Resin-immobilized palladium nanoparticle catalysts for organic reactions in aqueous media: Morphological aspects

Molecules 2015, 20, 18661-18684; doi:10.3390/molecules201018661

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molecules

ISSN 1420-3049

www.mdpi.com/journal/molecules

Article

Resin-Immobilized Palladium Nanoparticle Catalysts for

Organic Reactions in Aqueous Media: Morphological Aspects

1

,

1

Piero Mastrorilli *, Maria M. Dell’Anna , Antonino Rizzuti , Matilda Mali ,

Mauro Zapparoli and Cristina Leonelli ³

2

1

Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e di Chimica (DICATECh),

Politecnico di Bari, via Orabona 4, Bari 70125, Italy;

E-Mails: mariamichela.dellanna@poliba.it (M.M.D.A.); antonio.rizzuti@poliba.it (A.R.);

matilda.mali@poliba.it (M.M.)

2

Centro Interdipartimentale Grandi Strumenti (C.I.G.S.), Università di Modena e Reggio Emilia,

via Campi 213\A, Modena 41125, Italy; E-Mail: mauro.zapparoli@unimore.it

3

Dipartimento di Ingegneria “Enzo Ferrari”, Università di Modena e Reggio Emilia,

via Pietro Vivarelli 10, Modena 41125, Italy; E-Mail: cristina.leonelli@unimore.it

*

Author to whom correspondence should be addressed; E-Mail: p.mastrorilli@poliba.it;

Tel.: +39-80-596-3605; Fax: +39-80-596-3611.

Academic Editors: Nicola Cioffi, Antonio Monopoli and Massimo Innocenti

Received: 24 September 2015 / Accepted: 8 October 2015 / Published: 14 October 2015

Abstract: An insight into the nano- and micro-structural morphology of a polymer supported

Pd catalyst employed in different catalytic reactions under green conditions is reported.

The pre-catalyst was obtained by copolymerization of the metal-containing monomer

−

Pd(AAEMA)

2

[AAEMA = deprotonated form of 2-(acetoacetoxy) ethyl methacrylate] with

ethyl methacrylate as co-monomer, and ethylene glycol dimethacrylate as cross-linker.

This material was used in water for the Suzuki-Miyaura cross-coupling of aryl bromides,

and for the reduction of nitroarenes and quinolines using NaBH

4

or H , as reductants.

2

TEM analyses showed that in all cases the pristine Pd(II) species were reduced in situ to

Pd(0), which formed metal nanoparticles (NPs, the real active species). The dependence of

their average size (2–10 nm) and morphology on different parameters (temperature, reducing

agent, presence of a phase transfer agent) is discussed. TEM and micro-IR analyses showed

that the polymeric support retained its porosity and stability for several catalytic cycles in all

reactions and Pd NPs did not aggregate after reuse. The metal nanoparticle distribution

throughout the polymer matrix after several recycles provided precious information about

Molecules 2015, 20

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the catalytic mechanism, which was truly heterogeneous in the hydrogenation reactions and

of the so-called “release and catch” type in the Suzuki coupling.

Keywords: polymer supported Pd nanoparticles; morphological analysis; TEM; micro-IR

spectroscopy; recyclable catalyst; quinoline transfer hydrogenation; water solvent; nitrobenzene

reduction; quinoline hydrogenation; Suzuki-Miyaura coupling

1

. Introduction

The value of nanoparticles (NPs) is one of the discoveries that has recently been developed to an

extent that scientists could have never imagined a century ago, also in the field of catalysis [1–4].

The main problem arising with the use of metal nanoparticles as catalysts is their tendency to form

agglomerates, which decreases their activity. To avoid aggregation, NPs are usually stabilized by

inorganic or polymeric supports. Several methods are used nowadays to prepare metallic NPs entrapped

in polymeric supports for use in gas sorption, separation, and heterogeneous catalysis [5]. Recently, it

has been proven that high thermal and chemical stability can be attained when the polymer frame is

made up of tunable building units with only covalent bonds [6–8]. Additionally, it has been reported that

to avoid nanoparticle aggregation as in chemical vapor deposition [9–12], solid grinding [13–17],

impregnation [18,19], and immobilization [20–23], the NP formation should occur concurrently with the

reticulation of polymer matrices [24,25]. An alternative way to prepare highly dispersed polymer

stabilized nanoparticles consists in the copolymerization of metal-containing monomers with suitable

co-monomers [26], followed by reduction of the supported metal. In this field, for example

Pomogailo et al. [27] carried out the thermal polymerization of Pd(H

presence of inorganic supports, such as SiO , Al , C, obtaining hybrid polymer inorganic composites.

