Ind. Eng. Chem. Res. 2008, 47, 4049–4055
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Catalytic Wet Oxidation of Lactose Yah Nan Chia, Michael P. Latusek, and Joseph H. Holles* Department of Chemical Engineering, Michigan Technological UniVersity, 1400 Townsend DriVe, Houghton Michigan 49931
The catalytic wet oxidation of lactose to carbon dioxide/water and to a value-added product, lactobionic acid, has been demonstrated in a flow reactor. Lactose (milk sugar) is a low value byproduct of the dairy industry and makes up the largest part of the solids in cheese whey. Costs associated with cheese whey disposal are driving the need to develop alternative disposal methods. Pt/Al2O3, CeMn mixed-metal oxides, and Pt/CeMn catalysts have all been shown to effectively convert lactose to carbon dioxide and water at temperatures up to 443 K and pressures of 100 psig. Pt/CeMn demonstrated the lowest level of side-product formation. A BiPd/C catalyst was shown to convert essentially all lactose to lactobionic acid at similar temperature and pressure. Lactobionic acid selectivity was a strong function of oxygen concentration in the feed. The BiPd/C also produced a high yield of lactobionic acid at lower pH and higher temperatures than previously reported. Introduction Lactose (C12H22O11) is the main ingredient in cheese whey, which is a byproduct of cheese production.1 Cheese whey typically contains 5.2 wt % lactose in water together with minor amounts of ash, fat, and protein (each less than 0.5 wt %).1,2 In 2005, total U.S. cheese production was 9.13 billion pounds.3 For each pound of cheese, up to 9 lbs. of wastewater are created as a side effect.4 The majority of this liquid wastewater is known as whey, which can be processed and sold for human consumption. In 1998, approximately 2% (by weight) of the whey was recovered and processed.4 This is less than one-third of the solid matter in whey (by weight). Unfortunately, the remainder of this biomass is either sent to a wastewater treatment facility or simply field spread as a means to dispose it. Treatment at a wastewater facility to remove the lactose and other solids has associated costs. While field spreading is not as expensive, it can lead to significant runoff issues as this nutrient enters the watershed. This includes the eutrophication of water bodies due to the high biological oxygen demand of lactose. As a result, Wisconsin, a major dairy state (contributes 27% of the nation’s cheese manufacturing3) has recently instituted strict controls on field spreading. To minimize the concerns associated with disposal of lactose, two possible solutions can be considered, the degradation of lactose to a species with fewer disposal concerns or the conversion of lactose to a value-added product. For both of these solutions, catalytic wet oxidation (CWO) represents a potential technology. Wet oxidation is defined as an aqueous-phase oxidation process carried out when an organic is mixed thoroughly with a gaseous source of oxygen. This usually occurs at elevated temperatures to increase the reaction rate and elevated pressure to enhance oxygen solubility and control liquid evaporation.5,6 Traditionally, this process has been used to treat hazardous, toxic, and nonbiodegradable waste streams.7,8 With the addition of a catalyst to further increase the rate, the process becomes CWO. CWO is an ideal technique for this situation since the species of interest (lactose) is present at a low concentration in an aqueous medium. A gas-phase system would require significant additional energy input in order to vaporize the large amount of water in this system. * Corresponding author. Tel: 1 906 487 1956. Fax: 1 906 487 3213. E-mail:
[email protected].
The complete oxidation of lactose yields carbon dioxide and water according to the following stoichiometric equation: C12H22O11 + 12O2 f 12CO2 + 11H2O While no literature data for complete oxidation of lactose was found, literature data on other carbohydrates was found. Patrick and Abraham examined the CWO of glucose and cellulose over a Pt/Al2O3 catalyst and found that that the degradation of glucose resulted in the formation of small organic acids in addition to the desired complete conversion products.9 This work was performed in a batch reactor, and conversions did not exceed 80%. In addition, CoBi and CeMn mixed-metal oxide heterogeneous catalysts and precious metal (Ru, Rh, Pt, Ir, and Pd) catalysts supported on Ce were investigated for the CWO of polyethylene glycol (PEG) and acetic acid.10,11 The MnCe catalyst demonstrated better performance than the CoBi catalyst for degradation of acetic acid while their performances for PEG were similar.10 For the Ce-supported precious metal catalysts, Ru was found to have slightly better performance than Pt.11 Again, both of these studies occurred in batch reactors. Lactobionic acid (Figure 1) represents one of many potential value-added products that can be derived from lactose. The partial oxidation of lactose to lactobionic acid occurs according to the following equation: 1 O f C12H22O12 2 2 This selective oxidation reaction requires only 1/24th the oxygen required for complete oxidation. Lactobionic acid has commercial applications in pharmaceutical preparations,2 launC12H22O11 +
Figure 1. Lactobionic acid.
