Heterogeneous catalysis for thioanisole oxidation using hydrogen peroxide and Copper, Nickel and Zinc tungstates prepared by the polymeric precursor method


 The oxidation of sulfides to sulfoxides and sulfones provides valuable sulfur-containing chemical compounds that are used in the pharmaceutical agrochemical industry. Although some tungsten catalytic systems have been applied to sulfide oxidation, the most desirable heterogeneous catalytic protocols have not been described. The copper, nickel and zinc tungstates powders obtained by the polymeric precursor method and the evaluation of their catalytic activity in thioanisole oxidation were investigated. Thioanisole was oxidized by hydrogen peroxide to sulfoxides and sulfones and the presence of catalyst accelerates the conversion. Of the three catalysts, copper tungstate was the most efficient in the conversion process followed by nickel tungstate and finally zinc tungstate. The copper tungstate has higher hydrogen consumption, indicating a higher oxygen content on the surface and the ability to increase surface mobility, which increases the conversion and selectivity of the process. The addition of 0.1 mL of hydrogen peroxide enhanced the conversion and increased the amount of sulfone produced. The ideal reaction time was 12 hours and the optimum temperature was 75 °C.


INTRODUCTION
The oxidation of sulfides to sulfoxides and sulfones is an important reaction for the synthesis of many valuable sulfur containing chemical intermediates and products.
These compounds are utilized in industry to produce for example, pharmaceuticals, agrochemicals and polymeric materials [1]. The catalytic oxidative desulfurization process (sulfoxidation reaction) is regularly carried out in industry to remove sulfur-based impurities from crude oil [2]. The selective detoxification of organosulfide-containing chemical nerve agents to less toxic sulfoxide compounds also has important applications [3]. Efforts have been made towards the development of catalytic methods using homogeneous catalysts for the oxidation of sulfides to sulfoxides and/or sulfones [4][5][6][7][8][9][10].
Some of tungsten-based catalytic systems have proved to effectively promote sulfide oxidation in a homogeneous medium [22,23]. However, the heterogeneous catalytic protocols are more desirable given their potential to be recovered and reused in further cycles.
Metal tungstates (general formula MWO4, where M symbolizes a bivalent cation) have been applied to catalysis due to their peculiar physical and chemical properties [24].
The present work involved the synthesis of CuWO4, NiWO4 and ZnWO4 powders by applying the polymeric precursor method, which is a low-cost processing procedure, when compared to other chemical methods, besides ensuring high material reproducibility. The method has been successfully used in the synthesis of nanoparticles of different oxides, and is based on the chelation of cations by a hydrocarboxylic acid (normally citric acid), followed by polyesterification using a glycol (normally ethylene glycol [25]. The formed precursor resins contain cations randomly distributed throughout the polymer [26]. Herein, some initial results pertaining to the application of MWO4 (M = Cu, Ni and Zn, respectively) to catalyze thioanisole oxidation with aqueous H2O2 is reported.

2.1.Synthesis of tungstates
Dissolution of H2WO4 was carried out in distilled water at 70 o C, with pH of 8-11.
Next, an aqueous solution of citric acid was added to produce a tungsten citrate solution.
The polymeric resin was synthesized by adding Cu(NO3)2.3H2O or Ni(NO3)2.6H2O or Zn(NO3)2.6H2O to this solution, under constant stirring and at the temperature of 100 o C.
The polyesterification reaction was promoted after the addition of ethylene glycol. The citric acid:metal molar ratio was 3:1, while the citric acid:ethylene glycol ratio was 3:2 (mass ratio). The polymeric precursor solution was heat-treated at 400 ºC for 4 h at a heating rate of 10 o C.min −1 . The obtained powder was grounded in an agate mortar in order to deagglomerate the powder, hereafter called the precursor powder. Precursor powders were subsequently treated at 700 °C.

2.2.Characterization of samples
Thermal decomposition and crystallization of this powder were studied by TG the output of the laser was maintained at 500 mW, with ~14 mW reaching effectively the sample. Hydrogen temperature programmed reduction (H2-TPR) measurements were carried out in a Quantacrhome ChemBET-TPR/TPD equipment, at temperature range from 50 to 1100 °C, heating rate of 10 °C.min -1 , using a 5% H2/N2 mixture (25 mL.min -1 flow rate) as reducing gas and 50 mg of sample. The resulting curves were analyzed using Origin 9.0.0 software and the observed peaks were adjusted using Gaussian model.

