Advanced Zeolite Science and Applications (eBook)

J.C. Jansen, H.G. Karge, Michael Stocker, J. Weitkamp (Herausgeber)

eBook Download: PDF

1994 | 1. Auflage
690 Seiten
Elsevier Science (Verlag)
978-0-08-088695-4 (ISBN)

Infos

Zeolites and related microporous materials are used in oil processing and in the fine and petrochemical industries on a large scale. New applications of zeolites contribute to environmentally friendly processes and refined zeolites such as catalytic zeolite membranes and zeolites containing exhaust-pipe reactors are being introduced. Recent diversity in zeolite research has been fueled by the increase in number of microporous materials and the combination with interfacing science areas. The possibility to accommodate ions, large molecules or nanostructures in the crystalline matrix has been explored and the performance of electronic, acoustic and photonic modified response of the materials has been tested.

This volume provides up-to-date information on new zeolite and related materials and composites, their applications, testing of new processes and techniques, and promising laboratory results as well. A vast amount of work from a fundamental aspect is incorporated. In particular, the combination of science and application offers useful information for readers interested in molecular sieves.

Inhalt

Front Cover 1
Advanced Zeolite Science and Applications 4
Copyright Page 5
Contents 12
Preface 6
List of contributors 8
Chapter 1. Sol-gel chemistry and molecular sieve synthesis 18
Metal cations in aqueous solutions 18
Condensation of hydrolyzed precursors 25
Complexation of cationic precursors 39
Alkoxide precursors 46
Conclusion 57
References 58
Chapter 2. Effects of seeding on zeolite crystallization, and the growth behavior of seeds 60
Introduction 60
Theoretical foundations 64
Seeded synthesis results 73
Conclusions 85
References 88
Chapter 3. The opportunities of the fluoride route in the synthesis of microporous materials 92
Introduction 92
Crystallization of silica-based zeolites 93
Synthesis of AlPO4-based materials 113
Synthesis of gallophosphates 118
Conclusion 126
References 127
Chapter 4. Alkali metal and semiconductor clusters in zeolites 132
Introduction 132
Semiconductor clusters in zeolites 133
Electron solvation in zeolitic structures: alkali metal clusters in zeolites 143
References 156
Chapter 5. Nonlinear optical effects of dye-loaded molecular sieves 162
Introduction 162
Frequency doubling 163
High density optical data storage 170
Outlook 187
References 188
Chapter 6. Metal ions associated to the molecular sieve framework: possible catalytic oxidation sites 194
Introduction 194
Titanium-containing silicalites 196
Vanadium-containing silicalites 212
Other transition metal-containing silicalites 221
Transition metal-containing aluminophosphates 222
Conclusive remarks 224
References 226
Chapter 7. The preparation of coatings of molecular sieve crystals for catalysis and separation 232
Introduction 232
Theory 235
Experiments 244
Conclusions 265
References 266
Chapter 8. The catalytic site from a chemical point of view 268
Introduction 268
A covalent characterization of acid hydroxyl groups in zeolites 269
Catalytic transformations of olefins 273
Catalytic transformations of paraffins and the nature of adsorbed nonclassical carbonium ions 283
Conclusion 288
References 288
Chapter 9. Theory of Brnsted acidity in zeolites 290
Abstract 290
Introduction 290
The proton bond 291
The proton affinity differences 297
Proton-weak base interaction 302
Proton-transfer 305
Concluding remarks 309
References 310
Chapter 10. Analysis of the guest-molecule host-framework interaction in zeolites with NMR-spectroscopy and X-ray diffraction 312
Guest-host system in zeolite science 312
Guest host analysis in the porosil system and in porosil-like materials 313
Conclusion 342
References 342
Chapter 11. The preparation and potential applications of ultra-large pore molecular sieves: A review 346
Definition, nomenclature and scope 346
Introduction 346
Characterization 349
Materials 351
Theoretical Framework 368
Applications: conventional and emerging 369
References 370
Chapter 12. Advances in the in situ 13C MAS NMR characterization of zeolite catalyzed hydrocarbon reactions 374
Introduction 375
Controlled - atmosphere magic-angle spinning NMR experimental techniques 375
The study of catalytic reactions 378
Conclusions 403
References 404
Chapter 13. Practical aspects of powder diffraction data analysis 408
Introduction 408
Data collection 409
Rietveld refinement 423
Conclusion 443
References 444
Chapter 14. Review on recent NMR results 446
Introduction 446
High resolution solid state NMR - general aspects 448
Relevant nuclei for high-resolution solid-state NMR spectroscopy 458
Two-dimensional (20) solid-state NMR spectroscopy 459
The structure of zeolites and AlPO4 molecular sieves 467
Adsorbed molecules in zeolites and AlPO4 molecular sieves 504
NMR studies of catalytic reactions on zeolites and SAPO molecular sieves 507
Catalyst acidity 511
Mesoporous materials 513
Concluding remarks 514
References 515
Chapter 15. Supported zeolite systems and applications 526
Introduction 526
Preparation methods for supported zeolites 527
Structured catalysts by in situ growth of zeolites onto supports 533
Zeolite-based membranes by in situ growth of zeolites onto supports 538
Synthesis of MFI membranes on porous ceramic supports 541
Porous metal supported zeolite membranes 545
Catalytic zeolite membranes 554
Miscellaneous 557
Conclusions 557
References 558
Chapter 16. The intersection of electrochemistry with zeolite science 560
Introduction 561
Electrochemical considerations 563
Intersecting electrochemistry and zeolites 576
Electrochemistry of zeolites modified with electroactive guests 584
Advanced applications of zeolites in electrochemistry 590
Conclusion 597
References 598
Chapter 17. Past, present and future role of microporous catalysts in the petroleum industry 604
Introduction 604
Trends in oil refining 606
Catalytic cracking 609
Hydrocracking 623
Paraffin isomerization 637
Other zeolite-catalyzed processes in petroleum refining 643
Manufacture of synthetic fuels 643
Concluding remarks 645
List of abbreviations 646
References 646
Chapter 18. Application of molecular sieves in view of cleaner technology. Gas and liquid phase separations 650
Introduction 650
Adsorption of VOCs on hydrophobic zeolites 651
Examples of applications 657
Conclusions 666
References 667
Chapter 19. Crystalline microporous phosphates: a family of versatile catalysts and adsorbents 670
Introduction 670
AlPO4’s and GaPO4’s 670
Isomorphic substitution 684
References 698
Keyword index 704
Studies in Surface Science and Catalysis 710

