Intramolecular hydroalkoxylation of non-activated C=C bonds catalysed by zeolites: An experimental and theoretical study

Article abstract of DOI:10.1002/cssc.201300173

The high activity and selectivity of zeolites in the cyclisation of unsaturated alcohols is reported for the first time; the details of a reaction mechanism based on quantum chemical calculations are also provided. The high efficiency of zeolites MFI, BEA and FAU in the cyclisation of unsaturated alcohols (cis-decen-1-ol, 6-methylhept-5-en-2-ol and 2-allylphenol) to afford oxygen-containing heterocyclic rings is demonstrated. The best catalytic performance is found for zeolites with the optimum concentration of Bronsted acid sites (ca. 0.2 mmol g^-1) and the minimum number of Lewis acid sites. It is proposed that the efficiency of the catalysts is reduced by the existence of the so-called dual site, at which a molecule of unsaturated alcohol can simultaneously interact with two acid sites (an OH group with one and the double bond with the other Bronsted site), which increases the interaction strength. The formation of such adsorption complexes leads to a decrease in the catalyst activity because of (i) an increase in the reaction barrier, (ii) an unfavourable conformation and (iii) diffusion limitations. A new procedure for the preparation of tetrahydrofurans and pyrans over zeolite catalysts provides important oxygen-containing heterocycles with numerous applications. The zeolite fantastic: The high efficiency of zeolites MFI, BEA and FAU in the cyclisation of unsaturated alcohols to afford oxygen-containing heterocyclic rings is demonstrated. It is proposed that the efficiency of the catalysts depends on the existence of the so-called dual site, at which a molecule of an unsaturated alcohol can simultaneously interact with two acid sites. Copyright

Full text of DOI:10.1002/cssc.201300173

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DOI: 10.1002/cssc.201300173
Intramolecular Hydroalkoxylation of Non-Activated C=C
Bonds Catalysed by Zeolites: An Experimental and
Theoretical Study
[a, b]
[a, c]
[d]
[d]
[c]
Elena Pꢀrez-Mayoral,
Ines Matos,
Petr Nachtigall, Miroslav Polo zˇ ij, Isabel Fonseca,
[a]
[a]
ˇ
Dana Vitvarovꢁ-Prochꢁzkovꢁ, and Ji rˇ ꢂ Cejka*
The high activity and selectivity of zeolites in the cyclisation of
unsaturated alcohols is reported for the first time; the details
of a reaction mechanism based on quantum chemical calcula-
tions are also provided. The high efficiency of zeolites MFI, BEA
and FAU in the cyclisation of unsaturated alcohols (cis-decen-1-
ol, 6-methylhept-5-en-2-ol and 2-allylphenol) to afford oxygen-
containing heterocyclic rings is demonstrated. The best catalyt-
ic performance is found for zeolites with the optimum concen-
called dual site, at which a molecule of unsaturated alcohol
can simultaneously interact with two acid sites (an OH group
with one and the double bond with the other Brønsted site),
which increases the interaction strength. The formation of
such adsorption complexes leads to a decrease in the catalyst
activity because of (i) an increase in the reaction barrier, (ii) an
unfavourable conformation and (iii) diffusion limitations. A new
procedure for the preparation of tetrahydrofurans and pyrans
over zeolite catalysts provides important oxygen-containing
heterocycles with numerous applications.
ꢀ
1
tration of Brønsted acid sites (ca. 0.2 mmolg ) and the mini-
mum number of Lewis acid sites. It is proposed that the effi-
ciency of the catalysts is reduced by the existence of the so-
Introduction
Zeolites are well known as excellent acid catalysts, the perfor-
mance of which can often be fine-tuned by selecting the most
suitable channel architecture from a wealth of known struc-
Regarding sustainable chemistry and reactions with high
atom economy, zeolites are suitable catalysts (with adjustable
acidity) for the cyclisation of unsaturated alcohols to afford
oxygen-containing heterocycles, such as furans and pyrans.
Tetrahydrofurans and pyrans are important oxygen-containing
heterocycles often present in a large variety of natural prod-
[1–5]
tures that differ in channel size, shape and connectivity.
Zeolites are the most important industrial catalysts in oil refin-
[
2]
ing, and they have been extensively investigated as catalysts
in green processes, which includes the synthesis of precursors
for the preparation of pharmaceuticals and related com-
[13–15]
[16]
[17]
ucts,
which include polyether ionophores,
lignans,
[18]
macrodiolides and some other synthetic compounds that ex-
[
4,6,7]
[19]
pounds.
Recently, an extension of zeolite structures from
hibit biological activity. In addition, they are widely used in
[20]
3
D to 2D systems has been reported by direct and post-syn-
cosmetic formulations, perfumes and flavours. Some exam-
ples of biologically active derivatives of furans and pyrans are
shown in Figure 1. Mucoxin is a natural product that shows po-
[
8–12]
thetic approaches,
which offer new possibilities to catalyse
processes that involve bulky molecules that do not enter the
channel systems of standard 3D zeolites.
