Molybdenum oxide/γ-alumina: An efficient solid acid catalyst for the synthesis of nopol by Prins reaction

Article abstract of DOI:10.1039/c5ra12106j

Prins condensation of β-pinene with paraformaldehyde was carried out over MoO_x/γ-Al₂O₃ catalyst in liquid phase. MoO_x/γ-Al₂O₃ catalysts with different loadings were synthesized by impregnation method and characterized by XRD, N₂ sorption, NH₃-TPD, UV-DRS, TEM and SEM. The surface area, TEM and UV-DRS measurements reveal complete coverage of alumina support by MoO_x was achieved between 9 and 11 wt% loading in which the molybdenum is present as octahedral and tetrahedral species in the form of a monolayer domain and small clusters of MoO_x. The effect of MoO_x loading and calcination temperature of the catalysts on total acidity and catalytic activity in the Prins reaction were correlated. The total surface acid density of MoO_x/γ-Al₂O₃ increased with an increase in MoO_x loading until 11 wt% and further loading decreased the acidity. Among the different supports, ZrO₂, TiO₂, SiO₂ and Nb₂O₅, γ-Al₂O₃ exhibited the best performance. The influence of solvent on the Prins reaction in terms of dielectric constant, acceptor and donor number was investigated. Solvents with both AN and DN in the range of 10 to 20 were suitable to facilitate the reaction. The effect of reaction temperature, catalyst amount, reactant mole ratio and reusability of the catalyst for the Prins reaction were investigated. The 11 wt% MoO_x loaded on γ-Al₂O₃ showed the best performance with 96% β-pinene conversion with 86% nopol selectivity.

Full text of DOI:10.1039/c5ra12106j

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Molybdenum oxide/γꢀAlumina: An efficient solid acid
catalyst for the synthesis of nopol by Prins reaction
Vijaykumar S. Marakatti, Dundappa Mumbaraddi, Ganapati V. Shanbhag, Anand
B. Halgeri and Sanjeev P. Maradur*
Materials Science Division, Poornaprajna Institute of Scientific Research (PPISR),
Bidalur Post, Devanahalli, Bangalore-562110, India.
*Corresponding Author: Email: sanjeevpm@poornaprajna.org,
Fax: +9123611836; Tel: +91 27408552
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Abstract:
Prins condensation of ßꢀpinene with paraformaldehyde was carried out over MoOx/γꢀ
Al2O3 catalyst in liquid phase. The MoOx/γꢀAl2O3 catalysts with different loading were
synthesized by impregnation method and characterized by XRD, N2 sorption, NH3ꢀTPD, UVꢀ
DRS, TEM and SEM. The surface area, TEM and UVꢀDRS measurement reveal complete
coverage of alumina support by MoOx achieved between 9 and 11 wt % loading in which the
molybdenum is present as octahedral and tetrahedral species in the form monolayer domain and
small clusters of MoOx. The effect of MoOx loading and calcination temperature of catalysts on
total acidity and catalytic activity in Prins reaction were correlated. The total surface acid density
of MoOx/γꢀAl2O3 increased with increase in MoOx loading till 11 wt % and further loading
decreased the acidity. Among the different support such as ZrO2, TiO2, SiO2 and Nb2O5, γꢀAl2O3
exhibited best performance. The influence of solvents on the Prins reaction in terms of dielectric
constant, acceptor and donor number was investigated. The solvents with both AN and DN in
the range 10 to 20 were suitable to facilitate reaction. The effect of reaction temperature, catalyst
amount, reactant mole ratio and reusability of catalyst for the Prins reaction were carried out.
The 11 wt% MoOx loaded on γꢀAl2O3 showed better performance of 96 % ßꢀpinene conversion
with 86 % nopol selectivity.
Keywords: Prins reaction; MoOx/γꢀAl2O3; Nopol; ßꢀpinene; Paraformaldehyde.
