Zirconia supported phosphotungstic acid as an efficient catalyst for resorcinol tert-butylation and n-heptane hydroisomerization

Article abstract of DOI:10.1016/j.molcata.2004.06.021

The alkylation of resorcinol with tert-butanol was carried out using zirconia supported phosphotungstic acid (PTA) as catalyst in liquid phase conditions. Among the different PTA loaded catalysts, the 15% PTA/ZrO₂ calcined at 750 °C was found t

Full text of DOI:10.1016/j.molcata.2004.06.021

Journal of Molecular Catalysis A: Chemical 221 (2004) 113–119
Zirconia supported phosphotungstic acid as an efficient catalyst for
resorcinol tert-butylation and n-heptane hydroisomerization
a
a,∗
b
b
Biju. M. Devassy , S.B. Halligudi , S.P. Elangovan , S. Ernst ,
b,1
c
M. Hartmann , F. Lefebvre
a
Inorganic Chemistry and Catalysis Division, National Chemical Laboratory, Pune – 411008, India
b
Department ofChemistry, University ofKaiserslautern, Chemical Technology, P.O. Box 3049,D-67653 Kaiserslautern, Germany
c
Laboratoire de Chemie Organometallique de Surface, CNRS-CPE, Villeurbanne Cedex, France
Received 6 April 2004; received in revised form 23 June 2004; accepted 23 June 2004
Available online 12 August 2004
Abstract
The alkylation of resorcinol with tert-butanol was carried out using zirconia supported phosphotungstic acid (PTA) as catalyst in liquid
◦
phase conditions. Among the different PTA loaded catalysts, the 15% PTA/ZrO
2
calcined at 750 C was found to be the most active and
yielding 4-tert-butyl resorcinol and 4,6-di-tert-butyl resorcinol as the major products under the optimized reaction conditions.
In addition to the above, hydroisomerization of n-heptane was studied under vapor phase conditions using bifunctional catalysts comprising
Pt, Pd and Pt–Pd clusters on 15% PTA/ZrO . The catalytic activity of the bimetallic Pt–Pd catalyst is higher than that of the monometallic
2
Pt and Pd catalysts. The catalysts prepared from acetylacetonate complexes were more active than those prepared from amine complexes.
Addition of Pd has no beneficial effect on the catalytic performance when the bimetallic catalyst is prepared from acetylacetonate complexes.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Zirconia; Phosphotungstic acid; Resorcinol; tert-Butylation; n-Heptane hydroisomerization
1
. Introduction
acidic strength and relatively high thermal stability. A se-
rious problem associated with the use of this type of ma-
terials as heterogeneous catalysts is their low surface area
(5–8 m g ). The use of HPA in supported form is prefer-
able because of its high surface area compared to the bulk
material. Recently, we have shown that zirconia supported
PTA is an efficient solid acid catalyst for alkylation and acy-
lation reactions [5].
Tert-Butylation of dihydroxy benzenes is important, since
butylated dihydroxy benzenes are widely used as anti-
oxidants and polymer stabilizers. In spite of its importance
very few works are reported on tert-butylation of resorcinol
using solid acid catalysts. Narayanan and Murthy [6] studied
alkylation of resorcinol with tert-butanol using modified clay
as catalysts.
Acid catalyzed reactions are among the most important in
2
−1
the chemical industry. Alkylation and isomerization are two
classes of such reactions. These reactions are usually cat-
alyzed by homogeneous Lewis acids (AlCl3, BF3) or strong
mineral acids (HF, H2SO4), which are highly toxic, gener-
ate a substantial amount of waste and cause severe corrosion
problems. Considerable efforts are being made to find suit-
able solid acid catalysts, which can use successfully to carry
out the above type of transformations [1,2]. Heteropoly acids
(
HPAs) especially Keggin type HPAs are strong Br o¨ nsted
acid catalysts [3,4]. Phosphotungstic acid (PTA) is the usual
catalyst of choice out of various HPAs because of its high
Skeletal isomerization of alkanes has industrially impor-
tant applications because branched alkanes are useful as clean
high-octane fuels. These reactions are usually achieved over
bifunctional catalysts containing fine dispersed noble metal
∗
Corresponding author. Tel.: +91205893300; fax: +91205893761.