[a] Prof. E. Pꢀrez-Mayoral, Dr. I. Matos, Dr. D. Vitvarovꢁ-Prochꢁzkovꢁ,
ˇ
Prof. J. Cejka
J. Heyrovsk y´ Institute of Physical Chemistry
Academy of Sciences of the Czech Republic v.v.i.,
Dolej sˇ kova 3, 182 23 Prague 8 (Czech Republic)
E-mail: Jiri.Cejka@jh-inst.cas.cz
[b] Prof. E. Pꢀrez-Mayoral
Departamento de Quꢂmica Inorgꢁnica y Quꢂmica Tꢀcnica
Facultad de Ciencias, UNED
Paseo Senda del Rey 9, 28040 Madrid (Spain)
Figure 1. Oxygen-containing heterocycles that exhibit biological activity.
[c] Dr. I. Matos, Prof. I. Fonseca
REQUIMTE, FCT-UNL
Lisboa (Portugal)
[21]
tential antitumor activity for breast carcinoma, and brevis-
[
d] Prof. P. Nachtigall, M. Polo zˇ ij
amide is a recently isolated marine drug with a high neurotox-
Faculty of Science
Charles University in Prague,
Hlavova 8, 128 43 Prague 2 (Czech Republic)
[
22]
icity.
Furans 1 and 2a and pyran 3 are known as fla-
[23–25]
[26]
vours,
and furan 2b exhibits insecticidal properties.
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Apparently, small structural differences result in substantial
changes in their properties.
The cyclisation of various unsaturated alcohols catalysed by
acid zeolites is systematically investigated in this contribution.
In contrast to mesoporous molecular sieves and some metal–
organic frameworks that show no activity, zeolites with BEA,
MFI and FAU framework topologies are highly active and selec-
tive. These zeolites are studied herein with a focus on under-
standing the effects of the acid-site concentration, their type
and strength, and the role of the channel architecture. Our ex-
perimental investigation is accompanied by a quantum chemi-
cal study of the reaction mechanisms of the alcohol cyclisation.
Based on a combination of experimental and theoretical inves-
tigations, a reaction mechanism is proposed and the prefer-
ence for particular reaction products is explained.
The broad potential applications of furan and pyran deriva-
tives briefly outlined above show clearly that there is a need
for efficient green catalysts for the synthesis of these com-
pounds. Synthetic approaches that lead to tetrahydrofurans or
pyrans start from different substrates and often employ non-
[
27,28]
environmentally friendly catalysts.
The intramolecular oxy-
mercuration of unsaturated alcohols has been recently applied
[
29]
for the synthesis of mucoxin. The simplest synthetic strat-
egies to provide cyclic ethers consist of the intramolecular cyc-
lisation of suitable unsaturated alcohols or by the dehydration
of 1,4-diols. These synthetic approaches often require the use
of Brønsted acids and in some cases over-stoichiometric
[
30,31]
amounts must be used.
Coulombel et al. recently reviewed
the use of the trifluoromethane sulfonates TfOH, Al(TfO) and Results
3
Sn(TfO) as Lewis super acids for the intramolecular cyclisation
4
[
32]
[33]
Catalyst characterisation
of alcohols of non-activated olefins. In this sense, AgTfO
and lanthanide triflates are efficient catalysts for this transfor-
mation, which can be used in combination with ionic liquids at
All zeolites investigated in this study exhibited a high crystal-
linity as evidenced by XRD and SEM. The size and shape of the
zeolite crystals were determined by SEM and are reported in
Table 1; selected images are shown in Figure 2. The specific
[
34]
room temperature. Qian et al. described the intramolecular
hydroalkoxylation of g- and d-hydroxyolefins catalysed by
[
35]
[
PtCl (CH =CH )] in the presence of [P(4-CF C H ) ].
2 2 2 2 3 6 4 3
Only a few examples of heterogeneous catalysts, such as
[
36]
heteropoly acids in ionic liquids as solvents, metal(IV) phos-
Table 1. Brunauer–Emmett–Teller specific surface areas (SBET), typical crys-
tal sizes, and concentrations of Lewis and Brønsted acid sites in the zeo-
lites under investigation.
[
37]
[38]
phates and Amberlyst H-15, have been reported for the
intramolecular cyclisation of non-activated unsaturated alco-
hols. Titanium silicalite 1 (TS-1) has been used for the prepara-
tion of furans by the epoxidation/cyclisation of unsaturated al-
[
a]
[b]
Zeolite
CL
CB
[mmolg
SBET
Crystal size
[mm]
ꢀ
1
ꢀ1
2
ꢀ1
[mmolg
]
]
[m g
]
[
39]
cohols in the presence of H O .
