Tuning supported Ni catalysts by varying zeolite Beta heteroatom composition: effects on ethylene adsorption and dimerization catalysis

Article abstract of DOI:10.1039/d1cy00308a

The influence of zeolite heteroatom composition on the electron density and catalytic activity of a supported Ni cation is examined. Ni-[X]-Beta catalysts, where X = Al, Ga, Fe, or dealuminated, were synthesized and characterized with probe molecule adsorption with FTIR spectroscopy and C₂H₄dimerization catalysis. It was observedviaCO adsorption that supported Ni cations were increasing in electron density in the order: [Fe] > [Ga] > [Al]. C₂H₄dimerization activity increased with increasing electron density of the Ni cation. Despite similarities in reported acid site strength, the acid sites on [Fe]-Beta in this work had significantly lower activity than those on [Ga]-Beta for the skeletal isomerization of linear butenes as well as C₂H₄dimerization. Introducing H₂as a reactant resulted in a decrease in dimerization activity for Ni-[Al]-Beta and Ni-[Ga]-Beta but an increase for Ni-[Fe]-Beta. The selectivity and activity of Ni-[DeAl]-Beta changed dramatically with the introduction of H₂, which subsequently converted all C₂H₄withca.100% selectivity towards C₂H₆(even with a lower space velocity relative to without H₂). These results demonstrate the ability of heteroatom composition to tune catalysis by using C₂H₄dimerization catalysis as a test reaction with zeolite Beta supported Ni catalysts.

Full text of DOI:10.1039/d1cy00308a

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Tuning supported Ni catalysts by varying zeolite
Beta heteroatom composition: effects on ethylene
adsorption and dimerization catalysis†
a
Cite this: DOI: 10.1039/d1cy00308a
and Ron C. Runnebaum *ab
Michael Meloni
The influence of zeolite heteroatom composition on the electron density and catalytic activity of a
supported Ni cation is examined. Ni–[X]-Beta catalysts, where X = Al, Ga, Fe, or dealuminated, were
synthesized and characterized with probe molecule adsorption with FTIR spectroscopy and C
dimerization catalysis. It was observed via CO adsorption that supported Ni cations were increasing in
electron density in the order: [Fe] > [Ga] > [Al]. C dimerization activity increased with increasing
2 4
H
2 4
H
electron density of the Ni cation. Despite similarities in reported acid site strength, the acid sites on [Fe]-
Beta in this work had significantly lower activity than those on [Ga]-Beta for the skeletal isomerization of
linear butenes as well as C
dimerization activity for Ni–[Al]-Beta and Ni–[Ga]-Beta but an increase for Ni–[Fe]-Beta. The selectivity and
activity of Ni–[DeAl]-Beta changed dramatically with the introduction of H , which subsequently converted
all C with ca. 100% selectivity towards C (even with a lower space velocity relative to without H ).
These results demonstrate the ability of heteroatom composition to tune catalysis by using C
dimerization catalysis as a test reaction with zeolite Beta supported Ni catalysts.
2 4 2
H dimerization. Introducing H as a reactant resulted in a decrease in
Received 19th February 2021,
Accepted 27th March 2021
2
H
2 4
H
2 6
2
DOI: 10.1039/d1cy00308a
2 4
H
rsc.li/catalysis
catalytic activities in acid catalyzed reactions such as alkane
cracking or methanol dehydration on the basis of the identity
Introduction
8
,9
Zeolites are widely used as supports for transition metal
catalysts because of their size selecting features, highly
uniform crystalline structure, and extra-framework cation
of the heteroatom.
Experimental measurements of the
intrinsic acidity of a zeolite can be difficult to interpret due
to convoluting effects when using probe molecules, such as
1
–3
10
exchange sites.
composed of TO
be substituted with trivalent cations such as Al , Fe , Ga ,
Zeolite frameworks are regular structures
tetrahedra, where T is generally Si but can
interactions with the zeolite framework.
However, the
4
+
4
theoretically calculated deprotonation energy (DPE) has
3
+
3+
3+
appeared as a good descriptor, correlating well with activity
3
+
4
8,11,12
and
B
(referred to as the heteroatom). The charge
of MFI zeolites for methanol dehydration.
In methanol
4
+
3+
imbalance between Si
compensated by an extraframework cation such as Na .
Zeolites are frequently used as solid acid catalysts with H as
the charge compensating extraframework cation. These cation
exchange sites can also become the ligands of a transition
metal dispersed on a zeolite. Zeolite supported metal catalysts,
such as Zn, Cu, and Fe, have a wide variety of applications and
are commonly studied catalysts for relevant reactions including
and the
T
heteroatom is
dehydration, this outcome is because the rate constant in the
zero-order kinetic regime is insensitive to van der Waals
+
+
interactions. The differential DPE is related to the ability of
−
the surface oxygens on the TO
4
tetrahedra to stabilize the
negative charge when various trivalent cations are substituted
into the framework. Zeolite used as a solid acid catalyst with
different heteroatoms have been observed to catalyze
reactions such as methanol dehydration, alkane cracking,
and toluene alkylation differently depending on the
5
–7
alkene upgrading and selective catalytic reduction.
