on the formation of the acid sites in lanthanum exchanged x zeolites used for isobutane/cis-2-butene...
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Microporous and Mesoporous Materials 83 (2005) 309–318
On the formation of the acid sites in lanthanum exchangedX zeolites used for isobutane/cis-2-butene alkylation
Alexander Guzman 1, Iker Zuazo, Andreas Feller, Roberta Olindo,Carsten Sievers, Johannes A. Lercher *
Lehrstuhl II fur Technische Chemie, Technische Universitat Munchen, Lichtenbergstr. 4 D 85747 Garching, Germany
Received 13 January 2005; received in revised form 25 April 2005; accepted 25 April 2005
Available online 16 June 2005
Abstract
The acid sites generated at different steps during the preparation of La-H-X zeolites were characterized by physicochemical meth-
ods. The resulting materials were tested in isobutane/cis-2-butene alkylation in a continuously operated stirred tank reactor, under
industrially relevant conditions.
The concentration and strength of acid sites depend subtly on the ion exchange procedure. Especially, the rehydration of mate-
rials calcined for the first time after ion exchange changes the distribution of hydroxyl groups, the Brønsted acidic bridging hydroxyl
groups (3640 cm�1) being strongly affected. Rehydration leads to dealumination and, as consequence, the concentration of silanol
groups (3740 cm�1) and of Lewis acid sites increases. This in turn results in an enhanced stability towards the subsequent thermal
treatments and rehydration processes of the rare earth zeolite in the next steps of catalyst preparation. The strength of the Brønsted
acid sites was shown to be a function of the hydrolysis of hydrated lanthanum cations and removal of sodium cations.
The catalytic activity in isobutane/cis-2-butene alkylation and the fraction of strong Brønsted to total Brønsted acid sites are
directly related. Catalysts with similar concentration of strong Brønsted acid sites and higher concentration of weak Brønsted sites
showed shorter lifetime.
� 2005 Elsevier Inc. All rights reserved.
Keywords: IR spectroscopy; Ion exchange degree (IExD); Zeolitic hydroxyl groups; Alkylation
1. Introduction
Isobutane/butene alkylation is an important refiningprocess in which butenes and isobutane are converted
into a complex mixture of branched alkanes (alkylate),
which is an excellent blending component in the gasoline
pool [1]. The catalysts used in commercial processes are
1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2005.04.024
* Corresponding author. Tel.: +49 89 289 135 40; fax: +49 89 289 135
44.
E-mail address: [email protected] (J.A. Lercher).
URLs: http://www.unipamplona.edu.co (A. Guzman), http://thor.
tech.chemie.tu-muenchen.de/ (J.A. Lercher).1 Present address: Universidad de Pamplona, Km 1 vıa a Bucara-
manga, Pamplona-Colombia.
sulfuric and hydrofluoric acids [2]. The development of
new alkylation technologies based on more environmen-
tally friendly solid catalysts has seen high interest overthe past decades. In this context, highly Brønsted acidic
zeolites and especially large pore zeolites are a potential
alternative able to overcome the problems related to
liquid acids. However, the industrial application of
zeolites is constrained by their rapid deactivation.
The overall cycle in the alkylation reaction com-
prises the addition of linear butene (1- or 2-butene) to
a tert-butyl carbenium ion to form a secondary octylcarbenium ion, which can undergo isomerization to a
tert-octyl carbenium ion. Finally, the octyl carbenium
ion is removed from the acid site by hydride transfer
from isobutane leading to a tert-butyl carbenium ion
+
+
Olefin addition
Hydride transfer
Isomerization+
++
++ ++
Scheme 1. Simplified alkylation mechanism.
310 A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318
[3] (Scheme 1). Competition between hydride transfer
and addition of butene to a carbenium ion determines
the lifetime of the catalyst. A high ratio of hydride trans-
fer vs. olefin addition leads to enhanced trimethylpen-tane formation and reduction of catalyst deactivation [4].
Among the large pore zeolites, rare earth exchanged
faujasites have been shown to have a good ability to cat-
alyze hydride transfer in alkylation reaction. This is re-
lated to two factors, i.e. (i) the high concentration of
aluminum in the framework leads to an optimum
strength of the bond between the zeolite oxygen and
the secondary or tertiary carbon atom of the alkoxygroups being the ground state for the carbenium ions
in the transition state of olefin addition or hydride trans-
fer and (ii) the resulting high concentration of strong
Brønsted acid sites allows to generate a (relatively) high
concentration of alkoxy groups/carbenium ions increas-
ing the probability of hydride transfer over olefin addi-
tion for the individual alkoxy group. Both factors help
to compensate the higher energy barrier required forthe hydride transfer in solid catalysts compared to liquid
acids [5–7].
The nature of the acid sites in the rare earth ex-
changed FAU zeolites has been extensively studied. It
has been proposed that the rare earth ions exchanged
in X or Y zeolites are hydrolyzed upon calcination.
The resulting protons generate Brønsted acid sites
[8–10]: [La(H2O)n]3+ = [La(OH)(H2O)n� 1]
2+ + H+.Direct evidence for hydrolysis is deduced from IR
spectra showing O–H stretching bands that are attrib-
uted to OH groups attached to the exchanged metal cat-
ion and to the aluminosilicate framework [11]. Similarly,
inelastic neutron diffraction has provided direct evidence
for metal cation hydrolysis [12].
