Biomass-based carbon molecular sieve with high ethyne / ethylene selectivity, and preparation method and application thereof

Abstract

The application relates to the field of chemical separation technology, and discloses a biomass-based carbon molecular sieve with high acetylene / ethylene selectivity and a preparation method and application thereof. The method mainly comprises the following steps: dispersing sucrose, a nitrogen-containing organic small molecule and a trace amount of inorganic acid in an aqueous solution, and then transferring to a reaction kettle to perform self-assembly in a closed environment. The carbon precursor is placed in a temperature control tube furnace, pyrolysis carbonization is carried out in a temperature range of 500-900 DEG C under the protection of an inert atmosphere, and through the synergistic effect of a thermal decomposition reaction and a thermal polycondensation reaction, a biomass-based carbon molecular sieve material is synthesized. The carbon molecular sieve material prepared by the application is stable in structure, simple in preparation process, has high acetylene / ethylene separation selectivity, and is particularly suitable for deeply removing trace acetylene impurities in ethylene raw materials in industry.

Description

Technical Field

[0001] This invention belongs to the field of adsorption and separation materials, specifically relating to a method for preparing and applying a low-cost, structurally stable biomass-based carbon molecular sieve with high acetylene / ethylene separation selectivity. Background Technology

[0002] Ethylene, as one of the world's largest producers and consumers of organic chemical raw materials, accounts for over 75% of petrochemical products in its derivative polymers. Currently, the industrial production of ethylene mainly comes from naphtha steam cracking, which simultaneously generates approximately 1% acetylene impurities. The presence of acetylene can cause irreversible poisoning and deactivation of the Ziegler-Natta ethylene polymerization catalyst, reducing the quality of subsequent polymerization products. Furthermore, acetylene is highly corrosive to equipment components such as pipes, containers, and valves; long-term operation can lead to equipment failure and leaks. Moreover, acetylene reacts with metals such as copper and silver to form acetylenides, which can easily clog pipelines and cause explosions. Therefore, to meet the production demands for polymer-grade ethylene and ensure the safe operation of downstream processes, deep removal of acetylene impurities is crucial.

[0003] Traditional industrial processes typically employ two acetylene removal methods: The first is organic extractive distillation, which uses organic solvents such as acetone and N,N-dimethylformamide to absorb acetylene under high pressure and low temperature conditions, followed by removal under low pressure and high temperature. This method usually requires large amounts of organic solvents, easily causing environmental pollution. The second method utilizes precious metal catalysts such as palladium and platinum to catalytically hydrogenate acetylene to ethylene. Most of the ethylene plants in operation in my country use this process. However, palladium / platinum catalysts are expensive and prone to over-hydrogenation to ethane, limiting the further development of traditional acetylene hydrogenation processes. Furthermore, the production, transportation, and storage of hydrogen increase industrial production costs and energy consumption. To further reduce energy consumption and achieve efficient acetylene / ethylene separation, there is an urgent need to develop advanced chemical technologies that can efficiently remove acetylene under ambient temperature conditions.

[0004] Compared with solvent extraction and traditional catalytic hydrogenation, the adsorption separation of acetylene / ethylene using porous adsorbents does not involve phase transitions or high temperatures, offering advantages such as low energy consumption, high operational flexibility, and equipment friendliness, making it a highly sought-after gas separation and purification technology. The core of adsorption separation lies in the adsorbent. By designing and controlling the pore structure of the adsorbent material to adapt to the physical and chemical properties of different guest molecules (such as molecular dynamic diameter, polarizability, dipole moment, etc.), highly selective adsorption separation can be achieved. Currently reported adsorbents for separating acetylene / ethylene mainly include metal-organic frameworks (MOFs) and porous polymers. For example, patent CN202210649033.3 reports a class of MOF materials used for the selective adsorption separation of acetylene / ethylene. Specifically, TJE-2 exhibits a saturated acetylene adsorption capacity of 3.62 mmol / g at 298 K and a separation selectivity of 16.0 for acetylene / ethylene at a 1:99 (volume ratio). Patent CN202211612712.X reports a hyperbranched ionic porous organic polymer material for separating acetylene / ethylene, with adsorption capacities of 1.1 mmol / g for acetylene and 0.25 mmol / g for ethylene at 298 K and 1 bar, respectively. MOFs and porous polymers typically possess characteristics such as tunable pore size and strong chemical modifiability, exhibiting good acetylene / ethylene separation performance. These MOFs and porous polymers generally have uniform pore sizes and relatively matched surface chemistry; however, for MOFs and porous polymers to become industrially viable adsorbents, significant challenges remain, including improving material structural stability and reducing preparation costs.

