Green Synthesis of Pd Nanoparticles for Sustainable and Environmentally Benign Processes

: Among transition metal nanoparticles, palladium nanoparticles (PdNPs) are recognized for their high catalytic activity in a wide range of organic transformations that are of academic and industrial importance. The increased interest in environmental issues has led to the development of various green approaches for the preparation of efﬁcient, low-cost and environmentally sustainable Pd-nanocatalysts. Environmentally friendly solvents, non-toxic reducing reagents, biodegradable capping and stabilizing agents and energy-efﬁcient synthetic methods are the main aspects that have been taken into account for the production of Pd nanoparticles in a green approach. This review provides an overview of the fundamental approaches used for the green synthesis of PdNPs and their catalytic application in sustainable processes as cross-coupling reactions and reductions with particular attention afforded to the recovery and reuse of the palladium nanocatalyst, from 2015 to the present. The catalytic activity, the recyclability and the robustness of the Pd-nanocatalyst obtained via a green route have been evaluated in sustainable C-C coupling reactions and the reduction of nitroarenes. Many Pd-nanocatalysts have presented excellent catalytic activity and recyclability with a very low Pd leaching in solution, promising applicable developments for sustainable and environmentally friendly industrial productions.


Introduction
Currently, growing environmental concern requires the development of green and environmentally sustainable strategies for the preparation of metal nanoparticles [1,2].
Metal nanoparticles (MNPs) with a high surface to volume ratio, serve an important role in a wide range of disciplinary field such as catalysis, bio-diagnostics, medicine, drug-delivery, pharmacology, energy production and environmental remediation.
Among the various MNPs, due to their unique electronic and chemical properties, palladium nanoparticles (PdNPs) have attracted major attention for their high efficiency as heterogeneous nanocatalysts for various organic transformations such as C-C coupling reactions, hydrogenation of alkenes and alkynes, oxidation reactions, reduction of nitroarenes and degradation of dyes [3][4][5][6][7]. Pd nanocatalysts not only enhance the synthetic performance of a chemical process but allows the use of milder conditions and the development of a greener chemistry. Thus, clean and non-toxic synthetic routes for Pd nanoparticles production are highly desirable for the development of environmentally friendly processes.
The catalytic and biological activity of PdNPs are closely related both in their shape and size and in the nature of the dispersing/capping agent. Moreover, the synthetic procedure for PdNPs preparation plays a significative role in defining unique properties such as their chemical, physical, optical and electronic properties.
To date, many synthetic approaches have been explored for the preparation of Pd nanoparticles, including different physical, chemical, electrochemical and biological methods ( Figure 1) [8,9].
Physical methods often require the use of sophisticated equipment but it is possible to finely control the size and the dispersion of metal nanoparticles on the surface of the support. Moreover, both ultrasonic and microwave radiation have been widely used as alternative energy source for the synthesis of PdNPs in solution. The electrodeposition process allows high purity and the precise control of size of the metal nanoparticles simply by monitoring the key parameters of the applied current, the voltage and time. Chemical procedures are simplest and they are the most widely used methods for Pd nanoparticle's synthesis in solution. The reduction of Pd(II) metal ions to Pd(0) requires a reducing agent in the presence of a suitable capping or dispersing agent to nucleate Pd(0) with a welldefined size and shape. The chemical methods require the use of an over-stoichiometric amount of a strong reducing agent or an excessive amounts of solvents. Thus, biological methods that use biogenic materials as a reducing and capping or a dispersing agent are emerging as a new green procedure for the production of Pd nanoparticles in an aqueous medium [9][10][11][12][13][14][15]. As is known, Pd catalysts play a key role in the chemical industry production of complex molecules such as fine chemicals and pharmaceuticals, and in recent year, much attention has been afforded in designing efficient, low-cost and environmentally sustainable PdNPs nanocatalysts via green methods.
Indeed, the implementation of sustainable chemical processes requires the use of a catalyst that is safe, economical, simple to prepare, easily recoverable and reusable with minimal Pd leaching. This approach affects sustainability in different ways: (i) the catalyst can be recovered and reused and this reduces the amount of waste generated and the amounts of resources used (ii) The leaching in the product is limited and this reduces the product purification processes, which is very important in pharmaceutical preparations, because a rigid regulation regarding the maximum amount of metal permissible exists.
This review provides an overview of the different approaches used for the green production of PdNPs from 2015 to the present. Also, the synthetic applications of such synthesized Pd nanocatalyst in fundamental organic processes, mainly C-C coupling (such as Suzuki-Miyaura, Sonogashira, Heck-Mizoroki) and reductions, in environmentally safer reaction conditions will be discussed ( Figure 2). Particular attention will be paid to the recovery and reuse of the Pd nanocatalysts and to Pd leaching in solution. From a mechanistic point of view, both homogeneous catalysis in which PdNPs serve as a reservoir of active molecular species (release and catch mechanism), and a heterogeneous pathway in which PdNPs exhibit a surface reactivity, or a combination of both processes can be operative. Of course, the nanoparticle size, the nature of the reducing and stabilizing agent, nature of support, the reaction conditions such as temperature, reaction medium and the nature of reagents contribute toward determining the catalytic mechanism.
For the purpose of elucidating the mechanistic pathways, several studies such as kinetics, a hot filtration test, polymer test, mercury test, and catalyst characterization after catalysis were used.
In almost all the examples presented in this review, a heterogeneous catalysis is reported by the authors, and the few exceptions will be discussed.

PdNPs Preparation by Pd(II) Chemical Reduction
When considering the green synthesis of palladium nanoparticles by chemical reduction of the corresponding Pd(II) ion salt, there are three key factor to be considered: (i) the rection medium, (ii) the reducing agent and (iii) the stabilizer or dispersing agent.
