Dimerization of Linear Butenes on Zeolite-Supported Ni2+

Article abstract of DOI:10.1021/acscatal.8b03095

Nickel- and alkali-earth-modified LTA based zeolites catalyze the dimerization of 1-butene in the absence of Br?nsted acid sites. The catalyst reaches over 95% selectivity to n-octenes and methylheptenes. The ratio of these two dimers is markedly influenced by the parallel isomerization of 1-butene to 2-butene, shifting the methylheptene/octene ratio from 0.7 to 1.4 as the conversion increases to 35%. At this conversion, the thermodynamic equilibrium of 90% cis- and trans-2-butenes is reached. Conversion of 2-butene results in methylheptene and dimethylhexene with rates that are 1 order of magnitude lower than those with 1-butene. The catalyst is deactivated rapidly by strongly adsorbed products in the presence of 2-butene. The presence of π-allyl-bound butene and Ni-alkyl intermediates was observed by IR spectroscopy, suggesting both to be reaction intermediates in isomerization and dimerization. Product distribution and apparent activation barriers suggest 1-butene dimerization to occur via a 1′-adsorption of the first butene molecule and a subsequent 1′- or 2′-insertion of the second butene to form octene and methylheptene, respectively. The reaction order of 2 for 1-butene and its high surface coverage suggest that the rate-determining step involves two weakly adsorbed butene molecules in addition to the more strongly held butene.

Full text of DOI:10.1021/acscatal.8b03095

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Article
2+
Dimerization of linear butenes on zeolite supported Ni
Andreas Ehrmaier, Yue Liu, Stephan Peitz, Andreas Jentys, Ya-Huei (Cathy) Chin,
Maricruz Sanchez-Sanchez, Ricardo Bermejo-Deval, and Johannes A. Lercher
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03095 • Publication Date (Web): 29 Nov 2018
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ACS Catalysis
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Dimerization of Linear Butenes on Zeolite Supported Ni2+
Andreas Ehrmaier[1], Yue Liu[1], Stephan Peitz[2], Andreas Jentys[1], Ya-Huei (Cathy) Chin[1,3], Maricruz
Sanchez-Sanchez[1] *, Ricardo Bermejo-Deval[1]*, Johannes Lercher*[1,4]
[1] Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, D-85747
Garching, Germany
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[2] Evonik Performance Materials GmbH, 45772 Marl, Germany
[3] Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 3E5, Canada
[4] Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United
States
ABSTRACT: Nickel and alkali earth modified LTA based zeolites catalyze the dimerization of 1-butene in the absence of Brønsted
acid sites. The catalyst reaches over 95% selectivity to n-octenes and methylheptenes. The ratio of these two dimers is markedly
influenced by the parallel isomerization of 1-butene to 2-butene, shifting the methylheptene/octene ratio from 0.7 to 1.4 as the con-
version increased to 35 %. At this conversion, the thermodynamic equilibrium of 90 % cis- and trans 2-butenes was reached. Con-
version of 2-butene results in methylheptene and dimethylhexene with rates that are one order of magnitude lower than with 1-butene.
The catalyst is deactivated rapidly by strongly adsorbed products in the presence of 2-butene. The presence of π-allyl bound butene
and Ni-alkyl intermediates were observed by IR spectroscopy, suggesting both to be reaction intermediates in isomerization and
dimerization. Product distribution and apparent activation barriers suggest 1-butene dimerization to occur via a 1’-adsorption of the
first butene molecule and a subsequent 1’- or 2’-insertion of the second butene to form octene and methylheptene, respectively. The
reaction order of two for 1-butene and its high surface coverage suggest that the rate determining step involves two weakly adsorbed
butene molecules in addition to the more strongly held butene.
KEYWORDS: Linear alkenes; Nickel Lewis acid zeolite; dimerization; Nickel alkyl and LTA zeolite
INTRODUCTION
To achieve high rates and to avoid side reactions, Ni2+ cations
have to be well separated and the support should be free of other
strong acid sites, in particular Brønsted acid sites (BAS). Mes-
oporous and microporous crystalline aluminosilicates and
amorphous aluminosilicates are supports that can potentially
achieve such high Ni dispersion,22-29 provided that two Si-O-Al
exchange sites are sufficiently close to stabilize a Ni2+ cation.
Indeed, zeolites with high framework Al concentrations (Si/Al
~ 1-3) have been shown to be well suited. However, ion ex-
change with Ni2+ still leads frequently to isolated BAS that cat-
alyze alkene dimerization to dimethylhexene, but also lead to
isomerization, oligomerization and cracking.30-33
Linear and single-branched alkenes and in particular octenes are
important intermediates in the synthesis of high value products,
such as co-monomers for low density polyethene.1 The pathway
to produce these molecules via dimerization is attractive, be-
cause it utilizes abundantly present light olefins. Nickel-based
catalysts have been identified as the most promising family of
catalytic materials, exhibiting a high activity and selectivity to
linear alkenes.2-7 These catalysts favor the formation of dimers
and limit successive C-C bonding formation.
Mechanistically, α-alkene formation has been reported to occur
via a Ni-alkyl complex, following the Cossee-Arlman mecha-
nism, a metallacycles route, and proton-transfer mechanisms8-
10. Catalysts based on Ni dispersed on solid supports, including
molecular organic frameworks, were proposed to follow the
former mechanism.6, 11-18 19
The importance to avoid isomerization is best illustrated with
the fact that an additional pathway to branched alkenes is avail-
able for isomers with internal C=C bonds.13, 15 In the case of
dimerization of butenes, isomerization of 1-butene to 2-butene
opens this pathway to undesired products.34 Therefore, mini-
mizing the relative rate of double bond isomerization is a criti-
cal challenge for dimerization of alkenes with more than three
carbon atoms. Therefore, it is essential to combine a high con-
centration of isolated Ni2+ with the absence of Brønsted acid
sites in order to achieve high selectivity to linear and single-
branched alkenes. We have addressed this by choosing an LTA
zeolite to host the Ni2+, adjusting the other exchange cations not
only to minimize or eliminate the presence of Brønsted acid
sites, but also to adjust the electronic state of Ni2+.
