, a2 is the level of deactiva-
tion in the reaction, and a3 is the rate at the steady state (in
this case a quasi-steady state). The following values of para-
meters were obtained a1 = 17.2, a2 = 0.03, a3 = 11.52 for
the estimated rate in Fig. 3.
The rates at steady state were used for the determination
of the kinetic data. The reaction orders in hydrogen and hy-
drocarbon were determined in the temperature range from
137 to 227 ◦C. The experiments were made with intervals
of 10 ◦C, and the orders with respect to hydrogen and hy-
drocarbon were calculated at each temperature. The orders
in 4-TBP increased from −0.7 to −0.35 in the temperature
interval (Table 1). In this case, increase in 4-TBP pressure
was proven to retard the reaction rate. Previous reports in
the literature have shown orders with respect to the aromatic
compound very close to zero [12] or slightly negative [20] at
different temperatures. The possible reason for the tempera-
ture dependency of reaction orders could be due to a change
Fig. 4. Selectivity of products as a function of hydrogen partial pressure.
◦
4-TBP hydrogenation, T = 177 C, p
= 0.0034 bar.
4-TBP

64
A. Kalantar et al. / Journal of Catalysis 227 (2004) 60–67
Pt/Al2O3–Cl and Pt/Al2O3–nitrate, respectively) compared
to the Pt/SiO2 catalysts (21.6 × 10−4 mol/g(Pt)s) were ob-
served at 177 ◦C (pH = 0.13 bar, p4-TBP = 0.0034 bar).
2
Moreover, the hydrogenation of 4-TBP over Pt/Al2O3 cat-
alysts mainly resulted in the formation of hydrogenoly-
sis products and no 4-TBCHOL was observed over the
Pt/Al2O3 catalyst prepared from Cl-containing precursor.
The catalyst prepared from Pt–nitrate salt at higher hydro-
gen partial pressures yielded small amount of alcohols. It
appears, that alumina, being a more acidic carrier than sil-
ica enhances hydrogenolysis. The XPS and EDXA analy-
ses of the Pt/Al2O3 catalyst prepared from H2PtCl6 have
clearly indicated the presence of residual surface chlorine
even after the reduction at 400 ◦C [16], which results in
an enhanced surface acidity. According to [17], one might
anticipate that such surface acidity could stir the stereos-
electivity of the reaction toward cis-4-TBCHOL (although
one should differentiate between the “surface acidity” and
that of the “media”). Unfortunately, the yield of alcohols was
very low (to enable us a reasonable determination of cis- and
trans-isomer stereoselectivity), preventing a reasonable de-
termination of cis to trans ratio.
Fig. 5. Cis-to-trans ratio as a function of hydrogen partial pressure. 4-TBP
◦
hydrogenation, T = 177 C, p
= 0.0034 bar.
4-TBP
Calcium-doped Pt/SiO2 catalysts were tested in order to
investigate the effect of catalyst acidity on the hydrogena-
tion activity and product distribution. Liquid-phase hydro-
genation in acid solutions leads to the formation of the cis-
alcohol, whereas the formation of trans-alcohol is favored
in basic media [18]. The acidity of the catalyst carrier is re-
ported to have a strong effect on the selectivity toward the
formation of cis- and trans-4-TBCHOL, with the selectiv-
ity toward the formation of the cis isomer being enhanced
by the carrier acidity, which is in accordance with the the-
oretical predictions [17,18]. Since the higher acidity of the
carrier also results in hydrogenolysis reactions, the use of
additives can direct the formation of a given stereoisomer,
while suppressing the hydrogenolysis [17,21]. Addition of
Ca (1 wt%) to Pt/SiO2 catalysts resulted in a decreased hy-
drogenation rate (Fig. 8); however, further increases in the
Ca content did not result in an additional rate decrease.
Fig. 6. Rate of by-product (tBBZ + toluene + TBCHENE) formation as
a function of temperature. 4-TBP hydrogenation. p = 0.13 bar (ꢀ),
H
2
0.1084 bar (ꢁ), 0.078 bar (ꢂ), 0.052 (•). p
= 0.0034 bar.
