Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey’s research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

Catalysis Volume 15

A Review of Recent Literature

By James J. Spivey

The Royal Society of Chemistry

Copyright © 2000 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-219-7

Contents

Chapter 1 Strong Solid Bases for Organic Reactions By Yoshio Ono and Toshihide Baba, 1,
Chapter 2 Catalysis by Solid Bases 40 By Eric J. Doskocil, Shailendra Bordawekar and Robert J. Davis, 40,
Chapter 3 Solid Sorbents for Catalytic NOx Removal By Masato Machida, 73,
Chapter 4 Partial Oxidation of Light Alkanes in Short Contact Time Microreactors By P. Aghalayam, Y. K. Park and D. G. Vlachos, 98,
Chapter 5 Indirect Liquefaction -Where Do We Stand? By Yongqing Zhang and Burtron H. Davis, 138,
Chapter 6 Partial Oxidation of Methane Over Silicomolybdic Acid Catalysts By Akifumi Ueno, 185,

CHAPTER 1

Strong Solid Bases for Organic Reactions

BY YOSHIO ONO AND TOSHIHIDE BABA

1 Introduction

Carbanions are important intermediates in many organic reactions such as isomerizations, additions, alkylations, and cyclizations. They are formed by abstraction of a proton from a C-H bond of an organic molecule by a base.

These organic reactions often require a stoichiometric amount of liquid base to generate carbanions and produce a stoichiometric amount of metal salts as a by-product. For example, the methylation of phenylacetonitrile with methyl iodide proceeds in the presence of base under a phase-transfer condition.

[FORMULA NOT REPRODUCIBLE IN ASCII]

In this case, more than a stoichiometric amount of sodium hydroxide is required to neutralize the hydrogen iodide produced and to keep the system basic. Furthermore, a stoichiometric amount of sodium iodide is inevitably formed and has to be disposed of in an appropriate manner. Organometallic compounds such as Grignard reagents and alkyl lithium serve as donors of carbanion-like species. Here, again, a stoichiometric use of these reagents is required. Therefore, there is a need to develop solid bases to avoid these problems.

Solid base catalysts have many advantages over liquid bases. They are non-corrosive and environmentally benign, presenting fewer disposal problems, while allowing easier separation and recovery of the products, catalysts and the solvent. Thus, solid base catalysis is one of the economically and ecologically important fields in catalysis and the replacement of liquid bases with heterogeneous catalysts is becoming more and more important in the chemical industry. Furthermore, high activities and selectivities are often obtained only by solid base catalysts for various kinds of reaction.

Since the ability of bases to abstract a proton from a C-H bond is directly connected to the base strength, stronger bases are in general more effective in forming carbanions. Alkaline earth oxides such as magnesium oxide are strongly basic when properly pretreated. Extensive works by Tanabe, Hattori and their co-workers have been carried out using these materials.

Recently, other strong base catalysts have been reported. Potassium amide supported on alumina (KNH2/Al2O3) is effective for a number of base-catalysed reactions. Even toluene is activated to react with silanes at 329 K,8 and the isomerization of 2,3-dimethylbut- l-ene proceeds even at 201 K. Potassium fluoride supported on alumina (KF/Al2O3) has been used by organic chemists for a long time, but Tsuji and Hattori revealed that this catalyst becomes much more active when pretreated at 573-673 K under vacuum. Yamaguchi et al. reported that catalysts prepared by loading alkali- metal salts such as KNO3, followed by heating at 773-873 K, were very strongly basic and active for the isomerization of cis-but-2-ene at 273 K. Fu et al. used alkali-metal compounds supported on alumina for the reaction of catechol with dimethyl carbonate and found that the rate and selectivity depended strongly on the catalyst used. Furthermore, modified zeolites and calcined hydrotalcites are often reported as strong bases.

In this review, we will describe the preparation and characterization of these strong bases. Then, application of these catalysts to a variety of catalytic reactions is described. The reactions include the isomerization of alkenes and alkynes, the dimerization of alkynes, aldol reactions, and the formation of Si-C, Si-N and Si-O bonds.

2 Role of Solid Base and Basic Sites as a Catalyst

2.1 Abstraction of Protons – On the surface of solid bases, there are specific sites or centers, which function as a base. Basic sites (centers) abstract protons from the reactant molecules (AH) to form carbanions (A-).

[FORMULA NOT REPRODUCIBLE IN ASCII]

Here, the basic site B- on the solid surface acts as a Bronsted base. Stronger bases can abstract a proton with molecules with higher pKa values.

2.2 Activation of Reactants without Proton Abstraction – Reactants such as ketones and aldehydes are often activated by bases without proton transfer, as expressed by the following equation.

[FORMULA NOT REPRODUCIBLE IN ASCII]

Here, the basic sites B- act as a Lewis base.

It should be noted that a same surface site can serve as a Bronsted base as well as a Lewis base, depending on the nature of the adsorbate.

2.3 Cooperative Action of Acidic and Basic Sites – Magnesium oxide is active for the hydrogenation of 1,3-butadiene. It is assumed that hydrogen heterolytically dissociates in the presence of a pair of a coordinatively unsaturated Mg2+ and an oxide ion. Hydrogen adsorption is schematically expressed as:

[FORMULA NOT REPRODUCIBLE IN ASCII]

3 Base Strength of Basic Sites

3.1 H_ Acidity Function – The H_ acidity function is defined as a measure of the ability of the basic solution to abstract a proton from an acidic neutral solute.

[FORMULA NOT REPRODUCIBLE IN ASCII]

To determine the H_ value of a solution, the concentrations of AH and A- have to be measured accurately. When half of a solute AH is deprotonated in the solution, i.e.,]A-]= [AH], the H_ value of the solution is equal to the pKa value of AH. The basic strength of a solution is stronger when a neutral molecule of larger pKa value is deprotonated.

Tanabe proposed transferring this concept to solid bases as a measure of their strength. The base strength of solid bases is expressed by means of the H_ value, equated to the highest among the pKa values of the adsorbates from which the basic site is able to abstract a proton.

Tanabe defined solid superbases as materials with H_ values higher than 26. This value, like that of superacids (H_ ≤ -12), is 19 units from a neutral solution of 7.

In the use of this concept for solid bases, two important points should be noted:

(a) In the discussion of solid bases, the H_ value is treated as a parameter to describe the nature of individual basic sites. It is often assumed that there are a certain number of basic sites on solid surfaces and that each of the sites has its own basic strength. In the original definition, H_ scale is used to describe basic property of the solution, not that of individual basic molecules (or ions) in the solution.

(b) In principle, the idea of the H_ scale is only applicable to the Bronsted base. It is not, at least directly, related to the ability of the sites to function as Lewis bases, as shown in eqn. ( 1.5).

3.2 Indicator Method – The H_ values of basic solutions are determined by using indicator molecules. 18 If the pKa value of the indicator AH is known, the H_ value can be calculated by determining the ratio of [AH]/[A-]. To cover a wide range of the H_ scale, a series of indicators with different pKa values have been selected to obtain the accurate value of [AH]/[A-].

