EPA/600/R-98/041
Apnl 1998
Dehydrogenation Catalyst for
Production of MtBE
by
Josephine M. Hill, Randy D. Cortright,
Dale F. Rudd, and James A. Dumesic
University of Wisconsin, Madison
Madison, WI 53706
CR-822939
Project Officer
Paul Randall
Sustainable Technology Division
National Risk Management Research Laboratory
Cincinnati, OH 45268
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
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Notice
The U.S. Environmental Protection Agency through its Office of Research and Development
partially funded the research described herein under Cooperative Agreement CR-822939 to the
University of Wisconsin - Madison. It has been subjected to the Agency's peer and administrative
review, and has been approved for publication as an EPA document. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's
land, air, and water resources. Under a mandate of national environmental laws, the Agency
strives to formulate and implement actions leading to a compatible balance between human
activities and the ability of natural systems to support and nurture life. To meet this mandate,
EPA's research program is providing data and technical support for solving environmental
problems today and building a science knowledge base necessary to manage our ecological
resources wisely, understand how pollutants affect our health, and prevent or reduce
environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for investigation of
technological and management approaches for reducing risk from threats to human health and the
environment. The focus of the Laboratory's research program is on methods for prevention and
control of pollution to air, land, water and subsurface resources; protection of water quality in
public water systems; remediation of contaminated sites and ground water; and prevention and
control of indoor air pollution. The goal of this research effort is to catalyze development and
implementation of innovative, cost-effective environmental technologies; develop scientific and
engineering information needed by EPA to support regulatory and policy decisions; and provide
technical support and information transfer to ensure effective implementation of environmental
regulations and strategies.
This publication has been produced as part of the Laboratory's strategic long-term research plan.
It is published and made available by EPA's Office of Research and Development to assist the
user community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
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Abstract
The objectives of this project were to better understand the effect of different catalyst preparation
parameters, the effect of different catalyst treatment parameters, and the mechanism of deactivation.
Accordingly, catalysts were made using various preparation methods and with a range of platinum
(Pt) and tin (Sn) contents. Catalysts were also tested under a variety of reaction conditions.
Catalysts were characterized using kinetic studies, chemisorption, transmission electron microscopy
and temperature programmed oxidation. The results indicate that Pt/L-zeolite catalysts are not
selective without Sn. Sn reduces the size of the Pt ensembles thus inhibiting the hydrogenolysis,
isomerization and coking reactions. At high Sn loadings, or after long periods at high temperature
(-873 K) the surface Pt ensembles become enriched with Sn and metal sintering occurs; thus, the
number of exposed Pt atoms and subsequently the activity, are decreased. It is difficult to reverse
the results of these processes. Reacting isobutane at a lower temperature (-798 K) in the absence
of feed hydrogen reduces the Sn enrichment and the metal sintering. At these conditions the main
mechanism of deactivation is coking, a process which is relatively easy to reverse with flowing
oxygen at a moderate temperature.
This report was submitted in fulfillment of grant CR-822939 by J.A. Dumesic and co-workers
under the sponsorship of the United States Environmental Protection Agency. This report covers
a period from October 1994 to October 1997.
IV
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Table of Contents
Foreword iii
Abstract iv
List of Figures vi
List of Tables viii
Acronyms and Abbreviations , ix
Acknowledgments
Chapter 1 Introduction 1
Chapter 2 Conclusions >
Chapter 3 Recommendations ^
Chapter 4 Experimental Procedures *
Catalyst Preparation 5
Kinetics/TPO apparatus 5
Chemisorption apparatus 7
Transmission Electron Microscopy ^
Description of Measurement Parameters 7
Quality Assurance 8
Chapter 5 Results and Discussion 11
Catalyst Preparation 11
Catalyst Treatment 17
Catalyst Deactivation 22
Summary of Preferred Conditions ... 29
Discussion 29
References 34
V
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List of Figures
1 - 1. The production of MtBE from methanol and isobutane.. [[[ .2
4-1. Schematic of kinetics apparatus [[[ 6
4-2. Schematic of reactors [[[ 6
4-3. GC calibration curves for methane, ethane, propane and n-butane 9
4-4. GC calibration curves for isobutane and isobutene [[[ 9
4-5. Duplicate kinetic runs [[[ 10
5-1. Effects of preparation methods of 1:2 Pt/Sn/L zeolite on isobutane conversion at 873 K, 1
atm, and a H2/i-C4H10 ratio of 2 [[[ 12
5-2. Effects of preparation methods of 1 :2 Pt/Sn/L- zeolite on isobutane dehydrogenation
selectivity at 873 K, 1 atm, andaH2/i-C4H10ratio of 2 [[[ 12
5-3. Effects of platinum content on isobutane dehydrogenation reaction rates at 873 K, 1 atm,
and a H2/i-C4H10 ratio of 2 over Pt/Sn/L- zeolite (0.83 wt% Sn) ....................................... 13
5-4. Effects of platinum content on isobutane dehydrogenation reaction rates at 873 K, 1 atm,
and a H2/i-C4H10 ratio of 2 over Pt/Sn/L-zeolite (0.42 wt% Sn) ...................................... 13
5-5. Effects of tin content on isobutane dehydrogenation reaction rates at 873 K, 1 atm, and a
H2/i-C4H10 ratio of 2 over Pt/Sn/L-zeolite (0.1 wt% Pt) [[[ 14
5-6. Effects of tin content on isobutane dehydrogenation reaction rates at 873 K, 1 atm, and a
H^i-C4H10 ratio of 2 over Pt/Sn/L-zeolite (0.5 wt% Pt) [[[ 14
5-7. Comparison of the isobutane dehydrogenation rates at 873 K, 1 atm, and a H2/i-C4H10 ratio
of 2 [[[ 15
5-8. Comparison of the isobutene selectivity at 873 K, 1 atm, and a H2/i-C4H10 ratio of 2 ......... 16
5-9. Comparison of the isobutane conversion at 873 K, 1 atm, and a H2/i-C4H10 ratio of 2.. ..... 16
5-10. Comparison of the isobutane dehydrogenation rates at 873 K, 1 atm, and a fL/i-C^H^ ratio
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5-14. Comparison of the isobutane dehydrogenation rates at 798 K over 1:2.5 Pt/Sn/L-zeolite
with different feed streams 21
5-15. Comparison of the isobutane conversion at 798 K and in i-C4H10 only over 1:2.5 Pt/Sn/L
andPt/L. 21
5-16. Comparison of the isobutane conversion at 798 K and in i-C4H10 only over 1: 1.4 Pt/Sn/L-
zeolite andl:2.5 Pt/Sn/L-zeolite 22
5-17. Comparison of the TPO spectra after 4 h reaction at 873 K ..1 24
5-18. Isobutane conversions and dehydrogenation selectivities at 798 K in isobutane 26
5-19. Comparison of the TPO spectra after 4 h reaction at 798 K, 1 atm, and in isobutane only for
L-zeolite- and silica-supported catalysts 26
5 - 20. Isobutene turnover frequencies at 798 K in isobutane 28
5-21. The activity of 1:2.5 Pt/Sn/L-zeolite at 873 K, 65 1 Torr He and 109 Torr iC4H10 before and
after regeneration in 25% 0, in He at 300ฐC for 120 minutes 28
5 - 22. Effects of regeneration on isobutane dehydrogenation reaction 29
vn
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List of Tables
5-1. CO uptake on fresh catalysts and after reaction at 873 K in 2: 1 H2:iC4Hio 23
5-2. CO uptake versus reduction temperature for Pt/L 23
5-3. CO uptakes and extents of carbon deposition on catalysts 24
5-4. Isobutane uptake on catalysts 25
5-5. Preferred preparation and treatment conditions.. 29
vin
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Acronyms and Abbreviations
LaB6 Lanthanum Boride
MtBE Methyl tert-butyl ether
QA Quality assurance
SCCM Standard cubic centimeters per minute
TEM Transmission electron microscopy
TOF Turnover frequency
TPO Temperature programmed oxidation
WHSV Weight hourly space velocity
IX
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Acknowledgments
We would like to acknowledge and thank Jane Bare and Paul Randall, EPA Project
Officers, for their help in this project.
Several undergraduates in Chemical Engineering at the University of Wisconsin have
helped with data collection. We would like to acknowledge the help of Janice
Schneider, Kyle Lewis, Lisa Gracyalny, Mike Recupero and Willie Wiyatno.
X
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Chapter 1
Introduction
Industrial catalytic reaction pathways for the production of methyl text-butyl ether (MtBE), an
important motor fuel oxygenate, typically involve 1) the production of synthesis gas from
hydrocarbon feedstocks, 2) the conversion of synthesis gas into methanol, 3) the production of
isobutene by the cracking, alkylation, isomerization and dehydrogenation of light hydrocarbons, and
4) the conversion of methanol and isobutene into MtBE (see Figure 1-1). The weak link in this
sequence of steps is the production of the isobutene, a presently inefficient reaction that requires a
chromium-based catalyst that must be regenerated frequently (approximately every half hour).