Textauszug

Sol-Gel Chemistry and Molecular Sieve Synthesis

J. Livage    Chimie de la Matière Condensée, Université Pierre et Marie Curie 4 place Jussieu-75252 Paris-France

Sol-gel chemistry provides a new approach to the preparation of oxide materials [1]. Starting from a solution, a solid network is progressively formed via inorganic polymerization reactions. The term sol-gel chemistry could actually be used in a broader sense to describe the synthesis of inorganic oxides by wet chemistry methods such as precipitation, coprecipitation or hydrothermal synthesis. Two routes are currently used depending on the nature of the molecular precursor. The inorganic route with metal salts in aqueous solutions and the metal-organic route with metal alkoxides in organic solvents. In both cases the reaction is initiated via hydrolysis in order to get reactive M-OH groups. This reaction can be simply performed by adding water to an alkoxide or by changing the pH of an aqueous solution [2]. Condensation then occurs leading to the formation of metal-oxygen-metal bonds. Condensed species are progressively formed from the solution leading to oligomers, oxopolymers, colloids, gels or precipitates. Oxopolymers and colloidal particles give rise to sols which can be shaped, gelled, dried and densified in order to get powders, films, fibres or monolithic glasses [3].

This paper describes the basic chemical reactions involved in sol-gel syntheses from both inorganic and metal-organic precursors.

1 METAL CATIONS IN AQUEOUS SOLUTIONS

Molecular sieves are usually prepared via hydrothermal methods from aqueous solutions [4]. This route has already been used for a long time in industry for the synthesis of catalysts and ceramic powders. Literature provides many data describing the hydrolysis of metal cations in dilute solutions [5] but very little is known about the formation of polynuclear species at concentrations greater than about 1 mol.l- 1, although these conditions are generally relevant to the synthesis of solid phases. One of the main problem arises from the fact that water behaves both as a ligand and a solvent. A large number of oligomeric species are formed simultaneously. They are in rapid exchange equilibria and it is not obvious to predict which one would nucleate the solid phase. The key parameter is usually the pH of the aqueous solutions but anions, cations or templates are often added in order to obtain the desired product [4][6].

The whole synthesis occurs in an aqueous medium in which water exhibits very specific properties, both as a molecule and as a solvent.

The water molecule has a high dipolar moment, μ = 1.84 Debye, and liquid water exhibits a high dielectric constant (ε = 80). Water is therefore a good solvent for most ionic compounds. It breaks polar bonds (ionic dissociation) and dipolar water molecules solvate both cations and anions.

The water molecule is a Lewis base via its 3a1 molecular orbital. It reacts with metal cations M2 + (Lewis acids) via acid-base reactions giving OH2, OH- or O2 - ligands. These hydrolysis equilibria are responsible for the behavior of metal cations toward condensation. It is therefore very important to know the chemical nature of aqueous species as a function of parameters such as pH, temperature or concentration.

