Direct hydroxylation of benzene to phenol in a microstructured Pd-based membrane reactor [Elektronische Ressource] / von Laurent Bortolotto

Laurent Beau

Informations

• Publié par: karlsruher_institut_fur_technologie
• Publié le: 01 janvier 2011
• Nombre de lectures: 20
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

Direct hydroxylation of benzene to phenol in a
microstructured Pd-based membrane reactor



zur Erlangung des akademischen Grades eines
DOKTORS DER INGENIEURWISSENSCHAFTEN (Dr.-Ing.)

der Fakultät für Chemieingenieurwesen und Verfahrenstechnik des
Karlsruher Institut für Technologie (KIT)
vorgelegte



genehmigte
DISSERTATION


von
Dipl.-Ing. Laurent Bortolotto
aus Toulouse, Frankreich




Referent: Prof. Dr.-Ing. Roland Dittmeyer
Korreferent: Prof. Dr.-Ing. Georg Schaub
Tag der mündlichen Prüfung: 06.05.2011

Foreword
In early 2007, I bid farewell to the ever-changing and demanding world of the automotive industry in
Stuttgart to join the Technical Chemistry workgroup, under the leadership of Prof. Dittmeyer, at
DECHEMA in Frankfurt, as it provided me with the opportunity to return to research & development
work and further develop my knowledge in the field of palladium membranes, which I had previously
carried out research on at the DaimlerChrysler R&T of EADS in Friedrichshafen. Both promising and
challenging, the aim of the project was to reproduce and study the much discussed gas phase direct
hydroxylation of benzene to phenol in a Pd-membrane reactor, which was postulated as a new phenol
route by the AIST researchers (Japan) in 2002. The project involved many activities from differing
technical fields, such as reactor design (realization of a laboratory-scale planar reactor), materials
engineering (membrane preparation on a commercial support), processes (dosage of gases through
membranes into microstructured channels) and chemical reaction engineering (performance of a gas
phase reaction over a metal catalyst) and in turn would allow me to expand my knowledge in these
domains. The project DI696/6-1, which was financed by the German Research Foundation (DFG) and
had a duration of 3 years, commenced in April 2007 based on the project proposal, this thesis
represents the findings and conclusions determined over the course of the project. The project was
supervised by Prof. Dittmeyer, at the DECHEMA in Frankfurt and later from the Karlsruhe Institute of
Technology, whose extensive expertise on and in-depth research in the field of Pd-membranes for in-
situ hydrogen extraction is internationally acknowledged.
















1
Acknowledgements
A work of this magnitude would, of course, not be possible or at the very least be very difficult without
the support of many persons both within and outside of work. I would particularly like to thank my
fellow members of the Technical Chemistry workgroup and those members of the DECHEMA staff,
who supported me during the period I worked on the DI696/6-1 project. I would like to expressly thank
Prof. Dittmeyer for the invaluable support and advice, which he has provided me with over the course
of this project as well as his commitment, which has, time and again, helped me gain new insights as
well as push myself harder when it came to overcoming and dealing with the challenges and obstacles
that arise as part of such a project. I would also like to mention Walter Jehle from DaimlerChrysler
Research & Technology, who was the first person to introduce me to the field of Pd-membranes
applied to onboard hydrogen extraction systems. I would also like to acknowledge the contribution of
Mrs. U. Gerhards from IMVT (KIT, Karlsruhe) and Dr. M. Armbrüster (MPI, Dresden) with regard to the
ESMA and XRD analysis of the membranes. Last but not least my family in France. Every
achievement is for them and would be unattainable without them.


















