Diels-alder reaction between isoprene and methyl acrylate over different zeolites: Influence of pore topology and acidity

Article abstract of DOI:10.1002/cplu.201300157

The Diels-Alder reaction between isoprene and methyl acrylate over several zeolites was thoroughly investigated. ZSM-5 zeolites provided the highest productivity in methyl 4-methylcyclohex-3-enecarboxylate isomer, achieving 0.219mmol product per mmolH

Full text of DOI:10.1002/cplu.201300157

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DOI: 10.1002/cplu.201300157
Diels–Alder Reaction between Isoprene and Methyl
Acrylate over Different Zeolites: Influence of Pore
Topology and Acidity
Claire Bernardon, Benoꢀt Louis,* Valꢁrie Bꢁnꢁteau, and Patrick Pale*[a]
In memory of Detlef Schrçder
The Diels–Alder reaction between isoprene and methyl acrylate
over several zeolites was thoroughly investigated. ZSM-5 zeo-
lites provided the highest productivity in methyl 4-methylcy-
clohex-3-enecarboxylate isomer, achieving 0.219 mmol product
per mmol H+ in 1 hour. In addition, this study highlights the
influence of ZSM-5 zeolite’s Brønsted acidity and crystal size on
its performance in the Diels–Alder reaction. An optimal config-
uration between the two reactants within the medium-pore-
sized ZSM-5 framework is obtained, thus suggesting the pres-
ence of a confinement effect.
Introduction
The Diels–Alder reaction, a cycloaddition between an olefin
and a diene,[1] is commonly used to synthesise six-membered-
ring compounds in fine chemistry and total synthesis of natu-
ral products.[1b,2] Cycloadducts are usually obtained with high
regio- and stereoselectivities with complete atom economy.
High pressures,[1b,3] high temperatures,[1b,4] ultrasonication[1b,5]
or microwave activation[1b,6] are sometimes used to enhance
reaction rates and/or selectivities, but non-conventional media
proved beneficial as well: ionic liquids,[1b,7] water[1b,8] and super-
critical CO2.[1b,9] However, most Diels–Alder reactions are accel-
erated and more selective upon acid activation. Such activa-
chemical stabilities.[16] Furthermore, their porous structures ex-
hibit confined nanospaces, which favour contact between the
reactants and the zeolite active sites, often related to the
acidic properties. Though zeolites possess two types of acidity,
Brønsted and Lewis,[17] with strength close to superacidity, they
also render possible peculiar molecular diffusion, adsorption
and reactivity thanks to the so-called confinement effect. Intro-
duced by Derouane in 1986,[18] this concept explains how each
molecule present within the pores is affected by the zeolite.
Indeed, non-covalent interactions between zeolite and sub-
strate are formed, guided by the presence of non-equivalent
adsorption sites, surface curvature and pore topologies. These
interactions induce modifications in the conformational land-
scape of molecules to raise substrate/zeolite interactions.[19]
The adsorption energy of one molecule on the zeolite surface
is therefore strongly enhanced, by about eight times, with re-
spect to a planar surface.[20] In addition, it has been shown that
Lennard–Jones interactions are favoured if the molecular size
fits the pore aperture.[21,22]
[11]
tions are usually performed by Lewis acids,[10] such as AlCl3
or LiClO4,[12] often used in (sur)stoichiometric amount and thus
generating hazardous wastes, but also by protic acids, such as
triflic acid[13] or phosphonic acids,[14] also leading to safety and
waste concerns.
To avoid and minimise these problems, the replacement of
homogeneous acids by easily separable and recyclable hetero-
geneous solid acids in Diels–Alder reactions has been exten-
sively investigated. In earlier studies, numerous solid acid cata-
lysts, such as zeolites, clays, alumina and silica, have been re-
ported.[15]
The aim of the present study is to highlight the influence of
the zeolite framework and acidity in the Diels–Alder reaction
between isoprene 1 and methyl acrylate 2 (Scheme 1). These
Zeolites, which belong to the class of tectosilicates, offer
many advantages with respect to their homogeneous homo-
logues: they are non-corrosive, non-toxic, easy to handle and
recoverable, as well as having high thermal, mechanical and
[a] C. Bernardon, Dr. B. Louis, Dr. V. Bꢀnꢀteau, Prof. P. Pale
Laboratoire de Synthꢁse et Rꢀactivitꢀ Organiques
Institut de Chimie, UMR 7177 CNRS
Universitꢀ de Strasbourg
Scheme 1. Diels–Alder reaction between isoprene and methyl acrylate.
