Ind. Eng. Chem. Fundam. 1986, 25, 43-52
43
Catalytic Hydrogenation of Fatty Oils Jacques W. E. Coenen Catholic University, Nymegen, The Netherlands
A long period of research into the fundamentals of the process for hydrogenation of edible fatty oils and of the silica-supported nickel catalysts, used in this process, is reviewed, adding recent: findings to a large structure of fundamental insight built up in the past during the author’s employ in Uniiever Research, Vlaardingen, The Netherlands. The driving force for this research was the need for fundamental understanding as a means to achieve improvements in catalysts, process, and products. Even though fat hydrogenation is a structure-insensitlve reaction, so that catalytic activity should be proportional to nickel surface area, this fact is not easy to demonstrate. Mass transport limitation within the pore structure of the catalysts masks the fundamentally simple relations. Triglycerkles are large molecules, and their diffusion can be impeded in narrow pores. In the structure of the catalysts, formation of basic nickel silicate plays an important role a s structure stabilizer. Remnants of this compound in the reduced catalyst provide bonding between the small nickel crystallies and their silica support. Also, the transport of hydrogen, especially across the gaslliquid interface, is of crucial importance, both for the rate behavior and for selectivity. The transport of the large fat molecules in the pore structure also plays a dominant role in the attained selectivity.
Introduction The process of edible oil hydrogenation, invented by Wilhelm Normann a t the beginning of this century (Nomann, 1902,1903), filled a great need to alleviate raw material shortage in the margarine industry, which until then used mainly animal fats. Although the importance of the process, by which more abundant liquid oils could be converted to higher melting fats, was recognized immediately, the skills required for large-scale application were lacking in the food industry. A chemical and soapmaking industry, Joseph Crosfields in Warrington, England, put the first full-scaleplant on stream in 1907. Since then, application has quickly spread throughout the fat industry. Up to World War I1 the process was hampered by variable and poor activity and selectivity of catalysts, impurity of process hydrogen, inadequate prerefining methods, and poor equipment. Lack of insight in the rate-limiting factors resulted in very long reaction times and variable product characteristics. During and after World War I1 the application of many new investigating techniques led to improved insight in many catalytic processes and their catalysts. In fat hydrogenation many process improvements were gradually introduced, but the nickel catalyst, prepared by empirically developed methods, remained the dark horse for a long time. In the following the search for insight into the workings of catalyst and fat hydrogenation process will be reviewed, combining older data with newer findings. Although other catalytic materials have occasionally been used, nickel catalysts remain overwhelmingly important and we will limit ourselves to nickel. The original combination of nickel with kieselguhr is still with us, as in Normann’s days. The purpose in hydrogenating fats is twofold: to increase the melting point and to improve flavor stability. The former is easier to quantify and rationalize than the latter, which is often difficult to measure, controversial, and proprietary. We will therefore discuss only the former effect. Necessarily, fat hydrogenation is a three-phase process: solid catalyst, liquid oil, and hydrogen gas, which has low solubility in the oil. Interphase mass transport is therefore of crucial importance as we will see in the following. Almost always the process is performed in some form of stirred tank reactor, generally in batch, and sometimes in a cascade for continuous operation. Now the process 0196-4313/86/1025-0043$01.50/0
hydrogen is produced by steam reforming of natural gas. It is of exceptional purity and a far cry from the gas produced formerly by the steam iron process, which contained residual CO as well as Nz, COS, H2S, HzO, and COz. Prerefining and drying of raw materials are much improved. The combined result of improved reactors and purer oil and hydrogen is a shortening of reaction time from a former 12 h to 1/2-2 h now. The process is now more predictable and closer to what can be observed in a laboratory reactor. We will now discuss the steps which gradually led to an improved understanding of catalyst structure and performance and of the manner in which process conditions influence the latter. Catalyst S t r u c t u r e a n d Catalytic Activity Catalytic Activity. The activity test was done in a standardized metal reactor of 150-g-oil capacity, thermostated a t 180 OC, with a standardized stirrer running a t 3000 rpm, 60 L of H2/h passing through the oil, using well-refined sesame oil. Conversion after 30 min is divided by similarly obtained conversion for standard catalyst, so that activity is expressed as percent of standard. Nickel Crystallite Size, Crystallite sizes of face-centered cubic (fcc) nickel metal in the reduced and passivated catalysts were determined from X-ray line broadening. For details see Coenen (1958, 1970). Activity vs. Crystallite Size. For a number of nickel/kieselguhr catalysts we found no correlation a t all between activity and crystallite size. The question then arises whether all catalytically active nickel metal is taken into account in the crystallite size determination. Some of the nickel metal might be amorphous to X-rays. We therefore did a more extensive X-ray investigation. As already reported by de Lange and Visser (1946)and by van Eyck van Voorthuysen and Franzen (19511, we found that in the unreduced catalyst part of the nickel is present as a basic silicate, nickel antigorite, of which the structure is shown in the top half of Figure 1. This structure is closely related to that of nickel hydroxide, as shown in the bottom half of the figure. Nickel hydroxide is also present in the unreduced catalysts. The silicate, formed by interaction of nickel compounds with the silica of the kieselguhr, is ideally suited to act as an epitaxial attachment layer of nickel hydroxide to silica. The light green precipitate, the unreduced state of the Catalyst, was activated by reduction a t 450 OC for 4 h in flowing hydrogen. A t the reduction
0 1986 American Chemical Society
44
Ind. Eng.
Chem. Fundam., Vol.
