Monolithic stirrer reactor: The selective lactose oxidation in liquid phase over Au/Al2O3 nanostructured catalysts

1 Monolithic stirrer reactor: The selective lactose oxidation in liquid phase over Au/Al2O3 nanostructured catalysts. Authors: S.A.Regenhardt1, C.I.Meyer1, O.Sanz2, V.Sebastian3, S. Ivanova4, M.A. Centeno4, J.A. Odriozola4, M. Montes2, A. J. Marchi1, T.F.Garetto1,* 1GICIC (Catalysis Science and Engineering Research Group) – INCAPE (UNLCONICET), Colectora Ruta Nac. Nº 168 Km 0 (3000) Santa Fe, Argentina. 2Department of Applied Chemistry, Faculty of Chemistry, University of Basque Country (UPV/EHU), 20018 - San Sebastián, Spain. 3Department of Chemical Engineering, Aragon Institute of Nanoscience (INA), University of Zaragoza, Campus Río Ebro - Edificio I+D, C/ Poeta Mariano Esquillor S/N, 50018-Zaragoza, Spain. 4Sevilla Institute of Materials Science, Department of Inorganic Chemistry, University of Sevilla-CSIC, Américo Vespucio 49, 41092, Sevilla, Spain. Corresponding author Prof. Teresita.F. Garetto Predio CCT - Conicet Santa Fe Colectora Ruta Nac. 168 3000 Santa Fe Tel. 54 342 4511546 e-mail: [email protected] [email protected] 2 Abstract The performance of rotating metallic monolith stirrer reactor was studied for selective lactose oxidation in liquid phase at 65 ºC, atmospheric pressure and with air as oxidant agent. The Au/Al2O3deposition on metallic substrates was performed by wash-coating, producing catalyst coating thicknesses between 5 and 20 m. Monoliths with different configuration (channel size between 0.36 and 1.06 mm) were used as stirrer blades in a batch reactor. Internal and external mass transfer limitations were observed during liquid phase lactose oxidation. For stirring rates equal or higher than 600 rpm there were no important external diffusional restrictions and this was also independent of the monolith configuration. Coating with thickness higher than 15 m presents loss of catalyst effectiveness due to internal diffusional restrictions. Excellent stability in the catalytic tests was obtained after three regeneration-reaction cycles. Regeneration was carried out at 400 ºC in air flow. Gold particle size distribution in the monolith washcoat, determined by TEM before and after reaction, was homogeneous with a medium size of around 5 nm. This is in agreement with the very good reproducibility and stability obtained in the catalytic tests. After calcination at 500ºC, some sintering and a heterogeneous distribution of metal particle size was observed, accompanied by a slight loss in catalyst activity. It is concluded that metallic monolith stirrer reactors are a promising application for selective lactose oxidation in liquid phase. Keywords: Selective oxidation, lactose, gold nano-structured catalyst, metallic monolith, lactobionic acid 3 1. Introduction The recovery of waste from the agri-food industry is a highly strategic way of meeting the challenge of the circular economy [1]. However, despite the studies carried out, the number of products on the market derived from this waste remains limited. A case of high interest is whey; the liquid effluent from the cheese production.Nonprocessed whey causes an environmental problem due to its high biochemical (BOD) and chemical oxygen demand (COD) [2]. Lactose (LA) is an important by-product of the cheese and casein manufacture, whose production and global market had been estimated to about 1.4 million tons per year [3]. The relatively low water solubility and sweetness of LA, limits its use in many applications. Another restricting factor is the inability of LA intolerant people (who have a low level of lactase enzyme in the body) to digest milk sugar limiting its use in many applications [4]. However, as a consequence of its worldwide surplus and low cost, there is a great interest in the research of innovative processes for transforming LA, expanding its applications in the food and pharmaceutical industries as value-added derivatives. Some significant developments include the production of highly valued pharmaceutical products and functional food ingredients such as lactulose, lactitol, lactobionic acid, lactosucrose, and galacto-oligosaccharides [1-4]. Obtaining higher value-added products through catalytic processes, using LA from the cheese whey as raw material, is a challenging task. By selective oxidation of LA, lactobionic acid (LBA) can be obtained (Scheme 1). This acid has antioxidant, humectant and emollient properties, so it is widely used in formulations of the pharmaceutical, cosmetic and medicinal industry [4]. In addition, it is used in solutions for the preservation of human