Philippe Schulz

Mesopore formation during steam dealumination of zeolites: influence of initial aluminum content and crystal size

The texture of a series of dealuminated mazzites, with different compositions and crystal sizes, prepared by combined steam and acid treatments has been investigated by N2 and hydrocarbon sorption, TEM and 129Xe NMR spectroscopy. Crystal size is shown to affect the fraction of mesopores with ink-bottle openings leading to a steep nitrogen desorption at a relative pressure pp0 of 0.42. The composition of the parent zeolite determines the volume and the size of the mesopores, both of them decreasing with the aluminum content. Results are compared to those obtained on five additional zeolite structural types and lead to a unique relation between the mesopore volume generated by dealumination and the aluminum fraction in the starting solid.

Synthesis of Aluminium-rich Zeolite Beta

Hydrothermal synthesis of zeolite beta carried out at low alkalinity levels(OH-SiO2<0.25) from aluminium-rich media(SiO2/Al2O3<19) yielded zeolite beta with an aluminium content higher than 8 atoms per tetragonal unit cell(SiO<2/Al2O3=16). 29Si and 27 Al MAS NMR spectroscopy indicate that all aluminium occupy framework tetrahedrally coordinated sites. The framework negative charge of these aluminium-rich solids is compensated for by sodium and tetraethylammonium cations.

Origin of the optimum in catalytic activity of zeolite beta

Acidity and catalytic properties of zeolite beta depend on the stability of framework aluminium sites. Optimal performances are obtained with crystals of average radius 0.15–0.5 µm and an aluminium content m=Al/(Si+Al)=0.085. H-beta zeolites with such an aluminium content correspond to as-synthesized zeolites in which tetraethylammonium and aluminium contents are equal. Smaller crystals with higher aluminium contents are formed under synthesis conditions that generate tetrahedra distortions and silanol defects, with a significant decrease of the thermal stability.

Insertion of boron in tectosilicate frameworks in the presence of large alkali cations

Abstract Borosilicates of rubidium and cesium with the analcime structure can be formed under mild hydrothermal conditions. The formation of a solid solution is possible for any intermediate composition between borosilicate and aluminosilicate pollucite. K-danburite is obtained in the presence of potassium. The easy formation of borosilicates in the presence of the largest alkali cations is related to the organic-templated synthesis of borosilicate zeolites and confirms the relevance of a hydrophobic effect in the insertion of born in tetrahedral lattice sites.

Effect of Pt particle size on H/D exchange of methane over alumina- and zeolite-supported catalysts

Deuterium exchange in methane was studied over a series of Pt/Al2O3 and Pt/mazzite catalysts. The reaction proceeds by a simple stepwise mechanism with CH3D being the primary product. Large and small metal particles located in zeolite and alumina catalysts have different catalytic activity with respect to deuterium exchange in methane. The rate of reaction per Pt atom was the same on the catalysts investigated for particles > 15–20 A and considerably lower than the rate of reaction on samples with smaller Pt clusters. The activity of small clusters of Pt (diameter < 15–20 A) depended on the nature of the support.

Diffusion of Xylene Isomers in Dealuminated Mazzite Zeolites by the Frequency Response Technique

Abstract The diffusion of o-, m- and p-xylene isomers in a series of mazzite zeolites in their H + form has been studied using the frequency response technique. The mazzite samples have all been prepared by steam dealumination followed by acid leaching of two parent materials synthesised with different aluminium contents: one contains 9 Al atoms per unit cell (Si/A1 = 3) and the other 7 Al atoms per unit cell (Si/Al = 4). The dealuminated materials exhibit similar Si/Al framework ratios but contain differing amounts of extra-lattice aluminium. Analysis of the adsorption/desorption nitrogen isotherms reveals large differences in the size and volume of the mesopores created by dealumination depending on the composition of the parent zeolite. Dealumination of the aluminium rich parent sample yields mesopores 100A in diameter and whose volume amounts to 0.05-0.06 ml/g. The aluminium poor parent material generates smaller mesopores (60-80A) representing a volume of 0.03-0.04ml/g. Frequency response measurements show that the diffusion in these solids is controlled not by the amount of non-framework aluminum but by the number, volume and distribution of mesopores (created during steaming). At 373K and 1 torr, the self diffusion coefficients of p-xylene in the mazzite samples are of the order 0.7-3×10 −11 m 2 s −1 .

Synthesis of high-silica zeolites with unidirectional medium pores systems using nitrogen-free templates.

Zeolites TON, MTT and ZSM-48 have been prepared in the presence of low alcohols and glycols. The nature of the organic agent controls the selectivity of crystallization. The activation pattern indicates a weak organic-lattice interaction.