# 5.2: Gas Chromatography Analysis of the Hydrodechlorination Reaction of Trichloroethene


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

## 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

### 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

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

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}).

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}.

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

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