Isoprene by Prins condensation over acidic molecular sieves

Article abstract of DOI:10.1006/jcat.1997.1745

The Prins condensation of isobutylene with formaldehyde over different catalysts (HY, USY, H-ZSM-5, H-Boralite, H-B-MCM-41) having various pore sizes and acidities has been investigated in a pulse microreactor. The results show unambiguously that high sel

Full text of DOI:10.1006/jcat.1997.1745

JOURNAL OF CATALYSIS 170, 150–160 (1997)
ARTICLE NO. CA971745
Isoprene by Prins Condensation over Acidic Molecular Sieves
,1
E. Dumitriu, D. Trong On,† and S. Kaliaguine†
Laboratory of Catalysis, Technical University of Iasi, 71 Prof. D. Mangeron, Iasi-6600 Romania; and †Department
of Chemical Engineering, Laval University, Quebec, G1K 7P4 Canada
Received December 2, 1996; revised April 7, 1997; accepted May 5, 1997
EXPERIMENTAL
The Prins condensation of isobutylene with formaldehyde over
Y zeolites. Ultrastable Y zeolites with different Si/Al
different catalysts (HY, USY, H-ZSM-5, H-Boralite, H-B-MCM-41)
having various pore sizes and acidities has been investigated in
a pulse microreactor. The results show unambiguously that high
selectivities to isoprene are obtained only in the presence of weakly
c
ratios were provided by PQ Zeolites B.V. (Table 1). HY
zeolites were prepared by calcination of an ammonium-
exchanged sample. These samples (except USY-28) were
described in a previous publication (18).
acidic Brønsted acid sites.
1997 Academic Press
H-ZSM-5 zeolites. Two samples with respective Si/Al
of 17.5 and 37.5 were purchased from PQ Zeolites in their
ammonium form. Samples with higher Si/Al ratios ranging
from 110 to 500 were synthesized by a hydrothermal pro-
cedure, using an aerosil as the silicon source, aluminum hy-
droxide, Al(OH)3 (Merck) as the aluminum source, sodium
hydroxide and TPABr as the template. These samples were
calcined at 873K and ion exchanged three timesat 363K in a
0.1 M solution of ammonium chloride, filtered and washed,
and then dried and recalcined at 873 K for 6 h (7).
INTRODUCTION
The Prins condensation of olefins with aldehydes is con-
sidered to be an important organic reaction, since it allows
one to obtain various unsaturated alcohols, glycols, acetals,
and other valuable compounds. This condensation is typ-
ically catalyzed by strong mineral acids such as H2SO4 in
homogeneous catalysis (1), but in some applications, e.g.,
the synthesis of isoprene, an increasing interest in hetero-
geneous catalysis must be noted (2–7).
For isoprene synthesis the most promising results have
been obtained using zeolites as catalysts. The reaction net-
work shown in Fig. 1 was suggested for the reaction of
isobutylene and formaldehyde using 12- and 10-ring zeo-
lites (8).
If, over 12-ring zeolites route (a) is favored, when the con-
densation is performed over 10-ring zeolites, in accord with
expected shape-selective constraints, less 4,4-dimethyl-1,3-
dioxane is formed, and route (b) can be considered as
favored. The condensation reaction usually proceeds by
route (c) at low temperatures. Moreover, as previously
shown (6, 7), this network is complicated by a parallel net-
work involving the oligomerization-cracking reactions of
the olefins and dienes, as well as their aromatization.
Therefore, producing isoprene selectively is a difficult
task, since the ideal catalyst must assure a high selectiv-
ity not only for the primary Prins condensation to isoprene,
but for the entire process. The aim of the present study
is to compare the relative importance of acidic properties
and pore geometry to catalyst selectivity in this reaction.
These parameters are varied by changing the catalyst from
Y zeolites to H-ZSM-5, boralites, and boron-MCM-41.
Boralites. A series of boralites with different boron
contents were prepared from tetraethylorthosilicate
(TEOS), boric acid (H3BO3), tetrapropylammonium hy-
droxide (TPAOH), and water by a method described pre-
viously (9, 10). The composition of the alkali-free gel was
–
SiO2 xB2O3–0.40 TPAOH-35 H2O (where x = 0.1 0.5).
The gel was charged in a Teflon-lined autoclave and main-
tained under hydrothermal conditions for 5 days at 448 K.
The solids were filtered, washed with distilled water, dried
at 357 K, and calcined in a horizontal tubular quartz reac-
tor under a continuous flow of either dry ammonia or air at
823 K for 6 h (heated from room temperature to 823 K at
the heating rate of 2 K/min).
Ammonium and sodium exchange were carried out by
stirring 0.5 g of boralite (which was treated in NH3 at 450 C
for 4 h) in 30 ml of 1 mol liter 1 aqueous ammonium nitrate
and sodium bromide solutions, respectively, for 24h at room
temperature.
B-MCM-41. The hydrothermal synthesis of B-MCM-
41 was carried out using gels with the molar composi-
–
–
–
–
–
tion SiO2 0.05B2O3 0.27CTACl 0.13Na2O 0.26TMAOH
–
0.27NH4OH 60H2O (CTACl, cetyltrimethyl ammonium
chloride; TMAOH, tetramethylammonium hydroxide). A
typical synthesis procedure was described earlier (11, 12).
1 To whom correspondence should be addressed.
150
0021-9517/97 $25.00
c
Copyright
1997 by Academic Press
All rights of reproduction in any form reserved.

