Tuning secondary building unit of zirconium-based MOF for enhanced separation of light hydrocarbons

doi:10.11949/0438-1157.20210683

Abstract

Abstract:

The recovery of C₂/C₃ light hydrocarbon components from natural gas has important industrial value. Adsorption separation technology can realize the recovery of light hydrocarbons under normal temperature and pressure. Tuning the secondary building unit (SBU) optimizes pore chemistry and develops new adsorption sites of MOF, while the well-defined framework can be inherited. s-Triazine (TZ) was used to replace the coordinated water molecules from the SBU of Zr-TBAPy and construct an alkaline surface chemical microenvironment that has a stronger restriction on light hydrocarbon adsorbates in its pores to obtain a highly selective TZ@Zr-TBAPy adsorbent. On the one hand, the introduction of TZ improves the surface roughness of the pores on the molecular scale; on the other hand, it strengthens the confinement effect on light hydrocarbon adsorbates, thereby increasing the adsorption capacity and selectivity for alkanes of absorbent. Under standard ambient temperature and pressure, the adsorption capacity of TZ@Zr-TBAPy for propane reaches 10.08 and 4.19 mmol?g^-1, which is 27% and 9% higher than that of Zr-TBAPy. It is currently among the top adsorbents concerning propane adsorption capacity in the world. Moreover, the IAST selectivity of propane/methane is 1518, which is 6.27 times that of raw materials; the IAST selectivity of ethane/methane is 11.7, which is 22% higher than that of raw materials. In addition, the isosteric adsorption heats of TZ@Zr-TBAPy for the three alkanes follow the propane>ethane>methane, indicating that propane has a stronger affinity than methane and ethane, which is related to the preferential adsorption of propane of the material. More importantly, the fixed-bed adsorption process with TZ@Zr-TBAPy adsorbent as the core can realize the one-step separation and recovery of ethane and propane in natural gas at room temperature and pressure.

Key words: hydrocarbon, adsorption, selectivity, metal-organic framework, SBU-tuning

摘要：

从天然气中回收C₂/C₃轻烃组分具有重要的工业价值，吸附分离技术可在常温常压下实现轻烃的回收。对MOF材料进行次级结构单元（SBU）调控，可在继承其晶体结构和发达孔道的同时，优化孔道化学微环境并引入新的吸附位点。使用三嗪（TZ）取代Zr-TBAPy(NU-1000)SBU中的配位水分子，在其孔道内构筑对轻烃吸附质具有更强限域作用的碱性表面化学微环境，得到了高选择性的新型TZ@Zr-TBAPy吸附剂。TZ的引入在分子尺度上提高了孔道的表面粗糙度，同时强化对轻烃吸附质的限域作用，提高材料对烷烃的吸附容量和选择性。常温常压下，TZ@Zr-TBAPy对丙烷和乙烷的吸附容量分别为10.08和4.19 mmol?g^-1，比Zr-TBAPy提高了27%和9%，是目前国际上已报道的丙烷吸附容量最高的吸附剂之一。此外，丙烷/甲烷的IAST选择性为1518，是原材料的6.27倍；乙烷/甲烷的IAST选择性为11.7，比原材料提高了22%。更为重要的是，以TZ@Zr-TBAPy吸附剂为核心的固定床吸附过程可实现在常温常压天然气中乙烷和丙烷的一步分离回收。

关键词: 碳氢化合物, 吸附作用, 选择性, 金属-有机骨架, 次级结构单元调控

CLC Number:

TQ 028.1

Sa WANG, Yijing WEN, Danyu GUO, Xin ZHOU, Zhong LI. Tuning secondary building unit of zirconium-based MOF for enhanced separation of light hydrocarbons[J]. CIESC Journal, 2022, 73(2): 730-738.

王洒, 温怡静, 郭丹煜, 周欣, 李忠. 锆基MOF次级结构单元调控及轻烃吸附分离性能增强[J]. 化工学报, 2022, 73(2): 730-738.

Figures/Tables 12

Fig.1 Structure of TZ@Zr-TBAPy for the separation of ethane and propane from natural gas(Enlarged: water molecules on the SBU of Zr-TBAPy were replaced by TZ to yield TZ@Zr-TBAPy)

Fig.2 Definition of pore parameters in molecular simulation(a)； Fixed-bed permeation device(b)

Fig.3 XRD pattern (a), FT-IR spectra (b), and SEM images[(c),(d)] of Zr-TBAPy and TZ@Zr-TBAPy

Fig.4 N2 adsorption-desorption isotherms at 77 K (a) and pore size distribution (b) of Zr-TBAPy and TZ@Zr-TBAPy

Table 1 BET surface area and pore volume of Zr-TBAPy and TZ@Zr-TBAPy

吸附剂	BET比表面积/(m²·g^-1)	总孔容,V_t/(cm³·g^-1)	微孔孔容，V_micro/(cm³·g^-1)	介孔孔容，V_meso/(cm³·g^-1)
Zr-TBAPy	2283	1.30	0.31	0.99
TZ@Zr-TBAPy	2441	1.24	0.30	0.94

Fig.5 CH4/C2H6/C3H8 adsorption curves of Zr-TBAPy and TZ@Zr-TBAPy at 298 K

Fig.6 The heat of adsorption of CH4, C2H6 and C3H8 on Zr-TBAPy (a) and TZ@Zr-TBAPy (b)

Table 2 The fitting parameters of DSLF model and the corresponding correlation coefficients

参数	Zr-TBAPy			TZ@Zr-TBAPy
参数	C₃H₈	C₂H₆	CH₄	C₃H₈	C₂H₆	CH₄
q_m,1/(mmol·g^-1)	8.04	4.49	1.48	9.49	6.03	1.66
b₁	0.036	0.0043	0.0023	0.037	0.0043	0.0018
n₁	1.08	0.88	0.89	1.38	0.88	0.81
q_m,2/(mmol·g^-1)	4.87	3.30	1.34	6.21	3.15	0.60
b₂	0.010	0.0032	0.0016	0.021	0.0030	0.0010
n₂	1.02	0.82	1.10	0.85	0.82	1.04
R²	0.999	0.999	0.999	0.999	0.999	0.999

Fig.7 IAST selectivity of C3H8/CH4 (a) and C2H6/CH4 (b) for Zr-TBAPy and TZ@Zr-TBAPy; C3H8/CH4 adsorption selectivity vs C3H8 absorption curve (c); C2H6/CH4 adsorption selectivity vs C2H6 adsorption (d) (The test temperatures of UTSA-35a and MFM-202a are 296 K and 293 K, respectively)

Fig.8 Adsorption energy (a) and channels of Zr-TBAPy and TZ@Zr-TBAPy (b)

Table 3 Pore parameters of molecular model

材料	GCD/?	PLD/?	LCD/?
Zr-TBAPy	28.84	27.74	28.84
TZ@Zr-TBAPy	26.13	21.70	26.12

Fig.9 Breakthrough curves for fixed bed of Zr-TBAPy and TZ@Zr-TBAPy to methane/ethane/propane mixed gas

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