The Partial Charge Model

The Partial Charge Model (PCM) will be used as a guide to describe the aqueous chemistry of metal cations [7]. It is based on the electronegativity equalization principle stated by R.T. Sanderson as follows: “when two or more atoms initially different in electronegativity combine, they adjust to the same intermediate electronegativity in the compound” [8]. The main consequence is that both the electronegativity χx of a given atom X and its partial charge δx vary when the atom is chemically combined. These two parameters must be related. A linear relationship is usually assumed as follows:

x=χx0+ηxδx

  (1)

where ηx is the hardness of atom X as introduced by Pearson. Hardness is related to the softness σx = 1/ηx which provides a measure of the polarisability of the electronic cloud around X. Softness increases with the size of the electronic cloud, i.e. with the radius r of X. Therefore hardness varies as 1/r. According to the Allred-Rochow scale, electronegativity is proportional to Zeff/r2[9]. Hardness may then be approximated as:

=k√χ0

  (2)

where k is a constant that depends on the electronegativity scale, k = 1.36 when Pauling electronegativities are expressed in the frame of Allred-Rochow′s model (Table 1).

Table 1

Electronegativities χi and softness σi of atmos Xi according to [7]

The total charge “z” of a given chemical species is equal to the sum of the partial charges of all individual atoms z = Σδi. This together with equations (1) and (2) leads to the following expressions for:

meanelectronegativityχ=Σi√χi0+1.36zΣi1/√χi0

  (3)

thepartialchargeδi=χ−χi0/1.36√χi0

  (4)

q.4canalsobewritenasδi=σiχ−χi0

  (5)

σi=1.36√χi0−1

  (6)

The Partial Charge Model provides an easy way to work out the mean electronegativity of chemical species and the charge distribution on each atom. In the case of the water molecule for instance equation (3) and (5) lead to χ(H2O) = 2.49, δH ≈ + 0.2 and δO ≈ − 0.4

Hydrolysis of metal cations

The word “hydrolysis” is used here to describe those reactions of metal cations with water that liberate protons and produce hydroxy or oxy species [5]. In aqueous solutions this reaction results from the solvation of positively charged cations by dipolar water molecules. It leads to the formation of [M(OH2)N]z + species. Water is a Lewis base and the formation of a M ⇐ OH2 bond with the metal cation (Lewis acid) draws electrons away from the bonding σ molecular orbital of the water molecule. This electron transfer weakens the O-H bond and coordinated water molecules behave as stronger acids than the solvent water molecules. Spontaneous deprotonation then takes place as follows:

OH2NZ+hH2O⇒MOHhOH2N−hz−h++hH3O+

  (7)

Where “h” is called the hydrolysis ratio. It indicates how many protons have been removed from the solvation sphere of the metal cation. The acidity of coordinated water molecules increases as the electron transfer within the M-O bond increases. In dilute solutions this leads to a whole set of more or less deprotonated species ranging from aquocations [M(OH2)N]z + (h = 0) to neutral hydroxydes [M(OH)z]0 (h = z) or even oxyanions [MON′](2N′-z)- corresponding to the case when all protons have been removed from the coordination sphere of the metal. The electron transfer within the M ⇐ OH2 bond increases with the oxidation state of the metal cation Mz + and coordinated water molecules become more acidic as z increases (Table 2).

Table 2

Partial charge distribution in [M(OH2)6]z + species

Mg2 +

2.625

+ 0.86

-0.34

+ 0.27

Al3 +

2.754

+ 0.78

-0.29

+ 0.33

Ti4 +

2.875

+ 0.76

-0.25

+ 0.39

V5 +

3.023

+ 0.51

-0.19

+ 0.47

W6 +

3.283

+ 0.31

-0.09

+ 0.60

A convenient rule-of-thumb shows that the hydrolysis ratio h of a given precursor [M(OH)h(OH2)N-h](z-h)+ mainly depends on two parameters, the pH of the solution and the oxidation state of the metal cation Mz +. A charge-pH diagram can then be drawn in order to show which aqueous species predominate (Fig.1). Two lines corresponding to h = 1 and h = 2 N-1 respectively separate three domains in which H2O, OH or O2 - ligands are observed [10].

Figure 1 Charge-pH diagram showing the evolution of hydrolyzed species [10]

Determination of the hydrolysis ratio

In very dilute aqueous solutions, metal cations exhibit several hydrolyzed monomeric species in the pH range 0-14. The problem is then to know whether it is possible to predict the chemical nature of these aqueous species at a given pH.

Following the electronegativity equalization principle it can be stated that deprotonation (eq.7) goes on until the electronegativity χh of hydrolyzed...

Produkt-Details

• Erscheint lt. Verlag: 19.7.1994
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Physikalische Chemie
• Themenwelt: Naturwissenschaften ► Chemie ► Technische Chemie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Mineralogie / Paläontologie
• Themenwelt: Technik ► Bauwesen
• Themenwelt: Technik ► Maschinenbau
• Themenwelt: Technik ► Umwelttechnik / Biotechnologie
• ISBN-10: 0-08-088695-7 / 0080886957
• ISBN-13: 978-0-08-088695-4 / 9780080886954