ISOPRENE BY PRINS CONDENSATION
151
at room temperature on a Bruker ASX 300 spectrometer
at a resonance frequency of 96.25 MHz. 11B MAS–NMR
spectra were obtained with a 90 -pulse duration of 2.5 s,
repetition time of 2 s, and spinning rate of 3.5 kHz. Chem-
ical shifts were determined relative to BF3 O(C2H5)2.
Reagents. Tert-butyl alcohol (TBA), Fluka p.a., was
used to generate in situ isobutylene (iB). Fresh aqueous
solutions of formaldehyde were prepared by the thermal
depolymerization of paraformaldehyde. The concentration
of formaldehyde (34 wt% ) was determined by chemical
methods.
Catalytic tests. The condensation reaction was carried
out in a pulse microreactor under the following conditions:
weight of catalyst 0.04 g (granules of pure sieves, 0.25–
0.5 mm grain size); nitrogen flow rate 20 ml/min, unless
otherwise indicated; pulse size 0.4 l; equimolar reaction
mixture, unless otherwise stated. The liquid reagents were
fed to the reactor via an injector maintained at the reaction
temperature. Separate tests were performed, which showed
that for all tested catalysts a pulse of pure TBA is totally de-
hydrated to isobutylene at temperatures above 443 K. The
reaction products were analyzed using an on-line GC with
FID or TCD and various columns. A detailed description
of the catalytic tests is given in an earlier paper (6).
FIG. 1. Reaction pathways for Prins condensation of formaldehyde
and isobutene over various acidic 12- and 10-ring zeolites.
The homogeneous gel was transferred into a Teflon-lined
autoclave and heated to 423 K for 48 h. The solid products
were filtered, washed with distilled water, dried in air at
353 K, and finally calcined in flowing air at 813 K for 6 h
(heated from room temperature to 813 K at the heating
rate of 1 K/min). The pure-silica MCM-41 was synthesized
using the same procedure, except that no boron was
added.
Elemental analysis was performed by atomic absorption
spectroscopy for sodium and silicon, whereas ICPAES (in-
duced coupled plasma atomic emission spectroscopy) was
used for boron estimation. Powder X-ray diffraction pat-
terns of the samples were recorded on a Rigaku D-MAX
II VC X-ray diffractometer using nickel-filtered CuK
CHARACTERIZATION RESULTS
As mentioned above, descriptions of the Y and the
ZSM-5 samples were reported previously (7, 18). Results
obtained with these samples are reported here for compar-
ison with H-boralites and H-MCM-41. Thus only a detailed
characterization of the latter two types of catalysts is re-
ported here.
˚
( = 1.5406 A) radiation. Surface area measurements were
carried out using an Omnisorp-100 apparatus following the
BET procedure. IR spectra were recorded on a Digilab
Boralites
2
FTS-60 spectrometer, using wafers of 10 mg cm treated
in a vacuum cell at 773 K for 4 h. The samples were cooled
to room temperature under vacuum and then immediately
exposed to pyridine vapour for 15 min. Desorption of pyri-
dine was carried out by evacuation for 4 h at different tem-
peratures. 11B MAS–NMR measurements were performed
X-ray diffraction. The X-ray diffraction (XRD) pattern
of the boralite sample is characteristic of the orthorhom-
bic MFI structure. Considering the boron-free silicalite as
100% crystalline, the crystallinity of boronsilicalite sam-
ples was found to be 95 5% . The unit cell parameters
were calculated from Rietveld refinement of the powder
patterns using silicon as an internal reference. The 2.9%
3
˚
boron leads to a decrease of the unit cell from 5335 A for
TABLE 1
3
˚
the boron free silicalite to 5280 A (Table 2), which is consis-
tent with previous reports (13). This observation is logical
The Main Characteristics of Y Faujasites
˚
–
–
on account of B O bond being shorter (1.47 A) than Si O
SiO2/Al2O3
molar ratio
Unit cell size
Surface area
1
Sample
(A)
(m2 g
)
(1.61 A) and thereby supports the incorporation of boron
˚
˚
in the framework lattice.
HY
4.6
5.2
11.5
28.2
80
Framework IR. Typical IR spectra of the as-synthesized
and calcined boralite samples are given in Fig. 2. In all cases,
the IR spectra were typical of pentasil zeolites. The well-
USY-5
USY-11
USY-28a
USY-80
24.33
24.33
660
730
24.25
780
1
defined IR bands at 800 and 455 cm and the saturated
one in the 1000–1300 cm 1 range are characteristic of SiO4
a By steaming of USY-11.

152
DUMITRIU, TRONG ON, AND KALIAGUINE
presence of the tetracoordinated framework boron and it
TABLE 2
is observed in all boron pentasils. However, for the calcined
sample a strong band also appears at 1380 cm 1, which can
be assigned to tricoordinated framework boron (9, 10, 15).
The transformation of the absorption band at 920 cm 1 to
one at 1380cm 1 upon calcination islogicallyassigned to the
change of boron coordination from tetrahedral to trigonal.
On the other hand, after the NH3 treatments (Fig. 2b), fol-
lowed by Na+-exchange (Fig. 2c) and calcination (Fig. 2d),
the IR spectra do not show the characteristic BO3 absorp-
tion at 1380 cm 1 consistently with the literature (15). It is
concluded that tetrahedral framework boron is stable when
NH+4 or Na+ ions are counterbalancing the framework neg-
ative charge and unstable when the counterion is the proton
(Fig. 2e) (9, 15, 19).
