Application of tetramethylammonium modified EMT zeolite molecular sieve in CH4 / N2 adsorption separation

Abstract

The invention belongs to the technical field of gas separation, and particularly relates to application of a tetramethylammonium-modified EMT zeolite molecular sieve in CH4 / N2 adsorption separation. According to the invention, tetramethylammonium ions (TMA < + >) are introduced into the zeolite molecular sieve EMT, so that the EMT which has no performance originally shows efficient CH4 / N2 separation performance. Due to the introduction of TMA, the SBET value of the EMT molecular sieve is obviously reduced compared with that of the original EMT molecular sieve, but the adsorption capacity of methane is increased. Due to the introduction of TMA, the interaction affinity of CH4 and a zeolite framework is enhanced. Under the conditions that the K is 298 K, the pressure is 1 bar and the pressure is 10 bar, the EMT-TMA shows relatively high CH4 / N2 selectivity of 5.43 and 7.48.

Description

Technical Field

[0001] This invention belongs to the field of gas separation technology, and specifically relates to the application of a tetramethylammonium modified EMT zeolite molecular sieve in CH4 / N2 adsorption and separation. Background Technology

[0002] Natural gas, as a clean and efficient energy source, has long been considered an effective alternative to traditional fossil fuels. With the continuous growth in energy demand, developing unconventional natural gas resources such as coalbed methane (CBM) to ensure a stable supply is of significant practical importance. However, CBM typically contains a large amount of nitrogen impurities in addition to methane, which significantly increases the difficulty of direct utilization, often resulting in direct combustion or emission into the atmosphere, thus wasting resources. On the other hand, CH4, as one of the major greenhouse gases, has a global warming potential approximately 21 times that of CO2; therefore, large-scale emissions of CBM will exacerbate climate change. Based on this, achieving methane enrichment (CH4 / N2 separation) in CBM has become a key technological challenge for improving energy efficiency and reducing environmental impact.

[0003] However, CH4 and N2 have similar physical properties, making their separation extremely difficult. Currently, the separation of CH4 and N2 is mainly achieved through low-temperature distillation, which utilizes the difference in their boiling points—a high-energy-consuming technique. In contrast, pressure swing adsorption (PSA) technology utilizes the difference in forces between the adsorbent and the adsorbed gas to achieve gas separation, offering advantages such as environmental friendliness, low energy consumption, and simple equipment, making it a focus of research. In PSA, the choice of adsorbent is crucial. Currently, commonly used CH4 selective adsorbents include activated carbon, metal-organic frameworks (MOFs), and zeolite molecular sieves. Among these, zeolite molecular sieves, constructed from tetrahedrons such as SiO4 and AlO4 as basic structural units, are crystalline porous materials with advantages such as low cost, high stability, tunable structure, and mature synthesis technology, making them the preferred industrial adsorbent. However, most traditional zeolite molecular sieves exhibit poor CH4 / N2 gas separation performance, failing to meet the industrial demand for efficient CH4 and N2 separation. Therefore, creating highly efficient CH4-selective zeolites and improving the CH4 / N2 separation selectivity is key to achieving the enrichment and utilization of methane in low-concentration coalbed methane.

[0004] In recent years, research on molecular sieves for CH4 / N2 adsorption and separation has mainly focused on the modification of zeolites with ultramicroporous structures (pore size <7 Å). Because this pore size provides a strong confinement effect on larger-diameter CH4 molecules, the interaction potential between the structure and CH4 molecules is significantly enhanced, thereby improving its selectivity for CH4 and N2 molecules with similar properties. Although the ultramicroporous structure has advantages in improving selectivity, the strong spatial confinement often limits the amount of CH4 adsorbed, leading to a trade-off between selectivity and adsorption capacity. Based on this, this invention modifies macroporous zeolites with a low silica-to-alumina ratio by introducing organic groups with strong CH4 adsorption activity into their pores. Through the synergistic effect of CH4 adsorption sites and pore size, the affinity for CH4 is enhanced; simultaneously, the interaction between metal cations and N2 is effectively suppressed, thus achieving a CH4 adsorbent with both high selectivity and high adsorption capacity. Summary of the Invention

[0005] The purpose of this invention is to provide an application of tetramethylammonium-modified EMT zeolite molecular sieve in CH4 / N2 adsorption and separation. By modifying macroporous zeolite EMT with tetramethylammonium chloride, the affinity of the adsorbent for CH4 is enhanced; by controlling the amount of organic ammonium added, ideal pore sizes are constructed, thereby synthesizing a CH4 adsorbent with both high selectivity and adsorption capacity.

