Indian Journal of Chemistry VoI.18A, November 1979, pp. 385-387 Studies on the Oxidation of Propylene on AMo0 4 Molybdates of Mn (II), Co (II), Ni (II) Cu (II) Type & Zn (II) B. VISWANATHAN*, C. V. -BHUVANA & M, V. C. SASTRIt Department 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 or CO. formation have been correlated with Mo-O bond order and the relationships obtained have been explained on the basis of the requirement of an optimum bond order for acrolein formation. D IFFERENT concepts have been developed regarding the factors that control the activity and selectivity of a catalyst for the oxidation of hydrocarbons 1. Attempts have been made to relate selectivity with both microscopic" and macroscopic parameters" like intrinsic acidic/basic nature of the sites, the position of the Fermi level, the width of the forbidden band, the position of IR absorption band due to Mo-O double bond-, the extent of reducibi- lity", or the heat of formation per oxygen atom equivalent". These attempts have not yet provided a unified framework on the basis of which either the selectivity variations among the various catalysts can be accounted for or a catalyst selection could be made. The present study, attempts to formulate a working hypothesis for the rationalisation of oxidation activity and selectivity of AMo0 4 type molybdates where A = Mn H , COH, Ni2+, Cu 2 + and Zn 2 + on the basis of correlations between rate constants and observed Mo-O bond orders in these catalysts. AMo0 4 type molybdates with known crystal struc- tures have been chosen for the present study in order to define more exactly the parameters chosen for correlation with activity/selectivity as well as to cal- culate the M-O bond orders from the known bond lengths. Materials and Methods The molybdates of COH, Ni 2 +, Cu2+ and Zn 2 + were prepared by the co-precipitation method by mixing either metal chloride (C02+, NiH) or metal nitrate (Cu 2 +, Zn 2 +) and sodium molybdate (C02+, Ni 2 +) or ammonium molybdate (Cu 2 +, Zn 2 +) stoi- chiometrically. The precipitate was dried at 100 0 and calcined at 500 0 for 15 hr. MnMo04 was ob- tained by solid state reaction between the component oxides at 580 0 (ref. 7). The kinetic studies were carried out using a differential microreactor coupled with an on-line gas chromatographic unit. The reactants (CaH6 and oxygen) and the diluent nitrogen, purified by standard procedures and thoroughly tMaterials Science Research Centre. Indian Institute of Technology. Madras 600036. mixed, were fed at required flow rates into the reactor. The effluent unreacted propylene and the products of oxidation were analysed by on-line gas chromo- tography. Results and Discussion Kinetics of the oxidation reaction - The order of th reaction and the orders with respect to individual reactants namely polpylene and oxygen were evalua- ted by carrying out a series of experiments keeping the total flow rate constant (100 ml/min) and varying either the flow rate of oxygen or of propylene as the case may be. Typical plots of the percentage conversion (the amounts of acrolein and CO 2 formed) versus partial pressure of propylene or oxygen on cobalt molybdate are given in Fig. 1. The plots of -log(1-x) vs reciprocal of space velocity (where x is the degree of conversion) or log (rate) vs log(partial pressure of propylene) were found to be linear indi- cating that the reaction is first order with respect to propylene. The percentage conversion is inde- pendent of the partial pressure of oxygen on all molybdates studied thus showing that the reaction is zero order with respect to oxygen. The effect of temperature on the overall reaction, as well as on acrolein or CO 2 formation has been studied and the values of Arrhenius parameters (activation energy, E and frequency factor, k o ) together with the tempera- ture range studied for each of the molybdates are given in Table 1. These kinetic parameters have been evaluated under the conditions where the influence of the products of the reaction is negligible (i.e., from initial rate data) as the products of the reaction especially water and acrolein are known=" to affect the reaction rate. Since the reaction is first order with respect to propylene, the effect of the variation of flow rate of propylene on product distribution was studied and typical results in the case of CoMoO 4 are given in Table 2. The following trends were noticed from the results obtained : (i) At any particular temperature the selectivity with respect to acrolein increases slightly 385
Transcript
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
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
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
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Fig. 2 - Plot of activation energy vs logarithm of frequencyfactor (I, total reaction; 2, acrolein formation; 3, COI
formation).
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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
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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
,(
References1. HIGGENS, R. & HAYDEN, P., Specialist periodical reports,
Catalysis. 1 (1977), 188.2. AI, M. & SUZUKI, S., J. Catalysis. 30 (1973), 362; AI, M.,
J. Catalysis. 40 (1975), 327.3. MORRISON, S. R., J. Catalysis, 34 (1974), 462 .4. TRIFIRO, F. & PASQUON, I., J. Catalysis, 12 (1968), 412.5. SACHTLER, W. M. H. & DE BoER, N. H., Proc. 3rd Inter.
Congr. Catalysis. 1(1965), 252.6. MORO-OKA, Y., MORIKAWA, Y. & OZAKI, A., J. Catalysis,
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