Highly active and recyclable metal oxide catalysts for the prins condensation of biorenewable feedstocks

Article abstract of DOI:10.1002/cctc.201402160

Metal oxides such as Nb₂O₅, Cr₂O ₃, and especially a Zn^II-Cr^III mixed oxide are demonstrated to be highly active and recyclable heterogeneous catalysts for Prins condensation, which provides a clean, high-yielding route for the synthesis of nopol through the condensation of biorenewable β-pinene with paraformaldehyde. Zn-Cr mixed oxide with an optimum Zn/Cr atomic ratio of 1:6 gave 100% nopol selectivity at 97% β-pinene conversion, with the catalyst easily recovered and recycled. The acid properties of Nb₂O ₅ and Zn-Cr mixed oxide were characterized by the diffuse reflectance IR Fourier transform spectroscopy of adsorbed pyridine and ammonia adsorption microcalorimetry. An appropriate combination of acid-base properties of Zn-Cr mixed oxide is believed to be responsible for its efficiency.

Full text of DOI:10.1002/cctc.201402160

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DOI: 10.1002/cctc.201402160
Highly Active and Recyclable Metal Oxide Catalysts for the
Prins Condensation of Biorenewable Feedstocks
Vinꢀcius V. Costa,[b] Hossein Bayahia,[a] Elena F. Kozhevnikova,[a] Elena V. Gusevskaya,[b] and
Ivan V. Kozhevnikov*[a]
Metal oxides such as Nb2O5, Cr2O3, and especially a ZnIIÀCrIII
mixed oxide are demonstrated to be highly active and recycla-
ble heterogeneous catalysts for Prins condensation, which pro-
vides a clean, high-yielding route for the synthesis of nopol
through the condensation of biorenewable b-pinene with par-
aformaldehyde. ZnÀCr mixed oxide with an optimum Zn/Cr
atomic ratio of 1:6 gave 100% nopol selectivity at 97% b-
pinene conversion, with the catalyst easily recovered and recy-
cled. The acid properties of Nb2O5 and ZnÀCr mixed oxide
were characterized by the diffuse reflectance IR Fourier trans-
form spectroscopy of adsorbed pyridine and ammonia adsorp-
tion microcalorimetry. An appropriate combination of acid–
base properties of ZnÀCr mixed oxide is believed to be respon-
sible for its efficiency.
Introduction
Terpenes are important renewable feedstock for the perfumery,
food and pharmaceutical industries.[1–3] Acid-catalysed transfor-
mations of terpenes, such as isomerisation, hydration, esterifi-
cation and condensation, are widely used for the industrial
production of value-added terpenic compounds.[1–3] These re-
actions are often performed as batch processes using large
amounts of mineral acids as homogeneous catalysts, which
give moderate yields and have an adverse impact on the envi-
ronment. The development of sustainable chemical processes
using renewable raw materials through the environmentally
benign heterogeneous catalysis is a major challenge for
academia and industry.[4,5]
Our previous search for efficient solid acid catalysts for vari-
ous terpene conversions was focused on heteropoly acid cata-
lysts.[13–18] We now report that certain metal oxides such as
Nb2O5, Cr2O3 and especially a ZnIIÀCrIII mixed oxide are highly
efficient and recyclable heterogeneous catalysts for Prins con-
densation [Eq. (1)]. To our knowledge, little research on the use
of metal oxides as catalysts for the Prins reaction has been
reported so far.[10]
Nopol is an optically active bicyclic primary alcohol used in
the agrochemical industry to produce pesticides as well as in
soap fragrances, detergents and other household products
owing to its balsamic odour.[3] Nopol is generally produced
through the Prins condensation of b-pinene and paraformalde-
hyde [Eq. (1)] with homogeneous acid catalysts.[6] However,
these methods suffer from severe reaction conditions, low Results and Discussion
yields and unwanted by-products. Several heterogeneous cata-
Catalyst characterisation
lysts, such as mesoporous materials comprising Sn, Zn, Zr and
Fe, zeolites, sulfated zirconia, FeÀZn metal cyanides, ZnCl2 sup-
ported on montmorillonite and Sn(OH)Cl, have been used to
overcome these problems (see Refs. [7–12] and references
therein).
