CATALYTIC CONVERSION OF METHANE TO MORE USEFUL CHEMICALS AND FUELS: A CHALLENGE FOR THE 21st CENTURY

The quest for an efficient process to convert methane efficiently to fuels and high value-added chemicals such as olefins and aromatics is motivated by their increasing demands and recently discovered large reserves and resources of methane. Direct conversion to these chemicals can be realized either oxidatively via oxidative coupling of methane (OCM) or nonoxidatively via methane dehydroaromatization (MDA), which have been under intensive investigation for decades. While industrial applications are still limited by their low yield (selectivity) and stability issues, innovations in new catalysts and concepts are needed. The newly emerging strategy using iron single sites to catalyze methane conversion to olefins, aromatics, and hydrogen (MTOAH) attracted much attention when it was reported. Because the challenge lies in controlled dehydrogenation of the highly stable CH4 and selective C–C coupling, we focus mainly on the fundamentals of C–H activation and analyze the reaction pathways toward selective rou...

Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects

The article discusses oxidation catalysis by substitutional cation doping of binary oxides. Substitutional cation doping is not the only possibility. One can imagine that replacing some anions with other anions may also be beneficial. There is evidence that the presence of small amounts of halogen in the feed or on the oxide surface improves its catalytic activity. It is very likely that doped oxide catalysts have been used before the concept was formulated explicitly. Most oxide catalysts have low levels of impurities that may be substitutional dopants. If they segregate at the surface, they can affect the catalytic activity without our knowledge even though their net concentration is very low. It is also possible that the 'as-prepared' catalyst is a doped oxide that, under reducing reaction conditions, is converted to very small metallic dopant clusters supported on the host oxide. The physical and chemical properties of such clusters are different from those of a bulk metal, and it is difficult to distinguish them from a doped oxide.

Catalysis by doped oxides.

Oxidation catalysis not only plays a crucial role in the current chemical industry for the production of key intermediates such as alcohols, epoxides, aldehydes, ketones and organic acids, but also will contribute to the establishment of novel green and sustainable chemical processes. This review is devoted to dealing with selective oxidation reactions, which are important from the viewpoint of green and sustainable chemistry and still remain challenging. Actually, some well-known highly challenging chemical reactions involve selective oxidation reactions, such as the selective oxidation of methane by oxygen. On the other hand some important oxidation reactions, such as the aerobic oxidation of alcohols in the liquid phase and the preferential oxidation of carbon monoxide in hydrogen, have attracted much attention in recent years because of their high significance in green or energy chemistry. This article summarizes recent advances in the development of new catalytic materials or novel catalytic systems for these challenging oxidation reactions. A deep scientific understanding of the mechanisms, active species and active structures for these systems are also discussed. Furthermore, connections among these distinct catalytic oxidation systems are highlighted, to gain insight for the breakthrough in rational design of efficient catalytic systems for challenging oxidation reactions.

Recent advances in heterogeneous selective oxidation catalysis for sustainable chemistry

The partial oxidation of methane into more useful chemicals such as methanol, ethylene and benzene has been investigated extensively, although yields for these products have been poor1–4. Moreover, in several of these processes the required oxidant is N2O rather than O2. Recent work5 in our laboratory has demonstrated that lithium-doped magnesium oxide (Li/MgO) in the presence of O2 has high activity for abstracting H from CH4 to form ·CH3 radicals. This suggests that C2H6 and C2H4 (C2 compounds) are produced by a coupling between two gaseous ·CH3 radicals formed on this catalyst. We report here our success in converting CH4 to C2 compounds in high yields in conventional catalytic conditions.

Synthesis of ethylene and ethane by partial oxidation of methane over lithium-doped magnesium oxide

When methane was passed over MgO at temperatures of approximately 500/sup 0/C, methyl radicals were produced on the surface, released into the gas phase, and trapped downstream in a solid argon matrix where they were analyzed by EPR spectroscopy. Significant differences in initial activity were observed, depending on whether the MgO was pretreated under vacuum or a flow of oxygen. Vacuum conditioning led to essentially no activity while oxygen conditioning resulted in substantial radical production. The oxidant of choice was also critical. Nitrous oxide resulted in a continuous decline of activity while in the presence of oxygen the formation of radicals was at a steady state. Doping of MgO with lithium, sodium, or iron was also examined. Lithium was found to greatly increase the activity up to a doping level of approximately 15.0 wt%. Two pathways are believed to be responsible for the radical formation. Over pure MgO, intrinsic cattion vacancies react with molecular oxygen to given an O/sup -/ center which can abstract a hydrogen atom from methane to produce the methyl radical. For the lithium-doped samples, substitutional Li/sup +/ ions react with molecular oxygen to form a (Li/sup +/O/sup -/) center which is also capable of abstracting amore » hydrogen atom from methane. 28 references, 5 figures, 3 tables.« less

Formation of gas-phase methyl radicals over magnesium oxide

The formation of surface-generated gas-phase radicals during the reactions of methane, ethane, ethylene, and propylene over Bi/sub 2/O/sub 3/, ..gamma..-Bi/sub 2/O/sub 3/ x MoO/sub 3/, PbO, MgO, and Li/MgO was examined by EPR matrix isolation spectroscopy. Allyl radicals were detected during the reaction of propylene over all of the oxides examined. Gas-phase methyl and ethyl radicals were detected during the reactions of methane and ethane, respectively, over MgO and Li/MgO while Bi/sub 2/O/sub 3/, ..gamma..-Bi/sub 2/O/sub 3/ x MoO/sub 3/, and PbO were essentially inactive. Gas-phase vinyl radicals were not detected during the reaction of ethylene over any of the oxides. These differences in reactivity are attributed to differences in reactivity of the oxide ions on these surfaces towards hydrogen atom abstraction. 28 references, 1 figure.

Gas-phase radical formation during the reactions of methane, ethane, ethylene, and propylene over selected oxide catalysts

Activation and oxidative dimerization of methane over lithium-promoted zinc oxide

Publisher Summary This chapter discusses the formation, detection, and role in catalysis of surface-generated gas-phase radicals. The presence of surface-generated gas-phase radicals is well established for many catalytic decomposition and oxidation reactions. The radicals generally are observed at reaction temperatures in excess of 400° C, and in some cases temperatures of approximately 800°C are required. The surface-generated radicals may be significant as (1) initiators of chain reactions, (2) intermediates in gas-phase coupling reactions, (3) a means for the transport of intermediates from one surface to another, (4) experimental evidence for a radical mechanism, and ( 5 ) a means of determining the rate limiting step of a catalytic cycle. Even qualitative results are useful in several of these applications; however, more quantitative data are needed to establish whether gas-phase radical coupling is indeed a primary pathway for product formation. The practical utilization of this source of gas-phase radicals, in many cases, require the design of reactors and catalyst-bed configurations so as to maximize the desired reactions. It may, for example, be important to operate at short contact times, with the catalyst at much higher temperatures than the gas phase, or, in order to achieve high selectivity for the partial oxidation of alkanes, it may be necessary to operate in a pulse mode so that the alkyl radicals do not react with gas-phase molecular oxygen.

Daniel J. Driscoll

Papers

Formation of gas-phase methyl radicals over magnesium oxide

Gas-phase radical formation during the reactions of methane, ethane, ethylene, and propylene over selected oxide catalysts

Activation and oxidative dimerization of methane over lithium-promoted zinc oxide

Surface-Generated Gas-Phase Radicals: Formation, Detection, and Role in Catalysis

Methanol synthesis over palladium supported on silica