Homogeneous Catalysis of Transition Metals with Nanoparticles
Department of Chemistry, McGill University, 801 Sherbrooke Street West, H3A2K6
Received December 4th, 2015; E-mail [email protected]
Abstract – Nanotechnology is a technology, science, and engineering that is conducted by creating and modifying the properties of particles in the range of 1-100 nm. The field has explored the traditional fronts of chemistry and brought them forward to completely new horizons. Homogeneous catalysis is another domain of conventional chemistry that has been explored using transition metal nanoparticles (NPs). Usage of nanoparticles in the domain of green chemistry is an area of further exploration and requires an understanding of efficiency, sustainability, stability and recyclability. Development of conventional processes involving hydrogenation/dehydrogenation, hydrosilation oxidation, homogenous and heterogeneous cross-coupling reactions using nanoparticles catalysts would provide a new pathway for these reactions and greener technological advancements. Besides these core factors in nanocatalysis, it is vital to have environmentally benign and sustainable solvents and stabilizers as well. This paper will address the incorporation of different transition metals in homogeneous catalysis domain on nanotechnology fronts. Furthermore, the properties of transition metal nanoparticles are also observed for developing a highly selective and active catalyst for different reactions and reaction conditions.
Table of Contents
TOC o “1-3” h z u 1.Introduction PAGEREF _Toc436752845 h 22.Discussion PAGEREF _Toc436752846 h 22.1System of Ideal Nanoparticle Catalysis PAGEREF _Toc436752847 h 22.2Solvents and Catalyzed Nanoparticles PAGEREF _Toc436752848 h 62.3Metal, Stabilizer and Solvent: Relationships and Interactions PAGEREF _Toc436752849 h 72.4Roles of Transition Metal PAGEREF _Toc436752850 h 82.5Bimetallic Nanoparticles and Cross-Coupling PAGEREF _Toc436752851 h 152.6Nanoparticle Size PAGEREF _Toc436752852 h 162.7NPs Morphology PAGEREF _Toc436752853 h 182.8Magnetic Nanoparticles and Catalysis PAGEREF _Toc436752854 h 192.9Stabilizers and Catalysts Nanoparticle PAGEREF _Toc436752855 h 203.Conclusion PAGEREF _Toc436752856 h 234.References PAGEREF _Toc436752857 h 24
Table of Figures
TOC h z c “Scheme” Scheme 1: Hydrosilation Reaction (Yan et al, 2010) PAGEREF _Toc436325660 h 10Scheme 2: Methyl Formate production using Cu Nanoparticles (He et al, 2008). PAGEREF _Toc436325661 h 12Scheme 3: Epoxidation of ethylene using Ag NPs (Jia and Schuth, 2011). PAGEREF _Toc436325662 h 12Scheme 4: 95% Selective Acetone Hydrogenationvia Ir NPs (Ozkar and Finke, 2005). PAGEREF _Toc436325663 h 13Scheme 5: Cinnamaldehyde Hydrogenation Pathways (Yu et al, 1999). PAGEREF _Toc436325664 h 14Scheme 6: Cyclohexane Oxidization catalyzed by Ru NPs (Launay et al, 1998). PAGEREF _Toc436325665 h 15Scheme 7: C-Z and C-C bonds cross-coupling (Beletskaya and Ananikov, 2011). PAGEREF _Toc436325666 h 17Scheme 8: Z-Z and Z-H Bond Addition Cross-Coupling (Beletskaya and Ananikov, 2011). PAGEREF _Toc436325667 h 17
IntroductionNanocatalysis has undergone tremendous development during the last ten years. One of the most common branches of nanotechnology includes nanocatalysis in the liquid phase. The pioneering discovery of colloidal chemistry dates back to the 19th century in which Pt nanoparticles (NPs) were used for decomposition of hydrogen peroxide (CEN, 2015). Since the 1990s, the transition metal nanoparticles (NPs) have become a core component of homogeneous catalysis or quasi-homogeneous catalysis domain (Astruc, 2015). Homogeneous catalysis is a traditional area of catalysis in which the catalyst and along with reacting mixtures are in the same phase. A number of transition metals are available that can be used as soluble nanoparticles (Dupont and Meneghetti, 2013). Such a variety of transition metals enables the interaction of different in situ techniques involving in-situ NMR and IR (Meier, 2009). The paper will provide an outlook of transition metal nanoparticles and their potential applications along with their relations of green solvents.
