Abstract
Nanocatalysts, as a part of nanotechnology, have been seen very useful for various fileds of applications capturing a large contribution of the world market. Indeed, several unsolved issues of catalysts have been reconsidered by employing the new nanocatalysts including single core metal atoms and ions with surrounding holes. Moreover, it was expected that the future of catalytic reactions, especially those organic ones, will deal with the nanocatalyst applications. To this aim, the features of catalytic nanoparticles and magnetic nanocatalysts regarding evaluation of their advantages and applications in organic reactions were investigated in this work. Developments of catalytic nanoparticles and magnetic nanocatalysts were discussed in this work regarding the novel applications of such materials at the nanoscale for approaching advantageous features. Increased availability, activity, and stability are very important for applications of the catalysts in various organic reactions. Therefore, it is a must to discuss features of such nanocatalytic systems to provide more information about their advantages and even disadvantages of their applications.
Introduction
For pushing forward the reactions in chemistry related processes, employing catalytic systems is essential for approaching the reaction products and efficiency [1–5]. Developments of catalytic systems are mainly dependent on employing advanced materials for selectively catalyzing the specific chemical reactions to approach the desired product [6–10]. Moreover, the employed catalytic system should be recoverable after producing the product through a simple separation process [11]. The innovation of nanotechnology and related nanosubstances has raised considerable attention to help explore such nano-based catalytic systems for employing in various reactions [12–20]. Furthermore, availability of composite structures of nano-based systems has led to the innovation of multi-component materials in this case [21–25]. The wide area surfaces of nanosubstances has also made them suitable for employing in different adsorption applications of other substances with regard to both chemical and physical interactions [26–30]. In this regard, providing such metal-composite structures led to the innovation of magnetic nanoparticles with the advantage of controllability by an external magnetic field [31–33]. Indeed, several features of nanosubstances have been recognized by exposing to other irradiating sources even normal light [34–36]. To this goal, considerable attempts have been dedicated to help synthesize novel magnetics nanoparticles for specified applications such as employing in catalytic systems [37–39]. Several types of procedures have been used to help characterize the features of nanoparticles (Fig. 1) for providing information about their further use [40–42]. Putting various atomic decorations of nanostructures has yielded specified applications for various purposes, in which the role of such atomic decoration was dominant for this purpose [43–45]. Although several works have been reported on this topic to this time, but the problem is still open for further investigation [46–48]. As a consequence, this work was prepared to discuss about various features of such catalytic nanoparticles and magnetic nanocatalysts in addition to their applications in organic reaction processes.
Main features of a nanoparticle.
The cases of magnetic nanoparticles have been seen very interesting especially for better recovering possibility of the catalytic system from the reaction process [49]. The magnetic nanoparticle could be well dispersed in the reaction media for providing a suitable surface area for pushing forward the reaction in the absence of an external magnetic field exposure [50]. However, these magnetic nanoparticles could be easily separated from the product during the final step of reaction process by employing a simple magnetic separation process [50]. By the general definition of a catalyst to increases the rate of reactions, but features such as activity, selectivity, controllability, and recoverability besides low energy consumption and long lifetime are still important for introducing an efficient catalyst [51]. To achieve such purpose, controlling variables such as size, structure, spatial and electron distribution, surface composition, thermal and chemical stability could determine the performance of a catalyst, in which the nanocatalysts have been seen suitable in this regard [52]. Moreover, the nanocatalysts have been usually made as a composite structure by performing chemical modifications for economic and optimization reasons [53]. Accordingly, the research field of developing nanocatalysts has always been one of the most important topics of green chemistry and nanochemistry areas to produce safe and high-yielding products excluding serious disadvantages [53]. Besides importance of separation of both of homogeneous and heterogeneous catalysts from the reaction media, providing a wide surface area is a must [54]. To this aim, nanotechnology could provide catalytic substances with very active surfaces for applications in the reaction media, better than heterogeneous catalytic systems but not very much better than homogenous catalytic systems in terms of overall performance [55]. Hence, such nanocatalytic systems could be categorized somewhere between the heterogeneous and homogeneous catalyst systems [56]. It is important to note that the nanocatalysts have still their important significance for pushing forward the reactions better than several other conventional catalytic systems [57]. The main goal of this work is also focused on discussing the features of catalytic nanoparticles and magnetic nanocatalysts regarding their applications in organic reactions.
