RU2794181C1 - Porous composite absorbent for selective gas separation and method for its production - Google Patents

Abstract

FIELD: absorbent technology.

SUBSTANCE: group of inventions relates to technology for producing absorbents and can be used for sorption and selective separation of gas mixtures, including for purification of natural gas from carbon dioxide, concentration of exhaust or industrial carbon dioxide. A method for producing a porous composite absorbent for selective gas separation is presented, which includes the stage of preparation of the initial components, consisting of the preparation of component 1 by dissolving 0.5 mmol of a zinc acetate salt of the formula Zn(OCOCH₃)₂×2H₂O in a 10% solution of ethanol in water with the addition of 33.33 mmol of a carbon absorbent - activated carbon, with a micropore volume of 0.30-35 cm³/g and a mesopore volume of 0.15-0.25 cm³/g, mixing component 1 with a magnetic stirrer at a speed of up to 350 rpm at a temperature of plus 70°C for 2 hours and preparation of component 2 by dissolving in a separate container 0.03 mmol of 5,10,15,20-tetra(4-pyridyl)porphyrinate zinc (II) in dried chloroform, the stage of formation of the structure of the composite absorbent in the Pickering emulsion obtained with portion-wise addition of component 2 to component 1 with vigorous shaking and subsequent stabilization for 30 minutes in an ultrasonic bath at a temperature of plus 35°C and ultrasonic frequency up to 40 kHz, with keeping the finished Pickering emulsion for 2 days in an oven at a temperature of plus 70°C, the stage of isolating the finished composite absorbent precipitated at the stage of formation of its structure, including vacuum filtration of the precipitate, its repeated washing with a 10% solution of ethanol in water until a colourless filtrate is formed, the drying stage, carried out in a lyophilic dryer at a temperature of plus 25°C and a pressure of 20 mbar for 12 hours, followed by sealing the finished composite absorbent and storing it in an opaque container at room temperature. In another variant, a porous composite absorbent is provided for the selective separation of gases containing carbon dioxide produced by said process.

EFFECT: group of inventions ensures the creation and preservation of the structure of a composite absorbent, consisting of mineral and organic parts, having a developed inner surface, represented by pores of micro- and mesopore sizes; uniform distribution of MOF crystals inside the activated carbon matrix; imparting mechanical strength and selectivity to carbon dioxide to the composite material; reduction of material costs in the implementation of the emulsion method for obtaining a composite absorbent by selecting available components, using the same solvents at different stages, using low-temperature freeze-drying at the activation stage.

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Description

SUBSTANCE: group of inventions relates to a technology for producing adsorbents, in particular porous composite materials, namely a porous composite adsorbent for gas separation, and can be used for sorption and selective separation of gas mixtures, including for purification of natural gas from carbon dioxide, concentration of exhaust or industrial carbon dioxide.

Porous composite adsorbents are a new class of hybrid materials consisting of an inorganic matrix and impregnated into it from solutions of the corresponding salts of metal-organic framework structures (MOFs) with their subsequent crystallization. Metal-organic framework structures are themselves hybrid materials, consisting of polydentate organic ligands connected by metal ions or clusters through coordination or ionic bonds. The patent (US No. 7202385, IPC C07C 41/03; C07C 43/11; C08G 18/28; C08G 65/26; C08G 65/28) provides complete information on the IOC received to date.

MOFs have a number of unique properties: a developed specific surface area (with an area of up to 5900 m ² /g [AR Millward, O.M. Yaghi // Metal-Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature, 2005, 127, 17998-17999]), a narrow pore size distribution, and a porous crystalline structure not found in most organic materials. The combination of these properties makes MOFs more attractive materials for use as adsorbents, sensors, and catalysts [M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O'Keeffe, OM Yaghi // Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage, 2002, 295, 469 -472], compared to traditional activated carbons and zeolites.

Significant progress has been made in the field of application of MOFs for selective gas sorption and use in filters and sensors [H.-S. Wang // Metal-organic frameworks for biosensing and bioimaging applications, 2017, 349, 139-155]. At the same time, the disadvantage of metal-organic frameworks is that they often have low strength and resistance to external influences such as high temperatures and pressures [U. Mueller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt, J. Pastre // Metal-organic frameworks-prospective industrial applications, 2006,16, 626-636]. The combination of the properties of MOFs and a solid inorganic matrix (for example, activated carbon) makes it possible to combine the advantages of both traditional adsorbents and new MOFs [R.K. Prabhakaran, J. Deschamps // Doping activated carbon incorporated composite MIL-101 using lithium: impact on hydrogen uptake, J. Mater. Chem. A, 2015,13,7014-7021], namely, high resistance to external influences, mesoporosity and selectivity of MOFs with respect to various adsorbents due to the chemical affinity of the organic components of MOFs for certain compounds. The disadvantage of this method is the need for alloying with lithium and careful control of this process, since high concentrations of lithium destroy the frame structure of the MOFs.

