Hierarchical structure ZSM-5/SBA-15 composite with improved hydrophobicity for adsorption-desorption behavior of toluene

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Highlights

  • Hierarchically structured ZSM-5/SBA-15 with different morphologies were prepared.

  • Multi-pores shortened diffusion pathway and increased toluene adsorption capacity.

  • The composites possess stronger hydrophobicity due to higher amount of Q4 groups.

  • The composites breakthrough time were increased by 2.5–4.6 times compared with ZSM-5.

  • ZSM-5/SBA-15-hexagonal prism exhibited the best adsorption performance of toluene.

Abstract

Hydrophobic zeolites have been considered as one of the most promising adsorbents for capturing volatile organic compounds (VOCs). In this contribution, hierarchically structured ZSM-5/SBA-15 composites with different morphologies including platelet, hexagonal prism, long-hexagonal prism, and long-rod were prepared. All materials were characterized using XRD, SEM, and TEM-EDX, which confirmed that the high dispersion of two zeolites within the composites. Due to the presence of intracrystalline mesopores that could shorten the diffusion pathway of toluene, the composites resulted in increased effective diffusion and higher adsorption capacity. The composites also possess higher hydrophobicity, leading to much better toluene adsorption performance under humid conditions. The water contact angle of ZSM-5 could be increased from 15.6° to 44.9° by forming well dispersed zeolite composites. The 29Si MAS NMR results confirmed that the composites possess higher amount of Q4 groups (75.3%–80.1%), which is the intrinsic reason for higher hydrophobicity. Notably, ZSM-5/SBA-15-hexagonal prism resulted in the best adsorption performance and cycling stability when compared with pure ZSM-5, the breakthrough time was increased from 20.8 to 49.4 min under dry conditions and from 5.9 to 27.3 min in the presence of 50% relative humidity. This work represents an important scheme for making hydrophobic zeolite-based VOCs adsorbents with also improved toluene diffusion properties due to the unique hierarchical porous structure.

Introduction

The rapid development of urbanization and industrialization have been leading to a sharp increase in volatile organic compounds (VOCs) emission over the past decades, and have received public concerns because of their health risks and great contribution to air pollution [1]. VOCs refer to all organic compounds with a boiling point in the range of 50 °C to 260 °C and a saturated vapor pressure more than 133.3 Pa at room temperature [2], [3]. In general, emissions of VOCs are extensively from the outdoor source (e.g., factories and industrial applications), and indoor source (e.g., application of paints, fuels, glues, cosmetics, stain removers) [4]. Moreover, VOCs can spread far from their pollution sources due to their strong evaporation and volatility of VOCs causing extensively direct and indirect influences on the environment and humans [5]. Especially, the emission of benzene compounds is most harmful to human health. Benzene, toluene and xylene have been all listed as priority pollutants by US EPA, which are strong carcinogens and hematotoxic organic compounds [6]. Therefore, it is imperative to develop available technologies to control VOCs. Recently, both adsorption and catalytic oxidation have been regarded as effective technologies for VOCs removal [7], [8], [9], [10]. Notably, adsorption method as one of the most promising technologies for cleaning VOCs has been established for many years [11], [12], [13], [14]. In the field of air purification in the industrial circumstance, the treatment of gas emission with VOC concentrations below 1000 ppm is an important and ongoing challenge with long-term environmental and economic relevance [15], [16], [17].

As the promising inorganic porous material, zeolite was widely used in catalysis, adsorption and other important fields, especially in VOCs adsorption control [18], [19], [20], [21]. Among the more than 200 types of existing zeolites, lots of them have been found to be effective under dry environment due to their high specific surface area, good thermal stability, relative low-cost production process, as well as the ability to easily exchange extra-framework cations [22], [23]. In fact, the emission of VOCs takes the ambient air as carrier gas, which will inevitably be accompanied by the existence of relative humidity. Therefore, the competitive adsorption of target VOCs and water molecules on the surface of adsorbent material must be considered. Due to the hydrophilicity of zeolites, the adsorption of VOCs by zeolites was found to be inefficient in practice under humid condition [16].

