Al2O3 hydrogenation catalysts

Applied Catalysis, 52 (1989) 19-32 Elsevier Science Publishers B.V., Amsterdam -

19 Printed in The Netherlands

Enantioselective Hydrogenation of a-Keto Esters: Temperature-Programmed Reduction Study of Liquid-Phase Pt/Al,O, Hydrogenation Catalysts H.U. BLASER, H.P. JALETT and D.M. MONTI* Central Research Laboratories, Ciba-Geigy AG, CH-4002 Basle (Switzerland) and J.T. WEHRLI Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Ziirich (Switzerland) (Received 10 October 1988, revised manuscript received 8 March 1989)

ABSTRACT The effect of thermally pretreating Pt/A1203 catalysts in hydrogen on enantioselectivity and activity in the liquid-phase hydrogenation of cY-ketoesters and the structure and oxidation state of the catalysts was studied. Thermal treatments in hydrogen at 673 K gave 1520% higher optical yields of the Lu-hydroxyesters produced. In addition, the activity of the catalysts was up to three times higher for the activated catalysts. The processes taking place during the pretreatment of commercial and laboratory-made catalysts were characterized by temperature-programmed reduction (TPR), carbon monoxide and nitrogen adsorption and X-ray diffraction. The TPR profiles showed three well resolved peaks which were assigned to a platinum oxide shell on a platinum core (200-400 K), residual platinum salts (400-550 K) and carbon deposits that originate from the liquid-phase preparation processes (600-700 K). The amount and reducibility of the platinum oxide was determined by TPR and correlated with the activity of the catalysts in the sense that strongly oxidized catalysts show much lower activities. There is no simple correlation between the improvement in selectivity and the processes taking place during thermal activation, but there are indications that residual platinum salts have a negative influence on enantioselectivity.

INTRODUCTION

In the first reports on modified platinum catalysts for the enantioselective hydrogenation of a-keto esters, Orito and co-workers [ 1,2] showed that pretreating the platinum on carbon catalysts in either hydrogen or nitrogen gave higher optical yields for the resulting cu-hydroxy esters. This effect was later confirmed for Pt/A1203 catalysts [ 31. In the hydrogenation of ethyl benzoyl formate with a Pt/A1203 catalyst, the optical yield was only 34% without preheating in hydrogen. Thermal treatment of the same catalyst in hydrogen at temperatures above 573 K gave enantioselectivities higher than 80% with a maximum of 84% for treatment at 673 K. In a later systematic study [4] of

0166-9834/89/$03.50

0 1989 Elsevier Science Publishers B.V.

20

0 5% WA1203 RI

0 SUBSTRATE

R~= cn3, c6nScnZcn2 uz= cnZcn3

MODIFIER

R3=CHXH2 R3 =

ckiZcn3

Cinchonidine IO,ll-Dihydrocinchonidine

Fig. 1. Enantioselective

hydrogenation of a-keto esters.

Pt/C catalysts that had been treated with acetic acid in the last preparation step, thermal treatment in hydrogen only gave an increase in selectivity from 83% to 89%. In spite of the obvious advantages of the thermal pretreatment step, little work has been done to investigate the reasons for the effects observed. The only explanation given by Orito et al. [3] was that oxygen adsorbed on platinum is removed. We have recently reinvestigated this modified catalyst system [ 51 (see Fig. 1) and our results confirmed that the pretreatment step is more important for Pt/Al,O, than for Pt/C catalysts as far as selectivity is concerned. In addition, it was shown that after thermal treatment the reaction proceeded up to ten times faster. However, we also found [6] that very high temperatures ( > 773 K) in either hydrogen or air, where sintering effects become important, must be avoided in order to maintain optimal performance of the catalysts. This was surprising because under controlled preparation conditions both activity and enantioselectivity are higher for catalysts with larger platinum particles [6]. These observations prompted us to carry out the investigations described here. Pt/A1203 catalysts have been extensively investigated by temperature-programmed reduction (TPR) [ 7-121. However, most of the investigations were carried out on catalysts for petrochemical and automotive exhaust applications with low platinum loadings. Yao et al. [lo] and Huizinga et al. [ 121 studied Pt/A1,03 with higher platinum contents (up to 21.1% and 5.2%, respectively). Liquid-phase hydrogenation catalysts with platinum concentrations around 5% have received much less attention. These catalysts are commonly reduced in the liquid phase under relatively mild conditions. As a result, the dispersion of these catalysts is usually medium to low (CO/Pt < 0.5). The purpose of this investigation was to study the processes that take place during the thermal treatment in hydrogen and to identify the changes that have an effect on selectivity and activity in the enantioselective hydrogenation of cu-keto esters.

