Catalysis: Volume 15

Catalysis: Volume 15

by Burtron H Davis
     
 

There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series

Overview

There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series provides systematic and detailed reviews of topics of interest to scientists and engineers in the catalysis field. The coverage includes all major areas of heterogeneous and homogeneous catalysis and also specific applications of catalysis such as NOx control kinetics and experimental techniques such as microcalorimetry. Each chapter is compiled by recognised experts within their specialist fields and provides a summary of the current literature. This series will be of interest to all those in academia and industry who need an up-to-date critical analysis and summary of catalysis research and applications. Catalysis will be of interest to anyone working in academia and industry that needs an up-to-date critical analysis and summary of catalysis research and applications. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading experts in their specialist fields, this series is designed to help the chemistry community keep current with the latest developments in their field. Each volume in the series is published either annually or biennially and is a superb reference point for researchers. www.rsc.org/spr

Editorial Reviews

Booknews
Specialist Periodical Reports provide systematic and detailed review coverage of progress in the major areas of chemical research, written by experts in their specialist fields, for the benefit of active research chemists. This volume presents five topics of current interest in the field of catalysis: toward supported oxide catalysts via solid-solid wetting; model catalyst studies of supported metal sintering and redispersion kinetics; techniques for measuring zeolite acidity; applications of raman spectroscopy to heterogeneous catalysis; and oxidative coupling of methane. The detailed table of contents substitutes for an index. Annotation c. Book News, Inc., Portland, OR (booknews.com)

Product Details

ISBN-13:
9780854042197
Publisher:
Royal Society of Chemistry, The
Publication date:
12/31/2000
Series:
Specialist Periodical Reports Series, #15
Pages:
228
Product dimensions:
6.00(w) x 9.00(h) x 0.69(d)

Read an Excerpt

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



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.

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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-).

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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.

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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:

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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.

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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).

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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:

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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.

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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.

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

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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.

Meet the Author

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.

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