Zeolites of the same framework (e.g. MFI, BEA) but with
different heteroatoms have been observed to exhibit different
1
1,13,14
framework composition.
+
+
When Na or H is replaced by a transition metal, one
would expect changes in electron density of the supported
metal. Because the support oxygens can be considered
a
15
Department of Chemical Engineering, University of California, Davis, CA, 95616
ligands on the supported metal,
this interaction is
USA. E-mail: rcrunnebaum@ucdavis.edu
b
analogous to changing the ligands on an organometallic
complex to alter the properties of the metal center. The
resulting changes in electron density of the metal are
anticipated to be consistent with reported differences in
Department of Viticulture & Enology, University of California, Davis, CA, 95616
USA
†
Electronic supplementary information (ESI) available: ESI includes additional
FTIR spectra, N physisorption, and pXRD. See DOI: 10.1039/d1cy00308a
2
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electron donating/withdrawing properties of the oxygens on
the TO4 when the identity of the heteroatom is varied. Other
was combined with 50 mL of 0.25 M Ga(NO ) in a thick walled
3
3
−
glass round bottom pressure vessel. The solution was stirred at
80 °C for 96 h, then centrifuged with DI water, NH ion
methods that have been explored to influence a supported
metal cation involve changing the zeolite framework or using
4
exchanged, and then calcined at 400 °C in 10% O to obtain H–
2
1
5
a metal oxide support. Because cation exchange sites are
[Ga]-Beta. [Fe]-Beta was synthesized using a procedure from Raj
1
6
35
similar within a given zeolite framework, such as BEA,
framework composition enables the creation of
et al. Typically 0.0775 g of NaOH and 0.0575 g of KOH were
added to 10.6 g of 35% TEAOH. 3.00 g of fumed SiO was added
2
extraframework cation sites that are geometrically similar
between chemically different zeolite supports.
to the solution and stirred for 4 h. A solution of 0.306 g ferric
sulfate (ACROS Organics) dissolved in 3.68 g of H O was slowly
2
C2H4 upgrading is an important class of reaction that
enables the creation of longer hydrocarbon molecules from
relatively simple building blocks, including those obtained
added to the previous solution over 30 minutes. The solution
(10SiO :5.0TEAOH:0.25Fe O :0.39NaOH:0.21KOH:41H O) was
then transferred to a 23 mL Teflon liner in a Parr autoclave
2
2
3
2
1
7–20
from more sustainable sources.
The process can be
vessel, hydrothermal synthesis took place statically at 140 °C for
environmentally friendly given C H can be more sustainably
produced from the conversion of ethanol or methanol.
15 days. The resulting white zeolite was washed with H O then
2
4
2
19,21,22
2
calcined in 10% O at 480 °C for 4 h with a 1 h hold at 120 °C
−1
Many of the catalysts used are transition metals dispersed on
zeolites such as Zn, Mo, and Rh for alkene/alkane
and ramp rate of 2 °C min . [Al]-Beta zeolite with Si/Al = 19
(NH –[Al]-Beta) was purchased from Zeolyst (CP814C). Zeolites
4
5
,23,24
aromatization and C
H
2 4
dimerization respectively.
While
4
were converted to NH –[X]-Beta via ion exchange by two
2
5
Rh-based catalysts are highly active for C H dimerization,
consecutive exchanges mixing the zeolite with 1 M NH NO for
2
4
4
3
the relatively high cost of Rh motivates the investigation of
other transition metal catalysts. Ni catalysts supported on
aluminosilicate zeolites such as Beta, Y, and MCM-41 have
24 h at room temperature. Samples were converted to H–[X]-
Beta by calcination in 10% O (zero air, Praxair), 90% N
(99.999%, Praxair) at 600 °C and 400 °C for 6 h for NH –[Al]-
2
2
4
26–28
been studied for C
H
2 4
dimerization.
Selectivity of the
4
Beta and NH –[Fe]-Beta respectively. H–[X]-Beta zeolites were
catalyst depends, in part, upon the mechanism of dimerization,
which can favor 1-butene or a mixture of linear butenes.
transferred, without exposure to ambient air, to an Ar-filled
glovebox after calcination. [DeAl]-Beta was synthesized by
30,31
Additionally, the presence of acid sites can catalyze the skeletal
isomerization of linear butenes to isobutene. In this work, we
report the impact of framework elemental composition of
Beta zeolites on the electron density of supported Ni cations,
as well as the resulting catalytic activity and selectivity when
used for C H dimerization catalysis.
dealuminating NH
The sample was washed with water, then calcined in 10% O
at 600 °C for 4 h with a 1 h hold at 120 °C and a ramp rate of
4 3
–[Al]-Beta in 13 M HNO at 80 °C for 16 h.
2
−
1
2 °C min before Ni deposition.