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EXPERIMENTAL

Catalysts Two commercial Pt/A1203 catalyst (Engelhard E 4759 and Johnson Matthey JMC 5R94), which showed a very good performance for this hydrogenation reaction, were used as received. As an example of a well characterized catalyst, the standard Pt/SiO, catalyst EUROPT-1 [ 131, was ground into a fine powder and used in the hydrogenation experiments. Three laboratory catalysts were made as follows on a commercial y-A1203 (Alox GX, Martinswerke) with a surface area of 130 m2/g and a pore volume of 0.21 ml/g [6]. The catalyst precursors were prepared by the incipient wetness method from aqueous H2PtC1,*6H,0 (Ventron) solutions, resulting in a final platinum loading of 5% [ 61. The dry precursor material was reduced in the liquid phase with sodium borohydride (catalyst A), sodium formate (B) and formaldehyde (C). The reduction conditions were as follows: catalyst A, 0.1 g sodium borohydride, 45 ml water, 2 g precursor, 278 K (30 min) and 298 K (1 h); catalyst B, 0.523 g sodium formate, 30 ml water, 3 g precursor, 363 K (1 h); catalyst C, 0.174 ml formalin (40% formaldehyde), 20 ml water, 1.38 ml sodium hydroxide (20% ) and 1.5 g precursor, the precursor being added at 328 K, then heated for 2 h at reflux. All catalysts were filtered and washed free from sodium with water and then with 20% acetic acid at 323 K as described by Orito et al. [4]. Before the catalysts were dried at 393 K in air, they were thoroughly washed again with water. In a series of control experiments, the commercial catalyst JMC 5R94 was also treated with acetic acid, washed with water and dried in the same way as the laboratory samples. For the identification of TPR peaks, reference precursor materials were prepared on the same support. The salts used were K,PtCl,, H,Pt(OH),, [ ( NH3 14Pt] (NO, ) 2 and [ ( NH3 ) qPt ] C12.These precursors were dried at 393 K and used in the unreduced state for the TPR experiments. Hydrogenation experiments A detailed description of the hydrogenation experiments was given in previous papers [ 5,6]. Activity and selectivity were determined under the following standard conditions: 0.4 mol ethyl pyruvate, 0.5 g catalyst (standard thermal treatment: 2 h in hydrogen at 673 K), 0.1 g cinchonidine added directly to the reaction solution, 75 ml ethanol, 293 K, constant hydrogen pressure 70 bar and agitator speed 1200 rpm. In some experiments ethyl 2-oxo-4-phenylbutyrate was used instead of ethyl pyruvate under the following conditions: 0.2 mol ester, 0.2 g catalyst, 0.012 g lO,ll-dihydrocinchonidine, 75 ml toluene, 293 K, constant hydrogen pressure 100 bar and agitator speed 1200 rpm. Conversion was checked by gas chromatography (GC) (2-m OV-101 column, 423 K). The enantiomeric excess was determined from the optical rotation of the distilled

22

product with a reference of [a] 546= 12.4” (2O”C, neat) for @)-ethyl lactate and [cu]n= -11.4” (20°C neat) for ethyl (R)-2-hydroxy-4-phenylbutyrate. The optical yield is expressed as the enantiomeric excess (ee) of the Renantiomer:

