4560
Langmuir 199410, 4560-4565
Adsorption of Optically Pure Alanine on Silica-Supported Nickel and the Consequent Catalytic Enantioselectivity Mark A. Keane The Department of Chemistry & Biochemistry, University of Guelph, Guelph, Ontario NlG 2W1, Canada Received March 18,1994. I n Final Form: September 22, 1994@ The interaction of aqueous solutions of optically pure alanine (AL)with samples of an 11.9% (w/w) Ni/SiOz catalyst, prepared by homogeneousprecipitatioddeposition, has been examined, and the resultant modified catalysts were used to promote the asymmetric hydrogenation of a prochiral P-keto ester (methyl acetoacetate) to a P-hydroxy ester ((R)-(-)-methyl3-hydroxybutyrate).The dependence of the level of AL adsorption from the liquid phase on the duration and temperature of the treatment and on the initial concentration of AL in solution was studied. The adsorption of AL on the surface nickel is corrosive and data on the extent of nickel dissolutionare provided. The build up ofAL on the active surface is graphically illustrated and is related to the observed enantioselectivities;an optimum active surface coverage in the range 0.6-0.8 is identified. Modification of the catalysts with AL not only induced enantioselectivitybut also increased the apparent reaction rate. The nature of the interaction of AL with the surface nickel is considered and the catalytic consequencesare discussed. Data on catalyst modification by the adsorption of tartaric acid are also included for comparative purposes.
Introduction The ability of a catalyst to generate desired products with a high selectivity is often the ultimate yardstick by which catalyst performance is measured. A special case in point is the conversion of a prochiral molecule to a n optically active product mixture where one of the enantiomers is formed in excess. Such a reaction is represented by the asymmetric hydrogenation of a prochiral /?-keto ester (methyl acetoacetate, MAA) to a /?-hydroxy ester (methyl 3-hydroxybutyrate, MHB) which can exist in two enantiomeric forms, i.e. (R)-(-)-MHB and (S)-(+)-MHB.1,2 In order to achieve metal-catalyzed enantioselectivity, the reaction must take place in a chiral environment, and as conventional heterogeneous catalysts contain no inherent chirality, the hydrogenation of MAA on a bare metal surface generates a racemic p r o d ~ c t . ~It- ~is now well established that a chiral environment can be achieved by adsorbing a n optically pure isomer or "modifier" onto the active phase of the ~ a t a l y s t . ' - ~ , To ~ - ~date, ~ the best Abstract published in Advance ACS Abstracts, November 15, 1994. (1)Izumi, Y. Adu. Catal. 1983, 12, 215, and references therein. (2) Blaser, H. U. Tetrahedron: Asymmetry 1991,2,843and references therein. (3) Bennett, A.; Christie, S.; Keane, M. A.; Peacock, R. D.; Webb, G. Catal. Today 1991,10, 363. (4) Keane, M. A.; Webb, G. J . Catal. 1992, 136, 1. (5) Keane, M. A. Zeolites 1993, 13, 14, 22, 330. (6) Harada, T.; Yamamoto, M.; Onaka, S.; Imaida, M.; Ozaki, H.; Tai, A.; Izumi, Y. Bull. Chem. SOC.Jpn. 1981,54, 2323. (7) Hubbell, D. 0.; Rys, P. Chimia 1970,24, 442. (8)Zubareva, N. D.; Chernysheva, V. V.; Grigorev, Yu. A.; Klabunovskii, E. I. Bull. Acad. Sci. USSR, Diu. Chem. Sei. 1987, 476. (9) Chernysheva, V. V.; Murina, I. P.; Klabunovskii, E. I. Bull.Acad. Sci. USSR, Diu. Chem. Sei. 1983, 689. (10)Smith, G. V.; Musoiu, M. J . Catal. 