V.K. Díez

Base catalysis for the synthesis of α,β-unsaturated ketones from the vapor-phase aldol condensation of acetone

Abstract The vapor-phase aldol condensation of acetone was studied over MgO promoted with 0.7–1.0 wt.-% of alkali (Li, Na, K and Cs) or alkaline earth (Ca, Sr and Ba) metal ions. The basic properties of the samples were characterized by chemisorption of carbon dioxide. The basicity of MgO increased on addition of the promoter following the basicity order of the promoter oxide: the stronger the electron donor properties of the promoter, the greater the generation of surface basic sites. Major reaction products were mesityl oxide (MO), isomesityl oxide (IMO) and isophorone (IP). The selectivity to (MO + IMO + IP) over unpromoted MgO was practically 100%, thereby showing that magnesium oxide is suitable for selectively obtaining α,β-unsaturated ketones. The reaction was totally inhibited by co-feeding acetic acid along with acetone whereas the co-injection of pyridine did not affect the acetone conversion. This indicated that the self-condensation of acetone over MgO-based catalysts is catalyzed by basic sites. The promoter addition increased the activity of the MgO catalyst and a good correlation was obtained between catalyst activity and the concentration of basic sites. Such a proportionality between activity and surface basicity was an additional evidence that the rate-determining step in the aldol condensation mechanism is controlled by the surface base property. All the catalysts exhibited similar IP/(IMO+MO) selectivity ratio, except the Li/MgO sample which produced substantially larger amounts of isophorone. Because the tricondensation of acetone to give isophorone requires strong basic sites, the higher selectivity toward isophorone was indicative of the presence of stronger surface basic sites in the Li/MgO sample. Results from carbon dioxide chemisorption confirmed that Li/MgO exhibited the strongest basic properties. The generation of high-strength basic sites was explained by assuming that the addition of lithium causes a structural promotion of the MgO sample by replacing the Mg 2+ ions by Li + in the MgO lattice. The replacement would result in strained Mg O bonds and formation of [Li + O − ] species, which causes the generation of stronger basic sites.

Acid-base properties and active site requirements for elimination reactions on alkali-promoted MgO catalysts

Fil: Diez, Veronica Karina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Santa Fe. Instituto de Investigaciones en Catalisis y Petroquimica ; Argentina

Chapter 1:Basic catalysis on MgO: generation, characterization and catalytic properties of active sites

The generation, characterization and catalytic properties of MgO active sites were studied. MgO samples stabilized at different temperatures were used to control the distribution of surface base sites; specifically, MgO was calcined at 673 K, 773 K and 873 K (samples MgO-673, MgO-773 and MgO-873). The nature, density and strength of MgO base sites were characterized by temperature-programmed desorption of CO2 and infrared spectroscopy after CO2 adsorption at 298 K and sequential evacuation at increasing temperatures. MgO samples contained surface sites of strong (low coordination O2− anions), medium (oxygen in Mg2+-O2− pairs) and weak (OH− groups) basicity. The density of strong basic sites was predominant on MgO-673. The increase of the calcination temperature drastically decreased the density of strong base sites and to a lesser extent that of weak OH− groups, while slightly increased that of medium-strength base sites. The catalytic properties of MgO samples were proved for the aldol condensation of citral with acetone to yieldpseudoionone, the hydrogen transfer reaction of mesityl oxide with 2-propanol to obtain the unsaturated alcohol 4-methyl-3-penten-2ol, and the synthesis of monoglycerides via the transesterification of methyl oleate with glycerol. The effect of calcination temperature on the MgO catalytic properties depended on the basicity requirements for the rate-limiting step of the base-catalyzed reaction. The activity for both the aldol condensation of citral with acetone and the glycerolysis of methyl oleate diminished with the MgO calcination temperature because these reactions were essentially promoted on strongly basic O2− sites. In contrast, the synthesis of 4-methyl-3-penten-2ol by the hydrogen transfer reduction of mesityl oxide with 2-propanol increased with calcination temperature because the reaction intermediate was formed on medium-strength Mg2+-O2− pair basic sites. Additional information on the role played by the MgO active sites on the kinetics of base-catalyzed reactions was obtained by performing molecular modeling studies on our MgO catalysts using Density Functional Theory (DFT) for the glycerolysis of methyl oleate, an unsaturated fatty acid methyl ester (FAME). The molecular modeling of glycerol and FAME adsorptions was carried out using terrace, edge and corner sites for representing the MgO (100) surface. In agreement with catalytic results, calculations predicted that dissociative chemisorption of glycerol with O–H bond breaking occurs only on strong base sites (edge sites) whereas nondissociative adsorption takes place on medium-strength base sites such as those of terrace sites. Results also indicated that glycerol was more strongly adsorbed than FAME. The glycerol/FAME reaction would proceed then through a mechanism in which the most relevant adsorption step is that of glycerol.

