Aromatics dehydroaromatization

MoZSM-5 Methane dehydroaromatization Higher activity and selectivity to aromatics Higher tolerance to coking [75]... 

The first example illustrates how a 1,4-dehydroaromatic system with cyclohexane ring having two double bonds may be also disconnected according to a retro-Diels-Alder to give a diene and an acetylene as the dienophile [25]. The second example makes clear that even an aromatic double bond may be -in some instances-involved in a retrosynthetic pericyclic disconnection [26]. In the synthetic direction, the polycyclisation involves a conrotatory electrocyclic cyclobutene ring opening, (16 15) followed by an intramolecular Diels-Alder addition (see Scheme... 

The dehydroaromatization of light alkane feeds (methane to butanes) into aromatics has come into prominence as a method of converting the unreactive light paraffins into useful chemical precursors. In many of the world s markets, light alkanes are very undesired off-gasses which can not be used other than as fuel. To accomplish this difficult transformation, catalysts typically are bifunctional, containing a dehydrogenating component such as Pt, Ga, Zn or Mo with an acidic zeolite. 

Metal oxides on zeolites have also found use as redox catalysts. High-temperature (700-750 °C) dehydroaromatization of methane under nonoxidizing conditions has been explored with a number of zeolitic catalysts modified with transition metal ions. Although coke formation at these high temperatures is a problem, calcined molybdate-impregnated ZSM-5 shows unparalleled activity of up to 8 % methane conversion with 100 % selectivity towards aromatics. Surface studies of these Mo HZSM-5 catalysts indicate that M0O3 crystals are on the external zeolite surface [123]. 

We undertook to investigate the reaction of olefins with oxide-free carbon surfaces in the hopes that chemisorption would occur in ways which could be related to the known chemistry of aromatic free radicals and dehydroaromatics, and that suitable manipulation might provide routes to homogeneously functionalized surfaces. High surface-area carbon fibers were heated to approximately 1000°C under vacuum to remove the surface oxides (evolution of H2O, C02, and CO). The samples were cooled to room temperature and exposed to vapors of various different substrates. The quantity of substrate adsorbed was determined and corrected for the quantity of physisorbed material which could be pumped off at room temperature. Some typical results are reported in Table II. In certain cases the reactivity towards oxygen was redetermined after exposure to the organic substrate. 

On the basis of the results obtained one may draw a conclusion that there exists a certain relationship between the concentration of acidic sites of different types of zeolite and Mo content. In this coimection, to produce a catalyst e diibiting a high activity in the process of methane dehydroaromatization, it is necessary to optimise the relationship between the acidic sites number of a zeolite and the number of active sites connected with different Mo forms. The highest methane conversion per one run and maximal yield of aromatic hydrocarbons are reached for the sample containing 4.0 mass% of Mo nanopowder. The development of the mesoporous zeolite structure is an important factor promoting the activity of Mo-ZSM-5 in the reactions of the formation of high-molecular aromatic compounds. 

In this section we shall discuss an example that is described in detail in the literature [29, T40], An attempt was made to develop a catalytic process for the production of aromatics from olefins by means of an oxidative dehydroaromatization... 

Dehydroaromatization of methane Hierarchical Mo/HZSM-5 Enhanced selectivity to aromatics because of a larger tolerance to coke [180]... 

From a mechanistic point of view, in the initial step, the (Z)-l,2,4-heptatrien-6-yne, or compounds containing an equivalently unsaturated core, undergoes a mild, thermal electrocyclization reaction to form an a,3-alkylbenzenediyl, a diradical intermediate with substantial polar character. Dehydroaromatic intermediate 7, when trapped by the solvent or compounds (e.g., 1,4-cyclohexadiene) present in the reaction medium, forms than aromatic products of type 8, 9, and 10. In methanol, a mixture of hydrogen atom abstraction and polar addition product are obtained. ... 

The Mo-containing MFl-type core-shell HZSM-5-Silicalite-l s (HZ5 S1) materials with various core-shell ratios prepared by the epitaxial growth of Silicalite-1 on HZSM-5 demonstrated the high shape selectivity to aromatics and stability in methane dehydroaromatization [66]. The silicalite-1 layer covering the HZSM-5 core can eliminate the external acid sites, and thus prevent the formation of active Mo species associated with Brpnsted acid sites on the external surface of catalysts. However, the overgrowth of the Silicalite-1 shell may lead to a severe inhibition of the Mo species migration into zeolite pores and consequent anchoring on the Brpnsted acid sites of the HZSM-5 core. Therefore, the catalytic performance is dependent on the core-shell ratio. 

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