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R E V I E WCarbon-Based Material-Supported Palladium Nanocatalystsin Coupling Reactions: Discussion on their Stability andHeterogeneityMing Zhao1,2|Yaxing Wu1|Jing-Pei Cao11Key Laboratory of Coal Processing andEfficient Utilization (Ministry ofEducation), China University of Mining &Technology, Xuzhou, 221116Jiangsu,China2Pizhou Economic and TechnologicalDevelopment Zone, Pizhou, 221300,ChinaCorrespondenceMing Zhao, Key Laboratory of CoalProcessing and Efficient Utilization(Ministry of Education), China Universityof Mining & Technology, Xuzhou 221116,Jiangsu, China.Email address:Email: Funding informationNational Natural Science Foundation ofChina, Grant/Award Number: 21908241Scientific interest in carbon-based materials (CBMs) has grown dramaticallyover the past few decades. Due to a variety of atomic orbital hybrid forms (sp,sp2and sp3hybridization), carbon can form a variety of materials with diversestructures and characteristics. CBMs used as efficient catalyst supports showextensive promise in organic reactions, which is attributed to their structuralsimilarity with organics, large specific surface area, chemical stability, andphotocatalytic properties. This review presents the synthesis of CBM-supportedpalladium nanocatalysts based on impregnation, template methods, etc. TheCBMs include activated carbon (AC), graphene, carbon nanotubes (CNTs),and their functionalized products, as supports for improving the activity andrecyclability of simple Pd nanocatalysts. After surveying the literature wherethese catalysts have been utilized for carboncarbon coupling reactions, thereis a particular emphasis on Suzuki, Heck, and Sonogashira reactions. The cata-lytic mechanism of these Pd nanocatalysts (surface heterogeneous catalysis orhomogeneous catalysis caused by Pd leaching) is discussed in detail, especiallythe effect of Pd leaching on the stability of the catalyst.KEYWOR DScarbon materials, coupling reaction, heterogeneous catalyst, nanocatalyst, palladium1|INTRODUCTIONCarbon has played an important role in the developmentof human society since ancient times (Figure 1). In 1895,Acheson discovered artificial graphite while developingsilicon carbide, which started the production of moderncarbon-based materials (CBMs). Since the 1960s, manyspecial CBMs have emerged, such as glassy carbon, car-bon fiber, expanded graphite, and carbon monofluoride.With the further development of CBMs, some novelallotropes, including fullerene, carbon nanotubes, andgraphene,havebeendiscovered.Currently,CBMsareusedinmachinery,electronics,andmedicinalapplications; they are also used in a variety of fields,including aerospace, metallurgy, and many others.1,2Catalytic processes account for more than 90% ofchemical transformation processes, and the roles of a cat-alyst include improving the reaction rate and controllingthe selectivity of the particular chemical conversion. Theapplication of CBMs in heterogeneous catalysis has along history because they can satisfy most of the desirablecharacteristicsrequiredforexcellentsupport.3,4Recently, due to the large specific surface area, chemicalstability, easy recyclability and electronic conductivity ofCBMs,5,6they have been widely used as catalyst sup-ports for organic synthesis and electrocatalysis.79Received: 26 October 2019Revised: 8 January 2020Accepted: 11 January 2020DOI: 10.1002/aoc.5539Appl Organometal Chem. 2020; 2020 John Wiley & Sons, Ltd.1 of 28https:/doi.org/10.1002/aoc.5539C-C coupling reactions have a critical impact on thechemical and pharmaceutical industries.10,11These reac-tions are usually catalyzed by homogeneous Pd inorganic solvents or aqueous media with the help ofligands.12However, there are some remaining chal-lenges when using homogeneous catalysis for pharma-ceutical synthesis due to the difficulty in recoveringcatalysts and the potential contamination of the drug byresidualheavymetals.13,14Palladiumnanoparticles(NPs) deposited on a solid support (e.g., high surface areasilica,15metal oxides,16,17polymers1820) as a hetero-geneous catalyst offer an ideal alternative for addressingthe separation issue. The use of magnetic NPs as a sup-port can also enable, in specific cases, efficient and easyseparation of the catalysts from a reaction mixture withan external magnet.2125However, the active sites on Pdcatalysts immobilized on a solid support are not alwaysaccessible for the desired reactions because the contactbetween the support and reactants is always weak. Previ-ous studies in this field have shown that in addition toheterogeneous surface Pd atom-catalyzed C-C couplingreactions, Pd NPs deposited on solid supports can alsoserve as a reservoir for active Pd species that catalyzecouplingreactionsviaarelease/redepositionmechanism.2628The homogeneous process is usuallyaccompanied by a loss of Pd species and a change in par-ticle size and morphology of the Pd NPs, resulting in asignificant decrease in catalytic activity after severalrepeated uses of the