The majority of synthetic procedures employ the use of highly reactive reducing agents such as sodium borohydride (NaBH 4 ), hydrazine (N 2 H 4 ), formaldehyde and hydrogen (H 2 ), a capping agent such as surfactants, synthetic polymers, and an organic solvent.
For green perspectives, the use of environmentally friendly solvents, non-toxic chemicals and biodegradable capping agents are high desirable [27].
A number of analytical techniques were used to confirm the formation of palladium nanoparticles as well as to establish their size, morphology, surface characteristics and oxidation state. Commonly, PdNPs are characterized by ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and thermal gravimetric analysis (TGA).

Natural Reductant
Currently, the use of benign chemicals is one of the crucial issues to consider when designing a green process. Natural antioxidants such as vitamin C, monosaccharides and gallic acid have been used as reducing agents for the green synthesis of metal nanoparticles [28]. Some recent examples on the preparation of Pd nanoparticles are provided below.
In 2019, the use of sodium ascorbate as an environmental and biocompatible reductant, conjugated with bio-compatible phosphonic acid stabilizers was reported by Guénin and co-workers [29]. The described methodology leads to the formation of highly stable and dispersible Pd-NPs in aqueous media by using microwave heating. The effect of the stabilizers and the variation of the pH of the stabilizer solution was studied. In basic conditions, monodispersed small sized PdNPs (from 2 nm to 7 nm depending on the stabilizer) were obtained. The great versatility of these nanocatalysts has been evaluated for different important reactions such as C-C coupling, reduction and cyclization in an aqueous or aqueous/ethanolic (1:1) medium without any additives and display a high efficiency. A very low Pd loading (0.002 mol%) was used for the complete conversion in the Suzuki-Miyaura reactions of aryl iodide with tolyl boronic acid under microwave irradiation at 80 • C for 30 min (TOF~130,000 h −1 ).
Bora and co-workers report a facile methodology for the Suzuki-Miyaura coupling in water by using gallic acid, a natural and abundant phytochemical, as a reducing agent and stabilizing agent to produce in-situ mono-dispersed palladium nanoparticles from PdCl 2 [30]. A TEM image of the reaction mixture after the completion of the Suzuki reaction indicated the formation of uniform palladium nanoparticles with an overage dimension of 15 nm. After the extraction of the biphenyl product, the recovered catalytic solution was reused for up to three cycles, following the addition of fresh reactants, with only a slight decrease in activity (99→91% yield). There was no observed leaching of Pd in the product.
In the same year, Camp and co-workers described the use of glucose for the in-situ formation and stabilization of Pd nanoparticles during Heck, Sonogashira and Suzuki crosscoupling reactions in an aqueous medium in the presence of PdCl 2 [31]. The recyclability of the aqueous in-situ formed palladium nanoparticles was evaluated in the Heck reaction of iodobenzene with methyl acrylate. A drop-off in reactivity was observed after the 4th cycle (97→61% yield). The decrease in yield was most likely caused by the low level of sugar residue on the surface of the nanoparticles which lead to an increased aggregation and deactivation of the PdNPs.

Plant Extracts
Natural reducing agents from leaves, flowers, fruits, seeds, back and roots plant extract have been widely used for the green synthesis of Pd nanoparticles [32]. The phytochemical components contained in the extracts such as polyphenol, flavonoids, terpenoids, polyols, glycosides, vitamins, amino acids and carboxylic, act as alternative reducing and capping agents. Indeed, these compounds possess hydroxyl and carboxylic groups that can coordinate and reduce the Pd(II) metal ion to zero-valent state. The chelate Pd(0) atoms act as nucleation centers leading to a growth process which forms stabilized PdNPs. (Figure 3). The concentration of the phytochemical in the extracts changes considerably according to the type of plant extract, and this affects the size and shape of metal nanoparticles and consequently affects its chemical and biological activity. Also, other reaction conditions such as reaction time, temperature and pH have a considerable impact on the size, morphology and stability of synthesized nanoparticles [10].
The Pd nanoparticles were prepared in an aqueous medium by mixing the plant aqueous extract with an aqueous solution of Pd(II) ion precursor such as PdCl 2 , Pd(OAc) 2 , Na 2 PdCl 4 , and the mixture was stirred in thermal conditions across a wide range of temperatures (r.t.-100 • C) and reaction times (few minutes-48 h) or under ultrasonic vibration [47] or microwave irradiation [49]. The average size of the Pd nanoparticles varies considerably with the nature of the extract and the reaction conditions used for the preparation.
The Pd nanocatalysts have been employed in various organic transformations such as C-C coupling, reduction and oxidation reactions usually performed in a sustainable green solvent such as water or a water/ethanol mixture. The catalysts have been recovered and reused for several cycles and in general, a slight decrease of catalytic activity was observed. However, in some cases, the catalytic performances decline drastically at the second/third cycle and PdNPs aggregation was observed [65,72].
High catalytic activities were observed by Hekmati and co-workers in the Suzuki reaction in a water/ethanol solution catalyzed by PdNPs@Thymbra spicata [60] and by Veisi and co-workers in the Suzuki reaction in solely water catalyzed by PdNPs@Rosa canina [74] or by PdNPs@Stakys lavandulifolia [76]. The catalysts were recovered and reused for 7-8 cycles without a loss of catalytic activity. Moreover, a hot-filtration test and the very low Pd leaching in solution suggested that the prepared catalysts were stable and a heterogeneous mechanism has been proposed by the authors.
In 2020, the use of PEG-400 as an alternative green solvent for the Suzuki reaction catalyzed by PdNPs@CA (Coleus amboinicus) under ultrasound irradiation, was reported by Kim and co-workers [47]. An excellent reaction yield was obtained in only 30 min and the catalyst was recovered and efficiently reused for seven cycles without a loss of activity (98→95% yield).
In 2018, Patil and co-workers reported the use of PdNPs@Piper nigrum in the Hiyama reaction in ethylene glycol as a green solvent. The catalyst was recovered and reused for 11 cycles with only a slight decrease of catalytic performance (98→88% yield) [59].
