Alumina is an essential catalysts support used in heterogeneous catalysis in the refining industry. This material has a multi-scale hierarchical porosity that extends from nanometer to micron-sized pores.
What operations are required to convert boehmite into alumina?
The alumina support results from the calcination (topotactic transformation) of boehmite pastes and it has been shown that its porous structure originates from the structure of boehmite nanocrystal aggregates in aqueous phase.
Upstream, these pastes are the result of unit operations, such as synthesis, drying, peptization, filtration and shaping (by drying or by extrusion).
Boehmite paste: a structure that demands further study
The anisotropic nature of boehmite nanocrystals means these systems – according to their concentration and salinity – have an original phase diagram that shows (as isolated nanocrystals) a transition between an isotropic phase and a phase close to a nematic liquid state.
Naturally, the rheology depends on the structure and amount of the solid content in the paste. While studies have been conducted on paste structure at the scale of a few nanocrystals, more intensive work needs to be carried out at larger scales.
Two stages of research work
This research theme involves all the partners (PHENIX, IPCMS, IFPEN, CRMN). The objectives are to:
- understand the relationship between the initial structure of boehmite aggregates and the morpho-structural characteristics of alumina nanocrystals in the final extrudate through the multi-scale three-dimensional characterization of the porous network of the samples and in-situ monitoring of the transformation;
- study the multi-scale dynamics of the fluids confined within the materials by analyzing the interaction of molecules with the surfaces of the pores and, at a larger scale, the relationship between the transport properties and the architecture and topology of the porous media.
Using a few selected boehmites, we will conduct a combined structural and molecular transport analysis on samples ranging from a more or less concentrated colloidal dispersion to a shaped boehmite (by drying or kneading-extrusion).
First, we will study the impact of extrusion on the structure of boehmite pastes. Indeed, the extrudate is obtained by applying a mechanical stress and this needs to be studied in order to better understand the transition between concentrated colloidal suspension and the porous solid state before calcination.
To achieve this, the research will focus on two areas:
Focus 1: monitoring the structural and textural evolution of the system
Initially, as part of a first thesis project (Nivedita Sudheer) and a post-doctoral internship (Sumit Mehan), we will monitor the structural and textural evolution of the system, particularly before and after extrusion.
A multi-scale study based on 4 different experiments
In order to carry out a multi-scale study of this complex material (illustration below), we plan to compare the findings of four types of experiments, i.e.:
- small-angle scattering (X-rays or neutrons) conducted by PHENIX and IFPEN;
- high-resolution electron microscopy and tomography at IPCMS;
- cryo TEM (for the observation of suspensions) at IPCMS;
- x-ray tomography, recently developed at the SOLEIL facility on the ANATOMIX line, conducted by PHENIX and IFPEN.
Time resolved studies by using DLS, an approach that has already produced particularly interesting results, will be implemented. Comparing these different techniques will enable us to monitor the structure in direct and/or reciprocal space, ranging from 0.5 nm to µm.
At the same time, the rheological properties of the different paste states will be measured to establish the structure-transport relationship in these highly concentrated environments.
According to the results of the multi-scale study, we will conduct extrusion experiments coupled with tomographic (X-ray and/or neutron) analyses to highlight the role of boehmite particle orientation and segregation at the liquid/solid interface…
Liquid cell electron microscopy in a cell closed by two electron-transparent membranes should enable us to obtain new information (in the direct space) about the transformation undergone by the initial boehmite that is at the origin of the final extrudate.
Illustration of the proposed multi-scale 3D approach to be used to determine how the morphology and texture of the boehmite change during the various alumina preparation steps
Focus 2: studying the multi-scale dynamics of confined fluids
Secondly, as part of a thesis project starting in the autumn of 2020 (Alice Ducroix), we will study the multi-scale dynamics of fluids confined within these materials. The fluids may be water, alcohols or simple organic solvents such as toluene. The aim is to gain a better understanding of how these molecules interact with the surface of the porous medium.
Understanding the interaction between fluids and porous materials at a large scale
For larger scales (µm or more), we will need to quantify the diffusive transport properties in relation with the architecture and topology of the porous material, particularly when the system changes its state from concentrated boehmite suspension to extrudate.
This second study will essentially use:
- the nuclear magnetic resonance techniques (NMR) available at PHENIX, IFPEN and CRMN;
- in-situ liquid transmission electron microscopy at IPCMS.
Concerning NMR technics, two protocols will be used. First, the low field-cycling NMR relaxometry (NMRD) will be used , providing information about solvent dynamics near the porous interface; Second, the pulsed field gradient technique, able to analyze the molecular diffusion propagator and test the transport capabilities at a large scale in these materials.
Gathering information at a smaller scale
At a more local scale, we plan to use quasi-elastic neutron scattering ("time-of-flight" or "spin-echo") to obtain information about the solvent's molecular dynamics for times less than the nanosecond.
Finally, recent NMR techniques used at IFPEN and PHENIX ("2D T2-D-T2") will enable to conduct a mesoscopic transport analysis focusing on the exchange between families of pores of different sizes.