2382
Ind. Eng. Chem. Res. 2004, 43, 2382-2390
Hydrogenation of Sunflower Oil with Novel Pd Catalysts Supported on Structured Silica Me´ lanie Plourde,† Khaled Belkacemi,*,‡ and Joseph Arul† Departments of Food Science and Nutrition and of Soil Science and Agri-Food Engineering, Universite´ Laval, Sainte-Foy, Que´ bec, Canada G1K 7P4
Hydrogenations of sunflower oil over novel structured catalysts with pore sizes ranging from 3 to 20 nm, Brunauer-Emmett-Teller specific surface areas of 710-1200 m2/g, and metal concentrations on the support ranging from 0.7 to 5.0% (w/w) were investigated and compared to a commercial Ni catalyst. Catalyst supports with pore diameters between 7 and 8 nm were more active than supports with lower pore diameters. At the same metal loading and mean pore diameter, the activity was also higher with supports having smaller pore volumes and surface areas, suggesting that the pore depth and geometry of the supports may play an important role in the activity of the supported catalysts. The activity of hydrogenation depended on the Pd content, with maxima in the concentration range of 0.8-1.2% (w/w). A Pd catalyst at a Pd loading of 1% (w/w) had as much activity as a Ni catalyst but was more selective toward cis-monoenes with similar selectivity toward TFA at equal IV reductions. Hydrogenation of sunflower oil in the catalysts was described by a lumped kinetic model. Introduction Hydrogenation of vegetable oils is a major fat modification process in the fats and oils industry for the conversion of liquid oils into higher melting fats with physical characteristics suitable for various food-fat applications and higher oxidative stability. Unsaturated triacylglycerols (TAGs) are reduced by hydrogenation with the Raney Ni catalyst. However, a Ni catalyst produces a significant amount of trans-unsaturated (TFA) and saturated (SFA) fatty acids. There is also some concern regarding the toxicity of Ni salts formed from traces of Ni leaching out in the oil.1,2 The excessive consumption of TFA and SFA is considered unhealthy because of their potential adverse health effects. Apprehension and public awareness have arisen regarding the potential health hazards of TFA intake in the human diet, which has been linked to unhealthy shifts in serum lipid profiles and coronary heart disease.3 For some food-fat applications, it is necessary to hydrogenate the feedstock in a controlled manner so that maximum amounts of linoleic and oleic acids are maintained, while some other applications require hydrogenated oil containing high contents of TFA to achieve the desired solid-fat profiles. In both of these situations, the undesirable very high melting tristearin should be prevented or at least kept to a minimum because high levels of tristearin in the hydrogenated fat render it unpleasant with a “sandy and soapy” taste. A catalyst supplier is thus faced with several constraints in the formulation of a catalyst that should exhibit a high selectivity and high activity as well as desirable filtration characteristics.4 Hydrogenation of TAGs is a chemical reaction involving mass and heat transfers where maximization of the interface between hydrogen, catalyst, and oil is re* To whom correspondence should be addressed. Tel.: (418) 656-2131 ext. 6511. Fax: (418) 656-3327. E-mail: khaled.
[email protected]. † Department of Food Science and Nutrition. ‡ Department of Soil Science and Agri-Food Engineering.
quired.5 Transport limitations on hydrogen and TAGs to the active sites of the catalyst have a strong influence on both selectivity and TFA production.6,7 To facilitate the transport of TAGs through porous supported catalysts, pore dimensions and geometry should be optimized adequately. The mass transfer of H2 and TAGs to the catalyst surface can be improved not only by the optimization of the process parameters such as temperature, H2 pressure, catalyst concentration, and agitation rate but also by the architecture of the catalyst comprising suitable pore geometries with a mean pore size appropriate for effective diffusion of TAGs.8 As an unavoidable side reaction occurring during the catalytic hydrogenation, the geometric isomerization leading to the formation of TFA can be influenced by the type of the dispersed metal on the solid support7 and the surface characteristics of the support. Noble metals supported on various solids including alumina, silica, and activated carbon have been studied for their activity, selectivity, and TFA formation.9,10 These studies have shown that noble-metal catalysts are sensitive to intraparticle diffusion gradients attributable to the high activity of such metals. Although they may appear to be cost-prohibitive, their extremely high activities and the possibility of reuse may offset the cost limitation, and they could be a viable alternative to Ni.9,10 As a possible candidate, Pd was shown to exhibit an activity of 80-100 times that of Ni.11,12 Thus, the catalyst formulation is an important factor contributing to the TFA and SFA production during hydrogenation, besides process conditions.8,13 It is desirable to design catalysts with tailored surface characteristics, having higher selectivity for the formation of the monoene fatty acids with a cis configuration and lower selectivity for production of SFAs. The objective of this study was to evaluate the effect of a novel supported Pd catalyst on the activity and selectivity toward cis-monoenes in the hydrogenation of sunflower oil. We report the effect of the Pd concentration on the activity and the selectivity of the optimized
