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ournal of Catalysis 211, 445450 (2002)oi:10.1006/jcat.2002.3720

Kinetics of One-Stage Wacker-Type Oxidation of C2C4 OlefinsCatalysed by an Aqueous PdCl2Heteropoly-Anion System

A. Lambert, E. G. Derouane, and I. V. Kozhevnikov1

The Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom

Received April 2, 2002; revised May 10, 2002; accepted June 11, 2002

The steady-state kinetics of the one-stage Wacker oxidation of

aseous olefins such as ethene, propene, and 1-butene by oxygen

: 2 stoichiometric mixture of O2 and olefin) to form acetaldehyde,

cetone, and methyl ethyl ketone, respectively, was studied in a

micontinuous reactor. The catalytic system involves an aqueous

olution of Pd(II) chloride (0.052 mM Pd(II), [Pd(II)]/[Cl

]=: 50)and Keggin-typeheteropolyanions [PMo9V3O40]6 (50mM),

2050C. Under these conditions in a steady state, the reactiv-

y of olefins increases in the order ethene 1-butene

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46 LAMBERT, DEROUANE, AND KOZHEVNIKOV

ifferent reaction mechanisms are operative in these twoxtreme cases (7). However, little is known regarding thexidation of olefins in the presence of an intermediate levelf chloride except Catalyticas results for the oxidation ofthene (10, 11). Alternatively, the oxidation of olefins cane carried out as a one-stage process, i.e., when steps [1][3]ccur simultaneously in one reactor. Such a process has thedvantage of a much lower concentration of HPA-n in theatalyst, and the stability of the catalyst may be better thanhat in the two-stage process. To the best of our knowledge,o kinetics for the one-stage oxidation of C2C4 olefins by

he O2/Pd(II)/HPA-n system has been reported.The aim of this work is to study the kinetics of the one-

tage oxidation of ethene, propene, and 1-butene by oxygeno acetaldehyde, acetone, and MEK, respectively, using aatalytic system based on Pd(II)/HPA-3, similar to that de-eloped by Catalytica.

EXPERIMENTAL

Materials. The gases ethene, propene, and 1-butene of99% purity were purchased from BOC (N2.0 grade).-Butene (99+%) was purchased from Aldrich. A 50 mMqueous stock solution of the sodium salt of HPA-3,

Na4H2[PMo9V3O40], pH 1.3, was prepared as describedlsewhere (10, 11). A 2.0 mM Pd(II) chloride aqueous solu-on with a molar ratio Pd(II)/Cl= 1 : 50 (NaCl) containing0 mM HPA-3, pH 1.3, was used as a source of palladium.

Kinetic measurement. Oxidation of gaseous olefins wasarried out at atmospheric pressure in a specially designedemicontinuous reactor with intense stirring, which allowedvoidance of diffusion control of reaction rates. The reac-or was a 125-cm3 double-jacketed cylindrical glass vessel

with baffles on the inside wall of the reactor to enhanceasliquid mixing (Fig. 1). A gas flow (olefin + O2) wased into and removed from the reactor at a steady rate,

whereas the products were accumulated inside the ves-el. The reactor was heated by circulating thermostated0.05C) water, and the temperature wascontrolled with a

FIG. 1. Schematic diagram of the reactor setup used for the study ofinetics of gaseous olefin oxidation by O2/Pd(II)/HPA-3 system.

thermometer (notshown). The reaction mixture was stirrusing a PTFE blade attached to a glass rod connected tooverhead stirrer. Mass flow controllers were used to contthe oxygen and olefin flows into the reactor. The gas flout of the reactor was monitored by online GC analy(Varian 3380 instrument, 2-m Carbopak C packed columand TCD) through an automated valve, and the flow rwas measured using a manual bubble flow meter. For retions with 1-butene, the olefin feed line was heated to 40to prevent condensation of 1-butene in the line.