Catalyst characterization

The catalysts were characterized by TPR, pulse carbon monoxide chemisorption, X-ray diffraction (XRD) and total surface area measurements (BET). The combined TPR and pulse adsorption apparatus was similar to that described by Serrano and Carberry [ 141. The apparatus consisted of a gas dosing section equipped with electronic thermal mass flow controllers for hydrogen (99.9997% ) , argon (99.9999% ), helium (99.9996% ) and air. All gases were of ultra-high purity and were passed over Oxisorb cartridges to eliminate traces of oxygen and water. With the help of the manifold section, the operation mode (TPR, TPD, pulse chemisorption) could be selected and the desired gas for pulse adsorption loaded into the sampling valve (Valco, six-port valve with calibrated volumes from 0.25 to 5 ml). The reactor section consisted of either a Pyrex or a quartz glass U-tube reactor. The inlet tube of the reactor was of 8 mm I.D. and the outlet tube (after the sample) 2 mm I.D. Care was taken to minimize the volume of tubing between the reactor and the detector. After passage through a U-tube filled with glass beads, which was used to trap water from the reduction processes, the gases were directed to the analysis section. A thermal conductivity detector (Gow Mac, Model 10-454) thermostated at 428 K was used, the signal from which was recorded by a data acquisition and control unit (HP 3421A) which was linked to an HP 86B computer. This arrangement allowed direct data acquisition and data reduction (integration, determination of heating rates, calculation of dispersions, etc.). The detector was periodically calibrated by reducing known amounts of dry copper (II) oxide (Ventron, ultrapure) and a very good reproducibility ( + 2% ) was maintained. TPR experiments were run under the following standard conditions: 6% hydrogen in argon; total flow-rate, 1.25 ml/s; sample weight 0.5 g; and heating rate, 0.167 K/s. For noble metal catalysts that had passed through a reductionreoxidation cycle, reduction started below room temperature. Huizinga et al. [ 121 suggested a method to determine the hydrogen consumption during the switching period from argon to hydrogen-argon. In our experiments, however, we found that in most instances a starting temperature of 195 K was low enough to avoid this problem. Carbon monoxide pulse chemisorption was carried out at 298 K on samples

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that were reduced with hydrogen at 673 K for 2 h. A surface stoichiometry CO/ Pt, = 1 was assumed. BET surface areas were measured at 77 K with nitrogen and an area of 16.2 A” was assumed for an adsorbed nitrogen molecule. Bulk structural changes were characterized with a Philips diffractometer and Cu Kcu radiation. RESULTS AND DISCUSSION

Hydrogenation experiments The influence of the standard thermal treatment in hydrogen on the activity and selectivity of the catalysts is summarized in Table 1. For the commercial catalysts E 4759 and JMC 5R94, the reaction was two to three times faster and the optical yield 1519% higher for the thermally treated samples. The same trend was observed for the laboratory-made catalysts A-C, which had much lower dispersions and lower rates, and for the EUROPT-1, but the changes were not as pronounced. Microanalyses of the residual chlorine and the carbon content are also reported in Table 1. It can be seen that the chloride-ion content is unaffected by the thermal treatment, whereas the carbon content is lowered to about 10% of the original value. TABLE 1 Effect of thermal treatment in hydrogen (2 h, 673 K) on activity and selectivity in the liquidphase hydrogenation of ethyl pyruvate (standard conditions), Cl content and C content of the catalysts. Catalyst

CO/Pt

Thermal treatment in H,

Initial rate (mol/kg sl

e.e. (W)

Cl content (wt.-%)

C content (wt.-%)

Pt content (wt.-%)

E 4759 E 4759 JMC5R94 JMC 5R94 Cat. A Cat. A Cat. B Cat. B Cat. C Cat. C EUROPT-1 EUROPT-1

0.24 0.22 _

No Yes No Yes No Yes No Yes No Yes No Yes

0.66 2.22 1.65 3.42 0.08 0.13 0.10 0.16 0.27 0.33 1.95 2.67

53 72 55 70 48 60 49 64 58 72 52 65

0.13 0.14 0.41 0.40 0.41 0.39 -

_

5”

9.09 0.04 0.08 0.60

“Nominal content as given by manufacturers.

