Solvent-Free Methods in Nanocatalysis E-Book
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Solvent-Free Methods in Nanocatalysis
Discover solvent-free approaches for the synthesis of nanocatalysts as well as various catalytic transformations in this unique one-stop resource
Solvent-free methods have attracted wide attention in organic synthesis and catalysis as a promising approach towards “greener” and more sustainable chemical transformations. In this regard, nanocatalysis has seen particular growth in recent years.
Solvent-Free Methods in Nanocatalysis gives an in-depth overview of nanocatalysts and their catalytic applications using solvent-free methods. After a brief introduction, it covers various synthetic techniques for the preparation of nanocatalysts and supports using solvent-free methods, e.g. ball-milling, microwave- and plasma-assisted methods. The book discusses in detail different catalyst classes, such as metal oxides, doped and functionalized nanocarbons, as well as nitride- and silica-based materials to help researchers to understand the efficiency and nature of these catalysts/supports based on their chemical structure. In the book readers will also find:
- A brief account of the history, challenges, and recent advances in the field
- Detailed discussion of advantages and disadvantages of solvent-free techniques for nanocatalyst preparation
- Treatment of important solvent- and catalyst-free organic transformations (i.e. oxidation, coupling and multicomponent reactions)
- A chapter on supported ionic liquids for solvent-free catalysis
Written by leading researchers in the field, Solvent-Free Methods in Nanocatalysis is a useful reference for researchers and students working in organic synthesis, catalysis, and nanomaterials science.
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Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Introduction: Scope of the Book
1.1 Introduction: Green Chemistry, Solvent‐free Synthesis, and Nanocatalysts
1.2 Topics Covered in this Book
1.3 Solvent‐Free Synthesis of Nanocatalysts
1.4 Solvent and Catalyst‐Free Organic Transformations
1.5 Solvent‐Free Reactions Using NCs
1.6 Present Status and Future Direction
References
2 Strategies for the Preparation of Nanocatalysts and Supports Under Solvent‐Free Conditions
2.1 Introduction
2.2 Mechanochemistry
2.3 Thermal Treatment
2.4 Plasma‐Assisted Methods
2.5 Deposition Method
2.6 Conclusion and Future Perspective
Acknowledgments
References
3 Solvent‐ and Catalyst‐Free Organic Transformation
3.1 Introduction
3.2 Solvent‐ and Catalyst‐Free Organic Transformations
3.3 Conclusion
References
4 Metal Oxides as Catalysts/Supports in Solvent‐Free Organic Reactions
4.1 Introduction
4.2 Different Metal Oxides as a Catalyst/Support in Solvent‐Free Reactions
4.3 Conclusion
References
5 Silica‐Based Materials as Catalysts or Supports in Solvent‐Free Organic Reactions
5.1 Solvent‐Free Reactions Over Silica Gel
5.2 Silica Nanoparticles and its Applications
5.3 Zeolites and Hierarchical Zeolite Structures
5.4 Conclusion
References
6 Carbon‐Based Materials as Catalysts/Supports in Solvent‐Free Organic Reactions
6.1 Introduction
6.2 Solvent‐Free Catalysis Using Carbon‐Based Materials
6.3 Summary and Future Perspectives
References
7 Nitride‐Based Nanostructures for Solvent‐Free Catalysis
7.1 Carbon Nitride
7.2 Boron Nitride
7.3 Molybdenum Nitride
7.4 Aluminum Nitride
7.5 Conclusion
References
8 Supported Ionic Liquids for Solvent‐Free Catalysis
8.1 Introduction
8.2 Supported Ionic Liquids
8.3 Building Blocks of SILs
8.4 SIL Catalytic Systems
8.5 Supported IL Solvent‐Free Catalysis
8.6 Solvent‐Free Hydrogenation of Olefins
8.7 Solvent‐Free Heck Reaction
8.8 Solvent‐Free Multicomponent Reactions
8.9 Solvent‐Free Condensation Reactions
8.10 Solvent‐Free CO
2
Conversion Reactions
8.11 Solvent‐Free Oxidation Reactions
8.12 Miscellaneous Solvent‐Free Organic Reactions
8.13 Conclusion
References
9 Present Status and Future Outlook
9.1 Summary
9.2 Future Outlook
Acknowledgments
References
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 TiO
2
‐based catalyst/catalyst support for solvent‐free reactions....
Table 4.2 SnO
2
‐based catalyst/catalyst support for solvent‐free reactions....
Table 4.3 MnO
2
‐based catalyst/catalyst support for solvent‐free reactions....
Table 4.4 ZnO‐based catalyst/catalyst support for solvent‐free reactions.
Table 4.5 Al
2
O
3
‐based catalyst/catalyst support for solvent‐free reactions....
Table 4.6 Iron oxide‐based catalyst/catalyst support for solvent‐free react...
Chapter 5
Table 5.1 One‐pot Wittig‐type olefination of benzaldehyde in the presence (...
Chapter 7
Table 7.1 List of some common methods for synthesis of modification of CNs....
Table 7.2 Comparing PECVD versus thermal methods for MoN synthesis.
Table 7.3 List of some common methods for synthesis of AlN.
Chapter 9
Table 9.1 Solvent‐assisted and their corresponding solvent‐free reactions....
List of Illustrations
Chapter 1
Figure 1.1 12 principles of green chemistry.
Figure 1.2 Introduction to (a) solvent‐free synthesis, (b) nanomaterials, (c...
Figure 1.3 Representative solvent‐free methods used for the synthesis of NCs...
Figure 1.4 Representative examples of solvent and catalyst‐free organic tran...
Figure 1.5 Representative examples of solvent‐free nanocatalytic procedures ...
Figure 1.6 Representative examples of silica‐supported nanocatalytic systems...
Figure 1.7 Saluted examples of carbon‐based nanocatalysts for organic transf...
Figure 1.8 Representative examples of nitride‐based materials as catalysts/s...
Figure 1.9 Representative examples of ionic liquid (IL)‐based materials as c...
Chapter 2
Figure 2.1 Synthesis strategies for nanoparticles and supports by solvent‐fr...
Figure 2.2 (a) Schematic representation of CZTS NPs synthesis, (b) TEM image...
Figure 2.3 (a) Schematic representation of the synthesis of Bi
2
S
3
NPs, TEM i...
Figure 2.4 Synthesis and characterization of Pt/m‐Al
2
O
3
‐400. (a) Schematic r...
Figure 2.5 (a) Schematic representation of the synthesis of B/g‐C
3
N
4
, (b) TE...
Figure 2.6 (a and b) TEM images, (c) size distribution graph of NiP NPs (Ni
x
Figure 2.7 (a) The sample of α‐Ni(OH)
2
, (b and c) TEM images of α‐Ni(OH)
2
....
Figure 2.8 (a) Schematic synthesis representation of AgCu bimetallic NPs, (b...
Figure 2.9 Synthesis route of Co@NC‐MOF‐900 electrocatalyst.
Figure 2.10 Schematic representation of the synthesis of Co‐based zeolite....
Figure 2.11 (a) Schematic representation of the PdAg alloy synthesis, (b) TE...
Figure 2.12 (a) Schematic representation of the synthesis of SNCNPs, (b) TEM...
Figure 2.13 (a) TEM image, (b) HRTEM image of 20 w% CuO/TiO
2
nanocomposite....
Figure 2.14 Schematic representation of the synthesis of Cu‐based NWs.
Figure 2.15 Schematic representation of the synthesis of Cu
3
N cube nanostruc...
Figure 2.16 (a) Schematic representation of NPs synthesis, (b) SEM image, (c...
Figure 2.17 Process of particles formation in a plasma state.
Figure 2.18 (a) Schematic diagram of the preparation of PtO
a
PdO
b
NPs@Ti
3
C
2
T
x
...
Figure 2.19 (a) The schematic diagram of the synthesis process of NiO nanocu...
Figure 2.20 (a) Schematic representation of
DC transferred arc thermal plasm
...
Figure 2.21 (a) Schematic setup Ag NPs preparation via plasma‐assisted hot‐f...