DiscussionSystem of Ideal Nanoparticle CatalysisIn order to exemplify the ideal nanoparticle catalysts behavior, some potential aspects should be considered. The NPs in solution are considered heterogeneous in nature. Nevertheless, they are usually homogeneous instead of heterogeneous because when the NPs are well dispersed for effective diffusion and better catalyst activity, they create a homogenized reaction mixture. This very feature allows catalytic nanoparticle systems to be quite similar to homogeneous catalytic systems (Albonetti et al. 2014). Most of the homogenous nanocatalysis systems involve transition metal complexes, thereby allowing good selectivity and applicability at low temperatures. However, these nanocatalysis systems have severe issues such as catalyst degradation, and separation of catalysts from reactants and products in the nanocatalysis homogenous system. Unfortunately, nanoparticle (NP) catalysts also have similar problems associated with it.
On the other hand, the higher turnover frequency (TOF) of homogeneous catalysts is in the order of thousands while, heterogeneous catalysts have lower TOFs. The turnover frequency determines the intrinsic stability of catalysts. In order to analyze the approach of ideal NP catalysts, the criteria of stability, efficiency, recyclability and sustainability should be discussed in ample detail.
Considering the efficiency of NP catalysts, high efficiency implies that the catalyst would have a higher amount of selectivity at greater conversion rates. It is one of the salient features of NP catalysts as compared to different heterogeneous catalysts. As per Corma and Garcia (2006), homogenous catalysts are more selective and active, and they can easily be optimized for different system’s usage as compared to heterogeneous catalysis systems. Furthermore, nanoparticle catalysts have the same features in the solvent. The enhanced efficiency of NP catalysts is mainly because of their approach to a metal core that includes the screening of the metal, the structure of the surface and size of the particle. In this regard, the efficiency of catalysis requires NP catalysts and their systems to behave like catalytic cascade and one-pot process. A catalytic cascade reaction involves one or more than one series of catalytic reaction whereas the one-pot reaction is the reaction that does not require intermediate separation. The core advantages of using one-pot process systems are that they provides time and energy saving opportunities (Tao
and Kazlauskas, 2011).
The next concern is sustainability. For enhancing the green prospects of NP catalysis systems, it is essential that the catalyst is also sustainable. Sustainability comprises of a number of considerations, including the sustainability of transition metal associated with NPs, because different metals have significantly different market constraints as compared to other. Among them, the rare earth metals and metals in actinides and actinides series are becoming quite rare (Pitkethy, 2003). In this regard, the elements in the platinum group are considered to be future alternate energy resources and are used as fuel cell catalysts. Nevertheless, these reserves are depleting at the faster pace and cannot fuel all the world’s cars in the future (Graedel, 2011). Hence, it is essential to consider transition metals that are quite abundant while exploring options of nanoparticle catalysts. The first row of transition elements is quite commonly ignored for nanoparticle catalysis studies. As per Webster et al (2004), the other factors that are considered for sustainability of NP catalysts are bioaccumulation, persistence and toxicity for the solvent, reactants, and their respective products. Considering that some of the substrates are risky on environmental grounds, they should be replaced by less harmful materials and their intermediates. A careful consideration of sustainability is an essential step towards green catalysts.
Stability of NP catalysts is another important concern. NPs are kinetically stable and the stability is crucial for many processes. However, the stability of NP should be in the moderate range because one of the key aims is to protect the nanoparticles against different aggregations via surface sites of nanoparticles and stabilizers’ functional groups (Raveendran et al. 2003). This very protection can be magnified using multi-site interactions that involve surface sites having weak interactions. Nanoparticles have the colloidal range from 1 nm to 1 µm (Alexandridis, 2011). Under this consideration, the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory was put forward that originally explained the aqueous colloidal systems; however, it can also be used for explaining NP stability. The DLVO theory is used for rationalization of forces between the interfaces and for interpretation of planar substrate particle deposition. Moreover, it is worthwhile to note that DLVO does not explain the interaction and behaviors of NPs in all aqueous systems (Borkovec, 2015).
Recyclability is another domain that dictates the industrial or mass-scale production of NP -related catalysts in solution phase. The enhanced dispersion of NP catalysts in a solvent is a key design consideration. It results in catalytic systems having high activity in mild conditions of reaction. On the other hand, high dispersion makes the separation of catalyst from the solvent highly difficult task (Wang et al., 2011). Another specific problem arising from the NP catalyst is metal contamination of polymeric and pharmaceutical products. For these complications, the NP catalytic system having good recyclability becomes quite important because of environmental constraints. However, in 2006, Van Leeuwen has suggested methods for converting homogenous catalysts to heterogeneous catalysts as part of solving this issue. But, it is not quite feasible for NP catalysts. NPs are three-dimensional particles that possess significantly high catalytic activity and surface area that is negatively influenced by the heterogeneous immobilization (Corma and García, 2002). Hence, considering the usage of NP catalysts as traditionally used catalysts in a fluid bed reactor for mildly improving their three – dimensional limitation would result in significant activity loss. Nevertheless, regarding the recyclability issue, hydroformylation process is being developed and refined for its use on the industrial scale for biphasic and organic-aqueous systems (Van Leeuwen, 2006).