Chromium is a heavy metal that is known to be toxic. Substantial advances in pollution prevention
could be achieved by the discovery of a clean and efficient catalytic technology for the production
of isobutene. The annual productions of MtBE and isobutene were 17.62 and 3.23 billion Ibs in
1995[1].
Our research group has recently invented a new class of dehydrogenation catalysts that is capable
of achieving these pollution prevention goals in our laboratory. This new catalyst, the UW
Dehydrogenation Catalyst, is highly selective and extremely active for the dehydrogenation of
isobutane to isobutene, with the formation of virtually no pollutant by-products, such as methane,
ethane, propane, n-butane and carbon on the catalyst. Specifically, the catalyst achieved a greater
than 99% selectivity to isobutene at an average conversion of 55% over a 10 day period. This is
better than that achieved by the commercial catalyst used in UOP's Oleflex process - 92%
selectivity and 50% conversion[2]. Furthermore, this non-chromium catalyst requires only
infrequent regeneration. A patent will be issued for this new catalytic technology through the
Wisconsin Alumni Research Foundation in the spring of 1998.
The favorable catalytic properties of the UW Dehydrogenation Catalyst are a result of a unique
combination of metals, modifier, and support. Each of the four components of the Pt/Sn/K-L-
zeolite dehydrogenation catalyst is important in allowing this system to be highly selective and
stable for isobutane dehydrogenation. Platinum (Pt) catalyzes the removal of hydrogen from
isobutane to produce isobutene. The addition of tin (Sn) reduces the size of the surface platinum
ensembles and suppresses isomerization and hydrogenolysis reactions. The presence of potassium
(K) neutralizes the support, which inhibits the acid-catalyzed isomerization and coking reactions;
moreover, potassium suppresses the competing isomerization and hydrogenolysis reactions, and
enhances the dehydrogenation rate. The L-zeolite micropore structure supports small Pt/Sn
particles and perhaps also stabilizes adsorbed isobutyl species.
While it is known that each of the four components of the UW Dehydrogenation Catalyst are
necessary, it is not known how the various catalyst preparation and treatment parameters influence
the reactivity of the Pl/Sn/K ensembles in the L-zeolite. For example, it is known that the addition
of both tin and potassium are necessary to produce a selective and active dehydrogenation catalyst,
but the optimum amounts of these components have not yet been established. In addition, different
process conditions may require different catalyst formulations. While the UW Dehydrogenation
Catalyst exhibits good stability under reaction conditions, it still deactivates. It is necessary,
therefore, to also investigate the mechanism for deactivation.
The objectives of this project were to better understand the effect of different catalyst preparation
parameters, the effect of different catalyst treatment parameters, and the mechanism of deactivation.
Accordingly, catalysts were made using various preparation methods and with a range of Pt and Sn
contents. Catalysts were also tested with a variety of reaction conditions. The parameters by which
the performance of the catalyst was measured were isobutane activity, dehydrogenation selectivity
and stability. Chemisorption measurements and transmission electron microscopy (TEM) analysis,
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as well as kinetic studies, were used to characterize the catalysts. Temperature programmed
oxidation (TPO) was used to determine the extent of carbon deposition on the catalysts after
reaction.
n-Butane
I
Isomerization
3
Isobutane
Dehydrogenation
iC4H10->IC4H8 + H2
I
Methanol Isobutene
1
MTBE Process
Hydrogen
Figure 1-1. The production of MtBE from methanol and isobutene.
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Chapter 2
Conclusions
The metal particles of the Pt/Sn/K-L UW Dehydrogenation Catalyst are contained within the pores
of the L-zeolite. This catalyst is very selective for isobutane dehydrogenation because the Sn
reduces the size of the Pt ensembles, thus hindering hydrogenolysis, isomerization and coking
reactions. Potassium also reduces the size of the Pt ensembles and neutralizes the support,
inhibiting the acid catalyzed isomerization and coking reactions.
In preparation of the catalyst, Sn must be introduced prior to Pt and no acidity should be introduced
by the metal precursors. A Pt/Sn ratio of 1:2.5 and a Pt content of 0.5 wt% were found to be
optimal in terms of activity, selectivity and stability. Pt/Sn ratios lower than 1:2.5 (i.e. the addition
of more Sn) had better stability but did not significantly improve the selectivity and did decrease the
activity. With less Sn the catalyst is not very selective.
In the pretreatment of the catalyst, a slow heating rate of less than 2 K/min should be used to obtain
higher activities. Calcination of the catalyst during pretreatment results in significant particle
sintering.
A feed of isobutane only (i.e. in the absence of hydrogen) can be used for the reaction at 798 K but
the stability of the catalyst is related to its selectivity. For example, the unmodified Pt/L catalyst is
not very selective and its activity decreases to essentially zero within minutes on stream in isobutane
only. Conversely, the highly selective 1:2.5 Pt/Sn/L catalyst maintains its activity in isobutane for at
least 20 h on stream.
At 873 K and in the presence of hydrogen, the Pt/Sn/K-L catalysts deactivate by coking, enrichment
of the surface with tin, and/or sintering of the metal particles. At 798 K and in the absence of
hydrogen, the major mode of deactivation for the Pt/Sn/K-L catalyst is coking. The addition of Sn
and K to supported Pt catalysts improves the resistance of these catalysts to coking. Both additives
decrease the size of the Pt surface ensembles, thereby suppressing the hydrogenolysis,
isomerization and coking reactions. The presence of Sn may also help to transport coke
precursors from the active sites to the support.
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Chapter 3
Recommendations
Based on the deactivation studies, the UW catalyst is most suited for use at temperatures of 798 K
and below. Low temperature applications for this catalyst may include the dehydrogenation of
higher paraffins. This application would be limited by the size and shape of paraffins that can fit
into the pores of the Pt/Sn/K-L-zeolite catalyst. Another application, in which Sn would play an
active role in the chemistry as opposed to having only a geometric effect (i.e. reducing the size of
the Pt ensembles), is the hydrogenation of saturated aldehydes to saturated alcohols.
The role of potassium should be further investigated. It may be possible to take advantage of
potassium to reduce the carbon deposition on a working catalyst. With less carbon deposition the
catalyst will have to be regenerated less frequently.
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Chapter 4
Experimental Procedures
Catalyst Preparation
The standard procedure for preparing a Pt/Sn/K-L-zeolite catalyst is as follows. The K-L-zeolite
(Tosoh) was calcined in dry air at 873 K for 20 h, dried in flowing He at 673 K for 2 h, and
transferred to a dry nitrogen atmosphere. Tin was then added to the L-zeolite using incipient wetting
impregnation with a solution of tributyltin acetate (Aldrich) in methanol. The catalyst was then
dried in air at 390 K for 2 h and treated at 573 K in a flowing mixture of 25 mol-% oxygen in
helium for 1 h. The Sn/L-zeolite was then transferred to a dry nitrogen atmosphere and the
platinum added using an aqueous solution of tetraammineplatinum nitrate (Aldrich). After
treatment at 533 K in a flowing mixture of 25 mol-% oxygen in helium for 1 h, the Pt/Sn/L catalyst
was reduced in hydrogen at 873 K for 6 h (temperature ramp of 2 K/min), transferred to a glass vial
in a dry nitrogen atmosphere and stored for further use. The concentrations of the Sn and Pt
solutions were adjusted to obtain the desired weight percents of each metal. The absolute amount
of Pt was varied between 0.1 and 0.5 wt%, while the Pt/Sn ratio was varied between 1:0 and 1:14.
Variations in this procedure have been used to determine the effect of certain preparation variables.
Each of the following variations was done independently: 1. the Sn precursor was changed from
tributyltin acetate in methanol to an aqueous solution of stannic chloride, and 2. the order of
impregnation of the metals was reversed (i.e. Pt before Sn).
To determine the effect of the support on the deactivation of the catalyst, silica-supported platinum
catalysts were prepared by ion-exchange, using the method of Benesi et al. [3]. After ion-exchange,
the Pt/silica catalysts were filtered, washed with deionized water and dried overnight in air at 390 K.
A portion of the catalyst was treated in flowing oxygen at 573 K for 2 h, followed by reduction in
flowing hydrogen at 673 K for 2 h. Tin was added to the remaining uncalcined Pt/silica catalyst by
evaporative impregnation of a tributyltinacetate/pentane solution, after which the catalyst was dried
overnight in air at 390 K. Potassium was added to some of the Pt/Sn/silica catalyst by incipient
wetness impregnation with an aqueous solution of KOH, and the catalyst was then dried in air at
390 K. The Pt/Sn/silica and Pt/Sn/K/silica catalysts were calcined with flowing oxygen at 673 K
for 2 h and subsequently reduced in flowing hydrogen at 773 K for 7 h.
