Catalytic Selective Oxidation - American Chemical Society

Chapter 23

Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 24, 2014 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch023

Promotion and Deactivation of Platinum Catalysts in Liquid-Phase Oxidation of Secondary Alcohols T. Mallat, Z. Bodnar, and A. Baiker Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, Zürich CH-8092, Switzerland

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenylethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Pt ratio the yield increased from 18 to 99 %. 0

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The transformation of alcohols to the corresponding carbonyl compounds or carboxylic acids is one of the few examples in which a heterogeneous (solid) catalyst is used in a selective, liquid phase oxidation (1,2). The process, which is usually carried out in an aqueous slurry, with supported platinum or palladium catalysts and with dioxygen as oxidant, has limited industrial application due to deactivation problems. There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of 0

0097-6156/93/0523-0308$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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"chemical" deactivation has been ascribed to the formation of acidic or polymeric compounds during the reaction (6-8). It has been discovered that the performances of platinum and palladium catalysts may be improved by promotion with heavy metal salts. However, there is little information available about the role and chemical state of the promoter (8,9). We have recently found that a geometric blocking of active sites on a palladium-onactivated carbon catalyst, by lead or bismuth, suppresses the by-product formation in the oxidation of l-methoxy-2-propanol to methoxy-acetone (10). In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (11,12). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. The air-oxidation of 1-phenylethanol to acetophenone in an aqueous alkaline solution has been chosen as a model reaction. The catalytic experiments were completed with the application of an in-situ electrochemical method for studying catalyst deactivation and the role of promoters. The potential of the catalyst, which was considered as a slurry electrode, was measured during the oxidation reaction. More details of the method can be found elsewhere (13,14).

Materials and Methods Distilled water (after ion exchange) and purum or puriss grade reagents were used for the experiments. For the preparation of an unsupported Pt powder, 12 mmol H PtCl in 100 cm water was dropped into 300 cm 0.4 M aqueous NaHC0 solution at 95 °C. After refluxing it for 3 h the slurry was cooled to 30 °C and treated with hydrogen for 3 h. The catalyst was filtered off, washed to neutral with water and dried in air. The metal dispersion was 0.052 determined from the hydrogen region of a cyclic voltammogram (75). The 5 wt% Pt-on-alumina was a commercial catalyst (Engelhard 4462). The metal dispersion (D = 0.30) was determined from TEM pictures. Different fields were examined and about 1000 particles were counted and their size determined. The degree of dispersion was calculated from the surface average diameter (76). Before bismuth-promotion the Pt-on-alumina catalyst was pre-reduced in water with hydrogen. The pH was decreased to 3 with acetic acid and the appropriate amount of bismuth nitrate dissolved in water (10~ - 10" M) was added into the mixed slurry in 15-20 min, in a hydrogen atmosphere. Promotion of unsupported Pt was carried out similarly. The metal composition of the bimetallic catalysts was determined by atomic absorption spectroscopy. The oxidation reactions were performed in a 200 cm glass reactor, equipped with gas distributer, condenser, thermometer, measuring and reference electrodes. The mixing frequency of the magnetic stirrer was 1500 min" . 75 mg Pt or 450 mg Pt-onalumina catalyst was prereduced in nitrogen atmosphere at 60 °C with 3.67 g or 3.00 g 1-phenylethanol, respectively. The solvent composition was 35 cm water + 2

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In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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0.32 g Na C0 + 0.37 g dodecylbenzenesulfonic acid Na salt + 5 cm dioxane (for unsupported Pt). After 30 minutes the alcohol was oxidized with air (7.5 cm min" ) at 60 °C. Conversion and selectivity were determined by GC analysis. Only alterations from this procedure will be indicated in the text. All the potentials in the paper are referred to a Ag/AgCl/KCl electrode (E=197 mV). The electrochemical cell and polarization method used for cyclic voltammetric measurements have been described previously (9). 2 mg catalyst powder on a carbon paste electrode was polarized with 1 mVs" scan rate in a 0.085 M aqueous Ns^C0 solution at 25 °C. For the measurement of the open circuit potential of the catalyst during the oxidation reaction a Pt rod measuring electrode and a Ag/AgCl/KCl reference electrode were applied (13,14). 2

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Promotion of Platinum-on-Alumina We have found that platinum catalysts quickly loose their activity in the selective oxidation of aliphatic and aromatic secondary alcohols to the corresponding ketones in aqueous solutions. Acceptable yields could be achieved with an extremely high catalyst loading or with long reaction times. Catalyst deactivation could be suppressed by depositing a promoter metal submonolayer onto the surface of the platinum particles. For example, total conversion with more than 99 % selectivity could be reached in 6 hours in the oxidation of 1-phenylethanol to acetophenone with Bi-Pt/alumina catalysts. It is shown in Table I that even a moderate coverage of platinum by bismuth had a substantial influence on the final conversion. Lead-promotion gave similar results, but gold, tin or ruthenium were less efficient. The general sequence of promoter efficiency in suppressing catalyst deactivation in the partial oxidation of secondary alcohols was: Bi >• Pb >• Sn - Au - Ru.

