Influence of the pore architecture on the selective conversion of ethene to propene and butenes over medium pore zeolites

Article abstract of DOI:10.1039/c5nj03668b

This contribution focuses on the conversion of ethene to propene and butenes over medium pore zeolites with different pore architectures and similar n_Si/n_Al-ratios. The aim of this systematic study was to explore the factors which govern the yield/selectivity of propene/butenes from ethene and to optimize the catalytic properties for this reaction. All used zeolites were prepared by hydrothermal synthesis and characterized by various techniques. Their catalytic properties (with respect to increased propene yield/selectivity) were explored in a fixed-bed flow-type reactor operating under atmospheric pressure. One major result of these investigations is that medium pore zeolites with a one-dimensional channel system offer catalytic properties superior to the well-known zeolite ZSM-5 with its three-dimensional pore system. Selectivities to propene of 43% and to butenes of 24% could be achieved over zeolite HZSM-23 used as the catalyst at medium conversions. This demonstrates the effective suppression of by-product formation over a wide range of reaction conditions due to shape selectivity effects in the tubular pore structure of HZSM-23.

Full text of DOI:10.1039/c5nj03668b

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Influence of the pore architecture on the selective
conversion of ethene to propene and butenes
over medium pore zeolites
Cite this:DOI: 10.1039/c5nj03668b
S. Follmann and S. Ernst*
This contribution focuses on the conversion of ethene to propene and butenes over medium pore
zeolites with different pore architectures and similar nSi/nAl-ratios. The aim of this systematic study was
to explore the factors which govern the yield/selectivity of propene/butenes from ethene and to
optimize the catalytic properties for this reaction. All used zeolites were prepared by hydrothermal
synthesis and characterized by various techniques. Their catalytic properties (with respect to increased
propene yield/selectivity) were explored in a fixed-bed flow-type reactor operating under atmospheric
pressure. One major result of these investigations is that medium pore zeolites with a one-dimensional
channel system offer catalytic properties superior to the well-known zeolite ZSM-5 with its three-
dimensional pore system. Selectivities to propene of 43% and to butenes of 24% could be achieved over
zeolite HZSM-23 used as the catalyst at medium conversions. This demonstrates the effective
suppression of by-product formation over a wide range of reaction conditions due to shape selectivity
effects in the tubular pore structure of HZSM-23.
Received (in Montpellier, France)
11th January 2016,
Accepted 29th March 2016
DOI: 10.1039/c5nj03668b
www.rsc.org/njc
New strategies, the so-called on-purpose processes, were
developed to meet the increasing demand for propene indepen-
Introduction
The world-wide consumption of light olefins in the chemical dently from its production via steam cracking. Among them are
industry is steadily increasing. It is expected that especially the fluid catalytic cracking (FCC),3 the metathesis of ethene with
high demand for polymers will lead to a continuing strong butenes,4 propane dehydrogenation5 and the methanol-to-
growth rate of about 4% for ethene and 5% for propene. In the propylene (MTP) or methanol-to-olefin (MTO) route.6 All these
last few years, the market for propene has grown faster than the processes are already available on an industrial scale. A novel
one for ethene, which resulted in the so-called propene gap.1 on-purpose technology is the direct conversion of ethene to
So far, most of the light olefins required by the petrochemical propene and butenes (ETP-reaction). The advantage of the ETP-
industry were obtained via steam cracking of different feedstocks. reaction is a high feedstock flexibility, because ethene can not
Depending on the location of the steam cracker and feedstock only be obtained from crude oil as feedstock (after steam
availability, naphtha or ethane are predominantly used as feed- cracking) but also from other sources like shale gas and natural
stocks, beside gas oil and NGL (natural gas liquids). The shale gas gas (after dehydrogenation of ethane), as well as coal (via syngas)
boom in certain regions of the world promotes an increasing use and biomass (via bioethanol).7
of shale gas-derived ethane as a raw material due to low feedstock
The development of more efficient zeolite catalysts in the
prices. This trend is affecting the availability of propene, butenes ETP-reaction requires more detailed studies of the factors
and aromatics in a negative manner. With respect to the product influencing their activity and selectivity. Various catalysts have
distribution, a significantly higher yield of ethene is obtained been described in the literature, including supported metal
by steam cracking of ethane as compared to other feedstocks. oxide catalysts,8 mesoporous M41S materials9 and microporous
Only small amounts of propene, butenes and pyrolysis gasoline are zeolites.10–13 Particularly zeolites are promising catalysts due to their
produced in comparison to naphtha steam cracking.2,3 It is obvious versatile properties like tunable pore structure, shape selectivity,
that one of the biggest upcoming challenges in the petrochemical acidity and thermal/hydrothermal stability.
industry is the future supply of propene and butenes.
