Aiming at the identification of a suitable synthesis route to zeolite ZSM-5 with acidic properties and crystal diameters of about 100 nm or less, we have evaluated four different preparation methods. Three of these have been adopted from the literature, and one was developed by ourselves. The products, all of which synthesised from mixtures with a fixed Si/Al ratio of 60, were inspected by means of X-ray diffraction, scanning electron microscopy, laser-Doppler anemometry and temperature-programmed desorption of ammonia. Two methods turned out to be successful and well reproducible. The first one comprises hydrothermal crystallisation from clear solutions under autogenous pressure and has been described previously by van Grieken et al. The other approach is based on the use of colloidal silicalite-1 seed crystals in an open-vessel crystallisation at atmospheric pressure. The crystal size distribution and the Si/Al ratio of the products can be well controlled. The products from both synthesis types could be transferred into the H-form by conventional means without causing collapse of the crystal structure.

Many catalyst shaping procedures such as spray drying or extrusion involve aggregation of zeolite crystals with particles of the binder or matrix in the liquid phase. A better control of these processes should result in catalyst bodies with improved diffusional and mechanical properties. We have studied the attractive and repulsive interactions between zeolite crystals in aqueous sols and suspensions by monitoring the zeta ( ) potential and the mean aggregate size at varying pH values. The zeolites under investigation were of the structure types MFI, *BEA, and OFF, with single-crystal diameters ranging from 65 to 3500 nm and with Si/Al ratios between 5 and >2000. The following major observations were made: (i) at the isoelectric point (IEP, defined as the pH at which ) 0 mV), zeolites undergo spontaneous aggregation; (ii) framework aluminum causes a shift of the IEP to lower pH values; (iii) the IEP shifts to very low values as the template of zeolites containing aluminum is removed; and (iv) aggregation and peptization are completely reversible. Among the samples under investigation, no effect of the structure type could be observed. The size of the crystals can affect the slope of the -potential curve as a function of the pH, but not the position of the IEP.

Dedicated to Professor Andre M. Braun on the occasion of his 60th birthday Recovery of zeolites from aqueous media can be a very difficult operation, when the crystals have colloidal dimensions. The solid-liquid separation can be facilitated by processing at the respective isoelectric point (IEP) where the sol is unstable and the crystals form aggregates. The addition of salts results in a further improvement, because the zeolite agglomerates become larger, and the pH range of flocculation is broadened. This wider operational window, in particular, is of importance for the recovery of zeolites that would dealuminate or collapse at their respective IEP.Introduction. ± Handling of aqueous sols or suspensions of zeolite crystals is a frequently recurring operation in the manufacturing of zeolite-based catalysts or adsorbents. Starting with the zeolite synthesis in aqueous medium, the typical workup comprises repeated ion exchange in aqueous solutions and, finally, a wet shaping process during which bodies of suitable size, shape, strength, pore texture, and site distribution must be formed. Large agglomerates of crystals are beneficial, when zeolites must be recovered from their mother liquor or from ion-exchange solutions, whereas isolated zeolite single crystals, dispersed in the porous matrix of a shaped particle, are highly desirable when catalytic properties are to be optimized. These opposing requirements for efficient solid-liquid separations, on one hand, and optimum product quality, on the other hand, demand for means to control the particle size in a reversible manner.It is well-known that the stability of particles with respect to aggregation depends on the balance between attractive London-Van der Waals and repulsive electrostatic forces. The magnitude of the electrostatic repulsion as a function of the distance from the particle surface can be influenced. It depends on the ionic strength in the diffusive layer and on the surface potential (Nernst potential), which, in turn, can be altered by adjusting the pH value. While the Nernst potential is not experimentally accessible, the electrokinetic potential at the shear plane, the zeta (z) potential, can be monitored. Dispersions can be regarded as stable, when the zeta potential is higher than ca. j 30 mV j , whereas the particles tend to form aggregates near the isoelectric point (IEP), which is defined as the pH at which the zeta potential is zero. The measurement of the zeta potential, therefore, is a widely used tool to characterize the stability of disperse systems. Electrokinetic data of numerous materials, in particular, of inorganics are documented in the literature.

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The term quality is used with regard to the assessment of media content. Such assessments can have various foci—for example take different subjects, use different criteria, and reach different results, that is, be positive or negative. In any case, quality is not an isolated characteristic; it is a relation between a characteristic and a set of values. This set of values differs according to the perspective of the assessment. One can broadly differentiate between two perspectives: the perspective of society as a whole, where the requirements for the evaluation of quality are derived from universal social norms and where criteria like relevance, diversity, or objectivity are used; and the perspective of single individuals, where the connection between recipients' assessments of quality and their overall usage behavior is of particular interest.