Pt/[Fe]ZSM-5 modified by Na and Cs cations: An active and selective catalyst for dehydrogenation of n-alkanes to n-alkenes

Article abstract of DOI:10.1039/b715543c

Pt clusters within [Fe]ZSM-5 channels provide active and stable sites for the selective catalytic dehydrogenation of n-alkanes to n-alkenes. Cs and Na cations titrate acid sites and inhibit skeletal isomerization and cracking side reactions. The Royal Soc

Full text of DOI:10.1039/b715543c

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Pt/[Fe]ZSM-5 modified by Na and Cs cations: an active and selective
catalyst for dehydrogenation of n-alkanes to n-alkenes
Xuebing Li and Enrique Iglesia*
Received (in Cambridge, UK) 9th October 2007, Accepted 15th November 2007
First published as an Advance Article on the web 27th November 2007
DOI: 10.1039/b715543c
Pt clusters within [Fe]ZSM-5 channels provide active and stable
sites for the selective catalytic dehydrogenation of n-alkanes to
n-alkenes. Cs and Na cations titrate acid sites and inhibit
skeletal isomerization and cracking side reactions.
pressure using a tubular flow reactor with plug-flow hydro-
dynamics. Samples (0.1 g, 180–250 mm diameter pellets) were
3
21
treated in 40% H
Praxair, 99.999%, 0.30 cm s ) at 673 K for 2 h. Reactant and
(Praxair, 99.999%, 0.20 cm s ) in He
3
2
21
(
product concentrations were measured by gas chromatography
(HP 5890 II). n-Pentane and n-heptane reactants were introduced
using a high-pressure syringe pump (Teledyne Isco, Model 500 D).
Dehydrogenation rates were calculated from measured rates, by
correcting for approach to equilibrium for each alkene isomer
Linear alkenes are essential intermediates in the synthesis of
1
useful chemicals, such as lubricants and detergent-range alkyl-
2
aromatics. Alkane dehydrogenation reactions are endothermic
2
and favored by high temperatures and low H pressures; these
8
conditions lead, in turn, to cracking side reactions and fast
deactivation, especially for larger alkanes. Pt-based catalysts are
using thermodynamic data.
Fig. 1 shows pentene formation rates (per Pt) and selectivities
3
among the most active and selective for alkane dehydrogenation.
(on a carbon basis) on Pt/Na–[Fe]ZSM-5 at 673 K during contact
Until this study, high dehydrogenation rates and catalyst stability
with reactants for 140 h. The selectivity to pentenes increased
from 85% to 96% during this period and cracking selectivities
concurrently decreased to y1%. The selectivity to aromatics
also decreased from 8% to y2%. This inhibition of cracking
and aromatization reactions reflects the slow deactivation of
acid sites by unreactive organic deposits during dehydrogenation
have required the concurrent presence of H
2
(.200 kPa) and the
4
use of promoters or stabilizers (e.g. Cl to enhance Pt dispersion,
5
6
Sn to improve stability, and Li to neutralize support acid sites ).
We previously reported that Pt/Na–[Fe]ZSM-5 materials,
prepared by ion-exchange of Pt onto Na–[Fe]ZSM-5, showed
7
unprecedented rates and stability in the selective dehydrogenation
7
of C –C4 alkanes. These materials, when used as catalysts
reactions. These lower selectivities may also reflect, in part,
2
smaller contributions from secondary reactions as the alkane
conversion level decreases with time on stream. Pentene formation
for larger alkanes, gave significant isomerization selectivities,
apparently because acidic OH groups form during reduction of
rates (corrected for approach to equilibrium) decreased from
21 21
exchanged Pt cations by H
these protons with Cs after reduction of Pt cations leads to Pt/Na–
Fe]ZSM-5 materials with high selectivity for dehydrogenation of
2
. Here, we report that the titration of
5
.7 mol (g-atom Pt)
s
21 21
(n-pentane conversion: 25%) to
1
.5 mol (g-atom Pt)
s
(n-pentane conversion: 11%) over
21
40 h. Deactivation was initially rapid (kd = 0.025 h ), but
[
1
n-alkanes to linear n-alkenes and also with high rates and excellent
became slower with time, with a first-order deactivation constant
21
of 0.004 h after 20 h. These deactivation rates resemble those
catalyst stability. Low deactivation rates (first-order deactivation
2
1
rate constant: 0.003–0.006 h ) were achieved at much lower H2
pressures (7 kPa) than required in previous reports (.200 kPa)
and even without H2.
