PL240270B1 - Method for electrosynthesis of silver (II) sulfate (VI) and the product obtained by this method - Google Patents

Description

PL 240 270 B1

Description of the invention

The subject of the invention is a method of electrochemical synthesis of high-purity coarse-crystalline silver (II) sulfate and the product obtained in this way.

Silver (II) sulfate is a black, moisture-sensitive solid first obtained in 2009 [Angew. Chem. Int. Ed, 49 (2010) 1683]. It is an antiferromagnetic semiconductor material with a narrow band gap (1.4 ± 0.3 eV), with the highest absolute value of constant magnetic overexposure (217 K) among the known transition metal sulphates [Angew. Chem. Int. Ed, 49 (2010) 1683], and with the lowest temperature of thermal decomposition among known metal sulphates [Chem. Eur. J., 17 (2011) 10524). Silver (II) sulfate is the only currently known inorganic divalent silver compound that does not contain fluorine atoms.

Silver (II) sulphate (VI) shows very strong oxidizing properties due to the content of silver atoms in the +2 oxidation state. The redox potential of the Ag (II) / Ag (I) pair is approximately 2 volts relative to a standard hydrogen electrode. Due to its very strong oxidizing properties, silver (II) sulphate must be stored in a water-free protective atmosphere in vessels made of perfluorinated synthetic materials.

Silver (II) sulphate can be used as a selective oxidizing agent in organic synthesis [Polish patent application no. P.413922] or for the disposal of hydrocarbon waste and hazardous materials. The product of the reduction of silver (II) sulfate is silver (l) bisulfate (VI), less often silver (l) disulfate (VI).

Two methods are known for the chemical synthesis of silver (II) sulfate [Angew. Chem. Int. Ed, 49 (2010) 1683]. Both methods involve the chemical modification of precursors containing silver (II) atoms.

The first method involves a metathesis reaction between Ag (SbF6) 2 and K2SO4 in an anhydrous hydrogen fluoride solution:

Ag (SbF6) 2 (aHF) + K2SO4 (aHF) ^ AgSO4 Φ + 2 KSbF6 (aHF)

The product of this reaction contains silver (II) sulfate contaminated with significant amounts of KSbF6, which cannot be removed from the product. Silver (II) sulphate obtained by this method is not suitable for any chemical use due to the presence of impurities.

The second known method for the synthesis of silver (VI) sulphate (VI) assumes a volatile acid displacement reaction between AgF2 and H2SO4 in a solution of anhydrous sulfuric acid (VI):

AgF2 + H2SO4 ^ AgSO4 4 + 2 HF m

The product of this reaction contains silver sulfate (II) contaminated with silver compounds (I) such as Ag2S2O7 and Ag2SO4. As with the previous synthesis method, silver (II) sulfate cannot be cleaned of impurities present in the product. However, it should be noted that the purity of silver (II) sulphate obtained in the reaction of AgF2 and H2SO4 is much higher than the purity of the reaction product Ag (SbF6) 2 and K2SO4.

Both the previously known reactions for the synthesis of silver (II) sulphate do not allow the product to be obtained in a pure form. The product itself is a fine crystalline solid. In addition, a significant disadvantage of these synthesis methods is the need to use hazardous elemental fluorine in the synthesis of the precursors of the reaction [AgF2 or Ag (SbF6) 2]. The undoubted disadvantage of both synthesis methods is the generation of fluorine-containing waste, which is difficult to utilize.

There is an unmet need for a method for the synthesis of silver (II) sulfate which allows a product free of impurities to be obtained. The synthesis process should not require the use of precursors obtained with the use of hazardous chemical reagents and should not generate waste difficult to utilize. The method according to the invention overcomes the disadvantages known from the state of the art.

The essence of the invention

The method of electrochemical synthesis of silver (II) sulphate (VI), according to the invention, is characterized in that the electrolysis of silver (l) sulphate (VI) or silver (l) bisulphate (VI) dissolved in an aqueous solution of sulfuric acid (VI) is carried out with a concentration of more than 90%, preferably a solution of more than 95%, most preferably a solution of 96%, using a three-electrode system where the working electrode is an electrode made of doped tin oxide:

PL 240 270 B1

ITO or FTO, an electrode made of silver or platinum sheet is used as the auxiliary electrode, and the reference electrode is a saturated silver sulfate electrode in 100% sulfuric acid with a potential of 815 mV compared to the standard hydrogen electrode (NHE), the electrolysis being carried out in in an inert atmosphere, preferably in an argon or nitrogen atmosphere, doped with sulfur hexafluoride (VI), at a potential higher than 2.5 V relative to a standard hydrogen electrode (NHE), preferably at a potential in the range of 2.6-3.8 V relative to a standard electrode Hydrogen chloride (NHE), and the obtained solid electrolysis product is washed with liquid anhydrous hydrogen fluoride to obtain a coarse silver (II) sulfate in the form of a black powder with a crystallite size in the range of 20-1000 µm, preferably 20-500 µm.

