Document jmV8LG1E2kz221n5rMKqoq0ZZ

'/f 7XU> Kay 1974 EKS(Ol) Photochemical Reactivity of Vinyl Chloride R.A. Cox, A.E.J, Eggleton and P.J, Sandalls Environmental and Medical Sciences Division, AERZ, Harwell, Oxfordshire, 0X11 ORA. ucc 095553 . Contents 1. Introduction 2. Photo--oxidation Experiments 5. Hydroxyl Radical Attack on Vinyl Chloride A. Comparison of Reactivity Data with Other Investigators 5. Products of Vinyl Chloride Photo-oxidation 6. Eye Irritation 7. Conclusions , 6. References Table Z Rate parameters in photo-oxidation of vinyl chloride and some hydrocarbons Table II *m--b.* 4k z Relative photochemical reactivities Ccnvcr.trcticr. time curvcc fer the phota-cxidation of vinyl chloride in the presence of NO Plots shoving the removal of olofin and HO during photo oxidation Fig. 3 Ozono formation during tho photo-oxidation of hydrocarbons and vinyl chloride Fig. 4 The effect of added vinyl chloride on tho photo-dissociation of nitrous acid Fig. 5 Plot of the rate data from the photolysia of HN02~olefin mixtures according to equation (i) Pa^e 1 2 4 7 7 8 9 10 11 12 13 14 15 16 17 M Ucc 095554 1. Introduction The formation of 'photochemical smog1 in pollutod 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 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 NO2 /. \ ,, . \v/ tuu rate ox uyuxucaroun yuQuumptxun, emu (c) the rate of ozone formation The following general order 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 tho hydrocarbon-NG^ photo oxidation have suggested that the major free radical species involved in the initial attack on the hydrocarbon is the hydroxyl radical, OH. There is accumulating experimental evidence which confirms this. In particular the rate of OH reaction with aliphatic hydrocarbons corresponds closely to the empirically 11 determined 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 -1 - UCC 095555 to account for tho 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 ppm concentrations of vinyl chloride, ethylone, propylene and crans-2-buteno in tho presence of 1 ppm NO in air were measured and the rates compared. Secondly, the reactivity of these four olefins with hydroxyl radicals wa3 measured by a technique recently developed in these laboratories^ 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 olefins 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 NO^ on this material (loss rate < hr"1). Ozone loss rates were measurable ('''1($ 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. Tho Tedlar film is transparent throughout this region. The light intensity was approximately 75> of tJiat of natural eunli^it (zenith L = 40) in this spectral range, as measured from the rate of photolysis of NOg in pure nitrogen (k^fNOgJci 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 (oxygen free grade). The trace gases, olefins and NO were added to the Ng stream during filling. After allowing ten minutes for thorough mixing, the mixtures were irradiated and the concentra- i 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 approximately 20 +, 3$ and the temperature 22 ^ 2C, Analysis of NO and NOj was carried out using a chemiluminescence NO^ analyser -2- ucc 095556 (TECO Modol 12a). Ozone wag measured on a Nederbragt type ethylene chemiii 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 ppm levol vas better than +, %. hatorialn: Nitric oxide wan tekon from a standard mixture containing 110 ppm NO in Hj. Ethylene (99.G$), propylono (99$), traps-2-butene (99$) and vinyl chloride (99.95) were taken from 'lecture bottle' cylinders (BDIJ Ltd). No impurity was detected either by gas chromatographic or infra-red spectroscopic analysis of the vinyl chloride. (b) Results A. Photo-oxidation of vinyl chloride in the presence of NO Fig. 1 shows tho concentration time curves for the reaction of 2.21 ppm CgHjCl with 0.970 ppm NO under continuous irradiation. A typical, though rather slow, 'photochemical smog' type reaction is observed; after a short induction period, oxidation of NO to NOg commences with accompanying consumption of the vinyl chloride and as the NO ia depleted, the concentra tion of ozone risee. Even after six hours irradiation, oxidation of NO was incomplete and only 26/5 of tho vinyl chloride had beon consumed. After prolonged irradiation (22 hours), 79$ 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 p. long 2,erid 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 wore 1.02, -3- UCC 095557 0.91 0.97 ppa respectively. Fig. 2 shews the concentration tine curves for removal of NO and tho olefins. Clearly the reactivities of trana-2-butene i 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 ((N^)^ = 0.10 ppm for compared < 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:- (A) The average rate of NO oxidation to 50$ NO consumption (b) Tho 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 oxidation of NO. Tho numerical values for these parameters, estimated