[0006] Furthermore, the preparation method of the tetramethylammonium modified EMT zeolite molecular sieve includes the following steps: placing the EMT zeolite molecular sieve into a tetramethylammonium chloride solution, heating and stirring in a water bath at 60°C for 4 hours, washing with deionized water 3 times, and drying in an oven at 80°C overnight to obtain the EMT-TMA-X sample.

[0007] Furthermore, the molar composition of the EMT zeolite molecular sieve is 10 SiO2: 1 Al2O3: 2.4 Na2O: 1 SDA (18-crown-6 ether): 140 H2O.

[0008] Furthermore, the concentration of the tetramethylammonium chloride solution was 0.02-0.5M, and the exchange rate of the EMT-TMA-X sample was 15-43%.

[0009] Furthermore, the concentration of the tetramethylammonium chloride solution was 0.2M, and the exchange rate of the EMT-TMA-X sample was 34%.

[0010] Furthermore, at 298K and pressures of 1 bar and 10 bar, EMT-TMA-34% exhibited CH4 / N2 selectivity of 5.43 and 7.48, respectively.

[0011] Furthermore, the preparation method of EMT zeolite molecular sieve includes the following steps: dissolving NaAlO2 in an aqueous solution of NaOH and stirring for 30 min; then slowly adding silica sol and 18-crown ether-6 (SDA) to the mixture; aging the initial gel at room temperature for 24 h; transferring the prepared aluminosilicate gel to a 25 ml polytetrafluoroethylene liner; placing it in a stainless steel autoclave; crystallizing under static conditions at 110 °C for 7 days; cooling the solid product; washing it with deionized water until the pH value is close to neutral; drying it overnight in an oven at 80 °C; and calcining the dried powder in air at 550 °C for 5 h to finally obtain EMT zeolite molecular sieve.

[0012] Beneficial technical effects of the present invention: The present invention, through simple modification, reduces the tetramethylammonium ion (TMA) + The introduction of TMA (tissue-modified organic molecule) into zeolite molecular sieves (EMT) enables the previously ineffective EMT to exhibit highly efficient CH4 / N2 separation performance. The introduction of TMA further enhances the S-reduction properties of the EMT molecular sieve. BET The adsorption value was significantly lower than that of the original EMT, but the methane adsorption capacity increased. This is because the introduction of TMA enhanced the interaction affinity between CH4 and the zeolite framework, a conclusion verified by molecular simulation and isothermal adsorption heat experiments. At 298 K and pressures of 1 bar and 10 bar, EMT-TMA exhibited high CH4 / N2 selectivity of 5.43 and 7.48, respectively. Simulation calculations also showed that after TMA modification, the adsorption density of CH4 in the EMT molecular sieve increased, which greatly improved the CH4 / N2 adsorption and separation capacity of the EMT molecular sieve. Breakthrough experiments further confirmed that this TMA-modified zeolite EMT possesses excellent CH4 / N2 separation performance. Its highly efficient separation characteristics make it a promising adsorbent material for CH4 / N2 separation. Attached Figure Description

