Relations between Pore Structure Parameters and Their Implications for Characterization of MCM-41 Using Gas Adsorption and X-ray Diffraction

Abstract

Structural parameters for ordered mesoporous materials are shown to be strongly interrelated as a result of their well-defined structures. Equations for mesoporous materials with hexagonal arrays of uniform pores (e.g., MCM-41 and SBA-15) are presented, which can be used to calculate the pore size, pore wall thickness, and specific surface area on the basis of several quantities, which are easily available from X-ray diffraction and gas adsorption data (i.e., the interplanar spacing, primary mesopore volume, and micropore volume). The influence of assumptions about the pore shape, pore wall density, and the presence of microporosity or disordered nonmesostructured domains on the evaluation of structural parameters is examined. It is suggested that because of very large specific surface areas and primary mesopore volumes for many MCM-41 materials, the existence of extensive disordered domains is not a common feature of MCM-41. Examination of X-ray diffraction (XRD) patterns and nitrogen adsorption isotherms led to a conclusion that a detailed characterization of ordered mesoporous materials requires application of both gas adsorption and XRD, since these techniques provide complementary information. To facilitate a comparison of experimental results from different laboratories, some recommendations are made for the reporting of adsorption data and their application in the calculations of specific surface areas and pore size distributions.