E-mail addresses: halligudi@cata.ncl.res.in (S.B. Halligudi),
hartmann@rhrk.uni-kl.de (M. Hartmann).
1
Co-corresponding author.
1
381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.06.021

1
14
B.M. Devassy et al. / Journal ofMolecular Catalysis A: Chemical 221 (2004) 113–119
clusters on a matrix, which contains Br o¨ nsted acid sites. Dur-
ing the reaction, the noble metal catalyzes hydrogen transfer
reactions (hydrogenation–dehydrogenation), while isomer-
ization and hydrocracking occur on the Br o¨ nsted acid sites
chloride with ammonia solution and the precipitate obtained
was washed free of chloride and dried at 120 C for 24 h. A se-
ries of catalysts with different PTA loading (5–20 wt.%) were
prepared by suspending dried zirconia powder in a methanol
solution of PTA. This mixture was stirred for 8–10 h and the
excess of methanol evaporated to dryness. The resulting sam-
◦
[7,8]. For a catalyst where the metal function and the acid
function are well balanced, isomerization and hydrocrack-
ing are consecutive reactions and the rate limiting step is
the skeletal rearrangement of the alkenes obtained via de-
hydrogenation over the metal sites, which takes place on the
Br o¨ nsted acid sites. However, the practical application of this
◦
◦
ples were dried at 120 C and calcined in air at 750 C for 4 h.
Metal supported catalysts were prepared by impregnation
of Pt(NH3)4Cl2 and Pd(NH3)4Cl2 to obtain a catalyst with
1 wt.%Ptand0.54 wt.%Pd, respectively. Themetalprecursor
was dissolved in 50 ml of distilled water and the solution
was added drop wise to 1.5 g of 15% PTA/ZrO2 calcined
process has been only confined to C4/C alkanes. The skele-
6
tal isomerization of n-heptane, for which the octane num-
ber RON is 0 and this, could be enhanced to 42–112. But,
enhancement of RON of n-heptane is generally difficult be-
cause the isomerization is usually accompanied by undesir-
able cracking. Catalysts with sufficiently good balance of
metal and acid functions under the suitable reaction condi-
tions are generally needed to suppress cracking in order to
achieve high isomerization selectivity for long chain alkanes.
The exact value of isomerization maximum is expected to be
depending on the balance between two catalytic functions,
viz. density and the strength of Br o¨ nsted acid sites and the
nature, amount and dispersion of the metal. There are a num-
ber of reports on the hydroisomerization of n-heptane over
bifunctional catalysts such as Pt and/or Pd supported on zeo-
◦
at 750 C slurred in 50 ml of distilled water. After stirring
the solution for 2 h at 40 C, the water was removed in a
rotary evaporator at 60 C under vacuum. The samples were
then dried at 120 C for 12 h. The resulting materials were
◦
◦
◦
pressed, crushed and sieved to obtain catalyst particles with
a diameter of 0.25–0.355 mm. In order to study the effect
of metal precursor on the catalytic activity in isomerization
reaction, metal supported catalysts were also prepared by
Pt-acetylacetonate and Pd-acetylacetonate using benzene as
solvent.
2.3. Characterization
lites [9–15], SAPOS [16–18], SO42 –ZrO2 [19], WO3–ZrO2
−
31P MAS NMR spectra (Bruker DSX-300) were recorded
at 121.5 MHz with high power decoupling using a Bruker
4 mm probehead. The spinning rate was 10 kHz and the delay
between two pulses was varied between 1 and 30 s to ensure
that a complete relaxation of the 31P nuclei occurred. The
chemical shifts are given relative to external 85% H3PO4.
Acidity of the catalysts were measured using temperature
programmed desorption of ammonia. NH3 TPD was carried
[20,21], CS2.5H0.5PW12O40 [22,23] and on MoO3 [24,25].
The present study deals with preparation of zirconia sup-
ported PTA catalyst and its characterization by 31P MAS
NMR spectroscopy and acidity measurements by tempera-
ture programmed desorption (TPD) of NH3. These catalysts
were used in the alkylation of resorcinol with tert-butanol.
