Para-Xylene alkylation

Benzene and para-xylene are the most sought after components from reformate and pygas, followed by ortho-xylene and meta-xylene. While there is petrochemical demand for toluene and ethylbenzene, the consumption of these carmot be discussed in the same way as the other four. Toluene is used in such a large quantity in gasoline blending that its demand as a petrochemical pales in comparison. Fthylbenzene from reformate and pygas is typically dealkylated to make benzene or isomerized to make xylenes. On-purpose production of petrochemical ethylbenzene (via ethylene alkylation of benzene) is primarily for use as an intermediate in the production of another petrochemical, styrene monomer. Ethylbenzene plants are typically built close coupled with styrene plants. 

Other examples of systems that are likely to be governed by product shape selectivity effects include toluene disproportionation to para-xylene -i- benzene in favor of other xylenes r- benzene [61]. Toluene alkylation by methanol to give para-xylene in favor of other xylenes is yet another such example [76],... 

In the case of toluene alkylation with methanol an opportunity exists for para selectivity. Para-xylene ortho-xylene ratio was 3.1 over MFl and 0.6 over BEA framework types. 

Similar chemistry been used by Faigl and Schlosser in an elegant and simple synthesis of ibuprofen 632 using only superbase chemistry (Scheme 245). Starting with para-xylene 630, two successive metallations and alkylations give 631, which is once more metallated at the less hindered benzylic site and carbonated to give ibuprofen 632. 

When toluene is alkylated by methanol with a ZSM-5 catalyst, increase in the ciystallite size from 0.5 to 3 pm approximately doubles the amount of para-xylene produced. Suggest a possible explanation. 

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) ... 

Harmer et al.196 used 1,1,2,2-tetrafluoroethanesulfonic acid in the alkylation of para-xylene with 1-dodecene. The silica-embedded catalyst prepared by the sol-gel method showed much higher activity than the neat acid (almost complete conversion in 15 min at 100°C over the sol-gel-derived material versus 10% conversion, using the same molar amounts of acid). Practically no leaching was detected and the catalyst could be recycled with a slight decrease in conversion. It is in sharp contrast with silica-supported triflic acid, which showed much lower activity due to the loss of volatile triflic acid. 

Friedel-Crafts alkylation of benzene,220 221 toluene,222para-xylene,220 and naphthalene223 with benzyl alcohols have been studied over Nafion-silica nano-composite catalysts, including the kinetics of alkylation.221,223 In most cases, 13% Nafion-silica showed the highest activity, testifying again to the much higher accessibility of the active sites. Complete conversion of para-xylene was found in the presence of triflic acid, and it was the only reaction when ether formation as side reaction did not occur. 

Fujiwara et al.227 tested a nanocomposite material having Nafion immobilized in MCM-41 mesoporous silica in Friedel-Crafts alkylations with benzyl alcohol. Whereas Nafion-MCM-41 showed lower activity in the alkylation of toluene than 13% Nafion SAC-13 under identical conditions, it exhibited increased activity when used in the alkylation of para-xylene. 

The alkyl groups can also be made to pass from one hydrocarbon to another, toluene in the presence of alnminium chloride yielding both benzene and meta- and para-xylene. 

Diphenyl carbonate from dimethyl carbonate and phenol Dibutyl phthalate from butanol and phthalic acid Ethyl acetate from ethanol and butyl acetate Recovery of acetic acid and methanol from methyl acetate by-product of vinyl acetate production Nylon 6,6 prepolymer from adipic acid and hexamethylenediamine MTBE from isobutene and methanol TAME from pentenes and methanol Separation of close boiling 3- and 4-picoline by complexation with organic acids Separation of close-boiling meta and para xylenes by formation of tert-butyl meta-xyxlene Cumene from propylene and benzene General process for the alkylation of aromatics with olefins Production of specific higher and lower alkenes from butenes... 

W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein, and S.A. Butter, Selective Alkylation of Toluene with Methanol to Produce para-Xylene. J. Catal., 1981, 67, 159-174. 

Besides the production of cumene and ethylbenzene, there are a number of recent reports on the production of linear alkylbenzene, precursors to detergents, via the alkylation of benzene with C6-C18 olefins. One process uses suspension CD and essentially 100% conversion of olefin at low temperatures of 90-100°C was obtained. An HF-treated mordenite used in the alkylation of benzene and C10-C14 olefins was foimd to give a 74-84% selectivity to linear alkylbenzene containing 80% 2-phenyl isomer. A new patent on the alkylation of aromatic hydrocarbons such as benzene and cumene with straight-chain C6-C20 olefins on acidic catalyst such as zeolites or fluorine-treated zeolite catalyst packed in a Katamax-type packing was granted. A patent application on the manufacture of xylenes from reformate by RD also appeared and higher than equilibrium amounts of para-xylene were claimed. 

