Article
All-silica zeolites screening for capture of toxic gases from molecular simulation

https://doi.org/10.1016/j.cjche.2018.02.025Get rights and content

Abstract

The exhaust gases, including SO2, NH3, H2S, NO2, NO, and CO, are principal air pollutants due to their severe harms to the ecological environment. Zeolites have been considered as good absorbent candidates to capture the six exhaust gases. In this work, we performed grand canonical ensemble Monte Carlo (GCMC) simulations to examine the capability of 95 kinds of all-silica zeolites in the removal of the six toxic gases, and to predict the adsorption isotherms of the six gases on all the zeolites. The simulation results showed that, H2S, NO, NO2, CO and NH3 are well-captured by zeolite structures with accessible surface area of 1600–1800 m2·g 1 and pore diameter of 0.6–0.7 nm, such as AFY and PAU, while SO2 is well-adsorbed by zeolites containing larger accessible surface area (1700–2700 m2·g 1) and pore diameter (0.7–1.4 nm) at room temperature and an atmospheric pressure. However, at saturated adsorption, zeolites RWY, IRR, JSR, TSC, and ITT are found to exhibit better abilities to capture these gases. Our study provides useful computational insights in choosing and designing zeolite structures with high performance to remove toxic gases for air purification, thereby facilitating the development and application of exhaust gas-processing technology in green industry.

Introduction

Air pollution has received a widespread attention on account of the uncontrolled combustion and emission of fossil fuel in daily life, chemical industry and military war [1], [2]. The damages of ecological environment result in many severe respiratory and nervous diseases [3], [4]. Principal components in exhaust gases are NOx, SOx, and CO, which are hazardous compounds for human health [3], [5]. To solve the worsening ecological crisis, people have strived to develop novel technologies to capture the toxic gases for a post-processing of air purification [6]. Physical adsorption is one of the effective ways to prevent the release of toxic gases in the air due to its low energy consumption, convenient reuse of adsorbent and simple technological design [7]. Numerous studies on the physical adsorption of toxic gases, including NH3 [8], [9], [10], [11], [12], H2S [13], [14], [15], [16], SO2 [15], [17], NO [17], and CO [18], were reported.

Zeolites, which have been used broadly in chemical industry as adsorbents or catalysts, are promising materials for the separation or storage of gas due to their large surface area, abundant pore structure and high thermal stability [2], [19], [20], [21], [22]. Experimentally, the preparations of all zeolite materials and the measurements of adsorption capacity of the zeolites on toxic gases are time-consuming and expensive. Therefore, researchers conducted some non-experimental methods for material design, such as molecular simulation [15], [17], and classical density functional theory (CDFT) [23], [24]. To rationally design good zeolite structures to capture toxic gases, we conducted molecular simulations, which have achieved tremendous successes in gas adsorption and separation, to examine the adsorption behaviors of six toxic gases on 95 kinds of all-silica zeolites.

Numerous computational studies regarding adsorption and diffusion of small molecules on zeolites have been reported [16], [17], [18], [22], [25], [26], [27]. To remove the sulfur from a sulfur-containing mixed gas, Shah et al. [16] performed Gibbs ensemble Monte Carlo (GEMC) simulations to investigate the adsorption behaviors of binary mixtures containing H2S and CH4 on seven all-silica zeolites, viz. CHA, DDR, FER, IFR, MFI, MOR, and MWW. Their simulation results revealed that except for MOR, the other zeolites have an increasing selectivity in H2S as the H2S concentration increases due to favorable sorbate  sorbate interactions. Sun et al. [17] computationally studied the adsorption behaviors of SO2, NO2, and NO on four all-silica zeolites, viz. LTA, FAU, DDR and MFI, and found that the loading of the three gases on the four zeolites at 313 K and 100 kPa is ranked as SO2 > NO2 > NO. Many experiments have also been conducted to study the properties of different zeolites and use their excellent properties to apply to industry process [18], [21], [28], [29]. Maghsoudi et al. [29] investigated selectivities for H2S over methane on zeolite CHA, and their results show that this zeolite has good performance to remove acid gases, such as H2S, from methane. Therefore, it can be regarded as a promising candidate to be utilized as a zeolite membrane for the removal of acid gases from methane. And they found that all-silica zeolites could perform well to adsorb acid gases even in the presence of water due to their hydrophobic nature. By combining experiments and simulations, Matito-Martos et al. [2] investigated the capture of SO2, CO2 and CO by different zeolites as well as the separation processes of these gas mixtures. These studies much more concerned on selectivity of gases in several specific zeolites, however, to the best of our knowledge, there are rare results about screening large number of zeolites to remove toxic gases. By using both experiments and simulations, Matito-Martos et al. [2] screened zeolites for the removal of SO2.

In this paper, 95 all-silica zeolites were selected to remove six toxic gases, including H2S, SO2, NO, NO2, CO and NH3, through computational screening. We aimed to screen zeolites with high adsorptive performance and determined the relationship between the adsorption behavior and the zeolite structure.

Section snippets

Simulation Details

Structures of the 95 all-silica zeolites are from International Zeolite Association (IZA) [30] and their pores' geometry and topology are calculated using Zeo ++ codes [31]. The pore void space, which is crucial for the identification the type of zeolite structures can be determined through a given probe of 0.14 nm to access the surface of zeolites [31]. Relevant important parameters regarding zeolites, such as the diameter of the largest included sphere (Di), the largest free sphere (Df), and

Results and Discussion

The adsorbed behaviors of zeolites for six toxic gases were investigated, and the data was summarized in Supporting Information (SI2). In the following sections we chose two typical states to discuss the adsorption capacity of zeolites, i.e. atmospheric pressure adsorption and saturated adsorption. And more data from low pressure (about 0.01 kPa) to high pressure (> 1000 kPa) at 298 K are provided in Supporting Information (SI2 file). So the adsorption capacity of zeolites is still screened by any

Conclusions

We used molecular simulations to investigate the adsorption of H2S, SO2, CO, NO, NO2 and NH3 in 95 zeolites and screened top 10 zeolites for the removal of each gas at 298 K with 100 kPa or saturated pressure. Our calculations showed that AFY and PAU performed well for the removal of all selected gases at ambient temperature and pressure. Zeolites with good performance to remove all selected gases according to saturation adsorption are RWY, IRR, JSR, TSC and ITT. At ambient temperature and

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    Supported by the National Natural Science Foundation of China (21406172) and the Natural Science Foundation of Hubei Province, China (2016CFB388 and 2013CFA091).

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