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Ind. Eng. Chem. Res. 2006, 45, 3248-3255
Applications of Pore-Expanded Mesoporous Silicas. 3. Triamine Silane Grafting for Enhanced CO2 Adsorption Peter J. E. Harlick and Abdelhamid Sayari* Centre for Catalysis Research and InnoVation (CCRI), Department of Chemical Engineering and Department of Chemistry, UniVersity of Ottawa, Ottawa, Ontario, K1N 6N5, Canada
Conventional MCM-41 and pore-expanded MCM-41 (PE-MCM-41) silicas have been used as supports for grafting 3-[2-(2-aminoethylamino)ethylamino]propyl trimethoxysilane (TRI) and tested for CO2 adsorption. The effects of the quantity of triamine silane added to the grafting mixture on the CO2 adsorption capacity and apparent adsorption rate have been examined. The results showed that when both supports were grafted under the same conditions, PE-MCM-41 was grafted with slightly larger quantities of amine than MCM-41, for all controlled silane additions. Based on the adsorption performance of the materials using a dry 5% CO2/N2 feed mixture, the optimal quantity of triamine silane added to the grafting mixture was determined to be ca. 3.0 cm3/g(SiO2), for both MCM-41 and PE-MCM-41. The CO2 adsorption capacity of TRI-PEMCM-41 was significantly higher than that of TRI-MCM-41. Furthermore, the dynamic adsorption performance of TRI-PE-MCM-41 was far superior to TRI-MCM-41. In comparison to 13X zeolite, TRI-PEMCM-41 exhibited higher adsorption capacities in the initial time frame of exposure, even though the 13X zeolite exhibited a higher equilibrium adsorption capacity. The result of this behavior is largely due to the rapid CO2-amine interaction and the open pore structure of TRI-PE-MCM-41 over that of the 13X zeolite. When these adsorbents were exposed to a humid stream of 5% CO2/N2 (28% relative humidity), both grafted materials exhibited a slight increase in the adsorption capacity, whereas, 13X zeolite did not retain any significant CO2 adsorption capacity. These results suggest that the TRI-PE-MCM-41 material may be most suitable for use in a rapid cyclic adsorption process under humid feed conditions. Introduction
Scheme 1. CO2 Reaction Pathways with Primary Amines
Although there are several compounds that contribute to the greenhouse effect, carbon dioxide has received the most attention, because of its abundance as an effluent in industrial processes. Therefore, because of the global concern over rising atmospheric temperatures, the recent literature has shown that there is a keen interest in developing materials and processes that can efficiently and economically capture and isolate the effluent CO2. Although the present state-of-the-art methods for CO2 removal allow for such a process to be applied, the economics of the process may not be favorable enough to offset the capture cost. Carbon dioxide scrubbing is currently practiced on a large scale for the purification of industrial gases (natural gas, syngas, etc.) and also in life support systems in confined spaces such as submarines, space vehicles, and other inhabited vessels for space exploration platforms). These processes use mainly alkanolamine aqueous solutions,1 the most common being monoethanolamines and diethanolamines (MEA and DEA) and N-methyl diethanol amine (MDEA). The process is reversible and can be represented as shown in Scheme 1. Because the reactions are exothermic, the formation of carbamate and bicarbonate is favored at low temperature, whereas their dissociation into amine and CO2 prevails at elevated temperatures.2 The use of aqueous solutions of low-molecular-weight alkanolamines suffers many drawbacks.3,4 Under typical scrubbing conditions, (i) a fraction of the amine and its decomposition products is lost by evaporation, which, in addition to reducing the absorption capacity, may cause problems because of their * To whom correspondence should be addressed. E-mail:
[email protected].
toxicity; (ii) the amine undergoes oxidative degradation, leading to decreased capacity, increased viscosity, and excessive foaming; and (iii) excessive corrosion occurs, thus posing severe operational problems. In addition to liquid-phase systems, there were attempts to use reactive solids or amine-impregnated solids, particularly for air revitalization in manned space shuttles.5-7 Some of the drawbacks of these impregnated materials were the quantity of amine that could be occluded. Because of the nature of the support material, only limited quantities could be loaded, and, thus, the materials did not exhibit sufficiently high adsorption capacities. In our previous work,8 we developed a material that could occlude a large quantity of amine (up to 7.5 mmol(N)/g, based on diethanol amine), and thus exhibit a high adsorption capacity. However, since the amine loading was achieved through impregnation, the material could only be applied in lowtemperature applications, i.e., 0.80 min. This behavior demonstrated the dynamic processing ability of the PE-MCM-41 support over the standard MCM-41 support. Furthermore, the desorption of CO2 (not shown) was also rapid and complete at 100 °C with a N2 purge (purge/feed ratio of 1.0). Typically,
