Exploring the Keggin-Type Heteropolyacid-Catalyzed Reaction Pathways of the Β-Pinene with Alkyl Alcohols

Article abstract of DOI:10.1007/s10562-019-02808-5

Abstract: In this work, we investigated the activity of Keggin heteropolyacid catalysts (i.e., H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀ and H₄SiW₁₂O₄₀) in β-pinene reactions with alkyl alcohols (i.e. methyl, ethyl, propyl, sec-propyl, butyl and sec-butyl alcohols), and exploring the different aspects that drive the selectivity of this process. We have found that carbon skeletal rearrangements and isomerization providing intermediate carbocations that controlling the reaction selectivity. β-pinene was preferentially converted to α-terpinyl ion which undergoes a nucleophilic attack of alcohol providing alkyl alcohol. Bornyl ion was converted to bornyl and fenchyl ethers. The other secondary products were β-pinene isomers obtained from bornyl and α-terpinyl carbocations. Phosphotungstic acid (i.e., H₃PW₁₂O₄₀) was the most active catalyst and selective toward the main product (α-terpinyl alkyl ether); the highest conversion (ca. 96%) and ether selectivity (ca. 61%) was achieved in the reactions with β-pinene. Although having also been alkoxylate, α-pinene was less reactive (ca. 40%), while camphene and limonene remained unreactive under reaction conditions studied. An increase of temperature resulted in an improvement on conversion of β-pinene and selectivity toward α-terpinyl methyl ether. Similarly, the H₃PW₁₂O₄₀ concentration played a crucial role on reaction selectivity. This work presents positive features such as a short reaction time, high atom economy, mild reaction conditions (i.e., low temperature and room pressure). Even though soluble the catalyst was easily recovered by liquid -liquid extraction and efficiently reused. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02808-5

Catalysis Letters
https://doi.org/10.1007/s10562-019-02808-5
Exploring the Keggin‑Type Heteropolyacid‑Catalyzed Reaction
Pathways of the β‑Pinene with Alkyl Alcohols
Henrique Priori Polo1 · Neide Paloma Goncalves Lopes1 · Márcio José da Silva1,2
Received: 19 October 2018 / Accepted: 4 May 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
In this work, we investigated the activity of Keggin heteropolyacid catalysts (i.e., H3PW12O40, H3PMo12O40 and H4SiW12O40)
in β-pinene reactions with alkyl alcohols (i.e. methyl, ethyl, propyl, sec-propyl, butyl and sec-butyl alcohols), and exploring
the diferent aspects that drive the selectivity of this process. We have found that carbon skeletal rearrangements and isomeri-
zation providing intermediate carbocations that controlling the reaction selectivity. β-pinene was preferentially converted to
α-terpinyl ion which undergoes a nucleophilic attack of alcohol providing alkyl alcohol. Bornyl ion was converted to bornyl
and fenchyl ethers. The other secondary products were β-pinene isomers obtained from bornyl and α-terpinyl carbocations.
Phosphotungstic acid (i.e., H3PW12O40) was the most active catalyst and selective toward the main product (α-terpinyl alkyl
ether); the highest conversion (ca. 96%) and ether selectivity (ca. 61%) was achieved in the reactions with β-pinene. Although
having also been alkoxylate, α-pinene was less reactive (ca. 40%), while camphene and limonene remained unreactive under
reaction conditions studied. An increase of temperature resulted in an improvement on conversion of β-pinene and selectivity
toward α-terpinyl methyl ether. Similarly, the H3PW12O40 concentration played a crucial role on reaction selectivity. This work
presents positive features such as a short reaction time, high atom economy, mild reaction conditions (i.e., low temperature
and room pressure). Even though soluble the catalyst was easily recovered by liquid -liquid extraction and efciently reused.
Graphical Abstract
Keywords Keggin heteropolyacids · Terpenic ethers · β-pinene · Catalytic alkoxylation
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

H. P. Polo et al.
high turnover due to theirs very strong Brønsted acidity
[20]. On the other hand, pure HPAs have low surface area
and have been used as heterogeneous catalysts supported
on matrixes such as silica or polymeric membranes [21].
