Ethylene Dimerization and Butene Isomerization in Nickel-Containing MCM-41 and AlMCM-41 Mesoporous Molecular Sieves: An Electron Spin Resonance and Gas Chromatography Study

Article abstract of DOI:10.1021/jp9602181

Nickel-containing mesoporous materials MCM-41 and AlMCM-41 have been prepared by synthesis and ion exchange.These materials are catalytically active for ethylene dimerization and 1-butene isomerization.The catalytic activity for the dimerization reaction is shown to be due to nickel(I) species, while the 1-butene dimerization is dominated by the acidic properties of the MCM-41 materials.The ethylene dimerization activity and product distribution are different in nickel-exchanged Ni-MCM-41 and Ni-AlMCM-41 in contrast to Ni/MCM-41, where Ni(II) was introduced into the synthesis mixture.The activity for ethylene dimerization decreases in the order Ni/MCM-41 > Ni-AlMCM-41 > Ni-MCM-41, which reflects the concentration of the catalytically active nickel(I) species.The product distribution is dominated by the acidity of the MCM-41 materials, which drives the isomerization of the primary reaction product 1-butene into cis- and trans-2-butene.Electron spin resonance studies show that an isolated Ni(I) species can be stabilized, which converts to a Ni(I)-C2D4 complex after ethylene adsorption.After subsequent heating to 343 K two new species are observed, which can be ascribed to Ni(I) product complexes.

Full text of DOI:10.1021/jp9602181

9
906
J. Phys. Chem. 1996, 100, 9906-9910
Ethylene Dimerization and Butene Isomerization in Nickel-Containing MCM-41 and
AlMCM-41 Mesoporous Molecular Sieves: An Electron Spin Resonance and Gas
Chromatography Study
Martin Hartmann, Andreas P o1 ppl, and Larry Kevan*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641
X
ReceiVed: January 23, 1996; In Final Form: April 4, 1996
Nickel-containing mesoporous materials MCM-41 and AlMCM-41 have been prepared by synthesis and ion
exchange. These materials are catalytically active for ethylene dimerization and 1-butene isomerization. The
catalytic activity for the dimerization reaction is shown to be due to nickel(I) species, while the 1-butene
dimerization is dominated by the acidic properties of the MCM-41 materials. The ethylene dimerization
activity and product distribution are different in nickel-exchanged Ni-MCM-41 and Ni-AlMCM-41 in contrast
to Ni/MCM-41, where Ni(II) was introduced into the synthesis mixture. The activity for ethylene dimerization
decreases in the order Ni/MCM-41 > Ni-AlMCM-41 > Ni-MCM-41, which reflects the concentration of the
catalytically active nickel(I) species. The product distribution is dominated by the acidity of the MCM-41
materials, which drives the isomerization of the primary reaction product 1-butene into cis- and trans-2-
butene. Electron spin resonance studies show that an isolated Ni(I) species can be stabilized, which converts
to a Ni(I)-C
2 4
D complex after ethylene adsorption. After subsequent heating to 343 K two new species are
observed, which can be ascribed to Ni(I) product complexes.
Introduction
Ni(I) ions.12 To evaluate the acidity of the different MCM-41
materials, the acid-catalyzed dimerization of 1-butene is ex-
plored. The reactions are monitored by gas chromatography
The new crystalline mesoporous materials M41S, first
reported in 1992, are attracting increasing attention.1 Among
the different members of the M41S type of materials, the so-
called MCM-41 family, which exhibits a hexagonal array of
uniform mesopores, has been the focus of most studies. An
exciting property of these materials is the possibility to control
the internal diameter of the mesopores between 2 and 10 nm
(GC), electron spin resonance (ESR), and electron spin echo
modulation (ESEM) to detect different reaction products and
their interaction with the catalytically active Ni(I) center.
Experimental Section
2
,3
by the chain length of the micellar surfactant template.
