Journal of Nanomaterials
Volume 2019 (2019), Article ID 1562130, 17 pages
https://doi.org/10.1155/2019/1562130

Review Article

Recent Developments in Nanostructured Palladium and Other Metal Catalysts for Organic Transformation

S. M. Shakil Hussain,¹ Muhammad Shahzad Kamal,¹ and Mohammad Kamal Hossain²

¹Center for Integrative Petroleum Research, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
²Center of Research Excellence in Renewable Energy (CoRERE), King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

Correspondence should be addressed to Muhammad Shahzad Kamal; shahzadmalik@kfupm.edu.sa

Received 16 April 2019; Revised 25 August 2019; Accepted 12 September 2019; Published 20 October 2019

Guest Editor: Sally El Ashery

Copyright © 2019 S. M. Shakil Hussain et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Nanocatalysis is an emerging field of research and is applicable to nearly all kinds of catalytic organic conversions. Nanotechnology is playing an important role in both industrial applications and academic research. The catalytic activities become pronounced as the size of the catalyst reduces and the surface area-to-volume ratio increases which ultimately enhance the activity and selectivity of nanocatalysts. Similarly, the morphology of the particles also has a great impact on the activity and selectivity of nanocatalysts. Moreover, the electronic properties and geometric structure of nanocatalysts can be affected by polar and nonpolar solvents. Various forms of nanocatalysts have been reported including supported nanocatalysts, Schiff-based nanocatalysts, graphene-based nanocatalysts, thin-film nanocatalysts, mixed metal oxide nanocatalysts, magnetic nanocatalysts, and core-shell nanocatalysts. Among a variety of different rare earth and transition metals, palladium-based nanocatalysts have been extensively studied both in academia and in the industry because of their applications such as in carbon-carbon cross-coupling reactions, carbon-carbon homocoupling reactions, carbon-heteroatom cross-coupling reactions, and C-H activation, hydrogenation, esterification, oxidation, and reduction. The current review highlights the recent developments in the synthesis of palladium and some other metal nanocatalysts and their potential applications in various organic reactions.

1. Introduction

After realizing the unique morphological, structural, and optoelectrical characteristics of nanomaterials, their wide range of applications has been explored in various fields [1]. These include environmental, energy harnessing, biomedical sector, and catalysis [2–7]. The chemical process that involves the use of nanomaterials as a catalyst can be termed as nanocatalysis, while the nanomaterial can be termed as nanocatalyst. Based on their morphologies, nanocatalysts can be classified into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures [8]. The control dimensions of these materials induced specific physicochemical characteristics, which make them special for the catalysis industry [9]. More recently, researchers show significant inclination to use nanocatalysts in advance heterogeneous and homogenous catalysis applications [10]. A number of reviews in the area gave an insightful view into the prospects of nanostructured catalysis [11–14]. The catalyst system composed of nanoparticles/nanocomposites showed greater catalytic activity and selectivity because of its morphology and nanodimensional characteristics. Though many materials have been utilized as nanocatalysts in industries, transition metal NPs have received significant attention due to their unique physicochemical characteristics, abundant availability, and more importantly, consumer-friendly costs. It is well established that the size, morphology, and solvents play a key role in the catalytic activity, selectivity, and stability of the nanocatalysts [15].

The present review is an attempt to realize the current development and prospects in nanostructured catalysts, especially for organic synthesis. The beginning portion of the review is dedicated to the effect of various factors on the overall performance of nanocatalysts. This is followed by a critical overview of organic transformations, with a few case studies on Pd, Pt, Fe, Cu, Ag, Au, and Zn NPs, as well as other examples. In the later part of the review, some insights are provided through the Conclusion along with a few future recommendations about the future potential of nanostructured catalysts.

2. Factors Affecting the Performance of Nanocatalysts

There are several factors that affect the performance of nanocatalysts. However, this section will mainly focus on three important factors which include particle size, particle shape, and solvent.

