Characterization of Asphaltenes in Solution and Inside the Pores of Catalysts by 1 H NMR Relaxometry
Abstract
The upgrading of heavy petroleum fractions needs the development of more and more efficient heterogeneous catalysts. One of the major issues of these processes is the diffusion of asphaltenes to the active site through the porous network of the alumina support. The catalytic efficiency is deeply impacted by the transport phenomena and the interfacial interactions. The aim of this work is to capture the extent to which low-field 2D 1H NMR relaxation time correlations can contribute to a better understanding of the dynamics of asphaltene in solution and within the pores of catalyst supports. 2D T1-T2 maps for asphaltenes in solution in toluene exhibit several T1-T2 contributions, varying with the asphaltene concentration and the size of the asphaltenic fractions obtained by ultrafiltration separation. According to the nano-aggregate structure proposed by the Yen-Mullins aggregation model of asphaltenes, it was possible to unravel the different asphaltenic proton relaxation behaviors. By the use of NMR relaxometry, we have confirmed that water is a wetting solvent of the alumina instead of the toluene which is a nonwetting solvent. The presence of macropores in catalyst clearly boosts the toluene mobility through the porous network. 2D T1-T2 maps for asphaltenes inside the pores show various types of protons, all of them with a severe constrained dynamics. Asphaltene nano-aggregates and clusters can be seen as large entities jammed into the pores, slowly mobile and affecting the solvent (toluene) mobility. When macroporosity exists in the support, the asphaltene overcrowding is less sensitive, enabling a faster dynamics of asphaltenes and toluene.