Applied-Catalysis 47 115

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Ap plied Cc&&is, 47 (1989) 115-123Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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Effec t o f An t im on y (IV) Ox ide , B ism u th P h osph a tea n d T in (I V) O x id e o n t h e C a t a ly t ic P r o p e r t i e s o fC om p o u n d O xid e C a t a l ys t s i n t h e O xi d a t i veD e h y d r o ge n a t i o n o f n -B u t e n eFENG-YAN QIULanzhou Institute of Chem ical Physics, Ch inese Academ y of S ciences, L anzh ou (China )andLU-TAO WENG, P. RUIZ and B. DELMON*Catalyse et Chim ie d es Mat driau x Divi&s, Place Croix d u S ud 1, 1348 Louv ain-la-Neuve(Belgium)(Received 1 July 1988, revised manuscript received 11 October 1988)

ABSTRACTThe synergism between oxide phases in the oxidative dehydrogenation of n-butene to butadiene

was investigated. Pure compound oxides (single phase) ZnFe,O,, FeSbO,, Fe, (MoO,):~, NiMoO,CoMoO, and Bi2Mo~I0,2 were prepared using different methods, then mixed mechanically withSb?O,, BiPO, and Sn02. It was found that (i) pure compound oxides, ZnFe.,O,, FeSbO, andBi2Mo,3012, have a high activity and selectivity; (ii) Sb?O,, BiPO, and SnO,, in the mixed pow-ders, have a positive effect on the properties of ZnFe,O,, particularly at 450C; and (iii) theproperties ofFez(MoO,), can be improved by the addition of Sb,O, and BiPO,. Within the frame-work of the remote control mechanism. this shows that the active centres on the compound oxidescan be modified (improved) owing to the action of spillover oxygen produced by Sb,O.,, BiP04and sometimes SnO,.

INTRODUCTIONThe oxidative dehydrogenation of n-butene to butadiene is one of the most

challenging selective oxidation reactions owing to the two major industrial in-terests it represents. First, the main product, butadiene, is an important inter-mediate for the production of plastics, nylon, polybutadiene, maleic anhydride,etc. and second, the st.udy of this reaction may serve to develop new reactionswith similar mechanisms, such as the dehydrogenation of Z-methylbutene toisoprene, which are important in the petrochemical industry.

The catalysts used in this reaction are usually mixed oxides such as variousferrites, molybdates or oxides containing tin. These catalysts usually containseveral phases. For instance, the analysis of ferrite catalysts indicates that a

0X6-9834/89/$03.50 0 1989 Elsevier Science Publishers B.V.


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good cata lyst cont ains a pur e ferr ite with a spine1 str uctur e an d some impur ity,a-Fe,O, [ 11, th e pr esence of which seem s to impede the deactivat ion of th ecatalyst. We interpret the results of other work on oxide mixtures by assumingth at at least one of th ese pha ses car ries th e fun ctions nesessa ry for r eactionwhile the others may be thought to be necessary to improve the properties ofthe active phase, e.g., protecting the active phase from deactivation.

This paper present s preliminar y resu lts concern ing mecha nically mixed ox-ides. Its orginality resides in the fact t ha t we sta rt ed with compound oxides,which are supposed to be good or even excellent catalysts and which are fre-quent ly used in th e oxidat ion of hydrocar bons, an d investigat ed wh ether anextern al s ingle oxide could m odify t heir cat alytic a ctivity a nd selectivity. Th eextern al phases include an tim ony (IV) oxide (Sb,O,), bismu th phospha te(BiPO,) an d t in (IV) oxide (SnO,), th e influence of which on th e dehydr at ionof N-eth ylforma mide ha s been explained by th eir role as effective donors forth e pr oduction of mobile oxygen species [a]. The cat alyst s exam ined wer emecha nical mixtur es between compound oxides a nd th e above added pha ses.EXPERIMENTAL

