Relationships among Pore Size, Connectivity, Dimensionality of Capillary Condensation, and Pore Structure Tortuosity of Functionalized Mesoporous Silica

Abstract

Seven mesoporous forms of silica were prepared by controlled and gradual functionalization of the original SiO2 surface with silano-(trimethoxy)-propyl-imidazole groups. The degree of surface functionalization was n = 0.00, 0.23, 0.30, 0.40, 0.52, 0.60, and 0.85 and was controlled by previous knowledge of surface acidity, determined by temperature-programmed desorption of NH3. From N2 adsorption/desorption measurements, the specific surface area Sp (m2 g-1), the specific pore volume Vp (cm3 g-1), and the corresponding pore size distributions (PSDs) were determined. The connectivity c of the solids was also calculated according to the method of Seaton, and the dimensionality of capillary condensation Dcc was found using the thermodynamic method of Neimark. The increase of functionalization resulted in a linear drop of Sp and Vp, and the maximum Dmax of the PSD and the full width at half-maximum, fwhm ∼ 2σ, of the distribution drop in a regular way whereas the ratio (Dmax/2σ) remains practically constant. The connectivity c also decreases from c = 12.5 at n = 0 to c = 3 at n = 0.60−0.85, presumably because of blocking of channels connecting various pores. The Dcc values decrease with increasing n values. Next, the co-called corrugated pore structure model, CPSM, was employed for the estimation of tortuosity τ of the porous solids and the simulation of the experimental adsorption/desorption isotherms. From those CPSM simulations, the corresponding specific surface area SCPSM (m2 g-1), the specific pore volume VCPSM (cm3 g-1), and the corresponding pore size distribution PSDCPSM were estimated. The tortuosity τ of the system drops with the degree of functionalization from τ = 4.22 at n = 0 to τ = 3.37 at the initial functionalization (n = 0.23) and subsequently remains practically constant at about τ = 3.40 ± 0.10, for the same reasons which affect the connectivity, that is, blocking of various pore channels. The comparison between the parameters Vp and VCPSM is quite satisfactory. The SCPSM values appear systematically higher by 8−23% compared to the Sp ones. The dimensionality of capillary condensation Dcc is related to the variance 2σ of the PSD. The reasons and the limits of this relationship are discussed.