In the work [O. Fleker, A. Borenstein, R. Lavi, L. Benisvy, Sh. Ruthstein, D. Aurbach // Preparation and Properties of Metal Organic Framework/Activated Carbon Composite Materials, Langmuir, 2016, 32.4935-4944] a composite adsorbent based on activated carbon and MOF assembled from benzene-1,3,5 molecules was obtained -tricarboxylic acid and Cu ²⁺ ions, which has high electrical conductivity due to the carbon matrix and high porosity due to the IOC. The disadvantage of this system is the high content of MOF in the composite adsorbent (about 30%), which significantly increases its cost.

An interesting approach to the formation of composite adsorbents of the type MOC-carbon matrix is shown in [Z. Bian, J. Xu, S. Zhang, X. Zhu, H. Liu, J. Hu // Interfacial Growth of Metal Organic Framework/Graphite Oxide Composites through Pickering Emulsion and Their C02 Capture Performance in the Presence of Humidity, Langmuir, 2015 , 31, 7410-7417]. As a carbon base, the authors chose graphite oxide, which forms Pickering emulsions from organic and aqueous phases, which leads to the formation of a carbon framework on which MOF crystals are located. The disadvantage of this composite adsorbent is the high content of the frame structure, which significantly increases its cost.

The closest in essence and the achieved result to the claimed is the method described in [Z. Zhang, H. Wang, X. Chen, C. Zhu, W. Wei Y. Sun // Chromium-based metal-organic framework/mesoporous carbon composite: synthesis, characterization and CO2 adsorption, Adsorption, 2015, 21, 77-86 ], which indicates the protective effect of the carbon matrix, which retains open vacancies on metal ions in MOFs, which contributes to a significant improvement in carbon dioxide sorption. The disadvantage of this method is its complexity and difficulty in scaling, as well as the low content of the carbon matrix (about 2-4%).

Traditional solvothermal synthesis is the most common way to obtain MOFs. However, its use for the preparation of composite adsorbents consisting of materials that are heterogeneous in chemical nature and physical properties is associated with the formation of agglomerates with low porosity and nonuniform structure. The emulsion method for obtaining a composite adsorbent based on carbon matrices and MOFs was used in isolated cases and must be adapted to the tasks set.

The objective of this group of inventions is to obtain a porous composite adsorbent with the adsorption properties of MOK-AU, consisting of an organometallic frame and a carbon base, which is activated carbon, combining the advantages of two types of adsorbents, selective with respect to carbon dioxide, by a method based on the formation of a porous a composite adsorbent in an emulsion, with simultaneous or sequential lyophilization of the carbon matrix with a metal salt to form MOF crystallites on the inner surface of activated carbon particles.

To achieve this task, a method is proposed for obtaining a porous composite adsorbent by mixing a zinc acetate salt of the formula Zn(OCOCH3)2×2H2O, an organic dye 5,10,15,20-tetra(4-pyridyl)porphyrinate zinc (II) and activated carbon in the presence of immiscible solvents, followed by heating under the action of convection heating with the release of the target product and its drying. As solvents, an emulsion of distilled water and chloroform is used, the process is carried out in a closed reactor at a temperature of 70°C, and drying is carried out at reduced or normal pressure and room temperature. The duration of the emulsion synthesis of the composite adsorbent MOK-AU is 2 days, the duration of subsequent drying is 6-12 hours.

The technical result to be achieved by the group of inventions is:

- creation and preservation of the structure of a composite adsorbent, consisting of mineral and organic parts, with a developed internal surface, represented by pores of micro- and meso sizes;

- uniform distribution of MOF crystals inside the matrix of activated carbon;

- imparting mechanical strength and selectivity to carbon dioxide to the composite material;

- reduction of material costs in the implementation of the emulsion method for obtaining a composite adsorbent by selecting available components, using the same solvents at different stages, using low-temperature freeze-drying at the activation stage.