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

The effect of grain size is not always observed experimentally. Table V reports some data about the alkylation of toluene with methanol as a function of the grain size of ZSM-5 and ZSM-11 zeolite samples. It clearly appears that, as expected, a higher selectivity to para-xylene is obtained when the grain size of the ZSM-5 zeolite increases. This does not seem to hold true for ZSM-11, samples prepared in our laboratory. However, a detailed high resolution electron microscopy (HREM) study of the morphology of the grains has shown (63) that ... 

The only aromatic components that appeared at reaction temperatures below 300 °C were toluene and p-xylene. In the case of the small-crystalline H-ZSM-5(M), however, some m- and o-xylene were present in the product mixture even at 245 °C (WHSV = 6 h- ). This can be explained by xylene isomerization at the outer zeolite surface. At conditions where the para-selectivity was high (more than 90% para), the amount of p-ethyl-toluene (PET) in the product were one order of magnitude greater than that of any other Cg-component, but when it was low, the ratio 1,2,4-trimethyl-benzene (124TMB) PET was found to be about 10 1. These experimental facts indicate that 124TMB is mainly formed by secondary xylene alkylation with methanol. Toluene, p-xylene, PET and perhaps ethyl-benzene are more likely to be the primary aromatic products formed in the MTG-reaction. To confirm this suggestion the molar product ratios EB/PX,... 

B-ZSM-5 samples (A and C) were observed to be inactive for the alkylation, even at 600°C, with a very low methanol conversion yielding light olefins. The SAPO-11 sample (H) gave very low alkylation conversion with thermodynamic para-xylene equilibrium selectivity. The latter result obviously arises from the pore dimensions (6 x 6.2 A), larger than for MFI samples. 

The production of para-xylene is of interest to the petrochemical industry because of its use as monomer in polyester production. In addition to Cg aromatic isomerization, there are a number of important routes to para-xylene including the alkylation of toluene with methanol and the disproportionation of toluene. The catalytic properties of the SAPO molecular sieves for toluene methylation reactions have been described(11). While both large and medium pore SAPO s were active for the alkylation reaction, the medium pore materials were distinguished by remarkably high selectivity for methylation reactions, with disproportionation of the toluene feed representing less than 2% of the total conversion. By comparison, large pore SAPO-5 had nearly 60% disproportionation selectivity and the zeolite reference LZ-105 had nearly 80% disproportionation selectivity. The very low disproportionation activity of the medium pore SAPO s, attributed to their mild acid character, resulted in reduced losses of toluene to benzene and increased xylene yields relative to LZ-105 and SAPO-5. 

One of the most important industrial alkylations is the production of 1,4-xylene from toluene and methanol (Reaction 2). ZSM-5, in the proton exchanged form, is used as the catalyst because of its enhanced selectivity for para substituted products. para-Xylene is used in the manufacture of terephtha-lic acid, the starting material for the production of polyester fibres such as Terylene. The selectivity of the reaction over HZSM-5 occurs because of the difference in the rates of diffusion of the different isomers through the channels. This is confirmed by the observation that selectivity increases with increasing temperature, indicating the increasing importance of diffusion limitation. The diffusion rate of para-xylene is approximately 1000 times faster than that of the meta and ortho isomers.14... 

The substitution of a hydrocarbon side chain into a benzene ring is called aromatic alkylation. Using zeolite ZSM-5, modified by the inclusion of phosphate ions (P04 ), at 500 °C, para-xylene has been synthesized selectively with a purity of up to 97%. A typical aromatic alkylation of monoalkyIbenzenes uses a toxic and hazardous catalyst, such as AICI3 or FeCl3, and results in predominantly ortho-para substitution. Not only that, but rapid secondary alkylation reactions usually generate a mixture of products with two, three or more alkyl substituents (Figure 4.1). 

Employing the xylene dibromides (ortho, meta, para) as alkylating reagent under identical reaction conditions, the ditopic 1,3,5-trithianes were synthesized (from /)-xylene dibromide 162 cf Scheme 43) <1993RTC370>. The coordination properties of the alkylated 1,3,5-trithianes were investigated by conductometry. 

Figure 4.17. Zeolite transition-state selectivity. Toluene alkylation with methanol catalyzed by H-MOR showing the energies of the key reaction intermediates . Reaction energy diagram for ortho-, meta- and para-xylene are compared.

More recently, methods have been developed for reducing the effective pore and channel dimensions. These techniques employ both physical treatments and chemical reagents. They have provided the basis for para-selective alkylation catalysts (18). These modified zeolites permit discrimination between molecules of slightly different dimensions. As a result, the para-isomers of the xylene or ethyltoluene products with the smallest minimum dimensions (Table 4) are able to diffuse out of the catalyst pores at rates about three orders of magnitude greater than those for the corresponding ortho- and meta-isomers (20). This discrimination capability is schematically represented in Figure 1, where the effective size of a para-selective catalyst pore is shown by the dashed line. 

---