FTIR and UV-Vis Spectroscopic Study of Interaction of 1-Butene on H-Ferrierite Zeolite

Article abstract of DOI:10.1021/jp992117j

The interaction of 1-butene with zeolite H-ferrierite has been studied at increasing temperatures (between 300 and 670 K) by using IR and UV-vis spectroscopies, with the aim to isolate in situ the species precursors to isobutene and high-temperature coke. The bimolecular mechanism of conversion of butene to isobutene on the fresh catalyst has been confirmed, since low branched C8 chains are observed. At 300 K, the main products of the interaction are the butene isomers 2-cis- and 2-trans-butene. At temperatures between 300 and 393 K, the presence of monoenic allylic carbocations is observed, and at temperatures between 473 and 573 K, neutral and carbocationic polyenes are present. At temperatures ≥ 623 K, the polyenyl unsaturated chains cyclize to form mono- and polycyclic aromatics. These experimental features are explained on the basis of mechanisms in which the protonated intermediates (formed by protonation of butene) can produce, by further reaction with butene molecules, mainly two kinds of products: (i) polyenyl allyl carbocations (by a butene hydride abstraction) and (ii) octane carbocations (by dimerization), which then evolve (by cracking) to isobutene.

Full text of DOI:10.1021/jp992117j

9978
J. Phys. Chem. B 1999, 103, 9978-9986
FTIR and UV-Vis Spectroscopic Study of Interaction of 1-Butene on H-Ferrierite Zeolite
,†
‡
†
,‡
Costanza Paz e` ,* Birsel Sazak, Adriano Zecchina, and John Dwyer*
Centre for Microporous Materials, Chemistry Department, P.O. Box 88, Manchester M60 1QD,
United Kingdom, and Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali,
UniVersit a` di Torino, Via P. Giuria 7, I-10125, Torino, Italy
ReceiVed: June 24, 1999; In Final Form: September 8, 1999
The interaction of 1-butene with zeolite H-ferrierite has been studied at increasing temperatures (between
300 and 670 K) by using IR and UV-vis spectroscopies, with the aim to isolate in situ the species precursors
to isobutene and high-temperature coke. The bimolecular mechanism of conversion of butene to isobutene on
the fresh catalyst has been confirmed, since low branched C8 chains are observed. At 300 K, the main products
of the interaction are the butene isomers 2-cis- and 2-trans-butene. At temperatures between 300 and 393 K,
the presence of monoenic allylic carbocations is observed, and at temperatures between 473 and 573 K,
neutral and carbocationic polyenes are present. At temperatures g 623 K, the polyenyl unsaturated chains
cyclize to form mono- and polycyclic aromatics. These experimental features are explained on the basis of
mechanisms in which the protonated intermediates (formed by protonation of butene) can produce, by further
reaction with butene molecules, mainly two kinds of products: (i) polyenyl allyl carbocations (by a butene
hydride abstraction) and (ii) octane carbocations (by dimerization), which then evolve (by cracking) to isobutene.
Introduction
nature and formation of coke during the reaction are necessary
to comprehend the basis for enhanced selectivity with time on
Because of the need for isobutene in the synthesis of
methyltertiarybutyl ether (MTBE), the isomerization of n-butene
to produce isobutene has been in recent years the focus of many
investigations.1 This reaction proceeds with particularly high
selectivity and yield over H-ferrierite,1 a zeolite with a 10-
stream.
A study of the species present on the catalyst at temperatures
lower than that of the reaction has, to our knowledge, never
been made. A study at lower temperature could, in principle,
provide (i) information about the multiple steps of the reaction
and (ii) information about the nature of the coke formed which
is reported to increase the selectivity of the reaction. As far as
-23
,3
ring and an 8-ring system of channels, in which cavities of
4-28
0
.6-0.7 nm in diameter are present.2
Two kinds of mech-
anism have been proposed, depending on whether the reac-
tion occurs on the fresh or on the aged catalyst. On the fresh
catalyst a bimolecular mechanism, involving an initial oligo-
merization of n-butene to octene (or higher olefins) followed
by a second step involving the cracking of octene (or higher
(i) is concerned, steps that proceed at a very fast rate at higher
temperatures might be observable at lower temperatures (low
reaction rates). Moreover, although the isomerization of 1-butene
to isobutene on H-Fer has been investigated using a great
variety of techniques, a complete vibrational and electronic
characterization is still unavailable. A comparative study using
both techniques is interesting, because they are particularly
sensitive to different functional groups. This study, then,
concerns the reflectance FTIR and UV-vis-NIR characteriza-
tion of the interaction between 1-butene and zeolite H-ferrierite
at increasing temperatures, from 300 to 653 K.
9,10
olefins) to isobutene, has been proposed.
This mechanism
leads to the formation of many side products, including pro-
pene, pentenes, and n-butane.20 With time on stream, the
mechanism becomes monomolecular, where the direct isomer-
ization of n-butene proceeds via formation of a primary
carbenium ion intermediate (a protonated cyclopropane inter-
8,10
mediate is also involved).