The catalyst with the highest activity was also used for the
hydroisomerization on n-heptane after loading with Pt and/or
Pd. The effect of the metal precursor on the hydroisomeriza-
tion activity was also examined.
◦
after 0.1 g of the catalyst sample was dehydrated at 500 C
in dry air for 1 h and purged with helium for 0.5 h. The tem-
◦
perature was decreased to 125 C under a flow of helium
and then 0.5 ml NH3 pulses were passed through the samples
until no more uptake of ammonia was observed. NH3 was
◦
2
. Experimental
desorbed in He flow by increasing the temperature to 540 C
◦
−1
at 10 C min measuring NH3 desorption using a TCD de-
tector.
2
.1. Chemicals
Zirconyl chloride (ZrOCl2·8H2O), ammonia (25%),
2.4. Catalytic experiments
resorcinol and tert-butanol were procured from S.D.
Fine Chemicals Ltd., Mumbai. Phosphotungstic acid
2.4.1. Resorcinol tert-butylation
(
H3PW12O40·21H2O), Pt-acetylacetonate [Pt(acac)2], Pd-
The liquid phase tert-butylation of resorcinol was carried
out in a 50 ml Parr autoclave. A typical reaction mixture con-
sists of 6.6 g of resorcinol, 13.4 g of tert-butanol together with
0.66 g of catalyst. After a definite time interval, the reaction
is stopped and the reaction mixture is diluted with 5 ml of
methanol and 1 g of the product is taken and it is again diluted
with 5 ml of methanol. It is then filtered and the filtrate is ana-
lyzed using a Shimadzu 14B gas chromatograph using SE-52
packed column, coupled with FID. The product identification
was carried out using GC-MS. Conversion was defined as the
percentage of resorcinol converted into products.
acetylacetonate [Pd(acac)2] and n-heptane were purchased
from Aldrich. Methanol was obtained from E. Merck India
Ltd., Mumbai. All the chemicals were used without further
purification.
2
.2. Catalyst preparation
The catalysts were prepared by impregnation method us-
ing zirconium hydroxide as the support. The support was
prepared by the hydrolysis of an aqueous solution of zirconyl

B.M. Devassy et al. / Journal ofMolecular Catalysis A: Chemical 221 (2004) 113–119
115
2
.4.2. Hydroisomerization ofn-heptane
n-Heptane hydroisomerization was carried in a fixed-bed
flow type apparatus with on-line gas chromatographic analy-
sis of the reaction products. The experiments were conducted
at a hydrogen pressure of 1 MPa and the pressure of n-heptane
was adjusted to 10 kPa to obtain an H2/n-heptane molar ratio
of 100. Changing the temperature of the reaction modified the
conversion of n-heptane. The mass of the catalysts and the
hydrogen gas flow were adjusted to achieve a residence time
−
1
Wcat/Fn
= 400 g h mol . Prior to the catalytic exper-
−heptane
◦
iments, the catalysts were dehydrated at 250 C for 2 h in
argon flow, activated in oxygen at 300 C for 4 h and flushed
with argon at 400 C for 4 h. Finally, the metal clusters were
obtained by hydrogen reduction at 310 C for 4 h.
◦
◦
◦
Fig. 2. TPD pattern of (a) 5%, (b) 10%, (c) 15% and (d) 20% catalysts.
3
3
3
. Results and discussion
of phosphorous in the catalyst depends on PTA loading. For
low loadings, a broad signal above −20 ppm is observed. In
agreement with literature data, this peak is attributed to phos-
phorous in the Keggin unit [5,26]. For high loadings, a second
signal appears below −20 ppm, which can be attributed to
phosphorous oxide resulting from the decomposition of the
polyoxometalate [27]. This phosphorous oxide represents 20
and 45% of the total phosphorous for 15 and 20% PTA load-
ing, respectively.
.1. Characterization ofthe catalysts
.1.1. 31P MAS NMR
The 31P MAS NMR spectra of the catalysts with 5–20%
PTA loading are shown in Fig. 1. It can be seen that the state
3.1.2. NH3 TPD
Acidity of the catalysts were measured by TPD of NH3.