1979, 60, 184. (11) Brunner, H.; Muschiol, M.; Wishert, T.; Wiehl, J. Tetrahedron: Asymmetry 1990, I , 159. (12) Harada, T.; Haraki, Y.; Izumi, Y.; Muraoka, J.; Ozaki, H.; Tai, A. Proceedings of the 6th International Congress on Catalysis, London, 1976; Bond, G. C., Wells, P. B., Tompkins, F. C., Eds.; The Chemical Society: London, 1977; p 1024. (13) Nitta, Y.; Imanaka, T.; Teranishi, S. J . Catal. 1983, 80, 31. (14)Klabunovskii, E. I. Russ. J . Phys. Chem. 1973,47, 765. (15) Lipgart, E. N.; Petrov, Yu. I.; Klabunovskii, E. I. Kinet. Katal. 1971, 12, 1491. (16) Gross, L. H.; Rys, P. J . Org. Chem. 1974,39, 2429. (17) Ozaki, H.; Tai, A.; Kobatake, S.; Watanabe, H.; Izumi, Y. Bull. Chem. SOC.Jpn. 1978,51, 3559. (18) Hoek, A.; Sachtler, W. M. H. J . Catal. 1979, 58, 276. @
studied enantioselective system involves the modification of nickel catalysts by optically active tartaric acid (TA), using the hydrogenation of MAA as a model reaction. Catalyst modification by TA adsorption has been applied to a variety of supported3,4,'8-20,28,30,31,37,38 and unsupported'-3,11,28,29~32,36 nickel systems and the enantioselective hydrogenation reaction has been conducted principally in the liquid phase,3,4,11,19,20,27-32,34,35,37,38 but gas phase transformations have also been reported.23,26p30p36 Despite the body of information which has been accumulated in the course of these studies, there remains little consensus of opinion regarding the actual source of the enantiodifferentiation and the nature of the asymmetric hydrogenation site. While a number of tentative stereochemical models have been proposed' to explain the various phenomena which have been observed over such diverse (19) Nitta, Y.; Sekine, F.; Sasaki, J.; Imanaka, T.; Teranishi, S. J . Catal. 1983, 79, 211. (20)Nitta, Y.; Utsumi, T.; Imanaka, T.; Teranishi, S. J . Catal. 1986, 101, 376. (21) Nitta, Y.; Kawabe, M.; Imanaka, T. Appl. Catal. 1987,30, 141. (22) Osawa, T.; Harada, T.; Tai, A. J . Catal. 1990, 121, 7. (23) Hoek, A.; Woerde, H. M.; Sachtler, W. M. H. Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980; Tanabe, K., Sieyama, T., Eds.; KodanshdElsevier: Tokydhsterdam, 1981;p 376. (24) Wittmann, G.; Bartok, G. B.; Bartok, M.; Smith, G. V. J . Mol. Catal. 1990, 60, 1. (25)Keane, M. A.; Webb, G. J . Mol. Catal. 1992, 73, 91. (26) Yasumori, I.; Yozeki, M.; Inoue, Y. Faraday Discuss. 1981, 71, 385. (27) Richards, D. R.; Kung, H. H.; Sachtler, W. M. H. J . Mol. Catal. 1986, 36, 329. (28) Keane, M. A.; Webb, G. J . Chem. SOC.,Chem. Commun. 1991, 1619. (29) Brunner, H.; Amberger, K.; Wiehl, J. Bull. SOC.Chim. Belg. 1991,100,571. (30) Fu, L.; Kung, H. H.; Sachtler, W. M. H. J . Mol. Catal. 1987,42, "" 1Y. (31) Bostelaar, L. J.; Sachtler, W. M. H. J . Mol. Catal. 1984,27,387. (32) Tatsumi, S. Bull. Chem. Soc. Jpn. 1970, 41, 408. (33) Fish, M. J.; Ollis, D. F. J . Catal. 1977, 50, 353. (34) Tanabe, T. Bull. Chem. Soc. Jpn. 1973,46, 1482. (35) Harada, T.; Tai, A.; Yamamoto, M.; Ozaki, H.; Izumi, Y. Proceedings of the 7thInternational Congress on Catalysis, Tokyo,1980; Tanabe, K., Sieyama, S., Eds.; KodanshdElsevier: Tokyo/Amsterdam, 1981; p 364. (36) Yasumori, I.; Inoue, Y.; Okabe, KProceedings of the International Symposium on the Relations Between Heterogeneous and Homogeneous Phenomena, Brussels, 1974; Elsevier: Amsterdam, 1975; p 41. (37) Nitta, Y.; Imanaka, T.; Teranishi, S. J . Catal. 1986, 96, 429. (38)Nitta, Y.; Utsumi, T.; Imanaka, T.; Teranishi, S. Catal. Lett. 1984, 1339.