NMR. Figure 3 shows the 11B NMR spectra of as-
synthesized and calcined BS-3.0 samples. A single narrow
peak observed at 4.0 ppm for the as-synthesized sample
confirms that most of the boron has been incorporated as
tetrahedral BO4 units in the framework. Two very broad
lines of much weaker intensity around 2 ppm correspond
to framework-linked trigonal boron (9, 10, 15). Upon re-
moval of the template after calcination in flowing oxygen
the intensity of the line representing the tetrahedral frame-
work boron is decreased and is slightly shifted to 3.7 ppm,
while the trigonal framework boron lines are relatively in-
creased, with an extra hump appearing at 6 ppm (Fig. 3b).
This transformation is due to the loss of framework boron
during calcination (Fig. 3 and Table 2). The additional hump
at 6 ppm can be assigned to extraframework trigonal
boron, consistently with the same signal obtained for sil-
icalite impregnated with boric acid B(OH)3. The observed
Concentrations of the Boron Species in Boralites (BS), as
Determined by 11B MAS–NMR and Chemical Analysis
NMR
Line 1b
3.7 ppm) (
Line 2
Line 3
Chemical
Samplea
(
2.0 ppm) ( 6 ppm) Total analysis
B/u.c
B/u.c.
B/u.c.
B/Si
(% )
4.9
B/Si
(% )
8.5
[as-synth]BS-8.5
3.8
3.5
0.3
0.4
0.6
—
[as-synth]BS-4.0
[NH+4 ]BS-3.9
[H+]BS-3.8
4.2
4.0
3.9
3.8
3.4
[Na+]BS-3.4
[as-synth]BS-3.0
[H+]BS-2.9
2.8
2.1
1.7
0.1
—
—
—
0.1
—
3.2
2.3
1.8
3.0
2.9
2.4
[H+]BS-2.4c
[as-synth]BS-2.0
[H+]BS-2.0
2.1
2.0
[as-synth]BS-1.5
[Na+H+]BS-1.5
[H+]BS-1.5
1.4
1.0
0.1
0.4
—
—
1.6
1.5
1.5
1.5
1.5
a
Sample referred to as [f] BS-r, where f and r are the form and B/Si
ratio (% ) of boralite, respectively.
b Amounts estimated by using the calibration curve for framework tetra-
hedral boron at 3.7 ppm. The values of the line at
be quantitative and are given as trend.
c Washed with distilled water.
2 ppm might not
1
tetrahedron units, while the vibrational mode at 555 cm
attests the presence of the five member rings of the pentasil
silicalites. In addition to the intense absorption of pentasils,
the as-synthesized sample exhibits weak bands (1470, 1455,
and 1380 cm 1) due to TPA+. Upon ammonia treatment
(Fig. 2b), residual TPA+ (1380, 700, and 670 cm 1) is also
1
seen (14). The band at 920 cm can be assigned to the
FIG. 2. IR spectra of the boralites: (a) as-synthesized BS-40, (b)
[NH+4 ]BS-3.9, (c) [Na+]BS-3.4 before calcination, (d) [Na+]BS-3.4 after
calcination at 823 K, and (e) [H+]BS-3.8 before calcination.
FIG. 3. 11B MAS–NMR spectra of boralite, BS-3.0: (a) as-synthesized
and (b) calcined sample.

ISOPRENE BY PRINS CONDENSATION
153
small shift at lower fields (from 4.0 to 3.7 ppm) for the
tetrahedral boron signal after different treatments can be
explained by a change in the chemical environment (10, 15).
As the NMR line at
3.7 ppm is very sensitive to the
local structure and the electronic environment, a quantita-
tive determination of tetrahedral boron content in the stud-
ied samples can be made by measuring the intensity of the
NMR line at 3.7 ppm and using a calibration curve such
as the one in Ref. (10). Table 2 reports semiquantitative in-
formation on other estimated boron states. The total boron
content does not change much during the different treat-
ments as observed by chemical analysis. The extraframe-
work and trigonal boron are, however, strongly affected by
various postsynthesis treatments as seen from 11B NMR.
They can for example be extracted from the MFI chan-
nels by washing at room temperature with distilled water
(Table 2, sample BS-2.4).
The 11B NMR spectra are similar for the as-synthesized
and NH3 calcined samples. The amount of trigonal boron
only slightly increases during the NH3 treatment; NH4+
cation is charge balancing and, consequently, BO4 is stable
(Table 2). After Na-exchange with aqueous 1 M NaBr at
room temperature, the trigonally coordinated framework
boron signal disappears. A small amount of boron is, how-
ever, extracted again after the ammonia treatment. No loss
of framework boron can be observed in the sodium form
before and after calcination.
FIG. 4. 11B MAS–NMR spectra of B-MCM-41: (a) as-synthesized
and (b) calcined sample.
sample, indicating only the presence of tetrahedral boron
species. The 11B MAS–NMR spectrum of the calcined sam-
ple shows the simultaneous presence of both tetrahedral
and trigonal framework boron.
