CET Volume 86 DOI: 10.3303/CET2186113 Paper Received: 26 August 2020; Revised: 19 February 2021; Accepted: 4 April 2021 Please cite this article as: Car F., Susec I., Tomasic V., 2021, Preparation and Testing of Cordierite Monolithic Catalysts for Oxidation of Aromatic Volatile Organic Compounds, Chemical Engineering Transactions, 86, 673-678 DOI:10.3303/CET2186113 CHEMICAL ENGINEERING TRANSACTIONS VOL. 86, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-84-6; ISSN 2283-9216 Preparation and Testing of Cordierite Monolithic Catalysts For Oxidation of Aromatic Volatile Organic Compounds Filip Car*, Ivan Sušec, Vesna Tomašić University of Zagreb, Faculty of Chemical Engineering and Technology, Marulićev trg 19, HR-10 000 Zagreb fcar@fkit.hr This work reports the results of catalytic oxidation of aromatic volatile organic compounds (VOCs). A gaseous mixture of benzene, toluene, ethylbenzene, and o-xylene (BTEX) in nitrogen was used as representative of VOCs. Reactions were carried out in a monolithic reactor at different temperatures, with a constant initial concentration of reactants and a constant ratio of BTEX and oxidant (synthetic air). Analysis of the oxidation products was carried out on-line before and after reaction using gas chromatography. The work involves the preparation of catalysts and their application on an inert cordierite monolithic carrier, using different combinations of mixed manganese oxides (manganese with copper, iron, and nickel) and palladium as a representative of noble metals. The prepared cordierite monolithic catalysts showed great mechanical stability and high performance regarding oxidation of the mixture of aromatic compounds. High conversion values (90 %) of all components of the mixture were achieved at temperatures below 200 °C, depending on the chemical composition of the catalytic layer. A comparison with the activity of a commercial monolithic catalyst (Purelyst PH-304), which contains Pt and Pd as catalytically active components, using toluene as a model component. A comparison of the prepared manganese oxides showed that MnFeOx and MnCuOx had the best catalytic activity. When Pd was used in combination with MnFeOx in a form of a single monolith, where Pd was applied to ¼ of the overall monolith length and MnFeOx to ¾ of overall length, achieved 90 % conversions (T90) were up to 20 °C lower when compared with the same MnFeOx monolithic catalysts without Pd. Based on the comparison of the prepared ceramic monolithic catalysts with a commercial catalyst and because the comparison was made only based on toluene conversion, it was concluded that the prepared catalysts can be considered as alternatives to commercial monolithic catalysts by introducing a low concentration of Pd. 1. Introduction Volatile organic compounds (VOC) due to their use in industry (oil refineries and petrochemical plants), transport and households, are present in an aquatic environment, soil and atmosphere. Although biogenic sources are a large source of volatile organic compounds in the atmosphere, the impact of emissions from human activities is much more significant. This is a major environmental and sociological problem today, as it not only leads to many detrimental impacts on human health and living standards but also disrupts the state of ecosystems and causes global climate change. There are many exhausts and waste gas after-treatment technologies. One of the most suitable processes concerning environmental and economic criteria is catalytic oxidation. Given the great interest in expanding existing areas of application of monoliths, great attention of researchers is focused on improving existing and developing new procedures for the preparation of monolithic catalysts. In catalytic reaction engineering, the monolith represents a structure with well-defined and constant geometry, which most often serves as a carrier of catalytic material, through whose channels reactants and products are transferred by convection. There are also so-called integral versions of monoliths that do not contain an inert carrier but consist entirely of a catalytically active component or mixtures of active components. The geometry of the channels determines the total active surface area and therefore affects the mass and heat transfer processes and thus the catalytic efficiency. 673 There is no ideal channel geometry, but it depends on the specific problem and the chemical reaction. The first step in the preparation of monolithic catalysts usually involves a selection of basic structural material, so- called carrier (e.g. cordierite). Sometimes it is necessary to apply a second layer (support) between the carrier and catalyst layer which improves adhesion and application properties of monolithic catalysts (most often Al2O3 and zeolites). It may itself contain catalytically active components. The third (outer) layer contains catalytically active components (e.g. noble metals and transition metal oxides) that participate in chemical reactions and have the greatest impact on obtained conversions. This work aims to prepare ceramic monolithic catalysts for catalytic oxidation of aromatic compounds using mixed oxides of transition metals (manganese, copper, iron, and nickel) and a noble metal (palladium) as catalytically active components. In this study, a mixture of benzene, toluene, ethylbenzene, and o-xylene (BTEX) was chosen as a representative of volatile organic compounds. The reactions were carried out in a monolithic reactor. The main advantages of such catalytic reactor design are a small pressure drop, ease of construction, and specificity of the reactor design itself which ensures favorable hydrodynamic operating conditions and transfer to a larger scale (scale-up). The work includes devising a monolithic catalyst, which includes the preparation of mixed metal oxides and the development of a method for applying such a catalytic layer to an inert ceramic monolithic carrier. Testing of catalytic properties of the monolithic catalyst thus prepared was carried out at different temperatures, with a constant initial concentration of reactants (BTEX), a constant ratio of reactants, and oxidant (synthetic air) with an approximately constant mass/thickness of the catalyst layer. 