Reaction on the aged catalyst is
characterized by higher yield and selectivity than on fresh
catalyst. The reason for improved selectivity with time on
stream has been the object of much debate, and explanations
concerning the role of the coke in (a) modifying pore struc-
Experimental Section
H-ferrierite (Si/Al = 6.5; ammonium sample kindly supplied
by BP) was prepared by heating the ammonium form of the
zeolite at 723 K in a N flow for 20 h. The interaction of the
zeolite with 1-butene was studied by means of FTIR and UV-
vis-NIR spectroscopy at temperature steps of 300, 353, 393,
423, 473, 523, 573, 623, and 673 K. FTIR spectra in the
temperature range 353-393 K were collected in the transmission
mode after cooling the sample (in pellet form) at room
temperature, while spectra in the 423-653 K range were
collected at the reaction temperatures using a Spectra-Tech
HTEC 0030-103 diffuse reflectance cell (sample in powder
form). Since IR reflectance spectra obtained at different tem-
peratures are not directly comparable, a process of normalization
5
,12
ture, (b) poisoning of nonselective and/or the strongest acid
2
sites on the external surface,5
,6,13,15,17
(c) modification of the
active site from the acidic structural zeolitic proton to benzylic
carbocations associated with coke (pseudomonomolecular mech-
2
0,21
anism),
have been invoked. Consequently, studies of the
*
To whom corrispondence should be addressed. Phone:+44 161
2
004528. Fax:+44 161 2367677. E-mail: John.Dwyer@umist.ac.uk.
Phone: +39 011 6707537. Fax: +39 011 6707855. E-mail: zecchina@
ch.unito.it.
†
‡
Universit a` di Torino.
Centre for Microporous Materials.
1
0.1021/jp992117j CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/21/1999

Interaction of 1-Butene on H-Ferrierite Zeolite
J. Phys. Chem. B, Vol. 103, No. 45, 1999 9979
was made at each temperature using a clean thermally activated
sample of H-ferrierite for calibration. The spectra obtained after
interaction with butene were then normalized.
FTIR spectra were obtained using a Nicolet 460 instrument
-
1
with 2 cm resolution. For the IR experiments each dosage of
-butene was followed by a flow of pure N2 to eliminate the
1
gaseous and weakly sorbed species until stationary conditions
were reached at each temperature. NIR-UV-vis spectra were
obtained using a quartz cell. The cell, containing zeolite, was
evacuated at 723 K and loaded with 1-butene (6.6 kPa), sealed,
and transferred to the NIR-UV-vis spectrometer (Cary 5). For
the NIR-UV-vis experiments, a single dose of 1-butene was
used and the cell was heated at increasing temperatures and
cooled to RT prior to taking spectra. High-purity 1-butene was
supplied by BOC Gases and was dosed directly from a manifold
permanently attached to the measurement cell.
To avoid the problems associated with IR reflectance
technique in the study of zeolite powders in the region of
frequencies between about 2000 and 1300 cm-1 (i.e., formation
of a very intense false band due to a specular reflectance
component), parallel experiments with a sample of H-ferrierite
diluted in NaCl (1:1 ratio) were made. The results of the
experiments show that, under the conditions used, even at the
highest temperatures NaCl does not exchange the zeolitic
protons, leaving all the spectral characteristics unaffected; at
Figure 1. FTIR spectra of 1-butene sorbed on H-ferrierite at 300 K:
spectrum 1, unperturbed H-FER; spectra 2-11, decreasing amounts
of 1-butene (maximum pressure ) 2 kPa).
-1
the same time, the region between 2000 and 1300 cm (region
of the CdC vibration and of CH3/CH2 bending modes (δ))
becomes more clearly observable, because the specular reflec-
tance (false) bands are almost completely eliminated.
-
1
Fits were made in the region between 3000 and 2800 cm
by means of the Origin 4.10 software program, to evaluate the
degree of branching of aliphatic species (from the ratio I(CH3)/
I(CH2)). To estimate the ratio of the extinction coefficients (ꢀ)
of CH3 and CH2 symmetric and asymmetric stretching modes
(
ν) of aliphatic molecules in the zeolitic sorbent (for the
2
9
Figure 2. UV-vis (part a) and FTIR (part b) spectra of interaction of
condensed phase a ratio ꢀCH3/ꢀCH2 = 2-3 is reported ), a
preliminary fit of the components of n-butane and n-hexane
adsorbed on H-ferrierite was made. The results show that, in
H-ferrierite, a ratio of 2 is reasonable for the ratio ꢀCH3/ꢀCH2.
In the effected fits, only four bands were considered, to simulate
the bands associated with vCH3asym, νCH3sym, νCH2asym, νCH2-
sym modes. This means that the components at 2910-2920
1
-butene at increasing temperatures: (1) T ) 300 K, 15 min after
interaction; (2) T ) 300 K, after 30 min; (3) T ) 353 K; (4) T ) 393
K. The dotted line in part a is the UV-vis spectrum of unperturbed
H-ferrierite, while the dotted line in part b is the FTIR spectrum
obtained 1 min after the interaction with 1-butene. Asterisks indicate
the bands which are due to the presence of monoenic carbocationic
allylic species.
-1
cm , due to Fermi resonance effects with δCH3, δCH2 bending
29
ring channels, in cages in 8-ring channels, in eight-membered
rings, and in six-membered rings.
modes (often present in the hydrocarbon spectra ), are ignored.
This choice does not seem to affect the quality of the fits
significantly and has the advantage of limiting the number of
components in the fit.
The IR spectra for the interaction of 1-butene with H-fer-
rierite at 300 K are shown in Figures 1 and 2b. The main features
observed are as follows.
-
1
(
i) The intensity of the peak with maximum at 3600 cm ,
Results
due to the unperturbed ν(OH) mode of the bridged strong
Brønsted groups, is reduced upon dosage of 1-butene.
(ii) Upon adsorption of 1-butene, a very broad and composite
The results of the interaction of H-ferrierite with 1-butene
are described at increasing temperatures. For the sake of brevity,
results obtained at different temperatures but showing similar
features are described in a parallel way.
