Catalytic Olefin hydroalkoxylation by nano particles of pollucite

Article abstract of DOI:10.1071/CH14492

The catalytic hydroalkoxylation of α,β-unsaturated esters, nitriles, and ethers with aliphatic and aromatic alcohols over pollucite using thermal and microwave-assisted methods was investigated. To study the effect of the alcohol structures on the mechanism of the hydroalkoxylation reaction, different alcohols, such as methanol to butanol, cyclohexanol, phenol, and 2-ethylhexanol were used. The activities of pollucite, in contrast to other basic solids, were scarcely affected by the presence of air and moisture. The correlation between alcohol acidity and reaction activity is discussed. The prepared pollucite was characterized by X-ray diffraction, volumetric nitrogen adsorption surface area analysis, and CO₂ temperature-programmed desorption. Scanning electron microscopy analysis revealed that the size of the modified nano catalyst particles was under 40nm.

Full text of DOI:10.1071/CH14492

CSIRO PUBLISHING
Aust. J. Chem. 2015, 68, 981–986
Full Paper
http://dx.doi.org/10.1071/CH14492
Catalytic Olefin Hydroalkoxylation by Nano Particles
of Pollucite
A
A B
,
Sara Zamanian and Ali Nemati Kharat
A
School of Chemistry, University College of Science, University of Tehran,
Tehran 13145-1357, Iran.
B
Corresponding author. Email: alnema@khayam.ut.ac.ir
The catalytic hydroalkoxylation of a,b-unsaturated esters, nitriles, and ethers with aliphatic and aromatic alcohols over
pollucite using thermal and microwave-assisted methods was investigated. To study the effect of the alcohol structures on
the mechanism of the hydroalkoxylation reaction, different alcohols, such as methanol to butanol, cyclohexanol, phenol,
and 2-ethylhexanol were used. The activities of pollucite, in contrast to other basic solids, were scarcely affected by the
presence of air and moisture. The correlation between alcohol acidity and reaction activity is discussed. The prepared
pollucite was characterized by X-ray diffraction, volumetric nitrogen adsorption surface area analysis, and CO2
temperature-programmed desorption. Scanning electron microscopy analysis revealed that the size of the modified nano
catalyst particles was under 40 nm.
Manuscript received: 11 August 2014.
Manuscript accepted: 5 October 2014.
Published online: 1 December 2014.
Introduction
such as alkali metal alkoxides and basic anionic resins, have
been reported for preparing of EEP.[27] Likewise, low conver-
sion and poor selectivity are the main problems in using such
soluble catalysts. So, various hydroalkoxylation catalysts based
on different metals such as Cu,[28–30] Ag,[31] Au,[32–36] Ru,[37,38]
Rh,[39] Pd,[40–45] and Pt[46–49] have been developed to overcome
the limitation of using acid base catalysts and to achieve the
more selective products. There are several reports of hydro-
alkoxylation of olefins on heterogeneous catalysts such as
anion exchange resins,[50–54] zeolites,[55–57] alumina-supported
KF,[58] modified layered double hydroxides (LDHs),[59–64] and
aluminium-exchanged montmorillonite clays.[65]
Strong basic catalysts are not suitable for hydroalkoxylation
of sensitive unsaturated compounds, such as acrylate esters, so
choosing a well-balanced base is essential for achieving the
selective products. Published papers showed that caesium salts
can provide this balance,[66] and for this reason they are very
suitable for use in organic syntheses such as C–C, C–O, C–N,
C–P, and C–S bond formation.[67–71] Pollucite is an ANA-type
zeolite with distorted 8-ring pores. Pollucite, the most important
caesium-bearing ore, is a caesium alumino silicate (CsAlSi2O6)
zeolite. Pollucite, unlike the alkali metal alkoxide or anionic
basic resins, is not sensitive to moisture and can be used as a
basic catalyst several times without loss in activity.