2.1. Particle Size Effect of Nanocatalysts

Over the past few years, significant research has been conducted to identify the effect of nanoparticle size on catalytic performance for various chemical transformations [16]. As the particle size decreases, their surface area-to-volume ratio is enhanced, allowing more atoms on the surface to take part in the reaction [17]. As a result, improved catalyst activity and selectivity can be achieved. The particle size is very important for the development of highly active and selective catalysts as well as for the reduction of catalyst loading. Yoo et al. found better electrocatalytic and electronic properties by decreasing the size of Pt/TiO₂ nanocatalysts [18]. Bond and Thompson discovered that the catalytic activity of gold nanoparticles depends on their size, support system, and synthesis methods [19]. The gold catalyst is composed of very small-sized particles (<5 nm) and is supported by TiO₂. Before such discovery, gold was assumed to be the least catalytically active metal. Li et al. synthesized palladium nanoparticle-graphene hybrids and investigated the catalytic activities of such Pd-graphene hybrids in Suzuki reaction under aerobic and aqueous conditions [20]. It was observed that a palladium-graphene hybrid with a 4 nm particle size of palladium gave a 100% yield along with 95.5% selectivity. However, a palladium-graphene hybrid with a 15 nm particle size of palladium gave a 93.7 yield along with 95.2% selectivity.

2.2. Particle Shape Effect of Nanocatalysts

The shaped-controlled synthesis of catalytic materials is widely regarded to control some important physicochemical properties of nanocatalysts [21]. Many reports are available in the literature in this regard. For instance, hemispherical gold nanoparticles gave better results as compared to spherical-shaped gold nanoparticles for the oxidation of carbon monoxide (CO) even at low temperature [22]. In 2005, Henry reported a brief review with practical examples regarding the effect of nanoparticle shape on their properties in numerous developing technologies [23]. Narayanan and El-Sayed synthesized tetrahedral- and cubic-shaped platinum nanoparticles and studied the relationship of shape reactivity [24]. Shape control is significantly reported for photocatalytic applications in the literature. Khan and Qurashi synthesized highly controlled platelet-shaped copper vanadate nanocatalysts for PEC water splitting and compared the results with NPs of conventional shape. It was determined that the shape-controlled copper vanadate enhanced the light trapping properties of the catalyst and hence enhanced the photoelectrochemical performance of the catalyst [25].

Similarly, shape-controlled NPs are also found useful in organic conversion to some extent. As reported by Luo et al., shape-controlled synthesis of Rh-based nanocrystals and supported Rh-based nanocatalysts was found efficient in heterogeneous conversions such as in methane conversion and olefin hydroformylation [26].

2.3. Solvent Effect on Nanocatalysts

A solvent has a significant impact on the reaction pathway, reaction energy, and activation energy. The geometric structure and the electronic properties of nanoparticles can be influenced by the interaction of a solvent and metal atoms. Dufour et al. reported the effects of solvents on the electronic and structural properties of small gold clusters [27]. Li and Liu demonstrated the geometrical, electronic, and photocatalytic properties of titania anatase nanoparticles in aqueous media [28]. A comprehensive study of the effects of polar and nonpolar solvents on electronic and geometrical properties of nanocatalysts was conducted by Hou et al., and it was noted that a polar solvent has a great impact on the properties of nanocatalysts. It was observed that the ionization potential decreased by increasing the polarity of the solvent. Therefore, it was easier for neutral species to donate an electron in the solvent. Moreover, the electron-donating ability of a neutral species is considerably increased in a polar solvent as compared to that in a nonpolar solvent [29]. Recently, Chowdhury et al. synthesized palladium nanoparticles in an aqueous dimethyl formamide (DMF) solvent with a changing composition of DMF. Different shapes and geometries of palladium nanoparticles, such as hexagonal, cuboidal, and triangular plates, were obtained by varying the composition of DMF. It was emphasized that the various geometries of palladium nanoparticles are due to the blocking and interaction of DMF to some planes of nanoparticles leading to different geometries [30].

3. Major Nanocatalysts for Organic Synthesis

A huge number of metals have been investigated for organic transformations, and these metals showed better results in the formation of pharmaceuticals, fine chemicals, and new materials [31]. Among the varieties of different rare earth and transition metals, palladium is one of the most widely used transition metal for carbon-carbon coupling reactions, and palladium-based nanocatalysts have been extensively studied both in academia and in industry because of their applications such as in sensors, fuel cell catalysts, hydrogen storage, dechlorination, and organic transformations. Therefore, we mainly focused on the current development of palladium-based nanocatalysts for cross-coupling reactions. However, some other metals which have been successfully applied in this field have also been highlighted.

