Preparation of ceramic and metallic monoliths coated with cryptomelane as catalysts for VOC abatement

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Preparation of ceramic and metallic monoliths coated with cryptomelane as catalysts for VOC abatement ⁎

Diogo F.M. Santosa, Olívia S.G.P. Soaresa, José L. Figueiredoa, Oihane Sanzb, , Mario Montesb, ⁎ Manuel Fernando R. Pereiraa, a b

Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and Materials (LSRE-LCM), Faculty of Engineering, University of Porto, Porto, Portugal Department of Applied Chemistry, University of the Basque Country, San Sebastián, Spain

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

manganese oxides • Cryptomelane-type were prepared by a solvent-free method.

and metallic monoliths were • Ceramic washcoated with cryptomelane. catalytic activity of the struc• High tured materials was observed. conductivity was the key • Thermal factor to understand the results. different supports did not sig• The nificantly change the activity of the catalyst.

A R T I C LE I N FO

A B S T R A C T

Keywords: Manganese oxide VOC oxidation Cordierite monolith FeCrAlloy monolith

Ceramic and metallic monoliths coated with cryptomelane-type manganese oxides were prepared and tested as catalysts for the oxidation of ethyl acetate. Several preparation conditions were optimized in order to enhance the activity of the monoliths. Monoliths with different cell sizes, prepared with the same number of immersions in the washcoating solution or with similar loadings were tested in order to evaluate the effect of this parameter on the adherence of the active phase and on the catalytic activity of the final monolith. Several characterization techniques were used to assess the properties of these monoliths and it was found that almost all properties of the active phase were maintained after the washcoating process. The thermal conductivity of the structured supports must be taken into account in order to interpret the catalytic results in highly endothermic or highly exothermic reactions. The location of the measurement thermocouple is decisive when analyzing the results since it can give rise to apparent differences in activity. In our specific case, the available data suggest that the specific activity of the catalyst is not significantly altered by the deposition process or by the properties of the supports used.

1. Introduction Recently, the control and prevention of air pollution is beginning to

⁎

gain importance, with several European Directives appearing to enforce the monitoring and control of air quality. Volatile organic compounds (VOCs) are a group of air pollutants with major contributions to several

Corresponding authors. E-mail addresses: [email protected] (O. Sanz), [email protected] (M.F.R. Pereira).

https://doi.org/10.1016/j.cej.2019.122923 Received 4 June 2019; Received in revised form 27 August 2019; Accepted 20 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Diogo F.M. Santos, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.122923

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environmental problems, which impact not only the environment, but also human health [1–3]. As such, these compounds need to be removed from gaseous effluents before being released to the atmosphere. Several technologies can be employed to remove VOCs from gaseous effluents [4,5]. Catalytic oxidation is a very efficient VOC removal technology, allowing for their conversion into CO2 and water at relatively mild temperatures [6,7]. Noble metals (mainly platinum and palladium) supported on metal oxides are the most common catalysts due to their high catalytic activity for the oxidation of several VOCs [8–12]. Nevertheless, their commercial applicability is limited due to their high cost [13]. More affordable materials, such as transition metal oxides, started to be studied as catalysts, and high activities have already being achieved [14–17]. Cryptomelane-type manganese oxides were found to present high activity in the oxidation of several VOCs [18–20]. The activity is normally ascribed to the mixed valence state of manganese and the high mobility of the lattice oxygen, favoring the Mars-van Krevelen mechanism [21,22]. Peluso et al. [19] compared the activity of cryptomelane with other manganese-based catalysts (Mn2O3 and β-MnO2) in the oxidation of ethanol. Cryptomelane presented much higher activity than the other manganese-based catalysts. The higher activity was ascribed to the mixed valence and low crystallinity of the cryptomelane catalyst, opposed to the single valence and defined crystalline structure of the other materials. Soares et al. [23] also assessed the activity of several manganese oxides, prepared by different synthesis methods, for the oxidation of ethyl and butyl acetate. Cryptomelane-type manganese oxide prepared by solid state reaction was the most active. The differences in the performance of the materials were related to the average oxidation state, ability to donate lattice oxygen, reducibility of the material and available surface area. For the efficient application of these materials in real scale, the catalyst should be prepared on a macro-structured support, like monoliths, to allow the treatment of high gas volumes with high flow rates at low pressure drop [24,25]. Ceramic monoliths (mostly made of cordierite) are the most utilized for environmental applications; however, metallic monoliths can also be used for low temperature reactions like VOC oxidation [26]. Ceramic monoliths are cheaper than metallic ones and generally are able to achieve higher loadings of active phase. However, metallic monoliths can be made with smaller wall thickness (allowing for lower pressure drop) and present higher thermal conductivity, facilitating heat transfer and achieving a more homogeneous thermal profile [27–29]. To obtain a suitable washcoating of the monolith wall, the rheological properties of the catalyst containing slurry need to be studied and optimized. The particle size of the active phase [30], as well as the pH [31], the viscosity [32] and the solid content [33] of the slurry strongly affect the quality of the coating obtained, and thus the catalytic activity. Aguero et al. [33] studied the effect of the solid content (MnOx) on the quality of the monolith coating and the resulting catalytic activity. They found that not only a higher loading but also a more homogeneous and stable coating are necessary to reach high catalytic activities. In this work, ceramic and metallic monoliths washcoated with cryptomelane-type manganese oxide catalyst were prepared. The cryptomelane catalyst was prepared by a solvent-free method [23], which is known to produce cryptomelane with high catalytic activity and stability for VOC oxidation [34]. The main goal of this work was to produce highly active monolithic catalysts using cryptomelane as active phase for the oxidation of ethyl acetate. For this purpose, several preparation conditions (size of the monolith cells, initial coating with alumina, solid content of the slurry and number of immersions) were optimized.