The technical result is achieved due to the fact that in the method for obtaining a porous composite adsorbent for selective gas separation, according to the method, the stage of preparation of the initial components is included, consisting of the preparation of component 1 by dissolving 0.5 mmol of a zinc acetate salt of the formula Zn(OCOCH ₃ ) ₂ × 2H ₂ O in a 10% solution of ethanol in water with the addition of 33.33 mmol of a carbon adsorbent - activated carbon, with a micropore volume of 0.30-35 cm ³ /g and mesopores of 0.15-0.25 cm ³ /g, stirring magnetic stirrer at a speed of up to 350 rpm at a temperature of plus 70°C for 2 hours and preparation of component 2 by dissolving 0.03 mmol of 5,10,15,20-tetra(4-pyridyl)zinc porphyrinate (II) in a separate container in dried chloroform, the stage of formation of the structure of the composite adsorbent in Pickering's emulsion, obtained by batch addition of component 2 to component 1 with vigorous shaking and subsequent stabilization for 30 minutes in an ultrasonic bath at a temperature of plus 35°C and an ultrasound frequency of up to 40 kHz, with keeping of the finished Pickering emulsion for 2 days in an oven at a temperature of plus 70°C, the stage of isolating the finished composite adsorbent precipitated at the stage of its structure formation, including vacuum filtration of the precipitate, its repeated washing with 10% ethanol solution in water until formation colorless filtrate, drying stage, carried out in a lyophilic dryer at a temperature of plus 25°C and a pressure of 20 mbar for 12 hours, subsequent sealing of the finished composite adsorbent and its storage in an opaque container at room temperature.

The technical result is also achieved due to the fact that in the above described method for obtaining a porous composite adsorbent for selective gas separation, the drying stage is carried out for 6 hours at a temperature of minus 50°C and a pressure of 2×10 ^-3 mbar.

The technical result is achieved due to the fact that in the above described methods, a porous composite adsorbent is obtained for the selective separation of gas mixtures containing carbon dioxide.

The choice of activated carbon as a carbon matrix according to GOST 8703-74 provides the necessary pore characteristics (the presence of micro- and mesopores), a developed inner surface, and acceptable strength of the composite material.

The group of inventions is illustrated by a table and illustrations:

Table - Sorption characteristics of composite adsorbent samples

Fig. 1 - X-ray patterns of composite adsorbents MOK-AU obtained under the conditions of examples 1-4.

Fig. 2A-D - SEM photographs (scanning electron microscopy) of composite adsorbents MOK-AU synthesized under the conditions of examples 1 and 4, respectively. On FIG. 3B,D - images obtained in the ESB mode (with a backscattered electron detector with output energy selection, which provides an image with high chemical contrast).

Fig. 3A-D - SEM photographs of composite adsorbents MOK-AU synthesized under the conditions of examples 3 and 4, respectively. On FIG. 4B and D are images taken in the ESB mode.

Fig.4 A-D - Coefficients of separation of gas mixtures CO ₂ /CH ₄ for various concentrations 30/70, 50/50, 70/30% vol. on a sample of example 1 at a temperature of 253 K depending on the pressure of the mixture.

Fig. 5 - Adsorption isotherms of the gas mixture CO ₂ /CH ₄ at a concentration of 50/50, 30/70 and 70/30% vol. and individual components of the mixture on a sample of example 1 at a temperature of 273 K depending on the pressure of the mixture.

Fig. 6 - Isotherms of adsorption - desorption of standard nitrogen vapor at 77 K obtained in examples 1-4 samples of composite adsorbents and the original AC.

Fig. 7 - Differential curve of pore size distribution in the original matrix of activated carbon and in samples of the composite adsorbent obtained in accordance with examples 1-4.

The invention is illustrated by the following examples:

Example 1

A glass ampoule (reactor) 2 cm in diameter and 8.5 cm high was loaded with PO mg (0.5 mmol) of zinc acetate solid crystalline powder Zn(OCOCH ₃ ) ₂ × 2H ₂ O (GOST 5823-78) and dissolved in 10 ml a mixture of ethanol (10% vol.) (GOST 18300-87) and distilled water (90% vol.). In the resulting solution was added 400 mg (33.33 mmol of carbon) carbon adsorbent-activated carbon brand AP-B (GOST 8703-74), with a micropore volume of 0.30 cm ³ /g and mesopore 0.25 cm ³ /g , stirred with a magnetic stirrer with an intensity of up to 350 rpm at a temperature of 70°C for 2 hours, the reactor was hermetically sealed, after which the resulting mixture of components was left for 7 days at room temperature and atmospheric pressure. Next, a portion of 5,10,15,20-tetra(4-pyridyl)porphyrinate zinc (II) weighing 20 mg (0.03 mmol) was dissolved in a separate container in 10 ml of chloroform (GOST 20015-88), previously dried by distillation over hydride calcium. The resulting solution was poured into the reactor in portions to the previously prepared mixture of zinc acetate with activated carbon, the reactor was hermetically sealed, and it was vigorously shaken for several minutes. The Pickering emulsion prepared in this way (oil-in-water type) was additionally stabilized by keeping it in an ultrasonic bath for 30 minutes at a temperature of plus 35°C and an ultrasound frequency of up to 40 kHz, after which it was loaded into an oven preheated to a temperature of plus 70° C, where it was kept for 2 days to form the structure of the composite adsorbent.

