I SCIENCE/TECHNOLOGY

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 structure—for 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 group—which included Mobil researchers Michael E. Leono-wicz and Mae K. Rubin as well as Law-ton's son Jeffrey, a summer intern at the laboratory—selected 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 Lawton—that 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 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 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