Process for Chloroform Decomposition: Nonthermal Plasma

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Applied Chemistry

Process for chloroform decomposition: Non-thermal plasma polymerisation with methane and hydrogen Vaibhav Gaikwad, Eric M. Kennedy, John C Mackie, Clovia I. Holdsworth, Thomas S. Molloy, Sazal Kundu, Michael Stockenhuber, and Bogdan Z Dlugogorski Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01413 • Publication Date (Web): 23 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Industrial & Engineering Chemistry Research

Process for chloroform decomposition: Non-thermal plasma polymerization with methane and hydrogen Vaibhav Gaikwad* a, Eric Kennedy b, John Mackie b, Clovia Holdsworth c, Thomas Molloy b, Sazal Kundu d, Michael Stockenhuber b and Bogdan Dlugogorski e ––––––––– a Centre

for Sustainable Materials Research and Technology (SMaRT), School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia b

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

Centre for Organic Electronics, Chemistry Building, School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia. d

School of Engineering Cluster, Department of Chemical Engineering, RMIT, Melbourne, VIC 3000, Australia.

e

School of Engineering and Information Technology, Murdoch University, Murdoch, WA 6150, Australia. *[email protected]

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Industrial & Engineering Chemistry Research 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

Abstract This paper describes an alternative process for chloroform decomposition via non-thermal plasma polymerization at atmospheric pressure and investigates the effect of methane and hydrogen addition on the process. The effect of both additives was assessed separately, where experiments were conducted in a double dielectric barrier discharge reactor under non-oxidative conditions. The most profound impact of the additives was a significant increase in the yield of non-crosslinked polymer produced compared to that in their absence. The addition of methane resulted in a 120 % increase in polymer yield, while in hydrogen the increase was 31 %. Critical parameters such as effect of the methane and hydrogen concentration on the conversion of chloroform at various applied voltages, the product distribution, mass balance and polymer characterization are elucidated in this paper. Single pass conversions of 61 % and 68 % (with corresponding mass balances of 98 % and 95 % respectively) were achieved for CHCl3+CH4 and CHCl3+ H2 feed scenarios respectively. Furthermore, a polymerization mechanism which explains the formation of major chain structures as well as structural defects in the polymer is expounded in the paper.


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1. Introduction Chloroform is a chemical with a multitude of applications in the industry, including as a solvent in the formulation of resins, lacquers and rubbers.1 In addition, it is used in the preparation of pesticides, dyes and as an intermediate in manufacturing of chlorinated polymers.1 However, the majority of chloroform produced globally is used in manufacture of HCFC-22.2 Apart from being it being an important raw material, chloroform is a toxic chemical and is readily absorbed in human tissue. The US EPA classifies it as a possible carcinogen at concentrations that lead to cytotoxicity i.e. at concentrations which are toxic to cells.3 Industrial spills, wastewater treatment plants’ effluent and the chloroform manufacturing industries are some of the sources through which chloroform emissions to environment have been reported.2,3 Chloroform is also recognised as significant constituent of vinyl chloride monomer (VCM) heavy ends, a well-known effluent from the vinyl industry.4,5 Chloroform has a higher specific gravity than water and is sparingly soluble in it. Thus, if a chloroform spill reaches a water source, it tends to settle at the bottom of the body of water, making its remediation exceedingly difficult.2 The production of HCFC-22 is designated to be phased out, in accordance with the Montreal protocol, by 2020 and this will directly affect the commercial demand for chloroform. The global consumption of chloroform in 2015 is expected to be less than half of what it was in 2011,2,6 and these circumstances seem likely to lead to the creation of stockpiles of unused and waste chloroform. This scenario, along with the hazards of chloroform exposure, necessitates the development of technologies that can safely treat chloroform and convert



