Catalytic Ring Opening of Decalin – Bifunctional Versus Hydrogenolytic Pathways J

The Future Role of Hydrogen in Petrochemistry and Energy Supply DGMK Conference October 4-6, 2010, Berlin, Germany Catalytic Ring Opening of Decalin – Bifunctional versus Hydrogenolytic Pathways J. Weitkamp*, S. Rabl*, A. Haas*, D. Santi*, M. Ferrari**, V. Calemma** *Institute of Chemical Technology, University of Stuttgart, Germany ** Eni R&M Division, San Donato Milanese, Italy Abstract Ir/silica, Pt/La-X and Rh/H-Beta were prepared and tested in the hydroconversion of cis- decalin at different temperatures. The catalytic tests were carried out under hydrogen in a high-pressure flow-type apparatus at 5.2 MPa. On the three catalysts open-chain decane yields up to 20 % were achieved, which is much higher than the yields reported so far in the literature. Pt/La-X and Rh/H-Beta behave as bifunctional catalysts with a high tendency for skeletal isomerization. On these catalysts the so-called paring reaction via carbenium ions occurs, leading to iso-butane and methylcyclopentane as main hydrocracked products. On Ir/SiO2, carbon-carbon bond cleavage occurs through hydrogenolysis on the noble metal without prior isomerization. As a consequence the product spectrum is less complex than on the bifunctional catalysts which makes the system particularly amenable to mechanistic studies. Introduction Polynuclear aromatics in diesel fuel bring about various undesirable properties, such as poor ignition characteristics and cetane numbers, an increased propensity for soot formation and unfavorable cold-flow properties. For these reasons, the content of polynuclear aromatics in diesel fuels is limited by legislation, and certain refinery streams which are notoriously rich in these undesired components can be blended into diesel fuels only to a limited extent. The selective ring opening of polynuclear aromatics into high-value diesel components, in particular mildly branched alkanes, without degradation of the carbon number, continues to be among the great challenges of catalysis. It is generally agreed upon that such a ring opening must be preceded by a complete ring hydrogenation, and the resulting multi-ring naphthenes would be the true precursors of breaking up the rings. Prior work with model hydrocarbons, typically decalin, revealed that ring opening (or, synonymously, hydrodecyclization) can occur on acidic, bifunctional or metallic catalysts. Acidic zeolites were found to open one ring of decalin so that alkylnaphthenes with a single ring are formed, but at elevated conversions these catalysts tend to degrade the carbon number into C9- hydrocarbons and to deactivate rapidly [1, 2]. Bifunctional catalysts consisting of the acid form of a large-pore zeolite, mostly faujasite or Beta, and a noble metal were predominantly used in hydrodecyclization studies [3-9]. Monofunctional metallic catalysts on non-acidic supports have been scarcely used for ring opening of multi-ring naphthenes. However, among these investigations is a particularly thorough one devoted to the hydrogenolysis of various bicyclic naphthenes over iridium on non-acidic supports. Interestingly, while opening of one ring in bicyclic naphthenes is described in all above- mentioned reports, opening of both rings to the particularly desired alkanes with the carbon number of the feed is even not mentioned - with two exceptions: McVicker et al. [10] and Daage et al. [11, 12] observed traces of open-chain decanes (OCDs) and nonanes (OCNs) in the hydrogenolysis of decalin (bicyclo[4.4.0]decane) and perhydroindan DGMK-Tagungsbericht 2010-3, ISBN 978-3-941721-07-4 77 The Future Role of Hydrogen in Petrochemistry and Energy Supply (bicyclo[4.3.0]nonane), respectively, on 0.9 wt.-% Ir on Al2O3, and noticeable amounts of octanes in the hydroconversion of bicyclo[3.3.0]octane on the same catalyst. Very recently, Mouli et al. [13] reported on the formation of OCDs from decalin on a bifunctional catalyst, viz. Ir,Pt/H-Y zeolite, but their best OCD yields (4 %) and selectivities (5 %) were low. We recently developed experimental techniques tailored for the study of catalytic hydrodecyclization of two-ring naphthenes. The aim of this paper is two-fold: Using a non- acidic 2.6Ir/SiO2 catalyst and two bifunctional zeolite catalysts (1.0Pt/La-X and 5.0Rh/H- Beta), it will be demonstrated that significantly higher yields and selectivities of OCDs can be obtained from decalin than hitherto reported both by hydrogenolytic and bifunctional hydrodecyclization. Moreover, we will outline the very different reaction paths involved in the hydrogenolysis of decalin on iridium and its hydrodecyclization on the bifunctional zeolite catalysts. Experimental Section Preparation and Characterization of the Catalysts Within the frame of this study, three catalysts were prepared, namely non-acidic Ir/SiO2 and two bifunctional zeolites Pt/La-X and Rh/H-Beta. Table 1 gives some important physico- chemical properties of the supports and the metal-loaded catalysts. Table 1: Physico-chemical properties of the supports and catalysts. Sample Specific Pore volume / Metal loading / Metal dispersion surface area / cm3 g-1 wt.