Upgrading of diols by gas-phase dehydrogenation and dehydration reactions on bifunctional Cu-based oxides

Biomass-derived short-chain polyols can be transformed into valuable oxygenates used as building blocks. The gas phase conversion of a model molecule of 1,3-diols (1,3-butanediol), was studied on bifunctional Cu–Mg, Cu–Al and Cu–Mg–Al mixed oxide catalysts that exhibit surface Cu0 particles and acid–base properties. A series of ZCuMgAl catalysts (Z = 0.3–61.2 wt.% Cu, Mg/Al = 1.5 molar ratio) was prepared by coprecipitation and thoroughly characterized by several techniques such as BET surface area, TPR-N2O chemisorption, XRD and TPD of CO2. The ZCuMgAl catalysts promote the upgrading of diols by a series of dehydrogenation and/or dehydration reactions proceeding at reaction rates that depend on the copper content (Z). The overall activity increases linearly with the amount of surface Cu0 species thereby confirming the participation of metallic sites in the rate-limiting steps. Besides, surface Cu0 sites favor the reaction pathway toward 1,3-butanediol dehydrogenation. Thus, the dehydrogenation/dehydration selectivity ratio increases with Z as a result of the enhanced amount of exposed Cu0 particles. ZCuMgAl catalysts with Z 8 wt.% have higher activity and yield valuable multifunctional C4 oxygenates such as hydroxyketones and, to a lesser extent, unsaturated alcohols and ketones. A strongly basic Cu–Mg catalyst promotes the C–C bond cleavage reaction giving short carbon chain oxygenates at low rates; an acidic Cu–Al catalyst converts the diol into C4 saturated ketones and olefins.

Deactivation of MgyAlOx mixed oxides during aldol condensation reactions of ketones

Abstract The deactivation of Mg-Al mixed oxides during the gas-phase self-condensation of acetone was studied. Main aldol condensation reaction and secondary coke-forming reactions take place on both basic and acidic sites. Although deactivation is caused by carbon deposition, it was found that coke formed on basic sites rather than on acidic sites is responsible for the activity lost.

Acid-base site requirements for elimination reactions on alkali promoted MgO

Base-catalyzed elimination reactions were studied on MgO and alkali-modified MgO catalysts using 2-propanol as a probe reactant. The effect of the acid-base properties on the catalyst activity and selectivity was investigated by modifying the surface properties of MgO with 1 mole% of alkaline metals. Group IA metals promote the formation of medium- and high-strength basic sites, thereby increasing the basic site density and strength. The addition of alkaline promoters improves the MgO activity for 2-propanol conversion reactions. The selectivity toward dehydration or dehydrogenation products depends on the base site chemical nature, so that intermediate-strength base sites promote acetone formation whereas highstrength base sites selectively yield propylene. 2-Propanol decomposition to acetone and propylene is proposed to take place via an E1CB mechanism in two parallel pathways sharing a common isopropoxy intermediate.