METHOD FOR THE BASE-CATALYZED CONVERSION OF SILICONS WITH HYDROTHERMAL OR SURCRITICAL WATER

Description

[0001] The invention relates to a base-catalyzed process for reacting silicon with hydrothermal or supercritical water.

[0002] CN 111 498 854 discloses a process for the production of silicon dioxide by reacting silicon powder with supercritical water in a basic environment, wherein the temperature is 374°C to 450°C. The reaction time is 0.1 to 4 hours. The advantage of the process is, among other things, that a product of high purity is obtained.

[0003] When cured silicone sealants are disposed of at the end of their life cycle, they usually end up in landfills, where they decompose very slowly. They are then often incinerated, which causes major problems because they silicify in the filters and clog them. The dust is produced along with the ash as a byproduct and must be disposed of in a complex and costly manner.

[0004] More sustainable solutions for silicone recycling include acid- or base-catalytic equilibration, which allows cyclic siloxanes to be removed from the process. This involves only Si-O-Si bond cleavage, but not Si-C bond cleavage. However, residues, heavily contaminated rubbers, or resins are unsuitable for this method. Furthermore, the residue is highly toxic due to the strongly alkaline reaction conditions.

[0005] Hydrothermal treatment of silicone rubbers, i.e., the reaction of silicone rubber with water at high temperatures and pressures, allows for the selective cleavage of the Si-O-Si bond, in addition to the Si-C bond cleavage achievable through equilibration, forming hydrocarbons and silica. However, this process is very slow and therefore unsuitable for industrial application.

[0006] The invention relates to a process in which silicon is reacted in the presence of a base with hydrothermal water of at least 200°C or supercritical water to form silica and hydrocarbon.

[0007] In this context, hydrothermal refers to non-supercritical water at temperatures above 200°C, which exists in a liquid state due to pressure greater than or equal to the vapor pressure corresponding to that temperature. Because of the prevailing pressure conditions, the water remains liquid even at temperatures well above 100°C. Supercritical water is defined as water at temperatures and pressures higher than or equal to the critical point at 374.12°C and 22.1 MPa.

[0008] Surprisingly, it has been possible to cleave Si-C and Si-O bonds in silicones through hydrothermal reactions or reactions with supercritical water, yielding hydrocarbons, particularly methane, as well as silicic acid. The base accelerates this process, achieving complete cleavage of the silicone within a few hours. Additionally, the base converts amorphous silica into crystalline silica. These products can then be reintroduced into the silicone manufacturing process, promoting a circular economy approach. Depending on the amount of base added, it is also possible to form water glass (alkyl silicates) from the silicic acid produced by the silicone reaction and any silicic acid present as a filler. Water glass can be used as an additive in many sectors (e.g., the construction industry).

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

[0010] The silicones preferably contain at most 1 wt.%, particularly preferably at most 0.1 wt.%, in particular at most 0.01 wt.% of halogens, especially chlorine.

[0011] Compositions containing organosiloxanes may also contain, for example, fillers, catalysts, binders, and pigments. Examples of fillers include pyrogenic and / or precipitated silica, silicone resin, chalk, and quartz.

[0012] The use of silicone rubber filled with silica or silicone resin is preferred. Silicone rubber containing a filler selected from silica and silicone resin is particularly preferred.

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

[0014] The process can be carried out discontinuously or continuously.

[0015] The solid mixture obtained in the process preferably contains silica modifications selected from quartz, keatite and cristobalite.

[0016] The solid mixture obtained in the process preferably contains at most 2 wt.%, particularly preferably at most 1 wt.%, and in particular at most 0.5 wt.% carbon.

[0017] In the case of methyl-containing silicones, the resulting gas mixture contains methane as a hydrocarbon. Up to 550 g of methane can be produced per kg of silicone used. Silicones with phenyl or higher alkyl groups form benzene or the corresponding alkanes during decomposition, which then exist as an organic phase alongside the aqueous phase.

[0018] The preferred temperature in the process is 250°C to 500°C, particularly 280°C to 400°C, and especially preferably 300°C to 390°C. The preferred pressure in the process is 10 to 400 bar, particularly preferably 20 to 320 bar, and especially preferably 50 to 280 bar, the lower limit of which is the vapor pressure of the reaction mixture corresponding to the reaction temperature. The preferred residence time in the process is 1 minute to 48 hours, particularly preferably 5 minutes to 24 hours, and especially preferably 10 minutes to 12 hours.