Indeed, ethene oligomerization over Ni exchanged zeolites has
been proposed to proceed via formation of a 1’ alkyl carbenium
ion followed by a migratory insertion, while the products desorb
with a ß-hydride elimination (Cossee-Arlman mechanism).20 It
has been suggested that the density of liquid-like ethene is re-
quired to aid the desorption of butene, limiting C4 isomerization
and further polymerization via ethene addition.21
The present manuscript explores, therefore, partially Ni2+ ex-
changed Ca-LTA for butene dimerization. The narrow pores of
LTA allow only the conversion at pore mouth or on the outer
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surface. The rates of dimerization and competing isomerization
molC/g/s/bar2). Thus, Ni-Ca-LTA has a higher selectivity for tri-
1
2
3
4
are studied in the absence of Brønsted acid sites, combining
characterization of adsorbed species by IR spectroscopy and ki-
netic measurements. This study shows a unique chemical envi-
ronment for Ni+2, weakening its Lewis acidity by coadsorption
of a third butene molecule.
mers than for dimers in the oligomerization of 1-butene. This
selectivity is higher than observed for Ni-aluminosilicates in the
presence of Brønsted acid sites.26 Note that the operating tem-
perature below 200°C prevented cracking of 1-butene and prod-
ucts, as was observed with similar catalysts.25
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RESULTS AND DISCUSSION
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1-Butene dimerization on Ni-Ca-LTA catalysts
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At the start, a series of Ni-Ca-LTA-based catalysts with differ-
ent Ni-loadings was prepared and characterized with respect
morphology and crystallinity in order to select an adequate cat-
alyst for mechanistic studies. Details are provided in the Sup-
porting Information. Broadening of the XRD diffraction bands
of LTA was observed with increasing Ni loadings (see SI, Fig-
ure S1), indicating certain loss of crystallinity due to the ion
exchange.35-36 While a hysteresis for the N2 sorption was not
observed for Ca-LTA (Figure S2), the sorption volume of the
Ni containing catalysts showed important differences during
adsorption and desorption, pointing to an increase of mesopore
and a decrease of the micropore volume (Table S1). To mini-
mize the impact of local lattice destruction, the loading of Ni
was limited to 6 wt.% on Ni-Ca-LTA. Changes in the particle
size were not observed by SEM (Figure S3 and Table S2).
Figure 1. Reaction of 1-butene over a 6 wt.% Ni-Ca-LTA. Butene
consumption rate (black squares) and formation rates of dimeriza-
tion products over time on stream with respect to moles of carbon.
T = 160 °C, p = 50 bar, WHSV = 38 h-1.
Analysis by X-ray absorption spectroscopy (XAS) was used to
establish the oxidation state of the Ni in Ni-Ca-LTA in contact
with 1-butene. Variations in the Ni-K edge energy (8333 eV)
prior and after exposure to 1-butene were not observed in the
X-ray absorption near edge structure (XANES). This indicates
that Ni is present as Ni2+ in the acting catalyst (Figure S4 A) and
Product distribution with Ni-Ca-LTA catalysts
11,
we conclude that Ni2+ is the active center for dimerization.
The oligomerization of 1-butene on a 6 wt.% Ni containing
LTA catalyst was conducted at 50 bar, 160 °C and space veloc-
ities ranging from 6 to 244 h-1 (Figure 2). Results show higher
than 80 % selectivity towards octenes, with less than 20 % of
higher oligomers – mostly trimers (not shown). Within the di-
mer fraction, the main products were 3-methylheptenes and n-
octenes (Figure 1 and Figure 2 A). Only small amounts (less
than 5 % among the dimers) of 3,4-dimethylhexenes were
formed. The thermodynamic dimer equilibrium at the given
conditions predicts a ratio of dimethylhexene / methylheptene /
octene of 34 / 60 / 6 (Figure S6 in SI), i.e., the products observed
are quite distant from equilibrium.
37,38
The weight based rate of 1-butene dimerization was determined
for catalysts containing 2-6 % Ni (Figure S5, Table S3). The
rate of dimerization increased linearly with the Ni concentra-
tion. This led us to conclude in turn that all (or a constant frac-
tion) of Ni2+ is accessible to the reactants at this level of ion ex-
change. Therefore, the detailed mechanistic studies were per-
formed on a 6 wt.% Ni exchanged Ni-Ca-LTA catalyst.
The butene conversion rates and product rate distribution upon
time on stream for a 6 wt.% Ni exchanged Ca-LTA are shown
in Figure 1. Rates in butene conversion were the highest within
the first five hours (1.5·10-4 molC /g/s). These rates were less
than an order of magnitude lower than rates reported for zeolites
25-27
and mesoporous aluminosilicates with BAS.11,
Subse-
quently, slight deactivation was observed with time on stream.
For the mechanistic and kinetic studies in this work, the re-
ported activity data were extrapolated to zero time on stream
(TOS). Rates to octene and methylheptene were comparable (6-
7·10-5 molC /g/s), while rates to dimethylhexene were of one or-
der of magnitude lower. To compare the intrinsic dimerization
and trimerization activity of Ni-Ca-LTA, we assumed an over-
all second order reaction for each pathway, i.e., second order in
butene for dimerization and first order in both butene and octene
for trimerization. A higher apparent second reaction order con-
stant was observed for trimerization (ktri
=
=
5.67·10-9
1.67·10-9
molC/g/s/bar2) than for dimerization (kdi
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ACS Catalysis
bond isomerization, i.e., the increasing concentration of 2-bu-
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tene, limits the conversion level.
