New Materials for Catalytic Applications

By Vasile I. Parvulescu and Erhard Kemnitz

New Materials for Catalytic Applications proposes the use of both new and existing materials for catalytic applications, such as zeolites, metal oxides, microporous and mesoporous materials, and monocrystals. In addition, metal-oxides are discussed from a new perspective, i.e. nano- and photocatalytic applications.

The material presents these concepts with a new focus on strategies in synthesis, synthesis based on a rational design, the correlation between basic properties/potential applications, and new catalytic solutions for acid-base, redox, hydrogenation, photocatalytic reactions, etc.

• Presents organometallic concepts for the synthesis of nanocatalysts
• Provides a synthesis of new materials following the fluorolytic sol-gel concept
• Covers electronic and photocatalytic properties via synthesis of nano-oxide materials
• Details the nature of sites in MOFs generating catalytic properties immobilization of triflates in solid matrices for organic reactions

• Language: English
• Publisher: Elsevier Science
• Release date: Jan 28, 2016
• ISBN: 9780444635884

Vasile I. Parvulescu

Vasile I. Parvulescu is Founder and director of the Catalysts and Catalytic Processes Research Center in Romania and Professor at Bucharest University, he is an internationally well-known expert in catalysts and catalytic processes, particularly for environmental protection and green chemistry. He was an "Alexander von Humboldt" fellowship in 1997 and a visiting professor at several educational institutions worldwide. He is a member of the SUNERGY/SUNER-C Board and of several Permanent International Conferences and catalysis associations. He made many plenary and keynote lectures worldwide. He authorises several books, over 400 publications, and various patents.

Book preview

New materials for catalytic applications

Edited by

Vasile I. Parvulescu

Erhard Kemnitz

Table of Contents

Cover

Title page

Copyright

List of Contributors

Chapter 1: Double Metal Cyanides as Heterogeneous Catalysts for Organic Reactions

Abstract

1. Introduction

2. Epoxide Ring-Opening Reactions

3. Esterification, Transesterification, and Hydrolysis Reactions

4. Addition and Coupling Reactions

5. Oxidation Reactions

6. Conclusions

Chapter 2: Metal Organic Frameworks as Catalysts for Organic Reactions

Abstract

1. Introduction

2. Stability Issues

3. Zeolites Versus MOFs as Heterogeneous Catalysts

4. Active Sites in MOFs

5. MOFs for Oxidation Reactions

6. C–C Coupling Reactions

7. Hydrogenation Reactions

8. Concluding Remarks

Chapter 3: On the Use of Organometallic Chemistry Concepts for the Synthesis of Nanocatalysts

Abstract

1. Introduction

2. Methods for the Synthesis of Metal Nanoparticles and their Characterization

3. Selected Examples of Metal Nanoparticle Synthesis and their Application in Catalysis

4. Conclusions and Outlook

Acknowledgments

Chapter 4: Catalysts on Metallic Surfaces: Monoliths and Microreactors

Abstract

1. Overview

2. The Metal Choice

3. Catalyst Coating

4. Catalytic Layer: Influence of the Metallic Substrate and Deposition Procedure

5. Transport Phenomena in Structured Catalysts

6. Conclusions

Acknowledgments

Chapter 5: Modulation Excitation Spectroscopy with Phase-Sensitive Detection for Surface Analysis

Abstract

1. Introduction

2. ATR-IR/MES

3. DRIFTS/MES

4. PM-IRRAS/MES

5. EXAFS/MES

6. Conclusions

Chapter 6: Nanoscaled Metal Fluorides in Heterogeneous Catalysis

Abstract

1. Introduction

2. Sol–Gel Synthesis of Nanoscaled Metal Fluorides

3. Metal Fluorides as Exciting Heterogeneous Catalysts

4. Conclusions

Chapter 7: Heterogeneous Catalysts Used for Large-Scale Syntheses of Selected Chlorohydrocarbons and Fluorohydrocarbons: Fluorinated Chromia and eta-Alumina