BEA/12.5
BEA/25
BEA/37.5
BEA/68.5
MFI/15
MFI/25
MFI/75
MFI/140
FAU/15
FAU/40
0.45
0.19
0.11
0.07
0.21
0.07
0.03
0.01
0.34
0.04
0.14
0.33
0.17
0.08
0.37
0.35
0.15
0.01
0.20
0.02
674
680
642
650
400
425
396
392
769
774
0.3
0.3
0.2
0.1
0.7–0.9
0.9
1.0
1.0
1.0
2
2
The intramolecular cyclisation of cis-4-decenol (4) in nitro-
ethane as solvent catalysed by several zeolites has been re-
ported recently; BEA, MFI, FAU and STF zeolites showed much
[
40]
higher activities than montmorillonite K-10. The highest con-
version and yields were achieved with BEA, MFI and stabilised
FAU with Si/Al ratios of 12.5, 15.0 and 15.0, respectively. The
ring closure proceeded according to Markovnikov’s rule took
place with >95% selectivity to provide furan 2a as the major
compound, and the yield of pyran 5 was always <5%
1.0
[
[
a] Si/Al ratios determined by inductively coupled plasma MS (ICP-MS).
b] SBET values provided by the supplier.
(Scheme 1).
surface areas of the zeolites under study were in the range of
2
ꢀ1
7
69–392 m g ; zeolites BEA and FAU, which have large pore
sizes, exhibited higher BET surface areas than zeolite MFI
Table 1). The acidic properties of the zeolites under study
(
Scheme 1. Synthesis of 2a by the intramolecular cyclisation of 4.
were characterised by FTIR spectroscopy, and the concentra-
tions of Lewis and Brønsted acid sites, C and C , respectively,
L
B
The understanding of the mechanism of the cyclisation reac-
tion (Scheme 1) is important from a wider perspective, in par-
ticular, with respect to the formation of new CꢀO bonds. The
formation of CꢀO bonds plays a critical role in many transfor-
mations of organic compounds, for example, in the transfor-
mation of methanol to hydrocarbons, in which dimethyl ether
is the primary product, as well as in further reaction steps such
are reported in Table 1.
Characteristic FTIR spectra of BEA/37.5, MFI/15 and FAU/15
in the regions of the OH and CN stretching vibrations are
shown in Figure 3. The IR spectrum of zeolite BEA shows ab-
ꢀ
1
sorption bands at 3610 and 3745 cm , which are assigned to
Brønsted and terminal silanol groups, respectively. The IR spec-
trum of activated MFI exhibits two intense bands at 3610 and
[
41–43]
ꢀ1
as olefin alkylation with methanol.
3745 cm (the same assignment as above) and two shoulders
ꢀ
1
at 3724 and 3667 cm , which are assigned to internal silanol
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Figure 3. IR spectra in the OH (A) and CN (B) stretching regions of a) H-BEA/
Figure 2. SEM images of A) BEA/37.5, B) MFI/15, C) FAU/15 and D) FAU/40.
The scale bar in (A) corresponds to 0.2 mm whereas the others correspond
to 1 mm.
37.5, b) H-MFI/15, c) H-FAU/15; A: spectra of activated samples (red) and
spectra after [D ]acetonitrile adsorption followed by 20 min desorption at
3
room temperature (black).
[
44]
groups and OH groups linked to extra-framework Al.
For
Qualitatively similar results were obtained for the cyclisation
of 4 to 2a catalysed by zeolite MFI with various Si/Al ratios
(Figure 4B and C at 358 and 337 K, respectively). The efficiency
of the MFI catalysts at 337 K to yield 2a increases in the order
ꢀ
1
zeolite FAU, two bands are observed at 3565 and 3630 cm ,
attributed to Brønsted OH groups inside the six-rings and to
[
44]
those pointing into the super cage, respectively. The absorp-
tion bands of the Brønsted acid OH groups disappeared upon
the adsorption of [D ]acetonitrile (which evidences the accessi-
3
bility of all of the Brønsted sites) and new bands in the 2350–
Table 2. Catalytic performance of BEA zeolites in the cyclisation of 4 at
358 K.
ꢀ
1
2
250 cm region appeared, which were attributed to the ad-
ꢀ
1
sorption of [D ]acetonitrile on the Lewis (2325 cm ) and
3
Catalyst
t
Yield of 2a
ꢀ1 [44]
Brønsted acid sites (2300 cm ).
[h]
[%]
BEA/12.5
BEA/25
BEA/37.5
BEA/67.8
1.5
1.5
0.75
1.0
95
99
99
97
Catalytic performance
[
40]
Based on our preliminary results, we have investigated ef-
fects of the zeolite topology and composition, the re-
action temperature dependence and the role of the
reactant size on the reaction rates in the cyclisation
reactions. In addition, these experimental results are
combined with a computational investigation, and
a reaction mechanism on Brønsted acid sites in zeo-
lites is proposed.
Effects of zeolite topology and Si/Al ratio
The catalytic behaviour of the zeolites under study in
the cyclisation of 4 is strongly dependent on the
type and concentration of Brønsted and Lewis acid
sites (Si/Al ratio). The yield of 2a as a function of the
reaction time is shown in Figure 4. As observed from
the results shown in Table 2 and Figure 4A, all of the
BEA zeolites investigated here provide close to 100%
conversion to 2a within 90 min. Although all samples
show similar yields at 90 min, at shorter reaction
times the efficiency of the zeolite increases in the
order BEA/12.5<BEA/25<BEA/67.8<BEA/37.5. Nota-
bly, BEA/12.5 and BEA/25, which have a significantly
Figure 4. Kinetic profiles of the cyclisation of 4 catalysed by zeolites with different Si/Al
ratios. A) BEA/37.5 ( ), BEA/67.8 ( ),
), BEA/25 (~) and BEA/12.5 (~) at 358 K. B) MFI/75 (
MFI/25 (
) and MFI/15 (~) at 358 K. C) MFI/75 ( ), MFI/140 (~), MFI/25 ( ) and MFI/15 (~)
at 337 K. D) FAU/15 ( ) and FAU/40 ( ) at 358 K.
higher concentration of active sites (both C and C )
B
L
&
&
&
than BEA/37.5 and BEA/67.8, show lower conversions
than zeolites with a lower Al concentration.
