Digital simulation of diffusion in a γ−Al2O3 porous network and characterization by PFG-NMR

The scope of this thesis is to better understand the world around us and our perception of its phenomena. This study is embedded in a wide sphere of interest that comprehends mathematical modeling, porosity, diffusion and catalysis. The goal is to understand how Fickian diffusion may be simulated into a reproduced porous network of γ−Al2O3 with an innovative analysis by a PFG-NMR characterization. A proper description of the topology of the porous network is in fact vital to understand how to accurately design the catalyst, enhancing the prediction of reactions. γ-Alumina is the most important phase of alumina. It presents the higher surface area, porosity and it has been studied and exploited for the acidity of its surface that promotes catalytic performances. The industrial significance of this phase is evident as it is used as a catalyst support in numerous fields where heterogenous catalysis is involved: from automotive to petroleum refining or fuel cells applications. Because of its role, since 1935 alumina has been deeply studied in order to understand its structure and how to reproduce it. Numerous experimental strategies can be adopted to represent its topology and, in this work, PFG-NMR is used. Pulsed Field Gradient- Nuclear Magnetic Resonance, PFG-NMR is a technique that allows to study molecular dynamics in chemical systems in porous media. Nuclear magnetic resonance creates the possibility to look inside the porosity like a "spy" providing information of the internal structure and what is happening within, without any external interference on the macroscopic properties. Because of its nature, PFG-NMR can be used to study diffusion in porous. The experimental signals obtained by PFG-NMR can be in fact correlated to a self-diffusion coefficient (which is an apparent diffusion coefficient) that when related to the ideal Fickian coefficient allows to address the tortuosity of the system. The project is developed in this field but the innovation is the digital reproduction of both the porous network and the PFG-NMR analysis. Some modifications and initial hypothesis were needed, but the signals of the characterization and the results of the diffusion coefficient, allowed to calculate the tortuosity of the model. The outcomes of the digital simulations achieved have been subsequently correlated to the results of empirical experiences that have been both found in literature and in the results of doctorates studies at the research institute IFP-EN, situated in Lyon, that has also supervised this project.