Fig. 2 gives the TPD patterns of the various catalysts while
Fig. 3 represents the amount of sorbed ammonia per square
nanometre as a function of the PTA loading. It is evident
from these data that there is initially an increase of the acid-
ity until 15% loading and then a decrease for higher loadings.
If we take into account the above result, these result can be
explained easily: at low loadings, the polyoxometalate re-
tains its structure (and its acidity) while for higher loadings
it decomposes, at least partially into its oxides. The highest
acidity corresponds to a monolayer of polyoxometalate as it
Fig. 1. 31PMAS NMR spectra of (a) 5%, (b) 10%, (c) 15% and (d) 20%
catalysts.
Fig. 3. Acidity of the catalysts as a function of the PTA loading.

1
16
B.M. Devassy et al. / Journal ofMolecular Catalysis A: Chemical 221 (2004) 113–119
is evident from 31P MAS NMR. So, we can describe the evo-
lution of the catalysts as follows: until a PTA loading of ca.
1
5%, the heteropolyanion is well dispersed on the zirconia
surfaceanditretainsitsintegrity. Forhigherloadings, thecov-
erage is higher than the monolayer and the polyanion is not
◦
stabilized by zirconia and decomposes by heating at 750 C.
3
3
.2. Catalytic activity
.2.1. Resorcinol tert-butylation
The liquid phase alkylation of resorcinol with tert-butanol
was carried out using zirconia supported PTA as catalysts.
The analysis of the products showed that the reaction resulted
in the formation of ring alkylated products, 4-tert-butyl re-
sorcinol (4-tbr) and 4,6-di-tert-butyl resorcinol (4,6-dtbr) and
O-alkylated products like resorcinol mono tert-butyl ether
(rmtbe) and 4-tert-butyl resorcinol mono tert-butyl ether (4-
tbrmtbe). In a first step, the reaction was studied as a function
of PTA loading. The catalyst with optimum loading was then
used to study the effect of temperature, catalyst concentration
and reaction time.
Fig. 4. Effect of PTA loading on resorcinol conversion and product selec-
tivity. Conditions: resorcinol = 6.6 g; tert-butanol = 13.4 g; catalyst weight
◦
= 0.66 g; temperature = 70 C; time = 1 h; resorcinol/tert-butanol (mol ratio)
=
1:3.
Fig. 4 gives the variation of the conversion and of the se-
lectivities (after 1 h reaction) as a function of the PTA loading.
The conversion shows a maximum for a 15% PTA loading,
which corresponds to the catalyst with the highest acidity,
as determined by NH3 TPD. The selectivities are also vary-
ing as a function of the PTA loading. First of all, it must be
pointed out that, in contrast to what is observed on modified
clays [6], O-alkylated products (rmtbe and 4-tbrmtbe) are
formed in addition to C-alkylated products (4-tbr and 4,6-
dtbr). In addition, the products result from one (rmtbe and
alkylations). Fig. 4 shows that until the PTA monolayer, the
rate of the C-alkylation increases more rapidly than that of
the O-alkylation. For higher loadings, it is seen that the rate
of C-alkylation decreases while that of O-alkylation remains
constant.
Scheme 1 represents the formation of different prod-
ucts during liquid phase tert-butylation of resorcinol. The
different pathways corresponding to the transformation of
O-alkylated products into C-alkylated products have been
shown in Scheme 1. Indeed, it has been shown that such re-
arrangements occurred on heating or by contact with an acid
catalyst [28]. It is clear from the reaction scheme that rmtbe is
4
-tbr) or two successive reactions (4-tbrmtbe and 4,6-dtbr)
corresponding to a C- and O-alkylation or two C-alkylations
we did not observe the product corresponding to two O-
(
Scheme 1.

B.M. Devassy et al. / Journal ofMolecular Catalysis A: Chemical 221 (2004) 113–119
117
Fig. 7. Effect of time on resorcinol conversion and product selectivity. Con-
ditions: resorcinol = 6.6 g; tert-butanol = 13.4 g; catalyst weight = 0.66 g;
temperature = 70 ◦C; resorcinol/tert-butanol (mol ratio) = 1:3.
