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Catalysts based on nickel salt heteropolytungstates for selective oxidation of diphenyl sulfide

  • Mercedes Muñoz EMAIL logo , Matthieu Greber , Karima Ben Tayeb , Carole Lamonier , Carmen I. Cabello and Gustavo P. Romanelli EMAIL logo
Published/Copyright: July 6, 2023
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Green Processing and Synthesis
From the journal Green Processing and Synthesis Volume 12 Issue 1

Abstract

Nickel salts of Keggin heteropolytungstates with the general formula Ni x A y W12−y O39or40 (A = Si/P) were synthesized and studied as bulk catalytic materials or supported ones by deposition on modified and functionalized clay minerals (pillared layered clay and porous clay heterostructure). Characterizations by Raman, 31P and 29Si-NMR, and ESEM-EDS techniques showed that pure and supported systems preserved the Ni/W ratio and the expected structural properties of heteropolyanions. These materials were evaluated as catalysts in the selective oxidation of sulfides to sulfoxides or sulfones, using aqueous hydrogen peroxide and mild reaction conditions. The bulk materials, with a higher content of Ni, displayed a remarkable catalytic behavior in the oxidation of diphenyl sulfide (Ni3PW11NiO40H, 90% conversion in 15 min at 75°C, 100% sulfone selectivity in 3 h). Supported catalysts, particularly the non-functionalized PCH (Ni2SW12O40/PCH), showed excellent activity, with also being selective in the oxidation of sulfide to sulfoxide (87% conversion, 88.9% sulfoxide selectivity). The reuse of these materials was studied in the optimum reaction conditions, resulting in similar activity and selectivity.

Graphical abstract

Selective oxidation reaction of DPS to DPSO2 with hydrogen peroxide as oxidant in presence of nickel salts of Keggin heteropolytungstates as catalysts

Keywords: phosphoro/silicoheteropolyanions; Ni(ii) Keggin heteropolytungstates; modified clays; clean oxidation; diphenyl sulfide

1 Introduction

Particularly over the past few years, ongoing and strict amendments to environmental regulations have been directed for reducing the sulfide content in petroleum products [1,2,3]. The hydrodesulfurization (HDS) process generally used to remove sulfur is a high-cost process that involves high temperatures and pressures, larger volume reactors, and the use of very active catalysts [4,5,6]. Regarding the treatment of petroleum products, studies on oxidative desulfurization (ODS), a strategy proposed as an alternative or complementary to the HDS, have been reported. The oxidation of aromatic sulfides could be an interesting method to comply with the forthcoming regulations to obtain “0” sulfur fuels [7]. Moreover, another major problem facing the chemical industry is the enormous amount of waste that contains organic sulfur compounds, so the oxidation of sulfides has recently attracted enormous interest [8].

On the other hand, the selective oxidation of aromatic compounds is also important for the pharmaceutical industry to obtain commodity chemical products such as sulfone and/or sulfoxide substances, which are easily removable by conventional separation methods. Both sulfoxides and sulfones are important intermediate compounds in the pharmaceutical industry, food industry, etc. They are used as additives in crop protection, veterinary drug, and industrial lubricants [9,10,11,12,13,14]. For example, sulfone derivatives have been used as precursor in the synthesis of algaecide, bactericide, and fungicide; in the treatment of diabetes, leprosy, and malaria; and in dermatology or precursors of antibiotics [9,10,11,12,13,14,15].

Environmentally safe methods need to be developed for selective oxidation or sustainable oxidation of sulfides, especially with mild procedures [15].

Peroxides are oxidants where oxygen is easily accessible and produce nonpolluting residues, and they are easy to store and are inexpensive. Processes including oxygen donors such as H2O2, which allows the performance of reactions in aqueous solution (in batch) or tert-butyl hydroperoxide (t-BuOOH), have recently been reported to improve environmental efficiency [7,10,16,17,18]. However, hydrogen peroxide alone is a relatively weak electrophile, and catalytic activation is additionally required [9].

The reaction efficiency increases notably in the presence of a catalyst. Homogeneous catalysts were commonly used in the ODS process due to the high efficiency of the one-phase catalytic system. The main types of homogeneous catalysts reported in the literature are carboxylic acids (formic and acetic acids) as well as homogeneous metallic systems such as polyoxometallates (POMs). The POM catalysts used in ODS process are salts of transition metals (V, W, Mo) that form peroxo complexes, improving catalytic performances, but sometimes POMs have heavy metals, such as Cr and Mn salts, which are environmentally unfriendly and generate a massive amount of harmful waste products [19]. Oxo–peroxo complexes of tungsten(vi) ion with a closed-shell electronic structure and d0 configuration are important for their utilization as catalysts in the oxidation of phenols, and sulfides, as well as in the epoxidation of olefins. Generally, the oxo–peroxo complexes of transition metals have attracted special attention due to their importance in different fields [7,8,14]. By comparing the efficiency of heteropolyanions in the catalytic system, it was found that tungsten-based heteropolyanions were more active than those based on molybdenum, and another advantage of tungsten compounds over other heavy metals is their low toxicity [8,9]. The main disadvantage of POMs as catalysts is their high solubility in polar solvents, so they are difficult to be recovered and reused. This problem was overcome by their immobilization in appropriate porous materials with mean pore diameter and specific surface area [13,14].

In recent years, for oxidation reactions, attention has turned to heterogeneous catalytic systems. One of the advantages of heterogeneous systems is related to the easy product and catalyst separation, high productivity, and eco-compatibility.

In heterogeneous catalysts, the use of transition metals (molybdenum, vanadium, and tungsten) as active phases dispersed on various supports having acid properties has been reported. Various supports have been considered, including alumina, silica, and zeolites. There is also growing interest in molecular sieves such as mesoporous silicates, as well as activated carbon [18,19].

It has recently been observed that the incorporation of vanadium in the structure of Keggin-type silicopolyoxotungsto compounds improves their catalytic activity in sulfide oxidation due to the redox and acid properties of vanadium [14].

In recent work about WO x /ZrO2 and W-bronze catalytic systems, high conversion of diphenyl sulfide (DPS) and selectivity to diphenyl sulfone (DPSO2) were obtained. These studies suggested that the presence of sites with different acid and redox characteristics gives these species a bifunctional character that favor the formation of a peroxo tungstate intermediate and the subsequent nucleophilic attack of the sulfur atom in the sulfide by peroxo species. In the same way, the existence of W intermediate peroxo species and their efficiency in this type of reaction has been proven [7,10].

Previously, a series of NiW-based heteropolyanions supported on amorphous silica-alumina has been studied as sulfide precursors and has proven to be efficient in “Hydrocracking” reactions [20,21]. Several tungstic, silico-tungstic, and phospho-tungstic heteropolyanions, substituted or not by Ni(ii), such as Ni3/2PW12O40, Ni2SiW12O40, Ni3H2W12O40, Ni3PW11NiO40H, Ni4SiW11O39 (Ni x A y W12−y O39or40 (A = Si or P)), were selected to be used in ODS reactions.

