Patent Application
20240238754
This invention is related to the field of materials engineering, specifically in the development of an electrospun anion-doped metal oxide sorbent, wherein the sorbent is used at room temperature for capturing CO2 from air.
Constant increase in greenhouse gases has now become a global issue as it leads to a diverse set of direct and indirect effects on all living beings. One of the most well-known effects of greenhouse gases is global warming where the rising level of atmospheric CO2 is the main contributor. CO2 is a major anthropogenic greenhouse gas, and according to the National Oceanic and Atmospheric Administration of the United States (NOAA), the average CO2 concentration in the atmosphere was around 412.30 ppm by the beginning of 2020, which is 21.69% higher than that recorded in 1980 of 338.80 ppm. Therefore, scientists are devoted to finding solutions for reducing the levels of CO2 in the atmosphere. Another major concern of CO2 gas is that it is highly toxic and is prone to leakage in high quantities. This may result in indoor air becoming largely unbreathable.
Although the world is now moving towards using more renewable energy sources aiming to reduce CO2 emission, in the long run, this approach will not be sufficient to reduce the CO2 level in the air or in the atmosphere. Therefore, it is vital to find alternative methods that are capable of adsorbing CO2 in air. Nano wires, nano fibres of metal oxides, organic and inorganic membranes are the current materials of interest by many in this field of research. The main challenge of these materials is the complexity and significantly high costs in mass production of such materials in the industrial sector at large.
The current CO2 capture and separation techniques can be divided into three groups, namely pre-combustion capture, post-combustion capture and oxyfuel combustion which captures CO2 from power plants and other industrial scale factories. Unlike liquid and membrane adsorbents, solid adsorbents are able to function at a wider temperature range and possess the ability to desorb the CO2 without major hazards. Solid adsorbents used in industrial exhaust gases have proven to be effective in capturing concentrated CO2 for subsequent storage instead of direct emission to the environment.
In recent years, solid adsorbents such as zeolites, carbon, metal organic frame works, alkali metal and amine adsorbents have been investigated for promising CO2 capture properties. Generally, many metal oxides are considered to be promising candidates for adsorbing CO2. However, their natural structural and morphological properties prevent them from achieving high adsorption capabilities that are useful on an industrial scale. Further, conventional adsorbents often require high temperatures to operate, which is highly energy consuming.
In view of the above, there is a need to provide a material and method for capturing CO2 that at least partially ameliorates the disadvantages discussed above.
In an aspect, there is provided a material comprising an anion-doped alkaline earth metal oxide.
Advantageously, the material as defined above may be used to effectively and efficiently adsorb CO2 from air at room temperature. In particular, the material may comprise an oxide of an alkaline earth metal such as magnesium that has been doped with an anion such as a halide, sulphate, or phosphate. Further advantageously, the doping may improve the CO2 adsorption properties of the oxide of the alkaline earth metal, by preventing the formation of side-products such as carbonates on the surface of the material. This may significantly increase the CO2 adsorption capacity of the material as defined above over time.
Advantageously, the material as defined above may have high porosity and high surface area, thereby having an increased number of carbon capture sites to improve CO2 adsorption. This may facilitate adsorption of CO2 at room temperature whereby the material may chemically bind with CO2 to make it more stable. The material as defined above may exhibit higher CO2 adsorption capacities at room temperature compared to conventional sorbents, and may be suitable for commercial use as a room temperature CO2 sorbent.
Further advantageously, the material as defined above may store CO2 within the material. Further advantageously, the adsorbed CO2 may be desorbed, and this process of adsorbing and desorbing CO2 may be repeated multiple times, facilitating the repeated use of the material over a longer period of time. Advantageously, the material may be stable over multiple adsorption-desorption cycles.
In another aspect, there is provided a method for preparing the material as defined above, comprising the step of contacting a hydroxide salt of an alkaline earth metal with a second salt of the alkaline earth metal.
Advantageously, the material as defined above may be prepared in a cost-efficient manner by the method as defined above. Due to its high cost-effectiveness, the method may advantageously be used in up-scaled or mass production of the material.
In an example, the method may comprise the step of forming the material as defined above into nanofibers, which may be performed by electrospinning. Previously known methods for synthesizing MgO sorbents were high-cost processes. Electrospinning, in contrast, may be more economical both in terms of time and costs. Electrospinning may also advantageously be more robust, durable, versatile, adaptable and scalable. Electrospinning may facilitate formation of structures in 1-dimension, 2-dimension as well as amorphous nanomaterials. Moreover, the resultant nanomaterials made by electrospinning may easily be functionalized by employing both chemical and physical surface treatments. Advantageously, electrospinning may facilitate up-scale or mass production of the material as defined above.
In another aspect, there is provided a material obtainable by the method as defined above.
In another aspect, there is provided a method for adsorbing CO2 from an environment, the method comprising the step of contacting the material as defined above with the environment.
In another aspect, there is provided the use of the material as defined above to adsorb CO2 from an environment.
Advantageously, the material as defined above may be used for CO2 adsorption at room temperature, thereby avoiding the CO2 adsorption process to be performed under highly energy consuming high-temperature conditions.
The following words and terms used herein shall have the meaning indicated:
The term “doped” for the purposes of this disclosure refers to the introduction of impurities such as anions into the lattice structure of the oxide of the alkaline earth metal or minerals thereof, so as to alter their phase stability and physical and chemical properties of the formed phases. The term “doping” should be construed accordingly.
The term “mineral” for the purposes of this disclosure refers to compounds comprising an oxide or hydroxide of an alkaline earth metal, water and a carbonate.