The effect of the calcination temperature on the
migration of lanthanum ions from the supercages (theinitial location of La3+ after ion exchange) to the soda-
lite cages has been likewise investigated [13]. It has been
reported that at temperatures higher than 333 K the hy-
drated La3+ cations begin hydrolyzing water and
migrating to sodalite cages. However, during calcina-
tion, the hydrolysis of the rare earth cations produces
not only Brønsted acid sites, but also Lewis acid sites
[8]. Potential processes involving the framework during
calcination have not been well characterized. Especially,
it is unclear under which conditions water may lead to
the partial or full hydrolysis of framework aluminum
atoms and what role the rare earth metal cations play
in this process.
Here, primarily IR spectroscopy is used to provideinformation about the formation of the acid sites in lan-
thanum exchanged X zeolites and to investigate the
influence of water on the OH groups of the calcined
materials. A series of lanthanum exchanged X zeolites
was examined to establish, how the acidic properties
(concentration and strength) of the modified materials
vary with the lanthanum exchange degree. These find-
ings have been correlated with the catalytic activity inalkylation of isobutane with cis-2-butene.
2. Experimental
2.1. Material preparation
The samples in this study were prepared from a Na-Xzeolite obtained from Chemische Werke Bad Kostritz
(Si/Al = 1.2). The Na-X material was ion exchanged
with 0.2 M lanthanum nitrate solution with pH = 3
(from La(NO3)3 Æ 6H2O, Fluka, puriss. p.a., P99.0%),
using a liquid-to-solid ratio between 5 and 10 mL/g.
The ion exchange was carried out at 353 K for 2 h. This
step was performed one or two times. After washing and
drying, the resulting materials were calcined in 100 mL/min flowing air for 1 h at 723 K (‘‘first calcination’’). To
achieve a higher lanthanum ion exchange degree the
materials were further exchanged, between one and
three times, following the procedure described above
including a second calcination at 723 K (‘‘second
calcination’’).
2.2. Characterization of materials
The ion-exchanged materials were characterized by
XRD (determination of the unit cell size UCS by ASTM
D-3942-85 [14]) and 1H MAS NMR. The ion exchangedegree [IExD = (1-Na/Al)*100] was determined by AAS.
The effect of the calcination steps (‘‘first and second
calcinations’’) during the catalyst preparation was mon-
itored by mass spectrometry and IR spectroscopy. Two
different calcination procedures were employed: calcina-
tion directly in the IR flow cell (‘‘in situ calcination’’)
and calcination in a shallow bed type oven (‘‘ex situ cal-
cination’’ with respect to the IR spectroscopic character-ization). The temperature program and the air flow used
during these calcinations were practically identical. Typ-
ically, the sample was heated with 5 K/min to 393 K and
kept at this temperature for 1 h. Then, the temperature
was further increased up to 473 K with an increment
of 0.17 K/min. Finally, the sample was heated with
18,
a.u
.
600
700
800
re, K393 K
A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318 311
3 K/min to 723 K and kept at this temperature for 1 h.
IR measurements were performed with a Bruker IFS-
88 spectrometer equipped with a flow cell and operated
at atmospheric pressure.
The effect of the IExD on the acidity of the materials
at different preparation steps was studied via IR spec-troscopy of adsorbed pyridine. The spectra were col-
lected on a Perkin–Elmer system 2000 spectrometer
equipped with a vacuum cell (residual pressure
10�4 Pa). The samples were pressed into a self-support-
ing wafer (with a density between 5 and 10 mg/cm2) and
activated in vacuum at 723 K for 1 h. After lowering the
temperature to 423 K, an IR spectrum was recorded
(‘‘IR spectrum of the activated sample’’). Pyridine witha partial pressure of 1 Pa was introduced into the cell.
After saturation, the cell was evacuated at 423 and
723 K for 1 h, and IR spectra were recorded at 423 K.
The concentrations of Brønsted and Lewis acid sites
were quantitatively derived from the bands at 1540
and 1454 cm�1 corresponding to the pyridinium ions
and pyridine coordinatively bound to Lewis acid sites,
respectively. The extinction coefficients were taken fromEmeis [15]. The Brønsted and Lewis acid site concentra-
tions measured after outgassing at 423 K are referred as
BAS423 (‘‘total Brønsted acidity’’) and LAS423 (‘‘total
Lewis acidity’’), and after outgassing at 723 K as
BAS723 (‘‘strong Brønsted acidity’’) and LAS723(‘‘strong Lewis acidity’’), respectively.
2.3. Catalytic experiments
The alkylation of isobutane with cis-2-butene was
performed in a stirred tank reactor operated in continu-
ous mode. The liquefied gases were received from Mes-
ser with a purity of 99.95% (isobutane) and 99.5%
(cis-2-butene). The catalyst (typically 1–3 g) was acti-
vated in situ within the alkylation reactor at 443 K for
16 h in 100 mL/min flowing hydrogen. After coolingdown to reaction temperature (348 K), the reactor was
pressurized with H2 to 2 · 103 kPa and then filled withisobutane. The stirring speed was about 1600 rpm. The
reaction started by feeding a mixture of isobutane/cis-
2-butene (10/1 molar ratio). The catalyst lifetime was
defined as the time on stream at which the olefin conver-
sion starts to be lower than 99%. Details on the reaction
conditions are described elsewhere [4].
