Methanation zeolite stability

A further possibility for side-chain alkylation of toluene is oxidative methylation with methane. Catalysts with occluded alkali metal oxides, prepared by impregnating zeolites with alkali metal hydroxides followed by calcination, usually exhibit better performance.441 Further enhancement was achieved by impregnating ion-exchanged zeolites 442 Significant improvements in stability and the yields of Cg hydrocarbons were also observed when NaX was impregnated with 13% MgO which was found to increase the amounts of active sites.443... 

Although nickel is not the most active metal, for reasons of cost and stability, commercial catalysts are based on this metal. These catalysts are in general supported on alumina and contain relatively high metal loadings. There is, however, interest in the development of more thermally stable, sulfur resistant, and possibly even regenerable catalysts in the future. It may be for these reasons that zeolite-based methanation catalysts have recently attracted more interest. 

The production of hydrogen from methane over zeolite supported metal catalysts can be examined as an alternative to steam reforming because the concomitant aromatization reactions can increase the economic potential of the process. For methane aromatization, Mo/ZSM5 catalysts have been intensively studied since their first report in 1993 (/, 2). In 1997 (3), the promotional effect of ruthenium over Mo/ZSM5 catalysts was reported. Other second metals have also been studied to improve catalyst activity and stability and a review on this topic is available 4). 

Iron complexes stabilized in a ZSM-5 zeolite matrix produce a new form of surface oxygen (a-oxygen) upon decomposition of N20[58], a-Oxygen selectively oxidizes methane and benzene ... 

The nature of active sites of In zeolites and their function and behavior in the SCR of NOx with methane has been investigated. In zeolite structures (mordenite, ZSM-5) were prepared by various methods. The catalysts were additionally promoted by oxidic components, above all CeOx, to improve their activity as well as to stabilize their performance in the presence of water vapor. The obtained composite materials showed promising activity under reaction conditions. The promoting effect of the oxidic components will be discussed shortly. 

A synergistic effect leading to the increased catalyst activity and selectivity in selective catalytic reduction (SCR) of NO with methane or propane-butane mixtures was found when cobalt, calcium and lanthanum cations were introduced into the protic MFl-type zeolite. This non-additive increase of the zeolite activity is attributed to increased concentration of the Bronsted acid sites and their defined location as result of interaction between those and cations (Co, Ca, La). Activation of the hydrocarbon reductant occurs at these centers. Doping the H-forms of zeolites (pentasils and mordenites) with alkaline earth metal and Mg cations considerably increased the activity of these catalysts and their stability to sulfur oxides. 

The presence of NO in the gas phase may interfere with the methane combustion activity. Low-loading Pd zeolites may stabilize Pd " species, inactive for methane combustion, when NO is present. These catalysts exhibit very low combustion activity, but are effective for the selective reduction of NO. 

Defect free DD3R supported membranes have been newly prepared by NGK insulators (Japan).The great advantage of this zeolite type with respect to the SAPO-34 and T zeolites, should be the chemical and thermal stability because of its all silica structure. Kapteijn and co-workers studied the permeation of various gases (carbon dioxide, nitrous oxide, methane, nitrogen, oxygen. 

Groothaert MH, Smeets PJ, Sels BF, Jacobs PA, Schoonheydt RA (2005) Selective oxidation of methane by the Bis(/-oxo)dicopper core stabilized on ZSM-5 and mordenite zeolites. J Am Chem Soc 127 1394... 

Zeolite membranes show high thermal stability and chemical resistance compared with those of polymeric membranes. They are able to separate mixtures continuously on the basis of differences in the molecular size and shape [18], and/or on the basis of different adsorption properties [19], since their separation ability depends on the interplay of the mixture adsorption equilibrium and the mixture. Different types of zeolites have been studied (e.g. MFI, LTA, MOR, FAU) for the membrane separation. They are used still at laboratory level, also as catalytic membranes in membrane reactors (e.g. CO clean-up, water gas shift, methane reforming, etc.) [20,21]. The first commercial application is that of LTA zeolite membranes for solvent dehydration by pervaporation [22], Some other pervaporation plants have been installed since 2001, but no industrial applications use zeolite membranes in the GS field [23]. The reason for this limited application in industry might be due to economical feasibility (development of higher flux membranes should reduce both costs of membranes and modules) and poor reproducibility. 

The supramolecular environment can not only protect an active species but also stabilize it through local constraints. Cu-ZSM-5, a Cu-loaded zeolite where Cu ions are incorporated into the walls, was proposed to generate a mono-(/z-oxo)-dicopper core upon activation at 723 K under an O2 flow, as evidenced by UV experiments. Aerobic oxidation of methane was carried out at 448 K and CH3OH was detected as the only product, trapped inside the host. The nature of the host (Al/Si and Cu/Al ratios)... 

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