Methoxylation of α-pinene over heteropolyacids immobilized in silica

Article abstract of DOI:10.1016/j.apcata.2009.11.006

The methoxylation of α-pinene was studied using heteropolyacids immobilized on silica as catalysts, at 60 °C, being the α-terpinyl methyl ether the main product. Tungstophosphoric acid (PW), molybdophosphoric acid (PMo), tungstosilicic acid (SiW) and molybdosilicic acid (SiMo) were immobilized on silica by sol-gel method. It was observed that the catalytic activity of the silica-supported heteropolyacids decreases in the series: PW2_S > SiW_S > PMo_S > SiMo_S. A series of PW immobilized on silica with different PW loading were prepared. It was observed that the catalytic activity increases with the amount of PW immobilized on silica. However, at high amount of PW on silica, a decrease of the catalytic activity was observed. Good values of selectivity to α-terpinyl methyl ether (about 60% near complete conversion) were obtained with all catalysts. Catalytic stability of the PW2_S was evaluated by performing consecutive batch runs with the same catalyst sample. After the third batch it was observed a stabilisation of the initial activity. A kinetic model was developed assuming that the α-pinene is consumed according to the parallel reaction network. It was observed that the kinetic model fits the experimental concentration data quite well.

Full text of DOI:10.1016/j.apcata.2009.11.006

Applied Catalysis A: General 373 (2010) 140–146
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Methoxylation of
a
-pinene over heteropolyacids immobilized in silica
b
b
a,
b,**
*
D.S. Pito a, I. Matos b, I.M. Fonseca
, A.M. Ramos , J. Vital , J.E. Castanheiro
a
´
´
´
Centro de Quı´mica de Evora, Departamento de Quı´mica, Universidade de Evora, 7000-671 Evora, Portugal
b REQUIMTE, CQFB, Faculdade de Cieˆncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
A R T I C L E I N F O
A B S T R A C T
Article history:
The methoxylation of a-pinene was studied using heteropolyacids immobilized on silica as catalysts, at
60 8C, being the a-terpinyl methyl ether the main product. Tungstophosphoric acid (PW), molybdopho-
sphoric acid (PMo), tungstosilicic acid (SiW) and molybdosilicic acid (SiMo) were immobilized on silica
by sol–gel method. It was observed that the catalytic activity of the silica-supported heteropolyacids
decreases in the series: PW2_S > SiW_S > PMo_S > SiMo_S.
Received 17 May 2009
Received in revised form 3 November 2009
Accepted 4 November 2009
Available online 10 November 2009
A series of PW immobilized on silica with different PW loading were prepared. It was observed that
the catalytic activity increases with the amount of PW immobilized on silica. However, at high amount of
PW on silica, a decrease of the catalytic activity was observed.
Keywords:
Methoxylation
a
-Pinene
Good values of selectivity to
obtained with all catalysts.
a-terpinyl methyl ether (about 60% near complete conversion) were
Heteropolyacids
Silica
Catalytic stability of the PW2_S was evaluated by performing consecutive batch runs with the same
catalyst sample. After the third batch it was observed a stabilisation of the initial activity.
A kinetic model was developed assuming that the
a-pinene is consumed according to the parallel
reaction network. It was observed that the kinetic model fits the experimental concentration data quite
well.
ß 2009 Elsevier B.V. All rights reserved.
1. Introduction
area (1–10 m2/g), separation problem from reaction mixtures and
solubility [11–13]. In order to increase the specific area of the
a
-Pinene is a renewable raw material, which constitute real
heteropolyacids, a variety of supports like, zeolite [14–17],
building blocks for the synthesis of new important chemicals for
use as fragrances, flavours and pharmaceuticals intermediates [1–
activated carbon [18–20], silica [21–31], magnesia [32], aluminas
[33] and polymers [34–37] have been used as support to
immobilize HPAs.