Ni was dispersed on H–[X]-Beta zeolites by mixing the
given support with Ni(acac) (acac = acetylacetonate; STREM
2
4
2
Chemical, min 95%) and n-pentane in a Schlenk flask such
that the Ni loading was 1 wt% (±0.02%, exact Ni loading was
calculated for each sample). The solution was stirred for 24 h
and then the solvent was evacuated for 24 h. After
Experimental
Catalyst synthesis
Zeolites will be referred to as M–[X]-Beta, where M is the
extraframework cation and X is the framework heteroatom.
Na–[B]-Beta was synthesized by adapting a procedure from
2
evacuation, the sample was calcined in 10% O at 550 °C for
−
1
4 h with 2 °C min ramp rate, then transferred without
exposure to air to a glovebox. All work was done in a glovebox
or with a Schlenk line to exclude moisture and oxygen.
Catalyst structure was determined by powdered X-ray
3
2
Reddy et al. Typically 12.5 g of colloidal SiO
Aesar) was added to 15.8 g of tetraethylammonium hydroxide
TEAOH 35% in water, Alfa Aesar). A solution of 0.398 g
BO (Fischer Scientific) dissolved in 9.19 g of H O was
2
(40 wt%, Alfa
(
2
diffraction (PXRD) and N physisorption with details reported
H
3
3
2
in the ESI† (Fig. S11–S14, Table S3). Catalyst structure was
confirmed by powdered X-ray diffraction (PXRD) and N2
physisorption with details reported in the ESI† (Fig. S11–S14,
Table S3). The XRD patterns confirm the zeolite Beta crystal
added to the previous solution and stirred for 1 h. A solution
of 0.346 g of NaOH (Honeywell) dissolved in 7.46 g of H O
2
was then added to the previous solution. The resulting gel
(
10SiO :7.3TEAOH:0.77H BO :1.0NaOH:110H O)
turned
structure for all catalysts. N adsorption experiments before
2
3
3
2
2
opaque and viscous upon addition of the final solution, then
was stirred overnight at room temperature. After stirring
overnight, the solution was clear and was seeded with ca. 50
mg of Na–[B]-Beta before being transferred to a 125 mL
Teflon lined Parr autoclave vessel. Hydrothermal synthesis
took place statically at 140 °C for 11 days. Template removal
and after Ni loading, in which relatively small changes in
surface area are observed, suggest the support is not
significantly affected by the metal deposition.
FTIR
was done by calcination in 10% O at 600 °C for 6 h, with a 1
Samples stored in a glovebox were pressed into self-
2
−
1
h soak at 120 °C and 2 °C min ramp rate. [Ga]-Beta was
synthesized via heteroatom re-insertion done by post-synthetic
supporting wafers, with approximately 25 mg of sample, then
loaded into a Harrick high temperature transmission cell.
Diffuse reflectance cell measurements were performed with
3
3,34
treatment of Na–[B]-Beta.
Typically, 1.0 g of Na–[B]-Beta
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3
6,37
pure zeolitic samples. Fourier transform infrared
spectrometry (FTIR) spectra were taken by a Bruker Tensor II
with 2 cm resolution and 128 scans collected per spectrum.
assigned to alkyl ligands,
reported in Table 1, at 2957,
−
1
2923, 2872, and 2846 cm . In addition to these bands the
−
1
Ni–[Ga]-Beta exhibits transient bands at 3013, 2944, 2890 and
−
1
For subtraction spectra, a spectrum (in N ) before reactive
2864 cm , shown in Fig. S1,† which are shared with Ni–[Fe]-
Beta. Ni–[Fe]-Beta is also characterized by bands, which are
listed in Table 1, at 3013, 2968, 2943, 2925, 2890, and 2863
2
gas (CO, NO, or C H ) treatment was subtracted from the
2
4
resulting spectra. N
2
2 4
(99.999%, Airgas) and C H (99.998%,
−
1
Matheson) were passed through traps containing reduced Cu
and molecular sieve to remove oxygen and moisture
respectively. CO (99.999%, Matheson) was passed through a
trap containing γ-Al O and molecular sieve to remove trace
cm . These bands likely correspond to butene adsorbed to
3
8
silanol and Ni sites. The Ni–[Ga]-Beta, Ni–[Al]-Beta, and Ni–
[Fe]-Beta samples showed a decrease in absorbance due to
terminal and bridging acidic silanols (3741 and 3636[Fe],
2
3
−
1
39–41
moisture and metal carbonyls.
