Carbon dioxide adsorbents

Abstract

A carbon dioxide adsorbent including a hierarchical zeolite. The hierarchical zeolite defines micropores having a pore width between about 0.4 nm and about 2 nm, and at least one of: mesopores having a pore width between about 2 nm and about 50 nm; and macropores having a pore width greater than about 50 nm.

Claims

1. A method of removing carbon dioxide from a gas stream comprising carbon dioxide, the method comprising: contacting a first gas stream comprising carbon dioxide with a carbon dioxide adsorbent comprising a hierarchical zeolite; and adsorbing carbon dioxide from the first gas stream onto the carbon dioxide adsorbent, thereby removing some of the carbon dioxide from the first gas stream to yield a second gas stream, wherein the second gas stream has a lower concentration of carbon dioxide than the first gas stream, wherein the hierarchical zeolite comprises anatase and defines: micropores having a pore width between about 0.4 nm and about 2 nm; and one or both of: mesopores having a pore width between about 2 nm and about 50 nm; and macropores having a pore width greater than about 50 nm.

2. The method of claim 1, comprising desorbing at least some of the carbon dioxide from the carbon dioxide adsorbent to yield a third gas stream comprising desorbed carbon dioxide.

3. The method of claim 2, wherein desorbing at least some of the carbon dioxide from the carbon dioxide adsorbent comprises changing the gas pressure on the carbon dioxide adsorbent, the temperature of the carbon dioxide adsorbent, or both.

4. The method of claim 2, wherein the carbon dioxide adsorption capacity of the carbon dioxide adsorbent after desorbing at least some of the carbon dioxide from is at most about 15% less than the initial equilibrium adsorption capacity of the carbon dioxide adsorbent before contacting the first gas stream with the carbon dioxide adsorbent.

5. The method of claim 2, wherein the third gas stream has a carbon dioxide concentration of at least about 30 mol %.

6. The method of claim 1, wherein the first gas stream comprises flue gas, biogas, or natural gas.

7. The method of claim 1, wherein the first gas stream is produced from an industrial process or a waste combustion or breakdown process.

8. The method of claim 1, wherein the first gas stream comprises up to about 70 mol % carbon dioxide.

9. The method of claim 1, wherein the second gas stream comprises up to about 30 mol % carbon dioxide.

10. The method of claim 1, wherein the first gas stream comprises water, and further comprising removing some of the water from the first gas stream before contacting the first gas stream with the carbon dioxide adsorbent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows powder X-ray diffraction (XRD) patterns of samples prepared as described in Example 1 (3.0 Na.sub.2O:Al.sub.2O.sub.3: 4.0 SiO.sub.2: 32.4H.sub.2O and canola oil/H.sub.2O (v/v)=1.0) heated at 60° C. for 12, 24, 30, 36, 48, and 54 hours, with the simulated patterns of faujasite (NaX) and anatase (an impurity).

(2) FIGS. 2A and 2B show scanning electron microscope (SEM) images and FIGS. 2C and 2D show transmission electron microscope (TEM) images of samples prepared as described in Example 1 with canola oil heated at 60° C. for 54 hours.

(3) FIG. 3A shows N.sub.2 sorption isotherms of samples prepared as described in Example 1 with and without canola oil heated at 60° C., with NaX as a reference. FIG. 3B shows Barret-Joyner-Halenda (BJH) pore distributions of samples prepared as described in Example 1.

(4) FIG. 4 shows variable temperature CO.sub.2 adsorption isotherms for a sample prepared as described in Example 1.

(5) FIG. 5 shows kinetics of CO.sub.2 adsorption of a sample prepared as described in Example 1, another hierarchical zeolite sample, and a 13X reference (from Sigma Aldrich) at 25° C.

(6) FIG. 6 shows CO.sub.2 sorption isotherms for a sample prepared as described in Example 1 in the first two cycles at 0° C.

(7) FIG. 7 shows Ideal Adsorbed Solution Theory (IAST) CO.sub.2-over-N.sub.2 selectivity of the sample prepared as described in Example 1 at 20° C. in a mixture of 15 mol % CO.sub.2 and 85 mol % N.sub.2.