A
Butene
The dimer selectivity as a function of 1-butene conversion is
shown in Figure 2 B. Methylheptene and octene were the pre-
dominant products of 1-butene dimerization, while dime-
thylhexene was formed in significantly lower concentrations.
The changes in dimer product selectivity and the double bond
isomerization with butene conversion show that octene is
formed solely from 1-butene, and that dimethylhexene is solely
formed from 2-butene dimerization.
Octene
Methylheptene
Dimethylhexene
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Methylheptene can be formed from both 1- and 2-butene.
Scheme 1 shows the reaction pathways proposed for the dimer-
ization of 1-butene. The adsorption of the first butene molecule
on the Ni active site is hypothesized to take place at the primary
(1’-adsorption) or at the secondary carbon atom (2’-adsorp-
tion).20, 39-42 The insertion of the second butene for C-C bond
formation, can also take place at the primary (1’-insertion) or
secondary (2’-insertion) carbon. The modes of adsorption and
insertion and their probability determine the selectivity to the
dimer isomers (octene, 3-methylheptene and 3,4-dimethylhex-
ene). The obtained product distribution indicates that the main
route in dimerization with Ni-Ca-LTA takes place either by 1’-
adsorption of 1-butene and 1’-insertion or 2’-insertion of 1-bu-
tene, because n-octene and methylheptene were formed prefer-
entially.
0
0
0.1
0.2
Space time / h
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B
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20
0
Octene
Methylheptene
Dimethylhexene
At higher conversions, the increase in selectivity to methylhep-
tene and dimethylhexene is attributed to a higher contribution
of the 2’-adsorption and/or 2’-insertion pathway. This is specu-
lated to be also the result of a higher 2-butene concentration
from the double bond isomerization of 1-butene (the options for
2-butene are shown in green color in Scheme 1). Therefore, oc-
tene and methylheptene are concluded to be primary products,
dimethylhexene to be a secondary product. It is noted in passing
that, the absence of Brønsted acid sites (BAS) is responsible for
the low dimethylhexene selectivity. In presence of the latter for-
mation of dimethylhexene (selectivity above 30%) is favored
by the formation of secondary carbenium ions. 11, 28, 37, 43
0
20
40
Conversion / %
Figure 2. A) Effect of space time on catalytic conversion rate of 1-
butene (black squares) and dimer distribution, and B) Dimer selec-
tivity as a function of the conversion of 1-butene over a 6 wt.% Ni-
Ca-LTA catalyst. T = 160 °C, p = 50 bar, WHSV = 6-245 h-1.
The 1-butene consumption rate with varying space time is
shown in Figure 2 A. Upon higher space times, 1-butene con-
sumption leveled off at a rate of 5·10-5 molC /g/s, corresponding
to ca. 35 % conversion. It was not possible to surpass this level
of conversion by an increase in space time. However, this value
is far from the dimerization thermodynamic equilibrium (close
to 100 % 1-butene conversion as shown in Figure S6 in SI). As
the conversion increased, 1-butene was isomerized to 2-butene,
reaching the thermodynamic equilibrium (Figure 3 A) at the
point (Figure 2 A) when the 1-butene conversion rate in dimer-
ization leveled off, at approx. 5·10-5 molC /g/s (conversion val-
ues of ~30-35 %, grey bar in Figure 3 A). Figure 3B shows the
approach to equilibrium for the two parallel reactions: dimeri-
zation and butene double bond isomerization (defined as the ra-
tio of the different isomers, see SI for details). Isomerization
increased rapidly to the value of one with butene conversion,
indicating equilibration, while dimerization did not exceed the
value of 0.35 (Figure 3B ). Therefore, it is suggested that double
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A
80
60
40
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0
trans-2-Butene
cis-2-Butene
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1-Butene
20
0
40
Conversion / %
Scheme 1. Possible pathway for the formation of the dimer iso-
mers from 1-butene (black) and 2-butene (green). Adapted from
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0.5
0
B
Cossee-Arlman mechanism.39
In order to understand the low activity of Ni-Ca-LTA in the di-
merization of 2-butenes, the feed was switched between 1- and
2-butene (Figure 5). The first transient from 1-butene to cis-2-
butene (Figure 5A) led to a rapid decrease in butene conversion
rate. In a second experiment, the reactor was first fed with 2-
butene and subsequently switched to 1-butene. This transient
increased the dimerization activity, but to a level that was ap-
proximately one order of magnitude lower than that starting
from 1-butene (Figure 5B, and comparison to 5A). Thus, we
conclude that 2-butene induced irreversible changes to the most
active sites of Ni-Ca-LTA.
Dimerization
Isomerization
0
0.1
Residence time /h
0.2
Based on the above results, we conclude that 1-butene isomer-
izes to 2-butene, inducing rapid deactivation, especially at
higher conversions. The higher concentration of 2-butene com-
pared to 1-butene (90 % 2-butene and 10 % 1-butene at conver-
sions above 30 %, Figure 3 A) leads also an increase in the se-
lectivity to products formed from the 2’-adsorption pathway,
i.e., methylheptene and dimethylhexene (Figure 2 B). However,
the selectivity to n-octene is still higher than that to dime-
thylhexene, indicating a significantly faster dimerization for 1-
butene than for 2-butene.
Figure 3. A) Reactant isomerization behavior over 6 wt.% Ni-Ca-
LTA. The dotted lines give the equilibrium at 50 bar and 160 °C.
B) Approach-to-equilibrium for dimerization and isomerization.
Reaction conditions: T = 160 °C, p = 50 bar, WHSV = 6-245 h-1.