A number of important applications of MCM-41 materials
Sample Preparation. MCM-41 and AlMCM-41 were
synthesized by hydrothermal synthesis using the procedures
4
5
both in catalysis and as advanced materials have already been
identified. The catalytic properties of molecular sieves can be
controlled by the incorporation of transition metal ion species
leading to specifically tailored catalytic application of these
materials. Isomorphous substitution of a fraction of the
6,13
described in previous publications.
was performed by adding 40 mL of 1 mM NiCl2 solution to
.25 g of MCM-41 or AlMCM-41. The resulting mixture was
Liquid state ion exchange
0
stirred for 18 h at room temperature. Finally, the samples were
washed with deionized water and filtered. These samples are
designated as Ni-MCM-41 and Ni-AlMCM-41. In both ion-
exchange procedures the amount of Ni(II) added initially to the
3+
framework silicon by Al leads to a significant increase in the
exchange capacity for transition metal cations.6,7 Isomorphous
substitution is often accompanied by the formation of Br o¨ nsted
acid sites. Depending on the nature and number of trivalent
-4
MCM-41 or AlMCM-41 materials corresponded to 1.5 × 10
3
+
3+
3+
mol/g.
framework cations, such as Al , B , or Fe , both the density
and the strength of the acid sites may be varied.8
In another preparation method, NiCl was introduced into the
2
synthesis mixture of MCM-41. The synthesis was performed
Transition metal ions like Ni(I) or Pd(I) can be active sites
in catalytic reactions such as ethylene and propylene dimeriza-
as follows: 2.2 g of NaOH was dissolved in 25 mL of H2O.
Subsequently 14.5 mL of 25 wt % cetyltrimethylammonium
chloride (CTAC) solution was added slowly and the resulting
mixture was stirred for 30 min. Then 0.206 g of NiCl2 was
added and stirred another 30 min. Finally, 20.5 mL of triethyl
orthosilicate (TEOS) was added to the solution. The obtained
gel was stirred for 1 h, transferred into a Teflon bottle, and
kept at room temperature for 72 h. After the reaction the product
was filtered and washed thoroughly with deionized water and
ethanol. Powder X-ray diffraction patterns taken before and
after calcination confirm that the hexagonal MCM-41 phase was
formed in this sample. The XRD spectra showed an intense
peak at 2θ ) 2.17°, indicating the existence of the hexagonal
tion as well as acetylene cyclomerization.9
-11
However, ap-
plication of transition metal ion species such as Ni(I) in micro-
or mesoporous materials requires information about the forma-
tion of the active species and a detailed characterization of the
metal ion environment. Additionally, the acidity of MCM-41
material has to be investigated since it has a significant impact
on the overall catalytic performance of a bifunctional catalyst.
In this study, we ion-exchanged Ni(II) ions by liquid state
reaction into pure siliceous (MCM-41) and into aluminum-
containing (AlMCM-41) materials. The Si/Al ratio of the latter
sample was 16. For comparison, a sample was prepared by
adding NiCl2 to the synthesis mixture of MCM-41. As a test
reaction, ethylene dimerization is used, which is catalyzed by
6
phase with a spacing of about d100 ) 40 Å. After calcination
the intense peak is shifted to 2θ ) 2.5° corresponding to d100
X
14
Abstract published in AdVance ACS Abstracts, May 15, 1996.
) 35 Å. Such a shift is frequently observed after calcination.
S0022-3654(96)00218-3 CCC: $12.00 © 1996 American Chemical Society

Nickel Containing Mesoporous Molecular Sieves
J. Phys. Chem., Vol. 100, No. 23, 1996 9907
TABLE 1: Ethylene Dimerization on Various MCM-41
Materials after 24 h at 343 K
catalyst
wt % conversion
wt % selectivity
MCM-41
0.3
1.7
5.0
0.4
4.5
>99
>98
>99
>99
>99
Ni-MCM-41
Ni/MCM-41
AlMCM-41
Ni-AlMCM-41
This sample was named Ni/MCM-41 where the Ni site is
unknown and is probably not in the framework.
Sample Treatment and Measurements. Prior to the
catalytic reaction the samples were dehydrated at 723 K in
vacuum [p < 10- (hPa)] for 18 h. Ni(I) is formed during this
activation process from Ni(II) by water or hydroxyl group
4
15
desorption. The catalytic ethylene dimerization was performed
3
inside a glass reactor of total internal volume of 51 cm using
1
00 mg of sample material. Ethylene (250 hPa) was adsorbed
on the activated sample at 343 K and 1200 hPa of helium was
also introduced.
