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Particle size effect on enantioselective hydrogenation of methylacetoacetate over silica-supported nickel catalyst
Journal of Molecular
Catalysis,
42 (1987)
29 - 36
29
PARTICLE SIZE EFFECT ON ENANTIOSELECTIVE HYDROGENATIONOFMETHYLACETOACETATEOVER SILICA-SUPFO RTED NICKEL CATALYST L. FU, H. H. KUNG and W. M. H. SACHTLER Zpatieff Laboratory, IL 60201 (U.S.A.)
Department
(Received July 11, 1986;accepted
of Chemistry,
Northwestern
University,
Evanston,
April 14, 1987)
Summary
The effect of metal particle size on the enantioselective hydrogenation of methylacetoacetate (MAA) to methyl 3-hydroxybutyrate (MHB) in the gas and the liquid phases was studied over Ni/SiO? modified with R,Rtartaric acid. The Ni particle size of the catalyst at controlled Ni load was increased from 3.3 nm up to about 11 nm by Ostwald ripening, due to the formation and decomposition of gaseous Ni(C0)4. The experimental data showed no significant particle size effect on enantioselectivity within this size range. Atomic absorption analysis of the solution after modification showed that up to 50% of the Ni in the catalyst can be leached out during modification under severe conditions.
Introduction
Nickel modified by immersion in a solution of an optically active material catalyzes the enantioselective hydrogenation of molecules with a prochiral center. An example is the enantioselective hydrogenation of methylacetoacetate (MAA) to 3-hydroxybutyrate (MHB) over nickel, modified with R,R-tartaric acid, as the catalyst. In fact, this reaction has been extensively studied. Enantioselectivity has been reported for Ni, Pt, Ru, Cu and some alloys [l, 21. The effect of varying the modification conditions (pH, modifier concentration, presence of co-modifier, time and temperature), and the reaction conditions (liquid or gas phase, pressure, temperature, presence of other materials such as sulfates or halides in the reaction solution) has been extensively studied [3 - 61. Several groups have used unsupported metals [ 31, in particular Raney nickel, as the catalyst, while other researchers preferred silica-supported metals which are reduced with hydrogen at elevated temperatures (whereas Raney Ni is simply prepared by leaching a NiAl alloy in alkali) [5]. The supported catalysts permitted a systematic study of the effects of metal 0 Elsevier Sequoia/Printed
in The Netherlands
30
loading on the conversion and enantioselectivity. Vedenyapin et al. and Nitta et al. found in such studies that the en~tioselectivity can depend markedly on the metal loading, and they assumed that this observation reveals a particle size effect; in other words, they assume that enantioselectivity is a structure-sensitive phenomenon [ 7 - 91. The latter conclusion is, however, not necessarily correct, as it can be argued that even in the absence of any structure sensitivity the enantioselectivity may vary with the metal loading of the supported catalysts. It should be remembered that catalyst modification with an aqueous solution of tartaric acid is a pore diffusion-controlled process. Even with very long modification time, some metal particles will never be in contact with the modifying solution if gas plugs prevent the liquid from entering narrow pores. It is also well known that modification is accompanied by metal leaching [lo], and it has been shown that in particular at low metal loading much metal is leached from the peripheral zone of the catalyst pellets. Furthermore, the hydrogenation reaction in the liquid phase under the conditions usually used is a pore diffusion-controlled process, i.e., limited by the diffusion of dissolved hydrogen. As a consequence, it is conceivable that for catalysts of high metal loadings, the well-modified metal particles in the pellet periphery contribute most to the