Indian Journal of ChemistryVol. 35A, February 1996, pp. 93-101

Dynamic instability in gas-liquid reactors-Oscillatoryaerial oxidation of propionaldehyde

R P Rastogi" & G P MisraFermentation. Technology Division, Central Drug Research Institute, Lucknow 226001, India

andIshwar Das & Kiran Jaiswal

Department of Chemistry, Gorakhpur University, Gorakhpur 273 001, India

Received 28 November 1994; revised 16 June 1995; accepted 18 July 1995

New experimental results on the oscillatory aerial oxidation of propionaldehyde in acetic acid/watermedium in the presence of cobalt(ll) acetate and sodium bromide under controlled conditions are re-ported, Experiments have been performed in semi-batch reactor and continuously flow stirred tankreactor and the results have been compared with the corresponding results for benzaldehyde. The mod-el of Guslander et aL as well as an eleven variable model based on amplified mechanism of Roelofs etaL suitable for semi-batch reactor have been used for simulating the oscillatory features of aerial oxida-tion of aliphatic aldehydes. The rate constants have been assigned keeping in view the wide differencein the rate of formation of C2HsCO', and the reactivity of C2HsCO', C2HsCOi and C2HsC03 as com-pared to corresponding radicals related with benzaldehyde. Both the models predict oscillations in batchreactor in several variables including [Br "], However, latter model predicts qualitatively correctly thedependence of onset time/time period on [NaBr], [Co2+] and [C2HsCHO] whereas the former modelonly correctly predicts qualitatively the dependence oftime period on [C2HsCHO].

Jensen and coworkers I reported oscillations duringaerial oxidation of benzaldehyde catalyzed by co-balt and bromide ions in acetic acid/water medi-um. Roelofs et aU subsequently proposed an ele-ven step mechanism which simulates oscillatorybehaviour, which was elaborated in a subsequentpublication", Rastogi and Das" observed oscill-ations in [Br -] and reported preliminary results incontinuous flow stirre'd tank reactor (CSTR).Oscillations are also observed when benzaldehydewas replaced by acetaldehyde', Oscillations in[Br-] have also been observed in aerial oxidationof cyclohexanone", although very recently a halog-en free system 7 based on O2 oxidation of cyclo-hexanone in acetic acid, catalyzed by cobalt ionhas been reported. However, there has been noreport of this kind with respect to O2 oxidation ofbenzaldehyde. Electron paramagnetic resonancestudies made by Roelofs and Jensen" show thatfree radicals are produced in the reaction and sig-nals suggest the possible formation of benzoyl rad-ical complexed to cobalt or benzaldehyde radicalcation. Yuan and Noyes? have proposed an altern-ative mechanism for the oscillatory catalyzed acri-

aI oxidation of benzaldehyde. Reimus et al." madedetailed studies on the oxidation of benzaldehydein a continuous gas-liquid stirred cell reactor. Asimpler three variable model proposed by them 10

predicts the behaviour quite well in agreementwith experimental results almost in the same man-ner as the more complicated model of Roelofs etal.: Some new experimental results on oscillatoryaerial oxidation of aromatic II and aliphatic' alde-hydes have been recently reported. However, Co-lussi et al:" feel that some of the observations arenot consistent with mechanism of Roelofs et al.'Quite recently, another skeleton model+ has beenproposed for benzaldehyde oxidation. .

The object of this paper is to examine whetheran analogous mechanism of aerial oxidation ofbenzaldehyde also holds good for the aerial oxida-tion of aliphatic aldehydes and if so, to what ext-ent. Although some results on aerial oxidation ofacetaldehyde have been reported earlier, the re-sults are not fairly detailed. Accordingly, we re-port in this paper experimental results for aerialoxidation of propionaldehyde for wide range ofconditions using a semi-batch reactor. For numeri-

94 INDIAN J CHEM. SEe. A, FEBRUARY 1996

>E

-'~zw~oe,)(

8w _

'" 50mVa

Acetaldeh de

-SOmV lOOmVb c

Propionaldehy<le Benzaldehyde

Br- POTENTIAL (mV)

