Selective Oxidation of Anthracene

G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation

0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

N.T. Do, R. Kalthoff, J.

247

Laacks, S . Trautmann and M. Baerns

Ruhr-University Bochnn, POB 10 21 48, D-4630 Boch\Hn

s-

The oxidation of anthracene was studied on different unsupported V/Mo/P oxides catalysts as w e l l as on almina- and silica-supported catalysts a t 673 and 723 K. Selectivity of the reaction t o anthraquincne and phthalic anhydride was affected by catalyst c a p x i t i o n , support mterial and temperature. A kinet i c reacticn scheme was set up and the kinetic parameters were determined. Adsorbate structures of anthracene, anthraquincgle and phthalic anhydride an both the supported catalysts and m the pure support materials were derived fran ins i t u FTIR transmission spectnsscopic measurermlts.

INTRca3ucTIoN

Polycyclic aranatic hydrwarbns such as anthraoene, phenanthrene and fluom e being formed in coal pyrolysis may be used as feedstocks f o r producing guinones and dicarboxylic anhwides. ccmversion of s a w of the mopounds by heterqenmus catalytic gas-phase oxidation or in the liquid phase with chrun.ic acid are w e l l knckJn technologies (ref. 1).For the gas-phase reaction, mixtures of vanadium oxide and the oxides of molykdenm, mganese (ref. 21, tungsten (ref. 3) , iron, alkali ( r e f . 4) and phosphorus are used. as oatalytic active canp e n t s ; al-, silica and titania are often applied as support materials (refs. 5-7). In the present work anthracene has been subjected t o catalytic gas-

phase oxidation t o study the effect of catalyst ccnq?ositian and of the s u p p r t material on selectivity; furthemre a kinetic m c t i m scheme is proposed for characterization of the catalysts by kinetic pameters. Finally, adsorbate structures of anthrame, anthraquinone and phthalic anhydride were determined fran in-situ I R spectroscopic measurements. The investigations a h a t a better understandkg of the fundamentals of t h e anthracene oxidatian. EwERlMENTAL

Preparation of catalysts Vanadium oxide was med as base c a p n e n t for the catalysts; it was W i f i e d by adding m0lyMenum oxide arid phosphoric acid; in sane instances silica and almina were applied as support materials. Vanadium oxide was dissolved in concentrated hydrochloric acid as reducing agent a t 80'C for 2 h. Sukequently, m e lybaenum oxide and phosphoric acid were added. When preparing fllpported catalysts the carrier material was dispersed in the afore mentioned solution. After

248

solvent evaprization t h e solid material was dried a t 1 2 0 ' ~and subsequently calcined a t 500'C for 16 h. The canpositions and surface areas of the unsupported catalysts and the supported ones used in the oxidation of anthracene are given in Table 1. Apparatus

A schematic diagram of the apparatus used for catalytic testing is given in Fig. 1. Anthracene was oxidized in an electrically heated fixed-bed quartz react o r (length 300 mn, I . D . 8 m n ) . Axial t a p e r a t m e profiles in the catalyst bed were measured by a mvable thermxouple. Anthracene and s a w of t h e o w e n a t e s were analyzed by on-line GC. A l l condensable products of the effluent fran the reactor were collected a t room tmperature and analyzed by off-line GC and HPLc (ref. 8). The carbon oxides a3 and CO, were determined by cn-line Gc.

'* '+

Heated cwriw oil

I

Fig. 1. Scheimtic diagram of the apparatus for catalytic testing (A: capillary flaw meter, R: fixed hed reactor, F: separator, S: saturator for anthracene). For measuring I R transmission spectra a FTIR s p e c t m t e r (Perkin-Elmer &el 1710) was used. T?m . identical I R cells made of quartz were incorporated into the spectrcmeter. A schematic diagram of the I R cell which could be used as a react o r when the catalyst was inserted is sham in Fig. 2. The I R cell was c a p x e d of a cylindrical quartz tube (length 100 mn, E.D. 35 mn) which was sealed on both ends by NaCl wind-. The catalyst sample, hold in a guartz frame, was kept i n a fixed position by guide ledges containing heated fihments for direct heating of the catalyst up t o 773 K. The catalyst pm3er was pressed a t 32 bar t o a 10x30 mn specimen of about 1030 n q / a n 2 . A continuous gas stream of about 30 l/h loaded w i t h anthrame ( 0 . 1 vol.%) was passed through both cells one containing the catalyst sample. Both cells =re alternativelymved into the I R beam; hereby the spectrum of the gas

