Conjugated porous polymers for gaseous toluene adsorption in humid atmosphere

doi:10.1016/j.reactfunctpolym.2020.104804

Reactive and Functional Polymers

Volume 159, February 2021, 104804

https://doi.org/10.1016/j.reactfunctpolym.2020.104804 Get rights and content

Highlights

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Two triphenylamine-based conjugated porous polymers were successfully fabricated via Schiff-base reaction.
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The prepared TFB-TPA has high adsorption capacity and rapid adsorption speed for toluene in humid atmosphere.
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The adsorption capacity decreases with the increase of relative humidity.
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Physical adsorption plays an important role in the adsorption process.
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TFB-TPA exhibits well regeneration performance during 5 cycling tests.

Abstract

Two triphenylamine-based conjugated porous polymers, 1,3,5-triformylbenzene-4,4′4″-triaminotriphenylamine (TFB-TPA) and 2,4,6-triformylphloroglucinol-4,4′4″-triaminotriphenylamine (TFP-TPA) fabricated via Schiff-base reaction with π-conjugated skeletons and hierarchical pore structures were explored as adsorbents for the removal of gaseous toluene under humid environment. The effects of humidity, initial toluene concentration, adsorption temperature and surface functional groups on the adsorption capacity were investigated. The equilibrium isotherms, adsorption kinetics and structure-activity relationship were further analyzed. Experiment results suggest that the as-prepared TFB-TPA exhibits acceptable adsorption performance for gaseous toluene in humid atmosphere owing to the aromatic backbone and the hierarchically micro-meso-macroporous structure and the Freundlich equation agrees very well with the isothermal data, the pseudo-second-order kinetic model is more suitable to describe the adsorption kinetics. This study may not only show the application potential of triphenylamine-based conjugated porous polymers in toluene adsorption, but also provide a detailed understanding of the adsorption process of toluene for practical application.

Graphical abstract

Introduction

Toluene, a very volatile organic compound (VOC), is widely used as solvent for paints, coatings, adhesives and as an important component of a variety of industrial and household products. Its emission to atmosphere, especially to the indoor environment from furnishing and consumers productions has caused growing concern regarding the impact on human health [1,2]. The long-term exposure to toluene may bring fatal risk on the respiratory system, nervous system and hematopoietic system of human being owing to its strong toxicity and carcinogenicity [3]. Considering the toxicity of toluene and the possibility of exposure to gaseous toluene, it is urgent to develop effective elimination technologies to remove toluene in the air.

A variety of technologies including adsorption [1,4], thermal/catalytic oxidation [5,6], photocatalytic degradation [[7], [8], [9]], plasma catalysis [10,11], biological degradation [12,13] and membrane separation [14,15] have been employed for the removal of toluene. Taking the practicality and cost into consideration, adsorption has been recognized as a preferred method for toluene removal owing to its low cost, high efficiency and simple unit operation. The key factor in an adsorption process is the adsorbent which would directly affect removal efficiency and operating cost. A series of materials with porous structure and high surface area have been developed as adsorbents for toluene removal. For porous materials such as activated carbons (ACs) [[16], [17], [18]], zeolites [19,20]and silica [21,22] without π-conjugated structure, their adsorption of toluene mainly depend on the pore filling action. The specific surface area and pore structure have important influence on their toluene adsorption. Besides the pore filling adsorption, porous materials with π-conjugated structure and metal center involve the π-π interaction and cation-π bonding between toluene molecule and adsorbent, which will further enhance their adsorption capacity for toluene. Therefore, porous materials such as, metal-organic frameworks (MOFs) [23,24] and organic polymers [25] have been widely concerned as toluene adsorbents. However, these adsorbents still have some disadvantages including low adsorption capacity in humid environment and cost-consuming regeneration procedures, limiting their practical applications [4,26]. Thus, many efforts have been devoted to the development of novel reusable adsorbents that can effectively adsorb toluene under humidity conditions.

As an important branch of porous materials fabricated by organic building blocks via covalent bonds, conjugated porous polymers (CPPs) show great potential in the field of gas adsorption due to their unique properties. The abundant porous structure and high specific surface area of CPPs can improve the adsorption of pollutants. Meanwhile, a variety of functional groups or different pore sizes can be obtained on the polymer skeleton via predesigned or post-modification methods for the improvement and selective adsorption of target pollutants [27,28]. Moreover, taking advantage of their satisfactory physicochemical stability, the reuse performance of CPPs is expected to be improved to save the cost. A number of CPPs have been investigated for the adsorption of hydrogen [29,30], methane [31], carbon dioxide [[32], [33], [34], [35]] and iodine [36,37]. Recent studies have proven that the application of CPPs in adsorption of VOCs is very promising. For example, Han's group designed and synthesized two microporous polymers with π-conjugated skeletons and found that owing to their aromatic nature, these polymers exhibit good affinity toward aromatic molecules including toluene and benzene [38]. Xu and co-workers developed two porphyrin (Porp)-based conjugated microporous polymers, and the high porosity and aromatic backbone of the obtained Porp-TPE-CMP and Porp-Py-CMP make them be ideal candidates for toluene vapor [39]. However, while CPPs are of great interest for the adsorption of various VOCs, research on CPPs adsorbents has mainly focused on the development of new materials and their adsorption properties in anhydrous vapor environments. The study involving a detailed understanding of the adsorption process such as the effect of humidity on adsorption performance, adsorption isotherms, adsorption kinetics and the recycling performance is far from enough. However, water vapor, which often exists in the actual environment, may decrease the toluene adsorption owing to its competitive adsorption. It is of great significance to study the adsorption behavior of toluene by adsorbents under humidity conditions. On the other hand, triphenylamine and its derivatives are capable of forming star structure with three phenyl groups linked to the central nitrogen atom. Such a unique structure helps to prevent the aggregation of polymer chains and provides a higher specific surface area. Herein, two conjugated porous adsorbents, TFB-TPA and TFP-TPA, will be fabricated via Schiff-base reaction. Static adsorption experiments of toluene on the synthesized polymeric products including the effect of humidity, initial toluene concentration and adsorption temperature will be investigated. After that, the adsorption characteristics, kinetic behavior and structure-activity relationship will be further analyzed and the cyclability of adsorbent will be evaluated to provide the basic data for the application of TPA-based CPPs in the field of gaseous toluene elimination.

