Spatial distribution of coke residues in porous catalyst pellets analyzed by field-cycling relaxometry and parameter imaging

The distribution of coke residues inside porous catalyst pellets was investigated on the molecular as well as the macroscopic scale. The presence of coke on the pore surface affects the relaxation properties of adsorbed liquid species; these were determined by field-cycling relaxometry for different polar and nonpolar liquids in metal-doped and metal-free catalyst carrier materials. The presence of metal in the Al2O3 matrix had only a minor influence on the dispersion behavior, while the interaction of the adsorbates with the coke layer leads to considerable changes in the relaxation times at low Larmor frequencies. Lowering the temperature to well below the bulk freezing point of dimethyl sulfoxide resulted in a slightly stronger frequency dependence of T1. Not only relaxation times but also the diffusion coefficient is affected by the presence of coke residues in the pores. For macroscopically heterogeneous samples, they offer the possibility to generate maps of the local coke concentration by introducing appropriate filters into NMR imaging sequences. High-temperature regeneration of coked catalysts leads to such heterogeneous distributions which is visualized by T1 parameter imaging.