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1. Introduction
The reaction of alkene with an aldehyde to produce 1,3ꢀdioxanes, 1,3ꢀdiols and
unsaturated alcohols is known as Prins reaction. Nopol is an unsaturated, bicyclic, monoterpinic
primary alcohol obtained from naturally occurring ßꢀpinene and formaldehyde. Nopol has a wide
application in perfumery industry as fragrance, detergent industries as aroma and as pesticide in
agrochemical industries.1 Prins reaction is an acid catalyzed reaction and proceeds with
electrophillic addition of activated aldehyde to olefin. Traditionally, nopol was produced from
Prins condensation of ßꢀpinene and paraformaldehyde under anhydrous conditions or using
homogenous catalyst ZnCl2.2,3 The disadvantages associated with these process are the use of
drastic conditions like high temperature, pressure, low yield and formation of unwanted side
products. To overcome the following problems different heterogeneous catalysts were designed
and studied for the Prins reaction. The most widely explored catalysts for Prins reactions are Snꢀ
beta, SnꢀMCMꢀ41, SnꢀSBAꢀ15, ZrꢀSBAꢀ15 and ZnꢀMCMꢀ41, where metal ions are introduced
by isomorphous substitution, wet impregnation and chemical vapor deposition methods.4ꢀ9
A
good to moderate yield for nopol has been reported for the aforementioned catalysts. However,
the use of these catalysts may be limited due to the tedious synthesis procedures, leaching of
metal ions and use of toxic metal tin. In accordance with the insight of green chemistry, some of
the catalysts like sulfated zirconia, mesoporous iron phosphate, FeꢀZn double cyanide complex,
metal organic framework and MWW type zeolite have been applied for the Prins reaction to
produce nopol.10ꢀ13 Most of the catalysts studied, indicate the requirement of either Lewis acid
sites or weak to moderate Brönsted acid sites for the selective synthesis of nopol. 4ꢀ13
Supported oxides of transition metals are widely used as catalysts for various organic
transformations.14 Due to the dispersion of one metal oxide phase over the other; these are
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known as “monolayer type” of catalysts. The molybdenum oxide is impregnated on to inorganic
supports like TiO2, SiO2, Al2O3 and ZrO2 to improve catalytic activity, life and mechanical
strength of catalyst. The MoOx supported on silica has been studied as an acid catalyst for the
Prins reaction for the synthesis 1.3ꢀdioxolanes and 1.3 dioxanes.15 MoO3/TiO2 supported catalyst
has been widely studied as desulfurization catalytic process; MoO3/ZrO2 supported catalyst was
studied for several reactions such as alkane isomerization, synthesis of furfural etc.14ꢀ20 Among
the different support, γꢀAlumina has attracted much attention because of its large surface area,
high stability and porosity. Molybdenum oxide supported on γꢀalumina has been studied for the
catalytic processes such as epoxidation of allyl alcohol, oxidative dehydrogenation of propane,
21ꢀ24
hydrodesulphurization of thiophene.
However, there are only few reports discussing the
relation between the acidic properties of MoOx/γꢀAl2O3with catalytic activity of organic
reactions.14 To further explore on the acidic properties, MoOx/ γꢀAl2O3 catalyst has been chosen
to study for the Prins reaction.
Recently, we reported Sn(OH)Cl catalyst containing weak Brönsted acidic sites for the
Prins reaction of βꢀpinene with paraformaldehyde to produce nopol.12 There are only few
Brönsted acidic catalyst applied for the Prins reaction of βꢀpinene to produce selectively nopol.
Since, the acidic properties of the MoOx/γꢀAl2O3 catalyst differ from weak to moderate upon
variation of loading and calcination temperature, study of the following acid catalyst for Prins
reaction is interesting. To the best of our knowledge, Prins reaction of ßꢀpinene with
paraformaldehyde to produce nopol has not been studied over MoOx/γꢀAl2O3 catalysts. In the
present work, a series of MoOx /γꢀ Al2O3 was prepared to study the effect of MoOx loading and
calcination temperature of catalysts on the catalytic performance in Prins reaction. In order to
study the effect of support on catalytic activity of molybdenum oxide catalyst, MoOx was loaded
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on different supports like TiO2, SiO2, ZrO2 and Nb2O5. Furthermore, the influence of solvent in
Prins reaction in terms of their acceptor number, donor number and dielectric constant was
studied. The effects of reaction parameters like reaction temperature, catalyst amount and
reactant mole ratio were studied. The physiochemical properties of the catalyst were correlated
with catalytic activity.
2. Experimental
2.1 Materials
Paraformaldehyde (hereafter PF) was obtained from the Loba Chemie, India. βꢀpinene
was purchased from SigmaꢀAldrich, USA. Nopol, αꢀpinene, camphene and limonene were
purchased from Alfa Aesar, USA. Pseudoboehmite was a gift sample by SüdꢀChemie India Pvt
Ltd. All the chemicals were used without further purification.
2.2 Catalyst preparation
The support γꢀalumina was prepared by calcining pseudoboehmite at 550 ˚C for 4 h.
MoOx/ γꢀAl2O3 was prepared by the impregnation method using γꢀAl2O3 and a solution of
ammonium molybdate tetrahydrate [(NH4)6Mo7O24. 4H2O] as reported in the literature method.14
In a typical procedure, 10 g of γꢀAl2O3 was dissolved in 50 ml of solution containing required
amount of ammonium molybdate. The solution was stirred at 80 °C for 2 h followed by water
evaporation. Finally, the catalyst was dried at 120 °C for 4 h followed by calcination at 800 °C.
A series of MoOx/γꢀAl2O3 catalysts with MoOx ranging from 0.2 to 25 wt % on γꢀAl2O3 were
prepared. Similarly, MoOx loaded onto different supports like TiO2, SiO2, ZrO2, Nb2O5 and
pseudoboehmite were prepared. For the comparison, Al2Mo3O12 was prepared by solꢀgel method
as reported elsewhere.30
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2.3 Catalyst characterization
The crystallinity and phase purity of all the catalysts after calcination were measured with
an Xꢀray powder diffractometer instrument (Bruker Dꢀ2 phaser) using CuꢀKα radiation (λꢀ
1.542A˚). The samples were scanned in the 2θ range of 10ꢀ80 with scanning rate of 0.02 degrees
per second.