Fig. I-Phase-plane plots for the following systems in a batchreactor

(a) Acetaldehyde(4.7xlO-'M) +

(20.0 x 10- ~M) + sodium bromideCobalt(lI) acetate (2.3xlO-zM);

temp.=Sl ± 1°C(b) Propionaldehyde (20.0 x lO-~M) + sodium bromide(7.3X IO-.IM) + Cobalt(lI) acetate ·(I.SX IO-~M);

temp.-76± 1°C(c) Benzaldehyde (80.0 x lO-zM) + sodium bromide(4.0xlO--'M) + Cobalt(lI) acetate (2.0XIO-2M);

temp. = SI ± °CP and B denote pink and brown stage of colours respectively.

cal analysis, Roelofs model appropriately modifiedfor a semi-batch reactor has been used.

Materials and MethodsCobalt acetate (Sarabhai Chemicals, LR), sodi-

um bromide (BDH, LR), propionaldehyde (E.Merck, Germany) and glacial acetic acid (SarabhaiChemicals,LR) were used as such without furtherpurification.

Monitoring of oscillations and phase-plane plotsExperiments were performed in semi-batch

reactor and continuous flow stirred tank reactor(CSTR) as described below:

(i) Redox potential oscillations were measuredwith a bright platinum electrode in conjunctionwith calomel electrode used as reference.

(i.) Bromide potential oscillations were mea-sured with a bright platinum electrode in conjunc-tion with calomel electrode as reference.

(iii) Phase-plane plots were obtained for ben-zaldehyde, acetaldehyde and propionaldehyde sys-tems in a batch reactor" for the sake of com pari-sion by recording redox potential and Br - poten-tial simultaneously with the help of a XY It recor-der. Results are recorded in Fig. 1.

TIME(s)

Fig. 2-Redox potential changes as a function of time for var-ying [NaBr) in semi-batch reactor. System: cobalt(lI) acetate(1.8 x 10- Z M) + propionaldehyde (19.8 x 10 - ~M) + sodiumbromide concentration equal to (a) 1.6 x lO-.IM. (bl(2.7x JO-JM), (c) 4.7x JO-JM, (d) 7.3x IO-JM, (e)13.0x IO-·'M, (f) 16.0X IO-.IM, (g) IS.OX JO-·'M. (h)20.0 x 10 - ..1M and (i) 22.5 x 10 - .'M. Temperature = 75 ± OSC,

Flow rate of air = lO ccl s

A: Semi-batch reactorSince oxygen is an important reactant in the ae-

rial oxidation, its continuous flow to the liquidmixture at a fixed rate is neceessary hence an alt-ernative to batch reactor (in which air first diffusesin from the top) was to be chosen, which we callas semi-batch reactor.

In the semi-batch reactor", 43;2 ml of cobaltacetate (0.05M) and 15.6 ml sodium bromide(0.1M) were taken in the reaction cell. All solu-tions were prepared in acetic acidlwater (90:10)medium. The solution was vigorously stirred withthe help of a magnetic stirrer and air was passedcontinuously into the cell. At 75°C, 1.7 ml prop-ionaldehyde was added to the reaction mixture.The reaction mixture turns brown on addingpropionaldehyde and oscillates between brownand pink colours. Experiments were performed atdifferent concentrations and at different tempera-tures. Results are shown in Figs 2-5.

B: Continuous flow stirred tank reactor (CSTR)The design of a continuous flow stirred tank

reactor (CSTR) used in the present investigation isshown in Fig. 6. It was a modified version of semi-

RASTOGI et al: OSCILLATORY OXIDATION OF PROPIONA LDEHYDE

I')()mv 2~~I')()mY ~ (01

(0)

(II)

:; (e)e !-' (d) -'<% <%~ ;::z zw WI- •...0 ~no

'" (d)0 ••0 0!oJ C0: W

0:(.)