Potassium salts of lacunar Keggin P/Si[W11O39] heteropolyanions were also used for comparison purposes.

Moreover, research on the utilization of clay minerals has a long history. As reported by Mao et al., although clay minerals are natural catalyst materials, many disadvantages, such as poor thermal stability and wide pore-size distributions, limit their applications. To bridge the gap between the properties of clays and the need for catalysts in the industry, materials based on modified clay minerals are necessary. Clays and their derivatives have become indispensable materials in many fields, such as high-technology areas, and industrial and agricultural production [22,23,24,25].

The layered alumino-silicates with swelling ability, such as bentonite, always contain negative charges on their layers because of the isomorphous substitution of the central atoms in the octahedral/tetrahedral sheet by cations of lower valence. These negative charges must be compensated by inorganic cations (e.g., Na(i) and Ca(ii)). These inorganic cations are exchangeable and can be replaced with polycations and organic cations, and the resulting materials are denoted as pillared layered clays (PILCs) and organoclays, respectively. To improve thermal stability, porous clay heterostructures (PCHs) were obtained by incorporation of organosilane cations to organoclays and thermal treatment. The PCHs obtained are microporous materials of high specific area. The PILCs show affinity toward hydrophilic inorganic compounds, and the organoclays toward hydrophobic organic compounds [22,26,27]. Natural and modified clays find extensive applications in organic synthesis. These materials, unlike other conventional catalysts, enjoy considerable advantages such as ease of handling, recyclability, low cost, and easier modulation of acidity levels by suitable exchange of cations, thus contributing to the main subject of “green chemistry” [22,23,28,29,30,31]. The easy work-up procedure and the reproducibility of the PCH synthesis make this system attractive for its potential application to large-scale operations.

In the present work, natural alumino-silicate bentonite type clay, chemically modified to be functionalized, was selected as support for the heteropolytungstate. A series of bulk NiW-based and clay-supported catalysts was studied in the ODS of a diphenyl compound to compare the effect of the presence of P or Si heteroatom, nickel content, and the use of supported and unsupported systems.

2 Materials and methods

2.1 Preparation of bulk catalysts

The preparation and characterization of Keggin-type phases substituted with Ni, Ni3/2PW12O40, Ni2SiW12O40, Ni3PW11NiO40H, Ni4SiW11O39, and Ni3H2W12O40 have recently been described [32]. Starting from the well-known heteropolyacid (HPAs) H3PW12O40 and H4SiW12O40, barium salts are synthesized according to Eq. 1 [33,34].

(1) ( 3 H + , [ PW 12 O 40 ] 3 − ) + ( 1.5 Ba 2 + , 3 OH − ) → ( 1.5 Ba [ PW 12 O 40 ] 3 − ) + 3 H 2 O

To obtain the nickel salts, the procedure consisted of successive incorporations of NiSO4 solutions in stoichiometric amounts with respect to the corresponding solutions of the Keggin derivatives, under continuous stirring and at room temperature, according to Eq. 2 [32].

(2) ( 1.5 Ba 2 + , [ PW 12 O 40 ] 3 − ) + ( 1.5 Ni 2 + + SO 4 2 − ) → ( 1.5 Ni 2 + , [ PW 12 O 40 ] 3 − ) + 1.5 BaSO 4

The resulting solutions were then filtered to eliminate the BaSO4 precipitate. Then, the solvent was evaporated at room temperature to obtain the Ni(ii) salt.

The [SiW12O40]4− nickel salt was synthesized by the same procedure as that for [PW12O40]3− nickel salt. The formation of the lacunary species was due to the addition of 3.5 equivalent of barium hydroxide (instead of 1.5). Indeed, the pH of the solution increased to 3, and the most stable species was [PW11O39]7− [32].

2.2 Synthesis of supports based on PCHs

The preparation was carried out according to conventional methods [27]. The starting material was bentonite-type natural clay, extracted from an Argentine deposit that was enriched in the montmorillonite fraction by the Stokes’ method described previously [35,36]. This species is an aluminosilicate belonging to the group of smectites with a 2:1 laminar structure. Its structure is made up of two tetrahedral coordinated layers of (Si,Al)O4 intercalated with an octahedral layer of Al(O,OH)6 in which part of the Al is replaced by ions such as Mg or Fe. These 2:1 layers are negatively charged; in the clays, the neutrality is maintained by exchangeable cations such as Na(i) or Ca(ii) [35].

The synthesis of the heterostructure material was conducted in several stages:

  1. The natural material was first ion-exchanged to introduce sodium cations and then obtain sodium-based bentonite. For this, 2.00 g clay was suspended in 250 mL of NaCl solution (1 M) for 24 h, under stirring at room temperature. Then, the suspension was centrifuged, washed until a negative chloride reaction was obtained, and dried.

  2. A treatment of the sodium-based bentonite with cetyltrimethylammonium bromide (Sigma-Aldrich; 95% HDTMA-Br) was applied to generate a cationic bentonite denoted as B+. For this, 1.5 g Na-based bentonite was added to 150 mL of 0.5 mM solution of HDTMA-Br (NaB/HDTMA ratio = 1.5). The suspension was kept at 50°C under stirring for 24 h, then it was separated by centrifugation and washed until it reached pH = 7.

  3. A reaction of the B+ with Si precursor was performed. The B+ obtained was suspended in a mixture of dodecylamine (DDA) and tetraethyl orthosilicate (TEOS; both from Aldrich) in B+/DDA/TEOS (1.5 g B+/7.56 g DDA/69.5 mL of TEOS). The suspension was stirred for 4 h at room temperature. The solid was separated by centrifugation and dried in air.

  4. The material obtained in (3) was calcined at 550°C for 2 h to obtain the porous clay heterostructure material. The specific surface was determined using Micromeritics ASAP 2020 apparatus at the temperature of liquid nitrogen (−196°C) using a relative pressure range of 0.01–0.99. The specific surface area was estimated by the Brunauer–Emmett–Teller (BET) method. X-ray powder diffraction (XRD) patterns were collected using Philips PW-1732 equipment (Cu Kα radiation, Ni filter).

2.3 Functionalization of the surface area of PCHs (PCH-F)

A functionalization process of the PCH systems [24] was conducted to increase their adsorption capacity to become an efficient support for HPA adsorption. The first step was a pretreatment of the materials to promote the availability of silanol groups; to this end, PCH was suspended in water/ethanol (50:50) for 24 h at room temperature and dried at 80°C. In the functionalization stage, the pretreated PCH (1.00 g) was mixed with the surfactant 3-aminopropyltrimethoxysilane (F) in toluene as solvent at a rate according to the molecular weight of the surfactant and the SBET of the support (S BET PCH = 705 m2·g−1, PCH/F = 1 g/6.09 g·F).