10.1021/ie030708x CCC: $27.50 © 2004 American Chemical Society Published on Web 04/07/2004
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2383
supported catalysts. We also present a comparative evaluation of the hydrogenation of sunflower oil carried out with the Pd and commercial Ni catalysts and the kinetics of hydrogenation described by a lumped kinetic model. Experimental Section Materials. Sunflower oil was purchased from Pokonobe Industries Inc. (Montreal, Canada). The commercial Ni catalyst (Pricat 9920) was graciously supplied by SYNETIX (Emmerich, Germany). The Pd catalysts were prepared by impregnating different mesoporous silica supports with an aqueous solution of palladium nitrate, followed by drying at room temperature for 24 h, calcination in air at 540 °C for 5 h, and reduction under a hydrogen flow at 400 °C for 3 h. Mesoporous silica supports in a solid white powder form were prepared by polymerization of silica precursors on cylindrical micelles of quaternary ammonium surfactants via cooperative assembly, yielding a composite structured material, hexagonal ordered MCM-41,14 and expansion of MCM-41 pores with amine expanders,15 and through the self-assembly of triblock copolymer PEO-PPO-PEO (Pluronic 123) under acidic conditions, yielding a hexagonal ordered material, SBA-15.16 Following the synthesis, the occluded surfactant was removed by calcination to clear the pores of MCM-41, expanded MCM-41, and SBA-15 silicas. Surface Characteristics. The Brunauer-EmmettTeller (BET) surface area was determined from the N2 adsorption-desorption isotherms of the sample catalysts at 77 K using a volumetric adsorption analyzer (model ASAP 2010; Micromeritics, Norcross, GA). Before the adsorption analysis, the samples were degassed for 2 h at 473 K in the degassing chamber of the adsorption apparatus. The pore volume and its distribution were calculated using the Barrett-Joyner-Halenda theory from the adsorption isotherm.17 Hydrogenation Reaction. Reactions were carried out in a 600-mL Parr pressure reactor model 4560 (Parr Instrument Co., Moline, IL), equipped with both a pressure transducer for monitoring the pressure of the reactor and thermocouples for reaction temperature measurements. In a typical experiment, 200 g of sunflower oil and an amount of reduced catalyst at a concentration of 0.005 g of metal/100 g of oil were charged into the reactor. A vacuum exhaust line was used for venting of the reactor. Before introduction of hydrogen (H2) in the reaction system, dissolved and residual air present in the reactor was stripped out by applying a vacuum and reestablishing atmospheric pressure with nitrogen (N2). For safety considerations, this procedure was carried out twice, allowing the reactor to operate in the absence of oxygen (O2). The temperature of the system was increased gradually and maintained at 110 °C with an external controller at an agitation rate of 1500 rpm. This stirring rate was found adequate in minimizing gasliquid as well as liquid-solid mass-transfer constraints. When the temperature reached 100 °C, H2 was introduced into the reactor, and the pressure was maintained at 5 atm for the duration of the reaction. The time “zero” of reaction corresponded to the moment when the temperature of the reactor reached 110 °C, and the reaction was terminated after 60 min. Oil samples were drawn at regular intervals for fatty acid analysis. All of the hydrogenation reaction tests were duplicated to
estimate the reproducibility of the results obtained from the hydrogenation process. For this purpose, a new batch of catalysts was used for each duplicated test. Fatty Acid Composition and Iodine Value (IV). The fatty acid composition of the modified oils was determined by gas chromatography. The reaction products were converted into their methyl esters (FAME) and analyzed on a 5890 series II gas chromatograph (Hewlett-Packard, Palo Alto, CA) equipped with a flame ionization detector and a split-splitless capillary inlet. The injector and detector temperatures were 250 and 275 °C, respectively. Helium and hydrogen (ultrapure carrier, 99.999%, Praxair Canada Inc., Mississauga, Ontario, Canada) were used as carrier gases. Gasliquid chromatography separation was performed on a BPX70 capillary column (60 m × 0.25 mm i.d. × 0.25 µm film thickness; SGE, Melbourne, Australia) according to the method of Thompson.18 The oven temperature was kept constant at 230 °C for 5 min under a pressure of 160 kPa. The linear velocity through the column was typically 20 cm/s (helium) or 35 cm/s (hydrogen) (adjusted while at operating temperature). Sample injections (1-4 µL) were made with a 10-µL syringe (Hamilton 7101) using a split ratio of approximately 100:1. FAME standards were purchased from Sigma Chemical (St. Louis, MO), Nu Chek Prep (Elysian, MN), and Matreya (Pleasant Gap, PA). A positional cis-trans isomer mixture consisting