10.1021/ie701779u CCC: $40.75 2008 American Chemical Society Published on Web 05/14/2008
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dry detergent formulations, antiaging cosmetics, and as a preservation fluid during organ transplant. Lactobionic acid formation from lactose has been shown to occur over a Bipromoted Pd catalyst on a carbon support.12 High selectivity in the pH range of 8-10 at temperatures up to 333 K was demonstrated for the Bi-Pd/C catalyst in a batch reactor. The use of a high pH level allows for the immediate conversion of lactobionic acid to the lactobionate salt which precipitates out of solution. Pd-modified Pt electrodes have also been shown to electrocatalytically oxidize lactose to lactobionic acid,13 and electrochemical oxidation is the method of commercial production.2 More recently, electrochemical potential was used as a method for studying the selective oxidation of lactose. In these studies, Au was found to be more effective than Pd at catalyzing the first oxidation product (lactobionic acid), while Pd (without Bi) was more effective at producing the second oxidation product (2-keto-lactobionic acid) at a pH of 8.14–16 Similar conversion of other carbohydrates into acids has also been demonstrated in the literature. Glucose conversion to gluconic acid has been extensively studied over Bi-promoted Pd and Pt catalysts.17–20 Sucrose conversion to 6-carboxysucrose over Pt catalysts has also been investigated.21,22 The goal of the current research program is to develop methods of minimizing lactose waste disposal issues using the CWO process. The present investigation examines the ability of a Pt/Al2O3 catalyst to completely degrade lactose to carbon dioxide and water. Since CeMn catalysts have been shown to decompose small organic acids, which are the main byproduct of glucose CWO, the ability of this catalyst and a combined CeMn-supported Pt catalyst to completely decompose lactose in one step will also be investigated. Finally, the ability of a BiPd/C catalyst to convert lactose to lactobionic acid at neutral pH, higher temperature, and under flow conditions will also be examined. Experimental Section A feed with a concentration of 0.08 M lactose was prepared by dissolving about 317.1 g of R-D-lactose monohydrate (ACS reagent, Sigma Aldrich) in 11 L of distilled water in a glass container. The feed was charged into the reactor system using pulseless rotary gear pump heads. System pressure was maintained at 100 psig to maintain a liquid phase. The reactor temperature was varied between 150 and 170 °C. The reactor had a dimension of 5 cm in diameter by 10 cm in length and consisted of a section of 316 stainless-steel tubing connected to the heater and heat exchanger. Typically, about 4 g of catalyst was used and the average length of the catalyst packed bed was less than 0.5 cm. The desired oxygen gas feed rate was maintained by a mass flow controller (Omega FMA 700 Series). For complete degradation runs, the typical oxygen feed rate contained 40% excess oxygen on a mole basis. To produce a 5% platinum on Al2O3 catalyst, 0.5693 g of platinum(II) 2,4-pentanedionate (Alfa Aesar) and 4.7645 g of γ-aluminum oxide (99.97%, Alfa Aesar) were mixed together in 150 mL of toluene. The solution was stirred for 2 h and rotovapped to dryness. The sample was dried overnight in an oven at 120 °C and calcined in flowing air at 250 °C for 6 h, similar to the procedure of Patrick and Abraham.9 The CeMn composite oxide catalyst was prepared by coprecipitation and adapted from the procedures of Imamura et al.10,11,23 Manganese(II) nitrate solution was prepared by dissolving 23.3487 g of manganese(II) nitrate, Mn(NO3)2 (99.98% Alfa Aesar), in 247.5 mL of distilled water. Then, 13.7808 g of cerium(III) chloride, CeCl3 (99.9% Alfa Aesar), was added