249

phase surrounding the catalyst m u l d be eliminated. The adsorbate spectrum was then obtained by dividing the transmittances obtained for the catalyst plus the

adsorbate by the respective transmittances of t h e clean catalyst, i.e. without adsorbate as measured before adsorption (refs. 9, 10).

dn

10

1: quartz cylinder 2: sample holder 3: heated filament

I Ill I

4: guide 1 5: thermocouple 6: gas i n l e t s / o u t l e t s 7: NaCl windaw 8: graphite washer 9: v i a ring 10 : pole 11 : Al-ring

10,

Fig. 2. In-situ I R cell. RESULTS AND DISCUSSI@I

Catalytic testing Oxidation of anthracene (0.25 vol.3 in a i r ) was carried out using the catalysts listed in Table 1 (grain size: 0.5 t o 0.7 mn) a t 673 and 723 K. The concentrations of anthracene and of t h e products were m u r e d as a function of contact time (qarfi). Therefran, the depdence of the s e l e c t i v i t i e s on anthracene conwrsion was derived. Moreover, a reaction scheme w a s set up. Assming a

LO

-

a:

20

5

10 0 '0 Anthracene

+ 9, 10-Anthraquinone

o 1, 4-Products

*Phthalic anhydride

20

LO

60

80

100

XI%

miV

+ 9, 10-Anthraquinone XCO,

Fig. 3. Dependence of the partial pressures on amtact th? at 723 K. Catalyst: V:kb = 4.17; P:V = 0.11.

1, 4-Products

t Phthalic anhydride x

cox

Fig. 4. Wpendence of the selectivities on anthracene mversicn a t 723 K. Catalyst: V:Mo = 4.17; P:V = 0.11.

250

first-order reaction with respect to the hydrocarban canpourdis the kinetic paramters were detennined characterizing catalyst p e r f o m c e . For illustraticn, a typical dependence of the partial pressures on contact the is given in Fig. 3 for a selected catalyst (synbols: mebsufed data, lines: calculated according to the kinetic data, which are reprted further belaw); the corresponding depenaenCe of t k selectivities on anthracene conversion is presented in Fig. 4. The pattern of the relaticnships shc%-in in Fig. 3 and 4 indicate that 9,lOand lf4-anthraquinoneas w d l as the carbon oxides can be considered as prirrary prcducts. With increasing anthracene conversion 9,lO-anthraquinone is further oxidized to phthalic anhydride under simultaneous fonnaticn of carbon oxides. Reacticn scheme The oxidation of anthracene can occur in the 9,10- and/or 1,4-pitim. An attack of the oxygen in the 9,lO-pitions leads to 9,lO-anthraquinone while an 1,4-attack results in 1,rl-anthraquinOne.The 9,lO-anthmqumm ‘ ereactsfurther to phthalic anhydride while the lf4-anthraquinoneis oxidized further to 2,3naphthalic anhydride arad finally to pyranellitic anhydride. A sinplified reaction scheme for the anthracene axidation as derived fran the kinetic relatimskips is presented in Fig. 5.

1: Anthracme 2: 9,lO-AnthraquI-

ncne

” \

3: Phthalic anhydride 4: 1,4-An--

none

5: 2,3-Naphthalic anhydride 6: Fyrawllitic anhydride

Fig. 5. Readion scheme of the anthracene oxidation. catalyst perfomlance The effect of the V/t% ratio on the selectivity of different catalysts with a ccnstant Pfl ratio of 0.11was studied at 673 arid 723 K; the selectivities are canpared at an anthracene conversion of about 80%.An increase in temperature fran 673 to 723 K results in a higher 9,lO-anthraquinone selectivity. The results presentd in Fig. 6 s h that the selectivity of 9,lO-anthraquinone decreases with increasing V/bb ratio.