0 2 4 6 8 10 12Calcination time, h
MS
sign
al m
/e
300
400
500
Tem
pera
tu
573 K
423 K
Fig. 1. MS signal for m/e 18 recorded during the in situ first
calcination of a two times lanthanum exchanged Na-X zeolite.
3. Results
3.1. Hydroxyl group formation on La3+ exchanged
Na-X zeolites
The acidity of La-X zeolites has been attributed tothe protons that are generated, when water of the hy-
drated lanthanum cations is hydrolyzed at temperatures
between 333 and 573 K [8–10]. The transformations dur-
ing the activation procedure of a Na-X zeolite ex-
changed two times with lanthanum cations were
monitored by mass spectrometry (MS) analysis of the
outlet of the calcination oven and by in situ IR spectros-
copy. Fig. 1 shows the MS signal for m/e 18 (water) re-corded during calcination. Two sharp and intense
maxima of water evolution were observed at 393 and
573 K, and one of low intensity at about 423 K. Three
possibilities for releasing water from the sample exist,
i.e., (i) release of physisorbed water, (ii) dehydration of
the lanthanum cations and (iii) dehydroxylation of the
zeolite. The peak at 393 K is attributed to desorption
of molecularly adsorbed water, that at 423 K to furtherdesorption of water more strongly bound to the catalyst
surface and finally the peak at 573 K is tentatively
attributed to the zeolite dehydroxylation, i.e. recombi-
nation of the La-OH group with a proton at a bridging
hydroxy group or recombination of two neighbor bridg-
ing hydroxyl groups.
Fig. 2 shows the in situ IR spectra of this sample re-
corded during calcination in flowing air (‘‘in situ calcina-tion’’). Below 393 K, it was not possible to distinguish
well-defined bands in the hydroxyl region. At 393 K, five
bands were observed corresponding to terminal silanol
groups (3743 cm�1), bridging hydroxyl groups (3640
and 3600 cm�1), physisorbed water (3570 cm�1 [16]),
and La-OH groups (3520 cm�1). When the calcination
temperature reached 573 K, the bands at 3570 cm�1
and the deformation band of water at 1630 cm�1 disap-peared completely. For calcination temperatures above
573 K, the intensities of the bands corresponding to hy-
droxyl groups at 3600 cm�1 and to La-OH groups at
3520 cm�1 decreased progressively. The intensity of the
band at 3640 cm�1 also decreased slightly until approx-
imately 593 K and then increased for higher tempera-
tures. In the whole temperature range studied, the
silanol band at 3743 cm�1 was very weak.The activation of the ‘‘ex situ’’ calcined zeolite (stored
in ambient air) was followed by IR spectroscopy while
3800 3700 3600 3500 3400
Tem
pera
ture
, K
Abso
rban
ce
Wavenumber, cm-1
3300
393 K
723 K
593 K
3640
3600
3570
3520
3743
-
3500
673723
35203640
3800
573623
T, K
cm-1
3600
Fig. 2. IR spectra recorded during the in situ first calcination of a two
times lanthanum exchanged Na-X zeolite. In the inset comparison of
IR spectra corresponding to calcination temperature of 573, 623, 673
and 723 K.
312 A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318
increasing the temperature up to 723 K in flowing air.
The relative intensities of the IR bands differed mark-
edly from those of the sample calcined in situ. With
the in situ calcined samples, the intensity of the hydroxylgroups at 3640 cm�1 was higher than the intensities of
the La-OH groups at 3520 cm�1 and of the hydroxyl
groups at 3600 cm�1. The band of the silanol groups
at 3743 cm�1 was less intense. An example of the IR
spectra (recorded at 303 K) of two samples calcined
(ex situ and in situ) at the same temperature (723 K) is
shown in Fig. 3. Similar differences were observed for
the entire range of calcination temperature investigated.1H MAS NMR measurements of these two samples
activated under controlled conditions led to similar re-
sults. The NMR spectra showed four distinguishable
peaks at ca. 6.3, 4.0, 2.9 and 1.8 ppm. These correspond
3600Bridging OH
3520La-OH
3640BridgingOH
3743
Wavenumber, cm-1
Abso
rban
ce
3800 3700 3600 3500 3400 3300
Si-OH
-
Fig. 3. Comparison between IR spectra (recorded at 303 K) of a two
times lanthanum exchanged Na-X zeolite calcined at 723 K (first
calcinations) in situ (gray line) and ex situ (black line) with respect to
the IR cell.
to OH groups bound to lanthanum cations La-OH,
bridging hydroxyl groups SiOHAl in large cavities of
the zeolite, OH bound to extra-framework aluminum
species AlOOH and silanol groups, respectively [17].
For the sample calcined in situ the intensity of the pro-
tons associated to bridging hydroxyl groups at ca.4 ppm was higher than that of the protons associated
with the La-OH groups at ca. 6.6 ppm, while the inten-
sity of the protons corresponding to silanol groups at ca.
