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Hydrogenation of Lactose over Sponge Nickel CatalystssKinetics and Modeling Jyrki Kuusisto,*,† Jyri-Pekka Mikkola,† Mona Sparv,† Johan Wa1 rna˚ ,† Heikki Heikkila1 ,‡ Riitta Pera1 la1 ,§ Juhani Va1 yrynen,§ and Tapio Salmi† Laboratory of Industrial Chemistry, Process Chemistry Centre, Department of Chemical Engineering, Faculty of Technology at Åbo Akademi UniVersity, Biskopsgatan 8, FI-20500 Turku, Finland, Department of Applied Physics, UniVersity of Turku, Vesilinnantie 5, FI-20014 Turku, Finland, and Danisco Sweeteners, Sokeritehtaantie 20, FI-02460 KantVik, Finland
Kinetics of lactose hydrogenation to lactitol, an alternative sweetener, over Mo-promoted sponge nickel slurry catalyst in aqueous solutions was studied. Hydrogenation experiments were carried out batchwise in a threephase laboratory-scale reactor (300 mL, Parr Co.), operating at 20-70 bar and between 110 and 130 °C. The main hydrogenation product was lactitol, while small amounts of lactobionic acid, lactulose, lactulitol, sorbitol, and galactitol were detected as byproducts. The lactitol selectivity within the experimental range varied from 90 to 99%. The selectivity values improved as the hydrogen pressure increased and the reaction temperature decreased. The effect of catalyst loading and catalyst deactivation during consecutive hydrogenation batches was also studied. Fresh and recycled catalysts were characterized by nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), hydrogen temperature-programmed desorption (TPD), and particle-size analysis. Recycled sponge nickel catalyst was able to adsorb less hydrogen compared to fresh one, indicating active site poisoning. Of all byproducts, only lactobionic acid had an inhibiting effect on lactose conversion rate and deactivated the catalyst. By lactobionic acid, deactivated catalyst can be regenerated by controlled alkali wash. The kinetic data were modeled by Langmuir-Hinshelwood-HougenWatson (LHHW) kinetics, assuming surface reaction steps are rate determining. Noncompetitive adsorption of molecular hydrogen and lactose was assumed. The fitting of the experimental data to the kinetic model was carried out by a combined Simplex-Levenberg-Marquardt method. The model predicted the experimental concentrations of lactose and lactitol very well. A reasonably good description of the byproducts was obtained. 1. Introduction Lactose, a milk sugar, is a reducing disaccharide consisting of glucose and galactose moieties. In aqueous solution at 20 °C, lactose exists in two anomeric forms: 62.7% as β-pyranose and 37.3% as R-pyranose.1 The lactose contents of milks from different mammals vary between 0 and 9%, e.g., cow’s milk contains ∼4.9% and human milk ∼6.7% of lactose.2 The estimated annual worldwide availability of lactose as a byproduct from cheese manufacture is several million tons.2,3 However, only ∼400 000 t/a lactose is processed further from cheese whey.4 Nonprocessed whey causes an environmental problem because of its high biochemical and chemical oxygen demand.3 The relatively low solubility of lactose limits its use in many applications. Another restricting factor is the inability of lactoseintolerant people, who have such a low level of lactase enzyme in the body, to digest milk sugar.2 Thus, it has been necessary to develop novel lactose derivatives. Lactitol, lactulose, and lactobionic acid are the industrially most important lactose derivatives.5-8 Moreover, the hydrolysis products of lactose, D-galactose and D-glucose, can be oxidized for valuable raw materials (L-ascorbic acid, D-ascorbic acid, and erythorbic acid), which are used by the pharmaceutical industry.9 Furthermore, tagatose is manufactured by hydrolyzing lactose to galactose and glucose and isomerizing galactose to tagatose.32 Lactitol is a sugar alcohol, derived by reduction of the glucose part of the disaccharide, lactose. The lactose hydrogenation scheme is displayed in Figure 1. Lactitol is suitable for development of sugar-free, reduced-calorie, and low-glycemic* Corresponding author. E-mail:
[email protected]. † Åbo Akademi University. ‡ Danisco Sweeteners. § University of Turku.