5” 0.45 0.06 0.58 0.06 0.65 0.06 _

4.2 4.6 4.1 6.3

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Temperature-programmed reduction (TPR) The reduction profiles of the two commercial 5% Pt/Al,O, catalysts are presented in Fig. 2. Three well resolved peaks, which could be integrated separately, with maxima near 320,470 and 650 K were found for both samples. The hydrogen consumption is expressed as the ratio of hydrogen consumed per metal atom (HJPt value). The differences between the two catalysts are small, suggesting that similar processes are taking place during thermal activation. The reduction of the “as-received” EUROPT-1 catalyst takes place in one step with a peak maximum at 340 K and a H,/Pt value of 0.9, in accordance with results obtained by Bond and Gelsthorpe [ 151. The TPR profiles of the catalysts A, B and C are presented in Fig. 3. Catalyst A (reduced with NaBH,) was the only one showing a three-peak pattern comparable to those in Fig. 2. For the catalysts reduced with sodium formate (B) and formaldehyde (C ) the peak at 450-500 K was not observed. All three catTPR

0.40

PM

300

400 TEW

500

em

700

CKI

Fig. 2. TPR profiles of two commercial Pt/A1203 hydrogenation catalysts: 5R94. H2/Pt values are given for each peak.

(A) E 4759; (B) JMC

TPR 1.99, I

TEMP

\ \

tK1

Fig. 3. TPR profiles and H,/Pt values of the three laboratory-prepared catalysts, A, B and C.

25

alysts showed a large high-temperature peak with a maximum at 670-680 K. The amount of hydrogen consumed for this peak was 4-5 times higher compared with the commercial samples. Assignment of TPR peaks Low-temperature peak For the identification of the peak at low temperatures the following reduction-oxidation experiments were carried out. Catalyst JMC 5R94 was reduced with a TPR run up to 730 K and then cooled to 298 K, at which three pulses (5 ml each) of dry air were passed over the sample. After cooling to 195 K a TPR profile was recorded (trace A in Fig. 4). The second profile (B ) was obtained after oxidation at 623 K in an air stream for 2.5 h. In both instances only the low-temperature peak reappeared but with TM shifted to lower temperatures (TM= 233 and 244 K, respectively, compared with TM=306 K for the untreated catalyst ). This temperature shift has also been reported by Huizinga et al. [ 121. In agreement with their model, the TPR peaks below 400 K are assigned to the reduction of a platinum oxide shell on the surface of a metallic platinum core. The H2/Pt values for the low-temperature peaks were always below 1. If we assume a surface oxide stoichiometry of Pt : 0 = 0.5, this means that even after long oxidation times more than half of the platinum is still in the metallic form. Medium-temperature peak Based on the reference TPR profiles presented in Fig. 5 and the data for the unsupported salts published by Hurst et al. [ 161, we ascribe the TPR peaks in the medium-temperature region (400-500 K) to the reduction of residual platinum salts originating from the preparation process.

ZLJO

300

400 TEMP

500

600

700

IKI

Fig.4. TPR profilesandH,/Pt valuesof JMC5R94: (A) afterreoxidation at 298 K withthree5mlairpulses;(B) afterreoxidationat 623 K in a flowof airfor 2.5 h.

26 TPR

0.0

~~..‘~~~.‘~.~~I’~““‘.*‘.‘.“.~~.‘..’c-l 300

350

400

450

TEMP

500

550

600

650

700

[Kl

Fig. 5. TPR profiles of dried precursor materials. (A) [ ( NH3)4Pt] Cl,/Al,O,; (B) H,PtCl,/Al,O,; (C) KzPtCl,/AW~; (D)HJ’t(OHMAW,; (El [(N&)&l (NO&/A&.

