Document 0JZBYn8O5vM49QKY285jXVdxR

7? 7*A0 Key 1974 BS(Ol) Photochemical Reactivity of Vinyl Chloride R.A. Cox, A.E.J. Eggleton and F.J. Sandalla Environmental and Kedical Sciences Division, AERE, Harwell, Oxfordshire, 0X11 ORA. t 9> o UC 03 7 , The Photochemical Reactivity of Vinyl Chloride. Contents 1 Introduction 2. Photo-oxidation Experiments 3. Hydroxyl Radical Attack on Vinyl Chloride 4. Comparison of Reactivity Rata with Other Investigators 5. Products of Vinyl Chloride Photo-oxidation 6. Eye Irritation , 7* Conclusions , 8. References t Table I Rate parameters in photo-oxidation of vinyl chloride and aom hydrocarbons Table II Relative photochemical reactivities ZJ.X--C- for the photo-cxidatioa of viay Pig, 2 Pig. 3 Pig. 4 Pig. 5 Plots Bhoving the removal of olofin and NO during photooxidation Ozone formation during the photo-oxidation of hydrocarbons and vinyl chloride The effect of added vinyl chloride on the photo-dissociati n of nitrous acid Plot of the rate data from the photolysis of HN02~olsfI& mixtures according to equation tees. 1 2 4 7 7 13 15 17 (i) uce 037645 1 Introduction The formation of 'photochemical smog' in polluted atmospheres results from the oxidation of hydrocarbon substances in a photochemically initiated reaction Involving oxides of nitrogen. The oxidation products characteristic of photo chemical smog include oxidants (mainly ozone), aldehydes, CO, organio nitrogen compounds and nitric acid. Tho relative importance of the various hydrocarbons which are emitted into the atmosphere in producing photochemioal smog in a given area depends on the rate at which they undergo photo-oxidation. Investigators have drawn up an empirical scale of reactivity which is based on th measurement of certain rate parameters for the oxidation of individual hydrocarbons in laboratory experiments, carried out under simulated atmospheric conditions. Th parameters most widely used for comparison are (a) the rate of conversion of NO to KO2 (b) lite rale of iiydrucarbun cunuumpiiun, and (c) the rate of ozone formation The following general ordor of reactivity has been established(' l * 2*) 1 Internal > polysubstituted ;> terminal y mono alkyl > paraffins olefins benzenes olefins benzenes * The reactivity of a given hydrocarbon may be ascertained by comparing measured values of the above parameters with those for other hydrocarbons which have known reactivity. Recent theories concerning the mechanism of the hydrocarbon-NC^ photo oxidation have suggested that the major free radical specieB involved in the initial attack on the hydrocarbon is the hydroxyl radical, OH. Ther is accumulating experimental evidence which confirms this. In particular the rate of OH reaction with aliphatic hydrocarbons corresponds closely to the empirically determined photochemical reactivity for both unsaturated and saturated compounds* Although a similar correspondence la found for atomic oxygen and oz no roaction with olefins* the reactivity of 0 and 0^ with saturated hydrocarbons la too si w ^1 ucc Q37646 to acc unt for tho observed photochemical reactivities of this class f hydro carbons. In order to determine the photochemical reactivity of vinyl chloride two series of experiments have been carried out. Firstly, tho rates of photo oxidation of ppm concentrations of vinyl chloride, ethylene, propylene and trans-2-butene in tho presence of 1 ppm NO in air were measured and th rates compared. Secondly, the reactivity of these four olefins with hydroxyl radicals was measured by a tochnique recently developed in these laboratories(3)' which uses the photolysis of gaseous nitrous acid as a source of hydroxyl radicals. 