Experimental Study on the Reaction of CCl3F and CH4 in a Dielectric

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Experimental Study on the Reaction of CCl3F and CH4 in a Dielectric Barrier Discharge Non-equilibrium Plasma Reactor Sazal K. Kundu, Eric M. Kennedy, John C Mackie, Clovia I. Holdsworth, Thomas S. Molloy, Vaibhav V. Gaikwad, and Bogdan Z Dlugogorski Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04010 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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Industrial & Engineering Chemistry Research

Experimental Study on the Reaction of CCl3F and CH4 in a Dielectric Barrier Discharge Nonequilibrium Plasma Reactor Sazal K. Kundu,† Eric M. Kennedy,*† John C. Mackie,† Clovia I. Holdsworth,§ Thomas S. Molloy,† Vaibhav V. Gaikwad,† and Bogdan Z. Dlugogorski‡ †

Process Safety and Environment Protection Research Group, School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia

§

Discipline of Chemistry, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia

‡

School of Engineering and Information Technology, Murdoch University, Murdoch, WA 6150, Australia

ABSTRACT: The reaction of CCl3F (CFC-11) with CH4 (in an argon bath gas) in a dielectric barrier discharge non-equilibrium plasma was examined. Oxygen and nitrogen were excluded from the feed stream and the reactions resulted in the production of fluorine-containing polymers, as well as a range of gaseous products including H2, HCl, HF, C2H3F, C2H3Cl, C2H2ClF, CHCl2F, CCl2F2, CH3Cl, CH2Cl2, CHCl3 and C2Cl4. The polymeric material synthesised during reaction is characterised as being non-crosslinked and random in nature, containing functional groups including CH3, CH2, CHCl, CHF, ACS Paragon Plus Environment

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CF2 and CF3. The conversion level of CCl3F increased from 37 % to 63 % as the input energy density increased from 3 kJ L-1 to 13 kJ L-1 (the applied voltage range is 14.1 to 15.2 kV, peak-peak). The electrical discharge was characterised and found to be a slight modification of filamentary discharge towards a diffuse discharge which was resulted by the inclusion of the relatively low concentration of CCl3F and CH4 (less than 2 % each) in argon. A reaction mechanism is proposed describing the formation of gas phase as well as polymeric products.

1. INTRODUCTION CFC–11 (CCl3F), a notorious member of chlorofluorocarbons (CFCs), is a compound that has the highest ozone depleting potential among all refrigerants used commercially and has very high global warming potential with a value of 4680 and an atmospheric lifetime of 45 years.1 Its application was widespread as a blowing agent in the manufacture of polyurethane foams until 1995.2 It was also used as a solvent, chemical intermediate, dry cleaning agent, aerosol propellant and in fire extinguishers until 1995, when its manufacture was banned by the Montreal Protocol.3

In recent years, there have been a number of articles reporting the amount of CFC–11 existing in polyurethane foams in waste refrigerators or retained in refrigerators earmarked for disposal. Yazici et al. recently estimated the amount of CFC–11 in polyurethane foam based refrigerators in Turkey (estimation was based on the working refrigerators at the time of their research) as between 717–1237 tons.4 In China, refrigerators using CFC–11 based polyurethane foam and reported as waste contain a total of approximately 3300 tons of CFC–11 up to the year 2009, according to the published article by Ruan et al.5 There are many other countries around the world that have refrigerators with CFC–11 based polyurethane foams requiring proper disposal. ACS Paragon Plus Environment

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Polyurethane foams, separated from the refrigerators, are normally heated in an enclosed shredder in order to extract the CFC–11. There are several methods developed to recover CFC–11; they include: 1) adsorption of CFC–11 on activated carbon, followed by its subsequent desorption between 120 to 160 °C, 2) liquefying CFC–11 by cryogenic condensation below –100 °C. 4-7

Once the CFC–11 is recovered from the polyurethane foams, it requires treatment. Several approaches can be found in literature to treat CFC–11; however, the focus of most approaches is on its destruction. Cheung et al. studied the sonochemical destruction of CFC–11 in aqueous solution8 while Uneo et al. applied a cement kiln to study the destruction of CFC–11.9 Apart from these approaches, researchers also applied plasmas to study the destruction of CFC–11. Inaba et al. used a DC (direct current) arc thermal plasma for CFC–11 destruction,10 while Föglein et al. compared the destruction of CFC–11 applying a RF (radio-frequency) thermal plasma and SED (silent electric discharge) non-thermal plasma.11 Jasiński et al. and Mizeraczyk et al. employed microwave discharge non-thermal plasma for CFC–11 destruction.12, 13