catalyst.In this review, the synthesis of one-dimensional(1D)carbonnanotubes(CNTs),two-dimensional(2D)graphene,amorphousactivatedcarbon(AC),mesoporous carbon(MC), and otherfunctionalizedCBM-supported palladium nanocatalysts is introduced.Their utilization for Pd-catalyzed carboncarbon cou-pling reactions and the heterogeneity and stability of thevarious carbon-based nanocatalysts in their catalytic pro-cesses are discussed. Furthermore, a comparative analysisis presented for the effects of reactants, bases, organic sol-vents, etc. that are associated with the catalysis systems ofPd/CBMs.2|EFFECTS OF SIZE AND SHAPEON NANOCATALYST ACTIVITYIn the field of nanocatalysis (catalysts with a support oractive sites in nanoscale dimensions), catalysts with goodactivity, stability and selectivity can be designed by sim-ply controlling the size, shape and morphology of thecatalyst.2931It is well known that as an entity becomessmaller, its surface area to volume ratio increases. There-fore, because the size of nanocatalysts is nanoscale, theyhave a large surface-to-volume ratio. In addition, theavailable surface area of the active components of thenanocatalystislarge,thussignificantlyincreasingthe contact between the catalyst and the reactant mole-cules. This enhanced interaction improves the catalyticperformance in a heterogeneous system, bringing itcloser to a homogeneous counterpart, and helps achievebetter reaction rates.In addition to the size of the NPs, their shape also hasa critical effect on catalyst activity. Surface atoms of NPshave elevated surface energy, and the atoms at theiredges and corners have even higher surface energy, espe-cially for three-dimensional particles. Therefore, theseatoms and the entire nanoparticle are highly reactive.In 2005, El-Sayed and coworkers prepared Pt NPswith different shapes using a hydrogen reduction method(tetrahedral, cubic and spherical)32and then tested theprepared NPs as catalysts for several electron transferreactions.33,34They found that the tetrahedral Pt NPswere the most catalytically active, with a minimum acti-vation energy of 14 kJ/mol. In contrast, cubic NPs werethe least catalytically active, with an elevated activationenergy of 26.4 kJ/mol. The spherical NPs had a catalyticFIGURE 1History of carbonmaterial development2 of 28ZHAOET AL.activity between those of the cubic and tetrahedral sys-tems. The reason why the catalytic activity is so differentis because of the difference in the exposed facets, so thereare also differences in the number of catalytically activesurface atoms. Tetrahedral NPs were composed of (111)facets with a large fraction of the surface atoms on cor-ners and edges compared to cubic NPs, which were com-posed of (100) facets with small fractions of their surfaceatoms on the corners and edges. Therefore, tetrahedralNPs with more surface atoms are more catalyticallyactive than cubic NPs.This was also apparent from the study by Pelzer andcoworkers, wherein they prepared Pt NPs with a diame-ter of 2 nm that contained approximately 200 atomscovered with n-octylsilyl groups in a cuboctahedralshape, which had edges with a length of three atoms(Figure 2).35From the TEM images of these NPs, differ-ent atoms located at edges, planes, and corners can beclearly observed.3|HETEROGENEITY OFNANOCATALYSTSHeterogeneity and stability are key indicators for judgingthemeritsofheterogeneouscatalysts,andtheirmacroscopic performance is the catalyst cycling capacity.The recycling of Pd nanocatalysts in coupling reactionshas important general issues to be discussed. In mostcases, consistently high yields in a few repeated runs areused as strong evidence to demonstrate the potential of ananocatalyst. However, it is often the case that the yielddrops suddenly after the fifth or sixth repeated use. Moreimportantly, in some examples of successful reuse of thecatalyst, although the reaction has a sustained high yieldor conversion to some extent, it does not necessarilydemonstrate consistently high activity in longer uses.Therefore, the evaluation of the recycling ability of acatalyst in the long run is problematic and requireshigher quality kinetic studies.Measuring the TOF (turnover frequency) values ofincomplete conversion with kinetic curves is a reliabletool for gathering information about the properties ofcatalyticallyactivematerials.Kineticdataprovideinformation on the induction period, if any, and allow acomparison between the initial rates determined in suc-cessive reactions. In nanocatalysis, it is very important todetermine what is the active catalytic species, whichinvolves many kinetic studies. Generally, there are twopossible catalytic modes: one is catalysis on the NP sur-faces, and the other is leaching of solid NPs to produceactive catalytic species, which can be verified by catalystFIGURE 2TEM images of the PtNPs (Si/Pt = 0.5) with Si (n-C8H17)fragmentsCARBON-BASED MATERIALS-SUPPORTED PD NANOCATALYST IN COUPLING REACTION3 of 28recovery. As described by Gladysz,36,37a recycled solidcatalyst