of a margarine extract with methyl eicosanoate and methyl docosanoate was purchased from Supelco (Bellefonte, PA) and Matreya Inc. (Pleasant Gap, PA). Other details on fatty acid analysis were reported elsewhere.18 The IV was calculated from the fatty acid composition using the AOCS method.19 Solid-Fat Profile. The solid-fat content (SFC) was determined from melting curves by the Mortensen method.20 Melting curves were performed on a Dupont model 990 thermal analyzer (Dupont Instruments, Toronto, Ontario, Canada) by the Timms method.21 Results and Discussion Catalyst Performance. A comparative study of the sunflower oil hydrogenation was done using seven novel supported Pd catalysts and a conventional Ni catalyst under the same reaction conditions of temperature (110 °C), hydrogen pressure (70 psi), and agitation (1500 rpm). For the purpose of data treatment, average values of fatty acid concentrations obtained for two replicates were used. The accuracy of the determination of the fatty acid concentrations for the replicates represented at most 10% of the average values. Hydrogenation activity was monitored by the decay of IV, which indicates the level of saturation of double bonds. It is well-known that the saturation of double bounds follows a first-order simple kinetics with respect to the IV decay:
1 d(IV)t ) -kr(IV)t m dt
(1)
The integration of eq 8 using the initial condition that (IV)t ) (IV)0 at t ) 0 gives
(IV)t ) (IV)0 exp(-mkrt)
(2)
Therefore, the hydrogenation specific activity was estimated by the determination of the overall reaction constant kr at specific reaction conditions.
2384 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 1. Surface Characteristics and Hydrogenation Activity of Novel Pd Catalysts Supported on Structured Silica and Commercial Ni Catalysts
catalyst Pd 1 2 3 4 5 6 7 Ni
support type MCM-41 pore- expanded MCM-41 SBA-15 SBA-15 SBA-15 SBA-15 SBA-15
metal loading (wt %)
total pore volume (cm3/g)
mean diameter (nm)
BET surface area (m2/g)
mkr (min-1)
kr [(kg of oil)‚ (g of metal)-1‚min-1]
Pd (5.0) Pd (5.0)
1.142 1.928
3.9 7.8
1184 986
0.0007 0.0018
0.014 0.036
Pd (0.8) Pd (1.0) Pd (1.2) Pd (1.5) Pd (5.0) Ni (21.0)
1.209 1.209 1.209 1.209 1.209 0.476
7.0 7.0 7.0 7.0 7.0 19.2
750 750 750 750 750 99
0.0100 0.0088 0.0043 0.0030 0.0037 0.0140
0.200 0.176 0.086 0.060 0.074 0.067
Table 1 summarizes the surface characteristics of the catalysts as well as their activity. The Pd catalysts used structured silica supports with different pore geometrical shapes, sizes, and other surface characteristics. The catalyst supports with pore diameters between 7 and 8 nm (catalysts 2-7) were more active than supports with lower pore diameters (catalyst 1). Catalyst 1 with a pore diameter of 3.9 nm exhibited a very low activity. This pore diameter, being approximately only twice the dimension of TAGs (1.5-2.0 nm), constrained the reaction to be controlled by the diffusion of reactants into the catalyst pores (intraparticular diffusion control regime). Limitation of the reaction rates within a porous supported catalyst is predominantly attributable to the size-exclusion effect of the diameter of the pores of the catalyst structure.22 Furthermore, it is also conceivable that a truncated or cone geometry of the pores for a given pore dimension, as opposed to a cylindrical geometry, will not allow considerable penetration of TAGs within the pore to sustain hydrogenation. Catalysts 2 and 7 exhibited a significant difference in their activity, even though their pore diameters and their Pd loading are comparable; presumably the supports of these two catalysts are different in their internal pore structure and geometrical shape. This observation shows that the pore size is not likely the only geometrical parameter having an effect on the activity of the supported catalysts. The support used in the preparation of catalyst 7 exhibited a higher activity compared to those of catalysts 1 and 2. Optimization of Pd loading on the same support as that used in the preparation of catalyst 7 was carried out to formulate catalysts 3-6 at Pd loadings of 0.8, 1.0, 1.2, and 1.5 wt %, respectively. The activity increased by more than 14-fold with a supported catalyst (3) with a pore diameter of 7-8 nm compared to that with a pore diameter of 3.9 nm. Coenen8 summarized the data on diffusion limitations and reported a 50% decrease in the activity for narrow-pore catalysts (mean pore diameter d e 4 nm) relative to medium-pore (4 e d e 6 nm) and wide-pore catalysts (d g 8-10 nm), which is in agreement with our observation. Catalyst 3 with a Pd loading of 0.8 wt % demonstrated an activity, mkr, of 0.010 min-1. The Ni catalyst showed an activity of 0.014 min-1. However, when the catalysts are compared on the basis of their specific activities, kr, The Pd catalyst with a metal loading of 0.8% was the most active in hydrogenating sunflower oil, exhibiting a specific activity of 0.200 (kg of oil)‚(g of metal)-1‚min-1 at a metal concentration of 0.005 kg of Pd/100 kg of oil, compared to Ni with a metal loading of 21.0%, exhibiting a specific activity of 0.067 (kg of oil)‚(g of metal)-1‚min-1
Figure 1. Effect of the catalyst Pd loading on the activity of the new supported Pd catalysts.
