Platinum Metal-Free Catalysts for Selective Soft Oxidative Methane → Ethylene Coupling. Scope and Mechanistic Observations

Abstract

Using abundant soft oxidants, a high methane-to-ethylene conversion might be achievable due to the low thermodynamic driving force for over-oxidation. Here we report on the oxidative coupling of methane by gaseous S2 (SOCM). The catalytic properties of Pd/Fe3O4 are compared with those of Fe3O4, and it is found that high ethylene selectivities can be achieved without noble metals; conversion and selectivity on Fe3O4 are stable for at least 48 h at SOCM conditions. SOCM data for 10 oxides are compared, and ethylene selectivities as high as 33% are found; the C2H4/C2H6 ratios of 9-12 observed at the highest S2 conversions are significantly higher than the C2H4/C2H6 ratios usually found in the CH4 coupling with O2. Complementary in-detail analytical studies show that, on Mg, Zr, Sm, W, and La catalysts, which strongly coke during the reaction, lower ethylene selectivities are observed than on Fe, Ti, and Cr catalysts, which only coke to a minor extent. Further catalyst-dependent changes during SOCM in surface area, surface composition, and partial conversion to oxysulfides and sulfides are discussed. Evidence concerning the reaction mechanism is obtained taking into account the selectivity for the different reaction products versus the contact time. CH4 coupling proceeds non-oxidatively with the evolution of H2 on some catalysts, and evidence is presented that C2H4 and C2H2 formation occur via C2H6 and C2H4 dehydrogenation, respectively.

The Reactive Sites of Methane Activation: A Comparison of IrC3+ with PtC3+

“…Methane has attracted attention as the main component of natural gas and the conversion of methane into value-added chemicals is very important [ 1 , 2 ]. However, methane is extremely stable, with high C–H bond strengths (439 kJ/mol), negligible electron affinity and low polarizability [ 3 ].…”

Design and tailoring of advanced catalytic process for light alkanes upgrading

“…As a consecutive effort, they further studied the OCM by S 2 (SOCM) over 10 oxides. 73 It was found that the Mg, Zr, Sm, W, and Labased oxide catalysts suffered from severe coking and delivered limited ethylene selectivity, while Fe, Ti, and Cr-based oxide catalysts coked to a minor extent and exhibited much higher selectivity. Notably, the highest ethylene selectivity of 33% was achieved by further optimizing the reaction conditions, which was comparable to that of noble metal catalysts.…”

“…By further optimizing the operation conditions, for example, high CH 4 /S ratio, short contact time and the provision of a support, the efficiency of CH x intermediates coupling was significantly promoted with ethylene selectivity approaching 20%. As a consecutive effort, they further studied the OCM by S 2 (SOCM) over 10 oxides 73 . It was found that the Mg, Zr, Sm, W, and La‐based oxide catalysts suffered from severe coking and delivered limited ethylene selectivity, while Fe, Ti, and Cr‐based oxide catalysts coked to a minor extent and exhibited much higher selectivity.…”

Alternative Oxidants for the Catalytic Oxidative Coupling of Methane

“…A follow‐up study examined simple metal oxides as catalyst precursors for SOCM [61] . Since the oxides transform into the corresponding metal sulfides following pretreatment in an H 2 S/S 2 mixture, the resulting metal sulfide was considered to be the active phase.…”