Novel CO2 Adsorbent Prepared with ZSM-5/MCM-48 as Support: High Adsorption Property and Its Mechanism

Abstract

Adsorption by solid amine adsorbent is a promising technology for decarbonization of flue gas. However, adsorption properties of many solid amine adsorbents need to be enhanced, and it is necessary to further study the CO2 adsorption mechanism. A novel CO2 adsorbent with high capacity was obtained by grafting 3-aminopropyltriethoxysilane (APTES) on a micro-mesoporous composite molecular sieve ZSM-5/MCM-48 as the support, and then impregnated with tetraethylenepentamine (TEPA) or polyethyleneimine (PEI). The maximum adsorption capacity of APTES-ZSM-5/MCM-48-TEPA-60 (A-ZM-T60), loaded with 60% (in mass) TEPA, for CO2 reaches 5.82 mmol·g–1 at 60 ℃ in 15% (in volume) CO2. Carbamate, alkyl ammonium carbamate and carbonate are generated during the chemical adsorption, which is dominant for CO2 adsorption because of the reaction between CO2 and amino groups on the adsorbent, simultaneously accompanied by weak physical adsorption. All above data confirm that these composites display an outstanding adsorption performance with a bright future for CO2 capture from flue gas after desulfurization.

1. Introduction

Carbon capture and sequestration is often considered as a key technology for decarbonization of the global energy system, with applications ranging from power generation to industrial production. Especially in the industrial sector, carbon capture and sequestration is currently the unique technology that can drastically reduce carbon emissions[1]. Up to now, three strategies have been proposed for capturing CO2: post-combustion, pre-combustion, and oxy-fuel CO2 capture[2]. Post-combustion capture equipment can be easily integrated into existing infrastructure without substantial changes to the basic combustion technology[3]. CO2 adsorption is one of the common post-combustion capture methods, which has the advantages of easy regeneration, low energy consumption, low equipment corrosion, and environmental friendliness[4]. Common solid adsorbents include activated carbon[5], metal-organic framework-based adsorbents[6], MgO-based adsorbents[7], and molecular sieves[8].

ZSM-5 with large surface area and total pore volume is more favorable for the physical adsorption of CO2 [9]. However, the small pore size of microporous molecular sieves would hinder the transport and diffusion of macromolecules[10]. This limitation has been addressed by using mesoporous molecular sieves as supports, which have larger pore size and can improve their gas diffusion performance[11]. MCM-48 is one of the extensively investigated mesoporous molecular sieves, which possesses the structural characteristics of high surface area, large pore volume and adjustable pore size, and has faster mass transfer velocity compared with other mesoporous materials[12]. Therefore, the adsorption capacity of micro-mesoporous composite molecular sieve for CO2 is higher compared with the single microporous or mesoporous material. In addition, less liable to pore blocking, MCM-48 provides easy access to guest molecules provided from amines[13].

Amino-modified adsorbents are commonly used to further improve the adsorption performance of CO2. The methods for preparing amine-modified adsorbents include impregnation[14], grafting[15], direct synthesis[16], and amino-bifunctionalization[17]. Among them, amino-bifunctionalization combines physical impregnation and chemical grafting, overcoming the disadvantages of lower adsorption capacity and amine utilization efficiency in a single method. The adsorbent prepared through amino-bifunctionalization exhibits high amino efficiency[18], which has attracted more attention. However, the adsorption properties of a lot of solid amine adsorbents are not satisfactory, and there is little research on their adsorption mechanism. In this work, the novel ZSM-5/MCM-48 (ZM) composite was firstly grafted with APTES and then impregnated with PEI or TEPA for amino-bifunctionalization, and the CO2 adsorption performance of amino-bifunctionalized composites was evaluated. In situ Fourier transform infrared (FT-IR) spectroscopy was used to characterize the adsorption products for exploring the adsorption mechanism.