Process for preparing porous silicon

Abstract

The present application provides a low-temperature process to prepare porous silicon using the magnesiothermic reduction reaction. For example, the process comprises combining Mg with a first silica and a second silica to provide a mixture (a)- (1), or combining Mg with a first silica to provide a mixture (a)-(2), wherein the first silica comprises quartz; and (b) grinding the mixture (a)-(1) followed by heating under conditions to provide the porous silicon; or grinding the mixture (a)-(2) to provide a mixture (a)-(3) followed by adding the second silica to the mixture (a)-(3) and heating under conditions to provide the porous silicon.

Description

PROCESS FOR PREPARING POROUS SILICONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of priority from co-pending U.S. provisional patent application no. 63 / 537,679 filed on September 11 , 2023, the contents of which are incorporated herein by reference in their entirety.FIELD

[0002] The present application relates to processes for the preparation of porous silicon, in particular using the magnesiothermic reduction reaction at lower temperatures than previously reported.BACKGROUND

[0003] Porous silicon has been applied in gas storage, sensing, drug delivery, batteries, hydrogen generation, and optics. It is typically manufactured using carbothermal reduction which involves temperatures of >1500 °C. After the carbothermal reduction additional, energy and cost extensive steps are used to make the silicon porous. Recent published work on the use of magnesiothermic reduction has allowed the typical temperature for the production of porous silicon to decrease to 650 °C and can be used to produce porous silicon in one step. Magnesiothermic reduction involves the reaction of magnesium with silica that results in an interwoven composite product of magnesia (MgO) and silicon according to reaction (1):SiO2+ 2Mg Si + 2MgO (1)The magnesia is easily removed from the reaction product using an acid such as hydrochloric acid (HCI), leaving behind a silicon material that is porous.SUMMARY

[0004] The present application reports a process for preparing porous silicon via the magnesiothermic reduction reaction which occurs at lower temperatures than previously reported, providing a process that uses reduced energy input and therefore lower costs than other processes known in the art. This process uses quartz, a lower cost form of silica. Additionally, this process can use any form of other silica which offers feedstock flexibility (for example, glass of any type, clays, silica gels, precipitated silica, diatomaceous earth and the like).

[0005] Accordingly, the present application includes a process for preparing porous silicon comprising:(a) combining Mg with a first silica and a second silica to provide a mixture (a)-(1), or combining Mg with a first silica to provide a mixture (a)-(2), wherein the first silica comprises quartz; and(b) grinding the mixture (a)-(1) followed by heating under conditions to provide the porous silicon; or grinding the mixture (a)-(2) followed by heating to provide a mixture (a)-(3) and adding the second silica to the mixture (a)-(3) and heating under conditions to provide the porous silicon.

[0006] The process of the present application addresses the large energy demand and corresponding cost associated with the production of porous silicon material.

[0007] The present application also includes porous silicon prepared using the processes of the application and various products comprising this porous silicon and uses thereof.

[0008] Other features and advantages of the present application will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the application, are given by way of illustration only and the scope of the claims should not be limited by these embodiments, but should be given the broadest interpretation consistent with the description as a whole.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The embodiments of the application will now be described in greater detail with reference to the attached drawings in which:

[0010] Figure 1 shows the silicon yield data in mole percent for an exemplary process of the application.DETAILED DESCRIPTION OF THE APPLICATIONI. Definitions

[0011] Unless otherwise indicated, the definitions and embodiments described in this, and other sections are intended to be applicable to all embodiments andaspects of the present application herein described for which they are suitable as would be understood by a person skilled in the art.

[0012] The present application refers to a number of chemical terms and abbreviations used by those skilled in the art. Nevertheless, definitions of selected terms are provided for clarity and consistency.

[0013] In understanding the scope of the present application, the term “comprising” and its derivatives, as used herein, are intended to be inclusive or open- ended terms that specify the presence of the stated features, elements, components, groups, integers, and / or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and / or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives.

[0014] As used herein, the word “consisting” and its derivatives, are intended to be close ended terms that specify the presence of stated features, elements, components, groups, integers, and / or steps, and also exclude the presence of other unstated features, elements, components, groups, integers and / or steps.