&
&
&
&
&
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MFI/15<MFI/25<MFI/140<MFI/75 (except for the shortest re-
action time reported). The FAU/15 catalyst gives a higher yield
of 2a than FAU/40 at shorter reaction times (Figure 4D). The
kinetic profiles of the cyclisation catalysed by MFI/75, BEA/37.5
and FAU/15 (which have the optimum Si/Al ratio for cyclisation
as determined by the results shown in Figure 4) are compared
in Figure 5. The yield of 2a after 20 min is 100, 79 and 38% for
MFI/75, BEA/37.5 and FAU/15, respectively.
Table 4. Reaction rates for the cyclisation of 4 catalysed by zeolites at
358 K.
Entry
Catalyst
Si/Al ratio
37.5
t
Reaction rates
4
ꢀ1 ꢀ1
[
min]
[10 molL
s
]
1
2
3
4
5
6
7
8
9
BEA/37.5
5
7.866
4.438
3.920
3.256
2.424
1.851
12.633
7.239
5.079
0.867
0.860
0.851
10
15
20
30
45
5
10
15
15
30
45
MFI/75
FAU/15
75
15
10
1
1
12
Effect of the zeolite channel-architecture on the cyclisation re-
action
To study the influence of the structural characteristics of zeo-
lites on the cyclisation reaction of unsaturated alcohols, the
cyclisation of other substrates, such as secondary alcohol 6
and o-allyl phenol 7 (Schemes 2 and 3, respectively), was inves-
Figure 5. Kinetic profiles of the cyclisation of 4 at 358 K for catalysts with an
optimum Si/Al ratio for the particular framework type: MFI/75 (
) and FAU/15 (~).
&
), BEA/37.5
(
&
Temperature dependence and reaction kinetics
The influence of the reaction temperature on the cyclisation of
catalysed by various MFI zeolites is apparent from Figure 4B
Scheme 2. Cyclisation of 6 catalysed by zeolites.
4
and C, which show kinetic profiles at 358 and 337 K, respec-
tively. The conversion of 4 catalysed by BEA/12.5 and MFI/15 at
313–373 K are summarised in Table 3. It is apparent that the re-
action proceeds even at temperatures as low as 313 K, with
a lower yield of 2a, on BEA/12.5 in particular.
Scheme 3. Cyclisation of 7 catalysed by zeolites.
Table 3. Conversion of 4 catalysed by H-BEA and H-MFI with similar Si/Al
ratios at different reaction temperatures.
tigated for the zeolites that showed the highest conversion for
the cyclisation of 4. The results of the cyclisation of unsaturat-
ed alcohols catalysed by different zeolites are summarised in
Table 5.
Entry
Catalyst
T
t
[h]
Yield of 2a
[%]
[K]
1
2
3
4
5
6
7
8
BEA/12.5
373
358
337
313
373
358
337
313
0.5
1.5
6
100
95
92
49
100
99
Although MFI/75 shows a higher activity in the cyclisation of
4
than BEA/37.5 at 358 K (with an excellent yield of 91% in
24
MFI/15
0.75
2
just 15 min) the cyclisation of 6 almost did not take place over
MFI/75. A similarly low activity was observed previously for 4
24
24
99
39
[41]
over STF/38.5.
Zeolites BEA/37.5 and FAU/15 both gave pyran 8 in a rela-
tively high yield (95% after 15 and 90 min, respectively) even
at a low reaction temperature of 313 K. Notably, the H atom
on C-5 of 4 is substituted by a methyl group in 6, which results
in the change of product to pyran 8 in agreement with Mar-
kovnikov’s rule.
The kinetic rate constants for the cyclisation of 4 to 2a cata-
lysed by various zeolites were evaluated by assuming first-
order kinetics. The data reported in Table 4 confirm that the re-
action catalysed by zeolites in nitroethane at 358 K follows
first-order kinetics. The kinetic rate constant for the cyclisation
The cyclisation of 7 catalysed by BEA/37.5 and FAU/15 at
358 K afforded benzofuran 9 in almost quantitative yields in 60
and 90 min, respectively (Figure 6). However, it was necessary
ꢀ1
catalysed by MFI/75 and BEA/37.5 is 0.0001 s in both cases.
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1
0 (Scheme 4). Each of these adsorption complexes (reaction
Table 5. Cyclisation of unsaturated alcohols catalysed by zeolites.
precursors) can lead to two main products: either the Markov-
nikov product 2-methyltetrahydrofuran (11) or the anti-Mar-
kovnikov product tetrahydropyran (12).
