Journal of Molecular Catalysis, 71 (1992) 317--333 317 M2811

I n f l u e n c e o f a l l o y i n g p l a t i n u m f o r t h e h y d r o g e n a t i o n o f p - c h l o r o n i t r o b e n z e n e o v e r PtM/A1203 c a t a l y s t s w i t h M = Sn, Pb , Ge, A1, Zn

Bernard Coq*, Amina Tijani and Franqois Figudras Laboratoire de Chimie Organique Physique et Cindtique Chimique Appliqudes, URA 418 CNRS; ENSCM, 8 rue de l'Ecole Normale, 34053 MontpeUier Cedex (France)

(Received April 9, 1991; accepted October 20, 1991)

A b s t r a c t

Hydrogenation of p-chloronitrobenzene (CNB) has been studied, in methanol suspension, at 303 K and atmospheric pressure, over a series of Pt/A1203 and PtM/A1203 catalysts (M = Sn, Pb, Ge, A1, Zn). The bimetallic catalysts were prepared by the so-called controlled surface reaction method by reacting organometaUic compounds on the well-dispersed Pt/A1203 parent sample. The kinetics obeys a modified Langmuir-Hinshelwood model with competitive adsorption between CNB and hydrogen. The competition of adsorption no longer holds at high CNB concentration: on the Pt surface saturated by CNB, there are still Pt sites available for the chemisorption of the smaller hydrogen molecule. A rate law is then proposed which allows both adsorption and rate constants to be determined. Upon addition of the second metal M, the specific activity per Pt surface atom goes through a maximum value around a chemical composition M]Pt - 0.1-0.2. It is suggested that an electron-deficient species of the second metal promotes the reaction rate by activating the nitrogen-oxygen bond; ionic tin species are better promoters for this purpose. At high CNB conversion (> 98%) the yield of the desired product, p-chloroaniline (CAN), increases from 82% on pure Pt/Al2Os to 97.5% on PtSn/AI2Os (Sn/Pt= 0.36). The improvement of selectivity to CAN is not due to a decrease in CAN dehalogenation rate, which is not affected by alloying. Actually, the promoting effect of the second metal decreases the relative strength of adsorption between CAN and CNB up to a factor of 10: CAN is then easily removed from the surface by CNB, thus preventing hydrode- chlorination.

I n t r o d u c t i o n

Aromatic haloamines are of great interest in the chemistry of herbicides and pesticides. The traditional routes for their production involve Bechamp's reaction and a metal acid system, or selective hydrogenation over hetero- geneous catalysts. The latter route is now preferred, owing to its lower impact on the environment since no acid effluents are produced. Several formulations of heterogeneous catalysts have been claimed active and selective for these reactions by manufacturers. The most widely used heterogeneous

*Author to whom correspondence should be addressed.

0304-5102/92/$5.00 © 199 2 - Elsevier Sequoia All rights reserved

318

catalysts are based on platinum and Raney nickel. Two approaches have been developed in order to tune the catalytic system to produce high yields of the desired products: either dedicated preparation of the catalyst (alloying, metal/support interaction, etc.), or the use of specific additives (promoters, inhibitors) in the reaction medium. These respective approaches can be exemplified by two recent studies on hydrogenation of p-chloronitrobenzene over Raney Ni [1, 2]. Baumeister et al. [1] have improved the selectivity to p-chloroaniline by adding amidine derivatives to the reaction medium. E1 Alj [2] reached the same goal by modifying Raney Ni with chromium during the catalyst synthesis. The tendency now is to simplify the reaction medium, thus making easier the recovery of products and reducing extra processing s teps for t reatment of liquid effluents.

Therefore, optimisation of the solid catalyst requires detailed knowledge of the modifier action. This scope is of general interest, but the study of platinum-based catalysts, which are among the best candidates for producing aromatic haloamines, was particularly attractive.

The catalytic propert ies of supported metal catalysts can be modified by several means: size of the metallic particles, interaction between metal particles or precursors and the support, and doping and alloying procedures. Several examples exist in the literature proving that the addition of a second metal, such as Cu [3], Ru [4] or Sn [5], to Pt increases the yield of the desired aromatic amine. We have focused our attention on this last point, which has been widely studied for reforming catalysts and upgrading of olefin cuts.

First of all, in order to study the influence of alloy formation it is advisable to avoid any effect of particle size, support or precursor which may interfere. In particular, the nature of supports and precursors can greatly affect the propert ies of the final catalyst. The hydrogenation of crotonaldehyde gave only butyraldehyde when processed over Pt/SiO2, but large amounts of crotyl alcohol (3796) over Pt/TiO2 reduced at high temperature. W]smeUer et al. [6] claimed that the selective hydrogenation of citronellal to citroneUol requires a chlorine-free RufriO2 catalyst. The yield of p-chloroaniline from p-chloronitrobenzene increases when Pd or Pt is supported on a basic support (BaC03) rather than an amphoteric one (A1203) [7]. The use of alumina and organometallic precursors appeared to be efficient routes in order to minimize these effects for Rh- and Ru-based catalysts [8, 9]. Pt/A1203 catalysts of widely varying dispersion were prepared by this method for the study of the effect of Pt particle size on the hydrogenation of p-chioronitrobenzene [10]. The specific activity (per surface Pt atom) increases by one order of magnitude when the dispersion of P t decreases. The product selectivity also depends on the Pt particle size.At high conversion the best selectivity inp-chloroaniline is reached on large Pt particles. This was interpreted by a lower relative strength of the amine adsorption on poorly dispersed catalysts.