25 1

sI %

S/%

70 r

LO

I20

*" 51%

-I

I

'

0833 167

286

L17

v . Mo

+ 9, 10-Anthraquinone 0 cox

556 6.90 833

* Phthalic anhydride

Fig. 6. Effect of the ratio V:b@ on the selectivities at 723 K at an anthracene canversion of about 80%.

"

unsupported catalyst

Si02-

A120,supported catalyst

I9.10-Anthroquincne El Phthalic anhydride

COX

Fig. 7. Effect of support material on the selectivities at 673 K at an anthracene mversion of abxt 80%.

When using a support material for the catalytic CanpoUIlds, catalyst activity increases for the oxidation of anthracene. The selectivity is differently affected depending on the support applied. The effect of a silica and an a l h support on the selectivity loaded with catalytic material (Vm= 0.83 and P/V = 0.11) at 673 K and at an anthracene conversion of abcplt 80% is sham in Fig. 7. The unsupported and the Si02-supported catalyst show alnrxt the same selectivity behaviour; 9,lO-anikmqunm ' e selectivity decreased, haever, markedly when using a l h as support material.

Kinetic chracterization of the catalysts A statistical discrimination betdifferent kinetic models based on different reaction scfiemes shawed that the total oxidation of the oxygenates, i.e., 9,1O-anthraquinone,phthalic anhyd.ride and the other 1,4-products could be neglected as a first approximation up to anthracene conversions of about 90%; for simplification all the prcducts formed by the 1,4-attack of anthracene were 1 as a pseudc-canpcprent (1,4-products; cp. Fig. 5). All the reactim steps were assumed to be first-order with respect to anthracene and to the various oxygenates; this justified because of the l m ccncentration of these cunpurh. For catalyst characterization various ratios of rate ccnstants were defined. The dependence of these values on the V/Mo/P atanic ratio arid on the support material used for the catalytic materials are sham in Tab. 1.

252

TABLE 1. Ratio of t h e rate constants for the oxidation of anthraene al P:V = 0 . 1 1 Temperature

673 K

723 K

V:W 0.83 1.67 2.86 4.17 5.56 6.90 8.33

1.4

1.2 0.8 0.5

0.1 1.4 0.6

0.2 0.3 0.4 0.4 0.6 0.5 0.8

0.5 0.5 0.5 0.4 0.4 0.4 0.4

0.4 0.4 0.4 0.4 0.4 0.5 0.5

0.3 0.3 0.2 0.2 0.4 0.2 0.2

0.6 0.7 0.7 0.7 0.6 0.5 0.6

0.3 0.2 0.2 0.2 0.3 0.4 0.3

0.5 0.5

0.2 0.6

0.8 0.4

0.2 0.4

0.6 0.5

0.3 0.4

b) P:V = 0.4 1.67 8.33

0.3 0.8

0.6 0.6

0.4 0.4

0.5 0.5

1110 mass-% of catalytic material, 90 mass-% support material 2)Alon-C/Degussa; SBET = 95 mz/g 3)~e~osil-200/De9ussa; SBET = 140 nP/g The ratio k2/kl is a measure for the consecutive oxidation of the primary

product 9,lO-anthracpinone t o phthalic anhydride while the ratios k, /(kl tk3+k, ) and k3/(kl+k3+k4) represent the extent of the selective reaction route t o 9,lOantluxpinone and of the total oxidation t o CO, respectively (cp. Fig. 5 ) . The follawing results can be derived by a canparison of the numrical values: i) The consecutive oxidation of 9,lO-anthracpinone t o phthalic anhydride increases with an increase of the V/Mo ratio a t the lm reaction temp rature of 673 K and a t the low P/V ratio of 0 , l l . ii) Total oxidation is more marked a t 673 K than a t 723 K . iii) Silica as support material results i n better selectivities than almina. iv) No significant effect of the P/V ratio was observed i n the range fran 0 . 1 1 t o 0.40. These results are i n agreement with the qualitative data described above.