The following conditions were used for measuring treaction kinetics: [Pd(II)]= 0.050.2 mM; [Pd(II)]/[Cl]1 : 50 (NaCl); [HPA-3]= 50 mM; pH 1.3; stoichiometric mlar ratio O2/olefin= 1:2inthegasfeed(4.0cm

3/min O2 a8.0 cm3/min olefin); T= 2050C; and the volume of retion solution, 100 cm3. The pH was adjusted to 1.3 at eatemperature by adding NaOH or H2SO4. In a typical expiment ([Pd(II)]= 0.10mM),95cm3 of HPA-3 stock solutwas introduced into the reactor thermostated to a requir

temperature with slow stirring (250 rpm), and a 1 : 2 sichiometric gas mixture of oxygen (4 cm3/min) and ole(8cm3/min) was fed into the reactor to replace air until costant gas-phase composition was achieved, as monitoredGC. Once the system equilibrated, the stirring was stoppand 5.0 cm3 of the 2.0 mM Pd(II) stock solution was addStirring was then resumed at a speed 1200 rpm, and Ganalysis was automatically performed every 5 min. The flow out was measured every 5 min as well.GC analysis wabsolute calibration for O2 and olefins in conjunction wflow measurements in and out of the reactor permitted t

calculation of the amount of olefin and oxygen consumin the reactor, using the perfect-gas equation. The rategaseous olefin or O2 consumption (mol/dm

3 min) was cculated as a difference of moles of the reactant in and oof the reactor per unit time divided by the volume of treaction mixture (0.100 dm3). The oxidation products wnot studied in detail. Occasional GC tests confirmed ththe main products were acetaldehyde, acetone, and MEfrom, respectively, ethene, propene, and 1-butene, as pected. Small amounts of acetic acid were also found.

RESULTS AND DISCUSSION

Catalyticas typical catalyst solution for the two-stageidation of ethene at 110130C and 10-bar pressure consof an aqueous solution with 0.1 mM Pd(II), 525 mM Cand 300 mM HPA-n (n= 23)atpH1 (10, 11). The HPAsolutionis a complex equilibrium mixture of Keggin polyions containing one to four vanadium atoms (10, 11, 13, 1Hence the formula {PMo12nVn} refers only to the averacomposition of the solution. It was suggested (7,10) thatmolybdophosphate matrix serves two purposes, namely

solubilise high levels of V(V) in acidic aqueous soluti(VO+2 has a limited solubility at low pH) and to provide

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WACKER-TYPE OXIDATION OF OLEFINS 4

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120 140

Time, min

p

,

Period used for rate measurement

FIG. 2. Amount of 1-butene () and oxygen () consumed versus

me. An induction period for O2 consumption can be seen. Reaction con-

tions: 50 mM HPA-3, 0.10 mM Pd(II), 5.0 mM Cl, 30C, 1500-rpm

irring speed, 4.0 cm3/min oxygen, 8.0 cm3/min 1-butene.

he rapid oxidation of V(IV) by O2 (VO2+ is oxidised veryowly at low pH).The typical catalyst solution used in this study for the

ne-stage oxidation consisted of 0.10 mM Pd(II), 5.0 mMl, and 50 mM HPA-3. The ratio Pd(II)/Cl= 1 : 50 gave

he best results in Catalyticas system. The HPA-3 concen-ation was lowered to 50 mM, which was sufficient for thene-stage oxidation. This allowed for a greater solubilityf olefins and oxygen in the catalyst solution compared toatalyticas system due to reducing the salt-out effect on

he gas solubility.

Figure 2 shows typical plots for the consumption of-butene and O2 as a function of time under the precedingonditions. An induction period of 5 to 10 min for the O2onsumption is observed, as expected for a consecutive re-ction. The flow out drops immediately once the Pd(II)as been introduced (zero time) (Fig. 3). As seen from GCounts, this is mainly because of olefin absorption, whichauses a compensating increase in O2 concentration in the