2. Photo-oxidation Experiments (a) Procedure Mixtures containing part-per-million concentrations of olofins and nitric oxide in synthetic air were made up in a 200 1 flexible bag constructed of Tedlar film. There was no detectable adsorption of olefins, NO or NOg on this material (loss rate ( 1$ hr~^). Ozone loss rates were measurable (^1 Q$ hr"*) but not serious. The bag was irradiated by two banks of fluorescent lamps which hod a broad spectral intensity in the blue-UV region (300 - 450 nm) with maximum t intensity at 365 nm. The Tedlar film is transparent throughout this region. The light intensity was approximately 75$ of that of natural sunlight (zenith L * 40) in this spectral range, as measured from the rate of photolysis of NOj in pure nitrogen (k^NOgJci 0.27 min**^). Synthetic air was made up by introducing 50 1 breathing grade oxygen to the bag and filling to 240 1 with nitrogen (oxygen free grade). The trace gases, olefins and NO were added to the N2 stream during filling. After allowing ten minutes for thorough mixing, the mixtures were irradiated and the e ncentrat tions of the olefin, the oxides of nitrogen NO and NO2 and the ozone was determined as a function of time. The relative humidity of the air in the bag was approximate 20'+ 3$ and the temperature 22 2C. Analysis of NO and NOg was carried out using a chemiluminescence N0X analyser t -2- ucc (TECO Model 12a). Ozone was measured on a Nederbragt type ethylene chemi- luminescence ozone detector# and the olefins were measured by gas chromatographic analysis using a flame ionisation detector* The minimum detectable concentrations using each of theso techniques was of the order of 1 ppb and the precision at th 1 ppm level was better than 5$. Materials: Nitric oxide was taken from a standard mixture containing 118 ppm NO in Ng. Ethylene (99*8$)# propyleno (99^), trans-2-butene (99^) and vinyl chloride (99*9$) were taken from 'lecture bottle* cylinders (BS^ Ltd)* N impurity was detected either by gas chromatographie or infra-red spectrosc pic analysis of the vinyl chloride. 0>) Results A. Photo-oxidation of vinyl chloride in the presence of NO Fig* 1 shows the concentration time curves for the reaction of 2.21 ppm C^HjCl with 0*970 ppm NO under continuous irradiation. A typical* though rather slow, 'photochemical smog* type reaction is observed; after a short induction period# oxidation of NO to NOg commences with accompanying consumption of the vinyl chloride and as the NO is depleted# the concentra tion of ozone rises. Even after six hours irradiation* oxidation of NO was incomplete and only 26$ of the vinyl chloride had been consumed* After prolonged irradiation (22 hours)* 79$ of the vinyl chloride had been consumed and the ozone concentration had increased to 0,60 ppm. Thus* significant ozone concentrations result from the photo-oxidation of vinyl chloride in air but only after a long period of irradiation. B. Comparison of the rates of photo-oxidation of vinvl chloride with hydrocarbons Similar experiments to those described above were carried out for ethylene* propylene and trans-2-butene with initial concentrations of 2.36* 1.67* 2.10 ppm respectively. Initial NO concentrations were 1.02* 3 0.91 0.97 ppa respectively. Pig. 2 shows the concentrati n tine curves for removal of NO and the olefins. Clearly the reactivities of trane-2-butene and propylene are much higher than that of vinyl chloride which is similar to ethylene. The absence of a noticeable induction period for NO oxidation with propylene arises from the presence of a higher initial concentration of NO^ in this experiment ((NO^Jq * 0.10 ppm for compared with < 0.03 for the other hydrocarbons). The some order of reactivity i also evident from the plots for ozone formation shown in Pig. 3. A quantitative comparison of the photochemical reactivity can he made on the basis of a number of parameters. For the present discuss! n we will consider the following:- U) The average rate of NO oxidation to 50$ NO consumption (b) The amount of reactant consumed after a given time (4 hours) (c) The maximum rate of ozone formation . (d) The final ozono concentration after essentially complete xidatlon of NO. The numerical values for these parameters, estimated from the concentration time curves are given in Table I. On the basis of parameter A, the reactivity of vinyl chloride is rather close to that of ethylene but in terms of hydrocarbon reaction rat (B) vinyl chloride oxidation is significantly slower. Both compounds are considerably less reactive than propylene and trans-2-butene. For all four substances the final ozone concentration was approximately the same, showing that the chlorinated