The textural properties (surface area, pore volume and pore di-
ameter) of the Cr2O3, ZnO, ZnÀCr and Nb2O5 catalysts studied
are listed in Table 1. The surface area and the pore volume of
ZnÀCr mixed oxides increased significantly with the increase in
the Cr content: from 12 m2 gÀ1 at 0% Cr content to 243 m2 gÀ1
at 100% Cr content. ZnÀCr mixed oxide with an optimum Zn/
Cr atomic ratio of 1:6 that demonstrated the best catalytic per-
formance in nopol synthesis (see below) had a surface area of
230 m2 gÀ1, a pore volume of 0.32 cm3 gÀ1, and an average
pore diameter of 55 ꢁ. This catalyst demonstrated a narrow
pore size distribution peaked at a pore diameter of 37 ꢁ
(Figure 1). The surface area of Nb2O5 decreased with the in-
crease in the calcination temperature: from 243 m2 gÀ1 at
110 8C to 90 m2 gÀ1 at 5008C.
[a] H. Bayahia, Dr. E. F. Kozhevnikova, Prof. I. V. Kozhevnikov
Department of Chemistry
University of Liverpool
Liverpool L69 7ZD (UK)
Fax: (+44)151-794 3588
E-mail: kozhev@liverpool.ac.uk
[b] V. V. Costa, Prof. E. V. Gusevskaya
Departmento de Quꢀmica
Universidade Federal de Minas Gerais
Belo Horizonte 31270-901 (Brazil)
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Table 1. Nopol synthesis catalysed by metal oxides.[a]
[c]
Catalyst[b]
SBET
Pore volume[d] Pore size[e] Conv.[f] Select.[g]
[m2 gÀ1
]
[cm3 gÀ1
]
[ꢁ]
[%]
[%]
Nb2O5 (1108C)
Nb2O5 (2008C)
Nb2O5 (3008C)
Nb2O5 (4008C)
Nb2O5 (5008C)
Cr2O3
ZnÀCr (1:6)
ZnÀCr (1:1)
ZnÀCr (10:1)
ZnO
243
238
226
172
90
243
230
136
43
0.25
0.27
0.27
0.16
0.18
0.26
0.32
0.11
0.10
0.03
0.27
0.26
0.32
0.86
41
50
49
36
33
31
40
42
8
3
0
100
92
97
94
95
96
95
95
94
100
100
100
100
0
45
47
38
82
42
55
32
90
98
47
42
55
164
Figure 2. XRD spectra of a) Cr2O3, b) ZnÀCr (1:6), c) ZnÀCr (1:1), d) ZnÀCr
(10:1) and e) ZnO catalysts. All catalysts were calcined at 3008C under N2
flow for 5 h.
12
[h]
Nb2O5
Cr2O3
226
243
230
209
88
[h]
100
100
100
ZnÀCr (1:6)[h]
[h,i]
ZnÀCr (1:6)/SiO2
[a] Reaction conditions: 0.1 g of the catalyst, 10 mmol of paraformalde-
hyde, 5.0 mmol of b-pinene, 5.0 mL of acetonitrile, T=808C, t=6 h;
[b] Catalysts were calcined at 3008C under N2 flow for 5 h (Nb2O5 calcined
at 110–5008C in air for 3 h) and ground to 45–180 mm particle size;
[c] BET surface area; [d] Single-point total pore volume; [e] Average BET
pore diameter; [f] b-Pinene conversion; [g] Nopol selectivity; [h] 0.50 g of
the catalyst, 30 mmol of paraformaldehyde, 5.0 mmol of b-pinene, 5.0 mL
of acetonitrile, t=10 h; [i] 20%ZnÀCr(1:6)/SiO2 catalyst calcined at 4008C
under N2 flow for 5 h.
