Elsevier

Applied Surface Science

Volume 509, 15 April 2020, 145378
Applied Surface Science

Full Length Article
Constructing Fe-based bi-MOFs for photo-catalytic ozonation of organic pollutants in Fischer-Tropsch waste water

https://doi.org/10.1016/j.apsusc.2020.145378Get rights and content

Highlights

  • The MIL-88A@MIL-88B photocatalyst was successfully prepared by internal extended growth method.

  • This bi-MOFs material is with larger surface area, adequate active sites, and strong interfacial coupling.

  • Fast FeIII/FeII conversion and catalyst’s recover.

  • Avoid the loss of Fe ions in the bi-MOFs structure.

  • h+ and radical dotO2 are the predominant active radicals.

Abstract

Fischer-Tropsch waste water (FTW) is with complex organic compositions, and is toxic/stinking odor. So far, FTW is very difficult to be treated in industry. In this work, Fe-based bi-MOFs heterojunctions, i.e., MIL-88A(Fe)@MIL-88B(Fe) are synthesized through internal extended growth of rod-like MIL-88A on spindle-like MIL-88B. The compositions, structure and characteristics are systematically investigated. By virtue of larger surface area, rich active sites and strong interfacial coupling, this bi-MOFs catalyst establish better h+-e separation/transfer. As expected, this bi-MOFs hybrid materials shows remarkable photo-catalytic performance toward FTW remediation by the help of ozone. The COD removal in bi-MOFs/O3 system is 4.2 times higher than that of ozonation alone. Furthermore, h+, radical dotO2 and radical dotOH are the predominant active radicals for FTW remediation. Bi-MOFs displays acceptable stability after 5 cycles. This work provides a new way for photo-catalytic ozonation and may drive the development of MOFs-based photo-catalytic ozonation for waste water treatment, not limited in FTW.

Introduction

Fischer-Tropsch (FT) process transforms CO/H2 (mainly derived from coal) into liquid fuel with iron/cobalt catalysts. In China, just from coal to oil technology, up to 15.0 million tons liquid fuel are produced by FT process in 2018 year [1]. However, water is a major product of the FT synthesis, 1.5 tons waste water produced following one ton liquid fuel [2]. The Fischer-Tropsch waste water (FTW) are composed of ~1 wt% of oxygenated products, including monohydric alcohols, monocarboxylic organic acids, ketones and aldehydes etc, has follow characters: large amount, complex organic species, high COD (30 g COD/L) and low pH (pH = 2.0–3.0), lacked nutrient element (N,P) [3], [4]. Especially, this waste water is toxic/stinking odor, and has a mortal effect on the bacterial strains of biological remediation. Hence, it is desired to develop a high-efficiency/low-costed method to remove the organic pollutants in FTW.

Chemical method treats FTW is high efficient. Zhu et al. [5-] have done pioneer works, they hydrogenolysis organic pollutants in FTW to fuel gas. But their processes are H2 consumption and require harsh reaction condition (9.8 Mpa and 200 °C). Besides, advanced oxidation processes (AOPs) has been regarded as a powerful technology to deal with organic pollutants in waste water at ambient conditions. AOPs could be accelerated by catalytic ozonation/light/semiconductor photo-catalysts [9], [10]. Hence, choosing an efficient and stable semiconductor photo-catalysts becomes a key for the FTW remediation.

Over past years, Fe-based metal-organic frameworks (MOFs) have been widely used in AOPs [11], [12]. This materials can be designed with dimensions and specific structures to the requirements of a specific application [13]. So far, reported Fe-based Fe-MOFs include MIL-88A(Fe), MIL-88B(Fe), MIL-53(Fe), MIL-100(Fe) and MIL-101(Fe), etc. [14], [15], [16], [17]. However, single Fe-based MOF photo-catalyst suffers from inferior photo-catalytic activity, for fast recombination of h+-e pairs [10]. Therefore, it usually coupled with the other semiconductors, such as, ZnO/MIL-100(Fe) [18], TiO2/MIL-88B(Fe) [19], GO/MIL-88A(Fe) [20], [21], g-C3N4/MIL-88A(Fe) [22], which have been prepared for photo-catalytic reaction and pollutants removal.

