Development of Novel Monolithic Catalyst for BTEX Catalytic Oxidation Using 3D Printing Technology

Abstract

Four differently shaped monolithic catalyst supports were made using 3D printing technology. Two catalytically active mixed oxides, MnFeO_x and MnCuO_x, were applied to the monolithic supports using the impregnation technique. Catalysts were characterized using an adhesion test, field emission scanning electron microscopy, X-ray diffraction, and Raman spectroscopy in a manner similar to the density functional theory model. Excellent mechanical stability of the catalyst layer was obtained, with catalyst mass loss under 2% after 30 min of ultrasound exposure. SEM analysis revealed that the catalyst layer was rough but homogeneous in appearance and ~6 μm thick. The presence of double oxides—FeMnO₃ and CuMn₂O₄—as well as single oxides of Mn, Fe, and Cu was established via XRD and Raman spectroscopy. Additional theoretical calculations of Raman spectra for FeMnO₃ and CuMn₂O₄ were performed in order to aid in the interpretation of Raman spectra. The catalytic activity of the prepared catalysts for the catalytic oxidation of a gaseous mixture of benzene, toluene, ethylbenzene, and o-xylene (BTEX) was investigated. The monolithic support with the most complex shape and, consequently, the greatest surface area proved to enable the highest efficiency, while both catalysts performed well having similar conversions.

1. Introduction

Air pollution is a serious problem due to its harmful effects on human health and well-being, as well as numerous undesirable consequences for the environment. Due to rapid industrial development and anthropogenic activities, volatile organic compounds (VOCs) are released into the environment in significant quantities. VOCs are organic compounds whose boiling point is less than or equal to 250 °C, measured at a standard pressure of 101.3 kPa [1,2]. They remain in the atmosphere for several hours to months, spreading rapidly over long distances during this time.

Aromatic hydrocarbons, such as benzene (C₆H₆), toluene (C₆H₅CH₃), ethylbenzene (C₆H₅CH₂CH₃), and xylene (C₆H₄(CH₃)₂), or BTEX for short, account for 20–40% of total VOC emissions [3]. Although these compounds also occur naturally, the main sources of BTEX are anthropogenic, and the largest emissions are from motor vehicle exhausts [2,3,4]. These BTEX compounds have adverse effects on human health and the environment, and the effects themselves depend on several factors such as the amount, duration of exposure, and the type of BTEX compound. Acute and prolonged exposure can cause neurological and respiratory problems as well as damage to the central nervous system [5,6,7]. There are a number of technologies and processes aimed at reducing or mitigating the emission of VOCs to the atmosphere, with catalytic oxidation proving to be one of the most efficient and economically acceptable processes [8,9,10,11]. Numerous research efforts have been devoted to various types of catalysts for the catalytic oxidation of the aromatic VOCs, in particular noble metal-based catalysts, non-noble metal oxide catalysts, perovskites, spinels, dual-functional adsorbent-catalysts, etc. BTEX are very stable compounds due to the presence of a stable benzene ring and a corresponding substituted group on benzene (methyl or ethyl). The order in which BTEX are oxidized is usually determined by the strength of the weakest C-H bond. Depending on catalyst type and operating conditions, a possible pathway for toluene, xylene, and ethylbenzene oxidation likely involves the break-off of the methyl or ethyl groups on the aromatic compound to produce benzene and some intermediates, which are further oxidized during the ring-opening process and mineralized into H₂O and CO₂ as final products of total oxidation. It is also important to note that the oxidation of BTEX mixtures may differ from the oxidation of a single BTEX compound due to the various affinities of the aromatic compounds to the catalyst surface, resulting in variations in oxidation efficiency and product formation. Although extensive research has been carried out on treating gaseous BTEX in recent years, little attention has been paid to the development of monolithic catalysts, which are generally used to remove VOCs and other gaseous pollutants from exhaust gases. Monolithic catalysts used for catalytic oxidation are usually produced by traditional extrusion methods. Recently, the process intensification methodology has been used to develop more advanced techniques for the fabrication of structured catalysts. In this context, 3D printing technology is utilized to manufacture products with complex geometries that are very difficult or impossible to produce using traditional technologies [12,13,14,15]. 3D printing technology is based on manufacturing by deposition of the selected material layer by layer. It enables rapid and precise fabrication of complex-shaped catalysts and chemical reactors, including monolithic structures that can be used for the catalytic oxidation process. Over the last 40 years, this process has been intensively developed and researched; today, numerous polymer [16,17], metal [18,19], composite [20,21,22], and ceramic materials can be used to fabricate monoliths [23,24].