Fig. 1. Metallic (FeCrAlloy®) and ceramic (cordierite) monoliths, before and after washcoating.

2. Experimental 2.1. Catalyst preparation 2.1.1. Powder catalysts Cryptomelane-type manganese oxide catalyst was synthesized by a solvent free method [23,34]. Mn(CH3COO)2·4H2O and KMnO4, were mixed with a molar ratio of 3:2, and milled in a Retsch® PM 100 without any gas flow, during 1 h with a rotation speed of 450 rpm. The direction of the rotation changed every 2 min. The resulting solid was kept at 80 °C for 4 h and then washed with distilled water until near neutral pH. The solid was then dried at 120 °C overnight and calcined at 450 °C for 4.5 h in air. For the preparation of the coating solution, the material was not calcined. 2.1.2. Monolithic catalysts Monoliths of two different materials were prepared, ceramic (cordierite) and metallic (FeCrAlloy®), both cylindrical with a diameter of 2 cm and a length of 4.3 cm (Fig. 1). Ceramic monoliths with 400 cpsi (cells per square inch) were bought (Corning GmbH) and cut into the specified shape. The monoliths were heated to 900 °C in air to remove possible contaminants. The metallic monoliths were prepared by rolling flat and corrugated sheets alternatively around a spindle, producing monoliths with sinusoidal shaped channels. A calcination was then performed at 900 °C in air for 22 h in order to form non-porous alumina whiskers [24,32], allowing a better interaction between the surface of the monolith and the washcoat. Metallic monoliths with 3 different cpsi were prepared: 289, 465 and 1330. The pre-treated monoliths were then washcoated by immersion in a catalyst slurry containing the powder cryptomelane catalyst, colloidal alumina (NYACOL, AL20) and distilled water. Different weight ratios and pH values were used in order to achieve a stable slurry. The monoliths were immersed in the slurry for 1 min at an immersion/ withdrawing velocity of 3 cm min−1. The monoliths were then centrifuged at 300 rpm for 1 min to remove the excess slurry inside the cells. In the case of the monoliths with higher cpsi (1330) blowing with compressed air after centrifugation was also necessary to remove the excess slurry. The washcoated monoliths were dried in air at 120 °C for 1 h to solidify the catalyst layer. Several immersion/drying steps were performed to achieve the desired catalyst loading. Afterwards, the monoliths were calcined at 450 °C in air for 2 h. Before coating with the catalyst, some monoliths received a primer 2

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highest layer thickness. The morphology of the monoliths was studied by field emission scanning electron microscopy (FEG-ESEM) on a FEI Quanta 400FEG/ EDAX Genesis X4 M with a Schottky Emitter at an accelerating voltage of 10 kV.

with alumina. This was performed by immersing the monoliths in a 1:4 NYACOL AL20: water (v:v) solution under the same conditions as before (immersion/withdrawing velocity, duration, removal of excess liquid and drying). The monoliths were then calcined at 500 °C for 2 h with a heating rate of 2 °C min−1. Washcoating with the catalyst slurry was then performed as in the case of the monoliths without alumina coating. The adherence of the catalyst coating was evaluated by ultra-sonication tests. The calcined monoliths were immersed in petroleum ether and sonicated for 30 min. The materials were then dried at 120 °C for 1 h and calcined at 450 °C for 2 h in air. For characterisation purposes, the catalyst slurry was dried and calcined in the same conditions as the monoliths.

