Sequential improvement of activated carbon fiber properties for enhanced removal efficiency of indoor CO2

https://doi.org/10.1016/j.jiec.2020.06.011Get rights and content

Highlights

  • A new carbon based adsorbent (ACF) was developed.

  • Low level CO2 of indoor spaces could be reduced by an activated carbon fiber.

  • Chemical modification enhanced CO2 adsorption capacity.

Abstract

For effective capture of low-level indoor CO2, a polyacrylonitrile (PAN) solution was doped with potassium hydroxide (KOH) before electrospinning to increase the specific surface area and microporosity of the resulting activated carbon nanofiber (ANF). Additionally, the KOH-doped ANF was treated with tetraethylenepentamine (TEPA) in ethanol to introduce nitrogen functionalities favorable for CO2 adsorption on the surface. The sequential improvements of the physical and chemical properties of ANF were carried out at a PAN: KOH mass ratio of 1:0−0.05 and TEPA solution concentrations of 1, 2, and 3%. The effects of KOH and TEPA on the structure and chemical properties were investigated using a Monosorb instrument, a field-emission scanning electron microscope, a thermogravimetric analyzer, and an x-ray photoelectron spectrometer. In terms of the textural improvements resulting from KOH doping, a sample of 0.03-ANF showed a significant increase in specific surface area: from 94.59 for pristine ANF to 469.09 m2/g after treatment. The XPS(X-ray photoelectron spectroscopy) examinations also showed a large increase in the content of the nitrogen-containing functional groups on the sample treated with TEPA, thereby increasing the selective adsorption capacity of 0.3% CO2 by 21-fold. The combination of textural enhancement by KOH impregnation and surface chemistry enhancement provided by the TEPA doping improved the low-level CO2 capture capability of ANF.

Introduction

Aside from being the main anthropogenic contributor to global warming, CO2 is harmful at a relatively high concentration, especially in confined indoor spaces such as offices, schools, subways stations, cars, airplanes, and submarines [1], [2]. According to the survey of Park et al. [1] and Satish et al. [3], school classrooms frequently exceed 1000 ppm of CO2 and sometimes 3000 ppm. An office meeting room has shown 446−1452 ppm of CO2 with normal ventilation, and increased the maximum concentration to 4917 ppm in the absence of ventilation [4]. The CO2 level inside the subway cabins, which are not crowed, ranges from 550 ppm to 2680 ppm [2]. Monitoring and capturing of CO2 at an indoor concentration level of 1000 ppm or higher, as set by the Environmental Protection Agency (EPA), require efficient technologies for long-duration human operations in confined spaces [5].

Among various techniques for CO2 capture, dry adsorption is the only feasible process for public spaces. A high affinity for CO2 molecules, hydrophobicity, sanitary safety, mechanical stability and easy regeneration are required to be useful in indoor air control. This process should operate under room temperature and ambient conditions. In the past two decades, researchers have intensified efforts to develop adsorption technology for CO2 removal at room temperature. Popular among the approaches is use of amine-based or amine-functionalized adsorbents. These adsorbents are obtained by tethering amine groups onto a support through a chemical reaction known as grafting or by impregnation and immobilization of liquid amines onto the pores and external surfaces of a support [6]. Adsorbents engineered by impregnation have some advantages over those produced by grafting including easier preparation, reduced corrosiveness, and higher CO2 adsorption capacity [7]. Impregnation of amines is mainly a wet process whereby the amine is physically adhered onto a support via non-covalent attachment [8]. Jian Jiao et al. [9] investigated the effects of three types of organic amines, diethylenetriamine, triethylenetetramine, and tetraethylenepentamine (TEPA) on CO2 adsorption capacity. The results showed that TEPA with higher molecule weight provided more amino groups, which resulted in higher CO2 adsorption capacity. In addition, TEPA has the advantages of low cost and low toxicity. To achieve optimal wet impregnation, the support should have certain characteristics in terms of surface area, pore size, pore structure, and surface pH. Generally, a porous support with relatively high specific surface area and large pore volume has good potential for organic amine immobilization [9]. TEPA is a linear molecule containing two primary amines (RNH2) and three secondary amines (R2NH). The mechanism by which amine reacts with CO2 was shown in Eqs. (1), (2), (3) and (4) [9].CO2 + 2RNH2 ↔ RNH3+ + RNHCOO¯CO2 + 2R2NH ↔ R2NH2+ + R2NCOO¯CO2 + RNH2 + H2O ↔ RNH3+ + HCO3¯CO2 + R2NH2 + H2O ↔ R2NH2+ + HCO3¯

However, moisture was found to play a promoting effect on adsorption of high-concentration CO2 compared to low-concentration CO2 removal [10]. In addition, another study reported that CO2 was more highly adsorbed on TEPA film prior to H2O when exposed to a flow of CO2/H2O [11]. In addition, for indoor gas capture, hydrophobic solids adsorbents are better in general.

