Investigation of 3,3′,5,5′-tetra-tert-butyl-4,4′-Stilbenequinone based Catalyst in the Reaction of Liquid-phase Oxidation of Inorganic Sulfides
Hoang Hien Y 1*, Akhmadullin Renat Maratovich 3, Akhmadullina Farida Yunusovna 1, Zakirov Rustem Kayumovich 1, Dinh Nhi Bui2, Akhmadullina Alfiia Garipova 3, Gazizov Almir Sabirovich 4
1Kazan National Research Technological University, Department of Industrial Biotechnology, K. Marks str. 72, Kazan, Russia FederationDepartment, University, City, Country
2Viet Tri University of Industry, Tien Son st, 9 — Viet Tri, Viet Nam
3R&D Center «AhmadullinS — Science & Technology», 34 Syberian Tract, building 10, Kazan, Russian Federation.
4A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre Russian Academy of Sciences, Laboratory of Elementoorganic Synthesis, Arbuzov str. 8, Kazan, Russia Federation.
*Corresponding author: Hoang Hien Y — Graduate student of Department of Industrial Biotechnology, Kazan National Research Technological University; 32a st. Tovarischeskaya, Kazan, Russia, 420097; Phone: +79656027868; Email: firstname.lastname@example.org.
Investigation of 3,3′,5,5′-tetra-tert-butyl-4,4′-Stilbenequinone based Catalyst in the Reaction of Liquid-phase Oxidation of Inorganic Sulfides
In this paper, the intermediate and final reaction products of catalytic oxidation of inorganic sulfides in the presence ofdissolved in the kerosene fraction3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone were investigated. The thiosulfate and sulfate are major products of the oxidation of sodium sulfide in the presence of catalyst based on 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone. The intermediate and final products in the catalytic oxidation of sulfide sulfur do not affect on the rate of its oxidation. The yield of catalytic oxidation products depends on the nature of sulfides and pH of the solution. The catalytic cycle for the sulfide oxidation in the presence of a 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone was shown. The role of 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone is to create new and more effective way of electron transfer from the reducing agent (sulfide) to the oxidant (oxygen).
Keywords:inorganic sulfides, 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone, sodium thiosulfate, sodium sulfate, catalytic mechanism.
One of the main environmental problems in oil refining and petrochemical industry is the treatment and utilization of highly toxic sulfurous alkaline wastes (SAW) with high concentrations of inorganic sulfides. The current effluent guidelines do not allow the discharges of these pollutants into water or treatment together with other industrial wastewater even after their considerable dilution. It is advisable to create separate systems for the collection and the purification of SAW .
A number of processes have been developed for treating SAW. Some of the inorganic sulfide can be removed by simple stripping, especially at high pH values (≥9), oxidation of the sulfide is very necessary for complete removal.
The chemical oxidation of the sulfide can be achieved by strong agents such as potassium permanganate, potassium dichromate, chlorine, bleaching powder, ozone, hydrogen peroxide. However, the cost of the chemicals and possible pollution by the oxidation agents are serious disadvantage .
Nowadays, a common method of treatment the SAW is their liquid-phase catalytic oxidation in air atmosphere in the presence of various catalysts . Many authors have investigated the oxidation of sulfide in water by oxygen in the presence of activated carbon [3-4]. The reaction rate strongly depends on the pH of the solution and the mass transfer of oxygen to the aqueous suspension of activated carbon
The liquid phase oxidation of sulfide was found to be much faster in the presence of transition metal complexes [5-7]. Currently, transition-metal oxides based on a polymer matrix are the main heterogeneous catalysts used in the oxidation of sulfide and hydrosulfide anions. Transition-metal oxides were distributed evenly on the catalyst surface and were strongly adhered. During the catalyst process, the complexes between transition-metal oxides and sulfide ions on the catalyst surface were formed .
The main disadvantages of the proposed methods in previous studies are the low activity of catalysts and the contamination of the treated effluents. In order to eliminate the above disadvantages, we propose a new heterophase process in the presence of a catalyst based on 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone (here in after stilbenequinone) that is dissolved in kerosene fraction. The hydrocarbon phase (kerosene fraction) simultaneously performs both the role of the catalyst carrier and the role of the oxygen deposition. The purpose of this study is to investigate the role of stilbenequinone based catalyst in the liquid-phase oxidation of inorganic sulfides and evaluate factors that affect the reaction.