7400000

7500000

7600000

7700000

7800000

7900000

8000000

8100000

8200000

0 20 40 60 80 100 120 140

Time, min

GCCountsforButene

5

6

7

8

9

10

11

12

13

GasFlowOut,cm

3/m

in

1320000

1340000

1360000

1380000

1400000

1420000

1440000

0 20 40 60 80 100 120 140

Time, min

GCCountsforOxygen

5

6

7

8

9

10

11

12

13

3

FIG. 3. 1-Butene (left) and oxygen (right) concentrations in the gas flow (GC counts) versus time online. Reaction conditions: 50 mM HPA10 mM Pd(II), 5.0 mM Cl, 30C, 1500-rpm stirring speed, 4.0 cm3/min oxygen, 8.0 cm3/min 1-butene.

gas flow. The red catalyst solution turns dark green, indicing reduction of HPA-3. The system reaches a steady stain 4050 min with an average degree of HPA-3 reducti 0.98) were used for calculating the reactirates.

To eliminate diffusion control the reaction was pformed at different stirring speeds. At speeds 600 rpno significant increase in reaction rate was observed fany of the olefins at 2050C (Fig. 4), indicating that tdiffusionof olefins into thecatalyst solutionwas fast enouto avoid mass-transfer limitation. However, the oxygen d

fusioncouldstill affectthe reaction regimeat stirringspeeover 600 rpm. This was shown by an incremental increaof stirring speed while monitoring the gas flow rate acomposition (Fig. 5). Each increment caused a drop in bothe flow rate and O2 composition, the latter being offset an increase in olefin content. An additional amount of Owas therefore consumed to reach another steady state wa lower degree of reduction of HPA-3. This effect weaened as the stirring speed increased, and at stirring spee1000 rpm stirring ceased to affect the reaction regime. Fthis reason, rate measurements were performed at stirri

speeds1200 rpm. The slower O2 transfer comparedto thof olefin was also found by Grate et al. (10).The oxidation of 1-butene was complicated by t

double-bond migration to form 2-butene, especially lower temperatures. The 1-butene feed did not contain a2-butene. In the steady-state oxidation at 20C, the flow ocontained 1-butene and 2-butene in a ratio of 5 : 1 to 10which was determined by GC. In the absence of palladiu

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48 LAMBERT, DEROUANE, AND KOZHEVNIKOV

0 200 400 600 800 1000 1200 1 400 1600

0.0

0.2

0.4

0.6

0.8

1.0

1.2

ReactionR

ate,mmol/dm

3min

Stirring speed, rpm

FIG. 4. Olefin ()andoxygen() reactionratesversus stirring speed:thene (broken lines), propene (solid lines), and 1-butene (dotted lines).

eaction conditions: 50 mM HPA-3, 0.10 mM Pd(II), 5.0 mM Cl, 30C

50C for butene), 4.0 cm3/min oxygen, 8.0 cm3/min olefin.

o double-bond migration was observed which rules outhe acid-catalysed isomerisation of 1-butene. The butenes

were also collected from the flow out by saturating a CDCl3olution and subjected to 1H NMR analysis, which clearlyhowed the presence of both isomers. The double-bond mi-ration is suggested to occur via-allyl complexes (Eq. [7])4). The formation of-allyl Pd(II) complexes may affecthe kinetics of 1-butene oxidation (see the following discus-ion):

[7]

Table 1 shows thekinetic data for theoxidation of ethene,ropene, and 1-butenewith O2.Mostofthesedataaremeanalues of three or more parallel measurements. The results

nclude the steady-state rates of absorption of olefins andO2 as well as the initial rates of olefin absorption in theemperature range of 2050C and [Pd(II)]= 0.050.2 mM.

The reactions were typically carried out until 12 V(V)urnovers (7501500Pd(II) turnovers). For comparison, theate of stoichiometric oxidation of olefins by Pd(II)/HPA-3n the absence of O2 was measured. The measurement oflefin absorption was more accurate than that of O2, hence

he rates discussed here were typically obtained from thelefin absorption unless stated otherwise.

Figure 6 shows the time course for the stoichiometric oxi-ation of olefins by Pd(II)/HPA-3 in the absence of O2.

