Mia Tielen

PREPARATION OF HIGH-SILICA FAUJASITES BY TREATMENT WITH SILICON TETRACHLORIDE

NaY has been dealuminated in highly controlled conditions by SiCl4. The materials thus obtained, after subsequent washing and ammonium exchange, were characterized by chemical analysis, thermogravimetry, X-ray diffraction, infrared spectroscopy of the framework and surface hydroxyl groups and 29Si nuclear magnetic resonance spectroscopy.In order to obtain highly crystalline materials in a reproducible way, a very specific dealumination and washing procedure has to be followed. The mechanism of the dealumination is complex and always combines direct isomorphic substitution with acid leaching of aluminium. The materials obtained have continuously varying properties as far as unit-cell dimensions and framework vibrations are concerned. In contrast, the amount of extra-lattice aluminium retained in the zeolite reaches a maximum at intermediate degrees of dealumination.

Active sites in zeolites: Part 6. Alcohol dehydration over alkali cation-exchanged X and Y zeolites

2-Propanol and 2-butanol have been dehydrated over alkali cation-exchanged zeolites X and Y. The first reaction was carried out in a differential continuous-flow reactor, the second one in a recirculation reactor, using deuterated alcohols. From isopropanol, propylene and diisopropyl ether are formed as dehydration products. The kinetic data can be fitted to a Langmuir-Hinshelwood equation of the form: r = kKAPA(KAPA + KWPW)−1, where the rate of the reaction is slowed down by the desorption of water. The inter- and intramolecular dehydrations are parallel reactions catalyzed by weak hydroxyl groups. The selectivity is determined by the concentration and acidity of these groups. A detailed reaction mechanism (of the E1 type) is advanced which accounts for the observations.

Redox behaviour of transition metal ions in zeolites. Part 4.—Kinetic study of the reduction and reoxidation of copper–Y zeolites

The uptake of hydrogen and oxygen by (Cu, Na)–Y zeolites has been followed kinetically in a volumetric system. The temperature region investigated was between 273 and 773 K. The influence of several parameters on the reducibility of Cu2+ has been investigated: the degree of cation exchange, the presence of Cu2+ complexing ligands, the procedure of sample preparation and degassing.The formation of Cu+ ions is determined mainly by the mobility of Cu2+ ions in the zeolite. The preparational and degassing procedure has a drastic influence on the reducibility of the Cu2+ species. The rate of formation of Cu° is determined by the diffusion of the reactant through the pores.Cu+ ions and small Cu° clusters are reoxidized reversibly, the mobility of Cu+ ions determining the rate of the process. The reoxidation of Cu° external to the zeolite consists of a homothetic attack of oxygen on the Cu° particles.

Relation Between Paraffin Isomerisation Capability and Pore Architecture of Large-Pore Bifunctional Zeolites

Abstract In this study long-chain n-paraffins in the range from nonane to heptadecane were converted over bifunctional zeolite Y, USY, ZSM-3, ZSM-20 and BETA catalysts in the presence of hydrogen. n-Paraffins can be selectively converted into branched isomers, provided the hydrogenation-dehydrogenation function is in balance with the acid function of the catalyst. The equilibration of the two catalytic functions depends on the catalyst as well as on the carbon number of the feed. For each zeolite there exists an optimum loading with noble metal which depends on the chain length of the n-paraffin feed. If the geometry and dimensions of the zeolite pores are such that hydrocracking is suppressed by molecular shape-selectivity, the yield of isomerisation is improved substantially. The presence of mesopores in zeolite Y crystals is reflected in the isomerisation yield.