Section snippets

Materials

4,4′4″-triaminotriphenylamine (TPA) was purchased from Tokyo Chemical Industry (Tokyo, Japan). 1,3,5-triformylbenzene (TFB) was purchased from J&K Chemical Technology (Beijing, China). 2,4,6-triformylphloroglucinol (TFP), sodium bromide (NaBr) and lithium chloride (LiCl) were supplied by Shanghai Titan Scientific Co.,Ltd. (Shanghai, China). Toluene in air gas as reference material was obtained from Sichuan Zhongbiao Technology Co., Ltd. (Chengdu, China) and mixed with air for preparing the

Characterization of the adsorbents

In order to gain a better insight into the structure-activity relationship between surface chemistry and pore structure with toluene adsorption capacity, two polymers, imine bond TFB-TPA and β-ketoenamine-linked TFP-TPA, were synthesized via Schiff base reaction. The chemical structure of the synthesized TFB-TPA and TFP-TPA was first investigated using FT-IR spectroscopy. As shown in Fig. 1(b), the attenuation of the aldehyde band (1696 cm⁻¹), amino band (3405 cm⁻¹, 3335 cm⁻¹) as well as the

Conclusions

Two porous TPA-based CPPs were successfully fabricated and investigated for their toluene adsorption performance. The functional groups and pore size distribution of the polymer have a significant impact on their adsorption performance. Taking advantage of the rich π aromatic framework, large specific surface, the hierarchically porous structure as well as the satisfactory chemical stability, the as-prepared TFB-TPA has acceptable adsorption performance for the removal of toluene under humid

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Lidan Lan: Conceptualization, Data curation, Writing - original draft. Yun Huang: Validation, Investigation. Yi Dan: Conceptualization, Methodology. Long Jiang: Methodology, Writing - review & editing.

Declaration of Competing Interest

There are no conflicts of interest to declare.

Acknowledgements

We acknowledge the National Natural Science Foundation of China (No. 51573109), the State Key Laboratory of Polymer Materials Engineering (sklpme 2019-2-08) and the Fundamental Research Funds for the Central Universities (No. 20826041C4193) for financial support of this research.

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Citation Excerpt :
The correlation (R2) of the kinetic fitting results can describe the degree of correlation between the adsorption process and the kinetic model. The relevant results showed that the R2 of the adsorption results fitted by the pseudo-first-order model of all samples (0.987, 0.978, 0.982, 0.980, 0.982 and 0.977, respectively) was higher than that of the pseudo-second-order model (0.982, 0.973, 0.979, 0.978, 0.980 and 0.973, respectively), indicating that the adsorption process is mainly physical adsorption [44]. The Bangham model is often used to describe channel diffusion processes [5,41–44].
HY zeolites are widely used as adsorbent for volatile organic compounds removal due to their acceptable stability and high specific surface area. However, low adsorption capacity of toluene over HY zeolites at humid conditions limits their industrial application. In present work, the hydrophobicity of HY zeolite modified by CeO₂ was enhanced successfully via a two-step process of high-temperature hydrothermal treatment and then acid treatment. It can be found that CeO₂ introduction can significantly increase the adsorption capacity of toluene over HY zeolites. Typically, HY with 2 wt% CeO₂ (2CeO₂/HY-del) presented a remarkable adsorption amount of toluene (166.3 mg/g_cat) and a satisfied regeneration performance at humid environment, which was higher than those of most reported modified Y zeolites (about 148.5 mg/g_cat). The suppression of dealumination and maintaining of specific surface area during the harsh two-step treatment could be mainly responsible for the appropriate toluene adsorption capacity over the HY zeolites. Additionally, dynamic adsorption analysis revealed that the adsorption process belonged to the first-order adsorption kinetics, indicating that the adsorption process was mainly defined as a physical process. This work would guide the development of high toluene adsorption capacity at humid conditions.

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Perspective ArticleConjugated porous polymers for gaseous toluene adsorption in humid atmosphere

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Characterization of the adsorbents

Conclusions

Data availability

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

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Perspective Article
Conjugated porous polymers for gaseous toluene adsorption in humid atmosphere