Methyl 4-toluate

Various ways of overcoming the PTA oxidation problem have been incorporated into commercial processes. The predominant solution is the use of high concentrations of manganese and cobalt ions (2,248—254), optionally with various cocatalysts (204,255,256), in the presence of an organic or inorganic bromide promoter in acetic acid solvent. Operational temperatures are rather high (ca 200°C). A lesser but significant alternative involves isolation of intermediate PTA, conversion to methyl/)-toluate, and recycle to the reactor. The ester is oxidized to monomethyl terephthalate, which is subsequentiy converted to DMT and purified by distillation (248,257—264). 

Herm/es/Djnamit JS obe/Process. On a worldwide basis, the Hercules Inc./Dynamit Nobel AG process is the dorninant technology for the production of dimethyl terephthalate the chemistry was patented in the 1950s (67—69). Modifications in commercial practice have occurred over the years, with several variations being practiced commercially (70—72). The reaction to dimethyl terephthalate involves four steps, which alternate between liquid-phase oxidation and liquid-phase esterification. Two reactors are used. Eirst, -xylene is oxidized with air to -toluic acid in the oxidation reactor, and the contents are then sent to the second reactor for esterification with methanol to methyl -toluate. The toluate is isolated by distillation and returned to the first reactor where it is further oxidized to monomethyl terephthalate, which is then esterified in the second reactor to dimethyl terephthalate. 

Eigure 3 is a flow diagram which gives an example of the commercial practice of the Dynamit Nobel process (73). -Xylene, air, and catalyst are fed continuously to the oxidation reactor where they are joined with recycle methyl -toluate. Typically, the catalyst is a cobalt salt, but cobalt and manganese are also used in combination. Titanium or other expensive metallurgy is not required because bromine and acetic acid are not used. The oxidation reactor is maintained at 140—180°C and 500—800 kPa (5—8 atm). The heat of reaction is removed by vaporization of water and excess -xylene these are condensed, water is separated, and -xylene is returned continuously (72,74). Cooling coils can also be used (70). 

The oxidation reactor effluent and methanol ate sent to the esterification reactor, which operates at up to 250°C and a pressure sufficient to maintain the Hquid phase. This latter is about 2500 kPa (25 atm). The oxidation products are converted to methyl -toluate and dimethyl terephthalate without a catalyst. Excess methanol is suppHed, and steam and vaporized methanol ate removed and enter a methanol recovery column. The esterification products flow to a cmde ester column, which separates the toluate from the terephthalate. The overhead stream of methyl -toluate is returned to the oxidation reactor, and the bottoms stream of dimethyl terephthalate goes to a primary distillation. The distillate is dissolved in methanol, crystallized, and sohd dimethyl terephthalate is recovered. The dimethyl terephthalate can then be either recrystallized or distilled to yield the highly pure material needed for the polyesterification reaction. 

Methyl esters of short-chain dicarboxylic acids Loss of — OCH2CH2NH2 -C6H10(CO2CH3)2 O-methyl toluates... 

The DuPont patent (US 5866622 A, 1999) describes dissolving the polyester in molten dimethylterephthalate, methyl-/ -toluate or dimethylisophthalate and separating the polyester from non-polyester components. The polyester can subsequently be used as a feedstock for methanolysis to form dimethylterephthalate (DMT) and alkylene glycol. The DMT can be subsequently hydrolysed to recover terephthalic acid. 

Description A mixture of p-xylene and p-methyl toluate is oxidized (2, 3) with air in the presence of heavy metal catalysts to produce p-toluic acid and monomethyl terephthalate. These adds are then converted to p-methyl toluate and DMT. No reaction solvent is necessary during oxidation. The p-xylene and p-methyl toluate are continuously oxodized at 140°C to 170°C and 4 to 8 bar ... 

