The liquid-phase hydrogenation of methylacetoacetate using nickel-exchanged Y zeolite catalysts: Part I. Structure sensitivity

The liquid-phase hydrogenation of methylacetoacetate using nickelexchanged Y zeolite catalysts: Part I. Structure sensitivity Mark A. Keane

Chemistry Department, Glasgow University, Glasgow, Scotland

The liquid-phase hydrogenation of methylacetoacetate (MAA) over NiNaY and NiKY zeolites to a racemic methyl 3-hydroxybutyrate (MHB) product has been studied in a stirred reactor under a constant hydrogen purge. The variation of MHB formation with time was monitored and reaction rates accurate to +2% are reported. The effects of variations in the supported nickel metal particle sizes (in the range 16-75 nm) on the hydrogenation activity were examined by altering either the degree of nickel exchange or the temperature of reduction. It is shown that the hydrogenation of MAA at 343 K is structure-sensitive and a correlation between reaction sensitivity and particle size is presented. Data on the reaction over Ni/SiO2 catalysts are also provided for comparison purposes. Both the zeolite and silica-based catalysts were reusable and could be stored in the reaction solvent for extended periods of time without an appreciable loss of activity. Keywords: Ni-exchanged Y zeolites; rnethylacetoacetate hydrogenation; liquid phase; Ni crystallite size; structure sensitivity; catalyst durability

INTRODUCTION It is well established j that the heterogeneous hydrogenation of the prochiral [3-keto ester, methylacetoacetate (MAA), over nickel catalysts yields the [3-hydroxy ester, methyl-3, hydroxybutyrate (MHB), as the product. The reaction has been carried out over a variety of supported 2-12 and unsupported l. 10.11,13--15 nickel systems, principally in the liquid phase, 2-s,1°-14 but gas-phase transformations have also been reported. 5,9,~6 Whereas the use of nickel metal as the hydrogenating agent generates a racemic product, 12 treatment of the catalysts with a solution of optically active material induces enantioselectivity with the preferential formation of one of the possible enantiomers. Modification of Ni/SiO2 catalysts with tartaric acid 1°-12 and alanine ~7 has been shown to yield a range of enantioselectivities that are strongly dependent on the modification conditions. The rate of hydrogenation has been reported 18-2° to decrease upon modification due to the occlusion of a part of the active metal surface by the adsorbed modifier. Alternatively, modification has been shown 1°'12'17 to promote the reaction rate due to the more efficient transfer of hydrogen to the adsorbed reactant. Address reprint requests to Dr. Keane at the Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2Wl, Canada. Received 27 April 1992; accepted 18 May 1992 © 1993 Butterworth-Heinemann

14 ZEOLITES, 7993, Vol 13, January

A number of workers 6'9'2x'22 noted difficulty in obtaining reproducible reaction rates that may stem from diffusion limitations inherent in the particular reaction system. Nevertheless, the concentration of MHB produced has been observed to vary with the course of the reaction, 2°'23'24 the catalyst/substrate ratio, 18'21'23-25 reaction temperature, 18,24,25 and hydrogen pressure. 25 Nitta et al-.3'26'27 observed variations in activity with metal loading and catalyst activation conditions and attributed these to a particlesize effect. Nitta and co-workers 3 even suggested that as the selective hydrogenation of MAA is highly structure-sensitive the reaction may be used as a probe for the crystallite-size distribution of nickel metal supported on a heterogeneous catalyst. Regardless, Nitta et al. 3 and Brunner et al. 13 reported a decrease in the degree of hydrogenation with repeated catalyst use. A significant application of molecular sieve science and technology has been in shape-selective catalysis,28 which utilizes the unique microporous zeolite architecture. Although Corma et al. ~9 recently developed zeolite-supported chiral rhodium complexes for the asymmetric hydrogenation of Nacyldehydrophenylamine, the use of zeolites as enantioselective catalysts has not yet been explored. This author could find no instance in the literature where nickel zeolites were applied to the hydrogenation of MAA to either a racemic MHB product or to a product that exhibits an excess of one enantiomer. In

Hydrogenation of rnethylacetoacetate: I: M.A. Keane

this paper, the nonselective formation of MHB is examined for two families of well-characterized s°-ss nickel Y zeolites whose hydrogenating power has already been tested s4-sv in the reduction of benzene and toluene. With regard to the existing papers on MAA hydrogenation over unsupported nickel and nickel supported on amorphous carriers, reaction rates have either been omitted or included as addenda to the enantioselectivity data. In this paper, accurate MAA hydrogenation rates are reported for the first time over a wide range of NiY zeolites and the effect of variations in the metal particle size is examined. The conversion of MAA over these nickel zeolites is also compared with data obtained for silica-supported catalysts.