Entry
Alcohol
Catalyst
T
t
Product
Yield
[%]
[K]
[min]
1
2
3
5
4
BEA/37.5
MFI/75
FAU/15
BEA/37.5
358
358
358
313
45
15
240
15
30
3
360
15
30
90
60
2a
2a
2a
8
8
8
8
8
8
8
99
91
89
94
99
<5
<5
62
75
95
99
65
98
The reaction takes a different course starting from complex I
or II (Scheme 4). The relative energies (with respect to the
non-interacting reactant 10 and the model catalyst) of the
minima and transition states on the reaction paths are given in
Table 7. The interaction energy of the H-bonded complex II is
6
7
3
13
6
7
8
MFI/75
MFI/75
FAU/15
313
358
313
nearly twice as large as that of complex I, and the E values
int
ꢀ1
are ꢀ84 and ꢀ45 kJmol , respectively.
3
3
13
13
The reaction starting from complex I is described first. The
p-bonded precursor I can be transformed directly in a single
step either to adsorbed 11 (III) or adsorbed 12 (IV) with activa-
tion barriers (with respect to the energy of I) of 205 and
10
BEA/37.5
MFI/75
FAU/15
358
358
358
9
9
9
1
1
2880
90
12
ꢀ1
1
60 kJmol , respectively. Thus, the formation of 12 is more
likely than the formation of 11 if the reaction starts from pre-
cursor I.
The situation is different for the reaction that starts from
precursor II. In this case, the formation of III proceeds in one
step by the direct electrophilic attack of the proton from the
OH group to lead immediately to ring formation. The transition
state is depicted in Figure 7. Adsorbed III thus formed can sub-
sequently desorb or it can undergo an additional reaction step
to lead to adsorbed 12. We were unable to find a direct reac-
Figure 6. Kinetic profiles for the cyclisation of 7 catalysed by BEA/37.5 (
and USY/15 ( ) at 358 K.
&
)
&
to prolong the reaction to 48 h to yield 9 in 65% if using zeo-
lite MFI/75, which has medium-sized pores, and diffusion limi-
tations are likely the main reason of the low efficiency ob-
served in the cyclisation of 7.
Figure 7. Reaction mechanism for the conversion of 10 into 11. The reactant
interacts with the Brønsted site (complex II); the activated proton of the al-
cohol OH group electrophilically attacks the p-orbital and, following Markov-
nikov’s rule, this proton transfers to C-5 while the CꢀO bond is formed be-
tween the C-4 and O atoms (the transition state TS-O1); thus formed 11 re-
mains adsorbed on the Brønsted site (complex III). Colour scheme:
H=white, C=light grey, O=red, Si=dark grey, Al=black. Geometries ob-
tained at the B3LYP/6-311G(d,p) level.
Theoretical investigation of the reaction mechanism
To understand the reaction mechanism of the cyclisation of un-
saturated alcohols in more detail, a theoretical inves-
tigation of the cyclisation catalysed by Brønsted acid
sites was performed. The results obtained from a clus-
Table 6. Relative energies of stationary points for different reaction paths. Energies
were calculated at the MP2/cc-pVTZ level by using B3LYP/6-311G(d,p) geometries.
ter model (Table 6) are qualitative, and the effect of
the zeolite framework is discussed below based on
the calculations with a periodic model. The cyclisa-
tion mechanism was investigated computationally by
using pent-4-en-1-ol (10) as a model reactant. Com-
pared with 4 used in the experiments, the pentyl
group on C-5 is missing in 10 (this choice was led by
computational simplicity and is discussed below).
Compound 10 may interact with Brønsted acid
sites in two different ways to form two different reac-
tion precursors: complex I stabilised by the interac-
tion between the Brønsted site and the p bond of
Reaction
intermediate
C complex
structure
O complex
structure
E
rel
E
rel
[kJmol ]
ꢀ
1
[a]
ꢀ1 [a]
[
kJmol
]
formation of 11
reactant!11
adsorbed 11
TS-C1
III
11
160
TS-O1
III
11
47
ꢀ154
ꢀ61
ꢀ154
ꢀ61
11 +zeolite
formation of 12
reactant!12
III!12
TS-C2
–
IV
115
–
ꢀ148
ꢀ63
–
–
103
ꢀ148
ꢀ63
TS-O2
IV
12
adsorbed 12
1
2+zeolite
12
10, and complex II stabilised by the H bond between
[
a] Relative energies with respect to isolated 10 and catalyst model.
the Brønsted site of the zeolite and the OH group of
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Although the calculated pref-
erences are in agreement with
our experimental results, the
suitability of the model used still
needs to be tested. To prove the
suitability of the model, two as-
pects were tested: (i) the effect
of the substituent on C-5 of 10
and (ii) the effect of the catalyst
representation. The difference
between 10 and 4 is in the alkyl
chain on C-5. The results ob-
tained for hex-4-en-1-ol (14) are
reported in Table 7. The methyl
substituent does not significant-
ly change the interaction ener-
gies of the reaction precursors,
and their relative energies are
not affected. Therefore, we
expect that even the elongation
of the hydrocarbon chain should
not considerably change the rel-
ative interaction energy. The
Scheme 4. Reaction mechanisms for the conversion of 10 to 11 and 12. The reaction may proceed via adsorption
complex I or II. The corresponding relative energies and barriers are reported in Table 6.
effect of the zeolite catalyst model was investigated by using
a periodic model of H-ZSM-5. The interaction energies of I and
II obtained at the vdW-DF2 level are reported in Table 7. The
Table 7. Comparison of interaction energies of the reactants using differ-
ent models.