Kinetics/TPO apparatus
The kinetics and TPO apparatus is constructed of stainless-steel, with an attached Gas
Chromatograph (GC) and Residual Gas Analyzer (RGA). Figure 4-1 is a schematic of the kinetic
apparatus. The gas out can be flowed to the RGA or directly vented if the mass spectrometer is not
used. The inlet gases are flowed through mass flow controllers (Hastings). Isobutane (Liquid
Carbonic, 99.5%) is passed over a bed of reduced nickel on alumina at 333 K to remove sulfur
impurities. Hydrogen (Liquid Carbonic) is flowed through a Deoxo unit (Engelhard) and a bed of
molecular sieves (13X) at 77 K. Helium (Liquid Carbonic) is flowed through Cu turnings at 423 K
and a bed of molecular sieves (13X) at 77 K. The reactor inlet and outlet gases are analyzed by an
HP-5890 GC with a TCD detector and a 30 ft, 23% AT Chromosorb PAW column at 333 K. The
RGA is a Quad 25 OB Residual Gas Analyzer with an electron multiplier detector. The RGA is
usually operated at pressures of 10~3 Pa with an electron energy of 70 volts, and an emission current
of 150 [lamps. Calibration of the Mass 44 signal is done using a gas mixture of 5% CO, in He
(AGA).
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Figure 4-2 is a schematic of the reactors used. Both reactors are made from quartz. The U-tube
has a lower surface area and thus, minimizes gas phase reactions. The fritted reactor has a larger
catalyst capacity.
Gas Out
He
DeOx
1' S k
(liq.N2)
Reaction
Gas Out
Furnace
iC4
Ni/AI2O3
OxyTrap
Figure 4-1. Schematic of kinetics apparatus.
a.
Figure 4-2. Schematic of reactors: a. U-tube, b. fritted reactor.
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Chemisorption apparatus
The vacuum apparatus used for chemisorption measurements is constructed from glass, with a
stainless steel dosing section and a Baratron capacitance manometer. Carbon monoxide (Liquid
Carbonic) is purified by passage through a heated quartz wool trap at 573 K to remove metal
carbonyl compounds, and then dried by passage through molecular sieves at 77 K. Isobutane
(Liquid Carbonic) is purified with three freeze/pump/thaw cycles immediately before starting
adsorption measurements. The gas uptake amounts ((imol/g) are calculated by extrapolating the
plateau of the isotherm to zero pressure.
Transmission Electron Microscopy
Transmission electron microscopic studies were conducted using a high resolution microscope
(Phillips CM 200 equipped with a LaB6 filament) at a point resolution of 0.2 nm and a
magnification range of 25x to l,600,000x. A spot size of 120 nm was used to minimize damage to
the specimens. Samples were prepared by grinding the catalyst in a mortar and pestle with filtered
water (18 Mohm deionised water). A drop of the solution was then placed on specially prepared
copper grids (1000 mesh, Ted Pella Inc.). Before adding the drop of solution, the grids were
dipped in an adhesive solution made by dissolving the adhesive from double sided tape in naphtha.
The sticky grid with the drop of catalyst solution was then placed over a hot plate to evaporate the
water. The resulting grid contains many particles which overhang the edge of the grid bars and are
transparent to the electron beam.
Description of Measurement Parameters
The following list is a definition of the terms used to describe the experimental conditions and
results.
Pt-Sn ratio- Wp' * 118-69
Pt^n ratio. ^^
Weight hourly space velocity (WHSV):
fT' * v
r xiC.Ht
I F" * r \
I r -*ic H
Isobutane conversion: 1-7-^ijs- *100%
K + kGCiCH
-------
where:
F inlet flow rate (seem)
Fฐ outlet flow rate (seem)
GC^mo GC area of isobutane peak
k,K conversion factor for the GC
M mass of catalyst (g)
msites moles of sites per gram of catalyst (mol/g) (as defined by CO adsorption)
nt number of carbon atoms in the product compound
wn mass of Pt per gram of catalyst (g/g)
wSn mass of Sn per gram of catalyst (g/g)
Xj mole fraction of products (methane, ethane, propane, propene, isobutene, butenes)
xiC4Hซ mฐle fraction of isobutene
x;c4Hio mole fraction of isobutane
Quality Assurance
Quality assurance (QA) objectives have been outlined for this project in order to ensure that
comparisons can be made between kinetic runs. These quality assurance objectives have been met
and thus, there exists a high degree of confidence in the conclusions drawn from this study.
Calibrations of the equipment (e.g. mass flow controllers, GC and Mettler balance) have been done
on a regular basis. Figures 4-3 and 4-4 show representative GC calibrations for the C1-C4
hydrocarbons.
As an additional verification of the performance of the equipment, carbon balances are calculated
and checked for each kinetic run. In all cases, the carbon balance was 100+-2.5%. The carbon
balance is given by
_, , , , Moles of Carbon to Reactor ^,__,
Carbon balance = * 100%
Moles of Carbon out of Reactor
Duplicate kinetic runs have been done. Figure 4-5 is representative of the results from duplicate
kinetic runs.
This has been an exploratory project designed to investigate the effects of a number of process and
catalyst preparation variables on the performance of the UW Dehydrogenation Catalyst system. As
such, there should be no legal or regulatory ramifications if the QA objectives were not met.
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.016
0014
f~
g 0.012
& 0.01
^0.00*
3 0.006
0.004
0.002
ฐ I
y
-y
/
\//'-
iiii
) 5
n nnndists j_ 1 QnROo riPv R
v.vvv'tlooo + I .yUD n .0
^ . -ฐ .---'''
_ *
.-*'
,ป
i i , , , i i , , , i .
105 1106 1.5 106
GC Area
Ooonfi
0.99998:
OQQQdl-
. yyyij .
1
i
-j
-
2 1
O6
Figure 4-3. GC calibration curves for , methane, Q, ethane, 0, propane and , n-butane.
0.4
0.35
0.3
c
S 0.25
| 0.2
I 0.15
0.1
0.05
0
y = -0.0117 + 6.279e-09x R= 0.9966
= -0.002368 + 6.603e-09x R= 1
0 1 10' 2 10' 3 10' 4 10' 5 10' 6 10
GC Area
Figure 4-4. GC calibration curves for , isobutane and G, isobutene.
-------
20
c
o
-------
Chapter 5
Results and Discussion
Catalyst Preparation
It has been suggested that the favorable properties of Pt/Sn/L catalysts stem partially from the
interaction between the metals and the pores of the zeolite[4, 5]. Thus, one of the first tasks on this
project was to determine if indeed the metals were within the pores of the zeolite. The TEM was
used to examine a 1:2.5 Pt/Sn/L catalyst. Very few particles were visible on the external surface of
the catalyst. It is then assumed that because the catalyst is active but particles are not visible, that the
metals must be within the pores. Subsequently, each catalyst was examined in the TEM following
preparation as a quality control check. If significant numbers of particles were visible then the
catalyst would be rejected and a fresh sample prepared.
The activity profiles (turnover frequencies (TOP) versus time plots) are obtained using
approximately 5 mg of catalyst in a U-tube reactor with an isobutane conversion of less than 15%
of the equilibrium value for the dehydrogenation reaction. The TOF is on a total Pt basis unless
otherwise stated. Conversion and selectivity profiles are obtained using between 0.2 and 1 g of
catalyst in a U-tube or fritted reactor with an initial isobutane conversion close to or at the
equilibrium value for the dehydrogenation reaction.
The effect of tin precursor and impregnation sequence is shown in Figures 5- 1 and 5-2. The
WHSV was 13.2 h'1 for these experiments and the catalysts contained 0.5 wt% Pt. When the
catalyst was prepared using a stannic chloride precursor rather than tributyltinacetate, both the
conversion and selectivity decrease (see Figures 5-1 and 5-2). The chlorine from the SnCl4
solution introduces acidity to the catalyst, which promotes the isomerization and hydrogenolysis
reactions, thus lowering the selectivity of the catalyst. One of the features of K-L-zeolite is that it is
a neutral support.
The effect of impregnation sequence is also shown in Figures 5-1 and 5-2. When Sn is
impregnated first, the activity of the catalyst is significantly higher than when Pt is impregnated
first. The latter catalyst has better selectivity for the first 5 hours on stream, after which time the
selectivities are similar. It is likely that when Sn is impregnated second, it blocks a large fraction of
the Pt sites and the Pt that is exposed is in very small ensembles. Small Pt ensembles are desirable
because the dehydrogenation reaction is structure insensitive while the undesired side reactions,
isomerization and hydrogenolysis, are structure sensitive. Hydrogenolysis, isomerization and
coking reactions require several adjacent Pt surface sites[6, 7]. Thus, the dehydrogenation reaction
will be favored on small Pt ensembles.