TIME ( s )Fig. 3-Redox potential changes as a function of time for var-ying [cobalt(lI) acetate] in semi-batch reactor. System: propion-aldehyde (19.S x 10 2M) + sodium bromide (13.0 x 10- J M)+ cobaltrll) acetate concentration equal to (a) 12.5 x IO-JM,(b) (14.1 x IO-JM), (c) 15.4x IO-JM, (d) IS.Ox IO-JM, (e)22.1 x IO'M, (f) 26.3 x 10 ·'M, (g) 34.6 x 1O-3M and (h)41. 7 x 10' IH. Temperature = 75 ± 0.5°C, Flow rate of

air= 10 cc/s

l')()mv

>E

)(

ocwa:

TIME ( s )Fig. 4-Redox potential changes as a function of time for var-ying [propionaldehyde] in semi-batch reactor. System; co-balt(II) acetate (18.0 x 10 - J M) + sodium bromide(13.0 X \0 -.1M) + propionaldehyde concentration equal to (a)5.8x lO-zM, (b) ut.s x lO-zM), (c) 19.8x W-zM, (d)23.2 x IO-'M, (e) 29.0x IO'M and (f) 32.4 x 1O-'M. Temper-

ature = 75 ± O.5°c, Flow rate of air = 10 cc/s

95

TIME (5)

Fig. 5-Redox potential changes as a function of time for var-ying temperatures in semi-batch reactor. System: cobalt(lI)acetate (IS.OX 10 3M) + sodium bromide (13.0 x 1O-3M) +propionaldehyde (19.8 x 10 - 'M). Temperature (a) 60.0°, (b)65.00, (c) 72.0°, (d) 75.0°, (e) 77.0°, (f) so.o- and (g) 83.0°C.

Flow rate of air = 10 cel s

batch reactor used earlier', It consists of a corningglass reaction cell fitted with a platinum electrode(P), a calomel, electrode (Cj) and thermometer(T,). Since the reaction is to be perfortned at hightemperatures, a condenser (e2) has been attachedwith the reactor in order to promote condensationof the reactants thereby preventing any change inreactant concentration. An atmosphere of air in-side the reaction cell was maintained by the influxof air in the reaction cell through a nozzle (0) us-ing a compressor (e}). The compressor was con-nected to the dimerstat (D,) to control the flow ofair. The outlet of the nozzle (0) was not dippedinto the solution but kept above the surface of thesolution. The solution was continuously fed to thereactor from the reservoir (G) at a desired flowrate and maintained with the help of stopper (S}).The reservoir contained sodium bromide, cobaltacetate and propionaldehyde solutions mixed in anappropriate ratio and kept at room temperature(20°C). At this temperature oscillations in redoxpotential were not observed, The solution wasstirred with the help of a magnetic stirrer. Thereaction cell was put in a water thermostat main-tained at 75.5 ± OSc. The flow rate of air wasseparately determined by water displacementmethod.

96 INDIAN J CHEM. SEe. A. FEBRUARY 199fl

Fig. 6-Experimental set-up of continuous flow stirred tankreactor (CSTR). R = recorder; T I and T 2 = thermometer;P=platinum; CI = calomel electrode; C2=condenser; O=noz-zle, S I = reaction mixture; W= water; H = immersion rod;B = bulb; S1 = simerstat; D h D 2 and D J = dimerstat;C,=compressor; MI, M2 and MJ = mains; M=magnetic stir-

. rer; Q = mechanical stirrer; J = water bath; L= glass lid; G = res-ervoir; SJ = stopper; F = suction pump; Z I' Z 2 = tube

In CSTR experiments, the reactants sodiumbromide, cobalt acetate and propionaldehyde were.taken in the reservoir G at room temperature andcontinuously fed into the reactor at a definite flowrate through polyethylene tubings. Volume of thereaction was kept constant by a continuous out-flow of solution using a suction pump (F). Initiallya lowering of temperature ( - 1°C) was noticed dueto influx of solution from the reservoir but after afew minutes, temperature attained a constant va-lue. Principle utilized is that at lower temperatureno reaction would occur. Visual oscillations in co-lour (pink •..• brown) were also observed in thiscase.

Experiments at various ko values have been car-ried out and results are recorded in Fig. 7. Fre-quency of oscillation increased with increase inflow rate. With very limited data no inference re-garding amplitude can be made.