The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the invention.
Unless specified otherwise, the terms “comprising” and “comprise”, and grammatical variants thereof, are intended to represent “open” or “inclusive” language such that they include recited elements but also permit inclusion of additional, unrecited elements.
As used herein, the term “about”, in the context of concentrations of components of the formulations, typically means +/−5% of the stated value, more typically +/−4% of the stated value, more typically +/−3% of the stated value, more typically, +/−2% of the stated value, even more typically +/−1% of the stated value, and even more typically +/−0.5% of the stated value.
Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.
Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of
There is provided a material comprising an anion-doped alkaline earth metal oxide.
The alkaline earth metal oxide may be an oxide of an alkaline earth metal. The oxide of the alkaline earth metal may be doped with an anion.
The material may be a sorbent suitable for adsorbing CO2.
The alkaline earth metal may be selected from the group consisting of Mg, Ca, Sr, Ba and any mixture thereof.
Alkaline earth metals and their oxides may be readily and cheaply available as a by-product of sea water desalination, facilitating the cost-effective use of the material as defined above. Further, oxides of alkaline earth metals may facilitate the adsorption of CO2 at room temperature by chemically binding with CO2 to make it more stable.
The alkaline earth metal may be magnesium. The oxide of the alkaline earth metal may be MgO.
Magnesium (Mg) based minerals may advantageously be non-toxic and abundant, as they may be prepared in large scale with relatively low cost.
MgO or a mineral thereof, such as MgO—Mg(OH)2—H2O, may have a high theoretical CO2 capture capacity of 1100 mg CO2/g.
The anion may be selected from the group consisting of halide, sulfate, nitrate, phosphate and any mixture thereof.
The halide may be selected from the group consisting of fluoride, chloride, bromide, iodide or any mixture thereof.
Under dry and high temperature conditions, MgO may react with CO2 to form MgCO3 in dry, high-temperature environments, as shown below:
At lower temperatures, under moist conditions in the presence of H2O, MgO may react with H2O to form intermediate products or hydrates which may then in turn adsorb CO2. H2O molecules may further compete with CO2 to occupy the adsorption sites on the surface of MgO. Furthermore, this reaction may occur on the surface of the MgO. That is, over time, the surface of MgO may be saturated with MgCO3, causing a decrease in CO2 capture sites and rendering it inactive.
Advantageously, by doping the oxide of the alkaline earth metal with an anion, the formation and accumulation of intermediate and side-products on the surface of the material may be prevented.
Generally, structure basic sites may favour reversible CO2 adsorption, as represented in the following equation:
Advantageously, an increase in basic sites by incorporating the dopants, may result in better adsorption of CO2 than without any dopants.
The material may comprise MgO doped with an anion selected from the group consisting of chloride, sulfate, phosphate and any mixture thereof.
The material may comprise the alkaline earth metal oxide, hydrates thereof, and/or a hydroxide salt of the alkaline earth metal.
The material my further comprise water. The water may be in the form of a hydrate. The material may comprise a hydrate of the alkaline earth metal oxide or a hydrate of the anion-doped alkaline earth metal oxide. Advantageously, due to a large number of hydrates present in the material, they may facilitate to anchor a large number of hydrogen bonds (H-bonds) from the water molecules on the surface of the material, which may eventually form bonds with CO2 via chemisorption.
In the presence of water, by doping a MgO—H2O binary system with Cl−, four main reaction phases may be formed in the resultant ternary MgO—MgCl2—H2O system.
The four phases may be:
The water resistance and thermal stability of the four phases above, doped with Cl−, may be improved when reacting the material with CO2 from air to form magnesium chlorocarbonates.
The four ternary compounds may have improved water resistance, thermal stability, and CO2 capture compared to MgO or Mg(OH)2 when reacted with CO2 in air.
The anion-doped alkaline earth metal oxide as defined above may comprise the phase 2 ternary compound, 2Mg(OH)2. MgCl2.4H2O. The doped MgO may comprise impurities which may contribute to CO2 adsorption and mineralization, due to formation of magnesium chlorocarbonates.
The hydrate of the alkaline earth metal oxide or the hydrates of the anion-doped alkaline metal oxide may be selected from the group consisting of MgO—Mg(OH)2—H2O, MgO—MgCl2—H2O, 2Mg(OH)2·MgCl2·4H2O, 3Mg(OH)2·MgCl2·8H2O, 5Mg(OH)2·MgCl2·8H2O, 9Mg(OH)2·MgCl2·5H2O, Mg(ClO4)2·xH2O wherein x may be 1 or 2, Mg2Cl2CO3·7H2O, C8H20Cl2MgO6, MgCl2(H2O)+, Mg2CO3(OH)2·3H2O, Mg2CO3(OH)2·3H2O, Mg6O5SO4·8H2O, C3H2MgO4·2H2O, Mg3(PO4)2·22H2O, Mg5(CO3)4(OH)2(H2O)4, and any mixture thereof.
When MgO reacts with H2O to form Mg(OH)2, lower CO2 capture efficiency may be observed due to the replacement of CO2-philic MgO by CO2-phobic Mg(OH)2. However, the CO2-phobic Mg(OH)2 may act to transport or repel CO2 away from the MgO sites designed to adsorb CO2, allowing more CO2 to penetrate the MgO/Mg(OH), composite surface/interface and diffuse into the bulk of the material. The formed hydrates may therefore aid in increasing the CO2 capture capacity of the material.
The material may be in the form of nanofibers, flakes, rods, sheets, powder or any mixture thereof.