reaction. At low metal loadings, however, when much metal is leached from the outer zone, the poorly modified metal particles in the interior of the pellet contribute substantially to the hydrogenation. The result is low enantioselectivity at low metal loading irrespective of any true particle size effects. An even more complicated pattern is expected if one also considers the diffusion of dissolved nickel tartrate, which is the reaction product of the modification process. The concentration of nickel tartrate in the liquid phase far from the catalyst is initially zero. Thus the concentration gradient of dissolved tartrate has a sign opposite to that of tartaric acid at short modification times and also to that of hydrogen during the hydrogenation. In order to decide whether a true effect of metal particle size and/or shape is involved in enantioselectivity, it is therefore imperative to vary the particle size at constant metal loading. This is the objective of the research described in the present paper. We have chosen to change the size of the nickel particles in a given Ni/SiOz catalyst by using ‘Ostwald ripening’, in which the larger particles grow at the expense of the smaller particles [ 111. For this purpose, the formation and decomposition of nickel tetracarbonyl are used: Ni+4COe
Ni(C0)4
In earlier work [12] we had verified that this reaction is sufficiently rapid at > 50 “C to specifically remove the Ni particles of high free energy, while the decomposition of the carbonyl on particles of low free energy causes them to grow. We have monitored the growth of the average size of the nickel particles by hydrogen adsorption (after the catalysts had been used for reaction), and
31
by the broadening of X-ray diffraction lines. The enantioselectivity under standardized conditions was then measured for samples of different Ni particle sizes, but constant metal loading.
Experimental Catalyst preparation Wide pore (40 - 60 nm) silica beads (2.5 mm in diameter, Koninklijke/ Shell Laboratorium, Amsterdam) with a BET surface area of 40 m2 g-i were washed with dilute NaOH and HNO, and finally with demineralized water to pH 7, then dried in an oven at 62 “C. 27 g of the washed silica was suspended in a 600 ml solution of 20 g Ni(NOs),-6H2O (Aldrich Chemical Company, Inc.) to which 50 ml NH,OH (American Scientific Products, Mallinckrodt) (29.4 wt.%) was added at 40 “C. The suspension was kept at this temperature for 13 h. The resulting catalyst had an average nickel particle size of 3.3 nm. Reduction About 0.5 g catalyst was reduced by hydrogen using temperatureprogrammed reduction. A H2 flow rate of 100 ml min-’ was used and the temperature was increased by 3 “C min-’ from 25 to 450 “C and kept at 450 “C for 11 h. Ostwald ripening CO purified with A120s at 200 “C and 1 atm was introduced into a reactor with a volume of about 50 ml containing 0.5 g freshly reduced catalyst at room temperature. After sealing the reactor from the rest of the system, the temperature was increased to 80 “C and kept at this temperature for various periods of time. Modification After Ostwald ripening, the catalysts were reduced again with the same TPR method mentioned above. A 2 wt.% tartaric acid (Aldrich Chemical Company, Inc.) solution was used in modification, the pH of the solution was adjusted to 5 with NaOH. Three modification conditions were used: 1. 20 ml solution at room temperature for 2 h; 2. 20 ml solution at room temperature for 13 h; 3. 100 ml solution at 100 “C for 2 h. Doubly distilled water and MeOH were used to wash the catalyst after modification, and the catalyst was then transferred into the reactor under the protection of MeOH. The solution used in the modification was analyzed using atomic absorption spectroscopy to determine the amount of metal leached from the catalyst during modification.