3.1. Palladium-Based Nanocatalyst

3.1.1. Palladium-Based Nanocatalysts for Carbon-Carbon Cross-Coupling Reactions

Palladium-assisted nanocatalysts for carbon-carbon bond formation including Suzuki [32], Heck [33], Sonogashira [34], Negishi [35], Stille [36], Kumada [37], and Hiyama [38] cross-coupling reactions (Scheme 1) have made a huge impact on organic reactions because of mild reaction conditions and tolerance to various functional groups [39]. Such kinds of reactions showed extensive applications in the formation of pharmaceuticals, agrochemicals, and other important industrial products [40].

Scheme 1: Palladium-based nanocatalysts for carbon-carbon cross-coupling reactions.

Among the various carbon-carbon coupling reactions, Suzuki, Heck, and Sonogashira reactions are the most important reactions and play a central role in the formation of natural products, pharmaceutical, and agrochemicals [41]. Table 1 lists the various metal nanoparticles for catalyzing Suzuki, Heck, and Sonogashira cross-coupling reactions.

Table 1: Palladium-based nanocatalysts for carbon-carbon coupling reactions.

Catalyst (amount)
Substrate (1 mmol)
Substrate (2 mmol)
Coupling product
Base
Solvent
(°C)
Time (h)
Reaction name
Yield (%)
Ref.

Pd-SMU-MNPs (8 mg)
PhI (1)

K₂CO₃
DMF
120
1.5
Heck
95a
[42]

Fe₃O₄@CS-Schiff-based Pd catalyst (10 mg)
PhI (1)

Et₃N
DMF
120
0.3
Heck
98a
[43]

Stabilized Pd-NPs (1 mol%)
ArI (0.5)

CsCO₃
DMF
110
2
Heck
95a
[41]

Pd/NH₂-SiO₂ (0.05 mol%)
PhI (1)

K₂CO₃
DMF
110-120
2
Heck
95a
[44]

Pd-PVP-Fe (0.004 g)
PhBr (1)

K₂CO₃
DMF
RT
0.5
Heck
91a
[33]

OXDH-Pd-NPs (0.0091 mmol)
ArI (6.3)

Na₂CO₃
NMP : H₂O (1 : 1)
80
0.5
Heck
97a
[45]

Au/Pd-NPs (0.3 mol%)
PhI (1)

K₂CO₃
H₂O
80
8
Heck
89c
[46]

Fe₃O₄@SiO₂/isoniazide/Pd (10 mg)
PhI (1)

K₂CO₃
EtOH : H₂O (1 : 1)
25
0.5
Suzuki
96a
[47]

Pd-NP-HNG (0.025 mol%)
PhI (1)

K₂CO₃
EtOH : H₂O (1 : 1)
60
2.5
Suzuki
98a
[48]

PdCo ANP-PPI-g-graphene (0.004 g)
PhI (1)

K₂CO₃
None
25
1
Sonogashira
99b
[49]

Pd tripods (2 mol%)
PhI (0.49)

KOH
H₂O
100
6
Sonogashira
93a
[50]

Pd@MWCNTs (1 mmol%)
PhI (1)

K₂CO₃
MeOH : H₂O (3 : 1)
Reflux
2.5
Sonogashira
71a
[51]

Pd/NH₂-SiO₂ (0.05 mol%)
PhI (1)

K₂CO₃
EG
120
2
Sonogashira
98a
[44]

Stabilized Pd-NPs (1 mol%)
PhI (0.5)

CsCO₃
MeOH
90
22
Sonogashira
91a
[41]

aIsolated yield. bGC yield. cYield after work-up.

Within the framework of carbon-carbon cross-coupling reactions, the Suzuki reaction is the most extensively used reaction and it has been the benchmark for identifying the catalytic activity of newly prepared metal nanoparticles. In 2008, Kim et al. synthesized bimetallic nanoparticles (Pd-Ag, Pd-Ni, and Pd-Cu) on carbon support through the γ-irradiation technique for Suzuki and Heck cross-coupling reactions [52]. The catalytic efficiency of these supported bimetallic nanoparticles in Suzuki reaction were in the order of based on the reaction yield , respectively.

The various metal nanocatalysts used in the Suzuki reaction are listed in Table 2.

Table 2: Suzuki reaction with various metal nanocatalysts .

Nanocatalyst (amount)
PhI
PhB(OH)₂
Base
Solvent
(°C)
Time (h)
Yield (%)
Ref.