Fluorocarbons (chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons and halons) are chemically unreactive substances and therefore destruction is the conventional disposal route. Established technologies for treatment of these materials include thermal plasma14 and high temperature thermal destruction15 methods. The approach taken by various researchers to convert fluorocarbons to value-added materials primarily involves conventional thermal reactor based technologies. Previously, we have disclosed that fluorocarbons can be converted to a value-added material, vinylidene fluoride (VDF) in a thermal reactor.16-20

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The products targeted for the application of non-equilibrium plasmas in the conversion of fluorocarbons are mainly polymers. Integration of catalysts with these reaction systems is not meaningful, as depositions rapidly bury the catalysts, inhibiting activity. The most accessible reaction parameters available for control and manipulation of the product distribution are input energy density and applied voltage. Utility of these controls has been explored in the present investigation.

In the present research, a dielectric barrier discharge (DBD) non-equilibrium (also called non-thermal) plasma was employed for the conversion of CFC–11 following a non-destructive approach. DBD plasma reactors can generate a non-equilibrium plasma at atmospheric pressure. Although dielectric barrier discharge reactors provide a proven methodology for ozone generation, these reactors are also used in other applications including pollution control, surface modifications and flat plasma screen displays.21-24

In this research work, we have employed methane as a co-reactant with CFC–11. The reaction of CFC–11 with methane can result in the formation of highly corrosive hydrohalogen gases; therefore, an alumina reactor, of cylindrical geometry, was chosen for the construction of the DBD reactor. The cylindrical geometry ensures a uniform plasma is developed and is then used to facilitate reaction between the feed reactant gases. This ensures a high probability of reactant activation the plasma zone at a given input energy density.

In the present study, the reaction of CFC–11 and methane was conducted in argon bath gas, where oxygen and nitrogen were excluded from the reaction environment to avoid the formation of the toxic gases COF2 or COCl2. Under these conditions, the


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reactions produce gas phase as well as polymeric products. Here we focus on the conversion of CFC–11 under various conditions, the characterisation of electrical discharge, structural analyses of polymers and a detailed reaction mechanism.

2. EXPERIMENTAL A dielectric barrier discharge reactor, cylindrical in configuration and consisting of two concentric alumina dielectric tubes (purity of alumina – 99.8 %), was employed for this investigation. The dimensions of the outer alumina tube are 23 mm OD and 2 mm wall thickness while the dimensions of the inner alumina tube are 10 mm OD and 1 mm wall thickness. A helix of overall length 30 mm, formed from copper wire, was used as the high voltage electrode, while a copper shim of 24 mm length outside an outer dielectric was used as ground electrode. The entire reactor assembly was located inside a dedicated fume cupboard. A detailed description of the experimental facility can be found in our earlier publications.25-28

The power supply, employed in this investigation, is capable of delivering up to 20 kV (rms) at a constant frequency of 21.5 kHz. The power input to the plasma reactor was determined by the enclosed area of a voltage-charge Lissajous figure. A detailed description of our procedure for calculating power input to the reactor can be found in our earlier publication.29 The input power can then be converted to estimate the input energy density (input energy density = P/F, where P is the average power dissipated by the reactor and F is the total volumetric feed rate). The input energy density range for this investigation was 3–13 kJ L-1 and the corresponding applied voltage range was 14.1–15.2 kV (peak–peak). A J-type thermocouple (bead-shaped) was used to measure the temperature of the outer electrode in the plasma zone. This measurement can be


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considered to be proxy for the wall temperature of the outer dielectric. A mica sheet and an aliquot of zinc oxide based thermal transfer compound were used to isolate the thermocouple from electrical contact while maintaining thermal contact with the outer electrode.

