Phase disproportionation

In this orthorhombic structure, superconductivity is not observed, and two inequivalent Bi sites are observed. A cubic, but non-superconducting specimen has been reported (10) for x =. 13. In this structure, all Bi sites are equivalent, and a metallic or delocalized electronic structure would be expected to exist. This structure apparently persists into the superconducting compositions, where it has been observed for x =. 374 (4) which displays a Tc of 30.5 K. A cubic phase has also been reported for x =. 6, with Tc = 34K. The limit of this solid solution which may be at or beyond this x =. 6 composition, may be dependent upon synthesis temperature and technique. Indications of phase disproportionation at these high x values have been published by Jones et al. (11). 

Baidum.G Lu Deactivation Kinetics of the Catalyst TA-43 Mordenite for the Vapor-Phase Disproportionation of Toluene Huadong-Huagong Xuequan Xuebao 18 (1990) 538... 

There are three N,C-substituted dimethylanilines, the N-methyltoluidines. Of these, only the para isomer has been studied calorimetrically26 in the acid-catalyzed liquid-phase disproportionation reaction 5, X = CH3. The reaction is exothermic by only 2 kJmol-1, essentially thermoneutral, as was the case for the related reaction for Af-methylaniline22. 

It is not obvious how to ascertain the reasonableness of the previous two silver acetylide values because there seems to be insufficient data to make meaningful comparisons. Yet, using organic thermochemical data from Reference 12 for diphenylacetylene, the formal, organometalhc, solid phase disproportionation reaction 3... 

This approach is tested in the gas-phase disproportionation of toluene and in the de-hydratation of methanol into water and dimethyl ether. In the first case study the most competitive configuration is reported to be the reaction zone in the stripping and feed sections. The effect of the reaction equilibrium constant is analyzed in the second example, resulting in feasible configurations with a single reactive stage. 

As has been pointed out already several times in this section, the h-quartz s.s. crystals often form an intermediate metastable phase when glass ceramic base glasses are devitrified. At elevated temperatures or prolonged heating times these crystals transform into the more stable keatite s.s. crystals as is evidenced, for example, from Figs. 2.21 and 2.23. What processes initiate this transformation has up to now not been understood. In several cases [2.72, 74, 75] the primary nucleating phase disproportionates into rutile and... 

The liquid region boundary displays (as expected) a sharp kink at the intersection of the Krafft boundary with the hexagonal liquid crystal solubility boundary. The hexagonal phase disproportionates at its upper... 

A second Mobil process is the Mobil s Vapor Phase Isomerization Process (MVPI) (125,126). This process was introduced in 1973. Based on information in the patent Hterature (125), the catalyst used in this process is beHeved to be composed of NiHZSM-5 with an alumina binder. The primary mechanism of EB conversion is the disproportionation of two molecules of EB to one molecule of benzene and one molecule of diethylbenzene. EB conversion is about 25—40%, with xylene losses of 2.5—4%. PX is produced at concentration levels of 102—104% of equiHbrium. Temperatures are in the range of 315—370°C, pressure is generally 1480 kPa, the H2/hydrocatbon molar ratio is about 6 1, and WHSV is dependent on temperature, but is in the range of 2—50, although normally it is 5—10. 

In the absence of air, TEE disproportionates violently to give carbon and carbon tetrafluoride the same amount of energy is generated as in black powder explosions. This type of decomposition is initiated thermally and equipment hot spots must be avoided. The flammability limits of TEE are 14—43% it bums when mixed with air and forms explosive mixtures with air and oxygen. It can be stored in steel cylinders under controlled conditions inhibited with a suitable stabilizer. The oxygen content of the vapor phase should not exceed 10 ppm. Although TEE is nontoxic, it may be contaminated by highly toxic fluorocarbon compounds. 