Figure 3. XRD spectra of ZnÀCr (1:6) oxide calcined at 4008C under N2 flow
for 5 h.
Fourier transform (DRIFT) spectroscopy of adsorbed pyridine
from the characteristic IR bands at 1540 cmÀ1 (Brønsted acid
sites) and 1440–1450 cmÀ1 (Lewis acid sites).[23] Because these
bands have approximately equal extinction coefficients,[23] the
ratio of their integral intensities gives directly the ratio of
Brønsted and Lewis acid site densities (B/L). ZnÀCr (1:6) mixed
oxide as well as its parent ZnO and Cr2O3 oxides predominantly
had Lewis acid sites, as indicated by the strong adsorption
band at 1440–1450 cmÀ1 in their spectra (Figure 4). ZnÀCr (1:6)
Figure 1. Pore size distribution of ZnÀCr (1:6) oxide.
ZnÀCr oxides can be amorphous or crystalline, depending
on the calcination temperature and Zn/Cr atomic ratio.[19–22]
Zn-rich oxides are crystalline, which demonstrate the ZnO
phase. The oxides calcined at higher than 3508C also demon-
strate Cr2O3 and ZnCr2O4 spinel crystalline phases, the relative
amounts of which depend on the Zn/Cr atomic ratio. The XRD
spectra shown in Figure 2 revealed that ZnO and ZnÀCr oxide
with an optimum Zn/Cr atomic ratio of 10:1 demonstrated the
ZnO wurtzite phase (JCPDS file no. 36-1451). The ZnÀCr (1:6)
oxide catalyst calcined at 3008C was amorphous; however,
after calcination at 4008C, it demonstrated the pattern of the
Cr2O3 phase along with a little of ZnCr2O4 spinel (Figure 3). The
crystalline material had a lower surface area, and it demon-
strated lower catalytic activity in nopol synthesis than the
amorphous material.
Figure 4. DRIFT spectra of adsorbed pyridine on a) ZnO, b) ZnÀCr (1:6) and
The nature of acid sites and their relative amounts in ZnÀCr
oxide catalysts was determined by the diffuse reflectance IR
c) Cr2O3 catalysts. The catalysts were calcined at 3008C under N2 flow for
5 h.
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mixed oxide and Cr2O3 also had Brønsted acid sites, as indicat-
ed by the band at 1540 cmÀ1, with B/Lꢀ0.1 in these oxides.
However, ZnO did not show any Brønsted acid sites of suffi-
cient strength to protonate pyridine. In contrast to ZnÀCr
oxide, Nb2O5 has been found to possess predominantly Brønst-
ed acidity, with the relative amount of proton sites decreasing
with the increase in the calcination temperature (B/L=1.32
after calcination at 3008C and B/L=0.99 after calcination at
5008C).[24]
The acid strength of the catalysts was measured by using
ammonia adsorption microcalorimetry in terms of the differen-
tial enthalpy of ammonia adsorption at zero coverage (DH).
This technique does not discriminate Brønsted acidity and
Lewis acidity and thus provides a measure of the total catalyst
acid strength. The results are summarised in Table 2. Nb2O5,
sion, not shown in Table 1). Nb2O5 demonstrated high activity
in Equation (1), with 94–96% selectivity towards nopol (pino-
carveol, pinocarvone and nopol isomers myrtenal and myrte-
nol were formed as byproducts). The activity of Nb2O5 de-
creased with the increase in its calcination temperature: from
50% conversion at 1108C to 31% at 5008C. This decrease
could be due to the loss of proton sites upon catalyst calcina-
tion. Cr2O3 was slightly less active, but demonstrated a superior
nopol selectivity of 100% (Table 1). Interestingly, its activity
was increased by adding ZnII to the CrIII oxide, although ZnII
oxide was practically inactive in Equation (1). ZnIIÀCrIII mixed
oxide with an optimum Zn/Cr atomic ratio of 1:6 performed
better than pure Cr2O3.