from the concentration time curves are given in Table I. On the basis of parc^tcr 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 ozono concentration "m 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* ' Hydroxyl Radical Attack on Vinyl Chloride Procedure Mixtures containing 7 ppm gaseous nitrous acid together with approximately -4- UCC 095558 0.3 ppm each of NO and NOg, diluted in a m^I^ur0 (2:l) were made up in tho Todlar bag. The mixturo unu drawn from this rooorvoir at a conatant flow 3 rate through a 27 cm cylindrical photolysis cell irradiated with 330 -- 330 nm light from a mercury arc source. The concentrations of NO, NOg and HNO2 at the extent of photolysis of HNOg was approximately $>, (b) Reaulta Fig. 4 shows the effect of added vinyl chloride on the rates of NO, NOj and total NO + NO2 foimation in the photolysio of HNOg* The rates are normalised to unit KNOg concentration. It will he seen that the addition of increasing amounts of vinyl chloride leads to a fall in the rate of NO formation, an increase in tho rate of NO2 formation and a less pronounced decrease in the total rate ^NO + NO2' Similar offacts wore also obtained for tho hydrocarbons ethylene, propylene and trans-2-butene. The mechanistic interpret a t;i.on of the results in Fig. 4 is complex and Ik, ^ subject to considerable uncertainty. Howe .-or, on the basis of the following simplified scheme, the data can give an estimate of tho relative reactivity of the added hydrocarbons with OH. In the absence of additive the photolysia of KNOg proceeds by :nro2 = on + no OH + HN02 = H20 + N02 <0 (2) Thus equal rates of NO and NO2 formation are expected in the photolysis. The slightly lower rate of NO,, formation with zero C^^Cl Bhown in Fig. 4 is due to the side reaction of NOg with OH to give HNOj which was not measured. When a compound, R, la present which reacts with OH radicals, reaction (3) than competes with reaction (2), e 5 UCC 095559 OH + H > free radical product P (p) Tho free radical product from (?) reacts with molecular oxygen which is present in great exooes to yield a poroxy radical which can oxidise NO to NOg (p)62 + NO no2 + (p)o Some of the (p)0 radicals nay then he lost by recombination or undergo further reactions leading to the formation of N0^. Some of the (p)<32 radicals may also be lost by recombination. The radical los3 processes are reflected in the decline in the total rato R HO + NO' with 'norcasing additive (Fig. a). 1'n the simple case of H s CO, then (p)62 and (p)6 are 110- and OH respectively und it has been (3) shown that the above mechanism fits the observations for the photolysis of ENOj-CO mixtures. Furthermore, the relative rate constants for OH reaction can be obtained from a plot of the equation: AA NO + (HO + SOg) 'Xj[r] ^1 " ^(NO + H02) k^ [r] k2 N0*-] where and + no ) rePreoent the differences between the and ^NO + va^ue3 respectively in the absence and presence of additive, ^ is tho dissociation rate of HNOg, kg and k^ are the rate constants for reactions (2) 'and (3) respectively*and (NOj = (NO + N02 + HNOg]. Fig. 5 shows a plot of the data for vinyl chloride, C^, C^Hg and t-C^-2 according to equation (i). The slopes of the plots give a measure of the ratio kj/k2 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 k2 previously determined^, relative to the well known rate constant for the reaction of OH irith CO, values of kj of 9.4 x 10-12 and --12 3 -l --1 5*6 x 10 in car molecule b units are derived for C2H^ and C^l^Cl respectively from tho above elopes. Itecent determinations of the absolute value of the rate constant for the reaction of OH with ethylene all lie in the region -12 3 -1 -i(4} of 3 z 10 cm molecule a . 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 CjH^. A comparison of UCC 095560 the kj values indicates a 3toichiometry factor of about 3. 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 tho true value is probably a factor of 2 - 3 lower than the value given. 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 Alt3huller and Bufalini^. Tho OH reactivity data ar compared with those of Morris and Niki^. There ia 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 tho different experimental conditions and measurement methods used in the various investigations. A close correspondence between relative reactivity toward OH and reactivity, in thp photochemical oxidation system is also ovidont. This correspondence has also boen noted by Morris and Niki on the basis of their OH reaction measurements, with which the present i estimates show good agreement considering tho uncertainty in the stoichiometry mentioned abovo. It is also of interest to note that tho reactivity of trichlorothylcno ia similar to that of vinyl chlorido and othyleno. 5. Products of Vinyl Chloride Photo-oxidation In the present study no investigation of the products of the photo-oxidation of vinyl chloride ha3 been made. The nature of tho oxpected major products may bo deduced by 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 C.