The absorption of olefins was calculated both from theombined GC and flow-rate measurements and from theow-rate measurements only. Both methods gave practi-

ally the same results, which proves the accuracy of ourmethodology. The curves for ethene and propene fit well

TABLE 1

Kinetics of Oxidation of Gaseous Olefins

by O2/Pd(II)/HPA-3 Systema

Reaction rateb (mmol/dm3 min)Pd(II)

(mM) T (C) Ethene O2 Propene O2 1-Butene

0.05 30 0.203 0.116 0.613 0.339 0.243 0

0.10 20 0.145 0.09 1.06 0.510.10c 20 0.16 1.17

0.10 30 0.413 0.243 1.24 0.685 0.475 0

0.10c 30 0.461 1.56 0.878

0.10c,d 30 0.632 1.58 0.929

0.10 40 1.12 0.70 1.97 1.16 0.55 0

0.10c 40 1.48 2.64 1.27

0.10 50 2.07 1.26 3.16 1.74 0.622 0

0.10c 50 2.73 3.56 1.47

0.20 30 0.755 0.439 2.29 1.19 0.860 0

a Reaction conditions: 50 mM HPA-3, [Pd(II)]/[Cl]= 1 : 50 mol/m

4.0 cm3/min oxygen, 8.0 cm3/min olefin, 12001500-rpm stirring speedb The rates measured by consumption of olefin and O2 for a time

stream of 6075 min.c Initial rate for the time onstream of 015 min.d Reaction in the absence of O2: 50 mM HPA-3, [Pd(II)]/[Cl

1 : 50 mol/mol, 4.0 cm3/min nitrogen, 8.0 cm3/min olefin.

1250000

1270000

1290000

1310000

1330000

1350000

1370000

1390000

0 50 100 150 200 250 300 350 400 450

Time, min

GCCounts

forOxygen

7

8

9

10

11

12

13

250 rpm 500 rpm 750 rpm 1000 rpm

6050000

6100000

6150000

6200000

6250000

6300000

6350000

0 50 100 150 200 250 300 350 400 450

Time, min

GCCountsforPro

pene

7

8

9

10

11

12

13

250 rpm 500 rpm 750 rpm 1000 rpm

FIG. 5. Gas flow out and its oxygen (top) and propene (bottocontents as a function of time and stirring speed. Reaction conditio

50 mM HPA-3, 0.10 mM Pd(II), 5.0 mM Cl, 50C, 4.0 cm3/min oxyg

8.0 cm3/min propene.

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WACKER-TYPE OXIDATION OF OLEFINS 4

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250

Time, min

Vanadium (V)

Capacity

Propene

Ethene

Butene

FIG. 6. Olefin consumption from GC and flow measurement () andow measurement only () as a function of time for the stoichiomet-

c oxidation of olefins by Pd(II)/HPA-3 system. Reaction conditions:0 mM HPA-3, 0.10 mM Pd(II), 5.0 mM Cl, 30C, 1500-rpm stirring

eed, 4.0 cm3/min nitrogen, 8.0 cm3/min olefin.

ith zero-order kinetics in vanadium ([Pd(II)]= const) un-l almost complete reduction of vanadium(V) in HPA-3,s expected. Similar results were obtained for the oxida-on of ethene by Pd(II)/HPA-3 by Grate et al. (10, 11).he oxidation of 1-butene follows a more complex courseith a monotonous decrease of the reaction rate, althoughreaches the complete conversion of V(V) to V(IV). The

ifferent behaviour of 1-butene may be explained by theomerisation of 1-butene to less-reactive 2-butene and by

he formation of relatively strong -allyl Pd(II) complexes

Eq. [7]), which could decrease catalyst activity toward-butene oxidation.