3618[Ga], 3605[Al] cm , respectively)
dimerization products adsorb to acid sites at room
temperature. C adsorption onto Ni–[DeAl]-Beta resulted in
bands at 3016, 2961, 2925, and 2857 cm ; 3016 cm is
assigned to π-bonded C while the rest are assigned to
suggesting that
Catalysis: C H hydrogenation
H
2 4
2
4
−
1
−1
Catalysts, stored in an Ar-filled glovebox, were weighed
directly into a 6.35 mm OD quartz tube, and were diluted with
2
H
4
4
2
alkyl ligands. Experiments with the bare support, which
lacks dispersed Ni, did not show decreased silanol
absorbance after dosing C H , providing more evidence for
2 4
the adsorption of dimerization/hydrogenation products to
silanols. The control experiments without dispersed Ni also
provide evidence that extraframework heteroatoms are not
interacting directly with C H . C H adsorption gives some
1
2 3
00 mg of α-Al O . The samples in the quartz tube were
transferred from the glovebox to a three-zone furnace with the
exclusion of air and moisture. Prior to catalysis, samples were
heated to 350 °C for 2 h in N
to cool to 180 °C, they were used for C H dimerization. C H
4
2
. After the samples were allowed
2
4
2
dimerization was performed with all catalysts at 180 °C and
.16 kPa C , or in a separate experiment with 8.66 kPa H
99.999%, Praxair) at the same temperature and C H partial
2
4
2
4
2
2
H
4
2
2 4
insight into how the Ni sites interact differently with C H .
(
2
4
Ni–[Al]-Beta and Ni–[Ga]-Beta have similar bands
representative of alkyl ligands on the Ni sites. In addition to
these alkyl bands, Fig. S15† shows the bands corresponding
to isolated silanols and bridging acidic silanols (3740 and
618 cm , respectively for Ni–[Ga]-Beta, 3740 and 3607 cm
respectively for Ni–[Al]-Beta)
2 4
pressure. C H conversion was calculated via carbon balance
by using a gas chromatograph equipped with a flame
ionization detector. Calibration was done with a custom gas
2 4
mixture from Matheson. In all catalysis experiments the C H
−
1
−1
3
conversion was kept between 2–6% unless otherwise noted.
Blank reactions were run with the bare support; any activity
was subtracted from the Ni based catalysts by assuming a
4
0,43
decreased. No increase or
2 4
decrease of OH bands was observed when C H adsorption
+
2+
was performed with the bare support, suggesting that products
of dimerization could be adsorbed to various silanol sites. An
alternative explanation is that silanols near Ni sites are
participating in the adsorption/catalysis of C H . However,
isolated silanol bands decreased as well and by lack of
proximity should not be interacting with the Ni sites; therefore
this observed decrease in absorbance is more consistent with
consumption of 2H per Ni cation.
Results and discussion
2
4
C
2
H
4
adsorption
C H adsorption spectra of Ni–[X]-Beta zeolites are shown in
2
4
Fig. 1. Ni–[Ga]-Beta and Ni–[Al]-Beta share a set bands
2 4
the adsorption/hydrogenation of products rather than C H .
CO and NO adsorption: electron density of metal center
The relative electron density of the Ni cations was probed
with CO and NO adsorption because these probe molecules
are sensitive to the electron density of Ni. A reduction was
+
done before CO adsorption because Ni carbonyls, and not
2
+
Ni carbonyls, are sensitive to the electron density of the Ni
4
4
cation. CO adsorption was performed at 30 °C on all
catalysts after reducing the Ni sites in 10% CO, balance N
2
,
at 300 °C for 15 minutes and cooling in N to 30 °C. CO was
2
then dosed into the cell and spectra were recorded as CO was
purged out of the cell with N
2
. The spectra for Ni–[Al]-Beta,
Ni–[Ga]-Beta, and Ni–[Fe]-Beta, shown in Fig. S2–S4,†
Fig. 1
C
2
H
4
adsorption at room temperature onto oxidized Ni–[X]-
2+
+
respectively, follow similar patterns with Ni –CO and Ni –
CO) present initially. Then with increasing time after
evacuation of gas phase CO, Ni –CO desorbs while Ni –(CO)
Beta catalysts. Bands uniquely in the Ni–[Fe]-Beta spectrum are
assigned to adsorbed butene. The bands in the Ni–[Al]-Beta and Ni–
(
2
2
+
+
[
Ga]-Beta spectra are assigned to alkyl ligands of various length. Bands
2
+
position and assignments are reported in Table 1.
decomposes to Ni –CO. Band positions and assignment of
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Table 1 Bands after C
H
2 4
adsorption at room temperature onto oxidized Ni–[X]-Beta catalysts. Bands with marked with * are transient and decrease
with increasing time after C
H
2 4
exposure
Catalyst
Butene bands
Alkyl bands
Ni–[Al]-Beta
Ni–[DeAl]-Beta
Ni–[Fe]-Beta
Ni–[Ga]-Beta
—
—
—
—
2957, 2923
3016, 2961
—
2872, 2856
2925, 2857
—
3013, 2968, 2943
*3013, *2944
2924, 2890, 2863, 2854
*2890, *2864
2960, 2932, 2873
2
+
the Ni carbonyls are given in Table 2. In addition to Ni
are likely Ni –(NO) . However, due to the complexity of the
2
carbonyl bands, Ni–[Fe]-Beta, shown in Fig. S4,† also exhibits
a band at 2183 cm . This band corresponds to Fe –CO
spectra and relatively small amount of literature these
−
1
2+
assignments are tentative. The Ni–[DeAl]-Beta NO adsorption
−
1
likely formed from the reduction of extraframework Fe O .
has bands at 1879, 1869 and 1834 cm which have been
2
3
2
+
Fig. S5† shows CO adsorption onto Ni–[DeAl]-Beta. Bands
previously assigned to Ni –NO of Ni reoccupying silanol
−
1
47
at 2197, 2136, 2125, 2074, and 2090 cm are assigned to
nests.