Results and DiscussionRedox potential changes as a function of time

during the aerial oxidation of propionaldehyde inthe presenc~ of cobalt( II) and bromide ions inacetic acid medium have been monitored and typi-cal traces obtained at various experimental condi-tions are shown in Figs 2-5. Limit cycles are indi-cated by Fig. 1. Supercritical bifurcation may oc-

1100 mV

>E

TIME (I)

Fig. 7-Redox potential changes as a function of time for dif-ferent k" in CSTR. System: cobaJt(Il) acetate (18.0 x 10'M)+ sodium bromide ( 10.0 x 10- J M) + - prop ionaldehyde(llJ.llxlO'M). k., equal to (a) 1.66XIO-'min-l, (b):U/)xlo-'min-1 and (c) 3.5xlO-2min-l• Tempera-

ture = 75.5 ± OSC and flow rate of air = 10 ccl s

cur in the case of (f) and (g) in Fig. 2 whereas inthe upper part of Figures, saddle loop bifurcationmay occur where periods become very long. Be-low lower limit no oscillations are observed. Os-cillations were only observed between1.6 x lO-1M and 22.5 x lO-3M concentration ofBr- in a semi-batch reactor (Fig. 2). Figure 3 .:shows the variation of redox potential with time at -various [Co(II)] in a semi-batch reactor. Lower li-mit of [Co(II)] below which no oscillations are ob-served was 12.5 x lO-3M. In a semi-batch reactorexperiment, propionaldehyde concentration andtemperature were also varied. Propionaldehydeconcentration was varied in the range 58-324 x lO-3M. An upper limit above which no os-cillations are observed was found to be324 x lO-1M (Fig. 4). Temperature was varied inthe range MHO ± O.5°C. Time period decreaseswith increase in temperature (Fig. 5). Results .onthe influence of temperature can be comparedwith the batch reactor results of Jensen I on ben-zaldehyde oxidation, who has also observed dra-matic increase in oscillation frequency as the tem-perature is increased i.e. from 0.2 cycles/min at

"

-1.89

-1.90

RASTOGI et al.: OSCILLATORY OXIDATION OF PROPIONALDEHYDE 97

(b)

(c.)

-8.00

-12.00

(~)-1.91-2.00

150 450 750TIME (9)

1050

-16.00

-5.00

-10.00-7.50

-12.50-3.50

(F>

-4.50

-7.00

-3.00

-6.00':'6.500.00

-0.80

-1.00-8.00

-5.00•......•--N2-t7' -10.00~ -1.75

---+ -2.001.§' -2.25

-12.00

-16.00 150 450 75u 1050

TIME (9)

Fig. /l-Plot of computed values of (a) log [Br "], (b) log[(CoJ+),Br-), (c) log[(CoHh). (d) log[CzH;CHO). (e) log[C ,H~CO,I. (f)log[C2H;C02), (g) log[C,H;CO"J, (h) log[C,H;CO)H). (i) log[Co3+) (j) log[O,(1)) and (k) log[C02+] versus time. Initial

[CHsCHO] = 19.8 x 10-2 M, [Br "]> 13,0 x 10-3 Mand [Co2+]= 18.0 x 10-3 M

55°C to 3.2 cycles at 100°e. Results plotted inFig. 5 show that whereas at 60·C, the frequency inthe beginning of oscillations is found to be 4.6 cy-cles/hour while at 100°C, the same is of the orderof 78 cycles/hour. Although the concentrations intwo cases are not similar, in general it is foundthat the frequency in the case of propionaldehydeis less. The amplitude is more' or less the sameand it does not change significantly with the in-crease in temperature.

Experimental results obtained in semi-batchreactor show that onset time and time period

(a) decrease with increase in NaBr concentra-tion (Fig. 2).

(b) increase with increase in Co(II) concentra-tion. At low Co(U) concentration 12.5-15.4 x 10 -.1

number of oscillations and 'lifetime were found tobe considerably low but increase with increase inCo(II) concentration (Fig. 3).

(c) decrease with increase in propionaldehydeconcentration (Fig. 4). Above 23.2 x 10- 2 M, am-plitude and lifetime also decrease.

(d) decrease with increase in temperature. Thenumber and amplitude of oscillations were foundto be much smaller at 83.0 ± OSC (Fig. 5).