2. Experimental part 2.1. Characterization of catalytically active components Characterization of used mixed manganese oxides was performed by Duplancic et al (2020). It included differential scanning calorimetry, nitrogen adsorption-desorption analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy, temperature-programmed reduction of hydrogen, X-ray diffraction, and X-ray photoelectron spectroscopy. 2.2. Preparation of monolithic catalysts Monolithic catalysts were prepared by applying catalytically active components to the walls of an inert monolithic cordierite structure using the impregnation method. Two types of square-shaped monoliths were used that differed in channel size. Monolith channel dimensions were 1 mm x 1 mm for smaller channels (SCC) and 2 mm x 2 mm for larger channels (LCC). The length of the monolithic catalysts was 40 mm. When the combination of transition metal oxides and palladium oxide was used, the length of the monolith containing manganese oxides was 30 mm (¾ of overall length) while the length of the monolith containing palladium oxide was 10 mm (¼ of overall length). After the cordierite substrate was cut to the desired shape and size suitable for experimental measurements, it was washed in ethanol to remove impurities, and then dried at 120 °C for 1 hour. For applying mixed metals on the cordierite carriers 1 M water solutions (initial solutions) of manganese (II) nitrate tetrahydrate, copper (II) nitrate trihydrate, iron (III) nitrate nonahydrate, and nickel (II) nitrate hexahydrate were prepared. To compare the efficiency of mixed metal oxides with noble metals, monolithic catalysts containing palladium were prepared using palladium (II) nitrate hydrate (0.1 M). Solutions for impregnation were prepared by mixing initial solutions in volume ratio 1:1. When the combination of all transition metals was user volume ratio was Mn: Fe: Cu: Ni = 3: 1: 1: 1. Carriers were submerged in impregnation solutions for 30 minutes. This was followed by drying at 120 °C for 1 h and then calcination at 500 °C for 2 h. 2.3. Adhesion test Testing of the mechanical stability of the catalytic layer (adhesion test) was performed by exposing the prepared monolithic catalysts to ultrasonic vibrations and measuring the weights before and after the procedure, similar to how it is shown by Barbero et al. and Wu et al. An Elmasonic S 30 H ultrasonic bath with an ultrasonic frequency of 37 kHz and a working volume of 1.9 L was used. The monolithic catalyst was immersed in petroleum ether and then exposed to ultrasonic vibrations for 30 minutes at room temperature. Drying was then carried out at 120 °C for 1 h after which monoliths were cooled to room temperature and weighted. 2.4. Catalytic oxidation of BTEX compounds Catalytic oxidation of BTEX was tested at atmospheric pressure, at different temperatures, and with a constant total flow of the reaction mixture (92 cm 3min-1). During the experiment, the reaction mixture was passed from bottom to top of the reactor, and the monolithic catalyst was placed between two layers of quartz wool. 674 The initial concentration of benzene, toluene, ethylbenzene, and o-xylene was 52.1, 52.4, 49.9, and 55.4 ppm, respectively in nitrogen. The temperature inside the reactor was controlled using a thermocouple located in the central part of the reactor above the catalyst. Analysis of the reaction mixture at the reactor outlet was performed by on-line gas chromatography. The course of the reaction was monitored by determining the total conversion of BTEX components after reaching steady-state conditions. The economic goal was to achieve as high conversions as possible at the lowest possible temperatures, and therefore the possibility to catalytically oxidize volatile organic compounds. 3. Results and discussion 3.1. Mechanical stability of prepared monolithic catalysts The results of performed adhesion test are summarized in Table 1. As can be seen, the adhesion of the catalytic layer is excellent due to the mass loss ranging between 0 and 0.018 %. Such a low amount of lost catalyst suggests great mechanical stability of the prepared monolithic catalysts which is one of the key properties that monoliths must have before further tests. Significant loss of catalyst would suggest that catalyst and carrier are not compatible and that the application of support layer, a different combination of carrier and catalytically active components, or using additives for enhancing adhesion could be mandatory. Table 1: Mechanical stability of monolithic catalysts Catalyst Mass [g] Mass loss [%] Before ultrasound After ultrasound MnFeOx 0.6621 0.6220 0.016 MnCuOx 0.5609 0.5608 0.018 MnNiOx 0.6095 0.6094 0.016 MnFeCuNiOx 0.5994 0.5994 / 3.2. Catalytic oxidation of BTEX The results of the measurements are showed in Figures 1 and 2 and summarized in Table 2. Figure 1. Comparison of tested SCC monolithic catalysts with mixed manganese oxides and palladium 0 10 20 30 40 50 60 70 80 90 100 120 130 140 150 160 170 180 190 200 X A [% ] T [°C] MnFeOx MnCuOx MnNiOx MnFeCuNiOx 675 Based on the measurements made, it can be seen that the curves showing the dependence of the conversion on temperature took on a characteristic S-shape. Such curves are