Document 7R1LRzdYXpjXZEaOYpJaRaRej

URL 18420 Kay 1974 EMS(Ol) Photochemical Reactivity of Vinyl Chloride R.A. Cox, A.E.J. Eggleton and F.J. Sandalle Environmental and Kedical Sciences Division, AERE, Harwell, Oxfordshire, 0X11 ORA* ituJJt The Photochemical Reactivity of Vinyl Chloride Contents 1. Introduction 2 Pho to-oxidation Experimen te 3 Hydroxyl Radical Attack on Vinyl Chloride 4< Comparison of Reactivity Data with Other Investigators 5* Products of Vinyl Chloride Photo-oxidation 6. Eye Irritation 7* Conclusions , 6. References Table I Table II w_ m * Fig. 2 Fig* 3 Fig* 4 Fig* 5 Rate parameters in photo-oxidation of vinyl chloride and sons hydrocarbons Relative photochemical reactivities Cc~=cr.traticn tine curves for the photo-cxid&tioa of chloride in the presence of MO Plots showing the removal of olefin and MO during photo oxidation Ozone formation during the photo-oxidation of hydrocarbons and vinyl chloride The effect of added vinyl chloride on the photo-dissociation of nitrous add Plot of the rate data from the photolysis of HM02**<'lefin mixtures according to equation \i) ms-741 oi) 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, organic nitrogen compounds and nitric acid. The relative importance of the various hydrocarbons which are emitted into the atmosphere in producing photochemical 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 the measurement of certain rate parameters for the oxidation of individual hydrocarbons in laboratory experiments, carried out under simulated atmospheric conditions^ The parameters most widely used for comparison are * (a) the rate of conversion of NO to NOg me rate ox Jiyurucarixm uonuumpvAon, anu (c) the rate of ozone formation The following general order of reactivity has been established(I. 2) Internal > polysubstituted olefins benzenes > terminal y mono alkyl olefins benzenes > paraffins 33 %Xto-J 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-NG^ photo- oxidation have suggested that the major free radical species involved in the initial attack on the hydrocarbon is the hydroxyl radical, OB. There is accumulating experimental evidence which confirms this. In particular the rate of OH reaction with aliphatic hydrocarbons corresponds closely to the empirically detemined photochemical reactivity for both unsaturated and saturated compounds. Although a similar correspondence is found for atomic oxygen and ozone reaction with olefins, the reactivity of 0 and 0^ with saturated hydrocarbons Is too slow to account for the observed photochemical reactivities of this class of 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 ppa concentrations of vinyl chloride, ethylene, propylene and trans-2-butene in tho presence of 1 ppm 170 in air vere measured and the rates compared. Secondly, the reactivity of these four olefins with hydroxyl radicals was measured by a technique recently developed in these laboratories^ which uses URL 18423 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 olefins and nitric: i oxide in synthetic air were made up in a 200 1 flexible bag constructed of Tedlpr film. There was no detectable adsorption of olefins, NO or NOg on this material (loss rate < 1# hr"1). Ozone loss rates were measurable hr"1) but not serious. The bag was irradiated by two banks of fluorescent lamps which had a broad spectral intensity in the blue-UV region (300 - 450 nm) with maximum intensity at 365 nm. The Tedlar film is transparent throughout this region. The light intensity was approximately 79^ of that of natural sunlight (zenith L = 40) in this spectral range, as measured from the rate of photolysis of NOg in pure nitrogen (k^NO^od 0.27 min"1). Synthetic air was made up by introducing 50 1 breathing grade oxygen to the bag and filling to 240 1 with nitrogen (o^pgen free grade). The trace gases, olefins and NO were added to the N^ stream during filling. After allowing ten minutes for thorough mixing, the mixtures vere irradiated and the concentra- tions of the olefin, the oxides of nitrogen NO and NOg and the ozone was determined as a function of time. The relative humidity of the air in the bag was approximate. 