Pd(II) was then reduced under H to give supported Pd NPs. In this procedure, the preparation of the

2

C = CHCONH

2

)(NO

3

2

) in the

2

O

3

2

suitable Pd organometallic monomer to be linked with co-monomer units is critical, and a second

reduction treatment was necessary. A reduced number of monomers could be easily bonded to Pd ion

ligands, but when the ligands themselves could reticulate in a stable polymer, the preparation of the Pd

NP catalyst was successful [28–32]. The convenience of NP formation concurrent with monomer

reticulation has undoubtedly important implications as far as the process sustainability is concerned; the

choice of cross-linking ligands is an additional step forward, but the lowest environmental impact in the

catalytic process can be reached when water substitutes for the organic solvent as reaction medium.

In fact, water has been recently viewed as an eco-friendly alternative to common organic solvents

because it is non-toxic, non-flammable, has a high heat capacity, is cheap and renewable. In some cases,

due to its high polarity, its acid-base properties and its hydrogen bonding ability, water may influence

the reactivity and the selectivity of certain catalytic systems. However, sometimes the use of neat water

depresses the catalytic efficiency, as it has been reported, for example, for Pd NPs stabilized by various

polymers, such as poly(N-vinyl-2-pyrrolidone) [33–35], polystyrene-β-poly(sodium acrylate) [36] and

poly(N,N-dihexylcarbodiimide) [37] in the Suzuki-Miyaura reaction. In many cases, the polymeric

support does not swell in water, thus hampering the migration of the reactants to the active sites.

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Neverthless, a number of suitable polymeric matrices have been developed so far for C–C couplings [38],

also under green conditions [39].

In other cases (e.g., in the selective reduction of quinolines), the formation of hydrogen bonds

between the substrate and the water hydroxyl groups may inhibit the absorption of the reagent on the

surface of the catalyst, thus lowering its activity [40]. For this reason, despite the advantages of using

water as solvent, only a limited number of heterogeneous catalysts [41–44] have been employed for

promoting the hydrogenation reactions of quinolines in water.

The authors have already described a palladium based pre-catalyst for the Suzuki-Miyaura

cross-coupling [45] as well as for the reduction of nitroarenes [46] and quinolines [47,48] in water as

solvent. In order to obtain a material with a uniform distribution of the catalytically active sites, the

pre-catalyst was not synthetized by classical immobilization of palladium centres onto a prefabricated

support, but it was synthetised in a rather unconventional way, by co-polymerization of the metal-containing

−

monomer Pd(AAEMA)

2

[AAEMA = deprotonated form of 2-(acetoacetoxy)ethyl methacrylate] with a

suitable co-monomer (ethyl methacrylate) and cross-linker (ethylene glycol dimethacrylate) (Pd-pol

pre-catalyst, Scheme 1) [28,29]. Pd-pol was then reduced in situ under reaction conditions forming Pd NPs

(

the catalytically active species) immobilized and stabilized by the reticular and macro porous polymeric

support. It is known that the properties of the polymeric support playing a crucial role in catalysis are

the swelling capability, the porosity and a mild reticular structure [49]. Our Pd-pol swells in water and

in many organic solvents, is macroporous and is characterised by a rather low degree of cross-linking,

thus it was found active and recyclable for several palladium catalysed reactions [50–55] in conventional

organic solvents as well as in water [56].

In this work, the catalyst recovered after several cycles of the Suzuki-Miyaura cross-coupling and of

the hydrogenation of organic substances carried out in water, was studied in depth from the morphological

point of view by high resolution transmission electron microscopy (HRTEM), electron diffraction and

micro-IR spectroscopy.

The structural and chemical properties of Pd-pol before and after its use in catalysis, that is before

and after Pd(II) reduction, were already addressed in our previous work [29] but a real-time monitoring

of the evolution of the Pd NPs during the reduction process was missing, and the effect of different

reducing agents (temperature, phenyl boronic acid, dihydrogen, NaBH ) was never explored. The in situ

4

investigation of Pd NP formation is of great interest because it is well-known that size and shape of metal

NPs depend on the kinetics of the nucleation and growth processes [57]. Hence, understanding the

phenomena behind NP formation is fundamental in order to tune the NP properties, and therefore their

catalytic performances [58].

In addition, our interest in investigating the re-used catalyst stemmed from the observation that the

catalyst stability still represents the weak point of the heterogeneous catalysis that demands the reuse of

the material even when reaction conditions are harsh.

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Scheme 1. Synthesis of Pd-pol pre-catalyst.

2

. Results and Discussion

.1. Catalytic Efficiency and Re-Use

.1.1. Suzuki-Miyaura Cross-Coupling

Pd-pol was active, selective and recyclable in the Suzuki-Miyaura cross-coupling of aryl bromides

and activated aryl chlorides in water at 100 °C in the presence of K CO [45]. Table 1 reports the catalytic

2

3

results obtained in the Suzuki coupling between 4-bromoacetophenone and phenyl boronic acid taken as

a model reaction. The use of a transfer phase additive, such as tetrabutylammonium bromide (TBAB) [59]

accelerated the reaction (Table 1, entry 2) for two reasons: it facilitated the dissolution of the organic

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reactants in the water phase and it permitted the formation of smaller size (more active) nanoparticles

see below).