Pd-SMU-MNPs (6 mg)
1
1
K₂CO₃
PEG
50
0.5
97a
[42]

Cu-C (0.1 mol%)
1
1.2
K₂CO₃
H₂O
50
3
96a
[53]

Fe₃O₄NPs/IL/Pd(0) (0.2 mol%)
1
1.1
K₂CO₃
H₂O/EtOH (1 : 1)
RT
0.25
96a
[54]

Au-graphene (1 mol%)
1
1.2
NaOH
H₂O
100
4
85b
[55]

Pd/TiO₂ (0.7 mol%)
1
1.1
Na₂CO₃
NMP : H₂O (2.5 : 1)
120
4
97a
[56]

Ru/Al₂O₃ (5 mol%)
1
1.5
NaOH
DME/H₂O (1 : 1)
60
1
96a
[57]

Pd@MWCNTs (1 mmol%)
1
1.2
K₂CO₃
MeOH : H₂O (3 : 1)
Reflux
2.5
84b
[51]

PVP-stabilized Pd-NPs (0.07 mol%)
0.5
0.75
K₃PO₄
H₂O : EtOH (3 : 1)
90
24
97b
[58]

Pd/NH₂-SiO₂ (0.05 mol%)
1
1.2
K₂CO₃
H₂O (2 mL)
60-70
1
96a
[44]

Fe₃O₄@CS-Schiff-based Pd catalyst (10 mg)
1
1.2
K₂CO₃
PEG
80
0.17
98a
[43]

Pd-Ni@Fe₃O₄ (0.0026 mol% of Pd and 0.001 mol% of Ni))
0.5
0.75
K₂CO₃
EtOH
80
0.25
94a
[59]

Pd/r-GO NP thin film
1
1.2
K₂CO₃
H₂O
80
0.25
>99a
[60]

LDH-DS-Pd(0) (0.5 mol%)
1
1.2
K₂CO₃
DMF/H₂O 6 mL (5 : 1, /)
80
5
93a
[61]

Pd@MTiO₂ (0.03 g)
1
1.2
K₂CO₃
H₂O
70
3
99b
[62]

Carbon nanocomposite Pd catalyst (1 mol%)
0.5
0.6
K₂CO₃
DMF/H₂O (2 : 1)
100
1.5
97b
[63]

Core-shell-like Ni-Pd/CB catalyst (5.5 mg, 0.1 mol% Pd)
2.5
2.75
K₂CO₃
EtOH/H₂O (1 : 1)
30
0.5
90a
[64]

Au-G nanocomposite (0.05 g)
1
1.5
K₂CO₃
H₂O
RT
4
99b
[65]

Sr/Alg/CMC/GO/Au (0.005 mol%)
2
2.4
NaOH
H₂O
80
4
98a
[66]

OXDH-Pd-NPs (0.0091 mmol)
5.41
4.92
Na₂CO₃
1,4 Dioxane/water (1 : 1)
80
1
98a
[45]

Stabilized Pd-NPs (1 mol%)
0.5
0.75
KF
DMF : H₂O (1 : 1)
RT
24
93a
[41]

aIsolated yield. bGC yield.

3.1.2. Mechanism of Cross-Coupling Reactions

The reactants meet on a palladium atom and become so close together that reaction takes place. The major role of palladium and the other metals is to enable and encourage two coupling partners to undergo a chemical reaction. In 1972, Kumada et al. suggested that the catalytic cycle of a cross-coupling reaction occurs in three steps including oxidative addition, transmetalation, and reductive elimination (Scheme 2) [67].

Scheme 2: The proposed mechanism of cross-coupling reactions.

The reaction mechanism usually begins with the zero-valent palladium (Pdo) which undergoes oxidative addition (step 1) by reacting with an organic electrophile to form a Pd (II) species [68]. Usually, step 1 is the rate-determining step in this three-step catalytic cycle. Subsequently, transmetalation (step 2) occurs in the presence of a base for the transfer of towards a less electropositive metal. In this step, both coupling partners join the same metal center while removing the functional groups. At the end (step 3), reductive elimination occurs which leads to the formation of a new carbon-carbon bond as well as the regeneration of a zero-valent palladium species which is ready for another cycle. An unsaturated organic species was found to undergo a faster coupling reaction by following the order .