(8) FIG. 8 shows IAST CO.sub.2-over-CH.sub.4 selectivity of a sample prepared as described in Example 1 at 20° C. in a mixture of 50 mol % CO.sub.2 and 50 mol % CH.sub.4.

(9) FIG. 9 shows heat of CO.sub.2 adsorption of a sample prepared as described in Example 1.

DETAILED DESCRIPTION

(10) CO.sub.2 adsorbent materials and methods for removal of CO.sub.2 adsorbed to the adsorbent materials are described.

(11) Zeolites are typically described as crystalline aluminosilicates having ordered channel and/or cage structures and containing micropores (“zeolitic micropores”) which are typically smaller than about 0.9 nm. Zeolites such as NaX, NaY, and zeolite A are inexpensive and have high CO.sub.2 uptake and selectivity due to the high electrical field gradients in their micropores. These electrical field gradients are induced by the negatively charged framework and the charge balancing cations of the zeolites. Aluminum-containing zeolites are able to adsorb CO.sub.2 largely due to the negative aluminum framework charge that is compensated for by exchangeable cations (often alkali metal cations) in the pore space. These alkali metal cations enable zeolites to adsorb acidic gases such as CO.sub.2. Ideally, a low Si/Al ratio is favored to promote the presence of more cations.

(12) Adsorption of CO.sub.2 via zeolites is well suited for VSA/PSA/TSA processes due to their favorable kinetics and capacities at mild operating conditions. Zeolites have relatively high adsorption capacities at mild operating conditions (0-100° C., 0.1-1 bar CO.sub.2). Adsorption capacities for zeolites at mild operating conditions range from 0.09-5 mmol CO.sub.2/g zeolite. Although adsorption using zeolites is classified as physisorption, studies have found that some CO.sub.2 does irreversibly chemisorb on the zeolite surface in the form of carbonates and carboxylates. If desorption is performed using a pressure swing process, these chemisorbed CO.sub.2 molecules generally cannot be liberated from the surface. This results in a reduction in capacity up to about 0.5 mmol CO.sub.2/g zeolite. The zeolite adsorbents can typically be fully regenerated only by heating the adsorbent (around 623 K) under high vacuum, which is generally too energy-demanding to apply in practice.

(13) Hierarchical zeolites are zeolitic materials having a hierarchical porosity with at least two levels of pore sizes. That is, hierarchical zeolites possess, in addition to the typical and uniform zeolitic micropores, a secondary porosity. The secondary porosity may include pores with different widths extending from supermicropores (i.e., pores with a pore width less than about 0.4 nm) to mesopores (i.e., pores with a pore width between about 2 nm and about 50 nm) or macropores (i.e., pores with a pore width greater than about 50 nm). In this disclosure, the terms “pore width,” “pore size,” and “pore diameter,” are used interchangeably. As used herein, micropores have a pore width between about 0.4 nm and about 2 nm. In most cases the secondary porosity of a hierarchical zeolite lies within the mesopore range, with the pore size generally determined by the synthetic strategy used to form the zeolite. As described herein, CO.sub.2 adsorbent materials are hierarchical zeolites with micropores and at least one of mesopores and macropores (i.e., hierarchical zeolites “exhibit” or “define” micropores and at least one of mesopores and macropores).

(14) The diffusion rate of molecules through the zeolite micropores depends at least in part on the size of the molecules, and decreases as the size of the molecules approaches the dimensions of the zeolite channels (configurational diffusion). Consequently, the intracrystalline transport becomes the rate-controlling step in many adsorption and catalytic processes over zeolites. This limitation can be overcome with hierarchical zeolites, as diffusion through the secondary porosity (e.g., mesoporosity) is faster, taking place by mechanisms closer to the classical Knudsen diffusion. The net effect regarding the transport within the zeolite micropore is a significant reduction of the length of the diffusional pathway. Since the overall diffusion rate is inversely proportional to the square of the diffusional pathway, this may result in an increase of the diffusion rate by 2 to 3 orders of magnitude.

