Kinetic studies of propane oxidation on
Mo and V based mixed oxide catalysts
vorgelegt von
M. Sc. Chemiker Lnrd-Istvn Csepei
aus Zalau/Zilah/Zillenmarkt (Rumnien)
Von der Fakultt II Mathematik und Naturwissenschaften der Technischen Universitt Berlin
zur Erlangung des akademischen Grades Doktor der Naturwissenschaften
- Dr. rer. nat. -
genehmigte Dissertation
Promotionsausschuss: Vorsitzender: Prof. Dr. A. Thomas Berichter/Gutachter: Prof. Dr. R. Schomcker Berichter/Gutachter: Prof. Dr. R. Schlgl Berichter/Gutachter: Prof. Dr. M. Muhler Tag der wissenschaftliche Aussprache: 19. August 2011
Berlin 2011 D 83
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Acknowledgements/Danksagung
The work presented in this thesis was carried out in the time interval between February
2007 and June 2011 at the Inorganic Chemistry Department of the Fritz Haber Institute of
the Max Planck Society in Berlin.
Foremost, I would like to thank to Prof. Dr. Robert Schlgl for giving me the opportunity
to carry out the doctoral studies at this Institute. In the same time, I would like to express
my gratitude for giving me this interesting topic and the constructive criticism during the
discussions. I also would like to thank Dr. Annette Trunschke for introducing me in the
topic of the present thesis, for the fruitful discussions and new ideas and for contributing
to my professional development.
The substantial contributions of the department members to this work are also
acknowledged. I thank Yury Kolen`ko, Almudena Celaya Sanfiz, ZiRong Tang and Olaf
Timpe for catalyst synthesis; Gisela Lorenz for the nitrogen physisorption experiments;
Gisela Weinberg and Wei Zhang for the SEM-EDX and the STEM measurements; Edith
Kitzelmann for the XRD measurements; Sabine Wrabetz for the microcalorimetric
experiments; Raoul Blume, Michael Hvecker and Detre Teschner for the XPS
experiments and data analysis; Benjamin Frank, Kazu Amakawa, Pter Schnrch, Tom
Cotter, Manfred Schuster, Anton Nagy and Sylvia Reiche for the helpful discussions.
Special thanks are addressed to Siegfried Engelschalt and Raoul Naumann d`Alnoncourt
for their help in setting up the reactor system and to Frank Girgsdies for the in-situ XRD
experiment and data analysis on the effect of steam and the redox potential. Finally, the
help of all the members of the department and the workshops is acknowledged.
Last but not least, I would like to express my gratitude to my parents and my former
supervisor Dr. Csaba Bolla for encouraging me to undertake the doctoral studies abroad
and for their continuous moral support along these years.
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To my parents
iii
Eidesstattliche Versicherung Hiermit erklre ich, dass ich die Dissertation selbst angefertigt habe. Die Arbeit enthlt auch in Anteilen keine Kopien andere Arbeiten. Verwendete Hilfsmittel und Quellen sind vollstndig angegeben. Die Namen alle Wissenschaftler die mit mir zusammengearbeiten haben, sind in den Anlagen vollstndig genannt.
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Abstract
The present work concentrates on the systematic kinetic study of the one-step propane
oxidation to acrylic acid over a well defined, phase-pure M1 MoVTeNbOx catalyst. The
bulk structural stability of the catalyst is a key issue for kinetic studies. The stability of
the phase-pure M1 MoVTeNbOx catalyst under various conditions (steam-containing,
steam-free, net reducing, stoichiometric and net oxidizing feed compositions) was
evidenced by an in-situ XRD experiment which suggested that the bulk structure is
homogeneous and constant under reaction conditions. Thereby, the heterogeneously
catalyzed reactivity is exclusively determined by the surface properties, which in turn, are
controlled by the chemical potential of the gas phase.
A kinetic study on the reaction variables (temperature, steam content and redox potential)
was carried out. Stable catalytic performance was observed for all the conditions. Cycling
experiments showed the reversibility of the conversion and selectivity decrease upon
exposing the catalyst to dry and reducing feed, respectively. Further catalytic experiments
revealed that the reactivity spans over 5 orders of magnitude in the order of acrolein
oxidation>>propylene oxidation>propane oxidation>>carbon monoxide oxidation~water
gas shift reaction. The negligible CO oxidation activity suggested that the CO and CO2
are formed via two independent pathways in propane oxidation over M1. The stage-wise
addition of oxygen lead to an improvement of the catalytic performance by 5% compared
to the conventional single-tube reactor. Further experiments in the two-stage reactor
revealed that the phase-pure M1 is not reoxidized by N2O. The addition of propylene in
the two-stage reactor revealed a slight competitive adsorption on the active sites with
propane, which observation was supported by the results of microcalorimetric
experiments. On the other hand, the addition of CO and CO2 in the two-stage reactor
showed that these products do not adsorb competitively with the educt or intermediates.