Some remarks concerning the nature of these Pt species should be made. The precursor material obtained from HzPtCls showed a peak with a TM of 485 K and a H2/Pt value of 1.58 (Fig. 5, trace B). The regular shape of this peak suggests that the platinum species exhibit uniform reactivity towards hydrogen. Lieske et al. [ 111 proposed that the oxychloride surface species [Ptn (OH),Cl,], and [PtrVO,C1,], lead to TPR peak maxima at 533 and 563 K, respectively. Owing to a water content of 4-5% we propose that a partially reduced hydroxy species is present in our reference material prepared from H,PtCl,. The medium-temperature peak was never observed for thermally pretreated samples or for catalysts that were reoxidized after a first TPR run. We assume that during these treatments the adsorbed platinum salts are converted to platinum metal. High- temperature peak During the TPR experiment with catalyst C the effluent gas was analysed by GC. Methane was the main organic component in the temperature range 680-700 K. The high-temperature peak was particularly pronounced for the laboratory-made catalysts washed with acetic acid. A sample of JMC 5R94 that had been washed with acetic acid or ethanol prior to TPR also showed an enhanced intensity of this peak. On the other hand, if only the support material without platinum was treated with ethanol there was no signal around 670 K.

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These experiments and the decrease in carbon content observed after thermal treatment (see Table 1) lead to the conclusion that the TPR peaks in the 650-700 K range are due to the hydrogenation of carbon deposits which can originate either from the reduction step during catalyst preparation or, in the case of the laboratory-made catalysts, from the acetic acid treatment. Minor amounts of organic matter can also be adsorbed from the atmosphere when the pretreated catalysts are stored without special precautions. Concerning the location of the carbon deposits, it is possible to draw some conclusions from a combination of TPR and carbon monoxide chemisorption experiments summarized in Table 2. The following effects were observed: (i) The amount of CO adsorbed did not change when the catalyst JMC 5R94 was reduced in hydrogen for 2 h at 473,573,673 or 773 K. (ii) After the acetic acid treatment of the same catalyst and reduction at 473 K (i.e., below TIMof the high-temperature peak), the high-temperature peak is still present. The amount of carbon monoxide absorbed for this sample was CO/Pt = 0.23, which is the normal value. (iii) After reduction at 673 K (i.e., above TM of the high-temperature peak) of the acetic acid-treated catalyst, no signals were detected in the TPR experiment and the carbon monoxide uptake was the same as for the other samples. From this we conclude that the improved performance of the catalysts is not a consequence of an increase in platinum particle size. Sintering of platinum at 773 K in hydrogen is obviously not fast enough for this catalyst to result in a measurable decrease in dispersion [ 61. These results also show that hydrogenation of the carbonaceous deposits occurs only at or above temperatures used for the thermal pretreatment and that these deposits either do not cover TABLE 2 Effect of different pretreatments on the TPR profiles and CO chemisorption of catalyst JMC 5R94 Treatment” of catalystb

(H,/Pt),

(H,/Pt),

Hz, 473 K H2,573 K HP, 673 K H,, 773 K CH,COOH (50°C) CH,COOH + HB,473 K CH,COOH + HP, 673 K

_ _

_

0.54 0.04 0

_ 0 0 0

(Hz/Pt)s

co/pt 0.21 0.25 0.22 0.22

2.34 1.91 0

0.23 0.21

OH*,473 K indicates reduction in hydrogen at 473 K for 2 h. bCH,COOH (50°C) indicates that the catalyst was treated with 20% acetic acid at 50°C for 5 min, washed with water and dried at 120°C for 16 h. “1, Low-temperature peak; 2, medium-temperature peak; 3, high-temperature peak.