The focus of this systematic study was to explore the factors
which govern the yield/selectivity of propene/butenes from
ethene and to optimize the catalytic properties for this reaction.
For this purpose, zeolites with different pore sizes/architectures
(viz. Beta, ZSM-5, ZSM-11, ZSM-22, ZSM-23, EU-1) were synthesized,
Technische Universitat Kaiserslautern, Department of Chemistry,
Erwin-Schroedinger Str. 54, Kaiserslautern, Germany.
E-mail: ernst@chemie.uni-kl.de
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transformed into their acid forms and explored for their out using a Setaram Setsys 16/18 instrument. In a typical procedure,
catalytic properties in a fixed-bed flow-type apparatus.
approximately 40 mg of the material was heated in air from
40 1C to 900 1C with a heating rate of 10 1C minÀ1. An observed
weight loss in the range of 400À750 1C was assumed to result
from coke burn-off. Atomic absorption spectroscopy (AAS) was
performed using a Perkin Elmer AAnalyst 400 instrument
to determine the nSi/nAl-ratios of the synthesized zeolites.
Scanning electron micrographs (SEM) were collected using a
Jeol microscope (JSM-6490LA) to characterize the size and
morphology of the materials used in this study. The samples
were sputtered with a thin layer of gold (15 nm) to increase
contrast and thereby also the possible magnification.
Experimental
Synthetic procedures
The zeolites used in the present study were synthesized using
hydrothermal methods. In order to crystallize zeolites ZSM-5 and
ZSM-11, tetra-n-propylammonium bromide and tetra-n-butyl-
phosphonium bromide were used as structure directing agents.14
Zeolites ZSM-22 and ZSM-23 were synthesized using procedures
described by Ernst et al. using 1,6-diaminohexane and pyridine,
respectively, as template molecules.15,16 Zeolite EU-1 was prepared
with hexamethonium bromide and zeolite Beta was synthesized
using tetraethylammonium hydroxide as an organic template
according to the published procedures.17
Results and discussion
Catalyst characterization
Powder X-ray diffraction pattern of the as-synthesized zeolites
are shown in Fig. 1. All materials exhibit a good crystallinity
which is indicated by high signal intensities. No indications of
phase impurities or amorphous materials could be observed.19
The specific pore volumes and the specific surface areas
of the acid catalysts were determined using the BET-method.
The results are summarized in Table 1. The BET-surface areas
and the total pore volumes are in the expected range for the
investigated materials.
The as-synthesized materials were washed with distilled
water and dried at 120 1C over night. The dry materials were
heated to 560 1C with a rate of 1 1C minÀ1 and kept at the final
temperature for 24 h. The acid zeolites were prepared via ion-
exchange using a 0.25 molar ammonium nitrate solution for
three hours at 80 1C. This procedure was repeated twice. After a
further calcination step at 560 1C, the catalysts were pelletized,
crushed and sieved and the particle size fraction between 250
and 355 mm was used for the catalytic experiments.
Scanning electron microscopy was used to determine the
morphology and size of the zeolite crystallites. The results are
shown in Fig. 2.
Zeolite Beta consists of small crystallites with a mean size
below 1 mm. Zeolite ZSM-5 shows the typical morphology, viz.
Catalytic experiments
The conversion of ethene was carried out in a fixed-bed flow-
type reactor under atmospheric pressure. For each catalytic
experiment, 500 mg of the fresh catalyst was transferred into a
quartz glass reactor and dried in a vertical furnace at 600 1C for
0.5 h before cooling down to the desired reaction temperature.
Ethene was diluted with high purity nitrogen (499,99%) as a
carrier gas and fed into the reactor via a mass flow controller.
Unless otherwise stated, the partial pressure of ethene typically
amounted to 30 kPa and the total flow to 30 ml minÀ1. The
time-on-stream behavior of all catalysts was followed over
at least 24 h. Detailed product distributions are given for
0.5 h time-on-stream. The product composition was analyzed
via gas chromatography (Agilent 6890) equipped with a flame
ionization detector (FID). The separation of hydrocarbons was
carried out using an HP-PLOT M capillary column (length 50 m,
inner diameter 0.53 mm). All tubes and valves following the
reactor outlet were heated to 180 1C, to prevent condensation of
heavier hydrocarbon products.
