Catalysis : an Integrated Approach to Homogeneous ...

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4 - BONDING AND ELEMENTARY STEPS IN CATALYSIS 149 Fig. 4.63. Dependence of acid catalyzed reaction on zeolite AI/Si ratio. proton concentration is proportional to the Al/Si ratio. Once the Al/Si ratio exceeds a critical value (Al/Si)o the intrinsic acidity of a zeolitic proton starts to decrease. This effect dominates over the increased number of protons and, as a consequence, the overall rate of the reaction decreases. 4.4.3.2 Mechanism of protonation Protonation of a molecule creates charge separation. A positive charge is generated on the protonated molecule and a negative charge on the zeolite lattice. Charge separation is an energy-consuming process, because of the increase in potential energy. In solutions ions become stabilized by hydration or complexa- tion with the solvent molecules. The dielectric constant of a zeolite material is low. In a zeolite the interaction between cation and negatively charged zeolite wall takes over the role of solvation of cations in liquids. The strength of the interaction with the zeolite wall depends on the cation type. The stability of a cation depends on the type of carbon atom that is protonated. CH3 + + I H~C-CHZ < CH3-CH-CH3 < CH3--C--CH3 + A primary carbenium ion is less stable than a secondary and a tertiary ion is most stable. The interaction of various carbenium ions with the zeolite wall decreases in the same order. A primary carbenium ion forms covalently bonded ethoxy species with the zeolite wall. As illustrated in Fig. 4.64, when an ethylene molecule approaches a proton or weak x: hydrogen bond is formed initially. Upon proton transfer a stable (J ethoxy species is formed.
150 4 -BONDING AND ELEMENTARY STEPS IN CATALYSIS R -bonding tran&on skate 0-bonding Fig. 4.64. Protonation of ethylene; n- and 0-bonded ethylene [126]. To understand differences in acidity of zeolites one has to consider not only changes in the bond strength of the OH bond, but also zeolite 'solvation' effects on the positively charged carbenium or other ions generated by protonation. Figure 4.65 illustrates protonation of NH3 [127,128]. Protonation proceeds via the initial formation of hydrogen-bonded NH3, the analogue of the 7c complex formed with ethylene. Proton transfer only proceeds when the NI&+ ion can s c v i ammonium form Fig. 5.65. Protonation of ammonia. 0
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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 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 WlTH TRAN
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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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7 - CATALYTIC REACTION ENGINEERING
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7 - CATALYTIC REACTION ENGINEERING
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7 -CATALYTIC REACTION ENGINEERING I
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Ca ta I ys t preparation
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Chapter 8 Preparation of catalyst s
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8 - PREPARATION OF CATALYST SUPPORT
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8 -PREPARATION OF CATALYST SUPPORTS
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Chapter 9 Preparation of supported
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PREPARATION OF SUPPORTED CATALYSTS
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Catalyst characterization
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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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11 -TEMPERATURE PROGRAMMED REDUCTIO
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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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