sustainablecatalysis · 2017. 12. 25. · vi contents 3 heterogeneousroomtemperature...

Sustainable Catalysis

Sustainable Catalysis

Energy-Efficient Reactions and Applications

Edited by Rafael Luque and Frank Leung-Yuk Lam

The Editors

Prof. Rafael LuqueUniversidad de CórdobaDepartamento de Química OrgánicaCarretera Nacional IVA, Km. 396Edificio C-314014 CórdobaSpain

Prof. Frank Leung-Yuk LamThe Hong Kong University of Science &TechnologyChemical and Biomolecular EngineeringClear Water BayKowloonHong Kong

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v

Contents

1 Introduction to Room-Temperature Catalysis 1Eduardo J. Garcia-Suarez and Anders Riisager

1.1 Introduction 11.2 Room-Temperature Homogeneous Catalysts 21.2.1 Ionic-Liquid-Based Catalytic Systems at Room Temperature 21.2.2 Transition Metal Homogeneous Catalysts 61.2.2.1 Group 9-Based Homogeneous Catalysts (Co, Rh, Ir) 61.2.2.2 Group 10-Based Homogeneous Catalysts (Ni, Pd, Pt) 71.2.2.3 Group 11-Based Homogeneous Catalysts (Ag, Au) 101.3 Room-Temperature Heterogeneous Catalysts 101.3.1 Group 9-Based Heterogeneous Catalysts (Co, Rh, Ir) 111.3.2 Group 10-Based Heterogeneous Catalysts (Ni, Pd, Pt) 131.3.3 Group 11-Based Heterogeneous Catalysts (Cu, Pt, Au) 231.4 Conclusions and Perspectives 29

References 31

2 Functionalized Ionic Liquid-based Catalytic Systems withDiversified Performance Enhancements 35Shiguo Zhang and Yanlong Gu

2.1 Introduction 352.2 Functionalized ILs for Enhancing Catalytic Activity 362.3 Functionalized ILs for Improving Reaction Selectivity 382.4 Functionalized ILs for Facilitating Catalyst Recycling and Product

Isolation 402.5 Functionalized ILs for Making Relay Catalysis 432.6 Cation and Anion Synergistic Catalysis in Ionic Liquids 452.7 Functionalized ILs for Aqueous Catalysis 462.8 Catalysis by Porous Poly-ILs 472.9 Functionalized IL-Based Carbon Material for Catalysis 492.10 Summary and Conclusions 54

References 54

vi Contents

3 Heterogeneous Room TemperatureCatalysis – Nanomaterials 59Liyu Chen and Yingwei Li

3.1 Introduction 593.2 Solid-Acid-Based Nanomaterials 603.3 Grafted-Metal-Ions-Based Nanomaterial 653.4 Metal NPs-Based Nanomaterial 673.4.1 Metal NPs Stabilized by Ligands 673.4.2 Metal NPs@Polymers 683.4.3 Metal NPs@Metal Oxides 703.4.4 Metal NPs@Carbonaceous Support 723.4.5 Metal NPs@Siliceous Base Support 743.4.6 Metal NPs@MOF Nanocomposites 773.5 Metal Oxide NPs-Based Nanomaterial 823.6 Summary and Conclusions 83

References 84

4 Biocatalysis at Room Temperature 89Ivaldo Itabaiana Jr and Rodrigo O. M. A. De Souza

4.1 Introduction 894.2 Transaminases 904.2.1 General Features 904.2.2 Transaminase Applications at Room Temperature 904.3 Hydrolases 984.3.1 General Features 984.3.2 Application of Hydrolases at Room Temperature 1004.3.2.1 Lipases 1004.3.2.2 Aldol Additions 1014.3.2.3 Michael Addition 1024.3.2.4 Mannich Reaction 1024.3.2.5 C-Heteroatom and Heteroatom–Heteroatom Bond

Formations 1034.3.2.6 Epoxidation 1034.3.2.7 Synthesis of Heterocycles 1044.3.2.8 Kinetic Resolutions 1054.3.3 Cutinases 1074.4 Laccases 1084.4.1 General Features 1084.4.2 Applications of Laccases 1104.5 Enzymes in Ionic Liquids 1154.5.1 General Features 115

References 125

5 Room Temperature Catalysis Enabled by Light 135Timothy Noël

5.1 Introduction 1355.2 UV Photochemistry 136

Contents vii

5.3 Visible Light Photoredox Catalysis 1395.4 Room Temperature Cross-Coupling Enabled by Light 1415.5 Photochemistry and Microreactor Technology – A Perfect

Match? 1445.6 The Use of Photochemistry in Material Science 1465.7 Solar Fuels 1495.8 Conclusion 151

References 151

6 Mechanochemically Enhanced Organic Transformations 155Davin Tan and Tomislav Frišcic

6.1 Introduction 1556.2 Mechanochemical Techniques and Mechanisms: Neat versus

Liquid-Assisted Grinding (LAG) 1566.3 Oxidation and Reduction Using Mechanochemistry 1606.3.1 Direct Oxidation of Organic Substrates Using Oxone 1606.3.2 Mechanochemical Halogenations Aided by Oxone 1626.3.3 Reduction Reactions by Mechanochemistry 1636.4 Electrocyclic Reactions: Equilibrium and Templating in

Mechanochemistry 1656.4.1 The Diels–Alder Reaction: Mechanochemical Equilibrium in

Reversible C—C Bond Formation 1656.4.2 Photochemical [2+2] Cycloaddition during Grinding: Supramolecular

Catalysis and Structure Templating 1676.5 Recent Advances in Metal-Catalyzed Mechanochemical

Reactions 1686.5.1 Copper-Catalyzed [2+3] Cycloaddition (Huisgen Coupling) 1686.5.2 Olefin Metathesis by Ball Milling 1696.5.3 Mechanochemical C—H Bond Activation 1706.5.4 Cyclopropanation of Alkenes Using Silver Foil as a Catalyst

Source 1716.6 New Frontiers in Organic Synthesis Enabled by

Mechanochemistry 1716.6.1 Synthesis of Active Pharmaceutical Ingredients (APIs) 1726.6.2 Reactivity Enabled or Facilitated by Mechanochemistry 1736.6.3 Trapping Unstable Reaction Intermediates 1756.7 Conclusion and Outlook 176

Acknowledgments 176References 176

7 Palladium-Catalyzed Cross-Coupling in Continuous Flow atRoom and Mild Temperature 183Christophe Len

7.1 Introduction 1837.2 Suzuki Cross-Coupling in Continuous Flow 1847.3 Heck Cross-Coupling in Continuous Flow 1927.4 Murahashi Cross-Coupling in Continuous Flow 199

viii Contents

7.5 Concluding Remarks 202References 202

8 Catalysis for Environmental Applications 207Changseok Han, Endalkachew Sahle-Demessie, Afzal Shah, Saima Nawaz,Latif-ur-Rahman, Niall B. McGuinness, Suresh C. Pillai, Hyeok Choi, DionysiosD. Dionysiou, and Mallikarjuna N. Nadagouda