3.1.3. Palladium-Based Nanocatalysts for Carbon-Heteroatom Cross-Coupling Reactions

Palladium nanocatalysts have been successfully applied in carbon-heteroatom cross-coupling reactions such as in Buchwald-Hartwig amination. Recently, Panahi et al. reported an immobilized palladium nanocatalyst on a silica-starch substrate (PNP-SSS) as an effective catalyst for carbon-nitrogen cross-coupling reactions through Buchwald-Hartwig amination with excellent catalytic activity and reusability [69] (Scheme 3).

Scheme 3: Buchwald-Hartwig amination using PNP-SSS [69].

Most recently, Hajipour et al. studied the efficiency of a palladium nanocatalyst supported on cysteine-functionalized magnetic nanoparticles for N- and O-arylation reactions in environmentally friendly conditions [70]. The authors claimed that the synthesized palladium catalyst system exhibited excellent recyclability with no substantial deactivation even after ten cycles (Scheme 4).

Scheme 4: N- and O-arylation using a cysteine-supported palladium nanocatalyst.

Similarly, Veisi et al. also reported a carbon-heteroatom cross-coupling reaction using a palladium nanocatalyst immobilized on carbon nanotubes and observed no change in catalytic activity for up to six cycles (Scheme 5) [71].

Scheme 5: C-N cross-coupling reaction using a palladium nanocatalyst immobilized on carbon nanotubes.

3.1.4. Palladium-Based Nanocatalysts for Carbon-Carbon Homocoupling Reactions

The biaryl formation is a very important reaction in the field of catalysis, total synthesis, fine chemicals, and supramolecular chemistry [72]. The bond between two aryl groups is often available in natural products, dyes, medicine, and agrochemicals. The copper-catalyzed homocoupling reaction is a well-known method for the construction of biaryls, but it requires harsh reaction conditions. Movahed et al. reported palladium nanoparticles on nitrogen-doped graphene (Pd-NP-HNG) for an Ullmann-type homocoupling reaction in water (Scheme 6) [48].

Scheme 6: Ullmann homocoupling reaction using Pd-NP-HNG nanocatalysts [48].

Recently, Rafiee et al. reported the synthesis of a palladium nanocatalyst immobilized on a magnetic few-layer graphene support which they applied on cross- and homocoupling reactions [73]. The catalyst system was found to be active up to six runs with no loss of its catalytic activity (Scheme 7).

Scheme 7: Homocoupling reaction of 4-chloro-1-bromo benzene using a Fe₂O₃@FLG@Pdo catalyst.

Liu et al. prepared a series of polyaniline-supported palladium nanocatalysts for the Ullmann homocoupling reaction of aryl iodides to form biaryls. It was observed that the catalyst activity can be tuned by introducing electron-donating groups (Scheme 8) [74].

Scheme 8: Ullmann homocoupling reaction using a Pd@PANI-H catalyst.

3.1.5. Palladium-Based Nanocatalyst for Hydrogenation Reactions

A palladium catalyst has faster hydrogenation and dehydrogenation processes and are also used in petroleum cracking. A variety of hydrogenation reactions are conducted by palladium nanocatalysts. A palladium nanocatalyst has the capability to combine with a wide range of ligands for highly selective organic reactions. Research is more focused on supported palladium nanoparticles due to their excellent efficiencies and faster rate of reaction. Chang et al. reported on palladium nanoparticles entrapped in aluminum oxyhydroxide for the hydrogenation of nitroaromatics and solid alkenes (Scheme 9) [75].

Scheme 9: Hydrogenation reaction in the presence of (Pd/AlO(OH)) nanocatalysts [75].

The same catalyst (Pd/AlO(OH) was also used by Fry and O’Connor with different concentrations for the hydrogenation of unsaturated esters [76]. The palladium nanoparticles entrapped in aluminum oxyhydroxide were found to be selective without reducing other functionalities in the molecule.

3.1.6. Palladium-Based Nanocatalysts for the Dichromate Reduction Reaction

In 2013, Tu et al. synthesized polyvinylpyrrolidone-stabilized palladium nanoparticles (PVP-Pd) through a chemical reduction protocol for Pd-catalyzed dichromate reduction [77]. Chromium exists in two oxidation states which are (Cr-VI) and (Cr-III) (Scheme 10). Among these two oxidation states, hexavalent chromium (Cr-VI) is a highly toxic and carcinogenic species. However, trivalent chromium (Cr-III) is comparatively nontoxic and even small quantities of (Cr-III) are required by the human body as an essential nutrient. Many reports appeared in the literature on the reduction of (Cr-VI) by using iron nanoparticles, aluminum oxide, titanium oxide, mixed transition metal nanoparticles, palladium nanoparticles, etc. [78]. Yang et al. demonstrated the application of tobacco mosaic virus-templated palladium nanoparticles for the reduction of (Cr-VI) and claimed that such a nanocatalyst system can be applied in different kinds of catalytic reactions [79].

