Synthesis method of alkoxysilanes

Abstract

The direct depolymerization of biogenic and other high surface area silica sources uses both simple and hindered diols to produce alkoxysilanes in one or two steps that can be separated and purified directly from the reaction mixture by distillation, extraction or filtration followed by solution modification and distillation or extraction. The alkoxysilanes can take the form of spirosiloxanes or simple alkoxysilanes or oligomers thereof. Thereafter they can be treated with acid to produce colloidal or precipitated silica or aerosolized and combusted to provide fumed silica without the intervention of SiCl.sub.4.

Claims

1. A synthesis method of alkoxysilanes, comprising: a first step, in which freshly produced or aged biogenically concentrated silica in a milled or un-milled form is mixed with a liquid polyol including ethylene glycol, 1,2-diol, 1,3-diol, 2,3-diol, 1,4-diol, triols, triethanolamine, trishydroxy-methylamine, or a mixture thereof; a second step, in which sufficient base including LiOH, NaOH, KOH, CsOH, RbOH or a mixture thereof in a catalytic amount ranging from 0.25-50 mol % is added so that a mixture is heated to a temperature where water and a by-product of a dissolution process distill out; a third step, in which addition of a diol that forms a 4,5 or 6 member ring with silicon forms an alcoholate and a spirosiloxane that is distilled out directly from the polyol solution before or after filtration to remove undissolved biogenic silica to recover the solution of alkoxysilane and the alcoholate substantially free of solids followed by distillation, extraction, or separation through a semipermeable membrane that selectively removes the spirosiloxane leaving anionic and polyol species to remain in the original reaction mixture followed by addition of simple ROH alcohols including MeOH and EtOH and stirred to effect exchange with the chelating diol to produce Si(OR).sub.4 which can be separated, being also possible to slightly acidify the solution using an organic acid or acid anhydride or non-aqueous inorganic acid or acid anhydride or ion exchange resin or HCl to catalyze the exchange reaction followed by purification, said recovered spirosiloxane or alkoxysilane then being combusted to produce fumed silica or treated with sufficient acid to produce colloidal dispersions of or precipitated silica; and a fourth step, in which any residual base present as alcoholate in the recovered alkoxysilane or rice hull ash is neutralized by adding a non-aqueous acid including HCl gas and NH.sub.4Cl to eliminate the residual alkali metal base present in the form of a salt that can be removed by filtration or precipitation leaving a pure alkoxysilane or spirosiloxane free of metal impurities; alternately these impurities can be removed using ion exchange materials.

2. The synthesis method according to claim 1, wherein no additional base is added.

3. The synthesis method according to claim 1, wherein additional base is added.

4. The synthesis method according to claim 1, wherein NaOH or KOH is used.

5. The synthesis method according to claim 1, wherein a 1,2-diol, 2,3 or 1,3-diol or 1,4 diol is used to produce tetraglycoxysilanes and/or spirosiloxanes and the solution is concentrated to remove solvent.

6. The synthesis method according to claim 1, wherein a simple alcohol ROH is added to the filtrate before or after concentration and equilibrated and any product Si(OR).sub.4 is then recovered by distillation, extraction with a solvent that is a non-solvent for the filtrate medium or by membrane selective separation.

7. The synthesis method according to claim 1, wherein the solution is subsequently acidified using an anhydrous acid or acid anhydride or ion exchange resin to a nominal pH of 2-6.8 to promote formation of tetraalkoxysilanes and/or spirosiloxanes that thereafter can be further purified by distillation or extraction with a solvent that is a non-solvent for the filtrate medium or by membrane selective separation.

8. The synthesis method according to claim 1, wherein reaction is effected at temperatures of 140-250° C. at atmospheric pressure but preferably at temperatures of 160-220° C.

9. The synthesis method according to claim 1, wherein a second liquid such as benzene or toluene for example is used that can azeotrope water formed during the reaction such that the reaction temperature can be substantially reduced from the boiling point of the diol chosen.

10. The synthesis method according to claim 1, wherein reaction is effected at temperatures of 140-250° C. under pressures of 2-200 atmospheres but preferably 2-50 atmospheres and most preferable at 2-15 atmospheres.

11. The synthesis method according to claim 1, wherein the recovered spirosiloxane or alkoxysilane is combusted in the gas phase to produce fumed silica.

12. The synthesis method according to claim 1, wherein the recovered spirosiloxane or alkoxysilane is combusted in the gas phase to produce fumed silica.