The synthesis of nopol was further improved by increasing
the paraformaldehyde/b-pinene molar ratio to 6:1 (Figure 5)
Table 2. Acid properties of the catalysts.
Catalyst[a]
Acid site
type[b]
DH[c]
[kJmolÀ1
]
Nb2O5 (1108C)
Nb2O5 (2008C)
Nb2O5 (3008C)
Cr2O3 (3008C)
ZnÀCr (1:6) (3008C)
ZnO (3008C)
B+L
B+L
B+L
B+L
B+L
L
À115
À123
À129
ꢀÀ193[d]
À170[d]
ꢀÀ127[d]
[a] Catalyst pre-treatment: Cr2O3 and ZnÀCr (1:6) were calcined at 3008C
under N2 flow for 5 h and Nb2O5 at 110–3008C in air for 3 h; [b] Brønsted
(B) and Lewis (L) acid sites were determined by using the DRIFT spectros-
copy of adsorbed pyridine; data for Nb2O5 were taken from Ref. [24];
[c] Initial enthalpy of ammonia adsorption at zero adsorption coverage
(mean absolute error: Æ3 kJmolÀ1); [d] Estimated from Ref. [20].
Figure 5. Effect of the b-pinene/paraformaldehyde molar ratio on nopol
conversion. Reaction conditions: 0.1 g of ZnÀCr (1:6), 5.0 mmol of b-pinene,
5.0 mL of acetonitrile, T=808C, t=6 h.
and the amount of the catalyst to 8 wt% per total reaction
mixture as well as by increasing the reaction time from 6 to
10 h. For ZnÀCr (1:6) mixed oxide, this resulted in an excellent
nopol yield of 97% (97% b-pinene conversion at 100% nopol
selectivity). Notably, pure Cr2O3 gave a 92% yield under the
same reaction conditions (Table 1).
calcined at 110–3008C, possessing mainly Brønsted acidity,
demonstrated the DH values in the range of À115 to
À129 kJmolÀ1, which are largely characteristic of the proton
sites in niobia. The acid strength of niobia predictably in-
creased with the increase in the calcination temperature. The
acid strength of ZnÀCr oxides, possessing mainly Lewis acidity,
increased significantly with the increase in the Cr content, with
the DH values varying from À127 to À193 kJmolÀ1, which
mostly represent the strength of Lewis acid sites in ZnÀCr
oxides.[20] Therefore, given the different nature of acidity in
ZnÀCr and Nb oxides, it is not possible to quantitatively char-
acterise the strength of each type of acid sites in these cata-
lysts by using ammonia adsorption microcalorimetry.
Supporting ZnÀCr (1:6) oxide on silica (AEROSILꢃ 300; SBET
=
300 m2 gÀ1) also improved the nopol synthesis. The ZnÀCr(1:6)/
SiO2 catalyst with a ZnÀCr oxide loading of 20 wt% performed
as well as bulk ZnÀCr oxide per total catalyst weight, which
gave a 94% nopol yield (Table 1). The silica support was practi-
cally inactive in this reaction.