-% m2 g-1 Silica 391 1.07 - - La-X 529 0.39 - - Na-Beta 513 0.96 - - Ir/silica - - 2.59 1.02 Pt/La-X - - 1.00 0.43 Rh/H-Beta - - 4.99 0.42 A Varian optical emission spectrometer with an inductively coupled plasma (ICP-OES) Vista- MPX CCD was used for chemical analysis of the samples. The metal loading of all samples is defined as the mass of the metal per mass of dry catalyst. To detect the mass of the dry catalyst, it was first stored in a desiccator over a saturated aqueous solution of calcium nitrate for at least 24 h. The precise water content of the resulting samples was then measured by means of a Setaram Thermogravimetric Analyzer (TGA) Setsys TG-16/18. In the TGA experiment, the sample was heated in a nitrogen flow from room temperature to 600 °C with a heating rate of 20 K min-1. For the determination of the noble-metal dispersion the amount of irreversibly adsorbed hydrogen was measured in a Quantachrome Autosorb-1- C instrument by static volumetry. The samples were reduced similar to the treatment prior to the catalytic experiments and evacuated. After cooling, two isotherms were measured at T = 313 K. The first isotherm was considered to be a combination of physi- and chemisorption, and the second isotherm, measured after evacuating the sample, was interpreted as physisorption only. The difference of these two isotherms originating from irreversibly and strongly adsorbed molecules was applied for calculating the noble-metal dispersion with an assumed adsorption stoichiometry of nH / nnoble metal = 1. Porous properties were measured by N2 adsorption at -196 °C in a Quantachrome Autosorb-1-C instrument after degassing the samples at 350 °C for 16 h. For the calculation of BET specific surface areas p / p0 values 78 DGMK-Tagungsbericht 2010-3 The Future Role of Hydrogen in Petrochemistry and Energy Supply between 0.1 and 0.3 were applied. Ir/SiO2 was prepared by electrostatic adsorption [14] of [Ir(NH3)5Cl]Cl2 onto silica (Aerosil 380, Degussa/Evonik). By addition of NH4OH to a suspension of silica in demineralized water a pH ≈ 10 was obtained in order to deprotonate the silanol groups. Subsequently, an ion exchange with an aqueous solution of [Ir(NH3)5Cl]Cl2 was possible, followed by filtration and washing with demineralized water. The Aerosil particles had an average particle size of 7 nm and were pressed without a binder and again crushed and sieved to a size between 0.20 and 0.32 mm, before they were used in the flow apparatus. The complex was then decomposed in situ in a flow of synthetic air at 150 °C for 3 h, whereupon the metal was reduced in flowing hydrogen at 400 °C for 2 h. The starting material for Pt/La-X zeolite was Na-X (Strem Chemicals) with nSi / nAl = 1.21. It -1 was two times ion-exchanged with an aqueous solution of 0.076 mol l La(NO3)3 for 2 h at 80 °C. After each ion exchange, the zeolite was heated in an air flow at 450 °C in order to allow the La3+ ions to migrate into the small cages [15]. At this stage 90 % of the sodium ions were exchanged with lanthanum ions. The zeolite was suspended in demineralized water, and an aqueous solution of [Pt(NH3)4]Cl2 was added dropwise under vigorous stirring. Afterwards, the suspension was kept at 80 °C under stirring for 4 h, and the resulting solid was filtered off, washed with demineralized water and dried at 80 °C in air. Next, the complex was decomposed in an air flow at 300 °C. The noble metal was reduced in situ in flowing hydrogen at p = 5.2 MPa and T = 380 °C for 2 h. Before using this catalyst in the flow-type apparatus the 2 to 3 μm particles of zeolite X were pressed, crushed and sieved to a size fraction between 0.20 and 0.32 mm. Zeolite Beta with nSi / nAl = 14.0 was synthesized from colloidal silica (Ludox HS-40), aluminum sulfate 18 hydrate and tetraethylammonium hydroxide solution as a template via the dry-gel conversion method [16]. To remove the template, the as-synthesized zeolite was heated in a nitrogen flow from room temperature to 450 °C with a rate of 1 K min-1, holding at 450 °C for 24 h, then switching the gas flow to synthetic air and holding at 450 °C for another 24 h. Subsequently, a two-fold ion exchange was carried out at 80 °C with a 1 mol l-1 solution of NaNO3. Zeolite Na-Beta was washed nitrate-free and suspended in demineralized water at 80 °C, and an aqueous solution of [Rh(NH3)5Cl]Cl2 was added dropwise under vigorous stirring.

Load more