Elsevier

Chemical Engineering Journal

Volume 356, 15 January 2019, Pages 329-340
Chemical Engineering Journal

Construction of crystal defect sites in N-coordinated UiO-66 via mechanochemical in-situ N-doping strategy for highly selective adsorption of cationic dyes

https://doi.org/10.1016/j.cej.2018.09.060Get rights and content

Highlights

  • UiO-66 with defect pores and basic N surface was prepared by mechanochemistry.

  • Alkaline N-coordination to UiO-66 enhanced crystal stability under basic solution.

  • Alkaline N compounds formed sp2 and σ-π hybridization with Zr cluster in UiO-66.

  • N-doped UiO-66 showed high adsorption capacity and fast diffusion for rhodamine B.

  • N-doped UiO-66 exhibited 558 selectivity of RhB/ST (1:1 ratio), 223 times as UiO-66.

Abstract

Alkaline N-compounds (pyrrole, dopamine and 2-methylimidazole) were applied to induce crystal defects on UiO-66 via mechanochemical in-situ N-doping strategy and their role on its selective adsorption for cationic dyes i.e. rhodamine B (RhB) and safranine T (ST) were investigated systematically. Alkaline N-compounds coordination were proved to simultaneously modulate pore sizes and intensify surface alkalinity of the original UiO-66. The crystal defects constructed 3-D multi-point adsorption structure, which dramatically enhanced specific adsorption for RhB and ST in binary system. Results showed that pyrrole coordinated UiO-66 possessed 30% enhancement in surface area (1549.1 m2/g) with micropores at 9 and 12 Å (larger defects in UiO-66). Furthermore, temperature programmed desorption (H2O and NH3) and corrosion resistance test concluded that N-doped UiO-66 possessed improved alkali-resistance and higher alkaline surface compared to pristine UiO-66. Separation performance revealed that pyrrole-doped UiO-66 showed two times enhanced adsorption capacity for RhB (384.1 mg/g), and 223 times higher selectivity for equimolar RhB/ST than that of parent UiO-66. Textural characterization, DFT simulation and electronic factors concluded that proper defect size and alkaline surface endow the novel defective UiO-66 excellent selectivity, adsorption and recycling performances. Thus, our in-situ N-doping strategy has guiding significance to design MOFs with special and useful defects for unique selective adsorption system beyond the circle of organic dyes on industrial level.

Introduction

Organic dyes are important chemicals widely used in paper, food, and textile industrial [1]. However, their highly toxic and non-biodegradable nature have led to serious environmental problems [2]. Among these, classical cationic dyes like rhodamine B (RhB) and safranine T (ST) contain benzene, naphthalene, and benzoquinone groups, which are carcinogenic and mutagenic to humans [3]. Attributed to these harmful impacts, their removal from waste water has been widely studied by various approaches [4]. Among these reported abatement methods, adsorption has received much attention endorsing to its high efficiency, low operation cost and easy regeneration/recycling of spent adsorbent. The vital aspects for designing an exceptional adsorbent include high selectivity and adsorption capacity, fast kinetics and good recycling properties [5].

Metal organic frameworks (MOFs) are three-dimensional porous crystal materials linked by transition metals with multitooth organic ligands [6], [7]. Their unconventional porosity and flexible chemical environment facilitate easy tuning of stability and performance [8], and hence have earned widespread interests in gas separation [9], dyes adsorption [1], [10] and sensing [11]. Among these MOFs, UiO-66 has gained great attention [12] due to its relatively higher thermal stability. Among UiO-66 series, Zr-based UiO-66 possess high surface area (>1000 m2/g) and ultra-microporous structure [1]. Many researchers have focused on UiO-66 post-modification and hybridizing strategies to enhance its adsorption abilities [13]. For instance, J. F. Yao used HCl promoted UiO-66 with adsorption capacity of methyl orange and RhB of 84.8 and 13.2 mg/g respectively [1]. X.T. Liu et al. reported low adsorption efficiency (96.45 mg/g at 298 K) for cationic methylene blue over ammonia modified UiO-66 [14]. However, post-modification strategy is hindered by factors like non-uniform modification, partial disruptions of pore structure and stability.

