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Article

An Investigation of MnOx and K/MnOx-Based Catalysts on MnO2 and Fe3O4 Supports for the Deep Oxidation of Cyclohexane

by
Md Sarwar Kamal
1,
Catherine B. Almquist
2,* and
David L. Tierney
3
1
Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, OH 44106, USA
2
Department of Chemical and Biomedical Engineering, Miami University, Oxford, OH 45056, USA
3
Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056, USA
*
Author to whom correspondence should be addressed.
Processes 2025, 13(3), 634; https://doi.org/10.3390/pr13030634
Submission received: 7 January 2025 / Revised: 11 February 2025 / Accepted: 21 February 2025 / Published: 24 February 2025
(This article belongs to the Special Issue Metal Oxides in Heterogeneous Oxidation Catalysis)
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Abstract

:
K/MnOx catalysts on MnO2 and Fe3O4 supports were synthesized and compared for the deep oxidation of cyclohexane. The presence of potassium (K) on the catalysts enhanced the catalytic activity compared to catalysts with similar composition but without K. Interestingly, the lowest loading of K/MnOx used in this study (0.63 mmoles/g support) performed better than those with higher loadings. The presence of K on the catalysts increased water adsorption, decreased the extent of sintering, and inhibited changes in crystal phase of the catalyst support, as evidenced by TGA, XRD, and BET surface area analyses. The XRD profiles of the catalysts showed mixed crystal phases of MnOx and FeOx species, and EPR results support the presence of mixed valence states of Fe and Mn. The activation energies for MnOx-supported catalysts and FeOx-supported catalysts were approximately 50 kJ/mole and 53 kJ/mole, respectively.

Graphical Abstract

1. Introduction

Volatile organic compounds (VOCs) have detrimental impacts on both human health and the environment [1,2,3]. According to Ian Tiseo (2024), the United States emitted approximately 12.5 million tons of volatile organic compounds (VOC) in 2023 [4]. Not only do VOCs directly contribute to high healthcare costs [5], but VOCs actively participate in atmospheric photochemical reactions that result in secondary pollutants, such as ozone and peroxyacetyl nitrates (PANs) [6,7]. These compounds can be more harmful to human health than VOCs, and the resulting urban haze that they create can be damaging to the environment.
There are several methods used to remove VOC emissions from industrial sources, including those that recover the VOCs, those that destroy the VOCs, and combinations of both [8]. The selection of emissions control for VOCs often depends upon the volumetric flow rate of the gas stream, the concentration of the VOCs, safety, and ultimately, the relative cost effectiveness of the VOC control strategies. VOCs can be recovered using adsorption, absorption, condensation, and membrane separation processes. VOC destruction methods are often used for high-volume low-concentration streams, and they include thermal and regenerative thermal oxidation, catalytic oxidation, and biological treatment [8]. The most typical method used for VOC destruction in air is regenerative thermal oxidation [9]. In regenerative thermal oxidation, heat energy is recovered by cycling the inlet gases and combustion effluent gases through packed beds of ceramic materials. However, thermal oxidation requires very high temperatures, up to 1200 °C for some hazardous waste incinerators [10], and the combustion of fossil fuels is required to achieve such high temperatures. Additionally, thermally formed nitrogen oxides (NOx) can be formed in high-temperature combustion processes.
Catalytic oxidation of VOCs is attractive as a method to destroy VOCs because it reduces the amount of energy required compared to thermal oxidation. A catalyst reduces the activation energy of a chemical reaction, which allows the reaction to proceed at lower temperatures. Consequently, the lower temperatures in catalytic oxidation of VOCs reduce the consumption of fossil fuels and lower the formation of thermal NOx.
The catalytic oxidation of VOCs has been studied using a variety of catalysts, and several review articles have been published [11,12,13,14,15,16,17,18]. The catalysts investigated for the catalytic oxidation of VOCs include noble metals, non-noble metal oxides, and mixed-metal oxides. Noble metal catalysts have very high catalytic activity for VOC oxidation. However, they are expensive and can become deactivated during industrial use by sintering and poisoning. Therefore, non-noble metal catalysts are more attractive for removing VOC emissions due to their comparatively lower cost and stability.
Among various non-noble metal oxide catalysts, manganese oxide (MnOx) catalysts demonstrate significant activity for VOC oxidation [14,15,16,18,19,20,21,22,23,24,25,26]. The high activity can be associated with multiple valence states of Mn, mobile surface oxygen species, and lattice oxygen participation in the reaction [16,18,19,20,21,22,23,24,25,26]. The Mars-van Krevelen (MVK) catalytic mechanism for VOC oxidation is often used to describe MnOx catalysts because of the mobility of the lattice oxygen atoms in their structure [26]. In this mechanism, the VOC interacts with a lattice oxygen atom and removes it from the catalyst surface [19,25,26]. This creates oxygen vacancies on the catalyst surface, which are subsequently filled by gaseous oxygen molecules.
Potassium (K) doping of catalysts has been shown to enhance the stability and improve the catalytic activity of catalysts for VOC oxidation [27,28,29,30,31,32]. Promotion effects include increasing surface-active species, improving the dispersion of the active metal catalyst, improving redox properties, modifying the crystal structure of the catalyst, and improving the thermal stability of the catalyst. However, the promotional effects of K on MnOx catalysts are dependent upon the extent of K loading and their intended application. For example, in 2020 Lai et al. [31] found that the catalytic activity for the catalytic oxidation of toluene on Mn/Ce0.65Zr0.35O2 increased with increasing loadings of K up to 0.2 K/Mn molar ratio but decreased at higher K loadings [31]. They concluded that the addition of K promoted the reducibility of the catalysts, facilitated the formation of more surface adsorbed oxygen on the catalyst surface, generated more oxygen vacancies, and enhanced the oxygen mobility. At K/Mn molar ratios higher than 0.2, the catalytic activity likely decreased due to blocking of the active sites on the catalyst [31]. In 2023 Greluk et al. [32] found that K-doping enhanced the stability of the Co/MnOx catalysts for steam reforming of ethanol by lowering the carbon build-up on the catalyst surface [32]. They concluded that a role of K-doping was to enhance the adsorption–dissociation of water molecules, and hence, the direct oxidation of carbonaceous species on the surface of the catalyst [32].
In further support of the promoter effect of K on MnOx catalysts, it has been found that cryptomelane is an active VOC oxidation catalyst [32,33,34,35,36,37,38,39,40,41,42,43,44]. Cryptomelane is an octahedral molecular sieve (OMS-2) that has a 2 × 2 square tunnel structure with a pore size of about 4.6 Å, and it has a stoichiometric formula of approximately KMn8O16 [45]. The activity of cryptomelane has been reported to be a result of the mixed valence state of the manganese (Mn) and the mobility of the lattice oxygen in the cryptomelane surface structure [32,33,34,35,36,37,38,39,40,41,42,43,44]. In a review by Suib [45], it is stated that mixed valence is important in electron transfer, energy transfer, and redox catalysis and that oxygen vacancies are related to potential catalytic activity.
Supported MnOx catalysts have also been studied for VOC oxidation reactions [20,26,35,46,47,48,49,50,51,52]. Catalyst supports can have a significant impact on the activity of MnOx catalysts for many reasons, including changes in surface area; distribution of active species; catalyst-support interactions; reducibility; adsorption–desorption properties for the VOCs, oxygen, water vapor, and CO2; and changes in surface oxygen species and their mobilities. However, the application in which the supported MnOx is used must be considered. For example, Liu et al. [48] compared the catalytic activity for the oxidation of chlorobenzene of supported MnOx catalysts on TiO2, Al2O3, and SiO2 and found that MnOx/TiO2 had the highest activity. The dispersion of Mn and its stability in the presence of chlorinated compounds was provided as possible reasons for this observation. However, Gallardo-Amores et al. [49] found that supported MnOx on anatase TiO2 has less activity for the oxidation of 2-propanol than unsupported MnOx catalysts. It was concluded that the dissolution of Mn active sites by TiO2 was the reason for the lower activity of the supported catalysts compared to the unsupported catalysts [49]. Jung, et al. [47] investigated the role of the support (Al2O3, SiO2, and TiO2) for MnOx catalysts for the oxidation of benzene, toluene, and xylene vapors. They found that MnOx supported on γ-Al2O3 had a higher catalytic activity than MnOx on the other supports that were investigated. They concluded that defect oxides played a predominant role in the catalytic activity compared to surface area, acidity, and lattice oxygen mobility [47].
FeOx-supported MnOx catalysts have been investigated for VOC oxidation [50,51,52,53]. Duplancic et al. [50] investigated the catalytic oxidation of toluene on MnOx catalysts on FeOx, NiOx, and CuOx. They found that Mn/FeOx had the highest catalytic activity for toluene oxidation of those catalysts studied, and the activity was comparable to a commercial Pt/Al2O3 catalyst. The observed catalytic activities correlated with the predicted binding energy of toluene adsorption on the surface of the catalysts [50]. Gong et al. [53] found that the composite of FeOx/MnOx had higher catalytic activity for the degradation of chlorobenzene than MnOx alone. The FeOx altered the redox properties of the MnOx catalyst [53]. Soltan et al. [52] found that MnOx/FeOx composite catalysts had higher activity for toluene oxidation than MnOx or FeOx alone. They concluded that the synergistic effects of dual oxygen vacancies enhanced the catalytic activity for toluene oxidation [52]. This is supported by Li et al. [54] who used XPS results to show that redox reactions between Fe and Mn may occur at the surface of MnOx/FeOx composite catalysts [54].
In this study, the catalytic activities of K/MnOx catalysts and MnOx catalysts were compared for the deep oxidation of cyclohexane vapor. While the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA oil) has been extensively studied in support of the production of adipic acid (an important precursor to nylon), the deep oxidation of cyclohexane vapor has been published less often in the recent literature. In this study, cyclohexane vapor was used as a representative VOC for the comparison of catalyst activities.
The specific objectives of this research were as follows: (1) compare the effects of the support (Fe3O4 and MnO2) on the performance of K/MnOx-based catalysts; (2) investigate the effect of K on catalyst characteristics and catalytic activities; and (3) investigate the effect of K/MnOx loadings on catalytic performance for Fe3O4- and MnO2-supported catalysts. Efforts to meet these objectives are described in the ensuing sections.

