Tailored catalytic propene trimerization over acidic zeolites with tubular pores

Article abstract of DOI:10.1002/anie.200463045

(Graph Presented) Slim through spatial constraints: A higher content of linear and monobranched propene trimers is obtained with ZSM-22 as catalyst than with many other zeolite catalysts (see picture). This is explained by the constraints imposed by the diameter of the tubular pores of ZSM-22. ZSM-22 also brings an important environmental benefit compared with the presently used industrial catalyst silica-supported phosphoric acid.

Full text of DOI:10.1002/anie.200463045

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Industrial Catalysts
chain branchings (NCB) of the alkene dimers is high and
similar to that of processes using SSPA. In oxyfunctionaliza-
tion processes, olefins with two and especially with three or
more chain branchings show decreased reactivity.[11] We now
report the advantageous use of ZSM-22 for the synthesis of
propene trimers with appropriate low NCB.
DOI: 10.1002/anie.200463045
Tailored Catalytic Propene Trimerization over
Acidic Zeolites with Tubular Pores**
Zeolite pore architecture is characterized by the dimen-
sionality and size of the channel system, the latter being
determined by the number of oxygen atoms in the ring
circumscribing the opening. Candidate zeolite topologies
categorized according to micropore architecture were
screened in the propene trimerization reaction using a
mixture of 12 wt% of propene in propane fed in supercritical
state at 6.8 MPa and 473–513 K. Contact time was adapted to
reach at least initially a high conversion level. With respect to
trimerization yield (YC9) and limitation of NCB, zeolites with
a tubular 10-membered ring (10-MR) channel system devoid
of large lobes or intersections outperformed all others
(Figure 1, bottom). On the majority of zeolite structure
Johan A. Martens,* Wim H. Verrelst,
Georges M. Mathys, Stephen H. Brown, and
Pierre A. Jacobs
Oligomerization of short alkenes is applied in large-scale
petrochemical processes for a variety of products including
liquid hydrocarbon fuels, lubricants, detergent precursors,
solvents, and plasticizers.[1–4] Whereas longer-chain linear
alkenes result from homogeneous catalytic processes that
use transition metal complexes,[5] chain-branched alkenes in
the C6–C12 range stem from the oligomerization of C3 or C4
alkenes using a fixed bed of silica-supported phosphoric acid
(SSPA),[1–4,6] at temperatures and pressures amounting to
about 520 K and 12 MPa, respectively.[7,8] Despite drawbacks
related to the maintenance of a critical feed hydration level,
catalyst clogging, and corrosion problems caused by phos-
phoric acid leaching, SSPA processes are still widely oper-
ated.[1–4] We have now discovered that acidic ZSM-22 zeolite
allows the synthesis of propene trimers of intermediate
degree of chain branching, suitable as feedstock for oxy-
functionalization processes. The narrow tubular pores of this
zeolite impose preferential formation of the slimmer linear
and monobranched oligomers. Though the in-time stability of
ZSM-22 is similar to that of SSPA, the former catalyst does
not present the numerous disadvantages encountered with
the latter.
Difficulties in finding a viable substitute for SSPA are not
only related to the fact that cheap, impure alkene feedstocks
are used, but also to the critical product specification with
respect to molecular weight and chain branching of the
oligomers. Acidic zeolites through their molecular shape
selectivity properties can meet these stringent catalyst
requirements. For fuel and lubricant applications, a technol-
ogy based on ZSM-5 has been developed.[9] More recently,
ZSM-57 with its unique lobate pore structure was reported to
be a stable and selective dimerization catalyst for C4–C6
alkenes.[10] With a ZSM-57-type catalyst, the number of
Figure 1. Influence of the micropore system of the acidic zeolite at pro-
pene conversions exceeding 80% on propene trimer yield (YC9) and
number of chain branchings (NCB).
types known today, with intersecting 10-MR channels, side
pockets, or pronounced lobes or with 12-MRs,[12] YC9 at high
conversion was reduced, while the NCB was close to 2.0
(Figure 1, top), a value typical for SSPA processes.