25, No. 1, 1986
/
Ds
,/
= 1.096DX
c mpregnates 0
0 1 2 3 4 5 i
~
1 LJ ~
1
i
O 2 r
coprecipitates
6 8 1'0 12 li 16 18 20 22 2 i 26 20 D,nm
Figure 2. Good 1:l correlation between crystallite size D,, determined from X-ray line broadening, and D,, calculated from nickel surface area.
3 123453\
-- [ll]
Figure 1. Three projections of the structure of nickel antigorite (top) and of nickel hydroxide (bottom).
temperature the nickel hydroxide first dehydrates to the oxide, which is then reduced to fcc nickel metal. The nickel antigorite is much more difficult to reduce and partly remains at the applied reduction temperature, where reduction is never complete. For a subsequent X-ray investigation, the reduced catalysts were carefully passivated with a 1% 02-in-N2mixture. Quantitative X-ray Analysis of the Passivated Catalysts. The phase composition of the passivated catalysts was determined by quantitative X-ray analysis. Details have been published (Coenen, 1958; Coenen and Linsen, 1970). The line intensities could be interpreted as stemming from fcc nickel metal, nickel oxide, and nickel antigorite. The sum of nickel as metal, as oxide, and as silicate equaled the total nickel content determined chemically within a few percent. At the same time, it was found that the content of crystalline nickel metal was equal to the content of' zerovalent nickel, as determined chemically. We may thus conclude that the reduced catalyst does not contain amorphous nickel metal. Thus, the question remains: why is there no correlation between crystallite size and activity? Crystallite Size vs. Nickel Surface Area. The question we have to answer is this: is the crystallite size as determined from X-ray line broadening a true measure of nickel dispersion? Hydrogen chemisorption was measured a t 20 "C and l bar of hydrogen on the catalysts reduced at 450 "C for 4 h and pumped at 450 " C for 2 h. We assumed monolayer adsorption to be reached a t 1 bar, because a decrease in magnetization due to chemisorption, still significant above 300 torr, is hardly observable above 700 torr. To allow for some 5% slow adsorption, we observed equilibrium after 16 h. We made the usual assumption of one H per surface Ni atom. On various forms
of silica no hydrogen adsorption could be measured, so we hoped the assumption of no spillover to be justified. We further assumed a combination of the three densest crystallographic planes in the surface, yielding an average area of 0.063 nm2 per surface nickel atom. Combining these assumptions and the observed adsorption values to get nickel surface areas for the same range as catalysts as before, we found again absolutely no correlation with activity. Thus, the question remains: what is wrong, our estimates of nickel dispersion or our activity assessment? We first checked whether crystallite size and nickel surface area showed a sensible relationship. To do so, we need to assume a model for which the following reasoning was used: For a dispersion of nickel metal on a support, generated at high temperature, some form of minimumenergy situation should be attained, lying somewhere between two extremes: (i) spherical crystallites: optimum lattice energy, minimum surface energy, no attachment energy; contact area 0 X SNi.(ii) uniatomic nickel layer on support: no nickel lattice energy, optimum attachment energy; contact area 1 x SNi. Neither of these extremes looks very plausible. Halfway between these extremes, attachment area 'I2X SNi, we find the hemisphere model with exposed nickel area 2nr2 and contact area nr2 = SNi/2. In the following we will find much evidence for a strong interaction between nickel and silica, so that the model of a hemisphere attached with the equatorial plane looks like a reasonable approximation. Using this model, we can now calculate the crystallite size from the nickel surface area. We define D,, similar to D,, as the cube root of the crystallite volume. For catalysts prepared from nickel sulfate, which retain some sulfur, a correlation was made. Roberts and Sykes (1957) demonstrated that residual sulfur after reduction is concentrated as sulfide on the nickel surface. We showed that the surface stoichiometry is close to one S per two Ni, and we allowed 0.126 nmz per atom of S. Next to nine catalysts, supported on kieselguhr, 13 further catalysts, made by impregnating nickel nitrate on various forms of silica, were included. For the 22 catalysts, the relation between D, and D, is shown in Figure 2, showing conclusively that the two measures of nickel dispersion are highly consistent. D, values for the impregnates were determined only in duplicate, whereas for the precipitates 12-fold determinations per catalyst were made. This explains the greater scatter for the impregnates. The Location of the Components of the Catalyst. Quantitative X-ray analysis showed the presence of nickel oxide and nickel silicate, next to nickel metal. The quantity of oxide proved proportional to nickel surface area, which suggests that the oxide was formed in the
Ind. Eng. Cham. Fundam.. Vol. 25. No. 1. 1986 45
Figure 3. Relation between nickel crystallite size and the fraction of nickel present in the reduced catalyst as silicate.
