Process for the base-catalyzed reaction of silicone with hot water or supercritical water

Abstract

The present invention provides a process for reacting silicone with hot water or supercritical water at at least 200 °C in the presence of a base to form silica and hydrocarbons.

Description

Technical Field

[0001] The present invention relates to a base-catalyzed process for the reaction of silicone with hot water or supercritical water.

Background Art

[0002] When cured silicone compounds are discarded at the end of their life cycle, they usually end up in landfills where they decompose very slowly. Usually, they are then incinerated. This leads to a major problem because they silicate and clog the filter. In this case, dust is obtained as a by-product together with ash, which requires disposal and is costly and inconvenient.

[0003] Examples of more sustainable solutions for silicone recycling include acid or base-catalyzed equilibria that can discharge cyclic siloxanes from the process. What occurs here is only the cleavage of the Si-O-Si bond, and no cleavage of the Si-C bond occurs. However, residues, highly contaminated rubber or resin are not suitable for this. In addition, the residue is highly toxic due to the strongly basic reaction conditions.

[0004] The hydrothermal treatment of silicone rubber, i.e., the reaction of silicone rubber with water at high temperature and high pressure, also makes it possible to selectively cleave the Si-C bond to form hydrocarbons and silica in addition to the Si-O-Si bond cleavage available by equilibration. However, this process is very slow and thus not suitable for transfer to an industrial process.

Summary of the Invention

[0005] The present invention provides a process for reacting silicone with hot water or supercritical water at at least 200° C. in the presence of a base to form silica and hydrocarbons.

[0006] In this context, water referred to as hot water is subcritical water with a temperature exceeding 200 °C and exists in a liquid state as a result of a pressure above the vapor pressure associated with the temperature. Due to the prevailing pressure conditions, the water remains liquid far above 100 °C. Supercritical water refers to water at a temperature and pressure above the critical point at 374.12 °C and 22.1 MPa.

[0007] Surprisingly, success has been achieved in cleaving Si-C bonds and Si-O bonds in silicone and generating hydrocarbons, particularly methane, and silica by means of a hot water reaction or a reaction with supercritical water.

[0008] Bases accelerate this process and enable complete cleavage of the silicone within a few hours. In addition, amorphous silica is converted by the base into crystalline silica. These products can be fed back into the silicone manufacturing process in the context of a circular economy. Depending on the amount of base added, it is further possible to form water glass (alkyl silicate) from the silica formed by reacting the silicone and from any silica present as a filler, and this water glass can be used as an additive in many sectors, for example in the construction industry.

[0009] The term silicone encompasses oligomeric or polymeric organosiloxanes in which silicon atoms are bonded via oxygen atoms and at least some of the silicon atoms have one or more organic substituents, and compositions containing organosiloxanes such as silicone rubber.

[0010] Silicones preferably contain a maximum of 1 wt%, particularly preferably a maximum of 0.1 wt%, especially a maximum of 0.01 wt% of halogen, in particular chlorine.

[0011] Compositions containing organosiloxanes can contain, in addition to the organosiloxanes, for example fillers, catalysts, binders and pigments. Fillers are, for example, fumed silica and / or precipitated silica, silicone resins, chalk and quartz.

[0012] It is preferable to use silicone rubber filled with silicone resin or silica. It is particularly preferable to use silicone rubber containing a filler selected from silica and silicone resin.

[0013] Examples of suitable bases are alkali metal hydroxides and alkaline earth metal hydroxides such as LiOH, NaOH, KOH, RbOH, CsOH, Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2, alkali metal carbonates and alkaline earth metal carbonates such as Li2CO3, Na2CO3, K2CO3, alkali metal hydrogen carbonates and alkaline earth metal hydrogen carbonates such as LiHCO3, NaHCO3, KHCO3, alkali metal phosphates and alkaline earth metal phosphates such as Li3PO4, Na3PO4, K3PO4, Ca3(PO4)2, amines such as ethylenediamine, diethylenetriamine, amides such as sodium amide and potassium amide.

[0014] Inorganic bases, particularly alkali metal hydroxides, alkali metal carbonates and alkali metal hydrogen carbonates are preferred.

[0015] This process can be carried out batchwise or continuously.

[0016] The solid mixture obtained by this process preferably contains a silica modification selected from quartz, keatite and cristobalite.

[0017] The solid mixture obtained by this process preferably contains up to 2% by weight, particularly preferably up to 1% by weight, especially up to 0.5% by weight of carbon.

[0018] In the case of methyl-containing silicone, the gas mixture obtained contains methane as the hydrocarbon. It is possible to produce up to 550 g of methane per kg of silicone used.

