Mini-Review: Syngas Production Via Partial Oxidation of Methane Reaction and Its Potential Catalyst

Methane as a light gas was generally found in natural gas, which was burned freely to gain a high quality of petroleum. This action truly impacted the worst condition in nature, namely the greenhouse effect. This brief review described a fundamental theory of the crucial process in methane conversion from natural gas into value-added chemicals such as syngas (CO+H 2 ). The methane conversion reaction was commonly divided into direct and indirect reactions. The indirect reaction such as partial oxidation of methane was mostly chosen due to the intermediate product (syngas) can easily generate many raw materials of petrochemicals. This paper also described a potential catalyst to be applied in heterogeneous types, such as perovskite oxide, metal oxide, and zeolite.


Introduction
Indonesia has abundant natural gas reserves which methane as the main component with the 11 th largest reserves rank in the world and a reserve to production value (R/P) of 59 years at a stable price [1]. In addition, natural gas also accounts for about 20% of long-chain hydrocarbons and nonhydrocarbon gases such as N2, CO2 and H2S [2]. The highly fuel content in natural gas makes it widely consumed as a source of energy for industry and households.
Although the reserves are in abundant amounts, the use of natural gas is limited by the distribution constraint due to its form is gas. The long-chain hydrocarbon gas component can be compressed into a liquid by applying high pressure known as CNG (Compressed Natural Gas), which makes it easier to control and move. However, its greatest contain is methane, is a gas that can not be compressed so that its use really can only be done in the form of gas. Transport constraints lead to the most effective and efficient utilization only when close to natural gas sources. Based on the description, the increase in the benefits of natural gas will be expanded if methane in natural gas is converted into longer chain hydrocarbons that can be melted and become more value-added petrochemical materials such as polymer, fertilizer and plastic raw materials [3].  In addition, syngas is also used as fuel gas and produce value-added chemicals as shown in Figure 2  Both sources have the largest methane composition with a number of differences in secondary compositions as shown in Table 2 [12]. Meanwhile, the high methane composition in nature has a serious impact in radiation up to 21 times greater than carbon dioxide (CO2), greenhouse effect and rising earth temperature. This is because methane is very easy to react photochemically with OH radicals derived from the reaction between water vapor and ozone (O3) in the troposphere so that the amount of O3 decreases [13].
Therefore, natural gas in mining areas contributes the most significant percentage of methane content which is preferably burned to carbon dioxide.
Nurherdiana, S.D., dkk. Akta Kimia Indonesia 6(2), 2021, 187-201 On the other hand, natural gas can be utilized as household fuel for large-scale purposes such as industry rather than burned in vain. However, the usage can be utilized directly and close to natural gas sources to maintain the exergy efficiency. However, most of the natural gas reserves are scattered in the offshore areas as seen in Figure   compared to the production of 37.8 [14]. The value-added products from natural gas need to be increased such as chemicals to balance the operating costs of the energy industry. Thus, a process of methane conversion to syngas is essential to be optimized.

Oxidation of Methane (POM) Reaction.
Syngas can be obtained through a methane conversion reaction. In order to the methane conversion process by direct and indirect conversion as mentioned above, the process has each advantage and disadvantages in terms of the operation process and operational costs. As described in Table 3, the syngas production process using POM reaction is mainly recommended to other conventional processes. In addition, POM reaction can be applied on a small scale (laboratory) to large scale such as industrial production to be commercialized [15]. POM reaction occurs exothermic and irreversible which produce 36 kJ per mole of heat thus the reaction is being considered due to high efficiency of exergy.
Followed by water gas shift reaction: And CO2 R reaction to form syngas: The selectivity of syngas can be reduced due to the carbon deposition formation which increases the deactivation of the catalytic site of the catalyst by the following Boudouard reaction: The carbon fraction can be gasified by heating or oxygen by the reaction: C + O 2 ↔ CO 2 (11) Therefore, the major product of syngas can be obtained by particularly control of the O2:CH4 ratio as feedstock and the temperature conditions more than 1000 °C with a very short thermodynamic contact time. In addition, the catalyst can be used to control syngas production such as metal oxides, zeolites and perovskite oxides [21].

Active Catalyst Systems for Pom Reaction
As mentioned before, the catalyst plays an  Table 4 The metal from metal oxides catalysts is derived from two types: noble metal catalyst (Rh, Ru, Pt, Pd, Ir) and non-noble metal catalysts (Ag, Pt, Ni, Co) [25]. The noble metal catalyst has higher catalytic activity and is hardly poisoned by carbon deposition compared to a non-noble metal catalyst.
According to an economical subject, nonnoble metals are more economic and widely recommended in the industry [27] Nurherdiana, S.D., dkk. Akta Kimia Indonesia 6(2), 2021, 187-201 Ni + CH 4(g) → CH x(s) + (4 − x)H (s) Nurherdiana, S.D., dkk. Akta Kimia Indonesia 6(2), 2021,  [40]. Given the extreme reaction conditions (high temperature and pressure), the usable oxygen ion-conducting membrane needs to include the criteria as shown in Figure 1 One of the perovskite oxide membranes which has a general arrangement of ABO3 and high oxygen permeation is Cobased as LaCoO3-δ which was first studied by Teraoka et al [37]. In an effort to increase the conductivity of ions and electrons, each side  [44]].
In addition to the LSCF (La1-xSrxCo1-yFeyO3) group, LSM (La1-xSr3MnO3) is also extensively studied for POM reaction due to both structure materials are able to initiate the lattice oxygen to active the CH4 [ [45]]. The oxygen ion conductivity of LSM membrane at 900 o C is 10 -7 S.cm -1 , which is lower than LSCF ~0.2 S.cm -1 . However, LSM has higher reduction activity of O2 with an electron conductor of 300 S.cm -1 than LSCF of 230 S.cm -1 [46]. Therefore, many researchers develop LSCF and LSM-based perovskite oxides, particularly La0.7Sr0.3Co0.2Fe0.8O3 and La0.7Sr0.3MnO3 as the potential catalyst for methane conversion, primarily on syngas production.