The effects of Pt and Sn content on activity were studied by varying both the absolute amount of Pt
and the PttSn ratio. The results are shown in Figures 5-3 to 5-6. At low PtSn ratios the activity
decreases significantly. The catalyst activity is stable, although low, with a sufficient amount of Sn.
A Pt content of 0.5 wt% appears to be optimal for the catalyst in terms of activity.
11
-------
0s-
o
c
100
80
60
3D OO O
8
o
n an
0
10 15
Time (h)
20
25
Figure 5-1. Effects of preparation methods of 1:2 Pt/Sn/L zeolite on isobutane conversion at 873
K, 1 atm, and a H2/i-C4H10 ratio of 2; sequential impregnation of: 0, Sn then Pt; , SnQ4
then Pt; and Q, Pt then Sn.
Q Q
nnn
10 15
Time (h)
20
25
Figure 5-2. Effects of preparation methods of 1:2 Pt/Sn/L-zeolite on isobutane dehydrogenation
selectivity at 873 K, 1 atm, and a H2/i-C4H10 ratio of 2; sequential impregnation of: 0, Sn
then Pt; , SnCl4 then Pt; and Q, Pt then Sn.
12
-------
20
0>
O
CD
Q
0
n n
, I . .
0
10 15
Time (h)
20
25
Figure 5-3. Effects of platinum content on isobutane dehydrogenation reaction rates at 873 K, 1
atm, and a H2/i-C4H10 ratio of 2 over Pt/Sn/L-zeolite (0.83 wt% Sn); ,0.5 wt% Pt (1:2.7
Pt/Sn); and Cl, 0.1 wt% Pt (1:14 Pt/Sn).
LL
O
C
O
ง>:
CD
Q
20
15
0
Time (h)
20
25
Figure 5-4. Effects of platinum content on isobutane dehydrogenation reaction rates at 873 K, 1
atm, and a H/i-C^ ratio of 2 over Pt/Sn/L-zeolite (0.42 wt% Sn); ,0.5 wt% Pt (1: 1.4 Pt/Sn);
and Cl, 0.1 wt% Pt (1:7 Pt/Sn).
13
-------
03
O)
o
0)
Q
20
15
10
0
0
10 15
Time (h)
Figure 5-5. Effects of tin content on isobutane dehydrogenation reaction rates at 873 K, 1 atm,
and a H/i-C4H10 ratio of 2 over Pt/Sn/L-zeolite (0.1 wt% Pt); O, 0.15 wt% Sn (1:2.5 Pt/Sn); ,
0.42 wt% Sn (1:7 Pt/Sn); and Q, 0.83 wt% Sn (1:14 Pt/Sn).
20
O
O)
-------
Based on the results of the previous studies with various Pt and Sn contents, three catalysts were
prepared and studied in more detail (see also the results in the Catalyst Deactivation section). The
catalysts all contain 0.5 wt% Pt and Pt/Sn ratios of 1:0 (Pt/L), 1:1.4 (1:1.4 Pt/Sn/L) and 1:2.4
(1:2.4 Pt/Sn/L). The activities of these catalysts are shown in Figure 5-7. Note that the TOF is on
a per site basis, where the number of sites was determined by CO chemisorption. Without Sn the
Pt/L catalyst is much less active for the production of isobutene.
The selectivities of these catalysts are compared in Figure 5-8 and the corresponding conversions in
Figure 5-9. Although the isobutane conversions are similar over all three catalysts, the selectivities
are different. Note that the reactor configuration used for these experiments was designed for the
TPO experiments and significant gas phase reactions may have occurred. Therefore, the absolute
selectivities are lower than if the experiments had been designed to minimize gas phase reactions (as
in Figure 5-2). It is evident that a certain amount of Sn is required to achieve a reasonable
selectivity as the unpromoted Pt catalyst is very unselective to isobutene formation and instead,
methane, ethane, propane and coke are formed.
100
O
60
40
a.
"O ,_ -
t ^
Q
0 0.5 1 1.5 2 2.5
Time (hr)
3.5 4
Figure 5-7. Comparison of the isobutane dehydrogenation rates at 873 K, 1 atm, and a
ratio of 2 over A, Pt/L-zeolite, ,1:1.4 Pt/Sn/L-zeolite and ,1:2.4 Pt/Sn/L-zeolite.
15
-------
100
c
05
JD
O
E 20 I
0
c
0 0.5 1
.4....V..A ..
1.5 2 2.5 3
Time (h)
3.5 4
Figure 5-8. Comparison of the isobutene selectivity at 873 K, 1 atm, and a H2/i-C4H10 ratio of 2
over A, Pt/L-zeolite, , 1:1.4Pt/Sn/L-zeoliteand ,1:2.4Pt/Sn/L-zeolite.
c
O
'w
c'
CO
100
80
60
40
20
(}
0
0 0.5 1 1.5 2 2.5 3 3.5 4
Time (h)
Figure 5-9. Comparison of the isobutane conversion at 873 K, 1 atm, and a
over A, Pt/L-zeolite, , 1 : 1 .4 Pt/Sn/L-zeolite and , 1:2.4 Pt/Sn/L-zeolite.
ratio of 2
16
-------
Catalyst Treatment
The properties of the catalyst are affected by both the pretreatment (i.e. the reactivation during
which the catalyst is taken from being exposed to air at room temperature to being fully reduced at
the temperature of reaction) and the reaction conditions. It has been established that supported
metal catalysts must be heated slowly to avoid sintering the metals and to avoid damage to the
support by the evaporating absorbed water (from exposure to the air) [8-10]. A study was thus
undertaken to determine how best to pretreat or reactivate the Pt/Sn/L catalysts.
The following pretreatments were used with a 1:2.5 Pt/Sn/L catalyst (0.5 wt% Pt):
1. The catalyst was heated in a 10% H2 in He flow from room temperature (RT) to 400 K over 30
min, held for 1 h at 400 K, heated to 873 K over 2 h and then held at 873 K for 1 h in H2 (4 h 30
min pretreatment).
2. The catalyst was heated in a 10% Hj in He flow from room temperature (RT) to 873 K at 2
K/min and then held at 873 K for Ih in Hj (6 h pretreatment).
3. The catalyst was heated in He to 400 K over 1 h, held at 400 K for 2 h, heated to 533 K over 1 h
10 min, held at 533 K for 1 h, cooled to RT, and then reduced by heating from RT to 873 K over 16
h in a 10% H2 in He flow, held at 873 K for 1 h in H2 (22 h pretreatment).
4. The catalyst was heated in 25% 0, in He to 533 K over 2 h, held for 1 h at 533 K, cooled in He
to RT, and then reduced by heating from RT to 873 K over 16 h in a 10% H2 in He flow and held at
873 K for 1 h in H2 (20 h pretreatment).
The kinetic measurements were performed with approximately 5 mg of catalyst in a quartz U-tube
reactor. After each pretreatment, the samples were exposed to a 2: 1 ratio of F^ to isobutane at 873
K and the activity measured. For TEM analysis, approximately 0.04 g of catalyst was placed in a
fritted reactor. Instead of being exposed to reaction conditions after the pretreatment, the samples
for the TEM were cooled to RT in He and passivated with a flowing stream of 0.2% 0, in He for
30 min.
The 1:2.5 Pt/Sn/L catalyst is essentially free of particles after preparation. After the pretreatments,
however, particles are visible on the catalyst. The number and size of particles depends on the
pretreatment. The pretreatment involving calcination before reduction, treatment #4, resulted in the
greatest amount of particle migration and agglomeration with 80% of the catalyst having evidence of
particles, while the drying at 533 K followed by a 16 h reduction (treatment #3) resulted in the least.
In the latter case, metal particles were only visible on approximately 10-20% of the catalyst. A few
more particles were visible on the 6 h pretreated (treatment #2) sample and considerably more
particles were visible on the 4 h 30 min pretreated (treatment #1) sample, with 50% of the catalyst
having evidence of particles.
The activities of the catalyst after the pretreatments are compared in Figure 5- 10. Included in this
comparison, is the activity of the catalyst immediately after preparation (i.e. after the 6 h reduction at
873 K, the reaction was started without cooling to RT and without exposing the catalyst to air). The
catalysts that have been passivated and reactivated are significantly less active than the catalyst that
had not been exposed to air. It has also been observed that if the Pt/Sn catalysts are exposed to air
for extended periods after the Sn or Pt impregnation during preparation, significant particle
agglomeration occurs. This agglomeration does not happen on a Pt catalyst without Sn, which
suggests the oxidized Sn species may be the cause for the metal agglomeration.
17
-------
The activity of the catalyst pretreated for 4 h 30 min (treatment #1) is lower than that of the other
pretreatments. In this pretreatment, the catalyst is heated at 4K/min from 400 K to 873 K. A
decrease to 2K/min in the 6 h pretreatment appears to improve the activity of the catalyst.