A gas chromatograph (Shimadzu GC-17A), equipped with a Molesieve 13X column and a thermal conductivity detector used with argon as carrier gas, was employed to quantify the amount of hydrogen in the product stream. A second gas chromatograph (Varian CP 4900), online with the experimental facility, equipped with Molesieve 5A, PoraPLOT Q columns and thermal conductivity detectors, was used to quantify carbon containing gas phase products. A gas chromatograph/mass spectrometer (GC/MS: manufacturer–Shimadzu, GC system: GC-17A, equipped with an AT-Q column, MS system: QP5000) was employed to identify the carbon containing gas phase products. The acid gases, HCl and HF, were quantified by a Fourier transform infrared (FTIR) spectrometer (Perkin Elmer, Spectrum 100). A detailed description of the FTIR spectral analysis can be found elsewhere.26

A gel permeation chromatograph (GPC: manufacturer–Waters, model–GPCV 2000), equipped with a refractive index (RI) detector and three Styragel columns (HR5E, HR3 and HR0.5) and operating at 40 °C, was used to quantify the molecular weights of polymers and estimate the polydispersity index (PDI) of the polymer. The GPC was calibrated using polystyrene standards, with a molecular weight range of 470 to 2,300,000 g mol-1 (the standards were purchased from Polymer Standards Service) and consequently, the reported values of molecular weights of polymers in this article are based on polystyrene standards.


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The structural analyses of the polymers were conducted by nuclear magnetic resonance (NMR) and by FTIR spectroscopy. A Brüker Avance 400 MHz or 600 MHz spectrometer was used for NMR experiments. Brüker Topspin 3.1 software was used for the processing of the NMR spectra. Analysis of polymers was also conducted by an ATR (attenuated total reflectance) FTIR (Perkin Elmer Spectrum 2).

In order to produce enough polymeric material for analyses, all experiments were conducted continuously for a total of 90 min duration. The reactor was subsequently purged with argon and the reactor tubes were then repeatedly rinsed with tetrahydrofuran (THF) solvent (99.9 %, Merck). The rinsed solution contains polymeric mixture and the molecular weights of polymers were determined by analysing this THF/polymer solution by GPC. For analysing polymers by ATR–FTIR, the THF/polymer solution was primarily dried in a fume cupboard and the obtained solid polymeric material was then further dried in a vacuum oven operating at 40 °C. A sample for NMR analysis was prepared by collecting polymers from four repeat experiments under identical conditions and the material was then dried in the fume cupboard at room temperature. A concentrated polymer solution was then prepared by dissolving the polymeric material in THF solvent. The concentrated polymeric solution was then transferred dropwise into a beaker containing methanol (99.9 %, Scharlu). Approximately one third of the solution was then removed from the top of the beaker while the remainder was initially dried in a fume cupboard and then in a vacuum oven operating at 40 °C. This procedure enhanced the concentration of the high molecular weight fraction in the dried polymeric mixture. A detailed discussion of this procedure can be found elsewhere.30 The dried material was dissolved in deuterated chloroform solvent (99.96 atom % D, 0.03% v/v TMS, Aldrich). The high molecular weight


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polymeric fraction enrichment procedure assists in obtaining NMR spectra that clearly highlight the features attributable to both high (broad peaks) and low (sharp peaks) molecular weight fractions.

In this investigation, CFC–11 (> 99 %, Pacific Chemicals Industries Pty Ltd), methane (99.95 %, Linde) and argon (99.999 %, Coregas) constituted the feed gas components. The vapour pressure of CFC-11 is insufficient to enable the use of a mass flow controller at room temperature (boiling point of CFC-11 is 23.77 °C31). Therefore, an indigenous heater, constructed from an insulated box fitted with a heating element and a temperature controller operating at 49 °C was used to vapourise CFC–11. A detailed description with a schematic can be found elsewhere.26 The residence time (L/(F/A), where L is the length of the electrode, F is the total volumetric feed rate and A is the annular cross sectional area bounded by the dielectrics) was 2.95 s for all experiments. Data presented in this article were collected from the investigations on the reactions of 1.25 % CCl3F and 1.25 % CH4, made up to a constant feed rate of 100 cm3 min-1with argon. Any variation of concentration of the reactants is stated in the related context. Experiments were repeated and the data thus presented are depicted by error bars representing the values of the two repeat experiments.

3. RESULTS AND DISCUSSION 3.1. Conversion of CCl3F. In a non-equilibrium plasma, the temperature of kinetic electrons reaches several thousand degrees Celsius, while the temperature of the bulk gas remains relatively low, and thus the system remains far from thermodynamic equilibrium32 and this phenomenon is evident in Figure 1. While the temperature of the