2
+
+
+
+
+
Ni –CO, Ni –(CO) ,
Ni –(CO) , Ni –(CO)
Ni –(CO),
The relative electron density of the Ni cations was probed
with CO and NO adsorption. Band assignments for CO
adsorption are listed in Table 2, with the desorption spectra
2
2
2
respectively, re-occupying empty silanol nests that were
−
1
previously occupied by Al, where the 2125 and 2136 cm
+
bands are assigned to complex specified carbonyls. All
assignments agree with previous literature, except for the
shown in Fig. S2–S5.† The Ni –(CO)
bands for the Ni–[X]-
as
2
−
1
Beta catalysts are 2096, 2093, and 2091 cm for X = [Al], [Ga],
and [Fe] respectively. The decrease in wavenumber of this
band is consistent with a more electron dense Ni cation in
the order [Fe] > [Ga] > [Al]. NO adsorption onto oxidized
catalysts follow the same trend as CO adsorption with Ni–NO
bands decreasing in the order [Fe] < [Ga] < [Al]. Although we
did not directly probe the oxidation state of the Ni sites
−
1
43
2
074 and 2090 cm bands which will be discussed. At high
CO coverages, shortly after the CO pulse, the bands at 2125
−
1
−1
cm and 2074 cm dominate the spectra and decrease with
−
1
increasing time after the CO pulse. The band at 2090 cm
grows as the 2125 and 2074 cm
isosbestic point at 2083 cm indicates a direct conversion of
a dicarbonyl (Ni –(CO)
−
1
bands decrease. An
−
1
+
−1
+
2+
2
), 2074 cm , to a monocarbonyl (Ni –
during catalysis, it is likely the Ni is the active oxidation
−
1
(
CO)) at 2090 cm . These assignments are the opposite of
state given the recent experimental and theoretical reports of
2
+
+
27,29,30,48
those reported for low temperature CO adsorption, however
we believe the clear isosbestic feature in our data confirms
our assignment. NO adsorption was performed at room
temperature on previously oxidized catalysts. A comparison
of NO adsorption spectra, characterizing Ni–[X]-Beta catalysts,
is shown in Fig. S6.† NO was repeatedly dosed into the FTIR
cell and spectra were recorded. The NO adsorption bands for
all catalysts are summarized in Table 2. Ni–[Al]-Beta has
Ni being active while Ni is a spectator species'.
However, it is worth noting that there are reports of the C
dimerization activity correlated with concentration of Ni
2
H
4
+
4
9
sites.
C
2
H
4
dimerization activity and selectivity
Butene formation turnover frequency in the absence of H
2
is
−
1
−1
bands at 1896, 1888, and 1874 cm , 1896 cm has been
presented in Fig. 2. Ni–[Al]-Beta and Ni–[Ga]-Beta exhibit
activation periods at short times-on-stream, approximately
0.5 h, while the other catalysts do not show this behavior.
When comparing the activity of these catalysts for C H
2
+
previously been assigned to Ni –NO in extraframework
cation exchange sites. Bands at 1888 and 1874 cm are
likely due to Ni –(NO) as they are similar to those assigned
4
3
−1
2
+
2
2
4
in Ni–[Al]-MFI but we have insufficient evidence to make
definitive assignments. It is worth noting that there is lack of
consensus on the assignment as the Ni–[Al]-MFI bands have
dimerization, summarized in Table 3, the relative activity per
mole of supported Ni is in the order Ni–[Fe]-Beta > Ni–[Ga]-
Beta > Ni–[Al]-Beta > Ni–[DeAl]-Beta ranging from 2.12 ×
10 –1.00 × 10 [mol C H (mol Ni s) ]. Dimerization in the
4 8
2
presence of H , shown in Fig. 3, follows the same trend in
2
+
2+
45,46
−3
−4
−1
been separately assigned to Ni –(NO)
2
and Ni –NO.
−
1
Analogously, Ni–[Fe]-Beta bands at 1889, 1865, and 1841 cm
2
+
2+
2+
are likely due to Ni –NO and Ni –(NO) , Ni –(NO)
relative activity, however an activation period is only observed
2
2
respectively. Bands observed on Ni–[Ga]-Beta at 1903, 1898,
and 1893 cm are likely Ni –NO, while 1886 and 1839 cm
with Ni–[Al]-Beta. Ni–[DeAl]-Beta is omitted from this because
the activity for dimerization in the presence of H is essentially
2
−
1
2+
−1
Table 2 Summary of carbonyl bands after samples were reduced in 10% CO at 300 °C and nitrosyl bands of NO adsorption onto oxidized materials
2
+
+
+
+
+
2+
Catalyst
Ni –CO
Ni –(CO)2ss
Ni –(CO)2as
Ni –CO
Ni –(CO)
x
Ni –(NO)
x
Ni–[Al]-Beta
Ni–[Ga]-Beta
Ni–[Fe]-Beta
Ni–[DeAl]-Beta
2212
2209
2207
2197
2138
2136
2133
2096
2092
2089
2112
2107
2103
2090
—
—
—
1896, 1888, 1874
1893, 1903, 1898, 1886, 1839
1889, 1865, 1841
2137, 2124, 2074
1879, 1834, 1869
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Fig.