[0015] The term “consisting essentially of”, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and / or steps as well as those that do not materially affect the basic and novel characteristic(s) of these features, elements, components, groups, integers, and / or steps.

[0016] Terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.

[0017] As used in this application, the singular forms “a”, “an” and “the” include plural references unless the content clearly dictates otherwise. For example, an embodiment including “a second silica” should be understood to present certain aspects with one second silica or two or more second silica. In embodiments comprising an “additional” or “second” component, such as an additional or second silica, the second component as used herein is chemically different from the othercomponents or first component. A “third” component is different from the other, first, and second components, and further enumerated or “additional” components are similarly different.

[0018] The term “and / or” as used herein means that the listed items are present, or used, individually or in combination. In effect, this term means that “at least one of” or “one or more” of the listed items is used or present.

[0019] The term “quartz” as used herein refers to any crystalline form of silica (SiO2), including both alpha and beta forms.11. Processes of the Application

[0020] The present application includes a process for preparing porous silicon comprising:(a) combining Mg with a first silica and a second silica to provide a mixture (a)-(1), or combining Mg with a first silica to provide a mixture (a)-(2), wherein the first silica comprises quartz; and(b) grinding the mixture (a)-(1) followed by heating under conditions to provide the porous silicon; or grinding the mixture (a)-(2) followed by heating to provide a mixture (a)-(3) and adding the second silica to the mixture (a)-(3) and heating under conditions to provide the porous silicon.

[0021] In some embodiments, the first silica is quartz.

[0022] In some embodiments, the second silica is any suitable amorphous silica, crystalline silica, porous silica or non-porous silica. In some embodiments the second silica is any type of amorphous silica. In some embodiments, the second silica is sand, naturally occurring rocks, rice husk, glass, clay, silica gel, precipitated silica, or diatomaceous earth, or a mixture thereof. In some embodiments, the second silica is a synthetic silica, a precipitated silica, a monolithic silica or a silica from agricultural waste.

[0023] In some embodiments, the first silica consists essentially of quartz. In some embodiments, the first silica is quartz.

[0024] The melting point of Mg is 650 °C, and the vapour pressure of Mg at 428 °C is 1 Pa. This enables solid Mg and silica to be placed separately in a reaction vesseland the Mg gas to diffuse to reduce the silica and / or liquid Mg to flow over the silica for reaction during the process.

[0025] In some embodiments the mole ratio of the Mg to the total amount of silica in mixture (a)-(1 ) is at least 4.4:1 , that is, for 1 mole of silica, a minimum of about 4.4 moles of Mg is used. In some embodiments, the mole ratio of the first silica to the second silica in the mixture (a)-(1 ) is 0.1 :0.9 to 0.9:0.1 , or about 1 :1.

[0026] In some embodiments, the mole ratio of the Mg and the first silica in the mixture (a)-(2) is about 5-10:1 or about 7:1.

[0027] In some embodiments, the second silica is added to the mixture (a)-(3) in an amount to provide a mole ratio of Mg:silica of about 2-2.5 :1 , or about 2.2:1 .

[0028] In some embodiments, the heating of the mixture (a)-(2) is at a temperature of about 300°C to about 450°, or about 375°C to about 400°C, for about 30 minutes to about 2 hours, or for about 1 hour.

[0029] In some embodiments, the heating of the mixture (a)-(1) to provide the porous silicon is at a temperature of about 300°C to about 475°, or about 400°C to about 425°C, for about 30 minutes to about 9 hours, or for about 3 hours.

[0030] In some embodiments, the heating of the mixture (a)-(3) after adding the second silica to provide the porous silicon is at a temperature of about 450°C to about 550°, or about 500°C, for about 1 hours to about 5 hours, or for about 3 hours.

[0031] In some embodiments, a flow of an inert gas is used throughout heating treatments. In some embodiments, the inert gas is argon. In some embodiments, the flow rate of the inert gas is about 100-150 mL / min or about 125 mL / min.

[0032] In some embodiments, the heating is done in a rotary furnace or a rotary kiln.

