Search Engines for Shape Selectivity
Zeolites have been used successfully for over 30 years to separate small molecules of almost identical physical and chemical properties, relying on shape differences such as normal from branched paraffins. Many zeolites have approximately circular windows and these are the only ones used in current applications. As of today, the structures of 176 zeolites are available; many of them have windows that may be described as elliptic or irregular, presenting many opportunities for shape selective catalysis or adsorption. A molecule approaching a window may take a number of orientations, and it is difficult to visualize the three-dimensional shape and the best fit to the window. Search engines are needed to accelerate the discovery of one or more zeolite windows that can separate a given pair of molecules. A great deal of progress has already been made to construct such search engines, and innovative industrial processes may follow in the future.
KeywordsShape selectivity Zeolites Molecular flexibility Strain energy
The shapes of molecules have been utilized for catalysis  and for industrial separations  by zeolites. Shape selective separation is particularly useful when two molecules are closely related in physical and chemical properties but have different shapes, such as the case of normal and branched paraffins in the catalytic de-waxing by SAPO-11. A linear normal paraffin can be rotated to a slim configuration so that its projection onto the plane of the window, or “footprint,” is small enough to penetrate the window with ease. On the other hand, a branched paraffin has a footprint that is larger regardless of the rotation angles [3, 4]. Another industrial example is the separation of the narrower p-xylene from the wider o- and m-xylenes by ZSM-5. Such a separation can be due to one of three mechanisms: steric separation is achieved when only small and properly shaped molecules can diffuse in the pores but larger molecules are excluded; equilibrium separation is achieved when the equilibrium adsorption concentrations are selectively reduced by different amounts; and kinetic separation is achieved when one molecule adsorbs faster than the other due to different speeds of diffusion.
1 Hard Sphere Models and Footprints
It is easy to search for a circular window that would separate two spherical molecules of different diameters. For instance the noble gases vary in their van der Waals diameters from 2.5 Å for helium to 4.0 Å for xenon, so a 3A zeolite with a window diameter of 3.0 Å will allow the passage of helium and neon, but retain argon, krypton and xenon. However, most molecules are not spheres. Molecules with C∞v symmetry, such as HCl and CO, and with D∞h symmetry, such as hydrogen and acetylene, do produce circular footprints . The optimal rotation is to point the narrowest footprint towards the window; that is, to place the molecular axis perpendicular to the window plane.
2 First Flexible Model and Activation Energy
The hard sphere model of the atoms with rigid bond angles and lengths.
The Lennard-Jones radii of atoms σ are flexible, with an energy parameter of ε/R governing the softness of the atoms. On the window side, only the larger oxygen atoms are involved as the smaller T-atoms of silicon or aluminum are turned towards the outside of the frame.
The bond angles and bond lengths of the window are also flexible, and this involves only the T–O–T bonds.
The bond angles and bond lengths of the molecules are also flexible; this is a much more complex problem as there are many more types of bonds to consider.
The entropy effects due to the possibility that the approach of a molecule towards the window is turned back, resulting to the same molecule coming back for another approach.
Parameter values used
Atomic van der Waals radii in Å 
Molecular Lennard-Jones parameters, ε/R and σ 
T–O–T bond length and bond angle parameters 
kTO (kJ/mol Å2)
kOO (kJ/mol Å2)
The best separation is reached when A has free entry and B is hindered; that is, EA = 0 and EB > 8 kJ/mol, so that a very sharp separation (γAB > 0.95) can be carried out at room temperature. However, if the entry of both A and B are hindered, to reach a separation selectivity of γAB = 0.2 we would recommend zeolites that correspond to activation energy values EA < 1 and EB > 2 (kJ/mol). If a higher separation selectivity of γAB = 0.8 is desired, we would recommend EA < 0.1 and EB > 5 (kJ/mol).