In the present work we have studied the bimetallic effect of PtM/AI2Os catalysts (M--Sn, Pb, Ge, A1, Zn) prepared from organometallic precursors on the hydrogenation of p-chloronitrobenzene in the liquid phase.

319

Experimental

Reactants Hydrogen of high purity grade (99.99%) was used for the catalytic

experiments, and hydrogen of ultra-high purity (99.9995%) for adsorption measurements, p-Chloronitrobenzene (Aldrich, purity > 99%) and methanol (SDS, > 99%) were used without further purification. Platinum acetylacetonate (Pt(acac)2, Strem Chemicals) was used as precursor for the catalysts. This precursor was chosen in order to avoid the use of chlorine which could affect the chemisorption or catalytic properties [ 9 ]. The precursor was dissolved in anhydrous toluene (purity > 99.5%). The second metal was introduced as: tetra-n-butyltin (Merck, purity >95%), tetra-n-butyllead (Alfa, purity >98%), tetra-n-butylgermanium (Alfa), triethylaluminium (Alfa) and di- ethylzinc (Alfa). These organometallic compounds were dissolved in n-heptane (Fluka, purity >99%). The carrier was T-alumina from Rh6ne-Poulenc (GC054C805 type, surface area 200 m 2 g - l , mean pore d i a m e t e r = 8 - 9 nm, impurities: Na20<20 ppm, SIO2<140 ppm, Fe208<30 ppm, S < 5 0 ppm, CaO < 30 ppm, MgO < 20 ppm).

Preparation of platinum catalysts The monometallic parent Pt/A1203 sample was prepared by adsorption

of Pt(acac)2 on T-alumina. The requisite amount of Pt(acac)2 was dissolved in toluene, then contacted with T-A1203 at room temperature for several hours. Afterward, the solution was evaporated. The solid was dried at 298 K under vacuum, then reduced at 623 K overnight.

The bimetallic PtM/A1208 samples (M = Sn, Pb, Ge, A1, Zn) were prepared according to the so-called 'controlled surface reaction' method [11-13] by reacting the Pt/Al203 parent catalyst with the desired amount of (C4Hg)4Sn, for instance, in n-heptane solution and under bubbling hydrogen. First, the Pt/A1203 was reactivated in situ under flowing hydrogen at 623 K overnight. The solid was then cooled to room temperature under hydrogen, and the solution of (C4Hg)4Sn in n-heptane was pushed through the catalytic bed with hydrogen. The controlled surface reaction was carried out under hydrogen atmosphere at 363 K, with a solid/liquid ratio of 1 g per 10 cm s. The formal reaction is:

(C4H9)4Sn -b Pt/Al203 H2 > PtSn(C4Hg)x/Al203 + (4 - x ) C 4 H I o

These solids were dried under vacuum at room temperature and sub- sequently reduced under hydrogen at 623 K for 4 h: The samples were then stored under air.

Cha rac t e r / za t / o~ In order to characterize the dispersion of the platinum phase, the

chemisorption of hydrogen was carried out in a conventional volumetric apparatus, at 298 K in the 0 -20 kPa pressure range. The sample was first reactivated under flowing hydrogen at 573 K overnight, then evacuated to

320

1.2 × 10 -4 Pa, at the same temperature, for 3 h. Hydrogen was then adsorbed. The linear port ion of the isotherm, usually between 10 and 20 kPa, was extrapolated to zero pressure to determine the hydrogen uptake. By assuming a stoichiometry for hydrogen adsorption on surface platinum atoms (Pt~) of unity, the dispersion of platinum is given by D -- Pts/Ptt-- I-I/[~. It was repor ted recently that the stoichiometry of hydrogen adsorption changes with metal particle size, and can exceed unity on small particles [14]. This introduces an uncertainty in the number of surface Pt atoms, which can reach 20% on well-dispersed samples [10]. The size of the metallic particles was checked by transmission electron microscopy using a JEOL 100CX apparatus. The agreement with chemisorption is good for the Pt/Al2Oa parent catalyst.

The chemical composit ions of the solids were determined after dissolution by elemental analysis at the Service Central d'Analyse (CNRS, Solaize, France). The main characteristics of the catalysts and their designations are included in Table 1.

Catalyt ic tests Hydrogenat ion of p-chloroni trobenzene (CNB) was carried out in a 50

ml four-necked flask connected to a cooler, a dropping funnel, a hydrogen gas line and a pipe used to sample the reaction mixture. The catalyst was reactivated in s i tu under flowing hydrogen at 523 K overnight. After cooling to the reaction temperature (Tr= 303 K unless otherwise specified), 25 ml of the reactant solution (CNB in methanol) was introduced through one arm of the flask. This solution was previously purged with bubbling hydrogen. The reaction mixture was magnetically stirred at 700 rpm, and the reaction was carried out at atmospheric pressure under H2 flow.