13. The synthesis method according to claim 1, wherein the recovered spirosiloxane or alkoxysilane is treated with sufficient water and acid to produce a pH sufficient (<4) to stabilize a colloidal dispersion of silica.

14. The synthesis method according to claim 1, wherein the extracted alkoxysilane is treated with sufficient water and acid to produce a pH sufficient (<4) to stabilize a colloidal dispersion of silica.

15. The synthesis method according to claim 1, wherein the recovered spirosiloxane or alkoxysilane is treated with sufficient water and acid to produce a pH sufficient (<4) to precipitate the silica.

16. The synthesis method according to claim 1, wherein the recovered spirosiloxane or alkoxysilane is treated with sufficient water and acid to produce a pH sufficient (<4) to precipitate the silica.

17. The synthesis method according to claim 1, wherein the acid or acid anhydride used is nonaqueous for example HCl, CO.sub.2, trifluoroacetic acid or anhydride or other common anhydrous organic acid or acid anhydride such that no water is introduced and the acidity is insufficient to cleave CH.sub.2OH bonds to generate water unless precipitated or colloidal silica is desired.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing TGA-DTA of silica depleted RHA after the work-up of spirosiloxane;

(2) FIG. 2 is a graph showing GPC of tetraethoxysilane, Gelest; tetramethoxysilane, Sigma-Aldrich; TEOS from Si(2-methyl-2,4-pentanediolato).sub.2 (SP); TMOS Si(2-methyl-2,4-pentanediolato).sub.2 (SP); TEOS from glycoxysilane (GS); and TEOS from Si(1,4-butanediolato).sub.2 (BSP);

(3) FIG. 3 is a photograph showing that colloidal silica at pH 1 is transparent with no light scattering;

(4) FIG. 4 is a photograph showing that gelled silica at pH 7 after neutralization with Na.sub.2CO.sub.3 and for standing 30 min shows extensive scattering but likely due to bubbles;

(5) FIG. 5 is a photograph showing that addition of 10 mL of ethanol to silica at pH 7, shows scattering from disruption of diol chelation;

(6) FIGS. 6(a), 6(b), and 6(c) are photographs showing TEM images of fumed SiO.sub.2, wherein FIG. 6(a) is LF-FSP of compound I, FIG. 6(b) is LF-FSP of TEOS, FIG. 6(c) is commercial; and

(7) FIGS. 7(a) and 7(b) are graphs showing FTIR of fumed SiO.sub.2, wherein FIG. 7(a) is LF-FSP, FIG. 7(b) is commercial.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(8) Hereunder, preferred embodiments of the present invention will be explained with reference to the accompanying drawings.

Example 1. 2-methyl-2,4-pentanediol Dissolution Reaction Just Run, ˜45% Conversion

(9) RHA (1000 g, 85 wt. % silica content, 14.16 moles of silica) was dissolved in 10 L of 2-methyl, 2,4-pentanediol (hexylene glycol, HG) and placed in a 22 L flask, equipped with a heating mantle and a mechanical stirrer. Then, catalyst (10 mol. % NaOH) dissolved in 900 mL of ethanol was added to the reaction flask. The reaction mixture was slowly heated and refluxed for 2 days. Then the distillation started—first the ethanol/water was distilled out, and then the temperature was increased to start the SP/HG distillation. SP was distilled out and fresh HG added. The distillation was carried about 40 h and ˜9 L of distilled SP was collected, and then worked up (addition of hexane and three water washing steps). After addition of hexane, the solution formed two immiscible layers (diol and hexane) that were separated prior the washing steps. Then the hexane layer (containing the spirosiloxane product) was washed with water three times, dried over sodium sulfate and collected. In the final step the hexane was removed on a rotary evaporator to yield the product (1624 g of spirosiloxane). This means that we were able to extract ˜45% silica from the starting RHA. The theoretical yield for 45% silica dissolution is 1657 g (98% yield).

Example 2. Ethylene Glycol Dissolution Reaction Just Run With HG Added 40% Conversion

(10) RHA (630 g, 7.87 moles of silica) was placed in a 12 L flask, equipped with a heating mantle and a mechanical stirrer. Catalyst (10 mol. % NaOH) was added with 7 l of EGH.sub.2 and distillation started. Silica dissolution rates are seen in Table 1.

(11) TABLE-US-00002 TABLE 1 Percent silica dissolved (by LOI) from processed RHA with 10 mol. % NaOH. Time, h Silica Dissolution  6 28.2% 12 31.7% 18 35.4%  24* 37.1% *At 37% dissolution, the reaction was converted to synthesize spirosiloxane (SP).