The catalytic nucleophilic addition of alcohols to olefins,
hydroalkoxylation, has been of interest to synthetic and
medicinal chemists because of the formation of key compounds,
which consist of C–O bonds, that have importance in biological
research and industrial fields.[1–3]
The intermolecular hydroalkoxylation of unactivated olefins
with different alcohols for the synthesis of different types of
new ethers has been recognized and employed by several
research groups to improve synthesis techniques.[4–6] One of
the most important applications of these synthetic compounds
is fuel ethers, which are added to gasoline as oxygen-bearing
compounds. Direct addition of ethanol to gasoline increases
the risk of corrosion and phase separation problems.[7] For this
reason, it could be converted into value-added molecules such
as ethers by reacting with alkenes.[8] On the other hand, addi-
tion of the oxygenated compounds to the gasoline can decrease
harmful exhaust emissions and increase octane rating.[9] Classic
catalysts that have been used in hydroalkoxylation reactions
are strong acids[10–19] and bases such as phosphines and
amines.[20,21] For example, vapour phase synthesis of ethyl
tert-butyl ether from ethanol and isobutene over different acidic
zeolites as heterogeneous catalysts has been reported by
Collignon and Poncelet.[22] In the presence of acids, the stability
of the carbocation intermediate controls the regioselectivity of
the reaction products,[23,24] and the yields of reactions are
,50 %. In this situation, polymerization and rearrangement of
alkenes could occur. On the other hand, a series of non-toxic
industrial solvents, such as ethyl 3-ethoxypropanoate (EEP) and
methyl 3-methoxypropanoate (MMP), can be obtained from the
reaction of alcohols with methyl and ethylacrylate in the
presence of a strong acidic catalyst.[25,26] Strong basic catalysts,
In this paper, we prepared the above-mentioned ether com-
pounds by the reaction of activated olefins with alcohols over
nano-sized pollucite zeolite. In our previous work, we reported a
simple method of the preparation of nano-sized pollucite,[72] and
here we investigated its catalytic activity for hydroalkoxylation
of ethyl acrylate (EA), methyl acrylate (MA), acrylonitrile
(AN), and isobutylvinyl ether (IBVE). As mentioned above,
methanol and ethanol are the most common alcohols that are
Journal compilation Ó CSIRO 2015
www.publish.csiro.au/journals/ajc

982
S. Zamanian and A. N. Kharat
used in industrial etherification. For expanding the application
of this heterogeneous catalyst, other alcohols, such as propanol,
cyclohexanol, phenol, and 2-ethylhexanol were used in hydro-
alkoxylation reactions.
The caesium content of the catalysts was determined using
an inductively coupled plasma optical emission spectroscopy
(ICP–OES; Varian Vista-MPX). This sample contained 40.2 %
of caesium.
Microwave reactions were carried out using a Samsung
domestic oven, operating at 2.45 GHz in a 15 mL PTFE closed
container.
Experimental
Materials
All the reagents were used without further purification. Pollucite
was prepared as we reported in the previous work.[72] Before
running the catalytic test, pollucite was dried overnight at 1008C
and calcined at 6008C for 4 h.
Results and Discussion
The XRD diffraction pattern of the as-synthesized pollucite is
shown in Fig. 1. All peaks are well matched in position and
relative intensity to a standard XRD pattern for crystalline
pollucite (PDF No. 29–0407, International Center of Diffrac-
tion Data (ICDD), Newton Square, PA). The diffraction pattern
showed that the sample was monophasic with no traces of other
phases. No peaks at ,2y ¼ 278 were recorded in ICDD standard
29–0407. This peak is related to the excess of caesium ions that
was shown in Fig. 1.
The surface area and pore volume of the synthesized pollu-
cite were 194 m2 gꢀ1 and 0.02 cm3 gꢀ1, respectively. The basic
character of pollucite in comparison with NaY zeolite was
confirmed by CO2 temperature-programmed desorption. This
technique is a suitable tool for obtaining information about the
activity and distribution of active sites in zeolites. In Fig. 2, the
TPD profile for pollucite was shown and compared with that of
zeolite NaY. The first maxima of pollucite appeared at 5808C,
Reaction Procedure
Alcohols (38 mmol, excess amount), olefin (9.5 mmol), and the
catalyst (0.03 g) were reacted in a suitable vessel. No additional
solvent was used except for the reaction of phenol.
For reactions performed under reflux conditions, a 10-mL
two-neck round-bottom flask equipped with a reflux condenser
and argon inlet and for reaction under autogenously pressure a
5 mL, a high pressure stainless steel reactor was used. Reactions
with microwave irradiation were performed in a 15 mL poly-
tetrafluoroethylene (PTFE) container at 900 W for 30 min. After
the reaction, the solid catalyst was separated by centrifugation
(5040 g for 5 min) and the products were analyzed by gas
chromatography mass spectrometry (GC–MS). Conversions
were determined by using o-xylene as an internal standard. In
some cases, the used catalysts were washed with ethanol, dried
at 1008C overnight, and calcined at 3008C for 4 h. Catalytic
reactions were performed three times in the presence of the
recovered catalysts that showed no loss in activity.
