Subscriber access provided by LOCK HAVEN UNIV
Article
Single-Walled Carbon Nanotube-Polyamidoamine Dendrimer Hybrids for Heterogeneous Catalysis Francesco Giacalone, Vincenzo Campisciano, Carla Calabrese, Valeria La Parola, Zois Syrgiannis, Maurizio Prato, and Michelangelo Gruttadauria ACS Nano, Just Accepted Manuscript • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Nano is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Nano
Single-Walled Carbon Nanotube-Polyamidoamine Dendrimer Hybrids for Heterogeneous Catalysis Francesco Giacalone,*† Vincenzo Campisciano,† Carla Calabrese,† Valeria La Parola,‡ Zois Syrgiannis,§ Maurizio Prato,*§,&,# Michelangelo Gruttadauria*† † Dipartimento Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche (STEBICEF) Università degli Studi di Palermo, Viale delle Scienze, Ed. 17, 90128 Palermo (Italy) ‡ Istituto per lo Studio dei Materiali Nanostrutturati ISMN-CNR, Via Ugo La Malfa 153, 90146Palermo (Italy) § Centre of Excellence for Nanostructured Materials (CENMAT), INSTM, unit of Trieste, Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, via L. Giorgieri 1, 34127, Trieste (Italy) & CIC BiomaGUNE, Parque Tecnológico de San Sebastián, Paseo Miramón, 182, 20009 San Sebastián (Guipúzcoa), Spain # Basque Foundation for Science, Ikerbasque, Bilbao 48013, Spain
KEYWORDS. Carbon nanotubes – PAMAM dendrimers – Palladium nanoparticles – heterogeneous catalysis – C-C Cross coupling– TEM
ACS Paragon Plus Environment
1
ACS Nano
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 32
ABSTRACT
We report the synthesis and catalytic properties of single-walled carbon nanotube— polyamidoamine dendrimers hybrids (SWCNT-PAMAM), prepared via a convergent strategy. The direct reaction of cystamine-based PAMAM dendrimers (generations 2.5 and 3.0) with pristine SWCNTs in refluxing toluene, followed by immobilization and reduction of [PdCl4]2-, led to the formation of highly dispersed small palladium nanoparticles homogeneously confined throughout the nanotube length. One of these functional materials proved to be an efficient catalyst in Suzuki and Heck reactions, able to promote the above processes down to 0.002 mol% showing a turn over number (TON) of 48,000 and a turn over frequency (TOF) of 566,000 h-1. In addition, the hybrid material could be recovered and recycled for up to 6 times. No leaching of the metal has been detected during the Suzuki coupling. Additional experiments carried out on the spent catalyst permitted to suggest that a “release and catch” mechanism is operative in both reactions, although during Heck reaction small catalytically active soluble Pd species are also present.