The powder may be a fine powder. The fine powder may have a particle size in the range of about 0.5 μm to about 5 μm, about 0.5 μm to about 1 μm, about 0.5 μm to about 2 μm, about 1 μm to about 2 μm, about 1 μm to about 5 μm or about 3 μm to about 5 μm.
The nanofiber may be ground to form the fine powder.
The fine powder may comprise sheet-like structures or rod-like structures.
There is also provided a method for preparing the material as defined above, comprising the step of contacting a hydroxide salt of an alkaline earth metal with a second salt of the alkaline earth metal.
The second salt of the alkaline earth metal may not be the same as the hydroxide salt of the alkaline earth metal.
The hydroxide salt of the alkaline earth metal may be Mg(OH)2.
The hydroxide salt of the alkaline earth metal may be a precursor of the oxide of the alkaline earth metal. The contacting of the hydroxide salt of the alkaline earth metal with the second salt of the alkaline earth metal may result in the formation of the anion-doped oxide of the alkaline earth metal.
The second salt of the alkaline earth metal may be selected from the group consisting of an alkaline earth metal halide, an alkaline earth metal sulfate, an alkaline earth metal nitrate, an alkaline earth metal phosphate and any mixture thereof. The second salt of the alkaline earth metal may be selected from the group consisting of MgCl2, MgSO4, Mg3PO4 and any mixture thereof.
The ratio between the hydroxide salt of the alkaline earth metal and the second salt of the alkaline earth metal may be in the range of about 99.9:0.1 to about 80:20 by weight, about 99.9:0.1 to about 99.5:0.5, about 99.9:0.1 to about 99:1, about 99.9:0.1 to about 98:2, about 99.9:0.1 to about 95:5, about 99.9:0.1 to about 90:10, about 99.5:0.5 to about 99:1, about 99.5:0.5 to about 98:2, about 99.5:0.1 to about 95:5, about 99.5:0.5 to about 90:10, about 99.5:0.5 to about 80:20, about 99:1 to about 98:2, about 99:1 to about 95:5, about 99:1 to about 90:10, about 99:1 to about 80:20, about 95:1 to about 90:10, about 95:1 to about 80:20 or about 95:10 to about 80:20 by weight.
The contacting step may be performed in a solvent.
The solvent may comprise acetic acid, water, or a mixture thereof.
The water may be deionized water.
The solvent may comprise glacial acetic acid or a mixture of acetic acid and water. The acetic acid may be present in a concentration in the range of about 0.2 M to about 17.5 M, about 0.2 M to about 0.5 M, about 0.2 M to about 1 M, about 0.2 M to about 2 M, about 0.2 M to about 5 M, about 0.2 M to about 10 M, about 0.2 M to about 15 M, about 0.5 M to about 1 M, about 0.5 M to about 2 M, about 0.5 M to about 5 M, about 0.5 M to about 10 M, about 0.5 M to about 15 M, about 0.5 M to about 17.5 M, about 1 M to about 2 M, about 1 M to about 5 M, about 1 M to about 10 M, about 1 M to about 15 M, about 1 M to about 17.5 M, about 2 M to about 5 M, about 2 M to about 10 M, about 2 M to about 15 M, about 2 M to about 17.5 M, about 5 M to about 10 M, about 5 M to about 15 M, about 5 M to about 17.5 M, about 10 M to about 15 M, about 10 M to about 17.5 M or about 15 M to about 17.5 M.
The choice of solvent may determine the property of the material, for example the morphology, chemical structure, spinnability and viscosity of the material. If MgO, Mg(OH)2 or any other magnesium related anions are used as the alkaline earth metal oxide or alkaline earth metal hydroxide, then acetic acid may be the most suitable solvent for dissolving the oxide or hydroxide.
The contacting step may further comprise a polymer.
The polymer may be polyvinyl acetate or polyacrylonitrile.
The polymer may be present in the form of an aqueous solution.
The ratio between the hydroxide salt of the alkaline earth metal and the polymer may be in the range of about 1:5 to about 1:20, about 1:5 to about 1:5 to about 1:7, about 1:5 to about 1:10, about 1:5 to about 1:12, about 1:5 to about 1:15, about 1:7 to about 1:10, about 1:7 to about 1:10, about 1:7 to about 1:12, about 1:7 to about 1:15, about 1:7 to about 1:20, about 1:10 to about 1:12, about 1:10 to about 1:15, about 1:10 to about 1:20, about 1:12 to about 1:15, about 1:12 to about 1:20 or about 1:15 to about 1:20 by weight.
The hydroxide salt of the alkaline earth metal and the second salt of the alkaline earth metal in the solvent may be contacted with the aqueous solution of the polymer.
The contacting step may comprise the step of mixing.
The method may further comprise the step of forming the material into fibres.
The forming step may be performed by electrospinning. The electrospinning may be performed using the mixture of the hydroxide salt of the alkaline earth metal, the second salt of the alkaline earth metal and the polymer, in the solvent.
The presence of the polymer may facilitate the electrospinning step. The polymer may act as a medium for the hydroxide salt of the alkaline earth metal and the second salt of the alkaline earth metal to be electrospun.
The polymer may not participate in any chemical reactions, and may only be present to facilitate the electrospinning process. The polymer may be used to tailor the fluid viscosity and dielectrics of the material during the electrospinning step. If acetic acid is present in the solvent, polyvinyl acetate, which contains acetate groups, may be a suitable polymer.
The method may further comprise the step of removing the solvent.