Decreasing the heating rate to 0.6K/min and preceding the reduction by drying at 533 K (22 h
pretreatment) does not seem to significantly improve the activity, even though fewer particles were
visible on the catalyst. Also, the activity of the calcined sample (20 h pretreatment) is higher than
that of the catalyst pretreated for 4 h 30 min, even though significantly more agglomeration
occurred on the former catalyst. The TEM analysis may be somewhat misleading because the
particles that were seen were probably only a small fraction of the total number of particles in the
catalysts.
The other pretreatment variable that has been studied is the time for which the catalyst is reduced
immediately before reaction. A 1:2.7 Pt/Sn/L and a 1:1.4 Pt/Sn/L catalyst were reduced for 1 or 16
h and their dehydrogenation activities compared (see Figures 5-1 1 and 5-12). A significant loss in
activity occurs when either catalyst is reduced for 16 h compared with 1 h. This loss of activity
could be due to sintering of the metal particles or enrichment of the Pt particles with Sn. A 1:2.5
Pt/Sn/L catalyst was held at 873 K for 24 h in He and then reacted with 651 Torr He and 109 Torr
isobutane (see Figure 5-13). In He, the reduction of the Sn should be significantly reduced. Again,
a decrease in activity occurs after the high temperature soak. This suggests that the temperature is
causing some sintering of the metals. TEM examination of a Pt/Sn/L catalyst after a 16 h soak at
873 K in H,, confirms that particle agglomeration does occur.
Finally, the reaction conditions have been varied. Figure 5- 14 is a comparison of the activity of a
1:2.5 Pt/Sn/L catalyst at 798 K in a pure isobutane feed, a 2: 1 hydrogen/isobutane feed and a 2: 1
helium/ isobutane feed. The activity is lower in the presence of hydrogen because the reaction is
negative order in hydrogen (i.e. the hydrogen inhibits the production of isobutene). The catalyst,
however, is more stable in the presence of hydrogen. In fact, a Pt catalyst without Sn has essentially
no activity when used in the absence of hydrogen (see Figure 5-15). A 1:1.4 Pt/Sn/L catalyst is
active for isobutane conversion for a few hours on stream only, compared to a 1:2.5 Pt/Sn/L catalyst
which maintains reasonable conversion for 20 h on stream (see Fig 5-16). Thus, the catalyst must
be highly selective to be used with an isobutane feed stream that does not contain hydrogen.
18
-------
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P
1
C
O
C
0
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o
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-o
JZ
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ou
25
20
\n 15
10
1 W
5
n
\j
ti i i i i i i
A^
AAA A
* * A -
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^^^^^^^^^^^V^r ^t ^& ^B ^m ^1
0AA__ ~ V ^^
-I-J- *^^ ^ A A
++-H-++ + + I T '
T T -f +
.
1 1 1 1 1 1 1
0 0.5 1 1.5 2 2.5 3 3.5 4
Time (h)
Figure 5-10. Comparison of the isobutane dehydrogenation rates at 873 K, 1 atm, and a H,/i-
C4H10 ratio of 2 over 1 :2.5 Pt/Sn/L-zeolite. Pretreatment: A, none, , 22 h, , 6 h, , 20 h, and +,
4 h30
U_
O
H
c
0
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a
o
o
>
-------
o
c
o
O)
0)
Q
20
15
0
0
o0
Time (h)
20
25
Figure 5-12. Effects of reduction time on isobutane dehydrogenation reaction rates at 873 K,
atm, and a H/i-C4H10 ratio of 2 over 1: 1.4 Pt/Sn/L-zeolite (0.5 wt% Pt, 0.42 wt% Sn); 0, 1 h
reduction and ,16 h reduction.
LL
O
C
o
0)'
o
(D
Q
15
0
1111
1 1 1, 1, , . . ,, 1 1 1, 1, . , ,
0.5
1.5
2.5
Time (h)
3.5
Figure 5-13. Effect of He soak on isobutane dehydrogenation reaction rates at 873 K, 1 atm, and
a 651 Torr He and 109 Torr i-C4H10 over 1:2.5 Pt/Sn/L-zeolite (0.5 wt% Pt, 0.83 wt% Sn); , 1 h
reduction, and O, 24 h He soak.
20
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ca
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0)
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'c/T
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^_ -
'co
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80
60
40
20
0 t
_ i i i ' ' i ' ' ' ' i ' ' ' ' i ' ' ' ' i ' ' ' ' .
L ~
?AAAA :
P AAAAA A A A A A :
-flJMH ^B _ T
ง5 i
"** ~
""
.
', ... 1 .... 1 .... 1 .... 1 , ... 1 .... 1 .... 1 ... I
i n n i i <; 9 71 3 ? 1 i
Time (h)
Figure 5-14. Comparison of the isobutane dehydrogenation rates at 798 K over 1:2.5 Pt/Sn/L-
zeolite with different feed streams. A i-C4H10 only, ,2:1 H2/i-C4H10, and ,2: 1 He /i-C4H10.
100
ง 80 -
'w
CD
> 60
o
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CD 40
20 -
0
J-
L
n
i i 1 i . i i ! . . i i 1 i .. 1 i i i i 1 i 1 i i ^
D n n n n n c
! ,1 ! ! . 1 k. 1 ! ill 1 1 .^ 1 1 . 1^1 1 . 1 lj 1 1 1 -A
0 0.5 1 1.5 2 2.5 3 3.5 4
Time (h)
Figure 5-15. Comparison of the isobutane conversion at 798 K and in i-C4H10 only over A, Pt/L-
zeolite, and Q, 1:2.5 Pt/Sn/L-zeolite.
21
-------
CcT
tf^
c
o
'eo
CD
c
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0
CD
^ซ
OS
"ZJ
.Q
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80
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1
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.
.
_
.
~
^
- "" "
^_ :
M . . *f i . . . . i . , , . I , . . i
) 5 10 15 2
Time (h)
Figure 5-16. Comparison of the isobutane conversion at 798 K and in i-C4H10 only over '
Pt/Sn/L-zeoliteandl ,l:2.5Pt/Sn/L-zeolite.
1,1:1.4
Catalyst Deactivation
It is evident that deactivation is occurring in the activity profiles of the Pt/Sn/L-zeolite catalysts (for
example, see Figures 5-10 and 5- 11). As previously mentioned, coking, metal sintering and Sn
enrichment of the Pt particles can all contribute to the deactivation of the catalyst. Accordingly, the
catalysts were analyzed with TPO and TEM after 4 h of reaction at two conditions - 873 K in a 2: 1
hydrogen to isobutane feed stream and 798 K in a pure isobutane feed stream. The former
condition is similar to commercial conditions, and the latter condition maximizes coking while
minimizing sintering.
For the experiments at 873 K, approximately 0.75 g of catalyst was loaded into a quartz sample cell
which could be transferred between the kinetics/TPO apparatus and the chemisorption apparatus
without exposing the catalyst to air. The catalyst was first reduced in Hj by heating overnight in a
10% H2 in He flow to 873 K, held at 873 K for 1 h in H,, and evacuated for 30 min at 673 K. The
CO uptake was then measured at 298 K, after which the catalyst was heated to 873 K over 2 h in Hj
and treated for 4 h under reaction conditions (Hj to iC4H10 ratio of 2 at 873 K). After purging the
catalyst with He at 673 K for 1 h to remove adsorbed Hj, the catalyst was evacuated at 298 K for 2
h and the CO uptake was measured. Finally, the catalyst was heated to 873 K at 10 K/min in a 2%
O2 in He flow to obtain the TPO spectrum. Any carbon that has accumulated on the catalyst during
reaction will be removed as CO, when the catalyst is heated in the presence of oxygen. During the
collection of the TPO spectrum, the exit gas stream from the catalyst was analyzed using a mass
spectrometer which monitored the mass 44 signal (mass 44 corresponds to the signal for CO2).
Calibration of the mass 44 signal was achieved using a gas mixture of 5% CC>2 in He (AGA).
22
-------
At the low space velocity of these experiments, the isobutane conversions were initially equal to
75%, 65% and 55% for the 1:1.4 Pt/Sn/K-L, Pt/K-L, and 1:2.4 Pt/Sn/K-L, respectively, and these
values decreased to -40% after 4 h on stream. The CO uptakes before and after reaction are given
in Table 5-1. An increase in the amount of Sn in the catalyst causes a decrease in the number of
sites on the fresh catalyst, as seen in other studies[ 1 1-13]. The CO uptake is low on the Pt/L
catalyst, in comparison with previous studies of this catalyst system[ 14]. Therefore, CO uptakes
were measured on a Pt/L catalyst that was reduced at 673,798 and 873 K for 1 h (temperature ramp
of 2K/min). The corresponding CO uptakes are listed in Table 5-2. Higher temperatures of
reduction lead to a decrease in the CO uptake, which accounts for the low uptake on the fresh Pt/L
catalyst. Similar results have been observed by Vaarkamp and co-workers[ 10]. After reaction, the
CO uptake decreases on all catalysts; however, the relative decrease in the number of sites is smaller
as the percentage of Sn in the catalyst increases.
