ARTICLE pubs.acs.org/Langmuir
Adsorption, Desorption, and Thermodynamic Studies of CO2 with High-Amine-Loaded Multiwalled Carbon Nanotubes Fengsheng Su, Chungsying Lu,* and Hung-Shih Chen Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan ABSTRACT: Commercially available multiwalled carbon nanotubes (CNTs) were functionalized with a high mass load of 3-aminopropyltriethoxysilane (APTS) to study their behaviors in the cyclic CO2 adsorption as well as the associated thermodynamic properties. The breakthrough curve showed a fast kinetics of CO2 adsorption resulting in percentage ratios of working capacity to equilibrium capacity greater than 80%. The adsorption capacity of CNT(APTS) was significantly influenced by the presence of water vapor and reached a maximum of 2.45 mmol/g at a water vapor of 2.2%. The adsorption capacities and the physicochemical properties of CNT(APTS) were preserved through 100 adsorptiondesorption cycles displaying the stability of CNT(APTS) during a prolonged cyclic operation. The heat input required to regenerate spent CNT(APTS) was determined, and the result suggests that adsorption process with solid CNT(APTS) is possibly a promising CO2 capture technology.
1. INTRODUCTION Use of CO2 capture, utilization, and storage (CCUS) technologies on flue gas are considered to be a useful means of lessening global warning.1 Various CO2 capture technologies, including absorption, adsorption, cryogenics, membranes, and so forth, have been investigated.2 Among them, the design of a full-scale adsorption process might be feasible. Therefore, the development of a promising material that would adsorb CO2 with a high adsorption capacity and able to be regenerated with low energy input will undoubtedly enhance the competitiveness of an adsorptive separation system in a flue gas application.3 Low-temperature solid adsorbents reported in the literature4 include physical adsorbents (carbon-based materials,58 zeolites,915 and metalorganic frameworks1619) and amine-functionalized adsorbents.2025 Even through each family of adsorbents has its advantage, physical adsorbents were found to be unsuitable, mostly at high temperature and low pressure, and in the presence of moisture while the stability of amine-functionalized adsorbents has been questioned.4 Therefore, a continuous effort is being developed to avoid such disadvantages. Amine-functionalized carbon nanotubes (CNTs) have gained increasing attention for use in CO2 capture based on adsorption due to their unique physicochemical properties as well as high thermal and chemical stabilities.26 These CNT sorbents include single-walled CNTs27 and multiwalled CNTs28,29 functionalized with various amine solutions. Under similar operating conditions, the amine-functionalized CNTs not only possess good performance of CO2 adsorption as compared to the amine-functionalized high surface area activated carbon29 but also show low theoretical regeneration energy and superior cyclic stability.2931 Nevertheless, a high CO2 adsorption capacity at low CO2 partial pressure and relatively high temperature (e.g., 50 °C) is required to make them become suitable in the flue gas treatment.4,32 r 2011 American Chemical Society
This article therefore functionalized commercially available multiwalled CNTs with a high mass load of 3-aminopropyltriethoxysilane (H2NCH2CH2CH2Si(OCH2CH3)3, abbreviated as APTS) to enhance their adsorption selectivity of CO2 from gas streams in the 25100 °C. Cyclic CO2 adsorption on CNT(APTS) via a thermal/vacuum swing operation was conducted to evaluate their repeated availability during extensive adsorptiondesorption cycling. The regeneration heat input of the spent CNT(APTS) was also determined and compared with that of the rich CO230% monoethanolamine (MEA) mixture, which has been recognized as the most maturated CO2 capture process so far,2 to evaluate their practicability in the flue gas treatment.
2. EXPERIMENTAL SECTION 2.1. Preparation of the APTS Functionalized CNTs. Commercially available multiwalled CNTs (CTube-100) were provided by CNT Co. Ltd. in Korea. The physical properties of CNTs showed that specific surface area, specific pore volume, and average pore diameter respectively are 74.38 m2/g, 0.025 cm3/g, and 0.26 nm for pore sizes