Interaction of 1-Butene with H-Ferrierite in the Tem-
perature Range 300-393 K. IR and NIR Regions of Fre-
quency. The spectrum of unperturbed H-ferrierite is shown in
Figure 1, line 1. The band at 3744 cm is due to the OH
stretching vibration of silanols prevalently located on external
surfaces, while the band at about 3600 cm is due to the
presence of the strong Brønsted sites Si(OH)Al. This peak is
asymmetric on the low-frequency side, which implies hetero-
-
1
absorption between 3450 and 2600 cm develops. A clear
-
1
isosbestic point also appears at 3526 cm , showing that a
correlation between (i) and (ii) is present. The gradual erosion
-
1
of the 3600 cm peak and the formation of the broad and
-1
complex absorption in the 3450-2600 cm range is associated
-
1
with the formation of the hydrogen bonded species shown in
3
2,33
Chart 1.
A weak reversible band (νOH) is also present at
-
1
-1
about 3500 cm , due to H-bond interaction of a residual
fraction of bridged Brønsted sites with the aliphatic part of
butene molecules.
3
0,31
geneity of the Brønsted sites. Zholobenko et al.
deconstructed this peak into four components at 3609, 3601,
3
have
(iii) In the ν(CH) stretching region, bands due to νCH2 and
νCH3 stretching modes, respectively, of the unsaturated (3016
-1
-1
-1
587, and 3565 cm , assigned respectively to hydroxyls in 10-
cm , ν()CH2)) and saturated parts (2967 cm , νasymCH3; 2939

9980 J. Phys. Chem. B, Vol. 103, No. 45, 1999
Paz e` et al.
CHART 1
-
1
-1
-1
cm , νasymCH2; 2921 cm , νsymCH3; 2860 cm , νsymCH2) of
mainly) adsorbed butene molecules (complex in Chart 1) are
present.
iv) In the 1500-1350 cm- frequency region (Figure 2b,
dotted line and curve 1), bands due to δCH2 and δCH3 bending
(
1
(
-
1
modes, respectively, of the unsaturated (1410 cm , in plane
δ()CH2)) and saturated parts (1440 cm , δasymCH3; 1465 cm ,
δCH2; 1372 cm , δsymCH3) of mainly adsorbed butene
-1
-1
-
1
molecules (complex in Chart 1) are present.
(
v) In the CdC stretching region, three clearly distinguishable
-
1
components are visible at 1657, 1643, and 1629 cm . By
comparison with low-temperature selective adsorption of 2-butene
cis-trans isomers on other protonic zeolites,34 they can be
assigned respectively to the 2-trans-butene, 2-cis-butene, and
1
-butene H-bond adducts with strong Brønsted sites (Chart 1).
As already observed for several other adducts isolated in zeolites,
these frequencies are all reduced from the gas-phase values by
-
1
Figure 3. NIR spectra of interaction of 1-butene with H-ferrierite at
increasing temperatures: (1) H-ferrierite; (2) interaction with 1-butene
at 300 K; (3) 353 K; (4) 393 K; (5) 423 K; (6) 473 K; (7) 523 K; (8)
about 15-20 cm , due to weakening of the CdC bond by
H-bond interaction with the proton.32
With increasing (butene) contact times (0-30 min), a band
5
73 K; (9) 623 K; (10) 673 K.
-
1
is formed at 1580 cm (line 1, Figure 2b), and its formation is
not reversible. Its presence in a region of frequency typical of
aromatic ring modes cannot be explained by the formation of
aromatic molecules at this low temperature (300 K), however,
can be explained by the presence of allyl carbocations. In fact
it has been shown from IR measurements of allyl carbocations
in cryogenic superacid matrixes (supported also by ab initio
calculations),35 that allyl carbocations are characterized by a
but it gives much poorer information about molecules having a
different distribution of double bonds. In this latter case, the
only accessible information is present in a very limited range
-
1
of frequencies (about 1500-1650 cm ), other low-frequency
molecular modes being strongly covered by the zeolitic frame-
work bands. On the other hand, UV spectroscopy has two
important advantages when dealing with double bond systems:
(i) the π-π* transitions of different systems generally absorb
in quite distinguishable ranges (although they can overlap); (ii)
the absorption coefficients of electronic transitions of unsaturated
organic compounds in the visible and near UV are usually at
least 1 order of magnitude more intense than those associated
with vibrational transitions.
-
1
main band at about 1580 cm (νasymC-C-C). As discussed
subsequently, the presence of allylic carbocations is also
revealed by UV-vis spectra.
(vi) In the NIR region of frequency (Figure 3), two main
bands are present in the unperturbed zeolite, at 7048 and 4656
-
1
cm , due respectively to overtone, 2ν(OH), and combination,
ν(OH) + δ(OH), modes of bridged strong Brønsted sites. From
the frequency values for these modes, observed in the mid-IR
region, the average δ(OH) in plane mode of bridged hydroxyls
The UV-vis spectrum of H-ferrierite is shown in Figure
2a (dotted line). By interaction with 1-butene, two bands appear
-
1
at about 50 000 and 32 300 cm . The first band is easily
assigned to π-π* transitions of butene in the complex shown
in Chart 1, shifted to lower frequencies with respect to the gas
-
1
in H-ferrierite is estimated to be at about 1050 cm . Upon
-1
sorption of 1-butene the bands at 7048 and 4656 cm are almost
fully consumed and new absorptions appear in the regions
-
1
phase. The band centered at 32 300 cm can be assigned, by
-1
36-38
5
972-5563 and 4480-4206 cm due, respectively, to 2ν(CH)
comparison with previous publications,
to monoenic allyl
and ν(CH) + δ(CH) methyl combination modes. Finally, the
bands at 4154 and 4084 cm-1 are tentatively assigned to
vibrational combination modes of allylic carbocations. The first
is assigned to combinations of components at 3117 and 1043
carbocation species. The increase in intensity of the band at
-1
32 300 cm is time dependent (spectra not shown for brevity).