Abstract

1. Introduction

2. Fluorinated Chromia and the Montreal Protocol and its Successors and the Kyoto Protocol

3. Production of Chloromethane: A Green Perspective

Chapter 8: Mesoporous Materials Incorporating Metal Triflates

Abstract

1. Importance of Triflates in Catalysis

2. Triflates of Water-Sensitive Metals

3. Triflates of Water-Insensitive Metals

4. Importance of Materials Incorporating Triflates

5. Nature of the Acid Sites in Metal Triflates and Triflimidates

6. Materials Containing Metal Triflates

7. Application of Materials Incorporating Triflates in Catalytic Processes

8. Conclusions

Chapter 9: Nitrogen-Doped Carbon Composites as Metal-Free Catalysts

Abstract

1. Introduction

2. Chemical Functionalization and Doping of Carbon Nanomaterials

3. Method for Synthesis of Nitrogen-Containing Carbon Nanotubes

4. Properties of CNTs with Different Functionalization Groups on the Surface

5. Catalytic Process Applications

6. Conclusions and Outlook

Acknowledgments

Chapter 10: Metal Sulfides: Novel Synthesis Methods and Recent Developments

Abstract

1. Introduction

2. The Promotion Effect in Transition Metal Sulfides

3. Preparation of Transition Metal Sulfides

4. Structure and Properties of Transition Metal Sulfide Catalysts

5. Structure–Function Relationships in Sulfide Catalysts

6. Surface Composition

7. Theoretical Approach to the Comprehension of TMS

8. Hydrodesulfurization Mechanisms

9. Novel Applications

10. Conclusions

Acknowledgments

Index

Copyright

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ISBN: 978-0-444-63587-7

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List of Contributors

Josep Albero,     Instituto Universitario de Tecnología Química CSIC-UPVC, Univ. Politecnica de Valencia, 46022 Valencia, Spain

Pedro Amoros,     Department of the Inorganic Chemistry Institut de Ciencia dels Materials, Universitat de Valencia, Valencia, Spain

Tuğçe Ayvali,     Laboratoire de Chimie de Coordination, CNRS; Université de Toulouse, UPS, INPT, LCC, Toulouse Cedex, France

Housseinou Ba,     Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 CNRS-Université de Strasbourg, Strasbourg Cedex 02, France

Gilles Berhault,     Institut de Recherches sur la Catalyse et l’Environnement (IRCELYON), CNRS, Université Lyon I, Villeurbanne, France

M.A. Centeno,     Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Sevilla, Spain

Simona Coman,     Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, Bucharest, Romania

Dirk De Vos,     Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems, University of Leuven, Belgium

M.I. Domínguez,     Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Sevilla, Spain

Cuong Duong-Viet,     Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 CNRS-Université de Strasbourg, Strasbourg Cedex 02, France

Ha-Noi University of Mining and Geology, Dong Ngac Tu Liem, Ha-Noi, Vietnam

Hermenegildo García,     Instituto Universitario de Tecnología Química CSIC-UPVC, Univ. Politecnica de Valencia, 46022 Valencia, Spain

Pascal Granger,     Unité de Catalyse et de Chimie du Solide (UCCS), UMR 8181 CNRS-Université de Lille-1, Bâtiment C3, Université Lille 1, Villeneuve d’Ascq Cedex, France

Erhard Kemnitz,     Chemistry Department, Humboldt-Universität zu Berlin, Berlin, Germany

O.H. Laguna,     Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Sevilla, Spain

David Lennon,     School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, Scotland, UK

Yuefeng Liu,     Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 CNRS-Université de Strasbourg, Strasbourg Cedex 02, France

Jean-Mario Nhut,     Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 CNRS-Université de Strasbourg, Strasbourg Cedex 02, France

J.A. Odriozola,     Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Sevilla, Spain

Vasile I. Parvulescu,     Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, Bucharest, Romania

Cuong Pham-Huu,     Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 CNRS-Université de Strasbourg, Strasbourg Cedex 02, France

Karine Philippot,     Laboratoire de Chimie de Coordination, CNRS; Université de Toulouse, UPS, INPT, LCC, Toulouse Cedex, France

Ryan M. Richards,     Department of Chemistry and Geochemistry, Colorado School of Mines and National Renewable Energy Laboratory, Golden, CO, USA

Sarah Shulda,     Department of Chemistry and Geochemistry, Colorado School of Mines and National Renewable Energy Laboratory, Golden, CO, USA

Jean-Philippe Tessonnier,     Department of Chemical and Biological Engineering, Iowa State University, Ames, IA, United States

Lai Truong-Phuoc,     Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), UMR 7515 CNRS-Université de Strasbourg, Strasbourg Cedex 02, France

Pieterjan Valvekens,     Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems, University of Leuven, Belgium

John M. Winfield,     School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow, Scotland, UK

Chapter 1

Double Metal Cyanides as Heterogeneous Catalysts for Organic Reactions

Pieterjan Valvekens

Dirk De Vos    Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems, University of Leuven, Belgium

Abstract

Double metal cyanides (DMCs) are inorganic solids containing two or more metal ions linked by cyanides. They have been used since a long time for the industrial epoxide polymerization, but their potential as Lewis acid catalysts for other organic reactions has been far less studied. This chapter provides an overview of the current knowledge on the role DMCs can play in organic transformations.