^, 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 -7- ucc 095561 in air. Normally it decompooes to KC1 and CO which will undergo further oxidation only slowly in the photochemical system. 6. ffie Irritation Nhile there ig a strong correlation between the various chemical parameters nood to characterise the reactivity of hydrocarbons in the photochemical system, no ouch correlation exists idth the eyo irritation index^*^. This is no doubt <uio :.o 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 trichloro-ethylene^^ and there is unfortunately some conflict between two separate investigations. Trlchloro-ethylene lies between propylene and othylene in photochemical reactivity and gives an eye-irritation index reported to bo either somewhat greater than propylene^ or somewhat less than ethylene^. Taking tho mors 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 taking into account the somewhat lowor 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 thono for many other hydrocarbons. The eyo-irritation was assessed by a panel after 4 mins, 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 laborstory con centra- (7) tions resulting in equal eye-irritation, according to Schuck and Doyle' and there is much uncertainty surrounding the subject. -8- UCC 095562 7. Conclusions The results diacus3cd :,bo/c riww 'f (n) Vinyl chloride undergoos photo-oxidation in a similar manner to other hydrocarbon compounds when CP~iIi~Cl~hO-air mixtures are exposed to UV 1 o,`ci.on of `.rnvolrr f.h end in'.o icily oimilar to that of solar radiation u "r i'io onri.li' n c, (b) Vhr> pho Lochcmicn 1 reactivity of vinyl chloride, as measured from a number of rate parameters in the photo-oxidation reaction and also from it3 reactivity toward OH radicals, is similar to or alightly less than ethylene. Vinyl chloride is, therefore, only a moderately reactive precursor to photochemical smog, being loss reactive than propylene and higher olefins, but more reactive than the normal paraffins. (c) The rate constant for the reaction of OH with vinyl chloride has an UDner limit of t3.6 x cm^ molecule-^ at ^OO^. The true value io probably a factor of 2 - J lower than this.i i \ t / -9- UCC 095563 8. References 1. A.P, Altshullor mid J.J. lufallnl, 'Photochemical Aapects of Air Pollution* A Roviow', Environ. Sci. mid Tech. 39 - 64 (1971). 2 J.H, Heuos and V.A. Glascon, 'Hydrocarbon Reactivity and fjye Irritation1, Environ. Sci. and Tech. 2, 1109-1116 (1968). P.A. Cor, !:o bo published in J. Photochemistry. ' * -n, '.hr"! ! and h, .hih.l, ' Per c tin. by of Hydroxyl Radieala with Olefins', P. Phya. GU-tv. 75, 3640 - 3641 (1; 7 \). 5. O'.Iit. Nopc~ynski, unpublished. rmVta (lS68) reported in Ref. 1 p, 4B. 6. K.H. Hiloon, G.J. Doyle, D.A. Hansen and R.D. Englext, Symposium of ACS Division of Organic Coating and Plastic Chemistry, Hew York, Sept. 1969. See aleo Environ. Sci. and Tech. JJ, 896 (1969) and ibid p. 1224 together with Ref. 1 p, 48. 7* E.A. Schuck and G.J. Doyle, 'Photo-oxidation of Hydrocarbons in fixtures Containing 0xide3 of Nitrogen and Sulphur Dioxide', Report No. 29, Air Pollution Foundation, San Marino, Calif. (1959) see also Ref. 1 p, 56. 10 UCC 095564 TABLE I Rate parameters in photo-oridation of vinyl chloride and some hydrocarbons 1 : ; ror.=tn: A -- A (hi vf' ''i 0 ) ^ re ctant con after 4 hours C (d(b^dt)roax (ppm/min x 10^) D final [oj (ppm) Vinyl Chloride Ethylene Propylene Trans-2-butene 0.31 0.34 1.40 3.0 13 25 . 68 (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 (60 min) :'r hydrocarbon conoumed after 30 minutea / M'inK'ma re to not achl oved reaction timo ueod r tJmc at w'jch final onono r-nrcntr,.t.'!on jr.ensured. / - 11 UCC 095565 TAB!,?; II J?o!i n.t3 v7 V;' <' t ~ cr.i cnl ronctj.\ri ti G3^ ! | !' v n r< j f C2i;4 C3H6 0.25 TKn 1 ; (1 ` ` ' . 0.20 1.0 1.0 o; -,1 "-ity 0,29 1.0 Altnhullor & Bufalini^ A KO oxidation B reactant consumption 0.4 0.1 1.0 1.0 Morris. & NikiUl C OH reactivity 0.1 1.0 ^4V2 2,1 5.8 ^ 3.0 2 ^6 4.2 <- J CJJC3* 0.25 0.15 0.17 - - " - 0.5 .!------------------------ 1_______________I_____________ L i 0.45 - 'M' fiotivinro rolnti-r, prcpyleu-;. '.;1Joh is arbitrarily oat at unity. - 12 - UCC 095566 Concentration tiaio curvoo for tho photo-oxidation of vinyl chloride in the prcnonce of I.T0. A' A/ a. J a=X: ;oo IMS' 2.c>o 3 00 - 13 - UCC 095567 CONC'-MTR/^Tl 0M ( Pp (4) 7* *0/< * *J*j Or. one forc-.ation during the photo-oxidation of hydrocarbons and vinyl chiorj. (filled points). Fi# 4 l'2 The Gi'fect of aduea viny>. *uQi` on r..o tyi^uacjoidtion of mti'ouj acid. The plot shows the rates of formation of .N'O and h'C^ (%0 and and the total rate Rj.'O+h'Og* expresood per unit HN02 concentration as a function of the concentration ratio [CjjiljClj/lKNC^]. RAT*/ |>noz3 (Waid*) O.Z 0 -0.2. - . 16 - IJ CC 095570 UCC 095571