Methane aromatization

The direct catalytic conversion of methane has been actively pursued for many years. Much of the emphasis has been on the direct production of methanol via selective partial oxidation (8), coupling of methane to ethylene (9), or methane aromatization (10). At this time none of these technologies has been demonstrated commercially due to low yields of desired products due to combustion by-products or low equilibrium conversion at reasonable process temperatures and pressures. The potential benefits of a hypothetical process for the direct partial oxidation of methane to methanol (11) are presented as an example. 

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). 

Inter- and intramolecular oxidative addition of C-H bonds into Re(PM63)2Cp or Re(L)(PMe3)Cp (L = CO, PMe3) occurs on photolysis of an appropriate complex. Primary, cyclopropyl, methane, aromatic, and vinyl C-H bonds are attacked but not secondary centres. 

Luzgin MV, Rogov VA, Arzumanov SS, Toktarev AV, Stepanov AG, Parmon VN. Understanding methane aromatization on a Zn-modified high-silica zeolite. Angew Chem Int Ed 2008 47 4559-62. 

Starting with the early publications in 1980s, quite many attempts have been made to develop Mo/zeolite catalysts for methane aromatization ... 

Mo-modified catalysts Mo/TNU-9 [65] perform in the best way in methane aromatization at Si/Al=50, 6 wt.% MoOj loading, and reaction temperature of 973 K. The Mo/TNU-9 catalyst showed a higher CH conversion and selectivity to benzene compared with Mo/ZSM-5. 

Mesoporous ZSM-5-S and ZSM-5-M samples prepared by using ordered mesoporous carbon (CMK-3) and disordered carbon rods (C-MCM l) as the hard template, respectively, were compared with the conventional ZSM-5-C in methane aromatization [67] The Mo-ZSM-5-S and Mo-ZSM-5-M catalysts showed similar conversions of methane, but higher yields of aromatics compared with Mo-ZSM-5-C. The mesoporous catalysts were also more stable than Mo-ZSM-5-C. The formation of the secondary mesoporous system within the zeoUte crystal, which may lead to easier access to the active sites for reactants and be favorable for the diffusion of larger molecules formed in the microporous channels during the methane aromatization reaction, account for the better stability of the mesoporous systems. 

Zn/HZSM-5 catalysts known as efficient systems for ethane and propane aromatization (the UOP Cyclar Process) also showed some activity in methane aromatization [69]. At 723 K, methane can easily be activated in the presence of ethylene in the feed and converted to higher hydrocarbons (C -C ) and aromatics (Cg-C,g), through its reaction over rare metals modified Zn/HZSM-5 zeolite catalysts. The CH conversion of 37.3% was obtained and the catalysts showed a longer lifetime than usual metal supported HZSM-5 zeolite catalysts without adding any rare-earth metals. 

The effect of silver on the selectivities of Mo/ZSM-5 catalysts in methane aromatization. 

Coupling the methane aromatization with other reactions, such as Boudouard reaction and methane reforming or oxidative coupling of methane,... 

The conversion takes place at high temperature (820-850°C) and very short residence time (hundredth of seconds) in the presence of steam. The by-products are hydrogen, methane and a highly aromatic residual fuel-oil. 

The high acidity of superacids makes them extremely effective pro-tonating agents and catalysts. They also can activate a wide variety of extremely weakly basic compounds (nucleophiles) that previously could not be considered reactive in any practical way. Superacids such as fluoroantimonic or magic acid are capable of protonating not only TT-donor systems (aromatics, olefins, and acetylenes) but also what are called (T-donors, such as saturated hydrocarbons, including methane (CH4), the simplest parent saturated hydrocarbon. 

The finding that highly deactivated aromatics do not react with N02 salts is in accord with the finding that their greatly diminished TT-donor ability no longer snffices to polarize NOi. Similarly, (j-donor hydrocarbons such as methane (CH4) are not able to affect such polarization. Instead, the linear nitronium ion is activated by the superacid. Despite the fact that is a small, triatomic cation, the 11011-... 

Under these first-order conditions the rates of nitration of a number of compounds with acetyl nitrate in acetic anhydride have been determined. The data show that the rates of nitration of compounds bearing activating substituents reach a limit by analogy with the similar phenomenon shown in nitration in aqueous sulphuric and perchloric acids ( 2.5) and in solutions of nitric acid in sulpholan and nitro-methane ( 3.3), this limit has been taken to be the rate of encounter of the nitrating entity with the aromatic molecule. 

It is convenient to divide the petrochemical industry into two general sectors (/) olefins and (2) aromatics and their respective derivatives. Olefins ate straight- or branched-chain unsaturated hydrocarbons, the most important being ethylene (qv), [74-85-1] propjiene (qv) [115-07-17, and butadiene (qv) [106-99-0J. Aromatics are cycHc unsaturated hydrocarbons, the most important being benzene (qv) [71-43-2] toluene (qv) [108-88-3] p- s.y en.e [106-42-3] and (9-xylene [95-47-5] (see Xylenes and ethylbenzene) There are two other large-volume petrochemicals that do not fall easily into either of these two categories ammonia (qv) [7664-41-7] and methanol (qv) [67-56-1]. These two products ate derived primarily from methane [74-82-8] (natural gas) (see Hydrocarbons, c -c ). 

Steam Reforming. When relatively light feedstocks, eg, naphthas having ca 180°C end boiling point and limited aromatic content, are available, high nickel content catalysts can be used to simultaneously conduct a variety of near-autothermic reactions. This results in the essentiaHy complete conversions of the feedstocks to methane ... 

Hydrocarbons, compounds of carbon and hydrogen, are stmcturally classified as aromatic and aliphatic the latter includes alkanes (paraffins), alkenes (olefins), alkynes (acetylenes), and cycloparaffins. An example of a low molecular weight paraffin is methane [74-82-8], of an olefin, ethylene [74-85-1], of a cycloparaffin, cyclopentane [287-92-3], and of an aromatic, benzene [71-43-2]. Cmde petroleum oils [8002-05-9], which span a range of molecular weights of these compounds, excluding the very reactive olefins, have been classified according to their content as paraffinic, cycloparaffinic (naphthenic), or aromatic. The hydrocarbon class of terpenes is not discussed here. Terpenes, such as turpentine [8006-64-2] are found widely distributed in plants, and consist of repeating isoprene [78-79-5] units (see Isoprene Terpenoids). 

The carbon black (soot) produced in the partial combustion and electrical discharge processes is of rather small particle si2e and contains substantial amounts of higher (mostly aromatic) hydrocarbons which may render it hydrophobic, sticky, and difficult to remove by filtration. Electrostatic units, combined with water scmbbers, moving coke beds, and bag filters, are used for the removal of soot. The recovery is illustrated by the BASF separation and purification system (23). The bulk of the carbon in the reactor effluent is removed by a water scmbber (quencher). Residual carbon clean-up is by electrostatic filtering in the case of methane feedstock, and by coke particles if the feed is naphtha. Carbon in the quench water is concentrated by flotation, then burned. 

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