In the case of solid bases, the color change of indicator molecules upon adsorption is taken as a measure of basic strength. If the color change of the indicator is observed, the H_ value of the basic sites on the solid is higher than the pKa value of the indicator. Similarly, if the indicator does not change color upon adsorption, the H_ value of the sites is judged to be less than the pKa value of the indicator. By using indicators of different pKa values, the H_ value of the basic sites can be determined. It is important to know that the color change is due to proton abstraction by basic sites and not due to other types of interactions such as charge-transfer between the adsorbate and the surface.

3.3 Other Methods for Determining Basic Strength – Temperature programmed desorption of carbon dioxide is often used. When the interaction between basic sites and carbon dioxide is stronger, the molecule desorbs at higher temperature. One disadvantage of using carbon dioxide is that this molecule adsorbs on solid surfaces in several different forms. For example, as revealed by infrared spectroscopy, carbon dioxide is adsorbed on alkaline earth oxides to form a unidentate complex as well as a bidentate complex. Since the interaction of carbon dioxide with the surface does not involve a proton-transfer process, the result may not be directly related to the Bronsted basicity of the sites.

The XPS binding energy value of elements depends on the charge carried by the atom. The binding energy of O1s is then expected to decrease with increasing negative charge on the oxygen. The O1s binding energy of X- and Y-type zeolites decreases with increasing Si/Al ratio and decreasing electronegativity of the counter cation. Since the XPS binding energy is measured as the average of those for all of the O atoms in the material, the method is not applicable for the materials where only a fraction of the oxygen ions are active as basic sites, as in most alkaline earth oxides.

Infrared spectroscopy of adsorbed molecules is often used for characterizing surface bacisity. Pyrrole is an amphoteric molecule. The basic strength may be estimated from the shift of NH vibration upon its interaction with basic sites through hydrogen bonding. For example, when pyrrole interacts with framework oxygen ions in zeolites, the NH vibration is shifted to low wavenumbers from 3430 cm-1 in the pure liquid to around 3200 cm-1. The extent of the shift increases with the negative charge on oxygen, which is calculated from Sanderson’s electronegativities. Though pyrrole adsorption has become a popular technique, the spectra are rather complex and the molecule is not always stable on the surface; it polymerizes or dissociates on some oxides. The shift of C-D stretching mode of adsorbed CDCl3 is also a measure of the basic strength. Berteau et al. examined the base properties of modified aluminas by IR spectroscopy of probe molecules and CO2 TPD. Acetylene and substituted acetylenes have also been used as probe molecules for surface basicity.

Bosacek proposed the use of 13C NMR of adsorbed methyl iodide for the basicity of zeolites. Methyl iodide heterolytically dissociates and the methyl group attaches to the lattice oxygen. The chemical shift of the carbon, therefore, reflects the actual electronegativity of the oxygen. With this method, Hunger et al. confirmed that strongly basic sites were created by incorporating alkali metal oxides into the zeolite pores.

4 Base Strength and Catalytic Reactions

Catalytic reactions provide an accurate measure of basic strength, especially when the reaction starts by formation of carbanions by abstraction of a proton from the reactant, since the ease of carbanion formation depends on the pKa value of reactant. In Table 1.1, pKa values of various compounds are listed.

Isomerization of alkenes such as but-1-ene proceeds through the formation of allylic anions, which are formed by abstraction of a proton from but-1-ene, as shown in reaction (1.6).

[FORMULA NOT REPRODUCIBLE IN ASCII]

Since the pKa value of alkenes is high (pKa of C3H6 = 35), strong bases are required to activate the alkene molecule. Thus, alkene isomerization is an appropriate test reaction for strong solid bases. Moreover, the reaction is mechanistically simple. This makes the interpretation of experimental results straightforward.

Table 1.2 shows the catalytic activities of various solid bases for the isomerization of 2,3-dimethylbut- l-ene (DB- I) to 2,3-dimethylbut-2-ene after 20 h. The activities vary significantly from one catalyst to the other, reflecting a wide variety of the base strength and the number of the basic sites. KY (K+- exchanged Y zeolite) has no activity, indicating that no strong basic sites exist on KY. On the other hand, there are groups of catalysts that have very high activities: alkali amides on Al2O3, alkali compounds on Al2O3, CaO and MgO. Since the conversion over these catalysts is close to the equilibrium value at 313 K, it is hard to know the relative ranking of activities for these materials from Table 1.2. Table 1.3 shows the results when the isomerization was carried out at much lower temperature, i.e. 201 K, and a shorter reaction time on these materials, RbNH2/Al2O3, a series of alkali hydroxide supported on Al2O3, and K loaded on Al2O3 prepared by the deposition of K vapor (K/Al2O3), being added to the list. The reaction is very fast over RbNH2/Al2O3, KNH2/Al2O3, CsOH/Al2O3 and CaO even at 201 K. The order of the activities for the most active class of solid base catalysts is as follows:

[FORMULA NOT REPRODUCIBLE IN ASCII]

The pKa value of propene is 35 and we can assume that the pKa value of DB-I is not far from this value. Therefore, all of these catalysts can be classified as solid-superbases. CaO was reported to have basic sites stronger than H_ = 26 by an indicator method.

As mentioned above, KY does not show the catalytic activity for the isomerization of DB-I, whereas it catalyses a Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate.

[FORMULA NOT REPRODUCIBLE IN ASCII]

This indicates that basic sites of KY are able to abstract a proton from the latter, which has a pKa value of 8.6. Therefore, when this reaction proceeds over a solid catalyst, it is judged that the solid has basic sites stronger than H_= 8.6.

Though the H_ values of a base catalyst can be used to decide whether a reactant can be activated to form carbanions by the solid base or not, it is not absolute. As stated above for alkali-exchanged faujasite, although the H_ values of alkali-exchanged zeolites are estimated to be 10-13 from the rates of Knoevenagel condensations at 363-443 K, they can catalyse the reaction of phenylacetonitrile with dimethyl carbonate at 533 K.

[FORMULA NOT REPRODUCIBLE IN ASCII]

Rb- and Cs-exchanged X-zeolites can catalyse the side chain alkylation of toluene with methanol at 700 K.

[FORMULA NOT REPRODUCIBLE IN ASCII]

These facts clearly show that these catalysts can activate phenylacetonitrile (pKa = 21.9) and toluene (pKa = 37) at 533 and 700 K, respectively. Base strength increases with temperature. Weak bases (as measured at room temperature) can be catalysts for a variety of reactions at higher temperatures, as long as they are stable.

