Method for base-catalyzed reaction of silicone with hydrothermal or supercritical water

Abstract

A process is provided for reacting a silicone with hydrothermal water or supercritical water of at least 200 DEG 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 silicones with hydrothermal or supercritical water. Background Art

[0002] If cured silicone compounds are discarded after the end of their life cycle, they usually end up in landfills, where they can only decompose very slowly. They are usually then incinerated, which causes serious problems because they silicify in filters and thus clog them. In this case, dust is obtained as a by-product together with the ash and has to be discarded, which is expensive and inconvenient.

[0003] Examples of more sustainable solutions for silicone recycling include acid or base catalytic equilibration, by which cyclic siloxanes can be expelled from the process. Here only the Si-O-Si bonds are broken, without any breakage of Si-C bonds. However, residual, heavily contaminated rubber or resin is not suitable for this process. In addition, due to the strongly alkaline reaction conditions, the residues are highly toxic.

[0004] The hydrothermal treatment of silicone rubber, i.e. the reaction of silicone rubber with water at high temperature and high pressure, makes it possible to selectively cleave Si-C bonds to form hydrocarbons and silicon dioxide, in addition to the cleavage of Si-O-Si bonds achievable by equilibrium. However, this process is very slow and therefore not suitable for transfer to industrial processes. Summary of the invention

[0005] The present invention provides a process in which silicone is reacted with hydrothermal or supercritical water at a temperature of at least 200° C. in the presence of a base to form silicon dioxide and a hydrocarbon.

[0006] Water referred to as hydrothermal in this context is non-supercritical water at a temperature above 200° C., which exists in a liquid state due to a pressure greater than or equal to the vapor pressure associated with this temperature. Due to the pressure conditions that exist, water remains liquid at temperatures significantly above 100° C. Supercritical water refers to water at a temperature and pressure greater than or equal to the critical point of 374.12° C. and 22.1 MPa.

[0007] Unexpectedly, cleavage of Si-C and Si-O bonds in silicones by hydrothermal reactions or reactions with supercritical water has been successful in producing hydrocarbons (particularly methane) and silica.

[0008] The alkali makes it possible to accelerate this process and achieve complete cleavage of the silicone within a few hours. In addition, amorphous silicon dioxide is converted into crystalline silicon dioxide by the alkali. In the sense of a circular economy, these products can be returned to the silicone production process. Depending on the amount of alkali added, water glass (alkyl silicate) can also be formed from the silicon dioxide formed by reacting the silicone and any silicon dioxide present as filler, which can be used as an additive in many industries, such as the construction industry.

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

[0010] These silicones preferably comprise up to 1% by weight, particularly preferably up to 0.1% by weight, in particular up to 0.01% by weight, of halogen, in particular chlorine.

[0011] In addition to the organosiloxane, the composition comprising the organosiloxane may comprise, for example, fillers, catalysts, binders and pigments. Fillers are, for example, pyrogenic and / or precipitated silica, silicone resins, chalk and quartz.

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

[0013] Examples of suitable bases are alkali metal 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 and alkaline earth metal carbonates, such as Li2CO3, Na2CO3, K2CO3, alkali metal and alkaline earth metal hydrogen carbonates, such as LiHCO3, NaHCO3, KHCO3, alkali metal 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] Preference is given to inorganic bases, in particular alkali metal hydroxides, alkali metal carbonates and alkali metal hydrogencarbonates.

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

[0016] The solid mixture obtained in the process preferably contains a modification of silicon dioxide selected from the group consisting of quartz, keatite and cristobalite.

[0017] The solid mixture obtained in the process preferably comprises at most 2% by weight, particularly preferably at most 1% by weight and in particular at most 0.5% by weight of carbon.

[0018] In the case of methyl silicones, the resulting gas mixture comprises methane as hydrocarbon. Up to 550 g of methane can be produced per kg of silicone used.

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

[0020] Preferred temperatures in the process are 250° C. to 500° C., in particular 280° C. to 400° C. and particularly preferably 300° C. to 390° C. Preferred pressures in the process are 10 to 400 bar, particularly preferably 20 to 320 bar, in particular 50 to 280 bar, the lower limit for the pressure being in each case the vapor pressure of the reaction mixture in relation to the reaction temperature. Preferred residence times in the process are 1 minute to 48 hours, particularly preferably 5 minutes to 24 hours, in particular 10 minutes to 12 hours.