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outer electrode, presented in Figure 1, is expected to be different from the temperature of the reacting bulk gas, the measured temperatures of the outer electrode can be considered as a proxy for the reacting gases in the plasma zone. In a conventional reactor, several hundred degrees (usually in excess of 700 °C) is required to convert CCl3F as can be found in the literature.9 In contrast, a non-equilibrium plasma converts CCl3F at relatively low temperatures (< 110 °C in the present investigation). At an input energy density of 3 kJ L-1, the conversion of CCl3F and CH4 are 37 % and 35 % respectively, where the temperature of the outer electrode reaches 36 °C. At the highest input energy density investigated (13 kJ L-1), the temperature of the outer electrode measured was 103 °C and the corresponding conversion levels of CCl3F and CH4 were 63 % and 68 % respectively. The direct electron impact reactions are the primary reactions responsible for the conversion of the reactants in a plasma reactor, and the rates of these reactions increase with increasing input energy density (a detailed explanation can be found in the reaction mechanism section). Therefore, the conversion levels of both CCl3F and CH4 increase with the input energy density as it was observed in this investigation and presented in Figure 1.

Additives often have a substantive effect on the conversion of a targeted reactant. In the present investigation, CH4 was employed as a co-reactant for the conversion of CCl3F. In undergoing reaction in the plasma, CH4 fragments and forms atomic hydrogen which initiates abstraction reactions with CCl3F as well as other radical recombination reactions such as reactions with the fragmented species of CCl3F. The abstraction reactions lead directly to an increase in the conversion level of CCl3F, and radical combination reactions indirectly increase the conversion of CCl3F. This is a


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result of reducing the rates of recombination reactions of fragmented products of CCl3F, in other words, reducing the rate of the reverse reaction of CCl3F decomposition.

An investigation was undertaken to better comprehend the effect of additive on the conversion of CCl3F by varying the concentration of CH4 while maintaining a constant CCl3F concentration, and the data thus obtained are presented in Table 1. This examination includes data collected at 5 kJ L-1 for the reaction of 1.25 % CCl3F with a gradual increase of CH4 from 0.75 % to 1.75 %, made up to a constant feed rate of 100 cm3 min-1 with argon. Clearly, the conversion of CFC-11 increases with an increase in the concentration of methane in the feed. This observation is consistent with the hypothesis presented previously and the investigation of Ogata et al., who studied the conversion of CClF3 in argon bath gas in a DBD reactor and reported that the addition of H2 with CClF3 elevates the conversion level of CClF3.33

Table 2 summarises the effect of increasing the concentration of CH4 and CCl3F in the feed gas but maintaining a feed gas ratio of 1:1 on the conversion of the reactants. This table presents data obtained at 5 kJ L-1 for the reaction of CCl3F and CH4 with a gradual increase of both reactants from 0.75 % to 1.75 % and a constant total feed rate of 100 cm3 min-1. As the concentration of both reactants increases, the conversion level of both reactants decreases. The breakdown of reactant molecules via direct electron impact reactions does not increase in proportion to the change of reactants. As a consequence, the conversion of the reactants drops as their concentration in the feed increases.

3.2. Characterisation of Electrical Discharge. A study on electrical discharge of pure argon, conducted earlier and presented in our publication,25 shows that the ACS Paragon Plus Environment

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appearance of electrical discharge in pure argon at 21.5 kHz is filamentary which is consistent with other researches.34,

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The ionisation energies of CCl3F and CH4 are

11.77 eV36 and 12.62 eV37 respectively, and therefore, the lowest energy of metastable argon atoms (11.55 eV38) is not sufficient to ionise either CCl3F or CH4. Some fragmentation products from the reaction of CCl3F and CH4 have lower ionisation energies (e.g., CH3–9.80 eV,36 CH2–10.40 eV,36 CF3–9.25 eV,39 CF–9.20 eV,40 CHCl– 9.84 eV,36 CHF–10.06 eV36); however, they are not primary species and are formed by single or multiple fragmentation processes or by the coupling of fragmented species; therefore, their effect on electrical discharge is low compared to gas mixtures like ammonia in argon – where the addition of ammonia transforms the filamentary discharge of pure argon to a diffuse discharge.41 The ionisation energy of argon (15.76 eV42) is higher than that of CCl3F and CH4; therefore, the inclusion of 1.25 % CCl3F and 1.25 % CH4 in argon at 100 cm3 min-1 feed rate alters the electrical discharge of pure argon as CCl3F and CH4 preferentially ionise (Figure 2). The discharge does not develop fully into a filamentary regime, as the streamer formation is hindered by the sparse population of reactant molecules, as well as the low concentration fragmented species. Therefore, the discharge can be explained as a slight shift from filamentary to diffuse discharge for the given admixture.