2.3. Experimental procedure The catalytic oxidation of ethyl acetate was performed in a continuous flow reactor placed in a temperature-controlled electric furnace. The monolith was placed inside a quartz cell on top of a small amount of quartz wool to prevent eventual clogging of the reactor. The external part of the monoliths was also surrounded by quartz wool to ensure that all the reaction gas goes through the monoliths. Before reaction, the monolith was pre-treated in air at 400 °C for 1 h. A feed gas with an ethyl acetate concentration of 1000 ppmv was used in the catalytic tests. The experiments were performed in 3 steps: i) heating the reactor from ambient temperature to 400 °C: ii) an isothermal step at 400 °C for 1 h; iii) cooling the reactor until 50 °C. The heating and cooling rates used were 2.5 °C min−1. The temperature was measured in the external surface of the monolith at mid height. The composition of the gas phase was analysed on-line with a CO2 detector (Vaisala CARBOCAP® Carbon Dioxide Transmitter Series GMT220) and a gas chromatograph (DANI Master Fast Gas Chromatograph with a Cp-Wax-52CB column). The conversion of ethyl F acetate to CO2 ( XCO2 ) was calculated as XCO2 = (4 × FCO2 ) , where FCO2 is VOC , in the molar flow rate of CO2 at the outlet, FVOC , in is the molar flow rate of the VOC at the inlet, and 4 is the number of carbons in ethyl acetate. Two cycles of heating and cooling were performed to assess the reproducibility of the results and stability of the material. The values presented in the results (temperatures needed for 50 or 90% removal of the VOC or conversion into CO2) are from the cooling step to avoid coupling between the adsorption-desorption phenomena observed during the heating step [35]. For all catalysts tested, a maximum difference of 4 °C was observed between cycles and between different experiments with the same catalyst. During the reaction, the pressure drop in the reactor was measured by a manometer that allows the detection of an increase in pressure of 0.01 bar. No increase in the pressure was detected during the reactions over the structured supports, and a maximum of 0.6 bar was recorded in the experiments over the powdered catalyst.

2.2. Catalyst characterization Several techniques were used to characterize the materials prepared, as well as the slurries used. The particle size of powder cryptomelane was measured using a MasterSizer 2000 from Malvern Instrument Limited. Three measurements were performed to ensure the reproducibility of the analysis. The dX value was calculated by the Malvern Instrument software. This value represents the diameter below which x% of the population lies. The zeta potential was studied using a MALVERN Zetasizer 2000 Instrument. To plot the zeta potential curve as a function of the pH, 6 solutions containing 20 mg of the powder catalyst and 50 mL of 1.0 mM NaCl were prepared. Their pH was adjusted to different values (between 2 and 12) using nitric acid or ammonium hydroxide. Slurry viscosity was determined at 25 °C using an AR 1500ex rheometer from TA Instruments with a rotor HA AL Recessed (diameter of 28 mm, length of 42 mm). Approximately 7 mL of the sample were subjected to an increasing shear rate between 3 and 3715 s−1. The viscosity values presented were taken at a shear rate of 100 s−1. X-ray diffraction (XRD) patterns of the powdered cryptomelane and powdered catalyst slurry were obtained with a Bruker Instruments (Model D8 Advance 500) and a Philips X’Pert MPD diffractometer with copper Kα radiation (40 kV, 30 mA) over a 2θ range of 5° −85° with a step size of 0.05° and a step time of 5 s. The textural properties of both powder and monolithic materials were studied by N2 adsorption isotherms measured at −196 °C with an ASAP 2020 instrument from Micromeritics. Before the analysis, the samples were degassed at 150 °C for 5 h. The surface area (SBET) was determined by the BET (Brunauer-Emmett-Teller) method. The microporous volume was determined by the t-method. The total pore volume (Vp) was determined from the amount of nitrogen adsorbed at P/ P0 = 0.95. The pore size distribution and average pore diameter (dP) were analysed by the BJH (Barrett-Joyner-Halenda) method from the desorption branch of the isotherm. The reducibility of the cryptomelane was measured by temperature programmed reduction (TPR) in a Micromeritics AutoChem II. The TPR analysis was performed with a 5% H2/Ar reducing gas mixture (70 cm3 min−1) in a temperature range of 35–600 °C with a heating rate of 10 °C min−1. In the case of the monoliths, the thermocouple was placed in the centre of the material. The loading of cryptomelane adhered to the monoliths was calculated by the difference in weight between the coated and bare monolith, multiplied by the percentage of cryptomelane in the coating slurry. The density of the washcoat was determined in a Micromeritics AutoPore IV. The average catalyst layer thickness (δ ) was then calculated as δ = ω/ ρω × GSA, where ω is the catalyst loading (g), ρω is the washcoat density (g cm−3), and GSA is the geometric surface area of the monoliths (cm2). It must be noted that the determined value of the average catalyst layer thickness may not be entirely correct, due to the nature of the characterization technique; however, the relationship between the values of the different materials should be correct, i.e., the material with the highest calculated value should, in fact, have the