[0019] In a preferred embodiment, the reaction mixture is depressurized to atmospheric pressure after the reaction is complete, and the resulting vapor is used for heat recovery or preheating of the reactants. This allows a large portion of the heat to be recovered.

[0020] The following analytical methods and instruments are used for characterization: Examples Gas chromatography for determining the composition of the gas phase

[0021] The gas phase analysis is performed on an Agilent 6890 GC gas chromatograph using a CP molecular sieve column with dimensions of 25 m, 0.32 mm, and 30 µm. The gas composition is evaluated by integrating the detected signals. NMR spectroscopy for the determination of silicon-containing components in the aqueous phase

[0022] The measurement of 1H NMR and 29Si NMR spectra is performed in dmso-d6 or D₂O using a Bruker Avance 500 or Ascend 500 (500 MHz for 1H NMR spectra and 99.4 MHz for 29Si NMR spectra). All measurements are referenced against TMS as an external standard. The relative proportions of the components in the aqueous phase are determined by integrating the respective signal sets. The absolute concentration in water is determined taking into account the mixing ratios of the aqueous phase to dmso-d6. Analysis of the isolated solid CHN analysis

[0023] The determination of oxygen and hydrogen is performed on an ONH836 elemental analyzer. The carbon content is determined on a CS844 elemental analyzer. For the analysis, 10-20 mg of sample are required and digested with the application of energy. Hydrogen is then determined as water, and oxygen as CO and CO₂, by IR absorption. Carbon is first reduced, then reacted under an oxygen atmosphere to form CO and CO₂, and detected by IR absorption. Calibration is performed against SiO₂ for oxygen, against TiH₂ for hydrogen, and against acetanilide, sodium bicarbonate, and calcium carbonate for carbon. XRD analysis

[0024] To determine the composition of the crystalline components of the solid, 5 g of powder are ground and then analyzed using a PANalytical Empyrean X-ray diffractometer. The analysis is performed by copper X-ray diffraction (I = 1.54 Å). Reaction process:

[0025] All experiments were conducted in an Inconel® pressure autoclave equipped with temperature and pressure sensors. The reaction progress was recorded using pressure-temperature curves, and the reaction products were analyzed after the reaction was complete.

[0026] The silicone rubber was used in the form of cubes with an edge length of 0.5 cm. Example 1 (not according to the invention):

[0027] 24 g of silicone rubber (polydimethylsiloxane rubber WACKER ELASTOSIL® < 401 / 60E) and 100 ml of water were weighed out and heated to 374 °C for 12 h. The pressure autoclave was then cooled to room temperature. A residual pressure of 9 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 18.1 g of solid were obtained. Example 2 (not according to the invention):

[0028] 24 g of silicone rubber (WACKER ELASTOSIL® < 401 / 60E) and 100 ml of water were weighed out and heated to 374 °C until a constant pressure was reached (120 h). The pressure autoclave was then cooled to room temperature. A residual pressure of 20.2 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 18.7 g of solid were obtained. Example 3 (according to the invention):

[0029] 24 g of silicone rubber (WACKER ELASTOSIL® < 401 / 60E), 70 mg of potassium hydroxide, and 100 ml of water were weighed out and heated to 350 °C for 12 h. A constant pressure was observed after only 4 h. The pressure autoclave was then cooled to room temperature. A residual pressure of 24.2 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 20.7 g of solid were obtained. Example 4 (according to the invention):

[0030] 24 g of silicone rubber (WACKER ELASTOSIL® < 401 / 60E), 70 mg of potassium hydroxide, and 100 ml of water were weighed out and heated to 300 °C for 12 h. A constant pressure was observed after approximately 5.5 h. The pressure autoclave was then cooled to room temperature. A residual pressure of 24.0 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 20.1 g of solid were obtained. Example 5 (according to the invention):

[0031] 24 g of silicone rubber (WACKER SilGel® < 612), 70 mg of potassium hydroxide, and 100 ml of water were weighed out and heated to 350 °C for 12 h. A constant pressure was observed after only 3 h. The pressure autoclave was then cooled to room temperature. A residual pressure of 30.2 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 29.6 g of solid were obtained. Example 6 (according to the invention):

