Ethylene Oligomerization to Select Oligomers on Ni-ETS-10

Article abstract of DOI:10.1002/cctc.201800650

The oligomerization of short alkenes (ethylene and propylene) can be used for producing commodity chemicals. Various catalysts have been used for alkene oligomerization, among which ordered microporous catalysts are thermally and mechanically stable and are already established for large-scale industrial applications. In this work, we demonstrate ethylene oligomerization reaction on a microporous titanosilicate ETS-10 (Engelhard Titanosiliate-10) exchanged with Ni²⁺ (Ni-ETS-10). We demonstrate a template-free and fluoride-free ETS-10 synthesis method that does not produce impurities commonly seen in hydrothermal ETS-10 synthesis. Ni-ETS-10 showed high C₂ conversion rate, high selectivity to C₄ and high stability comparing to other microporous catalysts investigated in this work for ethylene oligomerization reaction.

Full text of DOI:10.1002/cctc.201800650

Accepted Article
Title: Ethylene oligomerization to select oligomers on Ni-ETS-10
Authors: Jay Thakkar, Xinyang Yin, and Xueyi Zhang
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Ethylene oligomerization to select oligomers on Ni-ETS-10
Jay Thakkar, Xinyang Yin, and Xueyi Zhang*[a]
potentially be utilized for ethylene oligomerization reaction.[31]
Abstract: The oligomerization of short alkenes (ethylene and
propylene) can be used for producing commodity chemicals. Various
catalysts have been used for alkene oligomerization, among which
In this work, we present the synthesis and catalytic behaviors of
ordered microporous catalysts are thermally and mechanically stable,
Ni-ETS-10. We demonstrate a template-free and fluoride-free
and are already established for large-scale industrial applications. In
synthesis method that produces ETS-10 without impurities
this work, we demonstrate ethylene oligomerization reaction on a
commonly seen in other systems. Ni-ETS-10 is shown to be
microporous titanosilicate ETS-10 (Engelhard Titanosiliate-10)
more active for ethylene oligomerization, and has shown higher
exchanged with Ni2+ (Ni-ETS-10). We demonstrate a template-free
stability and higher selectivity to C4 than other microporous
and fluoride-free ETS-10 synthesis method that does not produce
catalysts (Ni-CIT-6 and Ni-MOF-74) compared in this study.
impurities commonly seen in hydrothermal ETS-10 synthesis. Ni-
ETS-10showed high C2 conversion rate, high selectivity to C4 and
high stability comparing to other microporous catalysts investigated
in this work for ethylene oligomerization reaction.
Oligomerization reaction of ethylene has been studied on
various heterogeneous catalysts such as, metal oxides and
sulfates,[1-5] micro and mesoporous aluminosilicates,[1,6-17] and
metal organic frameworks (MOFs).[18-23] Compared to other solid
catalysts for ethylene oligomerization reaction, nickel (Ni2+)
containing microporous materials (such as zeolites and MOFs)
offer the opportunity to control and optimize reaction rate and
selectivity of these reactions.[6-23] ETS-10 (Engelhard
Titanosilicate-10) is a thermally stable microporous crystalline
titanosilcate (Si/Ti=5) containing [TiO6] units, which generate a
-2 charge (due to Ti4+),[24,25] capable of loading significant
amounts of divalent cations for ion exchange and water
treatment applications.[26,27] Since ETS-10 is capable of loading a
broad spectrum of divalent transition metal cations,[26,27] this
versatile ion-exchange capability of ETS-10 was therefore
exploited in this study, and Ni2+ was exchanged into the
framework for ethylene oligomerization. The framework structure
of ETS-10 consists of periodic large 12-member ring pores (7.6Å
x 4.9Å) and aperiodic 18-member ring pores (14.3Å x 7.6Å) due
to stacking faults.[24,25,28] Additionally, ETS-10 can be
synthesized using a template-free procedure which eliminates
the need of sacrificing the organic structure-directing agents by
calcination.[28] To the best of our knowledge, although there have
been numerous attempts at utilizing zeolites and MOFs for
ethylene oligomerization, gas phase ethylene oligomerization
Figure 1. (a) TEM image of ETS-10 particles synthesized using 3.4Na2O:
1.5K2O: 1TiO2: 5.5SiO2: 6.95 HCl: 140.7 H2O, where both 12-member rings
and 18-member rings (from stacking faults) are visible in the TEM image;
reaction in continuous mode on Ni2+ exchanged ETS-10 (Ni-
ETS-10) has not been studied before.[1,6-23,29] In order to
(b-e) Element maps of Ni(6.85wt%)-ETS-10 samples showing uniform
distribution of elements.
