Research on the Production of Methyltrioxorhenium and Heterogenous Catalysts from Waste Materials

Abstract

This paper presents the research results on the synthesis of rhenium catalysts MTO, Re₂O₇/Al₂O₃, and M-Re₂O₇/Al₂O₃ (where M = Ni, Ag, Co, Cu) from rhenium compounds (ammonium perrhenate, perrhenic acid, nickel(II) perrhenate, cobalt(II) perrhenate, zinc perrhenate, silver perrhenate, and copper(II) perrhenate) derived from waste materials. Methyltrioxorhenium (MTO) was obtained from silver perrhenate with a yield of over 80%, whereas when using nickel(II), cobalt(II), and zinc perrhenates, the product was contaminated with tin compounds and the yield did not exceed 17%. The Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ catalysts were obtained from the above-mentioned rhenium compounds. Alumina obtained in a calcination process of aluminum nitrate nonahydrate was used as a support. The catalysts were characterized in terms of their chemical and phase composition and physicochemical properties. Catalytic activity in model reactions, such as cyclohexene epoxidation and hex-1-ene homometathesis, was also studied. MTO obtained from silver perrhenate showed >70% activity in the epoxidation reaction, thus surpassing commercial MTO (1.0 mol% MTO, room temperature, and reaction time—2 h). Ag-Re₂O₇/Al₂O₃, Cu-Re₂O₇/Al₂O₃, and H-Re₂O₇/Al₂O₃ catalysts were inactive, while Co-Re₂O₇/Al₂O₃ and Ni-Re₂O₇/Al₂O₃ showed low activity (<43%) in the hex-1-ene homometathesis reaction. Only Re₂O₇/Al₂O₃ catalysts achieved >70% activity in this reaction (2.5 wt% Re, room temperature, and reaction time—2 h). The results indicate the potential of using rhenium compounds derived from waste materials to synthesize active catalysts for chemical processes.

1. Introduction

Homogeneous and heterogeneous rhenium catalysts play a crucial role in oxidation, metathesis, and epoxidation reactions. Due to their high activity and selectivity, rhenium compounds are the subject of intensive research focused on the development of new catalytic materials. Methyltrioxorhenium (CH₃ReO₃, MTO) is one of the best-known homogeneous rhenium catalysts, characterized by high activity and selectivity [1,2,3,4]. MTO is used in the oxidation of alkenes [5], alcohols [2,6,7], sulfides [8], amines [9], imines [3], and N-sulfinyl imines [10], as well as in olefin metathesis and epoxidation reactions [1,11,12,13]. MTO was discovered by Beattie and Jones in 1979 as a decomposition product of either trimethyldioxorhenium or tetramethyloxorhenium [14]. In 1988, Herrmann and his team developed a synthesis method for MTO through a reaction of Re₂O₇ with tetramethyltin [15,16]. Due to the toxicity of organic tin compounds and the formation of side products, research was conducted on alternative synthetic methods using metal perrhenates, such as silver perrhenate, zinc perrhenate, potassium perrhenate, sodium perrhenate, and magnesium perrhenate [17,18]. Ultimately, a synthesis method for MTO was developed using silver perrhenate, which is stable to moisture and air, in contrast to Re₂O₇, which is sensitive to moisture and prone to reduction [17]. In olefin epoxidation reactions, methyltrioxorhenium (MTO) is typically used in combination with an oxidizing agent, most commonly aqueous hydrogen peroxide (H₂O₂). In the presence of H₂O₂, MTO is converted into an active peroxo complex (e.g., [CH₃ReO(O₂)]), which is capable of transferring an oxygen atom to the double bond of an alkene, resulting in the formation of an epoxide [13]. However, due to the propensity of aqueous H₂O₂ to promote epoxide degradation, alternative oxidants such as urea–hydrogen peroxide (UHP) have been increasingly adopted as more stable and efficient oxygen sources [4,19]. A key challenge in these reactions is the Brønsted and Lewis acidity of rhenium, which promotes ring-opening of the epoxide, leading to the formation of diols [20]. Sharpless and co-workers addressed this issue by introducing pyridine as an additive, which was found to suppress epoxide decomposition [21]. Further studies confirmed that Lewis bases—particularly nitrogen-donor ligands—effectively inhibit epoxide ring-opening by coordinating to the rhenium center [21,22]. Espenson and colleagues demonstrated that both steric and electronic factors influence the coordination of pyridine and its derivatives to the MTO center [22]. The use of Lewis bases such as pyrazole, cyanopyridine, and pyridine N-oxides has proven effective in minimizing epoxide degradation under those conditions [23].