In addition to fabricating a monolithic catalyst support, the development of a monolithic catalyst includes the deposition of catalytically active components on the support, testing the mechanical stability of the catalyst layer, and testing the activity of the catalyst under specific operating conditions. Kim et al. [25] and Liotta et al. [26] showed numerous examples of the use of noble metals (Pt, Pd, Rh, Ag, Ru, and Au) as catalytically active components for the catalytic oxidation of VOCs. Santos et al. [27] used Pt, Pd, Ir, Rh, and Au on TiO₂ (Degussa P 25) as a support for the oxidation of CO, ethanol, and toluene. The incipient wet impregnation technique (IMP)—a similar technique to the one used in this work—and the liquid phase reduction deposition (LPRD) technique were both used to deposit the catalyst layer onto the surface of the support. Based on the obtained results, they conclude that the catalytic trend in VOC oxidation did not change with respect to the preparation method chosen. The Pt/TiO₂ was found to be the most active catalyst for the oxidation of VOCs, followed by Pd/TiO₂, Rh/TiO₂, and Ir/TiO₂, while Au/TiO₂ showed the lowest catalytic activity. Abbasi et al. [28] also used Pt for the oxidation of BTX (benzene, toluene, and xylene) but in the form of a nanostructured Pt/Al₂O₃-CeO₂ catalyst. The same technique as in this work (wet impregnation) was used to deposit Pt onto the support particles. The authors showed that the prepared catalyst was highly active and could completely remove toluene and xylene at 250 °C; however, only a 78% conversion of benzene was observed at the same temperature. This was mainly attributed to the greater stability of the aromatic ring of benzene. Morales-Torres et al. [29] presented a similar trend in the catalytic oxidation of BTX, where Pt and Pd were deposited onto different carbon-based supports such as aerogels, nanofibers, coatings, and structured monoliths with different porosity and surface chemistry. For all catalyst and support combinations used, complete conversion of BTX components was achieved at temperatures below 190 °C. On the other hand, Li et al. [30] used metal monoliths made of FeCrAl fibers onto which they deposited Pt by spraying nanoparticles and then dried and calcined them (5 h at 500 °C) to form a stable catalyst layer on their surface. The authors reported that the Pt loading was 0.1 wt% and the T₉₀ of all volatile organic compounds tested (toluene, hexane, isopropyl alcohol, acetone, and dimethylformamide) was reached at 300 °C or below, except for ethyl acetate, where the T₉₀ was reached at 374 °C. Based on that research, it can be concluded that noble metals are great catalysts in terms of activity; however, they also have their shortcomings, such as high cost and the possibility of poisoning, which significantly affect their use in catalytic activities.

Possible substitutes for precious metals are the oxides of transition metals (M = Mn, Fe, Cu, Ni, Co, etc.), which, although they have lower catalytic activity compared with precious metals, are much cheaper and less prone to cause poisoning [31,32,33,34]. Morales at el. [35] prepared powdered Mn-Cu oxides using the co-precipitation technique with different aging times (4, 18, and 24 h). These catalysts were used for the oxidation of ethanol and propane. The results showed that the prepared mixed oxides of Mn and Cu had higher catalytic activity than the pure Mn₂O₃ and CuO oxides. The authors found an interesting correlation between the catalyst aging time and catalyst efficiency, as the catalysts that were aged longer exhibited higher catalytic activity. This correlation was attributed to the formation of the Cu_1.5Mn_1.5O₄ species—whose formation is favored with increasing aging time—and to the reduction of the CuO species—which are less catalytically active. Duplančić et al. [36] also worked with the Mn mixed oxides (mixed oxides of Mn and Ni) in powder form and compared them with the Mn-Ni oxides deposited onto metallic catalyst supports such as the Al/Al₂O₃ plates. The prepared mixed oxides showed remarkable activity for the oxidation of toluene at low temperatures. The presented results showed that the monolithic catalyst (Al/Al₂O₃-MnNi) achieved a 13-fold higher conversion of toluene with a 20-fold lower catalyst mass compared with MnNi in the form of a powder. Conversions of over 90% were achieved at temperatures around 200 °C, which is crucial for the practical application of such catalysts. Qin et al. [37] investigated iron oxide as a promoter for the catalytic oxidation of toluene over Fe-Mn catalysts supported on γ-Al₂O₃. The authors presented the effects of Mn and Fe loading of the tested supports on toluene conversion at different reaction temperatures. They found that after 5–20% MnO₂ loading of γ-Al₂O₃, the toluene conversion was higher compared with pure MnO₂, but the toluene oxidation was not significantly promoted when the MnO₂ loading was increased above 15%. For γ-Al₂O₃ with 15% of MnO₂, a toluene conversion of greater than 92% was observed at 300 °C. When the Fe (in the form of Fe₂O₃) was introduced, it was found that the optimum Fe₂O₃ loading was 10%. With γ-Al₂O₃ loaded with 10% of Fe₂O₃ and 15% of MnO₂, a 95% conversion of toluene at 300 °C was achieved, and it was found that the addition of Fe increased the stability and moisture tolerance of the tested catalyst. Einaga et al. [38] compared mixed oxides of Mn with Cu, Co, Fe, and Ni on SiO₂ supports for the oxidation of benzene using ozone as a oxidant in a fixed-bed flow reactor. The steady state catalytic activity decreased in the following order: Mn-Cu > Mn-Fe > Mn-Ni > Mn-Co. The catalysts were prepared using the impregnation technique, and the authors tested the effects of different calcination temperatures (400–600 °C) on the catalytic activity. The authors reported that the optimum catalyst was obtained at a Mn-Cu ratio of 1:1 and a calcination temperature of 600 °C.