3]. The main product of the acid-catalysed methoxylation of
a-
pinene is -terpinyl methyl ether [4], which smells grapefruit-like
a
Heteropolyacids have been used as acid catalyst in reactions
and might be used as flavour and fragrance and as additive for
pharmaceuticals and food industry [5]. Traditionally, sulfuric acid
has been used in this reaction, as catalyst [6]. However, solid
catalysts have many advantages over liquid acids, such as they do
less harm to the environment, can be easily separated from liquid
where terpenes are involved, such as, hydration of
[34,35,38], limonene [38,39] and camphene [40], isomerization of
-pinene [41], acetoxylation of -pinene [38], limonene [38,39]
and camphene [40] and cyclization of pseudoionone [42].
In this work, we report the methoxylation of -pinene over
heteropolyacids immobilized on silica. The influence of various
reaction parameters, such as, temperature, molar ratio of -pinene
a-pinene
a
a
a
phase and can be reusable. The
a-pinene methoxylation has been
carried out over beta zeolite [7], acidic cation exchange resins [8]
and sulfonic-modified mesoporous silica [9].
a
to methanol and catalyst loading on the activity of the most active
catalyst, is also studied. A simple kinetic model was developed. It
Recently, PVA with sulfonic acid groups were used as catalysts
in the
a
-pinene methoxylation [10].
was assumed that the a-pinene is consumed according to the
Heteropolyacids (HPAs) are typical strong Bro¨nsted acids,
which catalyse a wide range of reactions. The major disadvantages
of HPAs as catalysts are their low thermal stability, low surface
parallel reaction network.
2. Experimental
2.1. Catalysts preparation
* Corresponding author. Tel.: +351 266745311; fax: +351 266744971.
** Corresponding author. Tel.: +351 212948385; fax: +351 212948385.
E-mail addresses: iss@dq.fct.unl.pt (I.M. Fonseca), jefc@uevora.pt
(J.E. Castanheiro).
Tungstophosphoric acid (PW), molybdophosphoric acid (PMo),
tungstosilicic acid (SiW) and molybdosilicic acid (SiMo) were
immobilized on silica by sol–gel method. The catalysts samples
0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2009.11.006

D.S. Pito et al. / Applied Catalysis A: General 373 (2010) 140–146
141
Table 1
(PW2_S, PMo_S, SiW_S and SiMo_S) were prepared according to
Physicochemical characterization of silica-included HPAs.
Izumi et al. [24]. A mixture of water (2.0 mol), 1-butanol (0.2 mol)
and heteropolyacid (5.0 ꢀ 10ꢁ4 mol) was added to tetraethyl
orthosilicate (0.2 mol) and stirred at 80 8C during 3 h. The
tungstophosphoric acid amount (mol) used to prepare PW1_S,
Sample
HPA loada
Surface areab
(m2/g)
Microporous
volumec
(cm3/g)
Total pore
(g/gsilica
)
volumed
(cm3/g)
PW3_S and PW4_S catalyst was 1.7 ꢀ 10ꢁ4
,
8.3 ꢀ 10ꢁ4 and
PW1_S
PW2_S
PW3_S
PW4_S
SiW_S
PMo_S
SiMo_S
0.025
0.042
0.065
0.084
0.057
0.037
0.039
489
458
254
141
332
478
466
0.23
0.24
0.11
0.07
0.16
0.25
0.24
0.24
0.25
0.12
0.07
0.16
0.26
0.25
11.7 ꢀ 10ꢁ4 mol, respectively. The hydrogel obtained was dehy-
drated slowly at 80 8C for 1.5 h in vacuo (25 Torr). The dried gel
obtained was extracted in a sohxlet apparatus with methanol
during 72 h, and dried at 100 8C, overnight. The silica-included
heteropolyacid was dried at 100 8C for 3 h prior to use in the
catalytic reactions.
a
HPA load determined by ICP analysis.
b
c
BET.
t-Method.
(p/p8) = 0.98.