0743-7463/94/2410-4560$04.50/00 1994 American Chemical Society
Alanine-Modified Catalysts
Langmuir, Vol. 10, No. 12, 1994 4561
aqueous solutions of tartaric acid (TA), the details of which are catalyst systems, the mechanism of enantioselective available elsewhere.3~4 hydrogenation remains unknown. There is, however, The nickel content ofthe postmodifier solution, resulting from general agreement1s3,4*20*25,31 that the overall values of the corrosiveadsorption ofAL on the surface metal, was measured enantiomeric excess reflect the relative contributions of Nickel metal by AA spectrophotometry as described previ~usly.~ the modified "selectivenand the bare nickel "nonselective" dispersions, before and after modification, were determined by metal sites. carbon monoxide chemi~orption.~ The amount of AL adsorbed With regard to the existing data, quantitative comon selected samples was determined by analyzing the pre- and parisons of different enantioselective systems (originating postmodification solutions by HPLC (Spectra-Physics);an Econosphere 5 p C8 packed column (Alltech Associates, 250 x 4.6 mm) from different laboratories) are difficult to make because was usedwith a 65%acetonitrile/35%water mixture as the mobile of the many permutations of experimental variables phase and detection by U V (A = 187 nm). To enhance sensitivity involved in the catalyst preparation, activation, modificaand accuracy (which was within *4%), the signal from the tion, and reaction steps. This effect is further compounded integrator was digitized and stored on disk for subsequent by the fact that efforts to relate optical yield to the coverage analysis using a WINner LABNET routine. The amount of AL of the catalyst surface by the modifier have been largely adsorbed was calculated as the difference between the values unsuccessfu1,20~27~33~35~36 with the result that no direct obtained for the solutions before and after modification. The correlation exists between the amount of adsorbed modifier experimental procedure, method of analysis, and HPLC calibraand the optical activity. In addition, the corrosive nature tion have been described in detail elsewhere.4 of the modification step has only been taken into considCatalytic Procedure. The liquid phase hydrogenation of eration in some s t ~ d i e s , ~ whereas , ~ , ~ the ~ ~majority ~ ~ ~ ~ ~ ,MAA ~ ~ (0.009-0.5 mol) either neat or in 1-butanol (0.01-0.5 mol) was carried out at 343 f2 Kin a 250-cm3glass vessel fitted with of the published data relates the observed asymmetric a condenser, hydrogen inlet, and thermocouple well. A 60 cm3 yields to the metallic function in the premodified catalyst. min-l stream of purified hydrogen was bubbled through the Furthermore, several workers have noted a difficulty in which was kept under constant agitation at 600 rpm. obtaining reproducible product compositions10J2J8~23~24,31suspension Details of the catalytic apparatus are available e l ~ e w h e r eThe .~ which may be attributed to diffusion limitations inherent extent of hydrogenation was determined by HPLC using a Pirkle in these catalytic systems. type 1A 5 p reversible column (250 x 4.6 mm) with a 10% IPA/ Asymmetric catalysis is still in a formative stage and 90% hexane mixture as the mobile phase. The overall degree of hydrogenation was converted to mol % MHB using a 21-point the possible reagents used to induce enantiocontrol have calibration plot; a quadratic equation was used to fit these data not, as yet, been fully characterized. For instance, the to better than *l%.The amount of MAA adsorbed on selected a s a possible catalyst use of the amino acid alanine (AL) samples was also measured by HPLC using the same analytical modifier has largely remained unexplored. In the only system. In each case, the MAA/butanol/catalyst suspension was two studies of AL modification known to the author,l2pZ5 agitated (600 rpm) for 0.5 h a t the desired temperature. Arange the use of AI, has been reported to yield higher (in the of MAA/butanol mixtures were used to construct a 30-point case of Ni/Si02)25and lower (in the case of Raney nickel)12 calibration curve and the MAA concentrations measured were enantioselectivities when compared with the TA treatreproducible to better than *2%. The amount of MAA adsorbed ment. In this paper, the progress of AL adsorption from was then calculated as the difference between values obtained before and after contact with the catalyst. Optical yields (O.Y.) the liquid phase onto the catalyst surface and the extent were determined from measurements of optical rotation (AA-10 of nickel leaching which accompanies the adsorption step Automatic Digital Polarimeter) using the equation are presented as a function oftime, temperature, modifier concentration, and catalyst pretreatment. The fractional O.Y.= ~a12[a1: = 100a~al;fic surface coverage by AL is, in turn, related to catalytic activity and enantioselectivity. For comparative purposes, relevant data on TA adsorption under identical experiwhere [a]: is the specific rotation of the product solution mental conditions are included. measured at the sodium D line and temperature T (293 f 3 K), [a]!is the specific rotation of the pure enantiomer under the same conditions (-22.95" for R-(-)-MHB), a is the measured Experimental Section