[0032] 24 g of silicone rubber (WACKER ELASTOSIL® < 401 / 60E), 80 mg of sodium hydroxide, and 100 ml of water were weighed out and heated to 350 °C for 12 h. A constant pressure was observed after only 1 h. The pressure autoclave was then cooled to room temperature. A residual pressure of 23.3 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 19.8 g of solid were obtained. Example 7 (according to the invention):

[0033] 24 g of silicone rubber (WACKER ELASTOSIL® < 401 / 60E), 80 mg of lithium hydroxide, and 100 ml of water were weighed out and heated to 350 °C for 12 h. A constant pressure was observed immediately after heating. The pressure autoclave was then cooled to room temperature. A residual pressure of 24.1 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 18.9 g of solid were obtained. Example 8 (according to the invention):

[0034] 24 g of silicone rubber (WACKER ELASTOSIL® < 401 / 60E), 80 mg of sodium bicarbonate, and 100 ml of water were weighed out and heated to 350 °C for 12 h. A constant pressure was recorded after 5 h. The pressure autoclave was then cooled to room temperature. A residual pressure of 24.1 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, the solid was separated from the aqueous phase and dried. 19.7 g of solid were obtained. Example 9 (according to the invention):

[0035] 24 g of phenyl-containing silicone rubber (WACKER ELASTOSIL® < 490 / 55 OH), 80 mg of KOH, and 100 ml of water were weighed out and heated to 350 °C for 12 h. A constant pressure was detected after approximately 4 h. The pressure autoclave was then cooled to room temperature, and a residual pressure of 22.3 bar was measured. Analysis of the gas showed the formation of methane. After releasing the overpressure, the solid was separated from the liquid and dried. 18.3 g of solid were isolated. The liquid showed a two-phase mixture consisting of an aqueous phase and a benzene phase (approximately 1 ml), which were separated and identified by NMR spectroscopy. Example 10 (according to the invention):

[0036] 24 g of silicone rubber (WACKER ELASTOSIL® < 401 / 60E), 10 g of sodium hydroxide, and 100 ml of water were weighed out and heated to 350 °C for 12 h. A constant pressure was observed immediately after heating. The pressure autoclave was then cooled to room temperature. A residual pressure of 23.1 bar was measured. Analysis showed the formation of methane. After releasing the overpressure, an aqueous solution was isolated. The solids content of the aqueous phase was 28.5%. The resulting sodium silicate could be isolated by drying the aqueous solution and showed a residual carbon content of 0.07%. Table 1 Example Aqueous phase: Solid: m(Silanols) EA (C in %) XRD 1* 870 mg 14,2 - ‡< 2* 170 mg 3, 4 - ‡< 3 14 mg 0,10 52.2% Quartz, 22.8% Cristobalite 24% Keatite 4 68 mg 0,13 100% Keatite 5 30 mg 0,14 29.4% Quartz, 70.6% Keatite 6 17 mg 0,03 98.5% Quartz, 1.5% Keatite 7 32 mg 0,11 100% Quartz 8 48 mg 0,10 31% Quartz, 69% Keatite 9 15 mg 0,12 42% quartz, 33.2% cristobalite, 24.8% keatite 10 - - - * not according to the invention * Sample is amorphous

Claims

1. A process in which silicone is reacted in the presence of a base with hydrothermal water of at least 200°C or supercritical water to form silica and hydrocarbon.

2. The process as claimed in claim 1, in which the term silicone encompasses oligomeric or polymeric organosiloxanes in which silicon atoms are bonded via oxygen atoms and in which at least some of the silicon atoms bear one or more organic substituents, and compositions comprising organosiloxanes.

3. The process as claimed in one or more of the preceding claims, in which the term silicone means silicone rubber comprising filler which is selected from silica and silicone resin.

4. The process as claimed in one or more of the preceding claims, in which the silicones comprise at most 1% by weight of halogens.

5. The process as claimed in one or more of the preceding claims, in which the base is selected from alkali metal and alkaline earth metal hydroxides, alkali metal and alkaline earth metal carbonates, alkali metal and alkaline earth metal hydrogencarbonates, alkali metal and alkaline earth metal phosphates, amines and amides.

6. The process as claimed in one or more of the preceding claims, in which the temperature is 300°C to 400°C.

7. The process as claimed in one or more of the preceding claims, in which the pressure is 10 to 400 bar.

8. The process as claimed in one or more of the preceding claims, in which methyl-containing silicone is used and the hydrocarbon formed is methane.