The gas phase over the samples was analyzed periodically
by withdrawing an aliquot into a Varian Model 3300 gas
chromatograph equipped with a thermal conductivity detector.
A 6 ft column of 0.085 in. i.d. packed with 0.19 wt % picric
acid supported on 80/100 mesh graphic-GC support was used,
and all runs were conducted isothermally at 308 K. For the
butene isomerization reaction 500 hPa of 1-butene was adsorbed
on the samples. The reaction was also performed at 343 K.
All ESR spectra were recorded with a Bruker ESP-300
X-band spectrometer at 77 K. The magnetic field was calibrated
with a Varian E-500 gaussmeter. The microwave frequency
was measured by a Hewlett Packard HP 5342A frequency
counter. ESEM spectra were measured at 4 K with a Bruker
ESP 380 pulsed ESR spectrometer. Three pulse echoes were
measured by using a (π/2-t-π/2-T-π/2) pulse sequence as a
function of time T to obtain the time domain spectrum. The
deuterium modulations were analyzed by a spherical ap-
proximation for powder samples in terms of N nuclei at distance
R with an isotropic hyperfine coupling Aiso. The best fit
simulation of an ESEM signal is found by varying the
Figure 1. Ethylene dimerization products vs reaction time on (a) Ni-
MCM-41, (b) Ni/MCM-41 and (c) Ni-AlMCM-41 (b, 1-butene; 2,
cis-2-butene; 9, trans-2-butene). Reaction conditions: catalyst 100 mg,
T ) 343 K, p(ethylene) ) 250 hPa.
Figure 2. 1-Butene isomerization products vs reaction time on Ni-
AlMCM-41 (b, 1-butene; 2, cis-2-butene; 9, trans-2-butene). Reaction
conditions: catalyst 100 mg, T ) 343 K, p(1-butene) ) 400 hPa.
16
parameters until the sum of the squared residuals is minimized.
Ethylene, 1-butene, and cis/trans-2-butene were obtained from
Union Carbide Linde Division while ethylene-d4 (99% D) was
obtained from Cambridge Isotope Laboratories. All gases were
purified before use by repeated freeze-pump-thaw cycles.
TABLE 2: 1-Butene Isomerization on Various MCM-41
Materials after 24 h at 343 K
catalyst
wt % conversion
cis/trans ratio
MCM-41
AlMCM-41
Ni/MCM-41
Ni-MCM-41
Ni-AlMCM-41
5.97
15.78
15.67
12.53
21.52
0.81
0.71
0.72
0.78
0.53
Results
Catalytic Activity of Ni-Containing MCM-41 Materials.
The parent materials MCM-41 and AlMCM-41 are found to be
essentially inactive for ethylene dimerization under the condi-
tions applied. However, nickel-containing MCM-41 materials
are catalytically active for this reaction (Table 1). Analysis of
the gas phase after 24 h at 70 °C shows a total turnover of 5.0
wt % in Ni/MCM-41, 4.7 wt % in Ni-AlMCM-41, and 1.7 wt
To understand these differences we have also studied the
isomerization of 1-butene in all our samples at a reaction
temperature of 70 °C. An example of the time dependence of
this reaction is given in Figure 2. All catalysts were found to
be active for isomerization of 1-butene to cis- and trans-2-butene
(Table 2). The isomerization activity increases in the order
MCM-41 < Ni-MCM-41 < Ni/MCM-41 < AlMCM-41 < Ni-
AlMCM-41. The cis/trans ratio also decreases in this order.
ESR and ESEM. Prior to any treatment the nickel-
containing samples do not show any ESR signals at 77 K. Thus,
the nickel species exist in the form of Ni(II). Ni(I) can be
generated and stabilized in Ni-AlMCM-41 by thermal or
%
in Ni-MCM-41. In all cases the selectivity for the formation
of n-butenes is close to 100%.