2 4
previously reported on Pt/Na–[Fe]ZSM-5 during C –C alkane
Pt/Na–[Fe]ZSM-5 catalysts were prepared as previously
7
+
4
reported. NH –[Fe]ZSM-5 was exchanged thrice with 0.1 M
NaNO solutions (EMD Chemicals, .99%) at 353 K for 15 h
3
to prepare Na–[Fe]ZSM-5 precursors. Pt cations were then
exchanged onto these samples by contact with [(NH ) Pt](NO )
3 2
3
4
2
5
solutions (6.25 6 10 M; Aldrich, 99.995%) for 12 h at 353 K,
filtered, washed with deionized water, and treated in flowing dry
3
21
21
air (1.67 cm s ) for 12 h at 723 K (0.017 K s ). These samples
contained 0.12 wt% Pt, 0.28 wt% Fe, and a Si/Fe atomic ratio of
3
40 (by inductively-coupled plasma emission spectroscopy).
Fe]ZSM-5 crystals had an average diameter of 0.5 mm (from
transmission electron microscopy). n-Pentane (25 kPa, Aldrich,
9%) and n-heptane (25 kPa, Aldrich, 99%) dehydrogenation
[
9
rates and selectivities were measured at 673 and 723 K at ambient
Fig. 1 Forward rates and selectivities for pentene isomers during
n-pentane dehydrogenation on ($,m) Pt/Na–[Fe]ZSM-5 and (#,n)
Department of Chemical Engineering, University of California,
Berkeley, CA, 94720, USA. E-mail: iglesia@berkeley.edu;
Fax: +1 510 642 4778; Tel: +1 510 642 9673
2
Cs–Pt/Na–[Fe]ZSM-5 at 673 K in the absence of H . n-Pentane partial
21 21
pressure: 25 kPa; space velocity: 12 mol (g-atom-Pt)
s .
5
94 | Chem. Commun., 2008, 594–596
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a
Table 1 n-Pentane dehydrogenation on Pt/Na–[Fe]ZSM-5 and Cs–Pt/Na–[Fe]ZSM-5 at 673 K
b
g
n-Pentane Pentene
conv. (%) sel. (%) 1-Pentene 2-Pentene Pentene 1-butene 1-butene 2-butene formation rate formation rate
trans-
cis-2-
3-Methyl- 2-Methyl- 2-Methyl- Net pentene
Forward pentene
c
c
Catalyst
Pt/Na–[Fe]ZSM-5 21
Cs–Pt/Na–[Fe]ZSM-5 13
89
97
0.44
0.53
0.42
0.50
0.36
0.43
0.39
0.01
0.35
0.004
0.38
0.005
2.8
1.8
b
4.5
3.5
a
21 21
s
Reaction conditions: 673 K, 25 kPa n-pentane, balance: He; space velocity: 12 mol (g-atom Pt)
.
Approach to equilibrium from
c
n-pentane. mol (g-atom Pt)
21 21
s .
7
dehydrogenation and correspond to a mean catalyst lifetime of
skeletal isomerization than their smaller homologs. n-Heptane
dehydrogenation at 673 K formed linear heptenes with high
selectivity (87% initial combined selectivity to 1-heptene, trans-2-
heptene, cis-2-heptene, trans-3-heptene, cis-3-heptene). Selectivities
to linear heptenes increased with time on stream (to 92% after
100 h) (Fig. 2). Heptadienes were the most abundant by-products,
as a result of sequential dehydrogenation of initial n-heptene
products; their selectivities decreased with time on stream (from
initial 12% to 7% after 100 h) because of the concomitant decrease
in n-heptane conversion with time and of the resulting smaller
contribution from sequential reactions of primary n-heptene
products. Reaction rates were lower with n-heptane than with
n-pentane in the linear region of the rate–time data shown in Fig. 1
and 2, apparently because of the faster initial deactivation observed
for the larger alkanes at the high conversion and high diene
concentrations prevalent during the initial contact of the catalyst
with reactants.