2. Experimental Section
The catalytic component of 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone was synthesized by the following method: 30 g of 2,6-di-tert-butyl-4-methylphenol, 3 g of potassium iodide, and 120 ml of isopropanol were charged in a glass cylindrical reactor with a volume of 500 ml and heated to 70 oC with stirring. 42 ml of 35% aqueous solution of hydrogen peroxide was added after heating for 30 min, and the reaction was continued for 9 hours at 70-75 °C. The obtained mixture was cooled to room temperature, and the precipitated crystals were filtered and dried. The yield of 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone was 98%. 1H NMR spectra of stilbenequinone were recorded on a Bruker Avance 600 spectrometer at an operating frequency of 600 MHz (Figure 1): δ ppm: 1.42 (s, 36H, C(CH3)); 6.54 (s, 2H, = CH); 7.19 (s, 4H, C6H2). An additional confirmation is the melting point of stilbenequinone (315 °C), which was determined on a Buchi M-560 device .
[Figure 1 near here]
A solution of sodium hydrosulfide was prepared according to the procedure of US №4439411 . During the experiments, the following reagents were used: Na2S (Russia, GOST 2053-77), O2 (Russia, GOST 5583-78), and kerosene fraction (Russia, GOST 10227-2013).
Catalytic oxidation of inorganic sulfides was conducted in a glass reaction vessel (Figure 2). 40 ml of inorganic sulfide aqueous solutionand 20 ml of kerosene fraction were added into the reactor in the presence of certain amount of the catalyst. Oxygen from the cylinder was injected into the reaction solution at 300 h-1. The solution inside the reactor was stirred at a speed of 1400 rpm. The temperature of the reaction solution was maintained at 90 oC with a thermally controlled magnetic stirrer.
[Figure 2 near here]
The quantitative content of sulfides was determined by potentiometric titration in accordance with GOST 22985-90. The concentration of thiosulfate and sodium sulfite was determined by the method proposed by Curtenacher and Wollack , and the concentration of sodium sulfate was determined by spectrophotometric method . Infrared spectra (IR) of substances was recorded on a Perkin Elmer Spectrum Two FTIR spectrometer.
3. Results and Discussion
3.1. The Reaction Products of the Liquid-Phase Oxidation of Sodium Sulfide in the Presence of Catalyst Based on Stilbenequinone
The process of liquid-phase oxidation of sulfide sulfur with oxygen depends on a number of factors: temperature, pH, the oxygen concentration, the nature of inorganic sulfides, the nature of catalyst, etc., where upon various products are formed (Scheme 1). In the course of this reaction, the oxidation number of sulfur varies from -2 to +6.
[Scheme 1 near here]
In the presence of catalysts, the main products of the sulfide sulfur oxidation are thiosulfate, sulfite and sulfate . Figure 3 shows the change in sodium sulfide concentrations and products of its catalytic oxidation in the presence of stilbenequinone. As can be seen from Figure 3, sodium thiosulfate and sodium sulfate are the main products of the oxidation of sulfide sulfur in the presence of stilbenequinone. The mass ratio of sulfate to thiosulfate is 4: 3 after oxidizing for 600 min.
[Figure 3 near here]
Oxidation of sulfide sulfur in the presence of stilbenequinone proceeds through two stages: the first stage is the oxidation of sulfide sulfur with stilbenequinone to 3,5,3′,5′-tetra-tert-butyl-4,4′-dihydroxy-1,2-diphenylethylene (here in after diphenylethylene); the second stage is the regeneration of the catalyst by oxidation of diphenylethylene to stilbenequinone in alkaline medium (Scheme 2).
[Scheme 2 near here]
To confirm the mechanism, the possibility of the reaction of sodium sulfide oxidation with stilbenequinone in the range 70-110 °C in an oxygen-free environment was studied with subsequent analysis of the light yellow powder precipitated after the reaction at room temperature that presumably was diphenylethylene (Table 1). It is shown that the reaction rate of the oxidation of sodium sulfide with stilbenequinone increases with increasing temperature.
[Table 1 near here]
Figure 4 shows two IR spectra: stilbenequinone and proposed diphenylethylene. In the stilbenequinone spectrum (Figure 4), there are peaks of the stretching vibrations of the conjugated diene (Ar=C C=Ar) 1605 cm-1 and the carbonyl group (C=O) 1605 cm-1, which are absent in the spectrum of the proposed diphenylethylene. It is characterized by absorption bands due to stretching vibrations of the hydroxyl group (OH-) 3627 — 3607, 1420 and 1231 — 1133 cm-1 and the double bond (-C=C-) 960 cm-1 .
[Figure 4 near here]
An additional confirmation is that the melting point of diphenylethylene is 240 °C . Thus, all the obtained results confirmed the proposed mechanism of the catalytic oxidation of sulfide sulfur in the presence of stilbenequinone (Scheme 2).