Uniform 10-MR tubular pores are encountered in ZSM-
22, ZSM-23, SAPO-11 with TON, MTT, and AEL topology,
respectively,[12] as well as in the ZSM-48 family of zeo-
lites.[13,14] With ZSM-22, initially all propene was converted.
Though the activity showed some decay with time-on-stream,
an amount of propene corresponding to 400 times the catalyst
weight could be converted easily (see Experimental Section).
The three other zeolites showed decreased initial activity,
with similar (ZSM-23) or higher rates of decay (ZSM-48 and
SAPO-11).
[*] Prof. J. A. Martens, Dr. W. H. Verrelst, Prof. P. A. Jacobs
Centre for Surface Chemistry and Catalysis
Catholic University of Leuven
Kasteelpark Arenberg 23, 3001 Leuven (Belgium)
Fax: (+32)16-32-19-98
E-mail: johan.martens@agr.kuleuven.ac.be
Dr. G. M. Mathys, Dr. S. H. Brown
ExxonMobil Chemical Europe, Inc.
European Technology Center
Hermeslaan 2, 1831 Machelen (Belgium)
Under the presently used reaction conditions, typical for
industrial processes, the tubular 10-MR zeolite catalysts
yielded mainly dimers, trimers, and tetramers as reaction
products. This is evident from Figure 2, which also clearly
[**] J.A.M. and P.A.J. acknowledge the Flemish Government for support
in the frame of a Concerted Research Action on Catalysis (GOA), as
well as the Federal Services of Science Policy for support in the
frame of a IAP-PAI Network on Supramolecular Catalysis.
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Figure 3. Skeletal distribution of the dimer fraction obtained at increas-
ing propene conversion with ZSM-22.
branched isomers (Figure 3). With ZSM-22, methyl shifts on
alkylcarbenium ions were found to be substantially slower
than de-branching reactions in which one branch vanishes.[15]
Linear dimers observed over the entire conversion range
(Figure 3) stem from fast de-branching reactions of 2-
methylpentenes. Above 40% propene conversion, linear
and methyl-branched dimers are found at equilibrium with
ZSM-22 as catalyst (Figure 3). Dimers with dimethyl-
branched carbon skeleton were obtained only at high feed
conversion. Similar dimers fractions were obtained on the
other 10-MR tubular zeolites, except for the more pro-
nounced formation of dibranched dimers on SAPO-11 and
ZSM-48.
The NCB in the trimer fractions obtained with 10-MR
tubular zeolites is displayed in Figure 4 and compared with
that of a typical trimer fraction obtained on SSPA, represen-
tative for an oligomerization without any confinement in
micropores. The large majority of trimers show a dibranched
carbon skeleton (Figure 4), which can be explained by acid-
catalyzed oligomerization causing an additional branching
upon addition of a novel propene unit to a methyl-branched
dimer. Especially with ZSM-23 and ZSM-22, the fraction of
Figure 2. Yield (Y) of propene dimers, trimers, tetramers, and of total true
oligomers (di-to hexamers, without products with intermediate carbon numbers
obtained by cracking and recombination of fragments) against propene conver-
sion (X) with ZSM-22, ZSM-23, ZSM-48, and SAPO-11.
indicates that on all four catalysts the true oligomers remain
the largest product fraction up to high conversion—for ZSM-
23 and ZSM-22 even more pronounced than for SAPO-11 and
ZSM-48. Despite the high conversion, also oligomer chain
growth was limited. On ZSM-22, the dimer yield peaked at
around 80% propene conversion; the trimer yield close to
100% (Figure 2). The highest trimer yields were obtained
with ZSM-22. Chain growth was more pronounced on ZSM-
23, SAPO-11, and ZSM-48, as noted from the more abundant
formation of tetramers and the occurrence of dimer and
trimer yield maxima at lower propene conversion.
Given the multitude of possible double-bond and skeletal
isomers, the molecular reaction network for propene oligo-
merization is complex. As double bond positions in the
individual dimers were always at equilibrium, selectivity
discussions can be limited to the rationalization of the
branching of the alkane skeleton obtained after saturation
of the double bond. Chain branching was analyzed using gas
chromatography with sample hydrogenation (see Experimen-
tal Section). In the dimer fraction obtained on ZSM-22
(Figure 3), primary products dominating at low conversion
have a carbon skeleton with a methyl branch at C2, as
expected for an acid-catalyzed alkylation [Eq. (1)].
