堿金屬改性對ZSM-5結構和吸附甲苯特性的影響及機理

崔永康; 邢奕; 蘇偉; 王嘉慶

doi:10.13374/j.issn2095-9389.2020.07.25.001

堿金屬改性對ZSM-5結構和吸附甲苯特性的影響及機理

doi: 10.13374/j.issn2095-9389.2020.07.25.001

崔永康^{1, 2},
邢奕^{1, 2},
蘇偉^{1, 2, ,},
王嘉慶^{1, 2}

1.
北京科技大學能源與環境工程學院, 北京 100083
2.
工業典型污染物資源化處理北京市重點實驗室, 北京 100083

基金項目: 國家自然科學基金資助項目（51770438）

詳細信息

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Email：suwei3007@163.com

中圖分類號: X513
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出版歷程
- 收稿日期: 2020-07-25
- 網絡出版日期: 2020-10-09
- 刊出日期: 2021-11-25

Influence and mechanism of alkali-metal modification on ZSM-5 structure and toluene adsorption

CUI Yong-kang^{1, 2},
XING Yi^{1, 2},
SU Wei^{1, 2
, ,},
WANG Jia-qing^{1, 2}

1.
School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China
2.
Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, Beijing 100083, China

More Information

Corresponding author: E-mail: suwei3007@163.com

摘要

摘要: ZSM-5是一種常用來吸附甲苯的微孔吸附劑，選擇三種堿金屬Li、Na和K對ZSM-5進行改性，結合表征手段和數學模型的方式研究引入ZSM-5中的堿金屬對微孔結構和吸附甲苯的影響。在此實驗中，分別從吸附容量、放熱能量、擴散阻力和脫附活化能四方面深入探討堿金屬對吸附甲苯的影響規律。基于實驗結果得知：堿金屬的引入改變了ZSM-5分子篩的微孔結構并呈現出一定的規律。隨著離子半徑(Li⁺<Na⁺<K⁺)的升高，ZSM-5的孔徑、比表面積和孔體積隨之降低，影響規律為Li?ZSM-5 > Na?ZSM-5 > K?ZSM-5。靜態飽和吸附量呈Li?ZSM-5（0.363 mmol·g^?1）>Na?ZSM-5（0.360 mmol·g^?1）>K?ZSM-5（0.325 mmol·g^?1）排序。恒定濃度波模型很好的描述甲苯在ZSM-5上的吸附擴散行為，空間位阻和靜電束縛力分別在高低進氣濃度條件下對甲苯在ZSM-5孔道中的擴散占據主導作用，較高進氣質量濃度(155 mg·m^?3)條件下，堿金屬改性對擴散阻力影響規律為Li?ZSM-5<Na?ZSM-5<K?ZSM-5；較低進氣質量濃度（25 mg·m^?3）條件下，影響規律為Li?ZSM-5>Na?ZSM-5>K?ZSM-5。結合脫附動力學分析，Na?ZSM-5因具有較大的孔徑和適中的吸附強度，表現出更好的再生潛能。本研究從空間位阻和吸附強度兩方面系統闡述了堿金屬改性對甲苯吸附行為的影響機理，為在復雜的實際環境應用中選擇合適的吸附劑提供了一定的參考意義。
- 分子篩 /
- 甲苯 /
- 空間位阻 /
- 吸附強度 /
- 堿金屬改性
Abstract: The microporous adsorbent ZSM-5 has been extensively applied for toluene adsorption. In this work, ZSM-5 was modified with alkali metals Li, Na, and K for toluene adsorption. The effects of the alkali metals introduced into ZSM-5 on the ZSM-5 microporous structure and toluene adsorption were studied via characterization techniques and mathematical modeling. Moreover, the influence of alkali metals on toluene adsorption was investigated from four aspects: adsorption capacity, exothermic energy, diffusion resistance, and desorption activation energy. The experimental results show that the introduction of alkali metal affect the ZSM-5 microporous structure in different aspects. The pore size, specific surface area, and pore volume of the modified ZSM-5 were of the following order: Li?ZSM-5 > Na?ZSM-5 > K?ZSM-5, corresponding to increasing ionic radius of the metals (Li⁺ < Na⁺ < K⁺). Likewise, the static saturated adsorption capacity was of the order: Li?ZSM-5 (0.363 mmol·g^?1) > Na?ZSM-5 (0.360 mmol·g^?1）> K?ZSM-5 (0.325 mmol·g^?1). The constant concentration wave model could well fit the adsorption and diffusion behaviors of toluene onto ZSM-5. The steric hindrance and electrostatic binding force played a dominant role in toluene diffusion in the ZSM-5 channel at high and low inlet gas concentrations, respectively. At a higher inlet concentration (155 mg·m^?3), the influence of alkali metal modification on the internal diffusion resistance for the three adsorbents was of the order: Li?ZSM-5< Na?ZSM-5 < K?ZSM-5, whereas at a lower inlet concentration (25 mg·m^?3), the trend was Li?ZSM-5 > Na?ZSM-5 > K?ZSM-5. The desorption kinetics analysis show that Na?ZSM-5 exhibite a better regeneration potential, due to its large pore size and moderate adsorption strength. In this study, the mechanism of alkali-metal modification of the adsorption behavior toward toluene was systematically investigated from two aspects: steric hindrance and adsorption strength to provide a certain reference for selecting a suitable adsorbent in complex practical environments.
- zeolite /
- toluene /
- steric hindrance /
- adsorption strength /
- alkali-metal modification