3 Model Refinements
During our computations, it is important to use appropriately tuned values for the atom radii parameters. It should be noted that the van der Waals values were derived from viscosity data, and the Lennard-Jones values were derived from equations of state and gas compressibility data; thus, they are not necessarily the best to describe the interaction of molecules in contact with walls. Particularly questionable is the value of 1.40 Å for oxygen atomic radius, as it would predict that aromatic molecules could not penetrate ZSM-5, which is not the case. We have studied the effect of atomic radius of oxygen on the penetration of the three xylenes in ZSM-5, and concluded that the appropriate radius is actually between 0.95 and 1.10 Å (so that the windows become larger and more accommodating). On the other hand, the penetration of nitrogen and methane in Sr-ETS-4 at 573 K would be better explained if the effective radius is 1.30–1.40 Å .
Commercial uses of zeolites always involve several modification steps to enhance selectivity, such as by changing the cation from sodium to calcium, by polymer additives, by coke deposition, and by steaming. All of these would have effects on the size and shape of window opening, and consequently the activation energy and equilibrium concentrations.
4 Future Opportunities
C/C0 predictions for separators of lingo-cellulosic biomass fermentation inhibitors 
Comparison between zeolites and other classes of sorbent materials
MW of potential sorbates
Complete list of investigated molecules
He, Ne, Ar, Kr, Xe, Rn
H2, N2, O2, CO, CO2, NH3
H2O, H2Se, H2S, H2Te
H2CO3, HNO3, H3PO4, H2SO4, HF, HCl, HBr, HI
CH4, C2H6, C2H4, C2H2, C3H8, C3H6, Propadiene, nC4H10, iC4H10, iso-butylene, alpha-butylene, cis-beta-butylene, trans-beta-butylene, butadiene, methylacetylene, nC5H12, iC5H12, neoC5H12, isoprene, spiropentane, diethyl-pentane, C10H22, C15H32, C20H42, TetraDecaneMethane (MOBIL1)
Benzene, toluene, o-xylene, m-xylene, p-xylene, mesitylene, ethyl-benzene, benzocyclobutene, biphenyl, napthalene, anthracene, phenanthrene, napthacene, triphenylene, pyrene, fluoranthene, tetraphene, chrysene, benzopyrene, coronene
CH3OH, C2H5OH, C3H7OH, isopropyl-alcohol, isobutyl-alcohol, tertamyl-alcohol, phenol, ethylene-glycol, propylene-glycol, glycerol
HCHO, CH3CHO, acetone, CH3COCOOH, C6H5COH, benzophenone, methyl-urea, urea
Carboxylic acids and anhydrides
HCOOH, CH3COOH, C2H5COOH, CH2CHCOOH, CH3CH2CH2COOH, C6H5COOH, oxalic-acid, malonic-acid, succinic-acid, maleic-acid, glutaric-acid, adipic-acid, salicylic-acid, citric-acid, maleic-anhydride, phthalic-anhydride
Propyl-methanoate, ethyl-ethanoate, propyl-ethanoate, ethyl-propanoate, methyl-butanoate
Cholesterol, progesterone, testosterone, l-lactic-acid, d-lactic-acid, estrone, insulin
Lauric-acid, stearic-acid, oleic-acid, elaidic-acid, linoleic-acid, alpha-linolenic-acid, gamma-linolenic-acid, arachidonic-acid, erucic-acid
Retinol (Vitamin-A), riboflavin (Vitamin-B2), niacin (Vitamin-B3), ascorbic-acid (Vitamin-C)
Sugars and sweeteners
Fructose, glucose, sucrose, aspartame, cyclamate, saccharin