Figure 2.22 (a) Schematic representation of hot‐wire Au NPs synthesis, inclu...
Figure 2.23 (a) Schematic process for Pd NPs‐decorated Hydrogen plasma‐treat...
Figure 2.24 (a) Schematic representation of the synthesis of Fe
3
N NPs on a x...
Figure 2.25 (a) Schematic representation of ALD‐based synthesis of BMNPs on ...
Figure 2.26 (a) Schematic representation of Pd/CM‐TiO
2
‐H catalyst synthesis,...
Figure 2.27 (a) Schematic representation of the preparation process of ZnO N...
Figure 2.28 (a) Synthesis of CNT forest via CVD route, (b) SEM image and the...
Chapter 3
Figure 3.1 Most common equipment used for mechanochemical reactions. (a) Mor...
Figure 3.2 Solvent and catalyst‐free Ugi reaction using twin screw extrusion...
Figure 3.3 Solvent‐ and catalyst‐free Biginelli reaction using twin screw ex...
Figure 3.4 Four solvent‐ and catalyst‐free condensation reactions using twin...
Figure 3.5 Solvent‐ and catalyst‐free synthesis of
N
,
N
′‐disubstituted thiour...
Figure 3.6 Solvent‐ and catalyst‐free synthesis of (2‐amino‐3‐cyano‐4
H
‐chrom...
Figure 3.7 Solvent‐ and catalyst‐free synthesis of imidazo[1,2‐
a
]pyridine de...
Figure 3.8 Solvent‐ and catalyst‐free synthesis of octahydroquinazolinone de...
Figure 3.9 Solvent‐ and catalyst‐free Michael addition of 1,3‐dicarbonyl com...
Figure 3.10 One‐pot solvent‐ and catalyst‐free Wittig reaction via
in situ
f...
Figure 3.11 Solvent‐ and catalyst‐free fluorination of 1,3‐diketones with Se...
Figure 3.12 Solvent‐ and catalyst‐free FeCl
3
‐mediated cyclization of fullere...
Figure 3.13 Solvent‐ and catalyst‐free bromofunctionalization of olefins usi...
Figure 3.14 Synthesis of chiral vicinal diamines through solvent‐ and cataly...
Figure 3.15 Solvent‐ and catalyst‐free synthesis of 2‐substituted benzimidaz...
Figure 3.16 Solvent‐ and catalyst‐free synthesis of oxazolo[5,4‐
b
] quinoline...
Figure 3.17 Solvent‐ and catalyst‐free synthesis of spiropyridine derivative...
Figure 3.18 Solvent‐ and catalyst‐free tandem one‐pot synthesis of thiazolid...
Figure 3.19 Solvent‐ and catalyst‐free synthesis of
N
‐(cyanomethyl)urea usin...
Figure 3.20 Solvent‐ and catalyst‐free one‐pot synthesis of tetrahydrobenzo[
Figure 3.21 Solvent‐ and catalyst‐free syntheses of indolizinoindoles and as...
Figure 3.22 Solvent‐ and catalyst‐free synthesis of 2,4‐disubstituted‐1,2‐di...
Figure 3.23 Solvent‐ and catalyst‐free synthesis of highly functionalized py...
Figure 3.24 (a) Solvent‐ and catalyst‐free Petasis reaction using microwave ...
Figure 3.25 Solvent‐ and catalyst‐free synthesis of several biologically act...
Figure 3.26 Solvent‐ and catalyst‐free benzannulation of aryl[4‐aryl‐1‐(prop...
Figure 3.27 Solvent‐ and catalyst‐free synthesis of tryptanthrins using micr...
Figure 3.28 Solvent‐ and catalyst‐free multicomponent synthetic reaction of ...
Figure 3.29 Solvent‐ and catalyst‐free synthesis of imidazo[1,2‐
a
]pyridines ...
Figure 3.30 Solvent‐ and catalyst‐free synthesis of hydroxylated 2,4,6‐triar...
Figure 3.31 Solvent‐ and catalyst‐free synthesis of 1,2,4‐trisubstituted imi...
Figure 3.32 Solvent‐ and catalyst‐free synthesis of pyrano[3,2‐
c
]chromenones...
Figure 3.33 Solvent‐ and catalyst‐free synthesis of 4‐(quinolin‐2‐yl)phenols...
Figure 3.34 Solvent‐ and catalyst‐free synthesis of phthalazinones using cla...
Figure 3.35 (a) Solvent‐ and catalyst‐free cyanosilylation of aldehydes, and...
Figure 3.36 Solvent‐ and catalyst‐free synthesis of organophosphorus derivat...
Figure 3.37 Solvent‐ and catalyst‐free synthesis of 4‐hydroxyquinolin‐2(1
H
)‐...
Figure 3.38 Solvent‐ and catalyst‐free synthesis of cycloalkenyl phosphonate...
Figure 3.39 Solvent‐ and catalyst‐free synthesis of 3‐aminoimidazo[1,2‐
a
]pyr...
Figure 3.40 Solvent‐ and catalyst‐free acetylation reaction of amines with i...
Figure 3.41 Solvent‐ and catalyst‐free synthesis of bis(indolyl)methane deri...
Figure 3.42 Solvent‐ and catalyst‐free synthesis of pyrazole[3,4‐
b
]thieno[2,...
Figure 3.43 Solvent‐ and catalyst‐free stereoselective synthesis of (
E
)‐trif...
Figure 3.44 Solvent‐ and catalyst‐free diastereoselective synthesis of nitro...
Figure 3.45 Solvent‐ and catalyst‐free synthesis of 2‐substituted benzothiaz...
Figure 3.46 Solvent‐ and catalyst‐free synthesis of 6
H
‐chromeno [4,3‐
b
]quino...
Figure 3.47 Solvent‐ and catalyst‐free synthesis of aminonaphthoquinones usi...
Figure 3.48 Solvent‐ and catalyst‐free synthesis of imidazo‐fused thiazoles ...
Figure 3.49 Solvent‐ and catalyst‐free
N
‐formylation of amines using ultraso...
Figure 3.50 Solvent‐ and catalyst‐free
N
‐Boc protection of various amines us...
Figure 3.51 Solvent‐ and catalyst‐free
O
‐TMS protection of various alcohols ...
Figure 3.52 Solvent‐ and catalyst‐free synthesis of
N
,
N
‐diarylsubstituted fo...
Figure 3.53 Solvent‐ and catalyst‐free synthesis of α‐aminophosphonates usin...
Figure 3.54 Solvent‐ and catalyst‐free synthesis of α‐sulfamidophosphonates ...
Figure 3.55 Solvent‐ and catalyst‐free synthesis of 1‐ferrocenyl‐3‐amino car...
Figure 3.56 Solvent‐ and catalyst‐free synthesis of 2,2′‐(1,4‐phenylene)bis[...
Figure 3.57 Solvent‐ and catalyst‐free synthesis of phosphonamide derivative...
Chapter 4
Figure 4.1 Applications of metal oxides.
Figure 4.2 Heterogeneous catalyst utilization in industrial applications....
Figure 4.3 Effect of solvent on isatin condensation reaction with
o
‐phenylen...
Figure 4.4 Synthesis of sulfonic acid‐functionalized TiO
2
catalyst.
Figure 4.5 Mesoporous SnO
2
as a catalyst for different reactions.
Figure 4.6 Dehydration of sorbitol to isosorbide.
Figure 4.7 ZnO nanoflower synthesis mechanism.
Figure 4.8 KI‐loaded Fe
3
O
4
catalyst for the synthesis of β‐nitroalcohol via ...
Chapter 5
Figure 5.1 Schematic structure of the surface of silica gel.
Scheme 5.1 The nitration of
m
‐cresol using 69% nitric acid in the presence o...
Scheme 5.2 Ozone‐mediated amine oxidation in continuous flow.
Figure 5.2 Scope of the continuous ozone‐mediated reactions.
Figure 5.3 Michael addition mediated by silica gel, and scope of the Michael...
Scheme 5.3 Synthesis of trisindoline using silica gel and HCl loading.
