Ketones, mesityl acetone self-condensation

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). 

Hydrated lime is used as an alkaline catalyst to promote the self-condensation of acetone to form diacetone alcohol (4-hydroxy-4-methyl-2-pentanone) [31.26]. Diacetone alcohol is used as a solvent for natural and synthetic resins. It is also used as an intermediate in the produetion of mesityl oxide, methyl isobutyl ketone and hexylene glycol. 

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. 

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). 

Firstly, we have the acetone aldol self-condensation reaction over basic sites to give diacetone alcohol (DAA). Dehydration of this alcohol yeilds mesityl oxide (MSO) winch, in turn, can be selectively hydrogenated over reduced metal sites to finally give methyl isobutyl ketone (MIBK). In addition to the aldol condensation route, the acetone carbonyl functional group can also be directly hydrogenated over reduced metal sites yielding 2-propanol. Other reaction by-products such as methane, propane, diisopropyl ether and diisobutyl ketone have been detected in some experiments, but in very low amounts, lower than 2% of the total reaction products. 

The first example of this reaction, the acid-catalyzed self-condensation of acetone to give mesityl oxide, was reported more than one and a half centuries ago by Kane [4]. The condensation of an aromatic aldehyde with an aliphatic aldehyde or ketone, obviously the first example of an aldol condensation under basic conditions, was reported by Schmidt [5] and by... 

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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