Stephen L. Lawton

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.

The Framework Topology of ZSM-18, a Novel Zeolite Containing Rings of Three (Si,Al)-O Species

ZSM-18 is the first known aluminosilicate zeolite to contain rings of three (Si,Al)-O species (3-rings). Its framework topology has been determined by hypothetical model building, subsequent constrained distance and angle least-squares refinements of atomic coordinates, and x-ray powder diffraction pattern simulations. Its channel structure is characterized by a linear unidimensional 12-ring channel, with an approximate pore opening of 7.0 angstroms. In addition, the channels are lined with pockets that are capped by 7-rings with dimensions of 2.8 angstroms by 3.5 angstroms. An intraframework packing model of the organic moiety used in the synthesis suggests that a strong templating effect may be responsible for the formation of this unusual zeolite structure.

Synthesis and proposed framework topology of zeolite SUZ-4

Crystalline SUZ-4, with anhydrous composition K5Al5Si31O72, is hydrothermafly synthesized in a reaction mixture of SiO2, Al3, KOH, tetraethylammonium hydroxide and quinuclidine; its proposed framework topology is related to zeolites ferrierite and ZSM-57 and contains straight channels having apertures defined by rings of ten (Si,Al)–O species; a novel cage may as the site for non-exchangeable potassium ions.

Sorbate-induced changes in the 29Si MAS NMR spectrum of highly siliceous MCM-22

Abstract Sorption of organics, such as 2,4-pentanedione and toluene, cause distinct and reversible changes in the 29 Si MAS NMR spectrum of highly siliceous MCM-22 that are characteristic of perturbations in local geometry. These subtle spectral features further substantiate the presence of the modified dodecasil-1H cage in the MCM-22 framework.

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