Based on our previous research [24,39], we designed our present study with the aim of developing a new generation of monolithic catalysts for the catalytic oxidation of BTEX compounds using stereolithography (SLA) technology to fabricate the monolithic catalyst supports. The catalytically active layer consists of mixed MnFe and MnCu oxides with a Mn to Fe or Cu ratio of 1:1. The catalytic properties were tested at different temperatures, a constant flow rate of the reaction mixture, a constant initial concentration of the BTEX mixture, and at a constant mass or layer thickness of the catalyst layer. The fabricated ceramic monolithic catalysts with different geometries and surface areas were compared with a commercial cordierite catalyst support onto which the same catalytically active components were deposited (using the same application technique).

SLA has not been sufficiently explored as one of the possible procedures within additive manufacturing technologies for the production of ceramic catalyst supports. Therefore, this work not only aims to address this shortcoming but also to contribute to the implementation of SLA as an advanced technology for the development of novel monolithic catalysts with different geometries for potential application in environmental protection.

2. Materials and Methods

2.1. Preparation of Monolithic Catalyst Supports

Monolithic catalyst supports were fabricated using SLA technology using a Form 2 (Formlabs; Somerville, MA, USA) 3D printer and Ceramic Resin [40] obtained from the same manufacturer. 3D models were designed using Autodesk Fusion 360, and 3D printing parameters were defined using a PreForm slicer (Formlabs; Somerville, MA, USA). The 3D printing parameters that need to be defined include the layer thickness of 100 µm and correction (scaling) factors to ensure that the 3D-printed monolith maintains the desired dimensions after heat treatment and sintering of the ceramic particles. These correction factors were defined as described in our previous study [24]. In the z-axis direction, the so-called Z-scale factor was defined as 1.12, while in the x- and y-axis directions, the correction factor was 1.14. After 3D printing, the monolithic catalyst supports were separated from the raft, the support structure was mechanically removed, and the channels of the supports were washed with i-propanol (Gram-Mol d.o.o; Zagreb, Croatia) to remove residual uncured resin. The monolithic structure was then heat treated according to the manufacturer’s instructions [40]. Heat treatment began with heating to a temperature of 240 °C, at which point the polymer component of the resin thermally decomposed. A sufficiently long period of time was required for the polymer to be completely removed, as the duration depends on the wall thickness of the monolithic structure. Removal of the polymer was followed by further heating to a temperature of 300 °C to ensure thermal decomposition of all remaining particles of the polymer phase, followed by heating to 1270 °C, resulting in sintering of the ceramic particles. The last step involved slow cooling to room temperature in order to avoid cracks in the structure, i.e., heat shocks.

2.2. Deposition of the Catalytically Active Components and Characterization

The catalyst supports fabricated by SLA were used as inert supports for the catalytically active components, and four different geometries were used in this work. Simpler and more complex geometries with and without an outer shell were studied and compared. The prepared monolithic structures were first washed with 95% ethanol (Gram-Mol d.o.o; Zagreb, Croatia) to remove surface impurities and then dried in a laboratory oven at 120 °C for 30 min. Three 1 M aqueous solutions of nitrate salts (catalytic precursors) were prepared for the application—using the wet impregnation method—of three catalytically active components: manganese(II) nitrate tetrahydrate (Fischer Chemical; Loughborough, LE, UK), copper(II) nitrate trihydrate (Honeywell Chemicals; Morristown, NJ, USA), and iron(III) nitrate nonahydrate (VWR Chemicals BDH, Radnor, PA, USA). The solutions of the catalytic precursors were prepared with stirring using a magnetic stirrer (IKA RCT basic; Staufen, Germany). The impregnation solutions were prepared by mixing 10 mL of an aqueous solution of Mn(NO₃)₂ × 4H₂O and 10 mL of an aqueous solution of Fe or Cu. A total of 8 different monolithic catalysts were prepared, i.e., both combinations of catalytically active components were applied to all four different support geometries. In the last step of catalyst preparation, the monolithic supports were immersed in the previously prepared solutions of the catalytic precursors. The impregnation process was carried out for 30 min, and the monoliths were then dried at 120 °C (1 h) and calcined at 500 °C (2 h). During calcination, the monolithic catalyst turns black, indicating the oxidation of the precursors and the formation of mixed metal oxides. The prepared catalysts were named MnFeO_x and MnCuO_x; a pure MnO_x catalyst was used as the control.