Impact of 2-butene on the deactivation of Ni-Ca-LTA
The limiting butene conversion of 35 % (leveling off at a rate
of 5·10-5 molC /g/s, Figure 1) and chemically equilibrated dou-
ble bond isomerization, taken together, suggest that 2-butene
hinders further dimer formation (Figure 3). To test this hypoth-
esis, pure cis-2-butene was fed and oligomerized (Figure 4).
With cis-2-butene, rates were significantly lower and decreased
much more rapidly (Figure 4A), as well as higher selectivities
to methylheptene and slightly higher selectivities to dime-
thylhexene (Figure 4B) were observed. Therefore, following
the reaction pathway of Scheme 1, 2-butene can only adsorb via
2’-adsorption (Scheme 1, green color), leading to branched
products only. The fact that n-octene was also formed even at
lower conversions, suggests that a small fraction of 2-butene
was isomerized to 1-butene, which in turn acted as precursor to
n-octene.
The low activity in 2-butene dimerization is hypothesized to be
caused by the formation of a stable Ni-alkyl species either via
the 2’-adsorption or via the insertion of another alkene (1-bu-
tene or 2-butene) into the secondary carbon of the adsorbed 2-
butene. Thus, the deactivation over time observed in Figure 1 is
concluded to be caused by the high concentration of 2-butene
(ca. 2/3 of butene at 20% butene conversion), leading to the for-
mation of a surface species that is unable to desorb.
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switch to 2-butene
1-Butene
cis-2-Butene
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1-butene
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0.1
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Octene
switch to 1-butene
Methylheptene
Dimethylhexene
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Conversion / %
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Figure 5. Effect of cis-2-butene on butene consumption rate over a
6 wt.% Ni-Ca-LTA, at T = 160 °C and P = 50 bar, starting with 1-
butene and switched to cis-2-butene (WHSV = 50 h-1) (A), and ex-
periment started with cis-2-butene and switched to 1-butene
(WHSV = 50 h-1) (B).
Figure 4. Conversion rate, in terms of the moles of carbon (A), and
product distributions (B) of 1-butene and cis-2-butene dimerization
reaction over 6 wt.% Ni-Ca-LTA at 160 °C, 50 bar, WHSV graph
A = 12.5 h-1; WHSV graph B =12-245 h-1. Conversion of cis-2-
butene is shown as empty triangles, reaction of 1-butene is pre-
sented as filled squares, and so are the corresponding products. Col-
ors of products: n-octene (blue), methylheptene (orange), dime-
thylhexene (green).
To analyze the carbonaceous deposits resulting from exposure
to 2-butenes, spent catalysts were dissolved in HF and the re-
mainder was subsequently dried at 90 °C. The organic residue
was dissolved in hexane and analyzed by gas chromatography.
A higher concentration of C16+ was observed after 2-butene di-
merization, while concentrations of hydrocarbons between C12
and C16+ were similar to those obtained after 1-butene dimeri-
zation. This suggests that 2-butene dimerization forms larger
oligomers that block and deactivate Ni2+ sites.
Kinetics of 1-butene dimerization
The reaction orders obtained for the formation of octene and
methylheptene (Figure 6), with respect to 1-butene, were ap-
proximately 2 (Figure S7 C and D). This points to the C-C cou-
pling of the two weakly adsorbed butenes as the rate determin-
ing step (r.d.s.), in agreement to studies on ethene dimeriza-
tion.21, 44-46 The measured activation energies for the formation
of the specific dimers, as well as for the different oligomers, are
shown in Table 1 (Arrhenius plots are shown in Figure S7 A
and B in SI). The reaction order and activation energy were not
measured for dimethylhexene, since it is a secondary product.
Among the dimers, the values of apparent activation energies
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(Ea) determined for methylheptene and octene were similar (76 Exposing the activated zeolites to 1-butene and subsequent
1
2
3
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8
and 72 kJ/mol, respectively) and indicate that both products
stem from a pathway with a similar transition state. The values
were similar to the ones obtained by Zhang et al. with Ni-ZSM-
5.47 As previously shown in Scheme 1, the formation of dime-
thylhexene requires the 2’adsorption and 2’insertion of 1-bu-
tene or 2-butene. Therefore, prior to the formation of dime-
thylhexene, isomerization of 1-butene into 2-butene must take
place.
evacuation led to IR spectra of adsorbed 1-butene on Ca-LTA
and Ni-Ca-LTA that are compiled together with the assign-
ments in SI-5 (Table S4, Figure S8, and Figure S9). The spectra
of 1-butene adsorbed on Ca-LTA and Ni-Ca-LTA showed
bands at 1300-1700 cm-1 (Figure S8), which are characteristic
of C=C stretching, C-H bending and carbonate or carboxylate
vibrations. The assignments and the discussion on the carbonate
bands are provided in SI-5. The IR spectra of 1-butene adsorp-
tion on Ca-LTA and Ni-Ca-LTA in the 2700-3200 cm-1 region
are shown in Figure S9. Admission of butene leads to C-H
stretching bands on both samples. The bands at above 3000 cm-
1 are attributed to C-H stretching vibrations at C=C bonds, while
the bands below 3000 cm-1 are attributed to C-H vibrations at
C-C bonds of physisorbed alkenes.48-49
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4
1-Butene
3
After activation of Ni-Ca-LTA, 1-butene was adsorbed at con-
stant pressure (1 mbar) and IR spectra (Figure 7) showed bands
characteristic of bidentate carbonate as described in SI-5. The
most pronounced bands between 1620 and 1660 cm-1 are as-
signed to π-bonded butene on Ni2+. Directly after exposing the
sample to a stream of 1-butene, a band at 1622 cm-1 with a
shoulder at 1658 cm-1 (attributed to gaseous 1-butene50) was ob-
served. The wavenumber at 1622 cm-1 tentatively attributed to
π-bonded 1-butene.48, 51-52 After 10 min, a band at 1641 cm-1
evolved. It is attributed to an adsorbed alkene with an internal
double bond (π-bound 2-butene or a branched octene), since the
higher wavenumber indicates a stronger C=C bond for 2-butene
than for the external C=C bond observed for 1-butene (1622 cm-
1).50, 53 This is further supported by the variations of the relative
intensity of the CH vibrations at 1440, 1408, 3061 and 3005 cm-
1 (Figure 7), indicating an increasing number of =CH vibrations
in the surface species.