Table 5-1: CO uptake on fresh catalysts and after reaction at 873 K in 2: 1 H^iC^H,,).
Catalyst CO Uptake (|imol/g)*
Fresh After Reaction
"1:2.4 Pt/Sn/L
1:1.4 Pt/Sn/L
Pt/L (II)
5.0
7.5
7.8
3.4
4.1
1.2
*error is +0.5 (imol/g
Table 5-2: CO uptake versus reduction temperature for Pt/L.
Reduction Temperature CO Uptake (|imol/g)
673 K 21
798 K 15
873 K 10
The TPO spectra of the Pt/L, 1:1.4 Pt/Sn/L and 1:2.5 Pt/Sn/L catalysts after 4 h of reaction in
isobutane and hydrogen at 873 K are shown in Figure 5- 17. Peaks occur at approximately 680 and
780 K for the Pt/L and 1: 1.4 Pt/Sn/L catalysts. The TPO spectrum of the 1:2.5 Pt/Sn/L catalyst has
a broad peak between 600 and 760 K. The amount of carbon on each catalyst is given in Table 5-3.
Increasing the amount of Sn in the catalyst decreases the amount of carbon deposited under these
reaction conditions.
Analyses of the catalysts by TEM indicate that sintering of the metal particles occurs after reaction
at 873 K. Before the reaction, the L-zeolite-supported samples have few particles visible on the
external surface of the zeolite, and these visible particles are 1-2 nm in size. The 1:1.4 Pt/Sn/L
catalyst looks similar to the 1:2.5 Pt/Sn/L catalyst before and after reaction. After reaction some
particles, up to 5 nm in size, are visible. However, even though more particles are visible after
reaction, the majority of the zeolite specimen (-70%) does not show visible particles. The Pt/L
catalyst is similar to the Pt/Sn/L catalysts before reaction (i.e. few particles are visible), but after
reaction particles are visible on a significant fraction of the zeolite (-50%). The average size of the
particles appears to be smaller (e.g., 1 nm) on the Pt/L catalyst after reaction compared to the
Pt/Sn/L sample.
23
-------
Table 5-3: CO uptakes and extents of carbon deposition on catalysts.
Catalyst
Bsnf:n'W-O't-873 . K in iC^il(JH2
1:1.4 Pt/Sn/L
Pt/L(H)
Reaction at 798 K in iCJilo
1:2.5 Pt/Sn/L
Pt/UD
l:lPt/Sn/silica
l:l:3Pt/Sn/K/silica
Pt/silica
CO Uptake*
(jimol/g)
5.0
7.5
7.8
6.1
15
16
10
18
Carbon
(p.mol C/g)
90
170
500
260
560
770
210
2400
Deposition+
(|j.mol C/(imol site)
18
22
64
43
45
21
132
* error is +_0.5 ^imol/g
'error is +20%
,0)
E
o
c
0
o
w
CM
o
o
Pt/L
1:1.4Pt/Sn/
1:2.5 Pt/Sn/
300 400 500 600 700
Temperature (K)
800
900
Figure 5-17. Comparison of the TPO spectra after 4 h reaction at 873 K, 1 atm, and
ratio of 2 for Pt/L-zeolite, 1:1.4 Pt/Sn/L-zeolite and 1:2.5 Pt/Sn/L-zeolite.
The uptake of isobutane was measured on the 1:2.5 Pt/Sn/L catalyst before and after reaction. The
adsorption of isobutane occurs primarily on the L-zeolite (the Pt/Sn/L sample contains a relatively
small number of surface Pt sites), and this uptake is thus a measure of the accessible interior
surface area of the catalyst. The change in uptake is negligible within the error of the experiment
(see Table 5-4). This results is consistent with the observation that only a small amount of carbon
is deposited on the catalyst during 4 hours of reaction.
24
-------
Table 5-4. Isobutane uptake on catalysts.
Catalyst Isobutane Uptake ((imol/g)*
_ Fresh _ After Reaction
L-zeolite 713
Reaction at 798 K in iCJiw
Pt/L(I) 603 528
Reaction at 873 K in iCfl^ H2
1:2.4 Pt/Sn/L _ 568 _ 545
* error is +15
Measurable catalytic activity for isobutane conversion in the absence of Hj is maintained by all
catalysts for reaction times of 4 h, except for the Pt/L- zeolite catalyst (see Figure 5- 18). The
isobutane conversion of the 1:2.5 Pt/Sn/L catalyst is lower than that of the Pt/Sn/silica and
Pt/Sn/K/silica catalysts because the reactor contained only 3 [imol of sites compared to 8 and 5
(imols of sites, respectively, for the silica-supported catalysts. The activity of the Pt/L catalyst
decreases to essentially zero within 30 minutes on stream. For all catalysts except Pt/Sn/K/silica,
the selectivity increases significantly between 1 and 30 minutes on stream, as shown in Figure 5-18.
This increase in selectivity may be caused by the formation of coke which decreases the size of the
Pt ensembles, thus suppressing the isomerization and hydrogenolysis reactions compared to the
dehydrogenation reaction.
Figure 5-19 shows TPO spectra for catalysts after 4 h reaction at 798 K in isobutane. The TPO
spectrum of the Pt/silica catalyst has peaks at 7 10 and 790 K, while the 1: 1 Pt/Sn/silica catalyst
displays broad peaks at 690,760 and 820 K. The spectra of Pt/L and 1:2.5 Pt/Sn/L have peaks
near 680 and 780 K, while the 1: 1:3 Pt/Sn/K/silica has a peak around 680 K. The peak at 360 K in
the spectrum of the 1:2.5 Pt/Sn/L catalyst is from weakly bound carbon on the surface, that can be
removed by purging the catalyst with He at 673 K for 1 h. Comparing Pt/silica with Pt/Sn/silica,
and comparing Pt/L with Pl/Sn/L, it appears that Sn suppresses the extent of coke formation and it
appears to alter the relative areas of the high and low temperature peaks, (i.e., decrease the ratio of
the 680 peak to the 780 K peak). The addition of K to the Pt/Sn/silica catalyst significantly
decreases the amount of carbon deposited on the catalyst.
25
-------
c
a
>
c
o
O
CD
I
100
80
60
40
20
- 0
V
o
a
0.5
v
a
^
a
u
1.5 2 2.5 3
Time on Stream (h)
3.5
A A : x 11 % i j
' VI..M..;..!
j.... .^,,, i j-.... +... .^ .. .^ . ปj ~
100
90 ฎ
<
80 I
(Q
CD
70 |
6'
60 =
CD
50
40
30
Figure 5-18. Isobutane conversions (filled symbols) and dehydrogenation selectivities (open
symbols) over 1:2.5 Pt/Sn/K-L (circles), Pt/K-L (squares), 1:1 Pt/Sn/silica (diamonds), 1:1:3
Pt/Sn/K/silica (triangles), and Pt/silica (inverted triangles) at 798 K in isobutane.
CD
.4ป
'SP
m
C
0
LU
0.2
0.15
0.1
0.05
CM
O
o
Pt/L
1:2.5 Pt/Sn/L
1:1 PVSn/Silica
. . . .1;1,:3 Pt/Sn/K/SiNca,
300 400 500 600 700 800 900
Temperature (K)
Figure 5-19. Comparison of the TPO spectra after 4 h reaction at 798 K, 1 atm, and in isobutane
only for L-zeolite- and silica-supported catalysts.
26
-------
The total carbon accumulated on the catalysts was determined from the area under the TPO spectra,
where the baseline for integration was taken to be the CO, evolution at 300 K. Table 5-3 lists the
amounts of carbon deposited on the basis of catalyst mass and also based on the number of sites
determined by CO chemisorption on the fresh catalysts. The order of decreasing amount of carbon
per site is Pt/silica ป1:1 Pt/Sn/silica > 1:2.5 Pt/Sn/L > Pt/L >1:1:3 Pt/Sn/K/silica.
To obtain deactivation profiles corresponding to the above reaction conditions, approximately 0.005
g of catalyst was loaded into a quartz U-tube reactor with quartz wool. The deactivation profiles for
the 1:2.5 Pt/Sn/L and Pt/L catalysts are compared in Figure 5-20, at conditions for which the
isobutane conversion was less than 10% of the equilibrium value. The Pt/L catalyst deactivates
rapidly, whereas the Pt/Sn/L catalyst has a high initial activity, deactivates over the first 30 minutes
on stream, and then continues to deactivate at a slower rate. The deactivation behavior of the
Pt/Sn/L catalyst is typical for many catalysts[ 15-17].