An efficient catalyst for the reduction of 4-nitrophenol in water was reported in 2017 by Veisi. An aqueous extract of natural black tea leaves was used as a reducing and stabilizing agent for the synthesis of Pd nanoparticles. The as-prepared Pd@B.tea NPs catalyst was used for 9 cycles without a loss of its catalytic activity [63].
Moreover, plant extracts have been used to form and stabilize PdNPs on the surface of solid supports such as Fe 3 O 4 magnetic nanoparticles, graphite, organic polymers, silica, metal oxides and natural clays to increase the stability and the recyclability of as-prepared Pd nanocomposites.
Indeed, the presence of residual phytochemicals on the surface of a solid support not only acts as a reducing agent but also facilitates the homogeneous distribution of PdNPs on the surface of the support and enhances the aqueous dispersibility of the resulting Pd-composite.
Recent examples on the immobilization of PdNPs on the surface of solid supports stabilized by a plant extract, and their catalytic applications, are reported in Table 2. Some examples are briefly described below.
Among the various solid supports used for the immobilization of palladium nanoparticles, magnetic nanoparticles (MNPs) have emerged as ideal supports because of their high surface area, their stability, low cost and good biocompatibility. Moreover, MNPs are simply and efficiently removed from reaction mixtures with an external magnetic field, reducing the metal contamination of products and simplifying recovery and reuse, all of which are fundamental aspects for a green approach [5,99]. PEG Colocasia esculenta leaf extract Irregular shape and size Suzuki coupling reaction [98] In 2021, Veisi and co-workers presented the catalytic activity of Fe 3 O 4 @Fritillara/PdNPs, in the reduction of nitroarenes in H 2 O/EtOH (2:1) as a medium. The magnetic nanocatalyst was obtained using a Fritillaria imperialis flower extract as the reducing and capping agent in the in-situ formation of PdNPs on the surface of Fe 3 O 4 MNPs [80]. The catalyst was recovered and reused for 8 cycles without a noticeable loss in activity. The particles' size and shape were retained and no agglomeration was observed.
In 2018, the same authors reported the use of tannic acid as the reducing and stabilizing agent for the immobilization of PdNPs on Fe 3 O 4 MNPs [82]. The nanocatalyst has been used in the Suzuki coupling reaction and in the reduction of 4-nitrophenols in an aqueous medium. There was not a noticeable reduction of catalytic activity observed for up to 8 and 10 cycles, respectively.
In 2020, Hemmati and co-workers developed the in-situ formation of PdNPs over the surface of Fe 3 O 4 magnetic nanoparticles by using a Strawberry fruit extract under ultrasonic conditions [81]. The Fe 3 O 4 @Strawberry/PdNPs nanocatalyst has been used to promote the Suzuki reaction efficiently in H 2 O/EtOH (1:1) as green reaction medium. The preservation of the catalyst's nanostructure and a negligible leaching of Pd over the reaction runs has been observed.
In 2019, the aqueous extract of Artemisia abrotanum leaf was used by Yousefi and co-workers to produce PdNPs on the surface of reduced graphene oxide (rGO) [85]. Both the graphene oxide support and Pd(II) ions were reduced by an abrotanum leaf extract.
The TEM image displayed that rGO are covered with a thin layer of biomolecules which increase the PdNPs dispersibility. The catalytic activity of PdNPs coated on modified rGO was tested in the Suzuki reaction in H 2 O/EtOH (1:1) at 50 • C, displaying excellent reaction performances for up to 9 cycles with negligible levels of Pd leaching.
In 2020, Peng and co-workers used tea and coffee extracts to produce PdNPs on the surface of titanium nanotube arrays (TNAs) under microwave irradiation [90]. The nanocatalyst has been used successfully in photoelectrochemical experiments for the degradation of dyes in the anodic chamber and the simultaneous production of hydrogen in the cathodic chamber. It is shown that the photocurrent of Pd/TNAs-C, obtained by using coffee as the reducing agent, was higher than that of Pd/TNAs, obtained by hydrothermal methods.
The green synthesis of Pd/CuO NPs via the reduction of a PdCl 2 /CuCl 2 (2:10) aqueous solution with Theobroma cacao L. seeds extract and their catalytic application was reported by Nasrollahzadeh in 2015 [92]. The Pd/CuO NPs, with an average size of around 40 nm, presented a good catalytic activity in the Heck coupling reaction for up to six consecutive runs (98→92% yield). However, for the realization of an environmentally sustainable process, a greener solvent should be used instead of dimethylformamide (DMF).
Inorganic supports such as zeolite, hydrotalcites and montmorillonite have been used for the immobilization and stabilization of PdNPs [93][94][95][96]. In 2020, the environmentally friendly preparation of PdNP@Zeolite Type-Y [93] was explored by Manjare and Chaudhari, using an aqueous extract of Anacardium Occidentale shell as the reducing and stabilizing agents. Very small PdNPs have been obtained (1-2 nm) and high catalytic activity and recyclability up to 10 cycles has been observed in the Suzuki reaction of iodobenzene with aryl boronic acid in water at 90 • C with a 0.01 mol% Pd loading (TOF 6100-8400 h −1 ).
In 2017, Nasrollahzadeh and co-workers reported the use of an eggshell as an economic and environmentally friendly support for immobilizing PdNPs prepared using a barberry fruit extract as a reducing and capping agent [97]. The catalytic activity has been evaluated in the hydroxylation of phenylboronic acid and in the reduction of 4-nitrophenol and dyes. High yields and excellent recyclability were observed for up to six cycles in the reduction of 4-nitrophenol. There was no observed morphology alteration of Pd nanocomposite and there was a very low Pd leaching into solution (<0.1%).