investigate the catalytic outcomes of Ni-ETS-10, two other
similar microporous materials that can load divalent transition
metal with significant amount – CIT-6[30] and MOF-74[31] were
chosen. Zeolite CIT-6 has *BEA topology, but contains Zn2+as
Previous reports have shown that the final OH-concentration (or
heteroatoms, where the framework Zn atoms generate a -2
pH value) of the ETS-10 synthesis gel plays an important role in
charge per Zn.[30] The -2 charge can be used for loading Ni2+ for
the final product purity.[28] Therefore, a set of experiments were
oligomerization, similar to ETS-10.[30] Ni-MOF-74 contains a
carried out to minimize the impurities and maximize the mass
significant amount of coordinatively unsaturated Ni2+ which can
fraction of active catalyst. A molar composition of 3.4Na2O:
1.5K2O: 1TiO2: 5.5SiO2: 6.95HCl: 140.7H2O was shown to
produce the highest purity, where the impurities (quartz and
other dense SiO2 phases) were undetectable from powder X-ray
diffraction (Figure S1). The ETS-10 with the least impurity
(x=6.95, where x is the moles of HCl) was used for subsequent
characterization, ion exchange, and catalytic reactions. The as-
synthesized ETS-10 nanoparticles have a square bipyramidal
morphology and a size of less than 500nm (figure S2a). High-
[a]
Jay Thakkar, Xinyang Yin, Prof.Dr. Xueyi Zhang
Department of Chemical Engineering
The Pennsylvania State University
106 Greenberg Building
University Park, PA 16802, USA
E-mail: xuz32@psu.edu
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201800650
ChemCatChem
COMMUNICATION
resolution TEM image of the ETS-10 nanoparticle provides
further evidence of its crystallinity (figure 1a), where the typical
stacking faults of ETS-10 and the resulting 18-member rings are
visible. Elemental analysis shows that a Si/Ti ratio of 5.03 was
obtained from the as-synthesized ETS-10 (Table S1). ETS-10
particles with size greater than ~2.5µm were also synthesized in
order to study the effect of crystallite size on the activity of
catalyst in ethylene oligomerization reaction (Ni(4.12wt%,
2.5μm)-ETS-10).[32,33] Two different ion-exchange procedures
were used to obtain different levels of Ni2+ loading. After ion-
exchange with Ni2+, three samples with Ni/Ti molar ratios of 0.36,
0.35 and 0.56 were obtained, corresponding to Ni mass
percentages of 3.90%, 4.12% and 6.85%, respectively. There is
a uniform distribution of framework elements (Si, Ti) and
exchanged Ni2+ ions in Ni-ETS-10 as is evident from STEM-EDS
(Ni loading 6.85wt%, figures 1b-1e).
There has been no previous report of using Ni-ETS-10 for gas-
phase ethylene oligomerization. The catalytic performance of Ni-
ETS-10 was compared with two other ethylene oligomerization
catalysts with comparable particle sizes, Ni-CIT-6 and Ni-MOF-
74. Figure 2a shows that Ni-ETS-10 is a promising catalyst for
ethylene oligomerization with the obtained maximum turnover
frequency of 688.96 molC2/(molNi h), when the Ni loading was
3.9wt%. This rate is greater than those obtained from previously-
reported microporous catalysts such as Ni-CIT-6 (with
a
comparable Ni loading of 3.3wt%) and Ni-MOF-74 (figure 2a).
The Ni-ETS-10 nanoparticles exhibited higher ethylene
conversion rates than Ni-CIT-6 and Ni-MOF-74 over the entire
time-on-stream range investigated. Since the catalyst particles
have comparable particle sizes (figures S2a, c, and d), the
higher rate on Ni-ETS-10 may not be due to diffusion limitation,
but due to the property of the active sites. Ni-ETS-10 with lower
loading (Ni(3.90wt%)-ETS-10) showed higher initial TOF than
Ni-ETS-10 with higher Ni loading (Ni(6.85wt%)-ETS-10).
However, the ethylene TOF on Ni(3.90wt%)-ETS-10 reduced
over time on stream, and the TOF stabilized at a lower value
than that of Ni(6.85wt%)-ETS-10. On the other hand, the TOF
on Ni(6.85wt%)-ETS-10 did not significantly change as the
reaction progressed, indicating that a higher Ni loading is
desirable for catalyst stability. Ethylene oligomerization reaction
performed on pure, as synthesized ETS-10 (i.e. without Ni2+
ions) as a control experiment exhibited no catalytic activity –
indicating that the obtained activity for ethylene oligomerization
on Ni-ETS-10 is solely from exchanged Ni2+ cations. The effect
of ETS-10 particle size on ethylene oligomerization reaction was
also studied (figure 2a), which indicates that large particle size
resulted in lower turnover frequency. The obtained C4
selectivities over a period of 6 hours for catalysts are also shown
in figures 2(b and c) and figure S4. Ni(6.85wt%)-ETS-10 sample
showed the highest C4 selectivity, especially after the reaction
stabilized. Conventional aluminosilicate zeolites (such as zeolite
Beta normally have strong Brønsted acid sites,[34-36] which lead
to reduced C4 selectivity. Therefore, the high C4 selectivity on
ETS-10 may be attributed to the lack of strong Brønsted acid
sites.[29,34-41] This catalyst sample (Ni(6.85wt%)-ETS-10) also
showed higher 1-butene selectivity over the entire period of 6
hours when compared to Ni(3.9wt%)-ETS-10 and Ni-MOF-74
(figure S4). The higher 1-butene selectivity however decayed
Figure 2. Catalytic behavior of four Ni2+-containing catalysts (Ni-ETS-10,
Ni-CIT-6, and Ni-MOF-74) with time on stream:(a) Turnover frequency
(TOF) based on total C2 consumed and total Ni loading; (b) C4 selectivity;
(c) C4oligomer product distribution (all C4 alkenes normalized to 100%,
averaged over the entire TOS range).