(

Table 1. Suzuki reaction of 4-bromoacetophenone and phenylboronic acid in water under

air over five cycles ^a.

Cycle (% Yield)

Entry

Time

First

97

Second

94

Third

84

Fourth

82

Fifth

82

1

2 h

b

2

15 min

98

93

85

84

80

^a: 4-bromoacetophenone (1.0 mmol), phenylboronic acid (1.5 mmol), base (2.0 mmol), H

^b: in the presence of TBAB (1.0 mmol).

2

O (4.0 mL), T = 100 °C;

Investigations on the activity of the mother liquor collected during and after catalytic runs (the mother

liquor was catalytically active during the catalytic run, resulting inactive when collected after substrate

consumption), ICP analyses carried out on the mother liquors and on the supported catalyst during and

after the catalytic runs (Pd amount was negligible in the mother liquor recovered at end of reaction and

was always the same in Pd-pol recovered after each catalytic cycle) (see [45]) revealed that Pd-pol could

act as a “release and catch” catalytic system [60,61]. A “release and catch” catalytic system is prepared by

noncovalent immobilization of the catalytic moiety onto a suitable support, but differently from the usual

non-covalently supported catalyst, the catalytic moiety is released in solution during the course of the

reaction and it is recaptured at the end of the reaction. Such a “catalyst-sponge like” system (Scheme 2)

permits to combine the benefits of homogeneous (high catalytic activity) and heterogeneous catalysis (easy

separation and recycling).

Scheme 2. “Release and catch” mechanism for a Pd NPs based catalytic system.

2

.1.2. Hydrogenation and Transfer Hydrogenation of Organic Compounds

Differently from the case of Suzuki-Miyaura coupling, Pd-pol behaves as a truly heterogeneous

catalyst in nitroarene reduction in water (Table 2) [46], as well as in the selective hydrogenation of

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quinolines in aqueous medium under 10 bar H

2

at 80 °C (Table 3) [47] or at 60 °C in the presence of

NaBH

4

[48] as hydrogen source (Table 4).

Table 2. Transfer hydrogenation of nitrobenzene over twelve cycles ^a.

Cycle

Yield (%)

1st

2nd

3rd

4th

5th 6th 7th 8th 9th 10th 11th 12th

94 93 94 95 96 95 94 96

96

95

96

97

^a: Substrate (1.0 mmol), H

2 4

O (5 mL), NaBH (10.0 mmol), Pd-pol (0.0175 mmol of Pd), room temperature, 2 h.

Table 3. Hydrogenation of 8-methylquinoline to 8-methyl-1,2,3,4-tetrahydroquinoline over

nine cycles ^a.

Cycle

First

Second

Third

Fourth

Fifth

Sixth Seventh Eighth Ninth

99 97 98 98

, 9 h.

Yield (%)

99

98

97

98

^a: Substrate (1.0 mmol), H

O/CH

2

3

OH (4 mL/1 mL), Pd-pol (Pd: 0.5 mol %), 80 °C, 10 bar H

2

Table 4. Transfer hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline over seven cycles ^a.

Cycle (% yield)

Cycle

First

Second

Third

Fourth

Fifth

Sixth Seventh

93 94

Yield (%)

94

95

92

91

^a: Substrate (1.0 mmol), H

2 4

O (5 mL), NaBH (20.0 mmol), Pd-pol (Pd: 2.0 mol %), 60 °C, 9 h.

Interestingly, by carrying out the nitrobenzene hydrogenation using H

instead of NaBH , the reaction reached completion after 6 h in the first run and after 24 h in the second

run, revealing that Pd-pol catalytic system was more active and recyclable using NaBH

2

(1 atm) as reducing agent

4

.

In all reported reductions, the mother liquors hot filtered at 40% of substrate conversion were

catalytically inactive, and the supported catalyst recovered at the end of the first run and at 40% conversion,

respectively, contained the same amount of palladium as the pre-catalyst. The same palladium content

was also found in the catalyst recovered at the end of the last cycle of the reduction reaction. These

results rule out any possible contribution of homogeneous catalysis by leached palladium species and

demonstrate that the polymer matrix was able to retain all the Pd NPs.

In addition, the polymer matrix could be involved in the mechanism of the reduction. In fact, the

−

negative electrostatic charge on the surface of the support derived by the presence of AAEMA moieties

(

Scheme 1), could favor the heterolytic cleavage of H

2

(used as the reductant or formed in situ by NaBH )

4

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+

−

(

Scheme 3), and the resulting H /H pair could preferentially transfer to the C = N or N = O polar bonds [62].

The heterolytic splitting of H (assisted by the basic support) has been substantiated in some heterogeneous

2

quinoline hydrogenation [63–65].

