Research Article pubs.acs.org/journal/ascecg
Efficient Synthesis of Monodisperse Metal (Rh, Ru, Pd) Nanoparticles Supported on Fibrous Nanosilica (KCC-1) for Catalysis Mahak Dhiman, Bhagyashree Chalke, and Vivek Polshettiwar* Nanocatalysis Laboratories (NanoCat), Department of Chemical Sciences, Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Colaba, Mumbai, Maharashtra 400005, India S Supporting Information *
ABSTRACT: We report a simple and sustainable protocol for the synthesis of monodisperse rhodium (Rh), ruthenium (Ru), and palladium (Pd) metal nanoparticles supported on fibrous nanosilica (KCC-1). In this protocol, use of expensive dendrimers was replaced by inexpensive polyethylenimine (PEI) to produce highly monodispersed supported metal nanocatalysts. First, KCC-1 was covalently functionalized by PEI and then metal(II) salts were loaded on KCC-1-PEI material to have complexation of metal ions with amines of PEI. Reduction of metal(II) ions by NaBH4 yielded metal(0) nanoparticles supported on KCC-1. As-synthesized metal nanoparticles supported on PEI functionalized KCC-1, named KCC-1PEI/Rh, KCC-1-PEI/Ru, and KCC-1-PEI/Pd, were characterized by transmission electron microscopy (TEM) for particle size and their distribution, N2 sorption studies for surface area, pore sizes and pore volume, thermogravimetric analysis for PEI loading, and solid state NMR for its covalent attachment. These nanocatalysts were then evaluated for the hydrogenation of phenylacetylene and styrene. They showed good catalytic activities under mild pressure, at room temperature and notably in a very short period of time. Catalysts were also recyclable several times with negligible loss of activity, indicating their good stability that is due to PEI functionalization as well as fibrous nature of KCC-1 support. KEYWORDS: Fibrous nanosilica, KCC-1, Metal nanoparticles, Nanocatalysis, PEI, Hydrogenation, Green chemistry, Sustainable protocol
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INTRODUCTION
Recently, Asefa et al. developed a benchmark protocol for the synthesis of Pd nanoparticles supported on silica spheres using a dendrimer shell strategy.24 Although this is an excellent protocol, use of dendrimers, (which are expensive and difficult to synthesize) could make this protocol less sustainable. Thus, simple and practical protocol for supported metal nanocatalysts using cheaper and green chemicals is urgently needed, to make entire nanocatalysis sustainable. Another challenge is the accessibility of supported metal nanoparticles. Most of the metal supported nanocatalysts are based on porous silica such as SBA-15 or MCM-41, because of their high surface area. However, nanoparticles inside the pores are not always accessible, which in turn reduces their overall activity. Hence, use of fibrous nanosilica (KCC-1) can positively help in overcoming this issue. In continuation of our work on nanocatalysis,25−31 herein we report a simple and sustainable protocol for synthesis of monodisperse metal (Rh, Ru, Pd) nanoparticles supported on fibrous nanosilica (KCC-1), using polyethylenimine (PEI) as a pseudochelator. Transmission electron microscopy (TEM) images clearly show formation of ultrasmall and monodisperse metal particles on KCC-1 (Figures 1 and 2). This is a novel way
The use of nanocatalysts have been rapidly increasing due to their extraordinary efficiency and selectivity.1 Among them, noble metal nanoparticles (NPs) supported on high surface area silica have attracted great attention due to their exceptional catalytic activity (like homogeneous catalysts) and stability−recyclability (like heterogeneous catalysts).1−17 Although, the use of supported metal nanoparticles in catalysis is maturing now, challenges related to their synthesis still remain. To design catalytically efficient and sustainable nanocatalysts, the supported nanoparticles should be monodisperse, small, accessible, and at the same time leach proof and stable. Several synthesis methods are reported in the literature, but the above-mentioned aspects of catalysts design are not always met. Hence, there is a need for a simple and efficient protocol to prepare these metal nanocatalysts. Numerous excellent methods are available to design supported metal nanocatalysts, and one of the widely used protocols is by using dendrimer functionalization of solid support.18−24 Although the use of dendrimers has led to synthesis of stable nanoparticles, the major stumbling block in the widespread application of these dendrimer based catalysts is their complex synthetic procedure. These dendrimers also tend to passivate the surface of nanoparticles to a certain extent causing weak substrate−metal interaction. © XXXX American Chemical Society
Received: August 5, 2015 Revised: October 14, 2015
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DOI: 10.1021/acssuschemeng.5b00812 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 1. Design of KCC-1 supported metal (Rh, Ru, Pd) nanoparticles.