Diffuse discharge (also known as homogeneous discharge) in a dielectric barrier discharge reactor at atmospheric pressure can be classified as Atmospheric Pressure Townsend Discharge (APTD) and Atmospheric Pressure Glow Discharge (APGD).41, 43 The current density in an APTD is lower than that of APGD.43, 44 Taking this distinction of diffuse discharges into consideration, the electrical discharge of the present system may be explained as a state of slight transition of filamentary to homogenous (or


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diffuse) glow discharge; briefly, filamentary towards APGD. As the diffuse discharge is more uniform compared to filamentary discharge, the activation of reactant molecules are facilitated in diffuse discharge

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; and therefore, a higher level of conversion is

achieved in diffuse discharge.

3.3. Product Distribution and Mass Balance. The experiments were conducted in the absence of oxygen and nitrogen. This approach assisted to prevent the formation of many toxic species. For example, in contrast to the current investigation, Jasiński et al. observed the formation of CF3CN while investigating the destruction of CCl3F in nitrogen applying a microwave torch discharge and they reported the formation of COCl2, CF3CN and COF2 while studying the reactions of CCl3F in presence of air instead of nitrogen.12 Uneo et al. reported the formation of COCl2 while they were studying the destruction of CCl3F with water in a cement kiln.9 These undesirable products were not observed in the present study.

The gas phase products, found in this study, include H2, HCl, HF, C2H3F, C2H3Cl, C2H2ClF, CHCl2F, CCl2F2, CH3Cl, CH2Cl2, CHCl3 and C2Cl4. In addition to these gas phase products, solid products deposited on the dielectrics and their analyses disclosed that they are fluorine containing polymers. The increased input energy density increases the conversion of fluorocarbon and therefore, the formation of polymeric materials is also increased.

Elemental analyses of the polymer generated at 9 kJ L-1 showed that it contains 27.55 % carbon, 1.66 % hydrogen, 54.26 % chlorine and 14.02 % fluorine (in wt %). In mole percentage, the contents of carbon, hydrogen, chlorine and fluorine in the polymer are


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36.90 %, 26.68 %, 24.56 % and 11.86 %. A detailed mass balance is presented in Table 3, including gas phase species as well as polymeric materials. Elemental balances can be found in the supporting information.

3.4. Reaction mechanism. Initially, CCl3F and CH4 molecules ionise via collisions with kinetic plasma electrons and yield CCl3F+ and CH4+ ions.

e- + CCl3F → CCl3F + + 2 e-

(R01)

e- + CH4 → CH4+ + 2 e-

(R02)

CCl3F+ and CH4+, would be expected to undergo electron-ion dissociative recombination reactions. CCl3F + may follow electron-ion dissociative recombination reaction paths as presented in R03-R08.

e- + CCl3F + → CCl2F + Cl

(R03)

e- + CCl3F + → CClF + 2 Cl

(R04)

e- + CCl3F + → CF + 3 Cl

(R05)

e- + CCl3F + → CCl3 + F

(R06)

e- + CCl3F + → CCl2 + Cl + F

(R07)

e- + CCl3F + → CCl + 2 Cl + F

(R08)

The dissociative recombination reactions of CH4+ produce CH3, CH2, CH and H as described below in R09-R12.46

e- + CH4+ → CH3 + H


(R09)

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e- + CH4+ → CH2 + 2 H

(R10)

e- + CH4+ → CH2 + H2

(R11)

e- + CH4+ → CH + H2+ H

(R12)

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The radicals take part in abstraction and radical recombination reactions. The formation of H2, HF and HCl via atomic abstraction can be explained by the following reactions:

H + CH4 → CH3 + H2

(R13)

F + CH4 → CH3 + HF

(R14)

Cl + CH4 → CH3 + HCl

(R15)

F + H2 → HF + H

(R16)

Cl + H2 → HCl + H

(R17)

The formation of H2, HF and HCl via radical recombination reactions can be expressed by the following reactions:

H + H + (M) → H2 + (M)

(R18)

H + F + (M) → HF + (M)

(R19)

H + Cl + (M) → HCl + (M)

(R20)

where M implies a third-body molecule.

The formation of H2, HF and HCl was detected and quantitated in the presented study and the data presented in mass balance (Table 3).


