Please use this identifier to cite or link to this item: http://dx.doi.org/10.18419/opus-1411
Authors: Kley, Ines
Title: Katalytische Eigenschaften von Zeolithen des Strukturtyps MWW
Other Titles: Catalytic behaviour of zeolite structure type MWW
Issue Date: 2013
metadata.ubs.publikation.typ: Dissertation
URI: http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-88550
http://elib.uni-stuttgart.de/handle/11682/1428
http://dx.doi.org/10.18419/opus-1411
Abstract: Bei der nicht-oxidativen Propandehydrierung wurden nicht saure bimetallische Pt,Zn-Zeolithkatalysatoren verwendet. Durch das Einbringen von Zn in den Katalysator wurde die Bildung von großen Pt-Agglomeraten verhindert, so dass strukturinsensitive Reaktionen, wie die Propandehydrierung, im Vergleich zu anderen Nebenreaktionen bevorzugt ablaufen. Es konnte gezeigt werden, dass durch diese Verdünnung des Platins mit Zink eine nahezu vollständige Unterdrückung der Hydrogenolyse von Propan erzielt werden konnte. Bei der nicht-oxidativen Dehydrierung von Propan muss zwischen Primär- und Sekundärreaktionen unterschieden werden. Primärprodukte werden vom Porensystem und vom Umsatz von Propan unabhängig gebildet, Sekundärreaktionen, wie die Oligomerisierung und Aromatisierung, nehmen mit steigendem Umsatz von Propan zu. Durch die Variation der Kontaktzeit der Reaktanden mit dem Katalysator konnte gezeigt werden, dass die nicht-oxidative Propandehydrierung an Zeolith MCM-22 vor allem an der äußeren Oberfläche abläuft und somit nicht durch Diffusionshemmungen beeinträchtigt wird. Die selektive Verkokung der äußeren Oberfläche hat auf die katalytische Aktivität oder Selektivität zu den Produkten an Zeolith Beta im Vergleich zu Zeolith MCM-22 keine Auswirkungen. An den MWW-Strukturtyp-Katalysatoren kann eine Beteiligung der sauren Zentren in den halben Hohlräumen auf der Oberfläche der Zeolithe durch einen Vergleich der Reaktionen mit verkokten und unverkokten Katalysatoren gezeigt werden. Es konnte im Weiteren gezeigt werden, dass die Reaktion durch die Säurezentrendichte und die Verteilung der sauren Zentren im Innern des Zeoliths stark beeinflusst wird. Saure Zentren, welche während der Reduktion des Edelmetalls gebildet werden, reduzieren die Selektivität zu Propen. Eine geringere Konzentration an aktiven Säurezentren kann durch die Reduktion des Edelmetalls in einer wässrigen Natriumborhydrid-Lösung erreicht werden. Durch eine geringere Anzahl an sauren Zentren wird die Bildung von unerwünschten durch säurekatalysierte Reaktionen dargestellten Nebenprodukten verringert. Durch Untersuchungen der Zugänglichkeit der sauren Zentren an Zeolith Beta und der selektiven Verkokung der äußeren Oberfläche der verwendeten Katalysatoren wurde deutlich, dass durch unterschiedliche Säurezentrenverteilungen komplexere Reaktionen möglich sind und eine starke Deaktivierung des Katalysatorsystems vor allem zu Beginn der Reaktion vorliegt. Die Bildung von aromatischen Verbindungen erfolgt bevorzugt in großen Poren und Kreuzungspunkten des Porensystems des Zeoliths Beta, da diese ausreichend Platz für sperrige Übergangszustände zur Verfügung stellen und Zeolith Beta auch eine geringe