250 h, a value larger than typically reported at the severe condi-
tions required for high alkene yields in alkane dehydrogenation
reactions.
[
Fe]ZSM-5 zeolites have weaker Brønsted acid sites than their
9
Al homologs. Protons form in Pt/Na–[Fe]ZSM-5 during reduc-
tion of exchanged Pt cations. These acid sites catalyze skeletal
isomerization of n-pentenes at 673 K to form branched pentenes
(3-methyl-1-butene, 2-methyl-1-butene, 2-methyl-2-butene) and
lead to equilibrium distributions of pentene isomers (approach to
equilibrium from n-pentane y0.4 for all isomers; Table 1). These
protons were titrated after reduction of Pt/Na–[Fe]ZSM-5 by
impregnation with a solution of Cs acetate (Cs/Fe = 1). Cs was
chosen because it is less mobile than Na under hydrothermal
conditions prevalent during drying or regeneration thermal
treatments.
These Cs–Pt/Na–[Fe]ZSM-5 samples gave much higher selec-
tivities to linear pentenes (96%) than Pt/Na–[Fe]ZSM-5 (28%).
Cracking and aromatic products were not detected after Cs
introduction at any time on stream, consistent with the stoichio-
metric titration of Brønsted acid sites by Cs. Pentadienes (y3%
selectivity) became the predominant by-products, because of
sequential dehydrogenation reactions of linear pentenes. These
dienes react quickly on acid sites via Diels–Alder type reactions to
give aromatics and unsaturated organic residues and are quickly
scavenged on catalysts containing acid sites. Forward pentene
formation rates and selectivities are shown in Fig. 1 on Cs–Pt/Na–
The addition of small amounts of H (7 kPa) to n-heptane
2
reactants decreased the prevalent concentration of heptadienes and
improved catalyst stability. H typically increases catalyst stability
2
1
2
during dehydrogenation reactions, but also decreases equilibrium
conversion levels. We compensated for these thermodynamic
effects by slightly increasing reaction temperatures (from 673
to 723 K). Deactivation rate constants were slightly smaller
2
1
(0.003 h ) with 7 kPa H at 723 K than without adding H at
2
2
21
673 K (0.004 h ) (Fig. 2). These deactivation rate constants
2
1
resemble those reported on Pt–Sn/Al O –Cl–Li (0.003 h ); these
2
3
[
Fe]ZSM-5 at 673 K. The deactivation rate constant (k
d
=
latter values required, however, much higher H2 pressures
21
0
.007 h ) was slightly higher than that on Pt/Na–[Fe]ZSM-5
2
1
(
0.004 h ), apparently because of the higher concentrations of
polyunsaturated precursors (e.g. dienes), which are otherwise
scavenged by acid sites on Pt/Na–[Fe]ZSM-5 catalysts. The high
reactivity and stability of Pt domains in [Fe]ZSM-5 channels for
dehydrogenation reflects in part the small size of encapsulated Pt
clusters with 58% dispersion (defined as the percentage of Pt atoms
2
exposed at surfaces; estimated by H chemisorption at 293 K);
these dispersion values are significantly higher than in previously
1
0
reported catalysts based on Pt/Al O (20–35% dispersion). Small
2
3
Pt domains, prepared by anchoring Pt precursors onto exchange
sites of [Fe]ZSM-5, give higher reaction rates because of the
concomitant higher densities of exposed Pt atoms. These small
clusters also inhibit the formation of deactivating residues because
oligomerization reactions require larger ensembles and more open
environments than those present within the zeolite channels that
Fig. 2 Forward formation rates and selectivities for heptene isomers
during n-heptane dehydrogenation on Cs–Pt/Na–[Fe]ZSM-5; open
11
contain the active Pt clusters in these materials.