Catalystpreparation and characterizationCompound oxidesZnFe,O,, Fe, ( Mo0,)3, Bi2MosO12 an d BiPO, were pr epar ed us ing th e cit-rate method [31. The ingredients used as starting materials were

Fe(N03)3*9H,0, Zn(N03)2.5H20, Bi(NO,),.5H,O, (NH,),Mo,O,,*4H,Oan d ort hophosph oric acid. The citra te met hod h as been proved t o be su ccessfulin th e prepa ra tion of th e mixed oxide ph ases. The pr ocedur e for th e prepar a-tion of Bi,Mo,O,,, for example, was as follows. An aqu eous solu t ion of Bi3+and MO+ with a bismut h-to-molybdenu m at omic r at io of 2:3 was prepa redfrom Bi(N03)3-5Ha 0 an d (NH ,)6M07024.4H20, a sm all am oun t of 6.5 Mni-tr ic acid being added t o th e solut ion in order t o prevent precipitat ion. Citricacid was th en added in such a mann er th at th e num ber of moles of citr ic acidwas equal to th e tota l nu mber of bismut h an d molybdenum ions. Evaporat ionof th e solution t hu s obta ined was car ried out u nder redu ced pressu re in a rotar yevapora tor at 60 C un til the liquid becam e viscous an d at 90C un til solid. Anam orph ous organ ic precursor was obta ined. Decomposition of th e precursor at300 C for 16 h an d fina lly calcina tion at 500C for 18 h were p erform ed.

The oth er compoun d oxides, FeS bO,, N iMoO, an d CoMoO,, were pr epar edby coprecipita tion accord ing to pr ocedur es described pr evious ly [4-61.

Single oxidesTin (IV) oxide was pr epar ed by pr ecipita tin g t he tin (II) hydr oxide fromtin (II) chloride dihydr at e followed by wash ing Cl- with aqueous am monia


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TABLE 1Some char acterist ics of pur e oxidesSample Crystalline phase Surface area (m/g)SbpO,BiPO,SnO,ZnFenOjFeSbO,Fe, (MOO, 11NiMoO,CoMoO,Bi,MoBO,,

Orthorhombic 2 . 0Monoclinic 4.3Tetragonal 12.6Spine1 9.6Tetragonal 24.4Monoclinic 4.3Monoclinic 28.9Monoclinic 8.6Monoclinic 2.7

TABLE 2Ph ysico-chemical cha ra cter istics of mixtu rescontaining St1~0:~Sample XRD

Fresh Used*BET sur face ar ea (m2/g)Fresh Used

ZnF e,O,Sb,O, Spine1 + orth orhombic Iden .** 5.8 7.0FeSbO,-Sb,O, Tetra gona l + orth orhombic Iden . 13.3 12.3Fe,(MoO,),-Sb,O, Monoclinic + orthorhombic Iden . 3.5 3.1Bi,Mo,,O,,-Sb,O, Monoclinic + ort hor hombic Iden . 2.5 2.1*Used means after catalytic test of dehydrogenation of n-butene.**Identical.

solut ion, drying overn ight at 100 C and fina lly calcinin g at 600 C for 8 h.Ant imony (IV) oxide was pr epar ed by th e calcina tion of an tim ony (III) oxidein air a t 500 C for 20 h.

Cryst al str uctu res of th e sa mp les obta ined were checked by X-ra y diffra ction(XRD ) (Siemen s Krist allofex 805 diffractomet er, us ing nickel-filter ed Cu Kara diation). The sur face ar eas were determ ined gravimetr ically with a Seta ra mMTB 10-S microbalance conn ected to a vacuu m an d gas ha ndling system bynitr ogen adsorpt ion at 77 K. Some char acterist ics of th ese samples ar e re-ported in Table 1.

