Study On Functionalization Of Porous Materials For CO₂Capture

CO2capture by adsorption is one promising method. Recently, aqueous aminesimmobilized in/on mesoporous silica adsorbents have attracted considerable attention due totheir high efficiency and selectivity for CO2capture from a gas mixture. Specifically, orderedmesoporous materials offer structural characteristics such as large pore size, high surface area,and a large number of highly dispersed active sites on the pore surface, which facilitate thedistribution of amines throughout the pore space, thereby promoting the CO2adsorptioncapacity of the adsorbents. There are two main preparation methods to obtain the supportedamine sorbents:(i) porous supports impregnated with liquid organic polymer such aspoly-(ethyleneimine)(PEI) via the wet impregnationmethod and (ii) amines that could becovalently linked to a solid support via the use of silane chemistry.“Wet impregnation” ofmesoporous silica with amines could be a simple method to obtain a high capacity adsorbent.Several SBA-15silica materials with different pore structures were synthesized andfunctionalized with poly(ethyleneimine)(PEI). The results showed that the CO2adsorptioncapacity linearly increased with the total pore volume of the SBA-15phases in the testedtemperature range (R2>0.94). Temperature also showed a strong influence on CO2adsorptioncapacity. SBA-15material with the largest pore volume (1.14cm3/g) exhibited the largestCO2adsorption capacity (105.2mg/g adsorbent) with15.1v/v%CO2in N2at75oC andatmospheric pressure. Pore size was found not to be the main factor influencing the CO2adsorption capacity of these PEI-modified SBA-15materials. Adsorption-desorption cycles(12) revealed that the adsorbents with PEI loaded inside the pore channels were found to bequite stable, as they retained their CO2adsorption capacity with many cycles.Mesoporous silica SBA-15samples with and without controlled framework microporositywere prepared and used directly or impregnated with polymer amine as adsorbent for CO2.These samples were evaluated for their ability to adsorb CO2by obtaining their equilibriumadsorption isotherms using volumetric adsorption at three different temperatures of0oC,5oCand10oC. The data obtained were analyzed using Freundlich adsorption isotherm modelwhile the isosteric heats of adsorption were estimated by the Clausius-Clapeyron equation.Under comparable conditions, the adsorption performance of silica SBA-15was found to be strongly dependent upon the framework microporosity. However, the microporositycontribution to CO2adsorption in amine-modified SBA-15was not obvious as amine blockedmicropores and dominated CO2adsorption. The SBA-15sample with higher microporositydisplayed higher CO2uptake and the CO2uptake by amine-modified SBA-15samplescorrelated with their total surface areas, as expected. These findings revealed the importanceof surface area in designing an adsorbent for CO2.Three mesocellular silica foam (MCF) materials with different window sizes were preparedand functionalized with polyethyleneimine (PEI) for use as CO2adsorbents. CO2capture wasperformed in a fixed bed reactor operated at atmospheric pressure. The results showed that theCO2adsorption capacity increased with the window size of the MCF substrates. MCFmaterial that had the largest window size exhibited the largest CO2uptake of152.0mg/g ofadsorbent (304.0mg/g of PEI) with a50wt%PEI loading under the conditions of15.1%(v/v)CO2in N2at75C and atmospheric pressure. It is one of the highest capture capacities pergram of PEI reported in the literature thus far under the above conditions used. Repeatedadsorption/desorption cycles revealed that the MCF modified by PEI is a good adsorbent forCO2with good cyclic stability.Amine-functionalized clover leaf-shaped Al2O3extrudates (CA) were prepared for use asCO2sorbent. The as-synthesized materials were characterized by N2adsorption, XRD, SEMand elemental analysis followed by testing for CO2capture using simulated flue gascontaining15.1%CO2. The results showed that a significant enhancement in CO2uptake wasachieved with the introduction of amines into CA materials. A remarkably high volume-basedcapacity of70.1mg/mL of sorbent of this hybrid material suggests that it can be potentiallyused for CO2capture from flue gases and other stationary sources, especially those with lowCO2concentration. The novel adsorbent reported here performed well during prolongedcyclic operations of adsorption–desorption of CO2.As-prepared metal–organic framework (MOF), Cu3(BTC)2(HKUST-1) was treated usingdifferent solvents with or without an inorganic salt. The CO2uptake capacity of the sampletreated by ethanol and ammonium chloride at65oC showed the highest CO2adsorptioncapacity of11.6mmol/g at0oC and1atm CO2pressure, which is a drastic increase of61%compared to the original MOF sample. The modified MOF sample is not only an excellentCO2adsorbent but also exhibited good cyclic stability and it can be easily regenerated.