2.2. Catalysts characterization
d
The textural characterization of the catalysts was based on the
nitrogen adsorption isotherm, determined at 77 K with a Micro-
meritics ASAP 2010 apparatus.
microporous material. Similar results were also observed by Izumi
et al. [24]. Table 1 shows the values of BET specific area (SBET),
microporous volume (Vmicro) and the total pore volume (Vtot). The
specific surface area was determined using the BET method, while
microporous volume was determined by the t-method, using a
standard isotherm proposed by Gregg and Sing [43]. It was
observed a continuous decrease of the BET surface area, micro-
porous volume and total pore volume with the increase of the PW
loading on silica (PW1_S to PW4_S). Similar results were also
observed by Izumi et al. [44]. It is known that a lower acid
concentration results in a higher specific surface area due to an
increase in the cross-linking level [45,46]. It was also observed that
the total pore volume is similar to the micropore volume,
indicating that the catalysts have low mesoporous contribution.
The amount of Mo and W in silica catalysts was measured by
dissolving the catalyst in H2SO4/HF 1:1 (v/v) and analysing the
obtained solution using inductively coupled plasma analysis (ICP),
which was carried out in a Jobin-Yvon ULTIMA instrument. FTIR
spectroscopy in KBr pellets was carried out on a Bio-Rad FTS 155
spectrometer. The spectra were taken with a resolution of 4 cmꢁ1
in the range of 400–4000 cmꢁ1, by running 1000 scans.
The X-ray diffraction (XDR) patterns of the heteropolyacid,
silica and catalysts were obtained by using a Bruker powder
diffractometer with built-in recorder, using Cu K
a radiation, nickel
filter, 30 mA and 40 kV in the high voltage source, and scanning
angle between 58 and 708 of 2u at a scanning rate of 18/min.
Fig.
1
shows the FTIR spectrum of the heteropolyacids
nꢁ
2.3. Catalytic experiments
immobilized on silica. In Keggin anion structure [XM12O40]
there are four different kinds of oxygen atoms, according to the
location they take in that unit: oxygen atoms bound to three M
atoms and to the X heteroatom (Oa), bridging oxygen atoms (Ob
and Oc), and terminal oxygen atoms (Od). The symmetric and
asymmetric vibrations of different M–O bonds are observed in the
following regions of the vibration spectra: M–Od bonds (1000–
960 cmꢁ1); M–Ob–M bonds (890–850 cmꢁ1); and M–Oc–M bonds
(800–760 cmꢁ1) [47]. However, some peaks typical of the Keggin
structures of HPA are overlapped or partially overlapped with the
peaks of the silica matrix framework.
The catalytic experiments were carried out in a stirred batch
reactor at 60 8C. In a typical experiment, the reactor was loaded
with 50 cm3 of methanol and 0.2 g of catalyst. Reactions were
started by adding 3 mmol of
a-pinene.
A blank experiment was carried out, in the absence of catalyst,
being kept all other conditions.
Stability tests of the PW2_S were carried out by running four
consecutive experiments, using the same reaction conditions.
Between the catalytic experiments, the catalyst was separated
from the reaction mixture by filtration, washed with methanol and
dried at 100 8C overnight.
The XRD diffractograms of the catalysts (not showed) do not
indicate the presence of any crystalline phases related to
heteropolyacid. These results suggest that the particles are too
small or well dispersed to be detected by XRD. It is only observed a
A hot-filtration test was carried out for the catalyst sample
PW2_S. The reaction was carried out as described above; after 27 h
(a-pinene conversion 51%) the catalyst was removed by hot-
filtration. Sampling of the liquid was continued until 140 h, since
the beginning of the experiment.
Nonane was used as the internal standard. Samples were taken
periodically and analysed by GC, using a KONIC HRGC-3000C
instrument equipped with a 30 m ꢀ 0.25 mm DB-1 column.
The GC mass balance was based on the substrate charge. The
difference was attributed to the formation of oligomers, which
were not GC determinable. The products were identified by gas
chromatography mass spectrometry (GC–MS) using a FISONS
MD800 (Leicestershire, UK) instrument, equipped with
a
30 m ꢀ 0.25 mm DB-1 column.
