Simple, green approach for the synthesis of solid support‐embedded PdNPs for ligand exchange

Abstract

Green approaches have the potential to significantly reduce the costs and environmental impact of chemical syntheses. Here, the authors used green tea (GT) leaf extract to synthesise and anchor palladium nanoparticles (PdNPs) to silica. The synthesised PdNPs in GT extract were characterised by ultraviolet–visible spectroscopy, Fourier‐transform infrared spectroscopy, X‐ray diffraction, and transmission electron microscopy. PdNPs primarily formed as capped NPs dispersed in GT extract before reduction completed after 24 h. This capped phytochemical solution was employed as a green precursor solution to synthesise PdNP‐embedded solid supports. The morphology of PdNPs anchored to silica differed to that of PdNPs in solution. Silica‐embedded PdNPs was employed as a new ligand exchanger to isolate trace polycyclic aromatic sulphur heterocycles from a hydrocarbon matrix. The isolation efficiency of the new, greener ligand exchanger was the same as an efficient chemical ligand exchanger and may, therefore, hold promise for future applications.

1 Introduction

Green syntheses of metal nanoparticles (NPs) are growing as a simple, inexpensive, environmentally safe, and industry‐friendly techniques [1, 2, 3, 4]. As a result, green NP synthesis has replaced more expensive and physicochemical methods [5, 6, 7]. Most green biosyntheses are based on plant extracts and microorganisms [8], with polyphenols in plant extracts representing the main, environmentally benign active ingredients responsible for capping, reducing, and stabilising metal NPs [2, 3, 9]. The most common noble metals used in such syntheses are silver (Ag), platinum, gold (Au), and palladium (Pd) [10]. The simplicity of green NP biosynthesis, which can take as little as minutes [11] and result in specific NP morphology based on plant type, has a further fuelled interest [12]. NPs are important products with a wide variety of applications including in nanoscale sensors [13], catalysts [14], medical diagnostics [15], and other applications [16], and indeed green tea (GT) extracts rich in polyphenols have successfully been used to synthesise Pd and AgNPs [17, 18].

NPs have traditionally been synthesised using different chemical methods and embedded in solid supports [19]. Many strategies have been used to control the size and distribution of NPs on solid supports including ion exchange, impregnation, deposition reduction, and deposition precipitation [19]. Various experimental protocols rely on different active site formulations, porous solid support structures, and precursor solution characteristics to achieve the desired NPs. In catalysis chemistry, neutral or ionic Pd complexes either in an inert or other solvents, respectively, have been used as precursors [20] such as acetylacetonate complexes in benzene or toluene, allylic complexes in pentane, colloidal PdO particles, or ionic Pd complexes. These complexes strongly interact with the active sites of solid support, mainly through electrostatic interactions.

Here, we exploit the deposition reduction mechanism in a green synthesis [21, 22], given that biosynthesised NPs have been shown to be useful as catalysts, particularly in the Suzuki–Miyaura coupling reaction [23]. NPs anchored on different solid materials have been fabricated and applied as heterogeneous catalysts to separate catalysts from reaction mixtures. There is, therefore, growing interest in different experimental protocols to fabricate NPs embedded in solid supports. Green synthesised stabilised Au and AgNPs have been used in catalytic degradation of 4‐nitrophenols [24, 25, 26, 27].

Petroleum fractions are supercomplex mixtures that contain traces of heteroatom‐containing compounds (HACCs) such as sulphur, nitrogen, oxygen, and metals. HACCs have been employed as revisable capping and stabilising agents for NPs in different applications [4, 15, 19]. Accordingly, green synthesised NPs embedded in solid supports could be used to replace chemical ligand exchangers to isolate these HACCs from hydrocarbon matrices [28, 29]. Many studies have examined the isolation of polycyclic aromatic sulphur heterocycles (S‐PAHs) by chemical ligand exchangers [30], the most common ligand exchanger being Pd(II) ions chemically bonded to silica surfaces via 3‐mercaptopropanotrimethoxysilane [29, 30]. While chemical methods are relatively expensive and can require harsh reaction conditions, NP ligand exchange improves isolation and overcomes the disadvantages of irreversibility, degradation, and adsorption of bulk metals.