[0033] In some embodiments, the grinding of the mixture (a)-(1 ) or the mixture (a)-(2) is done manually in a mortar and pestle or a rubber-lined rock tumbler, or automatically in a ball mill or similar equipment. In some embodiments, the grinding is done using low energy mixing equipment, for example a rotary drum mixer / tumbler, with the addition of an abrasion resistant grinding aid, for example alumina or tungsten balls. In some embodiments, grinding is done for about 15 minutes to about 24 hours, about 20 minutes to about 12 hours, about 25 minutes to about 3 hours, or about 30minutes. In some embodiments, if grinding is done for longer periods of time, for example more than about 1 hour, or with higher energy equipment, for example using a high energy ball mill, then the grinding is done under inert atmosphere, such as under argon. In some embodiments, the grinding of the mixture (a)-(2), is done in a spiral ribbon mixer, or some other particulate solids mixer, such as a rotating screw mixer, or V-blender, to achieve homogeneity. A person skilled in the art would appreciate that a wide range of existing industrial solids mixing equipment can be used.

[0034] In some embodiments, following heating, the temperature of the mixtures and / or the porous silicon is reduced to room temperature under the inert atmosphere. In some embodiments, the temperature is reduced by natural cooling.

[0035] In some embodiments, the process of the application produces magnesia as a side product and the process further comprises washing the porous silicone with acid to remove the magnesia. In some embodiments, the acid is hydrochloric acid (HCI) or sulfuric acid (H2SO4). In some embodiments, the concentration of the acid is about 1 M to about 5M. In some embodiments, the acid washing is done in an industrial, agitated, acid-resistant liquid storage or reacting vessel. In some embodiments, this vessel is a stainless-steel vessel with glass or PTFE lining.

[0036] Without wishing to be limited by theory, during grinding, the quartz powder decreases the particle size of the magnesium powder by attrition, producing finer grains. This size reduction makes the magnesium more reactive, allowing it to react with quartz and other types of silica at temperatures much lower than the previous standard of 650°C.

[0037] In some embodiments, in the process of the invention wherein Mg is combined with the first silica to provide a mixture (a)-(2) which is ground and heated to provide mixture (a)-(3), wherein the first silica comprises quartz, the quartz completely reacts the Mg, converting it into an intermediate product, magnesium silicide (Mg2Si). Then, when the second silica is added to the mixture (a)-(3) the magnesium silicide reacts with the second silica, producing the porous silicon. In this embodiment, the porous silicon is advantageously free from unreacted quartz.

[0038] In some embodiments, the process is a continuous process wherein a ground mixture of Mg and the first silica (a)-(2) is heated in a rotary furnace under conditions to provide the mixture (a)-(3), followed by addition of the second silica into the same rotary furnace. In this embodiment, one single rotary furnace is used for the first and second heating steps and the rotation of the furnace / kiln is enough to homogenize the mixtures, removing the need for a separate mixing step for (a)-(2) and addition of the second silica. In some embodiments, the mixture (a)-(3) is cooled prior to addition of the second silica,

[0039] In some embodiments, the present application includes a process for preparing porous silicon comprising:(a) combining Mg with a first silica and a second silica to provide a mixture (a)-(1), wherein the first silica comprises quartz; and(b) grinding the mixture (a)-(1) followed by heating under conditions to provide the porous silicon.

[0040] In some embodiments, the present application includes a process for preparing porous silicon comprising:(a) combining Mg with a first silica to provide a mixture (a)-(2), wherein the first silica comprises quartz; and(b) grinding the mixture (a)-(2) followed by heating to provide a mixture (a)-(3) and adding a second silica to the mixture (a)-(3) and heating under conditions to provide the porous silicon.II. Products and uses of the Application

[0041] The present application also includes porous silicon prepared using a process of the present application and uses thereof.

[0042] The present application also includes products and materials comprising the porous silicon prepared using a process of the application. Such products and materials include, but are not limited to, lithium-ion battery anode materials, photocatalyst for chemical production and water treatment, carbon dioxide sequestration, and chemical hydrogen storage materials.