The solvent may be removed by drying. The drying may be performed at a temperature in the range of about 50° C. to about 70° C., about 50° C. to about 60° C., or about 60° C. to about 70° C., for a duration in the range of about 24 hours to about 72 hours, 24 hours to about 36 hours, 24 hours to about 48 hours, about 36 hours to about 48 hours, about 36 hours to about 72 hours, or about 48 hours to about 72 hours.
The method may further comprise the step of calcining the material. The calcining step may be performed after removal of the solvent.
The calcining step may be performed at a temperature in the range of about 250° C. to about 350° C., about 250° C. to about 300° C. or about 300° C. to about 350° C., for a duration in the range of about 1 hour to about 3 hours, about 1 hour to about 2 hours or about 2 hours to about 3 hours.
The calcining step may remove any organic matter from the material. The calcining step may remove the polymer from the material. Further, the calcining step may convert the hydroxide of the hydroxide salt of the alkaline earth metal to the oxide of the alkaline earth metal. After the calcining step, the material may comprise the alkaline earth metal oxide, hydrates thereof, and/or residual hydroxide salt of the alkaline earth metal.
The method may further comprise the step of grinding the material into a powder. The material may be the calcined material. The powder may be a fine powder.
The method may further comprise the step of aging the material. The aging may be performed by exposing the material in the form of a powder to ambient air at room temperature for a duration of time.
The temperature during the aging step may be in the range of about 20° C. to about 35° C., about 20° C. to about 25° C., about 20° C. to about 30° C., about 25° C. to about 30° C., about 25° C. to about 35° C. or about 30° C. to about 35° C.
Ambient air may be atmospheric air in its natural state. Atmospheric air may comprise nitrogen, oxygen and traces of other gases such as argon, helium, carbon dioxide and ozone. Atmospheric air may comprise about 78% nitrogen and about 21% oxygen.
The relative humidity of the atmospheric air may be in the range about 40% to about 70%, about 40% to about 50%, about 40% to about 60%, about 50% to about 60%, about 50% to about 70% or about 60% to about 70%. Relative humidity may refer to the ratio of the amount of moisture in the air at the temperature as defined above, to the maximum amount of moisture that the air may retain at the same temperature.
The duration of the aging step may be in the range of about 2 months to about 12 months, about 2 months to about 4 months, about 2 months to about 6 months, about 4 months to about 6 months, about 4 months to about 12 months or about 6 months to about 12 months.
The pressure during the aging step may be atmospheric pressure of about 101,325 Pa, 1,013.25 hPa or 1,013.25 mbar.
There is also provided a material obtainable by the method as defined above.
There is also provided a method for adsorbing CO2 from an environment, the method comprising the step of contacting the material as defined above with the environment.
The environment may be gas or liquid.
The gaseous environment may be the atmosphere.
The atmosphere may comprise air.
The liquid environment may be an aqueous solution. The aqueous solution may be sea water or wastewater. The material as defined above may adsorb CO2 dissolved in the liquid and may be useful in reversing acidification of the liquid.
There is also provided the use of the material as defined above to adsorb CO2 from an environment.
The CO2 adsorption using the material as defined above may be performed at room temperature, or at a temperature in the range about 20° C. to about 35° C., about 20° C. to about 25° C., about 20° C. to about 30° C., about 25° C. to about 30° C., about 25° C. to about 35° C. or about 30° C. to about 35° C.
The adsorbed CO2 may be stored in the material as defined above. The adsorbed CO2 may be stored in the material for a duration in the range of about 1 second to about 12 months, about 1 second to about 1 day, about 1 second to about 1 week, about 1 second to about 1 month, about 1 day to about 1 week, about 1 day to about 1 month, about 1 day to about 12 months, about 1 week to about 1 month, about 1 week to about 12 months or about 1 month to about 12 months. The adsorbed CO2 may be stored in the material for a duration in the range of about 1 week to about 3 months or about 1 week to about 6 months.
The adsorbed CO2 may be desorbed from the material as defined above.
CO2 may be desorbed from the material as defined above via the decomposition of the formed magnesium carbonate minerals, when the CO2 level falls below the equilibrium CO2 partial pressure, which is the reverse reaction of mineral formation.
There is also provided a method for storing CO2, the method comprising the steps of:
The steps (i) to (iii) may be repeated multiple times.
The accompanying drawings illustrate a disclosed embodiment and serve to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.
Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.
For sample synthesis, analytical grade polyvinyl acetate (PVA) (MW 89,000-98,000, 99+% hydrolyzed) and as the precursor, reagent grade Mg(OH)2 95% were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Analytical grade glacial acetic acid (AA) 99.8% was purchased from Scharlab (Barcelona, Spain). Magnesium chloride (MgCl2), magnesium phosphate 98+% (Mg3(PO4)2) and magnesium sulphate (MgSO4) analytical grade were purchased from Sigma-Aldrich (St. Louis, Missouri, USA), Acros Organics (The Hague, Netherlands) and Shanghai Macklin Biochemicals Co. Ltd (Shanghai, China), respectively. All the chemicals were used without further purification. The water utilized in the experiments was deionized water (18 MΩ·cm).
X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance X-ray diffractometer (Bruker Corporation, Billerica, Massachusetts, USA) with Cu—Kα radiation of 1.54 Å to evaluate powder's composition and phase. The scanning angle was adjusted between 2θ angles 10° to 140° with the X-ray generator running at applied voltage 40 kV and current 25 mA. CO2 capture behaviour was examined using a Thermogravimetric analyser (TGA) TGA Q50 analyser (TA Instruments, New Castle, Delaware, USA). Surface structure and morphology were examined by scanning electron microscopy (SEM) JEOL JSM-7600F (JEOL Ltd, Tokyo, Japan). The porosity of the samples was studied by Brunauer-Emmett-Teller (BET) ASAP 2020 Specific Surface Analyzer (Micrometrics Instrument Corporation, Norcross, Georgia USA). Brunauer-Emmett-Teller (BET) test condition was in low pressure and 200° C. using 0.5 g of powder samples. To study the CO2 gas sensing properties the prepared sensor was electrically connected to a Keithley 2400 Source meter (Keithley Instruments, Cleveland, Ohio, USA) and measured the properties.