5 Solid Base Materials

5.1 Alkaline Earth Oxides – Alkaline earth oxides (MgO, CaO, SrO and BaO) are active for a various types of reactions including isomerization of alkenes. To obtain a high activity it is essential to remove adsorbed molecules such as carbon dioxide and water. The catalytic activities of these oxides depend on the pretreatment temperature. The dependence of the catalytic activities of MgO on outgassing temperature for various reactions is shown in Figure I.I. When the pretreatment temperature is low (below 700 K), MgO shows no activity for the isomerization of but-l-ene. The catalytic activity develops at a pretreatment temperature of 800 K and declines at higher pretreatment temperatures. The maximum activity for H-0 exchange between CH4 and D2 appears at 923 K, while the activities for hydrogenations develop at a higher pretreatment temperature of 1200-1300 K. The dependence of the catalytic activities on pretreatment temperature indicates that there are at least three types of basic site on the surface of MgO. A model of MgO surface shows that there are several types of oxygen anions with different coordination number on the surface. It is plausible that each type of oxygen anion manifests its own basic strength, and changes in amount, with pretreatment conditions. Oxygen anions at low coordination numbers exist at corners, edges and high Miller index surfaces. As pretreatment temperature increases, desorption of adsorbed molecules such as carbon dioxide occurs and oxygen anions become available for the reactants. Desorption of adsorbed molecules starts from weaker basic sites and more severe pretreatment is required for generating stronger basic sites. At the same time, pretreatment at higher temperature causes the rearrangement of the surface structure. These two factors induce a complex dependence of the catalytic activities with pretreatment temperature.

(Continues…)Excerpted from Catalysis Volume 15 by James J. Spivey. Copyright © 2000 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey’s research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

Catalysis Volume 13

A Review of Recent Literature

By James J. Spivey

The Royal Society of Chemistry

Copyright © 1997 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-209-8

Contents

Chapter 1 Solid Electrolyte Electrochemical Cells for Catalyst Sensing By Ian S. Metcalfe, 1,
Chapter 2 Oxidation Reactions over Supported Vanadia Catalysts By Israel E. Wachs, 37,
Chapter 3 Zeolite-catalysed Alkylation of Polynuclear Aromatics By Yoshihiro Sugi and Yoshihiro Kubota, 55,
Chapter 4 Preparation and Characterization of Hexaaluminate Materials for High-temperature Catalytic Combustion By G. Groppi, C. Cristiani and P. Forzatti, 85,
Chapter 5 Catalytic Conversions in Water. An Environmentally Benign Concept for Heterogenization of Homogeneous Catalysis By Georgios Papadogianakis and Roger A. Sheldon, 114,

CHAPTER 1

Solid Electrolyte Electrochemical Cells for Catalyst Sensing

BY IAN S. METCALFE

1 Introduction

The development of sensors for industrial process monitoring and control is an area of increasing importance. In particular, there are relatively few sensors that are capable of monitoring the state of a catalyst despite the fact that catalyst state can have a very significant impact on overall process performance. Consequently, there is a need to develop new sensors for the in-situ monitoring of catalyst state. Solid electrolyte electrochemical cells show promise as sensors which could be used for intermediate and high temperature application (temperatures greater than about 200°C).

A solid electrolyte is a material in which the electrolytic, or ionic, conductivity is much greater than the electronic conductivity (for solid electrolytes to be practically useful the ratio of electrolytic to electronic conductivities should be of the order of 100 or greater). Solid electrolytes with conduction ions of O2- H+, Li+, Na+, Ag+, F-, Cl- have all been reported. Much attention has been devoted to oxygen-ion conducting solid electrolytes, many of which show appreciable oxygen-ion conductivities in the range of 200-1200°C.

At high oxygen partial pressures an equilibrium is established between the gas phase oxygen, interstitial oxygen ions and electron holes and conductivity is predominantly p-type due to electron-hole transfer.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.1)

Using the notation of Kroger and Vink, the symbol Oi” denotes an interstitial oxygen ion carrying an effective double negative charge and h· denotes an electron hole.

At low oxygen partial pressures an equilibrium is established between lattice oxygen, gas phase oxygen, oxygen ion vacancies and free electrons and conduction is n-type.

2Oo ? O2 + Vo” + 2e’ (1.2)

Oo denotes a lattice oxygen, Vo” denotes an oxygen ion vacancy with an effective double positive charge and e’ denotes an electron.

Over a large range partial pressures of oxygen ionic conductivity dominates and the material behaves as a solid electrolyte. Under these conditions there is an equilibrium established between oxygen ion vacancies, interstitial oxygen ions and lattice oxygen.

2Oo ? Vo” + Oi” (1.3)

Hence the partial pressure of oxygen and the temperature determine whether the solid will exhibit n-type, p-type or ionic conduction. Although the concentration of defects is important it is also necessary to consider the mobilities of the individual defects; higher ionic mobilities will result in a larger domain for electrolytic conduction. Figure 14 shows the dominant mode of conduction in some mixed oxide materials, exhibiting solid electrolyte behaviour, as a function of temperature and oxygen partial pressure.

Suitable solid electrolytes can be employed as the electrolyte in an electro-chemical cell. The electrolyte is used in the form of a membrane which is impermeable to gas phase transport. Electroactive materials, or electrodes, are deposited on both sides of the electrolyte to increase the rates of charge transfer across the electrolyte interface and it is important that the active molecules in the gas phase have easy access to the electrode/electrolyte interface where they can participate in the charge-transfer reactions. For this reason it is necessary, in most cases, to ensure that the electrode has a high porosity while, at the same time, remaining electrically continuous.

Solid electrolyte electrochemical cells can be operated in a variety of ways (the three modes of operation are illustrated schematically in Figure 2). Such a cell may be operated potentiometrically in order to investigate the behaviour of a catalyst of interest. This technique has become known as solid electrolyte potentiometry (SEP). The catalyst itself is deposited in the form of an electrode and is exposed to reaction conditions. The electrode on the other side of the solid electrolyte membrane is exposed to constant gas phase conditions and acts as a reference. The e.m.f. generated by such a cell is related to the state of the catalyst and can be indicative of the catalyst work function. 5 In the case of an oxygen-ion conducting solid electrolyte, the e.m.f. of the cell is a reflection of the thermodynamic activity of oxygen at the catalyst-electrode.

If an external circuit and a power supply are now used, a current can be passed through the cell resulting in a pumping of oxygen ions (in the case of an oxygenion conducting solid electrolyte) towards the electrode of interest or, if the current is reversed, a removal of oxygen ions from the electrode. Consequently, the surface of the catalyst-electrode can be investigated by the passage of current (amperometric techniques). Recently behaviour under amperometric conditions has been recognised to be more complex because of evidence that electrochemical oxygen pumping (EOP) may modify the behaviour of a catalyst.

Solid electrolyte electrochemical cells can also be operated as fuel cells. One electrode is exposed to a fuel while the other is exposed to an oxidant. The driving force due to the differences in oxygen chemical potential at the two electrodes causes electrons to flow around an external circuit and electrical power is generated. The temperature of operation of fuel cells must be greater because of the increased oxygen fluxes required. Fuel cells are commonly operated at temperatures in excess of 800°C. Fuel cells can also be used for the cogeneration of both useful chemicals and electrical energy if the reaction occurring at the anode is appropriate. Fuel cell technology as such is, however, not relevant for sensor development.

It is important to appreciate that solid electrolyte systems are different from aqueous systems in one important way. In a solid electrolyte system the charge transfer takes place across the catalyst/electrolyte interface whereas the chemical reaction takes place on the gas-exposed surface of the catalyst. In aqueous systems the surface at which charge-transfer occurs is the same as the surface over which any catalysis occurs. As a result care must be exercised when making analogies between the two types of cell.

