Acidic properties and the catalytic activity of 12-molybdophosphates
(M"nn+H3-,PM012040; M = Cu2+, BP+, Cr3+ and X = 1-3) have been studied. The results are discussed as the effect of these catalyst components on the partial charge on oxygen atom which is in a relation with the acidity. It is shown that the oxygen-hydrogen bond is weakened as the value of partial charge on oxygen (-bo) becomes more negative, while the vapor-phase dehydration activity of 2 propanol is explained on the basis of the reacting zone wideness taking into account the pseudo liquid phase nature of the heteropoly compounds. Correlation, the percentage conversion of 2-propanol with the fractional charge on molybdenum atom was successful to interpret the effect of the redox properties of these catalysts on their catalytic activity

A. M. El-Awad

K. M. Abd El-Salaam

English

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CROATICA CHEMICA ACTA CCACAA 60 (4) 677-686 (1987)CCA-1764YU ISSN 0011-1643UDC 546.77Original Scientific PaperAction of Salt Formation on the Bifunctional Nature of12-Molybdophosphoric Acid and Their Relationship to theCatalytic ActivityA. M. El-Awad and K. M. Abd El-SalaamChemistry Department, Faculty of Science, Assiut University Assiut, EgyptReceived February 17, 1987Acidic properties and the catalytic activity of 12-molybdopho-sphates (M"nn+H3-,PM012040; M = Cu2+, BP+, Cr3+ and X = 1-3)have been studied. The results are discussed as the effect of thesecatalyst components on the partial charge on oxygen atom whichis in arelation with the acidity. It is shown that the oxygen--hydrogen bond is weakened as the value of partial charge onoxygen (-bo) becomes more negative, while the vapor-phasedehydration activity of 2-propanol is explained on the basis ofthe reacting zone wideness taking into account the pseudo liquidphase nature of the heteropoly compounds. Correlation, thepercentage conversion of 2-propanol with the fractional chargeon molybdenum atom was successful to interpret the effect ofthe redox properties of these catalysts on their catalytic activity.INTRODUC'l'IONHeteropoly acids have recently attracted much attention as catalysts forvarious industrial processes, such as oxidation, dehydration, esterification, andetheration.' It' has been reported that metal salts of heteropoly acids aremore active and selective than the acid form in some acid-catalyzed reactions.F"Relationships between catalytic activity and either the acidic or redoxproperties of these heteropoly compounds have been reported. Niiyama et aPobtained a fair correlation between the activity for alcohol dehydration andelectronegativity of metal cations. Hayashi and Moffat found that the activityfor methanol conversion was related to the partial charge on oxygen atom,"while in considering the redox properties many schemes have been proposedt"!".Also, the participation of both the surface and bulk heteropoly salt moleculesin the catalytic processes has been investigated by several wor'kers.4,11.-13The present work, in an attempt to gain more insight into the bifunctionalnature of the heteropoly salts (the acidic and redox properties), is concernedwith an investigation of the acidity, dehydration activity and the electricalconductivity properties of the substituted 12-molybdophosphoric acid withBi3+,Cr3+ and Cu2+ cations.EXPERIMENTALBDH 12-molybdophosphoric acid was incorporated with the nitrates of Cu(II),Cr(III), and Bi(III). An aqueous solution containing the required amounts of these678 A. M. ElL,AWAD AND K: M. ABD :EL~S~LAAMcations to form a series of salts having the general formula Mn\/n H3-xPM0120'0(where X = 1-3) was added to a solution of 12-molybdophosphoric acid withvigorous strirr ing. The produced mixtures were dried over a water-bath, thenthe resultant solids were calcined at 300 -c for 4 h. in air stream.The acidity of the samples was estimated by measuring the libera ted amountsof acetic acid produced by the hydrolysis of ethyl acetate. The reaction was carriedout in a water thermostate at 600C using a 5.0% aqueous solution of ethyl actate.The extent of hydrolysis of ethyl acetate was determined by convention al titrimetrywith 0.1 N NaOH solution and phenolphthalein as an indicator.The catalytic activity of the heteropoly compounds was measured via thedehydration reaction of ž-propancl, ..it was carrieđ out using a conventional flowreactor. Purified air was used as carrier gas. The exit feed was analyzed usinga 104 series PYE UNI CAM gas Chrematograph with a polyethylene glycol 20%column coated on celite. .Electrical conductivity vmeasurements were carried out with a conductivitycell described by Chapman