The positively charged nature of at least part of the species
giving rise to this band is also confirmed by the fact that, after
interaction of the system with NH3 (a tested procedure to
-
1
cm , and the second to components at 1418 (*2) and 1267
-
1
39,40
-1
cm , which are reported for allylic species isolated in supera-
evidence charged species ), the band at 32 300 cm partially
disappears, since the positive charge is neutralized and the
resonant system is lost.
cidic cryogenic matrixes.35
When the system is heated at 353 K, the main difference
from spectra at room temperature is a strong intensification of
the component at 1580 cm-1, assigned to allyl carbocations
This assignment of the band at 32300 cm- in the UV spectra
1
strongly supports the assignment of the corresponding band at
-
1
(
Figure 2b, line 3). On the contrary, successive heating at 393
1580 cm in the IR (vide supra), since the intensities of the
-
1
-1
K reduces the intensity of this band (1580 cm ) (line 4).
UV Spectra. In studying hydrocarbon species sorbed in
zeolites, it is useful to compare the information given by IR
and UV-vis spectroscopy. Infrared spectroscopy is quite
powerful in distinguishing different saturated functional groups,
UV band at 32 300 and of the IR component at 1580 cm
increase with time at the same rate. Moreover, when the system
is heated at 353 K, the intensity of the component at 32 300
-
1
cm is strongly enhanced, while subsequent heating at 393 K
reduces intensity (Figure 2). This behavior is mirrored by that

Interaction of 1-Butene on H-Ferrierite Zeolite
J. Phys. Chem. B, Vol. 103, No. 45, 1999 9981
-
1
(
ii) The band centered at about 3500 cm , previously
assigned to the interaction of a residual fraction of strong
Brønsted sites interacting with the aliphatic part of butene
molecules, is no longer reversible.
(
iii) The intensity of the peak due to the silanols at 3744 cm-
1
is partially decreased, and a weak adsorption is contemporarily
-
1
-1
formed at about 3700 cm (∆ νj = 44 cm ).
iv) The presence of a weak component at 1499 cm is
-
1
(
observed, in a region of frequencies where conjugated double
bonds can absorb, presumably as a result of dehydrogenation
by bimolecular hydrogen transfer.
Features (i)-(iii) can be explained by a butene protonation
reaction (Chart 2). Further oligomerization reactions can occur,
for example, to obtain C8 oligomers. Similar protonation re-
actions have already been fully studied using IR spectroscopy
for interaction of olefins (ethylene and propene) with the acidic
zeolites H-mordenite and H-ZSM-5, where protonation is
Figure 4. FTIR spectra (νCH region) of interaction of 1-butene with
H-ferrierite at increasing temperatures, after removal of physisorbed
species (see Experimental Section): (1) T ) 423 K; (2) T ) 473 K;
32,33
already observed at room temperature.
The saturated chains,
(3) T ) 523 K; (4) T ) 573 K; (5) T ) 623 K; (6) T ) 673 K.
obtained as products, are believed to be bonded to the zeo-
lite by a bond having a prevalently covalent nature.41 Never-
theless, they are often represented also as ionic species since
they are reported to be precursors of carbocations. The
contemporary presence of long free saturated molecules formed
after successive hydrogen transfer is unlikely, since separate
experiments show that these species (up to C8) are not retained
at 423 K.
Free Brønsted sites can interact with the aliphatic chains and,
-
1
presumably, generate the band at 3500 cm . The magnitude
of the shift in frequency, in consequence of this H-bond
interaction is, in fact, closely comparable to that of the inter-
-1
action of H-ferrierite with n-butane (band at 3492 cm , spectra
not shown for brevity). The probability of a multiple interaction
between proximate hydroxyls and a single butene molecule
(such that one hydroxyl protonates the olefin and the other is
H-bonded to the aliphatic chain) is enhanced by the low Si/Al
ratio of the sample.
The hydrocarbon chains are located mainly inside the zeolite
channels, since a difference spectrum in the region between 2000
-
1
and 1500 cm reveals a change in the framework modes
4
2
(
typical of intrazeolite adsorbed molecules ). The presence of
some saturated hydrocarbons also on the external surface (or
grown in proximity to the external mouths of the channels) is
-
1
-1
suggested by the shift in the band at 3700 cm to 3478 cm ,
due to the interaction with external silanols.
The nature of these saturated products (chain length and
degree of chain branching) can be investigated by deconstruction
of bands in the region 3000-2800 cm , since the ratio of the
Figure 5. FTIR spectra (νCH, νCC region) of interaction of 1-butene
with H-ferrierite at increasing temperatures, after removal of phys-
isorbed species (see Experimental Section): (1) T ) 423 K; (2) T )
-1
integrated area (corrected by the proper extinction coefficients)
of the CH3/CH2 bands is different in chains of different length
and also for different isomers. For example, the fit at T ) 423
K gives a ratio (ICH3/ICH2) of 0.42, estimated both for the
asymmetric and for the symmetric modes, which could arise
from the presence of methylheptane chains. Nevertheless, it must
be remembered that, as anticipated in the Experimental section,
4
73 K; (3) T ) 523 K; (4) T ) 573 K; (5) T ) 623 K; (6) T ) 673 K.
-
1
of the IR band at 1580 cm . The presence of monoenic allyl
carbocationic species in H-ferrierite is therefore revealed by
-
1
the presence of bands at 32300, 1580 cm and is probably
-
1
responsible for bands at 4080-4150 cm .