2. Materials and Methods

2.1. Materials

The following materials were used in this study: cyclohexane, potassium permanganate and manganese (II) acetate tetrahydrate (Alfa Aesar, now Thermo Fisher Scientific, Waltham, MA, USA); iron oxide (Fe3O4), nano powder (<50 nm), and activated manganese (IV) oxide (β-MnO2) (Sigma-Aldrich, St. Louis, MO, USA).

2.2. Catalyst Synthesis Methods

The catalysts were prepared using a wet-incipient method. The process is shown in Figure 1. In brief, 2 g of metal oxide support was added to 50 mL of deionized (DI) water, and the slurry was stirred with a magnetic stirrer for five minutes. Fe3O4 and activated MnO2 were used as the metal oxide supports. Then, a measured amount of KMnO4 was added to the slurry. The slurry was heated to about 80 °C and stirred continuously until the slurry became a thick paste. This paste was then dried overnight at 125 °C. Using a mortar and pestle, the dried sample was crushed to a fine powder, and the resulting powder was calcined for two hours at 700 °C in a furnace. Following calcination, the powders were again crushed with a mortar and pestle to form a fine powder. The support materials and two additional catalysts prepared using manganese acetate (MAc) as the precursor on the Fe3O4 support were also processed in a similar manner. A summary of catalysts that were prepared for this study is provided in Table 1.
KMnO4 is an oxidizing agent, and so its addition to Fe3O4 and β-MnO2 will result in catalysts with Mn and Fe in mixed oxidation states. In the synthesis method, the potassium is retained and would be distributed onto the surface of the catalysts. The support structures change crystallinity and crystal phase following calcination. Therefore, the supports will be referred to as FeOx and MnOx, since the crystal phases of the supports following calcination are mixed oxides.

2.3. Catalyst Characterization

The catalysts were characterized to assess their chemical and physical properties. Each type of characterization that was conducted is described below.

2.3.1. Brunauer–Emmett–Teller (BET) Surface Area

A Micromeritics TriStar II surface area analyzer (Norcross, GA, USA) was used to measure the surface area, pore volume, and pore size of the catalysts after calcination and of the supports both before and after calcination. The BET surface area was determined using nitrogen adsorption at 77 K. Pore size and pore volume were determined by the Barrett–Joyner–Halenda (BJH) model.