CH3NH2, C2H5NH2, dimethyl-amine, trimethyl-amine, monoethanol-amine, methylethanol-amine, dimethylethanol-amine, diethanol-amine, methyldiethanol-amine, triethanol-amine, tris, bis–tris, tricine, aniline, diphenyl-amine, triphenyl-aniline, amphetamine, ethylene-diamine, diethylene-triamine, triethylene-tetramine
Cyclohexane, methyl-cyclohexane, nitrobenzene, chlorobenzene, benzyl-chloride, menthol, nitroaniline, nicotine, caffeine, uric-acid, alizarin, indigo
Aziridine, azetidine, pyrrolidine, piperidine, pyrrole, indole, pyridine, quinoline, isoquinoline, acridine, pyrimidine, purine, furan, benzofuran, furfural, hydroxymethyl-furfural, vanillin, thiophene, benzothiophene, dibenzothiophene, MDBT, DMDBT
Acetyl-salicylic-acid (ASPIRIN), amlodipine (NORVASC), ampicillin, atorvastatin (LIPITOR), barbituric-acid, clopidogrel (PLAVIX), cocaine, codeine, ephedrine, heroin, methadone, methamphetamine, morphine, naloxone, olanzapine (ZYPREXA), paclitaxel (TAXOL), paracetamol (TYLENOL), penicillin-G, phenacetin, phenylephrine, simvastatin (ZOCOR)
Explosives and propellants
H2O2, hydrazine, monomethyl-hydrazine, unsymmetrical-dimethyl-hydrazine, dinitrogen-tetroxide, picric-acid, trinitrotoluene
CCl4, CCl3F, CCl2F2, CClF3, CF4, CF3I, CBrF3, CBrClF2, CHF3, CHCl3, CHBr3, CHClF2, CH2F2, CH2Cl2, CH2Br2, CH3F, CH3Cl, CH3Br, C2Cl6, Cl3CCH3, Cl2FCCClF2, Cl2FCCH3, Cl2HCCH3, ClH2CCH2Cl, ClF2CCClF2, Cl3CCF3, ClF2CCF3, ClF2CCH3, ClFHCCF3, F2HCCF3, FH2CCF3, F2HCCHF2, F2HCCH3
Alanine, arginine, asparagine, aspartic-acid, cysteine, glutamic-acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, valine
Adenine, cytosine, guanine, thymine, uracil
SF4, SF6, UF6, S2F10
Adamantane, cubane, octanitrocubane, C20, C60
HCN, camphor, dimethyl-sulfoxide, perfluorooctane-sulfonate, perfluorooctanoic-acid
Openings are reported in terms of distances between nuclei of O-atoms. Actual openings are, thus, smaller by two O-atom radius (typically 1.40 Å).
The authors gratefully acknowledge financial support from the National Science Foundation.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
- 1.Chen NY, Garwood WE, Dwyer FG (1996) Shape selective catalysis in industrial applications. Marcel Dekker, New YorkGoogle Scholar
- 5.Baerlocher C, McCusker LB, Olson DH (2007) Atlas of Zeolite framework types, 6th edn, Structure Commission of the International Zeolite Association, Elsevier, Amsterdam, HollandGoogle Scholar
- 6.Eliel EL, Wilen SH (1994) Stereochemistry of organic compounds. Wiley, New YorkGoogle Scholar
- 7.Hypercube Inc (1996) HyperChem® Getting Started Release 5.0 for Windows. Gainsville, FloridaGoogle Scholar
- 8.Pauling L (1947) General chemistry. Dover, New YorkGoogle Scholar
- 9.McQuarrie DA, Simon JD (1999) Molecular thermodynamics. University Science Books, SausalitoGoogle Scholar
- 11.Gounaris CE, Ranjan R, Tsapatsis M, Wei J, Floudas CA (2009) Rational design of shape selective separation and catalysis: lattice relaxation and effective aperture size. AIChE J (submitted for publication)Google Scholar
- 12.Ranjan R, Thust S, Gounaris CE, Woo M, Floudas CA, Von Keitz M, Valentas KJ, Wei J, Tsapatsis M (2009) Adsorption of fermentation inhibitors from lignocellulosic biomass hydrolyzates for improved ethanol yield and value-added product recovery. Microporous Mesoporous Mater (in press)Google Scholar