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Preparation of tartaric acid modified Raney nickel catalysts: study of modification procedure
Applied Catalysis A: General 210 (2001) 237–246
Preparation of tartaric acid modified Raney nickel catalysts: study of modification procedure ∗ ˇ Pavel Kukula, Libor Cervený Department of Organic Technology, Institute of Chemical Technology-ICT Prague, Technická 5, 166 28 Prague 6, Czech Republic Received 18 July 2000; received in revised form 26 September 2000; accepted 26 September 2000
Abstract Chiral modification of Raney nickel using (2R,3R)-tartaric acid was studied. The prepared catalyst was used in enantioselective hydrogenation of methylacetoacetate (MAA) to methyl-(3R)-hydroxybutyrate. The influence of the most important modification parameters, such as pH, temperature, time, concentration of the modifier and the presence of a co-modifier, on the optical yield of MAA hydrogenation was systematically investigated. From the data obtained, a considerable influence of modifying conditions on the resulting enantioselectivity of the catalyst was evident. The optical yield increased with an increasing of the modifying temperature and time. Dependencies of the optical yield on the tartaric acid concentration and on the modifying pH passed through a maximum. Therefore, there exists an optimal value of modifying pH, at which a minimal catalyst amount is leached to the modifying solution. Furthermore, significant adsorption of tartaric acid and subsequent complex formation of nickel and tartaric acid occurs on the catalyst surface. It was found that the presence of sodium bromide in the modifying solution resulted in a lower degree of the nickel leaching, and a decrease of the residual aluminum content in the catalyst increased the optical yield of the reaction. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Enantioselective hydrogenation; Chirally-modified Raney nickel; Tartaric acid; Methylacetoacetate
1. Introduction Enantioselective hydrogenations can be used for preparation of a series of valuable bioactive compounds. Heterogeneous enantioselective catalysis in the course of a chirally-modified catalyst utilization [1] represents one of the possible ways of selective preparation of optically pure compounds. Unmodified metal catalysts display no inherent chirality and produce racemic mixtures. However, the chiral molecule ∗ Corresponding author. Tel.: +42-2-2431-0280; fax: +42-2-3119-657. ˇ E-mail address: [email protected] (L. Cerven´ y).
properly implemented on the surface of a metal catalyst can represent the source of chirality. One of these types of catalysts is the Raney nickel modified using tartaric acid, which is used for enantioselective hydrogenations of -ketoesters. Hydrogenation of methylacetoacetate (MAA) to methyl-3-hydroxybutyrate (MHB) over nickel catalyst modified using optically pure isomer of tartaric acid has become the model reaction for the study of different parameter influence on enantioselectivity of the catalyst [2,3]. Various types of nickel catalysts were used for enantioselective hydrogenations, e.g. Raney nickel [2,3], several supported nickel catalysts [6–14], bimetallic supported catalyst [15–22] and nickel powder [2,4,23].
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 1 2 - 7
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Chiral modification is one of the most important steps involved in the preparation of an enantioselective catalyst. During the modification process, formation of chiral active sites takes place and it affects the future catalyst enantioselectivity. The modification procedure is generally carried out in an aqueous solution of a suitable modifier. (2R,3R)-(+)-tartaric acid is the most commonly utilized and the most suitable modifier from the point of view of enantioselectivity. Modification is a typical corrosive process, characteristic by leaching of nickel to the modifying solution [5,11–14]. During the modification, it is necessary to optimize all of the modifying parameters, because they play ultimate role in the future catalyst asymmetric activity. It is known that the addition of co-modifier (mostly NaBr) to the modifying solution can rise the optical yields even by 10–30%. The effect of co-modifier is in the literature explained as a blocking of unmodified active sites, since the ratio of modified and unmodified sites determines the resulting optical yield [2,3]. One of the key problems in the course of the catalyst preparation, which is mentioned by many authors [24–28], is a poor reproducibility of measured reaction rates and optical yields over individual catalysts. Nevertheless, there is a good agreement that the catalyst enantioselectivity depends mostly upon the method of its preparation and on the mode of the modification procedure. The catalyst modification is possible to carry out in different ways, but the modifying conditions need to be optimized for each type of catalyst [2,3]. Keane [11–14,29], Nitta [7–10] and coworkers have studied in detail the effects of the modification process performance on the catalytic properties of supported nickel catalysts, especially Ni/SiO2 . Lesser attention has been paid to the study of Raney nickel modification [2,26,30–32]. Many diverse opinions on the influence of modifying conditions on the enantioselectivity of a modified Raney nickel can be found. Hence, this work is devoted to the study of Raney nickel modification with the use of (2R,3R)-tartaric acid. The paper is an attempt to describe in detail the Raney nickel modification procedure by a systematic investigation of the influence of the most important modifying parameters, such as pH, temperature, time, modifier concentration, etc., over a wide experimental range on the resulting optical yield.
2. Experimental 2.1. Catalyst Commercial Raney nickel, Actimet M (Egelhard) was used as the catalyst. The catalyst was stored under water in an alkaline environment. It was assumed during weighing of the catalyst suspension that the suspension contains about 50 wt.% of water. This fact was also confirmed experimentally by weighing out of the catalyst before and after drying in an inert atmosphere of nitrogen [33]. The total surface area of the catalyst was 75 m2 /g, the volume of pores 0.12 cm3 /g and the particle size ranged between 5 and 101 m. 2.2. Chemicals Optically pure (2R,3R)-(+)-tartaric acid (p.a., Lachema Brno) was used as the modifier. Antipodal enantiomer of tartaric acid, (2S,3S)-(−)-tartaric acid (p.a., Aldrich) was also tested. Furthermore, the following chemicals were used, sodium bromide, sodium hydroxide and methanol (all p.a., Lachema Brno), hydrochloric acid (p.a., Penta Prague), MAA (p.a., Aldrich), hydrogen 3.0 and helium 4.0 (LindeTechnoplyn a.s. Prague). 2.3. Apparatus and procedure First, the catalyst, mostly about 3 g, was three times washed with distilled water and then put to the so-called preliminary treatment, in which the catalyst was washed with 1% tartaric acid solution. This washing step was carried out under stirring, and the time and pH of the washing solution was monitored. After this, the catalyst was washed with 20 ml of distilled water and was introduced to the modifying solution for a proper modification. Modifying solution was prepared by dissolving of an appropriate amount of tartaric acid to 100 ml of distilled water. In some cases, sodium bromide was added to the modifying solution as a co-modifier. The modifying pH was adjusted to the required value by addition of 20% sodium hydroxide solution. The pH was measured by the digital pH meter WTW pH 583 with the combined electrode SenTix 97/T, WTW (Germany). Suspension of the catalyst and the modifying solution was stirred at a constant temperature for 1 h
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using the magnetic stirrer IKAMAG-RCT placed in Erlenmayer flask equipped with a reflux condenser. During the modification process, pH of the catalyst suspension and the modifying solution was measured. After decantation, the modifying solution was poured off, and the catalyst was subsequently washed with water and methanol (20 ml). Modified catalyst was then introduced to the hydrogenation. Hydrogenations were carried out in a liquid phase with a solvent (methanol) in a stainless autoclave with volume of 300 ml. The autoclave was supplied with a double-propeller with a magnetic transmission, a thermocouple probe, a pressure gauge and a heating mantle with an adjustable temperature. The temperature was recorded digitally. Most of the reactions were carried out starting by insertion of 50 ml (53.8 g, 0.46 mol) of MAA, 100 ml of methanol and about 2 g of the modified catalyst. All reactions proceeded at the same constant reaction conditions, temperature 100◦ C and overall pressure 10 ± 0.2 MPa. The reactions were conducted so that the total conversion of MAA would be achieved. After the reaction was completed, the catalyst and the solvent were removed from the reaction mixture by filtration and evaporation. The product was distilled under a decreased pressure (bp 62–63◦ C/2.4 kPa).