So far, many studies have been devoted to improving the hydrophobicity of zeolites to increase the VOCs adsorption performance under humid condition, which mainly include silylation modification [24], dealumination [25], and synthesis of core-shell composites [26], [27], [28]. For instance, Zhao et al. [24] modified MCM-41 by silylation using trimethylchlorosilane, and demonstrated that the hydrophilic MCM-41 with a type IV adsorption isotherm could be changed to highly hydrophobic with a type III adsorption isotherm after silylation modification. Cheng et al. [25] revealed that the trichloroethylene adsorption capacity of three dealuminated Y-type zeolites in the presence of steam decreased with the increase of surface cation density. Kang et al. [27] synthesized ZSM-5 nanoparticles coated with diverse thickness of microporous organic polymer. The water contract angles for pure ZSM-5 and the thickest ZSM-5@MOP increased from 0° to 152°, illustrating the induction of hydrophobicity by MOP coating. Liu et al. [26] prepared high alumina zeolite 13X coated with microporous or mesoporous silicas first and then silylated, by which the water contact angle can be increased from 0° to 90°. In addition, the hydrophobicity of zeolites can also be increased by vertically grown a layered double hydroxide layer. The toluene breakthrough time of such prepared core-shell composites could be enhanced from 6.4–10.8 min to 20.1–27.5 min comparing to pure zeolites [28].

Based on previous work, we believe that the preparation of well-mixed composite zeolites (i.e., ZSM-5/MCM-41 and ZSM-5/Silicalite-1), composing one microporous zeolite with high VOCs adsorption and one all-silicon zeolite with high hydrophobicity is another promising scheme for overcoming the water effect [29]. It is well accepted that the stability and adsorption property of SBA-15 is relatively higher than many other all-silicon zeolites (e.g., MCM-41 and Silicalite-1) due to the thicker silica walls and interconnection of mesopores with the existed micropores across the pore walls [30], [31]. SBA-15 can resist the effect of water molecule due to its all-silica property [32], [33]. In addition, the morphology of SBA-15 can be tailored by changing the synthetic temperature and reaction time [34], [35]. However, the influence of morphology on the toluene adsorption performance was not clear. Therefore, we used ZSM-5 seed as core and prepared ZSM-5/SBA-15 composites with different morphologies to structure the composite zeolites with better adsorption performance and reveal the influence of morphology on the adsorption performance and hydrophobicity of composite zeolites.

Section snippets

Materials

Sodium hydroxide (NaOH), tetrapropylammonium bromide (TPABr) and cetyltrimethyl ammonium bromide (CTAB) were acquired from Shanghai Macklin Biochemical Co., Ltd.; hydrochloric acid (HCl) was purchased from Beijing Chemical Works; tetraethylorthosilicate (TEOS) was bought from Aladdin; polyethylene oxide–polypropylene oxide–polyethylene oxide (P123) and colloidal silica were obtained from Aldrich Chemical Reagent Company.

Adsorbents synthesis

Preparation of hierarchically structured ZSM-5/SBA-15 composites was

Structure and morphology of the adsorbents

The XRD patterns, N2 adsorption-desorption and TEM image of ZSM-5 seed with different crystallization times were shown in Fig. 1. Fig. 1(a) shows that all the ZSM-5 seeds (crystallization time more than 12 h) appeared typical XRD patterns of MFI crystal structure, with the characteristic reflections of (1 0 1), (0 2 0), (5 0 1), (1 5 1) and (3 0 3) planes were observed [36]. Fig. 1(b) and (c) depicted the N2 adsorption-desorption isotherms and pore size distribution. The isotherms of ZSM-5 seed

Conclusion

Hierarchically structured ZSM-5/SBA-15 composites with platelet, hexagonal prism, long-hexagonal prism, and long-rod morphologies were synthesized and investigated for toluene adsorption and desorption. XRD, SEM, and TEM-EDX analyses confirmed that the ZSM-5 seeds are highly distributed within the composites. Adsorption results indicated that the adsorption property of toluene on all ZSM-5/SBA-15 composites without water vapor was significantly improved due to the presence of intracrystalline

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.

Acknowledgments

We gratefully acknowledge the Fundamental Research Funds for the Central Universities (2019YC17), the National Natural Science Foundation of China (U1810209), the International Science and Technology Cooperation Project of Bingtuan (2018BC002), and the Beijing Municipal Education Commission for their financial support through Innovative Transdisciplinary Program “Ecological Restoration Engineering”.

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