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610 J. Phys. Chem., Vol. 100, No. 11, 1996
Hartmann and Kevan
catalytically active sites. Recently, Ni(I) has been shown to be
active for ethylene dimerization in framework and nonframe-
work sites in SAPO-5 and SAPO-11.36 Based on our ESR
results and on previous investigations in zeolites, there is little
doubt that Pd(I) is the active site for ethylene dimerization in
PdH-SAPO-n materials. However, the turnover in these
materials seems rather low compared to Pd-exchanged zeolites.
But according to the lower ion-exchange capacity of silicoalu-
minophosphates, the initial Pd(II) concentration is about 10-
fold lower than the lowest concentration used in zeolites. By
increasing the reaction temperature to 100 °C, the overall
turnover can be increased to 10% in PdH-SAPO-5, but this
success is accompanied by a significant loss in selectivity.
Additional products formed include propane, butane, and
isobutene which need ethylene dimerization as their first
formation step. It has been reported29 that ethylene is initially
dimerized to 1-butene, which is subsequently isomerized to an
equilibrium composition of n-butenes with predominant trans-
study it has been shown that ethylene reduces cationic palladium
(Pd(II) and Pd(I)) in NaCaPd-Y to Pd(0) during ethylene
dimerization on the catalyst at 50 °C accompanied by a color
change of the catalyst from brown to black. The reduction
process is likely enhanced by temperature, which leads to a faster
deactivation at higher temperatures.
4
1
Conclusions
Ethylene dimerization occurs on palladium-exchanged SAPO
materials with different pore sizes. The catalytic activity
increases with the reaction temperature while the selectivity for
the formation of n-butenes decreases. The decrease of the
channel size from 14-ring SAPO-8 to 12-ring SAPO-5 and 10-
ring SAPO-11 causes a higher amount of side products
(isobutene, propane, and butane) to be formed, probably because
of a lower product diffusion out of the channels. ESR and
ESEM experiments support the catalytic measurement, showing
that paramagnetic Pd(I) species are observed prior to ethylene
dimerization. After adsorption of ethylene a Pd(I)-C2D4
complex is observed, which is converted into a Pd(I)-C4D8
complex after reaction. This is confirmed by different ESR
spectra and contrasting coordination properties from ESEM data.
All catalysts are ultimately deactivated due to the conversion
of catalytically active palladium(I) to inactive palladium(0) by
ethylene or butenes at later stages.
2
-butene. The temperature-dependent equilibrium is reached
after 3 h, showing that the isomerization reaction at the acid
sites of the SAPO materials is faster than ethylene dimerization.
Compared to the PdCa-X zeolites equilibrium is obtained more
slowly, most likely because of the lower acidity of the
silicoaluminophosphates. Especially at elevated temperatures
this double-bond shift competes with several side reactions, like
skeletal rearrangement and oligomerization (to form hexenes)
followed by cracking.35 This leads to a significant amount of
isobutane, propane, and butane in medium-pore materials like
SAPO-5 and SAPO-11 where the kinetic diameter of the butene
molecule (0.39 nm) is in the range of the channel size. With
further increase of the channel or cage size in PdH-SAPO-8
or Pd-X and -Y zeolites side reactions are more suppressed
even in materials with a higher acidity. The same trends were
Acknowledgment. This research was supported by the
National Science Foundation and the Robert A. Welch Founda-
tion.
References and Notes
(
1) Martens, J. A.; Jacobs, P. A. In AdVanced Zeolite Science and
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1994; pp 653-684.
previously found in NiH-SAPO-5 and NiH-SAPO-11 materi-
als in comparison to NiH-SAPO-8.3
4,36
Our results show that
(2) Minachev, Kh. M.; Gavanin, V. I.; Kharlamov, V. V.; Isakova, T.
the selectivity for the formation of n-butenes is higher in SAPO-
1 than in SAPO-5. SAPO-5 is a large-pore molecular sieve
A. Kinet. Katal. 1972, 13, 1101.
1
(3) Lapidus, A. L.; Mal’tsev, V. V. Acta Phys. Chem. 1978, 24, 195.
(
4) Michalik, J.; Narayana, M.; Kevan, L. J. Phys. Chem. 1985, 89,
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with a pore opening around 0.73 nm, while SAPO-11 is a
medium-pore molecular sieve with elliptical pore openings
around 0.63 by 0.39 nm. For the large-pore SAPO-5, further
oligomerization to hexenes is more important, and a larger
amount of cracking products is detected. Also, differences in
acidity between SAPO-5 and SAPO-11 have to be taken into
consideration. In silicoaluminophosphates the acidity not only
4
(
(7) Michalik, J.; Heming, M.; Kevan, K. J. Phys. Chem. 1986, 90, 2132.
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is a function of the Br o¨ nsted site concentration but also strongly
depends on the structure.38 Therefore, SAPO-5 is considered
(
(
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9
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In this work, all catalysts were found to deactivate at longer
reaction times. The deactivation is most pronounced at higher
reaction temperatures. The deactivation may be ascribed to one
of the following reasons: (i) instability of the active Pd(I)
species, (ii) pore blocking of the zeolite catalyst, and (iii) coke
deposition on the catalyst during the reaction. In the present
work various paramagnetic species were found previous to
ethylene dimerization. The paramagnetic species were found
to decrease at longer reaction times with simultaneous decrease
of catalytic activity. Simultaneously, the catalyst turned dark
gray in color. These results imply that the active palladium(I)
cation is unstable under the reaction conditions and is likely to
be converted to Pd(0), which is catalytically inactive for ethylene
dimerization. The increase of the framework defect species at
g ) 1.96 is also indicative of a reductive treatment of the sample
since this signal is usually only obtained after dehydration or
(
16) Davis, M. E.; Montes, C.; Hathaway, P. E.; Garces, J. M. In
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9; Elsevier: Amsterdam, 1991; pp 135-143.
(20) Hartmann, M.; Kevan, L. Manuscript in preparation.
6
(
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1
(
(
(
40
hydrogen reduction. In an X-ray photoelectron spectroscopic