A number of reviews have been written on the use of solid electrolyte electrochemical cells in catalysis. Vayenas and other workers have reviewed the area of SEP as have Gellings et al., Stoukides and Metcalfe. Vayenas et al. have also reviewed work on the modification of catalytic behaviour.

The aim of this review is to present and discuss recent work on solid electrolyte electrochemical cells relevant to in-situ catalyst sensing. Consequently, the area of SEP will be concentrated upon, however, appropriate closed-circuit or amperometric studies will also be discussed. This review is intended to also introduce the reader familiar with heterogeneous catalysis to the electrochemical concepts and techniques required to fully appreciate the research work in this field.

Much work with solid electrolyte electrochemical cells has involved the use of oxygen-ion conductors and it is therefore the use of these conductors which will be concentrated upon in this review article.

2 Electrochemical Principles

In this section both the open-circuit and closed circuit behaviour of electro-chemical cells will be briefly discussed. The mechanism of the charge-transfer process for oxygen-ion conducting systems will also be discussed.

2.1 Open-circuit Behaviour

2.1.l Single Charge-transfer Process– Consider an electrochemical cell consisting of two porous platinum electrodes contacting an oxygen-ion conducting solid electrolyte membrane. The two electrodes are exposed to two different chemical potentials of oxygen µo2′ and µo2″. The cell may be represented as,

µo2′, Pt|MO|Pt, µo2″ (2.1)

where MO represents the metal oxide electrolyte.

To determine the e.m.f. of the cell the idea of electrochemical potential will be introduced. The electrochemical potential of species i is defined as,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.2)

where Φ is the electrostatic potential, zi is the charge on the species and F is the faraday constant.

Under open-circuit conditions there can be no net flow of current through the cell. However, there may be individual currents due to the migration of each charge carrying species, given by,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.3)

where x is the distance co-ordinate and cri is the individual conductivity. The net current must be zero, so,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.4)

The charge-transfer reaction at both electrodes is assumed to be,

O2 + 4e- ? 2O2- (2.5)

Using equations (2.2) to (2.4) it can be easily shown that the e.m.f. of the cell will be given by,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.6)

where tion is the ionic transport number (equal to unity if there is no electronic conduction). If tion is less than unity the relationship between tion and µO2 must be known in order to integrate equation (2.6). In practice, if any appreciable electronic conduction is present, then electrons will migrate in the opposite direction to oxygen ions and, if diffusional processes are too slow, the result may be the build-up of oxide at the negative electrode and the appearance of metal at the positive electrode. In such a case, the oxygen potentials at the interfaces are no longer fixed and the cell will exhibit an unstable e.m.f. For solid electrolytes, as tion approaches unity, this is not a problem and the equation can be easily integrated,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.7)

If the oxygen at the interfaces is in equilibrium with the gas phase oxygen, i.e. there is no chemical reaction, and oxygen behaves as an ideal gas, this reduces to,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.8)

Equation (2.8) is, of course, the Nernst equation. From the e.m.f. of the cell, an unknown oxygen partial pressure can be determined. This has led to the use of solid electrolyte electrochemical cells as oxygen probes.

2.1.2 Mixed Potentials – When reactive gases other than oxygen are also present, it is possible for mixed potentials to occur. As an example, if a mixture of oxygen, carbon monoxide and carbon dioxide are now supplied to the cell,

O2,CO,CO2,Pt|MO|Pt,O2 (2.9)

there may be more than one charge-transfer process which can take place, e.g., both of the following reactions may proceed,

* + Oo ? O* + Vo” + 2e’ (2.10)

CO* + Oo ? CO2* + Vo” + 2e’ (2.11)

where * denotes an electrode site.

Under open-circuit conditions no net current can flow so that the total rate of the anodic reactions must equal that of the cathodic reactions. Applying this condition allows the e.m.f. to be determined if the current-voltage relationships are known for the charge-transfer processes.

Mixed potentials have been found to be important in reaction systems other than CO oxidation. Michaels and co-workers studied a platinum electrode exposed to nitric oxide, nitrogen dioxide and oxygen in the region of 600-800°C. Using e.m.f. data and closed-circuit work at low overpotentials (to avoid modifying coverages of electroactive species) it was shown that two charge-transfer reactions were important,

* + Oo ? O* + Vo” + 2e’ (2.12)

NO* + Oo ? NO2* + Vo” + 2e’ (2.13)

Mixed potentials have also been shown for the electrodes exposed to reacting mixtures of methane and oxygen.

2.2 Closed-circuit Behaviour – When a voltage is applied to such a cell there is a tendency for oxygen to migrate from one electrode to the other. As a result such arrangements have been used as oxygen pumps.

When the potential difference across the electrode/electrolyte double-layer is not at its equilibrium value then the interface is said to be polarised. When the interface is polarised a net current will flow, the magnitude of the current being dependent upon the difference in the total anodic and total cathodic currents,

i = iA-iC (2.14)

Take as an example the oxygen charge-transfer reaction

* + Oo ? O* + Vo” + 2e’ (2.15)

Oxygen from the electrolyte reacts with a vacant site on the electrode forming adsorbed oxygen and creating an oxygen vacancy. The reverse reaction involves oxygen adsorbed on the electrode undergoing electroreduction. The forward reaction is the anodic process and the reverse reaction is the cathodic process. Using transition state theory, anodic and cathodic current densities can be expressed in terms of the potential difference across the interface, the coverage of reactant and the relevant rate constant,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.16)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.17)

where i is the current density, k is the rate constant, the subscript ‘A’ refers to the anodic process, the subscript ‘C’ refers to the cathodic process, θ refers to fractional coverages on the electrode, is the symmetry factor, ΔΦ is the potential difference across the electrode/electrolyte interface. At equilibrium, the anodic and cathodic current densities are equal in magnitude and equal to what is called the exchange current density, i0.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.18)

Therefore, rearranging, the exchange current density can be expressed in terms of the rate constants and coverages,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.19)

The actual potential difference across the electrode/electrolyte interface minus the equilibrium potential difference across the interface is known as the electrode overpotential, 11. and is, in effect, the driving force for net charge-transfer,

η = ΔΦ – ΔΦ0 (2.20)

Equations (2.16) and (2.17) can be rewritten in terms of the exchange current density and the overpotential,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.21)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.22)

The net current can be written as,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.23)

which is an equation of the Butler-Volmer type.

When,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.24)

it is possible to make a low-field approximation for the Butler-Volmer equation,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.25)

and there is a linear relationship between the current density and the over-potential.

When,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.26)

it is possible to make a high-field approximation in a form known as the Tafel equation,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.27)

So far it has been assumed that charge-transfer is the limiting step in the overall process. However, other steps may influence the overall rate. Such steps would include any chemical reactions, adsorption and desorption, and diffusional resistances in the gas phase, on the electrode surface, or in the electrolyte lattice. When mass transfer (here mass transfer refers to any step other than the charge-transfer reaction) between the surface of the electrode and the gas phase is rate limiting, the surface coverages of adsorbed species may be modified by the passage of a current. Under these conditions the current-overpotential relationship is modified. It can easily be shown (assuming that the rate of mass transfer between the three-phase region and the gas phase is directly proportional to the effective oxygen concentration difference between these two regions – a complete derivation has been presented by Bard and Faulkner) that,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.28)

where ilA and ilc are the anodic and cathodic limiting currents. Again this equation can be linearized for low overpotentials,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.29)

Defining resistances,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.30)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.31)

where Rmt can be considered a resistance due to mass transfer and Rct can be considered as a resistance due to charge transfer. Equation

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.32)

and the interface can be modelled as a series of resistances.