4-TBP
Fig. 7. Product distribution, hydrogenation of 4TBP p = 0.13 bar,
H
2
p
= 0.0088 bar.
4-TBP
from Arrhenius plots, in the hydrogenation of 4-tBP was 66
and 53 kJ/mol in the temperature range 127–187 and 187–
217 ◦C, respectively.
3.3. Effect of carrier, precursor, and platinum content
Fig. 8. Effect of acidity on hydrogenation rate and cis/trans ratio as a func-
To study the effect of the carrier and the precursor, two
different Pt/Al2O3 catalysts with a metal content of 1 wt%
were prepared from H2PtCl6 and Pt(NO3)2. Much lower
initial rates (2.1 × 10−4 and 4.6 × 10−4 mol/g(Pt)s for
◦
tion of partial pressure of hydrogen. p
= 0.0034 bar, T = 177 C.
4-TBP
Cis/trans with no addition of Ca (ꢁ) and with addition of 1 wt% Ca
(ꢀ) as well as hydrogenation rate with no Ca (ꢃ) and with addition of
1 wt% Ca (3).

A. Kalantar et al. / Journal of Catalysis 227 (2004) 60–67
65
Table 2
Composition of the stream of products, initial reaction rate, and cis-to-trans ratio after 140 min of reaction for the calcined and noncalcined catalysts (T =
◦
177 C, p = 0.13 bar, p
= 0.0034 bar)
H
2
4-TBP
Catalyst
Particle size Metal dispersion Rate
CIS
TRANS cis/trans ONE PTBP TBBZ TOL TBCH
−4
(nm)
(%)
(10 mol/(g s)) (%)
(%)
(%)
(%)
(%)
(%)
(%)
Pt
1% Pt/SiQ
3.03% Pt/SiQ
5.57% Pt/SiQ
1.2
1.3
1.5
86
77
65
25
21.6
64.0
34.8
26.9
1.8
2.1
0.89
0.81
0.63
0.54
6
19
18
14
89
18
0
1
16
21
27
0
1
4
1
0
17
43
5
13.1 16.1
4.6 7.3
10.7 19.8
5.57% Pt/SiQ (calc.) 4.0
23
CIS, cis-4-TBCHOL; TRANS, trans-4-TBCHOL; ONE, 4-TBCHONE; 4-TBP, 4-TBP; TBBZ, TBBZ; TOL, toluene; TBCH, TBCH.
Different Pt/SiO2 catalysts (3.03 wt% Pt and 5.57 wt%
Pt) were prepared from Pt(NH3)4Cl2 and investigated for
their performance in the hydrogenation of 4-TBP at 177 ◦C.
The results indicate an activity increase with a higher Pt
content (Table 2). The catalyst containing 3 wt% platinum
exhibited the highest activity as well as the highest amount
of generated cis- and trans-4-TBCHOL. At the same time
a considerable amount of by-products was formed. A com-
plete conversion of 4-TBP was achieved over the 5.5 wt%
Pt/SiO2 catalyst.
The results indicated that for the 5.5 wt% Pt/SiO2 catalyst
subjected to heat treatment in air at 600 ◦C an increased par-
ticle size due to sintering resulted in a lower hydrogenation
Fig. 9. Hydrogenation rates of 4-TBCHONE vs temperature at different
rate.
As shown in Table 2, an increase in the catalyst Pt con-
hydrogen partial pressures, p
= 0.041 bar, (ꢀ) p = 0.13 bar,
H
4-TBCHONE
2
(ꢁ) p = 0.1 bar, (ꢂ) p = 0.078 bar, (•) p = 0.051 bar.
