An investigation of the physical structure of MCM-41 novel mesoporous materials using a corrugated pore structure model

Abstract

The preparation and pore structure characterization of MCM-41 materials with or without Al or Ti addition during synthesis are presented in this work. Such solids are intended for use as catalysts supports possessing advantageous pore structure, total acidity and thermal stability characteristics. BET surface areas up to 1483 m 2/g were determined and compared with cumulative surface areas obtained by methods like the conventional Roberts’ and the newly reported corrugated pore structure model-nitrogen (CPSM-nitrogen) [Ind. Eng. Chem. Res. I 39 (2000) 3747; Ind. Eng. Chem. Res. II 39 (2000) 3764]. CPSM predictions are in perfect agreement with estimates obtained by the BET monolayer adsorption variant. Pore size distributions (PSDs) were deduced by using both the Roberts’ and the CPSM methods. The superiority of the latter method was confirmed, since apart from the prediction of intrinsic PSDs, enabled the determination of tortuosity factors ranging ca. τ CPSM=1–2.35, for the studied materials [Ind. Eng. Chem. Res. 40 (2000) 721]. These values are lower than those for traditional catalysts ca. τ=3–10 [Mass Transfer in Heterogeneous Catalysis, 1970, p. 37] and reflect the ordered pore structure of MCM-41 solids. Addition of Al (i.e. Si/Al=5) caused a PSD shift towards the micropore region (i.e. D mean shift from 3.28 to 2.10 nm) in contrast to Ti addition (i.e. Si/Ti=5) that caused a PSD shift farther to the mesopore region (i.e. D mean shift from 3.28 to 23.74 nm). Lower additions of Al or Ti (i.e. Si/Al=20 and Si/Ti=10) caused a pore volume and surface area reduction but not a substantial shift of the intrinsic PSDs.