nanoparticles with an average size range of 1 to 3 nm for Rh, 0.5 to 2.5 nm for Ru, and 1.5 to 4 nm for Pd (Figure 2). In all the cases, a narrow particle size distribution was distinctly observed, indicating effectiveness of this synthetic protocol. TEM images clearly showed that nanoparticles are well-dispersed throughout the KCC-1, and no local accumulation or agglomeration was present. Dispersity of metal nanoparticles with such high loading can be attributed to the fibrous nature of KCC-1 and PEI-amine functionalization, which acts as pseudoligand. The X-ray diffraction (XRD) pattern (Figure S1) consisted of only one broad hump at 25°, attributed to the amorphous nature of silica support. Absence of any noticeable peak in diffractogram could be due to smaller sized nanoparticles that do not provide enough coherent diffraction to be detected indicating the formation of well-dispersed and small nature of nanoparticles. The percentage loading of metal nanoparticles on KCC-1 was calculated by EDX analysis. EDX was carried out at 10 different well-separated positions and from the mean of all the spectra the weight percentage was calculated as given in Table 1. Metal loading found in these catalysts were 6.1% of Rh, 7.0% of Ru, and 11.0% of Pd in KCC-1-PEI/Rh, KCC-1-PEI/Ru, and KCC-1PEI/Pd, respectively. Textural properties and surface area of the as-synthesized catalysts were investigated using nitrogen sorption measurements. The nitrogen sorption analysis of these materials showed type-IV isotherm with H1 type hysteresis (Figure 3a). After PEI functionalization, BET surface area of the material was reduced from 590 to 316 m2/g, whereas pore volume was reduced from 0.79 to 0.54 cm3/g (Table 1). The reduction in surface area and pore volume was due to the loading of PEI (19%) that infiltrated inside the channels of fibrous nanosilica. This was confirmed by the disappearance of channel (pore) around 6.4 nm (Figure 3b).
to produce such high quality supported metal nanocatalysts using PEI and KCC-1. Use of PEI makes this process more economical as compared to use of dendrimers as a chelator. Synthesized nanocatalysts were also evaluated for test reactions, hydrogenation of phenylacetylene and styrene, which resulted in good yield and selectivity. Reactions were very fast and invariably took only 10−15 min to achieve a moderate turnover number (TON). Recyclability of catalysts resulted in negligible loss of activity attributed to leach proof nature of nanocatalysts due to attachment of PEI and fibrous nature of KCC-1. KCC-1 was prepared using our reported protocol25 and was first functionalized with (3-glycidyloxypropyl)trimethoxysilane (GTMS), which was then reacted with polyethylenimine, to yield KCC-1 covalently functionalized with PEI (Figure 1). We then used these PEI amines, grafted on each fiber of KCC-1 as pseudoligands to produce and stabilize monodisperse metal nanoparticles. Metal (Rh, Ru, Pd) salts with metal(II) ions bind with amines of PEI, which on reduction with NaBH4 produce metal(0) nanoparticles.
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RESULTS AND DISCUSSION Grafting of amines on KCC-1 was carried out in a two-step process involving a functionalization of silica surface with 3glycidoxypropyltrimethoxysilane (GTMS) and then SN2 type reaction of amines of PEI with epoxy group of GTMS, to produce a covalently linked hydroxy−polyamine branched network on KCC-1 fibers (Figure 1). This material was then loaded with respective metal salts (rhodium chloride hydrate, ruthenium chloride, or sodium tetrachloropalladate) followed by reduction by NaBH4, to produce metal nanoparticles supported on KCC-1. TEM analysis of these catalyst revealed that the fibers of KCC-1 were fully loaded with well-dispersed Rh, Ru, and Pd B
DOI: 10.1021/acssuschemeng.5b00812 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 2. TEM images and particle size distribution of KCC-1-PEI/Rh (a, b, c, and d), KCC-1-PEI/Ru (e, f, g, and h), and KCC-1-PEI/Pd (i, j, k, and l), respectively.