Diffusionshemmung innerhalb der Poren aufweist. An Zeolith MCM-22 hingegen läuft die Bildung von BTX-Aromaten sowohl in den großen Hohlräumen im Innern des Zeoliths als auch auf der äußeren Oberfläche in den halben Hohlräumen als nicht-formselektive Reaktion ab. Die Bildung von aromatischen Verbindungen in den großen Hohlräumen führt zu deren vollständigen Verkokung und Deaktivierung. Bei der Alkylierung von Toluol mit 1-Dodecen in der flüssigen Phase an sauren Katalysatoren konnte anhand eines Vergleichs von Zeolith Beta mit den Zeolithen MCM-22 und ITQ-2 gezeigt werden, dass die Selektivität zu den gewünschten 2-Tolyldodecanen durch das Porensystem beeinflusst wird. Dies kann auf Unterschiede bei der Produktdesorption zurückgeführt werden. Dies kann auch mit Hilfe unterschiedlicher Kristallgrößen von Zeolith Beta bestätigt werden. An kleineren Zeolithkristallen ist die Diffusionshemmung der Edukte und Produkte geringer und somit auch die Aktivität des Katalysators höher als an großen Kristallen, jedoch ist die Selektivität zu den Produkten davon unabhängig. Die Deaktivierung des Katalysators spielt bei dieser Testreaktion eine entscheidende Rolle, da durch eine unzureichende Desorption der Produkte eine vollständige Blockierung des inneren Porensystems resultieren kann, so dass im Fall der Reaktion an Zeolith Beta, im Vergleich zu den Alkylierungsreaktionen an Zeolith MCM-22 und ITQ-2, keine zugänglichen sauren Zentren mehr zur Verfügung stehen. Wie auch bei der nicht-oxidativen Dehydrierung von Propan spielen bei der Flüssigphasenalkylierung von Toluol mit 1-Dodecen die sauren Zentren sowie deren Säurestärke eine entscheidende Rolle. Bei Reaktionen an Katalysatoren, deren saure Zentren eine mittlere Säurestärke aufweisen, kann die Selektivität zu den gewünschten 2-Tolyldodecanen erhöht werden. Des Weiteren haben unterschiedliche Kontaktzeiten des Eduktgemischs an den aktiven sauren Zentren des jeweiligen Katalysators keinen Einfluss auf die Reaktion; die WHSV ist somit unabhängig von der Reaktion und erlaubt keine Rückschlüsse auf das Porensystem des verwendeten Katalysators. Bei einem Vergleich der Reaktionen an Zeolith Beta und MCM-22 ist davon auszugehen, dass die sauren Zentren im Innern des Porensystems von Zeolith MCM-22 auf Grund der Diffusionshemmung eine untergeordnete Rolle spielen. Die Reaktion an den MWW-Strukturtyp-Katalysatoren läuft demnach fast ausschließlich an den sauren Zentren in den halben Hohlräumen auf der äußeren Oberfläche ab. Die in dieser Arbeit verwendeten Zeolithe des Strukturtyps MWW sind auf Grund ihres einzigartigen Porensystems sehr interessante Katalysatoren, wie am Beispiel der Dehydrierung von Propan und der Flüssigphasenalkylierung von Toluol mit 1-Dodecen gesehen. Anhand dieser Beispielreaktionen konnte gezeigt werden, dass MWW-Zeolithstrukturtypen vielseitige Einsatzmöglichkeiten sowohl bei Reaktionen in der Gasphase als auch bei Umsetzungen in der Flüssigphase aufweisen. Aus diesen besonderen Eigenschaften resultiert auch der Einsatz von Zeolith MCM-22 in einem kommerziellen Verfahren, dem so genannten EB-Max™-Prozess, bei welchem Benzol mit Ethen zu Ethylbenzol umgesetzt wird.