passivation, in which superficial oxidation of two atom layers deep was found to occur. The freshly reduced catalyst does not contain nickel oxide, which is not surprising since NiO is easily reduced a t 450 "C (cf. Coenen, 1979). In Figure 3 crystallite size D, is plotted against nickel present as silicate. Clearly, the silicate acts as an inhibitor to nickel sintering, which again suggests that the silicate acts as a bonding "glue" between nickel metal and silica. We will come back to this point later. For the assumed bonding function of the silicate we found an additional argument. As shown in Figure 1, nickel hydroxide can grow epitaxially on antigorite, which on one side has identical crystal structure. In both compounds the nickel atoms are arranged in a hexagonal sheet. In NiO and in fcc Ni metal the (111) plane has a similar arrangement. It is therefore attractive to m u m e that this epitaxial relation with the bonding layer of antigorite persists. The Ni-Ni distance in the metal is smaller than in the antigorite. For catalysts with about 3-nm crystallite size and high silicate content, we found the lattice parameter of the nickel metal enlarged by about 0.5%. F u r t h e r Evidence f o r S t r o n g Metal-Support Interaction. The top half of Figure 4 shows an electron micrograph of a small piece of kieselguhr at a magnification of 1OOOOOX. Of this form of kieselguhr a nickel catalyst was made. After reduction the nickel metal was extracted with CO as nickel carbonyl. The bottom half of Figure 4 shows a similar piece of kieselguhr recovered from the extracted catalyst. Clearly, 'fins" have grown from the rather solid silica. Undoubtedly, these are remnants of antigorite layers. The original surface area of the guhr, 22 m2/g, was found to have grown to 446 m2/g for the CO-extracted material. This system also provided an interesting confirmation of the hemisphere model: Before nickel extraction the reduced catalyst had a BET surface area of 161 m2/g and a silica content of 28.2%. Per gram of silica the surface area is thus 571 m2/g. After extraction the surface area was 446 m2/g of S O z , a loss of 125 m2/g of SiO? The reduced catalyst had a nickel surface area of 99 m2/g of Ni or 246 m2/g of Si02in the catalyst. If the hemisphere model is correct, this would lead us to expect an area loss of 1/2 X 246 = 123 m2/g of SOz. in excellent agreement with the loss of 125 m2/g of SiOz actually found. A P i c t u r e of S t r u c t u r a l Change i n N i / S i 0 2 Reduction. To summarize this &ion on catalyst structure, Figure 5 depicts the structural changes in the cause of reductive activation of a silica-supported catalyst with a high nickel loading. A nickel hydroxide layer, epitaxially attached to silica by means of an antigorite layer, breaks up by dehydrative shrinking into little blocks of nickel oxide, for the sake of simplicity shown as cubes, still attached to the support. These oxide blocks, in turn, are reduced to nickel metal crystallites, still attached to the support. For the shape of these we aasume the hemisphere
Figure 4. Two electron micrographs of a piece of kieselguhr: top, native guhr; bottom. recovered from Ni/whr catalyst by extraction of nickel metal from the reduced catalyst by carbon monoxide.
CATALYST ACT I V A T I O N
Figure 5. Schematic representation of the structural changes involved in the reduction of a Ni/SiO, catalyst with high nickel loading.
to he a reasonable approximation. We were able to prove from X-ray line broadening that one nickel crystallite on reoxidation yields one nickel oxide crystallite, and vice versa. The formation of additional surface area and pore volume in the dehydration of the nickel hydroxide layer could be used to calculate the size of the oxide blocks, and this proved to agree closely with the same size found from X-rays (Coenen, 1979). Relationship of Catalyst Activity to Nickel Surface Area. Catalytic activity must be assumed to be located in the nickel surface. From the foregoing we must conclude that our estimates of nickel dispersion are essentially correct. Why then no correlation with activity? To understand this, we should realize that triglycerides are large
46
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 Activity
A,S
120
80
t I"/ .
0
.
bo
.
o Impregnates
-
0"
0
Coprecipitater
Po
d
( s N >25A)/i ~ 2
L
6
8
10
12
14
16
Figure 6. Correlation between-catalyst activity and a corrected nickel surface area (SNi > 25 & / L , which takes pore width and pore length into account.
molecules, roughly of molecular weight 1OOO. We will have to explore the catalyst structure even further to get information on the accessibility of the nickel surface for these large molecules. The catalysts we have discussed have particle sizes of 1-10 pm, and these particles are porous. The nickel surface is hidden in the pores, which may have widths close to the molecular size of the triglycerides. We determined pore size distributions and cumulative surface areas for the complete range of catalysts. We further assumed the nickel surface area to be proportionally distributed over the support surface, so that we could estimate the nickel surface area in pores wider than any given size. By trial and error we found a borderline of 25 A to be highly significant. Among the series of Catalysts we found some to have 100% of their nickel surface area to be in these wider pores, whereas for some as little as 11% of the nickel area had adequate accessibility. We can visualize the effect of pore width on triglyceride transport as an effect on the effective diffusion coefficient, which appears to drop from the bulk value to zero in the range of pore widths between 35 and 15 A. Once we realize the importance of triglyceride transport, we have to consider not only pore width but also pore length. This is obviously related to particle size, so we divided by average particle size as determined by a coulter counter. Our parameter for accessible nickel surface area thus became (SNi > 25 A ) / L ,to be correlated with activity A,. The result is shown in Figure 6. We now find that all catalysts, precipitates, and impregnates, of vastly differing structure, fall into line. It is not a straight line, however. The slight "toe" near the origin is due to some slight poisoning effect. The reasons for the break in the curve near 70%-80% activity are somewhat more complex. It is associated with the selective character of fatty oil hydrogenation. Like many vegetable oils, sesame oil contains linoleic acid (L, two double bonds), in addition to oleic acid (0,one double bond) and stearic acid (S, saturated). L is hydrogenated preferentially and faster than 0. Thus, the most active catalyst will have consumed all available L, before the standard reaction time of 30 min has elapsed. For the remaining time it works a t a handicap, on 0. More Evidence on Nickel-Silica Interaction. Geus and co-workers (1967,1968) and Hermans and Geus (1979) worked out an ingenious method for precipitation of supported metal catalysts, homogeneous precipitation by means of urea hydrolysis. We used this method to make two series of catalysts, one on Degussa Aerosil 180 as support and the other with Crosfields Sorbsil. Both silicm are microspheroidal and have surface areas of about 200 m2/g.
e,
10 2 e ' 20
30
40
50
60
'
Figure 7. X-ray diffraction patterns (Cu Kcu radiation) for four states of a 25% Ni/Si02 catalyst (support material, aerosil).