Crude ester from the esterifier bottoms (4) is distilled in a column (9) where overhead p-methyl toluate is recycled to the oxidizer and bottom product is separated in a second column (10) into crude DMT overhead (11) and residue bottom. 

Based on statistical analyses it may be reasonable to propose that this structure reflects the idealized or low energy mode of coordination to carbonyls for aluminum-centered Lewis acids. It would be interesting to see how distortions away hrom such an arrangement may come about Imposition of steric congestion through bulky substituents is one way to address this question. Ibe crystal suucture of (2,6-di-f-butyl-4-methyl)phenoxydiethylaluminum/methyl toluate was recently reported (Figure 22). ... 

It may be recalled that an opposite stereochemical result is obtained by employing the bulky aluminum reagents MAD and MAT. This observation has been explained by invoking out-of-plane complexation of the Lewis acid in a direction which would prevent equatorial attack (Figure 48). The X-ray crystal structure of methyl toluate complexed with a bulky aluminum Lewis acid is fully consistent with tius model7 However, it is worth mentioning that a six-membered transition state, perhaps involving [Me2( ArO)Al] Li, has not been considered as an alternative mechanism. 

The Witten process, which is also known as the Imhausen or Katzschmann process [3a-f], involves the oxidation of p-xylene to p-toluic acid as a reaction intermediate. For activity reasons, the p-toluic acid is esterified with methanol to methyl toluate, which is then oxidized to monomethyl terephthalate. The two oxidation reactions take place simultaneously in one oxidation system likewise... 

Figure 22 Ciystal stiucture of Et2 Al(OAr)/methyl toluate complex...

Esterification of p-toluic acid by methanol to p-methyl toluate ... 

Purification. The effluent is sent to a series of distillations. The first (30 to 35 trays) separates a methanol/water mixture at the top, which is then sent to a dehydration tower (25 to 30 trays), while the bottom is sent to a column in winch the ester is fractionated under partial vacuum (40 to 45 trays). Methyl toluate and excess p-xylene leave at the top and are recycled to the oxidation reactor. The withdrawal, consisting of crude terephthalate, is redistilled under vacuum to remove heavy compounds (20 trays), and then sent to a vacuum crystallizer (40 to 50 kPa absolute) using methanol as solvent. This may be followed by a second crystallization, or a countercurrent washing with methanol to complete the purification. The dimethyl terephthalate is finally centrifuged, melted to remove residual methanol, and vacuum distilled (30 trays). The molar yield of the operation in relation to p-xylene is about 87 per cent... 

Fractionation of the polymer by solvent extraction, optical rotation ([a]o ) and IR absorption (D763/D732) of the fractions indicated that the more soluble polymer fractions consisted, as expected, of copolymer of (S)-monomers, whereas the last fraction was based on poly[(R)DMO)]. In the presence of the above high yield system, activated by Al(i-Bu)3 and p-methyl toluate (MT) as external base, DM0 and (S)3MP (3.8/1 molar ratio) gave results very similar to the conventional catalysL This indicates that the chiral recognition of active centers capable of polymerizing these sterically hindered branched olefins remains substantially the same even in the presence of the support (MgCl2) and the Lewis base. 

The protocol for the preparation of a catalyst from the patent literature is MgCl2/EB/PC/AlEt3/TiCl4-AlEta/MPT where EB, PC, and MPT are abbreviations for ethyl benzoate, p-cresol and methyl-/ -toluate, respectively. 

In the process shown in Fig. 22.45, a mixture of p-xylene and p-methyl toluate is oxidized, and the oxidate is esterified. The crude ester is separated by distillation, with the fractions containing p-methyl toluate and terephthalic aldehyde ester being recycled in the oxidation. The crude DMT is hydrolyzed in water at 250 to 280°C to terephthalic acid. 

TiCU/MgClj/EB/AlEtj/EA Ethyl benzoate (EB) complexes MgCb- Ethyl anisate (EA) or p-methyl toluate added with cocatalyst. 300 92... 

---