3. Results and discussion 3.1. Optimization of the catalyst slurry In order to prepare a slurry that will enable the formation of a homogeneous and well adhered coating on the monolith walls, several properties have to be studied, such as the particle size, the zeta potential and the viscosity. Fig. 2 presents the particle size distribution measured for the powdered cryptomelane at pH 4. The material presents a low amount of small particles, a d10 of 3.9 µm being observed. However, the majority of the particles present have significantly higher diameter, as observed by the d90 of 15.5 µm. Nevertheless, the preparation of a stable suspension should be possible. The zeta potential was also measured, in order to find the pH range in which the particles will not tend to agglomerate. It is known that to avoid agglomeration, allowing for a stable solution, an absolute value above 20 mV is normally needed [36]. Fig. 3 shows the plot of zeta potential as a function of the pH for a solution with 20 wt% solids with a cryptomelane/Nyacol solution ratio of 2.5. An isoelectric point of 8.5 was observed. At pH values of 6 and below, the zeta potential is slightly above 30 mV which should be sufficient to allow a good repulsion of the particles, contributing to the good stability of the slurry. As such, a pH of 6 was chosen as optimal for the slurry. 3

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the slurry almost solidifying after 38 h in stirring. For the other suspensions, only small variations in the viscosity were detected. As such, a solid content of 18.0 wt% was chosen as the optimum value, allowing for a more stable suspension and the adherence of a significant amount of material. For all suspensions prepared the amount of material adhered to the monolith decreased during the first few coatings, especially for the two higher solid contents. This was ascribed to the sedimentation of the material at the bottom of the suspension decreasing the actual solid content in the slurry during the immersion step. Higher than expected particle size (d90 of 55 µm with 18.0 wt% solid content) after overnight stirring appeared to be the cause. To avoid this problem, the cryptomelane powder was milled at 450 rpm for 30 min, decreasing the particles size in the slurry, d90 of 35 µm. This resulted in an increase in the slurry viscosity to 7.5 mPa.s, which is in a range often considered as optimum for the coating process [32]. This increase in the viscosity allowed a larger and more uniform deposition of material, around 70 mg per coating, without compromising the homogeneity of the catalytic layer. As such, the optimized solution was prepared with a solid content of 18.0 wt% and a cryptomelane/nyacol solution ratio of 2.5, using the powdered cryptomelane catalyst milled at 450 rpm for 30 min.

Fig. 2. Particle size distribution of the powdered cryptomelane.

3.2. Monolith washcoating After the optimization of the slurry, several monoliths were prepared. In order to study the influence of the monolith material and cpsi on the loading achieved, several monoliths were coated using 4 immersions in the optimized catalyst slurry. The monoliths were identified as CM (ceramic monolith) or MM (metallic monolith) followed by the cpsi of the monolith, the indication of an initial coating with colloidal alumina (A) or not (WA), and finally the number of immersions on the catalytic slurry. For example, the monolith CM_400_A_4 is a ceramic monolith with 400 cpsi, where an initial coating with colloidal alumina was performed, which was immersed in the catalyst slurry four times. For all monoliths prepared, weight losses between 0.5 and 4.5% of the total coating weight were observed after ultrasound test (higher for metallic monoliths with lower cpsi), which shows excellent adherence of the coating (95.5 and 99.5%). It is notorious (Table 2) that the presence of an initial alumina layer had a positive effect in the preparation of the ceramic monoliths, allowing for higher specific loadings (mass of cryptomelane per geometric surface area). A better interaction between the active phase and the support may be responsible for this result. A similar behaviour had already been identified in a previous work [34]. However, the opposite effect was observed for the metallic monoliths, where the alumina coating decreases the amount of active phase incorporated into the monolith. In this case, the alumina coating may be covering the whiskers formed during the pre-treatment at 900 °C [24], decreasing the roughness, and consequently the amount adhered. Barbero et al. [28] tested the effect of pre-coating a FeCrAlloy® monolith with an alumina

Fig. 3. Zeta potential of the powdered cryptomelane catalyst as a function of the pH. Table 1 Variation of the viscosity and mass adhered per coating with the solids content. wt% solids

Viscosity (mPa.s)

Mass 1st coating (mg)

12.0 17.0 17.5 18.0 18.5 20.0

1.8 3.5 3.3 3.9 48.7 70.6

13 21 22 42 132 156

Afterwards, several solutions were prepared with different solid contents, but always maintaining a cryptomelane/Nyacol solution ratio of 2.5. Table 1 shows the solutions prepared, their viscosity and the mass adhered to the monolith in the 1st coating. For these tests, FeCrAlloy® monoliths with 289 cpsi, a length of 3 cm and a diameter of 1.6 cm were used. As expected, the viscosity of the slurry increases with the solid content. The same behaviour is observed with the mass adhered to the monolith. When a low solid content is used, 18.0 wt% or lower, the mass adhered is very small, but high homogeneity is observed. On the contrary, for solid contents of 18.5 wt% and higher a large deposit of active phase is observed; however, the solution is much less stable, with fast sedimentation of cryptomelane during the coating process, affecting the quality of the coating. The value for the viscosity presented was obtained after stirring overnight (14 h); however, for the two higher solid contents, the viscosity increased rapidly with time, with

Table 2 Loadings and average layer thicknesses of the different monoliths prepared with 4 coatings in the optimized slurry.