1 rue Blaise Pascal, 67000 Strasbourg cedex (France)
E-mail: blouis@unistra.fr
two substrates were chosen on the basis of their limited reac-
tivity and for comparison purposes. Indeed, Onaka et al.[23] con-
ducted this reaction at À18C for 18 hours in hexane in the
presence of hexagonal mesoporous silica materials with high
ppale@unistra.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300157.
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Al content. They achieved the formation of adducts 3 and 4 in
a regioisomeric ratio of 97:3 but with yields depending on the
solvent (94% in hexane, 73% in toluene and 65% in dichloro-
methane). Meanwhile, in dichloromethane heated at reflux as
solvent and in the presence of BEA zeolite, Carlson et al. ach-
ieved a 99% selectivity toward 3.[24] Using other kinds of zeo-
lites (ZSM-5, mordenite, Y), they observed regioisomeric ratios
in the range 4:96–0:100 but associated with low yields (11–
48%). Nevertheless, no relationships between physico-chemical
properties of the solid acids and conversion or selectivity have
been provided.
Hence, the outcome of this study would be to settle a rela-
tionship between the zeolite structure (pore and acidity), the
size of the crystals and the activity/selectivity in the Diels–
Alder reaction.
Figure 1. Selectivity in isomer 3 and methyl acrylate conversion as a function
of temperature.
Results and Discussion
To evaluate the role and activity of zeolites in the Diels–Alder
reaction, commercial as well as specifically designed zeolites
were used as catalysts for the isoprene–methyl acrylate reac-
tion as model (Scheme 1). With the goal of having zeolites of
different crystal sizes, porosities and acidities, three H-ZSM-5
samples, named N1, N2 and N3, were prepared according to
our previously reported procedure involving sugar-cane bag-
asse residues.[25] Two other MFI-type samples, named G1 and
G2, have also been obtained by performing the zeolite synthe-
ses at neutral pH through the non-conventional fluoride route,
which led to large zeolite crystals.[26–29] For comparison purpos-
es, an H-Y zeolite, named Y1, was synthesised according to
a slightly modified reported procedure[30] (see the Supporting
Information for details).
Preliminary Diels–Alder experiments
To optimise the reaction conditions, preliminary Diels–Alder re-
actions between isoprene 1 and methyl acrylate 2 (Scheme 1)
were performed at different temperatures with commercial H-
ZSM-5 zeolite (CBV5020, Zeolyst) as catalyst. The methyl acry-
late conversion rose gradually with the temperature, achieving
nearly 60% at 908C (with nearly complete isoprene conver-
sion), whereas the selectivity toward regioisomer 3 decreased
slightly above 408C (Figure 1). It appears that a compromise
between high conversion and appreciable selectivity toward
the cycloadduct 3 was obtained at 758C. Indeed, 95% of iso-
prene was incorporated into the cycloadducts by reacting with
methyl acrylate, with nearly 90% selectivity in 3 achieved at
this temperature.
Figure 2. Degree of methyl acrylate conversion and selectivity in 3 as a func-
tion of time for H-ZSM-5 and H-USY zeolites.
commercial H-ZSM-5 and H-USY catalysts at iso-site conditions.
The results (Figure 2) revealed both a higher activity and selec-
tivity in regioisomer 3 on performing the reaction in the pres-
ence of the ZSM-5 zeolite.
Besides the major regioisomer 3, several products were also
detected by GC (Figure S3 in the Supporting Information) and
identified by GC–MS or by injection of reference compounds.
Diprene was observed as the major by-product (see mass spec-
trum in Figure S2). It is worth mentioning here that it has been
shown that the cycloadducts could react further with iso-
prene.[23]
A further refinement of the influence of the pore topology
was attempted by evaluating different commercial zeolite
structures and other solid acids in the reaction. Table 1 pres-
ents those results and further highlights the best performance
of the ZSM-5 zeolite structure (94% selectivity in 3 and 95%
estimated isoprene conversion; Table 1, entry 6). It appears
that the trend in the catalyst performance did not follow the
To select the more suitable zeolite catalyst, two experiments
(repeated twice) were performed at 758C in n-heptane with
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Commercial ZSM-5 zeolite provided the best re-
sults, maximising the selectivity toward regioisomer 3
at a high degree of methyl acrylate conversion, so
therefore it is worth investigating our as-prepared
MFI-type zeolites that exhibit different crystal mor-
phologies and Brønsted acid site densities, to further
attempt to establish structure and activity/selectivity
relationships.