3. Results and discussion
3.1. Characterization of catalyst
The textural characterization of the catalysts was determined
based on nitrogen adsorption and desorption measurements. The
introduction of HPA on silica by the sol–gel method created a
Fig. 1. FT-IR spectra of catalysts: (A) silica; (B) PMo_S; (C) SiW_S; (D) SiMo_S; (E)
PW2_S.

142
D.S. Pito et al. / Applied Catalysis A: General 373 (2010) 140–146
broad band centered around 2u = 258 (amorphous silica). A similar
´
phenomenon was also observed by Molnar et al. [31].
3.2. Catalytic experiments
The main product of
terpinyl methyl ether (2) being also formed
ether (3), -terpinyl methyl ether (4), terpinolene (5), limonene
(6), endo-bornyl methyl ether (7), -fenchyl methyl ether (8), exo-
bornyl methyl ether (9), bornylene (10) and camphene (11) as by-
products (Scheme 1). The alkoxylation of -pinene proceeds via
a
-pinene (1) methoxylation was
a-
g
-terpinyl methyl
b
b
a
two parallel pathways, one of which resulting in by-cyclic products
(route A) and the other in monocyclic products (route B). The
reaction initiates by protonation of the
a-pinene double bond to
form the pinyl ion (A). The pinyl ion can rearrange into the bornyl
ion (B) and into the terpinyl ion (C). The bornyl ion (B) can lose a
proton to directly yield bornylene. Alternatively, it can be
isomerized into camphene. The bornyl ion (B) can yield endo-
Fig. 2. Methoxylation of
-pinene; (~) -terpinyl methyl ether; (&) H lumps all the minor components.
The solid lines represent the model fitted to the data points. Reaction conditions:
a-pinene over PW2_S catalyst. Concentration versus time.
(
)
a
a
initial concentration of
loading = 0.2 g.
a
-pinene = 0.061 mol dmꢁ3; temperature = 60 8C; catalyst
bornyl methyl ether,
b-fenchyl methyl ether and exo-bornyl
methyl ether, after methanol addition. The pinyl ion can also
rearrange into the terpinyl ion (C), which can lose a proton to
directly yield terpinolene, limonene, and after methanol addition,
included heteropolyacids (PMo_S, PW2_S, SiMo_S and SiW_S) in the
-pinene methoxylation. The light bars represent the initial activity
referred to the HPA amount (mol), i.e., the maximum slope of the
experimental kinetic curve of -pinene divided by the HPA amount.
a
-terpinyl methyl ether,
methyl ether are formed (Scheme 1) [4].
Fig. 2 shows the concentration profiles obtained over the
PW2_S catalysts, in the -pinene methoxylation. In Fig. 2, H lumps
g
-terpinyl methyl ether and
b
-terpinyl
a
a
a
On its turn, the dark bars represent the initial activity referred to the
catalyst amount (HPA_silica), i.e., the maximum slope of the
experimental kinetic curve divided by the catalyst (HPA plus
support) amount. It was observed that the catalytic activity
increases in the series: SiMo_S < PMo_S < SiW_S < PW2_S, which
can be explained due to PW is the strongest acid of the Keggin series
of heteropolyacids [14].
all the minor products (monocycle ethers, bicycle ethers, mono-
cycle hydrocarbons and bicycle hydrocarbons). The solid lines
represent the model fitted to the data points, which assume that
the
network (Scheme 2).
The -pinene concentration profile obtained over PW2_S, SiW_S,
a-pinene is consumed according to the parallel reaction
a
PMo_S and SiMo_S catalysts are shown in Fig. 3A. It was observed
that the PW2_S catalyst shows the highest catalytic activity of the
Keggin series. Fig. 3A shows the initial catalytic activities of silica-
From the above results, we can conclude that the PW2_S
catalyst is the most efficient catalyst for the
a-pinene methoxyla-
tion. However, in order to optimise the amount of PW immobilized
Scheme 1.