Samples of the reaction mixture were withdrawn during the hydrogenations, and analyzed by gas chromatography using the gas chromatograph HP 5890 Series II Plus (Hewlett-Packard, USA) with a flame ionization detector (FID) and the enantioselective capillary column -DEX 325 (30 m×0.25 mm×0.25 m) of Supelco Co. The analyses were carried out using the temperature program from 80 to 160◦ C with a constant initial temperature for the duration of 7 min. The first temperature rate was 10◦ C/min up to the temperature of 95◦ C, and the second temperature rate was 70◦ C/min up to the temperature of 160◦ C. The carrier gas pressure p(N2 ) was 79 kPa, the flow rate was 1.0 ml/min and the split ratio was 1:100. Under such chromatographic conditions, it was possible to carry out the separation of the produced enantiomers of methyl-3-hydrobutyrate. Following the fastidious separation of enantiomers, analyses were carried out with the lowest possible concentration of the determined compounds with the use of a very low injection volume (0.1–0.2 l) of the reaction mixture and an adjusted maximal sensitivity of the detector. The enantioselectivity of the reaction was calculated by use of the following equation:
2.4. Analysis
Optical yield was determined polarimetrically by means of the optical activity measurement of the product in its concentrated state. Digital polarimeter DIP-370 of Jasco (Taiwan) was used for the measurements. Optical yield (e.e.) was then calculated using the following equation:
The amount of nickel and aluminum leached to the modifying solution was monitored by means of atomic absorption spectrometry using AAS spectrometer SpectrAA 300 of Varian (USA). In order to compare, the content of nickel and aluminum in the catalyst was monitored before and after the modification. Weighted amount of a catalyst was placed into a distillation flask equipped with a reflux condenser and quartz capillary reaching to the bottom of the flask. Then, 100 ml of distilled water and 2 ml of concentrated hydrochloric acid was added to the catalyst. Apparatus was supplied with a heater with an adjustable drain power. The nitrogen was fed into the outer end of the capillary and was bubbled through the layer of the catalyst, which caused its agitation. All samples were dissolved in this way within 30 min at the boiling point of the liquid phase. Content of nickel and aluminum was measured in such obtained solutions using atomic absorption spectrometry.
e.e. = oy (%) =
e.e. = oy (%) =
|[(R)-MHB] − [(S)-MHB]| × 100 [(R)-MHB] + [(S)-MHB]
[␣]20 d product × 100 [␣]20 d max
where [␣]20 d product represents the specific optical rotation of the product, and [␣]20 d max the specific optical rotation of a pure enantiomer in its concentrated state, i.e. −22.95◦ for this case of methyl-(3R)(−)-hydroxybutyrate.
3. Results and discussion Since the Raney nickel catalyst was stored in an alkaline environment, it was necessary to wash it
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several times with distilled water before modification. The pH of the washing solutions was monitored, and it was found that after three washing steps, it decreased to the value of 9.3. During preliminary treatment (washing the catalyst with 1% tartaric acid solution), which took about 10 min, an increase in pH of the washing solution by about 0.4–0.5 pH occurred. At the same time, the washing solution darkened to chartreuse color as a result of the nickel leaching. It was found that the preliminary treatment of the catalyst with tartaric acid solution had a positive effect on the resulting enantioselectivity. It could be explained by a removing of the residual alkali from the catalyst and by an activation of the catalyst surface with tartaric acid. After the preliminary treatment, proper modification was carried out, during which an increase of the modifying solution pH took place by 2–3 on an average. This increase was probably caused by an adsorption of tartaric acid on the catalyst surface. Alternative explanation could be a dissolving of the residual aluminum from the catalyst, followed by a formation of alkaline aluminum compounds, which gradated to the liquid phase. At the lower values of the modifying pH, the modifying solution became green due to the nickel leaching to the modifying solution. Hydrogenation of MAA with an unmodified catalyst proceeds to the formation of racemic mixture (50:50) of methyl-(3R)-(−)-hydroxybutyrate and methyl-(3S)-(+)-hydroxybutyrate. The catalyst modified with (2R,3R)-(+)-tartaric acid produced a higher content of methyl-(3R)-(−)-hydroxybutyrate, while the catalyst modified with (2S,3S)-(−)-tartaric acid produced a higher content of methyl-(3S)-(+)hydroxybutyrate. Using antipodal enantiomer under the same modifying conditions, the optical yield of the reaction was nearly coincident. During the study of the influence of the modifying conditions on the optical yield, (2R,3R)-(+)-tartaric acid was used as the modifier. 3.1. Influence of modifying conditions on optical yield of the reaction 3.1.1. Tartaric acid concentration Optical yield dependence on tartaric acid concentration passes through a maximum (Fig. 1). The highest values of enantioselectivity were achieved in the region of tartaric acid concentration of about
Fig. 1. Dependence of the optical yield on tartaric acid concentration, modifying conditions: pH = 5.1; T = 100◦ C; t = 60 min; without NaBr.
0.2 mol/l. At higher concentrations, the optical yield decreased, and at concentrations higher than 0.5 mol/l the optical yield became independent of the tartaric acid concentration. Apparently, there were several effects acting together. With an increasing of the tartaric acid concentration, higher degree of its adsorption on the catalyst surface occurred, and the catalyst surface coverage increased. Simultaneously, nickel from the catalyst was leached into the modifying solution. This could also be examined visually, as the color of the solution was changed from clear to light green. Thus, an increased nickel leaching to the modifying solution brings about a decrease in the catalyst activity. Therefore, there exists an optimal tartaric acid concentration at which a certain amount of the modifier is adsorbed on the catalyst surface, providing that in the same time, only a low nickel leaching is taking place. 3.1.2. Modifying temperature The modifying temperature has also an effect on the optical yield (Fig. 2). With an increasing of the modifying temperature, the optical yield of the reaction increases almost with the linear rate. The more the temperature was increased, the higher amount of tartaric acid was probably adsorbed on the catalyst surface, which resulted in higher optical yields. It was also found that the catalysts modified at higher temperatures had a lower activity, however reached
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the highest optical yields lay in the region of tartaric acid isoelectric point (pH = 5.1). Having used acidic environment at lower pH values, increased dissolving of nickel took place, followed by the formation of Ni2+ ions on the catalyst surface. Then, these ions are leached to the modifying solution. Tartaric acid probably forms a fixed surface complex with Ni2+ ions. Consecutively, this surface complex serves as a chiral site on the catalyst surface. However, the complex formation is subjected to a specific modifying pH, which explains an occurrence of the maximum at the optical yield dependence on the modifying pH.
Fig. 2. Dependence of the optical yield on modifying temperature, modifying conditions: pH = 5.1; cTA = 0.2 mol/l; t = 60 min; without NaBr.
higher values of enantioselectivity. This was caused by the fact that with an increasing of the temperature, more nickel dissolves into the modifying solution. The higher the modifying temperature was, the greener the solution had its color.
3.1.4. Time of modification Fig. 4 shows the optical yield dependence on the modification time. Initially, the optical yield steeply increased with the modification time. After 50 min, the optical yield nearly reached its maximal value and then remained constant. With an increasing of the modification time, adsorption of the modifier on the catalyst surface occurred. Equilibration of the concentration of the modifier, dissolved in the liquid phase and adsorbed on the catalyst surface, occurred when the steady state was achieved.
