Influence of Asphaltene Aggregation on the Adsorption and Catalytic Behavior of Nanoparticles

This study is a continuation of our previous works on the use of metal-based nanoparticles for the adsorption of asphaltenes and its subsequent catalytic thermal decomposition. In this study, we evaluated the effects of asphaltene aggregation on the adsorption process and the subsequent catalytic oxidation using fumed silica and nanoparticles of NiO and/or PdO supported on fumed silica. Adsorption isotherms were constructed through batch adsorption experiments at 25 degrees C by using mixtures of n-heptane and toluene in amounts of 0, 20% v/v n-heptane (Heptol 20), and 40% v/v n-heptane (Heptol 40) to obtain different aggregate sizes of asphaltenes. Subsequently, asphaltene oxidation in the presence and absence of the nanoparticles was carried out in a TGA/FTIR system to investigate the impact of adsorbed asphaltene aggregates on the catalytic activity of the selected nanoparticles. The adsorption isotherms were described by the solidliquid equilibrium (SLE) model, and the catalytic behavior of the nanoparticles was compared based upon the trend of effective activation energies using the isoconversional method of Ozawa, Flynn, and Wall (OFW method). The results showed that the K parameter of the SLE model for both nanoparticles followed the trend of Heptol 40 > Heptol 20 > toluene, indicating that, as the amount of precipitant in the solution increases, a higher degree of asphaltene self-association on the active site of the catalysts is found. On the other hand, the H parameter revealed higher adsorption affinities as the n-heptane in the solution increased. However, when different adsorbents were compared at a fixed asphaltene concentration from the same solution, it was found that the use of functionalized nanoparticles led to a lower degree of asphaltene self-association and a higher affinity. A correlation between the effective activation energies from the OFW model and the SLE parameters was developed, finding that, for a fixed adsorbent, Ea increases as the affinity and the degree of self-association of asphaltenes increases. However, when the same asphaltenes were compared using different adsorbents, it was observed that E-alpha increases as the affinity decreases and the degree of asphaltene self-association increases. Consequently, this work shows the effect of the adsorption process on the catalytic activity of the nanoparticles. The reported results should give a better context for the use of such nanoparticles for the upgrading of heavy and extra-heavy oil.