20 + 356 and the temperature 22 2C. / Analysis of NO and NO^ was carried out using a chemiluminescence NO^ analy/ -2 - (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 these techniques was of the order of 1 ppb and the precision at the 1 ppm level was better than 5$. Materials: Nitric oxide was taken from a standard mixture containing 118 ppm NO in N. Ethylene (99.8$), propylene (9950* trana-2-butene (99$) and vinyl chloride (99.9$) were taken from 'lecture bottle1 cylinders (BMJ Ltd), No impurity was detected either by gas chromatographic or infra-red spectroscopic analysis of the vinyl chloride. (b) Results A* Photo-oxidation of vinyl chloride in the presence of NO Fig* 1 shows the concentration time curves for the reaction of r 2.21 ppm CgH^Cl with 0.970 ppm NO under continuous irradiation. A typical, though rather slow, 'photochemical smog' typo 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- f tion of osone 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 osone concentration had increased to 0.60 ppm. Thus, significant osone 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 vinyl 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, URL 18424 3 URL 18425 0.91 # 0.97 ppa respectively. Pig* 2 shows the concentration 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 preaence of a higher initial concentration of NOg in this experiment ( = 0,10 ppm for compared with 0,03 for the other hydrocarbons). The same order of reactivity is also evident from the plots for ozone formation shown in Fig, 3* A quantitative comparison of the photochemical reactivity can be made on the basis of a number of parameters. For the present discussion we will consider the following: - (4) The average rate of NO oxidation to 50^6 NO consumption (b) The amount of reactant consumed after a given time (4 hours) (c) The maximum rate of ozone formation ; (p) The final ozone concentration after essentially complete oxidation 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 rate (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 but only after a much longer reaction time* * 3. Hydroxyl Radical Attack on Vinyl Chloride (a) Procedure Mixtures containing asI ppm gaseous nitrous acid together with approximately -4- 0*3 ppm each or VO and NO2 diluted in a N2"02 mixture (2sl) 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 330 - 380 nm light from a mercury arc source. The concentrations of NO, N02 and HN02 at the inlet and outlet of the cell were measured and the rates of foimation of NO and N02 (b^q and R^ ) 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 Rl.q and determined. The maximum extent of photolysis of HNOg was approximately 4#. (b) Results Fig. 4 shows the effect of added vinyl chloride on the rates of NO, NO^ and total NO + NOo foimation in the photolysis of HN0~. The rates are normalise*! to * dt t unit HNOg concentration. It will be seen that the addition of increasing ambunts of vinyl chloride leads to a fall in the rate of NO formation, an increase In the rate of NO2 foimation and a less pronounced decrease In the total rate *N0 + mo2* Similar offects were also obtained for the hydrocarbons ethylene, propylene and trans-2-butene. The mechanistic interpretation of the results in Fig. 4 is complex and 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. In the absuee of additive the photolysis of HNOg proceeds by HN0a-0H + N0 OH + HN02 = H20 + N02 (l) (2) URL 18426 Thus equal rates of HO and H02 formation are expected in the photolysis. The slightly lower rate of V02 formation with sero C^^Cl shown in Fig. 4 Is due to the side reaction of N02 with OH to give HHO^ which was not measured. When a compound, R, is present which reacts with OH radicals, reaction (3) then competes with reaction (?)eg. OH + H free radical product ? ($) The free radical product from (3) reacts with molecular oxygen which ia present in great excess to yield a peroxy radical which can oxidlae HO to NOg (p)62 + HO ho2 + (p)o Some of the (p)6 radicals nay then be lost by recombination or undergo further reactions leading to the formation of