[0005] Porous carbon materials are highly anticipated for industrial application due to their excellent structural stability and low preparation cost. In recent years, researchers have begun to develop porous carbon materials for acetylene / ethylene separation. For example, Cai et al. used coconut shell and metronidazole as carbon and nitrogen sources, and CO2 as the pore-forming gas to prepare a carbon material with an ethylene adsorption capacity of 3.81 mmol / g, which is higher than the acetylene capacity of 2.48 mmol / g. It belongs to the ethylene selective adsorbent category and cannot yet be applied to the acetylene / ethylene separation process (Chemical Engineering Journal, 2023, 477: 147220). Wang et al. used NH2OH·HCl to activate camphor shells to form pores, and the resulting ultramicroporous carbon adsorbent had a separation selectivity of 10.2 for acetylene / ethylene (AIChEJournal. 2023, 69(6), e18046). At present, the bottleneck problem of carbon materials for acetylene / ethylene separation is the low selectivity, which is still far behind MOF materials and porous polymers, thus restricting the efficient separation of acetylene / ethylene by carbon materials. The main reason is that carbon materials have an amorphous nature and usually have a wide pore size distribution, which can cause co-adsorption of acetylene and ethylene.

[0006] Therefore, to solve the above problems, this invention is proposed. A type of biomass-based carbon molecular sieve is prepared and applied to the adsorption and separation of acetylene / ethylene by utilizing the thermal decomposition and thermal condensation reactions of a carbon precursor during pyrolysis and carbonization. During the thermal decomposition reaction, non-carbon elements such as hydrogen and oxygen contained in the carbon precursor, as well as non-organic carbon and graphite-like microcrystalline edge carbon, are gradually vaporized and decomposed, expanding the pores to form microporous channels. When the carbonization temperature is increased, a thermal condensation reaction is initiated, during which carbon atoms continuously cyclize and aromatize, condensing to form a denser three-dimensional framework structure, driving the pores to shrink. Through precise control of the pore structure by the two forces of "pore expansion and pore shrinkage," highly selective separation of acetylene / ethylene is achieved. Summary of the Invention

[0007] The purpose of this invention is to provide a biomass-based carbon molecular sieve with high acetylene / ethylene selectivity and its preparation method. The carbon molecular sieve material prepared by this method exhibits excellent structural stability and high acetylene / ethylene adsorption selectivity, and the carbon source is inexpensive and renewable.

[0008] The objective of this invention is achieved through the following technical solutions.

[0009] A biomass-based carbon molecular sieve with high acetylene / ethylene selectivity, its preparation method, and its application, comprising the following steps:

[0010] (1) Preparation of carbon precursor: A trace amount of inorganic acid was added to distilled water to prepare an acidic aqueous solution with a pH of 1-3. Then, sucrose and nitrogen-containing organic small molecules were added in proportion and stirred thoroughly to disperse. The mixture was transferred to a reaction vessel and polymerized at a constant temperature of 180-230℃ for 12-24 hours to obtain a carbon precursor with high thermal stability.

[0011] (2) Synthesis of carbon molecular sieve: The carbon precursor obtained in step (1) is washed and dried, and then placed in a temperature-controlled furnace. Under the protection of an inert atmosphere, the temperature is programmed to be raised to 500-900℃ for pyrolysis and carbonization for 1-3 hours. Through the synergistic effect of thermal decomposition reaction and thermal polycondensation reaction, a carbon molecular sieve material with high acetylene / ethylene separation selectivity is obtained.

[0012] Preferably, in step (1), the nitrogen-containing organic small molecule is one or more of cyanamide, guanine, and 2,3-diamino-2-butenedionitrile.