32
Hydrogenation The gas phase hydrogenation was performed in a plug flow system, shown in Fig. 1, at 55 “C with a hydrogen flow rate of 100 ml min-‘. The partial pressure of MAA (Aldrich Chemical Company, Inc.) was 0.34 torr. The product was collected at liquid nitrogen temperature for about 50 h. The liquid phase hydrogenation was carried out in a reactor shown in Fig. 2. The reaction was carried out at 55 “C!under 1 atm of hydrogen using 5 ml MAA and 0.5 g of catalyst. Particle size measurement Both HZ chemisorption and XRD method were used to determine the Ni particle size after the hydrogenation. Temperature programmed reduction was used to reduce the catalyst before the measurements. For H, chemisorption, the catalyst was heated to 400 “C for 2 h to remove the tartrate on the surface before the reduction. The particle size was calculated from the dispersion measured with chemisorption by assuming that the crosssection area of the Ni atom is 0.065 nm* [ 131. The Scherrer formula: t=
0.9 x J31/2
~0s
6~
used to calculate the average Ni particle size from the half-width B,,? of the (111) diffraction peak [ 141, where B,,? = (Bi,*, exp2- Br2)lR B, is the instrument broadening which was measured with a Ni foil. Temperature-programmed reduction is essential to keep the Ni particle size unchanged during the reduction. It has been reported [15] that the was
Fig. 1. Schematic drawing of the system for gas phase hydrogenation. 1. flow meter, 2. Teflon valve, 3. water trap, 4. water-cooled MAA saturator, 5. flug-flow reactor, 6. furnace, 7. septa for sampling, 8. ball joints, glass with Viton O-ring, 9. product trap, 10. soap film meter.
33
-
r
i5
Fig. 2. Reactor for liquid phase hydrogenation. 1. Teflon valve, 2. Teflon cap, 3. 50 ml Pyrex flask, 4. aluminium water jacket, 5. motor.
water formed during reduction accelerates the sintering rate of Ni catalyst. Ni and SiOz can also form a spine1 in the presence of water and thus decrease the reducibility of the catalyst [ 161.
Results and discussion Table 1 shows the effect of metal loading after modification by the three different methods. TABLE 1 Metal loading of catalysts modified by various methods Modification method
Ni loading after modification (wt.%)
none 1 2 3
11.3 10.3 f 0.3 9.5 + 0.1 5.8 + 0.3
34
Evidently a significant amount of Ni was leached during the modification process. The amount increases with the severity of the modification conditions. Once again, this indicates that modification is a corrosive process. As the smallest Ni particles have the highest free energy [19], they will be leached out first. Indirectly this lends further support to our model that the selective site is formed by a corrosive chemisorption of Ni tartrate [lo, 17, 181. Tartaric acid reacts with Ni on the surface and forms a Ni tartrate complex; this complex can stay on the surface to form a selective site, or it can diffuse into the solution. Due to the inward diffusion of tartaric acid in the catalyst pores and the outward diffusion of dissolved Ni tartrate, the concentration of the latter will vary with time and space. The structure of the Ni tartrate complex on the Ni surface is not known, but that of three-dimensional Ni tartrate has recently been identified [ 171. If we assume that the selectivity is proportional to the coverage of the surface with Ni tartrate, some intermediate region inside each pore should have the highest selectivity. Table 2 shows the ripening time, the corresponding average particle size of Ni after modification and the enantioselectivity of the catalyst. The average particle size of the catalyst increased with increasing time of ripening. Clearly, larger Ni particles grew at the expense of smaller particles. TABLE 2 A summary of the Ni particle sizes, the conversion and the enantioselectivity catalysts Modification method
Ripening time (h)
Particle size (nm) from Hz chemisorption
from XRD line width
of different
e.e. (%)
Conversion (%)
Gas phase hydrogenation 1 0 2.75 7.25 9.25 1:
3.9
3.9
6.5 7.3 a.3 10.8
5.7 6.3 6.5 1.7
10.9 8.4 9.7 9.6 8.8
96.1 56.9 83.6 94.8 96.1
Gas phase hydrogenation 2 0 6.8 10.5
5.4 6.5 10.7
3.3 5.5 6.8
14.5 15.0 15.2
92.6 87.3 93.6
4.8 5.6 7.0 7.5
4.3 5.5 6.3 6.1
32.7 33.2 32.0 30.7
4.0 6.7 3.2 7.0
Liquid phase hydrogenation 3 0 4.5 8.15 10
35
lo-
P.S.
(“In)
6
te.e.1t te.e.)t=o
-
- 20 4,r
.
.
.
.
.
