Catalyst characterization
The formation of CaO active phase was confirmed via SIMS-TOF data via the occurrence of CaOH + and CaO + functional groups, as shown in Fig. 1a. Chloride distribution was observed to be present all over the catalyst surface and suggests an arrangement of chloride ions along the pore channels in eggshells. Furthermore, upon high-temperature activation, chloride was observed to migrate from FeCl3 to interact with calcium present in eggshell. Visual observation showed that increasing FeCl3 loading led to clumping of catalyst particles, possibly minimizing accessible active sites for the reaction. During catalyst preparation, aqueous FeCl3 may have reacted with CaCO3 in the eggshell to exchange chlorine as well as to change the pH substantially because of the release of HCl and CO2 (Eqs. 1 and 2):
$${\text{FeCl}}_{{3}} + {\text{ 3H}}_{{2}} {\text{O}} \rightleftharpoons {\text{Fe}}\left( {{\text{OH}}} \right)_{{3}} + {\text{ 3HCl}}$$
(1)
$${\text{CaCO}}_{{3}} + {\text{ 2HCl}} \rightleftharpoons {\text{CaCl}}_{{2}} + {\text{ CO}}_{{2}} + {\text{ H}}_{{2}} {\text{O}}{.}$$
(2)
The mass spectral images suggested that iron was arranged both as clusters and in a well-distributed planar way (Fig. 1b). Hence, calcium overlapping with iron was observed in both situations, as depicted in Fig. 1a and c.
Interestingly, the energy-dispersive spectroscopy (EDS) data collected via VPSEM analysis for 2% Fe-loaded eggshell catalysts indicated that the metal loading both in the form of a cluster as well as in a widely distributed manner on the surface (Fig. 2a) suggesting a heterogeneous surface. Unique surface morphology resembling a surface with settled molten metal was observed, perhaps due to the low melting point of FeCl3 (~ 300 ºC) which may have spread over the eggshell support. Furthermore, partial dissolution of iron clusters into the eggshell catalyst support is possible (El-Shobaky and Fahmy 2006). Besides, the analysis of the spent catalyst exhibited significantly deformed surface morphology (Fig. 2b, c) with enhanced porosity suggesting possible momentum effects of the feed gas.
Effect of metal loading
The experimental data suggested that at 2% loading, iron impregnation did not have any effect on the fractional conversion of methane when compared to plain calcined eggshell (control). However, with an increase in metal concentration, a substantial drop in methane conversion was observed, as shown in Fig. 3a. This may be attributed to the migration of chloride to Ca inherent to eggshell forming an inactive CaCl2 phase during catalyst preparation (Eqs. 1 and 2). Furthermore, the relatively lower melting point of FeCl3 would have enhanced the distribution of its molten phase, masking active CaO sites on the eggshell.
Furthermore, C2–C7 hydrocarbons (ethane, ethylene, propene, 1,3-butadiene, pentene, pentadiene, benzene, and toluene) along with H2 and COx were identified as part of the product spectrum. As suggested by Ibrahim et al. (2015), the interaction between the catalyst and the support material may have played a role in the selectivity of the catalyst. In addition, as observed by Lim et al. (2019), the basicity of the eggshell support may have also influenced the catalytic activity and selectivity. The results indicated that the production of CO2 did not vary considerably with iron concentration. However, 10% loading, despite possessing higher metal concentration, led to a drop in CO2 production. A drop in CO2 production was not compensated with enhanced selectivity for other products, and rather, the production of C2–C6 hydrocarbons dropped simultaneously, as shown in Fig. 3b, c. This phenomenon suggests the possible existence of an alternative pathway leading to products that were not detected by our analytical system. Based on our results, a 2% Fe-loaded catalyst was chosen for further analysis, since iron loading beyond 2% substantially reduced methane activation.
Effect of CH4:O2
When the effects of molar ratios were studied, the fractional conversion of methane increased with decreasing oxygen proportion in the feed, as shown in Fig. 4a. This may be explained by the fact that oxygen and methane in the feed compete to adsorb on the catalytic surface. Reducing oxygen concentration will pose minimal constraint for methane molecules to adsorb on the catalyst surface. This would have led to the enhanced conversion of methane. Furthermore, it is theorized that at a lower concentration of oxygen, Fe2O3/Fe-O clusters may also have contributed towards methane activation.
The selectivity for CO2 dropped rapidly with a reduction in oxygen concentration in the feed, suggesting dominant selective oxidation reaction (Fig. 4b). Similar behavior was observed with eggshell-supported catalysts (Karoshi et al. 2020). However, the decrease in CO2 selectivity was also accompanied by the drop in C2–C6 hydrocarbon selectivities (Fig. 4c), corroborating the observation of alternative pathways and products that were undetected by the gas chromatograph system employed in our research that was also evident from the carbon balance. Furthermore, these observations were also supported by the data collected from temperature experiments as will be discussed later in the article. With reduced oxygen concentration in the feed, the drop in production of ethylene was greater than the reduction in ethane production, as shown in Fig. 4d. Besides, measured high oxygen conversions of 95–97% suggest the possibility of the presence of oxygenates as part of unknown products.