Analyses of the catalysts by TEM before and after reaction in isobutane at 798 K indicate that little,
if any, particle agglomeration occurs during the reaction. Essentially no particles are visible on the
L-zeolite-supported catalysts after reaction at 798 K. The silica-supported catalysts have particles
visible with diameters from 2-5 nm on the support before and after reaction. Isobutane uptake was
measured on the Pt/L catalyst before and after reaction (see Table 5-4). After reaction, the uptake
decreases by 75 |imol/g. This result indicates that carbon deposition had occurred within the pores
of the zeolite, and this pore blocking may account for the rapid decline with time in catalyst activity.
Regeneration of the Pt/Sn/L catalyst has been attempted. Figure 5-21 shows the activity of a 1:2.5
Pt/Sn/L catalyst at 873 K in 65 1 Torr He and 109 Torr isobutane. After 23 hours on stream, the
catalyst was cooled to 573 K and held at this temperature for 2 hours in a flowing stream of 25%
O2 in He. The catalyst was then purged with He for 30 minutes, heated to 873 K at 8 K/min in 5%
H2 in He, held for 2h in H2 at 873 K and exposed to reaction conditions. Only a fraction of the
initial activity was recovered.
A different regeneration scheme was successful in reestablishing the performance of a 1:2.7
Pt/Sn/L catalyst, provided that the catalyst was reduced for 16 h prior to the initial reaction and after
each regeneration. Figure 5-22 shows isobutane dehydrogenation reaction rates over fresh 1:2.7
Pt/Sn/K-L-zeolite (0.5 wt% Pt) at 873 K, 1 atmosphere, and a Hj iC4H]0 molecular ratio of 2.0.
This catalyst was pretreated with H2 at 873 K for 16 h before reaction and was allowed to deactivate
over 80 h at reaction conditions. The catalyst was then regenerated with a 10% 0, in He mixture
for 2 h at 533 K, cooled to room temperature, reduced with a 10% H2 in He mixture by increasing
the temperature from room temperature to 873 K over 8 h, and then further reduced in pure H^ at
873 K. After this regeneration, Figure 5-22 shows that the catalyst exhibited essentially the same
dehydrogenation activity and stability. After running for 50 h, the catalyst was regenerated using
the same procedure for a second time. After the second regeneration, the catalyst exhibited slightly
lower dehydrogenation reaction rates compared to the fresh catalyst. The catalyst was allowed to
deactivate for 24 h after this second regeneration and then was regenerated a third time, The third
regeneration followed a procedure similar to the first two regenerations, but the catalyst was
regenerated with 10% 0, in He mixture for 2 h at 573 K instead of 533 K. The catalyst was then
exposed to isobutane at 873 K in the absence of hydrogen. The catalyst was regenerated a fourths
time using the same procedure as the third regeneration. After this fourth regeneration, the catalyst
exhibited slightly lower dehydrogenation activity compared to the fresh catalyst. However, there
was little change in the performance of the catalyst between the second and fourth regenerations.
27
-------
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60
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_
"
tmmm ฑmim*t i i i ซl i i i id i i i i W ' i ' 4' ' ' ' 4' ' ' ' 1
05 1
Time (h)
35
Figure 5 - 20. Isobutene turnover frequencies at 798 K in isobutane over O, 1:2.5 Pt/Sn/K-L, and
, Pt/K-L.
10 15
Time (h)
o
20
25
Figure 5-21. The activity of 1:2.5 Pt/Sn/L-zeolite at 873 K, 651 Torr He and 109 Torr iC4H10
before and after regeneration in 25% 0, in He at 573 K for 120 minutes; , fresh catalyst and 0,
regenerated catalyst.
28
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1 1
LL.
O
c
o
1-5
0 C-
o
o
.c
CD
Q
8
7
6
5
4
3
2
1
0
-I1IIIIII[-
0
20
40 60
Time (h)
80
100
Figure 5 - 22. Effects of regeneration on isobutane dehydrogenation reaction rates at 873 K, 1 atm,
and a H^/i-C^ ratio of 2 over 1:2.7 Pt/Sn/L-zeolite (0.5 wt% Pt, 0.83 wt% Sn); , fresh catalyst;
0, regenerated catalyst (1st Regeneration); A, regenerated catalyst (2nd Regeneration); and 0,
regenerated catalyst (4th Regeneration).
Summary of Preferred Conditions
The following table lists the preferred preparation and pretreatment conditions for the
dehydrogenation of isobutane using the UW Dehydrogenation Catalyst.
Table 5-5: Preferred preparation and treatment conditions for the catalyst.
Variable Preferred Value
PtSn Ratio
Pt Content
Sn Precursor
Impregnation Order
Pretreatment
1:2.5
0.5 wt%
Tributyl tin acetate
Sn first, Pt second
Heating rate < 2K/min
No calcination
1 h reduction in EL, at rxn temp
Discussion
The goal of this project was to learn more about why the UW Dehydrogenation catalyst is such a
good catalyst for isobutane dehydrogenation and how the preparation and pretreatment parameters
could be chosen to achieve maximum performance from the catalyst. It is known that small Pt
29
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ensembles are essential to achieve high activity, selectivity and stability[ 11]. Our results indicate
some of the factors which affect whether the catalyst will have and/or maintain small Pt ensembles.
The first task in this project was to determine the location of the metal particles on the zeolite
support. The TEM analysis indicated that the particles were within the pores of the zeolite after
preparation. The TEM results support other work which showed that Pt/Sn/L catalysts were 50
times more active than Pt/Sn/silica catalystsf 18]. One of the differences between silica and L-
zeolite is their pore structure. The silica used in the study was non-porous. L-zeolite, however, has
a unidimensional pore structure. It has been suggested that when the reaction occurs within the
pores of the zeolite, favorable interactions between reaction intermediates and the pore walls can
occur, thus promoting the rate of reaction.
As well as the location of the metals, the interaction between the metals is also important. Tin is
necessary to make the catalyst selective (see Figure 5-8) but the exact role of tin is not fully
understood[19]. It is likely that the interaction between the Pt and Sn changes during preparation,
during pretreatment and over the course of the reaction as the environment to which the catalyst is
exposed changes. Previous work on these catalysts has shown that platinum and tin form an alloy
after high temperature reduction[5, 6]. In the preparation of the catalyst, the final step is a 6 hour
reduction at 873 K, however, the catalyst is then exposed to air when it is transferred from the
storage vial to the reactor. Work on model Pt/Sn systems on silica films has shown that oxidation
will destroy a Pt/Sn alloy and cause Pt and Sn to separate[20]. The pretreatment is designed to
reduce the metals and reform the alloy. It is shown from the pretreatment studies herein that the
pretreatment method does affect the performance of the catalyst. In particular, the heating rate and
atmosphere are important (see Figure 5-10). Calcination at 533 K before the reduction results in
significant sintering of the catalyst particles. This result is in contrast to the preferred treatment for
Pt/L catalysts in which a calcination before reduction is required to achieve high dispersions[21-
23]. The higher activity observed with a slower rate of heating is consistent with the results of other
studies[8,9].
In a related investigation[ 11], it was shown that increasing the amount of tin relative to platinum on
K-L-zeolite above a 1:2.5 Pt/Sn ratio, marginally increases the dehydrogenation selectivity, but at
the expense of decreased catalyst activity and stability. The results of this study confirm those
results (see Figures 5-3 to 5-6). If the Sn content is too high, then the activity decreases
significantly. In terms of selectivity, however, the Sn content cannot be too low or the catalyst is not
highly selective (see Figure 5-8). The selectivity of the catalyst is also reduced if acidity is
introduced (see Figure 5-2). If the catalyst is less selective, it will also be less stable. The TPO
results show that the addition of Sn improves the resistance of the K-L-zeolite-supported Pt
catalysts to the formation of coke during reaction at 873 K (Figure 5-17 and Table 5-3). In
addition, Table 5-1 shows that the higher coke content blocks a higher percentage of sites for CO
adsorption on the Pt/K-L catalyst than on the Pt/Sn/K-L catalysts. Tin improves the resistance of
the catalysts to coking by reducing the Pt ensemble size, thus preventing the undesired reactions
which lead to coke formation.
Although the amount of carbon deposited on the catalyst has an important effect on catalytic
activity, it is also important to address the location of the carbon (e.g., whether carbon is on the
active sites or on the support) [24]. It has been shown that the lower temperature peaks in TPO
spectra generally correspond to carbon on Pt, while the higher temperature peaks correspond to
carbon on the supportf 15, 17, 24-26]. Liwu et al. [ 17] studied carbon deposition on Pt/A^Qj and
Pt/Sn/A^Os after the dehydrogenation of n-butane. They observed that Sn decreased the ratio of
the carbon deposited on the metal to the total carbon deposited. In agreement with this behavior, it
has been observed in the studies herein, that the addition of Sn decreases the amount of carbon
associated with Pt on both the silica- and L-zeolite-supported Pt catalysts. In addition, the CO
30
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uptakes before and after reaction (see Table 5-1) indicate that fewer sites are lost on the catalyst as
the Sn content is increased.
