Meta disproportionation

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

Aromatic hydrocarbons substituted by alkyl groups other than methyl are notorious for their tendency to disproportionate in Friedel-Crafts reactions. This tendency has previously limited the application of the isomerization of para- or ortho-) m ky -benzenes to the corresponding meta compounds. At the lower temperature of the present modification, disproportionation can be minimized. 

The compounds discussed in 6.2.3 are significant as examples of metal-meta bonding in group IIIB, but their value for chemical utilization is restricted by their insolubility and their tendency to disproportionate to E(0) or E(I) and E(III) states. 

Unsually short NMR T, relaxation values were observed for the metal-bonded H-ligands in HCo(dppe)2, [Co(H2)(dppe)]+ (dppe = l,2-bis(diphenylphosphino)ethane), and CoH(CO) (PPh3)3.176 A theoretical analysis incorporating proton-meta) dipole-dipole interactions was able to reproduce these 7) values if an rCo H distance of 1.5 A was present, a value consistent with X-ray crystallographic experiments. A detailed structural and thermodynamic study of the complexes [H2Co(dppe)2]+, HCo(dppe)2, [HCo(dppe)2(MeCN)]+, and [Co(dppe)2(MeCN)]2+ has been reported.177 Equilibrium and electrochemical measurements enabled a thorough thermodynamic description of the system. Disproportionation of divalent [HCo(dppe)2]+ to [Co(dppe)2]+ and [H2Co(dppe)2]+ was examined as well as the reaction of [Co(dppe)2]+ with H2. 

With the percentages of the three xylenes from the various sources differing so much, it s not likely that a company, or the industry for that matter, will produce just the amount of the xylene isomer it wants. Para-xylene has the biggest demand and meta- the smallest, but none of the processes, cat reforming, olefins plants, or disproportionation, have commensurate yields. 

As an example for aromatic transformation the mechanism for meta-xylene disproportionation to toluene -i- trimethylbenzene is illustrated in Figure 13.46. In the first step the zeolite extracts a hydride from meta-xylene to form a carbenium ion at one of the methyl groups, presumably the rate-controlling step. This mechanism is likely to involve a Lewis acid site. The carbenium ion then adds to a second... 

A volt-equivalent diagram for the water-soluble nitrogen species in acidic solution is shown in Figure 6.4. It shows that the nitrate(V) ion is the least stable species, but also indicates the meta-stability of nitrous acid, which is unstable with respect to disproportionation into oxidation states + 5 and zero ... 

Not all reactions leading to carbonylate anions require strong reducing agents. Some involve reduction of the meta by carbon monoxide already present in the metal carbonyl or disproportionation of the complex In fact, the first synthesis of u metal carbonylate involved the former procedure ... 

Further complications arise from the fact that the alkylation reactions sometimes are under equilibrium control rather than kinetic control. Products often isomerize and disproportionate, particularly in the presence of large amounts of catalyst. Thus 1,2- and 1,4-dimethylbenzenes (ortho- and para-xylenes) are converted by large amounts of Friedel-Crafts catalysts into 1,3-dimethyl-benzene (meta-xylene) ... 

In presence of LDA 2,3-dibromothiophenes rearrange to 3,5-dibromothiophenes230. Aluminum halides are well known catalysts for disproportionation of aryl halides which permit the transformation of the kinetic products ortho- and / ara-dihalobenzenes to equilibrium mixtures which contain significant amounts of meta-dihalobenzenes231,232. 

The primary isomer distribution, which is the result of the disproportionation reaction, may deviate significantly from the thermodynamic equilibrium composition, for two reasons. First, the reaction may be controlled by the kinetics rather than the thermodynamics, i.e. mechanistic reasons may exist which cause the reaction to proceed along a certain path. Second, in the case that the reaction obeys a bimolecular mechanism, it may pass through a transition state which would presumably favor the (taller) para isomer. Hence, it is possible that the primary product contains an enhanced fraction of the para isomer. The departure from the equilibrium composition then gives the driving force for the subsequent (monomolecular) isomerization reaction. This will reduce the fraction of the para isomer, provided the formation of the bulky ortho and meta isomers are not inhibited by sterical effects, i.e. when the micropore diameter is sufficiently large or there is a chance for the isomerization reaction to take place at the outer surface of the crystallites. Thus, the secondary isomer distribution may approach the thermodynamic equilibrium composition, as a limiting case. 

For these simulations, the primary isomer distribution is chosen according to the thermodynamic equilibrium (sec Table 6). Such a situation would be encountered in practice when neither the reaction mechanism kineti-cally favors a particular isomer nor restricted transition state shape-selectivity effects occur. The disproportionation reaction is assumed to be unaffected by diffusion (i.e. y < 0.01). The effective diffusivities of the ortho and meta isomers are fixed, and assumed to be equal, but by a factor of Ro smaller than the effective diffusivity of the para isomer. 