et al.14• The temperature was controlled with a Gallen-kamp temperature controller. Voltage was obtained via a 240A Keithley Instrumentssupply and the current was measured with a 410A picoammeter (Keithley Instru-ments).RESULTS AND DISCUSSIONAcidity measurement~.; For the prepared catalysts, the acidity was esti-mated using the hydrolysis of ethyl acetate, sincethe reaction is knownto be catalyzed by Bronsted acid sites.l" Therefore, kinetic studies were madeof this reaction over various catalysts as well as in different concentrationsof hydrochloricacid solution for comparison. In all cases, the hydrolysis rateobeyed good first-order : kinetics with respect to the concentration of ethylacetate. The representative plots of the first-order kinetics over several cata-lysts are shown in Figure 1. During the hydrolysis in various concentrationsof ReI solution, the .first-order rate constant [kHC1 (mintj] was proportionalto the concentratian of ReI and was expressed as(1)Renee, the acidities can. be expressed as the amount of ReI required to attainthe same reaction rates over the studied acid catalysts as appear in Table 1.TABLE IThe Specific Rata Constants of EthyL Acetate HydroLysis, Acidity of the CataLysts(Expressed in meq HCL), O!oConversion of 2,PropanoL at 150oC, and the CaLcuLatedF.ractionaL Charge on Oxygen (00)RateSuggested constant Acidity00Catalyst % Conv.formulation 103 X k meq-g?(min" gOl)I H3PMo12040 2.555 16.24 0.687 -0.1966II CU3(PMo12040)2 2.531 24.10 0.680 -0.1968III CuHPMo12040 2.464 21.07 0.662 -0.19675IV CU(H2PM012040l2 2.236 18.18 0.601 -0.1967V CrPMo12040 1.750 14.2 0.470 -0.1954VI Cr2(HPMo12040la 2.001 10.5 0.538 -0.1958VII Cr(H2PMo1204ola 2.200 8.2 0.59 -0.1962VIlI BiPMo12040 1.15 29.17 0.309 -0.1945IX Bi2(HPMo12040) 1.640 30.77 0.441 -0.1952X Bi(H2PMo12040)il 2.156 36.9 0.579 -0.195912-MOLYBDOPHOSPHATES150r-----------------------------------~679)(100~----~--~J-----~----~----~----~--~150'"'O1~5x 125100~----~----~----~------~----~----~--~140125100Il)1 )!v)( lU )Reaction time (hr)Figure 1. Representative plots of first order kinetics of ethyl acetate hydrolysisover pure (I) and substituted 12-molybdophosphoric acid (II-X).The data illustrated in Table I show that: i - The acidity of H3PMo12040continuously decreases by a stepwise replacement of H" by Bi3+ or Cr+ cations ..ii - Although substitution of one proton of the heteropoly acid with theequivalent amount of Cu2+ reduces the acidity, but it was found that anincrease of Cu2+ content rais es the acidity again. This means, that the numberof available sites for reaction (protons) is higher than expected owing to thepartial salt formation. Also, the acidic nature is still present even with theformation of »neutral« salts.680 A. M. EL-AWAD AND K. M. ABD ElL~SALAAMWe believe that the observed acidity behavior of 12-molybdophosphatescan be explained in terms of proton generation!" from the coordinated waterto the metal cations of the substituted 12-molybdophosphoric acid accordingto the following equation:M?" + mHp -7 [M(HP)m]"+ -7 [M(OH) (HP)m_,J<"-l)+ + H+ . . . (2)It is very plausible that reaction (2) is operative in metal salts of heteropolyacids leading to generation of new acid sites5,li but it provides nothing abouttheir strength. Therefore, it is of interest to rel ate the Bronsted acidity ofthese catalysts to a parameter indicative of acid strength where the fractionalcharge on the oxygen atom may suffice for this purpose. Sanderson-" hasdeveloped a simple semiemipirical method for estimating the fractional chargeson atoms found in a molecule. The partial chargeb=tJ.X!tJ.X(1) (3)is related to the change in electrorieaativity (~X) of an atom in becomingpart of a molecule and the correspondina change r~X(l)] in gaining or losingone electron. The latter term can be related to the electronegativity, X, ofthe isolated atom bytJ. X (1) = 2.08 X'/2 (4)values for ~ X for each of the molecules of interest are obtained from theprinciple of electronegativity equalization", where X of the different atomsare obtained from Ref. (20).The acidity dependence on the fractional charge on oxygen (-00) is seenin Fig. 2. It shows that the acidity increases continuously with increasing(-00)- the negative sign of charge means the electronegativity of oxygenatom decreases-this fact provides evidence that the acidity measured for oursamples depends not only on the number of acidic sites but also on theirstrength; the latter has been related to the net charge of the oxygen wheresmaller magnitude of negative charge, i. e. higher (-00), corresponds to themore mobille proton.P Accordingly, there is nothing strange in the fact thateven »neutral« salts of heteropoly acids have a certain amount of Bronstedacidity and their strength increases from Bi(III), to Cr(III) to Cu(II)-12-moly-bdophosphates.Dehydration activity of 2-Propanol. - The catalytic activity of the heteropolycompounds was tested using the vapor-phase dehydration of 2-propanol overvarious catalysts. The reaction conditions were as follows: catalyst weight500 mg; reaction temperature 150°C; concentration of 2-propanol 4.20/0 inair, total flow rate 180 ml (NTP)/min. The product were propylene and water.As shown in Table I, the percentage conversion of 2-propanol is inverselyrelated to the acidity of both Bi(III) and Cr(III) heteropoly salts. On the otherhand, there is a proportional relationship between the acidity and the dehy-dration activity for Cu(II) salts. These findings mean that when the kindof counter cation is changed the activity of the Mx/nn+H3_x PMo12040 catalystsfor dehydration of 2-propanol varies remerkably. Also, when the kind ofcounter cation is fixed and the amount of counter cation is chang ed, thedehydration activity varies in away indicating that the activity is affectednot only by the acidity of the catalysts, but also by other functions.2512-MOL YBDOPHOSPHA TESlo IViii681-tO20 IlSĐ....-i 15 - Q,f:O -;IIuOJ ~....-iIC o-.~ (lJE~IIE.::j- eO ',""....-iX >.~ 10 Q~O +)I'rl"o'rlo«5- 0.20O~~~--~~~--~~~O0.194 0.195 0.196 0.197 0.196-°0Figure 2. Specific rate constant s of ethyl acetate hydrolysis and the acidity expressedin meq. Hel versus the parti al charge on oxygen (--00) for catalvsts I-X.682 A. M. EL-AWAD AND K. M. ABD EL~SALAAM40 -_._--V/IIX(10 Xorloc:co ID-o~o..IN<i- 20oc;o'rlVJ V~<l!>cou~oQ.6080.50S 0.605 Q.60701/10Figure 3. Variation of % conversion of 2-propanol versus the partial chargeonmolybdenum (OMO) for catalysts I-XIn order to interpret more perfectly such results, it is necessary to takeinto account the number of active sites as well as the redox properties of theheteropoly compounds. The former can be explained in terms of Misono et a1.22description of »pseudo liquid phase. nature of the heteropoly compounds. Theystated that such compounds are present in two structures, the first is thestable »primary structure« (the Keggin unit, PM0J20403-) and the second issubjected to easy rearrangement and is called the »secondary structure« (the12-MOLYBDOPHOSPHATES 683three-dimensional arrangement of the heteropoly anion, water and the countercation). These characteristics make heteropoly compounds behave like asolution, and in some cases the catalytic reactions proceed in the interstitialspace of the bulk'". Accordingly, the higher activity of Bi(III) salts may beattributed to an increase of the reacting zone wideness available for thecatalytic reaction, more than takes place for the other salts. This opinion willbe confirmed by electrical conductivity measurements.A second explanation based on the redox properties23,24 of 12-molybdo-phosphates is obtained by correlating the percentage conversion of 2-propanolwith the fractional charge on molybdenum cation as the effective component.The results presented graphically in Figure 3, show that: i -- Substitutionwith unreducible cations increases the dehydration activity with increasing0Mo, ii - An inverse situation is obtained in case of the substitution withreducible cations. 'I'aking into account that increasing OMo means increasingthe electron affinity of M06+ and we suggest that the reaction proceedsaccording to the following steps: i - Attacking the M06+ cations by alcoho1icOH groups, simultaneously C-OH bond becomes weaker and M06+ will be ina slightly reduced state (Mon+ <6 +), ii - Complete separation of alcoholicOH- group, leaving the alcohol molecule as a carboniumion to be attachedto M06+ in a reduced state, Mo'". iii - Reoxidation of Mo5+ accompanied byseparation of the hydroxyl ion to forrn with the released hydrogen ion amolecule of water.From this proposed mechanism two contradicitive steps appeal' to beinvolved, the first depends on the reducibility of M06+, while the seconddepends on the reoxidizability oi M05+. As regards the relationship betweencMo and the dehydration activity (Figure 3) it is easy to decide that thereaction is predominantly affected by step 1 which is enhanced by increasingaMo as found for Cr(III), and Bi(III)-molybdophosphates. On the other hand,an additional factor for the activation of step 1 is obtained from the obser-vable higher catalytic activity for Cu(II) salts, which show a higher % eon-version of 2-propanol than corresponding to aMo of these salts. This ref1ectsa certain role of the presence of a reducible cation on the catalytic activity.The two explanation mentioned above may be supported by electrical eon-ductivity measurements.Elecf1-ical Conductivity Measurement. - It was carried out on the three»neutral« salts (compounds II, V, VIII) as well as on the free heteropoly acid(cornp. 