Heterogeneous catalysts cause less corrosion and can be
easily recovered and reused, making the catalytic pro-
cesses friendlier to the environment [22]. Nonetheless,
supported-solid catalysts can sufer deactivation due to
leaching problems triggered by the polarity of reactional
medium and the by-products such as the water generated
in course of the reactions.
1 Introduction
Monoterpenes are natural origin hydrocarbons produced
from the turpentine oil (i.e., a resin obtained from conifer-
ous trees) or as a by-product of the Kraft process for the
paper pulping [1, 2]. In special, their oxygenated deriva-
tives are a versatile bio-based feedstock to synthesize fa-
vors, fragrances, agrochemicals and pharmaceuticals [3,
4]. For these reasons, a lot of research has been developed
focusing on catalytic processes for a chemical upgrade of
these renewable raw materials, converting them to com-
pounds such as ketones, alcohols and esters [5–9].
Another alternative is converting the HPAs to insolu-
ble salts. To do it, their protons should be total or partially
exchanged by larger radius cations, such as cesium or ammo-
nium tert-butyl ions. Gusevskaya et al. assessed the catalytic
activity of a cesium exchanged partially phosphotungstic
acid salt (i.e., Cs2.5H0.5PW12O40) in alkoxylation reactions
of monoterpenes with short chain alcohols [23]. Those
authors have found that the cesium salts were very active,
achieving high TON (ca. 150-4200). However, the best
results were obtained in reactions at temperatures equal or
higher than boiling point of alcohols and under autogenous
pressure. Although the solid-supported HPA catalysts have
been extensively used in alkoxylation reactions, no deeper
study of the catalytic activity of Keggin HPAs with difer-
ent heteropolyanions has been carried out in homogeneous
conditions.
Monoterpene ethers have attractive organoleptic proper-
ties like the α-terpinyl derivatives alkyl ethers have dis-
tinctive characteristics depending of origin alcohol: methyl
ether has a grapefruit-like aroma and fnds application as
favor and fragrance for perfume and cosmetic products;
ethyl ether has a citrus fragrance; propyl ether has a grassy
favor, whilst the butyl ether has a woody scent [10, 11].
Since long time, the production of α-terpinyl alkyl
ethers from α-pinene has been extensively described in
literature, mainly in heterogeneous catalysis conditions.
Monoterpenes such as α-pinene are non-activated olefns,
which intermolecular addition of alcohols to provided
ethers generally requires excess of Brønsted acids or the
use of strong acidic solids such as β-zeolites [12, 13].
Catrinescu et al. has developed a selective methoxyla-
tion route of α-pinene to α-terpinyl methyl ether over Al3+
ion-exchanged clays [14]. They achieved high selectiv-
ity (ca. 65%) towards the main product, α-terpinyl methyl
ether, especially at high α-pinene conversions. This selec-
tivity is similar to that obtained by Pito et al., which used
poly(vinyl alcohol) containing sulfonic acid groups as
catalysts [15, 16] and silica-immobilized Keggin heteropo-
lyacids (HPAs) [17]. Mato et al. by using of mesoporous
and microporous carbons also achieved high α-terpinyl
ether selectivity (ca. 60%), but at a much higher reaction
rate [18].
In this work, we assessed the catalytic performance
of Keggin HPAs (i.e., H3PW12O40, H3PMo12O40 and
H4SiW12O40) in reactions of β-pinene with methyl alcohol
under mild reaction conditions. The efects of the main reac-
tion parameters were evaluated. The reactions were also per-
formed with several alkyl alcohols (i.e., methyl, ethyl, pro-
pyl, sec-propyl, butyl and sec-butyl alcohols). The reactivity
of β-pinene was compared with other monoterpenes (i.e.,
α-pinene, camphene, limonene). We paid a special focus on
reuse and recycle of the H3PW12O40 catalyst.
