A Comparative Investigation of Micro- and Mesoporous Materials On the Pressure Swing Adsorption Behaviors for N2 and CH4 Separation

A comparative investigation of micro- and mesoporous materials on the pressure swing adsorption behaviors for N₂ and CH₄ separation

Fengjuan Shi ^a, Hongxing Dai ^a,*, Lei Zhang ^a, Jiguang Deng ^a, Hong He ^a, Jian Li ^b, Yingshu Liu ^c, Kai Wang ^d

^a Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. Email: hxdai@bjut.edu.cn

^b Institute of Environmental Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

^c School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China

^d School of Resource and Safety Engineering, China University of Mining & Technology Beijing, Beijing 100083, China

There are mainly O₂, N₂, and CH₄ in natural gas or ventilation air methane (VAM), in which the concentrations of O₂ and CH₄ must be strictly controlled so as to avoid the explosion limit range. Hence, the separation of O₂/CH₄ and/or N₂/CH₄ is of significance in the utilization of natural gas or VAM. Pressure swing adsorption (PSA) is one of the important strategies for the purification and bulk separation of such a gas mixture. The key issue is the availability of effective adsorbents with sufficiently high selectivity and adsorption capacity. Micro- and mesoporous materials¹ are good candidates for the separation of O₂/CH₄ and/or N₂/CH₄. We herein report the adsorption behaviors of micro- or mesoporous materials for N₂ and CH₄ separation using the PSA technique. It is found that the rod-like SBA-15 and MCM-41 mesoporous materials showed exceptional performance of the separation of N₂ and CH₄.

All of the micro- or mesoporous materials were synthesized according to the procedures described in the literature^1a,1b,2. Their surface areas and pore structures were determined by the BET method. The isotherms of N₂ or CH₄ adsorption-desorption on the porous samples were recorded on the static PSA apparatus (Micromeritics ASAP 2050). Before measurement, 0.2 g of the sample was pretreated at -0.1 MPa and 150^oC for 2 h. The purity of N₂ and CH₄ was 99.99%. The adsorption temperature and pressure range was 25^oC and 0~6000 mmHg, respectively.

Table 1 summarizes the surface areas, N₂ and CH₄ amounts adsorbed at 380 or 1140 mmHg, and N₂/CH₄ separation factor (Alfa_N2/CH4) of the porous samples. The N₂ or CH₄ adsorption isotherm for each of the samples was Langmuir type (it obeys the equation: 1/q = q_m + (1/Bq_m) (1/p), where B is the Langmuir adsorption constant), the saturated adsorption amount (q_m) can be calculated according to the intercept and slope of the plot 1/q versus 1/p. For the two-component (x and y) gas mixture, the separation factor (Alfa_x/y) can be defined as Alfa_x/y = ((q_m)_xB_x)/((q_m)_yB_y). From Table 1, one can see that (i) there was no clear relationship of the Alfa_N2/CH4 value and N₂ or CH₄ adsorption amount with the surface area of the porous sample; (ii) among the 4 microporous samples, ZSM-5 showed the highest N₂ and CH₄ adsorption capacities and Alfa_N2/CH4 value (4.51); (iii) in the 6 mesoporous silica samples, the rod-like SBA-15 and MCM-41 gave unusually high Alfa_N2/CH4 vales (9.38 and 6.59, respectively) although their N₂ and CH₄ adsorption capacities were lower; and (iv) as for the two mesoporous carbon materials fabricated from differently morphological SBA-16, the polyhedral SBA-16-derived mesoporous carbon sample was superior in N₂/CH₄ separation to the spherical SBA-16-derived counterpart. The far discrepancy in N₂/CH₄ separation performance and their adsorption capacity of these micro- or mesoporous materials might be associated with their pore structure as well as the possibly residual impurities. Detailed investigations are in progress.

Table 1. Surface areas, N₂ and CH₄ amounts, and Alfa_N2/CH4 values of the porous samples.

No.	Sample	Surface area (m2/g)	N2/CH4 amounts adsorbed at different pressures (cm3/g)		AlfaN2/CH4
			380 (mmHg)	1140 (mmHg)
1	4A	-	4.9/7.1	7.7/19.0	2.42
2	ZSM-5	321	8.9/11.9	14.8/25.3	1.71
3	Zeolite b	580	5.1/6.3	8.6/14.0	4.51
4	Zeolite X	11	3.0/6.6	7.4/16.7	1.13
5	MCM-41	1028	5.1/2.5	2.7/5.8	6.59
6	MCM-22	375	2.1/8.7	3.2/17.0	1.33
7	Spherical SBA-15	708	4.3/2.2	8.9/4.3	1.90
8	Rod-like SBA-15	796	1.2/7.2	2.2/10.0	9.38
9	Spherical SBA-16	809	2.1/2.0	3.6/4.4	1.37
10	Polyhedral SBA-16	1011	5.1/8.8	8.6/11.2	3.18
11	Spherical SBA-16-derived mesoporous carbon	966	1.2/7.2	2.2/10.0	2.56
12	Polyhedral SBA-16-derived mesoporous carbon	1600	1.2/7.2	2.2/10.0	3.51

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