Recently, pre-modification approach has become a research hotspot to modulate pore size and surface property of Zr based UiO-66 [15], [16]. Using long linker of precursors can design larger pores in UiO-66. However, this process is inevitably associated with a considerable loss in surface area and their corresponding selective adsorption performance [17], [18]. Another easy and efficient strategy of defect designing in MOFs is the addition of single tooth organic ligands [19] or weakly coordinated compounds [20]. Based on preserving original skeleton of MOFs, in-situ grafting defects approach using “one-pot” synthesis is very attractive in this regard. Concurrently, its challenge is to synergistically modulate pore size and surface properties avoiding the any loss in surface area. Credited to these hindering factors in modified MOFs synthesis, rare studies have been focused on these aspects with systematic investigation.

In this work, we proposed three alkaline nitrogen heterocycles i.e. dopamine, pyrrole, 2-methylimidazole to modulate UiO-66 by a facile in-situ one-pot mechanochemical high energy attack for the construction of defects. This strategy resulted in higher proportion of larger micropore size, increase in surface area (from 1215.2 to 1549.1 m2/g) and basic surface property. The adsorption performance of N-modified UiO-66 were investigated for RhB and ST having similar chemistry but opposite acid-base groups. The adsorption capacity and diffusivity of RhB on pyrrolic N-coordinated UiO-66 was twice (384.1 mg/g) and 53 times as that of pristine UiO-66. Similarly, selectivity of RhB/ST on N-coordinated UiO-66 increased by two orders of magnitude compared to pristine UiO-66 from 5 to 714, which were attributed to effective charges transfer and Zr-N sp2 configurations. The coordination mode, binding force and binding energy of alkaline N compounds towards UiO-66 were investigated via density functional theory (DFT) simulation and surface characterization including X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD).

Section snippets

Reagents

All the chemicals were of analytical reagent (AR) grade and used without further processing. Terephthalic acid (H2BDC, 99.5%,) and zirconium chloride (ZrCl4, 99%) were purchased from Sigma-Aldrich, Shanghai, China. Dopamine, pyrrole and 2-methylimidazol were obtained from Sinopharm Chemical Reagents Co. Beijing, China. Methanol (CH3OH, 99.5%), N,N-dimethylformamide (C3H7NO, DMF, 99.8%), RhB, ST and HCl (37 wt%) were obtained from Nanjing Chemical Reagent Co. Ltd. Nanjing, China.

Preparation of N-doped NX-UiO-66

NX-UiO-66 were

Morphology of N-coordinated UiO-66s

SEM images of pristine UiO-66 and N-coordinated NX-UiO-66 in Fig. 1 suggested an irregular morphology of the latter as compared to UiO-66 synthesized by hydrothermal method [20]. This could be attributed to the incomplete crystal surface growth in UiO-66 due to fast growing (30 min) mechanochemical approach involving strong mechanical friction [25]. The crystal size of parent UiO-66 (about 152 nm in Fig. 1A) was increased to 362 nm along with enhancement in crystallinity (Fig. 1B-D).

Conclusions

In summary, a facile in-situ N-coordination mechanochemical strategy was proposed to create defects and modulate alkaline surface on UiO-66 for the preparation of novel MOFs for the selective adsorption from a hard separating binary dyes system with similar molecular size and 3-D structure. Characterization results confirmed that N-doped UiO-66 exhibited high surface area (1549.1 m2/g), uniform larger defect pores (11–13 Å) and better resistance to alkaline corrosion. Meanwhile, H2O- and NH3

Conflict of interest

The authors declare no competing financial interest.

Acknowledgments

This work was financially supported by National Natural Science Foundation of China (No. 21676059 & 21666004), Natural Science Foundation of Guangxi Zhuang Autonomous Region, China (No. 2017GXNSFFA198009 & 2016GXNSFAA380229), Scientific Research Foundation of Guangxi University (No. XGZ130963) and Innovation Project of Guangxi Graduate Education (NO.YCSW2018030). We thank Dr. Zhiqun Tian and all members of Guangxi Key Laboratory for Electrochemical Energy Materials for characterization analysis.

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    These authors contributed equally to this work and should be considered co-first authors.

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