it to a non-toxic product, preferably with a commercial value so that it can aid, at least in part, offsetting its treatment cost. Non-thermal plasma provides a methodology that can be potentially used in tandem with thermal destruction methods for treatment of chlorinated hydrocarbons such as chloroform. Most current studies focus on reaction of chloroform in a non-thermal plasma are directed towards its complete decomposition/destruction, and are undertaken under oxidative conditions which can lead to formation of toxic compounds such as COCl2 and CO. Various techniques have been used to generate the non-equilibrium plasma for chloroform decomposition. Indarto et al have employed the gliding arc technique for this purpose,7–9 while Schultz et al adopted a capacitively coupled RF plasma.10 Indarto et al conducted their study under oxidative conditions, using air as the carrier gas, while Schultz et al used hydrogen as a carrier gas for their experiments. Foeglein and co-workers used dielectric barrier discharge in their study on chloroform decomposition and provided a comparison between results from experiments under oxidative and non-oxidative (inert) conditions.11 In contrast to the non-equilibrium plasma field, there is abundant literature published on chloroform decomposition/pyrolysis via the thermal pathways. Some notable work in this field is by Won et al and Semeluk et al.12–14 Previously, we demonstrated that neat chloroform can be converted to a potentially valuable polymer.15 However, it was a preliminary study and the yield of polymer generated was low. In the present study, we explore the effect of CH4 and H2 on the treatment of chloroform in a non-equilibrium plasma. The most important outcome of adding CH4 and H2 to the feed is the notable increase in the amount of polymer produced as


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compared to the earlier experiments in their absence. To the best of our knowledge, the reactions of CHCl3+CH4 and CHCl3+H2 under non-oxidative conditions have not been studied in a non-equilibrium plasma operating at atmospheric pressure.

There are

additional subtle differences in the nature of the polymer obtained from chloroform treatment when the reaction is conducted in presence of additives. We have used the dielectric barrier discharge technique in our experiments since it is capable of providing a stable envelope of plasma and distribution of micro-discharges within the bounded excitation region at atmospheric pressure. The material for the dielectric tubing is quartz. All experiments were performed under non-oxidative conditions using argon as a carrier gas. We present the results of the effect of applied voltage on conversion levels of chloroform, methane and hydrogen, polymer characterization, polymerization mechanism and the product distributions for both CHCl3+CH4 and CHCl3+H2 experiments including detailed mass balances in this paper. The results from present study along with our previous research papers are also intended to provide baseline data for chloroform and other chlorinated compounds decomposition, which will assist in further development of this technology as an alternative process to treat VCM heavy ends. 15–17 2. Experimental and analytical setup Experiments were conducted in a double dielectric barrier discharge reactor with a cylindrical geometry. A detailed description of the experimental and analytical setup was provided in our prior publications.16–18 Concisely, the arrangement of the dielectrics allows for a 4.7 mm discharge gap, wherein the plasma is generated. The custom-built power supply can deliver voltages up to 20 kV at a frequency of 21.5 kHz. The flow of all gases was



regulated by independent MFC controllers (Brooks), while flow of chloroform was controlled by a syringe pump (SAGE 355). Argon was used as a carrier gas in all experiments performed in this study. Average power dissipated into the reactor was calculated using a methodology outlined in our earlier paper.16 For collection of the polymer, the quartz reactor is disassembled and rinsed with tetrahydrofuran (THF, 98%, Aldrich) as solvent. The polymer/THF solution is then transferred to a fume-hood to facilitate evaporation of the THF from the polymer. A portion of this polymer sample is used for GPC analysis, while the remainder is used for NMR and FTIR analysis. The aliquot of polymer used for NMR and FTIR analysis is subjected to further drying in a vacuum oven to enhance THF removal from the polymer in order to reduce or eliminate its signal in the NMR and FTIR spectra. No further processing is performed for FTIR analysis of the polymer, however for NMR analysis; the polymer is dissolved in deuterated chloroform (CDCl3) Identification of carbon containing gas phase products was performed on a GC-MS (Shimadzu QP 5000) and their quantitation was conducted via micro-GC (Varian CP-4900) analyses. Quantitation of hydrogen was achieved by using a GC (Shimadzu GC-17A), using argon as carrier gas and a 13X molecular sieve column. HCl was identified and quantified on a FT-IR (Perkin Elmer Spectrum 100) instrument with a teflon gas cell. In addition, an ATR FT-IR instrument (Perkin Elmer Spectrum Two) was used to ascertain the functional groups present in the polymers. NMR analyses (Bruker Avance 600 MHz/ 400 MHz) were performed to enhance elucidation of the polymer chain structure. GPC analysis (Waters GPCV 2000) was employed for molecular weight determination of the polymers. The instrument is equipped with 3 columns (Waters Styragel HR5E, HR3 and HR0.5) and covers


