Catalysis : an Integrated Approach to Homogeneous ...

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8 - PREPARATION OF CATALYST SUPPORTS AND ZEOLITES 313 The silica gels obtained in this way are mixed with a solution of the salts formed by the neutralization of the alkali by the acid (usually a sodium sulphate solution). These salts can be removed by filtering and washing the gel. The silica gel obtained in this way, in which the pores are filled with water, is called the aquagel. From this a xerogel is obtained when the water is removed by evaporation. The structure of the xerogel will be compressed and the porosity reduced at least to some degree by the surface tension forces as the water is removed. An aerogel is a special type of xerogel from which the liquid has been removed in such a way as to prevent any collapse or change in the structure as liquid is removed. Specifically, this is done by heating the liquid-filled gel, usually an alcogel, in an autoclave to above the critical point of the liquid, so that there is no liquid-vapour interface, and releasing the vapour. All silicas dried from water at less than 150°C have a fully hydroxylated surface in which the surface structure terminates in silanol groups. This is readily wettable by water and water-soluble organic molecules. According to Bohm the concentration of &-OH groups on the silica surface is rather constant, at a value of 6.6 OH groups per run2 [4]. Commercial silica gels for use as catalyst bases are, for example, produced by Akzochemie and Davison, having specific surface areas between 100 and 800 m2/g and pore volumes between 0.3 and 2.0 ml/g. These silica gels contain as main impurities S (sulphate), Na, Al, Fe and Ti. 8.2.2 Silica Precipitation from Vapour: Pyrogenic Silica Very pure silica can be obtained by the oxidation or hydroxylation of purified Sic14 at high temperature. Sic14 + 0 2 -+ SiO;! + 2C12 Sic14 can be burned in oxygen to yield SiOz and C12 but simpler equipment can be used when Sic14 is mixed with CH4 and burned to produce fine SiO2 and HC1. Sic14 + CH4 + 202 + SiOz + CO2 + 4HC1 The silicas made by this process are commonly known by their trademarks, ‘Aerosil’ produced by Degussa in Europe, and ‘Cabosil’ produced by Cabot in America.
314 8 -PREPARATION OF CATALYST SUPPORTS AND ZEOLITES Secondary particles of about 10-30 nm ide!?tified by TEM as primary particles Voluminous "secondary" structure icles of ca. I nm Fig. 8.3. Schematic representation of the structure of pyrogenic silica. The structure of these pyrogenic silicas has been discussed by Barby [5], particularly with reference to their specific surface area. It was concluded that the initially condensed particles are only about 1 run in diameter and that these are so closely packed (high coordination number) to secondary particles of 10 to 30 nm that only a small amount of nitrogen can penetrate the micropores between them. Thus the secondary particles are the ones that are commonly identified in electron micrographs and which determine the specific surface area. They are the 'primary' particles in the voluminous aggregate structure and have a low coordination number of about 3 (see Fig. 8.3). Because of the low level of impurities this type of silica is often used as catalyst support in fundamental studies. 8.3. PREPARATION OF ALUMINA CATALYST SUPPORTS 8.3.1 Preparation of y-Al2O3 and q-A1203 Because of the amphoteric character of aluminium, it is soluble in both acidic and basic solvents. In acidic solutions below pH = 2 aluminium will be present as solvated A13+ ions, while in basic solutions above pH = 12 it is present in the form of aluminate ions (AlO:-). Starting from an aqueous acidic A13+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (AIO(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. a-A1203 is formed at higher temperatures. If the initial microcrystalline boehmite gel slurry is aged at 40°C it is converted
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Studies in Surface Science and Cata
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Studies in Surface Science and Cata
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Preface Catalysis is a fascinating
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industrial context of the features
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Contents Preface ..................
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4.3.4 P-Elimination and Deinsertion
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7.4.1 Scope .......................
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12.3 Adsorption Isotherms .........