8.1 Introduction 2078.2 Ferrate (FeO4

2−) for Water Treatment 2088.3 Magnetically Separable Ferrite for Water Treatment 2098.3.1 Magnetic Nanoparticles 2098.3.2 Magnetic Recovery of Materials Used for Water Treatment 2118.3.3 Ferrite Photocatalyst for Water Treatment 2128.4 UV, Solar, and Visible Light-Activated TiO2 Photocatalysts for

Environmental Application 2128.5 Catalysis for Remediation of Contaminated Groundwater and

Soils 2158.5.1 Catalytic Oxidative Pathways 2158.5.2 Catalytic Reductive Pathways 2178.5.3 Prospects and Limitations 2188.6 Novel Catalysis for Environmental Applications 2188.6.1 Graphene and Graphene Composites 2198.6.2 Perovskites and Perovskites Composites 2218.6.3 Graphitic Carbon Nitride (g-C3N4) and g-C3N4 Composites 2228.7 Summary and Conclusions 223

Acknowledgments 224Disclaimer 224References 224

9 Future Development in Room-Temperature Catalysis andChallenges in the Twenty-first Century 231Fannie P. Y. Lau, R. Luque, and Frank L. Y. Lam

Case Study 1: Magnetic Pd Catalysts for Benzyl AlcoholOxidation to Benzaldehyde 237Yingying Li, Frank L.-Y. Lam, and Xijun Hu

1.1 Introduction 2371.2 Pd/MagSBA Magnetic Catalyst for Selective Benzyl Alcohol Oxidation

to Benzaldehyde 2391.2.1 Results and Discussion 2391.2.1.1 Characterization 2391.2.1.2 Effect of Reaction Temperature 2401.2.1.3 Effect of Pd Loading 2411.2.1.4 Recycling Test 2461.3 Summary and Conclusions 246

References 247

Contents ix

Case Study 2: Development of Hydrothermally StableFunctional Materials for Sustainable Conversion of Biomass toFuran Compounds 251Amrita Chatterjee, Xijun Hu, and Frank L.-Y. Lam

2.1 Introduction 2512.2 Metal–Organic-Framework as a Potential Catalyst for Biomass

Valorization 2542.3 Xylose Dehydration to Furfural Using Metal–Organic-Framework,

MIL-101(Cr) 2552.3.1 Xylose Dehydration Catalyzed by Organosilane Coated

MIL-101(Cr) 2552.3.2 Xylose to Furfural Transformation Catalyzed by Fly-Ash and

MIL-101(Cr) Composite 2582.3.3 Xylose to Furfural Transformation Catalyzed by Tin Phosphate and

MIL-101(Cr) Composite 2622.3.4 Role of Acid Sites, Textural Properties and Hydrothermal Stability of

Catalyst in Xylose Dehydration Reaction 2642.4 Conclusion 267

References 268

Index 273

1

1

Introduction to Room-Temperature CatalysisEduardo J. Garcia-Suarez1,2,3 and Anders Riisager1

1Technical University of Denmark, Centre for Catalysis and Sustainable Chemistry, Department of Chemistry,Kemitorvet, Building 207, 2800 Kgs. Lyngby, Denmark2Tecnalia, Energy and Environment Division, Parque Tecnológico de Álava, Leonardo Da Vinci, 11, 01510Miñano, Spain3IKERBASQUE, Basque Foundation for Science, Maria Diaz de Haro 3, 48013 Bilbao, Spain

1.1 Introduction

The world’s energy demand is expected to increase significantly in the comingyears as a result of the exponential economic growth of emerging countries,BRIC (Brazil, Russia, India, and China). Such an increased energy requestis closely associated with environmental concerns and deficiency in watersupply. These key challenges should be addressed by creating and maintainingconditions that allow humans and nature to exist in productive harmony. Onlysuch a sustainable direction will permit fulfilling the social, environmental, andeconomic requirements of present and future generations and avoid the worldpassing the point of no return [1].

Chemistry has always played a pivotal role in development of societies byimproving the quality of life, the lifespan, and so on. However, despite its manyimportant progresses, chemistry is often recognized more as a problem thanas the solution to our daily needs. Indeed, the task of changing the persistingvision that society and governments uphold about chemistry is one of thebiggest challenges of chemists for the 21st century; this challenge shouldstart from the design and development of benign and efficient manufactureprotocols. To improve chemical production efficiency and fulfill internationallegislation, a multidisciplinary approach aimed at reducing by-products/waste,optimizing energy utilization, controlling emissions (climatic change), and usingrenewable materials to avoid hazardous or toxic substances is mandatory. Inthis connection, the “Green Chemistry” concept, being a list of 12 principles, isone of the most exciting, innovative, and realistic approaches that has emergedin order to minimize the drawbacks of chemical processing and contribute tothe protection of the environment [2]. “Green Chemistry” advocates increas-ing research on new renewable feedstocks, environmentally benign solvents(preferably water), catalysis, and greener technologies, processes, and products.Among the “Green Chemistry” principles, the ninth, focused on catalysis, plays

Sustainable Catalysis: Energy-Efficient Reactions and Applications,First Edition. Edited by Rafael Luque and Frank Leung-Yuk Lam.© 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 1 Introduction to Room-Temperature Catalysis

a key role in certifying the world’s sustainability by improving processes in thechemical industry, making them more efficient and benign. The developmentof greener catalytic protocols through the rational design of new catalysts, bothhomogeneous and heterogeneous, as well as solvent choice is important as it willincrease valuation and understanding at the government level and in society.

A “catalyst” is a substance that increases the rate at which a chemical reactionproceeds without itself becoming permanently involved. There are many funda-mental parameters in a chemical reaction that can be controlled by selecting theappropriate catalyst, including, for example, energy consumption, selectivity,productivity, and atom economy. Accordingly, the development of new catalystsor catalytic systems can be considered as an important step toward establishing amore green and sustainable chemical industry. In this regard, the design of moreeffective catalysts and catalytic protocols that allow a chemical process to becarried out at room temperature is a highly beneficial way to minimize both theenergy demand and the risk (minimizing safety issues) for employees of a chem-ical plant. Furthermore, by decreasing the reaction temperature, the selectivitytoward the desired product normally increases, thereby minimizing undesiredside reactions and by-products. On the other hand, the reaction kinetics can besignificantly hampered at room temperature and the catalyst should thereforebe selected carefully to provide a system having a sufficiently low activationenergy that allows the reaction to proceed at an acceptable rate without auxiliaryenergy input. Such selected catalysts for room-temperature reaction protocolscan be both homogeneous (e.g., organometallic complexes, ionic liquids) andheterogeneous (e.g., metal nanoparticles, supported nanoparticles). Recently,excellent reviews by Lam and Luque have covered this topic in detail [3].