Keywords

double metal cyanides

Prussian blue analogues

Lewis acid

ring-opening polymerization

(trans)esterification

hydroamination

aldol condensation

Prins condensation

Chapter Outline

1 Introduction 1

2 Epoxide Ring-Opening Reactions 3

2.1 Ring-Opening Polymerization of Epoxides 3

2.2 Copolymerization of CO2 and Epoxides 4

2.3 Cycloaddition of CO2 to Epoxides 5

2.4 Ring-Opening of Epoxides with Amines 5

3 Esterification, Transesterification, and Hydrolysis Reactions 6

3.1 Transesterification Reactions 6

3.2 Esterification Reactions 7

3.3 Hydrolysis Reactions 8

3.4 Polyester Synthesis 8

4 Addition and Coupling Reactions 8

4.1 Hydroamination Reactions 8

4.2 Aldol Condensation 9

4.3 Prins Condensation 10

5 Oxidation Reactions 11

6 Conclusions 11

References 11

1. Introduction

Double metal cyanides (DMCs), also known as Prussian blue analogues, are a class of molecular salts built up of a crystalline metal cyanide framework [1]. As their name suggests, DMCs feature two different metal centers, one coordinating via the carbon atom of the CN− ligand and the other via the nitrogen atom. DMCs have a general structural formula M¹u[M²(CN)n]v. A typical selection of M¹ metals is Zn(II), Fe(II), Co(II), or Ni(II), whereas Co(III), Fe(II), Fe(III), Cr(III), or Ir(III) is often used as M² [2]. Typical u–v combinations are 3–2, 1–1, 4–3, etc.; depending on the valencies of the metal ions, n can be 4, 6, or 8, although 6 is most common. In practice, however, a number of additives, called complexing agents (CAs) and co-complexing agents (co-CAs), as well as an excess M¹Xw-salt, are added to the synthesis mixtures of DMCs to increase their catalytic activity, and the structures are usually hydrated. The true structural formula of an active DMC is therefore more complex and can formally be written as follows: M¹u[M²(CN)n]v · xM¹Xw · yCA · zH2O. Scheme 1.1 depicts schematically a DMC with M¹ = Zn(II), M² = Co(III), CA = tert-butanol, and co-CA = ROH. Throughout this chapter, the DMCs with M¹ and M² as constituting metal ions will be abbreviated as M¹-M²-DMC. The crystal structure of this Zn-Co-DMC is shown in Figure 1.1. Due to the charge imbalance between Zn²+ and [Co(CN)6]³−, one-third of all hexacyanocobaltates are absent in the cubic lattice, creating vacancies in the coordination spheres of the Zn²+ ions.

Scheme 1.1   Schematic Representation of a Zn-Co-DMC with tert-Butanol and ROH as Complexing and Co-Complexing Agents, Respectively.

Figure 1.1   Cubic Crystal Structure of a Zn-Co-DMC.

DMCs, and especially Prussian blue (Fe4[Fe(CN)6]3 · 14H2O), are among the oldest known coordination solids. Their use as dye dates back to the beginning of the eighteenth century [3]; their potential as catalysts for industrial applications was already recognized in the 1960s [4–6] and they have been industrially applied in the polymerization of epoxides ever since. Studies even suggest that DMCs could have played a role in chemical evolution and the origin of life [7,8]; yet to date, these compounds and their catalytic properties remain relatively unknown by the broad chemical community. Nonetheless, the past decade has witnessed the emergence of new catalytic applications and an increase of the reaction scope of DMCs. This chapter will first highlight the classic application of DMCs in polymerization reactions and will then continue with their use as catalysts in other organic reactions for the production of, for example, biofuels or chemicals.

2. Epoxide Ring-Opening Reactions

2.1. Ring-Opening Polymerization of Epoxides

The most important chemical process involving DMCs as catalysts is the ring-opening polymerization (ROP) of propylene oxide (Scheme 1.2). The catalytic activity of DMCs in these reactions was first discovered and industrially applied by the General Tire & Rubber Company during the 1960s [4–6]. Whereas early versions of the Zn-Co-DMC catalysts employed glyme as a CA, improved catalysts followed using tert-butanol as a CA, sometimes in combination with other co-CAs [9]. The high activity of these improved catalysts led to a decrease of the catalyst concentration (below 30 ppm), which rendered the expensive isolation of the catalyst from the product stream obsolete. While these materials are suitable for the polymerization of pure propylene oxide and mixtures of propylene and ethylene oxide, they are not suitable for the polymerization of pure ethylene oxide.

Scheme 1.2   Ring-Opening Polymerization of Propylene Oxide.

(*) End group.

A major benefit of the use of DMCs compared with that of conventional base-catalyzed systems in these reactions is that they produce high-molecular-weight polymers, with very narrow molecular weight distributions, very low levels of unsaturation, and lower viscosities. The downside to using DMCs however is that they require an activation period at elevated temperatures in the presence of initiator molecules, which causes an induction period at the start of the polymerization process [10]. After this induction period, the polymerization process proceeds very rapidly, making it a very important parameter to control. The CAs play an important role in the catalytic activity of the DMCs and in the activation procedure. The zinc sites with bound CAs can be seen as dormant catalytic sites [11]. After exchange of the CA, preferably tert-butanol, with initiator molecules such as poly(propylene glycol), the dormant sites are converted into the very active catalytic sites.

To decrease the induction period, DMCs have been combined with quaternary ammonium salts, montmorillonite clays modified with quaternary ammonium salts, or imidazolium ionic liquids [10,12,13]. Such additives were found to decrease the induction period by acting as nucleophiles in the catalytic cycles. As such, these compounds (1) facilitate the formation and activation of the zinc–monomer bond and (2) make the zinc–monomer bonds more active for polymerization, and/or they stabilize the polymerization centers and prevent their decomposition [10,13].