Catalyst Activation, Modification, and Characterization. An 11.9% (w/w) Ni/SiOz catalyst was prepared by the homogeneousprecipitatioddeposition of nickel onto a nonporous microspheroidal Cab-0-Si1 5M silica as described in detail e l ~ e w h e r eThe . ~ hydrated (water content '5% (w/w))supported precursor was sieved in the mesh range 150-125 pm and activated by heating in a 150 cm3min-l stream of deoxygenated and predried hydrogen at a fixed rate of 5 K min-l to a final temperature in the range 673-1073 f3 Kwhich was maintained for 18h. The reduced catalysts were flushed in a purified stream (200 cm3 min-l) of nitrogen, cooled to room temperature, and contacted with 100 cm3aqueous solutions (0.003-0.1 mol dm-3) of alanine (AL). The pH of the modifyingsolutionwas preadjusted to the isoelectric point (6.0) for AL using 1 and 0.01 mol dm-3 aqueous NaOH solutions for rough and fine adjustments, respectively. The modificationwas performed in air with constant agitation (600 rpm) for up to 2 h in the temperature range 273373 f 2 K. The AL used was of AnalaR grade and the prepared solutions were found to be better than 99% pure by HPLC; the concentrations ofthe AL solutions used in the modifications were reproducible to within f l % .After modification, the catalyst was decanted and washed with distilled water (1 x 25 cm3), methanol (2 x 25 cm3),and 1-butanol (2 x 25 cm3)before being stored in the latter prior to use. All the solvents and modifying solutions used in this study were thoroughly degassed by purging with purified helium and heating under vacuum. The activated catalyst was modified in a similar fashion by treatment with
optical rotation, 1 is the path length (20 cm) of the cell, and c is the solute concentration. Optical rotation us R-(-)-MHB concentration (in MAAmutanol) data were fitted t o a quadratic equation to better than f 2 % which was then used t o determine the optical yield of the product. In this paper, optical selectivity is expressed in terms of enantiomeric excess (e.e.1which is defined as, % e.e. = 100([R-(-)-MHB] - [S-(+)-MHB]}/
{[R-(-)-MHBl+ [S-(+)-MHBl}
Results and Discussion ModificationofNi/SiOz with& and TA is highly specific in that treatment with (S)-(+)-AL and (R)-(+)-TAyielded excesses of (R)-(-)-MHB in the product; replacement of the modifier by its antipode gave identical values of enantiomeric excess b u t with a reversal of the sign. Modification with optically inactive meso-AL or meso-TA yielded a virtually racemic (e.e. < 2%)product mixture. The data in this paper are related solely to catalyst modification by (S)-(+)-ALand (R)-(+)-TA. The amount of AL adsorbed as a function of time is plotted in Figure l a for three representative AL treatments. Each profile is characterized by a n initial rapid uptake of AL followed by the slow attainment of the
4562 Langmuir, Vol. 10,No. 12, 1994
0
30
60
Keane
90
120
270
300
t min
Figure 1. Effect of tmod on: (a)the amount of AL adsorbed per [ALI = 0.03 mol dm-3, Tmd= 298 K (A) gram of catalyst, (0) [ALI = 0.10 mol dm-3,T,, = 298 K; (0) [AL]= 0.10 mol dm-3, Tmod= 373 K, (b) the amount of AI, (0) and TA ( 0 )adsorbed per gram of catalyst, [modifier] = 0.03 mol dm-3, Tmod = 298
K.
maximum level of adsorption. It can be clearly observed that the extent of AL adsorption is dependent on the modification time and temperature and on the initial modifier concentration. Under the stated modification conditions, treatment times in excess of 50 min were necessary to achieve the final level of adsorption. The time required for maximum adsorption decreased markedly with increasing temperature in the range 273-373 K. In the absence of any form of agitation, treatment times of up to 5 h were required to attain a n equivalent (within 6%)AL uptake. The modification step is therefore a diffusion-controlled process and the extent of AL adsorption is determined by the rate of transport of AL to the adsorption sites. It is necessary to agitate the ALI catalyst suspension in order to extend the liquidsolid interface and minimize the distance that a n AL molecule has to diffuse through the aqueous medium in order to reach the catalyst surface. The rate of AL uptake was found to be independent of the stirring rate a t speeds in excess of 600 rpm. On subjecting the calcined (in a 120 cm3min-l stream of air at 723 K) precursor to a range of modification treatments, the adsorbed AL phase was less than 9 x lo1’ molecules/g of catalyst. This suggests that the principal site for AL adsorption is the nickel metal and not the silica support or unreduced nickel. Similarly, Groenewegen and S a ~ h t l e rin, ~an~ infrared spectroscopic investigation of amino acid adsorption on Ni/SiOz, concluded that AL is more strongly adsorbed on nickel than on silica. The progress of AL and TA adsorption from aqueous solutions of identical concentration, under the same modification conditions, is compared in Figure lb. While both profiles exhibit the same basic shape, it is clear that the catalyst adsorbs a higher concentration of AL and the maximum modifier uptake is achieved a t lower modification times for the AL treatment. It can therefore be said that in aqueous solutions, AL shows a higher affinity than TA for adsorption on the supported nickel metal. The effect of the modification temperature on the extent of AL adsorption is illustrated in Figure 2a for two (39) Groenewegen, J.A.;Sachtler, W. M. H. J.Catal. 1974,33,176.