The resulting suspension was stirred in a closed container at room temperature for 12 h and subsequently, the temperature was raised to 70°C for another 12 h. Finally, the suspension was filtered and washed with toluene and acetone. The solid was vacuum dried up to constant weight [24].

2.4 Preparation and characterization of catalysts supported on PCH and PCH-F

Supported catalysts were prepared by equilibrium impregnation in excess of pore volume, using 200 mg of PCH and PCH-F materials, and 2.4 mL of aqueous solutions of the phases containing W (100 mg·mL−1). To quantify the Ni on the PCH surface, the content of adsorbed Ni was calculated by mass balance from the initial and final content of Ni determined by atomic absorption spectroscopy (AAS) using Varian AA 240 equipment.

The solid (impregnated support) was dried in an oven at 70°C for 24 h and then it was used in an oxidation test. The catalyst Ni/W ratios were obtained by ESEM-EDS technique using a FEI Quanta 400 and EDAX Apollo 40 microscope, with ultra-thin energy dispersive window, which allows determination of light elements (Z > 5, from B). The analysis was conducted to corroborate textural properties and chemical composition.

2.5 Catalytic test

Catalytic tests were carried out using bulk and supported on PCH and PCH-F catalysts. The tests were based on the oxidation of DPS. The oxidation products potentially formed are diphenyl sulfoxide (DPSO) and DPSO2. The reaction was carried out in batch at 75°C, using 0.01 mmol of catalyst, 1 mmol (0.186 g) of DPS, 5 mL of acetonitrile, and 1 mL of H2O2 35% as oxidant. The catalytic assessment was followed by thin layer chromatography, analyzing samples at intervals of 15 min at the beginning of the analysis and 30 min in the final stage. Aliquots of 0.2 mL of the reaction mixture were also withdrawn at the same intervals. Each sample was extracted with dichloromethane/water (1 mL and 1 mL, respectively), and the organic phase was dried with anhydrous Na2SO4.

The organic phase was analyzed by gas chromatography (GC) on a Varian Start 3400cx chromatograph equipped with a Chrompack column CP-sil 5 CB (30 m) and FID detector. The GC conditions were as follows: initial temperature was 50°C, ramped up to 210°C (at a rate of 20°C·min−1 and temperature was constant for 1 min), and finally again ramped to 240°C (at a rate of 10°C·min−1 and temperature was constant for 5 min). With this method, the corresponding retention times were: 11.1, 13.9, and 14.5 min for DPS, DPSO, and DPSO2, respectively.

The reaction sample compositions were determined by area normalization method. From these values, the DPS conversion and selectivity values were obtained as a function of time.

3 Results and discussion

3.1 Catalyst characterization

The experimental Ni/W ratios determined by EDS in unsupported systems are reported in Table 1 and agree with the theoretical values based on the heteropolycompound stoichiometry.

Table 1

Data of theoretical and experimental Ni/W ratio for Ni x A y W12−y O40 (A = Si or P) HPA

HPA Ni/W (wt%/wt)
Theoretical EDS
Ni4SiW11O40 11.6 10.7
Ni3H2W12O40 8.0 7.6
Ni3PW11NiO40H 11.6 16.1
Ni3/2PW12O40 4.0 3.4
Ni2SiW12O40 5.3 5.1
H2O2/H2WO4/NiW3 8.0 7.5

Previous studies of Raman vibrational spectroscopy [32] in the systems Ni x A y W12−y O39or40 (A = Si or P) showed the preservation of the W–O terminal stretching modes at 1,011 and 998 cm−1 that are characteristic of the H3PW12O40 and H4SiW12O40 phases used as starting material [37]. Thus, Raman spectra of Ni(ii) salts (Figure 1) indicated that the Keggin structures [PW12O40]3− and [SiW12O40]4− were preserved after ionic exchange [32]. In Figure 1, Ni x A y W12−y O40 (A = Si or P) Raman spectra shows typical lines corresponding to W–Od stretching (terminal bond) at 1,010 cm−1, W–Ob (νsym and νasym) stretching between 1,000 and 960 cm−1, W–Ob–W bridges (890–850 cm−1), W–Oc–W bridges (800–760 cm−1), and lattice modes below 400 cm−1, as reported by Ben Tayeb et al. [32]. The Ni(ii) salt of [PW11NiO40H]6− did not exhibit the same line as that observed for Ni3/2PW12O40, Figure 1a and b (respectively), but both presented a characteristic line at 989 cm−1 corresponding to the W–Ot stretching mode of the HPA. In the preparation procedure of this system, Ba(OH)2 is added to raise the pH to pH < 3 where the Keggin anion is kinetically stable, and then a lacunary species is formed. The FT-IR spectroscopy (not presented in this work, see ref. [32]) confirmed the incorporation of Ni(ii) to the structure of the [PW11NiO40H]6− anion. This fact indicated that the HPA structure did not change during the ion exchange to form the Ni(ii) salt. A slight shift to lower frequencies can be observed when the H+ is substituted by Ni(ii). In the Ni4SiW11O39 and Ni3PW11NiO40H systems (Figure 1d and e), the most intense Raman line shows that the W–O stretching is affected by the Si/P substitution (lacunar system). The general trend is a broadening and a shift to lower frequencies of the W–O point lines due to a decrease in symmetry. This can be attributed to the P or Si polarizing power that produces an inductive effect, affecting the aforementioned type of W–O bonds [32].

Figure 1 Raman spectra of: (a) Ni3/2PW12O40, (b) Ni2SiW12O40, (c) Ni3H2W12O40, (d) Ni4SiW11O39, and (e) Ni3PW11NiO40H in aqueous solution.
Figure 1

Raman spectra of: (a) Ni3/2PW12O40, (b) Ni2SiW12O40, (c) Ni3H2W12O40, (d) Ni4SiW11O39, and (e) Ni3PW11NiO40H in aqueous solution.

31P-NMR (Figure 2a) spectra showed the signal with chemical shift at −15.4 ppm, in agreement with values already reported for the [PW12O40]3− Keggin entities [38]. 31P-NMR spectra (Figure 2b) for Ni3/2PW12O40 species showed two signals at −15.4 and −13.1 ppm corresponding to phosphorus in [PW12O40]3− and P2W18O62 6− species, respectively [38]. A low decomposition (5%) seems to occur during the ionic exchange to form nickel salts. However, the Keggin structure is mainly preserved in the nickel salt Ni3/2PW12O40. The 29Si signal for [SiW12O40]4− was observed at −85.05 ppm (Figure 3b). The broad peak at −110 ppm presents the glass signal of the NMR probe. 29Si-NMR analysis of the nickel salt of [SiW11O39]8− (Figure 3a) showed only one line at −85 ppm, clearly indicating that in the case of the Si based Keggin heteropolyanion, the paramagnetic nickel atom was not inserted in the HPA [39]. This chemical shift corresponds to Ni2SiW12O40 species (Figure 3c). In fact, a low decomposition of [SiW11O39]8− on [SiW12O40]4− in aqueous solution occurred, whereas the absence of signal for Ni4SiW11O39 lacunar species is due to the paramagnetic effect of Ni [21].

