An NMR technique for the analysis of pore structure: Application to mesopores and micropores

Abstract

The use of NMR spin-lattice relaxation experiments on water, or other proton-containing fluids, in a porous solid as a pore structure/size measurement tool has been recently reported for porous solids with pore size greater then ≈5 nm. This approach has numerous advantages, as compared to nitrogen sorption/condensation and mercury porosimetry, including no network/percolation effects, no pore shape assumption is required, wet materials may be analyzed, and small specific pore volumes may be studied. Assumptions used to relate the measured spin-lattice relaxation time, T 1, to the pore size limit the application of the method to pore sizes greater than 5 nm. In this work, we explore the extension of this NMR technique to porous solids with pores in the micropore and mesopore size ranges. By comparing previously published results of the relative distribution of surface- and bulk-phase water as a function of pore size (E. Almagor and G. J. Belfort, J. Colloid Interface Sci. 66, 146, (1978)) to predictions from various pore models, we have demonstrated that the “two-fraction, fast exchange” model may be applied to relate measured T 1 to pore size in the mesopore and micropore size range if the pore geometry is known (or assumed). For pore size greater than 5 nm, pore geometry is not important. Comparisons between conventional techniques, such as nitrogen sorption and mercury porosimetry, and the results of NMR spin-lattice relaxation experiments are complicated by the fact that different pore structure parameters are measured with these methods. Despite this, agreement between the methods is quite good for the five sol-gel-derived materials which we have studied.