Nicht-oxidative katalytische Dehydrierung von Propan an platinhaltigen Trägerkatalysatoren

Abstract

Ziel dieser Arbeit war es, geeignete Katalysatoren für die nicht-oxidative – also frei von Oxidationsmitteln ablaufende – Dehydrierung von Propan zu dem Kunststoffmonomer Propen zu finden und zu untersuchen. Eine direkte Propandehydrierung soll in naher Zukunft nennenswerte Anteile der weltweiten Propenproduktion von den heute üblichen Produktionsmethoden für Olefine, dem Steamcracken und dem Fluid Catalytic Cracking (FCC), übernehmen. Neben der chemischen Zusammensetzung sollten verschiedene Faktoren, welche Aktivität und Lebensdauer des Katalysators beeinflussen, sondiert werden. Als Standardsystem dienten Katalysatoren mit dem chemischen Grundaufbau Pt-Zn/SiO2. Die Verwendung mikroporöser Materialien mit Porenöffnungen, welche im Bereich der Molekülgröße von Edukt und Produkt liegen, bewirkte zunächst meist nur eine Verschlechterung der Selektivität zu Propen bei mit Platin und Zink beladenen Trägern. Grund hierfür waren verstärkte Tendenzen zum Ringschluss innerhalb der Kanäle und Käfige der verwendeten Zeolithe, so dass ein Großteil des gebildeten Produkts zu Benzol und anderen Kohlenwasserstoffen weiterreagierte. Die Verwendung von weiterporigen Zeolithen und vor allem von meso- und makroporösen Trägerstoffen wie Kieselgel verbesserten diese Situation dagegen erheblich. Weiterhin wurde der Einfluss der Acidität des Trägermaterials auf das katalytische Verhalten betrachtet. Während Trägermaterialien mit sauren Zentren, wie z. B. gemischtes Silicium-/Aluminiumoxid, eine unwesentlich gesteigerte Aktivität auf Kosten eines verstärkten Crackens bzw. einer verstärkten Aromatenbildung mit der Konsequenz einer höheren Verkokung aufweisen, scheint neutrales Kieselgel den besten Träger für Platin-Zink-Katalysatoren abzugeben. Basische Metalloxide wie Magnesiumoxid zeigen dagegen eine zum Teil rapide Desaktivierung oder sind völlig inaktiv. Je stärker sauer ein Träger ist, desto wirkungsvoller katalysiert er die Nebenreaktionen des Crackens und der Verkokung. Obwohl basische Zusätze, wie z. B. CsOH, im vorliegenden Katalysatorsystem eine Verkokung effektiv zu unterdrücken vermögen, führt ein alkalischer Charakter schnell zu einem Verlust der Aktivität. Des weiteren beeinflusst die Acidität des Trägermaterials auch die chemische Zusammensetzung des Kokses und die Reduzierbarkeit der Zinkkomponente. Nur wenige oxidische Stoffe erfüllen überhaupt die chemischen Anforderungen als Träger für dieses katalytische System. Eine weitere Möglichkeit, zu aktiven Platinkatalysatoren zu gelangen, besteht darin, statt Zink andere Haupt- und Nebengruppenmetalle als Promotoren zu benutzen. Außer mit Zinn erhält man z. B. auch mit Kupfer relativ laufzeitstabile Katalysatoren. Die chemische Natur des Promotors beeinflusst nicht nur die Laufzeitstabilität, sondern auch das Ausmaß von Nebenreaktionen, wie vor allem Cracken, Hydrogenolyse und Aromatenbildung. Andere Edelmetalle, wie z. B. Palladium, sind ebenfalls für die Dehydrierung aktiv, allerdings sind sie weniger selektiv. Während ein steigender Platingehalt im Bereich zwischen 0,1 und 1 Massen-% zu einer Absenkung der Produktselektivität führt, ist ein Anstieg des Zinkgehalts von 0,5 auf 7,3 Massen-% mit einem messbaren Rückgang der Ausbeuten aller Nebenprodukte verbunden. Die Wirkungsweise des Promotors Zink kann durch einen elektronischen oder durch einen geometrischen Effekt erklärt werden. Es genügen geringste Mengen davon, um zusammen mit Platin einen stabilen Katalysator zu erzeugen. Daten aus der temperaturprogrammierten Reduktion (TPR) deuten unter üblichen Reaktionsbedingungen auf eine reduzierbare platinreiche Zinkspezies hin, welche im Vergleich zu reinem Platin weniger edel ist. Zusätzlich aufgebrachtes Zink führt zu der Ausbildung einer weiteren unedleren Metallphase. Manche Pt-Zn-Katalysatoren bestehen nach langer Laufzeit bis zu einem Drittel ihrer Gesamtmasse aus Koks. Dennoch sind sie bis zum Ende ihrer Umsetzung gleichbleibend aktiv. Dies legt einen Mechanismus nahe, wonach der entstandene Koks wieder relativ rasch von den aktiven Zentren des Katalysators desorbiert und auf andere Plätze auf der Trägeroberfläche befördert wird, ohne dass es zu einer Vergiftung der aktiven Zentren kommt. Pt-Zn- und Pt-Sn-Katalysatoren mit Kupfer oder Silber als drittem Metall erwiesen sich als aktiv und umsatzstabil. Es scheint, als ob den Elementen Kupfer und Silber die Rolle von stabilisierenden Promotoren zukäme. Sie unterbinden auch den Austrag von katalytisch aktiven Metallen. Die höchsten Umsätze wurden an Pt-Ag-Sn/SiO2 als Katalysatoren erzielt. Durch Optimierung der Reaktionstemperaturen bis auf über 600 °C und der WHSV auf über 22 h-1 lassen sich mit Pt-Zn-Katalysatoren Umsätze von über 50 % bei Selektivitäten zu Propen von über 85 % erzielen. Neben dem Einfluss der verschiedenen Aktivierungsprozeduren auf die Dehydrierleistung wurden auch die chemische Stabilität der wichtigsten Katalysatoren gegenüber Wasserdampf und die Regenerierbarkeit dieser Katalysatoren untersucht.