In addition to acetonitrile, other solvents such as ethanol,
1,2-dichloroethane, and cyclohexane were tested in Equa-
tion (1) but were found to be less effective in comparison to
acetonitrile, which gave lower b-pinene conversions; this find-
ing was in agreement with the previous reports.[7–12]
Catalyst testing
We tested a wide range of metal oxides under the typical con-
ditions of nopol synthesis, that is, at 808C, 2 wt% catalyst,
a paraformaldehyde/b-pinene molar ratio of 2:1, and acetoni-
trile as a solvent. The representative results are summarised in
Table 1 along with the textural properties (surface area, pore
volume and pore size) of the catalysts. The reaction did not
occur in the absence of catalysts. g-Al2O3, ZrO2, SiO2 and TiO2
demonstrated only moderate catalytic activity (ꢁ15% conver-
The time course for Equation (1) in the presence of the sup-
ported 20%ZnÀCr(1:6)/SiO2 catalyst and bulk Nb2O5, Cr2O3, and
ZnÀCr (1:6) oxides is shown in Figure 6. Notably, the reaction
was complete in 2 h, followed by a rather slow increase in con-
version. Nb2O5 was the most active amongst these catalysts,
which gave 100% b-pinene conversion in 4 h. However, it was
less selective than Cr2O3 and ZnÀCr mixed oxide. In terms of
the nopol yield, ZnÀCr (1:6) oxide was the best (97%), followed
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Figure 6. a) Conversion of b-pinene and b) nopol yield versus time. Reaction
conditions: 0.50 g of the catalyst, 5.0 mmol of b-pinene, 30 mmol of parafor-
maldehyde, 5.0 mL of acetonitrile, T=808C. Catalyst pretreatment: Cr2O3
and ZnÀCr (1:6) were calcined at 3008C under N2 flow for 5 h, 20%ZnÀ
Cr(1:6)/SiO2 at 4008C under N2 for 5 h and Nb2O5 at 1108C in air for 3 h.
Figure 7. Reuse of the ZnÀCr (1:6) catalyst in the synthesis of nopol a) with
washing by warm ethanol and drying in an oven at 1108C for 4 h and
b) with washing by acetonitrile followed by heating at 3008C under N2 flow
for 5 h. Reaction conditions: 0.50 g of the catalyst, 5.0 mmol of b-pinene,
30 mmol of paraformaldehyde, 5.0 mL of acetonitrile, T=808C, t=4 h.
by 20%ZnÀCr(1:6)/SiO2 (94%), Cr2O3 (92%) and Nb2O5 (88%) in
a reaction time of 10 h. ZnÀCr oxide is on a par with the best
heterogeneous catalysts reported so far.[7–12] In addition, it is an
inexpensive robust material that is easy to prepare and handle.
After the completion of the reaction, the oxide catalysts
were easy to recover by filtration and reused. Their per-
formance upon reuse depended on catalyst treatment. Even
a minor treatment of the recovered ZnÀCr (1:6) oxide by wash-
ing with warm ethanol and drying in an oven at 1108C for 4 h
allowed sufficient catalyst recycling, with a decrease in conver-
sion from 93 to 61% at a constant nopol selectivity of 100% in
four successive reaction runs (Figure 7). Washing the catalyst
with acetonitrile followed by heating at 3008C under N2 flow
for 5 h led to better catalyst recyling, with a small decrease in
nopol yield from 93 to 75% in four runs (Figure 7). The de-
crease in yield was probably due to catalyst loss during filtra-
tion. Catalyst leaching during nopol synthesis was negligible as
determined from the inductively coupled plasma analysis
(0.003% loss of Cr and 0.07% Zn in the filtrate). Further im-
provement in the catalyst recycling may be possible. Unreact-
ed paraformaldehyde could also be recovered and recycled.
A plausible mechanism of nopol synthesis through Prins
condensation [Eq. (1)] is shown in Scheme 1.[25] This mechanism
involves protonation of the aldehyde followed by attack on
the methylene group of b-pinene, which forms a b-hydroxyl
carbocation intermediate. In the absence of a competing nu-
cleophile, which is the case in our system, this carbocation un-
dergoes proton elimination to yield nopol. With excess of alde-
Scheme 1. Proposed mechanism of the synthesis of nopol through the Prins
condensation of b-pinene and formaldehyde.
hyde in a homogeneous system, the b-hydroxyl carbocation
would add another aldehyde molecule to form 1,3-dioxane.[25]
No such product was observed in our case even with a sixfold
excess of paraformaldehyde over b-pinene, which indicates
that the formation of 1,3-dioxane on the surface of solid acid
catalysts is not feasible, probably owing to steric constraints.
