Amino-impregnated MOF materials for CO2 capture at post-combustion conditions
Graphical abstrct
Introduction
Nowadays, the combustion of fossil fuels at coal power plants is significantly contributing to the increase of atmospheric CO2 concentration with a widespread concern about its influence on the climate change over the last decades (Quadrelli and Peterson, 2007, Pachauri and Reisinger, 2007, Pachauri and Reisinger, 2007). This fact has motivated many research efforts towards the reduction of carbon dioxide emissions by means of chemical fixation, capture and storage (Yang et al., 2012). Currently, one of the most promising alternatives has been the separation of CO2 from exhausted flue gases in order to obtain CO2 streams of high purity for storage, also so-called post-combustion carbon capture (Figueroa et al., 2008). The low CO2 concentration of exhausted flue gases (ca. 15% in vol. in dry basis) makes necessary highly selective technologies for carbon dioxide separation such as absorption and adsorption processes (Granite and Pennline, 2002).
Absorption processes using aqueous solutions of amino compounds like monoethanolamine (MEA), diethanolamine (DEA) or methyldiethanolamine (MDEA), have been highly efficient for CO2 capture due to the strong chemical interaction between basic amino groups and acidic CO2 molecules (Danckwerts, 1979, Caplow, 1968, Rochelle, 2009). However, absorption processes have some important drawbacks, such as the large amount of energy required for regeneration of the absorbent,, the degradation of the absorbent due to the presence of oxygen, the corrosion generated in the equipment and the necessity to use operation temperatures below 50 °C to reduce the significant volatility of the absorbent (Rochelle, 2009). As result of these constraints, the adsorption of CO2 over solid adsorbents with high adsorption capacity, selectivity and easy regeneration, can be considered a promising alternative. Typical solids adsorbents such as zeolites, activated carbons or hydrotalcytes have shown low CO2 adsorption capacity and selectivity for the exhausted flue gases conditions, low CO2 partial pressure (ca. 0.15 bar) and temperatures higher than 25 °C (Li et al., 2009, Martin et al., 2010). In this context, the incorporation of basic amino-containing molecules over the adsorbent surface has been recently proposed in order to improve their adsorption capacity and selectivity at those conditions of flue gases.
Mesostructured silica materials have been widely functionalized with amino-based molecules due to its large pore diameters and the presence of reactive silanol groups on their surface, that enable the anchoring of aminated organosilanos like aminopropyl-trimethoxysilane or diethylenetriamino-trimethoxysilane (Hiyoshi et al., 2004, Harlickand and Sayari, 2007). More recently, metal-organic frameworks (MOFs) as hybrid organic–inorganic materials have been also explored for post-synthetic modification with amino-based molecules. Porous MOFs materials, characterized by metal clusters linked to organic molecules with tridimensional crystalline frameworks, provide also multiple reactive sites for immobilization of amino-containing molecules (Furukawa et al., 2010, Farha et al., 2012, Zhou et al., 2012). In this sense, MOF materials functionalized with unsaturated and exposed metal sites have been modified by grafting of amino-based molecules such as N,N-dimethylethylendiamine (mmen-Mg2(dobpdc)), tetraethylenepentamine (TEPA-Mg-MOF-74) and pentaethylenehexamine (PEHA-MIL-101) (McDonald et al., 2012, Anbia and Hoseini, 2012, Cao et al., 2013).
One of the main drawbacks of amino-grafting methods is the limitation of the reactive sites of the porous frameworks. By this reason, another interesting alternative is the use of impregnation methods by physical interactions. These interactions could be too labile, leading to significant losses of the amino molecules during the adsorbent regeneration as attested for silica mesoporous materials (Sanz-Perez et al., 2013, Tanthana and Chuang, 2010). However, the use of MOFs materials with hybrid inorganic–organic microporous networks could generate strong superficial interactions as well as restricted diffusion of the amino molecules during regeneration depending on the chemical composition of the structural framework. These facts might assure a stable confinement of the amino containing molecules for the typical adsorption/desorption conditions. Recently, MIL-101 impregnated with polyethyleneimine have shown very promising results for adsorption of CO2 at post-combustion conditions (Yan et al., 2013, Lin et al., 2013a).
Herein we present the incorporation of tetraethylenepentamine (TEPA) in three structurally different microporous MOF materials (HKUST-1, MIL-53(Al) and ZIF-8) by wetness impregnation, which has not been studied to the best of our knowledge. The main objectives have been to assess the capacity of MOF materials for hosting TEPA as amino-containing molecule and the application for CO2 post-combustion capture by combination of physical adsorption into the microporous channels of the MOF supports and chemical adsorption onto the amino groups. The studied MOF materials were selected considering the presence of unsaturated metal sites (HKUST-1) and flexible structures (MIL-53(Al)) in metal-carboxylated based-MOF materials, and MOF materials based on nitrogen-containing imidazole linkers such as ZIF-8 with an isomorphic zeolitic framework. A priori, these features seem to be interesting for favoring the attachment of amino molecules to the MOF surface. Moreover, these MOF materials are characterized by remarkable thermal stability and commercial availability (Park et al., 2006, Küsgens et al., 2009, Wang et al., 2002, Loiseau et al., 2004, Yu et al., 2012, Yazaydin et al., 2009). TEPA was chosen due to the high amino-content and linear molecular geometry (ca. 16.5×3.0 Å), which makes possible its diffusion across the microporous channels of MOF supports. Moreover, its low cost and toxicity were also decisive for its selection.
Section snippets
Experimental
Three different commercial Metal-Organic Frameworks have been used as supports for tetraethylenepentamine impregnation: Basolite®C300, Basolite®A100 and Basolite®Z1200 (Sigma-Aldrich). These samples correspond to HKUST-1, MIL-53(Al) and ZIF-8 materials respectively. The rest of reagents were commercially supplied and used without further purification.
Assessment of TEPA impregnated MOF materials
Amino impregnated MOF materials with TEPA loadings equivalent to 25% of pore volume were initially evaluated in order to study the effect of TEPA incorporation on the structural and textural properties of selected MOF materials. Commercial MOFs used as supports were previously characterized by powder XRD, nitrogen adsorption/desorption isotherms and thermogravimetric analyses (Figs. S1–S3 of SI) to confirm their characteristic structures, microporous type I isotherms and thermal stability.
Conclusions
The incorporation of amino-containing molecules as TEPA over HKUST-1 and MIL-53(Al) was not successful for enhancing the CO2 adsorption capacity of microporous MOF materials. The main reason was the interaction of amino groups of TEPA molecules with unsaturated copper sites of HKUST-1 or hydroxyl bridges (µ2-OH) of MIL-53(Al). This fact produced a decrease of pore volume and specific surface area, which influence the physical CO2 adsorption of the microporous materials as well as the
Acknowledgments
The authors wish to thank Spanish Ministry of Economy and Competitiveness for their financial support to the CICYT project (CTQ2012-38015).
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