[0043] The following non-limiting examples are illustrative of the present application. As is apparent to those skilled in the art, many of the details of theexamples may be changed while still practicing the methods, compositions and kits described herein.III. EXAMPLES

[0044] The following non-limiting examples are illustrative of the present application:Example 1

[0045] The reaction and processing steps for an exemplary magnesiothermic reduction of the present application is shown in Scheme 1 and Scheme 2, respectively.Scheme 1 : Chemical Reaction SchemeASiC>2 (s) + 2Mg (g) -» Si (s) + 2MgO (s)

[0046] Quartz silica was added to magnesium and synthetic precipitated silica (an amorphous silica), screened to 325 mesh (<45 pM) at the molar ratio for magnesium, quartz silica and synthetic precipitated silica of 2.2:0.5:0.5 to provide a mixture of powders. This mixture was ground and further mixed manually in a ceramic mortar and pestle or in a rubber-lined rock tumbler, and the resulting ground powder was heated in a furnace, such as a rotary furnace, ora rotary kiln, to 425°C for 6 hours under a flow of argon gas (at about 125 mL / min). The heated mixture was then cooled naturally in the furnace under the argon gas flow to room temperature and washed with 200 mL of 1 M hydrochloric acid (HCI) and dried to provide the porous silicon. The experimental results confirming the success of this process are shown in Figure 1 . The data confirms that, at a much lower temperature (425°C), nearly identical silicon yields (approximately 75%) were obtained with an exemplary process of the application. Additionally, it confirms the effectiveness of the grinding step. The final silicon product obtained is a brown powder.Example 2

[0047] The process in Example 1 uses a mixture of quartz sand, an amorphous type of silica and magnesium powder. While not wishing to be limited by theory, by using this mixture, quartz sand grinds the magnesium powder down, making the particles smaller and more reactive. This allows the reaction to proceed at temperatures around 400°C and achieve yields above 70%. In this process, the quartz does not completely react and thus remains in the final product and is not readily separated.

[0048] In this example quartz was mixed with magnesium at a mole ratio of 7:1 (Mg:SiO2) and the mixture was ground and further mixed manually in a ceramic mortar and pestle. A rubber-lined rock tumbler with a capacity of 1 lb was also used and provided the same results. This mixture was heated in a furnace to 400°C under a flow of argon gas (at about 125 mL / min) for 1 hour then naturally cooled to room temperature under the argon gas flow. This first step completely reacts the quartz, converting it into an intermediate product, magnesium silicide (Mg2Si). Then, synthetic precipitated silica was added in an amount to provide a mole ratio of 2.2:1 (Mg:SiO2) and mixing was performed in a spiral ribbon mixer. This mixture was then placed in the furnace a second time and heated to 500°C for 3 hours. In this second step, magnesium silicide reacts with additional silica, producing silicon in a yield of about 91 %. A summary of the differences between the process of Example 1 and Example 2 is described in Table 1 .

[0049] While the present application has been described with reference to examples, it is to be understood that the scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

[0050] All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. Where a term in the present application is found to be defined differently in a document incorporated herein by reference, the definition provided herein is to serve as the definition for the term.

Claims

CLAIMS:1 . A process for preparing porous silicon comprising:(a) combining Mg with a first silica and a second silica to provide a mixture (a)-(1), or combining Mg with a first silica to provide a mixture (a)-(2), wherein the first silica comprises quartz; and(b) grinding the mixture (a)-(1) followed by heating under conditions to provide the porous silicon; or grinding the mixture (a)-(2) followed by heating to provide a mixture (a)-(3) and adding the second silica to the mixture (a)-(3) and heating under conditions to provide the porous silicon.

2. The process of claim 1 , wherein the second silica is any suitable amorphous silica, crystalline silica, porous silica or non-porous silica.

3. The process of claim 1 , wherein the second silica is any suitable amorphous silica.

4. The process of claim 1 , wherein the second silica is sand, naturally occurring rocks, rice husk, glass, clay, silica gel, precipitated silica, or diatomaceous earth, ora mixture thereof.

5. The process of claim 1 , wherein the second silica is a synthetic silica, a precipitated silica, a monolithic silica or a silica from agricultural waste.

6. The process of any one of claims 1 to 5, wherein the first silica consists essentially of quartz.

7. The process of any one of claims 1 to 5, wherein the first silica is quartz.

8. The process of any one of claims 1 to 7, wherein the mole ratio of the Mg to the total amount of silica in mixture (a)-(1 ) is at least 4.4: 1.