Nanocatalysts with the homogeneous behavior
The prepared nanoparticles by assistance of the intermediate metals are colloidally dispersed in the reaction media using the stabilizer to prevent the nanoparticle aggregation in the homogeneous nanocatalyst classification. The employed nanoparticles could be separated from the final product of the reaction [58]. Two methods of chemical and electrochemical reductions are usually used for running the metal conversions of the nanocatalytic systems [59]. In the case of chemical reduction, the metal salt in solution is reduced to a metal atom; it is then converted to a metal nanoparticle by reducing agents such as alcohols and sodium borohydride. In the case of electrochemical reduction, electrons are accumulated on the surface of the electrode to exchange a chemical reducing agent using a cell consisting of anode (oxidation site, cathode) and a reduction site (and electrolyte) of electrically conductive salt solution.
Nanocatalysts with heterogeneous behavior
Unfortunately, most homogeneous catalysts are not usable in many fields because of difficulties of their separation from the reaction media. Moreover, catalysts are needed to be completely separated from the final products in the case of needed high-purities such as pharmaceutical products. Hence, developments of heterogeneous catalysts have been seen important in this regard. Accordingly, separation of heterogeneous solids-based catalysts from the reaction media has been seen applicable by employing simple separation processes. However, lowering the activity of a catalyst by placing it at a solid surface has been seen as a limiting factor in applications, in which the innovation of nanoparticle has helped to overcome such limiting factor by providing large surface area, separatability, and reusability [60]. To provide such heterogeneous nanocatalyst system, a nanocomposite substrate is required such as positioning a gold catalyst on the substrate surface of titanium dioxide or iron oxide to yield gold / titanium dioxide and gold / iron oxide nanocatalysts, respectively [60]. Interestingly, such nanocatalysts could easily oxidize the carbon monoxide species to the carbon dioxide ones reducing the poisoning risk effects of carbon monoxide of the investigated environments [61].
Nanocatalysts characteristics
Maximum of active level per unit of mass and volume
The availability of a higher surface area for a catalytic structure could reduce the use of nanocatalyst amount whereas it could increase the rate of reaction by involving more reactants in the reaction [62]. Indeed, increasing the surface area of catalytic systems could provide more reactive sites for increasing catalyst efficiency, especially for a heterogeneous catalyst.
Controllable size
Considering the most appropriate size of a nanoparticle is crucial to help achieve the maximum catalytic activity, in which methods of controlling such sizes have been under development for approaching further efficiency. In this regard, performing computer simulations could also help to optimize the system for achieving an optimum size [63]. Approaching a more accurate method of estimation of the most appropriate size of these nanoparticles could help to achieve the highest catalytic activity.
Separation from the reaction media
It has been mentioned above that the nanocatalysts could be separated easily from the products of reactions media. The nanocatalysts are mostly suspended in the reactions media instead of being solved such as magnetic nanoparticles, which could be separated easily by employing a suitable magnetic field for collecting the catalytic particles [64].
High selectivity and efficiency
An important advantage of employing the nanocatalysts is their selectivity feature to select the correct reactants for working in reaction pathway avoiding the participation of other unwanted substances in the reaction process. Accordingly, the efficiency of reaction production could be increased whereas the existence of other unwanted byproducts could be decreased [65].
Aggregation talent
The aggregation of nanoparticles, which is occurrs commonly for many types of nanoparticles, reduces the activity of nanocatalysts making them inactive in several cases. This is indeed a disadvantage of such aggregated nanocatalysts losing their nano dimensions and expected features [66].
High diversity and chemical modification capability
Disrupted organic groups could bind to the surface of nanocatalysts because of their high surface activity, in which such high surface activity leads to form composites of nanocatalysts including minerals as well. In this regard, the chemical modification of nanocatalysts by linking disrupted groups could create a great diversity in their performance and applications [67].
Source of preparation
Interestingly, some types of nanocatalysts such as Zeolites are naturally available whereas so many other types of synthetic nanocatalysts such as metal oxide nanoparticles could be prepared artificially for various fields of applications [68].