Additionally, the catalytic activity of Zn-Co-DMCs in this reaction was found to increase upon the addition to the synthesis mixture of excess ZnCl2 [14], generally regarded as the most effective precursor for the preparation of these catalysts. This addition is believed to decrease the crystallinity and crystal sizes of the DMC particles, similar to the impact of CAs on DMC synthesis. This results in a higher dispersion of active sites, increasing the catalytic activity of the materials. Doping DMCs with CaCl2 has proven to be an alternative strategy to increase their catalytic activity; yet, to date, no explanation for this phenomenon has been proposed [15].

2.2. Copolymerization of CO2 and Epoxides

DMC materials have also been widely studied for the copolymerization of CO2 and epoxides (Scheme 1.3), a reaction that leads to the long-term fixation of CO2 in valuable products [16,17].

Scheme 1.3   Copolymerization of Epoxides and CO2.

For polycarbonate, m = 1; for polyethercarbonate, m > 1; (*) end group.

A series of studies have been performed to gain insight into the effects of the different constituting elements and synthesis additives on the overall catalytic activity. Zhang et al. studied the influence of the metal ion (Co) and the role of the cyanide ligand by preparing different Zn-M-DMCs with various cyano-metal salts (with the number of cyanide ligands ranging from 4 to 8) [18]. Additionally Zn-Co-DMCs were prepared in which one in every six cyanide ligands was exchanged for Cl−, Br−, I−, NO2−, or N3−. Zn-Co-DMC was found to have the highest catalytic activities for this reaction, compared with other materials. Comparison of Zn-Co-DMCs with those in which the cyanide ligands are partially replaced by other anions also showed that the lower activities of these materials are likely due to a distortion of the octahedral coordination structure and the electron-donating effect resulting from the displacement of one cyanide ligand. Variation of the synthesis procedure of Zn-Co-DMC has also led to more insight into the structure–activity relationship for these materials. Sebastian and Srinivas found that an active Zn-CO-DMC was characterized by a high density of Lewis acid sites, a moderate to low crystallinity, low crystal symmetry, chloride anions, and CAs [19]. Although not all studies agree on the necessity of a co-CA to obtain an active catalyst, the choice of co-CA can exert an influence on the activity and selectivity of the DMC material [2]. The presence of chloride in the structure improved the interaction between the active site and the substrate by increasing the acidity of the catalysts. Dienes et al. also showed that by coprecipitating a Zn-Co-DMC with SiO2, a hybrid material featuring a semicrystalline DMC closely associated with excess zinc and silica can be obtained [16]. This material was highly active for the copolymerization of styrene oxide and CO2 and, moreover, the activity, selectivity toward polymer formation, and CO2 incorporation could be tuned by varying the synthesis pH.

2.3. Cycloaddition of CO2 to Epoxides

It is clear from the numerous accounts in literature that DMCs can efficiently catalyze the copolymerization of CO2 and epoxides. DMCs can however also be used to develop systems that selectively catalyze the CO2 cycloaddition rather than the copolymerization (Scheme 1.4) as is illustrated by the work of Dharman et al. [20]. By itself, a Zn-Co-DMC is an efficient catalyst for the copolymerization reaction. However, the addition of a quaternary ammonium salt to the reaction mixture switches the selectivity of the catalytic system toward the exclusive formation of the cyclic carbonate. The quaternary ammonium ion plays two important roles in the catalytic system: it accelerates the diffusion of CO2 into the reaction mixture and it favors a backbiting mechanism. As such, it hinders the growth of the polymer chain and it enables the selective cyclic carbonate production. Although most zinc-containing catalysts for this reaction are very sensitive toward water, Wei et al. have shown that, for example, the combination of Zn-Co-DMC with CTAB (cetyltrimethylammonium bromide) could even use water-contaminated epoxides as an epoxide feed [21].

Scheme 1.4   CO2 Cycloaddition to Styrene Oxide.

2.4. Ring-Opening of Epoxides with Amines

The ring-opening of epoxides with amines forming β-amino alcohols (Scheme 1.5) is a final example of an epoxide reaction catalyzed by DMCs [22]. Saikia et al. found that the Lewis acid Zn²+ sites in Zn-Fe(II)-DMC exhibit a good catalytic activity in these reactions at near-ambient, solvent-free conditions. The DMC catalysts display a high regioselectivity for the attack of primary amines at the most hindered position of the epoxide, as illustrated by reactions of aniline or butylamine with, for example, epichlorohydrin. The intrinsic activity of the catalyst rivals with and often exceeds that of many known catalysts for this reaction.

Scheme 1.5   Ring-Opening of Epoxides with Amines.