Mechanical mixtureMecha nical mixtu res of pa irs of oxides wer e pr epar ed by disper sion of th e

r espective powder s in n-pent an e for 10 min , followed by evaporat ion of th esolvent and drying at 80 C for 20 h. No furt her calcina tion was car ried out . Inth is stu dy, only mixtu res cont ain ing 50 wt.-% of each oxide were pr epar ed. In


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order to compare the results, the pure oxides were also treated in the samemanner, i.e. dispersion in n-pentane.

Some mixtur es cont aining Sb,O, were cha ra cter ized using XRD an d BETmethods. The results are reported in Table 2.

It can be observed t ha t (i) no new pha se was detected du ring XRD mea-sur ement s for eith er fresh or used sam ples and (ii) th e surface ar eas of th emixtures are almost the simple sum of those of the pure oxides and they werenot modified during the catalytic test.

The Fe, (MOO, ) ,-Sb,O, syst em was cha ra cter ized u sing X-ra y ph otoelec-tr on spectr oscopy for both th e fresh a nd used sam ples. No mu tu al cont am i-nation could be detected [ 71.Catalytic activity measurements

The oxida tive dehydr ogenat ion of n-but ene was car ried out in a conven-tiona l fixed-bed rea ctor syst em. The rea ctor was ma de of a glass tu be of 8 mmin diameter, into which a small tube of 4 mm in diameter was inserted to carrya th erm ocouple to measu re the tem pera tu re of th e cat alytic bed. The cat alystwas ground and the fractions between 500 and 800 ym were used. The catalystwas diluted with glass spheres 500 pm in diameter. The standard conditionswere as follows: n -but ene pr essu re, 76 mm Hg; oxygen pr essu re, 152 mm Hg;tota l pressu re, at mospheric; tota l flow-ra te, 100 ml/min; an d cat alyst ma ss,500 mg.The pr oducts an d rem aining rea cta nt s were ana lyzed by on-line gas chr o-ma togra phy (Int ersm at IGC 120 ML) with a column cont aining 17% sebacon-itr ile sup port ed on Ch romosorb P AW.RESULTS

The cata lytic a ctivity resu lts for t he pur e oxides an d mechan ical mixtu resare reported in Figs. 1 an d 2, respectively, showing th e but ad iene, CO2 yields,(Fig. 1) nd but ad iene selectivity (Fig. 2) at 400 an d 450C. The colum ns atth e bott om of th e figur es represent th e absolute values for the compound ox-ides an d th ose in the upper par t r epresent th e effect (or synergy) of Sb204,SnO, a nd BiPO, when a dded to th ese compoun d oxides. The synergy was cal-culated using the equationnY=Y*,-0.5(Y*+Y,)where YAB represents the yield or selectivity of mechanical mixture AB andYA an d Ys r epr esen t th e yield or selectivity of oxide A and B, respectively.

F r om Figs. 1 an d 2, it can be observed th at : for pu re compoun d oxides,ZnF e,O,, Bi,Mo,O,z an d FeSbO, ar e modera tely a ctive an d selective cat alyst sfor th e form at ion of buta diene wherea s the a ctivities a nd selectivities of th emolybdates of iron, nickel and cobalt are relatively low, It should be added that


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20-O-

d0z-20-d(+ lo-E o-Epo-VI

-40 -20-

902

BiPQ,

O-

-M-L

ZnFeP4 F e S b O 4 FeZ(HGJ3 HHoO~ CoMu01, Bi$W2Fig. 1. But adien e and car bon dioxide yields for compound oxides a nd effect of Sb204, SnO, a ndBiPO, ( .A Y) a t 400 3C (ha tched boxes) an d 450 C (open boxes).

the activities of the external phases Sb204, BiP04 and SnOz (Table 3) aremu ch lower tha n th ose of th e compoun d oxides.