[0019] During decomposition, silicones having phenyl groups or higher alkyl groups form benzene or the corresponding alkanes, which then exist as the organic phase in addition to the aqueous phase.

[0020] The preferred temperature in this process is from 250 °C to 500 °C, particularly from 280 °C to 400 °C, and especially preferably from 300 °C to 390 °C. The preferred pressure in this process is from 10 to 400 bar, particularly preferably from 20 to 320 bar, especially 50 to 280 bar, the lower limit of the pressure in each case being the vapor pressure of the reaction mixture in relation to the reaction temperature. The preferred residence time in this process is from 1 minute to 48 hours, particularly preferably from 5 minutes to 24 hours, especially 10 minutes to 12 hours.

[0021] In a preferred embodiment, after completion of the reaction, the reaction mixture is depressurized to atmospheric pressure and the resulting vapors are used for heat recovery or preheating of the reactants. As a result, most of the heat can be recovered.

[0022] The following analytical methods and analytical apparatuses are used for characterization.

Examples

[0023] Gas chromatography for determining the composition of the gas phase The gas phase is analyzed on an Agilent 6890 GC gas chromatograph using a CP-Molsieve 5Å column with dimensions 25 m, 0.32 mm and 30 μm. The gas composition is evaluated by integrating the detected signals.

[0024] NMR spectroscopy for determining the silicon-containing components in the aqueous phase 1 H NMR and 29 Si NMR spectra are recorded on a Bruker Avance 500 or Ascend 500 ( 1 for the H NMR spectrum at 500 MHz, 29For Si NMR spectra, measurements are carried out in DMSO-d6 or D2O at 99.4 MHz. All measurements are referenced to TMS as an external standard. The relative ratios of the components in the aqueous phase are determined by integrating the signals of each set. The absolute concentration in water is determined taking into account the mixing ratio of the aqueous phase to DMSO-d6.

[0025] Analysis of the isolated solid CHN analysis The measurement of oxygen and hydrogen is carried out with an ONH836 elemental analyzer. The carbon content is measured with a CS844 elemental analyzer. For the analysis, a 10 - 20 mg sample is required and decomposed by the supply of energy. Then, hydrogen is measured in the form of water and oxygen is measured by IR absorption in the form of CO and CO2. Carbon is first reduced and then converted to CO and CO2 in an oxygen atmosphere and detected by IR absorption. Calibration is carried out for oxygen against SiO2, for hydrogen against TiH2, and for carbon against acetanilide, sodium bicarbonate and calcium carbonate.

[0026] XRD analysis To determine the composition of the crystalline components of the solid, 5 g of the powder is ground and then analyzed with a PANalytical Empyrean powder X-ray diffractometer. The analysis is carried out by X-ray diffraction using copper radiation (I = 1.54 Å).

[0027] Reaction process All experiments were carried out in an Inconel(R) pressure autoclave equipped with a temperature sensor and a pressure sensor. The progress of the reaction was recorded by a pressure-temperature curve and the reaction products were analyzed after the reaction was completed.

[0028] Silicone rubber was used in the form of a cube with an edge length of 0.5 cm.

[0029] [Example 1 (Non-invention)] 24 g of silicone rubber (polydimethylsiloxane rubber WACKER ELASTOSIL(R) 401 / 60E) and 100 ml of water were weighed and heated at 374 °C for 12 hours. Then, the pressure autoclave was cooled to room temperature. The residual pressure of 9 bar was measured. As a result of the analysis, the generation of methane was confirmed. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 18.1 g of solid was obtained.

[0030] [Example 2 (Non-invention)] 24 g of silicone rubber (WACKER ELASTOSIL(R) 401 / 60E) and 100 ml of water were weighed and heated to 374 °C until the pressure became constant (120 hours). Then, the pressurized autoclave was cooled to room temperature. The residual pressure of 20.2 bar was measured. As a result of the analysis, the generation of methane was confirmed. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 18.7 g of solid was obtained.

[0031] [Example 3 (The present invention)] 24 g of silicone rubber (WACKER ELASTOSIL(R) 401 / 60E), 70 mg of potassium hydroxide and 100 ml of water were weighed and heated to 350 °C for 12 hours. After 4 hours, a constant pressure was already recorded. Then, the pressure autoclave was cooled to room temperature. The residual pressure of 24.2 bar was measured. As a result of the analysis, the generation of methane was confirmed. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 20.7 g of solid was obtained.

[0032] [Example 4 (The present invention)] 24 g of silicone rubber (WACKER ELASTOSIL(R) 401 / 60E), 70 mg of potassium hydroxide and 100 ml of water were weighed and heated to 300 °C for 12 hours. After about 5.5 hours, a constant pressure was already recorded. Then, the pressure autoclave was cooled to room temperature. The residual pressure of 24.0 bar was measured. As a result of the analysis, the generation of methane was confirmed. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 20.1 g of solid was obtained.