Table I. Bi-promotion of a Pt-on-alumina catalyst No

1 2 3 4

Bi/Pt at/at

Conv. %

%

0 0.1 0.15 0.2

18.4 96.1 99.5 99.9

99.1 99.5

s

Sel.

Role of Bismuth Promoter In general, the influence of promotion may be explained by a geometric (blocking) effect or by the formation of new active sites. We suggest, that in our case the suppression of by-product formation by blocking a fraction of Pt° active sites is of


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decisive importance. Unfortunately, it is difficult to prove or exclude the existence of new active centers after promotion as reaction rates are always distorted due to oxygen transport limitation (to avoid oxygen poisoning). We found that our reactor worked in a "mixed" regime, as the measured reaction rates were influenced by both the rate of oxygen supply and that of the surface chemical reaction. The correlation between the coverage of surface platinum atoms by bismuth adatoms (9 ) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine 0 and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. The anodic polarization curves of a Pt powder modified by adsorbed Bi are shown in Figure 1. The ionization of adsorbed hydrogen on unmodified Pt (curve a) ranges between -0.8 and -0.4 V: Bi


Bi

H

+ OH -» H 0 + e

ad

2

Above this value a further oxidation of the surface by OH adsorption occurs, which becomes considerable above -0.2 V. A simplified reaction route of the step-by-step surface oxidation of Pt by OH" is (77): Pt -» Pt OH -> Pt O s

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Bismuth promotion suppresses the hydrogen sorption on platinum (curves bd). The peak at -0.05 V indicates the oxidation of adsorbed bismuth, which overlaps the OH adsorption on uncovered platinum surface sites (18). Bismuth adatoms are discharged in the low potential region (Bi°) and occupy three platinum sites at low surface coverages. The structure of the oxygen-containing species are (BiOH) , (BiO) and [Bi(OH) ] (79). Bi does not adsorb hydrogen, thus a Bi/Pt coverage can be calculatedfromthe hydrogen chemisorption data. It is seen in Figure 2 that there is an excellent correlation between the Bi-coverage of Pt and the rate of 1-phenylethanol oxidation. It seems that the hydrogen chemisorption ability of Pt or the size of active sites ensembles has to be minimized to avoid deactivation. There are indications in the literature that the suppression of hydrogen sorption on a Pt electrode can eliminate the poison formation (20). ad

ad

2

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Oxidation State of the Catalyst during Reaction The measurement of open circuit potential of the catalyst during the liquid phase oxidation of alcohols provides a unique insight into the redox processes taking place on the catalyst surface. A Pt catalyst stored in air contains surface oxides and in an aqueous Na C0 solution it behaves as an oxygen electrode. Its potential is 250-280 mV when referred to a Ag/AgCl/KCl electrode (Figure 3). When the catalyst is 2

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(sal)




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Figure 2. Bi coverage of unsupported Pt and the rate of 1-phenylethanol oxidation as a function of the overall Bi/Pt ratio. s



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pre-reduced before the oxidation reaction to obtain Pt° active surface sites, its potential is shifted to the negative direction by more than 1 V. 1-phenylethanol is a good reducing agent and decreases the catalyst potential in nitrogen atmosphere close to the value measured after hydrogen reduction (1 bar). After the addition of alcohol, the measured value is a mixed potential determined by two (or more) electrode processes (21,22). The two main components are the hydrogen electrode process and the alcohol/ketone reaction. The oxidation state of the catalyst during the air-oxidation of 1-phenylethanol is shown in Figure 4. The anodic polarization curve of a bismuth-promoted platinum catalyst (Bi/Pt = 0.39) is taken from Figure lb, as a reference. The lower part of Figure 4 represents the conversion of alcohol as a function of the potential of an unsupported and an alumina-supported Bi-Pt catalyst. When air is introduced to the reactor, the open circuit potential of the Bi-Pt/alumina catalyst slightly increases to the anodic direction. The catalyst potential reaches the region of the oxidation of Bi only at the end of the reaction. Almost up to total conversion the potential of the active Bi-Pt/alumina catalyst remains in the "hydrogen region". This behaviour indicates that both platinum and bismuth are in a reduced (discharged) state and platinum is partially covered with hydrogen during the oxidation reaction. This is in a good agreement with the dehydrogenation mechanism of alcohol oxidation, according to which only the Pt° sites are active (23,24). A similar situation was found in the oxidation of several other types of secondary alcohols like diphenyl carbinol or a-tetralol. Different results were obtained when the reaction was catalyzed with unsupported Bi-Pt catalysts. A few minutes after introduction of air into the reactor the catalyst potential was around -400 mV (Figure 4b). It is clear from the anodic polarization curve above that at this potential the hydrogen coverage is close to zero. It is interesting that the oxidation of 1-phenylethanol occurs on alumina-supported BiPt catalysts covered by hydrogen up to total conversion, while the similarly prepared but unsupported bimetallic catalysts are practically free of hydrogen at above 2-4 % conversion. This behaviour is attributed to some by-product formation and strong adsorption which shifts the mixed potential of the unsupported catalyst by 400 mV to the positive direction. The presence of high boiling point by-products in the latter case was evidenced by GC analysis. s