4

Monolith

Loading (mg)

wt% of active phase

Specific loading (mg cm−2)

Average layer thickness (µm)

CM_400_A_4 CM_400_WA_4 MM_1330_A_4 MM_1330_WA_4 MM_465_A_4 MM_465_WA_4 MM_289_A_4 MM_289_WA_4

549 494 729 827 433 453 265 320

8.2 7.1 3.8 4.2 3.9 4.2 3.5 4.1

1.4 1.2 0.7 0.8 0.7 0.8 0.7 0.8

18 15 10 11 10 11 8 10

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layer prior to the coating with a MnCu catalyst, and observed that this layer decreased both the mass of active phase adhered and the catalytic activity. This effect was ascribed to a loss in surface roughness with the alumina coating. Comparing the ceramic monoliths with the metallic ones exhibiting similar cpsi (400 and 465, respectively), higher amount of active phase was adhered to the ceramic monolith, with a specific loading of 1.4 mg cm−2 for the CM_400_A_4, almost twice the value obtained with MM_465_WA_4. This was expected since it is generally accepted that coating ceramic monoliths with an inorganic material is easier than coating metallic monoliths [24]. Due to the ceramic porosity, the liquid can penetrate inside the pores, with the catalyst particles being deposited onto the walls (“slip coating” or “filter-cake formation”) [37]. The average layer thickness is also higher for the ceramic monolith. The total amount of active phase incorporated on the metallic monoliths was higher for higher cpsi values. This is due to the higher geometric surface area, providing more surface for the cryptomelane to adhere. Nevertheless, both weight percent as well as specific loading are relatively constant, around 4 wt% and 0.8 mg cm−2, for all metallic monoliths without initial alumina layer. The deposition of cryptomelane per area of metallic monolith was not influenced by the cpsi (cell size). With the optimized slurry and the washcoating conditions used, quite reproducible values were obtained with the different metallic monoliths. In order to study the influence of the monolith material and cpsi on the catalytic activity, different types of monoliths were prepared with similar mass of cryptomelane (350 ± 30 mg). Since the initial alumina layer only improved the adherence for the ceramic monolith, this coating was not performed on the metallic monoliths. The number of coatings needed to achieve the specified load was higher for the metallic monoliths with lower cpsi, due to their lower geometric surface area. This resulted in an increase in the specific loading and average layer thickness with decreasing cpsi (Table 3). The ceramic monolith only needed two coatings to achieve the same loading value even with only 400 cpsi, showing once again the higher adherence of cryptomelane to the ceramic support.

Table 4 Textural properties of the indicated samples. Sample

SBET (m2 g−1)

VP (cm3 g−1)

dP (nm)

Powdered cryptomelane Optimized slurry MC_400_A_4 MM_465_WA_4

157 141 160 163

0.47 0.47 0.48 0.48

9.3 8.9 9.2 9.3

Fig. 4. XRD patterns of the cryptomelane powder and optimized slurry.

3.3. Characterization 3.3.1. Textural properties The textural properties of the materials were analysed by N2 adsorption isotherms. Table 4 summarizes the results of this analysis. The powdered catalyst presented a surface area of 157 m2 g−1, significantly higher than what is normally observed for cryptomelane catalysts prepared by the most conventional method, the reflux method, where values between 25 and 85 m2 g−1 are normally reported [20,31,38–40]. The high surface area of cryptomelane prepared by ball milling had already been shown in a previous study [23]. The material was mainly mesoporous, with a micropore volume lower than 2% of the total pore volume. The optimized slurry presented similar values to the powdered catalyst showing that the preparation of the slurry did not significantly change the textural properties of the material. The monoliths also retained the textural properties of the powdered cryptomelane. Table 4 shows an example for a ceramic and a metallic

Fig. 5. TPR profiles of the cryptomelane powder, optimized slurry and typical ceramic and metallic monoliths.

monolith, but all (160 ± 10 m2 g−1).

Loading (mg)

wt% of active phase

Specific loading (mg cm−2)

Average layer thickness (µm)

CM_400_A_2 MM_1330_WA_2 MM_465_WA_3 MM_289_WA_4

333 374 329 320

5.4 1.9 3.0 4.1

0.8 0.4 0.6 0.8

11 5 8 10

tested

presented

similar

values

3.3.2. Phase purity The XRD pattern of the powdered catalyst showed all the main peaks of cryptomelane (Fig. 4) with no impurities detected. However, for the optimized solution, an extra peak at 2θ of 47° was observed, which corresponds to a Mn2O3 impurity. The presence of colloidal alumina during the calcination step seems to be responsible for the appearance of this impurity since the peak does not appear in the spectra of the non-calcined slurry (not shown). For both samples, the relative intensities were slightly different from the standard pattern of pure cryptomelane (KMn8O16, JCPDS 421348) most likely due to defects in the lattice, different amounts of potassium cations or preferential ordering of the crystals [41].