ACS Paragon Plus Environment
2
Page 3 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Nano
Over the last decades, hybrid nanomaterials have intensely attracted the interest of the scientific arena. The rationale relies on the fact that the combination of materials with different properties may give rise to composites with superior properties. The recent discovery of new carbon nanomaterials, such as fullerenes, carbon nanotubes and graphene, has introduced a great and interesting source of starting materials for novel architectures and properties.1 The main advantage of these materials is the possibility of downscaling the conventional technologies. Thus, several structured carbon nanoforms (CNFs) and metal nanoparticles (MNPs) have been combined into nanohybrids aimed at emerging applications such as catalysis, electronics and biosensors.2-4 since they can be tailored at will to meet specific requirements.2, 5-9 In particular, carbon nanotubes (CNTs) show excellent properties to be exploited for catalysis such as proper pore sizes, relatively high surface area, high mechanical and chemical stability, hence they are finding increasing application as supports for MNPs.10-14 Among others, palladium-based catalysts are of fundamental importance since they enabled a stream of development in synthetic chemistry because of their high activity and selectivity.15-18 Palladium-mediated reactions are nowadays very important synthetic tools often employed in keysteps for the preparation of pharmaceuticals, drugs, agrochemicals but also organic semiconductors.19-21 Nevertheless, the difficulties in separating the catalyst from the product and the formation of a palladium black precipitate strongly hamper the recycling of such expensive species. However, the recovery and reusability of the catalyst is of primary importance in the sustainability of any catalytic process. In this regard, a lot of work has focused on palladium nanoparticles (PdNPs) due to their high surface-to-volume ratio and to the high catalytic activity of the low coordinated atoms exposed at their surface.22-26 In order to avoid their aggregation during the reaction and hence the
ACS Paragon Plus Environment
3
ACS Nano
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 32
deactivation, and also to facilitate their separation and recovery from the reaction solution, Pd nanoparticles can be anchored to solid supports.27, 28 In this regard, pristine CNTs29,
30
and oxidized-CNTs31,
32
have been employed for the
immobilization of palladium for C-C coupling reactions. However, these materials often show large PdNPs with a broad size distribution and, in many cases, suffer from severe leaching phenomena. This is probably due to the lack of sufficient binding sites on CNTs surface for anchoring metal nanoparticles or the parent precursor metal ions, subsequently leading to scarce dispersion and agglomeration of MNPs. In addition, pristine CNTs tend to aggregate to form ropes and bundles decreasing the exposed surface. Thus the chemical functionalization of CNTs becomes an important prerequisite in order to achieve debundling for better dispersibility and eventually improve catalytic performance.33-35 A few examples of CNTs-PdNPs nanocatalysts for Suzuki and Heck reactions have been reported in which multi-walled and single-walled nanotubes were chemically modified in order to maximize the interactions between MNPs.36-40 Recently, we have developed new and simple methodologies for the functionalization of structured carbon nanoforms41-46 and herein, we report the straightforward convergent synthesis and characterization of a heterogeneous palladium nanocatalyst supported on PAMAMSWCNTs and its efficiency in promoting the Suzuki and Heck coupling reactions under air and in ligand-free conditions.
RESULTS AND DISCUSSION For a suitable support for PdNPs stabilization and immobilization, we used single-walled carbon nanotubes (SWCNTs) in combination with polyamidoamine dendrimers (PAMAM). The main
ACS Paragon Plus Environment
4
Page 5 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Nano
motivation of our choice relies on the fact that SWCNTs, among other nanotubes, offers larger surface area and show improved reactivity toward functionalization. On the other hand, PAMAM dendrimers are well known stabilizers for PdNPs especially for catalytic uses,47, 48 even though their recycling is a rather hard task. The chemical modification of multi-walled carbon nanotubes (MWCNTs) with 3rd generation PAMAM dendrimers and subsequent immobilization of palladium has been achieved through a “graft from“ divergent strategy.40 However, this sequential synthesis in twelve steps is a highly time consuming task. Consequently, we decided to adopt a convergent strategy by employing the novel approach for the easy chemical modification of CNTs with disulfides recently reported by our groups.44, 46 The preparation of the hybrids SWCNT-PAMAM 3 and 4 has been accomplished through an atom-economical strategy which involves the direct reaction of PAMAM dendrimers with cystamine core of generation 2.5 and 3.0 with pristine SWCNTs in refluxing toluene (Scheme 1). R R O
R
O
O
HN
O
N
O O N
HN R
O N
R
N
NH HN
S S
N
O
N
O
O
O
O
N
O HN
NH
O
R
S
O
1 : R = -OCH3 2 : R = -NH-CH2CH2-NH2
N
NH N
R
O HN
NH
R
N
O
R
NH N O
R O
O HN
NH
Pd
N
R R
O
R
N O
R
O
5 : R = -OCH3 6 : R = -NH-CH2CH 2-NH2
Scheme 1. Synthesis of SWCNT-PAMAM hybrids 3 and 4 and Immobilization of PdNPs for the preparation of nanocatalysts 5 and 6.