In this work, the catalytic properties of the different pore systems of zeolite MCM-22 were examined. The reasons for this choice were the unique catalytic properties and pore structure of zeolite MCM-22 during the alkylation of aromatic compounds with alkenes in the liquid phase. Especially, the alkylation of benzene with ethene and propene is the most interesting commercial process on zeolite MCM-22. Due to the special pore structure and the catalytic properties, zeolite MCM-22 is used in the so called EBMax™ process of Mobil-Raytheon, in which benzene and ethene are transformed to ethylbenzene in the liquid phase. The main advantages of zeolite MCM-22 are not only the high activity and selectivity to the desired products, but also the high catalyst stability. Furthermore, zeolite MCM-22 shows, in comparison to zeolite Beta and zeolite US-Y, higher selectivities to the monoalkylated products. In addition, no diffusion limitations of the products can be detected, as the reaction takes place in the outer hemicages of zeolite MCM-22. Due to the low deactivation rate and high stability of the catalyst, longer catalyst cycles can be achieved in comparison to the reaction on zeolite US-Y and zeolite Beta. However, zeolite MCM-22 can also be used as a catalyst for the reaction of benzene with propene to produce cumene. The unique catalytic properties of zeolite MCM-22 minimize oligomerization of propene and, at the same time, enable the high alkylation activity for benzene. In addition, zeolite MCM-22 is used for the alkylation of aromatic compounds with long-chain α-olefins. The special pore system of zeolite MCM-22 is responsible for its unique and fascinating catalytic properties. The catalytic performance combines both properties of a 10-ring pore system and a 12-ring pore system and is very promising for catalytic reactions. Thus, the classification in wide- and medium-size pore systems generally applied for zeolite structures is not possible for zeolite MCM-22. The structure of zeolite MCM-22, zeolite framework type MWW being built up of lamellar platelets, consists of two non-interconnected pore systems both accessible through 10-ring openings. The first pore system is disposed of elliptic 10-rings, with a 2-dimensional channel with a cross-section dimension of 0.40 x 0.59 nm. The second pore system consists of large supercavities, with an inner diameter of 0.71 nm and an inner height of 1.82 nm. In addition, half supercavities are located on the outer surface with an aperture built by a 12-ring and a depth of the half supercavities of 0.70 nm. It has been shown, that the liquid-phase alkylation takes place in the half supercavities, therefore no deactivation of the catalyst and no diffusion limitation have been detected. All of these pore systems exhibit cation exchange sites, because of that, it is possible that the active sites, i.e. the noble metal sites and the acid sites, can be located in all of these three pore systems. In most cases, it is not clear or examined in which pore system the respective reaction occurs. Different test reactions were used in this work, such as the non-oxidative dehydrogenation of propane and the liquid-phase alkylation von toluene with 1-dodecene. The non-oxidative dehydrogenation of propane in the gas-phase was employed to compare the catalytic performance of zeolite MCM-22 with zeolites ITQ-2 and Beta on an acidic and a bimetallic catalyst. Zeolite ITQ-2 was chosen because it also belongs to the zeolite framework type MWW. It is formed by delamination of a zeolite MCM-22-precursor, consists of very thin sheets, leading to a high external surface area. Because of delamination of the zeolite MCM-22-precursor, the large supercavities are halved and the amount of half supercavities on the outer surface is increased. Zeolite Beta was chosen as a reference for the catalytic performance of a catalyst with a 12-ring opening. Understanding the different side reactions of the non-oxidative dehydrogenation is of main importance. It has to be distinguished between primary and secondary products. Primary reactions are the dehydrogenation of propane to propene and hydrogen, and cracking of propane to ethene and methane. These reactions are simple reactions and are independent of the pore system. Furthermore, secondary products, such as ethane, butenes and aromatic compounds, are formed by oligomerization, aromatization and cracking. There is no evidence that selectivities to the primary products change with changing the catalyst system. To further investigate the influence of the nature of the active sites, the reaction on bimetallic catalysts was examined. The dehydrogenation on bimetallic catalysts occurs predominantly on metal sites, whereas undesired side reactions take place mainly on acid sites. The acid sites on bimetallic Pt,Zn-catalysts were formed during the reduction of the noble metal and/or an incomplete sodium ion exchange. It can be shown that the formation of secondary products, like aromatic compounds and butenes, is favored by presence of strong acid sites. The tendency to form undesired side products is on zeolite MCM-22 higher in comparison to that on zeolite ITQ-2. This is mainly caused by strong acid sites in the large supercavities in zeolite MCM-22. Zeolite ITQ-2 posseses, on the other hand, a lower density of acid sites, which have in addition, a lower strength which