Halides. Gold(III) chloride [13453-07-1] can be prepared directiy from the elements at 200°C (167). It exists as the chlotine-bridged dimer, Au2Clg ia both the soHd and gas phases under an atmospheric pressure of chlorine at temperatures below 254°C. Above this temperature ia a chlorine atmosphere or at lower temperatures ia an iaert atmosphere, it decomposes first to AuCl [10294-29-8] and then to gold. The monochloride is only metastable at room temperature and slowly disproportionates to gold(0) and gold(III) chloride. The disproportionation is much more rapid ia water both for AuCl and the complex chloride, [AuCy, formed by iateraction with metal chlorides ia solution. 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

The Tatoray process, which was developed by Toray Industries, Inc., and is available for Hcense through UOP, can be appHed to the production of xylenes and benzene from feedstock that consists typically of toluene [108-88-3] either alone or blended with aromatics (particularly trimethylbenzenes and ethyl-toluenes). The main reactions are transalkylation (or disproportionation) of toluene to xylene and benzene or of toluene and trimethylbenzenes to xylenes in the vapor phase over a highly selective fixed-bed catalyst in a hydrogen atmosphere at 350—500°C and 1—5 MPa (10—50 atm). Ethyl groups are... 

The Xylene Plus process of ARGO Technology, Inc. (95,96) and the FINA T2BX process (97) also use a fixed-bed catalyst in the vapor phase for transalkylation of toluene to produce xylenes and benzene. The Mobil low temperature disproportionation (LTD) process employs a zeoHte catalyst for transalkylation of toluene in the Hquid phase at 260—315°C in the absence of hydrogen (98). 

The most commonly used emulsifiers are sodium, potassium, or ammonium salts of oleic acid, stearic acid, or rosin acids, or disproportionate rosin acids, either singly or in mixture. An aLkylsulfate or aLkylarenesulfonate can also be used or be present as a stabilizer. A useful stabilizer of this class is the condensation product of formaldehyde with the sodium salt of P-naphthalenesulfonic acid. AH these primary emulsifiers and stabilizers are anionic and on adsorption they confer a negative charge to the polymer particles. Latices stabilized with cationic or nonionic surfactants have been developed for special apphcations. Despite the high concentration of emulsifiers in most synthetic latices, only a small proportion is present in the aqueous phase nearly all of it is adsorbed on the polymer particles. 

Dehydrogenation ofy -menthadienes and a-piuene ia the vapor phase over catalysts such as chromia—alumina produces y -cymene (70). / -Menthadienes can be disproportionated over a Cu—Ni catalyst to give a mixture of yvmenthane andy -cymene (71). 

Another example of catalytic isomerization is the Mobil Vapor-Phase Isomerization process, in which -xylene is separated from an equiHbrium mixture of Cg aromatics obtained by isomerization of mixed xylenes and ethylbenzene. To keep xylene losses low, this process uses a ZSM-5-supported noble metal catalyst over which the rate of transalkylation of ethylbenzene is two orders of magnitude larger than that of xylene disproportionation (12). 

Dehydrochlorination of 1,1,2-trichloroethane at 500°C in the presence of a copper catalyst gives a different product, ie, cis- and /n7 j -l,2-dichloroethylene. Addition of small amounts of a chlorinating agent, such as chlorine, promotes radical dehydrochlorination in the gas phase through a disproportionation mechanism that results in loss of hydrogen chloride and formation of a double bond. The dehydrochlorination of 1,2-dichloroethane in the presence of chlorine, as shown in equations 19 and 20, is a typical example. 

Nitrous acid, HNO2, has not been isolated as a pure compound but it is a well known and important reagent in aqueous solutions and has also been studied as a component in gas-phase equilibria. Solutions of the free acid can readily be obtained by acidification of cooled aqueous nitrite solutions but even at room temperature disproportionation is noticeable ... 

The dioxides of molybdenum (violet) and tungsten (brown) are the final oxide phases produced by reduction of the trioxides with hydrogen they have rutile structures sufficiently distorted to allow the formation of M-M bonds and concomitant metallic conductivity and diamagnetism. Strong heating causes disproportionation ... 

Typical conditions for the disproportionation reaction are 450-530°C and 20 atmospheres. A mixture of C0O-M0O3 on aluminosilicates/alumina catalysts can he used. Conversions of approximately 40% are normally used to avoid more side reactions and faster catalyst deactivation. The equilihrium constants for this reaction are not significantly changed hy shifting from liquid to vapor phase or hy large temperature changes. 

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