N-doped activated carbon from used dyeing wastewater adsorbent as a metal-free catalyst for acetylene hydrochlorination
Graphical abstract
Introduction
With industrial development, various sorts of artificial dyes have been synthesized excessively and applied to the manufacture of paper, rubber, textile, plastic, leather and printing industries, causing environmental contamination [1], [2], [3]. Large amount of dyes in the water body may affect the photosynthesis in aquatic systems, and more importantly, many dyes pose a grave threat to human health [4], [5]. Hence, it is of critical significance to develop efficient methods for dye removal from effluents.
Adsorption is the most attractive and diffusely used method among several techniques for diminishing the pollution, because of its low expenditure, high efficiency and easy exploitation. Activated carbon (AC) with high adsorption capacities and relatively inexpensive compared with some newly-developed adsorbents provides the possibility of industrialization. After adsorption, the exhausted AC containing concentrated pollutants is deal with burning, burying or regeneration, which might cause the waste of resources and secondary source of pollution to some extent [6], [7]. Therefore, the recovering utilization of the used AC adsorbents is the key to the sustainable development of adsorption technology for dyeing wastewater treatment.
Except for the adsorption performance, the carbon-based materials have prominent advantages in the catalytic field [8], [9], [10], [11], [12]. For the acetylene hydrochlorination reaction which is the mainly synthetic route to produce polyvinyl chloride (PVC) for coal-rich regions [13], the carbon support exhibits superior property to other support and the current industrial catalyst is carbon-supported mercuric chloride. However, mercuric chloride as the active component tends to sublimation and volatile at the reaction condition, and does harm to ecological environment and the human health. It is urgent to develop environmentally friendly non-mercury catalysts [14]. Although the metal-based non-mercury catalysts, such as Au [15], Ru [16], Pd [17] and Cu [18], have been extensively researched, the metal-based catalysts are generally restricted to its cost and reserves. Furthermore, sulfur and arsenic et al. stemming from the industrial feed are toxic to metal-based catalysts, leading the catalysts deactivation, whereas metal-free catalysts are insensitive to these substances. Therefore, many types of N-doped carbon materials have been developed as metal-free catalysts for acetylene hydrochlorination reaction. For example, Li et al. introduced the synthesis of the activated carbon supported g-C3N4 (g-C3N4/AC) catalyst and revealed hydrogen chloride molecule was more tend to adsorb on pyridinic nitrogen [19]. Bao and co-workers reported a nanocomposite by growing the N-C layer out of the preshaped silicon carbide granules (SiC@N-C) which manifested outstanding performance in the 150 h test. Through experimental and theoretical analysis, they revealed the carbon atoms adjacent to pyrrolic nitrogen species were the active sites [20]. Wei et al. reported N-doped carbon nanotubes (N-CNTs) were active for acetylene hydrochlorination and uncovered good linearity between the quaternary nitrogen content and activity [21]. Li’s group prepared N-doped carbon catalyst derived from ZIF-8 and mesoporous carbon catalyst with sulfur and nitrogen co-doping, and they proposed that carbon atoms bonded with pyridinic nitrogen were active sites [22], [23]. The above studies showed different conclusions on which type of N species is the most active site for the acetylene hydrochlorination. The structural and electronic properties of the carbon materials as well as the doping method may all influence the form of active sites.
In this work, we have investigated the adsorption of neutral red (NR, Fig. S1), a representative dye in the dyeing wastewater, by using different kinds of AC adsorbent materials. Then, the used coconut activated carbon (CAC) adsorbent which had the highest NR adsorption efficiency has been prepared to be N-doped carbon catalyst through a simple carbonization process at high temperature. The synthetic N-doped carbon catalyst showed a superior activity and stability of acetylene hydrochlorination. A series of experimental characterizations combined with ReaxFF molecular dynamics simulations as well as density functional theory (DFT) calculations have been carried out to reveal the mechanism of nitrogen doping process on the AC from the NR dye molecule and the catalytic mechanism of different N species for the acetylene hydrochlorination. This work creates a novel way for the resource utilization of the used AC adsorbent produced from the dyeing wastewater treatment, which also can improve the sustainable development of PVC industry.
Section snippets
Raw materials and reagents
Coconut activated carbon (CAC, 20–40 mesh) was procured from Fujian Sensen Activated Carbon Industry Science and Technology Co., Ltd. Neutral Red (NR, indicator pH 6.8–8.0, Mw = 288.78) was procured from Shanghai Aladdin Bio-Chem Technology Co., Ltd. The reactants C2H2 gas and HCl gas (purity all above 99%) were procured from Tianjin Dongxiang Special Gases distribute Co., Ltd. All general reagents and solvents are commercially available and used as received.
Adsorption procedure
A stock solution of 2000 mg L−1 NR
3.1.1 The optimal adsorbent among different kinds of AC materials
In order to screen out the optimal adsorbent, four types of AC materials which are commonly used in catalytic reactions were involved to explore the ability of dye adsorption. Fig. 1a illustrates the variation of the adsorption capacity of NR over the contact time with the initial concentration of 100 mg L−1 and 0.05 g absorbents. All of the coconut activated carbon (CAC), coal based activated carbon (CBAC), hollow activated carbon (HAC) and spherical activated carbon (SAC) could adsorb the dye
4 Conclusion
In this work, the adsorption capability of a series of AC materials for the treatment of the NR wastewater has been investigated. CAC was screened out to be the optimal adsorbent. The waste CAC adsorbent was then synthesized to the N-doped metal-free catalyst via a simple method. The N2 adsorption/desorption experiments and ReaxFF molecular dynamic simulation reveal that a porous carbon material generated on the surface of CAC which could facilitate the diffusion of the reaction gas. The ReaxFF
Acknowledgement
This work was supported by the Natural Science Foundation of China [grant number 21576205] and the Program for Changjiang Scholars and Innovative Research Team in University [grant number IRT_15R46].
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