3. Esterification, Transesterification, and Hydrolysis Reactions

3.1. Transesterification Reactions

The first report on the use of DMCs in transesterification reactions came from Srivastava et al. in 2006 [23]. In this account, dimethyl carbonate was synthesized from the transesterification of propylene carbonate with methanol (Scheme 1.6), using a Zn-Fe(II)-DMC as a catalyst. Similar to the catalytic activity of DMCs in ROP reactions [24], terminal Zn²+ cations at the outer surface of the catalysts are believed to act as the active sites. A Fourier transform infrared spectroscopy (FTIR) study of pyridine adsorbed on the catalysts as well as an NH3-TPD (temperature programmed desorption) study confirmed that Lewis acid Zn sites are the only plausible active sites in the material. Furthermore, catalysts synthesized in the presence of both CAs and co-CAs featured a higher specific surface area as well as higher acid site density. Use of these materials as catalysts therefore also gave higher conversions toward dimethyl carbonate. Finally, these DMCs were also shown to act as good catalysts for the transesterification of, for example, dimethyl carbonate with ethanol affording diethyl carbonate.

Scheme 1.6   Transesterification of Propylene Carbonate.

Later that year, the same groups reported the use of DMCs as catalysts for the production of biodiesel and biolubricants from vegetable oils (Scheme 1.7) [25]. Various M-Fe(II)-DMCs (M = Zn²+, Cu²+, Ni²+, Co²+) and one Zn-Fe(III)-DMC were characterized and closely examined for their catalytic activity in transesterification reactions. A Zn-Fe(II)-DMC synthesized in the presence of both CAs (tert-butanol) and co-CAs (preferably of high molecular weight) showed the highest catalytic activity. The catalytic activity of the material could again be attributed to coordinatively unsaturated Zn²+ ions in the structure of the catalyst and the number of these sites could be maximized using surfactant molecules in the catalyst synthesis. The hydrophobicity of the surface of this material resulted in a high tolerance toward water and free fatty acids present in the feed oils. As such, in contrast to other solid or homogeneous catalysts, the Zn-Fe(II)-DMC was found to be active for the simultaneous transesterification of triglycerides and the esterification of free fatty acids present in the feed mixture [25,26]. On a similar note, the use of Zn-Fe(II)-DMC as a catalyst for the production of lubricant oils by reacting methyl oleate with long-chain alcohols (C8–C12) also resulted in good product yield and avoided the production of undesired side products typically found when using other acid catalysts [27].

Scheme 1.7   Transesterification of Vegetable Oils.

Biodiesel: R2 = C1–C2; biolubricant: R2 = C6–C8.

Yang et al. studied the effect of adding transition metals (zirconium or manganese) or rare earth metals (lanthanum or cerium) to Zn-Fe(II)-DMC on the catalytic activity of the resulting materials in the transesterification of rapeseed oil with methanol [28]. The catalytic activity was found to increase on the addition of these metals to the synthesis mixture. The addition of up to 1 wt% of metal did not noticeably affect the molecular formula of the resulting materials, but the catalysts did show a smaller particle size, a lower crystallinity, and an increased surface area that could explain the higher activity of the catalysts. Highest catalytic activity was observed when adding 1 wt% of La(NO3)3.

3.2. Esterification Reactions

Several examples also exist on the use of DMCs as catalysts for esterification reactions. Zn-Fe(II)-DMCs have, for example, been reported to catalyze the esterification of free fatty acids with alcohols [29–31]. Comparison with other solid acid catalysts, such as sulfated zirconia, Al-MCM-41, or zeolite H-β, has shown that these alternative materials typically show faster reactions and lower apparent activation energies compared with DMCs due to their higher acid strength, specific surface area, and pore diameters. However, in contrast to these materials, the apparent activation energy for the esterification reactions on the Zn-Fe(II)-DMC catalyst was found to decrease with increasing chain length of the fatty acid. Moreover, the relative adsorption of methanol and fatty acid compared with that of water was higher on the DMC, indicating that the surface hydrophobicity is an important additional parameter for the esterification reaction of long-chain fatty acids.

Another example of a DMC-catalyzed esterification reaction is the esterification of free fatty acids with glycerol [32]. This reaction again used Zn-Fe(II)-DMC as a catalyst; yet the activity of the material could be tuned by varying the synthesis temperature. As such, DMCs with different acidities (as evidenced by NH3-TPD), crystallite sizes (calculated from the Debye–Scherrer equation), particle sizes (studied with scanning electron microscopy (SEM)), and BET (Brunauer, Emmett and Teller) specific surface areas were obtained. The highest catalytic conversions were observed for the catalysts synthesized at elevated temperatures; these materials featured both the highest specific surface area and the largest number of acid sites.

3.3. Hydrolysis Reactions

Besides esterification reactions and transesterification reactions, DMCs were also reported to act as catalysts in the hydrolysis of triglycerides from a variety of vegetable oils and animal fats, resulting in high yields of free fatty acids (Scheme 1.8) [26]. Moreover, comparison with other solid acids showed that Zn-Fe(II)-DMC features a superior catalytic activity compared with Amberlyst 70, SAPO-11, H-β, HY, MoOx/Al2O3, and sulfated zirconia. The superior catalytic activity of the Zn-Fe(II)-DMC has been attributed to its hydrophobic surface that favors the adsorption of the nonpolar triglyceride reactant molecules at the Zn²+ cations of the active site. The reaction time could be further decreased by adding solvents such as N,N-dimethylformamide (DMF), surfactants, or fatty acid product molecules to the reaction mixture as these components increase the miscibility of the oils and water.