Solvents and Catalyzed Nanoparticles Solvents in nanoparticles catalysis play quite a crucial role in formulating the green nature. As per a study conducted by Liu and Xiao (2007), it has been found that the solvents account for more than 50% of greenhouse gasses in different industries. NP catalysis systems can include some solvents that include ILs, carbon dioxide, fluorous solvents, and more importantly, water. Among these traditional solvents, water bears a substantially high level of attraction over different traditional solvents because of its non-toxic, non-flammable, and non-carcinogenic nature. Also, water is the least expensive solvent as compared to any other chemical because of its universal nature. Apart from that, scCO2 (Super Critical CO2) also bears different advantages like water. It is usually characterized by its low viscosity along with exceptional solvating properties for a number of organic solvents. Also, the ease of mass transfer and diffusive properties of supercritical fluid CO2 also enables its use in different industrial applications (Zhao et al. 2008). Because of the low impact of water and supercritical carbon dioxide, it is difficult to challenge the environmental benefits of these green solvents. The core reason for calling these two solvents green is because of their non-volatile, non-flammable and non-toxic nature (Yan et al. 2010). However, the implications of these solvents are being challenged because of their harmful impact on biological diversity and aquatic ecosystems thereby making them not completely environmentally benign. Considering other organic solvents, ILs are much greener and environmentally sustainable. The very nature of IL solvents embodies the core importance of developing tailor-made properties of solvents as per application and sustainability (Jeon et al. 2000).
Another class of solvents includes fluorous solvents that are usually represented by perfluorinated alkanes that possess quite an importance in nanocatalysis domain. It is mainly because of their good physicochemical properties that encompass high chemical and thermal resistance, lower dielectric constants along with the lower amount of toxicity as compared to ILs (Clark and Tavener, 2007). Besides, these novel properties, the fluorous solvents have severe environmental and biosafety concern. They are bioaccumulative and are also greenhouse gasses. In this group, perfluoro ethers are more environmentally friendly as compared to perfluorinated alkanes. Furthermore, solvents have the capability to make biphasic systems that enable to be reused thereby making separation quite feasible (Heldebrant and Jessop, 2003).
Apart from that, alcohols with the high boiling range such as PEG and glycerol, are also used for reaction media. Also, they are quite cheap, non-toxic, and have the capability of easily functionalization (Reichardt and Welton, 2011). Glycerol is one of the most common and widely used chemicals that is used in different cosmetics along with food additives. Similarly, PEG is also used by pharmaceutical and food industries (Sanguansri and Augustin, 2006).
Metal, Stabilizer and Solvent: Relationships and InteractionsTraditional systems are commonly characterized with basic division of solute and solvent in a homogenous phase. However, NP catalytic systems it involves a complex and delicate understanding of stabilizer (S), solvent (S) and metal (M). The metal core is the catalyst having the high selectivity that is supported by different stabilizers. The stabilizers have the capability of supporting and protecting the metal cores against detrimental aggregation. However, enhanced protection would result in a significant decline of catalyst activity (Yan et al. 2010). Solvent, on the other hand, is responsible for the dispersion of stabilizer and metal core. Nevertheless, the solubility of solvent in the NPs is not entirely controlled via solvent, but it is also dependent on the properties of the stabilizers. Hence, the solvent is mostly a carrier that transfers the reactants towards the metallic core and the products away from active sites of catalysis. Hence, the solubility of stabilizers, reactant(s) and the metallic core is responsible for the overall catalytic activity of nanoparticles. This cohesive relationship is also a key to developing a stable and effective NP catalysis system (Alonso et al. 2012).
Roles of Transition Metal
Different metals have different catalytic properties and it is mainly dependent on the d-block orbital filling properties. Based on catalytic behavior, these elements are divided into five categories as per (Yan et al, 2010).