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a number average molecular weight (Mn) range from 470 to 2,300,000 g mol-1. Since the instrument was calibrated on polystyrene standards, all the molecular weights reported in this study are polystyrene equivalents. The concentrations for both components of the CHCl3+CH4 experiments, i.e., CHCl3 and CH4 in the feed were maintained at 1 %. Similar conditions were adopted for the CHCl3+H2 experiments; the concentration of each reactant was 1 % in the feed. For both experiments the balance feed material (98%) was argon. The total flow rate was retained at 200 cm3 min-1 and the residence time was thus estimated to be 2.1 s. The duration of each experiment was 65 min. 3. Results and discussion 3.1. Conversion level of feed components The conversion levels of chloroform, methane and hydrogen increase with an increase in applied voltage. This observation is similar to that reported in our previous publication, which explored the reaction of chloroform in a non-equilibrium plasma in the absence of methane or hydrogen.15 At similar applied voltages and residence times, the conversion level of chloroform decreases significantly in presence of the additives compared to when it is the sole feed reactant. Under identical experimental conditions, the conversion level of neat chloroform is 66.8 % at an applied voltage of 16 kV,15 which is slightly higher than its conversion in presence of methane (60.8 %) or hydrogen (65.9 %).



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Table 1. Conversion level data for CHCl3+CH4 experiments Applied voltage

Power dissipated

CHCl3 conversion

CH4 conversion

(kV)

(W)

level (%)

level (%)

13

2.72

25.2

7.1

14

7.67

30.8

15.1

15

13.4

36.2

17.9

16

14.9

54.7

20.1

17

19.4

60.8

27.1

Table 2. Conversion level data for CHCl3+H2 experiments Applied voltage (kV)

Power dissipated (W)

CHCl3 conversion

H2 conversion

level (%)

level (%)

13

2.81

15.9

6.81

14

9.16

35.8

8.51

15

11.7

44.4

10.8

16

14.0

55.4

16.7

17

18.9

65.9

18.7

This effect is not unexpected, as the net concentration of reactants in the feed has increased in the latter two cases. This implies that there is an increase in the probability of collision of


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species such as argon metastable atoms and excited chloroform molecules with neutral methane and hydrogen, thereby leading to a decrease in the conversion level when chloroform reacts in presence of methane and hydrogen due to collisional relaxation. At similar levels of applied voltage, the conversion level of chloroform is notably higher than that of methane and hydrogen; and the conversion level of methane is, in turn, higher than that of hydrogen. This may be explained when considering the different ionization energies of the molecules. The ionization energy of chloroform is 11.37 eV, while that of methane and hydrogen is 12.61 eV and 15.42 eV, respectively.19–21 This suggests that, the chloroform molecules would be activated preferentially at a higher rate than methane and hydrogen, as would be the case in the conversion levels of methane and hydrogen. In addition, the corresponding bulk gas temperatures measured for CHCl3+CH4 and CHCl3+H2 experiments at 16 kV were 136 oC and 148 oC respectively, and in both cases, the temperature recorded is lower than the 151 oC observed in CHCl3 treatment alone.15 A direct comparison of conversion levels obtained in this study with other studies dealing with non-equilibrium plasma treatment of chloroform is not straightforward, since the experimental conditions used in other studies were considerably different. In addition, to the best of our knowledge, the study of reaction between chloroform and methane employing non-equilibrium plasma, more specifically dielectric barrier discharge reactor, has not been reported in literature. However, it is insightful to discuss some of the data published by other researchers. Indarto et al reported maximum conversion level of 97 % employing gliding arc plasma, where air was used as bulk carrier gas .8,9 Schulz et al reported a conversion rate in excess of 99 % for chloroform using a capacitively coupled RF hydrogen plasma under reduced pressure.10 Both, Indarto and Schulz reported an