Scheme 3. Heterolytic hydrogen splitting and ionic hydrogenation of polar substrates by Pd

−

NPs on AAEMA supporting polymer matrix.

2

.2. Micro-IR Analysis

Numerous investigations concerning incorporation techniques of metal nanoparticles into the polymer

matrix have been published, demonstrating the stabilization of the metal nanoparticles encapsulated in

the polymer matrix [58,66–68]. The approach used for the preparation of Pd-pol [26], starting from a

polymerizable Pd(II) β-ketoesterate complex [29], is particularly interesting in catalysis applications due

to the attainment of an intimate contact of the components of the composite material.

Figure 1. Micro-IR spectra of Pd-pol before (red line) and after (green line) 7 cycles of a

NaBH reduction of quinoline.

4

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18668

Micro-IR spectroscopy analyses of the pristine Pd-pol and of the Pd-pol recovered after seven cycles

of a NaBH reduction of quinoline (under the reaction conditions reported in Table 4) were performed,

4

with the aim of gaining insight into the structural stability and the chemical modifications of the matrix

after a catalytic process. IR spectra recorded before and after duty (Figure 1) are nearly superimposable,

indicating that the polymer matrix backbone is stable. The disappearance of the weak β-ketoesterate

−1

combination band signals (1602 cm and 1509 cm ) [28] on passing from the pristine to the used catalyst

is ascribable to the loss of metal coordination by the ligand, as a consequence of the reduction of the

palladium oxidation state from +2 to zero that triggers the formation of NPs. These NPs may be stabilized

by electro-static interactions with the oxygen atoms of the polymeric support [69].

2

.3. TEM Observations

Transmission electron microscopy (TEM) has opened up new possibilities to investigate the nanoscale

features of innovative catalysts with complex molecular structures via morphological and chemical

characterization. The technique can be used in the ex situ approach when the catalyst is separated from the

solution allowing a vast array of synthetic parameters to be varied without concern for the delicacies of

the TEM equipment. In this respect, careful catalyst separation from the reaction media whereby the

state of the nanoscopic features at the crystalline surface may be retained before transfer to the TEM can

be performed. This is crucial to prevent secondary processes caused by changing growth conditions upon

contact with air and extraction from the liquid media. TEM ex situ observations of nanoparticles in organic

support must be accompanied by an accurate sample thinning operation to prevent contamination,

deformation and particles release. When these requirements are accomplished, TEM gives a more direct

approach to determining distribution, growth and dissolution rates of metal NPs onto the catalyst surface

than the indirect reaction conversion rate or yield.

2

.3.1. Pristine Pd-Pol

The thermal co-polymerization of Pd(AAMA) resulted in the pre-catalyst (Scheme 1) characterized

2

by an atomic Pd(II) polymeric network. However, the formation of primary Pd NPs could also occur, albeit

in low extent, during the thermal decomposition of metal-containing precursors, in accordance with the

simulated process proposed by Rozenberg et al. [30].

TEM images of three different batches of Pd-pol pre-catalyst showing the morphological structure

are reported in Figure 2a–c. At high magnification, all pristine catalysts showed almost the same

microstructure consisting of a small number of round irregular polyhedral metallic Pd primary particles

of ca. 9 nm, gathered in small aggregates with a maximum diameter of 18–25 nm and embedded in the

polymeric matrix. This confirms the reproducibility of the pre-catalyst microstructure after preparation

by a thermal polymerization process. The elemental identity of the nanoparticles was determined by means

of Energy Dispersive Spectroscopy (EDS) and Selected Area Electron Diffraction (SAED) analyses. SAED

image apparently consists of a ring pattern due to the diffraction of the thin metallic Pd-crystallites.

Molecules 2015, 20

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Figure 2. (a–c) TEM bright field images of three different batches of the pristine catalyst slice

with PdNPs; (d) enlargement of a selected area of (a) accompanied by the corresponding

e) EDS spectrum and (f) SAED pattern.

(

Figure 3 shows a low enlargement TEM image of the pre-catalyst where a sponge-like nanostructure,

which is capable of assuring a good contact between the reactive solution and the catalytic centres, is

observed. As it will be discussed below, micro-IR and TEM analysis indicated that the structural and

chemical features were retained by the polymeric support also after several reaction cycles and no

heteroatoms deriving from the reaction substrates and/or products were detected in the porous

framework, accordingly to what observed in other preparations [5].

Figure 3. TEM bright field image of the pristine catalyst slice with Pd NPs describing the

sponge-like nanostructure of the polymer matrix.

Molecules 2015, 20

.3.2. Pd-Pol Catalysed Suzuki-Miyaura Cross-Coupling

Figure 4 shows TEM images of Pd-pol recovered after (a) 30 min of the first cycle (at 30% substrate

18670

2

conversion); (b) the first cycle (2 h) and (c) the fifth cycle of the Suzuki-Miyaura cross-coupling reaction

described in entry 1 of Table 1.