Octene
2
1
Methylheptene
0
0
20
40
60
p1-Butene / bar
Figure 6. Reaction of 1-butene consumption and formation of
methylheptene and octene over 6 wt.% Ni-Ca-LTA.. T = 160 °C, p
= 50 bar, WHSV = 15-156 h-1; 1-Butene feed mix diluted with pro-
pane: c1-butene = 20 – 86 wt.%.
The changes observed in the concentration of methyl groups of
the adsorbed hydrocarbons allowed us to follow double bond
isomerization and/or dimerization reactions of 1-butene. An in-
crease in the intensity of CH3 group vibration (1390 cm-1, Fig-
ure 7A), especially within the first 30 min, and in the CH3 bend-
ing vibration (2873 and 2967 cm-1, Figure 7B), suggests that
several CH3 groups were formed, as the combined result of
isomerization and alkene addition. Because the intensity of the
CH2 bands (2841 and 2939 cm-1) remained constant throughout
the experiment, we conclude that double bond isomerization
and the formation of surface alkyl species are the main surface
reactions under these conditions. Within the 30 min of expo-
sure, two additional bands appeared at 1585 and 1295 cm-1. The
lower wavenumber is attributed to a delocalized C=C bond
stretching vibration, tentatively assigned to π-allyl formation.54
In addition, the band at 1295 cm-1 is assigned to the stretching
of a carbenium ion (νas C+-C).55-57 The allyl species is hypothe-
sized to lead to isomerization, while the carbenium ion is sug-
gested to be a key intermediate in the dimerization (Scheme 2).
Table 1. Energy of activation for the butene consumption, as
well as for the formation of products at conversion levels be-
tween 0.5 and 3.5 %. WHSV = 150 h-1, p = 50 bar, T = 140-180
°C. Arrhenius plots are shown in SI (Figure S7).
EA
[kJ mol-1]
C4 consumption
Methylheptene
Octene
73
76
72
IR spectra of adsorbed and reacted butenes
IR spectroscopy is used to characterize the adsorbed species and
monitor the gradual isomerization of adsorbed butenes as reac-
tion proceeds. The basic nature of the catalysts stabilizes car-
bonates, which change in nature and concentration during reac-
tion. We hypothesize that these carbonates are largely specta-
tors to the catalytic transformations. However, their presence
will moderate the acid-base properties of the investigated cata-
lysts.
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Scheme 2. Surface intermediates detected upon butene adsorp-
tion on Ni-Ca-LTA and their proposed reactivity.
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Figure 7. IR spectra of 6 wt.% Ni-Ca-LTA at 40 °C exposed to 1
mbar of 1-butene after 1-30 minutes A) 1300-1700 cm-1 range, B)
2700-3200 cm-1 range.
Enthalpic contribution of adsorption of butenes on Ni-Ca-
LTA
The heat of adsorption of 1-butene on Ni-Ca-LTA was of 63 kJ
mol-1 (Figure S10). A Langmuir approach was used to fit the
isotherm, with a maximum surface coverage of 0.73 mmol1-bu-
tene/g and an adsorption constant K of 4.9·10-3 bar-1 (SI-6) at 40
°C. This adsorption constant at 40 °C gives the adsorption con-
stant of 6.0·10-6 bar-1 at 160 °C, according to van’t Hoff equa-
tion (SI-6). This adsorption constant translates to Ni2+ sites
nearly saturated with 1-butene, as predicted by Langmuir ad-
sorption isotherm (P1-butene = 42.5 bar). The high adsorption en-
thalpy of 63 kJ mol-1 indicates a strong interaction of the alkene
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with the Ni, leading to the formation of a Ni alkene complex,
which simplifies to an apparent 2nd order dependence at low sur-
potentially increasing the electron density at Ni2+. It should be
noted in passing that this hypothesized interaction was not man-
ifested in XANES at the Ni-K edge energy (8333 eV).
face coverages in the limit of low product concentration. The K
and k are the equilibrium constants for adsorption and elemen-
tary C-C bond formation rate constant of the kinetically relevant
step, as also defined in Scheme 3.
1
2
3
4
The adsorption of 2-butene was also examined (Figure S11). A
successive weight increase was observed within the first 12
hours at 0.03 mbar of 2-butene. The initial adsorption enthalpy
was, however, approximately 63 kJmol-1 and hence similar than
for 1-butene.
5
6
7
8
It should be noted that, according to Scheme 3, an internal hy-
dride shift (step 3a) is necessary, before the products can desorb
from the Ni site and regenerate the active site to form a new Ni-
alkyl complex. Brogaard et al.20 have estimated that Ni-alkene-
alkyl complexes of different alkyl chain length are quite close
in free energy in the Cossee-Arlman pathway. Similarly, the
proposed mechanism suggests the change to a larger adsorbate
for the Ni-alkene-alkyl complex, evolving from a Ni-alkene-al-
kylC4 to a Ni—alkene-alkylC8. The reason for this modified
Cossee-Arlman mechanism on Ni-zeolite is attributed to the ab-
sence of Brønsted acid sites.20 The lack of Brønsted acidity also
agrees well within the high yields to linear and mono-branched
C8 isomers, because dibranched dimer are favored by Brønsted
acid catalyzed dimerization.32-33
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Catalytic pathway of butene dimerization on Ni-Ca-LTA
The product distribution and the concluded formation of a Ni-
alkyl complex (Scheme 1) are compatible with the proposed di-
merization mechanism. The obtained product distribution indi-
cates the dimerization is mainly taking place by 1’-adsorption
of 1-butene and 1’-insertion or 2’-insertion of 1-butene, because
n-octene and methylheptene were formed preferentially. In con-
trast, a metallacycle mechanism would involve the formation of
the three dimers in comparable concentrations. Their ratio
would depend on the adsorption and insertion orientation of the
π-bound alkenes.58-59 The very low formation rates of dime-
thylhexene and its non-existence at 0 % conversion led us to
conclude that the metallacycle mechanism is not operative for
Ni-Ca-LTA.