Scheme 10: Reduction of hexavalent chromium (Cr-VI) to trivalent chromium (Cr-III).

3.1.7. Supported Palladium Nanoparticles

Palladium nanoparticles can lose their catalytic activity due to aggregation or precipitation. Therefore, stabilizers such as ligands, polymers, or surfactants are useful to control agglomeration and precipitation [80]. A variety of palladium nanoparticles that have appeared in the literature have described the advantages of supported systems such as carbon nanotubes [81], colloidal support [82], silica [83], metal nanoparticle support [84], polymers [85], carbon [86], and graphene [87]. Palladium nanoparticles supported onto different materials increase the surface-to-volume ratio of the composite and improve the catalytic activity and selectivity of the heterogeneous catalyst. Palladium nanoparticles either in colloidal form or deposited form have been successfully applied as a catalyst for different kinds of reactions. Liew et al. reported a new catalyst system of palladium nanoparticles (XL-HGPd) (Scheme 11) with the help of a cross-linking method [88]. Such a catalyst system was easy to recover and showed excellent recyclability with continuously high catalytic activities.

Scheme 11: Suzuki reaction catalyzed by an XL-HGPd nanocatalyst [88].

Liu et al. prepared palladium nanoparticles (1-5 nm) with the help of a helical backbone containing poly(N,N-dialkylcarbodiimide) (PDHC-Pd) as a polymeric gel for stabilizing a palladium nanocatalyst. Such a composite material was found to be very active for the Suzuki reaction under regular heating or microwave irradiation (Scheme 12) [89].

Scheme 12: Suzuki reaction in the presence of PDHC-Pd nanocatalysts [89].

The catalyst was recycled for the second, third, fourth, and fifth time and reaction yields were 93%, 95%, 92%, and 90%, respectively. Palladium nanocatalysts with carbon nanomaterial support have been successfully applied for glucose oxidation reaction [90]. Glucose is considered as an emerging energy source for fuel cell technology improvement in order to fulfill the green energy requirement.

3.2. Platinum-Based Nanocatalysts

Platinum catalysts have been extensively used in pharmaceutical, chemical, electronic, petrochemical, and fuel cell applications [91]. Such catalysts have shown excellent catalytic and electrical activities as well as corrosion-resistant properties. Platinum-based catalysts have been successfully applied in sensors [92], fuel cells [93], methanol oxidation [94], and petroleum industries [95]. Platinum-based nanomaterials have shown remarkable properties because of their stability in different conditions. Just like other metal nanocatalysts, the activities of platinum-based nanocatalysts also depend on the size and shape of the catalyst. Several methods are available in the literature for the synthesis of platinum nanoparticles such as physical methods [96], solvothermal [97] and hydrothermal [98] approaches, sol-gel [99], and an electrodeposition [100] process. The morphology and properties of a platinum-based nanomaterial such as optical, magnetic, and catalytic properties can be tailored by changing the starting material and reaction parameters [101]. Narayanan and El-Sayed reported the Suzuki reaction between iodobenzene and phenylboronic acid to catalyze using platinum nanocatalysts (Scheme 13) [102].

Scheme 13: Suzuki reaction between iodobenzene and phenylboronic acid catalyze using platinum nanocatalysts [102].

3.3. Iron-Based Nanocatalysts

Iron, as a backbone of infrastructure, received great interest because of excellent magnetic and catalytic properties [103]. Due to their magnetic property, iron-based nanocatalysts can be easily separated by an external magnet after the completion of a reaction [104]. Iron oxide nanoparticles with various structures and morphologies have been widely used for drug delivery [105], biosensor [106], medical [107], and water treatment [108] applications, as well as other applications. Iron oxide nanoparticles have multiple advantages because of their low price and inherent biocompatibility. The synthetic scheme of iron nanoparticles plays a key role in terms of morphologies and chemical and physical properties [109]. Within the framework of different nanoparticles, ferromagnetic iron and cobalt nanoparticles and their oxides and alloys were found to be the most favorable probes for different applications [110].