2.3.2. X-Ray Diffraction (XRD)

The crystal phases and crystallinity of the catalysts were assessed on the catalysts used in this study using a Bruker D8 Advance powder X-ray diffractometer (Billerica, MA USA). Data on the instrument was collected from 2θ = 5° to 80°, with a step size of 0.01° at 0.1 s per step. Bruker software, DiffracEva, and the Powder Diffraction File (PDF) database, were used for phase identification.
The crystallite sizes of the predominant Mn oxide species were calculated using the Scherrer equation (Equation (1)) and the XRD profiles at 2θ = 12.8°, which is representative of cryptomelane-based materials [55].
L = K λ F W H M ∗ cos ⁡ θ  
In Equation (1), L is the crystallite size (nm), λ is the incident wavelength of 0.154 nm, θ is the angle in radians, FWHM = full width at half maximum (radians), and K is a shape factor that was taken to be 0.9 for this study. To find FWHM, OriginPro 2024 (Northampton, MA, USA) was used, which utilizes the Lorenz system for nonlinear curve fitting from gadgets by employing Quick Fit.

2.3.3. Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometry (EDS)

A Zeiss Supra 35 variable pressure FEG SEM (Oberkochen, Germany) was used to investigate the morphologies of the catalysts. The images were taken at 75,000× magnification, 3 KeV EHT, 6 mm to 8 mm working distance (WD), and 30 µm standard aperture.
The SEM is equipped with an EDAX Genesis 2000 energy dispersive spectrometry (EDS) detector (MahWah, NJ, USA). EDS was used to provide information regarding the elemental composition of each catalyst. The spectra were taken at 15 KeV and 10 mm WD with an aperture size of 60 µm. Bruker Esprit v.2.1 software was used for elemental analysis by the quantification model of the Phi (Rho, Z) method. These data are provided in Supplemental Information.

2.3.4. Thermal Gravimetric Analyses (TGA)

Thermal gravimetric analyses (TGA) were conducted on all catalysts using a TA Instruments Q500 TGA (New Castle, DE, USA). The temperature was ramped at 10 °C/min from ambient temperature to 900 °C under a nitrogen gas atmosphere. A platinum pan was used as the sample holder.

2.3.5. Fourier Transform Infrared (FTIR) Spectrometry

FTIR experiments were conducted on a Thermo Nicolet NEXUS 670 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) to evaluate the functional organic groups on both fresh and previously used catalysts. The catalysts were scanned in the 400 to 4000 cm−1 range at a resolution of 4 cm−1 on the spectrometer. Finely powdered samples were dispersed in potassium bromide (KBr), which is optically transparent in FTIR. The FTIR spectra were analyzed by comparison with an Infrared Spectroscopy Absorbance table [56].

2.3.6. Electron Paramagnetic Resonance (EPR)

A Bruker EMX spectrometer (Billerica, MA, USA) equipped with an ER041xG microwave bridge and an ER4119-HS cavity at Miami University Ohio’s Advanced EPR Laboratory was used to collect CW-EPR spectra. EPR was used to study the electronic properties of the catalyst. The samples were placed in a quartz X-Band EPR tube of 4 mm OD and 250 mm in length. The spectra were recorded at room temperature by signal-averaging 2 scans with 7600 G central field, 15,000 G sweep width, 100 s sweep time, 5 G modulation amplitude (100 kHz), and 0.02 mW microwave power (9.4 GHz) [57].

2.4. Catalyst Performance Tests

The test system used in this study, depicted in Figure 2, consists of a stainless-steel tubular reactor (ID = 0.32 cm), a diffusion cell to generate cyclohexane vapors, a mass flow controller (MKS Instruments, model no. M100B12CS1BV, Andover, MA, USA) to control the flow rate of air, and a Lindberg/Blue Mini-Mite tube furnace to control the reaction temperature.
For each experimental trial, the performance of the catalyst was investigated using 50 mg of catalyst that was secured with quartz wool in the middle of the reaction tube. Air, flowing at 50 cubic centimeters per min (ccpm), was mixed with cyclohexane vapors from the diffusion cell and was directed through the tubular reactor. The flow of air was measured before and after each experimental trial using a bubble meter. A gas-tight 100 µL Hamilton syringe was used to collect the effluent from the reactor for analysis on an Agilent 6890N gas chromatograph (GC) (Agilent Technologies, Santa Clara, CA, USA) equipped with a flame ionization detector (FID). The first readings were taken in triplicates at 100 °C and were collected at least two hours after the reaction temperature was reached. Then, subsequent reaction temperatures were investigated over the range of 150 °C to 400 °C. The test system was allowed to equilibrate for at least 30 min at each reaction temperature before gas samples were collected. An empty reactor tube was analyzed with a similar reaction setup to evaluate cyclohexane degradation without a catalyst.
Chemstation software (Agilent Technologies, Santa Clara, CA, USA) was used to obtain chromatographs and peak areas from the flame ionization detector (FID). A calibration curve was made to correlate the peak areas with cyclohexane concentration in air. An Amprobe CO2-100 CO2 meter (Everett, WA, USA) was used to measure the concentration of CO2 in the reactor effluent. The CO2 measurements were recorded to determine the carbon balance. For selected catalysts, the catalyst activity was assessed for 95 h of continuous operation at 350 °C, 375 °C, and 400 °C.

