Article pubs.acs.org/IECR
Kinetics of Lactose Hydrogenation over Ruthenium Nanoparticles in Hypercrosslinked Polystyrene Valentin Yu. Doluda,† Johan War̈ nå,‡ Atto Aho,‡ Alexey V. Bykov,† Alexander I. Sidorov,† Esther M. Sulman,† Lyudmila M. Bronstein,§ Tapio Salmi,‡ and Dmitry Yu. Murzin*,‡ †
Department of Biotechnology and Chemistry, Tver Technical University, Tver, Russia Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Turku, Finland § Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States ‡
S Supporting Information *
ABSTRACT: Lactose hydrogenation is a complex chemical process characterized by formation of numerous side products. Therefore, the synthesis of efficient catalysts for lactose hydrogenation and the investigation of the kinetics of this process are important for increasing of lactitol yield. Synthesis of nanocatalysts based on ruthenium-containing nanoparticles (NPs) formed in the pores of hypercrosslinked polystyrene (HPS) modified with amino groups and their catalytic properties in the lactose hydrogenation are described in the current work. The Ru species were incorporated in HPS using wet impregnation of ruthenium(IV) hydroxychloride followed by reduction with hydrogen at 300 °C. The catalysts containing from 1.1 wt % to 4.9 wt % of Ru were studied by X-ray fluorescence analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, CO chemisorption, and liquid nitrogen physisorption methods. It was demonstrated that the NP sizes are controlled by the HPS pores. Several types of Ru species, Ru(IV), Ru(IV) × nH2O, Ru(0), and [RuO4]2− constituted the NP composition. The kinetics model developed is based on the concept of noncompetitive adsorption of hydrogen and organic molecules, because of the large difference in the sizes of sugar molecules and hydrogen, describing the experimental data well. The distribution and sensitivity of the parameters obtained were checked with the Markov−Chain Monte Carlo method.
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INTRODUCTION Hydrogenation of saccharides is an important reaction for the synthesis of sugar alcohols, which could be used as intermediates in the production of pharmaceuticals, as well as artificial sweeteners. Moreover, synthesized polyols can play a key role in the production of biofuels from renewable sources.1 The most important saccharide hydrogenation processes are glucose, xylose, maltose, and lactose hydrogenation to the corresponding alcohols.1−4 Lactose is a low cost, large-scale product of dairy industry: annually, several million tons of lactose are produced worldwide.5 Lactose is suitable for chemical syntheses5; its hydrogenation results in lactitol, which can be used for development of sugar-free, reducedcalorie, and low-glycemic-index products with noncariogenic properties.5 The hydrogenation is commonly performed batchwise in stirred tank reactors at temperatures ranging from 60 °C to 150 °C, hydrogen pressures of 30−80 bar with sponge nickel or transition metals supported on oxides used as catalysts.1 Lactose hydrogenation is a complex chemical process characterized by formation of numerous side products such as lactulose, lactulitol, and lactobionic acid, as well as sorbitol and galactitol,1−4 along with lactitol formation (see Figure 1).3 Therefore, the development of efficient catalysts for lactose hydrogenation and the investigation of the kinetics of this process are crucial for increasing lactitol yields. Industrially used sponge Ni catalysts are very selective, with selectivity reaching up to 96%−98%. However, Ni leaching and fast catalyst deactivation are the main problems of this type of catalysts.4 Hydrogenation over ruthenium supported on magnesia silica, © 2013 American Chemical Society
alumina and titania leads to increased lactulose and, to some extent, lactobionic acid formation; therefore, the process selectivity decreases to 20%−80%.3 Commercially available catalysts, such as Ru supported on different types of carbon, show good performance in lactose hydrogenation with selectivity up to 96−97%, while catalyst deactivation remains the main problem.3 However, ruthenium is more expensive, compared to nickel; therefore, ruthenium-based catalysts should have enhanced activity and stability to replace nickelbased catalysts in industrial applications. Polymer-stabilized transition-metal nanoparticles represent an attractive possibility for the development of efficient and stable catalysts.5−39 Stabilization of metal nanoparticles in rigid polymer matrices can be an alternative for commonly used oxides or carbon-supported catalysts, providing high target product yield along with improved catalyst stability.40−45 Hypercrosslinked polystyrene (HPS)46−49 is a rigid polymer with superior mechanical and chemical stability that can provide a good matrix for catalytically active nanoparticle synthesis and stabilization. Thermal stability of the polymer depends on the degree of crosslinking. The HPS used for catalysts synthesis is stable up to 350 °C under nitrogen. HPS contains both small and large mesopores and is characterized by high surface area and pore volume, promoting substrate Received: Revised: Accepted: Published: 14066
June 5, 2013 August 6, 2013 September 5, 2013 September 5, 2013 dx.doi.org/10.1021/ie401778y | Ind. Eng. Chem. Res. 2013, 52, 14066−14080
Industrial & Engineering Chemistry Research
Article
Figure 1. Scheme of possible reaction products in lactose hydrogenation.