Here, we propose a new, greener approach for the synthesis of solid matrix‐embedded PdNPs based on a deposition reduction mechanism, namely the green synthesis of PdNPs anchored to silica using GT leaf extracts as a Pd‐precursor solution. We also compared the characteristics of the synthesised NPs using different analytical techniques in solution and on solid surfaces. These new, greener ligand exchangers have comparable efficiency to chemical ligand exchangers so may, therefore, be viable inexpensive replacements.

2 Materials and methods

2.1 Raw materials and reagents

Pd chloride (PdCl₂, 99%) and silica gel 60–80 mesh were obtained from Sigma‐Aldrich (St. Louis, MO).

2.2 Petroleum samples

Oil samples were kindly provided by the Egyptian Petroleum Co.

2.3 Green syntheses of PdNPs

About 1 g of GT leaves thoroughly washed and finely cut were boiled in 100 ml of water in a 100 ml elementary flask. After boiling, the solution was decanted and filtered. Then, 20 mg PdCl₂ was added to 20 ml GT extract with vigorous stirring at 80°C for 20 min. The reaction mixture was allowed to settle at room temperature.

2.4 Green syntheses of silica‐embedded PdNPs

About 10 ml of freshly prepared Pd capping solution was added to 3 g silica in a centrifuge tube before being centrifuged at 5000 rpm for 5 min to deposit phytochemical‐capped metal on the silica surface. The obtained silica‐embedded NPs were left for 24 h for complete Pd(II) ion reduction and then treated with n ‐hexane to remove water before being activated at 50°C. About 3 g of silica‐embedded PdNPs was packed into a glass column measuring 20 cm × 0.8 cm and washed several times with the investigated eluents.

2.5 Chemical synthesis of Pd(II)‐mercaptopropano silica gel

Pd(II)‐mercaptopropano silica gel was synthesised as in [25].

2.6 Column chromatography

The procedure is described in detail in [25]. Briefly, 20 μl petroleum oil was applied to the surface of treated silica and then separated with the following eluents: a sulphur‐free fraction eluted with 40 ml cyclohexane: dichloromethane (9:1) and S‐PAHs eluted with 40 ml cyclohexane:dichloromethane (2:1) containing 1% isopropanol. Finally, the column was washed with 50 ml cyclohexane:dichloromethane (9:1) for regeneration.

2.7 Characterisation of synthesised NPs

A Shimadzu double beam ultraviolet–visible (UV–vis) spectrometer (Shimadzu Corporation Tokyo, Japan) was used to detect the local surface plasmon resonance (SPR) of the PdNPs and to confirm formation in the range 200–800 nm. The functional groups of extracted and protected biomolecules were recorded by Fourier‐transform infrared spectroscopy (FTIR). The shapes, sizes, and size distributions were determined with a JEOL‐1200 EX II transmission electron microscope (TEM, Jeol Ltd. Tokyo, Japan) operating at 120 kV. TEM grids were prepared by placing a drop of the particle solution on a carbon‐coated copper grid and drying at room temperature.

2.8 Gas chromatography–mass spectrometry

Gas chromatography–mass spectrometry (GC–MS) analyses were carried out on an Agilent GC1890B‐MSD5977A apparatus (Agilent Technologies, Santa Clara, CA). The conditions were an initial temperature of 60°C for 1 min, ramp at 5°C min⁻¹ to 300°C, hold for 20 min. The capillary column was an HP‐ms5 (30 m × 250 µm × 0.25 µm).

3 Results and discussion

GT extract contains powerful reducing phytochemicals (catechins; vitamins C and E) [4, 14]. The initial evidence for the formation of PdNPs in solution was the visual observation of a colour change from yellow to a darker colour on the addition of metal salts to plant extracts. Fig. 1 illustrates the UV–vis spectra of biosynthesised PdNPs and GT crude leaf extract. As reported previously, the absorption band for Pd(II) ions was at 415 nm [16], and the absence of this band indicated the reduction of Pd(II) ions into PdNPs by the polyphenols in the GT leaf extract. The strong absorption peak in the UV–vis spectrum was ascribed to these polyphenols.

Fig. 1.