Recently, a new strategy that constructing two types of MOFs (bi-MOFs) as the hybrid photo-catalyst has been reported [23], this is a wise method to enhance the photo-catalytic performance. So far, bi-MOFs reported include UIO-66@MIL-125 [24], UIO-66@MIL-101 [25], MIL-101@MIL-101 [26] and MIL-100/MIL-53 [23]. The slight differences in the band structure between two MOFs are enable to the formation of iso-type nanojunctions at the interface of different MOFs. Advantages of bi-MOFs heterojunctions are obtained as followed: (1) good compatibility without relying on extra semiconductors [27], [28]. (2) leading to the enhanced photo-catalytic activity from promoted charge separation [29]. (3) the Fe-based bi-MOFs could confine Fe ions, and update this material’s stability [23]. MIL-88B(Fe) has been reported to use as a photo-catalysis for pollutant removal (band gaps, 2.43 eV) [30], [31], and Fe33-oxo clusters in this material is constructed by the FeIII center and dicarboxylate ligand. In addition, MIL-88A(Fe) (band gaps, 2.58 eV [21]) are similar 3D hexagonal structure with MIL-88B(Fe). This two Fe-MOFs own similar cell parameters [32], thus they have good compatibility. Coupling this two Fe-MOFs together would possess better activity and stability for organic pollutants removal. However, this Fe-based bi-MOFs materials (i.e., MIL-88A(Fe)@MIL-88B(Fe)) have not been reported, especially, their application in the area of FTW remediation.

In this work, by microwave heating, novel types of Fe-based bi-MOFs, i.e., MIL-88A(Fe)@MIL-88B(Fe) were synthesized for the first time via internal extended growth method (IEGM). This Fe-based bi-MOFs catalysts are employed for treatment of Fischer-Tropsch simulated/actual waste water. Compared with pristine MIL-88A(Fe) or MIL-88B(Fe), the bi-MOFs not only provides better activity but also possesses higher stabilizing ability.

Section snippets

Catalyst preparation

Pristine MIL-88A(Fe) (denoted as MIL-88A) was prepared as report [20]. Specifically, 1.352 g FeCl3·6H2O and 0.580 g fumaric acid were dissolved in water. The mixture was transferred into a hydrothermal kettle, and heated to 70 °C for 4 h. Finally, the raw product was washed, dried overnight. Pristine MIL-88B(Fe) (denoted as MIL-88B) was synthesized by reported method [30].

MIL-88A(Fe)@MIL-88B(Fe) (denoted as MIL-88A@MIL-88B) are prepared via internal extended growth method (IEGM). Specially, as

XRD, IR, TG and BET

The XRD patterns of MIL-88A, MIL-88B and their hybrid structures are shown in Fig. 1. The XRD patterns of MIL-88A (Fig. 1A(a)) are very similar to previous reports [20], [21], the sharp peak, around 12°, was attributed to an interlayer spcacing of 0.74 nm according to Bragg’s Law [21]. Fig. 1A(e) shows three main peaks around 9.1°, 9.6° and 10.6° corresponding to the (0 0 1), (1 0 0) and (1 0 1) face of MIL-88B [30]. From Fig. 1A(b–d), and with the increasing of MIL-88B(Fe), the peaks at

Conclusions

In this work, we fabricated a novel hybrid bi-MOFs (MIL-88A@MIL-88B) by internal extended growth method, for FTW remediation by the help of light-driving ozanation. The bi-MOFs hetero-junctions permit fast charge separation and transfer across the interface. The coupled bi-MOFs showed remarkable catalytic performance for FTW remediation due to large surface area, rich active sites and effective charge separation. In addition, the constructed bi-MOFs hybrid materials showed an excellent

CRediT authorship contribution statement

Xiaoyuan Liao: Conceptualization, Methodology, Investigation, Writing - original draft. Fan Wang: Validation, Formal analysis, Investigation. Yinzhi Wang: Validation, Investigation. Yi Cai: Resources, Visualization. Haitang Liu: Investigation, Writing - review & editing. Xuefeng Wang: Funding acquisition, Investigation. Yulei Zhu: Resources, Methodology. Lungang Chen: Resources, Methodology. Yue Yao: Software. Qinglan Hao: Investigation, Software, Resources, Methodology, Writing - review &

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Project from the Tianjin Education Commission (2018KJ111), National Natural Science Foundation of China (31700516, 21872125, 21878290 and 21902118), Foundation of Tianjin Key Laboratory of Marine Resources and Chemistry at TUST (No. 201705), and Foundation of Jiangsu Key Laboratory for Biomass Energy and Material (JSBEM201914). X. Liao would like to thank Shiyanjia Lab (www.shiyan.jia.com) for ESR test and Synfuels China. Co., Ltd.

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