Characterization
Powder X-ray diffraction (XRD) pattern were collected using a
Siemens D5005 instrument using CuKa (40 kV, 30 mA) radiation to
control crystallinity and phase purity of the synthesized zeolites.
Nitrogen physisorption isotherms at À196 1C were obtained using
a Quantachrome Autosorb-1 apparatus. The determination of the
specific surface areas was done according to the BET procedure.18
For this purpose, samples were degassed at 200 1C for 24 h
under vacuum (10À1 Pa). Thermal analyses (TGA) were carried
Fig. 1 Powder X-ray diffraction pattern (XRD) of the synthesized zeolites.
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Table 1 Textural properties and chemical compositions of the synthe-
sized zeolites used in this study
Material
SBET/m2 gÀ1
Vpor./cm3 gÀ1
nSi/nAl-ratio
HBeta
880
470
500
300
340
470
0.38
0.21
0.24
0.15
0.19
0.19
27
37
35
35
34
33
HZSM-5
HZSM-11
HZSM-22
HZSM-23
HEU-1
hexagonal prisms with crystallite sizes of ca. 5 mm in length and
a thickness of ca. 1.5 mm. The crystallites of zeolite ZSM-11 are
spherical-like with mean diameters of ca. 1 mm. For zeolite
ZSM-22 rod-like crystallites can be observed with lengths up to
3 mm. Zeolite ZSM-23 as prepared in the present study consists
of radial fin shaped rods, which agglomerates to larger particles
with a size of about 1 mm. EU-1 consists of small elliptical
particles with a length of around 3 mm and a diameter of
approximately 2–3 mm.
Fig. 3 Time-on-stream behavior of acid zeolites with similar nSi/nAl-ratios.
Reaction conditions: T = 550 1C, p(ethene) = 30 kPa, Ftotal = 30 ml minÀ1
,
Wcat. = 500 mg.
Catalytic performance of 10-membered ring zeolites
result of the high coking tendency of ethene under the applied
Important criteria for the selection of a suitable catalyst for a reaction conditions. The amount of coke which was formed
given reaction are its activity, selectivity and the time-on-stream after a continuous run of 24 h was investigated by thermal
behavior. The focus is here on a structure/property relation- analyses (Table 2).
ship of different well-known zeolites for the ETP-reaction. The
It can be seen that the coke content after 24 h time-on-stream
following catalytic experiments were carried out under similar is considerably higher for zeolite Beta with its three-dimensionally
reaction conditions and with zeolite samples of comparable interconnected system of 12-membered ring pores as compared
nSi/nAl-ratios. In the first step we investigated the influence of to the 10-membered ring zeolites. The coke contents of one-
the pore architecture of the zeolite catalysts on the time-on- dimensional systems are slightly lower than those of the three-
stream behavior at 550 1C (Fig. 3).
dimensional channel systems: over zeolites with one-dimensional
The time-on-stream stabilities decrease in the following channel systems, small amounts of coke deposited at the pore
direction: HZSM-11 4 HZSM-5 4 HZSM-23 4 HZSM-22 4 entrances or in the pores can lead to a complete blockage of
HEU-1 4 HBeta. The results clearly indicate a continuous the channels and the intrazeolitic active sites, hence the fast
decrease of activity with extended time-on-stream which is a deactivation of the one-dimensional systems which can be seen
Fig. 2 Scanning electron micrographs (SEM) of the synthesized zeolites with a magnification of 10 000Â: (A) Beta, (B) ZSM-5, (C) ZSM-11, (D) ZSM-22,
(E) ZSM-23, (F) EU-1.