Experiments were also carried out in CSTR(Fig. 6) for various ko values where klJ is the in-verse of residence time, flow rate/ reactor volume.Results are recorded in Fig. 7. Oscillations can heobserved visually when the reactants are pumpedin the CSTR at a definite flow rate. On stabilisa-tion of oscillations. it was observed that frequency

INDIAN J CHEM. SEe. A. FEBRUARY 1990

Table l+-Values of rate constants used for computation

Rate Rate constants used Rate Rate constants usedconstant for reaction with constant for reaction with

propionaldehyde propionaldehyde12.35xlO.1lH-'s-' i; IxlO"M-'s-'

1.0xlO.1s-1 k, 8XlOhM-'s-12.0M-'s··' kx IxI07M-'s-'

I x IO"'M-'s" k. I X IO·M-~s-'2xlOhM-'s-' klO IxlO"M-'s-'2xlO"'M-'s-1 k_1I lxlO-~s-1

For rate constants k2 to kK, 100 times larger values were takenthan the values mentioned in reg. 3 whereas for k. - k" similar

values were used.

of oscillations increases with increase in flow rate.With very limited data, no inference regarding am-plitude can be made.

Computer simulationAttempt has been made to simulate the above

experimental observations numerically. For thispurpose, we considered the following reactionscheme for the oxidation of propionaldehydebased on the mechanism of Roelofs et al.' and al-so adopted by Reimus et al'" Aerial oxidation ofaldehydes is a free radical chain reaction. In thecase of propionaldehyde, CiHsCO' is the most im-portant radical species corresponding to theC"HsCO' postulated in the aerial oxidation ofbenzaldehyde.

k(C03+),+Br- <=1 (Co-'+hBr- ... (R1)- k_1

RCO' + Br ' + C03+

+ C02+ + H+ (R2)

... (R3)RCO' + O2(1) ~1 RCOl

R<::O'+ (C03 + }z +H20 .~ I RCOi + 2C02+ + 2H+... (R4)

." (R5)RCO' + RCO) ~~ 2RCOi

RCO} + RCHO ~6 RC03H + RCO'

RCO) +C02+ + H+~? RC03H +C03+

... (R6)

." (R7)

RCOi + RCHO ~K RC02H + RCO' ... (RS)

RC03H + 2C02 + + 2H + ~ RC02H + (C03 +h

+H~O (R9)

• E"~rim<!ntQt• Comput.d

G.OOO'IQOO10.01 O~ 1

(No BrJ 1M

~1000

~.r

100 'i';• 10 Cb) \,Jc510.01 0.1

[C2HSCHO] 1M

10000

~ 1000

~.1100-

(e) , -»·.JIr-i0 10

•10.0001 0.001 0.01 0.1

[Co~/M

10000~

11000 1

o !10

Fig. 9-(a) Plot of experimental and computed onset time(solid lines) and time period (broken lines) versus [NaBr]. Sys-tem: C2H;CHO (II.J.!\X x IO-'M) + CO'+ (IS.Ox 1O-.1M) +

NaBr

(b) Plot of experimental and computed onset time (solid lines)and time period (broken lines) versus [C,H;CHO]. System:

CzHsCHO + Coz+ (18.0XIO-JM) + NaBr(13.0xlO-JM)(c) Plot of experimental and computed onset time (solid lines)and time period (broken lines) versus [CotAcl.] System:CzH;CHO (19.8x 1O-2M) + Co2+ + NaBr (13.0x IO-JM).

Temperature = 75 .5°C

... (RIO)

02(g)!11 O2(1)k_tt

where R corresponds to C2Hs, CsHs·Considering Br - , (C03 +h, (C03 + }zBr- ,

C2HsCHO, C2HsCO', C2HsCO), C2HsCOi,C2HsC03H, C03 +, and O2 (1) as dynamic var-iables, Eqs (D1-D11) for the oxidation of propion-aldehyde are obtained on the basis of above me-chanism.

... (Rll)

RASTOGI et al: OSCILLATORY OXIDATION OF PROPIONALDEHYDE 99

8.00 1600.00

1400.001.00

'"1200.00

<,4.00 "0-

0.t;X.

