Green Chemistry with Zeolite Catalysts

    It is difficult to spend any time in the chemical process industry without hearing about zeolite catalysts.  "Zeolite" is the broad term used to describe a family of minerals called tectosilicates.  These minerals contain small pores which provide a generous surface area.  Zeolites are constructed of tetrahedral AlO₄^-5 and SiO₄^-4 molecules bound by oxygen atoms.  Currently, there are 40 known natural zeolites and in excess of 140 synthetic zeolites.  Zeolites can be custom made by manipulating the structure, silica-alumina ratio, pore size, and density.  Other metals can also be incorporated into zeolites to obtain specific catalytic properties.

The motivations for using zeolite catalysts are primarily profit and environmental regulation compliance.  Zeolites can help produce products at more mild temperatures and pressures which lowers operating costs.  They also are used for their superior control of reaction selectivity which saves on feed costs and by reducing waste streams, saves on treatment costs.

Specific Zeolite Accomplishments

1.  NOx emission reductions by selective catalytic reduction (SCR)
2.  Direct oxidation of benzene to phenol which eliminates cumene as an intermediate and uses nitrous oxide (N₂O) as a reactant.  Nitrous oxide is a typical waste stream from adipic acid production.
3.  Ethylbenzene to styrene conversion at moderate temperatures and pressures.   This route also eliminates xylene as a by-product which eliminates several purification steps.
4.  Caprolactam via oxidation which drastically reduces the number of processing steps as well as waste streams.
5.  Ability to regenerate/recycle a process' catalyst.  Many catalyst must be disposed of after they are spent, but the impregnating of zeolites is a process that can be repeated over and over.  Ironically, zeolites themselves help reduce waste.

References:

   Marcus, Bonnie K. and Cormier, William E., "Going Green with Zeolites", Chemical Engineering Progress, June 1999
    SRI Consulting's website, "Process Economics Program"