Whereas dendrimer 2.5 G 1 smoothly reacted with the nanotubes, the 3.0 G dendrimer 2 needed the presence of catalytic amounts of AIBN to proceed properly, according to an
ACS Paragon Plus Environment
O
NH
R
3 : R = -OCH 3 4 : R = -NH-CH 2CH2-NH2
R
Pd O
O
O N
HN
N
N O
N
R
O
S
Pd
NH
O O
O
N
R
R O 1. Na [PdCl ] 2 4 H 2O, 16 h, rt 2. NaBH 4 H 2O, 6 h, rt
O
NH
O O
O
HN
O
N
N
N O
N
O
NH
R
(AIBN for 2) Toluene / MeOH reflux, 72 h O R
NH
R
R
SWCNTs
R R
O
R
O
N O
NH
O
O
N
N
O
O
HN
R
O
O
NH
O
O
HN
O
N
R
NH O
R
O R
O
N N
R
R
5
ACS Nano
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 32
analogous reaction between thiols and several CNFs.49-52 In such a way highly functionalized nanotubes were achieved, as determined by thermogravimetric analysis (TGA) and Raman spectroscopy. The TGA in nitrogen shows for 3 at 700 °C a net weight loss of 75.4%, corresponding to a 0.52 mmol/g of loading, values in excellent agreement with that obtained previously for the same material.44 SWCNT-PAMAM 4 resulted to be less functionalized (net weight loss of 41.5% corresponding to a loading of 0.25 mmol/g) probably due to the low solubility of 2 in the reaction solvent mixture. In both cases, differential thermal analysis (DTA) clearly indicate that the PAMAM network is decomposed in the 150-500 °C range.53 In conjunction with TGA, Raman spectroscopy represents a powerful and accurate technique for the characterization of SWCNTs.54 The covalent functionalization of the CNTs can be detected from the enhancement of the area intensity of D-band with respect to the G-band in the Raman spectrum, due to the rehybridization of the carbon atoms from sp2 to sp3. The normalized AD/AG ratio from the AD0/AG0 ratio can be used as a proof for an effective attachment on the scaffold of CNTs. As shown in Figure 1b, Raman spectroscopy confirms the effective functionalization of the SWCNTs and the higher degree of functionalization of 3 (with a normalized AD/AG of 3.9) with respect to 4 (AD/AG =2.9).
ACS Paragon Plus Environment
6
Page 7 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Nano
Figure 1. a) TGA (solid lines) and TG-DTA (dotted lines) of pristine SWCNTs, SWCNTPAMAM 3 and 4. b) D- and G-bands from Raman spectrum of p-SWCNTs (black line) and the f-SWCNTs 3 and 4.
Next, palladium nanoparticles were immobilized by following an “in situ” strategy (Scheme 1). Sodium tetrachloropalladate has been firstly used as Pd(II) precursor for interacting with the dendron moieties, followed by reduction with sodium borohydride. The fine black powders so obtained have been characterized by means of TGA, DSC, XRD, XPS and HR-TEM. TGA analysis coupled with differential scanning calorimetry (DSC) carried out under air flow resulted to be a very useful technique for the exact quantification of palladium content in the hybrid materials (Figure 2). As a matter of fact, TGA of 5 and 6, in the 770-840°C range, present a small yet defined weight loss (~1.5% w/w) which DSC associates to an endothermic process. During the TGA experiment under air flow, all the organic material is burned down whilst the PdNps are oxidized to PdO. The small losses are caused by the quantitative transformation of PdO to Pd given that the former species is not stable over 800°C.55, 56 Subsequently, the weight
ACS Paragon Plus Environment
7
ACS Nano
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 32
losses are due to the oxygen linked to Pd atoms and, since molPd = ½ molO2, loadings of 9.6 and 10.0% w/w can be estimated for 5 and 6, respectively.