Figure 4. Skeletal distribution of trimer fractions at 30% propene con-
version, obtained on collidine-treated ZSM-22, ZSM-23, ZSM-22, ZSM-
48, SAPO-11, SSPA, and on successive layers of SSPA and ZSM-22.
Skeletal isomers with methyl branching at C3 are
secondary products obtained by methyl shifts on 2-methyl-
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trimers with monobranched or linear carbon skeleton is larger
than with SSPA. Propene trimers are methyl-branched at
carbon positions 3 and 4 (3,4-group isomers) unless the
branchings are shifted (2,6-group isomers) or eliminated to
give monobranched and linear isomers (monobranched-
group isomers).[16] Among the 10-MR tubular pore zeolites
the formation of monobranched-group nonene isomers
decreases in the series ZSM-23 > ZSM-22 > ZSM-48 >
SAPO-11 (Figure 5). The skeletal distribution of the trimer
fraction obtained with SAPO-11 is already very similar to that
encountered with SSPA (Figure 4).
of tetramers most likely occurs through propene addition to
linear trimers. As ZSM-23 exhibits a stronger tendency to
trimer linearization, it shows a more pronounced chain
growth. In ZSM-48 and SAPO-11, chain growth can probably
also proceed on monobranched trimer chains, explaining why
in these zeolites tetramer formation is also more pronounced
than in ZSM-22.
As the ZSM-22 crystals are micrometer-sized and the
specific external surface area amounts to about 40 m2 gÀ1,
pore mouth sites of ZSM-22 may contribute significantly to
catalysis, for example, in catalytic isodewaxing.[25] Acidic sites
on the external surfaces and pore mouths of ZSM-22 can be
selectively poisoned with collidine (2,4,6-trimethylpyridine),
a bulky amine that cannot penetrate the pores of 10-MR
zeolites. Whereas the collidine-poisoned ZSM-22 catalyst was
about five times less active than the parent sample, the trimer
fraction was very rich in monobranched nonenes (80%) and
contained a significant amount of linear nonenes (15%)
(Figure 4), confirming that catalytic sites located inside the
tubular pores are responsible for the formation of mono-
branched and linear trimers, and that sites accessible to
collidine are responsible for the formation of di- and
tribranched nonenes. ZSM-22 also shows “coke selectivation”
as partially deactivated catalysts were found to be more
selective for linear and monobranched nonenes than fresh
catalysts, owing to a gradual loss of the contribution of the
external sites.
ZSM-22 can be used as hydroisomerization catalyst for
converting long-chain n-alkanes into mono- and dibranched
skeletal isomers,[25] as it establishes the internal equilibrium
between the linear alkane, its monobranched isomers, and
specific dibranched isomers.[25] This property of ZSM-22 can
be exploited to reduce the NCB of the C9 products obtained
from propene oligomerization with SSPA as was shown for
the trimer fraction produced in a process with SSPA for which
the NCB was reduced from 2.0 to 1.7 during a typical catalytic
run (473 K, 6.8 MPa, W/F0 = 280 kgsmolÀ1). The reduction
resulted primarily from conversion of dibranched into
monobranched and linear nonenes (Figure 4, columns 6 and
7). Tribranched isomers have too little adsorption affinity in
pore mouths and are not converted. Dibranched isomers
undergo a first de-branching in pore mouths and eventually
entire linearization deeper inside the pores.
The ZSM-22 catalysis was also run using feedstock of
50 wt% propene in propane, similar to a process with SSPA.
Though with this propene-rich feed, pronounced exothermic-
ity in the catalyst bed was observed, deactivation was minor.
Propene conversion remained complete until an amount of
propene of at least 400 times the catalyst weight was fed. An
incremental increase of catalyst temperature was sufficient to
reestablish complete propene conversion. Finally, the experi-
ment was interrupted after the formation of more than 600 kg
of oligomer product per kg of catalyst, which is in the range of
oligomer yields with SSPA processes. ZSM-22 also brings an
important environmental benefit, as it is reusable many times
after regeneration and has a much longer lifetime than SSPA
which can only be used once. Thus, the use of ZSM-22 in
propene oligomerization adds to the sustainability of the
petrochemical industry.
