Effect of Electrolyte Addition on the Colloidal Stability of Aqueous Zeolite Sols

Abstract

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.

Colloidal Processing and Thermal Treatment of Binderless Hierarchically Porous Zeolite 13X Monoliths for CO2 Capture

“…2 show that the zeolite 13X particles have an isoelectric point (IEP) at pH 4.7. Previous work has shown that the IEP of zeolites depends on the elemental composition 34 . The reduction in the IEP of MFI zeolite with the increasing aluminum content was related to the higher acid strength for Al‐containing surfaces compared with silicalite crystal surfaces 34,35 .…”

Comparing synthesis routes to nano-crystalline zeolite ZSM-5

“…We have shown recently that it is possible to control the agglomeration and peptisation of such colloidal zeolites in aqueous media [10]. This is a prerequisite for their efficient solidliquid separation [11,12], and for the manufacturing of catalysts containing small zeolite crystals.…”

Small molecules induce mesocrystal formation: nanoparticle aggregation directed by self-assembling calixarenes

“…The zeta potential of the precipitated barium sulfate in the 50 presence of 2 (0.0134 mM) was measured at -17mV, which, while being slightly negative, it is still within the range of zeta potentials (±30mV) where the system is expected to be unstable and would spontaneously coagulate. 10 Thus, charge stabilization is unlikely to be a factor in barite mesocrystal formation in the 55 presence of 2. Stabilization of some kind must be operating in order to avoid random aggregative processes.…”