Toluene conversion

This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

The MTDP process, which is similar to the Tatoray process, produces an equilibrium composition of xylene isomers. A -xylene yield of 24% in the xylene product is formed at 42—48 wt % toluene conversion over the heterogeneous catalyst at 390—495°C, 4.2 MPa (600 psig), 1 2 Hquid hourly space velocity, and 4 H2/hydrocarbon molar feed ratio. A new ZSM-5 catalyst, which has higher activity and stability than the current catalyst, has been reported (93). 

The rate of chlorination of toluene relative to that of ben2ene is about 345 (61). Usually, chlorination is carried out at temperatures below 70°C with the reaction proceeding at a profitable rate even at 0°C. The reaction is exothermic with ca 139 kj (33 kcal) of heat produced per mole of monochlorotoluene formed. Chlorine efficiency is high, and toluene conversion to monochlorotoluene can be carried to about 90% with the formation of only a few percent of dichlorotoluenes. In most catalyst systems, decreasing temperatures favor formation of increasing amounts of -chlorotoluene. Concentrations of requited catalysts are low, generally on the order of several tenths of a percent or less. 

Benzyl chloride is manufactured by the thermal or photochemical chlorination of toluene at 65—100°C (37). At lower temperatures the amount of ring-chlorinated by-products is increased. The chlorination is usually carried to no more than about 50% toluene conversion in order to minimize the amount of benzal chloride formed. Overall yield based on toluene is more than 90%. Various materials, including phosphoms pentachloride, have been reported to catalyze the side-chain chlorination. These compounds and others such as amides also reduce ring chlorination by complexing metallic impurities (38). 

Consequently, as a result of increasing environmental pressure many chlorine and nitric acid based processes for the manufacture of substituted aromatic acids are currently being replaced by cleaner, catalytic autoxidation processes. Benzoic acid is traditionally manufactured (ref. 14) via cobalt-catalyzed autoxidation of toluene in the absence of solvent (Fig. 2). The selectivity is ca. 90% at 30% toluene conversion. As noted earlier, oxidation of p-xylene under these conditions gives p-toluic acid in high yield. For further oxidation to terephthalic acid the stronger bromide/cobalt/manganese cocktail is needed. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The excellent high para-selectivity can be explained by the selective escape of p-xylene from the H-ZSM-5 catalyst and inhibition of isomerization on the external surface of catalysts by silicalite-1 coating. In addition to the high para-selectivity, toluene conversion was still high even after the silicalite-1 coating because the silicalite-1 layers on H-ZSM-5 crystals were very thin. 

XfjT Conversion m-nitro toluene Conversion bromine Selectivity benzyl bromide Selectivity benzal bromide Selectivity side product... 

Complete toluene conversion is achieved when using 5.0 fluorine-to-toluene equivalents [13], but at a much decreased selectivity of 11%. 

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... 

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. 

Figure 1 Toluene conversion and selectivity of the products on 1,9Pd/H-MCM-22 (20) as a function of time on stream in the fixed-bed reactor.

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. 

At the longer contact times required to give practical toluene conversion, the primary product on reentering the zeolite... 

In view of the difficulty of measuring the diffusivity of o-xylene at the reaction temperature, 482°c, we have used the diffusivity determined at 120°C. For a series of ZSM-5 catalysts, the two D-values should be proportional to each other. Para-xylene selectivities at constant toluene conversion for catalysts prepared from the same zeolite preparation (constant r) with two different modifiers are shown in Figure 11. The large effect of the modifier on diffusivity, and on para-selectivity, is apparent. 

Figure 14. Effect of MgO content on the activity of MgO-HZSM-5 catalysts. Toluene conversion at 577°c,...

Fig. 16.33 Simultaneous (a) toluene conversion and (b) AQDS reduction by Amsterdam petroleum harbor sediment in anaerobic culture bottles containing bicarbonate-buffered basal medium, supplemented with 25 mM AQDS. The unsupplemented control was prepared in the same manner but without AQDS. The endogenous control (without toluene addition) contained the same amount of hexadecane (0.2% [vol/vol]) as that used for toluene addition. AQDS reduction was quantified spec-trophotometiicaUy as the increase in absorbance at 450 nm. Data are means and standard deviations for tiiphcate incubations in each treatment. Arrows indicate the addition of fresh medium containing AQDS and toluene in depleted bioassay mixtures. (Cervantes et al. 2001) Reprinted with permission. Copyright American Society for Microbiology...

Pig. 3 Dependences of the degree of toluene conversion via alkylation by ethylene (1) and the degree of isomerization of ethylto-luene formed (2) on the polarizing ability of cations and on the heats of CO adsorption on cations in ZSM-5 zeolite. 

Vanadia-titania ( 5 and other supported vanadia catalysts (9) can also be applied for the production of aromatic nitriles by ammoxidation of toluene and of the three xylene isomers allumina-supported V-Sb-based oxides seem to be the best catalysts (10). Detailed kinetic studies of toluene ammoxidation have been reported recently using different vanadia-titania catalysts ( 77,72). Ammonia inhibits toluene conversion, while benzonitrile yields (up to 80 % near 610 K) are mainly limited by... 

A reexamination of the ammonia-toluene coadsorption shows that ammonia prevents the formation of benzyl-species at room temperature in the presence of ammonia, in fact, toluene only weakly adsorbs at r.t. in a reversible form. This agrees with the strong inhibiting effect of toluene conversion by ammonia due to the competitivity of the two reactants on the same adsorption sites (77,72). The spectrum... 

Monsanto disclosed the manufacture of ethylbenzene through a different approach by the methylation of toluene in the side chain.318 A cesium-exchanged faujasite promoted by boron or phosphorus is used as the catalyst. Toluene and methanol (5 1) reacting at 400-475°C produce an ethylbenzene-styrene mixture at very high toluene conversion. About 50% of the methanol is converted to carbon monoxide and hydrogen, which is a disadvantage since such a plant should operate in conjunction with a methanol synthesis plant. 

Vapor-phase oxidation of toluene to benzaldehyde was studied with various Mo-, U—, and Sb-based mixed-oxide catalysts. The selectivity to benzaldehyde fell with increasing the toluene conversion. The best performances were obtained with Mo-P and U-Mo oxide catalysts the one-pass yield of benzaldehyde reached 40 mol% with a selectivity of about 60 mol%. The catalytic activity of the U-Mo oxides was more stable than that of the Mo-P oxides. The effects of the reaction variables on both the rate and selectivity were also studied. 

Product distributions. The reaction was conducted at 550°C by changing the contact time, while fixing the other conditions as presented under Experimental. The main products were benzaldehyde and carbon oxides. The formation of benzoic acid, acetic acid, and maleic anhydride was also detected, but their amounts were much smaller. The yields of each product are shown as a function of the toluene conversion in Fig. 1. The selectivities are given by the slopes from the origin (dashed lines). The selectivity to benzaldehyde decreases with an increase in conversion, while that to carbon oxides increases, indicating that the benzaldehyde formed initially is oxidized gradually to carbon oxides. 

Effect of temperature. The reaction was conducted by changing the temperature and the contact time, while fixing the other conditions. In order to compare the selectivity at the same level of the toluene conversion, the yields of benzaldehyde are plotted as a function of the conversion in Fig. 3. The selectivity increases with raising the temperature. This finding is in conformity with that obtained with V- and Mo-P-based oxide catalysts [19-21]. 

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