Chemical analysis of the products was performed by gas chromatography (Carlo Erba, model 2450), equipped with a FID detector and a J & W capillary

TABLE 1

Chemical composition and hydrogen adsorption of Pt/AI208 and PtM/AI2Oa catalysts (M = Sn, Pb, Ge, AI, Zn)

Sample M M/Pt H]Pt (at/at)

nature wt.%

PtXP - - 0 1.0 PtSnl Sit 0.09 0.11 0.71 PtSn2 Sn 0.21 0.25 0.64 PtSn3 Sn 0.31 0.36 0.41 PtPb Pb 0.31 0.21 0 .65 PtGe Ge 0.11 0.22 0.67 PtA1 AI 0.04 0.21 0.69 PtZnl Zn 0.07 0.15 0 .63 PtZn2 Zn 0.25 0 .54 0 .62

~The PtXI sample contains 1.4 wt.% Pt.

321

column (30 m × 0 . 3 2 mm i.d., DB1 apolar phase). Reactants and products were identified with authentic samples, and GC-MS coupling.

R e s u l t s

The modification of a parent Pt/A1208 sample, with Sn(C4Hg)4 for instance, leads to well-defined PtSn/A1208 catalysts. It was previously shown for PtSn [12], RhSn [13] and PdSn [15] that, in the absence of Pt, Rh or Pd, no tin was deposited on alumina by this method. After exposure to air at room temperature, the subsequent reactivation of the PtSn bimetallic catalysts prepared by this method led to a nearly pure Pt= Sn alloy phase with a very low amount of under-stoichiometric SnOy species [16]. In the range of metal composit ions studied, the addition of Sn, or another second metal, and the subsequent reduction of PtSn(C4Hg)JAI2Os at 623 K does not induce sintering or redispersion of the bimetallic particles (Fig. 1). This behaviour confirms previous reports on PdM [15], RhSn [17] and RuM [18] catalysts.

The hydrogenation of halonitrobenzenes follows the formal reaction pathways described in Scheme 1 [19, 20]. Figures 2 and 3 show the product distribution for the hydrogenation of CNB over Pt/A120s and PtZn/A1208

PtXI

1000

o.

~6

.0

500

1 1 50(]

0 2 4 6 8 10 2 k size of metallic parl"ictes (nm)

Fig. 1. Distribution of pla t inum particle size in PtXI and PtZn2 samples.

PfZn2

10

322

~ 0 2

TH2

Scheme 1.

0 C ' ~ - N = N - O - C I

H2 -=" ( I - @ - NHOH H2

÷ CI-~-NHOH

H2 ,- Ct-~-NHNH-~-Cl CL-~

100 I i i r

75

-5 E

50

o

25

0 50 100 150 200 time {rnin)

Fig. 2. HydrogenaUon of p-chloronitrobenzene over PtXI as a function of time. Tr= 303 K; CR=0.52 tool 1-1. (C))p-chloronit~obenzene; (®)p-cltloroani]ine; (0 ) aniline; (r-I)p-cldoron- itrosobenzene; traces ( < 1%) of cldorobenzene, nibrobenzene, p-ehloropheny]hydrox3zlamine, azo- and azoxydieh]orobenzenes.

323

o E

(31=

100

75

50

I I t

25

0 50 100 150 200 time (rain)

Fig. 3. Hydrogenation ofp-chloronitrobenzene over PtZn2 sample as a function of time. Tr ffi 303 K; CR=0.52 mol 1-1. Symbols as in Fig. 2.

samples as a function of time. The sigmoid-shaped curves indicate the occurrence of three different kinetic regimes depending on the CNB con- centration. A negative order at high CNB concentration, then a zero order dependency and finally a positive order at the end of the CNB transformation. The only products appearing in detectable amounts in the course of the reaction are chlorobenzene (CB) , aniline (AN), p-chloronitrosobenzene (CNSB), nl t robenzene (NB), p-chloroa~iline (CAN), p-chlorophenylhydrox- ylamine (CPH), azo- and azoxydichlorobenzenes. Actually, it seems that p- chloroni trosobenzene is formed mainly in the GC apparatus, owing to the absence in the UV spect rum of the reaction medium of the reference peak at 755 nm. Probably, CPH would be the prevailing intermediate present in solution, but would decompose to CNSB during GC an&lysis, as repor ted for phenylhydroxylamine [5, 21 ]. Figure 4 reports the concentrat ion of the postulated intermediates, CPH or CNSB, as a function of time. It appears clearly that tin addition favours their accumulation, whereas aluminium and zinc have the opposi te effect; lead and germanium do not exhibit any significant influence. However, when PtSn3 (see Table 1) undergoes a redox cycling

324

5

I

u

Z L,/

I i I

~ I " -I ~ - ~ ' - - - I X ~ - - - ~ v

411 m n l

25 50 75 100

[NB co.version (%)

Fig. 4. Distribution as a function of the reactant conversion of the postulated intermediate, p-cbloronitrosobenzene or p-ehlorophenylhydroxylamine, during the hydrogenation of p-chlo- ronitrobenzene over platinum catalysts; (O) PtXI, (O) PtSnl, (®) PtSn2, ( ~ ) PtSn3, ([3) PtSn3ROR, ( . ) PtZnl, (V) PtZn2, (V) PtGe, (A) PtA1, (A) PtPb; Tr=303 K, Ca=0.52 mol 1-1.