The aim of this chapter is to provide an overview and point out some of themost relevant catalytic systems that allow carrying out catalytic reactions atroom temperature. The catalytic systems will be divided in two main groupsdepending on the nature of the catalyst involved, namely, (i) ionic liquids and(ii) homogeneous and heterogeneous catalyst-containing transition metals fromgroups 9 to 11 of the periodic table.

1.2 Room-Temperature Homogeneous Catalysts

Homogeneous catalysts are often superior to heterogeneous ones in termsof activity and, in particular, selectivity. In addition, the reaction conditions(temperature, pressure, etc.) are usually milder. However, homogeneous catalysisis hampered by other important issues from an industrial or applicability pointof view, such as catalyst recovery and recyclability.

1.2.1 Ionic-Liquid-Based Catalytic Systems at Room Temperature

Ionic liquids are defined as salts only composed of ions, which melt with-out being decomposed. A special group of ionic liquids are the so-calledroom-temperature ionic liquids, which are liquid below 100 ∘C. The first knownionic liquid (ethanolammonium nitrate) was reported in 1888 by Gabriel andWeiner [4]. Later in 1914, Walden reported the synthesis of other ionic liquids

1.2 Room-Temperature Homogeneous Catalysts 3

O

R1 R2

NH2R3

P

R4O

OR4

R4O

BAIL+ P

O

OR4

OR4

R1 R2

NHR3

α−Aminophosphonate

SO3HN

NCF3SO3

OSO3HN

HSO4BAIL

(a) (b)

P

H

OR4

R4O

O

or

Figure 1.1 Brønsted acidic ionic liquids (BAILs) used as catalyst in the synthesis ofα-aminophosphonates in a one-pot, three-component reaction. (Adapted with permissionfrom Ref. [8]. Copyright (2014) Wiley.)

such as, for example, ethylammonium nitrate [5], but it was only in 1943 thatthe term “ionic liquid” was coined by Barrer [6]. In the 1970s to the 1990s, novelionic liquids were developed and studied by US military researchers to be appliedmainly as electrolytes in batteries [7]. In the past 15 years, ionic liquids havebecome of great importance for scientists due to their unique properties, mainlytheir low vapor pressure, solubility, easy functionalization (task-specific ionicliquids), and their successful applications in catalysis, nanoparticle stabilization,electrochemistry, medicine, analytical methods, benign reaction media, and soon. One main advantage of ionic liquids is the huge pool available. In principle,this allows the possibility of selecting just the right ionic liquid for a specificapplication. In catalysis, the selection of the ionic liquid is determined mainlyby solubility characteristics (providing often biphasic systems that allow therecovery of the employed catalyst), intrinsic catalytic properties, as well astheir thermal and chemical stability. Here, we overview some reactions thatare conducted at room temperature in the presence of ionic liquids as catalystand/or reaction media.

An important subgroup of ionic liquids are the so-called acidic ionic liquids,where a Brønsted or Lewis acid functionality is part of the ionic liquid ions.They have been used to replace traditional mineral acids (MeSO3H, H2SO4, HF)or traditional Lewis acids (AlCl3, FeCl3) successfully and, often, with superiorperformance. In organic synthesis, the acidic ionic liquids have been extensivelyused and numerous reports have come out in the past years concerning their useas solvents or catalysts at room temperature. Since it is not possible to surveyall these applications, representative examples will be pointed out to show thepotential of the acidic ionic liquids in organic synthesis.α-Aminophosphonates are compounds of great interest due to their bio-

logical and chemical applications (antibacterial, antitumor, antiviral, enzymeinhibitors). The synthesis of these compounds is normally carried out throughthe so-called Kabachnik–Fields reaction in the presence of a dehydrating agentand a Lewis acid. In 2009, Akbari and Heydari used a Brønsted acidic ionicliquid (BAIL) (Figure 1.1a) as catalyst instead of the Lewis acid for the synthesisof α-aminophosphonates through a one-pot, three-component (phosphite,


aldehyde or ketone, and amine) reaction [9]. They got excellent results in termsof yield (up to 98%) in short reaction times at room temperature. Furthermore,the employed BAIL catalyst could be recovered and reused up to six timeswithout any deactivation. In 2010, Fang et al. prepared a series of “halogen-free”BAILs to be tested as catalysts in the same reaction and obtained good resultsat room temperature in aqueous media [10]. In 2014, Peng et al. prepared adifferent BAIL based on the choline cation (Figure 1.1b), also to be used ascatalyst in the same one-pot, three-component reaction. They claimed thattheir synthesized choline-based BAIL was cheaper and less toxic than the onepreviously reported by Akbari and Heydari [9]. Excellent results were obtainedunder solvent-free conditions at room temperature in short time reactions withisolated yields up to 95%. The recyclability of the catalyst was also tested up tosix times without any decrease in activity or degradation of the BAIL [8].

In a recent work, Ying et al. [11] showed the effectiveness in terms ofactivity and recyclability of using multiple-acidic ionic liquids as catalysts forthe synthesis of α-aminophosphonates at room temperature under solvent-freeconditions. The same authors used the multiple-acidic ionic liquids in thesynthesis of bis-indolylmethanes (Figure 1.2), compounds with biologicalactivity and of great interest in the medical chemistry, under solvent-freeconditions and at room temperature. Among the applied multiple-acidic ionicliquids, [TEOA][HSO4] (triethanolammonium hydrogensulfate) showed thebest performance, giving the products in excellent yield (up to 90%) after a fewminutes of reaction. In addition, the catalytic system was reused up to five timeswithout showing any sign of deactivation [12].

The protection of hydroxyl groups is an essential task in organic synthesis toavoid unwanted reactions where, for example, Grignard or alkyllithium reagentsare involved. In this connection, acidic ionic liquids have shown to be alterna-tives to commonly used volatile organic solvents in the protection of alcohols atroom temperature with excellent yields in less than 5 min reaction, making theoverall process safer and greener [13]. The esterification of carboxylic acids withalcohols is a reaction of great interest because it yields esters that are valuableintermediates in the chemical industry. Chloroaluminate-based acidic ionicliquids, as substitutes of inorganic acids, were first tested in the esterificationreaction by Deng et al. [14]. The authors highlight two main advantages of using

NH

O

R1 R2

R +[TEOA][HSO4]

rt, solvent-free

[TEOA][HSO4] NH

HO

HO

HO

HSO4

NH

NH

R1 R2R R

Figure 1.2 Multiple-acidic ionic liquids in the synthesis of bis-indolylmethanes. (Adapted withpermission from Ref. [12]. Copyright (2014) Elsevier.)


chloroaluminate-based acidic ionic liquids instead of, for example, sulfuric acid:(i) The insolubility of the produced esters facilitate easy separation from thereaction media and (ii) the recovery and reuse of the employed ionic liquidafter removing the water formed during the reaction. Despite these advantages,the well-known high sensitivity of this kind of acidic ionic liquids to moistureor organic acids make them nonideal candidates for the esterification reactionsince one of the by-products is water. Esterification of alcohols with acetic acidanhydride was probed to proceed in the presence of BAILs at room temperature,achieving good conversion without detecting any side reactions. Furthermore,due to the insolubility of most of the esters into the ionic liquids, the catalyticsystem allowed to be recycled and reused up to five times with only a smalldecrease in activity due to the loss of ionic liquid during the recycling procedure[15]. Upgrading of bio-oil in order to extend the range of its application couldbe achieved through a Fischer esterification with alcohols in the presence ofdicationic ionic liquids with Brønsted acidity at room temperature, overcomingmoisture and acidity problems related to bio-oil (Table 1.1) [16].