organs intended for transplant procedures [2]. In previous works, the oxidation of LA using Au catalysts supported on Al2O3 has been studied, and high conversions and high selectivity towards the production of LBA 4 were obtained. Generally, these studies have been done by heterogeneous catalysis in liquid phase, using powder catalysts and semi-batch reactors operating under continuous stirring [5-7]. The stirrer is used to provide mechanical energy required to mix or blend the reactants in such a way that the desired reaction or transfer process occurs at the desired rate. When using a stirred tank reactor for heterogeneous catalysis, the catalyst is usually added to the reaction system in the form of fine particles or powder. However, the use of powdered catalysts in this type of reactors can produce loss of activity by active phase leaching, diffusion limitations and operational problems such as filtering and reuse [7-9]. The problems related to using a slurry catalyst can be reduced significantly if the catalyst is immobilized in structured substrates [10-12]. Fixing the catalyst to a structured substrate (either by wash-coating or impregnation) is a possible solution. Moreover, the use of a structured catalyst as stirred blades combines the advantages of a structured substrate and catalytic stirrer. As structured substrates, monoliths have a promising application for liquid phase reaction due to their high geometric surface area and high void fraction, which provides accessibility of reactants to the catalyst particles with low pressure drop [11]. Another important benefit is that the safety is improved, because in case of reaction runway fast shutting down is possible by stopping the impeller. The cordierite is the most common material for monolithic catalyst used as stirrer reactor [13- 15]. Nevertheless, metallic monoliths are very interesting catalytic devices due to the possibility of obtaining thinner walls allowing higher cell density and, thus, higher geometric surface area [16]. The decreasing catalyst layer thickness with increasing cell density proved to be beneficial for the performance of the monolithic stirrer reactor [14]. This is very important, because thin catalyst layers may prevent internal diffusion 5 limitations. Therefore, the tuning ability of the catalytic layer thickness allows designing the monolithic catalyst for optimal activity and selectivity [8,14]. Although there are several works where the viability of structured systems is studied in multiphase reactions (hydrogenation of 3-methyl-1-pentyn-3-ol [8], synthesis of propylene carbonate [16], selective oxidation of cyclohexene [17], hydrogenation of edible oil [18]), there are no papers in the open literature, which analyze the behavior of monolithic structured catalysts used as a stirrer for LA oxidation. The aim of this work is to evaluate the performance of rotating metallic monoliths in a batch reactor for the selective LA oxidation to LBA. Wash-coating process was studied to coat monoliths with nano-structured Au/Al2O3 catalyst. The effect of stirring speed, monolith channel size and catalyst coating thickness on monolithic catalyst activity is studied. In addition, the optimization of thermal regeneration and the catalyst re-usability are analyzed. 2. Experimental 2.1. Preparation of powder catalyst. Supported gold catalyst was prepared by the direct anionic exchange (DAE) method assisted by ammonia, as previously proposed by Ivanova et al. [19]. Firstly, the necessary quantity of HAuCl4 (Johnson Matthey, >99.5%) to obtain 2wt% of supported gold was dissolved in 2 L of distilled water and then heated to 70ºC. Once the final temperature was achieved, the support, a commercial γ-Al2O3 (Spheralite SCS 505, Procatalyse), was added to the solution and aged under vigorous stirring for another 20 min. Then, the suspension was cooled down and a concentrated ammonia solution, prepared with NH3 (Aldrich), was added. After 20 min, the solid was filtered, washed with distilled water, dried at 100 °C and finally calcined at 300 ºC for 4 h. 6 CAUTION NOTE: The use of ammonia can produce the gold ammonia complexes known as fulminating gold (olive green upon drying). The variation of the above mentioned procedure could be potentially dangerous. 