2.3. Testing the Mechanical Stability of the Catalyst Layer

Good mechanical stability of the catalyst layer is one of the most important properties that any monolithic catalyst must meet prior to the start of testing in the reactor system in order to ensure safe and representative performance of catalytic activity measurements. Testing of the mechanical stability of the catalyst layer was performed in an Elmasonic S 30 H ultrasonic bath (Elma, Koprivnica, Croatia) with an operating frequency of 37 kHz. The measurements were performed on a series of five plates with dimensions 2 cm × 1 cm × 2 mm, onto which MnFeO_x was deposited in the same way as in the preparation of monolithic catalysts. The test was performed by immersing each plate in a bottle of petroleum ether and sonicating it with ultrasound (US) for 30 min at room temperature in a water bath. The same principle of adhesion testing was used by Wu et al. [41], Barbero et al. [42], and Aguero et al. [43]. The plates were then dried in a laboratory oven (30 min) at 100 °C, cooled to room temperature, and weighed. The difference in mass before and after ultrasonic exposure provided information about the adhesion of the catalyst layer to the 3D-printed ceramic plates.

2.4. Testing of Morphology and Phase Composition of the Catalyst

The morphology and the thickness of the catalyst layer were examined using a JEOL JSM-7000F field emission scanning electron microscope (FESEM; JEOL, Tokyo, Japan). Phase composition of the mixed oxides was determined by X-ray diffraction using a Shimadzu (Tokyo, Japan) XRD 6000 instrument with Cu Kα radiation. Raman spectra were acquired using a Renishaw InVia confocal Raman microscope (laser wavelength: 785 nm, laser power: 0.1 mV, resolution: 1 cm⁻¹, exposition: 10 s, accumulation: 5).

To gain additional insight into the vibrational modes of the catalyst samples, the Raman spectra were theoretically modeled for the MnFeO_x and MnCuO_x crystal grains with at least three unit cells in each crystallographic direction. Frequency calculation was performed using the long-range corrected hybrid density functional theory (wB97XD) of Chai and Head-Gordon [44] with the triple zeta valence basis set (TZVP) of Schaefer et al. [45] as implemented in the Gaussian16 suite [46]. For modeling the electrical properties of a larger medium, the Self-Consistent Reaction Field approach was used in the Polarizable Continuum Model approximation, using the dielectric constant of ε₀ = 14 for CuMn₂O₄ and ε₀ = 20 for FeMnO₃ [47].

2.5. Catalytic Oxidation of BTEX Compounds

Catalytic oxidation of the gaseous BTEX mixture (Messer; Pančevo, Serbia) was carried out in a tubular reactor filled with a previously fabricated monolithic ceramic catalyst. The apparatus for catalytic oxidation consists of a reactor housing (stainless steel tube, 0.07 cm in diameter and 18 cm in length) with a monolithic catalyst and two mass flow controllers—MFCs—(for the BTEX Brooks SLA5850, Brooks Instruments; Hatfield, PA, USA), as well as synthetic air (Brooks 4850, Brooks Instruments; Hatfield, PA, USA), a temperature measurement and control system (TC), a gas chromatograph for on-line analysis of the reaction mixture at the reactor outlet, and computers for the acquisition and processing of experimental data.

A quartz infill (4 cm long rod) was placed at the reactor inlet at the bottom of the reactor between two layers of quartz wool in order to avoid dead volume in the reactor and to preheat the reaction mixture to the operating temperature. The monolithic catalyst was placed above the quartz filling, followed by another layer of quartz wool. For temperature control, a thermocouple was installed in the central part of the reactor and connected to the temperature control unit. At the outlet of the reactor, the reaction mixture was analyzed using a gas chromatograph (Shimadzu GC-2014; Tokyo, Japan) equipped with a flame ionization detector (FID) and an RTX-WAX (Restek; supplied by AnAs d.o.o., Zagreb, Croatia) capillary column. The conditions for chromatographic analysis were as follows: the support gas was nitrogen; the on-column injector temperature was 200 °C; the column temperature was 100 °C; and the FID temperature was 200 °C. The injected sample volume was 0.5 cm³, defined by the volume of the sample loop. The reaction was monitored by determining the conversion of the BTEX components after reaching a steady state (4τ) in the reactor. The starting reaction mixture of BTEX contained 50 ppm of benzene, toluene, ethylbenzene, and o-xylene in nitrogen. The catalytic oxidation was carried out at atmospheric pressure, at different temperatures (100–350 °C), and at a constant total flow rate (92 mL/min) of the reaction mixture. The reaction mixture of BTEX (80 mL/min) and synthetic air (12 mL/min) flowed from the bottom to the top of the reactor containing the monolithic catalyst.

3. Results and Discussion

3.1. Preparation of the Ceramic Monolithic Catalyst Supports, Impregnation, and Thermal Treatment

Table 1 show the geometries and corresponding geometric surface areas of the monolithic catalyst supports calculated using Autodesk Fusion 360.

The length of all 3D-printed monoliths is 40 mm, while their outer diameter is 7 mm. CAD models of the 3D-printed catalyst supports are shown in Figure S1. Although the main function of the auxiliary elements (on the outer shell of models PS and SS) is to prevent the movement of the monolith inside the metal tube of the reactor, these elements also increase the geometrical surface area of the catalyst, which has a favorable effect on the obtained conversions. Due to the heat treatment of the green parts, a shrinkage of 12–13% was observed in the direction of the x-axis and the y-axis, while the shrinkage in the z-axis direction was 20–22%. To achieve the desired dimensions of the catalyst supports after the heat treatment, dimensional correction factors (1.12 in the direction of the x-axis and the y-axis; 1.21 in the direction of the z-axis) were used to enlarge the 3D-printed green parts. Images of the prepared supports are shown in Figure 1, left.