While this is a plausible mechanistic pathway, the strong ad-
sorption of butene on Ni2+ and the chemical potential of the re-
acting butene suggest that the reaction starts from a Ni-alkyl
complex. This complex is equivalent to those observed in ho-
mogeneous systems,8 having ligands coordinated to Ni metal
altering the electronic charge of its d orbital. The observed sec-
ond-order and the full coverage requires to postulate the pres-
ence of this Ni alkene complex that moderates the coordination
of the reacting butenes with Ni2+.
The product distribution obtained on Ni-Ca-LTA points to a
stepwise mechanism for butene dimerization. We propose a di-
merization pathway adapted from Cossee-Arlman mechanism
(Scheme 3), which involves initially the formation of a Ni-alkyl
complex as the active site throughout the entire catalytic cycle
(Scheme 3). The full coverage of Ni by 1-butene and the reac-
tion order of 2 with respect to 1-butene support the formation
of a Ni-alkyl complex as the active site; this site is able to coor-
dinate to two additional butene molecules that lead to the dimer.
The catalytic cycle begins with the adsorption of 1-butene on
the Ni-alkyl complex, preceded by the formation of a covalent
bond with the Ni-alkyl complex (Step 1, Scheme 3), in chemical
equilibrium with the gas phase butene. We view the allyl com-
plex as the intermediate for the isomerization of π-bonded 1-
and 2-butene on Ni (Scheme 2). In the next step, a second bu-
tene molecule weakly adsorbs via  bond interaction. Consecu-
tively, the third step is the kinetically relevant C-C bond for-
mation that forms the C8 alkene product (step 3 in Scheme 3).
Desorption of the C8 alkene regenerates the Ni-alkylC4 complex,
making it ready for the next catalytic cycle. Pseudo steady-state
treatments on the intermediates, together with assumptions of
binding site and Ni-alkyl complex as the most abundant surface
intermediates (step 1, Scheme 3), give the rate expression of
(derivation at SI-4):
The insertion step defines the product selectivity upon the ori-
entation of the adsorbed butene molecule. The possible orienta-
tions of the butene molecules within the cyclic transition state
are shown in Scheme S1 in SI, as those proposed by theory with
Ni homogeneous complexes.44 In this case it is proposed a rela-
tively low stability of the primary carbenium ion that would be
formed upon a 2’-adsorption of 1-butene (Scheme S1 B in SI).
Hence, it is preferred an initial 1’-adsorption of pure 1-butene.
Once 2-butene is available from the double bond isomerization
of 1-butene, 2’-adsorption of 2-butene leads to a better stabi-
lized secondary carbenium ion, which opens another dimeriza-
tion pathway via this intermediate. In all cases, however, the
butene insertion step is associated with the highest activation
barrier.
CONCLUSION
1-Butene dimerization on a Ni-Ca-LTA derived catalysts was
highly selective to methylheptene and n-octene (95%), shifting
the methylheptene/octene ratio from 0.7 to 1.4, in the conver-
sion range up to 35 %. The main dimerization pathway is pro-
posed to take place via initial 1’-adsorption of 1-butene on Ni
and subsequent 1- or 2-insertion of a second butene, leading to
the high selectivity to methylheptene and n-octene dimers.
2
[
]
퐾1퐾2푘3 퐶4
2
[ ]
≈ 푘 퐶4
푟 =
[
]
[
]
[ ]
1 + 퐾1 퐶4 + 퐾2 퐶4 + 퐶8 /퐾4
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ACS Catalysis
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Scheme 3. Proposed reaction network for dimerization of linear butenes over Ni active site (Ni-alkyl in the zeolite). Firstly, 1-butene
(green) would adsorb on the Ni-alkyl complex (1). A second butene molecule (blue) weakly adsorbs on the Ni-alkylC4 complex (2).
Collectively, dimerization is taking place on the Ni-alkylC8 complex (3), being rate limiting. Finally, there is a subsequent internal
hydride shift, leading to product desorption from the Ni site and formation of the Ni-alkylC4 complex for a new catalytic cycle.
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hours, the precursor was calcined (8 h, rate: 5 °C/min, 500 °C,
air). Ni-Ca-LTA with different Ni wt.% was prepared by aque-
ous ion exchange of Ca-LTA-5A with Ni2+. The Ni2+ ion ex-
change was carried out with an aqueous Ni(NO3)2 (Sigma-Al-
drich) solution (20 gsolution / gzeolite) at 80 °C for 24 h. The molar-
ity of the solution varied between 0.00-0.06 M to obtain 0-6
wt.% Ni2+ in Ca-LTA (0 wt.% of Ni2+ was obtained in the case
of no Ni(NO3)2 in solution). During Ni ion exchange, the pH
value was 6-7 during the total time of exchange (Figure S12).
The charge balance was approximately 100% with respect to Al
atoms (Table S5). The solid was recovered and washed thor-
oughly with deionized water. After drying at 100 °C overnight,
the precursor was calcined (8 h, rate: 5 °C/min, 500 °C, air).