2.2. Preparation of metallic substrates and structured catalysts. Fecralloy® monoliths (25-mm diameter and 25-mm length) were prepared with different channel size. The geometric parameters of different monoliths are summarized in Table 1. Prior to coating, metallic monoliths were thermally treated in air during 22 h at 900 ºC. Monoliths were coated by wash-coating with an aqueous slurry of powder Au/Al2O3 catalyst (20 wt%), with AL20-NYACOL® colloidal alumina (8.3 wt%) and polyvinyl alcohol (2.1 wt%) as additives. The slurry pH was adjusted at 4 to obtain a stable suspension [20]. The coating was repeated several times with a drying step (120 ºC for 30 min) between coatings to deposit 250, 500 and 1000 mg of the catalyst on the metallic monoliths. Afterward, the wash-coated monoliths were calcined between 400 and 500 °C for 2 h in air. Finally, the average thickness of the catalyst layer was estimated as δ = wcat · ρcat−1 · Sg−1, where wcat is the amount of deposited catalyst, ρcat is the catalyst coating density (0.825 g·cm–3), and Sg is the geometric surface area of the monolith, resulting an average δ ranging from 5–30 μm (Table 1). Coated samples are referred to as RX_Y_Z, where RX is the type of monolith (see Table 1); Y is the nominal catalyst load in mg (250, 500 or 1000); and Z is the catalyst coating thickness (m). 2.3. Characterization techniques. 7 The zeta potential curve was determined with a ZETASIZER model NANO-Z from MALVERN INSTRUMENTS LTD. The powder catalyst (≈20 mg) was ultrasonically dispersed in an aqueous solution of 0.001 M NaCl and the pH was varied between 2 and 12 with nitric acid or ammonium hydroxide. A laser diffraction particle sizing technique (MALVERN MASTERSIZER 2000) was used to measure the powder catalyst particle distribution. 100 mg of the powder catalyst were dispersed in 20 mL distilled H2O and the pH adjusted to 4, and promptly submitted to ultrasounds for 2 h to avoid undesired agglomerations. The viscosity of the catalyst slurry was measured in a rotational rheometer (TA INSTRUMENTS AR 1500 EX). The adherence of the catalyst layer deposited onto metallic monoliths was evaluated introducing the monoliths in vials with petroleum ether, sonicating for 30 min [21]. Excess of petroleum ether was removed by drying (100 °C for 1 h) and then calcining in air (450 °C for 2 h). Adherence of catalyst coating is expressed as the percentage of the catalytic loading that remains after ultrasonic test. The textural properties of samples (powder and structured catalyst) were determined by N2 physisorption at 77 K with a Micromeritics ASAP 2020. Prior to analysis, samples were degassed at 180 °C under a vacuum of 1.33 Pa for 2 h. Metal loading of samples was determined by inductively coupled plasma (ICP) using a Perkin Elmer OPTIMA 2100 equipment. Electron microscopy observations were carried out at the LMA-INA-UNIZAR facilities using a Tecnai G2-F30 Field Emission Gun microscope with high angle annular dark field scanning transmission electron microscopy detector (HAADF-STEM) allowing a 0.2 nm point-to-point resolution and 0.1 line resolution operated at 300 kV. HAADF-STEM enables to acquire images with atomic number contrast for high scattering angles of the electrons (Z-contrast). Sample powder was dispersed in water by 8 ultrasonic bath, and a drop of this suspension was put over a copper grid coated with carbon film, and allowed to dry in air. 2.4. Catalytic tests. Lactose (LA) oxidation in aqueous phase was carried out at 65 °C and atmospheric pressure in a thermostated glass reactor of 750 mL, equipped with a mechanical stirring system. The monoliths were attached by screws to the stirring system, what allows an easy disassembled. The objective is that the monoliths works as stirrer paddles in order to improve the contact between the reactive medium and the catalyst active phase. For regeneration, after a reaction cycle, monoliths were taken apart by unscrewing the mechanical attachment and then placed in a muffle furnace at the corresponding temperature, depending on the case. The initial LA (Anedra, 99%) concentration (C0) was 0.014 M for all experiments and the catalyst mass to LA initial amount ratio (w/n0) was 30 or 95 g·mol-1. Once the reaction temperature was reached, purified air (21% v/v O2) was bubbled through the reaction mixture at a constant flow rate. The pH was adjusted at 9.0, which was already reported in previous works as the optimum pH for this reaction [5,6], using a 0.5 N NaOH (Cicarelli, 98%) aqueous solution. During the reaction, the pH value was kept constant using an automatic control system. Oxidation experiments were done varying the stirring rate between 200 and 700 rpm. Samples of reaction mixture were taken at regular intervals and analyzed using a Shimadzu HPLC equipment with refraction index detector. The chromatographic separation of LA and LBA was performed in a Phenomenex (Phenosphere 5 micras NH2, 80A 250mm, x 4.60 mm). A solution of acetonitrile and sodium phosphate buffer (50 mM, pH=5), both HPLC grade, were used as eluents. Only LA and LBA were detected and quantified in all of the experiments. 