Biopolymers as Supports
As mentioned, the catalytic properties of PdNPs are closely related both to the shape and size of the nanoparticles and to the nature of the stabilizer. The agglomeration phenomena increase the size of PdNPs and considerably reduce the catalytic activity during use. One proposed solution is to immobilize the PdNPs on the surface of a support with a high surface area. The chelation between the support and the PdNPs stabilizes the nanoparticles and prevents the agglomeration phenomena, consequently increasing the catalytic activity and facilitating its recovery and reuse. Various supports have been developed for anchoring and stabilizing PdNPs such as organic polymers, inorganic materials, organic-inorganic hybrids and carbon materials [4][5][6][100][101][102][103][104].
Recently, multiple literatures, that have been referenced, report the use of natural polymers as green supports for the sustainable preparation of PdNPs.
Biopolymers such as polysaccharides (cellulose, chitosane, pectine, agarose, starch), lignin, gum, protein and DNA are promising materials as green supports for PdNPs because they are non-toxic, biodegradable, incur a low cost and are abundant in nature and also possess excellent mechanical properties and thermal stability, easing chemical modifications and having a high chelating capacity with metal ions [32,105,106].
The skeletons of these biopolymers are rich in hydroxyl, amino and carboxylic groups which are responsible for the chelation of the Pd ions and for the stabilization of the formed PdNPs. Furthermore, these groups determine the solubility of the Pd nanocatalysts in the reaction medium, a very important property for chemical processes performed in an aqueous medium.
The biopolymers can be used in their native form or after modification in order to improve the metal coordination, nucleation and stabilization of the metal nanoparticles.
There are three different approaches for the preparation of biopolymer-supported PdNPs. These are (i) impregnating the polymer in its native form or after its modification with the Pd(II) ion salt and an in-situ reduction by hydroxyl groups of biopolymer; (ii) impregnating the biopolymer with the Pd(II) ion salt followed by the addition of an external reducing agent (iii) synthesis of colloidal PdNPs and then adsorption into biopolymer branches.
The first method is the most convenient and commonly used, in this case the biopolymer acts both as a support and as a reducing agent. The reduction of the Pd (II) ion is generally obtained by thermal heating in an aqueous medium or by using ethanol as a solvent and an in-situ reducing agent. When an external reducing agent is required, from a green perspective, a natural reducing agent from a plant extract is preferred. Table 3 presents recent reported examples on the use of biopolymers as a support for PdNPs, the method used for PdNPs production and the catalytic application. Some examples are briefly described below.
As presented in Table 3, both native and modified biopolymers have been used as a support for the immobilization of PdNPs, dispersion and stabilization. Generally, the reduction of Pd(II) occurs without the addition of an external reducing agent, the functional groups present in the biopolymer are responsible for the formation of Pd(0). When an external reductant is required, natural reductants from plant extracts or ethanol reduction are generally used. Only a few examples report the use of chemical reducing agents such as NaBH 4 . Some examples of the different approaches are described below.
In 2021, Mao and co-workers reported the use of nanocellulose, generated from waste cotton cloth, as a green support for the immobilization and stabilization of small sized PdNPs [107]. The Pd@NC NPs catalyst was generated in situ during the course of the Suzuki reaction in an aqueous medium in the absence of additional reagents. The catalyst showed excellent catalytic activity not only in the Suzuki reaction of aryl iodides and bromides but also with the least reactive aryl chlorides. After the reaction, the in situ generated catalyst was separated and reused for up to 11 cycles without the loss of catalytic activity. The high stability was also confirmed using a hot-filtration test and through an ICP-OES analysis; no leaching of Pd in the aqueous reaction medium was observed.
In 2020, Patil and co-workers reported the preparation of a Pd@cellulose fiber from a waste banana pseudostem [109]. Pd nanoparticles, that were biogenically prepared from a banana pseudostem extract, were immobilized on cellulose fibers from waste banana pseudostem. An excellent catalytic activity was observed in the Suzuki coupling reaction at room temperature, in H 2 O/EtOH (1:1) as green reaction medium with only a 0.004 mol% Pd loading. The catalyst has been recycled for up to 15 times without losing its catalytic activity. A hot filtration test and <0.01 ppm Pd leaching in the reaction media confirm that PdNPs were tightly trapped in the cellulose fibers. The same catalyst has also been used, by the same authors, in the denitrogenative cross-coupling reaction [110].
In 2020, Bai and Wang reported the preparation of bio-supported Pd nanoparticles where the powder of Clove leaves (RCL) was used as the support, reducing and stabilizing agents [113]. A high catalytic activity and a recyclability for up to 8 cycles has been observed in the Suzuki reaction in H 2 O/EtOH (1:1) at 60 • C with a 0.05 mol% Pd loading (TOF 5773.2 h −1 ). The pore structure and the oxygen-containing groups on the RCL greatly improve the stability and recyclability of the small-sized PdNPs and facilitates the mass transfer of the substrate, thus improving the catalytic activity. a Preformed biogenically synthesized palladium nanoparticles were immobilized on cellulose.
In the same year, Baran and Nasrollahzadeh reported a green procedure for the synthesis of PdNPs on biodegradable microbeads consisting of chitosan, agarose and beta-cyclodextrin (CAP) [114]. The pendant hydroxyl and amino groups on the matrices strongly interact with the palladium, producing a highly active, recyclable catalyst for the Suzuki reaction. High yields of biphenyls have been obtained by using a green procedure without the presence of solvents (solvent free conditions), under microwave irradiation for 6 min. After 7 cycles the catalyst retains its spherical surface and a negligible level of Pd leaching was observed.
In 2020, recombinant 45-amino acid long peptides that were fused to a green fluorescent protein (GFPuv) were used by Beyzavi and co-workers to control the size, morphology, and structure of the PdNPs formation [116]. Histidine residues present in the protein structure inhibit the growth of nanoparticles resulting in PdNPs with an average size of 2.4 nm. The catalytic activity was evaluated in the Suzuki and Stille coupling reaction in H 2 O/EtOH as the reaction medium. A slight decrease of catalytic performance was observed over 5 runs. Also, no aggregation or morphological changes were observed in the used catalyst after five cycles.