Figure 2 31P-NMR spectra of (a) H3PW12O40 and (b) Ni3/2PW12O40 in aqueous solution at 5 × 10−2 mol·L−1. * NiPW11Ni for comparative purposes.
Figure 2

31P-NMR spectra of (a) H3PW12O40 and (b) Ni3/2PW12O40 in aqueous solution at 5 × 10−2 mol·L−1. * NiPW11Ni for comparative purposes.

Figure 3 29Si-NMR spectra of Ni4SiW11O39 (a), H4SiW12O40 (b), and Ni2SiW12O40 (c) in aqueous solution at 5 × 10−2 mol·L−1.
Figure 3

29Si-NMR spectra of Ni4SiW11O39 (a), H4SiW12O40 (b), and Ni2SiW12O40 (c) in aqueous solution at 5 × 10−2 mol·L−1.

3.2 Support characterization

The XRD pattern of the selected bentonite corresponds mostly to sodium montmorillonite with the approximate formula: (Na,Ca) x (Al,Mg)2Si4O10(OH)2 4H2O (PDF 291498) [35]. The XRD patterns are shown in Figure 4a. For the raw clay, the d001 diffraction peak can be observed at 2θ  =  6.7°, which indicates that the cations located in the interlayer spacing are partially solvated [22]. In addition, the presence of narrower peaks also suggests the existence of impurities in the original bentonite. Thus, the presence of a peak located at around 2θ  =  7° is noteworthy, which is assigned for the existence of quartz, a phase that was eliminated during the Stokes purification process. In the PCH formation, the surfactant and TEOS treatments affect the total surface, leading to an increase in BET surface area and a decrease in crystallinity.

Figure 4 XRD patterns for (a) bentonite, (b) PCH, and (c) PCH-F materials.
Figure 4

XRD patterns for (a) bentonite, (b) PCH, and (c) PCH-F materials.

During the formation of PCH systems, the TEOS precursor is located between the clay basal planes due to an exchange reaction with the interlaminar cations Ca(II) and/or Na(I). After thermal treatment, precursor species were converted into “nanoscopic” oxide species that expand the distance between the alumina–silicate planes [35]. The diffraction peaks are less intense after the insertion of the pillars (Figure 4b), probably due to a partial delamination of the smectite, which causes a random displacement along the a and b axes leading to a house of cards structure [22].

In the process of PCH formation, the silica precursor (TEOS) generates an “in situ” polymerization with the surfactants intercalated between the clay films, and further treatment at 550°C removes the organic “templates” and high-surface area materials are generated, which are mainly microporous [35]. Thus, PCH materials of high surface area are obtained (Table 2), a noticeable increase in surface area occurred by organosilane incorporation. In previous studies of textural parameters, PCH porosity had an intermediate behavior between microporous materials, such as zeolites and pillared clays (pores <10 Å), and mesoporous materials (MCM-41, pores >20 Å). The IUPAC convention assigned a limit of 20 Å to micropore dimension [35].

Table 2

Textural parameters and EDS chemical data* of modified and functionalized clays

Type of clay S BET (m2·g−1) Pore volume (cm3·g−1) Si% Al% Si/Al
Bentonite 22.5 0.02 80.11 19.89 4.00
PCH 705 0.56 89.46 10.54 8.50
PCH-F 18 0.20 94.75 5.25 18.00

*Data calculated from a content of Si + Al = 100%.

The surface analysis of modified systems was carried out by ESEM-EDS microscopy (Table 2); the chemical treatment showed a Si/Al ratio variation. This ratio increased more than 2 times with the surfactant and TEOS treatment.

With the purpose of using PCH as support, the functionalization of these materials was conducted to achieve the surface adsorption of the heteropolyanions Ni x A y W12−y O40 (A = Si or P).

The surface Si–OH unities of the PCH materials was modified by functionalizing with 3-aminopropyltrimethoxysilane (F). The process follows the reaction of Scheme 1 [24].

Scheme 1 Representation of the silica-surface functionalization process.
Scheme 1

Representation of the silica-surface functionalization process.

The XRD analysis (Figure 4) revealed that the functionalization process did not generate visible structural modifications in the clay matrix [24]. However, the functionalized systems showed a reduction in the surface area compared with the starting material (from S BET = 705–18 m2·g−1), probably because the process did not eliminate the surface Si–OH groups completely. This provides a certain degree of hydro-affinity to the surface by steric effect of the functionalizing agent, which could act as an umbrella protecting the surface silanol groups and reducing the surface area [24].

3.3 Supported catalysts

Regarding the preparation of the PCH and PCH-F based catalysts, the study of the Ni adsorption of these systems was conducted through mass balance by quantification of Ni by AAS in the Ni x A y W12−y O39or40 (A = Si or P) solutions before and after the impregnation process in equilibrium.

Table 3 shows experimental Ni/W ratios for the studied supported phases. A good agreement is observed between the Ni/W ratio determined from the pure phases and that of the supported systems obtained from EDS analysis. In the Ni2SiW12O40 supported system, the Ni/W ratio is less reproducible due to the detection method capabilities when the concentration of the metals adsorbed in the matrix decreases. The adsorbed Ni(II) content increased by 10% (Table 3) in the cases in which functionalized systems were used as support.

Table 3

Bulk, theoretical, and experimental ratio of pure phases and supported on modified clays

Bulk catalyst Ni/W (wt%/wt) EDS Supported catalyst Ni/W (wt%/wt) EDS Cad* Ni (%) AAS
Ni4SiW11O39 10.7 Ni4SiW11O39/PCH-F 10.6 3.62
Ni3H2W12O40 7.6 Ni3H2W12O40/PCH-F 7.1 4.48
Ni3PW11NiO40H 16.1 Ni3PW11NiO40H/PCH-F 10.0 0.70
Ni3/2PW12O40 3.4 Ni3/2PW12O40/PCH-F 3.2 0.66
Ni2SiW12O40 5.1 Ni2SiW12O40/PCH-F 0.2 0.43
Ni2SiW12O40/PCH 0.2 0.33

*Cad: concentration of adsorbed Ni = {[(C i − C f) × V]/m} × 100.

3.4 Evaluation of the catalytic activity

The catalytic activity was investigated in the selective oxidation reaction of DPS in the presence of hydrogen peroxide as oxidant (35%), in acetonitrile as solvent, and assisted by the different catalysts under study (Scheme 2).

Scheme 2 Selective oxidation reaction of DPS to DPSO2 in the presence of hydrogen peroxide as oxidant.
Scheme 2

Selective oxidation reaction of DPS to DPSO2 in the presence of hydrogen peroxide as oxidant.