Scheme 5.4 Solvent‐free microwave (MW) synthesis of substituted imidazoles o...
Figure 5.4 Scanning (a) and transmission (b) electron microscopy image of me...
Scheme 5.5 Acetalization of glycerol with benzaldehyde using SG‐[C
3
ImC
3
SO
3
H]...
Figure 5.5 Acylation of aromatic amine, alcohols, and thiols with Ac
2
O under...
Scheme 5.6 Solvent‐free deoximation on ammonium persulfate on silica.
Scheme 5.7 Transamidation of amides in the presence of mesoporous silica und...
Figure 5.6 Structures of selected zeolites. Color coding: green = Zn, red = ...
Scheme 5.8 Mechanism for the solvent‐free liquid‐phase Knoevenagel condensat...
Scheme 5.9 Continuous‐flow catalytic acetalization of glycerol using a HZSM‐...
Figure 5.7 Preparation of silica‐supported sulphonic acid catalysts coated w...
Chapter 6
Figure 6.1 Schematic illustration of alkaline ACs catalyzed various types of...
Figure 6.2 (a) Cyclohexane to KA oil conversion in the presence of AC materi...
Figure 6.3 (a) Synthesis of 1,8‐dioxodecahydroacridines catalyzed by CBSA ca...
Figure 6.4 CBSA‐catalyzed synthesis of amidoalkyl naphthols.
Figure 6.5 Schematic illustration for (a) the nitration of aromatic rings, (...
Figure 6.6 (a) and (b) represent the direct and indirect reductive amination...
Figure 6.7 Preparation of 14‐aryl‐14‐
H
‐dibenzo[
a,j
]xanthenes and suggested t...
Figure 6.8 Synthesis of dihydropyrimidin‐2(1
H
)‐ones using CCBSA.
Figure 6.9 Chemoselectivity of green Pis‐SO
3
H catalyst in acetalization, acy...
Figure 6.10 A possible reaction mechanism for the oxidative esterification o...
Figure 6.11 The preparation process and catalytic evaluation of surface‐func...
Figure 6.12 Preparation of the heterogeneous catalytic system.
Figure 6.13 Proposed reaction mechanism for the CuO NPs/rGO composite cataly...
Figure 6.14 Proposed synthesis scheme for different metal‐graphene complex n...
Figure 6.15 (a) A schematic illustration of the preparation of rGO‐AO‐TO/Ru ...
Figure 6.16 Model structures of graphite oxide (GO) (a) and sulfonated reduc...
Figure 6.17 Proposed reaction mechanism of 5‐substituted‐3
H
‐[1,3,4]‐oxadiazo...
Figure 6.18 An illustration of proposed reaction pathways for mesoporous car...
Chapter 7
Figure 7.1 3D structures of carbon nitrides' motifs: (a) melamine, (b) melam...
Figure 7.2 Synthesis of polyimides: M‐FDA, M‐NTDA, and M‐ODPA from melem and...
Figure 7.3 Solvent‐free photocatalytic H‐transfer reaction.
Figure 7.4 One‐pot synthesis of xanthene derivatives through solvent‐free re...
Figure 7.5 Hydrosilylation of alkynes affording three possible isomeric viny...
Figure 7.6 Mpc/g‐C
3
N
4
‐promoted chemical fixation of CO
2
to cyclic carbonate....
Figure 7.7 Cycloaddition of CO
2
and styrene oxide to produce styrene carbona...
Figure 7.8 g‐C
3
N
4
‐NaOH‐utilized CO
2
cycloaddition reaction to epoxide.
Figure 7.9 The Sonogashira–Hagihara cross‐coupling reaction in the presence ...
Figure 7.10 g‐C
3
N
4
/Cu
2
O‐utilized synthesis of propargylamines.
Figure 7.11 One‐pot three‐component synthesis of pyrimidoindazoles applying ...
Figure 7.12 p‐CNN photocatalytic synthesis of DHPMs under solvent‐free condi...
Figure 7.13 p‐CNN photocatalytic reaction for the synthesis of 12‐phenyl‐9,9...
Figure 7.14 Photocatalytic p‐CNNs for the synthesis of 5‐phenyl‐1(4‐methoxyp...
Figure 7.15 Cycloaddition reactions of various epoxides with CO
2
catalyzed b...
Figure 7.16 Synthesis of quinoxaline derivatives applying CN‐Pr‐VB
1
as catal...
Figure 7.17 Ring‐opening of various epoxides promoted by magnetic support ca...
Figure 7.18 Selective oxidation of cyclohexane to cyclohexanol via Co‐g‐C
3
N
4
Figure 7.19 Ring‐opening reaction of diverse epoxides with arylamines in the...
Figure 7.20
n
‐butBr/mp‐C
3
N
4
utilized synthesis of propylene carbonate.
Figure 7.21 ompg‐C
3
N
4
/SO
3
H‐catalyzed reaction for the synthesis of 1,2‐dihyd...
Figure 7.22 Synthesis of 2,3‐dihydroquinazolin‐4(1
H
)‐one derivatives catalyz...
Figure 7.23 Phosphorous g‐C
3
N
4
catalyzed the synthesis of cyclic carbonates....
Figure 7.24 Synthesis of dihydropyrimidine derivatives through g‐C
3
N
4
@SO
3
Ch‐...
Figure 7.25 Photocatalyzed multicomponent Biginelli reaction.
Figure 7.26 Crystal structure of hexagonal‐layered BN adapted from [69] proj...
Figure 7.27 (a) Crystal structure of hexagonal MoN from different projection...
Figure 7.28 Schematic illustration for the preparation of PC‐PMo‐Mel‐800 thr...
Figure 7.29 Oxidative coupling of benzylamine promoted by modified MoN.
Figure 7.30 AlN‐catalyzed solvent‐free synthesis of α‐aminophosphonates.
Chapter 8
Figure 8.1 Advantages and disadvantages of different catalysis strategies.
Figure 8.2 Different types of IL‐support interaction. (a) Physisorption of t...
Figure 8.3 Common cations and anions used to prepare ILs. R, R
1
, R
2
, R
3
, R
4
,...
Figure 8.4 Classification of common support materials according to chemical ...
Figure 8.5 Schematic silica gel surface.
Figure 8.6 Grafting silylated ILs onto ordered mesoporous silicas.
Figure 8.7 Categorization of supported ILs according to the phase behavior. ...
Figure 8.8 Different types of supported IL catalytic systems. The red color ...
Figure 8.9 Different types of commonly used TSILs to prepare SILs.
Figure 8.10 Hydrogenation reaction of olefins using Pd NPs immobilized on mo...
Figure 8.11 Solvent‐free Heck arylation of olefins using SBA‐TMG‐Pd.
Figure 8.12 The cross‐linked polymer‐supported amine‐functionalized IL used ...
Figure 8.13 Heck cross‐coupling reaction catalyzed by Pd‐porphyrin‐functiona...
Figure 8.14 Solvent‐free Mizoroki–Heck reaction using clay‐supported functio...
Figure 8.15 Solvent‐free synthesis of benzoxanthenes using SMNP‐supported ac...
Figure 8.16 The MNP‐supported Lewis acidic IL used for solvent‐free synthesi...
Figure 8.17 The SMNP‐supported urea‐based IL used for solvent‐free synthesis...
Figure 8.18 The SMNP‐supported acidic pyridinium‐based IL was used to synthe...
Figure 8.19 Solvent‐free synthesis of henna‐based benzochromenes via SBA‐sup...
Figure 8.20 Solvent‐free synthesis of tetrahydrobenzo[
b
]pyrans via DAIL@SiO
2
Figure 8.21 Solvent‐free synthesis of 2‐amino‐3‐cyano‐4
H
‐pyran using support...
Figure 8.22 The (a: SB‐DBU
+
Cl
−
), (b: NSB‐DBU
+
Cl
−
), and (...
Figure 8.23 Different supported IL catalysts used for solvent‐free symmetric...
Figure 8.24 Solvent‐free synthesis of 1‐amidoalkyl naphthol derivatives usin...