Fig. 5. Effect of temperature on resorcinol conversion and product selectiv-
ity. Conditions: resorcinol = 6.6 g; tert-butanol = 13.4 g; catalyst weight =
0
.66 g; time = 1 h; resorcinol/tert-butanol (mol ratio) = 1:3.
products. Hence, for 5 wt.% of catalyst, the selectivity to O-
alkylated products is 60%, which decreased with the increase
in catalysts amount in the butylation reaction.
a primary product while 4-tbr is a primary and/or a secondary
product, depending on the kinetics of the butylation reaction.
First of all, increasing in temperature of the reaction
increases the substrate conversion and affects the product
distribution selectivities towards C-alkylated products in the
Finally, the effect of the reaction time is shown on Fig. 7.
When the reaction time increases, the isomerization of O- to
C-alkylation products also increases to give higher selectiv-
ities for C-alkylated products in the butylation reaction. The
results of butylation of resorcinol can be summarized as at
the conditions of lower reaction temperature, reaction time
and catalyst to substrate ratio results in higher selectivities
for O-alkylated products. C-alkylated products become ma-
◦
butylationreaction(Fig. 5). At100 C, onlyC-alkylatedprod-
ucts are formed like that of clays [6] and this can be due to the
rate of transformation of O- to C-alkylated products increases
more rapidly than that of the rate of the alkylation reaction.
A second parameter is the catalyst to substrate ratio. Fig. 6
gives both the conversion and the product selectivities as a
function of the amount of catalyst in the butylation reaction. It
isknownthatPTA/ZrO2 alsoactsascatalystforisomerization
and hence increase in the catalysts amount increases both
substrate conversions and isomerization of O- to C-alkyaletd
◦
jor at high temperature (100% at 100 C). Corma et al. [29]
also found a similar observation in the alkylation of phenol
by tert-butanol over the solid acid HNa-Y zeolites.
For heterogeneous catalysts, it is also important to study
the recycling of the catalyst. For this purpose, the reaction
was stopped when the conversion reached 60% and the cat-
alyst removed by filtration. The filtrate immediately reused
in the butylation of resorcinol and it is found that there was
no increase in substrate conversion. In addition, the catalyst
separated is reused after filtration with fresh reaction mixture
and is found no loss in its catalytic activity. These data are in
agreement with the absence of catalyst leaching and a purely
heterogeneous nature of the reaction.
3.2.2. Hydroisomerization ofn-heptane
The hydroisomerization of n-heptane, which is believed
to require stronger acidic sites than the alkylation of resor-
cinol was also studied with the 15% PTA/ZrO2 catalyst. It
is well known that for this reaction a bifunctional catalyst
is necessary which combines an acid and a metal function
on the same support. We studied platinum and palladium
as the metal components forming monometallic or bimetal-
lic clusters supported on 15% PTA/ZrO2 in hydroisomeriza-
tion of n-heptane under vapor phase conditions. The effect
of the nature of the metal precursor was also investigated
Fig. 6. Effect of catalyst weight on resorcinol conversion and product se-
lectivity. Conditions: resorcinol = 6.6 g; tert-butanol = 13.4 g; temperature
◦
=
70 C; time = 1 h; resorcinol/tert-butanol (mol ratio) = 1:3.

1
18
B.M. Devassy et al. / Journal ofMolecular Catalysis A: Chemical 221 (2004) 113–119
both increased hydrogenolysis activity of the metal and en-
hanced cracking over the acidic sites of the catalyst. At low
temperatures, isomerization is the dominant reaction, while
at elevated temperature cracking of the isomers dominates
resulting in the observed decrease of the isomer yield. The
maximum isomer yield and the temperature at which it is ob-
tained together with the cracking yield are given in Table 1.
At a given temperature, the n-heptane conversion over
platinum and palladium containing catalysts is similar, how-
ever, the platinum-containing catalysts is more selective to-
wards isomerization products than palladium-containing one.