EXPERIMENTAL The starting or parent zeolite was Linde molecular sieve LSZ-52, which can be represented by the molecular formula Nass(A102)58(SiO2)ls4(H20)260. A fully exchanged KY zeolite was prepared by exchanging out the parent Na + ions; a known weight of NaY, usually 250 g, was refluxed with 400 cm s of 1 mol dm -s solutions of KNOs for 24 h, after which the zeolite was vacuum-filtered and thoroughly washed with hot deionized water to remove the occluded salt. The zeolite was then air-dried at 383 _+ 3 K for a further 24 h. The partially exchanged sample, i.e., KNaY, was exchanged an additional nine times and stored over saturated NH4C1 solutions. Nickelexchanged samples were prepared by taking 100 g of NaY or KY and refluxing with a 400 cm s Ni(NOs)2 solution for 24 h. The pH of the NaY/deionized water suspension was 9.5. It has been reported in the literature s8 that many transition-metal salt solutions are sufficiently acidic to cause structural breakdown of the zeolite host. Consequently, in this study, dilute nickel nitrate solutions (< 0.1 tool dm -s) were used in which the pH of the zeolite/nitrate suspensions was in the range 6-7.5. Under these conditions, a single exchange cycle resulted in a maximum exchange of ca. 7 Ni'Z+/u.c., i.e., seven nickel cations per unit cell; repeated exchanges were necessary to prepare catalysts with higher loadings. A range of Ni/SiO2 catalysts was also prepared by the homogeneous precipitation/deposition of nickel onto a nonporous microspheroidal Cab-O-Sil 5 M silica of surface area 194 m 2 g -x. The precipitation was carried out at 361 + 3K in the presence of urea. The pH values of the silica/nickel nitrate suspensions were in the range 3.8--4.1, but were adjusted with nitric acid to pH 2.5 prior to heating to prevent premature hydrolysis; the pH increased to a value of 5.3 on completion of the precipitation step. The weights of nickel nitrate and urea were varied to obtain a range of nickel loadings, but the m o l a r ratios were kept constant at Ni(NOs)2.6H20/HzNCONH2 = 0.36. The experimental procedures used for the preparation of the nickel-exchanged Y zeolites and nickeldeposited silica catalysts are described in more detail elsewhere, x2's2 The dried catalyst precursors were

sieved in the mesh range 150-125 microns. Atomic absorption (Ni 2+ concentration) and flame emission (Na + and K + concentrations) techniques, using a Perkin-Elmer 360 spectrophotometer, were employed to determine the cation content to within +2%. From the measured ion concentrations, the precise mass of zeolite sample and the number of unit cells per gram of parent zeolite, the number of exchanged cations per unit cell were determined. Thermal analyses were also conducted on all the prepared samples using a Perkin-Elmer thermobalance operating in the t.g. mode to measure the water loss (% w/w) as a function of temperature. The ion-exchanged catalysts are labeled according to the percentage exchange of the indigenous alkali metal ions, e.g., NiKY-23.5 exhibits a 23.5% exchange of the original potassium content that corresponds to a nickel cation content of 6.8 Ni2+/u.c. The silicasupported catalysts are labeled according to the percentage by weight nickel. The hydrated-supported precursors were reduced, without a precalcination step, by heating in a 150 cm s min - l stream of hydrogen at a fixed rate of 200 K h -~ to a final temperature in the range 573-1073 + 2 K, which was then maintained for 18 h. The hydrogen gas (BOC, 99.99%) was purified by passage through water and oxygen traps (activated molecular sieve type 5A and 1% Pd on WO3) that were connected in series. The reduced catalysts were flushed in a purified stream of nitrogen (BOC, 99.99%) at 200 cm 3 min -1 for 3 h, cooled to room temperature, and evacuated to a pressure of 10 -4 Torr. The reduced, evacuated catalysts were then contacted with n-butanol and stored in 10 cm 3 of the alcohol prior to reaction. The n-butanol, which was AnalaR grade (BDH Chemicals), was doubly distilled and stored over activated molecular sieve type 5A and was thoroughly degassed by purging in purified helium and heating under vacuum. The degree of reduction of the exchangeable Ni 2+ cations was measured by iodometric titration and atomic absorption techniques that have been fully described in a previous p a p e r y The nature of the supported metal phase has also been comprehensively characterized by XRD line broadening, s° Selected activated nickel zeolites were also analyzed by electron microscopy using a Jeol JEM-1200EX transmission microscope. The reduced catalysts were first passivated in a 120 cm s min-1 flow of 1% 0 2 in N2 at 298 + 1 K, before being dispersed on standard copper grids; particlesize distributions were determined from the micrographs. Details of the techniques used to characterize the catalyst acidity are also available in an earlier publication, sl X-ray diffraction and infrared spectroscopy were used to monitor zeolite crystallinity before and after the catalyst activation and the hydrogenation steps according to criteria given elsewhere, s2 In the case of the Ni/SiO2 catalysts, nickel metal dispersions were determined by carbon monoxide chemisorption at 273 K, assuming a nickel s u r f a c e a t o m to a d s o r b e d carbon m o n o x i d e stoichiometry of 2:1.s9 The carbon monoxide chemi-