Grafting is the conventional method adopted when TEPA is used to basify the surface of activated carbon materials. However, in this study, the impregnation method was used to improve selective CO2 adsorption at low concentrations. Aside from being a simpler and less expensive option, impregnation is potentially a more effective approach for amine loading on a carbon-based support [12].

Our previous work showed that potassium hydroxide (KOH) activation for carbon fiber is an efficient technique to develop ultra- or super-micropores with a pore diameter less than 2.0 nm [13]. This was achieved by opening graphitic layers and inducing irregularities by scratching their surfaces [10]. In the current work, we strive to obtain a high specific surface area of activated carbon nanofiber (ANF). The ANF was prepared by electrospinning and was then doped with KOH by wet impregnation. Finally, TEPA was immobilized on ANF to incorporate basic amine groups. Treatment with KOH and TEPA led to textural improvements in surface area and pore structure as well as chemical enhancement in the basic nitrogen functionalities on ANF. These alterations improved the adsorbent’s CO2 adsorption capacity.

Section snippets

Materials

Polyacrylonitrile (PAN) and TEPA (reagent grade) were purchased from Sigma-Aldrich, Co. in Korea. N, N-dimethylformamide (DMF), KOH, and absolute ethanol were procured from a local company (Daejung Chemicals & Metals Co., Seoul, Korea), and all reagents were used as received.

Preparation of PAN-based carbon nanofibers

The polymer solution to be electrospun was prepared by dissolving PAN and KOH in DMF at a weight ratio (PAN: DMF: KOH) of 1:9:0∼0.05 g. Each mixture was stirred gently for 24 h at room temperature to ensure homogeneity. Each

Effect of KOH on surface texture of activated carbon fibers

KOH activation is an effective method to increase the SBET and pore volume of carbonaceous materials. This is achieved by a series of reactions (Eqs. (6) to (10)) that occur when KOH-doped fibers are exposed to high temperatures [15]:6KOH + 2C ↔ 2 K + 3H2 + 2 K2CO3K2CO3 + C ↔ K2O + 2COK2CO3 ↔ K2O + CO22 K + CO2 ↔ K2O + COK2O + C ↔ 2 K + CO

Instead of conventional KOH activation, specific amounts of KOH and PAN were simultaneously dissolved in DMF as an electrospinning solution in this experiment. During

Conclusions

A new KOH treatment method was developed to improve the physical properties of ANF compared to the conventional method of activation, thereby saving time and achieving ANF with a high specific surface area. When 0.03 g-KOH was added per 1 g-PAN, we obtained ANF with 469 m2/g specific surface area and 2.68 mmol/g adsorption capacity for pure CO2. These values are 4.96 and 1.37 times higher, respectively, than those of pristine ANF. The basic nitrogen-containing functional groups with high affinity

Conflict of interest

No conflict of interest exists.

Funding

No funding was received for this work.

Intellectual property

We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

Research ethics

We further confirm that any aspect of the work covered in this manuscript that has involved human patients has been conducted with the ethical approval of all relevant bodies and that such approvals are acknowledged within the manuscript.

IRB approval was obtained (required for studies and series of 3 or more cases)

Written consent to publish potentially identifying information, such as details or the case and photographs, was obtained from the patient(s) or their legal guardian(s).

Authorship

All listed authors meet the ICMJE criteria. We attest that all authors contributed significantly to the creation of this manuscript, each having fulfilled criteria as established by the ICMJE.

We confirm that the manuscript has been read and approved by all named authors.

We confirm that the order of authors listed in the manuscript has been approved by all named authors.

Acknowledgement

This work was financially supported by the National Research Foundation of Korea grant funded by the Korea government (MSIT, MOE) (No. 2019M3E7A1113077), and partially by basic research fund (NRF-2015R1D1A1A01060182).

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