UV–vis absorption spectra of GT crude leaves extract, mother liquor, and PdNPs deposit

By contrast, GT ‘mother’ liquor (after PdNP deposition) was characterised by a dampened absorption intensity compared with the crude extract, indicating the deposition of extracted polyphenols on PdNPs. Here, PdNP deposition was considered the main difference between GT and other colloidal NP solutions. Each NP shape had a specific SPR and, accordingly, different UV bands corresponding to different NP shapes. The symmetrical absorption peak at 402 nm for a shaken sample indicated spherical PdNP formation according to a single SPR [31].

Fig. 2 illustrates the IR spectra of GT extracts and PdNP deposits. For aqueous extracts, the absorption peak at ∼3325 cm⁻¹ was attributed to the presence of hydroxyl groups indicating the existence of polyols, while the peak at ∼1680 cm⁻¹ was ascribed to the existence of C=C stretching, indicating the presence of aromatic moieties. Fig. 2 a illustrates the IR spectrum of PdNP deposits without purification. Spectra of crude leaf extracts and PdNP deposits were similar, indicating the deposition of polyphenol‐protected PdNPs. Furthermore, the change in peak intensities C–H stretching at ∼2988–2901 cm⁻¹, scissoring at ∼1393–1373 cm⁻¹, rock at ∼1370 cm⁻¹, and overlapping =C–H bending and C–O stretching vibrations at ∼1066–1056 cm⁻¹ mode of vibrations indicated the Pd(II) ions capping functional groups. The increase in the intensities of different C–H vibration modes along with aromatic moieties at 892–562 cm⁻¹ indicated a decrease in electron densities shielding such types of vibrations. This decrease in electron density corresponds to the consumed electron density in reduction and capping of Pd(II) ions to PdNPs.

Fig. 2.

FTIR spectra of

(a) GT crude leaves extract, (b) PdNPs deposit

Generally, and according to electrochemical studies, GT polyphenols are characterised by two types of hydroxyl groups. Hydroxyl groups donate electrons to Pd(II) ions more easily than others during reduction according to biomolecule conformation [18]. Generally, the formed NPs were partially or completely suspended and not completely deposited in the investigated extract. The complete deposition of PdNPs in GT extracts in addition to their unbending behaviour to L‐lysine functional groups indicated natural polyphenol‐protected PdNPs. Furthermore, GT‐PdNPs were stable, and the polyphenol UV absorption band was eliminated after PdNP formation. The size and crystalline structure of the formed PdNPs were next determined by X‐ray diffraction (XRD). Fig. 3 shows XRD patterns of PdNPs and reveals the presence of three distinctive sharp peaks corresponding to (111), (200), and (220) planes Bragg reflection sat 2θ  = 40°, 46.88°, and 68.2°, which are the characteristic peaks of face‐centred cubic (FCC) crystalline Pd (Ref. JCPDS file: 46‐1043) [18]. Unknown sharp reflections at 28˚ possibly can be rendered to some protected molecules of the plant; this confirmed the average size of PdNPs as calculated by the Scherrer equation [32]: 21.86 nm for PdNPs, less than the value calculated by TEM and [4, 32].

Fig. 3.

XRD patterns of PdNPs

Chemical methods for the synthesis of NPs embedded in solid supports have been based on different precursor solutions and solid supports [21]. In green chemistry, capped NPs can be used as the Pd‐precursor solution. The Pd‐precursor solution deposited on the silica surface in the form of Pd‐capped polyphenols, and the bioreduction continued on the silica surface. Finally, the PdNP‐embedded silica surface was the product. Fig. 4 shows a TEM image of PdNP‐embedded silica, which clearly demonstrates the formation of PdNPs of different shapes anchored to the silica and of average size of 40.25 nm (range 18.53–61.44 nm). PdNPs were well‐distributed and deposited. Furthermore, Fig. 5 shows a TEM image of spherical PdNPs in solution, with an average size of 54.1 nm (range 32.5–75.6 nm). These images clearly illustrate a change in PdNP morphology on deposition on the silica surface, and the image of PdNPs in solution reveals a colourless collective matrix which may correspond to polyol capsule‐protected NPs. These capsules hosted and capped Pd(II) ions deposited on the silica surface in this precursor form, and confirmed that Pd is deposited on the silica surface as capped polyols with reduction occurring on the silica surface. Furthermore, deposition on the silica surface did not prevent the continuation of reduction.

