Fm. Bautista et al., ALPO4-AL2O3 CATALYSTS WITH LOW ALUMINA-CONTENT .3. SURFACE BASICITY OF CATALYSTS OBTAINED IN AQUEOUS AMMONIA, Catalysis letters, 19(2-3), 1993, pp. 137-142
The amount of basic sites of AlPO4-Al2O3 (APA1-A, 5-15 wt% Al2O3) cata
lysts at two basic strengths was measured by studying the liquid-phase
adsorption of two acidic molecules (benzoic acid (BA, pK(a) = 4.2) an
d phenol (PH, pK(a) = 9.9) from cyclohexane solutions, through the app
lication of a spectrophotometric method. The data obtained follow the
Langmuir adsorption isotherm and the monolayer coverage at equilibrium
(at 298 K), X(m), is assumed as the amount of basic sites correspondi
ng to the specific pK(a) of the acid used as titrant. The amount of ba
sic sites of any AlPO4-Al2O3 catalyst is higher than that of AlPO4, bu
t lower than that Of Al2O3. Besides, an increase in the Al2O3 content
from 10 wt% gradually increases the basicity of the APA1-A catalyst. M
oreover, calcination at increasing temperatures does not practically a
ffect the surface basicity of APA1-A-5 and APA1-A-10 catalysts. Howeve
r, for AlPO4 content higher than 10 wt% we observe a decrease in surfa
ce basicity, this decrease depends on alumina content, i.e. it is high
er as the amount of alumina increases. The basic sites of APA1-A syste
ms catalyze the Knoevenagel condensation of p-methoxybenzaldehyde and
malononitrile at room temperature and in the absence of solvent.