ZEOLITES, 1993, Vol 13, January 15

Hydrogenation of methylacetoacetate: I: M.A. Keane

sorption measurements were made in the pulsed flow mode trsing "50 IXl aliquots of CO in a 40 cm 3 min-1 stream of purified helium and a thermal conductivity detector. The amount of CO adsorbed on calcined (in 150 cm 2 min -1 stream of dry air at 723 K for 12 h) unreduced samples of the appropriate catalysts were also measured and subtracted from the corresponding amounts adsorbed by the reduced catalysts to give the total number of CO molecules adsorbed by the metal component. The liquid-phase hydrogenation of MAA (10 cma), using n-butanol as a solvent (40 cmS), was carried out at 343 +_ 2 K in a 250 cm a glass vessel fitted with a condenser, hydrogen inlet, and thermocouple well. A 60 cm a min -l stream of purified hydrogen was bubbled through the suspension, which was kept under constant agitation at 600 rpm. Over the range of experimental conditions studied, the reactions attained an equilibrium conversion level after 3040 h, at which point the catalyst was removed from the reaction mixture by filtration. The extent of hydrogenation was determined by HPLC using a Pirkle type 1A 5 Ix reversible column (250 x 4.6 mm) with a 10% IPA:90% hexane mixture as the mobile phase. To enhance sensitivity and accuracy, particularly in the case of samples exhibiting a low mole fraction of MHB (< 0.05), the signal from the integrator was digitized and stored on disk for subsequent analysis using the WINner LABNET routine. The overall level of hydrogenation was converted to mol% MHB using a 22-point calibration plot; a quadratic equation was used to fit these data to better than _+1%.

RESULTS A N D D I S C U S S I O N The chemical compositions of the ion-exchanged samples are given in Table 1. By and large, the exchange process was stoichiometric. T h e extent of hydrolysis, as inferred from the number of protons present in the zeolite lattice, was slight and was only detected at lower nickel exchanges (< 10 Ni2+/u.c.). On calcination at 723 K, these catalysts exhibited ill-defined infrared bands in the hydroxyl stretching region. At higher metal loadings, partially hydrolyzed nickel species were also incorporated into the zeolite lattice, resulting in a combined cation charge greater than the 58 positive charges per unit cell of the parent zeolite. Sample crystallinity was maintained for all the tabulated zeolites after the activation and catalysis steps. In general, heterogeneous catalytic reactions, using porous supports, that are carried out in the liquid phase are pore-diffusion-controlled processes. In the case of a hydrogenation system, the progress of the reaction is limited by the diffusion of dissolved hydrogen. 5 Zeolite Y can be classified as a large-pore zeolite, 4° where access to the intracrystalline supercage metal sites is via 0.7-0.8 nm diameter pore openings, while the hexagonal prisms are only accessible through pore openings of 0.20-0.25 nm. If the catalytic system is operating under diffusion

16

ZEOLITES, 1993, Vol 13, January

Table 1 Chemical compositions of the nickel-loaded zeolites prepared by ion exchange Zeolite sample

AM+/u.c. a

Ni2+/u.c.

H+/u.c.

% Ni (w/w)

% H20 (w/w)

NaY NiNaY-6.8 NiNaY-15.8 NiNaY-22.8 NiNaY-29o9 NiNaY-35.7 NiNaY-48.8 NiNaY-63.1 NiNaY-78.6

58.0 53.7 48.8 44.0 41.0 36.0 30.0 22.3 14.6

2.0 4.6 6.6 8.7 10.4 14.1 18.3 22.8

0.3 0.8 0.6 1.2 -

0.7 1.6 2.3 3.0 3.5 4.6 5.9 7.3

25.1 25.3 26.5 26.6 26.8 27.6 28.6 29.1 29.5

KY NiKY-5.2 NiKY-10.7 NiKY-23.5 NiKY-30.5 NiKY-35.6 NiKY-49.1 NiKY-62.5 NiKY-73.8

58.0 54.7 51.5 44.2 39.7 36.9 29.8 22.3 16.3

1.5 3.1 6.8 8.9 10.3 14.2 18.1 21.4

0.3 0.3 0.2 0.5 0.5 -

0.5 1.0 2.2 2.9 3.3 4.6 5.8 6.8

22.4 22.6 22.7 23.5 24.4 24.8 26.4 27.6 28.0

aAM+ ~-Na +orK +

constraints, some of the metal particles located in the zeolite cages do not come in contact with the reactant (MAA) molecules. One of the major difficulties associated with the liquid-phase hydrogenation of MAA has been the attainment of reproducible reaction r a t e s . 6'9"21'22 Indeed, preliminary studies using a range of nickel-silica and nickel-zeolite catalysts in a static atmospheric hydrogenation p r o c e d u r e generated a scatter of hydrogenation activities that were within + 12% of the mean value. Such a divergence in data may be attributed to diffusion limitations inherent in this system, which effectively reduce (in a random manner) the concentration of hydrogen at the active surface to an insufficient level. In contrast, the combination of bubbling hydrogen at a fixed rate through the thoroughly degassed catalyst/substrate suspension, which is kept under constant agitation, ensures that the dissolved hydrogen is kept in close contact with the intracrystalline metal sites and results in conversion levels that are reproducible to better than +2%. By contacting the evacuated activated zeolite with thoroughly degassed n-butanol prior to the reaction, the contribution of diffusion effects to the overall rate of hydrogenation is minimized. The reaction rates that are measured should then reflect the progress of the reaction proper at the metal sites located in the zeolite supercages or dispersed on the external surface. The variation in mol% conversion to MHB as a function of time over three representative catalysts is illustrated in Figure 1. All the reaction profiles exhibit the same basic shape, which is characterized by an initial induction period (treaction = 0-1 h), followed by a continuous increase in MHB formation (treaction = 1-35 h) and, ultimately, a steady-state equilibrium (treaction > 35 h). In the absence of any form of reactant/catalyst agitation, reaction times in excess of 70 h were necessary for equilibrium conversion but

