Light hydrocarbon gas conversion using porphyrin catalysts Page: 4 of 9
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this is not surprising. The concept of steric bulk preventing bimolecular catalyst destruction
would be more applicable to a P450-like mechanism and to bulkier porphyrin catalysts than those
investigated here. Any enhancement of the catalyst activity or selectivity by the cavity adjacent
to the metal center was unable to be verified when 02 is used as the oxidant. Rapid catalyst
decomposition makes it impossible to definitively determine whether or not the pocket provides
any beneficial effect. If it does, it is small enough to be canceled out by the poor stability and
small differences in porphyrin charge depletion. In any case, molecular modeling suggests that a
more enclosed cavity is required to achieve radical trapping, promote substrate binding, and
prevent bimolecular degradation reactions.
Current Progress. Our previous attempts to measure the activity of Sandia-developed porphyrins
as alkane oxidation catalysts have been hampered by rapid catalyst decomposition and low
apparent activities. Furthermore, such low activities are inconsistent with results reported by
other research groups, including scientists at Sun Co. At the invitation of Drs. James Lyons and
Paul Ellis, Margaret Showalter traveled to the Sun Co. to test some of the Sandia catalysts in the
Sun reactors. In this collaborative effort, two different Sandia porphyrins and one Sun
porphyrin, shown in Figure 1, were tested side-by-side using a catalyst screening experiment
developed at Sun for isobutane conversion. There are three major differences in the Sun
procedure and the procedure used previously at Sandia. First, catalyst testing at Sandia has been
performed in teflon-lined stainless steel reactors, while the reaction vessel employed at Sun is a
glass aerosol tube. Second, the scientists at Sun have found that when the axial ligand
coordinated to the metalloporphyrin is OH, the reaction is usually easier to initiate than when the
ligand is Cl. Thus Sun prefers to screen potential catalysts as the OH form; however the
porphyrins tested at Sandia have had Cl as the ligand. Before traveling to Sun, one of the Sandia
catalysts to be tested was converted from the Cl-porphyrin to the OH-porphyrin. Finally,
isopentane is the substrate used for screening at Sandia, since this alkane is a liquid and is thus
simple to handle in the laboratory. However, the Sun researchers typically screen oxidation
catalysts with isobutane as the substrate.
FF O O F
F CI CI F
F F O O F OF F F
CI N C, O N O Q NO
N---Fe---N N---f ie--N N-Fe-N
Cl N G N OF
F- O F F F
CI CI O "
Sun-OH Sandia l-OH Sandia2-C1
(1.00) (0.45) (0.59)
Figure 1. Structures of porphyrins tested at Sun Co.; -OH and -Cl indicate the axial ligand, not shown in
the structure. The numbers in parenthesis are the predicted activities for identical axial ligands
relative to the Sun-OH catalyst based on the sum of the Hammett para substituent constants).
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Showalter, M.C. & Shelnutt, J.A. Light hydrocarbon gas conversion using porphyrin catalysts, article, July 1, 1995; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc793372/m1/4/: accessed September 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.