Low-Temperature Selective Oxidation Of Methane Research Articles

Low-temperature selective oxidation of methane to methanol over a platinum oxide.

The low-temperature activation of methane is highly important as a reaction that can dissociate the strongest C-H bond and convert it into useful compounds. This study demonstrated that supported platinum oxide was found to activate methane near room temperature and selectively afford methanol in the presence of oxygen.

Breaking the scaling relationship in selective oxidation of methane via dynamic Metal-Intermediate Coordination-Induced modulation of reactivity descriptors on an atomically dispersed Rh/ZrO2 catalyst

Direct conversion of methane to value-added oxygenate products is of considerable importance for effective valorization of methane, but remains a grand challenge in heterogeneous catalysis due to the high energy barrier required for the first CH bond activation and facile overoxidation of products. Generally, there exists a scaling relationship in direct conversion of methane that is a lower activation energy for methane dissociation always accompanies with undesired lower activation energy for overoxidation. In this study, by combining theoretical calculations and experiments, we systematically investigated the CO-assisted low-temperature selective oxidation of methane using H2O2 under aqueous conditions over atomically dispersed Rh/ZrO2. The results reveal that the introduction of CO on Rh/ZrO2 breaks the scaling relationship, which not only facilitates methane activation and conversion benefiting from the Rh-CO coordination, leading to a substantial enhancement of oxygenate products yield, but also prevents the overoxidation of CH3 species, achieving the improvement of methane activation and suppression of overoxidation concurrently. The dynamic metal-intermediate coordination-induced reactivity modulation mechanism was unveiled, in which electronic state and catalytic property of Rh-O active center dynamically changes along with the change of Rh-intermediate coordination during the reaction, giving rise to the dynamic shift of reactivity descriptors towards more optimal values and consequently enabling the facilitation of the initial CH bond activation while suppression of the following overoxidation. This study opens up new perspectives to tune the catalytic performance and offers a comprehensive picture of the dynamics of atomically dispersed Rh-based catalysts in the field of selective oxidation of methane.

Methane Oxidation to Methanol in Water.

Methane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C-H activation and prevention of overoxidation to CO2. Indirect methods such as steam reforming produce CO and H2 chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years.The thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N2O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H2O2 as an oxidant (effectively O2 + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H2O2 is an expensive oxidant to use in the production of methanol.This observation of activity under mild conditions led to new approaches to utilize O2 as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C-H activation reactions using O2 and H2O2, but the rapid decomposition of H2O2 over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H2O2 consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O2 could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area.

Low-temperature selective oxidation of methane over distant binuclear cationic centers in zeolites

Highly active oxygen capable to selectively oxidize methane to methanol at low temperature can be prepared in transition-metal cation exchanged zeolites. Here we show that the α-oxygen stabilized by the negative charges of two framework aluminum atoms can be prepared by the dissociation of nitrous oxide over distant binuclear cation structures (M(II)…M(II), M = cobalt, nickel, and iron) accommodated in two adjacent 6-rings forming cationic sites in the ferrierite zeolite. This α-oxygen species is analogous to that known only for iron exchanged zeolites. In contrast to divalent iron cations, only binuclear divalent cobalt cationic structures and not isolated divalent cobalt cations are active. Created methoxy moieties are easily protonated to yield methanol, formaldehyde, and formic acid which are desorbed to the gas phase without the aid of water vapor while previous studies showed that highly stable methoxy groups were formed on isolated iron cations in iron exchanged ZSM-5 zeolites.

Determining Cu–Speciation in the Cu–CHA Zeolite Catalyst: The Potential of Multivariate Curve Resolution Analysis of In Situ XAS Data

The Cu–CHA zeolite today represents an attractive platform to design catalysts for deNOx applications by NH3-assisted selective catalytic reduction (NH3–SCR) and for the low-temperature selective oxidation of methane to methanol (MTM). Accessing a quantitative understanding of Cu–speciation in this material is a key step to unveil structure–performance relationships for these high-impact processes. Herein, we select Cu–CHA as a case study to demonstrate the potential of chemometric approaches, such as multivariate curve resolution (MCR) applied in combination with principal component analysis (PCA). We employ these methods to assist the interpretation of X-ray absorption spectroscopy (XAS) experiments in the near-edge (XANES) region, determining the spectroscopic signatures and concentration profiles of the pure Cu–species formed. We pinpoint the composition impact on the material reducibility and highlight Cu–speciation–productivity relationships for the MTM process. Furthermore, we report novel insights on the formation of O2-derived species in Cu–CHA, obtained from MCR analysis of high energy resolution fluorescence detected (HERFD) XANES data collected during thermal treatment of Cu–CHA in both He and O2 gas flow. Multivariate analysis, in combination with the superior energy resolution adopted, allows us to identify an additional Cu(II) species. This component, different from the previously characterized Z[Cu(II)OH] moiety, is only formed at significant concentrations in O2 and it is envisaged to play an important role in the MTM conversion.