Figure 8.25 Solvent‐free synthesis of 1,3‐thiazolidin‐4‐ones using MNPs@SiO
2
Figure 8.26 Solvent‐free synthesis of thiazoloquinolines using Fe
3
O
4
/SiO
2
/sa...
Figure 8.27 Synthesis of 2,4,5‐trisubstituted imidazoles using supported mul...
Figure 8.28 Solvent‐free synthesis of α‐aminophosphonates via SMNP‐supported...
Figure 8.29 Solvent‐free synthesis of polysubstituted quinolines catalyzed b...
Figure 8.30 The MPIL‐derived solid‐base catalyst used for Knoevenagel conden...
Figure 8.31 Solvent‐free diesterification of phthalic anhydride, maleic anhy...
Figure 8.32 Schematic representation of bV‐Imi‐NT used for solvent‐free synt...
Figure 8.33 The MIL‐101(Cr)‐TSIL catalyst used for solvent‐free cycloadditio...
Figure 8.34 The SBA‐15‐supported aminofunctionalized IL used for solvent‐fre...
Figure 8.35 Solvent‐free hydroxylation of aromatic compounds using silica‐su...
Figure 8.36 The solvent‐free diazotization–halogenation reaction of aromatic...
Figure 8.37 Solvent‐free N‐formylation of amines using SMNP‐supported imidaz...
Figure 8.38 Solvent‐free
N
‐aryl oxazolidin‐2‐ones using SMNP‐supported aceta...
Figure 8.39 Solvent‐free Michael addition of ketones to β‐nitrostyrene using...
Figure 8.40 Solvent‐free synthesis of ethers using SBA‐supported multilayere...
Figure 8.41 Solvent‐free synthesis of carbamates using nanostarch‐supported ...
Figure 8.42 Solvent‐free synthesis of
N,N
′‐diaryl‐substituted formamidines u...
Figure 8.43 Solvent‐free continuous flow cyanosilylation of carbonyl compoun...
Figure 8.44 The task‐specific SILLP system containing imidazolium‐sulfonic a...
Chapter 9
Figure 9.1 Green chemical principles for material synthesis and reaction pro...
Figure 9.2 Preparation of tailor‐made and sustainable catalysts for a given ...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Solvent‐Free Methods in Nanocatalysis
From Catalyst Design to Applications
Edited by Rafael Luque, Manoj B. Gawande, Esmail Doustkhah, and Anandarup Goswami
Editors
Prof. Rafael LuqueUniversidad de CórdobaDepartamento de Química OrgánicaCarretera Nacional IV‐A, Km. 396Edificio C‐314014 CórdobaSpain
Prof. Manoj B. GawandeDepartment of Industrial and Engineering ChemistryInstitute of Chemical Technology MumbaiMarathwada CampusAurangabad RoadJalna‐431213, MaharashtraIndia
Dr. Esmail DoustkhahKoç University Tüpraş Energy Center (KUTEM)Koç University34450 IstanbulTurkey
Prof. Anandarup GoswamiVignan's Foundation for Science Technology and Research (VFSTR)Department of Chemistry, School of Applied Science and HumanitiesVadlamudi522 213 GunturIndia
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Preface
This book is a collection of recent developments in the area of solvent‐free synthesis and catalysis. This book can be a suitable collection for students and researchers of green chemistry. We are witnessing a tremendous effort from green chemistry scientists in developing solvent‐free organic reactions since solvent‐free methods are assumed to be a promising approach to greening chemical transformations. Solvent‐free reactions are not only a green approach but also, in some cases, it is the only highly efficient and straightforward strategy and the last resort that chemists can take it to have an facile and highly efficient synthesis. In this regard, coupling techniques such as microwave, ultrasonic irradiation, and ball‐milling techniques have been the new gates in the development of solvent‐free techniques. Therefore, we have invited the outstanding researchers and professors of the field to contribute to this book to build a comprehensive collection of green chemistry to review and discuss the reports from very old to very recent.
In this era, we believe that chemists should move on to developing solvent‐free methods of catalysts syntheses. Although catalysts in some cases are being synthesized using solvent‐free methods – also known as a solid‐state synthesis methods – they are not genuinely highlighted in the green chemistry synthesis class. Hence, this part of the catalyst synthesis needs more attention and highlight.
Here, in this book, we have collected the solvent‐free methods that can be classified through the catalyst type, e.g., carbon or silica, and besides, all the solvent‐free and catalyst‐free approaches together in a separate chapter (Chapter 3). This classification helps chemists and researchers to understand the efficiency and nature of these catalysts based on the chemical structure of the catalysts. As the last word, we hope this book could be a small portion in confronting the environmental threats coming from the inevitable chemical processes. We wish this book could attract the students and researchers more toward green chemistry of catalysts synthesis and the reactions.
Esmail DoustkhahKoç University, TurkeyManoj B. GawandeInstitute of Chemical Technology, IndiaAnandarup GoswamiVignan's Foundation for Science, IndiaRafael LuqueUniversidad de Córdoba, Spain
1Introduction: Scope of the Book
Anil Kumar Nallajarla and Anandarup Goswami
Vignan's Foundation for Science, Technology and Research (VFSTR deemed to be University), Department of Chemistry, School of Applied Science and Humanities, Vadlamudi, Guntur 522 213, Andhra Pradesh, India
1.1 Introduction: Green Chemistry, Solvent‐free Synthesis, and Nanocatalysts
Since the realization that the future of chemical/industrial processes primarily depends on their sustainable quotient, the path of modern science has shifted toward the improvement of processes/products following green chemistry principles [1, 2]. Green chemistry as a branch of chemistry primarily deals with developing chemical processes using environmentally benign protocols, including inexpensive renewable less‐toxic precursors. In that respect, the “12 rules of green chemistry,” first formally introduced by Anastas and Warner in their book “Green Chemistry: Theory and Practices” [3], play a pivotal role in identifying the areas that should be focused to achieve the expected sustainability goals [4]. For chemical processes, the main aim of introducing these 12 principles is to save the environment and society by reducing the usage of toxic and hazardous chemicals and solvents without affecting the product yield/selectivity. While the 12 principles are quite self‐explanatory, as depicted in Figure 1.1, the emphasis in certain areas often depends on the convenience of implementing them for specific protocols and the outcome. Of these 12 principles, this book focuses on the historical and recent developments of strategies that minimize solvent use in a chemical process, often termed “solvent‐free synthesis,” and the associated catalytic procedures involving nanomaterials.
The use of a solvent in a reaction creates a homogeneous solution phase where the reactants can interact effectively. While ideally, any liquid can be used as a solvent, the focus is largely on solvents based on their polarity and protic nature (e.g. methanol, ethanol, chloroform, dichloromethane, dimethylformamide [DMF], dimethyl sulfoxide [DMSO], toluene). The primary reasons can be attributed to their ability to solubilize various organic reactants/products as well as to control the stability of the transition state/intermediates (leading to modifications of the thermodynamic and kinetic reaction parameters) [5]. However, with the growth of industrial chemical processes along with growing interest in developing sustainable protocols, emphasis has been shifted toward choosing the best solvent based on not only their solubilizing power but also their abundance, cost, and, last but not least, short‐ and long‐term impacts on environment [6]. In that context, water has long been considered a sustainable choice. However, the poor solubility of organic species in aqueous solution has limited its wide use primarily at the industrial scale [7, 8]. While some of the current choices, such as ionic liquids (ILs), often come as rescue options, their selection has remained an area of concern for the processes related to bulk scale production of materials [9]. Around this debate regarding the choice of solvent, the idea of “no solvent is best solvent” was also considered. However, it did not receive its due because of the lack of initial appreciation, especially for industrial purposes. However, with the advent of eco‐friendly and greener approaches, the idea of “solvent‐free synthesis” resurfaced, and presently, it is being explored as one of the viable options for the synthesis of chemicals as well as various materials (Figure 1.2a) [10]. Initially, synthesis under solvent‐free conditions was associated with the solid‐phase synthesis where the reactants were made to react in the solid phase. However, recent advances in the area of materials syntheses (which include thermal treatment, plasma etching, etc.) have extended its scope significantly [11, 12]. Modern‐day scientific and technological developments are primarily governed by the utilization of materials for specific purposes. Thus, the choice of their synthetic strategies is often determined by the type of materials, their subsequent use, and their sustainable quotient. Considering these, the class of “nanomaterials” has emerged as a crucial player. Hence, a brief introduction of nanomaterials with special emphasis on their catalytic applications seems timely before moving to a detailed discussion on their solvent‐free synthetic procedures.