increasing trend of chloroform conversion with respect to an increase in applied input power to the plasma.8–10 3.2. Product distribution and mass balance Qualitatively, the products obtained in the CHCl3+CH4 and CHCl3+H2 experiments are indistinguishable. Table S1 and Table S2 detail the product spectrum and mass balances obtained in these experiments. In both cases, the results reported are for the experiments conducted at 16 kV, where mass balances of 97.7 % and 94.7 % were obtained for the CHCl3+CH4 and CHCl3+H2 experiments, respectively. Since non-oxidative conditions were employed for the current study, the formation of gases such as CO, CO2 and COCl2 is precluded, and this is one of the major advantages of operating under non-oxidative reaction conditions. The product spectrum presented in our prior publication pertaining to non-equilibrium plasma treatment of CHCl3 in absence of any additives is similar to that encountered in the present study, where we outlined a possible reaction mechanism for formation of major gas phase species. However, there is a 120 % and 31 % increase in the amount of polymer produced due to addition of methane and hydrogen respectively, compared to that in their absence.15 As mentioned in Table 1 and Table 2, single pass conversions of 54.7 %and 55.4 % were achieved for CHCl3+CH4 and CHCl3+ H2 feed scenarios respectively at 16 kV. There are a limited number of studies in the plasma field detailing a comprehensive list of products formed during the reaction. The study on CHCl3 decomposition in hydrogen plasma by Schulz et al is probably the most exhaustive account available in the literature. Products such as CH2Cl2, C2H3Cl, C2H4, C2H2 and CH4 were detected in the present study, and


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these are similar to the product species as described by Schulz et al.10 However, unlike in the study by Schulz et al, in the present study no hydrocarbons, either chlorinated or nonchlorinated, were detected above C2. Foeglein et al in their study on CHCl3 decomposition in argon plasma reported mass balance of 96 % and noted the formation of products such as CCl4 and HCl, which were also detected in the present study.11 Indarto et al also reported CCl4 as one of the primary products formed in CHCl3 decomposition in a gliding arc plasma.9

Foeglein et al also described the product

distribution from equilibrium (thermal) plasma treatment of CHCl3 and these data are in stark contrast to that obtained in the present study as well as to their own results from non-equilibrium plasma treatment. Numerous C6 chlorinated and non-chlorinated hydrocarbons were observed in the equilibrium plasma experiments.11 3.3. Polymer characterization 3.3.1. Molecular weight determination Based on GPC analyses of the polymers obtained from the experiments, it was determined that the CHCl3+CH4 and CHCl3+H2 polymer consisted of high and low molecular weight fractions. However, the low molecular weight fractions eluted at the very end of the sample run, making its molecular weight determination unreliable. The high molecular weight fraction of the CHCl3+CH4 polymer has a number average molecular weight Mn=1120 and a weight average molecular weight Mw=2240, and is thus characterized as having a polydispersity index (PDI) of 1.9. Likewise, the high molecular



weight fraction of CHCl3+H2 polymer has Mn=1200 g mol-1 and Mw=1700 g mol-1, thus having a PDI of 1.4.

3.3.2. NMR analysis The NMR analysis of the polymers obtained in this study was directed towards disclosing and understanding the functional groups present in the polymer chain, and determining probable chain structures. Given the nature of the plasma process, it is unlikely that the polymer will have a uniform structure and an unambiguous correlation to a conventional, chlorinated polymer. In addition, chloroform does not satisfy the ideal definition of a monomer and is in fact widely used as a chain transfer agent, usually responsible for introducing branching into a polymer. These complexities, along with considerable overlaps in chemical shifts in the NMR spectra, make the explicit assignment of functional groups virtually impossible. However, using a combination of 1-D and 2-D NMR techniques, it is possible to deduce with reasonable confidence, the functional groups present in the polymer along with the polymer chain structure. NMR analyses disclose that the functional groups, as well the chain structures in polymers obtained from CHCl3+CH4 and CHCl3+H2 experiments are more or less similar. Since the characteristic absorptions of both the polymers lie in similar range of chemical shifts, their assignment to respective functional groups will be performed simultaneously. The polymer structure also bears resemblance to that of the polymer obtained from non-thermal plasma treatment of chloroform in absence of any additives, albeit with some subtle differences.


