I have always wondered how to visualize MCM-41 crystal structure. The literature says that MCM-41 is an arrangement of cilyndrical pores in a hexagonal shape, builded by amorphous silica. Nevertheless, MCM-41 does have a well-defined XRD pattern (peaks related with 100, 110 and 200 planes).
Sincerely, I have not been able to visualize this. How can MCM-41 have a crystaline structure when silica building blocks are suposed to be amorphously packed? How can I visualize the unit cell of MCM-41?
On the other hand, if the unit cell is hexagonal, Miller-Bravais (hkil) indices should be used instead of Miller (hkl) indices, but all literature I have found so far uses Miller indices.
Hi,
MCM-41 silicas are amorphous the diffraction in the form of peaks we see in the XRD spectrum is due to the hexagonal pores and not diffraction as observed in crystalline solids. If you look carefully the peaks are rather broad and not intense.
As it is one the most investigated materials by material scientists you will get huge no of reports about this material on google, scopus or scifinder. reporting every aspect of it.
Although invented and marketed by Mobil Crystal Corporation (MCC); Mobil Composition of Matter OR Mobile Crystalline Material (MCM)-41 this material is amorphous porous silica. If the pores are in the meso range according to IUPAC rules then they are called mesoporous silica.
Typical X-ray diffraction patterns of MCM-41 usually show four Bragg peaks at low 2 theta value indicating the long order present in the material (greater than that observed for lattice planes, thus obtained at lower 2 theta values). The Bragg peaks can be indexed assuming a hexagonal symmetry. For example, the repeating distance ao between successive 2D pore centers is calculated as
ao= (2/Ö3)d100 .
To get the pore diameter, the approximate value for the pore wall (1.0 nm), should be subtracted from the above calculated ao value.
Dear Casas-Orozco
A typical X-ray diffraction pattern of MCM-41 shows the hexagonal symmetry of the pore ordering (space group: p6m) and it typically contains four main reflection lines (d100, d110, d200 and d210) or more at low angles (2theta = 10). Since MCM-41 consists of amorphous silica, it has no crystallinity at the atomic level. Therefore, no reflections can be observed at higher degrees 2theta and as Dr.Jadhav mentioned these diffraction are due to the hexagonal pores.
Useful review: Microporous and Mesoporous Materials 125 (2009) 170–223.
MCM-41 is a porous material. After doing P-XRD you will get sharp peaks that characterize that it is crystalline in nature.
The diffraction peaks are due to the ordered hexagonal pore pattern. The silica filling is amorphous and not contributing to MCM XRD pattern. As the inter-pore distance is larger than inter atomic distances, the peaks are at low 2theta values.
MCM-41 composed of amorphous silica. However, the observed diffraction peaks are related to the spacing between pore walls of MCM-41 porous texture. Please refer to my attached article for indexing of the XRD peaks. Refer also the cited reference in the articles for more explanation of the MCM-41 structure
Article Direct formation of LaFeO3/MCM−41 nanocomposite catalysts an...
and
doi:10.1016/j.jcis.2007.07.030
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