Ester Recycling Route

Esters obtained from the degradation of poly(ethylene terephthalate) (PET) can be used as raw materials for PPA. PPA can be condensed after the degradation of the polyester used. The ester groups can be converted almost completely into amide groups. After removal of the alcohols and diols released, the materials can be processed in a conventional manner. 

In particular, to a mixture of PET bottle recyclate and PA 6, benzoic acid is added as chain stopper. Further, disodium phosphate serves as an antioxidant. The mixture is molten under inert gas. To the melt, HMD is charged. After discharging and cooling, the ethylene glycol released 

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The stoichiometric and the catalytic reactions occur simultaneously, but the catalytic reaction predominates. The process is started with stoichiometric amounts, but afterward, carbon monoxide, acetylene, and excess alcohol give most of the acrylate ester by the catalytic reaction. The nickel chloride is recovered and recycled to the nickel carbonyl synthesis step. The main by-product is ethyl propionate, which is difficult to separate from ethyl acrylate. However, by proper control of the feeds and reaction conditions, it is possible to keep the ethyl propionate content below 1%. Even so, this is significantly higher than the propionate content of the esters from the propylene oxidation route. 

The boric and sulfuric acids are recycled to a HBF solution by reaction with CaF2. As a strong acid, fluoroboric acid is frequently used as an acid catalyst, eg, in synthesizing mixed polyol esters (29). This process provides an inexpensive route to confectioner s hard-butter compositions which are substitutes for cocoa butter in chocolate candies (see Chocolate and cocoa). Epichlorohydrin is polymerized in the presence of HBF for eventual conversion to polyglycidyl ethers (30) (see Chlorohydrins). A more concentrated solution, 61—71% HBF, catalyzes the addition of CO and water to olefins under pressure to form neo acids (31) (see Carboxylic acids). 

A low-cost route to 1,4-butanediol and tetrahydrofuran based on maleic anhydride has been disclosed (Davy process).343,344 Here dimethyl or diethyl maleate is hydrogenated over a copper catalyst. Rapid saturation of the C—C double bond forms diethyl succinate, which subsequently undergoes further slower transformations (ester hydrogenolysis and reduction as well as dehydration) to yield a mixture of y-butyrolactone, 1,4-butanediol, tetrahydrofuran, and ethanol. After separation both ethanol and y-butyrolactone are recycled. 

A stimulating development of urea alcoholysis has been demonstrated very recently for better AE, in an innovative integrated process that incorporates fatty ester hydrolysis to co-amino-alkanoic acids [44], Within the scope of this chapter, the most interesting step of this process is the recycling of waste alcohol, formed by the hydrolysis step, for urea alcoholysis. Dialkyl carbonate is produced together with ammonia thereafter, the ammonia is engaged in the amination reaction to obtain the amino acids. The overall process avoids the storage of NH3 that is necessary for the amination route, and transforms a waste product-the alcohol-into the valuable dialkyl carbonate. 

A number of approaches to the chiral chloropentenoic ester 30 have been disclosed, with enantiomeric purity established either by chiral auxiliaries, diastereomeric salt resolution, or enzymatic resolution.35,36 The enzymatic resolution reported36 in WO 0209828 is particularly efficient and has been disclosed as part of the commercial route to aliskiren.9 The enolate of methyl isovalerate (36) is alkylated with ( )-1,3-dichloroprop-1-ene to give rac-30, which is resolved by pig liver esterase (PLE) to give 30 with the desired 25-stereochemistry in high enantiomeric purity. The undesired acid 37 is then recycled in a two-step, one-pot process involving esterification and racemization to return rac-30. 

On completion, water is added to the mixture after which it is fractionated. Cyclohexane (b.p. 81°C) containing some benzene is collected from the top of the column, and after hydrogenation of the benzene, is recycled. The cyclo-hexanol-cyclohexanone mixture consists of approximately equal volumes of cyclohexanol (b.p. 161°C), cyclohexanone (b.p. 156°C), plus a mixture of several esters and ethers. It is collected from the bottom with 80+% yields on cyclohexane. An alternative route to cyclohexanol used by some plants is to catalytically hydrogenate phenol. 

A kinetic resolution of an ester requiring separate steps to recover and recycle the less reactive enantiomer has been transformed into a dynamic resolution of the corresponding thioester. While the thioester route requires an additional synthetic step, the efficiency of this step renders the dynamic resolution route to (R)-l shorter. The development of an alternative route provides our company with additional options for commercial-scale roxifiban preparation. 

However, a source of the non-natural 9S isomer (2) was first required. The ready availability of natural crinitol made a racemization/resolution route, as illustrated in Scheme 1, attractive. Racemization was accomplished by Collins oxidation (16,25) to the dicarbonyl compound (14), followed by lithium aluminum hydride (LAH) reduction to give the racemic mixture (1 + 2). Resolution via diastereomeric derivatives seemed plausible. Esterification with enantiomerically pure a-methoxy-a-(trifluoromethyl) phenylacetic acid (MTPA) (17), followed by separation of diastereomers by recycle-HPLC (R-HPLC), had earlier been used to purify enantiomers of ipsenol and ipsdienol (26). A model system, the resolution of -3-nonen-2-ol, a secondary allylic alcohol naturally occurring in Rooibos tea (16,27), also worked satisfactorily. Therefore, the route using the bis-(MTPA) esters was selected for crinitol. 

Propane and light ends are rejected by routing a portion of the compressor discharge to the depropanizer column. The reactor effluent is treated prior to debutanization to remove residual esters by means of acid and alkaline water washes. The deisobutanizer is designed to provide a high purity isobutane stream for recycle to the reactor, a sidecut normal butane stream, and a low vapor pressure alkylate product. 

Early attempts at chain extension took the route of using esters of dicarboxylic acids which had greater reactivity towards the polyester chain ends than simplistic additives such as dimethyl terephthalate [191, 193, 194], but many of the more reactive species gave nonvolatile small-molecule by-products such as phenol, which were difficult to remove. Another early attempt [192] used diisocyanates, but this approach can give imdesirable branching, and the new linkage formed was thermally unstable. Later studies used diisocyanates to chain-extend recycled PET [178]. 

The traditional batch process follows the route in Figure 2.8. The catalyst, used in catalytic proportions, can be washed out at the end of the process. The excess methyl ester and methanol formed as the esterification proceeds is distilled off as a mixture. Separation of this mixture is usually not straightforward and recycle of the excess methyl ester is therefore difficult and expensive. Byproduct formation, leading to undesirable impurities which have to be removed, is also likely as reagents and end products, in the presence of a catalyst, exist together for extended periods at elevated temperatures. 

A general approach to chiral 3-alkyl-succinaldehyde-esters (114) is outlined in Scheme 37. For simple alkyl groups, chemical yields are ca. 70% but, more significantly, optical yields are ca. 90% in addition the starting chiral pyrrolidine can be recycled. A route to 3-aroylpropionates, apparently superior to the methods of Stetter and Hauser (which have aryl aldehyde cyanohydrins as intermediates), has been developed (Scheme 38). ... 

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