Due to urea hydrolysis, the pH rises very gradually, ensuring that all nickel is precipitated on the silica, which nucleates the precipitation. In Figure 7 x-ray diffraction patterns (Cu Ka radiation) are shown for four states of one of the catalysts, 25% Ni/aerosil. The top curve is of the dried unreduced precipitate, the second after heating for 4 h in air a t 450 "C, and the third after reduction for 4 h at 450 "C in flowing hydrogen and subsequent passivation. The bottom curve id of the reduced/passivated catalyst after its reoxidation in air a t 450 "C for 1h. The diagram of the unreduced state shows no evidence of nickel hydroxide; the diagram is typical of single-layer nickel antigorite, with some unreacted silica left. Ignition at 450 "C in air induces no change, which confirms the thermostable silicate structure. Nickel hydroxide would have decomposed to nickel oxide. Reduction in hydrogen at 450 "C and subsequent passivation do induce a change. There is faint evidence for nickel metal and more for nickel oxide. Finally, the reoxidation converts the nickel metal to oxide and we now get a well-defined pattern of microcrystalline nickel oxide. From the line width of the latter diagram, we find a NiO crystallite size of 22.5 A, from which we calculate a nickel crystallite size before passivation and oxidation of 20.6 A. Assuming hemispherical nickel crystallites, we calculate from the nickel surface area a nickel crystallite size of 20.0 A, in excellent agreement with the X-ray findings. This crystallite size implies that in the reduced catalyst 35% of the nickel atoms are in the surface. Bearing in mind our earlier finding that in passivation the nickel is oxidized about two atom layers deep, we may expect about 70% of the nickel to be oxidized in the reduced/passivated catalyst. It is then not surprising that the oxide dominates in the third pattern of Figure 7. In the top half of Figure 8 an electron micrograph is given of the same catalyst in the unreduced state a t a magni-
Ind. Eng. Cham. Fundam.. VOI. 25. No. 1. 1986 47
NVAerosil catalysts
:p/L '0
1
2
3
4
5
6
7
5fln0
re4dud/nmdlred.
--
I O
0
1
2
3
L
5
6
151kP
Figure 10. XPS intensities for a aeries of Nifaerosil catalysts
4
P
II" *
b
Figure 8. (tap) Electron micrograph of unreduced 25% Ni/Si02 catalyst. (bottam) Electron micrograph of aerasil, the support used. Magnification for both 150000X.
*Y
200
Figure 9. Increase of the silica support surface area as a result of nickel deposition, as a function of nickel loading.
fication of 150000X. The thin sheets of nickel antigorite cnn be clearly seen. Of the original microspheroidal aerosil structure, shown in the lower half of Figure 8, only traces remain, which confirms the X-ray evidence. Most of the aerosil has been eaten away to form antigorite. In our discussion of the guhr-supported nickel catalysts we mentioned the enormous increase in silica surface area. Although aerosil and sorbsil already have large surface area, this is still further increased in nickel catalyst preparation. In Figure 9 the support surface area, m2/g of SiO,, is plotted against the amount of nickel per gram
of silica, Q. For both support materials a silica surface area of more than 800 m2/g of SiO, is reached at the higher nickel loading. The wet precipitated silica sorbsil is more reactive than the aerosil, made by flame hydrolysis of silane. Electron Spectroscopy on the Catalysts. The aerosil-supported catalysts were also studied by XPS, both in the pyrolyzed and in the reduced/reoxidized state, so that the nickel was always present as N P . In the top half of Figure 10 the intensity of Si 2~,,,+~,,is given, and in the bottom half the intensity of Ni 2p3,, is given, both as a function of the nickel loading Q. Use of a logarithmic Q axis is trivial; it was used to get a more even distribution of the available nickel contents in the graph. We see the silicon intensity gradually declining with nickel loading, with no significant difference for the two states of the catalysts. This is understandable: with increasing nickel loading more screening of the silicon occurs, and it makes little difference whether this OCCUR with a complete layer of oxidic nickel in the antigorite structure or with lumps of nickel oxide, which will screen more where they are on the silica but which leave silica bare and unscreened in the places from which the nickel migrated. The nickel intensities do show a difference for the two states of the catalyst. This again is understandable. In the pyrolyzed state, the nickel in the antigorite layer is only screened by an incomplete oxygen layer. After reduction/reoxidation we have lumps of nickel oxide, and the deeper layers of that oxide are effectively screened by the upper layers. For the highest nickel loading the difference is slight, but from the X-ray p a t t e m we know that in the catalyst also the pyrolyzed state has lumps of nickel oxide (see Figure 7 and the visualization of Figure 4). The Bonding Layer in Homogeneously Precipitated Catalysts. These catalysts are not easily reduced completely, and it is interesting to enquire again, where is the unreduced nickel located? Six of the catalysts prepared with aerosil were reduced in a careful manner a t 450 'C. With the assumed hemisphere model we calculated the nickel crystallite size from the nickel surface area. By a chemical method-hydrogen evolution on dissolving the sample in acid-we determined the degree of reduction. The results are shown in Figure 11. Earlier, we put forward the idea that residual antigorite might serve as a
48
Ind. Eng. Chem. Fundam., Val. 25, No. 1, 1986
1.f
DR
Nickel Antigorite layer L5OoC
0.9-
H2 - H , O
+
Dehydroxylated portially reduced I
0.6-
0 . .