Then, the dark-violet precipitated composite adsorbent was separated from the mother liquor by vacuum filtration, washed many times with a 10% solution (total volume up to 300 ml) of ethanol in distilled water to remove excess zinc acetate, until a colorless filtrate was obtained. The washed composite adsorbent was dried in a freeze dryer at a temperature of plus 25°C and reduced pressure (20 mbar) for 12 hours, sealed and sent for storage. The use of freeze drying makes it possible to carefully remove the frozen solvent from the finished composite adsorbent, while maintaining the pore structure of both MOF and activated carbon, to obtain a developed multimodal inner surface of the composite adsorbent (Table).

The yield of the target product (composite adsorbent) was 365.7 mg (69%) by weight of the initial reagents. The resulting sample of the composite adsorbent is a loose coarse-grained dark purple powder. It was stored in a hermetically sealed opaque container at room temperature. Before using as an adsorbent, it is necessary to carry out its thermal vacuum activation at atmospheric pressure and a temperature of plus 130°C for at least 30 minutes.

On FIG. 1 shows X-ray patterns of samples of composite adsorbents MOK-AU, obtained under the conditions of examples, where it is seen that the obtained composite adsorbents retain their structure when varying within the claimed technological parameters of the conditions for the formation of the structure of the composite adsorbent, its isolation and purification.

On FIG. Figures 2A-D show SEM photographs of MOK-AU composite adsorbents synthesized under the conditions of Examples 1 and 4, respectively. As can be seen in the photo, the inner surface of the composite adsorbent is predominantly represented by a crystalline phase; between disordered crystals of various sizes and shapes, a small amount of an amorphous phase is contained. The images in Fig. 2B and 2D, obtained in the ESB mode, show the inner surface of the synthesized MOC-AU composite adsorbents, where the black AC matrix contains gray MOC inclusions in the pores. Shown in FIG. 3A-D on a smaller scale compared to FIG. 2, SEM photographs clearly show the uniform distribution of fine MOF crystals in the volume of the AC matrix, which fill both meso and micropores in it.

Shown in FIG. 4 and 5, the separation coefficients of CO ₂ /CH ₄ gas mixtures for various concentrations and their adsorption isotherms clearly illustrate the high selectivity of the resulting composite adsorbent with respect to carbon dioxide. Moreover, the adsorption activity in relation to CO ₂ exponentially depends on the content of carbon dioxide in the CO ₂ /CH ₄ mixture and increases as the pressure of the gas mixture increases. This selectivity is explained both by the presence of free vacancies of axial ligands in the zinc complex of the porphyrin, which are capable of binding _CO2 molecules, and by the possibility of weak specific coordination on the central metal ion of the porphyrin ring. According to the data obtained, the calculated separation/retention coefficient, which is defined as the ratio of the corresponding sorbed masses of gas, at atmospheric pressure is 7.

Shown in Fig. 6 adsorption-desorption isotherms of standard nitrogen vapor show that the total adsorption capacity of the composite adsorbent samples slightly decreased compared to the initial activated carbon. Obviously, this is due to a change in the properties of the surface of activated carbon upon interaction with MOF, which led to a decrease in the adsorption capacity with respect to hydrocarbons.

The inherent mixture of micro and meso porosity in carbon materials is illustrated in FIG. 7 of the differential pore size distribution curve for both the obtained MOK-AC samples and for the initial AC. Compared to AC, the distribution of the pore characteristics of the carbon matrix is preserved, but the pore diameters in the samples of the obtained composite adsorbents are somewhat reduced due to the fixation of microporous MOF crystals in the pores of the AC carbon matrix when using this method.

Example 2

It differs from example 1 in that from the obtained target product MOK-AU removal of residual solvents was carried out by sublimation using freeze drying at a pressure of 2×10 ^-3 mbar and a temperature of minus 50°C for 6 hours. The yield of the target product in this case amounted to 386.9 mg (73%) by weight of the initial reagents.

Example 3

It differs from example 2 in that the Zn(OCOCH ₃ ) ₂ × 2H ₂ O solution and the carbon adsorbent suspension (component 1) were kept for 2 hours with constant stirring and a temperature of plus 70°C for the preparation of the initial components, after which they proceeded to the stage formation of the structure of the composite adsorbent in the Pickering emulsion obtained by batch addition of 5,10,15,20-tetra(4-pyridyl)porphyrinate zinc (II) in chloroform (component 2) to component 1. The yield of the target product was 408.1 mg ( 77%) by weight of the initial reagents.