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the platinum surface (i.e., must be located on the support) or they are in a form that does no block many adsorption sites. Based on our experimental data it is not possible to speculate on the structure of the carbon deposits. A comparison with standard Pt/A1203 hydrotreating catalysts is not practicable because the source of the carbon and the catalyst preparation processes are completely different. Further investigations are needed to clarify this point. Other characterization methods We did not detect any changes in the total surface area or the pore volume before and after the standard thermal treatment. Also, the pore size distributions remained unchanged. Thermogravimetric analysis showed a weight decrease for the catalyst E 4759 of 4-5%, which was mainly due to desorption of water. XRD was the only method apart from TPR that showed a difference between the untreated and thermally treated catalysts. The intensity of the Pt (111) reflex measured after subtraction of the reference spectrum of the support material was about 30% higher for catalyst E 4759 that had been treated at 673 K in hydrogen, but there was no difference in the peak width at half maximum intensity. This again suggests that the size of the platinum particles did not change significantly, but it might indicate that treatment in hydrogen at elevated temperatures leads to an increase in the crystallinity of the platinum. Effect of slow reoxidation in air on TPR and performance In order to obtain more information on the processes involved, the pretreated catalyst was exposed to air at room temperature. Two samples were taken from this batch at regular intervals (typically 15-20 days) and both a test hydrogenation and a TPR experiment were run on the same day. The reaction and TPR data are summarized in Table 3. The low-temperature peak increased by a factor of 13 and its maximum shifted from 228 to 249 K during the go-day period. The high-temperature peak also reappeared but with low intensity and constant TM. The medium-temperature peak was not observed. In Fig. 6 the amount of platinum oxide, carbon deposits (from TPR peak integrations) and the hydrogenation time are plotted versus exposure time. It is interesting that both the hydrogenation time and the low-temperature peak did not increase immediately after the catalyst had been exposed to air but that it takes 45 days before a significant change occurs. This behaviour shows that a substantial amount (H,/Pt > 0.28) of platinum oxide must be present before the activity decreases. Not only the amount of platinum oxide but also its resistance to reduction increases after longer exposure times. The shift of the maximum of the low-

29

TABLE 3 TPR and reaction data as a function of time since thermal treatment in hydrogen (673 K, 2 h) Hydrogenation of ethyl Z-oxo-4-phenylbutyrate under standard conditions; catalyst, JMC 5R94. Time since pretreatment

T (&

(I-WY 11

228 232 231 241 249

0.028 0.086 0.151 0.282 0.359

TM,

W&‘t)s

Hydrogenation time (min)

e.e. (%)

0.033 0.029 0.055 0.081 0.129

20 20 40 120 220

86 84 84 82 79

(K)

(days) 0

15 45 65 90

TIME

651 647 653 651 650

:mys:

Fig. 6. H,/Pt value for the low-temperature peak (A ), H*/Pt value for the high-temperature peak ( 0 ) and hydrogenation time (0 ) of ethyl 2-oxo-4-phenylbutyrate (standard conditions) as a function of time after thermal treatment (hydrogen, 673 K, 2 h) (catalyst JMC 5R94).

temperature peak (Table 3) indicates that the reducibility of the platinum oxide gradually decreases. If a catalyst that has been exposed to air for 60-90 days is used in the hydrogenation reaction, it is expected that in situ reduction will be slower or that only a fraction of the platinum oxide is converted to the active metal. As a consequence, the activity of the catalyst will be lower than for a freshly reduced sample. From UHV studies it is well known that clean surfaces show an accumulation of carbon compounds when they are exposed to the atmosphere. It was therefore not surprising that the high-temperature peak also reappeared. Even though it also increases with time, we do not think that it is related to the performance of the catalyst. Interpretation of the observed pretreatment effects Prior to a description of the different processes that can take place during thermal treatment of the catalysts, we would like to emphasize the method used for the preparation of the catalysts. To our knowledge, all TPR investi-