The Diels–Alder reaction is an important organic reaction for the synthesisof natural products and physiologically active molecules, and any improve-ment in the reaction rate and/or the selectivity is of special interest. In 1999,chloroaluminate-based ionic liquids were tested in the room-temperatureDiels–Alder reaction between cyclopentadiene and methylacrylate as a testreaction. The employed ionic liquid showed to be superior to the traditionalorganic solvents employed so far, yielding high conversion and higher endo/exoselectivity [17]. In the Diels–Alder reaction between cyclopentene and methy-lacrylate, the selectivity toward the endo product was attributed by Welton andcoworkers to the ability of the employed ionic liquid to form H-bonds with thedienophile. The use of ionic liquids offers the potential to be used as solventsfor Diels–Alder reactions, substituting even the traditional Lewis acids andextending the application to other reagents that can be sensitive to moisture,oxygen, or to strong Lewis acids [18]. Chiral ionic liquids are an interestingkind of ionic liquids that theoretically could transfer their chirality to the final

Table 1.1 Properties of crude and upgraded oil.

Properties Crude oil Upgraded oil

Moisture (wt%) 32.8 8.2pH 2.9 5.1Kinematic viscosity (mm2 s−1) 13.03 4.90High heating value (MJ kg−1) 17.3 24.6C (%) 41.82 50.64H (%) 8.75 10.77O (%) 48.73 38.03N (%) 0.64 0.39

Source: Xiong et al. 2009 [16]. Adapted with permission ofAmerican Chemical Society.


product when applied as solvent, making them highly attractive for asymmetricsynthesis. In this sense, the asymmetric aza-Diels–Alder reaction betweenasymmetric amines and the Danishefsky’s diene was carried out successfullywith moderate to high diasteroselectivity at room temperature using a chiralionic liquid catalyst/reaction media without the presence of Lewis acids ororganic solvents [19]. Many other ionic liquids – with or without special modifi-cations – have also been used successfully at room temperature in other organicasymmetric and nonasymmetric reactions, such as Knovenagel condensation[20–22], asymmetric aldol condensation [23], and so on. In general, the use ofionic liquids has demonstrated their potential to make chemical reactions safer,greener, and energetically more efficient; and, in many cases, the use of the ionicliquids allows the formation of biphasic systems that make product separationand reutilization of the catalysts easy. However, their application at the industriallevel is still hampered by their relatively high viscosity and price level comparedto common reaction media, which give rise to concerns about mass transferlimitations and process economy due to the large amounts required. Hence, toovercome these problems, development of improved ionic liquids and associatedtechnology is needed in the future.

1.2.2 Transition Metal Homogeneous Catalysts

Transition metals, and mainly their organometallic complexes, represent abenchmark in homogeneous catalysis. The complexes are special because theirelectronic and steric properties – which normally rule the catalytic reaction – canbe modified and fine-tuned by choosing the appropriate metal and rationaldesign of the ligand. Transition-metal-based homogeneous catalysts are wellknown to catalyze many reactions of industrial interest, such as polymerization,carbonylation, hydrogenation, oxidation, cross-coupling, epoxidation, and soon, and have historically contributed to develop systems that work under mildconditions. However, the use of such homogeneous catalysts is hamperedby their recovery and separation from the reaction media, which normally isenergy intensive. Only a limited number of examples of transition-metal-basedcatalysts operating at room temperature are reported. Here, a few late transitioncatalysts – mainly with metals from the groups 9 to 11 – and their performancein reactions at room temperature are discussed.

1.2.2.1 Group 9-Based Homogeneous Catalysts (Co, Rh, Ir)Homogeneous hydrogenation of arenes at room temperature was performedin 1977 by Muetterties and coworkers with a cobalt complex catalyst,𝜂3-C3H5Co[P(OCH3)3]3 [24]. Such reactions were earlier dominated byheterogeneous metallic systems based mainly on Ni, Pd, Pt and Rh, but thesesystems were hampered by lack of stereo- and chemoselectivity. The use ofhomogeneous catalysts enabled the possibility of getting systems where goodstereoselectivity as well as chemoselectivity could be achieved.

A rhodium-based catalyst, RhCl(CO)(PMe3)2, was tested in the homogeneouscarbonylation of liquefied propane at room temperature yielding butanalwith high selectivity [25]. This catalytic system was shown to be a promising


alternative for the selective functionalization of gaseous alkanes. A series ofwater-soluble rhodium complexes obtained by reaction of rhodium precursorswith 1,3,5-triaza-7-phosphaadamantane (PTA) were used as catalysts in the iso-merization and condensation of allylic alcohol at room temperature in aqueousmedia, showing advantages over commercial methods [26]. The selectivity ofthe reaction was easily controlled by the amount of base added, obtaining oneof the products in quantitative yield. In addition, these water-soluble catalystsallowed also catalyst recovery and reutilization, making the overall processgreener and energetically efficient. Another water-soluble Rh(I)-complex,RhCl(CO)(TPPTS)2 (TPPTS=m-P(C6H4SO3Na)3), was used successfullyin the polymerization of terminal alkynes at room temperature under mildreaction conditions. The reactions were carried out under biphasic conditions,which allowed the recovery and reutilization of the catalytic system [27]. Thepolymerization of these alkynes yielded conjugated systems that are interestingdue to their photosensitivity and optical nonlinear susceptibility. The firstpolymerization of terminal alkynes was performed by Natta using Zieglercatalysts in 1958 [28]. The homogeneous hydrogenation of olefins and acetyleneswas carried out efficiently at room temperature and atmospheric pressureof hydrogen with an iridium-based catalyst, [Ir(σ-carb)(CO)(PhCN)(PPh3)](σ-carb= 7-C6H5-1,2-C2B10H10) [29].

1.2.2.2 Group 10-Based Homogeneous Catalysts (Ni, Pd, Pt)Among group 10 metals, Ni is more attractive and preferable as catalyst since itis cheaper than Pd and Pt. Ni(NO3)2⋅6H2O was used as a homogeneous catalystat room temperature for the production of 2-((1H-benzo[d]imidazol-2-ylamino)(aryl)methylthio)acetates in a multicomponent reaction (MCR) [30].