Scheme 1.8   Hydrolysis of Triglycerides.

3.4. Polyester Synthesis

The Lewis acidic properties of DMCs not only are limited to catalyzing the (trans)esterification of, for example, fatty acids for the production of lubricants or fuels but also lead to catalytic activity in the formation of polyesters. Sebastian and Srinivas reported the formation of biodegradable, hyperbranched polymers from succinic or adipic acid and glycerol using a Zn-Fe(II)-DMC catalyst [33,34]. Whereas the tetra-coordinated Zn²+ ions in the DMC are thought to be the active Lewis acidic sites in catalysis, the enhanced control over the gelation in the synthesis of these polymers compared with in that of other solid acids was attributed to micro–mesoporosity and surface hydrophobicity of the Zn-Fe(II)-DMC.

Finally, the catalytic activity of DMCs in esterification reactions can be readily combined with their catalytic activity in epoxide ring-opening polymerizations, as was reported by Suh et al. [35]. This study showed that the copolymerization of propylene oxide with cyclic acid anhydrides such as succinic, maleic, or phthalic anhydride, catalyzed by a Zn-Co-DMC, afforded polyester polyols characterized by a moderate molecular weight and narrow polydispersity index.

4. Addition and Coupling Reactions

4.1. Hydroamination Reactions

The use of DMC salts as heterogeneous catalyst for hydroamination reactions, the formal addition of an amine to an olefin or alkyne, was investigated by Peeters et al. [36,37]. In a first study, the catalytic activity of different Zn-Fe(II)-, Zn-Fe(III)-, Cu-Fe(II)-, Cu-Co-, and Zn-Co-DMCs was screened in the reaction of phenylacetylene and 4-isopropylaniline, affording the imine product (4-isopropylphenyl)-(1-phenylethylidene)amine (Scheme 1.9) [36]. The catalytic activity was shown to vary greatly with metal combination, starting metal salts, (co-)CA, and synthesis procedure. Overall, the most active DMC for this reaction was a Zn-Co-DMC synthesized using ZnCl2 as a Zn source and poly(tetramethylene ether) glycol as a co-CA, although DMCs synthesized in the absence of CAs and co-CAs also had a considerable catalytic activity.

Scheme 1.9   Hydroamination of Phenylacetylene with 4-Isopropylaniline.

As the pore size of these DMCs is too small to accommodate the reactant molecules, the catalytic activity of these materials is confined to the outer surface of the particles. The synthesis of nanoparticles using a reverse emulsion technique, therefore, further increased the catalytic activity of the materials. Comparison of these Zn-Co-DMC materials with the Zn²+-exchanged zeolite H-β as a reference material moreover showed that DMC materials were both more active and selective catalysts.

In a follow-up study, Peeters et al. examined the nature of the active sites present in these DMCs [37]. Careful analysis of the crystallinity of these materials showed that the most active catalytic materials all displayed a cubic crystal structure (Figure 1.1, vide supra), matching the cubic Fm3m model as proposed by Mullica et al. [38]. In contrast to the monoclinic phase of certain, less active DMCs, this model demands the presence of vacancies in the octahedral lattice due to the unequal charge of the hexacyanocobaltate anion [Co(CN)6]³− and the Zn²+ cation. This creates open coordination sites associated with Zn²+. The presence of such free coordination sites, acting as Lewis acid catalytic sites, was confirmed by both an FTIR study of pyridine adsorbed onto these sites and by an extended X-ray absorption fine structure (EXAFS) study.

Furthermore, this follow-up study also illustrated the broad applicability of DMCs for the hydroamination of various substrate molecules [37]. Zn-Co-DMCs could catalyze the hydroamination reaction of both aromatic and aliphatic alkynes with aromatic as well as aliphatic amines, a rare trait in heterogeneous hydroamination catalysts.

4.2. Aldol Condensation

Various Zn-Fe(II)-DMCs were investigated as catalysts for the solvent-free synthesis of jasminaldehyde by Patil et al. [39]. The activity of the catalyst increased when CAs (n-, iso-, or tert-butanol) were added to the synthesis mixture of the DMCs. However, when both CAs and co-CAs (Pluronic P123) were present in the synthesis mixture, the catalytic activity was again lowered. It was hypothesized that the presence of co-CAs led to a partial blocking of the coordinatively unsaturated Zn sites at the outer surface, which act as Lewis acid sites in this aldol condensation reaction of benzaldehyde and heptanal. This cross-aldol reaction is essentially in competition with the heptanal self-condensation to 2-pentyl-2-nonenal (Scheme 1.10). The selectivity toward jasminaldehyde was found to increase both with increasing reaction time and with an increasing excess of benzaldehyde in the reaction mixture.