Figure 1.1 12 principles of green chemistry.
Nanomaterials are the class of materials size that falls under 1–100 nm in at least one dimension (Figure 1.2b) [13, 14]. The exceptional growth in the development of nanomaterials can be attributed to high surface area, quantum confinement effect, and the possibility of fine‐tuning their surface properties utilizing relatively straightforward methods. While the natural origin of nanomaterials can be traced back to the time of big‐bang, Prof. Feynman, in his great lecture series “There is plenty of room at the bottom,” first introduced the enormous potential of nanomaterials [15, 16]. Since then, progress in nanomaterials has been significant, and in the modern world, it is nearly impossible not to encounter nanomaterials in daily lives [17, 18]. Among the various fields in which nanomaterials have been explored, one of the areas involves their catalytic applications due to their unique size and shape‐dependent surface properties [19, 20].
Figure 1.2 Introduction to (a) solvent‐free synthesis, (b) nanomaterials, (c) catalysts, and (d) nanocatalysis. CNTs = Carbon nanotubes.
The term “catalyst” (or “catalysis”) was first introduced in 1835 by Swedish chemist J.J. Berzelius [21, 22]. Since then, the catalyst is defined as a substance/material that improves the reaction rate by minimizing the activation energy of the process without being consumed during the process (Figure 1.2c). The initial developments in catalysis were concentrated on relatively expensive transition metal–derived systems primarily due to their intrinsic catalytic properties. However, the need for sustainable results has allowed the recent advancements to focus on less‐toxic, low‐cost, and highly abundant and recyclable catalytic alternatives [23, 24]. Though the categorization of catalysts may vary depending on the classification criteria, catalysts are commonly divided into two classes: homogeneous and heterogeneous [25]. Homogeneous catalysts are the compounds that remain in the same phase as reactants/products during the catalytic reactions (mostly in the presence of a suitable solvent). While homogeneous catalysts often exhibit higher catalytic activity, poor separation and recyclability appear significant challenges. In contrast, heterogenous systems involve different reactants/products and catalyst phases and ideally have better separation and reusability. However, due to various mass transfer and diffusion limitations, the catalytic activity of heterogeneous catalysts remains inferior to their homogeneous counterparts. Thus, a combination of advantageous factors for both systems is essential to overcome the existing challenges of both sides to achieve the desired goals. In that context, the utilization of nanomaterials (often termed “nanocatalysts,” [s], Figure 1.2d) either as catalysts or as support materials for various homogeneous/heterogeneous catalytic entities has opened newer avenues as they often exhibit the potential to overcome the respective challenges in homo‐ and heterogenous catalysts [26].
The synthesis of NCs does not deviate too much from the synthesis of nanomaterials. It hence can primarily be classified into “top‐down” and “bottom‐up” approaches [27, 28], each of which can further be divided based on specific techniques. In “top‐down” approaches, NCs are prepared from the bulk using various “cutting” techniques, whereas the “bottom‐up” approaches involve synthesis of NCs from their atomic and/or molecular precursors. Both approaches have their own advantages and disadvantages, and often, the choice of synthetic methods is dictated by the NCs' specific properties and applications. For instance, while various “top‐down” strategies are preferred for carbon‐based nanomaterials (e.g. graphene, nanotubes), metal‐oxide nanoparticles (NPs) are generally synthesized using “sol–gel” techniques [29]. Irrespective of the synthetic processes, “solvent‐free” methods are always preferred as they can be directly related to the goals of green and sustainable transformations.
This brief introduction provides a general idea about various related topics interlinked with a common theme of sustainability and hopefully allows the readers to have a smooth transition in the remaining parts of the chapter.
1.2 Topics Covered in this Book
The chapters in this book are carefully designed to provide a very in‐depth idea about NCs and their applications using solvent‐free methods. In Chapter 2, Gawande and coworkers provide an illustrative overview of the syntheses of nanocatalysts using solvent‐free methods. The idea of introducing this chapter is to make readers aware of various synthetic techniques and their advantages and disadvantages that can be used for the solvent‐free preparation of nanocatalysts. The remaining part of the book (except the conclusion chapter, Chapter 9) primarily deals with catalytic applications of nanocatalysts using solvent‐free methods. In Chapter 3, Prof. Zamani introduce the topic of solvent and catalyst‐free organic transformations with specific examples of academic and industrial importance to set up the stage for the next chapters. While Manyar and coworkers describe various solvent‐free organic transformations, catalyzed by metal/metal‐oxide nanocatalysts in Chapter 4, a separate chapter is dedicated to silica‐based nanomaterials as catalysts/support for solvent‐free organic reactions (Chapter 5). In Chapter 6, Prof. Torad emphasizes the importance of carbon‐based nanomaterials either as supports or as nanocatalysts for solvent‐free organic reactions, focusing on doped and functionalized nanocarbons. In Chapters 7 and 8, concentration has been deliberately steered toward current developments in the areas of solvent‐free reactions using nitride‐based and ionic liquid‐based nanocatalytic systems, respectively, because they have been explored recently as potential sustainable choices in comparison to the existing ones. In the book's concluding chapter (Chapter 9), Prof. Pieta summarizes the present status of solvent‐free synthesis of nanomaterials and solvent‐free nanocatalytic transformations, emphasizing their relevance with green chemistry and sustainability. The author also provides a brief account of the challenges related to current approaches and some possible solutions as an outlook. Starting from the introductory chapter to the concluding one, the primary focus of this book has been on providing readers with insights into the background, recent advances, and future possibilities regarding solvent‐free synthesis of nanocatalysts as well as solvent‐free nanocatalytic methods. We, as contributors, strongly feel that this book will be helpful to students and researchers who want to gain knowledge about these topics and pursue their research in those areas. Following that thought, the next sections highlight some solvent‐free methods used in the synthesis of NCs, followed by their specific catalytic applications.
1.3 Solvent‐Free Synthesis of Nanocatalysts
As the name suggests, solvent‐free methods for preparing uniform, monodisperse solid NCs essentially follow the synthetic protocols that do not use any solvent [27, 30]. The size and shape of the synthesized NCs vary depending on the methods and the reaction conditions. While the approaches used in these syntheses can also be considered as a part of traditional “top‐down” or “bottom‐up” based classification, the present literature examples tend to categorize them in terms of specific procedures. In that context, primarily four types of procedures along with their subclassifications have been reported: (i) mechanochemical, (ii) thermal, (iii) plasma‐assisted, and, last but not least, (iv) deposition techniques (Figure 1.3).
The mechanochemical process for the synthesis of nanomaterials primarily involves grinding of bulk precursors into nanoscale materials using mechanical force [31, 32]. In that category, the traditional “ball‐milling” approach has widely been used [33–35]. For example, Barcellos and coworkers synthesized CuO NCs using high‐energy ball milling, and the synthesized NCs were used for nitroarene reduction under aqueous media [36]. Recently, solid‐phase synthesis that involves grinding the precursor materials using a mortar and pestle has also generated much interest owing to its simple operational procedure [37]. For instance, gram‐scale synthesis of Au/chitosan was reported by Reddy et al. recently using a mortar and pestle. The nanocatalyst was used for catalytic homocoupling of phenylboronic acid and the aerobic oxidation of benzyl alcohol in water [38].