In 2019, Javanshir and co-workers reported the use of Isinglass, containing approximately 98% protein collagen, as a reducing and stabilizing agent for PdNPs production [122]. The catalytic activity of the Pd/IG biocatalyst was tested using the Suzuki reaction with water as the reaction medium. The recovery and reuse were successfully achieved in 5 successive runs. The TEM image of the recovered catalyst showed that both the morphology and size of PdNPs does not vary significantly.
In 2018, Baran and co-workers designed green chitosan/starch and chitosan/cellulose composites as supports for the stabilization of Pd nanoparticles obtained through the reduction of Na 2 PdCl 4 with NaBH 4 in an aqueous solution [126,127]. The obtained Pd@chitosan/starch and Pd@chitosan/cellulose nanocomposites were employed in a very low Pd loading (0.005 mol% and 0.004 mol%, respectively) in the Suzuki reaction under microwave heating, in solvent-free conditions, for 5 min. Further, an excellent reactivity was observed (TOF~300,000-170,000 h −1 ) for the less reactive aryl chloride. The recovery and reuse of Pd nanocatalysts has been explored: while Pd@chitosan/starch showed a decrease in catalytic activity already at the 4th run, Pd@chitosan/cellulose retained its catalytic activity and structures for up to 8 cycles. For a totally green strategy, a sustainable green reducing agent, in place of NaBH 4 , is highly desirable.
In some examples, a significant reduction of the catalytic activity was observed during the recovery and reuse tests. Pd leaching and/or the agglomerating of PdNPs are often responsible for the decreased conversion of desired products as suggested by the TEM images and leaching studies [108,111,115,125,128,131,135].
Recently, DNA has received a great deal of attention in the field of catalysis because of its ability to act as a ligand for metal complex formation or as a support for metal nanoparticles [140,141]. Indeed, the phosphate groups and nitrogen-rich bases that are present in DNA have a high affinity for transition metals.
In 2017, Alonso and co-workers prepared a PdNPs/DNA nanocatalyst for the copperand ligand-free Sonogashira coupling reaction [139]. The catalyst was easily recovered and reused in five cycles and a drop of the yield was observed in the 5th run. The palladium content in the solution was determined to be only 0.01 wt.% of the original amount.
More stable and monodisperse PdNPs can be obtained by using biopolymers as coating agent to modify the surface of solid supports such as magnetic nanoparticles, graphite, metal oxides and natural clays.
Recent examples on synthesis and catalytic application of PdNPs immobilized on biopolymer-solid support nanocomposites are reported in Table 4.  [162] Similarly, the reduction of Pd(II) ions is achieved through thermal heating in an aqueous medium without adding any reducing agent or by using ethanol as the solvent and reductant. In some cases, the addition of an external reducing agent is required. Some examples are briefly described below.
Biopolymer functionalized Fe 3 O 4 nanocomposite can assist in obtaining nanocatalysts with a high performance both in terms of catalytic activity and recyclability.
In 2021, Veisi and co-workers synthesized a novel PdNPs-adorned chitosan-starch encapsulated core-shell type magnetic nanocomposite (Fe 3 O 4 @CS-Starch/Pd) without any reducing agent, by applying ultrasound irradiations [142]. Excellent performance has been obtained both in the Suzuki coupling reaction and in the reduction of 4-nitrophenol under sonication. The catalyst was easily magnetically recovered and reused for 11 cycles, with a slight decrease observed only after 9th cycle. A hot filtration test and Pd content displayed that the catalyst was truly heterogeneous without a significant leaching of the active species.
In 2020, Hasan prepared a magnetic nanocatalyst through the immobilization of chitosan (CS) on a magnetic Fe 3 O 4 following the deposition of PdNPs on its modified surface by using NaBH 4 as the reducing agent [143]. A high catalytic activity was observed both in the Suzuki and Heck coupling reactions in H 2 O/EtOH (4:1). The catalyst was magnetically recovered and reused up to seven times in the Suzuki coupling reactions with 97% efficiency.
Conversely, in the same year, Heydari synthesized a magnetic nanocatalyst, immobilizing PdNPs on the surface of phosphine-functionalized cellulose (PFC) by using NaBH 4 as the reducing agent, followed by the deposition of PFC-Pd on the surface of the Fe 3 O 4 magnetic nanoparticles [144]. The catalytic performances have been evaluated in the Sonogashira and Suzuki coupling reactions in a K 2 CO 3 /glycerol (1:5) deep eutectic solvent (DES), used as a green solvent. The catalyst was simply recovered by magnetic filtration and reused for up to five cycles without a significant loss of activity.
Between 2019 and 2020, Baran and co-workers synthesized various palladium nanocatalyst anchored on magnetically separable lignin-chitosan beads [146], chitosan/activated carbon microcapsules [147] and chitosan/agar microcapsules [148]. The catalytic activity has been evaluated on a microwave-assisted Suzuki coupling reaction under solvent free conditions. High performances have been observed for all palladium magnetic nanocatalysts, but the greatest results were obtained using PdNPs@CS-AC/Fe 3 O 4 [147] where both chitosan and activated carbons with a high surface area and microporous structure provide the coordination sites for PdNPs. The Pd magnetic nanocatalyst retained its catalytic performance even after ten successive runs (99→95% yield) with only 0.5% Pd leaching. There was no change observed in surface morphology of microcapsules and the shape and size of PdNPs.