H
H
H
2
2
2
tent, caused by increased metal uptake, resulted in a slight
increase in the mean platinum particle size, and the con-
sequent decreased metallic dispersion (it should be noted
that the catalysts are prepared by ionic adsorption on high
surface area silica). The overall reaction rate is found to
pass through a maximum at ca. 3 wt% Pt content, whereas
the cis-to-trans isomers ratio is decreased with incremen-
tal Pt content. The decreased cis-to-trans ratio, hence, can-
not be attributed to the overall reaction rate (conversion
dependency of the selectivity) as they do not follow the
same pattern. The observed cis–trans stereoselectivities is
believed to be governed instead by the platinum particle
sizes. Larger Pt particles are needed for the trans-isomer
formation/transformation. TBP can adsorb on the Pt sur-
face (or Pt-carrier interface) either parallel (π-binding) or
through the phenolic group (σ-binding). The possibility of
simultaneous π- and σ-binding could not be excluded. The
relative contribution of these adsorption modes will stir the
stereoselectivity. The effect of Pt particle size on the rate of
trans-isomer formation/transformation (see reaction mecha-
nism section) is further manifested as the presintered 5 wt%
Pt/SiO2 catalyst (6 h at 600 ◦C in air), while exhibiting a
similar overall reaction rate as the 1 wt% Pt/SiO2 catalyst,
gave rise to the faster formation/transformation rate of the
trans isomer (Table 2).
trans-4-TBCHOL were present, which could imply that ei-
ther the ketone produces both isomers or the ketone produces
only one of the alcohol stereoisomers (cis isomer) [17] while
the trans isomer is produced by the epimerization or from
the ketone via an enol. The reaction order with respect to
hydrogen was decreasing, from 0.71 to 0, by increasing the
temperature. Fig. 9 shows the behavior of the rates of hy-
drogenation of 4-TBCHONE in the range of temperatures
studied, at different partial pressures. At higher H2 partial
pressures, the hydrogenation rate is also higher, indicating
a positive reaction order similar to hydrogenation of 4-TBP.
The apparent activation energy was 32.5 kJ/mol.
3.5. 4-TBCHOL dehydrogenation and isomerization
In order to better understand the reaction mechanism,
the dehydrogenation and isomerization of commercial 4-
TBCHOL were carried out. 4-TBCHOL dehydrogenation
was studied in the temperature interval 137–217◦C with dif-
ferent partial pressures of hydrogen (0.05–0.13 bar). Both
hydrogenation and dehydrogenation were found to take
place over the Pt/SiO2 catalyst. The temperature is the main
factor that affects the rate and product distribution. At lower
temperatures (up to 187 ◦C) the cis-to-trans isomerization
is dominant. Thereafter also the trans isomer starts to be
consumed, to form by-products (see Fig. 6). In Fig. 10
one can observe that the isomerization rate is in the same
order as the 4-TBP hydrogenation rates. The ketone hydro-
genation rates showed in Fig. 9 can also be compared to
3.4. 4-TBCHONE hydrogenation
Hydrogenation of ketone produced both cis- and trans-
4-TBCHOL. At temperatures below 200 ◦C both cis- and

66
A. Kalantar et al. / Journal of Catalysis 227 (2004) 60–67
Fig. 11. Effect of hydrogen in the alcohol isomerization and de-
Fig. 10. Rates of formation (—)/consumption (- - -) of main products
vs temperature. p
cis-4-TBCHOL (consumption), (ꢁ) rate of trans-4-TBCHOL (formation),
(ꢂ) rate of 4-TBCHONE (formation).
hydrogenation. (ꢂ) Rate cis-4-TBCHOL (consumption), (ꢀ) rate of
= 0.06 bar, p
= 0.051 bar, (ꢀ) rate
H
4-TBCHOL
2
trans-4-TBCHOL (formation), (ꢁ) rate of 4-TBCHONE (formation).
◦
T = 177 C, p
= 0.06 bar, p = 0.051 bar.
H
4-TBCHOL
2
the isomerization rates. Comparison at the same hydrogen
partial pressure (0.051 bar) indicates that the cis-to-trans
isomerization is faster than the ketone hydrogenation. It is
reasonable to assume that the ketone is hydrogenated to the
cis alcohol as previous studies of 4-TBP hydrogenation have
suggested [21], and then cis-4-TBCHOL undergoes epimer-
ization to form trans-4-TBCHOL. At temperatures above
200 ◦C, the consumption rate of both stereoisomers is high,
leading to the formation of the by-products. At tempera-
tures below 180 ◦C the cis-to-trans ratio was constant (about
0.55), although lower than in the hydrogenation of 4-TBP.