[0013] Figure 1 EMT zeolite molecular sieve via TMA + Schematic diagram of the adsorption and separation mechanism of CH4 and N2 after modification; Figure 2 (a) PXRD of EMT and EMT-TMA-X; (b) SEM images; (c) TGA curves; (d) FTIR spectra; Figure 3 (a) N2 adsorption-desorption isotherms of EMT and EMT-TMA-X at 77 K; (b) CH4 adsorption isotherms of EMT and EMT-TMA-X; (c) N2 adsorption isotherms of EMT and EMT-TMA-X; (d) CH4 / N2 adsorption selectivity of EMT and EMT-TMA-X; Figure 4(a) CH4 / N2 adsorption isotherms of EMT and EMT-TMA-34% at 10 bar; (b) CH4 / N2 adsorption selectivity of EMT and EMT-TMA-34% at 10 bar; Figure 5 (a) The accompanying isotherms of CH4 and N2 of EMT and EMT-TMA-34% at 298 K and 273 K; (b) The isotropic heat of adsorption of CH4 and N2 of EMT and EMT-TMA-34%; Figure 6 (a) CH4 / N2 breakthrough curves of sample EMT-TMA-34% at different flow rates at 298K and 1 bar (50 / 50, v / v); (b) CH4 / N2 breakthrough curves of sample EMT-TMA-34% at different flow rates at 298K and 1 bar (20 / 80, v / v). Figure 7 The lowest energy framework for the adsorption of CH4 and N2 in Si-Al-Na and Si-Al-TMA clusters; Figure 8 Energy distribution of CH4 and N2 during adsorption in Si-Al-Na and Si-Al-TMA clusters; Figure 9 Contour plots of the centroid probability distributions of methane (a)(b) and nitrogen (c)(d) adsorbed on EMT and EMT-TMA under conditions of 298 K and 1 bar. Detailed Implementation

[0014] This invention utilizes a simple modification method to perform ion exchange between TMA and EMT. The resulting adsorbent effectively enhances the adsorption of CH4 while weakening the adsorption of N2, thereby effectively improving the CH4 / N2 adsorption selectivity. Figure 1 ).

[0015] Example 1 NaAlO2 was dissolved in an aqueous solution of NaOH and stirred for 30 min. Silica sol and SDA (18-crown ether-6) were then slowly added to the mixture, and the initial gel was aged at room temperature for 24 h. The prepared aluminosilicate gel was transferred to a 25 ml polytetrafluoroethylene-lined container and placed in a stainless steel autoclave for crystallization at 110 °C for 7 days under static conditions. The solid product was cooled, washed with deionized water until the pH was near neutral, and then dried overnight in an oven at 80 °C. To obtain an SDA-free sample, the dried powder was calcined in air at 550 °C for 5 h, finally yielding EMT zeolite molecular sieves. The molar composition of the EMT zeolite molecular sieves was 10SiO2: 1 Al2O3: 2.4 Na2O: 1 SDA: 140 H2O.

[0016] Example 2 Preparation of EMT modified with tetramethylammonium cation (TMA) The EMT obtained in Example 1 was placed in a 0.02M tetramethylammonium chloride solution, heated and stirred in a water bath at 60°C for 4 hours, washed three times with deionized water, and dried overnight in an oven at 80°C to obtain the EMT-TMA-15% sample.

[0017] Example 3: Preparation of Tetramethylammonium (TMA) Modified EMT The EMT obtained in Example 1 was placed in a 0.06M tetramethylammonium chloride solution, heated and stirred in a water bath at 60°C for 4 hours, washed three times with deionized water, and dried overnight in an oven at 80°C to obtain the EMT-TMA-27% sample.

[0018] Example 4: Preparation of EMT modified with tetramethylammonium cation (TMA) The EMT obtained in Example 1 was placed in a 0.2M tetramethylammonium chloride solution, heated and stirred in a water bath at 60°C for 4 hours, washed three times with deionized water, and dried overnight in an oven at 80°C to obtain the EMT-TMA-34% sample.

[0019] Example 5: Preparation of EMT modified with tetramethylammonium cation (TMA) The EMT obtained in Example 1 was placed in a 0.5M tetramethylammonium chloride solution, heated and stirred in a water bath at 60°C for 4 hours, washed three times with deionized water, and dried overnight in an oven at 80°C to obtain the EMT-TMA-43% sample.

[0020] Table 1. Elemental composition of EMT and EMT-TMA-X samples

[0021]

[0022] (1) Characterization of the EMT molecular sieve obtained in the above examples: Figure 2 Image a shows the experimental results of EMT molecular sieves in simulated PXRD. The peak values ​​of EMT-TMA are consistent with those of the original EMT, confirming that the crystal structure of EMT remains unchanged after modification. SEM images of EMT and EMT-TMA-34% are shown below. Figure 2 As shown in b, the sample exhibits a hexagonal flake shape, and no significant morphological change was observed before and after modification compared to the original EMT. PXRD and SEM results indicate that the modification of zeolite by TMACl is mild. Figure 2c shows the TGA curves for EMT and EMT-TMA-34%. For sample EMT, there is only one weight loss step below 300℃, which is due to the removal of water adsorbed on the surface. In addition to the water weight loss peak before 300℃, EMT-TMA-30% also shows a weight loss peak at 400-600℃, which is due to the decomposition of TMA. Figure 2 d is the FTIR spectrum of EMT and EMT-TMA-34% at 1382 cm⁻¹. -1 1488cm -1 and 2793cm -1 The strong absorption bands are due to the in-plane bending and stretching vibrations of CH and the stretching vibrations of NC. Absorption bands in these three locations were not observed on EMT, so TMA can be used for qualitative characterization.