A 1.0
T2-
0
- w
1
I
I
I
I
2
4
6
8
10
t
12
(hr)
Fig. 3. Particle size (P.S.) and normalized condition indicated for P.S. only.)
e.e.
versus ripening
time
(t).
(Modification
The results in Table 2 show that over the Ni particle size range from - 4 - 11 nm, there is no systematic change in enantioselectivity, which remains constant to within a few percent for each modification method. This is illustrated in Fig. 3, where the enantiomeric excess, e.e.,, after ripening, normalized by e.e.,, ,, at zero ripening time, and the particle size are plotted uersus ripening time. There is a substantial variation in the enantioselectivity with the modification method. The selectivity for catalysts modified by method 2 is about 50% higher than those modified by method 1. This agrees with the assumption that a longer modification time results in better modification. The still higher selectivity for catalysts modified by method 3 may be a combined effect of higher temperature during modification and the use of liquid phase hydrogenation. The latter has been shown generally to yield a higher selectivity than gas phase hydrogenation [ 51. The absence of significant structure sensitivity is in agreement with the dual site model of enantioselectivity. This model assumes that the selective formation of one enantiomer takes place on Ni sites which are surrounded by one or two tartrate ligands. Five-membered chelate rings are formed, each consisting of the nickel atom (or ion), one oxygen atom of a carboxylate group, two carbon atoms and one oxygen atom of the hydroxy group of the tartrate ligand. This configuration is stereospecific, the methyl acetoacetate which is adsorbed on this nickel atom (presumably as an enolate) is in an asymmetric environment. Upon adding three hydrogen atoms to the enolate, one enantiomer or the product methyl 3-hydroxybutyrate is formed. This mechanism requires the stepwise additon of the hydrogen atoms. As nickel tartrate lacks the ability to dissociate dihydrogen, these hydrogen atoms have to be formed on the metal surface underneath the tartrate overlayer. Once the tartrate layer has been formed by reaction of tartaric
36
acid with nickel, the remaining nickel metal has only this simple function of dissociating dihydrogen. In terms of this model it is therefore quite natural that the structural requirements for the nickel surface under the tartrate overlayer are extremely facile, and no dependence of the enantioselectivity on the particle size is observed. This statement is, however, limited to the regime of particle sizes which survive the corrosion by the tartaric acid during modification without being completely converted to nickel tartrate. Smaller particles which are completely leached during the ‘modification process’ are, of course, totally ineffective for the hydrogenation.
Acknowledgment This work was supported by the National Science Foundation, under grand No. CHE-8216664.
References 1 E. I. Klabunovskii, J. Phys. Chem. (Russian), 47 (1973) 765. 2 E. I. Klabunovskii, A. A. Vedenyapin, B. G. Chankvetadze and G. C. Areshidze, Proc. 8th Znt. Congr. Catal., Berlin, 1984. 3 Y. Izumi, Adu. Catal., 32 (1983) 215. 4 T. Harada, M. Yamamoto, S. Onaka, M. Imaida, H. Ozaki, A. Tai and Y. Izumi, Bull. Chem. Sot. Jpn., 54 (1981) 2323. 5 A. Hoek, H. M. Woerde, W. M. H. Sachtler, in T. Seiyama and K. Tanabe, (eds.), Proc. 7th Znt. Cong. Catal., Tokyo, 1980. Elsevier, Kodansha, Amsterdam, 1981, p. 376. 6 A. A, Vedenyapin, B. G. Chankvetadze and E. I. Klabunovskii, React. Kinet. Lett., 24 (1984) 77. 7 A. A. Vedenyapin, E. I. Klabunovskii, Yu. M. Talanov and G. Kh. Areshidze, Zzv. Akad. Nauk. SSSR, Ser. Khim., 11 (1976) 2628. 8 Y. Nitta, F. Sekine, T. Imanaka and S. Teranishi, Bull. Chem. Sot. Jpn., 54 (1980) 980. 9 Y. Nitta, 0. Yamanishi, F. Sekine, T. Imanaka and S. Yeranishi, J. Catal., 79 (1983) 475. 10 A. Hoek and W. M. H. Sachtler, J. Catal., 58 (1979) 276. 11 P. W. Jolly and G. Wilke, in The Organic Chemistry of Nickel, Vol. 1, Academic Press, London, 1974. 