index products, showing, e.g., noncariogenic and prebiotic properties. Lactitol is metabolized independently of insulin, and as such, it is suitable for a diabetic diet. Lactitol can successfully replace sucrose in most applications due to many similar physical properties. Lactitol is a widely used ingredient for sugar-free chocolate, baked goods, and ice cream applications.10-12 Sugar alcohols, such as lactitol, xylitol, and sorbitol, are industrially commonly prepared by catalytic hydrogenation of corresponding sugar aldehydes over sponge nickel and ruthenium on carbon catalysts.13-18 The kinetics of glucose and xylose hydrogenation has been reported in many publications.19-23 Kinetic studies about lactose hydrogenation have been extremely scarce,13,24 and no kinetic modeling work about lactose hydrogenation has been published so far. Here, we present experimental lactose hydrogenation data and the kinetic modeling of it. 2. Experimental Section 2.1. Experimental Setup. The lactose hydrogenation experiments were carried out batchwise in a three-phase laboratoryscale reactor (Parr Co.) operating at 20-70 bar and between 110 and 130 °C. The reactor was equipped with a heating jacket, a cooling coil, a filter (0.5 µm metal sinter) in a sampling line, and a bubbling chamber (for removing dissolved air from the liquid phase prior to the hydrogenation experiments). The effective liquid volume of the reactor was ∼125 mL (total volume 300 mL), and it was equipped with a hollow-shaft concave-blade impeller to ensure efficient mixing and gas dispersion into the liquid phase. A Parr 4843 controller was used for the temperature control and for monitoring the impeller speed and the reactor pressure. The temperature and pressure profiles were stored on a computer. Lactose solutions were prepared by dissolving lactose monohydrate (Leprino Foods,
10.1021/ie0601899 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/22/2006
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Figure 1. Lactose hydrogenation scheme.
purity > 99.5% of dry substance and dry substance content 95%) in deionized water. Too-high lactose-dissolution temperatures were avoided to suppress lactose hydrolysis prior to the hydrogenation. The Mo-promoted sponge nickel slurry catalyst (ACTICAT) amount varied between 2.5 and 10 wt % (dry weight) of the lactose weight throughout the kinetic hydrogenation series. The median particle size of the fresh catalyst was 32.4 µm, the specific surface area was 89.7 m2/g, and the pore volume was 0.14 cm3/g. The pH of the liquid over the fresh catalyst was 11.1. No pH buffer was used at lactose hydrogenation experiments. Before each experiment, the aqueous lactose solution, the catalyst, and the reactor vessel were flushed with hydrogen. The hydrogen-saturated lactose solution was fed into the reactor rapidly, and the hydrogen pressure and reactor temperature were immediately adjusted to the experimental conditions. Simultaneously, the impeller was switched on. This moment was considered as the initial starting point of the experiment. No notable lactose conversion occurred before the impeller was switched on. 2.2. Analysis and Catalyst Characterization. The reactor contents were analyzed off-line with a high-performance liquid chromatograph (HPLC), equipped with a Biorad Aminex HPX87C carbohydrate column. CaSO4 (1.2 mM) in deionized water was used as a mobile phase, since calcium ions improved the resolution of lactobionic acid.33 A sample for pH measurement was withdrawn simultaneously as the HPLC sample was taken. An additional sample was withdrawn at the end of the hydrogenation batches to measure the amount of leached metals in the sugar solution. The dissolved metals were analyzed by the direct current plasma (DCP) technique. The state of fresh and recycled sponge nickel catalysts was investigated by means of several catalyst characterization techniques (nitrogen adsorption Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) surface analysis, scanning electron micro-
Figure 2. Influence of impeller rate on lactose conversion at 120 °C and 50 bar H2 over 5 wt % sponge nickel catalyst.
scopy-energy-dispersive X-ray analysis (SEM-EDXA), hydrogen temperature-programmed desorption (TPD), and particlesize analysis) to reveal the underlying phenomena causing catalyst deactivation. 3. Kinetic results 3.1. Mass-Transfer Resistance. The influence of external mass transfer was studied by hydrogenating 40 wt % aqueous lactose solution at 120 °C and 50 bar over 5 wt % sponge nickel catalyst by various stirring rates (600, 900, 1200, and 1800 rpm). The lactose conversion (Figure 2) and the lactitol selectivity (Figure 3) values were hardly affected by changing the impeller rate from 900 to 1800 rpm. The experiment at 600 rpm had clearly a lower reaction rate and gave a somewhat lower lactitol selectivity indicating, external mass-transfer limitations. Simula-
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Figure 3. Influence of impeller rate on lactitol selectivity in lactose hydrogenations at 120 °C and 50 bar H2 over 5 wt % sponge nickel catalyst.