D.S. Pito et al. / Applied Catalysis A: General 373 (2010) 140–146
143
Scheme 2.
on silica, catalysts with different amount of this heteropolyacid
were prepared. Fig. 3B compares the initial activity of PW_S
catalysts loaded with different PW amounts, on the
a-pinene
methoxylation. It was observed that the initial catalytic activity
increases with the HPA content until a maximum, which was
obtained with the PW2_S catalyst. However, with high hetero-
polyacid loading on silica, a decrease in the catalytic activity is
observed. At a low PW loading on silica, the increase of the
heteropolyacid amount leads to the increase of activity, probably,
due to the kinetic effect. However, the high amount of hetero-
polyacid on silica leads to a decrease of the catalytic activity. This
behaviour can be explained probably, due to a decrease of the
accessibility to the active sites. A decrease of SBET and microporous
volume with the amount of PW was observed (Table 1).
When compared to other results reported in the literature, the
catalytic activity (mol/h molH+) of the different solid catalysts
increases as follows: PMO-SO3H-g (0.07 mol/h molH+) [9] < ionic
+
resin
(0.43 mol/h molH+) < PVA-SO3H
(0.66 mol/h molH
)
[10] < Beta (2.81 mol/h molH+) [7] < PW2_S (6.86 mol/h molH+).
The dependence of the selectivity to the -terpinyl methyl
ether with the -pinene conversion, for PW1_S, PW2_S, PW3_S,
PW4_S, SiW_S, PMo_S and SiMo_S catalyst is shown in Fig. 4. It was
observed that all of the catalysts tested on -pinene methoxylation
-terpinyl methyl ether (about 60%
a
a
a
exhibited good selectivity to
near complete conversion).
a
In order to optimise the reaction conditions, the effect of
different parameters (catalyst loading, initial concentration of
a
-
pinene and temperature) on the methoxylation of
PW2_S catalyst was studied.
a-pinene over
3.2.1. Effect of the catalyst loading
In order to study the effect of the PW2_S catalyst loading on the
-pinene conversion, different catalytic experiments were carried
Fig. 3. Methoxylation of
activitiestakenasthemaximumobservedreactionrate,calculatedfromthemaximum
slope of the -pinene kinetic curve. Reaction conditions: initial concentration of
pinene = 0.061 mol dmꢁ3; temperature = 60 8C; catalyst loading = 0.2 g.
a-pinene over heteropolyacids immobilized on silica. Initial
a
out, at 60 8C. The molar ratio of
a
-pinene:methanol (1:436) was
-pinene versus
a
a-
kept constant. Fig. 5 shows the conversion of the
a

144
D.S. Pito et al. / Applied Catalysis A: General 373 (2010) 140–146
Fig. 6. Methoxylation of
concentration of
-pinene. Conversion versus time. ( ) C = 0.061 mol dmꢁ3; (&)
C = 0.122 mol dmꢁ3 C = 0.305 mol dmꢁ3
(~) Reaction conditions:
temperature = 60 8C; catalyst loading =0.2 g.
a-pinene over PW2_S catalyst. Effect of the initial
a
Fig. 4. Methoxylation of
a-pinene over heteropolyacids immobilized on silica.
;
.
Selectivity to
a-terpinyl methyl ether. ( ) PW1_S; ( ) PW2_S; (~) PW3_S (&)
PW4_S; (+) PMo_S; (ꢀ) SiW_S; (–) SiMo_S.
time. It was observed that when the catalyst loading increases, the
equilibrium conversion is achieved more quickly, which can be
explained due to the increase of the total number of the active sites.