Magnetic nanoparticles supports for catalysts
The small sizes of nanoparticles and their high surface area for adsorbing the reaction substances make them indeed difficult for separation. To overcome such issue, employing magnetic nanoparticles supports for catalysts could show advantages of controllability of such systems under exposing to a magnetic field [69]. In the absence of an external magnetic field, the magnetic nanoparticles could be dispersed well in the reaction media to provide wide surface areas for readily accessible for the reaction substances. By completing the reaction process, employing the external magnetic field could lead to effective separation and collection of magnetic nanocatalysts for the reaction media. The iron oxide magnetic nanoparticles, those important supportive substrates for the nanocatalytic systems help yield both features of high efficiency and controllability [70]. The synthesis of magnetic nanoparticles is usually done in aqueous solutions by the simultaneous deposition of intermediate metal ions such as iron-II and iron-III, in which the surface would be covered by the hydroxyl groups making them dispersible in aqueous suspensions only [71]. Additionally, thermal decomposition processes have been used for synthesizing several types of metal nanocrystals and metal oxides for applications in organic solvents [72]. Indeed, new methods of nanoparticles syntheses and surface modifications are still required for developing such supporting materials of nanocatalysts. In this case, several types of molecular and atomic surface modifications have been done to achieve desired nanoparticles for specified purposes, in which the results of earlier works indicated effects of employing such modifications in electronic and structural features of nanoparticles [73–75]. For arising the magnetic features, cationic ligands or silane molecules were used for modifying the surface of nanoparticles in addition to employing organic or inorganic polymers for such coating process as trapper of magnetic nanoparticles inside the organic or inorganic polymer structures [76].
Applications of magnetically recoverable nanocatalysts for organic reactions
Nanocatalysts have been widely used to catalyze a various types of organic reactions in both of laboratories and industrial scales, in which hydrogenation, carbon-carbon coupling, and oxidation reactions are some of such examples [77].
Hydrogenation reaction
The magnetically recoverable palladium nanocatalysts have been selectively used for catalyzing the hydrogenation of olefins, as one of the most important hydrogenation reactions of hydrocarbons [78]. Selectivity between two or more reducing functional groups is of great importance in performing an efficient catalytic hydrogenation reaction. To this aim, synthesized nanocatalysts by direct palladium adsorption at the surface of magnetic iron oxide nanoparticles have exhibited excellent selectivity in addition to high activity for employing in the hydrogenation of olefins [78]. Details of this method was the reduction of palladium by isopropanol at the surface of magnetic iron oxide nanoparticles during incubation at 5°C for 5 min under the sonication condition. By completing the process, the reaction mixture was filtered and the resulting solid was washed by acetone to separate the dried black powder of palladium / iron oxide magnetic nanocatalyst. Next, the hydrogenation reactions of different olefins were investigated under a solution containing 0.3 mmol of raw materials (various olefins) and 0.3% molar of the prepared magnetic nanocatalyst in 1 ml of solvent at the room temperature under the pressure conditions of a hydrogen gas (Hz) atmosphere. The reaction efficiency was determined by employing the proton NMR analysis, in which the results were summarized in Table 1 [79]. The achievements indicated that the employed magnetic nanocatalyst worked very well in both polar and non-polar solvents by approaching the product in a shorter time in the polar solvent. The hydrogenation reactions of one and two-fold alkenes were done after 1 h of reaction time with efficiency greater than 1% (cases 1 to 3). However, the hydrogenation reaction of triple-bonded alkenes such as 1-methylcyclohexene, yielded only about 1% efficiency even after 2 h (case 4). Additionally, significant selectivity was observed for the hydrogenation of two-factor alkenes containing carbonyl groups. However, in the case of unsaturated aldehydes such as trans-cinnamaldehyde, partial resuscitation of the carbonyl functional group was carried out yielding about one main product of 2-phenyl-1-propanol and 3% of 3-phenyl propanol (case 5). In this case, applications of magnetic nanocatalysts were seen suitable for employing in hydrogenation reactions for both of selectivity and efficiency features.