The temperature programmed desorption of ammonia was performed in the temperature
range of 100ꢀ600˚C with ramp rate of 12.5˚C/min. Before the TPD measurements, samples were
pretreated at 550 ˚C for 1 h in flow of helium gas. Then samples were saturated with anhydrous
ammonia gas (10 % NH3 + 90 % He) at 100 ˚C for 45 min and finally flushed with He gas at 100
°C for 1 h to remove physisorbed ammonia. Nitrogen sorption measurements were carried out at
77 K in a Nova 1000 Quantachrome instrument. Prior to analysis, the samples were heated to
°
200 C for 1 h. Specific surface areas of catalysts were determined by Brunauer–Emmett–Teller
(BET) equation. Average pore diameter and total pore volume were measured by BJH method.
The TEM images of the catalysts were analyzed by transmission electron microscope (TEMꢀ
JEOLꢀ2010) instrument with SAED.
2.4 Catalytic activity studies
The liquid phase Prins condensation reaction of βꢀpinene with paraformaldehyde was
carried out in the batch reactor in the presence of MoOx/γꢀAl2O3 solid acid catalyst. In a typical
experiment, required amounts of the reactants (βꢀpinene and paraformaldehyde) in the presence
of a solvent (benzonitrile) with an appropriate amount of catalyst (20 wt% with respect to total
weight of reactant ) were taken in a round bottom flask fitted with a condenser with continuous
circulation of cold water and heated over oil bath with continuous stirring. The required
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temperature of oil bath was controlled and monitored by the PID controller and thermometer
respectively.
During the course of reaction, samples were collected at certain intervals of time, the
reaction mixture was cooled and the product was analyzed by gas chromatograph (Shimadzuꢀ
2014, FID detector) equipped with RTXꢀ5 column (0.25mm I.D and 30m length). The oven
temperature programme was 80˚C (5 min), 80ꢀ240˚C (15˚C/min), 240˚C (5 min). Reaction
products were quantified using a multiꢀpoint calibration curve through the external standard
method. The yield was calculated by the GC analysis using the formula, product (mol %) =
[conversion (mol %) * selectivity (mol %)]/100.
3. Results and discussion
3.1 Characterization of MoOx/γꢀAl2O3 catalyst
The XRD pattern of MoOx /γꢀ Al2O3 catalysts calcined at 800 ˚ C with different MoOx
loadings are as shown in Fig. 1. There were no intense diffraction peaks corresponding to the
crystalline MoOx and Al2Mo3O12 phase detected for the catalyst loading less than 11 wt % of
MoOx. This clearly indicates that MoOx is highly dispersed on the support as amorphous MoOx
species. Further, increase in loading of MoOx up to 25 wt %, the diffraction peaks due to
formation of crystalline MoOx and Al2Mo3O12 were observed.31 The XRD patterns of 11 wt %
MoOx /γꢀ Al2O3 catalyst calcined at various temperatures are as shown in Fig. 2. Calcination of
MoOx/γꢀ Al2O3 catalyst below 700 °C retained the γ phase of alumina, whereas further
calcination at higher temperature resulted in the formation of αꢀalumina. However the αꢀphase
was found to be more predominant for the catalyst calcined above 900 ˚C. Typically, the γ phase
of alumina turns to αꢀphase at 1050 °C, but in the present case, αꢀ alumina was found to appear
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at 900 °C, which could be attributed to stabilization of alpha phase by MoOx present on γꢀ
alumina at lower temperature.15, 27, 28.
The BET surface area, pore volume and average pore diameter of MoOx/ γꢀAl2O3 catalyst
are tabulated in Tableꢀ1. The γꢀAl2O3 support showed a high surface area of 200 m2/g. The N2
adsorptionꢀdesorption isotherm of γꢀAl2O3 and MoOx/γꢀAl2O3 catalysts showed typeꢀIV isotherm
with H1 type hysteresis loop confirming the presence of mesopores. (Fig.3) As the MoOx loading
increased on γꢀAl2O3 support, the surface area of the catalysts decreased due to shrinkage of
alumina support and at 9 wt % of loading sudden decrease of the surface area was observed
which is in concurrence with the previous report.14 The average pore diameter and pore volume
showed reverse trend to that of surface area. With increasing MoOx loading, the pore diameter
and pore volume initially decreased up to 9 wt % and subsequently increased which could be
associated with the formation of crystalline MoOx.31 Based on the higher catalytic activity of 11
wt % of MoOx in Prins reaction, the effect of calination temperature was studied on the same
catalyst system. The calcination of 11 wt % MoOx/γꢀAl2O3 catalyst at higher temperatures
decreased the surface area of the catalysts as shown in Tableꢀ2. The decrease of surface area
below 800 °C is due to the shrinkage of alumina support, whereas abrupt decrease in surface area
above 800 °C is due to formation of αꢀphase of alumina and Al2Mo3O12 as inferred by XRD. The
decrease of surface area followed decrease in pore volume and pore diameter.
