Determination of Pore Size and Pore Wall Structure of MCM-41 by Using Nitrogen Adsorption, Transmission Electron Microscopy, and X-ray Diffraction

Abstract

Highly ordered MCM-41 materials were synthesized using single or mixed alkyltrimethylammonium and alkyltriethylammonium surfactants of alkyl chain lengths from 12 to 22. As determined using combined adsorption/powder X-ray diffraction (XRD) analysis, the obtained samples exhibited pore sizes from 3.2 to 4.8 nm with about 0.3 nm increments. Nitrogen adsorption isotherms for the samples under study exhibited remarkably sharp capillary condensation steps. The adsorption isotherm for the highly ordered 4.8 nm MCM-41 featured a narrow hysteresis loop with parallel adsorption and desorption branches. This behavior is probably a feature of nitrogen adsorption in ideal cylindrical pores of this size and is distinctly different from the previously reported behavior of MCM-41 with pore sizes in the range 4-5.5 nm, which had triangular hysteresis loops on their nitrogen adsorption isotherms. All MCM-41 materials had remarkably narrow pore size distributions, which showed that fine-tuning of the pore size is easily achievable for the synthesis procedure used. The pore wall thickness was found to increase as the pore size increased. The pore wall thickness determined using transmission electron microscopy (TEM) was somewhat larger than that obtained using the combined XRD/adsorption analysis, which was attributed to inherent features of these determination methods. TEM images showed that pores of MCM-41 samples were approximately hexagonal rather than circular, thus providing structural information especially important for application of MCM-41 as a model adsorbent. Low-pressure adsorption studies of uncalcined ethanol-washed MCM-41 samples revealed that external surfaces of their particles were covered by relatively dense layers of surfactant ions. This behavior is consistent with the generally accepted mechanism of the MCM-41 formation via self-assembly of silicate-surfactant ion pairs. Comparison of adsorption isotherms for uncalcined and calcined samples allowed us to exclude the possibility of any appreciable structural degradation during calcination, although TEM provided some indication of the lowering of structural ordering upon calcination.

Non-Hydrothermal Synthesis of Cylindrical Mesoporous Materials: Influence of the Surfactant/Silica Molar Ratio

“…In all cases, a good linear correlation coefficient was obtained (R 2 > 0.995). The data for the textural properties of the materials as determined from the nitrogen isotherms at 77 K are listed in Table 2 1 It should be noted that the specific surface areas previously reported for MCM-41 materials Kruk et al , 2000Lelong et al 2008) and for SBA-15 materials Kruk et al 2000Kruk et al , 2003 synthesized under hydrothermal conditions were lower than those obtained in the present work. .…”

Structure of Ni(II) and Ru(III) Ammine Complexes Grafted onto Mesoporous Silicate Sieve

“…3b A value of a 0 = 45 Å is obtained for the samples shown in Figure 1. Assuming a pore wall thickness of 10 Å, 20,21 this results in a pore size of 35 Å. assigned to a LMCT from a NH 2 -ligand. 22 The presence of the strongly charge-donating amido group shifts the symmetric deformation of the remaining ammine ligands -which is known to be very sensitive towards changes of the effective charge on the metal center 25 -to lower energy.…”

Large Pore Methylene-Bridged Periodic Mesoporous Organosilicas: Synthesis, Bifunctionalization and Their Use as Nanotemplates

“…The hysteresis loops, with parallel and nearly vertical branches in all of the isotherms except for PMO-Me (0.1 M), are type H1 hysteresis, according to IUPAC definitions, which is characteristic of materials with a high degree of pore size uniformity. [31][32][33] In contrast, the PMO-Me (0.1 M) material showed a complex N 2 sorption isotherm. The one-step capillary condensation in the range 0.7 e P/P 0 e 0.8, which occurs inside the ordered pores, in the adsorption branch and the two-step evaporation in the range 0.46 e P/P 0 e 0.7 in the desorption branch are indicative of uniform mesopores with windows caused by pore blocking.…”