CO2 capture capacity was examined by performing a CO2 adsorption experiment, using a Q50 (TA instruments, New Castle, Delaware, USA), by loading 5 to 8 mg of the sorbent (anion-doped MgO—Mg(OH)2—H2O—CO2 quaternary system) to a platinum (Pt) pan in the TGA unit. To avoid errors caused by impurities such as pre-adsorbed species, atmospheric CO2, water, and other impurities, samples were subjected to pre-calcination at 150° C. for 60 minutes under a flow of high purity N2 (40 mL min−1) with a ramp rate of 10° C. min−1. The temperature was then lowered to the desired adsorption temperature at a rate of 10° C. min−1, the gas was switched from N2 to CO2 with a constant flow of high purity CO2 (1 atm, 40 mL min−1), and the CO2 adsorption uptake was measured for 1.5 hours. The CO2 levels in TGA measurements approached a near plateau when the testing duration reached 1.5 hours, and adsorption was at the maximum CO2 capture capacity.
Alpha-terpineol was purchased from Sigma-Aldrich (St. Louis, Missouri, USA) as the binder and 0.01 g of Mg(OH)2 powder was mixed 0.01 mL of alpha-terpineol. Then, this mixture was coated on the electrode of the source meter using screen printing. Subsequently, the samples were dried at 60° C. for 30 minutes. Finally, to remove the remaining solvents, the samples were heat-treated at 250° C. for 1 hour.
To study the CO2 gas sensing properties of the sorbent, interdigitated Titanium (Ti) and Platinum (Pt) electrodes were deposited by direct current (DC) sputtering on the surfaces with thicknesses of 50 nm and 200 nm, respectively. The prepared sensor was electrically connected to a Keithley 2400 Source meter for the gas sensing measurements. Gas sensing characteristics were investigated using a horizontal-quartz heating chamber at a total flow of 500 sccm. Dynamic sensing data were recorded under a constant DC bias of 1 V. Output resistances were obtained in the presence of target gas (Rg) and air (Ra), and the sensor's response was defined as Ra/Rg. The CO2 sensing properties of the sorbent was studied by testing in 50-5000 ppm CO2 gas at different temperatures between 25° C.-300° C.
Aqueous polyvinyl acetate (PVA) (5% w/w) solution was first prepared by dissolving PVA powder in deionized water, at 90° C. for 2 hours. The solution was then cooled to room temperature (RT) and stirred continuously for another 12 hours.
Preparation 1 (PVA/Mg(OH)2/MgCl2 solutions (1% MgCl2, 5% MgCl2, 10% MgCl2))
1% Cl− solution was prepared by dissolving 0.0025 g MgCl2 and 0.2475 g Mg(OH)2 in 5 mL acetic acid via sonication in a water bath at 40° C. for 1 hour. Then, the solution was mixed with the 5% PVA as prepared above at a volume ratio of 15:100 (0.750 mL to 5 mL), with further sonication in a water bath at 40° C. for 20 minutes to eliminate precipitation. The 5% Cl− solution was prepared by using 0.0125 g MgCl2 and 0.2375 g Mg(OH)2, and the 10% Cl− solution was prepared by using 0.025 g MgCl2 and 0.225 g Mg(OH)2, followed by a similar procedure for the 1% Cl− sample described above.
Preparation 2 (PVA/Mg(OH)2/MgSO4 Solutions (1% MgSO4, 5% MgSO4, 10% MgSO4))
1% SO42− solution was prepared by using similar weights as mentioned for Preparation 1, but MgSO4 was used instead of MgCl2. The measured samples were dissolved in 4 mL acetic acid and 2 mL deionized water via sonication in a water bath at 30° C. for 1 hour to prepare. The solution was then mixed with 5% PVA as prepared above at a 15:100 ratio (0.750 mL to 5 mL), with the aid of sonication in a water bath at 30° C. for 20 minutes to eliminate precipitation. Similarly, the 5% SO42− solution was prepared by using 0.0125 g MgSO4 and 0.2375 g Mg(OH)2, and the 10% SO42− solution was prepared by using 0.025 g MgSO4 and 0.225 g Mg(OH)2 followed by a similar procedure as for the 1% SO42− sample described above.
Preparation 3 (PVA/Mg(OH)2/Mg3(PO4)2 Solutions (1% Mg3(PO4)2, 5% Mg3(PO4)2, 10% Mg3(PO4)2))
1% PO43− solution was prepared by measuring the similar weights mentioned in preparation, but Mg3(PO4)2 was used instead of MgCl2. The measured weights were dissolved in 7 mL of 5 moldm−3 acetic acid via sonication in a water bath at 30° C. for 1 hour. Then the aqueous solution was added to 5% PVA with a ratio of 3:28 (0.750 mL to 7 mL), with further sonication in a water bath at 30° C. for 20 minutes to eliminate precipitation. The 5% PO43− solution was prepared by using 0.0125 g Mg3(PO4)2 and 0.2375 g Mg(OH)2. The 10% PO43− solution was prepared by using 0.025 g Mg3(PO4)2 and 0.225 g Mg(OH)2 followed by a similar procedure as for the 1% PO43− sample described above.