(Continues…)Excerpted from Catalysis Volume 13 by James J. Spivey. Copyright © 1997 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey’s research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

Catalysis Volume 10

A Review of Recent Literature

By James J. Spivey, Sanjay K. Agarwal

The Royal Society of Chemistry

Copyright © 1993 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-614-7

Contents

Chapter 1 Toward Supported Oxide Catalysts via Solid-Solid Wetting By Helmut Knözinger and Edmund Taglauer, 1,
Chapter 2 Model Catalyst Studies of Supported Metal Sintering and Redispersion Kinetics By Calvin H. Bartholemew, 141,
Chapter 3 Techniques for Measuring Zeolite Acidity By George Marcelin, 83,
Chapter 4 Applications of Raman Spectroscopy to Heterogeneous Catalysis By Israel E. Wachs and Franklin D. Hardcastle, 102,
Chapter 5 Oxidative Coupling of Methane By Zbigniew Kalenik and Eduardo E. Wolf, 154,

CHAPTER 1

Toward Supported Oxide Catalysts via Solid-Solid Wetting

BY HELMUT KNOZINGER AND EDMUND TAGLAUER

1 Introduction

Supported oxides of transition metals, particularly of groups Vb (V), Vlb (Cr, Mo, W), and Vllb (Re) are widely used as catalysts for various reactions. These so-called “monolayer-type” catalysts are formed when one metal-oxide phase is dispersed on the surface of a second metal-oxide support. Typical catalyst supports in industrial applications are transition aluminas, silica, and titania. Alumina-supported molybdenum and tungsten-based catalyst precursors are extensively used in the petroleum industry in hydrotreating processes. Consequently, numerous studies have been carried out to analyze their function in hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodemetalization (HDM) of petroleum and coal products. The oxidation of hydrocarbons, carbon monoxide hydrogenation and the water gas shift reaction are also catalyzed by supported molybdena and tungsta. TiO2-supported vanadium, molybdenum, and tungsten oxide catalysts were found to be highly active for the selective catalytic reduction (SCR) of NOx with NH3. The vanadium oxide/TiO2 system is also widely used for selective catalytic oxidations of hydrocarbons. Supported Re2O7 effectively catalyzes the metathesis reaction and chromia-based catalysts are active for polymerizations (SiO2 supported) or redox reactions (Al2O3 supported).

Typically, this class of catalysts is prepared by impregnation of the support from an aqueous solution containing a suitable precursor compound or (less frequently) by gas-phase chemisorption of a volatile metal compound (e.g., Mo(CO)6) on a carrier. When catalysts are prepared by impregnation on an industrial scale, large volumes of solutions must be handled and eventually large volumes of wastewater must be disposed. As a consequence, there might be an interest to synthesize catalysts via alternative routes that would not require impregnation and precipitation steps. Solid-state reactions provide a significant potential in this context, since reactions between two (or more) solids necessarily must involve the interfaces between them. Several processes can occur when an active solid component undergoes reactive interactions with another solid, the support. The active component may (1) retain its chemical identity, the support simply acting as a dispersing agent, (2) dissolve in the support matrix to form a solid solution, or (3) form new surface and/or stoichiometric bulk compounds. Haber has strongly emphasized the role which surfaces and interfaces play in the reactivity of solids. In powder mixtures, depending on the relative rates of nucleation and nuclei growth on one hand and surface migration or gas-phase transport on the other hand, two principal routes for the reaction progress can be envisaged. If the nucleation and nuclei growth rates are much higher than migration rates, a solid-state reaction can only occur at intergranular contacts and will lead to the formation of a bulk compound (Route I). If, in contrast, the migration of one mobile component across the surface of another less mobile component is very fast, grains of the latter will be encapsulated by a thin layer of the former, so that the entire surface becomes the reaction interface (Route II). A schematic representation of the propagation of the reaction interface via routes I and II is given in Figure 1. Several examples of solid-state reactions proceeding via route II have been reported in the literature. If the rate of formation of a bulk compound across the reaction interface is negligibly small, the process may come to a close once the surface layer has formed.

The migration of one solid over the surface of another solid is frequently described as surface diffusion of constituents of the lattice in a concentration gradient. Haber and coworkers suggested the wetting of one solid by a second solid under the action of forces of surface tension as an alternative mechanism.

It is tempting to take advantage of these phenomena known from solid-state chemistry in the preparation of supported oxide catalysts, although this has been realized in practice only in exceptional cases. The increasing interest in this area is in fact documented by a recent review by Xie and Tang on spontaneous spreading, which covers the literature up to 1987. In the present review we are reporting on wetting and spreading phenomena in systems of particular interest for catalyst preparation, where mixtures of oxides will play a central role.

2 Theoretical Considerations

2.1 Thermodynamics of Wetting and Spreading. – The thermodynamics of wetting of a solid by a liquid is well established and discussed in detail in relevant textbooks. The same principles can be applied in the phenomenological treatment of the wetting of one solid by another solid, a phenomenon that also plays a major role in the redispersion of supported particles on the surface of an oxide carrier (e.g., supported catalysts). Sintering and redispersion in supported metal catalysts have been discussed by Ruckenstein in several papers and excellent review articles.

Redispersion of particles on the surface of a carrier is a phenomenon that has much in common with the spreading of one solid component over the surface of a second solid in the course of solid- state reactions as discussed in the introduction. In this case, grains of both components are contacting each other in powder mixtures and the spreading will be initiated from the contact zones. This same situation is apparent when supported catalysts are to be prepared by spreading from powder mixtures containing the support and the precursor of the final supported active phase, where the active phase is formed by spreading of the precursor. It is therefore important to define the conditions under which solid-solid wetting and spreading can be expected to occur. A schematic representation of wetting and spreading is shown in Figure 2.

The overall change in interfacial-free energy ΔF is given by Equation (1):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where γij denotes the specific surface-free energy between phases i and j, ΔA the change in surface/interface area, and subscripts a, s, and g denote active phase, support and gas phases, respectively. For wetting of the support by the active phase to occur, the interfacial-free energy change must be negative (ΔF<0), hence, the condition

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

or

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

if /ΔAa/ = /ΔAas/ = /ΔAs/ must be fulfilled. Hence, for predictions to be made of whether or not solid-solid wetting can principally occur in a given system, the specific surface and interface-free energies must be known for the experimental temperature and environmental conditions applied. Surface-free energies of pure binary oxides have been compiled by Overbury et al. The available data are typically measured near the melting point of the material and the temperature coefficients of the y-values are not known in most cases. Surface-free energies will also vary with the nature and composition of the gas phase in an unknown form. Therefore, the tabulated values can only be used for order-of-magnitude considerations.