3. Results

3.1. Catalyst Characteristics

The BET surface area, micropore area, pore width, pore volume, and average crystallite size data for each catalyst are summarized in Table 2. Following calcination, the surface areas of Fe3O4 and β-MnO2 decreased significantly. This result is expected since calcination at 700 °C would cause the catalyst particles to agglomerate and sinter. The addition of K/MnOx to the FeOx support decreased the extent of sintering, as indicated by the higher surface areas of the KM-FO-X (where X = 1, 2, or 3) catalysts compared to FO-C and MAc-FO-X catalysts. The pore widths of the catalysts with K/MnOx on FeOx were similar to that of the Fe3O4 as purchased, while the pore volumes decreased proportionally to surface area. The MnOx crystal size was not affected significantly with increasing K/MnOx loadings on FeOx. The addition of manganese acetate to the FeOx support appeared to have very little impact on catalyst sintering, since the surface areas, pore widths, and pore volumes of MAc-FO-1 and MAc-FO-2 catalysts are similar to those of FO-C.
The addition of KMnO4 to β-MnO2 had very little effect on the surface area following calcination, as indicated by the similar surface areas of MO-C and KM-MO-X catalysts. However, the micropore area, pore width, and pore volumes increased with the addition of K/MnOx compared to those of MO-C, and the crystal size of the K/Mn species (based on 2θ = 12.8°) decreased from 54 nm to 29 nm with increasing K loadings from 0.63 mmoles/g support to 2.53 mmoles/g support.
Figure 3 shows the XRD profiles of FeOx-based catalysts. The XRD profiles of Fe3O4 as purchased and that of the support following calcination at 700 °C for 2 h (FO-C) show that the support underwent a crystal phase transformation from Fe3O4 to α-Fe2O3. However, the XRD profiles of the KM-FO-X catalysts show peaks that are representative of both Fe2O3 (2θ = 24.2°, 33.2°, 41.2°, and 49.4°) and Fe3O4 (2θ = 30.1° and 43.2°), indicating that the Fe in the sample has mixed valence states. As K/MnOx loading increases, XRD peaks that are representative of Fe3O4 increase, while those representative of Fe2O3 decrease. In addition, peaks associated with K/MnOx species appear in XRD profiles of KM-FO-X catalysts at 2θ = 12.8° and 25.6°, and they increase with increasing loadings of K/MnOx. These peaks represent the formation of birnessite (δ-MnO2) on FeOx, which is supported by Birkner and Navrotsky (2017) who published XRD patterns for both birnessite and cryptomelane (α-(K)MnO2) [64]. At the highest K/MnOx loading in this study (KM-FO-3), Fe3O4 appears to be the dominant crystal phase in the FeOx support. These results indicate that the addition of K/MnOx suppresses the extent of crystal phase change from Fe3O4 to Fe2O3. Sintering and crystal phase changes occur by mass transport processes at the grain boundaries between particles [65]. The addition of K, then, may interfere with the grain boundary diffusion, thus inhibiting sintering and crystal phase changes.
In Figure 4, the XRD profiles of the catalysts prepared with manganese acetate on FeOx are shown. In these catalysts, the phase change from Fe3O4 to α-Fe2O3 occurs following calcination. There are no noticeable peaks associated with Fe3O4 in FO-C, MAc-FO-1, and MAc-FO-2 catalysts. In addition, there are no noticeable peaks that are associated with MnOx in the XRD profiles of MAc-FO-1 and MAc-FO-2. This indicates that the MnOx species on these catalysts are either amorphous or they are present below the detection limits of the XRD. A comparison of Figure 3 and Figure 4 shows that the addition of K to the catalysts inhibits phase transformation of the catalyst support and promotes the crystallization of the K/MnOx species on the surface of the catalyst.
Figure 5 shows the XRD profiles for the K/MnOx catalysts. The activated MnO2 (as purchased) is weakly crystalline (β—MnO2), as indicated by the relatively broad and weak intensity peaks in the XRD profile at 2θ = 37.5°, 42.5°, and 55°. However, after calcination, a crystal phase transformation to Mn2O3 was apparent in sample MO-C, represented by peaks at 2θ = 23.4°, 32.9°, 38.5°, and 57°. The XRD profiles for KM-MO-X catalysts show mixed MnOx crystalline species, with peaks representing both Mn2O3 and cryptomelane (2θ = 12.8°, 18.3°, 26°, 29°, 37.2°, and 42°). The cryptomelane peaks increased with increasing K/MnOx loading.
SEM images were acquired to investigate the morphological structures of the catalysts. Figure 6 and Figure 7 show the SEM micrographs of the FeOx-supported catalysts. FO-C and KM-FO-1 (Figure 6a,b) have similar surface structures resembling sintered particles in the shape of small elbows. However, the KM-FO-1 (Figure 6b) catalyst has more particle agglomeration than the FO-C catalyst, and it has some precipitate on its surface, indicated by orange-colored arrows. The KM-FO-2 (Figure 6c) catalyst also displays agglomerated particles and precipitate on the surface of the catalyst. Larger structures start to appear, as indicated by the green arrows. The KM-FO-3 (Figure 6d) catalyst has a slightly different surface than KM-FO-2. The precipitate over the surface is predominant, and the larger structures have become more apparent, again shown by the green arrows. These new plate-like structures likely indicate the presence of K-Birnessite, which is supported by the XRD profiles of KM-FO-X catalysts.
SEM micrographs MAc-FO-1 and MAc-FO-2 were also compared with FO-C, as displayed in Figure 7a–c. These images show the same morphology as FO-C (Figure 7a), but they appear to be more sintered than the KM-FO-X catalysts.
SEM micrographs of KM-MO-X catalysts are shown in Figure 8. MO-C (Figure 8a) has a porous structure, and it contains some rod-like crystal structures that extrude out from the porous matrix. As K/MnOx is added to the catalysts, the formation of nanorod-like structures become more prominent. These nanorod or needle-like structures are similar to those reported for cryptomelane [19,42], which supports the XRD data regarding the formation of cryptomelane with the addition of K to MnOx.
TGA analyses of KM-FO-X catalysts are shown in Figure 9a,b. Based upon the TGA profiles, as K/MnOx increases, the loss of mass from the catalysts increases with increasing temperature. Mass losses at temperatures < 200 °C represent the desorption of water, whereas the mass lost at temperature > 800 °C likely represent the loss of oxygen due to changes in crystal phase of the K/MnOx and MnOx structures in the catalysts. The TGA analyses show that the presence of K on the catalysts enhances the adsorption of water vapor. This is supported by Greluk, et al. [32], who concluded that a role of K-doping of catalysts was to enhance the adsorption–dissociation of water molecules on the surface of the catalyst. The observed mass losses between 200 °C and 750 °C are likely due to losses of lattice oxygen from the K/MnOx structures. This is supported by Almquist, et al. [42] who also observed similar mass losses with temperature on cryptomelane samples.
TGA analyses of MAc-FO-X catalysts are shown in Figure 10a,b. In these samples, less than 1% of the catalyst mass was lost when temperatures increased from room temperature to 900 °C. Based upon the TGA profiles, the thermal stabilities of the MAc-FO-X catalysts were similar to that of FO-C. A comparison of Figure 10a,b with Figure 9a,b indicates that the K/MnOx species on the catalysts are responsible for the mass losses with temperature more so than FeOx or MnOx alone. In addition, the loss of mass at temperatures higher than 800 °C is due to the loss of oxygen as Mn2O3 changes phase to Mn3O4 [66].
TGA analyses of KM-MO-X catalysts are shown in Figure 11a,b. Based upon the TGA profiles, as K/MnOx increases, the mass loss from the catalyst also increases. Similarly to the KM-FO-X catalysts, the presence of potassium appears to promote the adsorption of water, as indicated by mass losses at temperatures < 200 °C. While minor mass losses were observed from the KM-MO-X catalysts between 200 °C and 600 °C, the presence of K/MnOx on the catalysts appeared to decrease their thermal stability. At temperatures higher than 600 °C, there appeared to be three temperature regions at which mass was lost from KM-MO-X catalysts: 600–750 °C; 750–850 °C, 850–900 °C. According to a study by Stelmachowski, et al. [66], as temperature increases from 600 °C to > 900 °C, crystal phase changes in MnOx occur due to the loss of oxygen from the catalyst. For example, with increasing temperature, Mn2O3 → Mn3O4 → MnO and KMn8O16 → KxMn4O8. At temperatures near 900 °C, there may also be a loss of K from the catalysts [66].
Figure 12a compares the EPR spectra of KM-FO-1, MAc-FO-1, and FO-C. FO-C exhibits very little evidence of unpaired electrons. However, KM-FO-1 and MAc-FO-1 have broad peaks. The breadth of the signals gives evidence of unpaired electrons that are either delocalized or the density of unpaired electrons is sufficiently high that ferromagnetic resonance (FMR) may be occurring. MAc-FO-1 appears to break up the interactions responsible for the broad signal of KM-FO-1, consistent with the oxidation of an Mn-O unit, which could be viewed as a partial oxygen vacancy.
When compared to the EPR spectra for Fe3+ ions in glass [67], g-values of ~2.02 and g ≈ 4.2 were apparent in MAc-FO-1. For molecular or isolated Fe3+ species, the g ~ 4.2 is symbolic of randomly oriented, rhombically symmetric Fe3+, while a signal near g ~ 2 is more consistent with a well-defined structure. According to Song, et al. [68], the EPR signal for unpaired electrons due to oxygen vacancies occurs at around g = 2.0 [68], which is approximately the zero-crossing in both the KM-FO-1 and Mac-FO-1 spectra. Overall, the EPR spectra for KM-FO-1 and MAc-FO-1 give evidence of oxygen vacancies.
Figure 12b compares the EPR spectra of KM-MO-1 and MO-C. Both KM-MO-1 and MO-C catalysts give a similar peak at 3715 G magnetic field with g-values of 1.85. This g-value indicates that there is a possibility of oxygen vacancies around manganese [69]. The presence of K in KM-MO-1 did not appear to significantly affect the presence of unpaired electrons in the catalysts, neither of which gave evidence of a large density of unpaired electrons.