12 W. M. H. Sachtler, C. R. Kiliszek and B. E. Nieuwenhuys, Thin Solid Films, 2 (1968) 43. 13 J. S. Smith, P. A. Thrower and M. A. Vannice, J. Catal., 68 (1981) 270. 14 B. E. Warren, in M. Cohen (ed.), X-Ray Diffraction, Addison-Wesley, Reading, MA, 1969. 15 C. H. Bartholomew and R. J. Farrauto, J. Catal., 45 (1976) 41. 16 J. B. van Eijk van Voorthuijsen and P. Franzen, Recueil. 70 (1951) 793. 17 L. J. Bostelaar, R. A. G. de Graaff, F. B. Hulsbergen, J. Reedijk and W. M. H. Sachtler, Znorg. Chem., 23 (1984) 2294. 18 Y. Orito, S. Niwa and S. Imai, Yukigosei Kyokaishi, 35 (1977) 753. 19 W. M. H. Sachtler, C. R. Kiliszek and B. E. Nieuwenhuys, Thin Solid Films, 2 (1968) 43.
Catalysis,
42 (1987)
29 - 36
29
PARTICLE SIZE EFFECT ON ENANTIOSELECTIVE HYDROGENATIONOFMETHYLACETOACETATEOVER SILICA-SUPFO RTED NICKEL CATALYST L. FU, H. H. KUNG and W. M. H. SACHTLER Zpatieff Laboratory, IL 60201 (U.S.A.)
Department
(Received July 11, 1986;accepted
of Chemistry,
Northwestern
University,
Evanston,
April 14, 1987)
Summary
The effect of metal particle size on the enantioselective hydrogenation of methylacetoacetate (MAA) to methyl 3-hydroxybutyrate (MHB) in the gas and the liquid phases was studied over Ni/SiO? modified with R,Rtartaric acid. The Ni particle size of the catalyst at controlled Ni load was increased from 3.3 nm up to about 11 nm by Ostwald ripening, due to the formation and decomposition of gaseous Ni(C0)4. The experimental data showed no significant particle size effect on enantioselectivity within this size range. Atomic absorption analysis of the solution after modification showed that up to 50% of the Ni in the catalyst can be leached out during modification under severe conditions.
Introduction
Nickel modified by immersion in a solution of an optically active material catalyzes the enantioselective hydrogenation of molecules with a prochiral center. An example is the enantioselective hydrogenation of methylacetoacetate (MAA) to 3-hydroxybutyrate (MHB) over nickel, modified with R,R-tartaric acid, as the catalyst. In fact, this reaction has been extensively studied. Enantioselectivity has been reported for Ni, Pt, Ru, Cu and some alloys [l, 21. The effect of varying the modification conditions (pH, modifier concentration, presence of co-modifier, time and temperature), and the reaction conditions (liquid or gas phase, pressure, temperature, presence of other materials such as sulfates or halides in the reaction solution) has been extensively studied [3 - 61. Several groups have used unsupported metals [ 31, in particular Raney nickel, as the catalyst, while other researchers preferred silica-supported metals which are reduced with hydrogen at elevated temperatures (whereas Raney Ni is simply prepared by leaching a NiAl alloy in alkali) [5]. The supported catalysts permitted a systematic study of the effects of metal 0 Elsevier Sequoia/Printed
in The Netherlands
30
loading on the conversion and enantioselectivity. Vedenyapin et al. and Nitta et al. found in such studies that the en~tioselectivity can depend markedly on the metal loading, and they assumed that this observation reveals a particle size effect; in other words, they assume that enantioselectivity is a structure-sensitive phenomenon [ 7 - 91. The latter conclusion is, however, not necessarily correct, as it can be argued that even in the absence of any structure sensitivity the enantioselectivity may vary with the metal loading of the supported catalysts. It should be remembered that catalyst modification with an aqueous solution of tartaric acid is a pore diffusion-controlled process. Even with very long modification time, some metal particles will never be in contact with the modifying solution if gas plugs prevent the liquid from entering narrow pores. It is also well known that modification is accompanied by metal leaching [lo], and it has been shown that in particular at low metal loading much metal is leached from the peripheral zone of the catalyst pellets. Furthermore, the hydrogenation reaction in the liquid phase under the conditions usually used is a pore diffusion-controlled