NIO
Migration of Ni'. nucleation 0.61
1
I
0
1
3
2
1
1
4
dnm
a = v o l u m e 1Ni atom,b=arealNi atom
Assumptions:
1 . C r y s t a l l i t e s a r e h a l f spheres 2 . A l i N i 2 + i n "glue
layer":
3 . H monolayer a t 1 Rar
H2,
2 0 C, I6 h
Figure 12. Schematic representation of the structural changes involved in the reduction of nickel antigorite.
4 . E p i t a x y : Wi-Ni d i s t a n c e equal i n g l u e and nickel c r y s t a l l i t e C a l c u l a t e d c u r v e : DR = N i o / ( N i o D R = (&R3/a)/(&R3/a
3 d
+
+ Ni2+)
+
TR 2 / b ) = (!.R)/(!-R
3
3 = 1.28 R
=
R =
Hemisphere crystals,epitaxially attached w i t h NI++ "glue"
r e d u c t i o n pseudocomplete
0.78
3
Hydrogen bubble
a
E)
Oil bulk
E
Catalyst particle
d
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .=. . (. I. . +. . 0.39/d)" . . . . . . . . . . . . (A)
DR = (0.26d)/(0.26d + a / 2 b )
3R = (2nR3/3a
-
nR2/bl/(2nR3/3a:
= I
-
0.39/d
(B) I gas bubble
Figure 11. Relation between crystallite size D,and degree of reduction DR (and derivation of the theoretical dependence curves A and B).
"glue" a t the interface between nickel crystallite and silica support. If we assume this bonding layer to be a single antigorite layer, we can relate the degree of reduction to the crystallite size. The calculation is given in the lower half of Figure 11: Since the "glue" layer is of constant thickness, it will be a larger fraction of the total nickel, when the crystallite is smaller, and the degree of reduction will be lower. We used two models, as shown at the bottom of the figure. The agreement with the observed degrees of reduction is striking, further confirmation that the hemisphere model is a useful approximation of reality. Figure 12 suggests how reduction of nickel antigorite may take place. Nickel antigorite contains a layer of corner-sharing Si04 tetrahedrons, which have been indicated in the figure as triangles. We assume that, after elimination of the hydroxyl layer, gradual reduction of nickel will occur to the zerovalent state. As soon as the nickel ions are zerovalent, they may be considered to be physically adsorbed rather then chemisorbed. At the reduction temperature they will then be mobile and start clustering;remnants of still unreduced Ni2+rafta may serve as a nucleating agent, and they constitute the unreduced part of the nickel and the bonding layer between crystallite and support.
Rate Behavior of Oil Hydrogenation Mass Transport Effects. In the Introduction we mentioned that fatty oil hydrogenation necessarily occurs in a three-phase system. Consequently, interphase mass transport processes may well be important. We touched briefly on triglyceride transport in the pore system, which may impair catalyst activity. Equally important is the hydrogen transport.
h bubble film
oil
bulk
catalyst catalyst nickel fllm pores surface
Ind. Eng. Chem. Fundam., Vol. 25,
6
-
180°C, Mrpm, SFM
_-__-_
No. 1, 1986 49
Table I. Effect of Process Conditions on Hydrogenation of Sunflower Seed Oil" conditions* 180 "C, 180 OC, 180 O C , 150 "C, 120 "C, 750 rpm, 750 rpm, 1500 rpm, 1500 rpm, 1500 rpm, % Ni 5atm 3 atm 3 atm 3atm 3atm % Stearic Acid 0.025 0.05 0.10
10.5 8.1
0.025
34 38
6.4
8.2 6.9 5.6
9.8 8.1 7.1
12.7
16.9
10.4
15.0 12.0
9.8
% Trans Acid 2-
0.05 0.10
41
38 41 43
35 38 39
16 16
28 31 34
22
"Composition after IV reduction = 51. *Pressure values are absolute.
100
I
200
Figure 14. Dependence of hydrogenation rate on catalyst concentration. (top) Rates for three applied hydrogen pressures. Broken line denotes small amount of nickel poisoned. (bottom) Data from top figure in reciprocal plots. Intercept = bubble resistance. Slope = catalyst resistance for unit catalyst concentration. O i l bulk
5 as
C a t W Po r e film
Catalyst concertration none 'I.
..
I
Nipurface ,
....
\
Figure 15. Hydrogen concentration gradients across bubble film and catalyst film for different catalyst amounts. The converging point on the left denotes the saturation concentration Co for the applied pressure Poand temperature.