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continuously over time, while the selectivity of trans- and cis-2-
butene increased over time (figure S4a), indicating that the
generated 1-butene product oligomer might be undergoing
isomerization. Ni(6.85wt%)-ETS-10 also showed the lowest C6
selectivity over the entire period of 6 hours (figure S5). Overall,
the above observations indicate that Ni(6.85wt%)-ETS-10 is a
stable catalyst for ethylene oligomerization to C4 and that Ni-
ETS-10 is a promising catalyst for ethylene oligomerization.
Packed-bed reactor with online gas chromatograph
A packed-bed reactor was used for testing the catalysts for ethylene
oligomerization reaction in an isothermal, continuous gas flow mode. 50
mg of catalyst was diluted with pre-calcined (1000oC, 24h) 13.5g silica
gel in order to eliminate local hot-spots and temperature gradients across
catalyst bed during reaction. Heated flow lines (170oC) were used to
eliminate condensation of large oligomeric reaction products.
Catalysts were treated in-situ in helium flow at 180oC for 16 hours before
performing the reactions. (ETS-10 catalysts were additionally pretreated
in-situ under dynamic vacuum (2 mmHg absolute) for 12 hours at 450oC
before helium treatment.) The reactions were carried out at 180oC, 5atm
ethylene pressure and 90 gC2/(gcatalysth) space velocity (conversion <1%
for all catalysts, figure S6). Rates and selectivities were quantified
continuously using an online gas chromatograph.
In conclusion, we exploited the ion-exchange capability of ETS-
10 for catalytic reactions, where Ni2+ was exchanged into ETS-
10for ethylene oligomerization. The Ni-ETS-10 catalyst was
active for ethylene oligomerization, which shows higher rate
(based on total Ni) than other microporous catalysts. Ni-ETS-10
also showed highest selectivity to C4 and higher stability than
other microporous catalysts investigated in this work.
Acknowledgements
The authors acknowledge the financial support from the
Department of Chemical Engineering and the Institute for
Natural Gas Research (INGaR) at the Pennsylvania State
University. X.Z. acknowledges financial support from the John J.
and Jean M. Brennan Clean Energy Early Career Professorship.
Materials characterization was performed at the Materials
Characterization Laboratory, which is a partner in the National
Nanotechnology Infrastructure Network (NNIN) and the
Materials Research Facilities Network (MRFN), supported by the
U.S. National Science Foundation (award DMR-1420620). A U.S.
patent application by X.Z. and J.T. was filed on March 5, 2018
(Application No. 62/638,366).
Experimental Section
Catalyst synthesis and characterization
Microporous nanoparticle ETS-10 was synthesized using conventional
hydrothermal method based on a previous report.[28] NaOH (EMD Millipore),
KOH (EMD Millipore), TiO2 (P25, Acros Organics), sodium silicate solution
(EMD Millipore), and 37wt%HCl aqueous solution (Sigma-Aldrich) were used
without further purification. DI water (resistivity=18.2 m) was used in the
synthesis and subsequent washing of the final products. The NaOH, KOH,
sodium silicate solution, and P25 were sequentially added into DI water at
ambient temperature, into which HCl solution was added dropwise while
stirring at 500 rpm. A thick gel was formed at the end of the addition of HCl
solution. The following molar ratio of the components, 3.4Na2O: 1.5K2O:
1TiO2: 5.5SiO2: xHCl: (116.7+3.45x) H2O (x=6.6-8.1) was achieved in the end.
The gel was then heated in a convection oven at 230oC for 72 hours. The
product was washed with DI water until the pH of the supernatant is 9-10.
Micron-sized ETS-10 particles were synthesized according to the synthesis
method based on a previous report (SEM images in figure S2b).[32,33]
Keywords: catalysts • ETS10 • ethylene • heterogeneous •
oligomerization
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Table of Contents
ETS-10 loaded with Ni2+ is shown
to be active and stable for ethylene
oligomerization. Ni-ETS-10 showed
Jay Thakkar, Xinyang Yin, Xueyi
Zhang*
Ethylene oligomerization to select
oligomers on Ni-ETS-10
high
C4
selectivity
among
microporous catalysts for ethylene
oligomerization.
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