330
360
T K Figure 2. Effect of Tmod on (a) the uptake of AL per gram of catalyst and (b) the extent of surface nickel metal leaching resulting from treatments with 0.03 mol dm-3(A) and 0.10 mol dm-3 (0)aqueous solutions, tmod = 2 h. Table 1. Effect of the Initial Modifier Concentration on the Degree of Nickel Leaching and the Fractional Surface Coverage by AL and TA: Tmod = 343 K tmd= 2 h [modifier],
modification by AL
low2mol dm4
% Ni leached
6’
0.3 0.5 1.0 1.7 3.3 5.0 6.7 10.0
14 27 42 51 60 63 66 70
0.12 0.18 0.35 0.52 0.75 0.92
0.98 1.01
modification by TA % Ni leached
6‘
5
0.06 0.09 0.17 0.25 0.51 0.68 0.87 0.98
9 23 34 46 51 56 57
representative modifier concentrations. In the case of the more dilute AL solution, the uptake of modifier increased on elevating the temperature from 273 to 373 K, whereas the profile associated with the more concentrated AL treatment shows a discernible maximum a t 313 K. This may be explained by the dissolution of surface nickel which accompanies AL adsorption and which is quantified in Figure 2b and Table 1. The adsorption ofAL on supported nickel metal is a corrosive process and the extent of nickel leaching from the surface increases with increasing temperature and modifier concentration, in other words, with a n increased severity of the modification conditions. Gronewegen and Sachtle1-3~8~ have concluded that alanine is dissociatively adsorbed on nickel metal, forming a chelate ring with one nickel atom. From the data obtained in this study, it can be concluded that the chemisorption step is corrosive and the nickel atom may be “pulled out” of its lattice position to form a two-dimensional nickel salt of alanine which may either remain on the silica carrier or diffuse into solution. Furthermore, nickel concentrations ofup to 60 ppm were detected in the product mixtures resulting from the hydrogenation step. This suggests that a range of NiALJSi02 interactions was present on the modified surface and some of the weakly interacting NiAL species which remained in contact with the support throughout modification went into solution ~~
~
(40) Groenewegen, J. A.; Sachtler, W. M. H. Proceedings ofthe 6th
International Congress on Catalysis, London, 1976; Bond, G. C., Wells, P. B., Tompkins, F. C., Eds.; The Chemical Society: London, 1977; p 1014.
Langmuir, Vol. 10, No. 12, 1994 4563
Alanine-Modified Catalysts
0.7
a< A OV9
I
1
I
Table 2. Effect of Reduction Temperature on the Extent of Nickel Leaching, AL Uptake and the Dispersion and Average Particle Size of Nickel Supported on Pre- and Postmodified Ni/SiO2: [ALI = 0.01 mol dm+; Tmd = 343 & tmod = 2 h
I
p'
reduction %Ni temperature,K 10-19Na leached 42 673 6.0 5.9 37 773 5.7 33 873 30 973 5.6 1073 5.6 29 a
I 270
I min
300
330
I
360
T K Figure 3. Variation in the fractional surface coverage by AL (Om)with Tmod resulting from treatments with 0.03 mol dm-3 (A) and 0.10 mol dm-3 (0)aqueous AL solutions, tmd = 2 h. Inset: the buildup of AL on the active surface with time, [AL] = 0.10 mol dm-3, Tmod = 343 K.