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This is not unexpected, as the basic atomic elements in these experiments involve C, H and Cl. 3.3.2.1. 1H NMR The 1H NMR spectra of the polymers obtained from CHCl3+CH4 and CHCl3+H2 experiments are depicted in Fig. 1. The peaks (at a chemical shift of) around 1.2 ppm are assigned to CH2 groups in the polymer that are not in close proximity to a Cl-containing group.22 The CH2 groups that are coupled to Cl-containing functional groups are indicated by peaks in the range of 1.8-2.6 ppm.16,23,24 The peaks representative of CHCl and CH2Cl groups are observed in the region from 3.5-4.8 ppm. This region is complicated by the presence of overlapping peaks from the functional groups as is indicated by 2-D NMR analyses and discussed in a later section of this paper. Peaks corresponding to CH2 groups from a structural defect such as -CH=CH-CH2Cl may also be present in the region from 4-4.2 ppm.23 The CH group of this structural defect is likely to register its signal in the 5.8 to 5.9 ppm region of the spectrum.23



Figure 1. 1H NMR spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments In addition, the peaks in the region from 5.2-5.5 ppm suggest the possibility of another structural defect such as -CHCl-CH=CHCl.23,25 These peaks are due the CHCl group bonded to a CH group by a single bond. The CHCl group attached to the CH group via double bond manifests itself in the peaks between 6 to 6.5 ppm.23,25 The region from 6 to 6.5 ppm may also include peaks due to a structural defect such as -CHCl-CH=CH2. The peaks above 7 ppm can be attributed to aromatic species. 3.3.3.2. 13C and HMQC NMR Fig. 2 illustrates the 13C NMR spectra of the polymers. The peaks in the region from 20 to 30 ppm are indicative of CH2 groups not in the vicinity of a Cl-containing functional group,22,23 as CH2 groups coupled to a Cl containing functional group manifest in spectral region in the chemical shift range between 38 and 45 ppm. 16,22,23

Figure 2. 13C NMR spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments


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The CH2Cl and CHCl groups from the main backbone polymer chain exhibit peaks from 45 to 50 ppm and 55 to 65 ppm respectively,22,23 while peaks in the region from 103 to 107 ppm correspond to CH groups from structural defects such as -CHCl-CH=CHCl or -CH=CHCH2Cl. The peaks in the broad region from 120-140 ppm are likely to correspond to functional groups involving both CH and CH2, which can be attributed to aliphatic or aromatic moieties.16,22 This region might include peaks from structural defects such as CHCl-CH=CH2. The CHCl group attached to a double bond in the structural defect -CHClCH=CHCl will display a signal in this region .23,25 In addition, peaks from CH groups that are not in proximity of a Cl-containing functional group, but which may be constituent of one of the fragments present in the chain structures involving unsaturation, can also contribute to NMR absorbance in this region. Such a fragment may be a structural moiety such as –(CH2CH=CH-CH2)m-. The HMQC spectrum (see Fig. 3) provides the carbon/hydrogen (C-H) correlation in the polymer. The horizontal axis represents the 1H spectrum while the vertical represents the 13C.

The contours depict C-H correlation. Point A (1 to 3 ppm in H and 20 to 45 ppm in C)

exhibits the C-H correlation of both types of CH2 groups i.e., ones which are not in the vicinity of a Cl-containing functional group as well as those which are. Point B (3.5 to 4.8 ppm in H and 40-50 ppm in C) represents the correlation in the CH2Cl group while point C (3.5 to 4.8 ppm in H and 55-65 ppm in C) in CHCl. Point D (4.5 to 6 ppm H and 100 to 110 ppm in C) and point E (6 to 7.5 ppm in H and 120-135 ppm in C) represent the structure / structural defects mentioned earlier.



Based on the NMR analyses it is evident that the polymer is composed of more than one type of chain structures and fragments. Possible structures include –(CH2-CH2)n-, –(CH2CH=CH-CH2)m-, -(CH2-CHCl)n-. Terminal groups may include CH2Cl, or even unsaturated species such =CH2, =CHCl. It is likely that the polymers can be random copolymers with the blocks having structures such as those mentioned previously.