[Ni(PR2NR′

2)2(CH3CN)]2+ complexes (Scheme 1.1) are the first exampleof homogeneous catalysts employed successfully in the electrocatalytic oxi-dation of formate to be used in fuel cells [31]. Mechanistic studies showedthat the pendant amine plays the main role in the rate-determining stepthat involves the transfer of a proton from the formate to the amine. Theturnover frequencies (TOFs) for the catalyst employed were comparableto any other reported formate/formic acid oxidation catalysts. Asym-metric α-arylation of ketones with chloro- and bromoarenes has beencatalyzed by a homogeneous Ni(0)-complex, [(R)-BINAP]Ni(η2-NC-Ph)((R)-BINAP= (R)-(+)-(1,1′-Binaphthalene-2,2′-diyl)bis(diphenylphosphine)), intoluene at room temperature at high reaction rates with excellent yield (up to91% and above 98% of enantiomeric excess, ee) [32]. The advantage of runningthe reaction at room temperature is the attenuation of the decomposition of theNi(0) complex to form the less active Ni(I) species.

Water extract of rice straw ash (WERSA) was employed as reaction mediatogether with Pd(OAc)2 without the presence of any ligand, base, or promoterin the Suzuki–Miyaura cross-coupling reaction at room temperature of differ-ent bromoaryl compounds and arylboronic acids, yielding good to excellentconversions [33]. WERSA was prepared by burning the rice straw to ashes thatwere further suspended in water and filtered off (Figure 1.3). Since WERSAis composed of different cations and anions and due to its basic nature (Na+,

Ph

PhN

2+

+

PPh

2 R–PH2

2R′N

PR

NR′PR

PhN Cy

Cy

NiNCCH3NCCH3N

Ph

P

P

[Ni(CH3CN)6](BF4)2

[Ni(PR2NR′

2)2(CH3CN)](BF4)2

[Ni(PR2NR′

2)(PR″2NR′

2)(CH3CN)](BF4)2

[Ni(PCy2NPh

2)(PPh2NPh

2)(CH3CN)2+

4 CH2O2 R P

OH

OH

2 R′ NH2 R′NPR

(1)

(2)

(3)

R

N

R′ NP P

RN R′

R′

R″

P Ni PN

NC

CH32+

R′R″

P

NPhP

R

NR′

Scheme 1.1 Synthetic procedure for [Ni(PR2NR′

2)2(CH3CN)]2+ complexes. (Galan et al. 2012 [31]. Reproduced with permission of American Chemical Society.)


Rice straw Rice straw ashH2O + rice straw ash

Br

R1

Aryl halide

R1 R2

Biphenyl derivatives

Na

Na

OH

K

OH

K

K

OH K

OH

K

OH K OH

K

OH2

2

OHMg

Ca

OH

Water extract ofrice straw ash

Basicextract

Preparationof WERSA

WERSA

WERSA

Air Air

Pd(OAc)2

HO AtR1 = H, Me, OMe,

R1 = H, Me, OMe,

NO2, CN, CHO

CF3, NO2,COMe etc.

COMe etc. Aftert = 15 min

T = 25 °CB

R2

Phenyl boronicacid

Pd(OAc)2 in WERSA catalyzedsuzuki coupling reaction

Room temperature

HO

Figure 1.3 WERSA isolation procedure from rice straw. (Boruah et al. 2015 [33]. Reproducedwith permission of Royal Society of Chemistry.)

K+, Mg2+, Ca2+ and OH−), the addition of a base – essential for the Suzukireaction – was not needed. WERSA is an aqueous-based reaction mediumwhere the palladium can be immobilized and after extraction of the reactionproducts with a nonpolar organic solvent, such as diethylether, the WERSA-Pdsystem can be recovered and reused. In this manner, the system could be reusedup to six times without significant loss of activity after the fourth run, probablydue to metal leaching into the organic phase.

Catalytic oxidation of secondary alcohols to ketones have been carried outefficiently at room temperature or slightly higher temperature (38 ∘C) in 1977with a catalytic system formed by PdCl–NaOAc using molecular oxygen asoxidant [34]. This catalytic system substituted the previous PdCl–Cu(II)-saltcatalytic system, where higher temperatures (70–120 ∘C) were required.A series of five-membered P,C-orthopalladate complexes with differentmonodentante ligands were synthesized and tested in the Suzuki–Miyauracross-coupling at room temperature [35]. The palladium-based catalyst,[Pd(PPh3)(Cl){P(OPh)2(OC6H4)}], showed good to excellent activity for allthe substrates tested including the cheaper, more available, less reactive, andchallenging arylchlorides.

A homogeneous diplatinum complex, [Pt2(μ-dppm)] (dppm=Ph2PCH2PPh2)showed to be effective in the catalytic synthesis of dimethylformamide through


C

CPh

H

1a or 1b, 40 °C

= BH 2

O

N

PAg

tButBu

X

1a, X = OAc1b, X = NO3

B10H12(CH3CN)2

Scheme 1.2 Synthesis of carboranes with homogeneous silver catalysts. (El-Zaria et al. 2014[37]. Reproduced with permission of Wiley.)

the hydrogenation of CO2 at room temperature under mild reaction conditions.This catalyst substituted previous ones where the required reaction conditionswere more harsh [36].

1.2.2.3 Group 11-Based Homogeneous Catalysts (Ag, Au)A homogeneous silver catalyst was used for the synthesis of carboranes intemperatures ranging from room temperature to 40 ∘C [37]. Carbonanes areboron clusters with unique structural and electronic properties with potentialuse in creating new diagnostics, therapeutics, and electronically tunable mate-rials. The preparation of these compounds usually required high temperatures(80–120 ∘C) [38–40], but the tested homogeneous silver catalysts (Scheme 1.2)facilitated preparation of functionalized carboranes in good to excellent yield atreduced reaction temperatures in the range from room temperature to 40 ∘C.The lower reaction temperature opens up the possibility of preparing carboranesfrom thermally sensitive alkynes that otherwise undergo degradation or sidereactions at higher temperatures. McNulty et al. described the first examplesof homogeneous intramolecular hydroamination of 2-alkynylanilines using ahomogeneous silver catalyst leading to substituted indoles. The reactions wereperformed with low catalyst loading (1.0 mol%) and at room temperature.Normally the cyclization takes place under harsh reaction conditions and hightemperatures.

Gold-based homogeneous catalysts were reported for the first time in theepoxidation of aromatic alkenes at room temperature using sodium chloriteas stoichiometric oxidant [41]. More recently, in 2012, Corma and coworkersshowed the first application of a homogeneous gold-based catalyst in the oxida-tive homocoupling of terminal alkynes in excellent yields at room temperature.Their mechanistic studies demonstrated that at least two different Au–alkynecomplexes are needed for the homocoupling reaction (Scheme 1.3) [42].