The silver (II) sulfate obtained by the electrochemical synthesis method described above according to the invention is characterized in that it is a coarse-crystalline product with a crystallite size in the range of 20-1000 µm, preferably 20-500 µm.

The method of electrochemical synthesis of silver (II) sulphate and the product obtained by this method are described in detail below, in working examples, with reference to the accompanying drawing, in which:

Fig. 1 is a schematic diagram of an electrochemical system for the electrochemical synthesis of silver (II) sulfate by the method of the invention, where WE is the working electrode, CE is the auxiliary electrode and REF is the reference electrode;

Fig. 2 is a photograph of a wet product obtained by the electrochemical synthesis of silver (II) sulfate according to the invention;

Fig. 3 is a SEM photograph of a dry product obtained by the electrochemical synthesis of silver (II) sulfate according to the invention;

Fig. 4 shows the result of an elemental analysis (EDS) of a product sample obtained by the electrochemical synthesis of silver (II) sulfate according to the invention;

Fig. 5 shows the crystal structure of silver (II) sulfate in views along different crystallographic axes;

Fig. 6 shows a comparison of the powder diffraction patterns of silver (II) sulphate (VI) samples obtained by classical metathetic synthesis methods [Angew. Chem. Int Ed., 49 (2010) 1683] and a sample obtained by the method of the invention with a diffractogram generated on the basis of the crystal structure of silver (II) sulfate [CrystEngComm, 15 (2013) 192]; λ = 1.789 A;

Fig. 7 shows the Raman spectrum recorded under excitation of 632 nm radiation of a product sample obtained by the electrochemical synthesis of silver (II) sulfate according to the invention;

Fig. 8 shows the thermal decomposition profile of a product sample obtained by the electrochemical synthesis of silver (II) sulfate according to the invention.

Detailed Description of the Invention

The electrosynthesis of silver (VI) sulphate (VI) is carried out in a three-electrode system in a solution of twice-distilled concentrated sulfuric acid (VI). Optimally, the concentration of the sulfuric acid solution is at least 95%. As the working electrode, an electrode made of doped tin oxide (ITO, FTO) is used. An electrode made of silver sheet with a developed real surface larger than the real surface of the working electrode is used as the auxiliary electrode. The surface of the auxiliary electrode is modified prior to electrolysis by unrolling the real surface. A saturated silver sulfate electrode in 100% sulfuric acid with a potential of 815 mV relative to the standard hydrogen electrode is used as the reference electrode [J. Phys. Chem. C, 117 (2013) 20689).

The electrosynthesis of silver (II) sulfate is carried out in an inert atmosphere in order to prevent the product from coming into contact with atmospheric moisture, because silver (II) sulfate decomposes on contact with water. Argon or high-purity nitrogen is used as the protective gas. It was observed that coating the surface of the sulfuric acid solution with a layer of sulfur hexafluoride (VI) additionally reduces the risk of contact of the synthesized silver sulfate (VI) with atmospheric moisture. It is possible to carry out the electrosynthesis of silver (VI) sulphate using pure sulfur hexafluoride as protective gas.

The synthesis precursor is silver sulfate (IV) (l) or silver bisulfate (VI) (l). They are harmless chemicals that are easy to obtain and cheap. Silver sulphate (l) or silver bisulphate (l) dissolve well in sulfuric acid, which is favorable for the process.

PL 240 270 B1

The electrosynthesis of silver (II) sulfate (VI) is carried out at a strongly anodic potential exceeding the formal redox potential of the Ag (II) / Ag (I) pair. In 96% sulfuric acid (VI), the redox potential of the Ag (II) / Ag (I) pair is about 2.6 V [Chem. Comm., 49 (2013) 7463).

The electrosynthesis of silver (II) sulfate is preferably carried out in high-concentration, at least 95%, double-distilled sulfuric acid (VI). The double distillation of sulfuric acid is necessary due to the need to ensure high purity of the reaction medium during the electrosynthesis of silver (II) sulphate. It has been observed that the optimal process yield is obtained with 100% sulfuric acid. Due to the difficulty of obtaining 100% double distilled acid, the process was optimized to 96% sulfuric acid, which is a cheap and durable material.

As a result of electrolysis of a solution of silver (l) sulphate (IV) or silver (l) hydrogen sulphate (VI), a yellow solution of silver (ll) bisulphate (VI) solvated with sulfuric acid is formed, i.e. a solution containing complex groups Ag (HSO4) 2 (H2SO4) 2 [J. Phys. Chem., 117 (2013) 20689]. The half-life of the Ag (HSO4) 2 (H2SO4) 2 complex moiety is about 10 s in 100% sulfuric acid (VI). It was determined that this is the maximum half-life of the Ag (HSO4) 2 (H2SO4) 2 complex moiety. After exceeding the limit concentration, a black precipitate of silver (II) sulphate (VI) precipitates from the yellow Ag (HSO4) 2 (H2SO4) 2 solution and settles on the bottom of the vessel in which the synthesis is carried out. The electrosynthesis product is washed with anhydrous hydrogen fluoride to remove sulfuric (VI) acid.

