Indian Journal of ChemistryVoI.18A, November 1979, pp. 385-387

Studies on the Oxidation of Propylene on AMo04Molybdates of Mn (II), Co (II), Ni (II) Cu (II)

Type& Zn (II)

B. VISWANATHAN*, C. V. -BHUVANA & M, V. C. SASTRItDepartment of Chemistry, Indian Institute of Technology, Madras 600036

Received 27 January 1979; revised 9 April 1979; accepted 1 May 1979

Catalytic oxidation of propylene on AMoO, type molybdates [A = Mn(ll), Co(ll), Cu(ll), Ni(ll) or Zn(ll)]is first order with respect to propylene and zero order with respect to oxygen. The rate constants for acrolein orCO. formation have been correlated with Mo-O bond order and the relationships obtained have been explained onthe basis of the requirement of an optimum bond order for acrolein formation.

DIFFERENT concepts have been developedregarding the factors that control the activityand selectivity of a catalyst for the oxidation

of hydrocarbons 1. Attempts have been made to relateselectivity with both microscopic" and macroscopicparameters" like intrinsic acidic/basic nature of thesites, the position of the Fermi level, the width of theforbidden band, the position of IR absorption banddue to Mo-O double bond-, the extent of reducibi-lity", or the heat of formation per oxygen atomequivalent". These attempts have not yet provideda unified framework on the basis of which either theselectivity variations among the various catalystscan be accounted for or a catalyst selection could bemade. The present study, attempts to formulate aworking hypothesis for the rationalisation of oxidationactivity and selectivity of AMo04 type molybdateswhere A = MnH, COH, Ni2+, Cu2+ and Zn2+ onthe basis of correlations between rate constants andobserved Mo-O bond orders in these catalysts.AMo04 type molybdates with known crystal struc-tures have been chosen for the present study in orderto define more exactly the parameters chosen forcorrelation with activity/selectivity as well as to cal-culate the M-O bond orders from the known bondlengths.

Materials and MethodsThe molybdates of COH, Ni2+, Cu2+ and Zn2+

were prepared by the co-precipitation method bymixing either metal chloride (C02+, NiH) or metalnitrate (Cu2+, Zn2+) and sodium molybdate (C02+,Ni2+) or ammonium molybdate (Cu2+, Zn2+) stoi-chiometrically. The precipitate was dried at 1000

and calcined at 5000 for 15 hr. MnMo04 was ob-tained by solid state reaction between the componentoxides at 5800 (ref. 7). The kinetic studies werecarried out using a differential microreactor coupledwith an on-line gas chromatographic unit. Thereactants (CaH6 and oxygen) and the diluent nitrogen,purified by standard procedures and thoroughly

tMaterials Science Research Centre. Indian Institute ofTechnology. Madras 600036.

(

mixed, were fed at required flow rates into the reactor.The effluent unreacted propylene and the productsof oxidation were analysed by on-line gas chromo-tography.

Results and Discussion

Kinetics of the oxidation reaction - The order ofth reaction and the orders with respect to individualreactants namely polpylene and oxygen were evalua-ted by carrying out a series of experiments keepingthe total flowrate constant (100 ml/min) and varyingeither the flow rate of oxygen or of propylene asthe case may be. Typical plots of the percentageconversion (the amounts of acrolein and CO2 formed)versus partial pressure of propylene or oxygen oncobalt molybdate are given in Fig. 1. The plots of-log(1-x) vs reciprocal of space velocity (where x isthe degree of conversion) or log (rate) vs log(partialpressure of propylene) were found to be linear indi-cating that the reaction is first order with respectto propylene. The percentage conversion is inde-pendent of the partial pressure of oxygen on allmolybdates studied thus showing that the reaction iszero order with respect to oxygen. The effect oftemperature on the overall reaction, as well as onacrolein or CO2 formation has been studied and thevalues of Arrhenius parameters (activation energy,E and frequency factor, ko) together with the tempera-ture range studied for each of the molybdates aregiven in Table 1. These kinetic parameters have beenevaluated under the conditions where the influenceof the products of the reaction is negligible (i.e., frominitial rate data) as the products of the reactionespecially water and acrolein are known=" to affectthe reaction rate.

Since the reaction is first order with respect topropylene, the effect of the variation of flow rate ofpropylene on product distribution was studied andtypical results in the case of CoMoO 4 are given inTable 2. The following trends were noticed fromthe results obtained :

(i) At any particular temperature the selectivitywith respect to acrolein increases slightly

385

INDIAN J. CHEM., VOL. 18A, NOVEMBER 1979

12

.§VI

t 8~u..'"oc:•.u; 4...

°O~----~O~.1------~OL~------~~L3------~~L4------~0.5Partial pressure

Fig. 1 - Plot of percentage conversion vs partial pressure ofreactants in the catalytic oxidation of propylene on cobaltmolybdate (I, percentage conversion with respect to partialpressure of propylene; 2, percentage conversion with respectto partial pressure of oxygen; 3 & 5, percentage of CO2 oracrolein as a function of partial pressure of oxygen;4 & 6, percentage conversion to CO. or acrolein as a functionof partial pressure of propylene at total flow = 100 ml/min;

propylene flow = 40 ml/min).