ꢀ
1
Reactant
Catalyst
Method
E
int [kJmol
]
DEint
interaction energies of I and II are lowered by 77 and
ꢀ
1
I
II
[kJmol
]
ꢀ1
7
6 kJmol , respectively, as a result of the effect of the disper-
1
1
1
0
4
0
cluster
cluster
MFI
MP2
MP2
vdW-DF2
ꢀ45
ꢀ48
ꢀ84
ꢀ90
39
42
38
sion interaction between 10 and the zeolite framework.
Although this has a huge effect on the interaction energies,
their relative values are almost unaffected.
[
a]
ꢀ122
ꢀ160
[a] Relative energies of corresponding adsorption complexes formed by
The results summarised in Table 7 show that the analysis of
the potential energy surface for the conversion of 10 obtained
with the cluster model is relevant for the discussion of the ex-
perimental results reported above. In agreement with the con-
clusions of our experimental work, the calculations reported
herein show that the reaction proceeds preferably towards
substituted tetrahydrofurans. The selectivity is primarily deter-
mined by the relative stability of reaction precursors I and II.
1
4.
tion path between II and IV. The H atom from the hydroxyl
group needs to be transferred to C-5 during such an elementa-
ry step, and this is sterically strained process characterised by
a very large barrier. This effect is not because of the small size
of 10 used herein instead of 4; the alkyl substituent on C-5 of
4
does not reduce the steric strain. Thus, reaction precursor II
Discussion
can only undergo a change to III, a reaction step characterised
by an activation barrier of 131 kJmol . The conversion of III to
IV proceeds via the transition state characterised by an activa-
ꢀ
1
The cyclisation of unsaturated alcohols can easily proceed
inside the channel systems of medium- and large-pore zeolites.
The structure of the zeolite (size of channels and cavities, chan-
nel intersection geometries) is the controlling factor for suc-
cessful transformations of unsaturated alcohol to cyclic ethers.
With respect to this, 4 can be easily transformed inside the
channels of BEA, FAU and MFI with >90% yields (Table 5).
In contrast, much lower yields (65% after prolonged reaction
times) were obtained in the conversion of 6 over MFI, and
yields <5% were reached with secondary alcohol 7. Both 6
and 7 provide high yields over zeolites BEA and FAU (Table 5).
This clearly shows a strong effect of the channel size (structure)
in this reaction.
ꢀ
1
tion barrier of 257 kJmol . This process is not likely to take
place as the desorption of 11 is only approximately
ꢀ
1
90 kJmol .
From a theoretical point of view, the activation barriers of
the elementary reaction steps reported in Table 6 clearly show
that the selectivity of the reaction depends on the reaction
precursor: reactions starting from precursors I and II lead pref-
erentially to 12 and 11, respectively. Therefore, the relative sta-
bility of I and II is decisive for the actual reaction products.
The results of the cluster model calculations presented in
ꢀ
1
Table 6 show that II is more than 40 kJmol more stable than
I and, therefore, the reaction will proceed preferentially to-
wards 11, which is in agreement with our experimental results.
Although the structural effect on the cyclisation yield over
zeolites with different structural types is relatively clear, the
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effect of the acid-site concentration needs further discussion.
Usually, an increasing concentration of active sites in the zeo-
lite catalyst results in an improved performance. However, the
results reported above for different zeolites with various con-
centrations of Al do not show such a trend, and the relation-
ship between the catalyst efficiency and the concentration of
Brønsted and Lewis sites is more complex (see catalytic perfor-
mance section and Table 1). The catalyst efficiency increases
It has been shown recently that even molecules as small as
CO and CO may simultaneously interact with two adsorption
2
sites on zeolites once the concentration of adsorption sites
[45–47]
reaches a certain threshold.
In the case of CO adsorption
on MFI, such a simultaneous adsorption of CO on dual cation
[48]
sites was observed for samples with Si/Al=14. This thresh-
old clearly depends on the size of the adsorbate and the zeo-
lite topology. Both Brønsted and Lewis sites can interact either
with OH groups or with the p-electron density of unsaturated
alcohols. Thus, the simultaneous interaction of 2a with two
active sites can be expected for samples with a Si/Al ratio even
higher than 14. We propose that such simultaneous interac-
tions of the reacting unsaturated alcohol with two active sites
(dual sites) can explain the decrease in the reaction rates ob-
ꢀ
1
with increasing C in the range 0–0.2 mmolg and it abruptly
B
ꢀ
1
decreases for C above 0.2 mmolg . Therefore, it is assumed
B
that the reaction is primarily catalysed by Brønsted sites and
ꢀ
1
that there is an optimum value of C (0.15–0.20 mmolg ) for
B
each structural type of the zeolite. This assumption is further
supported by the fact that among the most active samples
with a particular topology (MFI/75, BEA/37.5 and FAU/15 all
have a similar C ), the activity decreases with increasing C (see
ꢀ
1
served for samples with C > 0.2 mmolg . The simultaneous
B
interaction of 10 with two Brønsted sites was modelled at the
vdW-DF2 level by using a periodic model of the MFI unit cell;
the interaction energy of the complex shown in Figure 9 is
B
L
Figure 5 and Table 1).