The reaction therefore involves both protonation and depro-
tonation and is thus subject to acid and base catalysis. This re-
action can be performed effectively with certain metal oxides
possessing both acid (Brønsted and Lewis) sites and base sites
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and can thus act as bifunctional acid–base catalysts.[26] ZnÀCr
mixed oxide possesses the required acid–base properties
(Table 2 and Figure 4). The high catalytic activity of ZnÀCr
oxide in nopol synthesis is related to an appropriate combina-
tion of its Brønsted and Lewis acidity and basicity. It can be hy-
pothesised that strong Lewis acid sites in ZnÀCr oxide act as
adsorption centres for the reactants whereas relatively mild
neighbouring Brønsted acid sites and base sites selectively
affect Prins condensation [Eq. (1)] in a concerted way by proto-
nating the aldehyde and deprotonating the b-hydroxyl carbo-
cation, respectively (Scheme 1). In this regard, the addition of
ZnII to Cr2O3 could increase the basicity of the catalyst, which
would enhance proton elimination from the b-hydroxyl carbo-
cation. ZnII oxide is practically inactive in Equation (1) owing to
the lack of Brønsted acidity in it (Figure 4). In contrast, Nb2O5
has predominantly Brønsted acidity, which appears to be quite
strong (Table 2). This property could increase its catalytic activi-
ty but could also lead to side reactions (e.g., isomerisation and
oligomerisation of b-pinene and nopol), which impairs its se-
lectivity towards nopol (Table 1). Similarly, the use of zeolites
possessing strong Brønsted acidity for the synthesis of nopol
has resulted in the formation of isomerised products.[27] How-
ever, the acid properties of the catalysts were measured at the
gas–solid interface (by using the DRIFT spectroscopy of ad-
sorbed pyridine and ammonia adsorption calorimetry) whereas
the reaction was performed in the liquid phase in a polar and
slightly basic acetonitrile, which levelled off the acidity of the
catalysts. Therefore, in the reaction system, the difference in
the acidity of the catalysts is likely to be less than that in the
gas–solid system and thus the small effect of catalyst acidity
on the reaction, as can be observed from Table 1 and Figure 6.
708C. The precipitates were filtered off, washed with distilled water
until free from ammonia, dried in air at 1108C overnight, and finally
calcined under N2 flow at 3008C for 5 h. The oxides were ground
into a powder with a particle size of 45–180 mm. The silica-support-
ed ZnÀCr (1:6) catalyst was prepared through co-impregnation of
ZnII and CrIII nitrates onto AEROSIL 300 (Degussa; SBET =300 m2 gÀ1
)
from the aqueous solution, followed by rotary evaporation of
water, oven drying at 1108C overnight, and calcination at 4008C in
air for 2 h to decompose the nitrates to oxides.
Nb2O5 was prepared by using the method described in the litera-
ture.[28] NbCl5 powder (5.0 g, 18.5 mmol) was dissolved in absolute
ethanol (10 mL). This solution was then added to aqueous ammo-
nia (200 mL, 0.3m) at RT, and a precipitate of niobium hydrous
oxide was obtained. The precipitate was filtered, washed with dis-
tilled water until free from chloride, dried in an oven at 1108C, and
calcined at a specified temperature in air for 3 h.
Catalyst characterisation
The specific surface area and porosity of catalysts were determined
by using the BET method from nitrogen physisorption measured at
77 K with a Micromeritics ASAP 2010 instrument. Before analysis,
the samples were evacuated in situ at 2508C for 3 h. The powder
XRD spectra of the catalysts were recorded on a PANalytical X’Pert
diffractometer using a monochromatic CuKa radiation (l=1.54 ꢁ)
in a 2q range of 20–808. The DRIFT spectra of adsorbed pyridine
were recorded on a Nicolet Nexus FTIR spectrometer, as described
elsewhere.[24] The catalyst samples were ground with KBr (10 wt%)
and pre-treated at 1508C and 0.01 kPa for 1 h. The samples were
then exposed to pyridine vapour at RT for 1 h, followed by pump-
ing out the physisorbed pyridine at 1508C for 1 h. Then, the DRIFT
spectra of adsorbed pyridine were recorded at RT. Differential
heats of ammonia adsorption on the catalysts were measured at
1508C by using a pulse method in a flow system with a Setaram
TG-DSC 111 differential scanning calorimeter, as described previ-
ously.[24] Catalyst leaching was assessed by using inductively cou-
pled plasma atomic emission spectroscopy elemental analysis,
which was performed with a Spectro Ciros emission spectrometer.
