A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

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

The synthesis of bulk amounts of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chemical compositions and textural properties of the catalyst material used in the chemical vapor deposition (CVD) of methane A series of catalysts are derived by systematically varying the catalytic metal compounds and support materials The optimized catalysts consist of Fe/Mo bimetallic species supported on a novel silica−alumina multicomponent material The high SWNT yielding catalyst exhibits high surface-area and large mesopore volume at elevated temperatures Gram quantities of SWNT materials have been synthesized in ∼05 h using the optimized catalyst material The nanotube material consists of individual and bundled SWNTs that are free of defects and amorphous carbon coating This work represents a step forward toward obtaining kilogram scale perfect SWNT materials via simple CVD routes

/pdf/large-scale-cvd-synthesis-of-single-walled-carbon-nanotubes-3se2jf33jy.pdf

Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes

The efficient use of natural gas will require catalysts that can activate the first C-H bond of methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At 1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The lattice-confined single iron sites delivered stable performance, with no deactivation observed during a 60-hour test.

Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen

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 dehydrogenation and aromatization of methane on modified ZSM-5 zeolite catalysts has been studied under non-oxidizing conditions with a fixed bed continuous-flow reactor and with a temperature programmed reactor. The results show that benzene is the only hydrocarbon product of the catalytic conversion of methane at high temperature (973 K). The catalytic activity of ZSM-5 is greatly improved by incorporating a metal cation (Mo or Zn). H2 and ethene have been directly detected in the products with a mass spectrometer during TPAR. A carbenium ion mechanism for the activation of methane is suggested.

Dehydrogenation and aromatization of methane under non-oxidizing conditions

The activation and aromatization of CH4 and C2H6 under non-oxidizing conditionswere investigated over Mo(VI)/HZSM-5 and W(VI)/HZSM-5 zeolite catalysts. Mo(VI)/HZSM-5 exhibits good activity and selectivity for the conversion of CH4 to aromatics, but for C2H6 conversion the catalyst displays high selectivity to CH4. W(VI)/HZSM-5 is inactive for CH4 activation, but exhibits high selectivity for C2H6 aromatization. The W(VI) species can act as a modifier to improve the activity and the stability of Mo(VI)/HZSM-5 for CH4 conversion.

Activation and aromatization of methane and ethane overMo(VI)/HZSM-5 and W(VI)/HZSM-5 zeolites catalysts

The conversion of C3-C9 paraffins to small olefins over ZSM-5 zeolite is investigated. The small olefins are primary products and are usually converted into other more stable secondary products such as aromatics on the ZSM-5 zeolites. Thermally treated HZSM-5, K/HZSM-5 and Ba/HZSM-5 catalysts were developed and favourable oxidative conditions were introduced for the conversion process to maximize selective conversion of light paraffins to small olefins at the relatively low temperature of 873 K. The role of K and Ba is to minimize bimolecular hydrogen transfer reactions and enhance the dehydrogenation activity of the catalysts. Meanwhile, the oxygen in the gas phase is effective to improve the olefin selectivity and yield. C2-C4 olefin selectivities of 70.4 and 66.8% have been obtained for propane andn-hexane feed-stocks, respectively, at a temperature of 873 K.

Conversion of light paraffins for preparing small olefins over ZSM-5 zeolites

Publisher Summary Methane is the major component of natural gas and its catalytic conversion to desired chemical products or liquid fuel is not only a promising approach for the utilization of natural gas resource but also a great subject to catalysis science that deals with the activation of small molecules. Obviously, direct methane conversion to higher hydrocarbons seems to be one reasonable way to overcome this economic problem. In recent years, the catalytic conversion of methane has been actively studied using dioxygen or other oxidants. The most extensive study has been the oxidative coupling of methane (OCM) since 1982. But the difficulty in meeting the conditions required by the industrial application of the oxidative coupling of methane is still left. Meanwhile, the conversion of methane to benzene over high silica zeolite based catalysts in a pulse-reactor was reported, but limited information was provided. In this chapter the optimizing procedure of methane aromatization over Mo/HZSM-5 is presented and the mechanism for methane activation and aromatization is proposed.

Longxiang Tao

Papers

Dehydrogenation and aromatization of methane under non-oxidizing conditions

Activation and aromatization of methane and ethane overMo(VI)/HZSM-5 and W(VI)/HZSM-5 zeolites catalysts

Conversion of light paraffins for preparing small olefins over ZSM-5 zeolites

The aromatization of methane over Mo/HZSM-5 zeolites without using oxidants