Dehydroaromatization

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. 

MFl is typically the preferred zeolite due to its coking resistance, but other zeolites have been used, as detailed in the tables below. The mechanism of the dehydroaromatization is covered in greater detail in Chapter 15, as are representative processes such as UOP s Cyclar process [2, 91]. 

---

The electron impact positive ion spectrum of l,2,5-oxadiazolo[3,4-/]quinoline IV-oxide 46 shows the loss of N2O2 from the molecular ion, a process that must be followed by a substantial rearrangement to enable the observed loss of propyne-nitrile. This remarkable result apparently arises through a series of H-atom shifts which relocate the dehydroaromatic moiety in the heteroring (890MS465). 

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

Dehydrobenzene or benzyne 158 can be trapped by all manner of species. 1,2-Dehydro-o-carborane 159 has been shown to undergo many of the same reactions as its two-dimensional relative, 1,2-dehydrobenzene. Although dehydroaromatic molecules can be formed in a variety of ways, synthetic pathways to 1,2-dehydro-o-carborane are quite limited. An effective procedure reported so far78 first forms the dianion by deprotonation of o-carborane with 2 equiv. of butyllithium. Precipitated dilithium carborane is then treated with 1 equiv. of bromine at 0°C to form the soluble bromo anion 160. Thermolysis of 160 with anthracene, furan, and thiophene as substrates leads to the adducts 161-164.79 80 1,2-Dehydro-o-carborane reacts with norbomadiene to give both homo 2+4 and 2+2 addition, leading to three products 165-167, in a 7 1 ratio79. An acyclic diene, 2,3-dimethyl-... 

Modulation of zeolite acidity by post-synthesis treatments in Mo/HZSM-5 catalysts for methane dehydroaromatization... 

Keywords methane dehydroaromatization, Mo/FIZSM-5, dealumination, acidity. 

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... 

Tessonnier, J.-P., Louis, B., Rigolet, S., Ledoux, M.J., and Pham-Huu, C. (2008) Methane dehydroaromatization on Mo/ ZSM-5 about the hidden role of Bronsted add sites. Appl. Catal. A,... 

Hu, B., Yang, Y., and Sayari, A. (2001) Non-oxidative dehydroaromatization of methane over Ga-promoted Mo/ HZSM-5-based catalysts. Appl. Catal. A, 214, 95-102. 

Smdy of methane dehydroaromatization on impregnated Mo/ZSM-5 catalysts and characterization of nanostructured molybdenum phases and carbonaceous deposits. Ind. Eng. Chem. Res., 46, 4063 074. 

Methane dehydroaromatization on zeolites Mo/HZSM-5 was also investigated by solid-state MAS NMR spectroscopy 162. Both variation of the state of the transition metal component and products (such as ethane, benzene, and ethylene) adsorbed in zeolite were observed after reaction at high temperature (900-1000 K). Molybdenum carbide species, dispersed on the external surface or in the internal channels of the zeolite catalysts, had formed during the reaction 162. ... 

Heptane has a defined octane number of 0 and when dehydroaromatization occurs toluene is formed with an octane number of 116. 

The formation of benzene by the dehydroaromatization coupled with isomerization of methyl cyclopentane also increases octane from 76 to 106. 

Hetarynes (dehydroaromatic heterocycles) can also act as dienophiles in Diels-Alder reactions. 2 This capacity and the inherent regiochemical problems are illustrated by two syntheses of the alkaloid ellip-ticine, which both feature a [4 + 2] cycloaddition of 3,4-pyridyne (533) (Scheme 126). 2 The first approach used the triazenylcarboxylic acid (534) as a precursor for hetaryne (533), which then underwent an addition to pyranoindolone (535) spontaneous CCh extrusion of the initial cycloadducts (536 X = N, Y = CH) and (536 X = CH, Y = N) gave directly a 50 50 mixture of ellipticine (537) and its undesired regioisomer (538). 2 ... 

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. 

Amination of heterocycles with hydroxylamine-O-sulfonic acid 820PP265. Cations and pseudobases, equilibrium constants 80H(14)20I5, Dehydroaromatization of heterocycles, one-electron transfer by ... 

During their studies on the hydrogenation mechanism of quinolines, Zhou and coworkers found that the dehydroaromatization reactions of 1,4 dihydropyr idines (Hantzsch esters) could be realized using their catalytic system, and hydrogen gas was produced in this reaction [29]. Subsequently, they reported the... 

---