The first group contains Mn, Ti, Nb, Zr, V, Cr, W and Mo and using these elements has a potential advantage of low price. However, they possess weak hydrogenation activity and most are not frequently used for hydrogenation on the large scale. Jia and Schüth (2011) have prepared Zr, Ti, Mn and Nb nanoparticles by K[Bet3H] reduction of metal halides. Their study has concluded that the Ti-NPs are significantly more effective as compared to the other within the same study group for hydrogenation reactions. Moreover, Ti NPs are also successfully used for McMurry coupling reaction catalysis (Petrii, 2015). As compared to the metallic states of these early transition metals, the oxides of these transition metal nanoparticles bear substantial importance because of their better capability to be used in catalyzed heterogeneous reactions. In this regard, the oxides of V, Mn, Mo and Cr are quite commonly used in the oxidation of alkanes. As pointed out by Jia et al. (2012) in the field of photocatalysis, a similar catalyst TiO2 provides an exceptional outlook for production of hydrogen from water along with degradation of environmental pollutants under UV light. However, the preparation of soluble oxide NP for these very reactions is a demanding challenge that lacks effective preparation strategies along with having stability issues of soluble oxide NP. The application and synthesis of these early transition metal groups possess greater possibilities of research in the upcoming future.
Another group of elements includes Co, Fe and Ni that have the advantages of being quite cheap and abundant metals for their potential usage in catalysis domain. These catalysts also have exceptional outlooks in quasi-homogenous reaction mixtures. Currently, these catalysts are used for hydrosilation, hydrogenation, oxidation, and C-C coupling reactions. Martino et al (1997) have made iron NPs by reduction of different iron ions with LiBH4 having reverse micelle solutions. The NPs used are active catalysts for naphthyl bibenzyl methane reactions. Similarly, Ni-NPs having 45 nm size can be prepared from Ni(CH3COO)2 using solvothermal process via hydrazine reduction. These NPs possess exceptional selectivity and activity in the process of nitrobenzene hydrogenation (Xu et al, 2007). Also, for hydrogenation of cyclohexane, the Ni-NPs have almost twice the activity of conventional Ni-based catalysts (Ma et al, 2010). Mertens et al. (2007) have also examined the usage of Co NPs in α,β-unsaturated aldehydes’ hydrogenation. Another famous reaction used in petrochemical industries is Fischer-Tropsch’s methane hydrogenation to produce alkanes (Scariot et al, 2008). The reaction is quite widely known to be used in transportation fuel production. Hydrosilation is another core reaction for production of silica embedded polymers. Following is the pictorial representation of reaction in scheme 1:
Scheme SEQ Scheme * ARABIC 1: Hydrosilation Reaction (Yan et al, 2010)These reactions are quite catalyzed by homogeneous complexes. However, recent advancement in the domain of transition metal and nanotechnology has showed that the Ni and Co NPs can be formed in the vicinity of reductive silane and can lead towards good catalyst activity. Moreover, Ni-NPs can also be used in C-C coupling reactions. Reetz et al (1998) have formed Tetradodecylammonium bromides by Ni-NPs while following an electrochemical method for production. Oxidation reactions can also be catalyzed by Fe, Co and Ni NPs. The production of adipic acid is a vital example that lead to the production of Nylon 6,6 and Nylon 6 (Launay and Patin, 1997). For their use as nanoparticles in quasi-homogenous phase, they are usually used for homogenous reactions. The formation of Fe NPs using reverse microemulsion route can provide better prospects of using Fe-NPs in catalysts in cyclooctane oxidation under mild conditions. Co, Ni and Fe have tremendous potential to be used in solution based catalysis regarding future research and development in nanocatalysis domain (Launay and Patin, 1997).
Ag, Cu and Au comprises of another group of elements that are usually used for redox reactions. Jia and Schüth (2011) have found that the reduction of copper acetylacetonate along with trialkylaluminum can be used for the preparation of Cu-NPs. These NPs are used for methanol synthesis due to their high activity. The particle size distributions are quite smaller and range between 3 and 5 nm. Furthermore, they exhibit exceptional activity above 130 oC. On a commercial scale, the reaction catalysis can also be compared with the traditional Cu/ZnO catalyst. Previously mentioned study has also found out the Cu-NPs are highly active in quasi-homogenous phase. The Cu/ZnO combination requires Zn to be highly active to achieve desired results. Another petrochemical product, methyl formate is quite commonly formed using the carbonylation reaction of methanol and is catalyzed via the strong base (CH3ONa), that is efficient but not environmentally sustainable (He et al. 2008). The usage of CH3ONa leads to different problems including deactivation of catalyst via H2O and CO2 impurities, corrosion along with problems involving byproduct formations. However, Cu-NPs does not require the presence of the strong base for the formation of methyl formate as describes in (He et al, 2008). Following is a reaction that shows reactions conditions and equation in scheme 2:
Scheme SEQ Scheme * ARABIC 2: Methyl Formate production using Cu Nanoparticles (He et al, 2008).Further improvements in above scheme would lead towards the development of much more environmentally friendly catalysts while replacing the unfriendly metal alkoxides as catalysts. Ag-NPs are also used for different hydrogenation and oxidation reactions. More specifically, in the production of ethylene oxide (Shiraishi and Toshima, 2000). The soluble mixture of water and ethanol is usually used as catalyst instead of Ag metal. Also, as per Jia and Schüth (2011), it has been found that the addition of Re(VII) and Ce(I) metals enhances the activity of reaction mixture. Following is the representation of Ag nanoparticles reaction along with the reaction conditions in scheme 3:
Scheme SEQ Scheme * ARABIC 3: Epoxidation of ethylene using Ag-NPs (Jia and Schuth, 2011).Different researches have been conducted regarding the catalytic properties of Au nanoparticles; however, their usage is predominantly in oxidation. The formation of gluconic acid from glucose and carbon dioxide from carbon monoxide are the novel example of Au NPs catalyzed reactions. Ag and Au NPs quite usually decompose to NaBH4 for the formation of H2 and NaBO2 (Wu et al, 2012).
