Selective adsorption and separation of xylene isomers by alkali-modified MIL-53(Cr)

doi:10.11949/0438-1157.20250004

Abstract

Abstract:

MIL-53 is an important material for the adsorption and separation of xylene isomers, in which MIL-53(Cr) has high selectivity for o-xylene (OX) and p-xylene (PX), but low selectivity for OX and m-xylene (MX). To improve the selectivity of MIL-53(Cr) to OX and MX, a strong alkaline site was directly introduced into MIL-53(Cr) through a redox action between the base precursor KNO₃ and the metal center Cr³⁺. This alkali-modified MIL-53(Cr) was then tested as an adsorbent for the liquid-phase separation of OX, PX, and MX. The results show that the introduction of basic sites in MIL-53(Cr) does not alter the unique structure of the material. However, because the newly introduced alkaline sites partially occupy the metal sites, the adsorption capacity for xylene decreases. The adsorption selectivity for OX/PX remains unchanged, while the material's resistance to adsorption of MX increases with the density of basic sites, resulting in a decrease in overall adsorption capacity. Notably, the OX/MX adsorption selectivity improved from 2.3 for MIL-53(Cr) to 4.1 for the modified K-MIL-53(Cr). K-MIL-53(Cr) demonstrates excellent stability and regeneration ability. After five adsorption cycles, its adsorption capacity only decreased by 5.7%, and the structural integrity of the material remained largely unchanged.

Key words: alkaline sites, metal-organic framework, xylene isomer, adsorption separation, selectivity

摘要：

MIL-53是一类吸附分离二甲苯异构体的重要材料，其中MIL-53(Cr)对邻二甲苯（OX）和对二甲苯（PX）具有较高的分离选择性，而对OX和间二甲苯（MX）选择性低。为了改善MIL-53(Cr)对OX和MX的选择性，利用碱前体KNO₃和金属中心Cr³⁺之间的氧化还原作用在MIL-53(Cr)上直接制备强碱性位点，以碱改性的MIL-53(Cr)为吸附剂，对OX、PX和MX进行液相吸附分离。结果表明，在MIL-53(Cr)中引入碱性位不改变材料的独特结构。由于引入的碱性位占用了部分金属位点，导致材料对二甲苯的吸附量有所下降。OX/PX的吸附选择性不变，而碱性位点的增加导致材料对MX的吸附阻力增大，吸附量减少，OX/MX的吸附选择性从MIL-53(Cr)的2.3提高到K-MIL-53(Cr)的4.1。K-MIL-53(Cr)具有良好的稳定性和再生能力，吸附循环5次，吸附量仅下降5.7%，且材料结构保持不变。

关键词: 碱性位点, 金属有机框架, 二甲苯异构体, 吸附分离, 选择性

CLC Number:

TQ 241.1

Xingliang PEI, Cuiping YE, Yingli PEI, Wenying LI. Selective adsorption and separation of xylene isomers by alkali-modified MIL-53(Cr)[J]. CIESC Journal, 2025, 76(S1): 258-267.

裴星亮, 叶翠平, 裴赢丽, 李文英. 碱改性MIL-53（Cr）选择性吸附分离二甲苯异构体[J]. 化工学报, 2025, 76(S1): 258-267.

Figures/Tables 11

Fig.1 XRD patterns (a) and FTIR spectra (b) of MIL-53(Cr) and K-MIL-53(Cr)

Fig.2 SEM images and EDS mapping of MIL-53(Cr) and K-MIL-53(Cr)

Fig.3 Time-dependent adsorption of single-component xylene on (a)MIL-53(Cr) and (b)K-MIL-53(Cr)；(c) Quasi-second-order kinetic model fitting for the xylene adsorption process of K-MIL-53(Cr)