1); the initial conductivities at 150DCin a flow of air were 1.26 X 10-10,0.59 X 10-10,1.20 X 10-10and 43.48 X 10-10n-I cm-I, respectively. When 2-pro-panol vapor was introduced the conductivity increased and attained constantvalues of 3.02 X 10-10,0.91 X 10-10,3.80 X 10-10,and 87.1 X 10-10n-I cm-I, re-spectively, after 2 h. The data illustrated above are represented as ((J/(Jo -1)- where (Jo is the conductivity before admission of 2-propanol and (Ji is thatmeasured at time ti after admission of alcohol-versus time, as shown inFigure 4.From Figure 4, complete agreement between our opinion on the effectof the reacting zone wideness on the catalytic activity and the conductivitybehavior can be seen from the order of increase of the relative conductivity(JJ(Jo which has the same sequence of increasing the cataJytic activity, Thisresult provides evidence of the parallel relationship between the conductivityri3i A. M. EL-AWAD AND K. M. ABD EL~SALAAMlO 20 30Time( .min. )VInon--2 .---- •...(:;-16, IIoVIf~O 120Figure 4. Relative change of electrical conductivity versus time 'of amdission of2-propanol over catalysts I, II, V, and VIII.and the number of active sites which increase with an increase of the reactingzone wideness, showing more current carriers available for conduction. Furthersupport can be obtained from the activation energy calculations using equat-ion (5).o = o., exp (-E/RT) (5)where Log o values have been plotted against the reciprocal absolute tempe-rature (Figure 5); the slope of the obtained lines was considered to give theactivation energy, E25.As appears in Table II, the order of decreasing D.E have the sequencecompound II <VIlI < I <V, exhibiting the same order of decreasing thecatalytic activity, where compound VIn> I > V the only exception is samplen. It shows the lowest value of ~ E confirming the acceleration role of thereducible cations on the electrons mobility.TABLE IIActivation Energy VaLues for 12-MoLybdophosphoric Acid and its NeutraL SaLtsWith Bi(III), Cr(III), and Cu(II).E/ev E/ev afterCatalyst admission t:..E!evin air flow of 2-propanolH3PMo12040 0.586 0.688 0.102BiPMo12040 0.639 0.700 0.061CrPMo12040 0.527 0.791 0.264CU3(PMo12040)2 0.688 0.703 0.01512-MOLYBDOPHOSPHATES 685Figure 5. Variation of log o versus l!T for catalysts I, II, V, and VII in air (solidlines), and with admission of 2-propanol (dotted lines).Finally, our conclusion is that the dehydration reaction of 2-propanol over12-molybdophosphates is affected by the reacting zone wideness, i. e. thenumber of active sites, and the activity of these sites is essentially dependenton the acidic nature of the catalyst and the redox properties of MoG+,theformer is related to the 00 while the latter is related to 0Mo.REFERENCES1. T. Ohara, Shokubai 19 (1977) 157; K. Onoue, Y. Mizutani, S. Aki-ya m a, and Y. I z u m i, Chemtech, July (1978),p. 432.2. T. B a b a, J. Sa kai, and Y. O n o, eou. Chem. Soc. Jpn. 55 (1982) 2675;Y. O n o, T. B a b a, J. Sa kai, and T. K e i i, J. Chem. Soc. Chem. Commun.(1982)400.3. T. H i b i, T. O k uha r a, M. M i s o n o, and Y. Yo n eda, Chem. LeU.(1982)1275.686 A. M. EL-AWAD AND K. M. ABD EL~SALAAM4. T. O k uha r a, N. Ha y a k a w a, A. Ka s a i, M. M i s o n o, and Y. YO-ne d a, J. Cata!. 83 (1983) 121.5. H. N i i y ama, Y. S a i t o, and E. E c 11 i g o y a, Proceedings, 7th, Inte1·-nationaL Congress on CataLysis. Tokyo, 1980, p. 1416. Elsevier; Amsterdam, 1981.6. H. Ha y ash i, and J. Mo f f a t, J. Catai, 81 (1983) 61.7. H. T s u nek i, H. N i i y ama, and E. E c h i g ova, Chem. Lett. (1978)654; 1183.8. K. E g u c h i, Y. To Y oza W a, K. F u r u t a, N. Y ama zoe, and T. Se i-y ama, Chem. Lett. (1981) 1253.9. M. A kim o t o, and E. E c h i g o y a, Chem. Lett. (1981) 1759.10. K. E g u c h i, N. Y ama z o n e, and T. S e i y ama, Chcm. Lett. (1982) 1341..11. M. Ai, J. Catat. 