Attempts to Rationalize the Distribution of Hydrocracked Products. III. Mechanistic Aspects of Isomerization and Hydrocracking of Branched Alkanes on Ideal Bifunctional large-pore Zeolite Catalysts

Detailed compositions of the isomerisation and hydrocracking products obtained in the reaction of 2-, 3-, 4- and 5-methylnonane, 2,6- and 3,5-dimethyloctane and 2,4,6- and 3,3,5-trimethylheptane over Pt/USY using a continuous flow reactor and on-line high-resolution gas chromatography are reported. These product distributions can be rationalised and understood by the reaction schemes established for the conversion of long chain normal alkanes over ideal bifunctional large-pore zeolite catalysts. The composition of the hydrocracked products from the mono-, and di-branched feed molecules mentioned is very similar and identical to those from n-decane, previously reported. This indicates that the feedstock molecules are equilibrated in all cases, before substantial cracking occurs. With the tribranched molecules mentioned, the situation is totally different, as the formation of ALPHA-ALPHA-GAMMA-branched isomers, which are cracked very rapidly, requires in the worst case only a few methyl-shifts.

Silicalite-1 Zeogrid: A New Silica Molecular Sieve with Super- and Ultra-Micropores

Spherical, micrometer-sized particles with a layered structure were obtained by precipitation of a Silicalite-1 zeolite nanoslab suspension upon addition of cetyltrimethylammonium bromide (CTMABr) and subsequent calcination. The material had a specific micropore volume of 0.69 cm3 g–1, distributed over super- and ultra-micropores. The formation process of this peculiar microporous solid was studied using X-ray diffraction (XRD), 29Si MAS NMR spectroscopy, thermogravimetry (TG), and nitrogen adsorption. In the precipitate, the Silicalite-1 nanoslabs were laterally fused into nanoplates and stapled into layers with intercalated surfactant molecules. Removal of the surfactant through calcination caused facial fusion, besides additional lateral fusion, of the nanoplates. Empty spaces left lying laterally between individual nanoplates were responsible for the super-microporosity. The ultra-micropores were zeolitic channels inside the fused nanoplates. The potential of these Silicalite-1 zeogrids as molecular sieves was demonstrated with pulse gas-chromatographic separation of alkane mixtures. The mass-transfer resistance of a packed bed of zeogrid particles was considerably lower than of compacted zeolite powder.

Comparison of acid- to metal-catalysed conversion of n-decane and cyclodecane on ZSM-5 and faujasite-type zeolites

Abstract The metal-catalysed and bifunctional mechanisms for the conversion of n-decane and cyclodecane are reviewed. It is also shown experimentally that the selectivities of both mechanisms can be manipulated by supporting the catalytic functions on zeolites. As an example, the selectivities obtained in a flow reactor using the feedstock molecules mentioned are discussed. Typical samples have been prepared, i.e. Al-free silicalite and totally dealuminated Y-zeolite, as well as high silica-alumina ZSM-5 and ultrastable Y-zeolite. Platinum incorporation via specific methods gives finely dispersed metal on an inert support for the first two materials or on a low-acidity support for the latter two zeolites.

Framework and Non-Framework Al Species in Dealuminated Zeolite Y

Abstract The hydrothermal dealumination of zeolite Y was followed by 29Si and 27Al MAS NMR. From deconvoluted MAS NMR spectra the amount of the different silicon and aluminium species, formed during steaming, could be determined. Next to well documented 27Al signals at 60 ppm of tetrahedral framework Al and at 0 ppm of octahedral Al, lines at 50 ppm, 30 ppm and in the region of 4–0 ppm are observed, the chemical nature of which is discussed. Zeolite Y with framework Si/Al ratio from 3.5 to 17 contains non-negligible amounts of extra-framework silicon and aluminium species, which complicate the determination of the framework Si/Al and non-framework:framework Al ratios. A considerable amount of extra-framework aluminium is missing in the 27Al MAS spectra. This ‘NMR-invisible’ aluminium, assigned to oligomeric alumina species, can be made ‘NMR-visible’ by complexation with acetylacetone under specific conditions.

Stereochemical Effects in Shape-Selective Conversion of Hydrocarbons on Zeolites

Abstract A literature overview is made on the conversion of mono-, bi- and tricyclonaphtenes on zeolites and new data are shown on the effect of pore size and geometry on the selectivity in the bifunctional conversion of cyclooctane over faujasite, ZSM-5, and ZSM-11 zeolites. The strcuture of the pores definitely induces remarkable changes in selectivity although interpretation of them in terms of spatio-stereo shape-selectivity or product diffusion is not always straightforward.