Table 1 Kinetic fitting results of xylene isomers adsorption on K-MIL-53（Cr）

二甲苯	C₀/(mol·L^-1)	准一级动力学模型 $l n Q e - Q = l n Q e - k 1 t$			准二级动力学模型 $t Q = 1 k 2 Q e 2 + t Q e$
二甲苯	C₀/(mol·L^-1)	k₁/(g·mg^-1·h^-1)	Q_e/(mg·g^-1)	R²	k₂/(g·mg^-1·h^-1)	Q_e/(mg·g^-1)	R²
OX	0.5	0.0773	138.02	0.7887	0.0005	497.51	0.9979
MX	0.5	0.0600	24.05	0.6740	0.0101	164.21	0.9996
PX	0.5	0.0601	27.39	0.6666	0.0291	187.82	0.9995

Table 1 Kinetic fitting results of xylene isomers adsorption on K-MIL-53（Cr）

二甲苯	C₀/(mol·L^-1)	准一级动力学模型 $l n Q e - Q = l n Q e - k 1 t$			准二级动力学模型 $t Q = 1 k 2 Q e 2 + t Q e$
二甲苯	C₀/(mol·L^-1)	k₁/(g·mg^-1·h^-1)	Q_e/(mg·g^-1)	R²	k₂/(g·mg^-1·h^-1)	Q_e/(mg·g^-1)	R²
OX	0.5	0.0773	138.02	0.7887	0.0005	497.51	0.9979
MX	0.5	0.0600	24.05	0.6740	0.0101	164.21	0.9996
PX	0.5	0.0601	27.39	0.6666	0.0291	187.82	0.9995

Fig.4 (a) Single component adsorption of K-MIL-53(Cr) for xylene at 25℃; (b) Impact of temperature; (c) Langmuir adsorption isotherm fitting curves

Table 2 Parameters of Langmuir and Freundlich adsorption model for single-xylene isomer on K-MIL-53（Cr）

吸附质	温度/℃	Langmuir吸附等温线 $Q e = Q m K L C e 1 + K L C e$			Freundlich吸附等温线 $Q e = K F C e 1 n$
吸附质	温度/℃	Q_m/(mg·g^-1)	K_L/(L·mol^-1)	R²	K_F/(mg·g^-1·(L·mol^-1)^1/ⁿ )	$1 n$	R²
OX	25	581.39	11.23	0.9985	562.34	0.23	0.9506
MX	25	164.20	6.91	0.9913	175.27	0.31	0.9007
PX	25	187.70	7.86	0.9962	213.87	0.37	0.9612

Table 2 Parameters of Langmuir and Freundlich adsorption model for single-xylene isomer on K-MIL-53（Cr）

吸附质	温度/℃	Langmuir吸附等温线 $Q e = Q m K L C e 1 + K L C e$			Freundlich吸附等温线 $Q e = K F C e 1 n$
吸附质	温度/℃	Q_m/(mg·g^-1)	K_L/(L·mol^-1)	R²	K_F/(mg·g^-1·(L·mol^-1)^1/ⁿ )	$1 n$	R²
OX	25	581.39	11.23	0.9985	562.34	0.23	0.9506
MX	25	164.20	6.91	0.9913	175.27	0.31	0.9007
PX	25	187.70	7.86	0.9962	213.87	0.37	0.9612

Fig.5 Xylene adsorption capacity of MIL-53(Cr) and K-MIL-53 (Cr) at 25℃

Fig.6 (a) Cycle performance of K-MIL-53(Cr) at 25℃; (b) XRD pattern of K-MIL-53(Cr) after 5 cycles

Fig.7 (a) N2 adsorption and desorption isotherms of MIL-53(Cr) and K-MIL-53(Cr) at 77 K;(b) Micropore size distribution of MIL-53(Cr) and K-MIL-53(Cr)

Table 3 BET specific surface area， pore volume and most probable aperture of MIL-53（Cr） and K-MIL-53 （Cr）

Item	MIL-53(Cr)	K-MIL-53(Cr)
specific surface area（N₂）/(m²·g^-1)	1004	832
pore volume/(cm³·g^-1)	0.53	0.49
most probable aperture/nm	0.69	0.69

Fig.8 XPS spectra of MIL-53(Cr) and K-MIL-53(Cr)

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