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Table 2 Characterization of spent catalysts after 24 h time-on-stream
Table 3 Influence of the pore architecture on the conversion and
selectivities in the ETP-reaction
Catalyst
SBET/m2 gÀ1
Vpor./cm3 gÀ1
Coke/wt%
Selectivity/%
HBeta
20
190
340
60
150
20
0.017
0.084
0.153
0.084
0.146
0.013
25.5
8.9
7.6
6.2
5.7
5.7
Catalysta
(nSi/nAl-ratio) Xethene/%
Conversion
C1–C4
Propene Butenes alkanes C5+ Aromatics
HZSM-5
HZSM-11
HZSM-22
HZSM-23
HEU-1
b
HBeta (27) 41.3
14.1
22.9
19.1
41.8
43.4
45.4
5.1
10.2
8.7
28.3
24.4
21.1
69.1
31.4
38.2
6.3
8.9
13.2
3.8 7.9
6.3 29.2
4.9 29.1
20.5 3.1
18.4 4.9
14.7 5.6
HZSM-5 (37) 81.8
HZSM-11 (35) 85.0
HZSM-22 (35) 45.5
HZSM-23 (34) 54.4
HEU-1 (33)
26.7
from Fig. 3. Particularly pronounced is the deactivation on the
12-membered ring pore architecture of zeolite Beta. The activity
decay from initially 41% after 0.5 h is almost complete after 3 h
of time-on-stream. The large void spaces in the pore structure
lead to a strong coking activity which causes high coke loadings
of 25.5% after 24 h. The pore system is totally blocked by coke
which is evident by N2-physisorption measurements of the
spent catalyst (Table 2). The large pores of zeolite Beta offer
enough space for the bulky transition states of the hydrogen
transfer, aromatization and alkylation reactions of the light
olefins. These secondary reactions promote the formation of
coke either inside the pore system or on the external surface
of the crystallites. The smaller pore openings of the three-
dimensional 10-membered ring pore structures of zeolites
ZSM-5 and ZSM-11 reduce the amount of secondary reactions,
whereby the activity and stability increase and the coke load-
ings decrease significantly. The initial ethene conversions over
ZSM-5 and ZSM-11 amounted to ca. 82% and ca. 85%, respec-
tively, with final coke loadings of 8.9% and 7.6%. The deactiva-
tion of both zeolites is very different despite very similar
structures and coke loadings. This experimental observation
is tentatively explained by the differences in the crystallite size
of zeolites ZSM-5 and ZSM-11 as revealed by scanning electron
microscopy. The corresponding one-dimensional 10-membered
ring zeolites with a tubular pore system exhibit a stronger
influence of coke deposition on activity and stability. This is
particularly evident in the case of zeolites ZSM-22 and ZSM-23
with coke contents of ca. 6.2% and ca. 5.7%, respectively and
initial conversions of the time-of-stream behavior of ca. 46%
and ca. 54%, respectively. The deposition of coke has a stronger
impact on conversion/selectivities over tubular pore structures
(as compared to three-dimensional systems) by pore blocking
effects leading to a faster catalyst deactivation. In addition,
diffusion problems of reactants need to be considered which
may have a pronounced influence on the catalytic performance
of tubular pore structures. Although Zeolite HEU-1 also belongs
to the family of one-dimensional 10-membered ring zeolites,
clear differences were observed in activity and stability for
ethene conversion. The activity of zeolite HEU-1 after 0.5 h of
time-on-stream was estimated to be 27% with a final coke
loading of 5.7% after 24 h. The low activity and the very fast
deactivation of zeolite HEU-1 compared to the other one-
dimensional pore structures can be tentatively attributed to
the tubular pore system with large 12-membered ring side
a
Reactions conditions: T = 550 1C, p(ethene) = 30 kPa, Ftotal = 30 ml
minÀ1, Wcat. = 500 mg, TOS = 0.5 h. C5+ denotes all hydrocarbons with
five and more carbon atoms (excluding aromatics).
b
The observed fast deactivation of zeolite catalysts is a well-
known phenomenon in acid-catalyzed hydrocarbon conversions
due to the formation of coke as an almost always inevitable side
reaction. In principle, therefore, no steady- or stationary-state for
ethene conversion can be observed. However, in order to compare
the catalytic behaviour of the catalysts, we selected the product
compositions which were obtained after 0.5 h time-on-stream.
The conversion of ethene to propene and butenes produces a
variety of reaction products. Depending on the formation
mechanism and the carbon number of these products they were
grouped in 5 fractions. These include propene and butenes as main
reaction products as well as saturated C1–C4 alkanes, aromatics
and aliphatic hydrocarbons with five and more carbon atoms
(C5+) as by-products. All tested zeolites possess high selectivities
for the formation of propene and butenes under the applied
reaction conditions (Table 3).
Among the zeolites studied in this work, the one-dimensional
10-membered ring pore structure showed the highest total
selectivities to propene and butenes in the range of 66% to
70%. The three-dimensional 10-membered ring zeolites showed
comparatively lower overall selectivities to these light olefins
with approximately 28% to 33%. The lowest selectivity to pro-
pene and butenes with 19% was observed with the very open
pore structure of zeolite Beta. These results clearly indicate the
key role of spatial constraints in the intracrystalline voids for
the selective formation of propene and butenes. This is also
reflected in the by-product formation, especially the formation of
aromatics and saturated C1–C4 alkanes which are clearly reduced
in one-dimensional systems. Increasing the dimensionality and
the size of the pore opening results in the increase in selectivities
for the formation of saturated C1–C4 alkanes (and aromatics).
