Xylene disproportionation intermediate

Large differences exist between the xylene disproportionation/isomerization ratios (D/I) found with acid catalysts. With zeolites the size of the space available near the acid sites was shown to play a determining role (2). The smaller the size of the intracrystalline zeolite cavities, the lower the ratio between the rate constants of disproportionation and isomerization 0.05 at 316°C with a FAU zeolite (diameter of the supercage of 1.3 nm), 0.014 and 0.01 with MOR and MAZ (0.08 nm). Steric constraints which affect the formation of the bulky bimolecular transition states and intermediates of disproportionation (Figure 9.4) would be responsible for this observation. However, the very low value of D/I (0.001) obtained with MFI (2), the channel intersection of which has a size of 0.85 nm, is also due to other causes limitations in the desorption of the bulky trimethylbenzene products of disproportionation from the narrow pores of the zeolite ( 0.6 nm) and most likely the low acid site density of the used sample (Si/Al=70 instead of 5-15 with the large pore zeolites). 

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 ... 

With mordenite catalysts on which disproportionation occurs through benzylic carbocations and diarylmethane intermediates, the rate of this bimolecular reaction was shown to be dependent on the acid site density the turnover frequency for disproportionation is roughly proportional to the square of the concentration of acid sites (Figure 9.8 (25)). As suggested for xylene disproportionation, this probably means that xylene disproportionation requires two protonic sites for its catalysis (the first one for steps 1, 2, 3 and the second for steps 4, 5 and 6 of Figure 9.4). 

Toluene disproportionation (TDP) is a well-known acid reaction, occurring through the same mechanism as xylene disproportionation (Figure 9.4). Like this latter reaction, toluene disproportionation requires most likely two protonic sites for it catalysis, hence the density of protonic sites has a very positive effect on the catalyst activity. Furthermore, the bimolecular intermediates (methyldiphenyl-... 

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... 

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. 

This side reaction leads to undesirable losses of xylenes. With REHY zeolite as catalyst, disproportionation occurs at a rate comparable to that of isomerization of m-xylene (8), e.g., 14% disproportionation at 16% isomerization. In fact, the product, trimethylbenzene, is postulated as an important intermediate in isomerization (8). 

We have examined the rate constants for disproportionation and isomerization for a variety of zeolites, using a commercial-type feed containing 70% m-xylene and 30% o-xylene in a fixed-bed flow reactor. The results, listed in Table I, show the exceptionally low disproportionation/isomerization selectivity of ZSM-5 relative to synthetic faujasite. Synthetic mordenite and ZSM-4 have intermediate selectivities. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

Figure 13.46 Mechanism for disproportionation of meto-xylene to toluene and trimethylbenzene via diphenlymethane intermediate.

As many organic compounds may transform simultaneously through mono molecular (intramolecular) and bimolecular (intermolecular) processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one. This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections (32). Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites (41). 

Fig. 1.5 Schematic representation of shape selective effects a) Reactant selectivity Cracking of an n-iso C6 mixture, b) Product selectivity Disproportionation of toluene into para-xylene over a modified HFMI zeolite, c) Spatioselectivity Disproportionation of meta-xylene over HMOR. The diphenylmethane intermediate A in formation of 1,3,5 trimethylbenzene is too bulky to be accommodated in the pores, which is not the case for B...

The typical operating conditions of xylene and EB isomerization processes are shown in Table 9.3. These conditions minimize the above side reactions. Pressure, temperature and H2/HC ratio are key parameters that define the partial pressure of C8 naphthenes intermediates for EB isomerization. Naphthene cracking and disproportionation/transalkylation are responsible for the C8 aromatics net losses that affect the overall pX yield. The C8 recycled stream from the isomerization unit to the separation unit is three times higher than the fresh feed stream (since there cannot be more than 24% of pX in the C8 aromatic cut after isomerization). This means that each percent of loss in the isomerization unit will decrease the pX yield by 3%. For example, when standard mordenite-based catalysts lead to 4% of net losses, the overall pX yield is roughly 88%. 

Figure 16. Front and side views of the transition states and intermediate for the isomerization reaction via disproportionation reaction pathway of xylene molecules catalyzed by an acidic Mordenite as obtained from the periodic calculations (American Chemical Society,...

A third type of control, called spatiospectffeity, occurs when both reactants and products pass the opening but reaction intermediates or transition states are restricted by the size of the cavity. In xylene isomerization processes, selectivity is lost through disproportionation to toluene and trimethylbenzene. Diphenylroethane intermediates are too large for ZSM-5... 

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. 