After the supports had been heat treated (Figure 1, left), they were examined to determine if any deformation or cracks occurred in the monolithic structure. The next step was the application of the catalyst layer with suitable precursors using the impregnation technique. After impregnation, the dried monoliths (Figure 1, center) were calcined (Figure 1, right) and a stable layer of mixed manganese oxides formed on the surface of the monolith with a characteristic black color.

3.2. Testing of the Mechanical Stability (Adhesion) of the Catalyst Layer

Before testing the catalytic activity of the monolithic catalysts, the mechanical stability of the catalyst layer, i.e., the mixed manganese oxides, was tested. Table 2 shows the test results for mixed manganese and iron oxides (MnFeO_x) and mixed manganese and copper oxides (MnCuO_x) deposited onto the test specimens (plates) prepared under the same conditions under which the monolithic catalysts were prepared. The mechanical stability test is of great importance for the potential application of the catalysts in real systems, where frequent and sudden changes in the operating conditions (volume flow rates of the gas mixture, temperature, etc.) may occur. The results obtained by applying ultrasound to the samples show that the mass loss of the catalyst layer is less than 2%, indicating excellent mechanical stability, i.e., adhesion of the catalyst. It was found that the mechanical stability of the catalyst is satisfactory, and it was confirmed that the applied method for the preparation of the catalyst layer is acceptable and was successfully performed.

Figure 2a shows the morphology of a heat-treated ceramic catalyst support without the mixed oxides applied. Because pores are rare, ceramics cannot be classified as porous, especially with pore sizes being roughly between 10 and 50 µm, i.e., macropores. As reported in our previous work [24], the specific surface area of the ceramic monolith samples, measured by the Brunauer–Emmet–Teller method (BET), was 0.76 m²/g, which is comparable to the specific surface area of commercially used cordierite monolithic catalyst supports. Figure 2b shows the morphology of the catalytically active layer on the surface of the ceramic support. The layer is rough but with a relatively homogenous appearance. The roughness and some surface irregularities can be attributed to the surface roughness of the support, as well as to the deposition technique followed by thermal treatment involving salt decomposition. The thickness of the catalyst layer has also been established via FESEM by cross-section analysis. As can be observed, the thickness can be roughly estimated to ~6 µm (Figure 2c). A comparison with the literature data [48,49] shows that this is a relatively thin catalyst layer, which makes it suitable for the oxidation of BTEX, especially since the reaction in the gaseous phase that occurs on the outer (geometric) surface of the catalyst is fast. For this reason, internal diffusion does not have a significant influence to the process rate; instead, the overall oxidation reaction rate depends almost entirely on the surface area of the catalyst.

3.3. Investigation of the Phase Composition of the Catalyst

Crystal phases were identified via X-ray diffraction analysis (Figure 3). The analysis confirmed the preparation of double oxides: FeMnO₃ (ICDD PDF #75–0894) in the MnFeO_x sample and CuMn₂O₄ (ICDD PDF #74–2422) in the MnCuO_x sample. Diffraction patterns also revealed the presence of α-MnO₂ (ICDD PDF No: 44–0141) and α-Mn₂O₃ (ICDD PDF No: 73–1826) in both samples. Finally, the diffraction pattern of the MnFeO_x sample showed diffraction lines of Fe₂O₃ (ICDD PDF #72–0469), while the diffraction pattern of the MnCuOx sample showed diffraction lines of CuO (ICDD PDF #80–0076). Judging by the peak, the broadness crystallites are fine.

Additional characterization of the catalysts was performed using Raman spectroscopy as a technique complementary to XRD. Raman spectra of MnFeO_x and MnCuO_x are shown in Figure 4. The Raman spectrum of MnO_x was also acquired for comparison and easier determination of characteristic peaks that appeared in both the pure and the mixed Mn oxides.

Raman spectra were measured in a range from 100 to 3500 cm⁻¹; however, no characteristic Raman bands were observed in the range from 1000 to 3500 cm⁻¹. Therefore, only bands in the 100 to 1000 cm⁻¹ range are shown. As can be seen in Figure 4, the MnO_x sample shows only very weak Raman bands at 303, 655, and 704 cm⁻¹. The bands at 303 and 655 are ascribed to Mn₂O₃ [50,51] However, the band at 655 cm⁻¹ could also be associated with Mn₃O₄ [52], as well as with MnO₂ [51,53,54,55]. According to the literature [53], strong bands at 183, 574, and 634 cm⁻¹ and weak bands at 330, 386, 512, and 753 cm⁻¹ were expected on the Raman spectrum for α-MnO₂. Julien et al. [56] distinguished three regions in the Raman spectra of Mn-based oxides: (a) the 200–450 cm⁻¹ range (skeletal vibrations); (b) the 450–550 cm⁻¹ range (deformation modes of the Mn-O-Mn chains in the octahedral lattice); and (c) the 550–750 cm⁻¹ range (stretching modes of the Mn-O bonds in MnO₆ octahedra). The absence of other expected peaks in the Raman spectra of MnO_x was initially attributed to the low laser power of 0.1 mW. Additional Raman spectra of the samples were acquired using a different laser wavelength of 532 nm and a greater laser power of 0.86 mW (Figure S2) yielding no significant improvement.