ad

Chemical Deactivation and Oxygen Poisoning In Figure 5 the conversion of 1-phenylethanol and the open circuit potential of alumina-supported catalysts are plotted as a function of reaction time. There is a striking difference between the curves of unpromoted (a, a') and bismuth-promoted (c, c') catalysts. When air is introduced to the reactor, the potential of the platinum-onalumina catalyst quickly increases to the anodic direction and after one minute the catalyst potential is above -300 mV. One may conclude that there is practically no hydrogen on the platinum surface and after a short period an increasing fraction of platinum is covered by OH. The influence of bismuth promotion is a higher reaction rate (final conversion) and lower catalyst potential during reaction. It seems that the reason of deactivation of the unpromoted Pt-on-alumina catalyst is the over-oxidation of the Pt° active sites, due to the high rate of oxygen


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E/mV oxidation

400

w

. . .*·-γ··-*··*--^ *·—~—— " Ng+alc. * —


0

αΑ

-400 A Ά A A

-800

±A+é*

I

0

A..... 50

100 Time/min.

Figure 3. The variation of the potential of the Pt powder catalyst during pretreatment and oxidation. J_(mA)

Ε imV)

Figure 4. Anodic polarization curve of a Bi-Pt catalyst (Bi/Pt =0.39) and the conversion - catalyst potential relationship in the oxidation of 1-phenylethanol, in an aqueous Na C0 solution; a - Bi-Pt/alumina, Bi/Pt =0.20, b - unsupported Bi-Pt, Bi/Pt=0.39. s

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Conv./%

0

100

200

300

400

Time/min.

Figure 5. The influence of Bi-promotion on the deactivation of a 5 wt% Pt/alumina catalyst in the oxidation of 1-phenylethanol; a,a'- Pt/alumina, 21 vol% 0 /N , 1500 min , b,b' - Pt/alumina, 5 vol% 0 /N , 750 min" , c,c' - Bi-Pt/alumina, Bi/Pt=0.20, 21 vol% 0 /N , 1500 min . 2

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supply to the catalyst surface related to the rate of its consumption in the surface chemical reaction. However, a decrease in the rate of oxygen supply by decreasing both the oxygen partial pressure and mixing speed did not solve the problem. There is only a short period during which the catalyst is partially covered by hydrogen and the conversion is still below 20 % after a twofold reaction time (Figure 5b and 5b*). When the air flow was temporarily substituted by a nitrogen flow for 15-20 minutes in the reaction represented by Figure 5a, the rate of alcohol oxidation did not increase. These experiments also prove that the reason of catalyst deactivation is not the over-oxidation of Pt° active sites, but a partial coverage of active sites by impurities (chemical deactivation). The best promoters of the partial oxidation of secondary alcohols are Bi and Pb. Neither of them adsorb hydrogen and a partial coverage of platinum by any of them decreases the hydrogen sorption on the bimetallic system. Nevertheless, there is no hydrogen on the platinum-on-alumina catalyst during the oxidation reaction, while a partial hydrogen coverage and a reduced state of the promoted catalysts were observed up to almost total conversion. The explanation of this apparent contradiction is the partial coverage of unpromoted platinum by impurities. A decrease in the number of Pt° active sites by a factor of 3-10 due to Bi or Pb promotion may still be advantageous compared to an almost complete coverage of them by irreversibly adsorbed species. We believe that the primary reason of deactivation is the formation of irreversibly adsorbed species and oxygen poisoning occurs when the blocking of active sites reaches a critical level. By-products can be formed during the oxidation reaction. A frequently observed side-reaction is the aldol-dimerization of the carbonyl compound and a further oxidation of the product. The process is catalyzed by bases, including the basic functional groups of a carbon support (25). Another important source of by-product formation is the dissociative adsorption of alcohol on the platinum surface. It was found that methanol adsorption on platinum in aqueous solutions is a step-by-step dehydrogenation process resulting in triply bound *C-OH surface species (26). The formation of adsorbed CO, COH and HCOH species and C-C bond cleavage of higher alcohols leading to the formation of alkanes were also confirmed by classical analytical and in-situ spectroscopic methods (27-29). As there is a positive potential shift of unpromoted platinum from the beginning of the reaction (Figure 5a) we propose that the initial adsorption of the alcohol substrate is the real reason of deactivation, and the side reactions during the oxidation reaction are of secondary importance. The product composition of the irreversible adsorption of alcohols on platinum depends on whether the alcohol molecule comes into contact with a "free" metal surface or with hydrogen or oxygen covered sites (29). This effect can be seen in Figure 3: the catalyst potential during the oxidation reaction is higher by about 110-150 mV, when the platinum oxide has been pre-reduced by the alcohol reactant itself, compared to the potential corresponding to the reaction after pre-hydrogenation. The potential difference is an indication of the different composition of adsorbed species on platinum or the different surface coverage of active sites by impurities.