1.8 ppm was lower. In addition, IR spectroscopy of pyr-
idine showed that the total Brønsted acidity of the in situ
and ex situ calcined samples was 0.77 and 0.50 mmol/g,
respectively, while the total Lewis acidity was 0.06 and
0.13 mmol/g.As the exposure to water could be the main difference
between the samples activated in situ and ex situ, the
sample calcined in the IR flow cell (in situ first calcina-
tion) was exposed to a stream of 0.01 mmol water/min
in synthetic air for 10 min at 303 K. Subsequently, the
cell was purged with pure synthetic air for 30 min and
the temperature was raised with 10 K/min to 723 K.
When the temperature reached 400 K (the temperatureat which the OH bands of the zeolite can be clearly dis-
tinguished), the IR spectra were similar to those ob-
tained from the sample calcined ex situ (Fig. 4), i.e.,
the band at 3640 cm�1 was much weaker than those at
3600 and at 3520 cm�1, while the band at 3743 cm�1
was more intense.
In order to investigate, if the adsorption of water
after the ‘‘second calcination’’ can also modify the rela-tive intensity of the hydroxyl groups, the same hydra-
tion–calcination experiment was performed with a
sample further exchanged with lanthanum (three times)
after the ‘‘first calcination’’. As shown in Fig. 5, signifi-
cant changes in the IR spectra between the samples cal-
cined in situ and ex situ (‘‘second calcination’’) were not
observed.
Wavenumber, cm-1
Abso
rban
ce
3640
36003520
3800 3700
393 K
723 K
3600 3500 3400 3300
3695
3743 Tem
pera
ture
, K
-
Fig. 4. IR spectra recorded during re-calcination after water adsorp-
tion on the two times lanthanum exchanged Na-X zeolite calcined in
situ (first calcination).
3800
Wavenumber, cm-1
Abso
rban
ce 3600
3520
3743
3700 3600 3500 3400 3300
3640
Fig. 5. IR spectra (recorded at 303 K) of a La-X zeolite calcined at
723 K (second calcination). Grey line: in situ calcination. Black line: ex
situ calcination (re-calcination after adsorption of water on the in situ
calcined sample).
A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318 313
3.2. Acidic properties of La3+ exchanged Na-X zeolites
at different preparation steps
The standard procedure for the preparation of lan-thanum exchanged Na-X zeolites consists of one or
two ion-exchange steps with lanthanum salt solutions
followed by a first calcination and equilibration at ambi-
ent conditions. The resulting material is further ex-
changed up to three times, calcined and again
equilibrated at ambient conditions. Samples at different
steps of this preparation procedure were systematically
collected. Their properties (ion exchange degree, unitcell size, acidity and catalyst lifetime in alkylation reac-
tion) are compiled in Table 1.
Fig. 6(a) shows the IR spectra recorded at 423 K after
activation in vacuum at 723 K for 1 h of samples with
different IExD (dotted line) and the corresponding spec-
tra after pyridine adsorption and outgassing at 423 K
(black line). In Fig. 6(b), the profiles obtained by sub-
traction of the spectra recorded after and before pyri-
Table 1
Physicochemical properties of different lanthanum exchanged Na-X zeolites: i
of Brønsted and Lewis acid sites determined by pyridine adsorption and ou
Sample identification Preparationa IExD
(1 � Na/A1)*100
UCS
Parent material calc. 0.0 24.9
1La 1La3+ 37.3 25.1
1LaC 1La3+ + calc. 37.3 25.0
2LaC 2La3+ + calc. 82.2 25.0
1LaC + 1LaC 1La3+ + calc. + 1La3+ + calc. 90.8 25.0
1LaC + 2LaC 1La3+ + calc. + 2La3+ + calc. 95.5 25.0
1LaC + 3LaC 1La3+ + calc. + 3La3+ + calc. 97.7 25.0
2LaC + 3LaC 2La3+ + calc. + 3La3+ + calc. 99.6 25.0
a The abbreviations describe the sequence of steps involved in catalyst prepa
before calcination. For ex. 2La3+ + calc. + 3La3+ + calc. describes a sample e
calcination, respectively.
dine adsorption are also shown. In the spectra of
activated materials, the band characteristic of the La-
OH groups was progressively shifted to lower wavenum-
bers (from 3533 to 3514 cm�1) as the ion exchange
degree increased. Simultaneously, the intensity of this
band increased exponentially. The bridging hydroxylband at approximately 3600 cm�1 also increased with
the IExD, though not exponentially, while the intensity
of the second bridging hydroxyl band at 3640 cm�1 was
almost constant.
After adsorption of pyridine, the intensity of the La-
OH band of the sample with 37.3% IExD was higher
compared to that of the activated sample. At 82.2%
IExD pyridine was not adsorbed on the La-OH groups,while it reacted in the samples with higher IExD, the ex-
tent of the interaction being higher for the samples with
higher IExD (90.8–97.7%). However, in the fully La3+
exchanged zeolite (99.6% IExD) the coordination of
pyridine to La-OH groups was hardly observed. This
suggests that all lanthanum hydroxyl groups are located
in the inaccessible sodalite cages.
The reactivity of the band at 3600 cm�1 increasedwith IExD and only for low IExD this band did not
disappear completely after pyridine adsorption. Addi-
tionally, the non-interacting fraction of this band at
low IExD was found at slightly lower wavenumber
(3584 cm�1). The findings are in agreement with the
lower strength of the acid sites due to the remaining
sodium cations [18].
For all samples studied, the band at 3640 cm�1 attrib-uted to bridging hydroxyl groups disappeared com-
pletely after admission of pyridine.