Interface-free energies Γas are practically always unknown. γas is given by Equation (4):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where Uint is the interaction energy between the two oxides per unit area and Ustrain is an energy term brought about by a possible mismatch of the lattice parameters of the two oxides in contact. The interaction energy Uint can be considered as the adhesion energy; it may, however, also contain contributions from “chemical” interactions in the interface.

Surface-free energies of several oxides, which bear relevance in the present context as either supports or active oxides, are summarized in Table 1 together with their bulk melting points Tmelt and Tammann temperatures TTam ≈ 0.5 Tmelt·

2.2 Dynamics of Spreading. – As mentioned in the introduction, the spreading of a solid on the surface of another solid has been described as surface diffusion of constituents of the lattice of the mobile solid in a concentration gradient. Haber et al. argued that in oxide systems surface diffusion should be slow in the temperature ranges frequently encountered in the solid-state synthesis of oxide systems due to the typically high values of the lattice energies of oxides. The concept of solid-solid wetting was therefore introduced. However, even if the surface-free energy gradient is responsible for the wetting phenomenon to occur, migration of one component over the surface of the second oxide requires mobility of the constituents of the lattice.

Diffusion in a concentration gradient can be described by the model of independent particle diffusion as schematically represented in Figure 3(A), where the arrows indicate the time-averaged particle displacements along the concentration gradient. In this case the individual particle must be separated from the mobile phase and it must overcome an activation energy in each elementary jump on the support surface. As a rule of thumb, the Tammann temperature TTam ≈ 0.5 Tmelt,bulk (in K) is considered to be sufficient to make atoms or ions of the bulk of a solid sufficiently mobile for bulk-to-surface migrations, while the Huttig temperature (approximately one-third of the bulk melting temperature) is enough to make the species already located on the surface adequately mobile to undergo agglomeration or sintering. Ruckenstein has demonstrated that the enhanced mobility can be associated with the two-dimensional melting of the surface of a solid particle, i.e., with the occurrence of a “liquid-like” behavior of the surface layer. A theory of two-dimensional melting has been advanced by Kosterlitz and Thouless which is based on the dislocation pairs model of melting. The two-dimensional melting temperature is given by Equation (5):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where m is the atomic mass, k the Boltzmann constant, h Planck’s constant, a the lattice parameter and ΘD the Debye temperature. It turns out that Tmelt,2-dim as given by equation (5) is proportional to the bulk melting temperature as obtained by Lindemann with the proportionality constant being close to 0,5, the value used in the definition of the Tammann temperature. Values of the 3-dimensional melting temperatures and Tammann temperatures of several binary oxides are summarized in Table 1.

The mechanism of spreading in powder mixtures may thus be described as migration of species from a “liquid-like” surface layer of one solid across contact boundaries between grains onto the surface of the support, where they may be immobilized again if the interaction energy Uint is sufficiently high. A thin film (possibly a single atomic or molecular layer) may thus extend from the contact boundary onto the support surface. Further transport of the active oxide material can then be envisaged to occur via migration of active phase species over the film surface toward the leading edge of the film where they would ultimately be trapped again on the support surface. This process may be described by the unrolling-carpet mechanism which is schematically represented in Figure 3(B). Depending on the individual properties of the interacting oxide materials, formation of thick films (several molecular layers) or islands (particles) with finite contact angle may also occur.

Baker observed mobilization of small particles of several metals and metal oxides on graphite at a temperature (so-called “mobility temperature”) that was identical with the Tammann temperature. Thus, in systems exhibiting relatively weak interactions with the support surface, particle mobility may be induced at this temperature which might ultimately lead to agglomeration rather than spreading.

2.3 Mixing of Powders. – Intimate mixing of the two powder components is required so as to obtain a homogeneous product. Therefore, grinding or milling is usually applied to the powder mixture prior to thermal treatments. Often these processes are not well controlled when powder mixtures are prepared for solid-state synthesis of bulk products or supported catalysts, although they must be expected to influence the reactivity of the powders very significantly. Grinding will certainly influence the grain sizes and grain-size distributions and thus the rates of spreading. Also the two-dimensional melting temperature should be dependent on the grain size. In addition, several phenomena occur in the very complex grinding mechanisms that must influence the spreading and reactivity behavior of powder mixtures. During grinding, several particles are simultaneously and repeatedly subjected to stress application in the grinding zone. With each stress application, several fractures may occur in each particle. Cracks will be initiated and will propagate; flaw interaction in a particle, secondary breakage, and interaction of particles with each other will occur. The physical and chemical interaction between particles and the grinding environment and the transport of material through the grinding zone will also affect the nature of the product obtained. Occasionally material transport between chemically distinct particles may already lead to spreading and wetting during the grinding or milling procedure. Even solid-state reactions in bulk phases can be induced by mechanical activation of solid materials and several tribochemical processes have found technological applications. Angelov and Bonchev have reported on the formation of a Cu-rich surface layer on Co304 by mechanically treating a powder mixture of CuO and Co3O4 in a friction grinder. These reactions are believed to occur due to strong local temperature increases which may lead to melting of microscopic zones within contact regions.

Although the discussion in this section is qualitative, it must be concluded that the mixing and grinding procedures in preparation of powder mixtures for catalyst synthesis via solid/solid wetting and spreading undoubtedly play an important role and must be carried out carefully and under controlled and reproducible conditions.

3 Experimental Evidence for Spreading

3.1 Alumina-supported Systems. – Alumina-supported systems are by far the most studied in relation to solid/solid wetting and spreading with several group Vb and VIb oxides being used as mobile phases. Among these molybdenum oxide (MoO3) has found particular interest. The spreading behavior of MoO3 on the surface of transition aluminas will therefore be reviewed in some detail, followed by a discussion of other mobile phases including V2O5, CrO2, and WO3, and several salts of interest for catalyst preparation.

(Continues…)Excerpted from Catalysis Volume 10 by James J. Spivey, Sanjay K. Agarwal. Copyright © 1993 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey’s research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

Catalysis Volume 9

A Review of Recent Literature

By J.J. Spivey

The Royal Society of Chemistry

Copyright © 1992 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-604-8

Contents

Chapter 1 Zeolite Catalysis in the Conversion of Methanol into Olefins By G.F. Froment, W.J.H. Dehertog, and A.J. Marchi,
Chapter 2 Deactivation and Regeneration of Naphtha Reforming Catalysts By J.M. Parera and N.S. Figoli,
Chapter 3 Deactivation of Stationary Source Air Emissions Control Catalysts By J.R. Kittrell, J.W. Eldridge, and W.C. Conner,
Chapter 4 Direct Conversion of Methane to Liquid Fuels and Chemicals By R.D. Srivastava. P. Zhou, G.J. Stiegel, V.U.S. Rao. and G. Cinquegrane,
Chapter 5 Effect of Deactivation on Catalyst Selectivity By D.B. Dadyburjor,