MCM-41 Materials
Acidity
The chemical composition of solids and the BET surface
area of B-MCM-41 and its pure silica analogue are summa-
rized in Table 3. BET surfaces were 950 and 900 m2/g for the
pure silica and B-MCM-41, respectively. The X-ray diffrac-
tion pattern of the calcined B-MCM-41 sample matches
well that of the calcined boron free silica MCM-41 and also
the patterns reported by Kresge et al. (16). One major peak
Figure 5 shows the IR spectra of pyridine adsorbed on
pure silica MCM41, B-MCM-41, Al-MCM-41 and Fig. 6
at 2
1.6 and three peaks at 2
2.8, 3.3, and 4.61 were
observed. Beck et al. (20) indexed these peaks for a hexa-
gonal arrangement of unidimensional pores.
11B MAS–NMR spectra of the as-synthesized and cal-
cined forms of B-MCM-41 are shown in Fig. 4. Only one
peak at 2.5 ppm was exhibited by the as-synthesized
TABLE 3
Chemical Composition (Atomic Ratio) and Structural Properties
of the Calcined MCM-41 Materials
M/Sia
(% )
Na/M
(% )
XRD d100
BET surface
area (m2/g)
˚
Catalyst
d-spacing (A)
Pure silica MCM-41
B-MCM-41
Al-MCM-41
—
2.4
4.5
—
30
50
57
55
60
950
900
650
FIG. 5. IR spectra of pyridine after desorption at (A) 323 K and (B)
373 K: (a) pure silica MCM-41, (b) B-MCM-41, and (c) Al-MCM-41.
a M = B and Al, as per catalyst designation.

154
DUMITRIU, TRONG ON, AND KALIAGUINE
1
B-MCM-41 IR spectrum exhibits a new band at 1462 cm
,
indicatinga higher amount ofLewissitesascompared to the
Al-MCM-41. The acid strength of B-MCM-41 sample was
found almost identical with that of the Al-MCM-41 sam-
ple. The B-MFI sample has a stronger acidity and a higher
amount of acid sites than B-MCM-41 and Al-MCM-41.
From the thermal stability of chemisorbed pyridine, the or-
der of acid strength is found to be pure silica MCM-41 < B-
MCM-41 Al-MCM-41 B-ZSM-5.
REACTION RESULTS
As shown previously (6, 7), the condensation of formal-
dehyde with isobutylene can proceed over solid acid cata-
lysts like zeolites, but it is well known that the acidity of a
zeolite is connected with its behavior as catalyst in a com-
plex fashion. Therefore, it is essential to analyze this behav-
ior in relation to the nature, the number, and the strength
of acid sites present in zeolites and molecular sieves.
FIG. 6. IR spectra of pyridine on boralite, BS-3.0 after desorption at
different temperatures: (a) room temperature, (b) 323 K, (c) 423 K, (d)
473 K, and (e) 573 K.
Prins Condensation over Faujasites
The ability of zeolites to catalyze the Prins condensation
was established for the first time by Venuto and Landis (2),
when this reaction was performed on Y faujasite. Unfortu-
nately, this catalyst has a poor selectivity, a wide distribution
of products being obtained. Table 4 shows a typical distri-
bution of the hydrocarbons and 4,4-dimethyl-1,3-dioxane
obtained when a USY sample was used.
It can be observed that in addition to the condensation
of formaldehyde with isobutylene to form isoprene (what-
ever the route of the reaction from Fig. 1) a large amount
of secondary products, such as aromatic hydrocarbons, re-
shows the IR spectra of boralites all pretreated under simi-
lar conditions in the IR cell. The [H+]BS-2.9 sample of bo-
ralite containing 2.9 at% B was studied for comparison with
1
the MCM-41 analogues. The IR broad band at 1380 cm
characteristic of framework trigonal boron as well as the
920 cm 1 peak characteristic of framework boron was ob-
served for this sample. It is seen that pyridine adsorbed on
the pure silica MCM-41 sample exhibits IR bands at 1441,
1
1446, and 1600 cm 1. The bands at 1441 and 1600 cm
,
–
sults. Also, a significant amount of C1 C3 aliphatic hydro-
which are characteristic of physisorbed pyridine, decreased
carbons is formed (mainly from the cracking of oligomers)
but their proportion decreases as the strong acid sites are
1
sharply upon evacuation at 373 K. The band at 1446 cm
has been assigned to hydrogen-bonded pyridine (hydroxyl
groups which do not protonate pyridine). The B-MCM-41,
Al-MCM-41 and B-MFI samples show additional bands
which are absent in pure silica MCM-41 (Figs. 5 and 6). Two
very weak bands at 1545 and 1455 cm 1, which are charac-
teristic of Brønsted-acid and Lewis sites were observed in
the IR spectrum of Al-MCM-41, as well as another band
due to contributions of both Brønsted-acid and Lewis sites
TABLE 4
Results of Prins Condensation over USY-80 (Si/Al = 40) at 573 K
Products
2nd pulse
10th pulse
Conversion of isobutylene (% ):
76.8
60.7
at 1492 cm 1. The B-MCM-41 sample exhibited a very weak Distribution of products (wt% ):
band at 1545 cm 1 and a new band at 1462 cm 1; the latter
has been assigned to strong Lewis sites generated by the
polarization effect of hydroxyl nests and/or an electrophilic
B atom (Figs. 5 and 6) (17).