After an initial period, 1-butene, cis-2-butene, and trans-2-
butene can be detected by gas chromatography. The distribution
of the n-butenes in the gas-phase changes with the reaction time
(
Figure 1a-c). In Ni-AlMCM-41 after a 24 h reaction period
the thermal equilibrium distribution of the n-butenes (∼10 %
-butene, 30% cis-2-butene, and 60% trans-2-butene) is ob-
1
1
3
tained, but in Ni/MCM-41 and Ni-MCM-41 the relative amount
of 1-butene in the gas phase is much higher (34 and 23%,
respectively).
hydrogen reduction and by γ-irradiation. The ESR spectrum
of Ni-AlMCM-41 after dehydration at 723 K is shown in Figure
3a. An isolated Ni(I) species A can be detected with g⊥ )

9
908 J. Phys. Chem., Vol. 100, No. 23, 1996
Hartmann et al.
Figure 3. ESR spectra at 77 K in Ni-AlMCM-41 after (a) dehydration
at 723 K and (b) ethylene adsorption and subsequent heating to 343 K
for 2 h.
Figure 5. Experimental (s) and simulated (- - -) three pulse ESEM
2
spectra at 4.2 K showing H modulation in Ni-AlMCM-41 after (a)
ethylene adsorption (g ) 1.92) and (b) 2 h of reaction (g ) 2.05). The
simulation parameters are in Table 3.
TABLE 3: 2H ESEM Parameters in Ni-AlMCM-41 after
Different Sample Treatments
catalysta
shell
N
R/nm
A
iso/MHz
Ni-AlMCM-41 D + ethylene
Ni-AlMCM-41 G + ethylene
Ni-AlMCM-41 D
after reaction
Ni-AlMCM-41 G
1
1
1
2
1
2
4
4
2
6
2
6
0.37
0.35
0.37
0.44
0.36
0.45
0.10
0.15
0.12
0.05
0.13
0.03
after reaction
a
D ) dehydrated sample; G ) γ - irradiated sample.
The analysis of the deuterium modulation of species B1 and
B2 shows the interaction of eight deuteriums with one nickel-
(I) center indicating butene formation (Figure 5b). The H
Figure 4. ESR spectra at 77 K of Ni-AlMCM-41 after (a) γ-irradiation
at 77 K and subsequent annealing at RT for 21 h, (b) after ethylene
adsorption onto (a) and (c) after heating of (b) to 343 K for 2 h.
2
ESEM fit parameters are summarized in Table 3.
2
.102. The parallel part of this species cannot be seen because
Discussion
of a very broad underlying species C (g ≈ 2.2), which can be
17
Ni(I) ions formed by reduction of Ni(II) can be stabilized in
ascribed to superferromagnetic Ni(0) clusters. A signal at g
2
0
18,19
-
zeolites and SAPO materials.
It has been shown that Ni-
∼
2.0 is attributed to O2 formed by water decomposition or a
1
8,19
(I) ions are catalytically active for ethylene dimerization in these
lattice defect as has also been observed in SAPO materials.
12,21
materials.
An isolated Ni(I) species A can also be stabilized
After ethylene adsorption and subsequent heating to 343 K,
several new species can be seen (Figure 3 b), but the signals
are distorted by superposition with species C. To identify these
species, γ-irradiation was used to generate Ni(I), since it
does not produce a large amount of superferromagnetic Ni(0)
clusters. After γ-irradiation, an isolated Ni(I) species A (g| )
in Ni-AlMCM-41 after thermal or hydrogen treatment or by γ
-
irradiation. Stabilization of Ni(I) in pure siliceous MCM-41
1
3
seems to be less efficient.
Species A was found to decrease significantly upon the
adsorption of ethylene accompanied by the formation of species
E with a rhombic g tensor indicating a lower symmetry for this
Ni(I) species E. As in zeolites and silicoaluminophosphates,
species E can be ascribed to a Ni(I)-C2D4 complex. ESEM
analysis indicates the formation of a π-bonding interaction
between Ni(I) and ethylene with a Ni(I)-D distance of 0.36 (
0.01 nm. After the sample is heated to 343 K, species E slowly
disappears and two new species B1 and B2 are seen. These
species can also be formed after n-butene adsorption on these
samples.
