Nanostructured Hydrotalcite-Supported RuBaK Catalyst for Direct Conversion of Ethylene to Propylene

Article abstract of DOI:10.1134/S1070427218060149

A novel nanostructured hydrotalcite-supported alkali-doped Ru-based catalyst (RuBaKT/HT) was applied for the direct gas-phase conversion of ethylene to propylene at 70°C and 1 atm. The maximum conversion of ca. 87% was achieved at the initial time on stream with a 65% selectivity to propylene and a 6% selectivity to butenes via consecutive reactions. The overall selectivity to propylene and butenes decreased to 32% after 24 min of operation, however, necessitating a reaction–regeneration cycle in a possible industrial practice.

Full text of DOI:10.1134/S1070427218060149

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2018, Vol. 91, No. 6, pp. 972−976. © Pleiades Publishing, Ltd., 2018.
CATALYSIS
Nanostructured Hydrotalcite-Supported RuBaK Catalyst
1
for Direct Conversion of Ethylene to Propylene
Mohammad Ghashghaee* and Vahid Farzaneh
Faculty of Petrochemicals, Iran Polymer and Petrochemical Institute, P.O. Box 14975-112, Tehran, Iran
*
e-mail: m.ghashghaee@ippi.ac.ir
Received October 12, 2017
Abstract—A novel nanostructured hydrotalcite-supported alkali-doped ruthenium-based catalyst is introduced
for the direct gas-phase conversion of ethylene to propylene at 70°C and 1 atm. The maximum conversion of
ca. 87% was obtained at the initial time on stream with a 65% selectivity to propylene and a 6% selectivity to
butenes via consecutive reactions. Both conversion and selectivity to olefinic products decreased after 24 min
of operation, however.
DOI: 10.1134/S1070427218060149
INTRODUCTION
facilitate the in situ dimerization and isomerization steps
prior to the metathesis reaction.
Ethylene and propylene, the most important building
blocks in the petrochemical industry, have long been
obtained from steam pyrolysis of hydrocarbons [1–7].
Nonetheless, the reduced availability of propylene and
butenes from the conventional sources has urged on-
purpose technologies, that are planned to provide 30% of
global propylene supply in 2025 [8]. A promising option
to bridge this gap is the direct conversion of the relatively
abundant ethylene to propylene through metathesis,
catalyzed most effectively by Re, Mo, W, and Ni catalysts
EXPERIMENTAL
Materials. RuCl ·xH O (x = 1–3, 37% Ru content),
3
2
KOH (98.0%), Ba(NO ) (99.0%), Mg(NO ) ·6H O
3
2
3 2
2
(98.0%), Al(NO ) ·9H O (98.0%), NaOH (98.0%),
3
3
2
and Na CO (99.0%) were used as the starting materials.
2
3
High-purity C H (99.99%), H2 (99.99%) and N2
2
4
(99.99%) were used as reactants.
Catalyst preparation. The hydrotalcite (HT) support
was synthesized using the co-precipitation method
explained previously [16]. The as-synthesized support (5
g) was successively impregnated with 4.3 mL of 0.34 M
Ba(NO ) , 2.5 mL of 0.8 M RuCl ·xH O and 2.5 mL
[9–12]. The reaction is more preferably carried out in
heterogeneous systems rather than homogeneous ones,
owing due to the simpler separation of the reaction
mixture from the catalyst, rather facile regeneration of
the catalyst, and decreased product contamination [13].
Although different inorganic supports can be envisaged
3
2
3
2
of 6.2 M KOH solutions using the incipient wetness
impregnation method. At each impregnation step, the
sample was dried at 110°C overnight. The final catalyst
powders were calcined at 500°C for 5 h.
[14], zeolites are not ideal choices for this purpose due
to their too small apertures (below 1 nm) to allow both
the grafting of active species and their accessibility to the
reactants; the ordered mesoporous materials would then
be more suitable [15]. The present contribution reports on
the preparation of a ruthenium-based catalyst supported
on a mesoporous clay material for the direct conversion
of ethylene to propylene. The MgO and Al O phases in
Catalyst and product characterization. The pre-
pared support and catalyst were characterized using SEM,
EDX, XRD, and N -adsorption techniques. The structure
2
and morphology were analyzed on a Tescan instrument.
The XRD patterns were recorded on a Siemens, D5000X-
2
3
ray powder diffractometer (CuK radiation) with a step
α
the layered double hydroxide material were intended to
size of 0.02° and an exposure time of 2 s per step. The
BET surface areas were measured with a Quantachrome
1
The text was submitted by the authors in English.
9
72