TABLE I - KINETIC PARAMETERSFOR THE OXIDATIONOF PRo-PYLENE ON AMo04 TYPE MOLYBDATES

Catalyst MnMoO. CoMoO. NiMoO( CuMoO. ZnMo04

Temp. range COc) 400-440 280-340 310-360 300-350 360-400

Activation energy 61.3* 51.2* 69.3* 47.9* 55.4*(kJ/mole) 53.8t 37.4t 6l.3t 47.9t 44.9t

st.z; 45.8t 47.9t 19.3t 50.8t

Frequency factor 32* 58* 2100* 20* II*(ko) X 10-5 5.0t 0.82t 1l0t 9.5t 8.7tt(moles min-1 m-I)! I.5t 1St 22t 0.(\26t 1.6t

*For total reaction; +for acrolein formation; tor CO.formation

TABLE2 - PRODUCTDISTRIBUTIONWITH RESPECTTO CONTACTTIME AND TEMPERATUREFOR THE OXIDATIONOF PROPYLENEON

CoMoO.

Feed composition(CaH. : O. : N.)

10 : 20 : 20

Temp. % % %(OC) Conversion Acrolein CO.

280 11.8 2.7 8.9300 13.3 3.1 10.1320 15.9 3.5 11.9330 17.5 3.9 13.4340 19.0 4.2 14.6

280 10.1 2.2 7.6300 11.6 2.6 8.8320 13.0 2.8 9.8330 15.7 3.4 12.1340 17.1 3.6 13.2

280 4.8 1.0 4.8300 6.2 1.3 4.8320 8.2 1.7 6.5330 11.2 2.4 8.7340 12.0 24 9.4

15 : 30 : 30

25 : 50 : 50

386

(

with contact time on most of the molybdatesstudied.

(ii) At any given flow rate the selectivity withrespect to acrolein remains almostconstant with increase in temperature .

(iii) The activity order on the basis of the valuesof rate constants at 3300 is : CoMo04 >NiMo04>CuMo04>ZnMo04>MnMo04•

(iv) The selectivity order with respect to acroleinis : MnMo04 > ZnMo04 > CUM004 >-NiMoO, > CoMo04 •

Physico-chemical correlations- The plots of activa-tion energy vs logarithm of the frequency factor(Fig. 2) were found to be linear denoting the opera-tion of compensation effect. The observance ofcompensation effect shows that though the nature ofsites involved on these catalysts may be similar, theirintrinsic activities are different and probably a spect-rum of sites with different activities are present oneach of these catalysts. The difference between anytwo catalysts depends upon the relative densities ofdifferent types of sites. In order to elucidate thenature of these sites the Mo-O bond orders in thevarious molybdates were calculated-? and correlatedwith the individual activities (Fig. 3). It was foundthat the activity with respect to CO2 fomation de-creases with increase in bond order c- 1), while theactivity with respect to acrolein formation shows amaximum when Mo-O bond order is around 1.4, indi-cating that the partial oxidation reaction requires anoptimum bond order. To account for these observa-tions the following hypothesis is proposed. The olefinmolecule activated on a cation site can undergo totaloxidation reaction when the bond order is low ( ,.....1)since the oxidising power of the site is high. Withincrease in bond order the oxidising power of the siteis decreased and therefore the activated hydrocarbon

6log ko

7 8(2) 5I i I

(3) 4 5 6(3) (2)8055 70

7045 60,

"g,0<

,;35 50~•.

c•..~a>

25 .40 ~

'"(5E,"'.60,.,co

~..c.2

~50u«

40L -:- -;;.- ~...:......-'1530

(1)6 8' 9log k.,(1)

Fig. 2 - Plot of activation energy vs logarithm of frequencyfactor (I, total reaction; 2, acrolein formation; 3, COI

formation).

\

c0

B~N

8,g

N

IE

Ie:'EIt)

~ 160E.S.u0MC'1

-;;80

C0Iiic0u.."a:

°10

VISWANATHAN et al. OXIDATION OF PROPYLENE ON MO.LYBDATES

oxidation reaction can take place by both consecutiveas well as parallel reaction routes. Since most of themolybdates used as catalysts contain more than oneMo-O bond lengths, (for example minimum four inthe case of AMo04 type molybdates) it is possiblethat these systems can simultaneously promote bothpartial and complete oxidation reactions and theextent of selectivity depends upon the density of thesites with appropriate bond order. Further exten-sive studies are required to establish the applica-bility of this hypothesis for other oxidation catalystsespecially other molybdates.

.------------------------.00

20

Fig. 3 - Correlation between Mo-O bond order and oxidationactivity.

o

~--~--~--~--~--~~~OU 1A U

Mo-O bond order

C40~

e:oo..-;;a:

molecule is provided with a limited supply of oxygenand this occurs in a controlled fahshion on a catalystwith optimum bond order so as to preferentiallypromote partial oxidation reaction. On catalystswith bond orders less than the optimum value, total

,(

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