Hydrogenation of methylacetoacetate: I: M.A. Keane 75

m -1-

Table 2 Correlation between the nature of the nickel metal phase supported on a range of NiNaY and NiKY zeolites and the hydrogenation rate and overall conversion to MHB Zeolite sample

50

m

o

25

0

10

20

30

40

th Figure 1 The variation in % tool conversion to MHB with time over (©) Ni/SiO2 (11.9% w / w Ni) and (A) NiNaY-22.8 reduced at 723 K and ([]) NiKY-49.1 reduced at 1073 K: Tr~act~on= 343 K. Inset: initial portion of the profile expanded; symbol as above.

the actual levels of hydrogenation were within experimental error. At the equilibrium conversion, the extent of MHB formation was greatest over the Ni/SiO2 catalysts and decreased with increasing nickel loading and reduction temperature. Taking the data presented in Figure I, the maximum conversion exhibited by the Ni/SiO2 catalyst was ca. 11% higher than that for NiNaY-22.8 under the same activation (Treduction = 723 K) and reaction conditions and ca. 16% higher than that resulting from hydrogenation over the sample NiKY-49.1 that had been reduced at a higher temperature (Tred,ction = 1073 K); in both instances, the difference in activity exceeded the combined experimental errors. By treating the variation in hydrogenation activity with time as a linear relationship, the reaction rate (expressed here as the number of moles of MAA reacted per hour) was determined from the slopes. In the initial stages of the reaction (treaction < 1 h), substrate/surface interactions predominate where MAA can be considered to adsorb on the surface metal as an O-bonded chelate, s This induction period is illustrated in the inset shown

db

% Ni 2+ reduction s

(nm)

Sc R x 10 -3 (m 2) ( m o l M A A h -1)

Mol% MHB d

NiNaY-6.8 NiNaY-15.8 NiNaY-22.8 NiNaY-29.9 NiNaY-35.7 NiNaY-48.8 NiNa¥-63.1 NiNaY-78.6

96 85 79 74 65 57 53 52

24.5 32.7 38.7 42.3 46.2 55.1 59.0 56.0

0.18 0.28 0.32 0.34 0.34 0.37 0.42 0.46

3.10 3.16 2.72 2.66 2.46 2.44 2.48 2.49

65.3 64.4 64.0 63.6 63.2 62.3 61.8 61.6

NiKY-5.2 NiKY-10.7 NiKY-23.5 NiKY-30.5 NiKY-35.6 NiKY-49.1 NiKY-62.5 NiKY-73.8

98 89 69 60 53 72 55 54

22.4 27.0 34.8 36.4 41.9 50.1 55.6 54.1

0.15 0.22 0.29 0.32 0.28 0.44 0.38 0.46

2.90 3.06 2.88 2.80 2.58 2.64 2.48 2.60

66.0 64.9 64.3 63.5 62.8 62.4 62.0 61.8

a From iodometric titration measurements 32 bAverage Ni metal crystallite diameter from XRLB measurements 30 c Surface area of Ni ° (gzeolite) -1 d Equilibrium conversion: trea=ion = 40 h: Treaction = 343 K

in Figure 1. The reaction rate associated with the initial portion of the profile does not, therefore, represent the true initial rate, but rather reflects the surface interactions and hydrogen transfer effects where the contribution of hydrogen diffusion cannot be ignored. It should be noted, however, that, in the relevant documented studies, the reaction rates quoted have invariably been "initial" rates that represented the measured hydrogen uptake within the first hour of the reaction. 3,18.21.22,26 In this study, the linear profile segment following the observed induction period extends (under the reaction conditions associated with the data presented in Figure I) to a t. . . . tion of 10 h. In this region, the reaction may be considered to proceed in the absence of diffusion constraints and the reaction rate measured may be said to reflect the rate of the surface reaction. The rates measured over this region are given in Tables 2-4 for a range of catalysts and activation conditions.

Table 3 Relationship between nickel loading, metal crystallite size, area, and dispersion, and the hydrogenation rate and overall conversion to MHB over a range of Ni/SiO2 catalysts Silica sample Ni/SiO2 Ni/SiO2 Ni/SiO2 Ni/SiO2 Ni/SiO2

I II Ill IV V

dc

Sd

% Ni (w/w) a

% Db

(nm)

(m 2)

R x 10 -3 (mol MAA h -1)

Mol% MHB e

1.5 6.1 11.9 15.2 20.3

73 54 40 33 27

1.4 1.9 2.5 3.1 3.7

7.1 22.2 31.8 33.0 36.2

3.55 3.45 3.38 3.30 3.27

78.8 77.8 75.1 74.6 74.0

a Water content < 5% w / w for all catalysts b Percentage dispersion of surface nickel metal atoms, from CO chemisorption measurements c Nickel metal crystallite diameter, from the relationship 43d = 101/D dSurface area of Ni ° (gcatalyst) -1 e Equilibrium conversion: treaetion= 40 h:Trea=ion = 343 K

ZEOLITE$, 1993, Vol 13, J a n u a r y

17

Hydrogenation of methylacetoacetate: I: M.A. Keane Table 4 Effect of reduction temperature on the zeolite-supported metal phase, the hydrogenation rate, and the overall conversion to MHB Reduction temperature (K) Catalytic parameter