when used at a concentration of 0.021 kg of Ni/100 kg of oil. Effect of Pd Catalyst Loading on the Hydrogenation Activity. Figure 1 shows the variation of the activity kr of Pd catalysts as a function of Pd loading. The activity was at a maximum at about 0.8-1 wt % Pd loading. However, a further decrease or increase in Pd loading led to a drastic decrease of the activity. At a Pd loading greater than 1 wt %, the formation of large metal crystallites built up and lowered the metal availability at the reaction surface area. This diminution of the catalyst dispersion presumably affected the activity. A decrease of the metal loading on the catalyst surface below 0.8 wt % increased metal dispersion because high metal dispersions can be obtained at low metal loadings.23 However, this was accompanied by a dramatic decrease of the activity. This phenomenon, called “antipathetic behavior”, was also observed in other supported catalytic systems.24-26 This indicates that the hydrogenation of unsaturated fatty acids is, at a high degree of catalyst dispersion, sensitive to the structure of the catalyst support and that the shape, geometry, and size of the metal crystallites deposited on the porous support, physical properties such as electronic densities of the support milieu, and the metal crystallites exert a substantial influence on the activity of the catalyst. Parts A-C of Figure 2 show the time course of fatty acid composition curves for the batch hydrogenation of sunflower oil with the two most active Pd catalysts, 3 and 4, and Ni catalyst, respectively. Pd loading in the oil was 0.005%, while Ni loading was 0.021%. Linoleic acid depleted within 60 min of reaction with Pd catalysts (Figure 2A,B). The level of oleic acid initially increased first, then leveled off, and subsequently decreased, while the stearic acid content remained low at 5-7% up to
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2385
Figure 3. Fatty acid composition as a function of the degree of IV reduction during hydrogenation of sunflower oil: (A) Pd catalyst 4; (B) commercial Ni catalyst. The dotted lines represent only a general tendency. Reaction conditions: temperature ) 110 °C; H2 pressure ) 5 atm; agitation ) 1500 rpm.
Figure 2. Time course profiles of reactants and products during hydrogenation of sunflower oil: (A) Pd catalyst 3; (B) Pd catalyst 4; (C) commercial Ni catalyst. The dotted lines represent only a general tendency. Reaction conditions: temperature ) 110 °C; H2 pressure ) 5 atm; agitation ) 1500 rpm; reaction time ) 60 min. The solid vertical line (C) represents the reaction time required with the Ni catalyst to achieve an IV equivalent to that of Pd after 60 min of reaction (B).
50 min of reaction but steadily increased to a final value of ca. 20% and 15% for catalysts 3 and 4, respectively. The monoenoic trans fatty acid (TFA) content increased quasi-linearly from 0.1% to 39% for catalyst 3 and to 34% for catalyst 4 after 60 min. Dienoic TFA profiles exhibited a different behavior characterized by an increase of up to 9.6%, followed by a decrease as a result of their conversion to stearic acid, and possibly also to trans-monoenoic acids. trans-Monoenoic acids were largely responsible for the increase of the total TFA level throughout the course of the reaction and represented ∼84% of TFA after 1 h of reaction for both Pd catalysts 3 and 4. Despite its slightly higher activity than Pd catalyst 4, catalyst 3 (Pd loading of 0.8 wt %) exhibited a high production of stearic acid as well as trans C18:1 than catalyst 4, which is not desirable. From the standpoint of health and technical functional properties, catalyst 4 is a better compromise between the activity and capacity to produce TFA and SFA.
