Yvonne Traa

Zeolite-based materials for the selective catalytic reduction of NOx with hydrocarbons

This review covers all aspects of the selective catalytic reduction of nitrogen oxides with hydrocarbons (HC-SCR) over zeolite catalysts, mainly from the viewpoint of their potential for a practical application in exhaust gases from lean-burn engines. Emphasis is placed on HC-SCR over metal-containing zeolites with the exception of copper zeolites. Other items which are addressed are the combination of various metals on zeolites, combined exhaust gas purification systems and mechanistic considerations.

Isobutane/butene alkylation on solid catalysts. Where do we stand?

Abstract Liquid-phase processes with concentrated sulfuric acid or hydrogen fluoride as catalysts are currently being used in petroleum refining for the manufacture of alkylation gasoline from isobutane and butenes. While the product, i.e., alkylate, is a most valuable gasoline component, the existing processes for its manufacture are less satisfactory. Replacement of the liquid catalysts by a solid acid is an important target of modern research. In the past two decades, a large number of solid acids have been scrutinized, and at least four developments were driven till the pilot plant stage. In this paper, an attempt is made to rationalize, on a mechanistic basis, the selectivity loss almost always encountered with solid acids after relatively short times-on-stream. Suggestions are made concerning a more target-oriented research on isobutane/alkene alkylation in the future.

An EPR study on the enantioselective aziridination properties of a CuNaY zeolite

A CuNaY catalyst was prepared and used to study the enantioselective aziridination of styrene, with PhINTs as the nitrogen source, in the presence of a bis(oxazoline) chiral modifier. The chiral modifier used was a diimine ligand, (S)-(−)-2,2′-isopropylidenebis(4-phenyl-2-oxazoline). EPR spectroscopy provides the first direct experimental evidence for the formation of a copper(II)-bis(oxazoline) complex inside the Y zeolite pores after stirring the calcined catalysts with the chiral ligand using acetonitrile as solvent. The copper complexes possess square pyramidal and square planar symmetries, with spin Hamiltonian parameters analogous to those of the equivalent homogeneous complex dissolved in solution. These copper(II) complexes accounted for at least 40% of all available copper within the ion exchanged CuNaY catalyst and represent one Cu(II)-bis(oxazoline) complex per supercage. The remaining uncomplexed Cu(II) ions remain solvated to the acetonitrile molecules. After the aziridination reaction was carried out in the presence of styrene and PhINTs, EPR evidenced the selective loss of the signal due to the copper(II)-bis(oxazoline) complex with square pyramidal and square planar symmetries but practically no loss in overall Cu(II) content. This was explained on the grounds of a changing co-ordination environment of the encapsulated complex. However when PhINTs was added separately to the catalyst a dramatic loss in Cu(II) signal intensity was observed. These results are discussed in terms of the reaction mechanism in operation.

Preparation, Characterization and Catalytic Properties of Cobalt Phthalocyanine Encapsulated in Zeolite EMT

The synthesis of cobalt phthalocyanine (CoPc) in the large cages of zeolite EMT via the ship-in-the-bottle strategy is described. A detailed physicochemical characterization together with mass balances reveals that loadings of up to one CoPc complex in every sixth supercage can be achieved. CoPc encaged in zeolite EMT is an active catalyst for the oxidation of ethylbenzene with molecular oxygen. Maximum turnover numbers (defined as n E-Bz, converted /n CoPc, encaged) of ca. 22 000 are observed.

Gallium-containing zeolites – valuable catalysts for the conversion of cycloalkanes into a premium synthetic steamcracker feedstock

Gallium is incorporated into zeolite H-ZSM-5 by reductive solid-state ion exchange, i.e., by heating a physical mixture of gallium oxide and H-ZSM-5 in a hydrogen flow at 600°C. The samples are characterized by AES/ICP, XRD, FTIR and MAS NMR spectroscopy. Methylcyclohexane is converted over these catalysts in a hydrogen atmosphere at 6 MPa and 250 to 400°C, yielding a high-quality synthetic steamcracker feedstock (consisting mainly of ethane, propane and n-butane). Mechanistic aspects of the reaction are discussed, and the importance of non-classical Haag-Dessau cracking is emphasized.

An alternative method for the production of second-generation biofuels

Direct coal liquefaction (DCL) is an industrial technology for the production of liquid fuels by high-pressure and high-temperature hydroconversion of coal. The technique can also be applied to biocoals produced by hydrothermal carbonization (HTC) of biomass waste. The biofuels produced are promising with regard to the H/C molar ratio, higher heating value and oxygen content.

Aromaten: Von wertvollen Basischemikalien zu Überschusskomponenten?