(PtSnsROR), its behaviour differs completely with respect to the reactivity of intermediates, which becomes very high, and only trace amounts of CNSB or CPH were detected. Whatever the catalysts, CB, NB, CPH and azo- or azoxydichlorobenzenes are always detected in small amounts ( < 196).

The monometallic Pt/Al2Os and a bimetallic PtZn/AleOs sample (PtZn2) were tested as catalysts for the hydrogenation of CNB at different reactant concentrations. Figure 5 reports the dependency of the initial rate for CNB hydrogenation (t=O) on the CNB concentration. The reaction rate goes through a maximum, then tends to an asymptotic value. An in-depth kinetic study on Pt/Al2Os of various dispersions was performed in a previous work [10]. It was concluded that the kinetics obeys a modified Lang- muir-Hinshelwood model with competitive adsorption between CNB and hydrogen. The adsorption competition no longer holds at high CNB con- centration: on the Pt surface saturated by CNB, there are still Pt sites available for the chemisorption of the smaller hydrogen molecule. A rate law (eqn. (1)) was then proposed which allows determination of both adsorption and rate constants:

50

325

40

o x

~, 3o -6 E

z

.~ 20

10

I i r i I I I 0 10 20 30 l+0 50

CNB concentration (mot/ l xlO0)

Fig. 5. Hydrogenat ion rate as a funct ion of pochloroni t robenzene concent ra t ion over PtXI and PtZn2 samples. Tr= 303 K; (O) PtXI; ( 0 ) PrY_m2.

r = k(AHPH)°'sAR CR/(1 + AR C~(1 - ~)/[3)/[(AHPH) °'s

+ A~ CR/(1 + AR CR(1 - ~ ) / ~ 3 ) ] 2 (1)

where r is the reaction rate (tool g-1 s-1), CR the concentration of CNB (mol 1-1), PH the hydrogen pressure (atm), k the rate constant (mol g - i s - l ) , AR the adsorption constant of CNB (1 tool - I), AH the adsorption constant of hydrogen (atm-1), /3 a nondimensional number (/3~< 1) representing the fraction of Pt surface covered by CNB at saturation.

For all the catalysts the initial reaction rate ( t - -0) was determined at high CNB concentration (CR----0.52 mol 1-I). These values are reported in Table 2, as well as the specific activity per surface metal a tom determined as the turnover number ( s - l ) . The latter was calculated by dividing the reaction rate (tool g-1 s-1) by the concentration of surface Pt atoms (tool g - 1). In Table 2 the activities Of two Pt/AL.O3 catalysts of different dispersion

326

TABLE 2

Reaction rate (tool g- i s - l ) and specific activity per Pt surface atom (s -1) for the hydrogenation ofp-chloronitrobenzene over Pt/A120 s and PtM/AI~.O3 catalysts (M = Sn, Pb, Ge, Al, Zn). Tr = 303 K; CR=0.52 mol 1-1

Sample H/Pt Reaction rate Turnover number (tool g-I s- i × 108) (s- i)

PtVI a 0.26 42.0 0.81 PtVIIP 1.26 2.5 0.24 PtXI 0.99 12.0 0.17 PtSnl 0.71 15.8 0.31 PtSn2 0.64 19.0 0.41 PtSn3 0.41 2.5 0.085 PtPb 0.65 8.8 0.19 PtGe 0.67 22.0 0.46 PtAI 0.69 28.8 0.58 PtZnl 0.63 31.3 0.69 PtZn2 0.62 12.8 0.29 PtSn3ROR b n.d. 8.8 n.d.

"Prom [10]. bSample PtSn3 which has been activated as follows: reduction 473 K--calcination 473 K-reduction 473 K.

[10] we re a lso r e p o r t e d . U p o n addi t ion o f the s e c o n d me ta l M, a c lea r inc rease in specif ic ac t iv i ty up to M / P t = 0.2 (at /at) is obse rved , e x c e p t fo r P tPb. This b e h a v i o u r c o m p a r e s well wi th the r e p o r t o f Ga lvagno e t a l . [5] on the h y d r o g e n a t i o n of n i t robenzene o v e r P tSn/nylon catalysts : a m a x i m u m of act ivi ty is f ound for S n / P t ~ 0 . 1 - 0 . 2 . F o r the h y d r o g e n a t i o n o f p -ch lo - r on i t r obenzene on RuM/AI2Os b imeta l l ic ca ta lys t s [22], we o b s e r v e d a m o n o - tonic e n h a n c e m e n t o f TON up to a ra t io Sn,Pb/Ru n e a r one.