The IR spectra of the B-MFI sample showed the bands
at 1446, 1462, 1490, 1545, and 1600 cm 1, even after evac-
uation at 573 K, while these bands had disappeared in
B-MCM-41 and Al-MCM-41 samples, upon evacuation at
lower temperature, ca. 373 K (Figs. 5 and 6). Finally, weak
and mild acid sites (Brønsted and Lewis) were detected in
the B-MCM-41 and the Al-MCM-41 samples. However, the
–
C1 C3
39.0
23.2
4.0
24.0
39.3
6.6
C4
Isoprene
–
C6 C8 aliphatics
3.7
2.9
Aromatics
DMDa
25.2
4.7
25.4
1.7
Distribution of aromatics (wt% ):
Benzene
Toluene
5.30
5.4
6.8
6.8
Xylenes + ethylbenzene
29.8
59.6
37.0
49.4
C9+
a DMD, 4,4-dimethyl-1,3-dioxane.

ISOPRENE BY PRINS CONDENSATION
155
framework aluminum from ZSM-5 is responsible for the
formation of strong acid sites which play the role of active
sites for various reactions and that the pore structure of
ZSM-5 leads to various types of shape selectivity: reactant
shape selectivity, product shape selectivity, transition-state
shape selectivity (19). Moreover, this zeolite has a longer
catalyst life than the Y faujasites due to its resistance to
coking.
Thus, Chang et al. (3) were the first to show that the for-
mation ofDMD can be suppressed due to the molecular size
constraint imposed by the pore system of ZSM-5. However,
secondary reactions of olefins cannot be avoided.
FIG. 7. Isoprene (I) to 4,4-dimethyl-1,3-dioxane (DMD) and aromat-
ics (Ar) weight ratio and Isoprene weight fraction in the products after
Prins condensation over H-Y zeolites of various Si/Al ratio, T = 573 K;
average value of 9–11th pulses.
Our results confirm these findings. In the experiments re-
ported in Table 5, for example, no DMD was detected but
the selectivity to isoprene is still low, even lower than in
the presence of USY-80. Compared to the data shown in
Table 4 for USY-80, the fraction of aromatics in the prod-
ucts is decreased as well as the percent of C9+ in the aro-
matics. The I/Ar weight ratio is still low but when the test
was conducted with H-ZSM-5 samples of higher Si/Al ratio
(Fig. 8a), this ratio increased dramatically, reaching a value
close to 6 at Si/Al = 500. The conversion of isobutylene is,
however, decreased due to the decreasing number of acid
sites (Fig. 8b). It is well known that as the silicon to alu-
minum ratio of H-ZSM-5 is increased, the concentration of
Brønsted acid sites decreases, but the strength of the acid
sites is only mildly affected.
progressively poisoned by coke (see the second column of
Table 4). The high content of hydrocarbons in the products
is due to the presence of rather strong acid sites. The signifi-
cant fraction of bulky C9+ aromatics must also be associated
with the supercages of faujasites, cages which provide suffi-
cient space for their formation. A part of these products re-
sults from the oligomerization and aromatization of olefins.
Another part could also result from the disproportionation
of xylenes (18).
In their early study, Venuto and Landis (2) have tested
a single sample of Y faujasite. Their initial conclusion was
that the desired selectivity would only be achieved by a
proper choice of pore size. As shown in Fig. 7, another fac-
tor, namely the Si/Al ratio has a very important influence
on selectivity.
Prins Condensation over H-Boralites
Taking into account the above results, further investiga-
tions were focused on boralites, since this kind of molecu-
lar sieve is recognized as a weak acid catalyst. Indeed, the
Prins condensation has been performed using samples of
H-boralites with different boron content (see Table 2) and
Our previous characterization of the same samples per-
–
formed by IR, XPS, and NH3 TPD showed that the concen-
tration of Brønsted acid sites decreases in the order HNaY
> USY-5.2 > USY-11.5 > USY-80 (18). In other words, as
expected, the Brønsted acid site concentration decreases as
the Al content is decreased. It is seen from Fig. 7 that both
the isoprene to dimethyldioxane (I/DMD) and the isoprene
to aromatics (I/Ar) weight ratios increase as the Si/Al ratio
increases. This suggests that a decreased concentration of
Brønsted acid sites favors the selectivity to isoprene. The
ultrastabilization process, however, changes not only the
Si/Al ratio but also the acid strength distribution. Both fac-
tors may therefore affect the selectivity. Moreover the low
value observed for the I/Ar ratio (well below 1) indicates
a severe aromatization of olefins. These results show that
a decrease in the strength of the Brønsted acid sites is re-
quired in order to improve the selectivity to isoprene in the
Prins condensation of isobutylene and formaldehyde.
TABLE 5
Results of Prins Condensation over HZSM-5
(Si/Al = 17.5) at 573 K
Products
2nd pulse
10th pulse
37.2
Conversion of isobutylene (% ):
Distribution of products (wt% ):
nd
–
C1 C3
C4
65.9
14.5
1.6
17.3
60.7
3.9
Isoprene
–
C6 C8 aliphatics
3.6
1.0
Aromatics
14.4
17.0
Distribution of aromatics (wt% ):
Benzene
Toluene
3.16
18.2
48.9
29.7
2.2
14.7
56.9
26.2
Prins Condensation over H-ZSM-5 Zeolites
Xylenes + ethylbenzene
Some improvement of the isoprene selectivity was
achieved by using ZSM-5 zeolites instead of Y zeolites as
catalysts for Prins condensation. It is well known that the
C9+
Note. nd, not determined.

