Disproportionation toluene

Figure C2.7.13. Schematic representation of diffusion and reaction in pores of HZSM-5 zeolite-catalysed toluene disproportionation the numbers are approximate relative diffusion coefficients in the pores 1131.

Toluene disproportionation (TDP) is a catalytic process in which 2 moles of toluene are converted to 1 mole of xylene and 1 mole of benzene this process is discussed in greater detail herein. Although the mixed xylenes from TDP are generally more cosdy to produce than those from catalytic reformate or pyrolysis gasoline, thek principal advantage is that they are very pure and contain essentially no EB. 

A breakdown of the mixed xylene supply sources in the United States is summarized in Table 1 (1). As shown in Table 1, the primary source of xylenes in the United States is catalytic reformate. In 1992, over 90% of the isolated xylenes in the United States were derived from this source. Approximately 9% of the recovered xylenes is produced via toluene disproportionation (TDP). In the United States, only negligible amounts of the xylenes are recovered from pyrolysis gasoline and coke oven light oil. In other parts of the world, pyrolysis gasoline is a more important source of xylenes. 

Xylenes Produetion Via Toluene Transalkylation and Disproportionation. The toluene that is produced from processes such as catalytic reforming can be converted into xylenes via transalkylation and disproportionation. Toluene disproportionation is defined as the reaction of 2 mol of toluene to produce 1 mol of xylene and 1 mol of benzene. Toluene transalkylation is defined as the reaction of toluene with or higher aromatics to produce xylenes ... 

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. 

Lyondell and Sun Oil Co. are the main producers of benzene by disproportionation. Eiaa Oil Co. of Texas has developed the Eiaa T2BX process for toluene disproportionation usiag a proprietary catalyst. The new catalyst is claimed to reduce hydrogen consumption and is suitable for feeds containing small amounts of moisture (53). A commercial production unit was started up ia the fall of 1985. 

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... 

Both catalytic reactions were investigated in the gas phase under atmospheric pressure using a glass fixed bed micro-reactor Prior to the reaction given amount of the catalyst (fraction 0.5-0.71 mm) was in situ activated at 500 °C for 2 h (N2 stream 40 ml min"1) In the case of toluene disproportionation the reaction temperature was 500 °C, WHSV 2-20 h"1, concentration of toluene in a stream was 18.5 mol%. Toluene alkylation was studied at the reaction temperature 250 °C. WHSV related to toluene was 10 h"1, concentration of toluene was 18.5 mol% and toluene to isopropyl alcohol molar ratio was 9.6. 

The reaction products were analyzed using an on-line gas chromatograph (HP 6890) equipped by a FID detector and a capillary column DB-5 for toluene alkylation while HP-INNOWax, was used for toluene disproportionation. 

The catalytic activity of SSZ-33 and SSZ-35 was compared with those of ZSM-5 and Beta in toluene disproportionation and toluene alkylation with isopropyl alcohol. The crystals size of SSZ-33 and Beta is about 0.2-0.3 pm while crystals of ZSM-5 and SSZ-35 are larger - around 1 pm. Thus, the size of all crystals provides a nice opportunity to compare the catalytic behavior of zeolites in toluene transformations. 

The initial conversions of toluene in toluene disproportionation carried out at 500 °C follow the order ZSM-5 < SSZ-35 = Beta < SSZ-33 (Fig. 1). This order cannot be directly related to the increasing pore size or connectivity of individual zeolites. In such case SSZ-35 should exhibit a lower conversion compared with ZSM-5 and toluene conversion over zeolite Beta should be higher or comparable with that over SSZ-33. 

The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

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. 

MTDP [Mobil toluene disproportionation] A catalytic process which converts 2 moles of toluene to 1 mole of mixed xylenes and 1 mole of benzene. The catalyst is the zeolite ZSM-5. Developed by Mobil Research Development Corporation and first commercialized in 1975. Supersedby MSTDP. 

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. 

TDP [Toluene disproportionation process] A general name for catalytic processes for converting toluene to a mixture of xylene isomers and benzene. One proprietary version is MTDP. 

Gnep et al. (91) investigated toluene disproportionation and coke formation over chemically dealuminated mordenite. 

They found that aluminum-deficient mordenite (SiO /Al O ratio of 18), pre-calcined in a flow of dry air, is twice as active for toluene disproportionation and coke formation as compared to regular mordenite. The aluminum-deficient form also has a higher rate of deactivation. It is assumed that the formation of new, strong acid sites during the process of moderate dealumination is responsible for the observed change in activity. 

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

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