Partial oxidation of methane to methanol over MxOy/ZSM-5 (Mn, Fe, Co, and Ni) hierarchical transition metal oxide catalysts

The selective oxidation of methane to methanol is a key challenge in catalysis. The catalytic activities of nickel, cobalt, iron, and manganese oxides supported by hierarchical ZSM-5 zeolites for the partial oxidation of methane to methanol were studied. Hierarchical ZSM-5 was synthesized using a double-template method, in which tetrapropylammonium hydroxide was used as an MF1 structure-directing agent and PDDA-M was used as a mesopore-directing agent. The synthesized hierarchical ZSM-5 was characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy with energy dispersive spectroscopy. MxOy/ZSM-5 catalysts were prepared using a wet impregnation method and were also extensively characterized. The catalytic test was performed at 423 K in a batch reactor with a CH4. to N2 ratio of 0.5:2 and 0.5 g of MxOy/ZSM-5 catalyst for 2 h. The hierarchical Fe2O3/ZSM-5 catalyst showed the highest yield of methanol, whereas Mn3O4./ZSM-5 tended to produce formic acid.


Introduction
Natural gas has been known as a clean and effective energy source since its combustion generates fewer greenhouse gases than coal or petroleum fuels. However, its storage and transport is not easy, making its derivatives uncompetitive with those of other fossil fuels [1]. Conventionally, an indirect route for conversion of natural gas to methanol was through the production of syngas. However, the process is an energy intensive process, (temperature reaction of 800 ℃-1000 ℃) and more than 25 % of the feed (natural gas) has to be burned to provide the heat of reaction [2]. Hence, such a sustainable direct conversion of methane to methanol is more desirable compared with the current technology [3].

ZSM-5 synthesis and MxOy/ZSM-5 preparation
The synthesis was carried out following method reported by Wang et al. [6] with some modifications. Afterward, this mixture was added dropwise by 1 g PDDA stirred for 24 h, then hydrothermal processed in a Teflon-lined stainless steel autoclave at 423 K for 144 h. The then obtained zeolite (white powder) was dried in room temperature before calcined at 820 K. Metal oxide insertion into the zeolite was carried out using a wet impregnation method, where 1 g of ZSM-5 was added to 0.2495 M metal solution prepared from metal nitrate hydrate and stirred for 24 h at ambient condition, followed by calcination at 823 K. Respectively, the hierarchical ZSM-5 and MxOy-ZSM-5 counterparts were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR, Shimadzu Prestige 21), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDX, SEM FEI quanta FEG 450), and atomic absorption spectroscopy (AAS, Shimadzu 13000).

Catalytic test: partial oxidation of methane
The methane partial oxidations were performed using each MxOy/ZSM-5 catalysts (c.a. 0.5 g) in a 200 mL stainless steel batch reactor. Prior to use, the catalyst (0.5 g) was activated at 773 K and flushed with flowing nitrogen gas. The catalytic reaction was carried out at 423 K for methane and nitrogen pressures of 0.75 and 2 bar, respectively. After the reactor was left to cool down to room temperature, reaction product was extracted from the zeolite using 3 mL of ethanol and analyzed using gas chromatography (GC Shimadzu 2010) with a Carbowax column and flame ionization detector (Shimadzu).

Results and discussion
3.1. Characterization of ZSM-5 3.1.1. XRD Pattern. The ZSM-5 zeolite was synthesized using TPAOH as an MFI structure-directing agent and PDDA polycation as a mesopore template. During the formation of zeolites with Si/Al ratios ranging from 10.5 to 36.5, competition between ZSM-5 and mordenite occurred during the hydrothermal crystallization process [5]. However, the positively charged PDDA interacts with the negatively charged ZSM-5 crystal-to-be-structure, so that the ZSM-5 framework could be established around the PDDA polication [6]. The XRD pattern of as-synthesized hierarchical ZSM-5 (figure 1) shows certain peaks in the 2θ range from 7° to 9° and from 22° to 25°, which is consistent with the standard ZSM-5 pattern [7], indicating that the ZSM-5 structure was successfully synthesized [6]. Figure 2 shows the FTIR spectra of ZSM-5 before (a) and after (b) calcination. The spectrum of ZSM-5 before calcination shows several extra peaks at 2960-2850 cm −1 (C-H stretching) and at 1470-1350 cm −1 (C-H bending) compared with that of ZSM-5 after calcination, which can be attributed to the templates used in the synthesis of zeolite, i.e., TPAOH and PDDA. Calcination was performed to remove the organic templates used in the synthesis of zeolites. It can be seen from figure 2b that the template was removed as indicated by the loss of the C-H stretching and bending vibration bands. Moreover, the pores and channels in the zeolite became unoccupied after calcination.

SEM-EDX.
To determine the crystal structure and the Si/Al ratio of the synthesized zeolite, EDX analysis was performed, from which the mass% of the elements in the zeolite can be determined, in which the Si/Al ratio was 37.4. Table 1 shows the mass% of the elements present in a ZSM-5 crystal.

Methane partial oxidation catalytic test
The results of methane partial oxidation using as synthesized ZSM-5 catalysts are shown in figure 4. ZSM-5 without metal oxides produced methanol in 6.8% yield. The impregnation of most metal oxides in ZSM-5 increased the % yield of methanol, which indicates that the metal oxide is capable of being the active site of the catalyst, thereby increasing the catalytic activity for oxidizing methane to methanol. However, when using Mn3O4-modified ZSM-5, formic acid was formed with higher yield than methanol, indicating Mn3O4/ZSM-5 has selectivity toward the formation of formic acid.

Conclusions
Catalysts based on ZSM-5 modified with four metal oxides have been successfully synthesized, as confirmed by XRD data. The presence of metal oxides increased the number of catalyst active sites, resulting in high yield of methanol. It was found that the type of metal oxide species influenced the yield of methanol. Among the four types of metal oxide-modified ZSM-5 catalysts, Fe2O3/ZSM-5 is the most promising for partial oxidation of methane to methanol with the highest yield of 30.5%, whereas Mn3O4/ZSM-5 has selectivity toward the formation of formic acid.