3.1.3. Modifying pH Modifying pH had a considerable effect on the optical yield of the reaction. The dependence of the optical yield on the modifying pH (Fig. 3) also passed through a maximum. Optimal pH value for achieving
3.1.5. Presence of co-modifier Second modifying reagent (co-modifier) was used as the sub-component of the modifying solution. It was found [5] that several sodium salts were effective
Fig. 3. Dependence of the optical yield on modifying pH, modifying conditions: cTA = 0.2 mol/l; T = 100◦ C; t = 60 min; without NaBr.
Fig. 4. Dependence of the optical yield on modifying time, modifying conditions: cTA = 0.2 mol/l; pH = 5.1; T = 100◦ C; without NaBr.
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Fig. 5. Dependence of the optical yield on NaBr concentration, modifying conditions: cTA = 0.2 mol/l; pH = 5.1; T = 100◦ C.
Fig. 6. Effect of tartaric acid concentration on the extent of leached nickel into the modifying solution, modifying conditions: pH = 5.1; T = 100◦ C; t = 60 min; (䊏) without NaBr; (䉬) with NaBr.
as second modifying reagents due to a strong enhancement of the enantioselectivity of the catalyst. Among them, sodium bromide was the best. In this work, the sodium bromide was used as the co-modifier. Fig. 5 shows the effect of the co-modifier addition to the modifying solution on the optical yield of the reaction. From the presented dependence, an increase in the optical yield occurring with an increase of the amount of sodium bromide in the modifying solution is perceptible. The optical yield increased with an increase of the amount of sodium bromide reaching a plateau. The constant value was obtained with a higher concentration of sodium bromide than 2 mol/l. Thus, the presence of sodium bromide in the modifying solution can raise the optical yield even by 25%, regardless of the sodium bromide having no enantio-differentiating ability itself. The effect of a co-modifier is explained in the literature [2,3] by a blocking of the unmodified active sites. However, an addition of the co-modifier has also an influence on an occurrence of additional phenomenon. Since, the concentration of the co-modifier in the modifying solution was considerably high, the solution was nearly saturated. Sodium bromide affects the ionic strength of modifying solution and decreases hydrogen ions concentration. It resulted in a lower possibility for nickel and its compounds leaching to the modifying solution and in better conditions for complex formation between nickel and tartaric acid. The presence of co-modifier has also probably an effect on the tartaric acid stereo-
chemistry, which could be reflected by the resulting optical yield. 3.2. Influence of modifying conditions on nickel leaching into modifying solution During the following study of the catalyst modification, leaching of nickel and aluminum into the modifying solution was monitored in detail. Figs. 6–9 depict the effects of the modifying conditions on the amount of the leached nickel for the modifications,
Fig. 7. Effect of modifying temperature on the extent of leached nickel into to the modifying solution, modifying conditions: pH = 5.1; cTA = 0.2 mol/l; t = 60 min; (䊏) without NaBr; (䉬) with NaBr.
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3.2.2. Modifying temperature Similar situation could be observed in the dependence of the leached nickel on the modifying temperature (Fig. 7), showing that an increasing of the temperature has also increased the amount of the leached nickel.
Fig. 8. Effect of modifying pH on the extent of leached nickel into the modifying solution, modifying conditions: cTA = 0.2 mol/l; T = 100◦ C; t = 60 min; (䊏) without NaBr; (䉬) with NaBr.
which were carried out with and without a co-modifier. It is noticeable from all the figures that during the modifications, which were carried out without a co-modifier, several times higher nickel leaching into the modifying solution occurred. 3.2.1. Tartaric acid concentration Fig. 6 shows the dependence of the nickel leached into the modifying solution on the tartaric acid concentration. With an increasing of the tartaric acid concentration, the amount of the leached nickel increased.
3.2.3. Modifying pH The pH of the modifying solution (Fig. 8) had the ultimate effect on the amount of the leached nickel. Considerable dissolution of the catalyst occurred under acidic conditions. The amount of the leached nickel was more than 30 times higher than the amount leached under the optimal modification conditions (cTA = 0.2 mol/l, T = 100◦ C, pH = 4.9 and t = 60 min). In the range of pH = 3, the rapid ordinal decrease could be examined. When the modifying pH was higher than 4.8, the amount of nickel leached decreased slightly. 3.2.4. Modification time The dependence of the leached nickel on the modification time is presented in Fig. 9. There is a noticeable break in the dependence at about 60th minute. The amount of the leached nickel increased with the modification time faster in the beginning, then the nickel leaching proceeded in a less rapid pace. This fact is probably caused by a certain concentration equilibration of the modifier in the modifying solution and on the catalyst surface. Furthermore, it was in accordance with the ascertained dependence of the optical yield on the modification time. Table 1 shows the amount of nickel and aluminum leached into the modifying solutions under the optimal modifying conditions. During the preliminary treatment of a catalyst using 1% aqueous solution of tartaric acid, 2.3 wt.% of nickel and 1.7 wt.% of
Table 1 The amount of leached nickel and aluminum into the modifying solutions mg Ni/gcat. %Ni mg Al/gcat. %Al Pretreatmenta Fig. 9. Effect of modifying time on the extent of leached nickel into the modifying solution, modifying conditions: pH = 5.1; cTA = 0.2 mol/l; T = 100◦ C; (䊏) without NaBr; (䉬) with NaBr.
Modification with NaBr Modification without NaBr a
23.4 1.8 9.2
2.3 0.2 0.9
1.2 44.3 43.9
Washing with 1 wt.% tartaric acid solution for 10 min.
1.7 63.3 62.7
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aluminum, related to the total amount of nickel and aluminum, was leached into the washing solution within approximately 10 min. It can be implied that in aqueous solutions, where only tartaric acid is present, uniform leaching of nickel and aluminum takes place. During the modification, aluminum leaching is preferred due to the complex of nickel and tartaric acid formation on the catalyst surface. In the course of the modification, two-third of the aluminum content in the catalyst was leached into the modifying solution. It was found that the amount of aluminum leached into the modifying solution has also an effect on the optical yield of the reaction. With more aluminum leaching from the catalyst, the higher optical yield was achieved. Thus, it is presumable that the residual aluminum contained in the catalyst could be responsible for some of the nonselective active sites. The influence of a co-modifier addition to the modifying solution on the nickel and aluminum leaching is also perceptible from Table 1. The presence of sodium bromide in the modifying solution resulted in a lower nickel leaching with preservation of the amount of the aluminum leached, which corresponded with the measured data. The presence of sodium bromide in the modifying solution could also stabilize the nascent complex of nickel and tartaric acid on the catalyst surface. This assumption could explain the practically zero effect of sodium bromide addition on the aluminum leaching. Comparison of the calculated and the measured amount of nickel and aluminum in the catalysts prior and after the modification is presented in Table 2. The amount of nickel and aluminum in the catalysts was calculated from known losses of both metals during the pre-treatment and the modification. The measured amounts of metals were acquired by dissolving appropriate catalysts and by determining their
Table 2 Comparison between computed and measured content of nickel and aluminum in catalysts Catalyst
RaNi TA-NaBr-RaNi TA-RaNi
Computed/measured content of Ni and Al Ni (wt.%)
Al (wt.%)
–/92.7 97.4/97.6 97.3/97.4
–/7.3 2.6/2.4 2.7/2.6
concentrations using atomic absorption spectrometry. From the acquired data, a very good agreement between the calculated and the measured contents of individual elements was noticeable. The verified data infer that the modification is a typical corrosive process characterized by leaching of nickel and aluminum to the modifying solution, which resulted in the production of a complex of nickel and tartaric acid predominantly chemisorbing on the catalyst surface. The amount of the leached nickel and the created complex depends on the modifying conditions. In order to form a tartrate complex on a catalyst surface, a sufficient driving force of the reaction between nickel and tartaric acid must exist. For this reason, it is necessary to optimize all the modifying parameters in such a way that a high production of tartrate complex together with a low nickel leaching into the modifying solution would be achieved as the catalyst subsequently looses its activity. The highest values of the catalyst enantioselectivity are possible to achieve by modifying the catalyst under pH of about 5, while the pH was adjusted by titration of 20% sodium hydroxide solution. Under these conditions, the change of both protons of tartaric acid to sodium ions occurs roughly up to 80%. Thus, sodium salt of tartaric acid is the component which is involved in the complex formation on the catalyst surface. The nickel atoms are coordinated by oxygen atoms of carboxyl and hydroxyl groups of tartaric acid in the complex. Ni2+ ions released into the modifying solution, form a complex with tartrate as well. This complex is dissolved in the liquid phase and slows the catalyst modification. The role of the sodium ions present in the solution consists of the decrease of hydrogen ions concentration followed by higher stability of nascent complex of nickel and tartaric acid. Each end of tartrate has three oxygen atoms available to form coordination bonds with a nickel atom. Two of them are involved in the formation of the tartrate complex, while the third oxygen atom could form a bond with an additional nickel atom on the surface, or the negative charge of the molecule could be balanced by a sodium ion. This way, it is possible to explain the low enantioselectivity of the catalysts modified at low pH. Lower pH values of the modifying solution resulted in an extensive nickel leaching, which decreased the catalyst activity and increased the formation of nickel–tartrate complex in the modifying solution, but not on the
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catalyst surface. Adsorption of the already formed nickel–tartrate complex is particularly poor. This was found when the catalyst was modified by a separately prepared complex [33]. Enantioselectivity of this catalyst was significantly lower in comparison to the catalyst which had the complex formed “in situ” on its surface.