Characterization
The structure and phase purity of synthesized pollucite was
monitored by powder X-ray diffraction (XRD). A Bruker D8
advance diffractometer using CuKa radiation was used. The
scanning range of 2y was set between 58 and 508. N2 adsorption–
desorption isotherms were performed at ꢀ1968C by a BELSORP
mini-II surface area measurement equipment after degassing the
samples under vacuum at 3008C overnight. The surface area of
the pollucite sample was obtained using the BET (Brunauer–
Emmett–Teller) methodology. The surface morphology of the
catalysts was obtained on a Zeiss DSM 960 A scanning electron
microscope, with an accelerating voltage of 15 kV.
0
10
20
30
40
50
2θ [Њ]
Fig. 1. Powder X-ray diffraction patterns of as-synthesized pollucite.
Temperature-programmed desorption (TPD) of the synthe-
sized catalyst was carried out using CO2 as a probe for deter-
mining the number and strength of active basic sites. CO2-TPD
analysis was performed on a ChemBET-3000 Quantachrome
instrument. The temperature-programmed desorption ramp of
the chemically adsorbed CO2 was as follows: from 508C to
10008C with a heating rate of 108C minꢀ1 using helium as carrier
gas. The sample (0.103 g) with a particle size of 60–80 mm was
located in a plug flow quartz reactor that was coupled with a
thermal conductivity detector (TCD). The catalyst sample was
degassed under a nitrogen flow (10 mL minꢀ1) at constant
heating rate (108C minꢀ1) to 5008C, and held at the final
temperature for 1 h. Then, a stream of He/CO2 with a 19 : 1 feed
ratio was passed over the catalyst bed for 20 min at 508C and
finally flushed with helium flow for 1 h. To identify and
determine the purity of products, a mass spectrometer (model
5975C) with a triple- axis detector and a gas chromatograph,
Agilent 7890A, coupled with a HP-5 capillary column and a
flame ionization detector (FID) was used.
9
8
7
Pollucite
NaY zeolite
6
5
4
3
2
1
0
0
500
1000
1500
Ϫ1
Temperature [ЊC]
Fig. 2. Comparison of the temperature-programmed desorption profiles of
adsorbed CO2 onto the synthesized pollucite and NaY zeolite.

Catalytic Hydroalkoxylation
983
which is much higher than that of NaY zeolite (2638C). The
higher maxima for the CO2 desorption temperature and the peak
area of pollucite catalyst confirm that the prepared zeolite has a
higher basicity and a higher concentration of basic sites with
respect to NaY zeolite.
The scanning electron microscopy (SEM) image of the
prepared pollucite shows uniform particles with sizes below
50 nm, Fig. 3.
The prepared catalyst was tested in hydroalkoxylation reac-
tions, Scheme 1. This catalyst has enough basicity to catalyze
reactions of alcohols with ethyl acrylate, methyl acrylate,
acrylonitrile, and isobutylvinyl ether.
The results of the hydroalkoxylation reactions are summa-
rized in Table 1. Reactions were performed under reflux condi-
tions and also under autogenous pressure in a high pressure
stainless steel reactor.
The conversions of alkenes in hydroalkoxylation reaction
with alcohols were influenced by the acidity of alcohols, the
electron density on the C¼C and the catalyst basicity. Alcohols
were activated after deprotonation by surface of basic catalyst
and converted into alkoxides. The resulting alkoxide was
attacked to C¼C of the surface-adsorbed alkene to produce
the product. According to the mechanism, Scheme 2, the rate of
the deprotonation step increased with acidity of alcohols. As it
was shown in Table 1, the linear alcohols have reactivity in the
order of methanol . ethanol . propanol . butanol, which is
consistent with the acidity strength of the alcohols. This means
that the more acidic alcohol show more activity in reaction with
alkenes. The performance of basic catalysts in abstracting the
hydrogen atom from the hydroxyl group of alcohol is the first
step of the hydroalkoxyation reaction (step 1, Scheme 2). In step
2, the alkene was adsorbed onto the catalyst surface and affected
by the high electrostatic field of the caesium ions in pollucite.