Re₂O₇/γ-Al₂O₃ is a well-known heterogeneous rhenium catalyst mainly used in olefin metathesis [24,25,26,27,28,29]. A typical catalyst is prepared by impregnating gamma alumina with a large specific surface area (180–200 m²/g) with an aqueous solution of ammonium perrhenate (NH₄ReO₄) or perrhenic acid (HReO₄). The catalyst precursor is then dried at 110–120 °C and calcined at 500–550 °C in a dry air stream [25,28,30,31,32]. During the calcination of the catalyst precursor, a yellow crystalline rhenium(VII) oxide (Re₂O₇) is formed, with a melting point of 297 °C. This compound becomes uniformly dispersed over the surface of the alumina support (Al₂O₃). Due to its high volatility and the ability to sublimate below its melting point, Re is partially lost during the calcination process. Re₂O₇/γ- Al₂O₃ shows high catalytic activity and selectivity in olefin metathesis reactions at low temperatures (20–100 °C) [24,25,28]. In order for the catalyst to be active in the metathesis of olefins containing functional groups, it is necessary to use a promoter, which is typically SnR₄ or PbR₄ compound (R = methyl, ethyl, butyl) [33,34,35,36,37]. Re₂O₇/Al₂O₃ catalysts promoted with SnR₄ compounds (R = methyl, ethyl, butyl) exhibit significantly higher activity in the metathesis of unsubstituted alkenes compared to the non-promoted systems [36,38]. However, these catalysts deactivate more rapidly, likely due to the accumulation of tin, most probably in the form of SnO₂, on the catalyst surface [38]. The catalytic performance of Re₂O₇/Al₂O₃ with low rhenium content (<10 wt% Re₂O₇) can be further enhanced by the following: (1) adding transition metal oxides—MoO₃, WO₃, or V₂O₅ [39,40,41]; (2) using mixed supports—SiO₂-Al₂O₃, Al₂O₃-B₂O₃, B₂O₃/SiO₂-Al₂O₃ [33,36,42,43,44,45,46,47,48]; and (3) modifying the support with ammonium hydrogen phosphate ((NH₄)₂HPO₄) [49]. Low rhenium content catalysts modified with metal oxides such as MoO₃, WO₃, or V₂O₅ have been shown to exhibit high activity and selectivity in the metathesis of methyl oleate [39]. Furthermore, the use of mixed supports, such as SiO₂–Al₂O₃ or Al₂O₃–B₂O₃, enhances the performance of rhenium-based catalysts in the metathesis reaction [33,36,42,43,44,45,46,47]. Mixed supports are characterized by higher acidity and a greater number of Brønsted active sites compared to the traditional Al₂O₃ support [33,36,42,43,44,45,46,47].

Heterogeneous rhenium-based catalysts are primarily utilized in the metathesis of gaseous and/or liquid alkenes, as well as acyclic alkenes bearing functional groups. The Re₂O₇/Al₂O₃ catalyst has been employed in various metathesis reactions, including those of hex-1-ene [24,50,51], propene [51], and but-2-ene and ethene, as well as the reverse reaction of propene [52]; terminal alkenes (pent-1-ene, hex-1-ene, hept-1-ene, oct-1-ene) [53]; ethene and pent-2-ene [27,43]; and but-1-ene and but-2-ene [54,55]. Additionally, Re₂O₇/Al₂O₃-SnR₄ catalysts (where R = methyl, ethyl, or butyl) have been widely used in the metathesis of unsaturated carboxylic acid esters and unsaturated fatty acid esters [46,56,57,58,59,60], as well as in the metathesis of olefins containing chlorine or bromine [61].

The Re₂O₇/Al₂O₃ catalyst was also used in industrial processes. In the FEAST process, developed by Shell, α,ω-dienes were produced via the ethenolysis of cycloalkenes using a Re₂O₇/Al₂O₃ catalyst promoted by Bu₄Sn, at temperatures between 0 and 20 °C and pressures of 1 or 2 bar [25,62,63]. This process was used, among other applications, for the production of hex-1,5-diene from cycloocta-1,5-diene and ethene [25,62,63]. The FEAST process was commercialized in 1987 when a production plant opened in France, with a production capacity of 3000 t/year of dienes. However, the plant was closed after a few years due to the lack of market demand for the products [25,63]. The Institut Français du Pétrole (IFP), based in Frannce, in collaboration with Chinese Petroleum Co. of Taiwan, developed the META-4^® process for propene production from but-2-ene and ethene, using Re₂O₇/Al₂O₃, at a low temperature of 35 °C and pressure of 60 bar [25,63,64]. A pilot plant with a propene production capacity of 15 kg/hour operated from 1988 to 1990, but the process was not commercialized due to the high cost of the catalyst and the requirement for high-purity reagents [63].

This paper presents the research results of the synthesis of rhenium catalysts, both homogeneous and heterogeneous, such as MTO, Re₂O₇/Al₂O₃, and M-Re₂O₇/Al₂O₃ (where M = Ni, Ag, Co, Cu), from rhenium compounds (ammonium perrhenate, perrhenic acid, nickel(II) perrhenate, cobalt(II) perrhenate, zinc perrhenate, silver perrhenate, copper(II) perrhenate) obtained from waste materials. These catalysts were characterized in terms of their chemical and phase composition, and their physicochemical properties were determined. The catalytic activity of the obtained catalysts, both homogeneous and heterogeneous, was evaluated in the model reactions, such as cyclohexene epoxidation to cyclohexene oxide and hex-1-ene homometathesis.

2. Materials and Methods

2.1. Materials

The materials used in this research are as follows: chlorotrimethylsilane (≥98.0%, Sigma-Aldrich, Poznań, Poland), tetramethyltin (for synthesis, Sigma-Aldrich, Poznań, Poland), hex-1-ene (98%, Thermo Scientific Chemicals, Waltham, MA, USA), cyclohexene (99% stab., Thermo Scientific Chemicals, Waltham, MA, USA), methyltrioxorhenium (98%, Thermo Scientific Chemicals, Waltham, MA, USA), aluminum nitrate nonahydrate (≥98.0%, VWR Chemicals, Gdańsk, Poland), acetonitrile (≥99.5%, VWR Chemicals, Gdańsk, Poland), chloroform-D (for NMR, 99.8% atom D, Thermo Scientific Chemicals, Waltham, MA, USA), tetramethylsilane (99.9%, Thermo Scientific Chemicals, Waltham, MA, USA), nitric acid (65%, VWR Chemicals, Gdańsk, Poland), sodium peroxide (p.a., VWR Chemicals, Gdańsk, Poland), and argon (99.5%, SIAD, Ruda Śląska, Poland).