I n -si t u I R spectroscopic identification of adsorbate structures When oxidizing anthracene Cox f o m t i o n was increased by the use of the alumina support material for the V/fao/P oxides catalysts while by the use of silica no significant change i n selectivity was oberved. To elucidate this behaviour the follawing experiments were conducted. Anthracene was adsorbed on b t h the supported catalysts between 573 and 723 K i n the presence of a i r . I R transnis-

253

623 K 62 5

723 K

693 K 673 K

673 K

823 K 673 K

Fig. 8. I R transmissicn spectra of the anthracene adsorbates on the SiO, -supported catalyst.

Fig. 9. IR transmission spectra of the anthracene adsorbates on the Al,o,-supported catalyst.

sicm spectra of anthrame adsorbates are shown in Figs. 8 and 9. AnthraquinOne (vC=O: 1672 an-'), phthalic anhydride (vC=O: c. 1850 and 1780 at+ ) and carboxylate ccmplexes (v,,coO- : 1543 and v,O- : 1431 an-l) (ref. 11) were observed as adsorbate structures on the surface of both supported catalysts. The intensity ratios of the carboxylate bands t o those of anthraquinone and phthalic anhydride bands respectively are larger on the Alp03-supportd catalyst than an the SiO, -supported one. Ftxthemre, a strong product adsorption was obSenred on the A l , O 3 - s u p r t & catalyst up t o 723 K while on the Si0,-supported catalyst no adsoption was ohserved any mre above 623 K. The adsorbate spectra of anthracene on the Al,03-supported catalyst shmed additional strong negative OH bands of Al,O, a t 3640 - 3740 an-1 ( r e f . 12) as w e l l as bridged OgI bands a t 3500 an-1 arid a s t m g CH band of adsorbates a t 3073 cn-1 while on the Si0,supported catalyst the negative OH band of SiO, a t 3741 an-1 and the CH band a t 3073 an-1 were very w d c ; the negative bands are ascribed t o an interaction of OH groups w i t h the reactants. Fmn these results it can k derived that the interacticn between the catalyst and the reactants, i.e. intennediates and products was stronger on the Al,O, -supported catalyst than on the SiO, -supported catalyst. This could be confirmed by desorption r n e a s u m ~ ~at ~ ~723 t s K: mnplete desorption was &en& w i t k i n less than 1minute on the Si02-supported catalyst while desorption on the Al,03-supported catalyst toak more than 30 minutes. The IR spectroscapic results indicate that non-selective axf o m t i o n is favored on the AL,03-supported catalyst due t o the formation of carboxylate structures which are considered as precursors t o oxidative degradation of phthalic anhydride k i n g a ecxlsecutive oxidation product of anthraquinone. Rxthemre, it

254

ms s h m that anthracene adsorbed between 573 and 723 K only on pure A.l,O,; no adsorpticn was observed on pure SiO, . When pure SiO, and the silica supported catalyst were w e d t o gaseous anthraquinone and phthalic a n h w i d e no adsorpt i o n was okerved while on pure U , O , and on t h e alumina-supported catalyst s t m g cdmxylate formation occured on the solid surface. a I N C L U S 1 m

Catalysts canposed of V/MD/p oxides are suitable f o r the oxidation of anthracene t o anthraquinones. For 9,lO-anthraquinone a maximum s e l e c t i v i t y of 65%was obtained ( T = 723 K, X = 85%); smming-up a l l t h e valuable prcducts, i . e . , 1,4anthraquinone, 2,3-naphthAic anhydride, p y r a w l l i t i c anhydride and phthalic anhydride a total selectivity of about 85%w a s achieved. The catalytic p e r f o m c e of the various s o l i d s used as a catalyst could be quantitatively described by first-order rate anstants. IR spectroscopic studies slm& that adsorbate structures of d i f f e r e n t mnaentrations e r e f o d on the catalyst surface when using alumina o r silica as support materials. The alumina support having higher surface a c i d i t y when mnpared t o silica resulted in an extensive formation of surface carboxylates which are considered t o be precursors t o oxidative degradation of the valuable oxygenates.