Figure 4. TEM bright field images of Pd-pol after (a) 30 min; (b) the first cycle; and (c) the

fifth cycle of the Suzuki-Miyaura cross-coupling reaction described in entry 1 of Table 1;

(

d) EDS specta of selected areas of (a).

TEM analyses ascertained the reduction of the Pd(II) pre-catalyst and the EDS spectra clearly indicated

the presence of Pd NPs (on copper grid) and allowed us to follow their morphology with reaction time.

Once more, no contamination from other elements was recorded. Pd reduction was both thermal (being

the reaction temperature 100 °C) [30] and chemical (being the excess of phenylboronic acid the reductant).

Figure 5 shows Pd NP size distributions in (a) pristine Pd-pol and in Pd-pol recovered after (b) 30 min;

(c) the first cycle and (d) the fifth cycle of the Suzuki-Miyaura cross-coupling reaction described in entry 1

of Table 1. Pd-pol recovered after 30 min reaction (at 30% substrate conversion) contained new Pd NPs

whose size ranged from 2 to 6 nm, thus being smaller than the Pd NPs already contained in the pristine

catalyst. The same Pd NP size distribution was observed at the end of the reaction (quantitative substrate

conversion). Comparing Figure 5c (first cycle) with Figure 5d (fifth cycle) shows that the mean Pd NPs

increase with the cycle, with diameters ranging from 2–6 nm to 4–10 nm, and the nanoparticle size

distribution becomes broader. Differently from what observed by Mallick et al. [70], no particle aggregation

Molecules 2015, 20

18671

was observed, indicating that the reaction temperature was not sufficient to allow atoms diffusion. The

nanostructure of the catalyst was substantially maintained during the re-cycles and the Pd NPs remained

embedded in the polymer bulk with a higher density of Pd NPs on the support surface, thus pointing to

a “release and catch” mechanistic pathway.

Figure 5. Pd NP size distributions in (a) pristine Pd-poland in Pd-polrecovered after (b) 30 min;

(

c) the first cycle and (d) the fifth cycle of the Suzuki-Miyaura cross-coupling reaction described

in entry 1 of Table 1.

The slight decrease in the catalytic activity of Pd-pol observed in the Suzuki-Miyaura coupling reaction

after the second catalytic cycle (entry 1 of Table 1) can hence be influenced by the increase of the supported

nanoparticle sizes during catalysis which may either render the nanoparticles less prone to be leached out

in solution, or may result in the leaching of bigger, and therefore less active, nanoparticles.

Figure 6 shows TEM images indicating the snapshots of the PdNPs formation after (a) 5 min (at 30%

substrate conversion); (b) the first cycle and (c) the fifth cycle of the TBAB assisted Suzuki-Miyaura

cross-coupling reaction (conditions described in entry 2 of Table 1).

Both Pd-pol samples recovered after duty in the Suzuki-Miyaura reaction carried out in the presence

and in the absence of TBAB, respectively, presented the same nanostructure. In particular, an increased

number of well distributed metal NPs with reduced aggregation during time and the catalytic cycles were

observed by TEM analyses. All particles had similar diameters indicating the same growth rate. Some

aggregation could be observed for a total number of few primary particles, being the aggregate diameter

lesser than 20 nm.

Molecules 2015, 20

18672

Figure 6. TEM bright field images of Pd-pol after (a) 5 min of the first cycle; (b) the first

cycle and (c) the fifth cycle of the TBAB assisted Suzuki-Miyaura cross-coupling reaction

described in entry 2 of Table 1; (d) EDS spectra of selected areas of (c) (irregular contamination

of silica was observed in Spectrum 3).

Figure 7 shows Pd NP size distributions in (a) pristine Pd-pol and in Pd-pol recovered after

(

b) 5 min; (c) completion of the first cycle and (d) the fifth cycle of the TBAB assisted Suzuki-Miyaura

cross-coupling reaction described in entry 2 of Table 1. Pd NP sizes are smaller than the sizes of Pd NPs

formed during the cross-coupling reaction carried out in the absence of TBAB.

In particular, after the first TBAB-assisted reaction cycle, a local nucleation of the finer Pd NPs was

observed as two dimensional ranges of nanoparticles, the first one with a size distribution centered around

3

nm and the second one with a size distribution centered at 6 nm. The smaller size of Pd NPs obtained

in the presence of TBAB as phase transfer agent justified in part the higher catalytic activity of Pd-pol

with respect to the same reaction carried out in the absence of TBAB (Table 1). However, in the presence

of TBAB the “release and catch” process of Pd active species during catalysis occurred mainly for the

first part of the phenomenon, being the metal leaching not negligible as revealed by elemental analyses

results [45]. This could be due to the strong stabilization capacity of tetraalkylammonium salts towards

metal nanoparticles dispersed in solution. On the contrary, in the absence of TBAB, Pd NPs are less

prone to be leached out in solution and are more inclined to be re-captured at the end of reaction by the

polymer support, which acts as stabilizer. TEM images of the recovered catalyst confirm this hypothesis,

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18673

showing a higher density of Pd NPs on the surface respect to the inner part of the resin, thus revealing

that the Pd NPs re-capturing process by the polymer matrix occurs in the Suzuki reaction carried out in

water with (in less extent) and without TBAB.