Nearly the same behavior was observed with Ni catalyst (Figure 2C). The cis C18:2 was converted by hydrogenation and isomerization, and its level decreased until total depletion after 1 h. The levels of trans C18:2 and oleic acids increased, reached a maximum value, and then decreased. Between the two catalysts, the Ni catalyst produced 10% more of oleic acid than the Pd catalyst at 1% Pd loading (catalyst 4) at comparable levels of IV (76) and produced about 9% less of trans C18:1 and 4-5% more of C18:0 than the same catalyst. Furthermore, the content of stearic acid increased monotonically until 40 min but thereafter started to build up significantly, reaching 30% of total fatty acids after 60 min with the Ni catalyst. Although differences in the fatty acid composition between the hydrogenated sunflower oil with the two catalysts were more or less similar, at the same level of saturation or IV, a smaller amount of stearic acid and a higher level of trans C18:2 were produced with the Pd catalyst compared with the Ni catalyst. Pd catalyst 4 exhibited an activity comparable to that of the Ni catalyst but with lower production of SFA. Parts A and B of Figure 3 show the fatty acid composition curves for hydrogenation with Pd catalyst 4 and the Ni catalyst as a function of the degree of reduction in IV (degree of hydrogenation), ∆IVreduction, expressed as
∆IVreduction )
(IV)0 - (IV)t (IV)0
× 100
(3)
At ∆IVreduction of less than 30%, both Pd catalyst 4 and
2386 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
the Ni catalyst exhibited a nearly similar behavior in the distribution of fatty acids. With further reduction of IV of over 30%, the Pd catalyst produced less stearic acid but a slightly higher level of trans C18:1. The lag in stearic acid production was slightly longer with the Pd catalyst than with the Ni catalyst, suggesting that a small difference exists between the two catalysts and that cis C18:2 is more selectively hydrogenated than hydrogenation of cis C18:1 with the Pd catalyst than with the Ni catalyst. The slightly higher selectivity exhibited by the Pd catalyst is also accompanied by a slightly higher isomerization of cis C18:1 to trans C18:1 than that of the Ni catalyst under the reaction conditions. Lower production of SFAs with Pd catalysts is attributable to the difference in the affinity of dienoic and monoenoic acids and their stronger adsorption on the dispersed Pd, thus favoring the formation of monoenoic acids rather than stearic acid.27,28 It appears that the pore geometry of the support for Pd catalysts may also facilitate a better penetration of polyunsaturated TAGs, hydrogenation, and subsequent desorption from the active sites of the catalyst. McLeod and Gladden29 investigated the adsorption-desorption phenomena as well as reaction with catalyst surfaces composed of regular and randomly shaped pores and found that the selectivity and activity of supported metal catalysts depended on the catalyst surface morphology. Ultimately, the factor determining the catalyst activity is the residence time of TAGs in the catalyst pores. The longer the residence time at the active site, the greater is the probability of occurrence for all reactions, i.e., hydrogenation, isomerization, or conjugation.30 All supported Pd catalysts shown in Table 1 exhibited greater pore volumes and smaller mean pore diameters than Ni catalysts, suggesting the presence of deeper pores in supported Pd catalysts, which may allow a longer residence time of unsaturated TAGs, thus promoting their isomerization and hydrogenation. Lumped Kinetic Model. Hydrogenation of vegetable oil is a complex network of chemical reactions involving several reactant species. The various reactants compete with each other for catalyst active sites. For a kinetic description of such systems, all reaction steps should be considered. However, from a practical standpoint, it is neither possible nor useful to monitor all species involved. For example, in sunflower oil, the level of C18:3 fatty acid is very low. Consequently, the overall hydrogenation reaction involves consecutive saturation of cis C18:2 to C18:1 in a cis configuration and subsequent saturation of cis C18:1 to C18:0 as well as parallel reversible isomerizations of cis C18:2 to trans C18:2 and cis C18:1 to trans C18:1. The reaction pathway may also involve the partial saturation of trans C18:2 to C18:1 in cis and trans configurations as well as a simple step hydrogenation of trans C18:1 to C18:0. A direct transformation of cis C18:2 to trans C18:1 is very unlikely because it involves simultaneous isomerization and hydrogenation. Preliminary calculations using the experimental data showed that the isomerization reactions of cis C18:2 to trans C18:2 and cis C18:1 to trans C18:1 are essentially irreversible reactions (k-2t ≈ 0 and k-1t ≈ 0) and simplify the reaction network as depicted in Figure 4. Hence, all of the positional and geometrical trans C18:2 isomer products can be lumped together into one imaginary substrate whose concentration can be easily determined, as C18:2trans. Similarly, all trans C18:1 positional isomers can be lumped as C18:1trans.
Figure 4. Lumped reaction path for the hydrogenation of sunflower oil. The dashed lines are for negligible reactions.