Interaction of 1-Butene with H-Ferrierite at 423 K. IR
and UV-Vis Regions of Frequencies. After interaction of
H-ferrierite with 1-butene at 423 K (spectrum not shown for
sake of brevity), the main features of the IR spectrum closely
resemble those of the spectra obtained at lower temperatures.
Nevertheless, after removal (under a nitrogen stream) of
physisorbed species, a few important differences can be noticed
-
1
the fit in the 2880-2920 cm
range is not completely
satisfactory because of the presence of Fermi resonance
components. Moreover, even tentative assignment to chains of
methyl heptane must be considered with care, because the ratio
of CH3 to CH2 actually represents an average value for a more
complex distribution of species adsorbed in the channels. What
undoubtedly can be said is that the adsorbed hydrocarbons are
characterized by a limited degree of branching.
(
Figures 4 and 5, spectrum 1).
-
1
(i) Three components appear at 2957, 2937, and 2863 cm ,
which are due to νCH3 and νCH2 asymmetric and symmetric
modes in aliphatic hydrocarbons.
The analysis of other regions of frequency in the spectra can
provide additional information. Among components in the

9982 J. Phys. Chem. B, Vol. 103, No. 45, 1999
Paz e` et al.
CHART 2
modes of molecules having conjugated double bonds, such as
29
39
aromatics and long-chain neutral and carbocationic alkenes,
which could be present, in H-ferrierite, due to dehydrogenation
by bimolecular hydrogen transfer reactions. Unfortunately, IR
data cannot distinguish, in this case, between all these different
species. However, UV results (vide infra) provide some
clarification on this point.
(ii) In the CH2/CH3 stretching region, the increase in
temperature leads to progressive band broadening of the
-
1
components in the region around 2800-3000 cm and to a
decrease in their intensities. Consequently, the deconstruction
of the bands into components becomes less certain, but it is
clear that the nature of the hydrocarbon chains in the zeolite is
still, mainly, aliphatic. At 573 K, a weak broad band at about
-
1
3
100 cm (ν()CH2)) reveals the presence of unsaturated
species (alkenes or aromatics).
iii) In the CH2/CH3 bending region, a band at 1368 cm
-
1
(
-
1
appears as a shoulder on the band at 1378 cm , achieving
maximum intensity at 523 K. This indicates clearly that the
degree of branching of the hydrocarbon chains is substantially
increased, since a relevant fraction of carbon atoms bonded to
two methyl groups (revealed generally by the presence of two
-
1 29
bands of equal intensity at 1385-1368 cm
with carbons bonded to one methyl group (1378 cm ).
iv) The band due to νOH of strong Brønsted sites is
progressively restored, as are the NIR components at 7048 and
) now coexists
-
1
(
Figure 6. UV-vis spectra of interaction of 1-butene with H-ferrierite
at increasing temperatures: (1) T ) 423 K; (2) T ) 473 K; (3) T )
-1
4
656 cm (Figure 3, spectra 6-8), while a broad band appears
-
1
in the 3200-3500 cm range.
5
)
23 K; (4) T ) 573 K; (5) T ) 623 K; (6) T ) 673 K (10 min); (7) T
673 K (24 h).
(v) At 523 K and above, isobutene is clearly evident (νCH2
mode at 889 cm ).
-
1
500-1350 cm- range, the absorption at 1382 cm-1 (δsym-
1
1
UV-Vis Region of Frequencies. The UV-vis spectra as-
sociated with the interaction of butene with H-ferrierite in the
temperature range 473-573 K are strongly modified with
respect to spectra at lower temperatures, in that at the higher
temperature several clearly distinct components are evident
(Figure 6, spectra 2-4). Two main groups of bands are present,
CH3) is particularly significant, since its position and intensity
can give information about the presence of carbon atoms bonded
respectively to one, two, and three methyl groups. Since the
29
-1
band at 1382 cm is slightly asymmetric on the high-frequency
-
1
side, and a small shoulder at 1370 cm is present, the carbon
atoms of the sorbed hydrocarbons are predominantly bonded
to one methyl group (C-CH3), coexisting with a minor fraction
of atoms bonded to two methyl groups. This result is in
agreement with the result for the deconstruction of the bands
for the stretching modes. Products of the protonation reaction
are, therefore, mainly unbranched or slightly branched “satu-
rated” octanes.
-
1
respectively, in the 20 000-30 000 and 30 000-40 000 cm
range. Their assignment can be confidently made on the basis
37-40,42-45
of numerous reported studies
on neutral and carboca-
tionic alkenes. It is well-known that oligomers having a different
number of conjugated double bonds generate, in the electronic
spectra, components at distinct frequencies. Moreover, in the
UV spectra, there is greater distinction between neutral and
carbocationic species, since for cations the highest of the π
bonding orbitals is only half filled, and this gives rise to an
allowed electronic transitions (with respect to those in neutral
species) having a lower frequency.