Figure 5. Compositional diagram showing the evolution of the skeletal
structures of propene trimers obtained with 10-MR tubular pore zeo-
lites at increasing conversions indicated by the arrows.
Among the known zeolite structure types, those with
tubular 10-MRs having a free diameter of around 0.5 nm
exhibit the highest heats of adsorption for linear hydro-
carbons.[17–22] The free diameters of ZSM-23 pores of 0.45 ”
0.52 nm2 [12] preclude methyl-branched alkanes from enter-
ing.[17] The penetration of the same methyl-branched alkanes
into the slightly wider ZSM-22 pores with cross sections of
0.44 ” 0.55 nm2 is possible, depending on conditions.[17–20]
SAPO-11 belongs to the AEL zeolite family, has a little
wider 10-MR of 0.40 ” 0.65 nm2, and readily adsorbs isoal-
kanes.[21,22] Sorption data on ZSM-48 with its relatively wide
10-MR of 0.53 ” 0.56 nm2 are not available. Catalytic test
reactions such as the determination of the constraint index[23]
and of the refined constraint index[24] reveal that there is less
steric limitation inside SAPO-11 and ZSM-48 pores than
inside ZSM-22 and ZSM-23 pores. Thus the observed
tendency of debranching of propene trimers in the zeolite
sequence of SAPO-11 < ZSM-48 < ZSM-22 < ZSM-23 (Fig-
ures 4 and 5) reflects the order of decreasing effective pore
width.
The stronger limitation of oligomer chain growth in ZSM-
22 compared with ZSM-23 (Figure 2) can be explained as
follows: Inside the pores of these two zeolites, the formation
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[10]J. A. Martens, R. Ravishankar, I. E. Mishin, P. A. Jacobs, Angew.
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[11]B. Cornils in New Synthesis with Carbon Monoxide (Ed.: J.
Falbe), Springer, Berlin, 1980, p. 96.
[12]C. Baerlocher, W. M. Meier, D. H. Olson, Atlas of Zeolite
Framework Types, 5th ed., Elsevier, Amsterdam, 2001.
[13]J. L. Schlenker, W. J. Rohrbaugh, P. Chu, E. W. Valyocsik, G. T.
Kokotailo, Zeolites 1985, 5, 355 – 358.
[14]R. F. Lobo, H. van Koningsveld, J. Am. Chem. Soc. 2002, 124,
13222 – 13230.
Experimental Section
Zeolite samples were synthesized according to published methods:
ZSM-22,[26] ZSM-23 was run 4 of reference [27], ZSM-48,[28] SAPO-
11[29] was sample S-11/2 of reference [30]. The Si/Al ratio of ZSM-22,
-23, and -48 was 42, 44, and 50, respectively. The phase purity was
confirmed by XRD and SEM. Gram quantities of as-synthesized
zeolite powder were pelletized (0.25–0.50 mm) by compression,
loaded in a quartz tube, and calcined, first under flowing nitrogen
at 4008C for 5 h and subsequently under oxygen at 5508C for 15 h.
The samples were exchanged with 0.5m aqueous NH4Cl solution and
calcined at 4008C. This procedure resulted in acidic zeolites with
more than 99.5% of the aluminum tetrahedrally coordinated
according to 27Al MAS-NMR (signal at 54 ppm against aluminum
[15]C. S. L. Narasimhan, J. W. Thybaut, G. B. Marin, P. A. Jacobs,
J. A. Martens, J. F. Denayer, G. V. Baron, J. Catal. 2003, 220,
399 – 413.
nitrate reference with
a Bruker Avance 400 MHz instrument;
[16]After hydrogenation, the 3,4-group comprises the following
isomers: 3,4-, 2,3- and 4,4-dimethylheptane; 3-ethyl-2-methyl-
and 3-ethyl-3-methylhexane; 2,3,4-trimethylhexane. The 2,6-
group comprises 2,6-, 2,5-, 3,5-, 2,4-, and 2,2-dimethylheptane.
