The discovery and demonstration of molecular shape-selective catalysis in zeolitic silicate structures has led to a large segment of new catalytic science as well as to chemical process technologies with broad impacts on industry and society. Manmade catalysts are beginning to acquire skills of molecular structure selectivity and activity seen in the world of enzymes.
All our activities, including those of life itself, are dominated by the presence and action of ever transforming molecules. Heat alone transforms them but, basically, decomposes them indiscriminately. Catalysts, on the other hand, can direct transformations with meticulous selectivity as if to preserve a continuing purpose and order, and life itself. However, manmade catalysts, although the major tool of the chemical industry, have been highly simplistic and crude in their discriminatory capabilities. For example, we have solid acidic catalysts called cracking catalysts that will break carbon-carbon bonds. They have been employed for decades in the petroleum fuels industries and represent the largest volume of any manufactured catalyst. They will rather indiscriminately break C—C bonds in nearly any aliphatic organic structure, with only some structural preference for tertiary over secondary over primary carbon links.
In another class of manmade catalysts, molecules with unsaturated bonds are hydrogenated by a variety of metal-based 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 toluene. The usual acidic catalysts will add the new methyl group in any and all of the three possible isomeric positions to produce ortho-, meta-, and para-xylenes.