In ethylbenzene disproportionation

In Figure 9 the catalytic performance of almost 100% exchanged La-Y, obtained by solid-state reaction (A) and conventional exchange (B), is illustrated. Both catalysts were, under the conditions chosen, see Figure 9, entirely selective in ethylbenzene disproportionation in that they yielded, after an induction period, benzene and diethylbenzenes in the ratio 1 1. The activity and stability of catalyst (A) were similar to or even better than those of catalyst (B). 

WeiB, U., M. Weihe, M. Hunger, H.G. Karge and J. Weitkamp, 1997, The induction period in ethylbenzene disproportionation over large pore zeolites, in Progress in Zeolite and Microporous Materials, eds H. Chon, S.-K. Ihm and Y.S. Uh, Vol. 105 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 973-980. 

It is interesting to note that La,Na-X showed a 1.6 times higher conversion than the La,Na-Y zeolite, yielding benzene to diethylbenzene ratios of imity. The ranking of the catalysts with respect to their total acidity did not correlate with the activity in ethylbenzene disproportionation, while the concentration of very strong acid sites (Ho < -8.2) correlated very well with the catalytic activity of the faujasite-type materials. Sr- and Ba-Y samples, which do not possess strongly acidic sites, were not active in this test reaction. 

Of course, other reaction types have been also investigated more recently, such as the Beckmann rearrangement [247,277,278] or ethylbenzene disproportionation [279, 280], just to name a couple. In situ NMR methods are expected to play a vital role in the future science of heterogeneous catalysis. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

The mechanism of ethylbenzene disproportionation depends on the zeolite pore structure (3). With large pore zeolites, this reaction occurs mainly through the carbocation chain mechanism proposed for xylene disproportionation (Figure 9.4) which involves benzylic carbocations and diarylmethane intermediates. With MFI zeolites in the pores of which steric constraints limit the formation of the bulky diarylmethane intermediates, ethylbenzene disproportionation occurs mainly through a successive dealkylation-alkylation process ... 

This difference in mechanism is clearly demonstrated by substituting bifunctional catalysts for acidic catalysts (29). The introduction of platinum in MFI catalysts leads to a large decrease in the rate of ethylbenzene disproportionation (divided by 6), which is due to a large consumption of ethylene by hydrogenation as shown by the large increase in the rate of dealkylation. On the other hand, the introduction of platinum in MOR catalysts leads to a limited change in the rates of disproportionation and dealkylation. 

MFI zeolites seem to be the most efficient for EB dealkylation, in terms of activity, selectivity and stability. In the 70s, on metal-free MFI catalysts, EB was disproportionated into benzene and diethylbenzenes. As indicated above, with MFI catalysts, ethylbenzene disproportionation occurs through a deethylation-ethylation mechanism, with ethylene as desorbed intermediate. The addition of a metal (carried out early 80s) allows a rapid and irreversible conversion of ethylene into ethane with a consequent shift of ethylbenzene transformation from disproportionation to hydrodealkylation. The selectivity is highly sensitive to temperature that must be in the range 380°C-460°C to limit both alkylation and naphthene cracking. 

These catalysts are sflica-aluminas whose cradcing and dispropordonatioq power has been altered by steam treatment, the use of an inhibitor, or of alui as containing a halogenated compound or fluorine. They are very rugged, are empfoyed without hydrogen and hence cannot isomerize ethylbenzene, which is therefore cracked or transformed by a disproportionation reacdon into benzene and Cio aromatics. Consequently, they can only be used with feeds poor in ethylbenzene. However, no naphthenic hydrocarbons are formed. 

Ethylbenzene disproportionation was performed in a stainless-steel tubular fixed-bed reactor. Mordenites were evaluated at 150 and faujasites at 200°C at atmospheric pressure. Ethylbenzene vapour was mixed with helium and introduced into the reactor at a partial pressure of 1.33 kPa with a total flow rate of 1.81-h-i [3]. 

Figure 3. The relative activity in the disproportionation of ethylbenzene as a function of the pretreatment temperature for NH4Y2.35 (l)x NH4Y2.8 (2), DY4.8 (3)...

A plot of 1/r versus [DEB]/[EB] provides Figtue 3. In fact, [DEB] is assumed to be the diethylbenzene concentration averaged over the catalyst bed. From Figure 3 it follows that equ. (3) presents indeed an appropriate description of the actual rate of ethylbenzene disproportionation. Extrapolation to [DEB]/[EB] = 0 provides the inverse of the non-inhibited rate, i.e., 1/r... 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

The intensities of the bands were similar to those of La-Y samples obtained by conventional ion exchange. TPD of ammonia showed that also the strength of the acidic sites were essentially the same as found with conventionally exchanged La-Y. Thus, the nature, density and strength of acidity of La-Y prepared via solid-state ion exchange are comparable to those of the conventionally obtained products and, therefore, similar catalytic behaviour in acid-catalysed reactions was expected. This was, indeed, found when La-Y was employed as a catalyst for ethylbenzene disproportionation. 

