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5.2: Gas Chromatography Analysis of the Hydrodechlorination Reaction of Trichloroethene

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  • Trichloroethene (TCE) is a widely spread environmental contaminant and a member of the class of compounds known as dense non-aqueous phase liquids (DNAPLs). Pd/Al2O3 catalyst has shown activity for the hydrodechlorination (HDC) of chlorinated compounds.

    To quantify the reaction rate, a 250 mL screw-cap bottle with 77 mL of headspace gas was used as the batch reactor for the studies. TCE (3 μL) is added in 173 mL DI water purged with hydrogen gas for 15 mins, together with 0.2 μL pentane as internal standard. Dynamic headspace analysis using GC has been applied. The experimental condition is concluded in the table below (Table \(\PageIndex{1}\)).

    TCE 3 μL
    H2 1.5 ppm
    Pentane 0.2 μL
    DI water 173 mL
    1 wt% Pd/Al2O3 50 mg
    Temperature 25 °C
    Pressure 1 atm
    Reaction time 1 h
    Table \(\PageIndex{1}\) The experimental condition in HDC of TCE.

    Reaction Kinetics

    First order reaction is assumed in the HDC of TCE, \ref{1}, where Kmeans is defined by \ref{2}, and Ccatis equal to the concentration of Pd metal within the reactor and kcat is the reaction rate with units of L/gPd/min.

    \[ -dC_{TCE}/dt\ =\ k_{meas} \times C_{TCE} \label{1} \]

    \[ k_{meas} \ =\ k_{cat} \times C_{cat} \label{2} \]

    The GC Method

    The GC methods used are listed in Table \(\PageIndex{3}\).

    GC type Agilent 6890N GC
    Column Supelco 1-2382 40/60 Carboxen-1000 packed column
    Detector FID
    Oven temperature 210 °C
    Flow rate 35 mL/min
    Injection amount 200 μL
    Carrier gas Helium
    Detect 5 min
    Table \(\PageIndex{3}\) GC method for detection of TCE and other related chlorinated compounds.

    Quantitative Method

    Since pentane is introduced as the inert internal standard, the relative concentration of TCE in the system can be expressed as the ratio of area of TCE to pentane in the GC plot, \ref{3}.

    \[ C_{TCE}\ =\ (peak\ area\ of\ TCE)/(peak\ area\ of\ pentane) \label{3} \]

    Results and Analysis

    The major analytes (referenced as TCE, pentane, and ethane) are very well separated from each other, allowing for quantitative analysis. The peak areas of the peaks associated with these compounds are integrated by the computer automatically, and are listed in (Table \(\PageIndex{4}\)) with respect to time.

    Time/min Peak area of pentane Peak area of TCE
    0 5992.93 13464
    5.92 6118.5 11591
    11.25 5941.2 8891
    16.92 5873.5 7055.6
    24.13 5808.6 5247.4
    32.65 5805.3 3726.3
    43.65 5949.8 2432.8
    53.53 5567.5 1492.3
    64.72 5725.6 990.2
    77.38 5624.3 550
    94.13 5432.5 225.7
    105 5274.4 176.8
    Table \(\PageIndex{2}\) Peak area of pentane, TCE as a function of reaction time.

    Normalize TCE concentration with respect to peak area of pentane and then to the initial TCE concentration, and then calculate the nature logarithm of this normalized concentration, as shown in Table \(\PageIndex{3}\).

    Time (min) TCE/pentane TCE/pentane/TCEinitial In(TCE/Pentane/TCEinitial)
    0 2.2466 1.0000 0.0000
    5.92 1.8944 0.8432 -0.1705
    11.25 1.4965 0.6661 -0.4063
    16.92 1.2013 0.5347 -0.6261
    24.13 0.9034 0.4021 -0.9110
    32.65 0.6419 0.2857 -1.2528
    43.65 0.4089 0.1820 -1.7038
    53.53 0.2680 0.1193 -2.1261
    64.72 0.1729 0.0770 -2.5642
    77.38 0.0978 0.0435 -3.1344
    94.13 0.0415 0.0185 -3.9904
    105 0.0335 0.0149 -4.2050
    Table \(\PageIndex{3}\) Normalized TCE concentration as a function of reaction time

    From a plot normalized TCE concentration against time shows the concentration profile of TCE during reaction (Figure \(\PageIndex{1}\), while the slope of the logarithmic plot provides the reaction rate constant (\ref{1}).

    Figure \(\PageIndex{1}\) A plot of the normalized concentration profile of TCE.
    Figure \(\PageIndex{2}\) A plot of ln(CTCE/C0) versus time.

    From Figure \(\PageIndex{1}\), we can see that the linearity, i.e., the goodness of the assumption of first order reaction, is very much satisfied throughout the reaction. Thus, the reaction kinetic model is validated. Furthermore, the reaction rate constant can be calculated from the slope of the fitted line, i.e., kmeas = 0.0414 min-1. From this the kcat can be obtained, \ref{4}.

    \[ k_{cat}\ =\ k_{meas}/C_{Pd}\ =\ \frac{0.0414min^{-1}{(5 \times 10^{-4}\ g/0.173L)}\ =\ 14.32L/g_{Pd}\ min \label{4} \]