Physical property of MoOx/Al2O3 catalyst calcined at 800˚C is as given in Table 2. S0 is
1
the surface area g(Al2O3)− of a carrier, and was calculated from the BET specific surface area
and MoO3 content. The surface area of MoOx/Al2O3 decreased with loading amount of MoOx
due to a shrink of alumina support and sudden decrease of surface area was observed at 11 wt%.
The area of alumina support (S0) decrease and the area occupied by MoOx units (Soccupied
)
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increase with loading amount of MoOx and Soccupied become almost similar and larger than S0 at
11 wt% and above loadings. The surface density of Mo atom on 8 and 10 wt% MoOx/Al2O3
2
calcined at 800 ˚C catalyst is found to be 3.4 and 5.6 atoms nm− , respectively.32 The surface
2
density of Mo atoms in monolayer coverage with MoOx was 4.5 atom nm− and the value was
larger than the surface density of Mo atom on 7 wt% and a little smaller than 11wt%. These
results indicate that MoOx was loaded as twoꢀdimensional molybdenum oxide until 7 wt% and
formation of threeꢀdimensional MoOx just started on 9 wt% MoOx/γꢀAl2O3. A detailed study on
the transmission electron microscopy (TEM) analysis of 11 wt% MoO3/γꢀAl2O3 catalyst was
carried out. Fig. 4(a & b) shows a large bright substrate with a dark shadow covering them with
the amorphous mass. The bright substrate was assigned to be γꢀAl2O3 as determined by their d
spacing values (d (311) = 0.258 nm) of the respective plane and EDAX.(Fig.4 c) The dark
shadow was amorphous MoO3 as confirmed by the EDAX measurements. Furthermore, no d
spacing values are observed indicating amorphous nature of MoO3 as shown in Fig 4 (c). The
selected area electron diffraction (SAED) pattern of catalyst exhibited the concentric rings
indicating the presence of crystalline Al2O3 with wellꢀdefined lattice planes (311), (400), (440)
of cubic γꢀAl2O3. No concentric rings due to MoO3 was observed in SAED pattern due to
amorphous nature MoO3 (Fig.4 d) The TEM image of 11 wt % of MoO3/Al2O3 additionally
indicates the MoOX is uniformly loaded on alumina in the form amorphous monolayer domains
and small clusters of MoOx.
The solid UVꢀDRS spectroscopic measurements were carried out to study the presence of
molybdenum species in the supported catalyst. The most of the supported catalysts showed the
absorption bands between 200 and 400 nm, (Fig. 6) which is assigned to the ligand to metal
charge transfer transitions.33 Furthermore, the loading of 5 wt % exhibited two bands at 230 and
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300 nm, which could be assigned to isolated monomeric species of tetrahedral and octahedral
species, respectively. Whereas, above 5 wt % loading, only one broad peak was observed due to
the overlap of tetrahedral and octahedral molybdenum species. In summary, surface area, TEM
and UVꢀDRS measurements indicated that complete coverage of alumina support by
Molybdenum was achieved between the 9 and 11 wt % loading in which the molybdenum is
present in octahedral and tetrahedral species in the form monolayer domain and small clusters of
MoO3.
To determine the amount and strength of acid sites, a temperature programmed
desorption (TPD) profile of all the MoOx /γꢀAl2O3 samples were measured (Fig. 7). The bare γꢀ
Al2O3 catalyst exhibited around 0.76 mmol of acidity. The total acidity decreased with increase
of Mo loading and weaker to moderate new acid sites of Brönsted type are generated as proposed
in the literature. 14, 29, 30 The decrease in total acidity could be due to the decrease of surface area.
After loading of MoOx on γꢀAl2O3, high temperature desorption peak (450 ˚C) reduced in
intensity and the low temperature desorption peak (250 ˚C) was found major in all loaded
catalysts, which could be due to the formation of weak to moderate Brönsted acid sites.30 The
effect of different calcination temperature on acidity was also determined as shown in Fig. 8. It is
observed that, the increase of calcination temperature from 450 to 1000 °C decreased the total
acidity, which could be due to decrease in surface area and change of γꢀAl2O3 to αꢀ Al2O3 as
shown in Tableꢀ3.
To study the morphological properties of catalyst, SEM images of γꢀAl2O3 and 11 wt%
MoOx/γꢀAl2O3, were taken (Fig. 9.). Both γꢀAl2O3 and 11 wt% MoOx/γꢀAl2O3 contained
spherical shaped particles with a particle size ranging from 5ꢀ10 ꢁm. The morphology of γꢀ
alumina remained same even after 11 wt % MoOx loading.