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Radicals may react and form other radicals via third body collisions. Some examples of these reactions are given below: CH + F + (M) → CHF + (M)

(R21)

CF + F + (M) → CF2 + (M)

(R22)

CF2 + F + (M) → CF3 + (M)

(R23)

CH + Cl + (M) → CHCl + (M)

(R24)

CCl + H + (M) → CHCl + (M)

(R25)

CClF + H + (M) → CHClF + (M)

(R26)

CCl2 + H + (M) → CHCl2 + (M)

(R27)

The formation of carbon-containing species, i.e., C2H3F, C2H3Cl, C2H2ClF, CHCl2F, CCl2F2, CH3Cl, CH2Cl2, CHCl3 and C2Cl4, may be described by the followings:

CH2 + CHF + (M) → C2H3F + (M)

(R28)

CHF + CHCl + (M) → C2H2ClF + (M)

(R29)

CH2 + CHCl + (M) → C2H3Cl + (M)

(R30)

CH3 + Cl + (M) → CH3Cl + (M)

(R31)

CHCl2 + CH4 → CH2Cl2 + CH3

(R32)

CHCl2 + H + (M) → CH2Cl2 + (M)

(R33)

CCl3 + CH4 → CHCl3 + CH3

(R34)

CCl3 + H + (M) → CHCl3 + (M)

(R35)

CCl2F + CH4 → CHCl2F + CH3

(R36)

CCl2F + H + (M) → CHCl2F + (M)

(R37)

CCl2F + F + (M) → CCl2F2 + (M)

(R38)


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CCl2 + CCl2 + (M) → C2Cl4 + (M)

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(R39)

The radicals, generated in gas phase reactions, then participate in polymerisation. The detailed polymerisation mechanism has been presented in our earlier publication30, 47 and is not discussed here.

3.5. Polymer characterisation. The plasma polymerisation of CCl3F with CH4 in argon bath gas was carried out between an input energy density range of 3 and 13 kJ L-1. The polymeric products, generated in that input energy density range, readily dissolve in both tetrahydrofuran and chloroform solvents, indicating that the polymer is noncrosslinked. A trace amount of polymeric products remained undissolved, produced at the highest input energy density investigated or 13 kJ L-1 suggesting that, under these conditions, some crosslinked polymers are formed. The polymeric products characterised here are from the non-crosslinked polymeric fraction.

The polymers are comprised of two portions. The number average molecular weight (Mn) of the low molecular weight fraction is 370 g mol-1 with a polydispersity index (PDI) of 1.2. The Mn value for the high molecular weight fraction is 3,600 g mol-1 with a PDI value of 1.2. The value of the high molecular weight polymer fraction is somewhat different from that given in our earlier publication.26 This is because the analysis in the earlier publication was affected by interaction with the filter material used for the GPC sample preparation step. GPC is a precise and reliable method;48 however, filtration of the solvated sample is critical and requires appropriate filter selection as the solvent, THF is highly aggressive. In the present study, the filtrate


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contamination problem which occurred earlier, was eliminated and a correct value of the molecular weight for high molecular weight fraction is now reported.

Figure 3 displays the FTIR spectrum of the polymeric material produced at 9 kJ L-1. The peak at 750 cm-1 is assigned as a CH2 rocking spectra49, 50 while the peaks at 820 and 960 cm-1 are likely to be C-Cl stretching of various C-Cl functional groups (e.g., CHCl2, CFCl, CHClF, CHCl etc.).51,

52

The peaks at 1035, 1065 and 1265 cm-1 are

assigned to C-F stretching for various functional groups that include C-F bonds (e.g., CF=CF, CF2, CF3 etc.).53-55 The peak at 1650 cm-1 is assigned to internal double bonds of CH=CF, while the peaks at 1730 and 1775 cm-1 are assigned to internal double bonds of CF=CF and CF=CF2.56 The absorptions at 2880 and 2980 cm-1 represent CH3 asymmetric and symmetric stretching vibrations.50 Further discussion of various functional groups can be found in the following part of this section on the analyses of NMR experiments.

The non-crosslinked nature of the polymers simplified to some extent their subsequent analysis by solution state NMR experiments.

13

C and

19

F NMR spectra (in

solution) were obtained in order to characterise the structure of the polymers, and these analyses were facilitated by NMR pulse techniques such as DEPT 135 (Distortionless Enhancement by Polarisation Transfer at 135° angle) and DEPTQ 135 (Distortionless Enhancement by Polarisation Transfer at 135° angle, including the detection of quaternary nuclei).57 The DEPTQ 135 spectrum distinguishes CH and CH3 peaks from CH2 and quaternary carbon peaks (groups that do not have any hydrogen; e.g., CF2, CF3 etc) while DEPT 135 spectrum distinguishes CH and CH3 peaks from CH2 peaks and the spectrum does not show any quaternary carbon signal. The relatively narrow peaks


17


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Page 18 of 36

in the NMR spectra are obtained from the low molecular weight component while the broader peaks are from the higher molecular weight polymeric material. The analyses presented here focus on high molecular weight polymer products.