Assuming that Brønsted sites are significantly more active
than Lewis sites for the cyclisation reaction (and our calcula-
tions clearly show the involvement of Brønsted protons in the
reaction), the adsorption of either the reactants or products on
Lewis sites may result in diffusion limitations and, thus, in a de-
ꢀ
1
ꢀ174 kJmol (cf. Table 7). This is a randomly selected pair of
Brønsted sites, and different pairs of Brønsted sites (or a pair of
crease in the zeolite activity. It is apparent that C in BEA and
L
MFI zeolites decreases if the Si/Al ratio increases (Table 1).
Turn-over frequencies (TOFs) versus C for BEA and MFI calcu-
L
lated from the corresponding turn-over numbers (TONs) at low
conversions (equal to yield in our case) are shown in Figure 8.
For both zeolite types, the highest catalytic activity in the
shortest reaction time was obtained for the zeolites that exhib-
Figure 9. Simultaneous interaction of 10 with two Brønsted acid sites on
ZSM-5. The OH group and the p bond interact with protons in the vicinity
of framework Al in the T12 and T1 positions, respectively. Colour scheme:
H=white, C=light grey, O=red, Si=dark grey, Al=black. Geometry opti-
mised by using the vdW-DF2 functional.
Brønsted and Lewis sites) will be characterised by a somewhat
higher or lower interaction energy. The decrease in the catalyt-
ic activity because of the presence of dual sites results from:
Figure 8. TOF versus C
37 K, respectively. TOFs were calculated from the corresponding TON
values by considering a 10% conversion to 2a. TON=10ꢄsubstrate [mmol]/
.3ꢄC [mmol]; TOF=TON/t [min].
L
determined for BEA (
&
) and MFI (
&
) at 358 and
3
(
i) the molecule that interacts with a dual site will be bound
ꢀ
1
more strongly (by 14 kJmol for the situation depicted in
Figure 9) and, consequently, the reaction barrier becomes
higher; (ii) each functional group (OH and p bond) of the mole-
cule interacts with a different Brønsted site, thus, the molecule
is stretched between two active sites and such a conformation
is unfavourable for the reaction because of steric reasons
(Figure 9); and (iii) molecules that interact with the dual site
will effectively block two active sites (they are not likely to un-
dergo the reaction) and, in addition, they will limit the diffu-
sion of other molecules through the zeolite channel-system.
The proposed effect of adsorption of 10 simultaneously on
two Brønsted sites in ZSM-5 can be related to the first reaction
0
L
ited the highest Si/Al ratios and, therefore, the lowest C , in ac-
L
cordance with our hypothesis. The catalytic behaviour of FAU
zeolites follows a similar trend.
As a result of differences in the catalytic behaviour of some
catalysts in the shortest reaction times (conversion to 2a <5%
for the first-order reaction observed for BEA), we calculated
TOFs from the reaction times that provided 10% conversion to
2
a. This conversion should guarantee a reasonable comparison
between the zeolites.
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step in methanol dehydration to dimethyl ether. Iglesia et al.
suggested that two sites in close proximity are a necessary
prerequisite for methanol dehydration over Keggin-type poly-
Table 8. Characteristics of zeolites used with 3D channel systems.
Zeolite
BEA
Origin
Channel
diameter [nm]
Si/Al
ratio
[
43]
entrances
12-12-12
oxometalate.
Zeolyst
0.64ꢄ0.76
.56ꢄ0.56
12.5
25
0
3
6
15.0
25.0
75.0
15.0
7.5
8.5
Conclusions
MFI
Conteca
Zeolyst
10-10-10
12-12-12
0.53ꢄ0.56
.51ꢄ0.55
The cyclisation of unsaturated alcohols to afford heterocyclic
compounds was investigated. Zeolites BEA, MFI and FAU show
both high catalytic activities and high yields. The catalytic ac-
tivity in the cyclisation of cis-decen-1-ol (4) to afford furan de-
creases in the order MFI/75>BEA/37.5>H-FAU/15. For bulkier
reactants (6-methylhept-5-en-2-ol (6) and 2-allylphenol (7), in
particular), the catalytic activity of MFI decreases significantly.