Regardin g th e but ad iene yield, Sb,O, ha s a positive effect on th e pr opert iesof ZnF e,O, and Fe,(MoO,),, a negat ive effect on t hose of FeSb04 an dBi2M03012 an d alm ost n o effect on t he nickel a nd cobalt molybda tes. It is in-ter est ing that the addition of Sb204 inh ibits th e pr oduction of car bon dioxide


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30-

O-

-4 i

ZllFeZO~ Festa FdMoOfJj NitllJOl+ cd-iool, &+wlZFig. 2. Butadiene selectivity for compound oxides and effect of Sbz04, SnO, and BiPO, (AS) at400C (hatched boxes) and 450 C (open boxes).

for a ll of th e mixed oxides investigat ed. It follows th at t he ad dition of Sb204favours the but adiene selectivity with ZnF e,O,, Fe,(Mo04)s an d Bi2M03012but it ha s almost no influen ce in th e oth er in sta nces.

The effect of SnO, is differen t from t ha t of Sb,O,. It has a negat ive effect onth e pr odu ction of but ad iene wit h all th e mixed oxides except ZnF e20, at 450C.At 400 C, it increa ses t he pr oduction of car bon dioxide with m olybdate cat a-


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121TABLE 3Cata lytic activity and selectivity for Sb,O,, BiPO, an d Sn02Sample

Sb,O,BiPO,SnO?

400CYield ( % )

0.082.78

13.40

Selectivity (% )0.597.51

37.33

450CYield(%)

0.7212.8420.28

Selectivity (% )4.12

23.7631.44

lysts but inh ibits it at 450C for a ll th e cat alysts investigat ed. Regar ding bu-ta diene selectivity, SnO, ha s a negat ive effect with m ost of th e mixed oxides,except ZnFe,O, an d CoMoO, a t 450 C.The ad dition of BiPO, favours bu ta diene form at ion an d its selectivity forZnF e,O,, Fe,(MoO,), an d NiMoO,, which is not th e cas e for FeS bO, an dBi,Mo,O,,. Like Sb,O, it inh ibits the form at ion of car bon dioxide in all in-sta nces an d th e inhibiting effect increa ses with increas ing reaction tem pera tu re.

DISCUSSION

Severa l processes ma y take place when t wo oxide ph ases ar e mixed together ,for inst an ce t he form at ion of a new phase or th e conta min at ion of one oxideby th e oth er. Therefore, th e explana tion for the results with mu lti-pha se cat -alysts ma y vary from one system to an oth er, depending on which phenomenonappears between phases. The preliminar y cha racterization results presentedhere do not allow for fur th er inter pret at ion un til fur th er cha ra cterization re-su lts are obta ined. However, it would st ill be useful t o discuss some pointsbased on the results obtained.

It is genera lly accepted th at th e oxidat ive dehydr ogenat ion of n-but ene op-era tes via a redox mecha nism, na mely th at t he cat alyst is redu ced by th e reac-ta nt an d reoxidized by th e gaseous oxygen. Ther efore, a good cat alyst mu st becapable of being redu ced a nd reoxidized eas ily, a crit erion usua lly best s at isfiedby mixed oxides. The pr esence of gaseous oxygen is necessar y to ma int ain th ecat alyst in an active and selective (not deactivat ed) form . However, it ma y beexpected that, in certain instances during reaction the catalyst may be deac-tivated owing eith er to th e reduction (e.g., the redu ction ra te of th e cat alyst ismore ra pid th an t ha t of reoxida tion) or to th e deposition of car bona ceous pr od-ucts, and the supply of oxygen species coming from other phases may improvethe activity and, more particularly, the selectivity of the catalyst. This may beth e rea son for th e positive effect of Sbz04, BiPO, an d SnO, on ZnF e,O, an d ofSb204 an d BiP04 on Fez (Moo,),. The deta iled invest igations of th e ZnF e,O,-


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122Sb204 system showed clearly t ha t a t 450 o C pur e ZnF e,O, was deactivat ed dur -ing rea ction owing t o its pha se segregation to ZnO wherea s th is segregat ion didnot tak e place when Sb,O, was presen t [ 81. This result indicates that oxygenspecies emit ted by Sb,O, can st abilize th e ZnF e20, phase. Anoth er ser ies ofresu lts demonstr at ing th e influence of oxygen concentr at ion on the syner gisticeffect for ZnF e,O,, Fe2(Mo04)3 with Sb,O, an d BiP04 confirms t he aboveinterpretation [ 731.