One of the most widely used and well‐studied approaches for the synthesis of NCs includes the preparation of NCs using heat as an energy source, and this strategy (often termed “thermal treatment”) primarily involves heating of molecular precursors at high temperatures under an oxidative or reductive environment to obtain nanomaterials [39–41]. For example, various metal oxides are routinely synthesized from their precursors using heat treatment under an oxidative environment (e.g. air, O2) [42]. However, for synthesizing carbon‐based nanomaterials, precursors are often pyrolyzed under an inert atmosphere (N2, Ar, etc.). The product yield, extent of doping, and degree of graphitization depend on the nature of precursors, temperature, heating rate, etc. [43, 44]. Goswami et al. showed that a hydrogen‐assisted thermal treatment could be used to convert metal precursors into metal NPs [45]. As opposed to conventional heating treatment, to make the synthetic process greener and more sustainable, alternative energy sources (such as microwave (MW) or ultrasound) or bio‐derived precursors/processes have also been used to prepare metal nanoparticles NPs [46–48].
With the advent of instrumental developments, several sophisticated “top‐down” approaches have been developed to synthesize NCs. Among them, plasma‐assisted strategies have shown great promise due to their environmentally benign nature, no additional requirements of solvent or stabilizing agents, etc. [49]. In this case, “feed materials” are transformed into atoms or molecules through vaporization with the help of plasma and thus NPs/NCs are formed. The size, morphology, and properties of the final materials are generally dependent on the absolute temperature of plasma, the kinetics of plasma formation, quenching process, and the size and composition of feed materials. Based on the internal energy of electrons used for plasma generation, this can be classified into two categories: (i) thermal plasma and (ii) cold‐plasma methods [50]. While plasma is used to generate an intense heat source in both cases, the significant difference between these two processes lies in the operating temperature. In the case of thermal plasma, feed materials are atomized at an operating temperature of a few thousand degrees. Nanomaterials/NCs are formed during the cooling process. For example, NiO nanocubes were prepared from bulk Ni metal utilizing thermal plasma with oxygen as a carrier gas [51]. On the other hand, cold plasma uses a low‐pressure, low‐temperature method where NCs can be synthesized even at room temperature [52]. This energy‐efficient method is primarily used to synthesize noble‐metal NPs (e.g. Pd, Ag, Au), as exemplified by recent reports [53–55]. Recently, Haye et al. have also utilized this method to synthesize non–noble‐metal‐based FeNPs (more precisely Fe3N) embedded on carbon support [56].
Figure 1.3 Representative solvent‐free methods used for the synthesis of NCs. NPs = Nanoparticles.
Another solvent‐free method for the synthesis of nanomaterials/nanocatalysts that gained tremendous attention is deposition techniques (more precisely, vapor deposition techniques) [57, 58]. In this method, nanomaterials/nanocatalysts are deposited on a substrate in the form of thin films from their atomic precursors. Depending on the nature of deposition, they can further be classified into physical and chemical vapor deposition (CVD), and among them also several subclassifications are made. In the context of solvent‐free synthesis of NPs, CVD techniques [59] and atomic layer deposition (ALD) techniques [60] have been widely explored. For instance, carbon‐based support materials such as carbon nanotubes (CNTs) and graphene are routinely synthesized using the CVD method [61–63]. In addition, metal NPs have been embedded onto a carbon matrix using the CVD method [64, 65]. On the other hand, the precursors copper(II)‐hexafluoroacetylacetonate [Cu(hfac)2] and diethylzinc [DEZ, (C2H5)2Zn] were used to synthesize Cu/ZnO‐50 nanocatalysts using the ALD method [66].
The aforementioned examples are the only selected ones chosen from the vast pool of synthetic strategies employed for the preparation of NPs while these are primarily representative of the solvent‐free protocols; a detailed discussion on this topic is included in Chapter 2.
1.4 Solvent and Catalyst‐Free Organic Transformations
To have a smooth transition from the solvent‐free synthesis of NCs to solvent‐free processes utilizing NCs; as an intermediate, a separate section has been devoted to solvent and catalyst‐free organic transformations (Figure 1.4). As opposed to the use of relatively toxic catalytic entities and solvents, solvent‐ and catalyst‐free organic transformations can be considered among the classes of reactions that aim to follow sustainability goals [78]. While these straightforward protocols have shown great promise, the desired success of these procedures is often restricted due to poor yield, significant energy investment, etc. However, in recent years, the advancement of alternative energy sources and increasing knowledge of fundamental reaction mechanisms enable them to get closer to the expected ideal outcome.
Figure 1.4 Representative examples of solvent and catalyst‐free organic transformations.
Source: Adapted from the references given in parentheses.
Among other processes, mechanochemical processes hold a special place because of the utilization of simple mechanical force/energy to drive the reactions forward and its underexplored potential for large‐scale production of organic compounds without complicated purification steps. Different types of mechanical processes have been developed depending on the types of reactions and their outcome, among which ball‐milling, twin‐screw, mortar and pestle methods have become very popular. For example, thiourea derivatives were synthesized using ball‐milling within 10–20 minutes [67], and various multicomponent reactions (MCRs) such as Ugi, Biginelli have been carried out using twin‐screw extruder (TSE) [68]. Additionally, mortar and pestle method has also been exploited for the synthesis of fused heterocycles [69]. These are representative of the enormous possibility of mechanochemical synthesis.
Conventional thermal heating under solvent‐ and catalyst‐free conditions is often considered the classical way of achieving the desired product for any chemical transformations. Despite the challenges related to the product selectivity and energy efficiency that have impacted the long‐term use of the process negatively, several heterocycles (including imidazoles [70], pyrazoles [71]), phosphonates [72] have been synthesized under solvent‐ and catalyst‐free thermal heating methods.
To tackle the challenges related to the conventional thermal heating process, alternative energy sources such as MW and ultrasound have given a fresh impetus to make the synthetic methods more energy efficient and greener [73, 74]. The use of MW under solvent‐ and catalyst‐free conditions has been explored to synthesize spiro compounds [75], N‐containing heterocycles [79], etc. The employed protocols have shown faster kinetics, better reactivity, higher selectivity, and broader substrate scope. The utilization of ultrasound irradiation to drive a chemical reaction is also considered one of the green approaches. In this context, a combination of solvent‐ and catalyst‐free method with ultrasound irradiation stands unique because of safer energy inputs, waste reduction, higher yields/selectivity, etc., compared to other traditional approaches [80]. Starting from simple formylation [76] or protection of amines [77] to multicomponent coupling of heterocycles [81], the use of ultrasound irradiation has proven to be highly efficient, as exemplified in recent examples.
The aforementioned methods provide a glimpse of solvent‐ and catalyst‐free approaches that have shown great promise compared to traditional ones. More details about these procedures/approaches are provided in Chapter 3.
1.5 Solvent‐Free Reactions Using NCs
Most of the solvent‐free reactions using NCs are largely focused on three major classes: (i) metal or metal‐oxide NPs (Figure 1.5), (ii) silica‐based NCs (Figure 1.6), and (iii) carbon‐based nanosystems as catalyst/support (Figure 1.7). Additionally, nitride‐based (Figure 1.8) and ionic liquid (IL)‐based NCs (Figure 1.9) have recently been reported. While such topics will be discussed in detail in subsequent chapters, several such reactions are highlighted to exhibit a variety of the responses that can be performed using these NCs.
Figure 1.5 Representative examples of solvent‐free nanocatalytic procedures using metal‐oxide NPs as catalysts/supports.
Source: Adapted from the references given in parentheses.
MNPs = Magnetic nanoparticles.
Figure 1.6 Representative examples of silica‐supported nanocatalytic systems for organic transformations.
Source: Adapted from the references given in parentheses.
Figure 1.7 Saluted examples of carbon‐based nanocatalysts for organic transformations under solvent‐free conditions.
Source: Adapted from the references given in parentheses.