[0013] 4. The method according to claim 1, wherein in step (1), the pH value of the acidic aqueous solution is 1 to 3.

[0014] Preferably, in step (1), the mass ratio of the nitrogen-containing organic small molecules to sucrose is (2-3):1.

[0015] Preferably, in step (1), the inorganic acid catalyst is one or two of HCl, HNO3 or H2SO4.

[0016] Preferably, in step (1), the temperature of the hydrothermal polymerization reaction is 200-230°C.

[0017] Preferably, in step (1), the hydrothermal polymerization reaction takes 14 to 20 hours.

[0018] Preferably, in step (2), the inert atmosphere is argon, nitrogen, or a mixture of the two gases in any ratio.

[0019] Preferably, in step (2), the pyrolysis carbonization reaction takes 2 to 3 hours.

[0020] Preferably, in step (2), the heating rate of the pyrolysis carbonization reaction is 2 to 6 °C / min.

[0021] A biomass-based carbon molecular sieve material was prepared by the method described above. The carbon molecular sieve material exhibits adsorption capacities of 2.7–4.3 mmol / g for acetylene at 298 K and 1 bar, respectively, and only 1.2–2.9 mmol / g for ethylene, demonstrating reverse adsorption and separation of acetylene and ethylene by the carbon material, thus exhibiting preferential adsorption characteristics for acetylene.

[0022] The carbon molecular sieve material described above is applied in an acetylene / ethylene separation system.

[0023] This invention proposes a biomass-based carbon molecular sieve with high acetylene / ethylene selectivity, its preparation method, and its application. The method uses inexpensive sucrose as a raw material. Under conditions where nitrogen-containing organic small molecules and inorganic acid catalysts coexist, a highly homogeneous and structurally stable carbon precursor is first prepared through polymerization. Then, through a pyrolysis activation process and system optimization, a carbon molecular sieve material with high acetylene / ethylene separation selectivity is prepared for the efficient removal of trace acetylene impurities from ethylene. Furthermore, this adsorbent possesses industrial advantages such as structural stability and low cost, showing broad prospects for industrial separation applications.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] Traditional carbon molecular sieves utilize the volatilization and thermal decomposition of unsaturated hydrocarbons such as benzene vapor and methane, depositing them on the outside of activated carbon pores. However, due to uneven carbon deposition and easy pore blockage in vapor deposition, achieving high-selectivity separation of acetylene / ethylene is difficult. In contrast, this invention provides a pore control method that combines thermal decomposition-induced pore formation with thermal polycondensation regulation, enabling high-selectivity separation of acetylene / ethylene. Furthermore, the adsorbent material prepared by this method uses inexpensive sucrose as a carbon source, offering advantages of being environmentally friendly and renewable. Moreover, compared to traditional energy-intensive and polluting methods such as catalytic hydrogenation and solvent absorption, the adsorption separation method provided by this invention is cleaner, more environmentally friendly, and requires less equipment investment, thus showing great promise for industrial applications. Attached Figure Description

[0026] Figure 1 The adsorption isotherm (298 K) of the carbon molecular sieve prepared in Example 1 for acetylene / ethylene.

[0027] Figure 2 The IAST selectivity (298K) of the carbon molecular sieve prepared in Example 1 for an acetylene / ethylene (1:99, v:v) mixture.

[0028] Figure 3 The adsorption isotherm (298 K) of the carbon molecular sieve prepared in Example 2 for acetylene / ethylene.

[0029] Figure 4 The adsorption isotherm (298 K) of the carbon molecular sieve prepared in Comparative Example 5 for acetylene / ethylene. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the present invention is not limited thereto.

[0031] Example 1

[0032] 834 μL of concentrated hydrochloric acid (36.5% by mass) was added to 100 mL of distilled water to prepare an aqueous solution with a pH of 1. Then, 2 g of sucrose and 5 g of 2,3-diamino-2-butenedionitrile were added and dispersed evenly. The mixture was then transferred to a reaction vessel and subjected to hydrothermal polymerization at 200 °C for 20 h. The resulting carbon precursor was thoroughly washed and dried. The carbon precursor was then placed in a ceramic boat and placed in a temperature-controlled furnace. Under nitrogen protection, the temperature was raised to 900 °C and pyrolyzed for 2 h. After natural cooling to room temperature, biomass-based carbon molecular sieve material was obtained.