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Acknowledgment Financial support of this work by the Swiss National Foundation (Support Program "Eastern Europe") is kindly acknowledged. Literature cited


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Heyns, K.; Blazejewitz, L. Tetrahedron, 1960, 9, 67. van Bekkum, H., in Carbohydrates as Organic Raw Materials, Lichtenthaler, F. W., Ed., VCH: Weinheim, Germany, 1990, pp. 289-310. Dijkgraaf, P. J. M.; Duisters, Η. A. M.; Kuster, B. F. M.; van der Wiele, K. J. Catal., 1990, 112, 337. Dirkx, J. M. H.; van der Baan, H. S. J. Catal., 1981, 67, 1. Dirkx, J. M. H.; van der Baan, H. S. J. Catal., 1981, 67, 14. Nicoletti, J. W.; Whitesides, G. M. J. Phys. Chem., 1989, 93, 759. Smits, P. C. C.; Kuster, B. F. M.; van der Wiele, K.; van der Baan, H. S. Appl. Catal., 1987, 33, 83. Mallat, T.; Baiker, A. Appl. Catal. A: Gen., 1991, 79, 41. Mallat, T.; Allmendinger, T.; Baiker, A. Appl. Surf. Sci., 1991, 52, 189. Mallat, T.; Baiker, Α.; Patscheider, J. Appl. Catal. A: Gen., 1991, 79, 59. Kolb, D. M. Adv. in Electrochem. and Electrochem. Eng., 1978, 11, 125. Szabo, S. Int. Rev. Phys. Chem., 1991, 10, 207. Baria, D. N.; Hulburt, Η. M. J. Electrochem. Soc., 1973, 120, 1333. van der Plas, J. F.; Barendrecht, E.; Zeilmaker, H. Electrochim. Acta, 1980, 25, 1471. Commission on Electrochemistry, Pure Appl. Chem., 1991, 63, 711. Mallat, T.; Petro, J. React. Kinet. Catal. Lett., 1979, 11, 307. Angerstein-Kozlowska, H.; Conway, B. E.; Barnett, B.; Mozota, J. J. Electroanal. Chem., 1979, 100, 417. Kadirgan, F.; Beden, B.; Lamy, C. J. Electroanal. Chem., 1983, 143, 135. Clavilier, J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem., 1988, 243, 419. Watanabe, M.; Horiuchi, M.; Motoo, S. J. Electroanal. Chem., 1988, 250, 117. Wagner, C.; Traud, W. Z. Electrochem., 1938, 44, 391. Koryta, J.; Dvorak, J. Principles of Electrochemistry, Wiley & Sons, Chich ester, England, 1987, pp. 367-369. DiCosimo, R.; Whitesides, G. M. J. Phys. Chem., 1989, 93, 768. Horànyi, G.; Vèrtes, G.; König, P. Acta Chim. Hung., 1972, 72, 179. Mallat, T.; Baiker, Α.; Botz, L. Appl. Catal. A: Gen., 1992, 86, 147. Bagotzky, V. S.; Vassiliev, Yu. B.; Khazova, O. A. J. Electroanal. Chem., 1977, 81, 229. Lopes, M. I. S.; Beden, B.; Hahn, F.; Léger, J. M.; Lamy, C. J. Electroanal. Chem., 1991, 313, 323. Iwasita, T.; Nart, F. C. J. Electroanal. Chem., 1991, 317, 291. Damaskin, Β. B.; Petrii, Ο. Α.; Batrakov, V. V., Adsorption of Organic Com pounds on Electrodes, Plenum Press, New York, U. S., 1971, pp. 321-349.

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Catalytic Selective Oxidation - American Chemical Society

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