CHAPTER 1

Zeolite Catalysis in the Conversion of Methanol into Olefins

BY G.F. FROMENT, W.J.H. DEHERTOG, AND A.J. MARCHI

1 Introduction

Light olefins are key components in the petrochemical industry. Conventionally, they are produced by thermal cracking of naphtha. The importance of the research efforts to viable routes in the production of basic chemicals, independent of oil, cannot be overlooked. Methanol, which can readily be produced from coal or natural gas via synthesis gas (CO + H2) by existing and proven technologies, offers an interesting alternative. Although methanol itself is a potential motor fuel or can be blended with gasoline, it would require large investments to overcome the technical problems associated with it. Mobil’ s announcement of a zeolite-based process for the conversion of methanol into gasoline provided a new route for the conversion of coal to gasoline. This methanol-to-gasoline (MTG) process was based on a new class of synthetic shape-selective zeolites differing from the classical small-pore and large-pore zeolites in their pore dimensions, which are intermediate, and their Si/Al-ratio, which can be very high. An excellent review on the MTG-process is given by Chang. The general reaction path of the methanol conversion to hydrocarbons is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Methanol is first dehydrated to dimethylether (DME). The equilibrium mixture thereof is then converted to light olefins. In the final steps of the reaction path, the C2-C5 olefins are converted to paraffins, aromatics, naphthenes and higher olefins by polycondensation and alkylation reactions. The importance of light olefins as intermediates in the conversion of methanol to gasoline was soon recognized. As a result, several attempts were made to selectively produce light olefins from methanol on zeolite catalysts, not only on medium-pore zeolites but also on small-pore and, to a lesser extent, on large-pore zeolites. The development of a new type of molecular sieve (silico-alumino-phosphates) with a zeolite-like framework structure offered interesting perspectives in the methanol conversion to olefins. This review describes and compares the various zeolitic catalysts and operational conditions that have been reported to influence the olefin selectivity in the methanol-to-olefins (MTO) process. The reaction mechanism, which has already been reviewed extensively, will be dealt with briefly.

2 Small-pore Molecular Sieves

2.1 Principal Characteristics. Molecular sieves with pore openings of about 0.45 nm show very interesting shape-selectivity properties for the conversion of methanol to olefins (MTO process). The small-pore molecular sieves studied in the MTO process are chabazite, erionite, zeolite T, ZK-5, ZSM-34, zeolite A, SAP0-17, SAP0-34, and SAP0-44. All of them can sorb only straight chain molecules, e.g. primary alcohols and linear paraffins and olefins, but no branched isomers and aromatics: the pore opening is smaller than the kinetic diameter of branched and aromatic molecules, but large enough to permit the access of linear molecules.

The pore openings of small-pore molecular sieves are 8-membered oxygen rings. The dimensions vary with the shape of the rings, but the effective size is always lower than 0.45 nm. The ring shape may be circular or puckered and elliptical (see Table 1).

The porous systems of small-pore molecular sieves are conformed by ellipsoidal or spherical cavities that share the 8-membered oxygen rings to generate a three-dimensional channel system. These cages are normally much larger than the connecting windows (see Table 1). The structures of chabazite, erionite, and zeolite A cavities are shown in Figure 1.

Chabazite has a rhombohedral symmetry and its typical composition in the hydrated form is (Ca,Na2)O.Al2O3.4SiO2.6-6.5H2O. Its framework consists of double-6-rings (D6R) arranged in layers in the sequence ABCABC. The hexagonal prisms formed in this way are linked by tilted 4-membered rings (see Figure l(a)). The resulting framework possesses large, ellipsoidal cages composed of D6R at top and bottom, six 8-rings in rhombohedral positions and six pairs of adjacent 4-rings. The cavities are interconnected to six others by the puckered elliptical 8-rings.

Erionite has a hexagonal symmetry and its typical formula can be written as (Ca,Mg,Na2,K2)O.Al2O3.6SiO2.6H2O. Its framework consists of D6R units, arranged in the sequence AABAAC. These Hexagonal prisms are linked by 4-rings and single 6-rings (cancrinite cages). The structure contains “supercages” that are supported by the columns formed by cancrinite units and the hexagonal prisms (see Figure 2). The result is a complex pore system interconnected by the 8-rings. The sorption cavity is the “supercage”. Molecules have access to this cavity through the six elliptical openings formed by 8-rings.

Zeolite A has a cubic symmetry and its typical formula is Na2 O.Al2O3.2SiO2.4.5H2O. Its framework can be understood as truncated octahedral units linked by D4R units (see Figure 3). The result is a large spherical cavity with twelve 4-rings, eight 6-rings, and six 8-rings. The three-dimensional porous structure is originated by the linkage of the large cavities through the a-rings.

ZK-5 structure is close to that of zeolite A. It consists of truncated cubooctahedra linked by D6R units. Its typical formula is (R,Na2)O.Al2O3 .4.0-6.0SiO2.6H2O, where R is [1,4-dimethyl-1,4-diazoniacyclo (2.2.2) octane)2+.

Some zeolites possess sorption properties close to those of small-pore zeolites, even when they have pore openings exceeding 0.45 nm. This is due to blockage of pores by large cations or structural dislocations. Offretite, zeolite T, ZSM-34, and clinoptilolite belong to this category.

Offretite is very closely related to erionite, but presents two important differences. The first one is that the D6R unit’s layer sequence is AABAAB. The second one is that the cancrinite cages are no longer rotated by 60° with respect to one another as in erionite. This results in the formation of large channels with a free diameter of about 0.65 nm. Thus, offretite has a complex porous structure that can be understood as the composition of two pore structures: one similar to erionite and another one with large pore openings. Few dislocations or obstructions suffice to prevent access to the wide channels. For example, when offretite is synthesized in the presence of Me4NOH, to obtain tetramethylammonium (TMA)-offretite, the bulky molecules of this compound are placed in the large channels preventing even the sorption of linear molecules as n-hexane. The partial substitution of TMA by potassium cations enables the molecular sieve to adsorb n-hexane. The removal of TMA cations by heating or exchange with ammonium cations leads to solids with higher accessibility for bulky molecules such as cyclohexane and m-xylene. In other cases, due to the similarity of offretite and erionite, it is possible to have solids in which some portions of the crystal are erionite while others consist of offretite. The erionite-offretite intergrowth leads to the obstruction of the large channels. Zeolite T and ZSM-34 are examples of this.

Zeolite T has a hexagonal symmetry and its typical formula is 0.3Na2O.0.7K2O.Al2O3 .6.9SiO2.7.2H2O. This zeolite is a disordered intergrowth of offretite and erionite. In zeolite T, the more open structure of offretite is interspersed at intervals with the tighter erionite units. In this way, the large pores of offretite are blocked by the 6-rings of erionite. A single unit cell of erionite at the end of the large pore of offretite is enough to have a complete blockage of it. Even though erionite is only a small portion of zeolite T structure, the erionite cages control the diffusion path by forcing the molecules to pass through the a-rings •

ZSM-34 seems to be another example of offretite-erionite intergrowth. The highest SiO2/Al2O3 molar ratio that has been reported for this zeolite is about 15.

Clinoptilolite has a monoclinic symmetry and its typical formula is (Na2,K2)O.Al2O3 .10SiO2.8H2O. Its porous structure may resemble mordenite. The dimensions of its channels are 0.75 x 0.30, 0.43 x 0.33 and 0.31 x 0.33 nm. However, the apparent pore size of the nondecationized clinoptilolite is close to that of small-pore zeolites. Its pore size can be enlarged by decationization and dealumination.