This simplification can be used a priori to estimate the selectivity of hydrogenation reactions. Lumping of this type is used extensively in the kinetic analysis of complex chemical reaction systems.31 To illustrate the procedure describing the overall hydrogenation pseudokinetics, all reactions were considered to follow a firstorder kinetics. Moreover, in the experimental hydrogenation conditions used in this work, the reactant H2 was in excess of the stoichiometric quantity required. Its concentration [H2] is considered to be constant over the duration of the hydrogenation process. Therefore, this concentration is imbedded in the rate constant kx as
kx ) kx[H2]
(4)
where k/x is the real and intrinsic rate constant for a given reaction step x involved in the reaction path shown in Figure 4. In the following equations, C18:2cis, C18:1cis, and C18:0 represent cis C18:2, cis C18:1, and C18:0 fatty acids, respectively. With these simplifications and assumptions, the rates of reactions depicted in Figure 4 can be expressed as
1 d[C18:2cis] ) -k2[C18:2cis] - k2t[C18:2cis] (5) m dt 1 d[C18:1cis] ) k2[C18:2cis] - k1[C18:1cis] + m dt k3[C18:2trans] - k1t[C18:1cis] (6) 1 d[C18:0] ) k1[C18:1cis] + k4[C18:1trans] m dt
(7)
1 d[C18:2trans] ) k2t[C18:2cis] - k3[C18:2trans] m dt k5[C18:2trans] (8) 1 d[C18:1trans] ) k1t[C18:1cis] - k4[C18:1trans] + m dt k5[C18:2trans] (9) where m represents the metal catalyst (Pd or Ni) concentration in the oil, with
[C18:2cis] ) [C18:2cis]0; [C18:1cis] ) [C18:1cis]0; [C18:0] ) [C18:0]0; [C18:2trans] ) [C18:2trans]0; at t ) 0 The vector of unknown parameters {k2, k1, k2t, k1t, k3, k4, k5} can be identified from eqs 5-9 by the minimization of the least-squares problem
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2387 N
χ2 )
([C18:2cis] - [C18:2cis])j2 + ([C18:1cis] ∑ j)1
[C18:1cis])j2 + ([C18:0] - [C18:0])j2 + ([C18:1trans] [C18:1trans])j2 + ([C18:2trans] - [C18:2trans])j2 (10) This identification requires a combination of a RungeKutta integration algorithm RKS4 and a quasi-Newton constrained optimization method based on a mixed quadratic-cubic line search procedure.32 In eq 10, symbols with bars are experimental quantities. Validation of the Model and Catalyst Selectivity. The validation of the kinetic model was performed by simultaneously solving and nonlinearly fitting differential eqs 5-9 with experimental data. Convergence was attained when the predicted variable lumps of fatty acids matched those of the experimental data through eq 10. Typical results of predicted profiles of fatty acids (solid lines) along with those experimentally obtained with Pd catalyst 4 (1.0 wt % Pd loading) and the Ni catalyst are shown in parts A and B of Figure 5, respectively. As seen, good agreement was found between the experimental and the predicted fatty acid lumps, as described by the lumped kinetic model. Table 2 summarizes the calculated rate constants when optimizing the least-squares criterion χ2 of eq 10 for different Pd and Ni catalysts. Reaction Patterns of Pd versus Ni. It is interesting to observe that mechanistically both types of catalysts operate under the similar overall reaction pathway of Figure 4. The hydrogenation of trans C18:2 to trans C18:1 (k5) was found to be negligible for both Pd (catalysts 3 and 4) and Ni catalysts. The reaction step of trans C18:1 saturation to C18:0 (k4) was low for all three catalysts but was insignificant for the Ni catalyst. The Ni catalyst favored the reduction of cis C18:2 to cis C18:1(k2), but it was moderate for Pd catalysts 3 and 4. The saturation of cis C18:1 to C18:0 (k1) was more pronounced with the Ni catalyst but was significantly lower for Pd catalyst 4, and it was intermediate between Pd catalyst 4 and the Ni catalyst for catalyst 3, while the isomerization of cis C18:1 to trans C18:1 (k1t) was favored by both Pd and Ni catalysts and the isomerization of cis C18:2 to trans C18:2 was highly significant for Pd catalysts 3 and 4 but not for the Ni catalyst. In addition, trans bond hydrogenation of trans C18:2 to cis C18:1 (k3) was highly significant with Pd catalysts 3 and 4 but not the Ni catalyst, and the latter favored the formation of cis C18:1 directly from the hydrogenation of cis C18:2. Even though both catalyst types operate kinetically and mechanistically under the proposed reaction pathway, the magnitudes for each reaction step involved were different. Selectivity toward Desired Products. In vegetable oil hydrogenation, the notion of selectivity is important. Usually, one aims at hardening linolenic (C18:3) and linoleic acids (C18:2), maintaining oleic acid (C18:1), and minimizing the formation of stearic acid (C18:0). Besides saturation of double bonds, migration and cis-trans isomerization of double bonds also occur during hydrogenation, leading to TFA production, which should also be minimized. To adequately monitor hydrogenation toward the desired products, it is useful to introduce some selectivities. A well-known and accepted method to determine the selectivity is from the ratio of reaction rate constants22 governing the reaction pathway as