Alternatively, Yadav et al. developed a Brønsted
acid-free route to obtain ethers from monoterpenes (i.e.,
α-pinene, β-pinene, limonene), where the substrates under-
went a smooth alkoxylation under mild reaction conditions
in the presence of the FeCl3 catalyst, producing a wide
range of the corresponding ethers with high selectivity
[19]. However, those reactions were carried out in the
presence of dichloromethane, an unfriendly environmen-
tally solvent and a large load of catalyst (ca. 20 mol %).
Among the solid-supported catalysts, Keggin HPAs
have been explored in alkoxylation reactions of monoter-
penes (i.e., mainly α-pinene and camphene), especially in
the works developed by group of Castanheiro et al. [15–18,
20, 21]. Solid HPAs are soluble in polar solvents achieving
2 Experimental Procedures
2.1 Materials and Physical Methods
All the chemicals and solvents were used as received.
Keggin-type HPAs (i.e., H3PW12O40·12·H2O (98.5 wt%);
H4SiW12O40·12·H2O (97 wt%); H3PMo12O40·12 H2O
(97 wt%), and monoterpenes (i.e., β-pinene (98 wt%), cam-
phene (99 wt%), limonene (99 wt%), α-pinene (99 wt%))
were acquired from Sigma-Aldrich. The alkyl alcohols
(methyl, ethyl, propyl, sec-propyl, butyl and sec-butyl alco-
hols, 99.5 to 99.8% w/w) were also purchased from Sigma-
Aldrich. The toluene used as internal standard was acquired
from Alfatech.
1 3

Exploring the Keggin‑Type Heteropolyacid‑Catalyzed Reaction Pathways of the β‑Pinene with…
2.2 Catalytic Runs
3 Results and Discussion
Catalytic runs were performed under air in a glass reactor
(ca. 25 mL) ftted with refux condenser and sampling sep-
tum, under heating and magnetic stirrer. Typically, monoter-
pene (3.8 mmol) was dissolved in the adequate alcohol (ca.
14.3 mL) containing internal standard (toluene, 0.6 mL),
and the reactor temperature was adjusted to 323 K. Then,
the reaction was started by adding the adequate HPA catalyst
(1.5 mol % of H+ ions in relation to the monoterpene).
The progress of the reaction was followed taking aliquots at
regular time intervals and analyzing them by gas chromatogra-
phy, in a Shimadzu 2010 plus gas chromatograph instrument
ftted with a FID detector and a RTX-WAX capillary chroma-
tographic column (25 m×0.32 mm×0.30 µm).
3.1 Efect of the Catalyst Nature on the Reaction
of β‑pinene with Methyl Alcohol
A frst screening of the catalytic activity of HPA catalysts in
reactions of β-pinene with methyl alcohol was performed at
room pressure and temperature below the boiling point of
methyl alcohol. It is noteworthy that all the Keggin HPAs
were used at the same H+ ions concentration.
In according with the pKa values reported in literature,
the acidity strength of these catalysts can be described as
follow: H3PW12O40 >H4SiW12O40 >H3PMo12O40 [25]. The
conversions achieved in the reactions obeyed this tendency
of acidity strength (Fig. 1).
To calculate the reaction conversion, an aliquot was col-
lected before the addition of the catalyst. This corresponding
GC peak area of the substrate was compared to the GC peak
areas of substrate measured along the reaction. The GC mass
balance was based on the β-pinene substrate charge.
Although the HPAs-catalyzed reactions have reached
diferent conversions (Fig. 1), the products selectivity was
very similar, notwithstanding the HPA catalyst (Fig. 2). The
reaction pathway followed a pattern that can be depicted in
the Scheme 1 [26].
Selectivity of main products was calculated following the
Eq. 1; Acons correspond to the diference between the initial
and fnal GC peak areas of the substrate, ΣAprod correspond
to sum of GC peak area of products and rf is the response
factor experimentally determined or calculated in according
to literature [24].