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圖 1 吸附甲苯實驗流程圖

Figure 1. Schematic of toluene adsorption system

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圖 2 改性ZSM-5的XRD圖

Figure 2. XRD patterns of modified ZSM-5

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圖 3 改性ZSM-5的孔徑分布（a）和吸/脫附等溫線圖（b）

Figure 3. Pore size distribution (a) and adsorption/desorption isotherm (b) of modified ZSM-5

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圖 4 298 K溫度下甲苯在改性ZSM-5的吸附等溫線

Figure 4. Adsorption isotherms of toluene on modified ZSM-5 at 298 K

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圖 5 298 K溫度下甲苯在改性ZSM-5的吸附等溫線

Figure 5. Model-fitting and experimental breakthrough curves for toluene on modified ZSM-5 at 295 K

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圖 6 甲苯在改性ZSM-5的TG和DTG曲線

Figure 6. TG and DTG curves of toluene on modified ZSM-5

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表 1 改性ZSM-5的離子交換程度和孔結構

Table 1. Ion-exchange rate and texture properties for modified ZSM-5

Materials	Ion-exchange rate /%	Pore-size distribution/ nm	Pore volume / (cm³·g^?1)	Specific surface area / (m²·g^?1)
Li-ZSM-5	91.43	0.58?0.75	0.216	335
Na-ZSM-5	87.67	0.58?0.75	0.198	302
K-ZSM-5	92.73	0.43?0.55	0.177	281

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表 2 吸附等溫線擬合結果

Table 2. Fitting results of adsorption isotherm models

Models	Parameters	Li?ZSM-5	Na?ZSM-5	K?ZSM-5
L?F	q_m	3.805	8.033	8.602
	K_L?F	0.003	0.002	0.001
	γ	0.388	0.471	0.501
	R²	0.981	0.993	0.994
Freundlich	K_F	0.363	0.360	0.325
	n	2.698	2.166	1.962
	R²	0.9919	0.9987	0.9968

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表 3 甲苯吸附在改性ZSM-5中的熱力學變化

Table 3. Changes in thermodynamics for toluene adsorption on modified ZSM-5

Materials	ΔH/ (kJ·mol^?1)	ΔS/ (J·mol^?1·K^?1)	ΔG/(kJ·mol^?1)
Materials	ΔH/ (kJ·mol^?1)	ΔS/ (J·mol^?1·K^?1)	288 K	298 K	308 K
Li?ZSM-5	?18.92	?46.31	?5.58	?5.12	?4.66
Na?ZSM-5	?17.81	?41.49	?5.86	?5.45	?5.03
K?ZSM-5	?15.46	?33.67	?5.76	?5.42	?5.09

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表 4 甲苯在改性ZSM-5上的吸附動力學參數

Table 4. Mass transfer parameters for toluene adsorption on modified ZSM-5

Adsorbents	C₀/ (mg·m^?3)	q₀/ (mmol·g^?1)	K_P/ (10^?2 s^?1)	K_fα/ (10⁴ s^?1)	K_Gα/ (10³ s^?1)	R²
Li?ZSM-5	25	0.233	8.97	2.16	4.08	0.993
Li?ZSM-5	155	0.106	3.98	2.16	4.91	0.993
Na?ZSM-5	25	0.225	8.90	2.16	3.95	0.983
Na?ZSM-5	155	0.095	4.69	2.16	5.15	0.981
K?ZSM-5	25	0.213	8.20	2.16	3.53	0.973
K?ZSM-5	155	0.081	5.60	2.16	5.20	0.913

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表 5 Kissinger方程估算脫附動力學參數

Table 5. kinetic parameters obtained by the Kissinger method

Sample	θ/%	β/(K·min^?1)	T_P/K	E_a/(kJ·mol^?1)	lnA	R²
Li?ZSM-5	7.8	10	325.56	44.56	15.78	0.9997
	7.6	20	338.48
	7.2	30	346.03
Na?ZSM-5	8.0	10	303.28	35.21	4.42	0.9932
	6.7	20	331.87
	7.0	30	350.23
K?ZSM-5	6.7	10	321.94	47.70	17.29	0.9110
	6.3	20	330.81
	6.8	30	340.72

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