Table 1. Screening of commercial zeolites and other solid acids for Diels–Alder reac-
tion (Scheme 1) at 758C for 24 hours.
Entry Pore size Catalyst
[ꢃ]
Selectivity in methyl 3/4
4-methylcyclohex-3-
enecarboxylate 3
[%][a]
Estimated Methyl
isoprene acrylate
conversion conversion
[%]
[%]
1
2
3
4
5
6
7
8
7.4
7.4
7
H-Y
H-USY
H-MOR
90
90
58
81
88
94
87
2
100:0 86
100:0 88
91:9 90
52
53
54
58
51
57
33
5.6/7.7 H-BEA
100:0 96
100:0 85
96:4 95
100:0 55
Material study
5.4
5.5
–
FER
H-ZSM5
–
nafion
SnO2/SO4
SiO2/Al2O3
X-ray diffraction (XRD) was used to assess the struc-
tures of as-prepared zeolites. Fingerprints of both
MFI and FAU structures could be evidenced for all as-
synthesised materials from their XRD pattern, as
shown for N1, G1 and Y1 zeolites in Figure 3 and
Figure 4, respectively. Notably, the different as-syn-
thesised ZSM-5 zeolites (N1–N3, G1 and G2) differed
significantly in their respective Si/Al ratio, but dis-
30
–
70[b]
42[b]
59[b]
58
2À
9
10
11
–
–
–
24
83
–
94:6 98[b]
100:0 96
Cs3HSiW12O40
–
0[c]
0[c]
[a] The selectivity is defined by the ratio between moles of product 3 formed divided
by the molar total amount of all products formed. [b] Polymerisation occurred. [c] No
reaction occurred.
specific surface area (SSA) of the different zeolites (SSA values
given in the Experimental Section). Indeed, ZSM-5 zeolite ex-
hibits roughly two-thirds of the SSA of FAU zeolites (Y or USY).
It is therefore sound to investigate parameters other than the
BET area.
The highest methyl acrylate conversion was achieved over
H-BEA zeolite (58%) as already reported by Eklund et al.,[24] but
at the expense of selectivity (Table 1, entry 4). Furthermore, the
screening of zeolites and related solid acids tends to demon-
strate the importance of acid catalysts possessing a highly or-
ganised microporous network in performing the Diels–Alder
reaction. Indeed, the two regioisomers could barely be ob-
tained, neither over strongly acidic Keggin-type heteropolyacid
(Table 1, entry 11) nor over Nafion-H grafted on MCM-41 meso-
porous silica (Table 1, entry 8). Likewise, the activity of highly
acidic sulfated tin oxide[31] also remained limited (Table 1,
entry 9). In contrast, amorphous silica–alumina, which also ex-
hibits strong acidity, led to good methyl acrylate conversion
(58%, Table 1, entry 10). However, its selectivity toward
isomer 3 remained lower than that obtained with medium-
pore-sized ZSM-5 zeolite.
Figure 3. XRD pattern of N1 and G1 zeolites (MFI structure).
played the same porosity and SSA values of 320–370 m2 gÀ1.
Notably, these zeolites exhibit a high crystallinity. To distin-
guish the morphological effects related to crystal size on the
catalyst performances, these materials were observed by SEM.
Figures 5–7 present the micrographs for N1–N3, G1 and G2,
and Y1 samples, respectively. By varying the synthesis duration,
different morphologies and sizes were obtained for the ZSM-5
samples (Figure 5). A micrometre-sized N1 material (nearly 4–
5 mm) was formed by the growth of several nanoplates nearly
100 nm in width and hundreds of nanometres in length (Fig-
ure 5a). In contrast, N2 and N3 samples exhibit a rather spheri-
cal morphology of several micrometres (6–8 mm), and are
formed by the assembly of French fries-shaped nanocrystals as
already reported elsewhere.[25] These materials exhibit a high
roughness, which indicates the presence of a high level of
mesoporosity.[32,33] Energy-dispersive X-ray (EDX) analysis cou-
pled with the SEM chamber confirmed the homogeneous dis-
tribution of Al, O and Si elements, with Si/Al ratios ranging
from 25 to 47 for these MFI materials (Table 2).