The yie lds in r eac t ion p r o d u c t s a t h igh CNB conve r s ion ( > 98%) are r e p o r t e d in Tab le 3 fo r all the cata lys ts . I t a p p e a r s tha t b e t t e r y ie lds in CA are r e a c h e d for the b imeta l l ic fo rmula t ions . In par t icular , y ie lds h igher t h a n 97% can b e o b t a i n e d on heavi ly l oaded Sn samples . This i m p r o v e m e n t in se lect ivi ty w a s n o t of fse t b y a l onge r r e ac t i on t ime, wh ich did n o t inc rease dramat ica l ly . Such a d e c r e a s e in deha l ogena t i on o f a r o m a t i c h a l o a m i n e s u p o n al loying the ca ta lys t h a s b e e n r e p o r t e d p rev ious ly fo r Raney Ni a l loyed wi th Cr [2, 23] or Pt wi th Cu [3].

D i s c u s s i o n

The resu l t s o f CNB h y d r o g e n a t i o n o v e r PtM/A12Os elicit severa l c o m m e n t s c o n c e r n i n g the kinet ics , the specif ic act iv i ty and the se lect ivi ty wi th r e s p e c t to the p rev ious da t a ob ta ined o v e r p u r e Pt/A1208 cata lys ts .

A s imilar k inet ic law s e e m s to a p p l y fo r m o n o m e t a l l i c PtXI and b imeta l l ic PtZn2 cata lys ts , a n d ve ry l ikely fo r o the r bimetal l ics .

II

c~

0

CO

A

0

0

o 0

0

O m O 0 I ~ I I I 0

o I C ~ I I I I I I I

~ Q Q ~ m o o o o ~ I I I I d

d f d l IIIIII

0 0 0 0 0 0 0 0 0 0

0

0

6

327

328

At low CNB concentration (AHPH >> ARCR), the limiting form of the rate equation (eqn. (1)) is:

r = kAR C R / ( A H P I O 0"5 (2)

At high CNB concentration, the assumption that hydrogen adsorbs on the sites left free by CNB leads to the rate equation:

r = kfl(1 - t ) (3)

The rate and concentration at the maximum are obtained from the first derivatives of eqn. (1):

Cma x ---- (AHPH)0"5/[(1 -- (AHPH)0"5(1 -- ~ ) / ~ ) A R l ( 4 )

"rma x = k / 4 ( 5 )

From eqns. (2)-(5) and the experimental data on reaction rates, the values of k, fl and AR/(AHPH) °'5 Can be deduced for PtXI and PtZn2 catalysts (Table 4). The rate constant k of the surface reaction can be expressed on a per site basis (K=k/(Pt)s; (Pt)s: concentration of Pt atoms per unit weight of catalyst in mol g-1).

When comparing large and small Pt particles, we observed an increase in the specific rate constant of the surface reaction and a decrease in the relative CNB adsorption coefficient AR/(AHPH) °'s [10]. These facts were explained by the higher density of states at the Fermi level on large Pt particles, and the occurrence of a transition state bearing a low negative charge, which will be stabilized on large metal particles [10]. An alternative interpretation was also suggested, based on the requirement of multiply- bonded species during the rate-determining step, these species being dis- favoured on small Pt particles. However, these small changes in catalytic properties between well and poorly dispersed Pt/AI203 catalysts seem to be better explained by subtle electronic modifications of the metal particles rather than by a geometric effect.

At first glance, alloying Pt with Zn has only a small influence on the final catalyst (PtZn2) for the initial kinetics of CNB hydrogenation. Indeed neither K nor A~/(AHPH) °'5 exhibits large changes.

TABLE 4

Characterist ic values of the ra te law (eqn. (1)) for the hydrogenat ion of p-chloroni t robenzene over Pt/A1203 and PtZn/A120 s (PtZn2) catalysts at 303 K

Sample H/Pt k × 10 e K AR/(AHPH) 0"5 (tool g - i s - l ) ( s - l ) (1 tool -1)

PtVP 0.26 290 10.5 0.90 PtVIIP 1.26 32 3.1 2.60 PtXI 0.99 190 2.65 4.95 PtZn2 0.62 140 3.14 4.80

0.88 0.93 0.93 0.90

"F~m [10l.

329

However, when comparing all the bimetallic catalysts it appears that the TON goes through a maximum for formulations where M f P t - 0 . 1 - 0 . 2 (at/at) (Table 2). Such a synergistic effect is usually explained by electronic modifications. In the case of the PtZn2 sample (Zn/Pt = 0.54), a compensation effect between the pre-exponential factor and the activation energy may occur which ¢~bscures the influence of Zn on activity.