643

698

773

873

973

1073

41.0 16.7 0.12 3.18 69.2

79.0 21.0 0.18 3.48 67.8

97.0 26.0 0.18 3.06 65.0

98.0 28.6 0.16 2.81 64.4

99.0 30.3 0.15 2.60 64.3

99.0 31.0 0.15 2.40 64.1

28.0 38.1 0.35 3.30 63.8

45.0 51.5 0.36 2.48 62.7

57.0 64.2 0.36 2.15 61.1

59.0 70.5 0.36 1.94 60.4

61.0 62.0 73.0 74.6 0.34 0.34 1 . 9 6 2.00 60.2 60.2

NiNaY-6.8 % Ni 2+ Redn. a d(nm) b S x 1017 nm z c R x 10 -3 d Mol%. MHB" NiNaY-63.1 % Ni 2+ Redn. a d (nm) b S X 1017 nm 2 c R x 10 -3 d MOI% MHB e

a From

iodometric titrametric measurements a2 bAverage Ni metal crystallite diameter from XRLB measurem e n t s 3°

c Surface area of Ni ° (gzeolite)-1 d MOI MAP, h -1 eEquilibrium conversion: treac=ion = 40 h:Treaction = 343 K

It must be stressed that the quoted rates are by no means absolute, but serve as a useful index for comparing the effects of certain experimental parameters on the rate of the surface reaction and can best be described as "apparent" rates. The nature of the nickel metal phase supported on NaY and KY carriers has been characterized in two previous reports. 3°-32 The degree of reduction of Ni 2+ cations in the catalyst precursor was measured by iodometric titrations and an atomic absorption technique s° and the relationship between nickel exchange and nickel reduction is given in Table 2 for both sodium and potassium-based zeolites. Volumeweighted average nickel metal particle sizes (d) were calculated from X-ray line-broadening measurements ~° using the Scherrer equation, correcting for instrumental line broadening according to the criteria of Jenkins and de Vries. 4] The metal surface area (,per gram of catalyst) was obtained from the equation:

cages are appreciably reduced before the surface acidity (which depends on the electronegativity of the charge-balancing cation and is lower for the KY support 31) begins to inhibit further reduction, ss The average diameter of the resultant nickel metal crystallites also increases with increasing nickel loading (Table 2). It should be noted that the lower limit of detection for the XRD line-broadening technique is 5 nm and, as the diameter of the supercage is 1.2 nm, it follows that the observed line broadening is essentially diagnostic of the presence of nickel metal located on the external surface. For particles with diameters greater than 100 nm, the line broadening is no longer detectable and, hence, 100 nm represents the upper limit of detection. From the data presented in Table 2, it is evident that the particles supported on the KY carrier are smaller when compared with NiNaY catalysts of similar nickel content. The larger K + ions are therefore more effective than are the Na + ions in blocking the path of the migrating nickel species and retarding the aggregation of metal particles on the external zeolite surface. However, the wide range of particle sizes is immediately evident from the representative histograms presented in Figure 2. The distribution of particle sizes is very broad, and from visual inspection of the micrographs, the particles exhibited a marked shape anisotropy. Nevertheless, the average particle diameter computed from the electron microscopy data was up to 8% lower than that measured by the XRD line-broadening technique. Although the average metal particle size is greater (and, hence, the dispersion is lower) for the nickel-concentrated samples, the surface metal area per gram of catalyst is also higher. Both the apparent reaction rate and the overall percentage mole conversion to MHB decreased as the level of nickel loading and nickel crystallite size increased. The apparent rate data are more sensitive to changes in metal site density and distribution than are the mol% MHB values that represent equilibrium conversions, 60

50

4O

S = 60M/d 9 where S stands for the surface area (m 2 g-l); M, the metal loading (% w/w Ni°); d, the average particle size; and P, the density of the metal, taken here to be equal to 8.9 g c m - 3; the metal crystallite sizes and surface areas exhibited by the zeolite systems are given in Table 2. The decrease in Ni 2+ reduction with increasing nickel exchange (shown in Table 2) has been attributed 33 to an accompanying increase in surface Br6nsted acidity that suppresses the progress of reduction. The exception to this relationship (exemplified by the NiKY-49.1 sample in Table 2) occurs for the higher .-z+ nickel-loaded KY zeolites where the first four Ni cations that locate in the super-

18 ZEOLITES, 1993, Vol 13, January

c~ 03 Q. o_ Z

30

20

10

I 5-19

20-29 30-39 4 0 - 4 9

50-59

60-69

70-79 80-89 90-99

d nm Figure 2 Crystallite-size histograms of (open bar) NiKY-5.2 reduced at 723 K, (solid bar) NiKY-62.5 reduced at 723 K, and (crosshatched bar) NiKY-62.5 reduced at 1073 K.

Hydrogenation of methylacetoacetate: I: M.A. Keane

prepared by the homogeneous precipitation/ deposition method, are characterized by a uniform dispersion of very small (< 6 nm) metal particles. The corresponding reaction rates and equilibrium conversion levels are considerably higher than the values observed for nickel zeolites of similar metal content (w/w) but lower metal dispersions. Within the family of prepared Ni]SiO2 catalysts, activity was also found to decrease with increasing crystallite diameter (Table

[] I

J~

3.5

n

< <

]E

[]

3.0

i

[]

O

E !