The ammonium perrhenate (APR) of catalytic purity (69.4 wt% Re, Innovator Sp. z o.o., Gliwice, Poland) was produced from recycled materials, mainly superalloy scrap. Subsequently, using ion-exchange technology developed at the Łukasiewicz Research Network—Institute of Non-Ferrous Metals, perrhenic acid was obtained from ammonium perrhenate. The resulting perrhenic acid contained 295 g/dm³ of Re, <0.0002% of NH₄⁺, and <0.0001% of each of the following impurities: Ca, K, Mg, Cu, Na, Mo, Ni, Pb, Fe, Bi, Zn, W, As, and Al (Innovator Sp. z o.o., Gliwice, Poland) [65,66]. Nickel(II) perrhenate, cobalt(II) perrhenate, and zinc perrhenate were synthesized in studies described in the publications [67,68]. Silver perrhenate was obtained in a reaction of perrhenic acid with a solution containing Ag, derived from the processing of waste originating from the zinc industry (according to the method described in a patent application) [69]. Copper(II) perrhenate was obtained from copper(II) oxide (according to the method described in a patent application) [70]. CuO was produced from the leaching of sludges generated during electrolytic zinc production. The chemical composition of the above-mentioned rhenium compounds is presented in

Table 1. Chemical composition of rhenium compounds derived from waste materials (where M = metal).

Rhenium Compound	Composition [wt%], * [g/dm3]
	Re	M
AgReO4	51.8	30.3
Ni(ReO4)2	65.5	11.3
Cu(ReO4)2	65.6	10.9
Co(ReO4)2	66.8	10.4
Zn(ReO4)2	64.0	12.5
HReO4	295.02 *	-

.

2.2. Analytical Techniques

The rhenium, silver, copper, cobalt, nickel, and zinc content in the compounds was determined using inductively coupled plasma mass spectrometry (ICP-MS; NexION 300D, PerkinElmer, Waltham, MA, USA). For rhenium compounds, such as ammonium perrhenate, nickel(II) perrhenate, cobalt(II) perrhenate, zinc perrhenate, silver perrhenate, and copper(II) perrhenate, a sample of the material (0.2 g) was dissolved in concentrated nitric acid (10 cm³) at a temperature not exceeding 100 °C until complete dissolution. The solution was then transferred to a volumetric flask and diluted to a final volume of 100 cm³ with distilled water. Samples prepared in this way were subsequently subjected to analysis. For the Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ (where M = Ni, Ag, Co, Cu) catalysts, a sample of the material (0.2 g) was fused with sodium peroxide (1.0 g) at 600 °C for 30 min in a corundum crucible. The crucible containing the resulting melt was placed in a glass beaker, to which concentrated nitric acid (10 cm³) was added. The beaker was then heated to 100 °C until complete dissolution of the melt. The resulting solution was transferred to a volumetric flask and diluted to 100 cm³ with distilled water. The prepared samples were then subjected to analysis. Three repetitions were performed for each sample.

X-ray diffraction (XRD) analysis of the obtained heterogenous catalysts was performed using a Rigaku MiniFlex600 (Rigaku Co., Tokyo, Japan) diffractometer equipped with an X-ray tube with a wavelength of 1.5406 Å and a stripe detector, silicon D/TeX, and a high-resolution 2.5 Soller slit on the primary and scattered beam. All powder samples were dried and ground correctly in an agate mortar to obtain a homogenous powder without lumps. Then the grounded powder was put in a zero-background monocrystalline Si zero-background sample holder and measured. Data is as it was measured, and for the amorphous phase dominant sample, the noise is visible on the pattern. Data visualization and phase content analysis were performed using the Rigaku PDXL2 Software Package (Version 2.2.2.0).

¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra of methyltrioxorhenium, substrates, and products of the catalytical reactions were recorded using a Unity Inova Plus instrument from Varian. Tetramethylsilane was used exclusively during the measurement of the ¹H and ¹³C NMR spectra of methyltrioxorhenium obtained from AgReO₄ and M(ReO₄)₂ (where M = Ni, Zn, or Co). The signal from chloroform-D was used as the internal reference for the ¹H and ¹³C NMR spectra measurements of methyltrioxorhenium, as well as the substrates and products of the catalytic reactions.

The specific surface area was measured using a 3Flex analyzer (Micromeritics, Norcross, GA, USA), with a measurement range of specific surface area up to 0.01 m²/g; total surface area from 0.1 to 300 m². The analysis performed using the 3Flex analyzer also included the determination of the following sample parameters, pore volume (g/cm³) and pore size (Å), both calculated using the Horvath–Kawazoe method.

2.3. Experiment Procedures

The support for heterogenous catalysts precursors (alumina oxide) and heterogenous catalysts of the Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ type were prepared in a tube furnace (Tub.Frc, Wösthoff, Bochum, Germany) equipped with a quart tube, a thermocouple, and automatic control (Figure 1). During the calcination process, the quartz tube was sealed on both ends with rubber plugs fitted with hoses. An inert gas was introduced from one end, facilitating the removal of gaseous byproducts generated during the calcination from the opposite end.

2.3.1. Preparation of Support for Heterogeneous Catalysts

Aluminum oxide, serving as a support for heterogeneous rhenium catalysts of the Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ type, was synthesized from aluminum nitrate nonahydrate via a calcination process. This procedure was conducted in the tube furnace under a continuous flow of dried air (flow rate: 30 dm³/h) at temperatures of 400 °C or 600 °C for a duration of 4 h.