to the aqueous solution of manganese(II) nitrate. A portion of 3 M sodium hydroxide solution was added to the CeMn solution, forming a white and brown precipitate, which was then stirred moderately. About 300 mL of NaOH solution was added until no additional precipitate was formed. This solution was then filtered, washed, dried, and calcined. As filtering was completed, all solids turned dark earthy brown. The resultant precipitate was then washed with distilled water and was dried in an oven at 100 °C overnight, followed by calcination at 350 °C in air for 3 h. The Pt/CeMn catalyst was synthesized using the CeMn from the procedure above. Platinum was then impregnated on the CeMn composite oxide catalyst using an aqueous solution of platinum(II) 2,4-pentanedionate. First, 3.6434 g of CeMn and 0.3852 g of platinum(II) 2,4-pentanedionate were added to a round-bottom flask containing toluene and stirred. The solution was then rotovapped to dryness. After drying overnight at 100 °C in an oven, the material was calcined at 250 °C for 6 h. The 5% Pd/C catalyst (Alfa Aesar) was first reduced before used. The bismuth-promoted carbon-supported palladium catalyst, BiPd/C, was synthesized by mixing 2.9918 g of Pd/C in 300 mL of distilled water. This mixture was heated at 313 K, stirred, and deaerated by bubbling with nitrogen gas for 20 min. A solution of bismuth(III) nitrate oxide, BiONO3, was prepared by dissolving 0.0208 g BiONO3 (99.999%, Alfa Aesar) in 0.905 mL of concentrated HCl. 500 mmol or 180.02 g of D-lactose monohydrate was then added to the Pd/C suspension and stirred for 10 min. After that, the BiONO3 solution was added dropwise to the Pd/C solution under continuous stirring. During all these operations, nitrogen gas flow was maintained and the temperature was kept at 313 K. The catalyst was then calcined as above. The percentage of metal atoms exposed (dispersion) was determined using H2 chemisorption in a Micromeritics ASAP 2020 system. Adsorption isotherms were obtained at 303 K. The hydrogen uptake was calculated from the difference between total chemisorption and reversible chemisorption. Surface area measurements were obtained in a Micromeritics ASAP 2020 system using nitrogen adsorption at 77 K. Elemental analysis was performed by Galbraith Laboratories, Inc., Knoxville, TN. Lactose and lactobionic acid analysis was performed by HPLC. The samples were separated on a Hypersil APS-2 NH2 (250 mm × 4.6 mm) column at room temperature. A mobile phase of acetonitrile-sodium phosphate buffer (50 mM, pH 5.0) (60:40) was applied, and the flow rate of the mobile phase was set at 1.0 mL/min. The instrument was calibrated using known solutions of lactose and lactobionic acid. All lactose and lactobionic acid analyses were repeated in duplicate. To analyze for decomposition products, a reverse-phase HPLC analysis with ultraviolet (UV) detector at 214 nm on a µBondpak C-18 (300 mm × 3.0 mm) column was used. A mobile phase of 18 mmol/L KH2PO4 buffer solution adjusted to pH 2.25 with diluted H3PO4 at a flow rate of 0.3 mL/min was used. The instrument was calibrated for malic and succinic acid as representative side products. Results Elemental analysis results for the four catalysts are reported in Table 1. The weight loading of the MnCe catalyst corresponds to a 70:30 molar ratio of Mn to Ce. This is consistent with the molar ratio of the MnCe catalyst most active for complete oxidation of acetic acid.10 For the BiPd/C catalyst, the molar ratio of Pd to Bi is approximately 700:1. Dispersion (percent
Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 4051 Table 1. Elemental Analysis Results catalyst
element
wt %
dispersion % or surface area
Pt/Al2O3 MnCe
Pt Mn Ce Pt Mn Ce Bi Pd
5.43 35.5 34.0 4.17 34.0 32.6 (142 ppm) 5.0
49.3 98.1 m2/g
Pt/MnCe BiPd/C
21.7 16.0