Fig. 4.

TEM images, histogram, and particle diameter distribution of green synthesised PdNPs embedded silica surface

Fig. 5.

TEM images, histogram, and particle diameter distribution of green synthesised PdNPs

The elements containing biosynthesised PdNPs embedded silica were detected by energy‐dispersive X‐ray spectroscopy (EDX) (Fig. 6 a). EDX spectrum confirms the presence of metallic Pd (0.30 mass%), also evident of the successful formation of PdNPs. O, Cl, Ca, and Na elements have also existed, which can be rendered to the deposited phytochemicals. Fig. 6 b illustrates electron scanning micrograph scanning electron microscopy (SEM) of PdNPs embedded silica. Although the Pd particles are clearly visible distributed on the smooth surface of silica granules, its shapes can be hardly definite compared with TEM images. The change of white colour of silica granules to grey may be due to the swelling due to the interaction of the PdNPs with support material.

Fig. 6.

Elements containing biosynthesised PdNPs embedded silica were detected by EDX

(a) EDX spectrum, (b) SEM micrograph (20 μm) of PdNPs embedded silica

Fig. 7 illustrates IR spectra of PdNPs anchored on silica surface and parent silica, comparatively. Silica surface has been investigated previously by IR. Generally, silica surface composed of concentrated free hydroxyls which bonded to Si atoms. The band at about 3306 cm^−l has been assigned to these hydroxyls (Fig. 7 a). The broadness of the band and ill‐defined maximum due to hydrogen bonds of different strengths. The IR bands in low‐frequency region 760–1000 cm⁻¹ due to bulk Si–O groups’ different modes of vibrations. Comparative investigation of Figs. 7 a and b demonstrated the absence of a broadband due to hydroxyls groups, which indicates that PdNPs deposition occurred on the surface and in the pores.

Fig. 7.

FTIR spectra of

(a) Silica surface, (b) PdNPs embedded silica

Many applications are based on the protection of NPs by different functional groups [4, 15, 17], with thiol the most common capping group. The main advantage of thiol capping for PdNPs is reversible Pd–S bonding interactions. The isolation of trace S‐PAHs from a hydrocarbon matrix is challenging. Chemically synthesised capped Pd(II)NPs represent the most common ligand exchanger for S‐PAH isolation [29]. In our paper, a green synthesised PdNP ligand exchanger was used to isolate trace S‐PAHs. Fig. 8 compares isolations using the commonly used chemical Pd(II)NP ligand exchanger [29, 33] and our green synthesised PdNPs for C1‐benzothiophenes (BT) and C2‐ and C3‐dibenzothiophenes (DBT) from petroleum oil. Both ligand exchangers had the same isolation efficiency according to the traces or the complete absence of hydrocarbons. Fig. 9 illustrates GC–MS spectra of the targeted S‐PAHs, which shows that interactions between S‐PAHs and green PdNP assemblies or chemical Pd(II)NPs are similar [33]. Biosynthesis of PdNPs is accompanied by a visual change in colour to black, whereas the chemically prepared Pd(II)NPs retain the red‐brownish PdCl₂ salt colour, indicating that the chemical synthesis is thiol‐Pd capping dispersion without reduction.

Fig. 8.

Comparative isolations of green (lower trace) and chemically [25] (upper trace) synthesised PdNP ligand exchange of C2‐BT, C1‐DBT, and C2‐DBT isomers from petroleum volatile oil

Fig. 9.

GC–MS spectra of

(a) C1, (b) C3‐dibenzothiophenes, (c) C1‐benzothiophenes isolated by green synthesised PdNPs

4 Conclusions

Here, we synthesised Pd using GT extracts. For the first time, PdNP deposition reduction in GT extract was observed and employed to prepare a green precursor solution from Pd‐capped polyphenols. This green Pd‐precursor solution was used to fabricate PdNPs embedded in solid silica support. Embedded PdNPs serving as ligand exchangers had the same isolation efficiency as the most commonly used chemical ligand exchangers. Green embedded solid supports will be of considerable interest and use in many practical applications.