Download Correlation Between Acidity, Basicity and Catalytic Performance of

Correlation Between Acidity, Basicity and Catalytic Performance of Binary SolGel MgO-Al2O3 Catalysts
L. F. Chen1, J. A. Wang1,*, T. López 2 and R. Gómez2
1
Laboratory of Catalysis and Materials, ESIQIE, National Polytechnic Institute,
07738 Mexico City, Mexico. 2Departamento de Química, Universidad Autónoma
Metropolitana-I, A. P. 55-534, 09340 México City, Mexico
Introduction
Recently, by using reaction of alcohols like isopropanol on metal oxide surface as
probe reaction, correlation between acidic-basic properties of the oxide catalysts and
reaction selectivity has been reported (1). The dehydration selectivity is postulated to
proceed at acidic sites, while, dehydrogenation selectivity is presumed to be catalysed
by basic sites or by both acidic and basic sites (2). However, care must be taken when
one uses such probe reaction as simple diagnostics of surface acid-base properties
since 2-propanol decomposition is recently found to be a function of not only the
surface properties but also the reaction condition employed (3,4). It is the purpose of
the present work to study the influences of experimental parameters on dehydration
and dehydrogenation selectivity of isopropanol decomposed over different sol-gel
MgO-Al2O3 catalysts. Reaction mechanism involving the added water function is
assumed.
Results and discussion
The sol-gel magnesia-alumina oxides show a surface area ranging from 237 to 323
m2/g with bimodel pore size distribution. Acid and basic sites coexist on the surface
of the samples, depending on the molar ratio of MgO/Al2O3.
When isopropanol was decomposed over the magnesia-alumina oxides, propene,
acetone and isopropylether were identified in the outline stream, showing that
dehydrogenation, dehydration and bimolecular polymerisation reaction occur. The
total conversion of isopropanol linearly increased with increasing of the reaction
temperature. Below 150°C, the main product was acetone; however, above 200°C,
propene was the dominant. The acetone selectivity was correlated to basicity of the
solids as well as the reaction parameters such as the reaction time and temperature.
However, the selectivity to propene is independent of acidity of the catalysts,
differing from the conclusion reported by Kckenzie (2).
When water was present in the reaction mixture, both the activity and selectivity of
the catalysts are remarkably altered (Table 1): (i) isopropanol conversion is increased
or decreased, depending on the reaction temperature; (ii) acetone concentration and
selectivity are profoundly enhanced in the whole reaction temperature range. Below
250 °C, the total conversion of isopropanol significantly increased, however, above
300 up to 400 °C, the conversion decreased when water stream is present. These
results show that water may directly involve in the reaction, promoting the activity
below 250 °C but inhibiting the activity at higher temperature. For explanation the
promotion/inhibition originated from the added water, it is assumed that below 250
°C, water molecule is adsorbed on the catalyst surface to form new active species
those are involved in the reactions, promoting isopropanol decomposition. However,
above 250 °C, a competitive adsorption between the reactant and water molecules
may occur, the strong adsorption of water molecules poisons some active sites,
decreasing isopropanol adsorption and therefore suppressing the total activity.
Table 1. Differences of isopropanol conversion and acetone concentration and
selectivity between the cases of water presence and absence in the stream.
____________________________________________________________________
∆Ca
∆Ca/Ca
Sd
Sd/Sa
Temp. (°C)
∆Ct
___________________________________________________________________
400
-2.63
0.96
0.93
1.01
0.98
350
-3.58
2.73
3.42
2.85
3.56
300
-2.23
5.81
7.57
7.91
7.91
250
33.80
13.07
107.03
26.80
28.82
200
17.99
12.24
200.66
56.95
6.40
150
4.85
3.96
69.47
18.02
0.28
100
1.10
0.91
9.29
11.05
0.14
___________________________________________________________________
*∆Ct=Ctw–Ct, (%), the difference of isopropanol conversion between the cases of water presence (Ctw)
and absence (Ct) in the stream; ∆Ca=Caw–Ca (%), the difference of acetone concentration between the
cases of water presence (Caw) and absence (Ca) in the stream; Sd =Saw-Sa (%), the acetone selectivity
difference between the cases of water presence (Saw) and absence (Sa).
Since the added water enhanced acetone concentration and selectivity even when
the total conversion decreased at higher temperature, it is a clear indication that the
diminishing of the total conversion is resulted from inhibition of dehydration
pathway. This indicates that acetone formation is through more than one pathway: a
normal dehydrogenation route and a hydroxyl-assisted mechanism. In the latter case,
it is assumed that the H atom within OH linked to isopropanol reacts with hydroxyls
on the catalyst surface to dehydrate molecular water and an alkoxide carbonion
species bounded on the cationic lattice ((CH3)2HCO-); and then the active α-H in the
alkoxide carbonion species reacts with the H atom linking on the oxygen lattice to
yield hydrogen and acetone molecules. Therefore the hydroxyl-assisted mechanism is
a modified dehydrogenation route accompanying dehydration.
References
1.
2.
3.
4.
C. F. Fishel, R. J. Davis, Langmuir 10 (1994) 5.
A. L. McKenzie, C. T. Fishel, R. J. Davis, J. Catal. 138 (1992) 547.
J. Rekoske, M.A. Barteau, J. Catal. 165 (1997) 57.
J. A. Wang, X. Bokhimi, O. Novaro, T. Lopez, R. Gomez, J. Mol. Catal. A, 145(1999) 291.