O 0

[]

2.5

A

A O

O

O

3).

o ¢x:

2.0 I

F

I

I

15

35

55

75

d nm Figure 3 The relationship between reaction rate (R) and the average sizes of the nickel crystallites (d) supported on a range of (A) NiNaY and (C)) NiKY catalysts reduced at 723 K and on samples of (i7) NiNaY-6.8 and (~) NiNaY-63.1 reduced in the temperature range 643-1073 K: Treactlo. = 343 K.

attained after 30-40 h. These observations agree with the findings of Nitta et al. ~'26'27 where the initial reaction rates over Ni/SiO2 and Ni/AI203 catalysts decreased as the nickel particle size was increased in the range 3-31 nm. Such a crystallite size effect reflects a strong influence of surface morphology on the reaction rate or even the possible role of the crystal structure of the metal surface, as it is known 42 that the fraction of the nickel (111) plane in the fee cubooctahedron crystallites increases with increasing crystallite size. The relationship between the nature of the metal phase supported on the silica carrier and the hydrogenation activity for a range of nickel loadings is illustrated in Table 3. In the case of the silica supports, the nickel particle size was determined from the dispersion measurements (by CO chemisorption) according to the relationship43: d = 101/D where D is the dispersion or fraction of exposed metal atoms and d is the surface weighted average crystallite diameter, assuming (spherical) particles less than 1 nm in size to be 100% dispersed. In determining the nickel metal surface area, a cross-sectional area of 0.065 nm 2 was assumed for a nickel atom. 43 The nickel metal on the silica carrier was found to be present in the form of much smaller crystallites and, as in the case of the nickel zeolites, crystallite size and metal area (per gram of catalyst) increased with increasing metal loading. Furthermore, these metal particles exhibit a very narrow size distribution, as shown by electron microscopy. 12 Indeed, the dispersion figures quoted in Table 3 are in good agreement with values reported in the literature 44 and are in accord with the generally accepted view45"-47 that nickel catalysts, with metal loadings up to 30% w/w,

The consequences of an increase in the temperature of reduction on the average nickel particle size are illustrated in Figure 2 and Table 4 for representative NiY zeolites. The degree of Ni 2+ reduction and the resultant metal crystallite size was increased as the activation temperature was elevated, in the range 643-1073 K, while the metal surface area (per gram of zeolite) passed through a maximum in the interval 673-773 K; no metal particles were discernible at Treduction < 573 K. Not unexpectedly, an increase in the level of Ni 2+ reduction is accompanied by an increase in the average size of the nickel particles generated as the greater concentration of nickel metal particles produced at the higher temperatures will increase the probability of particle agglomeration, resulting in the generation of larger supported crystallites. There was no evidence to suggest that metal sintering occurred after the equilibrium degree of precursor cation reduction had been attained. Taking a sample of NiKY-62.5 as a representative case, the effect of T r e d u c t i o n o n the particle-size distribution is shown in Figure 2. It can be seen that an increase in Treduction from 723 to 1073 K results in a broader range of particle sizes with the virtual absence of particles less than 40 nm in diameter. In an earlier report, 3° the sintering of NiKY (AEapp[623-723 K] = 20.2 kJ mo1-1) was shown to proceed via a crystallite migration mechanism in which mass transfer occurs by the momentary accumulation of metal atoms on one side of a particle, the net effect being the migration of the particle as a whole in that direction. As was observed for variations in nickel content (Tables 3 and 4), the apparent reaction rate and the equilibrium conversion to MHB was lowest at the highest recorded metal crystallite size. In contrast to the tartaric acid-treated Ni/SiO2 systems, ]°-12 the nickel content of the catalysts in this study remained unchanged after repeated use and the nature of the particle-size distribution was preserved. The relationship between particle size (varied in the range 16-75 nm by altering either the metal loading or Treduction) and the apparent reaction rate is illustrated in Figure 3. Although the data are considerably scattered, there is a distinct decrease in the apparent reaction rate with increasing particle size. This scatter is narrowed when the log of the specific activity or apparent rate per unit metal area is plotted against particle size, as shown in Figure 4. This graphical relationship shows unambiguously that MAA hydrogenation to MHB is a structure-sensitive reaction and that over the range of particle sizes considered

ZEOLITES, 1993, Vol 13, January 19

Hydrogenation of methylacetoacetate: I: M.A. Keane T Z

[]

0.5

0

•' -4.0 E T

0.6

q

-4.1.

li

T

-3.9

[] On

0.4

~o

LL

O

O

!--

AOoz~ 0

E -3.8

o~r

A

~r

[]D

[] 0

0.3

0.2

o~ - 3 . 7

o

I

I

I

I

15

35

55

75

I

I

I

I

10

20

30

40

0.1

SNi' (g.Ni°1-1 mz

d nm Figure 4

Log of reaction rate (expressed per surface nickel metal area) as a function of the average sizes of the nickel crystallites (d) supported on a range of (A) NiNaY and (O) NiKY catalysts reduced at 723 K and on samples of (r-I) NiNaY-6.8 and (~-) NiNaY-63.1 reduced in the temperature range 643-1073 K: Treaction = 343 K.

activity decreases with increasing nickel crystallite diameter. T h e variation in rate with particle size for 41 distinct catalyst systems, where differences in d are either due to variations in nickel exchange or in Treduction at a constant nickel loading, has been fitted in Figure 5 to an expression of the form R/d = A exp (-kd) = 3.34 × 10 -4 (+ 1.14 × 10-5) exp{-3.78 5< 10 -2 (+1.11 × 10-3)d}, where A is a constant and k can be considered to reflect the d e p e n d e n c y o f the apparent reaction rate on d. Deviations from the fit can be accounted for by a combination of experimental error and the assumptions used to esti-