2
Turnover frequency of butene formation for Ni–[X]-Beta
. No activation period is observed
Fig. 3 Turnover frequency of butene formation in the presence of H
for Ni–[X]-Beta catalysts at 180 °C, 2.16 kPa C , and 8.66 kPa H
The activation period is observed for Ni–[Al]-Beta but none of the
other catalyst test in the presence of H
2
catalysts at 180 °C and 2.16 kPa C
when [Fe]- or [DeAl]-Beta was used as the support.
2
H
4
2
H
4
2
.
2
.
zero, with a 99.9% selectivity towards C H hydrogenation.
2
4
Comparisons between individual catalysts with and without
can be seen in Fig. S7–S9.† Activity towards butene
be associated with the formation of the active site (Ni(II)-
3
0,36
H
2
hydride or Ni(II)-alkyl/hydride).
The lack of observation of
formation in the presence of H2 decreases for Ni–[Al]-Beta
and Ni–[Ga]-Beta but results in an average 1.2-fold increase
with Ni–[Fe]-Beta. Butene selectivity for catalyst in the
absence of H2 is shown in Fig. 4. At 3.67 h on stream, all
catalysts exhibit high selectivity, 92–98%, towards butene
formation but at shorter times on stream, a lower selectivity
for some catalysts of 0.30, 0.69, 0.90, 0.98 for Ni–[Al]-, [Ga]-,
the activation period with Ni–[Fe]-Beta and Ni–[DeAl]-Beta,
suggests that the formation of this active site is significantly
faster on these catalysts than on Ni–[Al]-Beta or Ni–[Ga]-Beta.
The activity of the catalysts increases in the order Ni–[Fe]-
Beta > Ni–[Ga]-Beta > Ni–[Al]-Beta which is also the order of
electron density of the Ni cations as observed with CO
adsorption. There are proposals of the active site being Ni(II)
OH for dimerization that is formed due to Ni exchanging
2
[DeAl]-, and [Fe]-Beta respectively. In the presence of H , the
50
selectivities, shown in Fig. 5, at 3.67 h time-on-stream
decrease for all catalysts to 0.10, 0.44, 0.0, 0.52 for Ni–[Al]-,
with a single Al site, instead of a paired Al site. We think it
is unlikely that Ni(II)OH would survive our oxidative
treatment before catalysis, as there is literature that suggests
Ni(II)OH condenses to a Ni oxo-form after thermal treatment
[Ga]-, [DeAl]-, and [Fe]-Beta respectively. All catalysts except
for Ni–[DeAl]-Beta show a similar trend of increasing butene
4
5
selectivity until ca. 1.25
h
time-on-stream and then
on Ni exchanged MFI. While we do not have enough
information to determine if the Ni cations are associated
with a single or paired Al site, the Ni/[X] ratios, presented in
Table S2,† are low enough, and within the range of Co
exchanges observed in Beta by Dědeček et al.; it is likely most
Ni are sitting in paired Al sites. However, due to the difficulty
of identifying whether a cation is at a paired or isolated site
we cannot definitively assign the Ni cations to isolated or
paired [X] sites. The increase in activity suggests that the Ni
sites in Ni–[Fe]-Beta are better at stabilizing transition states
mediating the formation of the Ni–butene complex proposed
decreasing butene selectivity.
