Single-Walled Carbon Nanotube− Polyamidoamine Dendrimer Hybrids for Heterogeneous Catalysis ACS Nano 2016.10:4627-4636. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/04/18. For personal use only.
Francesco Giacalone,*,† Vincenzo Campisciano,† Carla Calabrese,† Valeria La Parola,‡ Zois Syrgiannis,§ Maurizio Prato,*,§,∥,⊥ and 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, 90146 Palermo, 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 S Supporting Information *
ABSTRACT: We report the synthesis and catalytic properties of single-walled carbon nanotubepolyamidoamine 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 turnover number (TON) of 48 000 and a turnover 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. KEYWORDS: carbon nanotubes, PAMAM dendrimers, palladium nanoparticles, heterogeneous catalysis, C−C cross coupling, TEM
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proper pore sizes, relatively high surface area, and 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, and 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
ver 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 © 2016 American Chemical Society
Received: February 5, 2016 Accepted: March 14, 2016 Published: March 14, 2016 4627
DOI: 10.1021/acsnano.6b00936 ACS Nano 2016, 10, 4627−4636
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Scheme 1. Synthesis of SWCNT−PAMAM Hybrids 3 and 4 and Immobilization of PdNPs for the Preparation of Nanocatalysts 5 and 6
PdNPs especially for catalytic uses,47,48 even though their recycling is a rather hard task. The chemical modification of multiwalled carbon nanotubes (MWCNTs) with third generation PAMAM dendrimers and subsequent immobilization of palladium has been achieved through a “graft from” divergent strategy.40 However, this sequential synthesis in 12 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). 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 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 those 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 analyses (DTA) clearly indicate that the PAMAM network is decomposed in the 150−500 °C range (Figure 1a).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).
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-tovolume ratio and to the high catalytic activity of the low coordinated atoms exposed at their surface.22−26 To avoid their aggregation during the reaction and hence the 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 multiwalled and single-walled nanotubes were chemically modified in order to maximize the interactions between MNPs and the support.36−40 Recently, we have developed new and simple methodologies for the functionalization of structured carbon nanoforms,41−46 and herein, we report the straightforward convergent synthesis and characterization of a heterogeneous palladium nanocatalyst supported on singlewalled carbon nanotube−polyamidoamine dendrimers (SWCNT−PAMAM) and its efficiency in promoting the Suzuki and Heck coupling reactions under air and in ligandfree conditions.
RESULTS AND DISCUSSION For a suitable support for PdNPs stabilization and immobilization, we used SWCNTs in combination with PAMAM. The main motivation of our choice relies on the fact that SWCNTs, among other nanotubes, offer larger surface area and show improved reactivity toward functionalization. On the other hand, PAMAM dendrimers are well-known stabilizers for 4628
DOI: 10.1021/acsnano.6b00936 ACS Nano 2016, 10, 4627−4636
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Figure 1. (a) TGA (solid lines) and TG-DTA (dotted lines) of pristine SWCNTs, SWCNT−PAMAM 3 and 4. (b) D- and G-bands from Raman spectrum of p-SWCNTs (black line) and the fSWCNTs 3 and 4.
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.
The small dimensions of the particles could not be detected by X-ray diffraction (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 SWCNT−PAMAM 3.0-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). X-ray photoelectron spectroscopy (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) (Pd 3d5/2 BE = 337.5 eV) with a ca. 20% of reduced Pd (Pd 3d5/2 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 that in case of 6 the higher Pd/C ratio is due to an effectively 3.5 higher amount of
Next, palladium nanoparticles were immobilized by following an “in situ” strategy (Scheme 1). Sodium tetrachloropalladate has been first 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 HRTEM. 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, presents 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, while 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 losses are due to the oxygen linked to Pd atoms and, since molPd = 1/2 molO2, loadings of 9.6 and 10.0% (w/w) can be estimated for 5 and 6, respectively. High-resolution transmission electron microscopy (HRTEM) 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 and S2). The main difference relies in their dimensions given that whereas 6 presents 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. 4629
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Figure 4. High-resolution XPS spectra of Pd 3d region of 5 and 6.
Section for details), giving rise to no reaction within 5 h at 0.1 mol % and only partial coupling (