We have also examined the more demanding dehydrogenation
of larger alkanes (n-heptane) on Cs–Pt/Na–[Fe]ZSM-5. These
larger alkanes are more reactive in acid-catalyzed cracking and
symbols (#,n) 673 K in the absence of H
2
and solid symbols ($,m)
723 K in the presence of 7 kPa H . n-Heptane partial pressure: 25 kPa;
2
21 21
space velocity: 12 mol (g-atom-Pt)
s .
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Table 2 Catalytic performance of Pt-based catalysts for n-heptane dehydrogenation
Reaction conditions
Reactant
pressure/kPa
Space
T/K velocity conversion (%) formation rate selectivity (%) rate/h
n-Heptane
Net heptene
n-Heptenes
Deactivation
21
a
a
Catalyst
Ref.
b
Cs–Pt/Na–[Fe]ZSM-5 n-Heptane: 25 H
Pt–Sn/Al –Cl–Li n-Heptane: 31 H
2
: 7
723 12
1.7
13
8
1.6
0.14
85
90
0.003
0.003
This work
12
O
2 3
2
: 220 743
a
21 21 b
.
mol (g-atom Pt)
cis-3-heptene.
s
Fraction of n-heptene isomers: 10% 1-heptene, 29% trans-2-heptene, 28% trans-3-heptene, 16% cis-2-heptene, 17%
1
2
(
220 kPa) (Table 2). Heptene formation rates (compared
Notes and references
12
per Pt, because dispersion were not reported in the earlier study )
1
2
J. F. Knifton, J. R. Sanderson and P. E. Dai, Catal. Lett., 1994, 28, 223.
H. S. Bloch (UOP), US Pat., 3448165, 1969.
on Cs–Pt/Na–[Fe]ZSM-5 were almost ten-fold higher than
1
2
on Pt–Sn/Al O –Cl–Li catalysts at similar conditions (Table 2).
2
3 M. M. Bhasin, J. H. McCaina, B. V. Vora, T. Imai and P. R. Pujad o´ ,
Appl. Catal., A, 2001, 221, 397.
3
This latter catalyst is the only practical dehydrogenation
catalyst reported for the synthesis of linear alkenes from larger
n-alkanes. No branched heptenes were detected during n-heptane
reactions and linear heptene isomers were in equilibrium
with each other on the catalysts reported here at all reaction
conditions (Table 2).
4
H. Lieske, G. Lietz, H. Spindler and J. V o¨ lter, J. Catal., 1983, 81, 8;
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In summary, we provide evidence here for the unique catalytic
properties of Pt/Na–[Fe]ZSM-5 with residual acid sites titrated by
Cs after reduction of exchanged Pt cations for the dehydrogena-
tion of n-pentane and n-heptane. These materials catalyze
dehydrogenation of larger n-alkanes with unprecedented rates
and with very high selectivity to n-alkenes. The catalyst stability is
excellent in comparison with catalytic materials in previous
i
8 The approach to equilibrium (g, where i represents different isomers of
pentenes or heptenes) was calculated from thermodynamic data (C. L.
Yaws, Yaws’ Handbook of Thermodynamic and Physical Properties of
Chemical Compounds, Gulf, Houston, FL, 2006) and on pressures of
reactants and products prevalent during dehydrogenation reactions:
n-Alkane = Alkene (i) + H
2
2
reports, which required much higher H pressures for stable
½
PAlkene(i)ꢀ½PH ꢀ
1
2
gi~
operation. These catalysts provide effective routes for the synthesis
of useful linear alkenes using the corresponding n-alkanes as
feedstocks. Our study extends the applicability of these materials
½
Pn-Alkaneꢀ
Keq,i
Forward rates are given by:
2 4
beyond their initial disclosure for dehydrogenation of C –C
r
f,i = rnet,i/(1 2 g ).
i
alkanes. It also provides compelling evidence that Brønsted acid
sites must be rigorously excluded to preserve the desired linear
nature of the alkenes formed in dehydrogenation turnovers.
The authors acknowledge financial support from ExxonMobil
Research and Engineering Co. during the course of these
studies and also Dr James C. Vartulli (ExxonMobil) for the
synthesis and characterization of the [Fe]ZSM-5 precursor used
in this study.
9
1
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1
1
1
1
5
96 | Chem. Commun., 2008, 594–596
This journal is ß The Royal Society of Chemistry 2008