The reaction conditions (temperature, time, amount of catalyst and oxidant) were made as per previous studies [24]. The oxidation reaction in the absence of the catalyst occurred with very low sulfide conversions (10% after 3 h of reaction), and the resulting product was the intermediate sulfoxide DPSO, which showed incomplete oxidation.

Figure 5 shows the catalytic results of pure and supported Ni x A y W12−y O39or40 (A = Si or P) catalysts in the oxidation of DPS at different reaction times.

Figure 5 Dependence of conversion on the reaction time in the Ni x A y W12−y O40or39 (A = Si or P) bulk-catalyzed (a) and supported (b) selective oxidation of DPS.
Figure 5

Dependence of conversion on the reaction time in the Ni x A y W12−y O40or39 (A = Si or P) bulk-catalyzed (a) and supported (b) selective oxidation of DPS.

All the Ni x A y W12−y O40or39 (A = Si or P) systems used as bulk catalysts show very good conversions (over 90%), except for the Ni4SiW11O39 lacunar system, which showed lower conversion (70% in 3 h). In particular, the phase containing a higher Ni/W ratio showed the best performance. Likewise, all the systems were selective to DPSO2 (over 75%) in 3 h of reaction, (Table 4, entries 1–5).

Table 4

Selectivity (%) obtained at 3 h after DPS oxidation reaction in the presence of H2O2 using the bulk and supported Ni x A y W12−y O40 (A = Si or P) systems as catalysts

Catalyst DPS conv. (%) Selectivity, 3 h
DPSO DPSO2
1 Ni3H2W12O40 100 25.8 74.2
2 Ni2SiW12O40 98 0.8 99.2
3 Ni4SiW11O39 70 22.6 77.4
4 Ni3/2PW12O40 96 3.9 96.1
5 Ni3PW11NiO40H 100 — 100
6 Ni2SiW12O40/PCH 97 88.9 11.1
7 Ni2SiW12O40/PCH-F 100 36.5 63.6
8 Ni3H2W12O40/PCH-F 100 — 100
9 Ni4SiW11O39/PCH-F 94 56.7 43.3
10 Ni3PW11NiO40H/PCH-F 99 4.8 95.2

The evaluation of the conversion of DPS at short times (15 min), and under identical reaction conditions, shows that the Ni3PW11NiO40H catalyst was the most active (90% conversion, orange line in Figure 5), this effect being associated with Ni content determined by EDS (Table 3). This catalyst showed 100% selectivity towards sulfone after 3 h of reaction (Table 4, entry 5).

The Ni3H2W12O40 needed a longer reaction time to reach the maximum conversion, presenting the greatest induction period of all the systems. These results agree with the well-known effect of peroxide complex formation, such as (PO4[WO(O2)2]4) type, which are intermediate species that catalyze the oxidation reactions studied [10]. The sulfides are oxidized to sulfoxides by electrophilic oxidants. The interaction of peroxide with W catalytic system generates an electrophilic intermediate (peroxo oxygen/metal), which attacks the sulfur atom generating the corresponding sulfoxide. Mechanistically, it is believed that the electrophilicity of the peroxide oxygen of H2O2 is increased by an oxometal group (M═Od) in the Ni x (P/Si) y W12−y O40. And for the oxidation of sulfoxide to sulfone, the mechanism involves the nucleophilic attack of the oxygen in the sulfoxide to the tungsten atom in the Ni x (P/Si) y W12−y O40 with the formation of a Ni(P/Si)W-sulfoxide intermediate and then, a nucleophilic attack of H2O2 on the sulfur atom in Ni(P/Si)W-sulfoxide via an SN2 mechanism. The presence of Ni(ii) metals in the structure introduces redox sites in the Keggin structure, giving a bifunctional character to these species, favoring the formation of peroxo tungstate intermediates [10,14,24].

Functionalized PCHs were also used as support to promote the adsorption of heteropolyanions. Ni2SiW12O40 was also studied on non-functionalized PCH. Figure 5b shows the conversions for these supported systems, which are lower than those obtained for bulk catalysts after 1 h of reaction. After 2 h, better results are achieved for supported catalysts with conversion values above 80%. Moreover, the supported Ni4SiW11O39 system presents a higher conversion than that of the corresponding bulk heteropolycompound, 94% and 70% of conversion in 3 h, respectively (Table 4, entries 3 and 9). The dispersion of the bulk catalyst on the surface of the support generally leads to a decrease in activity; however, the Ni3H2W12040/PCH-F catalyst also showed a significant increase in the conversion of the substrate (38% vs 92% in 15 min of reaction, black line in Figure 5). This again can be associated with the Ni content determined by AAS (the highest value found for all supported catalysts), as presented in Table 3. Another aspect of interest is related to the incorporation of the functionalizing agent. It was observed that the said treatment improves the nickel adsorption capacity and consequently, the activity of the catalyst and its selectivity towards the formation of sulfone. However, the non-functionalized PCH (Ni2W12O40/PCH) showed excellent activity, being also selective in the oxidation of sulfide to sulfoxide (Table 4, entry 6).

Based on these results regarding the role of nickel in the catalytic system and knowing that the active species are the peroxo tungstate entities, we decided to conduct a comparative study of the Keggin lacunar phases with P or Si as heteroatom but containing K as countercation instead of Ni [10]. Finally, the evaluation of catalysts with a similar Keggin structure that do not contain Ni revealed that, although they are active (both bulk and supported catalysts), they are not selective for sulfoxide formation, the reaction product being sulfone. Table 5 and Figure 6 show the catalytic activity values under the same reaction conditions, applying potassium salts of lacunar Keggin structures.

Table 5

Conversion and selectivity (%) at 3 h for the oxidation reaction of DPS in the presence of H2O2, using pure and supported K x A y W12−y O39 (A = Si or P) systems as catalysts

Entry Catalyst DPS conv. (%) Selec. (%) at 3 h
DPSO DPSO2
11 K7PW11O39 100 1.7 98.3
12 K8SiW11O39 100 0 100
13 K8SiW11O39/PCH 93 3.0 97.0
14 K8SiW11O39/PCH-F 95 2.1 98.0
Figure 6 Dependence of conversion on the reaction time in bulk and supported K x A y W11O39 (A = Si or P) in the selective oxidation of DPS.
Figure 6

Dependence of conversion on the reaction time in bulk and supported K x A y W11O39 (A = Si or P) in the selective oxidation of DPS.

The reusability of the catalysts was evaluated using the optimum reaction conditions. The catalyst was isolated from the reaction mixture, washed twice with the solvent (2 mL), dried under vacuum at 25°C until constant weight, characterized by Raman spectroscopy, and then reused in a new test. Raman spectra of the reused catalysts for the selected systems (Ni3PW11NiO40H and Ni2SiW12O40) presented a good agreement in the position of the most intense lines (around 987, 974, and 909 cm−1), without substantial changes in their structure, indicating the conservation of the heteropolyanion (Figure 7).

