Molecular Sieve With Unique Structural Features Synthesized

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    Molecular Sieve With Unique Structural Features Synthesized

    A molecular sieve newly synthe-/ % sized at Mobil Research & De-

    J L J L velopment Corp., Paulsboro, N.J., has structural features not seen before in this type of material: two independent pore systems, each accessible through so-called 10-ring apertures.

    The unusual structure of the molecular sieve, called MCM-22, suggests that a large number of catalytic applications in petrochemistry are in the offing. A model for the structure of the molecular sieve was described recently in Science [264, 1910 (1994)].

    MCM-22 is the working name for a molecular sieve with the formula Ho.033^a0.043(^lo.005^0.07lSio.924)^2-Structural determinations were made with the aid of high-resolution electron micrograph lattice images and model building. Refinement of trial models eventually led to the structure.

    Molecular sieves, or zeolites, are crystalline materials with three-dimensional frameworks of tetrahedral atoms, such as silicon, aluminum, and boron, interconnected through oxygen atoms. The number of tetrahedral atom-oxygen species defines the pore structurefor example, 10-rings or 12-rings.

    According to Stephen L. Lawton, a group leader at Mobil Research & Development, preparation of MCM-22 followed the usual method of hydrothermal synthesis using a template or structure-determining organic molecule around which the crystallization occurred. In this case, the template was hexamethyleneimine.

    MCM-22 may be synthesized as either an aluminosilicate or a borosili-cate. Scanning electron micrographs show that the sieve crystallizes in very thin sheets, with the boron version having slightly thicker sheets. Lawton says that the groupwhich included Mobil researchers Michael E. Leono-wicz and Mae K. Rubin as well as Law-ton's son Jeffrey, a summer intern at the laboratoryselected the boron version for structural studies. It exhibited larger pore size, which provided x-ray diffraction patterns that are easier to interpret.

    The problems of structure determination, according to Stephen Lawton, were overcome with the aid of insightful analysis by Jeffrey, a student at Baylor College of Medicine, Houston. "He was not a victim of tradition like the rest of us/'

    Lawton says, in relating how the unique structure was determined. Jeffrey had attacked the problem without preconceptions, Lawton explains, and in the

    process uncovered a structure that had not previously been found in either natural or synthetic materials.

    In some ways, the structure of MCM-22 resembles that of the silicate clathrate, dodecasil-H (DOH). In MCM-22, the DOH cage is modified so that the orientation of tetrahedral atoms at certain apexes is reversed and they join inside the cage through a shared oxygen atom. This is the unique structure that was deduced by Jeffrey Lawtonthat is, coordination inside a cage.

    The MCM-22 structure allows formation of slightly elliptical 10-ring apertures to large supercages with inside free dimensions of 0.71 nm 0.71 nm x 1.82 nm. The supercages stack one above the other through double 6-rings. The net effect is that there is a system of interlayer supercages and an intralayer system of sinusoidal channels. The two systems are not interconnected and thus provide two independent structures in the same material.

    How MCM-22 will specifically be used by Mobil has not been disclosed. The company merely says that it "may find applications in the catalysis of car-benium ion-mediated reactions." Neither Stephen Lawton nor other officials at Mobil would speculate further.

    Lawton does say, however, that about two dozen ideas are being considered for use of MCM-22. They favor those reactions where the size of the reactant mol

    ecule or the struc-

    High-resolution electron micrograph lattice images of zeolite MCM-22 show pore structures in two different projections. The

    conesponding TV framework drawings of Xj MCM-22 are drawn

    showing only tetrahedral atoms. In the bottom

    projection, the micrograph is not perfectly aligned on

    one zone, resulting in minor ^/ distortions of hexagonal symmetry in the image.

    ture of transition-state intermediates would conform to the shape and steric constraints peculiar to MCM-22. As oil refineries become inexorably more like chemical plants, that defines a rather large group of molecules.

    Joseph Haggin

    JULY 4,1994 C&EN 23

    SCIENCE/TECHNOLOGYMolecular Sieve With Unique Structural Features Synthesized


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