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Figure 3. HMQC spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments 3.3.3. FTIR analysis The FT-IR spectra of the polymers obtained from the CHCl3+CH4 and CHCl3+H2 experiments are presented in Fig. 4. As is evident, the spectra depict a large number of similar characteristic peaks, albeit with some subtle differences. This observation is similar to the NMR analysis presented in earlier sections of the paper. The peaks between 730 and 850 cm-1 are characteristic of a C-Cl stretching vibration.26–29 A trans C-H bending vibration from a Cl substituted alkene, most probably from a polymeric structural defect is indicated by the peaks between 920-1050 cm-1.27,28 The peak at 1260 cm-1 can be attributed to a C-H bending vibration from a chlorinated alkane species such as CHCl27–30 while the peak at 1580 cm-1 can be due to a C=C stretching vibration from an aromatic species.31–33 The peak at around 1730 cm-1 can either be due to a C=C stretching vibration from a substituted alkene species, or to an artefact due to residual THF.30,34 The peak around 2900 cm-1 is indicative of a C-H stretching vibration from an alkane species, most likely a CH2 group.26–29

Figure 4. FT-IR spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments



The main difference between the two spectra is the presence of the peak at about 634 cm-1 in the CHCl3+H2 polymer, attributed to a cis C-H bending vibration from a Cl substituted alkene.27,28 This particular peak is absent in the FTIR spectrum of the CHCl3+CH4 polymer. Polymers with chain structures similar to those produced in the present study tend to have good thermal and chemical resistance properties.35 They can be used as polymer impact modifiers, additives in specialty paints and manufacturing of vapour barrier membranes.35 Additionally, their deposition as thin films on a substrate can influence its surface properties such as hydrophobicity and adhesion.34 3.4. Polymerization mechanism The polymerization mechanism presented in this paper is aimed at qualitatively explaining the formation of chain structures and some of the fragments and structural defects present in the polymer. This mechanism is not directed at being generic, as the structure of the polymer chains can vary a great deal, depending on the prevailing experimental conditions.36,37 The mechanism is based on the polymer structure as elucidated from NMR analyses, the products formed in gas phase and available literature for plasma polymerization at atmospheric pressure38–40 as well as from conventional polymerization techniques.23,35,41,42 We propose that ethylene is the most probable product to undergo polymerization in this reaction system. The increase in the amount of ethylene and the polymer produced in the CHCl3+CH4 and CHCl3+H2 experiments as compared to the CHCl3 only experiments also lends support to this hypothesis.15 At atmospheric pressure, the polymer formation is said to occur in the gas phase.39 However, since the polymer is deposited in the plasma zone, it


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is still being exposed to the plasma post-deposition and hence the possibility of reinitiation of polymerization on the surface cannot be excluded. The polymerization mechanism is divided into three main sections to aid in mechanism description and illustration, i.e., initiation, propagation and termination.

3.4.1. Initiation The gas phase in the plasma is abundant with free radicals (most notably, H, Cl and CH3), which can react with ethylene molecules, initiating polymerization as depicted below. Formation of some of the functional groups in the polymer chain, such as terminal CH2Cl groups, can be a result of such reactions.35,39,42 CH2=CH2 + Cl. + M

CH2Cl-CH2. + M

(R1)

where M is a third body CH2=CH2 + CH3. + M

CH3-CH2-CH + M

(R2)

CH2=CH2 + H. + M

CH3-CH2. + M

(R3)

3.4.2. Propagation and formation of chain structures The attack of free radicals on an ethylene molecule generates a propagating chain that is capable of reacting with more ethylene molecules, thereby forming the bulk of the polymer chain. In addition, Cl free radicals can react with the growing polymer chain as illustrated in



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reaction (R5) and lead to formation of another chain structure decpicted in reaction (R6).35,39,42 CH2Cl-CH2. + CH2=CH2 + M

CH2Cl-CH2-CH2-CH2. + M

(R4)

~CH2-CH2~ + Cl. + M

~.CH-CH2~ + HCl + M

(R5)

~.CH-CH2~ + Cl. + M

~CHCl-CH2~ + M

(R6)

3.4.3. Chain unsaturation It is unlikely that the building blocks of the polymer chain will be very uniform and repeat regulary, because of the nature of plasma polymerization. The polymer will most likely be a random block co-polymer, with structural defects such as unsaturation.