2
.521 and g⊥ ) 2.105) can be detected (Figure 4a). A Ni(I)-
C2D4 complex (species E) with g1 ) 2.677, g2 ) 2.464, and
g3 ) 1.916 is obtained directly after ethylene adsorption at
room temperature (Figure 4b). Further heating to 343 K leads
to the development of two new species B1 (g⊥ ) 2.023 and g|
)
2.051) and B2 (giso ) 2.07) (Figure 4c), which are also
obtained after n-butene adsorption. ESEM analysis at g ) 1.92
of the Ni(I)-C2D4 complex shows the interaction of four
deuteriums with a Ni-D distance of 0.35 nm indicating a typical
π-bonding interaction between Ni(I) and one ethylene molecule
ESEM analysis shows an interaction with eight deuteriums.
Due to the fact that there are three butene isomers with different
(Figure 5a).

Nickel Containing Mesoporous Molecular Sieves
J. Phys. Chem., Vol. 100, No. 23, 1996 9909
geometries, the complex geometry of the Ni(I)-C2D8 complex
is difficult to determine. Assuming a π-bonding interaction two
incorporation of aluminum into the walls of MCM-41 seems to
enhance the acidity. To test the acidity of the MCM-41
materials under the conditions present during ethylene dimer-
ization, the isomerization of 1-butene was used as a test reaction.
MCM-41 and Ni-MCM-41 are the materials with the lowest
1-butene turnover and the highest cis/trans ratio indicating the
lowest acidity for these materials. The increase in conversion
in nickel-containing MCM-41 is probably due to the formation
(cis/trans-2-butene) or three (isobutene) deuteriums should be
closer to Ni(I) than the other six or five deuteriums. This is
somewhat reflected by the ESEM results, which show two closer
nuclei with a Ni(I)-D distance of 0.36 nm and six more distant
deuteriums with a Ni(I)-D distance of 0.45 nm. Similar results
were also found in SAPO-5 and SAPO-11.2
1
2
9
We have shown in an earlier publication21 that in materials
of acid sites during the reduction of Ni(II) to Ni(I) or Ni(0).
The conversion of 1-butene is higher in AlMCM-41 than in
MCM-41 showing that the incorporation of aluminum into the
walls of MCM increases the acidity. This is also reflected in
the decrease of the cis/trans ratio. By ion exchange of Ni(II)
into AlMCM-41 the material again becomes more acidic, as
reflected by the increased turnover and a cis/trans ratio of 0.53
which is close to the thermal equilibrium value of 0.45. Ni/
MCM-41 seems to be comparable to AlMCM-41 in acidity,
indicating that Ni(II) might be incorporated into the walls of
the MCM-41 molecular sieve and therefore lead to a material
with enhanced acidity.
with smaller channels like SAPO-11 (0.63 by 0.39 nm) and
SAPO-5 (0.73 nm) Ni-C4D8 complexes with a large g-factor
anisotropy are obtained. The g-factor anisotropy decreases with
increasing channel diameter.21 Therefore, we expect a lower
anisotropy in this MCM-41 system. In fact, we obtained an
isotropic species and an axially symmetric species. These
species were also obtained in NiH-SAPO-8,22 a molecular
sieve with large straight channels of 0.87 by 0.79 nm diam-
eter. This is also consistent with results in X and Y zeolites,
which have cages of 1.2 nm diameter. In NiCa-Y zeolites there
are three different species with axially symmetric (g| ) 2.109
and g⊥ ) 2.023; g| ) 1.965 and g⊥ ) 2.620) and isotropic (giso
All samples deactivate after a longer dimerization reaction
time, most likely due to the reduction of the catalytically active
2.048) parameters.12 Surprisingly, none of these Ni(I) species
are found in MCM-41 materials or in silicoaluminophosphates.
)
2
1
Ni(I) to Ni(0) by ethylene or butene. Coke formation could
not be detected by ESR spectroscopy, which in this case should
show a strong isotropic signal at g ) 2.0 after a longer reaction
It has been reported that ethylene is initially dimerized to
23
1-butene over various transition metal ion exchanged zeolites.
3
0
time.