Figure 5. Experimental and predicted fatty acid lumps of sunflower oil hydrogenated with (A) catalyst 4 (1.0 wt % Pd) and (B) Ni catalyst. Reaction conditions: temperature ) 110 °C; H2 pressure ) 5 atm; agitation ) 1500 rpm. Table 2. Rate Constants Obtained by Minimization of the Least-Squares Criterion χ2 catalyst
k2
k1
k1t
k2t × 104
k3
k4
k5
χ2
3 4 Ni
156 202 491
44 10 77
183 193 148
149 140 23
612 444 173
20 22 1
0.01 0.01 0.01
20.09 18.67 24.01
described in Figure 4. The concept of “traditional” selectivity, as introduced by Coenen8 (1986), describing the preference for hydrogenation of polyenes rather than monoenes is estimated by the ratio of rate constants of the following consecutive reactions steps: k2
k1
C18:2cis 98 C18:1cis 98 C18:0 SI ) k2/k1
(11)
High values of SI mean that the rate at which cis 18:1 produced from more unsaturated fatty acids is greater than the rate at which cis 18:1 itself is hydrogenated to stearic acid. For industrial hardening, this selectivity is the most important parameter and hydrogenation is considered to be selective when SI is higher than 10.33 However, this selectivity does not take into account the isomerization reaction steps. In addition, the formation of cis C18:1 is not the result of hydrogenation of cis C18:2 only, but trans C18:2 hydrogenation is also a possible route. On the other hand, stearic acid can be produced by hydrogenation of cis C18:1 as well as trans C18:1. Taking into account the supplementary steps
2388 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 3. Selectivities SI, SoI , Si, and St for Pd Catalysts and the Ni Catalyst catalyst SI ) k2/k1 Pd 3 4 Ni
3.54 20.20 6.38
SoI ) (k2 + k3)/ Si ) (k1t + k2t)/ (k1 + k4) mkr St ) k2/k1t 12.00 20.19 8.51
3.40 3.80 1.24
0.85 1.04 3.33
leading to the production of cis C18:1 via trans C18:2 hydrogenation (k3) and saturation of trans C18:1 to C18:0 (k4), a more rigorous derivation of the overall selectivity, SoI , can be made and its expression is as follows:
SoI )
k2 + k3 k1 + k 4
(12)
Furthermore, because cis-trans isomerization occurs during hydrogenation, a high selectivity toward retaining the cis configuration of fatty acids is desired from the standpoint of nutrition. The proportion of double bonds isomerized to the trans configuration relative to those saturated with hydrogen provides a specific isomerization index, expressed as
Si )
k1t + k2t mkr
(13)
We may also define St as an index for the production of trans-monoenes analogous to SI:
St ) k2/k1t
(14)
Table 3 summarizes various selectivities for Pd catalysts 3 and 4 and the Ni catalyst. Pd catalyst 4 exhibited the highest SoI selectivity among the three catalysts, followed by Pd catalyst 3 and the Ni catalyst; i.e., Pd catalyst 4 was the most selective toward cis C18:1 formation. The isomerization index, Si, of Pd catalysts was 3.40 and 3.80 for Pd catalysts 3 and 4, respectively, compared to that of 1.24 for the Ni catalyst. The isomerization index is the ratio of the total isomerization activity to the saturation activity of the catalyst. The higher Si values of Pd catalysts indicate their higher isomerization activity for a given saturation activity, as seen by somewhat higher TFA production by these catalysts compared to the Ni catalyst. A similar conclusion is evident when the catalysts are compared on the basis of the St index. Pd catalysts, in particular, Pd catalyst 3, slightly favor the formation of trans-monoenes over the formation of cis-monoenes compared to the Ni catalyst. Melting Characteristics of Hydrogenated Oils. Figure 6 illustrates the profile of the SFC as a function of the temperature of hydrogenated fats with Pd catalyst 4 (1% Pd loading, 60 min of reaction) and the Ni catalyst at the same IV (76) as well as the Ni catalyst after 60 min of reaction (IV ) 55) and the original sunflower oil. Ni hydrogenated oil exhibited melting characteristics of a fat harder than the hydrogenated oil with the Pd catalyst for equal reaction times. At equal IVs of 76, both catalysts produced selective hydrogenation as seen from the SFC profile (Figure 6), where both SFC curves exhibited double slope softening with an increase in temperature. However, Ni hydrogenated fat exhibited a higher SFC and a higher end melting temperature
Figure 6. Melting characteristics of the original and hydrogenated sunflower oils obtained at IV equal to 76 or after 60 min of reaction. Symbols: (0) Ni catalyst after 60 min of reaction (IV ) 55); (9) Ni catalyst after 42 min of reaction (IV ) 76); (O) Pd catalyst 3 after 60 min of reaction (IV ) 76); (4) original sunflower oil.