Figure 7 Raman spectra after reaction for (a) Ni3PW11NiO40H and (b) Ni2SiW12O40 selected systems.
Figure 7

Raman spectra after reaction for (a) Ni3PW11NiO40H and (b) Ni2SiW12O40 selected systems.

No appreciable changes in conversion and selectivity values were detected when the treated catalyst was reused. The tests were performed with catalysts Ni3PW11NiO40H, Ni2SiW12O40, and Ni2SiW12O40/PCH.

Finally, although slight differences are observed in the conversion rate between bulk and supported catalysts, it is expected that in the systems supported on PCH-F, the active phases will be more efficiently anchored, generating catalysts that are likely to be reused in several catalytic cycles.

As a general conclusion, it can be indicated that the incorporation of Ni in the Keggin-type structure generates bulk and supported materials that present very good activity and selectivity in the oxidation of DPS toward sulfone or alternatively towards the respective sulfoxide.

4 Conclusion

Supported catalysts, based on different Ni Keggin-type heteropolytungstates immobilized on chemically modified and functionalized Argentinean bentonite, were prepared. In the heterostructure systems, PCHs, a marked increase in the surface area and a decrease in the pore diameter were observed. The functionalization process allows the effective anchoring of the active phases. These were used as regenerable solid supports of different Ni Keggin-type heteropolytungstates.

The synthesized materials were evaluated as catalysts in the DPS oxidation to DPSO or sulfone, using aqueous H2O2 35% w/v as green oxidant. They showed a great catalytic performance and selectivity under mild reaction conditions (75°C, using acetonitrile as reaction solvent). The evaluation of the conversion of DPS at short time of reaction revealed that the bulk Ni3PW11NiO40H catalyst was the most active (90% conversion, 100% sulfone selectivity), this result being associated with its Ni content. Dispersion of the catalyst on the surface of the support generally leads to a decrease in activity; however, in the case of the Ni3H2W12O40/PCH-F catalyst, a significant increase in the conversion of the substrate is observed when passing from bulk to supported catalysts (38–90%, 15 min). This can again be associated with the Ni content determined by AAS. The supported catalysts, particularly the non-functionalized PCHs, showed excellent activity and were also selective towards the formation of sulfone (97% conversion, 88.9% sulfoxide selectivity).

As a general conclusion, it can be indicated that the incorporation of Ni in the Keggin-type structure generates bulk and supported materials that present very good activity and selectivity in the oxidation of DPS towards sulfone or alternatively, towards the respective sulfoxide.

Advances in the activity of these Ni-based catalysts are beginning to be studied in our laboratory in relation to the recovery of building blocks present in biomass. Specifically, we have begun to study the selective oxidation reaction of furfural and 5-hydroxymethyl furfural, using eco-compatible oxidants such as hydrogen peroxide, and microwave radiation as an alternative source of energy.

Acknowledgements

We wish to thank Ms. Mariela Theiller for her technical support.

  1. Funding information: Authors thank the following institutions for the financial support: CONICET, PIP Project 0111, UNLP, and CICPBA.

  2. Author contributions: Mercedes Muñoz: investigation, methodology, supervision, formal analysis, writing – original draft, and writing – review and editing. Matthieu Greber: investigation and methodology. Karima Ben Tayeb: investigation, methodology, supervision, formal analysis, and writing – original draft. Gustavo P. Romanelli: conceptualization, methodology, formal analysis, supervision, project administration, writing – original draft, and writing – review and editing. Carole Lamonier: conceptualization, methodology, project administration, supervision, formal analysis, writing – original draft, and writing – review and editing. Carmen I. Cabello: conceptualization, methodology, formal analysis, supervision, project administration, writing – original draft, and writing – review and editing.

  3. Conflict of interest: Authors state no conflict of interest.

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Received: 2023-02-16
Revised: 2023-05-20
Accepted: 2023-05-23
Published Online: 2023-07-06

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  86. Synthesis of a silica matrix with ZnO nanoparticles for the fabrication of a recyclable photodegradation system to eliminate methylene blue dye
  87. Natural polymer fillers instead of dye and pigments: Pumice and scoria in PDMS fluid and elastomer composites
  88. Study on the preparation of glycerylphosphorylcholine by transesterification under supported sodium methoxide
  89. Wireless network handheld terminal-based green ecological sustainable design evaluation system: Improved data communication and reduced packet loss rate
  90. The optimization of hydrogel strength from cassava starch using oxidized sucrose as a crosslinking agent
  91. Green synthesis of silver nanoparticles using Saccharum officinarum leaf extract for antiviral paint
  92. Study on the reliability of nano-silver-coated tin solder joints for flip chips
  93. Environmentally sustainable analytical quality by design aided RP-HPLC method for the estimation of brilliant blue in commercial food samples employing a green-ultrasound-assisted extraction technique
  94. Anticancer and antimicrobial potential of zinc/sodium alginate/polyethylene glycol/d-pinitol nanocomposites against osteosarcoma MG-63 cells
  95. Nanoporous carbon@CoFe2O4 nanocomposite as a green absorbent for the adsorptive removal of Hg(ii) from aqueous solutions
  96. Characterization of silver sulfide nanoparticles from actinobacterial strain (M10A62) and its toxicity against lepidopteran and dipterans insect species
  97. Phyto-fabrication and characterization of silver nanoparticles using Withania somnifera: Investigating antioxidant potential
  98. Effect of e-waste nanofillers on the mechanical, thermal, and wear properties of epoxy-blend sisal woven fiber-reinforced composites
  99. Magnesium nanohydroxide (2D brucite) as a host matrix for thymol and carvacrol: Synthesis, characterization, and inhibition of foodborne pathogens
  100. Synergistic inhibitive effect of a hybrid zinc oxide-benzalkonium chloride composite on the corrosion of carbon steel in a sulfuric acidic solution
  101. Review Articles
  102. Role and the importance of green approach in biosynthesis of nanopropolis and effectiveness of propolis in the treatment of COVID-19 pandemic
  103. Gum tragacanth-mediated synthesis of metal nanoparticles, characterization, and their applications as a bactericide, catalyst, antioxidant, and peroxidase mimic
  104. Green-processed nano-biocomposite (ZnO–TiO2): Potential candidates for biomedical applications
  105. Reaction mechanisms in microwave-assisted lignin depolymerisation in hydrogen-donating solvents
  106. Recent progress on non-noble metal catalysts for the deoxydehydration of biomass-derived oxygenates
  107. Rapid Communication
  108. Phosphorus removal by iron–carbon microelectrolysis: A new way to achieve phosphorus recovery
  109. Special Issue: Biomolecules-derived synthesis of nanomaterials for environmental and biological applications (Guest Editors: Arpita Roy and Fernanda Maria Policarpo Tonelli)
  110. Biomolecules-derived synthesis of nanomaterials for environmental and biological applications
  111. Nano-encapsulated tanshinone IIA in PLGA-PEG-COOH inhibits apoptosis and inflammation in cerebral ischemia/reperfusion injury
  112. Green fabrication of silver nanoparticles using Melia azedarach ripened fruit extract, their characterization, and biological properties
  113. Green-synthesized nanoparticles and their therapeutic applications: A review
  114. Antioxidant, antibacterial, and cytotoxicity potential of synthesized silver nanoparticles from the Cassia alata leaf aqueous extract
  115. Green synthesis of silver nanoparticles using Callisia fragrans leaf extract and its anticancer activity against MCF-7, HepG2, KB, LU-1, and MKN-7 cell lines
  116. Algae-based green AgNPs, AuNPs, and FeNPs as potential nanoremediators
  117. Green synthesis of Kickxia elatine-induced silver nanoparticles and their role as anti-acetylcholinesterase in the treatment of Alzheimer’s disease
  118. Phytocrystallization of silver nanoparticles using Cassia alata flower extract for effective control of fungal skin pathogens
  119. Antibacterial wound dressing with hydrogel from chitosan and polyvinyl alcohol from the red cabbage extract loaded with silver nanoparticles
  120. Leveraging of mycogenic copper oxide nanostructures for disease management of Alternaria blight of Brassica juncea
  121. Nanoscale molecular reactions in microbiological medicines in modern medical applications
  122. Synthesis and characterization of ZnO/β-cyclodextrin/nicotinic acid nanocomposite and its biological and environmental application
  123. Green synthesis of silver nanoparticles via Taxus wallichiana Zucc. plant-derived Taxol: Novel utilization as anticancer, antioxidation, anti-inflammation, and antiurolithic potential
  124. Recyclability and catalytic characteristics of copper oxide nanoparticles derived from bougainvillea plant flower extract for biomedical application
  125. Phytofabrication, characterization, and evaluation of novel bioinspired selenium–iron (Se–Fe) nanocomposites using Allium sativum extract for bio-potential applications
  126. Erratum
  127. Erratum to “Synthesis, characterization, and evaluation of nanoparticles of clodinofop propargyl and fenoxaprop-P-ethyl on weed control, growth, and yield of wheat (Triticum aestivum L.)”
https://doi.org/10.1515/gps-2023-0026
Keywords for this article
phosphoro/silicoheteropolyanions; Ni(ii) Keggin heteropolytungstates; modified clays; clean oxidation; diphenyl sulfide
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Articles in the same Issue