(15) Changes in the selectivity, especially in the product selectivity in catalytic reactions, may also take place since the presence of the hierarchical porosity implies a variation in the length of the diffusional pathway. Reduction of the diffusional pathway may increase the relative importance of the chemical factors related to the nature of the active sites with respect to diffusional factors. Due to shortened diffusion pathways, shape-selectivity effects may be less pronounced in hierarchical zeolites, which may affect negatively their performance, for example, in catalysis in terms of product distribution. In some cases, product selectivity may be improved due to a shorter residence time within the zeolite micropores, thereby avoiding conversion of the primary products through undesired secondary reactions.

(16) If diffusional factors become less critical, hierarchical structures may improve the gas adsorption selectivity as well for the zeolites whose chemical nature favors selective adsorption of a specific gas, for example, CO.sub.2. However, additional effects may also be related to the secondary porosity of hierarchical zeolites. Due to the discontinuity of the crystalline framework at the secondary or mesopore surface, the chemical nature of this pore surface represents that of nanozeolites. That is, the Si and Al species present a lower T atom connectivity compared with the positions inside the channels, and a high population of silanol groups are also found on surfaces defined by the mesopores. Accordingly, hierarchical zeolites show in many aspects features intermediate between conventional zeolites and amorphous aluminosilicates.

(17) An amorphous skin layer at the secondary or mesopore surface may act as a diffusion barrier and thus negatively affect gas adsorption processes. For example, surface resistance has been shown to be far from negligible for the zeolite crystals that are no larger than 2 μm, for which surface resistance represents more than 60% of the total mass transfer resistance at 398 K. Furthermore, it has been speculated that surface resistance may not be located at the extreme surface of the crystals but in a layer of non-negligible thickness of distorted crystal structure around the crystals. In another example, it was found that nanosized zeolite A (˜200 nm) did not show an increased adsorption rate in comparison to larger zeolite A (4 μm), which was attributed to a skin layer that had formed on the nanosized zeolite A, as well as imperfections within the small crystals, remaining water, or other effects. Furthermore, the CO.sub.2-over-N.sub.2 selectivity was not affected by the crystal size. In some cases, hierarchical zeolites can be shaped into technically suitable geometries by granulation with a clay binder, retaining their chemical and textural properties.

(18) Various synthetic methods are available for production of hierarchical zeolites with different chemical structures and pore structures at various length scales. In some implementations, hierarchical zeolites are produced by using a geopolymer resin as a precursor. For example, WO 2013/044016, which is incorporated herein by reference, describes processes of producing hierarchical zeolites by mixing a geopolymer resin and canola oil.

(19) As used herein, “geopolymers” are generally referred to by a variety of terms, including low-temperature aluminosilicate glass, alkali-activated cement, geocement, alkali-bonded ceramic, inorganic polymer concrete, and hydroceramic. Despite this variety of nomenclature, these terms all describe materials synthesized utilizing the same chemistry, which can be described as a complex system of coupled alkali-mediated dissolution and precipitation reactions of aluminosilicates in an aqueous reaction substrate. Geopolymers are nanomaterials that exhibit a dense gel-like structure with 5 nm to 60 nm-sized amorphous aluminosilicate particles. Their chemical structure generally includes an amorphous, three-dimensional network of corner-sharing aluminate and silicate tetrahedra, with the negative charge due to Al.sup.3+ ions in the tetrahedral sites balanced typically by the alkali metal ions. Alkali-activated aluminosilicates are a type of geopolymer. Geopolymers can be prepared typically by curing geopolymer resins. In some cases, geopolymer resins are prepared by coupled alkali-mediated dissolution and precipitation reactions of silicate or aluminosilicate precursors in an aqueous media. The term “geopolymerization process” used herein includes all the chemical processes that provide a geopolymer. As used herein, a “geopolymer resin” is uncured or partially cured alkali-activated aluminosilicates from the geopolymerization process.