tions of the hydrogen content in the liquid phase indicated that a complete saturation is achieved at high conversions, while the hydrogen concentration is below saturation in the beginning of experiments. By increasing the impeller rate, the kL values can be improved and the role of external mass-transfer resistance is suppressed. Furthermore, the quasi-stationary concentration fronts move inside the catalyst particles, as the catalyst deactivation proceeds.35 Inefficient mixing in the hydrogenation reactor increased especially formation of lactobionic acid, galactitol, and sorbitol. Thus, the impeller rate was fixed at 1800 rpm in all of the kinetic experiments to ensure operation at the kinetically controlled regime. Previous studies (xylose hydrogenation over sponge nickel) with an analogous system indicated that the mass-transfer resistance inside the catalyst particles is negligible.22 In this study, we have a slower reaction than xylose hydrogenation and the same catalyst. The porosity-to-tortuosity factors are rather small (,0.5) in these experiments; thus, the effective diffusion coefficient becomes small and internal diffusion resistance starts playing a role for particles, with diameter >100 µm.35 Thus, the internal mass-transfer resistance is excluded. 3.2. Influence of Lactose Concentration. The effect of lactose concentration was studied in the hydrogenation experiments with 20, 30, and 40 wt % lactose solutions over 5 wt % sponge nickel catalyst at 120 °C and 50 bar. Lactose concentration did not have any significant influence either on reaction rate or selectivity at the experimental range, when catalyst-tolactose ratio was kept constant. For a fixed partial pressure of hydrogen, the hydrogen pressure is practically constant. Thus, the conversion rate of lactose (rL) can be expressed by eq 1:
rL ) -k′(cL)β(pH2)R, where k’ )
∑kj
Figure 4. Plot of -ln(1 - X) vs reaction time, showing the reaction being close to first order with respect to the lactose concentration; X ) lactose conversion.
Figure 5. Influence of catalyst loading on lactose (40 wt % in water) conversion at hydrogenations at 120 °C and 50 bar H2.
(1)
The first assumption was that the process is first order with respect to lactose, i.e., β ) 1. Then the mass balance of lactose in the reactor can be expressed by eq 2: R
dcL/dt ) FBrL ) -k′FB(pH2) cL
(2)
Further, at constant pressure, eqs 3 and 4 can be derived.
k′′ ) k′FB(pH2)R
(3)
-ln(cL/c0L) ) -ln(1 - X) ) k′′t, where X ) conversion (4)
Figure 6. Influence of catalyst loading on lactitol selectivity at hydrogenations at 120 °C and 50 bar H2.
Plotting -ln(1 - X) versus hydrogenation time gave straight lines for all these hydrogenation experiments, indicating the reaction is first order with respect to the lactose concentration (Figure 4). 3.3. Effect of Catalyst Loading. The influence of catalyst loading was evaluated by varying the catalyst-to-lactose ratio between 2.5 and 10 wt % for hydrogenation experiments at 120
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Figure 7. Influence of catalyst loading on byproducts formation at 120 °C and 50 bar H2.
Figure 8. Arrhenius plots of the initial reaction rates for the hydrogenation of 40 wt % lactose in water carried out at 20 bar (Ea ) 51.2 kJ/mol) and 70 bar (Ea ) 44.8 kJ/mol) and at the temperature range between 383 and 403 K.
°C and 50 bar. As expected, higher activities were obtained at increased catalyst loadings (Figure 5). Higher catalyst loadings led to decreased lactitol selectivities (Figure 6) at low lactose conversion levels because of increased formation of lactobionic acid, lactulose, and lactulitol. However, lactitol selectivities close to 100% lactose conversion level were to some extent higher when high catalyst loadings were used, with selectivity varying between 96.5 and 98% at the experimental range. Moreover, there was a clear difference at byproducts distribution at altered catalyst loadings (Figure 7). Low catalyst loadings led to increased lactose hydrolysis and, thus, to higher galactitol and sorbitol formation. One can conclude that acidity and slow reaction rate increase the lactose hydrolysis reaction. Higher catalyst loadings led, on the other hand, to an increased pH value of the reaction solution, which favored lactose isomerization, leading to higher lactulose and lactulitol formation. In dilute basic solutions, glucose epimerizes to fructose and mannose according to the Lobry de Bruyn-van Ekenstein reaction,25 explaining the effect of pH for lactulose and lactulitol formation. Because of higher pH and high reaction rate, galactitol and sorbitol formation diminished as the catalyst amount was increased. Furthermore, since increased catalyst loading led to a higher reaction rate and, thus, to a higher hydrogen consumption, it increased lactobionic acid formation. 3.4. Temperature Dependency. The effect of the reaction temperature on the lactose hydrogenation over 5 wt % sponge nickel catalyst was clear. An increased hydrogenation temperature clearly improved the reaction rate at the experiments between 110 and 130 °C. From the Arrhenius graph (Figure 8)
Figure 9. Influence of hydrogen pressure on lactose (40 wt % in water) conversion at 120 °C over 5 wt % catalyst.
