A First-Hand View of Shape-Selective Catalysis in Zeolites: New Science and Technology
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y over secondary over primary carbon links. In another class of manmade catalysts, molécules with unsaturated bonds are hydrogenated by a variety of metalbased catalysts. This addition of hydrogen proceeds nearly regardless of where the bond occurs in the structure. Or, take a synthesis reaction, such as that of xylenes from the addition of methanol to toluène. The usual acidic catalysts will add the new methyl group in any and ail of the three possible isomeric positions to produce ortho-, meta-, and para-xylenes. In contrast, the biological process, with its enzymes as catalysts, has been unique in its mastery of discrimination, i.e., of catalytic selectivity, to choose and transform only narrowly defined chemical structures into precisely predetermined others. A classical example 1 is the selectivity of a major enzyme, acetylcholinesterase. It will hydrolyze acetylcholine — an important chemical transformation in neurobiochemical metabolism — with structural selectivity. The importance of shape, in contrast to absolute chemical identity of the atomic constituents, is seen when a geometrically similar hydrocarbon structure is substituted for the choline composition (Figure l a ) . The e n z y m e is still similarly active. However, there is a loss
of activity when the structure of the target molécule is altered, e.g., by simply changing the size (length) of the alkyl chain (Figure lb).
Development of Zeolite Catalysts In the late 1950s at Mobil Corporation^ research laboratories, while my interests and curiosities darted between some of the wonders of biology and the challenges of a chemical industry, we became aware of the remarkable class of minerais we call zeolites. Most zeolites are crystalline s u b stances of the most common éléments in the earth's crust: silicon, aluminum, and oxygen. They are crystalline aluminosilicates, but the arrangement of the atoms is highly uncommon. They form crystals of a rather complex but precisely répétitive atomic network, which includes equally précise and répétitive channels and cavities, spaces just large e n o u g h to a c c o m m o d a t e " f o r e i g n " molécules of water. Heating them drives out the internalized water. They were termed zeolites, a name derived from the Greek words for "weeping stone." Scientists, a n d in particular, R.M. Barrer in Great Britain, had developed much knowledge on structure, composition, and even the synthesis of some zeolites. Some were shown to be able to take into their structure small molécules other than water. Some could include a variety of gases. Zeolite A became the first synthetic zeolite 33 to be commercialized by Union Carbide's Linde Division. Figure 2 represents a model of its structure. Each corner is a silicon or aluminum atom. The arrangement of tetrahedra leaves channels and cavities of molecular dimensions. The channels can admit water, many gases, and a few larger but linear, i.e., "slender" molécules, like nparaffin hydrocarbons. The total internai volume accessible by molécules from the outside can be some 10-20%
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