KCC-PEI used was same for synthesis of these catalysts. Covalent attachment of PEI with KCC-1 was confirmed by 29 Si CP-MAS NMR analysis of KCC-1-PEI. The NMR spectrum (Figure 3d) shows signals at −90.3, −97.8, and −106.8 ppm, characteristics of Q2, Q3, and Q4 sites of silica, respectively. The signals at −54.6 and −63.1 ppm are due to the T2 and T3 sites of silica respectively, which indicates the formation of silica−carbon bond via covalent attachment of PEI with KCC-1. On the basis of the above characterizations, we can clearly see the advantage of using this protocol to synthesize silica supported monodisperse metal nanoparticles. Because the use of expensive dendrimer is replaced by low-cost PEI, it makes the protocol simple and sustainable. Thus, combination of PEI and KCC-1 seems an excellent way to synthesize monodisperse metalnanocatalysts. To evaluate the catalytic activity and stability, these nanocatalysts, KCC-1-PEI/Rh, KCC-1-PEI/Ru, and KCC-1-PEI/Pd, were evaluated for hydrogenation reactions (Scheme 1), as test reactions. After optimization of various reaction parameters, such as reaction temperature, reaction time, and hydrogen pressure, the reaction profile of catalytic hydrogenation of phenyl acetylene and styrene, using the as-synthesized catalysts is shown in Figure 4. In blank reaction, no catalysis was observed when only KCC-1 and KCC-1-PEI were used as catalysts due to absence of any active metal sites. Using KCC-1-PEI/M, hydrogenation of
Table 1. Comparison of Textural Properties of Materials sample KCC-1 KCC-1PEI KCC-1PEI/Rh KCC-1PEI/Ru KCC-1PEI/Pd
BET surface area (m2/g)
pore sizes (nm)
BJH pore volume (cm3/g)
metal loading (%)
590 316
3.2 and 6.4 3.2
0.79 0.54
0.0 0.0
339
3.2
0.45
6.1
319
3.2
0.44
7.0
316
3.2
0.43
11.0
Although there was a reduction in surface area, but it was not as drastic as found when conventional porous silica is used as a support.1 This minimum reduction in surface area is due to fibrous nature of KCC-1, which cannot be clogged completely even after PEI infiltration in the channels due to its open structure. To estimate the percentage loading of PEI on KCC-1, thermogravimetric analysis (TGA) was carried out in air. After the initial loss of around 5%, KCC-1-PEI was stable up to 200 °C, after which the weight loss of about 19% was observed up to 650 °C, which can be attributed to the loss of covalently bonded PEI (Figure 3c). For all the catalysts, the weight loss was approximately consistent, 17, 19, and 18% for KCC-1-PEI/Rh, KCC-1-PEI/Ru, and KCC-1-PEI/Pd respectively, because the C
DOI: 10.1021/acssuschemeng.5b00812 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 3. (a) Nitrogen adsorption−desorption isotherm, (b) pore size distribution, (c) thermogravimetric profile of as-synthesized nanocatalysts, and (d) 29Si CP-MAS NMR spectrum of KCC-1-PEI.
(Figure 4b). This may be due to the more reactivity of Pd catalysts for hydrogenation of styrene to ethylbenzene. Similarly, for the hydrogenation of styrene, KCC-1-PEI/Rh showed the highest TON value of 225 as compared to KCC-1PEI/Pd, which showed a TON value of 122 (Figure 4c). Interestingly, KCC-1-PEI/Ru showed moderate catalytic activity for this reaction, with a TON value of 29. All the catalysts showed 100% selectivity, with no other biproduct formed. Notably, the highest conversion was achieved in less than 10 min, indicating faster kinetics. The difference in activity could be due to the difference in the adsorption coefficient of these metal atoms. Good catalytic activity of some of these catalysts indicates that even after PEI infiltration, active metal sites are accessible for the reactants, which was due to fibrous structure of KCC-1, which did not clogged even after PEI loading. To check whether PEI functionalization is really limiting the leaching as hypothesized, we have conducted recyclability studies of best catalysts, KCC-1-PEI/Rh (for hydrogenation of phenyl acetylene) and KCC-1-PEI/Ru (for hydrogenation of styrene). We observed that both of them can be recycled for at least five times (Figure 5), with nearly same catalytic activity, indicating their good stability which may be due to PEI functionalization as well as fibrous nature of KCC-1 support. Metal concentrations before (Rh-6.1 and Ru-7.0) and after the reactions (Rh-5.5 and Ru-6.6) as determined by energy-dispersive X-ray spectroscopy indicated negligible metal leaching during the reactions. TEM images of these catalysts after five cycles (Figure S2) indicated no agglomeration of nanoparticles.
