Diacetone alcohol dehydration

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

The variations of the activity for decomposition of diacetone alcohol, dehydration of 4-methyl-2-pentanol, and alkylation of phenol are shown as a function of the composition of the binary oxide in Fig. 3.52. The catalytic activity for decomposition of diacetone alcohol correlates with the number of base sites, while that for the dehydration of 4-methyl-2-pentanol correlates with the number of acid sites. The alkylation of phenol with methanol is most effectively catalyzed by a binary oxide possessing both acid and base sites, the oxide containing Ti02 and MgO in a 1 1 ratio being the most active. 

McjC = CHCOCH3. Colourless liquid b.p. 129"C, with a strong peppermint-like odour. Prepared by distilling diacetone alcohol in the presence of a trace of iodine. Converted to phorone by heating in propanone with dehydrating agents such as sulphuric acid. It is a solvent For cellulose acetate and ethyl-cellulose and other polymers. 

Diacetone alcohol is readily dehydrated by adding a very small quantity of iodine as catalyst and distilling slowly ... 

In the three-step process acetone first undergoes a Uquid-phase alkah-cataly2ed condensation to form diacetone alcohol. Many alkaU metal oxides, metal hydroxides (eg, sodium, barium, potassium, magnesium, and lanthanium), and anion-exchange resins are described in the Uterature as suitable catalysts. The selectivity to diacetone alcohol is typicaUy 90—95 wt % (64). In the second step diacetone alcohol is dehydrated to mesityl oxide over an acid catalyst such as phosphoric or sulfuric acid. The reaction takes place at 95—130°C and selectivity to mesityl oxide is 80—85 wt % (64). A one-step conversion of acetone to mesityl oxide is also possible. 

Ma.nufa.cture. Mesityl oxide is produced by the Hquid-phase dehydration of diacetone alcohol ia the presence of acidic catalysts at 100—120°C and atmospheric pressure. As a precursor to MIBK, mesityl oxide is prepared ia this manner ia a distillation column ia which acetone is removed overhead and water-saturated mesityl oxide is produced from a side-draw. Suitable catalysts are phosphoric acid (177,178) and sulfuric acid (179,180). The kinetics of the reaction over phosphoric acid have been reported (181). 

MIBK is a valuable industrial solvent used primarily in the paint and coating industry and in metallurgical extraction processes. It is also used as a precursor in the production of specialty chemicals such as pesticides, rubber anti-oxidants as well as antibiotics and pharmaceuticals (1). Historically, MIBK has been produced commercially from acetone and hydrogen feedstock in three stages. First, acetone is dimerized to produce diacetone alcohol (DAA). Second, DAA is dehydrated to produce MO and water. Finally, the carbon-carbon double bond of MO is selectively hydrogenated to produce MIBK. These consecutive reactions are outlined in equations (1-3). 

The aldol condensation of acetone to diacetone alcohol is the first step in a three-step process in the traditional method for the production of methyl isobutyl ketone (MIBK). This reaction is catalysed by aqueous NaOH in the liquid phase. (3) The second step involves the acid catalysed dehydration of diacetone alcohol (DAA) to mesityl oxide (MO) by H2S04 at 373 K. Finally the MO is hydrogenated to MIBK using Cu or Ni catalysts at 288 - 473 K and 3- 10 bar (3). 

The reaction mechanism is shown in Figure 4 and is adapted from work by Fiego et al. [9] on the acid catalysed condensation of acetone by basic molecular sieves. The scheme has been modified to include the hydrogenation of mesityl oxide to MIBK. The scheme begins with the self-condensation of acetone to form diacetone alcohol as the primary product. The dehydration of DAA forms mesityl oxide, which undergoes addition of an addition acetone to form phorone that then can cyclise, via a 1,6-Michael addition to produce isophorone. Alternatively, the mesityl oxide can hydrogenate to form MIBK. 

In the second step, the diacetone alcohol is dehydrated (the -OH group and a hydrogen atom are clipped off) to form mesityl oxide. The dehydration is done by mixing the diacetone alcohol with the water-loving catalyst sulfuric acid at 212 250 F,... 

The diacetone alcohol reaction is a catalytic liquid-phase aldol condensation. The subsequent reactions involve dehydration, hydrogenation, and esterification. 

The methods of making mesityl oxide fall into three classes (1) the action of condensing agents (hydrochloric acid, etc.) on acetone 1 (2) the dehydration of diacetone alcohol 2 (3) and from diacetonamine.3 The latter method was not considered since the amine is relatively difficult to prepare. The action of acid condensing agents on acetone is very unsatisfactory the yields are poor and considerable quantities of phorone and similar substances are invariably produced. The direct production... 

Several methods which have been proposed for the dehydration of diacetone alcohol were investigated. H. Hibbert s method 2 (using a very small quantity of iodine) is superior to either the action of concentrated sulfuric acid or aluminum phosphate. The reaction proceeds more easily and the product is purer this method has therefore been adopted. 

Industrially, a selectivity to DAA of between 90—95% can be achieved (64). The principal by-products are mesityl oxide and acetone trimers. j W-Triacetone dialcohol [3682-91-5] can form by condensation of acetone with diacetone alcohol (116). Dehydration of ry/ -triacetone dialcohol can yield semiphorone [5857-71-6] (6-hydroxy-2,6-dimethyl-2-hepten-4-one), which may in turn ring close to form 2,2,6,6-tetramethyl-y-pyrone [1197-66-6/, or ultimately dehydrate to phorone [504-20-1] (2,6-dimethyl-2,5-heptadien-4-one) (146). Similarly, an unsymmetrical acetone trimer can also be formed which dehydrates to 2,4-dimethyl-2,4-heptadiene-6-one. These impurities complicate the high purity recovery of DAA, and are thought to be responsible for a yellow discoloration of DAA. The addition of dibasic acid (147) or nitrogen containing carboxylic or phosphonic acids (148) has been patented as refined product stabilizing agents. 

In this case however the position of equilibrium is not in favour of the condensation product (which rapidly dissociates into the ketone in the presence of base), and a technique has to be employed which continuously removes the ketol product, as it is formed, from the presence of base. A satisfactory procedure for converting acetone into diacetone alcohol is described in Expt 5.213. The dehydration step to give mesityl oxide is subsequently effected by the presence of a trace of iodine. 

Propose a mechanism for the dehydration of diacetone alcohol to mesityl oxide (a) in acid (b) in base... 

The conversion of acetone to methyl isobutyl ketone (MIBK) also uses a combination of base catalysis with a hydrogenation catalyst [35], The base component converts the acetone to diacetone alcohol (DAA) via an aldol reaction, which is then dehydrated by the silica to give mesityl oxide (MO). The final step is the hydrogenation of the MO to MIBK over the metal component. The action of the base catalyst in the absence of the hydrogenating metal has been studied [36]. As well as the aldol condensation reactions shown below, the cesium oxide also hydrogenated MO to MIBK, albeit at a low level (Scheme 21.3). 

S-Hydfoxy ketones obtained by the condensation of ketones or by the crossed condensation of aldehydes with ketones are important sources of olefinic ketones. Dehydration is effected by warming the ketols with oxalic acid, dilute sulfuric acid, hydrobromic acid, phosphoric acid, or a trace of iodine. A typical example is the dehydration of diacetone alcohol obtained from the self-condensation of acetone. The product is an equilibrium mixture of the conjugated and unconjugated isomers, (CHj)jC=CHCOCHj and CHj=C(CHj)CHjCOCHj, in a ratio of 91 to 9, respectively. ... 

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