Raman spectra of MnFeO_x and MnCuO_x samples have more characteristic bands having greater intensity. The Raman spectrum of MnFeO_x has several contributions at 184, 230, 289, 577, and 643 cm⁻¹. The strong bands at 184, 577, and 643 cm⁻¹ can be attributed to MnO₂ [51,53,54,55], even though the band at 643 cm⁻¹ may also be evidence for the presence of Mn₂O₃ [50,51] However, the band at 184 cm⁻¹ could also assigned to the external lattice vibration, while the bands at 577 cm⁻¹ and 643 cm⁻¹ could be attributed to the stretching mode in the Mn³⁺O₆ octahedra along the chains of the MnO₂ framework and to the symmetric stretching vibration in the Mn⁴⁺O₆ octahedra, respectively [51]. According to the literature [54], the characteristic Raman bands for α-Fe₂O₃ appear in the range of 200–800 cm⁻¹ with strong bands at about 221 and 288 cm⁻¹. Thus, the Raman band at 289 cm⁻¹ can be attributed to α-Fe₂O₃. Saravanakumar et al. [55] attributed Raman bands at 478 and 640 cm⁻¹ to FeMnO₃. However, the Raman band at 478 cm⁻¹ was not observed in our Raman study, and a band at a similar wavelength (643 cm⁻¹) is attributed to MnO₂ or Mn₂O₃; therefore, these bands likely overlap. Three characteristic Raman bands at 318, 431, and 581 cm⁻¹ were detected in the Raman spectra of MnCuO_x. According to the vibrational spectra of CuO powder reported in the literature [57,58,59], the weak band at 318 cm⁻¹ can be assigned to CuO. The strong Raman band at 581 cm⁻¹ is attributed to MnO₂. The observed wavenumbers 431 and 581 cm⁻¹ are close to those reported in the literature (430 and 580 cm⁻¹) for Cu_xMn_3−xO₄ [58]. Therefore, the accurate identification of the experimentally observed bands is again hindered by overlapping.

3.4. Theoretical Modeling of Raman Spectra for FeMnO₃ and CuMn₂O₄

In order to shed some more light on the band positions of the two mixed metal oxides detected by XRD, theoretical calculations of the Raman spectra of FeMnO₃ and CuMn₂O₄ were carried out and compared with the experimentally observed spectra of samples MnFeO_x and MnCuO_x (Figures S3 and S4). It should be stressed that the intensities of calculated spectra depend largely on the details of the theoretical approach (the presumed dielectric continuum model value or the applied density functional). On the other hand, the vibration frequencies are more robust. As can be observed in Figure S3, some of theoretical model structure peaks of FeMnO₃ are at the approximate frequencies as the experimentally observed bands of the FeMnO₃ catalyst (184, 230, 289 and 577 cm⁻¹). The three lower frequencies are related to collective modes (Figure S5a–c) involving both Mn-O and Fe-O bond length changes, while the 577 cm⁻¹ mode (Figure S5d) is related to the symmetrical oxygen vibration around the manganese centers, as mentioned previously and as referred to in previous findings [51]. Similar results were obtained by theoretical calculations of the CuMn₂O₄ Raman spectra (Figure S4), where two characteristic modes (318 and 431 cm⁻¹) in the FeCuO_x spectrum are reproduced within the CuMn₂O₄ structure model. These are theoretically predicted to be the result of oscillations involving Mn-O and Cu-O bond length changes (Figures S6a,b), respectively. The band at 581 cm⁻¹ appears to have originated from the accumulated theoretically predicted vibrations in the 550–680 cm⁻¹ range. However, theoretical spectra should be noted with great caution; the shortcomings of the applied theoretical approach stem predominantly from the model structure, which is limited in size (three-unit cells in each direction) in order to ease the calculation burden, as well as the fact that only the single phase model is used. This may lead to theoretical predictions of false-positive vibrations, i.e., vibrations that would be much more strongly damped in the real crystal, or of false-negative results, i.e., not predicting vibrations that might be existing in the real structure. In the present case, due to the limited crystallite size used in the model, a relatively large number of vibrational modes is theoretically predicted, e.g., a number of strong bands in the theoretical spectra in the range of between 300 and 500 cm⁻¹ are likely an artifact produced by the small size of the theoretical crystal grain. Those modes will likely be strongly damped for larger-sized crystal grain samples, which limits the theoretical soundness of the model. Additional investigations are presently underway to correct these weaknesses.