4. Conclusions The work is engaged in the study of chiral modification of Raney nickel catalyst using (2R,3R)-(+)tartaric acid. From the acquired data, a considerable influence of modifying conditions on the resulting enantioselectivity of the catalyst is evident. As to achieve the highest optical yields of MAA hydrogenation, it is necessary to optimize the parameters of modification. These parameters have to be optimized to create the optimal amount of the complex of nickel and tartaric acid on the catalyst surface together with a low nickel leaching into the modifying solution as the catalyst subsequently looses its activity. Amount of the adsorbed tartaric acid on the catalyst surface increases with an increasing of its concentration, the modifying temperature and the period of modification. One of the most important modifying parameters is pH of the modifying solution. There exists an optimal value of the modifying pH at which a minimal catalyst amount is leached into the modifying solution, and at the same time a significant adsorption of tartaric acid and subsequent complex formation of nickel and tartaric acid on the catalyst surface takes place. The presence of sodium bromide in the modifying solution resulted in a lower degree of nickel leaching. Thus, sodium bromide affects the catalyst enantioselectivity not only by blocking the nonselective sites, but also by affecting the formation of selective sites. During the catalyst modification, a significant aluminum leaching occurs as well. With decreasing the residual aluminum content in the catalyst, the optical yield of the reaction increases. Thus, the remaining atoms of aluminum in the modified catalyst are responsible for some of the nonselective sites. The presence of sodium bromide in the modifying solution has no effect on the aluminum leaching. The data obtained in this work infer that a chiral modification of Raney nickel catalyst using tartaric
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acid is a very complex process, which depends especially on an accurate optimization of the modifying conditions. Acknowledgements This work was supported by the Grant Agency of the ˇ 104/00/1009) and Czech Republic (Grant no. GACR the Ministry of Education (CEZ:MSM 223100001). References [1] H.-U. Blaser, B. Pugin, in: G. Jannes, V. Dubois (Eds.), Heterogeneous Catalysis, Plenum Press, New York, 1995, p. 33. [2] Y. Izumi, Adv. Catal. 32 (1983) 215. [3] A. Tai, T. Harada, in: Y. Iwasawa (Ed.), Tailored Metal Catalysts, Reidel, Dordrecht, 1986, p. 265. [4] E.I. Klabunovskij, Izv. Akad. Nauk SSSR, Ser. Chim. 505 (1984). [5] G. Webb, P.B. Wells, Catal. Today 12 (1992) 319. [6] L. Fu, H.H. Kung, W.M.H. Sachtler, J. Mol. Catal. 42 (1987) 29. [7] Y. Nitta, T. Utsumi, T. Imanaka, S. Teranishi, J. Catal. 101 (1986) 376. [8] Y. Nitta, T. Imanaka, S. Teranishi, J. Catal. 96 (1985) 429. [9] Y. Nitta, F. Sekine, J. Sasaki, T. Imanaka, S. Teranishi, J. Catal. 79 (1983) 211. [10] Y. Nitta, F. Sekine, T. Imanaka, S. Teranishi, J. Catal. 74 (1982) 382. [11] M.A. Keane, Can. J. Chem. 72 (1994) 372. [12] M.A. Keane, G. Webb, J. Catal. 136 (1992) 1. [13] M.A. Keane, Catal. Lett. 19 (1993) 197. [14] M.A. Keane, G. Webb, J. Chem. Soc., Chem. Commun. 22 (1991) 1619. [15] A.A. Vedenjapin, N.G. Giorgadze, I.P. Murina, S.V. Jushin, G.O. Chivadze, B.K. Nefedov, V.M. Akimov, Izv. Akad. Nauk SSSR, Ser. Chim. 1 (1990) 11. [16] N.D. Zubareva, E.I. Klabunovskij, G.V. Dorokhin, Izv. Akad. Nauk SSSR, Ser. Chim. 8 (1991) 1769. [17] A.A. Vedenjapin, B.G. Chankvetadze, L.K. Civinskaja, V.M. Akimov, E.I. Klabunovskij, React. Kinet. Catal. Lett. 33 (1987) 53. [18] N.D. Zubareva, V.V. Chernysheva, P.A. Zhdan, G.D. Chukin, V.M. Akimov, B.K. Nefedov, E.I. Klabunovskij, Kinet. Katal. 27 (1986) 1264. [19] A.A. Vedenjapin, G.G. Chankvetadze, Izv. Akad. Nauk SSSR, Ser. Chim. 10 (1988) 2213. [20] N.D. Zubareva, I.A. Ryndakova, E.I. Klabunovskij, Kinet. Katal. 29 (1988) 1485. [21] N.D. Zubareva, E.I. Klabunovskij, Izv. Akad. Nauk SSSR, Ser. Chim. 5 (1988) 1172. [22] T.I. Kuznetsova, I.P. Murina, React. Kinet. Catal. Lett. 37 (1988) 363.
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[23] H. Brunner, M. Mschiol, T. Wischert, J. Wiehl, Tetrahedron Asymmetry 1 (1990) 159. [24] A. Hoek, W.M.H. Sachtler, J. Catal. 58 (1979) 276. [25] A. Hoek, H.M. Woerde, W.M.H. Sachtler, in: Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980, p. 376. [26] G. Wittmann, G.B. Bartok, M. Bartok, G.V. Smith, J. Mol. Catal. 60 (1990) 1. [27] L.J. Bostelaar, W.M.H. Sachtler, J. Mol. Catal. 42 (1987) 387.
[28] G.V. Smith, M. Musoiu, J. Catal. 60 (1979) 184. [29] M.A. Keane, G. Webb, J. Mol. Catal. 73 (1992) 91. [30] A. Bennet, S. Christie, M.A. Keane, R. Peacock, G. Webb, Catal. Today 10 (3) (1991) 363. [31] J. Masson, P. Cividino, J. Court, J. Mol. Catal. 111 (1996) 289. [32] T. Harada, M. Ymamoto, S. Onaka, M. Imaida, H. Ozaki, A. Tai, Y. Izumi, Bull. Chem. Soc. Jpn. 54 (1981) 2323. [33] P. Kukula, Ph.D. Thesis, ICT, Prague, 2000.