So, an electron-deficient centre at the b-carbon of the alkene
was formed and the produced alkoxide was attacked to this
positive centre. This new produced anion picks up the hydrogen
atom from surface of catalyst that was adsorbed in the first step
(step 3, Scheme 2), and finally the product was formed.
All the types of alkenes used in this study showed better
reactivity in the presence of the prepared nano pollucite catalyst
with respect to other basic or acidic catalysts such as alkali metal
alkoxides, anionic resins, sulfonic acids, and phosphoric acids
derivatives.[25–27] Low conversion and poor selectivity are the
main problems associated with the use of these types of soluble
catalysts. On the other hand, using expensive catalysts based on
transition metals has no advantages from an economic point of
view in comparison with pollucite.[31,32,37–40,46]
Fig. 3. Scanning electron microscopy image of synthesized pollucite.
O
R
R
CN
O
CN
O
O
O
O
O
O
O
Pollucite
ROH ϩ
The adsorption of olefin onto the surface of basic catalysts is
an important step for hydroalkoxyation reaction. Olefins with
low polarity show lower affinity to adsorption onto the surface.
Isobutylvinyl ether showed lower reactivity in comparison with
acrylates in the reaction with linear alcohols that is probably
related to its lower polarity and lower attraction for adsorption
onto the surface. In this case, the lowest acidity of butanol leads
to the production of the alkoxide anion with a more negative
charge density. So the existence of repulsive forces between the
alkene group and alkoxide anion leads to lower reactivity in this
R
R
O
O
O
O
O
R ϭ CH3, C2H5, C3H7, C4H9, Phenyl, Cyclohexyl, 2-Ethylhexyl
Scheme 1. Hydroalkoxylation reaction of activated alkenes with different
alcohols.
Table 1. Conversion of different alkenes in the presence of pollucite in various reactions with alcohols
RF, reflux; AC, autoclave
Alcohol
Acrylonitrile
Methyl acrylate
Ethyl acrylate
Isobutylvinyl ether
RFA [%]
ACB [%]
RFA [%]
ACB [%]
RFA [%]
ACB [%]
RFA [%]
ACB [%]
Methanol
Ethanol
99.7
99.1
99.5
95.1
3
99.9
99.9
99.9
99.9
5
76.2
65.3
53.5
41.9
8
98.3
98.2
87.4
69.7
14
88.2
58.4
47.6
40.7
6
96
87
81
62
11
8
32
16
30
14
54
22
75
63
25
57
22
86
32
99
Propanol
Butanol
2-Ethylhexanol
Cyclohexanol
PhenolC
2
11
4
9
2
75
99.9
55
99
79
99
AReaction conditions: alcohol (38 mmol), olefin (9.5 mmol), pollucite catalyst (0.03 g), reflux, 12 h.
BReaction conditions: alcohol (38 mmol), olefin (9.5 mmol), pollucite catalyst (0.03 g), reaction in autoclave, 1708C, 12 h.
CToluene was used as solvent.

984
S. Zamanian and A. N. Kharat
O
C
Csϩ
Step 1
O
CH2 CH2
RO
ORЈ
O
O
ROH
Ϫ
Si
Al
ROϪ
Step 4
RЈO
HC
C
ϩ
ϩ
O
H
H2C
Csϩ
Al
ϩ
RO
O
O
O
H
ORЈ
Si
Csϩ
O
HC
C
O
O
ϩ
Ϫ
O
O
C
Si
Al
HC
ϩ
H2C
CH
ORЈ
H
Csϩ
Ϫ
RO
O
O
O
Ϫ
Step 3
Si
Al
Step 2
Scheme 2. Plausible mechanism for hydroalkoxylation of acrylates with alcohols.
O
ϩ
O
ϩ
O
O
O
OH
a
b
Scheme 3. Products of hydroalkoxyation reaction of isobutylvinyl ether with phenol.
reaction, even under high temperature conditions in an auto-
clave system.
