The Inverse Isotope Effect of the Synthesis of Hydrocarbons from CO and H2.
BUCHANG SHI*1, CHUNFEN JIN1, MICK A. MILLER1, MUTHU K. GNANAMATI2,
SHENKE ZHENG3, YALI LIU3, JINLIN LI3, BURTRON H. DAVIS2. 1Department of
Chemistry, Eastern Kentucky University, Richmond, KY 40475. 2Center for Applied Energy Research,
2540 Research Park Dr., Lexington, KY 40511. 3Department of Chemistry, Central South University for
Nationalities, Wuhan, PRC
Hydrogen/deuterium (H2/D2) switching experiments were conducted in a small fixed bed reactor for
hydrocarbon production through Fischer-Tropsch (FT) synthesis catalyzed by a cobalt catalyst. The results
show that when syngas is switched from CO/H2 to CO/D2, the CO conversion is increased; when syngas is
switched back from CO/D2 to CO/H2, the CO conversion is decreased as shown in Fig. 1. The kH/kD is
calculated to be 0.84 – 0.88, an inverse isotope effect. Since the kH/kD is calculated based on the
measurement of CO conversion, independent of the measurements of H 2 or D2, it is clear that an inverse
isotope effect has been demonstrated for the Co catalyzed FT reaction.
To ensure that the CO/H2 and CO/D2 are reacted under exactly the same reaction conditions and to
obtain additional information about the rate controlling steps, we have conducted competition experiments
utilizing equal moles of H2 gas and D2 gas as the reagents. The H/D ratios for hydrocarbons with carbon
number 7 to 19 are determined. The H/D ratios of these compounds are less than 1, an indication of the
presence of an inverse isotope effect as shown in Fig. 2. We also found that the H/D ratios decrease as the
carbon number increases, an indication of an accumulative inverse isotope effect. The possible rate
controlling step or steps of FT synthesis catalyzed by a cobalt catalyst will be discussed based on these
observations.
Fig. 1: The CO conversions in a H2/D2 switching
experiment. In the period of CO/H2 run, the CO
conversion is about 84%; when it is switched to CO/D2
run, the CO conversion is increased to 96%; when it is
switched back to CO/H2 run, the CO conversion is
deceased to 80%.
120
CO% Conversion
100
80
60
40
20
0
30
50
70
90
110
130
Time on Stream (hours)
experimental
calculated based on 1:1 ratio of H/D
Mol%
15.00
10.00
5.00
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41
0.00
Number of Deuterium
.
Fig. 2: Isotopemer Distribution of Pentadecane
in a 1:1of H2/D2 run. The calculated value is
based on 1:1 ratio of H/D by assuming the
isotopmer distributed statistically.