3.5. Catalytic Activity of the Prepared Monolithic Catalysts

The influence of temperature on the conversion of benzene (B), toluene (T), ethylbenzene (E), and o-xylene (X) on the prepared monolithic catalysts of different geometries of the monolithic carriers was investigated. A comparison of the activities of monolithic catalysts of different geometries (expressed by geometric surface area) with respect to the chemical composition of the catalyst layer was also performed. Results of the catalytic activity tests are given graphically for benzene (Figure 5) and numerically for all VOCs tested (Table 3).

As can be observed in Figure 5 all curves showing the temperature dependence of the conversion had a characteristic S-shape (light-off curves). This was in line with expectations since such a curve shape is common for the oxidation of VOC compounds, as indicated by Soares et al. [60], Brunet et al. [61], Zedan et al. [62], and Gallegos et al. [63], where the catalytic oxidation of different VOC species was successfully performed with various catalysts. Table 3 shows the characteristic values of T₅₀ and T₉₀ (temperatures at which a 50% and a 90% conversion of BTEX components was achieved, calculated by using linear interpolation) that allow for a comparison of different catalysts. Data regarding the catalytic activity of a control group without the supports (empty reactor), using the same reactor set-up and chemical composition of the inlet gas, were published in our previous work [39]. The conversion values thus obtained show that benzene is the most persistent BTEX compound, followed by toluene, ethylbenzene, and o-xylene, respectively. Similar conversions were obtained with MnFeO_x and MnCuO_x, a result which is in good agreement with the experimental data presented in our previous work [39].

As expected, the data clearly show that the conversions increase with the increase in geometric surface area of the monolithic catalyst (Table 1). The greater catalytic activity is not only due to the greater surface but also to the greater mass of the catalyst. The masses of the catalyst layers (MnFeO_x and MnCuO_x) on the 3D-printed catalyst supports used for the catalytic measurements are given in Table 4. As expected, the mass of a catalyst strongly depends on surface size and shape, i.e., area of the support. An increase in the geometric surface area facilitates the deposition of the catalyst layer and ensures a larger and better dispersion of the active sites available for BTEX oxidation. Some differences (0.2 to 0.3 mg) were noted between masses of a different catalyst, applied on the same geometry of the supports. Those differences are due to the variances of chemical and phase compositions of catalysts. Finally, small differences were noted even for the same support and catalyst. This is a consequence of slight differences in surface morphology between 3D-printed supports of the same geometry. It should be emphasized that the last two noted differences have a negligible effect on the achieved conversions of BTEX compounds.

A comparison of the established catalytic activities with those obtained for the commercial cordierite monolithic support (square-shaped channels, with dimensions of 1 mm × 1 mm) with the same dimensions (7 mm in diameter, 40 mm in length), using the same catalyst [39], showed slightly better catalytic activity of the catalyst on commercial support. However, this was expected since the cordierite monolithic support had a larger geometric surface area (29 cm²) and used noble metals (Pt, Pd) as the catalytically active components. The data regarding the catalytic removal of the BTEX mixture also showed good agreement with the work of Abassi et al. [28], where a combination of Pt/Al₂O₃-CeO₂ in the form of a nanocatalyst was used to oxidize a gaseous mixture of benzene, toluene, and xylene. Compared with the results reported by Genuino et al. [64], where an octahedral molecular sieve of cryptomelane type (OMS-2), amorphous manganese oxide (AMO), and mixed copper-manganese oxide (CuO/Mn₂O₃) were used for the catalytic oxidation of BTEX. The correlation, in terms of the trend of BTEX conversions obtained, is also similar to the trend of conversions reported in this work. It should be noted that the 3D-printed monolithic catalyst supports used in this work had relatively simple geometry. However, in our further research, more complex monolithic supports with a larger surface area will be fabricated for the same purpose, which may prove to be more catalytically efficient than commercial cordierite catalyst supports.

4. Conclusions

The present paper has reported the fabrication of new, small-scale catalytic monoliths with two mixed oxides—MnFeO_x and MnCuO_x—as the catalytically active components. The catalyst supports (monoliths) were prepared by stereolithography (SLA), which proved to be an excellent method for the preparation of ceramic monolithic catalysts. The prepared monoliths differed from each other by their characteristic geometry, which resulted in different surface areas of the prepared monolithic catalysts. To the best of our knowledge, the combination of a 3D printer and photopolymer resin used in this work has not been successfully used before for the fabrication of ceramic monolithic catalysts suitable for the catalytic oxidation of aromatic volatile organic compounds. Catalytically active mixed oxides were successfully deposited onto the monolithic supports using the impregnation technique.

Adhesion tests showed excellent mechanical stability of the catalyst layer, with catalyst mass losses of less than 2% after 30 min exposure to ultrasound. SEM analysis revealed that catalyst layer was rough but homogeneous in appearance and ~6 μm thick. XRD confirmed the presence of double oxides—FeMnO₃ and CuMn₂O₄—as well as Mn, Fe, and Cu oxides in the catalysts. Other than confirming the presence of manganese oxides, the Raman spectra results were inconclusive. Additional theoretical calculations of Raman spectra for FeMnO₃ and CuMn₂O₄ were performed in order to aid in the interpretation of the Raman spectra.