2.3.2. Synthesis of Heterogeneous Catalysts of Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ Type

Aluminum oxide (7 g; 0.068 mol) was impregnated with an aqueous solution of a rhenium compound (perrhenic acid, ammonium perrhenate, silver perrhenate, nickel(II) perrhenate, cobalt(II) perrhenate, or copper(II) perrhenate), with the rhenium concentration adjusted to achieve a catalyst precursor containing 30 wt% of Re in the final stage. The catalyst precursor was stirred at room temperature for 2 h, followed by heating at a temperature not exceeding 110 °C, until complete evaporation of the water. Subsequently, it was dried at a temperature not exceeding 110 °C for 4 h. The calcination process of the catalyst precursor was conducted in the tube furnace under a continuous flow of dried air (flow rate: 30 dm³/h) at a temperature of 550 °C for 2 h. Upon completion of the calcination process, the resulting catalyst was cooled in a stream of dried argon. The heterogeneous rhenium catalyst thus obtained was utilized in the catalytic reaction.

2.3.3. Synthesis of MTO According to Modified Recipe from AgReO₄ [17]

Silver perrhenate (1.0 g, 2.79 mmol) was dissolved in acetonitrile (20 cm³) and placed in a round-bottom flask equipped with a magnetic stirrer. The reaction mixture was stirred vigorously at room temperature for 30 min. Subsequently, chlorotrimethylsilane (0.78 cm³, 6.14 mmol) was added, resulting in the precipitation of silver chloride. Tetramethyltin (0.43 cm³, 3.07 mmol) was then introduced to the suspension. The mixture was stirred at room temperature for 20 h. After this period, the reaction mixture was filtered to remove the precipitate, and the volatile fractions were evaporated from the filtrate using a rotary evaporator. The crude product was purified by sublimation at 60 °C under a pressure of 250 mmHg. MTO was obtained with a yield of 89% as a white solid (620 mg). ¹H NMR (400 MHz, CDCl₃): δ = 2.63 ppm (s). ¹³C NMR (100 MHz, CDCl₃): δ = 19.30 ppm (s). The experiments were conducted under a well-ventilated fume hood, in accordance with standard laboratory safety protocols.

2.3.4. Synthesis of MTO According to Modified Recipe from M(ReO₄)₂ (Where M = Ni, Zn or Co) [17]

The specific metal perrhenate (2.79 mmol) was dissolved in acetonitrile (20 cm³) and placed in a round-bottom flask equipped with a magnetic stirrer. The reaction mixture was stirred vigorously at room temperature for 30 min. Subsequently, chlorotrimethylsilane (1.56 cm³, 12.28 mmol) was added, resulting in the metal chloride formation. Tetramethyltin (0.86 cm³, 6.14 mmol) was then introduced to the suspension. The mixture was stirred at room temperature for 20 h. After this period, the reaction mixture was filtered to remove the precipitate, and the volatile fractions were evaporated from the filtrate using a rotary evaporator. The crude products were purified by sublimation at 60 °C under a pressure of 250 mmHg. The experiments were conducted under a well-ventilated fume hood, in accordance with standard laboratory safety protocols.

2.3.5. The Procedure for Carrying out the Epoxidation Reaction of Cyclohexene to Cyclohexene Oxide Using MTO

MTO and UHP [23] (235 mg, 2.5 mmol) were placed in a glass, screw-on reaction vial (Wheaton 33, 8 cm³ capacity, with a screw cap and septum), equipped with a cylindrical magnetic stirrer (8 × 3 mm). Dried and deoxygenated cyclohexene (0.253 cm³, 2.5 mmol) and chloroform-D (1 cm³) were successively added. A stream of argon was passed through the resulting reaction mixture for 5 min, followed by intensive stirring (with a speed of 450 rpm) at 20 °C or 40 °C for 1, 2, 4, or 8 h. Upon completion of the reaction, the catalyst was separated from the reaction mixture. The composition of the reaction mixture was determined using the ¹H NMR spectroscopic method.

2.3.6. The Procedure for Carrying out the Homometathesis Reaction of Hex-1-ene to Dec-5-ene

The Re₂O₇/Al₂O₃ or M-Re₂O₇/Al₂O₃ (M = Ni, Ag, Co or Cu) catalyst was placed in a glass, screw-on reaction vial (Wheaton 33, 8 cm³ capacity, with a screw cap and septum), equipped with a cylindrical magnetic stirrer (8 × 3 mm). Dried and deoxidized hex-1-ene (1 cm³, 0.008 mol) and chloroform-D (1 cm³) were successively added. A stream of argon was passed through the resulting reaction mixture for 5 min, followed by intensive stirring (with a speed of 450 rpm) at 20 °C, 40 °C, or 80 °C for 30, 60, 90, or 120 min. Upon completion of the reaction, the catalyst was separated from the post-reaction mixture by filtration. The composition of the reaction mixture was determined using the ¹H NMR spectroscopic method.

2.4. Calculations

The synthesis yield of methyltrioxorhenium (relative to the amount of rhenium) was calculated using Formula (1):

$$\mathrm{W}={\displaystyle \frac{{\mathrm{m}}_{\mathrm{M}\mathrm{T}\mathrm{O}}\times {\mathrm{z}}_2}{{\mathrm{m}}_{\mathrm{M}\left[\mathrm{R}\mathrm{e}{\mathrm{O}}_4\right]}{\times \mathrm{z}}_1}}\times 100\%$$

(1)

• $\mathrm{W}$—yield in relation to the amount of rhenium.
• ${\mathrm{z}}_1$—rhenium content in metal perrhenate.
• ${\mathrm{m}}_{\mathrm{M}\left[\mathrm{R}\mathrm{e}{\mathrm{O}}_4\right]}$—mass of metal perrhenate used in the reaction to obtain MTO.
• ${\mathrm{z}}_2$—rhenium content in MTO (a constant value was assumed—74.71%).
• ${\mathrm{m}}_{\mathrm{M}\mathrm{T}\mathrm{O}}$—mass of obtained MTO.

