Pomeroy and Betty Perry Smith Professor of Chemical Engineering Emeritus
Dean Emeritus of the School of Engineering and Applied Science
B.S., Georgia Institute of Technology, 1952
M.S., Massachusetts Institute of Technology, 1954
Sc.D., Massachusetts Institute of Technology, 1955
Room: ACE33 Engineering Quad
Honors and Awards
- Founders Award, American Institute of Chemical Engineers, 1990
- Warren K. Lewis Award, American Institute of Chemical Engineers, 1985
- American Academy of Arts and Sciences, 1982
- Academia Sinica, 1982
- William H. Walker Award, American Institute of Chemical Engineers, 1980
- National Academy of Engineering, 1978
- Professional Progress Award, American Institute of Chemical Engineers, 1970
- Petroleum Chemistry Award, American Chemical Society, 1966
I am interested in shape selective separation with zeolites and nanoporous materials. I am also interested in molecular structure-property relations, especially its relation to product engineering.
Shape Selective Separation. Zeolites are silica-alumina crystals that have pores with extremely precise diameters from 4 to 13 Angstroms, and they have been used industrially to separate molecules that have almost the same physical and chemical properties, but have different shapes, such as normal from branched paraffins, and o-xylene from p-xylene. MCM-41 is a new class of porous material that have precise and tunable diameters from 25 to 100 Angstroms. This range of pore diameters put them in the range for separation of drug molecules up to 400,000 in molecular weights. We have developed methods to compute the activation energies required to squeeze and to distort a molecule to enter a zeolite channel. We are compiling a large database of activation energies between many molecule-channel pairs, and investigating their potential use to design separation processes for the chemical and the pharmaceutical industries.
Melting points and molecular symmetry. The strength of internal cohesive energies of molecules comes from van der Waals forces, and gives rise to many important molecular properties including: density, boiling point, melting point, the constant "a" in van der Waals equation of state, surface tension, light refractive index, and many others. Rod shaped molecules such as pentane package better than Y shaped molecules such as isopentane, which package better than tetrahedral molecules such as neopentane. This explains why the densities of the three molecules are in the order of pentane > isopentane > neopentane, and so are their boiling points. But the melting point of neopentane is 150 degrees centigrade higher than the other two. The melting point of methane is actually higher than that of ethane, which is higher than that of propane. When a methyl group is added to benzene, the boiling point goes up but the melting point plunges down.
The reason behind these anomalies is molecular symmetry, which can be described by the symmetry number σ, which is studied by group theory. A molecule that has a high symmetry, such as methane and benzene where σ = 12, the entropy in the melt is decreased by an amount equal to Rln(σ) compared to an asymmetrical analog molecule. Since the enthalpy of melting is related to the entropy of melting and the temperature of melting by the equation Tm = ΔHm/ΔSm, a change in symmetry that reduces ΔSm without affecting ΔHm will cause Tm to increase. That is to say that a symmetric molecule will have a higher melting point than an asymmetric analog molecule, because it has a lower entropy in the melt.