The corresponding UV-vis spectrum is shown in Figure 6,
-1
curve 1. The intensity of the band at 32 300 cm , due to
carbocationic allyl groups, is much decreased with respect to
the intensity at lower temperatures, while a new component
-
1
Bands present in the 20 000-30 000 cm-1 are assigned to
absorption by carbocationic conjugated double-bond oligomers
having a different number (n) of conjugated double bonds, as
appears around 34 000 cm .
Interaction of 1-Butene with H-Ferrierite in the Tem-
perature Range 473-573 K. IR Region of Frequencies. In this
range of temperature, many modifications happen in the system
as the interaction temperature is increased (Figures 4 and 5,
spectra 2-4).
-
1
shown in Chart 3. In particular, the band around 27 000 cm
(present at 473 and 523 K) is assigned to dienic ions (Chart 3,
-
1
n ) 1), while that at about 23 000 cm (present at 523 and
573 K) is assigned to trienic ions (n ) 2). Finally, the tail present
(
i) The main feature of spectra in this temperature range is
-
1
-1
the band at 1500 cm , already present as a shoulder at 423 K,
which becomes more intense (and complex) as temperature
increases. The range of frequencies is typical of νC-C stretching
at lower frequency (20 000-15 000 cm ) is due to species
having n > 2. As the temperature increases, the extended double
bond conjugation in species of type A also increases.

Interaction of 1-Butene on H-Ferrierite Zeolite
J. Phys. Chem. B, Vol. 103, No. 45, 1999 9983
CHART 3
CHART 4
-
1
modes between 2100 and 1700 cm (spectra not shown for
-
1
47
Absorptions in the range 30 000-40 000 cm (which consist
brevity). Finally, it should be noticed that the band at 3500-
-
1
-1
of a band centered at about 35 000 cm , to which large
3100 cm cannot be also attributed to isobutene molecules
-
1
components at 36 000-39 000 cm which merge as temper-
remaining trapped within the smaller channels, even though this
might result in a similar shift of frequency, because no other
bands typical of isobutene are observable.
ature increases) are assigned3
9,43
to neutral conjugated double
bond oligomers having a different number (n) of conjugated
double bonds, as shown in Chart 4.
A straightforward distinction in the assignation of the
components at 35 000 cm and 36 000-39 000 cm cannot
be made, since the nature of the alkyl substituent R is able to
shift the absorption by the same order of magnitude of the
frequency shifts observed. Nevertheless, absorptions in the
5 000-40 000 cm range are consistent with species B having
a number (n) of 2 and 3 conjugated double bonds. Moreover,
UV region of Frequencies. UV spectra obtained at 623 and
6
73 K (Figure 6, spectra 5-7) are mainly characterized by three
-
1
-1
groups of absorptions, at 42 000-50 000, 27 000-42 000, and
-1
5
000-27000 cm , respectively; the well-defined strong com-
ponent observable, at lower temperatures, in the 30 000-40 000
cm range (vide supra), is not evident. The complexity of the
39
-1
-1
3
spectra reveals, as in the case of the IR spectra, a considerable
number of hydrocarbon species, both neutral and carbocationic,
having aromatic character. Since the electronic properties of
aromatic and policyclic molecules in superacidic environments
the assignment is of limited certainty because of the presence
-
1
of a broad absorption at 40 000-50 000 cm (vide infra).
Interaction of 1-Butene with H-Ferrierite at 623-673 K.
IR Region of Frequencies. The interaction of butene and
H-ferrierite at 623 and 673 K (Figures 4 and 5, spectra 5 and
48
are partially known, a general assignment of the components
can be given, but a precise assignment is not possible, due to
the contemporary presence of neutral and carbocationic species.
For example, it seems clear that, for each aromatic mole-
cule, two kinds of carbocation can be formed either as a result
of hydride ion abstraction from the side chain or by a proton
attack on the ring. Carbocationic and neutral aromatic molecules
6
) strongly modifies the spectrum with respect to spectra
obtained at lower temperatures. Results at the highest temper-
ature (673 K) reveal the following.
i) In the νCH region, vibrational modes due to long aliphatic
chains are not evident. Three components, at 2921, 2865, and
070-3000 cm , appear and can be attributed to the presence
of aromatics (CH stretching at 3070-3000 cm ) substituted
by methyl groups (2921 cm (sym. νCH3), 2865 cm (2 asym.
δCH3)).
(
can be characterized by electronic absorptions separated by
-
1
3
-1
1
5 000-20 000 cm . For example, the benzyl ion (carbo-
-
1
-1 49
cation of toluene) absorbs UV radiation at about 22 000 cm ,
while the lower energy absorption of toluene is present at
-1
-1
-
1 50
3
8 000 cm .
(
ii) A complex absorption is generated between 1350 and
Whereas absorptions at highest energies (42 000-50 000
-
1
1
650 cm . In addition to the weak components due to CH3/
-
1
cm ) can be easily assigned to neutral molecules having a
single aromatic ring (alkylated benzenes), absorptions in the
-
1
CH2 bending modes at 1462 and 1379 cm , absorptions at
617, 1601, 1580, 1551, 1511, and 1427 cm-1 are also evident.
1
-
1
region between 40 000 and 20 000 cm can be assigned to
carbocationic and neutral (the high-frequency part) species
These bands can all be attributed to the presence of an extremely
heterogeneous group of alkylated aromatic molecules (for
(aromatics but also linear) having an extensive conjugated
example o,m,p-xylene and toluene, but also to polycyclic
molecules). Andy et al.22 report a series of polycyclic aromatics
double bond system (e.g., molecules having six condensed rings
-
1
are characterized by a low-frequency energy at 20 000 cm ).