[0033] This example uses a MicroPort 3Flex gas adsorption analyzer to measure the adsorption isotherm of acetylene / ethylene on carbon molecular sieve materials. The results are as follows: Figure 1As shown, the material exhibits adsorption capacities of 3.2 mmol / g for acetylene at 298 K and 1 bar, while its adsorption capacity for ethylene is only 1.2 mmol / g, indicating that the carbon molecular sieve material shown in Example 1 has excellent separation selectivity for acetylene / ethylene.

[0034] In this embodiment, the adsorption selectivity of the carbon molecular sieve material for acetylene / ethylene in Example 1 was predicted using IAST theory. The results are as follows: Figure 2 As shown, the carbon molecular sieve material in Example 1 exhibits a separation selectivity of 22.1 for an acetylene / ethylene mixture (1:99, v:v) at 298 K and 1 bar.

[0035] Example 2

[0036] 834 μL of concentrated hydrochloric acid (36.5% by mass) was added to 100 mL of distilled water to prepare an aqueous solution with a pH of 1. Then, 2 g of sucrose and 2 g of guanine were added and dispersed evenly. The solution was then transferred to a reaction vessel and subjected to hydrothermal polymerization at 190 °C for 16 h. The resulting carbon precursor was thoroughly washed and dried. The carbon precursor was then placed in a ceramic boat and placed in a temperature-controlled furnace. Under nitrogen protection, the temperature was raised to 800 °C and pyrolyzed for 3 h. After natural cooling to room temperature, biomass-based carbon molecular sieve material was obtained.

[0037] This invention measures the adsorption isotherm of acetylene / ethylene in the carbon molecular sieve material of Example 2. The results are as follows: Figure 3 As shown, the material adsorbed 4.1 mmol / g of acetylene at 298 K and 1 bar, while adsorbing only 2.0 mmol / g of ethylene, indicating that the carbon molecular sieve material shown in Example 2 has good separation selectivity for acetylene / ethylene.

[0038] Example 3

[0039] 84 μL of concentrated hydrochloric acid (36.5% by mass) was added to 100 mL of distilled water to prepare an aqueous solution with a pH of 2. Then, 1 g of sucrose and 3 g of cyanamide were added and dispersed evenly. The mixture was then transferred to a reaction vessel and subjected to hydrothermal polymerization at 200 °C for 18 h. The resulting carbon precursor was thoroughly washed and dried. The carbon precursor was then placed in a ceramic boat and placed in a temperature-controlled furnace. Under nitrogen protection, the temperature was raised to 900 °C and activated for 1 h. After naturally cooling to room temperature, carbon molecular sieve material was obtained.

[0040] In this invention, the adsorption isotherm of acetylene / ethylene for the carbon molecular sieve material of Example 3 was determined. The material showed an adsorption capacity of 4.3 mmol / g for acetylene at 298 K and 1 bar, while the adsorption capacity for ethylene was only 2.9 mmol / g, indicating that the carbon molecular sieve material in Example 3 exhibits good separation selectivity for acetylene / ethylene.

[0041] Example 4

[0042] 834 μL of concentrated hydrochloric acid (36.5% by mass) was added to 100 mL of distilled water to prepare an aqueous solution with a pH of 1. Then, 1 g of sucrose and 2 g of cyanamide were added and dispersed evenly. The mixture was then transferred to a reaction vessel and subjected to hydrothermal polymerization at 190 °C for 14 h. The resulting carbon precursor was thoroughly washed and dried. The carbon precursor was then placed in a ceramic boat and placed in a temperature-controlled furnace. Under nitrogen protection, the temperature was raised to 900 °C and activated for 3 h. After natural cooling to room temperature, carbon molecular sieve material was obtained.

[0043] In this invention, the adsorption isotherm of acetylene / ethylene for the carbon molecular sieve material of Example 4 was determined. The material showed an adsorption capacity of 2.7 mmol / g for acetylene at 298 K and 1 bar, while the adsorption capacity for ethylene was only 2.1 mmol / g, indicating that the carbon molecular sieve material in Example 4 exhibits good separation selectivity for acetylene / ethylene.