1200·00

.~...2.00

800·00

0.00 +--....--.---.----.-----.-.0.00 1.00 1.20 1.40 1.60 1.80 2.00

m

Fig. IO-Phase diagram between g and m showing the oscilla-tory regime (shaded area)

The nature and structure of complex ions ofCo(Il) and Co(III) in the presence of aldehydesand NaBr is difficult to ascertain" since manytypes of complexes can be formed involving[Co(IIIh] complex units.

Since Br - is involved in the complex formed inthe present reaction'? as a first guess, one maypostulate the participation of Co(III) as suggestedin reaction (Rl) where Co(III)-Co(Il) bond lengthmay be linked to H20 molecules and/or Br " andRO' by coordinate linkage I3.14. In order to avoidthis complication, a simplified model has beensuggested by Guslander et a/.12 which will be dis-cussed later.

The appropriate. differential equations are asfollows.

d[B -]_r_= _ k, [(C03+hl[Br-]+ k-[(CoH)2Br-]

dt + k2[(CoH)2Br-][C2HsCHO] ... (Dl)

d[C 3+)]~ 2 = _ k)[(CoHh][Br-]+ L1[(CoHhBr-]

t _ k4[H20][C2HsCO'X(COH)2]+ k9[H "IC2HsC03H][ Co2+]+ klO[C03+F

d[(Co3+hBr-]dt

... (D2)

kl[(C03+MBr"']- k_I[(C03+hBr-]- k2[(C03+ hBr-nC2HsCHO]

... (D3)

- k2[(C03+bBr-][CzHsCHO]- k(,[CzHsC03][CzHsCHO]- kg[CzHsCO'][CzHsCHO) ... (D4)

600.00 +----.---.-----,---.0.00 200.00 400.00 600.00 800.00

[Br- ] • 104 / tit

Fig. II-Dependence of time-period on [NaBr),g = 3; m = 1.1

d[CzHsCO') = k2[(COH hBr-)[C2HsCHOI

dt - k3[C2HsCO'][02(1)]- k4[HzO)[CzHsCO'][(C03+ hI- ks[CzHsCO'][CzHsCO:'1+ k6[CzHsCOj][CzHsCHO]+ k8[CzHsCOi][CzHsCHO] ". (D5)

d[C2HsCO~] ][ ()]k3[C2HsCOj 021dt - ks[C2HsCO'][C2HsCOj]

- k(,[CzHsCOj][CzHsCHO]- k7[H+][C2HsCQj][Co2+] ' .. (D6)

d[C2:SCO~] = k4[H20][CzHsCO'j[(C03+ )21t + 2k,[CzH,CO'][C2HsCOJ]

- kX[C2HsCOi][CzHsCHO] ". (D7)

d[CzHsCO)H] = k6[C2HsCOj][C2HsCHO]dt + k7[H+][C2HsCOj][C02+]

- k9[H +][C2HsC03H][ C02 +J2... (D8)

d[C02+ 1= k,[(CoJ+ hBr-][C2HsCHO]

de .:. 2k4[H20][C2HsCO'][(Co3+ h]- k7lH"IC2HsCOj][C02+]- Zk9[H+][C2HsC03H][Co2+]2 '" (DIO)

d[02(1)] _ k)[C2HsCO,][02(1)]

dt + L II ([02 (1)]*- [O, (I)]) ... (Dll)

100 INDIAN J CHEM. SEe. A. FEBRUARY 1996

1120.00

1315·00

~ 1)10.oor----------- _~ -o.;::!~ .._ 1305.00~

1300.00 ~ I I Iii

0.00 200.00 400.00 600.00 800.00 1000.00

Co (tI)1 Co (m) x 104 1M

Fig. 12-Dependence of time-period on Co(lII) concentration.g=J; m= 1.1

where [02(1))* is the solubility constant of oxygenin acetic acid/water medium.

Numerical solution of the above set of differen-tial equations (DI-Dll) was obtained with HCLPC/ AT computer and using subroutine'S STIFF3.Difficulty arises in assigning the values of rateconstants for various steps of the above reactionscheme. It should be noted that when rate con-stants due to Roelofs et al.2 for the benzaldehydesystem is used as such for the propionaldehydesystem, oscillations are not obtained. It seems thatrate constants for the propionaldehyde systemhave to be changed. In this context, we have to as-sess the rate constants associated with the

(1) formation of CZH5CO' and

(2) reactivity of C2H5CO', C2HsCOi andC2HsCOj.