Table 3 Loadings and average layer thicknesses of the different monoliths prepared with similar loading. Monolith

monoliths

3.3.3. Reducibility The TPR profile of the powdered cryptomelane (Fig. 5) presented a large peak at 300 °C and a shoulder at low temperatures due to highly reactive oxygen species on the surface of the powder. The profile of the 5

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Fig. 6. Light-off curves obtained with the monoliths prepared with similar amounts of cryptomelane. A) Ethyl acetate oxidation; B) Formation of acetaldehyde; C) Conversion of ethyl acetate into CO2; D) Conversion of ethyl acetate into CO2 over the ceramic monolith during the heating and cooling step.

(16 W m−1 °C−1 for FeCrAlloy® [42] compared to 3 W m−1 °C−1 for cordierite [43]), for the same temperature measured by the equipment (in the centre of the monolith) the periphery of the monoliths will be at different temperatures (significantly higher for the ceramic monolith). This originated the differences in the temperatures of maximum hydrogen consumption between the two monoliths.

Table 5 Catalytic results (tempertures, in °C, for 50 and 90% conversion) with monoliths prepared with similar amount of cryptomelane. Base catalyst

X EtAc

CM_400_A_2 MM_1330_WA_2 MM_465_WA_3 MM_289_WA_4

X CO2

T50

T90

T50

T90

154 155 171 166

185 255 217 200

188 197 201 192

227 275 255 237

3.4. Catalytic activity tests The prepared monoliths were tested in the oxidation of ethyl acetate using a concentration of 1000 ppmv and a space velocity of 3000 h−1. An experiment using a ceramic monolith without active phase was also performed, achieving less than 10% conversion into CO2 at 400 °C. Near complete conversion of ethyl acetate into CO2 was achieved by all monoliths at temperatures lower than 300 °C. Nevertheless, acetaldehyde was identified as by-product of the reaction. The formation of acetaldehyde presented a peak between 160 and 190 °C, depending on the monolith, with the ceramic presenting the largest formation of this by-product. For example, when 90% oxidation of ethyl acetate (X EtAc – T90) was achieved, acetaldehyde selectivities between 12 and 49% were observed, with the ceramic monolith presenting the highest value, and the metallic monolith with lower cpsi presenting the lowest. This indicates that ethyl acetate is first oxidized to acetaldehyde and then further oxidized to CO2. Since acetaldehyde is harder to oxidize than ethyl acetate, higher temperatures are needed to achieve complete conversion into CO2 than complete removal of ethyl acetate. The formation of acetaldehyde in the oxidation of ethyl acetate over manganese oxide catalysts has already been reported [34,44]. Comparing the activity of the monoliths with that of the powdered optimized slurry in the same flow conditions and using the same amount of catalyst, it was observed that the powdered slurry presented higher activity. For example, a temperature of 185 °C was needed to achieve 90% conversion of ethyl acetate over the ceramic monolith, while the powdered slurry only needed 166 °C to achieve the same conversion. This could be related to differences in the thermal

Table 6 Calculated thermal conductivities for the different supports. Thermal conductivities −1

−1

Ke,a (W m °C ) Ke,r (W m−1 °C−1)

MM_1330

MM_465

MM_289

CM_400

2.79 1.59

1.72 0.94

1.37 0.73

0.20 0.11

optimized slurry was slightly shifted to higher temperatures, near 310 °C. A secondary peak at 400 °C was also observed. This should be related to the reduction of Mn3+ to Mn2+ [31], which suggests the presence of Mn2O3, as seen by XRD analysis. The TPR spectra of the monoliths also presented the same two peaks observed for the slurry sample; however, they were less well defined. This was likely due to the fact that, during the analysis, there was a gap between the TPR cell and the monolith which allowed part of the reduction gas to pass through without contacting with the monolith. Nevertheless, the hydrogen consumption for the slurry sample and the monoliths was very similar, 230 and 210 cm3 g−1, respectively. Comparing the two monoliths, the metallic one was shifted to higher temperatures. This displacement of the peaks for the ceramic and metallic monoliths can be explained by the different thermal conductivities of the two materials. In this analysis, the temperature was measured in the centre of the monoliths. Since the metal has a much higher thermal conductivity than cordierite 6

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Fig. 7. SEM micrographs of: A) MM_1330_WA_2; B) MM_289_WA_4.