However, when the catalyst amount increases from 0.2 to 0.3 g,
good values of
near complete conversion of
a
-terpinyl methyl ether selectivity (about 60%, at
-pinene).
a
3.2.3. Effect of the temperature
The effect of the temperature on the a-pinene methoxylation
only a slight increase on the
a-pinene conversion was observed. It
was also observed that the catalyst loading has not any effect on
the equilibrium conversion (Fig. 5).
was also studied. The catalytic experiments were carried out at
different temperature over PW2_S catalyst, while the molar ratio of
The effect of the catalyst amount on the selectivity of PW2_S to
a
-pinene:methanol (1:436) and the catalysts loading (m = 0.2 g)
was kept constant. Fig. 7 shows the influence of the temperature on
the -pinene methoxylation. It was observed that the -pinene
a
-terpinyl methyl ether was also studied. It was observed that all
the catalytic experiments over different amount of PW2_S yield
good values of -terpinyl methyl ether selectivity (about 60%, at
near complete conversion of -pinene).
a
a
a
conversion increases with the temperature, at a fixed contact time
under otherwise identical conditions.
a
The effect of the temperature on the selectivity to
a-terpinyl
3.2.2. Effect of the initial concentration of
The initial concentration of -pinene was varied from 0.061 to
0.305 mol dmꢁ3, while the reaction temperature (T = 60 8C) and
the catalyst loading (m = 0.2 g) were kept constant. The -pinene
methoxylation was carried out over PW2_S. The results are shown
a
-pinene
methyl ether was also studied. At the different temperatures
considered, good selectivity values (about 60%, at near complete
a
conversion of
a-pinene) were obtained.
a
3.2.4. Catalyst stability and reusability
in Fig. 6. At a fixed contact time, it was observed that the
conversion increases when the initial concentration of
increases, under otherwise identical conditions. This behaviour can
be explained due to the increase of the reaction rate with
concentration.
a
a
-pinene
-pinene
In order to study the catalytic stability of the PW2_S, different
batch runs with the same catalyst sample, at same conditions were
carried out. It was observed similar catalytic activity for all the
tests (3.00 ꢀ 10ꢁ4, 2.89 ꢀ 10ꢁ4, 2.97 ꢀ 10ꢁ4 and 2.97 ꢀ 10ꢁ4, in the
first, second, third and fourth experiments, respectively).
In order to confirm the catalyst stability, a hot-filtration
experiment was performed with the catalyst sample PW2_S. After
27 h of reaction the catalyst was separated from the reaction
a-pinene
The effect of the initial concentration of
selectivity of PW2_S to -terpinyl methyl ether was also studied. It
was observed that all the initial concentration of -pinene yield
a-pinene on the
a
a
Fig. 7. Methoxylation of
a-pinene over PW2_S catalyst. Effect of the reaction
temperature. Conversion versus time. (~) T = 40 8C; (&) T = 50 8C; ( ) T = 60 8C.
Reaction conditions: initial concentration of
loading = 0.2 g.
Fig. 5. Methoxylation of
Conversion versus time. (&) m = 0.1 g;
conditions: initial concentration of
-pinene = 0.061 mol dmꢁ3; temperature = 60 8C.
a
-pinene over PW2_S catalyst. Effect of the catalyst loading.
a
-pinene = 0.061 mol dmꢁ3; catalyst
(
)
m = 0.2 g; (~) m = 0.3 g. Reaction
a

D.S. Pito et al. / Applied Catalysis A: General 373 (2010) 140–146
145
Fig. 9. Methoxylation of
a
-pinene over PW immobilized on silica. Effect of the PW
Fig. 8. a-Pinene concentration profile obtained for the blank experiment compared
loading on the concentration profile of the
a-pinene. (~) PW1_S; ( ) PW2_S; (ꢀ)
to that obtained with the catalyst sample PW2_S. (~) hot-filtration test; ( ) normal
experiment; (&) with out catalyst experiment. Reaction conditions: initial
PW3_S; (&) PW4_S. The solid lines represent the model fitted to the data points.
Reaction conditions: initial concentration of
a ;
-pinene = 0.061 mol dmꢁ3
concentration of
loading = 0.2 g.
a
-pinene = 0.061 mol dmꢁ3
;
temperature = 60 8C; catalyst
temperature = 60 8C; catalyst loading = 0.2 g.