It has been suggested that Sn transports coke from the active sites to the support on Pt/Sn/alumina
catalystsf 17, 27, 28]. Lieske et al. [27] proposed a drain-off mechanism to explain the stabilizing
effect of Sn on Pt/alumina catalysts. The presence of Sn decreases the size of the surface Pt
ensembles so that the hydrocarbon cannot readily form multiple carbon-metal bonds. Because the
adsorbed species is attached less strongly to the metal surface, it is more mobile and can more
easily migrate to the support. This effect is evident by the change in the ratio of the peak heights in
the TPO spectra with the addition of Sn to the catalysts.
The ability of Sn to improve the resistance of the Pt catalyst to carbon accumulation becomes more
evident when the reaction is run with an isobutane feed in the absence of feed hydrogen. In the
absence of hydrogen, carbonaceous species can accumulate on the surface of the catalyst and block
the active sites. In agreement with this behavior, the TPO results shown in Figure 5-19 and Table
5-3 indicate higher levels of coke after reaction in the absence of hydrogen at 798 K compared to
reaction in the presence of hydrogen at 873 K. A Pt/L catalyst deactivates to near zero conversion
in minutes (see Figure 5-20), a 1: 1.4 Pt/Sn/L catalyst deactivates to essentially zero conversion in 4-
5 hours on stream, while the highly selective 1:2.5 Pt/Sn/L catalyst maintains over 20% conversion
(equilibrium conversion for these conditions is 42%) for 20 hours on stream in isobutane at 798 K
in the absence of feed hydrogen (see Figure 5-16). The TEM results showed that little, if any,
particle agglomeration occurs during the reaction at 798 K in the absence of hydrogen and so
coking is the main mechanism of deactivation. In the presence of hydrogen the stability of the
catalyst is better but the activity will be lower because the reaction is negative order in hydrogen (see
Figure 5-14). Hydrogen is produced by the dehydrogenation reaction and the reactor configuration
can affect the product stream (see Figure 5- 15).
The deactivation studies on the silica catalysts indicate that K, as well as Sn, improve the resistance
of the catalyst to carbon deposition (see Figure 5-19 and Table 5-3). As well as decreasing the size
of the surface Pt ensembles, K may also hinder the dehydrogenation of adsorbed species. It has
been shown that the addition of K to Pt/Sn/silica decreases the CO uptake but increases the H2
uptake[29]. Barbier [30] has shown that the addition of K to alumina catalysts decreases the peak
temperature in the TPO spectrum by 100 K, indicating that the carbon formed on the K-modified
surface is more weakly bound. It is possible that the ion-exchanged potassium present within the
L-zeolite may also help to suppress the formation of highly dehydrogenated carbonaceous species.
This behavior may at least partially explain why the peaks in the TPO spectrum are at lower
temperatures for the Pt/K-L sample compared to the Pt/silica catalyst.
It has been shown in previous studies that Pt/L catalysts are resistant to coking during reactions
with hydrocarbons such as n-hexane[ 14,3 1,321. In our experiments, the Pt/K-L catalyst
accumulates much less carbon than the Pt/silica catalyst, but the isobutane conversion over Pt/K-L
decreases to essentially zero within several minutes on stream. The L-zeolite support has
unidimensional pores with diameters of 0.7 1 nm, and our isobutane adsorption studies (see Table
5-4) indicate that some of the pores become blocked during isobutane dehydrogenation over Pt/K-L
in the absence of hydrogen. Thus, the coke that forms on the Pt/K-L catalyst may be present near
the pore mouths, and thereby block Pt sites deeper within the pores of the L-zeolite from reaction
with isobutane. In the aforementioned studies with n-hexane, the reaction was conducted in the
presence of hydrogen, with hydrogen to hydrocarbon ratios between 5.5 and 8[ 14, 31, 32]. In
agreement with these previous studies, the Pt/K-L catalyst maintains its activity for isobutane
conversion in the presence of hydrogen, although this catalyst is not selective for isobutene
production (see Figures 5-8 and 5-9). Hydrogen probably removes the highly dehydrogenated
carbonaceous species from the surface as by-products (e.g., hydrogenolysis products), such that the
31
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selectivity of Pt/K-L to isobutene is low. On the Pt/Sn/K-L catalysts, the deactivation profiles were
similar with and without hydrogen. This behavior indicates that the presence of Sn is sufficient to
prevent the formation of highly dehydrogenated surface species that lead to the formation of coke
and other unwanted by-products.
The results of the 16 h reduction and the deactivation studies indicate that the extent of Pt/Sn
interaction increases during the course of the reaction. After a long reduction in H2, the activity of
the catalyst has decreased (see Figures 5-11 and 5-12). In fact, the catalysts exhibited a similar rate
of isobutene production after treatment in hydrogen for 16 h as catalysts that were treated for 1 h in
hydrogen and were subsequently exposed to hydrogen and isobutane for 16 h. Mossbauer
spectroscopy has shown that Pt and Sn form alloy clusters in L-zeolite, but the majority of the Sn is
present as Sn2+[5], Studies on Pt/Sn/alumina [ 19,331 and Pt/Sn/NaY-zeolite[ 13] have shown that
the proportion of tin in metallic particles increased with metal particle size. Furthermore, Merlen et
al. [19] have suggested that metallic tin cannot be stabilized in small particles. Thus, the Sn2+ in the
Pt/Sn/K-L-zeolite catalyst may slowly become reduced in the presence of hydrogen during
isobutane dehydrogenation at 873 K and migrate to the Pt/Sn alloy particles, resulting in surface
enrichment with metallic Sn and the loss of accessible Pt sites. Thus, although the presence of Sn
improves the resistance of the catalyst to coke formation, it may adversely affect the sintering of the
catalyst, since larger particles were visible on the Pt/Sn/K-L catalysts compared to the Pt/K-L
catalyst after reaction at 873 K. The 24 h He treatment at 873 K(see Figure 5-13) indicates that
some activity is lost in a non-reducing atmosphere which suggests that particle sintering occurs at
high temperatures. TEM analysis of catalysts held at 873 K for extended periods (over 16 h) in He
or H2 (no iC4H10) indicates that sintering of the metal particles does occur.
The regeneration scheme that was successful for the Pt/Sn/L catalyst required that the catalyst be
reduced for 16 h prior to the initial exposure to isobutane and after each regeneration (see Figures
5-21 and 5-22). As discussed above, a longer reduction leads to increased Pt and Sn interaction
but also some metal sintering. The regeneration in oxygen removes the carbon and also oxidizes the
Sn, destroying the Pt/Sn alloy, which may have become highly surface enriched in Sn over the
course of the reaction. A long reduction, therefore is needed to reform the Pt/Sn alloy. It is
unknown whether this regeneration method redispersed the metals that had agglomerated on the
external surface of the catalyst.
In summary, the Pt/Sn/K-L-zeolite UW Dehydrogenation Catalyst is a highly selective catalyst for
isobutane dehydrogenation to isobutene. Through this project our research team has been able to
understand the interactions between the components of the catalyst - namely, Pt, Sn, K and L-
zeolite. Sn is necessary to make the catalyst selective by reducing the size of the Pt surface
ensembles, thus hindering the competing reactions, such as isomerization, hydrogenolysis and
coking, which require large Pt ensembles; K neutralizes the support; and L-zeolite provides a
support in which the Pt can be highly dispersed by depositing the metals within the pores. Typical
industrial conditions employ hydrogen in the feed stream to minimize the amount of coking. In the
presence of hydrogen, however, the equilibrium dehydrogenation conversion is reduced and thus
high temperatures are employed to maintain the conversion. At 873 K in a 2:1 hydrogen to
isobutane feed stream, the catalyst deactivates by coking, metal agglomeration and surface
enrichment of the Pt particles with Sn. It has been found, however, that Pt/Sn/L catalysts can be
operated for isobutane dehydrogenation at 798 K in the absence of feed hydrogen and maintain a
reasonable conversion, provided that the catalyst is highly selective. At these conditions, the
temperature is low enough that metal agglomeration and Sn enrichment are significantly
suppressed. Without hydrogen, the catalyst is more susceptible to coking but if the catalyst is
highly selective, excessive coking does not occur. The coking that does occur is much easier to
reverse than the metal sintering which occurs at higher temperatures. Thus, running the reaction in
isobutane only at 798 K is very feasible for Pt/Sn/L-zeolite catalysts. Based on the results of this
32
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study, this catalyst may also be applicable for the selective dehydrogenation of higher paraffins to
olefins. Further study of this application is being undertaken in our laboratory.
33
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