3.2. Catalyst Performance

The catalytic performance results for the deep oxidation of cyclohexane vapor are shown in Figure 13, Figure 14 and Figure 15. In Figure 13a,b, the effect of K/MnOx loading on FeOx and MnOx, respectively, are shown. The addition of K/MnOx to FeOx and to MnOx increased their catalytic activity. However, in both cases, as K/MnOx loadings increased from 0.63 mmole/g to 2.53 mmole/g, the catalytic performance decreased. These results demonstrate the promotional effects of K on MnOx catalysts at low concentrations, but excessive loadings of K are detrimental to catalyst activity. These results coupled with the XRD and SEM results show that K alters the crystal structures, increases deposits on the catalyst surface, and alters the surface chemistry of the catalysts.
Figure 14 compares KM-FO-1 and KM-FO-2 with MAc-FO-1 and MAc-FO-2, respectively. In both cases, the catalysts that contained K, but similar loadings of Mn performed better than catalysts without K loadings. Reasons for the higher catalytic activity for catalysts containing K may include higher surface areas, changes in Mn crystallinity, and differences in the distribution of Mn and Fe valence states. In addition, the catalytic activities decreased with increasing Mn loadings for both KM-MO-X and MAc-MO-X catalysts. This can happen because higher Mn loadings may result in greater aggregation, less dispersion, and, therefore, less Fe-Mn interaction on the surface of the catalysts.
Table 3 summarizes the light-off temperatures (T50) and activated energies calculated for the oxidation of cyclohexane over each catalyst. The light-off temperatures are defined here as the reaction temperatures at which 50% of the cyclohexane is oxidized with each catalyst. The light-off temperatures for KM-MO-X and KM-FO-X catalysts were lower than their corresponding supports (MO-C and FO-C, respectively). The trends in the light-off temperatures correlated with the catalytic performance results. Only one catalyst, MAc-FO-2, had a higher light-off temperature than its support, FO-C. Reasons may include that higher loadings of MAc to FeOx catalysts change the valence state of Mn and Fe at the catalyst surface, thus lowering the oxygen availability and mobility. This is supported by Chen et al. [70], who found an optimal Mn/Fe molar ratio in mixed-metal oxide composite catalysts for the oxidation of toluene. They concluded that the proper ratios of Mn/Fe can improve catalyst performance due to higher amounts of Mn3+ ions and higher concentrations of lattice defects and oxygen vacancies [70].
Cyclohexane reacts with oxygen according to the stoichiometric reaction:
C 6 H 12 + 9 O 2 → i n t e r m e d i a t e s → 6 C O 2 + 6 H 2 O
According to a study by Lui et al. [71], the catalytic oxidation of cyclohexane proceeds by either a chain-opening mechanism to form straight-chained alkenes and olefins, or by hydrogen abstraction to form cycloalkenes, aromatics, and aromatic oxygenates. Yadav et al. [51], also conducted a detailed spectroscopic kinetic study of the mechanism of cyclohexane oxidation. In that work, the adsorption, oxidation, and formation of cyclohexane and several oxidation by-products were considered, including cyclohexanol, cyclohexanone, and cyclohexene, among others. They concluded that their model follows a pseudo-first-order reaction with cyclohexane at low conversions [51]. Therefore, in this study, the activation energy was calculated by first assuming a first-order reaction (Equation (2)) with cyclohexane, because oxygen is present as a reactant in significant excess (~100 times the stoichiometrically required molar amount). The first-order reaction expression was integrated, assuming a plug-flow reactor, to calculate the reaction rate constant, k (Equation (3)). The reaction rate constant, k, was used in the Arrhenius equation (Equation (4)), which was linearized to calculate the activation energy (Equation (5)).
r a t e = − d C A d t = k C A
The rate of the reaction is in units of moles L−1 min−1, CA is the concentration of cyclohexane in air (moles/L), t is time (min), and k is the reaction rate constant (min−1).
Assuming a plug-flow reactor, where time is residence time in the reactor, the equation is integrated, and k is determined as follows:
k = l n C A C A o ( V Q )
where Q is the volumetric flow rate of the influent air stream (L min−1), and V is the reactor volume (L). Equation (4) is the Arrhenius equation, which equates the reaction rate constant, k, to the activation energy, EA, of the reaction.
k = A e − E A R T
In Equation (4), A is the pre-exponential factor (min−1), EA is the activation energy (kJ/mole), R is the gas constant (0.008314 kJ/mole/K), and T is the temperature (K). A linearized form of Equation (4) is provided in Equation (5):
ln ⁡ k = ln ⁡ A − E A R 1 T
For each catalyst, the ln(k) was plotted as a function of 1/T at conversions < 20%, and a straight line was obtained with a slope of −EA/R. The pre-exponential factor for this effort was estimated to be 3.27 × 106 min−1. The activation energy for cyclohexane oxidation over each catalyst was calculated based upon the slope of these graphs, and the results are provided in Table 3.
The activation energies of the FeOx-supported catalysts are generally higher than those of the MnOx-supported catalysts (~53 kJ/mole compared to ~50 kJ/mole), and the activation energies are relatively unaffected by the K/MnOx loadings. The activation energies calculated here are supported by those reported by Yadav et al. [51] for cyclohexane to cyclohexanol (44 kJ/mole) [51]. In addition, Zhang et al. [72] also found the activation energy for cyclohexane oxidation to be 46.4 kJ/mole and 68.7 kJ/mole on Au/NaX zeolite catalysts. In this study, the activation energies for the oxidation of cyclohexane over KM-FO-X and KM-MO-X catalysts are comparatively lower than those obtained for their corresponding calcined supports (FO-C and MO-C, respectively). One exception is that the activation energy of MAc-FO-2 is significantly higher (~60 kJ/mole) than those of other FeOx-supported catalysts. As discussed above, possible reasons include changes in the valence state of Mn at the catalyst surface and the lowering the oxygen availability and mobility [70]. In addition, the catalyst composition can affect the adsorption and desorption characteristics of reactants and by-products, which will ultimately affect the apparent catalytic activity of the catalyst.
The catalytic performance of KM-FO-1 and KM-MO-1 were investigated for 95 h of continuous operation at 350 °C, 375 °C, and at 400 °C, as shown in Figure 15. The inlet cyclohexane concentration was approximately 230 ppm for all trials. At 350 °C, KM-MO-1 was more active than KM-FO-1. This is contrary to the screening results, which showed KM-FO-1 performed better than KM-MO-1 at 350 °C (Figure 13a,b). A possible reason for this observation is that KM-FO-1 is initially more active as a fresh catalyst, but after a short time, the activity decreases, possibly due to changes in the catalyst properties with time and temperature, build-up of organic by-products, or adsorption of water vapor on the catalyst surface. An initial decrease in catalyst activity was also observed with KM-FO-1 at 375 °C, where the catalytic activity at 2 h was higher than that at 25 h of operation. KM-MO-1 showed no deactivation over 95 h of operation at 350 °C or 375 °C, as indicated by a relatively constant conversion of cyclohexane. At 400 °C, no indications of catalyst deactivation were observed for either catalyst, and the conversion of cyclohexane was greater than 99.5%.
Carbon balances were conducted for the longevity trials, and the results are summarized in Table 4. In these balances, only cyclohexane and CO2 were considered. The carbon balances at 350 °C and 375 °C were greater than 80%. Deviations from a perfect carbon balance may be due to the formation of partial oxidation by-products that were not measured or observed and error inherent in the CO2 m. However, at 400 °C, the carbon balances were significantly lower at 67% and 69% for KM-FO1 and KM-MO-1, respectively. The relatively low carbon balances at 400 °C are most likely due to the presence of partial oxidation products that were not measured in this study. For example, carbon monoxide was not measured in this study. Also, there were some observable organic by-products in the effluent stream, but they were not measurable due to their small peak areas (GC peak areas < 1 picoamp·min). In addition, carbon build-up on the catalyst may have occurred.
In Figure 16, the FTIR spectra of unused and used catalysts (after 95 h of continuous operation at 400 °C) are compared. The used catalysts have absorption peaks that represent organic functional groups on the catalyst surface. This indicates that partial oxidation products were formed and adhered to the surface of the catalysts. They are more prominent for KM-FO-1 than for KM-MO-1, indicating that there was more build-up of partial oxidation products on KM-FO-1 than on KM-MO-1. In a study by Einaga and Futamura (2005), several partial oxidation species of cyclohexane were identified using FTIR on alumina-supported manganese oxides. They included ketones, alcohols, carboxylic acids, and acid anhydrides, among others [73]. The organic by-products retained on the catalyst surface may have accounted for the decrease in apparent catalytic activity as well as the carbon balance. In addition, the broad peak between 3000 cm−1 and 3700 cm−1 on KM-FO-1 compared to KM-MO-1 suggests that KM-FO-1 adsorbs water to a greater extent, which may also account for the decrease in apparent catalytic activity. Water vapor would compete with cyclohexane for adsorption sites on the catalyst surface.

