Stray field gradient NMR reveals effects of hydrogen bonding on diffusion coefficients of pyridine in mesoporous silica

The diffusion of pyridine confined in mesoporous silica MCM-41 (dpore = 3.3 nm) was studied with stray field gradient (SFG) NMR diffusometry as a function of the filling factor of the mesopores at room temperature, employing a laboratory-built SFG setup. The translational diffusion of pyridine in MCM-41 is found to be anisotropic and the diffusion parallel to the pores' cylinder axes is much faster than that perpendicular to them. The parallel diffusion coefficient depends strongly on the filling level of the guest liquid inside the pores. For a filling level of 25%, which corresponds approximately to a monomolecular layer of pyridine molecules hydrogen bonded to surface—SiOH groups, a parallel diffusion coefficient of D|| = 1.0 × 10−9 m2 s−1 is found, which is slower than the diffusion coefficient of the bulk liquid (D = 1.6 × 10−9 m2 s−1). For higher filling factors the parallel diffusion coefficient increases and at a filling factor of 85% a diffusion coefficient of D|| = 6.8 × 10−9 m2 s−1 is reached. The perpendicular diffusion coefficient of D⟂ = 3.7 ± 2.0 × 10−11 m2 s−1 is independent of the filling factor. Employing additional 15N MAS data for the pyridine inside the mesopores, a microscopic model of the diffusion is proposed, which depends on the exchange of the slowly diffusing hydrogen-bonded surface pyridine molecules with fast-diffusing free pyridine molecules inside the pores.