[0021] In a preferred embodiment, the reaction mixture is decompressed to atmospheric pressure after the reaction is completed, and the resulting steam is used for heat recovery or preheating of the reactants. Thus, most of the heat can be recovered. DETAILED DESCRIPTION

[0022] The characterization was performed using the following analytical methods and instruments:

[0023] Example

[0024] Gas chromatography for determination of gas phase composition

[0025] The gas phase was analyzed on an Agilent 6890 GC gas chromatograph using a CP-Molseieve 5Å column with dimensions of 25m, 0.32mm, and 30µm. The gas composition was evaluated by integrating the detected signals.

[0026] NMR spectroscopy for the determination of silicon-containing components in the aqueous phase

[0027] In DMSO-d6 or D2O, on a Bruker Avance 500 or Ascend 500 (500 MHz, for 1 H NMR spectra, and 99.4 MHz for 29 Si NMR spectroscopy) 1 H NMR and 29Si NMR spectra. All measurements were referenced to TMS as an external standard. The relative ratios of the components in the aqueous phase were determined by integrating the individual signal groups. The absolute concentrations in water were determined taking into account the mixing ratio of the aqueous phase to DMSO-d6.

[0028] Analysis of isolated solids

[0029] CHN Analysis

[0030] The determination of oxygen and hydrogen was carried out on an ONH836 elemental analyzer. The carbon content was determined on a CS844 elemental analyzer. For analysis, 10-20 mg of sample was required and digested under energy supply. Hydrogen in the form of water and oxygen in the form of CO and CO2 were then determined by IR absorption. Carbon was first reduced and then converted into CO and CO2 in an oxygen atmosphere and detected by IR absorption. Calibration was performed on oxygen by SiO2, hydrogen by TiH2 and carbon by acetanilide, sodium bicarbonate and calcium carbonate.

[0031] XRD analysis

[0032] To determine the composition of the crystalline components of the solid, 5 g of the powder were ground and then analyzed on a PANalytical Empyrean powder X-ray diffractometer. The analysis was carried out by X-ray diffraction using copper radiation (I=1.54 Å).

[0033] Reaction process:

[0034] All experiments were carried out in an Inconel® pressure autoclave equipped with a temperature sensor and a pressure sensor. The reaction process was recorded by a pressure-temperature curve, and the reaction products were analyzed after the reaction was completed.

[0035] The silicone rubber was used in the form of a cube with a side length of 0.5 cm.

[0036] Example 1 (non-invention):

[0037] Weigh out 24 g of silicone rubber (polydimethylsilicone rubber WACKER ELASTOSIL ® 401 / 60E) and 100 ml of water 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 the positive pressure was removed, the solid was separated from the aqueous phase and dried. 18.1 g of solid was obtained.

[0038] Example 2 (non-invention):

[0039] Weigh out 24 g of silicone rubber (WACKER ELASTOSIL ®401 / 60E) and 100 ml of water and heated to 374 ° C until the pressure is constant (120 h). The pressure autoclave is then cooled to room temperature. A residual pressure of 20.2 bar is measured. Analysis shows the formation of methane. After the positive pressure is removed, the solid is separated from the aqueous phase and dried. 18.7 g of solid are obtained.

[0040] Example 3 (invention):

[0041] Weigh out 24 g of silicone rubber (WACKER ELASTOSIL ® 401 / 60E), 70 mg of potassium hydroxide and 100 ml of water and heated to 350° C. for 12 h. After 4 hours, a constant pressure was already recorded. The pressure autoclave was subsequently cooled to room temperature. A residual pressure of 24.2 bar was measured. Analysis showed the formation of methane. After the positive pressure was removed, the solid was separated from the aqueous phase and dried. 20.7 g of solid were obtained.