at the temperature range carried out at 383-403 K and pressure ranges of 20-70 bar, it was found that the apparent activation energy for lactose hydrogenation over sponge nickel was 4452 kJ/mol. Thus, the estimated activation energies were much larger than the activation energy of diffusion in liquids (12-21 kJ/mol),34 therefore confirming that the experiments were performed under chemical kinetics control. 3.5. Effect of Hydrogen Concentration. An increased hydrogen pressure also had a positive effect on the reaction rate, but the influence was clearly bigger at the low-pressure range (Figure 9). To reveal the rate dependence on hydrogen pressure, eq 3 was rewritten to a logarithmic form (eq 5).
ln k′ ) ln(k′FB) + R ln pH2
(5)
The lactose hydrogenation at 120 °C was proved to be of 0.7 order with respect to hydrogen (Figure 10). The lactitol selectivity decreased as hydrogen pressure was decreased or reaction temperature increased (Figure 11). The influence of hydrogen pressure and temperature on lactobionic acid, lactulose, lactulitol, and galactitol formation is displayed in Figures 12 parts a-d. Sorbitol formation was equal to galactitol formation. The pH trend during the same hydrogenation experiments is shown in Figure 13. 3.6. Distribution of Byproducts. From the performed experiments, the following conclusions about byproducts formation can be drawn. Lactulose is an alkali-induced isomerization product of lactose and is further hydrogenated to lactulitol and lactitol. In theory, hydrogenation of lactulose should follow a similar reaction mechanism as catalytic hydrogenation of
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Figure 10. Determination of the reaction order with respect to hydrogen pressure at 120 °C over 5 wt % catalyst; X ) conversion.
Figure 11. Influence of reaction temperature on lactitol selectivity at 20 and 70 bar over 5 wt % catalyst.
Figure 12. Influence of reaction temperature and hydrogen pressure on (a) lactobionic acid, (b) lactulose, (c) lactulitol, and (d) galactitol formation over 5 wt % catalyst.
fructose to sorbitol and mannitol. High reaction temperatures increase formation of lactulose and lactulitol, while the effect of hydrogen is less pronounced. Acidity, high reaction temperature, and slow reaction rate increase the lactose hydrolysis reaction and, thus, galactitol and sorbitol formation. Since
generally sugar alcohols are thermodynamically more stable than sugar aldehydes, hydrolysis of lactitol was assumed to be minor. Moreover, in lactose, the steric crowding at the anomeric oxygen of galactose is probably more severe than in lactitol, with the latter being more flexible because of its acyclic structure. This
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rj )
kjcApHnH2 2 (1 + KH2pHnH2 2)(1 + KAcA + KBcB)
rj )
Figure 13. Evolution of pH in lactose hydrogenation batches performed at different temperatures and pressures over 5 wt % catalyst.