Table 2. Conversion [%] of different alkenes in the presence of
pollucite in various reactions with alcohols using microwave irradiation
Our prepared catalyst showed 100 % selectivity for produc-
ing phenyl ester as the main product in the reactions of phenol
with acrylates, whereas in the reaction of IBVE with phenol,
phenyl ether (a in Scheme 3) was produced with 88 % and 71 %
selectivity under reflux conditions and in autoclave system,
respectively. In this reaction, related benzofuran (b in Scheme 3)
was produced as the minor product. There are some reports for
the reaction of phenol with olefins for producing benzofurans
and cumarines as the main products over transition metal
catalysts under acidic conditions.[73–76] Also, there is a report
for using alkali metal catalysts for producing phenol esters as the
main products.[77]
Phenol showed better conversion than cyclohexanol as
shown in Table 1. Phenol is more acidic and the phenoxide
ion was stabilized by resonance effect. It seems that the basicity
of pollucite is not enough to catalyze the reaction of cyclohex-
anol with acrylates. The more basic and acidic catalysts, such as
sodium oxide or sulfuric acid derivatives, have been reported for
the related cyclohexanol reaction.[78,79] Cyclohexanol showed
only a moderate yield with IBVE. Probably reaction of cyclo-
hexanol is very slow, then the heat-sensitive acrylates polymer-
ized during the course of the reaction.
As we mentioned before, pollucite is an ANA-type zeolite
with distorted 8-ring pores and classified as small pore zeolites.
It seems that cyclohexanol and 2-ethyl hexanol cannot enter the
pollucite small pores, and their activation was done only by
surface basic sites. So, the reactivity of this catalyst is not
enough to catalyze these two alcohols for reaction with the
desired alkenes and low conversions were observed. On the
other hand, the polymerization of acrylates occurred when we
used 2-ethyl hexanol as the reactant. Polymerization of acrylates
Alcohol
Acrylonitrile
Methyl
acrylate
Ethyl
Isobutylvinyl
ether
acrylate
Methanol
Ethanol
Propanol
Butanol
Phenol
98
77
64
0
52
38
38
0
47
33
29
0
0
0
0
0
0
0
0
0
also occurred when the reaction was done at high temperature
and pressure. Normally, using polymerization inhibitors, such as
methyl hydroquinone, nitrophenol, phenothiazine, and ferrous
ions can inhibit polymerization.[80] In this paper, we used FeSO4
to inhibit free radical polymerization by trapping free radical
intermediates. So, in the presence of ferrous ions we achieved
low conversions for AN, MA, and EA. It is interesting to
consider that the large alcohol, 2-ethylhexanol, is only success-
fully added to isobutylvinyl ether in the presence pollucite in an
autogenously pressure system with 86 % conversion.
To improve the reactivity and reduce the reaction time, a
microwave-assisted technique was employed for some hydro-
alkoxylation reactions. The use of a microwave oven is conve-
nient and cleaner when compared with other experimental
conditions. Results of the microwave-assisted hydroalkoxyla-
tion reactions are summarized in Table 2.
The higher dielectric constant of methanol (e ¼ 32.7) in
comparison with that of ethanol (e ¼ 24.3), propanol (e ¼ 20.1),
or butanol (e ¼ 19.5) leads to a higher reactivity under micro-
wave irradiation. Isobutylvinyl ether has a lower dielectric

Catalytic Hydroalkoxylation
985
Table 3. Conversion of acrylonitrile in the reaction with phenol using
microwave irradiation and in the presence of different solvents
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Time [min]
Conversion [%]
15
30
15
30
15
30
0
0
DMF
38
72
28
50
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Conclusions
Pollucite is an active heterogeneous catalyst for hydroalk-
oxylation of activated alkenes such as AN, MA, EA, and IBVE
with alcohols. The activity of this catalyst, unlike other basic
solids, was scarcely affected by the presence of air or moisture.
The reaction activity in addition to the catalyst basicity were
affected by the alcohol acidity. So, the highest conversions were
observed for the reaction of methanol with alkenes. On the other
hand, acrylates show higher conversion in comparison with
IBVE when linear alcohols were used, but the heavier alcohols,
such as cyclohexanol, 2-ethylhexanol, and phenol, showed
higher reactivity with IBVE with respect to the acrylates.
The SEM images of all the catalysts showed particles with
sizes of ,50 nm. Using microwave irradiation, the reaction
times of chosen alkenes with different alcohols were reduced to
30 min with good yields in the presence of pollucite.
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Mass spectra and a GC chromatogram are available on the
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Acknowledgement
The authors gratefully acknowledge financial support from the University of
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