The synthesis yield of heterogeneous rhenium catalysts (relative to the amount of rhenium) was calculated using Formula (2):

$$\mathrm{W}={\displaystyle \frac{{\mathrm{z}}_2}{{\mathrm{z}}_1}}\times 100\%$$

(2)

• $\mathrm{W}$—yield in relation to the amount of rhenium.
• ${\mathrm{z}}_1$—rhenium content in rhenium catalyst precursor.
• ${\mathrm{z}}_2-$rhenium content in the obtained rhenium catalyst after the calcination process.

The activity of the catalyst was determined by assessing the degree of conversion of the substrate to product(s) and calculated using Equation (3):

$$\mathrm{D}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{P}}}{{\mathrm{C}}_{\mathrm{S}}{+\mathrm{C}}_{\mathrm{P}}}}\times 100\%$$

(3)

• $\mathrm{D}$—degree of conversion of the substrate to product(s).
• ${\mathrm{C}}_{\mathrm{S}}$—the amount of substrate was determined based on the interpretation of spectrum ¹H NMR.
• ${\mathrm{C}}_{\mathrm{P}}$—the amount of product(s) was determined based on the interpretation of spectrum ¹H NMR.

3. Results

3.1. Synthesis and Characterization of Catalysts

3.1.1. Synthesis and Characterization of Methyltrioxorhenium

Studies were conducted on the synthesis of methyltrioxorhenium from silver perrhenate, nickel(II) perrhenate, zinc perrhenate, and cobalt(II) perrhenate, which were obtained from waste materials. MTO was synthesized in a two-step reaction, the scheme of which is presented in Scheme 1.

The specific metal perrhenate was dissolved in acetonitrile and an excess of chlorotrimethylsilane was added. Then, tetramethyltin was introduced into the resulting reaction mixture. The reaction mixture was stirred at room temperature for 20 h. After the given time had elapsed, the volatile fractions were evaporated from the post-reaction mixture using a rotary evaporator. When silver perrhenate was used, filtration was necessary; the precipitate—AgCl—was directed to silver recovery and the volatile fractions were evaporated from the filtrate. The obtained crude product was purified by sublimation—Figure 2. The pure product was obtained in the form of a white precipitate.

The results obtained from the tests on the synthesis of MTO using various metal perrhenates are summarized in

Table 2. Methyltrioxorhenium obtained from rhenium compounds produced from waste materials.

MTO-M	Mass of M[ReO4] [g]	Content of Re in M[ReO4] [wt%]	Mass of MTO [g]	Yield [%] 1
MTO-Ag	1.00	51.8	0.62	89
MTO-Ag 2	3.00	51.8	1.79	86
MTO-Ni	1.56	65.5	0.18 3	13
MTO-Zn	1.57	65.6	0.23 3	17
MTO-Co	1.56	66.8	0.21 3	15

M—metal from which MTO was obtained using metal perrhenate; ¹ synthesis yield was calculated using Formula (1)—Experimental Section (calculations); ² pilot scale; ³ product contaminated with tin compounds.

.

Using all the perrhenates obtained from waste materials, it was possible to synthesize methyltrioxorhenium. Pure methyltrioxorhenium (MTO-Ag) was obtained solely using silver perrhenate via a sublimation process, with a yield of >80% based on the rhenium content (Table 2). For the remaining rhenium compounds, the obtained MTO was contaminated with tin compounds and the process was characterized by a very low yield (≤17%). As part of the conducted studies, a pilot batch of MTO-Ag weighing ~1.8 g was obtained from silver perrhenate. The purity of the obtained MTO-Ag was confirmed using ¹H and ¹³C NMR spectroscopy (Figure 3 and Figure 4).

3.1.2. Synthesis and Characterization of Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ Type

Heterogeneous rhenium-based catalysts, Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ (where M = Ag, Ni, Co, Cu), were synthesized using rhenium precursors derived from waste materials. The synthesis involved the following rhenium compounds: ammonium perrhenate, perrhenic acid, silver perrhenate, copper(II) perrhenate, nickel(II) perrhenate, and cobalt(II) perrhenate. The catalyst preparation procedure consisted of several stages, including preparation of Al₂O₃ (the support material) from aluminum nitrate nonahydrate via calcination; preparation of a rhenium solution by dissolving the rhenium compound (or diluting perrhenic acid) in water; impregnation of the Al₂O₃ support with the aqueous rhenium solution; removal of water through evaporation; drying of the precursor in a laboratory drying oven; and calcination in the tube furnace to produce the heterogeneous rhenium catalyst. The flowchart illustrating the synthesis process of the heterogeneous rhenium catalysts is presented in Figure 5.

As a result of the conducted studies, seven heterogeneous catalysts were synthesized, exhibiting rhenium content conversion efficiencies ranging from 64% to 92% (

Table 3. Heterogeneous rhenium catalysts derived from waste materials.