20i o

T

Figure 6 The relationship between the turnover frequency (TOF) of MAA. molecules and the nickel surface area (per gram of supported nickel metal) for a range of (A) NiNaY and (O) NiKY catalysts reduced at 723 K and for samples of (r-I) NiNaY-6.8 and (~-) NiNaY-63.1 reduced in the temperature range 643-1073 K: Treaction = 343 K.

mate particle sizes. T h e structure sensitivity of this reaction is f u r t h e r illustrated in Figure 6 where the turnover frequency or n u m b e r o f MAA molecules converted per metal site is plotted against the surface metal area (expressed per gram of supported nickel metal). For both NaY and KY supports, the turnover n u m b e r increases smoothly with increasing metal surface area. It is a prerequisite for the commercial viability of a catalyst that it can be used over and over again in a n u m b e r of reaction cycles without an appreciable loss of catalytic activity or selectivity. Both Ni/SiO2 and nickel powder catalyst systems have been reported 3'13 to show significant losses of MAA h y d r o g e n a t i o n activity with repeated use. T h e effects o f submitting the catalysts associated with this study to a n u m b e r o f reaction cycles are shown in Table 5. In the case o f both Ni/SiO2 and NiY systems, there was a negligible

7

Table 5 Variation in the equilibrium conversion to MHB, over three representative catalysts, with repeated use

Mol% MHB Cycle no. 0

II

15

I

I

I

35

55

75

d nm Figure 5 The relationship between the reaction rate (expressed per nm of nickel metal) and the average sizes of the nickel crystallites (d) supported on a range of (&) NiNaY and (0) NiKY catalysts reduced at 723 K and on samples of (11) NiNaY-6.8, ( * ) NiNaY-63.1, and (0) NiKY-73.8 reduced in the temperature range 573-1073 K: Tre°ction = 343 K. The solid line represents the expression Rid = 3.34 x 10-4 (_+ 1.14 x 10 -s) e x p { - 3 . 7 8 x 10 -= (+ 1.11 x 10-a)d}.

20

ZEOLITES, 1993, Vol 13, January

Ni/SiO2 a

NiKY-5.2

NiKY-49.1

1 2 3 4 7 10 14 18 20

77.8 77.6 77.2 76.5 75.4 73.6 64.7 57.8 51.7

66.0 65.5 65.5 64.8 63.6 61.8 55.4 50.0 45.4

62.4 61.7 61.0 60.5 59.4 57.2 50.8 42.6 38.7

b

74.9

62.7

58.7

a6.1% w/w Ni bAfter the first reaction cycle, the catalyst was stored in n-butanol for 8 weeks

Hydrogenation of methylacetoacetate: I: M.A. Keane

drop in activity (< 3%) after the first five reaction cycles. However, after 20 consecutive reaction runs, the overall levels of hydrogenation dropped to ca. 60% of the original value. Interestingly, both the amorphous silica and crystalline aluminosilicatebased nickel catalysts exhibited similar levels of durability. Furthermore, as illustrated in Table 5, the catalysts can be stored for extended periods (up to 8 weeks) in n-butanol and reused to yield similar product compositions (within +_6%) to the original reaction products.

10 11 12 13 14 15 16

CONCLUSIONS From the data reported in this paper it can be concluded that (i) nickel-exchanged Y zeolites can be used to promote the hydrogenation of MAA to MHB in the liquid phase under conditions where diffusion limitations are rendered negligible; (ii) the conversion of MAA to MHB at 343 K is structure-sensitive and depends on the nature of the supported metal phase; (iii) the dependency of the rate on the average crystallite diameter can be represented by the expression, R/d = 3.34 × 10 - 4 (+ 1.14 × 10 -5) exp{-3.78 x 10 -2 (+-1.11 × 10-3)d}, where the rate increases with decreasing crystallite size; (iv) under the same activation conditions and at equivalent metal loadings (w/w), Ni/SiO2 catalysts exhibit higher metal dispersions and surface areas and, consequently, higher conversions than the nickel zeolite samples; and (v) both the NiY and Ni/SiO2 catalysts show similar durabilities and retain a high level of hydrogenation activity after storage in the reaction solvent.

ACKNOWLEDGEMENTS The author is grateful to Prof. B. Coughlan (University College Galway) for provision of the Y zeolite support and to Prof. G. Webb (Glasgow University) for the use of some experimental facilities.