surrounding the gas bubble and catalyst particle, into the pore system, and t o the nickel surface. Diffusional transport requires a downhill concentration gradient, proportional to the rate. The effective dissolved hydrogen concentration in the oil will be lower a t higher catalyst concentration, and it can be quite considerably less than the saturation concentration. The maximum attainable concentration difference across the bubble film,belonging to the asymptotic maximum rate, equals the saturation concentration. Then the dissolved concentration approaches zero. A large gaslliquid interface reduces the concentration difference, so that more intense stirring increases the dissolved concentration. The bubble film and the catalyst film may be considered as two series-connected resistances. In an inverse plot, shown in the bottom half of Figure 14, we find a straight-line relation. As further explained elsewhere (Coenen, 1978),the dissolved concentration under any set of conditions can easily be computed from graphs like Figure 14. In Figure 16 we have shown the situation a t 180 OC for the sunflower seed hydrogenations as a function of catalyst amount and stirring rate. On the vertical axis we could have put the relative dissolved hydrogen concentration C/Co. We have used instead PIPo, in which p o
Table 11. Effect of Process Conditions on Concentration of Dissolved Hydrogen and on Selectivity Aspects effect on increase of LCH, ST Si Sv increased supply pressure + - - stirring intensity + - - increased demand
temperature catalyst amount catalyst activity IV of oil
-
+ + + +
+ + + +
+ + +
+
is the applied hydrogen pressure and p is the virtual hydrogen pressure belonging t o the prevailing dissolved concentration C. In this manner we have computed the virtual pressure, p , for a great many hydrogenations; this proved to be a very valuable parameter to rationalize the selectivity behavior. Before we can discuss these effects, we must first introduce selectivity concepts. Selectivity Concepts. Fatty oils are complex mixtures of triglycerides, which contain a broad spectrum of fatty acid types. To a certain extent the fatty acid chains behave independently in hydrogenation. For the moment we will discuss the situation in terms of the fatty acid composition. The starting oil contains saturated, monoenoic, and polyenoic fatty acids. During hydrogenation the composition becomes more complex: in polyenes there is the choice of which double bond is hydrogenated. Furthermore, double bonds can shift in the chain and also isomerize from the cis to the trans configuration. We have to schematize and divide into classes: saturated (S), monoenes (O), polyunsaturates (L), and trans-monoenes (T). Two selectivity concepts suffice for the moment: (1) Selectivity I (SI) describes the preference for hydrogenation of polyenes over monoenes. If it is very high, no saturated acid, S, is formed until almost all polyene, L, has been hydrogenated. The iodine value reduction of 51 for the sunflower seed oil hydrogenations was chosen such that a t absolute SI all L would just have been converted to 0, without formation of s. For this block of hydrogenations we can state that SIis high when formation of S is low. (2) Specific isomerization (Si)describes the tendency for formation of trans isomers, defied as % T formed per unit IV reduction. For the block of sunflower seed oil hydrogenations the N reduction is the same. Then we can state that Si is high, when formation of T is high. We apply these concepts now to the block of sunflower seed hydrogenations. Some representative results are shown in Table I. From the data we can derive some simple rules for the dependence of SI and Si on reaction conditions. These rules, which we found to hold practically universally, are
50
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986
'13-12-11
-10-9
/
1 1 1 1 1
0 1
0
'
+ HJ I-H
79
I I
'
'
'
'
0.05
'
'
'
'
' .
0,lO
'
'
'
'
0.15
'
'
'
%Ni
'
(
Figure 16. Reduction of the virtual (acting) pressure P with respect to the applied pressure Po a function of catalyst concentration and stirring rate.
'13-12-11-10
1
1
jf -11-
1
11
/+'
12-11 -10-9
1
I
+HJ /-H 0\
+
12-11
I
/
--A
_lL.-L-.
c9tll r10cl2
-1 6211-10 -9' .I.-. .L Ic 12
I I
Figure 18. Conjugation mechanism for selective hydrogenation of linoleic acid esters (L). Figure 17. Correlation of selectivity I and virtual hydrogen pressure P. The stearic acid content in the product at fixed end N is an index of the lack of selectivity I.
summarized in Table 11. We also show in this table the effect of conditions on the dissolved hydrogen concentration (which is linked to the virtual hydrogen pressure p via Henry's constant). The effect of conditions on p is for the greater part self-evident, but the effect of temperature needs some explanation: As shown elsewhere (Coenen, 1978), the catalyst resistance is governed by a greater activation energy, about 31 kJ/mol, than the bubble resistance, about 17 kJ/mol. Therefore, an increased temperature boosts the demand more than the supply, with the result that p is lower at higher T. For the moment we will ignore the third selectivity concept mentioned in Table 11,the triglyceride selectivity ST,and we will concentrate on the parallel behavior of SI and Sion the one hand and the virtual hydrogen pressure, p , on the other. In Figure 17 we have related the stearic acid (S) content (low, when SIis high) with the virtual hydrogen pressure (p). We find that the effect of stirring intensity, pressure, and catalyst amount (all represented in the data) is now quantitatively allowed for. The Mechanism of Selectivity and the Effect of p . Hydrogen, dissolved in the oil, chemisorbs on the nickel surface and reacts from the adsorbed state. Since virtual pressure p is generally low and the temperature rather high, the degree of surface coverage of adsorbed hydrogen, OH,is generally s m d . We thus tend to operate in the steep part of the hydrogen adsorption isotherm, and changes in p are reflected in OH. Why then are SIand Sihigh when OH is low? We have proved in earlier investigations (Coenen and Boerma, 1968; Heertje and Boerma, 1971) that the basis of S I lies in surface monopolization by polyunsaturated acid, L. Thus, the question moves to the following: Why is preferential adsorptioa of L more pronounced at low OH? This can be understood from the mechanism of Figure 18.
'/a
by W t
-#Location
H$O
of &ubk bond
0 \~-(cH~),-cH=cH-cHZ-CH=CH-(CHZ~-CHJ / 9 12
Figure 19. Double-bond location in the trans and in the cis fraction of the monoene acids resulting from partial hydrogenation of linoleic acid ester, shown at the bottom.