156
DUMITRIU, TRONG ON, AND KALIAGUINE
FIG. 10. The influence of boron content of H-boralites on the conver-
sion to isoprene: 30 ml/min flowrate of N2.
tion reaction was carried out at two different temperatures,
namely 498 and 573 K. These results were compared with
those obtained over silicalite (the sample without boron),
and they are shown in Fig. 10 (average values of pulse 2–4).
The results in Fig. 10 indicate that the conversion to iso-
prene increases steeply as the boron content is raised to 4% ,
which is the isomorphous substitutional limit for boron in
boralites (10). Above this limit only a minor increase is
observed, indicating that extraframework boron has essen-
tially no activity.
FIG. 8. (a) Isoprene to aromatics (I/Ar) weight ratio and isobutylene
conversion vs Si/Al in H-ZSM-5 zeolites; T = 573 K, 10th pulse. (b) Overall
conversion of isobutylene vs Si/Al ratio at 300C, 10th pulse.
The influence of contact time on the conversion to iso-
prene was also investigated. The contact time was varied
by changing of the carrier gas flow rate, over the range
20–60 ml min 1. The results are shown in Fig. 11 for [H]BS-
2.9 and H-ZSM-5, and they indicate that the conversion
to isoprene generally increases as the contact time is de-
creased. There is, however, a difference in behavior be-
tween the two catalysts. Over H-boralite, the conversion
shows a uniform and slight increase as the contact time is
decreased. For HZSM-5 (SiO2/Al2O3 = 35), a maximum is
observed at about 40 ml min 1, after which a slight de-
crease in conversion occurs.
all samples showed a high selectivity to isoprene ( 100% )
over a large range of temperature.
Figure 9 shows the conversion to isoprene obtained
(average of second and third pulses) with the H-boralite
sample, [H]BS-2.9 at various temperatures over the
range 448–663 K. A maximum conversion is observed at
473 K. Above this temperature, the conversion decreases
smoothly. It must be underlined that these conversions are
also isobutylene total conversions as the selectivity is es-
sentially 100% for all these tests. Thus compared to ZSM-5,
boralites seem to be of much higher interest for industrial
application both because of their high selectivity and their
low operation temperature.
An increase in reaction rate with flow rate is usually di-
agnostic for external mass transfer limitations. This may
therefore be the case with the H-boralite sample, but the
A series of experiments have been conducted using bo-
ralite samples with various boron contents, the condensa-
FIG. 11. Conversion of isobutylene to isoprene over H-boralite and
H-ZSM-5 (Si/Al = 17.5) vs carrier gas flowrate, reaction temperature
501 K.
FIG. 9. Conversion to isoprene over H-boralites vs reaction temper-
ature; mole ratio FA/iB=1/1.

ISOPRENE BY PRINS CONDENSATION
157
situation is less clear with the H-ZSM-5 tests, where selec-
tivity to isoprene is not 100% and the secondary reactions
of olefins are also affected by the change is contact time.
The change in conversion with varying molar ratio of
reagents in the feed was further studied over [H+]BS-2.9
catalyst, at 498 K. In Fig. 12 the conversion to formaldehyde
is reported since isobutylene was in excess.
The conversion of formaldehyde increases significantly
(more than twice) as the mole ratio of isobutylene to
formaldehyde is increased. Thus both high conversions and
100% selectivity can be simultaneously achieved. This cata-
lytic performance is better than those previously reported
for H-ZSM-5 (7), and it may be considered as interesting
for industrial application.
FIG. 13. Conversion to isoprene over B-MCM-41 vs reaction temper-
ature; mole ratio FA/iB = 1/1.
prene in a manner similar for boralites, since as reported
in Fig. 13, a smoothly decreasing conversion is obtained as
temperature is raised. It is believed that this minor decrease
Prins Condensation over H-B MCM-41
MCM-41 material is one of the members of the relatively
new class of siliceous mesoporous materials M41S (16, 20).
This material has attracted much attention due to its hexag-
onal arrangement of unidimensional pores, varying in size
– –
is due to partial hydrolysis of some Si O B bridges under
the influence of the water vapor generated by the reaction
(11, 12).
˚
˚
from around 15 A to more than 100 A. By comparison with
zeolites, this material opens new possibilities for process-
ing large molecules. Moreover, its acid properties can be
modified by the substitution of silicon with boron (11, 12).
Therefore, taking into account both its very mild
Brønsted acidity and large pore opening, B-MCM-41 has
been tested as catalyst for the Prins condensation of
formaldehyde with isobutylene. The experiments were per-
formed over the range of temperatures 448–623 K, and the
conversion to isoprene results are shown in Fig. 13.
DISCUSSION
In order to appreciate the role of the acid catalytic sites in
the isoprene synthesis by Prins condensation, the reactivity
of the species involved in this process must first be consid-
ered. As shown in Fig. 14, the charge density of the most
important reactive species—formaldehyde, isobutene, and
isoprene—was calculated (CNDO/2), and based on these
data their reactivity has been interpreted.
The most important observation which could be drawn
from this Fig. 14 consists of the following order of the po-
larization of unsaturated linkages:
The conversion to isoprene is lower than the ones ob-
tained using boralites. The selectivity, however, is kept at
the highest level (100% ). It is interesting to note that even
though the catalyst has very large pores, 4,4-dimethyl-1,3-
dioxane was not observed as a reaction product. This result
suggests that the DMD route to isoprene (see Fig. 1) re-
quires stronger Brønsted acid sites than the one present in
B-MCM-41. Temperature influences the conversion to iso-
e
e
e
==
==
==
C CH2
% C O > % C CH2 >
==%
Since oxygen is more electronegative than carbon,
formaldehyde shows a dipole moment, and sometimes the
carbonyl group is best represented as a hybrid structure,
with charges at both atoms. Therefore, due to its bipolar
nature, the addition to the carbonyl linkage can be easily
initiated bythe attack ofan electrophile to the oxygen atom,
such as the proton from a Brønsted acid site, as depicted in
Scheme 1.