[17]J. F. Denayer, A. R. Ocakoglu, W. Huybrechts, J. A. Martens,
J. W. Thybaut, G. B. Marin, G. V. Baron, Chem. Commun. 2003,
15, 1880 – 1881.
[18]M. Schenk, B. Smit, T. J. H. Vlugt, T. L. M. Maesen, Angew.
Chem. 2001, 113, 758 – 761; Angew. Chem. Int. Ed. 2001, 40, 736 –
739.
[19]F. Eder, J. A. Lercher, J. Phys. Chem. B 1997, 101, 1273 – 1278.
[20]J. A. Z. Pieterse, S. Veefkind-Reyes, K. Seshan, J. A. Lercher, J.
Phys. Chem. B 2000, 104, 5715 – 5723.
[21]F. Eder, J. A. Lercher, J. Phys. Chem. 1996, 100, 16460 – 16462.
[22]H. Stach, K. Fiedler, J. Janchen, Pure Appl. Chem. 1993, 65,
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spinning frequency of 20 kHz). The FT-IR spectra of the zeolites
displayed absorptions at about 3595 and 3745 cmÀ1, assigned to
À
À
bridging Al OH Si groups with Brønsted acid character and frame-
work-terminating silanol groups, respectively. IR absorption bands at
about 3655 cmÀ1, which are characteristic of hydroxy groups on
aluminum atoms that are partially dislodged from the framework and
responsible for Lewis acidity, were absent.
The chemical composition of the SAPO-11 material corresponds
to (Si0.21Al0.43P0.36)02. The SAPO-11 crystals are composed of 28%
aluminosilicate (AS) and 72% silicoaluminophosphate (SAP)
domains.[30] The catalytic activity of SAPO-11 is mainly due to
Brønsted acid sites at the interphase between the AS and SAP
domains.
Catalytic experiments in a fixed-catalyst-bed continuous-flow
reactor: catalyst pellets of compressed zeolite powder with diameters
of 0.25–0.50 mm were loaded in a stainless steel reactor tube with an
internal diameter of 9 mm and a length of 300 mm. Empty volume
was filled with alumina beads of the same diameter. Reaction
conditions: T= 473 K; P = 6.8 MPa; feedstock: 12 wt% propene in
propane; W/F0 = 34 kgsmolÀ1 (W: catalyst weight; F0: molar propene
flow at reactor inlet); liquid feedstock delivered with a liquid mass
flow controller (Brooks Instruments). The reactor outlet was sampled
online at high pressure using a 4-port HPLC sampling valve (Valco)
with an internal volume of 0.1 mL, and analyzed with capillary gas
chromatography. Product NCB was determined offline on a gas
chromatograph with sample hydrogenation module.
[23]V. J. Frilette, W. O. Haag, R. M. Lago, J. Catal. 1981, 67, 218 –
223.
[24]P. A. Jacobs, J. A. Martens, Pure Appl. Chem. 1986, 58, 1329 –
1338.
[25]J. A. Martens, W. Souverijns, W. Verrelst, R. Parton, G. F.
Froment, P. A. Jacobs, Angew. Chem. 1995, 107, 2726 – 2728;
Angew. Chem. Int. Ed. Engl. 1995, 34, 2528 – 2530.
[26]S. Ernst, J. Weitkamp, J. A. Martens, P. A. Jacobs, Appl. Catal.
1989, 48, 137 – 148.
[27]S. Ernst, R. Kumar, J. Weitkamp, Catal. Today 1988, 3, 1 – 10.
[28]P. A. Jacobs, J. A. Martens, Stud. Surf. Sci. Catal. 1987, 33, 22.
[29]M. Mertens, J. A. Martens, P. J. Grobet, P. A. Jacobs in Guide-
lines for Mastering the Properties of Molecular Sieves (Ed.: D.
Barthomeuf), Plenum, New York, 1990, p. 1.
Received: December 23, 2004
Revised: February 23, 2005
Published online: August 1, 2005
[30]J. A. Martens, P. J. Grobet, P. A. Jacobs, J. Catal. 1990, 126, 299 –
305.
Keywords: oligomerization · propene · shape selectivity ·
.
silica-supported phosphoric acid · zeolites
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