In the top left-hand corner L, e.g., linoleic acid with two double bonds, comes in. In the first instance it is adsorbed with one double bond. If OH is high, then the route down in the mechanism toward multiply bonded species is not very attractive, since it requires two additional free surface sites. Straightforward hydrogenation, the route to the right, will yield mainly unsaturated cis-monoene. Also, the difference in adsorptive strength between L and 0 is not pronounced, so that monoene 0 can adsorb and be hydrogenated as well as L. Both SI and Siwill be low. On the other hand, if OH is low, the route down in the mechanism, via multiply bonded species, is open. These are now bonded much more strongly to the surface than 0, so that they monopolize the surface and exclude 0. We then pass through a conjugated adsorbed intermediate. The produced monoene composition is then characteristic, as shown on the lowest level but one, mainly cis on the original positions 9 and 12 and trans on the intermediate positions 10 and 11in the chain. This composition is found experimentally, as shown in Figure 19, which shows the product distribution in the monoene fraction of selectively hydrogenated L. Note, however, that this prevails for
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 COMPOSlTlON
100
C
g
M,%o
80
P
0
60
T I 0 N
40
5 I
0
2o
1MI
0
4./
C
R 0 N
/t\
/--A
P
i
51
/I
x
60
40
2o 20
40 60 80
100 2 0 40
X HYDROGENATED FULL LINES: CALCULATED. 1’ LINOLEIC
0
0 100
‘A
60 80 100
X HYDROGENATED POINTS AND DOTTED LINES EXPERIMENTAL
= OLEIC
5
= STEARIC
0
0
0.2
0.4
0.6-
c,,O&ERsla
Figure 20. Change of composition during hydrogenation of glyceryl and methyl linoleate (L)on catalysts A (wide pore) trilinoleate (L3) and B (narrow pore). Note that a t low selectivity, early formation of S, trans acid formation T remains low.
Figure 21. Computer simulation of the change in fatty acid composition in the course of hydrogenation of L3. Drawn lines give composition in the liquid bulk, and broken lines give concentrations a t the bottom of the pore. We see from top to bottom selectivity depression aggravated by increasing hydrogen pressure.
moderately selective conditions. A t even lower OH, oscillation between levels 5 and 6 in the mechanism, via an allylic intermediate, may produce high levels of trans, the thermodynamically more stable form. The step to the bottom level and formation of saturate S are then very unlikely and high SI and high Si go together. This mechanism applies equally well to glycerides containing L groups and to methyl linoleate. Now, however, we should consider a well-defined distinction between the two substrates. The Effect of Molecular Size on Selectivity. In Figure 20 are given the changes in fatty acid composition in the course of hydrogenation of glyceryl trilinoleate L, and methyl linoleate L, on two catalysts Al and B. Catalyst AI has wider pores than catalyst B. From the figure we note that on either catalyst hydrogenation of L is more selective than of L,. On catalyst B, selectivity I is still reasonable on L, though poorer than on catalyst A,. In hydrogenation of L3 on B the SI is dramatically poor. We note that here again SIand Sibehave in a parallel fashion. We already saw the effect of pore structure on catalyst activity. We now find that a narrow-pore structure also affects selectivity. We recall that for large triglyceride molecules there is a dependence of effective diffusion coefficient on pore width. When diffusional transport cannot cope with the reaction rate, we get steeper concentration gradients along the length of the pore. Already in the beginning of the reaction the deepest pore bottom then “seesnonly fully hydrogenated product, and the nickel surface area does not contribute to activity. Less deep in the pore intermediate product accumulates and original raw material is not present, so that from the very beginning the nickel surface which is located there produces saturated acid: the selectivity is poor. These effects are aggravated by a high (virtual) hydrogen pressure, which boosts the reaction rate and does not help diffusional transport. Thus, concentration gradients are steeper, effectiveness is less, and selectivity is poorer. We were able to describe these phenomena quantitatively (Coenen, 1978). A t the pore mouth the flux of hydrogen and that of substrate into the pore must be equal. We found that a parameter = D H C H / D L C L , a function of the bulk concentrations and effective diffusion coefficients of the two reactants, governs the selectivity. Values of /3 of
the order of unity give excellent selectivity. /3 is increased by a lowering of D L (larger molecules, narrower pores) and by an increase in the concentration of dissolved hydrogen. We were able to simulate the concentration inside a catalyst pore. Figure 21 shows how the concentration a t the pore bottom “runs ahead” of those of the bulk of the oil and how the hydrogen pressure (strictly the virtual pressure) governs the gravity of the situation: in the lower part of the figure we see that in the pore bottom the L is depleted already at the beginning of the hydrogenation, so that stearic acid formation occurs from the very beginning. The upper part of the figure shows that the same pore width operates quite selectively at a low virtual (and applied) hydrogen pressure. Earlier we discussed a chemical/mechanistic reason for depression of selectivity I by high hydrogen concentration. We have now added an additional reason for this effect. Another Aspect of Selectivity: Melting Behavior. Hydrogenated fats, being complex mixtures, do not have a sharp melting point but a rather long melting range. We can express this by the solid fat content (SFC) as a function of temperature. If the fat contains high-melting triglycerides, then the SFC/ T curve extends to relatively high temperature. Thus, we can use SFC,,, the solids content at 35 “C, as an index of the presence of highmelting triglycerides. We now recall the effect of pore width The diffusional transport problem may result in accumulation of intermediate product in the pore, with resultant depression of SI. But this effect does not only imply early formation of saturated acid, but also of fully saturated, high-melting triglycerides: A molecule, which once has penetrated a pore, does not manage to get out until it is fully saturated. As a result, the triglyceride Selectivity, ST,is poor. The early formation of tristearates leads to increased values of SFCs5, which can be used as a (reverse) index of ST. A series of nickel/kieselguhr catalysts was prepared and characterized by nickel surface area SNi, standard degree of reduction, DR, average pore radius, rp, activity, and SFC3> The activity data have been plotted against S Nin~ Figure 22. In this case the activity was determined in a different manner, such that it has linear character, unlike the A , we used earlier. We find that the catalysts can be divided into three groups: group A, normal, linear be-
52
Ind. Eng. Chem. Fundam., Vol. 25. No. 1, 1986
selectivity & S
..
t
pore sizelnm F i g u r e 23. Selectivity in another series o f Ni/guhr catalysts as a f u n c t i o n o f pore width. Selectivity is low when SFC i s high.