Another group of catalysts involves different noble metals with exceptional hydrogenation capabilities. This group comprises of Rh, Ru, Pt, and Ir in the order of decreasing catalyst activity of their metallic nanoparticles counterparts for hydrogenation of alkenes (Widegren and Finke, 2003). The hydrogenation of alkenes and more specifically,carbon-carbon double bonds via soluble NPs are relatively easier and have the vivid amount of research in this area (Roucoux et al. 2002). Among these reactions, benzene ring hydrogenation has significant researching grounds, and Rh and Ru have been used for their enhanced activities. Rh is relatively cheaper; however, Ru has a relatively higher activity. The use of Ru-NPs dates back to the 1980s; however, their practical implementation has become possible with the turnover number for benzene hydrogenation as high as 20,000 that makes it highly attractive for large-scale implementation (Yu et al, 2011). For hydrogenation reactions involving carbon-oxygen double bonds, Pt, Ir and Ru have showed their excellent potential. Ozkar and Finke (2005) have prepared Ir nanoparticles that have been used for acetone hydrogenation. As part of this study, they found that the temperature required by the traditional support-type catalyst range in 100-300oC, which is significantly higher as compared to Ir NPs catalyzed the reaction.
Scheme SEQ Scheme * ARABIC 4: 95% Selective Acetone Hydrogenationvia Ir NPs (Ozkar and Finke, 2005).The presence of C-C or C-O double bonds in a compound raise the issue of selectivity. It is easier for the C-C double bond to undergo hydrogenation as compared to C-O double bond (Molnár et al. 2001). Nevertheless, specific reactions required a C-O double bond hydrogenation. Cinnamaldehyde hydrogenation is an example of such reaction. Regarding the selectivity of catalyst metal core, Ru and Pt NPs are usually used for selective C-O double bond hydrogenation reaction. However, Pt is significantly active as compared to Ru (Yan et al, 2010).
Scheme SEQ Scheme * ARABIC 5: Cinnamaldehyde Hydrogenation Pathways (Yu et al, 1999).Considering the reaction to scheme 5, Yu et al (1999) have deduced that the selective hydrogenation of acetone can be enhanced by the addition of Co2+, Fe3+, and Ni2+ ions to enhance the alcohol yield due to the induction effect produced by the metallic ions that can lead to C-O double bond activation. Moreover, it has also been found that Ru NPs are very active as compared to traditional catalysts in Fisher-Tropsch alkylation reactions. Quek et al (2011) have noticed that a 35-fold increment in the activity of the catalyst was observed while using unsupported Ru at 150oC. It is also worthwhile to note that the hydrocarbon product is immiscible in water that leads to easy separation of catalyst-product after the formation of fuel.
Another important reaction involves the formation of NH2 intermediates through reduction of N=O bond. This reaction is particularly important in dye manufacturing industries. Similar to the case of C=O reduction, Pt and Ru NPs are quite favorable for N=O reduction reaction with Pt greater activity as compared to Ru (Liu et al. 1999). The dye molecules contain halogen atoms and hence, dehalogenation would become the primary side reaction. The reason for proposing usage of these catalysts is because of its capability to weaken N=O bond. On the other hand, Pt is also used for different hydrosilation reactions (Yang and Liu, 1997). Quite commonly, Pt – based organometallic complexes are used for this very purpose. These nanoparticles are formed by the in situ reduction of silanes from a range of organometallic precursors (Lewis and Lewis, 1986). Apart from that oxidation reactions are also quite possible using this group of nanoparticles. Yan et al (2010) have prepared Pt NPs that are soluble in glycol. These NPs are readily soluble and have the capability to providing enhanced activity for both activated as well as non-activated alcohols for some products including allylic alcohols, primary and secondary alcohols and aromatic alcohols under aerobic conditions without the usage of any bases (Tao et al, 2013). Scheme 6 represents such a reaction along with reaction products.