process, i.e., limited by the diffusion of dissolved hydrogen. As a consequence, it is conceivable that for catalysts of high metal loadings, the well-modified metal particles in the pellet periphery contribute most to the reaction. At low metal loadings, however, when much metal is leached from the outer zone, the poorly modified metal particles in the interior of the pellet contribute substantially to the hydrogenation. The result is low enantioselectivity at low metal loading irrespective of any true particle size effects. An even more complicated pattern is expected if one also considers the diffusion of dissolved nickel tartrate, which is the reaction product of the modification process. The concentration of nickel tartrate in the liquid phase far from the catalyst is initially zero. Thus the concentration gradient of dissolved tartrate has a sign opposite to that of tartaric acid at short modification times and also to that of hydrogen during the hydrogenation. In order to decide whether a true effect of metal particle size and/or shape is involved in enantioselectivity, it is therefore imperative to vary the particle size at constant metal loading. This is the objective of the research described in the present paper. We have chosen to change the size of the nickel particles in a given Ni/SiOz catalyst by using ‘Ostwald ripening’, in which the larger particles grow at the expense of the smaller particles [ 111. For this purpose, the formation and decomposition of nickel tetracarbonyl are used: Ni+4COe
Ni(C0)4
In earlier work [12] we had verified that this reaction is sufficiently rapid at > 50 “C to specifically remove the Ni particles of high free energy, while the decomposition of the carbonyl on particles of low free energy causes them to grow. We have monitored the growth of the average size of the nickel particles by hydrogen adsorption (after the catalysts had been used for reaction), and
31
by the broadening of X-ray diffraction lines. The enantioselectivity under standardized conditions was then measured for samples of different Ni particle sizes, but constant metal loading.
Experimental Catalyst preparation Wide pore (40 - 60 nm) silica beads (2.5 mm in diameter, Koninklijke/ Shell Laboratorium, Amsterdam) with a BET surface area of 40 m2 g-i were washed with dilute NaOH and HNO, and finally with demineralized water to pH 7, then dried in an oven at 62 “C. 27 g of the washed silica was suspended in a 600 ml solution of 20 g Ni(NOs),-6H2O (Aldrich Chemical Company, Inc.) to which 50 ml NH,OH (American Scientific Products, Mallinckrodt) (29.4 wt.%) was added at 40 “C. The suspension was kept at this temperature for 13 h. The resulting catalyst had an average nickel particle size of 3.3 nm. Reduction About 0.5 g catalyst was reduced by hydrogen using temperatureprogrammed reduction. A H2 flow rate of 100 ml min-’ was used and the temperature was increased by 3 “C min-’ from 25 to 450 “C and kept at 450 “C for 11 h. Ostwald ripening CO purified with A120s at 200 “C and 1 atm was introduced into a reactor with a volume of about 50 ml containing 0.5 g freshly reduced catalyst at room temperature. After sealing the reactor from the rest of the system, the temperature was increased to 80 “C and kept at this temperature for various periods of time. Modification After Ostwald ripening, the catalysts were reduced again with the same TPR method mentioned above. A 2 wt.% tartaric acid (Aldrich Chemical Company, Inc.) solution was used in modification, the pH of the solution was adjusted to 5 with NaOH. Three modification conditions were used: 1. 20 ml solution at room temperature for 2 h; 2. 20 ml solution at room temperature for 13 h; 3. 100 ml solution at 100 “C for 2 h. Doubly distilled water and MeOH were used to wash the catalyst after modification, and the catalyst was then transferred into the reactor under the protection of MeOH. The solution used in the modification was analyzed using atomic absorption spectroscopy to determine the amount of metal leached from the catalyst during modification.