Figure 7. Pd NPs size distributions in (a) pristine Pd-pol and in Pd-pol recovered after

(

b) 5 min; (c) the first cycle and (d) the fifth cycle of the TBAB assisted Suzuki-Miyaura

cross-coupling reaction described in entry 2 of Table 1.

2

.3.3. Pd-Pol Catalyst for Nitroarene Reduction with NaBH

4

and H

2

TEM images of Pd-pol (a) pristine; (b) after the first cycle and (c) after the twelfth cycle of nitrobenzene

reduction with NaBH are shown in Figure 8.

4

On passing from the pristine to the first cycle, the recovered Pd-pol showed both the previously

formed palladium nanoparticles and an increasing high number density of newly formed Pd NPs, now

ranging from 2 to 4 nm. As a consequence of such reactivity, Pd NPs showed a bimodal size distribution:

a first peak with a mean diameter centred at around 3 nm and the second one centred at ca. 10–12 nm

(

see Figure 9). Remarkably, in Pd-pol recovered after the twelfth run, the presence of large (10–12 nm

diameter) together with small (diameter equal to ca. 3 nm) palladium nanoparticles (Figure 9) did not

cause a significant decrease in the catalytic activity, because the smallest Pd NPs were still abundant.

High resolution (HR)TEM images showed lattice pattern with an interplanar spacing of 0.22 nm ascribable

to metallic Pd {1 1 1} plane [71] as depicted in Figure 8d,e, indicating the single crystallinity nature for

these Pd NPs. Attempts to obtain HRTEM images of Pd NP lattice pattern also in Pd-polsamples used

in the Suzuki-Miyaura reaction were unsuccessful. The absence of the Pd NP crystallinity features of the

former Pd-pol samples might be ascribable to the presence of a thick layer of amorphous polymeric matrix.

Molecules 2015, 20

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Figure 8. TEM images of Pd-pol (a) pristine; (b) after the first cycle and (c) after the twelfth

cycle of nitrobenzene reduction with NaBH , described in Table 2; (d) HRTEM image of

b) and (e) HRTEM image of (c).

4

(

Figure 9. Pd NPs particle size distributionsof Pd-pol (a) before; (b) after the first cycle and

c) after the twelfth cycle of nitrobenzene reduction with NaBH

(

4

.

As expected, the formation of catalytically active Pd NPs was observed also when the catalyst was

employed for the hydrogenation of nitrobenzene in water under 1 atm H , instead of NaBH (Figure 10).

These nanoparticles, generated by hydrogen reduction, were bigger (6–10 nm diameter sizes) compared

to those formed using NaBH as reducing agent, and consisted of single crystallites with an interplanar

spacing of 0.22 nm ascribable again to Pd metallic {1 1 1} plane [71]. It is known that the nature of the

2

4

Molecules 2015, 20

18675

reducing agent can strongly influence the nanostructure and the size distribution of the formed metal

nanoclusters [71]. The different NPs size distributions observed in the two Pd-pol samples, the first one

reduced by NaBH

catalytic activities showed by the two resins (the former system being more active than the latter one).

NaBH acts as a reducing agent of the metal in a dual way, by providing hydrides and by generating

4

2

and the second one reduced by 1 atm H , were presumably responsible for the different

4

hydrogen gas. In fact, the hydrides from sodium borohydride can replace negatively charged ligands

bound to Pd(II) (in the present case: the β-ketoesterato moieties, Scheme 1), generating Pd-hydrides which,

upon H elimination, give the reduced metal.

2

Figure 10. (a,b) TEM images and (c) Pd NPs particle size distribution of Pd-pol used in the

nitrobenzene reduction under hydrogen at room temperature.

In addition, the H

small size metal nanoparticles. On the contrary, in the case of Pd-pol stirred under 1 atm H

occurs only by hydrogen gas, which has to be transported to the water phase and then by solid phase

diffusion to the Pd-pol structure. Since the metal reduction by H requires longer times than the analogous

, larger palladium crystallites are formed in the reactions carried out under

2

formed in situ in water (Scheme 4) can reduce Pd(II) to Pd(0) with formation of

2

, the reduction

2

process induced by NaBH

4

dihydrogen gas.

Scheme 4. Metal catalyzed production of H

2

by NaBH in water.