and because the conductivity of the monoliths varies inversely with the void fraction of these, the higher the cell density, the higher the thermal conductivity, both axial and radial [29,46]. The thermal conductivities of the different supports were calculated using a method developed by Groppi et al. [47]. This method was developed to calculate the thermal conductivities of monoliths with square cells, which is not the case for the metallic monoliths; however, it allows to determine a satisfactory relationship between the catalysts. The calculated values are presented in Table 6, showing the impact of the cell density and the support material in the thermal conductivity. On the other hand, it must be taken into account that the combustion reaction of ethyl acetate is highly exothermic, and it can be calculated that, under the reaction conditions used, the adiabatic temperature increase is around 70 °C. Consequently, important temperature gradients will be produced within the monolith. These gradients will increase logically with conversion and will depend fundamentally on the monolith thermal conductivity. In a previous work, we observed experimentally that the average temperature depends on the thermal conductivity of the structured substrate and this behavior was confirmed by CFD modelling [29]. Consequently, the monolith overall conversion will be larger than that which would correspond to the outer point of the monolith where the temperature is monitored. When observing the ignition curves, we appreciate that at low temperature, the conversion differences between the samples are small but increase with temperature. In the end, the monoliths are ordered according to their conductivities. Thus, the most active is apparently the ceramic one and then the metallic ones in increasing order of conductivity. Another possible explanation of the differences in activity between the samples that must be analyzed is the modification of the catalyst during the deposition or by interaction with the structured substrate.

Table 7 Catalytic results (tempertures, in °C, for 50 and 90% conversion) with the monoliths prepared with 4 immersions in the catalytic slurry. Base catalyst

CM_400_A_4 MM_1330_WA_4 MM_465_WA_4 MM_289_WA_4

Loading (mg)

549 827 453 320

X EtAc

X CO2

T50

T90

T50

T90

145 174 158 166

178 303 192 200

188 217 193 192

236 303 240 237

conductivities between the two types of material, and also due to the better gas/solid contact with the powdered material in the conventional fixed bed, allowing for lower external mass transfer limitations. Fig. 6 D presents a comparison between the CO2 formation in the heating and the cooling steps over the ceramic monolith. It is notorious that the amount of CO2 formed in the heating step is higher than the theoretical limit. This increase in the CO2 formed during the heating step is due to the conversion of the VOC and surface intermediates adsorbed on the catalyst surface in addition to the gaseous VOC [35]. On the other hand, the monoliths prepared with similar amounts of cryptomelane presented significant differences in their catalytic activity (Table 5). The ceramic monolith presented the highest activity achieving 90% conversion to CO2 at 227 °C, between 10 and 77 °C less than the metallic monoliths. Moreover, comparing the different metallic monoliths, the catalytic activity decreased as the cell density increased (decreasing cell sizes). These results could be related to the thermal behavior of these monoliths, due to their different thermal conductivities. Ceramic monoliths present lower thermal conductivity than metallic ones [45],

Fig. 8. Light-off curves obtained with the monoliths prepared with 4 immersions in the catalyst slurry. A) Ethyl acetate oxidation; B) Conversion into CO2. 7

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are presented in Table 7 and Fig. 8. It can be observed that, for the monolith with the highest number of cells, the higher catalyst loading led to an increase of the catalytic activity. However, this is only true until CO2 conversions below 20%. Above 130 °C, the behavior of MM_1330_WA_4 is strange, and 100% conversion is not reached suggesting some by-pass. It is possible that the temperature increase during reaction and the consequent thermal expansion of the metallic monoliths, in particular for the monolith with the smallest cell sizes and the highest catalyst loading, may originate preferential pathways (due to the partial blocking of several channels) resulting in less exposed catalysts and, therefore, significantly lower activity, as observed. The catalytic activity of the monoliths prepared with 4 coatings was also assessed using the same flow rates (500 cm3 min−1), resulting in different space velocities in the monoliths (between 2500 and 3000 h−1, higher for monoliths with higher cpsi). As expected, lower space velocities (higher contact times) resulted in higher activities. However, this improvement was only notorious for the oxidation of ethyl acetate, around 15 °C less for MM_289_WA_4 with a space velocity of 2500 h−1 compared to 3000 h−1, for both T50 and T90 values. For the conversion into CO2, a maximum improvement of 4 °C was observed. For the ceramic monolith, no difference in the conversion of ethyl acetate into CO2 was found when decreasing the space velocity from 3000 to 2650 h−1.