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List of contributors E.B.M. Doesbur
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Introductory section
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Chapter 1 History of catalysis 1.1
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1 -HISTORY OF CATALYSIS 5 One of th
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1 - HISTORY OF CATALYSIS 7 was disc
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1 -HISTORY OF CATALYSIS exlenaed em
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1 -HISTORY OF CATALYSIS 11 aromatiz
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1 -HISTORY OF CATALYSIS 13 had a gr
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1 - HISTORY OF CATALYSIS 15 The new
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1 -HISTORY OF CATALYSIS 17 1960 law
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1 - HISTORY OF CATALYSIS TABLE 1.2
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1 -HISTORY OF CATALYSIS 21 1900 195
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Chapter 2 Catalytic processes in in
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2 - CATALYTIC PROCESSES IN INDUSTRY
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2 - CATALYTIC PROCESSES IN INDUSTRY
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flue gaa t c t air Vacuum gas ol 2
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2 -CATALYTIC PROCESSES IN INDUSTRY
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2 -CATALYTIC PROCESSES JN INDUSTRY
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Chapter 3 Chemical kinetics of cata
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3 - CHEMICAL KINETICS OF CATALYSED
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3 -CHEMICAL KINETICS OF CATALYSED R
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Fundamental catalysis
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Chapter 4 Bonding and elementary st
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4 -BONDING AND ELEMENTARY STEPS IN
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Chapter 5 Heterogeneous catalysis 5
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5 -HETEROGENEOUS CATALYSIS 161 a. _
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5 -HETEROGENEOUS CATALYSIS 163 appr
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5 -HETEROGENEOUS CATALYSIS 165 In r
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5 - HETEROGENEOUS CATALYSIS 167 lit
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5 - HETEROGENEOUS CATALYSIS 169 An
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5 -HETEROGENEOUS CATALYSIS 171 A ma
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5 - HETEROGENEOUS CATALYSIS 173 2 L
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5 - HETEROGENEOUS CATALYSIS 175 (3)
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5 -HETEROGENEOUS CATALYSIS 177 20 '
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5 -HETEROGENEOUS CATALYSIS 179 with
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5 - HETEROGENEOUS CATALYSIS 181 In
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5 -HETEROGENEOUS CATALYSIS 183 Info
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5 -HETEROGENEOUS CATALYSIS 185 0 20
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5 - HETEROGENEOUS CATALYSIS 187 TAB
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5 -HETEROGENEOUS CATALYSIS 189 i)U
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5 -HETEROGENEOUS CATALYSIS 600 560
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5 -HETEROGENEOUS CATALYSIS 193 b H
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Chapter 6 Homogeneous catalysis wit
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6 -HOMOGENEOUS CATALYSIS WlTH TRANS
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6 - HOMOGENEOUS CATALYSIS WlTH TRAN
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6 -HOMOGENEOUS CATALYSIS WITH TRANS
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6 - HOMOGENEOUS CATALYSIS WITH TRAN
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6 -HOMOGENEOUS CATALYSIS WTH TRANSI
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Applied catalysis
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Chapter 7 Catalytic reaction engine
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7 -CATALYTIC REACTION ENGINEERING 2
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Chapter 10 Ca ta I yst characteriza
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10 -CATALYST CHARACTERIZATION WITH
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10 - CATALYST CHARACTERIZATION WITH
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Chapter 11 Temperature programmed r
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Chapter 12 The use of adsorption me
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12 -ADSORPTION METHODS 421 pairs of
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12 -ADSORPTION METHODS 423 of multi
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12 -ADSORPTION METHODS 425 son of t
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12 - ADSORPnON METHODS 427 equation
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12 - ADSOWON METHODS / Fig. 12.4. T
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12 - ADSOFSTION METHODS 431 - a i-
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12 -ADSORPTION METHODS 433 uptake o
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12 - ADSOlU'TION METHODS 435 gives
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12 - ADSOFSTION METHODS 437 condens
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Future trends
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Chapter 13 Future trends The large
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13 - FUTURE TRENDS 443 control of t
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13 -FUTURE TRENDS 445 the conversio
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Subject Index x-Acceptor Iigands, 2
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SUBJECT INDEX 449 - see also Extend
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SUBJECr INDEX 451 Ethane, 143 Ethen
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SUBJECT INDEX 453 Hydrogen peroxide
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SDJJTT INDEX 455 Octane number, 24,
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SUBJECT INDEX 457 RBS, 383 Reaction
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SUBJECT INDEX 459 - see also Ruther
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STUDIES IN SURFACE SCIENCE AND CATA
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Volume 35 Volume 36 Volume 37 Volum
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Volume 70 Volume 71 Volume 72 Volum
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