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  3. High removal efficiency of volatile phenol from coking wastewater using coal gasification slag via optimized adsorption and multi-grade batch process
  4. Evolution of surface morphology and properties of diamond films by hydrogen plasma etching
  5. Removal efficiency of dibenzofuran using CuZn-zeolitic imidazole frameworks as a catalyst and adsorbent
  6. Rapid and efficient microwave-assisted extraction of Caesalpinia sappan Linn. heartwood and subsequent synthesis of gold nanoparticles
  7. The catalytic characteristics of 2-methylnaphthalene acylation with AlCl3 immobilized on Hβ as Lewis acid catalyst
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  12. Exergy analysis of a conceptual CO2 capture process with an amine-based DES
  13. Construction of fluorescence system of felodipine–tetracyanovinyl–2,2′-bipyridine complex
  14. Excellent photocatalytic degradation of rhodamine B over Bi2O3 supported on Zn-MOF nanocomposites under visible light
  15. Optimization-based control strategy for a large-scale polyhydroxyalkanoates production in a fed-batch bioreactor using a coupled PDE–ODE system
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  17. Geranium leaf-mediated synthesis of silver nanoparticles and their transcriptomic effects on Candida albicans
  18. Synthesis, characterization, anticancer, anti-inflammatory activities, and docking studies of 3,5-disubstituted thiadiazine-2-thiones
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  21. Enrichment of low-grade phosphorites by the selective leaching method
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  23. Characterisation of carbonate lake sediments as a potential filler for polymer composites
  24. Evaluation of nano-selenium biofortification characteristics of alfalfa (Medicago sativa L.)
  25. Quality of oil extracted by cold press from Nigella sativa seeds incorporated with rosemary extracts and pretreated by microwaves
  26. Heteropolyacid-loaded MOF-derived mesoporous zirconia catalyst for chemical degradation of rhodamine B
  27. Recovery of critical metals from carbonatite-type mineral wastes: Geochemical modeling investigation of (bio)hydrometallurgical leaching of REEs
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  29. Attenuation of di(2-ethylhexyl)phthalate-induced hepatic and renal toxicity by naringin nanoparticles in a rat model
  30. Novel in situ synthesis of quaternary core–shell metallic sulfide nanocomposites for degradation of organic dyes and hydrogen production
  31. Microfluidic steam-based synthesis of luminescent carbon quantum dots as sensing probes for nitrite detection
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  33. Preparation of Zr-MOFs for the adsorption of doxycycline hydrochloride from wastewater
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  38. Synergism between lignite and high-sulfur petroleum coke in CO2 gasification
  39. Facile aqueous synthesis of ZnCuInS/ZnS–ZnS QDs with enhanced photoluminescence lifetime for selective detection of Cu(ii) ions
  40. Rapid synthesis of copper nanoparticles using Nepeta cataria leaves: An eco-friendly management of disease-causing vectors and bacterial pathogens
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  42. Development of a fuzzy logic model for the prediction of spark-ignition engine performance and emission for gasoline–ethanol blends
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  44. Green synthesis of strontium-doped tin dioxide (SrSnO2) nanoparticles using the Mahonia bealei leaf extract and evaluation of their anticancer and antimicrobial activities
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  49. Environmentally friendly synthesis and computational studies of novel class of acridinedione integrated spirothiopyrrolizidines/indolizidines
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  52. A one-pot, multicomponent tandem synthesis of fused polycyclic pyrrolo[3,2-c]quinolinone/pyrrolizino[2,3-c]quinolinone hybrid heterocycles via environmentally benign solid state melt reaction
  53. Green synthesis of silver nanoparticles using durian rind extract and optical characteristics of surface plasmon resonance-based optical sensor for the detection of hydrogen peroxide
  54. Electrochemical analysis of copper-EDTA-ammonia-gold thiosulfate dissolution system
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  63. Biosynthesis of zinc oxide nanoparticles from molted feathers of Pavo cristatus and their antibiofilm and anticancer activities
  64. Clean preparation of rutile from Ti-containing mixed molten slag by CO2 oxidation
  65. Synthesis and characterization of Pluronic F-127-coated titanium dioxide nanoparticles synthesized from extracts of Atractylodes macrocephala leaf for antioxidant, antimicrobial, and anticancer properties
  66. Effect of pretreatment with alkali on the anaerobic digestion characteristics of kitchen waste and analysis of microbial diversity
  67. Ameliorated antimicrobial, antioxidant, and anticancer properties by Plectranthus vettiveroides root extract-mediated green synthesis of chitosan nanoparticles
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  69. Studies on the extraction performance of phorate by aptamer-functionalized magnetic nanoparticles in plasma samples
  70. Investigation of structural properties and antibacterial activity of AgO nanoparticle extract from Solanum nigrum/Mentha leaf extracts by green synthesis method
  71. Green fabrication of chitosan from marine crustaceans and mushroom waste: Toward sustainable resource utilization
  72. Synthesis, characterization, and evaluation of nanoparticles of clodinofop propargyl and fenoxaprop-P-ethyl on weed control, growth, and yield of wheat (Triticum aestivum L.)
  73. The enhanced adsorption properties of phosphorus from aqueous solutions using lanthanum modified synthetic zeolites
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  75. Visible-light-assisted base-catalyzed, one-pot synthesis of highly functionalized cinnolines
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  77. Highly efficient removal of tetracycline and methyl violet 2B from aqueous solution using the bimetallic FeZn-ZIFs catalyst