[0044] Comparative Example 5

[0045] The difference between Comparative Example 5 and Example 1 is that no nitrogen-containing small organic molecules were added during the preparation of the carbon precursor. The resulting carbon precursor was then placed in a temperature-controlled furnace and pyrolyzed at a relatively low temperature of 500°C for 2 hours under nitrogen protection. After natural cooling to room temperature, a porous carbon material was obtained. The adsorption isotherms of the carbon material in Comparative Example 4 for acetylene / ethylene were tested. The results showed that the adsorption capacity of the carbon material for acetylene at 298 K and 1 bar was 1.3 mmol / g, while the adsorption capacity for ethylene was 1.5 mmol / g. This indicates that the carbon material shown in Comparative Example 5 is an ethylene-selective adsorbent and cannot be used for acetylene / ethylene separation. A possible reason is that the lack of nitrogen-containing small molecules resulted in a poor degree of polymerization in the obtained carbon precursor, leading to significant defects. Furthermore, the pyrolysis at 500°C generated large micropores, allowing both acetylene and ethylene to enter the pores simultaneously, resulting in low separation selectivity.

[0046] It should be understood that the above detailed description of the technical solutions of the present invention with reference to optimized embodiments is illustrative and not restrictive. It should not be considered that the specific implementation of the present invention is limited to this. For those skilled in the art, any modifications to the technical solutions described in the embodiments or equivalent substitutions of some technical features without departing from the concept of the present invention should be considered as falling within the scope of patent protection defined by the claims submitted by the present invention.

[0047] The above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a biomass-based carbon molecular sieve with high acetylene / ethylene selectivity, characterized in that, Includes the following steps: (1) Preparation of carbon precursor: Inorganic acid is added to distilled water to prepare an acidic aqueous solution, then sucrose and nitrogen-containing organic small molecules are added and stirred and dispersed. The mixture is transferred to a reaction vessel and hydrothermal polymerization is carried out under medium temperature conditions to obtain a carbon precursor with high thermal stability. (2) Synthesis of carbon molecular sieve: The carbon precursor obtained in step (1) is washed and dried, and then placed in a temperature-controlled furnace. Under the protection of an inert atmosphere, the temperature is programmed to be raised to 500~900 °C for pyrolysis and carbonization. Through the synergistic effect of thermal decomposition reaction and thermal polycondensation reaction, a carbon molecular sieve material with high acetylene / ethylene separation selectivity is obtained. In step (1), the nitrogen-containing organic small molecule is one or more of cyanamide, guanine, and 2,3-diamino-2-butenedionitrile; wherein the mass ratio of the nitrogen-containing organic small molecule to sucrose is (1~3):1; the temperature of the hydrothermal polymerization reaction is 180~230℃; and the time of the hydrothermal polymerization reaction is 12~24 h. In step (2), the time for the pyrolysis carbonization reaction is 1~3 h; the heating rate of the pyrolysis carbonization reaction is 2~10℃ / min.

2. The method for preparing the biomass-based carbon molecular sieve with high acetylene / ethylene selectivity according to claim 1, characterized in that, In step (1), the amount of inorganic acid added satisfies the following condition: the pH value of the acidic aqueous solution is 1~3.

3. The method for preparing the biomass-based carbon molecular sieve with high acetylene / ethylene selectivity according to claim 1, characterized in that, In step (1), the inorganic acid is one or two of HCl, HNO3 or H2SO4.

4. The method for preparing the biomass-based carbon molecular sieve with high acetylene / ethylene selectivity according to claim 1, characterized in that, In step (2), the inert atmosphere is argon, nitrogen, or a mixture of the two gases in any ratio.

5. A carbon molecular sieve material with high acetylene / ethylene separation selectivity prepared by the method according to any one of claims 1 to 4, characterized in that, The carbon molecular sieve material exhibits an adsorption capacity of 2.7–4.3 mmol / g for acetylene and 1.2–2.9 mmol / g for ethylene at 298 K and 1 bar.

6. The carbon molecular sieve material according to claim 5 is used to remove low concentrations of acetylene impurities from ethylene gas.

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