Aromatics conversion processes

The following aromatics conversion techniques have produced developments leachmg the industrial level  

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As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

Intermediate pore zeolites typified by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes in the petroleum and petrochemical industry (2-4). This paper describes the selectivity characteristics of two different aromatics conversion processes Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two different principles (5,j6) are responsible for their high selectivity a restricted transition state in the first, and mass transfer limitation in the second. 

In future, catalytic processes will become increasingly important in the development of aromatic conversion processes. The prime objectives will include the highly selective production of pure grades of the desired product and an improvement in environmental protection. Catalysts with corrosive properties will be replaced by less corrosive ones, such as the zeolites. 

Synthetic jet fuel derived from coal is even more difficult and expensive, since the best of the conversion processes produces a fuel very high in aromatics. With hydrogenation, overall thermal efficiency is only 50%. Without additional hydrogenation, the gas turbine fuels would contain 60—70% aromatics. 

There is no clear-cut proof that a one-step Sn2 mechanism, so important at a saturated carbon, ever actually occurs with an aromatic substrate. The hypothetical aromatic Sn2 process is sometimes called the one-stage mechanism to distinguish it from the two-stage S jAr mechanism. A clean example of a Srn2 reaction has been reported, the conversion of 10 to 11 in methanol.Both the SrnI and Srn2 reactions have been reviewed. ... 

The Fischer-Tropsch (FT) catalytic conversion process can be used to synthesize diesel fuels from a variety of feedstocks, including coal, natural gas and biomass. Synthetic diesel fuels can have excellent autoigitition characteristics. The Fischer-Tropsch diesel is composed of only straight-chain hydrocarbons and has no aromatics or sulfur. The synthetic Fischer-Tropsch diesel fuel can provide benefits in terms of both PM and NO, emissions. 

Until recently the lifetimes of the triplet states of aromatic hydrocarbons in fluid solution at room temperature had been investigated exclusively by the technique of flash absorption spectroscopy. The lifetimes reported for many hydrocarbons, e.g., anthracene or phenan-threne, had been below 1 msec, and it had been assumed that radiationless conversion processes were so rapid under these conditions that the competing radiative triplet-singlet transition would be too slow to per-... 

An internal conversion process recently discovered in our laboratory may shed light on the subject. Murovla found that quadricyclene, 6, is a powerful quencher of the excited singlet states of naphthalene and other aromatic hydrocarbons. In the course of the quenching reaction, the quencher was extensively converted to bicyclo [2.2.1] hepta-2,5-diene, 7. The following mechanism was suggested. 

Guerin, M.R., Epler, J.L., Griest, W.H., Clark, B.R., Rao, T.K., "Polycyclic Aromatic Hydrocarbons from Fossil Fuel Conversion Processes," "Carcinogenesis-A Comprehensive Survey," Vol. 3., "Polynuclear Aromatic Hydrocarbons,"... 

Of course, the reduced crude conversion process is not 100% efficient. By this it is meant that to date no catalyst and operating conditions have been developed which completely remove saturates, monoaromatics, diaromatics, and alkyl substituents of polynuclear aromatics from the slurry oil. Therefore, to predict slurry oil plus coke yield one must determine what proportion of each molecular type present in the reduced crude feedstock remains in the slurry oil and coke. 

The transient T-T absorption in the gas phase has been measured recently for aromatic molecules such as naphthalene (119,211) and anthracene (80,81) using flash kinetic spectroscopy and tandem laser pulse absorption techniques. Particularly, the later technique (211) provides time-dependent absorption spectra of the "isolated" unrelaxed triplet molecules because of its capability for rapid monochromatic excitation and detection. It will certainly provide a wealth of Important kinetic and spectroscopic information about the evolution and decay of triplet states. Direct observation of the formation of transient hot ground-state (Sq) molecules through an internal conversion process has also been achieved with laser excitation and laser... 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

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