during the prolonged hydrogenation step. The residual nickel detected for the modification of the calcined, unreduced precursor was less than 15 ppm, regardless of the modification conditions, which again supports the contention that the supported nickel metal is the principal adsorption site. As illustrated in Figure 2a, the combined effects of the temperature related increase in AI, adsorption and the progressive disappearance of the supported nickel metal adsorption sites results, ultimately, in the case of the concentrated AL treatment, in a situation where a further increase in temperature reduces the concentration of the adsorbed phase. Modification of Ni/SiOz by TA has been demonstrated elsewhere to also result in a dissolution of surface n i ~ k e l . ~ , ~The p ~ corrosive * effects of both AL and TA under identical experimental conditions are compared in Table 1. The interaction of AL with the activated catalyst resulted in a greater loss of surface nickel than was the case for the TA treatment a t each modifier concentration. Under the most severe modification conditions 70% of the initial metal content was lost from the surface. The relationship between modificationtemperature and the fractional coverage of the surface metal by AL is illustrated in Figure 3. As AL is adsorbed from solution, there is a n accompanying removal of nickel from the solid into the liquid phase which in effect dilutes the concentration of surface adsorption sites. The fractional coverage by AL is therefore governed by the interplay between the uptake of AL from solution, the number of available adsorption sites, and the extent to which the adsorption sites are removed in the corrosive adsorption step. The increase in both the number of AL molecules adsorbed and the percentage loss of the initial metal content with increasing Tmd translates into the increase in 0.a to monolayer coverage as shown in Figure 3. As AL is progressively adsorbed from solution (Figure l ) , there is a corresponding increase in BAL which is most marked in the interval 35-55 min (insetto Figure 3)and which must correspond to a maximum in the rate of outward diffusion of the leached NiAL species. As a direct consequence of higher uptakes and increased Ni leaching, 0.a is higher than f?TA a t each modifier concentration, Table 1. The implications of varying the catalyst reduction temperature on the nature of the supported metal phase in the pre- and postmodified catalysts are given in Table 2. The nickel metal dispersion data were obtained from
dc
%Db (b) (a) (b)
(a)
Om
25 19 16 15 15
4.4 5.3 6.2 6.6 6.7
0.35 0.41 0.45 0.46 0.46
36 29 23 21 20
2.8 3.5 4.5 4.8 5.1
Number ofTA molecules retained per gram of catalyst. Nickel
metal dispersion before (b) and after (a) modification. Average nickel particle diameter (nm) before (b) and after (a)modification.
CO chemisorption and the metal dispersion is expressed as (Nisu,+JNibtdx 100%. The percentage nickel dispersion was observed to decrease with increasing reduction temperature. At higher activation temperatures, agglomeration of nickel atoms on the support to form larger crystallites is known to occur41 and this results in a decrease in the exposed nickel area. The supported nickel metal particle size was determined from the dispersion measurements according to the r e l a t i o n ~ h i p ~ ~ d = 101/D where D is the dispersion or percentage of exposed metal atoms and d is the surface weighted average crystallite diameter, assuming (spherical)particles 5 1nm to be 100% dispersed. In this study, reduction ofthe 11.9% (w/w) Ni sample in the temperature range 673-1073 K generated nickel particles with average diameters in the range 2.85.1 nm. Treatment of the catalyst with AL removed the smaller nickel particles from the silica carrier and shifted the average crystallite diameter to a higher value, Table 2. The smaller nickel particles which have a higher surface free e n e r d 3 are therefore preferentially leached from the catalyst and nickel particles smaller than 3-4 nm are particularly susceptible to the corrosive effects of AL.It follows that a high temperature sintering of the active metal inhibits nickel leaching; the percentage of the initial nickel content remaining on the surface after modification was increased in each case as particle growth (via increased reduction temperature) was promoted. The decrease in dispersion with increased reduction temperature was accompanied by a decrease in the concentration of the adsorbed AL phase. However, the fractional coverage of the more poorly dispersed adsorption sites by AL was greater, Table 2. The variation in mol % conversion to MHB as a function of time over a n AL modified and unmodified catalyst is illustrated in Figure 4. The reaction profiles for both the modified and unmodified system exhibit the same basic shape which is characterized by an initial induction period (t -= 1 h) followed by a continuous increase in MHB formation and the ultimate attainment of a final level of conversion (att 30 h). In the initial stages ofthe reaction, mass transport effects and reactanthrface interactions predominate where MAA can be considered to adsorb on the surface metal as an O-bonded chelate;40the induction period is better illustrated in the inset shown in Figure 4. In the absence of any form ofreactantkatalyst agitation, reaction times of up to 60 h were necessary for a n equivalent degree of hydrogenation. The level of MAA (41)Coughlan, B.;Keane, M. A.Zeolites 1991,1 1 , 2 . (42)Smith, J. S.;Thrower, P. A.; Vannice, M. A.J . CatuE. 1981,68, 270. (43)Sachtler, W.M. H.; Kiliszek, C . R.; Nieuwenhuys, B. E. Thin Solid Films l968,2,43.