In the literature much of the kinetic data was reported for ill-defined catalyst surfaces. In
contrast to that, the present work reports the kinetic study of propane selective oxidation
to acrylic acid on a well defined phase-pure and structurally stable M1 MoVTeNbOx
catalyst. This study may contribute to the better kinetic and mechanistic understanding of
the propane selective oxidation reaction.
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Zusammenfassung
Die vorliegende Arbeit enthlt systematische kinetische Untersuchungen zur einstufigen,
selektiven Oxidation von Propan zu Acrylsure an wohl definierten, phasenreinen M1-
MoVTeNbOx-Katalysatoren. Die Stabilitt der phasenreinen M1-Katalysatoren unter
verschiedenen Reaktionsbedingungen (in Wasserdampf, wasserdampffrei, netto-
reduzierende, stchiometrische und netto-oxidierende Feed-Zusammensetzung) konnte in
In-situ-XRD-Experimenten bewiesen werden. Da die Festkrperstruktur homogen ist und
bestndig unter Reaktionsbedingungen, kann die unterschiedliche Reaktivitt des
heterogenen Katalysators allein durch seine Oberflcheneigenschaften bestimmt werden,
welche wiederum stark vom chemischen Potential der Gasphase abhngen.
Es wurden kinetische Studien zu den Reaktionsparametern Temperatur, Wasserdampf-
anteil und Redoxpotential durchgefhrt, wobei die Systeme unter allen Bedingungen
stabile Katalysatorleistungen aufwiesen. Zyklische Experimente zeigten die Reversibilitt
des Umsatz- und Selektivittsrckgangs, sowohl unter wasserfreiem als auch
reduzierendem Feed. Zudem konnten in den Katalysetests Unterschiede in den
Reaktivitten von bis zu 5 Grenordnungen ermittelt werden, mit Acrolein >> Propylen
> Propan >> CO-Oxidation~Wassergas-Shift. Die bestimmte Oxidationsaktivitt von CO
war vernachlssigbar klein, was die Bildung von CO und CO2 auf zwei voneinander
unabhngigen Reaktionspfaden suggeriert. ber eine stufenweise Zufuhr von Sauerstoff
konnte eine Steigerung der katalytischen Aktivitt um 5% im Vergleich zum
konventionellen, einstufigen Reaktor erreicht werden. Die Versuche im zweistufigen
Reaktor zeigten auch, dass der phasenreine M1-Katalysator in N2O nicht reoxidiert.
Weiterhin konnte unter Zugabe von Propylen im zweistufigen Reaktor eine teilweise
kompetitive Adsorption zu Propan an die aktiven Zentren des Katalysators beobachtet
werden. Im Gegensatz dazu, stand die Adsorption von CO und CO2 nicht in Konkurrenz
mit der Adsorption von Edukten oder Zwischenprodukten.
Die kinetischen Untersuchungen der, im Gegensatz zu den meisten Systemen in der
Literatur, wohl definierten, strukturstabilen M1-MoVTeNbOx-Katalysatoren knnten
einen entscheidenden Beitrag zum Verstndnis von Kinetik und Reaktionsmechanismus
der Propanoxidation leisten.
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Table of contents
Acknowledgements/Danksagung ............i Eidesstattliche Versicherung...iii Abstract .....iv Zusammenfassung ....v Table of contents ..vi
Chapter 1 Introduction and motivation .....1 1.1 Introduction ......1 1.2. Overview on the literature results .......3
1.2.1. The selective oxidation of propylene .......3 1.2.2. Oxidative dehydrogenation of propane ....8 1.2.3. The direct oxidation of propane to acrylic acid ..13 1.2.3.1. Generalities ..13
1.2.3.2. Identification of propane selective oxidation pathways ..14 1.2.3.3. Active sites on MoVTeNbOx catalysts ........20 1.2.3.4. The effect of acid-base character of the catalyst .. ...24 1.2.3.5. The effect of steam ......25 1.2.3.6. The effect of redox potential of gas phase and oxygen species27
1.2.4. Reactor designs, operation modes ...31 1.2.4.1. Conventional laboratory scale reactors 31 1.2.4.2. Catalytic membrane- and multi-stage reactor designs .33
1.2.5. Reaction kinetics .37 1.3. Motivation ..38 References Chapter 1 40
Chapter 2. Experimental methods .44 2.1. Physico-chemical characterization of the catalysts 44
2.1.1. Nitrogen physisorption 44 2.1.2. X-Ray Diffraction ...44 2.1.3. Scanning Electron Microscopy (SEM/EDX) and Scanning Transmission Electron Microscopy (STEM) 45 2.1.4. X-Ray Photoelectron Spectroscopy (XPS) .45 2.1.5. Microcalorimetry 45
2.2. The experimental setup for propane oxidation ..46 2.2.1. The gas dosi