The steady-state reactions of all the olefins are first-ordern Pd(II) (Fig. 7). The reaction orders in olefin and O2 wereot measured here; Grate et al. (10, 11) found the order

0

0.5

1

1.5

2

2.5

0 0.05 0.1 0.15 0.2 0.25

[Pd(II)], mM

Rate,mmol/dm

3min

C2H4

C3H6

C4H8

FIG. 7. Reaction rate versus Pd(II) concentration (50 mM HPA-3,

10 mM Pd(II), 5.0 mM Cl, 30C, 4.0 cm3/min oxygen, 8.0 cm3/min

efin).

to be first in both olefin and O2 for the two-stage oxidtion of ethene. The reactivity of olefins in the steady-staoxidation is found to be in the following order: ethene1-butene < propene; but the total range is only a factor oat30C. The same order was obtained for the initial ratesstoichiometric oxidation by Pd(II)/HPA-3 (Table 1, Fig. These rates are close to the steady-state rates of the corrspondingcatalyticreactions, althoughsomewhatlower. Tdifference between the two rates is probably because pof palladium in the steady-state catalyst system is presentinactive Pd(0). The effect of olefin structure on the reactirate is quite unexpected. For the stoichiometric oxidatiwith PdCl24 at [Cl

] > 0.1 M and 25C, Henry (15) foua different order: ethene > propene > 1-butene; the torange was a factor of 6. This order is mainly governed the steric hindrance of the olefin structure to the Pd(attack. In our system, Pd(II) mainly exists as PdCl2 aPdCl3 (with [PdCl

3 ]/[PdCl2]= 1.3 at 5 mM [Cl

]) (10, 1ThesePd(II) species aremore electrophilicthanPdCl24 a

hence more reactive toward the more nucleophilic propeand 1-butene than toward ethene. Also at the low chloriconcentrations, more than one olefin might bind to Pd(producing an unreactive Pd(II) species, which would hapen more readily with ethene. It should be noted that inchloride-free system, the highly electrophilic Pd2+ aqua-ioxidises propene significantly faster than ethene, as fouby Matveev et al. (17).

From the steady-state reaction rates of gaseous olefinsthe temperature range of 2050C (Table 1) apparent acvation energies E were calculated to be 71 7, 29 4, a

11 4 kJ/mol for ethene, propene, and 1-butene, respetively. The corresponding E values estimated from the itial rates (before the steady state had been reached) are 730, and 24 kJ/mol. The two values for ethene and propeare quite close and also similar to the activation energi88 (18) and 44 kJ/mol (19) for the oxidation of ethene apropene in the Wacker system (PdCl2/CuCl2). The reasfor the low activation energy for the steady-state oxidatiof 1-butene is not clear. Diffusion control can be ruled obecause of the high efficiency of mass transfer in the sytem. Apparently, the complexity of the system associatwith the formation of the -allyl Pd(II) complexes, as eidenced by the double-bond migration in 1-butene, coube the cause. If the enthalpies of solubility of the olefiwere taken into account, the activation energies thus otained would be higher (by ca. 20 kJ/mol). Unfortunatethe solubilities of olefins in the catalysts solution are navailable. Other contributions to the activation energfrom a number of equilibria involved in the process, suas PdCl2 +Cl

PdCl3 , olefin-Pd(II) -complexes form

tion, etc., should be noted.Another interesting result is that in the steady-state sy

tem the oxygen absorption rates are always slightly high

(by 425%) than those expected from the 1 : 2 O2/olestoichiometry (Fig. 8). The surplus rate is in the ord

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50 LAMBERT, DEROUANE, AND KOZHEVNIKOV

0

1

2

3

4

0 0.5 1 1.5 2

O2 Absorption Rate, mmol/dm3

min

OlefinAbsorptionRate,mmol/dm

3m

in

ethene

propene

1-butene

FIG. 8. Olefin absorption rate versus O2 absorption rate (50 mMHPA-3, 0.10 mM Pd(II), 5.0 mM Cl, 2050C, 4.0 cm3/min oxygen,

0 cm3/min olefin). The straight line corresponds to 2 : 1 stoichiometry.

thene 1-butene > propene, which correlates with thexidisibility of the corresponding products (acetaldehyde MEK> acetone). This may be explained by the co-

xidation of HPA-3 blue and the products by O2, leadingo overoxidation of products. Indeed, some acetic acid wasound among the products by GC analysis. It was shownhat the oxidation of HPA-n blues with a low average de-ree of reduction (