[0023] (2) Gas adsorption Figure 3 Table 1 shows the N2 adsorption-desorption isotherms of EMT and EMT-TMA at 77 K. Both materials exhibit a sharp increase in adsorption under low pressure (P / P0 < 0.01), typically displaying a Type I isotherm, indicating a microporous structure. With modification by tetramethylammonium cation (TMA), the equilibrium adsorption capacity of EMT molecular sieves for N2 decreases significantly with increasing exchange degree. Surface area data are summarized in Table 2. It can be observed that the EMT-TMA-34% is significantly lower than that of EMT, with values ​​of 469.3 m² / g and 748.9 m² / g, respectively. This is because the larger organic groups in TMA occupy part of the pores and block some gas molecules.

[0024] Table 2. Specific surface area of ​​EMT with different ammonium exchange degrees

[0025] To investigate the adsorption and separation capabilities of the modified sample, adsorption isotherms for CH4 and N2 were measured at 298 K and 1 bar. Figure 3 As shown in Table 3, the modified sample showed an increased adsorption capacity for CH4. With increasing exchange degree, the adsorption capacity gradually increased, while the N2 adsorption capacity decreased significantly after modification, resulting in a substantial improvement in selectivity. However, at an exchange degree of 43%, the N2 adsorption capacity increased, leading to a deterioration in adsorption performance. Therefore, through calculations of the selectivity of the CH4 / N2 (50v / 50v) gas mixture, the optimal sample, EMT-TMA-34%, was selected for further study. The CH4 adsorption capacity was increased from 7.72 cm⁻¹. 3 / g increased to 9.12cm 3 / g, the adsorption capacity of N2 increased from 4.28cm 3 / g decreased to 1.98cm3 / g, compared with the original EMT powder, the adsorption selectivity of CH4 / N2 of this sample increased from 1.82 to 5.43, which is relatively high among most zeolites currently available.

[0026] Table 3. CH4 and N2 adsorption data of EMT and EMT-TMA-X samples at 1 bar pressure.

[0027] (3) Gas adsorption under high pressure The above tests revealed that the CH4 adsorption isotherm of sample EMT-TMA-34% showed a steeper upward trend in the low-pressure region, while the N2 adsorption isotherm showed a more gradual upward trend. Furthermore, the adsorption selectivity still exhibited a good upward trend. Therefore, if... Figure 4 As shown in ab and Table 4, we performed adsorption isotherm tests on the samples for CH4 and N2 at 298 K and 10 bar, and calculated the adsorption selectivity. Under high pressure, EMT-TMA-34% showed a CH4 adsorption capacity of 44.91 cm⁻¹ compared to EMT. 3 / g decreased to 33.71cm 3 / g, while the adsorption capacity of N2 increased from 26.08cm³. 3 / g decreased to 11.10cm 3 Compared to the original EMT powder, the CH4 / N2 adsorption selectivity of this sample increased from 2.12 to 7.38. Although the CH4 adsorption capacity decreased slightly under high pressure compared to the original sample, the N2 adsorption capacity decreased even more significantly. Therefore, while maintaining a high CH4 adsorption capacity, it also exhibits good CH4 / N2 adsorption selectivity. Meanwhile, compared to the low pressure region, the CH4 adsorption capacity of the EMT-TMA-34% sample increased from 9.21 cm⁻¹. 3 / g rose to 33.71cm 3 The adsorption selectivity of CH4 / N2 increased from 5.43 to 7.38, which can be attributed to the good adsorption potential of the 12-membered ring macropores of EMT in the high-pressure region.

[0028] Table 4. CH4 and N2 adsorption data of EMT and EMT-TMA-X samples at 10 bar pressure.