As shown in Scheme 1, β-pinene can be converted to ter-
tiary carbocation under the acid catalysis conditions. After-
ward, this carbenium ion is then rearranged to diferent car-
bocations with two distinct carbon skeletal (i.e., α-terpinyl
and bornyl ions), which can sufer a nucleophilic attack of
the alkyl alcohol generating the respective ethers (α-terpinyl
(a), bornyl (b) and fenchyl (c) ethers) (Scheme 1). The selec-
tivity results (Fig. 2) shows that this is a preferential route
of β-pinene conversion. Otherwise, both β-pinyl, bornyl and
α-terpinyl carbocations can also release H+ ions resulting
in the formation of the β-pinene isomers (i.e., α-pinene (f),
(
)
ꢀ
Selectivity (%) =
Aprod rf∕(Acons)∗100
x
(1)
2.3 Products Characterization
The main reaction products were characterized by GC/MS
analysis (Shimadzu MS-QP 2010 ultra mass spectrometer),
electronic impact mode at 70 eV, coupled to a GC (Shimadzu
2010 plus), ftted with a RTX®-WAX capillary chromato-
graphic column (25 m×0.32 mm x 0.30 µm).
100
80
2.4 Recovery and Reuse of Catalyst
60
After the end of the reaction, excess alcohol was removed
under reduced pressure and 20 mL of water was added. The
resulting mixture was three times extracted with ethyl acetate/
water/hexane. After decantation the aqueous phase containing
the catalyst, it was evaporated under heating plate to near dry-
ness and then dried at room conditions. The solid catalyst was
weighted and reused in another catalytic run.
40
20
0
H3PW12O40
H3PMo12O40
H4SiW12O40
0
30
60
90
120
150
180
time / min
Fig. 1 Efect of HPA on conversion and rate of β-pinene reactions
with CH3OH. Reaction conditions: β-pinene (3.8 mmol); CH3OH
(14.4 mL; 380.0 mmol); catalyst loads (1.5 mol% of H+ cations in
relation to the β-pinene), 323 K
1 3

H. P. Polo et al.
α
-pinene
α
-terpinolene
limonene
It is noteworthy that a high molar ratio methyl alcohol to
β-pinene was used in all of catalytic tests (ca. 93:1). How-
ever, in absence of catalyst no conversion of β-pinene to
ether was observed. It is reinforcing the role of catalyst.
Regardless the Keggin anion nature, all the catalysts
were equally selective toward α-terpinyl methyl ether, which
was always the main reaction product (ca. 61% selectivity,
Fig. 2). Conversely, the tungsten HPAs had higher conver-
sions than molybdenum one. This result agrees with Castan-
heiro et al., who studied the activity of these Keggin HPAs,
however, in that case the HPAs were immobilized over a
solid support (i.e., silica) [16]. Since the highest conversion
and selectivity was obtained in H3PW12O40-catalyzed reac-
tions, it was chosen to be the goal catalyst of the coming
parts of this study.
bornyl methyl ether
fenchyl methyl ether
α
-terpinyl methyl ether
100
80
60
40
20
0
100
80
60
40
α
20
H SiW
O
H PMo O
12 40
H PW
O
12 40
4
12 40
3
3
catalyst
Fig. 2 Efects of HPA catalysts on the conversion and selectiv-
ity of β-pinene reaction with methyl alcohol. Reaction conditions:
catalyst (1.5 mol% of H+ cations); β-pinene (3.8 mmol); CH3OH
(353.0 mmol; 14. 3 mL); 323 K; reaction time: 3 h
3.2 Efect of H3PW12O40 Concentration
on the Reaction of β‑pinene with Methyl
Alcohol
limonene (d) and α-terpinolene (e); Scheme 1). Fenchene
and bornene that are isomers possibly formed were not
detected in reactions.
To evaluate the efciency of the H3PW12O40 catalyst, the
reactions were accomplished within a range of concentration
from 0.5–1.9 mol% of H+ ions (Fig. 3).
