INVESTIGADORES
DIEZ veronica Karina
artículos
Título:
Base catalysis for the synthesis of a,b-unsaturated ketones from the vapor-phase aldol condensation of acetone.
Autor/es:
JUANA. I. DI COSIMO; VERÓNICA K. DÍEZ; CARLOS. R. APESTEGUÍA
Revista:
APPLIED CATALYSIS A-GENERAL
Editorial:
Elsevier
Referencias:
Lugar: Amsterdam; Año: 1996 vol. 137 p. 149 - 166
ISSN:
0926-860X
Resumen:
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 a,/3-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 z÷ 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.