The reason for this behavior is probably related to an increased
contribution of hydrogen transfer reactions with increasing
space available around the catalytic sites.
Influence of the temperature
For further studies on the influence of the reaction temperature
in the ETP-reaction, zeolites HZSM-5 and HZSM-23 were
selected as potential catalysts. The determined product distri-
pockets which make this zeolite prone to coke formation with butions with these two catalysts are shown for a temperature
concomitant pore blockage.
range of 300–600 1C in Fig. 4.
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significant changes in selectivity to C1–C4 alkanes can be observed.
The formation of aromatic products from ethene clearly starts
at a temperature above 500 1C. Zeolite ZSM-5, on the other
hand, produces large amounts of saturated C1–C4 alkanes and
aromatics over the whole temperature range.
Influence of the partial pressure
The influence of the partial pressure of ethene on the catalytic
activities and selectivities was studied in the range from 10 to
100 kPa. As shown in Fig. 5, conversion increases with increas-
ing ethene partial pressure of zeolites ZSM-5 and ZSM-23 up
to ca. 30 kPa. Only a slight further increase in conversion is
observed by increasing the ethene partial pressure to 100 kPa.
In both ‘‘regimes’’, the selectivities to propene increase with
decreasing pethene. On the other hand, with increasing partial
pressure of ethene the selectivities to C5+ hydrocarbons increase.
This is most probably a result of co-oligomerization between
ethene and propene, which becomes more pronounced at higher
molar fractions in the feed gas. The selectivities to saturated
C1–C4 alkanes and aromatics show only minor changes over the
Fig. 4 Influence of the reaction temperature on catalytic performance of
zeolites HZSM-5 (A) and HZSM-23 (B). Reaction conditions: p(ethene)
30 kPa, Ftotal = 30 ml minÀ1, Wcat. = 500 mg, TOS = 0.5 h.
=
The ETP-reaction consists of two elementary steps: the
exothermic oligomerization of ethene to longer hydrocarbon
chains and the endothermic cracking of higher hydrocarbons
to short-chain olefins. In total, an exothermic reaction occurs,
which means that thermodynamic equilibrium conversion
decreases with increasing reaction temperature. This behavior
is indeed observed with ZSM-5 and ZSM-23 (cf. Fig. 4). The
differences in the conversion levels especially at lower reaction
temperatures could be caused by mass transport limitations
within the one-dimensional pore structure of zeolite ZSM-23.
The strong influence of the reaction temperature on the two
competing reaction mechanisms (viz. mono- vs. bimolecular
transition states) has a crucial impact on the product distribu-
tion. For both zeolites increasing selectivities to propene
were observed with increasing reaction temperature while the
selectivities to butenes showed no appreciable changes over the
whole temperature range. Moreover, considerable amounts of
aliphatic hydrocarbons C5+ (without aromatics) are formed at
lower reaction temperatures, which then decrease significantly
with increasing temperature. In the case of zeolite ZSM-23, no
Fig. 5 Influence of the ethene partial pressure on conversion and selec-
tivities over zeolites HZSM-5 (A) and HZSM-23 (B). Reaction conditions:
T = 550 1C, Ftotal = 30 ml minÀ1, Wcat. = 500 mg, TOS = 0.5 h.
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investigated range of ethane partial pressures. Presumably, this Acknowledgements
is an effect of unspecific reactions on the outer surface of the
crystallites where no shape selectivity effects occur.
Financial support for this work from NanoKat (Center for Nano-
Structured Catalysts) and the Center for Resource Efficient
Chemistry and Feedstock Change (Supported by the Carl-Zeiss-
Foundation) at the University of Kaiserslautern is gratefully
acknowledged.
In contrast to zeolite ZSM-23, ZSM-5 showed clear differences in
the by-product formation. Neither an influence of ethene partial
pressure on the selectivity to C5+ hydrocarbons nor on those of
saturated C1–C4 alkanes and aromatics was observed. Considering
the selectivities to the saturated hydrocarbons it is clear that
the involved cracking mechanisms depend strongly on the mole
fraction of ethene in the gas feed. In this context, monomolecular
cracking is favored at low partial pressures of ethene whereas
bimolecular cracking and thus hydrogen transfer reactions are
promoted at high partial pressures of ethene. Interestingly, a
relationship between the formation of aromatics and propene
can be observed (Fig. 5). The results suggest that aromatics are
predominantly formed by secondary reactions of propene.
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