Figure 2. TGA/DSC analyses of (a) 5, and (b) 6 recorded under air flow (10 °C/min). In the inset, the weight losses due to PdO decomposition are magnified.
High-resolution transmission electron microscopy (HR-TEM) has been used in order to shed light on the presence, distribution and dimension of palladium nanoparticles. Interestingly, in both materials 5 and 6 the PdNPs are clearly visible and very well dispersed on the nanotube surface (Figure 3 and Figures S1-S2). The main difference relies in their dimensions given that
ACS Paragon Plus Environment
8
Page 9 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Nano
whereas 6 present very small PdNPs with a mean size of 1.6 ± 0.4 nm (n = 245) and a sharp distribution, in the hybrid material 5 the nanoparticles are bigger (3.4 ± 0.6 nm, n = 260) with a broader size distribution. The small dimensions of the particles could not be detected by XRD, in which no nanocrystallites larger than 4 nm were detected. Very likely this difference between the two materials is due to the higher number of stabilizing amino-groups present in SWCNTPAMAM3.0-Pd 6. TEM pictures also show that, although the nanotubes are well functionalized, they are still arranged in ropes and bundles and not individually dispersed on the TEM grid. Finally, analysis of high magnification images confirm the nanocrystalline nature of the PdNPs, with a clear d spacing of 0.22 nm attributable to Pd(111) lattice planes (Figure S3).
ACS Paragon Plus Environment
9
ACS Nano
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 32
Figure 3. Representative HR-TEM images of a) 5, and b) 6. The inset shows PdNPs size distribution.
XPS analysis was used to evaluate the degree of Pd reduction and the surface concentration of active element (see Figure S4 for the survey spectra of 5 and 6). As evidenced by Figure 4, showing the Pd 3d region for 5 and 6, Pd is present for both catalysts, mainly as Pd (II) (Pd3d5/2 BE=337.5 eV) with a ca. 20% of reduced Pd (Pd3d5/2 BE=335 eV).57,
58
The surface atomic
ratio of Pd/C is quite different in the two catalysts; in the case of 5 Pd/C = 0.006 while for 6 Pd/C = 0.02. Given that XPS is a surface sensitive technique whose analysis depth is 3-10 nm, the atomic ratio calculated suffer, besides the actual amount of the elements, of segregation, particles size or uneven elements distributions. Hence, the difference in the Pd/C atomic ratio may be here attributed either to a different Pd loading or to a different Pd particles size. Considering that by TGA the total Pd loading is the same in the two catalysts and by TEM material 5 shows bigger particles, which could account for the lower Pd/C atomic ratio, we argue
ACS Paragon Plus Environment
10
Page 11 of 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Nano
that in case of 6 the higher Pd/C ratio is due to an effectively 3.5 higher amount of exposed active sites which is promising for application in catalysis.
Figure 4. High resolution XPS spectra of Pd3d region of 5 and 6.
The catalytic activity of the hybrid nanomaterials was tested in C-C cross-coupling reactions. Firstly, 5 and 6 were employed as catalyst at 0.1 mol% loading in the benchmark reaction between phenylboronic acid and 4-bromobenzaldehyde. A quick screening of the reaction conditions indicated a scarce attitude for 5 to promote the title process whilst, in the case of 6, the mixture of ethanol/water 1:1 was the best medium, allowing nearly full conversion within 5 h at 50 °C (Table 1, entry 1). Under such conditions, EtOH guarantees a good solubility of the
ACS Paragon Plus Environment
11
ACS Nano
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 32
substrates while water ensures a right dispersion of the catalyst, improving the activity. Very likely, the difference in activity is mainly due to the differences in NPs size in the two catalysts. As a blank experiment, SWCNT-PAMAM3.0-Pd 6 was replaced by colloidal PdNPs stabilized in the presence of PAMAM dendrimer 2 (see Experimental for details), giving rise to no reaction within 5 h at 0.1 mol% and only partial coupling (