To summarise, these preliminary experiments have demon-
strated the importance of using a solid acid catalyst. However,
this is a necessary but non-sufficient condition because other
solid acids, heteropolyacids for instance, were rather inactive.
The microporous architecture of zeolites seems therefore to be
required, since perfluorinated Nafion-H grafted on mesoporous
silica led to 42% methyl acrylate conversion but with almost
no formation of the Diels–Alder products. With the latter cata-
lyst, the diffusion of the reactants throughout the mesoporous
network is a priori accelerated with respect to zeolites.[32,33] It
does, however, seem to have a detrimental effect on the reac-
tion. A compromise has therefore to be found between the
size of the reactants and the pore aperture. Hence, zeolite can
be regarded as a solid solvent, which possibly induces a con-
finement effect facilitating the reactant interaction.[34–38]
G1 and G2 samples exhibit the classical prismatic morpholo-
gy of MFI crystals, being highly elongated with sizes of approx-
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Figure 4. XRD pattern of Y1 sample (FAU structure).
imately 35 and 25 mm, respectively (Figure 6). These two MFI
zeolites can be considered as giant crystals. Figure 7 shows the
pyramidal FAU-type crystals of sample Y1, which have a size
comprised between 300 and 500 nm. For the latter, a high
density of aluminium, Si/Al=2.1, was confirmed by elemental
analysis.
ments between Al species from the zeolite precursors and OH
groups from the sugars during the crystal growth process.[28,43]
However, all these zeolites are highly crystalline and exhibit
the sole MFI topology.
Optimisation: rational design of MFI zeolite
Acidity measurements were conducted according to the iso-
tope exchange of OH/OD groups from the zeolite surface, thus
allowing the quantification of all the Brønsted sites present in
All our as-synthesised zeolites proved to be very active for the
Diels–Alder reaction optimised previously (758C for 24 h). The
selectivity in favour of regioisomer 3 was always between 87
and 91% for the five ZSM-5 materials and the Y1 sample,
whereas variations in methyl acrylate conversion were ob-
served depending on the nature of the zeolite (Figure 8). The
highest conversions were again achieved over ZSM-5 zeolites
(ꢀ52%), except for the N3 sample, which gave a slightly lower
conversion (48%).
Table 2. Crystal sizes, elemental composition and Brønsted acidity of as-
prepared zeolites.
Catalyst
Crystal size
building block
[mm][a]
Si/Al[b]
Number of
Brønsted acid sites
[c]
[mmolgÀ1
]
Despite its high number of Brønsted acid sites (5.3 mmolH+
gÀ1), the Y1 sample only led to a 46% methyl acrylate conver-
sion, in turn confirming that acidity alone is a non-sufficient
parameter. Among the zeolites N1–N3 prepared with sugar-
cane residues, it nevertheless appears that the conversion was
enhanced in line with an increase in the density of Brønsted
acid sites as follows: N3<N1<N2. In contrast, G1 zeolite with
approximately half of the acid sites (0.42 mmolH+ gÀ1) led to
an unexpected 55% conversion in view of its low acid site den-
sity. Such anomalous behaviour might be linked (at least par-
tially) to the larger crystal size and also to a higher crystallinity
of this zeolite prepared by the fluoride-mediated route.[27]
To go further and try to better understand and rationalise
the performances of MFI zeolites, we defined a site time yield
(STY) as the moles of produced adduct 3 relative to the
number of catalyst Brønsted sites per time unit. Figure 9
shows these productivity data for the five ZSM-5 samples. The
values for the N2 and N3 catalysts are in the range of the com-
mercial ZSM-5 zeolite (H-ZSM5 C) used in the preliminary ex-
periments. Interestingly, a roughly 50% higher productivity rel-
N1
N2
N3
G1
G2
Y1
0.1–0.2
0.3–0.4
0.3–0.4
35–40
25
31
25
47
100
90
2.1
1.07
1.33
0.73
0.38
0.42
5.31
0.3
[a] Building block refers to individual crystals. [b] Measured by EDX analy-
sis. [c] H/D isotope exchange technique.
the material.[39,40] Table 2 summarises these values along with
the Si/Al ratio and the crystal sizes of the different zeolites.