1.5.1 Different Metal Oxides as a Catalyst/Support in Solvent‐Free Reaction
1.5.1.1 Titanium Oxide
Titanium oxide (TiO2) and titanium oxide–supported catalysts are used for organic reactions, including solvent‐free methods, because of their unique properties and catalytic activity. In 2007, M. Hosseini‐Sarvari et al. reported TiO2 as a new and reusable nanocatalyst for the Knoevenagel condensation reaction, which exhibited good to excellent yields [100]. For efficient conversion of cyclohexylamine into cyclohexanone oxime (78.4%) with high selectivity (89.1%) under solvent‐free conditions, Liu et al. used mobil composition of matter (MCM)‐41‐supported TiO2 NCs [82]. After using for five cycles, no significant change was observed in catalytic efficiency, and characterization data showed the hydroxyl groups on titania acted as catalytic sites. Recently, Amoozadeh and coworkers synthesized nickel(II) Schiff base complex supported on nano‐titanium dioxide, and the supported NCs were used for the synthesis of 3,4‐dihydopyrimidin‐2(1H)‐ones through Biginelli reaction under solvent‐free conditions [83]. The catalytic activity was found to be superior to previous catalytic systems and showed diverse substrate scope.
1.5.1.2 Tin Oxide
Tin oxide nanoparticles (more specifically SnO2 NPs) exhibit excellent catalytic properties in various organic reactions to synthesize organic compounds. In addition to the high catalytic activity/selectivity of SnO2, the possibility of easy separation (or reusability) has made this oxide unique as exemplified in several instances, including ones that use solvent‐free conditions. SnO2 NPs as NCs were utilized to synthesize 2H‐indazolo[2,1‐b]phthalazine‐triones using an MCR of aromatic aldehydes 1,3‐cyclohexanedione and phthalhydrazide under solvent‐free conditions [84]. The catalytic results showed that the final products can be obtained in good yields with high selectivity. While initial research was more focused on the catalyst and yields, with more advancements in the catalysis, recyclability came into the picture. For example, 1,2,4,5‐tetra substituted imidazoles were prepared using silica‐supported tin oxide under solvent‐free conditions [85]. In addition to high catalytic performance (yield: 84–97%), the catalyst can be separated easily from the reaction mixture and recycled up to five cycles without any significant change in the catalyst's activity and/or composition. In another example, Ahmed et al. synthesized nanocrystalline sulfated tin oxide. They used them as NCs to synthesize coumarin derivatives under solvent‐free conditions using acetoacetate and m‐cresol as reactants [101].
1.5.1.3 Manganese Oxide (MnOx)
Manganese metal is widely used in catalysis because it is inexpensive, is easily available, is less toxic, and shows variable oxidation states ranging from +II to +VII. Its various oxides are also available in different forms such as 3D structure, chain‐like structures, layered or sheet structures. These materials are routinely used as heterogeneous catalysts for numerous chemical transformations. In that respect, graphene oxide–supported manganese oxide (GO/MnO2) was used as a catalyst for synthesizing chalcogens under solvent and solvent‐free conditions [86]. The NCs showed superior catalytic activity than their counterparts, confirming the synthesis between each compound. In another example, manganese oxide–doped magnesium oxide (MnO2/MgO) was employed as a NC to prepare ethyl cinnamate (mostly known as a flavoring agent) using a Witting reaction between benzaldehyde and tri‐phenyl phosphonium salts following a green mechanochemical solvent‐free approach [87].
1.5.1.4 Zinc Oxide
Zinc oxide (ZnO) NPs have also been routinely explored in various applications, including optoelectronic field, photoluminescence devices, and solar cells. In addition, the recent research efforts are also engaged in tuning the shape, morphology, and properties of ZnO NPs for their utilization in electronic and antibacterial applications. In the case of nanocatalysis (especially for organic transformations), ZnO NPs have also been exploited as novel and reusable catalysts. In the present context of solvent‐free reactions, ZnO‐NPs‐catalyzed Biginelli reaction was reported to synthesize dihydropyrimidinones using aromatic aldehydes, urea, or thiourea and acetoacetic esters under solvent‐free conditions [88]. The catalyst showed excellent catalytic activity (94–97% of yields). In another study, a wide range of chloroesters were synthesized from cyclic ethers and acyl chlorides using ZnO NPs as nanocatalysts under solvent‐free conditions at room temperature [102]. The catalyst can provide good yields (87–95%) and be recycled easily up to three cycles. The ZnO nanoflowers, derived from the peel of Musa balbisiana and zinc nitrate, were used as catalysts for synthesizing chalcones via the Claisen–Schmidt condensation reaction using MW irradiation under solvent‐free conditions [89]. The ZnO‐decorated GO nanocomposite acts as a highly efficient reusable catalyst for synthesizing xanthenedione from 1,3‐dicarbonyl compounds and aromatic aldehydes under neat reaction conditions [103]. The final products were obtained in excellent yields, and the catalyst can be used up to five cycles.
1.5.1.5 Aluminum Oxide
Aluminum oxide (Al2O3, commonly known as alumina) has been extensively employed in the separation and purification of organic compounds primarily due to the insolubility of aluminum oxide in both water and organic solvents and variable interactions between the alumina surface and the eluting compounds with different polarities. Moreover, due to alumina's high Lewis acidic nature, catalytic applications of aluminum oxide and supported aluminum oxide‐based materials have also been explored for various organic transformations. For example, alumina NPs were used for the MCR to synthesize dihydropyrimidinones under solvent‐free synthesis through the Biginelli reaction [90]. ZrO2‐stabilized aluminum oxide was also reported as recyclable NCs (up to six cycles) for O‐methoxymethylation reaction between substituted alcohols and dimethoxymethane under solvent‐free conditions [104]. In another case, Shetarian et al. synthesized phosphoric acid–supported aluminum oxide (H3PO4/Al2O3), which could be used as an efficient catalyst for MCRs to synthesize 2H‐indazolo[2,1‐b]phthalazinetriones, 2,3‐dihydroquinazoline4(1H)‐ones, and benzo[4,5]imidazo[1,2‐a]pyrimidines under solvent‐free conditions [91].
1.5.1.6 Iron Oxide
Iron oxide NPs exist in various forms depending on the oxidation states of iron, oxygen vacancy, etc., among which maghemite, magnetite, and hematite are the prevalent ones. Because of the magnetic nature of some of the iron oxide NPs, the magnetic separation of the catalyst from the reaction mixture becomes easy, resulting in better catalyst recovery for recyclability. In 2015, Habibi et al. used iron oxide NPs as catalysts for the synthesis of benzoxanthenes using the reaction between aryl aldehydes, dimedone, and 2‐naphthol [92]. The catalyst worked effectively with excellent yields (80–95%) and was reusable up to 20 cycles. Magnetic iron oxide NPs as support were also explored extensively for anchoring catalytic entities on them. For example, Abbasi et al. synthesized CuO@γ‐Fe2O3 (copper oxide–supported magnetic NPs) to synthesize substituted guanidines via the simple addition of amines to carbodiimides [93]. The separation of the catalyst was straightforward by applying an external magnetic field, and it was confirmed that the presence of copper improved the activity of the catalyst and produced the desired products in good yields (60–97%) under solvent‐free conditions. The catalyst could be reusable for four cycles without any significant loss of its activity.
1.5.2 Silica‐Based Materials as Catalysts/Supports in Solvent‐Free Organic Reactions
The use of silica and/or silica‐based nanomaterials as either NCs or supports mainly stems from their exceptional thermal and chemical stability, high surface area, straightforward synthetic protocols, and the possibility of surface functionalization to incorporate various organic and/or inorganic functionalities [94, 105]. Among the different silica materials, two major types are extensively used for catalytic applications either as catalysts or as support: (i) nonporous and (ii) porous silica. While nonporous silica is historically important, the high surface area of porous silica offers a significant advantage due to the enhanced accessibility of catalytically active sites. As the catalytic activity of silica‐based materials depends on the functionalities present on the surface, controlling the amount, distribution, and nature of surface functionalities is essential. Irrespective of the materials, most of the synthetic and/or functionalization strategies utilize “sol–gel” methods. Hence, the synthesis of silica‐based nanocatalysts hardly follows solvent‐free protocols [95]. However, post‐synthetic modifications are often performed under solvent‐free conditions to introduce and control surface functionalities [106]. In terms of catalytic activity, surface silanol groups with their Lewis acidic character assist the active catalysts whenever needed. In addition, surface functionalization, i.e. incorporating catalytically functional groups, becomes a pivotal step in developing better NCs. In general, organic functionalities are grafted on the silica surface using organosilanes, and they have been used as recyclable organocatalysts for organic transformations [107]. These supported NCs have been used in various organic transformations, including solvent‐free ones (Figure 1.6). For example, a sulfonic acid‐functionalized silica nanosphere (SAFSNS) catalyst was prepared and used under solvent‐free conditions to synthesize carboxylic acid ester [96]. To incorporate metal/metal‐based NPs on silica surface, suitable organic functionalities are grafted that act as anchoring sites for the metal precursors. Depending on the requirements, metals can be further reduced to their nanoparticle forms or oxidized to oxide forms. In that respect, selective oxidation of toluene was achieved using silica‐supported Au NPs where the performance of the nanocatalysts improved through a mild reductive deprotection strategy [108].
