2.6.2. Effect of Temperature

The active phase of iron-based catalysts is affected by the operating temperature of FTS. The iron oxide FeO becomes hematite Fe2O3, and then magnetite Fe3O4 when the temperature of the reactor rises [35].The iron particles of the promoted catalyst stabilize on the surface of the carbon matrix during the FTS reaction, preventing aggregation [36]. When using a carbon-supported iron catalyst without a promoter at higher temperatures, the number of non-oxygenates increased from 53% to 66%. (Fe-C). In a carbon-supported iron catalyst system with the promoter, the number of non-oxygenates increased from 81.7 percent to 82.8 percent at a higher temperature (Fe-C-K). Diesel generation increased from 60% to 80% at low temperatures (200 ◦C), with and without the potassium promoter. In the absence of a potassium promoter, however, the synthesis of gasoline increased from 36% to 72% at high temperatures (350 ◦C). The results of carbon chain distribution with and without oxygenates are shown in Table 3.

2.6.3. Carbon Chain Distribution

The FTS method may create C5–C11(gasoline) and C12–C22(diesel) hydrocarbon chains, making it a desirable transportation fuel. Various characteristics, such as the active phase and the quality of the support utilized, were used to evaluate the performance of FTS catalysts [37]. The FTS test was carried out with and without potassium promoters on a carbon-supported iron catalyst. After a reduction reaction at 450 degrees Celsius, water molecules were formed as a result of pre-adsorbed water molecules on the catalyst surface. When hydrogen and oxygen that have been adhered to the catalyst mix and begin to break down, they can produce water.

As a result, there were two main sources of adsorbed oxygen: first, polluted molecular oxygen from the gas phase, and second, the CO dissociation product. Fe5C2 was formed during the reduction reaction, which is considered the most active phase of the process [38,39]. Furthermore, the use of activated carbon increased the catalytically active phase's anchoring. The samples from the separators were collected in glass vials and sealed to prevent evaporation after the reaction. The lower white layer in Figure 7, which is oxygenated, and the higher brown layer, which contains C5+ hydrocarbons, were separated in the collected samples. Figure 8 depicts the results, which show various peaks and all potential chemicals included in the product. The overall percentage of different hydrocarbons in the mixture is shown in Table 3.

**Figure 7.** Sample collection.

**Figure 8.** Product composition: (a) Fe-C (LT-FTS), (b) Fe-C (HT-FTS), (c) Fe-C-K (LT-FTS), and (d) Fe-C-K (HT-FTS).

## 2.6.4. P's Influence

Because phosphorus causes choking during the reaction in a fixed bed reactor, and affects reactor performance, it is not considered as good for the FTS process. The GCMS data revealed the creation of phosphorus derived during the FTS reaction (C7H18NO3P, C8H18NO3P, and C18H15OP). The yield of the FT product will increase if other chemical activation agents are utilized in the synthesis of activated carbon. For making activated carbon in the future, activation agents that are KOH, K2CO3, or FeCl3 will be good. These activation agents will nothave an effect on the performance of a fixed bed reactor.

#### 2.6.5. Distribution of Oxygen

FTS produces a significant amount of oxygenate. The amount of oxygenated compounds produced is determined by the catalyst utilized and the process parameters. The aqueous phase typically contains oxygenates in the range of 3–25 wt% [40]. According to researchers [15], increased metal loading on the activated carbon-supported Fe/K catalyst resulted in greater selectivity for oxygenates. Because Lantana Camara has a lot of sesquiterpene hydrocarbon essential oil components, the amount of oxygenates, such as C3H4O2, C4H8O4, C6H6O, C7H14O, C10H20O, C16H34O, and many more found in this study, was the highest (46%) out of all the plants.

According to Anderson–Schulz–Flory (ASF) Distribution, diesel is produced as the major component, with a value of roughly 0.9 using LTFT. Gasoline requires slightly lower values of 0.7–0.8 under HTFT circumstances [41].

$$\kappa = \left( 0.2332 \cdot \left( \frac{\text{yCo}}{\text{yCo} + \text{yH}\_2} \right) + 0.633 \right) \cdot \left( 1 - 0.0039 \cdot \left( \left( \text{T} \left( ^\circ \text{C} \right) + 273 \right) - 573 \right) \right) \tag{11}$$

The ASF model is illustrated in Equation (11) to determine the "α". The "α" is determined using the gradient of the linearized expression in the log Mn/n against the n plot, which is given as Equations (12) and (13) [42]. Figure 6 shows the computed value "α" using the ASF model.

$$\frac{\mathsf{Mn}}{n} = (1 - \mathsf{a})^2 \cdot \mathsf{a}^{(n-1)} \tag{12}$$

$$
\ln \alpha = n \ln \alpha + \ln \left[ \frac{\left(1 - \alpha\right)^2}{\alpha} \right] \tag{13}
$$

Some of the recent studies about the Fe/AC catalyst demonstrate that the use of activated carbon (3–5 wt%) provides the majority of the C1–C6 hydrocarbon composition. In this study, as the weight percentage of activated carbons (10–15%) was increased, the formation of C1–C20 hydrocarbons was the main product of FTs. Furthermore, the role of activated carbon in Fe/Ac-based catalysts has to improve the anchoring of the catalytically active phase.

The addition of carbon as a support provides a great advantage in terms of high surface area and thermal and chemical stability. The carbon support made from biomass would result in an additional advantage from an economic perspective. The carbon support has a remarkable feature which allows the bonding of extra heteroatoms on its surface. The carbon support in the FTS process seems to be beneficial for the formation of iron-carbide species [5]. Lantana Camara is composed of isocaryophyllene (16.7%), germacrene D (12.3%), bicyclogermacrene (19.4%), andvalecene (12.9%), and caryophyllene isomers were detected in *Lantana Camara's* essential oil composition [43]. Table 4 shows the carbonsupported iron-based catalyst results. Recently, some research revealed that the addition of carbon as support enhanced the C5+. In this research work, the addition of carbon support made from Lantana Camara enhanced the formation of C5+ hydrocarbons by upto 90%.


**Table 4.** Hydrocarbon selectivity of different Fe-carbon-supported catalysts.