[0042] Example 4 (invention):

[0043] Weigh out 24 g of silicone rubber (WACKER ELASTOSIL ® 401 / 60E), 70 mg of potassium hydroxide and 100 ml of water and heated to 300° C. for 12 h. After about 5.5 hours, a constant pressure was already recorded. The pressure autoclave was subsequently cooled to room temperature. A residual pressure of 24.0 bar was measured. Analysis showed the formation of methane. After the positive pressure was removed, the solid was separated from the aqueous phase and dried. 20.1 g of solid were obtained.

[0044] Example 5 (Invention):

[0045] Weigh out 24 g of silicone rubber (WACKER SilGel ® 612), 70 mg potassium hydroxide and 100 ml water and heated to 350° C. for 12 h. After 3 h, a constant pressure was already recorded. The pressure autoclave was subsequently cooled to room temperature. A residual pressure of 30.2 bar was measured. Analysis showed the formation of methane. After the positive pressure was removed, the solid was separated from the aqueous phase and dried. 29.6 g of solid were obtained.

[0046] Example 6 (Invention):

[0047] Weigh out 24 g of silicone rubber (WACKER ELASTOSIL ®401 / 60E), 80 mg of sodium hydroxide and 100 ml of water and heated to 350° C. for 12 h. After 1 hour, a constant pressure was already recorded. The pressure autoclave was subsequently cooled to room temperature. A residual pressure of 23.3 bar was measured. Analysis showed the formation of methane. After the positive pressure was removed, the solid was separated from the aqueous phase and dried. 19.8 g of solid were obtained.

[0048] Example 7 (Invention):

[0049] Weigh out 24 g of silicone rubber (WACKER ELASTOSIL ® 401 / 60E), 80 mg lithium hydroxide and 100 ml water and heated to 350° C. for 12 h. Immediately after heating, a constant pressure was recorded. 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 removing the positive pressure, the solid was separated from the aqueous phase and dried. 18.9 g of solid was obtained.

[0050] Example 8 (Invention):

[0051] Weigh out 24 g of silicone rubber (WACKER ELASTOSIL ® 401 / 60E), 80 mg of sodium bicarbonate and 100 ml of water and heated to 350° C. for 12 h. After 5 hours, a constant pressure was recorded. 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 the positive pressure was removed, the solid was separated from the aqueous phase and dried. 19.7 g of solid was obtained.

[0052] Example 9 (Invention):

[0053] Weigh out 24 g of phenyl-containing silicone rubber (WACKER ELASTOSIL ® 490 / 55.OH), 80 mg KOH and 100 ml water and heated to 350°C for 12 h. A constant pressure was detected after about 4 h. Subsequently, the pressure autoclave was cooled to room temperature and a residual pressure of 22.3 bar was measured. Analysis of the gas showed the formation of methane. After the positive pressure was removed, 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 (about 1 ml), which was separated and identified by NMR spectroscopy.

[0054] Embodiment 10 (invention):

[0055] Weigh out 24 g of silicone rubber (WACKER ELASTOSIL ®401 / 60E), 10 g of sodium hydroxide and 100 ml of water and heated to 350°C for 12 h. Immediately after heating, the constant pressure was recorded. 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 the positive pressure was removed, the aqueous solution was separated. The solids content of the aqueous phase was 28.5%. The sodium silicate formed could be separated by drying the aqueous solution and showed a residual carbon content of 0.07%.

[0056] Table 1

[0057]

[0058] *Not invented

[0059] ‡ The sample is amorphous.

Claims

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

2. The method according to claim 1, wherein: The term silicone includes oligomeric or polymeric organosiloxanes in which silicon atoms are bonded via oxygen atoms and in which at least some of the silicon atoms carry one or more organic substituents, and compositions comprising organosiloxanes.

3. The method according to one or more of the preceding claims, wherein: The term silicone refers to a silicone rubber comprising a filler selected from silica and silicone resins.

4. The method according to one or more of the preceding claims, wherein: The silicone contains up to 1% by weight of halogen.

5. The method according to one or more of the preceding claims, wherein: The base is selected from the group consisting of 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 method according to one or more of the preceding claims, wherein: The temperature is 300°C to 400°C.

7. The method according to one or more of the preceding claims, wherein: The pressure is from 10 bar to 400 bar.

8. The method according to one or more of the preceding claims, wherein: Methyl-containing silicones are used and the hydrocarbon formed is methane.

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