Isophorone 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. 

An analogous process, involving the interaction of a cross-conjugated dienamine with a linear dienamine, has been suggested in the self-condensation of acetone in the presence of pyrrolidine and hydroiodic acid, via the intermediacy of the dienamines of isophorone and mesityl oxide, to give a spiro-diketone43 (Scheme 28). 

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). 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

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. 

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. 

First, two acetic acid molecules ketonize to form acetone, water, and CO2 over the oxide surface through an acetoacetic acid intermediate. Thereafter, through an aldol pathway, acetone undergoes self-condensation, generating the enone, mesityl oxide as the main product. Mesitylene and isophorone are also formed as minor by-products. Finally enone undergoes a C—C bond cleavage step, producing isobutene and acetic acid. 

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