R-OH
- H+
OR
-terpinolene
(e)
limonene
(d)
α
-terpinyl ion
α
-terpinyl ether
(a)
α
I
H+
rearrangement
-pinene
α
-pinyl ion
α
II
- H+
OR
OR
R-OH
bornyl ion
-pinene
(f)
bornyl ether
(b)
α
fenchyl ether
(c)
Scheme 1 Reaction pathway of β-pinene and alkyl alcohols in Brønsted acid-catalyzed reactions [ adapted from refs. 18, 26]
1 3

Exploring the Keggin‑Type Heteropolyacid‑Catalyzed Reaction Pathways of the β‑Pinene with…
α
-pinene
α
-terpinolene
limonene
100
bornyl methyl ether
fenchyl methyl ether
105
α-terpinyl methyl ether
100
80
60
40
20
0
80
90
75
60
60
0.5 mol %
40
20
0
1.0 mol %
1.3 mol %
1.5 mol %
1.9 mol %
45
30
15
0
0
30
60
90
120
150
180
0.5
1.0
1.3
1.5
1.9
catalyst load / mol %
time / min
Fig. 4 Efect of the catalyst concentration on the conversion and
products selectivity of the H3PW12O40-catalyzed β-pinene reac-
tion with methyl alcohol. Reaction conditions: β-pinene (3.8 mmol);
CH3OH (353.0 mmol; 14. 3 mL); 323 K; reaction time: 3 h
Fig. 3 Efect of H3PW12O40 concentration on the reactions of
β-pinene with CH3OH. Reaction conditions: β-pinene (3.8 mmol),
CH3OH (14.4 mL), 323 K
The initial rates of reaction were notably afected by the
increasing on catalyst concentration (Fig. 3). Nevertheless,
most of the reactions reached the maximum conversion within
frst 120 min of reaction. An increase of the catalyst concen-
tration resulted in higher conversions, indicating that they do
not reach the equilibrium within the reaction time studied. In
reaction conditions studied (ca. 323 K, molar ratio 1:93 olefn
to alcohol, 3 h) the highest TON obtained was 411. Literature
describe that H3PW12O40-catalyzed alkoxylation of camphene
with methyl alcohol reached TON of 1380, however, it was
achieved after 24 h of reaction at 373 K temperature [27].
We observed that there was an optimal catalyst concen-
tration in which was obtained the highest selectivity of the
α-terpinyl methyl ether (product “a”, Scheme 1) (ca. 61%),
with good conversion rate (ca. 94%) (Fig. 4). In this sense,
the selectivity towards α-terpinyl methyl ether obtained
herein was close or equal to the other ones achieved in reac-
tions carried out over other solid catalysts [15, 16, 18]. When
the terpinyl ether selectivity decreases, the bornyl ether and
limonene becomes the main products. The existence of two
selectivity maximum for the α-terpinyl methyl ether at dif-
ferent concentrations (ca. 0.5 and 1.5 mol%) suggest that
there is no linear correlation between the α-terpinyl methyl
ether selectivity and concentration of catalyst. Indeed, there
was a competition between the isomerization and etherifca-
tion reactions.
point of the alcohol [23, 26]. However, our intention herein
was to evaluate if these reactions could be also run at lower
temperatures. Therefore, the reactions were studied at tem-
peratures ranging of 298–328 K (Fig. 5).
An increase on reaction temperature resulted in a higher
initial rate of reaction, as well as the fnal conversion. This
late observation suggests that the reaction has an endother-
mic characteristic. On the other hand, the formation of the
possible intermediate carbocations (Scheme 1) looks have
been diferently afected by increase on temperature; while
the formation of ether derivative from the α-terpinyl car-
bocation was enhanced, the formation of bornyl ether was
298 K
308 K
318 K
323 K
328 K
100
80
60
40
20
0
3.3 Efect of Temperature
0
30
60
90
120
on the H3PW12O40‑Catalyzed β‑pinene Reactions
with CH3OH
time / min
Fig. 5 Efects of temperature on the H3PW12O40-catalyzed β-pinene
reactions with CH3OH. Reaction conditions: β-pinene (3.8 mmol),
CH3OH (14.3 mL), catalyst load (1.5 mol % of H+ cations)
In general, alkoxylation reactions have been done under
autogenous pressure and temperatures higher than boiling
1 3

H. P. Polo et al.
Table 1 Efect of the
Exp.