Note that all MFI crystals and FAU nanocrystals exhibit acidity
in line with the Si/Al ratio. The two MFI zeolites in the form of
giant crystals possess nearly the same acidities: 0.38 and
0.42 mmolH+ gÀ1 for G1 and G2, respectively. The lower densi-
ty of Brønsted sites in the latter two MFI zeolites has already
been reported for the fluoride-mediated route.[41,42] Surprising-
ly, N1, N2 and N3 materials exhibit rather different amounts of
Brønsted acid sites. The non-conventional sugar-mediated
route probably allowed different interactions and arrange-
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Figure 6. SEM images of (a) ZSM-5 G1 crystals and (b) ZSM-5 G2 crystals.
Figure 5. SEM images of (a) N1 ZSM-5 obtained after 1 day of synthesis,
(b) N2 ZSM-5 obtained after 2 days of synthesis and (c) N3 ZSM-5 obtained
after 6 days of synthesis.
Figure 7. SEM image of FAU-type zeolite crystals (Y1).
ative to N2 and N3 was achieved over N1 catalyst, possibly
owing to the formation of higher-quality crystals (Figure 5a).
More interestingly, a nearly four times higher productivity was
observed for the G1 sample (three times higher for G2) with
respect to N2 and N3 materials.
The outstanding performance demonstrated by G1 and G2
is probably and mainly a result of the extremely long diffusion
length for the reactants throughout the porous network. It is
worth reminding ourselves here that G1 crystals are approxi-
mately six times larger than N1 crystals. However, the reduced
acid site density (three to four times) may also play an impor-
tant role. In addition, the quantity of Brønsted acid sites of the
former is roughly three times lower than that of the latter. One
may therefore expect a high dispersion of these sites within
the giant MFI crystal.
In contrast, the re-adsorption probability should be facilitat-
ed in N1 zeolite, with the risk to perform consecutive reactions,
Figure 8. Methyl acrylate conversion over home-made zeolites. The experi-
ments were conducted under iso-mass conditions.
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Conclusion
Several commercial and specifically designed zeolites have
been studied as catalysts in the isoprene–methyl acrylate
Diels–Alder reaction. Correlations between crystal sizes, porosi-
ties and acidities of the different zeolites examined have been
observed, which led to unexpected but interesting results.
ZSM-5 proved to be the best catalyst, but the synthesis proce-
dure seems critical. Indeed, ZSM-5 exhibiting giant crystal size
combined with a low density of acid sites led to a four times
higher productivity into cycloadduct.
Further studies are in progress to investigate the influence
of the MFI zeolite intrinsic properties on its performance in
other Diels–Alder reactions.
Figure 9. Productivity in regioisomer 3 expressed in mmol per number of
Brønsted acid sites per hour.
Experimental Section
or inhibit the proper geometrical organisation of the two reac-
tants in the vicinity of one or two acid-site neighbours.
Catalyst design
Commercial zeolites H-Y (Aldrich, SSA=655 m2 gÀ1), H-USY (Zeolyst,
CBV500, 621 m2 gÀ1), H-MOR (Zeolyst, CBV20A, 558 m2 gÀ1), H-ZSM5
(Zeolyst, CBV5020, 425 m2 gÀ1), H-BEA (Zeochem, 620 m2 gÀ1), H-FER
(Petrobras, 400 m2 gÀ1) and Nafion-H (Aldrich, 200–250 m2 gÀ1) were
used in their H-form. Prior to use, these catalysts were activated in
an air atmosphere at 5508C during 15 h.