4

Molecules 2015, 20

.3.4. Pd-Pol Catalyst for Quinoline Reduction Using NaBH

18676

2

4

Pd-pol was also tested in the transfer hydrogenation of quinolines to 1,2,3,4-tetrahydroquinolines.

Figure 11 shows TEM images of Pd-pol recovered after (a) the first and (b) the seventh cycle of the

quinoline reduction using NaBH in water. The number of primary particles in Pd-pol recovered after

4

the first cycle resulted higher compared to those observed in the pre-catalyst, and they remained

homogeneously distributed throughout the polymer indicating limited diffusion growth mechanism.

As occurred in the nitroarene reduction, the Pd NPs had a small average diameter ranging from 2 to 4 nm.

The Pd NPs were not grouped in clusters but were individually distributed. On passing from the first

(Figure 11a) to the seventh cycle (Figure 11b), the number of smallest size (2 nm) Pd NPs increased, the

nanostructure of the catalyst was substantially maintained during the subsequent cycles and the Pd NPs

remained embedded into the polymer bulk. The observation of a homogeneous dispersion of the metal

NPs onto the polymeric bulk rules out a “release and catch” process during catalysis.

Figure 11. TEM high enlargement images of Pd-pol after the (a) first and (b) seventh cycle

of the quinoline reduction using NaBH

of Pd-pol after the (c) first and (d) seventh cycle of the quinoline reduction using NaBH

0 °C in water (reaction conditions reported in Table 4).

4

at 60 °C in water. Pd NPs particle size distributions

4

at

6

Molecules 2015, 20

18677

Indeed, TEM results are consistent with a truly heterogeneous mechanism probably assisted by the

polymeric support, as depicted in Scheme 3. The clusters of primary nanoparticles did not grow in size,

indicating absence of particle mobility under reaction conditions. The Pd NPs supported onto Pd-pol

remained isolated from each other and they did not aggregate with recycling, although a limited amount

of them grew slightly. This explains why the catalytic activity of the supported catalyst recovered after

seven cycles was comparable to that of the catalyst reused after the first run.

In order to evaluate the influence of the substrate on the morphological nanostructure of the catalyst,

Pd-pol was subjected to the same reaction conditions reported in Table 3 using NaBH in the absence of the

4

substrate. The morphological features of Pd-pol recovered after this test were similar to those of Pd-pol

recovered after the first cycle using the substrate, as shown by the TEM image reported in Figure 12. This

indicates a negligible influence of the substrate on the morphological features of the polymer supported

Pd NPs.

Figure 12. TEM images of Pd-pol subjected to the same reaction conditions reported in

Table 3 using NaBH but without substrate.

4

3. Experimental Section

3.1. General Considerations

Tap water was deionized by ionic exchange resins (Millipore, Billerica, MA, USA) before use. All

other chemicals were purchased from commercial sources and used as received. Palladium content in

Pd-pol was assessed after sample mineralization by atomic absorption spectrometry using a PerkinElmer

3

110 instrument (Perkin-Elmer Corporation, Waltham, MA, USA). Reactions were monitored by GLC

or by GC-MS. GC-MS data (EI, 70 eV) were acquired on a HP 6890 instrument (Hewlett Packard,

Palo Alto, CA, USA) using a HP-5MS crosslinked 5% PH ME siloxane (30.0 m × 0.25 mm × 0.25 mm)

capillary column coupled with a mass spectrometer HP 5973. GLC analysis of the products was performed

using a HP 6890 instrument equipped with a FID detector and a HP-1 (crosslinked methylsiloxane)

capillary column (60.0 m × 0.25 mm × 1.0 μm). Pd(AAEMA) and the supported catalyst (Pd-pol) were

2

prepared as reported in [55]. The procedure adopted for the Suzuki-Miyaura reaction catalyzed by Pd-pol

is reported in [45]. The reductions of nitrobenzene were carried out as reported in [46]. The quinoline

reduction using NaBH was carried out as reported in [4].

4

Molecules 2015, 20

.2. Morphological Analyses

18678

3

Pd-pol was recovered after duty by filtration (in the case of Suzuki-Miyaura reaction) or by centrifugation

in all other cases). The isolated catalyst was then repeatedly washed with water, acetone, and diethyl

(

ether and dryed under high vacuum.

The microstructure of the polymeric matrix embedded NPs was determined by TEM observations at

acceleration voltage of 200 kV (Model JEM 2010, Jeol, Akishima, Tokyo, Japan), equipped with X-ray

energy dispersive spectroscopy (EDS, Plano, TX, USA). The samples were prepared by dispersing the

powders in distilled water using an ultrasonic stirrer and then placing a drop of suspension on a copper

grid covered with a transparent polymer film, followed by drying and carbon coating. The particle size

distributions were obtained by TEM image analysis using the ImageJ software [72]. Each morphology

observation was accompanied by EDS analyses that confirmed the presence of Pd. Micro-FTIR spectra

(neat) were recorded using a PerkinElmer Spectrum 2000 instrument.