Catalysts	wt% Re 1	wt% M 1	Yield [%] 2
Re2O7/Al2O3_4 3	20.9	-	70
Re2O7/Al2O3	24.8	-	83
Ag-Re2O7/Al2O3	27.7	14.8	92
Cu-Re2O7/Al2O3	19.2	5.02	64
Co-Re2O7/Al2O3	24.6	4.15	82
Ni-Re2O7/Al2O3	24.1	4.11	80
H-Re2O7/Al2O3	22.3	-	83

¹ Content of the individual elements (Re or M—metal) to the mass of M-Re₂O₇/Al₂O₃; ² the synthesis yield of catalysts was calculated using Formula (2)—Experimental Section (calculations); ³ support calcination temperature—400 °C.

). These catalysts differed in their chemical composition due to the utilization of various rhenium compounds during the synthesis process.

The key parameter characterizing the Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ catalysts (where M = Ag, Ni, Co, Cu) in terms of their further application in catalysis is the rhenium content. Aluminum oxide (Al₂O₃), serving as the support material, possesses five surface OH⁻ groups: two types with basic character, one neutral, and two types with acidic character [27,49,59,71]. During catalyst preparation, a reaction occurs between the ReO₄⁻ ions from the rhenium precursor and the surface OH⁻ groups of the support. ReO₄⁻ ions are transformed into Re₂O₇, which is then deposited onto the support surface. The properties of the resulting Re₂O₇ and, consequently, the heterogeneous catalyst depend on the type of OH⁻ group with which the ReO₄⁻ ions from the rhenium precursor react [25,28]. At low Re concentrations (~3 wt%), ReO₄⁻ ions react with the most basic OH⁻ groups, forming inactive centers on the catalyst surface. In contrast, at high Re concentrations (~18 wt%), these ions react with the neutral and more acidic OH⁻ groups, resulting in the formation of active centers on the catalyst surface [25,28]. In the synthesis of heterogeneous rhenium-based catalysts (Table 3), the aforementioned aspects were taken into account, with the rhenium content in the catalyst precursor set at 25–30 wt%. During the calcination process of the rhenium catalyst precursor, volatile Re₂O₇ is formed, which easily sublimes at 297 °C. This phenomenon was observed on the walls of the quartz tube of the tubular furnace (Figure 6). As a result, the Re content in each of the obtained catalysts decreased compared to the precursor by 8–36% (Table 3).

XRD analysis was performed on all the synthesized catalysts to determine the form in which ReO₄⁻ is deposited on the surface of the support (Al₂O₃). Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13 presents the XRD patterns of the heterogeneous Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ (where M = Ag, Ni, Co, Cu) catalysts, with Re content ranging from 19.2 to 27.7 wt% (Table 3). Three catalysts, namely Re₂O₇/Al₂O₃_4, Re₂O₇/Al₂O₃, and H-Re₂O₇/Al₂O₃, primarily exhibited an amorphous and nanocrystalline phase of the high-temperature γ-Al₂O₃, with a varied crystallite size distribution (Figure 7, Figure 8 and Figure 13). This was evidenced by the XRD peaks corresponding to the stoichiometric Al₂O₃ and non-stoichiometric Al_2.66O₄ or (Al₂O₃)_1.333. Characteristic peaks were observed only in the XRD pattern obtained for the Ni-Re₂O₇/Al₂O₃ catalyst, originating from the crystalline, volatile Re₂O₇ deposited on the Al₂O₃ surface, as well as peaks corresponding to rhenium oxide in a lower oxidation state, ReO₃ (Figure 12). In the XRD patterns of the other catalysts (Ag-Re₂O₇/Al₂O₃, Cu-Re₂O₇/Al₂O₃, and Co-Re₂O₇/Al₂O₃), peaks corresponding to metal perrhenates and metal oxides were observed (Figure 9, Figure 10 and Figure 11).

For two selected catalysts, the physical properties were determined, namely BET surface area, pore volume, and pore size (

Table 4. Physical properties of heterogeneous rhenium-based catalysts—Re₂O₇/Al₂O₃.

Catalysts	wt% Re	Specific Surface Area [m2/g]	Volume Size [cm3/g]	Pore Size [Å]
Re2O7/Al2O3_4 1	20.9	80.3521 ± 1.7353	0.028979	7.148
Re2O7/Al2O3	24.8	74.1301 ± 1.9455	0.026899	7.418

¹ Support calcination temperature—400 °C.

). Both catalysts showed almost identical pore volume and pore size, despite significantly different Re wt% content (Table 4). In the case of the catalyst obtained through calcination of the support at a lower temperature, a slightly higher BET surface area was achieved (Table 4).

3.2. Catalytic Test

The homogeneous and heterogeneous rhenium catalysts obtained in this study were evaluated for their catalytic activity. An analysis was conducted to assess the influence of the following factors on the catalytic process: catalyst amount, reaction time, and temperature. The catalyst activity was determined by assessing the conversion degree of the substrate to the product(s), according to Equation (3)—Experimental Section (calculations).

3.2.1. Methyltrioxorhenium

MTO is a well-known homogeneous catalyst widely employed in oxidation reactions, particularly in the epoxidation of alkenes. MTO-Ag, synthesized from silver perrhenate, was tested for its catalytic activity in the model reaction of cyclohexene epoxidation to cyclohexene oxide, as depicted in Scheme 2.

The commercial MTO was also used in the performed catalytic tests. First, the effect of the amount of catalyst—MTO and MTO-Ag—(from 0.50 to 1.0 mol%) at room temperature for 2 h on the conversion of cyclohexene to cyclohexene oxide was examined (Figure 14a). The highest catalytic activity >70% was obtained using 1 mol% MTO-Ag (Figure 14a). In further studies, the influence of time (from 1 to 8 h) on the course of the epoxidation process was determined (conditions: 1.0 mol% MTO or MTO-Ag, room temperature)—Figure 14b. After 2 h, the catalytic activity was >70% for both catalysts, and after 8 h, the thermodynamic equilibrium of the reaction shifted even more towards the product—cyclohexene oxide (Figure 14b).