Preparation of tartaric acid modified Raney nickel catalysts: study of modification procedure ∗ ˇ Pavel Kukula, Libor Cervený Department of Organic Technology, Institute of Chemical Technology-ICT Prague, Technická 5, 166 28 Prague 6, Czech Republic Received 18 July 2000; received in revised form 26 September 2000; accepted 26 September 2000
Abstract Chiral modification of Raney nickel using (2R,3R)-tartaric acid was studied. The prepared catalyst was used in enantioselective hydrogenation of methylacetoacetate (MAA) to methyl-(3R)-hydroxybutyrate. The influence of the most important modification parameters, such as pH, temperature, time, concentration of the modifier and the presence of a co-modifier, on the optical yield of MAA hydrogenation was systematically investigated. From the data obtained, a considerable influence of modifying conditions on the resulting enantioselectivity of the catalyst was evident. The optical yield increased with an increasing of the modifying temperature and time. Dependencies of the optical yield on the tartaric acid concentration and on the modifying pH passed through a maximum. Therefore, there exists an optimal value of modifying pH, at which a minimal catalyst amount is leached to the modifying solution. Furthermore, significant adsorption of tartaric acid and subsequent complex formation of nickel and tartaric acid occurs on the catalyst surface. It was found that the presence of sodium bromide in the modifying solution resulted in a lower degree of the nickel leaching, and a decrease of the residual aluminum content in the catalyst increased the optical yield of the reaction. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Enantioselective hydrogenation; Chirally-modified Raney nickel; Tartaric acid; Methylacetoacetate
1. Introduction Enantioselective hydrogenations can be used for preparation of a series of valuable bioactive compounds. Heterogeneous enantioselective catalysis in the course of a chirally-modified catalyst utilization [1] represents one of the possible ways of selective preparation of optically pure compounds. Unmodified metal catalysts display no inherent chirality and produce racemic mixtures. However, the chiral molecule ∗ Corresponding author. Tel.: +42-2-2431-0280; fax: +42-2-3119-657. ˇ E-mail address: [email protected] (L. Cerven´ y).
properly implemented on the surface of a metal catalyst can represent the source of chirality. One of these types of catalysts is the Raney nickel modified using tartaric acid, which is used for enantioselective hydrogenations of -ketoesters. Hydrogenation of methylacetoacetate (MAA) to methyl-3-hydroxybutyrate (MHB) over nickel catalyst modified using optically pure isomer of tartaric acid has become the model reaction for the study of different parameter influence on enantioselectivity of the catalyst [2,3]. Various types of nickel catalysts were used for enantioselective hydrogenations, e.g. Raney nickel [2,3], several supported nickel catalysts [6–14], bimetallic supported catalyst [15–22] and nickel powder [2,4,23].
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 1 2 - 7
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Chiral modification is one of the most important steps involved in the preparation of an enantioselective catalyst. During the modification process, formation of chiral active sites takes place and it affects the future catalyst enantioselectivity. The modification procedure is generally carried out in an aqueous solution of a suitable modifier. (2R,3R)-(+)-tartaric acid is the most commonly utilized and the most suitable modifier from the point of view of enantioselectivity. Modification is a typical corrosive process, characteristic by leaching of nickel to the modifying solution [5,11–14]. During the modification, it is necessary to optimize all of the modifying parameters, because they play ultimate role in the future catalyst asymmetric activity. It is known that the addition of co-modifier (mostly NaBr) to the modifying solution can rise the optical yields even by 10–30%. The effect of co-modifier is in the literature explained as a blocking of unmodified active sites, since the ratio of modified and unmodified sites determines the resulting optical yield [2,3]. One of the key problems in the course of the catalyst preparation, which is mentioned by many authors [24–28], is a poor reproducibility of measured reaction rates and optical yields over individual catalysts. Nevertheless, there is a good agreement that the catalyst enantioselectivity depends mostly upon the method of its preparation and on the mode of the modification procedure. The catalyst modification is possible to carry out in different ways, but the modifying conditions need to be optimized for each type of catalyst [2,3]. Keane [11–14,29], Nitta [7–10] and coworkers have studied in detail the effects of the modification process performance on the catalytic properties of supported nickel catalysts, especially Ni/SiO2 . Lesser attention has been paid to the study of Raney nickel modification [2,26,30–32]. Many diverse opinions on the influence of modifying conditions on the enantioselectivity of a modified Raney nickel can be found. Hence, this work is devoted to the study of Raney nickel modification with the use of (2R,3R)-tartaric acid. The paper is an attempt to describe in detail the Raney nickel modification procedure by a systematic investigation of the influence of the most important modifying parameters, such as pH, temperature, time, modifier concentration, etc., over a wide experimental range on the resulting optical yield.
2. Experimental 2.1. Catalyst Commercial Raney nickel, Actimet M (Egelhard) was used as the catalyst. The catalyst was stored under water in an alkaline environment. It was assumed during weighing of the catalyst suspension that the suspension contains about 50 wt.% of water. This fact was also confirmed experimentally by weighing out of the catalyst before and after drying in an inert atmosphere of nitrogen [33]. The total surface area of the catalyst was 75 m2 /g, the volume of pores 0.12 cm3 /g and the particle size ranged between 5 and 101 m. 2.2. Chemicals Optically pure (2R,3R)-(+)-tartaric acid (p.a., Lachema Brno) was used as the modifier. Antipodal enantiomer of tartaric acid, (2S,3S)-(−)-tartaric acid (p.a., Aldrich) was also tested. Furthermore, the following chemicals were used, sodium bromide, sodium hydroxide and methanol (all p.a., Lachema Brno), hydrochloric acid (p.a., Penta Prague), MAA (p.a., Aldrich), hydrogen 3.0 and helium 4.0 (LindeTechnoplyn a.s. Prague). 2.3. Apparatus and procedure First, the catalyst, mostly about 3 g, was three times washed with distilled water and then put to the so-called preliminary treatment, in which the catalyst was washed with 1% tartaric acid solution. This washing step was carried out under stirring, and the time and pH of the washing solution was monitored. After this, the catalyst was washed with 20 ml of distilled water and was introduced to the modifying solution for a proper modification. Modifying solution was prepared by dissolving of an appropriate amount of tartaric acid to 100 ml of distilled water. In some cases, sodium bromide was added to the modifying solution as a co-modifier. The modifying pH was adjusted to the required value by addition of 20% sodium hydroxide solution. The pH was measured by the digital pH meter WTW pH 583 with the combined electrode SenTix 97/T, WTW (Germany). Suspension of the catalyst and the modifying solution was stirred at a constant temperature for 1 h
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using the magnetic stirrer IKAMAG-RCT placed in Erlenmayer flask equipped with a reflux condenser. During the modification process, pH of the catalyst suspension and the modifying solution was measured. After decantation, the modifying solution was poured off, and the catalyst was subsequently washed with water and methanol (20 ml). Modified catalyst was then introduced to the hydrogenation. Hydrogenations were carried out in a liquid phase with a solvent (methanol) in a stainless autoclave with volume of 300 ml. The autoclave was supplied with a double-propeller with a magnetic transmission, a thermocouple probe, a pressure gauge and a heating mantle with an adjustable temperature. The temperature was recorded digitally. Most of the reactions were carried out starting by insertion of 50 ml (53.8 g, 0.46 mol) of MAA, 100 ml of methanol and about 2 g of the modified catalyst. All reactions proceeded at the same constant reaction conditions, temperature 100◦ C and overall pressure 10 ± 0.2 MPa. The reactions were conducted so that the total conversion of MAA would be achieved. After the reaction was completed, the catalyst and the solvent were removed from the reaction mixture by filtration and evaporation. The product was distilled under a decreased pressure (bp 62–63◦ C/2.4 kPa).