4564 Langmuir, Vol. 10,No. 12, 1994
Keane
80
I
c
9
f i d
I
60
I
-
40
0
I
(Y (Y
I
20
0 0
1
2
3
4
t h
0
0
I
I
I
I
10
20
30
40
Figure 4. Variation in percentage mole conversion of MA4 to MHB with treaetion over an AL modified (A)and unmodified (A) catalyst: [AL]= 0.005 mol dm-3,Tmod = 343 K, tmd= 2 h. Inset: initial portion of the profile expanded, symbol as above.
c
I
c
a
50
100
270
300
330
360
390
T K
t h
0
0
150
200
MAAINi Figure 5. Effect of AL modification ([AL] = 0.005 mol dm+, Tmod = 343 K,tmod = 2 h) on the apparent reaction rate: (A)
modified catalyst; (A) unmodified catalyst.
conversion was greater over the modified catalyst throughout the duration of the reaction. The apparent reaction rate (expressed here as the number of moles of MAA reacted per hour) is obtained by treating the variation of hydrogenation activity with time (at t > 1h, where the contribution of reactant diffusion and reactanthrface interactions is negligible6) as a linear relationship and determining the slope of the reaction profile. The dependence of the apparent rate on reactant concentration, expressed here as a MAA/Ni molar ratio which takes into account the loss of nickel during AL adsorption, is plotted in Figure 5. The higher reaction rates exhibited by the modified catalysts over the entire range of MAA concentrations studied reinforce the relationship presented in Figure 4. This observation is in direct conflict with earlier reports7J2J7where the adsorption of a modifier was viewed as inhibiting hydrogenation activity by blocking the active surface metal. The enhancement of activity, observed in this study, may be attributable to a n increased concentration of surface adsorbed hydrogen. The variation in the number of MAA molecules adsorbed with temperature on a n unmodified and on representative AL and TA modified surfaces is shown in Figure 6. The number of molecules of MAA adsorbed far exceeded the total number of surface nickel atoms. Clearly, extensive multilayer
Figure 6. Temperature dependenceof hU.4 uptake (tads = 0.5 h) per gram of an unmodified (O), TA modified (A), and AL modified (0)catalyst: [modifier]= 0.03 mol dm-3, Tmod = 343 K tmod = 2 h.
adsorption of MAA occurs under catalytic conditions. Hence, the transport of gaseous hydrogen to and that of product MHB away from the surface active sites is achieved via multilayers of adsorbed MAA. The number of MAA molecules adsorbed was lower on the modified surfaces. While the adsorbed AL or TA phase restricts the access of the MAA molecules to the surface metal atoms, the hydrogen can readily adsorb on these sites. Consequently, there is a greater “reservoir”of dissociated hydrogen on the modified surface which serves to further promote the rate of hydrogenation. The active centers may be considered to be composed of both zero valent nickel metal and the nickel ions in the surface nickell modifier complex. The nickellmodifier environment is essential to promote enantioselectivity. Moreover, the adsorbed modifier may serve as a template through which activated hydrogen is transferred to the reactant as distinct from the hydrogen transfer from the bare metal surface. For both the modified and unmodified systems, the apparent reaction rate increased with a n increase in the initial MAA concentration up to a MAA/Ni ratio in the range 120-130, above which the rate dropped. The profiles illustrated in Figure 5 are typical of a LangmuirHinshelwood type mechanism involving the adsorption of both MAA and hydrogen on the surface with rate inhibition occurring at high MAA concentrations. A pretreatment of the activated catalyst with MAA prior to the reaction proper was observed to lower the initial rate but had anegligible effect on the final degree of conversion. As the interaction of MAA with the surface appears to be a time-consuming process under the stated experimental conditions, contacting the catalyst with MAA prior to the reaction should promote this interaction with a greater possibility of direct hydrogenation upon the addition of hydrogen. The observed decrease in activity may be attributed to an occlusion of the active surface metal by adsorbed MAA which serves to initially hinder access by the incoming hydrogen to metal sites. It may therefore be inferred that the adsorption of MAA and hydrogen on the catalyst surface is competitive. The relationship between O h and enantiomeric excess is illustrated in Figure 7 for 43 separate modification steps where either the time, temperature, or modifier concentration was varied and the remaining variables were kept constant. It can be seen that the maximum enantioselectivities are obtained a t values of Om in the range 0.60.8 while the enantiodifferentiation is negligible at Om < 0.2 and Om > 0.95. This situation is quite different from
Langmuir, Vol. 10, No. 1.2, 1994 4565
Alanine-Modified Catalysts 35
' '
0-0 A
0
25 c .
