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Study of BTEX oxidation using 3D printed ceramic monolithic catalysts

The term monolith in the technical sense refers to structures with well-defined and invariable geometry. Monoliths are mainly used as an inert substrate (carrier) for various catalytically active components that are subsequently deposited on their surface, but there are also numerous examples where monoliths are made directly from catalytically active material [1]. In the field of chemical engineering, the possibility of using additive manufacturing (AM) technology, i.e. 3D printing, as an advanced method of fabricating monolithic catalysts/reactors is being increasingly explored, especially at laboratory scale. The aim of this work was to prepare 3D printed monolithic ceramic catalysts for the catalytic oxidation of benzene, toluene, ethylbenzene, and o-xylene (BTEX). The first step was the 3D printing of ceramic catalyst carriers using stereolithography (SLA). Autodesk Fusion 360 was used to create a 3D model of the catalyst carrier, PreForm was used as a slicer to prepare the models for 3D printing, and the 3D printer used was Form 2 (Formlabs). The dimensions of the monolith were 4 cm in length and 7 mm in diameter to fit in the reactor system and for comparison with cordierite monoliths (with the same dimensions) used in our previous work [2]. A commercial photopolymer resin with ceramic particles, Ceramic resin (Formlabs), was used to fabricate the monoliths. After 3D printing, the monoliths were washed in isopropyl alcohol and heat-treated according to the manufacturer's instructions [3]. In our previous work, we investigated the preparation of ceramic catalyst carriers with the Ceramic resin and characterized them [4]. Most commercially available monolithic cordierite catalysts contain various noble metals (mostly Pt, Pd, and Rh) as catalytically active components deposited on their surface. Although noble metals have great catalytic activity, they are very expensive and prone to catalyst poisoning. Various compounds were investigated as potential replacements, and mixed manganese oxides were found to be a good substitute due to their low cost and resistance to poisoning, although their catalytic activity is not as high as that of noble metals. In this work, catalytically active components (MnCuOx oxides) were applied to the surface of the heat-treated monolith using the impregnation technique. 1M aqueous solutions of Mn(NO3)2 x 4 H2O and Cu(NO3)2 x 3H2O were used as catalyst precursors. A series of five 3D printed ceramic plates (2 cm x 1 cm x 2 mm) were used according to the same principle as 3D printing the monolith to determine the optimum impregnation time and to test the mechanical stability (adhesion) of the catalytic layer. The optimal impregnation time was determined (15 min), and the impregnated monolith was calcined at 500 °C for 2 h to fabricate a MnCuOx layer on its surface. Adhesion tests were performed using a laboratory ultrasonic bath (Elmasonic S 30 H) to investigate the stability of the catalyst layer before the monolith could be used in the reactor. The mass of the plates was weighed and compared before and after the use of ultrasound for 30 minutes at room conditions. It was found that the mass loss of the catalyst was < 2%, indicating good adhesion of the catalyst layer and that the fabricated monolith (Figure 1) can be used for catalytic oxidation of BTEX. Catalytic oxidation was carried out at atmospheric pressure, at different temperatures, and with a constant total flow rate of the reaction mixture (80 mL/min of BTEX components and 12 mL/min of synthetic air as an oxidant). As can be seen in Figure 2, the high activity of the monolithic catalyst was achieved. Benzene showed to be the most difficult BTEX component to oxidize due to its known stable structure. Benzene was completely oxidized (XA=100%) at 300 °C, toluene and o-xylene at 200 °C, and ethylbenzene at 190 °C.

Monolithic catalysts ; Catalytic oxidation ; 3D-printing ; Impregnation technique ; Adhesion

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