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3.2 Catalytic activity by MoOx/Al2O3 catalyst.
The Prins reaction of ßꢀpinene with paraformaldehyde was carried over different Mo
loaded γꢀAl2O3 catalysts. The γꢀAl2O3 exhibited ßꢀpinene conversion of 52 % with 86 % nopol
selectivity. The lower catalytic activity of γꢀAl2O3 could be due to the less acidity (0.7 mmol/g).
As the amount of Mo loading on γꢀAl2O3 increased up to 11 wt %, the conversion of ßꢀpinene
increased, and further increasing of MoOx loading to 25 wt % decreased the ßꢀpinene conversion
(Fig. 10.). This could be explained on the fact that surface acid density (µ mol/m2) increases with
increase of MoOx loading up to 11 wt % and above 11 wt %, the surface acidity decreased as
shown in the Fig.10. Similar kind of correlation of Brönsted acidity with the MoOx loading were
reported earlier, where Brönsted acidity increases with increase in loading of MoOx up to 11 wt
% and above 11 wt %, the Bronsted acidity decreases due to the formation of crystalline MoO3
and Al2Mo3O12.14 Similar, trend of correlation between the acidity and the catalytic activity was
observed in case of benzylation of anisole and isomerization of αꢀpinene.14 These above findings
clearly indicates the formation of new Brönsted acid sites on the surface of the γꢀAl2O3 which
can be noticed and correlated with the change of surface acid density after MoOx loading. From
literature reports it is concluded that, at lower Mo loading tetrahedral MoOx species are formed
and higher Mo density results in octahedral MoOx structures.23 The catalytic activity increased
with increase of Mo loading, and once the formation of Al2(MoO4)3 (mainly, contains tetrahedral
MoOx species) started the catalytic activity decreased due to the presence of tetrahedral Mo
species in the Al2(MoO4)3. These results clearly indicate that octahedral MoOx species are more
active in the Prins reaction. The decrease of catalytic activity for alpha alumina could be due to
its lower surface area. The nopol selectivity was found to be in the range of 82 to 92 %
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irrespective of the MoOx loading. The side products mainly consisting of αꢀpinene, camphene,
limonene and mixture of terpinenes and terpinolenes were formed due to the isomerisation of βꢀ
pinene.(Tableꢀ4) The 11 wt % of MoOx on γꢀAl2O3 exhibited a high ßꢀpinene conversion of 96 %
with nopol selectivity of 86%. For comparison, MoOx, Al2Mo3O12 and (NH4)6Mo7O24. 4H2O
were also screened for the Prins reaction, which exhibited lower activity as shown in Tableꢀ4.
The above result clearly indicates, loading of MoOx on γꢀAl2O3 support generated new acid sites
which are catalytically active in Prins reaction rather than MoOx, Al2Mo3O12 or ammonium
molybdate alone.
In order to study the effect of calcination temperature of MoOx/γꢀAl2O3 catalyst on the
catalytic activity, the 11 wt % MoOx/ γꢀAl2O3 catalyst was calcined at different temperature
ranging from 450 to 1000 °C. Increase of calcination temperature from 450°C to 800 °C
increased the βꢀpinene conversion from 83 to 96 %. Further, increase of calcinations temperature
of catalysts to 900 and 1000 °C resulted in lower catalytic activity which could be due to the
formation of αꢀAl2O3 phase with lower surface area. (Tableꢀ3) The calcination temperature was
well studied by T. Kitanoa et.al. Indicating Brönsted acid generation of aluminaꢀsupported
molybdenum oxide is at high temperatures in the range of 700ꢀ800 ˚C.14 Therefore the catalyst
with lower calcination temperature has shown to be less active even with high surface acid
density. Whereas, the calcination of catalyst above 800 ˚C exhibited lower catalytic activity due
to the phase change of Al2O3. The catalyst with 11 wt % MoOx/ γꢀAl2O3 calcined at 800 ˚C
showed higher catalytic activity with lower surface acid density value.
The Prins reaction was also carried over MoOx loaded on different supports such as TiO2,
SiO2, ZrO2 and Nb2O5 as shown in the Tableꢀ3. Among the different supported catalyst, SiO2 and
γꢀAl2O3 support showed high TON, whereas other supports like TiO2, pseudoboehmite, ZrO2 and
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Nb2O5 showed moderate to low TON values. Since most of the supports studied have different
surface areas, the surface acid densities of the MoOx loaded on different support (µmol/m2) were
determined for the catalytic activity correlation and are tabulated in the Table 4. In spite of high
surface acid density of Nb2O5, TiO2, ZrO2 supports, exhibited lower catalytic activity and TON
than γꢀ Al2O3. Conversely, the SiO2 support with lowest surface acid density exhibited lower
catalytic activity. The MoOx loaded on γꢀAl2O3 with high TON and low surface acid density was
found to be the best support among the others, wherein high ßꢀpinene conversion of 96 % and 86
% nopol selectivity was obtained for the Prins reaction.