Scheme SEQ Scheme * ARABIC 6: Cyclohexane Oxidization catalyzed by Ru-NPs (Launay et al, 1998).Pd is the last type of catalyst that possesses both exceptional potential for hydrogenation as well as dehydrogenation reactions. Moreover, Pd is also quite effective with different C-C coupling reactions (Hirai et al. 1984). Further details of coupling reactions of above – mentioned group will be provided in the next section. Nevertheless, these features make Pd an ideal candidate for specific applications as well. Pd has unique selectivity for hydrogenation reactions. It is used for alkynes and diene compounds hydrogenation (Yan et al. 2010). Similarly, Pd nanoparticles are also used for dehydrogenation reactions as well. The dehydroaromatization reaction that involves the conversion of limonene to para-cymene in a hydrogen atmosphere comprises of a number of reactions (Zhao et al, 2008). Pd – catalyzed reactions are quite commonly used; however, they are only active for active substrates.
Bimetallic Nanoparticles and Cross-CouplingThe usage of two dissimilar metals called bimetallic NPs is common for enhancement of catalytic properties (Sinfelt, 1987). It is because of the fact that the bimetallization can greatly improve the catalytic properties of individual metallic complexes. The usage of bimetallic catalysis is quite traditional; however, the usage of bimetallic NPs in different solvents can provide desired results with enhanced environmental sustainability. For the preparation of bimetallic NPs, different processes are used, including successive reduction, co-reduction of mixed ions, electrochemical approaches, and reduction of double complexes (Toshima and Yonezawa, 1998). The combinations of Pt-Fe, Pd-Au, Au-Fe and Pd-Pt NPs are quite commonly known; however, other combinations, like Cu-ZnO as forwarded by Sliem et al. (2010) are also being experimented.
The selective and most importantly, effective construction of C-S bonds in different transition NPs based transformation is quite rare; however, other methods involving carbon-heteroatom bonds are widely used in different chemical applications. Catalytic methods for bond formation of C-Se and C-S are in ample demand in pharmaceutical and general organic synthesis applications. For achieving the aim of developing transition metal catalyst NPs reactions, two methods were put forward by Beletskaya and Ananikov (2011).The first method is cross-coupling of RZH and R2Z2 with organic halides (Z = Se, S, Te). The chemistry of cross-coupling is widely known for formation of different bonds involving C-Z and C-C bonds. Scheme 7 describes the generalized mechanism of reaction:
Scheme SEQ Scheme * ARABIC 7: C-Z and C-C bonds bimetallic cross-coupling (Beletskaya and Ananikov, 2011).Another method comprises of formation of C-Z bonds that involves C-H and Z-Z bonds addition reaction with the triple alkene bonds. Scheme 8 describe the second method of cross-coupling.
Scheme SEQ Scheme * ARABIC 8: Z-Z and Z-H Bond Addition Cross-Coupling (Beletskaya and Ananikov, 2011).Nanoparticle SizeAnother important aspect of nanocatalysis includes nanoparticles size and for that nanoparticle diameter bears quite an importance. NP diameter significantly influences the catalytic activity, and hence, the size distribution relationship that includes the surface area of the atoms that are responsible for catalytic activity with the change of diameter. Diameter is an important parameter for influencing catalyst size; however, it involves different variables that play their role in relationship between catalyst activity and NP diameter (Zhou et al. 2006).
Among them is an example of Au NPs having their particle size and catalytic properties highly correlated. Since studies conducted by Haruta et al in 1989 of CO oxidation via Au NPs, wide examples of oxidation reactions propelled by Au NPs have been found afterward. A similar study has showed the size distribution modeling of Au/TiO2 simulation for size – selective catalysis (Valden et al, 1998). However, nanosize studies have opened pathways for carbon monoxide oxidation, propylene epoxide reactions and alcohol oxidation. Also, the size distribution had the significant influence on particle size distribution (Corain et al. 2011). Nevertheless, the complex interaction and particle size effects of catalyst and its support is not fully explored at that time. More recently, Tsunoyama et al (2009) have used monodispersed Au NPs that are stabilized by PVP for benzylic alcohols anaerobic oxidation. The study used seeded growth method for having Au NPs within the range of 1.3-10 nm size range. The results have found to have the sharp increase in activity of catalyst when the particle diameter falls below 2.5 nm because of increased surface area thereby leading to relatively easier oxygen activation in the reaction mixture. From the study, the concept of threshold diameter was also observed that reflects the diameter below which significant increase in activity is observed (Yan et al. 2010).