In 2021, the same authors used an eco-friendly and low-cost magnetic nanocomposite consisting of a chitosan and magnetic iron oxyhydroxide microsphere, δ-FeOOH, to immobilize palladium nanoparticles [154]. The prepared catalyst has been tested in the Suzuki reaction in H 2 O/EtOH (1:1) at 70 • C with 0.05 mol% Pd loading. High catalytic activity and a recyclability for up to 8 cycles has been obtained. FE-SEM, TEM, and FT-IR analyses indicate that the magnetic microspheres retain their structure without any significant changes compared with the fresh one.
In 2018, Farzada and Veisi used Fe 3 O4/SiO 2 nanoparticles, coated with polydopamine (PDA), obtained by simple green self-polymerization of dopamine on magnetic support, to immobilize PdNPs [155]. The phenolic hydroxyl and amine functional groups on the surface of PDA act as the adsorbent, reducing and stabilizing agent for the production of PdNPs. The Pd-nanocomposite was conveniently recovered and reused for twelve runs, with a minor reduction in catalytic activity after the tenth cycle, in the Suzuki coupling reaction in H 2 O/EtOH (1:1) at 40 • C.
Graphene quantum dots (GQDs) have emerged as a new class of promising organic nanomaterials due to their high stability, interesting surface area, low-toxicity and biocompatibility. In 2021, a new palladium nanocatalyst was obtained by Keshipour and co-workers by building graphene quantum dots on a citric acid modified chitosan via a citric acid self-condensation reaction and the successive deposition of PdNPs by the chemical reduction of Pd(II) with NaBH 4 [156]. The catalyst exhibited excellent catalytic activity and selectivity in the reduction of nitroarenes in a mild reaction condition using water as a green solvent. The catalyst has been recovered and reused for up to 5 cycles with a negligible loss of activity and very low Pd leaching in solution. GQD assists the stability of PdNPs because of extensive electronic system on its structure.
Natural clay such as kaolin, bentonite and montmorillonite has been used as support to increase the stabilization of PdNPs-decorated chitosan (CS) [160][161][162]. A comparative catalytic study in the Sonogashira reaction between aryl iodides and alkynes in the presence of CS/Pd and CS/MMT/Pd was reported by Qi and co-workers in 2015 [162]. The stabilization of montmorillonite (MMT) effectively improved the thermal stability and Pd leaching resistance of the hybrid catalysts. Indeed, the CS/Pd catalyst showed a decline of catalytic activity after only the 4th run while the stabilized CS/MMT/Pd can be recycled 10 times without a significant decrease in yield. Moreover, CS/MMT/Pd showed a much lower Pd leaching percentage than CS/Pd.
In 2020, Patil designed a Pd/DNA bio-nanocatalyst, exploiting the metal chelating properties of DNA, immobilized on silyl functionalized Fe 3 O 4 magnetic nanoparticles, to graft and stabilize PdNPs [153]. The prepared catalyst was tested in the Suzuki reaction in H 2 O/EtOH (1:1) at room temperature with a 0.02 mol% Pd loading. Excellent reactivity and recyclability have been obtained for up to six cycles, and a slight drop in the yield was observed thereafter.
In 2019, Hajipour and Khorsandi report the use of a DNA-modified multi-walled carbon nanotubes (MWCNTs) to immobilize and stabilize PdNPs [159]. The aromatic nucleobases of DNA interact through π-π stacking with the surface of carbon nanotubes and stabilize palladium nanoparticles. The evaluation of catalytic activity has been performed in a Suzuki and Sonogashira coupling reaction in H 2 O/EtOH (1:1) at 65 • C in the presence of a 0.024 mol% Pd loading. High yields were also obtained with less reactive aryl chloride. The catalyst was recovered and reused seven times with a slight decrease in the yield after the 6th cycle (98→86% yield). Neither a spectroscopic analysis nor Pd leaching measurements were performed on the recovered catalyst.

Carbonaceous Supports
Carbonaceous materials such as mesoporous active carbon, graphene, carbon nanotube and carbon quantum dots (CDQ), have attracted considerable attention due to some important advantages such as high availability and low cost, biocompatibility, high surface areas, their porous structure and high thermal, mechanical and chemical stability [6,103]. Their unique pore structure can allow to obtain very small and dispersed metal nanoparticles and can increase the performance of a nanocatalyst, thereby enhancing the contact between the reactants and the catalyst, imparting an overall synergistic effect. These unique features have made carbonaceous materials interesting supports for the immobilization of palladium NPs. As of quite recently, bio-wastes have been used as a source for the preparation of mesoporous carbon active supports for PdNPs catalysts that prove to be inexpensive, renewable and biodegradable. Table 5 presents recent examples of green carbonaceous materials used as supports for palladium NPs, listing the used bio-source and their catalytic application.  [173] In 2021, Vaccaro and co-workers report a simple protocol for the valorization of a pine needle urban biowaste [163]. The lignocellulosic biomass is efficiently transformed into biochar, a low-cost support for the immobilization of Pd nanoparticles without adding any reducing agent. The catalytic efficiency was evaluated in Heck and Hiyama coupling reactions in GVL (γ-valerolactone), a biomass-derived solvent, comparing the obtained results with commercial Pd/C catalyst. The biocatalyst presented a good catalytic efficiency for five consecutive runs in the Heck reaction with a very low average Pd leaching.
In 2019, biochar from dried chicken manure was prepared by Hajjami and co-workers [164]. Surface modified biochar was used as a support to immobilize PdNPs by using NaBH 4 as the reducing agent. The catalytic efficiency of the prepared bio-catalyst has been evaluated in a Suzuki and Heck reaction, using polyethylene glycol (PEG) as the green reaction medium. The catalyst was recovered and reused for up to seven successive reaction runs without a significant change in its activity. The amount of palladium in the recovered catalyst was in corresponded well with the fresh catalyst, confirming that leaching of the palladium did not occur.