3.3 Effect of solvent
The reaction of ßꢀpinene with paraformaldehyde in absence of solvent resulted in low
yield of nopol (30.2 %). This indicates that appropriate solvent is required for the generation of
formaldehyde from PF. Hence, the effect of different solvents on the Prins reaction of ßꢀpinene
over a MoOx /γꢀ Al2O3 was studied in detail. A range of solvents were screened for the Prins
reaction and the results are represented in Tableꢀ5.
The influence of dielectric constant (DC), acceptor number (AN) and donor number (DN)
of the solvents on the Prins reaction was studied. In the beginning, the effect of DC on the
activity was studied by choosing solvents with similar dielectric constants. Solvents such as
acetonitrile (DC=37.5) and nitrobenzene (DC=34.8) with comparable DC showed different
yields for nopol. The solvents, toluene (DC=2.4), cyclohexane (DC=2.02) and triethylamine
(DC=2.42) also gave different activity profiles. This clearly indicates that dielectric constant of a
solvent has no influence on the catalytic activity of MoOx /γꢀ Al2O3 for Prins reaction.
Further, study on the solvent parameters like AN and DN gave better understanding on
the role of solvent on the reaction. The AN and DN were first measured by Gutman, which
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assess the acidic and basic properties of solvents, respectively.34 The Acceptor (AN) and Donor
number (DN) are the measure of strength of the solvents as Lewis acids or bases. The AN is
based on the 31PꢀNMR chemical shift of triethylphosphine oxide in the solvent and DN is based
on the heat of reaction between the 'solvent' and SbCl5 in 1,2ꢀC2H4Cl2.34ꢀ35 These values have
been well established and used by several researchers to explain the role of solvent in chemical
process. 34ꢀ38 As the AN of the solvents increased (DN = 0) from cyclohexane to dichloroethane,
the ßꢀpinene conversion improved from 81.2 to 100 %, whereas selectivity was low (~ 40 %).
Further increasing the DN of the solvents (having similar AN) from dichloroethane to
acetonitrile, conversion dropped from 100 to 53 % and selectivity increased substantially from
40 to 90 %. However, increase of DN from acetonitrile to DMSO decreased both conversion and
selectivity. Finally, triethylamine with highest DN (AN = negligible) showed no activity for
Prins reaction.
From the above results, it is clear that solvent with only AN or DN like toluene,
triethylamine, results in low yield for nopol. Solvents with both AN and DN values in the range
of 10 to 20 (acetonitrile and benzonitrile) are the best to give high yield for nopol. The AN of a
solvent represents acidic property, which facilitates fast decomposition of PF to formaldehyde, as
a result, high conversion with low selectivity are obtained over the MoOx /γꢀ Al2O3. The low
selectivity could be due to the activation of Brönsted acid sites by the acidic solvents leading to
the formation of isomerization products such as limonene, αꢀpinene and camphene etc. The DN
of solvent represents the basic property of the solvents. Therefore, solvents with DN in the range
of 10 to 20 could bind to the strong Brønsted acidic sites of the catalyst resulting in low
conversions and high selectivity. Further increase of DN (> 14), solvent may preferentially bind
to most of the acid sites of the MoOx /γꢀ Al2O3 catalyst leading to the low conversion. The low
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activity in presence of solvents with only high DN (triethylamine) could be due to the
preferential adsorption on the acid sites leading to low generation of formaldehyde from PF.
Hence, solvents with both AN and DN in the range 10 to 20 are suitable to facilitate reaction by
balancing formation of side products and decomposition of PF. Among all the solvents taken in
this study, benzonitrile with AN (15.5) and DN (11.9) exhibited good conversion and selectivity.
3.4 Influence of reaction conditions
In order to optimize the reaction condition, influence of reaction temperature on the
conversion of βꢀpinene and selectivity towards nopol was studied using MoOx /γꢀ Al2O3 with βꢀ
pinene: PF mole ratio of 1:2 using 20 wt% of catalyst. The βꢀpinene conversion increased with
increase in temperature as shown in Fig.11a. The low conversion of ßꢀpinene at lower
temperature (60 °C) is due to less inꢀsitu generation of formaldehyde from PF. At 100 °C, the
formation of formaldehyde was faster and as a result higher conversion was achieved. The
reaction temperature also had an influence on selectivity of nopol. With increase in reaction
temperature from 80 to 100 °C, nopol selectivity decreased due to formation of side products
such as camphene, limonene, αꢀpinene. The reaction carried out at 90˚C gave better conversion
of βꢀpinene as well as high selectivity to nopol.
Further, to study the effect of different concentrations of reactants, Prins reaction was
carried out with βꢀpinene: PF mole ratios of 1:1 to 1:4 as shown in Fig.11b. The conversion and
selectivity improved with increase in βꢀpinene: PF ratio from 1: 1 to 1: 2 and further increase of
mole ratio did not have any effect on the catalyst performance. The less conversion of βꢀpinene
at low PF concentration is due to less availability of active reactant, formaldehyde. The βꢀ
pinene: PF molar ratio of 1: 2 showed high βꢀpinene conversion (92 %) and nopol selectivity (93
%).