Recently, the use of platinum-zinc catalysts for the non-oxidative dehydrogenation of propane was described in the patent literature. Such catalysts are claimed to possess high activity, long-term stability and a very high selectivity for propene. In the present study, experiments were carried out in order to examine the potential, the limits and the structural features of this catalytic system. Various supports, promoters and dehydrogenating metals were employed, and the catalytic behavior of the resulting catalysts was tested in the dehydrogenation of propane. The catalytic experiments were carried out in a continuously operated laboratory unit with a fixed-bed reactor, typically with a pure propane feed at ambient pressure, a reaction temperature of 555 °C and a WHSV of 3 h-1. The reaction products were analyzed at various times-on-stream by means of gas chromatography. The catalysts consisted of a small amount (0.5 wt.-%) of noble metal, typically platinum, as the active component and a promoter based, e. g., on zinc or tin, on a porous carrier. From the family of 10-membrered-ring zeolites, ZSM-5 (MFI) and ZSM-11 (MEL) were used as carriers for the active component and the promoter. If the zeolites were used in an aluminum-rich form, aromatization and hydrogenolysis occurred to a significant extent, even if the acid sites of the zeolite were neutralized, e. g., by a sufficient amount of zinc. A completely different catalytic behavior was displayed by catalysts containing silicalite-1 as carrier, i. e., the aluminum-free variant of ZSM-5: Dehydrogenation of propane now became the main reaction, and essentially all undesired side-reactions were suppressed. However, the propane conversion on such catalysts remained relatively low. The results obtained with catalysts based on 12-membered-ring zeolites as carriers can be classified into two categories as well: With zeolites Beta (*BEA) and mordenite (MOR) as carriers for platinum and a promoter, the catalysts showed dehydrogenation activity with a reasonably moderate deactivation; exceptionally good propene selectivities and a significantly less pronounced deactivation were found with catalysts in which zeolite L (LTL) was used as the carrier, especially on 0.6Pt/6.8Zn-L. One reason for this good performance may be the high potassium content of zeolite L which helps to neutralize any acid sites inside the zeolite pores. All catalysts with a zeolitic carrier underwent a more or less pronounced deactivation, presumably mainly by pore mouth plugging. Dehydrogenation catalysts based on mesoporous (MCM-41) or macroporous (SiO2, Al2O3, SiO2-Al2O3) carriers gave significantly better results. In particular, catalysts with silica carriers featured high propene selectivities up to 85 % at ca. 50 % conversion and a good stability. Interestingly, even at high coke loadings amounting to ca. one third of the total mass, such catalysts continued to dehydrogenate propane stably and at high selectivities. Particularly high coke loadings were found on catalysts containing platinum as the active component and zinc as a promoter. It is envisaged that, on such catalysts, coke or coke precursors are formed on the noble metal and subsequently move away from the active centers until they are deposited somewhere on the carrier. Platinum-zinc catalysts were generally found to build up carbonrich coke with a highly aromatic character. Catalysts based on carriers with surface acidity, such as acidic alumina or silica-alumina, exhibited high activities, but limited selectivities. The acid sites favored undesired side reactions, such as dehydrocyclization of propane or propene to benzene and even higher aromatics. Moreover, extensive cracking of the C3 hydrocarbons into methane and ethene (or ethane when hydrogen was present) took place on such catalysts. Finally, catalysts containing platinum on acidic carriers turned out to be more prone to coking and a concomitant deactivation. Catalysts containing platinum on basic carriers such as magnesia or zinc oxide were also tested. They deactivated very fast, even though the coke loadings were comparably low (ca. 7.6 or 0.5 wt.