Aldehyde condensation reaction catalyst

For the cross condensation between two different aldehydes or ketones, the selectivity is not high because of many side reactions in the non-catalytic process. It is necessary to make the selectivity of target products meet the requirements of industrial application through catalytic process. The catalysts used in the reaction can be divided into acid catalysts, alkaline catalysts and acid-base catalysts according to their acid-base active centers.

1, acidic catalyst

Commonly used acidic catalysts are (VO)2P207, α VOHPO4, niobic acid and MFI zeolite. At the cationic active center (Brnsted center or Lewis center) of acidic catalyst, aldehyde carbonyl group is activated to form enol carbonium ion, which leads to condensation reaction. The balance of acid-catalyzed enolketone can be expressed by the existing research results. The type, quantity and distribution of acid active centers on the surface of the catalyst will affect its catalytic performance. Proper acid strength can effectively promote the formation of carbon ions in the gas-phase aldol condensation reaction and improve the reaction activity.

Tanner et al. used vanadium phosphate oxides of (VO)2P2P7 and α VOHPO4 as catalysts to study the self-condensation of acetone and the cross-condensation of acetone with formaldehyde. The results show that the acidic active center of vanadium phosphate catalyst has good catalytic ability for this reaction, and the carbonyl structural group quickly completes the protonation and nucleophilic addition reaction on its surface.

Paulis et al. [4] used niobic acid (Nb2o5 nH2O) as catalyst to carry out vapor-phase aldol condensation of acetone, and found that the types of reaction products were closely related to the acid strength and acidity of the acid center of the catalyst. The results show that the Bronsted acidic central acid on the surface of niobic acid catalyst is strong, and it has good catalytic activity, selectivity and stability in acetal and ketal reactions.

Dumitriu et al. used MFI zeolites with different acidity in the gas-phase aldol condensation reaction of low-carbon aldehydes. By changing the proportion of Si Fe3+ in the catalyst, the acid strength and acidity of Bronsted acid center can be adjusted. It is found that the enhancement of surface acid strength can promote the gas-phase aldol condensation reaction of low-carbon aldehydes and improve the conversion rate.

2. Basic catalyst

Basic catalysts commonly used in aldol condensation reaction include basic compounds (oxides, hydroxides, bicarbonates, carbonates and carboxylates of alkali metals or alkaline earth metals), organic amines and anion exchange resins. In practical industrial applications, the basic catalysts used for aldol condensation reaction can be weak bases (such as sodium carbonate, sodium bicarbonate and sodium acetate) or strong bases (such as sodium hydroxide, calcium hydroxide, sodium hydride and sodium alkoxide). The former is generally used for condensation between highly active aldehydes, and the products are mostly β -hydroxy compounds; The latter is used for the condensation reaction of aldehydes or ketones with small activity and large steric hindrance, and the reaction is mostly carried out in aprotic polar solvents.

Alkali metal compound catalysts are often used in the reaction of aldol condensation to prepare aldol. The obtained product can be hydrogenated and purified to obtain diol or even polyol, such as 3- hydroxybutyraldehyde obtained by self-condensation of acetaldehyde. When aqueous caustic soda solution is used as catalyst, 1, 3- butanediol can be obtained by catalytic hydrogenation of crude product. Similarly, formaldehyde and butyraldehyde cross-condense to form 2,2-dimethylol butyraldehyde. Choosing the mixed solution of sodium carbonate and sodium hydroxide as catalyst can reduce side reactions and improve reaction selectivity.

Lopez et al. used NaBEA, KF/ alumina and La2O3 solid catalysts respectively to study the deactivation mechanism of catalysts in aldol condensation reaction of benzaldehyde and acetophenone. The results show that benzoic acid produced in the reaction process will greatly reduce the transfer rate of proton hydrogen in the reaction process, but the addition of amine has little effect on the rate, so it is considered that the basic active center of the catalyst can effectively catalyze the reaction, and the deactivation of the catalyst is also related to the loss of the basic active center. Organic amine is another basic catalyst widely used in aldol condensation reaction. For example, triethylamine is often used as condensation catalyst in the condensation reaction of formaldehyde and isobutyraldehyde to generate hydroxyl neopentyl glycol, and the condensation product is hydrogenated to obtain neopentyl glycol. Formaldehyde and n-butyraldehyde are condensed and then hydrogenated under the catalysis of triethylamine to produce high-purity hydroxymethylpropane. Patent [9] reports an organic amine salt condensation catalyst, which is used in the process of preparing 1 3 propylene glycol by aldol condensation.

Anion exchange resin is a new alkaline catalyst. Traditional alkali metal hydroxide solutions (such as nAOH and KOH) as catalysts have some disadvantages, such as difficult catalyst recovery, easy corrosion of equipment, complicated reaction process and long production cycle. However, anion exchange resin overcomes the above shortcomings on the basis of maintaining catalytic activity, which has attracted more and more researchers' attention.

The industrial production of 2,2-dimethylolpropionic acid mainly takes formaldehyde and propionaldehyde as raw materials, and generates 2,2-dimethylolpropionic acid through aldol condensation reaction under the catalysis of inorganic or organic bases, and then oxidizes it with H2O2. The latest research shows that the reaction effect depends on the specific surface area of spherical catalyst, the number of active groups, adsorption and desorption speed, etc. The catalyst exists in solid form, which avoids a series of problems of using lye as catalyst and ensures the conversion rate and selectivity of the reaction. In the industrial synthesis of 2- methyl -2- pentenal, NaOH aqueous solution is also widely used as a catalyst, and the yield is about 80%. However, NaOH aqueous solution will corrode the experimental equipment, and the product is not easy to separate. Tang Siping studied the new process of preparing 2- methyl -2- pentenal by bimolecular condensation of propionaldehyde with anion exchange resin as catalyst. The yield of the target product 2- methyl -2- pentenal can reach 93.54%.

There are also many reports about the application of anion exchange resin in aldol condensation reaction in China. Ou Zhize and others chose tributylamine aminated strongly basic anion exchange resin as phase transfer catalyst to catalyze the synthesis of benzylidene acetone. Under the optimized reaction conditions, the yield of benzylidene acetone can reach 98%, and the catalyst can be reused. Hu Wei et al. chose strongly basic styrene quaternary ammonium ion exchange resin as catalyst to prepare acetoethanol by condensation of acetone and formaldehyde, and then dehydrated in the presence of oxalic acid to obtain methyl ketene. Shi Xiumin et al. [14] developed and screened a new anionic catalyst suitable for catalytic distillation of diacetone alcohol, that is, macroporous strongly basic styrene anionic resin with high catalytic activity and selectivity.