Double bond isomerization of butene is also catalyzed by Ni2+
in Ni-Ca-LTA. The extent of dimerization plays an important
role for the dimerization pathway. High concentrations of 2-bu-
tene favor the 2’-insertion into 1’-adsorbed species, increasing
the selectivity to methylheptene. The 2’-adsorption is the only
possible mode for internal alkenes, causing a high concentration
of 2’-adsorbed species and lead to the formation of mono- and
dibranched dimers. Double bond isomerization equilibrium was
reached at about 30% conversion and limited the conversion
rate to dimers by almost one order of magnitude. Adsorbed 2-
butene or its dimerization products also lead to irreversible
changes at the active sites and blocking them by formation of
strongly bound oligomers.
Characterization methods. Two different methods were fo-
llowed in the adsorption of 1-butene on Ni-Ca-LTA via IR: 1-
butene was adsorbed, equilibrated and subsequently evacuated
stepwise; and the time resolved adsorption of 1-butene at a
constant pressure. In the first method, low pressure IR spectra
were recorded on a Vertex 70 spectrometer from Bruker Optics,
equipped with a liquid nitrogen cooled detector. A thin and self-
supporting wafer was installed and activated for 1 h at 450 °C
(rate: 10 °C/min) in vacuum. After cooling to 40 °C, butene was
adsorbed to 0.5 mbar and then later evacuated (P < 10-7 mbar).
Scans were taken with a resolution of 0.4 cm-1, with an average
of 120 scans per spectrum. In the second approach, for IR spec-
troscopy under constant pressure, the samples were prepared as
self-supporting wafer and first activated in vacuum at 450 °C
(rate: 10 °C/min) in situ for 2 h. After pretreatment, the acti-
vated samples were cooled to 40°C. Subsequently, 1-butene
was adsorbed at 1mbar. Spectra were recorded after 1-30
minutes on a Nicolet iS50AEM spectrometer from ThermoSci-
entific, equipped with a liquid nitrogen cooled detector at a res-
olution of 4 cm-1. Scans were taken from 1000-4500 cm-1 with
an average of 500 scans per spectrum.
IR spectroscopy shows that Ni-allyl and Ni-alkyl complexes are
formed as most abundant intermediates during isomerization
and dimerization. During isomerization, the adsorption-desorp-
tion is equilibrated and the reactants assume an allylic transition
state.
Dimerization of linear butenes on Ni-Ca-LTA follows a
Cossee-Arlman type mechanism. After formation of the Ni-al-
kyl complex, two butene molecules form a C-C bond. Ni sites
are interacting with 1-butene, forming Ni-alkyl complexes, at
which a second order kinetically-relevant C-C bond formation
occurs that leads to octenes. These findings suggest the Ni2+
species that catalyzes the reaction exists in a very special chem-
ical environment involving an additional coordinated butene.
EXPERIMENTAL
Catalyst preparation. Ca-LTA was obtained by stirring Ca-
LTA-5A (Sigma-Aldrich, pre-calcined: 4 h, rate 5 °C/min, 500
°C) with water (20 gwater / gzeolite) at 80 °C for 24 h (pH = 7-8
over total exchange time). The solid was recovered and washed
thoroughly with deionized water. After drying at 100 °C for 10
For determining the coke, spent catalysts (24 h on stream at 160
°C and 50 bar, WHSV = 50 h-1) were dissolved in a sufficient
amount (approx.. 20 ml) of HF at 80 °C. After evaporation of
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HF, the residue was collected in hexane and injected into a GC temperature is controlled by a eurotherm 2416, pressure is con-
1
2
equipped with a 50 m HP-1 column and a flame ionization de-
tector.
trolled using a Tescom backpressure regulator.
3
4
5
6
7
8
9
Prior to weighing, the catalyst was dried at 100 °C for 1 h. The
catalyst bed is diluted with SiC and fixed in the isothermal zone
of the reactor. After activation for 2 h at 450 °C (rate: 10
°C/min) in air, the system is purged with nitrogen and pressur-
ized to the desired pressure. Subsequently the system is flushed
with the feed mixture (5 ml/min) for 2 min. After the desired
flow rate is set, temperature program and GC measurements are
started.
X-ray diffraction (XRD) measurements were performed in a
PANalytical Empyreal System diffractometer, equipped with a
Cu- Kα radiation source (Kα1 line of 1.54 Å; 45 kV and 40
mA)). The diffractograms were measured by the usage of a sam-
ple spinner stage in a 2θ range between 5 ° and 70 ° (step size:
0.0131303/2) at ambient conditions.
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X-ray absorption (XAS) spectra were recorded at DESY in
Hamburg, Germany at PETRA III, P65. The monochromatic
photon flux was at 2x1012 s-1 at 9 keV and a beam size of 0.5x1
mm2. The sample was packed in a in-situ XAS setup capillary
and activated in 10 % O2 in He flow (5 ml/min) at 450 °C (rate:
10 °C/min) for 2 h. After cooling down to 160 °C, the system
was flushed with He, before 1-butene flow (1.5 ml/min) was
loaded for 4 h at 160 °C and ambient pressure. The spectra were
recorded between 8160 and 8700 eV and evaluated in Athena
software.60 For the linear combination fit, NiO and Ni foil were
applied as standards for Ni2+ and Ni0, respectively.
Standard measurement conditions were at 160 °C and 50 bar
with a flow rate of butene mixture between 0.04 and 0.12
ml/min. These reaction conditions resulted in higher activity
and lower deactivation. Catalyst loading was varied between 10
and 200 mg. WHSV was in the range 6 to 244 g g-1 h-1.
Activation energy was determined between 140 and 180 °C at
50 bar with a WHSV of 150 h-1. Reaction order was measured
at 50 bar total pressure, 160 °C and WHSV of butene of 25 h-1.