Scheme 1.10   Cross-aldol reaction of benzaldehyde and heptanal (top) and self-condensation of heptanal (bottom).

4.3. Prins Condensation

A final example of DMCs as catalysts for condensation reactions was reported by Jasra and coworkers, who studied the catalytic activity of Zn-Fe(II)-DMCs in the synthesis of nopol via the Prins condensation of β-pinene and paraformaldehyde (Scheme 1.11) [40]. Performing the reaction using DMCs synthesized either in absence or in the presence of different (co-)CAs showed that catalysts prepared with both CAs and co-CAs show higher conversions and nopol selectivities compared with those in which co-CA was absent during the synthesis. Additionally, although the overall conversion was found to be only weakly dependent on the choice of CA, variation of the CA did lead to strong variations in the selectivity toward nopol. Overall, the highest conversion and nopol selectivity were achieved for a catalyst synthesized using tert-butanol and Pluronic P123 as CAs and co-CAs, respectively.

Scheme 1.11   Prins Condensation Reaction of β-Pinene and Paraformaldehyde.

5. Oxidation Reactions

A final example of the broad reaction scope of DMCs as catalysts in organic reactions is their use as oxidation catalysts. García-Ortiz et al. synthesized a number of double (M3[Co(CN)6]2, with M = Cu, Ni, Co, or Mn) and mixed double (MxNy[Co(CN)6]2, with MxNy = MnCu2, Ni1.3Cu1.7, FeCu2, Mn1.2Fe1.8, Fe1.4Ni1.6, Co1.5Ni1.5, Co1.4Cu1.6, or Fe1.4Co1.6) metal hexacyanocobaltates and evaluated their catalytic activity in the aerobic oxidation of oximes to ketones (Scheme 1.12) [41]. Due to the small pore sizes of the materials, the reaction is expected to proceed at the outer surface of each of the materials. Overall, the mixed metal iron copper hexacyanocobaltate was found to be the most active catalyst for these reactions. An in-depth study of the reaction mechanism has shown that the high catalytic activity of FeCu2[Co(CN)6]2 likely arises from the fact that the Lewis acidic Fe²+ ions bind with the oxime, whereas the Cu²+ interacts with oxygen. As such, the DMC catalyst is able to strongly bind both the oxime and the oxidizing reagent, enabling the reaction to proceed.

Scheme 1.12   Oxidation of Cyclohexanone Oxime to Cyclohexanone.

6. Conclusions

Despite the fact that DMCs have been used as dyes for hundreds of years or as industrial catalysts for nearly half a century, they remain a relatively unknown class of heterogeneous catalysts. In the field of epoxide polymerization reactions, this class of materials is already well established as the catalyst of choice. Recent publications have however shown that DMCs offer a much wider applicability and that they can catalyze a broad array of organic reactions, undoubtedly guaranteeing the future applications of these materials in other industrial processes.

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Chapter 2

Metal Organic Frameworks as Catalysts for Organic Reactions

Josep Albero

Hermenegildo García    Instituto Universitario de Tecnología Química CSIC-UPVC, Univ. Politecnica de Valencia, 46022 Valencia, Spain

Abstract

Metal organic frameworks (MOFs) are crystalline porous materials constituted by the coordination of metal ions or clusters of metal ions with bipodal or multipodal rigid linkers. Due to the high metal content, large surface area, and porosity, these materials have attracted a considerable attention as solid catalysts of organic reactions in the liquid phase. In the present chapter, we first describe the main features of MOFs that make these materials very appropriate as catalysts and, then, describe briefly the use of MOFs as catalysts for organic reaction types. The final part summarizes the state of the art in the use of MOFs as catalyst and describes our view on future developments in the field.

Keywords

Green chemistry

heterogeneous catalysis

metal organic frameworks as solid catalyst

aerobic oxidations

heterogeneous cross-coupling reactions

metal nanoparticles embedded inside metal organic frameworks

Chapter Outline

1 Introduction 13

2 Stability Issues 16

3 Zeolites Versus MOFs as Heterogeneous Catalysts 17

4 Active Sites in MOFs 19

5 MOFs for Oxidation Reactions 23

6 C–C Coupling Reactions 28

7 Hydrogenation Reactions 34

8 Conclusion Remarks 37

References 38

1. Introduction

Metal organic frameworks (MOFs) are crystalline porous materials whose structure is constituted by metal ions or clusters of metal ions held in places by coordination with bipodal or multipodal rigid organic linkers [1–4]. Typical organic compounds employed for the synthesis of MOFs are aromatic polycarboxylates that coordinate by electrostatic and coordinative bonds with metal ions or metal clusters. The aromatic ring provides conformational rigidity of the linker, making possible the directionality of the interaction of the carboxylic acids with the metallic nodes. There have been reported MOFs for virtually all the transition metals and also for alkali earth and other nontransition metals [5–7].