Temperature
(K)
Conversion
(%)
Selectivity (%)a
temperature on the conversion
and product selectivity on the
β-pinene etherifcation reaction
with methyl alcohol
a
b
c
d
E
f
1
2
3
4
5
298
308
318
323
328
30
56
77
94
97
27
36
49
61
59
20
19
12
5
7
4
8
9
23
20
16
15
18
16
14
8
7
7
7
7
4
3
0
5
11
Reaction conditions: β-pinene (3.8 mmol), CH3OH (14.4 mL), catalyst load (1.5 mol% of H+ cations), tem-
peratures (from 298 to 328 K), 2 h reaction
aIdentifed by GC–MS analyses as: α-terpinyl methyl ether (a); bornyl methyl ether (b); fenchyl methyl
ether (c); limonene (d); α-terpinolene (e); α-pinene (f)
Table 2 Efect of the
Exp.
Alcohol
Conversion
(%)
Selectivity (%)a
temperature on the conversion
and product selectivity on the
β-pinene etherifcation reaction
with methyl alcohol
a
b
c
d
e
f
g
1
2
3
4
5
6
Methyl
Ethyl
Propyl
sec-Propyl
Butyl
sec-Butyl
96
90
93
39
77
67
61
47
24
13
21
8
5
9
9
6
3
6
7
15
13
25
41
33
36
7
7
12
22
14
18
3
5
5
11
4
0
3
3
10
8
16
13
16
14
9
14
8
Reaction conditions: β-pinene (3.8 mmol), CH3OH (14.3 mL), catalyst load (1.6 mol% of H+ cations), tem-
perature (323 K), 2 h reaction
aIdentifed by GC–MS analyses as: α-terpinyl methyl ether (a); bornyl methyl ether (b); fenchyl methyl
ether (c); limonene (d); α-terpinolene (e); α-pinene (f); complex mixture of minority products (g)
lowered (Table 1). Limonene (product “d”, Scheme 1) was
always the major secondary product regardless the reaction
temperature, have being formed with a selectivity of varying
of 15 to 23%.
methyl alcohol
propyl alcohol
sec-propyl alcohol
ethyl alcohol
butyl alchol
100
80
60
40
20
0
sec-butyl alchol
3.4 Efect of Alcohol on the H3PW12O40‑Catalyzed
β‑pinene Reactions
The impact of alcohol structure on conversion and selectiv-
ity of reactions with the β-pinene was assessed (Table 2).
The experimental data plots are presented in Fig. 6.
It was verifed that an increase of carbon chain size of C1
to C3 has not reduce the reactivity of alcohols; only the butyl
alcohol was less reactive (ca. 77% of conversion). Similarly,
the initial rate of reactions of methyl and propyl alcohols
were relatively close. On the other hand, secondary alcohol
hydroxyl groups (i.e., sec-propyl and sec-butyl alcohols)
have high steric hindrance and were the less reactive (Fig. 6).
The selectivity of α-terpinyl alkyl ether was drastically
afected by the nature of alcohol. An increase on carbon
chain size resulted in a decrease of ether selectivity as well
as increased a selectivity of the formation of the isomers
such as limonene.
0
30
60
90
120
150
180
time / min
Fig. 6 Curves from H3PW12O40-catalyzed β-pinene reactions with
diferent alkyl alcohols. Reaction conditions: β-pinene (3.8 mmol),
ROH (14.3 mL), catalyst load (1.6 mol % of H+ cations), temperature
(323 K)
1 3

Exploring the Keggin‑Type Heteropolyacid‑Catalyzed Reaction Pathways of the β‑Pinene with…
When alcohols less reactive were used, the selectivity of
other products, which included small amounts of tricyclene,
fenchene, polymer compounds and water addition products
(i.e., named “g”, Table 2) was calculated through the mass
balance; in general, it varied from 3 to 10%.
and heterogeneous catalysis conditions ((i.e., Cs3PW12O40).