[0029] (4) Calculation of adsorption heat of CH4 and N2 isostructure To assess the interaction between the adsorbent and adsorbate, isochoric heats of adsorption were calculated. Adsorption isotherms for CH4 and N2 measured at 298 K and 273 K are shown below. Figure 5 a and Table 5 are shown. Figure 5b shows the isostructural adsorption heats of CH4 and N2 for EMT and EMT-TMA-34%. The isostructural adsorption heat of CH4 for EMT-TMA-34% is higher than that for EMT, while the isostructural adsorption heat of N2 is lower than that for EMT, indicating that the modified sample has a higher affinity for CH4 than for N2. This result is consistent with previous adsorption isotherm tests of CH4 and N2 and calculations of the CH4 / N2 adsorption selectivity.

[0030] (5) Penetration test To further evaluate the actual separation performance of the samples, breakthrough experiments were performed in an adsorption column packed with 34% EMT-TMA. The experiments were conducted at 298 K and 1 bar, using binary mixtures of CH4 / N2 (50 / 50 and 20 / 80, v / v) as feed at flow rates of 3, 5, 10, and 15 mL / min. Figure 6 As shown in ab and Table 5, it can be observed that when the CH4 / N2 gas mixture is purged into the adsorption column, after a period of time, the poorly adsorbed N2 elutes from the adsorption column. The time before N2 breaks through is called the co-adsorption time. After that, CH4 continues to be adsorbed until it becomes saturated and then breaks through the adsorption column, eventually reaching equilibrium. The break-through time interval ratio is defined as the retention time divided by the CH4 break-through time, where the retention time is the CH4 break-through time minus the co-adsorption time. Although the break-through time interval ratio decreases with increasing inlet gas flow rate, it still maintains good separation performance. Therefore, the sample exhibits good practical separation performance.

[0031] Table 5. Gas composition, flow rate, time interval, and time interval ratio used in the penetration test of EMT-TMA-34%.

[0032] (6) Energy distribution of CH4 and N2 EMT-TMA exhibits significant advantages in CH4 adsorption capacity and CH4 / N2 selectivity. Therefore, molecular simulation methods were used to study the adsorption of EMT zeolite via TMA. + The reasons for the increased CH4 adsorption capacity and enhanced selectivity after ion exchange were investigated. The energy distributions of CH4 and N2 adsorption in Si-Al-Na and Si-Al-TMA clusters were simulated, respectively. Figure 7 and Figure 8 The lowest energy framework and energy distribution of CH4 and N2 adsorption in clusters are shown respectively. Figure 7 As shown, the lowest energy framework for the adsorption of CH4 and N2 by Si-Al clusters varies with cation type, indicating that different cations interact differently with CH4 and N2. Figure 8As shown, the peak values ​​for the interactions between CH4 and N2 with the Si-Al-Na cluster are 14.85 and 8.57 kcal / mol, respectively, while the peak values ​​for their interactions with the Si-Al-TMA cluster are 17.36 and 9.41 kcal / mol. This indicates that the interaction energy of CH4 in the Si-Al-TMA cluster is much higher than that in the Si-Al-Na cluster, suggesting that CH4 has a better adsorption affinity in the Si-Al-TMA cluster than in the Si-Al-Na cluster. The figure also shows that the N2 peak is sharp and high, while the CH4 peak is low and broad. This is because the interactions between the framework and the N2 molecule are not diverse and are weak, while the interactions with CH4 are diverse and stronger. Furthermore, compared to... Figure 8 As shown in a and 8b, after TMA modification, the CH4 peak is significantly broadened and the peak value is larger, indicating that the interaction between CH4 and the framework is further diversified and strengthened. The simulation results are consistent with the adsorption isotherm, and TMA modification improves both the CH4 adsorption capacity and the CH4 / N2 adsorption selectivity of EMT.

[0033] (7) GCMC calculation The separation mechanism of CH4 and N2 in EMT and EMT-TMA was investigated using grand canonical Monte Carlo (GCMC) simulations. Figure 9 The figure shows the adsorption densities of CH4 and N2 in EMT and EMT-TMA at 298 K and 1.0 bar. It can be seen that the adsorption density of CH4 in the 12-membered ring of the EMT molecular sieve modified with TMA is significantly higher than that of the unmodified EMT molecular sieve, and also much higher than that of N2. The calculation results show that the binding energies of CH4 and N2 in EMT-TMA are 20.49 kJ / mol and 17.12 kJ / mol, respectively, consistent with the experimental trends. This also demonstrates that the introduction of TMA greatly improves the adsorption efficiency of CH4 by the EMT molecular sieve.