17 18

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

REFERENCES 1 Izumi, Y. Adv. CataL 1983, 12, 215 and references therein 2 Nitta, Y., Sekine, F., Sasaki, J., Imanaka, T. and Teranishi, S. J. CataL 1983, 79, 211 3 Nitta, Y., Utsumi, T., Imanaka, T. and Teranishi, S. J. CataL 1986, 101,376 4 Richards, D.R., Kung, H.H. and Sachtler, W.M.H.J. Mol. Catal. 1986, 36, 329 5 Fu, L., Kung, H.H. and Sachtler, W.M.H.J. Mol. CataL 1987, 42, 29 6 Bostelaar, L.J. and Sachtler, W.M.H.J. MoL CataL 1984, 27, 387 7 Hoek, A. and Sachtler, W.M.H.J. CataL 1979, 68, 276 8 Groenewegen, J.A. and Sachtler, W.M.H., in Proceedings of the 6th International Congress on Catalysis, London, 1976 (Eds. G.C. Bond, P.B. Wells and F.C. Thompkins) The Chemical Society, London, 1977, p. 1014 9 Hoek, A., Woerde, H.M. and Sachtler, W.M.H., in Proceed-

39 40 41 42 43 44 45 46 47

ings of the 7th International Congress on Catalysis, Tokyo, 1980 (Eds. K. Tanabe and T. Sieyama) Kodansha/Elsevier, Tokyo/Amsterdam, 1981, p. 376 Bennett, A., Christie, S., Keane, M.A., Peacock, R.D. and Webb, G. Catal. Today 1991, 10, 363 Keane, M.A. and Webb, G. J. Chem. Soc., Chem. Commun. 1991, 1619 Keane, M.A. and Webb, G., J. Catal., 1992, 136, 1 Brunner, H., Muschiol, M., Wishert, T. and Wiehl, J. Tetrahedron: Asymmetry 1990, 1,159 Brunner, H., Amberger, K. and Wiehl, J. Bull. Soc. Chim. Belg. 1991, 100, 571 Blaiser, H.U. Tetrahedron: Asymmetry 1991, 2, 843 and references therein Yasumori, I., Yokezeki, M. and Inoue, Y. Faraday Discuss. 1981, 71,385 Keane, M.A. and Webb, G. J. Mol. Catal. 1992, 73, 91 Harada, T., Haraki, Y. Izumi, Y., Muraoka, J. Ozaki, H. and Tai, A., in Proceedings of the 6th International Congress on Catalysis, London, 1976 (Eds. G.C. Bond, P.B. Wells and F.C. Thompkins) The Chemical Society, London, 1977, p. 1024 Hubbell, D.O. and Rys, P. Chimia 1970, 24, 442 Ozaki, H., Tai, Ao, Kobatake, S., Watanabe, H. and Izumi, Y. Bull. Chem. Soc. Jpn. 1978, 51, 3559 Smith, G.V. and Musoiu, M. J. Catal. 1979, 60, 184 Wittmann, G., Bartok, G.B., Bartok, M. and Smith, G.V.J. Mol. Catal. 1990, 60, 1 Tanabe, T. Bull. Chem. Soc. Jpn. 1973, 46, 1482 Keane, M.A. and Webb, G. J. Catal., in press Klabunovskii, E.I. Russ. J. Phys. Chem. 1973, 47, 765 Nitta, Y., Kawabe, M. and Imanaka, T. Appl. Catal. 1987, 30, 141 Nitta, Y., Utsumi, T., Imanaka, T. and Teranishi, S. Chem. Lett. 1984, 1339 Chen, N.Y., Garwood, W.E. and Dwyer, F.G. Shape Selective Catalysis in Industrial Applications, Marcel Dekker, New York, 1989 Corma, A., Inglesias, M., del Pina, C. and Sanchez, F..J. Chem. Soc., Chem. Commun. 1991, 1253 Coughlan, B. and Keane, M.A. Zeolites 1991, 11, 2 Coughlan, B. and Keane, M.A.J. Colloid. Interf. Sci. 1990, 137, 483 Coughlan, B. and Keane, M.A.J. Cata/. 1990, 123, 364 Coughlan, B. and Keane, M.A.J. Catal. 1992, 136, 170 Coughlan, B. and Keane, M.A.J. Mol. Catal. 1992, 71, 93 Coughlan, B. and Keane, M.A. Catal. Lett. 1990, 5, 101 Keane, M.A. Ind. J. TechnoL 1992, 30, 51 Coughlan, B. and Keane, M.A. Zeolites 1991, 11, 12; 1991, 11,483 McDaniel, C.V. and Maher, P.K. Zeolite Chemistry and Catalysis, ACS Monograph 121, Am. Chem. Soc., Washington, DC, 1976, p. 285 Romanowski, W. Highly Dispersed Metals as Adsorbents, Halsted Press, Wiley, New York, 1987, p. 171 Davies, M.A. Chem. Ind. 1992, 137 Jenkins, R. and de Vries, J.L. Worked Examples of X-Ray Analysis, 2nd Ed., Philips Technical Library, MacMillan, New York, 1978 van Haardeveld, R. and Hartog, F. Surf. Sci. 1969, 15, 189 Smith, J.S., Thrower, P.A. and Vannice, M.A.J. Catal. 1981, 68, 270 Montes, M., Penneman de Bosscheyde, Ch., Hodnett, B.K., Delannay, F., Grange, P. and Delmon, B. Appl. Catal. 1984, 12, 309 Richardson, J.T., Dubus, R.J., Crump, J.G., Desai, P., Osterwalder, U. and Cale, T.S., Stud. Surf. Sci. Catal. 1979, 3, 131 Richardson, J.T. and Dubus, R.J.J. Catal. 1979, 54, 207 Hermans, L.A.M. and Geus, J.W. Preparation of Catalysts II (Eds. B. Delmon et al.) Elsevier, Amsterdam, 1983, p. 113

ZEOLITES, 1993, Vol 13, J a n u a r y

21