bQ
IC
4
15
Q)
I I
0
A
I
I
I
I
0.2
0.4
0.6
0.8
I
eAL Figure 7. Relationship between enantiomeric excess (e.e.)and the surface coverage by AL (ea) resulting from variations in either tmd(O), Tmd(A), or [AL] (O), in the following ranges: tmd = 0.1-2.0 h; Tmod= 273-373 K [&I = 0.003-0.1 mol dm-3.
TA modification where a fractional coverage of 0.2 has been identified as providing the surface conditions for optimum ~electivity.~ It is reasonable to assume that if both the modifier and the reactant are coadsorbed, the surface coverage of both components should be critical. In the case of the TA modification,the most favorable surface arrangement is one in which a hydrogen bond is formed between the OH group ofthe hydroxy acid and the methoxy oxygen atom of MAA.4,40In other words the interaction is chemical in nature and as shown in Figure 6 this interaction is sufficient to shift the temperature of MAA desorption from 323 to 343 K. The interaction of AL with MAA, on the other hand, had little effect on the desorption temperature for MAA, which suggests that the AL/MAA surface linkage is not as strong as that exhibited by TA. Assuming that both AL and MAA are coadsorbed on the surface, the skeletons of both chelates may be considered to be perpendicular to the surface and parallel to each other. The enantioselective site must then be determined by the mutual positioning of the two adsorbed chelates. The preferred arrangement, as identified by Groenewegen and S a ~ h t l e ris, ~one ~ where MAA and the methyl group of alanine are on opposite sides of the amino acid plane and the methoxy group of MAA is positioned as far away as possible from the alanine methyl group. In other words, the most favorable arrangement on the AL modified surface is steric in nature rather than chemical as in the
case of TA modification. The steric influence exerted by the AL modifier is enhanced a t high coverage (0, = 0.60.8) of the catalyst surface. In the case of TA treatment, is viewed as causing a destructive such a high value of stereochemical interference where an adsorbed MAA molecule may interact with two surface TA species. At a monolayer coverage,AL can no longer stericallyinfluence MAA to the same extent with the result that the optical selectivity is negligible. At monolayer coverages of TA the reactant and modifier can still interact via two-site intermolecular hydrogen bindings visualized by Harada et ~ 1in which . ~ MAA ~ is in the liquid phase. Such a linkage, however, generates a smaller excess of the preferred e n a n t i ~ m e r .For ~ both AL and TA modifications, a t very low surface coverages (0 < 0.2) only a fraction of the adsorbed MAA can interact with the modifier with the result that racemic hydrogenation is favored.
Conclusions Alanine (AL) adsorption from aqueous solutions onto activated Ni/SiOa takes place on the available nickel metal and the amount adsorbed increases with the duration and temperature of the treatment and with an increasing concentration of AL in the original solution. The chemisorption of AL is corrosive and up to 70% of the initial nickel content may be lost from the catalyst surface.Under identical modification conditions, AL shows a greater affinity than tartaric acid (TA) for adsorption on the surface nickel and the adsorption of the amino acid is more corrosive than that ofthe hydroxy acid. The removal ofthe supported nickel phase results in a lowering of metal dispersion and a shifting of the average metal particle size to higher values. Under severe modification conditions, the increased uptake of AL is outweighed by the increased loss of surface metal. Treatment of the catalyst with AL not only induces enantioselectivity but also increases the reaction rate. The hydrogenation of methyl acetoacetate (MAA) on both the modified and unmodified surfacesfollows typical Langmuir-Hinshelwood behavior, where the reactants are adsorbed competitively at the surface and the reaction rate passes through a maximum with increasing MAA concentration. The most favorable arrangement of MAA and AL on the surface is steric in nature rather than chemical, as is the case of the MAA/ TA interaction. The optical yield of (R)-(-)-MHB passes through a maximum a t a fractional surface coverage by AI, in the range 0.6-0.8. Acknowledgment. Part of this work was conducted a t the Chemistry Department of Glasgow University. The author is grateful to Professor G. Webb for the use of some experimental facilities.