transport to active sites.41−43,45 The use of the catalysts based on HPS containing Pd and RuO2 NPs was reported in earlier work for the partial oxidation of D-glucose.41,50 These catalysts demonstrated high selectivity and activity in the formation of target products. It is noteworthy that HPS used did not contain any functional groups; thus, NP precursor deposition and stabilization were provided solely by confinement in pores. In this paper, ruthenium-containing catalysts based on HPS functionalized with amino groups were synthesized. The catalysts were characterized using X-ray fluorescence (XRF) analysis, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, and CO chemisorption methods. The influence of the Ru content and the various reaction conditions on the activity and selectivity of lactose hydrogenation was studied. The catalyst stability was not in the primary focus of this work, however, preliminary experiments demonstrated that Ru-HPS catalysts are stable during the hydrogenation of sugars (i.e., glucose). The developed kinetic model was used to compare it with the experimental data through numerical data fitting. Identifiability of the developed parameters was checked with the Markov−Chain Monte Carlo method.
Ru-ACC), lactose (Danisco), lactitol (Danisco), lactulitol (Danisco), lactulose (Fluka), lactobionic acid (Acros Organics), sorbitol (Sigma−Aldrich), galactitol (Fluka), and methanol (J.T. Baker) were used as received. Ruthenium hydroxychloride (Ru(OH)Cl3) was purchased from Aurat, Ltd. (Moscow, Russia) and used without purification. Reagent-grade hydrogen of 99.999% purity was received from AGA. Distilled water was purified with ELGA Purelab Ultra water purification system. Catalyst Synthesis. The catalysts were prepared by impregnation of HPS with ruthenium hydroxychloride in a solution containing THF, methanol and distillated water. In a typical synthesis, 0.33 g Ru(OH)Cl3 was dissolved under nitrogen in 7 mL of the solvent mixture consisting of 5 mL of THF, 1 mL of water and 1 mL of methanol, to which 3 g of MN-100 were added. The suspension was continuously stirred for 10 min to allow adsorption of the solution by the polymer granules. Then they were dried at 75 °C for 1 h. The catalyst was washed with water at pH 6.4−7.0 and dried at 75 °C. The ruthenium content was found to be 4.9 wt % by XRF elemental analysis. The two other samples with ruthenium contents of 2.85% and 1.1% were prepared following the same methodology but using 0.198 g and 0.07 g of Ru(OH)Cl3, respectively. The samples were designated as HPS-Ru-4.9%, HPS-Ru-2.85%, and HPS-Ru-1.1%, respectively (see Table 1). The catalysts were reduced under hydrogen at 300 °C. Lactose Hydrogenation Methodology. The hydrogenation was conducted batch-wise in a Parr 4561 autoclave. 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 and for saturation of the liquid phase with hydrogen prior to the hydrogenation experiments). The effective liquid volume was 125 mL (total volume = 300 mL). The reactor was equipped
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MATERIALS AND METHODS Materials. Hypercrosslinked polystyrene (HPS) was purchased from Purolite Int. (U.K.) as Macronet MN 100 (functional groups−tert-amino groups, weak base capacity of 0.5 mol/L, 55%−62% moisture, swell factor = ±5% (max), specific gravity = 1.04 g/mL, specific surface area = 1000 m2/g). The 5−7 μm polymer granules were washed with acetone and water twice and dried under vacuum for 24 h. Sodium hydrogen carbonate (NaHCO3), reagent-grade tetrahydrofuran (THF), 5 wt % Ru on activated carbon (Fluka, designated as 14067