metal exposed) and specific surface area, as appropriate, are also reported in Table 1. The Pt/Al2O3 catalyst was first investigated for its ability to decompose lactose. The concentration of lactose as a function of time on-stream for the Pt/Al2O3 catalyst is shown in Figure 2. The system took approximately 2 h on-stream to approach 100% lactose conversion and then remained near 100% for the remaining 2.5 h of the experiment. Complete conversion under these conditions results in a specific activity of 3.3 × 10-4 mol/ gcat min or a turnover frequency of 0.04 1/s. Side-product formation as a function of time is shown in Figure 3, which plots peak heights for a number of different species (based on retention time) as a function of time on-stream. While total sideproduct concentration was low for the entire run, the lowest value was achieved after 100% conversion was achieved. On the basis of the calibration of malic acid as a side-product, the steady-state concentration of malic acid is 2.7 × 10-4 mol/l (13.3 min retention time). This concentration is about 0.3% of the initial lactose concentration. Due to the large number and low height of other peaks, no attempt was made to identify and quantify other compounds. However, if response factors similar to malic acid are assumed for the unidentified species, the total concentration of side products was only a few percent of the original lactose concentration. The ability of the CeMn catalyst to decompose lactose was tested under the same conditions. Lactose conversion as a function of time on-stream is shown in Figure 2. Similar to Pt/ Al2O3, the catalyst reached approximately 100% conversion after 2 h on-stream and conversion remained steady for the next 3 h. Complete conversion under these conditions results in a specific
Figure 3. Side-product peak heights as a function of time on-stream for Pt/Al2O3.
Figure 4. Side-product peak heights as a function of time on-stream for CeMn.
Figure 2. Lactose concentration as a function of time on-stream for Pt/ Al2O3 and CeMn. Conditions were 170 °C, 0.08 M lactose fed at 17.5 mL/ min, with 40% molar excess oxygen feed, and 4.27 g of catalyst for Pt/ Al2O3, and 170 °C, 0.08 M lactose fed at 18 mL/min, with 40% molar excess oxygen feed, and 4.5 g of catalyst for CeMn.
activity of 3.2 × 10-4 mol/gcat · min and a normalized rate of 5.4 × 10-8 mol/m2 · s. Side-product formation is shown in Figure 4. Initial side-product formation for the CeMn catalyst is higher than that for the Pt/Al2O3 catalyst (for comparison, all figures showing side-product formation are to the same scale); however, once 100% conversion is approached, side-product formation is very similar to that of the Pt/Al2O3 catalyst. The combination of Pt supported on a CeMn catalyst was also tested for its ability to decompose lactose. Lactose concentration as a function of time on-stream for two runs is shown in Figure 5. For run 1, the oxygen feed was in 40% molar excess for complete degradation. The oxygen feed was increased to 75% molar excess for run 2. Lactose concentrations for both runs were very similar and approached 100% conversion after about 2.5 h on-stream and remained at that level for an additional 2.5 h. Complete conversion under these conditions results in a specific activity of 5.5 × 10-4 mol/gcat min and a turnover frequency of 0.2 1/s. The effect of oxygen feed rate on selectivity is compared in Figure 6. The higher oxygen feed (Figure 6b) resulted in lower initial and steady-state levels of side products. The lowest amount of side products observed
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Figure 5. Lactose concentration as a function of time on-stream for Pt/ CeMn. Conditions were 170 °C, 0.08 M lactose fed at 22.5 mL/min, with 40 or 75% molar excess oxygen feed, and 3.29 g of catalyst.