hyddocarbon, vinyl chloride, can potentially produce as much ozone as the 'reactive' olefins hut only after a much longer reaction time* Hydroxyl Radical Attack on Vinyl Chloride Procedure Mixtures containing ppm gaseous nitrous sold together with approximately 4 uce 037649 0,5 ppm each of NO and N02, diluted in a N2-<>2 mil'ture (2t 1) were made up In the Tedlar bag. The mixture was drawn from this reservoir at a constant flow rate through a 27 cm^ cylindrical photolysis cell irradiated with 550 - 5S0 am light from a mercury arc source. The concentrations of NO. NOg and HNOg at the inlot and outlet of the cell were measured and the rates of fonnation of NO and NOg (R.jq and IL^ ) In the photolysis determined. Successive aliquots of vinyl chloride (or other olefins) were then added to the mixture and the effect of increasing olefin concentration on and determined. The maximum extent of photolysis of HNOg was approximately 4#. (b) Results Nig. 4 shows the effect of added vinyl chloride on the rates of NO, NOg and total NO + NO2 formation in the photolysis of HN02. The rates are normalised to unit HN02 concentration. It will be seen that the addition of increasing amounts of vinyl chloride leads to a fall in the rate of NO formation, an increase in the rate of N02 formation and a less pronounced decrease in the total rate RjjO + Similar offectB were also obtained for the hydrocarbons ethylene, propylene and trans-2-butene. The mechanistic interpretation of the results in Fig. 4 is complex and a. ,, subject to considerable uncertainty. However, on the basis of the following simplified scheme, the data can give an estimate of the relative reactivity of the added hydrocarbons with OH. * i* In the absence of additive the photolysis of HN02 proceeds by HN02 * OH + NO OH +. HN02 = H20 + N02 (0 (2) Thus equal rates of NO and N02 formation are expected in the photolysis. The sligbl lower rate of NOg formation with zero CgHjCl shown in Fig. 4 is due to the side reaction of N02 with OH to give HNO^ which was not measured* When a compound. K. 1 present which reacts with OH radicals, reaction (?) then competes with reaction (2), e -5- ucc 037650 OH + R free radical product P (j) The free radical product from (3) reacts with molecular oxygen which is present in great ezoeee to yield a poroxy radical vhiah can oxidise MO to M02 (P)6Z + HO ho2 + (P)0 Some of the (p)C radicals may then be lost by recombination or undergo further reactions leading to the formation of N02. Some of the (p)d2 radicals may also be lost by recombination. The radical loss processes are reflected in the decline in the total rato + with increasing additive (fig. 4) In the simple case of R s CO, then (p)d2 and (p)<5 are H02 and OH respectively and it has be n shown^ that the above mechanism fits the observations for the photolysis of HN02~C0 mixtures. Furthermore, the relative rate constants for OH reaction can be obtained from a plot of the equation: amq * A(ho + ho2) k3fr] kj fa]. ^1 " A(MO + N02) kxtNOxl k2 CHOx.] where and + jjo ) represent the differences between the RjJO/[hN02'] and Rjjq + ^^/{HN023 values respectively in the absence and presence of additive, ^ is the dissociation rate of HNO2, k2 and k^ are the rate constants for reactions (2) * and (3) respectively* and [NOj = [NO + N02 + HN02]. Fig, 5 "shows a plot of the data for vinyl chloride, CgH^, C^Hg and t-C^H0-2 according to equation (i). The slopes of tiie plots give a measure of the ratio ky^, i.e. the relative reactivity of the hydroearbone with OH. The order of reactivity Is til same as % that found in the photo-oxidation experiments. By using the value of k2 previously determined'', relative to th well known rate constant for the reaction of OH with CO, values of k^ of 9*4 x 10*' and 5.6 x 10"12 in aP molecule-* a-* units are derived for CgH^ and CgH^Cl respectively from the above slopes. Recent determinations of the absolute value of the rate constant for the reaction of OH with ethylene all lie in the region --12 3 --i(4) of 3 x 10 cm molecule s . , The apparently higher value obtained in the : present analysis almost certainly arises because more than one MO molecule is # oxidised In the reactions following the attack of OH on CgH^. A comparison f 1 ICC 037651 the k,, values indicates a stoichiometry factor of about 3. The stoichiometry 3 factor for vinyl chloride is unknown and therefore the rate constant value obtained can only be regarded as an upper limit. By analogy with ethylene the true value is probably a factor of 2 - 3 lower than the value given. 