Samples of the reaction mixture were withdrawn during the hydrogenations, and analyzed by gas chromatography using the gas chromatograph HP 5890 Series II Plus (Hewlett-Packard, USA) with a flame ionization detector (FID) and the enantioselective capillary column -DEX 325 (30 m×0.25 mm×0.25 m) of Supelco Co. The analyses were carried out using the temperature program from 80 to 160◦ C with a constant initial temperature for the duration of 7 min. The first temperature rate was 10◦ C/min up to the temperature of 95◦ C, and the second temperature rate was 70◦ C/min up to the temperature of 160◦ C. The carrier gas pressure p(N2 ) was 79 kPa, the flow rate was 1.0 ml/min and the split ratio was 1:100. Under such chromatographic conditions, it was possible to carry out the separation of the produced enantiomers of methyl-3-hydrobutyrate. Following the fastidious separation of enantiomers, analyses were carried out with the lowest possible concentration of the determined compounds with the use of a very low injection volume (0.1–0.2 l) of the reaction mixture and an adjusted maximal sensitivity of the detector. The enantioselectivity of the reaction was calculated by use of the following equation:
2.4. Analysis
Optical yield was determined polarimetrically by means of the optical activity measurement of the product in its concentrated state. Digital polarimeter DIP-370 of Jasco (Taiwan) was used for the measurements. Optical yield (e.e.) was then calculated using the following equation:
The amount of nickel and aluminum leached to the modifying solution was monitored by means of atomic absorption spectrometry using AAS spectrometer SpectrAA 300 of Varian (USA). In order to compare, the content of nickel and aluminum in the catalyst was monitored before and after the modification. Weighted amount of a catalyst was placed into a distillation flask equipped with a reflux condenser and quartz capillary reaching to the bottom of the flask. Then, 100 ml of distilled water and 2 ml of concentrated hydrochloric acid was added to the catalyst. Apparatus was supplied with a heater with an adjustable drain power. The nitrogen was fed into the outer end of the capillary and was bubbled through the layer of the catalyst, which caused its agitation. All samples were dissolved in this way within 30 min at the boiling point of the liquid phase. Content of nickel and aluminum was measured in such obtained solutions using atomic absorption spectrometry.
e.e. = oy (%) =
e.e. = oy (%) =
|[(R)-MHB] − [(S)-MHB]| × 100 [(R)-MHB] + [(S)-MHB]
[␣]20 d product × 100 [␣]20 d max
where [␣]20 d product represents the specific optical rotation of the product, and [␣]20 d max the specific optical rotation of a pure enantiomer in its concentrated state, i.e. −22.95◦ for this case of methyl-(3R)(−)-hydroxybutyrate.
3. Results and discussion Since the Raney nickel catalyst was stored in an alkaline environment, it was necessary to wash it
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several times with distilled water before modification. The pH of the washing solutions was monitored, and it was found that after three washing steps, it decreased to the value of 9.3. During preliminary treatment (washing the catalyst with 1% tartaric acid solution), which took about 10 min, an increase in pH of the washing solution by about 0.4–0.5 pH occurred. At the same time, the washing solution darkened to chartreuse color as a result of the nickel leaching. It was found that the preliminary treatment of the catalyst with tartaric acid solution had a positive effect on the resulting enantioselectivity. It could be explained by a removing of the residual alkali from the catalyst and by an activation of the catalyst surface with tartaric acid. After the preliminary treatment, proper modification was carried out, during which an increase of the modifying solution pH took place by 2–3 on an average. This increase was probably caused by an adsorption of tartaric acid on the catalyst surface. Alternative explanation could be a dissolving of the residual aluminum from the catalyst, followed by a formation of alkaline aluminum compounds, which gradated to the liquid phase. At the lower values of the modifying pH, the modifying solution became green due to the nickel leaching to the modifying solution. Hydrogenation of MAA with an unmodified catalyst proceeds to the formation of racemic mixture (50:50) of methyl-(3R)-(−)-hydroxybutyrate and methyl-(3S)-(+)-hydroxybutyrate. The catalyst modified with (2R,3R)-(+)-tartaric acid produced a higher content of methyl-(3R)-(−)-hydroxybutyrate, while the catalyst modified with (2S,3S)-(−)-tartaric acid produced a higher content of methyl-(3S)-(+)hydroxybutyrate. Using antipodal enantiomer under the same modifying conditions, the optical yield of the reaction was nearly coincident. During the study of the influence of the modifying conditions on the optical yield, (2R,3R)-(+)-tartaric acid was used as the modifier. 3.1. Influence of modifying conditions on optical yield of the reaction 3.1.1. Tartaric acid concentration Optical yield dependence on tartaric acid concentration passes through a maximum (Fig. 1). The highest values of enantioselectivity were achieved in the region of tartaric acid concentration of about
Fig. 1. Dependence of the optical yield on tartaric acid concentration, modifying conditions: pH = 5.1; T = 100◦ C; t = 60 min; without NaBr.
0.2 mol/l. At higher concentrations, the optical yield decreased, and at concentrations higher than 0.5 mol/l the optical yield became independent of the tartaric acid concentration. Apparently, there were several effects acting together. With an increasing of the tartaric acid concentration, higher degree of its adsorption on the catalyst surface occurred, and the catalyst surface coverage increased. Simultaneously, nickel from the catalyst was leached into the modifying solution. This could also be examined visually, as the color of the solution was changed from clear to light green. Thus, an increased nickel leaching to the modifying solution brings about a decrease in the catalyst activity. Therefore, there exists an optimal tartaric acid concentration at which a certain amount of the modifier is adsorbed on the catalyst surface, providing that in the same time, only a low nickel leaching is taking place. 3.1.2. Modifying temperature The modifying temperature has also an effect on the optical yield (Fig. 2). With an increasing of the modifying temperature, the optical yield of the reaction increases almost with the linear rate. The more the temperature was increased, the higher amount of tartaric acid was probably adsorbed on the catalyst surface, which resulted in higher optical yields. It was also found that the catalysts modified at higher temperatures had a lower activity, however reached
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the highest optical yields lay in the region of tartaric acid isoelectric point (pH = 5.1). Having used acidic environment at lower pH values, increased dissolving of nickel took place, followed by the formation of Ni2+ ions on the catalyst surface. Then, these ions are leached to the modifying solution. Tartaric acid probably forms a fixed surface complex with Ni2+ ions. Consecutively, this surface complex serves as a chiral site on the catalyst surface. However, the complex formation is subjected to a specific modifying pH, which explains an occurrence of the maximum at the optical yield dependence on the modifying pH.
Fig. 2. Dependence of the optical yield on modifying temperature, modifying conditions: pH = 5.1; cTA = 0.2 mol/l; t = 60 min; without NaBr.
higher values of enantioselectivity. This was caused by the fact that with an increasing of the temperature, more nickel dissolves into the modifying solution. The higher the modifying temperature was, the greener the solution had its color.
3.1.4. Time of modification Fig. 4 shows the optical yield dependence on the modification time. Initially, the optical yield steeply increased with the modification time. After 50 min, the optical yield nearly reached its maximal value and then remained constant. With an increasing of the modification time, adsorption of the modifier on the catalyst surface occurred. Equilibration of the concentration of the modifier, dissolved in the liquid phase and adsorbed on the catalyst surface, occurred when the steady state was achieved.
3.1.3. Modifying pH Modifying pH had a considerable effect on the optical yield of the reaction. The dependence of the optical yield on the modifying pH (Fig. 3) also passed through a maximum. Optimal pH value for achieving
3.1.5. Presence of co-modifier Second modifying reagent (co-modifier) was used as the sub-component of the modifying solution. It was found [5] that several sodium salts were effective
Fig. 3. Dependence of the optical yield on modifying pH, modifying conditions: cTA = 0.2 mol/l; T = 100◦ C; t = 60 min; without NaBr.
