Aqueous suspensions of colloidal boehmite particles are frequently used for the production of catalyst supports. The synthesis process involves an acidification step with nitric acid, turning the suspension into a colloidal gel due to the emergence of attractive forces between the particles. The suspension processability requires maintaining the sample's viscosity within a specific range, which is achieved by applying a mechanical shear. The latter impacts the microstructure of the colloidal network and affects the terminal characteristics of the catalyst support obtained after drying and calcinating the gel.<br/>In order to understand the mechanisms by which the shear modifies the structure of the gel after flow cessation, we study the rheological properties of a boehmite gel depending on the shear rate used to rejuvenate its microstructure. We have identified a critical shear rate that separates two distinct behaviors. Soft solids obtained following a rejuvenation step at low shear rates show a large elastic modulus with a glass-like viscoelastic spectrum and a low yield strain. On the contrary, a rejuvenation step at high shear rates leads to the formation of soft gels characterized by a three times lower elastic modulus and a higher yield strain. Such striking differences in the same sample's rheological properties indicate that two different microstructures are selected during the rejuvenation step. This scenario is confirmed by small-angle X-rays scattering measurements, which show that gels formed following a low shear rate display an anisotropic microstructure, whereas gels obtained following a rejuvenation step at high shear rates are isotropic.<br/>All in all, our results show that shear history is a promising candidate for tailoring the microstructure of catalyst supports obtained from these soft precursors.

Shear-induced memory effect in boehmite gels

ACS Spring 2022

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