As the frequency of the electronic transition decreases, the num-
observed in the coke extracted from the zeolite. At this stage,
a further assignment of IR frequencies is not possible; however,
the extremely low intensity of the bands associated with νCH
vibrations with respect to that for νCC modes suggests the
presence of condensed rings.
iii) The strong Brønsted sites are partially and irreversibly
consumed, and a broad band is formed between 3500 and 3100
cm . These two phenomena are strongly correlated since further
dosage of 1-butene causes their progressive enhancement
spectra not shown for brevity). Since the frequency shift is
5
0
ber of aromatic condensed rings increases. Prolonged heating
at 670 K (24 h, spectrum 7, Figure 6) leads to an increase of
the intensity of the components in the region between 35 000
-
1
and 5000 cm , which indicates highly polycyclic graphitic-
like coke.
(
-1
These results are in accord with those obtained by Andy et
22
al., who studied the composition of high-temperature coke after
destruction of the catalyst with HF and successive extraction
of carbon species by methyl chloride; nevertheless, this method
could not differentiate between neutral and charged aromatics.
In our experiments, the carbocationic nature is confirmed by
the dosage of NH3 on the coked system: all bands in the
(
46
similar to that obtained with aromatic molecules, the band at
-
1
3
500-3100 cm
can be attributed to hydrogen bonding
interaction between strong Brønsted sites and the aromatic
species forming the coke. This means that part of the coke is
certainly present in the internal surface regions, and this is
confirmed by the slight modification of framework combination
-1
40000-5000 cm range are then subjected to a partial decrease
of intensity.

9984 J. Phys. Chem. B, Vol. 103, No. 45, 1999
Paz e` et al.
CHART 5
Protonation of butene is a reversible step, since at 300 K an
outgassing procedure does not lead to the appearance of the
modes typical of saturated groups of species 1 (2800-3000
-
1
cm , see Figure 1). Evidently, the butene molecules interact
with all the strong Brønsted sites, located either in 10- and 8-
-1
ring channels (complete consumption of the band at 3600 cm ,
Figure 1).
(ii) Formation of Monoenic Allylic Cationic Species. The
formation at low temperatures (between 300 and 393 K) of
monoenic allylic cationic species is observed by both UV-vis
-
1
and IR spectroscopy (bands at 32300 and 1580 cm ). The
formation of these monoenic allyl groups (species 2 in Chart
6
) could be due to the reaction of charged complexes 1 with
further n-butene molecules (1-butene and cis- and trans-2-
butene), via hydride abstraction from butene. Butane is presum-
ably formed in parallel from species 1, according to reaction A
shown in Chart 6; the formation of butane has been in fact
reported. The formation of species 2 is a slow, but irreversible,
reaction at room temperature, and a maximal concentration is
Discussion
Coke, together with isobutene (plus small amounts of C2-
C5 species), is the main product of interaction of 1-butene with
H-ferrierite. Moreover, it is known that the influence of coke
on the isobutene selectivity of the system is of great interest.
Whereas the mechanisms of isobutene formation on fresh and
aged catalysts have been extensively studied, no general scheme
including the competitive reactions of coke and isobutene
formation has been fully developed.
20
observed at 353 K. At higher temperatures (393-423 K), it is
still observable but in reduced amount.
9
(
iii) Dimerization of Butene. According to Guisnet and
1
0
Meriaudeau, isobutene is formed, over fresh catalyst, by
dimerization of two butene molecules (according to path B of
Chart 6) to form C8 molecules. Further isomerization steps
The interaction of butene and H-ferrierite at increasing
temperatures is studied here with the aim to identify the
precursors of coke. The identification of coke precursors using
this procedure is strengthened by the clear correspondence of
the IR and UV spectra obtained after interaction of butene at
increasing steps of temperature, and after prolonged interaction
at the maximum temperature (spectra not shown for sake of
brevity). From the obtained spectroscopic data, the precursors
of isobutene and of aromatic high temperature coke can be
schematically distinguished as formed during four different
phases.
(
alkyl shifts) lead to the more branched oligomers. Finally,
cracking reactions (â scission) produce isobutene and other
products. This mechanism is broadly confirmed by the present
IR spectroscopic results, which indicate the formation of
methylheptane carbocations in a first step (fit in the 2800-
-
1
3
000 cm
region) and then the increase in degree of
branching, with maximum branching around 520 K, a temper-
ature at which the first 2-alkyl-1-alkene molecules (e.g., iso-
butene and 2-methyl-1-pentene) are observed (band at 889 cm ,
-1
δCH2).
These results indicate that reaction B in Chart 6 (present at
temperatures higher than 300 K) is characterized by a higher
activation energy than reaction A (slow conversion, at RT).
(
i) Protonation of Butene. The formation of cis- and trans-
-
1
2
-butene (νCdC modes at 1657 and 1643 cm ) is observed
after interaction with 1-butene at room temperature. The double
bond migration of olefins is known to be catalyzed by Brønsted
sites. Either in acid solutions or in acid zeolites, reaction
(iv) Formation of Neutral and Charged Allylic Resonant
Chains. The presence of carbocationic and neutral conju-
gated double-bond oligomers is the more important phenomenon
in the temperature range 473-573 K. A progressive lengthening
of the conjugated double bond chains is observed as the
temperature of interaction increases. The monoenic and poly-
enylic allyl cations are resonance stabilized. Their for-
mation can be explained by multistep reactions as shown in
Chart 7.