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Financial support by Dsutsche Forschungsgerrreinschaft (grant SFB-O218/B3) is gratefully acknowledged.

REFERENCES 1 Ullmanns ~CyclopSdieder Technischen chemie, Vol. 7 , 4th edn., Verlag c3laanie, Weinheim-New York, 1974, pp.578. 2 J. Vymetal and J. Norek, Czech., CS Pat. 205981 (1983). 3 J . E . Gemah, Catalytic Cmversion of Hydrocarbons, Academic Press, New York,

1969, pp.256. 4 W. Wettstein and L. Valpiana, Swiss Pat. 407079 (1966). 5 U l l m a n n s hCyclo@die der Tedmischen M e , Vol. 17, 4 t h edn. , Verlag Chgnie, Weinheim-New York, 1974, pp.510. chem. P m . , 34(9) (1984) 467. 6 J. Vymetal, J. Norek and V. Ce&, 7 H. Y a s i and K. Ota, J p . Kokai Towry0 Koho JP, 75, 108254 (1975). 8 A. Zeh and M. Baerns, J. chranat. Science, 27 (1989) 249. 9 A. Ranstetter and M. Baems, J. C a t a l . , 109 (1988) 303. 10 N.T. Do a n d M . Baems, Appl. Catal., 45 (1988) 9. 11 L.J. E~llamy,The Infrared spectra of caoplex Molecules, 3rd edn. , C h a v ard IW.1 Ltd. , Iondon, 1975. 12 A.V. Kiselev and V . I . Lyyin, Infrared Spectra of Surface CEmpoUnas, John Wiley & Sons, New York-Torcmto, 1975.

255 B. Delmon (Universitg catholiqe de Louvah, Belgium): You have obtained a very law selectivity when yaur V/Mo/P catalyst was supported an a1mi.m. It is k n m that m, i n its oxide fonn, has such a strong affinity for A 1 2 0 3 that it fonns

strongly adherhg mrmohyers . A very likely reason for the low activity of the Al,03-supported catalyst is that Moo3 segregates cut of the V b / P axqound for reacting with A1203 (phcsphorous, t o a certain extent, could do the same). I naw refer t o your I R - s p e c t r a of Fig. 9. Did you take, for a n p r i s o n , similar spectra for Mo0,/Al,O3 (and P 2 0 5 ~ A 1 2 0 3 )The ? fomatim of a &HI3 m o l a y e r could explain the presence of the species you detect. M. Baerns (Ruhr-University Bochum, W.-Germany): W e have no IR-spectra of

~ , / A l , O , or P,05/A1203 ht we studied the pure carrier materials under the sarne reactim conditicns. The adsorbates on Al,O, shcmd similar IR-spedra as the Al,03-supprted catalyst. Our mclusion was that the law selectivity of the

Al,03-supported catalyst is mainly a f f e c t 4 by the support material.

S. L. Kipennan (N. D. Zelinskii Institute of Organic Chemistry, A r x t d q of Scienes, USSR): F i r s t question: I n this mrk the authors have proposed that a l l the reaction s t e p are of f i r s t order with respect t o h y d r o c a r b carqxxlnds. This was aSSuI[YXZ to be possible as concentrations of reagents and their prcdu&s =re srrall; ht law gas phase concentrations do not mean that surface coverings =re also small. Therefore, the f i r s t order of a l l the reactions was not proved. what do the authors think about i t ? Second question: Do you have a possibility t o measure the surface concentrations of the reaction mnponents? M. &ems (Ruhr-University Eochum, W.-Germany): (1)W e have described

OUT kinet i c r e s u l t s w i t h i n the range of m d i t i o n s s t u d i e d by the first-order reactions; this was possjble since this type of rate equation was applicable and described our experimental data sufficiently. I f , a v e r , the reactant mcentratirms are varied over a w i d e r range this skplification cannot any longer be wed; more anplex kinetics of the Hougen-Watson type are required. (2) W e have no possibilityto measure the absolute surface concentrations of the reaction ampcmds. We d y can estimate the relative surface concentratims of the adsorbed species fmn the IR-spectra by carpcison of the areas k l a w the

bands.