At temperatures above 197 ◦C, this ratio increases, due to
transformation of trans-4-TBCHOL into TBBZ. The pres-
ence of hydrogen was essential for both dehydrogenation
and isomerization reactions. The reaction rates decreased
considerably when hydrogen flow was halted for 30 min and
balanced with argon. After reintroducing hydrogen, the ac-
tivity of the reaction is partially restored (Fig. 11). The effect
of hydrogen on the reaction rate shows that hydrogen plays
an important role, not only in the removal of carbon deposits
but also for participation in the epimerization mechanism
(Scheme 2). According to this mechanism cis-4-TBCHOL is
adsorbed parallel to the surface with hydrogen atoms form-
ing H bonding. Here the presence of hydrogen is essential.
A similar effect has been reported by Bragin et al. [19] and
Kalantar et al. [8,20].
Scheme 2. Cis-to-trans isomerization.
formation of tert-butylbenzene that can be further be hydro-
genated to tert-butylcyclohexane. Dehydrogenation of the
alcohol can as well form TBCH. The reaction network is de-
picted in Scheme 3. The epimerization of the products was
observed to take place and hydrogen was proven to be an as-
toichiometric compound in cis-to-trans isomerization. The
conversion of cis-4-TBCHOL takes place to form the ther-
modynamically more stable compound, trans-4-TBCHOL.
Reliable thermodynamic data for these compounds cannot
be found in the literature; therefore estimations were made
using the Yoneda method [22]. The results suggest that the
trans isomer is slightly more stable, since it has more nega-
tive Gibbs energy (Fig. 12), which is logical considering that
the isomerization follows this path and is in accordance with
relative stabilities of cis and trans dialkylcyclohexanes.
3.6. Mechanism
4. Conclusions
The detailed kinetic study of 4-TBP hydrogenation per-
formed in this work has enabled the proposal of the fol-
lowing reaction mechanism. 4-TBP is transformed into with
4-TBCHONE as an intermediate. In the first hydrogena-
tion step 4-TBCHENOL is formed as an adsorbed surface
intermediate. This enol undergoes tautomeric equilibrium
transformation to adsorbed 4-TBCHONE or can be hydro-
genated to trans TBCHOL. 4-tert-butyl-cyclohexanone is
hydrogenated into cis-4-TBCHOL [21], which further un-
dergoes epimerization to form trans-4-TBCHOL. At higher
reaction temperatures hydrogenolysis of 4-TBP results in the
Gas-phase hydrogenation of 4-tert-butylphenol was stud-
ied over a silica-supported platinum catalyst in the tempera-
ture range 127–227◦C and hydrogen and 4-tert-butylphenol
partial pressures of 0.052–0.013 and 0.009–0.0153 bar, re-
spectively. The main reaction products from gas-phase hy-
drogenation of 4-tert-butylphenol were 4-tert-butylcyclo-
hexanol (cis and trans) and 4-tert-butylcyclohexanone.
At temperatures exceeding 200 ◦C tert-butylbenzene and
tert-butylhexanone were also generated. A maximum in
the hydrogenation rate of 4-tert-butylphenol was observed

A. Kalantar et al. / Journal of Catalysis 227 (2004) 60–67
67
tone is hydrogenated to cis 4-tert-butylcyclohexanol and the
formed cis 4-tert-butylcyclohexanol undergoes epimeriza-
tion to form the trans-4-tert-butylcyclohexanol. The stere-
oselectivity toward the formation of cis- and trans-4-tert-
butylcyclohexanol is strongly affected by the support acid-
ity/basicity.
Acknowledgments
The authors are grateful to Dr. Hannu Karhu at the De-
partment of Physics, University of Turku, for his contribu-
tion to the XPS and EDXA analyses. This work is part of
the activities at the Åbo Akademi Process Chemistry Centre
within the Finnish Centre of Excellence Programme (2000–
2005) by the Academy of Finland.
Scheme 3. Reaction network for the hydrogenation of 4-TBP.
References
[1] D.H. Pybus, C.H. Sell, in: The Chemistry of Fragrances, Royal Society
of Chemistry, Cambridge, 1999, pp. 108–113.
[2] B. Silberová, L. Cervený, React. Kinet. Catal. Lett. 67 (1999) 29.
[3] J. Tobicík, L. Cerveny, J. Mol. Catal. A: Chem. 194 (2003) 249.