Prepared monolithic catalysts were used for the catalytic oxidation of a gas mixture of benzene, toluene, ethylbenzene, and o-xylene (BTEX). As expected, the catalyst with the support having the greatest surface area proved to be the most catalytically active. This can be attributed not only to the surface area of the catalyst support but also to the fact that masses of the catalysts deposited onto this support were greater compared with the other support geometries that were used. Regarding the chemical composition of the catalytically active layer, comparable results were obtained for both investigated catalysts. With the most active combination of 3D-printed support (with a surface area of 22 cm²) and catalyst (MnFeO_x), the T₉₀ for benzene was achieved at 212 °C, for toluene at 179 °C, and for both ethylbenzene and o-xylene at 177 °C.

In future work, more complex geometries of catalyst support will be investigated to further increase the geometric surface area of monolithic catalysts and to achieve better efficiency of the catalytic oxidation of VOCs. Monolithic catalyst supports with integrated static mixers will also be fabricated, which can be designed and prepared relatively easily using the same 3D printing technology.

Author Contributions

Conceptualization, F.C. and V.T.; methodology, F.C. and V.T.; software, V.G.; validation, F.C. and V.T.; formal analysis, F.C., D.V. and S.K.; investigation, F.C., V.G., D.V., V.T. and S.K.; resources, V.T.; data curation, F.C., V.G. and S.K.; writing—original draft preparation, F.C., V.G. and V.T.; writing—review and editing, F.C., V.T. and S.K; visualization, F.C., V.G. and S.K.; supervision, V.T.; project administration, D.V.; funding acquisition, D.V. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article and Supplementary Materials; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Heat-treated catalyst supports (left), catalyst support after impregnation with the catalyst precursors (center), and a monolithic catalyst after calcination (right).

Figure 2. Micrographs of (a) heat-treated ceramic catalyst support, (b) catalyst layer, and (c) cross-section of catalyst layer.

Figure 3. XRD diffraction patterns of catalysts.

Figure 4. Raman spectra of MnFeOx and MnCuOx catalysts in comparison with MnOx (laser wavelength 785 nm, power 0.1 mW).

Figure 5. Catalytic activity of the prepared monolithic catalysts (a) MnFeOx and (b) MnCuOx for benzene.

Table 1

Geometries of the 3D-printed catalyst supports with respective geometrical surface areas.

Support	P	PS	S	SS
Geometry of the monolithic catalyst support	[Image omitted. Please see PDF.]
Geometric surface area (cm2)	11	20	15	22

Table 2

Mechanical stability (adhesion) of the catalyst layer.

Catalyst	Plate	M (Plate with Catalyst) (g)	M (Plate After Test) (g)	Mass Loss (%)
MnFeOx	1	0.4840	0.4789	1.05
	2	0.4596	0.4522	1.61
	3	0.4792	0.4719	1.52
	4	0.4836	0.4771	1.34
	5	0.4776	0.4683	1.95
MnCuOx	1	0.4668	0.4580	1.89
	2	0.4699	0.4638	1.30
	3	0.4863	0.4810	1.09
	4	0.4759	0.4690	1.46
	5	0.4777	0.4696	1.69

Table 3

T₅₀ and T₉₀ values of BTEX components for supports with applied catalyst.

		T50 (°C)				T90 (°C)
Catalyst	Support	B	T	E	X	B	T	E	X
MnFeOx	P	215	186	173	174	243	200	195	195
	PS	207	186	171	172	238	198	194	194
	S	211	181	169	168	241	199	194	192
	SS	183	166	164	164	212	179	177	177
MnCuOx	P	219	186	177	175	252	202	196	196
	PS	203	174	167	166	236	195	189	189
	S	211	181	169	168	241	199	194	192
	SS	187	165	163	163	217	178	177	177

Table 4

Mass of the catalysts applied on the 3D-printed supports used for catalytic oxidation of BTEX compounds.

Support	P	PS	S	SS
Mass (mg) of MnFeOx	4.8–5.0	6.4–6.6	5.5–5.8	7.0–7.3
Mass (mg) of MnCuOx	4.9–5.2	6.2–6.5	5.6–5.8	7.1–7.2

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemengineering9010009/s1, Figure S0: Scheme of the monolithic reactor; Figure S1: CAD models of the 3D-printed catalyst supports (dimensions are given in mm); Figure S2: Raman spectra of the MnFeO_x and MnCuO_x catalysts in comparison with MnO_x (laser wavelength 532 nm, laser power 0.86 mW); Figure S3: Comparison of the theoretical Raman spectra of FeMnO₃ and the experimental Raman spectra of FeMnO_x; Figure S4: Comparison of the theoretical Raman spectra of CuMn₂O₄ and the experimental Raman spectra of CuMnO_x; Figure S5: Force vectors for experimentally observed Raman bands in the FeMnO_x sample as obtained by the theoretical model of FeMnO₃. (O = yellow, Mn = green, Fe = violet); Figure S6: Force vectors for experimentally observed Raman bands in the CuMnO_x sample obtained by the theoretical model of CuMn₂O₄. (O = yellow, Mn = violet, Cu = green).

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