%, respectively). It was concluded that deactivation must have had a completely different origin, here, one possible mechanism being sintering of the noble metal component. Catalysts composed of platinum, zinc and redox-active carriers generally exhibited an unsatisfactory performance: Either the activity was low or they showed a rapid deactivation. A commercially available catalyst consisting of platinum on charcoal, however, gave similarly good results as platinum-zinc/silica: Even at high coke loadings, it retained a high degree of activity. Instead of zinc, a variety of different elements can be used as promoters in catalysts consisting of platinum on a carrier. Tin, copper, silver, iron and chromium were tested in some detail. On Pt/SiO2, none of these elements led to a stable performance in the propane dehydrogenation. Tin as a promoter appears to need an alkaline stabilizer, such as sodium or potassium. Copper seems to be the second best choice as a promoter (after zinc), when catalyst stability is the main criterion. The total amount of coke deposited on the catalyst within a given time-on-stream and the molar hydrogen/carbon ratio of the coke were found to be largely determined by the nature of the promoter: Zinc brings about a relatively fast formation of coke which is poor in hydrogen and highly aromatic, whereas tin-containing catalysts tend to produce only little coke which is very rich in hydrogen and hence of an aliphatic nature. Catalysts with copper as a promoter exhibit an intermediate behavior. While an increase in the platinum content in Pt-Zn/SiO2 catalysts between 0.1 and 1.0 wt.-% led to a decline in propene selectivity, a variation of the zinc content between 0.5 and 7.3 wt.-% caused a gradual decrease of the yields of all byproducts. Temperature-programmed reduction (TPR) of the freshly platinumimpregnated samples indicated a platinum-rich metallic phase which was losing its noble character with increasing zinc content. At least one other less noble, zinc-rich metal phase was coming up with higher zinc contents. In all cases, only a very small part of the zinc present was reducible at temperatures of up to 600 °C. At higher temperatures the total reduction of zinc resulted in the loss of zinc metal from the catalyst. Even if not all zinc was available in a reduced form under reaction conditions, its promoting effect was evident. The loss of zinc metal under reductive conditions, even at lower temperatures, may cause a contamination of parts of the reactor. A high molar excess of zinc compared to platinum compensated the catalytic consequences due to the loss of zinc. In that manner, even low amounts (e. g. 0.5 wt.-%) of zinc were sufficient to create a stable catalyst. Even a loss of platinum metal from the catalyst was observed, but the amounts were lower compared to those of zinc. A promoter-free sample lost almost 90 % by weight of its platinum during 14 h on-stream, along with a strong deactivation, and a platinumzinc catalyst with only 0.1 % by weight of platinum showed some slight deactivation, too. Some advantageous effects could be achieved by using bimetallic promoter combinations. Although neither copper nor silver alone were good promoters, their combinations with either zinc or tin resulted in highly active, selective and stable catalysts. For example, Pt-Cu-Sn/SiO2 or Pt-Ag-Sn/SiO2 catalysts displayed long-term stabilities which alkaline-free Pt-Sn/SiO2 samples never reached. With Pt-Ag-Sn/SiO2, a propane conversion of 34 % and a propene selectivity of 95 % could be attained at TR = 555 °C. This is very close to the equilibrium conversion and represents the best result achieved in this work. Another positive effect of the use of copper or silver is a decrease of the metal loss. Even the loss of zinc was remarkably reduced. TPR data suggested a strong interaction between platinum and the promoter and stabilizer components. Both bimetallic and trimetallic alloys may be present under reaction conditions.

Finally, a variation of reaction parameters was carried out in order to obtain an optimization of the catalytic system Pt-Zn/SiO2. Since non-oxidative dehydrogenation represents a strongly endothermic reaction, higher reaction temperatures cause the thermodynamically limited paraffin / olefin equilibrium to shift towards higher olefin yields. Reaction temperatures of around 600 °C led to conversions near 50 % at a propene selectivity of ca. 85 %. At still higher temperatures, the deactivation rates of all tested catalysts increased significantly. Most catalysts reach their maximum yield of propene at reaction temperatures around 630 °C, if the temperature is gradually increased from 555 °C to 780 °C. In several experiments the influence of the activation procedure on the activity and stability of the catalysts was screened. Furthermore, the addition of steam to the propane feed was studied. Whereas zinccontaining catalysts showed some slight deactivation during steam contact under reaction conditions, the copper-tin- and silver-tin-promoted catalysts remained stable.