Fig. 4. Dependence of the optical yield on modifying time, modifying conditions: cTA = 0.2 mol/l; pH = 5.1; T = 100◦ C; without NaBr.
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Fig. 5. Dependence of the optical yield on NaBr concentration, modifying conditions: cTA = 0.2 mol/l; pH = 5.1; T = 100◦ C.
Fig. 6. Effect of tartaric acid concentration on the extent of leached nickel into the modifying solution, modifying conditions: pH = 5.1; T = 100◦ C; t = 60 min; (䊏) without NaBr; (䉬) with NaBr.
as second modifying reagents due to a strong enhancement of the enantioselectivity of the catalyst. Among them, sodium bromide was the best. In this work, the sodium bromide was used as the co-modifier. Fig. 5 shows the effect of the co-modifier addition to the modifying solution on the optical yield of the reaction. From the presented dependence, an increase in the optical yield occurring with an increase of the amount of sodium bromide in the modifying solution is perceptible. The optical yield increased with an increase of the amount of sodium bromide reaching a plateau. The constant value was obtained with a higher concentration of sodium bromide than 2 mol/l. Thus, the presence of sodium bromide in the modifying solution can raise the optical yield even by 25%, regardless of the sodium bromide having no enantio-differentiating ability itself. The effect of a co-modifier is explained in the literature [2,3] by a blocking of the unmodified active sites. However, an addition of the co-modifier has also an influence on an occurrence of additional phenomenon. Since, the concentration of the co-modifier in the modifying solution was considerably high, the solution was nearly saturated. Sodium bromide affects the ionic strength of modifying solution and decreases hydrogen ions concentration. It resulted in a lower possibility for nickel and its compounds leaching to the modifying solution and in better conditions for complex formation between nickel and tartaric acid. The presence of co-modifier has also probably an effect on the tartaric acid stereo-
chemistry, which could be reflected by the resulting optical yield. 3.2. Influence of modifying conditions on nickel leaching into modifying solution During the following study of the catalyst modification, leaching of nickel and aluminum into the modifying solution was monitored in detail. Figs. 6–9 depict the effects of the modifying conditions on the amount of the leached nickel for the modifications,
Fig. 7. Effect of modifying temperature on the extent of leached nickel into to the modifying solution, modifying conditions: pH = 5.1; cTA = 0.2 mol/l; t = 60 min; (䊏) without NaBr; (䉬) with NaBr.
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3.2.2. Modifying temperature Similar situation could be observed in the dependence of the leached nickel on the modifying temperature (Fig. 7), showing that an increasing of the temperature has also increased the amount of the leached nickel.
Fig. 8. Effect of modifying pH on the extent of leached nickel into the modifying solution, modifying conditions: cTA = 0.2 mol/l; T = 100◦ C; t = 60 min; (䊏) without NaBr; (䉬) with NaBr.
which were carried out with and without a co-modifier. It is noticeable from all the figures that during the modifications, which were carried out without a co-modifier, several times higher nickel leaching into the modifying solution occurred. 3.2.1. Tartaric acid concentration Fig. 6 shows the dependence of the nickel leached into the modifying solution on the tartaric acid concentration. With an increasing of the tartaric acid concentration, the amount of the leached nickel increased.
3.2.3. Modifying pH The pH of the modifying solution (Fig. 8) had the ultimate effect on the amount of the leached nickel. Considerable dissolution of the catalyst occurred under acidic conditions. The amount of the leached nickel was more than 30 times higher than the amount leached under the optimal modification conditions (cTA = 0.2 mol/l, T = 100◦ C, pH = 4.9 and t = 60 min). In the range of pH = 3, the rapid ordinal decrease could be examined. When the modifying pH was higher than 4.8, the amount of nickel leached decreased slightly. 3.2.4. Modification time The dependence of the leached nickel on the modification time is presented in Fig. 9. There is a noticeable break in the dependence at about 60th minute. The amount of the leached nickel increased with the modification time faster in the beginning, then the nickel leaching proceeded in a less rapid pace. This fact is probably caused by a certain concentration equilibration of the modifier in the modifying solution and on the catalyst surface. Furthermore, it was in accordance with the ascertained dependence of the optical yield on the modification time. Table 1 shows the amount of nickel and aluminum leached into the modifying solutions under the optimal modifying conditions. During the preliminary treatment of a catalyst using 1% aqueous solution of tartaric acid, 2.3 wt.% of nickel and 1.7 wt.% of
Table 1 The amount of leached nickel and aluminum into the modifying solutions mg Ni/gcat. %Ni mg Al/gcat. %Al Pretreatmenta Fig. 9. Effect of modifying time on the extent of leached nickel into the modifying solution, modifying conditions: pH = 5.1; cTA = 0.2 mol/l; T = 100◦ C; (䊏) without NaBr; (䉬) with NaBr.
Modification with NaBr Modification without NaBr a
23.4 1.8 9.2
2.3 0.2 0.9
1.2 44.3 43.9
Washing with 1 wt.% tartaric acid solution for 10 min.
1.7 63.3 62.7
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aluminum, related to the total amount of nickel and aluminum, was leached into the washing solution within approximately 10 min. It can be implied that in aqueous solutions, where only tartaric acid is present, uniform leaching of nickel and aluminum takes place. During the modification, aluminum leaching is preferred due to the complex of nickel and tartaric acid formation on the catalyst surface. In the course of the modification, two-third of the aluminum content in the catalyst was leached into the modifying solution. It was found that the amount of aluminum leached into the modifying solution has also an effect on the optical yield of the reaction. With more aluminum leaching from the catalyst, the higher optical yield was achieved. Thus, it is presumable that the residual aluminum contained in the catalyst could be responsible for some of the nonselective active sites. The influence of a co-modifier addition to the modifying solution on the nickel and aluminum leaching is also perceptible from Table 1. The presence of sodium bromide in the modifying solution resulted in a lower nickel leaching with preservation of the amount of the aluminum leached, which corresponded with the measured data. The presence of sodium bromide in the modifying solution could also stabilize the nascent complex of nickel and tartaric acid on the catalyst surface. This assumption could explain the practically zero effect of sodium bromide addition on the aluminum leaching. Comparison of the calculated and the measured amount of nickel and aluminum in the catalysts prior and after the modification is presented in Table 2. The amount of nickel and aluminum in the catalysts was calculated from known losses of both metals during the pre-treatment and the modification. The measured amounts of metals were acquired by dissolving appropriate catalysts and by determining their
Table 2 Comparison between computed and measured content of nickel and aluminum in catalysts Catalyst
RaNi TA-NaBr-RaNi TA-RaNi
Computed/measured content of Ni and Al Ni (wt.%)
Al (wt.%)
–/92.7 97.4/97.6 97.3/97.4
–/7.3 2.6/2.4 2.7/2.6
concentrations using atomic absorption spectrometry. From the acquired data, a very good agreement between the calculated and the measured contents of individual elements was noticeable. The verified data infer that the modification is a typical corrosive process characterized by leaching of nickel and aluminum to the modifying solution, which resulted in the production of a complex of nickel and tartaric acid predominantly chemisorbing on the catalyst surface. The amount of the leached nickel and the created complex depends on the modifying conditions. In order to form a tartrate complex on a catalyst surface, a sufficient driving force of the reaction between nickel and tartaric acid must exist. For this reason, it is necessary to optimize all the modifying parameters in such a way that a high production of tartrate complex together with a low nickel leaching into the modifying solution would be achieved as the catalyst subsequently looses its activity. The highest values of the catalyst enantioselectivity are possible to achieve by modifying the catalyst under pH of about 5, while the pH was adjusted by titration of 20% sodium hydroxide solution. Under these conditions, the change of both protons of tartaric acid to sodium ions occurs roughly up to 80%. Thus, sodium salt of tartaric acid is the component which is involved in the complex formation on the catalyst surface. The nickel atoms are coordinated by oxygen atoms of carboxyl and hydroxyl groups of tartaric acid in the complex. Ni2+ ions released into the modifying solution, form a complex with tartrate as well. This complex is dissolved in the liquid phase and slows the catalyst modification. The role of the sodium ions present in the solution consists of the decrease of hydrogen ions concentration followed by higher stability of nascent complex of nickel and tartaric acid. Each end of tartrate has three oxygen atoms available to form coordination bonds with a nickel atom. Two of them are involved in the formation of the tartrate complex, while the third oxygen atom could form a bond with an additional nickel atom on the surface, or the negative charge of the molecule could be balanced by a sodium ion. This way, it is possible to explain the low enantioselectivity of the catalysts modified at low pH. Lower pH values of the modifying solution resulted in an extensive nickel leaching, which decreased the catalyst activity and increased the formation of nickel–tartrate complex in the modifying solution, but not on the
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catalyst surface. Adsorption of the already formed nickel–tartrate complex is particularly poor. This was found when the catalyst was modified by a separately prepared complex [33]. Enantioselectivity of this catalyst was significantly lower in comparison to the catalyst which had the complex formed “in situ” on its surface.