1.5.3 Carbon‐Based Materials as Catalysts/Supports in Solvent‐Free Organic Reactions
Carbon‐based NCs are often considered highly promising due to their high thermal stability, higher surface area, and low cost. The traditional “organocatalysts” are excluded in this category as they have already been considered under “small molecule” homogeneous catalysis rather than a heterogeneous one. In most cases, acid‐ or base‐functionalized carbon‐based materials can be used as catalysts. Alternatively, carbon materials can also act as support for anchoring other catalytic entities. Recent progress in this area indicates that carbon materials can be synthesized by pyrolysis of biodegradable waste materials such as corn, coconut shells at higher temperatures [43, 109]. The amphiphilic carbon has been used as a recyclable catalyst for the reaction between glycerol and 2‐propanone to form solketal under solvent‐free conditions [97]. Zali et al. used a carbon‐based solid acid catalyst for the solvent‐free aldol condensation of aromatic aldehydes with ketones to obtain the desired products in good yields; the catalyst can be reusable up to five times without a decrease in yields [98]. The carbon‐based catalyst has also been explored for various MCRs. For instance, Tavakoli‐Hoseini et al. reported a carbon‐based solid acid catalyst for the synthesis of tetra‐substituted imidazoles via one‐pot reaction using benzil, primary amines, aromatic aldehyde, and ammonium acetate as starting materials [99]. Figure 1.7 represents some examples where carbon‐based nanocatalysts have been explored under solvent‐free applications.
1.5.4 Nitride‐Based Materials as Catalysts/Supports in Solvent‐Free Organic Reactions
The functionalized or modified graphitic carbon nitrides (g‐C3N4) are used as catalysts to synthesize several organic compounds under solvent‐free green synthetic methods. In 2021, Azizi and coworkers synthesized xanthene derivatives using sulfonic acid–functionalized graphitic carbon nitride under solvent‐free condition using the ball‐milling method (Figure 1.8) [110]. The catalyst showed excellent catalytic activity with high yields and short reaction time and can be easily separated and reused up to four cycles.
Figure 1.8 Representative examples of nitride‐based materials as catalysts/supports in solvent‐free organic reactions.
Source: Adapted from the references given in parentheses. TSCN = triplet shelled carbon nitride.
Sonogashira coupling reaction is well known for the formation of the carbon–carbon bond. Commonly, palladium‐based catalytic systems are used for Sonogashira coupling reactions. However, efforts have been devoted to finding a sustainable solution to replace expensive and scarce palladium‐based catalysts. In that context, Akhlaghinia group reported a new catalyst, i.e. cobalt oxide (Co3O4) embedded in a triplet shelled carbon nitride (TSCN) for the Sonogashira–Hagihara cross‐coupling reaction between aryl halides and substituted acetylenic compounds under solvent‐free conditions (Figure 1.8) [111]. Though some of the reactions took place in water, the catalyst exhibited good performance under solvent‐free conditions. The catalyst can be reusable up to five cycles without significant change in their conversion and rate.
In 2021, Liu and coworkers reported hexagonal boron nitride (h‐BN) nanoflakes and a titanium dioxide hybrid photocatalytic system for the oxidation of cyclohexane with oxygen to form cyclohexanone with better selectivity than cyclohexanol under solvent‐free conditions. The catalyst showed excellent catalytic activity up to four cycles without any change in the yield and selectivity (Figure 1.8) [112].
For controlling environmental pollution, conversion of carbon dioxide (CO2) into useful products is an efficient way of mitigating the hazardous impact of CO2. Chand et al. developed a catalytic system for converting carbon dioxide into cyclic carbonates at atmospheric pressure using boron‐doped graphitic carbon nitride as a catalyst under solvent‐free conditions [113]. The catalyst was synthesized using thermal condensation method (Figure 1.8). The catalyst's performance with respect to yield, selectivity, and turn‐over number (TON) was superior to the previously reported catalytic systems. The catalyst can be reusable for eight cycles without any change in the activity and efficiency. A similar work was previously reported by Yin's group in 2016 (Figure 1.8) [114]. They synthesized phosphorous‐modified carbon nitride and used it as a catalyst (in the presence of tetra butyl ammonium bromide [Bu4NBr] as cocatalyst) for the cycloaddition reaction between carbon dioxide and epoxides to obtain the cyclic carbonates in excellent yields. The catalyst was readily separated from the reaction mixture via centrifugation. After washing and drying, the catalyst could be recycled up to five cycles with above 90% yields and 100% selectivity of the product.
1.5.5 Ionic Liquid‐Based Materials as Catalysts/Supports in Solvent‐Free Organic Reactions
Ionic liquids (ILs) are organic salts that exist under ambient conditions. Generally, they are made of polyatomic inorganic anions and organic cations and used as solvents because of their wide range of solubility. Recently, they (either in their original forms or as functionalized derivatives) have been used as catalysts, especially under solvent‐free conditions. For example, Dadhania et al. prepared iron oxide nanoparticle‐supported acidic ionic liquid catalyst to synthesize quinolines and fused polycyclic quinolines by the Friedlander reaction under solvent‐free conditions (Figure 1.9) [115]. The catalysts can be used up to six cycles without any loss of efficiency.
Figure 1.9 Representative examples of ionic liquid (IL)‐based materials as catalysts/supports in solvent‐free organic reactions.
Source: Adapted from the references given in parentheses.
In recent years, synthesis of diphenyl carbonates has become extremely important due to their diverse applications, including the preparation of polycarbonates. Recently, Wang et al. reported that diphenyl carbonates could be successfully synthesized using a metal‐free, Santa Barbara Amorphous (SBA)‐15 IL hybrid catalyst under solvent‐free conditions [116]. By changing the functional groups, they prepared 14 different catalysts, among which [SBA‐15‐IL‐OH]Br with hydroxyl‐terminated groups showed superior performance than the previously reported transition metal–based catalysts. The catalyst was reused up to the sixth cycle without significant changes in conversion, yield, and selectivity.
The oxidation of hydrocarbons plays a crucial role in organic synthesis. Instead of using metal‐based compounds as catalysts (Co, Mn, etc.), Dobras et al. synthesized a catalyst using commercially available silica gel immobilized with N‐hydroxyphthalimide (NHPI), followed by a coating with ILs [117]. The catalysts have been used for the oxidation of ethyl benzene (EB) under solvent‐free conditions, resulting in three different products, namely, acetophenone (AP), ethylbenzene hydroperoxide (EBOOH), and 1‐phenylethanol (PEOH). The selectivity of the product changes with the reaction conditions. To the best of their knowledge, this was the first time this class of catalyst was used under solvent‐free conditions and recycled for three cycles [117].
IL‐catalyzed Knoevenagel condensation reaction between different types of aromatic aldehydes and malononitrile or ethyl cyanoacetate under solvent‐free conditions was reported by Yue et al. in 2008 [118]. With a minimal amount of the catalyst, the reactions were completed within less than an hour, and the catalysts exhibited high reactivity/selectivity and substrate scopes. The easy separation of the catalysts via simple filtration allowed the NCs to be recycled up to five cycles without any noticeable changes in the activity [118].