Y " " ' " ' " ' ~
0
50
100
"
I
150 SNl/(m'/gNi)
F i g u r e 22. A c t i v i t y d a t a p l o t t e d against n i c k e l surface area SNi. T h e catalysts have been d i v i d e d i n t o three groups. T a b l e 111. S t r u c t u r e and P e r f o r m a n c e Data of C a t a l y s t s of F i g u r e 22, O r g a n i z e d i n t o Three G r o u p s , with Wide, M e d i u m , and Narrow P o r e s "
A
SNi,m 2 / g
SDR, %
ii,,nm
activity, %
SFC, %
60 84 86 104 108 114
90 97 84 90 84 86 89
3.9 4.9 4.2 4.5 3.6 3.5 4.1
113 115 150 165 160 180
0.7 1.1
128 130 137
78 76 81 78
2.6 3.4 3.1 3.0
165 160 160
3.0 4.7 1.7 3.1
88 90 90 105 108
47 47 47 52 52 58 61 62 53
35 40 40 60 80 65
6.8 6.6
40
8.0 7.3 6.8
average
B average
C
110 120 128 average
2.0 1.8 2.2 1.9
95 2.0
1.8 1.2 1.2 1.4 1.2
5.2 7.2 6.5
7.0
Regrouped o n t h e basis o f activity/surface area correlation.
havior; group B, activity somewhat depressed; group C, activity badly depressed. In Table I11 the data have been arranged in the three groups, within each group according to ascending SNI.In group A we find average pore widths above 3.5 nm. We also find high degrees of reduction: off-transport of water, very important in reduction, is easier from wide pores. Group A is also quite selective: SF& around 1.2%. In group C, pores are considerably narrower, with widths around 2 nm. Activities are accordingly depressed, degrees of reduction are lower, and the selectivity is very poor, SF& about 7 % . Group B is intermediate in all respects. To underline the important selectivity aspect, Figure 23 shows yet another series of catalysts. In the pore width range between 3 and 2 nm the SFCS5shoots up: triglyceride selectivity is very poor. Concluding Remarks The investigations, described in this paper, cover many years and many aspects of fatty oil hydrogenation and of silica-supported nickel catalysts for that process. Common to all types of Ni/Si02 catalysts is that silica is not an inert support material. The silica, used for
catalyst preparation, undergoes drastic structural changes as a result of a strong chemical interaction between nickel compounds and silica. Always, some basic nickel silicate, nickel antigorite, is formed, and this has an important function as a sintering inhibitor for the nickel metal and as bonding compound to anchor the nickel crystallites to the support. In principle, the activity per unit mass of nickel seems to be proportional to the nickel surface area, so that fat hydrogenation can be considered as a structure-insensitive reaction. In many cases, however, this proportionality is not apparent, because part of the nickel surface area is insufficiently accessible for the large fat molecules. To accommodate the large nickel surface on a silica particle, which at the same time must have sufficiently large particles to be filtered from the oil after use, the particle must have an extensive pore network and a large internal surface area. This porous structure must satisfy specific criteria. Nickel surface in pores of 2 nm or less is not accessible and does not contribute to activity. Active surface, in pores wider than about 3.5 nm, is fully accessible and gives fast and selective hydrogenation. In the intermediate range of pore widths the reacting molecules can still penetrate, but due to mass transport problems, selectivity is impaired. In addition to the transport of triglycerides, the transport of hydrogen, from the gas phase to the oil, as well as in the pore structure, also plays an important role, which can be quantified by means of the virtual hydrogen pressure concept. The pore width ranges, mentioned above, can shift with a change in virtual hydrogen pressure.
Literature Cited Coenen, J. W. E. Ph.D. Thesis, Technical University, Delft, 1958. Coenen, J. W. E.; Boerma, H. Fefte, Seifen, Anstrichm. 1988, 7 0 , 8 . Coenen, J. W. E.; Llnsen, B. G. I n "Physical and Chemical Aspects of Adsorbents and Catalysts"; Linsen, B. G., Ed.; Academic Press: New York, 1970; pp 472-524. Coenen. J. W. E. Chem. Ind. (London) 1978, 709-722. Coenen, J. W. E. I n "Proceedings of the 3rd International Congress on the Scientific Bases of Catalyst Preparation"; Elsevier: Amsterdam, 1979; p 89. de Lange, J. J.; Visser, G. H. Ingenieur (The Hague) 1948, 58, 24. Geus, J. W. Dutch Patent Application 6 705 259, 1967; Dutch Patent Application 6813236, 1968. Heertje, 1.; Boerma, H. J . Catal. 1971, 21. 20. Hermans, L. A. M.; Geus, J. W. I n "Proceedings of the 3rd International Congress on the Scientific Bases of Catalyst Preparation"; Elsevier: Amsterdam 1979; p 113. Linsen, 8. G. Ph.D. Thesis, Technical University, Delft, 1964. Normann, W. German Patent 139457, 1902. Normann, W. German Patent 141 029, 1902. Normann, W. BrRish Patent 1515, 1903. Roberts, M. W.; Sykes, K. W. Proc. R . SOC.London, A 1957, 242, 539. van Eyck van Voorthuysen, J. J. 6.; Franzen, P. R e d . Trav. Chim. Pays-Bas 1951, 70. 793. R e c e i v e d for r e v i e w J u n e 21, 1985 A c c e p t e d N o v e m b e r 21, 1985