30

gations of Pt/Al,O, catalyst have been made either with unreduced platinum salts adsorbed on alumina or with catalysts that were reduced with hydrogen in the gas phase and then reoxidized in air or oxygen. In contrast, the catalysts described in this paper were reduced in the liquid phase under mild conditions, dried and stored in this “reduced” state, as is common for powder hydrogenation catalysts. This preparation method can no doubt lead to a different structure and reactivity of the platinum. Therefore, it was important to identify the peaks observed in different TPR profiles. Thermal treatment in a hydrogen atmosphere is probably one of the most common operations during the preparation of a catalyst. The following effects have been described: changes in the dispersion and/or morphology of the platinum crystallites; changes in the properties of the support (texture, OH groups, water content); reduction of platinum in the oxidized state (oxides, residual salts); and cleaning of the catalyst surface (carbon, other impurities). Our results show that the standard thermal pretreatment does not change the dispersion of the platinum, the texture of the support or the chloride-ion content. We did, however, detect a strong effect of this pretreatment step on the amount and reducibility of platinum oxide (i.e., oxidation state of the platinum), on the amount of carbonaceous material, on the residual platinum salts in the catalyst and the crystallinity of the platinum. Let us consider how these changes are related to the performance of the catalyst in the enantioselective hydrogenation of cu-keto esters. Amount and reducibility of platinum oxide The slow reoxidation experiments suggest that the oxidation state of the catalyst at the start of the hydrogenation experiment has an effect on the activity but not on the enantioselectivity. This could mean that the in situ reduction (during the liquid-phase hydrogenation reaction) of the platinum oxide is a slow process, especially for catalysts that are strongly oxidized or where the platinum oxide has a low reducibility. However, even for the activity there is no quantitative correlation between the amount of platinum oxide and rate, which probably means that other influences are also important. Amount of carbonaceous material We did not find a correlation between this parameter and the performance of the catalysts. This is expected if our interpretation is correct that the carbonaceous matter is located mainly on the support and has only a minor influence on the active sites on the platinum. The decrease of these deposits during thermal treatments is therefore simply a parallel effect without consequences for the hydrogenation reaction.

31

Residual platinum salts There might be a correlation between the selectivity and the medium-temperature TPR peak. It is well known that cinchonidine can form insoluble salts with PtCl,‘- [ 171. When equimolar solutions of H,PtCl, and cinchonidine are yellow 1: 1 precipitate mixed (solvent ethanol), a voluminous ( HzPtCls*C19H220N2) is obtained instantaneously. In the presence of residual platinum salts on an untreated catalyst, the voluminous salt could be formed when the modifier is added to the reaction mixture. If this salt is then deposited in the pore system of the catalyst, a negative effect on selectivity and on activity can result. Crystallinity of the plutinum Further investigations using high-resolution electron microscopy are planned in order to show whether the small increase in crystallinity of the platinum as detected with XRD leads to changes in the morphology of the platinum crystallites and whether the performance of the catalyst is related to this effect. We acknowledge that our results do not provide a general explanation for the observed phenomena. While our hypotheses can rationalize some of the experimental results, others are still poorly understood. This is not surprising because in the course of our investigations we have demonstrated that chirally modified heterogeneous catalysts are very complex systems. Because so many parameters have to be optimized in order to obtain good enantioselectivity we consider that we have made progress when we can shown that one specific parameter has no effect on the performance of the catalytic system. CONCLUSIONS

Thermal activation in hydrogen is essential for obtaining optimum activity and selectivity in the enantioselective hydrogenation of cli-keto esters using Pt/A120, catalysts. From the experiments described here we conclude that the activation step has the following effects on the Pt/Al,O, catalysts: (i) Platinum oxide on the surface of the relatively large platinum particles ( dpt= 3-4 nm) is reduced without affecting the crystallite size. The amount of platinum oxide in some instances correlates inversely with the activity of the catalysts. (ii ) Traces of unreduced platinum salts are converted to the metal. These salts can form insoluble complexes with the chiral modifier (cinchonidine). The reduction of the platinum salts might be responsible for the increase in enantioselectivity. (iii ) Carbon deposits that originate from organic reagents used during the catalyst preparation are removed. Because CO chemisorption experiments

32

suggest that these carbon residues are located on the support and do not affect active platinum sites, we conclude that this effect of the thermal treatment has no influence on the performance of the catalyst. ACKNOWLEDGEMENTS

The authors thank Prof. A. Baiker for valuable discussions and R. Gosteli and A. Balmer for their experimental contributions.

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