THERMAL DECOMPOSITION OF tert-BUTYL PEROXIDE IN A GAS CHROMATOGRAPHIC REACTOR
387
peroxide with GCR approaches designed to utilize both
types of reactors.
The reaction of tert-butyl peroxide inside the injec-
tion port was studied by varying the injector tempera-
◦
◦
Using the injection port as the reactor, the chro-
matographed reactant peak areas measured as a
function of the injector temperature are used in the
determination of the activation energy (Ea) for the first
order decomposition of the peroxide. This method is re-
ferred to as the Injection Port Reactor (IPR) approach in
this work. In addition, using “reaction chromatograms”
obtained from the on-column decomposition of the
compound, two different analyses are employed for the
determination of Ea. One is the Product Curve (PC)
method, originally described by Langer and Patton [4].
This approach utilizes the shape of the product peak
to obtain rate constants at various column tempera-
tures. The rate constants are subsequently plotted us-
ing the Arrhenius equation to determine Ea. The other
approach, referred to as the PACR (an abbreviation for
ture from 115 to 275 C (using 10 C increments), with
◦
the column temperature maintained at 90 C and the
column pressure held at 21.5 psi He. The on-column
decomposition of the compound was investigated with
◦
the oven temperature varied from 110 to 185 C (using
◦
5 C increments, isothermal runs) and the injector tem-
perature maintained at 195 C. In this case, the column
◦
pressure was held at 6.9 psi He.
For both sets of experiments, a 1.0 ꢀl injection vol-
ume and 50:1 injector split ratio were used. The FID
◦
TM
temperature was maintained at 250 C. ChemStation
(Agilent Technologies, Wilmington, DE) software was
used to capture the total ion chromatograms and mass
TM
spectra, while Turbochrom (Perkin Elmer, San Jose,
CA) software was used to collect chromatograms ob-
tained with the FID; the data acquired at a rate of 10
points per second (note: all chromatograms shown in
this work were acquired with the FID).
“peak area, column reactor”) method, uses measure-
ments of the reactant peak area as a function of column
temperature to obtain Ea directly. The mathematical
treatment described in this approach is relatively new;
it has been described by us in only a few recent GCR-
related works [6–8].
RESULTS AND DISCUSSION
IPR Approach
The purpose of this article is to evaluate the three
techniques in terms of their ease of implementation,
robustness (i.e. range of applicability), speed, and the
accuracy of the results obtained. The decomposition of
tert-butyl peroxide is used as a model chemical system
for the evaluation, as this material has been the subject
of extensive kinetic studies (e.g. [9]): the activation en-
ergy for the decomposition is known to be in the range
Using MS detection, the thermal decomposition of tert-
butyl peroxide (structure shown in Fig. 1) in the GC
injection port was found to evolve the following prod-
ucts in various quantities: acetone, ethane, 2-methyl-
1-propene, and oxygen. The mixture of products ob-
served is consistent with a highly reactive, free-radical
intermediate of the parent compound. Figure 2 shows a
plot of the chromatographic peak area of the reactant as
afunctionoftheinjectortemperature. Thegeneraltrend
of the plot indicates more decomposition at higher tem-
peratures. In order to extract kinetic information from
the data, an equation derived by Schmiegal et al. [10]
and Sethi et al. [11] is used. These researchers com-
bined the first order rate law with the Arrhenius ex-
pression and Charles’ law to yield the general Eq. [4]:
◦
1
55–160 kJ/mol and the enthalpy of reaction, ꢀH , is
approximately −1.39 kJ/g. The ultimate goal of this
work is to identify the best approach for the kinetic
characterization of elementary reactions by GCR. In
so doing, we wish to establish the clear superiority of
our PACR method over the IPR and PC methods.
EXPERIMENTAL
ln[T ln(A0/A)] = constant − Ea/RT
(1)
Thetestcompound, tert-butylperoxide(98%), waspur-
chased from Sigma-Aldrich (Milwaukee, WI) and used
without further purification. Gas chromatograms were
obtained in duplicate under isothermal and isobaric
conditions on an Agilent Technologies (Palo Alto, CA)
HP6890GCsystemequippedwithanHP5973massse-
lective(MS)detector, aflameionizationdetector(FID),
where A0 is the amount of analyte entering the injector,
A is the amount exiting, T is the injector temperature,
and R is the gas constant. (Note that A0 and A are
dual split/splitless injection ports, auto-sampler, and a
ꢀ
6
0 m Rtx-1 (100% dimethylpolysiloxane oil station-
ary phase, Restek Corporation, Bellefonte, PA) open
tubular column with 0.32 mm i.d. and 1.0 ꢀm film
thickness.
Figure 1 Chemical structure of the test compound, tert-
butyl peroxide.