2,4-Dichlorophenol is a toxic material and is present in both wastewater and soil. Li et al. successfully degraded 2,4-dichlorophenol by either Fenton oxidation or reductive dechlorination with the help of various iron-based nanoparticles [111]. In 2005, Park et al. reported a new synthetic way for the synthesis of monodisperse nanoparticles of iron oxide with a size of 6-13 nm [112]. The synthesis of 6-13 nm particle size was accomplished by the additional growth of the monodisperse nanoparticles of iron oxide. There are several methods available in the literature for the synthesis of iron nanoparticles; however, iron pentacarbonyl decomposition is the most widely used method because of ease of handling and because it only has carbon monoxide as a byproduct. Some other methods are also available in the literature such as the reduction of organic or inorganic salts [113], mechanical methods, and decomposition of other unstable iron compounds [114]. Jagadeesh et al. describe the synthesis of iron oxide-based nanocatalysts for the hydrogenation of nitroarenes to anilines with excellent activity and selectivity (Scheme 14) [115].

Scheme 14: Hydrogenation of nitrobenzene to aniline using iron nanocatalysts [115].

3.4. Copper-Based Nanocatalysts

Copper-based nanocatalysts have received considerable attention because of their high activity and low reaction temperature [116]. The activity of the Cu-based nanocatalyst can be influenced by synthetic protocol, composition, temperature, pressure, concentration, and reactor type [117]. Various methods are available in the literature to synthesize Cu-based nanocatalysts such as hydrothermal [118], coprecipitation [119], homogenous precipitation [120], and impregnation [121]. Recently Lamei et al. reported a green nontoxic catalyst material to comprise nanowires and nanoparticles embedded in a carbonaceous matrix [53]. Such a Cu-based ligand-free nanocatalyst system was applied to the Suzuki coupling reaction with excellent activity and no significant loss of activity observed even after four cycles (Scheme 15).

Scheme 15: Suzuki coupling reaction using different copper catalysts [53].

3.5. Gold-Based Nanocatalysts

Since the pioneering studies of Haruta et al. [122], gold nanocatalysts have become widely used nanoparticles for oxidation [123], reduction [124], hydrogenation [125], homocoupling [126], degradation of organic pollutants [127], and electrochemical sensor applications [128]. In order to expose more atoms on the surface, gold nanoparticles are usually dispersed on a suitable support such as activated carbon [129], starch [130], silica [131], metal oxide [132], and resin [133]. Gold along with magnetic nanoparticles such as catalytic support has gained much attention due to its superparamagnetic properties and environmentally friendly nature. The gold-magnetic nanocatalyst (Au-Fe₃O₄) has shown excellent catalytic activity in various organic reactions such as oxidation of CO [134] and reduction of H₂O₂ [135]. Lin and Doong synthesized Au-Fe₃O₄ nanocatalysts through iron-oleate decomposition in the presence of Au seeds. The catalyst system was successfully applied in the reduction of nitrophenol with excellent activity and selectivity (Scheme 16) [136].

Scheme 16: Reduction of nitrophenol using gold-magnetic nanocatalysts (Au-Fe₃O₄) [136].

3.6. Silver-Based Nanocatalysts

Silver nanoparticles have been successfully applied in optics, medicine, catalysis, and sensors [137]. Silver-based nanocatalysts are continuously being developed due to their strong absorption in the region of visible light which is easily detectable through a UV-visible spectrophotometer. In terms of organic reactions, silver nanocatalysts are used in reduction reactions [138], alkylation [139], degradation [140], reduction [141], and synthesis of fine chemicals [142]. Recently, Mandi et al. reported the synthesis of supported silver nanocatalysts via acrylic acid polymerization and subsequent immobilization with silver nanoparticles to form nanocomposite Ag-MCP-1. The nanocomposite material was used in a reductive coupling reaction of nitrobenzene with alcohols in the presence of a hydrogen source such as glycerol (Scheme 17) [143].

Scheme 17: Reductive amination reaction between 4-methylnitrobenzene and benzyl alcohol using a Ag-MCP-1 nanocatalyst [143].

Apart from colloidal Ag nanoparticles, 1D and 2D structures of Ag and their composites have also shown huge prospects in catalytic conversion. Ag nanowires and copper oxide-embedded Ag nanowires showed excellent and rapid catalytic activity [144]. The activity and selectivity of such composites were reported to be ecofriendly and distinguished. However, Ag nanowires possessed 40% conversion efficiency along with 95% selectivity, and copper oxide-embedded Ag nanowires showed much higher activity and stability compared to individual metal oxides or metal nanowires [145]. Such demonstration paves way for further efficient designs and innovative applications of metal oxide-embedded 1D and 2D materials as new nanocatalysts for organic conversion.