2.3.Catalytic measures
The liquid phase oxidation reactions of thioanisole (Sigma-Aldrich) were carried out (1), whereas selectivity of products using the following expression: Selectivity (%) = total moles of product formed the sum of total moles of all oxidation products formed ×100 (2). Figure 1 presents the TG, DTG and DSC curves of the CuWO4, NiWO4 and ZnWO4 precursor powder. The TG curves reveal a series of mass loss-related decomposition reactions, connected to different exothermic events as indicated by the DSC curves. Table   1 summarizes the thermal events and corresponding temperature ranges, as inferred from the TG and DSC data.       [29] and NiWO4 and ZnWO4 by direct electronic transitions [30,31]  In Figure 5 the PL spectra of CuWO4, NiWO4 and ZnWO4 using the excited wavelength of 350.7 nm demonstrated that these powders present broad emission range typical of systems in which relaxation occurs involving the participation of numerous states [35]. CuWO4 samples have two maximum emission peaks centered at 445 nm (blue region) and 532 nm (green region), NiWO4 PL spectrum is bluish-green spectral range with maximum at 450 nm. The PL emissions are attributed to WO4 -2 groups due the recombination of electrons in the 3 T1u state and holes in the 1 A1g ground state [36,37].

RESULTS AND DISCUSSION
ZnWO4 PL emission is centered at 637 nm (red region). According to the literature, ZnWO4 also has blue-green emission [38], however oxygen vacancies induce red shift emission due deep defects in the lattice [34,39].  °C) [40,41]. This fact is accordance with the XRD results, suggesting that the copper is present in the tungstate structure, and not in a secondary copper oxide phase. Similar results were observed for NiWO4 and ZnWO4 [42][43][44][45].  Table 3 shows the adjusted peaks and hydrogen consumption for the three MWO4 samples. The signals observed at temperatures higher than 700 °C for all catalysts could be associated to the reduction of WO3 or MWO4-x phases formed after the initial reduction of copper or nickel catalysts. For ZnWO4 there is no signal bellow 700 °C, since Zn 2+ is reduced above this temperature. For CuWO4 it can be observed a higher hydrogen consumption (and oxygen removal) than NiWO4 and ZnWO4 (Table 3). This fact can be directly associated to the higher reducibility of copper in comparison with nickel and zinc, and suggest that CuWO4 would probably be more active in oxidation reaction the others, especially in comparison with ZnWO4, due to its higher oxygen mobility.  CuWO4 was the most active catalyst, attaining 93.2% conversion, followed by NiWO4 with 80.0% whereas ZnWO4 was found to be the least efficient achieving 68.9% conversion of thioanisole.
The product selectivity's, was not significantly influenced by the nature of the catalysts (Figure 7b). Methyl phenyl sulfoxide was the major product formed after 12 hours of reaction with selectivity greater than 70.0% (Figure 7c). Comparing the results of BET, it could be concluded that specific surface areas had no influence on the catalytic activities presented in this work. The superior performance presented by CuWO4 was attributed to its higher reducibility and oxygen mobility (higher hydrogen consumption, according to H2-TPR measurements) [46]. The hydrogen consumption (Table 3) for CuWO4 is almost twice as high as ZnWO4 (the less active catalyst). Besides, the highest amount of oxygen vacancies observed for CuWO4 (Table 2) resulted in an increase in the oxygen mobility.
The bulk and surface oxygen mobility can play an important role in oxidation reactions of organic compounds. Higher oxygen mobility can favour the migration of active oxygen species across the catalyst structure, resulting in a greater oxidation activity [47]. Due to the highest activity presented by CuWO4 catalyst, this was employed in the following tests to understand the influence of some chosen reaction parameters (temperature, time and H2O2 volume) on thioanisole conversion and product selectivity values. Thioanisole conversion and selectivity towards the sulfone increased with increasing reaction time (Figure 7c). Thioaniole conversion increases dramatically with increasing reaction temperature (Figure 7d) and the increasing temperature also alters the ratio of methyl phenyl sulfoxide to the methyl phenyl sulfone, highlighting the oxidation sequence of sulfide to sulfoxide and then to sulfone [6]. Finally, the thioanisole conversion increased with H2O2 volume, reaching 93.2% in the presence of 1.0 mL of the oxidant, after 12 hours, due to availability of more oxidant. When increasing the H2O2 volume up to 1.0 mL, the selectivity towards methyl phenyl sulfoxide dropped from 100% to 78.1%, with concomitant increase in methyl phenyl sulfoxide formation (21.9%).
Greater availability of the oxidant resulted in over-oxidation to the corresponding sulfone [48].

CONCLUSIONS
In conclusion, pure metal tungstates were successfully and efficiently synthesized using the polymeric precursor method. This method promotes significant time and energy savings when compared to other conventional methods. The characterization results confirm that a monoclinic wolframite structure can be obtained. The catalytic tests revealed that thioanisole could be oxidized by H2O2 to sulfoxides and sulfones at ambient conditions in the presence of the described metal tungstates. The conversion and distribution of the products were dependent on the experimental parameters employed, in particular the oxidant volume and nature of the catalyst. Analyses of the material suggest that a higher surface oxygen content and an enhancement of surface oxygen mobility improved the activity and selectivities of the reaction.