Example 4

It differs from example 3 in that to obtain the target product MOK-AU used a carbon adsorbent - activated carbon - with a micropore volume of 0.35 cm ³ /g and mesopore 0.15 cm ³ /g, and heating the finished raw mixture at a temperature of plus 70 °C was carried out for 4 days. The yield of the target product was 397.5 mg (75%) by weight of the initial reagents.

The BET method was used to estimate the internal surface area for nitrogen for the synthesized examples of the MOK-AU composite adsorbent (Table). The composite adsorbent from example 4 has the best characteristics. The composite adsorbent MOK-AU obtained in this way is proposed as a selective adsorbent for carbon dioxide. Such an adsorbent can be used to purify natural gas from impurity carbon dioxide, as well as to retain and accumulate exhaust and industrial carbon dioxide.

The proposed method allows, at the stage of formation of the structure of the composite adsorbent, to form a Pickering emulsion stabilized by particles of activated carbon acting as a surfactant. The MOF organic solvent drops stabilized in this way remain mobile for a long time, they penetrate into the internal pores of the activated carbon matrix, where their crystals are gradually formed. At the stage of formation of a porous composite adsorbent in a Pickering emulsion, the nature of the interaction is ambiguous, but it is most likely that, first, the carbon matrix of activated carbon is successively lyophilized with one metal salt (zinc acetate) due to chemical grafting to AC active sites, and then, at the stage of structure formation on thus prepared active centers of MOF crystallites from another salt of the same metal (5,10,15,20-tetra(4-pyridyl)porphyrinate zinc (II)) are fixed (chemisorbed) on the inner surface of the carbon matrix - activated carbon particles.

The advantage of using the emulsion method is the uniform distribution of microporous MOF crystals in the micro and mesopores of activated carbon, the framework of which bears the main mechanical load of the future composite adsorbent. An important parameter is also the time of formation of the framework structure. Continuous heating increases the uniformity of the distribution of the MOF in the volume of activated carbon, and, as a result, forms a homogeneous structure of the resulting composite adsorbent. Careful drying and activation at low temperatures and pressures contributes to the preservation of the formed structure of the composite adsorbent. And giving the surface of the obtained composite adsorbent the ability to selectively adsorb carbon dioxide is explained both by the presence of free vacancies of axial ligands in the zinc complex of the porphyrin, which are capable of binding CO2 molecules, and by the possibility of weak specific coordination on the central metal ion of the porphyrin ring.

Claims (3)

1. A method for producing a porous composite adsorbent for selective gas separation, which includes the stage of preparing the initial components, consisting of preparing component 1 by dissolving 0.5 mmol of a zinc acetate salt of the formula Zn(OCOCH ₃ ) ₂ × 2H ₂ O in 10% ethanol solution in water with the addition of 33.33 mmol of a carbon adsorbent - activated carbon, with a micropore volume of 0.30-35 cm ³ /g and mesopores of 0.15-0.25 cm ³ /g, stirring component 1 with a magnetic stirrer at a speed of up to 350 rpm at a temperature of plus 70°C for 2 hours and preparation of component 2 by dissolving in a separate container 0.03 mmol of 5,10,15,20-tetra(4-pyridyl)porphyrinate zinc (II) in dried chloroform, stage formation of the structure of the composite adsorbent in the Pickering emulsion, obtained by batch addition of component 2 to component 1 with vigorous shaking and subsequent stabilization for 30 minutes in an ultrasonic bath at a temperature of plus 35°C and an ultrasound frequency of up to 40 kHz, with keeping the finished Pickering emulsion for 2 days in an oven at a temperature of plus 70°C, the stage of isolating the finished composite adsorbent precipitated at the stage of its structure formation, including vacuum filtration of the precipitate, its repeated washing with 10% ethanol solution in water until a colorless filtrate is formed, the drying stage , carried out in a lyophilic dryer at a temperature of plus 25°C and a pressure of 20 mbar for 12 hours, followed by sealing the finished composite adsorbent and storing it in an opaque container at room temperature. 2. The method according to p. 1, characterized in that the stage of drying is carried out for 6 hours at a temperature of minus 50°C and a pressure of 2×10 ^-3 mbar. 3. Porous composite adsorbent for selective separation of gases, obtained according to one of paragraphs. 1, 2 as a selective adsorbent for the separation of gas mixtures containing carbon dioxide.

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