As mentioned previously, for most of the bimetallic formulations the activity is enhanced at low M/Pt ratio. Such behaviour was also reported by Galvagno et al. [5] for nitrobenzene hydrogenation over PtSn/nylon catalysts; it was explained in part by a redispersion of the Pt particles. This interpretation cannot be applied in the present work since on the one hand the starting Pt/A1203 sample had a mean particle size of 12/~, on the other hand these Pt particles did not change in size when adding the second metal (Fig. 1). We prefer the second interpretation proposed by Galvagno et al. [5 ], suggesting that the tin species activates the N--O bond, which becomes more reactive towards the attack by chemisorbed hydrogen. This promoter effect is similar to that of electrophilic species on the C=O bond in the hydrogenation of a, fl-unsaturated carbonyls. The pioneer work of Tuley andAdams [24] revealed the positive influence of Fe 2+ and Zn 2+ on Pt. The addition of Fe [25] or Sn [26] to Pt, and Cr to Raney nickel [27] improves the ability of Pt or Ni to produce cinnamyl alcohol. A similar positive effect upon alloying Sn to Rh was reported for hydrogenation of geranial to geraniol [28]. The better selectivity to crotyl alcohol during crotonaldehyde hydrogenation on Ptfrio2 catalysts in the SMSI state was claimed to originate from activation of the C=O bond by TiOx adspecies [29]. Ionic, or partially reduced species, are suggested as the active centres by several authors [24-27, 29]. However, reduced tin was proposed to promote Rh by inducing an electronic modification at the rhodium atoms [28]. With the remarkable exception of this last study, in most of the other works the bimetallic catalysts were prepared according to classical methods by co- or successive impregnation of the support with inorganic salts. It is now well established that these last methods fail to yield homogeneous true bimetallic aggregates. Several studies [30-32] evi- denced that PtSn/AI~.Oa catalysts obtained according to these procedures contain separated Pt and Sn species with tin at various levels of reduction, as well as PtSn bimetallic aggregates with different compositions. The same is true for PtGe/A1203 samples [33]. The so-called 'controlled surface reaction' method used here allows the attainment of true bimetallic aggregates of homogeneous composition between Pt and Pb, Sn, Ge, A1 and Zn. Many studies dealing with the electronic state of both metals in these catalysts concluded that an 'electron transfer ' occurs from Sn [34], Pb [35] or Ge [36] to Pt. In the same way, Boccuzi et aL [37] showed that the reduction of Pt/ZnO at temperatures higher than 593 K leads to PtZn alloy formation, with an increased back-donation in the CO antibonding orbitals during adsorption of carbon monoxide, related to a ligand effect of Zn in the alloy. Thus, for the hydrogenation of CNB o v e r PtM/A1203 we can conclude that an electron-deficient M species is very likely to be the activity promoter at

330

low M/Pt ratio. Never theless , it s eems tha t the p r o m o t i o n of act ivi ty increases with the a p p e a r a n c e o f ionic t in species . This is sugges ted b y the h igher ra te o f hyd r ogena t i on on PtSn3ROR c o m p a r e d to PtSn3 (Table 2). The PtSn3ROR sample is ob ta ined f rom the pa ren t P tSn3 which has unde rgone a r e d o x cycle: r educ t ion in H2 at 473 K-ca lc ina t ion in air at 473 K - r e d u c t i o n in H2 at 473 K. Such a t r ea tmen t p r o d u c e s cer ta in ly superficial suboxide SnO~ tin spec ies in which Sn is in a part ial ly ionic state.

The m o s t in teres t ing po in t deals with selectivity. In the react ion, several undes i r ed p r o d u c t s m a y be formed, such as AN, CB, NB, azo- and]or azoxyd ich lo robenzenes . Wha teve r the na tu re of the s eco n d e lement M added to Pt, the yield in CAN is improved (Table 3). However , w h en compar ing PtSn3ROR and P tSn3 samples , ionic t in spec ies ap p ea r less efficient fo r such p romot ion . The i m p r o v e m e n t depends on the na tu re and the am o u n t of M. In all cases, the p romo t ing effect induces a lower deha logena t ion o f CAN to AN. This behav iour can have one of two explanat ions:

(1) e i the r the relat ive s t rength o f adsorp t ion be tween CNB and CAN increases whe n al loying Pt with the s econd meta l and hence , p-chloroani l ine is easi ly d e s o r b e d f rom the Pt sur face and dechior ina t ion is suppressed ;

(2) or a l loyed Pt par t ic les exhibi t a lower act ivi ty for C - C I bond hydrogenolys is .

In o r de r to check these hypo theses , two expe r imen t s were per formed: first, the hydrodech lo r ina t ion o f CAN over PtXI and PtZn2; second, the hyd rogena t ion o f CNB in p r e sence o f CAN on the same samples .

The ra te o f CAN hydrodech lo r ina t ion over PtXI and PtZn2 is r epo r t ed in Table 5. Small changes in the reac t ion ra te are obse rved u p o n alloying

TABLE 5

Reaction rate (tool g-i s-~) and specific activity (s -1) for the hydrodechlorination of p- chloroaniUne over Pt/A120a and PtZn/Al20s (PtZn2) catalysts, T,=303 K; Cc~=0.13 mol 1-1

Sample Reaction rate TON (tool g-i s-l) (s-l)

PtXI 4 X 10 -8 0.056 PtZn2 3.7 × 10 "e , 0.084

TABLE 6

Effect of p-chloroaniline concentration (Cc~) on the initial rate of p-chloronitrobenzene hydrogenation over Pt/A1203 and PtZn/A1203 (PtZn2) catalysts; Tr=303 K; CRffi0.13 mol 1-1