32
Hydrogenation The gas phase hydrogenation was performed in a plug flow system, shown in Fig. 1, at 55 “C with a hydrogen flow rate of 100 ml min-‘. The partial pressure of MAA (Aldrich Chemical Company, Inc.) was 0.34 torr. The product was collected at liquid nitrogen temperature for about 50 h. The liquid phase hydrogenation was carried out in a reactor shown in Fig. 2. The reaction was carried out at 55 “C!under 1 atm of hydrogen using 5 ml MAA and 0.5 g of catalyst. Particle size measurement Both HZ chemisorption and XRD method were used to determine the Ni particle size after the hydrogenation. Temperature programmed reduction was used to reduce the catalyst before the measurements. For H, chemisorption, the catalyst was heated to 400 “C for 2 h to remove the tartrate on the surface before the reduction. The particle size was calculated from the dispersion measured with chemisorption by assuming that the crosssection area of the Ni atom is 0.065 nm* [ 131. The Scherrer formula: t=
0.9 x J31/2
~0s
6~
used to calculate the average Ni particle size from the half-width B,,? of the (111) diffraction peak [ 141, where B,,? = (Bi,*, exp2- Br2)lR B, is the instrument broadening which was measured with a Ni foil. Temperature-programmed reduction is essential to keep the Ni particle size unchanged during the reduction. It has been reported [15] that the was
Fig. 1. Schematic drawing of the system for gas phase hydrogenation. 1. flow meter, 2. Teflon valve, 3. water trap, 4. water-cooled MAA saturator, 5. flug-flow reactor, 6. furnace, 7. septa for sampling, 8. ball joints, glass with Viton O-ring, 9. product trap, 10. soap film meter.
33
-
r
i5
Fig. 2. Reactor for liquid phase hydrogenation. 1. Teflon valve, 2. Teflon cap, 3. 50 ml Pyrex flask, 4. aluminium water jacket, 5. motor.
water formed during reduction accelerates the sintering rate of Ni catalyst. Ni and SiOz can also form a spine1 in the presence of water and thus decrease the reducibility of the catalyst [ 161.
Results and discussion Table 1 shows the effect of metal loading after modification by the three different methods. TABLE 1 Metal loading of catalysts modified by various methods Modification method
Ni loading after modification (wt.%)
none 1 2 3
11.3 10.3 f 0.3 9.5 + 0.1 5.8 + 0.3
34
Evidently a significant amount of Ni was leached during the modification process. The amount increases with the severity of the modification conditions. Once again, this indicates that modification is a corrosive process. As the smallest Ni particles have the highest free energy [19], they will be leached out first. Indirectly this lends further support to our model that the selective site is formed by a corrosive chemisorption of Ni tartrate [lo, 17, 181. Tartaric acid reacts with Ni on the surface and forms a Ni tartrate complex; this complex can stay on the surface to form a selective site, or it can diffuse into the solution. Due to the inward diffusion of tartaric acid in the catalyst pores and the outward diffusion of dissolved Ni tartrate, the concentration of the latter will vary with time and space. The structure of the Ni tartrate complex on the Ni surface is not known, but that of three-dimensional Ni tartrate has recently been identified [ 171. If we assume that the selectivity is proportional to the coverage of the surface with Ni tartrate, some intermediate region inside each pore should have the highest selectivity. Table 2 shows the ripening time, the corresponding average particle size of Ni after modification and the enantioselectivity of the catalyst. The average particle size of the catalyst increased with increasing time of ripening. Clearly, larger Ni particles grew at the expense of smaller particles. TABLE 2 A summary of the Ni particle sizes, the conversion and the enantioselectivity catalysts Modification method
