Argon adsorption at 77 K as a useful tool for the elucidation of pore connectivity in ordered materials with large cagelike mesopores

It is shown that argon adsorption at 77 K is convenient for the elucidation of pore connectivity in ordered materials with large cagelike mesopores and is capable of providing valuable structural information that may not be available from commonly used nitrogen adsorption at 77 K. The discussed approach is based on the phenomenon of pore blocking upon desorption. The nature of this phenomenon is that the capillary evaporation from a particular pore is retarded until either the pore develops an access to the surrounding gas through a continuous path of unfilled pores, or the lower pressure limit of hysteresis for the pore is reached. When the former of these behaviors is observed, the capillary evaporation pressure provides information about the widest continuous path of pore entrances connecting the given pore with the surrounding gas, that is, about the smallest pore entrance size along this widest path. In the case where the capillary evaporation is delayed to the lower limit of hysteresis, it is possible to determine only the upper boundary of the possible pore entrance size. It is shown that argon adsorption at 77 K, for which hysteresis in cylindrical pores is observed for a wider range of pore sizes than for nitrogen at 77 K, is often superior from the point of view of the pore connectivity elucidation, allowing one to probe pore entrance sizes down to similar to4.0 nm (compared to similar to5.0 nm in the case of nitrogen at 77 K), as estimated using ordered silicas with cylindrical mesopores. The merits of the use of argon at 77 K were demonstrated for the FDU-1 silicas. Conditions favorable for the formation of small pore entrance sizes in FDU-1 were further elucidated, and the lowering of the pore entrance size uniformity with the increase in the initial synthesis temperature was revealed, and bimodal distributions of pore entrances in the samples hydrothermally treated at or above 373 K for extended periods of time were uncovered. Argon adsorption at 77 K is potentially useful for the characterization of pore connectivity in many other large-pore materials.