(20) In some cases, forming a CO.sub.2 adsorbent includes heating a composition including a geopolymer resin up to about 100° C. to yield a heated composition, and separating a solid product from the heated composition to yield the CO.sub.2 adsorbent. The geopolymer resin typically contains water in an amount no greater than a mole fraction of about 0.85. In some cases, the composition also includes an organic liquid that can undergo a saponification reaction when the organic liquid is in contact with the geopolymer resin, thereby yielding the mixture. In certain cases, the composition is heated at a temperature no higher than about 100° C. for no longer than a week. Heating the composition may include heating the composition in a container closed to the environment (a “closed container”) at a temperature no higher than about 100° C. for no longer than a week. In certain cases, heat is removed from the heated composition before the solid product is separated from the heated.

(21) In some implementations, the hierarchical zeolites have a faujasite structure (FAU) and define mesopores. The FAU zeolites are typically synthetic (i.e., man-made rather than occurring in nature). In some cases, the hierarchical zeolites are aggregates of intergrown small crystallites of synthetic FAU zeolite, exhibiting textural mesopores, with the small crystallites having an average size not larger than about 200 nm. In some cases, the small crystallites have an average size not larger than about 100 nm, or not larger than about 50 nm. In certain cases, the hierarchical zeolites have a FAU structure and exhibit both mesopores and macropores. As used herein, “textural mesopores” generally refers to mesopores between the small crystallites.

(22) In some cases, the hierarchical zeolites have a zeolite A structure (LTA). In some cases, the hierarchical zeolites are aggregates of intergrown small crystallites of synthetic zeolite A, exhibiting textural mesopores, wherein the small crystallites have an average size not larger than about 200 nm. In some case, the hierarchical zeolites have a LTA structure and exhibit both mesopores and macropores.

(23) In some cases, the hierarchical zeolites have a Si/Al ratio not greater than about 3, not greater than about 2, or not greater than about 1.5. In some cases, the hierarchical zeolites have a crystallinity not lower than about 80%, about 90%, about 95% or about 99% based on the micropore volume estimated from gas sorption measurements.

(24) In some implementations, the hierarchical zeolites are ion-exchanged fully or partially with alkali metals, alkaline earth metals, transition metal, or rare earth metals including, for example, lithium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, or a combination thereof, some or all of which may be in ionic form, to modify the pore size and/or chemical nature of the zeolite micropores.

(25) In some implementations, the hierarchical zeolites have a CO.sub.2 adsorption capacity over about 4 mmol/g at 0° C. in a 99.999 mol % CO.sub.2 gas stream with a pressure of 100 kPa.

(26) In some implementations, the CO.sub.2 adsorption capacity of a hierarchical zeolite decreases no more than about 15% after the first desorption (i.e., a hierarchical zeolite loses no more than about 15% its CO.sub.2 adsorption capacity in its first adsorption/desorption cycle, when compared to the initial equilibrium adsorption capacity before the desorption step in the first cycle). In some cases, the CO.sub.2 adsorption capacity of a hierarchical zeolite decreases no more than about 10% or no more than about 5% after the first desorption.

(27) In some implementations, the hierarchical zeolites have a heat of CO.sub.2 adsorption not greater than about 40 kJ/mol for the capacities ranging from about 2.5 to about 5 mmol/g.

(28) In some implementations, the hierarchical zeolites have an ideal CO.sub.2-over-N.sub.2 adsorption selectivity greater than about 46 or greater than about 100 in a mixture of about 15 mol % CO.sub.2 and about 85 mol % N.sub.2 at 20° C. under a total pressure of about 100 kPa. As used herein, “about” refers to ±10% (e.g., about 100° C. refers to a range of temperatures between 90° C. and 110° C.)

(29) In some implementations, the hierarchical zeolites have an Ideal Adsorbed Solution Theory (IAST) CO.sub.2-over-N.sub.2 selectivity great than about 400 or greater than about 600 in a mixture of about 15 mol % CO.sub.2 and about 85 mol % N.sub.2 at 20° C. under a total pressure of about 100 kPa. In some cases, the hierarchical zeolites have an IAST CO.sub.2-over-N.sub.2 selectivity over about 600 in a mixture of about 15 mol % CO.sub.2 and about 85 mol % N.sub.2 at 20° C. under a total pressure of about 100 kPa.