3. Results and discussion. 9 3.1. Monolithic catalyst preparation. Wash-coating process by catalyst slurry is an excellent method to prepare structured catalyst [10]. First, the zeta potential was measured as a function of pH to indentify the pH region for optimum dispersion of the Au/Al2O3 powder catalyst (Figure 1). The isoelectric point of the suspension occurred at pH 8.2. Zeta potential values higher than 30 mV were observed under acidic conditions (pH < 5), which is indicative of good particle dispersion [22]. Therefore, pH = 4 was selected for the catalyst slurry preparation. Another important parameter to increase of the slurry stability is the average particle size. In the literature, it was recommended that the average particle size should be below 10μm [22]. In this regard, a dry-milling during 30 min was carried out to obtain the adequate particle size of Al2O3. Several preliminary tests of the slurry formulation for wash-coating the metallic structured substrates were carried out following an experimental procedure. The coating characteristics (specific load, homogeneity and adhesion) were analyzed to choose the best recipe. In this sense, the selection of binder and stabilizer is crucial for the washcoating of the structured substrates. It is well known that long-chain surfactants containing hydrophilic and hydrophobic groups, such as PVA, adsorb on the catalyst surface leading to steric stabilization of the slurry [23], They are also able to reduce surface tension during drying process avoiding coating cracks and detachments [24]. Furthermore, binders like inorganic oxide colloid (Al2O3, CeO2, ZnO, SiO2) are used as thickeners to stabilize the slurry and also to promote the adherence between the catalyst and the substrate surface [25]. The additives and the solid content affect the viscosity of the catalyst slurry and this is probably the main parameter controlling the washcoating [ref 17]. Low viscosity values ensure easy coating and homogeneity but give rise to extremely low loads, and then numerous repeated coatings are required to reach the 16 rates when thickness is less than or equal to 15 µm. Instead, for thicknesses greater than 15 μm (28 μm), the reaction rate values at 600 rpm is markedly higher than at 450 rpm. These results confirm the presence of diffusional limitations when the layer thickness is higher than 15 μm. The thickness of the coating layers decreases with increasing cell density. Hoek et al. [14] observed that the activity per reactor volume clearly increases with increasing cell density. The higher specific surface area produce thinner catalyst coating and thus the internal mass transfer of reactive is enhanced. For foam stirrer reactor, an activity drop was observed with coatings thicker than 5 m [8,29]. With this type of reactors, it is possible to enhance the gas-liquid mass transfer. However, the more restricted thickness limits and the low specific surface area of foam reactors respect to monolithics ones (1205 m2·m-3 against 3433 m2.m-3 for R4 configuration, see Table 1) do not allows the optimisation on the catalyst amount. Therefore, it can be said that catalyst coating thickness is a critical point in liquid phase reaction. The results obtained in this work show that there is a limit to the amount of catalyst that can be coated to the monoliths. The amount of catalyst can be tuned by changing the number, length and diameter, and cell density of the monoliths. However, Hoek et al. [14] observed that there is a length limitation. The activity per reactor volume increases with the length of the monolithic structures due to the increase of the total amount of catalyst. Nevertheless, if the activity is normalised by surface area, the activity decreases with increasing length of the monolithic structures, becoming constant at a length of 0.03 m. The explanation of this result is that liquid entrance effect is beneficial for the external mass transfer, being more important in shorter monoliths [14,16]. Therefore, the best way to improve the structured catalyst activity in rotating stirrer reactor is to use metallic 17 monolith with high geometric surface area to obtain thin catalyst coating without diffusion limitations. Conclusions The nano-structure Au/Al2O3 monolithic catalysts obtained in this work, incorporated to the stirrer in a batch reactor, were active, stable and selective in the oxidation of lactose to lactobionic acid in aqueous phase. Thermal treatment in air at 400 ºC, carried out