NANOSTRUCTURED HYDROTALCITE-SUPPORTED RuBaK CATALYST
973
Catalytic activity. The reaction was carried out in
a tubular reactor of 12 mm internal diameter in a setup
similar to that explained previously [17–20]. A guard
bed consisting of 4A molecular sieve, KOH, and silica
gel particles was employed upstream to the reactor
to remove unwanted impurities. A 0.15 g of catalyst
powders dispersed in 0.90 g of quartz beads was placed
between two plugs of quartz wool and activated in
a reducing environment (with an H /N molar ratio of
2
2
–
1
4
) at the WHSV of 4 h and 703 K for 3 h. The catalyst
bed was cooled down to the reaction temperature in pure
Ru–Ba–K/HT
60 70 80
–1
nitrogen (32 mL min ) and kept at this temperature for
–1
3
h. Subsequently, the ethylene feedstock (2 mL min )
2
0
30
40
50
was introduced.
2
θ, deg
Fig. 1. XRD patterns of the HT support and RuBaK/HT catalyst.
RESULTS AND DISCUSSION
Chem-BET 3000 analyzer at 77 K. The reaction prod-
ucts were analyzed on a GC equipped with capillary and
packed columns as well as FID and TCD.
The XRD patterns (Fig. 1) show the hydrotalcite
structure (JCPDS no. 70-2151) as the only crystalline
phase with an Mg/Al molar ratio of 2. In the case of the
(a)
(b)
2
0 μm
6 μm
(c)
(d)
2
0 μm
5 μm
Fig. 2. SEM images of the HT (a–b) and RuBaK/HT (c–d) samples.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 91 No. 6 2018

974
MOHAMMAD GHASHGHAEE, VAHID FARZANEH
1
0 μm
K Kα1
10 μm
Ba Lα1
10 μm
Ru Lα1
Sum spectrum
O
Mg
Al
K
Ru
Ru
Ba
Ba
6
0
2
4
8
10
12
14
keV
Fig. 3. EDX results and elemental maps of the RuBaK/HT catalyst.
impregnated catalyst, the diffractogram with well-defined
peaks at 11.5°, 23°, 34°, 61°, and 62° was again typical
of a hydrotalcite material, indicating that the RuBaK/
HT catalyst was successfully prepared with acceptable
dispersions ofAl, Ru, and Ba in the crystalline hydroxide
layers [21]. The XRD patterns also indicated the KOH
signals on the catalyst. The intensities of (003), (006),
surface. The elemental composition and the dispersion
of species on the surface of the samples are shown in
Fig. 3, which proved the satisfactory distributions of the
active moieties. The positive influence of barium oxide
on the dispersion of ruthenium on the surface has been
demonstrated previously [23]. The amounts of active
elements on the surface were close to the nominal values
in the precursor solution. It is worth mentioning, however,
that the Mg/Al ratios on the surface of HT and RuBaK/
HT were higher than the corresponding theoretical values
owing to the effect of pH during HT preparation as the
molar ratio of Mg/Al in a hydrotalcite prepared under the
pH of 11–14 is anticipated to be around 3.1–3.2 [24]. The
EDX data pointed to an Ru content of 4% on the surface.
(009), (110), and (113) planes being explicitly related to
the crystallinity of the material were observed to decrease
upon impregnation, however, due most probably to the
increase in the number of cations of higher ionic radii
in the brucite sheet [22]. The surface areas (and pore
volumes) of HT and RuBaK/HT were, respectively, 45.6
2
–1
3
–1
and 54.4 m g (and 0.81 and 0.96 cm g ), indicating
that the impregnation and final calcination steps increased
the initial surface area without changing the interlayer
spacing in the clay structure.
The table demonstrates the catalytic performance of
RuBaK/HT for the gas-phase conversion of ethylene.
As evident, the initial conversion of ethylene was ~87%
with a propylene selectivity of 65 wt %, a selectivity of
The surface morphologies are shown in Fig. 2,
which indicates the existence of lamellar nano-particles
shaping HT aggregates. On the contrary, the impregnated
catalyst exhibited agglomerated micro-clusters on the
~
6 wt % to butenes, and a ca. 2 wt% C5+ selectivity.
A three-step reaction can be contemplated [25–28]
as the main events occurring during this process. First,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 91 No. 6 2018

NANOSTRUCTURED HYDROTALCITE-SUPPORTED RuBaK CATALYST
975
Performance of the RuBaK/HT catalyst
under the mild conditions of 70°C and 1 atm. The overall
selectivity to propylene and butenes decreased to 32%
after 24 min of operation, however, necessitating a
reaction–regeneration cycle in a possible industrial
practice.
Reaction conditions and performance
data
Reaction temperature, °C
Pressure, atm
70
1
70
1
Time-on-stream, min
Ethylene conversion, %
Product selectivities, wt%
Propylene
1
24
ACKNOWLEDGMENTS
86.8
22.2
The authors appreciate the financial support from Iran
Polymer and Petrochemical Institute. Helpful discussions
with Prof. Andreas Seidel-Morgenstern of Max Planck
Institute in Magdeburg and Prof. Klaus-Joachim
Jens of Telemark University College are gratefully
acknowledged.
65.0
22.7
5.9
0.5
0.5
Propane
Butenes
31.5
56.8
10.8
32.0
Butane
4.6
C5+ Products
1.8
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