Carbon Dioxide Separation ... fuel burning, releasing large amounts of gases such as CO2 into the ... neutral template method7 using dodecylamine (DDA).
Hybrid Mesoporous Materials for Carbon Dioxide Separation Seamus W Delaney, Gregory P Knowles, Alan L Chaffee School of Chemistry, PO Box 23, Monash University, Victoria 3800, Australia Introduction Gas separation processes are becoming increasingly important in a world changing the way it looks at energy production and its emissions. Increased energy demand has led to an increase of fossil fuel burning, releasing large amounts of gases such as CO2 into the atmosphere. This is the major cause of several environmental phenomena, including the ‘greenhouse effect’1. Methods for long-term sequestration of CO2 have been proposed. Long term storage in used oil wells and deep ocean waters is being considered as an alternative to releasing CO2 into the atmosphere.2 There are also proposals to re-use CO2 produced from one process in another process such as dry reforming of natural gas.3 However, such applications would require CO2 to be ‘captured’ in concentrated form. One common method for CO2 separation from mixed gas streams is by absorption in aqueous solutions of alkanolamines4, for example monoethanolamine (MEA) or diethanolamine (DEA). It is widely accepted that CO2 becomes absorbed via the formation of both carbamates and bicarbonates. However, some of the problems associated with this absorption approach include: (a) the corrosive nature of the scrubbing solutions leading to the build up of corrosion by products; (b) vapourisation losses due to relatively high volatility of alkanolamines; and (c) energy intensive regeneration of the absorbent solutions, due to high heats of dissolution4. A recent report by Leal et al5,6 indicated that CO2 reversibly adsorbs on silica gel (Davidson grade 62) modified by the addition of 3-aminopropyl groups bonded to the surface. In this solid phase analogue of the liquid phase process the amine functional groups behave as active sites for CO2 chemisorption. Adsorption capacities of approximately 10 STP cm3 dry CO2 per gram of adsorbent (or 1.8% by weight) were achieved at room temperature and the CO2 could be desorbed at temperatures below 100ºC. The mechanism of adsorption appears to involve the formation of surface bound carbamates (see Figure 1). In dry CO2, this limits the adsorption capacity to 1 mole of CO2 for every 2 moles of surface bound amino groups. In the presence of water, however, the capacity for CO2 adsorption doubles since the possibility for proton transfer allows the carbon dioxide adsorption capacity to be doubled by the transformation of surface bound carbamate into bicarbonate.
Figure 1. Scheme of the surface reaction of carbon dioxide with modified HMS materials
The silica gel used by Leal et al has a substantial surface area (380 m2/g), but we reasoned that much improved CO2 adsorption capacities could be achieved with (a) the use of higher surface area support materials and/or (b) the use of surface modifying groups that contain more amine functional groups. In this study, solid phase hexagonal mesoporous silicas (HMS) of known porosity (pore diameter) were modified using aminopropyltrimethoxysilane and related compounds to provide very high surface area materials with varied concentrations of surface bound amine and hydroxyl functional groups (see Figures 2 and 3).
Figure 2. Modified HMS materials with increasing amine content employed in the present study.
Figure 3. Modified HMS materials with increasing hydroxyl group content employed in the present study. Experimental Preparation of HMS. HMS materials were prepared by the neutral template method7 using dodecylamine (DDA). The reaction involved the addition of tetraethyl-orthosilicate to the amine template in a water and ethanol solution, which was left to age at ambient temperature for 24 hours. The solvent was then removed from the crude "template filled" silica formed via evaporation under gentle airflow at ambient temperature. The template was subsequently extracted from the crude silica into hot ethanol. The remaining solids were then collected over filter, washed with hot EtOH and finally dried under vacuum at 150ºC to form an HMS product with a total pore volume of 0.53 mL/g. Preparation of Hybrid HMS. Five modified HMS materials were prepared as illustrated by Figures 2 and 3. Aminopropyltrimethoxysilane (APTS), aminoethyl-aminopropyl-trimethoxysilane (AEAPTS) and N-[3-(Trimethoxysilyl)propyl)diethylenetriamine (DAEAPTS) modified HMS were prepared in toluene solution (room temperature, 2h) using a published alkylsilylation procedure8.
Fuel Chemistry Division Preprints 2002, 47(1), 65
Ethylhydroxyl-aminopropyl-trimethoxysilane (EHAPTS) and diethylhydroxyl-aminopropyl-trimethoxysilane (DEHAPTS) modified HMS were prepared by N-alkylation of the APTS HMS using bromoethanol in ethanol solution (room temperature, 2h). The modified silica was then filtered and dried under vacuum at 150ºC. Characterization of Materials. N2 adsorption/desorption experiments were conducted at –196°C using a Micromeritics ASAP 2010 instrument. BET surface areas (SA) were determined from the adsorption isotherms over the partial pressure range 0.05 - 0.22. BJH pore size distributions were determined from the desorption isotherms. Mass % elemental N compositions were determined by CMAS P/L, Belmont Vic. Carbon Dioxide Adsorption. CO2 adsorption and desorption experiments were conducted on a Setaram Thermogravimetric analyzer (TGA). In a typical experiment, the adsorbent sample was heated to 150ºC (held 40 min) under flowing Ar (60 mL/min) to remove residual water. The sample was cooled to 20ºC before the gas flow was switched to CO2. CO2 adsorption capacities were determined from the weight increase observed after 75 minutes CO2 exposure. After 75 minutes, the gas flow was reverted to Ar so as to monitor the CO2 desorption. Approximately 90% of the adsorbed CO2 was desorbed immediately. The remaining CO2 was removed by heating the adsorbent to 65ºC. High purity CO2 (99.95%) was used with a manufacturer specified moisture content of