71 (1981) 88.12. Y. K o n i s h i, K. S a kat a, M. M i s o n o, and Y. Yo n eda, J. Cata!.77 (1982) 169.13. T. O k uha r a, A. K asa i, N. H a y a k a W a, M. M i s o n o, and Y. YO-ne d a, Chem. Lett. (1981) 391.14. P. C 11 a p man, R. G r i f fit h, and J. Mar s h, P1·0C.R. Soc. London, (1954)224, 419.15. S. Nam b a, N. H o s o n uma, and T. Y ash ima, J. Cata!. 72 (1981) 16.16. T. B a b a, H. W ata n a b e, and Y. O n o, J. Phys. Chem. 87 (1983) 2406.17. H. Niiyama, Y. Saito, S. Yoshida, and E. Echigoya, NipponKagaku Kaishi, (1982) 569.13. R. T. S and er s o n ChemicaL Bonds and Bond Energy 200 Ed., Academicpress, New York, 1976.19. R. T. S and e r s o n, Science 114 (1951) 670.20. F. A. Co t t o n and G. Will k i n s o n, Advanced In01"ganic Chemistry 2ndEd. Interscience, New York (1967) p. 103.21. J. Hi g h f i e 1d, B. Hod net t, J. Mc M o n a gle, and J. Mo ff a t,Proc. 8th Intern. Cong. Catal, Vol. V, (1984) p. 611.22. M. M i s o n o, N. Mi z u n o, K. Kat a m ura, A. Ka s a i, Y. K o n i s h i,K. S a kat a, T. O k uha r a, and Y. Yo n eda, Bul!. Chem. Soc. Jpn, 55(1982) 400.23. N. Mi z u n o, K. Kat a m ura, Y. Y o n eda, and M. M i s o n o, J. Catal,83 (1983) 384.24. M. M i s o n o, N. M i z u n o, and T. Kom a y a, Proc. 8th Intern. Cong. Cata!.Vol. V (1984) P. 487.25. S. R. Mo r r i s o n, The Chemical Physics of Suriace Plenum Press, New York,(1977) p. 70.SAŽETAI<:Katalitička aktivnost 12-molibdofosfornib kiselinaA. M. El-Awad i K. M. Abd El-SaLaamIspitivani su 12-molibdofosfati općeg sastava Mn\;n H3-, PM012040 gdje jeM = Cu2+, BP+, Cr3+ i x = 1 - 3. U radu se objašnjava utjecaj različitih fizičko--kemijskih osebina tih spojeva kao što je na primjer parcijalni naboj na kisikovomatomu, redoks potencijal itd. na katalitičku sposobnost proučavanih spojeva.

Action of Salt Formation on the Bifunctional Nature of 12- Molybdophosphoric Acid and Their Relationship to the Catalytic Activity

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CROATICA CHEMICA ACTA CCACAA 60 (4) 677-686 (1987)CCA-1764YU ISSN 0011-1643UDC 546.77Original Scientific PaperAction of Salt Formation on the Bifunctional Nature of12-Molybdophosphoric Acid and Their Relationship to theCatalytic ActivityA. M. El-Awad and K. M. Abd El-SalaamChemistry Department, Faculty of Science, Assiut University Assiut, EgyptReceived February 17, 1987Acidic properties and the catalytic activity of 12-molybdopho-sphates (M"nn+H3-,PM012040; M = Cu2+, BP+, Cr3+ and X = 1-3)have been studied. The results are discussed as the effect of thesecatalyst components on the partial charge on oxygen atom whichis in arelation with the acidity. It is shown that the oxygen--hydrogen bond is weakened as the value of partial charge onoxygen (-bo) becomes more negative, while the vapor-phasedehydration activity of 2-propanol is explained on the basis ofthe reacting zone wideness taking into account the pseudo liquidphase nature of the heteropoly compounds. Correlation, thepercentage conversion of 2-propanol with the fractional chargeon molybdenum atom was successful to interpret the effect ofthe redox properties of these catalysts on their catalytic activity.INTRODUC'l'IONHeteropoly acids have recently attracted much attention as catalysts forvarious industrial processes, such as oxidation, dehydration, esterification, andetheration.' It' has been reported that metal salts of heteropoly acids aremore active and selective than the acid form in some acid-catalyzed reactions.F"Relationships between catalytic activity and either the acidic or redoxproperties of these heteropoly compounds have been reported. Niiyama et aPobtained a fair correlation between the activity for alcohol dehydration andelectronegativity of metal cations. Hayashi and Moffat found that the activityfor methanol conversion was related to the partial charge on oxygen atom,"while in considering the redox properties many schemes have been proposedt"!".Also, the participation of both the surface and bulk heteropoly salt moleculesin the catalytic processes has been investigated by several wor'kers.4,11.-13The present work, in an attempt to gain more insight into the bifunctionalnature of the heteropoly salts (the acidic and redox properties), is concernedwith an investigation of the acidity, dehydration activity and the electricalconductivity properties of the substituted 12-molybdophosphoric acid withBi3+,Cr3+ and Cu2+ cations.EXPERIMENTALBDH 12-molybdophosphoric acid was incorporated with the nitrates of Cu(II),Cr(III), and Bi(III). An aqueous solution containing the required amounts of these678 A. M. ElL,AWAD AND K: M. ABD :EL~S~LAAMcations to form a series of salts having the general formula Mn\/n H3-xPM0120'0(where X = 1-3) was added to a solution of 12-molybdophosphoric acid withvigorous strirr ing. The produced mixtures were dried over a water-bath, thenthe resultant solids were calcined at 300 -c for 4 h. in air stream.The acidity of the samples was estimated by measuring the libera ted amountsof acetic acid produced by the hydrolysis of ethyl acetate. The reaction was carriedout in a water thermostate at 600C using a 5.0% aqueous solution of ethyl actate.The extent of hydrolysis of ethyl acetate was determined by convention