is certainly not an active catalyst when the fol-lowing conditions occur:i.Inductions periods are observed.ii.Activity remains in the separated reaction mixture(a phase different from that of the recovered species).iii. The recovered species continues to exhibit inductionperiods, together with decreased activity.If the induction period does exist in successive catalyticcycles, it indicates the regeneration of the active catalystin each run, which is called the catalyst resting state.It has been well documented that in heterogeneousPd-catalyzed coupling reactions, the active palladiumspecies undergo a process of leaching from the catalystsupport and then redeposit on the surface of the catalystsupport (dissolution-redeposition or release-redepositionmechanism, Figure 3).3841At the beginning of the cata-lytic cycle, a highly active soluble complex is produceddue to the oxidative addition of Pd(0) to an aromatichalide (Pd0! Pd2+). On the other hand, studies withsupported Pd2+complexes have indicated that these tendto decompose in the reaction to form a ligand-free solublePd(0) species (Pd2+! Pd0).42,43Whether these activePd(0) species are well deposited on the catalyst supportand maintain the particle size and morphology of the PdNPs are important indicators for judging the performanceof the heterogeneous catalyst.Relevant information about the leached active Pd spe-cies in heterogeneous catalysis can be collected by severalexperimental techniques listed below.i.In a hot filtration test, the reaction is interruptedwhen the substrate is not completely converted andthen continues after removal of the catalyst. If nofurther conversion is detected in the catalyst-freesystem, it can be concluded that even the formed sol-uble species formed are not active in the reaction.ii.In 1974, Rebek designed a three-phase test in whichone of the reactants is immobilized on a solid sup-port and then reacted with another reactant insolution in the presence of a solid catalyst.44Onlyin the presence of soluble Pd species can theanchored reactant react smoothly.iii. Finally, catalyst poisoning caused by amalgamationwith elemental mercury has also been used to testthe properties of active Pd species involved in acoupling reaction.45Quadrapure TU,46a thiourea-functionalizedpolymerand3-mercaptopropyl-functionalized silicas,28and poly(4-vinylpyridine)(PVPy) have been used as selective trapping agents todetect the involvement of leached Pd species.When nanocatalysts with supported Pd NPs are used,the important reasons for the decrease in catalyst activityafter several repeated uses are as follows47:i.In the catalyst recovered after recycling, the aggrega-tion of NPs results in a reduction in available activemetal species.ii.Leaching of palladium occurs in heterogeneous cata-lysts, as evidenced by atomic or optical emissionspectroscopy(ICP-AES/OES)andinductivelycoupled atomic absorption and mass spectroscopyFIGURE 3Sketch of the possible “releaseand redeposition” mechanism involved during acatalytic cycle4 of 28ZHAOET AL.(ICP-AAS/MS)measurements.Notably,inmostcases, the leached species are deposited again on thesupport. However, this process may still be accompa-nied by a reduction in the surface area of the metalcatalyst.iii. Poisoning of surface active sites and salt accumula-tion hinders substrate contact at the active site andredeposition of Pd species from solution. The overallresult is a diminishing concentration of active Pdspecies involved in the catalytic cycle.4|ACTIVATED CARBONAC is a good choice as a solid support because of itsexcellent stability and ability to act as an inert environ-ment for stabilizing transition metals. Commercial Pd/Cis a readily treatable catalyst and has been used undermild conditions to induce carboncarbon cross-coupling,although the yield is reduced after multiple cycles.48PdNP catalysts have been introduced in AC via severalmethods, such as in situ reduction, wetness impregnationand chemical vapor infiltration. The first use of a Pd/Ccatalyst for Suzuki coupling was reported in 1994 byMarck and coworkers, carrying out couplings betweenaryl bromides and trifluoromethanesulfonates by addingtriphenylphosphine as a ligand.49Later, many ligand-freePd/C-catalyzedcross-couplingreactionsweredeveloped.In a recent study of Pd/C catalysts, Pd NPs with diam-eters of 36 nm were fabricated by the decomposition ofPd2(dba)3(where dba denotes dibenzalacetone) and cap-tured by AC to prepare an assembled Pd/C catalyst.50The catalyst efficiently induced the coupling of aryl bro-mides (chlorides) with phenylboronic acids to affordproducts with 90% yields, which were over twice thoseobtained with IM-Pd/C prepared by an impregnationmethod. However, the product yield slowly decreasedafter three recycling tests (Scheme 1, entry 1).Generally, homogeneous Pd-catalyzed Suzuki reac-tions are performed under an inert atmosphere becausethe catalyst is sensitive to oxygen and/or moisture. How-ever, Suzuki co
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