Toluene disproportionation selectivity

Figure 1 also shows activity for toluene disproportionation, selectivity to benzene and xylenes, and the amount of coke deposited on the catalyst after the reaction. 

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). 

As illustrated in Figure 10.6, the high para-selectivity in the toluene disproportionation is caused by the selective removal of p-xylene from the silica-alumina particles, which leads to an apparent equilibrium shift between the xylene isomers. 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

The objective of this contribution is to investigate catalytic properties of zeolites differing in their channel systems in transformation of aromatics, i.e. toluene alkylation with isopropyl alcohol and toluene disproportionation. In the former case zeolite structure and acidity is related to the toluene conversion, selectivity to p-cymene, sum of cymenes, and isopropyl/n-propyl toluene ratio. In the latter one zeolite properties are... 

Figure 1. Dependence of toluene conversion and selectivity to p-xylene in toluene disproportionation.

Toluene alkylation with isopropyl alcohol was chosen as the test reaction as we can follow in a detail the effect of zeolite structural parameters on the toluene conversion, selectivity to cymenes, selectivity to para-cymene, and isopropyl/n-propyl ratio. It should be stressed that toluene/isopropyl alcohol molar ratio used in the feed was 9.6, which indicates the theoretical toluene conversion around 10.4 %. As you can see from Fig. 2 conversion of toluene over SSZ-33 after 15 min of T-O-S is 21 %, which is almost two times higher than the theoretical toluene conversion for alkylation reaction. The value of toluene conversion over SSZ-33 is influenced by a high rate of toluene disproportionation. About 50 % of toluene converted is transformed into benzene and xylenes. Toluene conversion over zeolites Beta and SSZ-35 is around 12 %, which is due to a much smaller contribution of toluene disproportionation to the overall toluene conversion. A slight increase in toluene conversion over ZSM-5 zeolite is connected with the fact that desorption and transport of products in toluene alkylation with isopropyl alcohol is the rate controlling step of this reaction [9]... 

In toluene disproportionation the highest toluene conversion was achieved over SSZ-33 due to a high acidity combined with 3-D channel system. High toluene conversion over SSZ-35 results from its strong acidity and large reaction volumes in 18-MR cavities. Toluene conversion in the alkylation with isopropyl alcohol is influenced by a high rate of competitive toluene disproportionation over SSZ-33. ZSM-5 exhibits a high p-selectivity for /7-isopropyl toluene, which seems to be connected with diffusion constraints in the channel system of this zeolite. 

The results from the batch reactor and the fixed-bed reactor agree in terms of activity and selectivity for the different zeolites. The activity of Pd/H-MCM-22 is higher than that of Pd/H-ZSM-5, however, the selectivity to ethyltoluenes is much higher for Pd/H-ZSM-5. The yield of the desired ethyltoluene products is also higher with the ZSM-5 catalyst since it is less active for toluene disproportionation. However, with regard to the achievable yields and selectivities of the desired ethyltoluenes, the batch reactor is clearly inferior to the fixed-bed reactor. The reason might be the excess of toluene in the batch reactor. 

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. 

As is apparent from the previous discussion on toluene disproportionation,the observation of high p-selectivity in STDP requires a dramatic change in selectivity. First, the primary product must be directed to be highly para-selective. Secondly, the subsequent isomerization of the primary p-xylene product must be selectively inhibited ... 

The general characteristics of toluene disproportionation are summarized by the data presented in Figure 8. With standard HZSM-5 catalyst, as indicated by the lowest curve, the xylenes produced contain essentially an equilibrium concentration of the para isomer (24%) and exceed it only slightly at low conversion. The other curves result from a variety of HZSM-5 catalysts modified in different ways and to different degrees. It is apparent that a wide range of para-selectivities can be obtained. At increasing toluene conversions, the para-selectivity decreases for all catalysts. 

Figure 10. Effect of crystal size, 2r, on p-xylene selectivity in toluene disproportionation by HZSM-5.

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

Figure 12 Relationship between the diffusion parameter, to.3 and p-xylene selectivity in toluene disproportionation. Temperature 550°C. Pressure 41 bar. Conversion 20%. tQ 3 time to reach 30% of amount sorbed at infinite time.

Kurschhner, U., Jerschkewitz, H.-G., Schreier, E., and Volter, J. (1990) Shape selectivity of hydrothermally treated H-ZSM-5 in toluene disproportionation and xylene isomerization. Appl. Catal, 57, 167-177. 

Other examples of systems that are likely to be governed by product shape selectivity effects include toluene disproportionation to para-xylene -i- benzene in favor of other xylenes r- benzene [61]. Toluene alkylation by methanol to give para-xylene in favor of other xylenes is yet another such example [76],... 

ZSM-5 is a Mobil-proprietary, shape-selective zeolite which is used commercially in synthetic fuels (methanol-to-gasoline), petrochemicals (xylene isomerization, toluene disproportionation, benzene alkylation) and in petroleum refining (lube and... 

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)] ... 

---