al titrimetrywith 0.1 N NaOH solution and phenolphthalein as an indicator.The catalytic activity of the heteropoly compounds was measured via thedehydration reaction of ž-propancl, ..it was carrieđ out using a conventional flowreactor. Purified air was used as carrier gas. The exit feed was analyzed usinga 104 series PYE UNI CAM gas Chrematograph with a polyethylene glycol 20%column coated on celite. .Electrical conductivity vmeasurements were carried out with a conductivitycell described by Chapman et al.14• The temperature was controlled with a Gallen-kamp temperature controller. Voltage was obtained via a 240A Keithley Instrumentssupply and the current was measured with a 410A picoammeter (Keithley Instru-ments).RESULTS AND DISCUSSIONAcidity measurement~.; For the prepared catalysts, the acidity was esti-mated using the hydrolysis of ethyl acetate, sincethe reaction is knownto be catalyzed by Bronsted acid sites.l" Therefore, kinetic studies were madeof this reaction over various catalysts as well as in different concentrationsof hydrochloricacid solution for comparison. In all cases, the hydrolysis rateobeyed good first-order : kinetics with respect to the concentration of ethylacetate. The representative plots of the first-order kinetics over several cata-lysts are shown in Figure 1. During the hydrolysis in various concentrationsof ReI solution, the .first-order rate constant [kHC1 (mintj] was proportionalto the concentratian of ReI and was expressed as(1)Renee, the acidities can. be expressed as the amount of ReI required to attainthe same reaction rates over the studied acid catalysts as appear in Table 1.TABLE IThe Specific Rata Constants of EthyL Acetate HydroLysis, Acidity of the CataLysts(Expressed in meq HCL), O!oConversion of 2,PropanoL at 150oC, and the CaLcuLatedF.ractionaL Charge on Oxygen (00)RateSuggested constant Acidity00Catalyst % Conv.formulation 103 X k meq-g?(min" gOl)I H3PMo12040 2.555 16.24 0.687 -0.1966II CU3(PMo12040)2 2.531 24.10 0.680 -0.1968III CuHPMo12040 2.464 21.07 0.662 -0.19675IV CU(H2PM012040l2 2.236 18.18 0.601 -0.1967V CrPMo12040 1.750 14.2 0.470 -0.1954VI Cr2(HPMo12040la 2.001 10.5 0.538 -0.1958VII Cr(H2PMo1204ola 2.200 8.2 0.59 -0.1962VIlI BiPMo12040 1.15 29.17 0.309 -0.1945IX Bi2(HPMo12040) 1.640 30.77 0.441 -0.1952X Bi(H2PMo12040)il 2.156 36.9 0.579 -0.195912-MOLYBDOPHOSPHATES150r-----------------------------------~679)(100~----~--~J-----~----~----~----~--~150'"'O1~5x 125100~----~----~----~------~----~----~--~140125100Il)1 )!v)( lU )Reaction time (hr)Figure 1. Representative plots of first order kinetics of ethyl acetate hydrolysisover pure (I) and substituted 12-molybdophosphoric acid (II-X).The data illustrated in Table I show that: i - The acidity of H3PMo12040continuously decreases by a stepwise replacement of H" by Bi3+ or Cr+ cations ..ii - Although substitution of one proton of the heteropoly acid with theequivalent amount of Cu2+ reduces the acidity, but it was found that anincrease of Cu2+ content rais es the acidity again. This means, that the numberof available sites for reaction (protons) is higher than expected owing to thepartial salt formation. Also, the acidic nature is still present even with theformation of »neutral« salts.680 A. M. EL-AWAD AND K. M. ABD ElL~SALAAMWe believe that the observed acidity behavior of 12-molybdophosphatescan be explained in terms of proton generation!" from the coordinated waterto the metal cations of the substituted 12-molybdophosphoric acid accordingto the following equation:M?" + mHp -7 [M(HP)m]"+ -7 [M(OH) (HP)m_,J<"-l)+ + H+ . . . (2)It is very plausible that reaction (2) is operative in metal salts of heteropolyacids leading to generation of new acid sites5,li but it provides nothing abouttheir strength. Therefore, it is of interest to rel ate the Bronsted acidity ofthese catalysts to a parameter indicative of acid strength where the fractionalcharge on the oxygen atom may suffice for this purpose. Sanderson-" hasdeveloped a simple semiemipirical method for estimating the fractional chargeson atoms found in a molecule. The partial chargeb=tJ.X!tJ.X(1) (3)is related to the change in electrorieaativity (~X) of an atom in becomingpart of a molecule and the correspondina change r~X(l)] in gaining or losingone electron. The latter term can be related to the electronegativity, X, ofthe isolated atom bytJ. X (1) = 2.08 X'/2 (4)values for ~ X for each of the molecules of interest are obtained from theprinciple of electronegativity equalization", where X of the different atomsare obtained from Ref. (20).The acidity dependence on the fractional charge on oxygen (-00) is seenin Fig. 2. It shows that