Besides Au-NPs, Pd-NPs have exhibited a similar trend in catalytic activity. Li et al (2002) have employed widely known Suzuki reactions while concluding that the catalytic activity of Pd NPs increases with the decrease of NP diameter for low coordination number (edge and vertex) atoms. However, the smallest Pd-NP were found to possess least catalytic activity because of strong adsorption of intermediates that leads to catalytic poisoning issues. A similar study was conducted by Gniewek et al (2005) that has enhanced the particle size distribution ranging from1.9 to 19.8 nm. The higher yield of products was observed with smaller Pd NPs with iodobenzenemethoxycarbonylation reaction. This study also reflected the smaller size leads towards better catalyst activity and subsequently, the higher yield of product. Another recent study conducted by Glöckler and his coworkers (2007) have prepared Rh-NPs with a narrow particle size distribution ranging from 2 to 6 nm. This study has also found that Rh-NPs are quite favorable in biphasic hydrogenation reactions for different olefinnic hydrocarbons along with selective aromatic compounds. The results reflect that the smaller particle sizes of Rh exhibited enhanced catalyst activity.
It is worthwhile to note that the particle size effects of NPs on catalytic activity is one of the major factors; however, factors involving intrinsic morphology of NPs also bear significant importance.
NPs MorphologyBesides size, the shape is another core factor that dictates the development of enhanced catalytic activity. With different shapes of NPs, there are different arrangements of particles on the surface of the NP catalyst having different electronic properties and coordination structures. Narayanan and El-Sayed (2004) have prepared Pd NPs with tetrahedral, spherical and cubic shapes through different reduction methods and stabilizer usage. The core uses of the Pd NPs were to provide an electron bridge for thiosulfate ions and hexacyanoferrate (III) ions. The activation energy was deduced be in decreasing order in cubic NP, spherical NP and tetrahedral NP. However, the rate constant has followed the reverse trend considering the activation energies. The study has further elaborated that the fraction of active – surface sites located on edges and corners plays an important role. It has been found that the larger percentage of atoms on edges and corners reflects the higher activity of the catalyst. Also, different reactions have been found to possess different responses in accordance with the morphological changes in the catalyst. Another study conducted by Narayanan and El-Sayed (2005) reflects that the core difference in catalytic activity is not only due to surface (edge and corner) atoms but also the electronic difference of each shape’s surface atoms.
Magnetic Nanoparticles and CatalysisMagnetic NPs are also quite commonly used with different applications in smart materials science, biochemistry, and catalysis. In the catalysis domain, magnetic catalysis provides a newly developed way for catalyst recycling and separation (Majewski and Thierry, 2007). Among these specialized catalysts, includes the use of magnetically stabilized bed reactor. In this specific type of reactor, the catalyst is loaded onto the magnetic materials, and it is operated within a highly intensive magnetic field. As part of catalysis, magnetic NPs are utilized for mainly two purposes as catalysts and as the real catalyst carrier (Arnaldos et al. 1985).
Some magnetic transition metals including cobalt, iron and nickel are intrinsically active catalysts for various reactions (Liu et al. 1991). Moreover, the NPs of these metals along with their oxides can be used effectively as recyclable and efficient catalysts. One of the examples of these nanoparticles includes Ni¬-NPs that are formed by nickel chloride’s hydrazine reduction reaction in the presence of ethylene glycol solution resulting in Ni-NPs ranging in the range of 10-12 nm diameter. These Ni-NPs can be used for hydrazine decomposition to nitrogen and hydrogen gasses at ambient conditions (Yan et al. 2010). It is worthwhile to note that Ni-NPs can be redispersed and recycled in ethylene glycol without any size change or agglomeration issues (Wu and Chen, 2003). Following the same course, Fe NPs are also obtained through the reduction reaction of FeSO4 through NaBH4 for H2 production.The Fe NPs produced through this route are highly stable at room temperature and are quite easy for recovery (Yan et al., 2008).
Nevertheless, the magnetic properties are only limited to a few of transition metals and their oxides. For harnessing complete advantages of magnetic nanoparticles, it is essential to use nanoparticle composites and using those as a single nano-entity to achieve desired results (Yan et al. 2010). The combination of nanoparticles with other catalytically active groups can also provide ease of recyclability. Siegwart et al. (2012) have reported that the magnetic nanoparticles possess core structure having magnetic iron oxide that is surrounded by the organic shell in its vicinity. This organic shell can enhance the magnetic cores from aggregation thereby offering the interface for catalyst immobilization prevention. Recently, the Ru-Fe2O3 nanoparticles were made for the purpose of catalyzing coupling reaction between alcohols and sulfonamides. The C-N bond was formed through highly selective reaction giving out water as the only by-product (Shi et al., 2009).