An int erestin g observat ion is th at the influence of SnO, cha nges su bstan -tia lly from 400 to 450 C. F or ins ta nce, SnOz increa ses th e pr odu ction of car bondioxide at 400C wher eas at 450 C it inh ibits it for molybda te cat alyst s. Th isma y be related to th e cha nge in th e na tu re of th e oxygen species adsorbed onSnO, at these two tempera tu res. It has been shown t hat the ma in oxygen spe-cies adsorbed on Sn Oa at 400C ar e electr oph ilic an d ar e su ita ble for car bondioxide production, an d th at when the tem pera tu re increa ses, th ese species ar econver ted to nucleophilic O*- su ita ble for selective oxidat ion [9,101. It couldbe expected that most of the oxygen species adsorbed on SnO, are nucleophilicat 450C.

Some of th e above system s, such as NiMoO,-Sb,O, an d FeSbO*-Sb204, ha vebeen invest igated in th e selective oxidat ion of isobut ene to met ha crolein [ 111,where a synergy was observed, cont ra ry to th e presen t resu lts. Although bothrea ctions ar e considered to opera te via th e sam e mechan ism for th e first st epof th e rea ction, i.e., abs tr action of a-hydrogen, th e oxidat ion of isobut ene wouldrequire the insertion of lattice oxygen in the main product and hence the reox-idation of the catalyst would be more substantial in this instance. It may beth at th is difference will explain t he cont ra dictory resu lts obta ined with thesereactions.

In conclusion, th ese preliminar y resu lts show th at th e activity an d/or selec-t ivity of compoun d oxides, s imila r ly to th ose of single oxides, can benefit fromcont act with Sb204, BiP04 an d somet imes Sn02. As regar ds th e rem ote cont rolmechan ism, th e results suggest th at th e active cent res on these compound ox-ides can be modified owing to the action of spillover oxygen.ACKNOWLEDGEMENT

The fina ncial su pport of F NRS (Belgium ) accorded to F.Y. Qiu for his stayin Louva in-la-Neuve is gra tefully acknowledged.

REFERENCES1 M.A. Gibson an d J .W. High tower, J . Cat al., 41 (1976) 431.2 S. Ceckiewicz and B. Delmon, Bull. Sot. Ch im. Belg., 93 (1984) 163.3 Ph . Cour ty, H. Ajot, Ch. Mar cilly and B. Delmon, Powder Technol., ? (1973) 21.4 R.G. Teller, J .F. Brazdil an d R.K. Grasselli, J . Chem. Sot., Far ada y Tran s 1,Sl (1985) 1693.5 U. Ozkan and G.L. Schra der, J . Cata l., 95 (1985) 120.


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6 U. Ozka n and G.L. Schr ad er, Appl. Cat al., 23. (1986) 327.7 F.Y. Qiu, L.T. Weng, P. Ruiz and B. Delmon, Appl. Cata l., submit ted for publicat ion.8 F.Y. Qiu, L.T. Weng, P. Ruiz and B. Delmon, t o be publish ed.9 SC. Chen g, J . Vat. Sci. Technol., 17 (1980) 366.

10 J . Haber, in 8th Int erna tional Congress on Cata lysis, Berlin, 1984, Vol. 1, Dechema , 1984, p.85.11 L.T. Weng, B. Zhou, B. Yasse, B. Doumain, P. Ruiz and B. Delmon, in M.J. Phillips and M.Terna n (Editors), 9th In ter na tional Congr ess on Catalysis. Calgar y, 1988, Vol. 4, ChemicalInst itut e of Cana da, Ott awa, 1988, p. 1609.

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