Evaluating the catalytic shape-selectivities of these materials by use of the disproportionation of ethylbenzene (23.24) at 523 K at a conversion of 2% in differential reactor mode, it was observed that larger crystals of sample A gave 85% para-diethylbenzene and 15% meta-diethylbenzene. The smaller crystals of sample B with the smoother aluminium gradient yielded 96% para-diethylbenzene and only 4% meta-isomer. In a second series, samples of crushed large crystals with mean sizes of 1-10 (m were examined. No increase in activity was observed as is expected when the reaction is controlled by the diffusion limitation of molecules in the large crystals. However, this treatment created larger non-selective external surface area and hence a smaller selectivity of 87% para-diethylbenzene for sample B was recorded. 

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...

Triolefin Also called Phillips Triolefin. A process for disproportionating propylene into a mixture of ethylene and 2-butene. The reaction takes place at 160°C over a cobalt-molybdenum catalyst on an alumina base. Developed by the Phillips Petroleum Company from 1963. A commercial plant was built by Gulf Oil Canada in 1966 and operated by Shawinigan between 1966 and 1972 before closing for economic reasons. See also Olefin Conversion Technology, Meta-4. 

It can be seen in Figure 16 that the non-participating methyl group for each of the disproportionation transition states has enough room to avoid steric constraints with the zeolite wall. The activation energies are -i- 164 kJ/mol, -i-174 kJ/mol, and -i- 187 kJ/mol for an alkylation/dealkylation to the ortho, meta, and para position respectively. 

At low temperature the ferrous-iron-rich brucite (Mg,Fe)(OH)2 can subsequently disproportionate into a magnesium end-member brucite and magnetite, which becomes the stable iron oxide in contact with water at low temperature. The formation of magnetite under certain conditions can result in the release of hydrogen gas. At elevated temperatures in hydrothermal and meta-morphic reactions (Equations (2) and (3)), magnesium or potassium can be removed from solution forming the minerals chlorite or sericite ... 

Epilepsy and osteoporosis are very common and frequently overlap. Nevertheless, the prevalence of low bone density appears to be disproportionately higher in patients with epilepsy, and patients with epilepsy have an excessive risk of fractures. A meta-analysis of 94 cohort studies and 72 case-control studies has shown that anticonvulsant treatment is highly associated with fractures (relative risk over 2) (117). Other risk factors were low body weight, weight loss, physical inactivity, consumption of corticosteroids, primary hjrperparathjroidism, type 1 diabetes melhtus, anorexia nervosa, gastrectomy, pernicious anemia, and age over 70 years. 

It has been recently demonstrated [4] that the disproportionation mechanism of n-propylbenzene essentidly depends on the nature of the catalyst it is mainly of Sn2 type on Y-zeolites and leads to a meta/para (m/p) ratio s 2. This has been also observed for sBB under atmospheric pressure [5]. The ratio m/p between the two di-sec-butylbenzene isomers can be assumed as an indication of the relative extent of the Sn2 and SnI mechanisms the higher the m/p ratio, the higher the Sn2 extent. At low time factor values, when DBA is present in small amount (Fig.2a), the m/p ratio observed is about 1.6 and is constant with time on stream (t-o-... 

Xylenes. Mixed xylenes are generally obtained by the catalytic reforming of petroleum fractions (see Section 6.2.1.7, above) or by the recovery of the Cg fraction from an aromatic concentrate (pyrolysis gasoline) stream. An increasingly important source of mixed xylenes is from the disproportionation of toluene. The separation of mixed xylenes into para, meta, and ortho isomers can be accomplished by several methods (e.g., fractional crystallization). 

Figure 11 is a highly simplified diagram of this process. Toluene enters the ZSM-5 crystal and disproportionates at the acid catalyst sites to benzene and the three xylene isomers para-, meta-, and ortho-. Because of their larger size, meta- and ortho-xylenes diffuse slower than para-xylene. As you expect, the longer they stay inside the ZSM-5 crystal, the richer the product will be in para-xylene. 

The disproportionation of toluene was the dominant reaction on Mo exchanged ZSM-5 and mordenite (0.4 wt % Mo). Compared with H-ZSM-5 and H-mordenite, the Mo exchanged samples had a much better resistance to deactivation, and this efftect was more pronounced on Mo mordenite, especially when was used as carrier gas. The variation in the distribution of xylene isomers with the conversion is plotted in Fig. 2. The Mo zeolites enhanced the relative yield of para + meta xylenes. 

The relative distribution of para + meta aromatics in methanol conversion was increased over Mo exchanged ZSM-5 but not on the pyridine poisoned sample. The same increase trend was also observed in the disproportionation reaction over Mo exchanged zeolites (Fig. 3), thus a reasonable explanation is the presence of internal Mo. Both methanol and toluene conversions performed on zeolites are molecular shape selective processes. Internal Mo will create diffusional hindrances which will favour the formation of para aromatics (product selectivity). 

The significant finding was that the xylene fraction was 99% para. The other fractions are not lost. Toluene can be disproportionated to p-xylene and benzene with H-ZSM-5 treated with a little hexamethyldisiloxane to give 99% p-xylene, so that the usual separation of the ortho- and meta isomers with another zeolite would not be required.177 Benzene can be transalkylated with the higher aromatics to give toluene. Ethylbenzene can be isomerized to p-xylene. Ethylbenzene can be alkylated with ethanol in the presence of a modified ZSM-5 catalyst to produce p diethylbenzene with 97% selectivity.178... 

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