As the formed cation (b) can be added to the double
bond of isobutylene, the reaction network from Fig. 1 is ini-
tiated: of course, other types of reactions are also initiated
such as the reactions of electrophilic substitution (e.g., the
alkylation of aromatic hydrocarbons, etc.) or the attack of
a nucleophile (Nu( ) or Nu:) on the carbon of the carbonyl
group.
Regarding the reactivity of isobutylene, due to the elec-
trons, the olefin might be adsorbed at a protonic acid site
even at low temperature, forming a very reactive complex
(see Scheme 2).
FIG. 12. Influence of mole ratio of reagents in feed on conversion to
isoprene over H-boralite ([H]BS-2.9) T = 500 K, carrier gas (N2) flowrate:
1
30 ml min
.

158
DUMITRIU, TRONG ON, AND KALIAGUINE
FIG. 14. Charge density of the reagents.
Carbocation (b in Scheme 2) may further react in the activity. The condensation performed on HY faujasite leads
following ways: addition to an unsaturated linkage, inter- to a large distribution of products (2). When the condensa-
molecular hydride transfer, attack of a nucleophile, etc. tion reaction is carried out over USY a wide distribution of
Continued structural transformations, such as cyclization, products also results, but the proportion of DMD and aro-
ring contraction, and ring expansion, can also occur. There- matic hydrocarbons decreases as the Si/Al ratio increases
fore, a parallel network of reactions involving the unsat- (see Table 4 and Fig. 7). An explanation for this behavior
urated compounds in various processes such as oligomer- is given below.
ization, cracking, aromatization, dealkylation, alkylation,
The proportion of weak and strong sites changes sub-
and isomerization could take place. All these pathways are stantially in the case of USY. Macedo et al. (22), as well
acid-catalyzed reactions which can easily proceed on zeolite as other authors (23, 24), studying HY zeolites dealumi-
catalysts.
nated by steaming, found that the strength of intermediate
The catalytic sites in aluminosilicate zeolites are asso- sites decreased with increased dealumination for Si/Al ra-
ciated with tetrahedral aluminum atoms in substitutional tios from 8 to higher than 100. Also, Yaluris et al. (25) found
positions in the framework of silica. The acid strength of from differential heats of ammonia adsorption that the acid
–
–
the bridging hydroxyls in zeolites, Si OH Al, depends both site strength distribution shifts toward weaker sites when
on the geometry of the bridge (bond distances and bridge the catalyst is steamed. Steaming results in the preferen-
bond angle) and on the number of Al atoms close to the tial loss of Brønsted acid sites. This conclusion agrees with
bridge. The effect of increasing OH dissociation energies earlier work (24) indicating that the sites of intermediate
for increasing Al content close to the bridging hydroxyl has strength in Y zeolitesare predominantlyBrønsted acidsand
been discussed by Kazansky (21). A given acidic zeolite their number and strength are reduced with steaming. As
usually does not have a single class of acidic sites, but nor- Brønsted acid strength decreases, the fractional carbenium
mally shows a distribution of acid strengths. Thus various ion coverage on USY decreases; strong Brønsted acidity is
molecular species can be activated and different reactions indeed necessary to stabilized carbenium ions on the sur-
may occur. In consequence, the selective synthesis of iso- face. Changes in coverage have a direct effect on selectivity.
prene, by the condensation of formaldehyde with isobuty- Also, a decrease of unit cell size by steaming, which affects
lene over acid molecular sieves, is a difficult task since both the selectivity, might be considered, even though this effect
the formation of DMD and aromatic hydrocarbons must is not likely to be significant.
be suppressed.
Taking into account the above-mentioned data, obtained
Y zeolite possesses a large distribution of acid strengths, over Y zeolites, two main factors could be considered in
and a system of supercages, connected by a tridimensional order to achieve a selective synthesis of isoprene: the size
array of large-diameter channels, which enables a much of pores, which must be smaller, and the acid properties of
easier diffusion of reactants and products, favoring a high the catalyst as the acidity of catalyst must be shifted toward
weaker acid strength.
SCHEME 1
SCHEME 2

ISOPRENE BY PRINS CONDENSATION
159
The influence of these factors on the selectivity in the is activated leading to the reactive cation (b in Scheme 1),
Prins condensation must now be examined for MFI cata- whereas the isobutylene is mainly -adsorbed on the sur-
lysts. It is well established that Al-ZSM-5 zeolite has two face of catalyst (see Scheme 2, species (a)). As is obvious
types of Brønsted acid sites (26), one strong and one weak from the high selectivity to isoprene observed, all secondary
site in smaller concentration. Therefore, a variety of reac- reactions of olefins are suppressed and the -adsorbed
tions from both reaction networks can be initiated. More- isobutylene can only react to produce the tertiary carbo-
over, it is known that due to the strong Brønsted acid sites cation described in Fig. 1. Moreover this cation must des-
the reactive olefins can react rapidly even at room temper- orb rapidly from the weak SiOHB site leaving no time for
ature to form large oligomers (27, 28). The reactivity of its reaction with either formaldehyde or water (routes (a)
the alkenes increases as the stability of the carbenium ion and (c) in Fig. 1). Thus only route (b) is responsible for the
formed is increased. The oligomers crack to form a large formation of isoprene.