(30) In some implementations, the hierarchical zeolites have an IAST CO.sub.2-over-CH.sub.4 selectivity greater than about 100 or greater than about 200 in a mixture of about 50 mol % CO.sub.2 and about 50 mol % CH.sub.4 at 20° C. under a total pressure of about 100 kPa.

(31) In some implementations, the adsorbents are a powder, a film, or a coating. In some cases, the adsorbents are granules, beads, or pellets formed with or without a binder.

(32) As described herein, a process for removing CO.sub.2 from a gas stream containing CO.sub.2 uses an adsorbent comprising hierarchical zeolites. The process may include, for example, (a) passing the gas stream through an adsorbent having a hierarchical zeolite material to adsorb the CO.sub.2 onto the adsorbent and to produce a gas stream containing less than about 10 mol %, less than about 5 mol %, or less than about 1 mol % CO.sub.2, and (b) optionally changing pressure or temperature of the adsorbent having CO.sub.2 adsorbed thereon to desorb at least some of the adsorbed CO.sub.2.

(33) Gas streams containing CO.sub.2 include, for example, a flue gas, a biogas, or a natural gas, a gas stream from an industrial process, and a gas stream from a waste combustion or breakdown process, any one of which may be pretreated to reduce the relative pressure of the moisture to less than about 0.01, about 0.001, or about 0.0004. Flue gases may be from power plants, furnaces in industries, blast furnaces or cement kilns. The industrial processes include manufacturing, petroleum refining, construction, mining, and agriculture. In some cases, the manufacturing processes may produce paper, food, chemicals, metals, minerals or cement. In some cases, the manufacturing processes may produce hydrogen, ammonia, ethanol, ethylene oxide or steel. The waste breakdown processes include microbiological decomposition of land-filled garbage.

(34) Isothermal gas adsorption experiments are typically carried out with a thermogravimetric analyzer (TGA) to measure the gravimetric adsorption capacity of an adsorbent. The kinetics of the gas uptake can be monitored using the same method. The IAST can be used to predict binary adsorption isotherms using the adsorption isotherms of pure gases. In IAST, selectivity of CO.sub.2 over N.sub.2 is defined within a two phase model as the ratio of mole fraction of CO.sub.2 in the adsorbed state (x.sub.CO.sub.2) over the mole fraction of CO.sub.2 in the gas phase (y.sub.CO.sub.2) divided by the relative fractions, x.sub.N.sub.2 and y.sub.N.sub.2, defined for N.sub.2:

(35) ${\alpha }_{{\mathrm{CO}}_2/N_2}=\frac{x_{{\mathrm{CO}}_2}.Math.y_{N_2}}{x_{N_2}.Math.y_{{\mathrm{CO}}_2}}.$

(36) An alternative measure of selectivity is the ideal CO.sub.2-over-N.sub.2 selectivity (S.sub.CO.sub.2.sub.IN.sub.2) which is estimated with a mixture gas that contains 15 mol % CO.sub.2 and 85 mol % N.sub.2 as the ratio of equilibrium mole fraction of CO.sub.2 adsorbed at 15 kPa (x.sub.CO.sub.2.sup.15) over the equilibrium mole fraction of N.sub.2 adsorbed at 85 kPa (y.sub.N.sub.2.sup.85):

(37) $S_{{\mathrm{CO}}_2/N_2}=\frac{x_{{\mathrm{CO}}_2}^{15}.Math.85}{y_{N_2}^{85}.Math.15}.$

(38) The isosteric heat of adsorption can be calculated using adsorption isotherms obtained at different temperatures relying on the Clausius-Clapeyron equation.