The feed was diluted with propane, which led to concentrations
of 1-butene between 32 and 86 wt.%.
For the determination of the particle size, the catalyst particles
were enhanced (magnification: x2000) by a high resolution
JEOL JSM-7500F scanning electron microscope (voltage: 2
kV) and subsequently measured. For the sample preparation a
small amount of catalyst was sonicated in EtOH, dropped on the
sample Cu grid and dried at ambient conditions.
i-Butane is inert under reaction conditions applied, and hence it
was used as internal standard for normalization of GC areas.
Conversion, selectivity and yield are calculated according to
following equations:
푛(푏푢푡푒푛푒)푖푛 − 푛(푏푢푡푒푛푒)표푢푡
푋 =
The gravimetric sorption isotherms of 1-butene and 2-butene on
Ni-Ca-LTA was measured in a Setaram TG-DSC 111 thermo-
analyzer connected to a high vacuum system. About 20 mg of
sample was placed in a quartz crucible and activated at 450 °C
(rate: 10 °C/min) for 1 h in vacuum (p < 10-4 mbar). After cool-
ing to 40 °C, the sample was exposed to 1-butene in small pres-
sure steps from 0.01 to 10 mbar. Both, the sample mass and the
thermal flux were monitored. The heat of adsorption was di-
rectly obtained by integration of the observed heat flux signal.
During the adsorption of 2-butene on Ni-Ca-LTA a successive
weight increase was observed during the first 12 hours at 03
mbar of 2-butene. The adsorption enthalpy was directly ob-
tained by integration of the heat flux signal for the same time
interval as 1-butene.
푛(푏푢푡푒푛푒)푖푛
|
|
휈푏푢푡푒푛푒
푛(푝푟표푑푢푐푡)표푢푡
푆 =
푛(푏푢푡푒푛푒)푖푛 − 푛(푏푢푡푒푛푒)표푢푡 휈푝푟표푑푢푐푡
|
|
푛(푝푟표푑푢푐푡)표푢푡 휈푏푢푡푒푛푒
푌 =
푛(푏푢푡푒푛푒)푖푛 휈푝푟표푑푢푐푡
ASSOCIATED CONTENT
Supporting Information. Supporting information is available
free of charge via the Internet at http://pubs.acs.org.”
The BET specific surface area and pore volume of the zeolite
were determined by nitrogen physisorption. The isotherms were
measured at liquid nitrogen temperature (77 K) using a PMI
Automatic Sorptometer. The catalyst was activated in vacuum
at 473 K for 2 h before measurement. Apparent surface area was
calculated by applying the Brunauer-Emmett-Teller (BET) the-
ory with a linear regression between p/p0 = 0.01 – 0.15. The
micro- and mesopores were determined from the t-plot linear
regression for t = 5-6 Å.
Additional graphs, experimental details, characterization data,
thermodynamic equilibrium data, considerations on approach to
equilibrium and reaction kinetics, additional reaction schemes.
AUTHOR INFORMATION
Corresponding Author
*m.sanchez@tum.de
*ricardo.bermejo@tum.de
*johannes.lercher@tum.de
Catalytic testing and reaction kinetics. Catalytic tests were
conducted in a fixed bed plug flow reactor (PFR (id = 3.9 mm)),
connected to an online GC analysis unit (Agilent HP 6890,
equipped with a 50 m HP-1 column). Prior to GC analysis, hy-
drogen is added to the product stream, which is hydrogenated
over a Pt/Al2O3 catalyst. A mixture of 15 % i-butane and 85 %
1-butene is introduced by a syringe pump (ISCO model 500 D),
ACKNOWLEDGMENT
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ACS Catalysis
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Propene oligomerization over Ni-exchanged Na-X zeolites. Journal of
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This work was funded by EVONIK Performance Materials GmbH.
1
2
3
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J.A.L. acknowledges support for their contribution by the U.S. De-
partment of Energy (DOE) (CN 269409), Office of Science, Office
of Basic Energy Sciences, Division of Chemical Sciences, Geosci-
ences & Biosciences for exploring transition metal cations for C-C
bond forming reactions. R.B.D acknowledges the Alexander von
Humboldt foundation for financial support.
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ABBREVIATIONS
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Al, aluminum; Å, Angström; BAS, Brønsted acid sites; BET,
Brunauer-Emmett-Teller; C4, butenes; C8, octenes; C12, dodecenes;
C16, hexadecenes; Ca, calcium; Ca-LTA, Ca-containing LTA, Ca-
LTA-5A, commercial zeolite 5 A, LTA, Linde Type A zeolite; Cu,
copper; Ea, energy of activation; EtOH, ethanol; eV, electron volts;
GC, gas chromatograph; g, gram catalyst; h, hour; HF, hydrogen
fluoride; IR, infrared; kdi, rate constant for dimerization; ktri, rate
constant for trimerization; LTA, Linde type A zeolite; mA, milli
Ampere; min, minutes; ml, milliliter; molC, mols on carbon base;
N2, nitrogen; Ni, nickel; Ni(NO3)2, nickel nitrate; Ni-Ca-LTA,
nickel exchanged Ca-LTA; p, pressure; pH, pH value; Pt, platinum;
Al2O3, aluminum oxide; S, selectivity; SEM, scanning electron mi-
croscopy; Si, silicon; SiC, silicon carbide; T, temperature; TGA,
thermogravimetric analysis; TOF, turnover frequency; TOS, time
on stream; WHSV, weight hourly space velocity; wt.%, weight %;
X, conversion; XANES, X-ray absorption near edge structure;
XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction; Y,
yield;
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Dimerization of linear butenes on zeolite supported Ni2+
84x41mm (150 x 150 DPI)
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