Figure 15 shows a comparison of the ¹H NMR spectra recorded after 1 and 8 h. The figure highlights the characteristic peaks originating from the substrate (cyclohexene) and the reaction product (cyclohexene oxide), based on which the conversion degree was determined.

Additionally, the tests were also performed at 20 °C (room temperature) and 40 °C (conditions: 1.0 mol% MTO, reaction time—2 h)—Figure 14c. It turned out that an increase in the temperature adversely affected the course of the epoxidation reaction. Figure 16 illustrates a comparison of the ¹H NMR spectra recorded at temperatures of 20 °C and 40 °C.

The conducted studies unequivocally demonstrated that MTO-Ag synthesized from waste materials exhibited high catalytic activity exceeding 70%. This performance is comparable to, and in some cases surpasses, that of the commercial MTO (Figure 14). MTO-Ag was characterized by high catalytic activity of >70% in the epoxidation of cyclohexene to cyclohexene oxide under the following conditions: 1.0 mol% MTO, room temperature, and reaction time—2 h.

3.2.2. Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ Type

Re₂O₇/Al₂O₃ is a well-known catalyst for olefin metathesis [25,28,71,72,73]. As a model reaction for conducting catalytic tests with the heterogeneous catalysts obtained in the present study—Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ (where M = Ag, Ni, Co, Cu)—the homometathesis of hex-1-ene to dec-5-ene was selected (Scheme 3).

The course of the homometathesis reaction of hex-1-ene was monitored using NMR spectroscopy. The effect of the catalyst amount (ranging from 2.5 to 10 wt% of Re) on the conversion degree of hex-1-ene was investigated (Figure 17a). The reaction was carried out at room temperature for 2 h. The results showed that the catalysts Ag-Re₂O₇/Al₂O₃, Cu- Re₂O₇/Al₂O₃, and H-Re₂O₇/Al₂O₃ were completely inactive, while the Co-Re₂O₇/Al₂O₃ and Ni-Re₂O₇/Al₂O₃ catalysts exhibited low catalytic activity (<43%) at 10 wt% of Re. In comparison, the Re₂O₇/Al₂O₃ catalyst demonstrated a catalytic activity of >70% at a loading of 2.5 wt% of Re, while Re₂O₇/Al₂O₃_4 exhibited >70% catalytic activity at a lower loading of 5.0 wt% of Re (Figure 17a). Further catalytic tests were conducted using catalysts that exhibited >70% catalytic activity.

In further studies, the effect of the reaction time (ranging from 30 to 120 min) on the homometathesis of hex-1-ene was evaluated (conditions: 2.5 wt% of Re, room temperature)—Figure 17b. As the reaction time increased, the conversion degree of hex-1-ene also increased, with Re₂O₇/Al₂O₃ demonstrating the highest catalytic activity, achieving >70% after 60 min (Figure 17b).

Additionally, the effect of the temperature (ranging from 20 to 80 °C) on the homometathesis reaction for the Re₂O₇/Al₂O₃ catalyst was investigated (conditions: 2.5 wt% of Re, reaction time—2 h)—Figure 17c. Increasing the reaction temperature resulted in a significant increase in the catalytic activity, reaching >96% for this catalyst (Figure 17c).

The analysis of the obtained results revealed that both the Re₂O₇/Al₂O₃ and Re₂O₇/Al₂O₃_4 catalysts exhibited catalytic activity of >70%, with catalyst loadings of 2.5 wt% and 5 wt% of Re, respectively (conditions: room temperature, and reaction time—2 h)—Figure 17.

In the final stage of the conducted studies, the composition of the post-reaction mixture obtained from the homometathesis of hex-1-ene using two rhenium catalysts, Re₂O₇/Al₂O₃_4 and Re₂O₇/Al₂O₃, both containing 10 wt% of Re, was determined. Based on the analysis of the ¹H NMR and ¹³C NMR spectra, two-dimensional (2D) NMR spectra, such as COSY and HMQC, and the presence of the main product, dec-5-ene, as well as numerous side products, was unequivocally identified in the post-reaction mixture. Among the side products, a terminal alkene (pent-1-ene) and internal alkenes (hex-2-ene, hept-2-ene, oct-3-ene, non-4-ene) were identified—Figure 18, Figure 19, Figure 20 and Figure 21.

The presence of pent-1-ene was not unexpected, as it is known to be produced as one of the metathesis products of hex-1-ene [51,53]. Terminal alkenes are formed in the metathesis reaction of alkenes containing internal carbon–carbon double bonds with ethene (ethyleneolysis) or another alk-1-ene [5,38]. The analysis of the reaction products, performed based on the NMR spectra (Figure 10, Figure 11, Figure 12 and Figure 13), revealed that initially, hex-1-ene reacted with the Re₂O₇/Al₂O₃ catalyst, leading to the formation of active sites via the cleavage of the carbon–carbon double bond. This process resulted in the formation of a carbene species localized on the Re ion [53]. By eliminating the smallest fragment derived from hex-1-ene, an alkylidene complex (Re=CH₂) and pent-1-ene were formed, followed by the generation of other alk-1-enes, which are not always detectable, partly due to their high volatility (Equations (4)–(8)) [53].

$${\mathrm{Re}}^{\mathrm{n}+}+{\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CH}}_2 \to \mathrm{Re}*={\mathrm{CH}}_2+{\mathrm{CH}}_3({\mathrm{CH}}_2{)}_2\mathrm{CH}={\mathrm{CH}}_2$$