Examples

(39) Example 1. Deionized water was used throughout the synthesis and purification. In a typical synthesis, 9.114 g of NaOH pellets (Sigma Aldrich) and 23.411 g of water glass (Sigma Aldrich) were dissolved in 16.382 g of water in a polypropylene beaker. After the solution had cooled, 11.467 g of metakaolin (MetaMax® from BASF) was slowly added into the solution while stirring. The resulting mixture was homogenized with a mechanical mixer (IKA® RW 60 digital mixer) at 800 rpm for about 40 min to obtain a visually homogeneous and free flowing resin (“geopolymer resin”) with the following final molar composition: 3.0 Na.sub.2O:1.0 Al.sub.2O.sub.3:4.0 SiO.sub.2:32.4H.sub.2O. For those samples prepared by the addition of canola oil, canola oil (The J. M. Smucker Company, Crisco®) was added to the geopolymer resin and the mechanical stirring at 800 rpm was continued for additional 10 min to give a homogeneous “resin-oil blend”. The geopolymer resins or resin-oil blends were poured into 50 ml polypropylene tubes and tightly closed with lids. The closed tubes were then placed in a laboratory oven at 60° C. or 90° C. for an appropriate length of heating time. After the heating, the products, having a consistency of pastes, were taken out from the tubes and washed first with hot water (90° C.) multiple times. The final products were collected via vacuum filtration using filter paper. The pH of the filtrates was about 8. The filtrates were then dried in a laboratory oven at 110° C. overnight and were stored in sealed glass vials at room temperature for further analysis. For the products prepared without adding oil, it was noticed that the products were ultrafine particles (nanoparticles) which could not be isolated via simple vacuum filtration. Therefore, the product pastes were first dispersed in water and purified by repeated centrifugation and redispersion in water until the pH decreased to a constant near-neutral value. The precipitates were then dried in a laboratory oven and were stored in sealed glass vials at room temperature for further analysis.

(40) FIG. 1 shows XRD patterns of samples prepared as described in Example 1 (3.0 Na.sub.2O:1.0 Al.sub.2O.sub.3: 4.0 SiO.sub.2: 32.4H.sub.2O and canola oil/H.sub.2O (v/v)=1.0) heated at 60° C. for 12, 24, 30, 36, 48 and 54 hours corresponding to plots 100, 102, 104, 106, 108, and 110, respectively. Simulated patterns of faujasite (NaX) and anatase (an impurity) are shown, with the anatase impurity having 2θ closest to 25.

(41) FIGS. 2A and 2B show SEM images and FIGS. 2C and 2D show TEM images of samples 200, 202, 204, and 206, respectively, prepared as described in Example 1 with canola oil heated at 60° C. for 54 hours. The scale bars in FIGS. 2A and 2B are 50 μm and 1 μm, respectively.

(42) FIG. 3A shows N.sub.2 sorption isotherms of samples prepared as described in Example 1 with and without canola oil heated at 60° C. shown as plots 300 and 302, respectively, with NaX as a reference shown in plot 304. FIG. 3B shows BJH pore distributions of samples prepared as described in Example 1, with plots 310, 312, and 314 corresponding to plots 300, 302 and 304, respectively.

(43) FIG. 4 shows variable temperature CO.sub.2 adsorption isotherms for a sample prepared as described in Example 1, with plots 400, 402, 404, 406, and 408 corresponding to 10° C., 20° C., 30° C., 40° C., and 60° C., respectively.

(44) FIG. 5 shows kinetics of CO.sub.2 adsorption of a sample prepared as described in Example 1, another hierarchical zeolite sample, and a 13X reference (from Sigma Aldrich) at 25° C. in plots 500, 502, and 504, respectively.

(45) FIG. 6 shows CO.sub.2 sorption isotherms for a sample prepared as described in Example 1 in the first cycle and second cycle at 0° C. in plots 600 and 602, respectively.

(46) Plot 700 in FIG. 7 shows IAST CO.sub.2-over-N.sub.2 selectivity of a sample prepared as described in Example 1 at 20° C. in a mixture of 15 mol % CO.sub.2 and 85 mol % N.sub.2.

(47) Plot 800 in FIG. 8 shows IAST CO.sub.2-over-CH.sub.4 selectivity of a sample prepared as described in Example 1 at 20° C. in a mixture of 50 mol % CO.sub.2 and 50 mol % CH.sub.4.

(48) Plot 900 in FIG. 9 shows isosteric heat of CO.sub.2 adsorption of a sample prepared as described in Example 1.

(49) Only a few implementations are described and illustrated. Variations, enhancements and improvements of the described implementations and other implementations can be made based on what is described and illustrated in this document.