2
Abstract

The gas phase direct hydroxylation of benzene into phenol with hydrogen and oxygen, as initially
described by Niwa in 2002, was realized in a newly developed double-membrane reactor. In the
concept described by Niwa, a dense Pd membrane is used for the (safe) dosage of dissociated
hydrogen species into the tubular reaction zone, where it reacts with gas phase oxygen to form
surface species capable of directly converting benzene into phenol in a single-step process. In the flat
reactor design used in this project, the second membrane, for the simultaneous dosage of oxygen,
allows for a better control of the reaction atmosphere all along the distributed reaction channels. In this
connection, a stable H /O ratio favors the desired benzene hydroxylation conditions. Similar to other 2 2
researchers in this field, the direct hydroxylation of benzene into phenol was also observed on a Pd
surface (the process parameters T=150°C and H /O =1.4 over a Pd/PdCu membrane resulted in the 2 2
-8highest selectivity and rate). The phenol rate with 1.3x10 mol/h is, however, far behind the CO rate 2
-6 -3(range of 10 mol/h) and H O (range of 10 mol/h) rate, observed in previous research in this domain, 2
which makes them the main products of the system. Furthermore, the C-based selectivity was limited
to 9.6%.
The phenol performance of the reactor was improved by sputtering an active catalyst onto the surface
of the PdCu membrane. The 3 systems studied in this connection were: PdGa, PdAu (both catalytic
layers) and V O /PdAu (particles for a higher active surface), PdAu provided the highest C-based 2 5
-7phenol selectivity (up to 67%) and rate (up to 7.2x10 mol/h). In terms of performance, the PdAu layer
was the catalyst tested for phenol, which has the most positive result as part of this study. Most
interestingly, the PdGa-modified surface was the catalytic system, which was the most successful in
restricting the byproduct formation and this in spite of its reduced H -permeation properties and 10-fold 2
lower phenol rates it achieved in comparison to the other two systems. We believe this is due to the
isolated active sites on the surface of this intermetallic alloy as it is assumed that the adsorbed
benzene molecules are only weakly bonded at these locations. In this sense, PdGa is a promising
catalyst when it comes to lowering the formation rate of the reaction byproducts involved in the
system. The influence of the main process parameters (temperature and partial pressures) was
investigated with different catalytic surfaces and discussed.
Despite its technical achievements (proven feasibility of gas phase benzene direct hydroxylation and
the advantages of the double-membrane dosage in comparison to tubular single-membrane systems),
an inefficient use of hydrogen for obtaining phenol and the high gas phase stability of benzene
(benzene conversion below 1%) unfortunately make the system impractical for an industrial
application. Based on these observations, the mismatch between our results and those first mentioned
by the reaction pioneers, which made industrial applications appear to be within grasp, must be
stressed. This work, nonetheless, throws up some interesting aspects for other chemical engineering
applications involving double-membrane dosage and the use of Pd Ga at.% as a catalyst, which 50 50
has the potential to reduce the influence of side-reactions in the presence of gas phase oxygen and
hydrogen.
In addition to the experimental part, a system simulation was realized with Matlab 2007b. The
evolution of the H /O ratio due to the double-membrane dosage and the low benzene consumption 2 2
were modeled in parallel to the product increase along the reactor axis. Measurements were carried
out to correlate the simulation data with in-situ sample extraction by means of micro-capillaries
implanted in the reactor sealing. A model based on adsorption and surface reaction mechanisms is
proposed to describe the molecular mechanisms acting in the system and to enable a prediction of the
product amounts in a larger process parameter range.
In light of the findings over the duration of the project, this work aims to contribute to a better
understanding of the phenomena involved in the gas phase direct hydroxylation of a given aromatic
compound, performed in a Pd-based membrane reactor. A topic, which has been much discussed
since 2002. It also implements an innovative concept in the field of membrane reactors with the
combined use of 2 gas distribution membranes in a single system.
3
Kurzfassung

Die direkte Hydroxylierung von Benzol zu Phenol in der Gasphase mit Wasserstoff und Sauerstoff,
vorgestellt durch Niwa [24] im Jahre 2002, wurde in einem neuen Doppelmembranreaktor
durchgeführt. In Niwas Konzept dient eine dichte Pd-Membran zur sicheren Dosierung von aktiviertem
Wasserstoff in den Reaktionskanal. Dort reagiert der aktivierte Wasserstoff mit dem Gasphasen-
Sauerstoff an der Pd-Membranoberfläche zu Hydroxyl-Radikalen. Diese können an der Membran
adsorbierte Benzolmoleküle zu Phenol hydroxylieren.
In unserem planaren Design wird eine zweite Membran eingesetzt um auch die Zufuhr und Verteilung
von Sauerstoff in den Reaktionskanälen besser kontrollieren zu können. Dadurch kann im gesamten
Reaktionsbereich das optimale H /O -Verhältnis für die Benzolhydroxylierung erm