Zeolite-supported rhodium sub-nano cluster catalyst for low-temperature selective oxidation of methane to syngas

Rh sub-nano clusters supported on zeolite are remarkably more active, selective, and durable than Rh nanoparticles for the conversion of methane to syngas at low temperature.

Low-temperature selective oxidation of methane to ethane and ethylene over BaCO 3/La 2O 3 catalysts prepared by urea combustion method

A series of BaCO 3/La 2O 3 catalysts have been prepared using urea combustion method, and tested their activity in oxidative conversion of methane to ethane and ethylene below 600 °C. For 5% BaCO 3/La 2O 3 catalyst, under integral conditions, CH 4 conversion of 31.8% with C 2 selectivity of 45.9% have been obtained at 320 °C. With the increase of BaCO 3 contents, CH 4 conversion and C 2 selectivity are improved. The temperature profiles of reactor have been described and found that the distribution of hot spots on catalyst bed is related to the low-temperature activity of catalyst.

Low-Temperature Selective Oxidation of Methane into Formic Acid with H2–O2 Gas Mixture Catalyzed by Bifunctional Catalyst of Palladium–Heteropoly Compound

The selective oxidation of methane catalyzed by heteropoly compounds having the formulas MxCs2.5H0.5−2x+yPVyMo12−yO40 (M=Pd2+, Rh2+, Ru2+, Pt2+, Mn2+, Hg2+, Fe3+, Co2+, Cu2+; x=0–3, y=0–3) was investigated. It was demonstrated that addition of Pd and incorporation of V had a strong influence on oxidation with a H2−O2 gas mixture and that Pd0.08Cs2.5H0.34PVMo11O40 showed the highest yield of formic acid. The reaction proceeded at temperatures as low as 423–593 K. Moreover, the reaction rate reached 1.2×10−4 mol h−1 g−1 at 573 K; this value is about 300 times higher than that with FePO4 catalyst. Addition of steam promoted the production of formic acid, and the yield reached a maximum at a partial pressure of steam of 9.1 kPa. The coexistence of H2 and O2 was indispensable for the selective oxidation of methane. It is suggested that an active oxygen species is formed by the reaction of H2 with O2 catalyzed by Pd and acidic sites of supports. Pressure dependencies were expressed by −dPCH4/dt=kPH21.0PO21.0PCH41.0, and is consistent with the idea that the reaction of an active species formed from H2 and O2 with CH4 is rate determining.

A Highly Catalytic Bimetallic System for the Low-Temperature Selective Oxidation of Methane and Lower Alkanes with Dioxygen as the Oxidant

This report describes a highly catalytic bimetallic system for the low temperature selective oxidation of methane, ethane, and butane with dioxygen as the oxidant. The catalytic system consists of a mixture of copper chloride and metallic palladium and operates in a 3:1 mixture (v/v) of trifluoroacetic acid and water in the presence of dioxygen and carbon monoxide. Methane was selectively converted to methanol. The dependence of the reaction rate on the partial pressure of methane was measured, and saturation kinetics was observed. The dependence of the rate on the partial pressure of carbon monoxide was measured at two different pressures of methane, and a first-order dependence on the partial pressure of carbon monoxide was observed in both instances. The activation parameters for the overall reaction were obtained under the reaction conditions when the rate was first order in both methane and carbon monoxide. The values obtained were A = 2 × 104 s-1 and Ea = 15.3 kcal mol-1. Both C−H and C−C cleavage products were observed for ethane and n-butane, resulting in the formation of methanol, ethanol, and acetic acid. The rate of formation of methanol from methane with the bimetallic system was ca. 65 × 10-4 M/min at 145−150 °C. This rate may be compared to the rate of formation of acetic acid from methanol in the benchmark Monsanto process: 380 × 10-4 M/min at 180 °C.

Low-temperature selective oxidation of methane to carbon monoxide and hydrogen over cobalt-MgO catalysts

Reduced CoO-MgO catalysts (Co/Mg mole ratio = 1–10) show high activity and selectivity in oxidative conversion of methane to carbon monoxide and hydrogen with extremely high space-time-yield at low temperatures (⩽ 700 °C, down to 300 °C).