Extrusion: playing the SAXS for success!

Alumina, the archetypal material used for hydrotreatment catalyst supports, has a porous structure at supramolecular scales, determining the transport of oil feeds within it. This structure is largely inherited from that of the boehmite pastes from which the supports are made.

These pastes are composed of a concentrated dispersion of (nano) particles. Their transformation is achieved via individual operations, such as: peptization^a, extrusion and calcination, the step at which the boehmite is converted into alumina.

However, the extrusion of boehmite pastes is sometimes difficult — or even impossible — for some dispersions, and it is important to understand why if we want to improve the materials of these supports.

Optical observation under polarized light of boehmite pastes reveals strong birefringence under flow conditions, a property that persists at rest. While this observation is a qualitative reflection of a change of organization within the pastes, it cannot adequately describe the latter on a particulate scale.

Boehmite pastes with increasing particle concentrations and controlled salinity were therefore observed in flow conditions in the SOLEIL synchrotron by small angle X-ray scattering (SAXS). The flow was imposed by a rheometer, simultaneously enabling precise control of shear conditions and measurement of rheological characteristics¹.

The images taken under the effects of shear (figure) demonstrate a preferential direction of the particles, which could then be quantified.

Examination of all the data shows that this direction is primarily controlled by the particle concentration of the pastes. To optimize their extrusion, more in-depth analysis is required in order to determine the relationship between the microscopic structure and the rheology of boehmite pastes.