[0034] Conclusion: In summary, through simple modification, the tetramethylammonium ion (TMA) can be converted into a more effective form. + The introduction of TMA (tissue-modified organic molecule) into zeolite molecular sieves (EMT) enables the previously ineffective EMT to exhibit highly efficient CH4 / N2 separation performance. The introduction of TMA further enhances the S-reduction properties of the EMT molecular sieve. BETThe adsorption value was significantly lower than that of the original EMT, but the methane adsorption capacity increased. This is because the introduction of TMA enhanced the interaction affinity between CH4 and the zeolite framework, a conclusion verified by molecular simulation and isothermal adsorption heat experiments. At 298 K and pressures of 1 bar and 10 bar, EMT-TMA exhibited high CH4 / N2 selectivity of 5.43 and 7.48, respectively. Simulation calculations also showed that after TMA modification, the adsorption density of CH4 in the EMT molecular sieve increased, which greatly improved the CH4 / N2 adsorption and separation capacity of the EMT molecular sieve. Breakthrough experiments further confirmed that this TMA-modified zeolite EMT has excellent CH4 / N2 separation performance. Its high-efficiency separation characteristics make it a promising adsorbent material for CH4 / N2 separation.

Claims

1. Application of a tetramethylammonium-modified EMT zeolite molecular sieve in CH4 / N2 adsorption and separation.

2. The application of the tetramethylammonium-modified EMT zeolite molecular sieve according to claim 1 in CH4 / N2 adsorption and separation, characterized in that: The preparation method of the tetramethylammonium modified EMT zeolite molecular sieve includes the following steps: EMT zeolite molecular sieve is placed in a tetramethylammonium chloride solution, heated and stirred in a water bath at 60°C for 4 hours, washed three times with deionized water, and dried overnight in an oven at 80°C to obtain the EMT-TMA-X sample.

3. The application of the tetramethylammonium modified EMT zeolite molecular sieve according to claim 1 in CH4 / N2 adsorption and separation, characterized in that: The concentration of the tetramethylammonium chloride solution was 0.02-0.5M, and the exchange rate of the EMT-TMA-X sample was 15-43%.

4. The application of the tetramethylammonium-modified EMT zeolite molecular sieve according to claim 3 in CH4 / N2 adsorption and separation, characterized in that: The concentration of the tetramethylammonium chloride solution was 0.2M, and the exchange degree of the EMT-TMA-X sample was 34%.

5. The application of the tetramethylammonium-modified EMT zeolite molecular sieve according to claim 4 in CH4 / N2 adsorption and separation, characterized in that: At 298 K and pressures of 1 bar and 10 bar, EMT-TMA-34% exhibited CH4 / N2 selectivity of 5.43 and 7.48, respectively.

6. The application of the tetramethylammonium-modified EMT zeolite molecular sieve according to claim 1 in CH4 / N2 adsorption and separation, characterized in that: The preparation method of EMT zeolite molecular sieve includes the following steps: NaAlO2 is dissolved in an aqueous solution of NaOH and stirred for 30 min. Then, silica sol and 18-crown ether-6 SDA are slowly added to the mixture. The initial gel is aged at room temperature for 24 h. The prepared aluminosilicate gel is transferred to a 25 ml polytetrafluoroethylene liner and placed in a stainless steel autoclave. It is crystallized at 110 °C for 7 days under static conditions. The solid product is cooled, washed with deionized water until the pH value is close to neutral, and then dried overnight in an oven at 80 °C. The dried powder is calcined in air at 550 °C for 5 h to finally obtain EMT zeolite molecular sieve.

7. The application of the tetramethylammonium modified EMT zeolite molecular sieve according to claim 1 in CH4 / N2 adsorption and separation, characterized in that: The molar composition of EMT zeolite molecular sieve is 10 SiO2: 1 Al2O3: 2.4 Na2O: 1 SDA: 140 H2O.

Images