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The effect of catalyst amount on βꢀpinene conversion and selectivity for nopol was
examined for Prins condensation reaction with different amount of MoOx/γꢀAl2O3 catalyst with
1:2 mole ratios of βꢀpinene: paraformaldehyde in the presence of benzonitrile solvent at 90˚C. As
the catalyst amount was increased from 15 to 30 wt%, the βꢀpinene conversion increased from 88
to 98 %. (Fig. 11c) Increase of catalyst amount above 20 wt % decreased the selectivity for nopol
due to the formation of side products. Hence, catalyst with 20 wt % was sufficient to produce a
good conversion of ßꢀpinene and nopol selectivity.
Effect of reaction time for Prins condensation of ßꢀpinene with PF was carried out under
optimized reaction conditions (Fig. 11d.). The conversion of ßꢀpinene after 1 h was 35 % and
further increase in reaction time, increased the conversion and reached to 96 % after 10 h. But
the conversion of ßꢀpinene / hour decreased as reaction time increased. With increase of reaction
time from 1 to 10 h decreased the selectivity for nopol from 91 to 86%.
3.5 Catalyst recyclability
The reusability studies were carried out by using 11wt % MoOx/γꢀAl2O3 catalyst calcined
at 700 ˚C. The used catalyst was filtered after reaction, washed with acetone and dried for 2 h at
120˚C In order to remove the excess PF adsorbed on the catalyst and then catalyst was
regenerated at 500˚C for 1 h. Catalyst gave good catalytic activity up to 3 cycles with slight
decrease in βꢀpinene conversion and nopol selectivity (Tableꢀ6). The XRD patterns of catalyst
after 3 recycle showed the intense peak of γꢀAl2O3 phase indicating the structural integrity of the
catalyst. (Fig. 12) The recycled catalyst and reaction filtrate was carefully analyzed for the ICPꢀ
OES. The reaction filtrate showed the absence of Mo under the detection limit of ≥0.1ppm and
there was no decrease in the Mo content of fresh and recycled catalyst, indicating catalyst is truly
heterogeneous at reaction conditions.
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3.6 Reaction mechanism
Plausible reaction mechanism for Prins condensation of ßꢀpinene with PF over MoOx/γꢀ
Al2O3 is as shown in Schemeꢀ1. The loading of MoOx onto γꢀAl2O3 generates weak to moderate
acidic sites which are due to the molybdenum oxide domain (MoꢀOH), three dimensional MoO3
(Moꢀ(OH)ꢀMo) and at the interface by MoO3 domain with γꢀAl2O3 support (Moꢀ(OH)ꢀAl).
These could be the main catalytically active sites for the Prins reaction over MoOx/γꢀAl2O3
catalyst.14 The formaldehyde generated from PF interacts with acid sites to form a carbocation
via polarization of carbonyl group, thereby generating an electrophile. The ß –pinene attacks the
electrophilic center followed by the allylic proton transfer completing the catalytic cycle with
formation of nopol. The presence of strong Brönsted acidity activates the C=C of ß –pinene to a
greater extent resulting in isomerized products such as camphene, limonene, αꢀpinene etc.
leading to lower selectivity for nopol”.
3.7 Comparison of MoOx/γꢀAl2O3 with reported solid acid catalysts.
Tableꢀ7 shows the comparison of catalytic activity of MoOx/γꢀAl2O3 with other reported
solid acid catalysts. The tin based catalysts like SnꢀSBAꢀ15, SnꢀMCMꢀ41 and Sn(OH)Cl showed
high conversion and high selectivity for nopol. The other Lewis acidic metal ions such as Zn+2 is
also found to be active sites for the Prins reaction. Znꢀmontmorillonite catalyst showed slightly
higher nopol yield (87 %) however, the leaching of the Zn was observed during the reaction.
Prins reaction with sulfated zirconia was reported to give high nopol yield (98 %). Znꢀbeta
catalyst showed nopol yield of 85.5 %. MoOx/γꢀAl2O3 catalyst in present work exhibited higher
nopol yield with lower amount of catalyst than previously reported catalysts such as SnꢀMCMꢀ
41, ZnꢀMCMꢀ41, ZrꢀSBAꢀ15, NaꢀITQ, Snꢀkenyaite. These results clearly indicate the potential
application of MoOx/γꢀAl2O3 as solid acid catalyst in Prins reaction.