Scheme 1. Hydrogenation of Phenylacetylene and Styrene
phenylacetylene and styrene was achieved at room temperature with only 15 and 100 psi hydrogen pressure respectively, with moderate turnover number (TON). All the catalysts showed different activity for same reaction. KCC-1-PEI/Rh showed highest TON value of 83 for phenylacetylene hydrogenation as compared to KCC-1-PEI/Pd and KCC-1-PEI/Ru, which showed TON values of 61 and 21, respectively (Figure 4a). The initial rate of the reaction although was faster for KCC-1-PEI/Pd than for the other two catalysts. The reason for the difference in catalytic activity of these metal catalysts, even after having their particle size nearly in same range, may be due to the difference in density of states (DOS) of their dorbital, hence the difference in adsorption coefficient of these metals for the reactants.33 Another possibility could be the passivation of these metals by PEI molecules, which may be of different strength for each metals as these metals have different adsorption coefficient. In terms of selectivity of the reaction, KCC-1-PEI/Rh and Ru showed around 80% selectivity for styrene and 20% for ethylbenzene (Figure 4b) and remain unchanged during the entire reaction time. However, KCC-1-PEI/Pd initially showed 15% selectivity for styrene and 85% for ethylbenzene (EtPh), which with time, changed dramatically to 100% for ethylbenzene
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CONCLUSIONS We have developed a simple and efficient protocol for synthesis of highly monodispersed metal nanoparticles supported on D
DOI: 10.1021/acssuschemeng.5b00812 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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salts, which on reduction by NaBH4 yielded highly dispersed metal nanoparticles with narrow particle size distribution. This protocol accomplished synthesis of nanocatalysts without the use of expensive and complex dendrimer. Moreover, the use of PEI over dendrimers made this protocol economical. These nanocatalysts also showed good catalytic activity because of their smaller sizes as well as higher accessibility due to fibrous nature of the support (KCC-1). The unique combination of PEI and KCC1, restricted the metal leaching, which in turn, increased the stability of these catalysts. In summary, we were able to develop a simple and sustainable protocol to prepare Rh, Ru, and Pd metal nanocatalysts supported on KCC-1, with good activity, high reaction rates, stability, and recyclability.
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EXPERIMENTAL SECTION
Preparation of KCC-1. KCC-1 was prepared using a microwave assisted hydrothermal technique.25 In a typical synthesis, cetyltrimethylammonium bromide (CTAB, 6 g, 0.016 mol) and urea (7.2 g, 0.12 mol) were stirred (1500 rpm) in water (300 mL) for 15 min. Tetraethyl orthosilicate (TEOS, 30 mL, 0.14 mol) was placed in cyclohexane (300 mL) and added dropwise to the above-mentioned solution. The reaction mixture was further stirred for 15 min, followed by dropwise addition of 1-pentanol (18 mL, 0.2 mol). The resulting solution was stirred for 20 min at room temperature and then transferred to a Teflon-sealed 1 L microwave (MW) reactor. The reaction mixture was exposed to MW irradiation (800 W power) at 120 °C for 1 h under moderate stirring. After completion of the reaction, the mixture was allowed to cool to room temperature and the solid product formed was isolated by centrifugation, washed several times with water and ethanol, and dried at 60 °C for 12 h. The as-synthesized material was then calcined at 550 °C for 6 h in air to yield calcined fibrous nanosilica (KCC-1). Preparation of KCC-1-PEI. KCC-1 (5 g) was degassed at 120 °C for 12 h under vacuum and then cooled to 60 °C in the presence of argon. (3-Glycidyloxypropyl)trimethoxysilane (GTMS, 5.5 g) dissolved in hot methanol (30 mL) was then added, and resultant reaction mixture was stirred for 1.5 h. PEI (polyethylenimine, ethylenediamine branched average Mw ∼ 800, 4.5 g) in methanol (30 mL, heated