Direct synthesis of oxygenates via partial oxidation of methane in the presence of O2 and H2 over a combination of Fe-ZSM-5 and Pd supported on an acid-functionalized porous polymer
Graphical abstract
Introduction
Direct conversion of methane into value-added products has been a challenge over the past decades. Recent interest in new chemical feedstock other than coal and oil and growing shale gas production has rekindled the need for direct conversion of methane [1]. Several methodologies are reported for the direct conversion of methane, including aromatization, coupling, pyrolysis, and partial oxidation of methane into oxygenates [2]. Among them, partial oxidation of methane is particularly attractive in that it directly converts methane under mild reaction conditions with less energy required than the syngas route.
The partial oxidation of methane in the gas phase has been frequently reported over Cu and Fe catalysts supported on zeolites or SiO2 [[3], [4], [5], [6]]. Generally, it requires N2O, O2, or H2O as the oxidant at a reaction temperature of 200 ℃ or higher [[7], [8], [9], [10], [11], [12]]. In comparison, the partial oxidation of methane in liquid phase can be conducted at room temperature. Various noble metals [[13], [14], [15], [16], [17], [18]] such as Pd, Pt, and Rh and non-noble metals [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]] such as Fe, Co, Cu, V, Os, Eu, and Hg have been used as catalysts. Oxidants include SO3, K2S2O8, hydroperoxide (organic peroxides and H2O2), and O2 with or without reducing agents [[30], [31], [32], [33], [34], [35], [36], [37]]. However, oxidants costing more than O2 are commonly utilized. Furthermore, protic acids such as sulfuric acid and trifluoroacetic acid are generally used as solvents in order to stabilize methanol, which consequently cause the instability of the heterogeneous catalysts used [38]. Aqueous-phase methane oxidation was also reported over Fe-ZSM-5, Au-Pd/TiO2, Au-Pd colloid and Au-Pd/ZSM-5 using H2O2 [[39], [40], [41]]. Since H2O2 is an expensive oxidant, utilization of O2 in the presence of H2 has also been investigated to oxidize methane in aqueous phase [42,43]. Recently, Xiao et al. reported that the concentration of H2O2 within the catalyst was enriched through substitution of carbon chains on the outer surface of Au-Pd nanoparticles encapsulated in ZSM-5, which led to an increase in methanol yield [44].
The direct synthesis of hydrogen peroxide from H2 and O2 has low product yields since decomposition and hydrogenation occur simultaneously within the reaction [46,47]. Low pH of reaction medium was reported to be beneficial for stabilization of H2O2 and inhibiting side reactions [48]. However, the use of strong protic acids can cause corrosion of the reactor or leaching of active metal into the reaction medium [49,50]. Thus, solid acids including insoluble heteropoly acid [51], zeolites [52], acid- functionalized carbon material [53], metal oxide [54], and acid-functionalized hyper-crosslinked porous polymer (HCPP) [45] have been applied to overcome these drawbacks.
Based on the fact that Fe-ZSM-5 is quite effective for partial oxidation of methane in water and that Pd catalysts supported on an acid-functionalized support are efficient for H2O2 generation from H2 and O2 [45], herein we report our work combining these two catalytic systems in order to oxidize methane over Fe-ZSM-5 with H2O2 generated in situ from H2 and O2 over a supported Pd catalyst (Scheme 1).
Section snippets
Chemicals
All materials such as iron sulfate, palladium(II) chloride, palladium(II) acetate, hydrazine hydrate, dichloromethane, 1,3,5-triphenylbenzene, 2,4,6-tris(bromomethyl)mesitylene, anhydrous aluminum chloride, sodium hydroxide, potassium permanganate, chlorosulfonic acid, sulfuric acid, Ferroin indicator and activated carbon (AC) were purchased from Sigma-Aldrich. Methanol, ethanol, acetone, and hydrochloric acid were received from Merck Millipore. Cerium(IV) sulfate was purchased from Kanto
Characterization of the catalysts
The physical properties of the supports and Pd catalysts were measured and reported in Table S1. The BET surface area of HCPP, c−HCPP, and c-s−HCPP was determined to be 1.84 × 103, 1.45 × 103, and 929 m2/g, respectively. The pore volume of HCPP, c−HCPP, and c-s−HCPP was determined to be 1.6, 1.3, and 0.7 cm3/g, respectively. These results indicate that the porous structure of porous polymer was partly degraded during the surface functionalization. On the other hand, there was no significant
Conclusions
In summary, we demonstrated the partial oxidation of methane in water over a combination of Fe-ZSM-5 and Pd/c-s−HCPP catalysts in the presence of H2 and O2. The acid-functionalized HCPP was essential for in situ generation of H2O2 over Pd catalyst in the absence of an added acid. Metal leaching of Fe-ZSM-5 was not observed due to the absence of liquid acid in the reaction system. The cooperation of Pd/c-s−HCPP and Fe-ZSM-5 obtained a TOF of 3.8 h−1 and selectivity to methane oxygenates of 94 %
CRediT authorship contribution statement
Jongkyu Kang: Validation, Investigation, Data curation, Writing - original draft. Pillaiyar Puthiaraj: Investigation, Data curation. Wha-seung Ahn: Writing - review & editing. Eun Duck Park: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2015M3D3A1A01064899).
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