1.3 Room-Temperature Heterogeneous Catalysts

Heterogeneous catalysis overcomes some of the main drawbacks in homogeneouscatalysis, such as the recovery and recycling of the catalysts. In traditional

1.3 Room-Temperature Heterogeneous Catalysts 11

(I)

(I) (I)

(III)

(I)

(I)

AuPPh3NTf2

(Base)

AuPPh3F + AuPPh3BF4

AuPPh3NTf2

R H

+

–NTf2

R

R

HNTf2

(Base)

Rate-determining step

R

R

+ l

l

F

AuPPh3

BF4

I-ox

AuPPh3

NBF4

–NCl

F

NN

–2BF4

Cl

+

+

+

R

H

Scheme 1.3 Plausible mechanism for the gold-catalyzed oxidative homocoupling of terminalalkynes. (Leyva-Perez et al. 2012 [42]. Reproduced with permission of American ChemicalSociety.)

heterogeneous catalysis, the catalytic system comprises a porous solid materialin which the metals are immobilized. The high surface areas of the solid materialsallow the catalytic metal to be deposited on the surface, becoming available forthe substrates.

1.3.1 Group 9-Based Heterogeneous Catalysts (Co, Rh, Ir)

From an economic point of view, cobalt is the preferred group 9 metal since itis the cheapest, but it is a challenge to develop an efficient and robust Co-basedcatalyst that could replace the precious metals-based catalysts in industrialapplications. A heterogeneous Co-based catalyst was prepared by immobiliza-tion of a Co(II) complex on mesoporous SBA-15 silica and successfully appliedin the synthesis of α-aminonitriles, which are compounds with useful biologicalactivities and important building blocks for peptides and proteins preparation,through the three-component Strecker reaction at room temperature undersolvent-free conditions [43]. The Co-based catalyst was tested in the reactionwith different aldehydes and amines obtaining the corresponding products inexcellent yields, typically in the range of 90–97%. For practical viability, therecyclability of the catalyst was also studied. The catalyst could be recycled up to10 times with unchanged catalytic activity and no detectable metal leaching byinductively coupled plasma (ICP) spectrometry.

Another Co-based catalyst consisting of a Co(II) complex supported onethylenediamine-functionalized nanocellulose (EDANC), which is a biodegrad-able solid support, was prepared and tested in the aerobic oxidation of variousbenzyl alcohols at room temperature [44]. The Co(II) complex was immobilizedthrough coordination of the metal to the amine-functionalized cellulose, achiev-ing a good distribution of cobalt on the cellulose surface. The Co(II)–EDANCcatalyst was effective in the oxidation of different benzyl alcohols with goodconversions in the range of 81–97% using both air and molecular oxygen asoxidants. The recyclability of the catalyst was also shown to be good, resultingin only a minor decrease in activity after the fifth reaction cycle. In comparisonwith homogeneous Co(II) catalyst and Co(II)-impregnated on nonmodifiedcellulose, the Co(II)–EDANC catalyst was also demonstrated to be superior. Thebetter performance compared to the other catalysts was due to the presence of


ethylenediamine in the support, which could act as a ligand activating the Co(II)toward the alcohol’s oxidation.

Rhodium nanoparticles prepared by chemical reduction of a RhCl3⋅3H2O pre-cursor in the presence of a surfactant (N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride, HEA16Cl) were immobilized by simple impregnation onsilica in a very easy way without any further hydrogenation or calcinationstep [45]. The obtained catalyst was tested in the hydrogenation of aromaticcompounds at room temperature. The hydrogenation of aromatic compounds isof great importance and an industrially challenging reaction applied in refineriesto reduce the volatile organic matter (VOM) emissions, normally under harshconditions. The authors suggested that the Rh(0) nanoparticles were anchoredto the silica via oxygen bonding through the silanol groups. The Rh/SiO2 catalystwas tested in the hydrogenation of a series of aromatic compounds such as, forexample, benzene, toluene, styrene, xylene, and methylanisole, in a triphasicsystem composed of water/hydrocarbon/silica, at room temperature underambient hydrogen pressure. All the tested substrates were fully converted intoproducts with TOFs ranging from 93 to 129 h−1. Notably, cumene – which isquite difficult to hydrogenate – was reduced with a TOF of 93 h−1 under the mildreaction conditions. Increasing the hydrogen pressure in the reaction to 30 bars,TOF as high as 6429 h−1 was achieved using anisole substrate. The Rh/SiO2catalyst was found to be around four times more active than other traditionalRh-based heterogeneous catalysts. In addition, good recyclability of the catalystwas achieved, allowing it to be used up to five times with only a slight decreasein activity (TOF decrease from 129 to 120 h−1) in the hydrogenation of anisol.

The aerobic oxidative homocoupling of aryl amines was carried out for thefirst time using heterogeneous rhodium catalysts at room temperature in thepresence of acids [46]. The reaction protocol offered an efficient methodologyfor the preparation of biaryl diamines via oxidative C—H activation. The studiedreaction is very sensitive to the pK a and the choice of acid is essential. Therole of the acid is to protonate the aryl amine resulting in an ammonium saltthat prevents its oxidation as well as side reactions, which typically makes thereaction difficult to control. The best acid for the reaction was trifluoraceticacid with a pK a of −0.2. The Rh/C heterogeneous catalyst was superior to otherheterogeneous metal-based catalysts or homogeneous Rh catalysts investigatedin the study. No recycling tests were done but a greener and more energeticallyefficient route than previously reported was developed.

Iridium(0) nanoparticles dispersed in a zeolite (FAU) framework (IrNPs@FAU)were employed as heterogeneous catalyst in the hydrogenation of aromaticmolecules under mild conditions at room temperature [47]. The hydrogenationof aromatic molecules is a key reaction in synthetic and petroleum chemistry.The catalyst was prepared by cation exchange of Na+ with Ir3+ in the zeolite,followed by reduction with NaBH4 at room temperature. The sodium cation siteswere completely restored after reduction by sodium cations from the NaBH4used as reducing agent. The micropore area and pore volume of the zeolite wasdecreased after incorporation of the rhodium nanoparticles due to the formationof the nanoparticles in the cavities and the external surface of the zeolite-Y.Benzene is challenging to hydrogenate due to the resonance stabilization from


the π-conjugation in the aromatic ring and, in consequence, its hydrogenationrequires normally high temperature and pressure. Thus, it remains a challengeto find new heterogeneous catalysts capable of hydrogenating neat aromatics atroom temperature. The IrNPs@FAU showed to be highly active and selective inthe hydrogenation of aromatics with TOF as high as 3215 h−1 in the hydrogena-tion of benzene to cyclohexane. Also, a high resistance to agglomeration andmetal leaching was obtained achieving a cumulative turnover number (TON)of 197 000 before complete deactivation was observed after 92 h. Despite theactivity and selectivity for the prepared catalyst being lower than that previouslyreported for the best catalyst (Ru–Ni/C; TOF= 7905 h−1), it was higher thanmost catalysts used for hydrogenation of benzene under these mild conditions.The major drawback of this catalyst was that reactivity was low toward bulkysubstrates such as, for example, mesitylene due to size restrictions of thezeolite-Y cages, making it impossible for the substrate to reach the active Rh sitesinside the pores. Accordingly, only the Rh on the zeolite surface contributed tothe catalytic activity.