Synthesis of Cl−, SO42− and PO43− Doped MgO Sorbent Samples
Electrospinning was carried out by using a needle-collector setup in top-down configuration with aluminum foil spread across the collector plate. All the samples were electrospun with an applied voltage of 20.5 kV and 21 G×½″ needle with the sharp end ground flat. The needle-collector distance was 12 cm, with a flowrate of 0.3 mL/hour. After electrospinning, the wet nanofiber layer deposited on the aluminum foil was oven-dried at 60° C. for 48 hours. This gave the solidified layer having a brittle consistency, which was then collected as flakes for calcination in a box furnace (Anhui Haibei 1100 model). During calcination, the furnace temperature was increased from 30 to 300° C. with a heating rate of 2° C./minute. The samples were heated at 300° C. for 2 hours, followed by air cooling to room temperature. The prepared doped MgO sorbent samples were then subjected to mechanical grinding using a mortar and pestle to obtain a fine powder.
The aging treatment was carried out by exposing the fine powder samples of Cl−, SO42− and PO43− doped MgO sorbent samples as prepared above to ambient air conditions for 3 months to 6 months.
Current research in CO2 sorbents adopt a trial-and-error approach, which is an approach that is not only time consuming but may also risk overlooking optimal doping chemicals. Bioinspired water harvesting materials have been known to take inspiration from a mechanism used by Namib Desert Beetles whose surface structures and chemistries enable them to collect water from fog by firstly facilitating nucleation and growth of water vapor molecules into droplets on wax-free hydrophilic bumps and then transporting the droplets towards the mouth of the beetle by the waxy hydrophobic surroundings. An important point to note is to separate the water harvesting action into two processes: (1) water droplet formation at hydrophilic sites, and (2) water droplet transportation along hydrophobic channels. This concept may be adapted in the present technology, to fill the gap between the current trial-and-error approach and bioinspired rational design strategies, in the development of MgO-based CO2 adsorbents.
To design a highly efficient CO2 adsorbent, it was necessary to develop a tool that may couple the surface CO2-philic and CO2-phobic properties to balance the nucleation and transportation process during CO2 absorption so that the surface of the adsorbent will not be completely blocked by the formed MgCO3, and therefore more CO2 molecules may be further adsorbed. Therefore, a chemical doping method was explored based on the so-called iso-diagonality in the Periodic Table of Elements. The iso-diagonal trend refers to a pair of elements (such as carbon/phosphorus and nitrogen/sulphur pairs) that have an adjacent upper left/lower right relative location in the Periodic Table of Elements, which are believed to have similar size and electronegativity, resulting in similar trends in properties. It is less explored than the vertical and horizontal trends. For instance, it is well accepted that nitrites (NO2−) promote CO2 adsorption in MgO based sorbents, since carbon and nitrogen are located close by in the same period of the Periodic Table of Elements. Accordingly, new dopants that may have similar properties as either carbon/oxygen (the two constituent elements of CO2) or nitrogen were explored, using the iso-diagonal relationship of carbon, nitrogen, and oxygen. Their respective iso-diagonal partner, phosphorous (P), sulphur (S), and chlorine (Cl), were therefore tested as new dopants to nano MgO—Mg(OH)2 composites in order to balance the surface CO2-philic and CO2-phobic properties for efficient CO2 absorption.
Each precursor solution was prepared by incorporating MgCl2, MgSO4 or Mg3(PO4)2 with Mg(OH)2 and adding different amounts of acetic acid and water as solvents. First, the sorbents were tested for its CO2 adsorption capacities using thermo gravimetric analysis (TGA). All sorbents developed had over 2 w % of CO2 adsorption capacity.
The Cl− doped samples calcinated at 300° C. recorded the highest capture capacity compared to the SO42− and PO43− doped samples calcinated at 300° C. Maximum adsorption capacity at 1.5 hours was shown by the 10% Cl− doped sample indicating an adsorption of 5.59 wt %, while 5% Cl− and 1% Cl− doped sorbent samples indicated an adsorption of 2.79 wt % and 2.97 wt %, respectively, at 1.5 hours, as shown in
However, the SO42− and PO43− doped sorbent samples recorded low adsorption capacities as shown in
To determine the CO2 adsorption capacities of the aged Cl− doped MgO samples, TGA analysis was carried out for 2 hours on the sorbent samples. The electrospun Cl− doped MgO samples were aged over 6 months and the CO2 adsorption capacity was evaluated via a thermogravimetric method at 30° C. (
The MgO sorbent samples doped with Cl− reported higher CO2 adsorption capacities before and after aging. The 1% Cl− doped sorbent sample exhibited increased absorption with increase in aging time for up to 6-months. The 1% Cl− doped sorbent sample recorded a higher adsorption capacity of 16.15% at 6 months of aging, which was a large increase from its initial value of 2.79 wt % measured before aging treatment, as shown in
The behavior of the 1% Cl− doped sorbent sample may be explained by the formation of C8 compounds such as nano-C8H10MgO10·4H2O. The 1% Cl− doped sorbent sample, having less Cl−, may not have hindered the formation of the C8 compound, unlike the 5% and 10% Cl− doped samples.
MgO doped with SO42− showed a slight increase in CO2 adsorption for the 1%, 5% and 10% doped sorbent samples, which showed 2.99 wt %, 3.19 wt % and 4.46 wt % CO2 adsorption, respectively, at their 3-month aging time point. However, it was noted that this trend was not observed for the PO43− doped sorbent samples, as the adsorption capacity of the 1% PO43− doped sorbent samples decreased slightly after 3-months of aging from 2.48 wt % to 2.15 wt % and the CO2 adsorption capacity of 5% PO43− doped samples did not change with aging, remaining at 2.64 wt %.
