Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere

J. Ind. Eng. Chem., Vol. 13, No. 3, (2007) 400-405 Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere Yang-Soo Won† Department of Environmental Engineering, Yeungnam University Gyeongsan City 712-749, Korea Received September 11, 2006; Accepted January 16, 2007 Abstract: The four chlorinated methanes, methyl chloride (CH3Cl), methylene chloride (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4), were used as model chlorocarbon systems with Cl/H ratios of 0.021 to 0.083 to investigate the thermal stability and hydrodechlorination of chloromethanes in excess hydrogen. The pyrolytic reactions were studied in an isothermal tubular reactor at a total pressure of 1 atm with reaction times of 0.3∼2.0 s at temperatures between 525 and 900 oC. The thermal stabilities of the chloromethanes, o o i.e., the temperatures for 99 % destruction within a reaction time of 1 s were 875 C for CH3Cl, 780 C for o o CH2Cl2, 675 C for CHCl3, and 635 C for CCl4. The less-chlorinated hydrocarbons were more stable, with CH3Cl the most stable chlorocarbon in this reaction system. This work focused on pyrolysis of CH3Cl in an excess-hydrogen reaction atmosphere. The observed hydrodechlorinated products were CH4, C2H4, and C2H6 at o temperatures above 850 C in the CH3Cl/H2 reaction system. The number and quantities of intermediate chlorinated products decreased with increasing temperature; the formation of non-chlorinated hydrocarbons increased as the temperature rose. One of main pathways for hydrodechlorinated products resulted from H atom cyclic chain reaction by abstraction. Product distributions along with preliminary activation energies and rate constants are reported. The pyrolytic reaction pathways that describe the important features of reagent decay and intermediate product distributions, based upon thermochemical and kinetic principles, are suggested. Keywords: pyrolysis, thermal stability, reaction mechanism, chloromethane, methyl chloride, methylene chloride, chloroform, carbon tetrachloride Introduction generally incinerated in an oxygen-rich environment, 1) with relative small amounts of hydrogen [5]. It is re- Chlorinated organic compounds are widely used in syn- ported that the combustion of chlorinated hydrocarbons thesis and in the chemical industry. Thermal treatment of under severe conditions converts all carbon to CO2 [6,7]. these chlorinated hydrocarbons provides a source of It is important to understand both the pyrolysis and oxi- chlorine atoms in the initial stages of the process; they dation of these compounds. are thought to be associated with the formation of ar- When oxygen is involved in the process, oxygen and Cl omatics, such as dibenzodioxins and dibenzofurans in in- both compete for the available fuel hydrogen, which is cinerators, which have gained much attention due to the one reason why chlorinated hydrocarbons serve as flame fact that some are toxic or, indeed, carcinogenic [1-4]. inhibitors [8]. Also, C-Cl bonds may persist in an oxy- Different technologies have been developed for the safe gen-rich system of limited hydrogen atmosphere [6,9], so destruction of chlorinated hydrocarbons. Thermal de- that the emission of toxic chlorinated organic products struction of organic pollutants in an oxygen-rich atmos- persists during oxygen-rich incineration, in which carbon phere is the method most often used in the chemical species are one of the more stable sinks for chlorine waste disposal industry. Chlorinated hydrocarbons are atoms. To obtain quantitative formation of HCl, as one of the desired and thermochemically favorable products, from † To whom all correspondence should be addressed. chlorinated hydrocarbons, one might use a straightfor- (e-mail: [email protected]) Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere 401 ward thermal conversion of these compounds under a file of the tubular reactor was obtained using a type-K more reductive atmosphere of hydrogen. The “non-oxy- thermocouple probe moving coaxially within the reactor gen” methods were developed to avoid the formation of under steady state flow. The temperature profiles shown undesired oxy-containing products, such as phosgene and with varied flow rates in Figure 2 resulted from carefully dioxins [10,11]. The chlorocarbon and hydrogen system adjusting the heat flux to the reactor at each different contains only C, H, and Cl elements; it is expected to flow rate. Tight temperature control resulted in iso- lead to the formation of light hydrocarbons and hydrogen thermal temperature profiles within ±3 oC over 75 % of chloride at the temperatures where complete reaction the furnace length for all of the temperature ranges of occurs. Under such a system, carbon can be converted to this study. CH4, C2H2, C2H4, and C2H6 [12,13]. In this study, pure-compound chloromethanes were used as a model chlorocarbon system to investigate the thermal stability and hydrodechlorination of chlorocar- bons with excess hydrogen. This