Ripening time (h)
Particle size (nm) from Hz chemisorption
from XRD line width
of different
e.e. (%)
Conversion (%)
Gas phase hydrogenation 1 0 2.75 7.25 9.25 1:
3.9
3.9
6.5 7.3 a.3 10.8
5.7 6.3 6.5 1.7
10.9 8.4 9.7 9.6 8.8
96.1 56.9 83.6 94.8 96.1
Gas phase hydrogenation 2 0 6.8 10.5
5.4 6.5 10.7
3.3 5.5 6.8
14.5 15.0 15.2
92.6 87.3 93.6
4.8 5.6 7.0 7.5
4.3 5.5 6.3 6.1
32.7 33.2 32.0 30.7
4.0 6.7 3.2 7.0
Liquid phase hydrogenation 3 0 4.5 8.15 10
35
lo-
P.S.
(“In)
6
te.e.1t te.e.)t=o
-
- 20 4,r
.
.
.
.
.
A 1.0
T2-
0
- w
1
I
I
I
I
2
4
6
8
10
t
12
(hr)
Fig. 3. Particle size (P.S.) and normalized condition indicated for P.S. only.)
e.e.
versus ripening
time
(t).
(Modification
The results in Table 2 show that over the Ni particle size range from - 4 - 11 nm, there is no systematic change in enantioselectivity, which remains constant to within a few percent for each modification method. This is illustrated in Fig. 3, where the enantiomeric excess, e.e.,, after ripening, normalized by e.e.,, ,, at zero ripening time, and the particle size are plotted uersus ripening time. There is a substantial variation in the enantioselectivity with the modification method. The selectivity for catalysts modified by method 2 is about 50% higher than those modified by method 1. This agrees with the assumption that a longer modification time results in better modification. The still higher selectivity for catalysts modified by method 3 may be a combined effect of higher temperature during modification and the use of liquid phase hydrogenation. The latter has been shown generally to yield a higher selectivity than gas phase hydrogenation [ 51. The absence of significant structure sensitivity is in agreement with the dual site model of enantioselectivity. This model assumes that the selective formation of one enantiomer takes place on Ni sites which are surrounded by one or two tartrate ligands. Five-membered chelate rings are formed, each consisting of the nickel atom (or ion), one oxygen atom of a carboxylate group, two carbon atoms and one oxygen atom of the hydroxy group of the tartrate ligand. This configuration is stereospecific, the methyl acetoacetate which is adsorbed on this nickel atom (presumably as an enolate) is in an asymmetric environment. Upon adding three hydrogen atoms to the enolate, one enantiomer or the product methyl 3-hydroxybutyrate is formed. This mechanism requires the stepwise additon of the hydrogen atoms. As nickel tartrate lacks the ability to dissociate dihydrogen, these hydrogen atoms have to be formed on the metal surface underneath the tartrate overlayer. Once the tartrate layer has been formed by reaction of tartaric
36
acid with nickel, the remaining nickel metal has only this simple function of dissociating dihydrogen. In terms of this model it is therefore quite natural that the structural requirements for the nickel surface under the tartrate overlayer are extremely facile, and no dependence of the enantioselectivity on the particle size is observed. This statement is, however, limited to the regime of particle sizes which survive the corrosion by the tartaric acid during modification without being completely converted to nickel tartrate. Smaller particles which are completely leached during the ‘modification process’ are, of course, totally ineffective for the hydrogenation.
Acknowledgment This work was supported by the National Science Foundation, under grand No. CHE-8216664.
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