makes the glycosidic bond in lactose weaker than the corresponding bond in lactitol. This, in turn, makes lactose hydrolysis easier than lactitol hydrolysis. On the other hand, results in Figure 12d indicate that even lactitol or lactobionic acid hydrolysis may occur, since quite substantial amounts of galactitol and sorbitol are formed at the end of the hydrogenations. Lactobionic acid was formed from lactose under “hydrogenpoor” conditions on the catalyst surface, which caused lactose dehydrogenation. This observation is supported by the fact that more lactobionic acid was formed, when hydrogen pressure in the reactor was low and initial reaction rate was high because of high catalyst loading or temperature. When reaction rate was kept at the same level in two separate lactose hydrogenation experiments at different pH levels (low vs high), it led to equal lactobionic acid formation. Obviously, it is not correct to denote the formation of lactobionic acid as a Cannizzarro reaction (as formation of aldonic acids are often claimed to be), since formation of lactobionic acid was proved to be only indirectly dependent on pH. This is because a higher pH level leads to increased reaction rate and, thus, to hydrogen-poor conditions, leading to increased lactobionic acid formation. Lactobionic acid was partly hydrogenated back to lactose and lactitol at a later stage of reaction. In a separate test, aqueous lactobionic acid solution was hydrogenated over sponge nickel catalyst, confirming lactitol is the main product, and only small amounts of lactose, galactitol, and sorbitol were detected during the reaction. The hydrogen concentration in the liquid phase plays a crucially important role for the hydrogenations: in the case of external mass transfer of hydrogen, the isomerization, hydrolysis, and dehydrogenation reactions, which don’t require any hydrogen, are favored.35 3.7. Kinetic Modeling. On the mechanistic level, rate equations can be based on the concepts of adsorption, surface reaction, and desorption. It has previously been proposed by Mikkola et al.22 that sugar hydrogenations follow a competitive adsorption model, where adsorbed atomic hydrogen is added pairwise to adsorbed organics. However, because of the large size difference between sugar molecules and hydrogen, it is, however, reasonable to assume that full competition in adsorption does not take place. Consequently, a semicompetitive adsorption model has been proposed in the literature,30 but it is more difficult to determine the parameters of that model. Moreover, one can assume hydrogen is active either in molecular (H2**) form or in dissociative (H*) form. Equations 6 and 7 show the derived rate equations for the noncompetitive (eq 6) and competitive models (eq 7),
kjcApHnH2 2 (1 + KH2pHnH2 2 + KAcA + KBcB)2
(6)
(7)
where nH2 ) 1/R (R ) 1 in the case of molecular hydrogen and R ) 2 in the case of dissociative hydrogen). Since hydrogen molecules are much smaller than lactose molecules, interstitial sites between adsorbed lactose molecules are assumed to remain accessible for hydrogen adsorption. Thus, the adsorption behavior is shifted toward the noncompetitive one. On the basis of the preliminary kinetic analysis, some simplifications can be made. The amount of byproducts in the liquid phase and, thus, also on the catalyst surface is minor, and the main reaction turned out to be of first order with respect to lactose. In addition, it is known that the adsorption affinity of sugar alcohols is much less than that of sugars. Furthermore, the product desorption step was excluded, and the adsorption constants KL and KH were presumed to be independent of temperature. The rate constants (kj) follow the law of Arrhenius (eq 8),
kj ) k0,j e-(Eaj/R)(1/T-1/Th )
(8)
where T h is the average temperature of the experiments. The final set of rate equations for a simplified reaction scheme (Figure 14) became then
r1 ) k1cL[pH2/(1+ KH2pH2)]
(9)
r2 ) (k2cLpH2)/(1+ KH2pH2)
(10)
r3 ) (k3cLpH2)/(1+ KH2pH2)
(11)
r4 ) (k4cLpH2)/(1+ KH2pH2)
(12)
r5 ) (k5cLpH2)/(1+ KH2pH2)
(13)
r6 ) (k6cLUpH2)/(1+ KH2pH2)
(14)
r7 ) (k7cLBpH2)/(1+ KH2pH2)
(15)
r8 ) (k8cLUpH2)/(1+ KH2pH2)
(16)
The rate equations for hydrolysis reactions r4 and r5 are equal. The above-derived rate equations were further used in following the mass-balance equations for the organic components,
dci ) dt
∑VijrjFB,
where FB )
mcat VL
(17)
The catalyst bulk density was assumed to be constant during the experiments, since the liquid withdrawn through sampling was negligible compared to the total liquid volume. The fitting of the experimental data to the kinetic model was carried out by Modest software31 by using a combined SimplexLevenberg-Marquardt method. The following objective function was used in data fitting,
Q)
∑(ci,exp - ci,calc)2wi
(18)
where the weight factors (wi) were selected as follows: w ) 1
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Figure 14. Simplified reaction scheme and some data fitting results: lactose (O), lactitol (x), lactobionic acid (+), lactulose (0), lactulitol (]), galactitol (*), and sorbitol (3).
for lactose and lactitol and w ) 5 for byproducts. Using higher weight factors for low-concentration components improved the estimation of them. The parameter estimation was performed for all experiments at different temperatures and pressures together. The results of parameter estimation are summarized in Table 1 and Figure 14. As revealed by the table, the parameters are rather well-identified, the parameters of the main reaction are extremely well-identified, and the parameter standard deviations are just ∼2%. The parameters of the side reactions are less wellidentified, but it should be kept in mind that the concentrations of the byproducts were very low,