Table 2
Model parameters obtained by fitting the model to experimental data.
mixture, and its composition was then followed for 113 h. Fig. 8
compares the -pinene concentration profiles obtained for the
k1 (dm3/h gcat
)
k2 (dm3/h gcat
)
a
Catalysts
hot-filtration experiment and a normal catalytic experiment, both
performed with the same catalyst sample. It was observed that the
reaction stops when the catalyst is removed, while it continues
proceeding in the normal experiment. When the methoxylation of
PW1_S
PW2_S
PW3_S
PW4_S
SiW_S
PMo_S
SiMo_S
0.00031
0.00527
0.00187
0.00097
0.00210
0.00049
0.00043
0.00010
0.00249
0.00143
0.00083
0.00172
0.00048
0.00040
a
-pinene was carried out in the absence of catalyst, no a-pinene
conversion was observed (Fig. 8).
3.2.5. Kinetic modelling
A simple kinetic model can be established based on the
following assumptions:
The fitting of the model to the data points is shown in Fig. 9. It
was observed that the kinetic model fits the experimental
concentration data quite well. The effect of the heteropolyacid
amount immobilized into silica support on the model parameters
is shown in Table 2. It was observed that the apparent kinetic
constants increase with the increase of the heteropolyacid amount
on silica (Table 1). However, at high heteropolyacid loading on
silica, a decrease on the apparent kinetic constants was observed.
This behaviour can be explained due to the presence of internal
limitation diffusion. Probably, a decrease of the accessibility to the
active sites may occur, once a decrease of SBET and total volume
with the amount of PW (Table 1) were observed. It was also
observed that the apparent kinetic constants decrease in the series:
PW2_S > SiW_S > PMo_S > SiMo_S. This behaviour can be due to
the stronger acidity of the PW catalyst [14].
1. Isothermal and isobaric reaction conditions.
2.
a
-Pinene is consumed according to the parallel reaction
network shown in Scheme 2, where P represents -pinene, E
represents -terpinyl methyl ether and H lumps all the other
species formed.
3. As in the present case
a
a
a
-terpinyl methyl ether seems not to be
consumed, is not necessary a more complex reaction network.
4. The concentration profiles of the reactant and products do not
exhibit any pronounced initial inductive period (Fig. 2). The
external and internal diffusion of the reactants and products on
the catalyst were not considered.
5. First order kinetics is assumed.
The reaction rate of these two pseudo elementary reaction are
expressed as:
4. Conclusions
Dodecatungstophosphoric acid, dodecamolybdophosphoric
tungstosilicic acid and molybdosilicic acid immobilized on silica
r1 ¼ k1Cp
r2 ¼ k2Cp
(1)
(2)
were used in the
a-pinene methoxylation. The main product of
a-
pinene methoxylation was
a-terpinyl methyl ether being also
formed bornyl methyl ether, fenchyl methyl ether, limonene and
terpinolene as by-products. It was observed that the catalytic
activity increases in the series: SiMo < PMo_S < SiW_S < PW2_S.
Catalysts consisting in the PW immobilized on silica with
different PW loading were prepared. It was observed that the
catalytic activity increases with the amount of PW immobilized on
silica. However, at high amount of PW on silica, a decrease of the
catalytic activity was observed.
For batch reactor, the mole balance equations may be written as
dCp
W
¼ ꢁ ðr1 þ r2Þ
(3)
(4)
dt
V
dCE
dt
W
¼
r1
V
All of the catalysts tested on a-pinene methoxylation exhibited
dCH
dt
W
V
¼
r2
(5)
good selectivity (about 60% at near complete
sion).
a-pinene conver-
The optimisation was carried out by the SOLVER1 routine in a
Microsoft Excel1 spreadsheet.
In order to optimise the reaction conditions, the effect of
various parameters, such as, reaction temperature, molar ratio of

146
D.S. Pito et al. / Applied Catalysis A: General 373 (2010) 140–146
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a
-pinene to methanol and catalyst loading of PW2_S were carried
(1992) 41–57.
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observed similar activity for all the catalytic tests.
A simple kinetic model, which fits the experimental concentra-
tion data quite well, has been developed.
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