Mit dem Inkrafttreten der zweiten Stufe des Auto-Ol-Programms der EU im Jahr 2005 muss der Aromatengehalt des Benzins erheblich gesenkt werden, was zu einem Aromatenuberschuss fuhren konnte. Die Quellen fur aromatische Kohlenwasserstoffe und deren Verwendung in der Petrochemie werden beschrieben. Moglichkeiten zur Vermeidung eines Aromatenuberschusses werden aufgezeigt, wobei sowohl die Reduzierung der Aromatenproduktion als auch das Wachstumspotential der Folgeprodukte von BTX-Aromaten diskutiert werden. Sodann werden Verfahren zur Ringoffnung von Aromaten beschrieben. Naher eingegangen wird auf ein neues Verfahren zur katalytischen Umwandlung von Aromaten aus dem Pyrolysebenzin von Naphtha-Steamcrackern in einen synthetischen Steamcracker-Einsatz bestehend aus Ethan, Propan und n-Butan. Diese Umsetzung der Aromaten mit Wasserstoff kann einstufig an bifunktionellen Zeolithen oder zweistufig durch Vorhydrierung zu Cycloalkanen mit anschliesender hydrierender Ringoffnung an sauren Zeolithen erreicht werden. Aromatics: From Valuable Base Chemicals to Surplus Components? With the advent of the second stage of the European Auto Oil Programme in the year 2005, the aromatics content of gasoline has to be reduced significantly, which could lead to an oversupply of aromatics. The sources of aromatic hydrocarbons and their petrochemical use are described. Options for avoiding a surplus of aromatics are discussed, i.e., diminishing the production of aromatics and intensifying their conversion into valuable products. Finally, a novel catalytic process for hydrogenative ring opening of aromatics is introduced which allows the conversion of pyrolysis gasoline from naphtha steamcrackers into a high-quality synthetic steamcracker feedstock composed of ethane, propane, and n-butane. There are two process variants, namely the direct conversion of aromatics on bifunctional zeolites or a two-stage process comprising a conventional ring hydrogenation to cycloalkanes followed by ring opening of the latter on acidic zeolites.

Hydroconversion of methylcyclohexane on TEOS-modified H-ZSM-5 zeolite catalysts: Production of a high-quality synthetic steamcracker feedstock

Abstract With the advent of the European Auto Oil Programme and comparable legislation in other parts of the world, the aromatics content of gasoline has to be reduced considerably. Therefore, new outlets for surplus aromatics have to be found. Mixed n -alkanes with two and more carbon atoms (C 2+ - n -alkanes) represent a high-quality synthetic feedstock for steamcrackers. Thus, the hydrogenation of aromatics to cycloalkanes followed by their hydroconversion into C 2+ - n -alkanes (predominantly ethane, propane and n -butane) could become a useful process option for converting surplus aromatics into a synthetic steamcracker feed. During hydroconversion of methylcyclohexane to C 2+ - n -alkanes, the catalytic properties of acidic zeolite H-ZSM-5 can be remarkably improved by chemical liquid deposition of tetraethoxysilane. Thereby, the yield of the desired C 2+ - n -alkanes at, e.g., 400 °C can be enhanced from around 77% to nearly 82%. Simultaneously, a drastic diminution of the concentration of external Bronsted acid sites and a significant decrease of the total concentration of Bronsted acid sites were observed by adsorption of, respectively, collidine and pyridine monitored by FTIR spectroscopy. Nitrogen adsorption experiments revealed a slight decrease of the micropore volume. These data are consistent with an extensive deactivation of the active sites on the external surface and in close proximity to the pore openings, the latter leading to pore mouth narrowing or, more likely, to partial pore mouth plugging.

Characterization of the Pore Size of Molecular Sieves Using Molecular Probes

This Chapter deals with the evaluation of the pore size of crystalline microporous solids with molecular probes. Only methods where the dimensions of the probe molecules and the pore width are similar are discussed. This means that the adsorption or the selectivities and/or conversions of the reaction depend, in an unambiguous manner, on the pore width. After the introduction, in Sect. 2, some general aspects are discussed that are important for the detailed understanding of the methods covered, namely dimensions of probe molecules and intracrystalline cavities as well as molecular sieving. Section 3 is devoted to adsorption, i.e., the use of molecular probes without chemical reactions. This includes mainly the characterization of various zeolites in comparison with one another and the discussion of molecular probes for zeolites with different pore sizes. Finally, in Sect. 4 shape-selective catalytic reactions are reviewed which have been employed for characterizing the width of micropores. As an introduction, the basics of shape-selective catalysis in microporous materials are discussed. The main test reactions dealt with are, for monofunctional acidic molecular sieves, the competitive cracking of n-hexane and 3-methylpentane (Constraint Index) as well as the isomerization and disproportionation of m-xylene. For bifunctional molecular sieves, the isomerization and hydrocracking of long-chain n-alkanes (Refined or Modified Constraint Index) as well as the hydrocracking of C10 cycloalkanes such as butylcylohexane (Spaciousness Index) are reviewed.

Oscillation of the NOx concentration in its selective catalytic reduction on platinum containing zeolite catalysts

Oscillations of the NOx concentration were observed in the selective catalytic reduction of NOx with propene on Pt-V/H-zeolites. It is assumed that the oscillations have their origin in a periodic transition of Pt between a metallic and an oxidic phase, which is catalyzed by V.