Michael E. Leonowicz

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

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 [435663] cage.

Twelve-ring pockets on the external surface of MCM-22 crystals

MCM-22 is a synthetic high-silica zeolite which crystallizes as very thin plates. Its internal structure contains two independent internal pore systems accessible through 10-ring apertures. A high concentration of external zeolitic pockets, with 12-ring openings and having an approximate depth of 7 A, covers the hexagonal faces of the thin crystals. Evidence for the presence of these pockets was established by high-resolution electron microscopy. Experimental evidence to support this was obtained from ion-exchange studies using tetramethylammonium and tetraethylammonium ions, and from dynamic sorption studies using n-heptane and 2,2,4-trimethylpentane. The results suggest that the surface pockets unique to the MCM-22 structure have zeolitic characteristics and may play a significant role in certain catalytic processes.

MCM-36: The first pillared molecular sieve with zeoliteproperties

Publisher Summary This chapter describes the physical characterization of MCM-36, which unequivocally establishes its existence as a novel large pore pillared material with zeolite properties. MCM-36 is a pillared material obtained from MCM-22 layers. The preparation of MCM-36 involves a lamellar intermediate, designated MCM-22 precursor, produced in a hydrothermal process. The structural information concerning pore system of MCM-36 is revealed by adsorption methods. The layers in MCM-22 posses two kinds of pore systems. One consists of 10-ring interconnected channels within the layers. The second are isolated 12-ring cages on the surface, which result in pockets on the outside of MCM-22 crystal and internal supercages, accessible through 10-ring apertures, inside the crystal. Accordingly, the pore size distribution plot obtained for MCM-22 by Ar physisorption shows two distinct peak in the 6–7 A region. The complex pore structure of MCM-22 is also reflected in the unique three step uptake profile of bulky 2,2-dimethylbutane (DMB) observed in the dynamic sorption experiment.

The discovery of ExxonMobil's M41S family of mesoporous molecular sieves

Publisher Summary This chapter discusses the discovery of ExxonMobils M41S family of mesoporous molecular sieves. The quest for new molecular sieves in the late 1980s led a team of mobil researchers to the discovery of a family of nanostructured mesoporous materials known as “M41S.” Mobil composition of matter (MCM)-41, the hexagonal phase, is undoubtedly the best known and most widely studied of this family of materials. Other discrete members of the M41S family are the cubic (MCM-48) and the lamellar (MCM-50) forms. Each is synthesized via a counterion initiated self assembled liquid crystal mechanism, involving oxide precursors, which form an inorganic equivalent to a liquid crystal-micelle structure. This manuscript describes the events that led to the discovery of M41S materials. It also summarizes the supporting characterization and mechanistic studies that led to a picture of how these materials are actually formed. The mechanistic and characterization studies involved many researchers from ExxonMobils Paulsboro and Princeton laboratories. ExxonMobil has very recently scaled-up the synthesis and commercialized MCM-41 for an undisclosed application.

The framework topology of zeolite MCM-22

Summary The framework topology of the molecular sieve MCM-22 has been determined from high resolution electron micrographs and refined with synchrotron X-ray diffraction powder data. MCM-22 crystallizes as very thin sheets. The borosilicate form of this material, having unit cell framework composition (Al 0.4 B 5.1 Si 66.5 )O 144 , may be indexed on a hexagonal lattice in space group P6/mmm (D 1 6h , No. 191). The refined unit cell parameters are a = 14.1145(8) and c = 24.8822(18) . The MCM-22 structure contains hexagonal sheets constructed by interconnecting modified DOH cages through shared 4-ring faces in a manner analogous to that found in DOH. This modified cage has a TO 3 cap on top (forming a small [4 3 ] unit) and a -T-O-T- chain passing through its center. The MCM-22 sheets bond together in two ways— one side involving an oxygen bridge between TO 3 caps and the other side through double 6- rings. The net effect is to produce two different, independent pore systems with 10-ring apertures. One of these pore systems is defined by two-dimensional, sinusoidal channels. The other consists of large supercages whose inner free diameter, 7.1 , is defined by 12- rings and whose inner height is 18.2