Facilely synthesized mesoporous polymer for dispersion of amino acid ionic liquid and effective capture of carbon dioxide from anthropogenic source

Highlights

• Facilely synthesized mesoporous PDVB was proposed as support of AAILs for CO₂ capture.

• [N₂₂₂₂][Gly] was synthesized and dispersed in mesoporous PDVB by physical impregnation.

• [N₂₂₂₂][Gly]@PDVB samples show strong ability to adsorb CO₂ from anthropogenic source.

• [N₂₂₂₂][Gly]@PDVB samples also show good selectivity and reversibility for CO₂ adsorption.

Abstract

Dispersing amino acid ionic liquids (AAILs) in porous supports is an effective method to overcome the viscous and corrosive issues of AAILs. Herein, a facilely synthesized mesoporous polymer—polydivinylbenzene (PDVB) was proposed as the support of AAILs for CO₂ capture. A simple AAIL—tetraethylammonium glycinate ([N₂₂₂₂][Gly]) was synthesized and then dispersed in PDVB by physical impregnation. The prepared [N₂₂₂₂][Gly]@PDVB samples were systematically characterized and examined for CO₂ capture performance. Owing to the large pore volume and rich mesoporisty of PDVB, the CO₂ capacities of [N₂₂₂₂][Gly]@PDVB samples are very impressive, with the highest value of 1.55 mmol/g at 75 °C for 10 vol.% of CO₂ balanced in N₂. The utilization ratios of dispersed [N₂₂₂₂][Gly] at 75 °C approach the theoretical value of 0.5, suggesting the efficient utilization of [N₂₂₂₂][Gly] for the capture of CO₂. The adsorption selectivity and reversibility of [N₂₂₂₂][Gly]@PDVB samples are also very good, justifying their promising application in the capture of CO₂ from anthropogenic source.

Introduction

The carbon cycle of the earth has been broken for centuries due to the anthropogenic emission of carbon dioxide (CO₂), especially energy harvesting process [1,2]. As a result, the concentration of CO₂ in the atmosphere has risen to an unprecedent level. CO₂ is a kind of greenhouse gas, and its accumulation in the atmosphere may lead to many climatic and environmental problems, including global warming, sea level elevating, green land degradation, etc.. Given this situation, the capture of CO₂ from anthropogenic source is proposed as a solution [3]. Currently, the most widely employed method for CO₂ capture in the industry is scrubbing with aqueous alkanolamines [4]. However, aqueous alkanolamines are very volatile, making the loss of solvents and consumption of energy considerably high. Besides, aqueous alkanolamines are very corrosive, making the investment on facilities and cost for maintenance also considerably high. Therefore, it is of great significance to explore alternatives to aqueous alkanolamines for application in CO₂ capture.

In recent years, ionic liquids (ILs) have attracted widespread interest as a new class of green solvents [5,6]. They are organic salts with the melting points at around or below room temperature, and have unique characteristics such as wide liquidus range and extremely low volatility. One of the most promising applications of ILs is in gas separation [7], [8], [9], [10], [11], [12], which provides an opportunity to address the volatile issue associated with traditional solvents. At the end of 20th century and beginning of 21st century, researchers determined the solubilities of CO₂ in many ILs [13], [14], [15], [16], [17]. It was found that the solubilities of CO₂ in ILs are much higher than those of other inert gases such as N₂ and CH₄, which establishes the fundament of CO₂ capture with ILs. However, normal ILs enable only physical absorption of CO₂, which is unsatisfying for the capture of CO₂ from anthropogenic source, the content of CO₂ in which is normally low [3]. Therefore, ILs enabling reversible chemical absorption of CO₂ are highly required.

Another interesting characteristic of ILs is adjustable chemical structure, because the combinations of anions and cations for ILs can be changed, and various functional groups can be grafted to the frameworks of ILs [18]. Based on this characteristic, researchers developed many kinds of functionalized ILs enabling reversible chemical absorption of CO₂, such as carboxylate-based ILs [19,20], amine-functionalized ILs [21], [22], [23] and aprotic heterocyclic anion-based ILs [24], [25], [26], [27]. As a subcategory of amine-functionalized ILs, amino acid ILs (AAILs) are particularly attractive because of the low cost and biocompatibility of amino acids [28,29]. However, the practical application of AAILs in CO₂ capture is still greatly limited by their high viscosities. In addition, AAILs suffer from dramatical increase in viscosities after CO₂ absorption. The viscous issue makes the capture of CO₂ by AAILs very slow, and the transportation of AAILs in pipelines very difficult. To address this issue, researchers diluted AAILs in other low-viscous solvents for CO₂ capture [30,31]. Nonetheless, the possible employment of volatile solvents diminishes the advantage of AAILs with extremely low volatility, and the corrosive issue associated with AAILs is still not addressed.

Instead, AAILs can also be dispersed in porous supports to overcome the viscous and corrosive issues of AAILs [32,33]. With such method, the capture of CO₂ by AAILs can be enhanced by increasing the efficiency for contact between AAILs and CO₂. Moreover, the transportation of AAILs in pipelines, and the direct contact of AAILs with metal facilities are avoided. The performance of dispersed AAILs for CO₂ capture is greatly dependent on the porous structure of supports. In this regard, supports with rich mesoporosity and large pore volume are most preferred, because the rich mesoporosity is favorable for the good dispersion of AAILs, and the large pore volume is favorable for the accommodation of more AAILs. Besides, the synthesis of AAILs and porous supports should be as simple as possible from the perspective of practical application.

In this work, a mesoporous polymer−polydivinylbenzene (PDVB)−that can be facilely synthesized by a one-step solvothermal route without the use of any templates [34], was used as the support of AAILs for CO₂ capture. A simple AAIL−tetraethylammonium glycinate ([N₂₂₂₂][Gly]) was synthesized for dispersion in PDVB by physical impregnation according to the literature [35], [36], [37], [38], [39]. The chemical structures of PDVB and [N₂₂₂₂][Gly] are shown in Scheme S1. The prepared [N₂₂₂₂][Gly]@PDVB samples were then systematically characterized and investigated for CO₂ capture performance. To simulate the anthropogenic source of CO₂ emission, low-content CO₂ balanced in N₂ was used for investigation.