According to Scheme 2, the products of catalytic oxidation of sulfide sulfur are determined by the first stage and according to the authors  the catalytic oxidation of sulfide sulfur in the presence of quinones may lead to non-catalytic oxidation. For this purpose, the change in the concentration of products of the sodium sulfide oxidation with stilbenequinone (1-stage) was studied at a series of different temperatures in an autoclave in an inert medium(Table 2).
[Table 2 near here]
Results showed a selective formation of sodium thiosulfate in the oxidation of sodium sulfide with stilbenequinone in an oxygen-free environment. It is assumed that the sulfate ion formed in the catalytic oxidation of sodium sulfide in the presence of stilbenequinone is a product of non-catalytic oxidation of thiosulfate ions by oxygen. The yield of sodium sulfate during the oxidation of sodium thiosulfate in the presence and absence of stilbenequinone was compared to confirm this thesis. The obtained data (Figure 5) showed that stilbenequinone does not affect the oxidation of sodium thiosulfate.
[Figure 5 near here]
Obviously, concentration of formed thiosulfate decreases after complete exhaustion of the sulfide in the solution (Figure 3). It should be noted that sulfide ions can be oxidized to sulfate ions in strongly alkaline media  by the following reactions:
It is known that reaction (7) proceeds much faster than reactions (5) and (6), thus it can be assumed that thiosulfate and sulfate are main products of the oxidation of sulfide sulfur .
3.2. Influence of the Nature of Sulfides on the Formation of Final Products in the Presence of Stilbenequinone
One of the most important factors affecting the formation of reaction products is the nature of the reactants. Figure 6 shows the kinetics of the change in the concentrations of products of the oxidation of Na2S, NaHS in the presence of a stilbenequinone-based catalyst.
[Figure 6 near here]
At the initial time, the amount of thiosulfate formed during the catalytic oxidation of Na2S is higher than the oxidation of NaHS. The reversible picture is observed with further oxidation, which is caused by the change in the pH of the solution. It is known that hydroxide ion (OH-) is a catalyst for the oxidation of hydroquinone to quinone [17-19], which means that the increase of the amount of OH- anions contributes to the catalyst regeneration process in the oxidation of sulfide sulfur, so the rate of the catalytic oxidation of Na2S (pH = 13.5) is higher than NaHS (pH = 9). Therefore, the amount of thiosulfate formed during the catalytic oxidation of Na2S is greater than the oxidation of NaHS at the initial time. On the other hand, the obtained thiosulfate during catalytic oxidation of Na2S is faster oxidized to sulfate compared with NaHS in a strongly alkaline medium (pH> 12) [20, 21], which is confirmed by the results of the oxidation of sodium thiosulfate by the oxygen in a strongly alkaline medium (Table 3). Consequently, the amount of thiosulfate ions yield by the catalytic oxidation of Na2S is less in comparison with NaHS with further oxidation. In addition, the possibility of the partial non-catalytic oxidation of sulfide sulfur in a strongly alkaline medium cannot be excluded [20, 21] with the formation of sulfate.
[Table 3 near here]
3.3. Effect of Reaction Products on the Liquid-Phase Oxidation of Sulfide Sulfur in the Presence of Catalyst Based on Stilbenequinone
[Figure 7 near here]
The intermediate and final products of the oxidation of sulfide sulfur can affect the process of the catalytic oxidation. Figure 7 shows the results of the effect of sodium thiosulfate and sodium sulfate on the rate of the catalytic oxidation of sodium sulfide. It can be seen that sodium thiosulfate and sodium sulfate do not affect the rate of oxidation of sodium sulfide in the presence of stilbenequinone.
The kinetics of the formation of products of the oxidation of sulfide sulfur in the presence of catalyst based on stilbenequinone was studied. It has been established that stilbenequinone is reduced to diphenylethylene during the oxidation of sulfide sulfur. It has been proved that intermediate and final products of the catalytic oxidation of sulfide sulfur do not affect the rate of its oxidation, and sodium thiosulfate and sodium sulfate are the final products of the oxidation of sulfides. Selective formation of sodium thiosulfate was established during the oxidation of sodium sulfide with stilbenequinone. It is shown that the kinetics of the formation of products of the catalytic oxidation of sulfide sulfur depends on the pH of the medium.
The authors wish to recognize the R&D Center «AhmadullinS — Science & Technology» and their colleague for the technical support in experiments. We also thank the Department of Industrial Biotechnology of Kazan National Research Technological University for helpful discussions and comments on the manuscript.
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