Selective lactose oxidation in liquid phase on Au/Al2O3 metallic monolith catalysts

REGENHARDT, SILVINA; IVANOVA, S.; MONTES, M.; SANZ, O.; ODRIOSOLA, J. A.; MARCHI, ALBERTO; MEYER, CAMILO; CENTENO, M. A.; GARETTO, TERESITA

San Sebastián

Congreso; 5th International Conference on Stuctured Catalysts and Reactors (ICOSCAR5); 2016

Universidad del País Vasco - Universidad de Sevilla - Universidad Pública de Navarra

Lactose (LA) is a major component of whey, the main by-product in cheese production in dairy industry. Cheese whey typically contains about 5 wt. % lactose, which can be converted to lactobiónico acid (LBA) by selective oxidation. This acid has antioxidant properties and its largest commercial use is as an important constituent of solutions to preserve human organs during transplantation procedures. LBA is also employed as acidulant, complexing agent and antioxidant in food and pharmaceuticals industries. Nowadays, LBA is produced by microbiological oxidation of LA. The main disadvantages of this process are the long times required to achieve high levels of conversion and the formation of hydrogen peroxide as a byproduct. Recently, there has been increasing interest in the LBA production by heterogeneous catalytic oxidation of LA. An active, selective and stable metal for LA oxidation to LBA in aqueous solution is Au, usually supported on SiO2, Al2O3 or carbon powders. In this work, we used for the first time an Au/Al2O3 monolithic catalyst to perform this reaction. Homemade parallel channel monoliths (25 x 25 mm) consisting of 50 μm Fecralloy® sheets (Goodfellow) corrugated using rollers producing 287cpsi were fabricated. The catalyst (2 % Au-Al2O3 prepared by direct anionic exchange method assisted with ammonia) was deposited on themonoliths by the washcoating method. Catalytic experiments were performed in a thermostated (65°C) stirrer reactor (1000 mL). In all of the experiments, the initial LA concentration was 0.013 M. The pH was kept constant at 9, while O2 was fed by air bubbling. The contact between reactants and catalyst surface was enhanced by incorporating the monoliths to the stirrer configuration. Samples at different reaction times were taken and analyzed by HPLC. The influence of the stirring speed on the catalyst activity was studied. When the stirring rate was raised from 200 to 430 rpm, the LA conversion also increased, and a 100% conversion level was attained at 430 rpm in 2 h. A further increment of the agitation did not result in a higher reaction rate because the system became unstable over 600 rpm. As a consequence, the conversion level attained at 620 rpm was rather lower than at 430 rpm. The initial reaction rate clearly goes through a maximum at 430 rpm. In consequence, this stirring speed was adopted to carry out all of the following experiments.In order to analyze the effect of the activation temperature on the catalytic activity, samples of fresh monolith catalyst were treated in air between 350 and 500ºC, during 2 h. After each catalytic experiment using activated fresh catalyst (AF), a first regeneration, at the corresponding activation temperature, was carried out (IR) and a second catalytic run was performed. Afterwards, a second regeneration (IIR) was implemented in the same conditions and a third catalytic run was done. The lowest initial catalytic activity was reached with the AF sample activated at 350 ºC, but amazingly the activity increased noticeably after the first regeneration at the same temperature. The increment in activity was less important between the IR and IIR samples. These results may be explained assuming that is necessary an activation time longer than 2 h in order to obtain the maximum activity of the metallic Au phase at 350 ºC. When the activation of fresh samples and the successive regenerations were carried out at 400, 450 and 500 ºC, the activity of the AF sample was quite similar to that of the IR and IIR samples. Furthermore, no leaching of metal Au phase was detected. These results indicate that: 1) an stable active phase can be reached after 2 h only when the activation temperature is at least 400 ºC; 2) it is feasible the reuse of this monolithic catalyst in the aqueous-phase oxidation of LA into LBA; 3) the optimum temperature for the activation and regeneration of the catalyst is 400 ºC. At this temperature, both the initial reaction rate and the time to obtain a conversion level of 50% were the optimum. When activation is carried out over 400 ºC, it is likely that the metallic Au phase suffers an irreversible change that affects negatively the catalytic activity.