MCM-22: A Molecular Sieve with Two Independent Multidimensional Channel Systems

Abstract

The molecular sieve MCM-22 contains structural features previously unobserved in this class of materials. Its framework topology, derived from high-resolution electron micrographs and refined with synchrotron x-ray diffraction powder data, contains two independent pore systems, both of which are accessed through rings composed of ten tetrahedral (T) atoms (such as Si, Al, and B). One of these pore systems is defined by two-dimensional, sinusoidal channels. The other consists of large supercages whose inner free diameter, 7.1 angstroms, is defined by 12 T-O species (12-rings) and whose inner height is 18.2 angstroms. These coexisting pore systems may provide opportunities for a wide variety of catalytic applications in the petrochemical and refining industries. Another structural feature is an unusual -T-O-T- chain that passes through the center of a modified dodecasil-1H [4(3)5(6)6(3)] cage.

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

“…33 The amounts of Brønsted acid sites (per unit cell) of HMCM-22 and Mo/ HMCM-22 catalysts are presented in Table 1. It is interesting to note that the parent HMCM-22 has 3.5 Brønsted sites per unit cell, which is almost the same with the tetrahedral aluminum determined by 27 Al MAS NMR, indicating one bridging OH group is associated with one framework 4-coordinated Al in the zeolite. The location of Mo species is a crucial issue in studying its catalytic function and there are controversial conclusions in the literature on whether the Mo species can be introduced to zeolitic channels.…”

“…The details were presented elsewhere. 8 Results and discussion 27 Al NMR The 27 Al MAS NMR spectra were recorded on HMCM-22 and Mo/HMCM-22 with different Mo loadings, together with the measurement conditions, are reported in our previous paper. 30 In brief, we proposed that there is a strong interaction between the introduced Mo species and the framework Al, resulting in the extraction of framework Al to extra-framework positions with very high molybdenum loading.…”

“…Furthermore, the relative integrated intensity of the pyridine adsorbed on Brønsted acid sites decreases sharply with Mo loading in the range of 0-6% indicating that a severe effect is imposed on the acid sites of the HZSM-5 support. It is further illustrated by 27 Al MAS NMR, ESR and TPD techniques that the Brønsted acid sites in HZSM-5 zeolite act as powerful traps for the Mo species to migrate into the channels. 34 Iglesia et al studied Mo/ HZSM-5 catalysts prepared by using mechanical mixtures of MoO 3 and HZSM-5 via solid exchange reaction.…”

“…In their molecular dynamics study Sastre et al claimed that the 10 MR sinusoidal channels of the HMCM-22 have very low diffusibility for benzene, whereas the intercages between the 12 MR supercages are available for benzene diffusion, particularly if it is activated at a high temperature. 42 The lower diffusibility of benzene in the 10 MR channels of HMCM-22 may be a result from the relatively small size of the 10 MR channels in the MWW structure compared with HZSM-5 (0.4 Â 0.55 nm vs. 0.53 Â 0.56 nm and 0.51 Â 0.55 nm 27,43 ), as well as from their high degree of tortuosity. On the basis of these considerations, together with the discussion in the above sections, it is reasonable to assign peaks at high temperatures to benzene adsorped on the two types of Brønsted sites in the supercages of HMCM-22 zeolite.…”

One-pot synthesis of nano-crystalline MCM-22

“…Since the first report in the early 1990s [1], MCM-22 zeolite has attracted considerable attention of the catalysis community. MCM-22 is a medium-pore zeolite, prepared at typical Si/Al ratios between 10 and 20.…”