NOg. Some of the (p)6g radicals may also be lost by recombination. The radical loss processes are reflected in the decline in the total rate ^ with Increasing additive (Fig. 4). In the simple case of H = CO, then (p)6g and (P)6 are H02 and OH respectively and it has been ahown^ that the above mechanism fits the observations for the photolysis of HNOg-CO mixtures. Furthermore, the relative rate constants for OH reaction can be obtained from a plot of the equation: ANO + A(NO + W02) fcgfrj ^5 JjQ_ ^1 " ^(RO + N02) kx^Kxl k2 P10*-] .....URL 18427 where and 4^^ + }to ) raPraaan* the differences between the R^^fHNQgJ and Rjjq + jjq^/[HN023 values respectively in the absence and presence of additive, ^ is the dissociation rate of HN02, kg and *3 are the rate constants for reactions (2) and (3) respectively*and {NOJ = [HO NOg + HNOg]. Fig. 5 shows a plot of the data for vinyl chloride, CgH^, and t-C^Hg-2 according to equation (i). The slopes of the plots give a measure of the ratio i.e* the relative reactivity of the hydrocarbons with OH. The order of reactivity is the same as that found in the photo-oxidation experiments. By using the value of kg previously determined^ ^, relative to the well known rate constant for the reaction of OH with CO, values of kj of 9.4 x 10-12 and 5*6 x 10"*2 in ca? molecule"* s"* units are derived for CgH^ and CgH^Cl respectively from the above slopes, fiecent determinations of the absolute value of the rate constant for the reaction of OH with ethylene all lie in the region of 3 x 10"12 cm'* molecule"* e~^ \ The apparently higher value obtained in the present analysis almost certainly arises because more than one NO molecule is oxidised in the reactions following the attack of OH on CgH^. k comparison of *k- it) A --s-t-- --l aw- ----v j . --- 1 * URU8428 the values indicates a stoichiometry factor of about 2 _ The. stoichiometry 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 investigateis which have been summarised by Altshuller and Bufalini^. The OH reactivity data are compared with those of Morris and Niki('4) There is reasonably good agreement between the relative reactivities of the various substances based on A, the rate of HO oxidation and <B, the consumption of reactant* The differences which are observed can probably be attributed to the different experimental conditions and measurement methods used 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 Hiki on the basis of their OH reaction measurements, with which the present estimates show good agreement considering the uncertainty in the etoichiometxy mentioned above. It is also of interest to note that the reactivity of trichlorethylene is similar to that of vinyl chloride and ethylene. 5* Products of Vinyl 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' hy analogy with ethylene for which the major products are formaldehyde, CO and COj* Thus fission of the C-C bond occurs in the oxidation reaction, and in addition to the other three products observed for CgH^, vinyl chloride would be expected to yield formyl chloride. Although formyl chloride hae apparently never been isolated as a stable compound, it may be stable at very low concentrations 7 URL 18429 in air. formally it decomposes to HC1 and CO which will undergo further oxidation only slowly in the photochemical 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 eye irritation index('2) . This is no doubt duo to the widely differing lachrymatoiy effects of the products fonaed 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 trichloro-ethylene^ and there is unfortunately some conflict between two separate investigations. Trichloro-ethylene lies between propylene and \ ethylene in photochemical reactivity and gives an eye-irritation index reported to be either somewhat greater than propylaie^ or somewhat less than ethylene^. Taking the more pessimistic value, thought to be more realistic 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 tAieing into account the somewhat lover photo-reactivity of vinyl chloride observed In the present investigation, then the data suggests that the eye-irritation index (2) for vinyl chloride should be similar to that for propylene. Heues and Glasson 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 mine, exposure as: 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- (7) tions resulting in equal eye-irritation, according to 3chuck and Doyle and there ie much uncertainty surrounding the mibject. -6- 7 Conclusions The results discussed above ehov that (a) Vinyl chloride undergoes photo-oxidation In a similar manner to other hydrocarbon compounds when mixtures are exposed to UV radiation of wavelength and intensity similar to that of solar radiation near the earth's surface* (b) The photochemical reactivity of vinyl chloride, as measured from a number of rate parametere in the photo-oxidation reaction and also from its reactivity toward OH radicals, is similar to or slightly less than ethylene. Vinyl chloride is, therefore, only a moderately reactive precursor to photochemical smog, being less reactive than propylene and higher olefins, but more reactive than the noxmal paraffins, f f (c) The rate constant for the reaction of OH with vinyl chloride has an unDer limit of 5.6 x 10TM*^ ea^ moleculeTM* aTM* at 500^. The true value is probably a factor of 2 - 5 lower than this* URL 18430 / \ 6, References 1* A.P. Altshullor and J.J. Bufalini, 'Photochemical Aspects of Air Pollution: A Rev-low*, Environ. Sei. and Tech. J, 39 - 64 (1971). 2* Bouse and V.A. Glasson, 'Hydrocarbon Reactivity and Ey* Irritation', Environ. Sci. and Tech. 2* 1109 - 1116 (1968). 3. R.A. Cox, to be published in J, Photochemistry. 4. E.D. Morris and H Niki, 'Reactivity of Hydroxyl Radicals with Olefins', J. Phys. Chem. 22. 3640 - 3641 (I97l). 5* S.L* Kopczynski, unpublished results (1968) reported in Ref. 1 p. 48. 6. K.W. Wilson, G.J. Boyle, D.A. Hansen and R.B. Engle?t, Symposium of ACS Division of Organic Coating and Plastic Chemiatzy, Hew Toxic, Sept. 1969* See also Environ. Sci. and Tech, jg, 896 (1969) and ibid p. 1224 together with Ref. 1 p. 48. 7. E.A. Schuck and G.J. Boyle, 'Photo-oxidation of Hydrocarbons in fixtures Containing Oxides of Nitrogen and Sulphur Dioxide', Report Ho. 29, Air; Pollution Foundation, San Marino, Calif/ (1959) see also Ref. 1 p. 56. { URL 18431 / - 10 - / TABLB T Rate parameters__in photo-oxidation of vinyl chloride and some hydrocarbons Reactant k - d tN0"]/dt (ppm/min x 10 ) B & reactant consumed after 4 hours c (d(O^dt)max A (ppm/min x 10^) D final p>5"] (ppm) Vinyl Chloride Ethylene Propylene Trans-2-butene 0,31 0*34 1.40 3.0 13 25 88 (13)* >100 (75)* >0.017^ >0.177' 0,58 3.75 0.60 (1300 min)** 0.47 (1300 min) 0.66 (240 min) 0.7* (80 din) hydrocarbon consumed after 30 minutes / maximum rate not achiered during reaction time used ** time at which final ozone concentration measured* URL 13432 TABLE XI Relative photochemical reactivities* Reactant C2S4 This York AI B HO * reactant oxidation consumption. 0.25 0*28 Altshuller & Bufaliui^ C OH reactivity 0.29 A HO oxidation 0,4 B reactant consumption 0.1 Morris.& HikiW C OH reactivity 0.1 C3H6 1.0 1.0 1.0 1.0 1.0 1.0 2*1 5.8 ^ 5.0 2 ' ^6 CgH^Gl 0.25 0.15 CgHCl, t.,. ............t ____ , l________________ i 0.17 ' 0.5 0.45 ______________i________________ I 4.2 ;C t C.C6OO. Reactivities are relative to propylene vhioh ie arbitrarily set at unity. / / / / /// 12 FSj. 1 Concentration time curvoo for tho photo--oxidation presence of NO. vinyl chloride in the H W itifi 13 - o--9 -ft cCo?, x o; .- CI> -- :> c. coo o ) li o *3 / D URU8435 <M P. oo f\) tHo t-- (Wdtf) NO.. . H Ti(;. ? Ozcno formation during tho photo-oxidation of hydrocarbons and vinyl chlorid (filled points). (p p m ) 0 7 . 0 N t C o U C C N T A h o IJ o i5 Tint- also'" - 15 - ` 3^ Pig. 4 j2 [* The effect of added vinyl chloride on the photodissociation of nitrous acid. The plot shove the rates of formation of NO and NOj (%o an(* Rjjq^) and the total rate Rfjo+IK^* expressed per unit HNO;> concentration as a function of the concentration ratio [CsHjClJ/^KNO^j. ^T*/j>No23 CsecrUlO1) * 16 - FiC* 5 Plot of tho rato data from tho photolyoia of HHOg-clofin mixtures According to equation (i) (see text). 'to ~A(mo+ncO