(4)

$${\mathrm{Re}}^{\mathrm{n}+}+{\mathrm{CH}}_3({\mathrm{CH}}_2{)}_2\mathrm{CH}={\mathrm{CH}}_2 \to \mathrm{Re}*={\mathrm{CH}}_2+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}={\mathrm{CH}}_2$$

(5)

$$2\mathrm{Re}*={\mathrm{CH}}_2 \to {2\mathrm{Re}}^{(\mathrm{n}-2)+}+{\mathrm{CH}}_2={\mathrm{CH}}_2$$

(6)

$$\mathrm{Re}*={\mathrm{CH}}_2+{\mathrm{CH}}_2={\mathrm{CH}}_2 \to {\mathrm{Re}}^{(\mathrm{n}-2)+}+{\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2$$

(7)

$$\mathrm{Re}*={\mathrm{CH}}_2+{\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2 \to {\mathrm{Re}}^{(\mathrm{n}-2)+}+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}={\mathrm{CH}}_2$$

(8)

However, as the process progressed, in addition to pent-1-ene, further metathesis products of hex-1-ene were formed, namely hept-2-ene, hex-2-ene, oct-3-ene, and non-4-ene, and the main product, dec-5-ene, as shown in Equations (9)–(13).

$${\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CH}}_2+{\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2 \to {\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CHCH}}_3+{\mathrm{CH}}_2={\mathrm{CH}}_2$$

(9)

$${\mathrm{CH}}_3({\mathrm{CH}}_2{)}_2\mathrm{CH}={\mathrm{CH}}_2+{\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2 \to {\mathrm{CH}}_3({\mathrm{CH}}_2{)}_2\mathrm{CH}={\mathrm{CHCH}}_3+{\mathrm{CH}}_2={\mathrm{CH}}_2$$

(10)

$${\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CH}}_2+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}={\mathrm{CH}}_2 \to {\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CHCH}}_2{\mathrm{CH}}_3+{\mathrm{CH}}_2={\mathrm{CH}}_2$$

(11)

$${\mathrm{CH}}_3({\mathrm{CH}}_2{)}_2\mathrm{CH}={\mathrm{CH}}_2+{\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CH}}_2 \to {\mathrm{CH}}_3({\mathrm{CH}}_2{)}_2\mathrm{CH}=\mathrm{CH}({\mathrm{CH}}_2{)}_3{\mathrm{CH}}_3+{\mathrm{CH}}_2={\mathrm{CH}}_2$$

(12)

$${\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CH}}_2+{\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}={\mathrm{CH}}_2 \to {\mathrm{CH}}_3({\mathrm{CH}}_2{)}_3\mathrm{CH}=\mathrm{CH}({\mathrm{CH}}_2{)}_3{\mathrm{CH}}_3+{\mathrm{CH}}_2={\mathrm{CH}}_2$$

(13)

Based on the reactions in Equations (4)–(13), which describe of the individual stages of the metathesis reaction of hex-1-ene with Re₂O₇/Al₂O₃, it can be concluded that alkenes with terminal carbon–carbon double bonds were formed as a result of the interaction between hex-1-ene and the catalyst. On the other hand, alkenes with internal carbon–carbon double bonds were products of the metathesis of the starting alkene or co-metathesis of this alkene with the main reaction products. According to the metathesis reaction mechanism involving heterogeneous catalysts, the key step of the process was the formation of the metal–carbene complex [53].

4. Conclusions

In this work, rhenium catalysts, including MTO, Re₂O₇/Al₂O₃, and M-Re₂O₇/Al₂O₃ (where M = Ni, Ag, Co, or Cu), were successfully synthesized from rhenium compounds obtained from waste materials. MTO was successfully obtained from silver perrhenate and isolated as a white solid with the yield exceeding 80%. However, MTOs obtained from nickel(II) perrhenate, cobalt(II) perrhenate, and zinc perrhenate were contaminated with tin compounds and showed low yield of synthesis (≤17%). The Re₂O₇/Al₂O₃ and M-Re₂O₇/Al₂O₃ catalysts were prepared using ammonium perrhenate, perrhenic acid, silver perrhenate, copper(II) perrhenate, nickel(II) perrhenate, and cobalt(II) perrhenate, with alumina obtained by calcination of aluminum nitrate nonahydrate acting as the support. MTO from silver perrhenate showed comparable or even higher catalytic activity compared to commercial MTO in the model epoxidation reaction of cyclohexene to cyclohexene oxide, reaching catalytic activity above 70% under the following conditions: 1.0 mol% of MTO, room temperature, and reaction time—2 h. The Ag-Re₂O₇/Al₂O₃, Cu-Re₂O₇/Al₂O₃, and H-Re₂O₇/Al₂O₃ catalysts were completely inactive, while Co-Re₂O₇/Al₂O₃ and Ni-Re₂O₇/Al₂O₃ showed low catalytic activity (<43%) in the model homometathesis reaction of hex-1-ene. Only Re₂O₇/Al₂O₃ catalysts showed catalytic activity above 70% in the homometathesis reaction of hex-1-ene under the following conditions: 2.5 wt% of Re, room temperature, and reaction time—2 h. The products of this reaction were terminal alkenes and “internal” alkenes. These results emphasize the potential of using rhenium compounds derived from waste materials to synthesize effective catalysts, in particular MTO and Re₂O₇/Al₂O₃, for use in catalytic processes. The use of rhenium compounds derived from waste in the production of catalysts is an important step towards sustainable development and recycling of raw materials.