As can be seen in Figure 4, CH3, CH2, CH, CHCl, CHCl2, CHClF and CClF groups are present in the polymer chain and their signals are visible at 19, 46, 53, 60. 70, 98 and 111 ppm in the 13C spectrum. Although the signal due to a CH3 group is masked in the

13

C spectrum, this group is evident in DEPTQ 135 (Figure 5) and in DEPT 135

(Figure 6) spectra. The group, CClF, is quaternary in nature and is identified in the DEPTQ 135 spectrum but is absent in DEPT 135 spectrum. The peak at 118 ppm in the 13

C NMR spectrum is also quaternary in nature and this assignment is confirmed by

comparing the associated DEPTQ 135 and DEPT 135 spectra. This peak at 118 ppm includes contribution from two quaternary groups, CF2 and CF3, and their presence is confirmed by 19F spectrum which will be discussed in the next section. The CH peak at 53 ppm and CHCl peak at 60 ppm of the 13C spectrum is observed as a merged peak at 60 ppm in DEPTQ 135 and DEPT 135 spectra. The functional group resulting in the absorbance at 130 ppm in the

13

C NMR spectrum may be described as two equivalent

CH groups bonded by a double bond (-CH=CH-). In contrast, the functional group at 153 ppm in the

13

C NMR spectrum may be explained as two equivalent CF groups,

connected by double bonds (-CF=CF-). According to FTIR analysis, there are also CH=CF- and -CF=CF2- present in the polymer. CH and CF groups, double bonded as CH=CF, may have signals in 13C spectrum where CH peak can be found at 130 ppm and CF peak at 153 ppm. Both CF and CF2 of -CF=CF2- is most likely present at 153 ppm in 13

C spectrum.


18

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7 presents the

19

F chemical shifts from the polymer, with the spectrum

dominated by three broad regions ~40 to ~80, ~85 to ~125 and ~125 to ~190 consistent with CF3, CF2 and CF signals. CF signal in

19

F NMR spectrum is a result of

contribution from CClF, CHClF and CF in CF=CF and CF=CF2. CF2 signal in 19F NMR spectrum may be from CF2 of CF=CF2 or from the CF2 group of the backbone of the polymeric chain. CF3 can be present as a branch or an end group with the polymeric chain.

Based on these NMR spectroscopic analyses, the polymers are characterised as random in nature where CH2, CHCl, CFCl, CF2, CH=CH, CH=CF and CF=CF are present in the backbone of the polymer. The CH peak at 53 ppm in 13C NMR spectrum is also present in the main chain, with CHCl2, CHClF, CF=CF2 and CF3 branches. The low molecular weight fraction of polymeric materials may have all or some of the groups identified for high molecular weight fraction; however, they are also random copolymer with a relatively short chain. The NMR spectral assignments were conducted with the assistance of published references.58, 59

4. CONCLUSION The conversion of CCl3F with CH4 in argon bath gas was examined in a dielectric barrier discharge non-equilibrium plasma reactor in the absence of oxygen and nitrogen. This approach avoided the possibility of formation of a number of toxic products such as COCl2, COF2, CF3CN etc. The reactants were converted at a relatively low bulk gas temperature (below 110 °C) and this result is consistent with the characterisation as the reaction occurring in a non-equilibrium plasma.


19


The electrical discharge can be described as a slight modification from filamentary to diffuse discharge due to the presence of low concentrations of CCl3F and CH4. This modification facilitates the activation of reactant molecules, therefore, higher levels of conversion of both reactants were observed.

The reactions of CCl3F and CH4 generated gaseous as well as solid products. According to the analyses, the solid products are mostly non-crosslinked polymers. The polymers include CH2, CHCl, CHF, CF2 and CF3 groups, and the groups are arranged randomly in the polymeric chain; therefore, they can be termed as random copolymers.

150

75

● Temperature of outer electrode ◊ Conversion of CCl3F ♦ Conversion of CH4

125

65

100

55

75

45

50

35

25

% Conversion

Temperature of outer electrode (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

25

0

3

6

9

12

15

Input energy density (kJ L-1)

Figure 1: Temperature of the outer electrode and conversions of CFC-11 and methane as a function of input energy density (feed conditions – 1.25 % CCl3F, 1.25 % CH4 in 100 cm3 min-1).