Interestingly, the highest conversions are not observed for
zeolites with the largest concentration of acid sites. Although
the cyclisation can be catalysed by both Lewis and Brønsted
acid sites, the experimental data indicate that the largest con-
version is achieved for zeolites that have the optimum concen-
0
FAU
0.74
4
0.0
Catalyst characterisation
The crystallinity of the zeolites was determined by powder XRD by
using a Bruker D8 X-ray powder diffractometer using CuKa radia-
tion in the Bragg–Brentano geometry. XRD patterns of all of the
zeolites provided clear evidence of good crystallinity and phase
purity. The size and shape of the zeolite crystals were determined
ꢀ
1
tration of Brønsted sites (0.15–0.20 mmolg ). It is proposed
that the decrease in the conversion of unsaturated alcohols
observed for zeolites with higher acid-site concentrations is be-
cause of the simultaneous interaction of the alcohol with two
acid sites (adsorption on dual sites).
by SEM by using a Jeol JSM-5500LV instrument. N adsorption iso-
2
therms at 77 K were recorded by using an ASAP 2020 (Micromerit-
ics) static volumetric apparatus to obtain textural characteristics.
All samples were out-gassed by using a turbomolecular vacuum
pump at 623 K overnight by using a heating program that allowed
the slow removal of most preadsorbed water at low temperature.
The concentration of Lewis and Brønsted acid sites was deter-
Our calculations show that the simultaneous interaction of
each group of reactants (OH group and p bond) with a pair of
acid sites at an appropriate distance from each other increases
mined by the adsorption of [D ]acetonitrile as a probe molecule
3
followed by FTIR spectroscopy (Nicolet 6700) using the self-sup-
porting wafer technique. A thin zeolite wafer was activated prior
to the experiment under a high vacuum (1.33 kPa) at 723 K over-
ꢀ
1
the interaction by at least 14 kJmol compared to adsorption
complexes in which the alcohol interacts with just one Brøn-
sted acid site. The increased stability of the adsorption com-
plex affects the reaction rate in various ways: (i) activation bar-
riers are increased, (ii) the adsorption precursor adopts an un-
favourable configuration and (iii) the increased stability of the
adsorption complexes on dual sites may be behind diffusion
limitation as a result of channel blocking.
night. The adsorption of [D ]acetonitrile proceeded at RT for
3
30 min at a partial pressure of 0.665 kPa followed by 20 min evacu-
ation. To obtain quantitative analysis, the molar absorption coeffi-
cients for [D ]acetonitrile adsorbed on Brønsted acid sites [ n˜ (CꢁN)
3
B
ꢀ1
at 2297 cm
,
molar absorption coefficient e =2.05ꢂ
B
ꢀ1
0
2
.1 cmmmol ] and strong and weak Lewis acid sites [( n˜ CꢁN) at
L1
ꢀ
1
ꢀ1
ꢀ1
325 cm and n˜ (CꢁN) at 2310 cm , e =3.6ꢂ0.2 cmmmol ]
L2
L
By adopting the existence of dual sites and the site-blocking
model, the efficiency of zeolite catalysts can be better under-
stood. The highest cyclisation activity can be expected for zeo-
lites with an optimal distribution of framework Al (which
avoids the presence of two acid sites close enough to establish
a dual site for a particular reactant) and the optimum channel
architecture—large-pore zeolites for bulkier reactants. Thus,
a large-pore zeolite with a homogenous Al distribution (frame-
work Al atoms distributed as far as possible from each other)
and the lowest possible Si/Al ratio in which the closest acid
sites are further apart than the OH and C=C bond in the reac-
tant. Although the synthesis of such materials remains a chal-
lenge, zeolite FER with exactly these properties has been ob-
[50]
were used.
Catalytic performance
cis-4-Decenol (4), 6-methylhept-5-en-2-ol (6) and 2-allylphenol (7)
were used to test the catalytic activity of different structural types
of zeolites. All reactants and solvents were obtained from Sigma–
Aldrich. The individual zeolite catalysts were activated in air at
ꢀ1
7
23 K for 90 min using a heating ramp of 10 Kmin before the re-
action. The catalyst (300 mg) was added to a solution of the corre-
sponding unsaturated alcohol in nitroethane (4 mL, 2 mmol) at the
given reaction temperature with stirring.
The reaction products were analysed by using a high-resolution
gas chromatograph, Agilent 6850, equipped with a flame ionisa-
tion detector (FID) and a non-polar HP-1 column (length: 30 m, di-
ameter: 0.32 mm, phase thickness: 0.25 mm) using mesitylene as an
[
49]
tained recently for the adsorption of CO2.
internal standard. NMR spectra were recorded by using a Varian
Experimental Section
1
Mercury 300 spectrometer. H chemical shifts (d) in CDCl are re-
3
Commercial zeolites BEA, MFI and FAU, which have different Si/Al
ratios and, therefore, concentrations of acid sites, were used in this
study. The structural and chemical characteristics of the zeolites
under study are given in Table 8.
ported with respect to tetramethylsilane. MS spectra were record-
ed by using a GS–MS ThermoElectron instrument. Spectroscopic
data for 2a, 7 and 8 are in good agreement with those reported
[51]
previously.
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Calculations
Keywords: cyclization
· density functional calculations ·
heterogeneous catalysis · reaction mechanisms · zeolites
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Si AlO H cluster model (Figure 10A), often denoted as a 3-T
model, is OH-terminated on Al atoms and H-terminated on Si
atoms to avoid the artificial interaction of reactants with the clus-
ter-terminating silanol groups.
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Received: February 21, 2013
Published online on May 23, 2013
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