[4] G.D. Yadav, P.K. Goel, J. Mol. Catal. A: Chem. 184 (2002) 281.
[5] J. Richardson, Principles of Catalyst Development, Plenum, New
York, 1989.
[6] G. Johansson, J. Hedman, A. Brendtsson, M. Klasson, R. Nilsson, J.
Electron Spectrosc. 2 (1973) 295.
[7] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of
X-Ray Photoelectron Spectroscopy, Perkin-Elmer, Physical Electronic
Division, USA, 1992.
[8] A. Kalantar Neyestanaki, P. Mäki-Arvela, H. Backman, H. Karhu, T.
Salmi, J. Väyrynen, D.Yu. Murzin, J. Catal. 218 (2003) 267.
[9] A.I. Allakhverdiev, N.V. Kul’kova, D.Yu. Murzin, Catal. Lett. 29
(1994) 57.
Fig. 12. Gibbs energy vs temperature for 4-TBCHOL.
[10] R. Hudges, Deactivation of Catalysts, Academic Press, London, 1984.
[11] M. Consonni, R. Touroude, D.Yu. Murzin, Chem. Eng. Technol. 21
(1998) 605.
[12] A. Kalantar Neyestanaki, H. Backman, P. Mäki-Arvela, J. Wärnå, T.
Salmi, D.Yu. Murzin, Chem. Eng. J. 91 (2003) 271.
[13] S. Smeds, Gas-phase hydrogenation of aromatic hydrocarbons—
kinetics, mechanism and catalyst characterization, PhD thesis, Åbo
Akademi, Turku, Finland, 1996.
[14] S. Smeds, D. Murzin, T. Salmi, Appl. Catal. A 125 (1995) 271.
[15] M.V. Rahaman, M.A. Vannice, J. Catal. 127 (1991) 251.
[16] H. Karhu, A. Kalantar, I.J. Väyrynen, T. Salmi, D.Yu. Murzin, Appl.
Catal. A 247 (2003) 283.
[17] D.Yu. Murzin, A.I. Allachverdiev, N.V. Kul’kova, Stud. Surf. Sci.
Catal. 78 (1993) 243.
[18] D.Yu. Murzin, S.R. Konuspaev, Kinet. Katal. 33 (1992) 40.
[19] O.V. Bragin, V.G. Tovmacian, D.B. Furman, A.L. Liberman, Isv.
Akad. Nauk USSR Ser. Khim. 12 (1976) 2718.
[20] A. Kalantar Neyestanaki, P. Mäki-Arvela, H. Backman, H. Karhu, T.
Salmi, J. Väyrynen, D.Yu. Murzin, J. Mol. Catal. A: Chem. 193 (2003)
237.
[21] S.R. Konuspaev, K.N. Zhanbekov, N.V. Kulkova, D. Murzin, Chem.
Eng. Technol. 20 (1997) 144.
[22] R.C. Reid, J.M. Prausnitz, B.E. Poling, in: Property of Gases and Liq-
uids, fourth ed., McGraw–Hill, Singapore, 1988, pp. 157–167.
at 187 ◦C and the overall hydrogenation rate was found
to increase with hydrogen partial pressures. The reaction
orders with respect to hydrogen and 4-tert-butylphenol
varied from 1.4 to 1.9 and −0.7 to −0.35, respectively,
within the studied temperature range. The 1% Pt/SiO2
catalyst favors the formation of the intermediate product,
4-tert-butylcyclohexanone, with a selectivity of 55%. The
cis-to-trans ratios of the alcohols were close to one at
the temperatures studied. In order to establish the reac-
tion mechanism, hydrogenation of the intermediate product,
4-tert-butylcyclohexanone, and its stereoselectivity orien-
tation were studied. Additionally the dehydrogenation and
epimerization of the products were observed to take place
and hydrogen was found to be necessary for the cis-to-
trans isomerization. Besides being an astoichiometric agent
hydrogen contributes to the removal of carbon deposition.
The isomerization rate can be compared to the hydrogena-
tion rate of 4-tert-butylphenol and to the hydrogenation
rate of 4-tert-butylcyclohexanone as well, entailing that ke-