Sample Reaction rate (tool g-i s-1)x 10~

Cc~=0 Cc~ ffi 1.3 mol 1-1

PtXI 18.8 0.7 PtZn2 13 3.9

331

platinum with zinc. It is dimcult to compare these results with literature data because hydrodechlorination in the aromatic series is not yet well documented. Fouilloux et al. [38] reported a two-fold decrease in the rate of dechlorination of 2,3,4,5-tetracldoroaniline when alloying Pd with Ag (Ag/ Pd: 60/40). For the removal of chlorine from chlorobenzene in the gas phase, alloying Pd with Sn (Sn/Pd-- 0.3) on a Pd/A1203 catalyst decreased the activity by a factor of five [39]. But the effect was lower for 1,2,4-trichlorobenzene hydrodechlorination when Pb, Ge or Sb were added to the s a m e Pd/A1203 catalysts [40]. These few results show that on small Pd particles, at low second metal content, C--C1 hydrogenolysis does not appear highly sensitive to alloying in Boudart 's classification [41 ], in which the rate can be changed by 102-104 for a demanding reaction.

The effect of CAN concentration on the initial rate of CNB hydrogenation over ~ X I and PtZn2 catalysts is reported in Table 6. It appears clear that the inhibiting effect of CAN is much higher on PtXI than on PtZn2 catalyst. A three-fold decrease in activity is observed on the former, whereas it reaches a value of 30 on the latter. To a first approximation we can conclude that the ratio of the adsorption constants ACAN/)tCNB is divided by 10 when Zn is alloyed to Pt. Similar behaviour was observed when increasing the mean particle size of Pt from 10/~ to 100/~, and the selectivity to CAN was higher on the latter [10]. In corollary, the selectivity to CAN was also improved. The decrease in the strength of adsorption of CAN upon alloying Pt with elements possessing electron-donor character suggests that the effect of these promoters of selectivity is to induce a higher electronic density at the Pt sites.

Finally, the preferential formation of CPH, or CNSB, over PtSn3 compared to PtSn3ROR (Fig. 4) would suggest that tin ionic species enhance the reactivity of these intermediates. In this case, it could be questioned whether A1 and Zn in PtA1, PtZn/A1203 catalysts are in a totally reduced state, since the appearance of CPH, or CNSB, during the course of the reaction is also low on these catalysts.

C o n c l u s i o n s

The kinetics of CNB hydrogenation obeys the same modified Langmuir- Hinshelwood model on pure and bimetallic Pt catalysts. Very likely electron- deficient species of the second element promote the turnover frequency of Pt atoms by activating the ni t rogen-oxygen bond. Ionic tin species are better activity promoters but improve the selectivity to CAN to a lesser extent than metallic tin does. Indeed, addition of the second metal improves the yield of aromatic haloamine from 8296 to 97.5% at high CNB conversion (> 98%). The enhancement of CAN selectivity on bimetallic catalysts does not arise from a lower reactivity of the C-C1 bond on these catalysts. It is better explained by a decrease in the relative strength of adsorption between CAN and CNB which favours the desorption of the aromatic amine and thus prevents its dehalogenation.

332

R e f e r e n c e s

1 P. Baumeister, H. U. Blaser and W. Scherrer, in M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. P6rot (eds.), Heterogeneous Catalysis and Fine Chemicals II, Proc. Int. Syrup., Poitiers, lP90, Elsevier, Amsterdam, 1990, p. 321.

2 K. E1 Alj, Thesis, Grenoble, 1990. 3 L. Cerveny, I. Paseka, V. Strnckly and V. Ruzicka, Collect. Czech. Chem. Commun, 47

(1982) 853. 4 Eur. Pat. 0 073 105 (1982) to J. R. Kosak. 5 S. Galvagno, A. Donato, G. Neri and R. Pietropaolo, J. MoL CataL, 42 (1987) 379. 6 A. A. V~rlsmeij~r, A. P. G. Kieboom and H. Van Bekkum, Appl. CataL, 25 (1986) 181. 7 P. N. Rylander, M. Kilroy and V. Coven, Engelhard Ind~ Tech. Bull., 6 (1965) 11. 8 B. Coq, A. Bittar, R. Dutartre and F. Figuerss, AppL CataL, 50 (1990) 33. 9 B. Coq, R. Dutartre, F. Figueras and T. Tazi, J. Catak, 122 (1990) 438.

10 B. Coq, A. Tijani and F. Figueras, J. Mol. CataL, 68 (1991) 331. 11 U.S. Pat. 4455775 (1984) to C. Travers, T. D. Chan, R. Snappes and J. P. Bournonville. 12 J. Margitfalvi, M. Hegediis, S. GSbS15s, E. Kern Talas, P. Szedlacsek, S. Szabo and F. Nagy,

in Proc. 8th Int. Congr. Catalysis, Berlin, lP84, Verlag Chemie, Weinheim, 1984, Vol. 4, p. 903.

13 C. Travers, J. P. Bournonville and G. Martino, in Proc. 8th Int. Congr. Catalysis, Berlin, 1984, Verl~g Chemie, Weinheim, 1984, Vol. 4, p. 891.