common for the oxidation of related compounds, such as CO and various hydrocarbons, and are also known as self-ignition or self-heating curves. Figure 2. Comparison of conversions achieved on tested LCC monolithic catalysts In the preliminary part of the study, the possibility of homogeneous oxidation in the gas phase at different temperatures without the presence of catalytically active components was investigated. For this purpose, measurements were performed in an empty reactor and measurements with inert monolithic support (without a previously applied catalytic layer). Common parameters that indicate the efficiency of catalytic systems are the temperatures at which 50 % (T50) and 90 % (T90) conversion is achieved. When comparing catalysts on that basis, lower values of T50 and T90 indicate a catalyst with better efficiency. Table 2. Temperatures at which T90 and T50 were reached for all BTEX components Catalyst T90 [°C] T50 [°C] B T E o-X B T E o-X Empty reactor >400 347 333 312 395 305 279 271 Carrier without catalyst 398 342 336 334 332 308 286 280 SCC MnFeOx 184 174 169 169 173 165 158 158 LCC MnFeOx 190 180 178 178 178 173 168 168 SCC MnCuOx 179 170 168 169 174 165 161 161 LCC MnCuOx 190 178 171 170 178 168 162 162 SCC MnNiOx 192 185 179 179 183 175 171 171 LCC MnNiOx 196 188 183 184 186 177 174 174 SCC MnFeCuNiOx 180 178 172 172 175 168 164 164 LCC MnFeCuNiOx 190 180 177 176 180 172 164 164 SCC MnFeOx/Pd 182 166 164 165 161 156 156 156 LCC MnFeOx/Pd 182 160 160 160 159 155 155 155 LCC Purelyst PH-304 - 128 - - - 119 - - 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 100 120 140 160 180 200 220 X A [% ] T [°C] Purelyst PH-304 MnFeOx MnCuOx MnNiOx 676 From the obtained results it can be seen that complete homogeneous oxidation of the mixture of benzene, toluene, ethylbenzene, and o-xylene occurs only at temperatures higher than 300 °C. In the case of oxidation in an empty reactor, complete conversion of only o-xylene was achieved at a temperature of 350 °C, while at the same temperature in the reactor with inert monolithic support, complete conversion of ethylbenzene, o- xylene, and toluene were achieved. That is expected because the reaction mixture stays at operating temperature in the reactor for a longer time when passing through a filled reactor. The lowest conversions were obtained for benzene, which can be attributed to its stable aromatic structure. The maximum conversion of BTEX components is achieved at temperatures up to 200 °C. In both types of ceramic monolithic catalysts concerning the size of the channels, it was observed that the best catalyst was the one with a catalytically active layer containing a combination of Pd and transition metals (¼Pd and ¾MnFeOx), and the one with the highest T90 values with the active layer of MnNiOx. Of the two-component and three-component mixed metal oxides that did not contain Pd, MnFeOx and MnCuOx proved to be the best, followed by MnFeCuNiOx and MnNiOx. In line with expectations, monoliths with smaller channels, regardless of the chemical composition of the catalytic layer, showed better results, i.e. lower temperature values at which 50 % and 90 % conversion are achieved. This can logically be explained by the larger outer or geometric surface area of the monolithic support, which results in a larger specific surface area of the catalytic layer available for the oxidation of the model components. It can be seen that the effect of a larger surface area in monoliths with smaller channels was not significant, from which it follows that the oxidation of BTEX components is probably very fast. When results thus obtained are compared with the same catalysts in form of powders and immobilized on a metallic carrier, conversions are similar. As shown in Duplančić et al. were MnNiOx in a form of powder had higher conversions when compared with the same catalyst that was applied on a surface of a metallic monolith, but when the conversions were expressed per unit mass of catalyst it was shown that the monolith had 9 times higher conversions using 20 times smaller amount of catalyst. 4. Conclusions The catalytic oxidation of BTEX mixture on ceramic monolithic catalysts with two characteristic channel sizes and different composition of the catalytically active layer was investigated. Mixed oxides of manganese with copper, iron, and nickel, and palladium were used as catalytically active components. Ceramic monolithic catalysts have been successfully prepared, and it has been confirmed that the catalytic layer shows great mechanical stability. Complete conversion of all components of the BTEX mixture on all ceramic monolithic catalysts was achieved at a temperature of 200 °C or less. Due to the larger total surface area on ceramic monolithic catalysts with smaller channels (SCC), complete conversions of the BTEX mixture were achieved at lower temperatures in comparison with monolithic catalysts with larger channels (LCC). A combination of manganese and copper, followed by manganese and iron, proved to be the most efficient for the oxidation of BTEX. Monolith containing a mixed metal oxide of manganese and iron (¾ monolith) with palladium (¼ monolith) improved the catalyst activity, reducing T90 for all components of the BTEX mixture compared to a catalyst to which only mixed metal oxide of manganese and iron was applied. Future work will include further improvement of activities of the tested metal oxides and proposing of kinetic and reactor models with varying degrees of complexity using the basic methodology of chemical engineering. Acknowledgment This work has been supported by Croatian Science Foundation under the project IN-PhotoCat (IP-2018-01 8669). 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