DOI: 10.1002/cssc.201300973 Synthesis and Carbon Dioxide Sorption of Layered Double Hydroxide/Silica Foam Nanocomposites with Hierarchical Mesostructure Liling Fu, [a] Genggeng Qi, [a] Osama Shekhah, [b] Youssef Belmabkhout, [b] Luis Estevez, [a] Mohamed Eddaoudi,* [b] and Emmanuel P. Giannelis* [a] Layered double hydroxides (LDHs) with a hierarchical meso- structure are successfully synthesized on mesoporous silica foams by simple impregnation and hydrothermal treatment. The as-synthesized LDH/silica foam nanocomposites show well-defined mesostructures with high surface areas, large pore volumes, and mesopores of 6–7 nm. The nanocomposites act as carbon dioxide (CO 2 ) sorbents under simulated flue gas conditions. They also exhibit significantly enhanced CO 2 ca- pacities under high-pressure conditions and high CO 2 /N 2 and CO 2 /CH 4 selectivities. Layered double hydroxides (LDHs) are an important class of in- organic compounds with a well-defined layered structure. They have been extensively studied for many applications including adsorption, [1] ion exchange, [2] and catalysis [3] due to the high mobility of interlayer anions and water molecules. Among dif- ferent LDHs, Mg-Al-LDHs have attracted much attention be- cause of their high basicity, ease of synthesis, and low cost. [4] These features make them good candidates for carbon dioxide capture and separation. Unfortunately, most of the systems in- vestigated to date exhibit relatively low CO 2 adsorption capaci- ties (usually less than 0.5 mmol g À1 ). Strategies to overcome these limitations have included modifying the composition of the LDHs, that is, changing the type and amount of lattice cat- ions or gallery anions, [5] or incorporating doping elements (e.g., alkali metals). [6] Nevertheless, these materials lack high surface areas, which is a prerequisite for separation agents. An alternative strategy is based on controlling the structure and morphology of LDHs (e.g., particle size and surface area). [7] Sev- eral hierarchical LDH structures, including 2D thin films, [8] 3D nanostructures, [9] core/shell structures, [10] and hollow spheres, [11] have been developed for specific applications. However, these structured LDHs are usually synthesized using a hard template or a surfactant, and the need for multiple reaction steps, tem- plate removal, or surfactant toxicity have limited their practical applications. Recently, we reported a simple and cost-effective method to synthesize silica foams with large mesopores (cell size: ca. 50– 120 nm). [12] In addition, we have demonstrated that the silica foam can be used as a template to synthesize mesoporous car- bons [13] and as a support for depositing metal–organic frame- works. [14] These results inspired us to use the silica foam as a mesoporous matrix for other materials, including LDHs. Mesoporous silica/LDH nanocomposites prepared by various synthesis routes have been demonstrated for catalysis [15] and for delivery systems. [16] For example, an LDH/SBA-15 nanocom- posite was developed via in situ assembly as a high-efficiency catalyst for aldol condensation reactions. [15b] Nanometer-scale “rattles” with an LDH core and mesoporous silica shell struc- tures showed controlled release profiles as a delivery vehicle. [16] However, these nanocomposites have relatively low surface areas (less than 350 m 2 g À1 ) and limited pore volume. Herein, we report a simple and scalable approach to synthe- size mesoporous LDH nanocomposites with high surface areas and large mesopores by using a mesoporous silica foam. The newly developed mesoporous LDH/silica foam nanocompo- sites show very promising results for CO 2 capture under simu- lated flue gas conditions and for CO 2 separation from N 2 and CH 4 with enhanced capacity and high selectivity compared to bulk LDHs. Our approach involves impregnating the silica foam with the appropriate LDH precursor, calcination, and recon- struction under hydrothermal conditions to avoid dissolution of the silica foam at the high pH typically required for direct synthesis of LDHs (Scheme 1). While the method is applicable to a variety of LDH systems, we focus here on Mg-Al LDH as a model system. The formation of the LDHs on the silica foam was confirmed by X-ray diffraction (XRD) and transmission electron microsco- py (TEM). Figure 1 shows XRD patterns of the silica foam, bulk LDH, and the mesoporous LDH/silica foam nanocomposite. The broad scattering peak in Figure 1 a indicates the absence of a long-range-ordered structure in the silica foam. The (003), (006), and (110) reflections in Figure 1b are characteristic of an LDH with an interlayer spacing of 0.76 nm (2q ~ 11.78) and a unit cell of 0.305 nm, [17,18] and is consistent with an XRD pat- tern of reconstructed LDHs reported by Rocha et al. [19] The same reflections superimposed on the broad silica amorphous peak confirm the formation of the LDH on mesoporous silica [a] Dr. L. Fu, Dr. G. Qi, Dr. L. Estevez, Prof. E.P. Giannelis Department of Materials Science and Engineering Cornell University Ithaca, NY 14853 (USA) Fax: (+ 1) 607-255-2365 E-mail : epg2@cornell.edu [b] Dr. O. Shekhah, Dr. Y. Belmabkhout, Prof. M. Eddaoudi Advanced Membranes & Porous Materials Center King Abdullah University of Science and Technology Thuwal 23955 (Saudi Arabia) E-mail : mohamed.eddaoudi@kaust.edu.sa Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300973.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 1035 – 1037 1035 CHEMSUSCHEM COMMUNICATIONS