Even though a 100% exchange in the system LaC /Na-Y was not yet achieved by solid-state ion exchange, the product obtained showed catalytic activity in acid-catalysed ethylbenzene disproportionation similar to that of conventionally obtained La-Y with an exchange degree of 74%. 

Zeolite ERS-10 has been characterised as acid catalyst for ethylbenzene(EB) disproportionation and benzene alkylation. In the disproportionation of EB, ERS-10 shows some similarity with ZSM-12. In the alkylation of benzene with propylene a peculiar behaviour is related to the by-production of diisopropylbenzenes (DIPBs). Like ZSM-5 and ZSM-12, the preferred isomer produced by ERS-10 is the para. A Spaciousness Index (SI) = 5.3 and the preferred formation of p-DIPB seem to indicate that the effective pore-width of ERS-10 is in the intermediate range between large and medium pore zeolites. 

Ethylbenzene disproportionation catalyzed by add zeolites was studied by Karge et al. [887, 888] and recommended as a versatile test reaction for acid (monofunctional or bifunctional) catalysts such as add zeolites or related materials and is frequently used for this purpose. Also, this reaction was studied in situ by IR spectroscopy in combination with gas chromatography for determination of conversion and selectivity [888]. In these experiments, a flow-reactor quartz glass cell as described in Ref. [152] was used, which could be operated imder ultra-high vacuum during the pretreatment of the thin catalyst wafers of pressed zeolite powder at, e.g., 670-870 K and 10 Pa. After pretreatment, the cell was used as a differential fixed-bed micro-flow reactor (cf. also [ 152,158]). Results are illustrated by Fig. 52. 

It was shown that solid-state ion exchange is also a suitable route to preparation of active acidic or bifunctional catalysts. Introduction of Ca or Mg into mordenite [21] or La " into Y-type zeolite, mordenite or ZSM-5 [22] by solid-state reaction yielded, after brief contact with small amounts of water, acidic zeolite catalysts which were, for instance, active in disproportionation and/or dealkylation of ethylbenzene or in cracking of n-decane [43]. The contact with water was essential to generate, after solid-state ion exchange, acidic Brpnsted centres (compare, for instance. Figure 2). In the case of solid-state exchange between LaClj and NH -Y an almost 100% exchange was achieved in a one-step procedure, and the hydrated La-Y reaction product exhibited a catalytic performance (selectivity in ethylbenzene disporportionation, time-onstream behaviour) comparable to or even better than that of a conventionally produced La-Y (96) catalyst [22,23]. In fact, compared to the case of NH -Y the introduction of La " " by solid-state reaction proceeded less easily and was frequently lower than 100% with H-ZSM-5 or H-MOR. 

Niu and Hofmann [ 164] related the activity of USY, H-ZSM-5, and H-MOR zeolite catalysts in ethylbenzene conversion to the acidity measured by IR spectroscopy using pyridine adsorption and TPD of NH3. Catalytic measurements at 300 °C did not show deactivation over all three catalysts, while increasing the reaction temperature to 400 °C led to a strong deactivation for H-MOR and some deactivation of USY. The conversion over H-ZSM-5 was not affected by deactivation. At 300 °C only disproportionation reactions were observed, while at 400 °C large contributions of cracking were found (i.e., benzene to diethylbenzene ratios above 1) over all three catalysts. 

A very high / ara-selectivity of inorganic-modified ZSM-5 was also manifested in the disproportionation of ethylbenzene. The results with ZSM-5 modified with magnesium and phosphorus compounds are shown in Table 4.10. The concentration of ara-isomer in the diethylbenzene products was over 99%. ... 

D.G. Pobedimskii showed that the homolytic reaction with the formation of free radicals occurs in parallel with heterolytic transformation. Manifestations of this phenomenon are diverse. The acceptor of free radicals, stable nitroxyl radical, is consumed in the reaction of hydroperoxide with phosphite. This reaction in ethylbenzene in the presence of oxygen is accompanied by chemiluminescence appeared by the disproportionation of peroxyl radicals. Chemical polarization of nuclei was found... 

The transalkylation reaction is essentiaHyisothermal and is reversible. A high ratio of benzene to polyethylbenzene favors the transalkylation reaction to the right and retards the disproportionation reaction to the left. Although alkylation and transalkylation can be carried out in the same reactor, as has been practiced in some processes, higher ethylbenzene yield and purity are achieved with a separate alkylator and transalkylator, operating under different conditions optimized for the respective reactions. 

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