The relative activity in terms of moles of butene produced
per mole of Ni increases in the order Ni–[Fe]-Beta > Ni–[Ga]-
Beta > Ni–[Al]-Beta. Catalysis was performed with the bare
zeolite, before Ni was dispersed, to determine the effects of
acid sites and/or extra-framework species on catalysis. H–[Al]-
Beta and H–[Ga]-Beta had measurable but relatively low
activity compared to Ni–[Al]-Beta and Ni–[Ga]-Beta. H–[Fe]-
Beta had no measurable activity indicating that the acid sites
and extra-framework species are not active for
2 4
C H
4
8
dimerization under these conditions. An activation period is
observed with Ni–[Al]-Beta and Ni–[Ga]-Beta, which is not
observed with the other catalysts. This activation period may
to be the rate limiting step. In comparison to the Beta-
supported Ni, Rh–Y catalysts are more active. When operating
−
3
at PC2H = 40 kPa, T = 30 °C, the TOF with Rh–Y is 10.8 × 10
4
−1
2 4 4 8
Table 3 Summary of catalysis with materials at 180 °C and 2.16 kPa C H at 3.7 h time on stream. TOF units: [mol C H (mol Ni s) ], selectivity
presented as moles of listed product per mole of total product formation
3
Catalyst
TOF × 10
Butenes
Ethane
Isobutene
2-Butenes/1-butene
TOF3.7/TOF
i
Ni–[Al]-Beta
Ni–[Ga]-Beta
Ni–[Fe]-Beta
Ni–[DeAl]-Beta
0.87
1.2
2.1
0.95
0.98
0.98
1.0
0.023
0.016
0.019
0.0
0.068
0.065
0.013
0.0
7.6
7.6
7.2
3.1
0.75
0.70
0.80
0.09
0.10
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is more active, ca. 1.9-fold, and has a lower isobutene
selectivity than Ni–[Ga]-Beta, 0.001–0.01 compared to 0.17–
0.06 respectively, indicating the acid sites in [Fe]-Beta are less
active for the skeletal isomerization of butene. The difference
between activity of the H–[Fe]-Beta and H–[Ga]-Beta despite
the similarities in deprotonation energy can likely be
rationalized by considering how DPE is calculated. The DPE
is calculated as the energy difference between a neutral
zeolite framework and a non-interacting gas phase proton
8
,12
and negatively charged zeolite framework.
In the case of
C2H4 dimerization it is likely that charged carbocation
intermediates interact with the zeolite framework causing
5
3
framework distortion of the heteroatom sites.
framework distortions are likely different between H–[Ga]-
Beta and H–[Fe]-Beta resulting in a change in activity for C H
4
These
Fig. 4 Selectivity of butenes relative to total product formation for
2
catalysts in the absence of H . Ni–[Ga] and Ni–[Al]-Beta exhibit similar
2
trends in butene selectivity, including isobutene, starting low and
increasing to ∼90% after 2 h TOS. This change in selectivity over time
is consistent with a TOF induction period observed during catalysis.
dimerization and butene isomerization despite similar acid
site strength. The zeolite heteroatom elemental composition
influences the activity of the supported Ni cation, as well as
the activity of the acid sites for C H dimerization.
2 4
Reaction conditions: 180 °C and 2.16 kPa C H .
2
4
The overall selectivity towards butenes and the ratio of
2-butenes to 1-butene were characterized in order to
understand more about the possible mechanism of
dimerization between the catalysts as well as the isomerization
activity of the catalysts. The insertion–coordination (Cossee–
Arlman) mechanism has been observed on Ni dispersed on
−
1
−3 −1
25
s
and 58.5 × 10
s
2
(with 10 kPa H ). When using [Fe]-
Beta as a support to tune dispersed Ni, which is Earth-
abundant and less expensive, a TOF within an order of
magnitude of that of Rh–Y was achieved in the absence of co-
fed H2.
The catalysts that we would expect to behave the most
similar are Ni–[Ga]-Beta and Ni–[Fe]-Beta on the basis of the
similarities in acid site strength between [Ga]- and [Fe]-MFI
zeolites observed through methanol dehydration and
30,54,55
aluminosilicates.
potential competing dimerization mechanism) selectively
The metallacyclic mechanism (a
30,31
produces 1-butene,
while the Cossee–Arlman mechanism
produces a mixture of 1-butene, cis-2-butene and trans-2-
butene. In addition to this distribution of isomers, acid sites
(as well as Ni sites) in aluminosilicates have been observed to
be active for the skeletal isomerization of linear butenes. The
2-butene/1-butene ratios (Fig. 6) are consistent with the
Cossee–Arlman mechanism for all the catalysts tested.
The relative amounts of isobutene produced can enable
some insight into the nature of the acid sites as they facilitate
butene skeletal isomerization. The only samples that
produced relatively significant amounts of isobutene were
8
,51
calculated deprotonation energies.
Despite the similarities
in acid site strength, differences in activity are observed with
Ni–[Fe]-Beta being more active and selective towards butene
formation. In addition, catalysis experiments with H–[Ga]-
Beta were active for dimerization, while H–[Fe]-Beta had no
measurable activity for dimerization. These results are
similar to those observed with propene oligomerization when
using H–[Ga]-MFI and H–[Fe]-MFI, in which the H–[Fe]-MFI
5
2
catalyst is significantly less active. Alternatively, Ni–[Fe]-Beta
Fig. 5 Selectivity to butenes on the basis of total product formation
for conversions in the presence of H
2
. Except for Ni–[DeAl]-Beta, each
Fig. 6 Top: Selectivity of isobutene relative to total butene formation.
catalyst exhibits the same trend in selectivity with increasing TOS.
Bottom: Ratio of total 2-butene formation to 1-butene. Results for
Reaction conditions: 180 °C, 2.16 kPa C
2
H
4
, and 8.66 kPa H
2
.
2
dimerization carried out in the absence of H .
