Selective toluene disproportionation

Selective Toluene Disproportionation. Toluene disproportionates over ZSM-5 to benzene and a mixture of xylenes. Unlike this reaction over amorphous sihca—alumina catalyst, ZSM-5 produces a xylene mixture with increased -isomer content compared with the thermodynamic equihbtium. Chemical modification of the zeohte causing the pore diameter to be reduced produces catalysts that achieve almost 100% selectivity to -xylene. This favorable result is explained by the greatly reduced diffusivity of 0- and / -xylene compared with that of the less bulky -isomer. For the same reason, large crystals (3 llm) of ZSM-5 produce a higher ratio of -xyleneitotal xylenes than smaller crystahites (28,57). 

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). 

Y. Y. Huang, M. P. Nicoletti, and R. A. Sailor, "The Mobil Selective Toluene Disproportionation Process (MSTDP)," 1990 Petrochemical evieiv DeWitt Company, Houston, Tex., Mar. Ill—70 1990. 

After the cmde BTX is formed, by reforming in this case, a heart cut is sent to extraction. Actually, the xylenes and heavier components are often sent to downstream processes without extraction. The toluene produced is converted to ben2ene, a more valuable petrochemical, by mnning it through a hydrodealkylation unit. This catalytic unit operates at 540—810°C with an excess of hydrogen. Another option is to disproportionate toluene or toluene plus aromatics to a mixture of ben2ene and xylenes using a process such as UOP s Tatoray or Mobil s Selective Toluene Disproportionation Process (STDP) (36). 

MSTDP [Mobil selective toluene disproportionation] A process for converting toluene to benzene and a xylene mixture rich in /7-xylene. The catalyst is the zeolite ZSM-5, selectively coked to constrict the pores and thus increase the yield of //-xylene produced. Developed and licensed by the Mobil Oil Corporation and first commercialized in Sicily in 1988. See also MTDP. 

STDP [Selective toluene disproportionation process] A process for converting touene to mixed xylenes, predominately />-xylene. It takes place in the presence of hydrogen over a ZSM-5-type catalyst. Developed by Mobil in the 1980s and first operated by Enichem. 

Structure-Selectivity Relationship in Xylene Isomerization and Selective Toluene Disproportionation... 

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. 

Figure 11. Effect of diffusivity on p-xylene selectivity. Toluene disproportionation at 550°C, 20% conversion o-xylene diffusivity at 120°C.

C, 10-50 atm). Xylene benzene ratios of 1-10 may be obtained. Metal catalysts were later replaced by zeolites.210,211,326-328 The most recent development is the Mobil selective toluene disproportionation process,329 which takes advantage of the high para shape selectivity of a zeolite catalyst.210 The catalyst activated by a novel procedure ensures a p-xylene content of up to 95%. After the successful com-mercialization at an Enichem refinery in Italy, the process is now licensed. The catalysts and technologies applied in toluene disproportionation may be also used for transalkylation324,325,331 [Eq. (5.74)] ... 

Selective toluene disproportionation Paraxylene-benzene HMFI... 

Description Dry toluene feed and hydrogen-rich recycle gas are pumped through feed/effluent exchangers and charge heater and into the PxMax reactor (1). Selective toluene disproportionation (STDP) occurs in the vapor phase to produce the paraxylene-rich xylene and benzene co-product. Byproduct yields are small. Reactor efflu-... 

One of the most discussed cases of shape selectivity involving transition state selectivity or product diffusional constraints is the production of p-xylene over chemically modified MFI zeolites [248]. Several processes exist which utilize the shape selectivity of these zeolites, for example the alkylation of toluene with methanol [249], xylene isomerization [250] and selective toluene disproportionation [251]. The first two of these examples shall be used to describe in detail the principal possibilities to tailor the reaction pathway by shape selectivity. 

An example of product selectivity is the ZSM-5 process, selective toluene disproportionation (Ref. 15), which is the second generation of our commercial toluene disproportionation process that produces xylenes and benzene from toluene. 

An industrial application is the Mobil Oil selective toluene disproportionation process (STOP) [T32]. 

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