The ring vibrations of pyridine were used to charac-
terize the varying concentrations of Brønsted and Lewis
acid sites. The Brønsted acid sites were identified by the
band at 1543 cm�1 (pyridinium ion), the Lewis acid sites
associated with accessible Al3+ by the band at 1454 cm�1
attributed to pyridine coordinatively bound to cationic
on exchange degree (IExD), unit cell size (UCS), acidity (concentration
tgassing at 423 and 723 K) and catalyst lifetime in alkylation
(A) BAS LAS BAS723/
BAS423
Cat.
lifetime (h)BAS423(mmol/g)
BAS723(mmol/g)
LAS423(mmol/g)
LAS723(mmol/g)
83 0.01 0.00 0.00 0.00 0.00 –
39 – – – – – –
35 0.17 0.02 0.01 0.00 0.13 –
15 0.49 0.18 0.13 0.09 0.37 –
19 0.51 0.17 0.06 0.03 0.34 3.0
22 0.59 0.24 0.03 0.02 0.40 4.0
19 0.52 0.26 0.09 0.04 0.50 13.5
09 0.44 0.24 0.11 0.06 0.56 12.0
ration. ‘‘nLa3+ + calc.’’ corresponds to n ion exchange steps carried out
xchanged with lanthanum 2 and 3 times before the first and the second
Wavenumber, cm-1
Abso
rban
ce
3800 3700 3600 3500 3400
IExD, %
97.7
95.5
82.2
37.3
90.8
0.0
99.6
-
Abso
rban
ce
3800
Wavenumber, cm-1
3700 3600 3500 3400
IExD, %
97.7
95.5
82.2
37.3
90.8
0.0
99.6
-1ba
Fig. 6. (a) IR spectra of activated samples (dotted line) and after pyridine adsorption (black lines) at different preparation steps. (b) Profile obtained
by subtraction of the spectra recorded after and before pyridine adsorption.
0.0
0.2
0.4
0.6
0 2 4 6Area band La-OH/mg
BAS 7
23 /
BAS
423
Fig. 8. Correlation between the fraction of strong Brønsted acid sites
(BAS723/BAS423) and the normalized area of the hydroxyl groups band
at 3520 cm�1.
0.12
0.17
, mm
ol/g
314 A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318
extra-framework aluminum species (EFAL). Fig. 7
shows that the total concentration of the Brønsted acidsites (BAS423) passed through a maximum at 95.5%
IExD, while the concentration of the strong Brønsted
acid sites (BAS723) increased continuously up to about
99.6% IExD (0.25 mmol/g). Thus, the fraction of strong
Brønsted acid sites (BAS723/BAS423) increased exponen-
tially with IExD. It is interesting to note that the inten-
sity of the La-OH band at ca. 3520 cm�1 and the
fraction of strong Brønsted acid sites (BAS723/BAS423)are positively correlated (see Fig. 8).
Fig. 9 shows the effect of the IExD on the concentra-
tion of all Lewis acid sites (LAS423). A minimum
(0.03 mmol/g) was observed for 95.5% IExD. The sam-
ple twice exchanged before the first calcination (82.2%
IExD) showed the highest overall concentration of Le-
wis acid sites (0.13 mmol/g). For the sample with
37.3% IExD Lewis acid sites were hardly detected.Another typical band observed in the spectra of ad-
sorbed pyridine on this series of samples appeared at
ca. 1444 cm�1. Lercher et al. [19] assigned a band at
1440 cm�1 to pyridine adsorbed on OH groups and a
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100IExD, %
BAS 4
23 a
nd B
AS 72
3
mm
ol/g
0.0
0.1
0.2
0.3
0.4
0.5
0.6
BAS 7
23 /
BAS
423
Fig. 7. Effect of the IExD on the concentration of total ( BAS423)
and strong Brønsted acid sites ( BAS723) and on the fraction of
strong Brønsted acid sites (h BAS723/BAS423).
0.00
0.04
0.08
70 80 90 100IExD, %
LAS 4
23
Fig. 9. Effect of the IExD on the concentration of total Lewis acid sites
(LAS423).
band at 1448 cm�1 to pyridine adsorbed on Na+ cations.The thermal stability was used as the most important
criterion to distinguish between pyridine adsorbed on
OH groups (low thermal stability) and on accessible cat-
ions (high thermal stability). However, the temperatures
used in the present study to distinguish between strong
0.0
1.0
2.0
3.0
4.0
0 20 40 60 80 100IExD, %
Area
ban
d at
144
4cm
-1/m
g
Fig. 10. Effect of the IExD on the normalized area of the IR band at
1444 cm�1.
A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318 315
and weak acid sites, 723 and 423 K, were higher than
those used in Ref. [19]. Thus, the band at 1444 cm�1
must be related to a relatively strong interaction, pre-
sumably related to strongly sterically hindered metal
cations (La3+ or Al3+). The normalized area of the band
at 1444 cm�1 decreased with IExD up to a minimum for
IExD between 90.8% and 95.5%. The intensity of thisband increased afterwards (see Fig. 10).