The method according to the invention allows for the reuse of the reduction product of silver (II) sulfate, which allows cyclical use of this material as an oxidizing agent without the need to incur additional material costs for its synthesis. The main cost of replacing silver (II) sulphate is the cost of electricity.

As a result of the electrochemical synthesis of silver (II) sulphate, no hazardous chemical compounds are formed, which would be difficult to utilize. The method of synthesis according to the invention does not require the use of substrates obtained with the use of hazardous chemicals such as elemental fluorine.

The method of electrochemical synthesis of silver (II) sulphate according to the invention allows to obtain the product with the efficiency reaching 69% in relation to the used electric charge. The method according to the invention is highly efficient and environmentally friendly.

Silver (II) sulphate obtained by the process of the invention is a coarse-crystalline product with a crystallite size in the range of 20-500 μm. The largest observed silver (VI) sulfate crystals obtained by the method according to the invention had a size exceeding 1 mm in length.

Silver (II) sulphate obtained by the method according to the invention is characterized by a very high purity, as evidenced by the results of the powder morphology tests and chemical, pectroscopic and structural analyzes presented in Table 1 and Fig. 3, Fig. 4, Fig. 6, Fig. 7 and Fig. 8. Thus, silver (II) sulphate can be used as a selective oxidizing agent in organic synthesis or as an oxidizing agent for the disposal of hydrocarbon waste and hazardous materials.

The invention is described below in an embodiment with reference to the drawing and the table below.

Example 1. The electrosynthesis of silver (II) sulfate was carried out by the method according to the invention. The electrolysis was carried out in glass apparatus. The apparatus consisted of two parts joined by a porous sintered glass (Fig. 1). The glass apparatus was washed with twice distilled sulfuric acid (VI), then washed with water, and then again with twice distilled sulfuric acid (VI). A three-electrode system was used: working electrode (WE) made of ETO, auxiliary electrode (CE) in the form of a silver plate with a real surface of 300 cm ² , reference electrode (REF): saturated silver sulphate electrode in 100% sulfuric acid with a potential of 815 mV against standard hydrogen electrode (NHE). The working and reference electrodes were placed in the part of the apparatus where electrolysis was performed; in the second part there was an auxiliary electrode. A solution of silver (I) sulfate in 96% sulfuric acid was introduced into the reaction system. The reaction system was sealed and an argon protective atmosphere of 6.7 purity was introduced. The surface of the solution was covered with a layer of sulfur hexafluoride (VI). The electrolysis was carried out at a potential in the range of 2.6-3.8 V against a standard hydrogen electrode (NHE). During the process, a colorless solution of sulfuric acid turned yellow, and then

Claims (2)

A black precipitate began to form from the solution which was collected at the bottom of the reaction vessel. Upon completion of the electrolysis, the wet product (Fig. 2) was washed with anhydrous hydrogen fluoride which was stripped in vacuo to give a dry end product (Fig. 3). The obtained product was subjected to chemical, spectroscopic and structural analyzes. The results of the analyzes are shown in Table 1 and in Fig. 4, Fig. 6, Fig. 7 and Fig. 8. The reaction product was identified as silver sulphate (II). No significant amounts of impurities were detected in the product. Table 1. The result of the elemental analysis (EDS) of the product sample obtained by the electrochemical synthesis of silver (II) sulphate (VWI) according to the invention. weight content the atomic content of the experiment theory mistake experiment silver theory 57.43% 52.89% 1.89% 19.81% 16.66% sulfur 16.16% 15.72% 0.59% 18.75% 16.66% oxygen 26.42% 31.38% 3.19% 61.44% 66.66% Patent claims 1. The method of electrochemical synthesis of silver (l) sulphate (VI), characterized by electrolysis of silver (l) sulphate (VI) or silver (l) bisulphate (VI) dissolved in an aqueous solution of sulfuric acid (VI) with a concentration greater than 90%, preferably in a solution with a concentration greater than 95%, most preferably in a solution with a concentration of 96%, using a three-electrode system where the working electrode is an electrode made of a doped tin oxide; ITO or FTO, an electrode made of silver or platinum sheet is used as the auxiliary electrode, and the reference electrode is a saturated silver sulphate electrode in 100% sulfuric acid with a potential of 815 mV compared to a standard hydrogen electrode (NHE), the electrolysis being carried out in in an inert atmosphere, preferably in an argon or nitrogen atmosphere, doped with sulfur hexafluoride (VI), at a potential higher than 2.5 V relative to a standard hydrogen electrode (NHE), preferably at a potential in the range of 2.6-3.8 V relative to a standard electrode Hydrogen chloride (NHE), and the obtained solid electrolysis product is washed with liquid anhydrous hydrogen fluoride to obtain a coarse crystalline silver (II) sulfate in the form of a black powder with a crystallite size in the range of 20-1000 µm, preferably 20-500 µm. 2. Silver (II) sulfate obtained by the electrochemical synthesis according to claim 1, characterized in that it is a coarse-crystalline product with a crystallite size in the range of 20-1000 µm, preferably 20-500 µm.