work focused on the intermediate product distributions and the major reaction pathways to form various products based on fundamental thermochemical and kinetic principles for the pyrolytic reaction of methyl chloride (CH3Cl) with excess hydrogen. We characterized the reactant loss and intermediate product formation as functions of time and temperature to describe the reaction process, and to investigate the feasibility of formation of light hydrocarbons, e.g., CH4 or C2H4, from the pyrolytic reaction of CH3Cl. Figure 1. Schematic diagram of the experimental system. Experimental Method The experimental apparatus and the procedures used in this study were similar to those used in our earlier studies [8,9,12]. Therefore, only a brief summary of these sub- jects is given. Pure chloromethanes were reacted with hydrogen (in the absence of O2) in an isothermal tubular reactor at 1 atm. The products of such thermal degradation were analyzed systematically by varying the temperature from 525 to 900 oC and the residence time from 0.3 to 2.0 s. A diagram of the experimental system is shown in Figure 1. Hydrogen gas was passed through a multi-satu- Figure 2. Reactor temperature profiles with tight control. rator train held at 0 oC to ensure saturation with chlor- methane at a constant reference temperature for accurate An HP 5890II on-line GC with FID was used to de- vapor pressure calculation. A second (dilutent) stream of termine the concentrations of the reaction products. The hydrogen gas was used to maintain throughout the de- GC used a 5-ft-long by 1.8-in. o.d. stainless-steel column sired mole fraction of 4 % chloromethanes the experi- packed with 1 % Alltech AT-1000 on graphpac GB as ment. The reagent with hydrogen gas was fed con- the column. A six-port gas sample valve with a 0.5-mL tinuously into the tubular flow reactor in the vapor phase. volume loop was used to inject the sample. Quantitative The mixture was preheated to ac. 200 oC before entering analysis of HCl was performed for each run. The samples the reactor to improve isothermal temperature control. for HCl analysis were collected independent from GC The reactor effluent was passed through heated transfer sampling. Reactor effluent was diverted to bubbler trains lines to the gas chromatograph sampling valves and containing 0.01 M NaOH before being exhausted to a exhaust. All gas lines to the analytical equipment were fume hood. The amount of HCl produced was then calcu- held at 170 oC to limit condensation. The quartz tube re- lated based on titration of the bubbler solution with 0.01 actor (8 mm ID) was housed within a three-zone electric M HCl to its phenolphthalein end point. tube furnace (32 in. long) equipped with three independent temperature controllers. The actual temperature pro- 402 Yang-Soo Won Results and Discussion dominant initiation decomposition path for CHCl3 from experimental results based upon product distributions [8]. Decay of Pure Compound Chloromethanes The decomposition of CCl4 was more sensitive to in- Figure 3 depicts thermal degradation profiles of chloro- creasing temperature, and the decay curve of CHCl3 o methanes for each pure compound as functions of tem- crossed at 570 C (40 % destruction) with CCl4 more o perature at 1-s reaction times under an excess hydrogen easily degraded above 570 C, as illustrated in Figure 3. atmosphere. The parent thermal stabilities (defined by the temperature required for 99 % destruction) were 875 Table 1. Bond Dissociation Energies for Chloromethanes o o o [14,15] C for CH3Cl, 780 C for CH2Cl2, 675 C for CHCl3, and o Energy Energy 635 C for CCl4. The bond dissociation energies of chlo- C-H Bond C-Cl Bond (kcal/mole) (kcal/mole) romethanes for C-H and C-Cl are listed in Table 1 CH -H 105 - - [14,15]. These characteristics of chlorinated hydrocarbon 3 CH2Cl-H 103 CH3-Cl 84 thermal decomposition can be attributed in part to the CHCl2-H 101 CH2Cl-Cl 81 weaker C-Cl bond strengths relative to C-H bonds. The CCl3-H 96 CHCl2-Cl 77 low strength of the C-Cl bond means that thermal unim- - - CCl3-Cl 70 olecular fission of Cl from C-Cl compounds occurs or- ders of magnitude faster than the similar loss of H from Table 2. Kinetic Parameters for Decomposition of Choloroform C-H. The trend of weakening C-Cl bond strengths oc- Reaction rate parameter curred with increased substitution of chlorine for hydro- Reaction A Ea k -1 o ref. rzn no. gen. The 99 % destructions of each pure chloromethane (s ) (kcal/mole) (at 600 C) -3 were in agreement with the Least Bond Dissocia- tion CHCl3→ :CCl2+HCl 2.5E16 74.6 5.3×10 8 (1a) Energy (LBDE) trend shown in Table 1. This situation CHCl3→CHCl2+Cl 1.6E14 56.0 1.53 8 (1b) implies that the less-chlorinated methanes are more stable, consistent with the bond strengths of C-Cl bonds on The acceleration of CCl4 decomposition with increasing chlorinated hydrocarbons, which increase with decreas- temperature resulted from several combinated effects: ing chlorination.

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