3.3. Catalytic activity over La3+ exchanged Na-X
zeolites at different preparation steps
In order to investigate the impact of the IExD on the
catalytic activity for isobutane/cis-2-butene alkylation,
the samples with IExD P 90.8% (Table 1) were testedin a CSTR type reactor. The lifetime of these catalysts
increased with IExD from 3.0 to 13.5 h. In Fig. 11(a),
the dependence of the integral selectivities (i.e., the prod-
uct distribution in the product collected until the end of
the catalyst lifetime) upon the catalyst lifetime is shown.
The octanes (C8 fraction) dominated in the alkylate. The
integral selectivity to n-butane increased with lifetime.
In the C8 fraction, the selectivities to 2,2,4-TMP(trimethylpentanes) and to 2,2,3-TMP + 2,5-DMH
(dimethylhexanes) increased, while the selectivities to
2,3,4-TMP and 2,3,3-TMP decreased (Fig. 11(b)).
0
20
40
60
80
100
0.0 4.0 8.0 12.0 16.0Catalyst lifetime, h
Inte
gral
sel
ectiv
ity, w
t%
0.0
1.0
2.0
3.0
4.0
n-bu
tane
in
tegr
al s
elec
tivity
, wt%
a
Fig. 11. Variation in selectivities with catalyst lifetime of the lanthanum excha
different product groups as a function of catalyst lifetime ( n-butane, C
individuals C8 products as a function of catalyst lifetime ( 2,3,3-TMP,
DMH, –n– 2,4-DMH, –s– 3,4-DMH, octenes).
As shown in Table 1, the catalyst lifetime and the
fraction of strong Brønsted acid sites (BAS723/BAS423)
are directly correlated.
4. Discussion
4.1. Hydroxyl groups formation on La3+ exchanged
Na-X zeolites
The presence of bridging hydroxyl groups (IR bands
at 3600 and 3640 cm�1) and of lanthanum hydroxyl
groups (IR band at 3520 cm�1) on lanthanum exchanged
Na-X zeolites after calcination at 393 K indicates thatthe hydrolysis of hydrated lanthanum cations starts al-
ready at such low temperatures. Upon further increase
of the calcination temperature, physisorbed water is re-
moved (decrease of the IR band at 3570 cm�1) and the
concentration of free hydroxyl groups, bridging and
associated with lanthanum, decreases up to approxi-
mately 593 K. From this temperature, the concentration
of bridging hydroxyl groups (3640 cm�1) increases, whilethose of La-OH groups (3520 cm�1) and of the second
type of bridging hydroxyl groups (3600 cm�1) decrease.
This suggests a gradual dehydroxylation of the zeolite
as the calcination temperature increases. The decrease
of the intensity of the IR bands is not due to artifacts
by recording the IR spectra at higher temperature (see
e.g., Ref. [20]), as confirmed by comparison of the IR
spectra recorded at the same temperature, before andafter heating step. Thus, the water evolution at 573 K
monitored by MS (see Fig. 1) during the first calcination
can be attributed to water from dehydroxylation of lan-
thanum hydroxyl groups and bridging hydroxyl groups
at 3600 cm�1. The presence of Lewis acid sites in the in
situ calcined sample supports this interpretation. As the
calcination temperature did not exceed 773 K, the dehy-
droxylation does not lead to extra-framework octahedralaluminum species, but mostly to accessible aluminum
still attached to the framework atoms [21]. The increase
in the hydroxyl groups with a band at 3640 cm�1
0
10
20
30
40
0.0 4.0 8.0 12.0 16.0Catalyst lifetime, h
Inte
gral
sel
ectiv
ity, w
t%
b
nged Na-X zeolites with IExDP 90.8%. (a) Integral selectivities to the
8 products, C5–C7 products, n C9+); (b) Integral selectivities to the
2,3,4-TMP, 2,2,4-TMP, –h– 2,2,3-TMP + 2,5-DMH, 2,3-
316 A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318
suggests that the hydrolysis of lanthanum cations is
incomplete at low calcination temperatures and that tem-
peratures above 593 K are required to force the exchange
with Na+ cations in the sodalite cages. Thus, two simul-
taneous processes occur during calcination at tempera-
tures between 573 and 723 K, i.e., the hydroxyl groupsformed by hydrolysis of the hydrated lanthanum cations
are lost in the form of water (either by recombination of
a La-OH group with a proton at a bridging hydroxy
group or recombination of two neighbor bridging hydro-
xyl groups), and new hydroxyl groups are formed from
hydrolysis of the remaining hydrated lanthanum cations.
The exposure to water after the ‘‘first calcination’’ has
a strong effect on the hydroxyl groups, as observed bycomparison of the IR spectra recorded after ‘‘in situ’’
and ‘‘ex situ’’ calcination (Fig. 3). After adsorption of
water, the band at 3640 cm�1 decreases in intensity,
while the SiOH band increases. This process is accompa-
nied by the decrease of the total Brønsted acidity from
0.77 to 0.50 mmol/g and by a significant increase of
the total Lewis acidity (from 0.06 to 0.13 mmol/g).
Therefore, we conclude that the zeolite is dealuminated,when it is contacted with humid atmosphere after the
‘‘first calcination’’. The preference of the dealumination
at aluminum–oxygen tetrahedra associated with the
3640 cm�1 OH band in comparison to those giving rise
to the band at 3600 cm�1 is tentatively attributed to
the more accessible or more labile aluminum–oxygen
bonds in the former tetrahedron.