4. Discussion

In a review by James Spivey (1987), generalizations were made regarding the catalytic activity of metal oxides for VOC oxidation [16]. This review highlighted that the high activity of metal oxides is related to the metal having more than one valence state and the catalyst having highly mobile surface oxygen. He further stated that Mn and Fe would be among the most active metal oxide catalysts due to their multiple valence states and redox properties [16].
The efforts of this study were to investigate K/MnOx catalysts supported on FeOx and MnOx for the deep oxidation of cyclohexane. There are two key aspects of this study: (1) the effect of K on MnOx catalysts, and (2) the effect of FeOx as a support for K/MnOx and MnOx catalysts. Each of these aspects are discussed in more detail below.

4.1. The Effect of K on MnOx Catalysts

In this study, the effects of K on MnOx catalysts included the following: inhibition of changes in the crystallinity and crystal phase of the catalyst support, increased propensity for water adsorption, formation of K/MnOx crystalline species on the catalyst support, decreased thermal stability, and enhanced catalytic activity over catalysts of similar composition but without K. These effects were evidenced by experimental observations in this study, and they are supported by previously published studies. XRD and EPR results give indications that the catalysts contain mixed valence states of Mn and Fe and oxygen vacancies, which could enhance catalytic activity. These results are supported by Zheng et al. [19], where the authors reported that ion-doping of MnOx can have significant effects on catalyst properties and activities. For example, the introduction of K to the catalyst can affect the electronic and geometric structures of the catalysts, and so enhance surface oxygen activity and facilitate the regeneration of surface hydroxyl species through activation of water. In a study by Greluk et al. [32], a significant increase in catalyst stability with the addition of K was observed. They concluded that the potassium favored the adsorption and dissociation of water molecules and, therefore, enhanced the oxidation of soot build-up on the catalysts. Lui et al. [71] reported that the promotion mechanism of K to Mn-based catalysts is the formation of defect oxides, which have higher mobility than lattice oxygen species, and hence, higher catalytic activity.
It was also observed in this study that as the K/MnOx loading on FeOx and MnOx increased from 0.63 mmoles/g support to 2.53 mmoles/g support, the catalytic activity decreased. This result is supported by Zheng et al. [19], who state that excessive K would cause oxygen to become more stable and less mobile. In addition, the interactions of K with adsorbed carbon dioxide (CO2) may decrease the activity by decreasing the rate of desorption of CO2 from the surface of the catalyst [19]. The addition of K also changes the morphology and crystal structure of the catalyst, which was evident in the SEM micrographs presented in Figure 6, Figure 7 and Figure 8. These, too, could be reasons for the catalytic activity to decrease with increasing K/MnOx loading.

