Solid Acids and Bases

Chapter 1 Solid Acids and Bases There are many definitions of acids and bases in the literature, notably those of Arrhenius,^ Franklin, 2) Br0nsted,3) Germann, 4) Lewis,5) Ussanowitch,6) Bjerrum,7) Johnson, 8) Lux, Flood et al, and Tomlinson, 9) Shatenshtein, 10) and Pearson. 1 υ We may understand a solid acid in general terms as a solid on which the colour of a basic indicator changes, or as a solid on which a base is chemically adsorbed. More strictly, following both the Br0nsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. These definitions are adequate for an understanding of the acidbase phenomena shown by various solids, and are convenient for the clear description of solid acid and base catalysis. In accordance with the above definitions a summarized list of solid acids and bases is given here in Tables 1-1 and 1-2. Later chapters will detail the amount, strength, and nature of acid and base centres on various solid surfaces. They will also cover the dependence of these characteristics not only upon the purity of the materials and the method of preparation, but also upon heat-treatment, compression, and irradiation. The first group of solid acids, which includes naturally occurring clay minerals, has the longest history. Some were investigated as long ago as the turn of the century, and especially since the 1920's there have been numerous studies of their catalytic activities, although only recently have investigations commenced on zeolites. The main constituents of the first group of solid acids are silica and alumina. The very well-known solid acid catalyst synthetic silica-alumina is listed in the 4th group, which also includes the many oxide mixtures which have recently been found to display acidic properties and catalytic activity. In the 5th group are included many inorganic chemicals such as the metal oxides, sulfides, sulfates, nitrates and phosphates. Many in this group have recently been found to show characteristic selectivities as catalysts. Of the solid bases listed in Table 1-2 special mention should perhaps be made of the alkaline earth metal oxides in the 4th group, whose basic properties and catalytic action have recently been investigated. The fact that alumina, zinc oxide and silica-alumina show both acidic and 1

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SOLID ACIDS AND BASES

Chapter 1

basic properties is of special significance for acid-base bifunctional catalysis. It is to be hoped that the remarkable progress made in methods of measuring acidic and basic properties (as described in the following chapters) will result in the discovery of many more solid acids and bases. TABLE 1-1

Solid acids

1. Natural clay minerals: kaolinite, bentonite, attapulgite, montmorillonite, clarit, fuller's earth, zeolites 2. Mounted acids : H 2 S0 4 , H 3 P0 4 , H 3 B0 3 , CH2 (COOH)2 mounted on silica, quartz sand, alumina or diatomaceous earth 3. Cation exchange resins 4. Mixtures of oxides: Si0 2 -Al 2 0 3 , Β 2 0 3 ·Α1 2 0 3 , Cr 2 0 3 · A1203, Μο0 3 ·Α1 2 0 3 , Z r 0 2 S i 0 2 , G a 2 0 3 S i 0 2 , B e 0 2 S i 0 2 , MgO · Si0 2 , CaO · Si0 2 , SrO · Si0 2 , Y 2 0 3 · Si0 2 , La 2 0 3 · Si0 2 , SnO · Si0 2 , PbO · Si0 2 , Mo0 3 · Fe2 (Mo04)3, MgO · B 2 0 3 , Ti0 2 · ZnO 5. Inorganic chemicals : ZnO, A1203, Ti0 2 , Ce0 2 , As 2 0 3 , V 2 0 5 , Si0 2 , Cr 2 0 3 , Mo0 3 , ZnS, CaS, CaS0 4 , MnS0 4 , NiS0 4 , CuS0 4 , CoS0 4 , CdS0 4 , SrS04, ZnS0 4 , MgS04, FeS0 4 , BaS04, KHS0 4 , K 2 S0 4 , (NH4)2S04, A12(S04)3, Fe2(S04)3, Cr2(S04)3, Ca(N03)2, Bi(N03)3, Zn(N03)2, Fe(N03)3, CaC0 3 , BP0 4 , FeP0 4 , CrP0 4 , Ti3(P04)4, Zr3(P04)4, Cu3(P04)2, Ni3(P04)2, AlP0 4 , Zn3(P04)2, Mg3(P04)2, A1C13, TiCl3, CaCl2, AgCl, CuCl, SnCl2, CaF2, BaF2, AgC104, Mg2(C104)2 6. Charcoal heat-treated at 300 °C TABLE 1-2

Solid bases

1. Mounted bases: NaOH, KOH mounted on silica or alumina; alkali metal and alkaline earth metal dispersed on silica, alumina, carbon, K 2 C0 3 or in oil; NR3, NH3, KNH2 on alumina; Li 2 C0 3 on silica 2. Anion exchange resins 3. Mixtures of oxides: Si0 2 · A1203, S i 0 2 M g O , S i 0 2 C a O , Si0 2 -SrO, Si0 2 · BaO 4. Inorganic chemicals: BeO, MgO, CaO, SrO, BaO, Si0 2 , A1203, ZnO, Na 2 C0 3 , K 2 C0 3 , KHC0 3 , (NH4)2C03, CaC0 3 , SrC0 3 , BaC0 3 , KNaC0 3 , Na 2 \V0 4 · 2H 2 0, KCN 5. Charcoal heat-treated at 900 °C or activated with N 2 0, NH 3 or ZnCl2-NH4Cl-

co 2

References

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REFERENCES 1. R. P. Bell, Acids and Bases, p. 5, Methuen, 1952. 2. E. C. Franklin, Am. Chem. J., 20,820 (1898); 47,285 (1912); /. Am. Chem. Soc, 27, 820 (1905); 46, 2137 (1924). 3. J. N. Bronsted, Rec. Trav. Chim., 42,718 (1923);/.Phys. Chem., 30,777 (1926); Chem. Rev., 5,231,284 (1928); Z. Phys. Chem., A169, 52 (1934). 4. A. F. O. Germann, / . Am. Chem. Soc, 47, 2461 (1925). 5. G. N. Lewis, /. Franklin Inst., 226,293 (1938) ; Valency and Structures of Atoms and Molecules, Chemical Catalog Co., 1923. 6. M. Ussanowitch, / . Gen. Chem. USSR (Eng. Transi.), 9,182 (1939); H. Gehlen, Z. Phys. Chem., 203, 125 (1954). 7. J. iBjerrum, Fys. Tidssk., 48, 1 (1950); B. Sansoni, Naturwissenschaften, 38, 461 (1951). 8. R. E. Johnson, T. H. Norris and J. L. Huston, / . Am. Chem. Soc, 73, 3052 (1951). 9. H. Lux, Z. Elektrochem., 45, 303 (1939); H. Flood and T. Förland, Acta Chem. Scand., 1,592,781 (1947); J. W. Tomlinson, The Physical Chemistry of Melts (A symposium on molten slags and salts), p. 22, Institution of Mining and Metallurgy, 1953. 10. A. I. Shatenshtein, Advances in Physical Organic Chemistry, vol. 1, p. 174, Academic Press, 1963. 11. R. G. Pearson, /. Am. Chem. Soc, 85, 3533 (1963).