A breakthrough in direct conversion of methane to oxygenates under mild conditions

Abstract

Methane is an attractive C1 resource for production of value-added chemicals. The current industrial utilization of methane involves indirect route via preliminary oxidation to syngas (a mixture of CO and H 2 ) that can be converted to desired products [1]. Such two-stage process is energy-intensive. Considerable efforts have been devoted to developing catalytic processes that can efficiently and directly transform methane through oxidative or non-oxidative pathways [2,3]. Since the pioneering work of methane dehydroaromatization under nonoxidative conditions in 1993 [4], a variety of catalysts based on metal ions (Mn, Fe, Zn, Mo, W, etc.) dispersed on zeolites have been tested to improve the efficiency and product selectivity of non-oxidative methane conversion [3]. Recently, lattice-confined single-iron catalyst embedded in a silica matrix has been developed to convert methane exclusively to ethylene and aromatics with total hydrocarbon selectivity exceeding 99% at methane conversion of 48.1% [5]. However, the catalysis requires an extremely high operating temperature of 1090°C. Oxidative conversion of methane to oxygenates over heterogeneous transition metal oxide catalysts (typically MoO 3 and V 2 O 5 ) are also generally performed under high temperatures (>350°C) [6,7]. Although homogeneous metal (e.g., Pt, Pd, Au, etc.) catalysis opened up the possibility of oxidizing methane at low temperatures (0-220°C) with high selectivity [8], the commercial application is hampered by the need for highly corrosive or expensive chemicals and the difficulty in separation/recycling of the catalysts. The natural methane monooxygenases (MMOs) can selectively catalyze the transformation of methane to methanol at ambient conditions [9]. Attempts to artificially mimic the activity of MMOs at low temperatures have been made by encapsulating the active binuclear centers of Cu and Fe, in a form analogous to MMOs, within the zeolite catalysts [10]. Nonetheless, those artificial catalysts are far from industrial application due to the low product yield (<200 μmol g cat -1 ). Recently, Shan, Li, and Flytzani-Stephanopoulos at Tufts University in collaboration with their co-workers from Oak Ridge and Argonne National Laboratories reported an exciting discovery that the atomically dispersed rhodium catalysts on zeolite (ZSM-5) and titanium dioxide (titania) can catalyze the direct oxidation of methane to oxygenates (acetic acid and methanol) using oxygen and carbon monoxide at aqueous solution [11]. Furthermore, the catalysts can be operated at low temperature of 150°C. The product yields of acetic acid over 0.5 wt% Rh-ZSM-5 and methanol over 0.6 wt% Rh-TiO 2 can reach~22,000 μmol g cat -1 and~230 μmol g cat -1 , respectively. The product selectivity ranges from 60% to 100%.The meticulous performance test of the catalysts revealed that the product selectivity is oxygen-limited and acidity-sensitive. For Rh-ZSM-5 catalysts, the product selectivity of methanol and acetic acid increases significantly upon decreasing the partial pressure...

Direct Conversion of Methane with Carbon Dioxide Mediated by RhVO3−Cluster Anions

“…Thesingle rhodium atom catalyst has received increasing interest in recent years in heterogeneous catalysis. [19] It was found that the isolated Rh atoms anchoring on the supports of zeolite, [19a-c] titania, [19a] and zirconia [19d] can catalyze the oxidation of methane to acetic acid or methanol by using strong oxidants such as O 2 and H 2 O 2 at 423 K. The dispersed Rh atoms in the cationic states (+ 1, + 3, and + 4) were proposed as the active sites to selectively activate the first CÀHb ond of methane to form the important CH 3 intermediate,w hich then reacted with the adsorbed O 2 and H 2 O 2 to produce the oxygenated products.I nc ontrast to the heterogeneous catalysis,o ur gas phase study reveals an ovel route for direct methane conversion:t he single Rh atom supported on av anadium oxide cluster (RhVO 3 À )c an mediate the catalytic oxidation of methane to methanol by using the soft oxidant of CO 2 .T he NBO analysis indicated that the electron configuration of the Rh atom in RhVO 3 À cluster is [5s(0.75)4d(8.17)5p(0.07)5d(0.01)].T he total number of electrons of such electron configuration is almost equal to those of bare Rh atom (5s 1 4d 8 ). Thus,t he selective methane activation by RhVO 3 À occurs over the Rh atom in the neutral state.Subsequently the formed CH 3 intermediate reacts with the hydroxyl group to generate CH 3 OH after CO 2 reduction by the Rh À Vb ond.…”