Conclusions
We have demonstrated that metal oxides such as Nb2O5, Cr2O3
and especially ZnÀCr mixed oxide with an optimum Zn/Cr
atomic ratio of 1:6 are highly active and recyclable heterogene-
ous catalysts for Prins condensation, which provides a clean,
high-yielding route for the synthesis of nopol through the con-
densation of b-pinene with paraformaldehyde. An appropriate
combination of acid–base properties of ZnÀCr oxide is be-
lieved to be responsible for its catalytic efficiency. The results
obtained indicate that metal oxides are promising environmen-
tally benign catalysts for the Prins condensation of biorenewa-
ble feedstocks.
Catalyst testing
Prins condensation was performed in a glass reactor equipped
with a magnetic stirrer and a reflux condenser. In a typical run, the
reactor was charged with b-pinene (5 mmol), paraformaldehyde
(10–30 mmol of HCHO), acetonitrile solvent (5–10 mL), and a cata-
lyst (0.1–0.5 g) and placed in an oil bath heated to 808C. The reac-
tion was followed by GC (Varian STAR 3400 CX gas chromatograph
equipped with
a 30 mꢄ0.25 mmꢄ0.25 mm ZB-1701 capillary
column and a flame ionisation detector) by taking aliquots of the
reaction mixture at appropriate time intervals and using dodecane
as a GC standard. By using the Weisz model,[29] internal mass trans-
port limitations were estimated to be negligible in this system,
given the relatively low reaction rate (>0.5 h half-time; Figure 6),
large pore diameters, and small particle sizes of the catalysts
(Table 1). The product nopol was identified by using GC with a stan-
dard nopol sample from Sigma–Aldrich. The mass balance defined
as (nopol yield on b-pinene)/(b-pinene conversion) was 1.00Æ0.04,
that is, complete within 96%. After the completion of the reaction
with ZnÀCr (1:6), pure nopol was isolated as a colourless oil. The
isolation procedure included filtration of the catalyst from the reac-
tion mixture, rotary evaporation of the acetonitrile solvent, extrac-
tion of the product from oily residue with hexane, followed by the
separation of nopol using column chromatography with silica gel
Experimental Section
Chemicals and catalysts
b-Pinene (99% purity) and other chemicals were purchased from
Sigma–Aldrich and used as received without further purification.
Cr2O3, ZnO and a series of ZnÀCr mixed oxides with various Zn/Cr
atomic ratios were prepared through (co)-precipitation of ZnII and
CrIII hydroxides.[19–21] This reaction was performed by adding drop-
wise aqueous ammonia (10 wt%) to a stirred aqueous solution of
a mixture of ZnII and CrIII nitrates ([Zn+2]+[Cr+3]=0.2m) at 708C
until pH 7.0 was achieved, followed by ageing the slurry for 3 h at
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[14] K. A. da Silva Rocha, P. A. Robles-Dutenhefner, I. V. Kozhevnikov, E. V. Gu-
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as a stationary phase and hexane as a mobile phase. Unreacted
paraformaldehyde could be recovered from the residue and
recycled.
[16] A. L. P. de Meireles, K. A. da Silva Rocha, I. V. Kozhevnikov, E. V. Gusev-
skaya, Appl. Catal. A 2011, 409, 82–86.
[17] V. V. Costa, K. A. da Silva Rocha, L. F. de Sousa, P. A. Robles-Dutenhefner,
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Keywords: heterogeneous catalysis · metal oxides · mixed
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Received: March 24, 2014
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