Hydrogenation of olefins [79]
Hydrogenation of olefins [79]
Varieties of natural products, medicinal, and organic chemicals could be obtained by means of palladium-catalyzed carbon-carbon coupling reactions, in which the Suzuki coupling reaction is among the most important reactions because of its possibility for synthesizing and fabricating the asymmetric aryls [80]. In the case of running the Suzuki coupling reaction by assistance of a palladium / iron oxide magnetic nanocatalyst, several attempts have been achieved to approach desired products [81]. As could be seen by the content of Table 2, the advantages of employing nanocatalytic systems for the Suzuki phenyl boric acid coupling reactions with various aryl phenomena were exhibited (cases 1 and 2). But, in the case of aryl bromide with less active in the Suzuki reaction than the aryl iodide, less catalytic activity was observed (case 3). Moreover, the employed nanocatalytic system indicated excellent catalytic activity for the Suzuki coupling reactions of various aryl boric acids coated with the iodobenzene (cases 4 and 5). As a consequence, the selectivity and efficiency of employing such palladium / iron oxide magnetic nanocatalytic system were observed for running the Suzuki carbon-carbon coupling reactions.
Suzuki aryl boronic acids coupling reactions with aryl halides [81]
Suzuki aryl boronic acids coupling reactions with aryl halides [81]
Oxidation reactions are commonly used for several fields of industrial applications in which the oxidation of alcohols to carbonyl compounds is of great importance for production of various compounds, especially those of pharmaceutically related ones [82]. To this aim, a cobalt / iron oxide magnetic nanocatalyst could work very well to approach the product [82]. For examining this catalytic process, different types of alcohols were incorporated in the oxidation conditions and the results were summarized in Table 3 [83]. The obtained results of solvent-free reactions showed that employing the cobalt / iron oxide magnetic nanocatalyst could be applicable to a wide range of structurally and electronically differing types of alcohols. The reactions were also successfully performed for the oxidation of benzyl alcohols containing electron donor groups, and the corresponding ketones were obtained with high efficiency. In this method of oxidation reactions, a variety of functional groups such as methoxy, chloro, bromo, methyl, amino, and nitro groups were kept unchanged. On the other hand, the aliphatic second-type alcohols such as cyclohexanol reacted well under these conditions to produce cyclohexanol with 85% efficiency. The effects of other functional groups at the ortho and para-benzyl alcohol positions on the oxidation reactions were almost negligible. As a consequence, the investigated method was a good catalytic system for the oxidation of benzene even without breaking the carbon-carbon bonds in other conventional methods.
Alcohol oxidations [83]
Alcohol oxidations [83]
As mentioned above about various features of nanoparticles and their catalytic applications in organic reactions, the topic has been a dynamic topic with varieties of types of investigations on modifications of nanoparticles for specified applications [84–90]. In this regard, several types of nanoparticles with magnetic features were recognized by combining various components in chemical and physical junctions [91–97]. Indeed, working on nanotechnology related topics is an “Art” as Professor George Whiteside, who could be called the father of nano, always mentioned [98]. Accordingly, the achievements of pioneering works introduced nano-related substances to be suitable for working in catalytic systems, in which such suitability has been still challenging due to some disadvantages such as poisoning and toxicity [99, 100]. In this regard, careful attentions are still required for dealing with the nano-related systems.
Conclusion
The innovation of nanoscale technology and related materials has provided excellent conditions for developments of catalyst science and technology. Existence of high surface area and appropriate selectivity and activity in the nanocatalysts increased the rate and reaction efficiency by employing such nanocatalysts systems. Moreover, the nanocatalysts structures are very diverse based on their initial synthesis or modifications by other atoms and molecules. By existence of such diverse structures, the nanocatalysts could be separated and collected from the reactions media. In this regard, developments of recyclable catalysts by supports of employing ferrite magnetic nanoparticles has attracted considerable attentions as a new practical methodology of preparing more advantageous heterogeneous catalytic systems in comparison with other conventional systems. The advantages of magnetic nanoparticles for being dispersed in the organic reaction media during the catalytic process besides possibility of separation at the final reaction step made these nanoparticles very suitable for their specified applications. Indeed, such magnetic nanoparticle-based catalytic systems could participate in various types of reaction mechanisms such as carbon-carbon coupling, hydrogenation and oxidation reactions, etc., by exhibiting excellent performance. Additionally, a simple magnetic separation process using an external magnetic field could be employed for recycling or separating the magnetic nanocatalysts form the reaction media. And finally, employing the appropriate surface modifications and particles trapping could lead to the production of more specified nanocatalytic systems besides overcoming several disadvantages of nanoparticles applications such as aggregation.