(45 °C) than Pd hydrogenated fat with an end melting point of 40 °C. The difference in the SFC profiles of the two hydrogenated oils is attributable to the differences in stearic acid and trans-monoene contents. The melting point of TAGs depends on their fatty acid composition and also on the distribution of fatty acids over the three positions. The melting point of fatty acids depends on the chain length, number, geometry, and position of double bonds. For example, oleic acid (cis C18:1) melts at 16.3 °C, its trans isomer, eleidic acid (trans C18:1), melts at 44 °C, and stearic acid (C18:0) melts at 72 °C.34 Because Ni hydrogenated oil contains a greater proportion of the very high melting stearic acid than the moderately high melting trans monomers, its solidfat profile is somewhat harder than Pd hydrogenated oil. The latter contains a slightly higher proportion of trans-monoenes but a lower proportion of stearic acid. According to the FDA definition of hydrogenated vegetable oils as reported by Brown,35 such fats are solid at room temperature and contain 15-25% TFA. Partially hydrogenated oils are liquid at room temperature and are lower in TFA. Depending on the process conditions used, the hydrogenated oils with Pd and Ni catalysts easily belong to the FDA definition of partial hydrogenation. Conclusion The novel Pd catalyst 4, supported on a mesostructured support, is active and selective for hydrogenation of sunflower oil under mild process conditions. For the same IV reduction, this catalyst produced about the same level of TFA but less stearic acid and was more selective toward cis-monoene formation. Pd catalysts at metal loadings between 0.8 and 1 wt % were as active as the commercial Ni catalyst. However, the Pd catalyst with a metal loading of 0.8% was the most active in hydrogenating sunflower oil, on the basis of specific activity. Surface characteristics of the support and the metal concentration on the support affected the activity and selectivity of the Pd catalysts significantly. Catalysts with pore diameters of 7-8 nm were most active for sunflower oil hydrogenation.
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The lumped kinetic model of the hydrogenation of sunflower oil over Pd as well as Ni catalysts was found useful in the prediction of the fate of reaction and product lumps. Moreover, kinetically and mechanistically, it seems that both catalyst types operate under the proposed reaction pathway but at different magnitudes for each reaction step involved. The results show that Pd catalysts require noticeably less metal loading than the Ni catalyst to achieve the same activity. This would constitute a substantial economy in the catalyst cost issue even if Pd is more expensive than Ni and may open a route for a possible switch to the utilization of very low Pd loading on a suitably structured low-cost silica support as an effective catalyst for vegetable oil hydrogenation. Further work is needed to evaluate the progress of the hydrogenation reaction with the Pd catalyst under various process parameters, covering the effects of the selectivity and formation of trans isomers. It would also be of interest to determine the identity of trans-monoene isomers because recent nutritional investigations have revealed that among trans-monoenes, trans-11,C18:1 (trans-vaccenic acid) was not associated with coronary artery disease, unlike trans-9,C18:1 (elaidic acid) or trans-10,C18:1.36 Acknowledgment Natural Sciences and Engineering Research Council of Canada provided funds for this research. The authors thank F. Destaillats, A. Boulmerka, and C. N’Go Duy for analytical help and S. Hamoudi for invaluable advice in the catalyst formulation. Nomenclature [C18:2cis] ) concentration of cis-linoleic acid (% total fatty acids) [C18:2trans] ) concentration of trans-linoleic acid (% total fatty acids) [C18:1cis] ) concentration of cis-oleic acid (% total fatty acids) [C18:1trans] ) concentration of trans-linoleic acid (% total fatty acids) [C18:0] ) concentration of stearic acid (% total fatty acids) [H2] ) hydrogen concentration [(mol of H2)‚L-1] IV ) iodine value kx ) rate constant of the reaction step x [(kg of oil)‚(g of metal)-1‚min-1] / kx ) intrinsic rate constant of the reaction step x [(kg of oil)‚(g of metal)-1‚min-1‚L‚(mol of H2)-1] kr ) overall reaction constant [(kg of oil)‚(g of metal)-1‚min-1] m ) concentration of the metal catalyst (Pd or Ni) in the oil [(g of metal)‚(kg of oil)-1] N ) number of experimental runs (eq 10) S1 ) selectivity toward cis C18:1 formation (S1 ) k2/k1) SoI ) proposed selectivity toward cis C18:1 formation (eq 12) Si ) specific isomerization index (eq 13) St ) index for the production of trans-monoenes (eq 14) t ) reaction time (min) Greek Symbols χ2 ) minimization criterion of the least-squares problem (eq 10) ∆IVreduction ) degree of IV reduction (eq 3) Subscripts and Superscripts 0 ) initial time (time “zero”)
x ) subscript of the rate constants indicating the reaction step no. 1,2,3,4,5,1t, 2t -1t, -2t ) subscripts of the rate constants, specifying the reaction steps 1,2,3,4,5,1t, 2t, -1t, and -2t, respectively
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Received for review September 10, 2003 Revised manuscript received February 18, 2004 Accepted March 1, 2004 IE030708X