4

. Conclusions

A morphological study on Pd NPs supported onto a polymer matrix (Pd-pol) was carried out by TEM,

EDS and micro-IR techniques. These analyses were performed on such material before, during and

after its exposure to prolonged catalytic runs under green conditions in water solvent in the following

organic reactions: the Suzuki-Miyaura coupling between aryl bromides and phenylboronic acid and the

hydrogenation of nitroarenes and quinolines, using dihydrogen or NaBH , as reducing agent. This study

4

shed light into morphological features of both metal NPs and polymeric insoluble support. Micro-IR

analyses revealed that the support was chemically stable over the recycles of the catalyst in all the

reactions tested, while TEM techniques showed that the macroporosity of the resin remained constant

after each run (with good swelling in water), an important requirement for insoluble catalytic materials

capable of facilitating the migration of reactants towards active sites.

Moreover, TEM analyses showed that in all cases the pristine Pd(II) polymer supported complex (the

pre-catalyst) was reduced in situ to Pd(0) forming nanoparticles (the real active species) under the reaction

conditions. The organic polymer was always able to stabilize Pd NPs during recycling, since metal NPs

agglomeration detected by TEM analysis was always negligible. In all cases, after several re-uses (ranging

from 5 to 12, depending on the reaction) the amount (as a percent) of the smallest Pd NPs (2–8 nm diameter)

still remained high, while the presence of larger NPs (10–15 nm diameter) might indicate growth

(

no agglomeration) of the smallest Pd NPs with recycling. However, the level of NP growth that we did

observe with reuse was not enough to cause a significant decrease of the catalytic activity.

The Pd NPs’ average size and morphology was affected by several parameters, such as: temperature,

presence of a tetralkylammonium salt, and reducing agent. High temperatures (80–100 °C) favoured the

thermal reduction of Pd(II) with formation of NPs of 9 nm average size, while at room temperature a

greater amount of smaller NPs (2–6 nm diameter) was observed.

The presence of a phase transfer agent (tetrabutylammonium bromide) caused the formation of

smaller size Pd NPs (4 nm average size) and metal leaching into solution, due to its well-known metal

NPs stabilizing capacity.

Molecules 2015, 20

18679

TEM observations allowed us to deduce the following considerations about the effect of the reducing

agent on size and morphology of Pd NPs. In the case of the Suzuki coupling, the effect of the reductant

(

excess phenylboronic acid) was masked by the temperature reaction (100 °C) and/or by the presence of

tetrabutylammonium bromide (when used). On the contrary, for the reduction of nitroarenes and quinolines

in water the choice of the reductant (H or NaBH ) was crucial for Pd NPs diameter size (and activity),

2

4

ranging from 6 to 10 nm under 1 atm dihydrogen and from 2 to 4 nm, in the presence of NaBH

4

, because

the latter caused the reduction of Pd(II) in a more effective way.

Finally, morphological features of the supported catalyst could provide important information about the

mechanism involved in the catalytic cycle. The “release and catch” process of active species, commonly

invoked by the scientific community for the heterogeneously catalysed C–C bond forming reaction could

explain why TEM images of Pd-pol recovered after duty in the Suzuki reaction showed the majority of

Pd NPs onto the surface of the polymer. On the contrary, when Pd-pol was employed in the hydrogenation

under H

2

and in the NaBH transfer hydrogenation of nitroarenes and quinolines, TEM images showed

4

a uniform distribution of Pd NPs in the polymer matrix, pointing to a truly heterogeneous catalytic

mechanism, as confirmed by other techniques (hot filtration test, ICP analyses, etc.).

All these results offer interesting perspectives in catalysis, since it is well documented that the catalytic

activity of metal NPs depends on morphological features, on size control and size distribution. Finally,

the present work demonstrates that, when dealing with the characterization of polymer supported metal

NPs, a suitable combination of multiple complementary techniques may allow one to obtain a clear picture,

not only of the entire process that leads from the metal ion precursor to metal NPs, but also of the catalytic

cycle promoted by the metal NPs catalyst.

Acknowledgments

We thank Simona Intini, Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e di

Chimica (DICATECh), Politecnico di Bari, for laboratory assistance and Italian Ministero dell’

Istruzione, dell’Università e della Ricerca (MIUR) (project PRIN 2010–2011 n. 2010FPTBSH_001) for

financial support.

Author Contributions

Piero Mastrorilli and Cristina Leonelli: Responsibles for research publication. Maria Michela Dell’Anna

and Matilda Mali: Catalysis studies. Antonino Rizzuti: TEM and micro-IR discussion. Mauro Zapparoli:

TEM analyses.

Conflicts of Interest

The authors declare no conflict of interest.

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http://creativecommons.org/licenses/by/4.0/).

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Resin-immobilized palladium nanoparticle catalysts for organic reactions in aqueous media: Morphological aspects

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