In contrast, for the 10% PO43− doped sorbent sample, an increase in absorption capacity from 2.98 wt % to 3.58 wt % was observed after 3 months of aging. However, unlike sorbent samples doped with Cl− or SO42−, PO43− doped sorbent samples showed an increase in adsorption capacities after 6 months of aging, of 4.80 wt %, 3.28 wt % and 3.33 wt % for 10%, 5% and 1% PO43− doped sorbent samples, respectively. A summary of the CO2 capture capacities of the sorbent samples is summarized in Table 1.
It is evident that the sorbent samples comprise excessive amounts of hydrates and carbonates, as shown by the XRD data as further discussed in Example 5. Extensive carbonation of sorbents may be a result of using them at room temperature conditions. The sorbents adsorb CO2 from the atmosphere in typical indoor spaces where the temperature is approximately 25° C. and CO2 levels are approximately 2200 ppm. The existence of H2O enhances the carbonation process of the sorbents and this promotes conversion of MgO in the sorbent samples to its hydrates. If the calcination temperature is increased to 500° C., all the residual Mg(OH)2 in the sorbent sample is converted to MgO, indicating the decomposition of Mg(OH)2. This may result in the high adsorption capacities of the doped sorbent samples after 6 months of aging. The residual MgCO3 in the doped MgO sorbent samples may form pores on the surface due to aging, promoting diffusion of CO2 and H2O to react with residual MgO present in the sorbent samples. This may increase CO2 adsorption capacities when aging the sorbent sample.
In order to investigate the chemical composition of the synthesized sorbent materials, the Cl−, PO43− and SO42− doped MgO samples which were calcinated at 300° C. were analyzed by X-ray diffraction (XRD) as shown in
Based on the sharp diffraction peaks in
Further chemical equilibria calculations were carried out with FactSage for the Cl− doped sorbent samples at 10%, 5% and 1% wt MgCl2 levels at a temperature of 300° C.
FactSage is a commercial chemical equilibrium calculation system that predicts/calculates the chemical equilibria at a given process condition such as temperature, pressure and the chemistry of initial reagents. The calculations may help to determine the chemical reactions of Cl− doped systems, and thus the role of the Cl dopants in the sorbent.
The CO2 threshold (at an equal CO2 to H2O ratio) for the formation of MgCO3 for the 10%, 5% and 1% wt MgCl2 doped samples was respectively >1.0, 0.104, and 0.112 (moles) for a 100 gram sample. Based on equation (1), no MgCO3 was formed even when 1 mole CO2 was added to the 10% wt MgCl2 doped sample, but MgCO3 was formed for 5% wt and 1% wt MgCl2 doped samples.
Therefore, it is unlikely that MgCO3 is stable in samples having 10% wt MgCl2 and 90% Mg(OH)2 whereas the MgCO3 is stable in the 5% wt and 1% wt MgCl2 samples, because of the much lower (<<0.5 atm) CO2 level in air.
It is interesting that based on thermodynamic calculation, the 5% wt Cl− doped sorbent sample had the lowest CO2 threshold and therefore the largest CO2 concentration gap with refer to the CO2 level in air, which correlated well with the heights of the glass phase XRD peaks from 10° to 40° (2θ).
To further investigate whether hydrates form in the 5% wt Cl− doped sorbent sample at 25° C., the following calculation was performed:
A new hydrate phase, MgCl2(H2O)4, was formed at 25° C., which supported the assumption that the hydrate glass phases may be represented by the XRD peaks from 10° to 40° (2θ)). Therefore, based on thermodynamics, it was evident that hydration was a competitive process to CO2 adsorption.
In
The PO43− doped sorbent samples showed poor sample correlation with MgO in comparison with Cl− doped sorbent samples and SO42− doped sorbent samples, as shown in
The surface morphologies of the sorbent samples were investigated with scanning electron microscopy (SEM) (JEOL JSM-7600F), using a voltage of 5 kV and a working distance of 8 mm as shown in
The 5% Cl− doped MgO sample displayed sheet-like structures with uniform surfaces, having a typical 2-dimensional diffusion mode. The 10% Cl− doped MgO sample showed a similar structure to the 5% Cl− doped MgO sample, having a sheet-like uniform structure. The changes in morphology observed as the dopant concentration was increased from 1, 5 and 10% wt Cl−, were similar to that observed for MgO grown from Mg(OH)2 using different alkali salts.
SO42− doped MgO samples displayed sheet-like structures after calcination for 2 hours at 300° C. As SO42− concentration was increased, the observed grain size decreased, which was consistent with previous observations that the loss of water during the decomposition of Mg(OH)2 resulted in the formation of a porous structure, which fills up with growth of newly formed MgO particles.
PO43− doped MgO samples also showed sheet-like structures under similar conditions, indicating a strong presence of heterogeneously grown hydrates. The morphology of the PO43− doped MgO samples were observed to be different than that of the Cl and SO42− doped MgO samples, due to the disappearance of the MgO phase, which may be the reason for their much lower CO2 adsorption capacity among the 3 dopants tested. All the samples showed a sheet size of 1-3 μm.