dx.doi.org/10.1021/ie401778y | Ind. Eng. Chem. Res. 2013, 52, 14066−14080
Industrial & Engineering Chemistry Research
Article
by mixing 0.1−0.2 g of HPS with 10−20 mg of standard Ru compounds. Liquid Nitrogen Physisorption. Nitrogen physisorption was conducted at the normal boiling point of liquid nitrogen using a Beckman Coulter SA 3100 apparatus (Coulter Corporation, USA). Prior to the analysis, samples were degassed in a Becman Coulter SA-PREP apparatus for sample preparation at 120 °C in vacuum for 1 h. The desorption branch was used for pore size distribution calculations, while the adsorption one for calculations of the surface area. TEM Analysis. Transmission electron microscopy was performed with a JEOL JEM1010 transmission electron microscope operated at accelerating voltage of 80 kV. Rucontaining HPS powders were embedded in epoxy resin and subsequently microtomed at ambient temperature. Images of the resulting thin sections (ca. 50 nm thick) were collected with the Gatan digital camera and analyzed with the Adobe Photoshop software package and the Scion Image Processing Toolkit. X-ray Photoelectron Spectroscopy Analysis. XPS data were obtained using Mg Kα (hν = 1253.6 eV) radiation with a ES-2403 spectrometer modified with an analyzer PHOIBOS 100 produced by SPECS (Germany). All the data were acquired at an X-ray power of 200 W and an energy step of 0.1 eV. Samples were allowed to outgas for 180 min before analysis and were sufficiently stable during the examination. The data analysis was performed by CasaXPS. Deconvolution of the XPS curves of the Ru-based catalysts was made by simultaneous fitting of Ru 3p and C 1s + Ru 3d energy levels. Chemisorption. The dispersion of the ruthenium nanoparticles was measured by CO chemisorption. A Micromeritics AutoChem 2910 was used in the measurements. Prior to the analysis the samples were reduced in situ at 300 °C for 2 h in a continuous flow of hydrogen. Thereafter, the temperature was decreased to 25 °C. Pulses of CO (10% CO in He) were introduced to the sample and the amount of nonadsorbed CO was measured. A ratio of CO:Ru = 1 was used in the calculations.
Table 1. Surface Areas and Pore Volumes for Ru-ACC, RuModified HPS before and after Lactose Hydrogenation, and Surface Area of Parent HPS catalyst
BET surface area (m2 g−1)
micropore surface area (m2 g−1)
pore volume (mL g−1)
644 156 814 624
427 0 707 654
0.60 0.33 0.53 0.36
558
500
0.24
626
458
0.48
424
327
0.31
780
700
0.51
308
250
0.19
Ru-ACC Ru-ACCa HPS HPS-Ru4.9% HPS-Ru4.9%a HPS-Ru2.85% HPS-Ru2.85%a HPS-Ru1.1% HPS-Ru1.1%a a
Spent catalysts.
with a hollow shaft concave blade impeller to ensure efficient mixing and gas dispersion into the liquid phase. The reactor was operated at an overall pressure of 10−50 bar and a temperature in the range of 120−150 °C. The catalyst concentration was 1.3 g/L in the majority of experiments. The lactose concentration varied between 0.1 mol/L and 0.5 mol/L. A suspension of the catalyst (75 mL) prepared at a predetermined concentration was placed in the reactor. A lactose solution (50 mL) saturated with hydrogen in the bubble chamber was rapidly fed into the reactor and the pressure and reactor temperature were immediately adjusted to the experimental conditions. The impeller rate was fixed at 1800 rpm in all of the kinetic experiments to ensure operation in the kinetically controlled regime. The median particle size of HPS-based catalysts was
