CATALYSIS BY BASIC METAL OXIDES

Materials Chemistry and Physics, 18 (1988)
533-552
533
CATALYSIS BY BASIC METAL OXIDES
H. HATTORI
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060 (Japan)
pressure 2SrO + O2 2BaO + 0 + CO2 2
The metal exposure
oxides
listed in Table
1 scarcely
exhibited
basic properties of
on
to atomosphere. and peroxides
log P vs. l/T plots for the decomposition of alkaline are very oxides Removing earth low elements are shown
dependent on the severity of the heat treatment on metal oxides.
535
-2
-3
T-l
Fig. 1. Equilibrium a 2Sr02e b 2Ba02~ c MgC03eMg0
x
lo3 / K-’
for decomposition. d CaC03e e SrCO e f BaCOz= cao + co2 SrO + CO2 BaO + CC2
INTRODUCTION Acid and base are paired concepts; a number of chemical interactions have been understood in terms of acid-base interaction. In contrast to extensive
studies of solid acid catalysts, less attention has been given to the studies of solid base catalysts. The study of solid base catalysts was first reported by Pines et al. that sodium metal dispersed on Al203 acted as an effective catalyst
for double bond migration to donate electrons, it of alkenes seems [l]. Considering that Na/Al 0 23 in the strong acts as tendency a solid of Na base
natural
GENERATION OF BASIC SITES Generation of basic sites requires highteperature metal oxide surfaces by removing H2, C02, and 02.
pretreatment to reveal
The catalytic activities are
separation
reaction
regeneration to reactor
after
reaction.
3. Noncorrosive
system.
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534 Table I. Solid base catalysts. Alkaline earth oxides ( MgO, CaO, SrO, BaO ) Rare earth oxides ( La203, CeO2, Nd203, Y2O3, etc.) Alkaline oxides ( Li20, Na20, K20, Rb20, Cs20, ) Other metal oxides (Th02, Zr02, ZnO, )
Compared have a
the catalysts of advantages are:
homogeneous practical
solid
nuห้องสมุดไป่ตู้ber
The main advantages from
of solid base catalysts mixture.
1. Easy catalyst 2. Easy catalyst
Received
May 15, 1987; accepted
August
31, 1987
ABSTRACT Catalytic properties of basic materials investigated in our laboratories are reviewed. Firstly, the types of metal oxides and the indications of their basic properties are presented. Secondly, the importance of removing H20, CO2 and 02
In addition to the advantages, specific catalytic behavior would be expected of solid base catalyst caused by surface reaction in which geometric factors are important in some cases. Following the discovery of Na/A1203 by Pines et al., certain metal oxides with a single component were found to be solid base catalysts in the absence of such alkali metals as Na and K. The representatives of single-component metal oxides known to act as solid base catalysts are summarized in Table I. In addition to the metal oxides listed in Table I, any metal oxides are in principle solid base catalysts. Metal oxides expose 0 atoms on the surfaces, and 0 atoms attractively interact with H+. Any 0 atoms would be able to abstract H+ from strongly acidic molecules. The metal oxides listed in Table I act as base toward most of the reagents and therefore are called solid bases. Four reasons for recognizing certain metal oxides as solid base catalysts are as follows. (a) Color change of indicators to basic colors. The acid-base indicators adsorbed on the surfaces, exhibit basic colors. Using a series of indicators having different pKBH values, the basic strengths of the solid surfaces can be expressed in H_ scale [2]. (b) Poisoning effects of acidic molecules on the catalytic activity. The catalytic activities are reduced or eliminated when acidic molecules such as H20 and CO2 are present in the reaction mixtures. (c) Similar activities to those of homogeneous catalyzed reactions" in homogeneous systems. The metal oxides catalyze the reactions well known as base-catalyzed reactions in homogeneous systems. (d) Indications of anionic intermediates participating in the reactions. Mechanistic studies of the reactions, product distribution, and spectroscopic observation of adsorbed species over the metal oxides strongly suggest that anionic intermediates are involved in the reactions. catalysts for "base-
catalyst. Numerous alkaline base-catalyzed are used reactions are known homogeneous with systems and in for