20

Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: Instantaneous current and applied voltage trace at 9 kJ L-1 for the reaction of 1.25 % CCl3F and 1.25 % CH4 in 100 cm3 min-1 feed rate.

Figure 3. FTIR spectrum for the polymer synthesised at 9 kJ L-1 from the nonequilibrium plasma reactions between CCl3F and CH4.


21


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Page 22 of 36

Figure 4: 13C NMR spectrum for the polymeric mixture synthesised at 9 kJ L-1 from the non-equilibrium plasma reactions between CCl3F and CH4.

Figure 5. DEPTQ 135 NMR spectrum for the polymeric mixture synthesised at 9 kJ L-1 from the non-equilibrium plasma reactions between CCl3F and CH4 (QC: quaternary carbon).


22

Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. DEPT 135 NMR spectrum for the polymeric mixture synthesised at 9 kJ L-1 from the non-equilibrium plasma reactions between CCl3F and CH4.

Figure 7. 19F NMR spectrum for the polymeric mixture synthesised at 8 kJ L-1 from the non-equilibrium plasma reactions between CCl3F and CH4.


23


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Page 24 of 36

Table 1: Conversion of reactants at an input energy density of 5 kJ L-1 with the variation of the reactants’ relative concentrations

Conditions

% Conversion

% Conversion

Rate of

Rate of

of CCl3F

of CH4

Conversion of

Conversion of

CCl3F

CH4

mmol h

-1

mmol h-1

1) 1.25 % CCl3F, 0.75 % CH4, 100 3

43

57

1.5

1.1

48

48

1.6

1.6

50

41

1.7

2.0

-1

cm min feed rate 2) 1.25 % CCl3F, 1.25 % CH4, 100 cm3 min-1 feed rate 3) 1.25 % CCl3F, 1.75 % CH4, 100 3

-1

cm min feed rate


24

Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Conversion of reactants at an input energy density of 5 kJ L-1 with the variation of the reactants’ concentrations (or the absolute amount of the reactive species in plasma).

Feed composition

% Conversion

% Conversion

Rate of

Rate of

and flow rate

of CCl3F

of CH4

Conversion of

Conversion of

CCl3F

CH4

mmol h-1

mmol h-1

1) 0.75 % CCl3F, 0.75 % CH4, 100

63

62

1.3

1.2

48

48

1.6

1.6

43

44

2.0

2.1

cm3 min-1 feed rate 2) 1.25 % CCl3F, 1.25 % CH4, 100 3

-1

cm min feed rate 3) 1.75 % CCl3F, 1.75 % CH4, 100 cm3 min-1 feed rate


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

Table 3: Overall mass balance of CCl3F – CH4 experiment conducted at 9 kJ L-1 for 90 min (total volumetric flow-rate: 100 cm3 min-1, concentrations of feed gases: 1.25 % CH4 and 1.25 % CCl3F).

Feed stream

Product stream

Mass Species

Total

Total mass

Mass

Mass

mass in

out

balance

mg

mg

mg

%

772

758

98.3

Species mg

CH4

80.4

CH4

31.5

CCl3F

691

CCl3F

283

C2H3F

0.25

CH3Cl

1.53

CCl2F2

0.97

C2H2ClF

1.07

C2H3Cl

0.31

CHCl2F

9.29

CH2Cl2

1.34

CHCl3

5.90

C2Cl4

8.60

H2

1.89

HF

19.3

HCl

172

Other gas products

1.15

Polymers

220


26

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGEMENT The authors would like to thank the Australian Research Council for financial support of this project (reference number: DP130101994). Sazal K. Kundu and Vaibhav V. Gaikwad are indebted to the Department of Education, Science and Training (DEST) of the Australian Government and the University of Newcastle, Australia for postgraduate scholarships. We thank Dr. Monica Rossignoli and Ms Azrinawati Mohd Zin at School of Environmental and Life Sciences, The University of Newcastle, Australia for their assistance with NMR and GPC analyses. We thank Mrs Viki Withers and Mrs. Sasha Melnitchenko of Microanalytical Unit, Research School of Chemistry, The Australian National University for undertaking the elemental analysis of the polymeric material.

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Table of Contents (TOC) Graphic 160x98mm (300 x 300 DPI)


Page 36 of 36

Experimental Study on the Reaction of CCl3F and CH4 in a Dielectric

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