Please use this identifier to cite or link to this item: http://dx.doi.org/10.14279/depositonce-598
Main Title: Partialdehydrierung von Ethylbenzol zu Styrol an Kohlenstoffmaterialien
Author(s): Maximova, Nadejda
Advisor(s): Lerch, Martin
Granting Institution: Technische Universität Berlin, Fakultät II - Mathematik und Naturwissenschaften
Type: Doctoral Thesis
Language: English
Language Code: en
Abstract: Seit der Entdeckung der Fullerene 1985 erfährt die Chemie sp2-hybridisierter, nanostrukturierter Kohlenstoffe zunehmendes Interesse, zum einen aus fundamentaler Sicht, zum anderen wegen potentieller Anwendungen. Inzwischen wurde eine Vielzahl neuer Fulleren-verwandter Materialien (Riesenfullerene, Nanoröhren, Nanokugeln, Nanokonen, Nanobündel, zwiebelähnlicher Kohlenstoff, etc.) synthetisiert. Die einzigartigen chemischen und physikalischen Eigenschaften dieser Verbindungen ermöglichen neue Anwendungen. Diese Kohlenstoffnanomaterialien besitzen wegen ihrer nahezu perfekten graphitischen und trotzdem stark gespannten Strukturen auch neue interessante katalytische Eigenschaften. Eine gravierende Einschränkung erfährt die direkte Dehydrogenierung von Kohlenwasserstoffen wegen des endothermen Charakters dieser Reaktion. Dehalb sucht man intensiv nach alternativen Syntheseverfahren. Für die Styrolsynthese, einer der zehn wichtigsten Industrieprozesse, ist die exotherme, oxidative Dehydrogenierung von Ethylbenzol eine elegante und vielversprechende Alternativreaktion, in der Kohlenstoffkatalysatoren bereits ihre Effizienz gezeigt haben. Die begrenzte oxidative Stabilität oberflächenreicher Kohlenstoffe und deren Porosität wirkt sich jedoch negativ auf die katalytische Wirksamkeit aus. Jedoch scheint die katalytische oxidative Dehydrierung über Kohlenstoffkatalysatoren mit guten Ausbeuten möglich zu sein. Kohlenstoffnanofilamente und Kohlenstoffnanoröhren zeichnen sich hierbei besonders durch ihre hohe Oxidationsstabilität aus. Die verbesserte Leistungsfähigkeit der Kohlenstoffnanofilamente und Kohlenstoffnanoröhren im Vergleich zu anderen Kohlenstoffformen ist ebenfalls auf eine optimierte Verteilung der Basalflächen und der Prismaflächen bei diesen Typen von Nanokohlenstoff zurückzuführen. Rationale Experimentplanung auf Grund einer funktionalen Analyse technischer Katalysatoren mit Hilfe oberflächenphysikalischer Methoden führte in kürzester Zeit gezielt zu einem hochtemperaturstabilen, aktiven und selektiven Katalysator für die oxidative Dehydrierung von Ethylbenzol.
In the present work, the different nanostructures, i.e. carbon black, graphite, nanofilaments, nanotubes, onions, ultra-dispersed diamonds, were tested as catalysts for the oxidative dehydrogenation of ethylbenzene to styrene. The comparative characterizations of carbons before and after catalytic tests with TEM, XPS, Raman- and IR-spectroscopy, TG/DTA, and BET surface area techniques allowed us to develop a structure-activity relationship and to propose a model of the reaction mechanism. The determination of the conditions, under which carbon catalysts develop their activity maximum, was done with Experimental Design. A screening of the experimental parameters was conducted with the theoretically lowest possible number of experiments according to the Box-Behnken Plan and Simplex method. The optimum reaction conditions for all carbons tested lied at the temperature range of 495-515°C. The oxygen content in the feed was found to be an insignificant parameter in the accessible mass flow rates and the chosen temperature region. It was found that sp2-bound carbon is required for the selective styrene formation, since sp3-bound carbon led to the production of benzene instead of styrene. It has been shown that the microstructure of sp2-bound carbon materials is of paramount importance in order to obtain high and stable efficiencies. Carbon nanofilaments have shown the highest styrene yields at the highest ethylbenzene conversions as compared to carbon black and graphite. The comparative study of carbon nanofilaments and nanotubes of different structure has shown that more perfect carbon nanotubes produced by the arc-discharge technique are the most active catalysts in terms of reaction rates. The onion-like carbon was found to be the most efficient catalyst for the oxidative dehydrogenation reaction on a mass-referenced basis. XPS results revealed that the surface of onion-like carbon, being oxygen-free before the reaction, contained surface oxygen groups after the reaction. The experiments with oxygen pretreatment confirmed the creation of functional groups on the onion-like carbon surface at 570°C. Due to the high formation temperature and the XPS binding energy of the oxygenated species, it was proposed that chinoidic carbonyl groups of strongly basic character are generated during the reaction. The reaction model suggested for the oxidative dehydrogenation of ethylbenzene to styrene over sp2 - carbon materials follows a Langmuir-Hinshelwood mechanism, in which both adsorbed ethylbenzene and adsorbed oxygen-species play an important role. According to this model, the reaction might occur via i) ethylbenzene adsorption at the graphite step edges, ii) ethylbenzene reaction with the oxygenated species also located at the graphite step edges leading to the dehydrogenation of ethylbenzene to styrene, iii) the simultaneous transformation of the dehydrogenating oxygen species to hydroxyl groups, which remain at the graphite edges, iv) the styrene desorption from the carbon surface, v) gas-phase oxygen activation on the basal planes of the graphene layers, vi) oxygen diffusion to the prismatic planes with the hydroxyl groups, vii) reformation of the basic, chinoidic oxygen functionalities from the activated oxygen and the hydroxyl groups, iix) water desorption. The catalytic reaction passes these steps cycle by cycle. The establishment of structure-activity relation by the catalytic tests and the characterisation of different carbon nanostructures allowed one to determine carbon nanostructure stable under oxidative reaction conditions. Carbon nanotubes and onions have shown a high and stable efficiency in the ODH reaction. A radius of curvature of the basic structural element of carbon nanotubes and onions and also their high aspect ratio seem to provide a high density of functional surface groups under reaction conditions. The perfectness of these carbon nanostructures provides also enough stability toward oxidation and is essential for gas phase oxygen activation. The simplicity of carbon and its unique property that deactivated surfaces gasify themselves in oxidative dehydrogenation reactions not only renders them well-suited model systems but also allow for realistic expectations for a technical application.
URI: urn:nbn:de:kobv:83-opus-5006
http://depositonce.tu-berlin.de/handle/11303/895
http://dx.doi.org/10.14279/depositonce-598
Exam Date: 20-Dec-2002
Issue Date: 4-Feb-2003
Date Available: 4-Feb-2003
DDC Class: 540 Chemie und zugeordnete Wissenschaften
Subject(s): Dehydrogenierung
Ethylbenzol
Katalyse
Kohlenstoff
Styrol
Dehydrogenation
Ethylbenzene
Catalysis
Carbon
Styrene
Usage rights: Terms of German Copyright Law
Appears in Collections:Technische Universität Berlin » Fakultäten & Zentralinstitute » Fakultät 2 Mathematik und Naturwissenschaften » Publications

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