Influence of the Mesoporosity of Silica Carrier Materials on the Performance of an Immobilized Organocatalyst in Heterogeneous Catalysis

While the immobilization of organocatalysts in mesoporous scaffolds brings significant benefits, the relationship between mesopore diameter and the performance of the catalyst material is still a matter of research. Here, we studied the esterification of α-tocopherol (TP) and 1-phenylethanol (PE) using a versatile and well-known organocatalyst, 4-(dimethylamino)pyridine-(DMAP), being immobilized in mesoporous spherical silica particles, which possess three different mesopore diameters (6, 10, and 30 nm), but an identical particle diameter (5 μm). In order to analyze the interplay of transport and reactivity, the mesoporosity was thoroughly studied by advanced physisorption analysis, especially desorption hysteresis scanning. Furthermore, we used pulsed-field gradient (PFG) NMR for the determination of the intraparticular self-diffusion behavior of the chosen alcohols as well as their acetalyzed forms and also to identify their interaction with the polar silica surface. The catalytic performance was tested in packed-bed columns (continuous flow) as well as under batch conditions. Based on catalytic parameters and experimental diffusion coefficients, the Weisz-Prater-Criterion parameter Φ_WP was calculated for identifying potential mass-transport limitations for each material, to evaluate the influence of the mesopore diameter on the catalytic properties. In conclusion, we demonstrate that the observed poor catalytic performance in the esterification reaction of α-tocopherol, in the case of the material with the smallest average mesopore size (6 nm), is due to hindered mass transport. The combination of physisorption and diffusion analysis suggests that this limitation is on the one hand caused by bottleneck-like connecting pores and on the other hand by liquid-surface interactions. However, quite high product yields are observed for the 10 nm- and 30 nm-mesoporous particles under flow conditions, which correlates well with the self-diffusion coefficients obtained from PFG NMR, showcasing the feasibility of immobilized organocatalysts in organic synthesis.