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4. Conclusions
MoOx/γꢀAl2O3 exhibited to be highly selective and reusable catalyst for the synthesis of
nopol from ßꢀpinene and paraformaldehyde by Prins condensation. The XRD shows the phase
transformation from MoOx/γꢀAl2O3 to MoOx/αꢀAl2O3 and Al2Mo3O12 at high calcination
temperature and high MoOx loading respectively. All the MoOx/ γꢀAl2O3 catalysts showed typeꢀ
IV isotherm with H1 type hysteresis loop confirming the presence of mesopores. The total
acidity of the MoOx/ γꢀAl2O3 decreased with increase of loading and calcinations temperature
due to decrease of surface area. The MoOx supported on γꢀAl2O3 showed best performance
among different types of supports used. As the loading of MoOx increased, the ßꢀpinene
conversion increased up to 11 wt %, substantial loading decreased the conversion which is well
correlated with surface acid density of the loaded catalysts. The high selectivity for nopol in the
range of 82ꢀ86 % was observed over the MoOx/γꢀAl2O3 catalyst. The solvents like acetonitrile
and benzonitrile with both acceptor and donor numbers in the range of 10 to 20 are necessary to
enhance the performance of the catalyst. The 11 wt % MoOx loaded on γꢀAl2O3 showed a better
performance of 96 % ßꢀpinene conversion and 86 % nopol selectivity. The catalyst was truly
heterogeneous and can be recycled thrice with negligible decrease in the yield of nopol.
Comparison of MoOx/γꢀAl2O3 with reported catalyst showed high remarkable activity indicating
its potential application as solid acid catalyst for Prins reaction.
5. Acknowledgement
Vijaykumar S.M and Dundappa Mumbaraddi acknowledge Admar Mutt Education
Foundation (AMEF) for a fellowship. The authors are thankful to BIT, Bangalore for the BET
measurements.
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FIGURE LEGENDS
Fig. 1. XRD patterns of MoOX loading on γꢀAl2O3 calcined at 800 °C a) γꢀAl2O3 b)1 wt % c) 7
wt % d) 11wt % e) 15 wt % f) 20 wt %.
Fig.2 XRD patterns of MoOX loading on γꢀAl2O3 calcined at different temperatures.
Fig. 3.Nitrogen adsorption–desorption isotherms and pore size distribution of 11 wt % MoOX/γꢀ
Al2O3 catalyst.
Fig. 4. TEM Images of 11 wt % MoOx/Al2O3
Fig.5. EDAX Pattern of 11 wt % MoOx/Al2O3 catalyst.
Fig.6. UVꢀDRS Spectrum of different loaded MoOx/Al2O3 catalysts
Fig.7. TPDꢀ NH3 profile of MoOX/γꢀAl2O3 catalyst with different Mo loadings.
Fig. 8. TPDꢀ NH3 profile of 11% MoOX/ γꢀAl2O3 calcined at different temperature.
Fig. 9. SEM images of a) γꢀAl2O3 b) 11 wt % MoOX/γꢀAl2O3
Fig. 10. Activity and selectivity of MoOX/γꢀAl2O3 catalyst calcined at 800 °C with various
loadings on Prins reaction.
Fig. 11.Influence of reaction conditions
a) Effect of temperature:
Reaction conditions: ßꢀpinene = 10 mmol, PF=20 mmol, catalyst amount =20 wt %, Benzonitrile
= 5ml, reaction time =10 h.
b) Effect of ßꢀpinene: PF mole ratio:
Reaction conditions: catalyst amount =20 wt %, Benzonitrile = 5ml, reaction temperature =90
°C, reaction time =10 h.
c) Effect of catalyst amount:
Reaction conditions: ßꢀpinene = 10 mmol, PF = 20 mmol, Benzonitrile = 5 ml, reaction
temperature = 90°C, reaction time =10 h.
d) Effect of reaction time: ßꢀpinene = 10 mmol, PF=20 mmol, catalyst amount =20 wt %,
Benzonitrile = 5ml, reaction temperature =90 °C.
Fig. 12. XRD pattern of 11 wt % MoOX/ γꢀAl2O3 catalyst a) Fresh catalyst b) After 3 recycle

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O
#
O- Al Mo O
2
3 12
#- MoO3
#
O
O
O
f
e
d
c
b
a
10
20
30
40
50
60
70
80
2
θ
Fig.1.

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#- Al2Mo3O12
γ
1000 °C
α
α
α
α
γ
#
#
γ
α
#
γ
#
900 °C
α
α
α
800 °C
700 °C
550 °C
γ
γ
γ
γ
450 °C
20 30 40 50 60 70 80
2θ
Fig.2.

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300
250
200
150
100
50
4
3
2
1
0
0
50
100
150
200
250
o
Average pore diameter (A )
Adsorption
Desorption
0
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
Fig. 3.

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Fig.4.

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Fig.5.

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5 wt %
9 wt %
11 wt %
15 wt %
20 wt %
200 225 250 275 300 325 350 375 400
Wavelength (nm)
Fig.6.

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a-γ−alumina
b-3 wt %
c-7 wt %
d-11 wt %
e-15 wt %
c
b
d
e
a
100 200 300 400 500 600 700
Temperature (°C)
Fig.7.