2.9 Stabilizers and Catalysts Nanoparticle
Stabilization is dependent on three different mechanism that includes electrostatic, electrostatic, steric, and coordination stabilization (Jansen et al. 1995). However, with the development of scientific horizons, more stabilizers would surface based on catalysis needs. Also, it has been found that the catalytic activity and stability are somewhat anti-correlated meaning that for the development of stabilized nanoparticles, it is essential for the nanoparticles to be least catalytically active. For achieving the trade-off between the efficiency and stability domains (discussed before), it is vital to cost some degree of catalyst activity. For having a better understanding of that, it is essential to know more about stabilization mechanisms along with their respective interaction with nanoparticles and its surface. As per the above-mentioned criteria it can be divided into three domains:
First, strong stabilizers include different phosphine and nitrogen (III) agents. Common examples of these include oleylamine and trioctylphosphine that can interact with the surface of nanoparticles in a much better fashion (Yan et al. 2010). Moreover, these stabilizers are also quite commonly used as ligands having exceptional electron donation capabilities as far as homogeneous catalysis is concerned. The core function of this stabilizer is the formation of covalent bonds onto the catalyst surface thereby reducing aggregation issues of nanoparticles (Schröder et al. 2000).
Secondly, medium stabilizers are composed of different ionic surfactants along with some of the simple ions. They are responsible for the formation of a double protective layer thereby preventing the nanoparticles from aggregation. They possess strong interactions with the surface molecules of nanoparticles; however, they are less effective as compared to strong stabilizers (Bosch et al. 2002).
The third category is weak stabilizers comprising of different non-coordinating polymers that have the capability to hinder different steric and physical obstructions for the protection of nanoparticles (Na and Rajagopalan, 1994). The main function of weak stabilizers is to become adsorbed on catalyst surface; however, their interactions with the core nanoparticle is considered the weakest as compared to other stabilizers.
Considering a simply catalyzed reaction through NPs, the first and foremost step is the adsorption of reactants on the surface of NP catalyst. This feature makes the availability of NP atomic surface a crucial parameter for obtaining the highly selective catalyst. Hence, it can also is self-explained about the negative correlation between catalyst activity and its selectivity. For the stabilizer with a strong coordination property is used, there would be lesser vacant sites or surface atoms available because of irreversible adsorption of stabilizers thereby leading towards a decline in catalyst activity (Colussi et al. 2004). It can be deduced that the more stable catalytic nanoparticles are, the lesser they are catalytically active. For the types of stabilizers having protection layers, increasing the thickness of protection layer (or protection ability) would lead to enhanced catalyst stabilization. However, it would not affect the catalyst activity to the greater extent because the active sites on NP catalyst are still available for different substrates. For this specific combination of stabilizer and catalyst, it is possible for the catalyst to be robust and highly active. The core way for avoiding the reduction in activity is by the participation of stronger stabilizers along with designing of different stabilizers that can amalgamate static-electrostatic or steric-weak coordinating stabilization mechanisms (Yan el al. 2010).
Recyclability and multi-functionality are also the two most common benefits of NP catalysts. The utmost control of nanoparticles catalysts has become a reality when the stimuli-responsive polymers have emerged as a prospect candidate for its use as stabilizers (Winnik, 1990). The main advantage of using these specific polymers is because of their flexibility with environmental parameters. These thermally sensitive polymers are capable of undergoing an inverse temperature transition that has enabled it to be used in nano-reactor manipulation, catalysis and different material synthesis (Urry, 1992). In the domain of multi-functionality, some of the NP catalysts can be used as support for traditional heterogamous catalytic systems. One of the common applications of it involves Pt/zeolite catalyst that is widely used in petroleum cracking. These catalysts are bifunctional having Pt providing hydrogenation capacity whereas the zeolite is responsible for activation of different alkane’s rearrangements (Li et al, 2002).
Catalysis is an area of chemistry that has entered into the domain of nanosciences and nanotechnology. The catalysis of nanoparticles have solution phases; however, different grounds of nanochemistry have yet to be explored. The criteria set forward for green catalysis is essential and includes the careful and thorough understanding of nanocatalysis and its application in hydrogenation, hydrosilation, homogenous and heterogeneous cross-couplings and oxidation reactions. For evaluating the suitability of a nanoparticle catalyst, some factors including efficiency, sustainability, stability and recyclability should have to be taken into account. Besides these basic factors, the issues of nanoparticles size and compatibility of solvent and stabilizers must also be considered outlining an understanding of potential prospects for nanoparticles in the future.
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