Multiple scale investigation of molecular diffusion inside functionalized porous hosts using a combination of magnetic resonance methods†
Mass transport of molecular compounds through porous solids is a decisive step in numerous, important applications like chromatography or heterogeneous catalysis. It is a multi-scale, hierarchical phenomenon: macrodiffusion (>μm) is influenced, in addition to parameters like grain boundaries and particle packing, by meso-scale (>10 nm, <μm) factors like particle size and the connectivity of pores. More importantly, meso-scale diffusion and macro-scale diffusion are first and foremost determined directly by processes on the molecular scale (<10 nm), which depend on numerous factors like pore-size, interactions of the host with the solid surfaces and with the solvent. Due to the high complexity of the latter and the fact that current analytical techniques enable only limited insights into solvent-filled pores with sufficient spatial and temporal resolution, the knowledge about the molecular origins of diffusive processes in porous materials is still restricted. The main focus of the current paper is on the development of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy into a tool shedding some new light on molecular diffusion inside mesoporous silica materials differing systematically in pore size and surface functionalities. The advantages of CW-EPR are that its spatial resolution fits ideally to the size of mesopores (2–10 nm), it is fast enough for spotting molecular processes, and any conventional solvent and the porous matrix are EPR silent. Diffusion coefficients have been calculated considering spin exchange occurring from the diffusive collision of radicals, and are compared to complementary analytical techniques like MAS PFG NMR (sensitive for meso-scale) and EPR-imaging (sensitive to macroscale diffusion). Our results show that the choice of surface bound functional groups influences diffusion much stronger than pore-size. There are indications that this is not only due to different guest–surface interactions but also due to an altered mobility within the solvent under confinement.