Although not fully understood yet, it seems that peculiar
conditions are brought together within giant MFI crystals to
maximise the interactions between the reagents and the cata-
lyst surface (possibly through long-distance non-covalent van
der Waals bonding). This is in line with the so-called confine-
ment effect favoured in large zeolite crystals. In parallel, it has
been recently shown by FTIR spectroscopy that these giant
MFI crystals exhibit few silanol defects in comparison with con-
ventional ZSM-5 samples.[44,45]
Confinement effects[34–38] in the pores as a result of the long-
range electrostatic field should permit the absorption of reac-
tant molecules in the solid solvent and guide them towards
the strong Brønsted acid sites. Intra-zeolite void volumes sur-
rounding those sites ensure the required “activation volume”
and therefore a tight fit between the reactants and the zeolite
framework.[46]
H-Y zeolite (Y1) was synthesised according to a slightly modified
procedure (see the Supporting Information).[29] In addition, several
H-ZSM-5 zeolites were prepared with different crystal sizes, porosi-
ties and acidities. Three MFI-type samples were prepared according
to our previously reported procedure involving biomass residues
(see the Supporting Information).[25] The samples N1, N2 and N3
were obtained after hydrothermal synthesis in an autoclave at
1708C for 24, 48 and 144 hours, respectively. To investigate the in-
fluence of the crystal size, we also prepared zeolites (G1 and G2) at
neutral pH by the non-conventional fluoride route (see the Sup-
porting Information).[26,42,50–52]
The catalytic data presented herein revealed that a confine-
ment effect may occur in the MFI framework for this Diels–
Alder reaction. However, it appears that a compromise has to
be found between the diffusion length within the crystal and
a proper density of Brønsted acid sites. Hence, a proper Al
pairing in the zeolite framework has to be tailored to optimise
the adsorption/desorption phenomena on the surface. Quan-
tum chemical calculations and statistical studies have demon-
strated the high probability for having a second Al atom in the
next-nearest-neighbour coordination spheres of one Al atom
in MFI zeolites with Si/Al ratios below 50.[47–49] One may there-
fore expect a drastically reduced probability in giant crystals
(G1 and G2) to possess two Brønsted sites in the vicinity of the
reactants, since their Si/Al ratios approach or equal 100
(Table 1). An optimum seems to have been found in these
giant crystals possessing fewer sites to allow a higher produc-
tivity in the Diels–Alder reaction. It is therefore shown that
a special configuration, along with a tight fit in sizes between
methyl acrylate and isoprene within the MFI pores, occurred.
Material characterisation
XRD (Figures 3 and 4), SEM (Figures 5–7) and EDX analysis (see
Table 2) were used to characterise the as-prepared materials. An
evaluation of the Brønsted acidity of the different catalysts was
performed through H/D isotope exchange according to the
method developed by Louis et al.[39,40,44,53] (see Table 2 and the
Supporting Information).
Diels–Alder procedure
Catalytic experiments were performed under iso-Brønsted acid site
conditions by adjusting the catalyst mass, according to the same
procedure applied with each catalyst (see the Supporting Informa-
tion). Under an argon atmosphere, H-USY zeolite (62 mg,
0.24 mmolH+), previously activated at 5508C during 15 h, was
poured into dry heptane (3 mL; or dry cyclohexane) under stirring
at room temperature. Methyl acrylate (0.375 mL) in dry heptane
(2 mL; or dry cyclohexane) was added to the former mixture con-
taining the catalyst. Finally, isoprene (0.25 mL) was slowly added
separately to dry heptane (1 mL; or dry cyclohexane) and the mix-
ture was stirred at 20–908C for 24 hours (see Figure 1). The catalyst
was then isolated by filtration over a Millipore membrane and the
filtrate was analysed by gas chromatography.
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[14] T. Akiyama, H. Morita, K. Fuchibe, J. Am. Chem. Soc. 2006, 128, 13070–
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[17] a) G. L. Woolery, G. H. Kuehl, H. C. Timken, A. W. Chester, J. C. Vartuli,
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Science, Amsterdam, 2009, pp. 237–258.
The non-limiting reagent, methyl acrylate, was used to determine
the conversion. As the isoprene signal was quite close to heptane
in the chromatograms, we decided to focus on methyl acrylate
concentration. The degree of conversion was therefore determined
based on the quantity of methyl acrylate reacted. In addition, an
extrapolated isoprene conversion is given, assuming its sole reac-
tion with the acrylate.
The selectivity toward the regioisomer 3 was calculated as the
ratio of its amount formed divided by the whole quantity of prod-
ucts formed (taking into account the GC response factor through
the use of an external standard). Methyl 4-methylcyclohex-3-ene-
carboxylate 3 appeared as the major product along with regioiso-
mer 4 obtained in lower amount. The yield of isolated 3 obtained
under optimised conditions (758C, 24 h of reaction time) was very
low, in the range of 7% (1–94% reported in the literature).[24] The
experiment was repeated three times but the high volatility of the
product drastically hindered the purification steps. The main by-
product was identified as the isoprene dimer, diprene.[54]
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Revised: July 4, 2013
Published online on August 2, 2013
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