for any of the catalysts was for the Pt/CeMn catalyst at the higher oxygen feed rate. Although not quantified, gas-phase analysis showed carbon dioxide as the only volatile product. Since the Pt/CeMn catalyst resulted in the lowest amount of side products, its behavior as a function of temperature was also investigated. Reaction rates were measured at 140, 150, and 160 °C with the flow rate varied to maintain differential conversion conditions (less than 10% conversion). The data are shown as an Arrhenius-type plot in Figure 7. Each data point in Figure 7 represents the average of the last four sample points for each condition. On the basis of Figure 7, the apparent activation energy for the decomposition of lactose over the Pt/ CeMn catalyst is 132.2 kJ/mol. The ability of a carbon-supported BiPd catalyst to convert lactose to lactobionic acid at neutral pH and under flow conditions was also examined. Lactose concentration as a function of time on-stream for three different oxygen concentrations (33 times molar excess, 5 times molar excess, and 3 times molar excess) is shown in Figure 8. Conversion approached 100% in 30 min for the intermediate oxygen concentration, approximately 2 h for the lowest oxygen concentration, and approximately 3 h for the highest oxygen concentration. Complete conversion under these conditions results in a specific activity of 3.3 × 10-4 mol/gcat min and a turnover frequency of 0.072 1/s. The yield to lactobionic acid is shown in Figure 9, which shows lactobionic acid concentration as a function of time on-stream. Following the start-up and while approaching the steady state, there are some oscillation in lactobionic acid production for high and intermediate oxygen levels. However, at steady-state conversion, the yields are more consistent. For the high and intermediate oxygen concentrations, there is some selectivity to lactobionic acid early in the run; at longer times (greater than 3 h) when the conversion consistently approaches 100%, the steady-state yield of lactobionic acid is less than 5%. For the lowest oxygen feed concentration, lactobionic acid concentration and yield increase as a function of time on-stream and reach approximately 100% after 4.5 h on-stream. The behavior of the BiPd/C catalyst as a function of temperature was also investigated. Reaction rates were measured at 150 and 160 °C with the flow rate varied to maintain differential conversion. The data are shown in Figure 10 as an Arrhenius-type plot. Again, each data point represents the
Figure 6. Side-product peak heights as a function of time on-stream for Pt/CeMn for (a) 40% molar excess oxygen feed and (b) 75% molar excess oxygen feed.
average of the last four samples points for each condition. The two data points at high temperature represent data from runs on two separate days. On the basis of Figure 10, the apparent activation energy for the conversion of lactose to lactobionic acid is 163.0 kJ/mol. Discussion For the desired complete oxidation of lactose to carbon dioxide and water, all three catalysts (Pt/Al2O3, CeMn, and Pt/ CeMn) performed well with 100% conversion and high selectivity to the desired product (>95%) under similar conditions. On a normalized basis, the Pt/CeMn catalyst performed slightly better since the same conversion was achieved with approximately one-third less catalyst and one-third higher flowrate. Compared on an active metal site basis, the Pt/CeMn catalyst was 5 times as active as the Pt/Al2O3 catalyst. However, the reaction rates in all three cases were conversion limited, and the catalysts may be capable of higher rates if the molar feed rate is increased. Due to the large number and low height of the peaks, no attempt was made to identify and quantify the side products.
Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 4053
Figure 7. Arrhenius-type plot for lactose degradation for Pt/CeMn catalyst with 0.08 M lactose and 75% molar excess oxygen feed.
Figure 8. Lactose concentration as a function of time on-stream for BiPd/ C. Conditions were 170 °C, 0.08 M lactose fed at 17.5 mL/min, with 33, 5, and 3 times molar excess oxygen feed, and 4.27 g of catalyst.
For glucose oxidation, Patrick and Abraham report oxalic acid, succinic acid, and malic acid as major side products with fumaric acid and 5-(hydroxymethyl) furfural as minor products.9 We identified malic acid in our runs but did not observe succinic acid. Since lactose and glucose have many similarities, some of the same side products may be assumed. The addition of the second carbohydrate ring for lactose may also lead to some more complex side products. More recent work examining the partial oxidation of lactose has shown a consecutive reaction from lactose to lactobionic acid and then to 2-keto-lactobionic acid.14–16 It is quite possible that this path also occurs in our system, as lactobionic acid was the only product observed during the low conversion runs used to develop Figure 7. The selectivity for all three catalysts was also very comparable. Under start-up conditions (conversion