This effect is more pronounced when the catalysts have been
prepared from Pt(NH3)4Cl2 or Pd(NH3)4Cl2. It is also found
that catalyst prepared by using metal acetylacetonate precur-
sors is more active resulting in higher n-heptane conversions
and isomerization yields.
The bimetallic Pt–Pd catalyst prepared from chlorides is
more active than the respective monometallic catalysts, while
the catalysts prepared using acetylacetonates do not differ
significantly in activity. For all catalysts prepared from the
later precursor a higher isomer yield is observed at a lower (by
◦
ca. 40 C) reaction temperature. Blomsma et al. [30] reported
asimilarincreaseinactivityon n-heptanehydroisomerization
with bimetallic Pt/Pd on H-beta and USY zeolite.
It is important to mention that the activity and selectivity
of the catalyst under study significantly depend on the metal
precursor used for their preparation. This can be explained
by the nature of the interaction between the metal complex
and the support. It is well known that the ZrO2 gel has an iso-
electric point (IEP) or zero point of charge (ZPC) between
six and seven [31]. In the case of supported metal oxides, a
change in ZPC is observed compared to the supports: Yori et
al. [32] reported a decrease in ZPC, for zirconia supported
tungsten oxide catalysts compared to ZrO2. A similar de-
crease in ZPC has been reported by Gil-Liambias et al. [33]
for V2O5 supported on TiO2 and Al2O3 compared to TiO2
and Al2O3. We can then reasonably assume that the ZPC of
15% PTA/ZrO2 catalyst is lower than that of pure ZrO2. For
the adsorption of cationic metal precursor, e.g. tetrammine-
platinum, the surface should be negatively charged but it is
probable that the charge on the surface of the catalyst is not
high (the surface charge depends on the pH of impregnating
metalprecursorsolutionandwillbesmallerforthePTA/ZrO2
catalyst compared to pure ZrO2). As a consequence, this will
limit the amount of complex adsorbed on the support and
bound to it by ionic interactions. The non-negligible amount
Fig. 8. n-Heptane conversion or yield (%) as a function of reaction temper-
ature for different catalysts. Conditions: H2 pressure = 1 MPa; n-heptane =
−1
1
0 kPa; residence time = 400 g h mol
.
by preparing catalysts from the deposition of acetylaceto-
nate complexes and dichloro–tetrammine complexes of Pt
and Pd. The results obtained with 1% Pt on PTA/ZrO2 pre-
pared using dichloro–tetrammine complexes and acetylacet-
onate complexes are shown in Fig. 8a and b, which gives the
n-heptane conversions and the yields of isomerization and
cracking products as a function of the reaction temperature.
The conversion of n-heptane and the yield of the cracking
products increase with temperature, while the isomer yield
reaches a maximum and declines thereafter. The continu-
ous increase in cracking yield with temperature is a result of
Table 1
Catalytic activity in hydroisomerization of n-heptane
Precursor
Pt and Pd(NH3)4Cl2
Pt and Pd (acac)2
Metal loading (wt.%)
Temperature ( C)
1.0Pt
290
0.54Pd
280
0.5Pt–0.27Pd
270
1.0Pt
250
0.54Pd
260
0.5Pt–0.27Pd
260
◦
n-Heptane conversion (%)
Isomer yield (%)
Cracking yield (%)
63.5
42.6
20.9
43.0
11.8
31.2
58.9
39.7
19.2
66.7
49.9
16.8
68.6
44.1
24.5
64.5
50.2
14.3
Conditions: H2 pressure = 1 MPa, n-heptane = 10 kPa, residence time = 400 g h mol 1
−
.

B.M. Devassy et al. / Journal ofMolecular Catalysis A: Chemical 221 (2004) 113–119
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◦
zirconia supported PTA calcined at 750 C as catalysts under
liquid phase conditions The catalyst with 15% loading was
found to have the highest activity under optimized reaction
conditions, whichgave4-tert-butylresorcinoland4,6-di-tert-
butyl resorcinol as the major products. The catalyst with 15%
PTA on zirconia was used in n-heptane hydroisomerization
after loading with Pt and/or Pd. The supported metal catalysts
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