Effect of flowrate
An initial increase in flow rate enhanced the fractional conversion of methane probably because of adequate mixing that facilitates easy access of catalytic sites for reactants. However, with a further increase in flowrate, a rapid drop in methane conversion was observed suggesting inadequate residence time for methane activation (Fig. 5a). Thus, both mass transfer and residence time constraints are critical in obtaining the desired methane conversion rate. The deposition of FeCl3 on eggshell support may mask CaO clusters to a certain extent as is evident from the earlier observations with different metal concentrations. Lower flowrate with high residence time may not supply adequate energy for reactant methane molecules to diffuse in the catalyst bed to access many CaO active sites and hence releasing most of them unreacted from the reactor. Additionally, FeCl3 could have been relatively more mobile because of the lower melting point (~ 300ͦC) when the feed gas was flown at higher flow rates. This could restrict the access of reactant methane molecules to CaO clusters which are believed to be the cause for methane activation. Such mobility and molten state settlement of FeCl3 were confirmed via VPSEM magnified images, as shown in Fig. 2.
Production of CO2 and C2 hydrocarbons decreased with an initial increase in flowrate and was enhanced with a further increment of flowrate but at a lower methane conversion (Fig. 5b). Interestingly, no quantifiable concentration > C3 hydrocarbons was observed at a higher flow rate of 1.2 L/min (Fig. 5c). This suggested that inadequate residence time would not have permitted multiple adsorption and desorption steps for reactants to form higher hydrocarbons. On the other hand, with a lower flow rate, the tendency of target products to overoxidize into carbon dioxide will prevail causing lower selectivity. In one of their reports, Roseno et al (2016) investigated catalytic partial oxidation of methane via perovskite-type catalysts (La-Co/Fe-O) under different experimental conditions. The authors observed a strong dependence of fractional conversion and product (CO, CO2, and C2) selectivity on residence time, indicating that optimal residence time is necessary for balancing fractional conversion and overoxidation. Similarly, Grunwaldt et al (2001) studied partial oxidation of methane on rhodium-impregnated on alumina catalysts and reported that the residence time influenced the product selectivity. Hence, balance between adequate residence time and sufficient turbulence is essential to enhance yields of target products. This conclusion was further evident from the data of variation in ethylene and ethane proportions in the product stream (Fig. 5d). With increasing flowrate, selectivity for ethane was greater than that for ethylene, indicating a lack of secondary selective oxidation step due to lower residence time. It was observed that carbon balance improved whenever CO2 production dominated product distribution.
Effect of temperature
The results suggested that the fractional conversion of methane for iron-impregnated eggshell catalysts decreased with increasing temperature (Fig. 6a). The methane conversion decreased by about 55% when the reaction temperature increased from 650 to 750 °C. Our results are in contrast to those reported by (Lim et al. 2019), who investigated perovskite-supported alkaline earth metal oxides for oxidative coupling of methane. The authors reported an increase in methane fractional conversion when temperatures were increased from 600 to 725 °C for all calcium, strontium, and barium-based catalysts. Besides, Michalkiewicz (2004) investigated partial oxidation of methane on zeolite-supported iron catalysts. It was observed that the fractional conversion of methane increased with an increase in reaction temperature from 350 to 650 °C, while the selectivity of methanol decreased due to overoxidation of methanol into formaldehyde and CO2. The substantial drop in fractional conversion at higher temperatures may be due to catalyst deactivation via sintering. Barbosa et al. (2001) studied iron-based catalysts for combustion of methane and reported the occurrence of sintering effects of the catalysts tested. In addition, as suggested by Ibrahim et al. (2015), the interaction between the catalyst support and the catalyst may have played an important role in the overall activity of the catalyst. In our research, the drop in methane conversion can also be attributed to both possible Ca–Fe interaction as well as to relatively higher mobility of FeCl3 because of its low melting point. With increasing temperature coupled with the physical impact of feed gas flow, the mobility of FeCl3 is believed to be enhanced significantly. Such mobility would cause FeCl3 to mask the CaO active sites inherent to eggshell support. Therefore, to understand the surface chemistry of the catalyst, additional systematic studies on the catalyst characterization via XRD, XPS, and TPD are suggested. Besides, increased CO2 production with a simultaneous drop in C2–C6 hydrocarbons was observed because of the overoxidation (Fig. 6b, c). While a slight increase in ethane concentration was observed, ethylene production remained nearly the same at all the temperatures tested (Fig. 6d).
Moreover, an overall carbon balance was also performed for the experiments at different temperatures. The overall amount of unaccounted carbon reduced with an increase in temperature, as shown in Fig. 6e. With a simultaneous increase in CO2 selectivity under the same conditions, overoxidation of reaction products seems possible. This further supports the idea of possible existence of reaction products that were not identified by the gas chromatograph.
Longevity
As depicted in Fig. 7a, the catalyst was found to achieve consistent methane conversion over multiple cycles. However, a considerable increase in the production of CO2 was observed after the first cycle of operation (Fig. 7b). Analyzing these results in the light of VPSEM images depicting high porosity in the spent catalyst and high mobility of FeCl3, it appeared that high porosity along with mobile FeCl3 would have provided an enhanced number of Fe2O3/FeO clusters, resulting in enhanced oxidation and CO2 production. Furthermore, secondary collisions of products within the catalyst may have caused complete oxidation to CO2 (Cullis 1967). Furthermore, as described by (Enger et al. 2008), the creation of hotspots and temperature gradient zones during oxidation of methane would also contribute to enhanced CO2 selectivity that was also enhanced via the heat entrapment due to the possible molten state of the catalyst surface. On the other hand, the selectivity for other target products remained nearly similar over time. Unaccounted carbon was observed to reduce with each cycle as the products were inclined to overoxidize because of secondary collisions and heat entrapment (Fig. 7c, d).