Step R2 produces C2H5CO·. In this reaction,propionaldehyde which contains an electron do-nating aliphatic group, reacts with an electron defi-cient species (<;;03 +hBr ". Hence the formation ofLzHsCO' would be facilitated and rate constantwould be higher as compared to the case wherereaction involves benzaldehyde which contains anelectron withdrawing phenyl group.

Further, due to electron donating nature of ali-phatic groups and electron withdrawing nature ofphenyl groups, the aromatic free radicals will bemore stable than the corresponding aliphatic freeradicals. Hence the rate constant for reaction steps

3000.00

ZSOO.OO

2000.00

~ 1'iOO.OOJI 1000.00;:

'iOO.oo

0.00 I , i I Ii'0.00 0.20 0.40 0.60 0.80 1.00 '1.20

[C2 H5 CHO] / M

Fig. 13-Dependence of time-period on [C1H5CHOj. g = 3;m= 1.1

where aliphatic free radicals are involved wouldbe greater than the corresponding rate constant ofaromatic free radicals. However, if it is the changeof one radical to another, the rates may be similar.Keeping in view these arguments, computationswere made by assigning the rate constants k2 - k8a value 100 times the magnitude assigned byRoelofs et al.'. Values of rate constants are givenin Table 1.

Computed results for time variation of [Br "],[(Co3+hJ, [(C03+hBr-), [C2H5CHO), [C2HsCO'),[C2H5C03), [C2HsCOi),' [C2HsC03H), [Co3+),[Co2+) and [02(1)) are plotted in Fig. 8 which giveevidence of oscillatory behaviour. It should benoted that oscillations in [Br -) in a batch reactorare also predicted from numerical simulation.

Computations were made by increasing themagnitude of k2 - kg by 30% and 50%, but no os-cillations were produced. However, with five timesincrease, oscillations were obtained after an onsettime of 2 hours. Calculations showed that if k2alone was increased 100 times, oscillations aregenerated. When k2 was increased 100 times;. k6ten times, k; five times and ks ten times, oscill-ations were still obtained. However, on eliminationof steps R4, R5 and R8 in this case, no oscill-ations were produced.

Computations have also been made for varyingconcentrations of NaBr, Co2 + and C2HsCHO.The computer results have been compared withexperimental data in Fig. 9. Although quantitativeagreement is poor the trend is in qualitative agree-ment.

RASTOGI et al: OSCILLATORY OXIDATION OF PROPIONALDEHYDE 101

We also 'tried computer simulation based on themechanism proposed by Guslander, Noyes andColussi 12 for the pool reactor condition which ishighly attractive on account of simplicity. Themodel is as follows,

02(g)F02 (soln.)

O2 (soln.) +R· ...•g Co(ll)

Co(ll) ...•R

Co(lII) +R· ...•m R

.,. (C1)

, .. (C2)

... (C3)

, .. (C4)

R represents the free radical from the aldehydewhereas· g and m represent the stoichiometric co-efficients. Using the values of k. = 10-4, L.= 1.0,k2 = 109, k3 =0.01 and k4 = 103, the coefficients gand m were varied. The oscillations are obtainedin the shaded area of Fig. 10. The values of g andm are reasonable but no onset time is predictedwhich is contraray to experimental observation.

For testing the model further, dependence oftime period on [NaBr], [Co(ll)] and [C2HsCHO]was investigated. The results are recorded inFigs 11-13. Here again only the dependence on[Co(III)] is correctly predicted.

AcknowledgementThanks are due to the UGC and the CSIR

New Delhi for supporting the investigation:Thanks are also due to Professor S. Giri, Head,

Chemistry Department, Gorakbpur Universitywhere the experimental part was done.

ReferencesI Jensen J H, JAm chem Soc, 105 (1983) 2639.2 Roelofs M G, Wasserman E, Jensen J H & Nader A E, J

Am chem Soc, 105 (1983) 6329.3 Roelofs M G, Wasserman E & Jensen J H, J Am chem

Soc,109(1987)4207.4 Rastogi R P & Ishwar Das, Indian J Chem, 23A (1984)

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