14 B. J. Kip, F. R. M. Duivenvoorden, D. C. Koningsberger and R. Prins, J. Catal., 105 (1987) 26.

15 H. G. Aduriz, P. Bodnariuk, B. Coq and F. Figueras, J. Catal., 119 (1989) 97. 16 C. Vertes, E. Talas, I. Czako-Nagy, J. Ryczkowski, S. GSbblSs, A. Vertes and J. Margiffalvi,

Appl. CataL, 68 (1991) 149. 17 J. P. Candy, A. E1 Mansour, O. A. Ferretti, G. Mabilon, J. P. Bournonville, J. M. Basset

and G. Ma~ino, J. Catak, 112 (1988) 210. 18 B. Coq, A. Bittar, R. Dutartre and F. Figueras, J. Cata£, 128 (1991) 275. 19 J. R. Kosak, in W. H. Jones (ed.), Catalysis in Organic Chemistry, Academic Press, New

York, 1980, p. 107. 20 W. Pascoe, in P. N. Rylander, H. Greenfield and R. L. Augustine (eds.), Catalysis of

Organic Reactions, M. Dekker, New York, 1988, p. 121. 21 G. Mestroni, G. Zassinovieh, C. del Bianco and A. Camus, J. MoL Catak, 18 (1983)

33. 22 F. Figueras, A. Tijani and B. Coq, Appl~ Cata£, 75 (1991) 255. 23 V. D. Simonov, T. V. Denisenko, V. I. Savchenko, S. Yu Sklyar and N. M. Ryazanova, Khim.

Prom., 8 (1977) 579. 24 W. F. Tuley and R. Adams, J. Am. Chem. Soc., 47 (1925) 3061. 25 D. Richard, P. Fouilloux and P. Gallezot, in M. J. Phillips and M. Ternan (eds.), Proc. Pth

Int. Congr. Catalysis, Calgary, 1988, The Chemical Institute of Canada, Ottawa, 1988, Vol. 3, p. 1074.

26 Z. Poltarzewski, S. Galvagno, R. Pietropaolo and P. Stalti, J. CataL, 102 (1986) 190. 27 T. Koscielski, J. M. Bonnier, J. P. Damon and J. Masson, AppL Catal., 49 (1989) 91. 28 B. Didillon, A. E1 Mansour, J. P. Candy, J. P. Bournonville and J. M. Basset, in M. Guisnet,

J.Barratdt, C. Bouchoule, D. Duprez, C. Montassier and G. P6rot (eds.), Heterogeneous Catalysis and Fine Chemicals II, Proc. Int. Symp., Poitiers, lPPO, Elsevier, Amsterdam, 1990, p. 137.

29 ,M. A. Vannice and B. Sen, J. CataL, 115 (1988) 65. 30 R. Bacaud, P. Bussiere and F. Figueras, J. Catak, 69 (1981) 399. 31 B. A. Sexton, A. E, Hughes and K. Foger, J. Cata/~, 88 (1984) 466. 32 J. M. Stencel, J. Goodman and B. Davis, in M. J. Phillips and M. Ternan (eds.), Prov. 9th

Int. Congr. Catalysis, Calgary, 1988, The Chemical Institute of Canada, Ottawa, 1988, Vol. 3, p. 1291.

333

33 J. N. Beltramini and D. L. Trimm, in M. J. Phillips and M. Ternan (eds.), Proc. 9th Int. Congr. Catalysis, Calgary, 1988, The Chemical Institute of Canada, Ottawa, 1988, Vol. 3, p. 1268.

34 H. Verbeek and W. M. H. Sachtler, 3.. Catal., 42 (1977) 257. 35 M. S. Kharson, G. B. Kadinov and A. N. Palazov, React. Kinet. CataL Lett., 10 (1979)

267. 36 J. Goldwasser, B. Arenas, C. Bolivar, G. Castro, A. Rodriguez, A. Fleitas and J. Gimm, J.

Catal~, 100 (1986) 75. 37 F. Boccuzi, A. Chiorino, G. Ghiotti, F. Pinna, G. Strukul and R. Tessari, J. CataL, 126

(1990) 381. 38 P. Fouilloux, G. Cordier and Y. Coleuille, in B. Imelik, C. Naccache, G. Coudurier, H.

Praliaud, P. Meriaudeau, P. Gallezot, G. A. Martin and J. C. Vedrine (eels.), Metal-Support and Metal-Additive Effects in Catalysis, Lyon~ 1982, Elsevier, Amsterdam, 1982, p. 369.

39 P. Bodnariuk, B. Coq, G. Ferrat and F. Figueras, J. Catal~, 116 (1989) 459. 40 B. Coq, F. F2gueras and P. Bodnariuk, in F. Gossio, O. Bermfidez, G. del Angel and R.

Gomez (eds.), Proc. 11th Iberoamerican Syrup. Catalysis, Guanajuato, 1988, Instituto Mexicano del Petroleo, Mexico, 1988, p. 1145.

41 M. Boudart, in G. C. Bond, P. B. Wells and F. C. Tompkins (eds.), Proc. 6th Int. Congr. Catalysis, London~ 1976, The Chemical Society, London, 1976, Vol. 1, p. 1.