34
proceeds (unless at very low temperatures ) via protonated
intermediates, according to Chart 5. On H-ferrierite, isomer-
ization of the double bond is evident at 300 K, which means
that, at this temperature, the corresponding protonated inter-
mediates (species 1 in Chart 5), even if not directly visible in
the infrared (their vibration modes being covered by those of
butene gas), are also formed, or at least can exist as an excited
state. In Chart 5, the interaction of the zeolitic proton with the
In the first step, the allylic species 2 interact with n-butene
to form octenyl carbocations; by successive hydride shift (∼H)
+
olefins is indicated by H ‚‚‚.
CHART 6

Interaction of 1-Butene on H-Ferrierite Zeolite
J. Phys. Chem. B, Vol. 103, No. 45, 1999 9985
CHART 7
TABLE 1
hydrocarbons
(
e.g., to obtain the more stable tertiary carbocations) and
δ-
IR (cm-1
UV-vis (cm-1
)
deprotonation (e.g., by the zeolitic SiOAl group, represented
as ZO in Chart 7) neutral dienes can be formed. Further hydride
abstraction by carbocationic species (R ) leads to the formation
)
-
H-bond butenes:
1-
50 000
+
1629
1643
1657
2
2
- cis
- trans
of carbocationic dienes. Iteration of these steps by further
reaction with butene molecules (followed by isomerization-
deprotonation) gives rise to longer carbocationic allylic systems
having more than three resonant double bonds. This mechanism
is similar to that hypothesized for interaction of propene with
H-ZSM-5 at high temperature.51
This mechanism well justifies the extensive (and apparently
anomalous) decrease of intensity of the UV components at 420
K (above all around 30 000-40 000 cm-1), and the succes-
sive strengthening of the intensities, in the same range of
frequencies, at higher temperatures (470-670 K) (Figure 6).
This feature can be explained by the transformation of species
aliphatic products
2957,2937,2863
378, 1368
1580, 4154, 4084
1450-1550
1450-1550
1
allyl carbocation
dienyl carbenium ions
trienyl carbenium ions
enyl carbenium ions, n g 3 1450-1550
neutral di- and trienes,
aromatics
32 300
27 000
23 000
20 000-15 000
35 000-39 000
50 000-10 000
1450-1550
3000-3070, 2921,
2865, 1650-1450
UV spectroscopic observations and that their presence in
particular steps of the hypothesized mechanism for coke
formation is directly connected to their early or late appearance
in the IR and UV-vis spectra at increasing temperatures.
Accepting the scheme in Chart 6 as a qualitative picture of
the initial process, it can be suggested that path A in Chart 6
2
(π-π* transition in the experimentally accessible UV region
of frequencies) to species 3 (with n ) 1, single double bond,
so that the π-π* transition absorbs at energies higher than
5
-1
0 000 cm ). Only the successive deprotonation step (see Chart
) causes the formation of species having two or more
7
(hydride abstraction) leads mainly to the coke formation, while
conjugated double bonds, which absorb radiation in the studied
UV region.
path B (dimerization) is responsible for the formation of iso-
butene (following a bimolecular mechanism on a fresh catalyst).
This classification cannot, however, be considered as rigid, since
for example further dehydrogenation of C8 carbocations (Chart
Finally, polyenyl ions can undergo cyclization, according to
a mechanism that could be similar to that observed in acid
solutions, in which 1,5-cyclization of dienylic ions has been
invoked. In the present study, evidence for cyclization is first
seen around 620 K and involves a large fraction of the neutral
and charged polyenylic species.
6
, path B) would generate again the unsaturated species here
classified as “coke” (Chart 6, path A). Nevertheless, it is clear
that the formation of allylic positive species is the deter-
minant step in coke formation on H-ferrierite by interaction
with 1-butene and that the competition between “coke” and
isobutene formation begins with the C4 charged species 1 (Chart
It is noteworthy that all the species shown in Charts 5, 6,
and 7 (except otherwise indicated) are supported by IR and/or

9986 J. Phys. Chem. B, Vol. 103, No. 45, 1999
Paz e` et al.
5
), which are formed in the very early stages by interaction of
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1
strong zeolitic protons with butene.
(
17) M e´ riaudeau, P.; Naccache, C.; Le, H. N.; Vu, T. A.; Szabo, G. J.
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(
1
The interaction of 1-butene with H-ferrierite at increasing
temperatures (between 300 and 673 K) has been studied using
IR and UV-vis spectroscopies. A summary of assignments is
shown in Table 1. Precursors of isobutene and coke have been
observed, and a mechanism is proposed to explain their
presence. First of all, the bimolecular mechanism of conversion
of butene to isobutene on the fresh catalyst has been confirmed,
since low branched octane chains are observed. The isomer-
ization of 1-butene (to 2-cis, 2-trans-butene) has been observed
at room temperature. By reaction of the protonated intermediate
with butene, two reaction paths are hypothesised: (i) a butene
hydride abstraction, with formation of dienic allyl carbocations,
and (ii) dimerization to form octane carbocations and (by
successive cracking) isobutene. The reaction of allylic carbenium
ions with butene molecules, followed by H transfer, leads to
the formation of long, unsaturated, resonant neutral and car-
bocationic chains. At temperatures g 623 K, these unsaturated
chains cyclize to form mono- and polycyclic aromatics.
(
(
(
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(
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(
Acknowledgment. The Centre for Microporous Materials
acknowledges EPSRC, BNFL, BOC, Engelhardt, and ICI. A.
Zecchina and C. Paz e` acknowledge Italian MURST support
Phys. Chem B 1998, 102, 2715.
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