Silicoaluminophosphates (SAPOs) are a new generation of crystalline microporous molecular sieves. They have been discovered by incorporating Si into the framework of the aluminophosphates (AlP04) molecular sieves. Several small-pore SAPO crystals have been synthesized. SAP0-17, SAP0-34 and SAP0-44 have pore openings of about 0.43 nm. SAP0-17 has an erionite-like structure, while SAP0-34 and SAP0-44 have a chabazite-like structure.

An interesting fact is that SAPO molecular sieves show mild acidity, while chabazite and erionite are strong acids in the protonic form (see Table 2).

In SAPO crystals the concentration of Bronsted acid sites increases as the Si/Al-ratio is raised. This is the opposite of what is accepted for zeolites. It may be explained on the assumption that a SAPO crystal is obtained by silicon substitution into a hypothetical aluminophosphate framework. The predominant mechanism appears to be silicon substitution by phosphorus, but it is also possible that a substitution of two silicons by an aluminum plus a phosphorus takes place. The first mechanism leads to SAPO crystals having frameworks with a net negative charge that are potential Bronsted acid sites • Thus, small-pore SAPO have porous and crystalline structures similar to those of small-pore zeolites but different acidic properties.

New crystalline microporous molecular sieves have been synthesized by incorporating other elements into the AlP04 framework. Some of these elements are Co, Be, Mn, and Fe. They carry the generic names MAPO and MeAPO molecular sieves. The acidity of MAPO and MeAPO molecular sieves can vary widely (see Table 2).

2.2 Methanol Conversion to Olefins. – Chabazite, erionite, zeolite T, and ZK-5 have been used by Chang et al. for the conversion of methanol into olefins. The C2-C4 olefin concentration in the hydrocarbon fraction was always less than 60 wt% at 100% methanol conversion. It follows from Table 3 that the hydrocarbon fraction becomes richer in c2 -c4 olefins as the conversion of methanol decreases. That is because the conversion of olefins to paraffins is lower. Hydrocarbon fractions with more than 80 wt% of C2-C4 olefins were attained with a dealumninated H-erionite, but the conversion of methanol was very low.

Cartlidge and Patel have investigated the properties and behavior of dealuminated chabazite-type catalysts in methanol conversion. They used different treatments in order to lower the aluminum content of chabazite zeolites. After synthesis, the chabazite crystals were ammonium ion-exchanged and steamed at 550, 630, and 720 °c. The samples steamed at 720 °c were treated with hot aqueous concentrated hydrochloric acid. Chabazite-type catalysts with SiO2/Al2O3 molar ratios between 5 and 21 were prepared this way. The surface areas of the samples increased after ammonium ion-exchange and steaming at 550 °c. The steaming at 720 °c lowered the zeolite surface areas by 30% and led to partial crystal destruction. A subsequent treatment in aqueous hydrochloric acid improved the crystallinity and surface area. Samples with SiO2/Al2O3 molar ratios up to 21 were attained in these cases. These samples gave molar selectivities toward c2-c5 olefins of up to 94% at 100% methanol conversion (see Table 4). After a few hours on stream, selectivities to methane and dimethyl ether increased, indicating deactivation by coke in the intracrystalline structure of the catalysts.

The acidic properties of chabazite and erionite may also be modified by cation-exchange. Wunder and Leupold have used a Mn-exchanged mixture of chabazite and erionite. A 70/30 vol% methanol-water mixture was fed at 400 °c and led to a hydrocarbon fraction containing 37.1% of ethylene, 26.5% of propylene, and 2.7% of butene at 90% methanol conversion. The selectivity toward ethylene and propylene is higher than when the normal H-form of these zeolites is used (see Table 3). This may be due to the fact that the exchange with polycharged cations reduces the strength of the acid sites responsible for the conversion of light olefins into paraffins and oligomers.

Singh et al. have also used cation-exchanged chabazites. They modified a commercial chabazite by ion-exchange with ammonium hydroxide and a rare earth chloride mixture. The yields of ethylene, propylene, and propane were 35, 30, and 25%, respectively. The deactivation by coke was fast. They claim that the initial activity can be retained for several hours during reaction if carbon disulfide is fed along with methanol at concentrations up to 3000 ppm. Previously, Froment et al. also reported the use of CS2 to prevent deactivation by coking in cracking catalysts. They also claim that initial catalytic activity can be reestablished by regeneration in air at 750 to 825 K.

Klyueva et al. have investigated the acidic properties of erionite modified by isomorphous substitution of a3+, Ga3+, and Fe3+ by Si4+ and Al3+. The incorporation of these elements in the aluminosilicate framework led to the generation of new acid centers. These acid centers have a lower concentration of aluminum cations than aluminosilicates, leading to samples with lower acidity. Consequently, the rate of reactions involving hydrogen transfer, like olefin conversion into paraffins, was lower on isomorphous-substituted erionite samples. Table 5 shows that this enhanced the selectivity toward light olefins. The production of aromatics may be explained by the presence of strong acid sites on the external surface of the catalysts.

Tsitsishvili et al. have carried out experiments of methanol conversion on B-offretite and TMA-offretite. TMA-offretite zeolites were calcined at 200 and 450 °C. H-offretite zeolites were prepared by ammonium ion-exchange and then calcined at 300 and 450 °C. TMA-offretite calcined at 200 °c was inactive, probably because the channels are blocked by the large Me4N+ ions so that the acid sites become inaccessible for methanol molecules. A hydrocarbon fraction containing principally propylene, propane, n-butane, and n-butene was obtained in the cases of TMA-offretite and H-offretite calcined at 450 °C. At reaction temperatures lower than 210 °C only dimethyl ether was detected. H-offretite zeolites are active in the isomerization of xylenes, indicating that the removal of TMA-cations enlarged the pore opening.

Ceckiewicz studied the methanol conversion between 25 and 400 °C on zeolite H-T (SiO2/Al2O3 molar ratio of 7.4) with different degrees of decationization and dealumination. By Fourier transform infrared spectroscopy (FT-IR) he found that CH30H molecules interact principally with the most active 3,600 cm-1 OH groups Corresponding to Al-OH bonds. No transformation of methanol occurred at 25 °c. The dehydration of methanol to dimethyl ether was the most important reaction between 200 and 300 °C. The conversion of methanol to hydrocarbons was evident at temperatures higher than 300 °c. The conversion of methanol was less than 100% in all the cases. After 1.5 min on stream, a sample decationized to 43% and dealuminated to 18% led to a methanol conversion of 84.5% at 400 °c and O .11 g methanol/g catalyst ·min. The products were ethylene (13 .1%), ethane (1.1%), propylene (29.8%), propane (7.6%), C4-C5 hydrocarbons (18.3%), and dimethyl ether (14.6%). The hydrocarbon yields and methanol conversion decreased markedly with time on stream. The conversion of methanol dropped from 85 to 30% after only 30 min on stream.

(Continues…)Excerpted from Catalysis Volume 9 by J.J. Spivey. Copyright © 1992 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.