4.2. The Effect of FeOx as a Support for K/MnOx-Based and MnOx-Based Catalysts

Catalyst supports can provide high specific surface area, a stable structure, and a pore structure that is conducive to dispersion of active sites on a catalyst surface [21]. In this study, however, the FeOx-supported catalysts generally had lower catalytic activity than the K/MnOx-based catalysts on MnOx supports, even though the BET surface areas of the KM-FO-X catalysts were higher than those of KM-MO-X catalysts. In addition, the lowest loadings of K/MnOx and MnOx on FeOx (0.63 mmoles/g support) had the highest catalytic activities of the FeOx-supported catalysts in this study. MAc-FO-1 and KM-FO-1 had similar activities at temperatures up to 325 °C, indicating that the MnOx-species on the FeOx support is the active species. The FTIR spectra of used catalysts, however, indicate that the FeOx-supported catalysts had more organic build-up following extended continuous operation than MnOx-supported catalysts.

5. Conclusions

This research underscores the relationship between catalyst composition, support material, K/MnOx loading levels, and catalytic performance of K/MnOx- and MnOx-based catalysts, providing insights for the design of more effective catalysts for volatile organic compound (VOC) oxidation. The presence of K on MnOx catalysts significantly enhanced the apparent catalytic activity for the deep oxidation of cyclohexane. Mixed crystal structures in the supports indicate that Fe and Mn have a distribution of valence states, which would result in redox reactions at the interface of FeOx and MnOx structures, oxygen defects, and ultimately enhanced catalytic activity for VOC oxidation.
In this study, the K/MnOx catalysts supported on MnO2 performed better than those with similar K/MnOx loadings supported on Fe3O4 for the deep oxidation of cyclohexane. The activation energies for MnO2-supported catalysts and Fe3O4-supported catalysts were approximately 50 kJ/mole and 53 kJ/mole, respectively. The catalysts containing the lowest loadings of K/MnOx (0.63 mmoles/g support) resulted in the highest catalytic activity for both MnO2− and Fe3O4− supported catalysts. Changes in surface structure, morphology, valence states of Fe and Mn, and water adsorption properties, as evidenced by experimental observations in this study, are possible reasons for a decrease in the apparent catalytic activity as K/MnOx and MnOx loadings increase.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13030634/s1.