[0033] [Example 5 (The present invention)] 24 g of silicone rubber (WACKER SilGel(R) 612), 70 mg of potassium hydroxide and 100 ml of water were weighed and heated at 350 °C for 12 hours. After 3 hours, a certain pressure had already been recorded. Then, the pressure autoclave was cooled to room temperature. A residual pressure of 30.2 bar was measured. The analysis result confirmed the generation of methane. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 29.6 g of solid was obtained.

[0034] [Example 6 (of the present invention)] 24 g of silicone rubber (WACKER ELASTOSIL(R) 401 / 60E), 80 mg of sodium hydroxide and 100 ml of water were weighed and heated at 350 °C for 12 hours. After 1 hour, a certain pressure had already been recorded. Then, the pressure autoclave was cooled to room temperature. A residual pressure of 23.3 bar was measured. The analysis result confirmed the generation of methane. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 19.8 g of solid was obtained.

[0035] [Example 7 (of the present invention)] 24 g of silicone rubber (WACKER ELASTOSIL(R) 401 / 60E), 80 mg of lithium hydroxide and 100 ml of water were weighed and heated at 350 °C for 12 hours. Immediately after heating, a certain pressure was recorded. Then, the pressure autoclave was cooled to room temperature. A residual pressure of 24.1 bar was measured. The analysis result confirmed the generation of methane. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 18.9 g of solid was obtained.

[0036] [Example 8 (of the present invention)] 24 g of silicone rubber (WACKER ELASTOSIL(R) 401 / 60E), 80 mg of sodium hydrogen carbonate and 100 ml of water were weighed and heated at 350 °C for 12 hours. After 5 hours, a certain pressure was recorded. Then, the pressure autoclave was cooled to room temperature. A residual pressure of 24.1 bar was measured. The analysis result confirmed the generation of methane. After removing the positive pressure, the solid was separated from the aqueous phase and dried. 19.7 g of solid was obtained.

[0037] [Example 9 (the present invention)] 24 g of phenyl-containing silicone rubber (WACKER ELASTOSIL(R) 490 / 55OH), 80 mg of KOH, and 100 ml of water were weighed and heated at 350 °C for 12 hours. A constant pressure was detected after about 4 hours. Thereafter, the pressure autoclave was cooled to room temperature, and a residual pressure of 22.3 bar was measured. As a result of gas analysis, the generation of methane was confirmed. After removing the positive pressure, the solid was separated from the liquid and dried. 18.3 g of solid was isolated. The liquid showed a two-phase mixture consisting of an aqueous phase and a benzene phase (about 1 ml), which were separated and identified by NMR spectroscopy.

[0038] [Example 10 (the present invention)] 24 g of silicone rubber (WACKER ELASTOSIL(R) 401 / 60E), 10 g of sodium hydroxide, and 100 ml of water were weighed and heated at 350 °C for 12 hours. A constant pressure was recorded immediately after heating. Thereafter, the pressure autoclave was cooled to room temperature. A residual pressure of 23.1 bar was measured. As a result of the analysis, the generation of methane was confirmed. After removing the positive pressure, the aqueous solution was isolated. The solid content of the aqueous phase was 28.5%. The sodium silicate formed can be isolated by drying the aqueous solution and shows a residual carbon content of 0.07%.

[0039] [Table 1]

Claims

1. A process of reacting silicone with hot water or supercritical water at at least 200 °C in the presence of a base to form silica and hydrocarbons.

2. The process according to claim 1, wherein the term "silicone" encompasses oligomeric organosiloxanes or polymeric organosiloxanes in which silicon atoms are bonded via oxygen atoms and at least some of the silicon atoms have one or more organic substituents, and compositions containing organosiloxanes.

3. The process according to one or more of claims 1 and 2, wherein the term "silicone" means a silicone rubber containing a filler selected from silica and silicone resin.

4. The process according to one or more of claims 1 to 3, wherein the silicone contains up to 1 wt% halogen.

5. The process according to one or more of claims 1 to 4, wherein the base is selected from alkali metal hydroxides and alkaline earth metal hydroxides, alkali metal carbonates and alkaline earth metal carbonates, alkali metal bicarbonates and alkaline earth metal bicarbonates, alkali metal phosphates and alkaline earth metal phosphates, amines and amides.

6. The process according to one or more of claims 1 to 5, wherein the temperature is 300 °C to 400 °C.

7. The process according to one or more of claims 1 to 6, wherein the pressure is 10 to 400 bar.

8. The process according to one or more of claims 1 to 7, wherein a methyl-containing silicone is used and the hydrocarbon formed is methane.

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