In the same year, Becht and co-workers reported the preparation of a mesoporous carbon support through a self-assembly route, starting with a solution containing a surfactant and a phenolic resin based on phloroglucinol-glyoxal [165]. The biocatalyst with uniformly dispersed ultra-small PdNPs (1.2 nm) presented an excellent catalytic activity in the Suzuki reaction in solely water with a very low Pd loading (10-100 µequiv, TOF 10,000 h −1 ). The Pd content in the final product was less than 0.25ppm.
In 2019, an activated mesoporous carbon was synthesized from a rice husk biomass by Banerjee and co-workers [166]. The carbon-based support was decorated with palladium nanoparticles by reducing PdCl 2 with NaBH 4 and the as-prepared biocatalyst, tested in the Suzuki, Heck and Sonogashira coupling reactions in water or EtOH using microwave irradiation. The catalyst was recovered and reused for up to eight runs but a loss in yield was observed after the fifth cycle. The reduction of catalytic activity has been associated with the agglomeration of PdNPs and a loss of porosity in the mesoporous carbon support.
In 2018, Burri and co-workers used rice husk waste to obtain a C-SiO 2 support for the deposition of PdNPs via its impregnation with PdCl 2 and a successive reduction with hydrazine [167]. The catalytic activity was evaluated in the carbonylative Suzuki coupling reaction in dioxane for the formation of biaryl ketones. Complete conversion was obtained for up to 4 cycles and only a very slight decrease (98%) was observed at the 5th cycle.
In the same year, Pourjavadi and Habibi synthesized porous carbon material through the carbonization of pomegranate peel waste [168]. The surface of the carbon support was decorated first with Fe 3 O 4 nanoparticles providing magnetically separable catalyst and then with PdNPs via the reduction of Pd(II) with sodium lauryl sulfate (SDS). High catalytic activity and recyclability was obtained in the Suzuki reaction in EtOH/H 2 O (1:1) in the presence of a 0.02 mol% Pd loading without a significant change in the morphology of the catalyst even after seven runs.
In 2015, Becht and co-workers reported the synthesis of a palladium-containing mesoporous carbon catalyst through a one-step green synthesis based on a soft templating approach, using tannin as the bio-source carbon precursor without the use of toxic crossliker and reducing agents [169]. The catalyst has been successfully used in the Suzuki coupling reaction in propane-1,2-diol as a green solvent in the presence of an extremely low Pd loading (30 µequiv.). A significant change in the yield was not observed for up to 5 runs.
Recently, carbon quantum dots (CQD) have emerged as a green, non-toxic, abundant, and an inexpensive new class of carbon nanomaterials. The carboxylic and hydroxyl groups on the CQDs can act as a reducing and capping agent and confer high water solubility as well as biocompatibility, making it an excellent support for the immobilization of palladium nanoparticles [174].
In recent years, Gholinejad and co-workers prepared CQDs using various green and low-cost sources such as citric acid [172], vanillin [171] or glycerol/urea [170] for the modification of magnetic Fe 3 O 4 NPs. The magnetically separable supports have been used to immobilize and stabilize PdNPs. The catalytic activity has been evaluated in the Suzuki coupling reaction in H 2 O/EtOH (1:1) and the optimum result were obtained for the palladium supported on the magnetic Fe 3 O 4 nanoparticles modified with nitrogendoped CQD obtained from glycerol and urea [170]. The catalyst were recovered and reused for up to 10 cycles with a very small decrease in performance. A hot filtration and poisoning test highlighted that the reactions occur via a release and catch mechanism in which leached active Pd(0) species catalyze the reaction, and the return of the Pd from the reaction medium to support is responsible for its recycling. The catalyst has also been used in the reduction of nitrophenol in H 2 O/EtOH (5:1) with a very low Pd loading (0.008 mol% at room temperature). An excellent yield and recyclability were obtained for seven consecutive runs.
In 2020, Zhang and co-workers developed a simple and green in situ reduction approach for the preparation of reduced graphene oxide supported palladium nanoparticles (Pd/rGO) [173]. Sodium dodecyl sulfate was used to disperse rGO in the solution and after heating, it decomposed to 1-dodecanol, which can reduce Pd(II) to Pd(0) by self-oxidization, to dodecanoic acid. The as-prepared catalyst has been used to promote Ullmann and Suzuki coupling reactions by using aryl chlorides as the reactants in an aqueous media using MW irradiation, presenting even better catalytic performances than homogeneous Pd(OAc) 2 . Moreover, a recycling test on the Ullmann reaction presented a high catalytic performance for up to five cycles and only a slight decrease in the yield was observed at the sixth run due to the partial agglomeration of PdNPs as displayed by the TEM images.

Polyol as Liquid Supports
Alcohols and polyols have been widely used as solvent, reducing and stabilizing agents for the synthesis of palladium nanoparticles. A recent review described the preparation of Pd based nanomaterials in a polyol medium, focusing on size, morphology and structure and their catalytic application in their coupling and a hydrogenation reaction in polyol solvents [175].
Very recently, Pires and co-workers designed glycerol-based solvents such as glycerol ethers and glycerol-based deep eutectic solvents (DES) for the immobilization and stabilization of Pd nanoparticles in liquid phases (PdNPs/solvent) and as a reaction media for catalytic application [176,177]. DES were obtained by mixing a glycerol-derived solvent with a bio-ammonium salt such as choline chloride or N,N,N-triethyl-2,3-dihydroxypropan-1-aminium chloride.
For the immobilization the Pd-NPs, two strategies have been used. The first involves contacting a dispersion of the Pd nanoparticles on poly-N-vinylpyrrolidone (PVP/PdNPs), prepared previously using EtOH as the reducing agent and PVP as the stabilizer, with the glycerol ethers or DES [176]. The dispersion of PdNPs in a glycerol-based solvent avoided a palladium agglomeration, allowing for the creation of a homogeneous PVP-Pd-NPs/solvent system which was tested using the Heck coupling reaction. After the extraction of the reaction product with n-pentane, the PdNPs that were immobilized onto a glycerol-derived solvent were applied to a new catalytic cycle by simply adding new reagents. The best results were observed with the DES obtained from glycerol and isopropyl glyceryl monoethers with N,N-triethyl-2,3-dihydroxypropan-1-aminium chloride, and a high yield and recyclability was observed for up to 5 cycles.