1.3.2 Group 10-Based Heterogeneous Catalysts (Ni, Pd, Pt)

In 1998, a novel supported nickel complex was employed as heterogeneouscatalyst in Baeyer–Villiger oxidation of cyclic and linear ketones to lactonesand esters, respectively, using molecular oxygen as oxidant at room temper-ature [48]. The catalyst was prepared in an elegant way at room temperature,where mesoporous silica in a first step was functionalized by reacting withtrimethoxy-3-aminopropylsilane yielding aminopropyl silica (AMPS) and there-after successively treated with terephthaldialdehyde and p-aminobenzoic acid.To an ethanoic suspension of the resulting solid a Ni complex was subsequentlyadded, yielding the desired heterogeneous catalyst. The catalyst proved to beefficient in the tested reaction, yielding good conversions and high selectivities.For example, cyclopentanone was selectively oxidized to valerolactone with 91%conversion and TON of 31 h−1 per catalytic site. Unfortunately, no recyclabilitytest of the catalysts was performed.

A macroporous polystyrene–divinylbenzene cross-linked Merrifield resin wasused as support for the covalent anchoraging of a homogeneous unsymmetricalSalen-type nickel(II) catalyst [49]. The heterogeneous catalyst was effective in theTamao–Kumada–Corriu cross-coupling reaction between a Grignard reagentand an organobromide compound at room temperature. In addition, the catalystwas able to be recycled several times and did not leach metal into solution. Thecatalyst stability was attributed to a combination of hard and soft donor atomsaround the nickel metal centers, the oxidation state of which varies between 0 and2. The same Ni(II) catalyst was immobilized onto silicon-hydride-functionalizedsilica in a two-step procedure [50]. The first step was an etherification and thesecond a hydrosilylation reaction catalyzed by a platinum catalyst. The preparedcatalyst showed high activity in the Tamao–Kumada–Curriu reaction between4-bromoanisol (organobromide) and phenylmagenesium chloride (Grignardreagent) at room temperature without addition of phosphine ligands. Thecatalyst could be recovered by simple filtration and reused several times. ICP


analysis revealed that very low amount of Ni was leached into the solution.Catalytic tests were performed in a mini-flow reactor system where conversioncould be achieved after a few minutes, while in batch conditions hours wererequired.

A room-temperature Suzuki–Miyaura cross-coupling reaction was performedin the presence of a heterogeneous palladium catalyst [51]. The catalyst con-sisted of palladium nanoparticles immobilized in multiwalled carbon nanotubes(MWCNTs) using the layer-by-layer approach [51, 52] (Figure 1.4).

The catalyst showed to be efficient in the coupling of different halogenated aro-matics with boronic acids under sustainable conditions. The more challengingarylchlorides were also tested, showing unprecedented results using heteroge-neous catalysts at room temperature with yields up to 89%. The catalyst waseasily recovered from the reaction media and reused up to five times with nodecrease in catalytic activity. Furthermore, transmission electron microscopy(TEM) analysis of the catalyst recovered after five cycles did not show majorchanges in its morphology.

(a)

(b) (c)

Cl

N

Me Me n

MWCNT

1111

7

OO OHOH

HOHO

N

OHOH

O

O

NH

11

7

OO OH

HO

N

OH

O

O

NH

Nitrilotriacetic acidNitrilotriacetic acidhydrophilic headhydrophilic head

Nitrilotriacetic acidhydrophilic head

Hydrophobic tail with centralHydrophobic tail with centralpolymerizable diacetylenepolymerizable diacetylene

Hydrophobic tail with centralpolymerizable diacetylene

DANTA

PDADMAC

PdNPs

Figure 1.4 (a) Overview of the PdCNT catalyst assembly; (b) structure of DANTA; (c) structureof PDADMAC. (Jawale et al. 2015 [51] http://pubs.rsc.org/-/content/articlehtml/2015/cy/c4cy01680g. Used under CC BY 3.0.)

http://pubs.rsc.org/-/content/articlehtml/2015/cy/c4cy01680g

http://pubs.rsc.org/-/content/articlehtml/2015/cy/c4cy01680g


A montmorillonite K-10 clay was used as support for immobilizing aPd(TPP)2Cl2 complex [53]. The immobilization was performed by additionof the clay to a solution containing PdCl2 and the triphenylphosphine (TPP)after stirring the mixture for 6 days. The incorporation of the complex inthe clay was confirmed by a decrease in the surface area of the clay byBrunauer–Emmett–Teller (BET) analysis, and the presence of the complex wasconfirmed by infrared (IR) spectroscopy with the appearance of signals at 426 and337 cm−1 for Pd–P and Pd–Cl vibrations, respectively. The catalyst was tested inthe coupling of different functionalized arylbromides and boronic acids at roomtemperature, achieving conversions between 90 and 99% in a variable reactiontime from 0.5 to 18 h depending on the functionalization of the substrates. Therecyclability of the catalysts was not successful, which was attributed to deacti-vation of the catalyst during the course of the reaction and recovery process.

Heterogeneous palladium catalysts were also prepared and applied in the roomtemperature Suzuki–Miyaura coupling of various activated and deactivatedchloroarenes, achieving excellent yields after 16 h of reaction [54]. The hetero-geneous catalysts were prepared in an original way. First, the silica-supportedtripod phosphine (silica-3p-TPP) was prepared containing a disiloxane linkage atthe para position on each phenyl ring of TPP, as shown in Scheme 1.4. Next, thebromobenzene derivative 1a, with a (iPrO)Me2Si substituent in the para posi-tion, was converted into the corresponding Grignard reagent. Subsequently, theGrignard reagent was treated with PCl3, yielding the soluble silyl-functionalizedPh3P species 3p-TPP in 80% yield. The obtained specie was then grafted ontosilica gel by refluxing in benzene in the presence of imidazole, yielding thesilica-supported tripod phosphane Silica-3p-TPP(SiOH). The latter was treatedwith Me3Si–imidazole in order to end cap the silanol groups with Me3Si, givingthe desired Silica-3p-TPP. Once the TPP grafted on silica was obtained, thepalladium catalyst was immobilized by reaction of the prepared silica-3p-TPPwith a palladium(II) complex (PdCl2(PhCN)2), and the coordination of Pd(II)was confirmed by solid-state nuclear magnetic resonance (NMR) spectroscopy.The catalyst was recovered from the reaction media by simple filtration andreused up to four times with conversion ranging between 95 and 99% in thecoupling of p-chloroanisol and phenylboronic acid. However, the fifth runshowed a decrease in activity, yielding only 74%.