They have found that camphene was efciently converted
to methyl bornyl ether only when the H3PW12O40-catalyzed
reactions were carried out during 7 or 10 h at 363 or 373 K
temperatures and under autogenous pressure [27].
When we observe the carbon skeletal of the α and
β-pinene, we can see after a protonation step both terpenes
could be converted to α-terpinyl ion, which then can undergo
a nucleophilic attack providing directly the α-terpinyl alkyl
ethers, or alternatively, sufer a carbon skeletal rearrange-
ment giving fenchyl or bornyl ions, which can be converted
to respective ethers (Scheme 1). Nonetheless, β-pinene was
much more reactive than α-pinene. It can be explained if we
considered that the frst has an exocyclic and disubstituted
double bond, which is more reactive than the endocyclic and
trisubstituted double bond of α-pinene (Scheme 1, Fig. 8).
Limonene has an exocyclic double bond, which is also
disubstituted, an aspect that should become it as reactive
as the β-pinene. However, limonene was almost inert under
our reaction conditions. When we examine the molecular
models of limonene and β-pinene, we can realize the more
stable structural conformation of the limonene result in a
high steric hindrance of methyl group on exocyclic double
bond (Fig. 9).
3.5 Efect of Terpenic Substrate
on the H3PW12O40‑Catalyzed Reactions
with CH3OH
Limonene has an exocyclic double bond that if protonate
could provide a tertiary carbocation (i.e., ion α-terpinyl),
which is an intermediate of α-terpinyl methyl ether. None-
theless, it was unreactive at our reaction conditions; the
same occurred with camphene. Conversely, α-pinene was
converted (ca. 40%), however, it was less reactive than
β-pinene (Fig. 7).
Literature has explored the alkoxylation of camphene,
α-pinene and limonene, nonetheless, for converting of these
substrates to alkyl ethers, more drastic reaction conditions
or higher excess of alcohol have been used. For instance, to
convert α-pinene to α-terpinyl methyl ether (ca. 62% selec-
tivity) in reactions over H3PW12O40/SiO2 catalyst, Castan-
heiro et al. used a high molar ratio alcohol to olefn (i.e.,
433: 1) [16]. Herein, the reactions were carried out using a
proportion of 92: 1 (Fig. 7).
This efect looks difcult the protonation of double bond
and the formation of terpinyl ion from limonene at tempera-
ture conditions studied herein (ca. 323 K). Probably, when
the reactions are carried out higher temperatures (ca. 373 K)
this efect can be minimized, and the limonene molecule
becomes more reactive [24].
Hensen et al. assessed the alkoxylation of limonene and
α-pinene over diferent zeolites, however, longer reaction
times (ca. 5 to 20 h) were required for that a high conversion
and selectivity could be achieved [25].
Gusevskaya et al. assessed the alkoxylation of camphene
with alkyl alcohols under homogeneous (i.e., H3PW12O40)
Similarly, the low reactivity of the camphene may be
also attributed to the low temperature used herein; indeed,
the camphene alkoxylation acid-catalyzed reactions pass
100
80
conversion
α-terpninyl methyl ether
bornil and fenchil methyl ethers
monoterpene isomers
100
80
60
40
20
0
α
β
-pinene
-pinene
60
40
20
0
limonene
camphene
0
30
60
90
120
150
180
time / min
β
-pinene
α
-pinene
limonene
Fig. 7 Efect of the monoterpene structure on the reaction with
methyl alcohol. Reaction conditions: substrate (3.8 mmol);
H3PW12O40 catalyst (1.5 mol% of H+ cations in relation to the sub-
strate); CH3OH (14.4 mL); 323 K
Fig. 8 H3PW12O40-catalyzed monoterpenes alkoxylation of monoter-
penes with CH3OH. Reaction conditions: substrate (3.8 mmol); cata-
lyst (1.5 mol% of H+ cations in relation to the monoterpene); CH3OH
(14.4 mL); 323 K; 3 h reaction
1 3

H. P. Polo et al.
Fig. 9 Ball and stick molecular models of monoterpenes with minimized energy. These structures with minimized energy were obtained using
software ChemBio3Dultra
through Wagner-Meerwein rearrangement, which after the
conversion
catalyst recovery
alcohol nucleophilic attack give bornyl and fenchyl ethers.