results in a lower amount of secondary products (chapter 6.1.2.4). However, the high stability of the conversion of propane combined with a high selectivity to propene on zeolite Pt/Zn,Na-Beta (chapter 6.1.2.4), can be ascribed to the reaction temperature of 555 °C. At this high reaction temperature, octahedral coordinated Al atoms are formed, which favor the non-oxidative dehydrogenation reaction. However, these octahedral coordinated Al atoms cannot are not active for undesired acid-catalyzed side reactions, which causes the high conversion of propane combined with a high selectivity towards propene on zeolite Beta. To further study the influence of the acid sites on the non-oxidative dehydrogenation of propane, a variation of the nNa/nAl ratio of the bimetallic Pt/Zn,Na-MCM-22 catalyst has been performed. The test has been made by increasing the nNa/nAl ratio and at the same time reducing the quantity of acid sites on the zeolite by bad-exchange of the acid sites with Na ions after activation and reduction of the metal. This results in a more stable conversion level of propane and a higher selectivity to propene. If the concentration of the acid sites of the catalyst is reduced, the strength of the single remaining acid site increases. The stronger acid sites favor the cracking reaction so that the selectivity to cracking products increases with a lower acid site concentration. In further studies, different reduction techniques of the noble metal have been examined. The use of NaBH4 as reducing agent caused a higher metal dispersion and a lower acid site concentration. This also resulted in a higher activity of the catalyst combined with an increase of the selectivity to propene. The procedure of introducing the Zn ions into the catalyst system influences the activity and stability of the zeolitic catalyst. A higher and faster deactivation rate can be observed by introducing the Zn ions to the catalyst by impregnation technique. With the use of Zn ion exchanged catalysts, the surface of the Pt particles was diluted by Zn species, the influence of the “ensemble effect” increases and structure insensitive reactions are favored. It was possible to demonstrate with pre-coking experiments, that the formation of propene on zeolite structure types MWW takes place at the outer surface in the half supercavities and at the pore openings of the zeolite. In comparison to pre-coked zeolite MCM-22, the tendency of forming propene is higher on pre-coked zeolite ITQ-2, because of the higher outer surface and the shorter pores. The diffusion limitations are lower combined with a shorter residence time of propane at the active sites of the catalyst. Further investigations relating to the formation of aromatic compounds on pre-coked catalysts were made. On zeolite Beta no influence has been observed, comparable to the results of the formation of propene. In contrary, the tendency and selectivity to BTX aromatics is increased during the reaction on pre-coked zeolite MCM-22. This confirms the assumption that the synthesis of complex secondary products occurs at the large supercavities at the inner pore system. The influence during the reaction on zeolite ITQ-2 is lower due to the lower pore volume and the large supercavities being absent. In order to further investigate the special and unique pore system of zeolite MCM-22, the liquid-phase alkylation of toluene with 1-dodecene was chosen. Zeolite MCM-22 shows superior results in the liquid-phase alkylation of benzene with alkenes, and this is why the alkylation of toluene with 1-dodecene in the liquid phase has been chosen as a second test reaction. The reaction temperature was varied; higher reaction temperature resulted in a higher conversion of 1-dodecene, but the selectivity to the desired 2-tolyldodecanes decreased. By increasing the contact time of the reactant mixture by decreasing the WHSV at the active sites of the acidic zeolite, the conversion of 1-dodecene can be increased, but the resulting selectivities to 2-tolyldodecanes remain unaffected. Furthermore, there is no influence of the pore system of the used zeolite. It can be summarized that the acid sites of zeolite MCM-22 play a key role in the non-oxidative dehydrogenation of propane and the liquid-phase alkylation of toluene with 1-dodecene. Comparing the bimetallic catalysts during the non-oxidative dehydrogenation of propane with the acid catalysts during the liquid-phase alkylation of toluene with 1-dodecene, it can be shown that the reactions at zeolite ITQ-2 and zeolite MCM-22 are mainly occurring at the active centers at the half supercavities at the outer surface of the catalyst. The active centers in the large supercavities play a minor role for the different test reactions, because of the available space being unselective and favoring undesired side reactions. The formation of undesired side products causes fast deactivation of the active sites located in the large supercavities so that they are not accessible for further reactions, which causes the deactivation of the catalyst. The desired reactions on zeolite MCM-22 mainly take place at the active sites at the half supercavities at the outer surface and at the pore openings of the 10-ring pore system of the zeolite structure. This results in a loss of the shape-selective properties of zeolite MCM-22.
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