(12) Then, 3.5 L of 2-methyl, 2,4-pentanediol (hexylene glycol, HG) was added and spirosiloxane distillation commenced. SP was distilled out (˜3 L) and collected, and then worked up (addition of hexane and three water washing steps). After addition of hexane, the solution formed two immiscible layers (diol and hexane) that were separated prior the washing steps. Then the hexane layer (containing the spirosiloxane product) was washed with water three times, dried over sodium sulfate and collected. In the final step the hexane was removed on a rotary evaporator to yield the product (spirosiloxane) giving ˜507 g spirosiloxane (˜80% yield).

(13) The remaining RHA was washed with ethanol and filtered off and analyzed by TGA-DTA, FIG. 1. TGA-DTA of RHA showed 43 wt. % silica content. This means that we were able to extract almost half of the silica from the starting RHA (74.9 wt. % silica content).

Example 3. Conversion of I to TEOS

(14) ##STR00009##

(15) To a flame dried 500 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜25 mL of activated 4 Å molecular sieves, 10 g (0.038 mol) of Si(2-methyl-2,4-pentanediolato).sub.2 (I), and 400 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 2.5 mL (0.015 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 400 mL of hexanes was added to the filtered solution and washed with water (3×150 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of Si(OEt).sub.4 as determined by GPC, yield 5.2 g, 65%.

Example 4. Conversion of I to TEOS

(16) ##STR00010##

(17) To a flame dried 250 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜10 mL of activated 4 Å molecular sieves, 5 g (0.019 mol) of Si(2-methyl-2,4-pentanediolato).sub.2 (I), and 200 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 0.625 mL (0.008 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 800 mL of hexanes was added to the filtered solution and washed with water (3×300 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of Si(OEt).sub.4 as determined by GPC, yield 2.3 g, 63%.

Example 5. Conversion of I to TEOS

(18) ##STR00011##

(19) To a flame dried 250 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜10 mL of activated 4 Å molecular sieves, 5 g (0.019 mol) of Si(2-methyl-2,4-pentanediolato).sub.2 (I), and 200 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 0.45 mL (0.006 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 400 mL of hexanes was added to the filtered solution and washed with water (3×150 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of Si(OEt).sub.4 as determined by GPC, yield 2.2 g, 61%.

Example 6. GS Conversion to TEOS

(20) ##STR00012##

(21) To a flame dried 500 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜25 mL of activated 4 Å molecular sieves, 10 g (0.036 mol) of glycolato silicate (GS), and 400 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 2.5 mL (0.015 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 800 mL of hexanes was added to the filtered solution and washed with water (3×300 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetraethoxysilane. Crude yield 2.5 g, 40%.

Example 7. GS Conversion to TEOS

(22) ##STR00013##

(23) To a flame dried 250 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜10 mL of activated 4 Å molecular sieves, 5 g (0.018 mol) of glycolato silicate (GS), and 200 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 0.625 mL (0.008 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 400 mL of hexanes was added to the filtered solution and washed with water (3×150 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetraethoxysilane. Crude yield 1.46 g, 40%.

Example 8. GS Conversion to TEOS

(24) ##STR00014##

(25) To a flame dried 250 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜10 mL of activated 4 Å molecular sieves, 5 g (0.018 mol) of glycolato silicate (GS), and 200 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 0.45 mL (0.006 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 400 mL of hexanes was added to the filtered solution and washed with water (3×150 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetraethoxysilane. Crude yield 2.1 g, 56%.

Example 9. GS Conversion to TEOS

(26) ##STR00015##

(27) To a flame dried 250 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜10 mL of activated 4 Å molecular sieves, 5 g (0.018 mol) of glycolato silicate (GS), and 75 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 0.625 mL (0.008 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 400 mL of hexanes was added to the filtered solution and washed with water (3×150 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetraethoxysilane. Crude yield 1.4 g, 40%.

Example 10. GS Conversion to TEOS

(28) ##STR00016##

(29) To a flame dried 250 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜10 mL of activated 4 Å molecular sieves, 5 g (0.018 mol) of glycolato silicate (GS), and 75 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 0.625 mL (0.008 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 400 mL of hexanes was added to the filtered solution and washed with water (3×150 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetraethoxysilane. Crude yield 2.1 g, 55%.