the acidity increases continuously with increasing(-00)- the negative sign of charge means the electronegativity of oxygenatom decreases-this fact provides evidence that the acidity measured for oursamples depends not only on the number of acidic sites but also on theirstrength; the latter has been related to the net charge of the oxygen wheresmaller magnitude of negative charge, i. e. higher (-00), corresponds to themore mobille proton.P Accordingly, there is nothing strange in the fact thateven »neutral« salts of heteropoly acids have a certain amount of Bronstedacidity and their strength increases from Bi(III), to Cr(III) to Cu(II)-12-moly-bdophosphates.Dehydration activity of 2-Propanol. - The catalytic activity of the heteropolycompounds was tested using the vapor-phase dehydration of 2-propanol overvarious catalysts. The reaction conditions were as follows: catalyst weight500 mg; reaction temperature 150°C; concentration of 2-propanol 4.20/0 inair, total flow rate 180 ml (NTP)/min. The product were propylene and water.As shown in Table I, the percentage conversion of 2-propanol is inverselyrelated to the acidity of both Bi(III) and Cr(III) heteropoly salts. On the otherhand, there is a proportional relationship between the acidity and the dehy-dration activity for Cu(II) salts. These findings mean that when the kindof counter cation is changed the activity of the Mx/nn+H3_x PMo12040 catalystsfor dehydration of 2-propanol varies remerkably. Also, when the kind ofcounter cation is fixed and the amount of counter cation is chang ed, thedehydration activity varies in away indicating that the activity is affectednot only by the acidity of the catalysts, but also by other functions.2512-MOLYBDOPHOSPHA TESlo IViii681-tO20 IlSĐ....-i 15 - Q,f:O -;IIuOJ ~....-iIC o-.~ (lJE~IIE.::j- eO ',""....-iX >.~ 10 Q~O +)I'rl"o'rlo«5- 0.20O~~~--~~~--~~~O0.194 0.195 0.196 0.197 0.196-°0Figure 2. Specific rate constant s of ethyl acetate hydrolysis and the acidity expressedin meq. Hel versus the parti al charge on oxygen (--00) for catalvsts I-X.682 A. M. EL-AWAD AND K. M. ABD EL~SALAAM40 -_._--V/IIX(10 Xorloc:co ID-o~o..IN<i- 20oc;o'rlVJ V~<l!>cou~oQ.6080.50S 0.605 Q.60701/10Figure 3. Variation of % conversion of 2-propanol versus the partial chargeonmolybdenum (OMO) for catalysts I-XIn order to interpret more perfectly such results, it is necessary to takeinto account the number of active sites as well as the redox properties of theheteropoly compounds. The former can be explained in terms of Misono et a1.22description of »pseudo liquid phase. nature of the heteropoly compounds. Theystated that such compounds are present in two structures, the first is thestable »primary structure« (the Keggin unit, PM0J20403-) and the second issubjected to easy rearrangement and is called the »secondary structure« (the12-MOLYBDOPHOSPHATES 683three-dimensional arrangement of the heteropoly anion, water and the countercation). These characteristics make heteropoly compounds behave like asolution, and in some cases the catalytic reactions proceed in the interstitialspace of the bulk'". Accordingly, the higher activity of Bi(III) salts may beattributed to an increase of the reacting zone wideness available for thecatalytic reaction, more than takes place for the other salts. This opinion willbe confirmed by electrical conductivity measurements.A second explanation based on the redox properties23,24 of 12-molybdo-phosphates is obtained by correlating the percentage conversion of 2-propanolwith the fractional charge on molybdenum cation as the effective component.The results presented graphically in Figure 3, show that: i -- Substitutionwith unreducible cations increases the dehydration activity with increasing0Mo, ii - An inverse situation is obtained in case of the substitution withreducible cations. 'I'aking into account that increasing OMo means increasingthe electron affinity of M06+ and we suggest that the reaction proceedsaccording to the following steps: i - Attacking the M06+ cations by alcoho1icOH groups, simultaneously C-OH bond becomes weaker and M06+ will be ina slightly reduced state (Mon+ <6 +), ii - Complete separation of alcoholicOH- group, leaving the alcohol molecule as a carboniumion to be attachedto M06+ in a reduced state, Mo'". iii - Reoxidation of Mo5+ accompanied byseparation of the hydroxyl ion to forrn with the released hydrogen ion amolecule of water.From this proposed mechanism two contradicitive steps appeal' to beinvolved, the first depends on the reducibility of M06+, while the seconddepends on the reoxidizability oi M05+. As regards the relationship betweencMo and the dehydration activity (Figure 3) it is easy to decide that thereaction is predominantly affected by step 1 which is enhanced by increasingaMo as found for Cr(III), and Bi(III)-molybdophosphates. On the other hand,an additional factor for the activation of step 1 is obtained from the obser-vable higher catalytic activity for Cu(II) salts, which show a higher % eon-version of 2-propanol than corresponding to aMo of these salts. This ref1ectsa certain role of the presence of a reducible cation on the catalytic activity.The two explanation mentioned above may be supported by electrical eon-ductivity measurements.Elecf1-ical Conductivity Measurement. - It was carried out on the three»neutral« salts (compounds II, V, VIII) as well as on the free heteropoly acid(cornp. 1); the initial conductivities at 150DCin a flow of air were 1.26 X 10-10,0.59 X 10-10,1.20 X 10-10and 43.48 X 10-10n-I cm-I, respectively. When 2-pro-panol vapor was introduced the conductivity increased and attained constantvalues of 3.02 X 10-10,0.91 X 10-10,3.80 X 10-10,and 87.1 X 10-10n-I cm-I, re-spectively, after 2 h. The data illustrated above are represented as ((J/(Jo -1)- where (Jo is the conductivity before admission of 2-propanol and (Ji is thatmeasured at time ti after admission of alcohol-versus time, as shown inFigure 4.From Figure 4, complete agreement between our opinion on the effectof the reacting zone wideness on the catalytic activity and the conductivitybehavior can be seen from the order of increase of the relative conductivity(JJ(Jo which has the same sequence of increasing the cataJytic activity, Thisresult provides evidence of the parallel relationship between the conductivityri3i A. M. EL-AWAD AND K. M. ABD EL~SALAAMlO 20 30Time( .min. )VInon--2 .---- •...(:;-16, IIoVIf~O 120Figure 4. Relative change of electrical conductivity versus time 'of amdission of2-propanol over catalysts I, II, V, and VIII.and the number of active sites which increase with an increase of the reactingzone wideness, showing more current carriers available for conduction. Furthersupport can be obtained from the activation energy calculations using equat-ion (5).o = o., exp (-E/RT) (5)where Log o values have been plotted against the reciprocal absolute tempe-rature (Figure 5); the slope of the obtained lines was considered to give theactivation energy, E25.As appears in Table II, the order of decreasing D.E have the sequencecompound II <VIlI < I<V, exhibiting the same order of decreasing thecatalytic activity, where compound VIn> I > V the only exception is samplen. It shows the lowest value of ~ E confirming the acceleration role of thereducible cations on the electrons mobility.TABLE IIActivation Energy VaLues for 12-MoLybdophosphoric Acid and its NeutraL SaLtsWith Bi(III), Cr(III), and Cu(II).E/ev E/ev afterCatalyst admission t:..E!evin air flow of 2-propanolH3PMo12040 0.586 0.688 0.102BiPMo12040 0.639 0.700 0.061CrPMo12040 0.527 0.791 0.264CU3(PMo12040)2 0.688 0.703 0.01512-MOLYBDOPHOSPHATES 685Figure 5. Variation of log o versus l!T for catalysts I, II, V, and VII in air (solidlines), and with admission of 2-propanol (dotted lines).Finally, our conclusion is that the dehydration reaction of 2-propanol over12-molybdophosphates is affected by the reacting zone wideness, i. e. thenumber of active sites, and the activity of these sites is essentially dependenton the acidic nature of the catalyst and the redox properties of MoG+,theformer is related to the 00 while the latter is related to 0Mo.REFERENCES1. T. Ohara, Shokubai 19 (1977) 157; K. 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Catal, Vol. V, (1984) p. 611.22. M. M i s o n o, N. Mi z u n o, K. Kat a m ura, A. Ka s a i, Y. K o n i s h i,K. S a kat a, T. O k uha r a, and Y. Yo n eda, Bul!. Chem. Soc. Jpn, 55(1982) 400.23. N. Mi z u n o, K. Kat a m ura, Y. Y o n eda, and M. M i s o n o, J. Catal,83 (1983) 384.24. M. M i s o n o, N. M i z u n o, and T. Kom a y a, Proc. 8th Intern. Cong. Cata!.Vol. V (1984) P. 487.25. S. R. Mo r r i s o n, The Chemical Physics of Suriace Plenum Press, New York,(1977) p. 70.SAŽETAI<:Katalitička aktivnost 12-molibdofosfornib kiselinaA. M. El-Awad i K. M. Abd El-SaLaamIspitivani su 12-molibdofosfati općeg sastava Mn\;n H3-, PM012040 gdje jeM = Cu2+, BP+, Cr3+ i x = 1 - 3. U radu se objašnjava utjecaj različitih fizičko--kemijskih osebina tih spojeva kao što je na primjer parcijalni naboj na kisikovomatomu, redoks potencijal itd. na katalitičku sposobnost proučavanih spojeva.

Action of Salt Formation on the Bifunctional Nature of 12- Molybdophosphoric Acid and Their Relationship to the Catalytic Activity

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