100
α
-terpinyl methyl ether
These reactions require temperatures higher than 323 K,
besides longer reaction times [27].
80
60
40
20
0
3.6 Recovery and Reuse of H3PW12O40 Catalyst
on the Reactions of β‑pinene with CH3OH
Although soluble, the H3PW12O40 catalyst is an easily recov-
ered solid from reaction solution by liquid–liquid extrac-
tion followed by the aqueous phase vaporization (water/ethyl
acetate/hexane mixture; see Sect. 2.4).
A recovery rate of 87–90% for the H3PW12O40 catalyst
was obtained. It was found a decrease of conversion after
successive cycles reuse (see Fig. 10), which was attributed
to the loss of acidic strength of catalyst. Nonetheless, no
decreasing of selectivity for the α-terpinyl methyl ether was
observed after three successive cycles. To check this point,
FT-IR of fresh catalyst and three times reused were recorded
and are displayed in Fig. 11.
fresh
first
second
third
runs
Fig. 10 Conversion, recovery rate of catalyst and selectivity for the
α-terpinyl methyl ether obtained in H3PW12O40-catalyzed reaction
of β-pinene with CH3OH. Reaction conditions: β-pinene (3.8 mmol);
catalyst (1.5 mol % of H+ cations in relation to the monoterpene);
CH3OH (14.4 mL); 323 K; 3 h reaction
The spectrum of the bulk H3PW12O40 present the fol-
lowing typical absorption bands: νas(P-Oa) at 1080 cm−1,
νas(W=Od) at 960 cm−1 an νas(W–Ob–W inter octahedral) at
886 cm−1. In addition, the vasW–O–W intra-octahedral was
also observed close to 800 cm−1. In a Keggin anion unit, Oa
refers to the oxygen atom common to PO4 tetrahedron and
one group W3O13; contrariwise, Ob links two W3O13 groups;
Oc binds two octahedral WO6 inside a W3O13 group and Od
is the terminal oxygen of Keggin anion. We have found that
after it be three times reused, there was a shift of band at
886 cm−1 toward low wave number and enlargement of band
at 800 cm−1. It is an evidence that Keggin anion undergone
some alterations that compromised its activity.
4 Conclusion
In summary, we describe herein the main aspects related
to the Keggin heteropolyacid-catalyzed β-pinene reac-
tions with alkyl alcohols. The alkoxylation reaction
proceeds in methyl alcohol solvent at moderate tem-
perature (ca. 323 K) in the presence of 0.5 to 1.9 mol%
of H3PW12O40, which was the most active Keggin HPA
catalyst. The α-terpinyl methyl ether was obtained with
selectivity of 53 to 61%. In general, bornyl and fenchyl
1 3

Exploring the Keggin‑Type Heteropolyacid‑Catalyzed Reaction Pathways of the β‑Pinene with…
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1.5 mol% of H3PW12O40. In addition, the reaction tem-
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Acknowledgements The authors are grateful for the fnancial support
from CNPq and FAPEMIG (Brasil). This study was fnanced in part
by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
- Brasil (CAPES) - Finance Code 001.
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
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H. P. Polo et al.
Afliations
Henrique Priori Polo1 · Neide Paloma Goncalves Lopes1 · Márcio José da Silva1,2
2
* Márcio José da Silva
Departamento de Química, Universidade Federal de Viçosa
(UFV), Campus Universitário, Avenida P.H. Rolfs, s/n,
Viçosa, MG 36570-000, Brazil
silvamj2003@ufv.br
1
Chemistry Department, Federal University of Viçosa,
Viçosa, Minas Gerais 36570-000, Brazil
1 3