The 10% Cl− doped sorbent sample was further evaluated for the influence of calcination temperature on CO2 adsorption at room temperature. The 10% Cl− doped sorbent sample was calcinated at 500° C. in a box furnace (Anhui Haibei Import & Export Co., Ltd., 1100 model, Hefei, Anhui, China). During the calcination, the furnace temperature was increased from 30° C. to 500° C. with a heating rate of 2° C./minute, and was then kept at 500° C. for 2 hours, followed by air cooling to room temperature. TGA analysis was subsequently performed to record the CO2 adsorbing capacity using the same procedure indicated in Example 1. The same procedure for calcination and TGA analysis was repeated for a sample calcinated at 700° C. The sample calcinated at 500° C. and 700° C. showed a CO2 adsorption capacity of 1.4 wt % and 1.33 wt %, respectively, within 1.5 hours, which was a low adsorption value compared to the 5.59 wt % of the same sample calcinated at 300° C. shown in
This significant decrease in CO2 adsorption capacity with an increase in calcination temperature may be related to the dissolution of the designed CO2-philic MgO and CO2-phobic Mg(OH)2 interfaces, along with the disappearance of the Mg(OH)2 phase at a high calcination temperature of 500° C. and above, as shown in their respective XRD spectra in
The XRD analysis of the sorbent samples showed the main 2θ peaks of MgO (ICDD 00-045-0946) to be 36.74, 42.72, 62.06, 74.33, 78.35, 93.70, 105.33, 109.36, and 126.65, which were consistent with MgO (111), MgO (200), MgO (220), MgO (331), MgO (222), MgO (400), MgO (331), MgO (420), and MgO (422), suggesting that the formation of MgO may have caused a slight shift towards the lower angles indicated in
The sample calcinated at 500° C. showed high intensity peaks compared to the sample calcinated at 300° C., suggesting better crystallinity in the sorbent sample calcined at 500° C. Morphology of the 10% Cl− doped sorbent samples calcinated at 500° C. (
To clarify the performance of the Cl− doped MgO/Mg(OH)2 samples, the samples were thoroughly characterized using BET analysis. The specific surface area of the Cl− doped MgO samples were calculated using the BET method.
A summary of surface areas is shown in Table 2, where the 5% Cl− doped MgO sample showed a higher specific surface area of 65.53 m2g−1 compared to the other sorbent samples. Although the 10% Cl− doped MgO sample had a higher CO2 capture capacity as discussed in Example 4, it was also shown to have the lowest specific surface area of 26.18 m2g−1.
The adsorption-desorption curves of the Cl− doped sorbent samples are shown in
Gas sensing test was carried for the Cl− doped sorbent sample which recorded the highest surface area, the 5% Cl− doped MgO sample, and the results are shown in
The Cl− doped sorbent samples were found to be a poor CO2 sensing material at room temperature, due to the chemisorption of CO2.
The measurement at 300° C. detected CO2 at 500 ppm levels because the thermal decomposition temperature (to MgCO3) is about 327° C.
To further analyse the sorbent samples, adsorption/desorption cycles were carried out on 10% Cl− doped sorbent samples, and the results are shown in
The adsorption/desorption curves showed that even though a large number of hydrates were present in the 10% Cl− doped sample, it was stable at 30° C. Therefore, the presence of the hydrates may enhance the performance of the Cl− doped sorbent materials.
It is well known that iso-diagonality is an important feature when considering properties of chemical substances. Phosphorus, sulfur and chlorine are iso-diagonal partners in period 3 of the Periodic Table of Elements, where both phosphorus and sulfur are multivalent nonmetals and chlorine is a monovalent nonmetal. All three of these elements form van der Waals bonds. In recent times, the diagonal properties have come into light as most cases of iso-diagonality involve covalently bonded species. The electronegativity of these elements is in the order of phosphorus<sulfur<chlorine. The electronegativities of carbon and phosphorus are very similar, i.e 2.05 and 2.06, respectively. The non-polar nature of the carbon-phosphorus bond results in the ability of phosphorus to replace carbon with insignificant changes in chemical reactivity. However, the electronegativity of sulfur (2.44) and chlorine (2.83) are closer to carbon than nitrogen (3.07) and oxygen (3.05). This may be the reason why the Cl− doped sorbent samples outperformed the PO4−3 and SO4−2 doped sorbent samples, while the SO4−2 and PO4−3 doped sorbent samples performed similarly to each other in terms of CO2 adsorption.
The following Table 4 summarizes the previously known works on using MgO for CO2 capture, in comparison with the sorbent of the present disclosure.
The material as defined above may be used in CO2 adsorbing devices, gas separation equipment, post- and pre-combustion CO2 capture, CO2 recovery and storage, catalysis, interconversion of hydrocarbons, natural gas purification, biogas upgrading, building materials including cement materials, fuel synthesis and in CO2 sensors.
The material as defined above may also be suitable for use in coastal protection engineering, to adapt to rising sea levels and acidification of sea water due to CO2 adsorption. Similarly, the material as defined above may be suitable for use in adsorbing CO2 from wastewater.
The material as defined above may be used to desorb CO2, which may be suitable for use in regulating CO2 concentrations both indoor and outdoors, as well as facilitating organism growth, for example in a marine environment.
The carbon captured by the material as defined above may also be used in the manufacture of carbon products such as carbon nanotubes and carbon powders, thereby making the material as defined above valuable for sourcing carbon.
It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10202101959P | Feb 2021 | SG | national |
This application is the U.S. national phase of International Application No. PCT/SG2022/050090 filed Feb. 24, 2022, which designated the U.S. and claims priority to SG Patent Application No. 10202101959P filed Feb. 25, 2021, the entire contents of each of which are hereby incorporated by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2022/050090 | 2/24/2022 | WO |