during regeneration-reaction cycles, conduct to a stable metallic phase in which particle size is between 3 and 10 nm. Regeneration at 500 ºC produces a selective sintering of Au metallic particles what slightly decrease the catalyst activity. The monolith cell density and the thickness of the catalyst layer influence the lactose oxidation rate in liquid phase due to external and/or internal mass diffusional restrictions. The reaction conditions and catalyst characteristics that allow working in chemical control were determined: stirring rates higher than or equal to 600 rpm and catalyst thickness less than or equal to 15 m. The catalyst thickness that can be obtained in monoliths is higher than those than can be reached on foams, what allows an optimization of the final catalyst load. Acknowledgments. We thank the Universidad Nacional delLitoral (UNL), Consejo Nacional de InvestigacionesCientíficas y Técnicas (CONICET) and Agencia Nacional de PromociónCientífica y Tecnológica (ANPCyT) from Argentina, the Basque Government (IT1069-16) and the Spanish MINECO/FEDER (ENE2015-66975-C3-3-R and CTQ2015-73901-JIN) for the financial support of this work. 18 19 References [1] A.S. Matharu, E.M. de Melo, J.A. Houghton, Opportunity for high value-added chemicals from food supply chain wastes, Biores Technol 215 (2016) 123-129. [2] R.E. Shepherd, Y. Issacson, L. Chensny, S. Zhang, R. Kortes, K. 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Derksen, H.E.A. van den Akker, Turbulent flow in a stirred tank with permeable impeller blades, Proceeding of FEDSM 2002 Montreal, Quebec, Canada. [29] M.A. Leon, R. Tschentscher, T.A. Nihjuis, J. van der Schaaf, J.C. Schouten, Rotating foam stirrer reactor: Effect of catalyst coating characteristics on reactor performance, Ind. Eng. Chem. 50 (2011) 3184-3193. 21 Table 1. Geometric parameters of the different Fecralloy® metallic structured substrates (monoliths) employed. Monolith type R1 R2 R3 R4 Geometric surface (cm2) 940 825 540 424 Specific surface area (m2·m-3) 8640 6965 4312 3433 Hydraulic diameter (μm) 361 475 827 1065 Porosity (%) 78 83 89 91 cpc (cells·cm-2) 363 214 72 45 22 Table 2. Textural properties of powder and coating catalysts. Sample Catalyst coating thickness (µm) SBET (m2·g-1) Vpore (cm3·g-1) Dpore (nm) Parent catalyst - 242 0.44 7.3 Slurry catalyst - 229 0.43 7.5 R4_250_7 7 227 0.42 7.5 R4_1000_28 28 229 0.43 7.5 23 Captions to Figures Scheme 1: Oxidation of lactose (LA) to lactobionic acid (LBA) Figure 1. Evolution of electrical Z-potential as a function of the Au/Al2O3 slurry pH. Figure 2. Amount of catalyst loaded over the different metallic structured substrates (R1 to R4) as a function of the number of coatings. Figure 3. Optic image of different metallic structured substrate configurations (monoliths) coated with ~500 mg of catalyst per monolith. Figure 4. Initial oxidation reaction rate of lactose (r0) in liquid phase as a function of calcination-regeneration temperature: ■ calcinated monolith (FA), ■ first regeneration (I- R), ■ second regeneration (II-R). Reaction conditions: 65 ºC, 450 rpm, w/n0= 95 g·mol- 1, pH= 9.0. Figure 5. TEM micrographs of nanostructured Au/Al2O3 deposited on metallic monoliths (A and B), and metallic particle size distribution histograms (C and D) after three cycles of reaction at 65 °C and regeneration at 400 °C. Figure 6. TEM micrographs of nanostructured Au/Al2O3 deposited on metallic monoliths (A and B) and metallic particle size distribution histograms (C and D) after three cycles of reaction at 65 °C and regeneration at 500 °C. 24 Figure 7. Lactose conversion as a function of time corresponding to three different pairs of monoliths subjected to cycles of reaction (65 ºC, 600 rpm, 0.011 M, 750 mL) and regeneration (400 ºC in air for 2 h). □ after first regeneration (I-R), ○ after second regeneration (II-R), Δ after third regeneration (III-R). Figure 8. Influence of stirring rate on initial lactose reaction rate corresponding to monolith configurations: □ R1_500_6, ○ R2_500_7, Δ R3_500_11 and R4_500_15. Reaction conditions: 65 ºC, w/n0 = 95 g·mol-1, pH = 9.0. Figure 9. Effect of catalyst layer thickness on initial lactose reaction rate. (A) Reaction conditions: 65 ºC, w/n0 = 30 and 95 g·mol-1, 600 rpm. (B) Reaction conditions: 65 ºC, w/n0 = 30 g·mol-1, ■ 450 and ■ 600 rpm. 25 Scheme 1 32 Figure 7 050 100 0.00 0.25 0.50 0.75 1.00 LA Conversion I-R II-R III-R Pair 1 050 100 Pair 2 Time (min) 050 100 Pair 3 33 Figure 8 150 300 450 600 750 0.03 0.06 0.09 0.12 0.15 r0 (mmol.g-1.min-1) rpm (min-1) 34 Figure 9 10 20 30 0.00 0.04 0.08 0.12 0.16 r0 (mmol·g-1·min-1) thickness (m) (A) 815 28 (B)