From the point of view of heterogeneous catalysis the key characteristics of MOFs are porosity and stability of the crystal lattice due to the relatively strong coordinative and Coulombic attraction between the nodes and the linkers [8]. One of the main characteristics of MOFs is the large surface area and open porosity of these materials [9,10]. Since in heterogeneous catalysis adsorption of substrates and reagents on the solid surface is the first step of the reaction, it comes out that large surface area is generally a prerequisite for highly active catalysts. In fact, many MOFs can reach specific surface area values higher than 1000 m²/g, the record being over 5000 m²/g. Due to these high surface area values MOFs rank at the top of the list of porous materials, having specific surface area much higher than conventional microporous zeolites (typically about 500 m²/g), mesoporous materials (surface area about 1000 m²/g), or even active carbons (surface area above 1000 m²/g) that are materials with high specific area values [10]. This high surface area is a consequence of the large porosity and low framework density of MOFs. In fact, MOFs are the solids with the lowest framework density, meaning lowest mass in the volume of the unit cell.

Another important characteristic of MOFs is the flexibility in the nature of linkers and metals that can be used in their preparation. Besides aromatic polycarboxylates, linkers having nitrogen or phosphorous atoms to coordinate with the metal site have also been reported. The structure of MOFs can be rationalized and even predicted considering the molecular structure and directionality of the organic component and the geometry of the coordination around the metal ion or clusters. In this regard it is frequently possible to anticipate the crystal structure of MOFs knowing the constituent metal nodes and the geometry of the linker [11]. In this way it has been demonstrated that very often replacement of an organic linker by other with the same directionality but larger molecular size results in the formation of series of isostructural MOFs with the same crystal lattice but increasing the dimension of the unit cell according to the size of the organic linker. Scheme 2.1 illustrates a possible structure of a MOF showing the two building units.

Scheme 2.1   Pictorial Illustration of Structure of a MOF Showing the Two Constitutive Building Blocks.

The importance of MOFs in heterogeneous catalyst derives from the consideration that most of the transition metal ions have catalytic activity as Lewis acids, in redox reactions and in coupling transformations. Very often these metal ions are used dissolved in a convenient solvent as homogeneous phase catalyst. However, from the point of view of the recovery of the catalyst from the reaction products it is very convenient to use heterogeneous catalyst that can be separated from the reaction mixture by filtration or other physical process. In addition, the use of heterogeneous catalyst makes easy to develop continuous flow reactions as well as the reuse of the catalyst in subsequent runs. Therefore, instead of using dissolved transition metal salts it is frequently advantageous to use insoluble solids containing the appropriate transition metal. Thus, it comes out that the MOFs can be the equivalent of soluble transition metal salts to perform heterogeneous catalysis. As already commented, besides the metal content characteristic of MOFs, the high porosity allowing diffusion of reagents and substrates through the internal voids of MOFs and the large surface area are other two characteristic features highly wanted in heterogeneous catalysis.

One prerequisite that has been considered as necessary for a MOF to be used as heterogeneous catalyst is that the metal ion or metallic clusters acting as nodes in the MOF structure should have a coordinately unsaturated or exchangeable position [12]. It is also common that solvent molecules form part, in addition to the organic linkers, of the coordination sphere of metal ions in MOFs. These solvent molecules can be removed by appropriate activation procedures, normally heating for a certain period under reduced pressure. This makes available free coordination positions around the metal ions that can later interact with substrates or reagents. As commented later on, another possibility that is being increasingly considered is that metal ions in structural defects are capable to act as active sites [13]. For instance, in the case of MIL-53(Al) the nodes are constituted by Al³+ ions octahedrally coordinated to three carboxylate groups of three different benzene carboxylate linkers. Therefore, in this structure there are no free coordinative positions around Al³+ and, accordingly, MIL-53(Al) should not be catalytically active. However, as will be presented later, MIL-53(Al) can promote hydrogenation of C–C multiple bonds using hydrazine as reducing agent and it has been proposed that the centers responsible for the catalysis are the defective Al³+ ions. In this regard, it should be commented that MOFs are highly crystalline materials as evidenced by X-ray powder diffraction (XRD) (see Figure 2.1 for a representative XRD) and there is a good agreement between the experimental XRD of the solids and the simulations for perfect crystals. However, it is very common in heterogeneous catalysis that just a few percentage of metals acting as active sites is sufficient for developing highly active catalysts. In this context, it should be commented that, related to the case of MOFs, zeolites containing framework metals have been very often used as heterogeneous catalysts [14,15]. For instance, one particular case is Ti-containing β zeolite and in this case the most active catalyst contains only a low percentage of Ti. In fact, increasing the Ti content beyond 3–5% can be detrimental for the catalytic activity because instead of isolated penta-coordinated titanols, Ti clusters start to develop and they are responsible for the decrease of the selectivity of the oxidation. A similar situation has been reported for Cr-containing zeolites or porous aluminophosphates (ALPOs) [16,17]. Also in zeolites, structural defects are generally involved as the active sites [18].

Figure 2.1   XRD pattern of UiO-66 (left) and a view of its crystal structure and the corresponding tetrahedral cages.

Since defects can act as active sites in MOFs, one strategy that has been reported and should be further explored is the creation of defects in