Author Contributions

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

Funding

This research received no external funding.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The following personnel at Miami University are acknowledged for their assistance as this study was conducted: the Chemical, Paper, and Biomedical Engineering Department at Miami University for research support; Marion Lytle for training and assistance with XRD analyses, and Douglas Hart for general laboratory support. We also acknowledge the support of the staff and resources from the Center for Advanced Microscopy and Imaging (CAMI) at Miami University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Overview of catalyst synthesis process.
Figure 1. Overview of catalyst synthesis process.
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Figure 2. Schematic of the test system used for catalyst performance analysis.
Figure 2. Schematic of the test system used for catalyst performance analysis.
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Figure 3. XRD profiles for FeOx-supported K/MnOx catalysts.
Figure 3. XRD profiles for FeOx-supported K/MnOx catalysts.
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Figure 4. XRD profiles of the FeOx-supported MnOx catalysts.
Figure 4. XRD profiles of the FeOx-supported MnOx catalysts.
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Figure 5. XRD profiles for unsupported K/MnOx catalysts.
Figure 5. XRD profiles for unsupported K/MnOx catalysts.
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Figure 6. SEM micrographs of (a) FO-C, (b) KM-FO-1, (c) KM-FO-2, (d) KM-FO-3. 75,000X; WD between 6 mm and 8 mm; EHT = 3 kV.
Figure 6. SEM micrographs of (a) FO-C, (b) KM-FO-1, (c) KM-FO-2, (d) KM-FO-3. 75,000X; WD between 6 mm and 8 mm; EHT = 3 kV.
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Figure 7. SEM micrographs of (a) FO-C, (b) MAc-FO-1, (c) MAc-FO-2. 75,000X; WD = 6 mm; EHT = 3 kV.
Figure 7. SEM micrographs of (a) FO-C, (b) MAc-FO-1, (c) MAc-FO-2. 75,000X; WD = 6 mm; EHT = 3 kV.
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Figure 8. SEM micrographs of (a) MO-C, (b) KM-MO-1, (c) KM-MO-2, (d) KM-MO-3.
Figure 8. SEM micrographs of (a) MO-C, (b) KM-MO-1, (c) KM-MO-2, (d) KM-MO-3.
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Figure 9. TGA analyses of KM-FO-X catalysts in a nitrogen atmosphere. (a) % mass remaining vs temperature; (b) derivative of mass loss with temperature as a function of temperature.
Figure 9. TGA analyses of KM-FO-X catalysts in a nitrogen atmosphere. (a) % mass remaining vs temperature; (b) derivative of mass loss with temperature as a function of temperature.
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Figure 10. TGA analyses of Mac-FO-X catalysts in a nitrogen atmosphere. (a) % mass remaining vs temperature; (b) derivative of mass loss with temperature as a function of temperature.
Figure 10. TGA analyses of Mac-FO-X catalysts in a nitrogen atmosphere. (a) % mass remaining vs temperature; (b) derivative of mass loss with temperature as a function of temperature.
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Figure 11. TGA analyses of KM-MO-X catalysts in a nitrogen atmosphere. (a) % mass remaining vs temperature; (b) derivative of mass loss with temperature as a function of temperature.
Figure 11. TGA analyses of KM-MO-X catalysts in a nitrogen atmosphere. (a) % mass remaining vs temperature; (b) derivative of mass loss with temperature as a function of temperature.
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Figure 12. EPR analyses of selected catalysts used in this study. (a) EPR of MAc-FO-1, KM-FO-1, and FO-C. (b) EPR of KM-MO-1 and MO-C.
Figure 12. EPR analyses of selected catalysts used in this study. (a) EPR of MAc-FO-1, KM-FO-1, and FO-C. (b) EPR of KM-MO-1 and MO-C.
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Figure 13. Effect of K/MnOx loading on (a) FeOx-supported catalysts. (b) MnOx-supported catalysts. Inlet concentration = 230 ppm, catalyst mass = 50 mg; WHSV = 60 L g−1 h−1, operating time at reaction temperature < 2 h.
Figure 13. Effect of K/MnOx loading on (a) FeOx-supported catalysts. (b) MnOx-supported catalysts. Inlet concentration = 230 ppm, catalyst mass = 50 mg; WHSV = 60 L g−1 h−1, operating time at reaction temperature < 2 h.
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Figure 14. Comparison of FeOx-supported MnOx-based catalysts with and without K. Inlet concentration = 230 ppm, 50 mg catalyst, WHSV = 60 L g−1 h−1.
Figure 14. Comparison of FeOx-supported MnOx-based catalysts with and without K. Inlet concentration = 230 ppm, 50 mg catalyst, WHSV = 60 L g−1 h−1.
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Figure 15. Catalytic activity of KM-FO-1 and KM-MO-1for 95 h of continuous operation. Inlet concentration = 230 ppm, 50 mg catalyst, WHSV = 60 L g−1 h−1.
Figure 15. Catalytic activity of KM-FO-1 and KM-MO-1for 95 h of continuous operation. Inlet concentration = 230 ppm, 50 mg catalyst, WHSV = 60 L g−1 h−1.
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Figure 16. FTIR spectrum of used and un-used KM-FO-1 and KM-MO-1 catalysts after 95 h of continuous operation at 400 °C. The organic functional groups corresponding to the FTIR peaks were identified by comparison with a FTIR reference table [56].
Figure 16. FTIR spectrum of used and un-used KM-FO-1 and KM-MO-1 catalysts after 95 h of continuous operation at 400 °C. The organic functional groups corresponding to the FTIR peaks were identified by comparison with a FTIR reference table [56].
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Table 1. Inventory of catalyst samples synthesized for this study.
Table 1. Inventory of catalyst samples synthesized for this study.
IdentificationSupport
(as Purchased)		K/MnOx Loading
(mmole/g Support)		Nominal Composition 1
(Molar Ratios)
K/FeMn/FeK/Mn
FeOx supported catalysts
FO-CFe3O4000N/A
KM-FO-1Fe3O40.630.0490.0491
KM-FO-2Fe3O41.580.1220.1221
KM-FO-3Fe3O42.530.1950.1951
MAc-FO-1Fe3O40.63 (Mn only)00.0490
MAc-FO-2Fe3O41.58 (Mn only)00.1220
MnOx-supported catalysts
MO-CActivated MnO20N/AN/A0
KM-MO-1Activated MnO20.63N/AN/A0.052
KM-MO-2Activated MnO21.58N/AN/A0.121
KM-MO-3Activated MnO22.53N/AN/A0.182
1 Nominal composition is based upon measured amounts of KMnO4 added to the catalyst support. EDS analyses are provided in the Supplementary Documentation to support the reported elemental molar ratios.
Table 2. Summary of surface areas, pore width, pore volume, and crystallite size.
Table 2. Summary of surface areas, pore width, pore volume, and crystallite size.
Catalyst IDBET
Surface Area
(m2/g)		Micropore Area 3
(m2/g)		Pore Width 4
(nm)		Pore Volume 4 (cm3/g)Mn-Species Crystallite Size 5
(nm)
FeOx-supported catalysts
FO-AP 1409.935.10.39N/A
FO-C 214.22.612.30.04N/A
KM-FO-119.62.234.50.1766
KM-FO-224.13.534.80.2062
KM-FO-322.83.834.50.1861
MAc-FO-112.41.113.70.04N/A
MAc-FO-211.60.714.10.04N/A
MnOx-supported catalysts
MO-AP 180.607.10.17N/A
MO-C 28.70.917.90.03N/A
KM-MO-19.20.826.90.0654
KM-MO-28.12.6400.0736
KM-MO-37.32.738.30.0729
N/A = no XRD peak at 2θ = 12.8°. 1 AP: as purchased. 2 all samples except those that are as purchased were calcined at 700 °C for 2 h. 3 calculation methods provided in supporting documentation [58,59,60,61,62,63]. 4 Based on BJH model using the desorption of N2 from the catalysts at 77 K. 5 Based on Scherrer Equation and XRD peak at 2θ = 12.8°.
Table 3. Light-off temperature (T50) and activation energy.
Table 3. Light-off temperature (T50) and activation energy.
CatalystT50 1 (°C)Activation Energy (kJ/mole)
FeOx-supported catalysts
FO-C37653.7
KM-FO-129352.8
KM-FO-235053.4
KM-FO-333751.8
MAc-FO-130652.4
MAc-FO-239160.7
MnOx-supported catalysts
MO-C32652.1
KM-MO-128149.2
KM-MO-231350.3
KM-MO-331450.4
1 T50 is the temperature at 50% cyclohexane degradation.
Table 4. Carbon balances for trials over 95 h of continuous operation.
Table 4. Carbon balances for trials over 95 h of continuous operation.
CatalystTemperature (°C)Cyclohexane (ppm)CO2 (ppm) 1Cout/Cin
InOutConversionOut
KM-FO-135020812440%25180%
3752012787%95793%
4002130100%86367%
KM-MO-13502316174%90492%
3752202887%108395%
400239199.6%99169%
1C = moles of carbon in cyclohexane and in CO2.
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Kamal, M.S.; Almquist, C.B.; Tierney, D.L. An Investigation of MnOx and K/MnOx-Based Catalysts on MnO2 and Fe3O4 Supports for the Deep Oxidation of Cyclohexane. Processes 2025, 13, 634. https://doi.org/10.3390/pr13030634

AMA Style

Kamal MS, Almquist CB, Tierney DL. An Investigation of MnOx and K/MnOx-Based Catalysts on MnO2 and Fe3O4 Supports for the Deep Oxidation of Cyclohexane. Processes. 2025; 13(3):634. https://doi.org/10.3390/pr13030634

Chicago/Turabian Style

Kamal, Md Sarwar, Catherine B. Almquist, and David L. Tierney. 2025. "An Investigation of MnOx and K/MnOx-Based Catalysts on MnO2 and Fe3O4 Supports for the Deep Oxidation of Cyclohexane" Processes 13, no. 3: 634. https://doi.org/10.3390/pr13030634

APA Style

Kamal, M. S., Almquist, C. B., & Tierney, D. L. (2025). An Investigation of MnOx and K/MnOx-Based Catalysts on MnO2 and Fe3O4 Supports for the Deep Oxidation of Cyclohexane. Processes, 13(3), 634. https://doi.org/10.3390/pr13030634

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