9. The process of any one of claims 1 to 8, wherein the mole ratio of the first silica to the second silica in the mixture (a)-(1) is 0.1 :0.9 to 0.9:0.1.

10. The process of claim 9, wherein the mole ratio of the first silica to the second silica in the mixture (a)-(1) is 1 :1.11 . The process of any one of claims 1 to 7, wherein the mole ratio of the Mg and the first silica in the mixture (a)-(2) is about 5-10:1 or about 7:1 .

12. The process of claim 11 , wherein the second silica is added to the mixture (a)-(3) in an amount to provide a mole ratio of Mg:silica of about 2-2.5: 1 ,13. The process of claim 11 , wherein the second silica is added to the mixture (a)-(3) in an amount to provide a mole ratio of Mg:silica of about 2.2:1 .

14. The process or any one of claims 1 to 13, wherein the heating of mixture (a)-(3) is at a temperature of about 300°C to about 450°, or about 375°C to about 400°C, for about 30 minutes to about 2 hours, or for about 1 hour.

15. The process or any one of claims 1 to 13, wherein the heating of the (a)-(1 ) to provide the porous silicon is at a temperature of about 300°C to about 475°, or about 400°C to about 425°C, for about 30 minutes to about 9 hours, or for about 3 hours.

16. The process of claim 14, wherein the heating of the mixture (a)-(3) after adding the second silica to provide the porous silicon is at a temperature of about 450°C to about 550°, or about 500°C, for about 1 hours to about 5 hours, or for about 3 hours.

17. The process or any one of claims 1 to 16, wherein a flow of an inert gas is used throughout the heating.

18. The process of claim 17, wherein the inert gas is argon.

19. The process of claim 17 or 18, wherein the flow rate of the inert gas is about 100- 150 mL / min or about 125 mL / min.

20. The process or any one of claims 1 to 19, wherein the grinding of the mixture (a)- (1) or the mixture (a)-(2) is done manually in a mortar and pestle, or automatically in a ball mill or a rubber-lined rock tumbler.

21. The process of any one of claims 1 to 20, wherein the grinding is done for about 15 minutes to about 24 hours, about 20 minutes to about 12 hours, about 25 minutes to about 3 hours, or about 30 minutes.

22. The process of claim 21 , wherein if grinding is done for longer periods of time, for example more than about 1 hour, or with higher energy equipment, for example using a high energy ball mill, then the grinding done under inert atmosphere.

23. The process of any one of claims 1 to 20, wherein the adding of the second silica to the mixture (a)-(3) is done in a particulate solids mixer, such as a spiral ribbon mixer, a rotating screw mixer, or V-blender.

24. The process of any one of claims 1 to 23, wherein following heating, the temperature of the mixtures and / or the porous silicon is reduced to room temperature under an inert atmosphere.

25. The process of any one of claims 1 to 24, wherein the process produces magnesia as a side product and the process further comprises washing the porous silicone with acid to remove the magnesia.

26. The process of claim 25, wherein the acid is hydrochloric acid (HCI) or sulfuric acid (H2SO4).

27. The process of claim 25, wherein the concentration of the acid is about 1 M to about 5M.

28. The process of claim 1 comprising:(a) combining Mg with a first silica and a second silica to provide a mixture (a)-(1), wherein the first silica comprises quartz; and(b) grinding the mixture (a)-(1) followed by heating under conditions to provide the porous silicon.

29. The process of claim 1 comprising:(a) combining Mg with a first silica to provide a mixture (a)-(2), wherein the first silica comprises quartz; and(b) grinding the mixture (a)-(2) to provide a mixture (a)-(3) followed by adding a second silica to the mixture (a)-(3) and heating under conditions to provide the porous silicon.

30. The process of claim 29, wherein the process is a continuous process wherein the ground mixture (a)-(2) is heated in a rotary furnace under conditions to provide the mixture (a)-(3), followed by addition of the second silica into the rotary furnace.31 . The process of claim 30, wherein the mixture (a)-(3) is cooled prior to addition of the second silica.

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