In the second approach the Pd(II) reduction and nucleation was performed in the glycerol-based solvent in the presence of PVP and H 2 as the reducing agents, or in a glycerol-derived DES, without adding any reduction or capping agent, thus improving the sustainability for the production of PdNPs. Indeed, the DES acts both as a stabilizer and a liquid support and the small amounts of water present in these hygroscopic solvents would be responsible for the reduction of the palladium ion to Pd(0) [177]. The as-prepared PdNPs/solvent systems presented high catalytic activity in the hydrogenation of conjugated and nonconjugated alkenes, alkynes, and carbonyl compounds. The DES obtained from methyl glyceryl mono-ethers with N,N-triethyl-2,3-dihydroxypropan-1-aminium chloride provided the best results and quantitative conversions were achieved for three consecutive cycles. Palladium agglomeration and a drop of the yield was observed from the fifth run onward.

Hydrothermal Coprecipitation
Hydrothermal coprecipitation is an efficient, one-pot and environmentally sustainable procedure that involves the simultaneous precipitation of the metal precursor and the support material without the addition of a reducing agent.
In 2018, Chatterjee and co-workers developed a convenient method to produce PdNPs on γ-Fe 2 O 3 from PdCl 2 and Fe 3 O 4 magnetic NPs under modified hydrothermal conditions in water at 180 • C for 48 h [178]. Magnetic Fe 3 O 4 reduced the Pd(II) ions to zerovalent Pd to produce a Pd-γ-Fe 2 O 3 magnetic nanocomposite. The catalyst was tested in a Suzuki reaction with water as the reaction medium and recycled for six runs without a significant loss of catalytic activity until the 5th cycle.
In 2016, Zhao and co-workers reported a one-pot and eco-friendly hydrothermal procedure for the preparation of a magnetically separable NiFe 2 O 4 @GO-Pd composite [179]. By using this strategy, the reduction of PdCl 2 to Pd NPs, the conversion of FeCl 3 and NiCl 2 to NiFe 2 O 4 magnetic NPs and the deposition on the surface of graphene oxide (GO) were performed simultaneously in an ethanol/ethylene glycol (49:1 v/v) at 120 • C for 24 h. The catalytic activity has been evaluated using a Heck coupling reaction in EtOH/H 2 O (1:1). Excellent catalytic activity and recyclability was reported for up to six cycles (99→95% yield) with a negligible level of Pd leaching (0.15 wt.%).

Ultrasonic and Laser Irradiation
Physical methods such as ultrasound, microwave and laser irradiation are considered to present promising green alternatives to produce PdNPs on the surface of solid supports [27].
In 2020, palladium nanoparticles supported on a magnesium ferrite (Pd/MgFe 2 O 4 ) were prepared by Dasari and co-workers using one-pot ultrasound assisted coprecipitation of FeCl 3 , MgCl 2 and PdCl 2 without any reducing and capping agents or surface modifiers [180]. The catalyst has been used to promote the Suzuki reaction in H 2 O/EtOH (1:1). A high yield and recyclability, with a negligible level of metal contamination, has been observed for up to 5 cycles.
A green synthesis of palladium NPs immobilized on zeolite-Y by ultrasound irradiation was reported by Tadjarodi in 2018 [181]. High catalytic activity was reported in the Suzuki coupling reaction in H 2 O/EtOH (1:1) with a very low Pd loading (0.02 mol%). Moreover, high yields were also obtained with less reactive aryl chlorides and a good yield (63%) was obtained with an inactive aryl fluoride. The catalyst was recycled for up to 10 runs without losing the catalytic activity.
In 2021, Tibbetts and co-workers reported the synthesis of uncapped PdNPs through a green procedure that involved a femtosecond laser photoreduction in water of precursor metal salts, such as K 2 PdCl 4 and Pd(NO 3 ) 2 [182]. Laser reduction produced ultrasmall colloidal spherical Pd nanoparticles (3.1 ± 1.0 and 1.2 ± 0.3 nm using the chloride and nitrate precursors, respectively) and further, anisotropic large aggregates with nanopopcorn and nanoflower morphologies were observed. An exceptionally high level of catalytic activity was obtained in the Suzuki coupling reaction and a reduction of 4-nitrophenol but the colloidal PdNPs proved to be stable for at least a day, therefore, a deposition on solid supports is necessary for future applications.

Conclusions
In recent years, a growing interest in the development of green and environmentally sustainable procedures has affected all fields of chemistry. Considering the great potential of palladium nanoparticles across many applications, studies regarding their preparation in a sustainable way are constantly growing. The green production of Pd nanoparticles allows for the enhancement of their biological compatibility, the lowering of production costs and the reduction of physiological toxicity and environment pollution.
In this review, the solvent, the reducing agent, the capping and dispersing agent and the synthetic method were the main aspects considered for the synthesis of Pd nanoparticles in a green method.
The use of natural, abundant, easily available and low-cost bio-products as a reductant, stabilizer or support for the immobilization PdNPs certainly responds to the demand for an environmentally friendly and more sustainable synthetic process. Moreover, the use of alternative energy sources such as ultrasound and microwave irradiation are considered for a green perspective.
The catalytic activity, the recyclability and the robustness of the Pd-nanocatalyst obtained via a green route have been evaluated in sustainable C-C coupling reactions and the reduction of nitroarenes. Many Pd-nanocatalysts have presented excellent catalytic activity and recyclability with a very low Pd leaching in solution, promising applicable developments for sustainable and environmentally friendly industrial productions.
Funding: This research received no external funding.