at 60 °C) was then added to this reaction mixture, and the solution was further stirred for 5 h at 60 °C.32 The material was then isolated by centrifugation, washed several times with hot methanol, and dried overnight at 80 °C under vacuum to yield KCC-1-PEI. Preparation of KCC-1-PEI/M Catalysts (M = Rh, Ru, Pd). KCC1-PEI (500 mg) was dispersed in distilled water (50 mL), and the mixture was sonicated for 5 min followed by 10 min of vigorous stirring at room temperature. To this reaction mixture were added dropwise metal salts, i.e., sodium tetrachloropalladate(II) (142 mg, 0.483 mmol) or rhodium(III) chloride hydrate (102 mg, 0.487 mmol) or ruthenium(III) chloride (103 mg, 0.496 mmol) dissolved in water (10 mL), and then the mixture was sonicated further for 30 min, followed by stirring for 2 h at room temperature. Freshly prepared NaBH4 solution (5 mL, 1 M in water) was added dropwise to this solution, and the mixture was stirred for 2 h. The resulting material was then isolated by centrifugation, thoroughly washed with water and ethanol, and dried under vacuum at 80 °C for 16 h. These catalysts were named KCC-1-PEI/Pd, KCC-1PEI/Rh, and KCC-1-PEI/Ru. Characterization. Transmission electron microscopy (TEM) analysis was performed on an FEI TITAN operated at an accelerating voltage of 300 kV. For sample preparation, powders were dispersed in ethanol with assistance of sonication and a drop of solution was dropped on carbon coated TEM grid of 200 mesh. Thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA/DSC2 Star instrument. X-ray diffraction patterns were recorded using a Panalytical X’Pert Pro powder X-ray diffractometer using Cu Kα radiation. The surface area was obtained using the Brunauer−Emmett−Teller (BET) theory from N2 physisorption data, recorded using Micromeritics Flex3 analyzer. About 100 mg of each sample was degassed at 120 °C for 12 h prior to N2 sorption analysis.
Figure 4. Hydrogenation of (a) phenylacetylene (TON), (b) phenylacetylene (Selectivity), and (c) styrene (TON).
Figure 5. Recycling study of (a) KCC-1-PEI-Ru for hydrogenation of styrene and (b) KCC-1-PEI-Rh for hydrogenation of phenyl acetylene.
fibrous nanosilica (KCC-1). Synthesis involved simple covalent functionalization of KCC-1 by PEI, followed by loading of metal E
DOI: 10.1021/acssuschemeng.5b00812 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering Catalytic Hydrogenation Reactions. The catalyst (15 mg) was first placed in a 50 mL glass reactor and charged with a 1:1 ratio of methanol and 1,4-dioxane (20 mL). The reactor was then flushed with hydrogen gas five times, pressurized by 15 psi of hydrogen gas, and stirred for 1 h at room temperature. After that, phenylacetylene (102 mg, 1 mmol) was added and the reactor was again flushed with hydrogen gas for five times. The reactor was then pressurized by 15 psi of hydrogen gas, and the reaction was carried out at room temperature. To monitor the progress of the reaction, sampling was done at regular intervals and conversion was analyzed by gas chromatography−mass spectrometry (GC−MS). The calculation of TON was based on the total metal loading on the KCC-1 as calculated by energy-dispersive X-ray spectroscopy (EDX). For the hydrogenation of styrene, a similar procedure was followed with the following changes: styrene (2 mmol); solvent, toluene (20 mL); hydrogen pressure, 100 psi.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00812. PXRD pattern of KCC-1-PEI based metal nanoparticles and TEM images of recycled catalysts (PDF).
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AUTHOR INFORMATION
Corresponding Author
*V. Polshettiwar. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Department of Atomic Energy (DAE), Government of India, for funding.
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DEDICATION Dedicated to Prof. Rajender Varma for his pioneering contribution to the field of green chemistry and sustainable nanotechnology.
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REFERENCES
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