3.7. Zinc-Based Nanocatalysts

The nanocomposite system containing zinc oxide mixed with other metal oxides has been a material of choice due to several applications such as the production of biodiesel [146], CO₂ conversion [147], aldehyde oxidation [148], hydrogen production [118], transesterification [149], wastewater treatment [150], azo dye decoloration [151], and chemoselective acetylation [152]. The activity and selectivity of the zinc oxide-based nanocatalysts rely on size and morphology of the synthesized material. Different methods are available in the literature to describe the synthetic procedures for controlling the size and morphology of the zinc oxide nanocatalysts such as coprecipitation [119], microwave assisted [153], combustion [154], ion exchange, and vapor phase transport [155]. In 2015, Saikia et al. reported the synthesis of zinc oxide nanocatalysts through the leaf extract of Carica papaya and its application in the synthesis of oxime derivatives [156]. The reaction was run without a solvent under microwave irradiation to form an oxime with an excellent yield and with a recycle capability up to 5th run (Scheme 18).

Scheme 18: Conversion of aldehyde/ketone into oximes using zinc oxide nanocatalysts [156].

4. Future Prospects and Challenges

Carbon-carbon cross-coupling protocols such as Suzuki, Heck, and Sonogashira reactions are industrially important reactions, and a review of the literature reveals that these reactions are catalyzed by precious metals including palladium or gold nanoparticles. Therefore, it is highly anticipated that the focus of research will be on the development of either metal-free or nonnoble metal nanocatalysts with high activity and selectivity for carbon-carbon cross-coupling reactions. Nanocatalysts are known to have high activity and selectivity, but they suffer from instability and reusability issues. One way to achieve high stability and reusability is for the nanocatalyst to have a strong interaction with the support system which can prevent an aggregation problem. To achieve this, the support system should have chelating properties to bind the nanoparticles more strongly. Severe conditions such as high-temperature reactions can cause the leaching of metal in the nanocatalysts. Initially, it was thought that the leaching process mainly occurs with nanocatalysts containing the palladium metal. However, in recent times, a number of reports appeared in the literature describing the leaching of other noble metals in nanocatalysts. Therefore, the development of new nanocatalysts that can bear harsh conditions is highly desirable. As depicted earlier, the shape and size of the nanocatalyst have a great impact on their catalytic properties and stabilities. Hence, new methods with a well-controlled size and shape of the nanocatalysts need to be developed. The multistep synthesis using a costly starting material and low yield hinders their commercial applications. Such synthetic protocols need to be replaced with a facile route, a green process, and large-scale production with high quality.

5. Conclusion

In this review, we highlighted the recent progress on the design and development of nanocatalysts and discussed their catalytic application in important organic reactions. The synthetic procedure of nanocatalysts contains various metals such as Pd, Pt, Fe, Cu, Au, Ag, and Zn and have been reviewed along with their important applications. Among the various metals, the palladium-based nanocatalysts are the most widely investigated material for coupling reactions. Palladium nanocatalysts either with a suitable support or as mixed metal oxides are known to increase the surface-to-volume ratio of the composite and improve the catalytic activity and selectivity of the heterogeneous catalyst. The industrially important organic reactions such as carbon-carbon bond coupling reactions, carbon-heteroatom bond coupling reactions, carbon-carbon homocoupling reactions, hydrogenation, reduction, and oxime formation reactions have been reviewed. Similarly, the Suzuki reaction has been a benchmark to explore the catalytic activities of newly synthesized palladium-based nanocatalysts. The future prospects that need to be addressed, on the basis of literature review, have also been highlighted at the end. Consequently, this review article may help on the design and development of new nanocomposite catalysts containing a well-defined shape and size with high activity, selectivity, stability, and reusability. This literature search will also help to identify the best support system for high-performance supported nanocatalysts. Due to the ease of synthesis, high activity, and selectivity, more nanocatalyst systems will be developed in the near future for organic conversion.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

The authors are thankful to the Center for Integrative Petroleum Research (CIPR) for the research start-up project SF-17003, the Center of Research Excellence in Renewable Energy (CoRERE), and the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) through project No. IN151003.

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