Figure 5. Route for formation of unsaturation in the polymer Since the formation of an internal double bond is evidenced in the NMR analyses, we postulate a probable pathway for its formation in this study. We propose that such unsaturation can be introduced in the polymer chain as a result of intermolecular H abstraction followed by β-scission. In the present case, we propose that during the


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polymerization process, a propagating chain structure abstracts a hydrogen atom from a (previously generated) polymeric chain (see Fig. 5) resulting in the termination of the propagating chain as well as in chain transfer to the polymeric entity i.e. generation of a free radical.23 When the free radical containing part of the polymeric entity undergoes the β-scission process, it produces an internal double bond or unsaturation and a H radical. Nonetheless, it is also possible that instead of undergoing β-scission, it might combine with another propagating chain resulting in formation of branching. 3.4.4. Termination Termination of polymerization process can occur via two routes, i.e. either by radical combination

(R7-R9)

or

by

disproportionation

(R10-R11).

Termination

by

disproportionation can lead to formation of terminal unsaturated groups such as =CHCl or =CH2, which were identified during the NMR analysis of the polymer (section 3.3.3.2).35,39,42 3.4.4.1. Radical combination ~CH2-CH2. + .CH2-CH2~

~CH2-CH2-CH2-CH2~

(R7)

~CH2-CHCl. + .CH2-CH2~

~CH2-CHCl-CH2-CH2~

(R8)

~CH2-CHCl. + .CHCl-CH2~

~CH2-CHCl-CHCl-CH2~

(R9)

3.4.4.2. Disproportionation ~CH2-CH2. + .CH2-CH2~

~CH=CH2 + CH3-CH2~


(R10)


~CH2-CHCl. + .CHCl-CH2~

~CH=CHCl + CH2Cl-CH2~

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(R11)

4. Conclusion An alternative method for chloroform decomposition via its non-thermal plasma polymerization in presence of methane and hydrogen at atmospheric pressure was proposed. The most significant effect of methane and hydrogen addition was a 120 % and 31 % increase in the amount of the useful and non-crosslinked polymer. Polymer characterization based on NMR analyses indicate presence of more than one chain structure such as –(CH2-CH2)n-, –(CH2-CH=CH-CH2)m-, -(CH2-CHCl)n- along with some structural defects. Results from FT-IR analysis of the polymer complement the NMR analyses. A polymerization mechanism was proposed to explain the formation of the polymer formed during the reaction. 5. Acknowledgement The authors are thankful to the Australian Research Council for financial support of this project. Vaibhav V. Gaikwad acknowledges the Department of Education, Science and Training (DEST) of the Australian Government and the University of Newcastle, Australia for his postgraduate research scholarship. We also recognise the support of Dr. Monica Rossignoli at School of Environmental and Life Sciences, The University of Newcastle, with the NMR analyses.


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Supporting Information Table S1: Product distribution and mass balance data for CHCl3+CH4 experiment Table S2: Product distribution and mass balance data for CHCl3+H2 experiment 6. References (1)

Cheremisinoff, N. Industrial Solvents Handbook, Revised And Expanded, second.; CRC press, 2003.

(2)

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(3)

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(4)

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Figure 1. 1H NMR spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments 73x65mm (300 x 300 DPI)


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Figure 2. 13C NMR spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments 69x58mm (300 x 300 DPI)



Figure 3. HMQC spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments 82x165mm (300 x 300 DPI)


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Figure 4. FT-IR spectra for polymer from CHCl3+CH4 and CHCl3+H2 experiments 38x17mm (300 x 300 DPI)



Figure 5. Route for formation of unsaturation in the polymer 31x11mm (300 x 300 DPI)


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For Table of Contents Only 84x47mm (300 x 300 DPI)


Process for Chloroform Decomposition: Nonthermal Plasma

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