These modifications can be both in their physical form and in their chemical nature. The negative effect of a heterogeneous distribution of the catalyst on the activity, and in particular the presence of large agglomerates, has been reported. Aguero et al. showed that large agglomerates that reached almost 1 mm produced a loss of activity that could be explained by diffusion limitations in the pores [33]. In our case, SEM images (Fig. 7) show certain differences in homogeneity. The monolith with the smaller cell density (MM_289_WA_4) presented a more homogeneous distribution than that with higher cell density (MM_1330_WA_2). The latter presents larger agglomerates and cracks along the catalytic layer. However, the size of the agglomerates does not reach 50 μm in any case, which would hardly explain diffusional limitations in their pores. The cracks could have a negative effect on the adhesion, but the values measured in the ultrasound test showed excellent adhesion values, greater than 95% in all the samples. Moreover, the cracks formed could improve the diffusion of reactants and products as observed in Fischer-Tropsch synthesis, where diffusion limitations are very important [48]. Regarding changes in the chemical nature of the active phase, it has been also reported that the activity of the catalyst can be affected by the interaction of the catalyst with the surface of the substrate [49–51]. However, analyzing the low conversion zone in Fig. 6 (where thermal effects are not important) no significant difference in activity between monoliths can be observed. This suggests that the specific activity of the catalyst does not change when it is deposited on these structural materials. Aguero et al. [33] tested the oxidation of ethanol, ethyl acetate and toluene (initial concentration of 4000 ppm of carbon) over MnOx supported on a metallic monolith. The monolith was prepared by washcoating with a solution containing the MnOx supported in Al2O3 in powdered form. The authors optimized the preparation of the monolith in terms of the presence/absence of alumina and solid content (30 and 40 wt%) in the washcoating solution and the number of immersions. The incorporation of Mn in the monolith was also tested, by preparing 2 other monoliths where the powdered MnOx/Al2O3 in the washcoating solution was replaced by a suspension of Al2O3 in manganese acetate solution. For the washcoating solution prepared with the powdered catalyst, increasing the solid content increased the activity of the catalysts for the oxidation of all VOCs tested when 1 immersion was performed. However, for 2 immersions the same did not happen. The activity decreased by increasing the solid loading. The authors mentioned the low adherence of the coating and low homogeneity for the higher solid content, as the reason for this result. The presence of alumina in the coating solution did not influence the activity. The preparation method with Al2O3 suspended in a solution of manganese acetate presented the best results, especially when a solid content of 40 wt% was used. The most active catalyst achieved 50% conversion of ethyl acetate at 230 °C. Frías et al. [50] prepared metallic monoliths incorporated with OMS-type manganese oxides and tested them in the oxidation of ethyl acetate (initial concentration of approximately 2000 ppm of carbon). Two different monoliths (stainless steel AISI 304 and FeCrAlloy) and two different incorporation methods (washcoating with the slurry from reflux preparation method and MnOx growth in situ by reflux method) were tested. All monoliths presented a similar amount of catalyst loading and low weight loss. The washcoating method presented a higher surface area than the in situ method for both types of monoliths. The catalyst prepared by washcoating over the stainless steel monolith presented the highest activity in the conversion of ethyl acetate, achieving 50% conversion of this VOC at 161 °C. The authors suggested that this result was due to the formation of mixed oxides favored by the presence of Cr, Mn and Fe oxides in the surface of the oxidized stainless steel. Monoliths prepared with 4 immersions in the catalyst slurry (different amounts of catalyst loaded) were also tested in the oxidation of ethyl acetate under the same conditions as before. The results obtained

4. Conclusions Cryptomelane-type manganese oxide catalysts were successfully washcoated onto ceramic (cordierite) and metallic (FeCrAlloy®) monoliths. The adherence of the coating was high, with less than 5% weight loss after ultrasound test. Most of the powdered cryptomelane properties were maintained during the monolith preparation process, the only significant difference being the presence of Mn2O3 in the monolith, which seems to be due to the calcination step in contact with colloidal alumina. The thermal conductivity of the structured supports must be taken into account in order to interpret the catalytic results in reactions with high enthalpy changes. The location of the measurement thermocouple is decisive when analyzing the results, since it can give rise to apparent differences in activity. In the case of the catalysts prepared and evaluated in this work, the differences in the activities between materials seem to be mostly related to the differences in the thermal conductivities of the supports. Changes in the deposition due to the different materials or different cell sizes are noticeable but not significant enough to explain the observed results. Acknowledgments This work is a result of: Project “AIProcMat@N2020 – Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, with the reference NORTE-01-0145-FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF); Associate Laboratory LSRE-LCM – UID/EQU/50020/2019 – funded by national funds through FCT/MCTES (PIDDAC). The authors acknowledge the Basque Government (IT1069-16) and the Spanish MINECO/FEDER (ENE201566975-C3-3-R, RTI2018-096294-B-C32 and CTQ2015-73901-JIN) for financial support. Diogo F.M. Santos acknowledges grant received from FCT (PD/BD/ 105983/2014). The authors also acknowledge Dr. Carlos M. Sá (CEMUP) for assistance with SEM analyses. References [1] W. Tang, X. Wu, S. Li, W. Li, Y. Chen, Porous Mn–Co mixed oxide nanorod as a novel catalyst with enhanced catalytic activity for removal of VOCs, Catal. Commun. 56 (2014) 134–138.

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