Cg Aromatic Reactions with Hydrogen. The mild acid nature of the family of aluminophosphate based sieves renders them selective for a number of rearrangements as observed in the reactions of olefins and paraffins described above. This property as well as their apparent low disproportionation activity observed in the alkylation of toluene suggests that they be evaluated as the acid function in bifunctional Cg aromatic isomerization. As described above, cyclo-olefins are most likely involved in the conversion of ethylbenzene to xylenes. Strong acid functions, such as in mordenite, actively isomerize cyclo-olefinic intermediates but also catalyze ring-opening reactions which lead to loss of aromatics. A more selective acid function must still effectively interconvert ethyl cyclohexene to dimethylcyclohexenes but must leave the cyclohexene rings intact. 

An active metal supported on a zeolite has a promotional effect in increasing the ratio of isomerization to disproportionation which could be due to the introduction of a bifunctional path which is composed of the steps of hydrogenation of m-xylene on the metal to form an olefin, isomerization of the formed olefin and dehydrogenation of the isomerized olefin to o-xylene. Also, the rate of disproportionation reaction is reduced due to hydrogen spilling over, and reacting with the carbocation intermediates in the disproportionation reaction and consequently decreasing the rate of the disproportionation reaction." ... 

In dehalogenating the phenylaluminum chlorides with sodium it is best to work with xylene as solvent. The complex salts which are formed as an intermediate react further at above 100°C. Triphenylalane is obtained in high yield in this way from the corresponding phenylaluminum chlorides, but the compound usually contains some chlorine. A substantially better method for preparing pure triphenylalane is by the reaction of dimethyl-aluminum chloride with sodium phenyl the resulting dimethylphenylalane disproportionates on distillation at reduced pressure to pure triphenylalane and trimethylalane (171) ... 

The second, which explains the disproportionation of xylenes, occurs through benzylic carbenium ions and diarylmethane intermediates. 

As para-xylene is the most valuable xylene for use as a polymer intermediate, it is useful that ortho- and meto-xylene can be isomerized using zeolite catalysts. Two routes are possible disproportionation, that is swapping methyl groups between xylene molecules and direct isomerization, involving sequential shifts of methyl groups around the benzene ring. 

With medium-pore zeolites (e.g. ZSM-5) very little disproportionation occurs, because the smaller cavities of ZSM-5 cannot easily accommodate this bulky intermediate. Much more of the product is formed through the isomerization reaction proceeding via a mechanism in which only a single molecule of xylene is involved. In this unimolecular xylene undergoes a 1,2-methyl shift... 

As mentioned earlier in section 2.2, a two-step mechanism via intermediate formation of methanol has been proposed by Adebajo et al, [21-23, 26, 36] for the oxidative mcthylation of benzene with methane over acidic zeolites in a high-pressure batch reactor. In view of this mechanism, a preliminary investigation has been carried out by these workers [24] on the reaction of toluene with methane over acidic ZSM-5 catalysts in a batch reactor containing residual air to determine the actual contribution of direct mcthylation (via intermediate methanol formation) to the observed reaction products. The reactions were carried out at 400 C and 6.9 MPa pressure. The major reaction products obtained by these wwkers were benzene and xylenes. Smaller amounts of ethylbenzene, trimethylbenzene and other higher aromatics were also produced. Over acidic catalysis, the conversion of toluene can, in principle occur through two different reaction pathways mcthylation by methane via methanol (as in the case of benzene mcthylation) and disproportionation, as shown in equations (4) and (S) below ... 

Oolite pore diameter and structure has a strong influence on xylene isomerization. It becomes evident that the reaction intermediate needed for disproportionation is higher than that needed for isomerization (Fig. 33). 

Then, in confined spaces such as in medium pore zeolites the bulky intermediate needed for disproportionation can not be "fitted , and then this reaction is strongly suppressed (170). This transition state selectivity would be responsible for the hi isomerization to disproportionation ratio observed in ZSM-5 zeolites. The effect of the pore dimension on the selectivity for xylene isomerization is given in Figure 33 A and B taken fi-om (170) and (185), respectively. 

While it was found by means of isotopic studies than on amorphous silica-alumina the reaction proceed by an intramolecular mechanism (194), in zeolite Y, the distribution of isomers in the trimethylbenzene fraction indicates that some of the isomers could be obtained by a bimolecular mechanism (172,175). In a very recent work (196,197) it has been demonstrated by means of isotopic studies, that on some 12 MR zeolites such as Y, and mordenite, xylenes are isomerized by both uni and bimolecular transalkylation mechanism. The ratio of the uni to bimolecular increases when increasing the Si/Al ratio, and decreases when increasing the reaction temperature, the partial pressure of the feed, and the contact time. Another 12 MR, Beta zeolite, while being able to disproportionate xylene, does not isomerize via the bimolecular mechanism. This was explained by space constraints to accommodate a xylene and a trimethylbenzene as a bimolecular intermediate in the channels of the zeolite. A medium pore zeolite (ZSM-5) does isomerize only through a unimolecular 1,2 methyl-shift mechanism. 

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