A composite PVP/KIT-5 (PVP=Poly(N-vinyl-2-pyrrolidone); KIT-5=cage-type mesoporous silica) was prepared by in situ polymerization methodand used as solid support for the immobilization of palladium nanopar-ticles (Figure 1.5). The resulting heterogeneous catalyst was used in theSuzuki–Miyaura coupling between arylhalides and phenylboronic acid at roomtemperature in water as solvent and under aerobic conditions [55]. The catalystwas recovered by filtration and reused up to eight times with good yield. Theexcellent performance in recyclability was attributed to the interconnectedlarge-pore, cage-type mesoporous KIT-5 with three-dimensional (3D) porousnetworks.

Selective and sequential reduction of nitroaromatics to nitroanilines and toadiamines was performed for the first time in quantitative yield at room temper-ature and ambient pressure in ethanol in 1989 using a Pd-based heterogeneous


1a

Br SiMe2(OiPr)

Imidazolebenzenereflux

P SiMe2(OiPr)

3p-TPP 80%

Silica gel

TMS-imidazole

60 °C

MeP

MeSi

O

O

OO

O

OO

O

OO

O

O

SiMe

Me

Si

Si

Si

OHOH

OO

OO

OO

Me MeSi

SiSi

SiO2

SiO2

Silica-3p-TPP(SiOH)

Silica-3p-TPP

MeP

Me

Me Me

MeMe

Si

O

O O

OO

O

O

SiO

SiSi

SiMe3

O

OO

OSi

SiSiMe3

O

OO

OO

OOSi

O

Si

3

(1) Mg THF, rt

(2) PCI3 THF, rt

Scheme 1.4 Preparation of Silica-3p-TPP. (Iwai et al. 2014 [54]. Reproduced with permission ofWiley.)

catalyst. The catalyst was prepared by complexation of H-Montmorillonite,previously synthetized, with PdCl2(PhCN)2. The catalyst showed to be active foraromatic systems but inert for aliphatic ones [56].

Encapsulated silica nanosphere decorated with palladium nanoparticles ina nanoporous silica shell was reported and tested in the hydrogenation ofvarious olefins at room temperature. The heterogeneous palladium catalystwas prepared in a smart way in various steps (Scheme 1.5). In the first step,the silica nanospheres were prepared through the Stöber method [58]. Inthe second step, the surface of the silica nanospheres were modified with3-aminopropyltrimethoxysilane. Then, previously prepared Pd nanoparticleswere chemisorbed on the silica nanospheres, whereafter the SiO2/Pd–NPcore–shell nanospheres were coated with a silica shell through a sol–gel process.In the final step, the controlled etching of the silica shell with NaOH gener-ated the desired Pd-based heterogeneous catalyst SiO2/Pd–Np/porous-SiO2core–shell–shell nanospheres, as shown in the TEM images (Figure 1.6) [57]. Thecatalyst was tested at room temperature in the hydrogenation of various olefins,reaching styrene hydrogenation in 25 min with a TOF h−1 of 5000. The reachedTOF was found to be three times higher than the one found for the most efficientpolymer-supported Pd heterogeneous catalyst at 35 ∘C. The recyclability of thecatalyst, through centrifugation, proved to be efficient, showing only a small


Figure 1.5 Encapsulation of PVP and Pd in the 3D interconnected pore channels of KIT-5.(Kalbasi and Mosaddegh 2011 [55]. Reproduced with permission of Elsevier.)

Silicananosphere

APTS Pd NPs

NaOH

TEOS /

NH4OH

NH

2

NH

2

NH2

NH22HN

NH2

NH 2N

H2

NH

2

NH2

NH2N

H2

Scheme 1.5 Schematic representation of the synthesis of SiO2/Pd–NP/porous-SiO2core–shell–shell nanospheres. (Wang et al. 2010 [57]. Reproduced with permission of RoyalSociety of Chemistry.)

decrease in catalytic activity attributed to loss of the catalyst during the recyclingtests.

Encapsulated palladium nanoparticles in silica MCM-48 were used as a noveland recyclable heterogeneous catalyst for chemo- and regioselective hydrogena-tion of olefins at room temperature [59]. Silica MCM-48 is more attractive assupport than the MCM-41 due to the presence of an interpenetrating networkof 3D pores that are expected to facilitate molecular transport of reactantsand products more efficiently than in the case of the 1D network found in


(a)

10

0 n

m

10

0 n

m

10

0 n

m

(b) (c)

Figure 1.6 TEM images of (a) SiO2/Pd–NP core–shell nanospheres with 20 nm Pd–NPchemisorbed on aminopropyl-modified silica nanospheres and the corresponding(b) SiO2/Pd–NP/SiO2 core–shell–shell nanospheres, and (c) SiO2/Pd–NP/porous-SiO2core–shell–shell nanospheres etched for 120 min. (Wang et al. 2010 [57]. Reproduced withpermission of Royal Society of Chemistry.)

MCM-41. The heterogeneous Pd catalyst was prepared by encapsulating thepreformed PdNPs into the cubic phase of MCM-48 following the Stöber method[58]. The obtained material was calcined, followed by reduction under H2, andcharacterized by means of powder X-ray diffraction (XRD), TEM, and BET. Thesurface area was found to be approximately 1800 m2 g−1 with a metal dispersionof around 22%. The catalyst was tested in the hydrogenation of various olefinsat room temperature, yielding good to excellent conversions with TOF as highas 4400 h−1. The good performance was attributed to the high metal surfacearea (approximately 95 m2 g−1) and the high dispersion of the nanoparticles.High chemoselectivity toward the reduction of double bonds was observedwith the prepared catalyst in the presence of carbonyl, acetate, and ketonegroups. Furthermore, the prepared catalyst exhibited good regioselectivity,hydrogenating selectively terminal alkenes. This selectivity behavior was com-pletely opposite to the previously reported Pd-MCM-48 catalyst, where the Pdnanoparticles were deposited on preprepared MCM-48. Analysis of this catalystrevealed that the surface area was around 637 m2 g−1 (three times less than theone prepared by the authors) and the nanoparticle dispersion was 12% (half ofthe one prepared by the authors). After testing different ways of incorporatingthe Pd nanoparticles onto the silica MCM-48 and evaluating the performance ofthe catalyst in the hydrogenation of different olefins, it was established that theselectivity was significantly influenced by the preparation method of the catalysts.

A heterogeneous Pd catalyst consisting of Pd(OAc)2 dissolved in ionic liquidand immobilized in the pores of an amorphous mercaptopropyl silica gel(Pd–SH–SILC) was employed successfully for the hydrogenation of olefinsunder very mild conditions (atmospheric pressure and room temperature) withexcellent results in terms of activity and selectivity [60]. The catalyst was reused,by means of simple decantation, up to 10 times without observation of anydecrease in conversion and/or selectivity. The catalyst was prepared by stirring asuspension of the modified mercaptopropyl silica gel in a solution of Pd(OAc)2and the ionic liquid in tetrahydrofuran (THF) followed by the removal of thesolvent under vacuum. The high content of Pd(OAc)2 in the mercaptopropylsilica compared with other silica supports was attributed to the presence of the

sustainablecatalysis · 2017. 12. 25. · vi contents 3 heterogeneousroomtemperature...

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