Phorone acetone self-condensation

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. 

Acetic acid is known to undergo a vapor-phase ketonization reaction with formation of acetone on Brpnsted acids in general, and on proton-zeolites in particular. On large-pore zeolites in their proton form, the ketonization reaction is followed by acid-catalysed self-condensation amounting to mesitylene, mesityl oxide and phorone as main products [1], the chemistry being essentially identical to that in mineral acids. In H-pentasil zeolites with suitable acid site density, phorone isomerises to isophorone, which is cracked to yield 2,4-xylenol [1]. With propionic acid a similar chemistry occurs, but the formation of phenolics is severely suppressed by transition-state shape-selectivity effects... 

All PdPn x(a) catalysts showed activity and selectivity for the synthesis reaction of MIBK from acetone (Table 1). The conversion increased with the Pd content, while the selectivity toward MIBK reached a maximum for catalysts with Pd loading in the range 0.2-0.5 wt-%. Indeed, at lower Pd loading, when the basic function of the support prevailed on the hydrogenating ability, condensation reactions took place condensation of acetone with mesityl oxide to phorone, isophorone and trimethyl-cyclohexanone, and condensation of MIBK with acetone or its self condensation to diisobutyl ketone (DIBK) or trimethyl nonanone (NONA), respectively (Figure 6). 

Self-condensation of acetone was carried out at 473 K and 100 kPa in a flow system with a differential fixed-bed reactor. Acetone was vaporized in H2 (H2/acetone =12) before entering the reaction zone. The standard contact time (6fc) was 0.84 g of cat h/g of acetone. Main reaction products were mesityl oxides (MO s), isophorone (IP) and mesitylene (MES). Traces of phorone and light hydrocarbons were also identified. The coke formed on the catalysts was characterized after reaction, ex-situ, in a temperature-programmed oxidation (TPO) unit. The TPO experiments were carried out in a microreactor loaded with 50 mg of catalyst and using a 3 % O2/N2 carrier gas. Sample temperature was increased linearly from room temperature to 973 K at 10 K/min. The reactor exit gases were fed into a methanator operating at 673 K to convert CO in methane and then analyzed by flame ionization detector. 

As an example, the dehydration of diacetone alcohol, the )3-hydroxy ketone produced during the self-condensation of acetone leads to mesityl oxide, the a,p-imsatruated ketone dimer of acetone, which can further react with another acetone molecule, as shown in Scheme 6. Since both, mesityl oxide and acetone are able to form a carbanion by a-hydrogen abstraction, two trimers of acetone can be obtained in a consecutive aldol-dehydration sequence 4,6-dimethyl hepta-3,5-dien-2-one (carbanion supplied by acetone) and phorone (carbanion formed from mesityl oxide). A water molecule is lost in both reactions, and both products are a,/ -unsaturated ketones (11,19). A third tiimer is also possible. A review on the operating conditions and catalysts needed for the production of a, -tmsaturated ketones from acetone can be formd in Ref (11), whereas a discussion on the reaction pathways on different catalysts is reported in Ref (20). 

The isomerization of phorone by 1,6 internal Michael rearrangement to give cyclic isophorone (19) is shown in Scheme 10 as an example of the intramolecular reaction. Phorone is one of the trimeric intermediates produced by consecutive condensations during the gas-phase self-condensation of acetone. The first step of the Michael reaction is the a-hydrogen abstraction by the catalyst and the carbanion formation. Then, the six-member ring product forms by the carbanion addition to the C—C double bond. 

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