I SCIENCE/TECHNOLOGY
Molecular Sieve With Unique Structural Features Synthesized A molecular sieve newly synthe/ % sized at Mobil Research & DeJ L J L velopment Corp., Paulsboro, N.J., has structural features not seen be fore in this type of material: two inde pendent pore systems, each accessible through so-called 10-ring apertures. The unusual structure of the molecu lar sieve, called MCM-22, suggests that a large number of catalytic applications in petrochemistry are in the offing. A mod el 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^ a 0.043(^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 crys talline materials with three-dimensional frameworks of tetrahedral atoms, such as silicon, aluminum, and boron, inter connected through oxygen atoms. The number of tetrahedral atom-oxygen spe cies defines the pore structure—for ex ample, 10-rings or 12-rings.
process uncovered a structure that had not previously been found in either nat ural 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 at oms 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 Lawton—that is, coordination inside a cage. The MCM-22 structure allows for mation of slightly elliptical 10-ring ap ertures to large supercages with inside free dimensions of 0.71 nm χ 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 chan nels. The two systems are not intercon nected and thus provide two indepen dent 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 carbenium ion-mediated reactions." Nei ther 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 reac tions where the size of the reactant mol ecule or the structure of transitionHigh-resolution electron micrograph lattice state intermediates images of zeolite MCM-22 show pore would conform to structures in two different projections. The the shape and steric conesponding constraints peculiar TV framework drawings of to MCM-22. As oil Xj MCM-22 are drawn refineries become showing only tetrahedral inexorably more atoms. In the bottom projection, the micrograph like chemical plants, is not perfectly aligned on that defines a rath one zone, resulting in minor er large group of ^/ distortions of hexagonal symmetry molecules.
According to Stephen L. Lawton, a group leader at Mobil Research & De velopment, preparation of MCM-22 fol lowed the usual method of hydrother mal synthesis using a template or struc ture-determining organic molecule around which the crystallization oc curred. In this case, the template was hexamethyleneimine. MCM-22 may be synthesized as ei ther an aluminosilicate or a borosilicate. Scanning electron micrographs show that the sieve crystallizes in very thin sheets, with the boron version having slightly thicker sheets. Lawton says that the group—which included Mobil researchers Michael E. Leonowicz and Mae K. Rubin as well as Lawton's son Jeffrey, a summer intern at the laboratory—selected the boron version for structural studies. It exhib ited larger pore size, which provided x-ray diffraction patterns that are eas ier to interpret. The problems of structure determina tion, according to Stephen Lawton, were overcome with the aid of insightful anal ysis by Jeffrey, a student at Baylor Col lege of Medicine, Houston. "He was not a victim of tradition like the rest of us/' Lawton says, in relating how the unique struc ture was determined. Jeffrey had attacked the problem without pre conceptions, Lawton explains, and in the
in the image.
Joseph Haggin JULY 4,1994 C&EN
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