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  80. Ultrasound-assisted green synthesis and in silico study of 6-(4-(butylamino)-6-(diethylamino)-1,3,5-triazin-2-yl)oxypyridazine derivatives
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  82. Biogenic synthesis of silver nanoparticles using Consolida orientalis flowers: Identification, catalytic degradation, and biological effect
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  84. Adsorption synergy electrocatalytic degradation of phenol by active oxygen-containing species generated in Co-coal based cathode and graphite anode
  85. Antibacterial, antifungal, antioxidant, and cytotoxicity activities of the aqueous extract of Syzygium aromaticum-mediated synthesized novel silver nanoparticles
  86. Synthesis of a silica matrix with ZnO nanoparticles for the fabrication of a recyclable photodegradation system to eliminate methylene blue dye
  87. Natural polymer fillers instead of dye and pigments: Pumice and scoria in PDMS fluid and elastomer composites
  88. Study on the preparation of glycerylphosphorylcholine by transesterification under supported sodium methoxide
  89. Wireless network handheld terminal-based green ecological sustainable design evaluation system: Improved data communication and reduced packet loss rate
  90. The optimization of hydrogel strength from cassava starch using oxidized sucrose as a crosslinking agent
  91. Green synthesis of silver nanoparticles using Saccharum officinarum leaf extract for antiviral paint
  92. Study on the reliability of nano-silver-coated tin solder joints for flip chips
  93. Environmentally sustainable analytical quality by design aided RP-HPLC method for the estimation of brilliant blue in commercial food samples employing a green-ultrasound-assisted extraction technique
  94. Anticancer and antimicrobial potential of zinc/sodium alginate/polyethylene glycol/d-pinitol nanocomposites against osteosarcoma MG-63 cells
  95. Nanoporous carbon@CoFe2O4 nanocomposite as a green absorbent for the adsorptive removal of Hg(ii) from aqueous solutions
  96. Characterization of silver sulfide nanoparticles from actinobacterial strain (M10A62) and its toxicity against lepidopteran and dipterans insect species
  97. Phyto-fabrication and characterization of silver nanoparticles using Withania somnifera: Investigating antioxidant potential
  98. Effect of e-waste nanofillers on the mechanical, thermal, and wear properties of epoxy-blend sisal woven fiber-reinforced composites
  99. Magnesium nanohydroxide (2D brucite) as a host matrix for thymol and carvacrol: Synthesis, characterization, and inhibition of foodborne pathogens
  100. Synergistic inhibitive effect of a hybrid zinc oxide-benzalkonium chloride composite on the corrosion of carbon steel in a sulfuric acidic solution
  101. Review Articles
  102. Role and the importance of green approach in biosynthesis of nanopropolis and effectiveness of propolis in the treatment of COVID-19 pandemic
  103. Gum tragacanth-mediated synthesis of metal nanoparticles, characterization, and their applications as a bactericide, catalyst, antioxidant, and peroxidase mimic
  104. Green-processed nano-biocomposite (ZnO–TiO2): Potential candidates for biomedical applications
  105. Reaction mechanisms in microwave-assisted lignin depolymerisation in hydrogen-donating solvents
  106. Recent progress on non-noble metal catalysts for the deoxydehydration of biomass-derived oxygenates
  107. Rapid Communication
  108. Phosphorus removal by iron–carbon microelectrolysis: A new way to achieve phosphorus recovery
  109. Special Issue: Biomolecules-derived synthesis of nanomaterials for environmental and biological applications (Guest Editors: Arpita Roy and Fernanda Maria Policarpo Tonelli)
  110. Biomolecules-derived synthesis of nanomaterials for environmental and biological applications
  111. Nano-encapsulated tanshinone IIA in PLGA-PEG-COOH inhibits apoptosis and inflammation in cerebral ischemia/reperfusion injury
  112. Green fabrication of silver nanoparticles using Melia azedarach ripened fruit extract, their characterization, and biological properties
  113. Green-synthesized nanoparticles and their therapeutic applications: A review
  114. Antioxidant, antibacterial, and cytotoxicity potential of synthesized silver nanoparticles from the Cassia alata leaf aqueous extract
  115. Green synthesis of silver nanoparticles using Callisia fragrans leaf extract and its anticancer activity against MCF-7, HepG2, KB, LU-1, and MKN-7 cell lines
  116. Algae-based green AgNPs, AuNPs, and FeNPs as potential nanoremediators
  117. Green synthesis of Kickxia elatine-induced silver nanoparticles and their role as anti-acetylcholinesterase in the treatment of Alzheimer’s disease
  118. Phytocrystallization of silver nanoparticles using Cassia alata flower extract for effective control of fungal skin pathogens
  119. Antibacterial wound dressing with hydrogel from chitosan and polyvinyl alcohol from the red cabbage extract loaded with silver nanoparticles
  120. Leveraging of mycogenic copper oxide nanostructures for disease management of Alternaria blight of Brassica juncea
  121. Nanoscale molecular reactions in microbiological medicines in modern medical applications
  122. Synthesis and characterization of ZnO/β-cyclodextrin/nicotinic acid nanocomposite and its biological and environmental application
  123. Green synthesis of silver nanoparticles via Taxus wallichiana Zucc. plant-derived Taxol: Novel utilization as anticancer, antioxidation, anti-inflammation, and antiurolithic potential
  124. Recyclability and catalytic characteristics of copper oxide nanoparticles derived from bougainvillea plant flower extract for biomedical application
  125. Phytofabrication, characterization, and evaluation of novel bioinspired selenium–iron (Se–Fe) nanocomposites using Allium sativum extract for bio-potential applications
  126. Erratum
  127. Erratum to “Synthesis, characterization, and evaluation of nanoparticles of clodinofop propargyl and fenoxaprop-P-ethyl on weed control, growth, and yield of wheat (Triticum aestivum L.)”
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