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Ni–[Al]-Beta and Ni–[Ga]-Beta. Although the theoretical acid
site strength of Fe substituted zeolites should be similar to
that of Ga substituted zeolites (as demonstrated for methanol
dehydration), Ni–[Fe]-Beta had higher selectivity towards
linear butenes, shown in Fig. 6, than Ni–[Ga]-Beta while
operating at a similar conversion. This suggests the residual
acid sites present in Ni–[Fe]-Beta are significantly less able to
catalyze the skeletal isomerization of linear butene to
isobutene. The Si/X ratio between [Fe]- and [Ga]-Beta are
similar as seen in Table S2† suggesting that there are not a
+
lack of H sites on Ni–[Fe]-Beta compared to Ni–[Ga]-Beta. In
addition, catalysis experiments with H–[Fe]-Beta showed no
activity for dimerization, in contrast with H–[Ga]-Beta which
had some activity for C H dimerization. The results show
that butene selectivity due to Ni sites, as well as H sites, is
altered by changing of the zeolite framework heteroatom
composition. Overall selectivity to butenes is enhanced,
especially at short times-on-stream, by using the zeolite
heteroatom to tune the dispersed Ni.
Fig. 8
C
2
H
6
production rate for the given catalysts in the presence of
2
4
+
H
2
(T = 180 °C; 2.16 kPa C
not true reaction rate due to the operation at 99.9%
conversion.
2 4 2
H , 8.66 kPa H ). The Ni–[DeAl]-Beta TOF is
a
2 4
C H
The addition of co-fed H
2
was examined because it has
hydrogenation. As shown in Fig. 7, the overall rate of
been demonstrated to enhance catalytic activity by some
butene production was ca. 0.9-fold lower than in the
5
6
30,54
catalysts anktive site for C H dimerization.
In addition,
absence of H at more than 2 h on stream for Ni–[Ga]-Beta
2
4
2
it is possible that co-feeding H
2
increases stability of the
and approximately the same for Ni–[Al]-Beta. The rate of
catalyst by creating conditions where products that would
2
deactivation decreases upon the addition of co-fed H for
otherwise deactivate Ni sites can be desorbed. Ni–[Fe]-Beta
all the catalysts. The rates of deactivation in the last 3 h of
catalysis for each catalyst are reported in Table S1.†
Mechanisms of deactivation have been proposed to be
accumulation of large hydrocarbon products that do not
desorb from Ni sites, which prevent the sites from
participating in further dimerization catalysis. It is likely
that the addition of H2 helps to prevent this buildup of
large molecular weight products by hydrogenating
intermediates before they can become too large to desorb
from active sites. The role of zeolite heteroatom resulted in
enhancing activity of supported Ni when the heteroatom
was Fe, while reducing activity when the heteroatom was Al
or Ga, and most dramatically shifting conversion completely
to hydrogenation products when dealuminated Beta was
used as the support.
was shown, in Fig. 7, to be ca. 1.2-fold more active when H
2
is present in the feed stream. The sample with the most
significant change in activity, shown in Fig. 8, was Ni–[DeAl]-
Beta, which switched from ca. 98% selectivity toward butenes
(a dimerization product) in the absence of H
2
to 100%
selectivity towards C H (a hydrogenation product) in the
2
6
presence of H
operated at nearly 100% conversion operating at W/F = 5.0 s
2 2
. In the presence of H , Ni–[DeAl]-Beta
−
1
−
1
compared to W/F = 8.7 s and 1.1% conversion in the
absence of H
2
.
Future work will elucidate details for this dramatic change
in activity of Ni–[DeAl]-Beta, but it is possible that cationic Ni
re-occupying empty silanol nests is highly active for C H
2 4
Conclusions
Ni was dispersed on Beta zeolites with different framework
heteroatoms: Al, Ga, and Fe. Differences in electron density
of the supported Ni cations were observed by CO and NO
adsorption experiments. Dosing of C H at room temperature
2
4
revealed differences in adsorbed species between the
catalysts. Ni–[Al]-Beta and Ni–[Ga]-Beta showed adsorbed
alkyl species of unknown degree of oligomerization, while
Ni–[Fe]-Beta exhibited bands representative of adsorbed
2 4 2
butenes. C H dimerization activity in the absence of H
increased in the order Ni–[Fe]-Beta > Ni–[Ga]-Beta > Ni–[Al]-
Beta > Ni–[DeAl]-Beta. Despite the similarities in the acid site
+
−
1
−1
strength between [Ga]-Beta and [Fe]-Beta, both the Ni and H
Fig. 7 The ratio of TOF [mol C
4 8 2
H mol Ni s ] in the presence of H
sites present in each catalyst have different activity and
to TOF in the absence of H . Ni–[Ga]-Beta and Ni–[Al]-Beta see a
2
decrease in TOF while Ni–[Fe]-Beta sees an increase.
selective for dimerization. Ni–[Fe]-Beta had higher activity
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and selectivity for butene formation than Ni–[Ga]-Beta. The
acid sites on H–[Ga]-Beta and H–[Fe]-Beta also exhibited
different activity for the dimerization of C H , as well as the
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Conflicts of interest
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There are no conflicts to declare.
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