4. Comparison of Reactivity Data with Other Investigators Table II shows a comparison of the relative reactivities of the substances under consideration with those obtained by other investigators which have been summarised by Altshuller and Bufalini^. The OH reactivity data are compared with those of Morris and Nlki^. There is reasonably good agreement between-the relative reaotivlti s of the various substances based on A, the rate of HO oxidation and >B, the c nsumption of reactant. The differences which are observed can probably be attributed to the different experimental conditions and moasuremont methods uood in the various investigations. A close correspondence between relative reactivity toward OH and reactivity in the photochemical oxidation system is also evident. This correspondence has also been noted by Morris and Hlki on the basis of their OH reaction measurements* with which the present i * *' estimates show good agreement considering the uncertainty in the stoichiometry S mentioned above. It is also of interest to note that tho reactivity of trichlorethyleno is similar to that of vinyl chloride and ethylene. 5. Products of Yinvl Chloride Photo-oxidation In the present study no investigation of the products of the photo-oxidation of vinyl chloride has been made. The nature of the expected major products may be deduced by analogy with ethylene for which the major products are f noaldehyde, CO and COj* Thus fission of the C-C bond occurs in the oxidation reaotion, and in addition to the- other three products observed for CgH^, vinyl chloride would be expected to yield formyl chloride* Although formyl chloride has apparently never been isolated as a stable compound, it may be stable at very low concentrations in air. normally it decomposes to HC1 and CO which will undergo further # xldatlon nly slowly in the phot chemical system. 6, Eye Irritation While there is a strong correlation between the various chemical parameters used to characterise the reactivity of hydrocarbons in the photochemical system, no such correlation exists with the eyo irritation index('2) . This is no doubt duo to the widely differing lachrymatory effects of the products formed from quite similar starting materials. In the absence of experimental data, any attempt * to assess the eye irritation index for vinyl chloride must therefore be largely speculative. The only chlorinated compound for which the eye irritation index has been reported is tiichloro-ethylene^ and there is unfortunately some conflict between two separate investigations. Trlchloro-ethylene lies between propylene and ethylene in photochemical reactivity and gives an eye-irritation index r ported to be either somewhat greater than propylene^ or somewhat less than ethylene^. Taking the more pessimistic value, thought to be more realiatie because of the possible formation of the strongly lachrymatory compounds phosgene and formyl chloride (the latter also being a potential product of vinyl chloride), and * taking into account the somewhat lower photo-reactivity of vinyl chloride observed in the present investigation, then the data suggests that the eye-irritation index for vinyl chloride should be similar to that for propylene. Heuss and Glasson(' 2) reported values of 0.3* 0.5, 1.2 and 3.0 for ethylene, trans-2-butene, propylene and 1,3-butadiene respectively, together with those for many other hydrocarbons* The eye-irritation was assessed by a panel after 4 mins, exposure ast none, light, moderate or severe and assigned numerical values of 0, 1, 2 and 3 * ;' respectively. It should be pointed out,' however, that atmospheric measurements of eye irritants are almost an order of magnitude lower than laboratory concentra tions resulting in equal qye-irrltation, according to Schuck and Doyle^ and there Is much uncertainty surrounding the subjeot. -8 ucc 037653 Conclusions Tho results discussed above show that (a) Vinyl chloride undergoes photo-oxidation in a similar manner t other hydrocarbon compounds when C^HgCl-NO-air mixtures are exposed to UV radiation of wavelength and Intensity similar to that of solar radiation near the earth's surface. (b) Tho photochemical reactivity of vinyl chloride, as measured from a i number of rate parameters in the photo-oxidation reaction and also from its reactivity toward OH radicals, is similar to or olightly less than ethylene. Vinyl chloride is, therefore, only a moderately reactiv precursor to photochemical omog, being less reactive than propylene . end higher olefins, but more reactive than the normal paraffins, (c) The rate constant for the reaction of OH with vinyl chloride has an UDDer limit of 5.6 x 10-*^ cm** molecule-* s"* at 300^. The true value is probably a factor of 2 - 3 lower than this* / .1 \ ucc 037654 8* References 1. A.P. Altshullor and J.J. Bufalinl, Photochemical Aspects of Air Pollution A Reviow', Environ. Sci. and Tech. Jji, 39 " 64 (l97t). 2. J.M. Heuss and W.A. Glasson, 'Hydrocarbon Reactivity and E&re Irritation', Environ. Sci. and Tech. 2, 1109 " 1116 (1968). 3. R..A. Cox, to be publiolied in J. Photochemistry. 4. E.J). Morris and II. Niki, 'Reactivity of Hydroxyl Radicals with Olefins', J. Phys. CUein. 3640 - 3641 (l97l). 5. S.L* Xopczynski, unpublished results (1968) reported in Ref. 1 p. 48. 6. K.W. Wilson, G.J. Doyle, D.A. Hansen and R.D. Englert, Symposium of ACS Division of Organic Coating and Plastic Chemistry, Hew Toxic, Sept. 1969* See also Environ. Sci. and Tech. JJ, 896 (1969) and ibid p. 1224 together with Ref. 1 p. 48. 7. E.A. Sehuck and G.J. Doyle, 'Photo-oxidation of Hydrocarbons in fixtures Containing Oxides of Nitrogen and Sulphur Dioxide', Report No. 29, Air Pollution Foundation, San Marino, Calif*' (1959) see also Ref* 1 p* 56* i r/ - 10 - ucc 037655 TABLE I Rata parameters in photo-orldation of vinyl chloride and Borne hydrocarbons Reactant A - d {VoT/dt 2 (ppn/ntn s 10 ) B $ reactant con3umod after 4 hours C (d(O^dt)max (ppm/min x 10) B final [Ojl (ppo) Vinyl Chloride Ethylene Propylene Trana-2-butene 0.31 0.34 1.40 3.0 13 25 . 88 (13)* >100 (75)* >0.017^ >0,17^ 0.58 3.75 0.60 (1300 min) *** 0.47 (1300 min) ' 0.66 (240 min) 0.73 (80 min) * hydrocarbon coHouaed after 30 minutes / maximum rato not achieved during reaction time used ** timo at which final ozono concentration measured. TABLE II Rclatlva photochemical reactivities* Reactant C2H4 C3H6 t-C4H3-2 A HO rxidatiou 0.25 1.0 2.1 This work reactant consumption 0.28 1.0 5.8 Altshuller St Bufolini^ C OH reactivity 0.29 1.0 A NO oxidation 0.4 * 1.0 B reactant consumption 0.1 1.0 ^ 3.0 2 * ^6 1 Morris Nikit* C OH roactiv: 0.1 1.0 4.2 W1 0.25 0.15 CgHCl, ---------- ------- mm ......... __________________________________ 0.17 - 0.5 t 0.45 * m ^IcoaotivitiGn aro rolativo to propylene which is arbitrarily set at unity. I - 12 - "f t ' ' 4 `*<t 'k' , ' ucc 037657 Fij. 1 Concentration timo curvoo for tho photo-oxidation of vinyl chloride in the presence of NO. ucc 13 037658 Pic* 2 Plot.-) showing the removal of olefin (filled points) uni SO (open points) Jarir.;; the thole-oxidation of vinyl chloride (c), ethylene (A), pro;yleno (O) and tran.!-?-butene (0), in the pris once of SO. The concentration axis for each hydrocarbon in shifted 3li{ji`.ly so that the initial EC concentrations correspond to 0.97, ppa EC1. . '(P P M ) . CO VJO fM TfW ldN So tA ta 0 ISO . ^ P p rA') OTjONl' Cot-iCCu ^ A i i o l l o 5 Or.ono formation during tho photo-oxidation of hydrocarbons and vinyl chlorid (filled points). - 15 - Fig* 4. The ffect of added vinyl chloride on the photodissociati a of nitrous acid. The plot shows the rates of formation of NO and NO2 (%o and Rjjq^ and. the total rate %q+N02* expressed per unit HN02 concentration as a . function of the concentration ratio [c2HjjClJ/[KNO2]. ^ [* RATc/ [.HN023 (`SecT** 101) - 16 - ucc 037661 FiC* 5 riot of tho rato data from tho photolyoio of HNOg-olofin mixturoo according to equation (i) (ace text). Atfo +A<No + Nox) " A(,|sJO + KlO^