After these structural changes, the zeolite becomesstable towards the subsequent thermal treatments and
rehydration processes. This is well demonstrated by
the fact that water adsorbed after the ‘‘second calcina-
tion’’ did not affect the IR spectrum (Fig. 5). UCS mea-
surements for these samples are in good agreement with
the variations in the intensities of the OH bands. Thus,
the UCS of the one time La-exchanged sample before
calcination (sample 1La), after the ‘‘first calcination’’(sample 1LaC) and the ‘‘second calcination’’ (sample
1LaC + 3LaC) were 25.139, 25.035 and 25.019 A,
respectively, indicating that dealumination is more pro-
nounced after the first calcination. In this context, it
should be mentioned that La exchange affects the UCS
increasing it from 24.983 A for the parent material
(Na-X) to 25.139 A for the exchanged and uncalcined
parent material (sample 1La).
4.2. Acidity of La-Na-X with different ion exchange
degrees
The fraction of strong Brønsted acid sites (those that
adsorb pyridine strongly) increased exponentially with
IExD. It should be noted, however, that samples with
IExD larger than P 97.7 % have the highest concentra-tions of strong Brønsted acid sites, while the total concen-
tration of Brønsted acid passes through a maximum at
95.5% IExD. As discussed above, dealumination is more
pronounced after the first calcination. However, the La
exchange plays indirectly also a role in this process. As
shown by the data in Table 1, the samples with the higher
ion exchange degree before either the first or the second
calcination presented the lower UCS (25.015 and25.009 A, respectively); this process is related to the higher
concentration of bridging hydroxyl groups formed by
hydrolysis of the La hydrated cations. Thus, with increas-
ing La exchange degree, more bridging OH groups
are formed. After rehydration some of them, those prefer-
entially bounded to aluminum–oxygen tetrahedra associ-
atedwith the 3640 cm�1OHband, lead to dealumination.
This results, on the one hand, in a decrease of the totalBrønsted acidity, and on the other hand, in an increase
of the strength of the remaining OH groups. As a conse-
quence, the fraction of strongBrønsted acid sites increases
exponentially while the total concentration of Brønsted
acid sites passes through a maximum.
The band of adsorbed pyridine at 1444 cm�1 is com-
plex to interpret. In the parent material, this band has to
be attributed to the Na+ cations. With increasing IExD,Na+ cations are replaced by La3+ cations. After the first
calcination, the hydrated lanthanum cations are con-
verted to La-OH and migrate irreversibly to the sodalite
cages. Therefore, the contribution of the La3+ cations to
the band at 1444 cm�1 due to pyridine interacting with
cations, must be small. After this first calcination of
the exchanged zeolite, extra-framework aluminum spe-
cies are formed during the re-hydration process andtheir concentration depends on protons, especially those
characterized by the band at 3640 cm�1, which in turn
are formed by hydrolysis of the hydrated La3+ cations.
Zeolites exchanged at higher level before the first calci-
nation generate more EFAL (extra framework alumi-
num) species.
After a 3-fold exchange with La3+ cations in the sec-
ond exchange step followed by a second calcination, thecontent of lanthanum has been the highest (IExD of
99.6%). Consequently, the sodium content of this sam-
ple is very low and cannot be responsible of the band
at 1444 cm�1. Thus, with very low concentration of
Na+ and La3+ being located in the small cavities of the
zeolite, the increase of the band at 1444 cm�1 has to
be attributed to pyridine interacting with EFAL species.
The two partially overlapping bands at 1444 and1454 cm�1 in the IR spectra of adsorbed pyridine have
been observed in zeolites containing extra-framework
aluminum in the form of oxidic debris and cationic alu-
minum species, respectively [22].
As indicated by the linear correlation between the
intensity of the IR band corresponding to La-OH
groups (3520 cm�1) and the fraction of strong Brønsted
acid sites (BAS723/BAS423), the strong Brønsted acidsites are generated by the dissociation of water on
La3+. As the IExD increases, more La-OH and bridging
A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318 317
hydroxyl groups are formed. When La3+ has migrated
to the sodalite cages the La-OH groups must also be lo-
cated there. The bridging hydroxyl groups remain in the
supercages of La-Na-X, as concluded from the full
accessibility to pyridine.
At exchange degrees above 97.7%, most of La3+ cat-ions are in the small cavities, while the concentration of
Na+ is very low. These interdependent factors are
responsible for the enhanced strength of the bridging hy-
droxyl groups in this zeolite. Although with divalent cat-
ions, for instance Mg2+ or Ca2+, a similar situation is
observed, the strength of the bridging hydroxyl groups
formed by hydrolysis of the exchanged divalent cations
is lower [8]. At present, it cannot be decided whether thishigher effect is related to a stronger polarizing effect of
the cation or whether this is related to the particular
location of the Brønsted acid sites.
4.3. Catalytic properties for alkylation of isobutane
with 2-butene
The catalyst lifetime in isobutane/cis-2-butene alkyl-ation is correlated with the fraction of strong Brønsted
acid sites (Table 1) [23]. It is interesting to note that
the absolute concentration of strong Brønsted acid sites
exerts a weaker influence than the fraction of strong acid
sites. This is well illustrated by the sample with 95.5%
IExD: it has a concentration of strong Brønsted acid
sites similar to that of samples with higher IExD, but
at the same time a higher concentration of weakBrønsted acid sites and a relatively short lifetime (4 h).