In a subsequent reaction step 1-butene is isomerized to an
Due to space limitations in zeolites and SAPO materials
n-butenes are the major dimerization products. In the larger
MCM-41 and AlMCM-41 materials the formation of larger
olefins could be more likely. However, the formation of
hexenes was not detected in the MCM-41 materials under
investigation. This is in contrast to previous results in the Ni-
equilibrium composition of n-butenes with predominant trans-
2
-butene (cis/trans ratio ≈ 0.5). In zeolites and SAPO materials
it has been shown that monovalent transition metal ions with a
9
d configuration like Ni(I) and Pd(I) are the catalytically active
sites for ethylene dimerization.1
2,24,25
The subsequent isomer-
ization to cis- and trans-2-butene is catalyzed by the acid sites
of the MCM-41 materials. Ethylene dimerization occurs in all
nickel-containing MCM-41 materials after dehydration at 723
1
1
(I)/SiO2 system, where the formation of hexenes is obtained
after a 24 h reaction period. One possibly relevant difference
between MCM-41 and silica is that the pore sizes are regular
in MCM-41 but have a significant distribution in silica encom-
pasing some much larger pores that could contribute to hexene
formation.
K, which produces Ni(I) by reduction of Ni(II) by desorbing
water or hydroxyl groups.15 The activity for the formation of
C4 olefins decreases in the order Ni/MCM-41 > Ni-AlMCM-
4
1 > Ni-MCM-41, which reflects also the order of initial Ni-
(II) concentration after ion exchange or synthesis. The differ-
Conclusions
ences between Ni-AlMCM-41 and Ni-MCM-41 are most likely
due to different ion-exchange sites leading to a higher Ni(I)
concentration in AlMCM-41 which is confirmed by our ESR
results. In MCM-41 Ni(I) replaces silanol protons, whereas the
introduction of alumina into the walls of AlMCM-41 produces
a net negative charge, which is initially balanced by sodium
ions. The different strengths and behaviors of these ion-
exchange positions were recently also shown in copper-
Ethylene dimerization occurs at 343 K in nickel-containing
MCM-41 and AlMCM-41 materials after dehydration at 723
K, but there are significant differences in the product distribu-
tion. In ion-exchanged Ni-MCM-41 and in synthesized Ni/
MCM-41 a higher relative concentration of 1-butene is detected
than in ion-exchanged Ni-AlMCM-41, where the thermal
equilibrium distribution of n-butenes is detected, suggesting that
the acidity of Ni-AlMCM-41 is higher. This assumption was
confirmed by investigating 1-butene isomerization, an acid-
catalyzed reaction. The turnover was maximal in Ni-AlMCM-
6
containing MCM-41 materials. The high turnover in Ni/MCM-
41 is somewhat surprising but is consistent with its higher initial
nickel concentration.
The isomerization of 1-butene to cis-2-butene and trans-2-
4
1, showing that this is the material with the highest acidity.
2
6
butene is catalyzed by acid sites of the molecular sieve. In
Ni-AlMCM-41 the thermal equilibrium distribution of n-butenes
is detected after ∼24 h, but in pure siliceous Ni-MCM-41 and
Ni/MCM-41 the amount of 1-butene in the gas-phase is much
higher (Figure 1, a and c). The thermal equilibrium distribution
is not even reached after 96 h of reaction. In NiCa-X the
The catalytic activity for the formation of n-butenes increases
with the incorporation of aluminum into the walls of the
MCM-41 structure. This is probably due to better stabiliza-
tion of the catalytically active Ni(I) in AlMCM-41. This is
supported by the ESR data, where significant amounts of
Ni(I) are only detected in the aluminum containing form.
After adsorption of ethylene a Ni(I)-C2D4 complex is obtained,
which is converted into a Ni(I)-C4D8 complex after the reaction.
The formation of these complexes is confirmed by ESEM
analysis.
1
2
equilibrium distribution is reached very rapidly (∼30 min),
which shows that X-zeolite has higher acidity compared to the
MCM-41 materials.
Preliminary, recently published results comparing the acidity
of AlMCM-41 and MCM-41 show that AlMCM-41 is slightly
more acidic than MCM-41.28 So far no detailed investigation
of the acidity of MCM-41 and AlMCM-41 has been published,
but our catalytic ethylene dimerization results show that the
Acknowledgment. This research was supported by the
National Science Foundation and the Robert A. Welch
Foundation.

9
910 J. Phys. Chem., Vol. 100, No. 23, 1996
Hartmann et al.
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