Example 11. (Butanediolato)4Si Conversion to TEOS

(30) ##STR00017##

(31) To a flame dried 500 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜25 mL of activated 4 Å molecular sieves, 10 g (0.026 mol) of Si(1,4-butanediolato).sub.4 (BSP), and 400 mL of dry 200 proof ethanol. The reaction mixture was allowed to stir for 1 h before addition of 2.5 mL (0.015 mol) of TFA. The reaction was left to stir at room temperature for 24 h. It was then filtered to remove molecular sieves and precipitated solids formed during the reaction process (ROP/silica byproducts). Then 800 mL of hexanes was added to the filtered solution and washed with water (3×300 mL) to remove TFA and diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetraethoxysilane. Crude yield 710 mg, 14%.

Example 12. I Conversion to TMOS (Biphase System)

(32) ##STR00018##

(33) To a flame dried 500 mL round bottom flask equipped with magnetic stirrer under N.sub.2 were added ˜25 mL of activated 4 Å molecular sieves, 10 g (0.038 mol) of Si(2-methyl-2,4-pentanediolato).sub.2 (SP), and 200 mL of dry methanol and 200 mL of dry hexanes (an immiscible mixture). The reaction mixture was allowed to stir for 1 h before addition of 2.5 mL (0.015 mol) of TFA. The reaction was left to stir at room temperature for 24 h. The mixture was then poured into a separatory funnel and the two layers were separated. The hexane layer was then filtered and washed with water (3×300 mL) to remove TFA and residual diol. The pH of the resulting hexanes solution was then checked for neutrality. The hexanes solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetramethoxysilane. Crude yield 3 g, 40%.

Example 13. Conversion of GS to TMOS+Oligomers

(34) To a dry 1000 mL round bottom flask equipped with magnetic stirrer were added 50 g (0.03 mol) of glycolato silicate (16.3 wt. %), and 300 mL of anhydrous methanol. Then 300 mL of hexane were added to the reaction mixture. The reaction was left to stir at room temperature for 24 h. Then the hexane and methanol layers were separated in a sep-funnel and the hexane layer was washed with distilled water (3×300 mL) to remove the diol. The hexane solution was then dried over Na.sub.2SO.sub.4 and filtered. Then the solvent was removed in-vacuo, resulting in a colorless oil of tetramethoxysilane.

Example 14. Conversion of Si(2-methyl-2,4-pentanediolato)2 to Colloidal or Precipitated Silica

(35) To a 250 mL round bottom flask equipped with magnetic stirrer were added 10 g (0.038 mol) of Si(2-methyl-2,4-pentanediolato).sub.2, 50 mL of 200 proof ethanol, 4 mL of H.sub.2O and 2 mL of 12N HCl such that the pH is <3. The reaction was left to stir at room temperature for 24 h, resulting in a transparent colloidal dispersion of silica particles as indicated by the lack of laser light scattering in FIG. 3. The colloidal silica appears to be stabilized by the presence of the 2-methyl-2,4-pentanediol. Addition of Na.sub.2CO.sub.3 to neutralize the solution results in slow gelation, FIG. 4. Alternately, the additional ethanol or hexanes causes silica to precipitate rather than gel as the 2-methyl-2,4-pentanediol appears to be solvated and removed from the silica surface.

Example 15. Fumed Silica

(36) Spirosiloxane I was synthesized using the method described above. Distilled I was used for all the following experiments. TEOS was prepared as in Example 3. Methanol, ethanol, and propanol were purchased from Decon Labs (King of Prussia, Pa.). TEOS was purchased from Sigma-Aldrich (Milwaukee, Wis.).

(37) LF-FSP.

(38) Methanol, ethanol or propanol solutions of I and TEOS were obtained by dissolving sufficient I and TEOS to make a 1, 3 or 5 wt % silica ceramic yield solution. The general methods for conducting LF-FSP have been described in references x, y, z.

(39) The properties of the as-produced fumed silica are identical to those of SiCl.sub.4 derived silica and typical particle sizes are as shown in Table 2. Comparative transmission electron micrographs of the silicas are shown in FIG. 5. Comparative FTIRs for fumed silica produced from I and SiCl.sub.4 are shown in FIGS. 6(a), 6(b), and 6(c).

(40) TABLE-US-00003 TABLE 2 SSA of LF-FSP produced silica. Precursor Solvent/Fuel Precursor concentration (wt %) SSA (m.sup.2/g) SS MeOH 1 230 3 190 EtOH 1 220 3 190 5 140 PrOH 1 210 TEOS EtOH 1 230 EtOH 3 180 EtOH 5 150