This short lifetime is related to the fact that the olefin
addition to an acid site to form a carbenium ion and
the subsequent insertion of other alkenes is much more
facile than hydride transfer. Catalysts with a high con-
centration of weak Brønsted acid sites may allow that
heavier olefins formed at these weak sites desorb and re-
act immediately with alkoxy groups at strong Brønstedacid sites leading to rapid chain growth. Thus, catalysts
with a high concentration of weak acid sites catalyze
mainly olefin oligomerization, leading to heavy alkylate
deposits, which in turn shorten the catalyst lifetime.
The catalysts with the longest lifetime showed also
the highest production of n-butane (Fig. 11). The selec-
tivity to n-butane depends on the relative rates of hy-
dride transfer and alkene addition in the initial cycleof butene addition. Thus, with zeolites with very strong
hydride transfer capabilities the sec-butyl alkoxy group
formed upon adsorption of linear butene on strong
Brønsted acid sites will undergo hydride transfer from
isobutane to form n-butane and a tert-butyl alkoxy
group before another butene molecule can react with
it. It is important to note that this can only occur, when
free Brønsted acid sites exist under reaction conditionsand hence also the tert-butyl alkoxy group can decom-
pose into isobutene and a free Brønsted acid site. Thus,
with higher hydride transfer rates and weaker adsorp-
tion of the olefins on strong Brønsted acid sites more
n-butane will be produced [23].
As already reported [23], the higher selectivity to
2,2,3-TMP (the slightly sterically hindered primary
product of alkylation) and to n-butane are the directconsequence of the improved hydride transfer with cat-
alyst lifetime. The higher concentration of 2,2,4-TMP
results from the availability of isobutene to alkylate a
tert-butyl alkoxy group. A higher concentration of
Brønsted acid sites is equivalent to a higher concentra-
tion of alkoxy groups. Thus, for a given flow of olefins
the probability that a carbenium ion/alkoxy group re-
acts sequentially with two alkene molecules decreases.This lowers in turn the danger of forming larger (surface
bound) alkenes/alkoxy groups. Note that the catalysts
with 97.7% and 99.6% IExD showed the most pro-
nounced hydride transfer.
It is interesting to note that among the catalysts
tested, the sample with 95.5% IExD has the lowest con-
centration of Lewis acid sites. Although it was shown
that a high concentration of Lewis acid sites is detrimen-tal for the activity in isobutane/cis-2-butene alkylation
[23], the present results seem to suggest that the concen-
tration of weak Brønsted acid sites has the most negative
impact on the catalyst lifetime.
The presence of weak Brønsted acid sites has been
mainly related to the presence of sodium cations in the
zeolites [18]. However, in the present system, the sodium
content is not the only factor that affects the acidic prop-erties of the final material. The lanthanum exchange de-
gree before the first calcination is also important. The
higher it is, the higher the concentration of acidic hydro-
xyl groups formed by hydrolysis at the La3+ ions. This
leads to a higher degree of dealumination during rehy-
dration of the calcined material in the first step. Thus,
the catalyst with 97.7% IExD (one exchange before the
first calcination and three exchange steps before the sec-ond calcination) had a longer lifetime (13.5 h) than the
catalyst with the lowest Na+ concentration (two ex-
changes before the first calcination and three exchange
steps before the second calcination; 12 h lifetime).
5. Conclusions
Hydrolysis of the hydrated lanthanum cations starts
below 393 K as evidenced by the bands corresponding
of La-OH (3520 cm�1) and bridging hydroxyl groups
(3600 and 3640 cm�1). As the calcination temperature
is further increased, dehydroxylation of the zeolite and
hydrolysis of the hydrated lanthanum cations take
place. While the La-OH (3520 cm�1) and one type of
bridging hydroxyl groups (3600 cm�1) decrease in con-centration, the other bridging hydroxyl groups
(3640 cm�1) increase.
318 A. Guzman et al. / Microporous and Mesoporous Materials 83 (2005) 309–318
Local changes of the structure take place during rehy-
dration of the La-Na-X zeolites after the first calcina-
tion. A considerable decrease of the bridging hydroxyl
band (3640 cm�1) and an increase of silanol band
(3740 cm�1) are observed after adsorption of water at
room temperature. These changes are attributed to thedealumination induced by the simultaneous presence
of water and acidic hydroxyl groups (especially those
at 3640 cm�1). The higher the IExD in the first exchange
step, the higher the concentration of protons produced
by water dissociation on La3+ cations and consequently
more extra-framework aluminum species are formed
during rehydration. It should be noted that this dealu-
mination only affects rather labile Al–O tetrahedra, asrehydration after the second calcination did not have a
recognizable effect on the acid sites.
The activity in isobutane/cis-2-butene alkylation is
mainly controlled by the fraction of strong Brønsted
acid sites and the space velocity of the olefin in relation
to the absolute concentration of these strong Brønsted
acid sites. Catalysts with similar concentration of strong
Brønsted acid sites and lower concentration of weakBrønsted sites showed higher lifetime.
Acknowledgements
Financial support from Sud-Chemie AG is gratefully
acknowledged.
The authors wish to thank Prof. Dr. Michael Hungerof University of Stuttgart for the NMR measurements
of some of our samples and for his discussions on struc-
tural properties of zeolitic materials.
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