number of products, beginning at 400 K, and this crack-
The instability of boron during calcination of template
ing temperature for the oligomers is an additional measure containing B-MFI is well known (36, 38, 39) and could be a
of the site activity (27). Since the cracking activity depends problem for industrial application of boralites in the Prins
on the strength of the acid site, it can be considered as a condensation. However, Kuehl (40) reported highly stable
measure for the acid properties of the catalyst. As previ- lattice boron (<5% loss) upon heat treatment of boron-
ously reported (6) the cracking activity disappears as the containing molecular sieves is an ammonia atmosphere.
strong acid sites are blocked by coke. Thus the selectivity This was also confirmed by van Bekkum et al. (15) and
increases as the acid strength decreases. Also we demon- Trong On et al. (10).
strated by progressive poisoning of acid sites by pyridine
Up to this point the selectivity to isoprene may have been
(6) that a high selectivity to isoprene can be achieved when considered to be governed by two factors: (i) the size of
the catalyst possesses only weak to medium acid strength. pores, which acts suppressing the formation of bulky prod-
This was also confirmed when the silica to alumina ratio was ucts such as DMD and C9+ aromatics, and (ii) the acid
increased (7), but unfortunately the conversion strongly de- strength of the active sites. However, as the results ob-
creased due to the decrease in active sites concentration. tained over B-MCM-41 showed, a high selectivity may be
The results obtained in the Prins reaction performed over achieved, even if the catalyst has large pores, but the active
H-ZSM-5 catalysts clearly revealed that the formation of sites are characterized by a weak acid strength. Therefore,
DMD can be suppressed (presumably due to the pore struc- we can assume that the acid strength of the catalytic sites
ture) and that most secondary reactions of olefins can be is the main factor which governs the selectivity in the Prins
diminished or suppressed by decreasing the acid strength of condensation.
the catalytic site. Like the faujasites, MFI catalysts having
strong acid sites activate both reagents to the correspond-
ing cations (see Schemes 1 and 2), and therefore various
reaction routes are opened. In conclusion, it is necessary
to modify the acidity of these zeolites in order to achieve
selectivity for isoprene in the Prins reaction and one way of
doing this is the isomorphous replacement of Al by other
trivalent ions.
CONCLUSION
The isoprene synthesis by Prins condensation of iso-
butene with formaldehyde was studied over a series of
catalysts characterized by various pore geometry and acid
properties. Results obtained with strongly acidic USY and
H-ZSM-5 catalysts showed low selectivity to isoprene and
suggested that both pore size and acid strength were two
factors affecting this selectivity. Work performed with 10
oxygen ring, weakly acidic boralites showed a spectacular
increase in selectivity. The absence of DMD in the prod-
ucts was observed in boralites and in B-MCM-41 as well
which indicated that the DMD route to isoprene was not
suppressed due to steric constraints or shape selectivity but
due to the low acid strength of the catalytic sites. Thus acid
strength appears as the most important factor affecting the
selectivity of this process.
It has been shown that the isomorphous substitution
of silicon by Al, Ga, Fe, and B resulted in a decreasing
–
–
Si OH M acid site strength in this series, as followed by the
reactions of paraffin cracking, temperature maxima of am-
monia desorption in the temperature-programmed mode,
–
–
and IR frequency of bridging Si OH M groups (29–36)
shifting to higher-frequency values. Quantum chemical
–
–
ab initio calculations of simple models of Si OH M groups
have confirmed (37) that the dissociation energy increased
–
–
and, thus, the acidity of the proton of the Si OH M group
decreased in the order Al > Ga > Fe > B.
It is believed that the high selectivity of isoprene
formation is reached when the Brønsted acid sites are
weak enough so that the protonation of formaldehyde
(Scheme 1) is made selectively and no olefin protonation
to carbocations (Scheme 2) occurs.
Our results obtained for Prins condensation carried out
over boralites reveal that the isoprene can be selectively
synthesized, with reasonable yields. The selectivity of bo-
ralites could be explained as follows. It can be assumed that
on the weak acid sites of boralites only the carbonyl group

160
DUMITRIU, TRONG ON, AND KALIAGUINE
The best isoprene yields have been obtained over bo- 19. Chen, N. Y., Garwood, W. E., and Dwyer, F. G., “Shape Selective
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achieved at low temperatures, around 498 K. It was also
Catalysis in Industrial Applications.” Dekker, New York, 1996.
20. Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T.,
Schmitt, K. D., Chu, T. U., Olsen, D. H., Sheppard, E. W., McCullen,
S. B., Higgins, J. B., and Schlenker, J. L., J. Am. Chem. Soc. 114, 10834
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the presence of an excess of isobutylene.
(1992).
21. Kazansky, V. B., “Structure and Reactivity of Modified Zeolites,” p. 61.
Elsevier, Amsterdam, 1984.
22. Macedo, A., Auroux, A., Raatz, F., Jacquinot, E., and Boulet, R., in
“Perspectives in Molecular Sieve Science” (W. H. Flank and T. E.
Whyte, Jr., Eds.), p. 98, ACS Symposium Series, Vol. 368. American
Chemical Society, Washington, DC, 1988.
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