Catalyst azeotropic dehydration

Low boiling substances are removed from the chilled reactor product by distilling up to a cut point of 80 °C. These low boilers are gaseous dimethyl ether, methyl acetate, acetaldehyde, butyraldehyde, and ethyl acetate. The bottoms are flash-distilled to recover the rhodium catalyst. Flash distilled acid is azeotropically dehydrated. In the final distillation, glacial acid is obtained. Traces of iodine that may remain in the finished acid may be removed by fractional crystallization or by addition of a trace of methanol followed by distillation of the methyl iodide that forms. Somewhere in the carbonylation reaction, a minute amount of propionic acid seems to be made. It typically is found in the residues of the acetic acid finishing system and can be removed by purging the finishing column bottoms. 

Azeotropic dehydration and condensation polymerization (route 2 in Figure 8.2) yields directly high molar mass polymers. The procedure, patented by Mitsui Toatsu Chemicals [13, 14], consists of the removal of condensation water via a reduced pressure distillation of lactic acid for 2-3h at 130°C. The catalyst (in high amounts) and diphenyl ester are added and the mixture is heated up to reflux for 30-40 h at 103°C. Polycondensated PLA is purified to reduce residual catalyst content to the ppm range [5,10,15]. 

Ajioka et al. [26] have synthesized PLA with a higher than 300 000 by azeotropic dehydrative polycondensation of LA in the presence of a catalyst and an organic solvent. These polymers have good mechanical properties and can be processed into products such as cups, film, and fiber, which can be used as compostable materials. 

Conventionally, enamines are formed using azeotropic distillation methods to remove the by-product water from a mixture of the ketone and appropriate secondary amine. Benzene and toluene are therefore generally the solvents of choice. Taguchi and Westheimer demonstrated that molecular sieves can have a beneficial effect on this reaction, acting not only as a dehydrating agent but also as a catalyst.45... 

DMA and TMA. Product ratios can be varied to maximize MMA, DMA, or TMA production. The correct selection of the N/C ratio and recycling of amines produces the desired product mix. Most of the exothermic reaction heat is recovered in feed preheating (3). The reactor products are sent to a separation system where firstly ammonia (4) is separated and recycled to the reaction system. Water from the dehydration column (6) is used in extractive distillation (5) to break the TMA azeotropes and produce pure anhydrous TMA. The product column (7) separates the water-free amines into pure anhydrous MMA and DMA. Methanol recovery (8) improves efficiency and extends catalyst life by allowing greater methanol slip exit from the converter. Addition of a methanol-recovery column to existing plants can help to increase production rates. 

Nitriles can be prepared by dehydration of primary amides (reaction 8) or of aldoximes (reaction 9), catalyzed by the Re trioxo compounds [ReOsX] (X = OSiMes, OReOs, OH) in azeotropic (e.g. toluene/water or mesitylene/water) reflux. Aqueous perrhenic acid (X = OH) is the most convenient catalyst in view of the moisture sensitivity of the others. The oxophilicity of the Re(VII) oxo-complexes is believed to play a determining role in the reactions, which are proposed to involve six-membered cyclic transition states formed upon preferential (9-coordination, relative to A-coordination, of the amide or oxime. ... 

The acid in reaction 3 functions as an esterification catalyst. The toluene in this reaction serves as a dehydrating agent (toluene forms an azeotrope with ethanol and water containing 12% water [18]). In addition, the toluene renders the reaction solution a poor solvent for the byproduct salts and thus facilitates their separation. 

This process uses a multi-tube reactor containing a supported platinum-based catalyst Heat is removed by a coolant which vaporizes the benzene feed, previously dehydrated by azeotropic distillation, and also produces steam at 1.10 Pa absolute. 

Enamines and metalloenamines provide a valuable alternative to the use of eno-lates for the selective alkylation of aldehydes and ketones. Enamines are a,p-unsaturated amines and are obtained simply by reaction of an aldehyde or ketone with a secondary amine in the presence of a dehydrating agent, or by heating in benzene or toluene solution in the presence of toluene-/7-sulfonic acid (TsOH) as a catalyst, with azeotropic removal of water (1.31). Pyrrolidine and morpholine are common secondary amines useful for forming enamines. All of the steps of the reaction are reversible and enamines are readily hydrolysed by water to reform the carbonyl compound. All reactions of enamines must therefore be conducted under anhydrous conditions, but once the reaction has been effected, the modified carbonyl compound is liberated easily from the product by addition of dilute aqueous acid to the reaction mixture. 

Other recent relevant developments have been the replacement of conventional acid catalysts with greener analogues. Direct esterification of carbo Q lic acids and alcohols continues to be a focus of attention. As examples, diphenylammonium triflate 8 and bulky diatylammonium sulfonates were shown to catalyse ester condensation of carboxylic acids and alcohols efficiently, and without the need for azeotropic water removal in the former case. Pentafluorophenylammonium triflate 9 was shown to be an efficient and cost-effective catalyst not only for esterification, but also for thioesterification, transesterification and macrolactone formation without requiring a dehydrating system. The superior catal)4ic efficiency of 9 relative to 8 was ascribed to the lower basicity of the pentafluoroaniline counter amine compared to diphenylamine. In related work, polyaniline... 

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