4. Conclusions The work is engaged in the study of chiral modification of Raney nickel catalyst using (2R,3R)-(+)tartaric acid. From the acquired data, a considerable influence of modifying conditions on the resulting enantioselectivity of the catalyst is evident. As to achieve the highest optical yields of MAA hydrogenation, it is necessary to optimize the parameters of modification. These parameters have to be optimized to create the optimal amount of the complex of nickel and tartaric acid on the catalyst surface together with a low nickel leaching into the modifying solution as the catalyst subsequently looses its activity. Amount of the adsorbed tartaric acid on the catalyst surface increases with an increasing of its concentration, the modifying temperature and the period of modification. One of the most important modifying parameters is pH of the modifying solution. There exists an optimal value of the modifying pH at which a minimal catalyst amount is leached into the modifying solution, and at the same time a significant adsorption of tartaric acid and subsequent complex formation of nickel and tartaric acid on the catalyst surface takes place. The presence of sodium bromide in the modifying solution resulted in a lower degree of nickel leaching. Thus, sodium bromide affects the catalyst enantioselectivity not only by blocking the nonselective sites, but also by affecting the formation of selective sites. During the catalyst modification, a significant aluminum leaching occurs as well. With decreasing the residual aluminum content in the catalyst, the optical yield of the reaction increases. Thus, the remaining atoms of aluminum in the modified catalyst are responsible for some of the nonselective sites. The presence of sodium bromide in the modifying solution has no effect on the aluminum leaching. The data obtained in this work infer that a chiral modification of Raney nickel catalyst using tartaric
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acid is a very complex process, which depends especially on an accurate optimization of the modifying conditions. Acknowledgements This work was supported by the Grant Agency of the ˇ 104/00/1009) and Czech Republic (Grant no. GACR the Ministry of Education (CEZ:MSM 223100001). References [1] H.-U. Blaser, B. Pugin, in: G. Jannes, V. Dubois (Eds.), Heterogeneous Catalysis, Plenum Press, New York, 1995, p. 33. [2] Y. Izumi, Adv. Catal. 32 (1983) 215. [3] A. Tai, T. Harada, in: Y. Iwasawa (Ed.), Tailored Metal Catalysts, Reidel, Dordrecht, 1986, p. 265. [4] E.I. Klabunovskij, Izv. Akad. Nauk SSSR, Ser. Chim. 505 (1984). [5] G. Webb, P.B. Wells, Catal. Today 12 (1992) 319. [6] L. Fu, H.H. Kung, W.M.H. Sachtler, J. Mol. Catal. 42 (1987) 29. [7] Y. Nitta, T. Utsumi, T. Imanaka, S. Teranishi, J. Catal. 101 (1986) 376. [8] Y. Nitta, T. Imanaka, S. Teranishi, J. Catal. 96 (1985) 429. [9] Y. Nitta, F. Sekine, J. Sasaki, T. Imanaka, S. Teranishi, J. Catal. 79 (1983) 211. [10] Y. Nitta, F. Sekine, T. Imanaka, S. Teranishi, J. Catal. 74 (1982) 382. [11] M.A. Keane, Can. J. Chem. 72 (1994) 372. [12] M.A. Keane, G. Webb, J. Catal. 136 (1992) 1. [13] M.A. Keane, Catal. Lett. 19 (1993) 197. [14] M.A. Keane, G. Webb, J. Chem. Soc., Chem. Commun. 22 (1991) 1619. [15] A.A. Vedenjapin, N.G. Giorgadze, I.P. Murina, S.V. Jushin, G.O. Chivadze, B.K. Nefedov, V.M. Akimov, Izv. Akad. Nauk SSSR, Ser. Chim. 1 (1990) 11. [16] N.D. Zubareva, E.I. Klabunovskij, G.V. Dorokhin, Izv. Akad. Nauk SSSR, Ser. Chim. 8 (1991) 1769. [17] A.A. Vedenjapin, B.G. Chankvetadze, L.K. Civinskaja, V.M. Akimov, E.I. Klabunovskij, React. Kinet. Catal. Lett. 33 (1987) 53. [18] N.D. Zubareva, V.V. Chernysheva, P.A. Zhdan, G.D. Chukin, V.M. Akimov, B.K. Nefedov, E.I. Klabunovskij, Kinet. Katal. 27 (1986) 1264. [19] A.A. Vedenjapin, G.G. Chankvetadze, Izv. Akad. Nauk SSSR, Ser. Chim. 10 (1988) 2213. [20] N.D. Zubareva, I.A. Ryndakova, E.I. Klabunovskij, Kinet. Katal. 29 (1988) 1485. [21] N.D. Zubareva, E.I. Klabunovskij, Izv. Akad. Nauk SSSR, Ser. Chim. 5 (1988) 1172. [22] T.I. Kuznetsova, I.P. Murina, React. Kinet. Catal. Lett. 37 (1988) 363.
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ˇ P. Kukula, L. Cerven´ y / Applied Catalysis A: General 210 (2001) 237–246
[23] H. Brunner, M. Mschiol, T. Wischert, J. Wiehl, Tetrahedron Asymmetry 1 (1990) 159. [24] A. Hoek, W.M.H. Sachtler, J. Catal. 58 (1979) 276. [25] A. Hoek, H.M. Woerde, W.M.H. Sachtler, in: Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980, p. 376. [26] G. Wittmann, G.B. Bartok, M. Bartok, G.V. Smith, J. Mol. Catal. 60 (1990) 1. [27] L.J. Bostelaar, W.M.H. Sachtler, J. Mol. Catal. 42 (1987) 387.
[28] G.V. Smith, M. Musoiu, J. Catal. 60 (1979) 184. [29] M.A. Keane, G. Webb, J. Mol. Catal. 73 (1992) 91. [30] A. Bennet, S. Christie, M.A. Keane, R. Peacock, G. Webb, Catal. Today 10 (3) (1991) 363. [31] J. Masson, P. Cividino, J. Court, J. Mol. Catal. 111 (1996) 289. [32] T. Harada, M. Ymamoto, S. Onaka, M. Imaida, H. Ozaki, A. Tai, Y. Izumi, Bull. Chem. Soc. Jpn. 54 (1981) 2323. [33] P. Kukula, Ph.D. Thesis, ICT, Prague, 2000.